Papers by Author: Masaru Miyayama

Abstract: Transition Edge Sensor (TES) is an energy dispersive X-ray detecting system with high energy resolution. The energy resolution of this system depends on the steepness of superconducting transition curve from normal to superconducting state, heat capacitance and the operating temperature. The TES is based on the dilution refrigerator cooled by about 100 mK. The energy resolution is calculated about 1-2 eV for the detector with maximum detecting energy as 10 eV. The energy resolution also depends on the superconducting current flowing through the TES device because the superconducting current is affected by the temperature stability of the refrigerator. The fluctuation of the superconducting current means the fluctuation of the X-ray spectrum peak center. We have developed the sensitivity correction system to stabilize the peak center of the X-ray spectrum. The peak center of X-ray spectrum correlates with heater power to keep the base temperature of TES device at a constant temperature. The peak center of X-ray spectrum is calibrated by monitoring the heater power at constant time interval using the correlation curve between the peak center of X-ray spectrum and heater power.

Abstract: High-quality La_0.84Sr_0.16Ga_0.26Mg_0.74O_3-δ (LSGM) epitaxial thin films were successfully grown on (100)-SrTiO₃ (STO) substrates at a temperature of 800 °C by a pulsed laser deposition (PLD) method with KrF excimer laser pulses at an ozone pressure of 1.3 × 10³ Pa. X-ray diffraction rocking curve measurements showed that the LSGM films had a full-width at half-maximum (FWHM) value of 0.11 °for out-of-plane 002 reflection, which was smaller than that reported for LaGaO₃ films grown by atomic layer deposition methods (0.18 ^o). The reciprocal spaces mapping of 103 refraction showed that the LSGM films had a slightly larger lattice parameter a (out-of-plane) of 0.393 nm than a_// (in-plane) of 0.391 nm.

Abstract: Ferroelectric (Bi_0.5Na_0.5)TiO₃ single crystals were grown by a top-seeded solution growth method at a high oxygen pressure of 0.9 MPa and their polarization switching dynamics were investigated at ex-situ electric fields along <100>_c. Synchrotron radiation single-crystal X-ray diffraction analyses showed that a splitting of each reflection into four spots were clearly observed during polarization reversal. This splitting is associated with the polarization states with the spontaneous polarization vectors along four directions of equivalent <111>_c It is concluded that the polarization reversal is achieved through non-180 ° (71 ° and 109 °) domain switching in the BNT crystals along <100>_c.

Abstract: Electrochemical properties of defect-introduces graphenes for lithium ion batteries were investigated. Graphene sheets (GSs) were prepared from graphite through treating with oxidizing agent followed by rapid thermal exfoliation. Defect concentration was controlled by selecting the number of times of oxidation of graphite. GSs electrodes derived from 1, 2 and 3 times-oxidized graphite oxides exhibited a high charge capacity of 1250, 1790 and 2310 mAh g¹, respectively, at the 20th cycle at a current density of 100 mA g¹. The enhanced capacity is assumed to be due to additional lithium storage sites such as defects and edges.

Abstract: The electrochemical capacitors (ECs) have attracted a great attention as a rechargeable strage device with a high power density and high safety. An increase in specific capacity is demanded to use ECs in various applications. MnO₂ are expected as electrodes of ECs because of their large oxidation state change (Mn⁴⁺ Mn²⁺), low cost and environmental compatibility. When all manganese ions in the MnO₂ are completely reduced to Mn²⁺ from Mn⁴⁺ over a potential window of 1.1 V, the theoretical capacity is estimated to about 2000 Fg^-1. However, the reported capacity of MnO₂ electrodes are 100250 Fg^-1 [1-2] for powders and around 700 Fg^-1 [3] for thin films. Tunnel structured MnO₂ are expected to show high capacities by utilizing high ionic mobility in the tunnel and high surface area of tunnel walls. Fig. 1 shows crystal structures of (a) Pyrolusite (Tunnel size: 1×1), (b) Hollandite (Tunnel size: 2×2) and (c) OMS-5 (Tunnel size: 2×4) and (d) MnO₆ unit. In the present study, the relationship between the tunnel size and the specific capacity was investigated for those tunnel structured MnO₂. In addition, Hollandite /carbon composites were synthesized to improve the electrode properties of Hollandite.

