A fine β-spodumene type amorphous powder has been prepared by the sol-gel process using LiOCH₃, Al(OC₂H₅)₃ and Si(OC₂H₅)₄ as a staring materials. By the rapid hydrolysis of alcohol solution of starting metal alkoxides in the presence of hydrochloric acid as a catalyst and with large excess of water, a gel powder was obtained. For the single-phase gel, the DTA curve closely resembled the amorphous to β-spodumene (Li₂O⋅Al₂O₃⋅4SiO₂) reaction. At 800°C, the crystalline phase comprised the major phase of β-spodumene and the minor phase of β-eucryptite (Li₂O⋅Al₂O₃⋅2SiO₂). For 1000°C and 1200°C calcination, the XRD patterns showed pronounced growth of β-spodumene. The diffraction lines of β-eucryptite disappeared at 1200°C.

View full abstract

Pb(Zr_xTi_1-x)O₃ films with good surface flatness were deposited by CVD using the gas mixture of Pb(DPM)₂-Zr(O⋅t-Bu)₄-Ti(O⋅i-Pr)₄-O₂. The epitaxially grown Pb(Zr_xTi_1-x)O₃ films were deposited on (100)MgO substrates with 300nm/min of the deposition rate. This deposition rate is the highest among the reported value. The existence of the grains with slight different angle along the film thickness was ascertained by TEM observation. It was attributed to the volume change at the Curie temperature and to the large temperature dependence of a- and c-axes lattice parameters of Pb(Zr_xTi_1-x)O₃ film compared with that of MgO substrate below the Curie temperature. The refractive index of this film was in good agreement with that reported for Pb(Zr_xTi_1-x)O₃ crystal without post heat-treatment and its dispersion can be fitted the Sellmeier dispersion formula.

View full abstract

The green microstructure and density of Al₂O₃ were correlated with kinetic stability and interfacial characteristics of colloidal suspension. The packing density of Al₂O₃ as a function of suspension pH showed that the stability ratio or the zeta potential should be larger than a certain critical value for a green body with high packing density. The incipient stability point in the zeta potential was estimated using the DLVO theory, and this was well agreed with the experimental critical zeta potential for a rapid increase in the packing density of Al₂O₃. Further analysis of data indicated that the potential energy barrier between two interacting colloid particles should be higher than its critical value for a fabrication of Al₂O₃ green body with dense and uniform microstructure. The incipient point of this critical potential energy barrier was well accorded with the point of the critical stability ratio.
The cumulative pore-size distribution of green bodies showed that the green compacts prepared using unstable colloid suspensions had pore volume increase due to nonuniform packing of the agglomerates larger than 1μm size. A rapid decrease in the most frequent pore diameter occured for the green compact prepared from the kinetically stable slip having the stability ratio larger than its critical value, demonstrating a direct influence of colloid/interface variable on pore microstructure of Al₂O₃ green body. The specimen fabricated using kinetically stable slip showed homogeneous microstructure with essentially pore free state after sintering at 1480°C for 4h.

View full abstract

PZT thin films were epitaxially grown on (100)Pt/(100) MgO substrates by CVD under partial oxygen pressure (P_O2) from 0.96 to 8.4Torr. The crystal structure and microstructure of the films were almost the same as those prepared on MgO substrates irrespective of P_O2. However, for films deposited below 1.6Torr of P_O2, the existence of PbPt_5-7 phase was ascertained by XRD measurement, and the electric leakage between top and bottom electrodes was observed. The X-ray pole figure measurement showed that this PbPt_5-7 was epitaxially related to the deposited PZT thin film and the (100)Pt/(100) MgO substrate. The relative dielectric constant, ε_r, of epitaxially grown Pb(Zr_0.5Ti_0.5)O₃ films was about 300, lower than that of non-oriented film. This phenomenon can be elucidated by the fact that the calculated ε_r of PZT crystals along the c-axis is lower than that perpendicular to the c-axis.

View full abstract

In classical packing theory, composite packing density of a bimodal mixture is increased by increasing the component fine or coarse packing density via particle size distribution broadening. In this study, the effects of particle size distribution shape of component fine and coarse particle distributions on composite packing density of bimodal mixtures were investigated. The results from the present study indicated that while composite packing density can be increased by increasing either the coarse or fines packing density, it is limited to mixtures in which the fines and coarse are significantly different in size. In practically sized mixtures with a fines to coarse size ratio Z of 1/10, the highest composite packing density is dependent not only on component fine and coarse packing density, but on the shape of the particle size distribution as well. From the present study, asymmetrically skewed distributions devoid of midsized particles give the highest packing density in practically size systems.

