Fabrication And Characterization Of Highly Porous SiOC Ceramics From Silicone Resin

SiOC porous ceramics are widely used in many fields due to a number of favorable properties, and its manufacture methods are developing all the time. In this paper, the current state of porous ceramic has been summarized, and the fabrication of SiOC porous ceramics through silicone resin self-blowing process was studied.At first, the foaming ability of the two silicone resin DC217 and DC249 was compared and found the silicone resin DC217 was better as the self-blowing precursor. And then the highly porous SiOC ceramics were fabricated in a simple step manufacturing process, including mixing, molding, crosslinking and pyrolysis, using in situ foaming of different fillers loaded silicone resin DC217. The effects of heating rate, thermal pre-curing, exterior gas pressure, variety of the fillers (SiC, SiOC), filler content and the mean particle size of filler on the porosity, compressive strength and microstructure of the porous ceramics were investigated. The results are as following:With the heating rate increased from 0.25℃/min to 3℃/min, the total and open porosity of porous ceramics increased firstly and then degraded, while keeping the silicone resin content at 90vol.% and SiOC powders as fillers. The maximum total and open porosity of 88.2% and 72.5%, respectively, was obtained at the heating rate of 0.5℃/min. However, the compressive strength decreased progressively from 2.3MPa to 1.0MPa. The ceramic foams which cross-linked at the heating rate less than 1℃/min had a three-dimensional web and regular pore structures. Adjusting different thermal pre-curing procedure (160℃, 190℃, 220℃) in the self-blowing process has a biggish influence on the final pore structure. But it has no obvious improvement on the compressive strength.With the exterior gas pressure increased from 1MPa to 4MPa, the porosity (total and open) decreased progressively while keeping the silicone resin content at 70 vol.% and the SiOC powders as fillers. It was possible to control the total and open porosity of porous ceramics within a range of 58.3-69.8% and 43.9-58.4% respectively, by altering the exterior gas pressure. The compressive strength increased with the exterior gas pressure increasing, and the average compressive strength of the porous ceramics was in the range of 3.9-14.9 MPa. In addition, with the exterior gas pressure increasing, the final pore structure of porous ceramics became more and more regular and equirotal.With the filler content (SiOC) increased from 0vol.% to 30vol.%, the porosity (total and open) increased firstly and then decreased. However, the compressive strength decreased firstly and then increased. The cell morphology of porous ceramics with the filler content less than 30vol.% have a well-defined open-cell and regular pore structure with spherical cells.Choosing the SiC powders as fillers, the cell morphology of porous ceramic cross-linked without exterior gas pressure was extraordinary irregular. However, the ones under a certain pressure (4MPa) have a well-defined and regular pore structure with spherical cells. Comparing with the porous ceramics which cross-linked under the same condition with the SiOC powders as fillers, the cell morphology was similar. However, the oxidation resistance of the former was better.With the mean particle size of SiC fillers increasing from 5μm to 10μm, the porosity (total and open) of the porous ceramic increased, and the compressive strength decreased. However, continuing increasing the mean particle size from 10μm to 15μm, the porosity, mean pore size and compressive strength of the porous ceramics showed no evident difference.The researchment showed that the method of fabricating SiOC porous ceramics through self-blowing process with silicone resin DC217 was simple and easy controlled. The porous ceramics with a higher porosity can be made by it. The process parameters can influence the forming process of bubbles in the polymer melt, which has nucleation, growth and stabilization. As the result, the final pore structure of the porous ceramics which we needed can be obtained by adjusting different process parameters.