Colossal Dielectric Constant Research Articles

Small polaron conduction in lead modified lanthanum ferrite ceramics

In the present work we have illustrated the physics of the electrical characteristics of nanocrystalline La1−xPbxFeO3, (0⩽x⩽0.2) powder prepared using auto-combustion synthesis. The effect of lead doping on the dielectric, impedance and ac conductivity characteristics of lanthanum ferrite has systematically been investigated. The synthesized powders were phase pure and crystallized into centro-symmetric Pnma space group. As compared to pure LaFeO3 ceramics (dielectric constant∼14,000), the dielectric constant is grossly increased (∼30,000) in Pb doped LaFeO3. The temperature dependence of dielectric constant of 10.0 at.% Pb doped LaFeO3 exhibits dielectric maxima similar to that observed in ferroelectric ceramics with non-centrosymmetric point group. For La0.8Pb0.2FeO3 ceramics, the frequency dependence of the dielectric constant and loss tangent at various temperatures (300–450K) exhibit typical colossal dielectric constant (CDC) like behavior. From the impedance spectroscopy we have estimated the grain and grain boundary resistance and capacitance of Pb doped LaFeO3 that follow a small polaron hopping conduction model. Long range movement of the charge carriers govern the CDC behavior.

Oxygen vacancy related defect dipoles in CaCu3Ti4O12: Detected by electron paramagnetic resonance spectroscopy

Oxygen vacancy associate defects were studied by electron paramagnetic resonance (EPR) for the perovskite oxide CaCu3Ti4O12 (CCTO), which have a colossal dielectric constant. It is found that the EPR line width of the Cu2+–host signal of CCTO correlates with its permittivity. A new signal was found in the second differential of the Cu2+–host signal, which we think is associated with a specific copper–oxygen vacancy defect. This signal shows a negative g factor shift with temperature, suggesting a contribution to the conduction. Assuming solid state reactions between the various defects and using the mass action law, we offer a more specific relation between permittivity, the content of oxygen vacancy and the new signal, which might be useful for an indirect but fast oxygen vacancy content determination.

Effect of in situ synthesized Fe2O3 and Co3O4 nanoparticles on electroactive β phase crystallization and dielectric properties of poly(vinylidene fluoride) thin films.

A simple and low cost in situ process has been developed to synthesize Fe2O3-Co3O4 nanoparticles (NPs) loaded poly(vinylidene fluoride) (PVDF) thin films. The electroactive β phase nucleation mechanism and the dielectric properties of the films have been investigated by X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry and using an LCR meter. Results confirmed that the electroactive β phase crystallization in the PVDF matrix is due to the fast nucleating or catalytic effect of the in situ NPs. Homogenous dispersion of in situ Fe2O3-Co3O4 NPs in the polymer matrix leads to strong interfacial interaction between the NPs and the polymer resulting in enhanced β phase nucleation in PVDF and a large dielectric constant of the thin films. The observed variation in the electroactive β phase nucleation by NPs (Fe2O3-Co3O4) and the dielectric properties of the thin films have been explained on the basis of surface charge, size, geometrical shape and extent of agglomeration of the NPs in the polymer matrix.

Modulus spectroscopy of grain–grain boundary binary system

Understanding various polarization mechanisms in complex dielectric systems and specifying their physical origins are key issues in dielectric physics. In this paper, four different methods for representing dielectric properties were analyzed and compared. Depending on the details of the system under study, i.e., uniform or non-uniform, it was suggested that different representing approaches should be used to obtain more valuable information. Especially, for the grain–grain boundary binary non-uniform system, its dielectric response was analyzed in detail in terms of modulus spectroscopy (MS). Furthermore, it was found that through MS, the dielectric responses between uniform and non-uniform systems, grain and grain boundary, Maxwell–Wagner polarization and intrinsic polarization can be distinguished. Finally, with the proposed model, the dielectric properties of CaCu3Ti4O12 (CCTO) ceramics were studied. The colossal dielectric constant of CCTO at low frequency was attributed to the pseudo relaxation process of grain.

