Crystal growth of ZrO₂ and crystallite size dependence of tetragonal to monoclinic ZrO₂ transformation were investigated in relation to the fracture toughness of glass-ceramics. Glasses of 2ZrO₂⋅3SiO₂, ZrO₂⋅SiO₂ and 3ZrO₂⋅2SiO₂ in molar ratio, prepared by the sol-gel process from metal alkoxides, were heat-treated to precipitate tetragonal (t-) ZrO₂ crystals. Tetragonal-ZrO₂ crystals grew in proportion to the cube-root of heat-treatment time, and the growth rate increased with increasing ZrO₂ content. Crystals of t-ZrO₂ larger than a critical size transformed into monoclinic (m-) ZrO₂ during cooling. Transmission electron microscopy revealed that the m-ZrO₂ particles showed twinning associated with t- to m-ZrO₂ transformation. Tetragonal to m-ZrO₂ transformation temperature was found proportional to the reciprocal ZrO₂ crystal size, which was consistent with Garvie's transformation equation. The critical sizes corresponding to the transformation temperature of 273K were estimated to be 90, 56 and 40nm for 2ZrO₂⋅3SiO₂, ZrO₂⋅SiO₂ and 3ZrO₂⋅2SiO₂ glass-ceramics, respectively. The interfacial and strain energies for the transformation calculated from Garvie's equation increased with decreasing ZrO₂ content. The fracture toughness (K_IC) of these glass-ceramics increased in proportion to the cube of t-ZrO₂ crystal size. From the critical crystal size of t-ZrO₂ in the glass-ceramics, the maximum K_IC was estimated to be 4.7, 4.7 and 5.0MN/m^3/2 for 2 ZrO₂⋅3SiO₂, ZrO₂⋅SiO₂ and 3ZrO₂⋅2SiO₂, respectively. After reaching the maximum, K_IC decreased abruptly on further heating, which was attributed to the occurrence of m-ZrO₂ crystals in the glass-ceramics.

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TiO₂ films up to about 2μm thick have been prepared on a nesa glass substrate by dip-coating technique using the TiO₂ sol solution derived from Ti(O-i-C₃H₇)₄. Photoelectrochemical properties of the TiO₂ film electrode thus obtained have been investigated as a potential candidate for solar energy conversion device. The results obtained are summarized as follows:
(1) The sol-gel derived TiO₂ film shows a very large photocurrent due to porous surface structure and a maximum (-14mA cm^-2) at about 1.8μm in thickness. A decrease in the photocurrent of the TiO₂ film above 1.8μm thick is due to an increase in the resistance.
(2) An additional heating at 500°C improves remarkably the photocurrent of the TiO₂ film, although too long heating rather deteriorates it. These results are explained on the basis of the changes in the surface structure and carrier concentration with the additional heating time.

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Amorphous complex colloid containing La³⁺ and Sn⁴⁺ ions with a stoichiometric ratio in a bulk solid particle was prepared by rapid hydrolysis of homogeneous aqueous solutions of tin (VI) and lanthanum chlorides under strongly basic conditions. Heating of the complex colloid at relatively low temperatures (>800°C) resulted in the direct formation of La₂Sn₂O₇ active powder from the amorphous colloid particles without forming any intermediate phase. Some powder properties were measured for the amorphous and crystallized particles. Possible structural correlations between the amorphous colloid and the crystalline particles were discussed on the basis of the results obtained from infrared absorption spectroscopic examination on the structural change occurring during heat treatment of the amorphous particles. The simple procedure employed in the present work to prepare complex colloid consisting of La³⁺ and Sn⁴⁺ ions is applicable to powder formation of the family of Ln₂B₂O₇ (Ln=lanthanide atoms, B=Sn or Ti).

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A new method for preparing homogeneous cordierite ceramics by a sol-gel process is described. Reactive and fine cordierite-type amorphous powder with high homogeneity was prepared by the sol-gel method. The hydrolysis of metal alkoxides used as starting materials was controlled. Namely, Si(OC₂H₅)₄, with a hydrolysis rate lower than that of Mg (OC₂H₅)₂ or Al (OC₃H₇ⁱ)₃, was partially hydrolyzed and then mixed or reacted with Mg and Al alkoxides which had been reacted in butyl-alcohol. The cordierite powders prepared by the sol-gel method were characterized by infrared spectroscopy, X-ray diffraction study, differential thermal analysis (DTA) and transmission electron microscopy (TEM) with an energy dispersion X-ray microprobe analyzer (EDX). Homogeneous cordierite powders were obtained from Mg(OC₂H₅)₂, Al(OC₃H₇ⁱ)₃ and Si(OC₂H₅)₄. The homogeneity of resultant oxides depended upon the hydrolysis method of alkoxides.

