Highways and byways in thermal analysis
Two thermoanalytical techniques, differential thermal analysis (DTA) and thermogravimetry (TG), have always held the limelight. However, there has been an increasing trend to introduce variants of these for specific purposes, to develop methods dependent on properties other than energy and mass, to employ several techniques simultaneously and to combine thermoanalytical and non-thermoanalytical methods. The main aim of these developments is to produce in the minimum time the maximum amount of relevant and accurate information on the systems studied.
Variants of DTA, obtained by altering specimen holder design, by changing furnace atmosphere, by modifying sample size or by dispensing with the reference material, can improve the accuracy of energy-change determination, permit measurement of various thermal constants and allow examination of materials that would not give interpretable results in conventional equipment. Variants of TG can be used to derive more accurate kinetic information. Differential scanning calorimetry (DSC) has a somewhat different basis from DTA and is fundamentally quantitative for energy changes; recent developments allow it to be used to about 750 °C, opening the possibility of its application to a much wider range of materials.
Changes observed in dimensions and in mechanical, optical, electrical, magnetic and acoustic properties on heating all assist in interpreting the significance of results obtained by DTA, DSC and TG.
Evolved gas detection and evolved gas analysis are gaining increasing recognition. Although usually coupled with DTA or TG measurements, they are sometimes used independently, as in thermal volatilisation analysis, which is of value in the study of polymers, and in rapid pyrolysis coupled with gas chromatography or mass spectrometry, which has yielded interesting information on the nature of organic materials.
The use of two or more methods to examine one sample simultaneously has several advantages, particularly in facilitating comparison and interpretation of results. Although the optimum conditions for one determination are not necessarily those for another and care must therefore be taken in assessing results, multiple techniques can serve a useful function in many investigations.
Future trends seem to be towards the increasing development of some currently less well established methods; more extensive application of computers in experimental control and in interpretation of results can also be expected. In certain applications, the links with calorimetry may well become closer.