Gold nanoparticles coated with a silica layer, which has successfully been controlled in layer thickness, were self-assembled on poly(diallyldimethylammoniumchloride) (PDDA) coupling glass substrate. Gold particles with about 30 nm in diameter are at first prepared by the thermal-reduction method using citrate, and then by addition of 3-aminopropyltrimethoxysilane (APTMS) and subsequently a sodium silicate solution, a silica layer surrounding the gold particles is formed. The thickness of silica layer was controlled up to 8 nm by a variety of the aging periods of the solution. The electric properties of single silica-coated gold nanoparticles with different silica thickness are measured utilizing atomic force microscope (AFM) in vacuum. Their current-voltage (I-V) curves show clear zero current regions due to Coulomb blockade. The width of the region, Coulomb gap, increases with an increase in the thickness of silica layer surrounding gold nanoparticle.

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To obtain colored gels and glasses by a sol-gel method, we prepared SiO₂ gels doped with organometallic compounds, ferrocene and ferrocenium ion salts. Ferrocene was dissolved into a starting solution consisting of tetraethylorthosilicate and so on. When the sol-gel reaction and the drying process of the system proceeded from sol, via wet gel, to dried gel, the orange color of the sample turned blue. We measured optical absorption and electron spin resonance (ESR) spectra to investigate this color change and found that ferrocene molecule (orange) has been oxidized to ferrocenium ion (blue) in the sample. The oxidation of ferrocene in the sample is considered to be caused by H⁺, which has been produced by ionization of a surface silanol group (the Bronsted-acid character of a silanol group). ESR spectra of heat-treated gels reveal that the ferrocenium ions in the gels exist up to 400°C, and fully decomposed into iron oxide at 900°C.

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Silica gel films were deposited on single-crystal Si substrates using starting solutions of mole ratios, Si(OC₂H₅)₄ : H₂O : HNO₃ C₂H₅OH=1 : x : 0.01 : 4, where x=2, 4 and 10. Cracking of the gel films was observed in situ on heating using a near infrared image furnace equipped with an optical microscope. The gel films were found to be cracked at lower temperatures when the H₂O/TEOS ratios in solutions were larger. In-plane stress in gel films was also measured in situ on heating with a thin film stress measurement apparatus equipped with an electric furnace. Tensile stress generated on heating, and higher stress evolved at larger H₂O/TEOS ratios, which was thought to allow cracking at larger H₂O/TEOS ratios. However, higher rates of increase in stress at larger H₂O/TEOS ratios were seen only up to 100°C, while the slope of the stress-temperature relation was similar over that temperature irrespective of the H₂O/TEOS ratios. It was thought to be the higher capillary pressure due to the high surface tension of H₂O that gave rise to the higher rates of increase in stress at larger H₂O/TEOS ratios observed below 100°C. In other words, higher surface tension of H₂O is the most possible cause of the lower cracking temperatures at larger H₂O/TEOS ratios. The shape of the cracks tended to shift from scale-like to worm-like, and then to linear shape as the cracking onset temperature increases. In other words, the cracks propagate curled at lower temperature and straight at higher temperatures.

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Titania gel films were deposited on single-crystal Si wafers using Ti(OC₃H₇ ⁱ)₄-HNO₃-H₂O-C₂H₅OH solutions with or without PVP and CH₃COOH. The gel films were heated up to 500°C, and the stress in films were measured in situ in the heating-up and cooling-down stages. The film prepared without PVP or CH₃COOH showed about 50% reduction in thickness and an increase in stress up to 230 MPa at 200°C. The film prepared with CH₃-COOH exhibited about 55% decrease in thickness when heated up to 250°C, accompanied by an increase in stress up to ca. 200 MPa. The rate of the increase in stress and of the decrease in thickness was smaller than that observed in the film prepared without PVP or CH₃COOH, suggesting that CH₃COOH is effective in suppressing the volume shrinkage and stress evolution in the heating-up stage at this temperature range. The film prepared with PVP, on the other hand, showed an increase in stress only up to ca. 70 MPa when heated up to 300°C in spite of the reduction in thickness by about 40%. This suggests that PVP is effective in suppressing the stress evolution via plastic flow as well as via suppressed hydrolysis and condensation reaction. Smaller elastic constant of the film in the presence of the organic polymer might have also contributed to the smaller stress. The decomposition and/or oxidation of PVP occurred at 300-380°C, leading to a further decrease in thickness and an increase in stress up to about 180 MPa. All the films were crystallized between 400 and 500°C, where significant reduction in stress was observed. In the cooling-down stage, the film prepared with PVP showed almost constant stress whereas the other two films exhibited evolution of thermal stress.

