In the Emanation Thermal Analysis the release of inert gas atoms previously incorporated in the sample is measured. Results
of the statistical modelling of the inert gas release during thermal decomposition of solids are presented. The updated model
supposing three components in the solid-state reaction system including the formation of an intermediate metastable component
on the surface of the newly formed component was proposed for the modelling of the ETA curves. The release of inert gas atoms
previously incorporated into the sample is used as a probe of microstructure changes. The random nucleation mechanism was
considered in the modelling. The model can be used in modelling ETA curves of solid-solid, solid-gas and solid-liquid interactions
where the existence of metastable intermediate component is supposed.
The theoretical curves in the coordinates a vs. time for isothermal, and avs. temperature for non-isothermal experiments are calculated as functions of three kinetic parameters: activation energyE, pre-exponentical factorA and theg(α) function describing the mechanism of thermal decomposition of solids. The results show that conclusions not taking into consideration all three parameters can lead to information of little value concerning the mechanism of the decomposition and kinetic calculations. A correlation between non-isothermal and isothermal experiments, important for determination of the thermal stabilities of the compounds, is impossible without a knowledge of theg(α) function.
Aspects of the theories that are conventionally and widely used for the kinetic analyses of thermal decompositions of solids,
crystolysis reactions, are discussed critically. Particular emphasis is placed on shortcomings which arise because reaction
models, originally developed for simple homogeneous reactions, have been extended, without adequate justification, to represent
heterogeneous breakdowns of crystalline reactants. A further difficulty in the mechanistic interpretation of kinetic data
obtained for solid-state reactions is that these rate measurements are often influenced by secondary controls. These include:
(i) variations of reactant properties (particle sizes, reactant imperfections, nucleation and growth steps, etc.), (ii) the effects of reaction reversibility, of self-cooling, etc. and (iii) complex reaction mechanisms (concurrent and/or consecutive reactions, melting, etc.). A consequence of the contributions
from these secondary rate controls is that the magnitudes of many reported kinetic parameters are empirical and results of
chemical significance are not necessarily obtained by the most frequently used methods of rate data interpretation. Insights
into the chemistry, controls and mechanisms of solid-state decompositions, in general, require more detailed and more extensive
kinetic observations than are usually made. The value of complementary investigations, including microscopy, diffraction,
etc., in interpreting measured rate data is also emphasized.
Three different approaches to the formulation of theory generally applicable to crystolysis reactions are distinguished in
the literature. These are: (i) acceptance that the concepts of homogeneous reaction kinetics are (approximately) applicable (assumed by many researchers),
(ii) detailed examination of all experimentally accessible aspects of reaction chemistry, but with reduced emphasis on reaction
kinetics (Boldyrev) and (iii) identification of rate control with a reactant vaporization step (L’vov). From the literature it appears that, while the
foundations of the widely used model (i) remain unsatisfactory, the alternatives, (ii) and (iii), have not yet found favour. Currently, there appears to be no interest in, or discernible effort being directed towards,
resolving this unsustainable situation in which three alternative theories remain available to account for the same phenomena.
Surely, this is an unacceptable and unsustainable situation in a scientific discipline and requires urgent resolution?
This analysis of interface phenomena considers the alternative processes that may result from heating a crystal, particularly including thermal decomposition, involving chemical reactions, and melting, involving loss of long-range structural order. Such comparisons are expected to provide insights into the factors that determine and control the different types of thermal changes of solids. The survey also critically reviews some theoretical concepts that are currently used to describe solid-state thermal reactions and which provides relevant background information to models used in a recently proposed theory of melting. Probable reasons for the current lack of progress in characterizing the factors that control chemical changes and mechanisms of thermal reactions in solids are also discussed.
It is concluded that some aspects of the macro properties of reaction interfaces in crystal reactions have been adequately described, including geometric representations of interface advance during nucleation and growth processes. In contrast, relatively very little is known about the detailed (micro) processes occurring within these active, advancing interfacial zones: reactant/product contacts during chemical reactions and crystal/melt contacts during fusion. From the patterns of behaviour distinguished, a correlation scheme, based on relative stabilities of crystal structures and components therein, is proposed, which accounts for the four principal types of thermal changes that occur on heating solids: sublimation, decomposition, crystallographic transformation or melting. Identifications of the reasons for these different consequences of heating are expected to contribute towards increasing our understanding of each of the individual processes mentioned and to advance theory of the thermal chemistry of solids, currently enjoying a prolonged quiescent phase.
Practical aspects of the studies of stages of thermal dissociation of solids, of the kinetics of the stages, and of utilization of general regularities of the process for verification of kinetic studies are discussed.
Elementary thermochemical calculations show that in all cases of formation of solid product in the process of the congruent
dissociative vaporization of reactants, the equilibrium partial pressure of the main product greatly exceeds its saturation
vapour pressure, and therefore causes the appearance of vapour oversaturation. The oversaturation is responsible for the formation
and growth of nuclei, their shape and position, the transfer of condensation energy to the reactant, the existence of induction
and acceleration decomposition periods, the reaction localization, the epitaxial/topotaxy effects and the nanocrystal structure
of the solid product. Variations in the energy transfer explain an increase of the molar enthalpy with temperature and the
decelerating influence of melting on the rate of decomposition.
Reliable thermodynamic parameters for the thermal decomposition of diammonium hydrogenphosphate [(NH4)2HPO4] may be obtained using a fluidized bed. For the same size of particle, at the same temperature, but for different carrier gases, the rate constant and activation energy increase in the following order: air, methane, hydrogen. For the same carrier gas (air) the rate constant increases when the particle size decreases.