Handbook of Thermal Analysis and Calorimetry

Vol 5: Recent Advances, Techniques and Applications

M. E. Brown and P. K. Gallagher, editors

2008 Elsevier B.V.

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Chapter 3

MICRO-THERMAL ANALYSIS AND RELATED TECHNIQUES

Duncan M. Price

Exhaust Management Systems, BOC Edwards, Kenn Business Park, Kenn Road,

Clevedon, North Somerset, BS21 6TH, UK

Email: Duncan.Price@bocedwards.com

1. INTRODUCTION

The major difficulty with conventional thermal analysis techniques is that they

measure the response of the whole sample. If one observes a broad change in

behaviour on heating a specimen, this could be the result of a genuine effect in a

homogeneous system or be due to a series of overlapping responses from a

heterogeneous system, where there may be a gradation in properties throughout

the sample. Alternatively, a weak effect seen in the entire sample could arise

from a strong response from a minority component (e.g. an impurity) within in

the bulk of the material. The same statements are true of other classical

analytical techniques which require moderately sized amounts of material for

sampling.

It is not surprising, therefore, that many investigators have combined studies by

thermal analysis with such techniques as optical [1] or electron microscopy. For

example, Price and Bashir [2] studied the thermal behaviour of

poly(acrylonitrile) which had been compression moulded in the presence of

water. Under conditions of high pressure, water acts as a solvent for this

otherwise intractable polymer (which normally degrades on melting) allowing

the material to flow. On cooling, the specimen has an open structure with the

water trapped in pores within the polymer mass. Differential scanning

calorimetry (DSC) was used to study the formation of ice within the pores on

cooling. Analysis of the shape of the DSC curves show that the pore size

followed an approximately bimodal distribution, whereas scanning electron

microscopy indicated that a third category of larger pores existed which could

not be detected due to the fact that they were incompletely filled with water [2].

Thermomicroscopy is the direct observation of a sample (usually) by optical

microscopy (often with polarised light) as a function of temperature [1].

Frequently called “hot-stage (optical) microscopy”, this is a relatively common

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technique which is used widely for the study of historical artefacts [3],

polymorphic transitions in drugs [4], pyrotechnics [5], and also for following the

crystallisation of polymers [6,7]. Other forms of high-resolution imaging such

as electron and scanning probe microscopies have been combined with the use

of a variable temperature sample stage. Of particular relevance to this

discussion is the use of atomic force microscopy at temperatures other than

ambient [8,9]. Studies by Grandy et al. [10] illustrate the potential of

mechanical property imaging above and below transition temperatures of two

phase polymer systems. Particularly novel is the application of a method of

image analysis which seeks to assign a statistical probability of each pixel

comprising the image to be one phase or the other (or an interphase). This is

based upon the assumption that the contrast can be described using overlapping

distributions of signal intensity arising from each component. Fasolka et al. [11]

have published similar work using resonant frequency (rather than forced

oscillation) non-contact atomic force microscopy where they also obtain images

at a temperature between the glass-rubber transitions of a diblock polymer

system.

The major disadvantage of any method of thermomicroscopy (whether optical,

electron or scanning probe technique) is that the whole sample is heated at the

same time. Changes in the sample which occur outside the field of view are not

observed and if these changes are irreversible (e.g. the sample crystallises,

degrades, two phases mix or separate) then a new sample must be prepared.

Ideally, one would desire a means to obtain a high-resolution image of a

specimen under ambient conditions and then carry out thermal analysis of a

specific region, phase or contaminant identified using the image as a guide.

Reading [12], in a patent application filed in May 1992, proposed an instrument

based upon a scanning thermal microscope modified to perform spatially

resolved modulated(-temperature) differential thermal analysis. Although such

an apparatus was never constructed, subsequent development in a collaboration

with Pollock and Hammiche resulted in a working instrument and three more

patents [13-15]. Development and prototyping of this concept took place

through collaboration between Loughborough and Lancaster Universities in the

UK in conjunction with commercial and UK Research Council support. TA

Instruments Inc. (Newcastle, Delaware, USA) launched this equipment as a

commercial product in 1998. The instrument received the Gold Award at the

1998 PittCon analytical instrumentation conference and the R&D 100 award for

innovation. In addition, the UK government awarded the instrument “Millenium

Product” status.

Before considering the development of what has become known as “micro-

thermal analysis”, it is appropriate to review the history, technology and

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applications of scanning thermal microscopy so as to place this family of

techniques in context.

2. SCANNING THERMAL MICROSCOPY (STHM)

The inventions of the scanning tunnelling microscope (STM) [16] and the

atomic force microscope (AFM) [17] have allowed sub-micrometre and, at

times, atomic-scale spatially-resolved imaging of surfaces. Spatially-resolved

temperature measurements using optical systems are diffraction limited by the

wavelength of the radiation involved, which is about 5-10 µm for infrared

thermography and about 0.5 µm for visible light [18]. The spatial resolution of

near-field techniques (such as AFM) is only limited by the active area of the

sensor (which in the case of STM may be only a few atoms at the end of a metal

wire).

The first experiments in scanning thermal microscopy (SThM) were carried out

by Williams and Wickramasinghe [20] who employed a heated thin-film

thermocouple fabricated from a conventional STM tip. As the tip approached a

surface, it was cooled due to tip-substrate heat transfer. By using the

temperature sensed by the thermocouple as a feedback to maintain a constant

tip-substrate gap, this scanning thermal profiler could overcome the limitations

of STM and be used to image electrically insulating surfaces with a lateral

resolution of 100 nm. Since the feedback signal was based on maintaining a

constant probe temperature, this device could not be used to obtain true thermal

images of surfaces. Instead, the system measured a convolution of topography of

the specimen and its thermal conductivity in a non-contact mode by means of

heat leak from the thermocouple to the sample through a small air gap.

In an attempt to overcome the limitations of this method of SThM, Majumdar

[21] described the use of an AFM cantilever fashioned from a pair of dissimilar

metal wires (chromel and alumel) which met to form a thermocouple junction at

the tip. In this way, the conventional AFM force feedback mechanism could be

used to measure surface topography whilst at the same time mapping the

temperature distribution of energized electronic devices with sub-micrometre

resolution. Since this demonstration, a number of different probe designs have

been developed and progress has been made towards the measurement of

absolute thermal conductivities and 3-dimensional tomographic imaging. A

general discussion of SThM can be found in the reviews by Gmelin [22] and

Majumdar [18,19].

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2.1. Instrumentation for SThM

The atomic force microscope forms the basis of both scanning thermal

microscopy and instruments for performing localised thermal analysis. A

schematic diagram of an AFM is shown in Figure 1 [23]. The instrument

consists of a sharp tip mounted on the end of a cantilever which is scanned

across the specimen by a pair of piezoelectric elements aligned in the x-y plane.

