Whiskers ceramic

Whiskers are normally obtained by vapor phase growth. Early in the 1970s, a new process was developed, starting from rice hulls, to produce SiC particles and 

Refractory or ceramic fibers have low thermal conductivity, low heat capacity and high chemical resistance. These characteristics make them attractive as 

F nre 6.32 Schematic of the lice hull process of makmg silicon carbide whiskers. 

Use of ceramic fiber burners with natural gas can result in reduced emissions of nitrogen oxide. Ceramic fiber burners can lead to a high radiative heat transfer and thus a lower flue gas temperature, which in turn results in lower nitrogen monoxide emission. The nitrogen oxide formation depends on the flue gas temperature in a more or less exponential manner. Thus, a reduced flue gas temperature results in a reduced NO, production. Ceramic fiber burners can thus lead to less NO, than conventional burners. 

High stiffness ceramic fibers such as alumina, alumina-silica, silicon carbide, boron, etc. are used as reinforcement fibers for polymeric, metallic, and ceramic matrix composites (Chawla, 1987). Silicon carbide whisker reinforced alumina composites are used as high speed cutting tools (Chawla, 1993). 

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

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... 

In the case of a metallic binder phase (e.g. Co, Ni, Fe, and its alloys), ceramic whiskers can also be added to the c-BN [265]. The process has been described in detail for Si3N4 whiskers [266]. 

The ribbon-like fibrous polymers formed from 233b exhibit properties similar to those of metallic and ceramic whiskers . The polymers are semiconductors and exhibit photoconductivity. SCF calculations indicate that polydiacetylenes have a nearly free-elcctron-like valence band and are best described as wide band-gap semiconductors . 

Toughening of the ceramic matrix (phase A) by ceramic whiskers (phase B) formed in a separate process ... 

The toughening of a ceramic by the addition of whiskers that are different from the matrix material is the most traditional and the best explored methodology. The advantage of this method is the broad spectrum of ceramics that can be prepared in whisker form. Consequently, there is the potential for a broad range of ceramic matrix-ceramic whisker combinations. Since the whiskers are manufactured in a separate process, it is possible to control their size, morphology, and surface chemistry without affecting the matrix material. Additionally, whiskers can be chemically or thermally treated to change their future interactions with the matrix. 

Despite the fact that many different ceramic whiskers have been produced in research laboratories [93] only SiC and Si3N4 have gained real commercial significance so far. Among whisker-reinforced ceramics the best results were obtained with Al203-SiC whiskers and Si3N4-SiC whiskers. 

In theory, the combination of ceramic powders (A) with components (BuB2,B3) that form ceramic whiskers (B) at elevated temperatures should make it possible to produce ceramics with in situ whiskers. After the whisker formation is completed, the greenware is densified at a higher temperature. The disadvantages of this approach are potential reactions between A, Bu B2,..B the difficulties in removing the metal catalysts required to grow whiskers and the space constraints that limit the ability of the whiskers to form high aspect ratio crystals. 

Adding ceramic whiskers in volume fractions above the percolation threshold has been found to improve creep resistance, often increasing the creep resistance by two orders of magnitude. One would expect a similar effect with fibers but, in some cases, the fibers have such a small grain size (for high strength) that they can show very poor creep resistance. Other important factors that can affect the creep rate of a material are composition, stoichiometry, defect density and environment, often through their dependence on diffusivity. 

The VLS mechanism has been identified in the growth of other ceramic whiskers including AI2O3, BeO, and B4C. It has been proposed as a method for producing nanotubes, and is often assumed to be the principal feature of such growth processes. 

Information Bulletin, Production of ceramic whiskers, Battelle, Geneva Research Center (1987). 

So-called ceramic whiskers received much attention in the 1950s and 1960s for their outstanding mechanical properties. They are generally short, fiber-shaped. 

Developing appropriate ceramic—matrix composites [henceforth CMCs] for possible aero-engine applications started more than a decade ago. CMCs continue to be developed for improved toughness, to overcome the inherently brittle nature of most of the monolithic ceramics. Many experiments have shown that long-term loading of CMCs (for thousands of hours) produce improved high-temperature properties in monolithic ceramics by causing the dispersion of ceramic whiskers or... 

Silicon-carbide whiskers comprise die bulk of all ceramic whiskers. Other whisker materials are produced on an experimental basis and include ... 

There are organic and inorganic whiskers. Organic whiskers include fibrin whiskers, polyCbutyl acrylate-phenylethylene) whiskers, and so forth, and are usually used in polymers. Inorganic whiskers include metal whiskers and nonmetal whiskers. Metal whiskers are mainly applied in metal matrix composite material. Ceramic whiskers, a kind of nonmetal... 

To specify powders for particular applications and products we need to be able to determine their physical and chemical characteristics, often with a high degree of accuracy and with statistical significance. In this chapter we will describe the different analytical teclmiques used for particle characterization and also indicate which technique works best. In addition to powders there are other important dimensionally constrained forms of ceramics. Whiskers and fibers are long in one dimension but restricted in the other two. Ceramics in these forms are important reinforcement phases in composites, such as... 

Ceramic whiskers have been produced for many years, as reported by Gordon (1978), though their use in epoxies seems to be limited. However, aluminum borate whiskers have been combined with thermoset polymer matrices by Liang and coworkers (Liang and Hu 2004 Tang et al. 2007). Ceramic nanotubes have been produced by coating carbon nanotubes with silicon carbide (Morisada et al. 2007). Silicon carbide nanofibers, which can be about 40 nm in diameter with lengths of up to several hundred microns, have also been produced (Zhu et al. 2002 Bechelany et al. 2007). 

All brittle materials contain a population of small cracks and flaws that have a variety of sizes, geometries, and orientations. When the magnitude of a tensile stress at the tip of one of these flaws exceeds the value of this critical stress, a crack forms and then propagates, which results in fracture. Very small and virtually defect-free metallic and ceramic whiskers have been grown with fracture strengths that approach their theoretical values. 

Man-made fibers ean be amorphous, polycrystalline, or ciystalline. The amorphous fibers inelude the man-made vitreous fibers typified by the insulation wools, which form the bulk of man-made inorganic fibers. Polycrystalline fibers include continuous carbon fibers used in composite material and specialty fibers such as Saffil. Crystalline man-made fibers include ceramic whiskers, such as silicon carbide and silicon nitride, which are used in reinforce metals and other composite materials. 

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