Dielectric relaxation and ac conductivity in magnetoelectric YCrO3 ceramics: A temperature dependent impedance spectroscopy analysis

Here, we report on the temperature and frequency dependent electrical conduction and dielectric behaviour of YCrO₃ ceramics. Dielectric studies reveal a peak in the dielectric constant ～230?K, suggesting presence of spin-charge coupling. Also, an additional broad peak found at ～450?K is reminiscent of a relaxor like behaviour for YCrO₃, attributed to a diffused phase transition. The nature of dc conductivity is of Arrhenius type and shows an abrupt change in the activation energy at ～230?K and ～450?K. The activation energy suggests that the polaronic hopping mechanism stabilizes at low temperature while, at higher temperatures, the process is associated with the diffusion of double ionized oxygen vacancies. However, ac conductivity suggests that the overlapping large polaron tunnelling conduction mechanism drives the ac conduction below 300?K and above 300?K, the conduction behaviour is consistent with the correlated barrier hopping conduction mechanism.     

Effect of nano-porous Al2O3 on thermal, dielectric and transport properties of the (PEO)9LiTFSI polymer electrolyte system

Thermal, electrical conductivity and dielectric relaxation measurements have been performed on (PEO)₉LiTFSI+10 wt.% Al₂O₃ nano-porous polymer electrolyte system. It is observed that the conductivity enhances substantially due to the presence of the filler particles with different surface groups. The highest enhancement is found for the filler particles with acidic groups followed by basic, neutral, and weakly acidic. The results reveal that the filler particles do not interact directly with poly(ethelene) oxide (PEO) chains indicating that the main chain dynamics governing the ionic transport has not significantly affected due to the filler. The results are consistent with the idea that the conductivity enhancement is due to the creation of additional sites and favourable conduction pathways for ionic transport through Lewis acid-base type interactions between the filler surface groups and the ionic species. This is reflected as an increase in the mobility rather than an increase in the number of charge carriers. A qualitative model has been proposed to explain the results.     

Structural and dielectric properties of CuO nanoparticles

The DC conduction and dielectric behaviour of copper oxide nanoparticles prepared by sol-gel method and sintered at 950 °C were studied in the temperature range of 200–526 K. The formation of single phase monoclinic CuO was confirmed by x-ray diffraction. Chemical composition of the CuO ceramic was investigated with X-ray photoelectron spectroscopy (XPS) technique. Although XRD analysis shows the formation of single phase CuO, XPS spectra revealed the presence of Cu³⁺ and Cu²⁺. Deviation from linearity ln (σ_DC) vs. 1/T plot at ~390 K was observed, which indicates that DC conduction in the CuO pellet is dominated by two different conduction mechanisms. The results obtained on AC conductivity indicate that AC conduction mechanism could be well explained by the multihopping model at low frequencies, while high frequency AC conductivity data can be described by small polaron tunnelling model. The dielectric relaxation mechanism in the CuO pellet was studied by impedance spectroscopy. It was found that while dielectric constant is an increasing function of temperature, it decreases with increasing frequency. The obtained impedance spectra indicated that the grain boundary effects and intergranular activities play a crucial role on the dielectric relaxation processes.     

The role of electronic and ionic conduction in the electrical conductivity of carbon fiber reinforced cement

Electrical conduction in carbon fiber reinforced cement with a fiber volume fraction below the percolation threshold involves electrons and ions. The fiber affects both the electronic conduction and the ionic conduction. The ozone treatment of the fiber surface helps the ionic conduction. Latex as an admixture helps provide a relatively high ionic conductivity; silica fume as an admixture helps provide a relatively high electronic conductivity. In the dry state (the state of practical importance attained by room temperature drying), electronic conduction is more significant than ionic conduction. In the wet state (water saturated state), ionic conduction dominates. When silica fume is present with the fiber, the fractional electronic contribution in the dry state is 0.99. When latex is present with the fiber, the corresponding value is 0.72-0.78. The ratio of the wet ionic conductivity to the dry ionic conductivity is much increased by fiber surface treatment and is higher when latex rather than silica fume is used. The wet ionic conductivity is much higher than the dry overall conductivity when latex is present, but is lower than or comparable to the dry overall conductivity when silica fume is present; the wet ionic conductivity is lower than the dry overall conductivity when the fiber is not treated and silica fume is present.     

