The discrete element method (DEM) has been used to simulate the influences of ζ-potential on the particle packing structures during centrifugal casting of submicron alumina powders. The long-range potential interactions according to the DLVO theory have been implemented into the DEM simulation program. While well-stabilized suspensions with high ζ-potential tend to form particle chains, suspensions with low ζ-potential form agglomerate structures. The mechanism of particle clustering is discussed.

View full abstract

Alumina composites containing two different sizes and various compositions of Cr₃C₂ were pressurelessly sintered at 1550°C for 2h in a purified Ar atmosphere. The chemical stability of chromium carbide, microstructure, densification and mechanical properties were studied. The phase stability of Cr₃C₂ was found quite sensitive to the temperature and oxygen content in the sintering environment. The addition of Cr₃C₂ played an important role in the grain growth of alumina. Coalescence of Cr₃C₂ was observed along the grain boundaries of alumina in samples containing high Cr₃C₂. By controlling the phase stability, and the amount and particle size of Cr₃C₂, a flexural strength of 500MPa, 57% enhancement, was obtained.

View full abstract

Al₂O₃/Al alloy hybrid composites (SiC/Al₂O₃/Al alloy hybrid composite) reinforced with chopped and continuous SiC fiber (Nicalon^TM) have been fabricated by directed metal oxidation of Al-Mg-Si alloys. Effects of additive elements, Mg and Si on the in situ formation of SiC/Al₂O₃/Al alloy composites were studied. In situ infiltration of the Al₂O₃/Al alloy composite phase into the SiC fiber was possible only when a small quantity of Mg (5 mass%) was used. No infiltration was observed for 10 mass% Mg alloys. Composites fabricated from Al⋅5Mg⋅5Si and Al⋅5Mg⋅10Si alloys showed incorporation of Al₂O₃/Al alloy composite into the SiC fibers. The silicon content of the alloy affected the fabrication of the composite by altering the growth rate of the oxide, and the fastest fabrication of the composite was possible using Al⋅5Mg⋅5Si alloys. It was concluded that aluminum alloys of lower Mg contents (<5 mass%) are recommendable as a starting aluminum alloy for the incorporation of Al₂O₃/Al alloy composite phase into the SiC fibers with void free structure. However, degradation of the fiber during incorporation should be considered the application of this process.

View full abstract

Oxyfluoride glass in the system Na-Mg-Si-O-F, which exhibits lower Rayleigh scattering than silica glass, has been developed. The glass forming region is extended for an F content of up to 30% of the anions. Scattering measurements for (1-x)(Na₂O⋅2SiO₂)-xMgF₂ glasses revealed that the scattering intensity decreases to 70% of the intensity of silica glass for a MgF₂ content of 5mol%. The factors affecting scattering were evaluated based on glass properties such as refractive index dispersion parameters and glass transition temperature. The results suggest that the scattering in this glass is dominated by the density fluctuation.

View full abstract

Amorphous carbon in membrane form containing a small amount of silicon uniformly in the structure was prepared by firing a new organosilicon precursor, poly (phenylsilylene)-ethynylene-1, 3-phenylene-ethynylene (MSP). The pyrolyzed products look like glassy carbon. The cross-linking processes before pyrolysis were effective to keep its shape during the heat treatment and were influenced by oxygen in an ambient atmosphere during cross-linking processes. The oxygen introduced into the precursor enhanced the cross-linking but decreased the residual weight after the pyrolysis. In the pyrolyzed products, silicon atoms appeared to distribute uniformly from the outer surface to interior core. The structure of the products was amorphous, but β-SiC precipitated beyond 1873K.

View full abstract

Densification of SiC with oxide additives is affected mainly by the amounts of free C and SiO₂ contained in the starting powders. In this study, the influence of oxidation of starting powders on densification was investigated. Oxidation of SiC powders, prior to liquid phase sintering, increased the oxygen content and decreased the carbon content remarkably, and led to higher sintered density for both α- and β-SiC. Higher sintered density was attributed to the formation of liquid at lower temperatures and the suppression of reactions between the free C and oxide additives.

View full abstract

The preparation of high-T_c superconducting magnetic shielding by the electrophoretic deposition method has been investigated. The calcined YBCO powder was suspended in an acetone bath, and was deposited on various cylindrical substrates of silver, stainless steel, ceramics and permalloy. The sample deposited with oxides particles was sintered at 920°C for 1h, and then annealed at 500°C for 1h. The magnetic attenuation ratio (H_in/H_ex) was measured at 77K. All samples with silver, stainless steel, ceramics, or permalloy substrate were able to shield the ambient magnetism (0.5Oe) at least. The magnetic shield on a silver substrate with the diameter of 15mm and length of 30mm had the attenuation ratio of 5.1×10^-4 at 7.6Oe.

