Dielectric Properties Of Composites Research Articles

Epoxy Resin/Carbon Black Composites Below the Percolation Threshold

A set of epoxy resin composites filled with 0.25-2.0 wt.% of commercially available ENSACO carbon black (CB) of high and low surface area (CBH and CBL respectively) has been produced. The results of broadband dielectric spectroscopy of manufactured CB/epoxy below the percolation threshold in broad temperature (200 K to 450 K) and frequency (20 Hz to 1 MHz) ranges are reported. The dielectric properties of composites below the percolation threshold are mostly determined by alpha relaxation in pure polymer matrix. The glass transition temperature for CB/epoxy decreases in comparison with neat epoxy resin due to the extra free volume at the polymer-filler interface. At room temperature, the dielectric permittivity is higher for epoxy loaded with CBH additives. In contrast, at high temperature, the electrical conductivity was found to be higher for composites with CBL embedded. The established influence of the CB surface area on the broadband dielectric characteristics can be exploited for the production of effective low-cost antistatic paints and coatings working at different temperatures.

Permittivity of Dielectric Composite Materials Comprising Graphene Nanoribbons. The Effect of Nanostructure

New lightweight, flexible dielectric composite materials were fabricated by the incorporation of several new carbon nanostructures into a dielectric host matrix. Both the permittivity and loss tangent values of the resulting composites were widely altered by varying the type and content of the conductive filler. The dielectric constant was tuned from moderate to very high values, while the corresponding loss tangent changed from ultralow to extremely high. The data exemplify that nanoscale changes in the structure of the conductive filler result in dramatic changes in the dielectric properties of composites. A microcapacitor model most explains the behavior of the dielectric composites.

Nickel‐multiwalled carbon nanotubes/polyvinylidene fluoride composites with high dielectric permittivity

ABSTRACTComposites with nickel particles coated multiwalled carbon nanotubes (Ni‐MWNTs) embedded into polyvinylidene fluoride (PVDF) were prepared by solution blending and hot‐press processing. The morphology, structure, crystallization behavior, and dielectric properties of composites were studied. The results showed that the crystallization of PVDF was affected by Ni‐MWNTs. With the increment of Ni‐MWNTs, the content of β‐phase in PVDF increased. The dielectric permittivity was as high as 290 at 103 Hz when the weight fraction of Ni‐MWNTs was 10%. The results can be explained by the space charge polarization at the interfaces between the insulator and the conductor, and the formation of microcapacitance structure. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3746–3752, 2013

Influence of low concentration silver nanoparticles on the electrical and electro-optical parameters of nematic liquid crystals

Thermodynamical, optical, dielectric and electro-optical characterisation of nematic liquid crystals (LCs) and silver nanoparticle (NP) composites have been carried out. Transition temperatures of pure and composites systems have been measured. Thermodynamical studies suggest increase of clearing temperature of the composite material as compared to the pure material. Threshold voltage for switching from bright to dark state and splay elastic constant of the pure and composite materials have been determined. From frequency dependence of dielectric measurements, permittivity, loss, relaxation frequency and dielectric strength of flip-flop mechanism of LC molecules in the nematic phase have been calculated. Dielectric properties of composites have been explained in reference of Maier and Meier theory. The effects of doping of NPs on dielectric and electro-optic properties of LC-NP composites have been discussed.

Broadband dielectric/electric properties of epoxy thin films filled with multiwalled carbon nanotubes

Many attempts have been made to fully explore flexibility, resistance to corrosion, and processing advantage of epoxy resin filled with carbon nanotubes (CNTs) as conductive filler, although sometimes with a certain degradation of polymers’ intrinsic properties. It is important to move the percolation threshold into the region of smaller CNTs’ concentration. The results of a broadband dielectric investigation of multiwalled CNT (MWCNT)/epoxy resin composites in wide temperature range from room temperature to 450 K were analyzed for percolation. Far below the percolation threshold (0.25 wt. % MWCNT) the dielectric properties of the composite are mostly determined by alpha relaxation in pure polymer matrix and the freezing temperature decreases due to the extra free volume at the polymer–filler interface. Close to the percolation threshold, the composite shows the negative temperature coefficient effect in the temperature region, where the pure polymer matrix becomes conductive. The activation energy of DC conductivity increases with the MWCNT concentration far below the percolation threshold and decreases close to it (1.5 wt. % MWCNT). The dielectric analysis of the MWCNT/epoxy resin reveals a significant influence of the polymer matrix on the temperature dependence of composite dielectric properties.

