Formation and development of extremely large pores in alumina ceramics were investigated. Alumina specimens were sintered at 1350-1600°C. They were thinned down to about 150μm and their internal structures were characterized with direct observation method using optical microscope in transmission mode. They are precisely characterized using SEM. The large pore observed in the specimen sintered at 1500°C consisted of many small pores.

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The internal friction of porous ceramics, which were sintered by different methods, were evaluated. For evaluation, Q^-1 that represents the degree of internal friction, was measured. The Q^-1 of conventional ones was higher than that of HIPed ones at a given porosity or Young’s modulus. This difference of Q^-1 is caused by the effect of neck growth.

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The molecular dhyamics method was used to simulate energy transport in α- and β-Si3N4 single crystals. The simulation data, in conjunction with the Green-Kubo formulation, was used to calculate the thermal conductivity of the single crystals, as a function of temperature.

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Thermal response in silicon nitride materials was investigated locally with a micro-thermal analyzer. Three grades of silicon nitride materials, SN-H, SN-L, and SN-A, were examined. SN-H was ultimately fired at 2473K, SN-L was fired at 2273K, and SN-A contains Al₂O₃ as a sintering aid. Bulk thermal conductivities of SN-H and SN-L were 132.3W/(m·K) and 104.7 W/(m·k), respectively. Micro-thermal analysis revealed that the local thermal conductivity of silicon nitride grains was higher in SN-L than in SN-H, contrary to the results seen for the bulk thermal conductivities. The discrepancy in grain and bulk thermal conductivities is discussed.

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Alpha type silicon nitride with Yb₂O₃ and MgO or MgSiN₂ as sintering additives was sintered at 1900°C for 2-48h under 0.9MPa N₂. The sintered bodies were evaluated density, weight loss, thermal conductivity and microstructure. Thermal conductivity of the sintered bodies increased with sintering time. The thermal conductivity of MgSiN₂ doped Si₃N₄ was higher than that of MgO doped Si₃N₄. The thermal conductivity of Si₃N₄ was improved by about 20 W/mK by doping with MgSiN₂ instead of MgO, and high thermal conductivity over 155 W/mK could be achieved in MgSiN₂ doped specimen sintered for longer time (≈ 48 h).

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Silicon nitride with Yb₂O₃ and MgO or MgSiN₂ as sintering additives was sintered at 1900°C for 2, 48h under 0.9MPa N₂. The inner part and the surface of sintered Si₃N₄ were observed by SEM and TEM. In addition, microstructure and composition of grain boundary phase were examined by HREM and STEM. The triple point of MgSiN₂ doped Si₃N₄ was different from that of MgO doped Si₃N₄.

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In our previous work, we synthesized β-sialon nano powders by carbothermal reduction-nitridation of SiO₂-Al₂O₃ mixture, and fabricated β-sialon nano ceramics using it by spark plasma sintering method. The purpose of this study is to evaluate its corrosion resistance in various aqueous solutions. The relative density of the sintered body was over 99% under firing condition of 1800.C, 5min. As a result of corrosion test, it was found to be better corrosion resistance in H₂SO₄ than commercial Si₃N₄ ceramics.

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Thermal oxidation of sintered beta-sialon ceramics was investigated at 13000°C-1600°C in Ar/O₂ (dry) and Ar/H₂O (wet) atmospheres. The weight changes due to oxidation were monitored by thermo-gravimetry. Oxidized samples were characterized by XRD, SEM and EPMA. The weight increase curves showed that water vapor accelerates the oxidation of beta-sialon. Oxidation products were mullite and cristobalite but the latter was not detected in the sample oxidized at 1600°C.

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The oxidation mechanism of SiC should be understood to use in combustion environments at high temperature. CVD SiC was oxidized in N₂-O₂ and Ar-O₂ atmospheres in the temperature range between 1670 and 2010 K. Passive oxidation were observed below 1985 K. The oxidation kinetics changed from parabolic to linear-parabolic with increasing oxygen potential. In N₂-O₂, this transition occurred at higher oxygen potentials than in Ar-O₂. Above 1985 K, bubbles formed independent of oxygen potentials at more than 1 kPa. In the oxygen potential range less than 1 kPa, the boundary oxygen potential between the bubble formation and passive oxidation increased with increasing temperature. In N₂-O₂, the boundary oxygen potential become about 1 order of magnitude lower than in Ar-O₂. Oxide scales were well crystallized in Ar-O₂, whereas amorphous SiO₂ remained in N₂-O₂.

