Runhua Fan

Tunable negative permittivity behavior of random carbon/alumina composites in the radio frequency band

A random metamaterial, carbon/alumina (C/Al2O3) composite, was prepared using a precursor infiltration and pyrolysis method, which has potential applications in novel antennas, microwave absorbing and shielding. The microstructures, radio-frequency dielectric property and conductivity behavior of the composites with different carbon contents were investigated in detail. It was found that the carbon membrane spread out on the pore walls of the alumina matrix. As the carbon content increased, the composites underwent a percolation phenomenon, and the conductive mechanism changed from hopping conduction to metal-like conduction due to the formation of conductive carbon networks. A negative permittivity behavior was observed in the composites above the percolation threshold, and this was ascribed to the low frequency plasmonic state produced by the carbon networks. The frequency dispersion of such negative permittivity efficiently agreed with the Drude model. The negative magnitude of permittivity in the testing frequency was small, ranging from −370 to −28, which originated from the lower carrier concentration in the conducting carbon networks. This work will greatly facilitate the practical application of random metamaterials with tunable electrical properties, and has great significance for the development of metamaterials.

Generation mechanism of negative permittivity and Kramers–Kronig relations in BaTiO3/Y3Fe5O12 multiferroic composites

Recently, negative parameters such as negative permittivity and negative permeability have been attracting extensive attention for their unique electromagnetic properties. Usually, negative permittivity is well achieved by plasma oscillation of free electrons in conductor-insulator composites or metamaterials, while some attention has been paid to attaining negative permittivity in ceramics to reduce dielectric loss. In this paper, negative permittivity in barium titanate and yttrium iron garnet composites are reported which was well fitted by the Lorentz model. Further, negative permittivity behavior was verified via Kramers-Kronig relations, and it revealed that the causal principle still valid for negative permittivity resulted from dielectric resonance. The interrelationships among negative permittivity, capacitive-inductive transition and ac conductivity are discussed.

Tunable negative permittivity and permeability of yttrium iron garnet/polyaniline composites in radio frequency region

The present work demonstrated the tunable negative permittivity and permeability properties of a two-phase composite material consisting of conductive matrix [polyaniline (PANI)] and magnetic filler [yttrium iron garnet (YIG)]. The negative permittivity observed in the YIG/PANI composites is accompanied by the formation of conductive PANI network, which induced the plasma oscillation of carriers in PANI obeying the Drude model. Through theoretical calculation, negative permeability can be realized and further adjusted by external magnetic field via the domain wall and spin resonances of YIG.

An impregnation-reduction method to prepare graphite nanosheet/alumina composites and its high-frequency dielectric properties

The nano-graphite sheet/alumina composites were prepared in situ by a facile impregnation-reduction process. The microstructure of the composites was analyzed by X-ray diffraction (XRD), and the final phase composition after reduction is Al2O3, metal Fe and graphite crystal. Scanning electron microscopy (SEM) images show that the particle size of Fe is about 20xa0nm, and the lamellae thickness of the graphite is about 30xa0nm. Then, the dielectric properties and conductive mechanism of the composites were investigated experimentally in the frequency range of 0.01–1.00xa0GHz by impedance analyzer. The results show that the real part of permittivity of composites increases with Fe3+ concentration, which is due to the increase in interfacial polarization between Fe and Al2O3 and the three-dimensional network of lamellar graphite formation. Therefore, tunable microtopography and electrical parameters of nano-graphite sheet/alumina composites can be realized by changing Fe3+ concentration.

Electromagnetic attenuation property of multiphase Fe–Fe3O4–Al2O3 cermets near percolation threshold

Cermets are widely applied as attenuating materials due to high electromagnetic loss and better mechanical properties. In this paper, multiphase cermets composed of iron, iron oxide and alumina were successfully prepared by a two-step in situ synthesis process, which includes pressureless sintering and a selective reduction in hydrogen atmosphere. The phase composition and microstructure of cermets were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. It is shown that nanosized cuboid Fe particles and octahedral Fe3O4 particles are distributed in alumina matrix. The permittivity and permeability of composites were tested with radio frequency impedance analyzer (0.01–1.00xa0GHz). The results show that permittivity presents obvious frequency dispersion. Furthermore, dielectric constants of multiphase cermets get enlarged due to the enhancement of interfacial polarization. On the other hand, there is a magnetic loss peak in permeability spectra, which indicates typical relaxation behavior. It is possible to achieve better electromagnetic attenuation property by adjusting process parameters.

Magnetic properties and special morphology of barium ferrite via electrospinning

Barium ferrite micro-/nanofibers with special morphology, nanowires with diameters of 100xa0nm, nanoribbons with diameters of 1xa0μm, and nanotubes with outer diameter of about 300xa0nm while inner diameter of 100xa0nm were successfully prepared via electrospinning using different solvents (dimethyl formamide (DMF), solutions of deionized water and ethyl alcohol, and solutions of deionized water and acetic acid, respectively). The barium ferrite micro-/nanofibers were characterized by scanning electron microscope (SEM), X-ray diffraction analysis (XRD), and vibration sample magnetometer (VSM). The results demonstrate that the pure BaFe12O19 ferrite phase is successfully formed. And the SEM results show excellent morphologies. The magnetic hysteresis loops demonstrate that their magnetic properties are quite different with different morphologies. The specific saturation magnetization is approximately the same (46.12–49.17xa0A·m2·kg−1), but the coercivity of the BaFe12O19 increases from wires (190.08 kA·m−1), ribbons (224.16xa0kA·m−1) to tubes (258.88xa0kA·m−1).