Bubbles are essential in the formation of opaque silica glass, because opacity results from light scattering by bubbles dispersed throughout the glass matrix. In this sintering study of slip casting of a silica compact, the formation of bubbles was observed. In the final densification stage, a large number of small bubbles formed at around 1600°C. With increasing sintering temperature, isolated bubbles became larger and, concurrently, the number of bubbles decreased. Opacity increased with increasing sintering temperature up to 1800°C. These results are discussed from the viewpoint of the critical bubble size which determines whether bubbles expand or shrink.

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ZnSb₂O₆ was revealed to be an n-type semiconductor and the carrier was generated by oxygen defects related to the reaction with ambient oxygen gas. From thermoelectric and thermogravimetric measurements, the carriers were revealed to be thermally activated and the concentration was found to decrease as temperature increased.
The substitution of zinc site by transition metal (cobalt, nickel, copper) lowers the carrier concentration and also changes the activation energy of conduction. A relationship between the activation energy and the length of the c-axis was observed which indicated that the conduction of these materials was affected by the distance of the MO₆ octahedra. Due to low electrical conductivity, ZnSb₂O₆ does not have sufficient energy converting efficiency.

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Four wet-chemical methods (normal strike co-precipitation, reverse strike co-precipitation, urea homogeneous precipitation, and a modified Pechini process) were adopted in the present work to synthesize the precursor for a sinterable YAG powder. The X-ray diffraction (XRD) results revealed that the phase development of the precursor during calcination and the formation temperature of pure YAG vary among the four methods. The lowest temperatures for pure YAG formation were determined by the XRD method to be 850°C, 950°C, 1200°C and>1200°C for the modified Pechini method, reverse strike co-precipitation, normal strike co-precipitation and urea homogeneous precipitation, respectively. The differences concerning YAG formation among the four methods were explained from the view point of the chemical homogeneity of the precursor. That is, higher chemical homogeneity resulted in the lower temperature of YAG formation. Except the case of modified Pechini method, the sinterability of the YAG powders increased with a decrease in the above temperatures for pure YAG formation because the YAG powder produced from the precursor with higher chemical homogeneity showed smaller particle size and weaker agglomeration. The YAG powder synthesized by the modified Pechini method showed the poorest sinterability due to its extremely hard aggregates caused by the direct pyrolysis of the mother salts. The most sinterable YAG powder, produced by the reverse strike co-precipitation method, densified up to 91.86% of its theoretical density after sintering at 1520°C with a constant heating rate of 8°C/min. While the YAG powder synthesized by the modified Pechini method only densified up to 69.2% of its theoretical density under the same sintering conditions.

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A silicon nitride-silicon carbide composite was fabricated from silicon and carbon powders by reaction-bonding and post-hot-pressing procedures. The fabricated specimens had a nanocomposite-like microstructure with β-silicon nitride and β-silicon carbide. For some specimens, middle-stage heat treatment was applied before hot-pressing, which resulted in microstructure formation with large and elongated silicon nitride grains. With this microstructure, the specimens subjected to the middle stage heat treatment exhibited much higher fracture toughness than those not subjected to it.

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Surface conditions of three types of commercial high purity alpha alumina powders produced by three different processes, in-situ chemical vapo deposition (A powders), hydrolysis of aluminum alkoxide (B powders), and a pioneer chemical vapor deposition (no longer in the market) (C powder) methods are evaluated by FT-IR (Fourier transform infrared) transmission and Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy. The spectroscopy results suggest that the surface characteristics of as-received high purity alpha alumina powders are not unique. In some of the B powders with small size, a mixture of aluminum trihydroxides like bands is observed. A specific band at 3472cm^-1 observed on the B and C powders does not belong to band spectra of the aluminum hydroxide references. Relating to the predicted band on infrared spectra of hydrated alpha alumina surface obtained from reported molecular dynamics calculation, this band may be associated with hydrogen bonded hydroxyl groups of a fully hydroxylated alpha alumina surface.

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CeO₂ powder prepared by isothermally aging cerium (III) nitrate solution at 303K in the presence of hexamethylenetetramine was sintered at 1723K for 2h. This process yielded polycrystalline CeO₂ having a relative density equal to approximately 97.6% of its theoretical density. Polycrystalline CeO₂, finely polished and examined by atomic force microscopy (AFM), was used to develop thermal grain boundary grooves at different temperatures (1473-1663K) and times (t=30-480min) in air. Subsequently, AFM was used to observe the surface morphological change in the annealed polycrystalline CeO₂ in order to measure the width, W, and the dihedral angle Ψ of the developed grain boundary grooves. AFM observation revealed logW-logt relationships that are approximately linear with slopes of approximately 1/4. An analysis according to Mullins' formulas showed that grain boundary grooving is controlled by surface diffusion. Finally, the surface diffusion coefficient of polycrystalline CeO₂ was calculated at different temperatures (1473-1663K), and the obtained diffusion coefficient was considered to correspond to that of a (CeO₂)_n group transportation.