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In order to analyze the fracture probability of brittle materials like graphite subjected to multi-axial stress state, where compressive stress is dominant, a new multi-axial distribution function for fracture was derived using series-parallel link model. A brittle fracture loci was analyzed from the above distribution function. A multi-axial fracture test was carried out using isotropic graphite (IG-12) specimen, which were machined to hollow cylinders, by imposing both internal pressure and axial compression. The experimental results showed the validity of the theory.

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Alumina bicrystals were made by joining two mirror-polished single crystals at 2173K in order to obtain symmetrical and asymmetrical grain boundaries of alumina. Their interfacial fracture toughness values were estimated by single edge V-notched plate tension method. The fracture toughness values of the symmetrical tilt boundaries were almost the same as those of the asymmetrical tilt boundaries, showing a concave curve as the tilt angle increased The fracture toughness values of symmetrical twist boundaries changed on another concave curve as the twist angle increased. There was also no significant difference in the fracture toughness between symmetrical and asymmetrical twist boundaries.

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Ultrafine-grained monoclinic ZrO₂ polycrystals (MZP) and 3mol% Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (3Y-TZP) were obtained by hot isostatic pressing (HIP). Both MZP and TZP were “high purity” materials with impurities less than 0.1 wt%. The deformation behavior was studied at 1273 K, which was lower than the monoclinic ⇔ragonal transition temperature. The stress exponent of 3Y-TZP with grain size of 60 nm was 2.7 in higher stress region, and increased with decreasing stress. The transition of the stress exponent with stress was similar to the deformation of high-purity Y-TZP with grain size of 200 - 400 nm at 1673 K - 1723 K. The deformation of MZP was characterized by a stress exponent of 2.5 over a wide stress range. The transition of stress exponent, which is peculiar to superplasticity of high-purity Y-TZP, is probably a result of yttrium segregation at grain boundary or the solution of yttrium into ZrO₂ lattice.

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Silicon nitride have excellent properties as structural ceramics, such as high strength, good wear resistance, high toughness. To improve the mechanical properties of Si₃N₄, we are fabricated the Si₃N₄/SiC nanocomposites using hot pressing technique. For fabricating dense Si₃N₄/SiC nanocomposites, Al₂O₃ and Y₂O₃ added 2wt% and 5wt% as sinter additive, respectively. The amount of the SiC powder was restrained to fabricate the intra-granular dispersed nanocomposites. The Si₃N₄/SiC nanocomposites are measured mechanical properties, such as relative density, Vickers hardness, fracture strength, and fracture toughness, and observed the microstructures using SEM and TEM. We discussed the relationship between fracture toughness and frontal process zone (FPZ) size.

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Addition of carbon black into a precursor pitch was used to make several C/C composites with different matrix textures. Their modal work-of-fracture values were estimated by single edge notched beam test. Optical microscopy revealed that their matrix textures were changed from flow-type to fine mosaic texture as carbon black content increased. The work-of-fracture values of tensile and interlaminar shear modes were strongly dependent on the matrix textures.

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It was found that fracture toughness K_Ic of single crystalline SiC depended on crystal orientation. K_Ic and surface energy γ_S have a close relation. This study was carried out to clarify the relationship between γ_S of crystal plane estimated by molecular dynamics (MD) method and crack propagation behavior. Estimated γ_S of (0001) was higher than that of other planes. Vickers indentation test showed that (0001) plane was highly resistant to crack propagation.

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Fracture toughness was measured by the indentation fracture (IF) method on brittle materials such as ceramics including alumina and zirconia in this study. A discussion was then made on the relationship between the fracture toughness and the fractal dimension of the indentation crack that was estimated by the box-counting method. The fracture toughness of brittle materials increased with increasing the fractal dimension, namely, with increasing the complexity of crack morphology. Further, a unique relationship was found between the fracture toughness and the fractal dimension, irrespective of the kinds of ceramics. The relationship between fractal nature of cracks and microstructures of the materials was also discussed.

