Editing Piezoelectricity (section)

==Materials==<!-- This section is linked from [[Crystal Oscillator]] -->
{{See also|List of piezoelectric materials}}
Many materials exhibit piezoelectricity. Examples include:
<!--===Bone===
Dry [[bone]] exhibits some [[Piezoelectric material properties|piezoelectric properties]]. Studies of Fukada ''et al.'' showed that these are not due to the [[apatite]] crystals, which are centrosymmetric, thus non-piezoelectric, but due to [[collagen]]. Collagen exhibits the polar uniaxial orientation of molecular dipoles in its structure and can be considered as bioelectret, a sort of dielectric material exhibiting quasipermanent space charge and dipolar charge. Potentials are thought to occur when a number of collagen molecules are stressed in the same way displacing significant numbers of the charge carriers from the inside to the surface of the specimen. Piezoelectricity of single individual collagen fibrils was measured using piezoresponse force microscopy, and it was shown that collagen fibrils behave predominantly as shear piezoelectric materials.<ref>{{cite journal |volume=20 |year=2009|page=085706|last1=Minary-Jolandan |first1=M. |last2=Yu |first2=Min-Feng |name-list-style=amp |journal=Nanotechnology |doi=10.1088/0957-4484/20/8/085706 |pmid=19417467 |title=Nanoscale characterization of isolated individual type I collagen fibrils: Polarization and piezoelectricity |issue=8|bibcode = 2009Nanot..20h5706M }}</ref>

The piezoelectric effect is generally thought to act as a biological force sensor.<ref>{{cite web |url=http://silver.neep.wisc.edu/~lakes/BoneElectr.html |title=Electrical Properties of Bone: A Review |first=Roderic |last=Lakes |publisher=[[University of Wisconsin–Madison]] |url-status=live |archive-url=https://web.archive.org/web/20131105105946/http://silver.neep.wisc.edu/~lakes/BoneElectr.html |archive-date=2013-11-05 }}</ref><ref>{{cite book |first=Robert O. |last=Becker |last2=Marino |first2=Andrew A. |title=Electromagnetism & Life |chapter=Chapter 4: Electrical Properties of Biological Tissue (Piezoelectricity) |publisher=State University of New York Press |location=Albany, NY |isbn=978-0-87395-560-7 |chapter-url=http://www.ortho.lsuhsc.edu/Faculty/Marino/EL/EL4/Piezo.html |year=1982 |archive-url=https://web.archive.org/web/20090802194455/http://www.ortho.lsuhsc.edu/Faculty/Marino/EL/EL4/Piezo.html |archive-date=2009-08-02 }}</ref> This effect was exploited by research conducted at the [[University of Pennsylvania]] in the late 1970s and early 1980s, which established that sustained application of electrical potential could stimulate both resorption and growth (depending on the polarity) of bone ''in vivo''.<ref>{{cite book|last1=Pollack |first1=S. R. |last2=Korostoff |first2=E. |last3=Starkebaum |first3=W. |last4=Lannicone |first4=W. |year=1979 |chapter=Micro-Electrical Studies of Stress-Generated Potentials in Bone|isbn=978-0-8089-1228-6 |title=Electrical Properties of Bone and Cartilage |editor1-last=Brighton |editor1-first=C. T. |editor2-last=Black |editor2-first=J. |editor3-last=Pollack |editor3-first=S. R. |publisher=Grune & Stratton|location=New York}}</ref> Further studies in the 1990s provided the mathematical equation to confirm long bone wave propagation as to that of hexagonal (Class 6) crystals.<ref>{{cite journal |last=Fotiadis |first=D. I. |last2=Foutsitzi |first2=G. |last3=Massalas |first3=C. V. |year=1999 |title=Wave propagation modeling in human long bones |journal=Acta Mechanica |volume=137 |issue=1–2 |pages=65–81 |doi=10.1007/BF01313145}}</ref>-->

