Defect induced weak ferroelectricity and magnetism in cubic off-stoichiometric nano bismuth iron garnet: effect of milling duration

The mechanical activation technique has been used to synthesize nanocrystalline multiferroic materials as it creates large density of crystal defects in these materials leading to novel magnetic properties of the nanostructured materials. In the present work, an attempt has been made to prepare nanocrystalline bismuth ferrite by mechanical activation process using high-energy planetary ball mill followed by sintering at 830 °C for 2 h. The milled powder was characterized using X-ray diffraction, scanning and transmission electron microscope, which revealed the formation of Bi_3?δFe₅O₁₂ instead of BiFeO₃. A giant dielectric peak has been observed at around the Curie temperature indicating magneto dielectric coupling which is a primary requisite for potential applications and transition temperature increases with the milling duration. Suppression in ferroelectric nature and enhancement in magnetic properties has been observed with the milling duration.     

SPICE model for lossy piezoelectric polymers

This work presents the transmission line equivalent model for lossy piezoelectric polymers and its SPICE implementation. The model includes the mechanical/viscoelastic, dielectric/electrical, and piezoelectric/electromechanical losses in a novel way by using complex elastic, dielectric, and piezoelectric constants obtained from the measured impedances of PVDF and PVDF-TrFE samples by nonlinear regression technique. The equivalent circuit parameters are derived from analogies between a lossy electrical transmission line and acoustic wave propagation. The simulated impedance and phase plots of various samples, working in thickness mode, have been shown to agree well with the measured data.     

Study of aliovalent modification on dielectric and ac conductivity properties in lead titanate nanoceramics

Substitution of aliovalent ions on Pb and Ti sites of ferroelectric lead titanate nanoceramics has been carried out by using a novel mechanical activation followed by a conventional solid-state reaction technique to modify the dielectric and electric properties. The Optimization of calcinations and sintering temperatures has been checked by thermal gravimetric analysis and repeated firing. High resolution X-ray diffractogram (HRXRD) analysis of some aspects of crystal structure showed that single phase compounds were formed exhibiting a tetragonal structure. The ferroelectric phase-transition of PLMT materials is studied using dielectric measurements. A shift in the transition temperature towards the lower-temperature side with increased doping concentration has been noticed. The electrical conductivity calculated from impedance data has been observed to increase as a function of Mn concentration. Conduction and charge carrier behaviors were studied based on study of frequency and temperature dependence of ac conductivity. The low temperature conductivity mechanism shows frequency dependence.     

From normal ferroelectric transition to relaxor behavior in Aurivillius ferroelectric ceramics

The influence of the barium concentration on the dielectric response of Sr_1?x Ba _x Bi₂Nb₂O₉ system, with x = 0, 15, 30, 50, 70, 85, 100 at.%, has been studied. The barium concentration dependence of T _m, as well as the temperature of the corresponding maximum for the real part of the dielectric permittivity, has suggested a cation site mixing among atomic positions, which has been supported by the structural analysis. A transition from normal ferroelectric–paraelectric phase transition to a relaxor behavior has been observed in the studied samples, when the barium concentration has increased. The relaxor behavior has been discussed.     

Operating principles of the ultracapacitor

The charge-storage mechanism and the design of the ultracapacitor are described. Based on a ceramic with an extremely high specific surface area and a metallic substrate, the ultracapacitor provides extremely high energy densities and exhibits low ESR (equivalent series resistance). The combination of low ESR and extremely low inductance provides the ultracapacitor with a very high power density and fast risetime as well. As a double-layer capacitor, the ultracapacitor is not constrained by the same limitations as dielectric capacitors. Thus, although its discharge characteristics and equivalent circuit are similar to those of dielectric capacitors, the capacitance of the ultracapacitor increases with the ceramic loading on the substrate and its ESR is inversely proportional to the cross-sectional area of the device. The ultracapacitor is composed of an inline stack of electrodes, which leads to an extremely low inductance device, and it exhibits interesting frequency dependence. The ultracapacitor principle has been extended to nonaqueous electrolytes and to a wide temperature range     