High-temperature colossal dielectric response in RFeO3 (R=La, Pr and Sm) ceramics

Polycrystalline RFeO3 (R=La, Pr, and Sm) ceramics have been synthesized by a modified Pechini method. All the refined X-ray diffraction patterns reveal that the materials crystallized in orthorhombic structure with space group Pbnm. Clear grain and grain boundary are evident in PrFeO3 and SmFeO3 ceramics after sintering at 1200°C for 4h by scanning electron microscopy. Mixed valence of Fe2+/Fe3+ has been determined using X-ray photoelectron spectroscopy. High-temperature dielectric behavior and impedance spectrum over the frequency range of 100Hz to 10MHz are investigated at different temperatures from 300 to 600K. These materials exhibit relaxation-like dielectric behavior and colossal dielectric constant in a wide temperature and frequency range. Impedance spectrum analysis indicates the ceramics to be electrically heterogeneous consisting of semiconducting grains and insulating grain boundaries (corresponding to high and low frequency electrical response, respectively), which played important roles in the high-temperature dielectric properties of RFeO3 ceramics.

Tuning of colossal dielectric constant in gold-polypyrrole composite nanotubes using in-situ x-ray diffraction techniques

In-situ x-ray diffraction technique has been used to study the growth process of gold incorporated polypyrrole nanotubes that exhibit colossal dielectric constant due to existence of quasi-one-dimensional charge density wave state. These composite nanotubes were formed within nanopores of a polycarbonate membrane by flowing pyrrole monomer from one side and mixture of ferric chloride and chloroauric acid from other side in a sample cell that allows collection of x-ray data during the reaction. The size of the gold nanoparticle embedded in the walls of the nanotubes was found to be dependent on chloroauric acid concentration for nanowires having diameter more than 100 nm. For lower diameter nanotubes the nanoparticle size become independent of chloroauric acid concentration and depends on the diameter of nanotubes only. The result of this study also shows that for 50 nm gold-polypyrrole composite nanotubes obtained with 5.3 mM chloroauric acid gives colossal dielectric constant of about 107. This value remain almost constant over a frequency range from 1Hz to 106 Hz even at 80 K temperature.

Impedance spectroscopic investigation of electro active regions, conduction mechanism and origin of colossal dielectric constant in Nd1 − xSrxFeO3 (0.1 ≤ x ≤ 0.5)

Impedance spectroscopic studies of single phase Nd1−xSrxFeO3 (0.1≤x≤0.5), synthesized by solid state reaction method, have been carried out to investigate the electro active regions, conduction mechanism and origin of colossal dielectric constant in the wide temperature (77–358K) and frequency (1–1.5×107Hz) range. These results show that three electro active regions, i.e., grains, grain boundaries and electrode semiconductor contacts are present in these materials. The conduction mechanism changes from Mott variable range hopping to adiabatic small polaronic hopping with the increase in temperature. Moreover, by increasing the Sr concentration the activation energy of conduction and the temperature at which conduction mechanism changes decrease, while the localization length increases. The permittivity of grains is from 8 to 10 while the high permittivity observed at higher temperatures is extrinsic, due to the formation of different Schottky barriers.

Effect of Sintering Temperature on Dielectric Properties of CaCu<sub>3</sub>Ti<sub>4</sub>O<sub>12 </sub>Ceramics Prepared by Mechanochemical Process

CaCu3Ti4O12 (CCTO) is well known to have colossal dielectric constant in the range of 105.It is widely accepted that this phenomenon may be attributed to internal layer barrier capacitance (IBLC) model. The dielectric properties of CCTO were reported to be strongly dependent on the processing conditions and grain size. In this work, CCTO samples with different grain sizes were produced by varying sintering temperature in order to investigate IBLC effect on dielectric properties of CCTO. The samples were sintered at four different temperatures, (T=1100°C, 1050°C, 1000°C and 950°C). Dielectric measurements were carried out for the samples in the frequency range of 102 – 106Hz using impedance spectrometer. Electron micrographs showed that increasing temperature promoted the grain growth of CCTO while sintering. The internal crystalline defects are seen to play major role by increasing the grain conductivity in dipole formation and increased the dielectric constant of the samples.

