Dielectric Curves Research Articles

Measurement of dielectric response of transformer moisture content

Over a transformer's operational lifetime, the moisture content of its insulation pressboard increases because of the ageing of its internal insulation. Thus, this moisture content becomes an important index for judging the ageing of this insulation. In this study, to analyse the moisture content of a transformer's insulation pressboard using the dielectric response insulation technique, the XY-equivalent model analysis method for classical transformers is combined with a dielectric-constant algorithm for mixed media. In addition, a new method for calculating the moisture content of a pressboard is obtained via cyclic iterative comparison calculations based on the dielectric response translation formula, the temperature, and the material activation energy. The XY-equivalent models of transformers with various proportions are derived under laboratory conditions. The moisture contents of the transformers’ insulating pressboards are obtained under the corresponding dielectric loss factor curves by using the proposed moisture content calculation method, which provides a result that is within 4.83% of that provided by the IDAX-300 moisture content analysis algorithm, thus demonstrating its validity.

Coexistence of ferroelectric phases and electric field induced structural transformation in sodium potassium bismuth titanate ceramics

Lead-free sodium potassium bismuth titanate (1−x)[Na0.5Bi0.5TiO3]-x[K0.5Bi0.5TiO3] (NKBTx) ceramics have been believed to be a rhombohedral crystal system with R3c symmetry up to x = 0.16 mole fraction, right now. For the first time, on the contrary, we have disclosed the existence of a low-symmetry (MA)-type monoclinic phase (69%) along with rhombohedral R3c (31%) in the NKBT0.10 sample from Rietveld studies on conventional and high-energy synchrotron powder diffraction data in the unpoled state. The peak splitting and reduced FWHM of {110}pc and {111}pc reflections after poling indicate the field-induced structural transformation. Rietveld analysis reveals that the monoclinic (Cc) phase is partially transformed to rhombohedral (R3c) at 50 kV/cm. Two distinct dielectric anomalies are noticed in the dielectric curve at 120 and 310 °C corresponding to freezing temperature (Tf) and glass-like transition temperature (Tm), respectively. The dielectric diffusivity (γ) estimated from the modified Curie-Weiss law reveals that the sample has a relaxor behavior. Structural inhomogeneity in the nanoscale region due to cation disorder in the A-site causes relaxing nature. A well-saturated square-like ferroelectric hysteresis loop is obtained with a remnant polarization and a coercive field of 35.8 μC/cm2 and 47 kV/cm, respectively. A piezoelectric constant (d33) of 85 pC/N is obtained for the sample which is somewhat higher than that of pure Na0.5Bi0.5TiO3 ceramics.

Effect of SiO2/B2O3 Ratio on the Crystallization Behavior and Dielectric Properties of Barium Strontium Titanate Glass–Ceramics Prepared by Sol–Gel Process

Ferroelectric glass–ceramics, with a basic composition 90 wt.% (Ba0.65Sr0.35)TiO3−10 wt.% (B2O3−nSiO2) (n = 0.5, 1, 3, 5) were synthesized by the sol–gel method and their phase development and dielectric properties were investigated by differential thermal analysis, x-ray diffraction, field emission scanning electron microscopy, dielectric temperature curves and impedance spectroscopy. From the differential thermal analysis, glass transition and crystallization behavior can be observed. From the x-ray diffraction study, two crystalline phases (Ba,Sr)TiO3 and Ba2TiSi2O8 were formed over the entire composition range of the glass–ceramics. In addition, the main crystal phase has undergone a transformation from (Ba,Sr)TiO3 to Ba2TiSi2O8 with the increase of n. A typical structure in which the crystal phase was surrounded by a glassy matrix has been observed in the scanning electron microscope images. As a result of temperature dependent dielectric property measurements, the dielectric constant increased obviously with the increase of n from 0.5 to 1. Further increasing n led to a reduction of the dielectric constant, which is in coincidence with the variation of the intensity of (Ba,Sr)TiO3 phase with n. According to the impedance spectroscopy analysis and the activation energy calculation, the relaxation peak in both Z″ and M″ data should be attributed to the crystal–glass interface, and the change of conduction mechanism with the increase of SiO2/B2O3 ratio may be attributed to the corresponding transition of the main crystal phase.

