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Piezoelectric Crystals
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Piezoelectric Crystals

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Inorganic Crystalline Materials".

Deadline for manuscript submissions: closed (30 April 2014) | Viewed by 122003

Special Issue Editor

Prof. Dr. Shujun Zhang

grade E-Mail Website
Guest Editor
ISEM/AIIM, University of Wollongong, Wollongong, NSW 2500, Australia
Interests: piezoelectricity; ferroelectricity; crystals; ceramics; transducers
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleague,

Significant advances in functional materials have highlighted the role of piezoelectricity. They are able to cooperate with other functional materials, such as those dedicated for magnetism or optics, which is primarily ascribed to polarization dynamics induced by electric, thermal, and stress fields. Intense research methods were implemented on polycrystalline ceramics in order to enhance piezoelectric performance through the construction of morphotropic phase boundary (MPB) and phase convergence regions in some lead-based and lead-free systems. For instance, this was proposed based on the outstanding piezoelectricity of lead-based systems near MPB, because of the monoclinic distortion and polarization rotation. Another example is the excellent strain response obtained in some lead-free polycrystalline ceramics, which resulted from the strong coupling between relaxor and ferroelectric compounds.

Crystal growth has also been proposed to improve the ferroelectric and/or piezoelectric response. On the one hand, the absence of grain boundaries in single crystals facilitates polarization switching and enables a larger volume of the material to contribute to the electromechanical response. On the other hand, engineered domains in ferroelectric single crystals can be formed by poling along a particular crystallographic direction. Finally, further research on piezoelectric single crystals is also motivated by their increased anisotropic macroscopic electrical performance.

This Special Issue aims at covering all of the relevant aspects of lead-based and lead-free piezoelectric crystals and ceramics. Both well-established, novel, or less common compounds, and piezoelectric mechanisms in single crystals will be covered. Therefore, the Issue welcomes original research and review manuscripts on the following main aspects:

  • Lead-based and lead-free single crystals and ceramics
  • Piezoelectricity
  • Morphotropic phase boundary
  • Phase convergence region
  • Development and design of piezoelectric single crystals and ceramics
  • Ferroelectricity
  • Domain engineering
  • Role of defects
  • Chemical characterization
  • Electromechanical characterization
  • Ab-initio, DFT, and first principles calculations on piezoelectric single crystals and ceramics

Dr. Philippe Veber
Guest Editor

Published Papers (14 papers)

