To improve mechanical properties of TiO₂-added zirconia-toughened alumina (ZTA), the microstructure and sinterability of ZTA composites fabricated from ZTA powders containing (Zr, Ti)O₂ and sintered in an atmosphere of N₂+5%H₂ were investigated. In the sintered ZTA, the concentration of Ti in grains of Al₂O₃ or (Zr, Ti)O₂, was analyzed with EDS attached to an AEM apparatus. Experimental results indicated that some Ti ions diffused to the grain boundary and into Al₂O₃ grains, thereby enhancing the densification of ZTA powders sintered in N₂+5%H₂, the grain growth is less than that of powders sintered in air. The solubility of TiO₂ in (Zr, Ti)O₂ in ZTA sintered in N₂+5%H₂ was smaller than that of ZTA sintered in air, which diminished the tetragonal zirconia phase in the ZTA sintered body.

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Nine kinds of composite powders in CaO-P₂O₅-SiO₂-Al₂O₃ system were prepared by the sol-gel process. Although the as-prepared powders were amorphous, hydroxyapatite (HAp) and tricalcium phosphate (TCP) were formed as a calcium phosphate phase when they were heated from 800 to 1300°C. Additionally, anorthite (CaAl₂Si₂O₈) was formed in the composites containing 25mol% of Al₂O₃ and 25mol% of SiO₂ at 1100°C. The dense composite of 2.69g·cm^-3 bulk density was obtained by sintering powders with compositions of 38.48CaO⋅11.52P₂O₅⋅25SiO₂⋅25Al₂O₃ at 900°C for 5h. When this composite was hydrothermally treated at 160°C for 24h, β-TCP was changed into HAp to form a porous layer with the -8μm thickness on the surfaces of composites. The porous composite of 2.33g·cm^-3 bulk density was obtained when the powders with the compositions of 40CaO⋅10P₂O₅⋅10SiO₂⋅40Al₂O₃ were sintered at 1200°C for 5h. Ricelike HAp particles were formed on the surfaces and fracture surfaces of the porous composite treated hydrothermally at 160°C for 12h.

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Two-dimensional computer simulations were conducted on percolative structure in which second phases of various aspect ratios were arranged in a matrix phase. The second phases were randomly arranged in an array with two different computational programs; one prohibiting an overlap among second phases and the other allowing the overlap. From the simulation prohibiting the overlap, it was predicted that a continuous path was formed at less amounts of the second phase with higher aspect ratios. In the simulation allowing the overlap, a continuous path throughout the array was formed by arranging the second phase of an aspect ratio of 100 with less than 4% in the array. These results agreed well with the conduction path formation in hotpressed ceramic composites consisting of an insulator matrix and electrically conductive whisker additives.

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As the electro discharge phenomena can only occur between electro conductive materials and electrodes, the insulating materials could not been machined by electrical discharge machining (EDM) method. In this report, the new machining system is proposed and the machining characteristics of Sialon insulating ceramics are discussed. It is clarified that the insulating ceramics are also able to be machined using the standard EDM technique with the adhesive metal plate and soft metal electrodes. It is accomplished by using one of the surface modification techniques for continuously making an electro conductive surface layer on the insulating surface.

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Interface states at the grain boundaries in La, Mncodoped Ba_0.5Sr_0.5TiO₃ ceramics have been characterized by using the isothermal transient capacitance method. The ceramic materials doped with 0.05, 0.06 and 0.075mol% Mn were used for the present experiments, all of which had the Curie point around -70°C and a room temperature resistivity>10⁷Ω·m with a positive temperature coefficient of resistivity effect of more than seven orders of magnitude. The results indicate the existence of grain boundary interface states at a level of about 1.0eV below the conduction band edge in all the materials. On the other hand, it has been found from an analysis of the resistivity-temperature characteristics in the materials that barrier layers with a potential height of about 0.85eV were built at the grain boundaries above the temperature giving the maximum resistivity in the respective materials.

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The feasibility of microwave sintering of lead zirconate titanate (PZT) ceramics was examined. Microwave sintering was performed at low temperature and short soaking time, using a heating control system with single mode cavity (2.45GHz). Higher density and electric properties were obtained compared with the conventional process, because the sintering reaction was accelerated by microwave radiation. Moreover, higher strength was obtained due to smaller grain size. The microwave sintering was expected to give both high strength and high electric properties.