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This paper describes the ultraviolet (UV) irradiation hardening of sol-gel-derived hafnia films containing oxalic acid as an organic ligand. Colloidal and viscous hafnia sols were prepared by treating hafnium hydroxide with an oxalic acid aqueous solution. The transparent gel films were hardened to scratch hardness of over 9H by UV irradiation using a high-pressure mercury lamp for 2.5-10 min. The films showed a maximum water droplet contact angle of 89°, indicating that the hardened films displayed hydrophobicity similar to that of fired hafnia films obtained at 550°C. The bonding structure of the oxalato ligands was analyzed on the basis of their FTIR spectra. It was found that an oxalato ligand bridges to Hf ions in a bond involving two metal ions. The hardening process of the hafnia gel films was investigated using the thermal programmed desorption (TPD) technique. Species of C, O, CO and CO₂ evolved from the oxalato ligands in a temperature range of 200-400°C. The amount of CO and CO₂ decreased markedly after UV irradiation, indicating that the oxalato ligands were eliminated by UV irradiation, resulting in pencil hardness of over 9H.

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Monodispersed micro polyorganosiloxane spherical particles synthesized from methyltrimethoxysilane in W/O emulsion consisting of sorbitantrioleate (SPAN85), n-octane, and H₂O were heat treated from 100 to 600°C in the air, and examined from SEM, NMR, FT-IR, and specific surface area measurements. The heating process is characterized as follows. Before the formation of the Si-O-Si linkage by oxidation of CH₃ in Si-CH₃ near 410°C, silica network in the particles consists of only T² and T³ units and the transformation of T² into T³ and the shrinkage of the particles were observed up to 400°C, specific surface area showing the maximum value (18 m² g^-1) for the particles heat treated at 300°C. We concluded that the origin of the change in thespecific surface area was related to the changes in both the degree of polymerization and the shrinkage. On the other hand, after the formation of the new Si-O-Si linkage by the oxidation, silica network consists of only Q units and the particles were thermally stable with almost constant diameter and specific surface area.

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Electrochromic WO₃ thin film was prepared by using tungsten metal solution in hydrogen peroxide as a starting solution and by sol-gel dip coating method. The chemical modification was analyzed by FT-IR spectroscopy and the thermal analysis was conducted by DTA/TG method. DTA/TG analysis and XRD pattern showed that tungsten oxide crystal phase formed at 400°C. SEM microstructure also showed that about 20 nm sized crystals appeared in amorphous matrix at 400°C. In the view of electrochemical property, WO₃ thin film which was heat-treated at 300°C and was amorphous had better than that of the crystalline phase.

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Development of high refractive-index (n_D>2.0) materials is needed for micro-optical devices to accomplish their optical performances. In this study, thin films of n_D>2.0 were fabricated by heating the organic-inorganic hybrid films. The hybrid films of 0.1-1.0 μm in thickness were prepared on glass substrates by the dip-coating technique at room temperature, using titanium tetra-n-butoxide (TTBu), diphenyldimethoxysilane (DPhDMS), phenyltriethoxysilane (PTES) and 3-glycidoxypropyltrimethoxysilane (GPTMS) as starting materials. Refractive indices and thickness of the films were measured by the optical interference method. Refractive indices increased with increasing the TTBu content, and n_D=1.74 was attained in the film of 80TTBu-10DPhDMS-10PTES composition. Subsequently, the films of 80TTBu-20(DPhDMS-PTES-GPTMS) compositions were heated up to 600°C under oxygen atmosphere, the transparent and amorphous films of n_D=2.45±0.1 was achieved. Refractive indices of the hybrid films can be controlled from 1.52 to 1.74 by chosing the composition, and from 1.70 to 2.5 by selecting the heating temperatures.

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Mesoporous titania has been prepared by the sol-gel method with surfactant templating. The wet gels, obtained by hydrolysis of Ti-alkoxide in alcoholic solutions with acid catalyst, were immersed in surfactant solutions, dried and annealed. The annealed gels were labeled with ²²⁸Th and ²²⁴Ra, and the release rate of ²²⁰Rn, produced by alpha-decay, was measured during heating and cooling to obtain the ETA-curves. The gels were also characterized with TGA-DTA, XRD, nitrogen adsorption, SEM and AFM. The pore size and the microstructure of the gels obviously depended on the size and shape of the surfactant micelles. The ETA results clearly showed the differences in the surface area and the thermal stability of the annealed gels. The evaluated ETA results and the structural changes of the gels during heating were discussed.

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The photoluminescence of trivalent europium (Eu³⁺) ions in the vicinity of nanometer-sized gold (Au) particles in glasses has been investigated. Au nanoparticles, synthesized in liquid, are introduced into a borosilicate (B₂O₃-SiO₂) glass matrix together with Eu³⁺ ions via a sol-gel route. Polyvinylpyrrolidone (PVP) is utilized to control the size and protect the Au colloids. It is revealed that a heat treatment of the gel at 400°C leads to a great enhancement of Eu³⁺ fluorescence under a long ultraviolet excitation, which is six times higher than without polymer-protected Au nanoparticles. We shall also discuss the effects of a broad photoluminescence due to carboynyl moieties (>C=O) attached in the protective polymer and energy transfer from >C=O to Eu³⁺ ions in enhanced local fields around Au nanoparticles.

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Silica-based thin films with micrometer-sized bumps and light scattering function were created on a glass substrate by inducing phase separation in an alkoxy-derived polysiloxane system. Drying evolution on the surface of wet film was observed by a microscope for the purpose of examining the details of the formation process. The bump formation process consisted of generation of small droplets, followed by their growth in size, and finally their fixation on the glass substrate by sol-gel transition as bumps. The angular distribution of the scattered light of the film was found to be suitable for a reflection-type liquid crystal display apparatus.