High dielectric properties in a three-phase polymer composite induced by a parallel structure

A novel three-phase composite, consisting of semiconductive bismuth sulfide (Bi2S3) nanorods and self-passivated aluminum flakes (AFs)11Aluminum flakes (AFs). embedded in a poly(vinylidene fluoride) (PVDF) matrix, was fabricated by simple but robust ball-milling and hot-pressing techniques. The dielectric properties of composites were studied before and after the addition of AFs as a function of Bi2S3 volume fraction and frequency. The Bi2S3 nanorods in PVDF matrix form a percolation path and increase the dielectric constant of the composite dramatically. And after adding the third phase AFs, a parallel structure forms during the hot-pressing progress. Parallel AFs in the PVDF matrix can largely reduce the leakage current due to the self-passivated insulating shell outside the AFs, thus an obvious reduction in the dielectric loss of the three-phase Bi2S3–AFs/PVDF composite is observed. Meanwhile, many microcapacitors formed with Al flakes as electrodes and Bi2S3/PVDF composite as the dielectric layer form, which makes an additional contribution to the high dielectric constant of the three-phase composite.

Ferroelectric and dielectric properties of ferrite-ferroelectric ceramic composites

Particulate composites of ferrite and ferroelectric phases with xNiFe2O4 (NF) and (1 − x)Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3 (where x = 2, 10, 20, 30, 50, 70, and 100 wt. %) were prepared in situ by sol-gel method. The presence of a diphase composition was confirmed by X-ray diffraction while the microstructure of the composites was studied by scanning electron microscopy revealing a good mixing of the two phases and a good densification of the bulk ceramics. The dielectric permittivity shows usual dielectric dispersion behavior with increasing frequency due to Maxwell-Wagner interfacial polarization. AC conductivity measurements made in frequency range 1 Hz-1 MHz suggest that the conduction process is due to mixed polaron hopping. The effect of NF phase concentration on the P-E and M-H hysteresis behavior and dielectric properties of the composites was investigated. At low NF concentration a sharp ferro-paraelectric transition peak can be observed at around 360 °C while for higher NF concentrations a trend to a diffuse phase transition occurs. All the composite samples exhibit typical ferromagnetic hysteresis loops, indicating the presence of ordered magnetic structure.

Dielectric properties of sawdust reinforced epoxy composites post-cured in microwaves

This study focuses on the measurement and evaluation of dielectric properties of composite materials with a view to benefiting these relevant industries. Previously published have been the mechanical and thermal properties of sawdust reinforced epoxy composites. The properties of a dielectric material can be controlled through the introduction of various fillers. This study was primarily undertaken to determine the effect of sawdust on the dielectric properties of the composites. The original contribution of this paper states that samples post-cured in microwaves have higher dielectric constant values but lower loss tangent values than their counterparts post-cured in an oven. For all percentages by weight of sawdust, the glass transition temperature for the microwave-cured samples was lower and the composite was softer; the opposite was true for the oven-cured sample. In addition, the storage modulus of all samples post-cured in microwaves is also lower than their counterparts. This is in line with the fact that it is a softer material and has lower storage modulus and lower glass transition temperature.

