High-temperature Dielectric Relaxation Research Articles

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.

Effects of Bi-Substitution on Dielectric and Ferroelectric Properties of Yttrium Iron Garnet Ceramics

Dielectric and ferroelectric properties of Y3−xBixFe5O12 ceramics were extensively investigated. All the samples show a low temperature dielectric relaxation which originates from the charge carrier hopping between Fe2+ and Fe3+ and a high temperature dielectric relaxation which is associated with conduction. The dielectric constant, dielectric loss and the activation energy of the hopping process between Fe2+ and Fe3+ of Y3-xBixFe5O12 ceramics decrease with increasing Bi substitution. The conductivity decreases and the remanent polarization increases as the amount of Bi increases.

Dielectric properties and high-temperature dielectric relaxation of Ba3Ti4Nb4O21 ceramic

A3B8O21 hexagonal perovskite-like Ba3Ti4Nb4O21 ceramics were synthesized by a conventional solid-state reaction technique, which exhibit high dielectric constant at room temperature. A high-temperature dielectric relaxation was observed in frequency range from 40 Hz to 1 MHz above the Curie temperature. The real and imaginary parts of the impedance (Z′ and Z″) as functions of frequency indicate the presence of two relaxation processes at high temperatures, which are attributed to grain and grain boundary responses. The conductivities of both grain and grain boundary obey the Arrhenius law with activation energies of 1.04 eV and 1.27 eV, respectively. The dielectric relaxation processes are related to the oxygen vacancies inside the ceramic, indicating that the Q × f value of Ba3Nb4Ti4O21 might be improved via eliminating oxygen vacancies.

Ferroelectric and dielectric properties in Ba5SmFe1·5Nb8·5O30 tungsten bronze ceramics

The dielectric and relaxor properties of Ba5SmFe1·5Nb8·5O30 tungsten bronze ceramics were investigated together with the crystal structure. Tetragonal filled tungsten bronze structure was determined in space group P4bm with lattice parameters of a = b = 12·527(6) Å, c = 3·955(2) Å. Three dielectric relaxations were observed in the temperature range from 128 to 623 K. A temperature stable dielectric constant plateau (∼2·5×103) was observed between the middle and high temperature relaxations. The low temperature dielectric relaxation (∼250 K) illustrated a typical relaxor ferroelectric behaviour. The middle temperature dielectric relaxation (dispersing from ∼150 to 450 K) and the high temperature dielectric relaxation (>500 K) were Debye type thermal active processes. The middle temperature relaxation had an active energy with a magnitude of 10−1 eV, and a relaxation frequency of ∼108 Hz was attributed to the inhomogeneous structure. The high temperature relaxation with an active energy ∼1·4 eV and a relaxation frequency ∼1016 Hz was attributed to the electrode polarisation. Introduction of Fe ions generates large and temperature stable dielectric constant between the middle temperature and high temperature relaxations, disturbs the polarisation ordering and depresses the ferroelectric relaxation temperature.

Dielectric response and origin in antiferromagnetic/ferroelectric (1 − x)BiFeO3-(x)BaTiO3 ceramics

Dielectric permittivity (ε) and conductivity (σ) have been measured in (1 − x)BiFeO3-(x)BaTiO3 (BFO-xBT) multiferroic ceramics for x = 0.0, 0.05, and 0.10 as functions of temperature and frequency. A one-dimensional across-barrier model with intrinsic barriers B every lattice constant a and extrinsic barriers B + Δ every distance d is introduced to describe the dielectric response and conductivity. The across-barrier hopping is responsible for the high-temperature conductivity and dielectric relaxation in the lower temperature region. Good qualitative fits of dielectric permittivity and conductivity are obtained as functions of temperature and frequency. BaTiO3 substitution can enhance the intrinsic barrier and reduce the hopping conductivity.