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The grain-size dependence of the hardness of polycrystalline materials is usually described by the well-known Hall-Petch or inverse Hall-Petch relations. The hardness also depends on the depth of an indentation. These dependences are, however, not clear enough for an indentation with nano-scale on ceramics consisted of nanocrystals. In this study, the grain-size and indentation depth dependence of the hardness of zirconia with very fine grains was examined by using nanoindentation technique

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The slope and curvature of unloading, which are usually utilized for Young’s modulus evaluation, are affected by not only the Young’s modulus of a material indented but also its Piosson’s ratio, yield stress, the Young’s modulus and truncated geometry of a pyramidal indenter, and so on. The latter effects cannot be compensated enough by using conventional analysis, for example Oliver&Pharr method. A novel analysis for the proper evaluation of Young’s modulus was attempted by using the finite element method.

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We analyze numerically the residual stresses in a hollow cylinder of functionally graded materials (FGMs), arising from a fabrication process using a multi-layered model. The alumina-tungsten and alumina-zirconia systems are observed as example. The effects of residual stresses on thermal stresses generated from thermal shock are also studied. It is found that a gradient exponent adjusted by processing parameter can control the residual stresses and the mismatch in thermal expansion coefficients between two constituents has a more significant effect on residual stress distributions than does that of Young’s moduli.

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High temperature deformation tests of α-Al₂O₃ single crystals oriented for basal slip were conducted in order to clarify the effect of preliminary deformation. Basal slip was dominant deformation mode at single 1400°C, while rhombohedral twinning was dominant at single 1200°C. In contrast, it was found that after the preliminary deformation at 1400°C, rhombohedral twinning in the additional deformation of 1200°C was considerably restrained and basal slip became dominant. Moreover, dislocation density introduced by this two step deformation was over 1.0×10⁹/cm². These indicate that glide dislocations introduced by preliminary deformation enhance slip deformation and restrain the formation of twinning.

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Three types of fine-grained β-silicon nitrides which have nearly same volume fraction and different composition grain-boundary phases were prepared by HIP. Compression tests were carried out at 1600°C and at initial strain rate of 5×10^-5 /s. Figure.1 shows that true-stress-true-strain curves are systematically changed. It is supposed that difference in flow stress in related to the difference in the melting temperature of glass.

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The new synthetic method of diamond was proposed, where the water worked as the electrode and 1-propanol vapor was introduced into the reaction chamber as the carbon source. The spectroscopic measurements exhibited the emission from H and OH in the plasma above the water surface and the appearance of CO, C₂ and CH with the addition of 1-propanol vapor. With the discharge current of 500 mA, the diamond particles exhibiting the distinct 1333 cm^-1 peak of Raman shift were found on n-Si(111) substrates.

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MCMB powders were treated in RF thermal plasmas under reduced pressures. The plasma-treated powders were characterized by XRD, SEM, TEM, BET surface area, chemical composition and Raman spectroscopy. Electrochemical measurement were performed in 1M LiClO₄/(EC+DEC) at the current density of 0.25mA/cm². The plasma treatment modified the surface morphology, structure, and chemical composition and led to the disordered surface on the MCMB particles. Plasma modification gave rise to an improvement in electrochemical performance such as discharge capacity and first charge/discharge efficiency. The plasma-treated powder without air exposure showed the further improvement in first charge/discharge efficiency.

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Mono-dispersed, uniform, and highly crystallized TiO₂ nanocrystallites (10-50 nm) with controlled phase structures (anatase and rutile) were synthesized by a novel process based on rapid oxidation of in-flight TiC micrometer-size powder in an aerosol reactor at the thermal plasma high temperatures (3,000-10,000 K). The TiO₂ nanocrystallites formed in the Ar-O₂ plasma consisted of isolated single crystal particles of anatase with spherical shapes and rutile grains crystallized in a hexagonal morphology. The fraction of anatase increased greatly with increasing oxygen flow rate in the Ar-O₂ plasma.