Thermal-induced structural transition and depolarization behavior in (Bi0.5Na0.5)TiO3-BiAlO3 ceramics

The depolarization temperature Td determines the upper temperature limit for the application of piezoelectric materials. However, the origin of depolarization behavior for Bi-based materials still remains controversial and the mechanism is intricate for different (Bi0.5Na0.5)TiO3-based systems. In this work, the structure and depolarization behavior of (1-x)(Bi0.5Na0.5)TiO3-xBiAlO3 (BNT-BA, x = 0, 0.02, 0.04, 0.06, 0.07) ceramics were investigated using a combination of X-ray diffraction and electrical measurements. It was found that as temperature increased, the induced long-range ferroelectric phase irreversibly transformed to the relaxor phase as evidenced by the temperature-dependent ferroelectric and dielectric properties, which corresponded to a gradual structural change from the rhombohedral to the pseudocubic phase. Therefore, the thermal depolarization behavior of BNT-BA ceramics was proposed to be directly related to the rhombohedral–pseudocubic transition. Furthermore, Td (obtained from thermally stimulated depolarization currents curves) was higher than the induced ferroelectric-relaxor phase transition temperature TFR (measured from dielectric curves). The phenomenon was quite different from other reported BNT-based systems, which may suggest the formation of polar nanoregions (PNRs) within macrodomains prior to the detexturation of short-range ferroelectric domains with PNRs or nanodomains.

Peculiar Properties of Phase Transitions in Na0.5Bi0.5TiO3-xBaTiO3 (0<x<6) Lead-free Relaxor Ferroelectrics Seen Via Acoustic Emission

Na0.5Bi0.5TiO3-xBaTiO3 (0<x<6) relaxor ferroelectrics crystals were investigated by means of dielectric and acoustic emission methods. Dielectric curves exhibit the slightly visible small maxima near the depolarization temperatures, Td, and the wide maxima at the temperatures of Tm, whereas the acoustic emission exhibits the sharp bursts, corresponding to Td, Tlm, which is known to be a temperature exhibiting a strong frequency dispersion, TRE, which is known to be a temperature above which a frequency dispersion vanishes, and the Tm and the Tp manifesting a transition to the paraelectric phase. Based on the AE data it was established that all these characteristic temperatures shift down as x increases, but with different slopes. A mechanism of such the differences is discussed.

Structure, dielectric and energy storage properties of BaTiO3 ceramics doped with YNbO4

(1−x)BaTiO3−xYNbO4 [(1−x)BT−xYNb] (x = 0%, 3%, 5%, 7%, 10%, 15%) ceramics were prepared by a traditional solid-state reaction method. A solubility limit for (1−x)BT−xYNb was determined to be less than 3%. The phase transition from tetragonal to pseudo cubic were manifested via the XRD and Raman spectra. When x = 3% and 5%, a second liquid-like phase appeared in SEM images. As the YNb additive increased, the dielectric constant-temperature curves shifted towards low temperature and the dielectric nonlinearity was suppressed. At x = 7%, the recoverable energy storage density of 0.93BT−0.07YNb achieved a maximum of 0.614 J/cm3, which was 2.4 times of pure BaTiO3, as well as the efficiency was up to 87%. These results indicated that the BaTiO3 ceramic doped with YNb would be a promising alternate material for high energy storage density capacitor.

Effects of nano-sized BCZT on structure and electrical properties of KNN-based lead-free piezoceramics

