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Home , Electroceramics, Ferroelectricity, Ferroelectric polymer, Lead zirconate titanate

erroelectric materials are ubiq- Fuitous in electrical and elec- tromechanical components and systems. is associated with large and piezoelectric coefficients, particularly when the composition is adjusted to position the near a phase boundary. This characteristic allows high volumetric efficiency dielec- tric charge storage, as well as high dis- placement actuators at modest voltages. The ability to reorient the spontaneous polarization between crystallographically- defined states is essential in allowing poling of materials to obtain net Impact of piezoelectric or pyroelectric responses.

Capacitors The largest industrial use of ferroelectric materials is in ferroelectricity multilayer ceramic . The poor availability of mica- based during World War II spurred development of By Susan Trolier-McKinstry air- and moisture-stable, high volumetric efficiency dielec- trics. The subsequent dawn of the electronics age led to pro-

duction of several trillion BaTiO3-based capacitors around Since the discovery of ferroelectricity 100 years ago, the world on an annual basis, with hundreds to thousands used in each current generation smart phone or computer. ferroelectric materials are everywhere in our electronics-based The size of this market is approximately $6 billion. Among society. Learn how they drive a $7 billion industry. the major suppliers around the world are Murata, Taiyo Yuden, Samsung Electromechanics, Kyocera (AVX), TDK, Yageo, and Kemet.

Medical ultrasound is the second most widely adopted imaging modality in medicine. It works thanks to ferroelectric materials.

22 www..org | American Ceramic Society Bulletin, Vol. 99, No. 1 The relatively closely-spaced ferroelectric phase transitions in BaTiO3 enable a high relative over a broad Before temperature range. Equally important is the processing science that fostered progressive miniaturization of the dielectric layers thickness over the last several decades—the so-called Moore’s law of capacitors—which has helped enable miniaturization of many electronic devices, including cellular phones. Industrial production is dominated by tape casting, elec- troding, and lamination to produce multilayer components at progressively decreasing case sizes. At present, commercial parts with layer thicknesses less than one micron with thin, highly-conductive nickel metal layers are readily available, with projected future miniaturization down to dielectric thicknesses of about 0.3 micron. The rich defect chemistry of these systems allows production of high reliability parts, even in the case where low oxygen partial pressure firing induces a high con- centration of oxygen vacancies.

Piezoelectric applications A second major use of ferroelectric materials is in piezo- electric ceramics, single crystals, and composites for actuators, sensors, and . In the future, it is possible that fer- roelectric piezoelectrics will also be widely adopted for kinetic After systems for distributed low-power systems, including emplaced sensors for the Internet of Things. The world-wide piezoelectric ceramics market is approximately $1B annually. This market is dominated by zirconate titanate- based (PZT) materials; numerous formulations are utilized to tailor the piezoelectric responses, the coupling coefficients, and the field-induced . Among the main uses of these piezoelectrics are precise positioners, systems, fish finders, medical ultrasound transducers, fluid flow meters, ultrasound systems for nonde- structive testing, high precision accelerometers, transformers, and many other applications. Of these, medical ultrasound is the second most widely adopted imaging modality in medicine and offers tremendous capability in high resolution imaging of subsurface features without necessitating ionizing radiation (see image of infant). At this point, an enormous number of lives have been saved through the use of medical ultrasound Credit: DeVRIES, R.C. and BURKE, J.E. (1957), Microstructure of ceramics. Journal the American Ceramic Society , 40: 200-206. doi:10.1111/j.1151-2916.1957.tb12603.x employing ferroelectric– composite transducers. Microstructure of barium titanate (a) before and (b) after Notably, the diversity of form factors and compositions needed application of an . The ferroelectric response of barium titanate causes reorientation of its domain for this range of applications of piezoelectric materials means structure, most easily seen in the circled areas. that production for any given component in the piezoelectrics markets tend to be smaller. There are numerous suppliers of piezoelectric ceramics, including Channel Technologies, tive temperature coefficient of resistance for self- Murata, American Piezoceramics, Piezo Kinetics, Meggitt limiting heaters; pyroelectric based room occupancy sensors (Ferroperm), Morgan , TDK, PI Ceramics, and fire detectors; and computing memory elements (including

Sinocera, and many others. In other cases, ferroelectric single PZT and SrBi2Ta2O9). crystals are used for domain engineered high strain piezoelec- trics, or for surface acoustic wave devices. About the author While capacitors and piezoelectrics comprise two of the Susan Trolier-McKinstry is professor of ceramic science and largest uses of ferroelectric materials, many other electrooptic engineering at The Pennsylvania State University and codirector of the Center for and Piezoelectrics. Contact Trolier- components are also of commercial importance, e.g., LiNbO3 for frequency doublers, optical modulators, and more; posi- McKinstry at [email protected]. n

