Yttria-stabilized zirconia

Yttria-stabilized zirconia (YSZ) 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" (ZrO₂) and "yttria" (Y₂O₃), hence the name.

Stabilization

See also: Zirconium dioxide § Engineering properties

File:YSZ.jpg

Pure zirconium dioxide undergoes a phase transformation from monoclinic (stable at the room temperature) to tetragonal (at about 1173 °C) and then to cubic (at about 2370 °C), according to the scheme:

monoclinic (1173 °C) $\leftrightarrow$ tetragonal (2370 °C) $\leftrightarrow$ cubic (2690 °C) $\leftrightarrow$ melt

Obtaining stable sintered zirconia ceramic products is difficult because of the large volume change accompanying the transition from tetragonal to monoclinic (about 9%). 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 tetragonal 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.

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 (and thus conduct an electric 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 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 (linearly for low dopant concentrations), then saturates, and then starts to decrease. The maximum ionic conductivity is obtained at Y₂O₃ concentration of about 8% (1000 °C).

Applications

File:Metallfreie Brücke Zirkoniumdioxid 2012 PD 1.JPG

Multiple metal-free dental crowns

YSZ has a number of applications:

• For its hardness and chemical inertness (e.g., tooth crowns).
• As a refractory (e.g., in jet engines).
• As a thermal barrier coating in gas turbines
• As an electroceramic due to its ion-conducting properties (e.g., to determine oxygen content in exhaust gases, to measure pH in high-temperature water, in fuel cells).
• Used in the production of a solid oxide fuel cell (SOFC). 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 (800 °C-1000 °C). 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 (see "cubic zirconia"), 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 (at mildly acidic pH). 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

Further reading

• de:Zirconium(IV)-oxid#Stabilisierung