We have successfully developed a computer simulation technique for microstructural development such as sintering and grain growth of nano/micro-size particles. The molecular dynamic (MD) method, the Monte Carlo (MC) method and the finite element method (FEM) have been used for the simulations. The MD simulation, which was applied to alumina base ceramics, demonstrated the atomic structures and excess energy at grain boundaries and interfaces. The MC simulations were performed for the array of two- or three-dimensional lattices. Plural mechanisms of mass transport were introduced in the MC simulations of sintering and grain growth in ceramic systems, which involve a liquid phase and second solid particles. The combination method between MC and FEM has been developed to simulate precise shrinkage behaviors during sintering process.

View full abstract

The thermal stability of solium aluminium silicates and their reactivity to alkali carbonates were investigated in view of their application as CO₂ gas sensors. A nepheline phase was very stable, since the peak position of the X-ray diffraction pattern was hardly influenced by either heat treatment or humidification up to 800°C. A distorted carnegieite phase (like-carnegieite) transformed into the nepheline phase at 500°C under a humid condition, while under a dry condition, it was stable up to 800°C but transformed into the nepheline phase at 900°C. For mixtures with alkali carbonates, the nepheline phase was most stable even under the humid condition at temperatures below 500°C. When the mixtures were heated at 600°C, the nepheline phase reverted to the like-carnegieite phase and alkali silicates, such as Na₂SiO₃, Li₂SiO₃ and Li₄SiO₄, formed as by-products. For Na₂O-Al₂O₃-4SiO₂ sintering mixtures with Li₂CO₃, the like-carnegieite phase transformed into the nepheline phase at 500°C. The nepheline phase was stable up to 1000°C even under the humid condition and Li₂SiO₃ formed as a by-product.

View full abstract

Second-harmonic voltage between a pair of electrodes, appearing due to nonlinearity, was theoretically calculated in piezoelectric rectangular vibrators with a pair of half electrodes which were driven by a sinusoidal constant current having the resonance frequency of length-extensional 1/2λ-mode vibration. The theoretical calculation was performed using the electrical equivalent circuit of a piezoelectric rectangular vibrator. A material constant as a nonlinear coefficient of second-higher term can be derived by using an equation, as that obtained in the present paper, showing the relationship between the magnitude of second harmonic voltage and the nonlinear coefficient of second-higher term.

View full abstract

Thermal transformation of a previously reported spherical titania/PDMS hybrid particles (J. Mater. Sci., Vol. 39, pp. 4131-4137 (2004)) is investigated in the temperature range from 500 to 1200°C, in air, by means of differential thermal analysis (DTA), thermogravimetric analysis (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), N₂ adsorption/desorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The hybrid particles had an islet-like nano-structure, which consisted of titania and PDMS domains of a nanometer size. The amorphous titania domains were transformed to anatase by a heat treatment above 500°C accompanying densification, while there was no significant change in the titania domain size by the heat treatment at 900°C for 2 h. The silicone domains of the hybrid particles were transformed to amorphous silica completely by the thermal decomposition of the organic groups of the silicone at 700°C for 2 h. After a heat treatment at 1200°C for 2 h, the titania crystallites in the particle partly transformed to rutile and grew by diffusion and fusion of the titania domains accompanying a morphological change of the particles. On the other hand, the silica domains remained amorphous.

View full abstract

A new synthetic route based on solution chemistry was developed for a compound semiconductor lead telluride (PbTe). Lead telluride was synthesized by reduction of PbTeO₃, which was prepared by hydrolysis of the corresponding metal organic solution as starting material. Two kinds of metal organic solution, metal bis(trimethylsilyl)amide and metal alkoxide were used as starting chemicals. PbTe-bisamide solution was prepared by the reaction of lead chloride with Li-bisamide in the presence of Te-bisamide. PbTe-alkoxide solution was prepared by the alcoholysis reaction of PbTe-bisamide with BuOH. Amorphous PbTeO₃ was obtained by hydrolysis of the PbTe-bisamide and PbTe-alkoxide. Heat-treatment of the resulting amorphous PbTeO₃ powder at 250°C and 400°C in air resulted in the transformation to a cubic PbTeO₃ phase and to a tetragonal PbTeO₃ phase, respectively. PbTe was obtained by heat-treatment of calcined amorphous and crystallized PbTeO₃ at 400°C-500°C in H₂. The Pb/Te ratios of the products were chemically stoichiometric.

