View source for Yttria-stabilized zirconia

{{short description|Ceramic with room temperature stable cubic crystal structure}}
{{Use dmy dates|date=February 2021}}
[[File:Yttria-stabilized zirconia (YSZ) diagram.jpg|thumb|300 px|Yttria-stabilized zirconia (YSZ) crystal structure]]
'''Yttria-stabilized zirconia''' ('''YSZ''') is a [[ceramic]] in which the cubic crystal structure of [[zirconium dioxide]] is made stable at room temperature by an addition of [[yttrium(III) oxide|yttrium oxide]]. These oxides are commonly called "zirconia" ([[zirconium|Zr]][[oxide|O]]<sub>2</sub>) and "yttria" ([[yttrium|Y]]<sub>2</sub>[[oxide|O]]<sub>3</sub>), hence the name.

== Stabilization ==
{{See also|Zirconium dioxide#Engineering properties}}
Pure zirconium dioxide undergoes a phase transformation from monoclinic (stable at room temperature) to tetragonal (at about 1173&nbsp;°C) and then to cubic (at about 2370&nbsp;°C), according to the scheme

: monoclinic (1173&nbsp;°C) ↔ tetragonal (2370&nbsp;°C) ↔ cubic (2690&nbsp;°C) ↔ melt.

During these transformations, zirconia can experience volume expansion of up to 5-6%.<ref>{{cite journal |last1=Makri |first1=H. |last2=Belhouchet |first2=H. |year=2015 |title=Zirconia transformation in multi-phases ceramic composites |journal=Journal of Australian Ceramic Society Volume |volume=51 |issue=1 |pages=60–72 |url=https://www.researchgate.net/publication/338108929 |access-date=Oct 7, 2024}}</ref> This change can induce internal stresses, leading to cracking or fracture in [[ceramic]] materials.<ref>{{cite web |url=https://www.stanfordmaterials.com/blog/the-role-of-yttrium-oxide-in-advanced-ceramics.html |title=The Role of Yttrium Oxide in Advanced Ceramics |last=Loewen |first=Eric |website=Standford Advanced Materials |access-date=Oct 7, 2024}}</ref>

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<sup>4+</sup> ions (ionic radius of 0.82&nbsp;Å, too small for ideal [[Fluorite structure|lattice of fluorite]] characteristic for the cubic zirconia) in the crystal lattice with slightly larger ions, e.g., those of Y<sup>3+</sup> (ionic radius of 0.96&nbsp;Å). The resulting doped zirconia materials are termed ''stabilized zirconias''.<ref name="Yan">H. Yanagida, K. Koumoto, M. Miyayama, "The Chemistry of Ceramics", John Wiley & Sons, 1996. {{ISBN|0 471 95627 9}}.</ref>

Materials related to YSZ include [[calcia]]-, [[magnesium oxide|magnesia]]-, [[ceria]]- or [[alumina]]-stabilized zirconias, or partially stabilized zirconias (PSZ). [[Hafnium(IV) oxide|Hafnia]]-stabilized zirconia has about 25% lower [[thermal conductivity]], making it more suitable for [[thermal barrier]] applications.<ref>{{cite journal | last1=Winter | first1=Michael R. | last2=Clarke | first2=David R. | title=Thermal conductivity of yttria-stabilized zirconia–hafnia solid solutions | journal=Acta Materialia| volume=54 | issue=19 | year=2006 | issn=1359-6454 | doi=10.1016/j.actamat.2006.06.038 | pages=5051–5059| bibcode=2006AcMat..54.5051W }}</ref>

Although 8–9&nbsp;[[Mole fraction|mol]]% YSZ is known to not be completely stabilized in the pure cubic YSZ phase up to temperatures above 1000&nbsp;°C.<ref name=":0" /> 

Commonly used abbreviations in conjunction with yttria-stabilized zirconia are:

* Partly stabilized zirconia ZrO<sub>2</sub>:
** PSZ – ''partially stabilized zirconia''
** [[Tetragonal polycrystalline zirconia|TZP – ''tetragonal zirconia polycrystal'']]
** 4YSZ: with 4&nbsp;mol% Y<sub>2</sub>O<sub>3</sub> partially stabilized ZrO<sub>2</sub>, ''yttria-stabilized zirconia''
* Fully stabilized zirconias ZrO<sub>2</sub>:
** FSZ – ''fully stabilized zirconia''
** CSZ – ''cubic stabilized zirconia''
** 8YSZ – with 8&nbsp;mol% Y<sub>2</sub>O<sub>3</sub> fully stabilized ZrO<sub>2</sub>
** 8YDZ – 8–9&nbsp;mol% Y<sub>2</sub>O<sub>3</sub>-doped ZrO<sub>2</sub>: the material is not completely stabilized and decomposes at high application temperatures, see next paragraphs<ref name=":0" /><ref name=":1" /><ref name=":2" />)

