Effects of addition of polyacrylic ammonium (shorten as PAN) on colloidal processing of ceramics were investigated with aqueous suspensions of α-alumina powder with an equivalent spherical diameter of 0.35μm. The added PAN was completely adsorbed onto the alumina surface below a critical amount of -3.5×10^-6mol carboxyl group per m² of alumina surface at pH9.0. The zeta potential of alumina powder (-24mV at pH9.0) reached -40mV with addition of the critical amount of negatively charged polymer, and free polymer above the critical amount didn't affect the zeta potential.
The viscosities of PAN-added alumina suspensions showed a first minimum at the critical amount of PAN, and increased with increasing free polymer content. Further addition of PAN caused the secondary minimum of viscosity as a function of PAN content. It was possible to prepare a highly concentrated fluid alumina suspension of ≈63vol% solids by addition of the critical amount of PAN. However, it was difficult to increase the solids content above 53vol% at pH2.5 in electrostatic stabilization with HCl solution.
The green densities of alumina compacts consolidated by filtration of electrostatically stabilized suspensions were independent of solids content in suspension stage, while the green densities for electrosteric stabilization with PAN showed a minimum at around solids content of 40vol%. The pore size distributions depended on both green densities and consolidation processes of powder (filtration or doctor blade method). A narrow pore size distribution was measured in a high-density green compact consolidated by filtration. The median size of pore in a green compact was small for electrostatic stabilization than electrosteric stabilization at a similar density, but PAN-addition was effective to produce a more continuous pore size distribution and a more uniform green structure.
Although the densification behavior was mainly dominated by green density, PAN-addition in suspension stage was effective in achieving a higher density and a more fine texture than electrostatic stabilization at the final stage of sintering. The densification of a high-density green compact proceeded with disappearance of closed pores which were formed at around 92-95% T. D. The low-density green compacts were densified without the significant formation of closed pores. The texture of sintered alumina was greatly affected by the pore size distribution of a green compact. A wide pore size distribution formed a small number of large pores at the grain boundaries of large grains, while a narrow pore size distribution gave a microstructure with many small closed pores located at the grain boundaries of small grains.

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TiB₂, ZrB₂, HfB₂, VB₂, NbB₂, TaB₂, and WB were found to be dispersible particles to achieve the fully densified composites.
By dispersing the above metallic borides into SiC up to 20mol%, the fracture toughness of the composite increased linearly from 3.2MPa⋅m^1/2 to about 5MPa⋅m^1/2. The dispersed boride for the most effective toughening was not yet clariefied due to the large standard deviation of the experimental data and the large differences in particles size.
TiB₂, ZrB₂, HfB₂, and TaB₂, which formed oxides of TiO₂, ZrSiO₄, HfSiO₄, and Ta₂O₅ through an oxidation reaction, induced severe corrosion. On the other hand, VB₂, NbB₂, MoB, and WB, which formed liquid phase of oxides such as V₂O₅, Nb₂O₅, and WO₃, or gas phase such as MoO₃ at 1500°C, showed a somewhat mild corrosion. The more preferable metallic borides seemed to be NbB₂ and WB which formed lower equilibrium vapor pressure oxides only than VB₂ and MoB.
In conclusion, NbB₂ and WB seem to have bright prospects for developing new SiC composite ceramics that simultaneously realize sintering, toughening, and the least degrading in corrosive oxidation at 1500°C.

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Enhancement of fracture toughness increment by fiber bridging in fiber-reinforced composites was theoretically predicted in the case of small scale bridging conditions. This calculation was based on the stress field analysis of fiber pull-out which takes account of the interfacial bond, interfacial residual clamping stress and the Poisson's contraction of fibers induced by applied tensile loads. Fracture toughness tests of unidirectionally SiC fiber-reinforced glass composites were also performed. The fracture toughness increment observed in the experiment was in good agreement with the present theoretical prediction. The optimum interfacial properties to maximize the fracture toughness in steady-state as well as the relationship between the interfacial debonding behavior and fracture toughness were addressed.

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The caustic method is an optical technique for determining the dynamic stress intensity factor of metals and plastics. In this study, the application of the caustic method to ceramics was investigated, and a technique is proposed here for determining the static stress intensity factor from a fine caustic image of a ceramic specimen. The degree of deviation of parallelism and surface roughness of a specimen are important factors in obtaining a clear caustic image of ceramics. A reliable method for measuring the diameter of a caustic by image processing was proved to be effective in this experiment. The stress intensity factor of Si₃N₄ with a BI (bridge indentation) crack was evaluated, and the residual stress intensity factor caused by a Vickers indentation for a starter crack was examined using this method.