===Crystalline materials===
* [[Langasite]] (La<sub>3</sub>Ga<sub>5</sub>SiO<sub>14</sub>)&nbsp;– a quartz-analogous crystal
* [[Gallium orthophosphate]] (GaPO<sub>4</sub>)&nbsp;– a quartz-analogous crystal
* [[Lithium niobate]] (LiNbO<sub>3</sub>)
* [[Lithium tantalate]] (LiTaO<sub>3</sub>)
* [[Quartz]]
* [[Berlinite]] (AlPO<sub>4</sub>)&nbsp;– a rare [[phosphate]] [[mineral]] that is structurally identical to quartz
* [[Potassium sodium tartrate|Rochelle salt]]
* [[Topaz]]&nbsp;– piezoelectricity in topaz can probably be attributed to ordering of the (F,OH) in its lattice, which is otherwise centrosymmetric: orthorhombic bipyramidal (mmm). Topaz has anomalous optical properties, which are attributed to such ordering.<ref>{{cite journal |last1=Akizuki |first1=Mizuhiko |first2=Martin S. |last2=Hampar |first3=Jack |last3=Zussman |year=1979 |title=An explanation of anomalous optical properties of topaz |journal=Mineralogical Magazine |volume=43 |pages=237–241 |doi=10.1180/minmag.1979.043.326.05 |issue=326 |bibcode=1979MinM...43..237A }}</ref>
* [[Tourmaline]]-group minerals
* [[Lead titanate]] (PbTiO<sub>3</sub>)&nbsp;– although it occurs in nature as mineral macedonite,<ref>{{cite journal |last1=Radusinović |first1=Dušan |last2=Markov |first2=Cvetko |name-list-style=amp |year=1971 |title=Macedonite – lead titanate: a new mineral |journal=American Mineralogist |volume=56 |pages=387–394 |url=http://www.minsocam.org/ammin/AM56/AM56_387.pdf |url-status=live |archive-url=https://web.archive.org/web/20160305091225/http://www.minsocam.org/ammin/AM56/AM56_387.pdf |archive-date=2016-03-05 }}</ref><ref>{{cite journal|last1=Burke |first1=E. A. J. |last2=Kieft |first2=C. |year=1971|title=Second occurrence of makedonite, PbTiO<sub>3</sub>, Långban, Sweden |journal=Lithos |doi=10.1016/0024-4937(71)90102-2 |volume=4 |issue=2 |pages=101–104 |bibcode=1971Litho...4..101B }}</ref> it is synthesized for research and applications.

===Ceramics===
[[Image:Perovskite.svg|thumb|Tetragonal unit cell of lead titanate]]
Ceramics with randomly oriented grains must be ferroelectric to exhibit piezoelectricity.<ref>{{cite book|first1=B. |last1=Jaffe |first2=W. R. |last2=Cook |first3=H. |last3=Jaffe |title=Piezoelectric Ceramics |location=New York |publisher=Academic |date=1971}}{{ISBN missing}}</ref> The occurrence of [[abnormal grain growth]] (AGG) in sintered polycrystalline piezoelectric ceramics has detrimental effects on the piezoelectric performance in such systems and should be avoided, as the microstructure in piezoceramics exhibiting AGG tends to consist of few abnormally large elongated grains in a matrix of randomly oriented finer grains. Macroscopic piezoelectricity is possible in textured polycrystalline non-ferroelectric piezoelectric materials, such as AlN and ZnO.
The families of ceramics with [[Perovskite (structure)|perovskite]], [[tungsten]]-[[bronze]], and related structures exhibit piezoelectricity:
* [[Lead zirconate titanate]] ({{chem2|auto=1|Pb&#91;Zr_{''x''}Ti_{1−''x''}]O3}} with 0&nbsp;≤&nbsp;''x''&nbsp;≤&nbsp;1)&nbsp;– more commonly known as PZT, the most common piezoelectric ceramic in use today.
* [[Potassium niobate]] (KNbO<sub>3</sub>)<ref>{{Cite journal|last1=Ganeshkumar|first1=Rajasekaran|last2=Somnath|first2=Suhas|last3=Cheah|first3=Chin Wei|last4=Jesse|first4=Stephen|last5=Kalinin|first5=Sergei V.|last6=Zhao|first6=Rong|date=2017-12-06|title=Decoding Apparent Ferroelectricity in Perovskite Nanofibers|journal=ACS Applied Materials & Interfaces|volume=9|issue=48|pages=42131–42138|doi=10.1021/acsami.7b14257|pmid=29130311 }}</ref>
* [[Sodium tungstate]] (Na<sub>2</sub>WO<sub>3</sub>)
* Ba<sub>2</sub>NaNb<sub>5</sub>O<sub>5</sub>
* Pb<sub>2</sub>KNb<sub>5</sub>O<sub>15</sub>
* [[Zinc oxide]] (ZnO)&nbsp;– [[Wurtzite crystal structure|Wurtzite structure]]. While single crystals of ZnO are piezoelectric and pyroelectric, polycrystalline (ceramic) ZnO with randomly oriented grains exhibits neither piezoelectric nor pyroelectric effect. Not being ferroelectric, polycrystalline ZnO cannot be poled like barium titanate or PZT. Ceramics and polycrystalline thin films of ZnO may exhibit macroscopic piezoelectricity and pyroelectricity only if they are [[Texture (crystalline)|textured]] (grains are preferentially oriented), such that the piezoelectric and pyroelectric responses of all individual grains do not cancel. This is readily accomplished in polycrystalline thin films.<ref name="DD1998" />