A novel cross-linked polyimide film: synthesis and dielectric properties

A novel cross-linked polyimide (CPI) has been prepared by imidization of cross-linked poly(amic acid) (CPAA). In this work, the Ac conductivity and dielectric properties of this polyimide are presented comparitively with those of conventional polyimide (PI), in the 0.2-100 kHz frequency range and 300-463 K temperature interval. Although the frequency and temperature dependencies of dielectric constant of both conventional and cross-linked polyimides show the same behaviour, the dielectric constant of CPI takes lower values. The Ac conduction studies suggest that electron hopping is responsible for conduction of the PI and CPI films. The activation energy calculated in 296-353 K temperature interval and the ß-relaxation was also observed for CPI.     

Epoxy polymers. V. Dielectric strength

This report deals with material properties which determine dielectric strength rather than phenomena that occur during breakdown. We propose that the determining factors are the same for liquids and polymers. We report dielectric strengths calculated from measured breakdown potentials and sample thicknesses for liquids, ionic solutions, and epoxy polymers doped with ionizable materials. The breakdown field (in liquids and polymers) is hypothesized to be that which generates some critical current density value in the dielectric. The reasons for this value being critical and characteristic of the material are unknown; however, the ionic current density that the measured breakdown field produces can be calculated from the Onsager theory of high-field electrolytic conduction, and this ionic current density is assumed to be the critical threshold quantity involved. Because of field effects on the dissociative ionization and conductivity of electrolytes in organic systems, the breakdown field varies with concentration and solute species, but the ionic current density associated with the breakdown field is essentially constant for a wide range of species and molar concentrations in a given solvent.     

Dielectric analyses of a series of poly(2-hydroxyethyl methacrylate-co-2,3-dihydroxypropyl methacylate) copolymers

The dielectric spectra of a series of copolymers of 2-hydroxyethyl methacrylate (HEMA) and 2,3-dihydroxypropyl methacrylate (DHPMA) were investigated. Recently, the full range dielectric spectrum of poly(2-hydroxyethyl methacrylate) was reported. This study looks at the effects on the dielectric behavior as a result of 2,3-dihydroxypropyl methacrylate addition. The dielectric permittivity, ε′, and the loss factor, ε″, were measured using a dielectric analyzer in the frequency range of 0.6 Hz to 100 kHz and between the temperature range of −150 and 275 °C. The electric modulus formalism was used to reveal the viscoelastic and conductivity relaxations present in the polymers. Several notable changes were observed as 2,3-dihydroxypropyl methacrylate concentration increased. It was observed through DSC and DEA that the glass transition temperature decreased as DHPMA content increased. The secondary dielectric relaxations were also affected as it was recorded that the activation energy for the γ transition increased and the β relaxation decreased with DHPMA content. Ionic conductivity data prove that DHPMA facilitates ionic mobility more efficiently than HEMA.     

Ionic conductivity in Bi2Sn2O7 ceramics

Impedance spectroscopy measurements, in the temperature range from room temperature to 600 K, were performed in order to investigate the dielectric and ionic properties of Bi₂Sn₂O₇ ceramics. The results show that the conductivity in this pyrochlore is associated with the hopping of ions. An activation energy of 1.26 eV was observed and the dielectric constant exhibits a strong contribution from ionic conduction.     

