Large dielectric constant in zirconia polypyrrole hybrid nanocomposites

Zirconia nanoparticles have been synthesized by a novel two-reverse emulsion technique and combined with polypyrrole (PPY) to form ZrO2-PPY nanocomposites. Complex impedance and dielectric permittivity of ZrO2-PPY nanocomposite have been investigated as a function of frequency and temperature for different compositions. The composite samples are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning and transmission electron microscopy. The composites reveal ordered semiconducting behaviour. Polypyrrole is the major component in electrical transport process of the samples. A very large dielectric constant of about 12,000 at room temperature has been observed. The colossal dielectric constant is mainly dominated by interfacial polarization due to Maxwell-Wagner relaxation effect. Two completely separate groups of dielectric relaxation have been observed. The low frequency dielectric relaxation arises from surface defect states of zirconia nanoparticles. The broad peak at high frequency is due to Maxwell-Wagner type polarization.     

High dielectric permittivity in silica-polyaniline nanocomposites

Complex impedance and dielectric permittivity of wide band gap silica-conducting polyaniline nanocomposite have been investigated as a function of frequency and temperature at different compositions. Grain and grain boundary contributions are observed in impedance spectra. The results are interpreted in terms of two series connected equivalent circuit. The large dielectric permittivity approximately 7500 is found. Large value of permittivity is well described by Maxwell-Wagner polarization. Broad and asymmetric dielectric spectra are analyzed by Havriliak-Negami relaxation function.     

Low frequency ac conduction and dielectric relaxation in poly(N-methyl pyrrole)

The ac conductivity and dielectric constant of poly(N-methyl pyrrole) thin films have been investigated in the temperature range 77–350 K and in the frequency range 10 ²-10² Hz. The well defined loss peaks have been observed in the temperature region where measured ac conductivity approaches dc conductivity. These loss peaks are associated with the hopping of the charge carriers. The frequency and temperature dependence of ac conductivity have been qualitatively explained by considering the contribution from two mechanisms; one giving a linear dependence of conductivity on frequency and other having distribution of relaxation times giving rise to broad dielectric loss peak. Paper presented at the 5th IUMRS ICA98, October 1998, Bangalore.     

Dielectric behaviour of strontium tartrate single crystals

Strontium tartrate trihydrate (STT) crystals have been grown in silica hydrogel. Various polarization mechanisms such as atomic polarization of lattice, orientational polarization of dipoles and space charge polarization in the grown crystals have been understood using results of the measurements of dielectric constant (έ′) and dielectric loss (tan δ) as functions of frequency and temperature. Ion core type polarization is seen in the temperature range 75–180°C, and above 180°C, there is interfacial polarization for relatively lower frequency range. One observes dielectric dispersion at lower frequency presumably due to domain wall relaxation.     

Low-frequency dielectric permittivity of C60

Complex dielectric response measurements have been made on a C₆₀/C₇₀ mixture pellet. The frequency and temperature ranges cover from 20 Hz to 1 MHz and from 30 to 300 K. Results show that the real part of the dielectric constant is 5 with a weak thermally activated polarization contribution. This thermal polarization is believed to be related to reorientation of C₆₀ molecules recently observed by many techniques such as NMR, sound velocity, thermal conductivity, and others. Our data yield relaxation frequencies for reorientation, indicating that the presence of an additional electric dipole moment of the orientationally ordered C₆₀ molecules at low temperature contributes 0.2–0.3 to the static dielectric constant.We wish to thank William Jenks and Chris Rey for their assistance and advice in performing the measurements. We appreciate helpful discussions with E. Manousakis and J. Cioslowski. One of us (G.C.) acknowledges the support by the National Science Council of R.O.C. under grant No. NSC82-0208-M-194-030.     