The (1 − x)(Li0.03Na0.5K0.47)(Nb0.92Sb0.05Ta0.03)O3–xBa0.85Ca0.15Zr0.1Ti0.9O3 (LNKNST–BCZT, x = 0, 0.5, 1.5, 2.5, and 3.5%) lead-free piezoceramics were synthesized by the conventional solid-state reaction method with adding nano-sized Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) powder to investigate their influences on crystal structure and electrical properties. The X-ray diffraction (XRD) analysis reveals that all the sintered LNKNST–BCZT ceramics exhibit pure perovskite structure with the mainly rhombohedral (R) phase or the coexistence of rhombohedral (R) phase and tetragonal (T) phase. The composition of the LNKNST–1.5%BCZT ceramics locates around the critical point, i.e., the ceramics changing from the coexistence of rhombohedral (R) phase and tetragonal (T) phase to purely rhombohedral (R) phase, and presents the most densified microstructure morphology, which can be confirmed further by the field emission scanning electron microscope (FESEM) observation. There are two dielectric anomalies in the temperature dependent dielectric properties curves of the LNKNST–BCZT ceramics, which correlate with the ferroelectric rhombohedral (R) phase changing to the ferroelectric tetragonal (T) phase, and then to the paraelectric cubic (C) phase. Both the Curie–Weiss law and the power law fittings confirm the diffuse phase transition ferroelectrics characteristic of the LNKNST–1.5%BCZT ceramics, which is considered as correlating with the polar nanoregions (PNRs). Due to the densification effect caused by the chosen amount of nano-sized BCZT doping and the construction of R–T polymorphic phase boundary, the LNKNST–1.5%BCZT ceramics exhibit the best ferroelectric and piezoelectric properties. The complex impedance spectroscopy analysis confirms that the extrinsic electrical conduction mechanism at high temperatures is dominated by the oxygen vacancies induced by the evaporation of the alkali metals during sintering.

Study of enhancement in the dielectric and electrical properties of WO3-doped LiF nano-composite

We report the dielectric and electrical behavior of thermally evaporated lithium fluoride (LiF)–tungsten trioxide (WO3) (x wt% WO3-doped LiF; x = 0, 1, 3, 5, 7, 10) nano-composite thin films at room temperature over a wide range of frequencies from 0.1 Hz to 1 MHz. Among the various doping concentrations of WO3, the 5 wt% WO3-doped LiF nano-composite thin film reaches to the maximum dielectric constant ~ 25, compared with those of pure LiF ~ 9 thin film. The electrical studies of the films have been done by complex impedance spectroscopy and show the presence of grain and grain boundaries contribution in the films for all doping concentrations. The relaxation behavior in the nano-composite films has been observed in the dielectric curve at low frequencies which is superimposed by electrode polarization. The non-Debye type of relaxation in the films has been observed in the impedance and modulus curve. The peaks appearing at low frequencies for each doping concentration in loss tangent spectrum show the presence of relaxing dipoles in the films. An increment in the ac conductivity has been observed with doping concentration up to 5%. The ac conductivity curves obey the jump relaxation power law where an electrode polarization effect can be seen at low-frequency region.

Microstructure, dielectric, and energy storage properties of BaTiO3 ceramics prepared via cold sintering

BaTiO3 (BT) ceramics with fine grains were sintered at an ultra low temperature of 180°C by means of a novel sintering method named as cold sintering (CS). For CS, the hydrothermal precursor solutions were incorporated into the ceramic powders, and pressure was also applied during sintering. After annealing at 900°C, the obtained samples exhibit dense microstructures and relative densities higher than 96%. The mean size of grains in the annealed samples is as low as 122nm. Curie temperature of the ceramics is 116°C. Dielectric constant and dielectric loss at 1kHz and at room temperature are 2332 and 0.01, respectively. Compared to BT ceramics synthesized via a conventional solid-state sintering method, the BT ceramics in this work show increased peak width and decreased peak value on the dielectric constant-temperature curves around Curie temperature, suggesting diffuse phase transition character. The obtained BT ceramics show high breakdown strength (90kV/cm), high energy storage density (1.45J/cm3), and high energy storage efficiency (85.6%). The results indicate that CS is an efficient method for low-temperature sintering BT ceramics with fine grains as well as good performance in energy storage.

Dielectric Behavior and Electro-Magnetic Coupling at Room Temperature in BiFeO3/PVDF and CoFe2O4/PVDF Composites

The dielectric properties of composites formed by dispersions of two types of ceramic particles, BiFeO3 (multiferroic) and CoFe2O4 (magnetic), in poly(vinylidene fluoride), PVDF, (which can present a ferroelectric phase) were studied at room temperature as a function of particle concentration. The characterization studies include determination of dielectric polarization curves in the presence of external magnetic fields, H (that is, P–E plots in the presence of H, where P is the dielectric polarization and E the external applied electric field). Impedance spectroscopy analysis shows the presence of several distributions of dipolar relaxation processes, some of which are assigned to charge rearrangements at the different interfaces of the system, for example at the ceramic-polymer interfaces. Negative magneto-electric coupling was observed at low ceramic proportion (where leakage currents are lower than 20 nA/cm2), that is, decrease of P when applying H, (∂P∂H)E<0. The negative coupling is opposite to prev...

