Enhanced microwave absorbing performance of hydrogenated acrylonitrile–butadiene rubber/multi-walled carbon nanotube composites by in situ prepared rare earth acrylates

Rare earth oxides (REO = Gd₂O₃, Dy₂O₃, Tm₂O₃) and acrylic acid (AA) were in situ reacted in hydrogenated acrylonitrile–butadiene rubber (HNBR) to prepare HNBR/multi-walled carbon nanotube (MWCNT)/REO/AA composites. The HNBR/MWCNT/REO/AA composites have higher permittivity and dielectric loss than HNBR/MWCNT composite, leading to significantly enhanced microwave absorbing performance of the HNBR/MWCNT/REO/AA composites. Dielectric permittivity analysis reveals that the HNBR/MWCNT/REO/AA composites have longer dielectric relaxation time and higher conductivity than the HNBR/MWCNT composite. The HNBR/MWCNT composite has the minimum reflection loss of −15.1 dB, while the HNBR/MWCNT/REO/AA composites have the minimum reflection loss of −48.8 dB. The improvement of microwave absorbing performance is attributed to the stronger interfacial polarization and higher conductivity after formation of in situ prepared rare earth acrylates.     

Enhanced dielectric and ferroelectric properties induced by TiO2@MWCNTs nanoparticles in flexible poly(vinylidene fluoride) composites

Flexible polymer based composites containing multi-walled carbon nanotubes (MWCNTs) have been reported to present high dielectric constant. However, the composites generally exhibit high dielectric loss and low dielectric breakdown strength, which prohibits their practical use in electronic and electric industry. MWCNTs were coated with a continuous layer of TiO₂ nanoparticles (TiO₂@MWCNTs) by a simple hydrothermal process and TiO₂@MWCNTs/poly(vinylidene fluoride) (PVDF) composites were prepared by a solution casting method. Compared to the pristine MWCNTs/PVDF composites, the TiO₂@MWCNTs/PVDF composites presented enhanced dielectric constant and lower dielectric loss. Additionally, the breakdown strength of the TiO₂@MWCNTs/PVDF composites was also improved, which is favorable for enhanced ferroelectric properties in the composites.     

Enhancement of dielectric and electrical properties in BT/SiC/PVDF three-phase composite through microstructure tailoring

A polymer composite with high dielectric permittivity was prepared by embedding silicon carbide (SiC) whisker with an average diameter of 500 nm–1 μm in poly(vinylidene fluoride) (PVDF). However, the high dielectric loss and electrical conductivity of the two-phase composite prohibits its potential applications. Barium titanate (BT) particles with average diameter of 100 nm and 1 μm were incorporated as a third phase to fabricate a three-phase composite. The morphology structure, dielectric and electrical properties before and after the addition of BT particles were investigated. The three-phase composite exhibits largely suppressed dielectric loss and electrical conductivity without sacrificing the high dielectric permittivity, which was extremely hard to be realized for two-phase composite. It is also found that the nano-size BT is more favorable in achieving high dielectric permittivity than the micro-size BT, where their dielectric loss and electrical conductivity are similar. Furthermore, electric modulus analysis confirms the largely suppressed electron conduction process which results in the enhanced dielectric and electrical properties in three-phase composite.     

Effect of MgWO4 content on properties of Ba0.5Sr0.5TiO3 composite ceramics for tunable microwave applications

xMgWO₄-(1 − x) Ba_0.5Sr_0.5TiO₃ (x = 0.0, 5.0, 15.0, 25.0 and 35.0 wt%) composite ceramics were prepared via solid state reaction processing. Their structural and dielectric properties were systematically characterized. A significant increase in grain size was observed with increasing MgWO₄ content, which was accompanied by obvious variations in dielectric properties of the composite ceramics. It is found that the permittivity peaks of the samples gradually shifted to low temperatures with increasing MgWO₄ content. At the same time, tunabilities of the composite ceramics decreased, but their Q values increased. The sample with 35 wt% MgWO₄ possesses a high tunability of 16.8% (∼10 kHz), a low permittivity of 65 and an appropriate Q value of 309 (∼4.303 GHz), which meet the requirements of high power and impedance matching, thus making it a promising candidate for applications as electrically tunable microwave devices.     

