Quantitative thermal analysis V. Role of the flowing gas phase in the analysis of disperse materials

Abstract

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.