Styrene butadiene rubber/aluminum powder composites—mechanical,morphological and electrical behaviors

Different weight fractions of aluminum (Al) powder viz., 10, 20, 30, 40, 50, 60 and 70 phr were incorporated into styrene butadiene rubber (SBR) matrix. The Al powder filled and vulcanized SBR composites have been characterized for mechanical properties such as tensile strength, tensile modulus and surface hardness. A drastical improvement in tensile strength and tensile modulus with increase in filler content of the composites was noticed. The electrical properties such as dielectric constant, tan delta and dielectric loss were measured for all the four compositions. The effect of volume fraction (0–70 phr) of conducting filler, frequency (100 kHz–30 MHz), temperature (25–75°C) and relative humidity on dielectric constant, dielectric loss and tan delta values of the composites were studied.     

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 (ε′), 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, ε′, 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.     

Conductive carbon black-filled ethylene acrylic elastomer vulcanizates: physico-mechanical,thermal, and electrical properties

The effect of conductive carbon black (CCB) on the physico-mechanical, thermal, and electrical properties have been investigated by various characterization techniques. Physico-mechanical properties of the vulcanizates were studied with variation of filler loading, which revealed that the tensile strength increased up to 20 phr (parts per hundred rubber) CCB loading, whereas at higher filler loading it decreased marginally. Furthermore, tensile modulus, tear strength, and hardness gradually increased with increase in filler loading. The compression set and abrasion loss decreased with increasing CCB loading. The bound rubber content (Bdr) of unvulcanized rubber was found to increase significantly with increasing CCB content. The crosslink density increased, whereas the swelling decreased with CCB loading. The thermal stability of the vulcanizates evaluated by thermogravimetric analysis (TGA) showed a minor increment with increase in CCB content. It is observed from the dynamic mechanical thermal analysis (DMTA) that the storage modulus (E′), loss modulus (E″), and glass transition temperature (T _g) of ethylene acrylic elastomer (AEM) matrix increased by incorporation of CCB. The dielectric relaxation characteristics of AEM vulcanizates such as dielectric permittivity (ε′), electrical conductivity (σ _ac), and electric moduli (M′ and M″) have been studied as a function of frequency (10¹ to 10⁶ Hz) at different filler loading. The variation of ε′ with frequency and filler loading was explained based on the interfacial polarization of the fillers within a heterogeneous system. The ε′ increased with increasing the CCB loading and it decreased with applied frequency. The frequency dependency of σ _ac was investigated using conduction path theory and percolation threshold limit. The σ _ac increased with increase in both CCB concentration and applied frequency. The M′ increased with applied frequency, however, it decreased above 30 phr filler. The M″ peak shifted towards higher frequency region and above 20 phr filler loading the peaks were not observed within the tested frequency region. The electromagnetic interference shielding effectiveness (EMISE) was studied in the X-band frequency region (8–12 GHz), which significantly improved with increase in CCB loading.     

Electrical and thermal properties of PTFE-Sr<Subscript>2</Subscript>ZnSi<Subscript>2</Subscript>O<Subscript>7</Subscript> composites

A new polymer-ceramic composite was prepared using PTFE and low loss Sr₂ZnSi₂O₇. The dielectric properties of the composite were studied in the microwave and radiofrequency ranges. The relative permittivity (ε_r) and dielectric loss (tan δ) increased with the filler loading from 0.10 to 0.50 volume fractions (v_f). The observed values of ε_r, thermal conductivity and coefficient of thermal expansion (CTE) were compared with the corresponding theoretical predictions. The ability of the composite towards moisture absorption resistance was studied as a function of filler loading. It was also found that the variation of ε_r was less than 2% in the temperature range 25–90 °C, at 1 MHz. For a filler content of 0.50 v_f, the PTFE/Sr₂ZnSi₂O₇ composite exhibited ε_r = 4.4_, tan δ = 0.003 (at 4–6 GHz), CTE = 38.3 ppm/°C, thermal conductivity = 2.1 W/mK and moisture absorption = 0.09 wt%.     

