Universal Dielectric Response Research Articles

Electrical properties of AC3B4O12-type perovskite ceramics with different cation vacancies

AC3B4O12-type perovskite CaCu3Ti4O12 (CCTO), □0.34Pr0.67Cu3Ti4O12 (PCTO), □1Cu3Ta2Ti2O12 (CTTO), □2Cu2Ta4O12 (CTO) ceramics with different concentration cation vacancies were prepared through traditional solid state reaction method. X-ray diffraction analysis indicated that CCTO and PCTO are perovskite cubic with space group Im-3 (no. 204) while CTTO and CTO are Pm-3 (no. 200). Cell parameter of the samples dramatically increases with the increase of cation vacancies. Dielectric permittivity of them maintains very high value of ∼104 from room temperature to 550K but the dielectric loss is depressed with the increase of cation vacancies in the same space group. The dielectric properties and conductivity behavior were described by the Debye relaxation and the universal dielectric response, respectively. The effect mechanism of cation vacancy and crystal structure on carrier transposition were discussed.

Single crystal growth by gel technique and characterization of lithium hydrogen tartrate

Single crystal growth of lithium hydrogen tartrate by gel encapsulation technique is reported. Dependence of crystal count on gel density, gel pH, reactant concentration and temperature are studied and the optimum conditions for these crystals are worked out. The stoichiometric composition of the grown crystals is determined using EDAX/AES and CH analysis. The grown crystals are characterized by X-ray diffraction, FTIR and Uv-Visible spectroscopy. It is established that crystal falls under orthorhombic system and space group P222 with the cell parameters as: a=10.971Å, b=13.125Å and c=5.101Å; α=90.5o, β=γ=90°. The morphology of the crystals as revealed by SEM is illustrated. Crystallite size, micro strain, dislocation density and distortion parameters are calculated from the powder XRD results of the crystal. UV–vis spectroscopy shows indirect allowed transition with an optical band gap of~4.83eV. The crystals are also shown to have high transmittance in the entire visible region. Dependence of dielectric constant, dielectric loss and conductivity on frequency of the applied ac field is analyzed. The frequency-dependent real part of the complex ac conductivity is found to follow the universal dielectric response: σac (ω)~ωs. The trend in the variation of frequency exponent with frequency corroborates the fact that correlated barrier hopping is the dominant charge-transport mechanism in the present system.

Dielectric relaxation at high temperatures induced by oxygen vacancies at grain boundary in Na-doped barium strontium titanium ceramics

The temperature and frequency dependence of dielectric properties of A-site lower valence cation Na-doped Barium strontium titanate (Ba1−xNax)0.9Sr0.1TiO3−δ (x=0.01, 0.03, 0.05, 0.08, 0.1, 0.15, 0.2 and 0.3) ceramics have been investigated up to 450°C from 100Hz to 1MHz. Other than dielectric properties at low temperature, the relaxation behavior above 150°C for all the compositions was observed with broad dielectric peaks ε´(T), tan δ(T) and ε´(f). The universal dielectric response associated with the activation energy of 0.79–0.98eV and complex impedance properties indicate that with the increasing of Na, the concentration of oxygen vacancies inhabiting grain boundary increases and under thermal stimulation the tunneling effect of oxygen vacancies strengthens, which dominate for the relaxation at high temperatures.

Dielectric spectroscopy of polymer-metal composites across the percolation threshold

Polymer (polar/nonpolar)/metal composites (PMC) were prepared under different process conditions. In polar PMC, dipolar relaxation plays a predominant role below percolation threshold (fc) and anomalous low frequency dispersion (ALFD) becomes dominant above fc while ALFD is the only likely possibility for nonpolar PMC above fc. The magnitude of relaxation exponents "m", "p" and "n", evaluated from the experimental results using Jonscher's universal dielectric response (JUDR) laws, falls within the universal limit ~ [0, 1] with additional feature of strong dependence on volume fraction of conductor (f con ). The decrease in the relaxation exponent "m" with an increase of f con is directly linked with decrease in the number of dipoles of the polymer in the composite and is accompanied by a distribution of relaxation time due to increased heterogeneity of the system. The magnitude of the relaxation exponent "n" decreases at fc, due to the prevalence of Maxwell–Wagner–Sillar polarization contributed by uncorrelated electrons.

