Zirconium dioxide has become an important raw material for structural and functional materials because of its excellent high temperature resistance, wear resistance and corrosion resistance. However, due to the polycrystallinity of zirconia and its volume change during heating and cooling, as well as its small thermal conductivity and large thermal expansion coefficient, the mechanical, electrical and thermal shock resistance properties of pure ZrO2 are very poor, which results in that pure ZrO2 can not be directly used to manufacture large-scale and special-shaped products, greatly limiting the application of zirconia. Therefore, the first step is to stabilize pure ZrO2.
- Phase Stabilization of Zirconium Dioxide
At present, the mechanism of phase stabilization of zirconium dioxide is not very thorough. Generally speaking, chemical doping or physical methods are used to make the tetragonal and cubic crystal structure of high temperature phase produce irreversible phase transition, so as to stabilize its structure and obtain room temperature stable tetragonal or cubic ZrO2 materials.1.1 Chemical doping stabilization refers to adding stabilizer to zirconia, inhibiting the transformation of ZrO 2 crystal form, keeping tetragonal or cubic high temperature phase at room temperature, showing a metastable state.
1.1.1 The radius size effect of doped ions stabilizes the radius size effect of doped ions. It is a tetravalent oxide, such as UO 2 and CeO 2, whose radius of doped ions is larger than that of zirconium ions in ZrO 2, to increase the radius ratio of cations to anions. The stability of tetragonal or cubic ZrO2 at low temperature is affected only by the increase of cation radius, but the effect is not obvious.
1.1.2 Low-valent metal oxides doped stably with low-valent metal oxides are doped stably with alkaline earth metal oxides or rare earth metal oxides, such as MgO, CaO, Y2033 and Sc2 O3, whose valence is lower than tetravalent in ZrO2. The difference of cation radius between these oxides and Zr4 + radius is less than 12%. They are highly soluble in ZrO 2. After high temperature treatment, these low-valent cations will take the place of Zr4 +. In order to maintain the local electrical neutrality of the material, oxygen vacancies are introduced into the lattice, which are distributed in the zirconia body. The vacancies around the zirconium ions reduce the repulsion between local oxygen and oxygen, resulting in larger distortion of the coordination layer, thus forming cubic or tetragonal lattice displacement solid solutions that can maintain stability at room temperature. The eutectoid decomposition of the solid solution can be avoided by rapid cooling, and the metastable state can be maintained at room temperature.