Vapor transport

Chemical Vapor Deposition. In chemical vapor deposition (CVD), often referred to as vapor transport, the desired constituent(s) to be deposited are ia the form of a compound existing as a vapor at an appropriate temperature. This vapor decomposes with or without a reducing or oxidizing agent at the substrate— vapor interface for film growth. CVD has been used successfully for preparing garnet and ortho ferrite films (24,25). Laser-assisted CVD is also practiced. 

In pack cementation, the part to be coated is placed in a retort and surrounded with a powdered pack consisting of the coating component and an activator the latter reacts with the coating component to form the carrier vapor, usually a haHde or an inert diluent, to prevent the pack from sintering together and to permit vapor transport of the alloying component through the pack. 

Mercuric iodide crystals grown by physical vapor transport on Spacelab 3 exhibited sharp, weU-formed facets indicating good internal order (19). This was confirmed by y-ray rocking curves which were approximately one-third the width of the ground control sample. Both electron and hole mobiUty were significantly enhanced in the flight crystal. The experiment was repeated on IML-1 with similar results (20). 

H. Wiedemeier, Vapor Transport ofHgCdTe in Microgravity, NASA Conference PubHcation 3272, Marshall Space Flight Center, Alabama, May 1994, p. 263. 

L oss of Catalyst by Vapor Transport. The direct volatilisation of catalytic metals is generally not a factor in catalytic processes, but catalytic metal can be lost through formation of metal carbonyl oxides, sulfides, and hahdes in environments containing CO, NO, O2 and H2S, and halogens (24). 

Hanna, L. M. (1983). Modeling of heat and water vapor transport in the human respiratory tract. Ph.D. Dissertation, University of Pennsylvania, Philadelphia. 

The hydrogen reduction of the metal halides, described in Sec. 1.2, is generally the favored reaction for metal deposition but is not suitable for the platinum-group metals since the volatilization and decomposition temperatures of their halides are too close to provide efficient vapor transport. 1 1 For that reason, the decomposition of the carbonyl halide is preferred. The exception is palladium which is much more readily deposited by hydrogen reduction than by the carbonyl-halide decomposition. 

ZnSe is deposited by the reaction of hydrogen selenide with zinc vapor transported in argon, at a deposition temperature of700-750°C and at a pressure <100 Torr 1 0... 

Growth of single crystals. Crystals of the aluminum selenide halides (needles, maximum length 15 mm) were grown by vapor transport in sealed ampoules between two temperatures (380 and 320°C for Al-SeCl, and 350 and 300°C for AlSeBr and AlSel) over a period of two months. A large excess of the halogenide was used (266). 

The gas-phase methods usually applied to the crystal growth of borides are two chemical vapor deposition (CVD) and chemical vapor transport (CVT). 

Crystal Growth of Borides by Chemical Vapor Transport. 

The main problem in the system design is the heat and vapor transport in and out of the adsorbent. Advanced heat exchanger technologies have to be implemented in order to keep up the high energy density in the storage, which would be reduced by the amount of inactive heat exchanger material. 

Shibata, T. Muranushi, Y. Miura, T. Kishi, T. 1991. Electrical characterization of 2H-SnS2 single crystals synthesized by the low temperature chemical vapor transport method. J. Phys. Chem. Solids 52 551-553. 

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