Activation behavior and dielectric relaxation of nanocrystalline zinc ferrite

We report the ac conductivity and dielectric relaxation studies of nanocrystalline zinc ferrite prepared by the ball milling. The real part of the complex ac conductivity is found to follow the universal dielectric response σ′(f)∝Tnfs and values of both s and n show an anomalous behavior. Both the magnitude of the temperature exponent ‘n’ and thermal activation energy for ac conduction strongly depends on frequencies. The dielectric constant exhibits strong temperature dependence at higher temperature and lower frequencies. The dielectric properties of the samples have been analyzed in terms of electric modulus vector. The complex impedance has also been modeled by an ideal equivalent circuit consisting of grain and grain boundary resistances and capacitances. Considering the formation of Schottky diode at the metallic electrode and semiconductor junction, the diode parameters like built in voltage and space charge density have been extracted from the capacitance-voltage characteristics.

Pseudo-relaxor behavior in Na1/2Bi1/2Cu3Ti4O12 ceramics

High temperature dielectric behaviors have been explored in Na1/2Bi1/2Cu3Ti4O12 ceramics. The broad dielectric peaks could be fine fitted by the modified Curie–Weiss law and the Vogel–Fulcher relationship. Based on the universal dielectric response law and Maxwell–Wagner relaxation, the occurrence of the polarization mechanism transition from the grain boundary response to the electrode one with temperature is clearly evidenced in the low frequency range. It is likely to propose that the apparent relaxor-like behavior is related to an extrinsic effect instead of ferroelectricity.

Femtosecond tunneling of polarons in Pb5Cr3F19

The complex dielectric constant/ac electrical conductivity was investigated as a function of frequency and temperature in Pb 5 Cr 3 F 19. The system undergoes a ferroelectric phase transition at higher temperatures. At lower temperatures the real part of the complex ac electric conductivity was found to follow the universal dielectric response (UDR) σ′ ∝ νs, typical for hopping or tunneling of localized charge carriers. A detailed analysis of the temperature dependence of the UDR parameter s in terms of the theoretical model for tunneling of small polarons revealed that, at low temperatures, this mechanism governs the charge transport in Pb 5 Cr 3 F 19. The value of the inverse attempt frequency τ0 indicates the femtosecond tunneling of polarons in the system similar to the Büttiker–Landauer transversal time.

Frequency And Temperature Dependent Impedance Study In 50% BaTiO3- 50% La0.7Sr0.3MnO3 Nanocomposite

We report the impedance spectroscopy (IS) behavior of the ferroelectric- ferromagnetic (50% BaTi O3-50% La0.7Sr0.3MnO3) nano-composite prepared by sol-gel method. Frequency and temperature dependence of the complex impedance and conductivity has been measured over the temperature range 310 K-430 K. A distribution of relaxation times and decentralization of the semicircle has been observed from the Cole–Cole plots of real and imaginary parts of the complex impedance. Non-Debye type relaxation has been observed in the investigated composite system. An equivalent circuit has been constructed to describe the IS. Interestingly, a typical universal dielectric response in the frequency-dependent conductivity at different temperature has been found. The frequency dependent ac conductivity at different temperature indicates that the conduction process is thermally activated. The activation energy has been obtained from the Arrhenius fitting. The obtained dc conductivity showed that the system undergoes a positive temperature coefficient resistance (PTCR) to negative temperature coefficient resistance (NTCR) near 350 K. The results would help to understand deeply the relaxation process in these types of materials.