Large electric-field-induced strain in B-site complex-ion (Fe0.5Nb0.5)4+-doped Bi1/2 (Na0.82K0.12)1/2TiO3 lead-free piezoceramics

In order to obtain a new system of (Bi1/2Na1/2)TiO3 (BNT) based lead-free incipient piezoceramics with large strain for practical applications of actuators, we investigated the effect of B-site complex-ion (Fe0.5Nb0.5)4+ (FN)-doped Bi1/2 (Na0.82K0.12)1/2TiO3 ceramics on the phase structure, dielectric, ferroelectric, piezoelectric and electric-field-induced strain properties. All samples exhibited single perovskite phase with pseudocubic symmetry. The room temperature electric-field-induced polarization (P-E) and strain (S-E) hysteresis loops indirectly illustrated ferroelectric-to-relaxor (FE-RE) phase transition. The increasing content of FN doping decreased the FE-RE phase transition temperature, TF-R to below room temperature and induced the reversible FE-RE phase transition, giving rise to a large strain of 0.462% with a normalized strain, d*33 of 660pm/V at a critical composition of x = 5. A fluctuation of the dielectric curve for BNKT-5mol% FN ceramics in the spectra around 80°C before and after polarization suggested that the large strain response can be induced via delicate mixing of the FE and RE phase.

Low temperature magnetization and anomalous high temperature dielectric behaviour of (1-x) YMnO3/xZnFe2O4 composites

We synthesized YMnO3 and ZnFe2O4 composites, (1-x)YMnO3/x(ZnFe2O4) with x = 0, 0.05, 0.10, and 0.15 by high temperature sintering. X-ray diffraction (XRD) patterns indicate the successful formation of composites. Weak ferromagnetism is manifested below Néel temperature (TN) for pristine YMnO3, according to (M-H) study performed at 10 K. For (1-x)YMnO3/xZnFe2O4 (x = 0.05, 0.10, 0.15) a thin coercivity is observed in all compositions, due to short range magnetic ordering at low temperature after the insertion of ZnFe2O4. For pristine YMnO3 explicit divarication between FC-ZFC curves is observed, with crimps observed in both FC and ZFC curves at 75 K, which is the TN of YMnO3. For 1-x(YMnO3)/x ZnFe2O4 composites (x = 0.05, 0.10, 0.15) crimps are perceived only in ZFC curves at slightly varying values of 39.8, 42.32 and 45.63 K respectively. Anomalous peaks are observed in high temperature dielectric curves above 400 K for 1-x(YMnO3)/xZnFe2O4 (x = 0, 0.05, 0.10, 0.15) composites due to Maxwell-Wagner relaxation effect.

Simulations of statistical domain switching model of PIC151 based on self-adaptive differential evolution algorithm

The nonlinear properties of piezoceramic materials reflect their performance in actual application. Researches of nonlinear properties of piezoceramics are committed to improve piezoelectric properties and to predict the change of material performance. In this study, a two-dimensional model established by using a circular distribution to quantify certain domain properties is focused. Self-adaptive differential evolution algorithm is proposed to optimize spontaneous polarization and spontaneous strain which effects the model a lot for a commercial-grade soft PZT (PIC151). An inverse problem solving is proposed to retrieve dielectric coefficient and piezoelectric coefficient. The optimal results and inverse problem solving values are compared with dielectric and butterfly curves by experiment in literature. The algorithm converges quickly and a good agreement is achieved between retrieved values and experiment results, which means the simulation can character piezoceramic materials properties well.

