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The Pennsylvania State University
The Graduate School
Department of Physics
A SOLID-STATE HEAT PUMP USING ELECTROCALORIC
CERAMIC ELEMENTS
A Dissertation in
Physics
by
Matthew G. Hilt
c© 2009 Matthew G. Hilt
Submitted in Partial Fulfillmentof the Requirements
for the Degree of
Doctor of Philosophy
May 2009
The dissertation of Matthew G. Hilt was reviewed and approved* by the following:
J. D. MaynardProfessor of PhysicsDissertation AdviserChair of Committee
G. D. MahanProfessor of Physics
Peter SchifferProfessor of Physics
Victor SparrowProfessor of Acoustics
Jayanth BanavarProfessor of PhysicsHead of the Department of Physics
*Signatures are on file in the Graduate School.
iii
Abstract
The thermoacoustic cycle is a robust thermodynamic cycle that can be generalized
to describe and develop an all-solid-state heat pump using generic caloric elements.
Ferroelectric barium strontium titanate (BST) and relaxor lead magnesium niobate -
lead titanate (PMN-PT) are two candidate materials for the caloric elements using the
electrocaloric effect. I developed a procedure to repeatably produce high quality BST and
PMN-PT ceramics so that the electrocaloric and dielectric properties could be accurately
measured. The measured electrocaloric properties serve as the baseline numbers for
calculating the performance of a proposed all-solid-state cooler based on thermoacoustic
principles.
iv
Table of Contents
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii
Chapter 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1 Early history . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.2 Modern Refrigerators . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Thermoacoustics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2.1 Thermoacoustics, quantified . . . . . . . . . . . . . . . . . . . 9
1.3 Solid-state refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4 Perovskite ferroelectrics . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4.1 Ferroelectrics . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.2 Relaxors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.3 Ferroelectric thermodynamics . . . . . . . . . . . . . . . . . . 15
Chapter 2. General comments about ceramics and their processing . . . . . . . . 20
2.1 Preparing ceramic-grade powder . . . . . . . . . . . . . . . . . . . . 21
2.1.1 Mixing the components . . . . . . . . . . . . . . . . . . . . . 24
2.1.2 Calcining the powder . . . . . . . . . . . . . . . . . . . . . . . 27
v
2.1.3 Grinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.2 Preparing ceramic samples . . . . . . . . . . . . . . . . . . . . . . . . 31
2.2.1 Binder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.2.2 Pressing ceramic pellets . . . . . . . . . . . . . . . . . . . . . 33
2.2.3 Sintering the pellets . . . . . . . . . . . . . . . . . . . . . . . 35
2.3 Preparing samples for measurement . . . . . . . . . . . . . . . . . . . 38
2.3.1 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.4 Measurements on ceramic wafers . . . . . . . . . . . . . . . . . . . . 40
2.4.1 Physical property measurement methods . . . . . . . . . . . . 41
2.4.2 Electrical property measurement methods . . . . . . . . . . . 42
Chapter 3. Electrocaloric effect in barium strontium titanate . . . . . . . . . . . 47
3.1 Ferroelectric-Paraelectric transition in BST . . . . . . . . . . . . . . 48
3.2 Sample preparation protocol . . . . . . . . . . . . . . . . . . . . . . . 50
3.3 Electrical properties measurement . . . . . . . . . . . . . . . . . . . 56
3.4 Comparison to other results . . . . . . . . . . . . . . . . . . . . . . . 61
Chapter 4. PMN-PT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.1 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.2 Relaxor transition in PMN-PT . . . . . . . . . . . . . . . . . . . . . 67
4.2.1 Transition temperature for PMN-PT . . . . . . . . . . . . . . 67
4.3 Single crystal PMN-PT . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.4 Ceramic PMN-PT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.4.1 XRD of PMN-PT powder . . . . . . . . . . . . . . . . . . . . 75
vi
4.4.2 Electrical property measurements on ceramic PMN-PT . . . 76
4.4.3 Results for (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15 . . . . . 79
4.5 Comparison to other measurements . . . . . . . . . . . . . . . . . . . 82
Chapter 5. An electrocaloric solid state heat pump . . . . . . . . . . . . . . . . . 90
5.1 A theoretical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5.1.1 A quick review of thermoacoustics . . . . . . . . . . . . . . . 93
5.1.2 Generalizing the thermoacoustic cycle . . . . . . . . . . . . . 94
5.2 Description of device . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
5.3 Predicted performance using ferroelectric ceramic elements . . . . . . 102
Chapter 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Appendix A. Protocol for preparing ceramics . . . . . . . . . . . . . . . . . . . . 108
A.1 BST Protocol, July 2007 . . . . . . . . . . . . . . . . . . . . . . . . . 108
A.2 PMN-PT Protocol, July 2008 . . . . . . . . . . . . . . . . . . . . . . 109
A.2.1 Stage I: MgNb2O6 preparation . . . . . . . . . . . . . . . . . 109
A.2.2 Stage II: 0.92PMN- 0.08PT preparation . . . . . . . . . . . . 110
A.2.3 Stage III: Pressing 1/2” ceramic disks . . . . . . . . . . . . . 111
Appendix B. Perkin Elmer 4400 Operator’s Reference . . . . . . . . . . . . . . . 113
B.1 Start up procedure: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
B.1.1 Start up from shutdown: . . . . . . . . . . . . . . . . . . . . . 113
B.1.2 Start up from overnight: . . . . . . . . . . . . . . . . . . . . . 113
B.2 Automatically controlled operating procedures: . . . . . . . . . . . . 114
vii
B.3 Sputtering Procedures: . . . . . . . . . . . . . . . . . . . . . . . . . . 115
B.3.1 DC Sputter Deposition: . . . . . . . . . . . . . . . . . . . . . 115
B.3.2 RF Sputter Deposition: . . . . . . . . . . . . . . . . . . . . . 115
B.3.3 RF Sputter etching: . . . . . . . . . . . . . . . . . . . . . . . 116
B.4 Shutdown procedures: . . . . . . . . . . . . . . . . . . . . . . . . . . 117
B.4.1 Overnight: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
B.4.2 Longterm shutdown: . . . . . . . . . . . . . . . . . . . . . . . 117
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
viii
List of Tables
2.1 Compositional analysis of PMN-PT crystal . . . . . . . . . . . . . . . . 22
2.2 Furnace instructions for cleaning crucibles . . . . . . . . . . . . . . . . . 23
2.3 Powder to make 50 gm Ba0.67Sr0.33TiO3 . . . . . . . . . . . . . . . . . 25
2.4 Powder to make 50 gm MgNb2O6 . . . . . . . . . . . . . . . . . . . . . 25
2.5 Powder to make 50 gm (PbMg1/3Nb2/3O3)0.92 − (PbTiO3)0.08 . . . . 25
2.6 Furnace instructions for calcining BST . . . . . . . . . . . . . . . . . . . 29
2.7 Furnace instructions for calcining magnesium niobate . . . . . . . . . . . 29
2.8 Furnace instructions for calcining PMN-PT . . . . . . . . . . . . . . . . 29
2.9 Furnace instructions for binder burnout . . . . . . . . . . . . . . . . . . 33
2.10 Furnace instructions for sintering BST . . . . . . . . . . . . . . . . . . . 36
2.11 Furnace instructions for sintering PMN-PT . . . . . . . . . . . . . . . . 37
2.12 Furnace instructions for annealing PMN-PT ceramics . . . . . . . . . . 39
3.1 Expected and observed XRD angular peaks in BST . . . . . . . . . . . . 51
3.2 Corresponding grain sizes of different sintering conditions . . . . . . . . 56
4.1 Effect of crystal orientation on the electrocaloric effect in PMN-PT . . . 70
4.2 Compositional analysis of PMN-PT crystal . . . . . . . . . . . . . . . . 74
5.1 Cooling power at a given temperature span (lift) of a thermoacoustic-like
device operating at 30 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.2 SSHP performance using 1.0 K caloric elements . . . . . . . . . . . . . . 102
ix
5.3 SSHP performance using BST and PMN-PT ceramic elements . . . . . 103
x
List of Figures
1.1 The vapor compression cycle[1] . . . . . . . . . . . . . . . . . . . . . . . 6
1.2 The thermoacoustic cycle along a solid-fluid interface . . . . . . . . . . . 8
1.3 Perovskite crystal structure[27] . . . . . . . . . . . . . . . . . . . . . . . 12
1.4 Ferroelectric and relaxor polarization . . . . . . . . . . . . . . . . . . . . 14
2.1 Die used to press ceramic pellets . . . . . . . . . . . . . . . . . . . . . . 34
2.2 Setup to measure the electrocaloric properties of BST . . . . . . . . . . 43
2.3 Schematic of dielectric constant measurement . . . . . . . . . . . . . . . 44
3.1 A compilation of measured Curie temperatures in BST systems . . . . . 49
3.2 XRD analysis of BST powder . . . . . . . . . . . . . . . . . . . . . . . . 52
3.3 Particle size analysis of BST powders . . . . . . . . . . . . . . . . . . . . 54
3.4 SEM micrograph showing carbon contamination in BST ceramics . . . . 55
3.5 SEM micrographs of fracture cross-sections of BST ceramics . . . . . . . 57
3.6 Dielectric constant of BST ceramics with different electrodes . . . . . . 59
3.7 The dielectric constant of Ba0.67Sr0.33TiO3 . . . . . . . . . . . . . . . . 62
3.8 Electrocaloric effect in BST at 1 MV/m . . . . . . . . . . . . . . . . . . 63
3.9 Electrocaloric effect in BST as a function of temperature and applied field 64
4.1 Phase diagram for PMN-PT mixtures[62] . . . . . . . . . . . . . . . . . 68
4.2 Mask for electroding rectangular plates . . . . . . . . . . . . . . . . . . . 71
4.3 High temperature electrocaloric measurement setup . . . . . . . . . . . . 73
xi
4.4 Electrocaloric effect in single crystal (PbMg1/3Nb2/3O3)0.8 − (PbTiO3)0.2 74
4.5 XRD of calcined 0.90PMN-0.10PT powder . . . . . . . . . . . . . . . . . 77
4.6 XRD of a sintered 0.90PMN-0.10PT ceramic . . . . . . . . . . . . . . . 78
4.7 Effect of different electrodes on dielectric constant in ceramic 0.85PMN-
0.15PT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.8 Effect of different electrodes on electric effect in ceramic 0.85PMN-0.15PT 81
4.9 Dielectric constant of ceramic (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15 . 83
4.10 Electrocaloric effect in ceramic (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15 84
4.11 Strip chart recording of the electrocaloric effect in PMN-PT . . . . . . . 86
4.12 Strip chart recording of joule heating in a ceramic sample . . . . . . . . 87
5.1 A heat switching heat pump . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.2 The thermoacoustic cycle . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5.3 A generalized Stirling-like heat pump cycle . . . . . . . . . . . . . . . . 95
5.4 Proposed design of all solid state heat pump . . . . . . . . . . . . . . . . 101
xii
Acknowledgments
I would like to thank my advisor and my committee for their guidance and their
assistance throughout my graduate research. I would like to thank Omega Piezo Tech-
nologies, Inc. for technical support and the use of their facilities while preparing mate-
rials. I would like to thank the technicians at the Materials Research Lab for countless
hours of discussion and help refining my sample preparation and characterization tech-
niques. Finally, I would like to thank my patient wife for supporting me through the
completion of this research.
1
Chapter 1
Introduction
Perhaps the most influential technological developments in human history are
agriculture, medicine, and the atomic theory of matter. Supplementing all of these great
advances, refrigeration has emerged as one of the greatest recent advancements. Refriger-
ated storage allows for better food preservation and transportation over longer distances,
mitigating the effects of seasonal variance in farming conditions. Local droughts and re-
gional crop failures no longer lead to mass starvation and famine. The preservation
of medicines and vaccines have led to radical advances in the treatment and even the
elimination of many diseases. Study of matter at its coldest temperatures has given us
insight in to fundamental interactions of the structure of matter. Despite this progress,
problems arising from our implementation of refrigeration are threatening each of these
advances.
For over one hundred years vapor compression techniques have been the gold
standard for the design of refrigeration and air conditioning systems. The most efficient
fluids to drive these systems were Freon gases or chlorofluorocarbons (CFCs). As CFCs
entered the atmosphere they catalyzed a reaction with the atmospheric ozone, depleting
the natural barrier against ultraviolet (UV) radiation. The rise in UV radiation exposure
has led to increased rates of skin cancers, impacting public health[9]. Eventually the
world adopted the Montreal Protocol effectively banning CFC use.
2
Though the replacement for CFCs, hydrochlorofluorocarbons (HCFCs), do not
deplete the ozone, they feed a growing problem: atmospheric global warming. While
carbon dioxide is the most publicized contributor to global warming, HCFCs have global
warming potential 3000 times that of an equivalent volume of carbon dioxide[14]. New
advancements in refrigeration are needed to extend the benefit of artificial cooling, but
also limit, eliminate, or hopefully reverse the disastrous environmental consequences.
Liquids and solids are attractive candidate media for new refrigeration techniques, both
because their higher mass density allows a higher energy density, and because they avoid
the release of harmful gases. The main drawbacks to current alternatives are twofold:
energy efficiency and scalability. This thesis explores an all-solid-state alternative to
existing refrigeration technology with several target goals:
1. Identify an appropriate solid-state caloric effect which could be used in a scalable
refrigeration device.
2. Find a specific material which maximizes the magnitude of the solid-state caloric
effect.
3. Determine a design for a device incorporating the material so that its caloric prop-
erty results in a temperature lift and cooling power suitable for a real refrigerator.
To acheive target 1, the remainder of this chapter reviews various refrigeration
schemes and shows that a property of solids referred to as the “electrocaloric effect” could
be used for refrigeration. For target 2, a search of the literature indicated that there
were some electrocaloric materials which could be used to make a real, practical refriger-
ator. Specifically, materials literature implied or reported extremely large values of the
3
electrocaloric effect in both barium strontium titanate, BaxSr1−xTiO3, (BST) and lead
magnesium niobate-lead titanate, (PbMg1/3Nb2/3O3)x−(PbTiO3)1−x, (PMN-PT). At
this point it was believed that one could simply locate a company capable of making
these materials and order a quantity suitable for research. With considerable suprise,
it was discovered that no such company is currently in existance worldwide; a company
proposed to use its “best effort” to make the desired material but fell considerably short.
The results are documented in Chapter 2.
A search to collect the necessary information for fabricating these materials “in
house” was undertaken; this proved to be far more difficult than it should have been.
Many papers omitted what turned out to be critical steps or reported procedures that
were at odds with every other protocol in the field. At least one thesis presumably con-
taining detailed steps was not made available even after both a direct, informal request
and a formal request through the library of the granting institution. Despite the diffi-
culties, excellent materials, which were in many aspects superior to those found in the
materials literature, were made in house. Contrary to what seems to be the apparent
practice in the field of materials research, the full details, as well as general concepts of
the materials processing are provided in Chapter 2 and in Appendix A.
It was with further dismay that the values of the electrocaloric effect measured in
the high quality materials of this thesis were somewhat smaller than values claimed in
some of the materials literature. This seems to be another feature of materials literature:
papers which report unbelievable results should, in fact, not be believed. The reputation
of the research lab preparing the material must be known before any result reported in the
4
materials literature can be accepted. Another problem with some results in the literature
may have been poor practices used to measure and report the material properties.
In this thesis, the methods used to characterize the materials followed the high
standards of condensed matter and low temperature physics, and are reported in full
detail in latter part of Chapter 2. Furthermore, the raw data are presented unmodified,
and any calculations based on the data are fully explained. Data from measurements on
BST samples are presented in Chapter 3 and data from PMN-PT samples are presented
in Chapter 4.
Concerning target 3, a device based on the “thermoacoustic effect” [discussed
below] was designed. However, setbacks encountered from having to reproduce and
verify the materials literature precluded the construction of an actual device. A first
principles theory of operation was developed to predict the performance of the device
and an analysis of using BST and PMN-PT as caloric elements in such a device is
presented in Chapter 5.
To properly introduce a conceptually new heat pump, one must have a background
on refrigeration techniques and the relevant thermodynamics of the electrocaloric effect,
and this is presented next.
1.1 Refrigeration
Any physical mechanism which can displace heat further than it will naturally
diffuse can be adapted to form a heat pump or refrigerator. Many thermodynamic effects
exist which can repeatedly generate changes in heat. A most fundamental difference
5
between all forms of heat engine or pump is the manner in which heat is used to do work
or the manner in which work is done to move heat.
