Aluminum oxide ceramics strength

Alumina is used in ceramic products in varying amounts. However, discussion is usually limited to high alumina, which refers to those bodies containing 80 percent or more aluminum oxide. Ceramics with less than 80 percent alumina but still predominantly alumina are classified as porcelain. The most common aluminas are those containing 85,90, 94, 96, 99, 99.8, and 99.9%. Strength and other properties improve as the alumina percentage increases, bnt so do cost and complexity of processing. The properties are dependent not only on the alnmina content, but also on microstructure and porosity. 

Alumina, or aluminum oxide [1344-28-17, has a thermal conductivity 20 times higher than that of most oxides (5). The flexural strength of commercial high alumina ceramics is two to four times greater than those of most oxide ceramics. The drawbacks of alumina ceramics are their relatively high thermal expansion compared to the chip material (siUcon) and their moderately high dielectric constant. 

Pulpstones. Improvements have been made in the composition and speed of the grinding wheel, in methods of feeding the wood and pressing it against the stone, in control of power to the stones, and in the size and capacity of the units. The first pulpstones were manufactured from quarried sandstone, but have been replaced by carbide and alumina embedded in a softer ceramic matrix, in which the harder grit particles project from the surface of the wheel (see Abrasives). The abrasive segments ate made up of three basic manufactured abrasive siUcon carbide, aluminum oxide, or a modified aluminum oxide. Synthetic stones have the mechanical strength to operate at peripheral surface speeds of about 1200—1400 m /min (3900 to 4600 ft/min) under conditions that consume 0.37—3.7 MJ/s (500—5000 hp) pet stone. 

A third class of bioceramics are based on Zr02, stabilized by Y2O3 or Ce02. These materials are close to aluminum oxide materials in terms of biocompatibility but exhibit a higher bend strength and crack resistance, though with lower compressive strength. Zirconium dioxide ceramics can be used for many of... 

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... 

Sialons, as ceramic alloys of silicon nitride and aluminum oxide, were developed as an economically and fimctionally superior alternative to H PSN (see above). This alloying imposes increased high-temperature mechanical (flexural and tensile strengths, fracture toughness, hardness, wear resistance), thermal (thermal shock resistance) and chemical properties (corrosion and oxidation resistance) compared to unalloyed silicon nitride (see Table 11.13). 

Physical strength Ceramic coating (aluminum oxide) has inherent hardness and will without damage to the covering itself. 

Among non-oxide ceramics, aluminum nitride developed recently. Its mechanical properties (hardness, breaking strength) are modest, bnt its thermal conductivity is very high. This conductivity associated with high electrical resistance, good dielectric property and a coefficient of expansion close to that of silicon, which varies linearly with the temperature, make it an excellent material for substrates for power micro-electronics (Figure 7.12). 

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. 

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