Aluminum oxide whiskers

Figure 21.6 TEM images of different nanomaterials with a variety of sizes, shapes, and particle interactions. The upper images show high aspect ratio nanomaterials, including aluminum oxide whiskers (a) and iron oxide rods and tubes (b). The lower images show spherical particles, including iron (c) and titanium oxide highly agglomerated nanoparticles (d).

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. 

Aluminum borate whiskers are produced commercially by an external flux method. Chlorides, sulfates, or carbonates of alkali metals are added to alumina and boric oxide (or boric acid) and the mixture is heated to 800°C-1000°C to produce aluminum borate whisker (length 10-30 pm and diameter 0.5-1.0 pm). It has a melting point of 1440°C, a very low coefficient of thermal expansion, and an excellent chemical resistance toward acids. The aluminum borate whisker was reported to be effective in improving not only the thermal degradation but also the glass transition temperature of epoxy76... 

Fig. 4.5 Comparison of flexural creep of whisker-reinforced A1203 with that of whisker-free aluminum oxide. A sharp increase in the stress exponent is observed at the higher stresses for the whisker-reinforced materials tested at 1400°C. Figure from Ref.

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... 

Both hydrated and anhydrous metal borates have numerous industrial uses. Some of the major uses of hydrated metal borates are the manufacture of glasses, ceramics, and industrial fluids, and as micronutrient fertilizers, fire retardants, and biostats. Anhydrous borates find use as heterogeneous catalysts, scintillation hosts, and in the fabrication of optoelectronic devices. Anhydrous aluminum borates are used as additives in oxide ceramics to promote the formation of desirable phases. Also, aluminum borate whiskers are used as reinforcing additives in composite materials. Specific nses of sodium, calcium, zinc, and barium borates are discussed in sections below. 

SiC-whisker-reinforced aluminum oxide ceramics have been developed especially for the... 

Figure 3 shows tool life and cutting torque while milling Inconel 718 with three different ceramics aluminum oxide with titanium-carbide (CM), SiC-whisker-reinforced aluminum oxide (CR) and SiAlON (CN). 

H. Campbell, M. Rilhle, B. J. Dalgleish and A. G, Evans, "Whisker Toughening A Comparison Between Aluminum Oxide and Silicon Nitride Toughened with Silicon Carbide," J. Am. Ceram. Sac., 73 [3] 521-30 (1990). 

Metal monoliths were obtained from Emitec (Germany). They were subjected to high-temperature treatment by the supplier. The cell density of the monoliths used is approximately 4(X) cpsi. The monoliths consist of an iron-chrome-aluminum alloy which provides the surface with a textured whisker structure after suitable treatment. These whiskers, shown in Figure 8, act as anchors for the washcoat when deposited onto the substrate. Tbe whiskers consist of aluminum oxide, completely covering the metal surface. This is shown by the data in Table 2, giving the results of EDX and XPS analyses of the whiskers-covered metal surface. 

The effect of particle shape on particle size distribution was investigated by Naito et al. (1998) with commercial particle size analyzers based on five different measuring principles electrical sensing zone, laser diffraction and scattering, x-ray sedimentation, photosedimentation, and light attenuation. The particles used are blocky aluminum oxide and barium titanate particles, flaky boron nitride particles, and rodlike silicon whisker ceramic powders. Altogether, four commer-... 

Sensing technique Aluminum oxide Barium titanate Boron nitride Sflicon nitride whisker... 

Abdullayev E, Joshi A, Wei W, Zhao Y, Lvov Y (2012) Enlargement of halloysite clay nanotube lumen by selective etehing of aluminum oxide. ACS Nano 6(8) 7216-7226 Alexandre M, Dubois P (2000) Polymer-layered silicate nanocomposites preparation, properties and uses of a new class of materials. Mater Sci Eng R Rep 28(1-2) 1-63 Aice A, Earle MJ, Katdare SP, Rodriguez H, Seddon KR (2007) Phase equilibria of mixtures of mutually immiscible ionie liquids. Fluid Phase Equilib 261(l-2) 427 33 Azizi Samir MAS, Alloin F, Dufresne A (2005) Review of recent research into cellulosic whiskers, their properties and their application in nanocomposite field. Biomacromolecules 6(2) 612-626... 

Additives used in finai products Fillers aluminum nitride, barium titanate, aluminum nitride, antimony trioxide, aramide fiber, attapulgite, carbon fiber, carbon nanofiber, carbon nanotubes, clay, glass fiber, graphite, molybdenum sulfide, montmorillonite, PTFE, silica, smectite, titanium oxide whisker Plasticizers diethylene glycol dibenzoate, dimethyl phthalate, triallyl phthalate, diethynyldi-phenyl methane, phenylethynyidiphenyl methane, 4-hydroxy-benzophenone Antistatics antimony-containing tin oxide, carbon black, carbon, nanotubes, indium oxide microspheres, polythiophene Release polyethylen wax, PTFE, silicone oil, zirconium chelate ... 

Additives used in final products Fillers barium and strontium ferrites, boron carbide, calcinated clays, calcium carbonate, carbon black, carbon-silica dual phase filler, clays, dolomite, fumed silica, iron oxide, magnesium aluminum silicate, magnesium carbonate, mica, montmorillonite, nickel zinc ferrite, nylon fibers, pulverized polyurethane foam, quartz, silica carbide, soapstone, talc, zinc oxide Plasticizers naphthenic oil, polybutene, aromatic oil, esters of dicarboxylic acid Plasticizers adipates, aromatic mineral oil, paraffin oil, phosphates, phthalates, polyethylene glycol, processing oil, sebacates Antistatics dIhydrogen phosphate of 8-amlnocaprolc add. Iodine doping Antistatics carbon black, quaternary ammonium salt, zinc oxide whisker Antiblocking diatomaceous earth Release propylene wax Slip erucamide+stearamide ... 

Additives used in final products - Fillers aluminum borate whiskers, barium sulfate, glass fiber, montmorillonite, PTFE, zinc oxide, zirconium oxide ... 

Whiskers of aluminum oxide, several alkali halides, several sulfides, and graphite have been grown in the laboratory. 

The superalloys, as well as alloys of aluminum, magnesium, titanium, and copper, are used as matrix materials. The reinforcement may be in the form of particulates, both continuous and discontinuous fibers, and whiskers concentrations normally range between 10 and 60 vol%. Continuous-fiber materials include carbon, silicon carbide, boron, aluminum oxide, and the refractory metals. However, discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibers of aluminum oxide and carbon, or particulates of silicon carbide and aluminum oxide. In a sense, the cermets (Section 16.2) fall within this MMC scheme. Table 16.9 presents the properties of several common metal-matrix, continuous and aligned fiber-reinforced composites. 

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