Electrical Properties Of Grains Research Articles

Dielectric relaxations without colossal permittivity in Mg and Nb co-doped BaTiO3 ceramics

Magnesium (Mg) and Niobium (Nb) co-doped BaTiO3 ceramics are prepared using a solid-state reaction method under an air atmosphere, and their crystal structures, microstructures, valence states and dielectric properties are studied. The ceramics adopt a cubic perovskite structure, and the lattice parameter increases with doping content owing to the larger ionic radii of dopants. The complex permittivity and electric modulus as a function of frequency and temperature are analyzed. Dielectric relaxations without colossal permittivity are observed. The relaxation processes are thermally activated and their variations with temperature are analyzed. The activation energy values for the relaxation processes are determined by the Arrhenius relationship and increase with doping content. By analyzing the electric modulus, the activation values for high-frequency relaxation are found to be associated with the electrical properties of grain, and the contribution of grain boundary could be neglected based on internal barrier layer capacitor (IBLC) model. The electron hopping between the different Ti valences, which is confirmed by X-ray photoelectron spectroscopy, is supposed to play an important role in high-frequency relaxation. The permittivity plateaus are related to the inhomogeneous structures of grain and grain boundary, and the low-frequency permittivity plateau is attributed to the space-charge polarization at grain boundaries. By comparing with Cu/Nb co-doped BaTiO3, the absence of colossal permittivity is ascribed to high resistivity of the samples. Charge density and migration as well as the insulating grain boundaries are critical for the formation of space charge and a high permittivity.

Charge transport and dielectric characteristics of Sm x Bi1−x FeO3 thin films from the perspective of grain and grain boundary properties

Samarium-substituted bismuth ferrite (Sm x Bi1−x FeO3) compositions comprise a system of important materials due to their combination of multiferroic properties. Several dielectric and charge transport reports in literature can be found in this system. However, as a typical polycrystalline electroceramic, their grains and grain boundaries (GBs) are expected to possess very different properties. To this date, these distinctions have not been determined for this system. In this work, through measurements via impedance spectroscopy on Sm x Bi1−x FeO3 thin films, we show that using a brick layer model allows the separation of the electrical properties of grains and GBs. Results indicate that grains have dielectric permittivity and electrical conductivity much higher than GBs. Their properties mostly control the characteristics observed in the studied thin films. The introduction of samarium reduces the electrical conductivity and increases the activation energies for charge transport in grains and GBs. In turn, dielectric permittivity is reduced in grains and subtly increased in GBs.

Significantly enhanced dielectric properties of Y3+ donor-doped CaCu3Ti4O12 ceramics by controlling electrical properties of grains and grain boundaries

This paper focuses on the influence of Y3+ donor doping on optimizing dielectric properties of CaCu3Ti4O12 (CCTO) ceramics by controlling electrical properties of grains and grain boundaries (GBs). The Y3+ ions donor-doped Ca1-xYxCu3Ti4O12 (CYCTO, x = 0, 0.01, 0.02, 0.03, 0.05 and 0.07) ceramics were prepared by the conventional solid state reaction method. The influence of doping content on phase structure, morphologies and dielectric properties of CYCTO ceramics was investigated. It was found that Y3+ donor doping scarcely influenced the phase structure of CYCTO, but grain growth in CYCTO ceramics was depressed by Y3+ ions. Interestingly, although the grain size of CCTO was depressed, an enhancing value in dielectric constant (εr) with simultaneously reducing dielectric loss tangent (tanδ) was successfully achieved as Y3+ doping content x increased from 0.01 to 0.03. Impedance spectroscopy analysis further revealed that Y3+ donor doping can improve conductivity of semiconducting grain in CYCTO ceramics with x increasing up to 0.05, which was well consistent with the rising tendency of εr. The resistance of insulating GBs was improved with x increasing up to 0.03, which resulted in a considerable reduction in tanδ. The donor doping type of using Y3+ substitution for A site Ca2+ into CCTO in this work should provide an effective route for further optimizing properties of CCTO ceramics by reducing resistance of grains (Rg) and simultaneously enhancing resistance of GBs (Rgb).