As the height of the sample changes, the deflection of the tip in contact with the

surface is monitored by an optical lever arrangement formed by reflecting a laser

beam from the back of the cantilever into a photodetector [24]. The tip is then

moved up and down by a feedback loop connected to a z-axis piezo which

provides the height of the sample at each x,y position. Besides the topographic

information provided by rastering the tip across the sample, other properties can

be obtained by measuring the twisting of the cantilever as it is moved across the

sample (lateral force microscopy) [25]. This provides image contrast based on

the frictional forces generated from the sample-tip interaction. Other imaging

modes, such as force modulation and pulsed force modes can indicate the

stiffness of the sample [26]. An advantage of the AFM over the scanning

electron microscope is that little sample preparation is required because the

sample is not exposed to a high vacuum and electrically insulating materials can

be examined.

Figure 1. Atomic force microscope (schematic) [23]. (With permission from

Elsevier.)

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2.2. Probe design

Three methods have been used to combine the conventional AFM cantilever

with a means of localised thermometry.

2.2.1 Thermocouple cantilevers

The use of AFM with a thermoelectric element at the tip has been described

above. In an effort to improve the performance of a bare thermocouple tip,

Majumdar et al. [27] cemented a diamond shard to the junction so as to give a

harder tip with improved spatial resolution and reduced thermal resistance. The

same group also describe depositing successive layers of different metals so as

to make thermocouple pair on top of a standard “A-frame” AFM cantilever [28].

Fish et al. [29] borrowed technology from near-field scanning optical

microscopy to make a thermocouple derived from gold-coated glass

micropipettes containing a platinum core. Workers at Glasgow University

[30,31] have fabricated thermocouple probes using electron beam lithography

and silicon micromachining in order to deposit one or more thermocouple

junctions at the AFM tip. Such work leads to the possibility of building

thermopile sensors (with the incorporation of a heater) analogous to a miniature

heat flux calorimeter [32,33].

2.2.2 Resistance thermometry

Figure 2. Schematic diagrams of resistive SThM probes: a) Wollaston wire

type [34,35], b) micro-machined coated Si cantilever [37] and c) “data storage”

doped Si probe [55].

In 1994, Dinwiddie and Pylkki [34,35] described the first combined

SThM/AFM probes that employed resistance thermometry to measure thermal

properties. These were fashioned from Wollaston process wire. This consists of

a thin platinum/5% rhodium core (about 5 µm in diameter) surrounded by a

thick (about 35 µm) silver sheath. The total diameter of the wire is thus about

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75 µm. A length of wire is formed into a “V” and the silver is etched away at

the apex to reveal a small loop of Pt/Rh which acts as a miniature resistance

thermometer (Figure 2(a)). A bead of epoxy resin is added near the tip to act

serve as mechanical reinforcement and sliver of silicon wafer is glued to the top

of the probe to act as a target mirror for the laser employed in the cantilever

deflection detection mechanism [36]. This can be operated in two modes: a) as a

passive thermo-sensing element (by measuring its temperature using a small

excitation current) or b) as an active heat flux meter. In the latter case, a larger

current (sufficient to raise the temperature of the probe above that of the surface)

is passed through the wire. The power required to maintain a constant

temperature gradient between the tip and sample is monitored by means of an

electrical bridge circuit. In essence, this is equivalent to a power compensation

calorimeter. Mills et al. [37] describe similar probes in which the resistance

element is deposited across the apex of a silicon nitride pyramid similar to a

conventional AFM cantilever (Figure 2(b)).

In a passive mode, such devices function like thermocouple probes described

above. These can be used (for example) to map the temperature distribution in

energised electronic devices simultaneously with their topography [38,39]. If

the surface is illuminated with infrared radiation, the photo-thermal effect

arising from the absorption of energy specific to the infra-red (IR) active modes

of the specimen may be used to obtain the sample’s IR spectrum [40-47]. In the

active mode, the heat flow from the tip can be used to detect surface and sub-

surface defects of different thermal conductivity than the matrix [48,49].

Other resistive probe designs have been reported. For example, Li and

Gianchandani [50] have fabricated SThM probes using polyimide rather than

silicon as a substrate in a process similar to that of Mills et al. [37] except that it

exploits the mechanical flexibility of the polyimide to implement an assembly

technique that eliminates the need for probe removal or wafer dissolution. Use

of polyimide rather than silicon as a substrate offers a greater degree of thermal

isolation since its thermal conductivity is three orders of magnitude lower than

silicon [51]. These probes have been manufactured in a differential arrangement

with one probe being used as a reference. Edinger et al. [52,53] describe a

sensor consisting of a nanometre-sized filament formed at the end of a

piezoresistive atomic force microscope type cantilever. The freestanding

filament is deposited by focussed electron beam deposition using

methylcyclopentadienyl trimethyl platinum as a precursor gas. The authors

claim a spatial resolution of < 20 nm and, due to its small thermal mass, a high

sensitivity and fast response time. Leinhos et al. [54] have fabricated thermal

elements from silicon with a Schottky diode integrated into the probe tip. One

design of such a probe may also be used for simultaneous scanning near-field

optical microscopy as well as SThM and AFM.

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Chui et al. [55] also describe high resolution silicon (piezo)resistive cantilevers

designed not for SThM, but for high-density data storage. An array of probes

are used to “write” 10-50 nm diameter pits in a spinning polymer coated silicon

disc [56]. Read-back is achieved by making use of the increased cooling of the

tip when it encounters the indentations when the probes are operated in SThM

mode. Such an approach is essentially the reverse of micro-thermal analysis in

that the marks produced as a consequence of localised thermomechanical

experiments are detected by the effect of their topography on the heat flux rate

between the probe and the sample [57]. Schematic diagrams of the Wollaston,

micromachined silicon and “data storage” probes are shown in Figure 2(a-c).

The spatial resolution of these probes is of the order of 2, 0.2 and 0.02 µm

respectively. All of these designs are now commercially available.

2.2.3 Bimetallic sensors

Nakabeppu et al. [58] describe the use of composite cantilevers made from tin

or gold deposited on conventional silicon nitride AFM probes to detect spatial

variations in temperature across an indium/tin oxide heater. Differential thermal

expansion of the bimetallic elements causes the beam to bend. This movement

is monitored using the AFM optical lever deflection detection system. In order

to separate thermal deflection of the beam from displacement of the cantilever

caused by the sample topography, an intermittent contact mode of operation is

employed. Measurements were made under vacuum so as to minimize heat loss.

A more practical use of this technology is in the form of miniature chemical and

thermal sensors [59]. This approach has been used to perform thermal analysis

on picolitre volumes of material deposited on the end of a bimetallic cantilever

[60]. Arrays of such devices have applications as highly sensitive electronic

“noses”.

