Alpha alumina: Aluminum oxide, or alumina, crystallizes in the corundum structure (mineralogical term) to form sapphire monocrystals. We are referring here to white sapphire, like the one used for the manufacture of scratchproof “glasses” for watches, whereas in gemmology sapphire is the blue sapphire, which is alumina with a little Ti4+; ruby being alumina colored in red by Cr3+. This is the alpha variety (Al2O3-?). The corundum structure belongs to the rhombohedric system. The crystal can be described as a compact hexagonal stacking of O2- anions, in which the two-thirds of the octahedral interstices are occupied by Al3+ cations. We note that this reference to anions and cations insists rather misleadingly on the ionicity of the bond: it is considered that the alumina bond is ionic for two-thirds and covalent for the remaining third part. The strong iono- covalent bonds and the rhomboedric structure combine to restrict the dislocation movement, the sapphire monocrystal remains brittle until very high temperatures and the glide slip is only possible beyond 900°C. The plasticity of the polycrystal comes primarily from the effects of intergranular diffusion or flow of vitreous phases. These are mechanisms that depend on the purity of the material and whose occurrence temperature thresholds are lower than 800°C for impure aluminas and higher than 1,100°C for aluminas with purity exceeding 99%. Alumina is one of the best electrical insulators, hence its dielectric applications. Electric conduction depends primarily on impurities which act as acceptors (for example, Mg, Fe, Co, V or Ni) or as donors (for example, H, Ti, Si, Zr or Y), the balance between electronic and ionic contributions depending on the temperature and the partial pressure of oxygen. Among the impurities, sodium deserves a particular mention because the synthesis of alumina by the Bayer process brings into play sodic mediums; sodium aluminate (Na2O11Al2O3), also called beta alumina, is an ionic conductor with very high conductivity, which is why this compound is envisaged for solid electrolyte applications. With an optical gap of about 9.9 eV,
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