Microstructure And Properties Of Highly Porous Al₂O₃ Ceramics By Freeze-casting

Highly Porous ceramics have many merits, such as low density, high porosity, high temperature, moderate strength, low thermal conductivity and thermal stability in high temperature. They are extremely widely applied in the aerospace, high temperature insulation, active cooling, heat exchange, biomedical, catalytic filters and other fields.In this paper, high purity sub-micron alumina powders, camphene, Texaphor 963 were adopted as the ceramic substrate, the slurry solvent, the binder respectively. By the constant grinding, we obtained evenly dispersed alumina/camphene ceramic slurry. Using freeze-casting method, the ceramic green body was fabricated. After drying and sintering, we obtained highly porous alumina ceramics with three-dimensional network of pore structure. We analyzed the effect of the process conditions of freeze-casting method on the pore structure of porous alumina ceramics and thermal/mechanic properties systematicly. A reliable freeze-casting process condition was obtained, and the formation mechanism of the frozen camphene-based ceramic slurry crystal and the dense layer were discussed and analyzed. In the experiment, the Archimedes method, scanning electron microscopy, confocal laser microscopy, mercury method and compressive strength tests were used to test and characterize the pore shape, pore size, compressive strength, thermal conductivity and other properties.In this paper, the effect of freezing temperatures, sintering condition, initial solid loadings on the pore structure and mechanical properties of highly porous alumina ceramics were studied systematicly. The results showed that the dense layers of porous alumina ceramics became thicker in lower freezing temperature, and with the freezing temperature decreasing, the pore size increased; With the increasing of the sintering temperature, holding time and the initial solid loading, the porosity and density decreased resultly; Compressive strength improved with the sintering temperature, holding time and initial solid loading increasing respectively.In the freeze-casting process, the frozen crystal solidification of alumina/camphene slurry occurred. Camphene grew dendriticly, alumina particles were excluded from the camphene dendrite, distributing around the camphene dendrite. As the above crystallization mechanism, in the green body, the camphene dendrite area and powder particles accumulation area were found and analyzed. After the green body dried, camphene volatiled from the body, and the positions occupied by camphene were replaced by pores, forming a three-dimensional network pore structure. Because of the crystallization mechanism of the camphene-based slurry, highly porous alumina ceramics prepared by freeze casting had a three-dimensional interconnected pore structure.Highly porous alumina ceramics prepared by freeze casting had a thin dense layer.In the process of freeze-casting, because of apparent temperature difference between surface region and central region, camphene closed to the mold wall began to crystallize immediately, forming alined crystal. The ceramic particles in slurry closed to the mold wall were rejected to the mold center, forming a high solid-loading slurry layer.After green body dried,the camphene in the outermost layer evaporated completely. Through high temperature sintering, the surface layer was densified and formed a dense layer, while other regions in the sintered samples retained porous structure.