| Silicon oxycarbide(Si-O-C)porous ceramic have been used in thermal insulation,adsorption catalyst,electrode dielectric,high temperature filter and other fields,and the application prospect was broad due to its thermostability,low density,high specific strength,high specific surface area and excellent dielectric properties.In this thesis,silicone resin was used as preceramic polymer and derieved to Si-O-C porous ceramics.The composition and micostructure were analyzed by SEM-EDS,FT-IR,Raman and NMR,and the propertise were tested.The main contents and conclusions show as follows:(1)Silicone resin RSN-6018 were selected and the ceramicization process and its structure evolution mechanism was clarified.The result shows that RSN-6018 was the most suitable as preceramic polymer for the preparation of Si-O-C porous ceramics because it can be self-foaming to abtain a uniform porous body at low temperature.The Si-O-C ceramics could be formed by RSN-6018 cross-linking,curing and pyrolysis.(2)After pre-curing of part silicone resin,the effect of pre-cured silicone resin on the structure and properties of Si-O-C porous ceramics was studied.The result shows that the pore structure changed from the through hole to the particle "lap" through hole and then into the particle accumulation hole with the increase of the content of precured silicone resin,The free carbon content in the matrix pyrolysates increased with the increase of the amount of pre-cured silicone resin.The addition of pre-cured silicone resin could reduce the linear shrinkage of Si-O-C porous ceramics and improve the ceramic yield,the bulk density decrease,but porosity increase.And the compressive strength was between 27.9 and 17.5 MPa.(3)The effect of pyrolysis temperature and holding time on the microstructure and properties of Si-O-C porous ceramics have been studied.The results shows that: when the content of pre-cured silicone resin is 95 wt%,the free carbon content in the Si-O-C porous ceramic composition decreased,however,[SiO4],[SiOC3] and [SiC4] structural units increased with the increase of the pyrolysis temperature.SiO2 was the main phase.When the content of pre-cured silicone resin is 70 wt%,with the increase of the pyrolysis temperature,the pore size of porous ceramics decreased.The free carbon and[SiO4] structural units decreased,however,[SiC4] increased in the porous ceramic composition with the increase of the pyrolysis temperature.The crystsallization degree of cristobalite was higher then its when the pre-cured silicone resin is 95 wt%,andβ-SiC was generated at 1400 ℃.The structure of free carbon changed from amorphous to graphite nanocrystalline.With the increase of the pyrolysis temperature,the ceramic yield of Si-O-C porous ceramics decreased,linear shrinkage increased and the porosity was between 29% and 32%.the compressive strength of porous ceramic was the highest to 28.3 MPa at 1300℃.(4)When the Si-O-C porous ceramic is pyrolyzed by at 1400 ℃,the pore size decreased,the pore density increased and the pore size was thinner with the prolonging of the holding time.The nanowires in the pores were roughly straight and twisted.Morphology,When the holding time is prolonged,the top of the nanowires have liquid beads.The prolongation of the holding time contributes to the transformation of the quartz and β-SiC crystals,and the structure of free carbon first changes from the amorphous to the graphite nanocrystalline,and then the degree of graphitization gradually increases.When holding 2 h,the linear shrinkage of Si-O-C porous ceramics was the highest,the ceramic yield and bulk density were the smallest,and the porosity was 26.9%,and the compressive strength decreased from 90.8 MPa to 62.8 MPa. |
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