View full abstract

A novel synthetic route to preparation of GaN particles from Ga₂O₃ and Li₃N under milder condition than other conventional methods has been explored. We have found that a reaction of Ga₂O₃ and Li₃N at temperatures ranging from 500°C and 700°C in nitrogen atmosphere of 0.4 MPa yields a mixture of GaN and Li₂O powder, from which Li₂O is subsequently removed by dissolving the reaction products in water. The GaN single crystals prepared by this method were hexagonal wurtzite type, no cubic phase was found. The crystal sizes were about 1 μm. This technique seems to have applicability also for the synthesis of other nitride particles.

View full abstract

The PZT thin films of about 100 nm or less in thickness on the Pt/IrO₂ and Pt/TiO_x bottom electrodes were prepared by the novel chemical solution deposition process using zirconium oxyacetate as starting material of Zr. In addition, the effects of the excessive Pb to the precursor solution were examined on the crystallinity and the P-V property. PZT thin films prepared from the precursor solutions lowered the temperature starting to make perovskite phase after annealing at 450°C and 550°C for Pt/IrO₂ and Pt/TiO_x, respectively, which was lower than usual one. Among the bottom electrode structures tested, the Pt/TiO_x/SiO₂/Si structure with about 75 nm Pt in thickness was chosen from the viewpoint of the flatness. PZT thin films of 100 nm in thickness were deposited on the annealed Pt/TiO_x/SiO₂/Si optimally by using a novel precursor solution involving excessive Pb. Root-mean-square (RMS) roughness of PZT thin film increased from 2.59 nm to 6.67 nm with increasing excess Pb content from 0 to 15%. Remanent polarization (Pr) began to increase when the excess Pb exceeded 4%. On the other hand, coercive field (Ec) went through a minimum at 7% excess Pb before increasing much at 10%. Considering from these results, the Pb content optimal for the PZT thin film deposited on Pt/TiO_x/SiO₂/Si was suggested to be 7% in excess of the stoichiometry.

View full abstract

Synthetic mica has more high thermostability than natural mica and can be controlled expandability and cation-exchange-capacity (CEC) by changing magnitude of effective-layer-charge (ELC). In this study, we prepared lithium-type tetra silicic mica (Li-TSM) from Na-TSM by ion exchange method. This mica is one of the expansible synthetic fluoro mica but a few details had been attempted. The negative charge in silicate layer of Li-TSM can be controlled by adjusting the amount of Al³⁺-fixation, and the CEC is controllable. When the CEC of Li-TSM had been controlled by partially fixation of Al³⁺ to Li-TSM ((Al, Li)-HTSM), their swelling ability increased and their introducable capacity of organic compounds into interlayer also increased more than Na-TSM. Furthermore, the composites of (Al, Ag)-HTSM, which is Ag⁺-exchanged complex from (Al, Li)-HTSM, and (Al, Ag)-HTSM-TBZ (thiazole) complex, which has antifungal activity, was synthesized. The rate of elution of Ag⁺ from this TBZ composite was inhibited than (Al, Ag)-HTSM. We thought that this was caused by chelation of Ag⁺ and TBZ in interlayer. These composites had an antimicrobial and antifungal activity and this activity was observed for samples heat-treated up to at 400°C where TBZ in TSM interlayer decomposed completely.

View full abstract

Femto-second laser pulses (800 nm, 11 nJ/pulse, 82 MHz) were irradiated on the surface of Co- or Cu-doped silicate glasses (70SiO₂•20NaO₂•10Al₂O₃ doped with 20 mol% of CoO or CuO, respectively) for 1 s to modify its surface, which was observed by a scanning probe microscope and an ellipsometer. Some samples exhibited refractive index reduction without any structural modification, which was brought about by local increase in fictive temperature through spot heating and quenching. With the aid of a computer simulation using a finite element method, such a modification is found to occur when the maximum surface temperature is less than 1000°C for the Cu-doped glass.

View full abstract

When asbestos (chrysotile: Mg₃Si₂O₅(OH)₄) was heated at 600°C for 3 h, dehydration reaction of it had been almost finished, changing to an amorphous phase in crystallography. Then the remaining fiber became brittle, because the structure of fiber could be easily ground till <1 μm diameter in grain size using a mortar. When the compact body made by pressing the ground asbestos fiber at 80 MPa was heated at 1300°C for 1 h, the relative density of the compact reached 90%. Then the crystal phase of the compact was mainly forsterite (Mg₂SiO₄), but a small amount of chrinoenstatite (MgSiO₃) was also detected.

