Abstract
While various portable electrical equipments are designed to pursue for thinner and multifunctional characterizations, the undesired electromagnetic interferences (EMI) have become the seriously environmental disturbances that affect the functionalities of electric circuit in the nearby electrical equipments. These troubled electromagnetic interferences emitted from the different sources may result in malfunctions of the nearby electrical equipments due to misinterpretations of transferred data and information loss. Among all absorbers, the ferrites with the spinel lattice have the outstanding magnetic properties to attenuate electromagnetic wave energy within a GHz frequency range. The objectives of this investigation aim to discuss the concentration effect of Ni-related additives on the crystallographic structure, surface morphology, magnetic and electrical characterizations, electromagnetic transport and electromagnetic wave absorption for the functional ferrite composites which the NiCuZn and MnZn ferrite granules dispersed in polymer matrix. Ferrite materials are systematically synthesized by a standard double sintering technique and then ferrites are granulated with the insulation polymers. This research demonstrates a special finite-elemental-analysis (FEA) method to simulate precisely the concentration effect of ferrite granules on effective permeability for hybrid ferrite composites. In comparisons with the other empirical mixing rules, the results simulated by this FEA method are more accurate than those calculated by empirical mixing rules, especially at the higher ferrite concentrations (F/P?0.73). Electromagnetic properties of the ferrite composites in microwave region were characterized by the electrical S-parameters with a bidirectional transmission/reflection method in a coaxial airline method. Additionally, the characterized electromagnetic properties of ferrite composites are utilized to design such a single-layer or multilayer absorber to investigate absorbing performances by a computer aided computation method.Initially, we optimize the concentration dependence of Zn additives in NiZn ferrites to acquire the excellent saturation magnetic flux density, which the higher flux density is contributive to miniaturize the electrical equipments. The experimental results show that while Zn concentration continuously adds to x=0.5 at 1200℃, the NiZn ferrite achieve the highest magnetic flux density (~4400 G) and the densification structure (~5.22 g/cm3). The effect of CuO concentration on microstructure and electromagnetic properties in NiZn ferrites were also studied. Return losses of heterogeneous composites which the Ni0.1Cu0.4Zn0.6Fe1.9O4 ferrite embedded in epoxy resins are discussed. The results show that the maximal return loss of this heterogeneous composites with 8 mm thickness is estimated as -48 dB at 4.6 GHz for F/P ratio = 0.55 using the equation for a single-layer absorber with a back conductor. In additions, the electromagnetic wave absorptions of Ni0.1Cu0.4Zn0.6Fe1.9O4 ferrite are improved greatly by the additions of the nano-sized Ni fillers into the ferrite matrix. The nanosized Ni fillers play a significant role to enhance magnetic exchange interaction between the ferrite granules to increase effective permeability and microwave absorption of hybrid composites. Absorbing loss mechanism (<0.7GHz) of Ni-filled NiCuZn ferrite composites is predominated by the dielectric loss while the magnetic loss occurs at higher frequency. While the Ni concentration adds to 0.15, the single-layer absorber with 7.2mm thickness exhibit that the maximal return loss is -53.9 dB at 1.46GHz with an absorption bandwidth of 1.72GHz. Microwave absorbing performances of Ni-filled NiCuZn ferrite composites are more excellent than those of dielectric or conductive materials. Besides, the double-layer absorbers are also designed, which shows the high magnetic loss/ high dielectric loss structure posses the higher absorption performances. In additions, we also investigate the effect of NiO concentration on crystallographic structures, magnetic/electrical characterization, electromagnetic transports and microwave absorptions for MnZn ferrites. The Ni-substituted MnZn ferrites (x?0.24) have a soft ferrimagnetic behavior with the lower Hc and the higher Ms due to the absence of the antiferromagnetic hematite and the less inter/intra pores. Another interesting phenomenon is observed that crystallographic growths of the uniaxial structures occur on the surface, which it is attributed that Zn evaporations dramatically increase the surface-diffusion rate. Return loss of the heterogeneous Ni-substituted MnZn ferrites with a 6 mm thickness is estimated as -32dB at 2.3 GHz for x=0.24. The Ni-substituted ferrite composites have a higher potential to be designed as a microwave absorber in the GHz range with a thinner thickness.