Spherical monodispersed (SM) silica particles of up to 0.8μm were obtained from ethanol solution of readily available Si(OEt)₄ containing seeded silica. Growth in polydispersed system was also carried out in the same manner. Successive polydispersed growths showed an equal rate of increase in all particle diameter modes, suggesting that seeded silica grew by a polynuclear (surface reaction controlled) mechanism. Nielsen's chronomal analysis of the monodispersed growth also suggested the polynuclear mechanism with an exponent of polynuclear p=3 or 4.

View full abstract

β-sialon·TiN composite was fabricated by hot-pressing of a spray-dryed mixture of Si₃N₄ powder, Ti(Oi-Pr)₄ and Al(Oi-Pr)₃/n-C₆H₁₄ solution. Flexural strength, K_IC, phase composition, and microstructure were investigated. Phases identified by XRD in sintered body were β-sialon, O′ sialon and TiN. Reaction of TiO₂ with Si₃N₄ yielded TiN and increased amount of O′ sialon compared with samples without Ti(Oi-Pr)₄. The room temperature strength by 3-point bending test was 890MN/m², lower than that (150MN/m²) of β-sialon which was made from Si₃N₄ powder and Al(Oi-Pr)₃ solution without Ti(Oi-Pr)₄, but higther than that (740MN/m²) of β-sialon·TiN made from Si₃N₄, Al₂O₃ and TiO₂ powders. The inproved strength is due to the finer and more homogeneous microstructure. The lower strength compared with β-sialon without Ti(Oi-Pr)₄ was ascribed to the rough surface formed by heat-treatment in air. The Vickers hardness of β-sialon·TiN was about 17GN/m² and the K_IC was about 4MNm^-3/2, both slightly higher than those of β-sialon without Ti(Oi-Pr)₄. The improvement is probably due to the existence of TiN grains.

View full abstract

Fine AlN powder was synthesized continuously at 1350°-1550°C by the floating nitridation technique. The nitriding temperature was reduced by using ultrapure Al and N₂ gas as raw materials. The obtained AlN powder was pure with trace amounts of metallic impurities and oxygen. The primary particle size of AlN powder obtained was 0.1μm with 86% secondary particles in the range from 0.1 to 0.8μm. The hollow spherical powder was obtained when unreacted-Al remained. The formation process of the fine AlN powder was presumed to be constituted by several elementary reaction processes, i.e., formation of a thin oxide layer on Al particles; disintegration of the thin oxide layer; formation of an AlN layer; disintegration of the AlN layer; eruption of ultrafine Al droplets and Al vapor from the inside of particle; and formation of fine AlN particles by reaction of these Al's and N₂ gas. This AlN powder had good sinterability, and transparent AlN ceramics were obtained without any sintering aids.

View full abstract

“Sinter-bonding”, a new coating method which employs hot isostatic pressing (HIP) has been developed. Partially stabilized zirconia (PSZ) and MgO powders were coated on several metals (SUS 304, SUS 304L, SUS 403, Fe and Ni). Metals and ceramic powders were sinter-bonded at 1373 K-101MPa for 1h. The interfaces were observed by optical micrography, SEM, EPMA and TEM. Pores and cracks were not observed in the interfacial area. The surface hardness (H_v) of coated ceramics was nearly equal to that of bulk ceramics. The bonding strength of the interface was measured by tensile test at room temperature. After the tensile tests, the fracture surfaces were observed by SEM and EPMA. The bonding strength of ceramic/Ni joints ranged from 1.9 to 7.8MPa. The highest strength, 24.1MPa, was achieved by removing the layer with internal stress in the interfacial region.

View full abstract

The Si₃N₄-ZrO₂ composite was fabricated by sintering at 1500°-1700°C using Y₂O₃ and Al₂O₃ additives. The needle-shaped particles precipitated in the sintered body. They were identified as α-Si₃N₄ single crystal whiskers by EPMA and TEM observation. The growth of whiskers depended on the amount of additives and the purity of starting powders. It was obvious that the transformable t-ZrO₂ and the precipitation of the whiskers contributed to the enhancement of the fracture toughness.

