Effect of nickel nanofillers on the dielectric and magnetic properties of composites based on rubber in the X-band

Nickel–rubber nanocomposites were synthesized by incorporating ferromagnetic nickel nanoparticles in a natural rubber as well as neoprene rubber matrix. Complex dielectric permittivity and magnetic permeability of these composites were evaluated in the X-band microwave frequencies at room temperature using cavity perturbation technique. The dielectric loss in natural rubber is smaller compared to neoprene rubber. A steady increase in the dielectric permittivity is observed with increase in the content of nickel in both the composites. The magnetic permeability exhibits a steady decrease with increase in frequency and magnetic loss shows a relaxation at 8 GHz. The suitability of these composites as microwave absorbers is modeled based on the reflection loss which is dependant on the real and imaginary components of the complex dielectric permittivity and magnetic permeability.     

AC conductivity and dielectric behavior of polyaniline/sodium metavenadate (PANI/NaVO3) composites

The conducting polyaniline/sodium metavenadate (PANI/NaVO₃) composites were synthesized by single step in situ polymerization technique by placing finely grinded powder of NaVO₃ during the polymerization of aniline. The formation of mixed phases of the polymer together with the conducting emeraldine salt phase was confirmed by spectroscopic techniques like FTIR. SEM images indicated a systematic morphological variation of particles aggregated in the composite matrix as compared to the pristine PANI. AC conductivity and dielectric behavior of these composites were investigated in the frequency range 50 Hz to 5 MHz. It is found that AC conductivity obeyed the power law index and the variation of conductivity with wt% of NaVO₃ could be related to conductivity relaxation phenomenon. These composites have shown high dielectric constant, which is related to polarization. It is seen that both dielectric constant and dielectric loss decrease with increase in frequency. Variations in measured parameters of AC response with increasing frequency of these composites are found to follow systematic trends that are similar to those observed with temperature and doping.     

Dielectric and magnetic properties of low-temperature fired NiCuZn-BaTiO3 composites

The composition effects on the dielectric and magnetic properties of NiCuZn-BaTiO₃ composites fired at low temperature were investigated. The coexistence of perovskite BaTiO₃ and spinel ferrite phases in the composites were observed; no significant chemical reactions occurred between BaTiO₃ and NiCuZn ceramics during sintering. The nanosized BaTiO₃ powders favored a decrease in grain size. The saturation magnetization, remanent magnetization and real permeability continuously decreased with increasing BaTiO₃ content. And the real permittivity continuously increased with the BaTiO₃ content. The Q-factor (quality factor) exhibited relatively high values with 20-30 wt% BaTiO₃. All composite materials exhibited a low dielectric loss below 100 MHz. Synthetically considerations, the composites with 20-30 wt% BaTiO₃ could obtain relatively high real permeability and real permittivity values, and the magnetic and dielectric losses were relatively low, so they were the best candidates to produce LC-integrated chip elements.     

Effective dielectric functions of alkali halide composites and their spectral representation

In the electrostatic approximation inhomogeneous samples with characteristic scale of inhomogeneities much smaller than the wavelength of light can be characterized by an effective dielectric function _eff. This paper analyzes in which cases simple mixing formulas can be used to calculate _eff from the dielectric functions of the consituents. These considerations are compared with reflectance measurements of alkali halide composites in the far infrared. In many cases it turns out that it is essential to employ the general ansatz of the Bergman spectral representation in order to describe all effects due to the geometrical arrangement of the phases in a proper manner. It is necessary to adjust Bergman's spectral density to measurements. This can be done by use of a Monte-Carlolike algorithm. Further on we try to answer the question how to extract the dielectric function of one of the constituents from measurements of the effective dielectric function.     

On a Mathematical Framework for the Constitutive Equations of Anisotropic Dielectric Relaxation

Three classes of time-domain non-relativistic anisotropic dielectric constitutive equations of increasing generality are discussed. In each class dissipativity is ensured by the choice of a class of convolution kernels in the D-to-E constitutive equation expressing the electric field E in terms of the electric displacement field D. Defining properties of the inverse (E-to-D) kernels and their Fourier-Laplace transforms (complex dielectric functions) are determined by inversion of the D-to-E constitutive equation. By this procedure it is shown that dielectric functions of the dipolar dielectrics are tensor-valued Bernstein functions while the dielectric functions of the Drude-Lorentz type are tensor-valued negative definite functions. The properties of the complex dielectric permittivities are also determined for either class. The theory is applied to an exhaustive review of empirical response functions of real dielectric materials encountered in the literature. Each class of convolution kernels is consistent with existence of a conserved energy, but in one case a strictly dissipative energy can be constructed.     

