Some Basics En Route to Isoconversional Methodology - Isoconversional Kinetics of Thermally Stimulated Processes - page 23

This is important as one of the ways to reveal the temperature dependence of the
activation energy.
Last but not least, when selecting the temperature range for kinetic experiments
one should be mindful of possible phase transitions (e.g., melting or solid-solid
transformations of the solid reactant) that a reactant may undergo within that range.
For example, the respective rates and Arrhenius parameters for solid- and for liquid-
state decomposition can differ substantially [ 45 ], although they may also remain
practically unchanged [ 46 ]. Significant changes in the reactivity may also be en-
countered due to the solid-solid phase transitions. This phenomenon is known as
the Hedvall effect [ 47 , 48 ]. Therefore, one needs to exercise great care when com-
bining kinetic data collected in the temperature ranges below and above the phase
transition temperature. To detect the presence of possible phase transitions, one has
to carry out a DSC run because they are undetectable by TGA.
The aforementioned techniques allow one to minimize deviations of the sample
(process) temperature from the reference (furnace) temperature. Nevertheless, some
smaller deviations would continue to be present as long as a process is accompanied
by a thermal effect. It is good practice to compare the sample temperature against
the reference one. Both values are usually made available by modern DSC and TGA
instrumentation. The smaller the difference between the two temperatures the bet-
ter but it should not exceed 1-2 ﾰC, if the kinetic calculations are to be conducted
by means of the isoconversional methods that rely on the reference temperature.
When the difference is larger, one should consider reducing the sample mass and/or
decreasing the heating rates of nonisothermal runs or the temperatures of isothermal
ones. Larger temperature deviations can be tolerated [ 42 , 49 ] when using the iso-
conversional methods that permit directly using the sample temperature. A detailed
discussion of such methods is provided in Chap. 2.
References
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