View full abstract

The fatigue behavior of tetragonal zirconia polycrystals (TZP) containing 2 and 4mol% Y₂O₃ was studied by measuring their fracture strength as a function of stressing rate (dynamic fatigue technique). The average grain sizes of 2 and 4mol% Y₂O₃-TZP were 0.46 and 0.56μm, respectively. Stress-displacement relationships except Z2 Y-I at the crosshead speed of 0.005mm/min at 250°C showed elastic behavior, while that of Z2 Y-I at the crosshead speed of 0.005mm/min at 250°C showed inelastic behavior. The crack growth parameter N of Z2 Y-I was 104 at 20°C and 21.9 at 250°C. The N value of Z4 Y-I was 32.3 at 250°C. The N values at 20°C decreased with increasing Y₂O₃ content from 2 to 4mol%, but were larger than 40 indicating little fatigue. The N values in the grain size group of more than 1μm at 250°C were less than 10 indicating remarkable fatigue. The N values in the grain size group of 0.5μm showed a maximum at 3mol% Y₂O₃, and decreased with decreasing Y₂O₃ content from 3 to 2mol% or with increasing Y₂O₃ content from 3 to 4mol%. The monoclinic zirconia content on the fracture surface decreased monotonously with increasing Y₂O₃ content for a certain grain size, while that for a certain Y₂O₃ content increased with increasing grain sizes.

View full abstract

Al₂O₃ composites containing 20-50 volume% of dispersed SiC particles of 0.28, 0.48 or 0.65μm in average diameter were hot-pressed at 1850°C for 1h under an applied pressure of 25MNm^-2 The SiC particles prevented the densification of Al₂O₃, more pronounced with smaller particles. Composites containing more than 40vol% SiC particles were not fully densified with the SiC particles of 0.28 and 0.48μm. On the other hand, the composite containing 50vol% 0.65μm-SiC particles sintered to a relative density over 99%. The erosive wear was tested using SiC and Al₂O₃ abrasives. The erosion rate of the composites depended largely on their hardness. The Vickers hardness of a composite composed of 50vol% 0.65μm-SiC particles was 26GNm^-2 and its erosion rate was 5.3×10^-4cm³/cm³.

View full abstract

Effects of carbon impurity on HIP'ed silicon nitride under 60 and 200MPa at 1823, 1873 and 1973K were studied. The density, hardness, fracture toughness, α-β phase ratio, heat capacity and thermal diffusivity were evaluated. The results were compared with those of silicon nitride prepared by the normal and hot press sintering. Carbon behavior of grain boundaries was investigated by X-ray diffraction and Auger electron spectroscopy (AES).
(1) Silicon carbide was not formed during HIP'ing of silicon nitride powder with carbon film. Thermodynamic analysis showed that carbon is stable as graphite under the HIP'ing conditions studied. Silicon carbide can be produced from silicon nitride during the normal and hot press sintering. The formation of silicon carbide is explained by the free energy change of the related reaction under high pressure (high pressure Ellingham diagrams).
(2) Silicon nitride was HIP'ed to full density. But carbon retards α to β phase transformation of silicon nitride in HIP sintering in contrast to normal or hot press sintering.
(3) The mechanical properties are not influenced by the carbon content, unlike the cases for normal or hot press sintering. This can be also explained the pressure of gas phase during sintering.
(4) Carbon decreases the thermal diffusivity and thermal conductivity. This may be due to the fact that carbon reduces the α to β phase transformation of silicon nitride in HIP'ing. Heat capacity is not influenced by the carbon content for HIP'ed silicon nitride.
(5) The thermal conductivity data showed that carbon dissolves into silicon nitride up to 1000ppm between 1823 and 1973K.

View full abstract

The correlation between the formation of α-quartz and lithium silicate in NaBr-LiCl flux and NaBr-lithium compound (other than LiCl) fluxes and the vitrification of the sample was studied. In NaBr-LiCl flux at the soaking temperature of 800°C, α-quartz formed in the non-vitrified powdery part in larger quantities than in the vitrified part, while the situation was reversed for lithium silicate. In non-vitrified powdery part in range of R₂ (silicagel/flux, molar ratio)≥0.10, only α-quartz crystals were formed. Use of NaBr-Li₆Mo₇O₂₄ flux inhibited the vitrification almost completely, α-quartz crystals being maximum 8.9μm in length at R₁ (NaBr/Li₆Mo₇O₂₄, molar ratio)=9.97/0.03, and the formation of lithium silicate was suppressed in NaBr-Li₃PO₄ flux.

