Differential thermal analysis

Abstract: In differential thermal analysis, the temperature at which the maximum deflection is observed varies with heating rate for certain types of reactions. An expression can be derived relating this variation with the kinetics of the reaction. By making a number of differential thermal patterns at different heating rates, the kinetic constants can be obtained directly from the differential thermal data. Measurements of the variation of peak temperature with heating rate have been made for several minerals of the kaolin group, the values of the kinetic constants determined, and these values compared with corresponding values obtained for both the same samples and similar material by conventional isothermal techniques. Some factors affecting the results are discussed. The method of differential thermal analysis (DTA) has been universally accepted by mineralogical laboratories as a rapid and convenient means for recording the thermal effects that occur as a sample is heated. Changes in heat content of the active sample are indicated by deflections shown by a line representing the differential temperature. It is conventional to represent an endothermic effect by a negative deflection and an exothermic effect by a positive deflection. The deflections, whether positive or negative, are called peaks.

Abstract: Several isothermal experiments are generally needed to determine the parameters of the Avrami equation which describe most of the heterogeneous solid state reactions. Differential scanning calorimeters are suitable for such experiments. While most differential thermal analysis (DTA) apparatus cover a wider temperature range than DSC apparatus they cannot be used to perform isothermal determinations. However, Kissinger has already shown how activation energy and frequency factor can be calculated from DTA experiments for the case of homogeneous reactions following first order kinetics. We derive in this paper an extension of the Kissinger method and show its applicability to heterogeneous reactions described by an Avrami expression. The new method will allow the study of the kinetics of metallic reactions at the higher temperature range obtainable with DTA. The transformation kinetics of the metastable equiatomic tin-nickel alloy are given as an example.

Abstract: Several methods are known for the evaluation of the main kinetic parameters related to a thermoluminescence (TL) curve: namely, the activation energy, the pre-exponential factor and the kinetic order. These methods can easily be applied, under certain conditions, to a series of related thermally stimulated phenomena which are governed by similar differential equations. These include thermally stimulated conductivity (TSC), thermally stimulated electron emission (TSEE), ionic conductivity (ITC) and thermally stimulated depolarization (TSD), thermal annealing (release of stored energy, measured thermally), partial thermo-remanent magnetization (PTRM), thermal absorption and evolved gas analysis (EGA), derivative thermogravimetry (DTG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The concept of a kinetic order which is neither first nor second is discussed as related to the various phenomena, as well as the temperature dependence of the pre-exponential factors. A special interest lies in performing simultaneous measurements of two (or more) of these effects, in particular, TL and TSC. A method is described for calculating the recombination probability from such simultaneous measurements and the cross-section for recombination therefrom.