American Ceramic Society Bulletin, Vol. 99, No. 1 | www.ceramics.org 23 bulletin cover story

Prenatal ultrasound (left) and cardiac ultrasound (above).

magine a young couple Ithat has just “seen” their Ferroelectricity— infant still in the womb for the first time after a routine ultra- sound scan. What joy! This news A revolutionary is worth sharing, so they click an image of the monitor on their smartphones and send it to grand- century of discovery parents, family, friends, and their By Geoff Brennecka, Rachel Sherbondy, Robert Schwartz, and Jon Ihlefeld entire social media networks. Meanwhile, across the medical campus, a recent retiree undergoes a cardiac procedure A century after the discovery of ferroelectricity, we look at the physics that guided with real-time ultrasound scans to repair makes ferroelectric materials so useful and the research that got us here. a blockage discovered, also with ultrasound. What relief! This patient just found out he will be a grandfather and now confidently awaits that day with anticipation.

Capsule summary

A SALTY START VERSATILE USES FUTURE FERRO University of Minnesota graduate student The switchable spontaneous polarization that In the past 10 years, two new structural

Joseph Valasek gave the first presentation on defines ferroelectricity directly enables applica- classes—including fluorite-structured HfO2 ferroelectricity in Rochelle salt in 1920. He tions such as nonvolatile ferroelectric random and wurzite-structured (Al,Sc)N—joined the could scarcely have predicted how his careful access memory, but many other applications, ferroelectric family and bring with them some measurements would end up playing a funda- such as multilayer ceramic capacitors, are potential technological superiority to tradi- mental role in many of today’s technologies. enabled by ferroelectric materials even without tional materials. directly using the switchable polarization.

24 www.ceramics.org | American Ceramic Society Bulletin, Vol. 99, No. 1 FERROELECTRIC These routine activities of modern life are possible because of piezoelectric ultrasound transducers, many thou- TIMELINE sands of ferroelectric multilayer ceramic capacitors, and a host of other electronics, sensors, actuators, and devices In the ferroelectrics based on ferroelectric materials. In the world Ferroelectrics have intrigued materials scientists since community their first report in 1920 and first publication the following 1912 1900 year.1 The label “ferroelectric” can be slightly misleading, as THE PREDICTION COMMERCIAL RADIO only a small fraction of ferroelectric compositions contain Erwin Schroedinger proposes the Radio receivers become more iron. Rather, the nomenclature was adopted in recognition word “ferroelectric” (“ferroelek- practical and radio becomes a of the parallels between the newly discovered phenomenon trisch” in German) as the analogy household item of ferroelectricity and the already-familiar , of ferromagnetism for spontaneous which originally was thought to be tied exclusively to iron. electrifying of liquids when cooling 1914–1918 The defining characteristic of a ferroelectric material is WWI 1920 World War I stretches from the existence of spontaneous dipoles within its struc- THE DISCOVERY July 28, 1914, to Nov. 11, 1918 ture whose direction can be reoriented by the application Joseph Valasek, graduate student of an electric field. In other words, all materials belonging at the University of Minnesota, gives 1939–1945 to a polar point are potentially ferroelectric, but it fi rst presentation on ferroelectricity WWII takes a demonstrated polarization reversal for the material in Rochelle salt at the Meeting of the World War II stretches from Sept. 1, to earn the title.2 (See sidebar, “Genesis of hysteresis…” ). American Physical Society on April 23 1939, to Sept. 2, 1945, stimulating This strict definition highlights an important conundrum: spending for the military sector a variety of phenomena can produce polarization-vs-field 1944–1946 measurements that suggest ferroelectricity, but they are PEROVSKITES ENTER 1942 instead artifacts masquerading as polarization reversal.3 At T. Ogawa and S. Waku (Japan, DIGITAL COMPUTERS The fi rst digital computer (as the same time, only a small fraction of the applications 1944), B. Wul and J.M. Gold- man (Russia, 1945), and A. von recognized by the U.S. Patent offi ce) enabled by ferroelectric materials directly take advantage of Hippel (U.S., 1946) investigate ferro- is completed. The Atanasoff-Berry the switchable polarization. Instead, as summarized here, computer (ABC) is started in 1937 in BaTiO3; R.B. Gray (U.S., the tremendous utility of ferroelectric materials in modern 1945) showcases fi rst operating and work continues until 1942 life typically arises from phenomena associated with, but poled BaTiO not necessarily directly leveraging, the full reorientation of 3 1947 this spontaneous dipole. 1950–1955 TRANSISTORS PZT Bell Laboratories successfully Discovery and early developments Groups, including those of demonstrates the fi rst transistor on In 1919, Joseph Valasek began his Ph.D. work at the Shirane, Takeda, and Sawaguchi in December 23 Japan, explore the PbTiO : University of Minnesota under professor W.F.G. Swan, 3 PbZrO solid solution system 1969 who suggested that Valasek investigate the curious crys- 3 ARPANET DEBUT tals of Rochelle salt (KNaC4H4O6∙4H2O). These crystals 1952 The Advanced Research Projects were relatively straightforward to grow and were already FERAM PROPOSED Agency Network (ARPANET) debuted known to be piezoelectric, pyroelectric, and optically active. Ferroelectric random access memory in the United States and made the Annoyingly, they were also very sensitive to humidity, and (FeRAM) is proposed in Dudley Allen technical foundation for the internet. nearly all of their interesting properties seemed to depend Buck’s master’s thesis Thus began the race for semicon- on everything, including temperature, electric field, and ductors and optics technology previous history. 1971 Valasek’s first task was to develop sensitive measure- HIGH VALUE PZT 1996 ments that could finally pin down the properties of this B. Jaffe’s group explains the HANDHELD COMPUTERS The fi rst handheld computer (Palm mischievous Rochelle salt. Valasek’s thesis and the associ- importance of phase transitions in PbTiO :PbZrO Pilot) is available for purchase ated publications are a veritable treatise on crystal physics 3 3 and careful measurements. It is interesting to note that of 2000s 2000 Valasek’s five papers in Physical Review on Rochelle salt, he LEAD-FREE FLASH DRIVES is the sole author on four of them, and the seminal paper1 Interest in identifying lead-free USB fl ash drives become an alter- reporting the first ever ferroelectric hysteresis loop received piezoelectric materials increases native to the fl oppy disc and CD for just over 200 citations in the ensuing century. The scien- across the globe portable data storage tific enterprise certainly has changed! Reading Valasek’s papers, it is clear he imagined this new phenomenon would lead to new functionality and new Figure 1. Timeline presenting development of ferroelectric materials devices, but he could scarcely have predicted how his care- alongside global advances at the time.

