Aluminium oxide (Al2O3) is an abundant and inexhaustible mineral which occurs naturally in large parts of the Earth's crust. It is a crystalline form of aluminium with the formula Al2O3. It has an extremely low electric resistivity and a high melting point, making it useful in electrical insulators.
It is also insoluble in water and has excellent chemical resistance against strong acid. This property makes it a popular material in engineering ceramics and many other applications.
Alumina is an ionic conductor at temperatures above 1000 degC. The value of its (ionic) conductivity depends greatly on the purity of alumina, but also dramatically on the temperature at which it is used.
In addition, it is a basic anhydride that reacts with acids and with strong reducing agents in redox reactions. The resulting alumina oxide layer can be thickened using anodizing, which adds hardness and abrasion resistance to the surface.
However, the ionic conductivity of alumina can decrease when it is added to 3Y-TZP/Al2O3 composites and, in particular, when it is introduced through grain boundaries. This is due to a combination of the increased height of the Schottky barrier and the interaction of the aluminum ions with the impurities in the zirconia.
Two series of 3Y-TZP/Al2O3 samples, which were sintered at different temperatures, were characterized by a wide range of phase compositions as shown in Figure 9. The tetragonal zirconia phase was predominant in the first group of samples while, for the second, minor amounts of monoclinic phase were found. The sintering procedure as well as the alumina content had an important impact on the final properties of the 3Y-TZP/Al2O3 materials.