The mechanism of oxidation of a low-oxygen SiC fiber prepared by the electron radiation curing method was investigated. The oxidation rates were measured with a thermobalance in O₂ at temperatures from 1373 to 1773K. The reaction products were examined by X-ray diffraction and SEM observation. The oxide film was amorphous silica below 1573K and crystallized to cristobalite above 1673K. The core crystallized to form a trace of β-SiC, accompanying the formation of amorphous free carbon. The oxide film suppressed the pyrolysis of the core. The oxidation rate obeyed the diffusion-controlled contracting-disc formula. The activation energy was 234kJ/mol. The oxidation is considered to be controlled by the gas diffusion through the micropores in the silica film. The reduction of oxygen content in the polymer-derived SiC fibers is effective not only for suppression of the high-temperature pyrolysis but also for improvement of the oxidation resistance.

抄録全体を表示

The sintering reaction for sialon composites composed of Y-α′-sialon and β′-sialon was investigated in the system Si₃N₄-Y₂O₃-AlN. It is assumed that Y-α′-sialon with higher solubility is formed by the reaction between AlN and liquid phase at the initial stage of sintering, and that Y-α′-sialon with lower solubility was formed by the reaction between Y-α′-sialon with higher solubility and the remaining Si₃N₄ at the final stage of sintering. Y₂SiO₅ and YAM (2Y₂O₃⋅Al₂O₃) were formed as intermediate compounds.

抄録全体を表示

Multilayer composites (MLCs) of alumina and yttriadoped tetragonal zirconia (YTZ) with the layer thickness of 50-200μm have been fabricated. A combination of centrifugal filter-pressing and rate-controlled sintering has been proposed for preparing MLCs with a strengthened layer interface. Centrifugal-pressing was efficiently used to strengthen the layer interface by introducing the compositional gradient. Rate-controlled sintering reduced interlayer stress generation during sintering, prevented delamination and contributed to the strengthening of the interface. It was considered that the strengthened layer interface contributed to the toughening of MLCs by means of (1) effective crack deflection in the alumina layers, (2) enhanced transformation toughening due to a deformed frontal zone shape in the YTZ layer constrained between the alumina layers, and possibly (3) crack pinning at the layer interface.

抄録全体を表示

Densification kinetics of Al₂O₃ and Y-doped ZrO₂ ceramics was quantitatively correlated with the kinetic stability of the corresponding colloidal suspensions. Analysis of the densification data showed that increasing the kinetic stability of suspension significantly enhances the rate constant of sintering in the linear densification regime. A concept of critical stability of colloidal suspension was applied to the interpretation of densification kinetics of Al₂O₃ and Y-doped ZrO₂, and this demonstrated a direct linear correlation of the stability ratio (W) with the reciprocal densification-rate constant with a transition behavior at the critical stability point.

抄録全体を表示

Carbothermal reduction and nitridation of diatomaceous earth were carried out in a flow of nitrogen containing various reducing gases (NH₃, H₂, CO, and CH₄) at 1350°C. Addition of 75mol% NH₃ to N₂ brought about a marked effect upon the inside and outside whisker formation. The ability of the various reducing agents in the reduction of SiO₂ was investigated, and found that a carbon plate or methane, can reduce SiO₂ in diatomaceous earth, but that neither H₂ nor CO can reduce SiO₂ at 1350°C. The reduction rate in the presence of both the carbon plate and H₂ was higher than that for either one of them. The formation of CH₄ in a blank experiment using no diatomaceous earth suggests that the CH₄ formed from NH₃ and the carbon plate plays an important role in the reduction of SiO₂ into SiO (g). Next, the effect of addition of powdered carbon to diatomaceous earth was studied. Since an excess amount of powdered carbon accelerates the reduction and nitridation of SiO₂ and SiO (g), the yields of inside and outside whiskers decreased with increasing amount of powdered carbon added. On the other hand, the yield of Si₃N₄ powder increased with increasing carbon content in the starting materials up to 20wt%. SiC was also obtained in the Si₃N₄ powder at carbon contents >20wt%. When only methane was used as a reducing agent, the maximum yield of inside whiskers was about 28% at 1.0vol% CH₄ in N₂. On the other hand, the yield of the outside whiskers was lower over the CH₄ concentration examined.