American Ceramic Society Bulletin, Vol. 99, No. 1 | www.ceramics.org 25 Ferroelectricity—A revolutionary century of discovery ful measurements of nonlinear dielectric tion of submarines. To this day, needs the size of components, and deploying response of finicky Rochelle salt crystals for improved materials for sonar remain them in new and novel applications. would end up playing such a fundamen- a strong driver of ferroelectric and piezo- Much of that research was—and contin- tal role in so many technologies a cen- electric materials development. In fact, ues to be—reported in ACerS publica- tury later. Figure 1 presents an approxi- World War II spurred development tions. The sidebar “Historically signifi- mate timeline of the development of of BaTiO3, Pb(Zr,Ti)O3, and Bi4Ti3O12 cant ferroelectrics papers” highlights key ferroelectric materials from prediction based ferroelectrics roughly simultane- papers over the years that advanced the in 1912 to discovery in 1920 to the first ously in isolated groups in the United science and art that led to the implemen- ceramic transducer in 1945 through to States, Japan, and Russia. This develop- tation of ferroelectric components in the present time’s search for eco-friendly ment, discussed in more detail below, devices we enjoy today. compositions. This history, juxtaposed is an excellent example of how the phe- It is worth noting that ferroelectric against global advances at the time, gives nomenon of ferroelectricity enables oth- , most notably polyvinylidene interesting context to the significance of erwise unachievable performance even fluoride, do exist and find significant ferroelectric devices to the evolution of when the switchable polarization itself is application. However, in this article we digital technology. only indirectly related to the application. focus on inorganic, nonmetallic (i.e., One thing that has not changed In the century since Valasek first ceramic) ferroelectrics, which are both significantly since Valasek’s time is discovered the ferroelectric effect, enor- more numerous and more widely used that much of the funding devoted to mous research effort went into under- than their squishier counterparts. development of new materials is driven standing these extraordinary materials by potential military applications. In and how to make them better, control- Fundamentals of ferroelectric Valasek’s case, the interest was in detec- ling their properties precisely, shrinking ceramics One of the most significant char- Genesis of hysteresis in ferroelectric materials acteristics of ferroelectric ceramics is their ability to exhibit single crystal-like Polarization reversal is the defining char- behavior even when fabricated as poly- acteristic of ferroelectric materials, and the B crystalline ceramics. In addition to typi- hysteretic response is often represented as a C cally being significantly less costly and plot of polarization vs. electric field, as shown D A Electric easier to produce than single crystals, schematically here. Diagrams A-G show how G field the domain structure responds to applied fabrication of polycrystalline ceramics field in a single grain at different stages on F also introduces opportunities for micro- the hysteresis loop. The colored regions E structure engineering and formation of represent domains in which all of the unit complex shapes. Use of polycrystalline cells are collectively polarized in the direction ceramics as piezoelectrics depends upon of the arrow. In the initial unpoled state (A), the ability to break full isotropic symme- the domain structure forms to minimize total try through the application of a poling energy, balancing charge, strain, and the field and thus relies on extrinsic effects energy penalty associated with formation of domain walls themselves. Application of an (domain wall motion) in ferroelectrics. electric field gradually aligns domains (B) via The sidebar “What does symmetry have the motion of domain walls to increase the to do with it” reviews the importance of volume of material polarized along with the crystallographic symmetry to piezoelec- external field; this maximum polarization is tricity and ferroelectricity.

