The Pyrolytic Regime

Chemical modeling of gas-phase and surface reactions, nucleation, and growth is based on concepts developed for surface reactions on heterogeneous catalysts. In order to understand the reaction rates and the microstructure of the products observed during CVD, the driving forces for the reaction, the kinetics in the pyrolytic regime, powder formation at high supersaturation, and the uses of thermodynamic calculations have to be discussed briefly. 

If diffusion and chemical reaction rate coefficients are high with respect to the flow of the bulk reactant gas supply (similar to the situation of a well-stirred reactor in which the contents are in thermodynamic equilibrium), so-called CVD diagrams or stability diagrams can be used to predict the process conditions necessary for 

Thermodynamic calculations allow estimates of the species in the gas and solid phases that are in equilibrium at the conditions specified. This is useful for determining the choice of process conditions necessary for ensuring a positive driving force for the reaction. 

For the calculation of thermodynamic equilibria the Gibbs energy of the system is minimized at given temperature and pressure as a function of the composition of all the reactants and reaction intermediates. Boundary conditions are the amounts of the reactants that enter the reactor. Also the stoichiometric coefficients of the reactions are constraints in the minimization. Commercial programs for routine thermodynamic calculations are SOLGASMIX, CHEMSAGE, or HSC. 

The deposition rate in the pyrolytic regime is proportional to the equilibrium constant K for small values of K for large K the reaction rate does not depend on K, but only on the mass flow rate. This will be shown for the case of a simple fictitious pyrolysis equilibrium AB g) A(s) -I- B(gf). The equilibrium constant is 

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The pyrolytic regime is distinguished from the (diffusion-limited) dialytic regime by the effect of the input gas flow rate on the deposition rate. Both transport-controlled regimes occur at high temperatures, where the slopes of the Arrhenius plot decrease (Figure 6.12). In the case of small equilibrium constants, the slope of the Arrhenius plot in the pyrolytic regime equals the reaction heat, as will be shown below. 

Codeposition in CVD means deposition of several solid phases simultaneously from the same precursor gas mixture. Making composite coatings by codeposition requires relatively high temperatures, i.e., working in the pyrolytic regime. The reason is that the deposition rates of the phases must be high and approximately comparable and must not depend on the substrate too much. This last condition is... 

A thermo-analytical study of untreated and CCA treated wood samples (more or less cylindrically shaped with diameter less than 2 mm) was performed in order to examine the influence of the presence of CCA on the pyrolytic behaviour of wood samples (TG study), as well as the release of metals (Cu, Cr and As) during the pyrolysis process in the kinetically controlled regime. Since arsenic is the most problematic compound during pyrolysis of CCA treated wood and almost all arsenic is present in the treated wood as CrAs04 (precipitated on cellulose or complexed with lignin) [9], a... 

M. L. Bernard et al, CRAcadSci, Ser C, 272 (26), 2112-15 (1972) CA.75, 101675 (1971) A generalization of the previously described (Bernard et al, 1971) ablation theory (for AP) combustion to include AP-based solid propints provides satisfactory fits to the exptl data of M. Summerfield et al (1960), with reproduction of the two distinct pressure regimes observed in the combustion of AP-based powders between 0 and 100 bars. The solid binding agent is assumed to be easily pyrolyzable, with each mol producing several combustible gaseous mols, as in the case of pyrolytically degrading polymers... 

Such systems are realized first of all in pilot plants for pyrolytic preparation of active carbon, for instance, by means of the fluidization of an oxidizing agent into the solid phase in a nonstationary regime. The related heat-transfer equation contains the convection term and that of the heat effects of the chemical reaction ... 

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