Ion mobility time of flight measurements: effect of experimental parameters on measurements in a non-hydrogen bonded system

An in situ measurement technique that isolates the mobility of charge carriers is described and analyzed. The technique allows significant improvement over conductivity measurements to monitor changes in the physical properties and state of a material as it cures. This is essential in systems where N_i, the number of charge carriers, cannot be assumed constant such as during cure of epoxies, urethanes and polyimides. Currently, there is an assumption made in the literature that the number of charge carriers present in a curing material is constant when conductivity is used as an in situ measurement technique to monitor changes in mobility (and thereby viscosity). This assumption is widely used, for example when dielectric conductivity measurements are correlated with changes in properties such as viscosity. Ion mobility, time of flight (ITOF) measurements, which are described here, are an appropriate technique to isolate and measure particle mobility due to changes in the state of the material. Furthermore, the ITOF technique, coupled with the measurement of σ, the dielectric conductivity, allows one to measure separately changes in the mobility and the number of charge carriers due to curing or changes in temperature. This is possible since conductivity is the product of the number of charge carriers and their mobility. Length of pulse, strength of applied field, sensor geometry, and temperature/viscosity are examined to determine optimum parameters of measurement for a simple, non-curing system: dimethacrylate of tetraethoxylated bisphenol A (D121). This paper seeks to show that with changes in viscosity, the pulse length and magnitude of voltage in relation to the distance between the electrodes should be varied to obtain accurate ITOF information on the changing mobility.     

Mixed electron and proton conductivity of polyaniline films in aqueous solutions of acids: beyond the 1000 S cm−1 limit

BACKGROUND: The application potential of conducting polymers depends on their conductivity. It is generally assumed that the conductivity determined in the dry state is a parameter that unambiguously characterizes them. RESULTS: The conductivity of polyaniline (PANI) films immersed in aqueous solutions of sulfuric acid may be more than 1000 times higher compared with that obtained by measurement of dry films in air, and is estimated to reach a value exceeding 3300 S cm^?1 in 1 mol L^?1 sulfuric acid. This is explained by the reduction of conductivity barriers between conducting PANI islands. CONCLUSION: The organized polymer chains in the conducting islands of a PANI film are separated by disordered regions of low conductivity in the dry state. The penetration of sulfuric acid solution into the disordered areas increases the overall conductivity of the PANI film by improving the electrical contact between the islands through ionic charge transport. The electronic conductivity of the PANI film in the dry state thus converts to mixed electron–proton conduction in acidic aqueous solutions, electron conductivity being dominant in ordered regions and ionic conductivity in disordered regions separating them. Weakly bound protons are the most important ionic charge carriers hopping along the PANI chains. Copyright © 2009 Society of Chemical Industry     

Suppression of high-temperature dielectric loss by designed thermal annealing treatment in (Bi1/2Na1/2)TiO3 ceramics

Dielectrics for automobile applications generally require high temperature reliability. Bi_1/2Na_1/2TiO₃-based materials are excellent high-temperature dielectric candidates with relatively large temperature-insensitive dielectric responses and a maximum dielectric permittivity temperature as high as 300 °C. However, they suffer from a high dielectric loss that increases exponentially above 200 °C due to ionic conduction from thermally activated oxygen vacancy migration. Here, we demonstrate that the impact of ionic conduction on dielectric loss can be effectively suppressed by introducing a sodium deficiency and thermal annealing An appropriate combination of both treatments elevated the temperature insensitivity of the dielectric loss up to ～ 300 °C. A systematic investigation using impedance spectroscopy correlated with microstructure analysis revealed that the sodium deficiency and thermal annealing affected the mobile oxygen vacancy concentration differently.     

Electrical and magnetic properties of double perovskite: Y2CoMnO6

In this communication, the structural, micro-structural, dielectric, electrical, magnetic, and leakage-current characteristics of a double perovskite (Y₂CoMnO₆) ceramic material have been reported. The material was synthesized via a high-temperature mixed-oxide route. The compound crystallizes in a monoclinic structure which is confirmed from preliminary X-ray structural study. The morphological study by using scanning electron micrograph reveals the almost homogeneous distribution of grains throughout the surface of the sample. The nature of frequency-dependence of dielectric constant has been described by the Maxwell-Wagner model. The occurrence of a dielectric anomaly in the temperature dependence of dielectric permittivity study demonstrates the ferroelectric-paraelectric phase transition in the material. From the Nyquist plots, we found the existence of both grain and grain boundary effects. The frequency dependence of conductivity was studied by the Jonscher’s Power law, and the conduction phenomenon obeys the large overlapping polaron tunneling model. By using the Arrhenius equation, the activation energy has been calculated which is nearly equal to the energy required for the hoping of the electron. Both impedance and conductivity analysis demonstrate that the sample exhibits negative temperature coefficient of resistance (NTCR) properties indicating the semiconducting type of material at high temperatures. The anti-ferromagnetic character of the material is observed from the nature of magnetic hysteresis loop. The leakage current analysis suggests that the conduction process in the material follows the space charge limited conduction phenomenon. Such material will be helpful for modern electronic devices and spintronic applications.     