2.3 Quantitative SThM

Small-scale measurements of thermal conductivity and thermal diffusivity

would benefit the semiconductor and other industries where thermal transport

properties are significantly different to, and cannot be inferred from,

measurements at higher scales. Examples of key areas of modern technology

and science which might be expected to benefit include microelectronics,

cellular biology, forensic, pharmaceutical and polymer science etc. In theory,

heated thermal probes are capable of measuring the absolute thermal

conductivity of materials by the heat flux between the tip and the surface. In

practice, heat losses also occur both within the probe and to the atmosphere.

Furthermore, the contact area between the tip and the specimen is usually

unknown. Ruiz et al. [61] developed a simple method for converting heat flux to

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thermal conductivity by using hard materials of known thermal conductivity to

calibrate the system. This procedure was used to determine the thermal

conductivity of diamond-like nanocomposites to a precision of ±15%.

Gorbunov et al. [62-64] measured the change in heat flux as the probe

approached the sample surface, or was ramped in temperature in contact with the

specimen, so as to derive its thermal conductivity – again by calibration with

samples of known response. This approach has been used to study IR receptors

in snakes with a view towards designing artificial sensors which mimic those

found in nature [65]. Fiege et al. [66] used AC heating of the tip to measure the

thermal conductivity of silver and diamond, using gold as a single-point

reference material in order to estimate the contact area of the tip. Blanco et al.

[67] have used SThM to investigate qualitative differences in thermal

conductivity of carbon-carbon composites arising from different processing

histories.

Figure 3. Illustration of the convolution of surface roughness on the apparent

thermal conductivity (k) measured by the tip during a line scan. Darker

coloured material (left) has lower bulk thermal conductivity than lighter

coloured material. See text for additional explanation.

The thermal conductivity contrast image obtained by scanning thermal

microscopy represents a convolution of the true thermal transport properties of

the specimen with artefacts arising from changing tip-sample thermal contact

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area caused by any surface roughness of the specimen [48]. When the probe

encounters a depression on the surface, the area of contact between the tip and

sample increases, resulting in increased heat flux from the tip to the sample.

More power is required to maintain the tip temperature at the set-point value and

this is recorded on the image as an apparent increase in the local thermal

conductivity. The opposite is true when the probe meets an asperity. Visual

comparison of the thermal image with the topographic image (recorded

simultaneously) often shows that “features” in the former are correlated with

changes in height of the specimen – particularly at edges of such features where

the surface relief changes rapidly. Even with careful sample preparation (e.g.

cutting or polishing) it is almost impossible to avoid artefacts in the thermal

image not attributable to topography. In cases where there is little inherent

thermal contrast between parts of a heterogeneous specimen, interpretation of

the thermal image can be problematic. An example of this is illustrated in

Figure 3 for three cases: 1 – a homogenous material flat surface; 2 – the same

material with a depression and an asperity; 3 – a smooth interface between low

thermal conductivity and high thermal conductivity phases & 4 as 3, but with

added surface roughness.

In theory, it would be possible to construct an analytical model of the probe-

surface interaction based upon the local geometry of the probe tip and surface

[62-64]. Rather than develop a general model for heat transfer, an alternative

approach can be developed using a neural net algorithm [68]. Each digitised

image consists of a grid of points (pixels) which define the height of the sample

and the heat flux from the probe to the sample. For each pixel the local

topography is characterised by subtracting its height from the heights of the

surrounding points. This defines whether the probe rests in a depression,

asperity, flat surface, etc. The training set for the neural network comprises

images acquired on materials of homogeneous thermal properties with a range of

surfaces roughnesses. From these samples a table of topographic parameters

correlated with the thermal measurement is obtained for a wide range of surface

roughness. The ‘true’ thermal measurement is the value obtained on a perfectly

smooth surface. In practice, an average value from the smoothest available

surface can be taken as the ‘true’ value. In each case, the required value is

given as the measurement on a smooth surface.

The usual procedures for training, testing and validation of a neural network

can be applied to result in a method to take a pair of images (thermal and

topographic) and effectively “subtract” artefacts arising from surface roughness.

As an alternative to lengthy training with a variety of specimens, it is feasible to

use the images of a test specimen itself to train the network. This method relies

upon the sample having discrete areas of homogenous thermal conductivity to

provide internal calibration values for network training. Figure 4 shows the

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results of carrying out this type of image processing on a sample of polyolefin

packaging film. Despite careful sample preparation, differences in mechanical

properties result in a non-flat sample after cutting with a microtome. This

generates false contrast in the raw thermal conductivity image which is largely

eradicated by processing with a neural net which was trained using a series of

polymers of differing surface roughnesses.

Figure 4. Left – topography of microtomed cross-section through a multi-

layer polyolefin packaging film. Centre – raw thermal conductivity contrast

image. Right – thermal conductivity image post-processing with trained neural

net program.

Other methods of image processing have been devised which apply a statistical

analysis of pixel intensity distribution to enhance image contrast. Royall et al.

[69] have used such a method to discriminate between the substrate and coating

of a pharmaceutical compact whereby each pixel is defined to be one or the

other component according to the heat flux from the tip to the sample. This

simple “on/off” data treatment can be extended to assign a probability (displayed

as a grey-scale level) of being a certain component based upon the position of

the pixel’s intensity within the total distribution of values. This procedure is

illustrated using a non-prescription analgesic tablet containing paracetamol

(acetaminophen) described below.

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Figure 5. (a) Raw thermal conductivity contrast image of an analgesic tablet

containing paracetamol [70], (b) histogram showing distribution of

pixel intensity fitted to two overlapping Gaussian peaks assigned to

the drug and filler components.

Figure 5(a) shows the raw thermal conductivity image of an analgesic tablet

containing the drug paracetamol (acetaminophen). Localised thermal analysis of

the sample indicates that the bright area in the bottom right of the image is some

form of thermally inert filler whereas the dark regions comprise the drug [70].

Inspection of a histogram of pixel distribution in Figure 5(b) shows that the

shape of this distribution can be modelled using two overlapping Gaussian peaks

corresponding to the drug and filler responses.

Figure 6. (a) Simple black and white separation of image shown in Figure

5(a) using a threshold of 1.66 mW, (b) Grey-scale separation of image shown in

Figure 5(a) using a more complete statistical analysis of the original histogram

distribution in Figure 5(b).

