Effect of different forms of poly o‐toluidine/poly vinyl chloride composites on dielectric properties in the microwave field

In this paper we report the preparation and dielectric properties of poly o‐toluidine:poly vinyl chloride composites in pellet and film forms. The composites were prepared using ammonium persulfate initiator and HCl dopant. The characterization is done by TGA and DSC. The dielectric properties including dielectric loss, conductivity, dielectric constant, dielectric heating coefficient, absorption coefficient, and penetration depth were studied in the microwave field. An HP8510 vector network analyzer with rectangular cavity resonator was used for the study. S bands (2–4 GHz), C band (5–8 GHz), and X band (8–12 GHz) frequencies were used in the microwave field. Comparisons between the pellet and film forms of composites were also included. The result shows that the dielectric properties in the microwave field are dependent on the frequency and on the method of preparation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2253–2260, 2007     

Microwave Characteristics of Polyaniline Based Short Fiber Reinforced Chloroprene Rubber Composites

This report presents the microwave characteristics of conducting polymer composites (CPCs) based on chloroprene rubber with special reference to dielectric properties. CPCs based on polyaniline (PANI), polyaniline-coated short nylon fiber (PANI-N) and chloroprene rubber (CR) were prepared by mechanical mixing. The important properties like dielectric permittivity, loss tangent, conductivity, and dielectric heating coefficient were evaluated and compared. It was found that PANI/CR composites had very good dielectric properties in the microwave range. The addition of PANI-N improved the mechanical properties of the composites with reasonably good dielectric properties. The CPCs were also found to have good microwave absorption.     

Temperature dependent microwave attenuation behavior for carbon-nanotube/silica composites

SiO₂-matrix composites filled with 2, 5 and 10 wt.% multiwalled carbon nanotubes (MWCNTs) were prepared to evaluate the dielectric properties and microwave attenuation performances over the full X-Band (8.2–12.4 GHz) at a wide temperature ranging from 100 to 500 °C. On the basis of the conductivity induced by the structure of the MWCNT, the transport of migrating and hopping electrons in the MWCNT micro-current network has been discussed, and the effects of MWCNT content and temperature on the electronic transport and conductivity have been investigated. These effects also have great influences on the dielectric properties, electromagnetic wave propagating and microwave attenuation performances of the composites. The behavior of electromagnetic interference (EMI) shielding and microwave absorption provide the technical direction for the design of microwave attenuation materials and also indicate that CNT-based composites could be promising candidates for microwave attenuation application.     

Development of Low Density,Heat Resistant and Broadband Microwave Absorbing Materials (MAMs) for Stealth Applications

The development of low density and broadband microwave absorbers are the need of the hour to cater for the needs of all military platforms for stealth technology. The low density and broadband properties can be inculcated in microwave absorbers using dielectric lossy materials (e.g. carbon fibres, carbon nanotubes, carbon black, fullerene, graphite, graphene and silicon carbide fibre). Therefore, we designed low density and heat resistant microwave absorbing materials (MAMs) using a novel approach of ceramic fibre board manufacturing technology. The microwave absorbing composites were prepared with varying percentage of milled carbon fibres, discontinuous aluminosilicate fibres and silicone resin as the matrix. The physico-mechanical properties of microwave absorbing composites were determined. Reflection loss of microwave absorbing composites was measured in the frequency range 2–18 GHz by unique single horn interferometry technique. The electromagnetic properties were measured in X-band using free space measurement system. Based on these properties the effect of thickness on the microwave absorbing properties in X-band was simulated The effect of weight % variation of milled carbon fibres on the microwave absorbing properties of composites have been studied in the frequency range 2–18 GHz.     

Dielectric behavior of aluminum hydroxide‐filled oil‐extended ethylene‐propylene‐diene‐monomer rubber composites in microwave fields

Oil‐extended ethylene‐propylene‐diene rubber composites were prepared with aluminum hydroxide (ATH) and high abrasion furnace carbon black. The dielectric properties at microwave frequencies of the samples were measured in the S (2–4 GHz) band using cavity perturbation technique. The thermal stability of the composites was studied using thermogravimetric analysis. The morphology of the composites was investigated by scanning electron microscopic studies. The fire retardancy of the composites was identified through the limiting oxygen index and horizontal burning test (UL94 HB). The dielectric permittivity, AC conductivity, and absorption coefficient of the highly ATH loaded systems are much greater than the unfilled and lower systems. At higher loading, the dielectric heating coefficient and skin depth were found to decrease significantly. The incorporation of ATH was found to improve the thermal stability and flame retardancy of EPDM. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Effect of BaTiO3 on the microwave absorbing properties of Co‐doped Ni‐Zn ferrite nanocomposites

