Yttria-stabilized zirconia (YSZ) is a ceramic which has a structure of zirconium dioxide (ZrO2) and stabilized at room temperature by addition of yttrium oxide (Y2O3) as depicted in figure 4. The addition of yttria to pure zirconia replaces some of the Zr4+ ions in the zirconia lattice with Y3+ ions. This produces oxygen vacancies because three O2− ions replace four O2− ions [6]. It also allows YSZ to conduct O2− ions and therefore conduct an electrical current, provided there is sufficient vacancy site mobility, a property that increases with temperature. This ability to conduct O2− ions makes yttria-stabilized zirconia well suited to be used in solid oxide fuel cells, although it requires high operating temperatures.
Figure 4. YSZ cubic fluorite structure [7]
Pure zirconium dioxide is stable at the room temperature in a monoclinic structure but undergoes phase transformation from monoclinic to tetragonal at around 1 000 °C and then to cubic at about 2 370 °C [8].
YSZ has a number of applications to illustrate. It is used in the production of a solid oxide fuel cell (SOFC) as the solid electrolyte which enables oxygen ion conduction while blocking electronic conduction. An SOFC with YSZ electrolyte must be operated at high temperature (600 °C - 1000 °C), while YSZ can still retain its mechanical robustness at this temperatures[9]. Moreover, it is used as an electroceramic due to its ion-conducting properties for various applications such as determining oxygen content in exhaust gases and measuring pH in high-temperature steam. Furthermore, its hardness make it suitable for jewelry and non-metallic knife blades.