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Defect spinel lithium manganese oxide and spinei LiMn₂O₄ compounds (which are of potential interest for lithium-ion battery cathode materials) with different crystallization degrees have been synthesized by a citric acid gel process using lithium acetate and manganese acetate as sources for lithium and manganese at various temperatures and in different atmospheres. Non-stoiciometric lithium-manganese-oxide spinel, Li_1-δMn_2-2δO₄, had been synthesized at temperatures as low as 200°C. The structure of the crystalline compounds was found to be cubic spinel in which the lattice parameter and full width at half maximum (FWHM) were sensitive to calcination temperature. As the calcined temperature increased, the oxygen was lost and cation defects were removed from the structure with better crystallization. At temperatures >700°C, LiMn₂O₄ underwent phase transition from a cubic to a tetragonal phase by removing oxygen ion from the surface of powder in air. However, the samples calcined in O₂ atmosphere did not show any phase transformation. These results highlight the importance of temperature and atmosphere control when synthesizing LiMn₂O₄ powder using a citric acid gel process. The specific surface area of the LiMn₂O_4-x powder prepared at 800°C for 24h was 11.6m²/g that is distinctly larger than that prepared by solid-state reaction. From the result of scanning electron microscope (SEM), the synthesized powders showed a spherical shape and a narrow particle size distribution.

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Particulate ceramic composites composed of Si₃N₄ matrix and dispersing MoSi₂ particles were sintered in 1MPa N₂ atmosphere at 1850°C for 1h. The physical and mechanical properties of these composites containing 10, 20, 30, 40 and 50vol% MoSi₂ were evaluated. The microstructure, strength, fracture toughness and electrical resistivity for the composite of MoSi₂/Si₃N₄ were investigated. The electrical resistivity of the composites decreases with increasing amount of dispersoids and reaches a minimum (-10^-2Ω·cm) in the 50vol% MoSi₂/Si₃N₄ composite. Such a conductivity could be machined by wire electric discharge machining (WEDM). The removal rate of MoSi₂/Si₃N₄ composite was found to be dependent on electrical conductivity for constant working voltage. This result demonstrated the feasibility of machining structural ceramics using EDM method through the incorporation of MoSi₂ conductive phase into ceramic matrix composite.

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The surprisingly low temperature (-323K) synthesis of the ceria-zirconia solid solution upon milling ceria powder with zirconia mill and zirconia balls (in ethanol and/or water) was studied. Solid solutions up to 60mol%ZrO₂ were obtained, whose formation was proved to result from solid phase reaction between ceria and zirconia powders, enhanced by contact stress (either shear or compressive one). Furthermore, the occurrence of large plastic deformation, breaking and mutual combining was found on the ceria powder before and after solid solution. The grain size of ceria-zirconia solid solution was ≤20nm, which would cause on easy rearrangement of low diffusion atoms of cerium and zirconium to form the solid solution at such a low temperature. In addition, the possible existence of a solid solution for which the migration of the constituents is much faster (or the stability of the solid at the composition is much higher) than that of other composition, was suggested around 50mol%CeO₂-50mol%ZrO₂.

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In the aim of the understanding of the coloration mechanism in Cr-doped inorganic pink pigments, the valences of Cr dopant in sphene-type (CaTiSiO₅-CaSnSiO₅), perovskite-type (CaTiO₃-CaSnO₃) and rutile-type (TiO₂-SnO₂) matrices were investigated by use of X-ray photoelectron spectroscopy (XPS). The valences of chromium were directly related to the color of the pigments. The valences were dependent on the Ti/Sn ratio in the mother crystals. Results of XPS showed that chromium ions doped in CaSnSiO₅ and CaSnO₃ matrices existed Cr⁴⁺, causing reddish purple color. With substitution of Ti for Sn in these matrices, the fraction of trivalent chromium increased and the color changed from reddish purple through purple red, red brown and finally to brown. A similar valence change of chromium was observed in the rutile-type pigments that changed lilac through brown to ocar colors. Purple red color was achieved in 2mol% Cr-doped sphene- and perovskite-type pigments containing 20mol%Ti.

