Directional Freeze Casting of Porous Ceramics: Effects of Processing Parameters on Pore Network Characteristics.

摘要

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, surface area, and tortuosity, 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. The process involves directional solidification of a slurry of ceramic particles and a liquid dispersion medium. During solidification the particles are ejected from the freezing front and compacted between adjacent crystals. The frozen dispersion medium is removed via sublimation and the particles that remain are sintered to form a robust porous monolith. Pores within the structure are negative copies of the frozen dispersion medium. Changes in slurry composition and freezing conditions can greatly affect the pore network, potentially offering a high degree of tailorability.;In this work the influences of the dispersion medium, freezing conditions, and ceramic solids loading, mean particle size, and chemistry on the pore network were studied to provide guidelines for tailoring pore networks for desired applications. Freeze-cast samples were characterized using X-ray computed tomography to obtain three-dimensional data which were used to measure the pore network characteristics in both two and three dimensions.;The velocity of the freezing front as it progresses during solidification is known to influence pore size in freeze-cast samples. Temperature within the slurry was recorded in situ to determine experimentally the temporal evolution of freezing front position and velocity, and was compared to mathematical models predicting spatial and temporal temperature fields and freezing front position and velocity as functions of the freezing parameters. The models were based on the Stefan problem for solidification and were modified to match the forms of the temperature data, which varied between dispersion media, and account for the influence of the experimental setup.;Pore microstructures of the freeze-cast samples were likened to microstructural features that develop during freezing of traditional materials like pure metals and alloys, allowing application of classical solidification theory to freeze casting of porous ceramics. These theoretical models for traditional solidification scenarios provide a formalism to explore morphology-dependent relationships between freezing kinetics, i.e., freezing front velocity and temperature gradients, and feature size such as dendrite arm spacing or lamellar wavelength. Morphology of the freeze-cast samples, comparable to the lamellae and dendrites formed in solidified metals, dictated which form of the solidification theory models were applicable for each dispersion medium.;Feature size prediction models were combined with the Stefan problem mathematical models of freezing front velocity as a function of temperature parameters to predict pore size from freezing conditions. Relationships between temperature parameters and pore size were extended to specific surface area data through geometric considerations, and porosity and tortuosity were analyzed using the relationships between pore size and freezing conditions as a foundation.