Yoko Suyama

Grain size dependence of dielectric properties of ultrafine BaTiO3 prepared by a sol-crystal method

Ultrafine BaTiO3 prepared by a decomposition of an organometallic crystal with unity of Ba/Ti ratio (sol-crystal method) has been characterized. While the as-prepared product resulting from the decomposition of the organometallic crystal at room temperature was BaTiO3 with pseudo-cubic structure, the well-crystallized tetragonal polymorph was obtained by firing the as-prepared product above 1000°C. Residual organic compounds, CO2-3 and OH- ions in the samples prevent the grain growth and tetragonal distortion of BaTiO3. We obtained quite higher room temperature permittivity (3700) at 1 kHz for the sample fired at 1200°C than that (630) prepared by conventional solid-state reaction starting from BaCO3 and TiO2. Such a high value was probably due to the accomplishment of homogeneous cation stoichiometry, which was achieved by this preparation method via the organometallic crystal with stoichiometric Ba/Ti ratio.

Spark-plasma-sintering of fine BaTiO3 powder prepared by a sol-crystal method

Spark-Plasma-Sintering (SPS) has been applied to fine BaTiO3 powder (several nanometers) prepared by a sol-crystal method. The starting powder was densified to more than 95% of the theoretical X-ray density, and the obtained SPS pellets had an average grain size of less than 1 μm, owning to the relatively low sintering temperature (900–1000°C) and short sintering period of ca. 3 min. The room temperature permittivity at 1 kHz for SPS samples sintered at 1000°C is ca. 10,000. This high room temperature permittivity value is attributed to electrical inhomogeneities within pellets; a resistive surface layer covers the inner pellet core that consists of oxygen-deficient BaTiO3. The reduced pellet core is characteristic of SPS pellets formed from powders that contain small amounts of residual organic matter.

Formation of BaTiO3 from a Barium Titanium Isopropoxide Complex

The mechanism for the formation of BaTiO3 from a single-crystal barium titanium isopropoxide complex with the presumed composition BaTi[OCH(CH3)2]6C6H6 was investigated in this study. When the alkoxide complex was treated in Ar at 29° C for 2.5 h, 80 wt% of the organic constituents in the crystal were eliminated as isopropanol, propene and benzene, thus forming BaTiO3; 95 wt% of the organic constituents were eliminated in the same manner at a temperature below 100° C. The remaining organic constituents were eliminated as alkyl benzene at around 460° C in Ar. Crystalline BaTiO3 with ultrafine particles, ranging in size from 5 to 7.5 nm, formed even at room temperature under these experimental conditions.

New Synthesis Process of Li, Na and K Niobates from Metal Alkoxides

New synthesis process to prepare nano-particles of lithium niobate, sodium niobate and potassium niobate by thermal decomposition of the constituent double metal alkoxides was developed. Single crystals of such double-metal alkoxides as Na-Nb, Li-Nb and K-Nb ethoxides were newly synthesized from a mixed solution of the constituent metal ethoxides. The doublemetal alkoxides of the Li-Nb, Na-Nb and K-Nb systems decomposed at low temperatures below 673 K to form nano-particles of LiNbO3, NaNbO3 and LiNbO3. The lattice constants and crystallite size of the obtained LiNbO3, NaNbO3 and LiNbO3 particles were elucidated. It was shown that this new synthesis process was useful for preparation of niobate nano-particles at low temperatures.

In Situ TEM Observation of Crystallization Process for LiNbO3 and NaNbO3

Fabrication of advanced electronic components requires high-quality powders. In this work, nano-powders of Li or Na niobates are synthesized from (Li or Na)-Nb ethoxide by a sol-crystal method. A single crystal of (Li or Na)-Nb ethoxide is decomposed to an amorphous matrix below 473 K. Next, small crystals are grown by heating at the appropriate temperature for each specimen. The sol-crystal method provides homogeneous quality and fine grains by heating at lower temperature. Structural analysis of the powders is performed by a transmission electron microscope (TEM) and X-ray diffraction. As a result, LiNbO3 turns to dense-powders, but NaNbO3 forms nano-porous powders. In order to understand this difference, we try to observe in-situ the crystallization and grain growth processes by high-temperature TEM. We successfully observe in-situ this processing and discuss the structural change and formation mechanism of LiNbO3, comparing these features with those of NaNbO3.