Epitaxial growth and in-plane dielectric properties of orthorhombic HoMnO3 films

Orthorhombic HoMnO3 (HMO) thin films were grown epitaxially on LaAlO3 (001) substrates by using pulsed laser deposition technique. The films showed perfect orthorhombic crystallization and were well-aligned with the substrates. The in-plane dielectric constant and loss of HMO films were measured as functions of temperature (80–300 K) and frequency (120 Hz–100 kHz) by using coplanar interdigital electrodes. Two thermally activated dielectric relaxations were found, and the respective peaks shifted to higher temperatures as the measuring frequency increased. The in-plane dielectric properties of epitaxial orthorhombic HMO films were considered as universal dielectric response behavior, and the dipolar effects and the hopping conductivity induced by the charge carriers were used to explain the results.

Dielectric responses of glass-added Ba0.95Ca0.05Zr0.3Ti0.7O3 ceramics for energy storage capacitors

The structure and energy storage properties of Ba0.95Ca0.05Zr0.3Ti0.7O3 ceramics with different contents of MgO–CaO–Al2O3–SiO2 glass additive added were examined. The dielectric properties of the specimens were studied as a function of temperature and frequency. The specimens displayed a relaxor-like behavior and the glass addition enhanced the diffuseness degree of the ferroelectric–paraelectric phase transition. With increasing glass additive content, the dielectric loss of the specimens steadily increased. Over several decades of frequency, the ac conductivity data of the specimens agreed with the power-law relation of the universal dielectric response (UDR) model, indicating the presence of hopping motion of polarization carriers. Complex impedance analysis confirmed that oxygen ions are the mobile carriers in the specimens. The origin of oxygen-ion conduction in the glass-added specimens is explained in relation to a chemical reaction between the perovskite grains and the glass additive. It is confirmed that, for the glass-added specimens, polarization response resulting from oxygen-ion hopping is responsible for an overall increase of their dielectric loss in a wide frequency range.

Dielectric Properties and Defect Chemistry of WO3-Doped K0.5Na0.5NbO3 Ceramics

The dielectric properties and conductivity behavior of WO3-doped K0.5Na0.5 NbO3 ceramics were investigated as a function of temperature (25°C to 600°C) and frequency (40 Hz to 106 Hz). The dielectric loss and direct-current (DC) conductivity of the ceramics depend strongly on the tungsten content. A high-temperature dielectric relaxation near temperature of 500°C was observed and analyzed using the semiempirical complex Cole–Cole equation. The activation energy of the dielectric relaxation was estimated to be ∼2 eV and increased with increasing WO3. The frequency-dependent conductivity can be well described by the universal dielectric response law. The activation energy obtained from the DC conductivity changes from 0.93 eV to 1.49 eV. A possible mechanism for the high-temperature dielectric relaxation and conductivity is proposed based on the activation energy value and defect compensation.

Electrical conductance through adsorbed water molecules on freshly fractured glass surfaces

Plate glass sheets of low emissivity glass were cut into small rectangles and scratched with a diamond tipped scribe from one edge to another. The scratch was deep enough to locally remove the conducting tin oxide surface and to fracture the glass beneath, leaving behind conducting electrodes on either side. They were connected by gold wires to a standard meter to measure the complex admittance at different relative humidities (14–79%) and frequencies (20Hz to 1MHz). The raw data was converted to the complex susceptibility after subtracting the signal due to the bulk glass substrate and analyzed using the universal dielectric response model developed by Jonscher. The change in the conductance and capacitance, as measured in a parallel mode, gave a measure of the humidity sensing ability of the scratch and roughly followed an exponential dependence. A control plain glass sample showed a very small effect. The Jonscher index “n” for the susceptibility (χ′) in the high frequency range was seen to be about +0.5, which is characteristic of ionic or hopping conductance and which we expect to be the Grotthuss process. The somewhat lower values of the index seen for the dielectric loss (χ″) data are typical of a lossy material. A combined plot of χ′ versus χ″ of all the data for all humidities and frequencies showed that the Jonscher model was well obeyed in the present work, with a log–log slope close to one as required by the model. Although one scratch sample is highlighted in this work, measurements are presented that show reproducibility between a number of samples as well as a minor amount of aging in repeat measurements on the same sample.