A simple means of highlighting the two phases might be to assign a zero grey-

scale intensity to pixels with an original value below the cross-over between the

two peaks at 1.66 mW and a grey-scale intensity of unity to all the pixels above

this threshold. Thus, points which are statistically more likely to be comprised

of the drug appear black and those which are more likely to be the filler appear

white (Figure 6(a)). This is the same, simple method of image processing that

has also been found to be particularly applicable to mechanical property imaging

modes (pulsed force mode atomic force microscopy) in combination with a

variable temperature sample stage [10]. A more refined approach is to generate

a grey-scale value for each pixel based upon the probability of it belonging to

the drug or filler component derived from the ratio of the value of the fitted

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peak height assigned to the filler to the total fitted histogram intensity. The

resulting transformation of the thermal image in Figure 5(a) via this process is

shown in Figure 6(b) and exhibits a more satisfactory discrimination of phases –

particularly at the interface between the two domains where there exists some

uncertainty over their assignment.

De Cupere and Rouxhet [71] have recently published a similar method of

image contrast enhancement based upon splitting a surface friction image into

two components using the pixel intensity histogram. The resulting image was

cleaned by selectively erasing any foreground pixel in contact with a

background pixel (“erosion”) and the final image was obtained by reconstruction

of the objects left after erosion. This procedure, which is similar to that

described above, was used to measure the surface fraction of spherulites grown

on amorphous poly(ethylene terephthalate) film after different annealing times.

2.3 Other SThM techniques

2.3.1 3-D tomographic imaging

The decay length of thermal waves produced by AC heating of a tip varies as a

function of the reciprocal of its frequency [72]. Thus, it is possible to detect

variations in thermal response at shallower depths by using a high frequency

temperature modulation superimposed on the conventional DC heating of the

tip. Several groups have employed this technique to study the thermal

diffusivity variations in materials [49,73,74]. Gomès et al. have examined this

process theoretically [75] and there is potential for the use of multiple frequency

modulated-temperature SThM as a means to provide non-destructive three-

dimensional imaging of optically opaque samples using similar principles to

those employed for medical imaging by electrical impedance tomography [76-

79].

2.3.2 Thermal expansivity imaging

An additional imaging mode has been demonstrated whereby the thermal

expansion of a specimen is detected whilst AC heating is applied to the probe as

it is scanned over the surface [23,70,80,81]. The resulting z-axis modulation of

the probe arising from thermal expansion and contraction of the surface is

detected and used to construct an image based on the thermal expansivity of the

sample. Although Majumdar [82-84] has also described similar measurements,

this approach does not require electrically conductive specimens and employs

the same configuration used for thermal conductivity imaging. Again, there is

the potential for using this approach for tomographic imaging.

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3. LOCALISED THERMAL ANALYSIS

3.1 Principles

Active, resistively-heated, thermal probes readily lend themselves to localised

thermal analysis. Although it is feasible to control the temperature of a

thermocouple-based probe by passing a current through it [86], the simultaneous

measurement of its temperature is non-trivial and requires filtering out of the

current providing the heating. Probes based on resistance thermometers are

more readily controlled and this technology forms the basis of the well-known

power-compensation differential scanning calorimeter [87]. Using a previously

acquired topographic and/or thermal image obtained by the SThM facility, it is

possible to place the probe at one or more selected locations in sequence on the

sample and program the tip’s temperature in order to make localised

measurements of transition temperatures. By monitoring the power required to

follow the temperature programme, a form of spatially resolved calorimetry may

be carried out [49,89]. In addition, since the vertical deflection of the tip can be

determined using the AFM stage, localized thermomechanical analysis (TMA)

may be performed concurrent with the calorimetry [72,90]. A linear temperature

ramp of the order of tens of degrees Celsius per second is the most commonly

employed programme, often with a superimposed sinusoidal modulation of the

probe temperature at kilohertz frequencies about the mean set-point. The use of

AC heating allows two extra calorimetric signals to be obtained – the AC power

and phase difference between the applied modulation and probe response – akin

to AC calorimetry [88]. Such high heating (and cooling rates) are possible as a

consequence of the small size of the probe and the region (typically a few µm

square) of material that it contacts. The apparatus developed by Fryer [91]

employs lower heating rates (of the order of a few degrees Celsius per minute)

because of manual control of the probe set-point temperature and lack of

automatic data logging.

At present, only the analogues of DSC and TMA are commercially available.

Localised dynamic mechanical analysis (DMA) has also been demonstrated

[23,70,72,80,92]. Here, the force between the thermal probe is modulated during

the temperature ramp. Such a procedure may also be used as an imaging mode

in order to obtain a map of variations in mechanical properties across the

specimen [80,92]. Localised modulated-temperature TMA has been performed

whereby the amplitude and phase difference of the modulation in z-axis

displacement of the probe is detected whilst a temperature ramp with overlaid

AC modulation is applied to the tip [81]. An indirect form of thermogravimetry

has been reported whereby the mass of material remaining adhered to the tip was

monitored indirectly using the AC calorimetric signal [70]. By employing

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stiffer cantilevers with integrated heaters, the mechanical resonance frequency

of the beam has also been used to measure the mass of material on the tip during

heating [93].

3.2 Calibration [94]

Localised thermal analysis has generally been used as a qualitative tool; that is,

it is primarily used to identify the material being examined or for examining

compositional gradients within materials. The temperature at which a transition

takes place is often of primary interest, although it may be possible to add

further semi-quantitative interpretation of the data from the magnitude of the

change (e.g. amount of probe penetration) observed. Like all thermal analysis

techniques, interpretation of results requires detailed temperature calibration

procedures and an understanding of the precision of the temperature

measurement.

A good reference material has a number of desirable properties including a

well-documented value, availability in a suitable form for analysis,

homogeneity, stability, low toxicity, and traceability to a national reference

laboratory (NRL). In traditional DSC and TMA, metals like indium, tin, and

zinc meet these criteria. These metals are not suitable for temperature calibration

for localised thermal analysis, however, as they may contaminate the probe tip

thus changing its resistance and defeating the object of calibration..

Organic calibration materials are more suitable for calibration as they do not

react with the probe and the tip may be easily cleaned at the end of an

experiment by heating to above 500 °C in air. This is sufficient to decompose

most organic substances. The British Laboratory of the Government Chemist

(LGC), a national reference laboratory, conveniently offers eleven organic

reference materials with melting temperatures ranging from 41 to 285 °C.

However, in these materials are generally unsuitable to be used directly and

large flat polycrystalline surfaces must be prepared by melting a small amount

of substance in a suitable holder (such as small cup aluminium foil) and then

cooled.

Many investigators, particularly those working in the field of polymer science,

prefer to use films of semi-crystalline thermoplastics such as poly(caprolactone

or polyethylene terephthalate) as melting point standards. Whilst less desirable

from a theoretical standpoint, temperature calibration with polymeric films

offers a number of ease-of-use advantages. Moreover, many polymers have

high melting temperatures that provide a calibration range of nearly 300 °C, a

range difficult to achieve with organic chemicals. Generally, a two point

temperature calibration is carried out spanning the experimental range of

interest. Whatever protocol the investigator adopts, it is essential that the

method is well-documented so that it can be replicated.

