A four-point bending test was carried out on sintered Si₃N₄ at room temperature to investigate the static fatigue behavior of ceramics. The feature of the overall fractured surface after the test was similar to that after the static test if the fracture stress was the same. Mirror-like regions were observed on the fractured surfaces after both tests. The character of mirror-like region after the static fatigue test was compared with that after the static test. The characters of both regions were almost the same. The mirror-like region was roughly semicircular, and the fracture propagated radially from the mirror-like region. The size of mirror-like region, which was determined from optical micrographs at a magnification of 25, was correlated with the applied stress. Stress intensity factor at the deepest point in the region was also almost constant independent of the depth of the region. It was considered that the mirror-like region was restricted to the subcritical crack growth under the applied stress, and that the failure occurred when its size reached a critical value which was dependent on the applied stress.

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Porous mullite ceramics were prepared by leaching glassy matrixes with a mixed acid from the sintered samples with compositions Al₂O₃/SiO₂=1/3.3, 1/2, 1/1 and 3/2 in molar ratio. The shape of the mullite crystals were granular for the samples from submicron powder mixtures of Al₂O₃ and SiO₂ sintered at 1400°-1600°C. Addition of CaO, MgO, BaO, TiO₂, or Fe₂O₃ (5wt%) to the Al₂O₃-SiO₂ system with composition Al₂O₃/SiO₂=1/3.3 promoted the formation of elongated or needle-like mullite crystals. Effects of these oxides on the formation of mullite grains with high aspect ratio were discussed on the basis of the ternary phase diagrams. Average pore sizes of the leached porous mullite ceramics were in the range of 0.4-3μm, and became larger in the order of CaO>MgO>Fe₂O₃>none>BaO>TiO₂ in additives. The pore size measured by mercury-penetration method agreed with that observed by SEM.

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To understand the fatigue behavior of ceramics, the equipment for the tension-compression axial cyclic fatigue testing of ceramics at high temperature was successfully developed. The tension-compression cyclic fatigue tests and the tensile static fatigue tests were carried out up to 1000°C using sintered silicon nitride. At high temperature, the cyclic fatigue did depend on frequency. The lower the frequency became, the less the fatigue resistance became. The failure diagram was constructed to show the effects of mean stress and stress amplitude on the fatigue strength. It is concluded that a design methodology proposed for the metal fatigue is applicable to the sintered Si₃N₄ examined in this study. Microstructural observations suggest that the progress of cyclic fatigue of the sintered Si₃N₄ is caused by slow crack growth and creep defomation.

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Barium titanate series dielectric ceramics, as an indispensable material for capacitor in electronic circuit, has been used in large quantities. Since the material is ferroelectricity, however, it has problems for high voltage use of power electric field, which are to let change capacitance sharply caused by its voltage depnedent characteristic, and to be unable to have long life caused by the material's peculiar property of high dielectric loss. It was already reported that the present material, which had been developed from (Sr, Ba)TiO₃ series ceramics for the elimination of problems, was remarkable for showing paraelectric characteristics and was moreover high dielectric constant. Furthermore, a new material with a higher dielectric constant than the present, has been required for the same purpose of power electricity. The new material, which fulfills the requirement, has been developed from (Sr, Pb)TiO₃ series ceramics, showing its dielectric constant of 2800. The explanation of the anthor's fundamental way of thinking in developing the new materials, and the study on the electric characteristics and the microstructure of the material are discussed in this paper.

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Joining of alumina ceramics was studied by the microwave heating method. Samples were the sintered 99% alumina rods, 6mm in diameter. The results are summarized as follows:
(1) The joining was carried out easily by covering the joining part with mullite insulating fire brick during application of microwave at the surface temperature on the joining part of higher than 1400°C and under the pressure at joining plane of 0.13-0.5MPa.
(2) There was a peak in the bending strength-heating time curve. The maximal value increased and the time to give the maximum decreased with increasing joining temperature.
(3) The hardness value of joined part was larger than that of the others. The grain size at the joined part increased during microwave heating.
(4) Joining was so good that the joined interface was hard to distinguish from other part by SEM observation of cross section of the joined part.

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Amorphous vanadium oxides were synthesized by the hydrolysis of tri-n-butoxy vanadile, and three types of products, gel, green precipitates and red precipitates were obtained. These products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), infrared absorption spectroscopy (IR), thermogravimetric-differential thermal analysis (TG-DTA) and chemical analysis. Properties of the products were correlated to the synthetic conditions, and the mechanisms of the formation of each product are proposed. (1) Gel has a network structure. It is formed when hydrolysis followed by condensation proceeds on three substitutional groups of tri-n-butoxy vanadile, (2) green precipitates are formed when the condensation reaction is inhibited. They are the aggregates of particles with small molecular weights and have short structure units with hydroxide groups at their ends, (3) red precipitates are caused by insufficient proceeding of hydrolysis. They have many alkoxy groups in their structure units and their molecular weights are between those of gels and green precipitates.

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The sensing mechanism of SnO₂ semiconductor gas sensors was studied by examining, via the pulse reaction method, the transient response characteristics of the conductivity of gas sensors for methane and isobutane, typical hydrocarbons. The study revealed the following facts:
(1) The relationship between gas sensor conductivity and inflammable gas concentration, both in steady and unsteady states, can be determined for hydrocarbons such as methane and isobutane, as for hydrogen using Equation (1) below: dσ/dt=aP_G+b/σ-cσ (1) σ: sensor conductivity P_G: atmospheric gas concentration a, b, c: constants t: time
(2) If atmosphere containing no inflammable gas is changed to one containing inflammable gas, the speed of change in sensor conductivity is highest in air containing hydrogen, followed by air containing isobutane, and is lowest in air containing methane. This order corresponds to the order of flammability, confirming the supposition that the change results from the reaction between flammable gas and adsorbed oxygen.
(3) If atmosphere containing flammable gas is changed to one containing no inflammable gas, the speed of change in sensor conductivity does not vary significantly with the kind and concentration of gas. This fact suggests that the change results from the adsorption of oxygen onto open sites on SnO₂.

