Studies On The Gelcasting Process Of Porous Sillicon Nitride Ceramics

Porous silicon nitride ceramic has the excellent overall properties.However,when used for molding of curved-surfacethin-wall construction,the traditional molding method of this material has many drawbacks such as inferior flexibility,non-uniform distribution of pore size,and poor mechanical properties.These limit its application and further development.Based on the principle of stability and uniformity of aqueous slurries for aqueous gelcasting of silicon nitride ceramic,this thesis introduces the hydrosoluble low-viscosity organic solution and thus develops the silicon nitride slurry with high-fraction solid phase,low viscosity,and excellent stability.By selecting the materials of sintering aid and pore former and studying their effects on ceramic,we develop the technology of property modulation of porous silicon nitride ceramic,and achieve the fabrication of porous silicon nitride ceramic with the density of 1.2g/cm³,porosity of 66.5%,and bending strength of?75MPa.This thesis analyzes the reaction of silicon nitride powder in the water and solves the issues,induced by a mass of degassing due to the hydrolysis of powder,of poor stability and non-uniform distribution of pore size in green body and ceramic.We study the effects of process parameters such as pH value,dose of organic solution and dispersing agent,content of solid phase,milling time,e.g,and achieve the optimized process parameters as follows:pH value of?9-10?tuned by the additive TMAH,the dose of?0.8-1.0?vol%for dispersing agent?PMAA-NH₄?,the milling time of 24 hours,the dose of?0.2-0.4?vol%for initiator,and the curing temperature of?50-800??.This thesis optimizes the parameters of body-curing,desiccation,baking in the process of gelcasting forming of porous silicon nitride ceramic,analyzes the mechanism and controllable technology of the transition from?-Si₃N₄ to?-Si₃N₄,investigates the correlation between the porosity of silicon nitride ceramic and its mechanical properties,and achieves the key technology of high mechanical strength at the high porosity by controlling the ratio of length to diameter of?-Si₃N₄columnar grain.We also measure the mass density,bending strength,and microstructure of material and find that the density and bending strength increases with the increasing fraction of solid phase,the increasing dose of pore former reduces the bending strength of material,and the bending strength decreases due to the larger pore diameter and its non-uniform distribution.According to these laws mentioned above,we recognize that the diameter and the ratio of length to diameter of?-Si₃N₄ columnar grain,the number of overlapping joints per unit area,diameter of pore,and the uniform distribution of pores are the key parameters to determine the mechanical properties of the material.This thesis achieves the property-controllable fabrication technology of aqueous gelcasting of porous silicon nitride ceramic,and can accurately control the density?1.75⁰.96?g/cm³,porosity?52⁷2.5?%,and bending strength?202⁴0?MPa of porous silicon nitride ceramic by tuning the concentration of pore former?0³0?wt%.