The surface microstructure of tricalcium silicate (C₃S) was observed by transmission electron microscopy (TEM). The tricalcium silicate specimen was dispersed in ethanol or isopropanol. It was then placed on a copper mesh with a collodion film that had an amorphous layer on the surface of each grain. The amorphous layer was caused by the hydration reaction of C₃S with water in the dispersants. The irradiation by an electron beam caused the layer to change into CaO crystallites with a concurrent formation of silica-rich domains.

抄録全体を表示

β-SiC conversion layers have been prepared on two different graphites with the “continuous” and “discontinuous” pore size distribution by the solid-solid (SS) and vapor-solid (VS) reactions in SiO₂-C system. X-ray diffraction patterns of converted specimens showed the noticeable differences in the conversion extent to SiC according to pore size distribution. It was thought that the high infiltration and high conversion rate of “continuous” (CBY) graphite were due to the continuous pore distribution in the range of 0.01-10.0μm. In cross-sectional morphology, the converted “continuous” graphites showed the SiC layer with the interlayer with the average SiC:C ratio of 1:3, but the converted “discontinuous” graphites showed the conversion layer and an interface boundary with the SiC:C ratio in the range of 1:3 to 1:4. The interlayer of converted “continuous” graphite seems to be responsible for the high SiO gas infiltration rate, the resultant active SiC nucleation behavior and the conversion behavior over limited volume of surface. The FEM stress field diagrams of the converted graphites revealed the residual stress fields associated with the thermal mismatch between the conversion layer and the substrate. It was assumed that the stress values of SiC conversion layers measured by X-ray method originated from the additional constraining effects by the stress relaxation across boundaries and the grain growth or grain combination effect in conversion area.

抄録全体を表示

A composite material containing La₂CuO₄ and CuO was directly synthesized by thermal decomposition of a heteronuclear CuLa (dhbaen) (NO₃)⋅nH₂O complex, where dhbaen is N, N′-bis (3-hydroxysalicylidene)-ethylenediamine. The mean particle size was of the order of 200nm for a sample decomposing at 800°C. The exposure to 7.8ppm H₂S at 50°C for 6d (the exposure gas was alternately changed from air to 7.8ppm H₂S at 6h intervals for 6d) resulted in the formation of copper sulfide and lanthanum sulfate on the surface. At 100°C, the surfaces were mostly covered with both sulfide and sulfate. Lanthanum sulfate and copper sulfide, without copper sulfate, were formed at temperatures below 200°C. Copper sulfate was detected when the sample was exposed at 250°C. For samples exposed at 100°C, the atomic ratio determined by X-ray photoelectron spectroscopy (XPS) for [S (sulfate)]/[La] was 1.6 on the surface and decreased to 0.01 with increasing depth of Ar sputtering on the surface. The ratio of [S (sulfide)]/[Cu] was 0.99 on the surface and decreased to about 0.45 upon Ar sputtering. However, [S (sulfate)]/[La] and [S (sulfide)]/[Cu] ratios on the surface were 0.03 and 0.39, respectively, for single-phase La₂CuO₄ prepared by the solid state reaction method after the exposure. It is concluded that the reactions of Cu and La in La₂CuO₄ with hydrogen sulfide are accelerated by the presence of well-dispersed CuO.

抄録全体を表示

An oxalate coprecipitation method was applied to produce rare-earth-doped ceria powders (Ce_0.8R_0.2O_1.9, R=Yb, Y, Gd, Sm, Nd and La). The powders calcined at 600°C were isostatically compacted and sintered at 1600°C in air to reach a relative density exceeding 98%. The electrical conductivity of rare-earth-doped ceria ceramics was dominated by the migration of oxygen ions in the bulk and was in the range of 6.7 to 13.6S·m^-1 at 800°C. The activation energies for diffusion of oxygen ions in the bulk ranged from 75.3 to 93.4kJ/mol. The lattice energy theory for the fluorite structure was applied to estimate the activation energy. The calculated activation energy was close to the measured value.

抄録全体を表示

Various semiconducting oxides are examined as alternative electrode materials for dye-sensitized solar cells. It was confirmed that Nb₂O₅-based electrodes exhibit a higher open-circuit voltage and fill factor, compared to TiO₂-based electrodes. It was also found that a mixing of oxides, with a higher flat-band potential level such as SrTiO₃, Ta₂O₅, and ZrO₂, with Nb₂O₅ led to a higher open-circuit voltage. By mixing Nb₂O₅ with TiO₂ in different ratios, it was revealed that these oxides form no chemical reaction products after a low-temperature heat treatment and a highest power output of the cells with the composite electrodes was obtained at Nb₂O₅:TiO₂=8:2 in mass. The power output with this composite electrode was higher than that with pure Nb₂O₅ or with pure TiO₂. The Nb₂O₅-TiO₂ composite electrode, in which the Nb₂O₅ particles are dispersed on the TiO₂ surfaces, exhibits high photoelectrochemical performance.