Electrical characteristics of three-component dielectric media

We study the effective parameters of a three-component plane dielectric medium with a doubly periodic arrangement of circular inclusions. The problem is solved in the one-dipole approximation. The computational results are compared with a two-component Rayleigh model. The general structure of the formulas for the effective parameters is discussed and the reciprocity relations for a three-component matrix system are determined. Explicit expressions are given for the dielectric permittivity when the concentration of circular inclusions is low, and their domain of applicability is determined. Under certain polarization conditions, the effective conductivity in a three-component medium is exactly equal to the dielectric permittivity of the matrix. Zh. éksp. Teor. Fiz. 114, 1121–1136 (September 1998)     

Electrical and optical properties of composites based on carbazole derivatives and silicon particles

The electrical and optical properties of polymer-silicon composites with particles incorporated by different means have been studied. It is shown that both when silicon particles are embedded in a carbazole-containing polymer matrix and in the case of a pure polymer, the I–V characteristics are nonlinear and asymmetric (the I–V characteristics of the carbazole-containing polyorganosiloxane, which has silicon atoms in the monomer link, behave in a more symmetric pattern). In all cases, the I-V characteristics can be fitted with power laws, I(V) ～ V ^p , with three different slopes for different voltage intervals, which remainds one of the pattern typical of the mechanism of space-charge-limited currents. It is shown that, in its luminescent properties, the carbazole-containing polyorganosiloxane is similar to a carbazole-containing polymer matrix with embedded silicon particles. The results obtained argue for charge transfer between the polymer and silicon nanoparticles if they are embedded in the matrix and for an formation of an interchain charge-transfer complex in the case of chemically bound silicon.     

Availability of water glass/Bi2O3 composites in dielectric and gamma-ray screening applications

ABSTRACT

Sodium silicate (Na₂Si₃O₇) also known as water glass is a very low cost material which is used in many industrial applications such as a builder in detergents, as a binder and adhesive etc. But so far the electrical properties of sodium silicate and its ability to screen radiation have never been investigated. In the present study, the frequency dependent electrical properties and gamma-ray shielding performance of water glass based bismuth oxide composites have been studied for the first time. In accordance with this purpose, Na₂Si₃O₇/Bi₂O₃ glassy composites have been prepared for searching their possible applications in electronics and radiation screening. The surface morphology of the samples have been determined by Scanning Electron Microscope (SEM). The frequency dependent electrical properties such as complex impedance, complex dielectric function and conductivity have been analyzed at room temperature between 1 and 40?MHz. As a result of alternative current (ac) electrical analysis, it has been determined that the Na₂Si₃O₇/Bi₂O₃ composites can be utilized as a dielectric layer in capacitors. On the other hand, since bismuth oxide is an anti-radiative material, the gamma-ray screening parameters such as mass attenuation coefficient, half layer and tenth layer values along with mean free path of the composites have been defined experimentally by using NaI(Tl) scintillation detector for the Ba-133 radiation source at 81 and 356?keV. The values of these parameters have also been checked by Monte–Carlo simulation. Since a good agreement has been assigned between experimental and Monte–Carlo simulation results, the related gamma ray shielding parameters have been determined by Monte–Carlo simulation for other gamma photon energies (140?keV, 208?keV, 468?keV, and 661?keV) which are generated from Tc-99, Lu-177, Ir-132, and Cs-137 sources. Ultimately, Na₂Si₃O₇/Bi₂O₃(35%) composite has been suggested as an eco-friendly, lead-free glassy structured material for the gamma radiation shielding in medical applications.     

Dielectric elastomer composites: small-deformation theory and applications

Abstract

We provide estimates for the effective response of Electro-Active Polymer Composites (EAPCs) consisting of aligned ellipsoidal inclusions of a stiff dielectric material which are distributed randomly in an soft elastomeric matrix with “ellipsoidal” two-point statistics. The derivation of the results for the electro-mechanical response assumes linearized deformations, but includes non-linear (quadratic) terms in the electric fields. We investigate three different physical mechanisms contributing to the macroscopic electro-mechanical response of the composite: the intrinsic effect of the particles on the Maxwell stress, the inter-particle (dipole) interactions which are accounted for by evaluating the effect of changes in the “shape” of the two-point probability functions with the deformation, and the effect of particle rotations and torques when the geometric and/or anisotropy axis of the particles are not aligned with the applied electric field. Several illustrative examples are provided to emphasize the relative importance of the different effects on the overall electrostriction of the composites. In particular, for the “compliant electrode” boundary conditions that are widely used in applications, it is shown that inter-particle interactions are synergistic with the intrinsic effect of the particles on the Maxwell stress, leading to significant enhancements in the electro-mechanical coupling of the EAPCs, especially at high particle concentrations. On the other hand, the effect of electric torques on non-aligned particles is generally deleterious for electrostriction.     