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Highly densified sintered Si₃N₄ bodies are conventionally obtained through liquid phase sintering by adding sintering aids to overcome difficulty of sintering due to the strong covalent bonding in Si₃N₄. Sintered Si₃N₄ ceramics contain a weak grain boundary phase, which is produced from a reaction between the sintering aids and the raw Si₃N₄ powder, especially oxide phase of surface contamination on the Si₃N₄ powder. Therefore, the grain boundary phase influences mechanical properties of Si₃N₄ ceramics. The present paper discusses a method to enhance mechanical properties by forming nano SiC particles in Si₃N₄ system with additives, especially in grain boundary phase. The Si₃N₄ ceramics with dispersed nano SiC particles were obtained by using a Si₃N₄ powder coated by carbon, which was obtained by thermal decomposition of methane (CH₄) gas. The nano SiC particles were formed by in-situ reaction between the oxide phase on the surface of the Si₃N₄ powder and the coated carbon. The sintered bodies were obtained by hot-pressing at 1800°C for 1h in N₂ atmosphere under 30 MPa of applied pressure. Results of mechanical property measurements showed that the bending strength and fracture toughness increased about 20-25%, and the high temperature mechanical strength was also improved by the carbon coating method. TEM observation showed that the precipitated SiC particles were dispersed in three different types of locations, i.e., in the grain boundary layer, at the Si₃N₄-Si₃N₄ interface penetrating the grain boundary layer and inside of the Si₃N₄ grain. SiC particles dispersed at the Si₃N₄-Si₃N₄ interface block grain boundary sliding and slow crack growth at high temperatures. For carbon coated samples, the quantity of grain boundary phase was smaller than the non-coated samples as shown by cryogenic specific heat measurements. The quantity of grain boundary phase decreases, which leads the improvement of high temperature mechanical strength also. Therefore, the high temperature strength was improved by precipitated SiC particles dispersion and decreasing the quantity of grain boundary phase.

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Aluminum titanate ceramics were fabricated from compacts of aggregated mixture of alumina and titania. The mixture was prepared by precipitation of aluminum hydroxide from aqueous solution of aluminum nitrate, containing rutile powder. After calcination, the mixture was crushed and classified to fractions' 2-5, 5-10, 10-20, 20-47 and 47-77μm sizes. The SEM observation of compacts of the aggregates revealed that the aggregates were not broken by the compaction. The fired bodies of the compacts consisted of oriented aluminum titanate grains. The sizes of the oriented regions were about 4 times larger than the initial aggregate sizes. The reasons for the formation of the oriented grains were considered as follows: the formation of aluminum titanate has low change in free energy, increases elastic energy, and is endothermic. Because of these reasons, grain boundary energy becomes significant in nucleation of aluminum titanate and, therefore, the nucleation occurs epitaxially to result in the oriented texture. The semi-quantitative estimation of change in energies during formation of aluminum titanate agrees with the argument.

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In relation to the joining of silicon nitride ceramics to metal, the reaction products and reaction mechanism between Si₃N₄ and Mn have been investigated in Ar or N₂ at temperatures from 1073 to 1573K. Using an Si₃N₄-Mn powder mixture, the reaction rate was determined thermogravimetrically, and the reaction products were examined by X-ray diffraction. Under an Ar atmosphere, from 1223 to 1423K, MnSiN₂, Mn_0.815Si_0.815 and Mn₃Si were produced, and above 1423K, MnSiN₂ disappeared. At higher temperatures and on prolonged heating, the suicide composition changed in the following order: Mn_0.815Si_0.185, Mn₃Si, Mn₅Si₃, and MnSi. Under an N₂ atmosphere, the reaction between Mn, Si₃N₄ and N₂ produced Mn nitrides and MnSiN₂. Below 1223K, the reaction products were Mn₆N_2.58 and Mn₄N. At higher temperatures, MnSiN₂ appeared, while Mn nitrides disappeared. Above 1423K, only MnSiN₂ was produced. By relating these results to TG measurements, a full understanding of the reaction mechanism between Si₃N₄ and Mn was acquired.

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Si-C-O long fiber (20vol%)/alumina matrix composites have been fabricated by infiltration of aqueous alumina suspensions into fiber-sheets and sintered at 1000-1400°C by firing in air, vacuum (0.01Pa) and hotpressing (39MPa). The sinterability of monolithic alumina and the composites was higher in an atmosphere of air rather than under vacuum. The flexural strength of composites sintered in vacuum showed a maximum (300MPa) at a sintering temperature of 1300°C. The vacuum sintering at 1200°C-1300°C produced a significant non-linear fracture behavior in stress-strain curves based on the pull-out of fibers from the matrix. Increase of sintering temperature of the composite to 1400°C resulted in brittle fracture behavior, suggesting that the interfacial strength increased with an increase in density of the composite. Sintering of the composite in air produced the intermediate layers of the SiO₂-Al₂O₃ system between the alumina matrix and the Si-C-O fibers due to oxidation of the fibers. The formation of SiO₂-Al₂O₃ layers reduced the strength of composite with brittle fracture behavior. Hot-pressing of the composite at 1200°C using Si-C-O fibers coated with fine BN particles led to an increase in strength (420MPa) as well as in non-linearity of the fracture behavior.