Effects of Rare-Earth Doping on Dielectric Properties of CaCu3Ti4O12Ceramics

The Ca1−xNdxCu3Ti4O12(x = 0.0%, 0.1%, 0.5% and 1.0%, respectively) ceramics were synthesized by the solid-state reaction method in the temperature range of 1000°C to 1100°C, and single-phase structures with space group Im3 were obtained up to x = 1.0%. The effects of Nd-substitution on the dielectric properties and conductivity of Ca1−xNdxCu3Ti4O12ceramics were investigated and discussed. The room temperature dielectric constant and dielectric loss of Ca1−xNdxCu3Ti4O12ceramics slightly decreased with x = 0.1% and 0.5%, but increased with x = 1.0%. The Nd-substitution has little effect on the giant dielectric constant step but a strong effect on the high-temperature dielectric relaxations.

Dielectric properties of Sm-doped CaCu3Ti4O12 ceramics

The effects of Sm substitution on structure, dielectric properties and conductivity of CaCu3Ti4O12 ceramics were investigated. Ca1−xSmxCu3Ti4O12 (x=0.0%, 0.5%, 1.0%) ceramics were synthesized by the solid-state reaction method. Single phase crystal of the ceramics with space group Im3 was obtained. With increasing Sm content, the dielectric loss of Ca1−xSmxCu3Ti4O12 ceramics improved but the dielectric constant also decreased significantly, with both the low- and high-temperature dielectric relaxations suppressed.

Oxygen-vacancy-related high-temperature dielectric relaxation and electrical conduction in 0.95K 0.5Na 0.5NbO 3–0.05BaZrO 3 ceramic

The crystal structure and dielectric properties of 0.95K 0.5Na 0.5NbO 3–0.05BaZrO 3 (KNN–BZ) ceramic have been investigated by X-ray diffraction and dielectric measurement. A rhombohedral distortion was caused and the dielectric permittivity near Curie temperature was significantly enhanced by introducing BZ into KNN. The dielectric and conductivity properties of the sample were studied by using AC impedance spectroscopy and universal dielectric relaxation law in detail. The typical high-temperature dielectric relaxation process was confirmed to be related to the oxygen vacancies inside the ceramic. The effect of lattice distortion on the activation energy for oxygen vacancy migration in KNN–BZ was discussed by comparing with KNN and KNN–BaTiO 3.

BiFeO 3 ceramics synthesized by spark plasma sintering

Phase-pure BiFeO 3 particles were synthesized by an improved solid state technique. High density BiFeO 3 ceramics were prepared using these particles by spark plasma sintering (SPS). The dielectric permittivity and loss of SPS samples were measured as functions of sintering temperature, frequency, and annealing conditions. Dielectric spectra of the ceramics annealed at 650 °C were characterized in a broad range of temperature (300–725 K) and frequency (100 Hz to 20 MHz). Two kinds of dielectric relaxation following the Arrhenius law were detected in low and high temperature ranges, respectively. The low temperature dielectric relaxation could be almost completely removed by annealing in vacuum and it should be assigned to be a valence fluctuation of Fe ions, while the high temperature dielectric relaxation was proposed to stem from the short-range motion of oxygen vacancies.

Temperature dependence of dielectric and piezoelectric properties of (1−x)(BiScO3–0.64PbTiO3)–xLiNbO3 high-temperature relaxor ferroelectric ceramics

(1−x)(0.36BiScO3–0.64PbTiO3)–xLiNbO3 (BSPT64–xLN) high-temperature relaxor ferroelectric ceramics near the morphotropic phase boundary (MPB) composition were investigated. X-ray diffraction showed a change in symmetry from MPB phase to rhombohedral phase with LiNbO3 content increasing. A change from normal ferroelectric features to relaxor ferroelectric features was observed with LiNbO3 substitution up to x = 0.06, while high-temperature dielectric relaxation was exhibited at T max ~230 to 383°C for 0.02 ≤ x ≤ 0.06. The BSPT64–xLN ceramics with x = 0.02 LiNbO3 exhibited good piezoelectric properties compared with BS–0.64PT ceramics: piezoelectric coefficients d 33 = 505pC/N, planar electromechanical coupling factors k p = 0.47, and remnant polarization P r = 40 μC/cm2, respectively. The annealing temperature dependence of piezoelectric response for BSPT64–xLN ceramics was measured and it was found that the piezoelectric properties decreased slightly before 210°C for x = 0.02 and 0.04, showing excellent thermal stability, which indicated that BSPT64–xLN relaxor ferroelectric ceramics can be used in the range of high temperature compared to Pb-based relaxor ferroelectrics.

