Yttria-stabilized zirconia
Encyclopedia
Yttria-stabilized zirconia (YSZ) is a zirconium-oxide
based ceramic, in which the particular crystal structure of zirconium oxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" (Zr
O
2) and "yttria" (Y
2O
3), hence the name.
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.
Related materials include calcia-, magnesia
-, ceria- or alumina-stabilized zirconias, or partially-stabilized zirconias (PSZ). Stabilized hafnia
is also known.
Zirconium dioxide
Zirconium dioxide , sometimes known as zirconia , is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral baddeleyite. The high temperature cubic crystalline form is rarely found in nature as mineral tazheranite O2...
based ceramic, in which the particular crystal structure of zirconium oxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" (Zr
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
O
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
2) and "yttria" (Y
Yttrium
Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...
2O
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
3), hence the name.
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.
Applications
YSZ has a number of applications:- For its hardness and chemical inertness (e.g., tooth crownsCrown (dentistry)A crown is a type of dental restoration which completely caps or encircles a tooth or dental implant. Crowns are often needed when a large cavity threatens the ongoing health of a tooth. They are typically bonded to the tooth using a dental cement. Crowns can be made from many materials, which...
). - As a refractoryRefractoryA refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
(e.g., in jet engines). - As a thermal barrier coating in gas turbines
- As an electroceramicElectroceramicsElectroceramics 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 magnetic properties have become of increasing importance...
due to its ion-conducting properties (e.g., to determine oxygen contentOxygen sensorAn oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It was developed by the Robert Bosch GmbH company during the late 1960s under the supervision of Dr. Günter Bauman...
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 electrolyteElectrolyteIn chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
, 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, overall the high temperature necessary is a disadvantage of current SOFCs, and is one of the major barriers to successful commercialization. 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 zirconiaCubic zirconiaCubic zirconia is the cubic crystalline form of zirconium dioxide . The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is a zirconium silicate...
"), it is used as jewelry. - As a material for non-metallic knifeCeramic knifeA ceramic knife is a knife made out of very hard and tough ceramic, often zirconium dioxide . These knives are usually produced by dry pressing zirconia powder and firing them through solid-state sintering. The resultant blade is sharpened by grinding the edges with a diamond-dust-coated grinding...
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 phosphorPhosphor thermometryPhosphor thermometry is an optical method for surface temperature measurement. The method exploits luminescence emitted by phosphor material. Phosphors are fine white or pastel-colored inorganic powders which may be stimulated by any of a variety of means to luminesce, i.e. emit light...
and a luminescent material. - Historically used for glowing rods in Nernst lampNernst lampNernst lamps were an early form of electrically powered incandescent lamps. Nernst lamps did not use a glowing tungsten filament. Instead, they used a ceramic rod that was heated to incandescence...
s.
Related materials include calcia-, magnesia
Magnesium oxide
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
-, ceria- or alumina-stabilized zirconias, or partially-stabilized zirconias (PSZ). Stabilized hafnia
Hafnium(IV) oxide
Hafnium oxide is the inorganic compound with the formula HfO2. Also known as hafnia, this colourless solid is one of the most common and stable compounds of hafnium. It is an electrical insulator with a band gap of approximately 6 eV. Hafnium dioxide is an intermediate in some processes that...
is also known.