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In applications of ceramics as a structural component, the forming technology and the strength reliability are the most important. One of the factors, which control the strength reliability of ceramics, is large pores as defect, and it is produced in forming process. In this study, the influence of granule properties on the structure of green bodies and the strength of sintered bodies in uniaxial press forming of alumina ceramics was investigated. The sintered bodies from several kinds of granules showed the bending strength of 450-310 MPa, and the granule with low breaking strength gave the sintered bodies with high bending strength due to a small number of large pores. The breaking strength of granule was affected significantly by the amount of dispersant and the type of binder.

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The solid-state reaction between La_1-xCo_1-yFe_yO_3-δ (x=0.0, 0.1; y=0.0, 0.5) and Al₂O₃ was investigated by X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy. LaAlO₃ and spinel phases formed from a mixture of LaCoO₃ and Al₂O₃ with the increase in the reaction temperature. However, substitution of Fe for Co and La-site deficiency in LaCoO₃ inhibited the formation of LaAlO₃ and spinel phases. Moreover, the electron diffraction pattern of LaCoO₃ after the reaction suggests the possibility that the solid-state reaction enhance the formation of the La-site deficiency of La_1-xCoO₃.

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To study the effect of process-related oxidation on microstructure and densification of β′-o′ sialon, green compacts of a silicon nitride powder and boehmite sol were prepared. With progressive oxidation of the silicon nitride powder, the amount of o′-phase increased and grain size of o′-phase was changed. The different densification was caused by o′-phase content. The experimental result showed that, the transition liquid phase in the specimen of low oxygen contents formed at higher tempeature than that of high oxygen contents.

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The effect of talc grain size on the microstructure of cordierite ceramics prepared from the talc, kaolin and alumina was studied. Six kinds of talc materials with various grain size distributions were prepared by classification and grinding. Enstatite formed by decomposition of fine talc grains contributed to the cordierite formation at a lower temperature of 1200°C without transformation to protoenstatite. When the grain size of the talc was larger than that of kaolin, behavior of melt of the talc grains was so complicated that the melt affected the pore formation of fired bodies. By grinding the talc to submicron size, it was possible to densify the bodies at a temperature lower than that of cordierite formation, and high strength of fired bodies was achieved.

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Various Ni²⁺-substituted tetrasilicic micas having high contents of Ni²⁺, such as KNiMgLi(Si₄O₁₀)F₂, KNi₂Li(Si₄O₁₀)F₂ and KNi_2.5(Si₄O₁₀)F₂, were synthesized at ca. 900°C through solid-state reaction techniques. Lattice constants a, b, c decreased with increasing Ni²⁺-substitutions, reflecting the decrease in the ionic radius of Ni²⁺ against Mg²⁺. UV-visible spectra of Ni²⁺-micas were very similar to those of [Ni(H₂O)₆]²⁺, This can be explained in terms of the nephelauxetic series F^-<H₂O<O_a (apical oxygen) as well as the spectrochemical series F^-<H₂O<O_a. IR spectra showed that the tetrahedral distortion changes with increasing Ni²⁺-substitutions, i.e., the interatomic distance of Si-O_a lengthens while that of Si-O_b (basal oxygen) shortens. Ni²⁺-micas exhibited paramagnetism when the Ni²⁺ content was smaller than 2.0 mol in octahedral sites, however, antiferromagnetism seemed to appear at larger substitutions of 2.5 mol Ni²⁺ per formula unit.

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The effect of microstructure in silicon nitride on fracture toughness and Subcritical crack growth (SCG) behavior at room temperature was investigated. Three kinds of silicon nitride materials containing 5 mass% of Y₂O₃ and Al₂O₃ were sintered under different conditions, and they showed different microstructures. Fracture toughness of the materials was obtained by single edge precracked beam (SEPB) method and controlled surface flaw (CSF) method with Knoop indentation-crack in order to investigate the dependence of the fracture toughness on microstructure. The fracture toughness data measured by the CSF method increased with increasing crack size for all kinds of the materials. The increment of the fracture toughness per crack length was more considerable for coarser grain sizes. It can be due to stress shielding at crack tip by such toughening mechanism as crack bridging. The static fatigue behavior of the materials was investigated by the dynamic fatigue technique at room temperature. The fatigue parameter n in SCG model (da/dt=AK_Iⁿ) increased with increasing as fracture toughness and grain size increased. Based on stress shielding model at crack tip, the fatigue crack growth parameters in SCG model were calculated. The predicted value agreed to the experimental results.

