Powder preparation from vapor phase

Powder Preparation. There are several routes to preparing SiC powders having variable purity levels, crystal stmcture, particle size, shape, and distribution. Methods that have been examined include growth by sublimation from the vapor phase, carbothermic reduction, and crystallization from a melt. 

A laboratory preparation of pure Li uses a finely powdered mixture of Li,0 and a noble metal (Pt, Pd or Ir), which is heated > 1100°C in a stream of Hj. The pure intermetallic phase so produced is then heated under vacuum and the Li can be isolated by condensation from the vapor phase. ... 

By this method, which had formerly been employed only for the alkali metals and iron, the magnesium derivative, Mg(C5H6)2, has recently been prepared in the vapor phase. Cyclopentadiene vapor was passed over magnesium powder at 500-600° and under suitable conditions crystals of mp 176° were obtained (4). Thallium and indium reacted similarly with cyclopentadiene vapor at 350°, giving the compounds TlCsHs and InCgH (55). Attempts to develop a more general method of preparation from cyclopentadiene and molten metals met with no success 121). 

Three times crystallized, dialyzed, and lyophilized hen egg white lysozyme powder from Sigma was used after a subsequent dialysis and lyophilization. Hydration of the dry powder was accomplished through the vapor phase by exposing the sample to a constant relative humidity for 5 days. In preparing the lysozyme-D20 sample, the lysozyme was twice dissolved in D2O and held at 40 deg C for 24 h both times before lyophilization. 

Some vapor-phase materials are prepared from hard starting materials, For nutshells, the procedure used is to calcine Che shells, gmauiate to size, and activate. In the case of ccel or charconl, the material is ground to powder, bonded with pitch and formed, calcined a( 500-700°C. and activated wirh steam or flue gas at 500-950°C. The high-temperature furnaces employed include kilns, multiple-hearth, and vertical retort. 

The topic of this chapter is how to produce particles of a particular shape, chemistry, and size and then how to characterize them. We are going to describe the methods used to produce ceramic powders, from the traditional ball-milling technique to more recent vapor-phase approaches that can produce nanometer-sized particles. It is worth remembering that powder processing is used to produce some special metals (e.g., tungsten filaments for incandescent lamps), it is used in the pharmaceutical industry, for making catalysts, and it is used to prepare many food ingredients. 

Concerning the preparation techniques, there are different approaches from vapor or liquid phase. The critical aspects, that have to be taken into account, are the reliability of the preparation process, the quality of the nanostructures prepared and the integration into final devices. Among the most promising techniques there are catalyst assisted vapor phase transport and thermal oxidation. Vapor phase technique consists in the evaporation of the oxide powder in a furnace with controlled atmosphere. In general the pressure is lower than lOOmbar to ease the vaporization of the oxide powder and an inert gas carrier is used to facilitate the mass transport from the source to the substrates, where the vapors condense in form of nanowires. 

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