Silicon carbide SiC whiskers

Finally, it can he added that very thin nickel films are used as seed layers for the growth of fullerenes and of silicon carbide SiC whiskers, and that C/Ni multilayers are used as x-ray mirrors. ... 

Among existing synthetic whiskers, silicon carbide (SiC) whiskers have the highest hardness, modulus, tensile strength, and heat resistance temperature and are divided into a-SiC and... 

Silicon carbide (SiC) whiskers are extremely anisotropic, short fibrous crystals and grow from SiC particles along the [111] plane through catalysis. At present, SiC whiskers are usually prepared by vapor reaction and solid methods, and the latter is more economical and more suitable for industrial production. Figure 3.1 shows the microstructures of SiC whiskers. Table 3.1 shows the chemical and physical properties of SiC whiskers. 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

Silicon carbide whiskers typically have diameters of a few micrometers and lengths up to 5 cm. They may be composed of either P-SiC or CC-SiC, the latter in one or more polytypes, and occur mosdy as hair- or ribbonlike crystals. Despite many attempts to produce SiC whiskers on a large scale at low cost, they have never acquired a wide importance. SiC whiskers have been reviewed (111—120). 

Silicon carbide has been described as a mild inhalation irritant (143). The threshold limit value for silicon carbide in the atmosphere is 5 mg/m3. Because of increased interest in SiC whiskers as a reinforcement for composites, the ASTM has established Subcommittee E34.70 on Single-Crystal Ceramic Whiskers to write standards for handling this form of SiC (144). 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

Results on other composite materials are similar to those obtained by Morrell and Ashbee.56 Creep asymmetry has been demonstrated for two grades of siliconized silicon carbide,35,60,61 SiC whisker-reinforced silicon nitride,53 HIPed silicon nitride,29 and vitreous-bonded aluminum oxide.29 Again, stresses required to achieve the same creep rate were at least a factor of two greater in compression than in tension. In two grades of siliconized silicon carbide,35,58-61 the stress exponent changed from 4 at creep rates below... 

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 bulk analysis of /3-SiC whiskers shows the least variation in chemistry. In some whiskers, the residual metals content can vary, most likely, as a result of additives that used as catalysts during synthesis. These include iron, cobalt, and chromium. Studies by Karasek et al. [56] have shown that the physical properties of silicon carbide whisker-reinforced composites do not correlate to the bulk properties of the whiskers significantly. This lack of significant correlation is mainly due to the fact that the important phase chemistry of the whisker-matrix interface is controlled by the matrix chemistry and the surface chemistry of the whiskers. There seems to be little impact of the diffusion of materials into or out of the bulk whisker material. 

It has already been well known for more than two decades that it is feasible to produce SiC powders and/or whiskers by pyrolyzing rice husk [178] under an inert atmosphere, since a considerable content of colloidal amorphous silica is incorporated in husks and straws of this vegetable. It means that nature by itself is supplying a similar intimate mixture of carbon-containing molecules (cellulose) and colloidal silica like those artificially made mixtures we discussed in the last section. Thus, also in this case, silicon carbide may be formed by a carbothermal reduction process. This process starts in the temperature range 1100-1400 °C ... 

Conclusive experimental studies on the effects of respirable fibrous SiC dusts (whiskers) are not yet available [255]. In the United States ASTM has published recommendations for safe handling procedures related to all ceramic fibers, including silicon carbide whiskers [256]. 

Rice hull hydrocarbons supply the carbon source and rice hull ash the silicon source. This is an early carbothermal process (Equ. 10a). At <900°C, coking (Equ. 10b and 10c) removes water and organics and yields a mixture of silica and carbon, and at <1700°C, pyrolysis yields silicon carbide whiskers (Equation lOd). The overall yield is 1 percent. Three variants of the rice hull process are known. Compacted rice hulls can be pyrolized at 1600°C without coking [14], thus directly yielding a mixture of graphite and SiC whiskers, 10-200 pm in length and... 

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. 

The highest modulus of a given substrate is obtained with a single crystal structure. Single crystal CVD-SiC whiskers (578 GPa) have a stiffen more highly ordered, structure than polycrystalline CVD-SiC fibers (190-400 GPa), and sapphire whiskers and fibers (415 GPa) are stiffer than slurry spun polycrystalline alumina fibers such as Fiber FP (380 GPa). Superimposed upon this relationship is a compositional factor. Fiber modulus and structural order generally also decrease with increasing compositional complexity, e.g., silicon carbide is intrinsically stiffer than silicon oxycarbide such as Nicalon, and slurry spun alumina fibers are stiffer than sol-gel or melt spun aluminate fibers. 

Silicon carbide whiskers n. These high-modulus SiC fibers are made by pyrolysis of organosilanes in hydrogen at 1500-2000° C... 

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