1.1.1 Early history
Early examples of heat engines based off natural work are waterfalls and wind-
mills. Evaporation and condensation do work against gravity, creating a reservoir of
water at a higher altitude. As the water flows downhill to a reservoir at a lower grav-
itational potential, the current is used to drive mechanical wheels. Windmills similarly
rely on rotors driven by a fluid flow, though the moving gas comes from nonuniformity
in atmospheric temperature and pressure.
The earliest forms of refrigeration were also based on natural mechanisms. An-
cient Egyptians used evaporative cooling systems[12]. Advances in shipping and land
transportation lead to a boom in the ice trade during the eighteenth and nineteenth
centuries[12]. While ice has been stored under insulation since ancient times, by the
eighteenth century it could be transported from colder regions. Ice was sold in large
blocks which were placed in chests, creating a reservoir at 0 C. A great boom in refrig-
eration came at the end of the nineteenth century with the advent of mechanical vapor
compression refrigerators.
1.1.2 Modern Refrigerators
The refrigerator as it is currently known started with the vapor compression cycle
in the 1850s with ammonia-based refrigerants. In the 1920s, CFC gasses were discovered
with very large thermal expansion coefficients near room temperature. This expansion
6
Fig. 1.1. The vapor compression cycle[1]
7
was exploited by compressing the gas in one location and then allowing the gas to expand
through a long network of tubes. A schematic of a vapor compression refrigerator is
shown in figure 1.1[1]. A mechanical compressor condensed the CFC gas, dumping heat
into the environment. The condensed gas was then forced through a long network of
coils wherein the gas expanded drawing heat into the gas from the coils. The gas was
then returned to the compressor. As the compressor is maintained at room temperature,
the free expansion of the gas in the coils cools the coils below room temperature.
Though the technology is over 150 years old, vapor compression refrigeration is
still the workhorse in most refrigeration applications. CFC gas has been banned by the
Montreal Protocol due to the catalyzing effect CFCs have in breaking down atmospheric
ozone[9]. A different class of gases, HCFCs, had less ideal thermodynamic and transport
properties; however, they were good substitutes for CFC gases in vapor compression
refrigerators. Unfortunately, because of the relatively low concentration of HCFC gases
in the atmosphere, the infrared absorption spectrum of HCFC gases make them 1000 to
3000 times worse than carbon dioxide as green house gases. As HCFCs are emitted, they
settle in the upper atmosphere. HCFCs are man-made; they have never been produced
naturally. The photoabsorbtion spectrum of HCFCs is different than carbon dioxide,
with the result that light that had previously passed through the atmosphere is now
being absorbed. As the atmosphere is normally 1% carbon dioxide, additional releases
of carbon dioxide are only slowly increasing the amount of light absorbed because it is in
the same frequency spectrum that has always been absorbed. More nations are focussing
on legislation to limit emissions to combat global warming, and more research will be
needed in alternate refrigeration cycles based on completely environmentally inert fluids
8
or solid state techniques. One technology using environmentally-safe fluids as a working
medium is thermoacoustics.
1.2 Thermoacoustics
A simple picture of a sound wave is a small pressure oscillation within a fluid
about some equilibrium coordinate. Considering the ideal gas law, PV = nRT , if
volume were held constant that pressure oscillation would correspond to a temperature
oscillation. This is not a useful means of transporting heat, however, because these
oscillations happen on a time scale faster than the heat is able to be displaced. Normal
sound propagation is therefore adiabatic.
Fig. 1.2. The thermoacoustic cycle along a solid-fluid interface
9
If the parcel of gas is placed next to a solid plate, parallel to the displacement
of the gas, the gas is able to transfer heat to and from the solid. The gas can expand
above the plate, drawing a small amount of heat from the solid. The gas then moves and
compresses further down the plate, dumping a small amount of heat into the solid. The
gas then returns to its initial position, repeating the cycle. This is the thermoacoustic
effect and the cycle is shown in figure 1.2. The displaced heat would naturally travel back
down the solid to restore thermal equilibrium. However, the gas displaces more heat per
unit time than the solid is able to transport, breaking a symmetry. On either side of
the gas parcel is another gas parcel, and each individual temperature oscillation acts as
a bucket brigade converting the small local temperature oscillations into a temperature
gradient along the length of the solid. Quantitatively, the presence of the fluid-solid
interface introduces a phase shift between the pressure oscialltion, pa, and the volume
oscillation, ua.
1.2.1 Thermoacoustics, quantified
Using the approximation that a fluid is oscillating parallel to a solid plate in a very
open channel, the cooling power for a thermoacoustic cycle is found by the relation[50]:
Q =12ΥδκTmβppaua(1−RT ) (1.1)
where Υ is the perimeter of the channel, δκ =√
2κ/ρω is the thermal penetration depth,
Tm is the average temperature, βp is the thermal expansion coefficient of the fluid, pa is
the amplitude of the pressure oscillation, ua is the amplitude of the velocity oscillation
10
and RT is the ratio of established temperature gradient to a characteristic or critical
temperature gradient.
The acoustic wave equations have been more properly solved including thermal
and viscous loss effects and geometries beyond open parallel plates[50, 51]. As devices
are being scaled to kilowatt levels of refrigeration power, the thermoacoustic equations
are being tested to incorporate non-linear effects such as mass streaming[46]. In addition
to switching to inert gases using thermoacoustic refrigerators, research into alternatives
to vapor compression refrigeration has focussed on solid-state refrigeration.
1.3 Solid-state refrigeration
Most existing solid-state coolers are based on one of two physical effects: thermo-
electrics and adiabatic demagnetization of magnetocaloric materials. The basis of the
thermoelectric effect is the work of Seebeck, Peltier and Thomson during the mid-1800s.
When a junction of dissimilar metals experience a temperature gradient, a small cur-
rent is generated. This effect is reversible, where the flow of a current through a metal
junction warms or cools the junction. Advances in the 1950s led to the discovery of
semiconductor-based thermoelectric materials which allowed the creation of coolers ca-
pable of operating at temperature spans of over 30 K. Ongoing thermoelectric research is
seeking to tailor the physical properties of semiconducting thermoelectric materials with
the ultimate goal of achieving larger temperature changes with smaller applied currents
in an effort to overcome the Joule heating limits of thermoelectric materials[30].
Magnetocalorics is the interaction of magnetism and heat. On the application
of an external magnetic field, the magnetic domains in a ferromagnetic material orient
11
along the direction of the applied field, raising the entropy and therefore the temperature
in an isolated system. As the magnetic field is removed, the magnetic dipoles are able
to relax and absorb some heat from the materials crystal lattice, lowering the material’s
temperature. The largest observed effects occur in gadolinium-based composites with 11
to 17 K magnetocaloric temperature changes occurring upon the application and removal
of up to 5 T magnetic fields[21, 54, 61, 69].
Cryogenic adiabatic demagnetization refrigerators have been built to continuously
maintain a sensor at 4 K, cooled from 10 K[11]. Current magnetocaloric refrigerators
need either large superconducting magnets or heavy permanent magnet arrays to gen-
erate the large magnetic fields needed to run the devices[21]. Even these devices can
only cool over a 20 to 30 K span. A promising alternative to magnetocaloric ferromag-
netic materials are electrocaloric ferroelectric materials. Most of the interesting new
electrocalorics come from a class of materials known as perovskites.
1.4 Perovskite ferroelectrics
Perovskite refers to both the mineral perovskite CaTiO3 and crystalline material
with cubic structure and the generic unit cell ABO3. Oxygen atoms form an octahedral
cage around the B-site ions while the A-site ions form a simple cubic lattice, as shown
in figure 1.3. The atoms in the B-site “cages” can often be pulled slightly above and
below the equilibrium position which give rise to polarizability and ferroelectric and
paraelectric properties.
12
Fig. 1.3. Perovskite crystal structure[27]
13
1.4.1 Ferroelectrics
Ferroelectricity is analogous to ferromagnetism in magnetic materials. Weak per-
turbative fields lead to the formation of permanantly polarized electric dipole domains
at low temperatures. As the temperature reaches the Curie temperature, the ferroelec-
tric ordering is quickly lost and the electric domains enter a paraelectric phase. In the
paraelectric phases, the electric dipoles are highly mobile and align in response to an
applied electric field, but will not maintain a net polarization when the field is removed.
1.4.2 Relaxors
What separates relaxor materials from similar ferroelectric materials is the nature
of the phase transition. As the temperature is lowered below the Curie temperature,
ferroelectrics see a sudden rise in permanent polarization that begins to saturate about
10 K below the Curie temperature. This rise manifests itself as a very sharp peak
in the dielectric constant, often occurring in the span of 20 K and almost no thermal
hysteresis. Relaxors, on the other hand, see a gradual rise in polarization at the Curie
temperature followed by a transition to a ferroelectric-like rise in polarization below the
Curie temperature. This manifests itself as a very broad peak in the dielectric constant
with a strong hysteresis and a dependence on the frequency at which the dielectric
constant is measured. These two behaviors are shown in figure 1.4. The data have been
scaled to normalize dielectric peak heights, but the temperature data is not scaled. The
ferroelectric dielectric constant data have been plotted as the dashed curve towards the
left hand side of the graph. The relaxor dielectric constant data are plotted as the solid
curve towards the right hand side of the graph. The ferroelectric has a Curie temperature
14
Fig. 1.4. Ferroelectric and relaxor polarization
15
of 21 C. The relaxor has a freezing temperature which occurs around 100 C and a Curie
temperature which occurs at 72 C. For both the ferroeletric and the relaxor, the lower
curve represents data taken upon heating the sample and the upper curve represents data
taken upon cooling the sample. Arrows have been imposed over the data to indicate the
curves corresponding to heating and cooling the sample. Both samples were measured
at a frequency of 1000 Hz.
Relaxors are analogous to a spin glass in magnetic systems. The Curie tempera-
ture refers to the temperature at which the onset of permanent polarization begins; the
temperature at which the transition to a ferroelectric-like rise in polarization occurs is
called the “freezing” temperature, a term borrowed from spin-glass literature.
Another characteristic of relaxors is a frequency dependence of the dielectric con-
stant. In the region of the phase transition, a larger dielectric constant is measured at
lower frequencies. Above the transition region, the dielectric constant is independent
of frequency. The divergent results typically begin at the Curie temperature and the
dielectric constant is largest at the freezing temperature.
1.4.3 Ferroelectric thermodynamics
In any material, there exists an electric susceptibility describing the capacitance
of a parallel-plate capacitor made with that material between the plates. The dielectric
constant, ε is found from the relation:
ε =
(∂D
∂Eq
)
T
(1.2)
16
where D is the electric displacement, Eq is the electric field[17].
Ferroelectrics exhibit very large dielectric properties near the phase transition. In
ferroelectric materials, there is a permanent polarization, Ps, in addition to the electric
displacement. In the following discussions on ferroelectrics, polarization P refers to the
total contribution, defined as:
P = Ps + κEq = Ps + (1 + ε)εoEq (1.3)
The temperature derivative of polarization is called the pyroelectric coefficient, pq, given
as:
pq =(
∂P
∂T
)
Eq
. (1.4)
Given that the polarization is the charge per unit area, consider polarization changing
as a function of time in the material. Expanding the derivative:
dP
dt=
1A
dq
dt=
dP
dT
dT
dt(1.5)
where A is the area. We call Ipyro = dq/dt the pyroelectric current. We can rearrange
this equation,
Ipyro = A(
∂P
∂T
)
Eq
dT
dt= Apq
dT
dt(1.6)
to show the dependence of the pyroelectric current on the pyroelectric coefficient and a
time-dependent temperature. The converse effect of pyroelectricity is the electrocaloric
effect.
17
The electrocaloric effect is derived by considering a temperature change resulting
from the application of an electric field at constant entropy. An electrocaloric coefficient,
Υq can be defined as follows:
Υq(Eq) =
(∂T
∂Eq
)
S
=∂(T, S)
∂(Eq, S))=
∂(Eq, T )
∂(Eq, S)∂(T, S)∂(Eq, T )
= T
1
T(
∂S∂T
)Eq
(∂S
∂Eq
)
T
=T
CEq
(∂S
∂Eq
)
T
(1.7)
where CEqis the heat capacity at constant electric field and S is the entropy. Through
a Maxwell relation we find that
(∂S
∂Eq
)
T
=(
∂P
∂T
)
Eq
= pq. (1.8)
To find an electrocaloric temperature change, ∆TEqresulting from the application of an
electric field, Eq, integrate the electrocaloric coefficient in equation 1.7,
∆TEC =∫ Eq
oΥqdE′
q=
∫ Eq
0
T
CEq
pq(E′q)dE′
q' T
CEq
∫ Eq
0pq(E
′q)dE′
q. (1.9)
It has been suggested[20] that below the ferroelectric transition temperature, the
pyroelectric coefficient is independent of electric field. Using this assumption, equa-
tion 1.9 simplifies to
∆TEC =T
CEq
pqEq (1.10)
and the observed electrocaloric effect should be linear with respect to the applied field.
18
Well above the critical region the material should be paraelectric and there should
be no remnant polarization. The only contribution from the polarization would then be
P = εεoEq (1.11)
where ε is the dielectric constant defined in equation 1.2 and εo is the permittivity of
free space. Integrating equation 1.9 using this assumption yields
∆T ' εo2
T
CEq
∂ε
∂TE2
q. (1.12)
Equation 1.12 implies that the electrocaloric effect should increase as the square of the
applied field above the transition temperature.
The polarization would be more properly defined by including the internal con-
tribution of the polarization using the electric displacement Dq defined as
Dq = εoEq + εεoEq. (1.13)
The dielectric susceptibility, χ, is defined as
χ = 1 + ε. (1.14)
Using χ to define the electrocaloric effect in the perovskite ceramics is not necessary,
however, because ε tends to be very large. At the Curie temperature in BST, ε is 20,000
or larger. Even well outside the critical region ε is typically over 1,000.
19
Just a fundamental description of the thermodynamics of ferroelectric materials
is not enough to produce electrocaloric materials. An extensive study in the process
of preparing high-quality ceramic materials was undertaken to produce electrocaloric
ceramics capable testing the electrocaloric effect as a refrigeration mechanism. A pro-
tocol to prepare two ferroelectric ceramics, barium strontium titanate (BST) and lead
magnesium niobate-lead titanate (PMN-PT), was developed. Samples of both BST and
PMN-PT were measured to determine the maximum possible electrocaloric effects. Fi-
nally, a complete theoretical framework and a design for an electrocaloric refrigerator
was developed to evaluate the practicality of using the electrocaloric effect in a solid-state
refrigerator.
20
Chapter 2
General comments about ceramics and their processing
To prepare research-grade ceramics, one must pay careful attention to seemingly
insignificant details. Often exotic elements with complex reaction mechanisms are com-
bined in a precise order to form a new material. Impurities on the order of 1 part in
1000 can radically alter ceramic properties. Even the materials forming electrodes for
measurements of electrical properties can be the difference between observing a physical
effect and observing practically nothing. Through an exhaustive process, a protocol to
reproduce high-quality ferroelectric ceramics was established. Protocols for both barium
strontium titanate (BST) and lead magnesium niobate-lead titanate (PMN-PT) were
developed through this thesis; step-by-step details of both protocols are presented as
Appendix A.
BST, BaxSr1−xTiO3, is made in a single calcination process using barium car-
bonate, BaCO3, and strontium carbonate, SrCO3, and titanium dioxide, TiO2. The
decomposition of the carbonates in to metal oxide and carbon dioxide regulates the re-
action of the barium oxide, strontium oxide and titanium dioxide so that the powder
ends up in the proper crystalline phase.
PMN-PT, (PbMg1/3Nb2/3O3)x−(PbTiO3)1−x, must be made in a double cal-
cination process to avoid unwanted crystalline phases. First, magnesium oxide, MgO,
and niobium (V) oxide, Nb2O5 are reacted to form magnesium niobate, MgNb2O5. The
21
magnesium niobate (MN) is then mixed with lead oxide, PbO, and titanium dioxide
and reacted to form PMN-PT. The preparation of high-quality ceramics begins with
preparing high-quality reacted powder.
2.1 Preparing ceramic-grade powder
Many considerations must be taken into account to make high-quality, ceramic-
grade powder. Proper stoichiometry is essential to ensure that there is no batch-to-batch
variance in sample properties. Many steps involve transfer of ingredients and exposure
to solvents, each of which runs a risk of contaminating the powder. Even uncontrolled
environmental factors, such as loss of air conditioning during powder processing can
produce unexpected results.
Ferroelectric properties, such as the phase transition temperature, depend highly
on mixing components in the proper ratio. Table 4.2 shows the full compositional analysis
of a PMN-PT single crystal that had a phase transition at an unexpected temperature.
