Chemicals transportation

Numerous teclmiques have been developed for depositing films from vapours, ranging from straightforward evaporation to advanced chemical transport in which reactions are activated by heat, light or plasma. These have been surveyed in two comprehensive reviews [8, 9] and two popular interdisciplinary textbooks [K), H]. The tliree most widely used chemically based teclmiques are ... 

Chemical Transportation Emergency Center, a pubhc service of the Chemical Manufacturers Association, 2501 M Street, N.W., Washington, D.C. 20037-1303. 

In most cases, CVD reactions are activated thermally, but in some cases, notably in exothermic chemical transport reactions, the substrate temperature is held below that of the feed material to obtain deposition. Other means of activation are available (7), eg, deposition at lower substrate temperatures is obtained by electric-discharge plasma activation. In some cases, unique materials are produced by plasma-assisted CVD (PACVD), such as amorphous siHcon from silane where 10—35 mol % hydrogen remains bonded in the soHd deposit. Except for the problem of large amounts of energy consumption in its formation, this material is of interest for thin-film solar cells. Passivating films of Si02 or Si02 Si N deposited by PACVD are of interest in the semiconductor industry (see Semiconductors). 

CCPS G-28. Guidelines for Chemical Transportation Risk Analysis. American Institute of Chemical Engineers, Center for Chemical Process Safety, New York. 

H. Schafer. Chemical Transport Reactions. Academic Piess, New York (1964). 

Formerly known as Chemical Manufacturers Association (CMA). A trade association of chemical manufacturers, representing more than 90 percent of the production for basic industrial chemicals in the US. Administers research in areas significant to chemical manufacturing such as air and water pollution control operates Chemical Transportation Emergency Center (CHEMTREC) to control and report chemical accidents. ACC is organized by industrial groups and also by issue groups that sen>e as coordinators and advocators in their fields of specializations. 

Chemical Transportation Emergency Center American Chemistry Council (ACC)... 

Chemical Specialties Manufacturers Association (CSMA), See Consumer Specialty Products Association, 270 Chemical Stnrcture Association (CSA), 270 Chemical Transportation Emergency Center (CHEMTREC), 270 Chemicals hic., 223 Chemhidustry.com, 308 Cheminova A/S, 154... 

The SLIM technique is described in detail in Embrey et al. (1984) and Kirwan (1990). The technique was originally developed with the support of the U.S. Nuclear Regulatory Commission but, as with TFIERP, it has subsequently been used in the chemical, transport, and other industries. The technique is intended to be applied to tasks at any level of detail. Thus, in terms of the HTA in Figure 5.6, errors could be quantified at the level of whole tasks, subtasks, task steps of even individual errors associated with task steps. This flexibility makes it particularly useful in the context of task analysis methods such as FITA. 

These iiicchanisnis can affect the near-term and ultimate fate of a chemical hazard. Recognition of these inechanisms can significantly assist in the identification of a chemical agent as a health hazard. In recent years, the understanding of chemical transport, chemical manipulation in the body, and response by animals luid humans to cheniicals has advtmccd to a point where it is possible to determine whether a chemical is indeed a health hazard. 

In this step, the assessor qiuuitifies tlie magnitude, frequency and duration of exposure for each patliway identified in Step 2. Tliis step is most often conducted in two stages estimation of exposure concentrations and calculation of intakes. The later estimation is considered in Step 4. In tliis part of step 3. the exposure assessor determines the concentration of chemicals tliat will be contacted over the exposure period. E.xposure concentrations are estimated using monitoring data and/or chemical transport and environmental fate models. Modeling may be used to estimate future chemical concentrations in media tliat are currently contaminated or tliat may become contaminated, and current concentrations in media and/or at locations for which tliere are no monitoring data. The bulk of the material in tliis chapter is concerned witli tliis step. 

Much of the difficulty in demonstrating the mechanism of breakaway in a particular case arises from the thinness of the reaction zone and its location at the metal-oxide interface. Workers must consider (a) whether the oxide is cracked or merely recrystallised (b) whether the oxide now results from direct molecular reaction, or whether a barrier layer remains (c) whether the inception of a side reaction (e.g. 2CO - COj + C)" caused failure or (d) whether a new transport process, chemical transport or volatilisation, has become possible. In developing these mechanisms both arguments and experimental technique require considerable sophistication. As a few examples one may cite the use of density and specific surface-area measurements as routine of porosimetry by a variety of methods of optical microscopy, electron microscopy and X-ray diffraction at reaction temperature of tracer, electric field and stress measurements. Excellent metallographic sectioning is taken for granted in this field of research. 

A CVD reaction can occur in one of two basic systems the closed reactor or the open reactor (also known as close or open tube). The closed-reactor system, also known as chemical transport, was the first typetobeusedforthe purification of metals. It is a hybrid process which combines vapor-phase transfer with solid-state diffusion. As the name implies, the chemicals are loaded in a container which is then tightly closed. A temperature differential is then applied which provides the driving force for the reaction. 

A "chemical transport reaction may be defined as the transference of a condensed phase through a gaseous phase by means of a chemical reaction in which gases B are involved. 

This definition of the process shows that it differs essentially from sublimation and distillation. A chemical-transport reaction is necessarily reversible a concentration gradient is induced, e.g., by means of a tem-... 

Generally, it is not important in which form the components are introduced into the transport tube, because the transport reaction transfers the whole system into a reversible state. In the preparation of NbS2Cl2, for example, Nb and S2C12, as well as Nb, S, and NbCls, have been used as starting materials (339). The principles of chemical transport reactions are treated in refs. 241, 336, 337, 338). 

Fig. 21. Reaction scheme of crystal growth by chemical transport A, transport furnace having two consecutive heating coils.

Preliminary X-ray work on CrSBr crystals, probably formed via a chemical-transport reaction, shows rhombic symmetry (181). Powder patterns of the iodides can be indexed with respect to a hexagonal lattice, and are similar to those of Cris (17, 181). 

BiTel has been grown by chemical transport from BizTea and Bils, using Br2 as the transport agent (218). 

Explosives, See also Explosive chemicals Transportation carriage, 447, 460, 594 compatibility groups, 456 hazard classification, 447, 455 Exposure limits. See Hygiene standards Eye protection, 435... 

Radioactive chemicals, See also Chemicals Transportation control measures, See Precautions exposure limits, 393 hazards, 391 monitoring, 393 types, 391... 

It was argued by Saito et al. [509] that a hydrogen plasma treatment as used in the LBL technique causes chemical transport the a-Si H film deposited on the cathode prior to the hydrogen treatment is etched and transferred to the anode. Hence, in the hydrogen plasma silane-related molecules and radicals are present. In fact, material is deposited on the anode under the same conditions as in the case of high dilution of silane with hydrogen. 

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