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Research

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771 KiB  
Article
Phase Energy Determined from Stress and Electric-Field-Induced Phase Transformations in [011]C Cut 0.24PIN-PMN-PT Single Crystals
by Dorinamaria Carka, John A. Gallagher and Christopher S. Lynch
Crystals 2014, 4(3), 377-389; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4030377 - 19 Aug 2014
Cited by 5 | Viewed by 6256
Abstract
The effect of composition and temperature on the large field behavior of [011]C cut and poled (d32-mode) rhombohedral relaxor ferroelectric 0.24PIN-(1−x)PMN-xPT single crystals was characterized under electromechanical loading and the relative phase energy determined. The [...] Read more.
The effect of composition and temperature on the large field behavior of [011]C cut and poled (d32-mode) rhombohedral relaxor ferroelectric 0.24PIN-(1−x)PMN-xPT single crystals was characterized under electromechanical loading and the relative phase energy determined. The electric field and stress induced polarization and strain response and field dependent material properties are reported for two concentrations of lead titanate (PT), with one PT concentration closer to the morphotropic phase boundary, at low and high temperature. A thermodynamic analysis to determine the relative energy levels of the rhombohedral and orthorhombic phases is based on the path integration of the measured data to determine external work done to drive the phase transition. The effect of heat generated by irreversible strain and electric displacement increments (hysteresis in the phase transformation) was removed from the work done during the loading cycle and the relative Helmholtz free energy density levels of the phases was determined. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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12884 KiB  
Article
Preparation and Characterization of Lead-Free (K0.5Na0.5)NbO3-LiNbO3 and (K0.5Na0.5)NbO3-LiTaO3 Ferroelectric Single Crystals
by Tao Chu, Chao He, Hamel Tailor and Xifa Long
Crystals 2014, 4(3), 296-305; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4030296 - 16 Jul 2014
Cited by 4 | Viewed by 6309
Abstract
Lead-free (K0.5Na0.5)NbO3-LiNbO3 (KNN-LN) and (K0.5Na0.5)NbO3-LiTaO3 (KNN-LT) ferroelectric single crystals, with the dimensions of 11 ´ 11 ´ 5 mm3 and 5 ´ 5 ´ 3 mm3, [...] Read more.
Lead-free (K0.5Na0.5)NbO3-LiNbO3 (KNN-LN) and (K0.5Na0.5)NbO3-LiTaO3 (KNN-LT) ferroelectric single crystals, with the dimensions of 11 ´ 11 ´ 5 mm3 and 5 ´ 5 ´ 3 mm3, were grown successfully using the top-seeded solution growth (TSSG) method, respectively. The crystal structures were analyzed by means of X-ray diffraction, showing orthorhombic symmetry for KNN-LN single crystals and coexistence of orthorhombic and tetragonal symmetry for KNN-LT single crystals at room temperature. The orthorhombic-tetragonal (TO-T) and tetragonal-cubic (TC) phase transition temperatures are 195 °C and 420 °C for the KNN-LN single crystals, and 130 °C and 280 °C for KNN-LT single crystals, respectively. The remnant polarization (Pr) is 27.8 μC/cm2 with a coercive field (Ec) of 17 kV/cm for KNN-LT single crystals. The two single crystals showed 90° domains with layers in (parallel) straight lines, while KNN-LT single crystals have a larger domain region. The actual stoichiometry deviates easily from the original composition in the process of crystal growth, thus, an appropriate nominal composition and optimized crystal growth method is desired to get high-quality crystals in the future. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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915 KiB  
Article
Temperature-Dependent Phase Transition in Orthorhombic [011]c Pb(Mg1/3Nb2/3) O3-0.35PbTiO3 Single Crystal
by Wenhui He, Qiang Li, Qingfeng Yan, Nengneng Luo, Yiling Zhang, Xiangcheng Chu and Dezhong Shen
Crystals 2014, 4(3), 262-272; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4030262 - 11 Jul 2014
Cited by 19 | Viewed by 9419
Abstract
Relaxor [011]c PMN-0.35PT single crystal phase transition characteristics are investigated through various methods including variable temperature dielectric properties, X-ray diffraction, bipolar ferroelectric hysteresis loops (P-E) and electric-field-induced strain (S-E) hysteresis loops measurements. The results reveal that two phase transitions exist within the [...] Read more.
Relaxor [011]c PMN-0.35PT single crystal phase transition characteristics are investigated through various methods including variable temperature dielectric properties, X-ray diffraction, bipolar ferroelectric hysteresis loops (P-E) and electric-field-induced strain (S-E) hysteresis loops measurements. The results reveal that two phase transitions exist within the range from room temperature to 250 °C: orthorhombic (O)-tetragonal (T)-cubic (C). The O-to-T and T-to-C phase transition temperatures have been identified as 84 °C and 152 °C, respectively. Diffuseness degree of the T-to-C phase transition for the unpoled single crystal has been calculated to be 1.56, implying an intermediate state between normal and relaxor ferroelectrics. Temperature-dependent remanent polarization (Pr), coercive field (Ec), saturation polarization (Ps), hysteresis loop squareness (Rsq), and longitudinal piezoelectric constant (d* 33) are also explored to learn the details of the phase transitions. Variable temperature unipolar Suni-E hysteresis loops avail additional evidence for the microstructure change in the as-measured single crystal. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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2237 KiB  