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Processing and mechanical properties of dense and porous composites were studied with the nonoxide fiber/oxide matrix system and the oxide fiber/oxide matrix system. Preparation of dense composites needs a high processing temperature (1500°-1700°C), resulting in degradation of fibers, formation of large residual stresses during cooling process and low pseudoductility. On the other hand, porous composites processed at a low temperature (500°-1100°C) provide a high strength and great damage tolerance, which are associated with significant delamination of layered fabric or fiber pullout. The chemical affinity and difference in the thermal expansion coefficients and Young's moduli between fiber and oxide matrix in the composite play important role on the mechanical properties.

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The design of products based on particulate structures and processing thereof are complex topics with roots in various chemical and physical phenomena, and these phenomena typically span several lengthscales. Multi-scale modeling can be a useful approach to investigating these phenomena across a range of scales. For example, a traditional engineering approach to bulk powder processing may use the bulk-scale correlations. On the other hand, a more detailed approach might consider an analysis of individual particles on a micro-scale. This paper describes the distributed properties of micro-scale particle assemblies using statistical analyses of meso-scale simulations and their application to materials science. The simulations provide a microstructural description of particulate, porous or dense virtual mixtures. The meso-scale approach enables the examination of large numbers of particles (order 10⁸) over a distribution of assemblies. In turn, this enables the practical investigation of particulate systems with broad size distributions and multi-component mixtures with significant differences in size. It is projected that the meso-scale simulation results can be used to develop distributed models for product functionality by linking the micro-scale particle characteristics with distributed structural and/or compositional features on a larger scale. This may be especially useful for materials where product performance can be designed on the basis of the tails of structural distributions rather than the average values.

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A novel fabrication process for porous alumina-based ceramics (calcium hexaaluminate (CA6)) was developed using hydraulic inorganic binder (calcium dialuminate (CA2)) and water. The CA2 binder and water acted as agents for direct consolidation, and water also acted as a fugitive material to create open pores. The green bodies formed with the CA2 binder and water had low green densities and high compressive strength. The porosity of the sintered CA6 ceramics increased with the content of water in the starting mixture: porosities could be adjusted to 15.7-33.3% with the addition of 20-80 wt.% water. The results of evolved gas analysis-mass spectrometry measurements showed the emissions from the novel fabrication process of the porous CA6 ceramics to be environmentally safe.

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Particle connection of two-component powder compact was quantitatively evaluated by electrical conductivity measurement. As a model system, an alumina powder (median size 0.33 μm, insulator) of isoelectric point pH 8.0 was mixed with an indium tin oxide powder (ITO, In₂O₃-SnO₂, median size 0.20 μm, electronic conductor) of isoelectric point pH 3.0, in aqueous solutions at pH 3.0-10.0 to make different kinds of microstructures of the consolidated powder compact. The rheological properties of the suspensions and the packing density of the powder compacts consolidated by filtration through gypsum molds, were greatly dominated by the dispersibility of matrix phase particles (Al₂O₃) rather than the dispersibility of second phase particles (ITO). The addition of polyacrylic acid (dispersant) to the basic suspensions (pH 10.0) including the agglomerated colloidal particles, enhanced the dispersibility and packing density of the colloidal suspensions. The particle connection between second phase particles (ITO-ITO) in the consolidated powder cake was influenced by the dispersibility of the matrix phase particles (Al₂O₃) and enhanced by increasing the packing density. When a well-dispersed suspension was consolidated, a continuous particle network of ITO-ITO was formed at 30 vol%ITO fraction of the Al₂O₃-ITO compact. This result was discussed with the collision frequency theory of colloidal particles in a suspension.

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Multilayered mullite/Mo functionally graded material (FGM) disks with different composition gradient were fabricated by a powder stacking method and a pulse electric current sintering (PECS) technique. Thermal shock tests on the FGMs were carried out using a newly developed infrared radiation/water flow technique. The thermal stress distribution in the FGM plates during thermal shock was estimated. The stress behavior on the ceramic surface was evaluated by taking into account the influence of residual thermal stress introduced during fabrication process. The present work show that the thermal stress generated during thermal shock loading and the sintering-induced residual thermal stress can be controlled by adjusting the compositional profiles in the FGMs. The results indicate that the thermal shock resistance was greatly influenced by the residual thermal stress on the ceramic surface.

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Heat capacities of Zr_1-xHf_xW₂O₈(x=0.25, 0.5 and 0.75) were measured from 1.8 to 70 K. The heat capacity of Zr_1-xHf_xW₂O₈ increased with increasing Hf content due to atomic mass effect. Frequency distributions of lattice vibrations were estimated through an analysis of the heat capacities for Zr_1-xHf_xW₂O₈ and indicated the presence of two Einstein modes in low-energy region. The two Einstein characteristic temperatures linearly depended on Hf content. Raman spectra of Zr_1-xHf_xW₂O₈(x=0.25, 0.5 and 0.75) at room temperature showed two characteristic optical phonon modes corresponding to the two Einstein modes obtained from heat capacity analyses. Mode assignment is made of these characteristic optical phonon modes involved in the negative thermal expansion.

