Origin Of Dielectric Relaxation Research Articles

Investigation of the dielectric relaxation mechanisms for Pb(Fe1/2Nb1/2)O3 single crystal based on the universal relaxation law

Large-sized multiferroics Pb(Fe1/2Nb1/2)O3 (PFN) single crystal was grown by Bridgeman method in order to understand the giant dielectric response by investigating the electrical conductivity and dielectric properties. The synchrotron radiation soft x-ray absorption spectrum revealed the valence states of Fe in PFN crystal. PFN exhibits semiconductor behaviors in the thermal active electrical conductivity investigation. Besides, the frequency-dependent dielectric constants were measured in the frequency range from 100 Hz to 1 MHz to study the dielectric relaxation behaviors. Two relaxation processes were found and described by Havriliak-Negami equation. The origin of dielectric relaxation in PFN with semiconductor characters was discussed based on the universal relaxation law for a dipolar system.

Origins of dielectric relaxations in AgNb7O18 ceramic

The origins of the relaxor behavior and low-temperature dielectric relaxation processes in AgNb7O18 ceramic were studied and a new relaxation mechanism was proposed through combined bond valence energy landscape (BVEL) calculation and equivalent circuit modeling (ECM). The BVEL calculation suggests that the Ag1 displacement subjects to a double-well potential that originates from the anisotropic coordination symmetry and low coordination number at Ag1 site. The relaxor ferroelectric behavior, observed at 200 K ≤ T ≤ 270 K, is sensitive to thermal history and is ascribed to the Ag1 hopping in double-well potential. The temperature dependence of the matrix capacitance (obtained by ECM) is very similar with that of b axis (obtained by in situ XRD) at 123 K < T < 174 K. Therefore, a phase transition being responsible for the low-temperature dielectric relaxation is assumed. The combined BVEL and ECM techniques performed in this work are therefore quite helpful to understand the complicated relaxor phenomena and explore new relaxors.

Origin of dielectric relaxations in KHSeO4 above room temperature

The dispersion curves of the dielectric response for KHSeO4 were obtained in the radio frequency range at several isotherms below the fast proton conducting phase (T < 415 K). The results reveal a distinct dielectric relaxation at low frequency, which is about 682 Hz at 320 K, and then, it shifts to higher frequencies (∼10 kHz) as the temperature increases. The f max vs. reciprocal T shows an activated relaxation process with an activation energy of 0.5 eV, which is in close agreement with that associated with transport of charge carriers. We suggest that the observed dielectric relaxation could be attributed to polarization induced by the proton jump and selenate tetrahedral reorientations. The displacement of mobile H+ proton accompanied by SeO 4 − 2 tetrahedra reorientations creates structural distortion in both sublattices which induce localized dipoles like HSeO 4 − .

Origin of dielectric relaxations in Na2SO4 in the intermediate temperature regime

The dielectric permittivity of Na2SO4 was measured in the frequency range 5Hz–13MHz. It shows a distinct relaxation at low frequencies at several isotherms (T≤377°C). For example, the relaxation frequency, fmax, is around 1MHz at 247°C, which shifts to higher frequencies (∼4.5MHz) as the temperature increases. The fmax vs. reciprocal T shows an activated relaxation process over the temperature range 247–377°C with activation energy Ea=0.38eV, which is close to that derived from the dc conductivity, Eσ=0.44eV. We suggest that the observed dielectric relaxation might be attributed to polarization induced by the sodium-ions hops and the sulfate-groups reorientations. The displacement of mobile Na+ ions accompanied by SO42− tetrahedra reorientations create structural distortion in both sublattices that induce localized dipole-like moments.

Comparative Dielectric Studies of NaNbO3–Based Lead-Free Relaxor Ceramics and Lead-Containing Relaxors

Complex dielectric permittivity ϵ* = ϵ′ -iϵ″ of lead-free (Na, Sr, Li)NbO3 ceramic composition exhibiting a relaxor-like dielectric behavior was studied in wide temperature (−180–500°C) and frequency (10 2 –10 6 Hz) range. Some features characteristic of relaxors were observed, namely fulfilment of quadratic and linear Curie-Weiss laws for ϵ ′ just above and well above the temperature T m of ϵ′ (T) peak respectively, the Vogel-Fulcher scaling of both ϵ′ (T) and ϵ” (T) peaks. However, in contrast to canonical lead-containing relaxors Curie-Weiss temperature is lower than T m and the scaling parameters differ for ϵ′ (T) and ϵ”(T) suggesting different origin of dielectric relaxation in these two groups of materials.

