Chemical vapor purification

High process temperatures generally not achievable by other means are possible when induction heating of a graphite susceptor is combined with the use of low conductivity high temperature insulation such as flake carbon interposed between the coil and the susceptor. Temperatures of 3000°C are routine for both batch or continuous production. Processes include purification, graphitization, chemical vapor deposition, or carbon vapor deposition to produce components for the aircraft and defense industry. Figure 7 illustrates a furnace suitable for the production of aerospace brake components in a batch operation. 

Purification of Silicon. Chemical purity plays an equally important role in the bulk of materials as on the surface. To approach the goal of absolute stmctural perfection and chemical purity, semiconductor Si is purified by the distillation of trichlorosilane [10025-78-2] SiHCl, followed by chemical vapor deposition (CVD) of hulk polycrystalline siUcon. 

This article presents a general discussion of actinide metallurgy, including advanced methods such as levitation melting and chemical vapor-phase reactions. A section on purification of actinide metals by a variety of techniques is included. Finally, an element-by-element discussion is given of the most satisfactory metallurgical preparation for each individual element actinium (included for completeness even though not an actinide element), thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, and einsteinium. 

Matlhoko L, Pillai SK, Moodley M et al (2009) A comparison of purification procedures for multi-walled carbon nanotubes produced by chemical vapor deposition. J Nanosci Nanotechnol 9 5431-5435... 

Unfortunately, current S3mthesis techniques, such as chemical vapor deposition, arc discharge, laser ablation, or detonation, typically lead to a mixture of various nanostructures, amorphous carbon, and catalyst particles rather than a particular nanostructure with defined properties, thus limiting the number of potential applications [1]. Even if pure materials were available, the size-dependence of most nanomaterial properties requires a high structural selectivity. In order to fully exploit the great potential of carbon nanostmctures, one needs to provide purification procedures that allow for a selective separation of carbon nanostructures, and methods which enable size control and surface functionalization. 

Besides chemical conversion and chemical vapor transport, the reduction process is a purification step, too. Trace impurities, always present in the oxide, may evaporate. On the other hand, foreign phases can be incorporated during the CVT growth of tungsten, finally leading to inclusions in the tungsten powder particles. This is of special interest in the production of non-sag tungsten wire used for incandescent lamps. 

Chemical vapor-phase transport is one of the earliest methods developed for the synthesis of new materials [19-26]. The method can also be used for the growth of high-quality single crystals or for the purification of solid-state compounds. The method has been used to synthesize a wide range of LMCs. 

Both types of carbon nanotubes are synthesized by three methods involving gas phase processing, namely, arc-discharge, laser ablation, and chemical vapor deposition (CVD) [58]. Subsequent purification procedures are required to remove impurities, such as catalyst particles, amorphous carbon, and nontubular fuUerenes, from the nanotubes [59]. 

---