known as the saturated polarization (Psat). Intrinsic is linear When the external field is removed, the under small fields, so applying a +5 V material relaxes (C) and some of the domain field in one direction will produce equal volume reorients to minimize strain and and opposite strain to applying –5 V in charge, but some residual net polarization is retained and is referred to as the remanent the same direction. In a typical sintered polycrystalline ceramic, even if each of polarization (Pr). When an external field of the opposite polarity is applied, some volume of the billions of individual grains is piezo- the domains reorient accordingly.The point at electric, they all collectively cancel out which the net polarization on the sample is their neighbors, and the net result is Credit: Ihlefed zero (D) is the coercive cield, Ec, referring to zero macroscopic piezoelectric response. the field required to coerce the sample into This fact is why piezoelectrics, switching. Further increases in this polarity of field lead to a P , and removal of the applied field sat such as those used for timekeeping in lands the polarization at a value of –P . Subsequent field reversal will trace out an approximately r watches, must be single crystals. symmetric hysteresis loop without ever returning to a stable net zero polarization unless, or until, the sample is ‘depoled,’ for example, by heating above the . Therefore, to use polycrystalline mate- rials as piezoelectrics, the macroscopic

26 www.ceramics.org | American Ceramic Society Bulletin, Vol. 99, No. 1 32 point groups 20 point groups 10 point groups Experimental Equilibrium subset Equilibrium Figure 2. Of the crystal- Equilibrium line or polycrystalline materials that exhibit piezoelectric behavior, some are pyroelectric, and of those, some are Crystalline materials Piezoelectric materials Pyroelectric materials ferroelectric. Materials (point symmetry) Non-centrosymmetric Polar Ferroelectric materials are classified into these (induced dipole) (permanent dipole) categories based on the (switchable dipole) symmetry of their unit Applied stress/Electric field cell, which in turn dictates how they interact with thermal, electrical, and mechanical energies. Applied and removed Equilibrium Electric dipole electric field Credit: Sherbondy random symmetry of the grains must be Historically significant ferroelectrics papers broken in some way. One option is to H. Jaffe, (1958) Piezoelectric Ceramics, JACerS, 41(11), 494–498. force the grains to all (or at least mostly) grow in a coordinated direction. This H.A. Sauer and J.R. Fisher, (1960) Processing of positive temperature coefficient thermistors, JACerS, 43(6), 297–301. approach is used for AlN thin films in G.H. Haertling and C.E. Land, (1971) Hot-pressed (Pb,La)(Zr,Ti)O ferroelectric ceramics for elec- MEMS resonators and clever ceramic 3 processing approaches such as templated troopic applications, JACerS, 54(1), 1–11. grain growth.4 Another option is to start R.B. Atkin and R.M. Fulrath, (1971) Point defects and sintering of , JACerS, with a crystallographically random poly- 54(5), 265–270. crystalline ceramic sample and break the K.A. Klicker, J.V. Biggers, and R.E. Newnham, (1981) Composites of PZT and epoxy for hydro- macroscopic symmetry in some way, such static transducer applications, JACerS, 64(1), 5–9. as with the application of a large electric N.H. Chan, R.K. Sharma, and D.M. Smyth, (1981) Non-Stoichiometry in undoped BaTiO3, JACerS, field. The applied field aligns crystallo- 64(9), 556-562. graphic dipoles, and this “poling” process A.I. Kingon and J.B. Clark, (1983) Sintering of PZT Ceramics 1: Atmosphere Control, JACerS, is enabled by the field-induced alignment 66(4), 253–256. of spontaneous dipoles, in other words, ferroelectricity (Fig. 2). S.L. Swartz, T.R. Shrout, W.A. Schulze, and L.E. Cross, (1984) Dielectric properties of lead- magnesium niobate ceramics, JACerS, 67(5), 311–315. The outstanding properties of fer- roelectric materials for piezoelectric and Y. Sakabe, (1987) Dielectric materials for base-metal multilayer ceramic capacitors, ACerS charge storage applications arise from a Bulletin, 66(9), 1338–1341. combination of intrinsic and extrinsic W. Pan, Q. Zhang, A. Bhalla, and L.E. Cross, (1989) Field-forced antiferroelectric-to-ferroelectric contributions. Ceramics based on barium switching in modified lead zirconate titanate stannate ceramics, JACerS, 72(4), 571–578. titanate (BaTiO3) dominate the capaci- R. Waser, T. Baiatu, and K.H. Härdtl, (1990) DC electrical degradation of type titanates tor market while lead zirconate titanate 1: Ceramics, JACerS, 73(6), 1645–1653. (Pb(Zr,Ti)O , PZT) based ceramics are the 3 W.L. Warren, K. Vanheusden, D. Dimos, G.E. Pike, and B.A. Tuttle, (1996) Oxygen vacancy motion most widely used piezoelectric ceramic for in perovskite oxides, JACerS, 79(2), 536–538. more than six decades, finding applica- D. McCauley, R.E. Newnham, and C.A. Randall, (1998) Intrinsic size effects in a barium titanate tions in medical ultrasound transducers, glass-ceramic, JACerS, 81(4), 979–987. sonar, micropositioners, and more. These materials are cubic and thus nonferroelec- C.A. Randall, N. Kim, J.P. Kucera, W.W. Cao, and T.R. Shrout, (1998) Intrinsic and extrinsic size tric at temperatures above T (the Curie effects in fine-grained morphotropic-phase-boundary lead zirconate titanate ceramics JACerS, C 81(3), 677–688. temperature) and transform to a lower symmetry polar state below this tempera- D. Damjanovic, (2005) Contributions to the piezoelectric effect in ferroelectric single crystals and ture. Figure 3 shows how the ferroelectric ceramics, JACerS, 88(10), 2663–2676. occurs in the prototypic J. Rödel, W. Jo, K.T.P. Seifert, E.M. Anton, T. Granzow, and D. Damjanovic, (2009) Perspective on the development of lead-free piezoceramics, JACerS, 92(6), 1153–1177. ferroelectric, BaTiO3 (Fig. 3).

American Ceramic Society Bulletin, Vol. 99, No. 1 | www.ceramics.org 27 Ferroelectricity—A revolutionary century of discovery

The change in shape and the result- P ing spontaneous polarization along the P P s s (by definition) c-axis occurs in every unit P s cell in the sample, and of course each Pr unit cell unit cell is influenced by those around it. Having all of the unit cells transition E in the same direction would be both a Ec tremendous coincidence and a higher energy state than having some disorder.

domain wall domain Thermodynamic free energy minimiza- tion drives the formation of domains