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Densification behavior of powder compacts having a cordierite composition prepared by the sol-gel process using metal alkoxides is studied. The calcination of the alkoxy-derived powder at 800°C for 12h gave the best sintering. Densification of the powder compacts occurred from 800° to 900°C. Dense cordierite ceramics with different crystalline phases or properties were obtained by sintering the powder compacts without any sintering aid. The sintering of the powder compact at 1300°C for 12h gave dense α-cordierite ceramics having the flexural strength of 120MPa and the critical stress intensity factor of 2.8MN/m^3/2.

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This paper describes a new preparation process of highly densified bodies of a γ_II-Li_3.4Si_0.4V_0.6O₄ solid solution, which is the best lithium ion conductor among those containing silicates, via sintering a xerogel powder derived from alkoxides. The xerogel powder consisting of spherical, amorphous aggregates was prepared by hydrolysis of alcoholic solutions of Si(OC₂H₅)₄, VO(OC₂H₅)₃ and LiOC₂H₅ at 70°C, followed by drying. The diameter was smaller than 0.1μm. The powder crystallized gradually into the β_II-solid solution with poor crystallinity above 500°C, and a γ_II-solid solution appeared above 650°C. The highly densified bodies of the γ_II-solid solutions were obtained by sintering the compact of the xerogel powder at a relatively low temperature, 900°C. The sintered bodies consisted of only the γ_II phase and contained no other phases such as Li₂SiO₃ which forms as a result of Li₂O evaporation during sintering. This method lowered the sintering temperature by 150°C compared with that for the conventional method. The ionic conductivities of the sintered bodies were determined by analysing their complex impedance diagrams in the frequency range of 5Hz-13MHz. The sintered bodies prepared by the conventional method were characterized by the presence of two distinct semicircles in the complex impedance diagram; one corresponds to AC response of the grain (bulk) and the other to that of the grain boundaries. However, the sintered bodies prepared by this method indicated only a single semicircle due to AC response of the grain. The total ionic conductivity of the sintered bodies, 3.4×10^-5Scm^-1 at 25°C rising to 4.7×10^-3Scm^-1 at 150°C, was about three times higher than that of the those prepared by the conventional method due to negligible grain boundary resistance.

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Alkali-free borosilicate glass films have been prepared from boron triisoproxide and silicon tetraethoxide by the sol-gel method. It was found that the optical transmission of these films decreased, accompanied with the increase in surface roughness, with increasing humidity of the atmosphere in the coating process. The X-ray diffraction patterns and the wavelength dependence of the optical loss proved that the above findings resulted from the deposition of crystalline H₃BO₃ by the attack of water in the atmosphere. Transparent B₂O₃-SiO₂ films containing up to 30 mol% B₂O₃ were obtained in the dry atmosphere below 20% relative humidity.

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Pure and fine barium hexaferrite particles were synthesized by hydrolysis of ethanol solutions of organometallic compounds, barium ethoxide and tris (acetylacetonate) iron (III) under controlled hydrolyzing conditions. Effects of substitution of cobalt ion in barium hexaferrite were examined with cobalt-substituted barium hexaferrite synthesized by the same way using bis (acetylacetonate) diaquacobalt (II). Heating the amorphous coprecipitated products at 800°C for 1h resulted in direct crystallization of barium hexaferrite partly substituted with cobalt ion without forming intermediate compounds such as barium monoferrite or cobalt ferrite. No significant difference in crystallization process was observed between samples with and without cobalt ion. Crystallinity of heat-treated sample was found to be insensitive to the substitution of cobalt ion, which reflects the saturation magnetization. The saturation magnetization of crystallized barium hexaferrite slightly decreased with cobalt substitution in the range of 0≤x≤1 in BaFe_12-xCo_xO₁₉. On the other hand, the coercive force changed drastically with increasing cobalt ion content. The coercive force could be controlled over the range of 1 to 5 kOe by cobalt ion content and heat treatment condition. Particle size of cobalt-substituted barium hexaferrite lay in 500 to 2000Å with single domain.

