Key Engineering Materials Vol. 485

Abstract: We investigated domain structures in Pb(Zr,Ti)O₃ (PZT) ceramics whose composition lies on the morphotropic phase boundary (MPB) using transmission electron microscopy (TEM) and evaluated the piezoelectric properties of PZT. We found that monoclinic nanosized domains (nanodomains), which form in tetragonal domains, strongly correlated with the piezoelectric properties of PZT. The degree of formation of nanodomains depends on the grain composition. Thus, controlling the homogeneity of grain composition in the ceramics is crucial for optimizing the piezoelectric properties of PZT.

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 effect of in-situ magnetic field during deposition (dynamic aurora PLD) was examined for SrTiO₃ thin film deposited on (La_0.5Sr_0.5)CoO₃/CeO₂ buffered YSZ(001) substrates. It was found that both in-plane and out-of-plane lattice parameters were changed by the in-situ magnetic field. Reciprocal space map measurement revealed that the in-plane and out-of-plane lattice parameters are 0.3817 and 0.3953 nm without magnetic field, meanwhile those are 0.3906 and 0.3974 nm with magnetic fields of 2 kG. This indicates that biaxial strain induced by the magnetic field. The effect of magnetic fields on the ferroelectric properties was examined.

Abstract: The effect of oxygen vacancies and their migrations on the dielectric responses of BaTiO₃ (BT) ceramics and (Ba, Ca)(Ti, Zr)O₃ (BCTZ) ceramics was studied using wideband dielectric spectroscopy. Both dipole and ionic polarization of BT ceramics after annealing in a reducing atmosphere markedly decreased. To elucidate the decrease of dipole and ionic polarization, we observed the domain width and the crystal structure of BT ceramics annealed in different PO₂. The mass fractions of constituted phases in 90^o domain were calculated using the refinement of XRD patterns to explain the degradation of ionic polarization by oxygen vacancies. From the results above, we judged that the decrease of dipole polarization was due to the domain wall clamping by oxygen vacancies, while that of ionic polarization was attributable to the lattice hardening by the incorporation of oxygen vacancies into the BT lattice. In the case of BCTZ ceramics, dipole polarization showed the same way as BT ceramics with annealing conditions but the ionic polarization was independent of PO₂ in annealing because of anti-reducing behavior of BCTZ ceramics. On the basis of the effect of oxygen vacancy in BT ceramics and BCTZ ceramics, we could clarify the oxygen vacancy migration in BCTZ ceramics under high direct-current voltage (DCV).

Abstract: Structural changes induced by oxygen vacancies in tetragonal phase BaTiO₃ are studied with first principles calculations within density functional theory. In our calculations, the incorporation of oxygen vacancies greatly decreases c/a ratio of the tetragonal phase BaTiO₃, and a phase transition from tetragonal to cubic phase occurs when the incorporation of the oxygen vacancies reaches about 4%. Our results also shows that the generation of the oxygen vacancies slightly increases the volume of BaTiO₃, and the increases are typically less than 0.5% even in heavily reduced conditions.

Abstract: We performed first-principles calculations to examine the interaction among rare-earth (RE), alkaline-earth (AE) elements and oxygen vacancy (V_O) in BaTiO₃, in order to clarify the combined effects of V_O trapping by two different elements. It was found that there is a synergistic effect of V_O trapping by RE at Ba site and AE at Ti site, so that the co-doping is effective to improve insulating reliability of BaTiO₃. We also verified that the local atomic structures around dopants obtained by calculations well agree with that obtained experimentally by extended X-ray absorption fine structure (EXAFS) analyses. The present work is the first one to clarify the structural environment around dopants including V_O by both theoretical and experimental approaches.

Abstract: The synthesis of Ba_1-xSr_xTiO₃ (BST) powders was attempted using KOH-KNO3 molten salt. The starting materials, Ba(NO3)2, Sr(NO3)2 ,TiO2,and the mixed salt of KOH-KNO3, were well-mixed. The mixture was heated at 300 to 500°C for 1 hour, and the product was thoroughly washed with de- ionized hot water. When the mixture containing excess TiO2 was used, single-phase BST powders with a particle size of 10~50nm were obtained.

Abstract: Relaxor characteristics of (1-x)BaTiO₃-xBi(Mg_1/2Ti_1/2)O₃ (x=0.1–0.7) ceramics were investigated. Microstructural observation showed second phases and no domain structure for the sample with x=0.6. Deviation from the Curie-Weiss behavior was found in temperature dependence of the inverse permittivity for all the samples. The stronger dielectric dispersion was found for x=0.6 and 0.7 and they were described by the Vogel-Fulcher relationship. The temperature dependence of the remanent polarization and coercive field indicated the freezing temperature was 100~150°C for x=0.6. The strong dielectric dispersion of x=0.6 is believed to be induced by the structural disorder due to the second phases.

Abstract: Dense barium titanate (BaTiO₃, BT) nanoparticle accumulations were prepared by an electrophoresis deposition (EPD) method using three kinds of BT nanoparticles with different particles sizes and tetragonality. First, the BT particles were dispersed into ethanol using ball-milling technique, and an optimum milling condition was investigated. As the results, it was confirmed that ball-milling led to a formation of mixture between single-dispersed nanoparticles and aggregated particles. Thus, the aggregated particles in the slurry were removed by a centrifuge in order to obtain narrow the size distribution. Using well-dispersed slurry, BT nanoparticle accumulations were deposited on paradigm electrode by the EPD method. Despite various BT nanoparticles, the dense BT nanoparticle accumulations were obtained at relative densities of around 60 %. After gold electrode deposition on the both surfaces of the accumulations by spattering method, dielectric properties were measured at 20 °C, the highest dielectric constants of around 160 was measured at 10 MHz.