Preparation and characterization of cordierite filled PTFE laminates for microwave substrate applications

Phase pure cordierite (2MgO · 2Al₂O₃ · 5SiO₂) powder was prepared through solid state ceramic route. Silane coated cordierite powder was filled in the PTFE matrix through SMECH process comprising of sigma mixing, extrusion, calendering, followed by hot pressing, to fabricate flexible microwave substrates. Filling fraction of cordierite in the PTFE matrix was varied from 10 to 70 wt% and its effects on density, dielectric properties, coefficient of thermal expansion and water absorption were investigated. The morphology and filler distribution of the filled composite were studied by SEM. Waveguide cavity perturbation technique was employed to measure the dielectric properties of the composites at X-band (8.2–12.4 GHz). Dielectric constant and loss tangent were found increasing with filler loading from 10 wt% (ε _r′ = 2.17, tan δ = 0.0007) to 60 wt% (ε _r′ = 3.17, tan δ = 0.0034).     

Evaluation of a.c. conductivity behaviour of graphite filled polysulphide modified epoxy composites

Composites of epoxy resin having different amounts of graphite particles have been prepared by solution casting method. Temperature dependence of dielectric constant, tan δ and a.c. conductivity was measured in the frequency range, 1–20 kHz, temperature range, 40–180°C for 0.99, 1.96 and 2.91 wt% graphite filled and unfilled epoxy composites. It was observed that the dielectric constant, tanδ and a.c. conductivity increase with increasing temperature. Near the transition temperature the materials show anomalous behaviour for the observed properties. Peaks of dielectric constant, tan δ and a.c. conductivity were observed to shift towards lower temperature with increasing frequency. Clear relaxation (tan δ) peaks around 169°C were observed in epoxy resin, which shifted to lower temperature side on increasing the frequency. Addition of 2.91 wt% graphite shifted the tan δ peaks towards higher temperature side by creating hindrances to the rotation of polymer dipoles. Addition of 2–91 wt% graphite leads to an increased relaxation time τ of dipoles in polysulphide epoxy from 1.44 × 10⁻⁵− 3.92 × 10⁻⁵ (s) at 90°C by creating the hindrance to the rotation of dipoles.     

Dielectric properties of Fe(OH)3 thin films formed at solution-gas interface

Dielectric properties of solution-gas interface-formed Fe(OH)₃ thin-film capacitors (Al/Fe(OH)₃/Al) of various thicknesses have been studied in the frequency range 10–10⁶ Hz at various temperatures (300–443 K). Dielectric constant, ε, increases with increasing film thickness (d) and temperature (T) and decreases with increase of frequency (f). The loss factor (tan δ), showing pronounced minimum with frequency, increases with rise of temperature, and tan δ_min shifts to a higher frequency. The large increase in dielectric constant towards low frequency region indicates the possibility of an interfacial polarization mechanism in this region.     

Study of dielectric relaxation behavior of electron beam-cured conductive carbon black-filled ethylene acrylic elastomer

Composites based on ethylene acrylic elastomer (AEM) filled with a special type of conductive carbon black (CCB) have been prepared by two-roll mixing mill. The compression-molded sheet of the prepared composites have been subjected to electron beam (EB) radiation dose up to 400 kGy to induce radiation crosslinked composites. The crosslinked density has been calculated according to Flory–Rehner equation and is found to increase with increasing EB dose and CCB loading. Chain scission-to-crosslink density has been calculated by Charlesby–Pinner equation, which shows decreasing trend with increasing radiation dose. The dielectric relaxation behaviors of different doses of EB-treated AEM/CCB composites have been extensively studied as a function of frequency of applied electric field (10¹–10⁶ Hz), CCB loading [0–30 phr (parts per hundred)], temperature (25–120 °C), and EB dose (50–400 kGy). It is observed that the dielectric permittivity (ε′) increases with CCB loading and temperature, but decreases with increasing EB dose. This can be explained on the basis of interfacial polarization. Based on dielectric loss tangent (tan δ) values, it is observed that the dielectric relaxation time decreases with increases in the filler loading and temperature. However, it increases with increase in the radiation doses. Both the real and imaginary parts of the impedance (Z′ and Z″) have been found to decrease with increase in conductive filler loading. The AC conductivity (σ_ac) increases with increase in the CCB concentration, test temperature, and radiation doses, which is attributed to the more pronounced hopping and tunneling mechanism. The percolation threshold (φ_crit) occurred in the range of 16 phr CCB loading. The dispersions of CCB phase in AEM matrix below and above percolation have been captured by the transmission electron microscope photomicrographs.     