缺氧YBCO多晶陶瓷的磁电耦合与磁电阻效应

将高温超导陶瓷YBa2Cu3O6+x经过真空热处理得到氧含量x=0.13的缺氧陶瓷样品,利用HP4294A精密阻抗分析仪测量了样品介电常数温谱图,在温度为410K附近发现了介电异常现象,认为是由反铁磁相变感应铁电相变引起的.铁电测量表明缺氧YBCO多晶陶瓷在室温下有一定的铁电性.在零磁场和外加磁场条件下,采用标准四引线法分别测量了样品电阻率随温度的变化关系,发现温度低于400K时样品的磁电阻MR约为60%且基本不随温度变化,在反铁磁相变温度410K附近出现异常,认为是由于样品大量本征载流子产生并且外加磁场对顺磁区域影响较小所致.缺氧YBCO(x=0.13)陶瓷样品的磁电耦合特性,显示出它可能成为一种新的室温多铁材料从而在传感、控制和信息储存等方面发挥重要作用.     

Electrical Relaxation in Mixed Alkali Bi2O3-K2O-Li2O-Fe2O3 Glasses

A new glass system(Bi2O3)50(Fe2O3)10(Li2O)x(K2O)40-x, where x changes in steps of 5 mole fraction between 0 and 40, was selected to study the electrical relaxation and the mixed alkali effect(MAE)phenomena.Measurements of ac conductivity σac, dielectric permittivity ε' and loss factor tanδ in the frequency range of 0.12～102 kHz and in the temperature range of 300～650 K were carried out. The temperature dependence of the ac conductivity shows a slow increasing rate at low temperature and high frequency and a rapid increase at high temperature and low frequency. At constant temperature, the ac conductivity is found to be proportional to ωs, where s is the frequency exponent, which is less than 1. Analysis of the conductivity data and the frequency exponent shows that the overlapping large polaron tunnelling(OLPT)model of ions is the most favorable mechanism for the ac conduction in the present glass system. The ac response, the dc conductivity and dielectric relaxation have the same activation energy and they originate from the same basic transport mechanism. The results of the dielectric permittivity show no maximum peak in the temperature and frequency range studied. This absence of maximum peak is an indication of non-ferroelectric behavior of all the studied samples. The MAE has been detected in the ac conductivity, which is the same as the classical MAE in the dc conductivity. The electrical parameters such as dielectric permittivity ε' and real dielectric modulus M'show a typical minimum deviation from linearity by about two orders of magnitude. The loss factor tanδ and the imaginary dielectric modulus M" are insignificantly dependent on composition even at the same transition temperature Tg.     

Temperature induced phase transition in nanocomposites based on low-density polyethylene matrix

The temperature dependence of the complex dielectric permittivity of composites based on a low-density polyethylene matrix containing dispersed nanoparticles of copper and zinc oxides has been studied. It is established that a phase transition takes place in the volume of a composite at a temperature of about 60°C. Exposure of the composite samples to a flux of 10-keV electrons leads to changes in the temperature dependence of the permittivity. Mechanisms are proposed that can explain the observed behavior.     

Dielectric properties of sol-gel derived barium-strontium-titanate (Ba(0.4)Sr(0.6)TiO(3)) thin films

The structural and dielectric properties of sol-gel derived barium-strontium-titanate (Ba(0.4)Sr(0.6)TiO(3 )) thin films have been investigated. The as-fired films are found to be amorphous, and films crystallize to a cubic phase after a post deposition annealing at 700 degrees C for one hour in air. The variation of dielectric constant with temperature and field was investigated as a function of film thickness. These films display a nonlinear dielectric response that can be described in terms of a power series expansion of the free energy in polarization as in Landau-Ginzburg-Devonshire approach. The measured room temperature dielectric constant (epsilon') of the film was about 320. The dielectric constant did not show any significant frequency dependence up to 100 kHz. The temperature dependence of dielectric constant exhibited a diffused ferroelectric to paraelectric phase transition at -60 degrees C. The room temperature dielectric constant and magnitude of the dielectric peak at the Curie point was dependent on the thickness of the film. The obtained dielectric data on sol-gel barium strontium titanate thin films on platinized substrates were analyzed in the light of a barrier layer model.     