View full abstract

Fine zirconia powder was added to the bimodal size distributed alumina powder mixtures and the influence of zirconia addition on the sintering behavior of the powder mixtures was investigated. At low firing temperature, that is, in the initial stage of sintering, zirconia retarded the densification of the powder mixtures and also enlarged the pore size of the compacts. At high firing temperature, the influence of zirconia addition was different depending on the packing structure of the green compact. When the coarse alumina particles were dispersed in the matrix of the fine particles, zirconia promoted the densification of the mixtures. This result was explained by the promotion of the densification between the fine and the coarse alumina particles in the intermediate stage of sintering and by the migration of zirconia and the hindrance effect of zirconia on the grain growth of alumina in the final stage of sintering. When the coarse alumina particles formed a skeletal structure, zirconia did not promote the densification of the mixtures. This result was explained by the dependence of the densification of the compacts only on the densification among the coarse alumina particles.

View full abstract

This paper discusses the thermal stability of phases in the system Si₃N₄(-SiO₂)-AlN-Y₂O₃. The total amount of AlN and Y₂O₃ varied from 5 to 11wt% and the molar ratio of AlN/Y₂O₃ was 0 to 11. Specimens were prepared by hot-pressing, and were then heat-treated for 50h. The phases formed by hot-pressing changed during the heat treatment. The fraction of α′-Sialon was decreased by the heat treatment. This decrease was remarkable, when the solubility of α′-Sialon was small. Due to the heat treatment, the solubility of α′-Sialon changed to a constant value, while that of β′-Sialon showed little change.

View full abstract

Submicron micropores were uniformly dispersed in an alumina ceramic matrix by using the volume shrinkage accompanied by the decomposition of aluminum titanate (AT) into corundum and rutile. The methodology for the fabrication of the pore-dispersed alumina includes (1) homogeneous mixing of 2-8wt% TiO₂ with alumina, (2) densification of AT-dispersed alumina, and (3) decomposition of AT into corundum and rutile with the formation of micropores. Such micropores are considered to release the residual thermal strain and to deflect cracks propagating in the alumina matrix.

View full abstract

Phosphate glasses containing very large amounts of TiO₂ have been prepared in the compositions of (3Na₂O⋅67TiO₂⋅30P₂O₅)+5SiO₂(mol%) (Glass-1) and (3Na₂O⋅72TiO₂⋅25P₂O₅)+5SiO₂ (mol%) (Glass-2). The thermal expansion coefficient (α) from 30 to 600°C were 7.7×10^-6°C^-1 and 6.9×10^-6°C^-1, respectively. The glass-ceramics prepared by heating Glass-1 at temperatures in 50°C intervals from 800 to 1000°C for 30h had α=0.7-2.2×10^-6°C^-1 and glass-ceramics prepared by heating Glass-2 at various temperatures from 900 to 1100°C for 30h had a=1.6-8.4×10^-7°C^-1 in the same temperature range. The bending strengths of the glass-ceramics from Glass-1 were larger than those from Glass-2. The low thermal expansions of these glass-ceramics were attributed to NaTi₂(PO₄)₃-like phase, TiO₂ and (TiO)₂P₂O₇ produced by crystallization, and the bending strengths were greatly influenced by the crystalline phases. The NaTi₂(PO₄)₃-like phase seems to be a solid solution of NaTi₂(PO₄)₃ and Si⁴⁺, Na_1+xTi₂P_3-xSi_xO₁₂, as judged from the differences between the lattice parameters of this phase and those of NaTi₂(PO₄)₃.

View full abstract

Temperature dependence of electrical resistivity has been investigated for α-SiC ceramics added with transition metal oxides in order to develop a heating element, whose electrical resistivity is positive or constant against temperature up to 400°C. Electrical resistivity of α-SiC with no addition is 1×10³Ω⋅cm at room temperature, and decreases by raising the temperature. The α-SiC samples added with Cr₂O₃, CoO or NiO at 5wt% show a resistivity less than 5Ω⋅cm at room temperature. Electrical resistivity of α-SiC added with Cr₂O₃ or CoO at 5wt% decreases by raising the temperature. On the other hand, the α-SiC sample added with NiO at 5wt% shows a constant resistivity in the temperature range from room temperature to 400°C. Electrical resistivity of the α-SiC samples added with NiO at less than 3wt% is more than 10²Ω⋅cm at room temperature, and decreases by raising the temperature. Electrical resistivity of the α-SiC samples added with NiO at more than 5wt% is less than 5Ω⋅cm at room temperature, and its temperature coefficient is constant or positive up to 400°C. The added NiO reacts with SiC to form Ni₃Si₂ at high temperature during the sintering process. The Ni₃Si₂ product remains at grain boundary with a metallic property of positive temperature coefficient, and compensates the negative property of the α-SiC matrix resulting in the constant resistivity of α-SiC/NiO against temperature.