Peculiar properties of phase transitions in Na0.5Bi0.5TiO3−0.06BaTiO3 lead-free relaxor ferroelectrics seen via acoustic emission

Na[Formula: see text]Bi[Formula: see text]TiO3-0.06BaTiO3 relaxor ferroelectrics (RFEs) ceramic samples were investigated by means of dielectric and acoustic emission methods in the temperature range of 70–350[Formula: see text]C and in dependence on frequency. Dielectric curve exhibits two anomalies: strongly dispersive the local maximum, within 130–180[Formula: see text]C and slightly dispersive the smeared high-temperature maximum within 300–310[Formula: see text]C. Acoustic emission exhibits the five groups of bursts: near 123[Formula: see text]C, 150–180[Formula: see text]C, near 225[Formula: see text]C, 300–310[Formula: see text]C and 327[Formula: see text]C. The first and third groups of acoustic emission bursts correspond to phase transitions of R[Formula: see text]c-P[Formula: see text]bm phases coexisting in this temperature, range, while the fourth and fifth groups of acoustic emission bursts correspond to P[Formula: see text]bm-P[Formula: see text] /mbm-Pm[Formula: see text] m phase transitions. The second group of acoustic emission bursts corresponds to strongly dispersive local maximum at which no phase transition has been proved in Na[Formula: see text]Bi[Formula: see text]TiO3-0.06BaTiO3. The existence and the location of the intermediate temperature, [Formula: see text], in Na[Formula: see text]Bi[Formula: see text]TiO3-0.06BaTiO3 compound are discussed.

Moisture determination of the main insulation in a transformer by frequency‐domain spectroscopy

The presence of moisture in the oil–paper insulation in a transformer drastically accelerates the aging process of the insulation and reduces the service life of the transformer. To analyse the influence of moisture on the dielectric response of the oil–paper insulation and to precisely determine the moisture content of the transformer by frequency‐domain spectroscopy, the modified Cole–Cole model is introduced to fit the frequency versus dielectric responses of the oil–paper insulation samples with different moisture contents, and the influence of the variation in moisture content on the parameters in the modified Cole–Cole model is investigated. Results show that the modified Cole–Cole model parameters, including the α and β relaxation components as well as the DC and hopping conductivity components, can be used to effectively fit the frequency versus dielectric response curve of the oil–paper insulation. The characteristic parameters τα and τβ in this model are found to be sensitive to changes in the moisture content. They can be used to quantitatively evaluate the moisture content in a transformer insulation system. A case study is presented to verify the effectiveness of the proposed technique in assessing the moisture content of onsite transformers. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.

The microstructure, ferroelectric and dielectric behaviors of BiFeO3-Na0.5Bi0.5TiO3 based solid solution thin films

Abstract BiFe0.98Zn0.02O3-Na0.5Bi0.5Ti0.98W0.02O3 solid solution thin films with two thicknesses (300 nm and 1.2 μm) were fabricated on indium tin oxide/glass substrates via metal organic decomposition. The effects of the thickness on crystallization, microstructure morphology, ferroelectric and dielectric properties were investigated. Compared with the 300-nm-thick film, 1.2 μm- film exhibits standard ferroelectric hysteresis loop with slim feature and larger dielectric constant due to the improvements of crystallinity and insulating property. Moreover, obvious aging behavior, manifested by pinched-like ferroelectric hysteresis loop and abnormal butterfly dielectric constant-electric field curve, can be observed in the film with 1.2 μm thickness. The aging behavior can be explained by the formation of the defect complex.

Optical, electronic and elastic properties of ScAlN alloys in WZ and ZB phases: Prospective material for optoelectronics and solar cell applications

To comprehend merits and demerits of Sc0.25Al0.75N alloys in WZ and ZB phases for optoelectronics and solar cell technology, we performed a comparative study of the elastic, electronic and optical properties of these alloys using first-principles methodology with density functional theory. The elastic constants (Cij, B, G, E, ν and B/G), longitudinal and shear wave velocities (vs, vl) and Debye temperature θD of ZB and WZ-Sc0.25Al0.75N alloys have been calculated for the first time. The calculated results show that ZB and WZ-Sc0.25Al0.75N alloys have the direct band gap and the ZB-Sc0.25Al0.75N alloy exhibit comparatively narrower energy gap than WZ. The density of states of W-Al0.75 Sc0.25 N and ZB-Sc0.25Al0.75N were analyzed. The peaks of imaginary dielectric function and absorption curves are at higher energy point for ZB-Sc0.25Al0.75N. The highest absorption of ZB-Sc0.25Al0.75N is 3.65 × 105/cm at 14.26 eV, which is larger than wurtzite of 3.32 × 105/cm at 11.22 eV. The average reflectivity of WZ-Sc0.25Al0.75N is lower than that of ZB-Sc0.25Al0.75N. Moreover wide energy gap of WZ-Sc0.25Al0.75N results in smaller values of the static refractive indices compared to ZB-Sc0.25Al0.75N. The comparison of the electronic and optical characteristics of the ZB-Sc0.25Al0.75N with those of WZ-Sc0.25Al0.75N endorses that ZB-Sc0.25Al0.75N can be a material of choice for optoelectronics and solar cell applications besides the WZ-Sc0.25Al0.75N.