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3.3 Features

There are several important differences between the localised calorimetric and

thermomechanical measurements and their more conventional “bulk” analogues.

Firstly, for the calorimetric measurements, the sample size is poorly defined.

This is because the contact area between the tip and specimen is ill-defined.

Furthermore, should the sample soften during the measurement, the probe will

sink into the specimen thus aggravating this effect. Therefore, there is often a

strong correspondence between the shape of the calorimetric response and the

displacement of the probe [95]. A second-order effect, not often considered, is

that the temperature gradient extending away from the heated tip will be affected

by the properties of the specimen and the heating rate employed during the

measurement. This is analogous to the modulated temperature SThM mode of

imaging which has been considered by Pollock and Hammiche [85] whereby the

depth of penetration of the thermal wave emanating from the tip decays more

rapidly for higher modulation frequencies. Slough [96] has considered the case

of a heat pulse lasting the duration of the temperature programme of the probe

for a typical polymeric sample. His calculations suggest that for an

instantaneous heat pulse of 200°C delivered for one second to the surface of a

sample of polystyrene at 25°C, the specimen’s temperature is not significantly

affected 10 µm from the probe. Inoue and Uehara [97] have modelled this

process at a more sophisticated level in the context of data writing and erasing in

a phase-change optical disk. Melting of a crystalline material at a part of the

surface causes surface rippling around the molten area and subsequent rapid

cooling generates an amorphous spot. This amorphous material is maintained at

low temperature and subsequent localised thermal analysis can be used to

determine the glass-rubber transition temperature of this region.

The way in which localised thermomechanical measurements are performed

also differs from conventional measurements. In the usual implementation of

these measurements, the feedback loop between the cantilever force and the z-

axis piezo control is disabled at the start of the experiment. Thus, the probe is

initially applied to the surface with a user-defined cantilever deflection which

will change (thus giving the displacement of the tip by means the optical lever

formed by reflecting a laser spot from the back of the probe into a

photodetector) during the course of the measurement. Should the specimen

soften during the experiment, the probe will indent into the sample and the force

on the tip will decrease (possibly to the extent of losing contact with the

specimen).

A final effect often observed during localised thermal analysis arises from the

high heating-rates that can be employed. Many thermal transitions are governed

by kinetic laws which define the time dependence of such processes as

70

devitrification and thermal degradation. As a consequence of this, some

transitions can appear at elevated temperatures when compared to bulk

measurements at more modest heating rates - even after careful calibration of the

instrument. This effect is not generally seen for melting phenomena, and the

rapid heating affords a means of measuring the melting temperature of

metastable materials without them undergoing rearrangement to more stable

forms [98].

Figure 7. Localised thermal analysis of semi-crystalline poly(ethylene

terephthalate) showing two consecutive measurements at the same location

(solid line is the probe displacement, broken line is the probe power, filled

symbols denote first upwards temperature scan, open symbols denote second

upwards temperature scan, heating rate 10 °C s-1).

Some of the above aspects of the technique are illustrated in Figure 7. This

shows two measurements performed consecutively on the same region of a

sample of poly(ethylene terephthalate) of approximately 40% bulk crystallinity

at a heating rate of 10 °C s-1. During the first upwards temperature scan, very

little effect is seen in the probe displacement until melting of the polymer occurs

around 250 °C. This is mirrored by an increase in probe power which is largely

arises from the increased contact area between the tip and the sample as it

indents the surface. A small decrease in probe power occurs around 125 °C,

well above the "normal" glass-rubber transition temperature of this polymer of

70-80 °C. This feature might be ascribed to devitrification of amorphous

material present. Immediately the first measurement was completed, the tip was

lifted clear of the (now molten) material. The polymer in this area is rapidly

quenched to room temperature by the surrounding specimen. On carrying out a

second scan of the same region, the glass-rubber transition is observed readily in

71

both signals, without any evidence for further rearrangement to crystalline

material above this temperature or subsequent melting of any crystals so formed.

3.4 Terminology

The nomenclature of this new method of analysis is still under development.

The originators (Hammiche, Pollock and Reading) devised the terms

“Calorimetric Analysis by Scanning Microscopy” (CASM) as a name for the

measurement of probe power, and “Mechano-thermal Analysis by Scanning

Microscopy” (MASM) as part of an effort to coin a systematic hierarchy of

terms for this family of techniques [72]. The commercial term for this field is

“µTA” which is a registered trade mark of TA Instruments Inc. (Newcastle,

Delaware, USA) who also holds trademarks on the terms “µDTA” and “µTMA”

corresponding to the calorimetric and thermomechanical methods [99]. Many

authors have used the terms: “micro-thermal analysis” (with or without the

hyphen, abbreviated to “micro-TA”) and corresponding terms “micro-DTA” and

“micro-TMA”. Such usage goes against the recommendations of the

International Confederation of Thermal Analysis and Calorimetry nomenclature

committee who claim that there is ambiguity over the term “micro” because it is

not clear whether the term is related to sample size (mass, volume), the size of

the instrument, or the value of the measured signal (which may be amplified) or

quantity detected [100]. (At least one company markets a “micro-DSC” which

is a high sensitivity instrument for measuring heat flow in the µW range.) The

general term which seems to be gaining popularity is “local(ised) thermal

analysis” and the derivations “local(ised) calorimetry” and “local(ised)

thermomechanical analysis” in order to differentiate this technique from

traditional forms of thermal analysis which measure the global response of the

sample [101]. It this discussion, the term “micro-thermal analysis” (or “micro-

TA”) is used to denote SThM and localised measurements, whereas the

individual measurements are described in full. Until there has been time for the

terminology to develop and become accepted, searching the growing literature

on this subject will be difficult.

3.5 Applications

Since its development and subsequent commercialisation, localised thermal

analysis has been employed in a number of different disciplines. A review by

Pollock and Hammiche [85], summarises the wide range of materials which

have been studied by these techniques. An earlier paper by Price et al. [102]

describes measurements on polymers, electronic components and biological

specimens in the same publication as an illustration of the wide applicability of

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such measurements. Articles in popular technical publications have also served

to illustrate this point [72, 90, 103-105].

3.5.1 Polymers

Localised thermal analysis has been used for the characterisation of multi-

component polymer thin films [101,109,110] or polymer blends [102,111-113],

for the investigation of surfaces and interfaces between materials [92,98,114-

126], and compositional gradients brought about by the specimen’s processing

history [90,126-131]. In particular, localised thermal analysis is useful for

probing the behaviour of polymers used for the fabrication of microelectronic

components, where the bulk response of the polymer may not be representative

of the same material when it is present in thin layers [126,132-136]. However,

care must be taken to ensure that an apparent elevation in transition temperature

is not due to the substrate acting as a heat sink, thus leading to errors in

temperature measurement [137]. One interesting application of localised

thermal analysis is to use the thermal probe for in-situ processing of materials,

whereby heat is used to cross-link or decompose the substrate [126,138,139].

