Crystals2014, 4(3), 351-376; doi:10.3390/cryst4030351 - published online 28 July 2014 Show/Hide Abstract
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 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.
Crystals2014, 4(3), 342-350; doi:10.3390/cryst4030342 - published online 16 July 2014 Show/Hide Abstract
Abstract: We propose a high-Q photonic crystal cavity formed by introducing random disorder to the central region of an otherwise defect-free photonic crystal slab (PhC). Three-dimensional finite-difference time-domain simulations determine the frequency, quality factor, Q, and modal volume, V, of the localized modes formed by the disorder. Relatively large Purcell factors of 500–800 are calculated for these cavities, which can be achieved for a large range of degrees of disorders.
Crystals2014, 4(3), 331-341; doi:10.3390/cryst4030331 - published online 16 July 2014 Show/Hide Abstract
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 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,  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.
Crystals2014, 4(3), 306-330; doi:10.3390/cryst4030306 - published online 16 July 2014 Show/Hide Abstract
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 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.
Crystals2014, 4(3), 296-305; doi:10.3390/cryst4030296 - published online 16 July 2014 Show/Hide Abstract
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, 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.
Crystals2014, 4(3), 273-295; doi:10.3390/cryst4030273 - published online 16 July 2014 Show/Hide Abstract
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 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.