View full abstract

The optimum conditions for mullite coating on SiC-gradient carbon substrates and its oxidation resistance were studied. A homogeneous precursor solution with the composition of Al/Si=3 was prepared by refluxing an ethanol solution of Al(NO₃)₃ and silicon ethoxide (TEOS) of 0.1mol/l at its boiling point for 12h. A mullite film with a thickness of 1.0μm without any cracks was obtained under the following conditions; the drawing up the substrate from the precursor solution at a rate of 0.05cm/s, keeping at room temperature for 24h and at 100°C for 48h in air, and then heating up to 1300°C at a rate of 200°C/h. The SiC-gradient carbon substrates coated with the mullite film showed the excellent oxidation resistance; no appreciable weight loss by oxidation at 1000°C for 15h was observed.

View full abstract

Several metal-oxide composites have been made by solid-state displacement reactions between powders of metals and oxides, and their structures have been examined metallographically. In the Cu₂O-Ni system, in which the sandwich-type reaction couple forms products with a typical layered arrangement, spherical NiO grains scattered in the Cu matrix; the size of oxide spheres was determined by the particle size of the metal powder used. In the NiO-Ti, Cu₂O-Fe and Cu₂O-Ti systems, in which the sandwich-type reaction couples form products with aggregate arrangements, metal and oxide product phases were finely interwoven; the microstructures were independent of the particle size of the metal powders used. Composition of metal and oxide products was controlled by adding another metal or oxide to the starting powder. Fusion of products by reaction heat is avoided by pre-annealing at low temperatures and adding inert substances as diffusion barriers.

View full abstract

The lattice constants, elastic modulus and fracture toughness of 12mol% CeO₂-containing tetragonal zirconia polycrystals with grain sizes of 2.8μm (sintered at 1500°C, Z12C-I) and 6.1μm (sintered at 1600°C, Z12C-II) were measured. Lattice constants (a_t and c_t axes) of tetragonal zirconia for Z12C-II were larger than those for Z12C-I. On the other hand, the axial ratio c_t/√2a_t for Z12C-II was smaller than that for Z12C-I. Therefore, it was estimated that tetragonal zirconia in Z12C-II was more stable than that in Z12C-I. Young's modulus at room temperature was 194-195GPa for both specimens. The effect of grain size on Young's modulus was not observed. The Young's modulus decreased with increasing temperature and was 179-180GPa at 400°C. Fracture toughnesses at room temperature were 6.4MPa⋅m^1/2 for Z12C-I and 6.7MPa⋅m^1/2 for Z12C-II. That is, fracture toughness with a large grain size was larger than that with a small grain size. The fracture toughness decreased gradually with increasing temperature and was 4.0MPa⋅m^1/2 for Z12C-I and 4.2MPa⋅m^1/2 for Z12C-II at 300°C. The monoclinic zirconia content on the fracture surface for Z12C-II with large grain size was comparable to that for Z12C-I with small grain size at room temperature. Fracture modes in Z12C-I was characterized by intergranular mode and that in Z12C-II, by transgranular mode. It is concluded that the fracture toughness for Z12C-II was larger than that for Z12C-I because of the difference in fracture modes.

View full abstract

Phase changes in the quenched R phase (ramsdellite structure Li₂Ti₃O₇) synthesized by the melt-quenching method were evaluated in terms of the electrical property change. The electrical resistivity and dielectric constant varied in accordance with the thermal expansion behavior, indicating their effectiveness for studying the phase changes. The effect of adsorbed water on the electrical properties of the R phase was observed in the temperature range of 100 to 400°C. A sharp increase in resistance was observed above 550°C, which did not coincide with the thermal expansion behavior. This phenomenon indicated the decrease in Liion conductivity taking place prior to the phase change from R to H phase. The single phase spinel structure Li₄Ti₅O₁₂ (S phase) was synthesized, and electrical measurements and crystallographic analyses were also conducted. Powder X-ray diffraction of the S phase gave rise to results different from those of Deschanvres et al. Its lattice constant and linear thermal expansion coefficient were precisely determined to be α=0.83588 (2) nm and α=1.49×10^-5K^-1, respectively. The S phase was confirmed to be an electric insulator.