Dielectric and impedance spectroscopy studies on sisal fibre-reinforced polyester composite

Frequency and temperature dependence of dielectric constant (ε_r), dielectric loss (tanδ), ac conductivity (σ_ac) and complex impedance spectroscopy studies on cured polyester matrix and sisal fibre-reinforced polyester composites (SFRPC) have been investigated in the frequency range from 180 Hz to 1 MHz and temperature range from room temperature to 200 °C. The experimental results showed that with the incorporation of sisal fibre, the values of ε_r, tanδ and σ_ac are increased. It is also found that the values of ε_r and tanδ for both cured polyester matrix and SFRPC are decreased with increasing frequency, which indicates that the major contribution to the polarization may come from orientation polarization and interfacial polarization. The increasing value of ε_r with increasing temperature at a particular frequency is due to free motion of the dipole molecular chains within the cured polyester matrix and SFRPC at higher temperature.     

Dielectric properties of NaF-B2O3 glasses doped with certain transition metal ions

Dielectric constant ε, loss tan δ, a.c. conductivity Σ and dielectric breakdown strength of NaF-B₂O₃ glasses doped with certain transition metal ions (viz. Cu²⁺, VO²⁺, Ti⁴⁺ and Mn⁴⁺) are studied in the frequency range 10²-10⁷ Hz and in the temperature range 30–250°C. The values of ε, tan δ, Σ_a.c. are found to be the highest for Cu²⁺ doped glasses and the lowest for Mn⁴⁺ doped glasses. Activation energy for a.c. conduction and the value of dielectric breakdown strength are found to be the lowest for Cu²⁺ doped glasses and the highest for Mn⁴⁺ doped glasses. With the help of infrared spectra, increase in the values of ε and tan δ of these glasses with frequency and temperature are identified with space charge polarization. An attempt has been made to explain a.c. conduction phenomenon on the basis of quantum mechanical tunneling model (QMT)/carrier barrier hopping model.     

Dielectric properties of epoxy/short carbon fiber composites

The dielectric properties of epoxy/short carbon fiber composites at different concentrations 0, 5, 10 and 15% by weight, different thicknesses 2 and 4 mm, and frequency in the range from 20 Hz to 1 MHz were characterized. Scanning electron microscopy and differential scanning calorimetry were utilized. The alternating current (ac) electrical properties (complex impedance, dielectric constant, dielectric loss, real part of electric modulus, imaginary part of electric modulus, electrical conductivity, and relaxation time) were determined. It was found that the applied frequency, filler concentrations, and composite thickness affected the ac electrical properties of the epoxy/carbon fiber composites. The dielectric behaviors of the interfacial polarization between epoxy matrix and carbon fibers could be described by the Maxwell–Wagner–Sillars relaxation. The analysis of the complex electric modulus in the frequency range from 20 Hz to 1 MHz revealed that the interfacial relaxation followed the Cole–Davidson distribution of relaxation times. The universal power-law of ac conductivity was observed in the epoxy/carbon fiber composites. The calculated power exponent (near unity) is physically acceptable within this applied model.     

Studies on electrical properties of ( x ) BaTiO3?+?(1? x ) Ni0.92Co0.03Mn0.05Fe2O4 ME composites

The (1−x) Ni_0.92Co_0.03Mn_0.05Fe₂O₄ + (x) BaTiO₃ magnetoelectric (ME) composite have been prepared using conventional double sintering ceramic process where x varies as 1.00, 0.85, 0.70, 0.55 and 0.00. The presence of both phases has been confirmed by X-ray diffraction and the microstructure study will be carried out by SEM technique. The dc resistivity and thermo-emf of the samples have been studied with variation in temperature. The variation of dielectric constant (έ) and loss tangent (tan δ) will be measured in the frequency range 100 Hz–1 MHz. The ac conductivity has been derived from dielectric constant (έ) and loss tangent (tan δ). The static value of magnetoelectric conversion factor dc (ME)_H has been studied as a function of intensity of magnetic field.     

