Whiskers metal catalyzed

Silicon carbide whiskers are generally manufaclured by a metal-catalyzed process, in which carbon and silicon are condensed from the gas phase. Another jtroccss utilizes rice husks decomposed at 500°C. At temperatures of 1800°C the carbon/Si02-mixture formed is converted into a-SiC. Potassium titanate whiskers are synthesized from potassium molybdate, potassium carbonate and litanium dioxide at temperatures above 1200°C. 

The vaporization-condensation of B4C at 2223 K in vacuum allows the growth of whiskers, metallic impurities catalyzing the growth " . 

Metal catalyzed chemical vapor deposition has become the most versatile and therefore most important whisker growth process. This process and other vapor phase processes facilitate the formation of uniform and reproducible products for demanding applications, where they offer new premium electrical, magnetic, dielectric and near theoretical mechanical properties [1-2]. 

Metal particle catalyzed chemical vapor deposition is the most versatile VLS process (Table II) yielding a wide range of single crystal whiskers and nanowhiskers [1-2] [5-6], short amorphous or polycrystalline fibers [1] [7], and nanotubes [8]. Laser ablation of selected metal alloys is a recent VLS process used for the synthesis of semiconductor nanowire [74]. Metal particle catalyzed carbothermal reduction, another VLS process, yields single crystal whiskers [9-10]. Metal catalyzed arc discharge [11], metal particle catalyzed laser ablation [12], and metal particle catalyzed plasma arc discharge [13] yield nanotubes by a VLS mechanism. 

Metal catalyzed caibo-thermal reduction Liquid metal and/or alloy droplets 0 SiC whiskers from SiOj and C o SiC whiskeis/powder from SiN... 

Except for initially producing a sheath/core SiOz/Si nanowhisker, the laser ablation process parallels the metal catalyzed chemical vapor deposition process (Chapter 2.2.3). In this process, the Si that is desired is generated by chemical vapor deposition and dissolved in molten metal droplets, e.g., Au or Fe. The molten alloy droplets, e.g., SIAu, which result in this process sequence, give rise to the grovrth of single crystal Si micro-whiskers by a similar overall VLS phase transformation. 

Silica gel, carbon furnace black and cobalt chloride yield silicon carbide whiskers, or Tokawhiskers [30], in a metal catalyzed process at >1450°C. A process variant [9] yields SiC whiskers >1350 C in a fixed bed percolated by a hydrogen flow. The addition of iron above 1450°C affords submicron whiskers ending with a silicon rich droplet. The iron seems to evaporate and condense below 1450°C leaving behind whiskers with silicon rich tip >1450 C. These processes use the same starting materials as the rice hull processes but they also use a metal particle catalyst. As a result, they are believed to proceed by a VLS phase transformation. 

Until 1995, micrometer scale whiskers of the lll-IV semiconductors have been grown only by metal-catalyzed or metal-organic CVD processes at relatively high temperatures, e.g., SiSOO C. Lack of useful crystallization mechanisms for highly covalent non-molecular solids has so far prevented their growth at lower process temperatures. A recent breakthrough [18]... 

VLS siiicon carbide whiskers with diameters ranging from <3 to 11 jm, and lengths ranging from 5 jm to <10 cm are readily obtained by metal catalyzed chemical vapor deposition. This process facilitates an exacting control over whisker shape and dimension in a batch or a continuous process. The diameter of the catalyst determines that of the whisker. Less well-defined whiskers have been obtained by vapor deposition, chemical mixing and carbothermal processes with diameters ranging from <3 pim to >30 nm. 

Short (or discontinuous) fibers are best prepared in a batch process, e.g., in a small cylindrical reaction chamber. The value of the technology, however, lies in its capability to facilitate the growth of continuous (potentially endless) fibers with a recently discovered automatic self-regulating growth mechanism [2], Finally, the diameter of the laser focus determines the diameter of fibers grown by laser assisted chemical vapor deposition, just as the diameter of the metal particles determines the diameter of the whiskers grown by metal catalyzed chemical vapor deposition. 

Mass transfer in metal catalyzed and in laser assisted CVD processes is driven by highly localized temperature gradients. The relatively small area of either a hot molten metal particle or of a hot laser focus affords whiskers [4] or continuous fibers, respectively [2] [18-19]. The transfer of an equal mass from the vapor to the solid phase in a conventional chemical vapor deposition results in a thin coating over the relatively large area of a hot surface, i.e., that of a flat complex shaped composites part. 

The structure property relationships observed in the Si-N system [17], are reminiscent of those observed for silicon nitride whiskers grown by metal catalyzed chemical vapor deposition (Chapter 2.2). The difficulty in obtaining binary (i.e., boron or silicon nitride) fibers with exactly required stoichiometry seems to have so far precluded the production of single crystal fibers by this route. 

The supersaturated metal droplet that is located on the tip of the growing whisker or short fiber sustains the directional growth of the localized solid phase. This continues to catalyze the reaction as long as an equilibrium is maintained between the diffusion of gaseous atoms or molecules from the vapor phase into the liquid phase and their subsequent extrusion as a solid whisker or short fiber. Growth stops if the catalyst is exhausted or sufficiently contaminated by impurities imported from the vapor phase. 

Metal particle catalyzed chemical vapor deposition Liquid metal and/or liquid metal alloy (Au-Si, Fe-FejC, other) droplets catalyze whisker growth o Si, SiC and other whiskers o Short SiC and SijN fibers o SiC nanowhiskers and batts o Short graphite and carbon fibers o Short carbon TiOj microcoils o Carbon nanotubes (Ugh yields)... 

Self catalyzed carbo-theimal reduction Spontaneous whisker formation o SiC whiskers w/o metal caps o SiC nanowhiskers and batts... 

A film is deposited in a conventional chemical vapor deposition (CVD) process when the gaseous reactants are presented with a large hot support surface. Supported growth of whiskers occurs also when the gaseous reactants are presented with discrete hot metal catalyst particles located on the surface of a suitable substrate. Unsupported whisker growth occurs when hot metal catalyst particles are freely interspersed with the gaseous reactants in the vapor phase. The most common mechanism for whisker growth is a vapor-liquid-solid transformation, and the most versatile VLS process is a metal particle catalyzed chemical vapor deposition. 

Whisker growth during pyrolytic-deposition process is generally considered as being catalyzed by metals." However, Haanstra et al. observed noncatalytic columnar growth of carbon on j8-SiC... 

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