Anomalous electrical transport properties of CdSe quantum dots at and below room temperature

Here we report the dielectric relaxation and anomalous ac conduction behavior of CdSe quantum dots in the temperature (T) range of 77K≤T≤300K and in the frequency (f) range of 100Hz≤f≤1MHz. Interfacial polarization exhibits strong temperature dependence only above 250K and dielectric loss spectrum undergoes a shallow minimum. Superlinear power law behavior of the ac conductivity was observed above certain frequencies, along with the universal dielectric response at comparatively lower frequencies. Extraction of the temperature dependent dc contribution, hopping frequencies, frequency exponents and activation energies values were done by theoretical fitting and was observed that for T=77–200K, classical hopping over barrier (HOB) is the prevailing charge transport mechanism whereas for T=250–300K, the charge transport mechanism is correlated barrier hopping (CBH). Analysis of the Nyquist plotting and non-Debye electric modulus spectra have also been conferred in this context.

Near constant loss regime in fast ionic conductors analyzed by impedance and NMR spectroscopies.

Universal dielectric response (UDR) and nearly constant loss (NCL) dispersive regimes have been investigated in fast ion conductors with perovskite and NASICON structure by using NMR and impedance spectroscopy (IS). In this study, the electrical behavior of La(0.5)Li(0.5)TiO3 (LLTO-05) perovskite and Li(1.2)Ti(1.8)Al(0.2)(PO4)3 (LTAP0-02) NASICON compounds was investigated. In both systems a three-dimensional network of conduction paths is present. In the Li-rich LLTO-05 sample, lithium and La are randomly distributed on A-sites of perovskites, but in LTAP0-02 Li and cation vacancies are preferentially disposed at M1 and M2 sites. In perovskite compounds, local motions produced inside unit cells are responsible for the large "near constant loss" regime detected at low temperatures, however, in the case of NASICON compounds, local motions not participating in long-range charge transport were not detected. In both analyzed systems long-range correlated motions are responsible for dc-conductivity values of ceramic grains near 10(-3) S cm(-1) at room temperature, indicating that low-temperature local motions, producing large NCL contribution, are not required to achieve the highest ionic conductivities.

Microstructure and Dielectric Properties of Barium-vanadate Glasses

The ac and dc conductivity in barium-vanadate glasses was investigated as a function of temperature and frequency with the use of impedance spectroscopy. The topography and microstructure of glasses were investigated by the means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. The XRD results show that all samples are amorphous but microscopy investigation reveals that glasses with greater amount of V2O5 contain nanostructures. The influence of V2O5 quantity on dielectric properties was checked. The activation energy of dc conduction process was evaluated. On the basis of Jonscher universal dielectric response the temperature dependence of conductivity parameters were determined and compared to theoretical models collected by Elliott.

Room temperature frequency-dependent complex impedance and conductivity behavior of BaTiO3–La0.7Ca0.3MnO3 composites

A series of composite materials (100 − x)BaTiO3 (BTO) − (x)La0.7Ca0.3MnO3 (LCMO) has been prepared using the solid state reaction route varying x in wt%. The influence of LCMO on the frequency-dependent complex impedance and conductivity of BTO has been investigated. The Cole–Cole plots of real and imaginary parts of the complex impedance indicates separate contributions of various factors, such as bulk effect, grain boundary, and interface effects in frequency-dependent conductivity in the composites. Non-Debye-like relaxation has been observed in the BTO–LCMO composites. Interestingly a typical universal dielectric response in the frequency-dependent conductivity at room temperature (300 K) has been found for all the compounds.

