Recent research activities of the author's group concerning the structure-property relationship and property design by defect control of bismuth layer-structured ferroelectrics (BLSFs) are described. The polarization measurements for various single-crystal BLSFs showed that the remanent polarization Pr and the coercive field Ec of these ferroelectrics are related to Curie temperature and the number of perovskite units in one layer, m, respectively. For SrBi2Ta2O9 (SBT), Bi substitution at the A site increased Pr, and rare-earth-element substitution at the A site decreased or increased Ec depending on the type and amount of rare-earth element used. These property changes were due to changes in lattice distortion induced by the substitution, and the softening and hardening of polarization property were attained in SBT. A decrease in the concentration of oxygen vacancies was found to be very effective in improving the polarization and insulating properties of Bi4Ti3O12 (BIT). Conductivity analysis and ab initio band-structure calculations showed that the doping of higher-valence cations at the B site and the substitution of rare-earth elements for Bi at the A site decrease the concentration of oxygen vacancies, which cause domain pinning, by charge compensation and lattice stabilization, respectively. Such defect control was demonstrated to be a promising approach to designing the polarization properties of BLSFs.
Unstabilized and yttria stabilized ZrO2 fibers were prepared from a monodispersed ZrO2 particles derived from hydrolysis of Zr(OBu)4. Unstabilized ZrO2 fibers with 71 to 151 μm of width and yttria stabilized ZrO2 fibers with 96 to 214 μm of width, were formed by drying the ethanol suspension of monodispersed ZrO2 particles with 116 nm in mean diameter. The morphology of the ZrO2 fibers were plate like and consisted of densely packed ZrO2 nano particles. The preparation conditions affected the fiber width. This was due to the lower concentration of ZrO2 particles in suspension, or the higher drying temperatures, which gave the narrower fiber width. The mean particle size of the 5 mol% Y2O3 doped ZrO2 fiber heat treatment at 1473 K was 0.14 μm and the fiber showed 11.0 GPa on the Vickers Hardness Test.
Oxidation behavior of MgO-based composites with 5 vol% Ni particle dispersoid was investigated at temperatures ranging from 1100 to 1300°C in air. Oxidized zone consisted of (Mg, Ni)O solid solution and included voids. Growth kinetics of the oxidized zone followed a parabolic law, which meant that mass transport in the oxidized zone was the rate-controlling process. An oxidation model of Ni-dispersed MgO composites was established as a basis of cation diffusion via vacancy diffusion with defect chemistry of formation of cation vacancy in the MgO-NiO system. The parabolic rate constant and its apparent activation energy estimated from the proposed model agreed with experimental results.
The changes in microstructure and properties of ZnO-added alumina upon sliding under water lubrication were studied. Although the frictional force was lower than that of conventional alumina in the early stages, it increased upon long-term friction testing. Of the acicular ZnO-nAl2O3 (n=3-9) and granular ZnAl2O4 phases dispersed in the samples, ZnAl2O4 with its low hardness was worn preferentially and pulled out during testing. The increase of frictional force during the sliding process was attributed to the pull-out of ZnAl2O4 particles and their traction on the sliding surface during the experiment.
The ferroelectric PbTiO3 (PT) thin films were deposited on MgO(100) and Si(001) substrates by Liquid Source Mist Chemical Vapor Deposition (LSMCVD) method. The Pb(DPM)2 and Ti(iOC3H7)4 (TTIP) were used as metal sources. Metal-organic sources were dissolved in 2-metoxyethanol. The PT thin films have (h00) and (00l) preferred orientations at lower metal source concentrations on MgO(100) substrate. The epitaxial PT thin film could be prepared on MgO(100) substrate at 0.02 mol/L solution at 600°C. The epitaxial PT thin films having stoichiometric composition and smooth surfaces were prepared on MgO(100) substrates. In this study, the solution concentration is an important process parameter to fabricate epitaxial PT thin films. The PT thin films which have polycrystalline phases on Pt(111)/TiO2/SiO2/Si(001) showed about 320 of dielectric constants and 3uC/cm2 of remanent polarization with 109.54 kV/cm of coercive field.
The phase transition of Zr1-xHfxV2O7 (x=0-1) solid solutions substituted for Zr(IV) sites was investigated using X-ray diffraction, TMA and DSC methods. X-ray diffraction experiments revealed that all compositions of Zr1-xHfxV2O7 (x=0-1) were of a single phase having a cubic structure. Lattice parameters of Zr1-xHfxV2O7 (x=0-1) at room temperature decreased linearly with increasing Hf content, due to the smaller ionic radius of the Hf(IV) ion than that of the Zr(IV) ion. Abrupt positive thermal expansions in TMA were observed for all samples at two phase transition temperatures in the vicinity of T1=350 K and T2=375 K, and negative thermal expansions were seen above about 380 K. DSC measurements clearly showed that all of the Zr1-xHfxV2O7 (x=0-1) samples also underwent two successive phase transitions between 330 and 390 K. Two transition temperatures and phase transition enthalpies of Zr1-xHfxV2O7 (x=0-1) samples decreased with the increase of x content, reflecting the decrease of superlattice structure. The sum of transition entropy for successive two phase transitions increased with increasing the normalized relative intensity of the superlattice line at room temperature.