Frequency and temperature dependent dielectric and conductivity behavior of 0.95(K 0.5Na 0.5)NbO 3–0.05BaTiO 3 ceramic

Dielectric and conductivity measurements were carried out on the 0.95(K 0.5Na 0.5)NbO 3–0.05BaTiO 3 (KNN–5BT) ceramic both as a function of temperature (∼350–850 K) and frequency (40 Hz to 100 MHz). A high-temperature dielectric relaxation above Curie temperature (∼590 K) was observed and analyzed with the Cole–Cole function. Frequency dependent conductivity was analyzed with an augmented Jonscher relation and exhibited a universal conductivity behavior. The activation energy of dielectric relaxation was estimated to be 1.09 eV. It could be attributed to the thermal motion of double ionized oxygen vacancy or to the formation of defect dipoles between the acceptor ion and charge compensating oxygen vacancies. The mechanism for both the high-temperature dielectric relaxation and the frequency dependent conductivity was proposed based on a possible mode of incorporation of Ba 2+ and Ti 4+ ions into the KNN lattice sites.

Structure and modified giant dielectric response in CaCu 3(Ti 1− xSn x) 4O 12 ceramics

The influence of Sn-substitution on structure, dielectric properties and conductivity of CaCu 3Ti 4O 12 ceramics was investigated. Single-phase structures with space group Im 3 ¯ were obtained up to x = 0.1 in CaCu 3(Ti 1− x Sn x ) 4O 12. The giant dielectric constant step became lower with Sn-substitution by depressing both the low and high temperature dielectric relaxations. The room temperature dielectric constant and dielectric loss of CaCu 3(Ti 1− x Sn x ) 4O 12 decreased with increasing Sn-substitution. The dielectric constant decreased from 79,846 to 31,324 when x changed from 0 to 0.1, while and the dielectric loss was suppressed from 0.083 to 0.062 (at 10 kHz). XPS analysis results confirmed the obvious decrease of Ti 3+/Ti 4+ and Cu +/Cu 2+ ratio, and the decreased giant dielectric response was primarily attributed to the modification of the mixed-valent structure of Ti 3+/Ti 4+ and Cu +/Cu 2+.

Multiferroic ceramics in BaO–Y2O3–Fe2O3–Nb2O5 system

Multiferroic ceramics in BaO–Y2O3–Fe2O3–Nb2O5 system were synthesized and their dielectric, ferroelectric and magnetic properties were evaluated. XRD results showed that the ceramic composite consists of a major phase of tetragonal tungsten bronze structured Ba2YFeNb4O15, and minor phases of monoclinic YNbO4 and hexagonal Ba3Fe2Nb6O21. Three dielectric relaxations were observed in the temperature range from 125 to 575 K. The relaxor dielectric behavior in the temperature range from 125 to 350 K was attributed to the random occupation of Fe3+ and Nb5+ ions at B site of the tungsten bronze structure. The electrode polarization and the inhomogeneous structure contributed to the high-temperature and middle-temperature dielectric relaxations, respectively. Both the ferroelectric hysteresis loop and the magnetic hysteresis loop were measured, which suggested that the synthesized ceramic composite was a promising candidate of multiferroics.

Oxygen-vacancy-related high-temperature dielectric relaxation in SrTiO3 ceramics

Quantum paraelectric SrTiO3 has resulted in many investigations because of the anomalous properties. Here, using the conventional solid-state reaction method, we fabricated polycrystalline SrTiO3 ceramics with pure cubic perovskite structure. A dielectric loss peak is observed at around 450 K and 100 Hz and it shifts to higher temperature with increasing frequency. The typical high-temperature dielectric relaxation process is confirmed to be related to the oxygen vacancies (OVs) inside ceramics. More interestingly, a Cole–Cole fitting to loss peaks reveals a weaker correlation among OVs for such dielectric materials compared with that of ferroelectrics.

