Vapor whisker

In all of these processes it is possible to increase the yield of whiskers by a dding metallic impurities, and the sublimation process requires such additions. The vapor—Hquid—sohd (VLS) growth mechanism is often thought to be involved. 

Nonpowder Synthesis. Many ceramic composites (qv) under iavestigation utilize reinforcing ceramic whiskers or fibers to achieve toughening (19). Whiskers (17,19,20) are produced by vapor-synthesis techniques. SiC whiskers can be produced by the rice hull or vapor—soHd (VS) method whereby rice hulls are pyrolyzed to produce a mixture of carbon, C, and Si02, and whiskers are produced by directional growth by vapor... 

Lackey, W., Hanigofsky, J., and Freeman, G., Experimental Whisker Growth and Thermodynamic Study of the Hafnium-Carbon System for Chemical Vapor Deposition Applications, 7] Amer. Ceram. Soc., 73(6) 1593-98 (1990)... 

Futamoto, M., Yuito, I, andKawabe, U., Hafnium Carbide and Nitride Whisker Growth by Chemical Vapor Deposition, /. Cryst. Growth, 61(l) 69-74 (Jan./Feb. 1983)... 

Other ceramic cutting-tool materials include alumina, Si-Al-0-N, alumina-carbide composites and, more recently, a composite of silicon nitride reinforced with silicon carbide whiskers. This last material can be produced by chemical-vapor infiltration (CVI) and has high strength and toughness as shown in Table 18.3.Cl... 

Direct metal deposition from metallic sources has been extensively used for model catalyst deposition for high-throughput and combinatorial studies. However, these methods are also increasingly used to deposit practical electrocatalyst materials. The best known approach is the one developed by 3M researchers have used physical vapor deposition to deposit Pt and Ft alloys onto nanostructured (NS) films composed of perylene red whiskers. The approach has been recently been reviewed by Debe. ... 

Fig. 2.23 The VLS mechanism of unidirectional growth of a fiber. Vapor condenses on a substrate and continues to localize at the site, providing the necessary material for whisker growth.

The compound is prepared by vapor phase reaction of gallium suboxide, Ga20 and phosphorus. It is produced in polycrystaUine form or as single crystals or whiskers in high purity grade for use in semiconducting devices. 

Almost defect-free GaSb whiskers are formed by the so-called VLS process vapor-liquid-solid) that was introduced by Ellis and Wagner [32]. Initially formed Ga droplets on the substrate, formed by pyrolysis of t-BusGa which is present in the gas phase to some extent due to the dissociation of the starting t-BusGa - SbRs adduct, catalyze the decomposition of t-BusGa and SbRs and... 

For example, spheres of narrow size distribution of polyl/Mm-butylstyrene) were obtained by exposing the corresponding monomer droplets to the trifluorometh-anesulfonic acid initiator vapor (Fig. 1.5.8). The polymer particles ranged in diameter from I to 3 p,m, and their uniformity was sensitive to the monomer-to-initiator mass ratio. Under certain conditions the normally smooth spheres appeared connected through polymer whiskers (Fig. 1.5.9) (67). Using styrene monomer and adjusting the boiler temperature, uniform polystyrene particles up 10 xm in diameter and spheres of 20 p,m of broader size distribution could be prepared (67). 

Whiskers have been produced in a range of fiber sizes and in three forms grown wool, loose fiber, and felted paper. The wool has a fiber diameter of 1-30 p,m and aspect ratios of 500-5000. The wool bulk density is about 0.03 g/cm, which is less than 1% solids. This is a very open structure, suitable for a vapor-deposition coating on the whiskers or direct use as an insulation sheet. Loose fibers are produced by processing the larger diameter fiber to yield lightly interlocked clusters of fibers with aspect ratios of 10-200. In felt or paper, the whiskers are randomly oriented in the plane of the felt and have fiber aspect ratios ranging from 250 to 2500. The felted paper has approximately 97% void volume and a density of 0.06-0.13 g/cm. ... 

Crystals formed under small driving force conditions (see Section 3.2) in a dilute ambient phase, such as the vapor phase or solution phase, will generally exhibit polyhedral forms, irrespective of their size. Even crystals of micrometer size, such as clay minerals, show polyhedral forms. However, there are crystals that show elongated needle forms that resemble whiskers, coils, hollow tubes, and even ice cream cones (see Figs. 2.2 (a), (b)) others exhibit tree-like polycrystalline aggregates of dendrites (see Fig. 2.2 (c)). 

When a solid particle of Au is placed on a Si substrate and Sil vapor is supplied in the heated system, whiskers of Si with Au droplets at the tips are formed. We know these are whiskers of Si, because only Si (S) grows in eutectic liquid droplets of Si-Au (L) formed by Au and Si supplied in the vapor phase (V). The Au-Si eutectic... 

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