Again, this technology lends itself to data storage and micro-machining [140].

Figure 8. Localised thermal analysis of multi-layer polyolefin film shown in

Figure 4 [105].

Two examples of the use of localised thermal analysis are provided in order to

illustrate the generic applications of this approach. Figure 8 shows localised

thermomechanical analysis of the surface of the multi-layer film in Figure 4.

Measurements were made at points within this image describing the bulk

73

polymer, the central gas-barrier layer and the thin tie-layer between this and the

bulk film. The melting transition temperatures are consistent with high density

polyethylene, poly(ethylene-co-vinyl alcohol) and medium density polyethylene

for the bulk, gas barrier and tie layers, respectively [101].

Figure 9. Optical micrograph of the cross-section “gel” particle embedded in

a 75 µm low density polyethylene film. Also visible are craters remaining

following localized thermal analysis of the specimen.

Another example is illustrated in Figures 9 and 10 for a blemish or “gel”

particle in a blown polyethylene film. Figure 9 shows an optical micrograph of

the film which has been carefully cross-sectioned across the feature. Also

visible are small craters in the film which are a consequence of a series of

localised thermal analyses along the specimen. These also serve to illustrate the

potential spatial resolution of the technique. Whilst the Wollaston wire probe

has a diameter of 5 µm at the tip and is capable of detecting thermal transitions

in the order of a few square micrometres, heat from the measurement process

spreads out and disrupts a large region (about 20 µm square) around the tip.

This ultimately restricts the proximity of a sequence of measurements on a

specimen. For the analysis of the sample shown in Figure 9 the specimen was

translated under the instrument using a micrometer stage. Over an order-of-

magnitude improvement (both in terms of sampling area and proximal

placement of multiple analyses) can be achieved using semi-conductor probes

such as those shown in Figure 2(c). These have recently become commercially

available and promise to move localised thermal analysis into the nano-scale

with sub-micrometre resolution.

74

Figure 10. Results from localised thermal analysis of the specimen shown in

Figure 9. The solid symbols denote measurements on the normal film whereas

the open symbols denote measurements made in the “gel” particle [103].

Figure 10 shows results measurements made on the bulk film and in the region

of the defect. Whilst it can be observed that the temperatures of the onset of

probe penetration into the low density polyethylene film are broadly similar for

all measurements. The degree of penetration into the sample into the “gel”

particle is much lower. This implies that the material here has a much higher

melt viscosity than the normal perhaps as a consequence of some problem

during polymer synthesis [104].

3.5.2 Pharmaceuticals

The applications of micro-thermal analysis within the pharmaceutical industry

have been reviewed by Craig et al. [141]. Most tablets are not composed of the

pure active ingredient, but contain the drug dispersed within an excipient

package (such as micro-crystalline cellulose, starch, glucose etc.) with added

processing aids (such as magnesium stearate) which act as fillers and lubricants

for the formation of a mechanically stable compact. Furthermore, the tablet

may be coated with a polymer or sugar film to prevent the drug being released

into the body before it enters the stomach [142]. Imaging by SThM can be used

to identify discrete phases containing these ingredients and their identification

can be confirmed by localised thermal analysis [23,69,70, 104,143-147]. Figure

11 illustrates this by means of measurements on the specimen shown in Figure

75

5(a). Localised thermal analysis detects the melting of the drug paracetamol

(acetaminophen) around 180 °C for the low thermal-conductivity region of the

image. No change in response is observed when an area from the high thermal-

conductivity region of the same image is examined. This suggests that the

material here is comprised of a filler or excipient - most probably micro-

crystalline cellulose. The distribution of drug within a pharmaceutical compact

can have very important implications for the dissolution of the tablet within the

digestive system and micro-thermal analysis is a useful characterisation tool.

Figure 11. Localised thermal analysis of the analgesic tablet shown in Figure

5(a) with measurements made in the low thermal conductivity (dark) region

(solid line) and the high thermal conductivity (bright) region (broken line) of the

image [70].

Many drugs can be produced in more than one crystalline modification. Micro-

thermal analysis has been shown to be a viable means of differentiating between

such polymorphs [148,149], or between crystalline and amorphous regions in

drugs [150,151]. Royall et al. [152] have employed localised thermal analysis

to detect surface segregation of progesterone encapsulated in poly(lactic acid)

microspheres (thus confirming earlier work by DSC), while Zhang et al. have

studied the distribution of poly(ethylene glycol) in the same polymer [153].

76

3.5.3 Biology

Micro-thermal analysis has been used to examine a number of specimens of

biological origin. Gorbunov et al. have reported the study of the infrared

receptors of snakes using SThM and localised thermal analysis, whereby the

thermal transport properties of the receptors were found to be lower than the

surrounding skin [65]. The surfaces of plant leaves have also been examined by

localised thermal analysis, to study the waxy coating which protects the leaf

from water loss. Melting of the cuticular wax has been detected [102] and the

effect of surfactant packages on this behaviour investigated as a means of

improving the absorbtion of agrochemicals [154]. Studies of historical

parchment derived from the dermis of animal skin have been employed to

measure the softening temperature of the material. It is a particular advantage of

the technique to be able to examine small quantities of material [155]. SThM

has been used in food science to image the surface of caramel [156]. In this

instance pulsed-force microscopy and infrared spectroscopy were employed to

characterise surface structures, although it would be a natural extension to use

the SThM to study local thermal transitions

3.5.4 Inorganic materials

This category includes metals, ceramics and electronic materials which are

typically of high thermal conductivity whereby the discrimination between

phases afforded by SThM can be compromised [157]. Furthermore, it is

difficult to envisage that point-source heating afforded by active thermal probes

would be sufficient to heat highly-conductive bulk specimens such as metals.

However, thin-film NiTi shape-memory alloys have been studied by localised

thermal analysis, whereby measurements of the martensitic to austenitic

transformation were made and the spatial variation in transition temperature

corresponding to compositional variations within the specimen identified [158].

Micro-thermal analysis has also been employed to resolve differences in thermal

conductivity and softening temperatures that arise during the processing of

carbon fibres [67,129,159]. These were related to the local oxygen content of

the fibre measured by electron probe micro-analysis.

77

Figure 12. Shaded topography (left) and thermal conductivity contrast (right)

images of a light emitting diode. Reproduced from reference [102] with the

permission of Akadémiai Kiadó.

Figure 13. Localised thermal analysis of the LED shown in Figure 12. The

upper set of curve show measurements made within the high thermal

conductivity central region and the lower set of curves show measurements

made outside this area [102].

As described earlier, passive SThM techniques have proven popular in the

microelectronics industry for the identification of hot-spots within components.

