Controllable Preparation,Microstructure And Properties Of Porous Y-Si-O Ceramics With Lightweight And High Strength

Porous ceramics exhibit many excellent properties, such as high porosity, high specific strength, low density, and low thermal conductivity. Moreover, an important aspect of the applications for porous ceramics is high-temperature thermal insulators. However, the common porous ceramics used as thermal insulation materials (such as porous SiO2, Al2O3, YSZ, and mullite) have some shortcomings, for instance, low strength and low operating temperature for porous SiO2, high thermal conductivity for porous Al2O3, serious linear shrinkage and high density for porous YSZ, and low strength for porous mullite. Thus, new materials with better overall performances are highly demanded. In this dissertation, γ-Y2Si2O7and X2-Y2SiO5have the excellent merits of high melting point, very low thermal conductivity and good thermal and chemical stability; therefore, they are chose as the matrix materials of new-style porous ceramics. Porous materials are prepared by polymeric sponge impregnation method and in situ reaction sintering foam-gelcasting method, and the microstructures of samples are characterized in detail. Then the relationships between the microstructure and properties of porous samples (compressive strength and thermal conductivity) are investigated, as well as high-temperature properties.Pure-phase porous γ-Y2Si2O7ceramic with high porosity is firstly fabricated by polymeric sponge impregnation method and in-situ reaction synthesis process using Y2SiO5powders and silica sol as starting materials. The as-prepared sample is a kind of white, open-cell, and reticulated porous ceramic. Moreover, porous sample has interconnected pores with sizes ranging from200to700μm and the uniform strut. By increasing the number of recoating times, the strength of samples could be greatly enhanced. Porous γ-Y2Si2O7ceramic pretreated by PEI possess better properties than others, for instances, the compressive strength, porosity and density are1.28MPa,84.5%and0.62g/cm3, respectively.Porous γ-Y2Si2O7ceramic with multiple pore structures is successfully fabricated by in situ reaction sintering foam-gelcasting method using Y2O3and SiO2powders as starting materials and N-hydroxymethyl acrylamide as monomer. The sample has large pores with the size of70～250μm and small pores in skeleton with the size of0.1-1.5μm. The formation of multiple structures are generated from the in situ reaction sintering process and the unique structure retains excellent thermal stability to1700℃. In addition, porous γ-Y2Si2O7ceramic with high performance is synthesized by in situ reaction sintering foam-gelcasting method using the environmentally friendly non-toxic gelatin as gelformer. Samples have good mechanical strength and thermal conductivities. When the porosity is64.3～89.3%, the corresponding compressive strength of sample is46.5～3.4MPa. Porous γ-Y2Si2O7shows low thermal conductivity of0.918～0.147W/(m·K) with the porosity of57.2～90.0%at room temperature and excellent mechanical property at high temperature, as well as low sintering shrinkage under heat circulation. The as-prepared porous γ-Y2Si2O7has the merits of light-weight, high porosity, relatively high specific strength, low thermal conductivity, and good thermal stability.Through studying the effects of preparation paths and sintering temperatures on the microstructure and properties of porous Y2SiO5by foam-gelcasting method, the reason why the reported porous Y2SiO5ceramics have very large shrinkage is found: using fine YaSiO5powders as starting materials, which sintering activity is very high; and low solid content of slurry. The solution is proposed subsequently, which is to fabricate porous Y2SiO5by in situ reaction sintering foam-gelcasting method with high solid content of slurry and using Y2O3and SiO2powders as starting materials. The as-prepared porous Y2SiO5ceramic has good overall properties, such as the low shrinkage of1.0～4.7%, low bulk density of0.79～0.88g/cm3, high porosity of82.1～80.1%, and high strength of3.54～6.90MPa with the sintering temperature ranging from1350to1550℃. Besides, the sample also has low thermal conductivity and excellent phase and thermal stability at high temperatures.High-performance porous Y2SiO5ceramic is successfully prepared by in-situ foam-gelcasting method using non-toxic gelatin as gelformer. The relationships between the structure and properties (strength and thermal conductivity) of porous Y2SiO5are also investigated, as well as high-temperature properties. The resultant porous sample has unique multiple pore structure and interconnected three-dimensional net-work structure. Samples exhibit excellent overall properties, including the high porosity of60.7%～88.4%, low linear shrinkage of6.3～4.5%, low density of1.74～0.52g/cm3, and high compressive strength of38.2～0.90MPa. Porous Y2SiO5sample possesses low thermal conductivity of0.513～0.119W/(m·K) with the porosity of60.2～87.1%at room temperature. Porous Y2SiO5still has excellent thermal stability and strength at high temperature.In this work, a controllable preparation method is developed for two kinds of porous Y-Si-O ceramics. By adjusting the technological parameters, the pore sizes or porosities of samples could be controlled, and then the porous ceramics with different properties would be obtained. The fabricated porous Y-Si-O ceramics have the advantages of light weight, low thermal conductivity, high porosity, high sterngth, good thermostability, and good mechanical properties at high temperatures, which indicates that they are well-suited for use as the promising high-temperature thermal insulator.