Electrical Relaxation in Mixed Alkali Bi2O3-K2O-Li2O-Fe2O3 Glasses

A new glass system(Bi2O3)50(Fe2O3)10(Li2O)x(K2O)40-x, where x changes in steps of 5 mole fraction between 0 and 40, was selected to study the electrical relaxation and the mixed alkali effect(MAE)phenomena.Measurements of ac conductivity σac, dielectric permittivity ε' and loss factor tanδ in the frequency range of 0.12～102 kHz and in the temperature range of 300～650 K were carried out. The temperature dependence of the ac conductivity shows a slow increasing rate at low temperature and high frequency and a rapid increase at high temperature and low frequency. At constant temperature, the ac conductivity is found to be proportional to ωs, where s is the frequency exponent, which is less than 1. Analysis of the conductivity data and the frequency exponent shows that the overlapping large polaron tunnelling(OLPT)model of ions is the most favorable mechanism for the ac conduction in the present glass system. The ac response, the dc conductivity and dielectric relaxation have the same activation energy and they originate from the same basic transport mechanism. The results of the dielectric permittivity show no maximum peak in the temperature and frequency range studied. This absence of maximum peak is an indication of non-ferroelectric behavior of all the studied samples. The MAE has been detected in the ac conductivity, which is the same as the classical MAE in the dc conductivity. The electrical parameters such as dielectric permittivity ε' and real dielectric modulus M'show a typical minimum deviation from linearity by about two orders of magnitude. The loss factor tanδ and the imaginary dielectric modulus M" are insignificantly dependent on composition even at the same transition temperature Tg.     

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 Sr3CuNb2O9 perovskite ceramics

The dielectric constant ? and loss tangent tanδ of Sr₃CuNb₂O₉ perovskite ceramics prepared by solid-state reactions have been measured at temperatures from 300 to 900 K and frequencies from 25 to 1 × 10⁶ Hz. The results demonstrate that the samples slowly cooled from the temperature of the final, high-temperature firing (1200°C) have relatively low permittivity (? ? 10) and dielectric losses (tanδ ? 0.005 at 1 kHz) at room temperature, with no strong dielectric dispersion and no prominent maxima in the temperature dependences of their permittivity and dielectric loss. The ceramics quenched from 1300°C exhibit a pronounced Debye-type low-frequency relaxation and strong dielectric dispersion in conjunction with high permittivity ? ? 2000 at low frequencies and/or high temperatures. The observed dielectric anomalies in the Sr₃CuNb₂O₉ ceramics can be understood in terms of Maxwell-Wagner relaxation at dielectric inhomogeneities associated with the quenching-induced difference in oxygen-vacancy concentration between the grain bulk and surface layer.     

Deposition and dielectric properties of CaCu3Ti4O12 thin films deposited on Pt/Ti/SiO2/Si substrates using radio frequency magnetron sputtering

Polycrystalline CaCu₃Ti₄O₁₂ thin films were deposited on Pt(111)/Ti/SiO₂/Si substrates using radio frequency magnetron sputtering. The phase formation and the physical quality of the films were crucially dependent on the substrate temperature and oxygen partial pressure. Good quality films were obtained at a substrate temperature of 650 °C and 4.86 Pa total pressure with 1% O₂. The dielectric constant (∼ 5000 at 1 kHz and 400 K) of these films was comparable to those obtained by the other techniques, eventhough, it was much lower than that of the parent polycrystalline ceramics. For a given temperature of measurements, dielectric relaxation frequency in thin film was found to be much lower than that observed in the bulk. Also, activation energy associated with the dielectric relaxation for the thin film (0.5 eV) was found to be much higher than that observed in the bulk ceramic (0.1 eV). Maxwell-Wagner relaxation model was used to explain the dielectric phenomena observed in CaCu₃Ti₄O₁₂ thin films and bulk ceramics.     

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.     

CaCu3Ti4O12中的晶界弛豫

研究了CaCu3Ti4O12在低温低频下的内耗曲线特征,发现金属中来源于晶界的内耗规律也适合CaCu3Ti4O12材料.测量了晶界弛豫激活能的大小和不同温度下该材料的特征弛豫时间.与常温的情况相比,低温下CaCu3Ti4O12的特征弛豫时间明显增加.分析表明:这种弛豫时间的增加来源于在畴区上极化弛豫的动态慢化效应.     