The dielectric temperature characteristics of the high-k (Ba1-xLax)(Ti1-x/4-0.05Ce0.05)O3 ceramics (BLTC: where x=0.01 to 0.05) with diffuse phase transition (DPT) behavior at different frequencies were studied. The ‘relaxor’ behavior of BLTC with respect to temperature (at 1 kHz) and frequency was quantified by means of some parameters, FWHM, δ, γ, and ΔTmf′, Δεmf′/εm′, repectively. DPT behavior and thermal hysteresis effect were discussed. The results indicated that the co-doping with La and 5 at% Ce causes strong diffusion of ferroelectric phase transition of BaTiO3 and pronounced rising of the permittivity peak (εm′>10000 for 0.02≤x≤0.05) in a frequency range 1 to 100 kHz. The rate that the permittivity peak shifts towards low temperature decreases slightly with increasing frequency, from 30.7°C/at% La at 1 Hz to 29.5°C/at% La at 100 kHz. This rate is faster than any BaTiO3 dopant reported previously. The x=0.03 sample meets Y5V and Z5U specifications at 1 kHz when the thermal hysteresis effect is considered. The increase in La concent (x) can markedly suppress the dielectric loss (tan δ).
Recently, the development of new nonsulfide phosphors with high efficiency is urgently needed for VFDs and FEDs. The fabrication for hexagonal Zn1-x MgxO (0.0≦x≦0.15) solid solution and the effect of partially substituted MgO on the cathode luminescence (CL) were investigated in this research. A limited range of (Zn1-xMgx)O solid solution was prepared using the thermal decomposition of complex Zn-Mg oxalate. The solubility of Mg in ZnO was estimated to be ca. 15%. In the samples annealed at 1373 K for 2 h in a reduced atmosphere, green to blue cathode luminescence was observed. It is evident that the partial substitution of MgO results in the shift of green emission band at low wavelength side. The CL material is expected as a low-cost and blue light-emitting phosphor for lowvoltage luminescence in flat panel displays.
Silicate gels were prepared by mixing K2O•3.9SiO2 (0.3 mol/L) and Al(NO3)3•9H2O (1 mol/L) solutions in various ratios, using potassium hydroxide solution (1 mol/L) to regulate the pH. Nine gel samples were thus prepared at a range of pH conditions, filtered, washed and air-dried. About 1 mol% of potassium was incorporated into the gels at low pH, but up to 20 mol% was incorporated under high pH conditions. On heating, characteristic DTA exotherms were observed at higher temperatures, possibly providing an indication of the degree of polymerization of the gels. The low-potassium gels exhibited a sharp exothermic peak at about 1000°C corresponding to mullite formation, whereas the high-potassium gels exhibited a small, gentle exotherm at about 1000-1100°C corresponding to the formation of KAlSiO4 (PAS, considered to be one of the modifications of kalsilite) and/or KAlSi2O6 (leucite), depending on the gel composition. The 29Si and 27Al MAS-NMR spectra showed that the potassium-poor gels contain predominantly tetrahedral Si(1-2Al) and Q1 silicate units, and both tetrahedral and octahedral Al3+, whereas the potassium-rich gels contain predominantly Si(4Al) units and solely tetrahedral Al3+. The implications of these results for the formation of potassium geopolymers are discussed.
We investigated the relation between the crystal structure and ferroelectric properties of Bi2SiO5, Bi4Si3O12 and Bi2O3-added Sr0.8Bi2.2Ta2O9. Moreover, we investigated that the effect of heat treatment using one of the Bi4Si3O12 added Sr0.8Bi2.2Ta2O9 compounds after a vacuum and high pressure oxygen treatment. The remanent polarization(Pr) increased with the added Bi4Si3O12 or Bi2O3, however, it decreased due to added Bi2SiO5. It was considered that the volume of the tetrahedrons, which makes the top of Bi, increased with the addition of Bi2O3 in the (Bi2O2)2+ layer, and the displacement of the atom becomes easy for the applied field due to the pseudo perovskite (SrTa2O7)2- received stress. It was noted that the Tc decreased with the decreasing tilting angle(αb) of the TaO6 octahedron by heat treatment of the reduced direction.
