Stdudy Of Technology And Theory Of Porous Ceramics By Heavy Gas-gelcasting Method

Porous ceramics with high porosity(porosity>80%)not only have the special advantages of high hardness,high temperature resistance and corrosion resistance of ceramic materials,but also has the characteristics of high specific surface area,controllable aperture shape and size,which has become an important dual-use material for military and civilian use,such as broadband wave-transparent radome for aerospace and chemical industry catalyst carrier and so on.Therefore,it is of great significance to study the porous ceramic preparation technology for its wider application.In view of the shortcomings of difficult sintering and serious pollution in the rigid template preparation technology widely used in the industry,the paper innovatively proposed a new preparation technology,namely heavy gas-gelcasting,for the preparation of porous ceramics by using basic principle of integrated free radical polymerization,gelcasting,surfactant stirring foaming and heavy gas upward exhaust collection based on the compressive deformation and flowability,and releasing the internal stress of the porous ceramic body during drying and sintering for the gas template.The Al2O3 and SiC porous ceramic materials with high porosity were successfully prepared and discussed the pore formation theory through investigating the influencing factors of agitation foaming equipment and foaming,and exploring the technological process of preparing porous ceramic materials by heavy gas-gelcasting technology.The obtained results were as following,1.The as-invented heavy gas-gelcasting method can be used to prepare porous Al2O3 ceramics,porous SiC ceramics and other porous ceramic materials,which are characterized by simple process,fast gel speed,thorough air isolation,uniform pore size distribution and controllable size of porous ceramics.2.Heavy gas-gelcasting technology is suitable for the controllable density preparation of porous Al2O3 ceramics with a density ranging from 0.54 g/cm3 to 1.18 g/cm3.The preparation of the porous Al2O3 ceramics is a remarkable performance,with a minimum density of 0.54 g/cm3,the compression intensity of 30 MPa,the dielectric constant of 1.7 and the dielectric loss of 10-4 orders of magnitude.3.Mg(NO3)2 can inhibit the growth of Al2O3 grain,while TiO2 can promote the growth of Al2O3 grain.Utilizing the difference between the two,Mg(NO3)2 and TiO2 composite sintering agent was used to promote the directional growth of porous Al2O3 ceramic grain.The directional growth and control model of Al2O3 ceramic crystal was proposed.4.The microstructure of porous SiC ceramics prepared by heavy gas-gelcasting technology is honeycomb shape,the pore size with about 100 ?m,and the thickness of the pore wall from 4 to 8 ?m.The pore wall is mainly composed of SiC and a small amount of Si,in which Si is distributed on the surface of the pore wall and SiC exists in the interior of the pore wall.High temperature vacuum can be used to remove the excess Si on the pore wall surface,and nitrogen can be used to convert the residual Si dispersed in the pore wall into Si3N4,which can reduce the content of Si in the matrix to 4.8%.5.During studying porous SiC ceramics prepared by heavy gas-gelcasting technology,special equipment for Si and C reaction was designed.Using Si and Cu composite powder as raw material,finally showedthe minimum temperature of Si and C reaction is between 1200?1250 ?,which provides an evidence for the analysis of Si and C reaction mechanism.6.The mechanism research of heavy gas-gelcasting technology showed that foaming by stirring was the key to prepare porous ceramics.By adjusting the foaming rate of slurry,the closed and open ceramic green bodies can be obtained respectively.7.Based on the drying shrinkage model of rigid template and experimental analysis results,a drying shrinkage model of gas template was proposed,and the role of flexible gas template pore former in the drying process of porous ceramics was explained.