Vapor-Solid Phase Reactions

All vapor phase epitaxial growth processes involve the interaction of the vapor with the surface of the solid phase, thereby demanding the inclusion of heterogeneous kinetics into the overall rate discussion. The typical description of surfaces is a model based on the hypothesis that surfaces are composed of a fixed number of sites on which 

In certain instances, reaction rates substantially decrease with increasing temperature. A classic example is atomic layer epitaxy (ALE). This is a result of adversely modifying the rate of desorption of the reactants to become greater than the concomitant increase in the surface reaction rate accompanying the increase in substrate temperature. One potential approach to address this specific issue is to move to alternate precursors (see Sect. 1.8). 

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Tubular reactors have been the main tools to study continuous flow processes for vapor or gas-phase reactions. These are also used for reaction in tv o flowing phases over a solid catalyst. When the catalyst is in a fixed bed, the contact between the liquid on the outside surface of the particulate is uncertain. For slurry-type solid catalyst the residence time of the catalyst or the quantity in the reactor volume can be undefined. 

Measurements of the true reaction times are sometimes difficult to determine due to the two-phase nature of the fluid reactants in contact with the solid phase. Adsorption of reactants on the catalyst surface can result in catalyst-reactant contact times that are different from the fluid dynamic residence times. Additionally, different velocities between the vapor, liquid, and solid phases must be considered when measuring reaction times. Various laboratory reactors and their limitations for industrial use are reviewed below. 

Polymer films can also be deposited on solid particles by vapor phase reaction or from a melt. The best example of vapor phase reaction is the deposition of Union Carbide s Parylene ,... 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

Gasification reactions of solids The reactions of solids with gas-phase reactants to form gaseous products are generally described in the same manner as are surface-catalyzed reactions. The reaction of carbon with water vapor is an example ... 

Solid deposition from gas- or liquid-phase reactants Solid-deposition reactions are important in the formation of coatings and fdms from reactive vapors (called chemical vapor deposition or CVD) and of pure powders of various solids. Examples are ... 

The consequences of the electrochemical reduction of high valence chromium species would be the precipitation of Cr203 solid phase at the cathode-electrolyte interface boundary. These led to the hypothesis that the degradation mechanism of LSM cathode is dominated by an electrochemical reduction of high valence vapor species of chromium (Cr03 and C OH O to solid phase Cr203 in competition with the 02 reduction reaction, followed by the chemical reaction with LSM to form (Cr,Mn)304 phases at the TPB, blocking the active sites [174-180], The process is written as follows [174] ... 

The chemistry of soil is contained in the chemistry of these three phases. For the solid phase, the chemistry will depend on the amount and type of surface available for reaction. In the liquid phase, solubility will be the most important characteristic for determining the chemistry occurring. In the gaseous phase, gas solubility and the likelihood that the component can be in the gaseous form (i.e., vapor pressure) will control reactivity. 

Investigators have used the words carbon and soot to describe a wide variety of carbonaceous solid materials, many of which contain appreciable amounts of hydrogen as well as other elements and compounds that may have been present in the original hydrocarbon fuel. The properties of the solids change markedly with the conditions of formation and, indeed, several quite well-defined varieties of solid carbon may be distinguished. One of the most obvious and important differences depends on how the carbon is formed carbon may be formed by a homogeneous vapor-phase reaction it may be deposited on a solid surface that is present in or near the reaction zone or it may be generated by a liquid-phase pyrolysis. 

In this case the reaction occurs only in the hquid phase (the oils have negligible vapor pressure), but reaction requires a solid catalyst such as finely divided Ni, which is suspended in the hquid. Thus we see that this is in fact a three-phase reactor containing H2 gas, organic hquid reactants and products, and sohd catalyst. 

Immediately after the isolation of macroscopic quantities of Cgo solid [298], highly conducting [299] and superconducting [141] behaviors were verified for the K-doped compounds prepared by a vapor-solid reaction (Haddon, Hebard, et al.). Crystallographic study based on the powder X-ray diffraction profile revealed that the composition of the superconducting phase is KsCeo and the diffraction pattern can be indexed to be a face-centered cubic (fee) structure with a three-dimensional electronic pathway [300]. The lattice parameter (a = 14.24 A) is apparently expanded relative to the undoped cubic Ceo = 14.17 A). The superconductivity has been observed for many A3C60 (A alkali metal), e.g., RbsCeo (Tc = 29 K... 

According to the relationship between the lattice volume and Tc as described, cubic CssCgo would be an ultimate candidate for a higher Tc superconductor, but the conventional vapor-solid reaction affords only the thermodynamically stable CsCso and CS4C60 phases. In 1995, noncubic CssCgo was obtained by a solution process in liquid ammonia, and the superconductivity was observed below 40 K under an applied hydrostatic pressure of 1.4 GPa [311]. 

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