November 18 , 2021
Alpha- Aluminium Oxide
Aluminium oxide can exist in one of several crystalline forms, including the most stable one – alpha-aluminium oxide. α-Al2O3 modification occurs in nature as corundum. Normal corundum is a non-transparent fine- or coarse-grained mineral, colourless or greyish; sometimes it may look like a large non-transparent crystal. Large transparent corundum crystals are precious stones. Alpha-modification crystals can also be red (ruby), blue (sapphire), violet or of other colours due to the admixtures of oxides of different metals. The Properties and Production of α-Aluminium Oxide Aluminium oxide can occur as separate or grouped crystals, grains, isolated impregnations or compacted granular aggregates. Corundum has a shine ranging from metallic to matted one. In some cases, the asterism effect is observed due to inclusions of fine oriented rutile crystalloids. Alpha-aluminium oxide is shaped as insoluble crystals, having a melting temperature of 2050°C, a density of 3,960 kg/m3 and a boiling temperature above 3,000°C. The material is resistant to chemical attacks and acid-insoluble. The artificial alpha-form of aluminium oxide is obtained from bauxites and other high-alumina minerals using a thermal treatment method. Commercial alpha-aluminium oxide can be produced by heating aluminium hydroxide and its salts to above 900°C. When aluminium salts are heated to temperatures above 600°C, a cubic gamma-aluminium oxide...
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