Development of magnetodielectric materials with low loss and high Snoek's product for microwave applications

• Online Access: http://hdl.handle.net/2047/D20398272

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, due to their moderate to high values of permeability and permittivity, high electrical and thermal resistivity, low magnetic and dielectric losses, and low fabrication cost. 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.As an important type of magnetodielectric materials, magnetodielectric composites are prepared by dispersing magnetic particles homogenously in an electrically insulating matrix. The effective medium theory, which has been widely applied to predict the macroscopic electromagnetic properties of multi-phase mixtures, is extended to include the effects of particle-size distribution and clustering of inclusions. The accuracy of the modified formulas is experimentally verified by two kinds of magnetodielectric composites over wide ranges of both particle concentration and frequency. The magnetic properties of microwave ferrites are closely related to their polycrystalline microstructure, which can be tailored through the sintering process. The two-step sintering technique is systematically studied for the preparation of hexaferrites. With optimal combinations of the sintering temperatures during the first and second steps, 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 and evaluate advanced magnetodielectric materials. A straightforward, explicit and noniterative method is proposed by removing the need for prior knowledge 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. Ferrites with hexagonal crystal structure, also known as hexagonal ferrites or hexaferrites, are another important type of magnetodielectric microwave 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, however, the dynamic magnetic properties and their temperature dependence of polycrystalline Mg-Zn 18H hexaferrites at microwave frequencies are reported. Owing to a remarkably low damping coefficient, the frequency dispersion of complex permeability reveals a narrow and strong resonance, and excellent loss tangent of 0.07 over 2-4 GHz. Narrow FMR linewidths in the range of 486-660 Oe are also measured. The temperature dependence of the damping coefficient is 0.0004 °C1, 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. One important application of magnetodielectric materials is for antenna miniaturization and bandwidth enhancement. Patch antennas on Mg 18H hexaferrite substrate are 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, the prototyped magnetodielectric patch antenna exhibits pronounced advantages for larger bandwidth and gain.