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 conduct...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 3ωmethod 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.展开更多
SiOC-based ceramics are considered promising electromagnetic wave-absorbing materials because of their lightweight,high-temperature resistance,and heat insulation properties.Herein,SiOC@C ceramic nanospheres were prep...SiOC-based ceramics are considered promising electromagnetic wave-absorbing materials because of their lightweight,high-temperature resistance,and heat insulation properties.Herein,SiOC@C ceramic nanospheres were prepared using a liquid-phase method combined with a polymer-derived ceramic(PDC)method,followed by heat treatment in N_(2) and Ar atmospheres at different temperatures.The morphology,microstructure,phase composition,and electromagnetic wave absorption performance of the SiOC@C ceramic nanospheres were investigated in detail.The SiOC@C ceramic nanospheres obtained in the Ar atmosphere showed a minimum reflection loss(RL_(min))of−67.03 dB,whereas the SiOC@C ceramic nanospheres obtained in the N_(2) atmosphere exhibited an RLmin value of−63.76 dB.The outstanding electromagnetic wave absorption performance of the SiOC@C ceramic nanospheres was attributed to the synergistic effect between conductive loss,interfacial/defect polarization loss,multiple reflections,and scattering.Therefore,this research provides valuable insights into the design and fabrication of SiOC ceramic-based electromagnetic wave absorbers.展开更多
基金This work is financially supported by Beijing Natural Science Foundation(3202020)National Natural Science Foundation of China(No.51876008)+2 种基金Beijing Nova Program(Z201100006820065)Interdisciplinary Research Project for Young Teachers of USTB(Fundamental Research Funds for the Central Universities)(FRF-IDRY-19-004)WU Jin acknowledgesfinancial support from Guangdong Natural Science Funds Grant(2018A030313400).
文摘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 3ωmethod 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.
基金supported by the National Natural Science Foundation of China(No.52360018)the Shiyanjia Lab(www.shiyanjia.com)for the support of thermogravimetric(TG)tests.
文摘SiOC-based ceramics are considered promising electromagnetic wave-absorbing materials because of their lightweight,high-temperature resistance,and heat insulation properties.Herein,SiOC@C ceramic nanospheres were prepared using a liquid-phase method combined with a polymer-derived ceramic(PDC)method,followed by heat treatment in N_(2) and Ar atmospheres at different temperatures.The morphology,microstructure,phase composition,and electromagnetic wave absorption performance of the SiOC@C ceramic nanospheres were investigated in detail.The SiOC@C ceramic nanospheres obtained in the Ar atmosphere showed a minimum reflection loss(RL_(min))of−67.03 dB,whereas the SiOC@C ceramic nanospheres obtained in the N_(2) atmosphere exhibited an RLmin value of−63.76 dB.The outstanding electromagnetic wave absorption performance of the SiOC@C ceramic nanospheres was attributed to the synergistic effect between conductive loss,interfacial/defect polarization loss,multiple reflections,and scattering.Therefore,this research provides valuable insights into the design and fabrication of SiOC ceramic-based electromagnetic wave absorbers.
