Dielectric Relaxation Mechanism Research Articles

Dielectrophoresis force spectroscopy for colloidal clusters

Optical trapping-based force spectroscopy was used to measure the frequency-dependent DEP forces and DEP crossover frequencies of colloidal polymethyl methacrylate spheres and clusters. A single sphere or cluster, held by an optical tweezer, was positioned near the center of a pair of gold-film electrodes where alternating current elecroosmosis flow was negligible. Use of amplitude modulation and phase-sensitive lock-in detection for accurate measurement of the DEP force yielded new insight into dielectric relaxation mechanisms near the crossover frequencies. On one hand, the size dependence of the DEP force near the crossover frequencies indicates that the dominant polarization mechanism is a volume effect. On the other hand, the power-law dependence of the crossover frequency on the particle radius with an exponent of -2 indicates the dielectric relaxation is more likely because of ionic diffusion across the particle surface, suggesting the dominant polarization mechanism may be a surface polarization effect. Better theories are needed to explain the experiment. Nevertheless, the strong size dependence of the crossover frequencies suggests the use of DEP for size sorting of micron-sized particles.

Ab initio calculations of neutral and charged impurity centers of manganese and chromium in strontium titanate

This paper presents the results of ab initio calculations of the equilibrium geometry, the electronic structure, and the spin and charge densities for neutral and negatively charged defects produced by the Mn and Cr impurities in the B position of the SrTiO3 structure. It has been shown that, in both cases, the neutral defect is an acceptor center, while the singly charged defect is a donor center. It has been found that doubly charged defects are polar, have the symmetry C 4v, and reside in the ionic configurations 5Mn3+ + 3Ti3+ and 4Cr3+ + 3Ti3+, respectively. In each case, there is a pair of almost energy-degenerate electronic states (4 B 1 and 6 B 1 for Mn and 3 A 1 and 5 A 1 for Cr), which differ only in the direction of the spin of the electron polaron localized at one of the neighboring titanium atoms. For the manganese impurity, the energy of the polar state 6 B 1 is only 0.174 eV lower than that of the state 6 A 1g (O h ) with the Mn2+ ion in the high-spin state. A new mechanism of dielectric relaxation in STO: Mn has been proposed.

Extrinsic and Intrinsic Frequency Dispersion of High-k Materials in Capacitance-Voltage Measurements.

In capacitance-voltage (C-V) measurements, frequency dispersion in high-k dielectrics is often observed. The frequency dependence of the dielectric constant (k-value), that is the intrinsic frequency dispersion, could not be assessed before suppressing the effects of extrinsic frequency dispersion, such as the effects of the lossy interfacial layer (between the high-k thin film and silicon substrate) and the parasitic effects. The effect of the lossy interfacial layer on frequency dispersion was investigated and modeled based on a dual frequency technique. The significance of parasitic effects (including series resistance and the back metal contact of the metal-oxide-semiconductor (MOS) capacitor) on frequency dispersion was also studied. The effect of surface roughness on frequency dispersion is also discussed. After taking extrinsic frequency dispersion into account, the relaxation behavior can be modeled using the Curie-von Schweidler (CS) law, the Kohlrausch-Williams-Watts (KWW) relationship and the Havriliak-Negami (HN) relationship. Dielectric relaxation mechanisms are also discussed.

Impedance spectroscopy studies on CuPc/n-Si hybrid solar cell

Impedance spectroscopy of copper phthalocyanine (CuPc)/n-Si hybrid solar cell was studied. The dielectric relaxation mechanism of the solar cell was analyzed by Cole–Cole plots, indicating the presence of a single relaxation mechanism. The equivalent circuit model consisting of a parallel resistor and capacitor in series with a resistor was found to give a well fit to the experimental data. The conduction mechanism was suggested to be a space-charge limited current with exponential trap distribution. Beside, the dielectric relaxation time of the device was decreased with increasing applied bias voltages.