抄録全体を表示

[Si-Y-Ti-O-C-N] multicomponent amorphous powders were synthesized by pyrolysis at 1000°C, in NH₃ flow, of chemically modified PHPS using Y(OCH(CH₃)₂)₃ and Ti(N(CH₃)₂)₄. The crystallization and microstructure de velopment were studied by comparison with [Si-Y-O-C-N] amorphous powders derived from PHPS chemically modified with Y(OCH(CH₃)₂)₃.
(1) Amorphous Si₃N₄ in the [Si-Y-Ti-O-C-N] powders was rather stable below 1400°C in N₂. Above 1600°C, Si-Ti-Y-O-N liquid phase accelerated the α-/β- phase transformation of Si₃N₄ and the [Si-Y-Ti-O-C-N] powders yielded β-Si₃N₄-Ti(C, N)-Y₂O₃ ceramics at 1800°C.
(2) Fully dense β-Si₃N₄-Ti(C, N)-Y₂O₃ ceramics were also synthesized by hot pressing at 1800°C in N₂. The resulting ceramics exhibited a uniform and fine-grained micro structure.
(3) Ti(C, N) phase was dispersed as particles having a size range of 10nm to 1.6μm and the large Ti(C, N) particles with diameters larger than 0.5μm were located at the β-Si₃N₄ matrix boundaries.
(4) Titanium in the [Si-Y-Ti-O-C-N] powders could act as a catalyst to accelerate α-/β-Si₃N₄ phase transformation as well as an in-situ source for nano/micro Ti(C, N) particles, which lead to microstructure uniformity and refinement of β-Si₃N₄-Ti(C, N)-Y₂O₃ ceramics.

抄録全体を表示

Mechanical properties of dense spinel/mica composites were investigated. The composites were fabricated by sintering powder mixtures containing spinel as a matrix and 5, 10 and 20mass% Fph-glass or Tm-glass. Fph-glass has the composition of fluorophlogopite. Tm-glass has the composition of tetrasilicic mica enriched in K₂O and SiO₂ components. The microstructures of the composites obtained by sintering powder mixtures containing 5, 10 and 20% Tm-glass (Tm-composites: Tm-5, Tm-10 and Tm-20) were fine and the intergranular phase among the spinel grains consisted of mica crystallites embedded in glass. The microstructures of the sintered composites obtained by sintering powder mixtures containing 5, 10 and 20% Fph-glass (Fph-composites: Fph-5, Fph-10 and Fph-20) were coarser and the intergranular phase among the spinel grains was crystalline. Strength values were in the range of 293-199MPa. In Fph-5 and Tm-5, crack propagation occurred in a brittle fashion through the spinel-spinel grain boundaries. High strength values were obtained only because Fph-5 and Tm-5 composites had smaller critical defect size. In Tm-10 and Tm-20, cracks also propagated mainly in the intergranular glassy region and brittle fracture occurred. In Fph-10 and Fph-20, deviation and branching of the cracks occurred along large-grown mica and spinel grains and led to toughening of the composites. Furthermore, the results of Hertz indentation tests showed that in Fph-10 and Fph-20, quasi-plastic deformation occurred and the post-indentation strength values of these specimens were higher than those of Tm-composites, which showed cone-like cracking.

抄録全体を表示

The purpose of this study is to manufacture the electrode material for high efficiency wet-type optical sensor. As for the electrode material, TiO₂ was found to be suitable. From current experimental results, it is also clarified that TiO₂ electrode manufactured by the light beam melting process is better to be used for wet-type optical sensor which can measure turbidity, temperature, and flow velocity in the liquid. In the present study, the improvement of electrode characteristics is performed by using TiO₂ electrode which is manufactured with the baking and the light beam melting method. Especially, when the methanol is used for rapid cooling, we can manufacture the better TiO₂ electrode for absorbing the UV light. This TiO₂ electrode can be used wet-type optical sensor.

