In thermal analysis of disperse materials, the gas filling the pores of the samples has an important effect on the determination of heats of phase transformations.Particularly significant errors, up to 25 ~, may arise in cases when thermal analysis is carried out in gas atmospheres having high thermal conductivities.Based on experimental data, a relationship expressing the dependence of peak area on the thermal conductivity of the gas and on the thermal conductivity and particle size of the studied material has been derived. This relationship allows to calculate the possible experimental error and hence to adopt measures for reducing its value.Owing to the inadequate elaboration of the quantitative thermal analysis of processes accompanied by the formation of a gaseous phase, in such cases researchers are often forced to apply relationships established for solid-phase processes.In our view, the particular features of heat exchange and mass exchange in a disperse material as a result of the appearance of gaseous products, the form in which they are reflected, the quantitative aspects of the effects, and hence wellfounded suggestions as to the necessity and means of eliminating or taking them into account, have not been discussed with satisfactory consistence in the thermal analysis literature [1][2][3][4][5][6][7].The fact that the presence of a gaseous phase in the pores of a disperse material is reflected in the magnitude of the peak areas corresponding to the phase transformations may be considered as reliably established. Gases with higher thermal conductivities increase, and gases with lower thermal conductivities decrease the peak areas [I-4].The quantitative aspect of this problem, however, i.e. the degree to which peak areas change as a function of the parameters of the disperse system and the gaseous medium, has been much less studied. Reports in the literature on the quantitative evaluation of the effect of the gaseous phase in the thermal analysis of disperse materials are insufficient for the establishment of general relationships. In many cases the data are contradictory, obviously because the experimental apparatus used by the different authors differed in design.The main objective of the present work was the quantitative evaluation of the effect of the gaseous phase on the quantitative characteristics of the thermal curves, and primarily on the phase-transition peak areas.

In the analysis of disperse materials, the thermophysical characteristics (rate, thermal conductivity, thermal capacity) of the gas flowing through the pores of the sample are of great importance in determinations of the heats of phase transformations. We have found that the peak areas corresponding to the phase transformations may either d~rease or increase with increasing flow rate of the gas. The largest errors (as high as 15%) caused by gas flow in the pores of the disperse material oc~:ur when the thermal conductivities of both the solid material and the gas in which the analysis is performed are low. The experimentally derived relationship between the peak area, the flow rate and thermal conductivity of the gas, and the dispersity and thermal conductivity of the solid phase permits calculation of the possible error, and hence application of measures for its reduction.Gas flow through a layer of disperse material occurs when the disperse material undergoes thermal dissociation or decomposition and the gas evolved leaves the sample. Gas fow through a layer of disperse material also takes place in thermal analysis in a flowing gas atmosphere, utilized as an independent technique in the study of many chemical reactions.According to existing concepts [1-4], the gas evolved in the course of the reaction has a substantial effect on the peak area of the DTA curve. The gas flow changes the heat transfer coefficient in the mass of the sample, and consequently changes the areas of the peaks in the differential curve utilized in calculations of phase transformation heats. There is reason to assume [4,5] that with increasing flow rate of the gas the thermal conductivity of the gas phase will increase, due to the increased heat transfer by convection. The rate of gas evolution in thermal dissociation (and hence the flow rate of the gas in the pores of the disperse material) will depend on the rate of heat transfer to the transformation front, i.e. on the heating rate.If, with increasing flow rate of the gas, the overall thermal conductivity of the sample also increases, then (for one and the same sample of the investigated substance) the peak area should decrease with increasing heating rate.