For example, Boroumand et al. have made measurements of the temperature

distribution across a polymer light-emitting diode (LED) [160]. Figure 12

shows the topography and thermal conductivity contrast image of a silicon-

based LED from a batch of components which had failed under testing.

78

Localised thermomechanical measurements of the centre and outside of

specimen are shown in Figure 13. Although no thermal transitions are observed,

the thermal expansivity is different between the two areas. This would lead to

thermal stresses building up when to the device was energised. This could

ultimately lead to failure of the LED. Measurements on a LED which passed

testing showed no variation in properties [102,104].

4. LOCALISED CHEMICAL ANALYSIS

The measurements physical properties afforded by existing forms of micro-

thermal analysis can be insufficient to discriminate between different materials.

Incorporation of some means of chemical analysis of the specimen is therefore

highly desirable. This has been achieved by two processes; localised pyrolysis-

evolved gas analysis, and near-field photothermal spectroscopy.

4.1 Localised evolved gas analysis

It has been demonstrated that the Wollaston wire probe tip used for localised

thermal analysis measurements can be heated rapidly and repeatedly to

temperatures in excess of 600°C. This is sufficient to bring about localised

pyrolysis of most organic materials [72,157]. This process generates a small

plume of gaseous decomposition products characteristic of the substrate.

Chemical analysis of these products can be performed via two alternative routes:

by absorbing them on a suitable substrate and subsequent investigation using

thermal desorption gas chromatography-mass spectrometry (td-GC-MS), or by

direct sampling by mass spectroscopy (MS) [23,70,106,162-165]. Furthermore,

the thermal probe may be used to soften and remove material from the surface of

the specimen for subsequent characterisation [70]. These approaches are

described in more detail in the following sub-sections.

4.2.1 Offline localised pyrolysis-td-GC-MS

Realisation of the first approach – trapping and offline analysis – employs a

miniature gas-sampling tube packed with a mixture of Tenax (molecular sieve)

and Carbopak (activated charcoal) absorbent material. Such tubes are

routinely used for environmental monitoring of hazardous industrial

atmospheres whereby operators during the normal course of their duties carry a

small tube (about the size of a pen) clipped to their clothes. A pump may be

used to draw gas through the tube at a controlled rate and, at the end of the work

period, the tube is sealed and sent for analysis. Heating the sorbent tube drives

79

off the trapped material into a gas chromatograph for separation and

quantification.

For micro-pyrolysis-td-GC-MS, the sorbent tube is modified to end in a short

section of stainless steel hypodermic tubing the open end of which can be placed

immediately adjacent to the heated thermal probe using a micro-manipulator.

As the tip is heated, a pump is used to draw gas through the tube. After

sampling, the tube is placed in a suitable carrier that fits into a standard thermal

desorption unit interfaced to a GC-MS system. Blank desorption runs of the

sorbent tube are carried out before and after each pyrolysis experiment to

confirm the cleanliness of the detection system. Such tubes are re-usable since

the thermal desorbtion process cleans them of trapped material. The lifetime of

such tubes is at least 1000 cycles.

A dedicated design of micro-manipulator is used for positioning the sampling

tube which can be interfaced with a variable-temperature sample stage upon

which the microscope is placed. Thus, the sample can be cooled or heated

independently of the thermal probe, using a small heated sample holder coupled

to a Dewar vessel containing liquid nitrogen for cooling the specimen.

Generally, the specimen is only required to be under ambient conditions.

Therefore, an x-y-z translator can be used in place of the heated stage. This

configuration is very versatile and affords independent positioning of the

sampling tube, microscope and sample.

One of the obvious benefits of off-line trapping and analysis of pyrolysis

products is the ability to take samples of evolved gases from more than one

location on the sample. For example, if the region of interest covers a

sufficiently wide area, then multiple points may be selected for pyrolysis and the

evolved gases gas can be trapped in the same tube, thus increasing the yield of

material for subsequent analysis. Alternatively, line or area scans of surfaces

may be made with a heated probe to drive off any volatiles into the tube [165].

This approach has particular benefits for filled systems, such as paints and

coatings, which may contain only a small fraction of organic binders.

Another benefit of analysis by GC-MS is to use the ability of gas

chromatography to separate the mixture of decomposition products yielded by

all but the simplest of substrates. This allows complex systems (such as paints

and coatings described above) to be identified or at least “fingerprinted” by the

characteristic mixture of volatile materials formed during thermal degradation

[166]. Alternatively, one may only be interested in the presence (or absence) of

a particular component at a specific location. For example, this technique has

been used to locate the source of camphor extracted from plant leaves [164].

It is possible to exploit this technique as a means of surface-specific pyrolysis.

For example, Figure 14 shows total-ion chromatograms for the decomposition

products from samples of the same household paint. The top curve (“macro-

80

EGA”) shows material collected from a bulk sample (about 1 mg) of paint using

a thermobalance to decompose the material and gas sampling tube in the purge

gas outlet to collect the evolved gases. The lower curve (“micro-EGA”) shows

material collected by scanning a hot thermal probe over at 25 × 50 µm area of

the surface of the paint. Both curves show common features arising from the

binder in the paint (polyamide and acrylic polymers), but also differences

between the surface and the bulk indicating a reduction in antioxidants and

volatile plasticisers (particularly the peak at 21.3 min due to dibutyl phthalate) at

the exposed surface [165].

Figure 14. Comparison of evolved gases from bulk (macro-EGA) and surface

(micro-EGA) of paint film [165].

4.2.2 Online localised pyrolysis-MS

As an alternative to analysis of evolved gases by GC-MS, mass spectroscopy

by itself can be used. This has the disadvantage of lacking the specificity given

by the chromatographic separation, but the advantage of considerably reducing

the time for analysis. One of the drawbacks of the td-GC-MS approach is the

time taken for the analysis of collected gases from the micro-pyrolysis

experiment. The thermal probe itself may be heated at over 100°C/s to the

required pyrolysis temperature and several locations may be examined within a

few minutes. Desorption and separation of the trapped material is limited by the

cycle time of the GC-MS system. Even with an optimised oven programme for

81

the GC, a typical analysis can take 30 minutes. Furthermore, it is advisable to

conduct a “blank” experiment with the tube prior to sampling in order to ensure

that no residues are present in the system.

One way around this problem is to dispense with the trapping and separation

stage by continuous sampling of the atmosphere around the thermal probe. This

can be achieved by using a small-bore silica-glass capillary transfer line which

also serves to reduce the pressure from atmospheric to that which could be

accommodated by the ion source of the mass spectrometer. The capillary tube

must be surrounded by a heated jacket for most of its length in order to prevent

condensation of volatiles on the walls of the tube. Only a few centimetres of the

end of the capillary are left unheated – these were passed through an empty

micro-sorbent tube (described above) so as to enable the same micro-

manipulator assembly to be used to position the end of the transfer line close to

the thermal probe.

