Preparation And Control Of Three-dimensionally Ordered Lamellar Alumina Porous Scaffolds

Porous ceramics are increasingly used in a variety of applications such as filters,thermal insulation,and energy devices because of their chemical stability,low thermal conductivity,and ionic activity,respectively,among other properties.Each application requires a unique set of pore characteristics,i.e.,porosity,pore size and morphology,which can be achieved by choice of the processing technique.Freeze casting,a sacrificial template method,is a simple technique that can be used to tailor pore networks.In this paper,we designed a dual-temperature gradient using a side metal plate and prepared a three-dimensional ordered layered Al₂O₃ porous ceramic by freeze casting method,revealing the temperature gradient control mechanism of the pore structure,and studying the solid content and initial freezing temperature.The influence of solid content,initial freezing temperature,thickness of side metal plate and material on pore structure was studied,and fine control of pore structure was achieved.The main research results obtained are as follows:?1?A dual temperature gradient freezing casting technique was designed with side metal plates.This technology has the advantages of simple device,adjustable temperature gradient and high refrigeration efficiency.Through this technology,large-size Al₂O₃ porous ceramics with three-dimensional ordered lamellar pore structures were successfully prepared.?2?The influence of initial solid content,initial freezing temperature and thickness of side copper plate on porous ceramic microstructure was elucidated.When the solid content was 15 vol.%,the layer was so thin that the layer deforms after sintering;when the solid content was 35 vol.%,the layer thickness increases significantly,and the layer structure disappeared at 25 mm away from the side metal plate.With the growth of ice crystals,the effect of initial freezing temperature and the thickness of the side metal plate on the pore characteristics gradually weakened.In the vicinity of the cooling surface,the ceramic particles were wrapped between the porous ceramic laminates with an initial freezing temperature of-60°C or a side copper plate of 6 mm.?3?The type of the side metal plate had a significant influence on the microstructure of the porous ceramic.With the decrease of the thermal conductivity of the side metal plates,the number of ceramic bridges between adjacent ceramic walls on the longitudinal section gradually decreased,and the length of the pores gradually increased.A parallel layer structure with a similar cross-section was obtained on the longitudinal section when a stainless steel plate was used as the cold surface.?4?In the three-dimensionally ordered lamellar Al₂O₃ porous ceramics,the dendritic structure was observed on the longitudinal section and the parallel lamellar structure was observed on the cross section.The significant difference of this pore structure was caused by the difference between the corresponding vertical temperature gradient?G_b?and the horizontal temperature gradient?G_a?.?5?The temperature gradient in the slurry played a significant role incontrolling the lamellar structure of the porous ceramic.When the temperature gradient was less than 0.30±0.06°C/mm,a dendritic structure with many ceramic bridges between the lamellae formed because the ceramic particles were squeezed by the adjacent ice crystals in the ice crystal growth direction during freezing.This structure was more prominent as the temperature gradient decreased.In contrast,a large-scale parallel structure of the ceramic lamellae evolved when the temperature gradient was greater than 0.40±0.04°C/mm.Between these two critical temperature gradients,the dendritic structure could coexist with the parallel structure.