Preparation,electric conductivity and dielectrical properties of Cu6PS5I-based superionic composites

Composite samples based on microcrystalline Cu₆РS₅I superionic conductors, embedded in matrices of polyvinylacetate, epoxy and conducting glue, are produced. Temperature and frequency studies of complex electric conductivity and dielectric permittivity as well as dielectric loss of the obtained composites are carried out. The effect of differences in the composite production technology on the temperature of a second-order phase transition in the superionic phase, values of electrical conductivity, activation energy and dielectric permittivity is shown.     

Study on dielectric properties of oil/water random composites

Considering the electric double layers between oil and water, a new “complex model” of dielectric constant of oil/water composites was built up. Starting from the Maxwell–Garnett theory and Bruggeman theory, the effective dielectric constant of oil/water random composites is presented. The nonlinearity of the theory is obvious. The model is especially suited to study the dielectric properties of oil/water composites of different nature. The model is also suited to study the dielectric properties of two-phase random composites with an interfacial shell. The theoretical results on dielectric properties of different kinds of oil/water composites are in good agreement with experimental data.     

Resonance dispersion of the longitudinal waves in disperse composites

Vibrations and longitudinal waves in a composite formed by a viscoelastic matrix and solid-state inclusions are considered. Vibration-wave processes in the long-wave approximation are described on the basis of the complex dynamic density taking into account inertial-elastic-viscous interaction of the matrix and inclusion-oscillators upon their forward vibrations. Resonance dependences for the dynamic density and translational viscosity of the composite at the moderate volume concentration of inclusions are presented. The formulas obtained are specified for composites with spherical and cylindrical inclusions and are compared with the known experimental results.     

A comprehensive review on surface modification,structure interface and bonding mechanism of plant cellulose fiber reinforced polymer based composites

Abstract

Plant cellulose fiber polymer composites are readily applied in wide range of applications due to ecological and economical alternative to traditional materials. The considerable amount of residues and organic wastes from agricultural process are still employed as lower energy resource. Organic materials are generally disposed in composting, landfilling or anaerobic digestion. The utilization of these wastes in plant fiber composites shows significant alternative and environmental friendly in nature. The production of plant cellulose fiber composite with higher structural properties is optimized by interfacial bonding between polymer and reinforced fiber. The interface plays a vital role in regulating mechanical properties by distributing bonds and stress transferring, which is one of least understood element of composites. This paper presents the comprehensive review of fiber structures, different modification techniques to reduce the incompatibility between matrix and fiber, assessment of structure interface and bonding, clarifies the interfacial adhesion of cellulose fiber composites.     

Dielectric and impedance studies of DBSA doped polyaniline/PVC composites

Polyaniline (PANI) was doped with dodecylbenzene sulfonic acid (DBSA) and then mixed with PVC by solution blending method to prepare DBSA doped PANI (PAND)/PVC composites. FTIR spectroscopy indicates the strong dipole–dipole interaction between the individual components of the composites. The ac electrical properties of the synthesized composites were investigated by complex impedance spectroscopy in the frequency range of 0.5–10⁶ Hz at room temperature. Both dielectric loss factor and permittivity increase with the decrease of frequency exhibiting strong interfacial polarization at low frequency. Addition of PAND in PVC reduces the charge trapping centers by increasing the number of conducting channels participating in the relaxation process; hence an increase in conductivity is observed.     

Interfacial aspects of carbon composites

Abstract

Carbon-based composites bring great promise for various practical applications ranging from aviation industry to advanced biomedical sensors. The interface chemistry and the ultimate conductivity of these composites are responsible for their functional applicability. The interfaces can be modified by various chemical and physical techniques. This article reviews the synthesis methods of carbon composites and discusses how the interface properties dictate their applicability.     

Dielectric properties of short sisal/coir hybrid fibre reinforced natural rubber composites

The dielectric properties, such as dielectric constant, volume resistivity and dielectric loss factor, of sisal/coir hybrid fibre reinforced natural rubber composites have been studied as a function of fibre loading, fibre ratio, frequency, chemical modification of fibres and the presence of a bonding agent. The dielectric constant values have been found to be higher for fibre filled systems than pure natural rubber. This has been attributed to the polarization exerted by the incorporation of fibres into the matrix. Dielectric constant values were observed to be decreased with increase in frequency due to the decreased interfacial and orientation polarization at higher frequencies. Whereas dielectric constant increases with fibre loading because of the increment in number of polar groups after the addition of hydrophilic lignocellulosic fibres. The volume resistivity of the composites was found to be decreased with fibre loading and a percolation threshold has been obtained at 15.6% volume of fibres. Fibre treatment, such as alkali, acetylation, benzoylation, peroxide and permanganate, were carried out to improve the adhesion between fibres and matrix. The dielectric constant values were lower for systems consisting of fibres subjected to chemical treatments due to the increased hydrophobicity of fibres. The addition of a two-component dry bonding agent consisting of hexamethylene tetramine and resorcinol, used for the improvement of interfacial adhesion between the matrix and fibres, reduced the dielectric constant of the composites. When the weight percentage of sisal fibre was increased in the total fibre content of the hybrid composites, the dielectric constant was found to increase. The added fibres and different chemical treatments for them increased the dielectric dissipation factor. A dielectric relaxation has been observed at a frequency of 5 MHz.     