Electric field effects on CNTs/vinyl ester suspensions and the resulting electrical and thermal composite properties

In this study, electrical conductivity of a vinyl ester based composite containing low content (0.05, 0.1 and 0.3 wt.%) of double and multi-walled carbon nanotubes with and without amine functional groups (DWCNTs, MWCNTs, DWCNT-NH₂ and MWCNT-NH₂) was investigated. The composite with pristine MWCNTs was found to exhibit the highest electrical conductivity. Experiments aimed to induce an aligned conductive network with application of an alternating current (AC) electric field during cure were carried out on the resin suspensions with MWCNTs. Formation of electric anisotropy within the composite was verified. Light microscopy (LM), scanning electron (SEM) and transmission electron microscopy (TEM) were conducted to visualize dispersion state and the extent of alignment of MWCNTs within the polymer cured with and without application of the electric field. To gain a better understanding of electric field induced effects, glass transition temperature (T_g) of the composites was measured via Differential Scanning Calorimetry (DSC). It was determined that at 0.05 wt.% loading rate of MWCNTs, the composites, cured with application of the AC electric field, possessed a higher T_g than the composites cured without application of the AC electric field.     

Dielectric properties of Poly(vinylidene fluoride)/CaCu3Ti4O12 composites

The possibility of obtaining relatively high dielectric constant polymer–ceramic composite by incorporating the giant dielectric constant material, CaCu₃Ti₄O₁₂ (CCTO) in a Poly(vinylidene fluoride) (PVDF) polymer matrix by melt mixing and hot pressing process was demonstrated. The structure, morphology and dielectric properties of the composites were characterized using X-ray diffraction, Thermal analysis, scanning electron microscope, and impedance analyzer. The effective dielectric constant (ε_eff) of the composite increased with increase in the volume fraction of CCTO at all the frequencies (100 Hz–1 MHz) under study. The dielectric loss did not show any variation up to 40% loading of CCTO, but showed an increasing trend beyond 40%. The room temperature dielectric constant as high as 95 at 100 Hz has been realized for the composite with 55 vol.% of CCTO, which has increased to about 190 at 150 °C. Theoretical models like Maxwell’s, Clausius–Mossotti, Effective medium theory, logarithmic law and Yamada were employed to rationalize the dielectric behaviour of the composite and discussed.     

Study on the semi-empirical model for the complex permittivity of carbon nanocomposite laminates in microwave frequency band

A semi-empirical model is proposed for the complex permittivity of composites containing electrical conductive carbon nanomaterials such as carbon black (CB), carbon nanofiber (CNF) and multi-walled carbon nanotube (CNT). The composites were fabricated with E-glass fabric/epoxy prepregs. The model is based on the percolation theory. The model is available for the composite of filler content over the percolation threshold and applicable within the high frequency band in which AC electrical conductivity of the composite is continuously proportional to the frequency. The proposed model is composed of the numerical equations of the scaling law in percolation theory and constants obtained from experiments to quantify the model. The model describes the complex permittivity as a function of frequency and filler content. The model was verified when compared with the measurements. The measurements for the complex permittivities of the composites were performed at the frequency band between 0.5 and 18.0 GHz using a vector network analyzer with a 7 mm coaxial air line.     

Electromagnetic characteristic and microwave absorbing performance of different carbon-based hydrogenated acrylonitrile–butadiene rubber composites

Hydrogenated acrylonitrile–butadiene rubber (HNBR) was mixed with carbon fiber (CF), conductive carbon black (CCB) and multi-walled carbon nanotubes (MWCNT) to prepare microwave absorbing composites, their complex permittivity was measured in microwave frequencies (2–18 GHz), and their electromagnetic characteristics and microwave absorbing performance were studied. The real part and imaginary part of permittivity of the composites increased with increasing carbon filler loading, showing dependency on filler type. The microwave reflection loss of the composites also depended on the loading and type of fillers. The matching thickness of the absorber layer decreased with increasing permittivity, while the matching frequency decreased with increasing layer thickness. The minimum reflection loss was −49.3 dB for HNBR/MWCNT (100/10) composite, while −13.1 dB for HNBR/CCB (100/15) composite and −7.1 dB for HNBR/CF (100/30) composite. The efficient microwave absorption of HNBR/MWCNT composites is accounted from high conduction loss and dielectric relaxation of MWCNT, and strong interface scattering.     