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Amorphous graphite slurries used in flushing operations were conditioned at pH4.2 in the presence of chelatin and reducing agents both to release graphite and clayey particles and to enhance the hydrophobicity of the graphite surface. Graphite particles were first extracted from the aqueous slurry using a kerosene-based oil phase, while clayey impurities (including kaolinite, illite, chlorite and quartz) were left in the water phase. Then, the residual clayey impurities in the graphite particles were extracted from the oil phase using an aqueous solution containing the same reagents as used for making the raw slurries. The ash content of purified products proportionally decreased with increasing amount of dissolved iron component, indicating the existence of iron (III) hydroxide compounds which aggregate the constituent particles of raw graphite. Furthermore, the efficiency of removing the impurities became greater as the particle size of the pulverized raw graphite decreased because of the improvement in liberation of clayey impurities from graphite particles. In a graphite sample with a mean particle size of 2μm, the ash content could be lowered from 8.4 to 2.8mass%.

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Porous alumina bodies were obtained by heat treating in air porous bodies, which were produced by polymerization and drying of polymerizable W/O-type emulsions containing alumina powder. The total porosity of the alumina bodies markedly increased with the water content of W/O-type emulsions. The bending strength and compressive strength of porous alumina bodies markedly decreased with the water content of W/O-type emulsions. Both macro-pores and micro-pores existed in porous alumina bodies by the W/O-type emulsion method.

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Amakusa pottery-stone was ground by a dry ball-mill process, and some properties of the ground powder, plastic body, and sintered body obtained by this process were investigated and compared with those of products by conventional stamp- and wet ball-mill process. Dry ball-mill powder produced relatively large amounts of particles <1μm and >30μm in size, with a particle size distribution similar to that found in the stamp-mill process. In the dry ball-mill powder, very fine particles (<1μm) contained clay minerals for about 90mass% and coarse particles (>30μm) containing quartz for about 92-95mass%. These components were similar to those found in the stamp-mill process. The plastic body of dry ball-mill had a high fluidity, similar to the stamp-mill body. Fluidity decreased in the dry ball-mill body as the water content decreased; however, the fluidity increased with increasing grinding time. Better properties were obtained in the sintered body by dry ball-mill as compared with that of a stamp-mill sintered body; however, these properties were less improved than in a wet ball-mill body. The relation between the value of sintered body properties and the amount of particles >30μm in each grinding powder obeyed a simple linear relation.

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Relationship between color of chromium green glaze fired at 1280°C under oxidation atmosphere and its chemical composition was successfully analyzed by a newly developed method measuring absorption and scattering coefficients. Sixteen glazes were prepared by adding 0.25mass% Cr₂O₃ into the base glazes with Segar formulars of 0.3KNaO, 0.7CaO, xAl₂O₃, ySiO₂ (x=0.35, 0.40, 0.45, 0.50; y=3.5, 4.0, 4.5, 5.0). Complicated changes in the apparent color of the glaze were observed because of the occurrence of several elemental changes in the absorption and scattering coefficients. The decrease in the scattering coefficient with increasing Al₂O₃ amount could be related to the amount of a Wollastonite crystalline phase, which had a significant influence on the apparent color of the glaze. Relationship between absorption coefficient and glaze composition could be clearly detected, which indicated a change in chemical state of Cr.

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Redox reaction of platinum ion in 50PbO⋅50SiO₂ (molar ratio) melt was analyzed electrochemically using cyclic voltammetry and chronopotentiometry. The platinum ion in the melt was reduced at around -250mV (Pt²⁺/Pt⁰), in the temperature range 800°C-1000°C. From the results of cyclic voltammogram, it is suggested that in the system of 50PbO⋅50SiO₂ dissolving Pt, the reaction rate of reduction of Pt ion is much higher than that of oxidation and the rate of electron transfer at electrode surface is slower than ion diffusion. It was found that the electron transfer number of platinum ion is 2, meaning that platinum ion is dissolved as Pt²⁺ and the reaction on the electrode is Pt²⁺+2e^-→Pt⁰. Various parameters such as the diffusion coefficient and the reaction rate were determined for the melt in which high concentration of platinum was disolved.

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The nickel metal joined to silicon nitride ceramics using silver-copper-titanium brazing and titanium filler was placed in a sulfuric acid solution at 323K for up to 214h. Joining strengths were measured by the 4-point bending test. Microstructures of the reaction layer and porous layer were investigated with a scanning electron microscopy and an electron probe microanalyzer. Fracture strength of the normal joint was 245MPa before soaking and decreased to 170MPa after 16h and to 100MPa after 144h in 1N H₂SO₄. Three regions were observed on the fracture surface: the region I at the reaction layer/brazing layer interface, the region II at the bonding interface neighborhood in porous layers (in silicon nitride) and the region III at the silicon nitride/reaction layer interface. In the porous layer of the silicon nitride was formed yttrium-oxide and aluminium-oxide elute by the soaking the sintering aid in the grain boundary of silicon nitride. The growth of the porous layers occurred on the joint surface of silicon nitride ceramics with different roughness due to the ground surface roughness of the test piece. The wedge-shaped porous layers were formed a residual stress on the interfacial side of the silicon nitride ceramics joints.

