Papers by Author: Yuji Noguchi

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: Single crystals of K0.47Na0.53NbO3 (KNN) and Mn-substituted KNN (Mn-KNN, K0.53Na0.47Mn0.004Nb0.996Oy) were grown by a flux method, and the influence of lattice defects on the polarization and leakage current properties was investigated. As-grown KNN did not show an apparent polarization hysteresis loop due to its large leakage current (~10-3 A/cm2). The substitution of 0.4%-Mn at the Nb site and subsequent annealing under a moderate oxidative condition were effective in suppressing the leakage current of KNN crystals. 0.4%-Mn-KNN annealed at 1100°C in air exhibited a low leakage current (~10-8 A/cm2), a relatively large remanent polarization of 40 μC/cm2 and a coercive field of 12 kV/cm at 25°C. The oxidation of Mn and Nb ions during annealing in air is found to play an essential role in the low leakage current of Mn-KNN.

Abstract: Single crystals of PbTiO3 (PT) were grown by a self flux method, and effects of lattice defects on the leakage current properties were investigated. While PT crystals annealed in air at 700 oC showed a leakage current density of the order of 10-5 A/cm2, annealing under a high oxygen partial pressure of 35 MPa increased leakage current density to 10-4 A/cm2. The increase in leakage current by the oxidation treatment provides direct evidence that electron hole is a detrimental carrier for the leakage current property of PT at room temperature. The vacancies of Pb are suggested to act as an electron acceptor for generating electron holes.

Abstract: Polarization switching and domain dynamics in unpoled and poled crystals of bismuth titanate by applying electric field along the crystallographic c axis were investigated through polarization measurements and domain observations by optical microscope and piezoelectric force microscope. Poled crystals showed a well-saturated polarization hysteresis with a remanent polarization of 4.4 μC/cm2 and a coercive field of 4.7 kV/cm. Domain observations reveal that lenticular domain acts as an initial nucleus during polarization switching. The sidewise motion of the lenticular-domain walls and resultant single domain state were easily established for the poled crystals, while the lenticular domains observed in unpoled crystals were clamped even though a high electric field was applied to them.

Abstract: Crystal structures of Bi4Ti3O12 ferroelectrics have been investigated by high-temperature neutron powder diffraction study. The ferroelectricity in Bi4Ti3O12 is shown to originate from the in-plane displacement of TiO6 octahedra with respect to heavy Bi ions, and that the off-center Ti displacement in the octahedra plays a minor role. Bond valence analysis shows that the underbonding of Bi with the adjacent oxide ions at the perovskite A site in high-temperature paraelectric state plays an essential role in the ferroelectric phase transition in the Bi4Ti3O12 system.

Abstract: Complex perovskite oxides of Sr0.5(Nb0.5Ta0.5)O3 were synthesized by reassembling (SrNb2O7)2− and (Sr1.5Ta3O10)2− nanosheets followed by topochemical dehydration with heat treatment. Atomic force microscopy (AFM) images showed that the thicknesses of the (SrNb2O7)2− and (Sr1.5Ta3O10)2− nanosheets were approximately 6 nm (7 unit layers) and 4 nm (3 unit layers), respectively. Raman scattering measurement has suggested that nanosheet-processed Sr0.5(Nb0.5Ta0.5)O3 with B-site-cation ordering has a superlattice-like structure composed of alternating layers of Sr0.5NbO3 and Sr0.5TaO3 along the c-axis. Nanosheet-processed Sr0.5(Nb0.5Ta0.5)O3 with B-site-cation ordering exhibited a dielectric permittivity of 42 (1 MHz), which was lower than that of solid-state-reacted samples (65) with disordered B-site cations.

Abstract: Large Bi0.5Na0.5TiO3 (BNT) single crystals with 10 mm × 10 mm × 5 mm were successfully grown by a flux method using BNT powder and the mixed flux composed of Bi2O3 and NaF. Dielectric measurements show a dielectric-permittivity peak at 310°C, which is the result of the gradual phase transition from rhombohedral (R3c) to tetragonal (P4bm) structure. Electric-field induced strain measurements along [100]C reveal that the effective piezoelectric strain constant (d33 *) was estimated to be 130 pm/V in a multidomain state.

Abstract: Influence of annealing on the domain structures of bismuth titanate (BiT) and rare-earth (La and Nd)-substituted BiT crystals (BLT or BNT) were investigated by optical microscope and piezoresponse force microscope (PFM). Annealing of BiT at 950°C in air significantly decreased the number of striped 90° domain walls, while charged 180° domain walls were still present in the crystals after the annealing. The annealing for the crystals of BLT and BNT at the same condition did not change their 90° domain structures. PFM observations indicated that antiphase domain boundaries in BLT play an important role in the formation of 90° domain structures.