Crystalline orientation dependent photoresponse and heterogeneous behaviors of grain boundaries in perovskite solar cells

Using photoconductive atomic force microscopy and Kelvin probe force microscopy, we characterize the local electrical properties of grains and grain boundaries of organic-inorganic hybrid perovskite (CH3NH3PbI3) thin films on top of a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)/ITO substrate. Three discrete photoconductivity levels are identified among perovskite grains, likely corresponding to the crystal orientation of each grain. Local J-V curves recorded on these grains further suggest an anti-correlation behavior between the short circuit current (JSC) and open circuit voltage (VOC). This phenomenon can be attributed to diffusion-limited surface recombination at the non-selective perovskite-tip contact, where a higher carrier mobility established in the perovskite grain results in an enhanced surface recombination and thus a lower VOC. In addition, the photoresponse of perovskite films displays a pronounced heterogeneity across the grain boundaries, with the boundaries formed between grains of the same photoconductivity level displaying even enhanced photocurrent and open circuit voltage compared to those of the adjacent grain interiors. These observations highlight the significance of controlling the microstructure of perovskite thin films, which will be a necessary route for further improving the efficiency of perovskite solar cells.

Significantly improved non-Ohmic and giant dielectric properties of CaCu3-xZnxTi4O12 ceramics by enhancing grain boundary response

The high permittivity of CaCu3Ti4O12 ceramics has attracted considerable attention in recent years due to their potential applications in electronic devices. However, their dielectric loss tangents are still too large. Here, we report of achieving a greatly decreased loss tangent with retaining high permittivity in Zn-doped CaCu3Ti4O12 (x=0–0.10) ceramics prepared using a modified sol–gel method. The influences of Zn2+ doping on the microstructure, giant dielectric response, and non–Ohmic properties as well as microscopic effects on the electrical properties of grain and grain boundary were systematically studied. In addition to their dielectric properties, their non–Ohmic properties were remarkable improved. Very–low loss tangents (tanδ~0.016–0.017) and high permittivities (ε′~6500–7700) at 1kHz were successfully achieved in CaCu3-xZnxTi4O12 ceramics with x=0.05–0.10. Substitution of Zn2+ ions into Cu2+ sites caused an increase in the total resistance, which was governed by the grain boundary resistance. The potential barrier height of CaCu3Ti4O12 ceramics was significantly increased by Zn2+ dopant ions. The reduced apparent tanδ and improved nonlinear electrical properties were primary attributed to the modified grain boundary response as a result of Zn2+ doping.

Dielectric constant versus voltage and non-Ohmic characteristics of Bi2/3Cu3Ti4O12 ceramics prepared by different methods

Bi2/3Cu3Ti4O12 (BCTO) ceramics were successfully prepared by traditional solid-state reaction method (BCTO-SS) and sol–gel method (BCTO-SG). Pure perovskite phase and dense structure were obtained in BCTO ceramics prepared by both methods. BCTO-SG ceramics showed a large dielectric constant of ~1.1×104 while BCTO-SS ceramics exhibited a low dielectric constant of ~3200. At 100kHz, the dielectric constant of BCTO-SS ceramics decreased with applied voltage increasing, while the dielectric constant of BCTO-SG ceramics increased with applied voltage increasing. Further study of the relationship between dielectric constant and voltage suggested BCTO-SG ceramics had larger defect concentration than BCTO-SS ceramics. The investigation of complex impedance indicated that the electrical properties of grain boundaries for all BCTO ceramics were evidently affected by applied voltages and the electrical properties of grains were independent of applied voltages. In addition, the non-Ohmic properties of BCTO ceramics were studied in detail. The non-linear coefficients of BCTO-SS and BCTO-SG ceramics were 1.65 and 1.01, respectively. The breakdown electric fields of BCTO-SS and BCTO-SG ceramics were found to be 1.21 and 0.48kV/cm, respectively. The potential barrier heights of BCTO-SS and BCTO-SG ceramics were calculated to be 0.549 and 0.485eV, indicating that the potential barriers at the grain boundaries for BCTO-SS and BCTO-SG ceramics are the Schottky-type barrier.

