Physical, chemical and mechanical properties of opaque silica glass fabricated by the slip casting method were evaluated and compared with commercial products manufactured by the fusion method. The slip casting sample contained a large number of small bubbles, and exhibited a higher light scattering coefficient and higher ability as a thermal radiation barrier. These features were well described in terms of the variation of the total surface area of bubbles per unit volume. Due to the smaller size of the bubbles, other properties such as bending strength, surface roughness and chemical durability, were considerably improved as compared to those of the fusion samples.

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Tin atoms were inserted into the skutterudite-type CoSb₃ host lattice under high pressure and temperature conditions. It was found that Sn atoms could be inserted into the body-centered vacant site, resulting in the formation of new filled (or partially filled) skutterudites, Sn_xCo₄Sb₁₂. The degree of insertion (x) increases with increasing applied pressure, and the vacant sites of the CoSb₃ host lattice is fully occupied by Sn atoms when the reaction is carried out at 8GPa and at 550°C. X-ray Rietveld refinement revealed that the inserted Sn atom showed an anomalously large isotropic atomic displacement parameter, indicating a large thermal vibration amplitude at the body-centered position. The Sn-filled skutterudites exhibit n-type semiconducting behaviour indicating that Sn filling causes the formation of a donor level.

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The dependence of the negative nonlinear absorption effect at 1.5μm on the sample temperature, the modulation frequency and the sample length were investigated in glasses containing amounts higher than 20mol% Er³⁺. Reversed-phase waveforms were observed in the temperature range -260-600°C and at modulation frequencies from 1kHz to 10GHz in an erbium-doped phosphate glass. With increasing temperature, the amplitude of the reversed-phase waveforms decreases exponentially according to two separate steps. Optical bistability in the transmitted laser was observed in the range of incident laser intensity of 30μW/cm²-2.4mW/cm². The transmitted laser intensity propagating through the glass decreased exponentially according to the well-known exponential law. However, the amplitude of the optical bistability increased logarithmically as the sample length was increased. The negative nonlinear absorption effect can be explained by considering an enhanced absorption model for a four-level system of the Er³⁺ ion.

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Reduction behavior of tungsten oxide mixed with alumina powder and its sintering processes were investigated to fabricate Al₂O₃/W nanocomposites. Submicron-sized WO₃ powder could be obtained by a conventional ball-milling technique. Hygrometry analysis of Al₂O₃/WO₃ mixture revealed that WO₃ powder was reduced in two steps at 873 and 973K. X-ray diffraction and transmission electron microscopy (TEM) analysis confirmed that Magneli phases, such as WO₂ and WO_x(x=2.89 to 2.92), were obtained by the reduction of WO₃ at 873K. However, WO₃ was completely reduced to metallic W when the reduction temperature was higher than 1173K. A thermodynamic analysis qualitatively agreed with the experimental result that WO₂ and WO_x coexisted at intermediate temperatures. Reduction followed by hot-press sintering of Al₂O₃/WO₃ mixtures was carried out to obtain Al₂O₃/W nanocomposites. Agglomeration and/or incomplete reduction was found at high W content; however, dense Al₂O₃/W composites with >98% of their theoretical density were obtained at low W content (i.e., <10vol%). A microstructural investigation of the dense composite revealed that it consisted of both micro- and nano-sized W dispersions. Furthermore, a large number of W particles of around 40nm were found to be homogeneously dispersed in the Al₂O₃ matrix. These particles were much finer than the WO₃ particles obtained by ball-milling.

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Spinel/mica composites were fabricated by sintering powder compacts containing 5, 10, 20, 50 and 80mass% fluorophlogopite-composition (Fph: KMg₃AlSi₃O₁₀F₂) glass and spinel (MgAl₂O₄). Mica precipitated in all the composites. For composites containing 5 and 10% glass, the maximal densification was achieved at 1400°C. A spinel skeleton was formed and fine mica crystals precipitated among spinel grains. For composites containing 20% glass, the maximal densification was achieved at 1300°C. Thick mica platelets precipitated among the spinel grains. For composites containing 50 and 80% glass, a porous house-of-cards structure, which consists of plate-like mica crystals, was formed. Flexure strength and Vickers hardness were measured.