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Two fluorite-type solid solutions of Pb_1-yBi_yF_2+y (y_??_0.5) and Pb_1-yBi_yO_0.5yF₂ (y>0.8) were synthesized hydrothermally from α-PbF₂ and β-BiF₃ at 250° to 500°C for 4h to 14 days under 100MPa using sealed Pt-capsules. The water molecules and/or hydroxyl ion (OH^-) in the products synthesized hydrothermally were not detected by Fourier transform infrared spectrophotometry. Pb_1-yBi_yF_2+y solid solution (β-PbF₂ss) was synthesized at 400°C from compositions containing 0 to 50mol% BiF₃. Single crystals of PbF₂ss of about 100μm were grown from the composition containing 10mol% BiF₃ at 400°C (ΔT≤10°C) for 7 days under 100MPa. Pb_1-yBi_yO_0.5yF₂ and a small amount of β-BiF₃ were produced from compositions containing more than 80mol% BiF₃. A fluorite-type BiO_xF_3-2x (x≅0.5) phase was obtained by the hydrolysis reaction of BiF₃. Therefore, the system studied was a pseudoternary system β-PbF₂-BiO_0.5F₂-HF(H₂O).

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The conductivity of rapidly quenched glassy flakes in the system Li₄SiO₄-Li₃BO₃ was measured. A maximum conductivity was observed at a composition near 50-60mol% Li₃BO₃; this behavior can be called the “mixed anion effect”. The enhancement of conductivity was shown to be related to the decrease in the activation energy for conduction. The relation between the “mixed anion effect” and the degree of supercooling of glass was discussed on the basis of the linear plots of the conductivity enhancement vs. the ratio (T₁-T_g)/T₁, where T₁ is the liquidus temperature and T_g the glass transition temperature.

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Amorphous materials have been prepared by rapid quenching, -10⁵°C/s, of the melt of the mixed oxides, 20wt% ZrO₂-80wt% (3.4Al₂O₃⋅2SiO₂) using an arc-imaging furnace and a twin-roller. This paper deals with the crystallization and the products of these amorphous materials obtained by repeating under various conditions. The crystallization of this amorphous material took place at -990°C precipitating t-ZrO₂ and mullite phases after showing glass-transition temperature at 919°C on heating by 10°C/min. The mullite had markedly larger lattice parameters than that of stoichiometric mullite (3Al₂O₃⋅2SiO₂) indicating higher (>83wt%) Al₂O₃ contents in the early stage of crystallization, then showed smaller lattice parameters on prolonged reheating. The TEM observation of the sample re-heated at 1200°C for 12h revealed that the product consists of mullite grains of 2-5μm in which fine (10-20nm) t-ZrO₂ crystals are contained. The characteristic arrangements of the fine ZrO₂ crystals in the mullite grains suggest that these t-ZrO₂ crystals are precipitated by exsolution on the crystallization of the mullite from the amorphous matrix. Reeating of the amorphous materials at 880°C reduced the glass-transition and crystallization temperatures and increased the lattice parameters of mullite. This suggests that the heat treatment at 880°C caused a phase separation in amorphous samples. The phase separation would yield the precipitation of Al₂O₃-rich mullite in the early stage of crystallization with the exsolution of t-ZrO₂ micro-crystals.

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Transparent and smooth LiTaO₃ thin films 1000nm thick were prepared at 600°-800°C on platinum and fused silica substrates by the dipping method. Uniform grains were distributed in the films, and the grain size increased with increasing temperatures. Dielectric properties of the film depended on the defects of the film such as crack or pinhole. The crack-free film prepared at 800°C showed a symmetric D-E hysteresis loop. The spontaneous polarization was 0.6μC/cm², remanent polarization was 0.4μC/cm² and coercive field was 60kV/cm. Dielectric constant and dielectric loss angle were about 50 and 0.05, respectively. The optical transmission of the films was about 40-80% and refractive index was 2.1 in the visible region.