View full abstract

W-doped Fe₂O₃ films have been fabricated on poly-crystalline allumina substrates by the RF magnetron sputtering method, and effects of annealing temperature on the NO₂ sensing properties have been examined. The crystal structure of the obtained film changed from Fe₃O₄ to α-Fe₂O₃ after annealing at temperatures of 500 to 700°C in air. An increase in the annealing temperature increased the particle size, resulting in a decrease in the NO₂-gas sensitivity. These results suggest that the NO₂ sensitivity of W-doped Fe₂O₃ film depends on the particle size.

View full abstract

Defocusing a laser beam is one of the most important factors to control the feature of drilling holes in ceramic materials. This suggests that laser beam energy is delivered and focused on the bottom of the drilling hole by multi-reflection from the internal wall surface of the hole. The deep hole drilling process of silicon nitride ceramics was simulated by the newly developed analysis model. The ray tracing method for a laser power intensity distribution analysis and difference method for thermal analysis was combined. The drilling hole is processed mainly by energy reflected from the internal wall surface of the hole. In this process laser beam intensity becomes higher in the center of the hole since the laser beam is focused by the wall-reflecting effect. The defocusing position from the material surface has direct effects upon the drilling process, which depends on reflection angle of a laser beam at the internal wall surface of the hole. The reflection angle controls the wall-focusing effect and determines the hole shape.

View full abstract

A high-T_c Bi-Pb-Sr-Ca-Cu-O superconductor (2223 phase) has been prepared using Bi(Sr_0.5Ca_0.5)O_z as a starting material. Mixed powder of Bi(Sr_0.5Ca_0.5)O_z, Ca₂PbO₄, SrCO₃, CaCO₃ and CuO was heated at 1133K. We examined the samples by X-ray diffraction, thermal analysis, thermal expansion measurement and scanning electron microscopy. The samples showed as high volume fractions of the 2223 phase and almost the same partial-melt temperature as obtained in the citrate method.

View full abstract

ITO thin films were prepared at 550°C by a sol-gel method using colloidal particles derived from a solution of indium nitrate and tin chloride. The colloidal particles were secondary particles with about 50nm in diameter and were dispersed by adding indium chloride as a peptizer to prepare a dipping solution. Firing of gel films at 550°C for 2min developed a film microstructure consisting of ITO particles through thermal decomposition of the colloidal particles. Indium chloride was also decomposed to form indium oxide by firing, and the oxide was found to crosslink the ITO particles in the film. ITO films with thickness up to 2μm were obtained by a single dipping-firing procedure. The sheet resistivity of the films prepared at 550°C for 30min was 500Ω/cm² and the optical transmittance was above 90% in the visible region.

View full abstract

Water-repellent films were applied on various glass substrates at room temperature using fluoroalkyl isocyanate silane as a starting material. These waterrepellent glasses were examined on initial water-repellent property and its mechanical durability against rubbing treatment. It was found that the difference in the initial water-repellent property among them was hardly discernible, but the mechanical durability was dependent on the surface state of glass substrates. The dependence of the mechanical durability on the substrate can be explained by the density of the absorbed waterrepellent molecules associated with the number of adsorption sites such as silanol group at the surface of glass substrates. Furthermore, another reason for the improvement of the mechanical durability is strong adhesion of the film to the substrate due to the formation of siloxane bond at the film/substrate interface. The durability mechanisms are described in detail based on the results of XPS, static SIMS and IR analyses.

View full abstract

Si-Ti-C-O (Tyranno) fiber reinforced SiC composites were fabricated at 1420°C by reaction bonding from phenolic resin and silicon powder. The flexural strength of the composites is about 200MPa. The flexural strength of the composites oxidized at 1000, 1200, 1300, and 1400°C for 1h in air is about 170, 145, 130 and 130MPa, respectively.

View full abstract

The tetragonality of submicrometer-sized crystals of barium titanate, BaTiO₃, has been investigated by the ultramicro single-crystal X-ray diffraction technique using synchrotron radiation. The c/a ratios have been determined using the splitting of Laue spots due to twinning to be 1.0072 (2) and 1.0106 (6) for crystals with conversion diameters of 0.56μm and 10μm, respectively. The c/a ratio of the 10μm crystal is approximately the same as that of the bulk crystal, while it is significantly small for the 0.56μm crystal, indicating that the size effect on the tetragonal-to-cubic phase transition is remarkable in the submicrometer region. No clear splitting due to tetragonal twinning was observed for the crystal with a conversion diameter of 0.25μm, suggesting that the critical size for the transition is close to this size.

View full abstract