The ratio of ions suggest that this is (PbMg1/3Nb2/3O3)0.82 − (PbTiO3)0.18, however,
using this concentration the sample has a magnesium deficiency of 22%. Theoretically,
the transition temperature should occur at 88 C; the transition in the sample occurred
near 141 C. Note, the calculation to find the composition assumes the number of oxygen
molecules in the unit cell and then finds the amounts of other ions present relative to
the assumed number of oxygen molecules.
Small levels of contaminants can cause other undesirable effects. Some of our
early ferroelectric samples exhibited the behavior of a semi-conductor. The resistivity of
BST should be greater than 1013 Ω−m. When measured under the small bias field of
22
Ion Number per unit cell Expected Percent differencePb 0.98 1.00 2.1Mg 0.21 0.27 21.8Nb 0.57 0.55 3.6Ti 0.20 0.18 8.3O 3 3 0
Table 2.1. Compositional analysis of PMN-PT crystal
a digital multimeter, the sample resistivity was 109 Ω−m or greater. Under an electric
field of 0.2 MV/m the resistivity dropped to 104 Ω−m. Further analysis showed these
samples were contaminated with < 0.5% mol of calcium. The calcium contamination was
eventually traced to crucibles that had been washed in State College water, a richer source
of calcium than milk. To avoid contamination, all tools, crucibles, storage containers and
mixing vessels that contact powder should be cleaned with the following procedure: wash
the item in reagent grade acetone (with a purity greater than 99.5%) then dry the item
with helium gas. Any component that will go inside a furnace, such as a crucible, should
be cleaned as follows: heat the item to 1200 C in a furnace and let it set for 6 hours at
that temperature. This procedure will help burn off any contaminants that may have
become bound to the surface of the object. Any object used in the making of lead-free
powders must only be used to make lead-free powders. If a furnace has ever been used in
the production of lead-based powders, it must be assumed to be contaminated with lead
and can be used for only the production of lead-based powders. Also, any tool, crucible,
storage container or other item used in the making of a lead-based powder must be used
only in the making of other lead-based powders.
23
All powders and ceramics were prepared in a Sentro-Tech model ST-1600 furnace
with a programmable interface. To clean crucibles the program in table 2.2 should be
used. The C0X instructions are temperature set points in C and the T0X instructions
are times in minutes. The programming logic is: temperature, time to next temperature,
repeat. A time instruction of -121 is the furnace’s code to shutdown.
Instruction SettingC01 0T01 200C02 1400T02 200C03 1400T03 200C04 0T04 -121
Table 2.2. Furnace instructions for cleaning crucibles
In addition to a clean working environment, high-quality ingredients are needed
to produce high-quality powder. All metal-oxide powders used to create the ceramic
powders should be reagent grade or better. All solvents throughout the process also
need to be reagent grade or better. No water should ever be allowed to contact powder
of any kind.
Even the water vapor in a humidity-controlled air conditioned environment can
degrade BST ceramics. If a BST ceramic wafer is left exposed to air for six months, the
skin of the pellet will change from a light brown color to a dark brown color, and then
24
eventually turn purple. All powders should be stored in sealed containers. Ceramics
without electrodes should be stored in a desiccator. With close attention to cleanliness,
purity and environmental control, the process to make ceramic samples begins with the
mixing of constituent metal oxide powders.
2.1.1 Mixing the components
Given all of the attention that must be paid to consistently produce powder with
similar properties, the mixing of the constituent metal oxide powders is perhaps the
riskiest step in the procedure. To minimize the risk of contaminating the powder, all
constituent metal oxide precursors should be massed onto a fresh waxed massing paper
using a balance sensitive to a tenth of a milligram. The powders required to make BST
are listed in table 2.3. The total amount adds up to more than 50 grams because the
recipe yields 50 grams of reacted powder and 10.1 grams of carbon dioxide is given off
as exhaust. Since PMN-PT is made in two stages, the mixture amounts for making
magnesium niobate are presented separately in table 2.4 for the mixture amounts for
0.92PMN-0.08PT in table 2.5. The MN numbers are slightly larger than 50 grams
because the calculation is performed for mixing in a ratio of 1 mol of MgO to 2 mol of
Nb2O5. It was previously found that in addition to a two-stage reaction the addition of
2% mol of MgO to the stoichiometric ratio almost prevents unwanted pyrochlore phases
of PMN-PT from forming[48, 49].
A 1 l Nalgene bottle should be prepared with a layer of duct tape wrapped twice
around both the top and bottom of the wide part of the bottle. This will aid in giving
the bottle better traction as the powder mixes. After the powders are massed they are
25
Component Mass (gm)BaCO3 30.364SrCO3 11.358TiO2 18.436
Table 2.3. Powder to make 50 gm Ba0.67Sr0.33TiO3
Component Mass (gm)MgO 6.715
Nb2O5 43.421
Table 2.4. Powder to make 50 gm MgNb2O6
Component Mass (gm)PbO 34.501
MgNb2O6 14.549TiO2 0.988
Table 2.5. Powder to make 50 gm (PbMg1/3Nb2/3O3)0.92 − (PbTiO3)0.08
26
placed in the prepared bottle. Once a bottle is used for mixing a particular powder, that
bottle should be used only for mixing that same powder in all future mixings. For BST,
150 ml of reagent- grade acetone should be added to the powder to form a slurry and
approximately 450 ml of yttria-stabilized zirconia spheres should be added to the slurry.
For MN and PMN-PT, 75 ml of reagent-grade ethanol should be added to the powder to
form a slurry and then approximately 220 ml of yttria-stabilized zirconia spheres should
be added to the slurry. In both cases, the level of the spheres should be just below the
level of the slurry. More solvent or spheres can be added if needed to get the proper
level.
Once the proper slurry level is achieved, the bottle should be sealed and placed
on a roller mill to mix the slurry. The mill should be set so that the bottle rotates
at about 22 rpm. For the 1 l Nalgene bottles, this is a roller speed of approximately
44 rpm. The roller mill has a spring-loaded nylon bolt to maintain a downward force
on the bottle to improve traction. Tension should be adjusted so that the bottle rolls
smoothly with minimal slipping. At the proper speed, the grinding media should be
pulled about one-quarter of the way up the side of the bottle as it rotates. The powder
should be mixed for at least 12 hours.
Once the mixing has completed, the slurry must be recovered from the bottle and
the media and the powder must be dried. The slurry should be poured from the Nalgene
bottle through a stainless steel strainer into a large Pyrex mixing bowl. The strainer
will separate the zirconia media from the slurry. The Nalgene bottle should be rinsed
twice with approximately 25 ml of solvent and the bottle should be drained through
the strainer. Additionally, about 50 ml of solvent should be drizzled over the zirconia
27
media in the strainer to wash any remaining powder off the media. It is important to not
use too much solvent because this will prolong the drying time, increasing the powder’s
exposure to possible contamination. The media in the strainer should be carefully shaken
over the Pyrex bowl to promote the drip drying of the media. The strainer should then
be set aside to allow the media to finish drying.
To dry the powder, the mixing bowl of slurry should be placed under a 60 W
incandescent lamp. As the powder dries, it must be stirred every 15 minutes to prevent
the constituent powders from separating as the slurry dries and settles. Once the slurry
has dried to a paste-like consistency, stirring frequency can be reduced to once per hour
to help complete the drying. Large chunks should be broken up with a stainless steel
spatula. Once the powder has finished drying, the dried chunks should be broken up in
an agate mortar. The BST and MN powder should be placed in a clean yttria-stabilized
zirconia crucible. PMN-PT should be placed in an alumina crucible, that has previously
been seasoned with lead-based powder. The powder is now ready for calcination.
2.1.2 Calcining the powder
Calcination is the process of heating mixed metal oxide powders to near their
melting points to catalyze a solid state reaction which results in the formation of small
crystals of BST, MN, or PMN-PT. The temperatures and duration of calcination is
chosen as the ideal conditions under which perovskite structured crystals will form[23,
48, 49]. All calcination was carried out in a Sentro-Tech Model ST-1600 furnace. The
BST calcination program is found in table 2.6. The calcination program for MN is found
in table 2.7. The calcination program for PMN-PT is found in table 2.8. Calcining for
28
longer times and at higher temperatures can result in the growth of larger crystals which
will take significantly longer to mill.
After the powder has been calcined, x-ray diffraction (XRD) measurements should
be performed on the powder to verify that the reaction has completed and that the pow-
der is not contaminated. According to the Powder Diffraction File (PDF-4) maintained
by the International Center for Diffraction Data (ICDD), BST powder should have an
XRD pattern with six strong peaks at angles of 2θ equal to 22.399o, 31.866o, 39.312o,
45.733o, 51.486o, and 56.802o. Extraneous peaks in BST data are often the result of
unreacted carbonate powders. Good BST powder should also have a white color.
According to PDF-4, PMN-PT powder should have one strong central peak at an
angle of 2θ equal to 32.3o and two smaller peaks at angles of 22.1o and 38.8o. Peaks at
angles from 30.2o to 30.5o often accompany powder with a red-yellow salt and pepper
color. These peaks are the result of unreacted lead oxide. Good PMN-PT powder
should have a faint yellow color. Non-ferroelectric pyrochlore phased material, such as
Pb6MgNb6O23 can also form during the PMN-PT reaction. This pyrochlore will show
up in the XRD spectrum with peaks at 29.2o and 33.9o. Small amounts of pyrochlore in
the starting powder often disappear during the sintering process. However, if pyrochlore
is present in the powder, XRD should be performed on sintered ceramics to verify that
this reaction has completed. The XRD results for BST are presented in Chapter 3 and
the results for PMN-PT are presented in Chapter 4. If the calcined powders pass both
visual and XRD inspection, they are ready to be ground to a fine size.
29
Instruction SettingC01 0T01 240C02 1200T02 360C03 1200T03 240C04 0T04 -121
Table 2.6. Furnace instructions for calcining BST
Instruction SettingC01 0T01 200C02 1000T02 360C03 1000T03 200C04 0T04 -121
Table 2.7. Furnace instructions for calcining magnesium niobate
Instruction SettingC01 0T01 160C02 800T02 600C03 800T03 160C04 0T04 -121
Table 2.8. Furnace instructions for calcining PMN-PT
30
2.1.3 Grinding
As the calcined powder comes out of the furnace, it is typically comprised of
polycrystalline granules several microns in size. In order to form a dense ceramic, the
reacted powder will need to be packed as tightly together as possible. The ideal packing
density is between 55 and 60% of the single crystal density. To accomplish this packing,
the powder must be ground to a fine size of 100 nm to 1 µm. A distribution of particle
sizes can help improve the packing packing density, as will be discussed in section 3.2.
The same 1 l Nalgene bottle in which the powder was mixed should be used for
grinding. The bottle should be free of unreacted powder and dry. Pour the calcined
powder into the bottle and add approximately 150 ml of zirconia spheres to the bottle.
Seal the Nalgene bottle and place it in an empty can on the vibratory mill. Secure the
bottle in place and grind for 4 hours. After 4 hours, stop the mill and add solvent to the
powder: 150 ml of acetone for BST or 75 ml of ethanol for either MN or PMN-PT. The
level of the zirconia media should be just below the level of the solvent; add more media
to the bottle if needed. Replace the bottle in the mill and grind for an additional 6 hours.
After grinding, retrieve the powder in the same manner as outlined in section 2.1.1. It is
not essential to frequently stir the ground, calcined powder as it should be homogenous
and settling is not an issue. Once the powder has formed a thick paste, however, it
should be stirred to complete the drying.
After the powder is dry, it should be in large, solid chunks. These chunks should
be broken up in an agate mortar and the powder should be filtered through a #50 sieve.
It may be necessary to grind the powder several times in the mortar to allow it to pass
31
through the sieve. Using a recipe to make 50 grams of powder, at least 45 grams of
reacted powder should be recovered after this step. After sieving, the ground powder
should be stored in a clean 125 ml Nalgene bottle. The lid should be secured tightly.
If the powder will not be used immediately or if the atmosphere is not environmentally
controlled, the powder should be stored in a desiccator or under vacuum.
2.2 Preparing ceramic samples
The first step in forming a ceramic from the calcined powder is to shape the
powder by pressing it into what is known as a “green pellet”. Here, “green” refers to
any pressed powder pellet that has not been fired in a furnace. To form a dense ceramic,
it is often necessary to create a green pellet with a density of 55 to 60% of the single
crystal density so that the powder is close enough to condense during firing. To aid
in the formation of high density green pellets, organic binders are often added to the
powders.
2.2.1 Binder
The addition of organic binder to ceramic powders as an aid to forming green parts
is a common industrial practice for the manufacture of most ceramics. The binder helps
lubricate both the powder and the die as the pellet is being shaped, minimizing stress
gradients in the green pellets and allowing tighter packing. Binders typically improve
cohesion of the unsintered pellets making them easier to handle.
The most common binders are poly vinyl alcohol (PVA) and poly acrylate (PA).
PVA is water soluble and often requires a dispersant to properly bond with ceramic
32
powder. Dispersants often contain elements such as sodium which can interfere with
electrical properties. PA is acetone soluble and requires no dispersants to bond to PMN-
PT. As a result, PA was used as the binder for making PMN-PT ceramics. Because
of the reactivity of barium, the process of adding binder was omitted for making BST
ceramics.
Binder typically constitutes as little as 1% of the total weight of pressed pellets
to as much as 40% of the weight of pressed pellets for some special ceramic applications.
For making PMN-PT, 4% weight of liquid binder is added to the powder, which becomes
1.5 to 1.7% of the total weight of the binder plus powder once the binder has dried.
To add the binder, first weigh 4% of the weight of the powder of binder and add
it to a small, clean Pyrex dish. Mix in just enough acetone so that the binder completely
dissolves. The more acetone that is added beyond this amount will increase the time
it takes the binder to dry. Stir the powder into the dilute binder using a stainless steel
spatula. Place the dish under an incandescent heat lamp and stir continuously until the
mixture becomes too tough to stir. It is important to keep stirring because the binder
can separate from the powder forming a skin on the mixture. If a skin forms, the binder
may not properly attach to the PMN-PT powder, eventually resulting in poor sintered
ceramics. If this has happened, the binder must be removed from the powder and the
process needs to be restarted.
To remove binder from powder, place the bindered powder in an open crucible.
Place the crucible in the furnace and run the program found in table 2.9. The 3 hours
at 325 C breaks down the compounds in the binder and the 10 hours at 500 C removes
the carbon ash from the powder.
33
Instruction SettingC01 0T01 300C02 325T02 180C03 325T03 90C04 500T04 600C05 500T05 20C06 0T06 -121
Table 2.9. Furnace instructions for binder burnout
If the addition of binder was successful, the Pyrex dish should contain several
large hard clumps of binder and powder. These clumps need to be broken up in an agate
mortar and the bindered powder should be passed through a #50 sieve to filter out any
large clumps of binder that may have formed. The bindered powder should pass readily
through the sieve compared to the non-bindered powder.
2.2.2 Pressing ceramic pellets
To form dense ceramics, the powder must first be shaped as compactly as possible
into a pellet. For the property evaluations carried out in this thesis, the powder was
pressed into 1/2” cylinders using a high strength steel die with high strength steel anvils
on a uniaxial press. A schematic of the die is presented in figure 2.1. The anvil is placed
inside the die. The powder is poured into the die and then the press forces the hammer
down on the powder, compacting the powder between the hammer and the anvil.
34
Anvil
Hammer
Die
Force gauge
Fig. 2.1. Die used to press ceramic pellets
35
To make BST ceramics, 1.5 grams of powder were pressed. The pressure was
slowly increased to 200 MPa, which corresponded to a force of 5700 lbs on the press.
The pressure was maintained for 30 seconds and then released. The compression cycle
was then repeated two more times.
To make PMN-PT ceramics, 0.7 grams of powder were pressed. The pressure was
slowly increased until the press was just starting to measure a force on the die. Then,
in one fluid motion, the pressure was raised to 150 MPa, a force of 4500 lbs, and then
immediately released. The compression was not repeated.
Once compressed the pellets were extracted from the die. First, the die was
inverted on the press and a stainless steel spacer was placed between the die and the
press. The anvil was pushed three-quarters of the way out of the die so that it could be
removed by hand. The inverted die was then placed back on the press with the spacer
and the hammer was pushed through the die until the sample was clear of the die. Then
a fine paint brush was used to push the green pellet onto a razor blade to be transported
to the sintering vessel.
2.2.3 Sintering the pellets
Sintering is the process of heating green pellets to just below the melting point
of the material so that the pressed powder condenses into a dense ceramic. All sintering
was performed in the same furnace as the calcination.
BST pellets were placed on a sheet of platinum foil in a yttria-stabilized zirconia
calcination tray. The tray was then covered with another tray and placed in the furnace.