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Steam electrolysis cells were constructed using proton conducting ceramics SrCe_0.95Yb_0.05O_3-α and SrZr_0.9Yb_0.1O_3-α as electrolytes. The removal of nitrogen oxide (NO) by the hydrogen produced by steam electrolysis cells was examined. When a Co/Al₂O₃ catalyst was set on the cathode of the cell, NO was effectively removed even in excess O₂. A cell with a SrCe_0.95Yb_0.05O_3-α electrolyte and a Co/Al₂O₃ catalyst could remove NO with about 100% efficiency in N₂ containing 8% O₂ and 1000ppm NO at 440°C. On the other hand, the removal efficiency of NO for only a Co/Al₂O₃ catalyst was less than 4% under the same conditions. It was found that the steam electrolysis cell in combination with a Co/Al₂O₃ catalyst was very effective for the removal of NO in excess O₂.

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The effect of magnesia addition on the microstructural development and the mechanical properties of commercial low-soda alumina was investigated in the context of sintered bodies highly toughened by plate-like abnormal grains. Magnesia addition did not affect sinterability and final density. The alumina bodies, with and without magnesia, sintered at just below the temperature for which fully dense materials are obtained with high bending strength (i.e., exceeding 600MPa). Very large plate-like grains were formed at higher sintering temperature in the sample without magnesia. These large plate-like grains are the reason of both a lowered bending strength and an extremely high increase in fracture toughness. Fracture toughness of more than 7MPa·m^1/2 and bending strength of 200MPa were achieved in a sample sintered at 1800°C without magnesia addition. The small amount of magnesia effectively inhibited abnormal grain growth in low soda alumina and even the sample sintered at 1800°C showed a bending strength higher than 300MPa and 5MPa·m^1/2 fracture toughness. The high fracture toughness obtained in these samples is attributed to crack bridging mechanism.

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In order to improve the characteristics of multilayer chip varistors, we have investigated the effects of additives on the varistor voltage and the nonlinear coefficient of ZnO-based chip varistor, whose fundamental components were ZnO:CoO:Pr₆O₁₁:Al₂O₃=100:1.0:0.1:0.005 (molar ratio). Selected additives were Cr₂O₃, MoO₃, WO₃, and Nb₂O₅, the extent of addition being from 0.05 to 0.1mol%, respectively. Thermal shrinkage and grain-growth rate were observed. The Capacitance-Voltage (C-V) method was applied for evaluating the electrical properties when the sintering temperature was varied from 1120 to 1230°C All the additives remarkably decreased the shrinkage rate above 1000°C. While the grain growth was controlled by the addition of any of the selected additives. its rate depended on each species. Additives were not soluble but segregated within the grain and/or at grain boundaries, irrespective of the kind of additive. While the varistor voltage was generally lowered and the nonlinear coefficient became higher with increasing sintering temperature, their absolute values depended on the kind of additive. Addition of Cr₂O₃ and MoO₃ made available a high nonlinear coefficient with a relatively high varistor voltage, thus much improved varistor characteristics. A factor which influenced the increase of the nonlinear coefficient was interpreted by considering a relative increment of the barrier height, based on the double Schottky barrier model.

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Some of the SiC fibers (Hi-Nicalon^TM) embedded in reaction-bonded SiC matrix composites were found to be altered during the reaction sintering process. The microstructures of the altered fibers were investigated using transmission electron microscopy. The altered fibers consisted of coarse SiC and carbon grains. On the other hand, undamaged fibers consisted of fine SiC grains with amorphous carbon between the SiC grains. In the carbon grains of the altered fibers, concentric lattice fringes were visible, and crystallinity was raised compared to the amorphous carbon of the undamaged fibers. The alteration of fibers was probably caused by liquid Si which infiltrated into the powder preforms during reaction sintering. It is thought that the coarsening of the SiC grains occurred through a solution-reprecipitation mechanism and that the change in the carbon phase was promoted by the catalytic graphitization of Si.

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The solid-liquid reaction of α-Ca₃(PO₄)₂ and Ba(H₂PO₄)₂ solution proceeds to successively precipitate BaHPO₄, “X” phase and DCPD (CaHPO₄⋅2H₂O), depending on the sluggish dissolution of α-Ca₃(PO₄)₂. The process experiences three exothermal flows accompanying a three-step decrease in electric conductivity. Chemical analysis of the liquid phase together with X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA) and scanning electron microscopy (SEM) of the residue were conducted to clarify the reaction mechanism. As a result a suitable method for isolating the new compound “X” was examined. The “X” phase was isolated, for example, by stirring a suspension composed of 0.6g α-TCP and a mixed solution, 500ml of 14.2mmol·dm^-3 Ba(H₂PO₄)₂(aq) and 300ml of 15.0mol·dm^-3 Ca(H₂PO₄)₂(aq), for several hours. The chemical formula of the “X” phase was BaHPO₄⋅CaHPO₄⋅H₂O and its crystal structure was monoclinic with unit-cell parameters, a=1.0656nm, b=0.7802nm, c=0.9605nm and β=104.26°.

