Preparation, High Temperature Mechanical And Insulation Thermal Properties Of Elongated Mullite Self-reinforced Porous Ceramics Via Foam-gelcasting Method

Porous ceramics are widely used as thermal insulators, gas/liquid filters, purification separation, catalytic supports, sound absorption materials, biomaterial and sensor materials. In this thesis, whisker/elongated mullite were synthesized via in situ solid-reaction method using Al₂O₃, SiO₂ or mullite(Al₆Si₂O₁₃) powders as raw materials, AlF₃·3H₂O and TiO₂ respectively as additives. Based on thermodynamic analysis of the reaction synthetic process, the effects of reacting temperature, types and contents of additives and reacting time on the synthesis of whisker/elongated mullite were investigated. According to density functional theory?DFT? calculation results of adsorption energies of AlF₃?g? and SiF₄?g? adsorbed on various surface of mullite, the growth mechanisms of whisker/elongated mullite were also proposed. Then, elongated mullite self-reinforced porous ceramics were prepared by foam-gelcasting technique. The stability, rheological properties and gelling behaviors of mullite slurries were investigated. The effects of foaming agent contents, AlF₃·3H₂O and TiO₂ contents, sintering temperature, and self-bonded phase contents on the mechanical properties were examined. The relationship between the high-temperature mechanical properties, thermal shock behaviors, thermal conductivity of porous mullite ceramics and temperature were discussed, and the insulation thermal properties of porous mullite ceramics were also predicted by ANSYS. The results indicated that:?1? The largest value of length and aspect ratio of mullite whiskers prepared using Al₂O₃ and SiO₂ powders as raw materials, AlF₃·3H₂O as additives at the reacting temperature of 1673 K for 5 h were 99.0 ?m and 54, respectively. Mullite whiskers were formed through vapor-solid reaction mechanism. Based on the results of DFT calculation, adsorption energies of AlF₃?g? and SiF₄?g? on various facet of mullite crystal followed the sequence: Eads?001?>Eads?111?>Eads?210?>Eads?120?>Eads?110? for AlF₃?g? and Eads?001?>Eads?111?>Eads?120?>Eads?110?>Eads?210? for SiF₄?g?, which indicated that superior adsorption of AlF₃?g? and SiF₄?g? on?001? facet rather than other surface promoted mullite whisker growth along?001? facet. Kinetic studies demonstrated that the growth index of mullite whisker over the length and diameter directions were 3.3 and 7.2, respectively.?2? Elongated mullite was synthesized via in situ solid-reaction method using mullite powders as raw materials, AlF₃·3H₂O and TiO₂ respectively as additives. The average length of elongated mullite synthesized at 1873 K for 5 h was respectively about 22.3 ?m and 27.5 ?m for the sample with 4 wt% AlF₃·3H₂O and 7 wt% TiO₂.?3? Porous mullite ceramics were prepared via foam-gelcasting technique using mullite powders as the main raw materials, Isobam-104 as the dispersing and gelling agent, triethanolamine lauryl sulfate as the foaming agent, and sodium carboxymethyl cellulose as the foam stabilizing agent. Even though the porosity of porous ceramics was as high as 76.0%, their compressive strength and flexural strength still remained up to 15.3 MPa and 3.7 MPa, respectively. The calculation model of thermal conductivity, i.e. for porous mullite ceramics with the porosity of 67.0⁷6.9% at various temperature was proposed based on the measured results.?4? Elongated mullite self-reinforced porous ceramics were respectively prepared via foam-gelcasting method using AlF₃·3H₂O?0⁸ wt%? and TiO₂?0⁹ wt%? as additives. The pore size and pore distribution of porous ceramics were obviously reduced with appropriate content of AlF₃·3H₂O, and their mechanical properties were significantly enhanced. The flexural strength of elongated mullite self-reinforced porous ceramics with the porosity of 67.0% had the highest value of about 13.9 MPa, the value was about 27% higher than that porous ceramics without elongated mullite.?5? The displacement deformation of as-prepared porous mullite ceramics at 298⁸73 K decreased firstly, then increased, and finally fractured under a cyclic compressive stress of 50 N-600 N-50 N. The high-temperature modulus of rupture?HMOR? of as-prepared porous mullite ceramics rises first, followed by a decrease with the increasing of temperature. HMOR of elongated mullite self-reinforced porous ceramics were much higher than the porous ceramics without elongated mullite at identical temperature. At 1473 K, HMOR?6.7 MPa? of elongated mullite self-reinforced porous ceramics was about 13.6% higher than that porous ceramics without elongated mullite?5.9 MPa?. Even at 1673 K, the HMOR of elongated mullite self-reinforced porous ceramics still remained as high as 2.8 MPa.?6? Self-bonded mullite porous ceramics were prepared at 1873 K for 2 h via foam-gelcasting using Al₂O₃ and SiO₂ as self-bonded phase. A large number of elongated mullite was generated in porous ceramics with addition of AlF₃·3H₂O, and thus the mechanical properties of porous ceramics were significantly improved. The compressive strength and flexural strength of elongated mullite reinforced self-bonded porous ceramics with porosity of 71.1% reached up to 28.2 MPa and 11.5 MPa, respectively, which were obviously superior to that self-bonded porous ceramics without elongated mullite with porosity of 70.3%?compressive strength of 11.6 MPa and flexural strength of 6.5 MPa?.