Synthesis, crystal structure, dielectric and optical properties of a new rare earth double perovskite: Ca 2CeNbO 6

In order to develope and understand the phenomena involved in producing advanced materials, a rare earth double perovskite oxide calcium cerium niobate, Ca 2CeNbO 6 (CCN) is synthesized for the first time. The x-ray diffraction pattern of CCN at room temperature (300K) shows orthorhombic perovskite structure, with the lattice parameters, a=9.36Å, b=6.61Å and c=5.88Å and α= β= γ= 90°. A scanning electron micrograph shows the formation of grains with average size ∼2μm. Impedance spectroscopy and Fourier transform infrared spectroscopy are applied to investigate the dielectric and optical properties of CCN. The frequency-dependent electrical data are analyzed in the framework of the conductivity and modulus formalisms. The experimental data of real part of dielectric permittivity ( ε′) and imaginary part of electric modulus ( M″) are fitted with Davidson-Cole equation to explore the idea of dielectric relaxation (conduction) mechanism in CCN. The frequency-dependent conductivity spectra follow a power law. The scaling behaviour of imaginary electric modulus ( M″) suggests that the relaxation describes the same mechanism at various temperatures.

Impedance spectroscopy of bilayered bismuth ferrite thin films

Bi0.90La0.10Fe0.85Zn0.15O3/Bi0.90La0.10Fe0.95Zn0.05O3 bilayers with different thickness ratios were deposited on Pt-coated silicon substrates by radio frequency sputtering. A polycrystalline structure with a random grain orientation is induced for all bilayers, owing to the introduction of the bottom Bi0.90La0.10Fe0.95Zn0.05O3 layer with a random orientation. Impedance spectroscopy shows that oxygen vacancies dominate the dielectric relaxation and electrical conduction of these bilayers, and their dielectric relaxation process gradually endures a transition from a long-range relaxation, the coexistence of long-range and localized relaxation, to a localized relaxation with increasing thickness ratios of Bi0.90La0.10Fe0.85Zn0.15O3. Scaling behavior indicates that their dielectric relaxation and conduction mechanisms are independent of the measurement temperature. Different electrical conduction behavior in impedance determines the electrical properties of these bilayers. The bilayer with a higher thickness ratio of Bi0.90La0.10Fe0.95Zn0.05O3 exhibits a higher remanent polarization owing to the involvement of a low defect concentration, as confirmed by the impedance spectroscopy and the dielectric behavior.

Investigation of thermally stimulated properties of SHI beam irradiated polycarbonate/polystyrene double layered samples

The double layered samples of polycarbonate/polystyrene (PC/PS) have been prepared by solvent casting method and irradiated with 55 MeV C 5+ beam at different ion fluences range from 1 × 10 11 to 1 × 10 13 ion/cm 2. The effect of swift heavy ion (SHI) beam in interfacial phenomena, phase change, dielectric relaxation, degradation temperature, stability, charge storage and transport mechanism of PC/PS pristine and irradiated double layered samples have been investigated by thermally stimulated discharge current (TSDC), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). TSDC show α, β-relaxation peaks shifted to the lower temperatures side with increase of fluence. The activation energy and relaxation time decrease, while the depolarization current and charge released increase with increase in the ion fluences. DSC curve show the glass transition temperature ( T g ) and heat capacity decreases with increase in the ion fluences. The TGA characteristics represent the thermal stability, which is found to be decreased with increase in the ion fluences.

Giant dielectric relaxation in TlGaTe2 crystals

The conductivity and temperature-frequency dependences of the permittivity of TlGaTe2 crystals have been studied. Strong dielectric relaxation has been revealed. It has been shown that the mechanism of dielectric relaxation is associated with hopping of Tl ions over vacancies in the thallium sublattice due to the transition of the system to the superionic state.

Lithium niobosilicate glass doped with samarium: Dielectric and optical analysis

AbstractLithium niobate (LiNbO3 – LN) is a large bandgap [3.78 eV at room temperature (RT)] semiconductor mainly known by its ferroelectric behaviour and excellent non‐linear optical, electro‐optical and piezoelectric properties. In this study 91SiO2–4Li2O–4Nb2O5–1Sm2O3 (mol%) glasses were prepared by sol–gel. Glass–ceramics were obtained through heat‐treatments (HTs) of the dried gel. The samples structure was studied by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The HT above 600 °C promotes the crystallization of LiNbO3 and SmNbO4. For heat HT above 800 °C, SiO2 crystallites also precipitate. SEM micrographs indicate that the size of the particles and aggregation phenomena is promoted with the increase of the HT temperature. The electric characterization was performed as a function of the temperature and frequency. The main contribution for the electric conductivity behaviour is the number of network modifier ions, which decreases with the increase of the HT temperature. The dielectric constant and loss value decrease with the increase of the HT temperature. Dielectric relaxation mechanisms were observed in the samples HT below 800 °C. At RT, the photoluminescence (PL) spectra, revealed a red light emission, due to the 4G5/2 → 6HJ = 5/2,7/2,9/2 transitions of Sm3+ ion. The results suggest that the intraionic emission is promoted when the lanthanide ions are inserted in the amorphous matrix. Auto‐absorptions of the samarium ion and large emission bands were detected and assigned to the crystalline and amorphous phases of SiO2. The obtained results reflect the decisive effect of HT temperature in the structural, electric and optical properties of these glasses.

