Oxide fibers whiskers

Directed Oxidation of a Molten Metal. Directed oxidation of a molten metal or the Lanxide process (45,68,91) involves the reaction of a molten metal with a gaseous oxidant, eg, A1 with O2 in air, to form a porous three-dimensional oxide that grows outward from the metal/ceramic surface. The process proceeds via capillary action as the molten metal wicks into open pore channels in the oxide scale growth. Reinforced ceramic matrix composites can be formed by positioning inert filler materials, eg, fibers, whiskers, and/or particulates, in the path of the oxide scale growth. The resultant composite is comprised of both interconnected metal and ceramic. Typically 5—30 vol % metal remains after processing. The composite product maintains many of the desirable properties of a ceramic however, the presence of the metal serves to increase the fracture toughness of the composite. 

Other forms ofvery fine oxide fibers exist or are being developed. Single crystal whiskers which pose health risks and also present difficulties for handling, because of their very small dimensions, have not found wide use. Nano-oxide fibers are being developed which may be fine enough for liquid crystal technology to be used so as to handle them but as yet remain a possibility for the future. 

This book serves as an introduction to advanced inorganic fibers and aims to support fundamental research, assist applied scientists and designers in industry, and facilitate materials science instruction in universities and colleges. Its three main sections deal with fibers which are derived from the vapor phase such as single crystal silicon whiskers or carbon nanotubes, from the liquid phase such as advanced glass and single crystal oxide fibers, and from solid precursor fibers such as carbon and ceramic fibers. 

Ceramic matrix composites (CMCs) can be thought of as an improved form of carbon matrix composite in which the carbon matrix is replaced with ceramics that are stronger and much more resistant to oxidation. CMCs employ a variety of reinforcements including continuous fibers, discontinuous fibers, whiskers, and particles. Continuous fibers provide the best properties. There are many different types of CMCs, and they are at various stages of development. 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Fibrous materials may be naturally occurring or synthetically manufactured by thermal or chemical processes (Fig. 1) (see Fibers, survey). Refractory fibers are generally used in industrial appHcations at temperatures between 1000°C and 2800°C. These fibers may be oxides or nonoxides, vitreous or polycrystalline, and may be produced as whiskers, continuous filaments, or loose wool products. 

Surface treatments of carbon fibers can in general be classified into oxidative and non-oxidative treatments. Oxidative treatments are further divided into dry oxidation in the presence of gases, plasma etching and wet oxidation the last of which is carried out chemically or electrolytically. Deposition of more active forms of carbon, such as the highly effective whiskerization, plasma polymerization and grafting of polymers are among the non-oxidative treatments of carbon fiber surfaces. 

Most minerals occur in a variety of morphologies. Although it is not exhaustive, the list we recorded as occurring in fibrous form (Appendix 1) contains more than 350 entries, each with a reference. The format follows that proposed in Dana s System of Mineralogy, (Palache, et al., 1944), one of the standard references in the field. The names of fibrous minerals are alphabetically arranged within each chemical group that is, elements, oxides, hydroxides, carbonates, sulfates, phosphates, and so on. A similar, parallel system has been adopted for the list of synthetic fibers (Appendix 2). The list of synthetics includes glassy fibers produced from natural materials, as well as whiskers. 

Non-whisker SiC fibers are also of imporfance in MMCs, and they are currently available in two commercial forms Tyranno and Nicalon . As with the whisker form, the primary advantages of SiC fibers is their oxidation resistance and high-temperature mechanical property retention relative to other fibers. The high-temperature strength of three commercially available Nicalon SiC fibers is shown in Figure 5.111. 

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. 

Use Monomer for acrylic and modacrylic fibers and high-strength whiskers ABS and acrylonitrile styrene copolymers nitrile rubber cyanoethylation of cotton synthetic soil blocks (acrylonitrile polymerized in wood pulp) organic synthesis adiponitrile grain fumigant monomer for a semiconductive polymer that can be used like inorganic oxide catalysts in dehydrogenation of tert-butanol to isobutylene and water. 

Electrospinning processes are used to prepare nanocomposite fiber mats. In order to prepare nanocomposite fiber mats, mixtures containing the polymer solution and cellulose whiskers solution are placed in the appropriated electrospinning setup. Peresin et al. [128] have produced nanocomposite mats of poly (vinyl alcohol) (PVA) reinforced with cellulose nanocrystals using this electrospinning technique. Smooth nonwoven mats with homogeneous nanofibers were obtained. Park et al. [129] have also incorporated cellulose whiskers into nanofibers of polyethylene oxide (PEO) by the electrospiiming process. 

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