Monosized spherical SiC, $Si_3N_4$, and $SiC/Si_3N_4$ composite powders were synthesized by carbothermal reduction of gel powders derived from sol-gel processs. Gel powder were synthesized by the hydrolysis of a mixture of phenyltrimethoxysilane and tetramethyl orthosilicate. The ratios of C/Si of gel powders, which were the ratios of carbon and silicon in a mixture of phenyltrimethoxysilane and tetramethyl orthosilicate except the carbon involved in methyl group, were 3.5, 4.0, 4.5, and 6.0. Monosized spherical gel powders were obtained in all the ratios of C/Si of gel powders.
Gel powder with the C/Si ratio of 3.5 became monosized spherical SiC powders of submicrometer size by heat-treating at 1450℃ for 4 h in an Ar atmosphere, but could not retain its spherical shape above 1550℃. This SiC powder was porous secondary particle composed of primary particles of nanometer size.
Gel powder with the C/Si ratio of 4.5 became monosized spherical $Si_3N_4$ powder by heat-treating at 1500℃ for 4 h in a $N_2$ atmosphere and monosized spherical SiC powder at 1550℃. This result is generally agreed that SiC is thermodynamically more stable than $Si_3N_4$ above certain critical temperature between 1500℃ and 1550℃. Obtained $Si_3N_4$ powder was porous secondary particle composed of primary $Si_3N_4$ particles and free carbon of nanometer size.
$Si_3N_4$ powder obtained by heat-treating gel powder with the C/Si ratio of 4.5 at 1500℃ for 4 h in a $N_2$ atmosphere, became monosized spherical $SiC/Si_3N_4$ composite powder by heat-teating at 1400℃ for 2 h in an Ar atmosphere. This $SiC/Si_3N_4$ composite powder was also porous secondary particle composed of primary $Si_3N_4$ and SiC particles of nanometer size.