Study On Preparation Methods?Microstructure And Properties Of Porous Mullite From Coal-series Kaolin

In this paper,we employ coal-series kaolin as raw materials preparing porous mullite thermal insulators,and the solid waste coal-series kaolin is recycled.We investigate preparation methods including space holder,foaming and freeze casting,microstructure and properties of porous mullite basically.The mullite phase appears after sintered at 1300-1500?in air for 3 hours from coal-series kaolin.Unreacted alumina and silicon dioxide remain after sintered at1300?.By increasing the sintering temperature up to 1400-1500?,the products present a single mullite phase almost,indicating further enhanced mullite reaction.The space holder method has advantages of simple and cheap,but disadvantages of pores distribute nouniformly and low porosity,which limts mullite's application.The space holder method conducted using carbamide as space holders.The mullite foams possess porosity of 60.2-19.8%,compressive strength of 0.4-49.8MPa and bulk density of 0.61-1.22g/cm3.Foaming process conduct high porosity and low thermal conductivity,however,it affected by foaming agent content,solid loading and gellant types easily.We prepare mullite with porosity of 63-82%,compressive strength of 1.2-13.7MPa when foaming agent content varies form 0.1wt% to 0.6wt% at the 45wt% solid loading.Keep foaming agent at 0.3wt% and control solid loading 20-40wt%,the porosity is74-91% with compressive of 0.2-7MPa.Mullite foams fabricated by starch based foaming method possess porosity of 71.8-88.2%,thermal conductivity of0.10-0.38W/m·K and compressive strength of 0.21-8.7MPa.Freeze-casting results in unidirectional pore structure and smaller pores usually.The freeze-cast mullite foams possess porosity of 60.2-83.4%,thermal conductivity of0.17-0.34W/m·K and compressive strength of 5.6-49.4MPa.TBA solvent results in unidirectionally aligned channels,while water develops cross-linked lamellar pores.Lower porosity and higher compressive strength are obtained by using TBA as solvent.Besides,lower freeze temperature benefits smaller pore structure,as-198? and-40? freeze temperature result in about 20?m and 100?m respectively.Large pore and small pore co-exist in as fabricated mullite foams by using three methods above-mentioned.Large pores originates from carbamide particles,gas bubbles and sublimation of solvent respectively.Small pores deriving from the package of mullite crystals.Besides,moderate pores appear in pore walls by starchbased foaming method,which result from the burnout of of starch particles.The moderate pores rise the connectivity of porous ceramics,which is potential and preferred in biomaterials and filtering separation.