Electromagnetic modelling of curved lossy dielectric surface and cylinder

A new electromagnetic model for curved dielectric surfaces and cylinder has been introduced. The model considers scatterers as a lossy dielectric body. The theoretical and simulation results have been compared with the literature and the model agrees with the published results of the hollow cylinder for which an exact solution exists. Figures of relative error versus ka and kT of hollow cylinder (a) single- (b) three-layered cylinder are used for the parameters. The curved dielectric scatterer is discretised to N small element, and then the contributions of each element are sum up coherently to find the total response. Finally, the simulations for curved cylinder and convex surfaces of arbitrary shape are presented.     

Electrical and dielectric properties of bismuth holmium cobalt titanate (BiHoCoTiO<Subscript>6</Subscript>): a complex double perovskite

A lead-free bismuth holmium cobalt titanate multiferroic(BiHoCoTiO₆) was synthesized at high temperatures by a solid-state reaction (a mixed oxide) route. Structural analysis of the compound is performed using X-ray diffraction data and an orthorhombic crystal system is suggested for the material. Study of room temperature scanning electron microigrapgh exhibited better morphology for the material. The uniform distribution of the small rod-type of grains with dimension of 1–2 µm length and 0.2–0.3 µm diameters was visible. Analysis of temperature-frequency dielectric data exhibited two dielectric anomalies or phase transitions: first transition temperature(t_c1) at 175?°C and the second one at 325?^°C. Study of frequency and temperature dependence of resistive characteristics (performed using complex impedance spectroscopy) has shown significant contributions of grains and grain boundaries, which in turn, helps in understanding the electrical conduction mechanism and microstructure behaviour of the material in a better way. The impedance or Nyquist plots were modelled with an equivalent circuit containing capacitance, resistance and related parameters due to grain (bulk), grain boundaries and capacitance. The transport properties, AC conductivity and electrical modulus of the material were also investigated and reported here.     

SPICE model for lossy piezoceramic transducers

A transmission line equivalent circuit for piezoelectric transducers has been modified to provide modeling of lossy piezoceramic transducers. A lossy transmission line is used to model the mechanical losses. The equivalent circuit parameters are derived from analogies between electrical transmission lines and acoustic wave propagation. Implementation of the equivalent circuit model in SPICE is shown. Simulations and measurements in the time and frequency domain of a low-Q material and a multilayered ultrasonic sensor using a low-Q piezoceramic transducer are presented.     

Dielectric properties and polarizability of molybdenum tellurite glasses

The dielectric behaviour of the [TeO₂]_1–x [MoO₃] _x , x=0.2, 0.3 and 0.45 mol%, glassy system is reported for the temperature range 300–573 K and the frequency range of 0.1–10 kHz. Both the static and high frequency dielectric constants for these binary tellurite glasses decrease with increasing MoO₃ content. The temperature dependence of the dielectric constants of these glasses are positive. The frequency dependence of the dielectric constant identifies a frequency dependence which does not show a flattening at low frequency. The room temperature static dielectric polarizability is discussed in terms of the MoO₃ concentration. The temperature dependence of the dielectric constant has been analysed in terms of the temperature changes of both volume and polarizability and also a volume change of the polarizability.     

Mechanical relaxation in simple molecular liquids from the liquid to the supercooled state

Attenuation and velocity of acoustic waves have been revealed at ultrasonic frequencies (2, 5 and 10 MHz) in some glass-forming liquids. The mechanical response has been studied following continuously the materials from the liquid to the supercooled state, using an experimental set-up developed to this purpose. A peak in the attenuation of longitudinal acoustic waves has been observed in a temperature region in which the liquids are supercooled. Correspondingly, the sound velocity shows a dispersion, increasing from liquid-like to solid-like values for decreasing temperatures. Both features develop above the calorimetric glass transition temperature (T_g). In the deeply supercooled liquids, nearly 10 K above their calorimetric T_g, also the propagation of transverse wave sound (which is a characteristic behaviour of solid-like materials) has been experimentally detected. Shear and longitudinal relaxation times are not decoupled in the time–temperature region investigated. Compared to the mechanical one, the dielectric relaxation studied as a function of temperature at the same frequency of the ultrasonic experiments shows a loss peak centred at the same temperature. Depending on the liquid investigated, the mechanical relaxation spectrum can be broader than the dielectric one, specially in the low temperature flank, suggesting that some dissipative processes at lower energies can contribute to the mechanical loss, even though they do not couple to the electric probe field.     