Crystal structure, morphological, optical and electrical investigations of Oxypeucedanin micro crystals: an isolated compound from a plant

The structural, optical and dielectric properties of micro-crystals of Oxypeucedanin (C16H14O5), isolated from ethyl acetate extract of the roots of the plant Prangos pabularia is presented. These isolated crystals were characterized by using X-ray diffraction (XRD), scanning electron microscopy, optical microscope, ultraviolet (UV)–visible spectroscopy and dielectric spectroscopy techniques. XRD of these crystals confirm that they have monoclinic structure (space group = P21/c). The frequency-dependent real part of the complex ac conductivity was found to follow the universal dielectric response: σac (ω)–ω s . The trend in the variation of the frequency exponent (s) with frequency (ω) supports the fact that correlated barrier hopping is the dominant charge transport mechanism for this compound. From the UV spectroscopy, the optical band gap (E g ) has been found to be around 3.76 eV and is following indirect allowed transition. The observed optical and electric properties predict these crystals as an open choice for modern flexible electronics.

Electronic conductivity in the SiO2–PbO–Fe2O3 glass containing magnetic nanostructures

The linear impedance spectra of iron–silicate–lead glass samples were measured in the frequency range from 1 MHz to 1 MHz and in the temperature range from 153 K to 423 K. The structure was investigated by means of XRD and atomic force microscopy. Local electrical and magnetic properties of the samples were tested with the aid of electrostatic force microscopy (EFM) and magnetic force microscopy (MFM). The obtained results show that the impedance spectra of all samples exhibit one large relaxation process and some additional small relaxation is present in glass containing 20 mol% and 25 mol% Fe2O3. The AFM micrographs show that in a subset of samples some nanostructures exist. Their size, shape, and quantity depend on the amount of Fe2O3 in the composition of the glass. The samples containing an amount of 15 mol% of Fe2O3 include the biggest nanostructures which are ball-shaped. EFM and MFM micrographs show that nanostructures exhibit different electrical and magnetic behaviors than the rest of the glass matrix. On the basis of Jonscher universal dielectric response the temperature dependence of conductivity exponent s was determined and compared to theoretical models proposed by Elliott. It was found that while in the glass containing less than 15 mol% iron oxide, there is only one process responsible for conduction mechanism, the overlap polaron tunneling. In the other samples, a different conduction mechanism may coexist: quantum mechanical tunneling between semiconducting granules.

In-plane dielectric properties of epitaxial orthorhombic HoMnO3 films grown on LaAlO3 substrates

Orthorhombic HoMnO3 thin films were grown epitaxially on LaAlO3 (001) substrates by using a pulsed laser deposition technique. The films showed perfectly orthorhombic crystallization and were well-aligned with the substrates. The in-plane dielectric constant and the dielectric loss of HoMnO3 films were measured as functions of temperature (80 ∼ 300 K) and frequency (120 ∼ 100 kHz) by using interdigital surface electrodes. Two thermally-activated dielectric relaxations were found, and the respective peaks shifted to higher temperatures as the measuring frequency was increased. The in-plane dielectric properties of epitaxial orthorhombic HoMnO3 films were considered to have a universal dielectric response behavior, and the dipoar effects and the hopping conductivity induced by the charge carriers were used to explain the results. The dielectric properties at low temperatures, which showed the multiferroicity, of the orthorhombic HoMnO3 films are discussed.

Polaronic behavior of La0.8Te0.2MnO3

We have analyzed recently published data of the ac electrical conductivity and dielectric constant in La 0.8 Te 0.2 MnO 3 (LTMO), and at temperatures below 500 K both quantities follow the universal dielectric response (UDR) typical of hopping or tunneling of localized charge carriers. A detailed analysis of the temperature dependence of the UDR parameter s in terms of the theoretical model for tunneling of small polarons revealed that below 500 K, this mechanism governs the charge transport in LTMO system.