A review of cement-based materials as electroceramics

The dielectric behavior of unpoled cured cement-based materials enables these materials to serve as electroceramics. The behavior entails the DC polarization (apparent DC electrical resistivity increase), permittivity (AC polarization, capacitance measurement) and DC electret (permanent electric dipole, voltage measurement). The dielectric behavior is not derived from functional admixtures such as the perovskite ceramics. The polarization involves charge-carrier polarization, with the carriers being primarily the ions in the pore solution. Dipolar polarization associated with the polar water molecules plays a minor role. Silica fume, if present, decreases the permittivity, partly due to the pore refinement. A polymer admixture, if present, increases the permittivity, with significant polarization resulting from the cement-polymer interface. Carbon fiber, if present, affects the electronic and ionic conduction, with the fiber’s ozone treatment promoting the ionic conduction and enhancing the permittivity. As the water/cement ratio increases, the permittivity increases, but the DC polarization decreases. The DC polarization occurs faster and more significantly than the subsequent depolarization. This reflects the electret, which discharges upon short circuiting (as in capacitor discharge) and subsequently charges back upon open circuiting. The temperature increases the permittivity or the electret’s electric field, whereas tension decreases the same, enabling capacitance-based/voltage-based self-sensing of temperature and stress/strain.     

Ionic conductivity of polymer electrolytes and future applications

Polymer electrolytes are solvent-free ion-conducting polymers and provide new and attractive materials in both polymer chemistry and electrochemistry. A proper understanding of ion dissociation and ion transport in such polymers is necessary for their application as solid electrolytes in electrochemical devices. Ionic conduction behaviour in polymer electrolytes is described here in relation to the characteristic properties. Of special interest is the ability of polymer electrolytes to include various kinds of electroactive molecules within them. The combination of this ability with their high ionic conductivity has enabled polymer electrolytes to be used as media for electrochemical syntheses and redox reactions.     

Dielectric Study of Tetraalkylammonium and Tetraalkylphosphonium Levulinate Ionic Liquids

Broadband dielectric spectroscopy in a broad temperature range was employed to study ionic conductivity and dynamics in tetraalkylammonium- and tetraalkylphosphonium-based ionic liquids (ILs) having levulinate as a common anion. Combining data for ionic conductivity with data obtained for viscosity in a Walden plot, we show that ionic conductivity is controlled by viscosity while a strong association of ions takes place. Higher values for ionic conductivities in a broad temperature range were found for the tetraalkylphosphonium-based IL compared to its ammonium homolog in accordance with its lower viscosity. Levulinate used in the present study as anion was found to interact and associate stronger with the cations forming ion-pairs or other complexes compared to the NTf₂ anion studied in literature. In order to analyze dielectric data, different fitting approaches were employed. The original random barrier model cannot well describe the conductivity especially at the higher frequencies region. In electric modulus representation, two overlapping mechanisms contribute to the broad low frequencies peak. The slower process is related to the conduction mechanism and the faster to the main polarization process of the complex dielectric permittivity representation. The correlation of the characteristic time scales of the previous relaxation processes was discussed in terms of ionic interactions.     