Thermally Stimulated Relaxation in Fe‐Doped SrTiO3 Systems: II. Degradation of SrTiO3 Dielectrics

Thermally stimulated depolarization current (TSDC) measurements were carried out on degraded SrTiO3 crystals and ceramics to investigate the relationship between depolarization effects and degradation phenomena. Using this technique, the origins of dielectric relaxations in Fe‐doped SrTiO3 crystals and ceramics have been verified; the physical origins of relaxation mechanisms were later linked to the transient leakage behavior of the samples undergoing degradation from oxygen vacancy migration. In the course of degradation in a single crystal system, the migration of oxygen vacancies, , and the build‐up of an internal bias are detected through the TSDC measurements on samples with different degradation levels. Using a curve fitting method, trap charge concentration on the order of 1014 cm−3 is obtained in the single crystal system. Other than those relaxation mechanisms identified in the single crystal system, extra relaxation mechanism was found in the polycrystalline systems and was attributed to the relaxation of oxygen vacancies across grain boundaries. Using the initial rise method, the activation energies estimated for the relaxation of defect dipoles, the in‐grain oxygen vacancies pile up at grain boundaries, and relaxation of oxygen vacancies across grain boundaries are 0.73±0.03, 0.86±0.07, and 1.1±0.09 eV, respectively. An ionic demixing model is applied to account for the evolution of TSDC spectra and to bridge changes to the leakage behavior of the degraded samples. In the case of the polycrystalline system, it is suggested that a strong degradation to the insulation resistance occurs when oxygen vacancies migrate across grain boundaries and start to pile up at the cathode region of metallic electrodes. Before that point, the vacancies accumulate at partial blocking grain boundaries in each of the crystallites.

Dielectric Behavior of Ba(Ti1 − xZrx)O3 Solid Solution

The dielectric permittivity as a function of temperature at frequencies from 10−2 to 105 Hz is studied and parameterized in the perovskite Ba(Ti0.675Zr0.325)O3 ceramics. The features characteristic of relaxor ferroelectrics are found, namely, the Lorenz-type temperature dependence at high-temperature slope of the real permittivity peak, the Curie-Weiss law observed at temperatures much higher than the temperature of the peak, the Vogel–Fulcher scaling of the temperature/frequency dependencies of the permittivity. However, scaling characteristics differ from those in classical complex perovskite relaxors suggesting different origin of dielectric relaxation.

Effect of bottom electrodes on dielectric relaxation and defect analysis of (Ba 0.47Sr 0.53)TiO 3 thin film capacitors

The dielectric relaxation and defect analysis of (Ba 0.47Sr 0.53)TiO 3 (BST) thin films deposited on various bottom electrodes, such as Pt, Ir, IrO 2/Ir, Ru, RuO 2/Ru before and after annealing in O 2 ambient was investigated. Through the measurement of dielectric dispersion as a function of frequency (100 Hz ≤ f ≤ 10 MHz) and temperature (27°C ≤ T ≤ 150°C), we studied the trapping dielectric relaxation and defect quantity of the films, and proposed an equivalent circuit on the basis of the capacitance, admittance and impedance spectra. A shallow trap level located at 0.005–0.01 eV below the conduction band was observed from the admittance spectral studies in the temperature range of 27–150°C. The origin of dielectric relaxation and defect concentration was attributed to the existence of the grain boundary defect, interface defect and shallow trap level in the films. An equivalent circuit was established which can well explain the AC response and identify the contribution of defects on electrical properties of BST thin film. From the viewpoint of trapping phenomena and dielectric relaxation analyses, we propose Ir as the optimum material for bottom electrode to withstand the post-annealing treatment.

Amorphous phase separation in polypropylene block copolymers as revealed by thermostimulated depolarization measurements. II. Thermal sampling analysis

We achieved a thermal sampling scanning of the β-relaxation region of polypropylene (PP) block copolymers together with the β process of isotactic polypropylene, high-density polyethylene, propylene-ethylene rubber, and part of the PP bloc copolymer soluble in xylene and insoluble in ether. The comparison of activation parameters determined in the above relaxations confirmed the hypotheses drawn from complex spectra studies concerning phase separation and the origin of dielectric relaxations in PP block copolymers. One of the dielectric relaxations of the part of PP block copolymer soluble in xylene and insoluble in ether could be attributed to polypropylene blocks. We related the preexponential factor of the relaxation time to chain environment. Then we discussed the compensation phenomenon in distributed relaxations and the relation between the compensation temperature and variation of thermal expansion coefficient through the compensating relaxation.