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Corrosion behaviors of sintered mullite bodies (99% in RD) with spherical and columnar shapes fabricated from high-purity mullite powders synthesized using a sol-gel processing have been investigated at 900°C, 950°C and 1000°C for 5-180min in KF melt. For both shapes, the reaction of the sintered mullite bodies with KF melt proceeded from the entire surface bodies towards the inner part uniformly at the interface. The thickness of corrosion layer formed at the interface increased relatively uniformly with increases in dipping temperature and time, and it was revealed that the corrosion proceeds parabolically. The corrosion layer acted as a protective layer against the corrosion reaction and suppressed the subsequent corrosion reaction. Using a geometrical shape-parameter depending on the initial sample shape, the process of corrosion progress could be expressed in terms of the diffusion controlled reaction rate equation based on the unreacted core model through the reaction layer.

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Microstructure-controlled ceramic composites composed of various ratios of hydroxyapatite (HAp) and tricalcium phosphate (β-TCP) were formed by reaction sintering of HAp and brushite. Analyses of the reaction by TG, DTA and other methods revealed that several reaction steps were involved. It was found that β-TCP formed at about 800°C and that initial stage sintering of HAp was enhanced by the presence of brushite. The ratio of β-TCP to HAp in the product can be controlled by changing the concentration of brushite in the initial mixture.

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Commercial 96% alumina substrates were evaluated. Nine commercial resistor pastes were printed and fired on seven commercial substrates and a sapphire plate, and the relation between sheet resistance and substrate properties was studied. The fluctuation in sheet resistance for low-and medium-resistance pastes was 30%, but that for high-resistance pastes was 10%. A high correlation was observed between sheet resistance and fired density of the substrates, and the correlation with SiO₂ and MgO contents was medium. No significant correlation of sheet resistance with grain size, surface roughness, thermal expansion coefficient and CaO content was observed. RuO₂-based resistors and pyrochlore-based resistors showed an opposite tendency; the substrate which gave a minimum resistance for RuO₂-base resistors yielded a maximum resistance for pyrochlore-base resistors, and vice versa. The resistance variation for RuO₂-resistors has been attributed to the local concentration of conductive particles, whereas that for pyrochlore-base resistors to the decomposition of pyrochlore to RuO₂. In both cases the resistance variation has been interpreted as a function of glass-substrate interaction.

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A precursor gel powder with the composition of MgTi₄(PO₄)₆ (MTP) was prepared by the sol-gel process. This powder was amorphous up to 700°C and changed into single phase MTP at about 800°C. The relative density of a sintered compact showed that the amorphous powder was more sinterable than the crystallized one. A sintered body with a 95% relative density was obtained from an amorphous MTP powder. Some precursor gels with excess amounts of P₂O₅(1-9wt%) and MgO(0.1-3wt%) were also prepared by the same process. A small amount of P₂O₅ and MgO did not improve the sinterability of the powder. A small amount of MgO, however, increased the bending strength. A relative density of 91% and bending strength of 150MPa was obtained with an addition of 1wt% of MgO.

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To develop a complicated shape forming method for the fabrication of Si₃N₄ engineering ceramic parts the vacuum-pressure assisted slip casting was studied. A well-dispersed and high solids containing slurry (77wt% solids) was prepared, and the consolidation kinetics of the slurry using the vacuum-pressure assisted slip casting system with a porous polymer resin mold and the mechanical properties of the sintered body were investigated. From the slurry containing 77wt% solids (92wt% α-Si₃N₄, 4wt% α-Al₂O₃, 4wt% Y₂O₃), 0.01wt% deflocculant, 0.1wt% defoamer, 4wt% binder, the high-density green body (2.01-2.07g/cm³) was formed in the pressure range of 0.1-0.7MPa. The rate of filter cake buildup (κ) was experimentally determined to be 2.25×10^-13P[m²/s]. On the other hand, the k was calculated to be 4.61×10^-13P[m²/s] using the Adcock and McDowall's equation. The difference between the experimental and calculated values of κ seemed to be mainly due to that the calculation of κ using the BET-surface area of α-Si₃N₄ powder in place of the “permeability surface area” resulted in the overestimation of κ although the actual solids had the wide particle size distribution. The green density was mostly independent on the forming pressure in the range of 0.2-0.7MPa. The Si₃N₄ sintered at 1770°C for 4h at about 0.9MPa N₂ with the addition of 4wt% binder showed a density of 3.22g/cm³, a flexural strength of 612MPa and a Weibull modules of 7.4. It was found that the vacuum-pressure assisted slip casting was useful for the forming of silicon nitride.