===Lead-free piezoceramics===
* [[Sodium potassium niobate]] ((K,Na)NbO<sub>3</sub>). This material is also known as NKN or KNN. In 2004, a group of Japanese researchers led by Yasuyoshi Saito discovered a sodium potassium niobate composition with properties close to those of PZT, including a high ''T''<sub>C</sub>.<ref>{{cite journal |title=Lead-free piezoceramics |first1=Yasuyoshi |last1=Saito |last2=Takao |first2=Hisaaki |last3=Tanil |first3=Toshihiko |last4=Nonoyama |first4=Tatsuhiko |last5=Takatori |first5=Kazumasa |last6=Homma |first6=Takahiko |last7=Nagaya |first7=Toshiatsu |last8=Nakamura |first8=Masaya |journal=[[Nature (journal)|Nature]] |volume=432 |issue=7013 |pages=81–87 |doi=10.1038/nature03028 |pmid=15516921 |date=2004-11-04 |bibcode=2004Natur.432...84S }}</ref> Certain compositions of this material have been shown to retain a high mechanical quality factor (''Q''<sub>m</sub>&nbsp;≈&nbsp;900) with increasing vibration levels, whereas the mechanical quality factor of hard PZT degrades in such conditions. This fact makes NKN a promising replacement for high power resonance applications, such as piezoelectric transformers.<ref name="GurdalUral2011">{{cite journal|last1=Gurdal|first1=Erkan A.|last2=Ural|first2=Seyit O.|last3=Park|first3=Hwi-Yeol|last4=Nahm|first4=Sahn|last5=Uchino|first5=Kenji|title=High Power (Na<sub>0.5</sub>K<sub>0.5</sub>)NbO<sub>3</sub>-Based Lead-Free Piezoelectric Transformer|journal=Japanese Journal of Applied Physics|volume=50|issue=2|year=2011|page=027101 |doi=10.1143/JJAP.50.027101|bibcode = 2011JaJAP..50b7101G }}</ref>
* [[Bismuth ferrite]] (BiFeO<sub>3</sub>) &nbsp;– a promising candidate for the replacement of lead-based ceramics.
* Sodium niobate (NaNbO<sub>3</sub>)
* [[Barium titanate]] (BaTiO<sub>3</sub>)&nbsp;– Barium titanate was the first piezoelectric ceramic discovered.
* [[Bismuth titanate]] (Bi<sub>4</sub>Ti<sub>3</sub>O<sub>12</sub>)
* [[Sodium bismuth titanate]] (NaBi(TiO<sub>3</sub>)<sub>2</sub>)
The fabrication of lead-free piezoceramics pose multiple challenges, from an environmental standpoint and their ability to replicate the properties of their lead-based counterparts. By removing the lead component of the piezoceramic, the risk of toxicity to humans decreases, but the mining and extraction of the materials can be harmful to the environment.<ref>{{cite journal |last1=Ibn-Mohammed |first1=T. |last2=Koh |first2=S. C. L. |last3=Reaney |first3=I. M. |last4=Sinclair |first4=D. C. |last5=Mustapha |first5=K. B. |last6=Acquaye |first6=A. |last7=Wang |first7=D. |title=Are lead-free piezoelectrics more environmentally friendly? |journal=MRS Communications |date=March 2017 |volume=7 |issue=1 |pages=1–7 |doi=10.1557/mrc.2017.10 }}</ref> Analysis of the environmental profile of PZT versus sodium potassium niobate (NKN or KNN) shows that across the four indicators considered (primary energy consumption, toxicological footprint, eco-indicator 99, and input-output upstream greenhouse gas emissions), KNN is actually more harmful to the environment. Most of the concerns with KNN, specifically its Nb<sub>2</sub>O<sub>5</sub> component, are in the early phase of its life cycle before it reaches manufacturers. Since the harmful impacts are focused on these early phases, some actions can be taken to minimize the effects. Returning the land as close to its original form after Nb<sub>2</sub>O<sub>5</sub> mining via dam deconstruction or replacing a stockpile of utilizable soil are known aids for any extraction event. For minimizing air quality effects, modeling and simulation still needs to occur to fully understand what mitigation methods are required. The extraction of lead-free piezoceramic components has not grown to a significant scale at this time, but from early analysis, experts encourage caution when it comes to environmental effects.