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Superconductive BaPb_1-xBi_xO₃ (BPB) single crystals could be grown in hydrothermal NaCl solution. The solubility of BPB in NaCl hydrothermal solution was much higher than that in KCl hydrothermal solution under the same condition. The optimum condition to grow BPB single crystals were as follows; temperatures at the dissolution zone in a gold capsule, 400° to 450°C; temperature difference ΔT, 20°-30°C; pressure, 100MPa. The composition of crystals grown in NaCl hydrothermal solution was found to deviate slightly in the Bi lean side from that of nutrients as well as in KCl solution. The hydrothermally grown single crystal with x=0.25 showed the transition temperature T_c of 11.4K to the superconducting state and the narrowest transition temperature width ΔT_c of 1.8K.

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Single crystals of ZnO were grown from molten ZnCl₂ in a wet nitrogen atmosphere and NaCl flux containing ZnCl₂ in wet air at 500°C. Mechanisms of the precipitation are as follows. On the surface of melts, ZnO formed by the hydrolysis of molten ZnCl₂ dissolves in the molten flux. As the hydrolysis proceeds, the concentration of ZnO in the flux reaches a super-saturation and then ZnO precipitate as single crystals. While the hydrolysis is continued, the ZnO dissolved may be supplied continuously to the ZnO single crystals. Granular crystals, prismatic crystals with pyramids, needle-like crystals, tabular crystals and radiatelamellar aggregates are obtained. The flux method supplying solute by such reactions is expected as a synthetic method of large single crystals.

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Eight kinds of commercial SiC powders were cold pressed isostatically, encapsulated in a silica glass tube, and HIPed in 150MPa-argon gas at 1900°-2000°C. The density, microstructure, hardness and K_IC were studied. The results are summarized as follows:
(1) The density of sintered specimens depends strongly on the mean particle size of starting powders and HIP temperature. High density (>99.7% T. D.) was obtained by HIPing above 1950°C using powders with the mean particle size less than 0.6μm. With 0.8μm α-SiC powder, the sintered density was about 95% T. D. with slight dependence on the HIP temperature.
(2) The microstructure of highly dense sintered materials was uniform with fine grains, but that of 95% T. D. material was characterized by coarse grains and intra-granular isolated pores.
(3) α-SiC powder sinters better than β-SiC powder for the same mean particle size.
(4) Fine SiC powders gave high Vickers hardness and low K_IC value in comparison to coarse powders.

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Four kinds of Si₃N₄ starting powders with different amounts of impurities and various specific surface areas, were cold pressed isostatically, encapsulated in a Vycor glass tube, and HIPed in 150 MPa-argon gas at 1800°-2000°C. The density, flexural strength, hardness and microstructure were investigated. The results are summarized as follows:
(1) Sintered materials with more than 99% relative density were fabricated by HIPing above 1900°C.
(2) The sintered materials were composed of equiaxed β-Si₃N₄ grains. A small amount of an intergranular glassy phase was detected using high resolution TEM technique.
(3) The sintered materials prepared from high purity powders showed excellent mechanical properties at elevated temperature, i.e. the flexural strength at 1400°C and Vickers hardness at 1300°C were 750MPa and 13.2GPa, respectively.

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The effect of N₂ gas pressure on the sintering of α-sialon(x=0.2 and 0.4) and β-sialon (z=2) was studied at 1700°-1900°C. In the pressureless sintering, the reaction between Si₃N₄ and oxides (SiO₂ or Al₂O₃) generated gaseous species, and dense compacts were not obtained, especially for β-sialon. In the gas pressure sintering, dense compacts were produced at increased N₂ gas pressure. However, N₂ gas pressure higher than 10MPa resulted in lower density because high pressure gas inside the closed pores inhibited densification in the final stage of sintering. This result suggests that sialon ceramics should be sintered at appropriate temperature and gas pressure to minimize decomposition and to promote shrinking after pore closure.

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Silicon nitride powders operated by oxidation in air at 1200°C for 70 to 210min, in which the oxygen content was from 2.8 to 8.9 wt%, were sintered without additives under the pressure of 3.0GPa. The influence of oxygen on the characteristics of the sintered bodies was studied. Various amounts of α phase remained in sintered bodies depending on the oxygen content in the operated powders. Vickers microhardness of the sintered body prepared from the powder containing 2.8 wt% oxygen was higher than that of the sintered body from the starting powder at elevated temperature up to 1300°C.

