Masaru Miyayama

Impact of defect control on the polarization properties in Bi4Ti3O12 ferroelectric single crystals

We have investigated the defect structure in bismuth titanate (Bi4Ti3O12) through high-temperature neutron powder diffraction analysis and ab-initio electronic structure calculations. It is shown that the vacancies of Bi and oxide ions are created preferentially in the perovskite layers rather than in the Bi2O2 layers. Measurements of the leakage-current properties of the single crystals demonstrate that electron holes arising from the incorporation of oxygen at the vacancies of oxide ions act as detrimental carriers for electrical conduction at room temperature. A crystal growth under high oxygen pressure is proposed to be advantageous for suppressing the vacancy formation and for attaining a large remanent polarization as well as a high insulating property of the Bi4Ti3O12 system.

Domain structure and polarization properties of lanthanum-substituted bismuth titanate single crystals

Single crystals of bismuth titanate (BiT) and lanthanum-substituted BiT [Bi4−xLaxTi3O12 (BLT)] were grown by a self-flux method. BLT (x=0.85) showed a smaller remanent polarization (2Pr) along the a(b) axis of 41u2002μC/cm2 than 98u2002μC/cm2 for BiT, which mainly originates in its smaller ionic displacements along the a axis. Striped 90° domain walls as well as head-to-head and tail-to-tail 180° domain walls were observed for both crystals, and La substitution led to a marked decrease in the 90° domain width from 8.5 μm (BiT) to 2.5 μm for BLT (x=1.20). Piezoelectric-force microscope observations suggested that antiphase domain boundary (ADB) is present only for BLT and the ADB plays an important role in the formation of the 90° domain.

Large electric-field-induced strain in Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 solid solution single crystals

Single crystals of ferroelectric (1−x)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3 (BKT) solid solution with rhombohedral structure were grown by flux method in the composition range of 0 0.02, piezoelectric strain constant (d33) was enhanced with increasing x up to 297 pm/V at x=0.14.

Sulfonated Polyether Ether Ketone-Based Composite Membranes Doped with a Tungsten-Based Inorganic Proton Conductor for Fuel Cell Applications

Sulfonated polyether ether ketone (SPEEK)-based composite membranes doped with hydrated tungsten oxide were prepared and studied for proton exchange membrane applications. Hydrated tungsten oxide (W O3 ·2 H2 O) was synthesized via acidic hydrolysis of sodium tungstate and its structure and physicochemical features were investigated by thermogravimetric analysis (TG), X-ray diffraction (XRD), and electrochemical impedance spectroscopy (EIS). SPEEK/ W O3 ·2 H2 O composite membranes were prepared by mixing proper amounts of SPEEK and hydrated W O3 in dimethylacetamide as casting solvent. The composite membranes were characterized by XRD, TG-DTA, EIS, and water uptake measurements as a function of the oxide content in the membrane. In particular, XRD patterns as well as TG measurements indicated the existence of a coordinative interaction between the water molecules of tungsten oxide and the SPEEK sulfonic acid groups. This interaction lead to the enhancement of the membrane proton conductivity, as well as of their properties, from the point of view of heat resistance and water solubility. In fact, the addition of tungsten oxide resulted in higher proton conductivity, improved heat resistance, and lower water solubility.

Lithium Intercalation Properties of Reassembled Titanate/Carbon Composites

The lithium intercalation properties of reassembled titanate/carbon (acetylene black or carbon fibers) composites were examined and compared with those of reassembled octatitanate simply mixed with carbon particles or conventional octatitanate. Composites of reassembled titanate and carbon particles were prepared by reassembling tetratitanate nanosheets, which were synthesized through exfoliation and reassembly, by a reaction with HCI in colloidal suspension mixed with carbon particles, followed by heat-treatment. The composites with acetylene black exhibited a smaller capacity and a lower high-rate capability than those with carbon fibers due to the large amount of hydrated water in titanate. The composites with carbon fibers showed a pseudocapacitive lithium insertion behavior. The reversible capacity of the composites with carbon fibers was 190 mAh g -1 , which was higher than those of conventional octatitanate or reassembled octatitanate. A relatively high capacity of 135 mAh g -1 was maintained at a high current density of 10 A g -1 , which corresponds to ∼70% of the discharge capacity at a relatively low current density of 100 mA g -1 . The composites with carbon fibers exhibited an excellent performance as a high-rate lithium insertion electrode.