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SrTiO₃ ceramic foams with spherical pores were obtained by polymer extraction from oxide green sheets containing polymer microspheres. Fine SrTiO₃ particles were prepared by hydrolysis of Sr(OC₃H₇ⁱ)₂ and Ti(OC₃H₇ⁱ)₄, and were well-dispersed by oily alkylbenzensulfonate type ainonic surfactant. Size and morphology of polymer spheres were reflected in pores of green sheets. Five kinds of the green sheets with various weight ratios of SrTiO₃ to polystyrene microspheres, 100/0, 100/50, 100/100, 100/150 and 100/200, were prepared. The polystyrene microspheres were extracted by benzene, and heat-treated at 1300°C. Linear shrinkage of the green sheets observed during the heat treatment increased from 31.1% to 41.2% with increasing of the polystyrene microsphere content. Porosity of the SrTiO₃ ceramic foams changed from 3.7% to 74.0% with the adding amount of polystyrene microspheres. Closed pore structures of the SrTiO₃ ceramic foam were transformed into open pore structures with the increase of the polystyrene microsphere content.

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A Copper hydroxide-vermiculite complex (CHVC), Cu_1.61(OH)_2.38(Si_2.95Al_1.05)(Mg_2.75Fe_0.22Ti_0.02)O₁₀(OH)₂⋅nH₂O, having a particle size in the range from 1 to 3mm was prepared from a Zimbabwe vermiculite by hydrothermally treating at 250°C for 40h in copper nitrate solution. X-ray diffraction patterns of the products were characterized by d₀₀₁ reflection of 1.4nm and their subsequent interger reflections. One-dimensional Fourier electron density analysis revealed that the interlayer structure of CHVC is similar to that of a natural chlorite. The thermal behavior of CHVC was characterized by high temperature X-ray diffractometry and TG-DTA technique. Copper hydroxide in the interlayer was dehydroxyrated in the temperature range from ca. 300 to 700°C. By heating at higher temperature, copper oxide deposited on the surface of the silicate layer.

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Dense zirconia-europia tetragonal solid solutions were obtained by sintering nanometer-sized powders produced by coprecipitation from solutions of zirconyl and europium nitrates hydrates. The tetragonal phase could be retained at room temperature for compositions of 2.0-4.0mol% Eu₂O₃. The fired materials had a fine microstructure, but those containing 2.0mol% Eu₂O₃ rapidly transformed from tetragonal into monoclinic cell upon cooling. The grain transformability in such polycrystals was dependent on the composition, because low doped samples had a critical grain-size for transformation, the lower the size the higher the content of Eu₂O₃. The ionic radius of the rare earth element used as a stabilizing oxide played an important role on the critical grain dimension for phase transformation. Eu³⁺ has a higher ionic radius than Er³⁺, but smaller than La³⁺. Accordingly, also its critical grain size was smaller than in the case of erbia, but higher than that of lanthana.

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Through a flushing process, natural amorphous graphite was highly purified using a commercial oscillating liquid-liquid continuous-extraction apparatus. Aqueous slurries of raw graphite with a mean particle size of 2μm were conditioned at pH 4.2 in the presence of chelating and reducing reagents, and the graphite particles were extracted using a kerosene-based oil phase. The graphite particles in the oil phase were returned to an aqueous dispersion with the addition of a nonionic surfactant and sodium carbonate, and were recovered by centrifugation. The ash in the graphite decreased from 16 to 2mass% as a result of the separation of clayey impurities. The excellent functionality of the extractor for refining amorphous graphite was attributed especially to its perforated cylinder-shaped impeller capable of inducing a high shearing stress. Not only are fine oil droplets created, which are favorable for the collection of graphite particles, but the aggregates of raw graphite are also released as individual particles.

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Changes in refractive index, thickness and structure of the poly (ethylene glycol), (PEG), -containing silica gel films, prepared under various concentrations of hydrochloric acid during the exposure to an ambient atmosphere at room temperature, have been investigated to determine the influence of preparation conditions on the hardening behavior of the films in a patterning process. The PEG-containing silica gel films, derived from silica sols which were prepared under lower hydrochloric acid concentration, showed a steep increase in refractive index and large shrinkage upon increasing the exposure time. The increase in refractive index and shrinkage of the films were found to reflect the decrease of Si-OH groups and the development of Si-O-Si network in the films, accoding to infrared absorption spectra. The process time to emboss micro-patterns was shortened by using the gel films derived from silica sols which were prepared under a low hydrochloric acid concentration. This result suggests that the silica gel film with a low degree of polymerization hardens rapidly.

