Yttria-stabilized zirconia is a ceramic in which the crystal structure of zirconium dioxide
is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly
called "zirconia" and "yttria", hence the name.
Stabilization
Pure zirconium dioxide undergoes a phase transformation from monoclinic to tetragonal and then to
cubic, according to the scheme: monoclinic tetragonal cubic melt
Obtaining stable sintered zirconia ceramic products is difficult because of the large
volume change accompanying the transition from tetragonal to monoclinic. Stabilization
of the cubic polymorph of zirconia over wider range of temperatures is accomplished by substitution
of some of the Zr4+ ions in the crystal lattice with slightly larger ions, e.g., those of
Y3+. 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. Hafnia stabilized Zirconia is also known.
Some of the abbreviations used in conjunction with stabilized zirconias are as follow:
Partly stabilized zirconia ZrO2: PSZ - Partially Stabilized Zirconia
TZP -Tetragonal Zirconia Polycrystal 4YSZ: with 4 mol-% Y2O3 partially Stabilized
ZrO2, Yttria Stabilized Zirconia
Fully stabilized zirconias ZrO2: FSZ - Fully Stabilized Zirconia
CSZ - Cubic Stabilized Zirconia 8YSZ - with 8 mol-% Y2O3 Fully Stabilized
ZrO2
Thermal expansion coefficient The thermal expansion coefficients depends
on the modification of zirconia as follows: Monoclinic: 7 · 10−6/K
Tetragonal: 12 · 10−6/K Y2O3 stabilized: 10.5 · 10−6/K
Effect of stabilization of ionic conductivity The addition of yttria to pure zirconia replaces
some of the Zr4+ ions in the zirconia lattice with Y3+ ions. This produces oxygen vacancies,
as three O2- ions replace four O2- ions. It also permits yttrium stabilized zirconia to
conduct O2- ions, 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 use in solid oxide fuel cells, although it requires that they operate at
high enough temperatures. The ionic conductivity of the stabilized zirconias
increases with increasing dopant concentration, then saturates, and then starts to decrease.
The maximum ionic conductivity is obtained at Y2O3 concentration of about 8%.
Applications
YSZ has a number of applications: For its hardness and chemical inertness.
As a refractory. As a thermal barrier coating in gas turbines
As an electroceramic due to its ion-conducting properties.
Used in the production of a solid oxide fuel cell. YSZ is used as the solid electrolyte,
which enables oxygen ion conduction while blocking electronic conduction. In order to
achieve sufficient ion conduction, an SOFC with a YSZ electrolyte must be operated at
high temperatures. While it is advantageous that YSZ retains mechanical robustness at
those temperatures, the high temperature necessary is often a disadvantage of SOFCs. The high
density of YSZ is also necessary in order to physically separate the gaseous fuel from
oxygen, or else the electrochemical system would produce no electrical power.
For its hardness and optical properties in monocrystal form, it is used as jewelry.
As a material for non-metallic knife blades, produced by Boker and Kyocera companies.
In water-based pastes for do-it-yourself ceramics and cements. These contain microscopic YSZ
milled fibers or sub-micrometer particles, often with potassium silicate and zirconium
acetate binders. The cementation occurs on removal of water. The resulting ceramic material
is suitable for very high temperature applications. YSZ doped with rare-earth materials can act
as a thermographic phosphor and a luminescent material.
Historically used for glowing rods in Nernst lamps.
As a high precision alignment sleeve for optical fiber connector ferrules.
See also Cubic zirconia
Sintering References
Further reading Green, D.J.; Hannink, R.; Swain, M.V.. Transformation
Toughening of Ceramics. Boca Raton: CRC Press. ISBN 0-8493-6594-5.