Preparation Of Mullite-based Porous Ceramics By Particle-stabilized Foam Method

Mullite-based porous ceramics posess high temperature resistance,corrosion resistance and low thermal conductivity,etc.,so they are often used as thermal insulators.With the development of science and technology,higher requirements for the comprehensive performance of light weight,heat insulation and strength has been put forward.At present,the commonly used preparation technology has certain deficiencies.The particle-stabilized foam method is a new process for preparing lightweight high-strength materials developed in recent years,but its research on the preparation of mullite porous ceramics was rarely reported.In this paper,Al₂O₃ and Si O₂ are used as raw materials,propyl gallate（propyl gallate,PG）is used as the surface modifier,and gelatin is used as the binder.A series of mullite-based porous ceramics were prepared by the combination of particle-stabilized foam method,freeze-drying and pressureless sintering.The effects of the ratio of aluminum to silicon,sintering aids and the introduction of anorthite powders on the composition,structure and performance of the materials were studied respectively.The research of aluminum to silicon（Al/Si）molar ratio in the raw material ratio shows:The increase of aluminum-silicon ratio is beneficial to the improvement of foam stability after ceramic foaming,up to 96.59%.As the Al/Si ratio increases from 2.0:1.0 to3.0:1.0,the mullite phase gradually decreases,and the total porosity and volume density change range is not large,the values fluctuate between 85.51～86.51%and 0.44～0.48g/cm³ respectively;The compressive strength firstly increases and then decreases,the variation range is 2.25～2.86 MPa;The vacuum thermal conductivity increases from 0.15W/（m·K）to 0.22 W/（m·K）.Sintering aid research shows:Using Al F₃·3H₂O as additives,the mullite grains in the prepared materials with the highest porosity（86.05%）are long columnar shape,and a maximum mullite phase content of 99.20 wt.%is achieved.The mullite phase content is the lowest with the addition of Ca Cl₂,but the liquid phase introduced by it tightly connects the mullite particles,thus the materials has the highest compressive strength6.31 MPa（total porosity is 83.02%）.Inside and outside the samples by adding Mn O₂,the composition and structure is very different,and the process controllability is poor.Using Ca Cl₂/Al F₃·3H₂O composite additives,a special rope-like structure occures in the skelton.As the amount of Ca Cl₂ in the composite additive increases,the rope-like structure becomes tighter.During the growth process,there are both liquid-solid（LS）and gas-solid（VS）mechanisms.This special structure makes the compressive strength of porous ceramics significantly higher.The samples with a total porosity of 79.11%have the high compressive strength of 10.23 MPa.Porous anorthite/mullite ceramics can be fabricated through the particle-stabilized foam method using anothite,Al₂O₃ and SiO₂ as raw materials.The prepared materials show lower thermal conductivity than porous mullite ceramics with similar porosity.Studies have shown that sintering temperature has a significant effect on the phase composition,microstructure and properties.The phase composition,micro-morphology and properties of the multiphase materials change abruptly beteen 1350℃ and 1400℃.As the sintering temperature increases from 1300℃ to 1500℃,the open porosity of samples is 75.12～86.44%,the bulk density is 0.41～0.73 g/cm³,the compressive strength is 1.09～15.18 MPa,and the thermal conductivity 0.17～0.35 W/（m·K）.