抄録全体を表示

hBN/Si₃N₄ composites using polysilazane as Si₃N₄ precursor were prepared by hot pressing at 1400 to 1900°C, 30MPa, N₂ atmosphere. The relative densities of the composites increased with increasing temperature to 92% at temperature of 1900°C, while the hBN/Si₃N₄ composite using only Si₃N₄ powder hardly sintered at the given temperature range. The resultant hBN/Si₃N₄ composites had an excellent oxidation resistance than hBN monolith. A hBN-polysilazane mixture involved by small amounts of oxygen, which may exist in amorphous SiO₂. The polysilazane thermally decomposed to α-Si₃N₄, a part of which reacted with amorphous SiO₂ to form Si₂N₂O, the other was converted into β-form at temperatures above 1600°C. X-ray diffraction studies indicated no formation of solid solution between hBN and Si₃N₄, and the densification process of the hBN/Si₃N₄ composites might be related to the above reaction.

抄録全体を表示

The relationship between critical impact velocities (V_c) on crack initiation and materials properties for silicon nitride having different microstructure was investigated. All silicon nitride ceramics showed Hertzian cone crack at impact over V_c^H and it showed clear relationship with K_IC according to Hertzian cone crack theory, however, the relationship between Young's modulus was not defined clearly. Relative to V_c^R for ring crack initiation does not show exact relationship between bending strength and Young's modulus. In the silicon nitride having higher K_IC, of which included 30-50μm needlelike grains, Hertzian cone crack growth was suppressed from surface ring crack and Hertzian cone crack extension were also suppressed.

抄録全体を表示

The effect of heat treatment on electrical properties of a synthetic silica glass was investigated systematically, and the following results were obtained. (1) The electrical conductivity decreased irreversibly by heat treatment at 1000°C. (2) The glass approached finally a state in which the electrical conductivity changed reversibly between high and low conductive states depending on the condition of the second heat treatment. (3) The irreversible and reversible changes started at the surface layer of the glass and spread to the interior. Such irreversible and reversible changes can be explained by considering the chemical equilibrium between OH group and molecular water, outward diffusion of molecular water and protonic conduction.

抄録全体を表示

Effects of grain size of quartz mixed to porcelain bodies on the mechanical strength of the fired bodies were already reported. In the present paper, effect of amount of the quartz on the strength was examined. There are no relation between content of mullite of the fired bodies and their bending strength, and amount of addition of quartz into the bodies and the strength. But for amount of quartz remained in the fired bodies, the bodies containing a few to 35% of such a quartz have as high bending strength as 170-180MPa. If the content increased beyond this range, their strength decreased. The bending strength of the fired bodies increased with increase of unit cell volume of quartz in the bodies. This result confirms that the prestress effect generated in the glassy phase surrounding the quartz grains is one of the most important factors raising their mechanical strength of the porcelain bodies. Increase in quartz content in porcelain bodies raises their maturing temperature comparatively, but thermal expansion coefficient of these fired bodies which showed highest strength in each body series, did not differ so much.

抄録全体を表示

Effect of Ca-compound addition on synthesis of AlN powder by carbothermal reduction-nitridation method was investigated, in which CaF₂, CaCl₂, CaCO₃, Ca(OH)₂, Ca(CH₃COO)₂ and CaC₂ were selected as additives. Each compound promoted the nitridation, and CaF₂ and CaCO₃ were the most excellent reaction promoters achieving full nitridation even at 1300°C. In case of CaF₂ addition, it was estimated that the reduction-nitridation proceeds through the formation of an intermediate compound, CaF₂⋅5Al₂O₃. In case of other additives, Ca-aluminates, CaO⋅xAl₂O₃ (x=1, 2, 6), were formed as intermediates. The value x in the aluminates was dependent on the chemical species of additives. The grain morphology of synthesized AlN varied with the species of additives; addition of CaF₂ and CaCl₂ resulted in spherical AlN particles. The reduction-nitridation behavior of CaO⋅Al₂O₃ and CaO⋅2Al₂O₃ was also investigated. The Ca-aluminates changed through the reaction step; CaO⋅2Al₂O₃→CaO⋅Al₂O₃→12CaO⋅7Al₂O₃→CaO, finally yielding AlN.

