Yttria-stabilized zirconia (YSZ) is a
ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, ...
in which the cubic crystal structure of
zirconium dioxide is made stable at room temperature by an addition of
yttrium oxide Yttrium oxide may refer to:
* Yttrium(II) oxide, YO, a dark brown solid
* Yttrium(III) oxide
Yttrium oxide, also known as yttria, is Y2 O3. It is an air-stable, white solid substance.
The thermal conductivity of yttrium oxide is 27 W/(m·K).
...
. These oxides are commonly called "zirconia" (
Zr O2) and "yttria" (
Y2 O3), 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) ↔ tetragonal (2370 °C) ↔ cubic (2690 °C) ↔ melt.
Obtaining stable sintered zirconia ceramic products is difficult because of the large volume change, about 5%, 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 Zr
4+ 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 Y
3+ (ionic radius of 0.96 Å). The resulting doped zirconia materials are termed ''stabilized zirconias''.
[H. Yanagida, K. Koumoto, M. Miyayama, "The Chemistry of Ceramics", John Wiley & Sons, 1996. .]
Materials related to YSZ include
calcia
Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "'' lime''" connotes calcium-containing inorganic m ...
-,
magnesia-,
ceria- or
alumina-stabilized zirconias, or partially stabilized zirconias (PSZ).
Hafnia-stabilized zirconia has about 25% lower
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
, making it more suitable for
thermal barrier applications.
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:
* Partly stabilized zirconia ZrO
2:
** PSZ – ''partially stabilized zirconia''
**
TZP – ''tetragonal zirconia polycrystal''
** 4YSZ: with 4 mol% Y
2O
3 partially stabilized ZrO
2, ''yttria-stabilized zirconia''
* Fully stabilized zirconias ZrO
2:
** FSZ – ''fully stabilized zirconia''
** CSZ – ''cubic stabilized zirconia''
** 8YSZ – with 8 mol% Y
2O
3 fully stabilized ZrO
2
** 8YDZ – 8–9 mol% Y
2O
3-doped ZrO
2: the material is not completely stabilized and decomposes at high application temperatures, see next paragraphs
)
Thermal expansion coefficient
The
thermal expansion coefficient
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kinetic ...
s depends on the modification of zirconia as follows:
* Monoclinic: 7·10
−6/K
[Matweb]
CeramTec 848 Zirconia (ZrO2)
Zirconium Oxide, Zirconia, ZrO2
/ref>
* Tetragonal: 12·10−6/K
* Y2O3 stabilized: 10,5·10−6/K
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:
:
meaning that 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 nanometre scale and, consequently, the electrical degradation during operation. The microstructural and chemical changes on the nanometre 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 hours. 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:
* For its hardness and chemical inertness (e.g., tooth crowns).
* As a refractory
In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...
(e.g., in jet engines).
* As a thermal barrier coating
Thermal barrier coatings (TBCs) are advanced materials systems usually applied to metallic surfaces operating at elevated temperatures, such as gas turbine or aero-engine parts, as a form of exhaust heat management. These 100 μm to 2 mm ...
in gas turbines
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...
.
* As an electroceramic Electroceramics is a class of ceramic materials used primarily for their electrical properties.
While ceramics have traditionally been admired and used for their mechanical, thermal and chemical stability, their unique electrical, optical and mag ...
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
A solid oxide fuel cell (or SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte.
...
(SOFC). YSZ is used as the solid electrolyte
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
, 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 (800–1000 °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
Cubic zirconia (CZ) is the cubic crystalline form of zirconium dioxide (ZrO2). The synthesized material is hard and usually colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is a zirco ...
"), 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
Potassium silicate is the name for a family of inorganic compounds. The most common potassium silicate has the formula K2SiO3, samples of which contain varying amounts of water. These are white solids or colorless solutions.Gerard Lagaly, Werner ...
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 lamp
The Nernst lamp was an early form of incandescent lamp.
Construction
Nernst lamps did not use a glowing tungsten filament. Instead, they used a ceramic rod that was heated to incandescence. Because the rod (unlike tungsten wire) would not ...
s.
* As a high-precision alignment sleeve for optical fiber connector ferrules.
See also
*
*
*
References
Further reading
*
{{DEFAULTSORT:Yttria-Stabilized Zirconia
Zirconium compounds
Refractory materials
Electrochemistry
de:Zirconium(IV)-oxid#Stabilisierung