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Compressive creep behaviors of in-situ single-crystal alumina/yttrium aluminum garnet (Al₂O₃/Y₃Al₅O₁₂(YAG)) and alumina/gadolinium aluminum perovskite (Al₂O₃/GdAlO₃(GAP)) eutectic composites were investigated in the temperature range 1723 to 1923 K and the stress range 100 to 450 MPa in air. The Al₂O₃/YAG and Al₂O₃/GAP systems exhibited a stress exponent of 5.4-10, indicative of compressive creep behavior characterizing a dislocation mechanism. The activation energy for creep deformation was 810-986 kJ/mol for the Al₂O₃/YAG system and 910-1033 kJ/mol for the Al₂O₃/GAP system. At a given stress, the Al₂O₃/GAP system indicated the creep rate larger by about twice-fifth than that for an Al₂O₃/YAG system.

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Thermal shock fracture tests of ceramics were performed using a newly developed water-flow cooling technique. This novel technique was proposed to obtain quantitative values of thermal shock parameters of ceramics. The surface of the specimen heated uniformly in an electric furnace was cooled by steady-state water-flow to remove the fluctuation of heat-transfer on the specimen surface. The transient thermal stresses in the thermal shocked specimens cooled by water-flow were calculated numerically using an experimentally obtained relation between the heat-transfer coefficients and the surface temperatures. The critical thermal stress at the onset of failure was calculated using the failure time measured by an acoustic emission technique. These water-flow cooling tests were performed for alumina ceramics, and the thermal shock strength defined by R_1c=λσ_c/Eα was estimated, where λ is the thermal conductivity, σ_c is the critical stress, E is the Young's modulus, and α is the thermal expansion coefficient.

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Zirconia based ceramics exhibiting transformation toughening via the tetragonal to monoclinic transformation remain intensely interesting. In-situ neutron diffraction during uniaxial compression has revealed the presence of transient effects highlighting a need for an in-situ method of determining the phase quantities. The two methods used to date i.e. the volume strain from strain gauges and in-situ neutron diffraction are compared for the same test data and their relative merit discussed. In most instances the volume strain gives an excellent measure of changes in the phase transformation. Circumstances causing potential misleading results are discussed.

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Ca_0.9Sr_0.1TiO₃ ceramics were prepared both by conventional liquid phase sintering and spark plasma sintering (SPS). Although the sintering of Ca_0.9Sr_0.1TiO₃ powder without sintering aid required a high temperature of 1400°C, they could be densified at 1200 and 1170°C by liquid phase sintering using Li₂Si₂O₅ and SPS, respectively. The optimum concentration of Li₂Si₂O₅ to obtain high density and high dielectric strength was 0.5 mass%. The sample prepared by SPS showed a higher fracture strength and Vickers hardness than those prepared by liquid phase sintering, while the fracture toughness and permittivity of both samples were almost identical.

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Transparent Nd³⁺-doped Y₃ScAl₄O₁₂ (Nd: Y₃ScAl₄O₁₂) ceramics with up to 5.0-at.% Nd³⁺-concentration (C_Nd) were fabricated by a solid-state reaction method using commercial 4N powders. Spectroscopic properties of this media, such as absorption and emission spectra, the fluorescence lifetime, and the influence of increasing C_Nd on these characteristics are discussed. It is shown that high absorption efficiency of the pump radiation that is necessary for efficient operation of a Nd: Y₃ScAl₄O₁₂ microchip laser can be obtained by increasing C_Nd. One-micron laser operation with 113-mW output power (7.7% optical-to-optical overall efficiency) and 9.6% slope efficiency was demonstrated from a 1.0-mm thick, 5.0-at.% Nd: Y₃ScAl₄O₁₂ uncoated sample under 808-nm pumping by a Ti: Sapphire laser. Further works are directed toward optimization of the fabricating process in order to reduce the optical losses that are found to increase with increasing C_Nd.

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The grain growth under hipping pressure has been investigated on some oxides; Ba(Ti_0.85Zr_0.15)O₃, TiO₂, MgAl₂O₄, MgO and α-Al₂O₃. The grain growth in Ba(Ti_0.85Zr_0.15)O₃ and TiO₂ was suppressed by hipping pressure. The hipping pressure accelerated the grain growth in MgO, α-Al₂O₃ and MgAl₂O₄. The results were discussed based on the equation of grain growth; D=Kt^1/n (D: grain size, K: constant, t: time, n: grain growth exponent). The grain growth exponent n was distributed over the wide range of 1 to 9, which depended on temperature and pressure. Particularly, the suppression of grain growth revealed large n value. Simple model was constructed to solve this problem, considering a coagulation model. It was deduced that the hipping pressure mainly decreased grain boundary diffusion to suppress the grain growth in Ba(Ti_0.85Zr_0.15)O₃ and TiO₂.

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Coarse grained ZrO₂-1.9 mol% Y₂O₃ specimens were prepared by a post-sinter high temperature annealing. The high temperature tetragonal phase of such specimens could be fully retained at room temperature by rapid cooling and allowed to isothermally transform into the stable monoclinic phase by heating at an intermediate temperature. Microscopic observation revealed that well defined thin plate martensite nucleated mostly at grain boundaries and longitudinally grew at considerably low speeds. The habit plane was identified to be near {013}_c, which was close to the previously reported orientations based on transmission electron microscopy works.