La1.7Sr0.3NiO4 nanocrystalline powders prepared by a combustion method using urea as fuel: Preparation, characterization, and their bulk colossal dielectric constants

La1.7Sr0.3NiO4 nanocrystalline powders were successfully prepared by a combustion method using a urea as fuel. A pure phase of La1.7Sr0.3NiO4 powders was obtained by firing the precursor at a relatively low temperature and short reaction time of 1000 °C for 2 h, whereas a pure La1.7Sr0.3NiO4 phase prepared by a conventional solid-state reaction method was obtained at a temperature higher than 1200 °C for 12 h. Sintered La1.7Sr0.3NiO4 ceramics were found to have a highly dense ceramic microstructure. Interestingly, all the ceramics exhibited very high dielectric constants of about 4.3 × 105–1.05 × 106 at room temperature and 1 kHz. Using impedance spectroscopy, it was observed that the low-frequency loss tangent and DC conductivity data were associated with the electrical response at grain boundaries. The extremely high dielectric behavior of the La1.7Sr0.3NiO4 ceramics was attributed to the small polaronic hopping process.

Extrinsic mechanism for colossal dielectric constant in CaCu3Ti4O12 ceramics evidenced by nanodomain

We studied the effect of various sintering temperature and dwell time on phase formation, microstructure and dielectric properties of CaCu3Ti4O12 ceramic synthesized by sol–gel method. The dielectric property was greatly influenced by sintering temperature and dwell time. We found colossal dielectric constant (∼153 816) with low dielectric loss (∼0.20) for 1100 °C/4 h sintered sample at room temperature and 1 kHz frequency. Impedance spectroscopy results support the grain boundary barrier layer capacitor model. Observation of nanostripe structure domains inside the grains of CaCu3Ti4O12 confirms the extrinsic mechanism for colossal dielectric response of this material.

A colossal dielectric constant of an amorphous TiO2:(Nb, In) film with low loss fabrication at room temperature

High-performance dielectric materials continue to arouse considerable interest due to their application in the field of solid state capacitors.

Transport characteristics and colossal dielectric response of cadmium sulfide nanoparticles

We report here the synthesis of ∼20 nm sized cadmium sulfide (CdS) nanoparticles via conventional solid state reaction at low temperature ∼200 °C and ambient pressure. X-ray diffraction and high resolution transmission electron microscopy analysis confirmed the synthesis of hexagonal phased nanoparticles. Impedance and electrical modulus investigations were carried out in the frequency range 20 Hz to 2 MHz and at temperature from 300 K to 400 K, which show the presence of bulk, grain boundary, and sub-grain boundary phases in CdS nanoparticles. Overlapped large polaron tunneling was the observed mechanism of charge carriers in used temperature range. The presence of colossal dielectric constant in the system is attributed to the Maxwell-Wagner type polarization. High and temperature dependent dielectric constants make the CdS nanoparticles efficient material to be used in capacitive energy storage devices.

IBLC effect leading to colossal dielectric constant in layered structured Eu2CuO4 ceramic

Dielectric (εr′) studies of phase pure T′-type Eu2CuO4 ceramics of two markedly different grain sizes (D), prepared by (i) conventional powder mixing and (ii) citrate complexation-Pechini process, have been carried out in the frequency range 0.1Hz to 1MHz, and at temperatures −100°C to 150°C. εr′ is found to be highly grain size dependent. For the sample with coarse bar-like grains (D2~17×6μm2) εr′ is >103, and for the finer grain size sample with bimodal distribution (D1~1μm, D2~3μm) εr′ is ~105; for both the samples, high εr′ value is nearly frequency independent over 500Hz≤f<100kHz and T≥30°C. The impedance spectroscopy (IS) study has clearly shown that both, the coarse- and the fine-grained samples consist of semiconducting grains and insulating grain boundaries that primarily lead to an internal barrier layer capacitance (IBLC) effect. And thus, manifest colossal dielectric constant (εr′>103) in Eu2CuO4 ceramics. The smaller grain size (Pechini) sample, with over an order higher number of grains and grain boundary network, showing over an order higher εr′ (~105) compared to the coarse grained one, further endorses the IBLC effect.