Peculiarities of high-temperature dielectric relaxation in vinylidene fluoride – hexafluoropropylene copolymers

A method of broad-band dielectric relaxation spectroscopy (10 −1–10 7 Hz) has been used to study the mechanisms of high-temperature relaxation, related to space charge and electrode phenomena, in extruded films of vinylidene fluoride – hexafluoropropylene copolymer. The analysis of dielectric data on films of different thickness showed qualitative agreement with Iwamoto theory. Quantitative correlation of temperature dependencies of the relaxation times with conclusions of this theory assumes that not only the film thickness, but also the diffusion coefficient must be taken into account. It was found that mobility characteristics of the space charge are dependent upon the electrode material.

Synthesis and dielectric characteristics of La0.5Bi0.5MnO3 ceramics

La0.5Bi0.5MnO3 ceramics with a single phase were prepared by a solid-state reaction method, and their dielectric properties were characterized. Two dielectric relaxations with a giant dielectric constant were identified in the temperature range from 125 to 350 K. The electron hopping between Mn3+ and Mn4+ was found to be the origin of the dielectric relaxation at low temperatures (125–200 K) with an activation energy of 0.18 eV. The high temperature (200–350 K) dielectric relaxation can be attributed to the conduction.

Dielectric relaxation mechanisms of BiMn2O5 ceramics

Dielectric properties of multiferroic BiMn2O5 ceramics were evaluated over broad temperature and frequency ranges. Two Debye-type dielectric relaxations were observed at low temperatures (130–250 K) and high temperatures (200–450 K), respectively. The low temperature relaxation with an activation energy of 0.18 eV was attributed to charge carrier hopping process between Mn3+ and Mn4+. The high temperature dielectric relaxation with an activation energy of 0.38 eV, which is similar to the activation energy of conductivity, was associated with oxygen vacancies related defect complex. The dielectric response at high temperatures was significantly suppressed and the dc conductivity increased after oxygen annealing.

Relaxorlike dielectric behavior and weak ferromagnetism in YFeO3 ceramics

Dielectric characteristics of YFeO3 antiferromagnetic ceramics were evaluated over broad temperature and frequency ranges. Two dielectric relaxations were observed at low and high temperatures, respectively, and a dielectric constant step was detected between them. The low temperature dielectric relaxation was an intrinsic thermally activated process following the Arrhenius law with the activation energy very close to that for electronic ferroelectrics, while the high temperature dielectric relaxation was related to the point defect since it could be significantly suppressed by O2 annealing. M-H hysteresis loop was detected at room temperature, and this indicated the weak ferromagnetism in YFeO3 ceramics.

10.1007/s11498-008-2013-z

The experimental data on high-temperature dielectric relaxation αcin linear PE samples with different thermal prehistories (slow cooling, quenching, and annealing) are described. The measurements are conducted at temperatures ranging from 0 to 80°C. Dielectric losses are measured with a high accuracy at frequencies varying from 10−2 to 10–106 Hz. These dielectric measurements allow one to reveal changes in the frequency dependence of losses with temperature. This effect of the thermal prehistory of the sample and applied stress is explained within the framework of the molecular model of chain diffusion between crystalline and amorphous phases in PE.

Dielectric relaxation in A2FeNbO6 (A = Ba, Sr, and Ca) perovskite ceramics

A2FeNbO6 (A = Ba, Sr, and Ca, abbreviated as AFN) ceramics were synthesized by a wolframite method with monoclinic FeNbO4 precursor. X-ray diffraction studies revealed that Sr2FeNbO6 and Ca2FeNbO6 had a orthorhombic structure, while Ba2FeNbO6 had a cubic symmetry. Dielectric properties of the AFN ceramics were characterized in a broad frequency range (1 Hz–10 MHz) at a temperature ranged from −120 to 300 °C. Two dielectric relaxations were observed to follow the Arrhenius law at both low and high temperatures for all samples. Polaron relaxation was proposed to explain the low temperature dielectric relaxations in AFN. The high temperature dielectric relaxations in AFN were a competitive phenomenon between the dielectric relaxation and the electrical conduction of the relaxing species.