In this paper, the three important aspects of SiC materials, that is, the powder synthesis, the sintering process, and the theory of sintering and grain growth are reviewed. They are summarized as follows.
(1) SiC precursors were derived from the liquid materials of silicon-tetraethoxide and phenolic resin. Homogeneous mixing of SiO₂ and C could be achieved in the precursor. The pure SiC powders without residual carbon were obtained by carbothermic reduction of the precursors. SiC powders were of the single-phase β (3C) type, and sintered with the addition of carbon and a small amount of B.
(2) The low-temperature sintering of SiC powder was developed using AlB₂ and C additives. These additives could sinter SiC powder at around 1900°C, and promoted elongation of α-SiC grains during sintering. Al atoms from the additive appear to have dissolved into SiC and partly transformed 6H-SiC into 4H-SiC. This transformation caused anisotropic grain growth. The fracture toughness of the sintered SiC materials increased as the aspect ratio and the size of grains increased.
(3) New models and theory for sintering and grain growth were proposed. The theory assumed that the total free energy on the surface and grain boundary of grains drove mass transport. The rate equations for sintering and grain growth were derived from the theory, and the sintering behavior involved in joining two grains was simulated. It is found that the grain boundary energy, which was dependent on the additives used, was important for sintering of SiC, and that low grain boundary energy was necessary for stimulation of sintering and suppression of grain growth.

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The transition behavior from active to passive oxidation and from passive oxidation to bubble formation for CVD SiC was studied by thermogravimetry at wide-ranged oxygen partial pressures in O₂-Ar, CO₂-Ar and CO-CO₂ atmospheres at temperatures from 1600 to 2000 K. In O₂-Ar, the mass loss rate (i.e., active oxidation rate) increased with increasing P_O2, and at a critical P_O2 the active-to-passive transition (i.e., mass loss to mass gain) was clearly observed. In CO-CO₂, the mass loss was significant at a low P_CO2/P_CO ratio and then the mass loss decreased with increasing P_CO2/P_CO ratio. The transition from mass loss to mass gain was gradual, being contrast to that in O₂-Ar. The transition mechanism in O₂-Ar and CO-CO₂ was explained by Wagner model, volatility diagram and numerical thermodynamic calculation.
The transition from passive oxidation to bubble formation was observed at more than 1985K in O₂-Ar at P_O2>5kPa. At P_O2<5kPa the transition temperature decreased with decreasing P_O2 in O₂-Ar and CO₂-Ar. The bubble formation might be initiated by the stoichiometric reaction between SiO₂ and SiC forming SiO and CO vapors at the SiO₂/SiC interface. The transition temperature could be calculated from the condition that the equilibrium SiO and CO vapor pressures exceed the ambient pressure.

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Novel mesolamellar niobium phosphates have been synthesized using neutral and cationic surfactants. Long range ordered mesostructured lamellar phase was obtained with hexadecylamine (HDA) as a surfactant. A cationic surfactant, cetyltrimethylammonium bromide (CTMABr) gives disordered lamellar structure. Influence of various parameters such as temperature, surfactant/Nb, H₃PO₄/Nb and method of preparation were studied. Mesolamellar spacing of compounds prepared using HDA is between 3.53 and 4.35nm, depending upon synthesis parameters.

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The present paper deals with the determination of optimal conditions for preparing spray-dried granules and slurries, based on the characteristics of granules and supplied slurries. These characteristics were evaluated by the compression and stress-relaxation method, and by the centrifugal consolidation method, respectively. Spray drying of the flocculated slurry resulted in low-density granules with a spherical and solid morphology, which ensures a high compressibility. Another important factor governing the press forming is to adjust the amount of binder so as to improve the visco-elastic deformation of granules; this leads to a better homogeneity of the resulting compacts.

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The oxidation behavior of reduced CeO₂ and CeO₂-ZrO₂ solid solution (CZS) was studied by measuring the weight and temperature change during oxidization, in the near-room-temperature range. A flaming oxidation at close to room temperature was observed in some of the reduced CZS. This oxidation takes place in the CZS reduced at much lower temperatures, 800°C and 1000°C, compared with CeO₂, for which the temperature of 1400°C or so is required for the occurrence of the flaming oxidation. The oxidation was not observed in the CZS reduced at a temperature higher than 1200°C. The material, which exhibited the flaming oxidation, possessed a high oxygen storage capacity (OSC) and a large specific surface area. The dissolution of ZrO₂ into CeO₂ results in an increase of OSC and in the retardation of grain growth. These two effects lead reduced CZS to a flaming oxidation at close to room temperature.