Study on dielectric behavior of the ferroelectric poly(vinylidene fluoride)/Li and Ti codoped NiO composites

Polyvinylidene fluoride (PVDF) matrix composites filled by Li and Ti codoped NiO (LTNO) with different Li/Ti chemical stoichiometric ratio in the LTNO fillers were prepared using a physical mixing technology and subsequent hot-molding process. The dielectric permittivity of two kinds of PVDF/LTNO composites was studied over a wide temperature and frequency range. The composite with Li_0.1(Ti_0.02Ni_0.98)_0.9O (LTNO-A) filler displays a high value in the real part of dielectric permittivity ε′ > 200 at 10³ Hz. Whereas as a comparison, the composite filled with Li_0.1(Ti_0.05Ni_0.95)_0.9O (LTNO-B) shows a remarkable difference in dielectric permittivity. The result could be attributed to a different physical characteristic of the LTNO fillers, which is from the change of composition and microstructure of LTNO fillers.     

Modified tunable dielectric properties by addition of MgO on BaZr0.25Ti0.75O3 ceramics

Phase composition, microstructure and tunable dielectric properties of (1 − x)BaZr_0.25Ti_0.75O₃-xMgO (BZTM) composite ceramics fabricated by solid-state reaction were investigated. It was found Mg not only existed in the matrix as MgO, there was also trace amount of Mg²⁺ ions dissolved in the BZT grains, which led to Curie temperature of the BZTM composites ceramics shifting to below −100 °C. Dielectric permittivity of the BZTM composite ceramics was reduced from thousands to hundreds by manipulating the content of MgO. Johnson's phenomenological equation based on Devonshire's theory was used to describe the nonlinear dielectric permittivity of the ceramics with increasing applied DC field. With increasing content of MgO, anharmonic constant α_(T) increased monotonously. Dielectric permittivity was 672, while dielectric tunability was as high as 30.0% at 30 kV/cm and dielectric loss was around 0.0016 for the 0.6BaZr_0.25Ti_0.75O₃-0.4MgO sample at 10 kHz and room temperature.     

Melt mixed nano composites of PA12 with MWNTs: Influence of MWNT and matrix properties on macrodispersion and electrical properties

Nanocomposites containing four different polyamide 12 (PA12) types and three grades of multiwalled carbon nanotubes (MWNTs) were prepared via small-scale melt processing to study the effect of different MWNTs and the influence of polymer properties on the dispersion of the fillers and the electrical properties of the composites. Under the selected mixing conditions the lowest electrical percolation threshold of 0.7 wt.% was found for Nanocyl™ NC7000 in low viscous PA12. Moreover, big influences of the end group functionality (acid or amine excess) and the melt viscosity of the matrix were found. Composites of PA12 with acid excess showed lower percolation thresholds than those based on amine terminated materials. At constant end group ratio low viscous matrices resulted in lower percolation thresholds than high viscous materials. The best MWNT dispersion was obtained in both high viscous PA12 composites. In these systems the mixing speed was varied indicating an optimum concerning electrical conductivity at 150 rpm as compared to 50 and 250 rpm.     

Dielectric and tunable characteristics of Ba0.4Sr0.6TiO3-BaWO4 composite ceramics for microwave applications

Phase compositions, microstructure and microwave dielectric properties, of BaWO₄ (BW)-Ba_0.4Sr_0.6TiO₃ (BST) composite ceramics, prepared by the traditional solid-state route, were systematically characterized. Meanwhile, mechanism of dielectric tunability of those materials was discussed. Dielectric properties of the BW-BST composites at a DC bias field near the phase transition temperature could be interpreted by using Johnson's phenomenological equation. The sample with x = 0.60 exhibited a tunability of 29.5%, a dielectric permittivity of 192 and a Q value of 231 (at 2.700 GHz), which make it a promising candidate for applications in electrically tunable microwave devices.     