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Powder precipitates and monolithic gels of lead zirconate-titanate (PZT) were prepared by hydrolysis of metal alkoxides in 1-propanol and acetylacetone/1-propanol solutions, respectively. The as-dried precipitates and gels were amorphous. Deposition of pyrochlore and perovskite at a relatively low temperature, 450°C, was observed for both the precipitate and gel. However, crystalline PbO was found in the gels heated at 300°C. The results showed that the homogeneity of the powder precipitate was better than that of the gel. After heating at 600°C, the diffraction peaks of only perovskite were found for both samples. Powder-compacts and gel compacts were sintered at temperatures higher than 900°C. The shrinkage of the gel-compacts after sintering was greater than that of the powder-compacts because of its small particle size.

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A transparent mixed alkali tellurite glass of 10Li₂O-10Na₂O-80TeO₂ containing LiNbO₃ crystals with a diameter of about 4μm and with the amount of about 6 mass% is prepared using an incorporation method, and LiNbO₃-doped tellurite glass fibers are drawn. LiNbO₃ particles in the fiber and bulk samples are well dispersed. The samples also have a good transparency. The mixed alkali tellurite glass with a high thermal stability against crystallization and with a high refractive index of n=1.98 is very suitable for incorporation of LiNbO₃ crystals and for LiNbO₃-doped fiber drawing. This technique will be applicable for fabrications of nonlinear optical glass fibers containing various nonlinear optical crystals.

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A commercial TiB₂ powder with the grain size of 2μm was milled by an attriter and the powder having the average grain size of 0.9μm and the oxygen content of 3.4 mass% was obtained. Sinterability of the powder without additives and mechanical properties of hotisostatically pressed compacts were investigated. The density of fired compacts improved with rising firing temperature. However, compacts with densities approaching 100% were difficult to obtain by pressureless firing. Abnormal grain growth with cracks and residual pores occurred in the compact fired at 2000°C and its density was 92%. A compact having 99.5% density could be obtained by firing at 1800°C and sequential hot isostatic pressing with encapsulation at 1700°C. Fracture toughness, 3-point bending strength and Vickers hardness of the hot-isostatically pressed compact were 3.3 MPa·m^1/2, 420MPa and 2500, respectively.

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The effect of thickness deviation on the apparent strength distribution through simultaneous loading measurement was estimated. Some thickness deviations were generated by computer simulation. An apparent strength was calculated from the intrinsic strength of a sample and thickness difference taking into account of the sample setup of the simultaneous loading measurement. For a hundred samples with thickness deviation which accords with a normal distribution, the apparent strength distribution was calculated based on Weibull statistics and compared to the ideal one. Almost equivalent strength distribution shape was proved to be obtainable from samples with standard deviation of thickness deviation up to 6.5μm in the case of titanium dioxide ceramics. This critical thickness deviation was larger than 1μm for all ceramic materials calculated, which could be controllable using a micrometer.

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Silicon nitride powder calcined with yttria-rich or alumina-rich additives (total 10 mol%) was alternately accumulated into layers by centrifugal filter-pressing; a combination of filter-pressing and centrifugal shape forming. The subsequent sintering at 1800°C in 1.0 MPa N₂ atmoshere provided sintered composites (SN-MLCs) with more than 98% density and 60μm thick of each layer. Elongated rodlike grains characteristic of yttria-rich additives were periodically observed in the SN-MLCs at the calcination temperature of 1400°C. At a high calcination temperature (1450°C), the difference in microstructure between layers became smaller. It was considered that the diffusion of a part of the additives through the layer interface reduced the interlayer stress to prevent delamination during sintering. The crack length propagated from Vickers indents on the SN-MLCs was shorter than that on the monolithic materials. The shorter crack length and interlayer fracture observed for SN-MLCs suggested a possibility of toughening.

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The effect of high voltage screening on ceramics parts composed of titania ceramics with relative densities of 60 and 90%, was examined. Before screening, strength distribution was clearly descriminated into two regions, high strength region and low strength region. The dielectric strength was measured on the low strength samples, and the electric field below which all of the low strength parts would break was estimated as a screening field. After electric screening, the mechanical strength was measured and compared with the original one. Ceramic parts composed of only high strength region were obtained by eliminating the low strength parts electrically.

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Highly c-axis oriented AIN thin films were prepared on the polycrystalline substrates of MoSi₂, Al₂O₃ and SiC by rf magnetron sputtering. Crystal orientation of the thin films was not influenced by the difference in the polycrystalline substrate materials. Crystal structure of the thin film was hexagonal and its texture was consisted of many columnar grains. It was thought that the thin films were c-axis oriented because the interaction between the thin films and the substrate surfaces was small and the growth of (100) and (110) planes of AlN was fast. The orientation of the thin films deposited on the polycrystalline substrates was worse than that of thin films deposited on single crystal substrates, however, the thin films showed appreciable piezoelectricity.

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Effect of additives on the sintering of calcite under the atmospheric pressure was studied. Calcite could be sintered at about 793K by adding a small amount of lithium fluoride. The microstructure of the sintered calcite was consistent with that of ceramics fabricated by liquid-phase sintering.

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