Chemistry Reference
In-Depth Information
Chapter 6
Mechanical and Thermal Dilution Methods Used
in the Thermographic and Thermogravimetric
Analysis of High-Temperature Decomposition
of Condensed Materials
Abstract
The need to substantially modify common nonisothermal methods such
as TGA, DTA and DSC when used to study the kinetics of fast exothermal and
endothermic reactions in condensed media is theoretically demonstrated. Modified
methods based on deep ballasting of the sample with inert heat-conducting material
are proposed. For homogeneous samples, “mechanical dilution” (mixing the sample
particles with chemically inert material in the ratio 1:100) is used. For heterogeneous
samples, a “thermal dilution” method where a thin sample is pressed between the
ends of two metal cylinders characterized by high thermal conductivity is used. An
analytical approach for determining kinetic constants from TGA, DTA and DSC
data is proposed. Use of the modified experimental cells was shown to increase
the upper limit of the reaction rates that could be measured by several orders of
magnitude, even when commercial TGA, TDA and DSC instruments are applied.
6.1 Introduction
Although they have been widely used in physicochemical studies for a long time,
differential thermal analysis (DTA, thermography), differential scanning calorime-
try (DSC) and thermogravimetric analysis (TGA, thermogravimetry) have only re-
cently been applied to obtain kinetic data (the most comprehensive information on
the theory and practice of DTA can be found in [1, 2, 3, 4, 5, 6]).
Compared to isothermal methods, DTA and TGA are more informative with
respect to the kinetic behavior of the system under study. In addition, the corre-
sponding equipment is quite simple. The application of DTA and TGA to study fast
high-temperature reactions in condensed media appears to be very promising.
When the experiments described here were begun, almost all methods of deter-
mining kinetic parameters from DTA-TGA data (except for a method used in [7])
were “discrete” ones [1]. As a rule, only some points on the experimental curve were
used for analysis: peak height, maximum reaction rate, furnace temperature corre-
sponding to these points, etc. When most or all of the experimental points were
used for the analysis, the analytical expression describing the kinetics of the system
was assigned a priori, which is characteristic of a “discrete” technique. For these