Preparation of high k expanded graphite/CaCuTi4O12/cyanate ester composites with low dielectric loss through controlling the interfacial action between conductors and ceramics

New composites with high dielectric constant and low dielectric loss, based on expanded graphite (EG), CaCuTi₄O₁₂ (sCCTO) and cyanate ester (CE) resin, were developed by controlling the interaction between EG and sCCTO. Difference from EG, surface modified EG (mEG) has an additional strong chemical interaction with sCCTO, this not only improves the dispersion of fillers, but also enhances the filler-matrix interfacial adhesion, leading to different micro-structures and dielectric properties. Specifically, the percolation thresholds of mEG/sCCTO/CE and EG/sCCTO/CE composites are 3.45 vol% and 2.86 vol%, respectively. When the loading of conductors approaches the percolation threshold, mEG/sCCTO/CE composite has much higher dielectric constant and lower dielectric loss than EG/sCCTO/CE composite. The nature behind these attractive data was revealed by building an equivalent circuit.     

Magnetic and dielectric properties of Ni0.8Zn0.2Fe2O4/HDPE composites for high-frequency application

A low loss high-frequency magnetic composite with Ni_0.8Zn_0.2Fe₂O₄ (NZO) ultrafine particles embedded in a high density polyethylene (HDPE) matrix was fabricated by using a simple low-temperature hot-pressing technique. The magnetic and dielectric properties of the as-prepared composites were investigated in details. The results indicate that as the volume of the ceramic fillers increase, the permittivity, permeability, dielectric and magnetic loss of the composite all increase. The cut-off frequencies of the composites are all above 1 GHz. Because of the low resistivity of NZO, the dielectric losses of the composites are big and decrease with frequency below 100 MHz. Good frequency stabilities of the permittivities and permeabilities, and low dielectric and magnetic losses within the measurement range have been observed. For the composite containing 30 vol% NZO, the permittivity, dielectric loss, permeability and magnetic loss are 3.7, 0.0025, 2.2 and 0.002 at 100 MHz, respectively.     

Microstructures and dielectric properties of Ba0.5Sr0.5TiO3–Zn2TiO4 composite ceramics with low sintering temperature for tunable device applications

Ba_0.5Sr_0.5TiO₃–Zn₂TiO₄ composite ceramics with low dielectric constant and high tunability are fabricated at a relatively low sintering temperature of 1200 °C via the conventional solid-state reaction route. Zn₂TiO₄ and Ba_0.5Sr_0.5TiO₃ can be friendly coexistent in the composite material system. The dielectric constant is tailored from 2500 to 83 by manipulating the addition of Zn₂TiO₄ content from 0 wt.% to 80 wt.% weight ratio. The dielectric loss still keeps around 0.002 and the tunability is 10.3% under a DC-applied electric field of 30 kV/cm at 10 kHz for the 80 wt.% Zn₂TiO₄ added Ba_0.5Sr_0.5TiO₃ composite ceramics. These composite ceramics are promising candidates for multilayer low-temperature co-fired ceramics (LTCC) and potential tunable devices applications.     

Thermal conductivity and dielectric properties of Al/PVDF composites

Polymeric composites with relatively high thermal conductivity, high dielectric permittivity, and a low dissipation factor are obtained in the present study. Three types of core-shell-structured aluminum (Al) particles are incorporated in poly(vinylidene fluoride) (PVDF) by melt-mixing and hot-pressing processes. The morphological, thermal, and dielectric properties of the composites are characterized using thermal analysis, a scanning electron microscope, and a dielectric analyzer. The results indicate that the Al particles decrease the degree of crystallinity of PVDF, and that the particle size and shape of the filler affect the thermal conductivity and dielectric properties of Al/PVDF. No variation in the dissipation factor is observed up to 60 wt.% Al. Thermal conductivity and dielectric permittivity values as high as 1.65 W/m K and 230, respectively, as well as a low dissipation factor of 0.25 at 0.1 Hz, are realized for the composites with 80 wt.% spherical Al.     