Oxygen was fed into the furnace to prevent oxygen depletion in the ceramics at high
36
temperatures. To set the oxygen flow rate, a tube from a regulated oxygen cylinder was
fed into a water bath. A needle valve on the regulator was adjusted until the oxygen
flowed out of the regulator at a rate sufficient to produce one to three 3 mm bubbles per
second in the water bath. The oxygen line was then connected to the oxygen inlet on
the furnace. The furnace was programmed with the program found in table 2.10. The
BST ceramics had to be cooled very slowly from high temperatures to avoid cracking
and other structural damage.
Instruction SettingC01 0T01 80C02 400T02 60C03 400T03 367C04 1500T04 360C05 1500T05 750C06 0T06 -121
Table 2.10. Furnace instructions for sintering BST
PMN-PT ceramics were first placed on platinum foil on a zirconia brick. The
brick was placed in the furnace and the binder burnout program from table 2.9 was
run. The pellets were then weighed to determine that they had lost approximately
1.5% of their weight and transferred to a piece of platinum foil on the lid of a small
37
alumina crucible that had been previously seasoned with lead oxide-based powder. The
pellets were covered with another piece of platinum foil, the crucible was placed over
the lid, covering the platiunum wrapped pellets. The crucible was placed in the furnace.
Oxygen was fed into the furnace in the same manner as described above. The pellets
were sintered according to the program in table 2.11. Lead oxide can evaporate out of
PMN-PT solutions at temperatures above 1000 C, so the ceramics had to be cooled as
quickly as possible to avoid changes in elemental composition. To combat this lead loss,
industrial ceramics are often sintered while buried in some sort of lead donor powder.
This was attempted with the PMN-PT samples, but this often lead to bizarre structural
deformations of the ceramics.
Instruction SettingC01 0T01 128C02 1280T02 120C03 1280T03 20C04 0T04 -121
Table 2.11. Furnace instructions for sintering PMN-PT
High quality sintered ceramics should have a lustrous shine. BST should be a
brown color and PMN-PT should be light tan to faint yellow color. Both BST and
38
PMN-PT should be translucent below a thickness of 1 mm. Samples meeting these
visual properties are ready to be prepared for electrical property measurements.
2.3 Preparing samples for measurement
PMN-PT is a more stable ceramic than BST and is therefore more forgiving in
the preparation of thin slices. PMN-PT ceramics thicker than 1.0 mm can be cut on
a wire saw. The ceramics should be mounted to an aluminum right angle jig with hot
wax. A small piece of graphite should be placed just below the ceramic to help support
the slices as they are cut and to keep the blade from falling into the aluminum jig. The
ceramic should be sliced into 400 µm or thicker slices and then each piece should be
polished to thickness. The wire saw cuts very slowly compared to the wafering saw and
the wire blade is more subject to travel. Sometimes the cut surface from a wire saw cut
sample is wedge shaped or bowled, which requires careful polishing.
Ceramics thinner than 1.0 mm should be hand polished down to a thickness of
250 µm. The ceramics should be mounted to a flat stainless steel polishing base by
heating a small amount of wax on the base and pressing the sample firmly into the wax.
Weight should be placed on the ceramic as it cools to ensure that the ceramic lies flat
against the steel base. After the wax has hardened, the sample can be polished using a
South Bay polishing jig, starting with 120 grit emery paper, followed by 400 grit emery
paper and finally with 9 µm grit paper. In all cases, distilled water should be used to
lubricate the polishing surface. Further polishing beyond 9 µm grit paper is usually
unnecessary because the friction between the polishing paper and the ceramic tends to
pull grains out of the ceramic rather than break down the ceramic grain, itself.
39
After polishing, the samples need to be annealed to help repair any scratches,
gouges, or microfractures that may have occurred during polishing. The samples should
be placed on platinum foil on a zirconia brick. The brick is placed in the furnace and
oxygen should be flowing into the furnace at a rate of one to three 3 mm diameter bubbles
of gas per second. The furnace program is shown in table 2.12. At a temperature of
900 C the material should not be in danger of decomposing. After the samples have
cooled, they are ready for the application of electrodes.
Instruction SettingC01 0T01 180C02 900T02 600C03 900T03 180C04 0T04 -121
Table 2.12. Furnace instructions for annealing PMN-PT ceramics
2.3.1 Electrodes
Electrodes turn out to have significant influence on the measurement of ferro-
electric properties. The dielectric constant and electrocaloric effect depend on a high
mobility of surface charges both at ceramic grain surfaces and along ferroelectric domain
boundaries. Industrially manufactured piezoelectric ceramics typically use a silver paint
40
electrode that is screen-printed, dried, then fired onto the ceramic. The organic backing
of the paint can leave behind a thin insulating film, spoiling electrical property measure-
ments. Because of the high reactivity of barium, it is best to minimize the contact of
BST with any possible surface contaminant. The ideal electrode solution is to directly
sputter metal film electrodes directly onto each surface of the polished ceramic disk.
The electrodes in the research were applied using a Perkin-Elmer model 4400
commercial sputter deposition system. Detailed operating instructions of the sputtering
system are found in appendix B. To form the electrode, a layer of chrome was deposited
for 10 minutes at a power of 500 W using an RF magnetron. Then, a layer of aluminum
was deposited for 10 minutes at a power of 1000 W using a DC magnetron. The samples
were then flipped over and a similar electrode was applied to the other side. The resulting
electrodes had a thickness of one to two microns and a surface resistance of 1 to 3 Ω.
Because the sputtered metal can redeposit as it accumulates on the sample, a thin bridge
of metal usually forms around the edge of the sample. After sputtering, this bridge of
metal needs to be polished from the sample with some fine grit emery paper to isolate
the two electrodes. With the application of the electrodes, the ceramic samples are ready
for measurement of electrical properties.
2.4 Measurements on ceramic wafers
Both physical and electrical properties were measured to evaluate the ferroelectric
ceramics. Density and structure of the materials were probed first to determine that
the ceramics compared to those found in the literature. After the physical parameters
were met, the dielectric constant and electrocaloric effect of the samples were measured.
41
Physical property measurement methods will be discussed in section 2.4.1. Electrical
property measurement methods will be discussed in section 2.4.2. The results of these
measurements will be presented and discussed in chapters 3 and 4.
2.4.1 Physical property measurement methods
The first test typically performed on the materials was x-ray diffraction (XRD)
As this was first performed just after calcination, this was discussed previously in sec-
tion 2.1.2. Occasionally, XRD was used as a spot check on ceramic wafers to ensure
that the phase and composition of the powder had not changed during processing. No
significant differences were ever found between powder or ceramic XRD measurements.
SEM microscopy was also used to characterize the structure of the grains within
the ceramic. SEM micrographs along fractures of ceramic samples were made to deter-
mine the average grain size and porosity of the samples. After a reliable method for
reproducing large grained, dense ceramics was established, most of the SEM microscopy
was deemed unnecessary.
The density of the ceramic samples is the fastest test outside of a visual inspection
that can provide clues to the quality of the sample. Good ceramics for ferroelectric
measurements should have a density 98 to 99 % of the single crystal density. The
single crystal density of BST is 5700 kg/m3 and the single crystal density of PMN-PT
is 8000 kg/m3. Lower densities suggest that the ceramic has either large open pores
between the ceramic grains or large empty regions inside the sample. Both high-porosity
and voids in the sample increase the risk of dielectric breakdown and spoil the electrical
properties.
42
2.4.2 Electrical property measurement methods
Electrical properties were directly measured using the setup shown in figure 2.2.
Copper leads were attached to the metal films on the ceramic wafers using a conducting
silver paint; one side was connected to ground and the other to a high voltage power
supply. A thermocouple was attached to the grounded electrode. Figure 2.2 shows
two PVDF sensors on the ground electrode of the sample. These were initially used
to check the thermocouple data. As the sample was heated, the pyroelectric PVDF
sensors produced a current corresponding to the heating rate. The heating rate from
the thermocouple was always in excellent agreement with the PVDF sensors, so for
simplicity, the PVDF sensors were deemed unnecessary for most ceramic samples. The
thermocouple was connected in series with a second thermocouple which was attached
to a large copper block with an alcohol thermometer attached to it. This block served
as an isothermal reference and all temperature measurements were made relative to the
isothermal block. Both dielectric and electrocaloric measurements were carried out under
high vacuum to minimize the risk of arcing when large electric fields were applied to the
sample and to maximize the thermal isolation between the sample and the environment,
giving the longest possible time constant to observe thermal effects before the sample
returned to thermal equilibrium.
The dielectric constant was measured by connecting the sample in series with a
56.6 kΩ resistor and applying a 100 mV AC electric field at a frequency of 1 kHz. A
schematic of the measurement is shown in figure 2.3. The voltage across the capacitor
was measured using a computer controlled digital volt meter. Using Vo as the applied
43
Fig. 2.2. Setup to measure the electrocaloric properties of BST
44
voltage, VC as the voltage across the capacitor, R as the series resistance, and ω as the
angular frequency of the applied AC field, the capacitance of the sample is given as
C =1
ωR
√(VoVC
)2− 1. (2.1)
Since the sample is a parallel-plate capacitor, the dielectric constant can be found from
ε =Ct
εoA(2.2)
where t is the thickness of the sample, A is the cross-sectional area of the sample and εo
is the permittivity of free space, 8.85 F/m.
V
Fig. 2.3. Schematic of dielectric constant measurement
45
The electrocaloric effect was measured by applying a large DC electric field across
the sample and recording the resulting temperature change observed on the thermocou-
ple. The high resistivity of the ceramics mitigates any possible Joule heating. Also,
Joule heating shows up like an exponential curve (as with an RC time constant) in the
temperature data; there is an initial rapid change in temperature that gradually levels off
as the system comes into thermal equilibrium. Under the conditions in this experiment,
the time constant for this equilibrium was on the order of one minute. The observed elec-
trocaloric temperature change established itself on the order of two seconds, the response
time of the thermocouples. As the electric field was kept on the sample, the tempera-
ture of the sample slowly returned to thermal equilibrium on a several minute thermal
time constant scale. If the electric field was removed after the system had returned to
thermal equilibrium, the temperature of the sample would instantly drop below room
temperature and recover to thermal equilibrium on a several minute time scale.
The electrocaloric effect can also be measured as the heat required to restore a
system to equilibrium in a calorimeter[26]. In the calorimeter, the sample and a reference
material are mounted to heaters with a compensation system to ensure that they remain
at the same temperature. To study phase transitions, the materials are heated at a slow
consistent rate; if the sample has a change in heat capacity, the reference heater must
change its output to match the effects in the sample.
To measure the electrocaloric effect, the sample and the reference are heated at
a slow, uniform rate. An electric field is applied to the sample, inducing a temperature
change in the sample. The reference heater must then do work to compensate for the
change in the sample. The power of the heater can be recorded to find the heat flow
46
in and out of the sample. It must be noted that this process is not inducing a phase
transition and this is not latent heat. The proper term for measuring the electrocaloric
effect through calorimetry is measuring the heat of polarization.
Both direct temperature change data and calorimetry data can be compared by
either converting the temperature change in an isolated environment to a heat of polar-
ization or by converting the heat of polarization to a temperature change in an isolated
environment. Assuming the temperature change occurs in isolation, the heat of polar-
ization is given from
∆hEC =cp
ρ∆TEC . (2.3)
Direct observation of the electrocaloric effect is not practical in thin film systems.
Polarization of thin films can be mapped as a function of applied external electric field
and temperature. This surface can then be fit numerically to calculate derivatives and
these can be integrated to determine electrocaloric temperature change. Unfortunately,
this calculation is only useful as an intellectual exercise and cannot be translated into
real-world devices. Any exploitation of the electrocaloric effect needs to consider the
thermodynamics of the entire system. If there is a 2 µm thin film on both sides of a
100 µm thick rigid substrate there is only a 4% electrocalorically active region. Ther-
modynamically, there is 25 times more material that will be heated but not positively
contribute to the electrocaloric effect. Any electrocaloric element based off a thin film
will have a useable temperature change of the theoretical thin film effect divided by the
ratio of electrocalorically inactive material to electrocalorically active material.
47
Chapter 3
Electrocaloric effect in barium strontium titanate
Recent trends have prompted global research for more environmentally friendly
sources of energy and refrigeration. While ozone-depleting chlorofluorocarbon refriger-
ants have been phased out, their replacements, hydrochlorofluorocarbons tend to have
global warming potentials 3000 times that of an equivalent amount of carbon dioxide[14].
Alternatives to vapor compression refrigeration suffer from serious technical challenges.
Thermoelectric technology provides an all-solid-state solution, but thermoelectric de-
vices have typical efficiencies around 7% of the Carnot limit. Although some recent
advances[30] are pushing the technology to near 10% of the Carnot limit, typical house-
hold refrigerators operate with efficiencies near 25% of the Carnot limit. As most elec-
trical power in the United States is provided by carbon emitting sources, the reduced
efficiency contributes to the potential for global warming. Magnetocaloric materials are
capable of very large temperature changes, greater than 17 K; however, they require mag-
nets capable of generating large fields (1-5 T) which are not conducive to scalability[21].
The electrocaloric effect is a possible replacement for thermoelectrics in the devel-
opment of all-solid-state heat pumps and refrigerators. Modern ceramic and single crystal
material can withstand large electric fields, and high voltages can be generated from in-
expensive sources. Most of the electrocaloric materials are perovskite ferroelectrics, and
the general properties of these materials will be discussed next.
48
3.1 Ferroelectric-Paraelectric transition in BST
Ferroelectric materials exhibit large electrocaloric effects near the ferroelectric-to-
paraelectric phase transition. Barium strontium titanate, (BST), has a phase transition
temperature which can be tuned from ∼ −233 oC, the transition temperature of SrTiO3,
to 118 C, the transition temperature of BaTiO3. The chemical symbol is usually given
as BaxSr1−xTiO3, where x is the molar concentration of barium.
The simple approximation of the transition temperature in C as a function of
doping can be found with
Tc = 371x− 253.0[3] (3.1)
This assumes a linear shift in transition from pure SrTiO3 to pure BaTiO3. Careful
study of the transition temperature shows multiple regions of interest in the transition
temperature[24, 58]. Below a Ba concentration of 20.0%, the Curie temperature increases
as x1/2 with the temperature in C given by[58]
TC = 274(x− 0.0002)1/2 − 273.15. (3.2)
Above a concentration of 20.0% a better approximation of the Curie temperature in C
is given by[24]
Tc = 339x− 221. (3.3)
Room temperature phase transitions occur with a doping of 0.65 < x < 0.70. A broad
survey of transition temperatures found throughout the literature is presented in figure
3.1[2, 3, 4, 7, 6, 8, 16, 18, 19, 22, 23, 24, 25, 26, 28, 29, 33, 37, 41, 44, 45, 47, 52,
49
Fig. 3.1. A compilation of measured Curie temperatures in BST systems
50
53, 55, 56, 57, 58, 59, 63, 65, 66, 68]. The dashed line plots the linear shift in phase
transition temperature that is predicted by equation 3.1. The solid curve plots the phase
transition temperatures predicted by equations 3.2 and 3.3. It should be noted that both
the theoretical curves usually tend to under-predict observed Curie temperatures. The
Curie temperature can be very sensitive to both small variations from the assumed
stoichiometric ratio of barium and strontium, and no paper surveyed made any effort to
verify the exact composition of the sample studied.
Small levels of impurity can have strong effects on the transition temperature
and physical properties near the transition. Small levels of nickel and calcium can lead
to strong changes in resistivity near the phase transition, creating a semiconductor-
like material. At large electric fields, if the resistivity drops below 1012 Ω−m, Joule
heating can begin to overwhelm the electrocaloric response. The electrocaloric materials,
therefore, need to be good insulators. To achieve consistent, high-quality electrocaloric-
grade BST ceramics, a precise preparation protocol is crucial. A considerable portion of
the research of this thesis was directed toward developing such a protocol.
3.2 Sample preparation protocol
BST powder was prepared from carbonate precursors; the procedure is docu-
mented in Chapter 2 and a step by step summary is presented in Appendix A. Briefly:
powders were mixed for 4 hours in an acetone solution using a roller mill and dried. The
dried powder was calcined at 1200 C for 6 hours. To verify the crystal phase of the BST
powder, X-ray diffraction (XRD) was performed on the calcined powder. The results of
the XRD analysis are shown in figure 3.2. Table 3.1 lists the angular locations of the
51
expected XRD peaks for BST found in the PDF-4 in addition to the measured locations.