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Static and cyclic fatigue of aluminum titanate (Al₂TiO₅: AT) ceramics, which contain many microcracks at their grain boundaries as a result of a large thermal expansion anisotropy during cooling after sintering, were investigated at room temperature and 700°C. The fatigue lifetime is related to the morphological changes of the microcracks. The fatigue lifetimes at room temperature and 700°C decreased when a cyclic stress was applied, and was much shorter at room temperature. During cyclic fatigue at room temperature, the microcracks around the main crack tip are thought to combine and propagate according to crack resisting-reactivating and wedging-microcracking mechanisms. In contrast, cyclic fatigue at 700°C is thought to be accelerated by the wedging effect of the glassy connections behind the crack tip. The microcracks underwent repeated initiation and self-healing via a glassy phase segregated at the grain boundaries.

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Alumina fiber-reinforced alumina composites using different fiber/matrix interface and matrix infiltration method were synthesized. The fiber/matrix interfaces were arranged as with and without monazite coating on fibers, and alumina infiltration into fiber bundle, or monazite/alumina mixture infiltration into non-coated fiber bundle. Maximum strength of the composites using different fiber/matrix interface showed no significant difference regardless of sintering temperature, however, pseudo-ductility of these composites showed quite different characteristics. The composite without fiber/matrix interlayer showed the lowest ductility, while the composite using monazite coated fibers showed the highest pseudo-ductility. Fiber debonding was also observed for the composites using the mixture of monazite/alumina suspension infiltrated fibers. Re-infiltration of matrix slurry after high temperature heat-treatment led to increase of density of composites.

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Regarded as an important structural and functional material, mullite is widely researched on electronic, optical and high-temperature structural applications. In this paper, dense ferromagnetic mullite-based composites with 30 mol% α-Fe₂O₃ were obtained at 1400 and 1450°C in the air by pressureless sintering technique, respectively. Due to the excess of solid solution limitation at sintering temperature, some spinel solid solution of iron oxide and Al³⁺ ion separated from the mullite/Fe₂O₃ solid solution matrix with the elevation of sintering temperature, which is confirmed by phase and microstructure analyses. The mechanical, magnetic and high temperature electrical conductive properties of the composite were measured. It is shown that the mechanical properties of composite are comparable with those of monothetic mullite. Furthermore, the composite also behaves ferromagnetic property at room temperature and good electrical conductivity at high temperature, which shows the possibility of adding multiple functions to mullite composite.

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The sinterabilities of fine zirconia powders including 5 mass% Y₂O₃ were investigated with emphasis on the effect of Al₂O₃ addition on densification at the initial stage. The powders were pressed into disks and sintered at 1100-1500°C in air. When a small amount of Al₂O₃ was added, high-density sintered bodies were obtained at a low temperature. The shrinkage of powder compact in the course of heating was measured by a dilatometric method, indicating that the addition of Al₂O₃ increased the densification rate with elevating temperature. The activation energies of sintering were determined by analyzing the densification curves. The activation energy at the initial stage of sintering decreased by Al₂O₃ addition. The analysis of isothermal shrinkage curves exhibited that the diffusion mechanism changed from grain-boundary to volume diffusions by addition of Al₂O₃. It is concluded that the Al₂O₃ addition enhanced the densification because of decreasing the activation energy of volume diffusion at the initial stage of sintering.

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Al₂O₃-ZrO₂ composite powder has been prepared using the coprecipitation technique. Al₂O₃ particles in the powder were surrounded with ZrO₂ particles whose sizes were about 50 nm. Al₂O₃-20 mass%ZrO₂(3 mol%Y₂O₃) bulk ceramics, made from the Al₂O₃-ZrO₂ composite powder, using hot isostatic pressing (HIP) after pressure-less sintering attained a mean bending strength of about 1.1 GPa. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for analyzing the microstructure of the composites. Both a homogeneous dispersion of ZrO₂ particles in the Al₂O₃-ZrO₂ composite ceramics and the existence of fine ZrO₂ particles remaining at grain boundaries of the Al₂O₃ matrix might be two important factors to strengthen the Al₂O₃-Al₂O₃ grains coupling.

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Lithium titanate (Li₂TiO₃) ceramics is among the most promising candidates for solid breeder materials of fusion reactors. So far, many thermal property data of Li₂TiO₃ have been reported. However, those data are different, from researcher to researcher and not appropriate enough for the blanket design. In the present study, thermal properties of Li₂TiO₃ ceramics having different compositions (Li₂O/TiO₂=0.90∼1.00) have been determined with the laser-flash method over a temperature range from 300 to 1100 K. The observed value of thermal diffusivity decreased with increase in non-stoichiometry. It was also found that the temperature dependence of the thermal diffusivity showed a fall at around 1100 K for all the samples, in accordance with the lattice parameters of the samples, which also showed variation at about 1100 K.