Migration Kinetics of Oxygen Vacancies in Mn-Modified BiFeO3 Thin Films

Migration kinetics of oxygen vacancies in BiFe(0.95)Mn(0.05)O(3) thin film were investigated by the temperature -dependent leakage current as well as the electric field and temperature-dependent impedance spectroscopy. The BiFe(0.95)Mn(0.05)O(3) is of an abnormal leakage behavior, and an Ohmic conduction is observed regardless of varied temperatures and electric fields. The temperature-dependent impedance spectroscopy under different resistance states is used to illuminate different leakage behavior between BiFe(0.95)Mn(0.05)O(3) and pure BiFeO(3). The impedance spectroscopy under a high resistance state shows that the first ionization of oxygen vacancies is responsible for the dielectric relaxation and electrical conduction of BiFe(0.95)Mn(0.05)O(3) in the whole temperature range of 294 to 474 K; the BiFeO(3) exhibits similar dielectric relaxation and electrical conduction behavior in the low-temperature range of 294-374 K, whereas the short-range motion of oxygen vacancies was involved in the high-temperature range of 374-474 K. The impedance spectroscopy under a low resistance state demonstrates that the dielectric relaxation and conduction mechanisms almost keep unchanged for BiFe(0.95)Mn(0.05)O(3), whereas the hopping electrons of Fe(2+)-V(O)(•)-Fe(3+) and Fe(2+)-Fe(3+) are responsible for its dielectric relaxation and conduction mechanism of BiFeO(3). Different impedance spectroscopy under low and high resistance states confirms that an abnormal leakage behavior of BiFe(0.95)Mn(0.05)O(3) is related to different migration kinetics of oxygen vacancies, obviously differing from that of BiFeO(3).

DIELECTRIC RELAXATION AND ELECTRICAL STUDY OF BaNb2O6 PREPARED BY SOL–GEL ROUTE

A gel was formed when an aqueous solution of BaCl 2, NbF 5 and citric acid in stoichiometric ratio is heated on a water bath at 100°C. This gel on decomposition at 700°C yielded the nano phase of BaNb 2 O 6 as confirmed by X-ray diffraction study. The crystal structure is orthorhombic with unit cell parameter a = 12.2136 Å, b = 10.2413 Å, c = 7.8831 Å. The estimated grain size is 88 nm. Dielectric studies reveal the dielectric relaxation mechanism. Conducting charges and free charges both contribute to the dielectric relaxation in this material. Electrical properties of the material were studied using impedance spectroscopy technique. Detailed analysis of impedance spectrum suggested that the electrical properties are strongly temperature-dependent. The relaxation is polydispersive and conduction is mainly through grains. AC conduction activation energies are estimated from Arrheneus plots and conduction mechanism is discussed.

Two dielectric relaxation mechanisms observed in lanthanum doped barium strontium titanate glass ceramics

The dielectric relaxation at temperatures ranging from 100 °C to 550 °C in lanthanum-doped barium strontium titanate glass ceramics has been investigated by measurements of both dielectric temperature curve and complex impedance spectroscopy. The temperature dependence of dielectric properties reveals two regimes. In the first regime, at temperatures lower than 350 °C, the charge carrier contributed to interfacial polarization is conduction electrons. In contrast, in the second regime, at temperatures higher than 350 °C, the dielectric relaxation was attributed to the thermally activated movement of oxygen vacancies. Thus, two relaxation mechanisms could be explained on the basis of defect chemistry and impedance analysis.

