Thesis Committee

Ulrike G.K. Wegst, Ph.D. (Chair)

Jifeng Liu, Ph.D.

Harold Frost, Ph.D.

Abstract

Highly porous materials with architectures that can be custom-designed for each application offer solutions to challenges posed by inadequacies in today’s technologies, such as an increasing demand for energy and the need for alternatives to fossil fuels, the growing population and its need for potable water, and increasing needs for biomaterials and medical devices that improve quality of life. A very promising technology for the manufacture of these highly porous materials is called freeze casting, a process that uses phase separation and controlled ice crystal growth to create, during solidification, well-defined pore sizes and networks in aqueous solutions and slurries. However, current freeze casting systems still lack in precision when it comes to ice nucleation and growth control to reproducibly achieve a particular material architecture. To achieve improved control and material structures, this research presented here focused on the design of a freeze casting system for the manufacture of materials with a monodomain texture. The chosen approach is immersion freeze casting which, instead of the more common cold finger, uses the controlled vertical motion of the sample into a freezing chamber to control ice nucleation and crystal growth conditions. Both experimental and computer models were employed in an iterative design of the equipment and process optimization that included different means of cooling, sample geometries, immersion speeds, and resulting freezing conditions. A great advantage of this freeze caster design is that, in addition to improved processing control, it also allows to use grain selection for the manufacture of a monodomain texture. After careful experimental analysis and the testing of the fundamental grain selection concepts in 2D, two critical parameters that control ice crystal growth dynamics were identified and tested also in 3D: the incident angle between ice crystal and mold wall, and a length-width ratio to isolate individual crystals. Applying the knowledge gained, an immersion freeze-casting prototype and a suitable grain selector were developed and tested successfully.  Freeze-cast alginate scaffolds show that a monodomain texture can be created with a high degree of reproducibility.