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A unidirectionally solidified Al₂O₃/GdAlO₃ eutectic composite has a unique microstructure, in which continuous networks of single-crystal Al₂O₃ phases and single-crystal oxide compounds GAP (GdAlO₃) interpenetrate without amorphous grain boundary phases. This material was called as Melt Growth Composite (MGC). Therefore, the MGCs have excellent high-temperature strength characteristics, creep resistance and thermal stability in an air atmosphere at very high temperature. For the purpose of ultra-high thermal efficiency and low NOx emission for gas turbine systems, non-cooling turbine nozzle vanes of Al₂O₃/GAP MGC were fabricated and those high temperature strength characteristics and thermal stability were evaluated. The MGC parts are thermally stable after heat treatment at 1700°C for 500 hours in an air atmosphere. This is attributed to the MGC's unique microstructure without amorphous grain boundary phases and its interfacial effect. These properties of MGCs seem to be prospective as an application to a gas turbine system.

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In this paper, a review on the synthesis of nanocrystalline nitride powders by direct nitridation of metal oxide nanoparticles, metal complex or salt in flowing ammonia gas is given. The nitrides include TiN, CrN, NbN, GaN, InN and BN. It is found that this method shows the great flexibility for preparing various kinds of nanocrystalline nitride powders with relatively simple production equipments, and is a promising route for mass production of nanocrystalline nitride powders.

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In order to obtain a large amount of highly pure, and nano-size ultra fine powders (UFP) of silicon carbide, much kind of trials have been performed. The presentation is a summery of molar ratio effects on product SiC manufactured via the plasma CVD method. Synthetic conditions are summarized as follows; frequency=4 MHz, maximum out-put power=15 kW, reactants=monosilane+methane/ethane, plasma working gas=argon, and sheath gas=argon. Powders were characterized to be ultra fine (UHP, 5∼100 nm), ultra pure (each cation impurity: ppb level), and spherical shape with a very narrow particle size distribution. Silicon carbide UFP were covered their surfaces with amorphous carbon, when UFP were prepared under excess carbon condition. Temperature and reaction during the formation of silicon carbide were monitored in-situ by measuring the emission spectra from plasma flame using an optical analyzer. The present experimental conditions established local thermodynamic equilibrium (LTE). Temperature distribution in the plasma flame and near the flame was consisted with that expected from plasma reactor design. In the region of 200∼550 nm, optical emission spectra of neutral Si, C, Ar, H, H₂, and C₂ were observed. As C/Si molar ratio of the reactant gases was increased, both of Si and C spectra became weak. Radial distributions of spectral intensities of Ar, Si, and C after Abel inversion showed that Si and C atoms were mainly excited at center of the plasma while Ar atoms were mainly excited at off-axis position. These obtained results were attributed to plasma reactor design.

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SiC properties change with grain size, orientation, and surface morphology. We prepared polycrystalline 3C-SiC on graphite by sublimation method using 6H-SiC powder compacts as source materials. The present study investigated the distance between the substrate and the SiC source, temperature gradients between the substrate and the SiC source=20-40°C/mm, under the constant experimental conditions (argon pressure=133 kPa, substrate temperature=1580°C, deposition time=30 minutes) Deposited crystals showed clear hexagonal habit and yellowish color. The maximum growth rate was obtained with preferred (111) orientation, and at 13 mm distance between the substrate and the SiC source.

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A polytitanocarbosilane (PTC, 20-30 mass%)-xylene solution was infiltrated into a porous laminated woven fabric of 31-38 vol% Si-Ti-C-O fibers including 24-27 vol% ceramic filler (mullite, alumina and silicon carbide), and decomposed at 1000°C in an Ar atmosphere. This polymer impregnation and pyrolysis method (PIP) was repeated 8 times to produce the laminated composite of 80-84% of theoretical density. All the laminated composites showed the elastic deformation in the initial stage of the stress-displacement curves, followed by the significant pseudoductility. The laminated composites with mullite and SiC filler showed a maximum deformation strength at around 0.5 mm of displacement. On the other hand, the composite with alumina filler deformed under a constant stress of about 100 MPa in the wide displacement range from 0.5 to 3.0 mm. The deformation strength became higher in the order of ceramic filler: alumina (92±20 MPa)<SiC (234±16 MPa)<mullite (312±17 MPa). The maximum deformation energy reached 20 kJ/m² in the composite with SiC filler, which showed a gradual decrease of the deformation stress at a larger displacement.

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This paper reports on the tensile fracture behavior of fiber bundle (yarn) and woven fabric of the Si-Ti-C-O fiber with 11 μm diameter, 3.6 GPa of tensile strength and 186 GPa of Young's modulus. The yarn of 662-765 filament and fabric showed a stable pseudoductility with the tensile strength of 752-1714 MPa and 522-1274 MPa, respectively. The elongation at fracture (1.2-1.9%), fracture energy (141-482 kJ/m²) and Weibull modulus (4.29-4.45) were similar for the yarn and fabric. The pseudoductility of yarn and fabric was analyzed using the fracture probability and Weibull modulus of the fiber in yarn.