Intrinsic electrical properties of LuFe2O4

We here revisit the electrical properties of LuFe${}_{2}$O${}_{4}$, compound candidate for exhibiting multiferroicity. Measurements of dc electrical resistivity as a function of temperature, electric-field polarization measurements at low temperatures with and without magnetic field, and complex impedance as a function of both frequency and temperature were carried out in a LuFe${}_{2}$O${}_{4}$ single crystal, perpendicular and parallel to the hexagonal $c$ axis, and in several ceramic polycrystalline samples. Resistivity measurements reveal that this material is a highly anisotropic semiconductor, being about two orders of magnitude more resistive along the $c$ axis. The temperature dependence of the resistivity indicates a change in the conduction mechanism at ${T}_{\mathrm{CO}}$ \ensuremath{\approx} 320 K from thermal activation above ${T}_{\mathrm{CO}}$ to variable range hopping below ${T}_{\mathrm{CO}}$. The resistivity values at room temperature are relatively small and are below 5000 \ensuremath{\Omega} cm for all samples but we carried out polarization measurements at sufficiently low temperatures, showing that electric-field polarization curves are a straight line as expected for a paraelectric or antiferroelectric material. Furthermore, no differences are found in the polarization curves when a magnetic field is applied either parallel or perpendicular to the electric field. The analysis of the complex impedance data corroborates that the claimed colossal dielectric constant is a spurious effect mainly derived from the capacitance of the electrical contacts. Therefore, our data unequivocally evidence that LuFe${}_{2}$O${}_{4}$ is not ferroelectric.

Applications of CCTO supercapacitor in energy storage and electronics

Since the discovery of colossal dielectric constant in CCTO supercapacitor in 2000, development of its practical application to energy storage has been of great interest. In spite of intensive efforts, there has been thus far, no report of proven application. The object of this research is to understand the reason for this lack of success and to find ways to overcome this limitation. Reported herein is the synthesis of our research in ceramic processing of this material and its characterization, particularly with the objective of identifying potential applications. Experimental results have shown that CCTO's permittivity and loss tangent, the two most essential dielectric parameters of fundamental importance for the efficiency of a capacitor device, are intrinsically coupled. They increase or decrease in tandem. Therefore, efforts to simultaneously retain the high permittivity while minimizing the loss tangent of CCTO might not succeed unless an entirely non-typical approach is taken for processing this material. Based on the experimental results and their analysis, it has been identified that it is possible to produce CCTO bulk ceramics with conventional processes having properties that can be exploited for fabricating an efficient energy storage device (EDS). We have additionally identified that CCTO can be used for the development of efficient solid state capacitors of Class II type comparable to the widely used barium titanate (BT) capacitors. Based on high temperature studies of the resistivity and the Seebeck coefficient it is found that CCTO is a wide bandgap n-type semiconductor material which could be used for high temperature electronics. The temperature dependence of the linear thermal expansion of CCTO shows the presence of possible phase changes at 220 and 770 °C the origin of which remains unexplained.

Effect of V2O5–TeO2 Glass Addition on Dielectric Properties of CaCu3Ti4O12 Ceramics Prepared by Solid State Method

CaCu3Ti4O12 (CCTO) is well known to have colossal dielectric constant in the range of 105. However, CCTO has considerably high dielectric loss values (tanδ > 0.1) at room temperature and 1 kHz. In this work, addition of glass to CCTO was suggested in order to improve its dielectric properties. The effect of V2O5–TeO2 (VT) glass addition on microstructure and dielectric properties of CCTO ceramics were investigated. Dielectric measurements were carried out for (1 − x) CCTO-(x) V2O5–TeO2 (x = 0, 0.03, 0.05, 0.10) samples in the frequency range of 102–106 Hz using impedance spectrometer. Electron micrographs showed that low melting VT glass addition facilitated the grain growth of CCTO. As VT glass amount increased, dielectric constant decreased. The drop, though within reasonable values (~105), may related to the presence of grain boundary glassy phase which itself has low dielectric constant. However, the dielectric loss of the composite has decreased after VT glass addition. The low melting V2O5–TeO2 glass aided in liquid phase sintering and improved the grain boundary resistance which resulted in decreased leakage currents.

