There are two coordination states of A1+ in glass, namely, located in tetrahedron or octahedron. When A1+ is located in aluminum-oxygen tetrahedron [A104], it forms a unified network with silicon-oxygen tetrahedron. It can reduce the crystallization tendency of glass, improve the chemical stability, thermal stability, mechanical strength, hardness and refractive index of glass, and reduce the erosion of glass on refractory materials. At present, most glasses add 1~3.5% A1O?. However, A12O? has an adverse effect on the electrical properties of glass. In silicate glass, when A12O. replaces SiO2, the dielectric loss and conductivity do not decrease but increase. Therefore, in vacuum glass, glass with high electrical performance requirements generally does not contain or contains less A1?O3. This is because aluminum-oxygen tetrahedron [A10] has a larger volume than silicon-oxygen tetrahedron, so the structure is loose and the gap is large, which is conducive to the activity of alkali metal ions, so that the dielectric loss and conductivity increase accordingly.
The right amount of A12O? can also reduce the expansion coefficient of glass. In the aluminosilicate glass used in gas discharge light sources such as high-pressure mercury lamps and halogen tungsten incandescent lamps, if the glass shell temperature is not too high (below 550℃), the A12O3 content can reach 25% by weight (excluding alkali), while when the glass shell temperature is high, the borosilicate glass used contains only 2% A12O3.
The raw materials for introducing alumina include industrial alumina, feldspar (such as potassium feldspar KA1SiOs calcium feldspar CaAl?Si2O8), kaolinite [A1(Si4O1)(0H)], aluminum hydroxide, etc.