== Thermal expansion coefficient ==
The [[thermal expansion coefficient]]s depends on the modification of zirconia as follows:
* Monoclinic: 7·10<sup>−6</sup>/K<ref name="matweb">Matweb: [http://www.matweb.com/search/DataSheet.aspx?MatGUID=05e29689151440d3904052baa788ae22 CeramTec 848 Zirconia (ZrO<sub>2</sub>)] & [http://www.matweb.com/search/datasheet.aspx?MatGUID=0742ddaddf80467fb6532e025c694e89 Zirconium Oxide, Zirconia, ZrO<sub>2</sub>]</ref>
* Tetragonal: 12·10<sup>−6</sup>/K<ref name="matweb"/>
* Y<sub>2</sub>O<sub>3</sub> stabilized: 10,5·10<sup>−6</sup>/K<ref name="matweb"/>

== Ionic conductivity and degradation ==
By the addition of yttria to pure zirconia (e.g., fully stabilized YSZ) Y<sup>3+</sup> ions replace Zr<sup>4+</sup> on the cationic sublattice. Thereby, oxygen vacancies are generated due to charge neutrality:<ref>{{Cite journal |last=Hund |first=F. |year=1951 |title=Anomale Mischkristalle im System ZrO<sub>2</sub>–Y<sub>2</sub>O<sub>3</sub>. Kristallbau der Nernst-Stifte |lang=de |journal=Zeitschrift für Elektrochemie und Angewandte Physikalische Chemie |volume=55 |issue=5 |pages=363–366|doi=10.1002/bbpc.19510550505 }}</ref>

: <math>\text{Y}_2\text{O}_3 \to 2\text{Y}_\text{Zr}' + 3\text{O}_\text{O}^\text{x} + \text{V}_\text{O}^{\bullet\bullet}
 \text{ with } [\text{V}_\text{O}^{\bullet\bullet}] = \frac{1}{2}[\text{Y}_\text{Zr}'],
</math>

meaning that two Y<sup>3+</sup> ions generate one vacancy on the anionic sublattice. This facilitates moderate conductivity of yttrium-stabilized zirconia for O<sup>2−</sup> ions (and thus electrical conductivity) at elevated and high temperature. This ability to conduct O<sup>2−</sup> 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 Y<sub>2</sub>O<sub>3</sub> content. It has a maximum around 8–9&nbsp;mol% almost independent of the temperature (800–1200&nbsp;°C).<ref name="Yan" /><ref name=":0">{{Cite book |url=https://publikationen.bibliothek.kit.edu/1000015724 |title=Yttria-doped zirconia as solid electrolyte for fuel-cell applications: Fundamental aspects |last=Butz |first=Benjamin |publisher=Südwestdt. Verl. für Hochschulschr. |year=2011 |isbn=978-3-8381-1775-1}}</ref> Unfortunately, 8–9&nbsp;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.<ref name=":1">{{Cite journal |last1=Butz |first1=B. |last2=Schneider |first2=R. |last3=Gerthsen |first3=D. |last4=Schowalter |first4=M. |last5=Rosenauer |first5=A. |date=1 October 2009 |title=Decomposition of 8.5 mol.% Y2O3-doped zirconia and its contribution to the degradation of ionic conductivity |journal=Acta Materialia |volume=57 |issue=18 |pages=5480–5490 |doi=10.1016/j.actamat.2009.07.045}}</ref> 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&nbsp;°C within 2500&nbsp;hours.<ref name=":2">{{Cite journal |last1=Butz |first1=B. |last2=Kruse |first2=P. |last3=Störmer |first3=H. |last4=Gerthsen |first4=D. |last5=Müller |first5=A. |last6=Weber |first6=A. |last7=Ivers-Tiffée |first7=E. |date=1 December 2006 |title=Correlation between microstructure and degradation in conductivity for cubic Y<sub>2</sub>O<sub>3</sub>-doped ZrO<sub>2</sub> |journal=Solid State Ionics |volume=177 |issue=37–38 |pages=3275–3284 |doi=10.1016/j.ssi.2006.09.003}}</ref> 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&nbsp;°C.<ref>{{Cite journal |last1=Butz |first1=B. |last2=Lefarth |first2=A. |last3=Störmer |first3=H. |last4=Utz |first4=A. |last5=Ivers-Tiffée |first5=E. |last6=Gerthsen |first6=D. |date=25 April 2012 |title=Accelerated degradation of 8.5 mol% Y<sub>2</sub>O<sub>3</sub>-doped zirconia by dissolved Ni |journal=Solid State Ionics |volume=214 |pages=37–44 |doi=10.1016/j.ssi.2012.02.023}}</ref>