Impedance spectroscopy properties of polypyrrole doped with boric acid

The electrical conductivity and dielectric properties of polypyrrole doped with boric acid have been investigated. The direct current conductivity (dc) of the polymer increases with increasing temperature. The alternating current (ac) conductivity of the polymer obeys the power law, i.e., σ ac ( ω) = Aω s . The alternating conductivity of polypyrrole doped with boric acid is controlled by the correlated barrier hopping model. The activation energy for alternating current mechanism decreases with increasing frequency which confirms the hopping conduction to the dominant mechanism as compared with the dc activation energy. The density of localized states N( E F ) for polypyrrole doped with boric acid was in the range of 2.5–9.2 × 10 22 cm −3 for various temperatures. The dielectric relaxation mechanism was explained on the basis of complex dielectric modulus. The imaginary modulus plot at different temperatures shows a dielectric mechanism with non-Debye relaxation. Boric acid can be a good candidate for controlling the electrical conductivity of the conducting polymer.

Dielectric relaxation and conduction mechanism in LaNi 3/4M 1/4O 3 (M = Mo, W) at low temperature

In order to investigate the electrical transport in LaNi 3/4Mo 1/4O 3 and LaNi 3/4W 1/4O 3, the dc conductivity and dielectric properties in these polycrystalline materials are investigated in the temperature range from 163 K to 383 K and frequency range from 50 Hz to 1 MHz. The X-ray diffraction patterns of the samples show monoclinic phase at room temperature. The homogeneity of the samples is determined by energy dispersive analysis of X-ray (EDAX) attached with a scanning electron microscope. The temperature dependence of dc conductivity shows the semiconducting nature of the materials. The complex impedance plane plots show that the relaxation (conduction) mechanism in these materials is purely a bulk effect arising from the semiconductive grains. The frequency-dependent electrical data are also analyzed in the framework of ac conductivity formalism. The ac conductivity spectra follow the universal power law. The activation energies required for bulk conduction is 0.143 and 0.165 eV for LNM and LNW respectively. The scaling behaviour of loss tangent suggests that the relaxation describes the same mechanism at various temperatures.

ZnO as a buffer layer for growth of BiFeO3 thin films

Multiferroic BiFeO3 thin film was grown on the ZnO-buffered Pt/TiO2/SiO2/Si(100) substrate by off-axis radio frequency magnetron sputtering, where the ZnO buffer layer gave rise to a strong (110) texture for the BiFeO3 thin film. The resulting BiFeO3/ZnO thin film exhibits diode-like and resistive hysteresis behavior, in which the resistive hysteresis and rectifying ratio are dependent on the applied voltage and temperature. The resistive switching behavior of the BiFeO3/ZnO thin film is shown to relate to the trap-controlled space charge limited conduction and interface-limited Fowler–Nordheim tunneling, while the polarization reversal takes place in the BiFeO3 layer of the heterostructure. The BiFeO3/ZnO thin film is also demonstrated with a higher remanent polarization (2Pr∼153.6 μC/cm2), a much lower dielectric loss (tan δ∼0.012), and a better fatigue endurance as compared to those of the BiFeO3 thin film without a ZnO buffer layer, where the much reduced leakage is largely responsible for the enhanced ferroelectric behavior. The ZnO as a buffer layer for BiFeO3 significantly changes the dielectric relaxation and conduction mechanisms, when the dielectric relaxation and electrical conduction are governed by the thermal excitation of carriers from the second-ionization and short-range motion of oxygen vacancies, respectively, while the relaxation process remains the same over the entire temperature range of 20 to 200 °C investigated in the present study.

A Study of Concrete Hydration and Dielectric Relaxation Mechanism Using Ground Penetrating Radar and Short-Time Fourier Transform

Ground penetrating radar (GPR) was used to characterize the frequency-dependent dielectric relaxation phenomena in ordinary Portland cement (OPC) hydration in concrete changing from fresh to hardened state. The study was experimented by measuring the changes of GPR A-scan waveforms over a period of 90 days, and processed the waveforms with short-time Fourier transform (STFT) in joint time-frequency analysis (JTFA) domain rather than a conventional time or frequency domain alone. The signals of the direct wave traveled at the concrete surface and the reflected wave from an embedded steel bar were transformed with STFT, in which the changes of peak frequency over ages were tracked. The peak frequencies were found to increase with ages and the patterns were found to match closely with primarily the well-known OPC hydration process and secondarily, the evaporation effect. The close match is contributed to the simultaneous effects converting free to bound water over time, on both conventional OPC hydration and dielectric relaxation mechanisms.

