Electric Field Dependence Of Permittivity Research Articles

Nonlinear transmission line implemented as a combined pulse forming line and high-power microwave source.

Nonlinear transmission lines (NLTLs) are typically driven by pulse forming lines (PFLs) or Marx generators to generate high repetition rate, high power microwaves (HPMs) with fewer auxiliary systems than conventional sources. This paper reports the development of an even more compact HPM system that utilizes a composite-based hybrid NLTL as the PFL and HPM generator in a single device. We designed the following three different combinations of nickel zinc ferrite (NZF) and barium strontium titanate (BST) inclusion volume loads in a polydimethylsiloxane host material to provide magnetic field dependent permeability and electric field dependent permittivity, respectively: 25% NZF, 10% BST/15% NZF, and 15% BST/10% NZF. By constructing the NLTL in a coaxial geometry, this device uses the capacitance and length of the NLTL to generate a fast rise-time high voltage pulse with microwave oscillations that occurred both during and after the pulse after exceeding a threshold charging voltage. The output frequency of the NLTLs ranged from 950MHz to 2.2GHz during the pulse for all volume loadings and was 1GHz after the pulse for the 10% BST/15% NZF and 15% BST/10% NZF volume loadings. The oscillations generated after the pulse were much higher in amplitude and achieved 160 kW at a 15 kV charging voltage for the 15% BST/10% NZF composite-based NLTL.

Control of polarization in bulk ferroelectrics by mechanical dislocation imprint.

Defects are essential to engineering the properties of functional materials ranging from semiconductors and superconductors to ferroics. Whereas point defects have been widely exploited, dislocations are commonly viewed as problematic for functional materials and not as a microstructural tool. We developed a method for mechanically imprinting dislocation networks that favorably skew the domain structure in bulk ferroelectrics and thereby tame the large switching polarization and make it available for functional harvesting. The resulting microstructure yields a strong mechanical restoring force to revert electric field-induced domain wall displacement on the macroscopic level and high pinning force on the local level. This induces a giant increase of the dielectric and electromechanical response at intermediate electric fields in barium titanate [electric field-dependent permittivity (ε33) ≈ 5800 and large-signal piezoelectric coefficient (d 33*) ≈ 1890 picometers/volt]. Dislocation-based anisotropy delivers a different suite of tools with which to tailor functional materials.

Enhanced tunability in ferroelectric composites through local field enhancement and the effect of disorder

We investigate numerically the homogenized permittivities of composites made of low-index dielectric inclusions in a ferroelectric matrix under a static electric field. A refined model is used to take into account the coupling between the electrostatic problem and the electric field dependent permittivity of the ferroelectric material, leading to a local field enhancement and permittivity change in the ferroelectric. Periodic and pseudorandom structures in two dimensions are investigated, and we compute the effective permittivity, losses, electrically induced anisotropy, and tunability of these metamaterials. We show that the tunability of such composites might be substantially enhanced in the periodic case, whereas introducing disorder in the microstructure weakens the effect of the enhanced local permittivity change. Our results may be useful to guide the synthesis of novel composite ceramics with improved characteristics for controllable microwave devices.

Electrostatic analysis of n-doped SrTiO3 metal-insulator-semiconductor systems

Electron doped SrTiO3, a complex-oxide semiconductor, possesses novel electronic properties due to its strong temperature and electric-field dependent permittivity. Due to the high permittivity, metal/n-SrTiO3 systems show reasonably strong rectification even when SrTiO3 is degenerately doped. Our experiments show that the insertion of a sub nanometer layer of AlOx in between the metal and n-SrTiO3 interface leads to a dramatic reduction of the Schottky barrier height (from around 0.90 V to 0.25 V). This reduces the interface resistivity by 4 orders of magnitude. The derived electrostatic analysis of the metal-insulator-semiconductor (n-SrTiO3) system is consistent with this trend. When compared with a Si based MIS system, the change is much larger and mainly governed by the high permittivity of SrTiO3. The non-linear permittivity of n-SrTiO3 leads to unconventional properties such as a temperature dependent surface potential non-existent for semiconductors with linear permittivity such as Si. This allows tuning of the interfacial band alignment, and consequently the Schottky barrier height, in a much more drastic way than in conventional semiconductors.

Nonlinear properties of antimony-doped BaTiO3 ceramics

Barium titanate nanopowders doped with various concentrations of antimony were synthesized by polymeric precursors method based on modified Pechini process. Obtained powders were pressed and sintered at 1,573 K for 8 h. The temperature dependence of dielectric permittivity pointed on the significant influence of added antimony on dielectric properties of obtained materials. The analysis of impedance spectrum in the temperature range 250–445 K has shown the presence of one semicircle that corresponds to the grain contribution but impedance spectrum at 773 K showed possible overlapping of two semicircles indicating also grain boundary resistivity. The total specific resistivity obtained from impedance analysis showed modest PTCR jump in the heavily doped barium titanate ceramics. The electric field dependence of permittivity of the ceramics was investigated in detail. The experimental data were analyzed using Johnson model.

