Understanding the Giant Seebeck Coefficient of MnO2 Nanoparticles

Costel Constantin

James Madison University

James Madison University, October 2012

- Crystal structures and semiconductor properties.

- Materials characterization methods.

- Giant Seebeck Coefficient Observed in Manganese Oxide Nanostructures?

Outline

Crystal Structures

WHAT KEEPS THE ATOMS TOGETHER INSIDE OF A CRYSTAL?

unique arrangement of atoms in a crystal. composed of a unit cell, which is periodically repeated in three dimensions on a lattice.

a = lattice constant

Types of Crystal Structures

Forces Between Atoms in a Crystal IONIC BONDS - electrostatic forces

between two oppositely-charged ions, e.g. alkali halogenides

METALLIC BONDS - electrostatic attraction between the metal atoms or ions and the FREE electrons, also called CONDUCTION electrons. e.g. Metals.

COVALENT BONDS - sharing of pairs of electrons between atoms, e.g. Semiconductors, Organic Molecules; C, Si, InSb.

VAN DER WAALS BONDS – arises from the polarization of molecules into dipoles.e.g. Noble Gas crystals, H2, O2. HOW ABOUT THE ENERGY LEVELS IN A CRYSTAL?

Electronic Band Structure in Solids

Electrons live in ENERGY ORBITALS = ENERGY LEVELS. ENERGY LEVELS in a crystal, where ions bond, form ENERGY BANDS.

HOW CAN WE UNDERSTAND THE DIFFERENCE BETWEEN METALS, INSULATORS, AND SEMICONDUCTORS IN TERMS OF ENERGY BANDS?

E

1s

2s 2p

3s 3d 3p

Metals, Insulators, Semiconductors

WHAT IS THE MECHANISM FOR SOME MATERIALS TO CONDUCT ELECTRICITY?

Define EF as the level below which all electrons fill up the states (little cups). METALS - Fermi energy level falls at the middle of the allowed band. INSULATORS and SEMICONDUCTORS - Fermi energy level falls at

the middle of the forbidden gap.

1s

2s 2p

3s 3d 3p

Conduction Bands, Valence Bands, and Band Gaps

VALENCE BAND - created by the outer shell electrons, and most of the states (cups) are occupied by electrons.

CONDUCTION BAND - free electrons coming from VB and able to conduct electricity.

BAND GAP - the width of the forbidden band.

1s

2s 2p

3s 3d 3p

Materials Characterization Principles and Techniques

X-ray Diffraction (XRD)

2sin( )nd

n = integer number for constructive interference. λ = the wavelength of the incoming and outgoing X-ray. = the diffraction angle. Great technique for identifying crystal structures

Seebeck Effect

Seebeck Effect

Seebeck Effect[a] Before Thermal Excitation [b] After Thermal Excitation

Seebeck coefficient, S = - DV/ DT. Typical values in the order of mV/(o). It can give an easy carrier type determination for semiconductor substrates.

How Do We Measure Seebeck Effect

Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM)

Fig. 1 Scanning Electron Microscope Fig. 2 Scanning Electron Microscope

Giant Seebeck Coefficient Observed in Manganese Oxide Nanostructures

Why Manganese Oxide Nanoparticles?

FangFang Song, Liming Wu and S Liang, Nanotechnology 23, 085401 (2012).

Einstein Prediction For Lowest Thermal Conductivity

XRD of our as-received MnO2 powder

Fig. 1 X-ray diffraction of as-received MnO2 powder Fig. 2 NIST MnO2 standard

Crystal Structure of our Manganese Oxide powder

Rutile structure. Gray atoms are Mn. Red Atoms are O.

http://en.wikipedia.org/wiki/File:Rutile-unit-cell-3D-balls.png

SEM and TEM images of MnO2 powder

1. FangFang Song, Liming Wu and S Liang, Nanotechnology 23 (2012) 085401 (4pp)

Fig. 1 Scanning electron microscope image of MnO2 [ref. 1]

Fig. 2 Transmission electron microscopy image of MnO2.

Figure of Merit and Harman Transient Method Thermoelectric materials are characterized by the figure of merit “ZT”. Sigma (s) – electrical conductivity. S – Seebeck coefficient. Kappa (k) – thermal conductivity.

ZT = VDC/VAC - 1

Preliminary ResultsZT vs. MnO2 particle stacking density

Preliminary ResultsSeebeck vs. MnO2 particle stacking density

Preliminary ResultsThermal conductivity vs. MnO2 particle stacking density

Conclusions MnO2 nanoparicles are promising for creating devices: Seebeck coefficient can be improved,

conductivity can be improved, and they exhibit very low thermal conductivity.

MnO2

THANK YOU

Giant Seebeck Coefficient Thermoelectric Device of MnO2 powder

FangFang Song, Liming Wu and S Liang, Nanotechnology 23, 085401 (2012).

How does the Figure of Merit behave as a function of temperature, particule size, and particule density?

By applying the transient Harman method we can find an answer to all these questions.

Important Problems to be Studied

Doping Semiconductors

INTRINSIC – do not conduct electricity because electrons are tightly bondedto the nucleus.

N-type – doping with Phosphorous atoms introduce an extra electron in the conduction band.

P-type – doping with Boron atoms introduce an extra hole in the conduction band.