We established a quantitative analytical method of nanoparticle dispersibility, which includes the following steps; 3D reconstructed image by TEM-CT, particle separation by 3D image analysis, and calculation of a dispersibility index. First the 3D image was reconstructed by a series of 121 projections collected along a single tilt axis on TEM. Second particles were separated from the reconstructed image by binarization. The distance between the gravity centers of particles was measured from the reconstructed images. Finally a dispersibility index was defined as the average of the shortest distances for adjacent nanoparticles divided by an ideal distance. The proposed method was applied for alumina nanoparticles in polymer nanocomposites under different synthetic processes, and the dispersibility of them was calculated by the index. The correlation between the dispersibility as a structure and the transmittance at 700 nm as a function was found to be linear. This method is applicable to evaluate the 3D dispersibility quantitatively for the nanoparticle arrays.
The viscoelastic properties of phosphate and borate glasses at and below the transition temperature (Tg) were investigated using the dynamic viscoelastic measuring method. The viscosity in these glasses was found to increase with decreasing temperature, from 1012 Pa•s at Tg to between 1015-1016 Pa•s at around 50°C. The free volume-viscosity relation in glass is discussed considering the free volume theory as applied to polymers. As a result, the free volume was found to be applicable to glass at below Tg. But in contrast to polymers, the free volume fraction was found to differ from one glass to another. Glass structure and bond strength are believed to affect the free volume fraction.
Reactivity of the mixed powder of Li2CO3 with several metal oxides (Y2O3, Al2O3, ZrO2, SiO2 and ZrSiO4) in synthetic air and 100%CO2 was examined to develop a high temperature repeatable CO2 absorbent. The mixture of Li2CO3 and Y2O3 in molar ratio of 1 : 1 was stable in CO2 up to 950°C but decomposed to form LiYO2 at around 850°C in synthetic air (N2 79%+O2 21%, CO2<0.5 ppm). The newly formed LiYO2 started to absorb the CO2 at 300°C with the reaction of 2LiYO2+CO2→Li2CO3+Y2O3. Since the mixture was fairly stable in CO2 up to 980°C, the reversed reaction directed to CO2 emission was not observed in CO2. LiYO2 is a new material for a CO2 absorbent available in a wide range of temperature.
The novel hydrothermal synthesis assisted with ball milling during hydrothermal treatment was developed. Barium titanate (BaTiO3) powder prepared by this method and the effect of the ball milling was evaluated. The starting materials were sub-micron sized TiO2 powder and Ba(OH)2. The synthesized BaTiO3 powders were characterized by X-ray diffraction (XRD), a laser scattering particle size distribution analyzer and a field emission scanning electron microscopy (FE-SEM). The XRD patterns showed that the BaTiO3 powders were synthesized at 100°C and for 5 h with little contamination. The present BaTiO3 powders observed by FE-SEM were regularly agglomerated. The agglomerated particle size estimated from SEM micrograph was 152±42 nm, which corresponded to the particle size distribution measured by a lazar scatter method. Furthermore, as for BaTiO3 synthesis from Ba(OH)2 and sub-micron sized TiO2 powder, the rate constant of BaTiO3 powder fabricated from assistance of ball milling process was increased (2.5×10-2), as compared to a conventional hydrothermal reaction.
A previously proposed method for depositing Ag nanoparticles on silica particles was extended to the preparation of ZnO-supported Pd (Pd/ZnO) catalysts. The method presented in this work consists of two steps: (1) Sn2+ ions are adsorbed on the surfaces of ZnO particles, and (2) Pd2+ ions added are reduced by oxidization of Sn2+ to Sn4+ and simultaneously adsorbed on the surface. TEM observations revealed that Pd nanoparticles with an average size of 2-3 nm were formed on the 20 nm ZnO particles. The Pd nanoparticles were transformed into a Pd-Zn alloy by partial oxidation and steam reforming of methanol, and consequently the Pd/ZnO attained CH3OH conversion efficiencies of 35-50% at reaction temperatures of 310-370°C.
Highly c-axis-oriented aluminum nitride (AlN) thin films were prepared on Inconel substrates using the RF magnetron sputtering technique. The AlN film characteristics were evaluated under pressures of 10.0 to 300.0 MPa and frequencies of 0.1 to 30 Hz at room temperature. The deviation from the linearity of charges with pressures for the AlN films were within 0.58% of a full scale at 300.0 MPa, which indicated a good linearity between 10.0 and 300.0 MPa. Furthermore the time dependence of charge for the AlN films hardly changed under pressure of ±0.8 MPa at 450°C in the time range from 0 to 54 h, which indicated a good durability. It is confirmed that the AlN thin films showed good potential as a pressure sensor under high temperatures and high pressures.
Organic-inorganic hybrid low-melting glasses with extremely low water-absorption were obtained by the complete removal of OH groups which are hydrophilic substituents from phenyl-modified siloxane glasses by heat-treatment. The saturated water-absorption was estimated to be below 0.1 mass%. The thickness, bulk density and refractive index of such glasses remained unchanged even after soaking in water at 23°C for 192 h.