Investigation of the ferroelectric–relaxor crossover in Ce-doped BaTiO3 ceramics by impedance spectroscopy and Raman study

BaCe x Ti1− x O3 (x = 0.06, 0.10, and 0.20) solid solutions were prepared via conventional the solid-state reaction and sintered at 1500°C for 4 h, resulting in dense single phase ceramics with homogeneous microstructures. The electric field dependence of permittivity of the BaCe x Ti1− x O3 ceramics was investigated in detail, together with the ferroelectric–paraelectric phase transition features. A transformation from normal to relaxor ferroelectrics was observed by increasing the Ce concentration. For low-Ce content, a substitution of Ce on both A and B site positions was proposed from the dielectric study and confirmed by Raman analysis.

Non-linear electric field response of permittivity of atomically smooth TiO2(rutile) single crystals studied by an electrochemical approach

Electric field-dependent permittivity of n-TiO2 (rutile) along different crystallographic axes was investigated by an electrochemical impedance spectroscopy (EIS) measurement. The C−2 versus U plots were fitted with a quadratic function of U, derived from a model that adds the effects of electric field dependent permittivity to the conventional Mott–Schottky equation. Only when atomically flat n-TiO2 single crystal electrodes were used, could the intrinsic behavior of the electric field dependent permittivity be observed. The principal components of permittivity in the ab-plane exhibited a weak electric field dependence, while a strong electric field dependence was confirmed along the c-axis direction. By taking the electric field dependent permittivity into account, we were able to evaluate accurately the flat-band potential and the donor density. Different photocurrent behavior between (110), (100), and (001) electrodes is discussed in terms of the electric field strength at the surface and the depletion layer width within TiO2.

Influence of Schottky Interfaces on Dielectric Properties in Perovskite-Type Oxide Thin-Film Capacitors

An estimation method of the influence of interfaces on properties in perovskite-type oxide thin-film capacitors is presented. We proposed a modified Schottky model that can be employed to explain the electric properties of metal/perovskite-type oxide junctions. The modified model considers the electric field dependence of permittivity and the flow of electrons from metal to defect states located in the band gap of the perovskite oxide. The simulation based on this model could successfully describe the results of capacitance-voltage measurements of junctions between metals (Pt, Au, and Ag) and Nb-doped SrTiO3, which were not explained by the conventional Schottky model. Additionally, a simulation for a thin-film capacitor with hysteretic behavior could depict an asymmetric capacitance-voltage curve.

The influence of metal/perovskite-type oxide interfaces on tunability of thin film capacitors

We propose a calculation model that can predict the influence of interface on capacitance–voltage characteristics of metal-insulator-metal type thin film capacitor with perovskite type oxide. The simulation could successfully describe the results of capacitance–voltage measurements of the barium strontium titanate thin film capacitors with top electrodes of Pt, Au, and Ag. It is found that the electrode dependent on the tunability is derived from the work function of the electrode metal. The simulation model is base on the Schottky model that can be employed to explain the dielectric properties of metal/perovskite type oxide junction. The modified Schottky model considers the electric field dependence of permittivity and the back flow of electrons from metal to defect states located in the band gap of the perovskite oxide. This flow forms negatively charged space at the interface and affects the electric properties of the thin film capacitors.

The modified model of the dielectric characteristics for porous Ba0.6Sr0.4TiO3 ceramics

The effect of direct-current (dc) biasing on the dielectric properties of porous Ba0.6Sr0.4TiO3 with various porosities, pore sizes, and different pore shapes has been studied in the paraelectric state. The existing model did not consider the effect of the change in grain size and pore shape, lead to the theory fitting the fine grained or nonspherical pore ceramics less well. The bias equation used in this literature to predict the electric field dependence of permittivity is modified by introducing a coefficient α. α is the microstructure parameter introduced to depict the influence of grain size and pore shape effect. Taking high porosity samples and nonspherical pore ceramic samples, for example, the modified bias equation gives a better fit to the experimental result in this study.