Dielectric response of Li0.4K0.6NbO3 crystal in the frequency domain

Complex impedance and dielectric constants of ferroelectric crystals Li_0.4K_0.6NbO₃ have been studied in the frequency domain at several temperatures. A strong low-frequency dielectric dispersion below 1 kHz was revealed in the high temperature range. The normalized behaviour of the complex dielectric constants satisfies the universal dynamic response with its power exponent n=0.04. In the high frequency region, the complex impedance was described by a modified Cole-Cole arc. An equivalent circuit for the transport process of high frequency region has been examined. The temperature-dependent exponent has its minimum at the ferro- to para-electric phase transition point and relaxation frequencies do not satisfy the Arrhenius law in the ferroelectric region     

Dielectric behaviour of erbium substituted Mn-Zn ferrites

Dielectric properties such as dielectric constant (ε′) and dielectric loss tangent (tan□δ) of mixed Mn-Zn-Er ferrites having the compositional formula Mn_0.58Zn_0.37Fe_2.05−xEr_x0₄ (where itx = 0.2, 0.4, 0.6, 0.8 and 1.0) were measured at room temperature in the frequency range 1–13 MHz using a HP 4192A impedance analyser. Plots of dielectric constant (ε′) vs frequency show a normal dielectric behaviour of spinel ferrites. The frequency dependence of dielectric loss tangent (tan δ) was found to be abnormal, giving a peak at certain frequency for all mixed Mn-Zn-Er ferrites. A qualitative explanation is given for the composition and frequency dependence of the dielectric constant and dielectric loss tangent. Plots of dielectric constant vs temperature have shown a transition near the Curie temperature for all the samples of Mn-Zn-Er ferrites. However, Mn_0.58Zn_0.37Er_1.0Fe_1.05O₄ does not show a transition. On the basis of these results an explanation for the dielectric mechanism in Mn-Zn-Er ferrites is suggested.     

Characterization of the granular ceramic materials corundum and mullite by impedance spectroscopy

The granular ceramic materials corundum and mullite are potential filter bed materials for electrically enhanced filtration at elevated temperatures. The dielectric response of the granular ceramics has been determined at temperatures between 420 and 920 K. It is shown that the response of these materials can be described by one equivalent circuit over the whole temperature and frequency (0.1 Hz–10 kHz) range. Thermally activated conduction in parallel with a dipole relaxation with a symmetrical logarithmic distribution of relaxation times (Cole-Cole relaxation) is the mechanism which determines the response at not too low frequencies.     

Synthesis and characterization of Na2Pb2Pr2W2Ti4Ta4O30

A new complex oxide (Na₂Pb₂Pr₂W₂Ti₄Ta₄O₃₀) of tungsten bronze structural family has been synthesized by a high-temperature solid-state reaction (mixed-oxide) route at 1,050 °C. Room temperature structural analysis shows the formation of a single phase new compound. Study of microstructure of the pellet sample, recorded by scanning electron microscope, exhibits the uniform distribution of different size and shape of grains (with a few small voids) on the surface of the sample. Detailed studies of dielectric properties as a function of frequency and temperature show a dielectric anomaly above room temperature suggesting the existence of a ferroelectric phase transition in the material. Impedance spectroscopic analysis and electrical conductivity of the material exhibit a strong correlation between microstructure and electrical parameters. The temperature dependence of dc conductivity of the compound follows Arrhenius equation. The frequency and temperature dependence of ac conductivity (with fittings) shows the signature of Jonscher’s universal power law. The existence of non-exponential-type of conductivity relaxation in the compound was confirmed.     

Fluctuation conductivity of tunnel junctions

Tunneling of electrons is studied at joints between two metals separated by a thin dielectric layer. The conductivity of electrons in the case of the same metals at temperatures higher (close) to the temperature of superconducting transitions has a strong temperature dependence. Tunneling of electrons between a superconductor and a normal metal separated by a thin dielectric layer at temperatures higher than that of the superconducting transition of the normal metal is also considered. The fluctuation correction to the current increases strongly when the temperature approaches the transition temperature and the voltage applied to the barrier becomes of the order of magnitude of that of the energy gap of the superconductor.