抄録全体を表示

The microstructure and mechanical properties of Si-Ti-C-O and Si-Zr-C-O fiber-bonded ceramics, produced by hot-pressing a laminated material of pre-oxidized Si-(Ti or Zr)-C-O fibers, were investigated. The Si-Ti-C-O fiber-bonded ceramic had a close-packed structure of fibers with a volume fraction of 85% and a SiO₂-based oxide matrix including TiC particles. On the other hand, the Si-Zr-C-O fiber-bonded ceramic consists of a close-packed structure with a volume fraction of 83% and a SiO₂-based oxide matrix without any precipitated phase. Moreover, an interfacial carbon layer is constructed around all the fiber elements in both the fiber-bonded ceramics during the hot-pressing. The high-temperature flexural strength was much higher in the Si-Ti-C-O fiber-bonded ceramic than in the Si-Zr-C-O one. This was thought to be due to the SiO₂-matrix in the Si-Ti-C-O fiber-bonded ceramic, which seems to be refrained from the softening effect by dispersed TiC particles at high temperature.

抄録全体を表示

The oxidation behavior of stoichiometric SiC fiber (Hi-Nicalon-S) was investigated and compared with those of Nicalon and Hi-Nicalon fibers. Hi-Nicalon-S fiber was oxidized in Ar-O₂ gas mixtures with 25-100% O₂ at 1273-1773K. The oxidation rates were measured with thermobalance experiments. The thickening in SiO₂ film produced a large decrease in the strength of the as-oxidized fiber. The SiO₂ film-removed fiber (unoxidized core) retained high strength independent of the SiO₂ film thickness. The oxidation rate obeyed the diffusion-controlled contracting-disc formula. The rate constant was proportional to the oxygen partial pressure of Ar-O₂ gas mixture. The activation energy was 79kJ/mol. The oxidation rate was considered to be controlled by the diffusion of oxygen molecule through the micropores in SiO₂ film. Compared to Nicalon and Hi-Nicalon, Hi-Nicalon-S is resistant to oxidation and has good strength in the oxidized state.

抄録全体を表示

The effect of synthesis condition, such as calcining temperature and grinding, on the sinterability and electrical conductivity were investigated for the perovskite-type oxides which are expressed as (La_1-xCa_x+y)CrO₃ (x=0-0.5, y=0-0.2). The sinterability depended on the calcium content (x), A-site excess amount (y), calcining temperature, and ball-milling procedure. (La_1-xCa_x)CrO₃ (x=0.2-0.3) without A-site excess (y=0) showed high sinterability by firing in air at 1300°C, provided that the calcining temperature was above 1000°C and that subsequent ball-milling was adopted. The electrical conductivity at 1000°C (1273K) also strongly depended on the calcium content (x) and A-site excess amount (y). The electrical conductivity was proportional to the calcium content (x) in the compositional range of x=0-0.4, y=0.

抄録全体を表示

This report represents results of ²⁹Si MAS NMR, investigation of the silicon carbide powders synthesized with a precursor method. The variety in particle morphology is controllable in the precursor method, and the synthesized powders show characteristic NMR spectra. Most of the powders show overlapping three broad signals at peak positions of -16, -21 and -25ppm in the NMR spectra, which indicate variation in second neighbors in coordination of individual silicon atoms. Such variety in the coordination is known to have close relationship to the stacking faults in the quite small and spherical SiC grains.
As an exceptional case, the powder synthesized from the SiO₂-2.1C precursor (C/SiO₂=2.1) at 1973K shows a large broad signal at -19ppm with a spike at -16ppm. This powder contains no residual carbon, and consists of large SiC crystallites of 100-200nm. The fluctuation in chemical shift in type A silicon, which is dominant coordination in β-SiC (3C) structure, should originate the broad signal at -19ppm.

抄録全体を表示

The present work is concerned with the thermal shock behavior of a ceramic matrix composite consisting of an alumina base containing 5vol% of copper particles. The composite, hot-pressed at 1550°C, exhibited increased flexural strength, enhanced toughness and higher resistance to thermal shock compared with monolithic alumina. Some mechanical and thermal properties relevant to thermal shock were discussed which gave plausible explanations for the differences between the composite and the monolithic alumina.

抄録全体を表示

An Al-free glass-ionomer cement with good workability and setting behavior was obtained by mixing CaO-Fe₂O₃-SiO₂ glass powder with polyacrylic acid solution. The cementation was speculated to proceed via the release of Fe³⁺ and Ca²⁺ ions from the glass, and subsequent formation of Fe (III) and Ca (II) polyacrylates.