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Si₃N₄ ceramic was joined to itself using Cu76.5Pd8.5Ti15 alloy filler at 1373 to 1573 K for 1.8 ks in a vacuum. The room temperature bending strength of the Si₃N₄/Si₃N₄ joint reached a maximum value of 155.8 MPa when the brazing temperature and holding time were 1423 K and 1.8 ks, respectively. The joints were composed of three parts: (1) TiN reaction layer (A) at the ceramic/filler interface; (2) reaction layer (B) (TiN, CuTi and Pd₂Si) connecting with the reaction layer A; (3) Cu based solid solution in the middle of the joint, and the reaction phases (Pd₂Si, PdTiSi, Ti₅Si₃, TiN) existing in the Cu solid-solution. With increasing temperature from 1373 K to 1423 K, the thickness of the reaction layers (A and B) and the amount of the reaction phases increased, which improved the bending strength of the joints. When the brazing temperature was over high, voids were formed in the joint because of the decomposition of the Si₃N₄ ceramic, leading to a decrease of the bonding strength.

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Silicon nitride ceramics with different amounts of aligned reinforcing grains were prepared by incorporating 3 wt%, 5 wt% and 7 wt%β-Si₃N₄ whiskers. The degree of alignment of the reinforcing grains was increased as the whisker content was increased as determined by the linear sintering shrinkage anisotropy. Silicon nitride ceramics with 3 wt% and 5 wt%β-Si₃N₄ whiskers exhibited bi-modal grain size distributions. The number of the reinforcing grains was increased as the whisker content was increased, but volume fraction occupied by those grains was the maximum for silicon nitride with 5 wt%β-Si₃N₄ whiskers. The fracture toughness and the flexural strength at room temperature were increased as the whisker content was increased in part due to the increased number of the aligned reinforcing grains. However, the flexural strength at 1673 K was the highest for silicon nitride with 5 wt%β-Si₃N₄ whiskers in part due to the maximum volume fraction occupied the reinforcing grains.

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Sintered boron carbide is famous for its ultimate lightness, rigidity and hardness. But boron carbide is difficult to sinter, so has been manufactured industrially by hot pressing only. So it is very expensive, and its shape and size are restricted to small tile or the like. In this study, new method of slip casting and pressureless sintering of boron carbide is proposed. This method realize crystal nano-structure without grain boundary layer, which makes its mechanical properties no less than hot-pressed one. This new method set boron carbide ceramics free from restriction of size and shape. And boron carbide is also famous for its ability of neutron absorption or shielding. This boron carbide with free size and shape is applicable to this nuclear field, and neutron shielding design for some nuclear installations has begun to be changed.

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Since Iijima first reported carbon nanotubes in 1991, a lot of researchers have conducted study on their mechanical and electrical properties. Recently, multi-wall and single wall nanotubes have been used as reinforcing material for composites in matrix of aluminium, SiC, epoxy and alumina. In this paper, alumina matrix composites containing 0.1 to 2 weight percent of multi-wall carbon nanotubes (MWNT) were successfully fabricated by pressureless sintering in air atmosphere. The composites sintered to nearly full density. Hardness, fracture toughness and bending strength were measured using Vickers indention method and the three-point-bending test. The hardness of the composite decreased with increasing weight content of the MWNT. The fracture toughness does not change much with the amount of MWNT, while the bending strength increased with the added amount of CNTs. Microstructure characterization, performed by SEM, showed that the tight bonding between CNTs and alumina matrix, the pullouts of CNTs from matrix and the thermal expansion mismatch are possible mechanisms leading to the improvement of the mechanical properties. The samples sintered by SPS (spark plasma sintering) were employed for comparison.

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The wettablities of Ni-20Si-xTi (x=0∼15) and Cu-5Si-xTi (x=0∼15) alloys against Si₃N₄ were investigatged by measuring contact angles in vacuum using a sessile drop method. The addition of 5 at% Ti or more to Ni-20Si alloys effectively lowers the contact angle against Si₃N₄. The Ni-20Si alloy containing 10 at% Ti shows the lowest contact angle (30 degrees at 1523 K after 3.9 ks) and the best wettability against Si₃N₄. The addition of Ti to Cu-5Si alloys effectively decreases the contact angle. Cu-5Si alloys containing 5 at% Ti or more show low wetting angles at 1473 K. The active metal Ti improves the wettability of Si₃N₄ by these alloys, because it moves to the metal-ceramic interface.

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In this work, the processing of sintered reaction-bonded silicon nitride (SRBSN) has been applied to the development of high thermal conductivity Si₃N₄ materials because of its moderate cost of production. The system of Y₂O₃-MgO was chosen as the sintering additives. The post-densification behaviour was dependent on the amount of sintering additives by pressureless sinteirng at 2073 K, but there is no such dependence by gas-pressure sintering ≥2173 K. The thermal conductivity is enhanced by the optimum amount of sintering additives, the increased sintering temperature and prolonged hold time. Thermal conductivity over 110 Wm^-1•K^-1 could be reached for SRBSN materials by optimising the additive amount and post-sintering conditions.

