The microstructure and dielectric properties of the graphene–epoxy composites prepared by cryomilling at liquid nitrogen temperature were studied by SEM and RF impedance material analyzer. The result indicated that both the dielectric constant and conductivity of the composites increased with the increase of the graphene content. The value of the dielectric constant of the composite with about 8 wt % of graphene was as high as 200. Moreover, the frequency dispersion behaviors of the conductivity within a certain frequency range accorded with the Jonscher's power law demonstrating that the conductive mechanism is hopping conduction. The negative reactance decreased with the increase of the testing frequency which indicated a capacitive character
Impregnation method was used for fabricating Co/YIG (Y3Fe5O12) composites under low temperature, and the electromagnetic properties were tuned by adjusting the content (0 wt% - 35 wt%) and microstructure of the metal in composites. The XRD and SEM were used to analyze the phase and microstructure. Electromagnetism properties including reactance, permittivity and permeability of the composites were tested using impedance analyzer (10 MHz~1 GHz). The results indicated that, the high cobalt content leads to the formation of three-dimensional cobalt networks on the pore walls of porous YIG gradually, and their electromagnetic properties have also been changed significantly with the Co content increasing. Especially, double negative properties were achieved in Co/YIG composites when the content of Co reached 35 wt% at the frequency band of 575 MHz-1 GHz. The appearance of double negative properties in cermets provides promising applications in electromagnetic stealth materials, microwave antenna, and traveling wave tubes.
The uniform hexagonal barium ferrite powders were synthesized by co-precipitation method using metal chloride. The effects of the amount of hexadecyltrimethyl ammonium bromide (CTAB), the water bath and calcination temperature on the phase formation, microstructure and density of barium ferrites were systematically investigated. The results showed that the formation of uniform hexagonal barium ferrite powders was significantly influenced by the amount of CTAB and the water bath could lead to the larger grain size and density. The SEM demonstrated that the BaFe12O19 powders had plate-like shape with crystallite sizes varing from 150 to 200 nm. When the amount of CTAB was 0.2g/100ml and the calcination temperature was 850 °C, the barium ferrite powders were uniform which indicated that the amount of surfactant and calcination temperature were very optimum.
In this paper, carbon nanotubes (CNT)/Fe powders were synthesized by a simple catalytic pyrolysis routine and the Fe-CNT/Al2O3 bulk composites were fabricated by hot-pressing sintering. The CNT/Fe powders were characterized by X-ray diffraction (XRD) and TGA-DSC. The dielectric properties of the Fe-CNT/Al2O3 bulk composites were measured by using Agilent E4991A impedance analyzer from 100MHz to 1GHz. Both real part and imaginary part of the permittivity increased as added CNT into the composites. The imaginary part of permittivity (ε'') ranged from 137 to 1300 with the frequency, which indicated a good dielectric loss properties.
In this paper, Cu/SiO2composites with different metal contents were successfully prepared by a selective reduction process. The phase composition and microstructure of composites were analyzed by XRD and SEM, respectively. The results show that spherical copper particles are uniformly distributed in silica matrix. The impedance and permittivity of composites were tested with RF impedance analyzer (0.1~1 GHz). It is indicated Cu/SiO2composites exhibit capacitive character. In addition, the dielectric constants get enlarged due to the enhancement of interface polarization.
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