Industrial Al2O3 used in the production of structural ceramics is calcined at a higher temperature to completely transform it into α-Al2O3. The shrinkage and local sintering during the crystallographic transformation causes a change in the particle size composition of Al2O3 – the number of fine particles increases. The calcination process also causes a significant change in the bulk density of industrial Al2O3, which is 0.85g/cm3 without calcination, 1.1-1.2g/cm3 after calcination at 1500°C, and 1.5-1.6g/cm3 after calcination at 1700°C. The bulk density of industrial Al2O3 is 0.85g/cm3 after calcination, 1.1-1.2g/cm3 after calcination at 1500°C, and 1.5-1.6g/cm3 after calcination at 1700°C.
High-purity alumina powder preparation method, Al2O3 products such as strength, heat resistance and electrical insulation and many other properties with the increase in impurity content and deterioration, so in the products require high strength and excellent light transmission properties, must use high-purity alumina powder raw materials, generally using the following methods to produce.
(1) organic aluminum salt with water decomposition method, that is, sol-gel method
Aluminum hydroxide is produced by decomposing the alcohol salt of aluminum with water, and the reaction formula is as follows
Al(OR)3+3H2O==Al(OH)3+3R-OH
The hydrolysis generated sol is washed, filtered, dried, and calcined at a suitable temperature to obtain pure Al2O3 powder which is easy to sinter.
(2)Thermal decomposition method of inorganic aluminum salt
alumina powder is prepared by thermal decomposition using refined aluminum sulfate, ammonium alum, aluminum ammonium carbonate salt, etc. Generally, reagent-grade aluminum sulfate and ammonium sulfate are heated and dissolved in pure water, and stirred and cooled to precipitate the ammonium sulfate salt (ammonium alum) crystals of aluminum obtained. In order to improve the purity of the final Al2O3 produced, the ammonium alum can be recrystallized 3 to 4 times, and then thermal decomposition, the purity of Al2O3 powder produced by this method can reach more than 99.9%, which can be used to manufacture transparent high-pressure sodium lamp tubes and other products.
(3) Discharge oxidation method
High-purity alumina powder is immersed in pure water, inserted into the electrode to produce high-frequency spark discharge, aluminum powder and the reaction with water to generate aluminum hydroxide, calcined to produce high-purity Al2O3 powder. In addition, Al2O3 powder can be prepared by hydrothermal precipitation method, vapor phase method, purified sodium aluminate solution, etc.
In addition, alumina powder can be prepared by hydrothermal precipitation method, gas phase method, purified sodium aluminate solution, etc.