When the content of α-Al2O3 is greater than 90wt%, we call this refractory material sintered corundum. Sintered corundum is a refractory clinker that uses Al2O3 (γ-Al2O3) as raw material and is calcined at a high temperature of 1900°C after being shaped. Sintered alumina is mainly prepared by direct combination of fine corundum grains. Its bulk density is above 3.60g/cm3, apparent porosity is below 3.0%, and its refractoriness is ≥1800℃. The α-Al2O3 crystals calcined at high temperature are well developed and have a plate-like structure, and there are more closed pores inside, which is beneficial to improve its thermal shock resistance and peeling resistance. Without the addition of additives, its sintered corundum has high purity, good volume stability, and minimal re-fire shrinkage, so it has the following properties:
1) High hardness, Mohs hardness 9;
2) High melting point, about 2040℃;
3) Corrosion resistance, except for hydrofluoric acid and phosphoric acid, does not react with most alkalis and mineral acids;
4) High thermal conductivity, excellent electrical performance at high temperature;
5) The single particle has high strength, so that the sintered corundum has high strength.
1. The sinterability of corundum
The interatomic bonding energy of α-Al2O3 in corundum is relatively large and sintering is difficult. Without the introduction of sintering aid, the sintering temperature of corundum reaches about 1800℃. Such a high sintering temperature will make the diffusion rate of grain boundaries faster than that of pores. The discharge speed of the grain boundary makes the pores difficult to discharge, which leads to the deterioration of the air tightness of the material and the decrease of the mechanical properties. For sintered corundum, two methods are currently used to reduce the sintering temperature. One is to reduce the temperature by obtaining monodisperse ultrafine powder with small crystal grains, large specific surface area and high surface activity. However, this method has limited ability to reduce the sintering temperature of Al2O3, and the crystals of the burned sample are smaller. Another method introduces an appropriate amount of sintering aids (TiO2, MgO, SiO2, etc.) to increase the diffusion coefficient and reduce the alumina sintering temperature by forming a solid solution or low-temperature liquid phase with alumina. Among them, the composite sintering aid has the best compacting effect, but the introduction of the second phase will increase, which will affect its corundum phase content and high temperature performance. Among the single additives, the better sintering aid is TiO2 sintering aid, and does not produce a liquid phase, which affects its high-temperature performance. Therefore, TiO2 is widely used as a sintering aid for corundum.
2. Types and applications of sintered corundum
At present, there are mainly two types of sintered and fused corundum on the market. The fused corundum mainly includes fused white corundum, fused chromium corundum, and fused dense corundum. However, the variety of sintered corundum is relatively simpler than that of fused corundum. As the price of electricity rises, the production cost of fused corundum will increase and the sintered corundum market will further expand. Therefore, the research and development of a variety of sintered corundum has become one of the current research hotspots. The following are several types of sintered corundum and their uses.
Lightweight sintered corundum is a porous refractory with a core-shell structure that is fired using transition phase alumina as raw material and using foaming technology. The pores of this porous corundum are mostly closed pores, and the density is controlled by controlling the amount of foaming agent added. Lightweight sintered corundum is mainly used in ladle castables, and its closed pores can significantly reduce thermal conductivity and improve thermal shock resistance.
Microporous sintered corundum is a refractory material with a lower bulk density than ordinary tabular corundum. Microporous sintered corundum has more intracrystalline and intercrystalline pores, and the pore size is maintained at the micron level, but its chemical composition is the same as that of sintered plate corundum. The pores of the microporous sintered corundum make it have the characteristics of small bulk density, low thermal conductivity, and high strength. According to this property, the microporous sintered corundum can be partially substituted for the sintered tabular corundum and used in the ladle to reduce heat loss.
Dense sintered corundum is a new type of high-density sintered corundum. Compared with ordinary sintered corundum, the volume density of dense sintered corundum is> 3.65g/cm3, and the apparent porosity is less than 3.0%. Compared with the original standard value, it has been greatly improved, which will effectively improve the performance of sintered corundum and extend its use life. Higher compactness makes it have better wear resistance and erosion resistance. This is for refractory products with more demanding conditions, such as skateboards, three pieces, breathable bricks, and ladle impact area castables, which require high erosion resistance. For refractory products, the use of dense sintered corundum can greatly increase its service life and safety factor.
Sintered titanium corundum is a kind of special corundum prepared by introducing TiO2 into alumina and calcining at high temperature. Because the lattice constants of Al2O3 and TiO2 are close, Ti4+ and Al3+ will undergo limited replacement, causing lattice distortion, activating the lattice, significantly increasing the sintering activity and reducing its sintering temperature. Sintered titanium corundum has high mechanical strength and good elasticity resistance. It can be used together with β-corundum in the molten pool of a glass furnace to improve the toughness under high temperature conditions.
Therefore, researching different types of sintered corundum, giving full play to the excellent properties of sintered corundum, reducing its sintering temperature, improving its properties after firing, and further increasing the application of sintered corundum has become the current development trend of sintered corundum.