Frequency-dependent dielectric and electric modulus properties of Li–Ni–Sm–Fe–O spinel embedded in conducting polymer

Conducting polymeric nanocomposite containing Li–Ni–Sm–Fe–O spinel was synthesized by the chemical oxidizing of aniline in the presence of LiNi_0.5Sm_0.08Fe_1.92O₄ particles. The dielectric and electric modulus properties of the as-prepared samples were investigated over a frequency range from 10⁶ to 10⁹ Hz. The dielectric constant (ε′), dielectric loss (ε″) and dissipation factor (tan δ) for all samples presented relatively high values at low frequency and were found to decrease with the frequency. The values of ε′, ε″ and tan δ of the nanocomposite were lower than that of the pristine PANI. Electric modulus analysis had been carried out to understand the electrical relaxation process. The dielectric relaxation time for the nanocomposite became longer due to the introduction of LiNi_0.5Sm_0.08Fe_1.92O₄ particles lowering the crystallinity of PANI.     

Study of ac electrical conductivity and dielectric properties of ortho-hydroxy acetophenone azine thin films

AC electrical conductivity measurements were carried out in the temperature range 290–473 K over the frequency range 0.1–20 kHz of vacuum deposited Ortho-hydroxy acetophenone azine films. It was found that the ac electrical conductivity increases with frequency according to the relation σ_ac(ω) = A ω^s. The values of the dielectric constant, , slightly changed in higher frequencies irrespective of temperature change, whereas its value increases in higher temperature with the decrease in frequency. Also, the dielectric loss, ɛ′′, and tan δ, has been found to increase with raise in temperature and decrease in frequency. The obtained experimental data has been analyzed with reference to various theoretical models. The analysis shows that the correlated barrier hopping (CBH) model is the most appropriate mechanism for the ac electrical conduction in these films.     

Synthesis and dielectric properties of MXTi<Subscript>7</Subscript>O<Subscript>16</Subscript> (M = Ba and Sr; X = Mg and Zn) hollandite ceramics

MXTi₇O₁₆ (M = Ba and Sr; X = Mg and Zn) ceramics have been synthesized by the conventional solid state ceramic route. The dielectric properties such as dielectric constant (ε_r), loss tangent (tan δ) and temperature variation of dielectric constant (τ_εr) of the sintered ceramic compacts are studied using an impedance analyser up to 13 MHz region. The strontium compounds have relatively high dielectric constant and low loss tangent compared to the barium analogue. The phase purity of these materials has been examined using X-ray diffraction studies and microstructure using SEM method.     

Effect of kaolin–metal oxides core–shell pigments on the properties of styrene–butadiene rubber composites

This study investigated the effects of core–shell kaolin–metal oxide pigments on the rheological, physico-mechanical and dielectric properties of styrene–butadiene rubber composites. In this way, newly prepared core–shell pigments based on kaolin as the core representing 90% of the whole pigment was covered with different metal oxides (CaO, MgO and CaO⋅MgO) comprising the shell which represents only 10% of the prepared pigments were incorporated with different concentrations in styrene–butadiene (SBR) rubber composites. Studying the different properties of pigmented and unpigmented SBR composites were done. Scanning electron microscopy (SEM) was used to feature out the surface morphology. Addition of the new pigments increased the tensile strength and strain energy, while elastic modulus was decreased. This study revealed that there is a significant effect of the new prepared pigments on SBR properties and the optimum pigment loading was 40 phr for CaO/kaolin, while it was 2.5 phr for MgO/kaolin. The dielectric results also showed that, the values of ε′ (relative permittivity) and ε′′ (dielectric loss) increased with increasing core shell content. Moreover, the samples containing MgO/kaolin and MgO⋅CaO/kaolin showed promising dielectric properties with low relative permittivity and electrical insulating properties. The different measurements showed good agreement in their results.     

Synthesis,dielectric, and hysteresis behaviour of Ba(Nb<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> ceramics and their solid solutions with BaZrO<Subscript>3</Subscript>

Solid state reaction technique was employed to synthesize Ba(Nb_0.2Ti_0.8)O₃ [BNT], and 0.9Ba(Nb_0.2Ti_0.8)O₃ + 0.1BaZrO₃ [BNT + BZ] samples. Sintered pellets were investigated for its dielectric (ε_r and tanδ) properties in the temperature range 100 K–380 K and in the frequency range of 100 Hz–1 MHz. The variation of ε_r and tan δ may be attributed to hopping of trapped charge carriers, which resulted in an extra dielectric response in addition to the dipole response. Hysteresis loop measurements were studied in the temperature regime 295 K–423 K. Loop area shrunk with the increase of temperature that may be due to phase transition from ferroelectric to paraelectric state.     

