Solid-vapor process

In a study by Kong et al. [7], single-crystal nanorings of ZnO were grown by the solid-vapor process. The raw material was a mixture ofZnO, indium oxide, and lithium carbonate powders at a weight ratio of 20 1 1, and it was placed in the highest temperature (1400 °C) zone available in a horizontal tube furnace. At such a high temperature and low pressures ( 10 Torr), ZnO decomposes into Zn + and O. ... 

Systems involving an interface are often metastable, that is, essentially in equilibrium in some aspects although in principle evolving slowly to a final state of global equilibrium. The solid-vapor interface is a good example of this. We can have adsorption equilibrium and calculate various thermodynamic quantities for the adsorption process yet the particles of a solid are unstable toward a drift to the final equilibrium condition of a single, perfect crystal. Much of Chapters IX and XVII are thus thermodynamic in content. 

The Clausius-Clapeyron equation The Clapeyron equation can be used to derive an approximate equation that relates the vapor pressure of a liquid or solid to temperature. For the vaporization process... 

Liquids with high vapor pressures at ordinary temperatures are said to be volatile. Methanol (vapor pressure 98 Torr at 20°C) is highly volatile mercury (1.4 mTorr) is not. Solids also exert a vapor pressure, but their vapor pressures are usually much lower than those of liquids because the molecules arc gripped more tightly in a solid than they are in a liquid. Nevertheless, solids vaporize in the process called sublimation (Section 6.11), which we can observe in the presence of some pungent solids—such as menthol and mothballs. 

Ans. (a) The change of a liquid into a gas. (b) The change of a solid into a liquid, U) The enthalpy change accompanying a vaporization process, (d) The enthalpy change accompanying a melting (fusion) process. 

Sample. This source places no restrictions on target material. Clusters of metals, produced. For example, polyethylene and alumina have been studied as well as refractory metals like tungsten and niobium. Molecular solids, liquids, and solutions could also be used. However the complexity of the vaporization process and plasma chemistry makes for even more complex mixtures in the gas phase. To date the transition metals(1-3) and early members of group 13 (IIIA) and 14 (IVA)( 11-16) have been the most actively studied. 

Each of the properties of the PCB isomers, listed above (Sect. 3.1.2) and either measured or calculated using various equations presented in Sect. 2.1, plays a role in the environmental distribution of these contaminants, especially at air-solid and water-solid interfaces. From the physical and chemical properties specific for PCBs and their isomers (Table 7, Figs. 2-8), the following information evaluates routes by which PCBs are lost from a particular source, spill or environmental compartment, that includes air-solid or aqueous-solid phase interfaces. These include vaporization (i.e., solid— air process), sorption/desorp-tion and partitioning (i.e., water <- solid processes) and biodegradation (i.e., water <- biosolid interactions). 

The physical nature of the process stream. Is it single-phase or two-phase Is it liquid, solid, vapor or slurry What is its temperature and pressure at the sampling point, and how far can these be allowed to change during sampling What is its viscosity at the appropriate sample measurement temperature The chemical nature of the process stream. Is it at equilibrium (a final product) or is it to be measured mid-reaction Is sample transport possible, or must the sample be measured in situ Is it corrosive, and what material and metallurgical constraints exist ... 

Gas-liquid relationships, in the geochemical sense, should be considered liquid-solid-gas interactions in the subsurface. The subsurface gas phase is composed of a mixture of gases with various properties, usually found in the free pore spaces of the solid phase. Processes involved in the gas-liquid and gas-solid interface interactions are controlled by factors such as vapor pressure-volatilization, adsorption, solubility, pressure, and temperature. The solubility of a pure gas in a closed system containing water reaches an equilibrium concentration at a constant pressure and temperature. A gas-liquid equilibrium may be described by a partition coefficient, relative volatilization and Henry s law. 

The desorption and vapor extraction system (DAVES) uses a low-temperature fluidized bed to remove volatile and semivolatile organics such as polychlorinated biphenyls (PCBs), polynuclear aromatic compounds (PAHs), pentachlorophenol (PCP), volatile inorganics (tetraethyl lead), and some pesticides from soil, sludge, and sediment. The process generally treats waste containing less than 10% total organic contaminants and 30 to 95% solids. The process does not treat nonvolatile inorganic contaminants such as metals. 

Equation (10.18) was derived for capillary rise or depression assuming complete wetting, that is, 6 = 180°. In the case of contact angles greater than 0° and less than 180°, equation (10.18) must be modified. As liquid moves up the capillary during capillary rise the solid-vapor interface disappears and the solid-liquid interface appears. The work required for this process is... 

Several formal and informal intercomparisons of nitric acid measurement techniques have been carried out (43-46) these intercomparisons involve a multitude of techniques. The in situ measurement of this species has proven difficult because it very rapidly absorbs on any inlet surfaces and because it is involved in reversible solid-vapor equilibria with aerosol nitrate species. These equilibria can be disturbed by the sampling process these disturbances lead to negative or positive errors in the determination of the ambient vapor-phase concentration. The intercomparisons found differences of the order of a factor of 2 generally, and up to at least a factor of 5 at levels below 0.2 ppbv. These studies clearly indicate that the intercompared techniques do not allow the unequivocal determination of nitric acid in the atmosphere. A laser-photolysis, fragment-fluorescence method (47) and an active chemical ionization, mass spectrometric technique (48) were recently reported for this species. These approaches may provide more definite specificity for HN03. Challenges clearly remain in the measurement of this species. 

Thermally induced deactivation of catalysts is a particularly difficult problem in high-temperature catalytic reactions. Thermal deactivation may result from one or a combination of the following (i) loss of catalytic surface area due to crystallite growth of the catalytic phase, (ii) loss of support area due to support collapse, (iii) reactions/transformations of catalytic phases to noncatalytic phases, and/or (iv) loss of active material by vaporization or volatilization. The first two processes are typically referred to as "sintering." Sintering, solid-state reactions, and vaporization processes generally take place at high reaction temperatures (e.g. > 500°C), and their rates depend upon temperature, reaction atmosphere, and catalyst formulation. While one of these processes may dominate under specific conditions in specified catalyst systems, more often than not, they occur simultaneously and are coupled processes. 

In this book we are concerned only with mass transport, or diffusion, in solids. Self-diffusion refers to atoms diffusing among others of the same type (e.g., in pure metals). Interdiffusion is the diffusion of two dissimilar substances (a diffusion couple) into one another. Impurity diffusion refers to the transport of dilute solute atoms in a host solvent. In solids, diffusion is several orders of magnitude slower than in liquids or gases. Nonetheless, diffusional processes are important to study because they are basic to our understanding of how solid-liquid, solid-vapor, and solid-solid reactions proceed, as well as [solid-solid] phase transformations in single-phase materials. 

The Young equation contains the surface tension of the liquid yi - which can easily be measured, and the difference of the surface tensions of the solid-vapor ysv and the solid-liquid interface ysL That the surface tension enters the Young equation is not beyond doubt. Linford I6 inserted the generalized intensive surface parameter ys, arguing that at the three-phase contact line elastic deformations take place. In accordance with Rusanov [I7 we use the surface tension, because the spreading of a liquid on a surface is a process similar to immersion or adsorption. Immersion is usually considered to effect the surface tension since no extension or contraction of the surface occurs. 

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