Hirohiko Kumagai

Enhanced Piezoelectric Properties of Potassium Niobate Single Crystals by Domain Engineering

The engineered domain configuration was induced into potassium niobate (KNbO3) crystals, and the piezoelectric properties were investigated as a function of domain size. First, single-domain treatment was investigated, and a combination of chemical etching and 2-step electric poling treatment was effective. Using the single-domain crystals, all piezoelectric related constants were determined. On the basis of these piezoelectric constants, the d31 and d32 surfaces were calculated. The piezoelectric properties of [001]c oriented KNbO3 single-domain crystals were measured and were completely consistent with the calculated properties. Finally, the engineered domain configurations were induced into KNbO3 crystals by the control of the temperature and the electric-field along the [001]c direction. The piezoelectric properties of these KNbO3 crystals with the engineered domain configurations showed much higher values than those of the single-domain crystal. Moreover, the piezoelectric properties increased with decreasing the domain sizes of the engineered domain configuration.

Growth and spectra characterization of Ce and Eu doped SBN crystals

Congruent Sr 0.61 Ba 0.39 Nb 2 O 6 (SBN : 61) crystals doped with Ce and/or Eu have been grown by Czochralski method, and characterized by transmission, reflection, emission, and excitation spectra. Both of the two doping ions substitute for Sr 2+ or Ba 2+ and occupy A 1 or A 2 sites in SBN. Ce 3+ and Eu 3+ are the dominant charge states. Compared with the undoped SBN, both Ce and Eu doped crystals exhibit shifts of the fundamental absorption edge towards the longer wavelength direction. Lattice distortion caused by the doping ions has been characterized by the change of lattice parameters obtained from X-ray diffraction patterns, and the width of exponential absorption band tail. In the Ce and Eu double-doped crystal, serious lattice distortion, location shift of Eu 3+ absorption, excitation, and emission peaks, and emission quenching of Eu 3+ ions have been observed. The doping defects and their effects on the lattice and energy band structures are also discussed.

Temperature evolution of domains in potassium niobate single crystals

The behavior of domain walls in KNbO3 single crystals with temperature variation from room temperature to 300 °C has been investigated in situ by a heating visualization system. It has been observed that domain walls show active behavior in a small temperature range from the phase transition temperature of 225 °C, the range being about 10 °C in the orthorhombic phase and 15 °C in the tetragonal phase. The 90° domain walls are generated randomly and extend themselves rapidly within the crystal in high density. The 60° domain walls, which are only observed in orthorhombic phase, do not appear randomly but are formed along the boundary of intersecting 90° domain walls. The results suggest that the 90° domain walls are most likely caused by microdefects within the crystal, and that dislocations at the junction of intersecting 90° domain walls supply nucleation sites for the 60° domain walls.

Pseudo-dendritic growth in lead molybdate single crystal by Czochralski technique

A pseudo-dendritic crystal growth of lead molybdate (PbMoO4) has been studied with the Czochralski technique. The observed dendritic growth is iso-directional, projecting from the circumference of the growth front, where the content of impurities, melt stoichiometry and temperature are expected to be varied. The dendrite arm length and inter-dendrite angle are closely correlated. It is found that the grown dendrite arms are bounded only by ten low-index crystallographic planes (001), (001), (101), (101), (101), (101), (011), (011), (011), and (011). Based on this work, a facet transition pattern existing among the dendritic arms has been revealed. Further analysis for crystal grains in as-grown PbMoO4 single crystals indicates that a similar dendritic growth can also occur under conventional growth conditions. It is further suggested that the constitutional supercooling is a major factor causing the dendritic growth in PbMoO4 single crystals.

Effect of ambient water on crystal morphology and coloration of lead molybdate

Vapour phase growth of two types of lead molybdate (PbMoO4) crystals has been studied in air under normal pressure. It has been found that both PbMoO4 crystal morphology and crystal coloration depend sensitively on water content in ambient air. Using dry air, the resultant PbMoO4 is mainly in form of needles growing along 〈001〉 directions of the crystal whereas in an atmosphere of moisturized air, the product is mostly terrace-like platelets, extending towards 〈110〉 directions. The PbMoO4 needle crystals are found with light-yellow colour while the platelet crystals are light-grey or colourless. Considerations of growth have also been given to the observed two types of PbMoO4 crystals.