Dielectric relaxation and thermal studies on dispersed phase polymer nanocomposite films

Dispersed phase polymer nanocomposite films (PNC) based on PMMA–LiClO₄+ n-YSZ, has been prepared. The effect of filler concentration on dielectric constant, tanδ and ac conductivity has been observed. For each PNC films the activation energy for relaxation (E_τ) is almost same as the activation energy for ion conduction (E_a). The dc conductivity, the hopping frequency of charge carriers have been obtained at different temperature from the analysis of the ac conductivity data. For all the PNC films, the concentration of charge carriers has been calculated at different temperature using Almond–West formalism. The estimated activation energies for the dc conductivity and the hopping frequency are different, which indicates that the both charge carrier mobility and concentration contribute significantly to the ionic conductivity of polymeric electrolyte. Contribution of charge carrier mobility to the total conductivity has also been confirmed from the differential scanning calorimetry analysis. Improvement in thermal stability has been noticed with filler addition.     

Effect of silver nanoparticle embedment on the frequency dispersive conductivity and electrical relaxation dynamics in dodecylbenzenesulfonic acid-doped polyaniline

The present work aims at studying electrical relaxations in silver–polyaniline nanocomposites using dielectric spectroscopy. The nanocomposites of dodecylbenzenesulfonic acid-doped polyaniline (PANI) with different concentrations of silver nanoparticles (~6–12 nm) are synthesized by simple wet-chemical route. The temperature dependence of dc conductivity in all the samples follows three-dimensional variable range-hopping conduction mechanism. The loss factor, after having subtracted the dc contribution, shows a relaxation peak which simultaneously attributes to the frequency dispersion in conductivity spectra. The observed dielectric relaxation is well fitted by the Havriliak–Negami function, and the fitting parameters are determined. The temperature dependence of characteristic relaxation frequency and dc conductivity is in line with each other and bear a resemblance to the origin of dc transport and dielectric relaxation in these systems. The temperature behavior of the relaxation strength confirms that an exclusive hopping conduction of polarons in the disordered PANI matrix can be considered as the origin of the observed electrical properties of the systems. Further, the self-similar behavior of the real part of normalized ac conductivity, within the measured temperature range, also confirms the above inferences. The variation of frequency exponent with temperature suggests that ac conduction is due to the correlated barrier hopping of polarons which strongly affected by the dispersion of silver nanoparticles within the PANI matrix.     

Temperature dependence of ferroelectric and dielectric properties of textured 0.98(0.94Na0.5Bi0.5TiO3–0.06BaTiO3)–0.02K0.5Na0.5NbO3 thick film

Textured 0.98(0.94Na_0.5Bi_0.5TiO₃–0.06BaTiO₃)–0.02K_0.5Na_0.5NbO₃ thick film was prepared by reactive templated grain growth (RTGG) method. The effect of temperature on ferroelectric and dielectric behaviors of the thick film was investigated. Its dielectric constant as a function of temperature displayed typical relaxation behavior, which was similar to that of NBT-based bulk ceramics. Remnant polarization, saturation polarization, and coercive field of the thick film all decreased with increasing temperature. Dielectric constant and tunability of the film were also dependent on temperature. Electric field dependence of dielectric constant of the thick film suggested a transition from ferroelectric to antiferroelectric phase at around the depolarization temperature. A strong increase in leakage current density with increasing temperature was observed, which could be related to the enhanced activity of conductivity carriers.     

Dielectric studies of PZT-polymer composites

Lead zirconate titanate (PZT)/Nylon 66 composites having 3-3 connectivity have been prepared by a relatively simple fabrication process. The d.c. conductivity and dielectric constant were measured from 30 to 220° C at various frequencies (0.1, 1, 10, 100 kHz). The dielectric constant increased with increasing temperature and PZT-constant of the composite up to 50 vol% PZT, but later showed a decrease due to increase in porosity in the samples. The dielectric constant also increased rapidly at higher temperature (> 100°C) for lower frequencies (0.1 and 1 kHz) due to intergrain polarization. The dielectric loss increased with increasing temperature and with decreasing PZT content of the composite. D.c. conductivity increased continuously with increasing temperature.     

Dielectric relaxation related to single-ionized oxygen vacancies in (Pb1−xLax)(Zr0.90Ti0.10)1−x/4O3 ceramics

The dielectric relaxation phenomenon has been studied in lanthanum modified lead zirconate titanate ceramics in the high temperature paraelectric phase. The high temperature dielectric response revealed an anomalous behavior, which is characterized by an increase of the real component of the dielectric permittivity with the increase of the temperature. At the same time, a similar behavior, with very high values, has been observed in the imaginary component of the dielectric permittivity, which can be associated with conduction effects related to the conductivity losses. The frequency and temperature behavior of the complex dielectric permittivity has been analyzed considering the semi-empirical complex Cole-Cole equation. The activation energy value, obtained from the Arrhenius’ dependence for the relaxation time, was found to decreases with the increase of the lanthanum concentration and has been associated with single-ionized oxygen vacancies. The short-range hopping of oxygen vacancies is discussed as the main cause of the dielectric relaxation.     

Studies on electrical conductivity and dielectric behaviour of PVdF–HFP–PMMA–NaI polymer blend electrolyte

Polymer blend electrolytes composed of poly(vinylidene fluoride-co-hexafluoro-propylene), poly(methyl methacrylate) and 1·0 M NaI as salt have been synthesized using solution caste technique by varying the PVdF(HFP)–PMMA blend concentration ratio systematically. A.c. impedance studies were performed to evaluate the ionic conductivity of the polymer electrolyte films. The highest ionic conductivity at room temperature for [PVdF(HFP)–PMMA(4:1)](20 wt%) – [NaI(1·0 M)](80 wt%) system is found to be 1·67 × 10^???2 S cm^???1. XRD studies reveal complete complexation of the salt in the polymeric blend systems. The temperature dependence conductivity has been performed in the range of 303–373 K and it is observed that it obeys the Arrhenius behaviour. It has been observed that the dielectric constant, ε _r and dielectric loss, ε _i, increases with temperature in the lower frequency region and is almost negligible in the higher frequency region. This behaviour can be explained on the basis of electrode polarization effects. Plot of real part, M _r and imaginary part, M _i vs frequency indicates that the systems are predominantly ionic conductors. The phenomenon suggests a plurality of relaxation mechanism.     

Structural and dielectric properties of parylene-VT4 thin films

58% semi-crystalline thin parylene-VT4 (–H₂C–C₆F₄–CH₂−)_n films, have been investigated by dielectric spectroscopy for temperature and frequency ranges of [−120 to 380 °C] and [0.1–10⁵ Hz] respectively. The study comprises a detailed investigation of the dielectric constant, dielectric loss and AC conductivity of this fluoropolymer. Dielectric behavior of parylene-VT4 is represented by a low dielectric constant with values in the range of 2.05–2.35 while the dielectric losses indicate the presence of two relaxation processes. Maxwell−Wagner−Sillars (MWS) polarization at the amorphous/crystalline interfaces with activation energy of 1.6 eV is due to the oligomer orientation. Electrical conductivity obeys to the well-known Jonscher law. The plateau in the low frequency part of this conductivity is temperature-dependent and follows an Arrhenius behavior with activation energy of 1.17 eV (deep traps) due to the fluorine diffusion. Due to its thermal stability with a high decomposition temperature (around 400 °C under air and 510 °C under nitrogen) and due to its good resistivity at low frequency (10¹⁵–10¹⁷ Ω m⁻¹), parylene-VT4 constitutes a very attractive polymer for microelectronic applications as low k dielectric. Moreover, when parylene-VT4 is subjected to an annealing, the dielectric properties can be still more improved.     

Dielectric relaxation and alternating current conductivity of lanthanum, gadolinium, and erbium-polyvinyl alcohol doped films

Fourier transform infrared (FTIR) spectrum dielectric constant, ε', loss tangent, tan(δ), electric modulus, M*, and ac conductivity, σ(ac), of pure polyvinyl alcohol (PVA) as well as La-, Gd-, and Er-PVA doped samples have been carried out. The dielectric properties have been studied in the temperature and frequency ranges; 300-450?K and 1?kHz-4?MHz, respectively. FTIR measurements reveal that La(3+), Gd(3+), and Er(3+) ions form complex configuration within PVA structure. Two relaxation processes, namely, ρ and α were observed in pure PVA sample. The first process is due to the interfacial or Maxwell-Wagner-Sillers polarization. The second one is related to the micro-Brownian motion of the main chains. For doped PVA samples, α-relaxation process splits into α(a) and α(c). This splitting is due to the segmental motion in the amorphous (α(a)) and crystalline (α(c)) phases of PVA matrix. Electric modulus analysis was discussed to understand the mechanism of the electrical transport process. The behavior of ac conductivity for all PVA samples indicates that the conduction mechanism is correlated barrier hopping.