Switchable diode-effect mechanism in ferroelectric BiFeO3 thin film capacitors

We investigate the mechanism of a switchable diode behavior observed in ferroelectric SrRuO3/BiFeO3 (BFO)/SrRuO3 capacitors. We experimentally demonstrate that the switchable diode effect observed in the capacitors is induced by the polarization reversal in the BFO film. The conductivity in an Ohmic region in different oxidation states provides direct evidence that electron hole acts as the majority carrier, delivering p-type conduction. Density functional theory (DFT) calculations show that the p-type conduction arises from an unoccupied gap state of Fe4+ in an FeO5 pyramid which is derived from Bi vacancy. Our experimental and DFT study leads to the conclusion that the switchable diode effect originates from an asymmetric band bending in the top and bottom depletion layers modulated by ferroelectric polarization and oxygen vacancies.

Cooperative effect of oxygen-vacancy-rich layer and ferroelectric polarization on photovoltaic properties in BiFeO3 thin film capacitors

Photovoltaic (PV) properties of ferroelectric SrRuO3/BiFeO3/SrRuO3 (SRO/BFO/SRO) epitaxial thin-film capacitors are investigated. The experimental results of the markedly reduced PV response caused by the doping of Mn provide evidence that the PV properties originate not from the bulk-derived PV effect but from an interfacial band bending. We show that the capacitors having a defective layer composed of oxygen vacancies at the SRO/BFO interface exhibit a markedly large photocurrent. Our study demonstrates that a cooperative effect between the interface charges arising from ferroelectric polarization and the oxygen-vacancy-rich layer enhances the PV response in capacitor form in the BFO system.

Giant photovoltaic effect of ferroelectric domain walls in perovskite single crystals.

The photovoltaic (PV) effect in polar materials offers great potential for light-energy conversion that generates a voltage beyond the bandgap limit of present semiconductor-based solar cells. Ferroelectrics have received renewed attention because of the ability to deliver a high voltage in the presence of ferroelastic domain walls (DWs). In recent years, there has been considerable debate over the impact of the DWs on the PV effects, owing to lack of information on the bulk PV tensor of host ferroelectrics. In this article, we provide the first direct evidence of an unusually large PV response induced by ferroelastic DWs—termed ‘DW’-PV effect. The precise estimation of the bulk PV tensor in single crystals of barium titanate enables us to quantify the giant PV effect driven by 90° DWs. We show that the DW-PV effect arises from an effective electric field consisting of a potential step and a local PV component in the 90° DW region. This work offers a starting point for further investigation into the DW-PV effect of alternative systems and opens a reliable route for enhancing the PV properties in ferroelectrics based on the engineering of domain structures in either bulk or thin-film form.

Field-induced strain behavior for potassium sodium bismuth titanate ceramics

Data are reported for the dielectric, piezoelectric, electrostrictive, and ferroelectric properties of potassium-substituted sodium bismuth titanate, [(K_xNa_1-x)_0.5Bi_0.5]TiO₃. For the morphotropic phase boundary composition x = 0.2, relaxor-type behavior was observed at room temperature with piezoelectric (effective d₃₃₃ = 325 ldr10^-12 m/V) and ferroelectric properties (P_R = 25 muC/cm², E_c = 30 kV/cm). A transition to a relatively frequency-independent, diffuse phase transformation region occurred with increasing temperature, with no remanent strain or coercive field. Above the transition temperature, the field-induced strain was consistent with contributions from elec-trostriction and field-induced piezoelectricity (M₃₃₃₃ = 1.9 ldr 10^-16 m²/V² and d₃₃₃ = 81ldr10^-12 m/V at 100degC). Information is given for the temperature dependence of properties, e.g., 0.14% strain induced at 50 kV/cm at 200degC. Higher potassium content x = 0.6 stabilized the ferroelectric piezoelectric region to temperatures above 200degC, with a relatively stable (d₃₃₃ = 150-145 ldr 10^-12 m/V between 25degC and 200degC. Pb-free KNBT ceramics appear competitive with PZT, especially for higher temperature electromechanical applications.

Evaluation of Electrode Performances of Single-Chamber Solid Oxide Fuel Cells

The interfacial resistances of La1-xSrxCo1-yFeyO3-δ (denoted LSCF(10(1-x)/10x/10(1-y)/10y)) cathodes, and the catalytic activities of a Ni-Ce0.85Y0.15O1.925 (Ni-YDC) anode and an LSCF(2/8/8/2) cathode of a single-chamber solid oxide fuel cells (SOFCs) were investigated. LSCF cathodes with high oxide ion conductivities gave low interfacial resistances. LSCF cathodes with suitable thermal expansion coefficients formed favorable interfacial structures with a ceria-based electrolyte. Ni-YDC showed a higher conversion efficiency for CH4 and a lower selectivity for CO2 than LSCF(2/8/8/2). The single-chamber SOFC based on the Sm-doped ceria electrolyte with the Ni-YDC anode and LSCF(2/8/8/2) cathode showed a maximum power density of 186 mW/cm2 at 800°C.