No abstract

Freeze casting under external fields (magnetic, electric, or acoustic) produces porous materials having local, regional, and global microstructural order in specific directions. In freeze casting, porosity is typically formed by the directional solidification of a liquid colloidal suspension. Adding external fields to the process allows for structured nucleation of ice and manipulation of particles during solidification. External control over the distribution of particles is governed by a competition of forces between constitutional supercooling and electromagnetism or acoustic radiation. Here, we review studies that apply external fields to create porous ceramics with different microstructural patterns, gradients, and anisotropic alignments. The resulting materials possess distinct gradient, core–shell, ring, helical, or long-range alignment and enhanced anisotropic mechanical properties.

Research is conducted into freeze-casting of surface-magnetized Fe 3 O 4 particles under uniform, low-strength magnetic fields (5.2 mT) to mimic the mechanical characteristics of natural human bone. Freeze-casting is a technique that fabricates porous materials by directionally freezing and sublimating an aqueous slurry. A novel, Helmholtz coil-based freeze-caster is developed and it is shown that, during freeze-casting, the use of this Helmholtz coil generates a more uniform magnetic field than permanent magnets. This uniform magnetic field, applied in the direction of ice growth, keeps particles from agglomerating and results in an increase of 55% in both the ultimate compressive strength and the elastic modulus of porous surfacemagnetized Fe 3 O 4 scaffolds. These increases can be linked to a reduction in the porosity that occurs due to magnetic interactions between particles in the presence of the field. These results offer a novel method for the fabrication of bone-inspired biomaterials and structural materials.