High-energy storage performance in lead-free (1-x)BaTiO 3 -xBi(Zn 0.5 Ti 0.5 )O 3 relaxor ceramics for temperature stability applications

Abstract In this paper, we prepared lead-free (1-x)BaTiO 3 -xBi(Zn 0.5 Ti 0.5 )O 3 (x=0.04, 0.08, 0.10, and 0.14) ceramics by a conventional solid-state reaction technique. Pure perovskite structures and dense microstructures were demonstrated for all the compositions. Interestingly, it was found that the sintering temperature tended to decrease with increasing the Bi(Zn 0.5 Ti 0.5 )O 3 content. It should be stressed that a low sintering temperature of 1050 °C was utilized for the composition of x=0.14. Moreover, the dielectric permittivity-temperature curve became more flat and the relaxor degree became stronger with the augment in Bi(Zn 0.5 Ti 0.5 )O 3 content. We also conducted a detailed study on the energy storage performance for all the compositions from 25 °C to 180 °C.We found that relatively temperature-stable energy storage performance could be obtained in the compositions with x=0.08, 0.10 and 0.14 regardless of the evolution of dielectric constant during the test temperature range. In particular, due to a higher field of 12 MV m −1 , the discharge energy storage densities of x=0.14 could reach 0.81 J cm −3 , 0.80 J cm −3 , 0.78 J cm −3 , 0.72 J cm −3 , and 0.67 J cm −3 with high efficiencies of 94%, 92%, 94%, 88% and 77% at 25 °C, 50 °C, 100 °C, 150 °C, and 180 °C, respectively. All these results demonstrate the (1-x)BaTiO 3 -xBi(Zn 0.5 Ti 0.5 )O 3 ceramics are quite promising for temperature-stable energy storage applications.

Quantitative diagnosis of moisture content in oil‐paper condenser bushing insulation based on frequency domain spectroscopy and polarisation and depolarisation current

Dielectric response techniques have been proven useful for evaluating the moisture content (K m.c) of oil-paper insulation. However, an accurate model for oil-paper condenser bushing has not been presented. This study designed a laboratory oil-impregnated paper (OIP) bushing model to simulate moistened defects in oil-paper bushings. Moreover, a 10 kV OIP bushing model was simulated using COMSOL multiphysics AC/DC to confirm that the distribution of potential meets the design requirements. Furthermore, this work investigates the effect of moisture content on the dielectric frequency response and polarisation and depolarisation current (PDC) of models of oil-paper bushings. A group of characteristic parameters was extracted from the dielectric dissipation factor curves and the DC conductivities of different moisture contents were computed by charge difference method from PDC curves. The obtained results suggest that the dissipation factor and the capacitance increase with the increase of moisture content in the whole frequency range, and the quantitative relationships among the characteristic parameters, DC conductivities, and the moisture content were established and follow an exponential law.

Study of Cs/NF3 adsorption on GaN (0 0 1) surface

To investigate the optoelectronics properties of Cs/NF3 adsorption on GaN (0 0 1) photocathode surface, different adsorption models of Cs-only, Cs/O, Cs/NF3 adsorption on GaN clean surface were established, respectively. Atomic structures, work function, adsorption energy, E-Mulliken charge distribution, density of states and optical properties of all these adsorption systems were calculated using first principles. Compared with Cs/O co-adsorption, Cs/NF3 co-adsorption show better stability and more decline of work function, which is more beneficial for photoemission efficiency. Besides, surface band structures of Cs/NF3 co-adsorption system exhibit metal properties, implying good conductivity. Meanwhile, near valence band minimum of Cs/NF3 co-adsorption system, more acceptor levels emerges to form a p-type emission surface, which is conductive to the escape of photoelectrons. In addition, imaginary part of dielectric function curve and absorption curve of Cs/NF3 co-adsorption system both move towards lower energy side. This work can direct the optimization of activation process of NEA GaN photocathode.