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A mathematical model for chemical vapor deposition in an impinging jet reactor is extended to consider growth of materials that are nonstoichiometric or that contain multiple solid phases. The model treats the fluid flow of the reactant gas mixture, multicomponent heat and mass transfer, and simultaneous gas-phase and surface reactions. For a given system, insight into the rate-limiting steps can be obtained by identifying a set of reaction rate constants that gives a match between theoretical results and experimental data. The calculations also provide quantitative information on the relationships between operating conditions, deposition rate, and deposit composition. Results for production of boron carbide from BCI,, CH,, and Hz are presented as an example. Under typical processing conditions, deposition of B,C is found to be controlled primarily by the rate of CH, decomposition and the extent of surface coverage by CH,. Theoretical calculations are presented for different sets of operating conditions to illustrate the predictive capability of the model. Also, the sensitivity of results to various reaction mechanisms and to values for solid-phase activities is evaluated. Analogies are drawn between the boron carbide process and deposition of other ceramics in which a carboncontaining reactant is used.

A mathematical model that describes chemical vapor deposition in an impinging jet reactor has been used to assess the sensitivity of theoretical predictions to physical property values for intermediate species. Uncertainties in thermochemical data for a species have greatest impact on predicted deposition rates when the composition level of that species near the surface is comparable to the local compositions of the reactants. Under these circumstances, relatively small changes in the values of thermodynamic quantities used for the species can produce significant differences in deposition rates calculated under equilibrium conditions. Also, with the modified property values, different values of the kinetic parameters may be required to fit available experimental data, and this can lead to different interpretations of the rate-limiting steps in the process. Quantitative information for chemical vapor deposition of boron from boron trichloride and hydrogen is presented.

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