A tetranuclear Cu4(μ3-OH)2-based metal–organic framework (MOF) with sulfonate–carboxylate ligands for proton conduction

A new tetranuclear Cu4(μ3-OH)2-based metal-organic framework (MOF) with sulfonate-carboxylate ligands features large hydrophilic channels. This MOF exhibits proton conductivity over 10(-3) S cm(-1) at 85-100 °C and 98% relative humidity and colossal dielectric constant.

Dielectric property and relaxation mechanism of CaCu3Ti4O12 ceramic

In this paper, the dielectric property of CaCu3Ti4O12 ceramic is measured by Novocontrol wide band dielectric spectrometer in a temperature range of -100-100 ℃ and frequency range of 0.1 Hz-10 MHz, and the corresponding dielectric relaxation mechanism is discussed. Firstly, on the basis of quantitative analysis of macroscopic shell-core structure, the possibility of colossal dielectric constant (CDC) originating from the surface insulated layer effect is rejected. Secondly, after the analysis of the nature of classical Maxwell-Wagner sandwich polarization and its activation energy, classical Maxwell-Wagner mechanism is also abandoned. Finally, a new model of trapped electron relaxation at the boundary of Schottky barrier is proposed. The new mechanism correctly reflects the essential connection between intrinsic point defects, conductivity and dielectric constant of CaCu3Ti4O12material.

Structural and dielectric properties of La0.8Te0.2MnO3

We have studied the structural and dielectric properties of La0.8Te0.2MnO3 pervoskite compound, has a rhombohedral structure with space group R-3c, at room temperature. Infrared spectrum shows two active bands located at 611 and 410cm−1, which can be ascribed to the internal stretching and bending phonon modes. The additional bands observed at 925, 969 and 1383cm−1 are attributed to the multiphonon scattering. The dielectric constant ε′ shows a step like relaxation behaviour and has been discussed with in the frame work of the Kramers–Kronig transformation model. The ac conductivity follows a universal dielectric response (UDR), and the results were discussed and fitted with the Jump relaxation model (JRM). The occurrence of giant or colossal dielectric constant is most likely due to electrode polarization or interface polarization effect. The depletion layers are arising due to the formation of Schottky barriers at the metallic contacts of semiconducting samples, which may be formed by grain boundaries, can give rise to Maxwell–Wagner type relaxation and apparently very high dielectric constants.

Colossal dielectric constant polymer nanocomposites: Role of charge injection at matrix–filler interfaces

Recent experiments show that certain self-assembled polymer nanocomposites exhibit colossal dielectric constants (∼4000 at 20 Hz), as compared to eight for the polymer matrix, even for small filler volume fractions (∼7%). However, reasons for this are not well understood. In this work, two possible explanations are systematically evaluated: (i) presence of interface zones with dielectric constants much greater than that of the matrix and (ii) charge injection from filler surfaces into the dielectric at matrix–filler interfaces. Linear, isotropic composites with periodic and random arrangements of spherical, conducting nanofillers are considered. While finite element analyses on unit cells are used to compute quasi-static dielectric properties of periodic composites, standard homogenization theories are used for investigating random composites. A novel two-parameter, non-local model is used to describe the charge injection. It is shown that both the effects improve the overall dielectric constants but in very different ways. While charge injection leads to a drastic increase in effective dielectric constant even for small filler volume fractions (∼1%), high dielectric constant interface zones significantly alter the overall properties only at percolation volume fractions. Charge injection appears to be the likely reason for the colossal dielectric constants observed experimentally in self-assembled nanocomposites.