View full abstract

This investigation is undertaken to discuss the influence of the structure of aggregates, interparticle interaction and particle shape on the spinnability of slurry. An increase of viscosity of glass bead model slurry in which the interaction among model flocs is negligible tends to decrease the spinnability. On the other hand, the interaction among kaolin particles increases, the spinnability of kaolin slurry in which the interaction among particles is extremely strong. The strong effect of orientation of needle-like kaolin particles on the spinnability is found experimentally. It is found that the orientation of needle-like kaolin particles affects the spinnability significantly.

View full abstract

Fracture energy and R-curve behavior of β′-Sialon-BN composites were investigated to elucidate the strength degradation caused by thermal shock. The fracture energy and R-curve behavior were obtained from compliance analysis on the load-displacement curves of the quasi-static fracture of 3-point bend specimen with chevron-notch. The critical temperature difference of strength degradation (ΔT_c) for the composite containing 30mass% BN was 700K and 900K for Z=1.0 and 2.0 (Si_6-zAl_zO_zN_8-z), respectively. These ΔT_c results agree well with the values calculated from Hasselman's equation. The steep reduction of strength by thermal shock and the disappearance of R-curve behavior were observed on the same thermal shock temperature difference ΔT. The relationship between the ratio of effective fracture energy to elastic fracture energy (γ_eff/γ_i) and retained strength was found to be roughly linear. It was presumed that the residual stress from thermal shock influences the crack-face contact shielding mechanism in the composites and is a possible inhibition factor to forming of the R-curve effect.

View full abstract

The addition of amines such as ethylenediamine, diethylenetriamine and meramine in the course of carbonation of basic calcium carbonate promoted the formation of a disk-like calcium carbonate which is vaterite and 1-1.5μm in diameter and 0.1-0.2μm in thickness. On the other hand, dicarboxylic acids, carboxylic chelate compounds, and amino acids promoted the formation of cubic or spindle-like calcite. Diethylenetriamine which was adsorbed on the surface of calcium carbonate plays an important role in the formation of disk-like calcium carbonate.

View full abstract

Semiconducting ceramics of p-type bismuth telluride was fabricated by plasma sintering using milled powders of (Bi₂Te₃)_0.25(Sb₂Te₃)_0.75+Se2.0mass%. The figure of merit and compressive strength were as large as 2.7×10^-3K^-1(25°C) and 6MPa, respectively. Their thermoelectric properties did not exhibit anisotropy due to their isotropic microstructure without grain orientation. The power factor increased with increasing sintering temperature and pressure, and was independent of the sintering time. Thermoelectric properties apparently have been generated possibly due to enhanced cleaning and activity of milled powder by spark plasma effect.

View full abstract

In order to develop an economical production process of stable and uniform thin film resistors, the RuO₂-based thin films prepared by MOD were studied with focussing on the three major factors to affect the properties of the RuO₂ resistors: the kinds of the substrate, suitable resin component for screen printing and effect of the cleanness of the substrate. As a result, it was found that (1) the films formed on amorphous glazed alumina plate were much more thermally stable than those on crystalline glazed plate, (2) rosin type paste with higher temperature thermal decomposition property gave better resistance uniformity than cellulose type paste, and (3) cleaning the substrate surface with fumed nitric acid was indispensable for making realization of the uniformity and reproducibility in the resistance of RuO₂ dot array. Studies on the properties of sputtered RuO₂ films were also made for comparison with the properties of MOD RuO₂ films.

View full abstract

A CIP hybridized process for fabrication of a single-mode optical fiber with a small core eccentricity error has been studied. As a result, a process for fabrication of an optical fiber with a core eccentricity error of less than 0.3μm has been developed. In this study, we also clarified the following. (1) A hybridized soot preform with an outer diameter noncircularity of less than 0.4% was prepared by packing granular silica particles with a high packing density into a rubber mold with a thick wall. (2) A soot preform with a noncircularity of less than 0.06% and an outer diameter fluctuation of less than 0.11mm was made by means of a grinding treatment in which the outer surface of the soot preform was ground around a core rod. (3) A single-mode optical fiber with a core eccentricity error of less than 0.3μm was also fabricated from the soot preform ground above.