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A new technique based on a self-propagating reaction is proposed for manufacturing ceramics by inducing the synthesis reaction and forming simultaneously. In this paper the effects of one-dimensional spring pressing on the densification of ceramics have been investigated. It is important for efficient densification to maintain the compacting pressure during processing in which a product shrinks significantly and also to apply the pressure along the propagating direction of the reaction front. The density of products in Ti-B system increased with increasing pressure and with decreasing spring constant for a given initial pressure. Final products with 95% relative density were produced from initial substances containing titanium more than 50% under the pressure of 20MPa. It has been found that crack-free products are produced by applying a large pressure at ignition time and the pressure of 10MPa after synthesis reaction in vacuum.

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The conditions for preparing WB by solid state reaction between tungsten and amorphous boron powders were investigated. The effect of added titanium or zirconium on the sinterability of the synthesized WB powder was examined under high pressure and temperature conditions. WB was obtained as a single crystalline phase, when a mixed powder containing boron 10 at% in excess of the stoichiometric composition WB, was pretreated at 500°C for 60min in a stream of hydrogen and subsequently treated at 1400°C for 60min in a stream of argon. The crystallinity and the amount of WB increased with increasing both heat treatment time and temperature. The atomic ratio B/W remained constant after the heat treatment below 1500°C. Sintering of the synthesized powder at 4GPa and 1500°C for 15min resulted in the density of 13.3g/cm³ and microhardness of 1400kg/mm². For 5 at% titanium addition, higher density (14.0g/cm³) and improved microhardness (2800kg/mm²) were attained by sintering at a lower temperature of 1300°C at 4GPa. The densif ication of reaction-bonded WB is promoted possibly by the formation reaction of TiB₂ from free amorphous boron and added titanium. Similar behavior was observed with the zirconium addition also.

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A new method for preparing ultra-fine alumina powders consisting of spherical particles based on combustion has been developed. A mixture of fuel gas, H₂+CH₄, and aluminum powder with particle size of several micrometers is burnt in a conventional burner for glass blowing. Examination of obtained products by electron microscopy and X-ray diffraction showed that they were alumina with two types of structures, θ- and δ-Al₂O₃. The present note describes an economical and efficient method for the production of alumina spherical particles.

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The nitridation of polycarbosilane started at 400° to 500°C in the heat treatment of polycarbosilane in NH₃ gas and was almost completed at about 800°C. Above 1000°C, the obtained nitride changed from amorphous to crystalline state, and at 1400°C, the crystalline phase was α-Si₃N₄. On the basis of the nitridation of polycarbosilane, oxidation-cured polycarbosilane fibers were heated in NH₃ gas between 1000° and 1400°C to obtain silicon oxynitride fibers. The silicon oxynitride fibers were in the amorphous state and transparent to visible light. The tensile strength and Young's modulus of the fibers obtained at 1300°C were 1.80±0.37GPa and 139±17GPa, respectively. The fibers obtained at 1400°C were heat-treated between 1000° and 1400°C in Ar gas and in air to examine the heat resistance of the fibers. The fibers retained their tensile strength and Young's modulus up to 1400°C in Ar gas and up to 1200°C in air.

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Titanium wires were exploded in air by electric discharge and properties of obtained particles were investigated. Ti wires (purity>99.5%) with diameters of 0.20, 0.508 and 0.762mm were cut to 50 or 80mm long. Exploding conditions were as follows: charged voltage 2.5-6.5kV, condenser capacity 40μF and charged energy 125-845J. Most particles formed were collected on the filter paper set in a vacuum cleaner. The powder was bluish white and consisted of spherical particles. Two types of particles were observed: large (>1μm) and very fine. These particles were identified as rutile- and anatase-type TiO₂, respectively. The amount of anatase increased from 60 to 90% with an increase in the energy ratio which was defined as the ratio of heat contents required for melting and vaporizing the Ti wire to charged energy. Anatase particles were considered to be formed by a reaction between metal vapor and oxygen gas because the amount of anatase almost agreed with that of metal vapor produced in the wire explosion. On the other hand, liquid metal drops were considered to be oxidized into rutile. Particle size distributions indicated a maximum frequency at about 0.3μm, but the distribution was considerably broad and particles larger than 2μm existed in the range from 10 to 20%. The average size of particles decreased from 0.74 to 0.37μm with increasing the energy ratio mainly due to an increase in the amount of metal vapor produced.