Article
Quasi-Linear Polarized Modes in Y-Rotated Piezoelectric GaPO4 Plates
by Cinzia Caliendo and Fabio Lo Castro
Crystals 2014, 4(3), 228-240; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4030228 - 07 Jul 2014
Cited by 6 | Viewed by 5576
Abstract
The propagation of both surface and flexural acoustic plate modes along y-rotated x-propagation GaPO4 piezoelectric substrates was studied for several y-cut angles: the phase velocity and coupling coefficient dispersion curves were theoretically calculated for two different electroacoustic coupling configurations. [...] Read more.
The propagation of both surface and flexural acoustic plate modes along y-rotated x-propagation GaPO4 piezoelectric substrates was studied for several y-cut angles: the phase velocity and coupling coefficient dispersion curves were theoretically calculated for two different electroacoustic coupling configurations. The investigation of the acoustic field profile across the plate thickness revealed the presence of thin plate modes having polarization predominantly oriented along the propagation direction, and hence suitable for operation in liquid environment. These modes include the linearly polarized Anisimkin Jr. and the quasi longitudinal plate modes, AMs and QLs, showing a phase velocity close to that of the longitudinal bulk acoustic wave propagating in the same direction. The temperature coefficient of delay (TCD) of these longitudinal modes was investigated in the −20 to 420 °C temperature range, in order to identify thermally stable or low TCD cuts. The power flow angle, i.e., the angle between the phase and group velocity vectors, was also estimated to evaluate the substrate anisotropy effect on the acoustic wave propagation. The GaPO4 intrinsic properties, such as its resistance to high temperature and its chemical inertness, make it especially attractive for the development of acoustic waves-based sensors for applications in harsh liquid environment. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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879 KiB  
Article
Orientation and Temperature Dependence of Piezoelectric Properties for Sillenite-Type Bi12TiO20 and Bi12SiO20 Single Crystals
by Chuanying Shen, Huaijin Zhang, Yuanyuan Zhang, Honghao Xu, Haohai Yu, Jiyang Wang and Shujun Zhang
Crystals 2014, 4(2), 141-151; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4020141 - 20 Jun 2014
Cited by 15 | Viewed by 7819
Abstract
The full matrix of electro-elastic constants of sillenite-type crystals Bi12TiO20 (BTO) and Bi12SiO20 (BSO) were determined by the resonance method, with d14 and k14 being on the order of 40–48 pC/N and 31%–36%, respectively. In [...] Read more.
The full matrix of electro-elastic constants of sillenite-type crystals Bi12TiO20 (BTO) and Bi12SiO20 (BSO) were determined by the resonance method, with d14 and k14 being on the order of 40–48 pC/N and 31%–36%, respectively. In addition, double-rotated orientation dependence of d33 was investigated, with the maximum values of 25–28 pC/N being achieved in ZXtl45°/54°-cut samples. The electrical resistivity of BSO was found to be two orders higher than that of BTO, being on the order of 7 × 105 Ω cm at 500 °C. The temperature dependence of dielectric and piezoelectric properties were investigated. BSO exhibited a high thermal stability in the temperature range of 25–500 °C, while BTO showed a variation of ~3% in the range of 25–350 °C. The high values of d14 and k14, together with the good thermal stability, make BTO and BSO crystals potential candidates for electromechanical applications in medium temperature range. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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1765 KiB  
Article
Tetragonal-to-Tetragonal Phase Transition in Lead-Free (KxNa1−x)NbO3 (x = 0.11 and 0.17) Crystals
by Dabin Lin, Zhenrong Li, Fei Li, Changlong Cai, Weiguo Liu and Shujun Zhang
Crystals 2014, 4(2), 113-122; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4020113 - 10 Jun 2014
Cited by 9 | Viewed by 7504
Abstract
Lead free piezoelectric crystals of (KxNa1−x)NbO3 (x = 0.11 and 0.17) have been grown by the modified Bridgman method. The structure and chemical composition of the obtained crystals were determined by X-ray diffraction (XRD) and electron [...] Read more.
Lead free piezoelectric crystals of (KxNa1−x)NbO3 (x = 0.11 and 0.17) have been grown by the modified Bridgman method. The structure and chemical composition of the obtained crystals were determined by X-ray diffraction (XRD) and electron probe microanalysis (EPMA). The domain structure evolution with increasing temperature for (KxNa1−x)NbO3 (x = 0.11 and 0.17) crystals was observed using polarized light microscopy (PLM), where distinguished changes of the domain structures were found to occur at 400 °C and 412 °C respectively, corresponding to the tetragonal to tetragonal phase transition temperatures. Dielectric measurements performed on (K0.11Na0.89)NbO3 crystals exhibited tetragonal to tetragonal and tetragonal to cubic phase transitions temperatures at 405 °C and 496 °C, respectively. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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Review