Coprecipitation and hydrothermal method were utilized for the synthesis of Co‐doped Ni‐Zn ferrite and barium titanate nanoparticles. The microwave absorption properties of Co‐doped Ni‐Zn ferrite/barium titanate nanocomposites with single layer structure were studied in the frequency range of 8.2–12.4 GHz.The spectroscopic characterizations of the nanocomposites were examined using X‐ ray diffraction, scanning electron microscopy, transmission electron microscopy and dynamic light scattering measurement. Thermogravimetric analysis indicated the high thermal stabilities of the composites. The composite materials showed brilliant microwave absorbing properties in a wide range of frequency in the X‐band region with the minimum return loss of ?42.53 dB at 11.81 GHz when sample thickness was 2 mm and the mechanisms of microwave absorption are happening mainly due to the dielectric loss. Compared with pure Co‐doped Ni‐Zn ferrite, Co‐doped Ni‐Zn ferrite/BaTiO₃ composites exhibited enhanced absorbing properties. The microwave absorbing properties can be modulated by controlling the BaTiO₃ content of the absorbers and also by changing the sample thicknesses. Therefore, these composites can be used as lightweight and highly effective microwave absorbers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39926.     

Electromagnetic radiation shielding by composites of conducting polymers and wood

Polyaniline or polypyrrole composites with fir or oak wood have been prepared by in situ polymerization of the corresponding monomers in an aqueous suspension of wood sawdust. The percolation threshold of compressed coated particles is located below 5 wt % of the conducting component and, above this limit, the conductivity of most composites was higher than 10^?3 S cm^?1. The conductivity of composites containing ca 30 wt % of conducting polymer was of the order of 10^?1 S cm^?1, an order of magnitude lower than that of the corresponding homopolymers, polyaniline and polypyrrole. The conductivity stability has been tested at 175°C. The polypyrrole‐based composites generally lasted for a longer time than pyrrole homopolymers, also on account of the improved mechanical integrity of the samples provided by the presence of wood. The reverse order was found with polyaniline composites. The dielectric properties of the composites were determined in the range of 100 MHz–3 GHz, indicating that thick layers of composite material, ～ 100 mm, are needed for the screening of the electromagnetic radiation below ?10 dB level in this frequency range. Nevertheless, considering the potential production cost of composites and their low weight, such composite materials could be of practical interest in the shielding of electromagnetic interference. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 807–814, 2005     

Polyaniline–antimony oxide composites for effective broadband EMI shielding

Conducting polyaniline (PAni)–antimony trioxide (Sb₂O₃) composites with different weight percentages (wt%) of Sb₂O₃ in PAni have been synthesized by in situ chemical oxidative polymerization. The composites were structurally and morphologically characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Measurements of electromagnetic interference (EMI) shielding, complex permittivity and microwave absorbing as well as reflecting properties of the composites were carried out in the frequency range of 8–18 GHz, encompassing the microwave X and Ku bands of practical relevance. All the computations are based on microwave scattering parameters measured by transmission line waveguide technique. It is observed that the presence of Sb₂O₃ in the PAni matrix affects the electromagnetic shielding and dielectric properties of the composites at microwave frequencies. The composites have shown better shielding effectiveness (SE) in both the X (SE in the range ?18 to ?21 dB) and Ku (?17.5 to ?20.5 dB) bands. ε′ and ε′′ values of the PAni–Sb₂O₃ composites are in the range of 64–37 and 63–30, respectively, in the frequency range of 8–18 GHz. Dielectric measurements indicated the decrease in dielectric constant with the increase in wt% of Sb₂O₃. The results obtained for the reflection and absorption coefficients indicated that PAni–Sb₂O₃ composites exhibit better electromagnetic energy absorption throughout the X and Ku bands. The results indicated that PAni–Sb₂O₃ composites can be used as potential microwave absorption and shielding materials.     

Structural,thermal and dielectric properties of AlN–SiC composites fabricated by plasma activated sintering

Highly dense AlN–SiC composites with various SiC additions (0–50?wt-%) were fabricated at 1800°C by plasma activated sintering. The effect of SiC addition on structural, thermal and dielectric properties as well as microwave absorbing performance of the composites was investigated. The thermal conductivity decreases with increasing SiC addition, from 68.7 W (m?K)^?1 for 0?wt-% SiC to 19.38?W (m?K)^?1 for 50?wt-% SiC. On the contrary, the permittivity and dielectric loss increase gradually, from 7.6–8.5 to 22–26.7 and from 0.02–0.1 to 0.2–0.53, respectively. AlN–SiC composite with better thermal and dielectric properties in 30?wt-% SiC, whose thermal conductivity and dielectric loss are found to be 24.88?W (m?K)^?1 and 0.15–0.74, respectively. Furthermore, the composite exhibits microwave absorbing performance with the minimum reflection loss (RL) of ?16.5 dB at 15.5 GHz and the frequency range of 2.6 GHz for RL below ?10 dB (90% absorption).     

Electromagnetic and microwave absorption properties of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size

The increasing electromagnetic interference problems have drawn much attention to microwave absorbing materials. To satisfy the needs of practical application, FeSiAl and flaky graphite filled Al₂O₃ composites were sintered by hot-pressing for microwave absorption application. The effect of FeSiAl particle size on the electromagnetic and microwave absorption properties was investigated in the X-band (8.2–12.4 GHz). The results show that the dielectric properties enhance significantly with increasing FeSiAl particle size, which is attributed to the increased interfacial polarization and conductance loss. As a result of the favorable impedance matching and appropriate electromagnetic attenuation, the reflection loss (RL) of the composites filled with 25–48 μm flaky FeSiAl achieves -15.2 dB at 10.6 GHz and the effective absorption bandwidth (RL < -10 dB) is 1.2 GHz in 10.0–11.2 GHz with a matching thickness of 1.0 mm. It indicates that FeSiAl and flaky graphite filled Al₂O₃ composites are potential candidates for thin-thickness microwave absorbing materials, and the microwave absorption properties can be enhanced by adjusting absorbent particle size.     

Effect of SiC nanowires on the high-temperature microwave absorption properties of SiCf/SiC composites

SiC-nanowire-reinforced SiC_f/SiC composites were successfully fabricated through an in situ growth of SiC nanowires on SiC fibres via chemical vapour infiltration. The dielectric and microwave absorption properties of the composites were investigated within the frequency range of 8.2–12.4 GHz at 25–600 °C. The electric conductivity and complex permittivity of the composites displayed evident temperature-dependent behaviour and were enhanced with increasing temperature. The composites exhibited superior microwave absorption abilities with a minimum reflection loss value of ?47.5 dB at 11.4 GHz and an effective bandwidth of 2.8 GHz at 600 °C. Apart from the contribution of the interconnected SiC nanowire network and multiple reflections, the excellent microwave absorption performance was attributed to dielectric loss that originated from SiC nanowires with abundant stacking faults and heterostructure interfaces. Results suggested that the composites are promising candidates for high-temperature microwave absorbing materials.     

Polymeric composites for use in electronic and microwave devices

The dielectric and conductive properties of thermoplastic (ABS) composites filled with ceramic powder (barium titanate), conductive powders (carbon black, copper) and conductive fibers (carbon, steel) were investigated for use in electromagnetic crystals and microwave devices. Barium titanate/ABS composites were produced that had dielectric constants over 8 and loss tangents of 0.01, which are the requirements for electromagnetic crystals. Carbon black/ABS and steel fiber/ABS composites were obtained with conductivities suitable for electromagnetic shielding (over 10^?3 S/cm). Fused decomposition modeling was tested as a method for building electromagnetic crystals and showed promising results. Polym. Eng. Sci. 44:588–597, 2004. © 2004 Society of Plastics Engineers.     

