Study On High-performance Silica-based Porous Ceramics Prepared By Rice Husk

Rice husk,from agricultural waste,contains the main elements of C,O,H and Si,and low content of impurities.while the main component of rice husk ash is a highly active amorphous SiO₂.After a series of pretreatment,carbonized rice husk with specific C/SiO₂ were chosen as silicon source,carbon source and pore forming agent in this work.So high performance silicon-based porous ceramics?including porous P-Si₃N₄ ceramics,porous a-Si₃N₄ ceramics,porous silicon carbide ceramics and porous silicon oxynitride ceramics?were prepared by carbon thermal reduction method.Porous ?-Si₃N₄ ceramics were fabricated by low temperature pressureless sintering,using carbonized rice husk and ?-Si₃N₄ powders as raw materials,and MgO-CeO₂ as sintering additives.The effects of CeO₂ concentration and sintering temperature on phase composition,microstructure,porosity and flexural strength of the sintered products were investigated.The results suggested that MgO-CeO₂ was a much more effective sintering additive for Si₃N₄ than MgO alone.When CeO₂ concentration was 4-5 wt.%and sintering temperatures were in a range of 1500 to 1550?,the obtained porous silicon nitride ceramics had the porosity of 45.78%-42.81%and flexural strength of 49.44-84.09 MPa.Moreover,when sintering temperature was 1550? and CeO₂ concentration was 5%,large elongated ?-Si₃N₄ grains were well developed.With carbonized rice husk and a-Si₃N₄ powders as raw material,effect of kinds and content of sintering additives,content of rice husk on preparation and properties of porous a-Si₃N₄ ceramics by low temperature pressureless sintering were studied.The results suggested that when La₂O₃ content is 2-4%and sintering temperatures were in a range of 1400? to 1450?,the main crystal phase of the sample are well developed a-Si₃N₄.When La₂O₃ content is 4%,the sintering temperature is 1450?,the comprehensive performance of porous ?-Si₃N₄ ceramics is best with the bending strength of the maximum?60.45 MPa?,while open porosity is 26.84%,and the dielectric constant is 4.65.While open porosity of porous ceramics is obviously improved with the increase of rice husk content can,dielectric constant also increased sharply.With carbonized rice husk and SiC as raw materials,effect of sintering temperature and Al₂O₃ content on preparation and properties of porous silicon carbide ceramics by low temperature pressureless sintering were studied.Porous silicon carbide ceramics were successfully prepared at 1550?,and have a porous microstructure with the six-party shape ?-SiC grains.When Al₂O₃ concentration was 4-6%and sintering temperatures were in a range of 1500 to 1550?,the obtained porous SiC ceramics sintered at 1550? had the porosity of 27.88-30.35%and flexural strength of 40.67-42.49 MPa.Porous Si₂N₂O ceramics were synthesized via carbothermal reduction nitridation?CRN?method at a relatively low temperature using carbonized rice husk and ?-Si₃N₄ powders as starting materials,and NaF,MgO and Al₂O₃ as sintering aids.The influence of sintering temperature and carbonized rice husk content on phase composition,open porosity,flexural strength,microstructure and oxidation resistance of the sintered porous ceramics were determined.The results showed the Si₂N₂O phase increases while the Si₃N₄ phase decreases when the sintering temperature rose from 1400 to 1500?.The major crystalline phases were Si₂N₂O and few Si₃N₄ phase was detected in the product after sintering at 1450?.Porous Si₂N₂O ceramics with high flexural strength?31.84 MPa to 53.76 MPa?and relative high porosity?42.76%to 27.83%?were fabricated,when carbonized rice husk content is 70%to 80%.The SEM images showed that the obtained Si₂N₂O ceramics had uniform pore structure with typical plate Si₂N₂O grains.Moreover,porous ceramics showed excellent oxidation resistance.