Nowadays, more complex ceramics like co-doped zirconia (e.g., with scandia) are in use as solid electrolytes.

==Applications==
[[File:Metallfreie Brücke Zirkoniumdioxid 2012 PD 1.JPG|thumb|right|Multiple metal-free dental crowns]]
YSZ has a number of applications:
* For its hardness and chemical inertness (e.g., [[Crown (dentistry)|tooth crowns]]).
* As a [[refractory]] (e.g., in jet engines).
* As a [[thermal barrier coating]] in [[gas turbines]].
* As an [[electroceramics|electroceramic]] due to its ion-conducting properties (e.g., to [[Oxygen sensor|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 (800–1000&nbsp;°C).<ref>{{Cite journal |last1=Song |first1=B. |last2=Ruiz-Trejo |first2=E. |last3=Brandon |first3=N. P. |date=August 2018 |title=Enhanced mechanical stability of Ni-YSZ scaffold demonstrated by nanoindentation and Electrochemical Impedance Spectroscopy |journal=Journal of Power Sources |volume=395 |pages=205–211 |doi=10.1016/j.jpowsour.2018.05.075 |bibcode=2018JPS...395..205S |doi-access=free|hdl=10044/1/60309 |hdl-access=free }}</ref> 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.<ref>{{cite journal |last1=Minh |first1=N. Q. |year=1993 |title=Ceramic Fuel-Cells |journal=Journal of the American Ceramic Society |volume=76 |issue=3 |pages=563–588  | doi = 10.1111/j.1151-2916.1993.tb03645.x }}</ref><ref>{{cite report |last1=De Guire |first1=Eileen |year=2003 |title=Solid Oxide Fuel Cells |publisher=CSA |url=http://www.csa.com/discoveryguides/fuecel/overview.php}}</ref>
* For its hardness and optical properties in monocrystal form (see "[[cubic zirconia]]"), it is used as jewelry.
* As a material for [[ceramic knife|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 [[Phosphor thermometry|thermographic phosphor]] and a luminescent material.<ref>{{cite book |author=American Ceramic Society |title=Progress in Thermal Barrier Coatings |url=https://books.google.com/books?id=dqCNlgDsu0EC&pg=PA139 |access-date=23 October 2011 |date=29 May 2009 |publisher=John Wiley and Sons |isbn=978-0-470-40838-4 |pages=139–}}</ref>
* Historically used for glowing rods in [[Nernst lamp]]s.
* As a high-temperature coating, produced by ZYP Coatings, Inc.
* As a high-precision alignment sleeve for optical fiber connector ferrules.<ref>{{Cite web |url=http://www.diamond-fo.com/en/products_catalogue_details.asp?section=2&group=e2000&source=Assemblies&family=10101 |website=DIAMOND SA |title=Fiber Optic Interconnect Solutions}}</ref>

==See also==
*{{annotated link|Cubic zirconia}}
*{{annotated link|Sintering}}
*{{annotated link|Superconducting wire}}

==References==
{{reflist}}

==Further reading==
* {{cite book |last = Green |first = D. J. |author2=Hannink, R. |author3=Swain, M. V. |year = 1989 |title = Transformation Toughening of Ceramics |location = Boca Raton |publisher = CRC Press |isbn = 978-0-8493-6594-2}}

{{DEFAULTSORT:Yttria-Stabilized Zirconia}}
[[Category:Zirconium dioxide]]
[[Category:Refractory materials]]
[[Category:Electrochemistry]]
[[Category:Yttrium compounds]]

[[de:Zirconium(IV)-oxid#Stabilisierung]]