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Piezoelectric materials have been developed since early 1900s and many research had been conducted on the composition and process to obtain higher piezoelectric constants (${d}_{33}$). Within composition research, lead perovskite relaxor piezoelectric single crystals (SCs) of Pb(Mg1/3Nb2/3)O3–lead titanate PbTiO3 type have been actively studied since 1990s because of their outstanding ${d}_{33} >1500$ pC/N compared with those of the conventional Pb(Zr,Ti)O3 ceramics. A major driving force of these SC research has been promoted by mass production of ultrasound transducers and array probes for medical diagnostic systems since early 2000s. However, higher ${d}_{33}$ material and process research for these ultrasound devices are almost saturated. In this review article, we present a brief overview of the history, current situation, and future perspective of piezoelectric SCs. The authors believe that the main research in the next century is high ${d}_{33}$ SCs with a high composition uniformity and low-energy SC growth methods, such as solid-state SC growth, low-loss SC transducer manufacturing technique, and improved poling process. This is a big technical challenge for all the scientists; however, the relatively large market of medical ultrasound has been expanded year by year, and we hope that the community is motivated to solve such technical problems in the near future. [ABSTRACT FROM AUTHOR]

With the great success on verifying its effectiveness on relaxor-PbTiO3 (PT) single crystals (SCs), alternating current poling (ACP) has been taking a center as a new domain engineering method in the last few years. Compared with the conventional direct current poling (DCP), ACP enables enhanced piezoelectric and dielectric properties. In this article, research progress in ACP and perspectives are introduced. Initially, optimized conditions of ACP for relaxor-PT SCs and unsolved issues on polycrystalline ceramics and spurious modes are reviewed. Second, the ferroelectric domain size dependence of piezoelectricity associated with ACP is discussed. We hypothesize that a tradeoff between domain and domain wall contributions exists for high piezoelectricity, suggesting an optimum 109° domain wall size, which is presumably dependent on compositions, crystallographic symmetries, and domain configurations. Finally, crystals synthesized by a solid-state crystal growth (SSCG) method are briefly introduced due to their unprecedented piezoelectricity obtained by ACP (${d}_{33} \sim 5500$ pC/N). We hope that this work helps to grasp the current status of ACP and to guide future tasks to be studied. [ABSTRACT FROM AUTHOR]

Various 2 inch SmxY1−xCa4O(BO3)3 (SmxY1−xCOB) crystals with good quality were grown successfully along the <010> axis by the Czochralski method. As the amount added increased, the unit cell dimensions for the obtained crystals were found to increase. At room temperature, the relative dielectric constants ε E11 /ε0, ε E22 /ε0 and ε E33 /ε0 were found to be 9.57, 11.87 and 9.58 for Sm0.05Y0.95COB, 9.66, 12.33 and 9.64 for Sm0.3Y0.7COB, 9.70, 12.78 and 9.75 for Sm0.5Y0.5COB, and 9.73, 14.10 and 10.14 for Sm0.75Y0.25COB, respectively. The electromechanical coupling factors and piezoelectric coefficients as a function of Sm content were studied, with maximum values found in the Sm0.75Y0.25COB crystal. All the as-grown crystals were observed to possess ultrahigh resistivities, and the values of the electrical resistivities of SmxY1−xCOB were approximately 1.1–3.1 × 108 Ω cm at 850 °C. In the range of the test temperature, the dielectric losses for the SmxY1−xCOB crystals were determined to be less than 45% at 840 °C, which together with the high thermal stability of the electromechanical and piezoelectric properties, with a variation of less than 5.5% at 850 °C, make the SmxY1−xCOB crystals important candidate materials for high-temperature piezoelectric applications. [ABSTRACT FROM AUTHOR]

Growth of Ca 3 NbGa 3 Si 2 O 14 (CNGS) has been attempted by the conventional RF-heating Czochralski technique with a platinum crucible (50 mm in diameter and height). Obtained crystals were generally prismatic with clearly defined face of (001). Transparent and colorless single crystals of CNGS up to about 30 mm in diameter and 50 mm in length were successfully grown from the melt of 1325°C under the optimum growth conditions of the pulling axis of y , a crystal pulling rate of 1.0 mm/h and rotation rate of 10 rpm. The dielectric constant ℇ 11 T and ℇ 33 T were 17.6 and 46.7, respectively. Elastic compliances s 11 E and s 44 E were 8.84 and 11.17 × 10 -12 m 2 N -1 , respectively. The coupling coefficient k 12 of 10% was somewhat smaller than that of La 3 Ga 5 SiO 14 (LGS) due to the crystal quality. [ABSTRACT FROM AUTHOR]

Ba 3 Ga 2 Ge 4 O 14 (BGG) has been synthesized by solid state reaction and then grown by the Czochralski method. Although BGG has a langasite structure and low melting point of 1030°C, it is very difficult to grow a large crystal of BGG, because of high viscosity in the melt. Syntheses of Ba 3 Ga 2 Ge 4-x Si x O 14 , Ba 3 Ga 2 Ge 4-x Sn x O 14 , Ba 3 Ga 2 Si 4-x Sn x O 14 have been attempted to improve their high viscosities by substitutions of Ge sites with Si or Sn in Ba 3 Ga 2 Ge 4 O 14 . They have been regretfully not langasite structures. On the other hand, Ba 3 Nb 2 Ga 4 O 14 , Ba 3 NbGa 3 Si 2 O 14 and Ba 3 Nb 0.5 Ga 2.5 Si 3 O 14 have been synthesized. Among them, only Ba 3 NbGa 3 Si 2 O 14 is a langasite structure and its melting temperature is 1260°C. Growth of this crystal has been attempted, but a single phase Ba 3 NbGa 3 Si 2 O 14 could not be grown because Ba ion is too large for A site in this system. Further, by substitution of Ba with Ca in Ba 3 NbGa 3 Si 2 O 14 , transparent and colorless Ca 3 NbGa 3 Si 2 O 14 crystals have been successfully grown at the melting point of 1320°C. [ABSTRACT FROM AUTHOR]