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TiSi₂ thin films have been prepared under different experimental conditions. Homogenous C54-TiSi₂ films with smooth TiSi₂/Si interface deposited at 850 °C of substrate temperature, 200W of RF power and 0.4 sccm of TiCl₄ flow rate. The film morphology became rough with coarse grains at 900 °C. Ti₅Si₃ phase formed at 700 °C. As the deposition temperature increased, the resistivity of the film decreased to reach a minimum of 20 μΩ-cm at 850 °C. Ti₅Si₃ and TiSi formed at the RF power increased up to 400 W. TiCl₄ flow rate significantly affected the film composition. TiSi₂ films showed high selectivity of deposition on Si rather than SiO₂.

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Chemically vapor deposited (CVD) SiC was oxidized by electron cyclotron resonance (ECR) plasma in N₂-O₂ and Ar-O₂ atmospheres in the temperature range between 473 and 1073 K. The effects of temperature, oxygen partial pressure and crystal surface (i.e. Si face and C face) on the oxidation rates were investigated. The oxide thickness on the C face was larger than that on the Si face in both N₂-O₂ and Ar-O₂ atmospheres. The structure of oxide films was amorphous SiO₂. The oxide formation in Ar-O₂ atmosphere proceeded at lower oxygen partial pressures than that in N₂-O₂ atmosphere. The ECR plasma oxidation for CVD-SiC was significantly effective to produce oxide films at low temperatures.

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LASER thermal CVD was newly developed and employed to synthesize YSZ films on alumina substrates. High deposition rate of several hundreds μm/h was achieved by supplying high concentrate MO gas on the substrate. Microstructres of deposited films were changed with incident LASER power. Effect of LASER power on the microstructures and the deposition rates was investigated.

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Copper nanosized powder covered by an organic film was prepared by pulsed wire discharge. Thin metallic Cu wire was discharged in a chamber filled with organic fumes and N₂ gas at 100 Torr. Nanosized powder was collected by evacuating the chamber through a filter. X-ray and electron diffraction patterns indicate that the main phase in the powder was Cu and was not oxidized for 3 months in air. From energy dispersive X-ray analysis results, a Cu particle was covered by an organic film, which may suppress the oxidation of the powder.

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Synthesis of fullerene was attempted by pulsed wire discharge in oxygen atmosphere. From results of X-ray diffraction (XRD), a crystallizedgraphite was of served together with stainless steel, Fe-Cr and FeCr₂O₄ which may come from electrodes.

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High purity aluminum nitride powders were successfully synthesized by a pulsed wire discharge using a newly designed continuous wire feeder system. The AlN content of the powders determined by X-ray diffraction analysis was as high as 98 wt.%. The AlN content was improved by increasing the discharge energy. The specific surface area of the powder was measured to be 65.5 m²/g by Brunauer-Emmett-Teller(BET) method. The ratio of the powder weight to the discharge energy was estimated to be 46 g/kWh. The average particle size measured to be 30nm by BET analysis and SEM observatioins

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ZnO films were prepared by a nearby sublimation chemical vapor deposition method using bis(2,4-pentanedionato)zinc [Zn(C₅H₇O₂)₂] as a source material. The deposition rate increases exponentially from 0.58 to 147 nm/min with the increasing substrate temperature (T_s). C-axis highly preferred orientation of the ZnO films were obtained at distance of substrate to source surface; D=5.0 mm and T_s =300°C.

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Thermally-activated Chemical Vapor Deposition (CVD) process was performed with imposition of magnetic field up to 5T (tesla) and iron crystals were deposited. Influence of the magnetic field on the deposition kinetics, the crystal morphologies and the orientations was investigated. Consumption of FeCl₃ increased as the increase of the magnetic field strength. The magnetic field influenced the shape and the orientation of the island-shaped crystals, markedly, however, the influence on the morphology and the crystal orientation was not observed in the film-state crystals.

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The developing texture is one possible way in order to improve the properties of ceramics. Susceptibility is very small in diamagnetic ceramics such as Al₂O₃ and TiO₂; therefore, the effects of high-magnetic field on these ceramics had been generally neglected. Recently, superconducting magnet technologies have developed and have been used for applications to such feeble magnetic materials. We demonstrate that the texture of titania whisker and alumina can be controlled by slip casting of well-dispersed suspensions of the particles in a high magnetic field.

