Preparation And Microwave Properties Of BST-based And FeCo-based Microwave Electromagnetic Materials

With the rapid development of wireless communications and electronics technology, microwave electromagnetic materials are increasingly demanding for miniaturization and low energy consumption of microwave devices. At the same time, the microwave devices with higher operating frequency, even reconfigurable frequency are developing very fast. In this dissertation, the preparation, tunable microwave dielectric performances of BST ferroelectric ceramics and microwave ferromagnetic properties of FeCo-based soft magnetic films(SMFs) were investigated. The calcined BST–MgO composites with high permittivity tunability, low leakage current and moderate dielectric constant were prepared by doping MgO and optimizing the sintering process. On the other hand, a well-defined uniaxial magnetic anisotropy is present in FeCoB films prepared by doping composition gradient sputtering(CGS) method. It is interesting to note that quasi-isotropy magnetic multilayers were obtained using the uniaxial magnetic anisotropy FeCo B layer as an unit cell via laminating six uniaxial magnetic anisotropy FeCoB sublayers, where the direction of the easy axis FeCoB sublayers were rotated 30° relative to that of the neighbor layer. This quasi-isotropy magnetic multilayer is especially suitable for the micro-magnetic inductors because it overcomes the limit of magnetic anisotropy of SMFs to the inductor shape. The major work of this dissertation was listed as follows:(1) The microwave performance of BST-MgO composites with various MgO doping ratio and their sintering process were investigated. The composite ceramic with nominal composition of 45.0 wt%(Ba0.5Sr0.5)TiO3–55.0 wt%MgO(acronym as BST-MgO) exhibits good microwave dielectric performances at X-band with appropriate dielectric constant ?r around 85, lower dielectric loss tan? lower than 0.01, and higher permittivity tunability 14.8% at 8.33 kV/cm. As a prototype, we fabricated a frequency reconfigurable aperture-coupled microstrip antenna using the BST-MgO composite slices to verifying the possible application of the tunable dielectric materials in frequency tunable antenna. It is exciting that an ultra-high electric field tunability of working frequency up to 11.0%(i.e., from 9.1 to 10.1 GHz with a large frequency shift of 1000 MHz) was obtained at a DC bias field from 0 to 8.33 kV/cm. at the same time, a considerably large center gain over 7.5 dB was achieved. These results demonstrate that BST-MgO composites are promising for the frequency reconfigurable antenna.(2) The micro-magnetic inductors are suffered from the limitation of magnetic anisotropy of high-frequency SFMs. It is desired for the isotropic high-frequency soft magnetic films. In this dissertation, we fabricated a quasi-isotropy magnetic(FeCoB/MgO)6 multilayers via laminating uniaxial magnetic anisotropy FeCoB sublayers. The FeCoB single layer exhibits a high anisotropy field HK of 400 Oe and ferromagnetic resonance frequency f FMR of 6.45 GHz. The 2 ? MgO separated(FeCoB/MgO)6 multilayers show a quasi-isotropic hysteresis loop and omnidirectional ferromagnetic resonance around 4.77 GHz. This quasi-isotropic multilayer provides flexible design for the micro-magnetic inductors since 100% hard-axis excitation can be achieved for inductors of any shape.