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49554 KiB  
Review
The Growth and Properties of Lead-Free Ferroelectric Single Crystals
by Xiaobing Li, Chao Chen, Hao Deng, Haiwu Zhang, Di Lin, Xiangyong Zhao and Haosu Luo
Crystals 2015, 5(2), 172-192; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst5020172 - 25 Mar 2015
Cited by 16 | Viewed by 10052
Abstract
Much attention is drawn to the preparation, structure and properties investigation of lead-free ferroelectrics for the next generation of piezoelectric devices. (Na0.5Bi0.5)TiO3-BaTiO3 (NBT-BT) lead-free solid solution piezoelectric single crystals with composition x in the range of [...] Read more.
Much attention is drawn to the preparation, structure and properties investigation of lead-free ferroelectrics for the next generation of piezoelectric devices. (Na0.5Bi0.5)TiO3-BaTiO3 (NBT-BT) lead-free solid solution piezoelectric single crystals with composition x in the range of 0–0.05 as a materials with high piezoelectric properties were successfully grown from platinum crucible by using the top-seeded solution growth (TSSG) method. The dimensions of NBT-BT crystal is Ø40 × 10 mm2. X-ray powder diffraction patterns reveal that the crystal structure of NBT-BT crystal changes from rhombohedral to tetragonal symmetry with increasing amounts of BT(x). The dielectric, ferroelectric and piezoelectric properties of NBT-BT crystals with different compositions near the morphotropic phase boundary (MPB) were studied systematically. Ions (Mn, Eu, Zn) doped NBT and NBT-BT 95/5 crystals were also grown and studied. In addition, their piezoelectric and ferroelectric properties are investigated. Further, a high-quality and large-sized (K0.25Na0.75)NbO3 (KNN25/75) single crystal has been achieved by a carefully controlled TSSG method. The dimensions of the as-grown KNN25/75 single crystal reached up to Ø30 × 10 mm2. The obtained KNN crystals provided us a superb material for the dielectric, piezoelectric, ferroelectric and electromechanical coupling property characterization along different orientations. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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2524 KiB  
Review
Relaxor-PT Single Crystal Piezoelectric Sensors
by Xiaoning Jiang, Jinwook Kim and Kyugrim Kim
Crystals 2014, 4(3), 351-376; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4030351 - 28 Jul 2014
Cited by 53 | Viewed by 12561
Abstract
Relaxor-PbTiO3 piezoelectric single crystals have been widely used in a broad range of electromechanical devices, including piezoelectric sensors, actuators, and transducers. This paper reviews the unique properties of these single crystals for piezoelectric sensors. Design, fabrication and characterization of various relaxor-PT single [...] Read more.
Relaxor-PbTiO3 piezoelectric single crystals have been widely used in a broad range of electromechanical devices, including piezoelectric sensors, actuators, and transducers. This paper reviews the unique properties of these single crystals for piezoelectric sensors. Design, fabrication and characterization of various relaxor-PT single crystal piezoelectric sensors and their applications are presented and compared with their piezoelectric ceramic counterparts. Newly applicable fields and future trends of relaxor-PT sensors are also suggested in this review paper. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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10872 KiB  
Review
Growth and Characterization on PMN-PT-Based Single Crystals
by Jian Tian and Pengdi Han
Crystals 2014, 4(3), 331-341; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4030331 - 16 Jul 2014
Cited by 11 | Viewed by 7964
Abstract
Lead magnesium niobate—lead titanate (PMN-PT) single crystals have been successfully commercialized in medical ultrasound imaging. The superior properties of PMN-PT crystals over the legacy piezoelectric ceramics lead zirconate titanate (PZT) enabled ultrasound transducers with enhanced imaging (broad bandwidth and improved sensitivity). To obtain [...] Read more.