Dielectric behaviour and relaxation processes of montmorillonite clay nano‐platelet colloidal suspensions in poly(vinyl pyrrolidone)–ethylene glycol oligomer blends

BACKGROUND: Intercalated and exfoliated montmorillonite (MMT) clay structures in polymer matrices improve the thermal, mechanical, electrical and pharmaceutical properties of organic–inorganic materials. Poly(vinyl pyrrolidone) (PVP)–ethylene glycol oligomer (EGO) blends are biocompatible and non‐toxic materials. The dielectric characterization of MMT clay nano‐platelet colloidal suspensions in PVP–EGO blends is important in understanding the ionic conduction behaviour in many complex phenomena occurring in biological systems, and in selective membranes and their use in controlled drug release systems and in liquid electrolytes. RESULTS: An investigation using dielectric spectroscopy in the 20 Hz to 1 MHz frequency range of MMT clay nano‐platelet colloidal suspensions in PVP–EGO blends confirmed that the PVP segmental motion, ionic conduction relaxation time, electric double layer relaxation time and direct current electrical conductivity are significantly influenced by the clay concentration and EGO chain length. In these materials, ionic motion and PVP segmental dynamics are strongly coupled. Intercalation of EGO structures in clay galleries and exfoliation of clay platelets by adsorption of PVP–EGO structures on clay surfaces are governed by hydrogen bonding interactions between the carbonyl groups of PVP monomer units, the hydroxyl groups of EGOs and the hydroxylated aluminate surfaces of the MMT clay. CONCLUSION: The dielectric behaviour of intercalated and exfoliated structures of MMT clay nano‐platelet colloidal suspensions in PVP–EGO blends provides a convenient way to obtain liquid organic‐inorganic polymeric nanocomposite electrolytes with tailored ionic conduction properties. Copyright © 2009 Society of Chemical Industry     

Sodium ion incorporated alumina - A versatile anisotropic ceramic

The present article is a review of crystal structure dependent anisotropic properties of β and β″-phases of sodium ion incorporated alumina. The anisotropy in electrical properties such as ionic conductivity and dielectric permittivity is due to the layered structure. Conducting plane between two consecutive spinel aluminas constituting loosely bound mobile sodium ions, promote ionic conductivity in the parallel direction. In contrary, the restricted movement of ions in the orthogonal direction brings about polarization giving it directional dielectric property. High ionic conductivity of 1.3 S/cm and large dielectric constant of ˜ 200 are reported. Exchanging sodium ions with different cations, such as potassium and lithium, results in similar anisotropy. The processing of β and β″-phases along with metastability of intermediate mullite phase is described in the current review. In addition, the applications of sodium ion incorporated aluminas, such as solid electrolyte in batteries, thin film transistors and gas sensors are discussed.     

Electrical properties of Cu(II) and Ni(II): N-salicylidene polymethacrylic acid hydrazide complexes

Different types of chelated polymer complexes have been synthesized to obtain improved electrical properties. Compact discs from powders of the chelated polymers were prepared and heated in a specially designed holder. Electrical conductivity and dielectric constant of Cu(II) and Ni(II): N-salicylidene polymethacrylic acid hydrazide samples were measured at a fixed frequency (1600 Hz) throughout the temperature range 25-150°C. The AC conductivity as well as dielectric measurements showed maxima at 85°C. The water molecules which were trapped in the polymer matrix are believed to play the main role in conduction and dielectric behaviour of the polymeric material. From the AC conductance and dielectric constant measurements, the dielectric losses of these polymeric materials were calculated as a function of temperature.     

Electrical transport properties of M2FeV3O11 (M=Mg,Zn, Pb,Co, Ni) ceramics

Multicomponent oxide systems have been widely studied in the last few decades and can be used as cathode materials in high-energy cells. However, the electrical characteristics have not yet been fully disclosed. We report the electrical conductivity, thermoelectric power, the I-V characteristics, conductance and dielectric spectroscopy measurements made for M₂FeV₃O₁₁ (M=Mg, Zn, Pb, Co, Ni) ceramics. This multicomponent oxide system was found to show semiconducting properties strongly thermally activated above room temperature, n-type conduction at higher temperatures, higher conductance for the ceramics containing Co²⁺, Ni²⁺ and Mg²⁺ ions as well as a strong dependence of relative dielectric constant and loss tangent on temperature and frequency. Moreover, the transition metal ions, which have unfilled 3d-shells strongly affected polarization and conductivity of the ceramics, while the effect of porosity could be neglected. These effects are discussed in terms of microstructure, thermal activation of charge carriers, small polarons as well as the Maxwell–Wagner polarization.