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High magnetic field was effective for fabricating grain oriented ceramics in Bi layered structured ceramics system. Slurry was prepared from raw powder, dispersant and distilled water. Slurry was set and dried in a high magnetic field(10T). The oriented grain structures in samples were examined by XRD analysis. a, b-axis direction in grains aligned along the direction of magnetic field in Bi layered structured ceramics system

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Microstructure with grain orientation has a high potential for greatly improving characteristics of ceramic. However, there has been strong restriction on processing and material for developing this structure. In recent years, much attention has been paid on the effect of high magnetic field on diamagnetic materials. Many ceramic materials are known to be diamagnetic. High magnetic field has a high potential to orient grains. This study uses alumina as raw materials, and produces alumina ceramics with orientated grain in high magnetic field (10T). The oriented grain structure in the samples was examined with XRD and novel polarized light microscopy analysis. Alumina grains were oriented by high magnetic field (10T). The c-axis of the alumina grain were aligned along the direction of magnetic field. This result can be identified by XRD and the novel polarized light microscope by the liquid immersion technique.

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A novel optical microscope has developed to investigate particles dispersed in high magnetic field. The tube length of this microscope was elongated on account of lens of infinity correction optics. This microscope has a high potential to understand microscopic phenomena in high magnetic field.

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Various studies have been reported on the chemical prcessing under high magnetic fields. In the present study, the ceramic compactions were sintered at 1000∼1300°C under a high magnetic field. The microstructures of ceramics sintered bodies were evaluated by XRD technique and SEM observation. The effect of a high magnetic field on microstructure of ceramics was mainly investigated.

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Ca_1-xLa_xB₆ crystals were prepared by the flux method. In the process of dissolving the flux in NaOh solution, iron impurity, a main impurity in the aluminium, attached to the crystal surface. The amount of the iron was increased by the lanthanum doping, and consequently caused ferromagnetism.

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The crystal structure of beta-rhombohedral boron is composed of B₁₂ ICOSAHEDRAL CLUSTERS WITH THREE KINDS OF INTERSTICES A₁, D, E which are large enough to accomondate transition metals. Transition metals occupy the intersticies alternatively. The properties arised by interactions between doped atoms via B₁₂ clusters are an issue of interest. In this study, we successfully synthesized Fe and Cu-codoped beta-boron by solid state reaction.

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High purity Fe₁₆N₂ powder was prepared by nitridation of α-Fe fine powder in ammonia at 120°C for various duration time. Crystallite size of Fe₁₆N₂ became smaller with a progress of nitridation. Saturation magnetization decreased with a reduction of the crystallite size and the value at 2K was 154emu/g, which was larger by 7.0% than that at room temperature. Coercivity showed a maximum of 2kOe at crystallite size of 22nm. Rf-suputter deposited α-Fe thin film was annealed in ammonia without any exposing to ambient air. Nitride formation temperature was reduced with the deposition time of α-Fe thin film.

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So far we have synthesized CaB₂S₄-I and CaB₂S₄-II, and have solved the crystal structure of CaB₂S₄-II. In this work, single phase of CaB₂S₄-I was obtained and its X-ray diffraction pattern can be indexed on a tetragonal unit cell (l4₁/a, a=13.75, c=13.20Å). The structural model of CaB₂S₄-I was deduced by the direct method using the program EXP0 and refined by the Rietveld method using the program RIETAN2000. Final R values were converged to be R_I=3.78 %, R_F=2.53 %, and S=1.36. It was revealed that CaB₂S₄-I is made up from isolated [B₁₀S₂₀]^10-- macrotetrahedral units.

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Aluminum nitride nanoparticles were successfully synthesized from transition alumina nanoparticles using an NH₃-C₃H₈ gas mixture as a reduction-nitridation agent. Phase-pure, nanocrystalline AlN powder with primary particle size of 30 nm and specific surface area of 36 m²/g was prepared under typical reaction condition. Produced AlN nanoparticles possess excellent sinterability, allowing full densification under conventional processing conditions even without the addition of sintering aids.