Rubber composites based on graphene nanoplatelets,expanded graphite,carbon nanotubes and their combination: A comparative study

Solution styrene butadiene rubber (S-SBR) composites reinforced with graphene nanoplatelets (GnPs), expanded graphite (EG), and multiwalled carbon nanotubes (MWCNTs) were prepared and the electrical and various mechanical properties were compared to understand the specific dispersion and reinforcement behaviours of these nanostructured fillers. The electrical resistivity of the rubber composite gradually decreased with the increase of filler amount in the composite. The electrical percolation behaviour was found to be started at 15 phr (parts per hundred rubber) for GnP and 20 phr for EG filled systems, whereas a sharp drop was found at 5 phr for MWCNT based composites. At a particular filler loading, dynamic mechanical analysis and tensile test showed a significant improvement of the mechanical properties of the composites comprised of MWCNT followed by GnP and then EG. The high aspect ratio of MWCNT enabled to form a network at low filler loading and, consequently, a good reinforcement effect was observed. To investigate the effect of hybrid fillers, MWCNT (up to 5 phr) were added in a selected composition of EG based compounds. The formation of a mixed filler network showed a synergistic effect on the improvement of electrical as well as various mechanical properties.     

Graphene sheets segregated by barium titanate for polyvinylidene fluoride composites with high dielectric constant and ultralow loss tangent

In this paper, we report a unique method to develop polyvinylidene fluoride (PVDF) composites with high dielectric constant and low loss tangent by loading relatively low content of graphene-encapsulated barium titanate (BT) hybrid fillers. BT particles encapsulated with graphene oxide (BT-GO) were prepared via electrostatic self-assembly and subsequent chemical reduction resulted in BT-RGO particles. SEM morphology revealed that RGO sheets were segregated by BT particles. The hybrid fillers have two advantages for tuning dielectric properties: loading extremely low content of RGO can be exactly controlled and individual RGO sheets segregated by BT particles would prevent leakage current. As a result, PVDF composites filled with BT-RGO displayed improved dielectric properties before percolative behavior occurred. Composites filled with 30 vol% BT-RGO have a dielectric constant and loss tangent (tan δ) value of 67.5 and 0.060 (1 kHz), respectively. By contrast, dielectric constant and tan δ of composites filled with 30 vol% BT-GO and BT were 57.7 and 38.3, 0.076 and 0.042 (1 kHz), respectively. The improvement of dielectric constant is attributable to the formation of microcapacitors by highly conductive RGO sheets segregated by BT particles. Meanwhile, the distance between adjacent RGO sheets is large enough to prevent leakage current from tunneling conductance, by which tan δ is remarkably constrained. The composites could achieve excellent dielectric properties by loading relatively low amount of ceramic fillers, which indicates that this method can be used as guideline for reduce the usage amount of ceramic fillers.     

Highly thermally conductive POSS-g-SiCp/UHMWPE composites with excellent dielectric properties and thermal stabilities

Polyhedral oligomeric silsesquioxane grafting thermally conductive silicon carbide particle (POSS-g-SiCp) fillers, are performed to fabricate highly thermally conductive ultra high molecular weight polyethylene (UHMWPE) composites combining with optimal dielectric properties and excellent thermal stabilities, via mechanical ball milling followed by hot-pressing method. The POSS-g-SiCp/UHMWPE composite with 40 wt% POSS-g-SiCp exhibits relative higher thermal conductivity, lower dielectric constant and more excellent thermal stability, the corresponding thermally conductive coefficient of 1.135 W/mK, the dielectric constant of 3.22, and the 5 wt% thermal weight loss temperature of 423 °C, which holds potential for packaging and thermal management in microelectronic devices. Agari’s semi-empirical model fitting reveals POSS-g-SiCp fillers have strong ability to form continuous thermally conductive networks.     

Electrical and dielectric properties of polypropylene nanocomposites based on carbon nanotubes and barium titanate nanoparticles

Functional polypropylene (PP) nanocomposites were prepared by melt compounding with multiwalled carbon nanotubes (MWNT) as the electrically conductive component and barium titanate (BT) spherical nanoparticles as the ferroelectric component. To make PP electrically conductive, more than 3 wt.% MWNT is required. Surface modification of either MWNT or BT with titanate coupling agent further improves the electrical conductivity of the PP/MWNT/BT ternary nanocomposites. Interestingly, by modifying both MWNT and BT, 2 wt.% MWNT are sufficient to make the ternary nanocomposite electrically conductive. In addition, the incorporation of MWNT greatly increases the dielectric permittivity of PP/BT nanocomposites. However, to retain a low dielectric loss, the MWNT loading should be slightly less than the percolation threshold of the nanocomposites. The improved electrical conductivity and dielectric properties make the ternary nanocomposites attractive in practical applications.