Study On Preparation Of Porous Silicon Carbide-based Ceramics By Freeze-casting

SiC porous ceramics are widely used in environmental protection,energy,chemical,biological and other fields due to their high temperature resistance,large specific surface area,wear resistance and corrosion resistance.In this paper,the process of preparing silicon carbide-based porous ceramics by water-based freezing casting method is explored and optimized.Based on the design of the mold,the effects of different freezing conditions on the structure and properties of porous ceramics were investigated.The change of pore structure along the direction of temperature gradient was studied by X-ray micro-computed tomography.The porosity of the porous ceramics in different directions was calculated.The micro-element model of porous ceramics after three-dimensional reconstruction was combined.The connection between the pore structure and the sheet layer was analyzed.The research results are as follows:（1）Using the difference in thermal conductivity between metal and polymer,a mold that achieved uniform and directional freezing was designed.The finite element analysis was used to compare the temperature distribution of the mold and internal samples in the freezing process under different freezing conditions.The freezing rate of the two ends of the sample was calculated by the measured cooling curve.Under different freezing conditions,the simulated cooling curve had a high consistency with the measured cooling curve,which verified the feasibility and effectiveness of the design of the mold.（2）The effects of freezing rate on the structure and properties of SiC-based porous ceramics were investigated.The results showed that the main crystal phases of porous ceramics prepared from raw materials such as silicon carbide,alumina and Guangxi white clay were 6H-SiC,α-Al₂O₃,cristobalite and mullite.The pore size and pore wall thickness were both large under slow cooling conditions.The pore size was about 30μm,and there were porous structures overlapping each other on the pore walls.On the contrary,the pore walls of porous ceramics prepared under rapid cooling conditions were more compact.The pore size and the pore wall thickness were small,and the pore size was about 20μm.The porous ceramic prepared under the rapid cooling condition had higher porosity and lower compressive strength.（3）The effects of different freezing methods on the structure and properties of porous ceramics were studied.The results showed that the top and bottom of the porous ceramics prepared by bidirectional freezing had little difference in pore size and wall thickness.The pore arrangement and connectivity were better.The directional pore structure which was more uniform and ordered had the highest apparent porosity.The pore size and wall thickness of porous ceramics prepared by unidirectional freezing varied greatly from top,middle and bottom.The pore size and wall thickness increased from tens of microns at the bottom to hundreds of microns at the top,while the number of channels decreased.The pore structure of homogeneously frozen samples was disordered and the direction of lamellae was different,but the compressive strength was the highest.（4）The effects of freezing temperature on the structure and properties of unidirectional freezing porous ceramics were studied.The results showed that the pore size and thickness of the top,middle and bottom of the porous ceramics were almost the same when the freezing temperature was-25^oC,and the pore size was between 30μm and 40μm.When the freezing temperature is-196^oC,the pore size and wall thickness of the top,middle and bottom of the porous ceramics varied greatly.The pore size of the top is about 20μm,while the pore size of the bottom is reduced to 5¹0μm.With the decreased of freezing temperature,the pore size and wall thickness of porous ceramics decreased,while the number of pore channels increased.The porosity decreased from82.3%to 78.2%with the freezing temperature decreased from-25^oC to-196^oC.Compared with-25^oC,the compressive strength of porous ceramics freezing at-65^oC and-196^oC is higher.（5）The pore structure changes of porous ceramics prepared by unidirectional freezing were studied based on micro-XCT images.The results showed that the porosity increased from 53.62%to 57.4%along the direction of temperature gradient.The shape of porous ceramic sheets was very irregular and the surface fluctuated greatly.The connection between the sheets was very complex.In some places,two or three layers were connected by surface contact,and in others,only by point contact.