Yttria-stabilized zirconia

Yttria-stabilized zirconia (YSZ) is a ceramic in which the cubic crystal structure of zirconium dioxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" (Zr O2) and "yttria" (Y2O3), hence the name.
Stabilization.
Pure zirconium dioxide undergoes a phase transformation from monoclinic (stable at room temperature) to tetragonal (at about 1173 °C) and then to cubic (at about 2370 °C), according to the scheme:
monoclinic (1173 °C) formula_1 tetragonal (2370 °C) formula_1 cubic (2690 °C) formula_1 melt
Obtaining stable sintered zirconia ceramic products is difficult because of the large volume change accompanying the transition from tetragonal to monoclinic (about 5%). Stabilization of the cubic polymorph of zirconia over wider range of temperatures is accomplished by substitution of some of the Zr4+ ions (ionic radius of 0.82 Å, too small for ideal lattice of fluorite characteristic for the cubic zirconia) in the crystal lattice with slightly larger ions, e.g., those of Y3+ (ionic radius of 0.96 Å). The resulting doped zirconia materials are termed "stabilized zirconias".
Materials related to YSZ include calcia-, magnesia-, ceria- or alumina-stabilized zirconias, or partially stabilized zirconias (PSZ). Hafnia stabilized Zirconia is also known.
Although 8–9 mol% YSZ is known to not be completely stabilized in the pure cubic YSZ phase up to temperatures above 1000 °C. 
Commonly used abbreviations in conjunction with yttria-stabilized zirconia are:
Thermal expansion coefficient.
The thermal expansion coefficients depends on the modification of zirconia as follows:
Ionic conductivity of YSZ and its degradation.
By the addition of yttria to pure zirconia (e.g., fully stabilized YSZ) Y3+ ions replace Zr4+ on the cationic sublattice. Thereby, oxygen vacancies are generated due to charge neutrality:
formula_4 with formula_5,
meaning two Y3+ ions generate one vacancy on the anionic sublattice. This facilitates moderate conductivity of yttrium stabilized zirconia for O2− ions (and thus electrical conductivity) at elevated and high temperature. This ability to conduct O2− ions makes yttria-stabilized zirconia well suited for application as solid electrolyte in solid oxide fuel cells.
For low dopant concentrations, the ionic conductivity of the stabilized zirconias increases with increasing Y2O3 content. It has a maximum around 8–9 mol% almost independent of the temperature (800–1200 °C). Unfortunately, 8-9 mol% YSZ (8YSZ, 8YDZ) also turned out to be situated in the 2-phase field (c+t) of the YSZ phase diagram at these temperatures, which causes the material's decomposition into Y-enriched and depleted regions on the nm-scale and, consequently, the electrical degradation during operation. The microstructural and chemical changes on the nm-scale are accompanied by the drastic decrease of the oxygen-ion conductivity of 8YSZ (degradation of 8YSZ) of about 40% at 950 °C within 2500 hrs. Traces of impurities like Ni, dissolved in the 8YSZ, e.g., due to fuel-cell fabrication, can have a severe impact on the decomposition rate (acceleration of inherent decomposition of the 8YSZ by orders of magnitude) such that the degradation of conductivity even becomes problematic at low operation temperatures in the range of 500–700 °C.
Nowadays, more complex ceramics like co-doped Zirconia (e.g., with Scandia, ...) are in use as solid electrolytes.
Applications.
YSZ has a number of applications: