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.     

Electrical response of microcellular EPDM rubber composites: complex dielectric modulus formalism and current–voltage characteristics

Electrical response of conductive carbon black (Vulcan XC 72)-reinforced microcellular EPDM rubber composites has been studied as a function of variation in blowing agent and filler loading in the frequency range of 10–10⁵ Hz. The data was analyzed by dielectric modulus formalism. The examined system exhibit a strong dependence of dielectric modulus on the applied frequency. A gradual increase of real part of dielectric modulus with frequency is observed for all fillers and blowing agent loadings. The imaginary part of the dielectric modulus exhibited one relaxation peak with frequency at each filler and blowing agent loading. With increase in filler loading the peak shifts toward higher frequency whereas, with blowing agent loading the relaxation peak shifts toward lower frequency. The relationship between real and imaginary part of dielectric modulus shows a semicircular trend followed by a linear increase for all filler and blowing agent loadings. Hence, the presence of non-Debye type of relaxations has been confirmed. The effect of variations in filler and blowing agent loading on current–voltage characteristics has also been investigated. It is observed that with increase in filler and blowing agent loading, the nonlinearity of the curves increases and the point from which this nonlinearity starts decreases to lower voltage values. It is also observed that the electrical current is free from time when the measuring voltage is low. But as the applied voltage increase to 30 and 40 V, the electrical current changes with time.     

Nonexponential Dynamics in Porous Glasses

The dielectric relaxation properties of porous glasses prepared from sodium borosilicate glasses are studied by dielectric spectroscopy over a wide range of frequencies (20 Hz–1 MHz) and temperatures (–100–300°C). The dielectric behavior reflecting the geometric disorder is analyzed within the models describing the non-Debye slowly damped dynamics. It is found that the dielectric response is very sensitive to microstructural and mesostructural features of the porous matrix and the properties of a material filling pores. The response contains information on the dynamics of water molecules in pores, which accounts for the interaction of these molecules with the pore surface.     

Medium dielectric constant and low-loss PTFE composites filled with MgO-LiF co-doped Li2TiO3 particles

In this work, we presented a simple strategy to fabricate medium dielectric constant and low-loss composites for microwave substrate applications. MgO-LiF co-doped Li₂TiO₃ (LT) powders were fabricated by the solid-state reaction route and modified by perfluorooctyltriethoxysilane (F8261). The LT/polytetrafluoroethylene (PTFE) composites were fabricated by cold pressing and hot treatment. The XPS and contact angle analysis indicated that the fluorinated group was introduced to LT particle successfully. The effects of modified LT powders content on the dielectric, thermal, and mechanical properties of composites were investigated. As the modified LT content increases, the dielectric constant, dielectric loss, and temperature coefficient of dielectric constant (τ_ε) increase while the bending strength and coefficient of thermal expansion (CTE) decrease, which is attributed to the higher dielectric constant of LT ceramic, more pores, stable τ_ε of LT ceramic, interface defects and low CTE of LT ceramic, respectively. The composites with 60 wt % LT exhibit the best microwave dielectric properties: ε_r = 6.8, tanδ = 0.001, τ_ε = −29.6 ppm °C⁻¹ at 8 GHz and acceptable coefficient of thermal expansion (28.3 ppm °C⁻¹). Therefore, modified LT powders filled PTFE composites are potential materials for high-frequency microwave substrate applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47980.     

短切SiC纤维含量对SiC_(sf)/LAS复合材料性能的影响

用热压烧结法制备了不同纤维含量的SiCsf/LAS玻璃陶瓷复合材料,研究了该复合材料的微观结构、力学性能和微波介电性能。结果表明:随着SiC纤维含量的增加,SiCsf/LAS材料的抗弯强度先增加后降低,最大值为104MPa。由于碳界面层的形成,SiCsf/LAS比LAS具有更高的介电常数。当SiC短切纤维的质量分数为1.5%时,SiCsf/LAS复合材料介电常数具有最大值,其实部ε′和虚部ε″均值分别为48和66,并具有明显的频散效应。     

Dielectric relaxations of lead zirconate titanate films up to millimeter-wave frequencies

Modern high-frequency applications critically depend on the availability of data on the dielectric properties of functional materials in the microwave and millimeter-wave range. This paper investigates the dielectric response of polycrystalline lead zirconate titanate (PZT) thin films prepared by solution deposition at frequencies between 10 MHz and 70 GHz and temperatures of 305–395 K by measuring the S-parameters of coplanar waveguides (CPW). The real and imaginary parts of the permittivity of the PZT film are de-coupled from the electrical properties of both the fused silica substrate and the copper electrodes. Two dielectric relaxations are identified: one around 850 MHz, with room-temperature permittivity dropping from 1080 to 560, and one around 36 GHz, with permittivity dropping below 100, respectively. The low-frequency relaxation shows a shift to lower frequencies with increasing temperature; it is explained by the conventional Arlt model of mechanical shear mode resonance across the film thickness. The high-frequency relaxation, which is practically independent of temperature, is attributed to the response of ferroelectric domain walls.     

Microstructure,luminescence, and dielectric properties of microwave-sintered Ce:LuAG nano-ceramics

Herein, phase pure and highly crystalline Ce:LuAG nano-ceramics were fabricated using a novel, ultra-fast microwave sintering approach. The influence of microwave sintering on the microstructural, photoluminescence, and dielectric characteristics of Ce:LuAG nano-ceramic powders was examined. Microwave-assisted sintering of Ce:LuAG nano-ceramic powders yielded high crystallinity, low lattice strain, and reduced grain size. The process also improved the sintering kinetics and enhanced the surface diffusion between the grains, resulting in enhanced luminescence and dielectric properties. The Cole-Cole impedance plots showed single semicircular arcs, indicating non-Debye relaxation and a high dielectric constant in the microwave-sintered Ce:LuAG nano-ceramic and highlighting its potential for use in optoelectronics.     

碳纤维/玻璃纤维复合纤维毡的介电和吸波性能

考察了碳纤维/玻璃纤维复合并浇注硅溶胶制成的纤维毡的介电和吸波性能。结果表明:碳纤维的分散对复合材料的介电性能有很大影响。复合材料的介电常数随着碳纤维含量的增加而增加,随频率的增加而降低,具有明显的频响效应。复合材料具有明显的双峰吸收性能,且最高吸收峰随着厚度的增加向低频移动。当w(碳纤维)=1%,复合材料厚度为6、7、8 mm时,反射率小于-10 dB的频段分别为4.2~10.5、3.9~9.3、3.8~7.7 GHz。     

The production of carbon nanotube/epoxy composites with a very high dielectric constant and low dielectric loss by microwave curing

Multiwalled carbon nanotube (MWCNT)/epoxy (EP) composites were developed using microwave curing (m-MWCNT/EP). They have a very high dielectric constant and low dielectric loss. For comparison, composites based on the same components were also prepared by thermal curing (t-MWCNT/EP). Results show that the two types of composites have greatly different dielectric properties. With the same content of MWCNTs, m-MWCNT/EP composites show a much higher dielectric constant and lower dielectric loss than t-MWCNT/EP composites. Specifically, the dielectric constant and loss at 100 Hz of m-MWCNT/EP composite with 0.04 vol% MWCNTs are about 2.5 and 0.05 times the corresponding value of t-MWCNT/EP composites, respectively, because of their different structures. Compared with t-MWCNT/EP composites, the nanotubes in m-MWCNT/EP composites not only have a better dispersion in the matrix, but also align in a direction. An equivalent circuit model was set up to evaluate the influence of dispersion and spatial distribution of MWCNTs on the dielectric properties. It shows that it is possible to control the dispersion and spatial distribution of carbon nanotubes using a different curing technique to obtain high performance composites with unexpected dielectric properties, especially those with very high dielectric constant and low dielectric loss.     

Remediation of activation energy anomalies,scaling distortions,and bandwidth limitations in superionic conductivity modeling of NZSP NaSICON synthesized by an augmented SSR method

Less constrained by bandwidth limitations and sampling scarcity, broadband profiling in a wide temperature range, starting at the cryogenic threshold at ?150 °C and extending to 200 °C, can be used to derive parameters of minimal variance for the Jonscher power law for ionic conductivity; these are employed to model the superionic regime over elevated temperatures and frequencies beyond the limits accessed by contemporary electrochemical impedance spectroscopy (EIS) equipment. We apply this technique to non-stoichiometric NaSICON based on the canonical NZSP formula with 5% excess sodium, synthesized by an augmented solid-state reaction (SSR) method. We thoroughly analyze broadband conductivity, dielectric permittivity, and electric modulus data over the extended temperature range. Activation energy anomalies and scaling distortions inherent to the Arrhenius approximation are investigated, and an alternative formulation based on linearized difference equations is proposed to remedy these issues. With Cole–Cole analysis establishing non-Debye relaxation behavior, dissipation analysis is employed to identify relaxation bands, used for extracting initial condition parameters for the Jonscher power law. Finally, simulations of the AC dispersion region at high temperatures and frequencies suggest the dominance of polaron tunneling mechanisms instead of the classical ion hopping mechanism assumed for NaSICON, in line with the latest insights on superionic conduction.     

POM/PU/carbon nanofiber composites produced by water‐mediated melt compounding: Structure,thermomechanical and dielectrical properties

Binary and ternary composites composed of polyoxymethylene (POM), polyurethane (PU), and carbon nanofiber (CNF) were produced by water‐mediated melt compounding. PU latex and/or aqueous CNF dispersion were introduced into the molten POM in laboratory kneader to prepare toughened and/or nanoreinforced POM composites. The crystallization of the POM‐based systems was studied by polarized optical microscopy. The dispersion of the CNF was inspected in scanning electron microscopy. The mechanical and thermomechanical properties of the composites were determined by dynamic‐mechanical analysis, thermogravimetric analysis, short‐time creep‐, stress relaxation‐, and uniaxial static tensile tests. The dielectric response of the nanocomposites was investigated by means of broadband dielectric spectroscopy at ambient temperature. CNF worked as reinforcement (i.e., increased the stiffness, resistance to creep, tensile strength, and reduced the elongation at break), and also improved the thermo‐oxidative stability of POM. PU alone had an adverse effect to the above listed properties, which could be enhanced again by additional incorporation of CNF. Dielectric spectroscopy proved to be a useful tool to get deeper understanding on morphological changes caused by the additives. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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.     

Thermodynamic and Electrical Effects of Residual Carbon in Glass–Barium Titanate Composites for MLCC Applications

The thermodynamic and electrical effects of residual carbon in glass–barium titanate composites were investigated. Thermodynamic arguments are made suggesting that new phases form through carbothermal reactions between carbon and titanate-based dielectrics. Experimentally, it was shown that the reduced dielectric separated by insulating glass in the grain boundaries gave rise to a dielectric relaxation at room temperature consistent with the Maxwell–Wagner model. The conductivity of the dielectric was calculated based on the relaxation frequencies, allowing an estimate of the local oxygen activity due to the presence of carbon. This activity was consistent with the equilibrium oxygen activity for carbon oxidation at the processing temperature, and eight orders of magnitude different from the oxygen activity in the furnace atmosphere.     

Electrical properties of polyimide composite films containing TiO2 nanotubes

A study on the dielectric behavior of polyimide composite films containing different amounts of TiO₂ nanotubes (TNs) was performed. The films were prepared by casting solutions resulting from direct mixing of a poly(amic acid) and TNs onto glass plates, followed by thermal imidization. The influence of TNs content on the properties of polyimide composites was investigated. AFM and SEM analyses showed good compatibility between the filler and polymer matrix. Dynamic mechanical analysis and broadband dielectric spectroscopy were used to evidence relaxation processes into the films. The electrical properties were evaluated on the basis of dielectric constant and dielectric loss, and their variation with frequency and temperature. At moderate temperature a secondary β relaxation was observed while incorporation of TNs decreased the activation energy and facilitated the appearance of an additional β ₁ process. An α relaxation and a conductivity process were evidenced at higher temperatures. The values of dielectric constant and dissipation factor increased with TNs amount, and the maximum of σ relaxation peak shifted to higher temperatures. POLYM. COMPOS., 38:2584–2593, 2017. © 2015 Society of Plastics Engineers     

AC and DC dielectric properties of some polypropylene/calcium carbonate composites

AC dielectric properties and thermally stimulated polarization (TSP) and depolarization (TSD) currents were studied in a series of CaCO₃-filled polypropylene composites. The filler content (0 to 50 weight percent) and the average particle size (3.0 to 16.1 μm) at constant filler content (30 weight percent) were varied in separate groups of samples. In a third group of samples the filler (20 to 40 weight percent) was surface treated with stearates. The AC dielectric behavior of composites containing untreated fillers is largely determined by a small amount of adsorbed water. Upon heating, the dielectric properties show maxima (increasing with decreasing frequency) which disappear on cooling. In the case of stearate-treated fillers the dielectric loss level is higher, the dispersion and loss curves on heating reflect a combination of dipolar and protonic processes with water desorption. In the dry state the onset of an audio frequency relaxation process is observed in the pre-melting zone. The thermally stimulated currents of the composites containing treated and untreated fillers are also different. In the case of the untreated fillers the TSP curves show maxima indicating water desorption which are increasingly intense and roughly exponential with filler content. The high temperature conductivity and the intensity of the pre-melting depolarization peak pass through a minimum as a function of filler content. Above 20 weight percent filler content the activation energy of high temperature conductivity decreases. In the case of the surface treated samples, the thermally stimulated response is different for “wet” and dried samples. The dry samples exhibit a relaxation between the amorphous and crystalline transitions of the matrix polymer which is probably due to interfacial relaxation caused by the enhanced surface conductivity of the stearate-treated fillers.     

Liquid crystalline (cyanoethylpropyl)cellulose and its optically anisotropic composites with acrylic polymers

Described herein are anisotropic polymer composites, made up of (cyanoethylpropyl)cellulose (CEPC) and acrylic polymer. A band texture (which can be seen with a polarisation microscope) forms in CEPC and its lyotropic solutions in acrylic monomer after orientation as a result of relaxation phenomena. The photopolymerisation of acrylic monomer (acrylic or methacrylic acid) in an oriented lyotropic solution of CEPC ‘freezes’ the orientation, and stable birefringent polymer composites are created. Thermooptical analysis shows that these composites exhibit thermally reversible anisotropy of optical properties. Molecular relaxations in such composites were investigated by means of dielectric and mechanical spectroscopies. The molecular relaxation processes, characteristic of pure components, are preserved in these composites. Two representations were used in dielectric spectroscopy: the temperature dependences of dielectric loss ε″ and of electric modulus M″. The latter was especially useful in the high temperature range, where ionic conductivity dominates the dielectric response.     

Effect of a carbon black surface treatment on the microwave properties of poly(ethylene terephthalate)/carbon black composites

A surface treatment was applied to carbon black to improve the electrical and microwave properties of poly(ethylene terephthalate) (PET)-based composites. Three different formamide solutions with 1, 2, and 3 wt % concentrations were prepared to modify the surface chemistry of carbon black. Microwave properties such as the absorption loss, return loss, insertion loss, and dielectric constant were measured in the frequency range of 8–12 GHz (X-band range). Composites containing formamide-treated carbon black exhibited enhancements in the electrical conductivity, electromagnetic interference (EMI) shielding effectiveness, and dielectric constant values when compared to composites with untreated carbon black. In addition, increases in the formamide solution concentration and carbon black content of composites resulted in an increase in the electrical conductivity, EMI shielding effectiveness, and dielectric constant values. The percolation threshold concentration of PET composites shifted from a 3 to 1.5 wt % carbon black composition with the surface treatment. The best EMI shielding effectiveness was around 27 dB, which was obtained with the composite containing 8 wt % carbon black treated with a 3 wt % formamide solution. Moreover, this composition gave the lowest electrical resistivity and the highest dielectric constant among the produced composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Microwave absorbing properties of ternary linear low‐density polyethylene/carbonyl iron powder/carbon black composites

Linear low‐density polyethylene (LLDPE) was used as polymer matrix, carbonyl iron powder (CIP) and carbon black (CB) were used as fillers, and ternary composites with microwave absorbing properties were prepared by melt blending. Transmission electron microscopy was used to characterize the prepared samples. The absorbing ability (reflection loss) of the prepared composites was measured using the arch method, and the electromagnetic parameters of composites were determined by the transmission/reflection method. The filler contents of CIP and CB have effects on the absorbing peak positions and reflection loss, and there is the optimum filler content in composites to obtain the maximum microwave absorbing. The microwave absorption of LLDPE/CIP/CB composites comes from the combining contributions of the dielectric loss and the magnetic loss. The synergistic effects of CIP and CB effectively improve the microwave absorbing properties of polymer composites. CIP and CB are uniformly distributed in the polymer matrix. The theoretical calculation results of the absorbing ability are in agreement with the experimental results using the transmission line theory. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Electrical Properties of Boron Nitride Matrix Composites: II, Dielectric Relaxations in Boron Nitride–Silicon Carbide Composites

A low-frequency Maxwell–Wagner relaxation, resulting from space-charge buildup, was observed when BN–SiC composites were subjected to an ac current. This paper discusses the Maxwell–Wagner relaxation and information that it reveals about the microstructure of the composite. A modified Maxwell–Wagner equation for the real part of the permittivity is used to model the data. SiC has a higher dielectric constant than that of BN. The microstructure of these composites is anisotropic, with BN platelets preferentially oriented perpendicular to the hot-pressing direction. Samples measured in this direction exhibit a lower dielectric constant than those measured parallel to the hot-pressing direction. In regard to frequency response, the permittivity of BN shows no dependence on frequency, whereas frequency dependence is seen with increasing SiC content.     

Microstructure and dielectric response of (Ba,Sr)TiO3 filler-dispersed resin composites

Resin-matrix composites dispersing low-loss dielectric ceramic filler have received a considerable interest for high-frequency application, because of their good shape flexibility and controllable dielectric properties. In this study, (Ba,Sr)TiO₃-type ceramic particles have been synthesized by KCl molten salt method to serve as filler particle for resin-matrix dielectric composite. Dielectric measurement confirmed that the composite fabricated by tape-casting demonstrated two times higher dielectric constant of 50.4 than the other composites fabricated by direct-casting using a metal mold. Pore-size distribution as well as ceramic filler content was strongly correlated with the formation of electrical flux in the composites to enhance dielectric constant.