Gaseous reactant

It is important to maintain a uniform reaction environment within the vessel chamber by using a stirrer to agitate and mix the reactants. Gaseous discharge is vented through filters to the outside environment. As the reactions may generate substantial heat (exothermic) and pressure, or may even be potentially explosive, special precautionary features are designed into the vessel. 

Mass transport of reactant gaseous species to vicinity of substrate ... 

With more advanced instruments available, it is possible to identity the reactant gaseous species of the chemical reactions. This in turn helps to improve the understanding of the underlying chemical reactions and the processes ... 

The reactant gaseous streams are irradiated by a continuous-wave C02 laser beam. The laser power is usually between 600 and 1500 W, and the laser beam in the left reaction zone is typically 10 to 11 mm in diameter, while the right one is 9 to 10 mm in diameter. The chemical reaction of the HMDS precursor results in a bright yellow flame. The powders are conducted into a filter holder for collection driven by the vacuum pump. 

Homogeneous chemical reactions occur in the bulk gas and within the boundary layer leading to the formation of various intermediates. Reactant gaseous species and intermediates are transported within the boundary layer to the surface of the substrate. 

The reactant gaseous species and intermediates are adsorbed on the surface of the substrate. First, the adsorbed species migrate to the preferred sites on the surface, then heterogeneous chemical reactions take place to form a solid deposit and to incorporate into the lattice of the substrate. 

To understand the temperature dependence of the growth rate Grove [44] proposed a simple model as shown in Figure 4.20. In this model it is assumed that the mass transport of the reactant gaseous species across the boundary layer only depends on the mass diffusion. As a result there is a concentration gradient of the gaseous species. The flux (Fj) of mass transport from the gas phase to the substrate surface is written as [45]... 

Assuming that the order of the chemical reaction is one, the flux (F2) of the reactant gaseous species consumed on the substrate surface through the heterogeneous reactions is represented by... 

Under steady conditions the flux of reactant gaseous species from the bulk of gas to the surface of the substrate is equal to the flux of the reactant gas consumed on the substrate surface, i.e. F, = F2. Then the relationship between CG and Cs is obtained as... 

This regime, shown in Figure 4.19, takes place at relatively low temperatures. It implies that the chemical reaction rate is much lower than that of mass transport, which can be described by ks hG. In such a case the reactant gaseous species can be sufficiently transported from the bulk gas to the substrate surface. Hence, there exists no concentration gradient in the boundary layer, i.e. CG = Cs. The growth rate (r) of the CVD coating can be expressed by... 

This regime appears at moderate temperatures. Compared with the chemical reaction regime the mass transport rate of the reactant gaseous species is much... 

For I-CVI processes, the driving force for mass transport is the concentration gradient of the reactant gaseous species as shown in Figure 5.3. During the densification process the precursor gases flow over the preforms at a reduced pressure. Then they diffuse into the porous and fibrous preforms, react and form a... 

For the infiltration of a pore with a large aspect ratio, a small Sc implies the entire surface of the deep pore can be uniformly coated. The reason is that the reactant gaseous species are easily transported to the bottom of the pores by... 

Noncatalytic gas-solid reactions (e.g., combustion and gasification of coal, roasting of p5irites). These reactions occur on the surface of the solid. The gaseous reactant is transported to the interface, where it reacts widi the solid reactant. Gaseous products are transported to the gas phase, and solid products (e.g., ash) remain in the solid. The overall reaction rate depends on the surface area available and the rate of transfer of the gaseous reactant to the solid surface. 

This is a general situation When reactants, gaseous or liquid, have to diffuse from one phase to another phase which may be a liquid or a porous solid with which these reactants react or in which they react catalyticaUy, this effed of penetration will be more or less marked depending on the relative rates of reaction and diffusion. Since many solid catalysts consist of porous grains or pellets with a large internal surface area, this phenomenon will be of importance in all catalytic processes using such solids. 

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