Abstract: Electrochemical properties of restacked nanosheets (RS-LDH) of nickel-aluminum layered double hydroxide were investigated for the first time. RS-LDH was prepared by delaminating nickel-aluminum layered double hydroxide powder (powder LDH) into nanosheets and restacking them. RS-LDH and powder LDH showed same discharge capacity of 230 mAhg¹ at a current density of 500 mAg¹, but RS-LDH showed a larger capacity than powder LDH at a large current density of 1000 mAg¹. This increase in capacity was assumed to be attributed to the increased specific surface area and ion conductivity.

Abstract: Single crystals of (K_,Na)Nb_1-xTa_xO₃ (KNNT) were grown by the top-seeded solution growth (TSSG) method under high oxygen pressure atmosphere (Po₂ = 0.9 MPa), and their dielectric properties were investigated along [10_cubic (the cubic notation). KNNT single crystals in a composition range of 0 x 0.65 were successfully obtained. Through the measurements of x-ray diffraction and dielectric properties, we propose a temperature (T)-composition (x) phase diagram in the KNNT system in which a successive phase transition from orthorhombic (Amm2), tetragonal (P4mm) to cubic (Pmm) occurs with increases in T and x.

Abstract: Top-seeded solution growth method under high-oxygen-pressure atmosphere has been developed for obtaining high-performance and large-sized single crystals of ferroelectric (Bi_0.5Na_0.5)TiO₃ (BNT). Crystals grown at 1000 °C at a Po₂ of 0.9 MPa exhibited a well-saturated hysteresis with a remanent polarization of 34 μC/cm² and a coercive field of 22 kV/cm along <100>_cubic. The spontaneous polarization of BNT along <111>_cubic is estimated to be 59 μC/cm² from the measured polarization properties along <100>_cubic of the crystals obtained. Domain observations using piezoresponse force microscopy revealed that the degraded performance of BNT crystals grown at a low Po₂ is attributed to unswitched 71° domains remaining even after applying a high electric field to the crystals .

Abstract: A single crystal of ferroelectric 0.88(Bi,Na)TiO₃0.12BaTiO₃ (BNTBT) solid solution with tetragonal P4mm structure was grown by the top-seeded solution growth (TSSG) method at a high oxygen pressure (PO₂) of 0.9 MPa. The crystals grown by the high-PO₂ TSSG method exhibited a large remanent polarization (P_r) of 54 μC/cm², which leads to a spontaneous polarization of 54 μC/cm². The large P_r compared with that of crystals grown at PO₂ = 0.1 MPa is suggested to originate from a low oxygen vacancy concentration for the crystals grown at a higher PO₂. The high-PO₂ TSSG method was demonstrated to be effective for obtaining BNTBT crystals with superior polarization and piezoelectric properties.

Abstract: Lithium intercalation properties of lithium tetratitanate obtained by nanosheets process (NS-LT4) was examined and compared with those of conventional lithium tetratitanate. NS-LT4 was prepared by restacking of tetratitanate nanosheets with LiOH aqueous solution. NS-LT4 exhibited a reversible capacity of approximately 140 mAh g^-1, which corresponds to approximately two Li insertions per formula unit. Two Li insertions per formula unit mean that half of the Ti atoms were reduced from a tetravalent state to a trivalent state. The quasi open-circuit voltage of NS-LT4 was comparable with that of conventional lithium tetratitanate, and the voltage change of NS-LT4 as the change in lithium composition was greater than that of conventional lithium tetratitanate. This potential behavior would be caused by the unique stacking structure with stacking fault and random rotation in nanosheet-plane generated during the restacking of nanosheets.