Ordinary sintered alumina is generally fired in a rotary kiln or tunnel transport. Due to temperature restrictions, a small amount of sintering promoters such as MgO and CaO are usually added to promote its densification. Therefore, the porosity of ordinary sintered alumina is low, and the crystals are fine and dense, resulting in poor thermal shock resistance.
Tabular alumina is a recrystallized α-Al2O3 that is quickly fired at a high temperature above 1900℃ and sintered thoroughly. It does not contain any additives and has the following microstructure characteristics:
⑴ It is composed of well-developed α-Al2O3 crystals;
⑵ α-Al2O3 crystals are coarse, with a median diameter of 40-200 μm, and their two-dimensional morphology is flat and interlaced;
⑶ α-Al2O3 crystals contain many 5~15μm round closed pores, while open pores are relatively few, generally 2%~3%;
⑷ There is β-Al2O3 in the structure, but it all grows at the grain boundaries of α-Al2O3 crystals.
Due to the above structural characteristics of tabular alumina, it has the following physical properties:
⑴ High melting point, about 2040℃;
⑵ The grain hardness is high, Mohs hardness is 9, Knoop hardness is 2000;
⑶ Resistant to chemical corrosion, except for hydrofluoric acid and phosphoric acid, most alkalis and mineral acids have no effect on tabular alumina;
⑷ Since there are no micro cracks and large internal pores, its strength is relatively high; at the same time, its strength does not drop much when subjected to thermal shock, so its thermal shock stability is good;
⑸ High thermal conductivity and high resistivity, with good electrical properties at high frequencies and high temperatures.
Tabular alumina is mostly used as refractory aggregate, and is also used as an additive to enter the matrix. When combined with other refractory raw materials, it has the following advantages: high refractoriness; high thermal load strength; small creep; high density; low permeability; good thermal shock stability and wear resistance; low thermal shrinkage; high purity, reducing the impact of impurities on the high temperature performance of the material.