Functional properties of BaTiO3–Ni0.5Zn0.5Fe2O4 magnetoelectric ceramics prepared from powders with core-shell structure

In the present work, diphasic ceramic composites with core-shell nanostructures formed by Ni0.50Zn0.50Fe2O4 core and BaTiO3 shell were investigated. Their properties were compared with those of composites prepared by coprecipitation. The core-shell structure was confirmed by microstructural powder analysis. Homogeneous microstructures with a good phase mixing and percolated dielectric phase by the magnetic one were obtained from coprecipitated powders. Less homogeneous microstructures resulted in ceramics produced from the powder prepared by core-shell method, with isolated small ferrite grains besides large ferrite aggregates embedded into the BaTiO3 matrix. Both the ferroelectric and magnetic phases preserve their basic properties in bulk composite form. However, important differences in the dielectric relaxation and conduction mechanisms were found as result of the microstructural difference. Extrinsic contributions play important roles in modifying the electric properties in both ceramics, causing space charge effect, Maxwell–Wagner relaxations and hopping conductivity, mainly due to the ferrite low resistivity phase. The conductivity and dielectric modulus spectra analysis allowed to identify different polaron contributions associated with the microstructural differences. It results that by using the core-shell method, improved dielectric properties and limited hopping contributions can be realized.

Ferroelectric and Impedance Behavior of La‐ and Ti‐Codoped BiFeO3 Thin Films

To study the effects of La and Ti cosubstitutions on ferroelectric and impedance behavior of BiFeO3, (Bi0.90La0.10)FeO3, Bi(Fe0.95Ti0.05)O3, and (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin films were deposited on LaNiO3‐buffered Pt/TiO2/SiO2/Si(100) substrates by off‐axis radio frequency sputtering. The (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin film exhibits an Ohmic conduction behavior in the range of the electric field investigated, and its leakage current at high electric fields is greatly suppressed. The Curie temperature of (Bi0.90La0.10)(Fe0.95Ti0.05)O3 decreases to ∼690°C due to La and Ti cosubstitutions, and a direct band gap of 2.88 eV is identified for the (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin film, demonstrating the increase of a direct band gap with La and Ti codoping. A high remanent polarization (2Pr∼102.6 μc/cm2 and 2Ec∼538.0 kV/cm) as confirmed by positive up negative down was obtained for the (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin film at room temperature because of the great inhibition of the leakage current density at high electric fields. The La and Ti cosubstitutions also improve the fatigue behavior of BiFeO3 thin film. Impedance analyses at different temperatures and frequencies show that the La‐ and Ti‐codoped BFO thin film exhibits rather different dielectric relaxation and conduction mechanism as compared with those of the undoped and La‐ or Ti‐doped BFO thin films, where oxygen vacancies appear to be involved in the dielectric relaxation and conduction processes of these thin films.

The electrical conductivity and dielectric properties of C.I. Basic Violet 10

The electrical and dielectrical properties of C.I. Basic Violet 10 were investigated using both DC and AC measurements. The DC electrical conductivity of the dye exhibited similar behavior to that of organic semiconductors and the mechanism of the AC conductivity the dye can be attributed to correlated barrier hopping, as confirmed by an observed reduction in activation energy. The density of localized states for C.I. Basic Violet 10 was 0.15 × 10 22 to 4.4 × 10 22 eV −1 cm −3 over the range of temperatures and frequencies employed. Variation of the dielectric constant with temperature at different frequencies showed a dielectric relaxation peak; with increasing frequency, the intensity of the dielectric relaxation peak decreased and became less pronounced. The obtained results suggest that C.I. Basic Violet 10 is an organic semiconductor and its dielectric relaxation mechanism is temperature-dependent.

Possible mechanism of charge transport and dielectric relaxation in SrO–Bi2O3–B2O3 glasses

Transparent SrBi2B2O7 glasses were prepared via melt-quenching technique and characterized using differential scanning calorimetry and x-ray powder diffraction. The ac conductivities of the glasses were studied as a function of frequency (100 Hz–10 MHz) at different temperatures. The frequency dependence of conductivity has been analyzed using Almond–West expression. The exponent n was nearly unaffected by temperature. Impedance and modulus spectroscopies were employed to further examine the electrical data. Dielectric relaxation exhibited a stretched exponential behavior with a stretching exponent β independent of temperature. From conductivity analysis we have proposed that the charge transport occurs through the participation of nonbridging oxygen (NBO), which switches positions in a facile manner. The stretched exponential behavior appears to be a direct consequence of the NBO switching mechanism of charge transport.