Alumina: also corundum

Af-dipropyl-p- toluidine, 2 550t a, a -dinitroanthraquinones, 9 315—316 a-alumina, 2 406t 14 103. See also Corundum transition to, 2 403 a-aluminum-iron—silicon alloys, 2 317 intermetallic phases, 2 316t a-aluminum oxide-hydroxide. 

See also Corundum transition to, 2 403 P-Alumina, 2 408 y-Alumina, 2 391, 403, 404, 406t y-Alumina, 2 403, 404, 406t 8-Alumina, 2 404, 406t 0-Alumina, 2 404, 406t K-Alumina, 2 404, 406t p-Alumina, 2 394-395 Alumina-aluminum titanate, 5 570 Alumina-chromia-thoria, catalytic aerogels, l 763t... 

The most common form of crystalline alumina is corundum, which has a rhombohedral Bravais lattice with a space group R-3c. These materials have been used in energy storage, alkali metal thermal-to-electric conversion cells, and gas sensors. In recent years, it has been reported that alumina can also reduce heat, humidity, light, ozone and gamma radiation, flame resistance, and the crack growth in NR. ... 

The red color of the ruby is also caused by the presence in it of a trace of chromic oxide, which distinguishes this costly gem from common crystalline corundum (alumina). Thus chromic oxide, according to F. H. Pough, is the most valuable commodity in the world when purchased in the form of a ruby (84). A beautifully illustrated article on synthetic rubies appeared in the Journal of Chemical Education for June, 1931 (85). 

Aluminum is a constituent of many minerals, including clay (ka-olinite), mica, feldspar, sillimanite, and the zeolites. Some of these minerals are discussed under the chemistry of silicon, in Chapter 31. Aluminum oxide (alumina), occurs in nature as the mineral corundum. Corundum is the hardest of aU naturally occurring substances except diamond it scratches all other minerals, but is itself scratched by diamond, and also by the artificial substances boron carbide, and silicon carbide, SiC. Corundum and impure corundum (emery) are used as abrasives. 

Abbattista et al. (26) found that phosphorus addition prevents crystallization of the y-alumina phase and the transformation from y- to a-alumina in the system AI2O3 —AIPO4 (Fig. 23). More precisely, Morterra et al. (77) reported that phosphates do not affect the phase transition from low-temperature spinel alumina (y-alumina) to high-temperature spinel aluminas 8 and 6 phases) but delay the transition of 8 and 9 to a-alumina (corundum). Stanislaus et al 46) also reported that phosphorus significantly improves the thermal stabihty of the y-alumina phase in P/Al catalysts. However, the same authors found that the positive effect of phosphorus seems to be canceled in the presence of molybdenum due to the formation of aluminum molybdate. Thermal treatments of MoP/Al catalysts at temperatures >700°C result in a considerable reduction of SSA and mechanical strength. The presence of phosphorus does not prevent the reaction between the molybdenum oxo-species and alumina since the interaction between molybdates and phosphates is weak. The presence of nickel does not obviously affect the positive effect of phosphorus in terms of thermal stability 46). On the other hand, Hopkins and Meyers 78) reported that the thermal stability of commercial CoMo/Al and NiMo/Al catalysts is improved by the addition of phosphorus. 

The (0001) surface of sapphire (a-alumina, corundum) is one of the most widely used substrate for the growth of metal, semi-conductor or high-temperature superconductor thin films. It is also used as a substrate in silicon on sapphire (SOS) technology. Moreover, its initial state is known to play a role on the overlayer properties [50]. 

The mineral corundum provides a good example. Corundum is an extremely hard substance. Small bits of corundum are part of the rock called emery which has been used since ancient times as an abrasive, to cut and grind metal and stone. Pure corundum is still used for this purpose today. Another property of corundum is that it remains solid and stable at very high temperatures, well past the melting point of iron. Therefore, masses of small corundum crystals pressed together are shaped into alumina firebricks, crucibles, and other apparatus to use in furnaces. Corundum is also the basis of several gemstones. 

The reaction phases are similar for the 70% alumina brick exposed to K20 however, corundum and potassium aluminate are also produced by potassia reaction. 

A review of First Principles simulation of oxide surhices is presented, focussing on the interplay between atomic-scale structure and reactivity. Practical aspects of the First Principles method are outlined choice of functional, role of pseudopotential, size of basis, estimation of bulk and surface energies and inclusion of the chemical potential of an ambient. The suitability of various surface models is discussed in terms of planarity, polarity, lateral reconstruction and vertical thickness. These density functional calculations can aid in the interpretation of STM images, as the simulated images for the rutile (110) surface illustrate. Non-stoichiometric reconstructions of this titanium oxide surface are discussed, as well as those of ruthenium oxide, vanadium oxide, silver oxide and alumina (corundum). This demonstrates the link between structure and reactivity in vacuum versus an oxygen-rich atmosphere. This link is also evident for interaction with water, where a survey of relevant ab initio computational work on the reactivity of oxide surfaces is presented. 

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