Impedance performances of SnO2–Zn2SnO4 composite ceramics

To investigate the formation of grain boundary barriers, composite SnO2–Zn2SnO4 varistor ceramics were prepared by traditional ceramic processing and, the electrical properties of grains and grain boundaries were evaluated by the measurement of complex impedances spectra. Calculated using the low frequency impedance data for each sample, a lower value about 0.09–0.32 eV and a higher value about 0.50–0.83 eV of activation energy Ea was obtained in the temperature range of 50–110 and 140–200 °C, respectively. The simulated results of complex impedances spectra by “Zsimpwin” software indicate that the equivalent circuit for SnO2–Zn2SnO4 composite ceramics is connected in series by one RC and two QR parallels. The relaxations in the dielectric dispersion spectra at low frequency of ∼1000 Hz suggest that one of the QR parallels is corresponding to oxygen vacancy. With increasing sintering temperature, the Ea values related to oxygen vacancy decreased whereas, the values related to grain boundary barriers increase obviously. The results suggest that the oxygen vacancy is the key factor to the semi-conductance of SnO2 grains and formation of grain boundary barriers.

The origin of giant dielectric relaxation and electrical responses of grains and grain boundaries of W-doped CaCu3Ti4O12 ceramics

The origin of giant dielectric relaxation behavior and related electrical properties of grains and grain boundaries (GBs) of W6+-doped CaCu3Ti4O12 ceramics were studied using admittance and impedance spectroscopy analyses based on the brick–work layer model. Substitution of 1.0 at. % W6+ caused a slight decrease in GB capacitance, leading to a small decrease in the low-frequency dielectric constant. Surprisingly, W6+ doping ions have remarkable effects on the macroscopic dielectric relaxation and electrical properties of grains. X-ray photoelectron spectroscopy analysis suggested that the large enhancements of grain resistance and conduction activation energy of grains for the W6+-doped CaCu3Ti4O12 ceramic are caused by reductions in concentrations of Cu3+ and Ti3+ ions. Considering variation of dielectric properties together with changes in electrical properties of the W6+-doped CaCu3Ti4O12 ceramic, correlation between giant dielectric properties and electrical responses of grains and GBs can be described well by the internal barrier layer capacitor model. This model can ascribe mechanisms related to giant dielectric response and relaxation behavior in CaCu3Ti4O12 ceramics.

Effects of Fe, Ti, and V doping on the microstructure and electrical properties of grain and grain boundary of giant dielectric NiO-based ceramics

We report the giant dielectric response and electrical properties of Li0.05B0.02Ni0.93O (B=Fe, Ti, and V) ceramics prepared by a polymer pyrolysis route. The giant dielectric response in these materials can be ascribed based on the Maxwell–Wagner polarization and thermally activated mechanisms. It is found that Fe, Ti, and V doping has a strong effect on the microstructure and the conduction of grains and grain boundaries of these NiO-based ceramic systems, which make large contribution to their dielectric properties.

Microstructure and dielectric properties of NaxTiyNi1−x−yO (x = 0.05–0.30, y = 0.02)

The microstructure of NaxTiyNi1−x−yO (abbreviated as NaTNO), a high dielectric material, has been investigated by scanning electron microscopy. Dielectric dispersion and complex impedance of NaTNO ceramics (in the frequency range 10–107 Hz) are discussed. The relatively lower value of the dielectric constant (ε′) of NaTNO compared with those of LixTiyNi1−x−yO (LTNO) and KxTiyNi1−x−yO (KTNO) is attributed to the typical distinguishing behaviour of its microstructure. Ac impedance spectroscopic studies indicate that the internal barrier layer capacitance effect, arising from differing electrical properties of grains and grain boundaries, is responsible for high dielectric permittivity of this non-ferroelectric material. The ratios of dielectric dispersion strengths, in the low (<100 kHz) and high (>100 kHz) frequency ranges, increase with an increase in the Na content. Deviation of the measured ε′ value of NaTNO from that predicted by the boundary layer capacitance mechanism is attributed to its typical microstructure.