Piezoelectric and Fatigue Properties of Pb(Zr0.53Ti0.47)O3 Thin Films on LaNiO3 Thin Film Electrode

This paper describes the electrical properties of the CSD-derived ferroelectric PZT thin films on the oxide electrode of LaNiO 3 (LNO), which can improve the electrical fatigue for the PZT thin film. We successfully control the orientation of the resulting LNO thin film electrode by the molecular design of the precursor solution to control the orientation of the PZT thin film with a MPB composition on the LNO. As a result, PZT/LNO thin film capacitors exhibited superior electrical properties with no fatigue, not only for remanent polarization but also for piezoelectric and dielectric properties.

TEM Zone Axis Critical Voltage in YBCO and Microanalysis of Oxygen Concentration

The zone axis critical voltage effect of YBa2Cu3O7-y is investigated, in order to detect a local change in oxygen content and to examine ionic states of constituent atoms.The critical voltage is determined with an accuracy of 1~3% by observing convergent-beam electron diffraction discs or measuring the characteristic X-ray intensities. The Critical voltages calculated theoretically do not fit the measured values due to an uncertainty of scattering factors for ions, in particular for an oxygen ion. The calculated critical voltage changes linearly with a change in oxygen content and the slope is almost constant irrespective of values of the scattering factors used. A relative local change in oxygen content can be detected with an accuracy of 0.3~0.7% and the spatial resolution better than 0.1μm, by measuring the critical voltage, although an absolute value of oxygen content is not determined. Applying this technique to a melt-textured sample we observed a local change of 0.8% in 7-y near a dislocation.

Dislocation loops in melt-textured YBa2Cu3O7-y

We find by transmission electron microscopy that dislocation loops are created in YBa2Cu3O7-y during electron irradiation or during subsequent preservation in a silica-gel desiccator. The loops are produced in a specimen prepared by Ar ion milling but not in the one prepared by crushing or chemical etching. They are associated with stacking faults formed by insertion of an extra layer of CuO or two extra layers of CuO and Ba or Y planes. The loops are produced near the surface regions in one of the two orthorhombic variants suggesting that formation of the loops is initiated by irradiation of Ar ions and they grow by electron-induced diffusion. Dislocation loops are also found to be generated during degradation near surfaces of chemically etched specimen but the distribution and size of the loops are distinctly different from those of ion-milled specimen. The dislocation loops generated during or after TEM specimen preparation and those produced during the crystal preparation are hardly distinguished in the high resolution TEM images, making their adequate characterization very difficult.

Microstructures Near The Y2BaCuO5 Particle in Melt-Processed YBa2Cu3Oy

Transmission electron microscopy (TEM) was used to investigate the microstructures near the Y2BaCuO5 (211)/YBa2Cu3Oy (123) interface. Lattices of the 123 phase are elastically strained around the particles of 211 phase due to thermal and elastic mismatch between the two phases. The strained region is far larger than that of a dislocation. Thorn-like twin platelets are formed from the surfaces of 211 particles and dislocations are generated from most of the 211 particles irrespective of whether a TEM specimen is prepared by dimple-grinding or not. Stacking faults are freguently observed in the vicinity of the 211 particle, although they are restricted on (001) planes which intersect with the particle. The stacking fault consists of excess single layer of CuO, or double layer of CuO and Ba or Y planes, giving rise to local deviation from the 123 stoichiometry. These lattice strain, dislocations and stacking faults may play an important role in flux-pinning and account for the observed superconductor characteristics: improvement in Jc under magnetic fields and an increase of Jc with the volume fraction of 211 particles.

TEM Observation of Microstructures in Melt-Processed YBa2Cu3Oy

The melt-processed YBCO with the high critical current density (Jc) was investigated through transmission electron microscopy in order to elucidate the relation between microstructure and improvement in the Jc. The sample consisted of single grain of 123 phase with small angle (2∼3°) tilt boundaries and fine particles of 211 phase embedded in the matrix of 123 phase. Every 211 particle accompanied many dislocations around it. Some of them formed a network or a spiral. Piled-up dislocations were also observed frequently. Most of dislocations were a perfect-type with Burgers vector of [100] or [010]. A local fluctuation of oxygen content was revealed from the selected area diffraction and the dark-field imaging near the critical voltage of second order reflection. It appeared from high resolution electron microscopy that a lattice spacing of 123 phase slightly varied in the vicinity of 211 particle. It is considered that the high Jc under a strong magnetic field of the melt-processed YBCO is attributed to the absence of usual grain boundaries and the presence of abundant dislocations around the 211 particles.