Credit: Geoff Brennecka to reduce both elastic (strain) and elec- Figure 3. During the phase transition that results upon cooling through the Curie tempera- trostatic (charge) energies. Domain sizes and distributions depend on boundary ture, the BaTiO3 unit cell expands by roughly 1% in the polar direction while shrinking accordingly in the two perpendicular directions. Displacement of the cation species (the conditions and defects, but the ability to Ba2+ and Ti4+) and anion species (O2-) in opposing directions results in a polar (tetragonal) configure and reconfigure these domains and the presence of a small electrical dipole, a spontaneous polariza- by applying electric fields is both the tion, within each unit cell. While the dimensional changes associated with this phase defining property of ferroelectrics and transition are relatively small (about 0.1 Å), the lower symmetry is the key to enabling the source of extrinsic contributions that ferroelectricity and its derivative properties. often dominate the properties. It is worth noting that in recent What does symmetry have to do with it? decades, researchers have found that elec- trically driving single crystal piezoelectrics Differences in atomic packing and bonding in different crystallographic directions influence a along nonpolar directions can produce material’s mechanical, thermal, electrical, magnetic, and optical properties, all of which can very large extrinsic contributions to elec- be direction-dependent. The symmetry principles that govern the categorization of crystal- tromechanical strain, taking advantage of line materials into seven crystal systems and 32 point groups also reflect in the symmetry of similar mechanisms to those described the material’s properties. This simple but powerful concept that the properties of a crystal- line material must exhibit at least the same symmetry as the underlying crystal structure is here for polycrystalline ceramics. Neumann’s principle.S1 Some of the same factors that enable large piezoelectric responses in ferroelec- All crystalline substances, neglecting quasicrystals, belong to one of 32 crystallographic trics also contribute to large permittivi- point groups. Eleven of those point groups possess an inversion center, which means that for any point (x,y,z) in the crystal, the corresponding point (-x,-y, -z) is completely identical. ties, which is of great importance for In other words, if you pick any point in space and turn the crystal “inside out” through that charge storage in capacitors. Relative point, it will look and behave exactly the same as when you started. Of the remaining 21 permittivity (also referred to as dielectric point groups, 20 are piezoelectric (the combination of other symmetry operators prohibits constant, though it is anything but con- piezoelectricity in point group 432), which simply means that applying a stress to the crystal stant in the materials discussed here) is will create a net separation of positive and negative charges in the crystal, hence the name proportional to the change in polariza- piezoelectric, which translates simply to pressure-electric. This polarization results in the tion with an applied electric field. When buildup of charges at the crystal surface. The converse is also true: applying an electric field a small electric field is applied to a ferro- to pull the positive and negative charges apart will produce a net strain in a piezoelectric electric, the intrinsic response discussed material. It is this intrinsic coupling of electrical and mechanical energies that makes piezo- electricity such a useful phenomenon. above results in a change in polarization that is typically much larger than in lin- Of the 20 piezoelectric point groups, 10 are spontaneously polarized, meaning they have a built- ear dielectrics, such as silica or alumina. in (spontaneous) dipole even without the presence of an applied field. These are the polar point This change in polarization to a groups, and because the magnitude of the spontaneous dipole changes with temperature, such polar materials are also known as pyroelectric (temperature-electric) materials. If the direction large and a high vol- of this spontaneous dipole can be reoriented through the application of an electric field, the umetric capacitance. Additional extrinsic material is a ferroelectric. This