Date of Award
Doctor of Philosophy (PhD)
Freeze casting is a physical process for the fabrication of porous anisotropic materials. In this method, an aqueous slurry of ceramic particles is frozen directionally, creating lamellar columns of ice that push particles between the growing crystals. Then, the frozen material is lyophilized to remove the ice and sintered to densify the ceramic. Scaffolds made by freeze casting often have significant strength in the solidification direction, while they lack sufficient strength in the transverse direction. To enhance strength in the transverse direction, magnetite particles are added to a slurry of paramagnetic particles, and an external magnetic field is applied during solidification. Interactions between the magnetite and paramagnetic particles compete with thermal and viscous forces, resulting in different colloidal behaviors. Under relatively weak magnetic fields, the particles are attracted to one another, forming aligned chains that are trapped by the ice front and result in bridges spanning the lamellar walls. When interactions between magnetite and paramagnetic particles are strong, the alignment of magnetite also results in alignment of the paramagnetic particles. Under stronger magnetic fields, however, a gradient magnetic force attracts particles toward the field’s poles, creating biphasic regions of iron-rich and iron-poor microstructures.
To further investigate the relationship between microstructure and mechanical properties observed, 3D printed scaffolds mimicking patterns observed in magnetic freeze casting were designed and fabricated for comparison. The 3D printed scaffolds were tested in compression in three orthogonal directions. To compare their performance, permutated radar charts were used to simultaneously analyze the strength, toughness, resilience, elastic modulus and strain to failure across each orthogonal direction.
Niksiar, Pooya, "Fabrication and Mechanical Properties of Micro-Architectured 3D Scaffolds" (2018). All Dissertations. 2570.