Effects of film thickness and preferred orientation on the dielectric properties of (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 films

(Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 (BZN) films with different thicknesses and preferred orientations have been fabricated on Nb doped SrTiO3 substrates by pulsed laser deposition. As the thickness increases, the permittivity increases, and the dielectric loss decreases, while the tunability only has a little variation. The asymmetric behaviour of the electric field dependent permittivity reduces gradually with the increasing thickness, which should be attributed to the decrease in the effect of the interfacial layer between the dielectric film and substrate (electrode). Furthermore, compared with the (1 0 0) oriented BZN film, BZN film with (1 1 1) preferred orientation exhibits high dielectric loss.

Finite element models of the application of stress-relieving dielectrics

The addition of ferroelectric fillers to a polymer can produce a material with electric field-dependent permittivity properties, which can be used for the purposes of electrical stress relief for applications under AC fields. The paper illustrates the electrical stress-relieving properties of these composite polymers through finite element models. The electric field-dependent permittivity characteristics of the stress-relieving dielectric have been based on a barium titanate filler in an acrylic resin, for a range of filler levels. Two sets of models are considered. The first set of models considers the case of a needle-point plane geometry and the electrical stress distribution in the stress-relieving dielectric for needles of different geometries. The second set of models considers the stress distribution in a coaxial cable using the stress-relieving dielectric. The effectiveness of the stress-relieving dielectric in each of these geometries is clearly illustrated.

C-axial oriented (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 thin film grown on Nb doped SrTiO3 substrate by pulsed laser deposition

A c-axial oriented (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 thin film has been grown on a (0 0 1) Nb doped SrTiO3 substrate by pulsed laser deposition. The permittivity, dielectric loss and tunability of the c-axial oriented film are 187, 0.002 and 6% (at 750 kV cm−1 biasing), respectively, indicating a figure of merit of 30. Moreover, an asymmetry behaviour is observed in the dc electric field dependence of permittivity, which could be attributed to the asymmetry of top and bottom electrodes.

Space-charge-limited leakage current characteristics influenced by field-dependent permittivity in high dielectric constant and ferroelectric thin films

The current-voltage relationship dominated by space-charge-limited conduction for high dielectric constant and ferroelectric thin films, including (Ba,Sr)TiO3 and Pb(Zr,Ti)O3, is derived taking into account the electric field dependence of permittivity. Typical current-voltage characteristics are given by the derived relationship with empirical values for its parameters. The obtained current characteristics might account for the high-field quasi-Ohmic region observed experimentally in the leakage current characteristics of a very thin BaTiO3 single crystal in the previous study [Morrison et al., Appl. Phys. Lett. 86, 152903 (2005)].

BZN THIN FILM CAPACITORS FOR MICROWAVE LOW LOSS TUNABLE APPLICATIONS

ABSTRACT Bismuth zinc niobate (BZN), a high dielectric constant material that combines low dielectric losses with an electric-field dependent permittivity, was implemented as the dielectric in metal-insulator-metal (MIM) capacitors. Dielectric properties up to 20 GHz were evaluated by measuring reflection coefficients with a vector network analyzer. The dielectric constant was around 180 and the quality factor remained greater than 100 up to 5 GHz for 64 μ m2 devices. No onset of dielectric relaxation, as seen in bulk ceramic data, could be detected in the measured frequency range. The results show that BZN thin films have great potential for low loss, tunable microwave devices.

Electric-field dependence of permittivity for PLZT ceramics and their application for laser pumping capacitors

Abstract DC bias dependence and under a DC high-voltage field of permittivity for variety of the PLZT ceramics of ferroelectricity (7/65/35, 8/65/35), pseudo-ferroelectricity (9/65/35), anti-ferroelectricity (10/80/20, 10/90/10) and paraelectricity (11/65/35) by controlling the composition of PLZT have been investigated. The 11/65/35 had a high permittivity under a DC biasing field and also having a low electric-field dependence, but the accumulated charge under DC high-voltage of 9/65/35 capacitor was larger than that of 11/65/35. The out put laser energy using 9/35/65 capacitor was as about 1.5 times larger as that of the 11/65/35 capacitor.

Phenomenological Treatment of Antiferroelectric Transition under Hydrostatic Pressure

By applying Kittel's antiferroelectric polarizations (Pa and Pb) and Cross' ferroelectric polarization Pm and antiferroelectric polarization Pn to the free energy function for ferroelectrics under hydrostatic pressure, the Gibbs function for antiferroelectrics under hydrostatic pressure is given in terms of the pressure, polarization and electric field. On the basis of this energy function, dielectric properties such as the pressure dependence of permittivity or spontaneous polarization and the electric field dependence of permittivity are clearly explained, and the so-called double hysteresis loop under hydrostatic pressure is explained fairly well for antiferroelectrics of the second and the first order transitions. Moreover, these analyses are confirmed to be appropriate by comparing with the experimental results for antiferroelectric Pb2MgWO6, etc.