The measured angles are all slightly below the expected angles for BST, however, it is
likely that the sample in the reference database has a slightly different stoichiometric
composition. The data from figure 3.2 are in good agreement with the expected XRD
signature from the PDF-4. Also, there are no additional peaks beyond the six anticipated
peaks. Large quantities of unreacted carbonate powder would show up as two additional
peaks. As the calcination appeared to be complete the BST powder was ground to a
suitable size for making ceramic pellets.
2θtheory [o] 2θobserved [o]22.399 22.35231.866 31.85039.312 39.28745.733 45.62351.486 51.41456.802 56.760
Table 3.1. Expected and observed XRD angular peaks in BST
The optimal procedure to grind the BST powder was found to be a 4 hour dry
grinding using yttria-stabilized zirconia media, followed by a 6 hour grinding in an ace-
tone slurry. A particle size analysis was performed using a laser diffraction particle size
analyzer. Figure 3.3 shows the distribution of grain sizes. The time listed is the total
time for which the powder was ground. The 2 hour and 4 hour ground powder was only
ground dry. The 6 hour, 8 hour and 10 hour all included both the 4 hour dry grinding
52
Fig. 3.2. XRD analysis of BST powder
53
in addition to acetone grinding to complete the balance of the time. As grinding time
is increased, the mean particle size decreases and the distribution of particle sizes tends
to increase. It is only at 10 hours of total grinding time that a significant fraction of
powder particles appear below 250 µm. It is important to maintain a broad distribution
of particle sizes to achieve optimal packing of powder when the powder is pressed into a
pellet.
Ceramic pellets were prepared by pressing 0.5 grams of BST powder in a 1/2”
diameter die to a pressure of 200 MPa. As barium is highly reactive, even in solid solu-
tion, no binder was used to hold the pressed powder together. While binder is typically
used in industrial ceramic manufacture, many binders are either water or solvent-based.
Barium is highly reactive with water; even limited exposure to water vapor in a humid-
ity controlled environment for more than three months can turn the surface of ceramic
BST from brown to purple. In addition to the problems presented by water, the organic
compounds that comprise the binder can inhibit the sintering of high-quality ceramics.
The presence of carbon inhibits grain growth and leaves large deposits throughout
the sample. Figure 3.4 shows the surface of a BST ceramic pellet made from powder
that has been contaminated with an organic compound. The open porosity and small,
poorly connected grains increase the occurence of dielectric breakdown.
It is suggested that the pyroelectric coefficient in BST increases as the ceramic
grain size increases, with optimal values occurring between 10 and 20 µm[23]. Figure 3.5
shows SEM micrographs of a cross-sectional fracture of BST ceramics. Table 3.2 shows
the estimation of the average grain diameters of these ceramics. The sintering time
54
Fig. 3.3. Particle size analysis of BST powders
55
Fig. 3.4. SEM micrograph showing carbon contamination in BST ceramics
56
and temperature is sufficient to reach the predicted sweet spot for optimal pyroelectric
activity.
Sintering conditions Average grain size ( µm)1460 C, 8 hours 12.71500 C, 9 hours 16.2
Table 3.2. Corresponding grain sizes of different sintering conditions
As grain size increases, the ceramics become increasingly brittle. Above a grain
size of 15 µm the ceramics could not be sliced thinner than 330 µm. Polishing of the
ceramics typically had a negative impact on the larger grained samples. As the grain size
increased, the strength holding the grain together seemed to be stronger than the strength
connecting the grains. This leads to grains pulling out of the ceramic causing a rough,
cloud-like texture on the surface. The rough surface can create an uneven thickness,
which leads to local spots of larger than average electric field. These hotspots increase
the likelihood of catastrophic dielectric breakdown. Once ceramic pellets exhibiting
good physical properties were crafted repeatedly, the pellets were prepared for electrical
properties measurements.
3.3 Electrical properties measurement
The sintered pellets were sliced to a thickness of 300-400 µm and approximately
1-2 µm thick chrome and aluminum electrodes were sputtered onto the wafers. Detailed
57
Fig. 3.5. SEM micrographs of fracture cross-sections of BST ceramics
58
instructions for operating the sputtering machine can be found in Appendix B. While it is
more traditional in the ceramic industry to apply electrodes by firing a conductive silver
paste onto the ceramic, the BST samples showed degraded properties after contact with
the silvering compound. This degradation is most likely due to either the reactivity of
barium, or a thin layer of the organic carrier of the paint remaining between the ceramic
and the electrode. This paint layer could degrade charge mobility from the ceramic to the
electrode, spoiling the measurement of dielectric and electrocaloric properties. Figure 3.6
shows a comparison of dielectric constants measured on samples prepared from the same
powder: the top trace is for a ceramic with sputtered electrodes, the bottom trace is
for a ceramic with silver paint electrodes. While the peak temperature is visible, the
sharp peak is muted, and the maximum value of dielectric constant in the paint electrode
samples are almost an order of magnitude smaller than the sputtered electrode samples.
The electrocaloric effect was directly measured using the setup shown in figure 2.2.
Copper leads were attached to the metal films on the BST wafers using a conducting silver
paint; one side was connected to ground and the other to a high voltage power supply.
A thermocouple was attached to the grounded electrode. Figure 2.2 shows two PVDF
sensors on the ground electrode of the sample. These were initially used to check the
thermocouple data. As the sample was heated, the pyroelectric PVDF sensors produced
a current corresponding to the heating rate. The heating rate from the thermocouple
was always in excellent agreement with the PVDF sensors, so for simplicity, the PVDF
sensors were deemed unnecessary for most ceramic samples.
The dielectric constant of Ba0.67Sr0.33TiO3 is shown in figure 3.7. The lower
curve was recorded as the sample was heated and the upper curve was recorded as
59
Fig. 3.6. Dielectric constant of BST ceramics with different electrodes
60
the sample cooled. Arrows have been imposed over the data to show the direction
in which the data were recorded. The difference in the two curves can be explained
by the process of measuring the dielectric constant. The polarization does not truly
spontaneously appear, especially because of the presence of the small AC electric field
present to measure the dielectric constant. Just as ”permanent” ferromagnets can be
degaussed with an oscillating magnetic field, the dielectric measurement fights the onset
of the ”permanent” polarization. Over time, small fluctuations will begin ordering the
electric dipole. As this remnant polarization sets in, the measured dielectric constant
slowly decays to the lower curve.
The sharp peak in the dielectric constant of just under 20000 occurs at a temper-
ature of 25 C; this is the Curie temperature. Below 18 C and above 35 C the dielectric
constant levels off. These upper and lower temperatures set the bounds for observing a
large electrocaloric effect.
Figure 3.8 shows the electrocaloric response of this sample under the application
and removal of a 1 MV/m electric field. The effect peaks at 24.5 C with an electrocaloric
temperature change of 0.38 K. The electrocaloric response is larger than 0.25 K from
20 C to 33 C. Outside of this range, the effect drops quickly.
The electrocaloric effect in the phase transition temperature region was mapped as
a function of both temperature and applied field and is presented in figure 3.9. As the field
is increased the maximum electrocaloric effect is observed at larger temperatures. This
is due to the nature of the polarization in this region. Below the transition temperature,
there is some net polarization, even at zero applied field. There is a limit to the ability
of the applied field to polarize the material; at low fields, the effect is linear with applied
61
field but the electrocaloric effect begins to saturate. Above the transition temperature,
the BST has become a paraelectric and there is a growing contribution from the square
of the electric field, as mentioned in equation 1.12. At much higher temperatures, the
electrocaloric response should scale exactly as the square of the applied electric field.
The largest absolute electrocaloric response observed in the BST sample was 0.45 K,
which occurred at 24 C and an applied field of 1.33 MV/m.
3.4 Comparison to other results
While this is not the first experiment to measure the electrocaloric effect in
BST[26], the measurements are superior to those found in the literature. Lin measured a
bulk BST ceramic using differential scanning calorimetry. Lin reports his electrocaloric
measurement as a latent heat; however, the terminology is not proper. The effect is
more properly called a heat of polarization[10], which can be converted to an equivalent
electrocaloric temperature change for easy comparison, as follows: assuming that the
electrocaloric effect occurs in isolation, the electrocaloric heat of polarization is given as,
∆hEC =cp
ρ∆TEC . (3.4)
The density of BST is 5700 kg/m3 and the specific heat is 2.7 MJ/m3 −K[5], the ob-
served heat of polarization is approximately 0.189 kJ/kg for a temperature change of
0.40 K. The only latent heat observations made on the electrocaloric phase transition in
BST observe a heat of polarization of 0.12 kJ/kg at the transition temperature under
62
Fig. 3.7. The dielectric constant of Ba0.67Sr0.33TiO3
63
Fig. 3.8. Electrocaloric effect in BST at 1 MV/m
64
Fig. 3.9. Electrocaloric effect in BST as a function of temperature and applied field
65
an applied field of 1.0 MV/m[26]. Lin’s measurement corresponds to an electrocaloric
temperature change of 0.25 K in an isolated sample.
Extrapolating the peak electrocaloric effect to 2.5 MV/m, close to the limit of
the onset of dielectric breakdown in bulk ceramics, we would anticipate to see an elec-
trocaloric temperature change of 0.65 K and a heat of polarization of 0.3 kJ/kg.
66
Chapter 4
PMN-PT
During the last 10 years, one of the most interesting perovskite compounds to
emerge is lead magnesium niobate-lead titanate, (PbMg1/3Nb2/3O3)x−(PbTiO3)1−x
(PMN-PT). Both ceramic and single crystal PMN-PT exhibit an interesting structure
which have led to the observation of extremely large electrocaloric and piezoelectric
properties. At lower PT concentrations, from x = 5 − 15%, PMN-PT has a near-
room-temperature pseudo-cubic to cubic phase transition, around which a very large
electrocaloric effect occurs. At higher PT compositions, a morphotropic region exists
between rhombohedral and cubic phases; in this region large piezoelectric effects are
observed, making PMN-PT an ideal material for sensors and actuators. This uniquely
structured material is at the forefront to revolutionize several technical fields[31].
4.1 Crystal structure
Though not obvious from its canonical chemical formula, PMN-PT has the same
perovskite unit cell structure as BST, ABO3, shown in figure 1.3. To highlight this
structure an alternate chemical formula, Pb(Mgx/3Nb2x/3Ti1−x)O3, could be used.
The A sites in the unit cell are clearly occupied by Pb atoms. The oxygen atoms form
octahedral cages about the B site atoms. The B sites are filled in proportion to the ratios
set within the parentheses; for example, for (PbMg1/3Nb2/3O3)0.9 − (PbTiO3)0.1, or
67
(Pb(Mg0.3Nb0.6Ti0.1)O3) in the alternate scheme, there would be 30% Mg atoms, 60%
Nb atoms and 10% Ti atoms occupying the B site. Below the Curie temperature, it
becomes energetically favorable for the B site atoms to settle either slightly above or
below the centers of the oxygen octahedra, leading to a remnant polarization at low
temperatures.
4.2 Relaxor transition in PMN-PT
Just as ferroelectrics exhibit large electrocaloric effects near the ferroelectric-to-
paraelectric phase transition, relaxors exhibit large electrocaloric effects in the region
of the freezing temperature. As the ferroelectric-to-paraelectric phase transition is very
sharp, the region of large electrocaloric effect in a ferroelectric such as BST is limited to a
range of about 10 K. Because the relaxor transition is very broad, a large electrocaloric
effect can be observed over a broader range, though the peak effect may be smaller
because the changes in the polarization are smoother.
4.2.1 Transition temperature for PMN-PT
As PMN is a relaxor and PT is a ferroelectric, the structural transition becomes
rather complicated. Room temperature phase transitions occur for two different con-
centrations of PT. At PT concentrations of 27-33%, PMN-PT exhibits a morphotropic
transition from the low-temperature rhombohedral phase to a low temperature tetrago-
nal phase. This transition allows very large piezoelectric coupling coefficients[15, 62, 64].
Figure 4.1 is based on the phase diagram presented by Zekria, et. al. At a composition
of 6-12% PT there is a transition from the rhombohedral phase to the high-temperature
68
Fig. 4.1. Phase diagram for PMN-PT mixtures[62]
69
cubic phase. It should be noted that diffuse neutron scattering experiments fail to show
the distinct change in the physical structure along this transition[61] that is seen in XRD
measurements[62, 64]. Gehring has speculated that this transition is really a skin effect,
the part sensitive to XRD, and that the bulk of the material, the part sensitive to neutron
scattering, does not experience any change[13]. This could explain some observations of
larger electrocaloric effects in thinner samples[31, 40].
4.3 Single crystal PMN-PT
Recent data suggests a strong dependence of crystal orientation on the observation
of large electrocaloric effects[39]. Table 4.1 shows the electrocaloric effect observed by
Sebald, et al., in three orientations of single crystal 0.75PMN-0.25PT as well as ceramic
PMN-PT. The orientations investigated were [111], [110], and[100]. While Sebald fails to
draw the connection between crystal orientation and electric field, the result is straight
forward. The energy in and electric dipole in an electric field is E = ~P · ~Eq, where ~P is
the dipole moment vector and ~Eq is the applied electric field vector. For a single crystal,
the dipole moment vector is the direction of net polarization, which seems to occur in
the [111] orientation. The column in table 4.1 labeled cos θ[111] is the cosine of the angle
between the listed crystal orientation and the [111] orientation. Sebald’s data show good
agreement with the simple estimate of maximum polarization occurring along the [111]
orientation.
Note that for the ceramic sample mentioned in table 4.1 the fraction of the po-
larization which remains when the bias field is removed is presented in lieu of the cosine
70
Orientation cos θ[111] ∆TEC,[111] cos θ[111] Observed ∆TEC (J/g)[39]
[111] 1.000 0.40 0.40[110] 0.817 0.33 0.32[100] 0.578 0.23 0.18
Ceramic 0.866 0.35 0.36
Table 4.1. Effect of crystal orientation on the electrocaloric effect in PMN-PT
of the rotation angle. Because the largest electrocaloric response seems to occur in
[111]− oriented, single crystal PMN-PT was initially investigated.
Single crystal PMN-PT plates were provided by Omega Piezo Technologies, Inc.
of State College, PA. The plates measured 18 mm by 22 mm by 0.2 mm thick. The plates
were very fragile and the edges could not be safely polished if metal from the electrode
redeposited around the side of the sample. A special deposition mask was prepared for
use in the sputtering system and is shown in figure 4.2. The edges of the mask overlap the
sample edges by 0.5 mm to maintain a large gap to prevent arcing through air between
the electrodes when large fields were applied to the plates. The mask had holes over one
corner of the plate to ease connection to the electrical leads in the proposed solid-state
heat pump.
When the first sample was mounted in the electrocaloric apparatus, no peak in
either dielectric constant or electrocaloric effect was observed below 80 C. A special
high temperature setup was constructed in an oven and is shown in figure 4.3. The
sample is still suspended by the electrical connections with a thermocouple attached
to the grounded electrode; however, the sample was not measured under high vacuum.
71
Fig. 4.2. Mask for electroding rectangular plates
72
The presence of air increased both the uncertainty of the absolute temperature and
diminished the size of the measured electrocaloric effect. These considerations are re-
flected by the large error bars in figure 4.4. The lower set of data, presented as boxes,
show the electrocaloric effect measured at 1 MV/m. The upper set of data, presented
as “X’s”, show the electrocaloric effect measured at 1.5 MV/m. The peak effect ob-
served was 0.56 K at 1 MV/m and 0.74 K at 1.5 MV/m, both occurring at 141 C. A
transition temperature of 141 C would suggest that the composition of the material is
(PbMg1/3Nb2/3O3)0.73 − (PbTiO3)0.27. The manufacturer of the sample quoted the
sample as having between 12 and 15% PT.
An electron backscatter measurement was made on the ceramic, and an analy-
sis of the electron absorption measured the relative concentration of constituent ions.
The results are presented in table 4.2. The magnesium, niobium and titanium ratios
suggest that the composition is closest to (PbMg1/3Nb2/3O3)0.8 − (PbTiO3)0.2 though
this material would be significantly magnesium deficient. This deficiency could explain
why the Curie temperature was observed to occur at 141 C when it should occur at
88 C in (PbMg1/3Nb2/3O3)0.8 − (PbTiO3)0.2. To help control compositional variance
and produce PMN-PT with a Curie temperature near room temperature, a protocol to
produce ceramic PMN-PT samples was developed.
4.4 Ceramic PMN-PT
PMN-PT powder was prepared in a two reaction process[48, 49]; the procedure
is documented in Chapter 2 and a step-by-step summary is presented in Appendix A.
