Abstract: A metal oxide powder except &agr;-alumina, comprising polyhedral particles having at least 6 planes each, a number average particle size of from 0.1 to 300 &mgr;m, and a D90/D10 ratio of 10 or less where D10 and D90 are particle sizes at 10% and 90% accumulation, respectively from the smallest particle size side in a cumulative particle size curve of the particles. This metal oxide powder contains less agglomerated particles, and has a narrow particle size distribution and a uniform particle shape.

Abstract: Alpha alumina powder having a regulated particle size, a uniform particle shape and a narrow particle size distribution and further a low halogen content is produced by calcining at least one starting material selected from transition alumina and alumina compounds in a halogen-containing atmosphere which comprises a hydrogen halide gas, a halogen gas or a mixture of a halogen gas and steam and contains at least 0.1% by volume of at least one halogen-containing gas selected from the group consisting of hydrogen halide gas and halogen gas, and removing halogen from a calcined material. The produced alpha-alumina powder is excellent in a packing property and homogeneously packed.

Abstract: A process for producing &agr;-alumina containing &agr;-alumina single crystal particles characterized in that at least one of a transition alumina and transition alumina precursor is calcined in an atmosphere containing at least 1% by volume of hydrogen chloride at a temperature of not less than 600° C., preferably from 600 to 1,400° C., and more preferably from 800 to 1,200° C. Aluminum hydroxide, alum, aluminum sulfate, etc. are used as the alumina raw material capable of being converted to transition alumina on heating. &agr;-Alumina comprising octahedral or higher polyhedral &agr;-alumina single crystal particles have a high alumina purity above a given level, are fine and homogeneous, have a narrow particle size distribution, are not agglomerated particles, and can be obtained from raw materials of various kinds, purities, shapes, sizes, and compositions.

Abstract: A metal oxide powder except &agr;-alumina, is disclosed comprising polyhedral particles having at least 6 planes each, a number average particle size of from 0.1 to 300 &mgr;m, and a D90/D10 ratio of 10 or less where D10 and D90 are particle sizes at 10% and 90% accumulation, respectively from the smallest particle size side in a cumulative particle size curve of the particles. This metal oxide powder contains less agglomerated particles, and has a narrow particle size distribution and a uniform particle shape.

Abstract: .alpha.-Alumina comprising .alpha.-alumina single crystal particles which are homogeneous containing no crystal seed inside the particles; have an octa- or higher polyhedral shape; have a D/H ratio of from 0.5 to 3.0, wherein D represents a maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of the particles, and H represents a diameter perpendicular to the hexagonal lattice plane; have a number average particle diameter of more than 5 .mu.m and not more than 30 .mu.m; have a sodium content of less than 5 ppm in terms of Na.sub.2 O; and have an alumina purity of not less than 99.90% by weight. .alpha.-Alumina in a powder form composed of .alpha.-alumina single crystal particles having a high punity, a fine and uniform composition, a narrow particle size distribution, and an octa- or higher polyhedral shape which are not aggromerates. The .alpha.

Abstract: A powdery .alpha.-alumina having an alumina purity of at least 99.8 wt. %, a sodium content of 40 ppm or less and an iron content of 20 ppm or less, which is obtained by grinding aluminum hydroxide which is produced by the Bayer process or a transition alumina which is obtained by precalcining aluminum hydroxide produced by the Bayer process, and calcining the aluminum hydroxide or transition alumina in an atmosphere containing 1 vol. % or more of a hydrogen chloride gas, or an atmosphere containing 1 vol. % or more of a chlorine gas and 0.1 vol. % or more of steam, at a temperature of 600 to 1400.degree. C.

Abstract: A process for producing .alpha.-alumina powder comprising the step of calcining at least one of transition alumina and a transition alumina precursor capable of becoming transition alumina on heating, in a gas atmosphere containing (1) a hydrogen halide, (2) a component prepared from a halogen and steam or (3) a halogen, in the presence of at least one of a seed crystal and a shape-controlling agent. The .alpha.-alumina powder is particularly useful as a raw material for abrasives, fillers, sinters or spacers which comprises .alpha.-alumina particles having a substantially octahedral or eicosahedral shape, a specific structure, and a narrow primary particle size distribution.

Abstract: Alpha alumina powder is produced by granulating at least one alumina starting material selected from group consisting of transition alumina and alumina compound to obtain particles and calcining the particles in a halogen-containing atmosphere which comprises a hydrogen halide gas, a halogen gas or a mixture of a halogen gas and steam and contains at least 0.1 % by volume of at least one halogen-containing gas selected from the group consisting of hydrogen halide gas and halogen gas.By granulating the alumina starting material powder in the form of particles to increase a bulk density and calcining them in the presence of halogen, .alpha.-alumina powder having a regulated particle size and shape and a narrow particle size distribution and containing fewer agglomerated particles is produced at a high calcining efficiency.