MAGNETIC AND DIELECTRIC PROPERTIES OF POLYMER-CERAMIC COMPOSITES SYNTHESIZED USING A MELT COMPOUND TECHNIQUE

Polymer composites have emerged as a new class of materials, which have attracted technologist as they display novel properties compared to traditional materials and dramatically improves the performance properties of polymer system. We have synthesized hexaferrite-polymer composites of Polyvinylidene Fluride (PVDF) with BaCa2Fe16O27 (2.0, 5.0 % wt/vol.) using a melt compounding technique. BaCa2Fe16O27 hexaferrite powder was prepared using a microemulsion technique and directly mixed in melted PVDF, following by a low temperature hot pressing. The composites of PVDF with various weight percent of BaCa2Fe16O27 (2.0 & 5.0% wt/vol.) were processed via melt mixing at 200°C, with 60 rpm for 5 minutes. The films of uniform thickness are about 0.5 mm is obtained by a compression molded instrument at 200°C under 5-ton pressure. The influence of BaCa2Fe16O27 hexaferrite contents on magnetic and dielectric properties of composite was investigated. The prepared PVDF- BaCa2Fe16O27 composite thick films were characterized for their magnetic; dielectric and thermal behavior employing magnetic, dielectric and thermal analysis (TGA/DTA). Maximum saturation magnetization was obtained for 5 % wt/vol. of barium calcium hexaferrite composite.

Influence of matrices chemical nature on the dynamic mechanical and dielectric properties of rubber composites comprising conductive carbon black

The study presents the effect that elastomeric matrices different in their chemical nature (a non-polar and crystallizing natural rubber and a polar and non-crystallizing acrylonitrile-butadiene rubber) have upon the dynamic mechanical and dielectric properties of the composites comprising different amounts of conductive carbon black. Dynamic mechanical thermal analysis (DMTA) and Dielectric thermal analysis (DETA) are the techniques used for studying the structure-properties relationships of the composites. The experimental results show that the matrices studied and their specific properties have a great impact upon both the dynamic mechanical and dielectric parameters of the composites based on them. The chemical nature, structure and specific characteristics of the matrix affect the storage modulus, glass transition temperature, elasticity behavior, high-elasticity, energy dispersion, dielectric permittivity and DETA tan δ of the composites investigated. The matrix effect dominates at lower filler amounts and determines the properties of the composites.

Dielectric and microwave properties of natural rubber-based composites tailored by the fillers specific features

This article presents the results on the effect of specific characteristics of four completely different carbon black types upon the dielectric and microwave properties of natural rubber based composites. It has been found that the proneness of carbon black particles to interact, aggregate and agglomerate affects the dielectric properties of the composites much more than their microwave ones. The proneness of carbon black particles to form chains, aggregates and agglomerates affects unfavorably the microwave properties of the composite thus filled. Fullerene carbon black having the lowest specific surface area and proneness to aggregate and agglomerate yields good microwave properties of the composites.The study demonstrates that the dielectric and microwave properties of rubber composites can be tailored via an appropriate selection of filler which specific chemical nature and structure are capable of yielding the parameters required for the particular applications.

Effect of annealing and biaxial deformation on the dielectric properties of composites of multiwall carbon nanotubes and poly(ethylene terephthalate)

The dielectric properties of composites of poly(ethylene terephthalate) (PET) with MWCNTs were investigated over a wide frequency and temperature range below and close to the electrical percolation threshold. In composites with 1 wt.% multiwall carbon nanotubes (MWCNT) inclusions, the dielectric properties below room temperature are mostly determined by β relaxation, as a consequence of the rotation of PET molecules. In stretched samples, the CNTs are oriented at about 45 deg to the stretch direction. Such deformation increased the potential for molecular rotation. However, annealing after stretching increased homogeneity of the composite and decreased the potential barrier for polymer chain rotation. Electrical conductivity effects and Maxwell-Wagner polarization mostly cause the dielectric properties of the samples with 2% MWCNT inclusions. The potential barrier for carrier tunneling is lowest in the annealed sample.

