The Effect of Frequency on Electromagnetic Field – Material Interactions

This thesis aims to improve the understanding of electromagnetic (EM) field – material interactions, with the end goal of introducing frequency as an experimental variable during processing of ceramics. Work towards this goal has been achieved through three main projects. The W-band EM characterization of Al2O3 explored the critical role that polarization mechanisms have on EM field-material interactions. The dominant polarization mechanisms in the W-band for Al2O3 were identified as electronic (~32%) and ionic (~68%) polarization. This is a step towards understanding the role of frequency in EM field-material interactions, especially at higher frequencies. Specific focus was then applied to the ionic polarization mechanism, which was studied by exploiting the dielectric anisotropy in the Al2O3 unit cell. By rotating and heating the Al2O3 sample with respect to the electric field direction, the strength of the ionic polarization was adjusted. The results of the W-band characterization were also used to develop a framework that can relate the polarizability of orientations to their dielectric permittivity at W-band frequencies and tune these permittivity values in a rapid and granular fashion by rotating or heating the sample/device.

Sintering of additively manufactured (AM) ceramics using various frequencies of microwave radiation (MWR) gave insight into the microstructure and how it is changing during EM field processing. This showed how to exploit EM fields at different frequencies to achieve processing goals (e.g., synthesis of metastable phases, sintering and crystallization at temperatures below those required in conventional processing). Through densification, microhardness, and microstructural characterization, the most effective method was determined to be MWR sintering using a frequency of 2.45 GHz. Split ring resonator (SRR) preliminary experiments showed exciting results on how to use SRRs as microscale tunable EM field or plasma generators/devices. By using an electrochromic powder, a proof-of-concept experiment proved that the volume of an SRR gap can be loaded with a powder and irradiated by an electric field with negligible thermal loading. Exploring these phenomena can help the EM field processing community at large in understanding the powerful role that frequency can have as an experimental variable during processing.