Osami Abe

Sintering process of Y2O3 and Al2O3-doped Si3N4

The sintering process of Y2O3- and Al2O3-doped Si3N4 has been investigated by dilatometry and microstructural observations. The densification progressed through three processes. The bulk density increased to 85% theoretical without the formation of β-Si3N4 in the initial process. The densification once terminated after the second process. The α/β transformation of Si3N4 and the related formation of prismatic grains reduced the densification rate in the second process, although the grain size and the aspect ratio were very small. The final process was the densification of β-Si3N4, where the fibrous grains grew remarkably. The kinetic order for the densification of α-Si3N4 indicated a diffusion-rate controlling mechanism with the activation energy of 244 kJ mol−1 (<1450 ° C) and 193 kJ mol−1 (>1450 ° C). The influence of heating rate on the grain growth was characterized by a parameter derived from kinetic parameters. The relationships between grain growth and densification behaviour have also been discussed.

Sintering process of Y2O3-added Si3N4

The sintering process of Y2O3-added Si3N4 has been investigated by dilatometry and microstructural observations. Densification was promoted above 1440 ° C by the formation of eutectic melts in the Y2O3-SiO2-Si3N4 triangle. However, the dilatometric curves indicated no shrinkage corresponding to the rearrangement process, despite liquid-phase sintering. The kinetic order for The Initial-stage sintering was 0.47 to 0.49. These values indicated that the phase-boundary reaction was rate controlling. The apparent activation energy (323 kJ mol−1) was smaller than the dissociation energy for the Si-N bond (435 kJ mol−1). ESR data and lattice strain indicated that the disordered crystalline structure of the Si3N4 starting powder promoted the reaction of Si3N4 with eutectic melts. After a period of initial-stage sintering, the formation of fibrous β-Si3N4 grains resulted in interlocked structures to interrupt the densification.

Preparation of translucent mullite ceramics

Des ceramiques de mullite hautement translucides ont ete preparees par filtrage sans pression ni additif, avec elimination des phases secondaires et agrandissement approprie de la taille des grains de mullite

Synthesis of intermetallic NiAl and Ni 3 Al fine powders through organometallic precursors

Fine powders of intermetallic NiAl and Ni 3 Al were synthesized through organometallic precursors, which were coprecipitated from aqueous solutions of NiCl 2 and AlCl 3 by the addition of ammonium benzoate and hydradinium monochloride as precipitants. Ni 3 Al and NiAl were synthesized by a two-step heat treatment of the precurors. Single phase powders of NiAl and Ni 3 Al with the particle size less than 3 μm were obtained above 1300 and 1400 o C, respectively

Sintering of silicon nitride with alkaline-earth nitrides

Abstract Hot-pressed silicon nitride containing 1·24-5·96wt% oxygen was prepared by using alkaline-earth nitrides as sintering aids. Dependencies of strength on oxygen content, microstructure and additives were discussed. The room-temperature strength increased with decreasing grain size irrespective of the oxygen content. The degradation of strength with rise of temperature was closely related to the amount of oxide grain boundary phases represented by the oxygen content. Relatively high strength was maintained above 1200°C at an oxygen content less than 3wt%. When the oxygen content exceeded 3wt%, the high-temperature strength degraded remarkably, where the grain boundary composition affected the degradation additionally. Calcination of powders did not improve the degree of degradation, although it increased the room-temperature strength.

The thermal stability of alkali and alkaline-earth metal hydroxide—nitrate systems

Abstract The thermal stability of alkali and alkaline-earth metal hydroxide—nitrate systems has been investigated by thermal analysis, voltammetry and observation with a high-temperature microscope. The melts of alkali metal hydroxide—nitrate systems are thermally stable to at least 300°C above the melting temperatures. The melts have a strong oxidizing ability caused by the strong chemical interaction between hydroxide and nitrate ions. The thermal stability of strontium and barium hydroxide—nitrate systems increases at some peculiar compositions. Although nitrates of strontium and barium decompose in the solid state, the thermal decomposition of the mixtures of these hydroxide—nitrate systems is initiated via the liquid state as a result of the increase in the thermal stability of liquids.