View full abstract

Non-Arrhenius temperature dependence of bulk dc conductivity was observed in La_1/3-xLi_3xTaO₃ which are Li-ion conductors derived from A-site deficient perovskites. In order to discuss the non-Arrhenius conductivity, several parameters relevant to the Li ion dynamics were determined by impedance spectroscopy and ⁷Li NMR. The activation energies of the dc conductivity in the high and low temperature ranges were nearly equal to that of the reciprocal correlation time (f_NMR) obtained from NMR measurement and that of the crossover frequency (f_p) extracted from a power-law analysis of conductivity spectra, respectively. A change in the activation energy of the dc conductivity occurred in the vicinity of temperature where f_p is equal to f_NMR. Because f_p relates to correlated hops of mobile ions and f_NMR to non-correlated hops, our result supports Ngai's explanation which premises presence of a microscopic time.

View full abstract

Mo₅Si₃ particle reinforced Si₃N₄ composite with crystallized grain boundary phase of Yb₂Si₂O₇ was synthesized in-situ by sintering a green compact that contained Si₃N₄, MoO₃, Ta₂O₅, and Yb₂O₃ powders. The reaction between Si₃N₄, MoO₂, Ta₂O₅, and Yb₂O₃ at high temperature generated the Mo₅Si₃ particles, the complex molybdenum and tantalum silicide particles, and the grain boundary phase of Yb₂Si₂O₇ simultaneously. In the Mo₅Si₃-Yb₂Si₂O₇-Si₃N₄ composite, most of Mo₅Si₃ particles residing at the grain boundary phase, and had an average size of 0.18 μm. The addition of Ta₂O₅ promoted the formation of Mo₅Si₃ particle in Si₃N₄ grain and the generation of the Yb₂Si₂O₇ nano-particle in the grain boundary phase. The grain boundary phase and the Si₃N₄ grains were reinforced by the sub-micrometer sized Mo₅Si₃ particles and Yb₂Si₂O₇ nano-particles, which improved the strength of Si₃N₄. The crystallization of grain boundary phase was almost completed by adding the MoO₃, Ta₂O₅, and Yb₂O₃, while a small amount of amorphous phase and the thin amorphous films were detected from the grain boundary pockets, and the interfaces between the Si₃N₄, Yb₂Si₂O₇, and Mo₅Si₃, respectively.

View full abstract

High-temperature characteristics (especially high-temperature strength and oxidation resistance) of gas pressure sintered Si₃N₄ ceramics using Yb₂O₃-SiO₂ additives were evaluated. The high temperature characteristic of Si₃N₄ depended on the Yb₂O₃/SiO₂ molar ratio of additives. High 1500°C bending strength of 395 MPa was obtained for sintered bodies with 10.0 mass% addition of additives with Yb₂O₃/SiO₂ molar ratio of 1.0. Also, small weight change (13.2 g/m²) during oxidation at 1300°C for 300 h was obtained in this sintered body. This is considered to be due to the precipitation of Yb₄Si₂N₂O₇ and Yb₂Si₂O₇ phase in the grain boundary glass phase, as induced by a crystallization process.

View full abstract

Tin dioxide (SnO₂) nanosized powder was synthesized by a simple wet process. The precipitate, which was formed from the addition of NH₄OH to SnCl₄ aqueous solution, was washed with distilled water, and was filtered. The obtained gel was mixed with carbon powder with a particle size of about 20 nm by a mechanically rotating mixer. The mixture was dried at 70°C in air. TEM observation of the dried sample revealed that the hydrated SnO₂ and the carbon powder were mixed at nano-scale level. The carbon powder was burned out by heat treatment at 600°C in air for 2 h, resulting in the formation of SnO₂ powder with a size of 10-30 nm.

View full abstract

Sol-gel derived SiO₂-TiO₂ coatings on indium tin oxide (ITO)-coated glass substrates were treated with hot water at 90°C under electric field aiming at the control of morphology and crystalline phase of the titania precipitates. The shape of the precipitates on 75SiO₂•25TiO₂ (mol%) coating at the negative electrode changed from granular to ramiform by applying an electric field to the substrates during the treatment, whereas such changes in the shape of titania nanocrystals with the electric field were not observed at the positive electrode. The granular and ramiform precipitates were identified as anatase (TiO₂) and hydrated titania (n(TiO₂)•mH₂O), respectively. The ramiform shape of the titania precipitates became significant with increasing the applied voltage, while the coatings gradually dark-colored due to the reduction of Ti⁴⁺ to Ti³⁺.

View full abstract

Wrong:Received October 1, 2001
Right:Received October 1, 2004

View full abstract