抄録全体を表示

Natural quartz, synthetic quartz, quartz glass, blue silica gel, white silica gel and cristobalite were heated with Al and pitch coke at temperatures from 873 to 2073K in nitrogen gas atmosphere. Effect of the SiO₂ type and addition of CoCl₂, Co, Fe and Ni on the reaction products was investigated. At 1673 and 1873K, the reaction products depended on the SiO₂ type. Judging from the reaction products at 1873K, the SiO₂ was classified into two groups. AlN was formed in the first group that consisted of natural quartz, synthetic quartz and quartz glass. However, AlN was not formed in the second group that consisted of blue silica gel, white silica gel and cristobalite. β-sialon was formed only from blue silica gel. On the other hand, β-SiC was formed from all the SiO₂ except blue silica gel. At 2073K, the reaction products were AlN⋅Al₂O₃ (spinel) and AlN irrespective of the SiO₂ type. CoCl₂, Co, Fe and Ni acted as a catalyst for β-sialon formation and Fe was the best catalyst. In the presence of additives, amorphous white silica gel was better source for β-sialon formation than synthetic quartz and cristobalite.

抄録全体を表示

SiC-SiC whisker composite was fabricated by infiltration of polycarbosilane (PCS) as a precursor for matrix into the whisker preform followed by the pyrolysis process. The density was improved by repetitive infiltration into the composite with low density. The bending strength at room temperature and 1200°C was also improved by increased density. The strength at high temperature was the same as that room temperature. Two liquids were employed for infiltration; n-hexane solution of polycarbosilane and molten polycarbosilane were infiltrated. The density was improved to ca. 2g⋅cm^-3 by the infiltration of these liquids. The composite modified by solution infiltration tended to have higher bending strength than that by molten PCS infiltration for the same density.

抄録全体を表示

Results from the present work on the preparation of Mn-Co-Ni-O thin films from. the corresponding metal naphthenates mixed solution by a dip-coating method are summarized as follws:
(1) The metal naphthenates decomposed thermally to become mixed oxides upon heat treatment at 450°C.
(2) A Mn-Co-Ni-O thin film of an appropriate composition for use as a thermistor has been prepared from the mixed metal naphthenates by further heating at temperatures above 700°C.
(3) The as-prepared thin film almost completely crystallized to form a complex spinel phase, a solid solution of MnCo₂O₄ and NiMn₂O₄, by further heat treatment at temperatures above 700°C for 1h.
(4) The sheet resistance of the film was markedly lowered after heat treatment at temperatures above 600°C.
(5) The B constant of the thin film is slightly higher than that of the bulk of the same compositions

抄録全体を表示

A PZT (PbZr_0.58Ti_0.42O₃) plate was bonded to MoSi₂-20wt%Mo₂B₅ ceramics with an adhesive. Electrical properties of the layered composite and voltage generated in the PZT by mechanical impacts were measured. The electromechanical coupling coefficient κ_p, dielectric constant ε_r and mechanical factor Q_m in the composite were lower values than those in the PZT, but the piezoelectricity of the PZT was found to be left in the composite. When cracks were introduced on the MoSi₂-Mo₂B₅ surface of the composite with the Vickers hardness tester, the voltage vibration above 250mV appeared. These results suggest a possibility for producing ceramic materials with a self-detecting function of crack generation and fracture.

抄録全体を表示

Organo-zirconium phosphate compound, containing phenyl group was prepared by the reaction between zirconium butoxide and phenylphosphonic acid. This compound was applied to the surface modifying reagent of calcium carbonate and homogeneous modification was achieved by using n-butylamine. Bovine serum albumin (BSA) was adsorbed on modified calcium carbonate based on hydrophobic interaction. The amount of BSA adsorbed on the modified calcium carbonate was about three times larger than that on nonmodified calcium carbonate.

抄録全体を表示

Superplastic diffusion bonding of a ZrO₂/Al₂O₃ composite was performed to study the effect of surface roughness on the strength of diffusion bonding. The diffusion bonding proceeded with increasing applied strain. High bonding strength was obtained when the roughness of bonding surface was smooth. The longwave length of surface asperity formed initial voids at the bonding interface. The contact process was completed at the applied strain of a few percent.

抄録全体を表示