Influence of Ce and Co doping ions on photorefractive effect of SBN:61 crystals

Abstract Ce-, Co-, and (Ce,Co)-doped Sr0.61Ba0.39Nb2O6 (SBN:61) crystals with different concentrations were grown by Czochralski technique in the congruent composition. It is the first report of double-doped SBN crystals with Ce and Co. The doping ions Ce and Co occupy 15- (or 12-) fold coordinated and 6-fold coordinated lattice sites in crystal, respectively. A little change of the lattice constants induced by doping ions were detected by XRD experiments. Absorption spectra show that Ce3+ and Co3+ are dominant valence status in SBN:61 crystal. Ce and Co doping enhances two-beam coupling gain coefficient and photorefractive response speed, due to these doping ions donating or capturing electrons during the photorefractive process.

Structural and dielectric investigation of doped Sr0.61Ba0.39Nb2O6 crystals

Undoped, Ce, and Co-doped Sr0.61Ba0.39Nb2O6 crystals were grown by the Czochralski method. The temperature dependence of dielectric constants was measured experimentally. The peak temperatures in dielectric constants of the doped crystals were detected to be lower by around 7 °C than that of the undoped crystal. Doping also results in the dispersion of ferroelectric phase transition due to the more disordered structure. Nonisoelectronic substitution of doping ions and lattice deformation are discussed by taking some properties of doping ion into account, such as radius, charge status, polarization, and concentration. The influences of the doping ions on the dielectric behaviors are also discussed. In Ce and Co double-doped crystals, the interaction between the two doping ions results in the compensation of the extra charges and counteraction of the structure deformation.

Top seeded solution growth and characterisation of ferroelectric potassium lithium niobate crystals

We have grown KLN crystals from different melts by the TSSG method at our laboratory. The melting and crystallisation behaviours of the raw materials have been studied by the DTA method. The crystal compositions were measured by chemical analysis and the solute segregation effects between the melt and crystal were discussed. The crystal structure and optical properties were characterised by means of XRD and optical spectrum measurement. The blue SHG effect of the as-grown crystals was also examined by a dye laser.

Effects of meniscus on the directional growth of potassium niobate single crystals

The study of meniscus effects on the directional growth of single crystals of potassium niobate (KNbO3) with a [110]pc oriented seed by the top-seeded-solution-growth technique has been described. The directional growth of KNbO3 single crystals has been illustrated for different crystallographic orientations of the crystal. Experimental results show that the shape of the solution flux meniscus, which is determined by a number of growth parameters, has a significant effect on growth rates on different 100pc, 010pc and 001pc type surfaces. Findings based on this work enable one to select a dominant growth direction and hence to engineer the geometrical shaping of resultant KNbO3 single crystals through careful control of growth parameters. Maximum crystal dimensions for the grown KNbO3 along [100]pc and [001]pc achieved in this work are 34.0 and 31.2 mm, respectively.

Preparation of potassium niobate single-domain crystals and their piezoelectric properties

Piezoelectric properties of potassium niobate (KNbO 3 ) crystals with the single-domain state were investigated. KNbO 3 crystals were cut, polished and sized into various shapes. The mechanical processing induced mechanical damage into the crystals. Removal of the stressed surface layers was very effective to achieve high mechanical quality factor ( Q m ). After this treatment, KNbO 3 crystals were poled by the 2-step poling method. As a result, the domain state was almost single-domain. Using poled KNbO 3 single-domain crystals, the piezoelectric properties with k 32 modes were measured using a conventional resonance method. As a result, the [001] o poled KNbO 3 crystals exhibited the electromechanical coupling factor ( k 32 ) of almost 40%, the piezoelectric constant ( d 32 ) of almost 18.5 pC/N, the dielectric constant ( l 33 ) of almost 48 and Q m over 7,600. Thus, the KNbO 3 crystals are piezoelectrics with low dielectric constants and high electromechanical coupling properties.