engleski

Formation of High-Temperature ZrO2 Polymorphs at Room Temperature

The combination of strenght, toughness and chemical resistance alows the application of zirconia ceramics in harsh enviroments under severe load conditions. Zirconia-based ceramics found use as cutting tools, dies for wire drawing or hot extrusion, seals in valves, chemical and slurry pumps, impellers, automotive engine parts, etc. Zirconia is a polymorphic material which, depending on the temperature, appears in three different forms: monoclinic, tetragonal and cubic. At room temperature (RT), pure zirconia is monoclinic (m-ZrO2), having a distorted fluorite-type (CaF2) structure, with the Zr atom having the coordination number seven. At temperatures above 1170 oC the tetragonal (t-ZrO2) polymorph becomes thermodynamically stable, while above 2370 °C up to the melting point (2680 °C) it is the cubic (c-ZrO2) polymorph. In both high-temperature phases the Zr atom has the coordination number eight, as in CaF2, but in t-ZrO2 form the O atom is displaced from its ideal position. Because of its high melting point, zirconia is an attractive refractory material. However, the volume expansion of the t-ZrO2→m-ZrO2 transition upon cooling from the sintering temperature causes crumbling of zirconia ceramics. This phase transition occurred with very small shifts of atoms and could not be suppressed by rapid cooling. However, beside the m-ZrO2 at RT often appeared a metastable t-ZrO2, and in some cases even a metastable c-ZrO2. Various factors which influence the appearance of a t-ZrO2 polymorph at RT have been extensively investigated. Several proposed models illustrate the role of anionic impurities (SO , OH–), the crystallite size (surface energy), structural similarities between the starting material and t-ZrO2, lattice strains, water vapour, lattice defects (oxygen vacancies), etc. In the investigations presented here various processing routes (ball-milling, thermal decomposition, precipitation or hydrolysis, hydrothermal treatment, the formation of solid solutions by incorporation of aliovalent undersized dopant cations) were used to obtain high-temperature polymorphs of zirconia at RT. The obtained results were used to discuss the proposed models of stabilisation.

T-zro2. M-zro2. Hydrous zirconia. Oxygen vacancies. Lattice strain. Xrd. Dsc.

nije evidentirano