Papers by Author: Masaru Miyayama

Abstract: High-performance single crystals of ferroelectric Bi₄Ti₃O₁₂ (BiT) have been successfully obtained via the top-seeded solution growth under high oxygen pressure (Po₂) atmosphere. Crystals grown at 960°C at a Po₂ of 0.9 MPa exhibited a well-saturated hysteresis loop with a remanent polarization of 48 mC/cm² and a coercive field of 29 kV/cm. The results of piezoresponse force microscopy indicate that polarization switching is accomplished throughout the BiT crystals obtained. Electric-field-induced strain measurements along the a axis yield a piezoelectric constant d₁₁* of 37 pm/V for BiT.

Abstract: (Bi_0.5Na_0.5)TiO₃ (BNT) single crystals were grown by flux method at a high oxygen pressure (Po₂) of 1 MPa, and their polarization properties were investigated. BNT single crystals exhibited a saturated polarization hysteresis with a remanent polarizations (P_r) of 31 μC/cm² along [100]_c, 44 μC/cm² along [110]_c and 54 μC/cm² along [111]_c. These results show that spontaneous polarization of BNT is approximately 55 mC/cm² or larger.

Abstract: The lithium ion conducting properties of lithium- and MⅣ (M = Al, In, Y)-doped zirconium pyrophosphates synthesized via solid state reaction were investigated. The ionic conductivity of the compounds increased with increasing Li content. The activation energy of LixMxZr1-xP2O7 decreased as the lattice parameter increased owing to the enlargement of the size of bottleneck between the cavities. Li0.55(Li0.15Y0.1Zr0.75)P2O7 with a high Li content and a large lattice parameter exhibited a conductivity of 1.7×10-3 S cm-1 at 350 °C, which is sufficient for its application as a solid electrolyte for sensors.

Abstract: 0.995(Bi1/2Na1/2)TiO3-0.005Bi(B’,B”)O3 solid-solution ceramics (B’,B” denote Zn, Mg, Ni, Ti, Nb) were fabricated and their ferroelectric properties were investigated. Based on the structural characteristics of Bi(B’,B”)O3 analyzed by first-principles calculations, a materials design for obtaining a large piezoelectric strain is proposed: an index Z= Ps∙(ε33*∙s33)1/2, where Ps is spontaneous polarization, ε33* is relative permittivity and s33 is elastic compliance. A positive correlation was observed between Z and effective piezoelectric constant d33* observed for strain measurements.

Abstract: Single crystals of x (Bi0.5K0.5)TiO3-(1-x)(Bi0.5Na0.5)TiO3 (BKT-BNT) solid solution were grown by a flux method, and their ferroelectric properties were investigated along [100]cubic (the cubic notation). While the flux growth soaked at 1250 °C led to the crystals with a composition of x less than 0.14, the crystals with x of 0.45 were obtained by decreasing soaking temperature to 1150 °C. The crystals of BKT-BNT (x = 0.45) exhibited a relatively large remanent polarization of 39  C/cm2 and a low coercive field of 23 kV/cm.

Abstract: SPEEK-based composite membranes containing various amounts of titania nanosheets (TNS) as inorganic fillers were investigated for proton exchange membrane fuel cell applications. The samples were characterized for water uptake, proton conductivity (EIS), and structural features (SEM and XRD). Composites at low inorganic additive contents exhibited improved properties in terms of proton conductivity and water uptake behavior. Best improvements were observed for the composite containing only 0.95 wt% of TNS. This result could be associated to the unique nature of the two dimensional nanostructure of the inorganic additive.

Abstract: 0.999Bi0.5Na0.5TiO3-0.001BaTiO3 (BNT-0.1%BT) single crystals with a maximum size of 1×1×1 mm3 were successfully grown by a flux method using BNT-6%BT powder and Bi2O3 as a flux. The crystals showed a remanent polarization of 33 μC/cm2 and coercive electric field of 40 kV/cm along [100]Cubic. Electric-field induced strain measurements along [100]Cubic revealed an effective piezoelectric strain constant d33 * of 80 pm/V.

Abstract: Superlattice-structured Bi4Ti3O12- PbBi4Ti4O15 single crystals were grown, and their properties of polarization hysteresis and leakage current along the a axis were investigated. Oxidation annealing led to a marked increase in leakage current, while annealing in N2 atmosphere yielded a marked decrease in leakage current at room temperature. These results show that electron hole is the dominant carrier for the leakage current. A well-saturated polarization hysteresis with a remanent polarization of 41 μC/cm2 was observed, which is suggested to originate from the peculiar ferroelectric displacement of Bi in the Bi2O2 layers.

Abstract: Magnetic-field-assisted electrophoretic deposition (B-assisted EPD) method has been applied for synthesizing a(b)-axis-oriented Bi5FeTi3O15 ceramics, and the effects of the B-assisted EPD on grain orientation and microstructures have been investigated. The sintering at 1100oC of the green compact obtained by the B-assisted EPD led to dense ceramics with a high relative sintered density of 98%. X-ray diffraction analysis shows that the a(b)-axis-orientation degree of the grain oriented ceramics evaluated by the Lotgering method was 45 %.

Abstract: Lithium-ion conducting properties were investigated for a layered perovskite oxide Li2SrTa2O7 (LST) and defect-controlled LST, synthesized via solid state reactions. The ionic conductivities of A-site solid solutions Li2[Sr-(La2/3□1/3)-(La1/2Li1/2)]Ta2O7 (□ denotes vacancy.) suggested that lithium ions migrate in the Li-layer. The conductivity of Li-deficient (Li2-z□z)(LazSr1-z)Ta2O7 increased dramatically from 4.2 × 10-6 S cm-1 (z = 0, LST) to 1.6 × 10-3 S cm-1 (z = 0.2) at 400°C with increasing Li-vacancy concentration. This result obviously indicates that the conductivity of LST originate from the Li migration through vacancies in the Li-layer.