fact means that ferroelectrics are an experimental subset of effects, small motions of domain walls, pyroelectric materials. If a material undergoes dielectric breakdown (essentially a lightning bolt) lead to even larger changes in polariza- before the applied electric field is sufficient to reverse its polarization, it is not ferroelectric, or at tion and even higher relative permittivi- least it was not, before you and your lightning bolt destroyed it. ties. These attributes make ferroelectrics Therefore, all ferroelectric materials are pyroelectrics, and all pyroelectrics are piezoelectric, but well-suited for many capacitor applica- not all piezoelectric materials are pyroelectric, and not all pyroelectrics are ferroelectric. If that tions. For example, the relative permit- is confusing, just remember that all squares are rectangles, but not all rectangles are squares. tivity of BaTiO3-based dielectrics in the S1. Neumann’s principle. Online Dictionary of . http://reference.iucr.org/dictionary/Neu- multilayer ceramic capacitors (MLCC, mann's_principle. Accessed Dec. 2, 2019. Fig. 4) that are present in our cell phones, computers, and other electron-

28 www.ceramics.org | American Ceramic Society Bulletin, Vol. 99, No. 1 ics, is greater (often much greater) than peratures approach- 1,000. For comparison, silica has a rela- ing their Curie tive permittivity value of just 3.9. temperature where the coercive fields Abundant applications are lower. Doping Sophisticated ceramic processing and with magnesium microstructure engineering enabled reduces the coercive scaling of ferroelectric-containing field and increases MLCCs, such that today our electronics the laser dam- operate with dielectric layer thicknesses age threshold of far less than one micron packaged into LiNbO3 and LiTaO3 capacitors with more than 100 active by compensating layers. Packaged MLCC devices today for the lithium defi- Credit: Wikimedia, Jens Both Elcap, CC BY-SA 3.0 can be many times smaller than a grain ciency inherent in Figure 4. Schematic structure of a multilayer . of salt. In fact, MLCC scaling over the melt-grown LiNbO Layers are (1) dielectric ceramic, (2) outer ceramic layer, (3) 3 electrode, and (4) contact surface. past 50 years rivaled—and often out- and LiTaO3 crystals. paced—the aggressive transistor scaling Creating an array single crystals fabricated into specific of the semiconductor industry known of inverted domains with periodicity geometries for best performance. To as Moore’s Law. related to the wavelength of an incident affect technologies at smaller size scales, As discussed in the sidebar “What laser beam enables a variety of clever such as millimeter-scale robotics, RF does symmetry…”, polar crystals exhibit manipulations of the beam as it passes switches, and actuators for inkjet print- the pyroelectric effect, or a change in through the PPLN or PPLT crystal, and ers, the device sizes must also decrease. polarization magnitude with a change in the high coercive fields ensure that once This requirement has driven significant temperature. While all polar materials the material is poled, it will stay that efforts into development of ferroelectric possess this property, ferroelectrics may way during operation, even under large piezoelectric thin films and micro-electro- have pyroelectric coefficients that are optical powers and occasionally large mechanical systems (MEMS) devices that much larger than their nonferroelectric applied electric fields. can perform at reduced dimensions—a counterparts, often in the proximity of One technology that utilizes the full field known as piezoMEMS. Figure 5 a ferroelectric phase transition. This switching of polarization, the very hall- shows an example piezoMEMS device property makes ferroelectric pyroelectrics mark of ferroelectricity, is ferroelectric designed to imitate dragonfly flight.6 particularly well-suited for thermal sen- random access memory, FeRAM. In The most broadly studied material for sors. A wide variety of ferroelectrics are this technology, the polarization of the piezoMEMS is PZT. In approximately used for pyroelectric sensors, but some ferroelectric represents a binary bit of 30 years of piezoMEMS study, the piezo- of the most common are single crystal information (i.e., 1 or 0). When coupled electric performance of PZT thin films

