2 Diffusion & Migration – Ionic Transport
2.5. Thermal Analysis (TA):
Foreword:
Thermal analysis, involves the measurement of certain physical and chemical properties as a function of temperature [77]. Applications of thermal analysis in electrochemistry are many and varied and include the study of reactions, phase transitions and the determination of phase diagrams. Most solids are “thermally active” [77][78] in one way or another and may be profitably studied by means of thermal analysis. The main techniques are thermogravimetry (TG), which records the mass of a sample as a function of temperature or time, and differential thermal analysis (DTA), which measures the difference of temperature, ∆T between a sample and an inert reference material as a function of temperature i.e. DTA therefore detects changes in heat content. A closely related technique is differential scanning calorimetry (DSC). In DSC, the equipment is designed to allow a quantitative measure of the enthalpy changes that occur. During the progress of this work, the author utilised two main techniques to study the thermal and physical behaviours of the materials under investigation.
Thermal Analysis - (DTA) & (DSC):
comp
until some thermal event, such as melting, decomposition or change in the “crystal” structure, tem
or leads (if the change is exothermic) the reference temperature. The reason for having both
referen fig
time as seen in Fig.
til the endothermic event occurs, e.g. melting at temperature Tc. The sample perature rem
applications, its main use historically has been in method of cooling curves which was used to 2.5.1.
In DTA, the temperature of a sample is ared with that of an inert reference material during a programmed change of temperature. The temperatures should be the same occurs, in which case the sample perature either lags behind (if the change is endothermic) sample and ce is shown in Fig. (2.5.1). In ure (2.5.1a) a sample is shown heating at a constant rate; its Ts monitored with a thermocouple, varies linearly with
(2.5.1.b) un
tem ains constant at Tc until the event is completed; the Tc. increases rapidly to catch up with the temperature required by the user. The thermal event in the sample at Tc
therefore appears as a rather broad deviation from the sloping baseline as seen in Fig.
(2.5.1b). Such a plot is insensitive to small heat effects. Furthermore, any spurious variations in the baseline, caused for example by fluctuations in the heating rate, would appear as apparent thermal events. Because of the insensitivity of this technique; it has limited
determine phase diagrams the sample temperature was recorded on cooling rather than on heating and since heat effects associated with solidification and crystallisation are usually
placed side by side on a heating block, which is heated or cooled at a constant rate; identical thermocouples are placed at each and are connected back to back. When the sample and When a thermal event occurs in the sample, a temperature difference, ∆T exists between sample and
large, they could be detected by this method.
The figure (2.5.1c) shows the arrangement used in DTA. Sample and reference are
reference are at the same temperature, the net output of this pair of the thermocouples is zero.
reference, which is detected by the net voltage of thermocouples. A third thermocouple is used to monitor the temperature of the heating block and the results are presented as ∆T against temperature as seen in Fig. (2.5.1d). A horizontal baseline corresponding to ∆T = 0, occurs and superimposed on this is a sharp peak due to the thermal event in the sample. The temperature of the peak is taken wheather as the temperature at which deviation form the baseline begins, T1 or as the peak temperature T2. While it is probably more correct to use T1, it is often not clear where the peak begins and therefore, it is more common to use T2 The size of the peak g to ∆T peak may be amplified so that the events with very small enthalpy changes may be detected.
The technique DSC is very similar to DTA. A sample and an inert reference sample are also used but the cell design is different. In some DSC, cells the sample and reference sample are maintained at the same temperature during heating and the extra heat input to the sample (or reference if the sample undergoes an exothermic change) required in order to maintain this balance is measured. Enthalpy changes are therefore measured directly. In other DSC cells, the difference in temperature between the reference is measured, similar in DTA.
Time Ts
Tc
Temperature + - (b)
(a)
Ts
Heat at a constant rate
∆T 0
T1
T2
Exothermic
Endothermic (d)
Ts
Sample
Tr
Reference
(c) + +
Fig. (2.5.1) DTA & DSC techniques
Fig. (2.5.2) Schematic of DTA curves showing melting of crystal on heating and a large hysteresis on cooling which gives Tg
Temperature
∆T
Exo
Endo
Heat
Cool Melting
Liquid
Melting Crystals
Supercooled liquid Tg
2.5.2.
TA – Applications:
An important use of DTA and DSC in glass, polymer, and liquid crystal science is the easur
in order to study new behaviours of the ubstituted materials.
m e of the glass transition temperature Tg. [11] [79 – 82] This appears as a broad anomaly in the baseline of the DTA curve, as seen in Fig. (2.5.2); the Tg value represents the temperature at which the glass of polymer transforms from a rigid solid super-cooled, to a very viscous liquid. The Tg represents the upper temperature limit at which the material under investigation can be used and also provides a readily measurable parameter for studying such aforementioned materials. For glasses and other materials, which are kinetically very stable, such as silica glasses, the glass transition is only a small thermal event observed in DTA since crystallisation it usually very slow in occurring [77]. For glasses, however crystallisation or devitrification may occur at some temperature above Tg and below the melting point Tm. Devitrification appears as an exotherm and is followed by an endotherm at higher temperature that corresponds to melting of these same crystals. DTA and DSC are both powerful techniques for the determination of phase diagrams, especially in conjunction with other techniques such as impedance spectroscopy. Both techniques are particularly useful during studies of ion mixtures and substitution reactions;
s
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Chapter 3
ctroscopy (EIS):
r otherwise known as the ac application of a periodic
small-ection {2.4.}, impedance of application of
onductors.
Characterisation of material; solid electrolyte and solid-state devices.
easured using a e question; what is the ideal
e [8]. ise between “high
ll known [8 - 12]
tics of more complex ber of