Crystallographically textured polycrystalline ceramics refer to non-cubic materials consisting of crystallites that are more or less ordered relative to their crystallographic axes. Textured materials can be engineered to possess anisotropic magnetic, electrical, mechanical, optical, and other properties. Permanent magnets are a prominent example of crystallographic texturing. The development of texture through the conventional ceramic powder metallurgy relies on the significant and anisotropic magnetic susceptibility of these substances. Prior to the die compaction by either uniaxial or isostatic pressing, powder particles are suspended in a solution and then rotated magnetically so that their easy axes become parallel to the applied magnetic field. Crystallographic texturing of ferrimagnets allowed for the creation of a self-biased circulator - a passive non-reciprocal device that does not need an external biasing magnetic field to operate.
At the core of the present study is the development of crystallographycally textured polycrystalline aluminum oxide. The focus is on its optical properties, namely the transparency, mostly in the infrared band. Diamagnetic alumina has a rhombohedral structure that results in the optical and magnetic anisotropy. Therefore, similarly to permanent magnets, alumina can be textured magnetically. However, its weak magnetic anisotropy requires the fields in excess of 10 Tesla and makes this method unpractical.
It was found that micron-sized, single-crystalline alumina platelets with large shape anisotropy can be embedded in a matrix of much finer alumina particles and aligned by either sheer forces or by a low magnetic field.
The latter is based on the shape anisotropy rather than on the anisotropic magnetic susceptibility, and achieved with the help of an iron oxide coating. The texture then develops during the subsequent sintering as a result of the templated grain growth.
The second half of the study is dedicated to the devices and methods that have been initially designed to characterize the properties of the materials discussed above, but later evolved as independent projects. The measurements of the dielectric permittivity, loss tangent, and ferromagnetic resonance linewidth are discussed.
Advisor: Professor Vince Harris
Professor Vince Harris
Professor Yongmin Liu
Dr. Yunume Obi