Freeze Casting and Freeze Drying
FREEZE CASTING: Lead by Team Member Theo Woodson
Freeze casting is a fabrication process in which a suspension of solid and liquid particles is frozen to create unique microstructural features. As the solvent, or liquid phase, in the material is frozen, the solidifying growth front separates the solid phase particles, segregating and templating the particles into a distinctive pattern. After the suspension is fully solidified, the frozen solvent is removed via sublimation. Sublimation is the direct transformation of a material from the solid phase to the vapor phase, without passing through the liquid phase. When the solvent is fully removed, a porous structure remains where the ice was formed. The goal of freeze casting, with regards to solid oxide fuel cell research, is to develop a porous layer that will allow gas to easily diffuse into the cell, increasing its efficiency and power output. The four basic steps in the Freeze Casting include: Material Preparation, Filling the Mold, Freezing, and Freeze Drying. Figure 1 shows the mold setup during freeze casting and freeze drying stages.
Preparation of the suspension includes mixing the ceramic materials, solvent and any other additives, adjusting pH levels and fine tuning slurry rheology. This mixture is called slurry, slip, or ink and are considered complex multiphase systems. After the slurry has been mixed, it is injected into the mold, frozen, and then sublimated to create a green body. The green body is sintered to burnout unneeded additives and to agglomerate the ceramic powders. The green body will act as the anode support layer, mechanically supporting the cell and providing gas delivery pathways, which will be further processed with electrolyte and cathode layers to complete the fuel cell.
Filling the Mold is an important step during freeze casting as the material must be processed by a freezing medium in order to solidify. Many different techniques are being studied for this procedure to best optimize the results of experimentation. Once the mold is filled, freezing can begin.
In the freezing step, the water or solvent is frozen into the slurry. During this step the solvent is solidified in order to template the ceramic material. Many process parameters affect the resultant microstructure including freezing temperature, casting time, freezing rate, solids loading and others. The freezing step is very important in developing the microstructure as the material is not able to revert form once frozen and the manner in which the solvent solidifies will dictate the formation of pores within the structure. After the freezing stage is complete, the mold is transferred to a freeze drying apparatus to remove the frozen solvent.
Sublimation occurs when a solid material transforms into a gas, skipping the liquid phase. Through sublimation, we are able to remove the solvent crystals from the slurry, leaving behind an empty space. This void is what gives the material porosity for gas diffusion. In the freeze drying step, pressure and temperature are controlled in accordance with the material’s phase diagram in order to sublimate the frozen solvent. Pressure is controlled by a vacuum, and temperature is controlled by a recirculating chiller. By controlling the freezing conditions and process parameters, the crystal growth of the solvent can be tailored and manipulated into different architectures to perform useful tasks thus demonstrating the usefulness of the technique. Images of developmental microstructures is shown in Figure 2.
Images showing microstructure development and acicular patterns
The research aim of this project is to enhance Tubular Solid Oxide Fuel Cell (T-SOFC) performance by decreasing gas diffusivity resistance and therefore increasing triple phase boundary (TPB) reactions. The TPB in SOFCs is where the gas, electric and ionic phases meet and electricity is converted. Enhancing the TPB will increase fuel cell performance and increase commercial viability. Freeze casting as a manufacturing technique can be used for many other purposes such as bone and dental implants, turbine blade manufacturing, permeable filtration membranes and many others. Freeze casting is considered to be a technique that uses minimally harmful chemicals and environmentally benign processing methods.