Fabricating lead-free piezoceramics faces the challenge of maintaining the performance and stability of their lead-based counterparts. In general, the main fabrication challenge is creating the "morphotropic phase boundaries (MPBs)" that provide the materials with their stable piezoelectric properties without introducing the "polymorphic phase boundaries (PPBs)" that decrease the temperature stability of the material.<ref>{{cite journal |last1=Wu |first1=Jiagang |title=Perovskite lead-free piezoelectric ceramics |journal=Journal of Applied Physics |date=21 May 2020 |volume=127 |issue=19 |doi=10.1063/5.0006261 |bibcode=2020JAP...127s0901W |doi-access=free }}</ref> New phase boundaries are created by varying additive concentrations so that the [[phase transition]] temperatures converge at room temperature. The introduction of the MPB improves piezoelectric properties, but if a PPB is introduced, the material becomes negatively affected by temperature. Research is ongoing to control the type of phase boundaries that are introduced through phase engineering, diffusing phase transitions, domain engineering, and chemical modification.

===III–V and II–VI semiconductors===
A piezoelectric potential can be created in any bulk or nanostructured semiconductor crystal having non central symmetry, such as the [[List of semiconductor materials#Types of semiconductor materials|Group]] [[Boron group|III]]–[[Nitrogen group|V]] and [[Group 12 element|II]]–[[Chalcogen|VI]] materials, due to polarization of ions under applied stress and strain. This property is common to both the [[zincblende]] and [[wurtzite]] crystal structures. To first order, there is only one independent piezoelectric coefficient in [[zincblende]], called e<sub>14</sub>, coupled to shear components of the strain. In [[wurtzite]], there are instead three independent piezoelectric coefficients: ''e''<sub>31</sub>, ''e''<sub>33</sub> and ''e''<sub>15</sub>.
The semiconductors where the strongest piezoelectricity is observed are those commonly found in the [[wurtzite]] structure, i.e. [[GaN]], [[InN]], [[AlN]] and [[ZnO]] (see [[piezotronics]]).

Since 2006, there have also been a number of reports of strong [[non linear piezoelectric effects in polar semiconductors]].<ref>{{cite conference |title=A Review of Non Linear Piezoelectricity in Semiconductors |first=Max |last=Migliorato |conference=AIP Conference Proceedings|volume=1590 |issue=N/A |pages=32–41 |doi=10.1063/1.4870192|display-authors=etal|series=AIP Conference Proceedings |year=2014 |bibcode=2014AIPC.1590...32M |doi-access=free }}</ref>
Such effects are generally recognized to be at least important if not of the same order of magnitude as the first order approximation.

