Abstract: The alumina powder contains: a first alumina particle having an average particle diameter of 0.1 µm or more and less than 1 µm; a second alumina particle having an average particle diameter of 1 µm or more and less than 10 µm; and a third alumina particle having an average particle diameter of 10 µm or more and 100 µm or less, wherein each of the average particle diameters is a particle diameter measured using laser light diffraction scattering particle size distribution analyzer, average sphericity of first alumina particle having projected area equivalent circle diameter of 0.1 µm or more and 1 µm or less as determined by microscopy is 0.80 or more and 0.98 or less, a specific surface area of first alumina particle is 1.9 m2/g or more and 20.0 m2/g or less, and content ratio of an ? crystal phase is 80% by mass or more.

Abstract: A carbon-containing alumina powder containing a carbon-containing alumina particle having a projected area equivalent circle diameter of 1 ?m or more and 100 ?m or less as determined by microscopy, wherein an average sphericity of the carbon-containing alumina particle is 0.85 or more, and a specific surface area is 0.05 m2/g or more and 1.0 m2/g or less, and a ratio B/A of a carbon content ratio B to a carbon content ratio A in the carbon-containing alumina powder calculated by using a specific measurement method is 0.20 or more and 0.90 or less.

Abstract: An alumina powder containing: a first alumina particle having average particle diameter from 0.1 ?m to 1 ?m; a second alumina particle having average particle diameter from 1 ?m to 10 ?m; and a third alumina particle having average particle diameter from 10 ?m to 100 ?m, wherein the particle diameters are measured using laser light diffraction scattering particle size distribution analyzer, average sphericity of first alumina particle having projected area equivalent circle diameter from 0.1 ?m to 1 ?m as determined by microscopy is from 0.80 to 0.98, and a ratio of D90/D10 of first alumina particle is from 2.0 to 8.0 wherein the ratio of D90/D10 is a ratio when particle diameter at cumulative value of 10% from fine particle side of cumulative particle size distribution on volume basis is D10 and particle diameter at cumulative value of 90% from fine particle side is D90.

Abstract: Provided is a spherical alumina powder which, when incorporated into resins in a high concentration, enables a resin composition to have low viscosity and which has reduced viscosity increase in the resin composition over time after surface treatment of the spherical alumina powder. The spherical alumina powder has an alkyl group(s) derived from a silane compound(s) on its surface, and is characterized in that an intensity ratio {I (CH3)/I (CH2)} of a peak (2960±5 cm?1) associated with asymmetric vibration of CH3 to a peak (2925±5 cm?1) associated with asymmetric vibration of CH2 in the alkyl group(s) of the silane compound(s) in the same spectral data obtained by infrared spectroscopic analysis measurement is 0.2 or more and less than 2.0.

Abstract: Provided is a spherical alumina powder which, when incorporated into resins in a high concentration, enables a resin composition to have low viscosity and which has reduced viscosity increase in the resin composition over time after surface treatment of the spherical alumina powder. The spherical alumina powder has an alkyl group(s) derived from a silane compound(s) on its surface, and is characterized in that an intensity ratio {I (CH3)/I (CH2)} of a peak (2960±5 cm?1) associated with asymmetric vibration of CH3 to a peak (2925±5 cm?1) associated with asymmetric vibration of CH2 in the alkyl group(s) of the silane compound(s) in the same spectral data obtained by infrared spectroscopic analysis measurement is 0.2 or more and less than 2.0.