Dielectric relaxation in metal-coated particles: the dramatic role of nano-scale coatings

Abstract

Insulating materials filled with conducting particles permit tailoring of electrical, electromagnetic and thermal properties of the resulting composite. When the filler particles are small and metallic, a dielectric relaxation due to interfacial polarization is commonly observed at optical or smaller wavelengths. Here, experimental results are presented in which the dielectric relaxation is shifted to microwave frequencies as a result of using metal-coated dielectric particles with a nano-scale coating thickness. The results are analysed in the context of effective medium theory adapted for multi-layer particles. Such a large shift in relaxation frequency, compared with that for a similar composite with solid metal filler particles, is shown to be a function of both the coating geometry and a thin-film-related reduction in the conductivity of the metal. The observed broadening of the relaxation peak is attributed to non-uniformity of the coating thickness and a consequent distribution of coating conductivity.

Effect of temperature on the electrical properties of copolymer/carbon black mixtures

“…In this theory, the electrical conductivity and the dielectric constant are expressed by scaling laws, near the threshold conduction [9,10]. The effective medium theory [11] and the mixture laws [12][13][14] have been proposed for calculating the complex permittivity as a function of composition and constituent properties. For very low particle concentrations, these laws give identical results, often in agreement with experiments [15].…”

Engineering of ferrite-graphite composite media for microwave shields

“…Paraffin in the frequency range below 10 GHz has dielectric properties very close to those of Teflon: its complex permittivity is b ε =2.25-j5.63⋅10 -4 at f=10 GHz [35,36]. Density of paraffin is 0.9 g/cm 3 .…”

Electro-magnetic properties of composites with aligned Fe-Co hollow fibers

“…Recently, it has been demonstrated, experimentally and theoretically, how hollow insulating particles (sphere or fiber) with nano-scale metal coatings enable this resonance to be shifted to a lower frequency regime (microwave range). 15,16 Consequently, the shape anisotropy characteristic and the cavitary structure of the Fe-Co hollow fibers, not only giving rise to a dielectric loss resonance and also adjusting its peak frequency, may be a pathway to useful EM wave absorptive devices in GHz frequency ranges.…”