Study of the kinetics of the mechanism of solid-state reactions at increasing temperatures

Many solid state reactions can be represented by equations of the type F(α) =kt, where α is the fraction of material reacted in time, t. These equations can be expressed in the form F(α) =A(t/t0.5) where t0.5 is the time for 50% reaction and A is a calculable constant depending on the form of F(α). Numerical tables are given of F(α) in relation to α, and to (t/t0.5), for nine equations corresponding to reactions which are diffusion controlled, or are reaction-rate controlled, or obey first order kinetics, or follow the equations of Avrami and Erofe'ev. The application of the tables to the analysis of experimental data is described.

Numerical Data for Some Commonly Used Solid State Reaction Equations

A method of comparing the kinetics of isothermal solid-state reactions based on the classical equation for analysis of nucleation-and-growth processes is described. In this method, plots of In In (1-α) vs In (time), where α is the fraction reacted, are used to distinguish reaction mechanisms. Even nonintegral slopes obtained for values of the fraction reacted from 0.15 to 0.50 may indicate whether the reaction rate is diffusion- or phase-boundary-controlled. The problems of ascertaining zero time and self-cooling (or heating) of the reacting sample can be observed in the analysis but do not cause severe difficulties in interpretation, as they can for analyses based on reduced-time plots. The analysis is applied to the dehydroxylation of kaolinite and of brucite and the decomposition of BaCO3.

Method of Comparing Solid‐State Kinetic Data and Its Application to the Decomposition of Kaolinite, Brucite, and BaCO3

This chapter deals essentially with the reaction between two solids A and B to form one or more product phases (A m B n ). During the course of this heterogeneous solid-solid reaction A m B n separates the reactants spatially. Therefore the progress of the reaction has to be attributed to a transport of the reactants across phase boundaries and through the reaction product.

Solid state reactions

Self-propagating high-temperature synthesis (SHS) and processes based on SHS are currently being developed the world over for the production of powders and near-net shape components of advanced materials. The research activities that have been and are being carried out in this field are reviewed here. Theoretical principles underlying SHS process, such as equlibrium computation and kinetics involving heat and mass transfer are described. General concepts about the SHS reaction mechanisms with a few illustrative examples are presented. Along with a detailed description of the processing techniques such as powder production,in situ consolidation and casting, a few of the novel techniques based on SHS are also elaborated.

Self-propagating high-temperature synthesis

A model for solid‐solid or solid‐gas reactions between spherical particles and a fine powder or gas has been developed. The oxidation of uniformly sized nickel spheres has been shown to fit this model to 100% reaction. Previously reported models are inadequate because they do not meet the boundary conditions set down and because the volume of the product was assumed to equal that of one of the reactants. The inadequacy of earlier experimental results has been explained by the failure to experimentally meet the boundary conditions imposed.

R. E. Carter

Papers

Kinetic Model for Solid‐State Reactions