Physical structure of brown corundum
Microscopic examination of brown corundum shows that it is composed of alpha alumina grains and is bonded together by a small amount of glassy slag. Zong Gangyu, which satisfies the typical technical conditions, is composed of more than 95% alpha alumina crystals, which is composed of Al2O3 solid solution containing Ti2O3. Most of the glass slag is composed of silicon dioxide and titanium dioxide, as well as other oxides in the arc furnace. These oxides form the glass phase, which has only a low solubility in the crystal structure of the alpha alumina grains. The number of glass slag and the distribution of the impurities in the alumina grains are the main factors that affect the properties of brown corundum. Alpha alumina gives the corundum material with high hardness and high melting point, while the presence of glass phase and impurities gives Zong Gangyu the toughness and resistance to breakage. To meet the requirements of the technical conditions of the typical Zong Gangyu, mixed proportion of glass phase of silicon dioxide and titanium dioxide, the theory about 1:1 silica glass phase is too much, the excess silica is not retained in the glass phase, but with a part of the reaction of alumina mullite (2SiO2 (3Al2O3)), which will reduce corundum toughness. On the other hand, if there is an excess of titanium oxide in brown corundum (TiO2), can not be retained in the glass phase, but the reaction of alumina and aluminium titanate (TiO2 (Al2O3)). Aluminum titanate is the third phase on the interface between the alpha alumina and the glass phase, which can cause the glass phase of the alpha alumina grains to weaken. The difference of the coefficient of thermal expansion between the three phases can also cause the intergranular crack.