For online micro-pyrolysis-MS, three modes of pyrolysis have been developed.

Firstly, the temperature of the probe may be rapidly pulsed to the required

temperature – the amount of material liberated depending upon the duration and

temperature of the heat pulse. Secondly, a conventional temperature ramp can

be applied to the probe whilst monitoring for evolved species. This approach

has the benefit that the operator can select several locations within the field of

view of the microscope in order to carry out compositional mapping via evolved

gas detection. Finally, a heated thermal probe may be brought into contact with

a specimen whilst monitoring gas evolution. As the heat source nears the

surface, material is progressively decomposed, affording a means of depth-

profiling though the sample so as to reveal buried layers beneath the surface

[106,164]. Alternatively, successive pyrolysis measurements may be made

using either of the first or second methods of heating the probe with either direct

sampling to the mass spectrometer, or the more time-consuming offline td-

GCMS sampling [106].

As stated earlier, online MS sampling of evolved gases lacks the separation

stage afforded by td-GC-MS and is unsuitable for complex systems.

Furthermore, the sensitivity of the technique is improved by monitoring for

specific species rather than collecting mass spectra across a wide mass range

during pyrolysis. For example, materials containing aromatic species often

evolve benzene amongst their decomposition products, whereas aliphatic

materials often give alkene fragments. These may be detected by monitoring

single-ion masses rather than acquiring a full mass spectrum. The ability to

examine a succession of points within a few minutes can enable a compositional

map of the specimen to be obtained, which shows the spatial distribution of

phases in a specimen via its pyrolysis products [165].

82

An example of this is shown in Figure 15 for a poly(methyl

methacrylate)/polystyrene laminate. Data from a 6 × 6 array of pyrolysis

measurements were used to reconstruct images based upon the ion yield of the

respective monomers. Unlike similar methods of chemical imaging (e.g.

secondary ion mass spectrometry and laser ionisation mass spectrometry), the

sample is examined under ambient conditions rather than high vacuum [167].

Three-dimensional tomographic imaging may also be considered by using the

probe to ablate the surface.

Figure 15. Left: topographic image of poly(methyl methacrylate)/polystyrene

laminate – polystyrene layer is to the right of the image. Centre: evolved gas

“image” reconstructed from multiple pyrolysis measurements monitoring for

methyl methacrylate monomer (molecular ion m/z 100). Right: similar image

reconstructed for styrene (m/z 104) [165].

4.2 Near-field photothermal spectroscopy

When a material is illuminated with infrared radiation it will tend to absorb

energy corresponding to specific infra-red (IR) active modes and increase in

temperature. This is known as the “photo-thermal effect” and can be exploited

to obtain the IR spectrum of a specimen. Passive temperature-sensing SThM

probes can be used to measure the specimen response to IR radiation over a very

small area below that realisable using conventional optics because the spatial

resolution is limited largely by the contact area of the tip and the thermal

diffusivity of the specimen being of the order of 1 µm which is at least one

order-of-magnitude improvement over conventional techniques. Near-field

photothermal spectroscopy using resistive probes of the types shown in Figure

2(a-b) has been exploited by Hammiche and co-workers [40-47], who used a

mirror system to bring the IR beam to focus on the specimen surface while it

was in the SThM. Two approaches have been used to generate the spectrum. In

the first method a high-intensity tunable IR source has been used to scan the

spectrum in a manner analogous to dispersive IR spectroscopy [42]. The

83

second approach uses a broadband source in an interferometer and subsequent

Fourier transform analysis to obtain the spectrum [40].

The results of this method, in addition to the work described above, have also

been combined with measurements by SThM and localised thermal analysis.

Polymers [106] and pharmaceuticals [23] represent the largest classifications of

materials that have been investigated, although this technique has been used in

cellular biology to monitor the life-cycles of cells [168].

4.3 Thermally-assisted micro-sampling

Figure 16. Online MS single-ion monitoring for methyl methacyrlate

monomer (using the CH2C(CH3)CO.+ ion fragment m/z = 69) removed from the

surface as a consequence of heating the probe in contact with the specimen.

The ability of the heated thermal probe to soften the surface of a specimen and

remove a small amount of material has also been demonstrated. By placing the

probe on the sample and raising its temperature, the substrate is softened [70].

As the probe is withdrawn from the sample, some material adheres to tip. This

residue can then be characterised by pyrolysis-evolved gas analysis (td-CG-MS

or MS). Figure 16 illustrates this process using online-MS monitoring for the

evolution of monomer from a specimen of poly(methyl methacrylate). The tip is

first heated rapidly to 1000 °C to ensure that it is free of any contaminants.

Then, the probe is brought into contact with the specimen and the tip is heated to

84

400 °C at 10 °C s-1, whereupon a small amount of monomer is evolved as the hot

tip is removed from the surface. A second probe-cleaning cycle results in a

larger amount of monomer being detected as a result of the decomposition of

material adhering to the tip. Subsequent cleaning of the tip by heating to 1000

°C does not result in the detection of any residue. Experiments indicate that it is

only necessary to heat the specimen above its glass-rubber transition or melting

temperature in order to transfer material from the surface to the tip. Analogous

measurements have been demonstrated using photothermal IR spectrometry to

detect material removed in this way [169]. Thermal dip-pen nanolithography

using a heated AFM tip has also been developed [170]. In this case, the tip is

coated with an organic material which is transferred to the surface by heating the

tip so as to melt the “ink”. This is essentially the inverse of the micro-sampling

process described above.

5. CONCLUSIONS

This chapter presents an overview of micro-thermal analysis. Scanning

thermal microscopy has gained acceptance in many areas of physical science.

The commercial availability of instrumentation will continue to broaden its

scope of usage into more areas of characterisation of materials. A better

understanding of the mechanisms of heat transport from the tip to the surface

can be expected to make routine measurements of absolute thermal properties

via scanning thermal microscopy possible. The thermodynamic limit of

measurement (kT) is about 10-21 J at room temperature [27]. The current spatial

resolution of scanning probe microscopy is around 10-10 m. The maximum

temperature resolution of the most sensitive thermal probes (bimetallic

cantilevers) is 10-5 K with an estimated sensitivity limit of ≈10

-12 J and a spatial

resolution of ≈10-7 m. There is, therefore, plenty of room for improvement.

Advances in thermal probe design are also expected to lead to applications for

localised thermal analysis, thus enabling the behaviour of materials to be

investigated over even smaller dimensions. The integration of chemical analysis

(by pyrolysis and/or near-field photothermal infrared microscopy) raises the

possibility of constructing a versatile instrument capable of performing a wide

range of analyses that exploits abilities of the thermal probe to act as a very

small heater and thermometer. Such an instrument might well be termed “the

laboratory on a tip”.

85

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