Research On Properties Of Silicon Nitride Based Porous Ceramic Prepared By Rice Husk At Low Temperature

The rice husks from agriculture waste were chosen as raw materials in this paper, After pretreatment, these rice husks were carbonized and then be used as silicon source,carbon source and pore forming agent of Si3N4 porousceramics, Sialon porous ceramics and Si2N2O porous ceramics) which were prepared directly by carbothermal reduction-pressureless reaction sintering at low temperature in nitrogen. The effects of carbonization process on properties of rice husk and The effects of Sintering temperature, C/SiO2, Al2O3 content and rice husk content on properties of silicon nitride based porous ceramics were studied. The porosity, density and flexural strength of sintered sample were measured by the Archimedes method, three points bending method. The phase composition and the microscopic morphology of sintered sample were characterized by XRD and SEM respectively.In this work the C/SiO2 of rice husk is 7.32, after pretreatment The C/SiO2 of carbonized rice husk was 0.4-0.9. The influence of C/SiO2 on the properties of porous ceramics were studied. The results revealed that a-Si3N4 phase and P-Si3N4 phase appeared in as-prepared samples when sintering temperature was in the range of 1450-1500℃ with C/SiO2 was 0.7. When sintering temperature was 1550℃, only P-Si3N4 appeared in samples, phase transition temperature was about 200 ℃lower than the literature records, the porosity and flexural strength of the sample prepared were 38.48% and 83.40 MPa respectively, and porous distributed uniformly with the size was about 2 μm. Also, β-Si3N4 phase grow in cluster with aspect ratio was 6-8.For further research on sintering additives affecting the performance of silicon nitride based porous ceramics, The porous Si3N4 ceramics were prepared with carbonized rice husk C/SiO2 was 0.7 and α-Si3N4 as the main raw material, changing the content of Al2O3. The results showed that in the range of 1450-1550℃ with Al2O3 content increased, the porosity of the sintered sample decreased, flexural strength and oxidation resistance improved. When Al2O3 content was 6%-9%, phase change process from a to β was completed at 1500℃, the main phase of sample was β-Sialon at 1550℃. When Al2O3 content was 6%, the porosity and flexural strength of β-Sialon porous ceramics was 35.98% and 103.75 MPa respectively. Also, with a good oxidation resistance.The Si2N2O powders were successfully synthesized when the C/SiO2 was 0.3, the content of Si3N4 was 5%, the sintering temperature was 1300℃ in the nitrogen gas. the median diameter D50 of Si2N2O powders was 3.50 μm. The Si2N2O porous ceramic were fabricated successfully using carbonized rice husk with C/SiO2 was 0.3 and Si3N4 as the main raw materials, at the sintering temperature ranging from 1450℃ to 1500℃.The influence of rice husk content on the properties of Si2N2O porous ceramics were studied. The results revealed that with the increase of rice husk content, the linear shrinkage, mass loss, porosity of samples increased, the flexural strength reduced. The main phase of porous ceramics was Si2N2O, the flexural strength was 67.31 MPa when sintering temperature was 1500℃ and rice husk content was 80%.