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.     

Electrical relaxations in polymeric particulate composites of epoxy resin and metal particles

Composites of epoxy resins and nickel particles in various amounts were prepared and their dielectric spectra were measured in the frequency range 5 Hz–13 MHz and temperature interval from ambient to 140°C.The formalism of electric modulus proved to be efficient in analysing and interpreting obtained data.For these composites two relaxation processes are revealed in the frequency range and temperature interval of the measurements.One is an interfacial dielectric relaxation (Maxwell–Wagner–Sillars), MWS and the other is a conductivity relaxation. They both follow the Cole–Davidson approach with the exponent γ reflecting a distribution of relaxation times with the characteristics of each process.AC conductivity of these composites is frequency and temperature dependant, it generally follows the exponential law σ_ac∼ω^s and reveals a conductivity relaxation process, in the low frequencies.     

Potential of short Si–Ti–C–O fiber-reinforced epoxy matrix composite as electromagnetic wave absorbing material

The effects of fiber electrical properties on electromagnetic wave absorbing potential in short Si–Ti–C–O fiber-dispersed epoxy matrix composites were studied. Six kinds of short Si–Ti–C–O fibers with different respective electrical resistivity were incorporated into an epoxy matrix and the dielectric properties of the composites in a frequency range from 1 MHz to 1 GHz were measured. The penetration depth of electromagnetic wave, which is defined as the distance to reduce 1/e of the incident electromagnetic wave power, is obtained from the measured dielectric properties. It is found that the dielectric properties of the composites are strongly dependent on the electrical resistivity of the fiber: the use of lower electrical resistivity fiber leads to a shorter penetration depth. Independent of the electrical resistivity of fiber, the penetration depth decreases with increase in the frequency. This result demonstrates the potential of the composite as a thin electromagnetic wave absorbing material.

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Studies of the temperature coefficient of capacitance (TCC) of a new electroceramic composite: Pb(Fe0.5Nb0.5)O3 (PFN)–Cr0.75Fe1.25O3(CRFO)

In this paper we did a study of the dielectric properties: capacitance (C), dielectric permittivity(ε), electric modulus(M) and dielectric loss(tangα)) in the radio-frequency(RF) and microwave (MW) frequency range as a function of temperature and the temperature coefficient of capacitance (TCC) of the composites [Pb(Fe_1/2Nb_1/2)O₃(PFN)]_Z–[Cr_YFe_2−YO₃(CRFO)]_100−Z where Z = 0, 10, 50, 90 and 100%. The compounds were prepared by the powder-sintering and the X-ray diffraction (XRD) was used in this study using the Rietveld procedure. The X-ray analysis shows that PFN phase is tetragonal where the CRFO phase belongs to a trigonal structure. The capacitance, dielectric permittivity, electric modulus and loss were studied in the frequency range of 100 Hz to 1GHz and temperature range of 25–100 °C. The values of TCC for all the samples are presenting positive values. The study of the electric modulus (M) as a function of frequency and temperature lead to the measurement of the activation energy (E_ac), which is directed linked to the relaxation process associated to the interfacial polarization effect in the samples. The study of the electrical properties in the RF and MW range of frequencies is important in view of possible applications of these composites as future components in RF and MW circuits.     

Dielectric properties of carbon black: SBR composites

A series of styrene–butadiene rubber (SBR) composites have been prepared with different weight ratios of polyacetylene based conducting carbon black (CCB) (0–90 phr). The SBR–CCB systems are characterized for dimensional stability which is enhanced by increasing the CCB loading because of enhancement in polymer-filler interaction. The electrical properties such as dielectric constant (ε_r), dissipation factor (tan δ) and dielectric loss (ε″) of the composites have been studied. The influence of different loading of CCB (0–90 phr), frequency of ac (100 Hz–30 MHz) and temperature (25–75 °C) on the electrical properties was studied. An increase in dielectric constant and tan δ of the SBR composites was observed with increase in CCB content and ac frequency. This is due to enhancement of filler–filler interaction and the increase in continuity of conducting phase. The surface morphology has been studied using scanning electron microscopy (SEM).     

Electrical impedance of ethylene-carbon monoxide/propylene-carbon monoxide (<Emphasis Type="Italic">EPEC</Emphasis>-<Emphasis Type="Italic">69</Emphasis>) thermoplastic polyketone

Electrical impedance technique was employed to investigate the electrical properties of ethylene-carbon monoxide/propylene-carbon monoxide terpolymer (EPEC-69). The measurements were performed in the frequency range 0.1–10⁵ Hz and in the temperature range 30–110 °C. The results reveal that the dielectric constant, loss factor, modulus, and ac conductivity are dependent of frequency and temperature. A Debye relaxation peak was detected in the plot of Z″ versus frequency where the activation energy was determined and found to be 1.26 eV. When the surface phenomenon effects were separated using the imaginary part of the complex admittance a second dielectric dispersion was observed in the low frequency region. Two models were proposed from the impedance measurements depending on temperature range.     

Dielectric properties and structural dynamics of melt compounded hot-pressed poly(ethylene oxide)–organophilic montmorillonite clay nanocomposite films

The dielectric properties of melt compounded hot-pressed nanocomposite films consisting of a poly(ethylene oxide) (PEO) and organophilic montmorillonite (OMMT) clay surface modified with trimethyl stearyl ammonium as filler with increasing amount up to 20 wt.% OMMT were investigated in a frequency range of 20 Hz–1 MHz at 30 °C. The predominance of OMMT exfoliated structures in PEO–OMMT nanocomposites were recognized by a decrease of the real part of complex dielectric function. OMMT concentration dependent dielectric and electric modulus relaxation times have revealed that the interactions compatibility between PEO molecules and dispersed OMMT nano-platelets in PEO matrix governs the PEO segmental dynamics. A.C. conductivity of these nanocomposites increases by two orders of magnitude in the experimental frequency range.     

Effect of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> on the dynamics of Li<Superscript>+</Superscript> ions in Li<Subscript>2</Subscript>O·P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Lithium ion transport has been studied in bismuth lithium phosphate glasses in the frequency range 20 Hz–1 MHz and in the temperature range 423–573 K using impedance spectroscopy. The addition of Bi₂O₃ in Li₂O·P₂O₅ glass is related to the modification of the glass structure and facilitates the Li⁺ ions migration. The ac and dc conductivities, activation energy of the dc conductivity and relaxation frequency are extracted from the impedance spectra. Conductivity of the present glass system is found to be ionic in nature. The electrical response of the glasses has been studied using both conductivity and electric modulus formalisms. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates the temperature independence of the dynamic processes for ions in these glasses. Nearly identical values of activation energy for dc conduction and for conductivity relaxation time indicate that the ions overcome same energy barrier while conducting and relaxing.     

Dielectric relaxation in double perovskite oxide,Ho<Subscript>2</Subscript>CdTiO<Subscript>6</Subscript>

A new double perovskite oxide holmium cadmium titanate, Ho₂CdTiO₆ (HCT), prepared by solid state reaction technique is investigated by impedance spectroscopy in a temperature range 50–400°C and a frequency range 75 Hz–1 MHz. The crystal structure has been determined by powder X-ray diffraction which shows monoclinic phase at room temperature. An analysis of complex permittivity with frequency was carried out assuming a distribution of relaxation times as confirmed by Cole–Cole plot. The frequency dependent electrical data are analysed in the framework of conductivity and electric modulus formalisms. The frequencies corresponding to the maxima of the imaginary electric modulus at various temperatures are found to obey an Arrhenius law with an activation energy of 0·13 eV. The scaling behaviour of imaginary part of electric modulus suggests that the relaxation describes the same mechanism at various temperatures. Nyquist plots are drawn to identify an equivalent circuit and to know the bulk and interface contributions.     

Dielectric dispersion and ac conductivity in—Iron particles loaded—polymer composites

Polymer composites of an epoxy resin matrix with randomly dispersed Iron micro-particles in various amounts were prepared and their dielectric spectra were measured in the frequency range 5 Hz–13 MHz and temperature interval from ambient to 140 °C. Obtained data were analysed by means of electric modulus formalism.

Interfacial or Maxwell-Wagner-Sillars relaxation process was revealed in the frequency range and temperature interval of the measurements, which was found to follow the Cole-Davidson approach for the distribution of relaxation times.

The examined systems exhibit strong dispersion with frequency. At low frequencies ac conductivity tends to be constant, while at higher becomes frequency dependent varying as a power of frequency. Conductivity increases with temperature in the low frequency regime, remaining almost unaffected at higher frequencies.     

Dielectric and magnetic properties of Ca(Zn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>/NiZn ferrite composites

Dense Ca(Zn_1/3Nb_2/3)O₃/NiZn ferrite composites with homogeneously fine microstructures were prepared through conventional solid-state method. The powder XRD patterns confirm the coexistence of the two phases. The dielectric properties in the low frequency range (100 Hz–1 MHz) follow the rule of Maxwell–Wagner interfacial polarization. The dielectric and magnetic properties in the high frequency range (10 MHz–1 GHz) are also reported. The results show that this kind of magnetic–dielectric composites could be used in high-frequency communications for the capacitor-inductor integrating devices such as electromagnetic interference filters and antennas.     

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.     

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.     

Dynamic magneto-electric multiferroics PZT/CFO multilayered nanostructure

Highly oriented PbZr_0.53Ti_0.47O₃/CoFe₂O₄ (PZT/CFO) multilayered nanostructures (MLNs) were grown on MgO substrate by pulsed laser ablation using La_0.5Sr_0.5CoO₃ (LSCO) as conducting bottom electrode. The effect of various PZT/CFO (PC) sandwich configurations having three, five, and nine layers while maintaining total thickness of PZT and CFO be identical has been systematically investigated. X-ray diffraction (XRD) and micro-Raman spectra revealed the existence of pure PZT and CFO phases without any intermediate phase. Intact MLNs were observed by transmission electron microscopy (TEM) with little inter-diffusion near the interfaces at nano-metric scale without any impurity phase. Impedance spectroscopy, modulus spectroscopy, and conductivity spectroscopy were carry out over a wide range of temperatures (100–600 K) and frequencies (100 Hz–1 MHz) to investigate the grain and grain boundary effect on electrical properties of MLNs. Temperature dependent real dielectric permittivity and dielectric loss illustrated step-like behavior and relaxation peaks near the step-up characteristic, respectively. Cole–Cole plots indicate that most of the dielectric response came from the bulk (grain) MLNs below 300 K, whereas the grain boundary and the electrode–MLNs effects are prominent at elevated temperatures. The dielectric loss relaxation peak shifted to higher frequency side with increase in temperature, it was out of the experimental frequency window above 300 K. Our Cole–Cole fitting of dielectric loss spectra indicated marked deviation from the ideal Debye-type of relaxation, which is more at elevated temperature. Master modulus spectra supported the observation from the impedance spectra; it also indicated that the magnitude of the grain boundary compared to grain becomes more prominent with increase in number of layers. We have explained these electrical properties of MLNs by Maxwell–Wagner type contributions arising from the interfacial charge at the interface of the ML structures. Three different types of frequency dependent conduction processes were observed at elevated temperatures (>300 K), which fitted well with the double power law, indicating that the low frequency (<1 kHz) conductivity may be due to long-range ordering (frequency independent), mid frequency conductivity (<10 kHz) may be due to short-range hopping, and high frequency (<1 MHz) conduction due to the localized relaxation hopping mechanism. Ferroelectric polarization decreased slowly in reducing the temperature from 300 to 200 K, with complete collapse of polarization at ~100 K, but there was complete recovery of the polarization during heating, which was repeatable over many different experiments. At the same time, the temperature dependent remanent magnetization of the MLNs showed slow enhancement in the magnitude till 200 K with three-fold increase at 100 K compared to room temperature. This enhancement in remanent magnetization and decrease in remanent ferroelectric polarization on lowering the temperature indicate temperature dependent dynamic switching of ferroelectric polarization. The magnetic and ferroelectric properties of MLNs were quite different compared to individual layers suggesting its improper ferroelectric characteristics. The fatigue test showed almost 0–20% deterioration in polarization. Fatigue and strong temperature and frequency dependent magneto-electric coupling suggest MLNs utility for Dynamic Magneto-Electric Random Access Memory (DMERAM).     

Carbon black/graphite nanoplatelet/rubbery epoxy hybrid composites for thermal interface applications

Hybrid composites were developed by dispersing carbon black (CB) nanoparticles and graphite nanoplatelets (GNPs) at 4–6 and 12–14 wt%, respectively, into rubbery epoxy resin. SEM analysis showed that CB particles improved the dispersion of GNPs in the hybrid composite. The thermal conductivity of 4 wt% CB/14 wt% GNP-15/rubbery epoxy hybrid composite, 0.81 W/m K, is ca. four times higher than that of rubbery epoxy. When silane-functionalised, the fillers reduced the viscosity of the hybrid dispersion and made the hybrid composite highly electrically insulating. Nevertheless, filler functionalisation decreased the composite’s thermal conductivity by only 16.6%. Compression testing showed that the hybrid fillers increased the compressive modulus and strength of rubbery epoxy by nearly two and three times, respectively. Overall, the hybrid composites with their thermal paste-type morphology, low viscosity, high compliance, improved thermal conductivity and, when fillers are functionalised, low electrical conductivity makes them promising materials as thermal interface adhesives.     

Frequency and gate voltage effects on the dielectric properties and electrical conductivity of Al∕SiO(2)∕p-Si metal-insulator-semiconductor Schottky diodes

The dielectric properties and electrical conductivity of Al∕SiO(2)∕p-Si (MIS) Schottky diodes (SDs) in the frequency range of 10 kHz to 10 MHz and the gate voltage range of -2 to 6 V have been investigated in detail using experimental C-V and G∕w-V measurements. Experimental results indicated that the voltage dependence of the real part of the dielectric constant (?') and loss tangent (tan δ) characteristics have a peak at each frequency. The values of ?' increase with decreasing frequency and tend to be frequency independent in the negative voltage region. However, the values of the dielectric loss (?″) increase with decreasing frequency at each voltage. In contrast, ?' and ?″ are almost found to decrease, and the ac electrical conductivity (σ(ac)) and the real part of the electric modulus (M') increase, with increasing frequency. In addition, the imaginary part of the electric modulus (M″) showed a peak that shifts to a higher frequency with increasing applied voltage. It can be concluded that interfacial polarization can more easily occur at low frequencies, and consequently the majority of interface states at the Si-SiO(2) interface contribute to the deviation of the dielectric properties of Al∕SiO(2)∕p-Si (MIS) SDs.     

Dielectric relaxation in complex perovskite oxide In(Ni1/2Zr1/2)O3

The dielectric study of indium nickel zirconate, In(Ni_1/2Zr_1/2)O₃ (INZ) synthesized by solid state reaction technique is performed in a frequency range from 500 Hz to 1 MHz and in a temperature range from 303 to 493 K. The X-ray diffraction analysis shows that the compound is monoclinic. A relaxation is observed in the entire temperature range as a gradual decrease in ?′(ω) and as a broad peak in ?″(ω) in the frequency dependent real and imaginary parts of dielectric constant, respectively. The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. The frequencies corresponding to the maxima of the imaginary electric modulus at various temperatures are found to obey an Arrhenius law with activation energy of 0.66 eV. The Cole-Cole model is used to study the dielectric relaxation of INZ. The scaling behaviour of imaginary part of electric modulus suggests that the relaxation describes the same mechanism at various temperatures. The frequency dependent conductivity spectra follow the universal power law.     

Thermoelectric power and AC electrical properties of PAN-based carbon fiber composites

The thermoelectric power and AC electrical properties of conductive polymer composites made of polycarbonate filled with randomly distributed PAN-based carbon fibers of different concentration: 0, 5, 10, 18, and 30 wt% were studied. The thermoelectric power was measured as a function of temperature in the frequency range from 200 kHz to 12 MHz. It was found that the observed Seebeck coefficient and thermoelectric activation energy depend on temperature, frequency and fiber concentration. The Seebeck coefficient calculated using the electrical transport theory of semiconductors decreases with both increasing temperature and carbon fiber content. The thermoelectric power results revealed that the composites function electrically as semiconductors. Dielectric constants and AC conductivity were calculated from impedance and phase angle measurements. It was found that both increase with increasing temperature. The activation energy and relaxation time decrease with increasing temperature measured and applied frequency. The thermoelectric power results indicated that electrical conduction in bulk composites is produced from a combination of transport processes involving: electrons, holes, ions and charged impurity motion in addition to protonic migration.     

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.