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Silicon powder in a reaction-sintered silicon nitride crucible was heated at 1600°C in a nitrogen atomosphere. Clustered β-Si₃N₄ single crystals were obtained in residual silicon metal after cooling. The silicon residuals were dissolved by chemical purification using a mixture of aqueous HF and HNO₃, and subsequently treated and with H₂SO₄. Scanning electron microscopy observation showed that large β-Si₃N₄ crystals without Si residuals could be successfully separated by this process.

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For undoped-SrTiO₃ and La-doped SrTiO₃ single crystals, X-ray photoemission spectroscopy (XPS) measurements were performed with monochromated Al Kα radiation, Valence band (VB) spectra of as-fractured SrTiO₃ exhibit two peaks, P1 and P2, arising from the O 2pπ and O 2pσ bonding orbitals, respectively. Upon adding 3.0atm% of La to SrTiO₃, the valley between P1 and P2 disappears in the VB spectrum. Calculated VB spectra obtained from a molecular orbital calculation reproduce well the experimental XPS spectra for the samples.

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Si₃N₄-TiN ceramics prepared from Si, TiC and TiO₂ mixed powder compacts were investigated. Si₃N₄ matrix and TiN particles were formed during the sintering process of the ceramics. TiN particles about 100nm in diameter were dispersed within β-Si₃N₄ matrix particles. Yttrium which originated from the sintering aid (Y₂O₃) was detected at Si₃N₄ matrix/TiN particle boundary, when the TiN particles were intragranularly located. It is suggested that the TiN particles were incorporated in the Si₃N₄ during the α→β transformation or grain growth of the Si₃N₄ grains.

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Sintering is an extremely old technology, which has been used since prehistory. However, it is so complicated that a quantitative study just began to start in the 1950's. A theory for sintering in a simple model was separately derived by Frenkel, Kuczynski, Kingery, and Coble. They elucidated the mechanism of material transport in sintering by the rate of interface growth and distance between the centers of two spherical particles. The equation derived was successfully applied to the shrinkage data of powder compacts. Since then, these so-called log-log plots of data have been widely used by many researchers in order to elucidate the mechanisms of the sintering shrinkage.
In such a situation, Moriyoshi and Komatsu pointed out that the log-log plot method was not necessarily suitable because of the effect of many factors such as the combined mechanisms of sintering, the size distribution of particles, particle shape, and so on. They reported this in a series of papers (Kinetics of Initial Sintering with Grain Growth, J. Am. Ceram. Soc., 53, No. 12, 621-675 (1970); Analysis of Initial Combined Sintering, Yogyo-Kyokai-Shi, 81 [3], 102-107 (1973); Initial Sintering of Glass, Yogyo-Kyokai-Shi, 82 [8], 426-429 (1974); A Method for Obtaining Surface Diffusion Coefficients from Initial Sintering Data, Yogyo-Kyokai-Shi, 86 [2], 130-136 (1978)), in which they suggested that various processes such as surface and volume diffusion, grain boundary and volume diffusion, and other processes proceeded concurrently during sintering. The above paper was the first one in the series, in which a different method was used to evaluate and to define a sintering mechanism. The paper also served as an important trial encouraging other sintering studies of ceramics.
Moriyoshi and Komatsu pointed out that the method of log-log plots was not necessarily reliable and that the decision of a mechanism at the initial stage of sintering should be carried out by the comparison of activation energies obtained from sintering data with those obtained from the tracer diffusion of ions. From these viewpoints, they derived theoretically an equation to obtain activation energies from shrinkage data at a constant rate of heating. As a result, it was shown that activation energies of sintering shrinkage can be simply obtained with precision by using the equation derived. Then, they indicated that the activation energies obtained in the shrinkage data of ZnO, NiO, and Fe₂O₃ compacts were in good agreement with the values obtained from tracer measurements. Based on these results, they discussed the rate determining steps in the sintering process.
This paper not only provides fundamental information to understand sintering processes such as sintering shrinkage, grain growth, and others, but also stimulates the studies concerning solid state reactions, erosion and corrosion of ceramics, creeps associated with ion diffusion, and so on. The study also gave basic information to fabricate ceramic materials.

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