View full abstract

Zeolite A was synthesized at 80°C from allophane in NaOH-KOH mixed solution. Single phase of zeolite A was obtained from mixed solutions with Na/Na+K molar ratios larger than 0.8. In the range between 0.6 and 0.8 for mole fraction of Na, chabazite type zeolite (K-G) and linde F(K-F) coexisted with zeolite A. Zeolite A did not form from mixed solutions with the mole fraction of Na less than 0.6. The rate of formation of zeolite A decreased with decreasing mole fraction of Na, which was explained by the decreases in nucleation rate and liner crystal growth rate. The crystal size of zeolite A increased with an increase in the K content in the mixed solutions.

View full abstract

The effects of atmospheres on foaming characteristics and properties of perlite were investigated. The foaming characteristics and properties of perlite were influenced by the thermal conductivity of the heating gas and the presence of oxygen in the heating atmosphere. The release temperature of H₂O in perlite was shifted to the higher temperature region by a rapid heating. In order to accelerate the heating rate of perlite, He gas with a high thermal conductivity was used for a heating atmosphere. It was found that the unit weight of perlite expanded by heating in He gas was smaller than that in a gas with a moderate thermal conductivity such as N₂. The unit weight of perlite expanded by heating in an oxidizing gas was larger than that in an inert gas. In the case of heating at 900°C which was near a softening temperature of perlite, the inhibition effect of oxygen on foaming characteristics was observed remarkably. This is because that the softening temperature is increased due to the oxidation of the glass phase.

View full abstract

Cu⁺-containing calcium phosphate glasses have been prepared by remelting Cu-containing glass (a) under a controlled atmosphere of low oxygen pressure, and (b) with silicon powder as a reducing agent. About 96% of copper became cuprous ion by remelting under an oxygen partial pressure of 10^-10Pa at 1400°C for 8h. However, further reduction of oxygen pressure was not effective but led to the vaporization of phosphor, resulting in an increase in the devitrification tendency of the melt. Remelting with silicon powder was much more effective in reducing Cu²⁺ to Cu⁺ state. More than 99% of copper could be reduced to cuprous state by the remelting at 1250°C for 0.5h when the ratio of silicon to copper, Si/Cu, was equal to 2.5 or larger. The emission intensity from the Cu⁺-containing glass became maximum when Si/Cu=2.5.

View full abstract

Effect of wet grinding of commercial aluminum hydroxide on the transformation to α-alumina during heating was investigated using vibration ball mill. The grinding with high purity alumina balls reduced the transformation temperature to α-alumina by 300°C. Such significant reduction in the transformation temperature was not observed in the samples obtained by the grindings with zirconia and low purity alumina balls. The formation of α-alumina from ground aluminum hydroxide at such a low temperature was possibly attributed to a seed-effect of fine α-alumina powder worn from the balls and its uniform mixing with the mother powder during grinding. The α-alumina powder made from ground aluminum hydroxide consisted of very fine particles, which enabled us to sinter at a relatively low temperature. The sintered bodies had fine-grained structure without abnormal grain growth.

View full abstract

Fracture resistance behavior with respect to crack extension is evaluated from indented strengths for both alumina and alumina matrix composite dispersed with 10% SiC particles. Since residual stresses due to the thermal expansion mismatch between SiC particle and Al₂O₃ matrix are expected to be exerted on crack surfaces regardless of crack length, the fracture resistance are assumed to be dependent on the square root of crack extension according to fracture-mechanical predictions. R-curves are obtained from the linearity between Vickers-indented strength and α_f^-1/2, where α_f is the critical crack length. A nearly constant fracture resistance curve is obtained for the alumina, while rapidly increasing fracture resistance for the composite. The fracture toughnesses measured independently by the controlled surface flaw method are consistent with the obtained fracture resistance. The role of residual stresses is discussed as a characteristic parameter for the fracture resistance of particle-dispersed composites. In addition, the technical aspects during the evaluation process of R-curve are also discussed.

View full abstract

Two methods for evaluating absorption coefficients of porcelain bodies in the visible wavelength range were proposed and applied to Japanese porcelain. The comparative merits of the methods were discussed. Apparent absorption coefficients could be obtained more precisely than conventional ones by measuring the transmittances for different sample thicknesses. Variations in translucency due to compositional change of the porcelain bodies could be explained quantitatively using these coefficients. The scattering and absorption coefficients of bodies were calculated from their reflectances and transmittances using the Kubelka-Munk equations. The accuracy of these coefficients was lower than that of the apparent absorption coefficient because of difficulties in the transmittance measurements. Nevertheless these coefficients were very useful for understanding the optical properties of porcelain bodies. The relationship between the reflectance and scattering coefficient of bodies was also discussed.

