Corundum (Al2O3) has excellent properties and is non-toxic and polluting. It is widely used in refractories. Al2O3 is an amphoteric compound. Its density is 3.94.0g/cm3, its melting point is 2050℃, and its boiling point is 2980℃. Its hardness is only Second to diamond; Al2O3 has a variety of crystal forms. α-Al2O3 is the most stable crystal form among all alumina crystal forms. It belongs to the corundum-type structure. The trigonal crystal system is R-3C space group. The unit cell is a sharp rhombohedron. . The structure has the most densely packed layer of oxygen atoms in the hexagon, and 2/3 of the octahedral gaps of oxygen atoms are filled with metal aluminum atoms. The aluminum atom is surrounded by six oxygen atoms to form an octahedral six-coordinate type.
Corundum crystals are ionic crystals, and the ionic bond strength is relatively strong. Corundum has high hardness and good thermal conductivity, especially at low temperatures. The dielectric constant is small and the insulation resistance is high. It has good light transmittance and is an excellent optical crystal material. Its chemical properties are stable, it is not corroded by acid at room temperature, and it is insoluble in water. Different metals have a large difference in the degree of infiltration of corundum. The wetting angle of lead, zinc, aluminum, copper and some iron alloys is greater than 90°, and the wetting angle of iron-manganese alloys, iron-chromium alloys, and some titanium alloys is less than 90°. However, corundum can interact with oxides in slag, glass, refractory materials and other materials to form compounds, so the corundum material is easily corroded by the slag. Corundum also has shortcomings such as low breaking strength, poor thermal expansion resistance, and poor creep resistance, which limit its application.
In order to improve the shortcomings of Al2O3 refractories, people add binding phases or in-situ synthesis reinforcement phases to Al2O3 refractories to prepare corundum composite refractories. In the last century, a wide variety of oxide-reinforced corundum refractories have been developed, including Al2O3-Cr2O3 refractories, Al2O3-MgO refractories, Al2O3-ZrO2 refractories and other oxide composite refractories. However, the oxide binding phase has the shortcomings of large thermal expansion coefficient and poor corrosion resistance, which gradually cannot meet the increasing demand for industrial technology. C can improve the thermal shock stability and corrosion resistance of Al2O3 refractories. People add C to Al2O3 refractories and develop Al2O3-C refractories. Nowadays, Al2O3-C refractories have been widely used in the field of steel smelting.
However, C has the problem of being easily oxidized during use, and the use of carbon-containing materials will cause the carbon increase of molten steel, which cannot meet the production requirements of modern clean steel technology. With the research and development of non-oxides such as Si3N4 and SiC in the ceramic field, especially the appearance of AlON and SiAlON in the Si-Al-ON and Al-ON systems in the 1970s, non-oxide combined phases have gradually been applied to Al2O3 refractories , Developed Al2O3-Si3N4 refractories, Al2O3-SiC refractories, and Al2O3-SiAlON refractories.
With the advent of cermet technology, it has been discovered that a small amount of metal in brittle inorganic ceramics or refractory materials can play a role in improving the brittleness and microstructure. People add metal Al and Si to Al2O3 refractories and sinter them in situ by reaction. Synthesizing SiAlON reinforcing phase, and a small or trace amount of residual metal can improve the brittleness or oxidation resistance of the material, so a new type of metal-non-oxide-oxide refractory with excellent performance has been developed. Corundum refractories combined with metals and non-oxides will have great application potential. Iron silicon nitride contains Si3N4 and Fe-Si intermetallic compounds. Therefore, the introduction of iron nitride into the corundum refractory can produce a metal-non-oxide-oxide refractory material, which has greater theoretical and practical significance.
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