Study of the formation of Co 3O 4 thin films using sol–gel method

Sol–gel derived coatings containing cobalt have been analyzed using impedance and reflection measurements. It is found that during the thermal treatments in air at temperatures in the range of 35–400 °C, cobalt migrates to the front surface of the coating where it is oxidized by the atmospheric oxygen and forms a top layer rich in nanocrystalline Co 3O 4. In samples heated above 260 °C, the current flows entirely through this top layer because it has higher conductivity than the rest of the coating. For these samples, the impedance spectra show two semicircles, related with the electrical properties of grain and grain boundaries of the cobalt oxide layer. Using the brick model, the grain boundary volume fraction was calculated as a function of the heating temperature.

Using cereal grain permittivity for sensing moisture content

A brief history of cereal grain moisture measurement by sensing the electrical properties of grain is presented. The basic principles are also described for using radio-frequency (RF) and microwave dielectric properties, or permittivity, of grain for sensing moisture through their correlation with moisture content. The development of density-independent functions of the permittivity is explained. The findings of recent research are summarized, which indicate that reliable density-independent moisture content determinations can be realized by measurements on grain at RF and microwave frequencies. Development of these techniques will provide useful instruments for on-line monitoring of moisture content in flowing grain to manage moisture in grain, prevent spoilage in storage and transport, improve processing, and provide information important for yield determinations in precision agriculture applications.

Effect of MnO on the Electrical Properties of Nb-Doped SrTiO3 Varistor

AbstractThe effect of MnO on the electrical properties of Nb-doped SrTiO3 was investigated to understand the electronic states of grains and grain boundaries. Grain size and dielectric constant were decreased with increasing Mn-doped content from 0 to 0.07 mol%. However, it was observed that the non-linearity coefficient of Nb-doped SrTiO3 was changed from 5 to 43 with Mn-doped content increasing from 0 to 0.03 mol%. Current-voltage (I-V) characteristics, capacitance-voltage (C-V) measurement, complex plane analysis and impedance spectroscopy were performed to obtain the information about the electrical properties of grains and grain boundaries. The potential barrier height increased and the donor concentration decreased with increasing Mn-doped content. This phenomena, that the electrical properties of Nb-doped SrTiO3 were changed with the addition of Mn, would be determined by the effect which Mn-2-3 substituted for Ti sites. It was observed that the breakdown voltage of single grain boundary was changed from 3.8 V to 8.9 V when measured by micro-electrode method.

ADVANCES IN SENSING GRAIN MOISTURE CONTENT BY MICROWAVE MEASUREMENTS

The history of information development on the electrical properties of grain and their use for sensing moisture content is briefly reviewed. Basic principles of grain moisture sensing by microwave measurements are presented, including techniques for moisture content determination independent of bulk density. Recent research findings on densityindependent microwave determination of moisture content in grain are briefly summarized. Three functions of the grain dielectric properties or the wave parameters are identified from which moisture content can be determined independent of bulk density, and the performance of these density-independent functions is compared for sensing moisture content in hard red winter wheat. Results show that standard errors of performance of about 0.3% moisture content or less can be achieved by using these functions with microwave measurements at frequencies between 11 and 18 GHz. Results of these and other cited studies indicate that measurement techniques for density-independent moisture sensing in grain should provide a sound basis for development of microwave moisture meters for on-line applications.

Ac Electrical properties of high-curie-point barium-lead titanate PTCR ceramics

High-Curie-point semiconducting barium-lead titanate PTCR ceramics of composition Ba 0.897Pb 0.10La 0.003TiO 3 were prepared. The electrical properties of grain and grain boundary of the sample were studied by a combination of complex-plane impedance analysis and impedance spectra methods. The PTC originated only in the grain boundary material.