===Polymers===

The piezo-response of [[polymer]]s is not as high as the response for ceramics; however, polymers hold properties that ceramics do not. Over the last few decades, non-toxic, piezoelectric polymers have been studied and applied due to their flexibility and smaller [[Acoustic impedance|acoustical impedance]].<ref name=":0">{{Cite book |title=Piezoelectricity : evolution and future of a technology |date=2008 |publisher=Springer |editor-last1=Heywang |editor-first1=Walter |editor-last2=Lubitz |editor-first2=Karl |editor-last3=Wersing |editor-first3=Wolfram |isbn=978-3540686835 |location=Berlin |oclc=304563111}}</ref> Other properties that make these materials significant include their [[biocompatibility]], [[Biodegradation|biodegradability]], low cost, and low power consumption compared to other piezo-materials (ceramics, etc.).<ref name=":1">{{Cite journal |last1=Sappati |first1=Kiran |last2=Bhadra |first2=Sharmistha |last3=Sappati |first3=Kiran Kumar |last4=Bhadra |first4=Sharmistha |date=2018 |title=Piezoelectric Polymer and Paper Substrates: A Review |journal=Sensors |volume=18 |issue=11 |pages=3605 |doi=10.3390/s18113605 |pmid=30355961 |pmc=6263872 |bibcode=2018Senso..18.3605S |doi-access=free }}</ref> Piezoelectric polymers and non-toxic polymer composites can be used given their different physical properties.

Piezoelectric polymers can be classified by bulk polymers, voided charged polymers ("piezoelectrets"), and polymer composites. A piezo-response observed by bulk polymers is mostly due to its molecular structure. There are two types of bulk polymers: [[Amorphous solid|amorphous]] and [[Semi-crystalline polymer|semi-crystalline]]. Examples of semi-crystalline polymers are [[polyvinylidene fluoride]] (PVDF) and its [[copolymer]]s, [[polyamide]]s, and [[Parylene|parylene-C]]. Non-crystalline polymers, such as [[polyimide]] and [[polyvinylidene chloride]] (PVDC), fall under amorphous bulk polymers. Voided charged polymers exhibit the piezoelectric effect due to charge induced by poling of a porous polymeric film. Under an electric field, charges form on the surface of the voids forming dipoles. Electric responses can be caused by any deformation of these voids. The piezoelectric effect can also be observed in polymer composites by integrating piezoelectric ceramic particles into a polymer film. A polymer does not have to be piezo-active to be an effective material for a polymer composite.<ref name=":1" /> In this case, a material could be made up of an inert matrix with a separate piezo-active component.

PVDF exhibits piezoelectricity several times greater than quartz. The piezo-response observed from PVDF is about 20–30 pC/N. That is an order of 5–50 times less than that of piezoelectric ceramic lead zirconate titanate (PZT).<ref name=":0" /><ref name=":1" /> The thermal stability of the piezoelectric effect of polymers in the PVDF family (i.e. vinylidene fluoride co-poly trifluoroethylene) goes up to 125&nbsp;°C. Some applications of PVDF are pressure sensors, hydrophones, and shock wave sensors.<ref name=":0" />