LiNbO3 or LiTaO3 owing to their com- in series with a transistor, the amount substantially improved through advances bination of high pyroelectric coefficient, of current flowing through the channel in processing and understanding of the low losses, and low relative permittivity, when the transistor is activated allows roles of mechanical boundary conditions. which increases the measurable voltage differentiation of the positive or nega- While PZT and other lead-based fer- for a given amount of charge produced. tive polarization (memory) state of the roelectric piezoelectrics demonstrate

LiNbO3 and LiTaO3 single crystals ferroelectric capacitor. A voltage pulse is the most utility for electro-mechanical also find tremendous use in the optics then used to set the polarization of the applications, the looming European sector, often in the form of periodically ferroelectric for the next stored bit. The Union Restriction on Hazardous poled (PPLN) and peri- result is a nonvolatile memory that can Substances (ROHS) requirements drive odically poled lithium tantalate (PPLT), maintain its state for timescales up to 45 the search for reduced lead in materials structures for waveguides, phase match- years.5 FeRAM has advantages over other and has also led to the study of lead-free ing, difference frequency generation, memory technologies in terms of the piezoelectric ceramics and films. The and many others. This periodic poling number of read/write cycles possible and piezoelectric coefficients of the lead-free takes advantage of the nearly strain-free the energy required for each switching materials, such as (K,Nb)NbO3 and 180-degree domain walls in these crys- event, though its physical scaling into Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 to date lag tals as well as their large coercive fields. the tens of nanometer range remains a behind those of lead-based systems and In fact, the coercive fields of as-grown significant challenge. represent a large area of current and

LiNbO3 and LiTaO3 crystals at room While ferroelectric and related materi- future study and growth. temperature are often very close to the als dominate the high strain piezoelec- breakdown strengths of the crystals, so tric technologies of sonar, ultrasound, Frontiers of ferroelectricity the pure materials are often poled as and micro- and nano-positioners, these In the first 90 years of ferroelectrics they are pulled from a melt or at tem- technologies rely on bulk ceramics or research, attention focused on a few

American Ceramic Society Bulletin, Vol. 99, No. 1 | www.ceramics.org 29 Ferroelectricity—A revolutionary century of discovery

oxides, but recent the scenes, facilitating critical functions efforts have identi- and systems. fied promising nitride ferroelectrics. About the authors

Suspension Primary among Geoff Brennecka is associate professor and Traces them is (Al,Sc)N.8 of metallurgical and materials engineer- MEMS resonators ing and Fryrear Chair for Innovation