Lead magnesium niobate—lead titanate (PMN-PT) single crystals have been successfully commercialized in medical ultrasound imaging. The superior properties of PMN-PT crystals over the legacy piezoelectric ceramics lead zirconate titanate (PZT) enabled ultrasound transducers with enhanced imaging (broad bandwidth and improved sensitivity). To obtain high quality and relatively low cost single crystals for commercial production, PMN-PT single crystals were grown with modified Bridgman method, by which crystals were grown directly from stoichiometric melt without flux. For ultrasound imaging application, [001] crystal growth is essential to provide uniform composition and property within a crystal plate, which is critical for transducer performance. In addition, improvement in crystal growth technique is under development with the goals of improving the composition homogeneity along crystal growth direction and reducing unit cost of crystals. In recent years, PIN-PMN-PT single crystals have been developed with higher de-poling temperature and coercive field to provide improved thermal and electrical stability for transducer application. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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3842 KiB  
Review
Advances in the Growth and Characterization of Relaxor-PT-Based Ferroelectric Single Crystals
by Jun Luo and Shujun Zhang
Crystals 2014, 4(3), 306-330; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4030306 - 16 Jul 2014
Cited by 46 | Viewed by 10239
Abstract
Compared to Pb(Zr1−xTix)O3 (PZT) polycrystalline ceramics, relaxor-PT single crystals offer significantly improved performance with extremely high electromechanical coupling and piezoelectric coefficients, making them promising materials for piezoelectric transducers, sensors and actuators. The recent advances in crystal growth [...] Read more.
Compared to Pb(Zr1−xTix)O3 (PZT) polycrystalline ceramics, relaxor-PT single crystals offer significantly improved performance with extremely high electromechanical coupling and piezoelectric coefficients, making them promising materials for piezoelectric transducers, sensors and actuators. The recent advances in crystal growth and characterization of relaxor-PT-based ferroelectric single crystals are reviewed in this paper with emphases on the following topics: (1) the large crystal growth of binary and ternary relaxor-PT-based ferroelectric crystals for commercialization; (2) the composition segregation in the crystals grown from such a solid-solution system and possible solutions to reduce it; (3) the crystal growth from new binary and ternary compositions to expand the operating temperature and electric field; (4) the crystallographic orientation dependence and anisotropic behaviors of relaxor-PT-based ferroelectriccrystals; and (5) the characterization of the dielectric, elastic and piezoelectric properties of the relaxor-PT-based ferroelectriccrystals under small and large electric fields. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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4648 KiB  
Review
Polarization Rotation and Monoclinic Distortion in Ferroelectric (Bi0.5Na0.5)TiO3–BaTiO3 Single Crystals under Electric Fields
by Motohiro Ogino, Yuji Noguchi, Yuuki Kitanaka, Masaru Miyayama, Chikako Moriyoshi and Yoshihiro Kuroiwa
Crystals 2014, 4(3), 273-295; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4030273 - 16 Jul 2014
Cited by 24 | Viewed by 10869
Abstract
The features of the crystal structures and spontaneous polarization (Ps) under an electric field (E) have been reviewed for (1 − x)(Bi0.5Na0.5)TiO3–xBaTiO3 (BNT–BT). In-situ measurements of high-resolution synchrotron [...] Read more.
The features of the crystal structures and spontaneous polarization (Ps) under an electric field (E) have been reviewed for (1 − x)(Bi0.5Na0.5)TiO3–xBaTiO3 (BNT–BT). In-situ measurements of high-resolution synchrotron radiation X-ray diffraction (SR-XRD) under electric fields show that single crystals with x = 0 (BNT) and 5% have a monoclinic distortion in space group Cc at 25 °C. The SR-XRD study combined with density functional theory (DFT) calculations demonstrates that BNT–5%BT exhibits a rotation of Ps in the monoclinic a–c plane by 2° under an E of 70 kV/cm along the <001> pseudo-cubic direction, which is much larger than BNT. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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1180 KiB  
Review
Rare-Earth Calcium Oxyborate Piezoelectric Crystals ReCa4O(BO3)3: Growth and Piezoelectric Characterizations
by Fapeng Yu, Xiulan Duan, Shujun Zhang, Qingming Lu and Xian Zhao
Crystals 2014, 4(3), 241-261; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4030241 - 08 Jul 2014
Cited by 26 | Viewed by 10542
Abstract
Rare-earth calcium oxyborate crystals, ReCa4O(BO3)3 (ReCOB, Re = Er, Y, Gd, Sm, Nd, Pr, and La ), are potential piezoelectric materials for ultrahigh temperature sensor applications, due to their high electrical resistivity at elevated temperature, high piezoelectric sensitivity [...] Read more.