Controllable synthesis of FeSe2/rGO porous composites towards an excellent electromagnetic wave absorption with broadened bandwidth

Due to the escalating demand for electronic dependability and defense security, there has been a surge in research into broadband and lightweight microwave absorbers. Porous composites that are lightweight and plentiful in interfaces have the potential to be high-performance absorbers due to their ability to attenuate waves in a balanced manner and match impedance. “Using a solvothermal technique we generated FeSe₂/rGO composites with a porous topology. By varying the weight of rGO, the electromagnetic properties of FeSe₂/rGO composites may be finely tuned. Impedance matching and attenuation capability are both improved as a direct result of the porous structure and the appropriate electromagnetic parameters. FeSe₂/rGO composites benefit from the tunable composition, porous structure, and strong synergistic effect between FeSe₂ and rGO sheets and display outstanding microwave absorption performance with an ultrabroad bandwidth approaching 5.2 GHz with a thin thickness of 1.6 mm which covers 75% of the studied frequency range. At the same thickness, a significant reflection loss of ?43.7 dB is attained. This work not only enables the tuning of electromagnetic parameters but also expands the use of high-performance microwave absorption devices. Remarkable microwave absorption ability, of the porous composites FeSe₂/rGO can be utilized as a high-performance microwave absorber.”     

Microwave Absorption Studies on Superparamagnetic Conducting Nanocomposites as Signal Coating Materials

In this paper, preparation and characterization of superparamagnetic nanoparticles and their polymer composites prepared by varing doping level of conducting polymer and their microwave absorption studies at radar system in 8–12 GHz frequency range have been discussed. These composites are conducting polymers have been widely used because of their lower density as well their good environmental stability as in the case of polyaniline (PAN). In the present work, in situ polymerization of aniline was carried out in the presence of 30 mole% Fe₃O₄ nanoparticles to synthesize polyaniline/Fe₃O₄(PAN/Fe₃O₄) composites in epoxy resin matrix. The composites, thus synthesized have been characterized by infrared (IR) spectroscopy and X-ray diffraction. The morphology of these composites was studied by scanning electron microscopy. The measurement of % absorption was carried out in X and K band microwave region.     

Electromagnetic interference shielding behavior of polyaniline/graphite composites prepared by in situ emulsion pathway

Polyaniline–graphite composites were prepared via in situ emulsion pathway, using different weight ratios of aniline to graphite. These composites were characterized for thermal, electrical, and spectral attributes. The thermal stability (～ 230°C) and electrical conductivity (67.9 S/cm) were improved significantly as compared with polyaniline doped with conventional inorganic dopants such as HCl (140°C and 10 S/cm). Scanning electron micrographs indicated a systematic change in morphology from globular to flaky with increasing amounts of graphite. The relative shifting of UV–visible bands indicates that some interactions exist between doped polyaniline and graphite. Absorption‐dominated total electromagnetic interference shielding effectiveness of the order of ?33.6 dB suggests that these materials can be used as futuristic microwave shielding materials. The good electrical conductivity and thermal stability make them ideal candidates for preparing conducting composites by melt blending with conventional thermoplastics such as polyethylene, polypropylene, and polystyrene, etc. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Hybrid effects of polyvinyl alcohol (PVA) and basalt fibers on microwave absorption of cement composites with fly ash

Polyvinyl alcohol (PVA) and basalt fibers have been overlooked in the microwave absorption enhancement due to their low electromagnetic parameters. In this study, we examined the hybrid effects of PVA and basalt fibers on the microwave absorption of cement composites with high-volume fly ash. The mechanisms of the enhanced absorption were explored by investigating microstructures, phase compositions, and chemical bonds. It was found that reflection loss is significantly enhanced with variation in fiber proportion. Also, the fibers affect the dielectric properties using the Debye relaxation. In addition, in this paper, the relationship between the microstructure and electromagnetic parameters is established. It was also found that, in cement composites with high-volume fly ash, the hybrid effects of PVA and basalt fibers can improve the polymerization of the C–A–S–H gel, which influences these cement composites’ microstructures, and in turn, their Debye relaxation.     

The use of carbon/dielectric fiber woven fabrics as filters for electromagnetic radiation

Fabrics that allow selected microwave frequencies to pass through, called frequency selective fabric composites (FSFCs), were fabricated by weaving carbon fibers and dielectric fibers in periodic patterns. Design parameters affecting the electromagnetic characteristics (EM) of the FSFCs were widely discussed with respect to electrical conductivity of carbon fibers, the type of dielectric fiber and matrix, and weaving patterns. Transmission coefficients of square FSFCs with the aperture sizes of 10 mm and 20 mm were investigated considering electrical conductivity of carbon rovings, fiber undulation, and aperture-to-cell ratio. Compared with metallic frequency selective surfaces (FSSs), lower electrical conductivity of the carbon rovings caused a partial transmission near resonance frequency. The fiber undulation made little effect on the electromagnetic property of FSFCs. In addition, as the aperture-to-cell ratio decreased, the transmission of microwaves through FSFCs substantially decreased around resonance frequencies. The distinct difference in the microwave property of FSFC and FSS near resonance frequency shows that FSFCs can be new candidates as impedance modifier for microwave devices, such as microwave absorbers.     

