Strength alumina-based ceramics

Although beryllium oxide [1304-56-9] is in many ways superior to most commonly used alumina-based ceramics, the principal drawback of beryUia-based ceramics is their toxicity thus they should be handled with care. The thermal conductivity of beryUia is roughly about 10 times that of commonly used alumina-based materials (5). BeryUia [1304-56-9] has a lower dielectric constant, a lower coefficient of thermal expansion, and slightly less strength than alumina. Aluminum nitride materials have begun to appear as alternatives to beryUia. Aluminum nitride [24304-00-5] has a thermal conductivity comparable to that of beryUia, but deteriorates less with temperature the thermal conductivity of aluminum nitride can, theoreticaUy, be raised to over 300 W/(m-K) (6). The dielectric constant of aluminum nitride is comparable to that of alumina, but the coefficient of thermal expansion is lower. 

Padture NP (1991) Crack resistance and strength properties of some alumina-based ceramics with tailored microstructures. Ph.D. Thesis, Lehigh University... 

Table 4.1 further shows essential mechanical properties of several products of CeramTec s BIOLOX family of alumina-based materials for femoral heads of hip endoprostheses as well as those of BIONIT manufactured by Mathys Orthopadie GmbH (Bettlach, Switzerland). It is evident that decreasing the grain size of the ceramic precursor powders increases both the flexural strength and the fracture toughness of the material dramatically. 

Figure 6. Effect of an annealing treatment on the residual tensile failure strength at room temperature of alumina-based fibers (a) for Nextel 440 fiber reproduced with permission from Elsevier Science Ltd.. The Boulevard, Langford Lane, Kidlirigton 0X5 1GB, UK, and (b) for fiber FP and PRD-166, according to ref. [65] and [29] reproduced with permission of The American Ceramic Society, PO Box 6136, Westerville, OH 43086-6136.

Figure 7. High temperature tensile strength in air of alumina-based fibers. Figure redrawn from [27] [64] with permission of the American Ceramic Society, Westerville, OH 43086.

Oxide fibers include glass fibers, mullite fibers, zirconia fibers and alumina fibers. Of these, a-alumina-based fibers have been used intensively for ceramic matrix composites. Fiber FP, manufactured by Du Pont in 1979, was the first wholly a-alumina fiber produced [34]. At present, Almax (Mitsui Mining Material Co. Ltd., Japan) and Nextel 610 (3M Co., USA) are commercially available a-alumina fibers. Almax contains 99.5% alumina and has an elastic modulus of 330 GPa, and Nextel 610 has a tensile strength of 2.4 GPa and an elastic modulus of 380 GPa [35]. 

Aluminum oxide-based ceramic insulators are a common construction material for a wide variety of electrical components, including vacuum tubes. Aluminum oxide is 20 times higher in thermal conductivity than most oxides. The flexure strength of commercial high-alumina ceramics is 2-4 times greater than that of most oxide ceramics. There are drawbacks, however, to the use of alumina ceramics, including... 

Remarkable characteristics of nanocomposites observed by many researchers are summarized " Drastic change of fracture mode is observed from intergranular fracture of monolithic ceramics to transgranular fracture of nanocomposites, especially in alumina-based nanocomposites. Several mechanical properties were also improved, such as fracture strength, " " fracture toughness, " thermal shock resistance, creep resistance, " hardness, " and wear resistance." ... 

The MF membranes are usually made from natural or synthetic polymers such as cellulose acetate (CA), polyvinylidene difiuoride, polyamides, polysulfone, polycarbonate, polypropylene, and polytetrafiuoroethylene (FIFE) (13). Some of the newer MF membranes are ceramic membranes based on alumina, membranes formed during the anodizing of aluminium, and carbon membrane. Glass is being used as a membrane material. Zirconium oxide can also be deposited onto a porous carbon tube. Sintered metal membranes are fabricated from stainless steel, silver, gold, platinum, and nickel, in disks and tubes. The properties of membrane materials are directly reflected in their end applications. Some criteria for their selection are mechanical strength, temperature resistance, chemical compatibility, hydrophobility, hydrophilicity, permeability, permselectivity and the cost of membrane material as well as manufacturing process. 

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