Briefly: magnesium niobate was prepared first by mixing magnesium oxide and niobium
73
Fig. 4.3. High temperature electrocaloric measurement setup
74
Fig. 4.4. Electrocaloric effect in single crystal (PbMg1/3Nb2/3O3)0.8 − (PbTiO3)0.2
Ion Number per unit cell Expected Percent differencePb 0.98 1.00 2.1Mg 0.21 0.27 21.8Nb 0.57 0.55 3.6Ti 0.20 0.18 8.3O 3 3 0
Table 4.2. Compositional analysis of PMN-PT crystal
75
(V) oxide for 12 hours in an ethanol solution using a roller mill and drying the powder.
The dried powder was calcined at 1200 C for 6 hours and ground for a total of 10 hours in
a vibratory mill. The magnesium niobate was then mixed with lead oxide and titanium
dioxide for 12 hours in an ethanol solution in a roller mill and dried. This powder was
then calcined at 800 C for 10 hours and ground for a total of 10 hours in a vibratory
mill. The ground powder was run through a #50 mesh sieve and binder was added to the
powder to aid the pressing of ceramic pellets. For the ceramics measured in this research,
a PT concentration of x = 0.15 was chosen. PMN-PT powders with concentrations of
x = 0.10 were also prepared; however, they were found to contain extremely high levels
of lead, and the ceramic samples were not studied in detail. Properly reacted PMN-PT
powder should have a uniform faint yellow color. Excess amounts of lead typically show
up as a reddish color throughout the powder. To ensure there is minimal excess lead
and that the PMN-PT powder has the proper crystalline structure, XRD must first be
performed on any calcined powder.
4.4.1 XRD of PMN-PT powder
To verify the crystal phase of the PMN-PT powder, XRD was performed on the
calcined powder. The results of the XRD analysis are shown in figure 4.5. The vertical
lines superimposed over the intensity data are the expected locations and relative intensi-
ties for the XRD peak in perovskite PMN-PT found in the PDF-4. Perovskite PMN-PT
has one very strong peak at an angle of 2θ equal to 32.3o and two small peaks at 2θ equal
to 22.1o and 38.8o. The small peaks at 29.2o and 33.9o are evidence of a small amount
of non-ferroelectric pyrochlore material in the powder, most likely Pb6MgNb6O23. The
76
small peak at 30.5o could be unreacted lead oxide powder. Figure 4.6 shows XRD data
from a sintered ceramic. The parasitic peaks have mostly disappeared, except for tiny
peak around 30.2o. The reaction to form PMN-PT does not always complete during cal-
cination but often will complete during the sintering process. With the proper chemistry
and structure ensured, the PMN-PT cermaics can be prepared for electrical property
measurements.
4.4.2 Electrical property measurements on ceramic PMN-PT
To form thin samples for measurement, sintered ceramics thicker than 1.0 mm can
be cut on a wire saw using the procedure from section 2.3. Sometimes the cut surface
from a wire saw cut sample is wedge shaped or bowled, which requires careful polishing
by hand. Sintered ceramics thinner than 1.0 mm should be hand polished down to a
thickness of 250 µm using the procedure documented in section 2.3..
Initially, electrodes were applied by silk screening a conducting silver paste onto
each face of the ceramic. These electrodes were then dried and fired onto the ceramic.
This process is standard in the piezoelectrics industry and avoided when working with
BST only because of barium’s known high reactivity. Later, electrodes were applied to
the PMN-PT ceramics using the same sputter deposition process that was developed for
BST. Figure 4.7 shows dielectric constant measurements in a PMN-PT sample with fired
silver electrodes and sputtered chrome/aluminum electrodes. The arrows represent the
direction in which the data were taken. Both curves show the heating and cooling asym-
metry typical of relaxors and both have the freezing and Curie temperatures occurring
at roughly the same temperature. However, the dielectric constant is three times larger
77
Fig. 4.5. XRD of calcined 0.90PMN-0.10PT powder
78
Fig. 4.6. XRD of a sintered 0.90PMN-0.10PT ceramic
79
when sputtered electrodes were used compared to fired silver electrodes. The effects were
not limited to the dielectric constant.
Figure 4.8 shows the electrocaloric effect measured in two identical samples, one
with sputtered electrodes, (the data are presented as the “X’s”), the other with fired
electrodes, (the data are presented as the crosses). With the exception of one data point
taken at high temperatures, the sputtered electrode sample consistently showed a 20%
larger electrocaloric effect. These electrical and thermodynamic effects arise from the
mobility of electric dipoles in the ceramics. In addition to dipole boundaries, there are
also grain boundaries within the ceramic. It is possible that large surface charges could
move over these grain boundaries to attempt to cancel changes in polarization. If a very
thin layer of non-ferroelectric insulating material, such as remnants of the organic carrier
of the silver paint, is left behind between the electrode and the ferroelectric material, it
is conceivable that surface charges in this insulating layer could screen the macroscopic
fields applied to the sample, mitigating the dielectric and electrocaloric measurements.
4.4.3 Results for (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15
Both the dielectric constant and electrocaloric effect were measured in a 0.85PMN-
0.15PT ceramic. The dielectric constant data are shown in figure 4.9. The lower curve
was taken heating the sample and the upper curve was taken cooling the sample. Ar-
rows have been imposed over the data to show the direction the data were taken. All
measurements were made at a frequency of 1 kHz; at lower frequencies larger dielectric
constant values would be expected below the Curie temperature. The curves begin to
diverge at a temperature of 90 C. The expected Curie temperature for 0.85PMN-0.15PT
80
Fig. 4.7. Effect of different electrodes on dielectric constant in ceramic 0.85PMN-0.15PT
81
Fig. 4.8. Effect of different electrodes on electric effect in ceramic 0.85PMN-0.15PT
82
is 70 C. The peak of just under 25000 occurs at a temperature of 72 C, the freezing
temperature for this sample. As this is where polarization changes most rapidly, this is
the temperature around which the largest electrocaloric effect is expected.
The electrocaloric data are shown in figure 4.10. The effect peaks at 60 C with a
temperature change of 0.375 K. The electrocaloric effect is larger than 0.35 K from 55 C
to 81 C, and is larger than 0.30 K from 50 C to above 95 C. While the absolute maximum
electrocaloric effect is about the same in PMN-PT as it is in BST, large electrocaloric
effects occur in a 45 degree temperature span rather than a 10 degree temperature
span. At lower PT concentrations, similar dielectric and electrocaloric effects would be
expected to occur at lower temperatures.
4.5 Comparison to other measurements
The largest reported electrocaloric is effect in bulk 0.85PMN-0.15PT is 1.8 K
under an applied electric field of 1.6 MV/m at 18 C[40]. This peak temperature con-
tradicts most reports of peak properties in 0.85PMN-0.15PT, which should occur above
63 C[15][62][64]. In Shaobo’s description of the electrocaloric effect, the large temper-
ature gradient establishes itself over a period of many seconds to several minutes [40].
However, the electrocaloric effect should be an almost instantaneous change in the tem-
perature of the sample. Figure 4.11 shows a typcial electrocaloric measurement taken on
a PMN-PT ceramic. The horizontal axis shows the time and the vertical axis shows the
temperature reading of the thermocouple. A DC electric field of 1 MV/m is applied for
25 seconds and removed. A DC electric field of -1 MV/m is then applied for 25 seconds
and removed. The circuit supplying the electrical field has a time constant of 20 ms to
83
Fig. 4.9. Dielectric constant of ceramic (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15
84
Fig. 4.10. Electrocaloric effect in ceramic (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15
85
prevent damage to the sample. The temperature response of the thermocouple occurs
with a time constant of less than a second. The temperature change provided by Shaobo
could, however, be described if the sample was joule heating because of the application
of the electrical field.
Ferroelectrics and relaxors are supposed to have very high resistance, though con-
taminents can change the structure of the material. Contaminated BST powder created
semiconductor-like ceramics, as was mentioned in section 3.1. While the low voltage
resistance measurement of a typical multimeter measured high resistance, a measure-
ment of the resistance of the contaminated sample under a large electric field showed
resistances less than 10 MΩ. When attempting to measure the electrocaloric effect in
this sample by applying a 200 V electric field, the result was a 5 mW of joule heating.
Figure 4.12 shows the thermocouple response due to joule heating of a ceramic sample.
The temperature rises quickly, but with a time constant on the order of several seconds.
Even though this particular sample was dissipating only 5 mW of power, this would have
led to a steady-state temperature gradient of 10 K if the electric field would have been
left on for several minutes. Other groups using measurement techniques similar to both
Shaobo and the techniques described in this research report electrocaloric temperature
changes ranging from 0.4 to 1.2 K under an applied electric field of 1.5 MV/m[67]; how-
ever, these data show very erratic behavior, large amounts of scatter and peak effects
outside the expected temperature range for the stated concentration.
One group has measured the electrocaloric effect in both single crystal and ce-
ramic 0.75PMN-0.25PT using a calorimetry technique[38, 39], which measures the heat
generated by the electrocaloric effect. In calorimetry-based electrocaloric measurements,
86
Fig. 4.11. Strip chart recording of the electrocaloric effect in PMN-PT
87
Fig. 4.12. Strip chart recording of joule heating in a ceramic sample
88
samples are slowly heated in a temperature regulated environment. A temperature-
regulated heater is connected directly to the sample. If the temperature of the sample
starts to drop, the power to the heater is increased; if the temperature of the sample
starts to rise, the power is decreased. The power input to the heater is recorded. When
an electrocaloric measurement is made, the power of the heater is measured and then
integrated to find the total heat released by the sample when the field is applied and
the total heat absorbed by the sample when the electric field is removed. In ceramic
PMN-PT a peak electrocaloric effect of 0.13 J/g was measured under an applied field of
1.0 MV/m at a temperature of 130 C[38]. This corresponds to an electrocaloric temper-
ature change of 0.35 K, slightly below the average peak electrocaloric effect of 0.38 K
reported in this dissertation. In [111]− oriented single crystal 0.75PMN-0.25PT the
largest reported electrocaloric effect is 0.23 J/g under an applied field of 1.0 MV/m at
a temperature of 110 C[39]. This corresponds to an electrocaloric temperature change
of 0.61 K, compared to the peak electrocaloric effect of 0.56 K reported in this disserta-
tion. While none of the samples in this research have the same compostition as Sebald’s
samples, the directly observed electrocaloric temperature is consistent with calorimetry
measurements in similar materials.
Other groups have estimated the electrocaloric effect of PMN-PT in thin-film
samples[31]. While the thin-films are too small to directly probe, they are grown on a
substrate with an electrode and the polarization is mapped as a function of temperature
and applied electric field. The presented data is the electric field integral of the temper-
ature derivative of the polarization. While the astounding result of a 12 K electrocaloric
89
effect is presented, this must be taken with a mine of salt. This estimation of the tem-
perature change in the thin film neglects the substrate, which account for nearly all of
the thermodynamics. The proper way to treat the thin film is as a latent heat source;
however, the heat capacity of the substrate plus the thin film must be considered to
derive the useful contribution. For a 1 µm thin film on a 100 µm substrate, there would
be a two order of magnitude reduction in the “useful” electrocaloric contribution.
Polarization measurements in PMN-PT suggest that it should be capable of pro-
ducing an electrocaloric effect greater than 1 K over a temperature span of 30 to 50 K.
While 1 K has yet to be reliably observed, single crystal PMN-PT is capable of pro-
ducing electrocaloric effects greater than 0.5 K over a 30 K temperature span. Better
ceramic processing techniques may allow ceramics to withstand electric fields greater
than 2.0 MV/m, increasing the electrocaloric effect. Dependence of the electrocaloric
effect on concentration of PT should also be studied. Even though the maximum elec-
trocaloric effect in a PMN-PT plate may be limited to 0.5 to 0.75 K, this may be more
than enough to serve as the working medium for a refrigerator capable of cooling an
object by 30 K.
90
Chapter 5
An electrocaloric solid state heat pump
While vapor compression refrigeration has tended towards safer, more environ-
mentally friendly working fluids, better environmental benefits would come from the de-
velopment of high-efficiency all-solid-state refrigerators. Currently thermoelectric heat
pumps are used as chip coolers for computer processors, and they have been adapted as
plug-in refrigerated chests. A key drawback to more widespread use of thermoelectric
technology are low efficiencies, typically 7 to 11% of the Carnot limit. Thermoelec-
tric research has focused on designing better materials with larger temperature changes
to raise efficiencies. While there is no theoretical upper limit to the performance of
thermoelectrics, few breakthroughs have occurred since the discovery of semi-conductor
thermoelectrics. The slow pace of discovery for thermoelectrics has helped spur research
into other solid-state cooling technologies.
The focus in cooler design has been to develop new materials capable of producing
the largest possible temperature changes. Larger temperature changes allow simpler
designs. If a working material can produce a 30 K temperature change, it can cool an
object almost 30 K through direct contact and relatively slow cycles. Figure 5.1 shows
a schematic of a typical solid state cooler. A heat switch connects a caloric element
to the object to be cooled as the element is engaged. As the two objects come into
thermal equilibrium, the switch connects the caloric element to the hot heat exchanger
91
Fig. 5.1. A heat switching heat pump
92
to recharge the caloric element. The process is then repeated. If the caloric elements
can produce temperature changes smaller than the desired temperature span, the caloric
elements could be run in series to create a step-up cooling system[36]. This simplicity
has focussed development towards a search for caloric elements with larger and larger
temperature changes.
Recent research into magnetocaloric and electrocaloric materials[21, 34, 35, 42,
43, 31, 39] has led to the discovery of many new compounds capable of generating large
temperature changes. Gadolinium-based materials exhibit a magnetocaloric temperature
change of 11 to 17 K under the application of 5 T magnetic fields[34]. Demonstration
coolers based off these materials can produce cooling spans of 30 to 50 K[21]. The limit
to magnetic refrigeration, however, is the ability to scale devices. Large magnetic fields
are not easy to create. Most 5 T magnets require superconducting wires with a separate
cryogenic cooling system, making it difficult to build a small, portable system. It is
also difficult to raise and lower a magnetic field quickly. Electrocaloric materials avoid
most of the problems with magnetocalorics; large electric fields are relatively cheap and
easy to produce and electrical power supplies are highly scalable. The drawback to the
electrocaloric effect has been the lack of discovery of materials capable of producing large
temperature changes. In this research, the largest measured electrocaloric effects were
0.75 K at electric fields of 1.5 MV/m in single crystal PMN-PT. The largest reports in
the literature of a directly measured electrocaloric effect is 1.8 K at 2.0 MV/m in lead
scandium tantalate at temperatures around 300 K[42]. The limitation may not be in the
thermal properties of the materials, but in the design of devices to exploit the cooling
power of these materials.
93
If the thermoacoustic effect discussed in section 1.2 could be generalized to de-
scribe a device using a generic thermally-active element in a process analogous to the
Stirling cycle, such a device could achieve greater cooling power and larger temperature
spans than current solid-state cooler designs. In this chapter, an all-solid-state heat
pump analogous to the thermoacoustic effect in fluids will be described. Though the de-
sign may be adapted to exploit one of many thermal effects, this heat pump is designed
to use electrocaloric elements as a working medium.
5.1 A theoretical model
So long as some physical symmetry is broken[60], a small, cyclical temperature
change can be used to establish temperature gradients much larger than the local tem-
perature change. Thermoacoustics provides a model for a novel thermodynamic cycle to
more effectively use solid-state thermodynamic effects.
5.1.1 A quick review of thermoacoustics
Fig. 5.2. The thermoacoustic cycle
94
Literally, thermoacoustics is the interaction of heat and sound. Specifically, ther-
moacousticians design closed-cycle heat engines using the inertia, compliance, and dissi-
pation of acoustic media to replace the mechanical pistons and linkages of conventional
heat engines and refrigerators. While normal sound is adiabatic, standing waves along a
solid interface can interact with the interface to pump heat. Figure 5.2 shows a diagram
of the thermoacoustic cycle.
In the first panel, the gas moves to the right and is compressed. In the next panel,
the compressed gas transfers a small amount of heat to the solid, compressing further. In
the third panel, the gas moves to the left and expands. In the final panel, the gas absorbs
a small amount of heat from the solid and expands. A small amount of heat has been
moved from left to right, and the cycle repeats. Just before and just after this parcel
of gas is another small parcel of gas, which allows a bucket-brigade-like effect to occur,
building a large temperature gradient from a small local temperature oscillation in the
gas. Schematically, it is easy to envision a generic temperature oscillation interacting
across an interface. In the next section I will generalize the heat transport equations in
thermoacoustics to present a theory for a general cycle analogous to the thermoacoustic
cycle.