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α-Si₃N₄ ceramics were sintered at a low temperature of 1773 K by using a spark-plasma-sintering method. Some of the ceramics were further heat-treated at different temperatures from 1773 K to 2173 K for 1 hour, to study the effect of heat-treatment on the microstructure and the thermal conductivity of the ceramics. The results show that the heat-treatment at temperatures above 1873 K can increase the thermal conductivity greatly. During the heat-treatment, the α-Si₃N₄ transforms into β-Si₃N₄ and the β-Si₃N₄ columnar crystal grows up largely as the heating temperature increases, but the phase transformation and the growth of the β-Si₃N₄ columnar crystal does not seem to have an obvious effect on the thermal conductivity of the ceramics

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Our previously proposed technique for estimating critical sizes of the frontal process zone (FPZ) in ceramics is improved using exact solutions of stress distributions around a crack and elliptical hole tips in an infinite plate for calculating critical local stress. In the present work, a three-point flexure test is carried out with V-notched ceramic specimens (Single-edge V-notched beam method), and the relation between the strength and notch depth is successfully established. The critical local stress at the crack propagation is calculated at a characteristic distance from the notch tip based on a local fracture criterion. Exact solutions for a crack or elliptical hole in an infinite plate are used to estimate the critical local stress. In this study, attempts were made to estimate the critical size of the FPZ, fracture toughness, and flexural strength of several ceramic materials. A comparative study between mechanical properties and the critical size of the FPZ indicates that the fracture toughness and product of strength and square root of the critical FPZ size holds a linear relation. This result suggests that both the strength and critical size of the FPZ must be increased to enhance the fracture toughness of ceramics.

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Recently, ceramic matrix composites are considered for a high performance brake disk in automobile industry as an alternative of the conventional cast iron disk mainly due to excellent thermo-mechanical properties as well as high strength to weight ratio. Generally, since the thermo-mechanical properties of fiber-reinforced ceramics are anisotropic and non-homogeneous, the effective properties are not easy to be included in computer analyses. In this paper, simulation procedures based on homogenization of 3-D woven fiber reinforced ceramics are presented to evaluate automobile disk brake performance. The simulation results include thermo-mechanical behavior of CMC disk brake as well as homogenized thermal and mechanical properties of CMC.

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Ternary carbides feature a unique characteristic of coexistence of covalent, metallic and ionic bonds in one crystal cell, resulting in unusual properties which combine the merits of both metals and ceramics. In the present work, single-phase ternary Ti₃AlC₂ was produced by combustion synthesis using Ti, Al and C powders as the raw materials. The experimental results showed that the addition of TiC and Ti₃AlC₂ affect the formation of the phases in the final product. Increasing addition of TiC and Ti₃AlC₂ in the raw mixtures and decreasing combustion temperatures resulted in enhancement of the amount of the produced Ti₃AlC₂.

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Polycarbosilane was synthesized from polydimethylsilane in the presence of zeolite as a catalyst. Characterization of synthesized polycarbosilane was performed with ²⁹Si Solid NMR, FT-IR, TG, UV and GPC analysis. The number average molecular weight (M_n) of the polycarbosilane were ranged between 2,000 and 2,500. A continuous pcs fiber was fabricated by melt spinning this precursor under an inert atmosphere. Fibers dispersed with nano-sized SiC crystallites were obtained by the pyrolysis of thermally cured pcs fiber at 1200°C for 1 h under Ar atmosphere.

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Amorphous PZT (Pb/Zr/Ti=1.1/0.54/0.46 in molar ratio) thin films were prepared by RF magnetron sputtering at room temperature, and heat-treated in N₂-O₂ mixture gases at 750°C. The crystal orientation of PZT films changed depending on the oxygen partial pressure in the N₂-O₂ atmosphere. An (100) orientation was significant at lower oxygen partial pressures, and an (111) orientation at higher oxygen partial pressures. The remnant polarization and coercive field of the (100) oriented films were 25.2 μC/cm² and 41.5 kV/cm, and those of the (111) oriented films were 27.6 μC/cm² and 50 kV/cm, respectively.

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Thickness dependence of coercive field (E_C) and remanent polarization (P_r) in ferroelectric thin films has been numerically simulated using four-state Potts model. In this model, four mutually perpendicular dipole-orientations result in four different kinds of domains. The thickness dependence is induced by the influence of the surface dipoles. These dipoles have slightly different physical parameters due to the interfacial effects. The simulation result shows the existence of a maximum coercive field at the critical thickness (d_C) for the coercive field against thickness (d) curve. For the thickness d<d_C, E_C increases with d. On the other hand, for d>d_C, an opposite trend can be obtained.

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Polycrystalline Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT) ferroelectric thin films were deposited on plantinized Si substrates by chemical solution deposition. The Bi-layered perovskite structure was achieved by rapid thermal annealing the spin-on films at 700°C for 3 min. Well-saturated hysteresis loops with remanent polarization (P_r) around 10 μC/cm² were obtained on Pt/BNdT/Pt capacitors. The effect of forming gas (FG: 5%H₂+95%N₂) anneal on electrical properties and microstructures of BNdT films were characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and electric measurements. P_r values were considerably suppressed after forming gas anneal at 400°C for 10 min. From structure characterization, it is deduced that the suppressed P_r was not due to decomposition of BNdT, but might be ascribed to the increased switching resistance due to formation of polar hydroxyl bonds by penetrated hydrogen ions bonded with oxygen ions.

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