Al2O3 ceramics often need to be colored in practical applications; that is, different colors can be presented by introducing coloring ion compounds. For example, alumina ceramics used as packaging tubes in semiconductor integrated circuits are required to have light-shielding properties, so the Al2O3 ceramics of the digital tube lining are also required to be black to ensure a clear digital display. For this reason, coloring oxides such as Fe2O3, CoO, Cr2O3, TiO2, and MnO can be introduced into Al2O3. Al2O3 ceramics appear black because Ti4+ in the ceramic material is partially reduced to Ti3+ under the action of reducing atmosphere (H2) and high temperature. Ti3+ can be regarded as Ti4+ with bound electrons, that is, Ti4+e-. This bound electron is a weakly bound electron and can be regarded as a “color center” in TiO2, so this type of ceramic appears black.
In black alumina ceramics, the interactions between coloring additives and the matrix are sometimes very complex. Some researchers have observed that the added colorants and the matrix alumina generate spinel crystals such as CoAl2O (dark blue), MnAl2O4 (yellow-brown), and CrAl2O1 (green), and there are also crystals such as Mn2TiO4 (brown), MnCr2O4 (gray-green), and CoCr2O4 (blue-green) generated between the additives. These crystals are either on the grain boundaries or attached to the grains of alumina. Through the selective absorption and reflection of light, they work together to make Al2O3 black.
Another commonly used red-purple Al2O3 ceramic is the introduction of Cr2O3 and MnO into Al2O3 ceramics. Al2O3 ceramics containing about 1% Cr2O3 often appear red because the Cr3+ ions dissolved in the α-Al2O3 lattice have strong selective absorption of the blue-green frequency band of visible light, which makes the porcelain appear blue-green complementary color, that is, pink.
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