Rare-earth calcium oxyborate crystals, ReCa4O(BO3)3 (ReCOB, Re = Er, Y, Gd, Sm, Nd, Pr, and La ), are potential piezoelectric materials for ultrahigh temperature sensor applications, due to their high electrical resistivity at elevated temperature, high piezoelectric sensitivity and temperature stability. In this paper, different techniques for ReCOB single-crystal growth are introduced, including the Bridgman and Czochralski pulling methods. Crystal orientations and the relationships between the crystallographic and physical axes of the monoclinic ReCOB crystals are discussed. The procedures for dielectric, elastic, electromechanical and piezoelectric property characterization, taking advantage of the impedance method, are presented. In addition, the maximum piezoelectric coefficients for different piezoelectric vibration modes are explored, and the optimized crystal cuts free of piezoelectric cross-talk are obtained by rotation calculations. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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3348 KiB  
Review
Potential Advantage of Multiple Alkali Metal Doped KNbO3 Single Crystals
by Hideo Kimura, Hongyang Zhao, Rumi Tanahashi, Lei Guo, Tingting Jia, Qiwen Yao and Zhenxiang Cheng
Crystals 2014, 4(3), 190-208; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4030190 - 26 Jun 2014
Cited by 6 | Viewed by 6521
Abstract
Potassium niobate crystal KNbO3 (KN) is a well-known crystal for lead free piezoelectric or nonlinear optical applications. The KN crystal has been studied in both single crystal form and in thin film form which has resulted in many review articles being published. [...] Read more.
Potassium niobate crystal KNbO3 (KN) is a well-known crystal for lead free piezoelectric or nonlinear optical applications. The KN crystal has been studied in both single crystal form and in thin film form which has resulted in many review articles being published. In order to exceed the KN crystal, it is important to study KN phase forming and doping effects on the K site. This article summarizes the authors’ study towards a multiple alkali metal doped KN crystal and related single crystals briefly from the viewpoint of crystal growth. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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563 KiB  
Review
Flux-Grown Piezoelectric Materials: Application to α-Quartz Analogues
by Pascale Armand, Adrien Lignie, Marion Beaurain and Philippe Papet
Crystals 2014, 4(2), 168-189; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst4020168 - 23 Jun 2014
Cited by 12 | Viewed by 8358
Abstract
Using the slow-cooling method in selected MoO3-based fluxes, single-crystals of GeO2 and GaPO4 materials with an α-quartz-like structure were grown at high temperatures (T ≥ 950 °C). These piezoelectric materials were obtained in millimeter-size as well-faceted, visually colorless [...] Read more.
Using the slow-cooling method in selected MoO3-based fluxes, single-crystals of GeO2 and GaPO4 materials with an α-quartz-like structure were grown at high temperatures (T ≥ 950 °C). These piezoelectric materials were obtained in millimeter-size as well-faceted, visually colorless and transparent crystals. Compared to crystals grown by hydrothermal methods, infrared and Raman measurements revealed flux-grown samples without significant hydroxyl group contamination and thermal analyses demonstrated a total reversibility of the α-quartz ↔ β-cristobalite phase transition for GaPO4 and an absence of phase transition before melting for α-GeO2. The elastic constants CIJ (with I, J indices from 1 to 6) of these flux-grown piezoelectric crystals were experimentally determined at room and high temperatures. The ambient results for as-grown α-GaPO4 were in good agreement with those obtained from hydrothermally-grown samples and the two longitudinal elastic constants measured versus temperature up to 850 °C showed a monotonous evolution. The extraction of the ambient piezoelectric stress contribution e11 from the CD11 to CE11 difference gave for the piezoelectric strain coefficient d11 of flux-grown α-GeO2 crystal a value of 5.7(2) pC/N, which is more than twice that of α-quartz. As the α-quartz structure of GeO2 remained stable up to melting, a piezoelectric activity was observed up to 1000 °C. Full article
(This article belongs to the Special Issue Piezoelectric Crystals)
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