Alumina And Silica Porous Ceramics With Controlled Microstructures Prepared By Freeze-casting Technique

Porous ceramics as a technological important material have been widely applied in various regions, because they are lightweight, can withstand high temperature and corrosion, and exhibit high specific surface aera. In the past few decades, a large number of techniques are developed for the fabrication of such materials. However, all processing routes suffer from themselves inherent limitation: pore characteristics of ceramics which prepared by these techniques are limited to a narrow range. Freeze-casting technique, a manufacture method which uses growth and sublimation of vehicle crystal, possesses controllable microstructure and properties and environmental friendliness, and supplys a new route to prepare porous ceramics.In view of the new formation technique, a stability condition of solid-liquid interface was determined, combined with effect of particles on temperature fields and concentration fields of solidification system. Based on the interaction between the moving interface and a particle, critical velocity of trapped particle was evaluated. Using the criteria condition of interface instability and particle expelled from interface and unidirectional solidification way under a constant cooling rate, porous materials with aligned channel morphology were fabricated. From the perspectives of slurry components and freeze-casting technique investigated effects of formation technics on microstructure and properties of prepared materials.Experiment results indicated that mechanical properties of alumina porous ceramics might be improved by introducing glycerol into raw slurries. The effects of glycerol on the freeze-casting process and thereby on the microstructure and compression strength of porous ceramics obtained were investigated. It was shown that the addition of glycerol increased both the slurry viscosity and sample sintered density, promoting transition of macroporous structure and connection between ceramic lamellae. This connection made as-prepared porous ceramics obtain high mechanical properties. For the 30 vol.% alumina slurry with glycerol, the axial and radial compression strengths reached to, respectively, 255.1 MPa and 105.8 MPa.In addition to changing slurry components, the change of freeze-casting technique could also adjust the microstructure and properties of porous ceramics. For instance, the ceramic with complex 3D interconnected porous structures could be fabricated by a combination of impregnating and unidirectional freeze-casting technique. In the process, the polyurethane sponge was impregnated in the mold with 20 vol% of aqueous alumina slurry, and then the bottom of the cast body was kept at a constant cooling rate of 6°C/min to induce solidification. After drying and sintering of the green part, porous ceramic with obvious lamellar architectures was obtained. The lamellae thickness and interlayer distance were as large as ~9μm and ~15μm, respectively. The large pores, which resulted from the burn-up of sponge struts, were homogeneously distributed in the sample. The existence of these large pores introduced some local interruption of the lamellar structures. However, high compression strength for the porous ceramics could still be remained.In the unidirectional freeze-casting process, introduction of an electric field could also be used to prepare dense/porous bilayered ceramics. The thickness of dense layer in the sample could be controlled by tuning the electric field intensity. When the voltage was increased from 15 to 90 V, the dense layer thickness was increased from 51μm to 155μm, correspondingly. On the other hand, all samples exhibited an identical and good connective lamellar structure in the porous region. These lamellae thickness and interlayer distance were as large as ~14μm and ~24μm, respectively.Because of practical application of materials in catalysis field and adsorption field, hierarchically porous silica ceramics with unidirectionally aligned channel morphology were in-situ synthesized by mineralization of soluble starch monolith. The procedure followed two steps. First, soluble starch monoliths with well defined uniaxial macroporous structures were prepared by unidirectional freeze-casting process of starch hydrosol or starch hydrogel. Second, these prepared artificial monoliths which used as templates were soaked into a surfactant-templated sol-gel solution to mineralize aligned channel structures in their tissues. It was shown that the macropore size, wall thickness, and macropore morphology of obtained starch materials may be controlled by adjusting the concentration of starch slurry. When initial concentration changed from 10 wt.% to 15 wt.%, the macropore size of soluble starch materials decreased, while the wall thickness increased. Through the second step, porous silica ceramics were obtained after drying and calcining of organic-inorganic hybrid silicate compounds from the solution. These resulting products preserved templates microstructure in great detail and exhibited narrow macropore size distributions. It was observed when the soaking time increased from 36 h to 84 h, the average pore size of silica ceramics was decreased from 4.1μm to 3.6μm. In addition, in nanopore regime, all of these silica monoliths presented the conexistence of uniform worm-like nanopore and lamellar phase feature, and large BET and microporous surface area.