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Small amount (less than 1.0vol%) of Al₂O₃ or MgO added lead zirconate titanate, Pb(Zr, Ti)O₃(PZT), ceramics were prepared by conventional processing routes. Two different kinds of starting PZT powder were used., i.e., one was pure Pb(Zr_0.52Ti_0.48)O₃ and the other was hard-type Pb(Zr_0.50Ti_0.50)O₃. The effects of added second phases on the microstructure and properties for the PZT were investigated. The addition of a small amount of oxides strongly affected the microstructure, and significantly improved the mechanical properties of the PZT. The effect of starting PZT powders on the piezoelectric properties of the PZT was also investigated. It would be considered that the difference in solid-solution state of Al⁺³ or Mg⁺² ions onto the PZT crystal structure is very critical for the piezoelectric properties of the composites. High strength (140MPa) and good electromechanical coupling factor K_p (60%) were both achieved in the hard PZT added with MgO.

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Toughening mechanisms in polycrystalline ceramics were discussed based on Griffith's energy equilibrium. These mechanisms were classified into three groups: (1) The frontal process-zone toughening mechanism, which is related to the intrinsic fracture energy of a material, and creates a damaged zone ahead of the crack tip. This mechanism generates intrinsic fracture toughness in a material. (2) The crack-surface-bridging mechanism, which is caused by stress shielding, resulting from bridging and/or interlocking in a process-zone wake, and creates an extrinsic increase in crack resistance. (3) The macroscopic crack deflection mechanism, in which the crack deflection decreases the energy-release rate and, consequently, the stress-intensity factor at the crack tip, thus apparently enhancing the fracture toughness.

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Static and cyclic fatigue tests were performed on alumina using smooth-surface flexure specimens under several constant stress levels. The fatigue lifetimes, including imperfect data, were analyzed statistically to estimate the original distributions of the lifetimes. A technique for estimating the static and cyclic fatigue limits was proposed using the residual strength of the specimens surviving after the running time. The effects of cyclic stress on fatigue crack extension were confirmed experimentally by comparing the stress-lifetime curves obtained in the static and cyclic fatigue tests on smooth-surface specimens.

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Few literature had been published on the solid solubility of oxides in nitrides before 1971. Oyama and Kamigaito originally reported that Al₂O₃ dissolves in Si₃N₄ in 1971. Jack and Wilson (1972) reported on the solid solution of Si₃N₄ with Al₂O₃ (β′-Si₃N₄) as well as a new material having the α-Si₃N₄ structure formed by the reaction of LiSi₂N₃ with Al₂O₃. The term SiAlON (Si-Al-O-N) well known these days was given by Jack and Wilson and the former material is called β-SiAlON and the latter α-SiAlON.
This paper reports in detail on the solid solubility of Al₂O₃ in Si₃N₄ that was reported firstly in 1971. This paper was submitted in January and published in August 1972. Jack and Wilson's paper was submitted in April and published in October 1972.
In both papers, almost the same figure, which relates the lattice constants of the solid solution and Al₂O₃ concentration in Si₃N₄, was given. The following facts are reported in this paper.
(1) Based from the specific gravity of Si₃N₄ss, the solid solution is thought to be a substitution type.
(2) The solubility limit of Al₂O₃ in Si₃N₄ss is estimated to be about 67mole% at 1750°C and 78mole% at 1850°C.
(3) The high-temperature oxidation resistance of Si₃N₄ss is excellent compared to that of Si₃N₄.
In our opinion, this paper should be honored from the standpoint that it has contributed much to the research on SiAlON globally. Credit is due to Oyama and Kamigaito for performing pioneering work in this field.
The phenomenon that nitrides dissolve oxides could not be understood at the time this paper was submitted. Incidentally, there is a story that this paper was nearly rejected by the editorial committee of Yogyo-Kyokai-Shi (Journal of the Ceramic Society of Japan).
Prior to 1980, at the dawn of the research on Si₃N₄ ceramics, several worldwide innovative research studies were reported in the following papers; the report on the excellent mechanical properties of reaction sintered silicon nitride, densification of Si₃N₄ ceramics with additives by hot-pressing, discovery of solid solubility of Al₂O₃ in Si₃N₄, the report on the conventional sintering of Si₃N₄, gas pressure sintering of Si₃N₄ and the patent on effectiveness of Y₂O₃ as a sintering additive of Si₃N₄ ceramics. Finally, it is worth mentioning that Japanese researchers performed three of these six globally famous and original works.

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