Study On The High-frequency Magnetic And Dielectric Properties In Ferrite Materials

With the rapid development of electronic information and communication technologies, especially the development and popularization of the internet of things technology and RF microwave technology, there is increasing need for further improving the miniaturization, lightweight and integration of electronic devices. Thus, the multifunctional magneto-dielectric information materials become an important research field. The magneto-dielectric information materials refer to the materials that having both high magnetic and dielectric properties. It is well known that the magnetic and dielectric properties are two mainly physical properties demanded in electronic devices, so various devices can be fabricated by using one kind of magneto-dielectric materials. This will make it more easily to obtain good matching and high-density integration among various devices, and thus promote the miniaturization and integration of electronic devices and systems.Recently, ferrite has become a research focus for application as magneto-dielectric materials due to its both high magnetic and dielectric properties as well as high resistivity. Especially, with the development of electronic devices and systems toward high frequency, it is very necessary to investigate the high-frequency magneto-dielectric properties of ferrites. In this dissertation, NiZn spinel ferrite, Z-type barium hexaferrite(BaCo-Z), and M-type barium hexaferrite(Ba M) were investigated, respectively, in order to modify their high-frequency magneto-dielectric properties. Research were also done to explore the key factors and physical mechanism that influence the magneto-dielectric properties, and to develop ferrites with excellent magneto-dielectric properties for application in relevant devices.Firstly, a series of NiZn spinel ferrite/BaCo-Z hexaferrite diphase composite ferrites have been fabricated by solid-state reaction method. It was found that the composite ratio of Ni Zn and BaCo-Z had obvious effects on the microstructures and sintering properties. The composite ferrites exhibit tunable magnetic-dielectric properties and lower loss than that of undoped NiZn and BaCo-Z. According to the effective medium theory, it was found that the Maxwell-Garnett model is not suitable to analyze the effective permeability for the composites with obvious change of grain size. However, the modified effective permeability formula by a variation factor was found to be valid, which indicates that the permeability of the composite ferrites is closely related to their microstructure. Based on the composite ferrite, a low loss ferrite magneto-dielectric material with almost equal μ’ and ε′in VHF band was successfully fabricated. The miniaturization factor is about 15 when the magneto-dielectric composite was applied as antenna substrate.Secondly, Ni Zn ferrites doped with nano-sized ZnAl2O4(ZA) were synthesized. It was found that ZA can fully solve into NiZn to form single-phase cubic spinel structure, which changes the crystal structure, microstructure and magneto-dielectric properties. BaCo-Z doped with ZA were fabricated and their magneto-dielectric properties were studied in the frequency range of 0.1?10 GHz. With the addition of ZA, the stable section of μ’ spectrum was extended over 1 GHz, and ε′ exhibit stable values in 0.1?10 GHz. When ZA addition is up to 3 wt%, an excellent magneto-dielectric material was obtained: tanδμ<0.15, tan δε is in the order of 10-3, μ’ and ε′ have almost equal values at 1 GHz. By using the magneto-dielectric substrate in UHF band antenna, the antenna miniaturization factor could achieve 8 with good impedance-matched to free space.Thirdly, the magnetic permeability and dielectric permittivity of BaM sintered at 920°C with different additive amount of Bi2O3 were studied in Ka-band frequencies(26.5~40 GHz). The results show that, with the increasing Bi2O3 from 0 to 5 wt%, there is not much difference in the permeability values, and μ’ is in the range of 1.1 ~ 1.6. However, the dielectric constant ε′ is improved from ~9 to ~18. Besides, the low-temperature sintered BaM exhibit lower dielectric loss at Ka-band frequencies compared with that sintered at high temperature.Finally, a measurement system for measuring the ferromagnetic resonance(FMR) linewidth ?H of microwave ferrites at X band and Ka band was designed and built. The system can operate automatically and measure ?H on multi-frequency points at Ka band. The building work includes design and fabrication of cavity test fixtures, design and fabrication of mechanical platform, system assembly, building microwave network and debugging, and developing testing software. Thereafter, the system achieved the measurement of ?H at 9.56 GHz in X band and at 27.75 GHz、31.04 GHz、34.30 GHz、38.25 GHz, respectively in Ka band. It was proved that the system exhibits obvious testing phenomenon and reliable test results. Also, FMR curves can be obtained by the system. This system is very useful for the high-frequency ?H measurement.