Thermal decomposition of silicon nitride along with powder characteristics.

窒化ケイ素の熱分解過程と粉末粒子中の酸素の存在状態の関係,および, 比表面積など他の粉末物性値との関係を検討した。窒化ケイ素の真空中での熱分解反応は1400℃まではおだやかであった。1450℃以上では一定期間の初期過程ののち, 急激に分解して遊離のケイ素を生成した。初期過程においては粉末の酸素含有量は減少したが, これをすぎると, 分解生成物中の窒化ケイ素に対する酸素のモル比はほぼ一定になった。このモル比からα-窒化ケイ素中への酸素の固溶限界は組成式Si3N3.932-3.94200.101-0.086で表わされた。熱分解の初期過程においては,窒化ケイ素粒子の酸素に富んだ表面層の分解除去によって遊離のケイ素の生成をともなうことなしに酸素含有量を低減することができた。しかしながら, 酸素含有量を固溶限界以下にすることは, 熱処理雰囲気を変えても不可能であった。また, 初期過程では窒化ケイ素のα→β転移は起こらなかった。非晶質粉末では,初期過程においても結晶化してα相になったが, 結晶性は悪かった。比表面積は熱分解反応の進行にともなって減少した。

Influence of size and shape on homogeneity of powder compacts formed by cold isostatic pressing (part 1): Press forming of thick cylinders

The influence of size and diameter/thickness (D/T) ratio on the homogeneity of cylindrical silicon carbide powder compacts formed by cold isostatic pressing has been studied. The degree of inhomogeneity was estimated by the measurement of spatial distribution of bulk density, Vickers microhardness and pore size distribution for the specimens cut out from the compacts. The degree of inhomogeneity in the density and hardness was relatively smaller than that in uniaxially die-pressed compacts. However, the distribution patterns of the density and hardness which were similar to each other showed the presence of high density and high hardness layer near the surface of the compacts. Furthermore, inhomogeneity was found locally in the inner part of the compacts. A considerable decrease in volume fraction of the smaller sized pores was recognized at the corner edge of the cylindrical compacts. The volume of the inhomogeneous layer near the upper surface in the compacts with a larger D/T ratio tends to be reduced with decreasing D/T ratio. For the same D/T ratio, however, the homogeneity did not depent on the size of the compacts. This behavior could be explained by the difference in damping behavior of the applied pressure between the radial and axial directions.

Sintering and mechanical properties of silicon nitride crystallized from amorphous powder

無添加非晶質窒化ケイ素粉末及びイットリアを10wt%添加した粉末を窒素中, 1200℃から1500℃で1時間結晶化した. 得られた粉末を1750℃, 1時間, 49MPaの条件でホットプレスし, 粉末の特性, 焼結性及び焼結体の機械的性質について検討した.イットリアの添加は, 非晶質窒化ケイ素のα相への結晶化及びα/β転移のいずれも促進する. 無添加で結晶化させた粉末は形や大きさが不規則になるが, イットリアを添加して結晶化させると微細で均一な粒径のα及びβ窒化ケイ素粒子となる. イットリアを添加後仮焼した場合, 焼結体のかさ密度, 室温及び1200℃での抗折強度は仮焼温度とともに増加するが, 窒化ケイ素を結晶化させた後イットリアを添加した場合は密度, 強度とも高温で仮焼するほど低下する. 仮焼した粉末のα相含有率は焼結体の密度及び強度にほとんど影響を及ぼさない. 非晶質窒化ケイ素にイットリアを添加して結晶化した場合, 焼結性, 機械的性質及び微構造を制御する要因は粉末の形態及び窒化ケイ素とイットリアの反応生成相であると考えられた.