Gradient Pore Structure Control And Performance Study Of Silicon Carbide Ceramic

SiC porous ceramics have many excellent characteristics,such as low density and thermal expansion coefficient,high thermal conductivity,good chemical stability and thermal shock resistance,etc.,which makes them widely used in food processing and petroleum concentration,high-temperature flue gas purification,wastewater treatment and other fields.However,most of the current preparation methods use particle accumulation or adding pore formers to create pores,which will cause the pore structure of the material to be uneven,the pore size is not easy to control,and the connectivity of the pores is poor,which severely limits the application of SiC porous ceramics in filtration and separation.In this thesis,the controllable design of the directional gradient pore structure was realized by adjusting the preparation process of the freeze-drying method,and then the feasibility of the application of SiC porous ceramics with the directional gradient pore structure in the field of filtration and separation was discussed.In addition,the reinforcement effect of C_f/SiC composites on SiC porous ceramics was also studied.The main results of this thesis are as follows:(1)Aiming at the problem that the freeze-drying method requires the slurry to have good fluidity and suspension stability,the influence of different solid content,p H value and the addition amount of TMAH on the characteristics of SiC slurry was discussed.Finally,it was determined that the slurry has better fluidity and stability when the solid content of the slurry is within 30～40vol%,the p H value is between10～11 and the addition amount of TMAH is 1.0wt%.(2)The effect of freeze-drying preparation process on the structure and properties of SiC porous ceramics was studied.The decrease in freezing temperature will reduce the pore size of the ceramic material and increase the strength.The solid content increased from 30vol%to 40vol%,the porosity of SiC porous ceramics decreased from71.3%to 63.2%,the pore size gradually decreased,and the strength increased from0.47MPa to 5.44MPa.The increase of the sintering temperature will cause the SiC particles to transform from the 6H crystal type to the 4H crystal type,resulting in a decrease in the pore size and a transformation of the pore structure from an oriented orderly to a random network structure;the optimal sintering temperature is 2100℃.The increase in the content of coarse particles makes the hierarchical structure of SiC porous ceramics more significant.It can be used as a ceramic membrane for the removal of suspended solids and colloidal particles;when the amount of coarse particles is 90wt%,the pure water permeability of SiC ceramic membrane is5.67×10~5L/(m~2?h?bar),and the rejection rate of ground calcium carbonate with an average particle size of 6.17μm is 96.1%.(3)C_f/SiC composite reinforced materials were prepared by CVI-PIP process,and the effect of different C_f/SiC composite additions on the microstructure and properties of SiC-based porous ceramics was studied.When the addition amount of C_f/SiC composite is 1.5wt%,the porosity is 60.08%,and the compressive strength reaches4.29MPa,which is 15%higher than the strength of the sample without C_f/SiC.