A sol of inorganic oxide particles is stably dispersed in a hydrophilic organic solvent containing a hydrocarbon such as a paraffinic hydrocarbon or a naphthenic hydrocarbon. The sol contains a dispersion medium containing an organic solvent containing a C.sub.6-18 paraffinic hydrocarbon, a C.sub.6-18 naphthenic hydrocarbon, or a mixture of these, a C.sub.4-8 alcohol having a carbon chain with a carbon-carbon bond in the molecule in an amount of 0.1 to 5% by mass in the entire dispersion medium, and inorganic oxide particles having an average particle diameter of 5 to 200 nm as measured by dynamic light scattering as a dispersoid, wherein the inorganic oxide particles contain a C.sub.1-3 alkyl group bonded to a silicon atom and a C.sub.4-18 alkyl group. The paraffinic hydrocarbon is a normal paraffinic hydrocarbon or an isoparaffinic hydrocarbon. The naphthenic hydrocarbon is a saturated aliphatic cyclic hydrocarbon substitutable with a C.sub.1-10 alkyl group.

A sol of inorganic oxide particles is stably dispersed in a hydrophilic organic solvent containing a hydrocarbon such as a paraffinic hydrocarbon or a naphthenic hydrocarbon. The sol contains a dispersion medium containing an organic solvent containing a C.sub.6-18 paraffinic hydrocarbon, a C.sub.6-18 naphthenic hydrocarbon, or a mixture of these, a C.sub.4-8 alcohol having a carbon chain with a carbon-carbon bond in the molecule in an amount of 0.1 to 5% by mass in the entire dispersion medium, and inorganic oxide particles having an average particle diameter of 5 to 200 nm as measured by dynamic light scattering as a dispersoid, wherein the inorganic oxide particles contain a C.sub.1-3 alkyl group bonded to a silicon atom and a C.sub.4-18 alkyl group. The paraffinic hydrocarbon is a normal paraffinic hydrocarbon or an isoparaffinic hydrocarbon. The naphthenic hydrocarbon is a saturated aliphatic cyclic hydrocarbon substitutable with a C.sub.1-10 alkyl group.

A preparation method for a spherical silica powder filler, comprises the following steps: S1, providing spherical polysiloxane comprising T units by means of a hydrolysis condensation reaction of R.sub.1SiX.sub.3, wherein R.sub.1 is hydrogen atom or an independently selectable organic group having 1 to 18 carbon atoms, X is a hydrolyzable group, and the T unit is R.sub.1SiO.sub.3—; and S2, calcining the spherical polysiloxane under the condition of a dry oxidizing gas atmosphere at a calcining temperature between 850° C. and 1200° C., so as to obtain a spherical silica powder filler having a low hydroxyl content. The spherical silica powder filler is composed of at least one selected from Q.sub.1 unit, Q.sub.2 unit, Q.sub.3 unit and Q.sub.4 unit, wherein Q.sub.1 unit is Si(OH).sub.3O—, Q.sub.2 unit is Si(OH).sub.2O.sub.2—,Q.sub.3 unit is SiOHO.sub.3—, Q.sub.4 unit is SiO.sub.4—, and the content of Q.sub.4 unit is greater than or equal to 95%.

A preparation method for a spherical silica powder filler, comprises the following steps: S1, providing spherical polysiloxane comprising T units by means of a hydrolysis condensation reaction of R.sub.1SiX.sub.3, wherein R.sub.1 is hydrogen atom or an independently selectable organic group having 1 to 18 carbon atoms, X is a hydrolyzable group, and the T unit is R.sub.1SiO.sub.3—; and S2, calcining the spherical polysiloxane under the condition of a dry oxidizing gas atmosphere at a calcining temperature between 850° C. and 1200° C., so as to obtain a spherical silica powder filler having a low hydroxyl content. The spherical silica powder filler is composed of at least one selected from Q.sub.1 unit, Q.sub.2 unit, Q.sub.3 unit and Q.sub.4 unit, wherein Q.sub.1 unit is Si(OH).sub.3O—, Q.sub.2 unit is Si(OH).sub.2O.sub.2—,Q.sub.3 unit is SiOHO.sub.3—, Q.sub.4 unit is SiO.sub.4—, and the content of Q.sub.4 unit is greater than or equal to 95%.

