Preparation,Mechanical And Insulation Thermal Properties Of Hierarchically Porous Mullite Ceramics Prepared Via Foam-gelcasting Method

Hierarchical porous materials enjoy the advantages compared to the conventional porous materials including high porosity,elaborate pore structure,good chemical inertness,low bulk density,large pore volume and surface area,and have been widely applied in thermal insulating materials,sensors,catalyst supports,etc.In this thesis,elongated mullite was synthesized via in-situ solid-reaction method using V₂O₅ as additive,and the effect of reacting temperature and additive contents on the synthesis of elongated mullite were investigated.Then,elongated mullite self-reinforced porous ceramics were prepared by foam-gelcasting technique.The effect of additive contents on physical properties and insulation thermal properties of elongated mullite self-reinforced porous ceramics were studied.The hierarchical porous mullite ceramics?macro-mesoporous?were synthesized via foam-gelcasting combined with pore former addition method using mullite(Al₆Si₂O₁₃)powders as raw materials,ZrSiO₄,submicron-sized CaCO₃ and SiC respectively as additives,The stability,rheological properties and gelling behaviors of mullite slurries were investigated.The physical properties and insulation thermal properties of porous ceramics were also investigated.The results indicated that:?1?Addition of V₂O₅,ZrSiO₄,CaCO₃ and SiC had no clearly effect on the stability,rheological properties of the mullite slurries;Furthermore,the addition of V₂O₅ and CaCO₃ accelerated evidently the slurry gelation process.?2?Elongated mullite was synthesized via in situ solid-reaction method using mullite powders as raw materials,and V₂O₅ as additive.The average length and aspect ratio of elongated mullite were respectively about 8.5?m and 3.5 for the sample with7wt%V₂O₅.?3?Elongated mullite self-reinforced porous ceramics were prepared via foam-gelcasting combined with in-situ solid-reaction method using V₂O₅?0⁴wt%?as additive.The flexural strength of elongated mullite self-reinforced porous ceramics had the highest value of about 13.9MPa,which value was about 45%higher than that porous ceramics without elongated mullite.In addition,the thermal conductivity of samples was 1.04W·m^-1·K^-1at 473K.?4?Mullite porous ceramics with 77%porosity were fabricated via foam-gelcasting technique using ZrSiO₄?0⁸wt%?as additive.Addition of ZrSiO₄ had no evidently effect on the pore size,pore distribution and physical properties of the porous ceramics,but significantly reduced their thermal conductivity.?5?Hierarchical porous mullite ceramics were synthesized via foam-gelcasting combined with pore former addition using Al₆Si₂O₁₃3 powders as raw materials,and CaCO₃?0⁴wt%?as additive.The average pore sizes of the porous ceramics were obviously reduced with addition of CaCO₃,and their mesopore volume were significantly improved.As-prepared porous mullite ceramics respectively showed high porosity of 72.98⁷9.07%,and high flexural strength and compressive strength of2.7MPa and 5.5MPa.Most importantly,they had low thermal-conductivity of 0.114W m^-11 K^-11 at 473K.?6?Porous mullite ceramics with closed porosity of 1.2².7%were synthesized via foam-gelcasting combined with pore former addition way using Al₆Si₂O₁₃3 powders as raw materials,and CaCO₃?0⁴wt%?as additive.As-prepared porous mullite ceramics with porosity of 69.9%showed lowest thermal-conductivity and higher flexural strength and compressive strength under the addition of adding 4wt% CaCO₃.