REACTION-RATE MODELING IN NONCATALYTIC GAS-SOLID SYSTEMS - SPECIES TRANSPORT AND MECHANICAL-STRESS

A detailed model to describe the overall reaction rate of the oxidatio n of titanium is developed. The mathematical model consists of two fac ets, the first of which involves a detailed description of species tra nsport that accounts for the formation of charged species. This is aug mented by a description of tile occurrence of mechanical stress due to a Pilling-Bedworth ratio that differs significantly from 1 as well as differences between precursor and product thermal expansion coefficie nts. A self-imposed electric field is formed across the oxide layer du e to different mobilities of the species considered. This field oppose s the transport of electrons and enhances rile transport of anion vaca ncies, thus increasing the overall reaction rate compared to a pure di ffusion process, while also ensuring that electrical charge is conserv ed. Large growth stresses result from the unmatched precursor and prod uct volumes, significantly affecting the overall process. These result s show that the incorporation of a consistent treatment of mechanical stress forms a necessary part of any accurate description of the overa ll behavior of a reacting particle.