Credit: U.S. Army Research Laboratory based on AlN thin at Colorado School of Mines. Rachel Figure 5. U.S. Army Research Laboratory’s prototype PZT films dominate the Sherbondy is a Ph.D. candidate at piezoMEMS dragonfly with stroke and pitch actuated wing wireless communica- Colorado School of Mines. Robert design. The image on the right shows a platform suspended on tions market since Schwartz is professor emeritus of ceramic 7 the dragonfly between individually controllable wings. the early 2000s, and engineering at Missouri University of primary crystal structures and materi- in recent years, such Science and Technology. Jon Ihlefeld is als populating those structures at the resonators showed improved piezoelectric associate professor at the University of forefront for commercial applications. response via alloying with ScN or other Virginia in the Department of Materials These include the perovskite ferroelec- transition metal nitrides. Increasing the Science and Engineering and the concentration of scandium in (Al,Sc)N Department of Electrical and Computer trics LiNbO3 and LiTaO3 described previously; layered structures, such as reduces the c/a ratio of the polar AlN Engineering. Contact Brennecka at wurtzite structure, gradually pushing the [email protected]. SrBi2Ta2O9; and dihydrogen phosphates, unit cell closer to the nonpolar hexagonal such as KH2PO4. However, in the past 10 years, two new structural classes have BN-type structure in which the cation References joined the ferroelectric family and bring sits in the same plane as the anions. At 1J. Valasek (1921) Piezo-electric and allied phenom- with them some potential technological sufficiently high scandium contents, ena in Rochelle salt, Phys. Rev. 17, 475. superiority to traditional materials on the amount of electric field required to 2S.B. Lang (2004) A 2400 year history of pyroelec- the basis of chemical compatibility with switch the polarity of this structure can tricity: from Ancient Greece to exploration of the be less than the breakdown strength of solar system, British Ceramic Transactions 103(2), dominant semiconductor technologies. 65–70. the sample, and the material is ferro- These include fluorite-structured HfO2 3J.F. Scott (2008) Ferroelectrics go bananas, J. Phys.: and wurzite-structured (Al,Sc)N. electric. In large part due to the size of Condens. Matter 20(2), 02100. the MEMS industry already established In 2011, Boscke et al. first reported 4G.L. Messing, S. Trolier-McKinstry, E.M. Sabolsky, ferroelectricity in fluorite-structured around AlN thin films for radio frequen- C. Duran, … K.S. Oh (2004) Templated grain 7 cy communications, ferroelectric (Al,Sc)N growth of textured piezoelectric ceramics, Critical silicon-doped HfO2 thin films. The observation of a switchable polarization films and their derivatives have attracted Reviews in Solid State and Materials Sciences 29(2), tremendous attention for a number of 45–96. in this material surprised the community 5 integrated devices, but the enormous N. Setter, D. Damjanovic, L. Eng, G. Fox, ... S. and generated a great deal of excitement. Streiffer (2006) Ferroelectric thin films: Review of This excitement was driven largely by the driving force for phase separation makes materials, properties, and applications, Journal of inherent silicon compatibility of hafnia reliable fabrication rather challenging. Applied Physics 100(5), 051606. and the fact that the first observation of Recent predictions suggest ferro- 6G.L. Smith, J.S. Pulskamp, L.M. Sanchez, D.M. electric behavior in nitride perovskites, Potrepka, ... R.G. Polcawich (2012) PZT-based ferroelectric response in HfO2 occurred 9 piezoelectric MEMS technology, Journal of the in films that were only 10 nm thick just including LaWN3. Calculations point to a polar structure with a sufficiently American Ceramic Society 95(6), 1777–1792. shortly after HfO2-based gate dielectrics 7 low energy barrier between anti-aligned T.S. Boscke, J. Muller, D. Brauhaus, U. Schroder, emerged in commercial integrated cir- and U. Bottger (2011) Ferroelectricity in hafnium polarities to make it a strong candidate cuits. HfO2-based ferroelectrics are poised oxide thin films, Applied Physics Letters 99(10), 10 to enable scaling of existing ferroelectrics- for ferroelectricity, but these await 102903. based technologies, such as FeRAM, to experimental confirmation. Similarly, 8S. Fichtner, N. Wolff, F. Lofink, L. Kienle, and even smaller dimensions. In addition ferroelectricity reported in several of B. Wagner (2019) AlScN: A III-V semiconduc- to demonstrated and commercialized hybrid halide perovskites took the photo- tor based ferroelectric, Journal of Applied Physics voltaic world by storm, though the verac- 125(11), 114103. ferroelectric thin film technologies, the 9 ity of the genuine ferroelectric nature R. Sarmiento-Pérez, T.F. T. Cerqueira, S. Körbel, integrated circuit process compatibility S. Botti, and M.A.L. Marques (2015) Prediction and the potential role(s) of the sponta- of HfO2 positions it to enable other of stable nitride perovskites, Chemistry of Materials new devices, such as negative differen- neous polarization on the properties of 27(17), 5957–5963. tial capacitance field effect transistors these materials remains controversial. 10Y.W. Fang, C.A.J. Fisher, A. Kuwabara, X.W. (NC-FETs) and ferroelectric-FETs, which We will not pretend to know exactly Shen, … C.G. Duan (2017) Lattice dynamics and may offer continued performance increases what lies ahead for the second century ferroelectric properties of the nitride perovskite LaWN , Physical Review B 95(1), 014111. ■ to silicon based integrated circuits. of ferroelectricity, but it is a safe bet that 3 All of the commercially relevant fer- ferroelectrics will continue to play the roelectric ceramics discussed so far are role of enabler, quietly operating behind

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