5.1.2 Generalizing the thermoacoustic cycle
While figure 5.2 shows a schematic of the actual thermoacoustic effect, figure 5.3
shows an abstraction of the effect. The fluid half of the interface has been replaced
by a solid capable of producing a temperature oscillation. Every other solid plate is
free to move parallel to the interface. Provided that the move plate travels farther
95
Fig. 5.3. A generalized Stirling-like heat pump cycle
96
laterally than the heat generated by the temperature oscillation, the resulting effect
should be completely analogous to the thermoacoustic effect. Two length scales would
need to be observed for this analogy to hold. First, the plates would have to be within
one thermal penetration depth of each other. This would allow heat to be transferred
between the plates within the time span of the oscillation. Second, as previously stated,
the relative displacement of the two plates would have to be much greater than the
thermal penetration depth, breaking a physical symmetry that would cancel out any net
heat transport.
For BST and PMN-PT, the thermal pentration depth for a 30 Hz oscillation is
about 100 µm. A 200 µm thick plate would be nearly totally thermally active. 2 cm
square, single crystal PMN-PT plates 200 µm thick have already been produced. Ceramic
PMN-PT and BST samples have been polished thinner than 250 µm, though the largest
areas made have been 1 cm cylinders. To break the heating symmetry, the plates would
need to be displaced a distance greater than 1 mm relative to each other. This could
easily be accomplished using a loudspeaker cone as an actuator. Thermoacoustics also
provides a framework to estimate the maximum possible cooling power of an analogous
relative motion device.
The cooling power for a fluid-based thermoacoustic cycle is found by the relation[50]:
Q =12ΥδκTmβppaua(1−RT ) (5.1)
where Υ is the perimeter, δκ =√
2κ/ρ2πf is the thermal penetration depth, Tm is
the average temperature, βp is the thermal expansion coefficient of the fluid, pa is the
97
amplitude of the pressure oscillation, ua is the amplitude of the velocity oscillation and
RT is the ratio of established temperature gradient to a critical temperature gradient
given by
RT =ρmcpua∆T
2πTmβppafl(5.2)
where ρm is the mean density of the fluid, cp is the specific heat at constant pressure,
∆T is the temperature span, f is the frequency of the oscillation and l is the length of
the interface. The critical temperature gradient
∆Tcrit =2πfTmβppa
ρmcpua(5.3)
is the point in thermoacoustics where temperature changes due to pressure oscillations
exactly cancel the temperature changes due to displacement oscillations[50].
The expressions for Q and RT can be elucidated with a few substitutions. First,
the pressure amplitude pa can be converted to a temperature oscialltion Ta with the
relation
Ta =Tmβp
ρmcppa (5.4)
The velocity amplitude ua can be expressed in terms of the displacement ampli-
tude ψa = ua/(2πf). Finally, the thermal working volume of the device is V = lΥδκ.
This simplifies the temperature gradient ratio to
RT =(∆T/l)(Ta/ψa)
(5.5)
98
The numerator in the temperature gradient ratio is the mean temperature gra-
dient established across the device while the denominator is an effective temperature
gradient given by the temperature oscillation amplitude Ta and the displacement ampli-
tude ψa.
The cooling power density Q/V can now be written as
Q
V=
(πfρmcpT 2
a
) 1∆T
RT (1−RT ) (5.6)
This density has a maximum value when the ratio RT = 1/2.
The maximum cooling density can then be defined as
Q
V=
π
4
(ρmcpTa
)f
Ta∆T
(5.7)
The factor ρmcpTa is the amount of heat per unit volume generated in one cycle
of the device. The frequency converts the heat per cycle to the heat per second. Ta/∆T
is the fraction of the total temperature lift ∆T provided by the temperature oscillation
Ta.
Table 5.1 shows the theoretical maximum cooling power density for a heat pump
described by equation 5.7. The maximum cooling power density and total span achieved
by the device are inherently trade-offs; the larger a temperature span sustained, the
less cooling power available for the device. As this is a first principles theory, loss
effects have been neglected. As a result, cooling power density is not reported when
it drops below 1.0 W/cm3 −K. Since the cooling power increases as the square of the
99
(Q/V )Max (W/cm3)Lift (K) 0.50 0.66 1.00 1.50 2.00 Ta
0.5 35.3 61.6 141 318 5651.0 23.5 30.8 94.3 212 3771.5 11.8 20.5 47.1 106 1892.0 8.8 15.4 35.3 79.5 1412.5 7.1 12.3 28.3 63.6 1133.0 5.9 10.3 23.6 53.0 94.34.0 4.4 7.7 17.7 39.8 70.75.0 3.5 6.2 14.1 31.8 56.67.5 2.4 4.1 9.4 21.2 37.7
10.0 1.8 3.1 7.1 15.9 28.312.5 1.4 2.5 5.7 12.7 22.615.0 1.2 2.1 4.7 10.6 18.920.0 1.5 3.5 8.0 14.125.0 1.2 2.8 6.4 11.330.0 1.0 2.4 5.3 9.440.0 1.8 4.0 7.150.0 1.4 3.2 5.7
Table 5.1. Cooling power at a given temperature span (lift) of a thermoacoustic-likedevice operating at 30 Hz
100
driving temperature oscillation Ta, even small improvements in the caloric elements could
significantly reduce the size of a potential device.
It is important to note that the theoretical performance summarized in table 5.1
is for any physical mechanism capable of producing temperature oscillation Ta at a fre-
quency of 30 Hz. While in the traditional thermoacoustic picture only the fluid is capable
of generating a temperature oscillation, one could double the effective temperature os-
cillation using two thermally active solids properly phased with each other. Hence, a
temperature oscillation of 0.5 K in one element translates to a working temperature
oscillation of 1.0 K.
5.2 Description of device
Figure 5.4 shows a plan for a demonstration solid-state heat pump. At the heart
of the heat pump is a stack of rectangular caloric plates. Every other plate is rotated 90o
so that the ends of the plates stick out slightly from each side. Beryllium copper flexures
are attached to the protruding ends of the rotated plates. A conductive paint is used to
electrically connect an electrode on the plate stack to the copper flexure. Four electrical
connections are needed to apply opposite electric fields on every other plate but have
no potential difference in the small gap between electrodes. Copper blocks are attached
to the stationary plates for structural support and also serve as heat exchangers. The
moving plates can be driven by a piezoelectric actuator, a loudspeaker with a linkage,
or some other type of piston. The caloric elements could be either BST or PMN-PT;
estimations of the performance of each of these materials in this proposed device are
presented in the next section.
101
Fig. 5.4. Proposed design of all solid state heat pump
102
5.3 Predicted performance using ferroelectric ceramic elements
Max Ta 2.0 KSpan at Q/V = 1.0W/cm3 150 K
Q/V at 30 K Span 9.4 W/cm3
Table 5.2. SSHP performance using 1.0 K caloric elements
Table 5.2 shows the performance characteristics of a hypothetical relative motion
heat pump using caloric elements capable of producing a temperature oscillation of 1.0 K.
If each layer could generate this temperature change and the device could be properly
phased, the total net temperature oscillation would be 2.0 K. Given this temperature os-
cillation, equation 5.7 predicts that the device could produce a temperature span greater
than 200 K at almost no cooling load. To sustain a cooling over a 30 K temperature
span, the device could theoretically achieve a cooling density of 9.4 W/cm3. Cooling the
volume the size of a human body would require about 150 W of cooling power. If the
caloric elements could achieve the 1.0 K oscillation, the proposed device would require
16 cm3 of active material to achieve this cooling over the 30 K span; this would require
a device with a length of 2.5 cm on each side.
While measurements made on BST and PMN-PT ceramics only observed max-
imum electrocaloric effects between 0.4 and 0.5 K, these ceramics may still be useful
103
BST PMN-PTMax ∆T 0.45 K 0.38 K
∆T > 0.25K 10 K 50 K∆T > 0.33K 5 K 35 K
Span at Q/V = 1.0W/cm3 15 K 30 KQ/V at 10 K Span 1.8 W/cm3 3.1 W/cm3
Q/V at 30 K Span 1.0 W/cm3
Table 5.3. SSHP performance using BST and PMN-PT ceramic elements
in designing and building small-scale coolers. Table 5.3 shows performance characteris-
tics of a relative motion solid-state heat pump using both BST and PMN-PT elements.
While the maximum electrocaloric effect observed in BST is slightly larger than that
measured in PMN-PT, the electrocaloric effect in BST drops off rapidly as the tempera-
ture differs from the peak effect temperature. PMN-PT has an electrocaloric effect larger
than 0.25 K over a 50 K temperature span compared to a 10 K temperature span in
BST. PMN-PT has an electrocaloric effect greater than 0.33 K over a 35 K temperature
span compared to only a 5 K temperature span in BST. This limits a BST-based heat
pump to a maximum operating span of 15 K whereas PMN-PT could operate over a
30 K span. Q/V > 1.0W/cm3 was chosen as the threshold of operation because the
theory of operation currently does not account for losses, such as thermal conduction,
which must be overcome in a practical device. Over a 10 K operating span, a PMN-PT
solid-state heat pump should be able to achieve a cooling density of 3.1 W/cm3. If the
device was constructed from 10 layers with dimensions of 22 mm by 18 mm by 0.2 mm,
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a demonstration cooler should be able to produce 12 W of cooling power about the same
power as other thermoacoustic demonstration coolers[50].
105
Chapter 6
Conclusion
Many challenges in the realm of physics, materials science and engineering exist
before a practical electrocaloric refrigerator is ready for the market. First and foremost,
we need to construct a working prototype to verify AC-to-DC temperature conversion.
With a working prototype, research in better electrocaloric materials will need to be
conducted to further understand ferroelectric properties.
To bridge the electrocaloric measurements in bulk materials and estimated elec-
trocaloric effect in thin films, a detailed study of polarization in bulk materials should
be conducted to confirm the integral relation. The mapping would need to include mea-
surements of the dielectric constant, ε(Eq, T ), as a function of temperature and external
electric field. In addition, the pyroelectric effect will need to be measured with no exter-
nal electric field to determine any temperature effects of the constant of integration. The
electrocaloric temperature change, ∆TEq(Eq, T ), could then be mapped as a function of
temperature and external electric field to confirm the relation. Thin films should also
be tested to try to directly observe any electrocaloric effect. Thin films can reportedly
withstand electric fields in excess of 20 MV/m[31, 32]. The mechanism which supports
larger fields should be investigated further, beacuse if thinner single crystal or ceramic
plates could be made which could safely accept electric fields larger than 2 MV/m, larger
electrocaloric effects may be measured in bulk BST or PMN-PT. While it is dubious to
106
calculate electrocaloric temperature changes from thin-film polarization measurements,
it may be possible to observe some temperature change.
If a thin film 2 µm thick can truly create a 12 K electrocaloric temperature
change[31], some direct temperature change should be observable on a 150 µm thick
substrate. Provided that the specific heats of both the thin film and the substrate
are comparable, a 12 K temperature change in an isolated film would translate to a
0.16 K temperature change in a thin film-substrate system. The direct electrocaloric
temperature measurement setup has a resolution capable of seeing temperature changes
as small as 0.4 K, so direct observations of the effect from a thin film are possible.
While thin films may lead to larger temperature changes in BST or PMN-PT, there are
other reported ferroelectrics capable of producing 1 K electrocaloric effects. If reports of
electrocaloric effects larger than 1 K in perovskite ferroelectrics such as lead scandium
titanate are true[42], these materials could open the door for better exploitation of
thermoacoustic-based solid state heat pumps.
In addition to a better understanding of the materials themselves, processing
techniques need to be developed to produce thinner samples with larger areas. Previous
research[23], confirmed by data in chapter 3, shows that better electrocaloric effects are
achieved by larger grained ceramics. However, larger grained ceramics tend to be more
brittle. A detailed study of mechanical properties such as Young’s modulus must be
performed to better understand the limits to aspect ratio of caloric plates for building
larger devices.
Achieving a sustained temperature gradient through a moving temperature oscil-
lation is still just a first step. Even with advances in elements themselves, an effective
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heat exchanger would need to be designed to convert the temperature gradient into use-
ful cooling power in a practical device. Each element added in the process of converting
work into cooling power introduces new loss mechanisms and engineering challenges.
While a caloric element capable of 30 K temperature changes remains the holy grail of
solid-state cooling, making better use of easier to control caloric effects can open up a
new field of development for all-solid-state devices capable of producing a greater cooling
density compared to current technology.
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Appendix A
Protocol for preparing ceramics
A.1 BST Protocol, July 2007
Preliminary steps, as necessary: a) Burn out oven, crucibles, and mixing/grindingmedia. b) Use high purity metal oxide and carbonate powders, 99.99% or better. c) Useonly reagent grade acetone (99.5% purity or better) to mix the powder or to clean anyvessel contacting the BST.
1. Weigh powder. For 50 g Ba0.67Sr0.33TiO2: 25.346 g BaCO3; 9.339 g SrCO3;15.313 g TiO2.
2. Form a slurry with 50 g powder and 150 ml acetone in a 1 l Nalgene bottle.
3. Add 500 ml Zr media; media should be just below level of slurry.
4. Mix on roller mill at 22 rpm for 4 hours.
5. Pour through SS strainer (to separate media) into a 1 l Pyrex bowl; rinse media instrainer with acetone once into bowl. Wash media, discarding acetone, for futureuse.
6. Dry slurry at 80 C; stirring occasionally to promote drying. Place under vacuumafter initial drying.
7. Grind to fine powder in agate mortar and pestle.
8. Sieve through 250 µm mesh into Zr trays.
9. Calcine in oven. Ramp up at 5 C/min to 1200 C for 6 hours; ramp down at5 C/min.
10. Put dry calcined powder in 1 l Nalgene bottle with 150 ml Zr media. Dry-grind inbalanced vibratory mill for 4 hours.
11. Perform x-ray diffraction (XRD) analysis on sample of dry-ground powder. Deter-mine if calcining eliminated non-BST materials.
12. Add 150 ml acetone to the grinding bottle, then add more Zr media to reach justbelow slurry level.
13. Wet-grind in balanced vibratory mill for 6 hours.
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14. Dry slurry as in step 6.
15. Grind to fine powder in mortar and pestle.
16. Clean die with lint-free tissue.
17. Weigh 0.5 g powder on small square of waxed massing paper and sieve through#50 mesh screen into 1/2 inch die.
18. Uniaxially press to 5700 lbs (29,000 psi = 200 MPa for 1/2 inch die) and hold for30 seconds.
19. Cold isostatic press (CIP) to 200 MPa (if available).
20. Place green pellets on Zr tray and cover with second tray; place in oven.
21. With O2 diverted to water in beaker, adjust needle valve for flow rate of 1 bubble/s;reconnect O2 to oven.
22. Heat oven at 5 C/min to 400 C and hold for 1 hour; heat at 3 C/min to 1500 Cand hold for 6 hours; cool at 2 C/min.
23. Cut 250 µm thick wafers of sintered pellets using commercial ceramic wafering saw.
24. Sputter Cr/Al electrodes on each wafer face using Perkin-Elmer 4400 sputteringmachine.
A.2 PMN-PT Protocol, July 2008
Preliminary steps, as necessary: a) Burn out oven, crucibles, and mixing/grindingmedia. b) Use high purity components, 99.9% or better.
To avoid the pyrochlore phase, the powder is made in two stages.
A.2.1 Stage I: MgNb2O6 preparation
1. Weigh powder. For 50 g MgNb2O6: 6.697 g MgO; 43.302 g Nb2O5. (2% mol MgOis added to inhibit the pyrochlore phase.)
2. Form a slurry with 50 g powder and 150 ml ethanol in a 1000 ml Nalgene bottle.
3. Add 500 ml Zr media; media should be just below level of slurry.
4. Mix on roller mill at 22 rpm for 12 hours.
5. Pour through SS strainer (to separate media) into 1000 ml Pyrex bowl; rinse mediain strainer with ethanol once into bowl. Wash media, discarding ethanol, for futureuse.
6. Dry slurry at 80 C. Stir occasionally to promote drying.
7. Grind to fine powder in agate mortar and pestle.
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8. Sieve through a #50 mesh into a zirconia crucible.
9. Calcine in oven. Ramp up at 5 C/min to 1000 C for 6 hours; ramp down at5 C/min.
10. Put dry calcined powder in 1000 ml Nalgene bottle with 150 ml Zr media. Dry-grind in balanced vibratory mill for 4 hours.
11. Perform x-ray diffraction (XRD) analysis on sample of dry-ground powder. Deter-mine if powder has the proper columbite phase and is free of unreacted powder.