Due to their flexibility, piezoelectric composites have been proposed as energy harvesters and nanogenerators. In 2018, it was reported by Zhu et al. that a piezoelectric response of about 17&nbsp;pC/N could be obtained from PDMS/PZT nanocomposite at 60% porosity.<ref>{{Cite journal |last1=Ma |first1=Si Wei |last2=Fan |first2=You Jun |last3=Li |first3=Hua Yang |last4=Su |first4=Li |last5=Wang |first5=Zhong Lin |last6=Zhu |first6=Guang |date=2018-09-07 |title=Flexible Porous Polydimethylsiloxane/Lead Zirconate Titanate-Based Nanogenerator Enabled by the Dual Effect of Ferroelectricity and Piezoelectricity |journal=ACS Applied Materials & Interfaces |volume=10 |issue=39 |pages=33105–33111 |doi=10.1021/acsami.8b06696 |pmid=30191707 }}</ref> Another PDMS nanocomposite was reported in 2017, in which BaTiO<sub>3</sub> was integrated into PDMS to make a stretchable, transparent nanogenerator for self-powered physiological monitoring.<ref>{{Cite journal |last1=Chen |first1=Xiaoliang |last2=Parida |first2=Kaushik |last3=Wang |first3=Jiangxin |last4=Xiong |first4=Jiaqing |last5=Lin |first5=Meng-Fang |last6=Shao |first6=Jinyou |last7=Lee |first7=Pooi See |date=2017-11-20 |title=A Stretchable and Transparent Nanocomposite Nanogenerator for Self-Powered Physiological Monitoring |journal=ACS Applied Materials & Interfaces |volume=9 |issue=48 |pages=42200–42209 |doi=10.1021/acsami.7b13767 |pmid=29111642 }}</ref> In 2016, polar molecules were introduced into a polyurethane foam in which high responses of up to 244&nbsp;pC/N were reported.<ref>{{Cite journal |last1=Moody |first1=M. J. |last2=Marvin |first2=C. W. |last3=Hutchison |first3=G. R. |date=2016 |title=Molecularly-doped polyurethane foams with massive piezoelectric response |journal=Journal of Materials Chemistry C |volume=4 |issue=20 |pages=4387–4392 |doi=10.1039/c6tc00613b }}</ref>

===Other materials===
Most materials exhibit at least weak piezoelectric responses. Trivial examples include [[sucrose]] (table sugar), [[DNA]], viral proteins, including those from [[bacteriophage]].<ref>{{cite journal|last1=Lee|first1=B. Y.|last2=Zhang |first2=J. |last3=Zueger |first3=C. |last4=Chung |first4=W. J. |last5=Yoo |first5=S. Y. |last6=Wang |first6=E. |last7=Meyer |first7=J. |last8=Ramesh |first8=R. |last9= Lee |first9=S. W. |title=Virus-based piezoelectric energy generation|journal=Nature Nanotechnology|date=2012-05-13|pmid=22581406|doi=10.1038/nnano.2012.69|volume=7|issue=6|pages=351–356|bibcode = 2012NatNa...7..351L }}</ref><ref>{{cite journal |title=Stable and Optoelectronic Dipeptide Assemblies for Power Harvesting |first1=Kai |last1=Tao |last2=et |first2=al |journal=Materials Today |volume=30 |pages=10–16 |doi=10.1016/j.mattod.2019.04.002 |pmid=31719792 |pmc=6850901 |year=2019 }}</ref> An actuator based on wood fibers, called [[cellulose fiber]]s, has been reported.<ref name=":1" /> D33 responses for cellular polypropylene are around 200 pC/N. Some applications of cellular polypropylene are musical key pads, microphones, and ultrasound-based echolocation systems.<ref name=":0" /> Recently, single amino acid such as β-glycine also displayed high piezoelectric (178 pmV<sup>−1</sup>) as compared to other biological materials.<ref>{{Cite journal|last1=Guerin|first1=Sarah|last2=Stapleton|first2=Aimee|last3=Chovan|first3=Drahomir|last4=Mouras|first4=Rabah|last5=Gleeson|first5=Matthew|last6=McKeown|first6=Cian|last7=Noor|first7=Mohamed Radzi|last8=Silien|first8=Christophe|last9=Rhen|first9=Fernando M. F.|last10=Kholkin|first10=Andrei L.|last11=Liu|first11=Ning|date=February 2018|title=Control of piezoelectricity in amino acids by supramolecular packing |journal=Nature Materials|language=en|volume=17|issue=2|pages=180–186|doi=10.1038/nmat5045|pmid=29200197 }}</ref>

[[Ionic liquid]]s were recently identified as the first piezoelectric liquid.<ref>{{cite news |last1=Choi |first1=Charles Q. |title=Liquid Salts Bring Push-button Lenses Into Focus – IEEE Spectrum |url=https://spectrum.ieee.org/piezoelectric-liquid |access-date=13 April 2023 |work=[[IEEE Spectrum]] |date=25 March 2023 |language=en}}</ref>