Thermal, dielectric and compressive properties of hollow glass microsphere filled epoxy-matrix composites

Four types of hollow glass microspheres, having the density of 0.125–0.60 g/cm3, were filled into epoxy-matrix, and volume fraction of hollow glass microsphere was varied from 0% to 60%. The thermal, dielectric and compressive properties of the composites were investigated. The results show that the thermal conductivity, dielectric constant (Dk) and loss (Df) and compressive modulus and strength of the composites show decreased trend with increasing hollow glass microsphere content or decreasing hollow glass microsphere density, which indicates that the properties of the composites are mainly dependent on the characteristics of hollow glass microsphere. By comparing the experimental data and theoretical predictions, it is found that the properties of the composites, especially for thermal conductivity, are also related to the voids in epoxy-matrix. To conveniently predict thermal conductivity and Dk in the investigated materials system, theoretical models reported in the literature are analyzed and compared with the experimental data. Finally, suitable models are recommended. In addition, the thermal conductivity and dielectric properties of the composites were investigated as a function of testing temperature. This work indicates that thermal, dielectric and compressive properties of epoxy-matrix composites can be tailored by adjusting hollow glass microsphere content and density, which makes hollow glass microsphere filled composites a promising candidate in related fields.

Effect of high aspect ratio filler on dielectric properties of polymer composites: a study on barium titanate fibers and graphene platelets

High aspect ratio fillers are predicted to increase the dielectric constant of polymer composites more efficiently than spherical fillers according to the rule of mixtures. Using high aspect ratio fillers is a promising route for creating high dielectric constant, low loss materials at a low filler volume fraction, for use as capacitor and electric field grading materials. In this work, two high aspect ratio fillers were mixed into a polymer matrix, and the dielectric properties of composites were studied. Barium titanate fibers were synthesized by electrospinning a sol-gel, followed by a heat treatment to obtain a perovskite crystal structure. The heat treatment conditions were found to be crucial for obtaining tetragonal barium titanate fibers with high dielectric constant. Graphene platelets were prepared by a thermal shock method, which was found to result in a larger dielectric constant. A combination of barium titanate and graphene platelets yielded the highest dielectric constant when used in a polydimethyl siloxane matrix. The increase in dielectric loss over the pure matrix was small when the volume fraction was below the percolation threshold of graphene platelets. Electric flux density-electric field (D-E) measurements showed a linear dielectric constant in barium titanate filled composites and higher loss when graphene was added. The ac breakdown strength was reduced compared to the neat polymer and was affected by filler aspect ratio. The mechanisms that lead to the observed phenomena are discussed.

Dielectric characterization of electromagnetic composite barrier materials with plasma layers on nonwoven fabrics

Composite materials are becoming more widely used in the technique of shielding electromagnetic field. Among these materials, textile materials with modified surfaces are of growing importance. Among them we can distinguish nonwoven fabrics coated with conductive plasma layers which are responsible for the phenomenon of reflection and attenuation of electromagnetic wave. These phenomena are related to the dielectric properties of composite material and its structure, in particular with the phenomena on the surfaces of interphase. In the paper, dielectric properties of plasma layer-polypropylene nonwoven fabric composite is reported. To determine these properties, measurements of transient and conduction currents were used. Dielectric response of the composite to an electric field is complicated, and is a function of both the physical structure of nonwoven and applied layer. Current measurements indicate the existence of build-in charge. Electrical conductivity is nonohmic but is controlled by several mechanisms, which include ion hopping, Schottky emission, space charge limited current, and Poole-Frenkel mechanism. The dielectric parameters vary depending on the morphology of substrate surface and on parameters of the plasma generating process.

Preparation and Characterization of Electrically and Thermally Conductive Polymeric Nanocomposites

The dielectric properties of composites and nanocomposites composed of epoxy resin as base matrix and AlN (Aluminum Nitride) as micro and nanofiller has been studied at variable loading of AlN. To improve the dispersion of the filler within the polymer matrix, AlN was surface modified with silane coupling agent (SCA). The thermal conductivity behavior of epoxy/AlN composites and nanocomposites has been studied at variable percentage of filler and temperatures. Test result indicated an increase of thermal conductivity of the composites at 20 wt% of AlN. Also, silane treated composites exhibited improved electrical conductivity properties wherein the electrical insulation property decreased in terms of di-electric strength and resistivity.

