NONCATALYTIC GAS-SOLID REACTIONS AND MECHANICAL-STRESS GENERATION

The reaction rate in gas-solid systems can be affected by mechanical s tresses that arise as the reaction proceeds. Stresses develop due to d ifferences between precursor and product molar volumes and thermal exp ansion coefficients. Experimental evidence on the interaction of react ion rate and mechanical stress for the Ti/N-2 and Ti/O-2 systems is pr ovided. A detailed and consistent mathematical model is developed for the reaction taking place at the constrained precursor/product interfa ce. An elastic formulation for the stresses is adopted, and stress gen eration due to mismatches in linear thermal expansion coefficients and equivalent volume (Pilling-Bedworth ratio) for the precursor and prod uct are considered. The effect of surface energy, which becomes signif icant for particle sizes below 1 mu m, is also included in the model. Both experimentally and theoretically, conditions exist where the mech anical stresses exceed the strength of the material, leading to mechan ical breakdown of the product layer, thus causing a discontinuity in t he observed reaction rate. The entire processing history including the reaction, temperature, and pressure profiles, plays an important role in determining the overall reaction kinetics of the powder.