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Surface silanol group (SiOH) has been analyzed using Static SIMS. In general, adsorption properties or surface reactivity of silicate glasses are influenced by the surface silanol group, though determination has been difficult up to now. In this study, detection of ⁴⁵SiOH⁺ cluster fragment ion has been successfully applied to estimate the surface silanol group in silicate glasses. The absolute determination was achieved by using a calibration curve obtained from measurement of a quartz glass as a reference sample. The surface silanol concentration of the quartz glass was determined by analysis of O1s photoelectron line provided by XPS. The developed technique was applied to analyze the soda-lime-silica glass and silica films. On the basis of this analysis, the role of the surface silanol group has been discussed in terms of surface adsorption.

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Hydroxyapatite was prepared from calcium nitrate and disodium hydrogenphosphate solutions. Heating this at 1000°C for 1h yielded round-shaped hydroxyapatite particles with diameters of about 0.5μm. A green compact of the hydroxyapatite (green compact-I) was fromed by an uniaxial compression process (300kg/cm²), and then a green compact II was made by an isotropic compression (1000kg/cm²) of the green compact-I. A hydroxyapatite sintered body (porous structure) was obtained by heating the green compact-II at 1150°C for 2h. Internal homogeneities of these green compacts and sintered bodies were evaluated from CT-values measured with a CT-scanner. Depending on the method proposed in this study to indicate homogeneity using the ratio of the standard deviation of linear absorption coefficient to linear absorption coefficient, the green compact-I was most homogeneous, followed by the green compact-II. The sintered body was comparatively inhomogeneous.

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C-t characteristics of ZnO varistors containing praseodymium oxide are examined. The effects of sintering temperature on the C-t characteristics are also discussed. C-t characteristics are found to be strongly connected with the electronic interface states at the grain boundaries and useful for characterizing the interface states. When samples were sintered below 1340°C, the interface states were discrete and monoenergetic at 0.8eV below the conduction band edge. However, samples which were sintered above 1360°C formed continuously distributed levels near 0.7eV below the conduction band edge. Moreover, the density of states and varistor voltage per grain boundary rapidly increased in this temperature range. These phenomena were explained by the oxygen evolution from Pr₆O₁₁ by the reduction to Pr₂O₃ during the sintering process.

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As one of the serial studies on fibrous calcium compounds, the synthesis of fibrous C₃A⋅3CaCO₃⋅32H₂O was studied by adding CaCl₂ solution (6-21×10^-3mol dm^-3) to the solution prepared by introducing CO₂ gas or by adding Na₂CO₃ solution (CO₃^2-concentration: 1-5×10^-3mol dm^-3) into NaAlO₂ solution (7-22×10^-3mol dm^-3). Characteristics of the product obtained by liquid-phase reaction in CaCl₂-NaAlO₂-CO₂ and CaCl₂-NaAlO₂-Na₂CO₃ systems were determined by means of X-ray diffraction, thermal analysis (TG-DTA), scanning electron microscopy, infrared spectroscopy and chemical analysis. The formation of fibrous C₃A⋅3CaCO₃⋅32H₂O was affected remarkably by synthetic conditions such as pH, initial Ca/Al atomic ratio and concentration of CO₃^2-ion. When the concentration of CO₃^2-was 1-3×10^-3mol dm^-3, the initial Ca/Al atomic ratio to form fibrous crystals of C₃A⋅3CaCO₃⋅32H₂O was less than 0.3 in CaCl₂-NaAlO₂-CO₂ system and less than 2.0 in CaCl₂-NaAlO₂-Na₂CO₃ system. Thus, the fibrous crystals were stable at pH above 11.7. Therefore, when the initial Ca/Al atomic ratio exceeds 0.3 in the system of CaCl₂-NaAlO₂-CO₂, pH because less than 11.7, yielding no fibrous crystals. While, the reaction in CaCl₂-NaAlO₂-Na₂CO₃ system to form the fibrous crystals progressed successfully because the pH value was kept over 11.7 even in the initial Ca/Al atomic ratio 0.3-2.0. It was confirmed that fibrous C₃A⋅3CaCO₃ 32H₂O crystallized as high crystalline fibers with aspect ratios above 300 elongated parallel to c-axis in hexagonal through an unstable amorphous phase which was precipitated in the initial step. The fibrous shape was stable in heating up to 300°C. Furthermore, the packing state of the fibrous crystals as filler for paper was discussed.