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The hot-working processes such as rolling, forging, extrusion etc. are widely used for metallic materials to make their shape and improve their properties. For ceramics, these processes are seldom carried out because of the lack of their plastic property. However, it is also well known that ceramics can deform plastically at high temperature. In this study we tried to examine the possibility of the hot-extrusion process for polycrystalline Ba-ferrite (BaO⋅6Fe₂O₃) that is easier to deform plastically than other oxide ceramics. Polycrystalline Ba-ferrite samples capsulated in stainless steel (SUS 304) were pre-heated from 1200° to 1430°C and then extruded by pressing the stem to the bullet at a maximum pressure of 800MPa. After extrusion, the body became dense and the grains became fine and orientated. The analysis by SEM and X-ray diffraction patterns showed that the polycrystalline grains had the tendency to orient their c-plane to the pressing direction and in a concentric circle. These results are discussed in view of strain rate, applied stress and slip plane. Besides, the control of the heat-transfer from the billet to the container was very important to carry out the hot-extrusion process smoothly.

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The nitriding reaction of floating Al particles in N₂ gas was studied at 1350°-1550°C. The hollow spherical and fibrous AlN's formed were examined by X-ray diffraction and scanning electron microscopy. The nitriding reaction of floating Al particles in N₂ gas proceeded in the following four steps: (i) floating Al particles melted and became spherical by surface tension, (ii) up to 1350°C the nitriding reaction was controlled by the diffusion of nitrogen through the AlN surface layer, (iii) cracks propagated in the AlN layer at 1400°-1550°C. The nitriding reaction rapidly proceeded with the eruption of internally melted Al, (iv) the eruption of Al also caused the nitriding reaction between vaporized Al and N₂ gas to form AlN, resulting in hollow spherical AlN and fibrous AlN. The synthesized AlN was white, indicative of high purity. Since the particle size of floating Al was determined by controlling the N₂ gas velocity, the classification of Al particles was possible. The mean size and size distribution of hollow spherical AlN particles formed were controlled within the range 4 to 12μm. The hollow spherical AlN particles were so brittle that they were easily crushed to fine particles even by fingers. Thus obtained AlN particles were very fine and had narrow size distribution of 0.1-0.2μm.

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Effects of NaF and NH₃ on the preparation of Si₃N₄ powders were investigated by nitridation of SiO₂-C-NaF system in a flow of NH₃/N₂ mixture gas at 1350°-1500°C. Reaction products were α-Si₃N₄ and β-Si₃N₄. A small amount of β-SiC formed at a higher reaction temperature. Also, a trace of Si₂ON₂ formed with an excess amount of NaF. The yield of Si₃N₄ powders increased remarkably with the addition of a small amount of NaF and/or NH₃. The α-phase content in Si₃N₄ powders also increased with the addition of a small amount of NaF, but was not influenced by the addition of NH₃. It was considered that NaF acted as flux and accelerated the reaction of SiO₂ with C, followed by the formation of SiO and Si, and that NH₃ promoted the reduction of SiO₂ to SiO or Si. Addition of excess amounts of NaF and NH₃ promoted the formation of Si₃N₄ whisker. From the thermodynamical consideration it was suggested that Si₃N₄ powders were formed by the following reactions:
3SiO+3C+2N₂→Si₃N₄+3CO and/or 3Si+2N₂→Si₃N₄.
The optimum condition to prepare Si₃N₄ powders was as-follows; SiO₂:C:NaF=1:20:(0.01-0.05) (molar ratio), NH₃/N₂=50(ml/min)/350(ml/min), 1350°C, 10h. Si₃N₄ fine powders with α-phase contents of 94-96% were prepared with more than 95% yield.

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Brushite and monetite powders were hardened by hydration in the presence of a “solid pH buffer”. The hardening was accompanied by the transformation of the starting calcium phosphates into carbonate-containing hydroxyapatite and/or octacalcium phosphate. The resulting hardened bodies had porosities of 71-81% and diametral tensile strengths of 0.1-1.5MPa. Microstructures of the hardened bodies showed that the hardening was caused by the entanglement of microcrystals.

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