Abstract: A process for producing .alpha.-alumina powder comprising the step of calcining at least one of transition alumina and a transition alumina precursor capable of becoming transition alumina on heating, in a gas atmosphere containing (1) a hydrogen halide, (2) a component prepared from a halogen and steam or (3) a halogen, in the presence of at least one of a seed crystal and a shape-controlling agent. The .alpha.-alumina powder is particularly useful as a raw material for abrasives, fillers, sinters or spacers which comprises .alpha.-alumina particles having a substantially octahedral or eicosahedral shape, a specific structure, and a narrow primary particle size distribution.

Abstract: An inorganic porous sintered body comprising inorganic particles of polyhedral crystals, said inorganic porous sintered body having pores formed by the inorganic particles, wherein said pores have a pore size distribution that a ratio of D10 to D90 is not more than 3 and D50 is 0.01-5 .mu.m when D10, D50 and D90 are pore sizes at 10%, 50% and 90% cumulation from the largest pore size of a cumulative pore volume distribution, respectively, and a porosity is 10-50%.

Abstract: A complex metal oxide powder having at least two metal elements, which contains polyhedral particles each having at least 6 planes, a number average particle size of from 0.1 to 500 .mu.m, and a D.sub.90 /D.sub.10 ratio of 10 or less in which D.sub.10 and D.sub.90 are particle sizes at 10% and 90% accumulation, respectively from the smallest particle size side in a cumulative particle size curve of the particles, and an yttrium-aluminum garnet powder containing polyhedral particles each having at least 6 planes, and a number average particle size of 20 to 500 .mu.m. These complex metal oxides contain less agglomerated particles, and have a narrow particle size distribution and a uniform particle shape.

Abstract: An alumina composition, an alumina molded article, prepared from the alumina composition, an alumina ceramic prepared by sintering the alumina molded article, and a process for producing the ceramic; the alumina composition comprising .alpha.-alumina particles having a ratio of the shortest diameter to the longest diameter of from 0.3 to 1; and a size distribution of not more than 5, as expressed in terms of a D.sub.90 /D.sub.10 ratio, wherein D.sub.10 and D.sub.90 represent a cumulative 10% diameter and a cumulative 90% diameter, respectively, of a cumulative distribution depicted from the small diameter side, or comprising .alpha.-alumina particles having a polyhedral shape; a D/H ratio of from 0.5 to 3.0, wherein D represents a maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of .alpha.-alumina, and H represents a particle diameter perpendicular to the lattice plane; and a size distribution of not more than 5, as expressed in terms of a D.sub.90 /D.sub.

Abstract: An aluminum nitride powder comprising polycrystalline particles which have clearly observed grain boundaries, an agglomerated particle size of 0.1 to 100 .mu.m when measured by a particle size distribution analyzer, a tapped bulk density of 0.55 to 2.0 g/cm.sup.3, and a ratio of a number average particle size calculated from a SEM photograph image to a particle size calculated from a BET specific surface area of 1.1 to 3.

Abstract: A process for producing .alpha.-alumina powder comprising .alpha.-alumina single crystal particles, the process comprising the step of calcining at least one of transition alumina and a transition alumina precursor capable of becoming transition alumina on heating, in a gas atmosphere containing (1) at least about 5% by volume, based on the total gas atmosphere, of a halogen selected from fluorine, chlorine, bromine, and iodine, (2) at least about 1% by volume, based on the total gas atmosphere, of a hydrogen halide selected from hydrogen fluoride, hydrogen bromide, and hydrogen iodide, or (3) a component prepared from at least about 1% by volume, based on the total gas atmosphere, of a halogen gas selected from fluorine gas, bromine gas, and iodine gas, and at least about 0.1% by volume, based on the total gas atmosphere, of steam.

Abstract: A method of preparing dense sintered bodies of beta alumina suitable for solid electrolytes, which comprises preparing a shaped body from a mixture containing an aluminum powder, a sodium compound, a structure stabilizer and a zirconia powder or from a mixture containing an aluminum powder, an alumina powder, a sodium compound, a structure stabilizer and a zirconia powder, and reaction-sintering the shaped body.

Abstract: A sintered body where alumina is dispersed in a zirconia matrix in which yttria has been doped to form a solid solution can be obtained by compacting a mixed powder comprising zirconia or yttria-stabilized zirconia and yttrium-aluminum complex oxide and sintering the resulting compacted body. This sintered body comprises a fine zirconia matrix in which fine alumina particles are dispersed to strengthen it and is excellent in flexural strength and fracture toughness and has a practically effective oxygen ionic conductivity.