Papers by Author: Takashi Nishida

Abstract: In order to perform the growth position control, PbTiO3 (x=0) (PTO) nanocrystals were deposited on atomically flat and non-atomically flat -Al2O3 substrates by RF magnetron sputtering. The atomically flat substrates with atomic steps and terrace are expected to induce the lateral growth along a terrace between the surface steps. In the case of the atomically flat, the nanocrystals got lined up along the atomic steps, and uniform nanocrystals were obtained on surface in same pitch.

Abstract: In this study, (Ba1-x,Srx)TiO3 thin films of various composition ratios were deposited on glazed Al2O3 substrates by the CSD method, and the temperature dependence of the dielectric property was analyzed. The dielectric property of the BST (80/20) thin film annealed at C was highly tunable, and the temperature dependence of the dielectric constant was stabilized at around room temperature. However, this BST (80/20) thin film exhibited intense ferroelectricity. Its tan intensified to 0.0511 in the high-frequency range (1 MHz). After the BST thin film was annealed at 600C, the grain size of the BST thin film was reduced to 40 nm and the ferroelectricity was alleviated. The dielectric constant, tan, and tunability of the BST thin film at 100 kHz were 158, 0.0170, and 39.6 (214 kV/cm, 6.0 V), respectively. The change rate of the dielectric constant, which indicates its temperature dependence, was ±7 or less in the range of -55 to 85C. Therefore, this BST thin film was considered to guarantee a wide operation temperature range for microwave tunable devices.

Abstract: A new oxide, Bi(Mg1/2Ti1/2)O3 was prepared using a high-pressure high-temperature technique at 8 GPa and 1000°C. As-sintered Bi(Mg1/2Ti1/2)O3 has the BiFeO3-type perovskite structure with the rhombohedral R3c. After ground, crystal structure changed to tetragonal P4mm. Structure parameters of tetragonal Bi(Mg1/2Ti1/2)O3 were refined from laboratory X-ray powder diffraction data (a = 4.033(10) Å, and c = 3.938(10) Å). Bi(Mg1/2Ti1/2)O3 does not decompress at ambient pressure, but the structure changed from rhombohedral to tetragonal on heating above 800°C. This phase transition was nonreversible. Bi(Mg1/2Ti1/2)O3 is expected to be one of the end members of lead-free ferroelectric materials.

Abstract: The (Ba0.6Sr0.4)TiO3 (BST) thin films were formed on a Pt bottom electrode/glazed-Al2O3 substrates by a chemical solution deposition (CSD) method. The BST films were annealed at temperatures between 600 and 800°C in a rapid thermal annealing (RTA) process and grown at each temperature with a random orientation. The grain size of the BST films enlarged and the electrical properties of the BST films improved as the annealing temperature rose. The grain size of the film annealed at 800°C enlarged to be 80 nm. The averaged surface roughness Ra was 2.927 nm, which resulted in a favorable degree of planarity. The dielectric constant and loss tangent of the film at 1 MHz were 403 and 0.049, respectively. The film also displayed a high degree of tunability, which was 58.3% (at 429 kV/cm). These results indicated that high-quality BST thin films could be formed on an extremely cheap glazed-Al2O3 substrates.

Abstract: (Ba,Sr)TiO3 (BST) thin films have been investigated because of significant interest in device applications of their nonlinear dielectric properties such as in frequency tunable devices. We have investigated frequency conversion devices using BST films as new nonlinear devices, such as frequency mixers and multipliers. In this paper, we report the evaluation of nonlinear properties of various capacitors, and the experimental equation of the properties and the equivalent circuit model for frequency converters were established. A frequency multiplier using the BST film capacitor was fabricated, and obvious third harmonic signals could be observed. A conversion efficiency of -20 dB was obtained with a low input power of 0.25 W. Analysis of the converter with an equivalent circuit was also carried out, and the calculated output voltage agreed well with the measured result. The model is expected to be used for the analysis and improvement of conversion devices.

Abstract: Growth of ferroelectric Bi2WO6 (BWO) mono-domain bulk crystals was attempted by the vertical-Bridgman (VB) method below the phase transition (ferro- to paraelectric) temperature of 940oC using Li2B4O7 as a flux. In this method, Pt crucibles with different shapes were used. The crucible with a wedged tip bottom produced BWO crystal with a thickness of over 4 mm along the crystallographic c-axis (perpendicular to the spontaneous polarization axis). Using BWO mono-domain crystals grown by slow cooling technique, on the other hand, their electric properties were characterized. The dielectric constants, ε ij, and electromechanical coupling factor, k33, of the crystals were 70-100 and 36% at room temperature, respectively.

Abstract: We report an effective substitution of aluminum for gallium in langasite-type La3Ta0.5Ga5.5O14 (LTG) crystals for use in a pressure sensor at high temperature. Al-substituted LTG (La3Ta0.5Ga5.5-xAlxO14; LTGAx) single crystals up to the solubility limit of x=0.5 have been grown by the conventional Czochralski technique. The electric properties of the LTGAx crystals were investigated and compared with those of LTG. By Al substitution, the piezoelectric constant d11 and the electromechanical coupling factors (k12) became slightly larger. The LTGAx crystals showed a lower temperature dependence of d11 and a higher electric resistivity ρ than those of the LTG crystals.