The present invention addresses the problem of evaluating the hydrophobicity of a powder. According to the present invention, a powder is charged into a mixed solvent composed of a lipophilic solvent and a hydrophilic solvent, the voltage rate R of the mixed solvent is measured at predetermined time intervals while adding a lipophilic solvent to the mixed solvent charged with the powder, a parameter x correlating with the concentration of powder is defined for an arbitrary voltage rate R, a continuous function HP(x) of the ratio of a lipophilic solvent corresponding to x is defined, and HP(x) for required x is set as a representative value of a lipophilic solvent ratio distribution and used as an index of hydrophobicity.

The present invention addresses the problem of evaluating the hydrophobicity of a powder. According to the present invention, a powder is charged into a mixed solvent composed of a lipophilic solvent and a hydrophilic solvent, the voltage rate R of the mixed solvent is measured at predetermined time intervals while adding a lipophilic solvent to the mixed solvent charged with the powder, a parameter x correlating with the concentration of powder is defined for an arbitrary voltage rate R, a continuous function HP(x) of the ratio of a lipophilic solvent corresponding to x is defined, and HP(x) for required x is set as a representative value of a lipophilic solvent ratio distribution and used as an index of hydrophobicity.

A silicon nitride powder for sintering which, despite of its fine powdery form, shows a very small increase in the oxygen concentration with time and features excellent storage stability. The silicon nitride powder for sintering has a specific surface area of 5 to 30 m.sup.2/g, and is characterized by having a hydrophobicity (M value) of 30 or more and an increase in the oxygen concentration of 0.30% by mass or less after left to stand in the air of a humidity of 90% and 20° C. for 48 hours. The silicon nitride powder for sintering can be obtained by dry-pulverizing aggregated masses of the silicon nitride in an inert atmosphere in the presence of a silane coupling agent.

A silicon nitride powder for sintering which, despite of its fine powdery form, shows a very small increase in the oxygen concentration with time and features excellent storage stability. The silicon nitride powder for sintering has a specific surface area of 5 to 30 m.sup.2/g, and is characterized by having a hydrophobicity (M value) of 30 or more and an increase in the oxygen concentration of 0.30% by mass or less after left to stand in the air of a humidity of 90% and 20° C. for 48 hours. The silicon nitride powder for sintering can be obtained by dry-pulverizing aggregated masses of the silicon nitride in an inert atmosphere in the presence of a silane coupling agent.

These surface-treated metal oxide particles are metal oxide particles surface-treated with a silane coupling agent having an alkoxy group, in which the metal oxide particles have an ultraviolet shielding property, a weight loss of the surface-treated metal oxide particles on drying at 105° C. for 3 hours is 0.5% by mass or less, a peak derived from the alkoxy group is not detected in a reflection spectrum of the surface-treated metal oxide particles in 900 cm.sup.−1 to 1300 cm.sup.−1, which is measured by a Fourier transform infrared spectrophotometer, and a value (D98/BET converted diameter) obtained by dividing a dry particle size D98 (μm) thereof by a BET-converted particle diameter (nm) thereof is 0.01 or more and 5.0 or less.

These surface-treated metal oxide particles are metal oxide particles surface-treated with a silane coupling agent having an alkoxy group, in which the metal oxide particles have an ultraviolet shielding property, a weight loss of the surface-treated metal oxide particles on drying at 105° C. for 3 hours is 0.5% by mass or less, a peak derived from the alkoxy group is not detected in a reflection spectrum of the surface-treated metal oxide particles in 900 cm.sup.−1 to 1300 cm.sup.−1, which is measured by a Fourier transform infrared spectrophotometer, and a value (D98/BET converted diameter) obtained by dividing a dry particle size D98 (μm) thereof by a BET-converted particle diameter (nm) thereof is 0.01 or more and 5.0 or less.