Study of the structural and dielectric properties of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> and PbO addition on BiNbO<Subscript>4</Subscript> ceramic matrix for RF applications

In this paper, the structural and dielectric properties of BNO (BiNbO₄) was investigated as a function of the external RF frequency and temperature. The BNO Ceramics, prepared by the conventional mixed oxide method and doped with 3, 5 and 10 wt. % Bi₂O₃–PbO were sintered at 1,025 °C for 3 h. The X-ray diffraction patterns of the samples sintered, shown the presence of the triclinic phase (β-BNO). In the measurements obtained at room temperature (25 °C) was observed that the largest values of dielectric permittivity (ε′ _r ) at frequency 100 kHz, were for the samples: BNO5Bi (5 wt. % Bi₂O₃) and BNO5Pb (5 wt. % PbO) with values ε′ _r ~ 59.54 and ε′ _r ~ 78.44, respectively. The smaller values of loss tangent (tan δ) were for the samples: BNO5Bi and BNO3Pb (3 wt. % PbO) with values tan δ ~ 5.71 × 10⁻⁴ and tan δ ~ 2.19 × 10⁻⁴, respectively at frequency 33.69 MHz. The analysis as a function of temperature of the dielectric properties of the samples, obtained at frequency 100 kHz, showed that the larger value of the relative dielectric permittivity was about ε′ _r ~ 76.4 at temperature 200 °C for BNO5Pb sample, and the value smaller observed of dielectric loss was for BNO3Bi sample at temperature 80 °C, with about tan δ ~ 5.4 × 10⁻³. The Temperature Coefficient of Capacitance (TCC) values at 1 MHz frequency, present a change of the signal from BNO (−55.06 ppm/°C) to the sample doped of Bi: BNO3Bi (+86.74 ppm/°C) and to the sample doped of Pb: BNO3Pb (+208.87 ppm/°C). One can conclude that starting from the BNO one can increase the doping level of Bi or Pb and find a concentration where one have TCC = 0 ppm/°C, which is important for temperature stable materials applications like high frequency capacitors. The activation energy (H) obtained in the process is approximately 0.55 eV for BNO sample and increase with the doping level. These samples will be studied seeking the development ceramic capacitors for applications in radio frequency devices.     

Dielectric behaviour of erbium substituted Mn-Zn ferrites

Dielectric properties such as dielectric constant (ε′) and dielectric loss tangent (tan□δ) of mixed Mn-Zn-Er ferrites having the compositional formula Mn_0.58Zn_0.37Fe_2.05−xEr_x0₄ (where itx = 0.2, 0.4, 0.6, 0.8 and 1.0) were measured at room temperature in the frequency range 1–13 MHz using a HP 4192A impedance analyser. Plots of dielectric constant (ε′) vs frequency show a normal dielectric behaviour of spinel ferrites. The frequency dependence of dielectric loss tangent (tan δ) was found to be abnormal, giving a peak at certain frequency for all mixed Mn-Zn-Er ferrites. A qualitative explanation is given for the composition and frequency dependence of the dielectric constant and dielectric loss tangent. Plots of dielectric constant vs temperature have shown a transition near the Curie temperature for all the samples of Mn-Zn-Er ferrites. However, Mn_0.58Zn_0.37Er_1.0Fe_1.05O₄ does not show a transition. On the basis of these results an explanation for the dielectric mechanism in Mn-Zn-Er ferrites is suggested.     

Effect of bismuth on the properties of Mn ferrite nanoparticles prepared by co-precipitation method

Nanoparticles of MnBi_xFe_2−xO₄(x = 0.0, 0.02, 0.04, 0.06) have been synthesized by a chemical co-precipitation method. X-ray diffraction analysis (XRD) reveals that bismuth exits in its Bi³⁺ and Bi⁵⁺ state. It was also observed that bismuth substitution enhances the grain growth and density. The dielectric constant (ε′) and the loss tangent (tan δ) increases with increase in bismuth content. Resistivity (ρ) and maximum magnetization (M) was found to decrease with increasing bismuth content.