Preparation,structure And Properties Of Ultra-high Porosity Porous Ceramics

Porous ceramics are widely used in metallurgy,chemical engineering,aerospace,and other fields due to their high temperature resistance,corrosion resistance,and chemical stability as well as the advantages of low density,low thermal conductivity,high specific surface area,controllable pore size and good connectivity conferred by their porous structure.For example,they can be used as catalyst supports due to their high specific surface area,as fluid filters due to their high porosity and interconnected pores,or as thermal insulation materials for aerospace applications due to their low density and low thermal conductivity.Porous ceramics with porosity higher than 90%are generally referred to as ultra-high porosity porous ceramics,which exhibit lower density,better permeability,and superior insulating properties and have become a hot research topic.Porous ceramics are commonly used as filters for molten metals.Zirconia-based ceramic filters are mainly used to filter high melting point and high-temperature alloys due to their high temperature resistance and good thermal shock resistance.They can effectively remove inclusions from liquid metals,improving the properties of metals significantly.However,they cannot meet the requirements of precision casting of high-temperature alloys because they are limited by low porosity,low high-temperature strength,poor thermal shock resistance of domestic products,and mainly relied on imports from abroad.Therefore,research on the preparation technology of domestically produced ceramic filters to solve this bottleneck and step up production is important.This work focuses on the aforementioned problems and mainly involves the following research results:Preparation of ceramic filters using partially stabilized zirconia as the raw material through organic foam impregnation method.Firstly,two different methods of directly grafting surface-active agents and grafting polymers onto polyurethane sponge templates after alkali treatment was used respectively to modify the hydrophilicity of the polyurethane sponge template resulting in increasing the slurry loading amount.The results showed that the two methods effectively improved the hydrophilicity of polyurethane sponge.The effect of different dispersing agents on the stability of the slurry was tested,and it was found that ammonium citrate,ammonium poly（acrylate）,and poly（etherimide）as dispersing agents can effectively improve the stability of the slurry.It was found that slurry with 75 wt%solids content has high solid content and low viscosity,which was the optimal solid content;in addition,the slurry hanging process was improved,and the second slurry hanging process was used to improve the mechanical properties of the samples.Partially stabilized zirconia foam ceramics were prepared using polyurethane foam templates with pore densities of 10PPI and 15 PPI respectively and consequently achieved ultra-high porosity of 85.6-91.2%and density of 0.51-0.84 g/cm³ and compression strength of 3.01-7.21 MPa,making them ceramic filters with superior comprehensive properties.Porous ceramics also play an important role in thermal insulation.With the rapid development of hypersonic aircraft,their flight speed is increasing,leading to surface temperatures of the aircraft reaching over 2000℃.Traditional insulation materials cannot meet the requirements of ultra-high temperature insulation,and thus new lightweight and ultra-high temperature insulation materials need to be developed.Ultra-high temperature ceramics are currently the materials with the highest melting point and the best temperature resistance,but they suffer from high intrinsic thermal conductivity and high density.This study utilizes a multi-element solid solution and super-high porosity strategy,introducing lattice distortion and porous structure to solve the above problems,and thus develops lightweight and super high-temperature ceramics for ultra-high temperature insulation.To achieve the goal of lightweight and insulation,Ti C,Zr C and Nb C carbide ceramics powders with low density and relatively low thermal conductivity were chosen as raw materials,and a foaming-gelcasting-freezing drying method was used to prepare the super-high porosity medium entropy carbonitride.Performance comparison with single-component porous Nb C ceramics showed that the prepared porous medium entropy(Ti_1/3Zr_1/3Nb_1/3)C ceramics possess ultra-high porosity（91.3-95.1%）,low density（0.31-0.55 g/cm³）,high strength（1.09-4.95 MPa）,and low thermal conductivity（0.141-0.191 W/（m·K））,meeting the requirements of lightweight,high strength,and thermal insulation.Silicon carbide as a sintering additive can effectively promote the sintering of the ultra-high temperature carbide ceramics,form a dense pore wall,improve the mechanical properties of the samples,and significantly improve the oxidation resistance of the material.In addition,compared with single-component porous Nb C ceramics,it was found that the strength of the ternary porous(Ti_1/3Zr_1/3Nb_1/3)C ceramics was higher and thermal conductivity was lower under the same porosity,which was attributed to solid solution strengthening and lattice distortion caused by multi-element solid solution,significantly improving the properties of the material.This study shows that the prepared super-high porosity medium entropy(Ti_1/3Zr_1/3Nb_1/3)C ceramics are potential as superior ultra-high temperature insulation materials.