Broadband Complex Dielectric Permittivity of Porous Aluminum Silicate–Pyrolytic Carbon Composites

The broadband (10 kHz–18 GHz) complex dielectric permittivity of electrically lossy aluminum silicate–pyrolytic carbon composites is studied as a function of carbon content. The composites are made from a commercial machinable ceramic that is treated by oxalic acid refluxing, followed by sucrose pyrolysis within the open porosity. The resulting composites have a slightly percolating conductor configuration that exhibits non-Debye dielectric properties. Such composites are useful as microwave absorbers in vacuum electronic amplifiers, for suppressing spurious oscillations, and for bandwidth control. Four contributions to the dielectric response were observed and analyzed. These consist of a direct current conductivity contribution that is significant only at frequencies below 1 MHz, an irregular low-frequency relaxation, a strong non-Debye, high-frequency relaxation with its upper portions extending into the microwave regime, and an underlying, broadband loss consistent with a universal dielectric response. The microwave behavior is dominated by changes in the position, intensity, and Cole–Cole exponent of the high-frequency relaxation as the carbon content increases.     

Plasticized/graphite reinforced phenolic resin composites and their application potential

Plasticized graphite (PG)/phenolic resin composites are candidates for positive temperature coefficient resistivity (PTCρ) thermistors, which are used for self‐recoverable elements that provide protection from overcurrents, gasoline sensors, and electrostatic charge and electromagnetic wave shielding in many kinds of electrical devices. The morphology and network structure of PG/phenolic resin composites have been characterized with scanning electron microscopy and with measurements of the crosslinking density, bound resin content, degree of crystallinity, viscosity, surface energy, thermal conductivity, enthalpy, and glass‐transition temperature. In addition, mechanical properties such as the tensile strength, Young's modulus, Shore A hardness, and elongation at break for resins filled with PG have been studied. The electrical properties of the composites have been measured to relate the PG volume fraction to the electrical conductivity. A large PTCρ value has been observed for all samples. The mechanism of the PTCρ effect in the materials is related to the thermal expansion and highest barrier energy of the composites. Switching behaviors of the current and voltage for all samples have been observed. The applicability of PG/phenolic resin composites for temperature controllers and gasoline gas sensors has been examined. The antistatic charge dissipation and dielectric constant as functions of the PG content have been studied. Finally, the experimental electromagnetic interference of the PG/phenolic composites has been investigated in the frequency range of 1–15 GHz and compared with a theoretical model. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 697–709, 2007     

Mechanical,thermal, and microwave properties of conducting composites of polypyrrole/polypyrrole‐coated short nylon fibers with acrylonitrile butadiene rubber

Pyrrole was polymerized in the presence of anhydrous ferric chloride as oxidant and p‐toluene sulfonic acid as dopant. Polypyrrole‐coated short nylon fibers were prepared by polymerizing pyrrole in the presence of short nylon fibers. The resultant polypyrrole (PPy) and PPy‐coated nylon fiber (F‐PPy) were then used to prepare rubber composites based on acrylonitrile butadiene rubber (NBR). The cure pattern, direct current (DC) conductivity, mechanical properties, morphology, thermal degradation parameters, and microwave characteristics of the resulting composites were studied. PPy retarded the cure reaction while F‐PPy accelerated the cure reaction. Compared to PPy, F‐PPy was found to be more effective in enhancing the DC conductivity of NBR. The tensile strength and modulus values increased on adding PPy and F‐PPy to NBR, suggesting a reinforcement effect. Incorporation of PPy and F‐PPy improved the thermal stability of NBR. The absolute value of the dielectric permittivity, alternating current (AC) conductivity, and absorption coefficient of the conducting composites prepared were found to be much greater than the gum vulcanizate. PPy and F‐PPy were found to decrease the dielectric heating coefficient and skin depth significantly. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012