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Diamond surface reforming was carried out using various silane coupling reagents (n-octyltrimethoxysilane, 3-aminopropyltrimethoxysilane, or 3-mercaptopropyltrimethoxysilane). In order to introduce the O-H group, which reacts with the methoxy group in the silane coupling reagents, on the diamond surface, hydrogenated diamond surfaces were treated with mineral acids (H₂SO₄, H₂SO₄+HNO₃, or aqua regia). The diamond powders treated with H₂SO₄ or H₂SO₄+HNO₃ were utilized as the diamond substrate for the reaction with the silane coupling reagents. After the treatment with the silane coupling reagents, peaks assigned to the chemical bonds in the silane coupling reagents appeared in the IR spectrum of the diamond powders. Moreover, these peaks remained even after washing 3 times with organic solvents. The oxidized diamond surface was changed from hydrophilic to hydrophobic upon treatment with n-octyltrimethoxysilane. From the results of this study, it was confirmed that the silane coupling reagent is effective for the surface reforming of diamond treated with mineral acid.

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Ferroelectric lead zirconate titanate (PZT) thin films containing crystalline seeds of barium strontium titanate (BST) nanoparticles were prepared by the complex alkoxide precursor method on indium titanium oxide (ITO) glass electrodes. The PZT films approximately 500nm thick at BST particle concentrations of 0-34.2mol% were fabricated with a spin-coating method and annealed at various temperatures. A non-seeded PZT film was crystallized into a perovskite structure by annealing at 500°C. The seeding with the BST particles promoted crystalline growth of PZT perovskite around the seeds, and lowered the crystallization temperature of the PZT films to 420°C.

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Nickel-zinc (NiZn) ferrite thin films were successfully prepared from nickel, zinc, and iron nitrates aqueous solutions at a substrate temperature between 350 and 500°C in air by a spray pyrolysis deposition method (SPD), where the film thickness was held at about 1μm and the deposition rate was varied between 100 and 200nm/min depending on the substrate temperature. The crystallinity of as-prepared NiZn ferrite films became higher with increasing substrate temperature, but it could not reach a suitable value to produce magnetic properties suitable for practical applications. It was then necessary to anneal the films at higher temperatures above 600°C. Magnetic properties comparable with those of bulk NiZn ferrite and applicable to practical magnetic devices were obtained by annealing at 750°C. Furthermore, we attempted the preparation of well-crystallized NiZn ferrite thin films by adding hydrogen peroxide to the starting solutions.

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CdSe microcryssals with various sizes was successfully embedded in SiO₂ glass matrix by magnetron rf-sputtering technique. The mean size of microcrystals was controllable by changing the relative surface area ratio of the CdSe chips to the matrix in the target, and increased with increasing the relative area. The negative real part of the third-order optical susceptibility was seen for all of the present films, and the magnitude of the absolute value of real part increased with increasing concentration of CdSe. The imaginary part due to two photon absorption also increased with the CdSe concentration. After normalized with respect to the concentration, it was revealed that actual optical third-order nonlinearity of the CdSe microcrystals increased as the mean size decreased, in other words as the quantum confinement effect increased.

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The differences in the kinds of zirconias used for dispersion, and the distribution of the zirconia particles both had an influence on the microstructure and the tensile ductility of the zirconia dispersed alumina. Every zirconia particle inhibited alumina from grain growth controlled by the Zener pinning model and zirconia grain growth rate. The pinning effect of each zirconia particle was different, because it could be dependent on the distribution of zirconia particles in the alumina. Finely and homogeneously the ZR-3Y particles dispersed in the alumina were quite effective in improving the microstructure and tensile ductility due to a suppression of the alumina grain growth.

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Different tar/pitch bonded Egyptian dolomite and dolomite-magnesite mixtures with magnesia content ranging between 30-50% to replace dolomite, were prepared. Three different types of tars with different pitch content (I, II and III) were used. The mixtures were moulded, tempered and attacked by BOF slag at temperatures up to 1700°C. The attacking process was carried out under load 3.5-5kg/cm² as a new method to study the deformation of the refractory to indicate the slag attack. The results showed that tar/pitch (III) with 80% pitch can resist load up to 4kg/cm² when attacked by BOF slag. At the same time, increasing the magnesia content of the tar/pitch (III)-bonded dolomite up to 50% magnesia can increase its softening under load up to 5kg/cm². The erect of the slag composition on the best-obtained mixture of the tar/pitch (III)-bonded dolomite-magnesite refractory was studied. Tempered tar/pitch (III)-bonded Egyptian dolomite was attacked by synthetic basic slag containing different iron oxide content. It was found that the increase of the iron oxide content of the slag is the main factor responsible for the attacking of refractories. It was concluded from this study that, this test could be considered as beneficial test because it simulates the application of the refractories in the furnace. It also can be used as fast criteria for the determination of the resistance of the refractories for slag attacking and can provide the manufacturer and the consumer valuable data about the behavior under load.

