Investigation of thermal characteristics of a DTA cell block

Abstract

Prior to evaluating DTA curves a thorough investigation of different types of specimen holders and cell blocks should be made, which means a quantitative appraisal of heat transfer conditions. The empirical approach makes use of the thermal analogue of Ohm's law, where R = thermal resistance. Calculation of R gave a graphical correction of about 1 ram, which stresses the merit of computer calculation. Very small differences of the values for R between platinum and alumina sample holders were observed. R was experimentally determined at 500 ~ and 1000 ~ by means of UO2 and BaCO3.A rigorous comparison of various approaches in investigations of kinetics is needed to clear up the controversy between isothermal versus dynamic experimental methods. DTA methods, which are simple and time-saving, might be of use in such a project. Prior to evaluating DTA curves a thorough investigation of different types of specimen holders and cell blocks must be accomplished. This means that a quantitative appraisal of heat transfer conditions existing in an actual block should be made. A close examination of the total peak area including the exponential tail of a DTA curve, i.e. that part returning from the peak to the baseline, may make it possible to calculate the parameters of the apparatus. Thus the kinetic effects might be separated from the main diffusivity of the system, and the existing theories, (e.g. Piloyan et al.) for kinetic calculations might be tested.There have been a number of mathematical treatments of DTA curves. The investigation of thermal characteristics of DTA cell blocks has received less attention.In 1954

Calculation of thermal effects in DTA

“…A similar parameter figures in [2][3][4] as a time constant. Its value can be obtained from a calibration experiment using a sample with a known heat capacity value, differing from that of the reference sample.…”

“…A calculation using a linear approach of the heat exchange between the sample being tested at the temperature Ts(z) and the block at the temperature Tb('C), of the heat exchange between the sample and a number of heat outlets with the temperatures Toj (j = 1, 2,..., k; z = time), and also of the heat exchange between the samples, leads to the following equation for the capacity absorbed in unit volume of the sample studied: dr i=1 (4) Assuming that the temperature change rate of the reference sample is constant, j=k dTr _ fl, and introducing the terms 3 = T~ -Tr and B = ~ Bj, we obtain from dr j=i Eqs (3) and (4)…”

“…The analytical description of the thermal processes in DTA will be carried out similarly to [1][2][3][4] and others, assuming that the temperature and other parameters of the sample under study and of the reference sample are constant over their volume at each moment of time. The experimental approach to this model is an instrument in which the heat exchange between the block and the samples takes place over sufficiently large heat resistances, so that temperature transfers in the interior of the samples are small in comparison to the transfers over the above-mentioned resistances.…”