So far, many crystalline states, or low-temperature and high-temperature types, have been discovered. The identified there are nine types of alumina: a, K, &, y, x, edge, γ, p, β, etc. β-Al2O3 is not a pure alumina.Its composition is Na20(K2O)-11Al2O3 containing alkali metal ions, but it is still customarily regarded as a kind of alumina.Al2O3 is not pure alumina. The X-ray diffraction patterns of these many alumina are shown in Figure 2-3.Among the other seven alumina, except for a-Al2O3. r, 8, which is a high-temperature type alumina have distinct diffraction patterns, while y, γ, x, and p have only broad diffraction patterns, or even little difference.When distinguishing intermediate alumina by X-ray, the diffraction pattern must be carefully studied.In alumina, there are two types of oxygen accumulation: the hexagonal most densely packed ABAB type and the cubic most densely packed ABCABC type.There are also two types of aluminum coordination: 6 oxygen octahedral center positions and 4 oxygen tetrahedral center positions.and 4 oxygen tetrahedral center positions. This combination gives rise to a variety of alumina structures.
Commonly used crystalline classes of alumina:
(1) a-alumina. Also known as corundum-type structure, it is the most stable of the alumina crystalline forms.It is the representative structure of M2O type (M: trivalent metal element) oxide, corundum type structure.It has a hexagonal layer of the most densely packed oxygen atoms, and 2/3 of the gaps in the octahedral coordination between the oxygen atoms are filled by the metal atoms.The metal atoms fill 2/3 of the gaps in the octahedral coordination between the oxygen atoms. In other words, α-Al2O3 is an ionic bond between aluminum ions and oxygen ions (Lonic Bond), the aluminum atom is surrounded by six oxygen atoms to form an octahedral hexahedral coordination pattern.
(2) Y-Al2O3 (y, γ, t). y-A2O3, η-Al2O3 are hydrous alumina ore and alumina hydroxide.E-Al2O3 is γ-Al2O3, a transition alumina generated during the dehydration of alumina hydrates such as alumina ore.E-Al2O3 is the heating product of γ-Al2O3. The crystalline system is different from α-Al2O3, and it is a square crystal system. The lattice constant is large, the specific weight, easily soluble in acid. After heating above 1000℃, it will be transformed into α-Al2O3 dense structure.The firing shrinkage is very large. It is structurally hollow, has good adhesion to other substances, and has a large surface positive, so it can be used as absorbent.
(3) β-Al2O3. strictly speaking, β-Al203 does not belong to alumina. The chemical composition can be approximated by Me0-6Al2O3 and Me2O-11Al2O3. where MeO refers to CaO, RsO, SrO, etc.
alkali metal oxides; Me2O refers to alkali metal oxides such as NaO, K2O or L2O. β-Al2O3 is just a class of polyaluminate compounds with high Al2O3 content, with obvious ionic conductivity and relaxation polarization phenomenon, large dielectric loss, poor electrical insulation properties. Although it can not be used as structural ceramics, in radio ceramics, but it can be used as an important fast ion conductor material for sodium-sulfur batteries.