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Fluidized-bed reduction-nitridation of potassium titanate (K₂O·8TiO₂) fibers were carried out at 800∼1400°C for 1∼4 h in N₂, N₂-NH₃ and/or N₂-NH₃-C₆H₁₄. Reduction-nitridation of K₂O·8TiO₂ fibers was not occurred in N₂. However, TiN fibers were synthesized in N₂-NH₃ and/or N₂-NH₃-C₆H₁₄. The content of resultant TiN in N₂-NH₃-C₆H₁₄, was larger than the ones in N₂-NH₃. The diameter of the resultant fivers increased with increasing reaction temperature above 1100°C.

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The fluidized-bed reduction-nitridation of monoclinic titania whiskers (diameter: 0.1∼0.5μm, length: 5∼10μm) was carried out at 900 ∼ 1450 °C for 30 min in N₂-NH₃-C₆H₁₄ gas mixture. Raw whiskers were fed on SiC filter placed at the center of the reactor. TiN was synthesized above 900 °C in N₂-NH₃ gas mixture. Conversion to TiN increased with increasing reaction temperature. The initial shape of whiskers were maintained at 900 to 1300 °C, however, it was found that needle-like fibers were synthesized at 1450 °C. Compacted whiskers after reduction-nitriding showed the highest electrical conductivity of 0.19 Ω cm.

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Li-M-Ti-O pseudo-ternary compounds (M=Cr, Ni, Fe) were prepared based on combinatorial chemistry by robot system which automatically prepares and evaluates many compounds in short time. It was found that the ramsdellite structure, which was promising cathode materials in Li-ion battery, was obtained at temperatur higher than 1273K.

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The niobic acid gel was prepared by adding ammonia aqueous solution into NbCl₅ ethanol solution. The niobic acid gel was peptized by H₂O₂ and the peroxo niobic acid sol was obtained. When the niobic acid sol was heated at 348 K for more than 24 h, the crystallized Nb₂O₅ nanoparticles whose average diameter was ca. 8 nm were obtained. The particle size and the specific surface area depended on the concentration of ammonia solution.

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TiO₂O powders were prepared by the hydrolysis of concentrated Ti(OH)₄-HCl-H₂O solutions changing with composition of solutions. Titania powders could be form easily on large quantity in a simple process at nearly room temperature. The synthesis rate of TiO₂ increased with decreasing H₂O. The specific surface area of produced powders increased with decreasing HCl. A highly sinterable powder composed of fine TiO₂ particles was obtained by hydrolysis of the solution at Cl^-Ti⁴⁺=10/10 in molar ratio. The relative density of the sintered body fired at 950°C for 2h was >95%.

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Geopolymer technique using alkaline silicate solutions as main source materials is preferrable for synthesing nm-sized silicate powders. Two kinds of sodiumu silicate were used in this study, Na20.SiO2 and Na20.2SiO2 solutions. Gel prepcipitates were obtained by dropwise of Al-nitrate solution. Caustic soda was used as pH regulation agent. Some gels obtained in alkaline region showed polymer character, when examined with MAS-NMR and DTA. The gels turned into crystalline after heating above 1000°C to form mulite and cristobalite. Chemical composition of a gel, for instance, showed 86% mullite and 13% crystobalite contents. Generally sodium contamination was less than 0.5 percent.

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The formation reaction of BaTiO₃ from BaCO₃ and TiO₂ has been interpreted as a solid phase reaction in many cases. The appearance of liquid phases were however confirmed when powder compacts containing BaCO₃ of coarse particles were heated above 950°C. It is predicted that the reaction mechanism is different, where a liquid phase participates in it or does not. Activation energies of the reaction were measured at the two stages of low and high temperatures. It was supported kinetically that the activation energy of the reaction was lowered by a participation of a liquid phase.

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The DTA curve of coarse particles of gibbsite showed an exothermic peak at 1234°C, corresponding to the phase transformation of θ-alumina to α-alumina. The exothermic peak disappeared on the DTA curve of the sample with an addition of 2mass% of AlF₃. The main crystalline phase of the heating samples has already transformed into α-alumina at 1000°C. The addition of AlF₃ caused to shift the phase transformation of alumina to lower temperature side and moreover the transformation phenomenon to α-alumina to be sluggish.

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