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The effect of HF on the decomposition of brucite was studied by differential thermal analysis (DTA) and X-ray powder diffraction (XRD). HF was added to brucite by addition of fluoride or by treatment with hydrofluoric acid solutions. In fluoride addition, AlF₃ was more effective in retarding the decomposition of brucite than MgF₂. HF gas was formed by the hydrolysis of fluoride and then sorbed on the surface of brucite. The endothermic peak of brucite shifted to high temperature from several to about 40 degrees with increasing AlF₃ content from 0.5 to 5mol%. Similarly, the endothermic peak of brucite treated with hydrofluoric acid solution also shifted to high temperature, about 70°C for the specimen treated with 0.15 HF (molar ratio of HF vs. brucite). The retardation of decomposition was due to the chemisorption of HF on the edge of brucite which would decompose from the edge.

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The effect of ball milling on the PTCR (positive temperature coefficient of resistivity) characteristics in porous semiconducting (Ba_0.9Sr_0.1)TiO₃ ceramics has been studied. Ball millings were used before and after the calcination of the starting powders, and their times were variously changed. The ball milling time before the calcination of powders influenced the PTCR effect much more significantly than that after the calcination. The ball milling time used for the powder processing in the present study has been confirmed to be effective to obtain materials with large, stable PTCR effects.

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The series of Ga-substituted fluorine micas, KMg_2+xLi_1-x(Ga_xSi_4-x-yGe_yO₁₀)F₂ and KMg_2+xLi_1-x(Ga_xGe_4-x-ySi_yO₁₀)F₂; x=0.0-1.0, y=0, 1, 2, 3, were synthesized and the changes in infrared spectra and lattice constants with Ga-substitution were studied. The larger flakes of mica crystals were obtained with increasing Ga- and Ge-contents. The infrared spectra of the synthesized micas varied sensitively with mica composition. An absorption band, which appeared in the range 740-705cm^-1 with increasing Ga-content, is attributable to Ga-O vibration. Continuous changes of lattice constants with increasing Ga-contents proved that there were complete series of solid solutions between the end member micas. Lattice constants a and b increased linearly or almost linearly with increasing Ga-content in all series. However, c⋅sinβ (basal spacing) decreased or remained almost unchanged in the case of Ga-for-Ge substitution, while it increased linearly in the case of Ga-for-Si substitution. These complicated changes of cell dimensions were shown clearly by the plots of c⋅sinβ vs. b-dimension and explained in terms of the tetrahedral rotation angle α, the octahedral flattening angle ψ, and the change of interlayer conformation, where α and ψ show the degree of structural deformation in mica structure. It is also indicated that the degree of structural deformation increased with increasing Ga and Ge contents.

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A super-absorbent polymer was fixed on alumina particles to prepare water-holding alumina powder. A mixture of alumina powder and water has various apparent viscosities between 800 and 90000Pa·s depending on the content and the condition of heat treatment of the super-absorbent polymer. This mixture was injection-molded at room temperature. The dried moldedbody has a high bending strength, about 30MPa, and densifies above 1350°C.

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Creep in mullite ceramics containing 71, 74 and 78wt% Al₂O₃ made from coprecipitated powder has been studied between 1350° to 1500°C. Microstructure of 71wt% Al₂O₃ mullite was characterized by rod-like and equiaxed grains (average grain size: 2.13μm), that of 74wt% Al₂O₃ mullite, by equiaxed grains (grain size: 5.44μm), and that of 78wt% Al₂O₃ mullite, by equiaxed mullite grains (grain size: 2.11μm) and Al₂O₃ particles of 1.31μm in grain boundaries. Creep rate in 74wt% Al₂O₃ mullite was the lowest and that of 71wt% Al₂O₃ was the highest. That of 78wt% Al₂O₃ mullite located in between. Activation energies for creep were 138, 168 and 119kcal/mol for 71, 74 and 78wt% Al₂O₃ mullite, respectively. Vertical offsets from grain sliding out of the specimen surface were observed after creep test. The number of grains indicating the vertical offsets increased and cavities evolved with increasing creep time or applied stress. A slight grain growth was observed for 71 and 78wt% Al₂O₃ mullites after creep tests. On the other hand, grain growth was not recognized for 74wt% Al₂O₃ mullite. It was proposed that the creep mechanism in mullite ceramics is grain boundary sliding accompanied by diffusion.

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The production of calcium carbonate by the thermal decomposition of calcium acetate was tried. Calcium acetate changed into calcium carbonate (calcite structure) at a calcining temperature about 400°C. The particles were large and tabular polycrystalline aggregates of 0.3-1.4mm long and 0.06mm across with low whiteness (under 40%), large oil absorption (95-110ml/100g) and small specific surface area (2-7m²/g).

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Silicon nitride containing 0.5-10mol% of Y₂O₃-Nd₂O₃ was fired at 1600°-1900°C in 10MPa N₂ to investigate the lowest limit of additives to densify Si₃N₄ using gas pressure sintering. One mol% oxide additive was found to be sufficient for densification. This material fired at 1900°C for 4h was densified to 97% of theoretical with the average bending strength of 784MPa.

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