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The treatment of MgO substrates by the H₃PO₄ wetetching method were carried out in order to remove the impurities (H₂O or CO₂) in the surface layer of a MgO single crystal. Pt thin films were deposited on MgO substrates by rf-magnetron sputtering method. To evaluate the effect of the treatment, orientation and crystallinity of the obtained films were investigated. The etching rate in this experiment was about 2.90nm/s. It was considered that crystallinity of the MgO substrates decreased due to H₂O or CO₂ in the surface region with a thickness of 1μm. The surface roughness of substrates and thin films increased with increasing etching time for the MgO substrates. All of the obtained films showed [100]-orientation. The measurement of the rocking curve of (100) peak of the Pt film deposited on the MgO substrate etched for 5min indicated that the half width was about 0.62 deg. and that the film had a good crystallinity. Pt films deposited on MgO substrates etched for 5 to 60min exhibited epitaxial [100]-orientation from the measurement of half width for X-ray rocking curves, TEM micrographs, electron diffraction patterns and spacing of lattice planes in lattice images.

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The grain size distribution of raw materials and microstructure of their fired bodies indicate that the grain size of Hizen commercial raw porcelain bodies is too coarse to obtain desirable mechanical properties. The strength of fired bodies from commercial raw porcelain bodies increased with milling, and it increased further by removing grains coarser than 20μm from the milled powder. These treatments increased the mean bending strength of commercial porcelain bodies from 830kgf/cm² to 1450kgf/cm². The results obtained with a commercial porcelain body are in agreement with those of previous works, in which the control of size and its distribution of quartz grains governed the mechanical properties.

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On the system of PVA-water-alumina, a new technique was developed to study quantitatively the surface segregation of PVA during drying of the slurry. This technique involves one-dimensional drying of slurry and determination of PVA content at various positions by sectioning. The samples containing 1.6 to 3.85wt% PVA were dried at 40° to 90°C. The drying temperature and the concentration of PVA affect the segregation markedly. With increasing concentration of PVA, the proportion of the constant-rate drying period to the entire drying period decreased. With increasing drying temperature, the proportion of the constant-rate drying period to the entire drying period decreased, and the surface segregation increased. The surface hardness increased with increasing content of PVA in the surface region. The surface segregation of PVA was discussed in terms of the behaviors of constant-rate drying, diffusion, and viscosity.

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Crystallization of glass with a composition of MgO-BaO-SiO₂-B₂O₃ was investigated with DTA, XRD, SEM, surface roughness measurement, Vickers hardness measurement and wear rate. In firing process of the glass, two crystal phases, i.e. 2MgO⋅B₂O₃ and BaO⋅2MgO⋅2SiO₂ were identified. Surface roughness and Vickers hardness of the partially devitrified glass board (glass-ceramics-board) changed with crystallization. The crystallization in the glass became cause of change of surface properties. Surface roughness and Vickers hardness of the glass-ceramics-board were, respectively, 0.16-0.2μm and 700-800Hv. The board was superior in a surface smoothness and hardness to any other customary porcelain enamel board (0.3μm, 300-600Hv). The glass-ceramics-board (fired at 880°C) was also superior in wear properties against SUS 420J2 pin. It was found that glass-ceramics-board thus obtaind can be applied to wear resistance material.

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Li-doped LaCrO₃ ceramics were prepared in air at 1600°C and a perovskite solid solution with 5% Li-substituted in the Cr site was obtained. The Li doping promoted the densification of LaCrO₃, resulting in dense ceramics of 98% theoretical density. The Li doping decreased the electrical resistivity, implying the creation of holes. The resistivity of Li-doped LaCrO₃, however, was higher than those of Sr- and Zn-doped LaCrO₃, which had lower relative densities. The densification mechanism of LaCrO₃ was discussed based on equilibrium of point defects, and the volume diffusion of oxygen ions was considered to be the rate-limiting process for the densification.

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R-curve properties for two kinds of SiC whisker reinforced Si₃N₄ ceramic composite were obtained by three-point bending test. Single edge notched beam specimens with sharp tip radius were fractured under a slow cross-head speed of 0.002mm/min in air. Crack extension Δa was calculated from the change of the specimen compliance. Crack extension resistance K_R was also calculated from the crack extension Δa and the fracture load during stable fracture. The extent of K_R increase in the R-curve was more remarkable in the composite containing whiskers with larger diameter. Some theoretical and technical problems for measuring the crack extension resistance of toughened ceramics were discussed in this letter.

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Strontium titanate (SrTiO₃) films with a thickness in the range of 100-200nm were prepared by the sol-gel method using metal alkoxides as starting materials. The perovskite phase was observed by X-ray diffraction analysis of the films heat-treated over 600°C. The dielectric constant was 30-60, which is lower than that of SrTiO₃ ceramics (250).

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High-strength β-calcium metaphosphate fibers for new applications to biomedical fields were successfully extracted from crystallized products of calcium ultraphosphate glasses by leaching with a dilute NaOH aqueous solution. The crystalline β-Ca(PO₃)₂ fibers obtained have high aspect ratios ranging from 30 to 120 with diameters of 1 to 5μm.

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