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Thermal shock fracture behavior of alumina ceramics containing pores was studied by measuring strength after water-quenching. The thermal shock fracture behavior was evaluated by a critical temperature difference (ΔT_c) and a survival strength (σ_r) after water-quenching test. The critical temperature difference increased with increasing porosity. On the other hand, an influence of pore size could not be recognized. From the comparison between measured and estimated values, it was suggested that an increase in critical temperature difference was originated from the adiabatic effect by air bubbles generating at surface pores. The survival strength after water-quenching test increased with increasing pore size, but increase became smaller in pore sizes over 10μm. The reciprocal of squared survival strength could be related with the elastic strain energy stored up to fracture.

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Aiming at the development of Si₃N₄ based ceramics with low frictional coefficient, Fe₅Si₃-particles-dispersed Si₃N₄ ceramics were fabricated from a mixture of powders containing Fe₃O₄, Si₃N₄ and sintering additives. Microstructure, frictional and mechanical properties were investigated of the obtained ceramics, and it was confirmed that Fe₃O₄ consumed and granular Fe₅Si₃ phase generated with in the Si₃N₄ matrix by the reaction of Fe₃O₄ with other components during sintering. Sintered bodies were densified and high strength of 900MPa was obtained, when the amount of Fe₃O₄ addition was less than 10mass%. Whereas Fe₃O₄ addition exceeded 20mass%, the strength fell off remarkably due to agglomerated Fe₅Si₃ and big pores. Moreover, the 10mass% Fe₃O₄ added Si₃N₄ showed the lowest frictional coefficient. According to data of heat of immersion and contact angle measurement, this result was interpreted as being provided by the lubricant film formed on the ceramic surface.

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Al₂O₃/SnO₂ multilayered films were fabricated via the sol-gel method using ⁱPrOH solutions of Sn(ⁱOPr)₄ and Al(ⁱOPr)₃, which were prepared by adding triethanolamine. Coating was accomplished by a dip-coating procedure. Doping with Pt and surface modification of SnO₂ films by Pt were also performed. The Al₂O₃ layer was coated on the SnO₂ film to fabricate Al₂O₃/SnO₂ multilayered films. The SnO₂ film, after five depositions and firing at 600°C, was about 400nm in thickness and 20-30nm in grain size. The Al₂O₃/SnO₂ multilayered gas sensor exhibited higher gas sensitivity than the monolithic SnO₂ film, particularly to CH₄. The increase in the sensitivity was due to the high resistivity in air atmosphere when the Al₂O₃ layer was overcoated. The doping of Pt into the SnO₂ film negligibly affected the sensitivity, except that to CH₄, though SnO₂ film surface-modified with Pt film exhibited high gas sensitivity even at low temperature. The multilayered film of Al₂O₃/Pt/SnO₂ exhibited high sensitivity to CH₄ and fast changing speed.

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Transparent gels obtained from polyhydroxoaluminum (PHA) solutions on drying were transformed into activated alumina by heating. To obtain activated alumina with uniform pore size, different amounts of tetrapropylammonium chloride (TP) or tetrapentylammonium chloride (TPn) were added to PHA solutions prior to drying to form composite gels. The γ-alumina obtained from the composite gels with ≥40% TP content showed a sharp unimodal pore size distribution in the range of 3 to 4nm, depending on the heat treatment temperature in the range of 400 to 900°C. The γ-alumina thus obtained gave the largest pore volume of 0.30cm³·g^-1 at 600°C. On the other hand, the pore properties of the products obtained from the PHA-TPn composite gels were similar to those obtained from PHA gels without additives, giving a bimodal pore distribution and a small surface area. These results indicate that the pore size of γ-alumina can be precisely controlled by the heat treatment temperature using composite gels with ≥40% TP content.

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Electrical conductivity measurements were applied to a 3Y-TZP/alumina laminate composite fabricated by electrophoretic deposition in order to evaluate characteristics of its laminar structure non-destructively. Electrical conductivity of the composite was measured in directions parallel and perpendicular to the alumina layers. The composite showed the electrical properties of 3Y-TZP in the direction parallel to the alumina layers, and those of alumina in the perpendicular direction. Electrical conductivity measurements well reflected the laminar structure of the zirconia/alumina composite.

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