| Permittivity (ε) and permeability (μ) are the basic parameters to describe the response of a material to the external electromagnetic field. In different application fields, the permittivity and permeability should satisfy requirements of corresponding conditions. For example, high permittivity was needed for capacitors, low permittivity was needed for wave-transmitting materials, and permittivity should match well with permeability for microwave absorbing materials. Therefore, much attention has been paid to the investigations on the adjustment of the electromagnetic parameters of materials. Generally, the ε and μ of a conventional material are both positive. In recent years, much attention has been paid to double negative materials (DNMs) with simultaneously negative permittivity and permeability. DNMs have many unique electromagnetic properties, such as negative refraction effects. And, they have various novel potential applications, such as novel cavity resonators, microstrip antenna and microwave attenuation, etc.It is worth noting that, the double negative property of DNMs is usually provided by artificial structures (metamaterials) rather than directly from the materials’ composition and micro structure. Therefore, it is also interesting to investigate the possibility of realizing double negative property from the point of "real" random materials rather than periodic artificial structures.In fact, metamaterial is a kind of ordered conductor-insulator composite structure. Therefore, cermets consisting of metal and ceramic could be a potential candidate for the realization of double negative property by tailoring the microstructure of the cermet. From this point of view, porous cermet is proposed to obtain double negative property in this work, and double negative property is realized in MHz frequency. In this work, cermets were prepared via a wet chemical process. The composition and microstructure of the cermets could be easily tuned.Porous ceramics are prepared using pore former method. Then, metallic phase is loaded into porous ceramics via impregnation-calcination technique, forming porous cermets. The composition and microstructure of the porous cermets can be tailored by changing experimental parameters. The composition and microstructure of the cermets are characterized using X-ray diffraction (XRD), Mossbauer spectra. Vibrating Sample Magnetometer (VSM) and Scanning Electron Microscope (SEM). The physical and chemical reactions during the calcinations and reduction processes are analyzed using Thermogravimetric and differential thermal analysis (TG-DTA) and Temperature Programmed Reduction (TPR). The dielectric and magnetic properties of the porous cermets are tested using impedance analyzer. The relationship between the microstructure of the cermets and its electromagnetic properties are investigated. Finally, the double negative mechanism of the cermets is analyzed using Drude model, equivalent circuits, HFSS simulation, and Effective Medium Theory (EMT), etc. The main contents are summarized as follows:(1) Carbon was selected as the pore former, and porous alumina and porous yttrium iron garnets with different porosities were prepared. The influences of pore former content, particles size and sintereing temperature on the porosity of the porous ceramic were investigared. Metals were loaded into porous ceramics using impregnation-calcination process. And the influences of impregnation solution, porous matrix composition and porosity, calcination temperature and time on the microsturctures of the supported metals were investigated.(2) Fe/Al2O3and Ni/Al2O3composites were prepared using impregnation-calcination process. For the composites with low metal content, the conductive mechanism is hopping conduction. For the composites exceeded the percolation threshold, metallic networks were formed, and the conductivity decreases with increasing frequency due to the skin effects. The plasma oscillation of delocalized electrons in the current loops leads to the negative permittivity. Meanwhile, the combined effects of the diamagnetic response of current loops and magnetic resonances of ferromagnetic metals lead to the negative permeability. Porous cermets can be prepared via low temperature reduction in the impregnation-calcination process, leading to small metal particls. The fano resonance frequency and magnetic resonance frequency can be tunable.(3) Silver and copper were hosted in prous YIG matrix using impregnation-calcination process, forming Ag/YIG and Cu/YIG composites. The negative mechanism of the YIG matrix cermet is the same as the alumina matrix ceremt. However, in addition to the diamagnetic response of metallic networks, negative permeability can also be obtained by the magnetic resonances of ferrimagnetic YIG matrix.(4) The impedance spectra of cermets were investigated using equivalent circuit analysis. The results indicated that, composites with low metal content can be simulated by an equivalent circuit model composed of a parallel combination of resistor R and capacitor C. For the composites with high metal content, current loops will be induced in the composites, and the composites can be simulated by an equivalent circuit model composed of inductor L, resistor R and capacitor C. The appearance of shunt inductances is the origin of negative permittivity. The HFSS simulation and Effectivbe Medium Theory (EMT) calculation results agrees well with the experimental results.(5) The permeability spectra of YIG in the frequency band of0.1MHz-10GHz were calculated. Negative permeability can be realized by the domain wall resonance and nature resonance of YIG, and the negative permeability is tunable with external bias magnetic field.This work was supported by National Natural Science Foundation of China (50772061,51172131), Program for New Century Excellent Talents in University (NCET-10-518), Independent Innovation Foundation of Shandong University (GIIFSDU-YZC12076, GIIFSDU-YYX10011), and Shandong S&T Plan (2007GG10003007),973Project of China (2012CB825702). |
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