Closed-space vapor transport

A PVD-type reactor can be one in which molecules reach the surface directly in a molecular beam from some source or sources in which raw materials are vaporized. At the pressures commonly used (<10 6 Pa), the vaporized material encounters few intermolecular collisions while traveling to the substrate. Historically, higher pressure processes, such as sputtering and close-spaced vapor transport, have been classified as PVD (I). These processes also use physical means to generate the gas-phase species. However, the transport phenomena that need to be modeled for such higher pressure processes are more similar to CVD than PVD because of the diffusive-convective nature of transfer from the gas phase to the substrate. 

Given the fact that the free enthalpy of the reactions ZnO + H2 Zn + H2O and ZnO + H2O o Zn + H2 + O2 are positive and that such reactions cannot consequently occur towards the right-hand side, we have tried to investigate the actual mechanisms of the CVT transport with hydrogen and other species both from close-spaced vapor transport (CSVT) experiments and from a numerical modeling. 

Other techniques include closed space vapor transport, sol-gel and other solution technologies, such as self-aligning molecular epitaxy. Closed space vapor transport relies on localized heating of sources, generally elemental, to evaporate them, and a cool zone at which a film or bulk crystal is deposited. A variation of this technique has been used to grow II-VI crystals on the space shuttle. This technique has difficulty controlling uniformity, dopant levels, and multiple layer structures among other issues. 

In both cases a vapor containing the initiaUy sohd/liquid element is produced, usually upstream from the substrate in the CVD reactor itself These reactions can run in reverse to redeposit the vaporized species. In some cases it is possible to simply evaporate a high vapor pressure compormd such as CdTe in the CVD reactor, although this yields an evaporation-like process more than a CVD process. This case is referred to as closed-space vapor transport and is generally distinct from evaporation as described in Chapter 11 because the substrate and source are in equilibrium with a relatively dense vapor of the source material in a closed environment. The case above for Ga requires an etchant because Ga has a naturally low vapor pressure. In the case of liquids with high vapor pressures a different method can be used. 

See, for example, Bailly, F. Cohen-Solal, G. and Mimila-Arroyo, J., Simplified theory of reactive closed-spaced vapor transport. y. Efectroc/jem. 5oc., 1979 126 1604-1608. 

The basic structure of polycrystalline cadmium telluride (CdTe) thin-film cells has a glass superstrate and a layer of TCO as front contact, a near-transparent n-type cadmium sulfide (CdS) window layer, p-type CdTe, and a metallic rear contact. The CdTe is usually deposited by three families of techniques. In the first group (vapor transport deposition, close space sublimation, physical vapor deposition, and sputtering) elemental vapors of Cd and Te condense and react on the substrate. In the second (electrodeposition), Cd + and HTe02" ions in acidic electrolyte are galvanically reduced at the surface ... 

The U. S. Department of Transportation has announced (9) that future labeling regulations for flammable liquids may be based on their TCC flash points. If this change is made, the empirical calculation technique proposed here is expected to be equally applicable to predicting TCC data if the closed cup flash points of the individual solvents are substituted in basic calculation data. No attempts have yet been made to predict closed cup data, but one might expect that the vapor space in the closed cup would be more easily simulated than that in the open cup tester. 

Vapor phase inhibitors (VPIs), also called volatile corrosion inhibitors (VCIs), are compounds that are transported in a closed system to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds such as morpholine or octadecylamine are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide (Boles et al. 2009). Compounds of this type inhibit corrosion by making the environment alkaline. In closed vapor spaces, such as shipping containers, volatile solids such as the nitrite, carbonate, and benzoate salts of dicyclohexylamine, cyclohexylamine, and hexamethyleneimine are used. 

Cylinders containing liquefied fuel gas should be stored or transported in a position so that the safety relief device is always in direct contact with the vapor space in the cylinder. All valves must be closed off before a cylinder is moved, when the cylinder is empty, and at the completion of each job. Low-pressure fuel-gas cylinders should be checked periodically for corrosion, general distortion, cracks, or any other defect that might indicate a weakness or render it unfit for service. 

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