View full abstract

The target of this research is to find the conditions for obtaining the valid fracture toughness value of nuclear graphite. The effects of the notch sharpness and size of specimen on the elastic-plastic fracture toughness values are examined. (1) The maximum notch root radius of curvature required for obtaining nearly constant fracture toughness values depends upon the kind of graphite, ca. 0.25mm for the fine-grained isotropic graphite; IG-110 and ca. 1.2mm for the coarse-grained near isotropic graphite, PGX. (2) The minimum specimen thickness for obtaining nearly constant fracture toughness values depends upon the kind of graphite, ca. 10mm for IG-110 and ca. 20mm for PGX. (3) The conditions that the specimen should satisfy for obtaining the valid fracture toughness values vary with the kind of graphite. (4) The linear elastic-plastic fracture toughness is applicable to IG-110 graphite, and evaluation by the elastic-plastic fracture mechanics is not always necessary for IG-110, but it is considered that for PGX the correct fracture toughness values are not obtained unless the elastic-plastic fracture mechanics approach is applied.

View full abstract

TiB₂-5% TaB₂-1% CoB materials obtained from fine powder have the porosity of 0.3-0.7vol%. Effect of TiC addition on the densification was studied for eliminating the residual pores. TiB₂-5% TaB₂-1% CoB-1.7% TiC materials have lower porosity of 0.1-0.2vol%. It is thought that lower porosity occurs when TiC, Ta and O₂ form a solid solution of (Ti, Ta) (C, O) during the sintering stage and the C which was percipitated from TiC deoxidated the raw powder at the same time. Also TiB₂-5% TaB₂-1% CoB-1.7% TiC materials have almost the same transverse rupture strength as those of TiB₂-5% TaB₂-1% CoB materials. It has been found that there are three factors that affect the transverse rupture strength. One is a lower oxygen content, which is a positive factor. The second is a larger average grain size of TiB₂, which is a negative factor. The third is the solid solution phase of (Ti, Ta) (C, O) described above, which is an unknown factor.

View full abstract

This work was undertaken to improve the strength of the conventional Si₃N₄ reaction-bonded SiC ceramics, a well known material by their very little size change in sintering stage. Metallic Si (average grain size 0.9μm) and SiC (average grain size 16μm) powders were mixed and molded using thermoplastic resin binder and sintered in the 0.88MPa nitrogen gas atmosphere. The maximum bending strength of 350MPa was obtained for the sintered body containing 60Si-40SiC (in weight) plus 9wt% binder. The size change in sintering was minimized to 0.13% under the above process conditions. Several properties of the newly developed ceramics such as high temperature strength, oxidation resistance and thermal diffusivity, were measured.

View full abstract

In order to improve workability of alkali-resistant BaO-SiO₂-TiO₂ glasses, glass-forming ability was evaluated for various glass compositions obtained by partial replacement of 30BaO, 30SiO₂, 40TiO₂ in mol% with Li₂O, Na₂O, CaO, SrO, Al₂O₃ and/or ZrO₂ (Tables 1-4). Here, a parameter K_gl was employed for estimating glass-forming ability. This was expressed as (T_c-T_g)/(T_l-T_c) by Hruby, where glass transition temperature T_g, crystallization temperatrure T_c and liquidus temperature T_l are determined on the differential thermal analysis curves (Fig. 2). It was found that the composition of 7.0Na₂O, 5.75CaO, 5.75SrO, 11.5BaO, 30.0SiO₂, 33.0TiO₂, 7.0ZrO₂ in mol% gives the largest K_gl value among the examined compositions. Melt of this composition could be drawn into a continuous long fiber 8-15μm in diameter and 50, 000m long with a specially designed double-crucible method (Fig. 3). Glass fibers of this composition were confirmed to be alkali-resistant and showed no diameter reduction even after exposure to either Portland cement aqueous phase solution or a 2N NaOH solution at 95°C for 18h.

View full abstract

Five kinds of silicon carbide ceramics were oxidized in flowing dry air at 1300°C for 360h and 1000h, and oxidation behaviour was studied by measuring the weight gain and thickness of oxide layer and of unoxidized substrate, in which the weight loss by evaporation of some species from the oxide layer during oxidation was taken into account. The change of thickness of substrate was complicated, and some samples expanded during oxidation. Therefore, no direct correlation is possible between evapolation and dimensional change of substrate. To evaluate the oxidation resistance correctly, the thickness, density and texture of the oxide layer must be examined together with the weight gain.

View full abstract

Y-Ba-Cu-O thin films were prepared at 800°C on yttria-stabilized zirconia substrates by chemical vapor deposition using β-diketonate chelates of Y, Ba, and Cu. An as-deposited film showed zero resistivity only at 10K. When annealed at 450°C for 1h in O₂ atmosphere, zero resistivity was observed at 30K. For a film with in-situ oxygen treatment during cooling after the deposition, the superconducting transition started at 83K and zero resistivity was obtained at 43K.

View full abstract