12. Add 150 ml ethanol to grinding bottle, then add more Zr media to reach just belowslurry level.
13. Wet grind for 6 hours in a balanced vibratory mill.
14. Recover the powder following steps 5 through 8.
15. Dry the powder in the oven at 200 C for 12 hours.
16. Transfer dry powder to a 250 ml Nalgene bottle.
17. Store powder under vacuum to prevent absorption of moisture.
A.2.2 Stage II: 0.92PMN- 0.08PT preparation
1. Weigh powder. For 50 g PMN-PT: 34.501 g PbO; 14.549 g MgNb2O6; 0.988 gTiO2. (5% mol of PbO is added to compensate for loss during ceramic processing.)
2. Form a slurry with 75 g powder and 150 ml ethanol in a 1000 ml Nalgene bottle.
3. Add 450 ml Zr media; media should be just below level of slurry.
4. Mix on roller mill at 22 rpm for 12 hours.
5. Pour through SS strainer (to separate media) into 1000 ml Pyrex bowl; rinse mediain strainer with ethanol once into bowl. Wash media, discarding ethanol, for futureuse.
6. Dry slurry at 80 C. Stir occasionally to promote drying.
7. Grind to fine powder in agate mortar and pestle.
8. Sieve through a #50 mesh into Al2O3 crucible and cover.
9. Calcine in oven. Ramp up at 5 C/min to 800 C for 4 hours; ramp down at 5 C/min.
10. Put dry calcined powder in 1000 ml Nalgene bottle with 100 ml Zr media. Dry-grind in balanced vibratory mill for 4 hours.
11. Perform x-ray diffraction (XRD) analysis on sample of dry-ground powder. Deter-mine if powder has pyrochlore phase.
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12. Add 150 ml ethanol to grinding bottle, then add more Zr media to reach just belowslurry level.
13. Wet grind in balanced vibratory mill for 6 hours.
14. Recover the powder following steps 5-7.
15. Sieve all the powder through a #50 mesh screen and store in a 250 ml Nalgenebottle.
16. Add 35 g of PMN-PT powder into a 200 ml Pyrex dish.
17. Measure out 4% of powder weight of binder resin into another 200 ml Pyrex dish.For 35 g of powder, this corresponds to 1.4 g of binder resin.
18. Dilute binder resin with acetone until the resin completely dissolves into a clearliquid.
19. Pour the diluted binder resin into the dish containing the powder and stir the liquidinto the powder using a stainless steel spatula.
20. Continue stirring until the acetone has completely evaporated. Transfer the powderto a mortar.
21. Grind the powder with a mortar and pestle, sieve through a #50 mesh screen andstore powder until it is ready to be pressed.
A.2.3 Stage III: Pressing 1/2” ceramic disks
1. Measure 0.7 g of binderized powder.
2. Wipe off die with lint-free tissue.
3. Increase the pressure in the press until the force gauge is just beginning to get areading.
4. In one motion, press to a force of 4500 lbs and hold for 10 seconds.
5. Remove the pellets from the die and measure their mass.
6. Place the pressed pellets on Pt foil on a Zr plate. Place the plate in the oven.
7. Burn out the binder by heating the oven at 1 C/min to 325 C and hold for 3 hrs.Heat oven at 2 C/min to 500 C and hold for 10 hrs to remove the ash. Turn offthe heating elements and let the oven cool.
8. Measure the mass of the pellets just after coming out of the furnace. The pelletsshould have lost about 1.5 to 1.7% of the initial mass. Measure the thickness ofthe pellets to compute the green density. The green density should be between4.4 g/cc and 4.8 g/cc.
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9. Place the pellets on Pt foil in the lid of an Al2O3 crucible. Cover with a secondsheet of Pt foil. Cover with the crucible and place in the oven.
10. Sinter the pellets heating the oven at 10 C/min to 1280 C. Hold the oven at 1280 Cfor 2 hours. Turn off the heating elements and let the oven cool.
11. Polish the sample on one face to create a flat, smooth surface.
12. Polish the sample’s other face until the sample has a total thickness of 300 µm orless.
13. Measure the mass of the pellets just after coming out of the oven. Measure thethickness and diameter to compute the density. The ceramics should have a densitywithin 2% of 8.0 g/cc.
14. Anneal the polished samples to remove any excess lead that may have built up inthe ceramic.
15. Sputter Cr/Al electrodes on each wafer face using Perkin-Elmer 4400 sputteringmachine.
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Appendix B
Perkin Elmer 4400 Operator’s Reference
B.1 Start up procedure:
B.1.1 Start up from shutdown:
1. Open the N2 purge gas regulator.
2. Turn on the cryopump water using the middle valve located on the wall between the twowindows about 10 ft high.
3. Turn on the compressed air line, located behind the sputtering unit about 12 feet high.
4. Make sure that all keys are set to ‘Manual’ and the all switches are set to off or closed.
5. Turn on the main breaker on the back of the sputtering unit under the load lock.
6. Locate the Ultek Auto Pumpdown Control (APC).
7. Turn on the mechanical pump.
8. Turn on the Hi-Vac pump.
9. Open the throttle valve.
10. Open the Plexiglas door on the lower right side of the unit. Locate the cryo-pump regen-eration control.
11. Turn the ‘cycle time minutes’ control knob to the right, slightly off the ‘0’ mark and pushthe red start button. At the end of the cycle, all adsorbed gases in the cryo-pump shouldbe vented and the temperature should reach 10 K.
B.1.2 Start up from overnight:
1. Turn on the network water by opening the valve below the cryopump water valve. Checkthe flow meters on the cryopump panel below the Ultek Lock Control (ULC) to make surethat water is flowing.
2. Make sure that the vent fan is on and the purge gas cylinders are open.
3. Check the key on the APC. If the key is set to ‘Manual’ or ‘Manual (interlock)’, set thekey to ‘Auto’. Pump down the chamber by holding the ‘Start’ button on the APC andpressing the ‘Pump’ button. When the trip light comes on the hi-vac valve should open.
4. When the convectron gauge reads 0-1 mTorr, turn on the ion gauge. If the chamber iskept under vacuum the ion gauge should read below 5.0 × 10−7 Torr after 2-3 hours. Ifthe unit has not been used in a while, the ion gauge may take a while to reach this value.
5. Vent the load lock. When the ‘ATM’ light turns on, close the vent line and set the ULCkey to ‘Auto’.
6. Open the gas regulators on the Ar and N2 cylinders.
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7. Turn on the target water using the valve located below the cryopump water valve.
8. Turn on the RF generator and the DC power supply located to the right of the sputteringunit. If the DC sputtering controller is beeping, press the blinking red light to silence thealarm. The light will continue to flash until the gas solenoid valve is opened.
B.2 Automatically controlled operating procedures:
1. With the ‘ATM’ light on, depress both ‘Up’ switches on the load lock lid. As the lid opens,the elevator will automatically raise the pallet for loading substrates.
2. Load substrates on the pallet. If working with a non-standard substrate, the total heightmust be kept below 3/4” or the pallet will not clear the gate valve.
3. Depress both ‘Down’ switches on the load lock lid. Wait for the elevator to fully lower andseat before completely closing the lid. When the lid closes, the automatic loading sequenceshould begin.
4. To interrupt the pumpdown and loading sequence hold down ‘Start’ and press ‘Standby’on the ULC. To resume loading hold ‘Start’ and press ‘Load’.
5. Monitor the progress of the pallet as it loads into the sputtering chamber. Look throughthe viewport to make sure that the pallet is evenly balanced on the table and that nothingwill catch on the chamber shutter as the table rotates.
6. If you need to use the DC sputtering target, wait for the chamber pressure to drop below7.0× 10−7Torr as read by the ion gauge.
7. To add gas to the sputtering chamber, hold down ‘Start’ on the APC, press ‘Gas’ and thenquickly press ‘Pump’. Pressure can build up behind the gas solenoid valve causing thechamber pressure to rise too quickly when the valve is opened. If the chamber pressurerises to much and the hi-vac valve closes, pump the system down by holding ‘Start’ andpressing ‘Pump’ on the APC. Wait for the trip light to illuminate and the hi-vac valve toreopen.
8. Repeat ‘Gas’, ‘Pump’, ‘Gas’ bursts until the convectron holds at 0 mTorr with the gaslight indicated. Open the flow controller cutoff valve of the desired sputtering gas, locatedon the right side of the sputtering unit. Make sure the flow controller of the desired gasis turned on and set to flow. Run the gas pressure up to the desired level for sputtering.The convectron should read 8-10 mTorr. For Ar, the flow controller should be set pointshould be 3.7.
9. With the gas running, make sure the motor speed control is set at the yellow mark. Look inthe viewport and confirm that the table is rotating. The pallet is now ready for sputtering.Detailed instructions can be found in the ‘Sputter Procedures’ section of this guide.
10. When you have finished sputtering, turn the flow controller set point back to 0.0. Whenthe convectron reads 0 mTorr close the flow controller cutoff valve. Shut the gas solenoidby holding ‘Start’ and pressing ‘Pump’ on the APC panel. Check the chamber viewport tomake sure that the pallet table has stopped rotating. If the table is still rotating 5 secondsafter turning off the gas, tap on the front of the APC to free the relay. If you cannot freethe relay, turn the rotation control all the way counterclockwise to stop the motor. Neverattempt to load or unload the pallet if the table is rotating as this can damage the system.Contact the sputtering unit supervisor immediately.
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11. With the pallet table stationary, unload the pallet by holding ‘Start’ and pressing ‘Unload’on the ULC panel. The pallet will automatically unload and the load lock will vent toatmospheric pressure.
12. The purge gas line will run until the load lock is opened. To conserve N2, open the loadlock as soon as the ‘ATM’ light is illuminated.
B.3 Sputtering Procedures:
B.3.1 DC Sputter Deposition:
1. Set the selector switches on the RF control panel to ‘Sputter Deposit’ and ‘Target 1’.
2. The DC power supply is controlled by an interface panel in the bottom right corner of thesputtering unit. After ‘Target 1’ has been selected an alarm on this panel should sound.Press the flashing red button to silence the alarm. If the button is still flashing, check thatgas is flowing and that water is flowing through the switches in the lower left panel.
3. Make sure that all of the yellow ‘Remote’ lights are out and that the regulation mode isset to ‘Power’ (left green regulation button lit).
4. Set the cathode power by holding down the ‘Level’ button and turning the knob. Forcoarse adjustments, the ‘Vernier’ button should not be lit. For finer adjustments, pressthe ‘Vernier’ button.
5. Set the power ramp up time by holding the ‘Ramp’ button and turning the knob. Thesystem can easily handle a ramp of several kV per minute.
6. Set the sputtering time by holding the ‘Ramp’ button, pressing the ‘Set Pt’ button, andturning the knob. Note: All time settings are displayed in 1
100 ’s of a minute.
7. If the target has not recently been used, clean the target as follows:
8. set the shutter position switch to ‘Close’ and wait for the light above the shutter positionswitch to go out. The open section of the shutter should be closest to the sputteringchamber viewport.
9. Begin sputtering by pressing the green ‘On’ button on the DC control panel. You canmonitor power, voltage, and current on the top monitor of the panel and power, voltage,current, energy (kW-hrs), and time elapsed on the bottom monitor.
10. When the process stops, press the blinking red ‘Off’ button to silence the alarm.
11. To sputter onto a substrate, set the shutter position switch to ‘Open’. Wait until the lightabove the switch goes out. The open section of the shutter should be toward the left sideof the chamber. The target is now ready for sputtering. If needed, adjust the depositionpower and time by following the above procedure.
B.3.2 RF Sputter Deposition:
1. Set the selector switches to ‘Sputter Deposit’ and either ‘Target 2’ or ‘Target 3’
2. Make sure the RF power knob is turned all the way down and the ‘Auto’ toggle switch onthe digital clock timer panel is down (off).
3. If the selected target has not been recently used, clean the target as follows:
4. Set the shutter switch to ‘Close’ and wait for the light above the shutter switch to go out.The open section of the shutter should be by the viewport of the sputtering chamber.
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5. Tune the system by turning on the ‘RF power on/off’ button and slowly turning up the‘RF power adjust’ knob to the desired forward power. Carefully monitor the ‘RF reflectedpower’ meter and adjust the ‘Load’ and ‘Tune’ switches to minimize reflected power, ad-justing the ‘RF power adjust’ as needed to maintain forward power. Keep the reflectedpower below 10 Watts.
6. When the system is tuned, press the ‘RF power on/off’ button to turn off the power. Flipthe ‘Auto’ toggle switch on. Set the sputtering time on the timer by holding down theblue button and punching in the desired time in tenths of seconds. Start sputtering bypressing the RF power button. The plasma impedance will vary slowly over time; monitorthe reflected and forward power and adjust tuning settings as necessary. The system willautomatically turn off when the timer has finished counting down. After the power hasbeen shut off, press the RF power button once to cycle the button.
7. Set the shutter switch to ‘Open’ and wait for the shutter indicator light to go out. Thetarget is now ready for RF sputtering. If necessary, forward power and deposition timecan be changed using the above procedures.
B.3.3 RF Sputter etching:
(Note: As the auto tune system is currently offline, Etch mode is not recommended atthis time)
1. Set the selector switches to ‘Sputter Etch’ and ‘Etch’
2. Make sure the RF power knob is turned all the way down and the ‘Auto’ toggle switch onthe digital clock timer panel is down (off).
3. During sputter etching mode, there is no way to prevent plasma from contacting thesubstrate. Keep in mind that the plasma starts bombarding the substrate as soon as it islit. It is best to use the auto tune system if there is concern of thermally damaging thesubstrate due to prolonged exposure to hot gas ions. To manually tune the system, seethe procedure for RF sputtering.
4. When the system is tuned, turn off the RF power. Flip the ‘Auto’ toggle switch on. Setthe etching time on the timer in tenths of seconds. Start etching by pressing the RF powerbutton. The system will automatically turn off when the timer finishes counting down.When the etch has finished, press the RF power button once to cycle the button.
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B.4 Shutdown procedures:
B.4.1 Overnight:
1. Turn off the RF generator and DC power supply.
2. Close all sputtering gas feed and make sure that the purge gas cylinder remains open.
3. Check that the pallet is unloaded.
4. Set the ULC key to ‘Manual’.
5. Pump down the load lock until the pressure falls below the trip point.
6. Turn off the ion gauge.
7. Check that the hi-vac and mechanical pumps switches are on and that only the throttlevalve is switched open.
8. Close only the target water line. Water must remain flowing through the cryo-pumpcompressor whenever the compressor is on to prevent damage.
9. If the cryo-pump temperature is above 50 K, the cryo-pump needs to be regenerated.Set the APC key to ‘Manual’ and follow the procedure outlined in the “start up fromshutdown” section to regenerate the cryo-pump.
B.4.2 Longterm shutdown:
1. Check that the RF generator and DC power supply are turned off.
2. Check that the pallet is unloaded.
3. Pump down the load lock until the pressure falls below the trip point.
4. Check that the ion gauge is turned off.
5. Set all keys to ‘Manual’.
6. Close all valves.
7. Turn off the hi-vac pump.
8. Turn off the mechanical pump.
9. Close all gas cylinders and shut off the regulators.
10. Turn off the vent fan and compressed air line.
11. Make sure that the target water line is closed.
12. When the cryo-pump compressor has cooled (after about 30 minutes) close the cryo-pumpwater line.
13. Turn off the main breaker on the power distribution box located in the back of the sput-tering system under the load lock.
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Vita
Matthew G. Hilt
EDUCATION:
May 2009 Ph. D. Physics The Pennsylvania State University
May 2002 B. S. Physics Worcester Polytechnic Institute
PRESENTATIONS:
Matthew G. Hilt, K. A. Pestka II, Jin H. So, and J.D. Maynard (2007, March). Elastics
constants and sound velocities in single crystal transition metal scandates. Paper presented at
the annual meeting of the American Physics Society, Denver, CO.
Matthew G. Hilt, K. A. Pestka II, G. D. Mahan, J. D. Maynard, D. Pickrell, B. Na,
and J. Tamburini (2006, May). Unconventional thermoacoustic heat engines (A). Journal of the
Acoustical Society of America, 119, 5, p. 3414.
J. D. Maynard, Matthew G. Hilt, and Logan Marcus (2005, September). Thermophysical
properties, as functions of pressure and temperature, for over 300 fluids, in vapor or liquid phase
(A). Journal of the Acoustical Society of America, 118, 3, p. 1927.
TEACHING EXPERIENCE:
2008 Instructor, Fluids and Thermal Physics (PHYS 213), The Pennsylvania State Uni-
versity
2008 Instructor, Wave Motion and Quantum Physics (PHYS 214), The Pennsylvania
State University
2005 Teaching Assistant, Mechanics (PHYS 211), The Pennsylvania State University
2004 Lab Instructor, Senior Labs (PHYS 458/402), The Pennsylvania State University