Freeze Casting | College of Aeronautics and Engineering | Kent State University

The Freeze Casting project is led by Research Team Member Theo Woodson.

photo Theo Woodson headshotThe goal of freeze drying is to perfect a porous tube that will allow hydrogen gas to easily diffuse into a fuel cell, increasing its efficiency. There are many steps to Freeze drying including: Preparation, Freezing, Primary Drying, and Secondary Drying.

Freeze casting is a wet shaping fabrication process in which a suspension of solid and liquid particles is frozen to create unique and controllable microstructural features. As the solvent or liquid phase in the material is frozen, the solidifying growth front mechanically separates the solid phase particles, segregating and templating the particles into a unique and 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 by sublimation, a porous structure remains where the ice or solvent particles once were. The goal of freeze casting, with regards to solid oxide fuel cell research, is to develop a porous substrate that will allow gas to easily diffuse into the cell, increasing its efficiency and power output. The four basic steps in Freeze Casting include: Material Preparation, Filling the Mold, Freezing, and Freeze Drying.

The Material Preparation step includes mixing the ceramic materials, solvent and any other casting or processing additives, adjusting pH levels and fine tuning slurry rheology. This mixture is called Slurry, Slip, or Ink depending on consistency, and is considered a complex multiphase system. After the slurry has been mixed, it is injected into a mold, frozen, and then sublimated to create a green body. The green body is sintered to burn out unneeded additives and to agglomerate the ceramic powders. The green body will act as the anode layer, which will be further processed with electrolyte and cathode layers to complete the fuel cell.

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 determining 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.

photo Fuel Cell Research freeze casting operation
Freeze casting operation

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 in order to optimize the results of experimentation. Once the mold is filled and freezing occurs, freeze drying can begin.

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 empty space 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.

photos microstructure development and acicular patterns
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 produced. 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 a technique that uses minimally harmful chemicals and environmentally benign processing methods.