Study On Preparation And Pore Structure Of Porous Ceramic Microbeads By Dripping-freeze Casting

Porous ceramic microbeads have a high specific surface area,chemical stability,excellent high temperature properties and are easy to recycle,therefore they have a wide range of applications in the fields of adsorption,filtration and catalyst carriers.In this paper,mullite（3Al₂O₃·2Si O₂）and alumina（Al₂O₃）were chosen to prepare stable low-viscosity ceramic slurry,and porous ceramic microbeads were prepared by freeze-forming combined with drip method to explore the mechanism of regulation of their pore structure.The porous mullite beads prepared by the drip-frozen forming method have good sphericity and a uniform and controllable particle size.The internal pore structure of the porous ceramic microbeads has a"dandelion"like structure,with laminar pores radiating from the centre of the sphere to the surface in an orderly manner.The solid phase content increases from10 vol%to 30 vol%and the porosity,pore size and whisker aspect ratio of the porous ceramic decrease.The pore size decreases with decreasing freezing temperature,and when the freezing temperature decreases from-25°C to-196°C,the pore size decreases and the pore wall thickness increases.Furthermore,it was found that:the morphology of mullite grains after sintering in open and closed crucibles was significantly different,with short rod-like and whisker-like structures respectively,mainly due to the volatilisation of Mo O₃ and Si F₄ at high temperatures which inhibits the generation of high aspect ratio mullite whiskers.Porous alumina microbeads were also prepared by the droplet-freeze forming method.And the internal pore structure of the microbeads was radially distributed,but with a clear concentric layered structure.As the solid phase content increased from 10 vol%to 30 vol%,the porosity of the porous alumina microbeads decreased from 79.63%to 42.51%and the pore size decreased from 10μm to 3μm.In addition,as the solid phase content increases,the grain size of some of the sintered grains increases and the uniformity decreases.This is because the pore pinning effect of samples with high solid phase content decreases during sintering and grain boundary migration between grains is more likely to occur to form large grain.In another study,porous ceramic beads were prepared using pristine Al₂O₃powder of different grain sizes.It was found that the smaller the grain size,the easier it was to be dislodged by ice crystals during the cooling process,giving it a higher cumulative density and denser pore walls after sintering.The two-step sintering method can obtain grains with smaller particle size and good uniformity.