Phase, microstructure and microwave dielectric properties of Zr-doped SrLa4Ti5−xZrxO17

The effect of Zr+4 substitution for Ti+4 on the phase, microstructure and microwave dielectric properties of compositions in the SrLa4Ti5−xZrxO17 (0 ≤ x ≤ 0.1) series was investigated. All the compositions formed single phase ceramics within the detection limit of in-house X-ray diffractometer when sintered in the temperature range 1,450–1,580 °C for 4 h in air. The substitution of Zr+4 for Ti+4 enabled processing of highly dense ceramics with a decrease in temperature coefficient of resonant frequency (τf) from ~117 to 70 ppm/°C, dielectric constant (er) from 60.8 to 57.3, with no significant effect on the quality factor. In the present study, optimum properties i.e. er ~ 57.3, τf ~ 70 ppm/°C and quality factor (Qu × fo) ~ 9,841 GHz, were achieved for SrLa4Ti4.9Zr0.1O17. The trend of the results demonstrates that further studies with increased Zr+4 content are required to achieve ultra low loss SrLa4Ti5−xZrxO17 ceramics.

Dielectric Composites Containing Core@shell Structure Particles

The composites with high permittivity and low dielectric loss were obtained. The fillers were the core@shell structure of the self-passivated aluminum (Al) particles (denoted as Al@Al2O3). The Al2O3 interface between Al and polymer matrix played important role in tailored dielectric properties of composite. The transmission electron microscopy (TEM) indicated that the Al2O3 shell was 2.7 nm. The self-passivated Al/polystrene (PS) composite displayed that the permittivity almost independed on frequency in wide range (102-107 Hz) and retained high-steady level, which is 27.4 times higher than that of a neat PS. On the other hand, the effect of size ratio value of core/shell in core@shell structure filler on dielectric performance of self-passivated Al/PS composites was also explained. The insulating oxide layer (Al2O3) restricts the electron transfer between Al particles. Moreover, the self-passivated Al/PS composites exhibited very steady dielectric performance at the wide temperature range (－60 ~60 °C). The composites have potential application in dielectric materials.

Impedance-spectroscopy analysis of oriented and mercerized bamboo fiber-reinforced epoxy composite

Bamboo fiber-reinforced epoxy composites were fabricated with untreated and alkali treated bamboo fibers. Dielectric, electric modulus, ac, and dc conductivity studies were carried out to rationalize the dielectric behavior of bamboo/epoxy composites. Composites of two fiber orientation parallel and perpendicular to the electric field were prepared. The dielectric behavior and electric modulus spectra of the composites were characterized using standard impedance analyzer. Dielectric properties were analyzed as a function of frequency (95 Hz–2 MHz) for temperatures in the range from 30 to 180 °C. Real part of dielectric constant (e′), conductivity, and dielectric dissipation factor (tan δ) of 0° oriented bamboo/epoxy composites were higher than that of 90° oriented composites. Conductivity activation energy, tan δ, e′, and volume resistivity decreased with increase in frequency at all the temperatures under study. Mercerization reduces the water absorption in bamboo fibers and thus improves corresponding dielectric properties of composites. Relaxation times 39.80 μs and 258.5 μs for 0° and 90° oriented bamboo/epoxy composites were calculated respectively from the relaxation peaks observed in electric modulus spectra at 180 °C.

Dielectric Properties of Ferroelectric Composites with TGS Inclusions

Dielectric properties of composites consisting of nanoporous aluminium oxide Al2O3 or glass SiO2 with inclusions of the ferroelectric triglycine sulphate (TGS) are investigated in the present work. Our investigations enable us to assume the presence of ferroelectric state in these materials.