抄録全体を表示

Effect of the slurry depth on slurry hydraulic pressure and cake growth rate during slip casting was examined for zirconia aqueous slurry of 50-75mass% solid. Buoyancy, which acted on three sorts of stainless steel sinkers in the zirconia slurries, was measured. Buoyancy in slurry increased linearly with the slurry density and the volume of the sinker. Buoyancy is generated by the slurry hydraulic pressure distribution. It is found that slurry hydraulic pressure increases proportionally to the slurry depth. The cake growth rate became higher at a higher head of the slurry.

抄録全体を表示

It was attempted to implant hydroxyapatite granules into a superplastic Ti-4.5Al-3V-2Fe-2Mo-alloy substrate. In order to improve the biocompatibility of the Ti-alloy, a pure titanium film was initially coated on the alloy substrate by reactive direct current (DC) sputtering method. Hydroxyapatite granules with 32-38μm in diameter were spread over the coated Ti-alloy substrate and then pressed to implant the granules in the substrate. Upon implanting the granules at the temperature of 750°C, many cracks on the coating film were observed centering around the implanted granules, and the HAp granules themselves were observed to be damaged, too. In the meanwhile, upon implanting the granules at the temperature of 800°C, a pure titanium coating film was observed to become thin partially at the shoulder of the side wall in the vicinity of HAp granules; however, the granules were implanted into the alloy smoothly without any crack at all on the coating film and any damage to the granules. The hydroxyapatite implanted Ti alloy is expected to be useful for biomaterials as artificial bones and dental roots.

抄録全体を表示

In the early 1980s, SiC ceramics with unique properties were developed by researchers in the laboratory of Hitachi, Ltd. These SiC ceramics have high thermal conductivity (>250W/mK) comparable to that of metals, as well as high electrical resistivity (about 4×10¹¹Ωm) close to that of insulators such as alumina. This development was epoch-making, since at that time, no other materials exhibiting both high thermal conductivity and high electrical resistivity were known except BeO and the development of SiC ceramics exhibiting these properties could not be expected because SiC is naturally a semiconductor. In addition, as the thermal expansion coefficient of SiC ceramics (4×10^-6/K) is nearly equal to that of Si, the application of this material in the field of semiconductors as a substrate and heat sink was anticipated.
This material is commercially used at present as a heat sink for semiconductor lasers globally and commands a high share of the market.
The period when this material was developed was the most active for the ceramics industry and development. Materials called “New Ceramics” or “Fine Ceramics” were attracting attention worldwide. The fact that high thermal conductivity and high electrical resistivity could be compatible in a popular material propelled ceramics researchers to try to find materials similar to these SiC ceramics. As a consequence, a remarkable increase of the thermal conductivity in AlN, which is more manageable than SiC for semiconductor packages, was subsequently achieved. The role of these SiC ceramics in promoting the development of these new materials is a major one.
These SiC ceramics are also important from the point of view of grain boundary phenomena unique to ceramics research. The SiC is produced by sintering SiC powders with small amounts of BeO additives. High thermal conductivity and high electrical resistivity result from the change of the physical and chemical states of grains and grain boundaries caused by the addition of BeO. Because of the low solubility of Be into SiC grains, the purity of SiC grains is enhanced as the impurities are emitted to grain boundary regions. The resulting decrease of phonon scattering is thought to be the cause of the high thermal conductivity. At the same time, the low solubility of Be into SiC grains results in a low carrier concentration in SiC grains and the formation of high potential barriers for the carriers at the grain boundaries. This is the reason for the high electrical resistivity. ZnO varistor is an example of “grain boundary controlled ceramics” and the varistor property of SiC ceramics has also been recognized. The dramatic changes of grain boundary characteristics exhibited by these SiC ceramics have led to the recognition of the importance of grain boundary control. Technological advances for grain boundary analysis and control, including chemical composition distribution analysis and direct measurement of electrical properties at grain boundaries, were hastened. SiC contributed to these advances.
Many papers have been written on these SiC ceramics. This paper details the effects of additives on thermal conductivity and electrical resistivity. The characteristics of SiC ceramics change markedly depending on the type of chemical additive. Of all the additives, only BeO results in anomalous thermal conductivity and electrical resistivity of SiC ceramics. This is due to the low solubility of Be into SiC grains. These are essential properties of SiC ceramics. This paper, describing these properties, deserves to be considered as one of “the leading papers on ceramics of the 20^th century.”
SiC ceramics with high thermal conductivity and high electrical resistivity are said to be the product of “serendipity.” The effect of BeO addition was discovered in the course of finding effective additives to develop SiC ceramics

抄録全体を表示