Tribological behavior of Al-based self-lubricating composites

Abstract

Aluminum-based composites containing either SiC (Al10%SiC) as the hard phase or a combination of SiC and MoS₂ (Al10%SiC4%MoS₂) have been synthesized following stir casting route. To overcome the poor wetting characteristics, magnesium was added in one of the composites (Al10%SiC4%MoS₂4%Mg) to improve the bonding between matrix and second phase. The results suggested an enhancement in hardness and strength of the composite containing SiC–MoS₂ and Mg, thus indicating the effectiveness of Mg addition in improving the interfacial bonding strength. Tribological performance of the composites has been examined by carrying out pin-on-disk wear tests under dry sliding conditions at different normal loads of 9.8, 14.7, 19.6, and 24.5 N and at a constant sliding speed of 1 m/s. Both the friction coefficient and the wear rate have been found to reduce with addition of MoS₂; however, bonding between the matrix and reinforcements was not good. Al10%SiC4%MoS₂4%Mg has shown the best tribological performance at all the loads in terms of the lowest friction coefficient and the lowest wear rate. The wear mechanism has been found to be a combination of adhesion and abrasion as indicated by the presence of some abrasive grooves and delaminated flakes at the worn surface and the X-ray examination of wear debris for all the materials used in the present investigation.     

Thermopower of composites containing metallic cobalt nanoparticles embedded in the Al<Subscript>2</Subscript>O<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> dielectric matrix

The concentration and temperature dependences of the thermopower of composites containing Co nanoparticles embedded in the Al₂O _n dielectric matrix are investigated. Below the percolation threshold, i.e., in the tunneling conduction region, the absolute values of the thermopower of the composites under investigation are less than those above the percolation threshold. It is revealed that, in the tunneling conduction region, the slope of the temperature dependences of the thermopower changes at a temperature of ～205 K. This can indicate that the thermopower is sensitive to a change in the mechanism of conduction from the Mott law ln(σ) ∝ (1/T)^1/4 to a power relation that corresponds to the model of inelastic resonant tunneling through a chain of localized states in the dielectric matrix. The introduction of oxygen in the course of sputtering brings about a decrease in the absolute values of the thermopower; however, the character of variation in the concentration and temperature dependences of the thermopower remains unchanged.     

Theory of the oscillatory dependence of the conductivity of two-component dielectric composites at room temperatures

It is predicted that at room temperatures a hopping mechanism of charge transfer plays a very important role and leads to temperature oscillations of the conductivity σ(T) of a dielectric composite. The dependence of the conductivity σ(ω) on the frequency of an alternating electric field is calculated. The relation obtained can be used to determine, first, the electron relaxation times and, second, and more importantly, the frequency of electron tunneling through the dielectric matrix from measurements of the conductivity in various frequency ranges. Zh. Tekh. Fiz. 69, 31–34 (March 1999)     

Complex permittivity,complex permeability and microwave absorption properties of ferrite–polymer composites

The complex permittivity (ε′–jε″), complex permeability (μ′–jμ″) and microwave absorption properties of ferrite–polymer composites prepared with different ferrite ratios of 50%, 60%, 70% and 80% in polyurethane (PU) matrix have been investigated in X-band (8.2–12.4 GHz) frequency range. The M-type hexaferrite composition BaCo⁺²_0.9Fe⁺²_0.05Si⁺⁴_0.95Fe⁺³_10.1O₁₉ was prepared by solid-state reaction technique, whereas commercial PU was used to prepare the composites. At higher GHz frequencies, ferrite's permeabilities are drastically reduced, however, the forced conversion of Fe⁺³ to Fe⁺² ions that involves electron hopping, could have increased the dielectric losses in the chosen composition. We have measured complex permittivity and permeability using a vector network analyzer (HP/Agilent model PNA E8364B) and software module 85071. All the parameters ε′, ε″, μ′ and μ″ are found to increase with increased ferrite contents. Measured values of these parameters were used to determine the reflection loss at various sample thicknesses, based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 80% ferrite content has shown a minimum reflection loss of −24.5 dB (>99% power absorption) at 12 GHz with the −20 dB bandwidth over the extended frequency range of 11–13 GHz for an absorber thickness of 1.6 mm. The prepared composites can fruitfully be utilized for suppression of electromagnetic interference (EMI) and reduction of radar signatures (stealth technology).