Absorption of Electromagnetic Waves by Fe\(_3\)O\(_4\)/Paraffin Composite Materials

Electromagnetic pollution in general and the harmful effects of microwave radiation in particular on environment is currently in the urgent stage. To eliminate the effect of electromagnetic energy the most common methods relies on the use of absorbing materials with large absorption capacities in wide frequency band. In this paper, we investigate the capability of absorption of electromagnetic waves of Fe3O4 nanoparticles in paraffin basis. The Fe3O4/paraffin composite materials were prepared with a weight ratio of 35%/65%. The dielectric constant (εr) and the magnetic permeability (μr) in the frequency (f) range from 8 to 18 GHz were measured for various sample thicknesses. The results indicated a large dielectric loss and strong thickness dependence of absorption capacity. The maximum absorption coefficient (RL) is of order -13.6 dB, corresponding to the achieved absorbance of 96,9% for the 2.6 mm of sample thickness. The experimental results are consisted with our simulation calculation.

Частотная зависимость угла диэлектрических потерь в неупорядоченных полупроводниках в терагерцовой области частот-=SUP=-*-=/SUP=-

Frequency dependence of the real part of the conductivity σ_1(ω) in the region of the transition from almost linear ( s < 1) to quadratic ( s ≈ 2) can indicate a change in the conduction mechanism (the transition from the variable-range to the fixed-range hopping with increasing frequency); in this case, the sharpness of the change in the slope of the frequency characteristic is related to the dependence of the preexponential factor of the resonance integral on the intercenter distance in the pair. The frequency dependence of the imaginary part of the conductivity σ_2(ω) has no kink in the vicinity of the transition frequency ωcr, remaining almost linear. A large dielectric loss angle |cotγ| = |σ_2|/σ_1 can indicate that the imaginary part of the conductivity at ω < ω_cr is defined by the larger zero-phonon contribution in σ_2^res the region of weak variation in the loss angle γ(ω), which significantly exceeds the relaxation contribution σ_2^res.

Effect of Multiple Substitutions (Pb, Ti, Zr) on Structural, Permittivity and Electrical Properties of BiFeO3 Ceramics

Three complex polycrystalline samples of Bi1-xPbxFe1-x Zrx-yTiyO3,with the combination of (i) x, y = 0.0 (BFO), (ii) x = 0.5 and y = 0.5 (BFPTO)and (iii) x = 0.5 and y = 0.25 (BFPZTO) were prepared by a standard mixed oxide (solid state reaction route) method at optimized calcinations (900oC) and sintering (930-950oC)temperatures. The effect of multiple (Pb, Ti and/or Zr) substitutions of different amount onthe permittivity, impedance and modulus characteristics of the materials has been investigated at different frequencies and temperature.Limitation associated with BiFeO3 (BFO) due to its semiconducting behavior acts as constraint in polling the material at room temperature leading to large dielectric loss. Attempts were made to minimizethe loss by substitution of Pb on the Bi site and Ti and/or Zr at the Fe site of BFO. The study indicates that the phase transition temperature of BFO is lowered,the degree of diffuse phase transition is enhanced and electrical parameters (dielectric constant, electrical resistivity, remnant polarization and maximum polarization) areincreased with a significant reduction in dielectric loss as a consequence of the substitution. Detailed studies of Nyquist plots with impedance and electric modulus data suggest that the existence of non-Debye type of relaxation phenomena in the materials. The ac conductivity study reveals that the conduction mechanism in the material obeys universal Jonscher’s power law.

Fabrication of high-k epoxy composites with low dielectric loss based on polymer shell-coated multiwalled carbon nanotubes

A core–shell microstructured hybrid was controllably synthesized by coating cross-linked polymer shells onto multiwalled carbon nanotubes (MWCNTs) via direct in situ free-radical polymerization and was compounded with epoxy to solve the problem of large dielectric loss.

DIELECTRIC BEHAVIORS OF ZnFe2O4 – SiO2 COMPOSITE THIN FILMS PREPARED BY SOL-GEL METHOD

The dielectric property of ZnFe2O4 – SiO2 composite thin films deposited on Pt - Ti -SiO2- Si substrates, prepared by sol-gel method, are investigated. It is observed that the thin films consist of ZnFe2O4 nanoparticles embedded in the matrix of SiO2. Such a composite structure exhibits a significantly enhanced dielectric constant with respect to SiO2 thin films without too large dielectric loss enhancement.

AN INVESTIGATION OF SOME BARIUM TITANATE COMPOSITIONS FOR TRANSDUCER APPLICATIONS

The most important factor limiting the use of barium titanate in high power electroacoustic transducer applications is the large dielectric loss in high exciting fields. It has been shown that the addition of cobalt to 5% calcium, 95% barium titanate compositions produces a large reduction in the loss in high fields without significantly affecting the piezoelectric constants of polarized ceramic.