Kinetic Investigations by Techniques of Thermal Analysis

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Abstract
Thermoanalytical techniques offer attractive ways of determining kinetic parameters. There are obvious convenience and economy in making a single set of measurements in a few hours and performing extensive computations while the equipment is generating data on another material. This apparent gain in efficiency has led to considerable effort in planning computations which lead to easy extraction of the activation energy, the apparent kinetic order, the rate constant from a single curve. To enable these computations, simplifying assumptions had to be made. The following statements provide a set of cautions based on past work. Nearly all solid decompositions are topo-chemical in nature. Many reactions have some reversible step which leads to effects due to changing atmospheres or pressures. The variation of k with τ does not provide an accurate measure of the activation energy except under very reproducible conditions. The derivative dlnk/d(1/τ) is a partial, not a full, derivative and there is generally no certainty that the other variables are held constant. The energy states in a solid material do not vary statistically because there is no way to establish an equilibrium distribution. Both the total energy and distribution of energy states are influenced greatly by the history of the sample. The added energy of the surface plus its immediate contact with the atmosphere plus its higher temperature during heating will cause it to react differently from the bulk. This also introduces a particle size effect. Decompositions do not necessarily proceed by a single mechanism; hence, different rate constants and activation energies would apply to different stages. These stages are not necessarily separated. Pressure effects are not easily isolated because gases need to diffuse through particles to reach or leave the reaction site. Reaction products and hence subsequent reactivity are affected by the atmosphere. Direct measurement of the sample temperature is vital for kinetic treatments. The self-cooling during any but very slow reactions can cause the temperature to deviate greatly from the programmed temperature. The general form of the DTA peak can be simulated by calculations based on homogeneous kinetics. The variability of real peaks indicates that other models are actually operative. Thermodynamic heats of reaction are in many cases the only limit on reaction rate. The activation energy has this enthalpy change as its lower limit. Heat transport and mass transport or a combination of them can simulate a chemical control on reaction velocity. Mechanical effects due to expanding or contracting crystal structure may limit the agreement with any single model. Modern methods of data acquisition make it possible for the experimenter to accumulate enough data from a single experiment to permit greatly extended data treatments. Enough data can be acquired and stored to offer hope of separation of contributing mechanisms.