Based on the chemical cross-linking method,this paper uses polydimethylsiloxane with various viscosities of 10 cSt,20 cSt,50 cSt,and 100 cSt to synthesize mesoporous and macroporous SiOC ceramics.Their thermal conductivities are measured by using 3ω method with high accuracy.Three typical models for their thermal conductivities,i.e.,series model (SM),maxwell-Eucken 1 model (ME1),and effective medium theory (EMT) model,are utilized to derive the empirical formula through the multi-parameter linear optimization algorithm,which agrees well with the experimental results.The effects of pore size and specific surface area on the overall thermal conductivity of the porous structure are explored.Interestingly,it is found that the thermal conductivities of both gas phase and solid phase inside the porous structure increase with the increasing pore size at the nanometer scale,but the overall thermal conductivity of the porous structure decreases with the increasing pore size.Scanning electron microscopy graphs corroborate that the extension of the heat transfer route and the barrier of more pores between the solid phases together cause the reduction of the gas-solid coupling thermal conductivity of SiOC ceramics with larger pore size.On the contrary,the miniaturization of individual particles through modulating the synthesis parameters can increase the number of small pores in the sample itself to meet the pseudo-lattice vibration conditions,which results in the increment of the gas-solid coupling thermal conductivity and the overall thermal conductivity of the porous structure.These findings would provide meaningful guidance for designing SiOC porous ceramic super-insulation materials with extremely low thermal conductivity.