ECE PhD Dissertation Defense: Qifan Li

PhD Dissertation Defense: Development of Magnetodielectric Materials with Low Loss and High Snoek’s Product for Microwave Applications

Qifan Li

Location: Teams Link

Abstract: Exhibiting both relative magnetic permeability and electric permittivity greater than unity, magnetodielectric materials have been attracting great attention in both academia and industry for next-generation communication, sensing, and radar applications. It is always of great interest for researchers to tailor the magnetic properties of magnetodielectric materials for high permeability, low magnetic loss and large Snoek’s product towards higher-frequency applications.
Hexagonal ferrites form an important group of magnetodielectric materials. Besides the six best known hexagonal structures, i.e., M-, W-, X-, Y-, Z- and U-type hexaferrites, some unique hexagonal structures, named 18H hexaferrites, were discovered in 1970s. For the first time, the dynamic magnetic properties and their temperature dependence of polycrystalline Mg-Zn 18H hexaferrites at microwave frequencies are investigated. Owing to a remarkably low damping coefficient, the frequency dispersion of complex permeability reveals a narrow and strong resonance. The Mg-Zn 18H hexaferrites show excellent loss tangent of 0.07 at 3 and 4 GHz. Accordingly, narrow FMR linewidths in the range of 486-660 Oe are measured. The temperature dependence of the damping coefficient is 0.0004 /°C, indicating a small variation of the intrinsic loss with temperature. These results are the best performance among the polycrystalline microwave ferrites reported so far for the S- and C-band applications.
Magnetodielectric composites, prepared by dispersing magnetic particles homogenously in an electrically insulating matrix, are another type of magnetodielectric materials. It is crucial to predict the effective magnetic properties of the multi-phase mixture. A modified effective medium theory is proposed by extending the traditional formulas with the effects of particle-size distribution and clustering of inclusions. Its accuracy is verified by two kinds of magnetodielectric composites over wide ranges of both particle concentration and frequency.
The magnetic properties of microwave ferrites are strongly affected by their polycrystalline microstructure, which is mainly controlled by the sintering process. The two-step sintering technique is systematically studied for the preparation of hexaferrites. With optimal combinations of sintering temperatures in each step, significant reduction in magnetic loss and enhancement in Snoek’s product are achieved with uniform and fine-grained structures.
Precise measurement of broadband permeability and permittivity is crucial to develop advanced magnetodielectric materials. A straightforward, explicit and noniterative method is proposed by eliminating the error from the direct measurement of sample position in the standard Nicolson-Ross-Weir method. Based on the results from two kinds of magneto-dielectric materials measured in two sets of test fixtures of different geometries, this method is theoretically and experimentally proven to have high and position-independent accuracy over a wide frequency range.
Finally, a patch antenna on Mg-18H magnetodielectric substrate is designed to operate at 3.6 GHz for 5G wireless communication. Benefiting from the large refractive index of the magnetodielectric material, the size of the patch antenna is significantly reduced. Moreover, compared to the dielectric substrate providing the same miniaturization factor, magnetodielectric antennas exhibit significant advantages for larger bandwidth and gain.