Chapter 5 Differential Thermal Analysis and Differential Scanning Calorimetry

Abstract

This chapter discusses the differential thermal analysis and scanning calorimetry. The observation of changes in materials during heating has provided the foundation of experimental chemistry and technology. The thermochemical nature of the events occurring during the heating or cooling regime is recognizable from Le Chatelier's temperature-time experiments. The use of differential thermal analysis (DTA) for estimating calorimetric quantities is dependent on the proper control, calibration, and operation of an instrument designed for quantitative work. A differential temperature control loop senses any difference between the sample and reference and supplies differential power to correct this, with due regard to the direction and magnitude needed. The chapter discusses calibration, filtering, signal sampling, A/D conversion, and baseline treatment in detail. In many of the older DTA apparatuses, the measurement thermocouples, which were more massive than those used today, were buried in the centre of the test materials. There were many reasons for these arrangements, the main one being the lack of means, whereby the small microvolt difference signals from the thermocouples could be satisfactorily amplified. When the temperature measuring devices are situated beneath the pans containing the test materials, as in heat flux differential scanning calorimetry (DSC), then a baseline shift only occurs from a change in the specific heat of sample and reference materials, assuming that the sample mass does not change. In power compensation DSC, the sample and reference holders are insulated from each other and have their own individual sensors and heaters. The theories of DTA and DSC are generally based on heat transfer by conduction and assume that radiation is insignificant.