Results of computations carried out on a model in which the samples are contained within cylindrical cavities in a highly conducting block, and in which centre temperatures are measured lead to the following conclusions.(1) The area under a DTA peak is directly proportional to the heat of reaction and mass of the sample and inversely proportional to the thermal conductivity of the material. The area, measured as AT vs. time is independent of the heating rate.(2) The temperature of a DTA peak increases for increasing radius of sample and the variation of sample peak temperature is less than that of the reference material. Hence DTA curves should use sample temperature as abscissa to reduce the influence of differing sample radius used by different experimental designs.The heating rate at the peak of the DTA curve is reduced from the nominal value in samples of large radius. Where the reaction is governed by an equation which is heating rate dependent, then erroneous peak temperatures will be recorded. Further, large radius samples distort the peak shape, and hence small radius samples are important to reduce or eliminate these two effects.(3) The DTA peak reference temperature increases with decreasing sample conductivity and increasing density and specific heat. The peak sample temperature is sensibly independent of the physical properties. The physical properties do, however, influence the peak temperature shift with heating rate leading to erroneous values of activation energies when using techniques outlined by Kissinger (12).The positioning of measuring thermocouples is not so critical in samples of small radius as it is in samples of large radius.(4) Heat loss via the measuring thermocouples causes a reduction in the area under the DTA peak and also lowers the peak temperature.Large heat losses can reduce the actual rate of heating of the centre of the sample even though the rise in block temperature is closely controlled. This fact must be borne in mind when thermocouples are changed, as a change of heat loss will affect the actual rate of heating of the sample.The heat loss may be minimized by the use of thin thermocouple leads, with the limitation that the wires should be conveniently handled when making connections.
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