View full abstract

The influence of bridging characteristics on the fracture strength of ceramics was investigated by numerical simulation. Factors influencing the fracture strength of ceramics include the defect size, bridging characteristics and the fracture toughness. The relationship between defect size and fracture strength has been studied by many researchers. However, there are few reports on the relationship between bridging characteristics and fracture strength, though this relationship is important for materials development. The purpose of the present investigation was to reveal the relationship between the bridging characteristics and the fracture strength of ceramics by numerical simulation. In the numerical simulation, the stress intensity factor at crack tip and the fracture toughness were used for the fracture criterion. It is assumed that the crack propagates when the crack-tip stress intensity factor reaches the fracture toughness. The effect of bridging on the toughening is not expected when the defect size as an equivalent crack length is smaller than 20 to 50μm. The reason is that the stable crack extension with bridging before a catastrophic fracture is very short for defects smaller than 20 to 50μm.

View full abstract

The suitable levels of fabrication factors such as the content (F/C) of the fine aggregates, the content (V_f) of the vinylon toughening fibers and the surface modification of the fibers for new development of the non-asbestos fiber-reinforced cement board (FRCB) having an excellent mechanical or a frost durable properties were determined. The values of F/C=0.5mass% and V_f=2.0vol% were the best for the mechanical properties of the FRCBs. The hydrophilic surface modification of the fibers was also effective to improve the mechanical properties due to an enhancement of the interfacial adhesion between hardened cement matrix and fibers. The frost durability was more effectively improved by the controlling of the pore size distribution than the surface modification of the fibers.

View full abstract

Alumina matrix composites mixed with titanium carbide whiskers up to 30vol% were fabricated by hot-pressing at 1600°C under 30MPa for 30min. Two kinds of titanium carbide whiskers having different diameters were used as a reinforcing agent. Mechanical properties, oxidation resistance and cutting perfomance for cast iron were studied. The addition of thin whiskers had a tendency to obstruct the densification of composite body. However, bending strengths at room temperature and 1000°C and oxidation resitance of the composites containing thin whiskers were higher than those of the composites containing thick whiskers. On the other hand, no remarkable difference in Vickers hardness and fracture toughness was observed concerning to the effect on the whisker diameter. The composites containing thin whiskers showed the cutting performance superior to those containing thick whiskers.

View full abstract

Birnessite-type lithium manganese oxide was obtained directly by reacting a Mn(NO₃)₂ solution with a mixed solution of H₂O₂ and LiOH at room temperature. The manganese oxide was characterized by X-ray, DTA-TG and chemical analysis. The manganese oxide had a layered structure with a single sheet of crystal water and lithium ions between two-dimensional edge-shared MnO₆ octahedral sheets. The basal spacing along the c-axis was 0.713nm. The lithium ions in the interlayer space were ion-exchangeable with Na⁺ and K⁺, but partially with Mg²⁺.

View full abstract

The relationship between the grain growth and the wettability of grains by a liquid phase was discussed on the basis of the observation of the microstructure of La-doped SrTiO₃ ceramics and the measurement of the contact angle of the liquid phase (Sr₁Ti_3.206La_0.328O_7.904) on the SrTiO₃ substrate using the sessile drop method. The contact angle of the liquid phase in SrTiO₃, which was the composition at 1490°C as determined from the SrO-TiO₂ phase diagram, on the SrTiO₃ substrate was 22°. The contact angle of the liquid phase containing La₂O₃ was 22°. The contact angles of the liquid phases in the cases of SrTiO₃ containing SiO₂ and a mixture of SiO₂ and Bi₂O₃ were 16° and 9°, respectively. The contact angle was decreased by the addition of SiO₂ and/or Bi₂O₃. The liquid phase of La-doped SrTiO₃ ceramics was considered to have relatively poor wetting and to localize at the triple grain junctions at high temperatures and then to be exuded from the inner part to the surface of the samples during cooling. The nucleation and grain growth occur at this spot where the liquid phase is present. Thus, exaggerated grain growth occurs. The number of nuclei increases with the increase of the volume of liquid phase upon increasing the Ti/Sr ratio, which leads to the inhibition of grain growth. On the other hand, since the liquid phase of La-doped SrTiO₃ containing SiO₂, Bi₂O₃ can uniformly wet the grains, the growth rates of all grains are almost the same and exaggerated grain growth is not observed.

View full abstract