Showing papers on "Dielectric published in 2009"

Realization of a High Mobility Dual-gated Graphene Field Effect Transistor with Al2O3 Dielectric

Abstract: We fabricate and characterize dual-gated graphene field-effect transistors (FETs) using Al2O3 as top-gate dielectric. We use a thin Al film as a nucleation layer to enable the atomic layer deposition of Al2O3. Our devices show mobility values of over 8,000 cm2/Vs at room temperature, a finding which indicates that the top-gate stack does not significantly increase the carrier scattering, and consequently degrade the device characteristics. We propose a device model to fit the experimental data using a single mobility value.

Realization of a high mobility dual-gated graphene field-effect transistor with Al2O3 dielectric

Abstract: We fabricate and characterize dual-gated graphene field-effect transistors using Al2O3 as top-gate dielectric. We use a thin Al film as a nucleation layer to enable the atomic layer deposition of Al2O3. Our devices show mobility values of over 8000 cm2/V s at room temperature, a finding which indicates that the top-gate stack does not significantly increase the carrier scattering and consequently degrade the device characteristics. We propose a device model to fit the experimental data using a single mobility value.

Polymer Composite and Nanocomposite Dielectric Materials for Pulse Power Energy Storage

Abstract: This review summarizes the current state of polymer composites used as dielectric materials for energy storage The particular focus is on materials: polymers serving as the matrix, inorganic fillers used to increase the effective dielectric constant, and various recent investigations of functionalization of metal oxide fillers to improve compatibility with polymers We review the recent literature focused on the dielectric characterization of composites, specifically the measurement of dielectric permittivity and breakdown field strength Special attention is given to the analysis of the energy density of polymer composite materials and how the functionalization of the inorganic filler affects the energy density of polymer composite dielectric materials

High Dielectric Permittivity and Low Percolation Threshold in Nanocomposites Based on Poly(vinylidene fluoride) and Exfoliated Graphite Nanoplates

Abstract: Ferroelectric polymers, such as poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) have great potential for applications in micro-electromechanical devices and high-charge storage capacitors. In order to realize these applications, it is highly desirable to substantially improve the dielectric constants of such ferroelectric polymers. Up to now, much work has focused on the preparation of 0–3-type composites based on polymers and ceramics of high dielectric constant, and the resultant composites usually possess a relatively high dielectric permittivity (about 100). Nevertheless, the high volume fraction (> 50 vol%) of ceramics, which was necessary to achieve the high dielectric constants, presents a number of limitations, in terms of high weight, low flexibility, and poor mechanical performance, as a result of the weak matrix-filler bonding and agglomeration of ceramic nanoparticles. Furthermore, most ceramics of high dielectric constant are lead-based, and potentially harmful to our health. To overcome the limitations of ferroelectric polymer/ceramic composites, very promising work has been carried out recently based on percolation theory, in which a small volume fraction of some conductive filler was added to the polymer matrix to achieve a high dielectric constant, thus preserving the mechanical flexibility of the polymer. For example, Dang et al. fabricated a new poly(vinylidene fluoride)/carbon-nanotube composite, with a percolation threshold of 8 vol%, possessing a dielectric constant of 600 (the dielectric loss value, tan d, is about 2) at 1000Hz. For a P(VDF-TrFE-CFE)/carbon-nanotube system, the dielectric constant increased from 57 to 102 (tand! 0.36) at 100Hz, by inclusion of only 2wt% (1.2 vol%) carbon nanotubes. A dielectric constant as high as 56 was observed in a PVDF/acetylene-black composite, when the acetylene-black concentration was in the neighborhood of the percolation threshold (about 1.3 vol%). Recently, Panda et al. reported that, for PVDF/Ni composites, a high effective dielectric constant of 2050 (tand1⁄4 10) at 100Hz was observed near the percolation threshold of 27 vol%. Along this line, in this communication, we propose a novel nanocomposite system consisting of poly(vinylidene fluoride) and exfoliated graphite nanoplates (PVDF/xGnPs). The xGnPs were selected as the conductive filler, because of their good electrical and thermal conductivity, high mechanical strength, and more importantly, large aspect ratio and unique layered structure with nanoscale thickness, which give advantages in the formation of a large number of parallel-board microcapacitors with low filler loading. Moreover, functional groups, such as C–O–C, C–OH, and C––O, existing on the surface of graphite nanoplates, can promote the interaction between the PVDF and the graphite nanoplates, leading to good dispersion of xGnPs in thematrix. It is well known that the homogenous dispersion of conductive fillers in a polymer matrix is a critical factor in achieving a high-performance nanocomposite. Therefore, it was expected that a much lower volume fraction of xGnP in the PVDF/xGnP nanocomposite could result in a greater increase in dielectric permittivity. xGnPs were obtained from subjecting natural graphite flakes to acidic intercalation, rapid thermal treatment, and ultrasonic powdering, in sequence (see S1 in Supporting Information). Natural graphite flakes (see S2 in Supporting Information), a naturally abundant and low-cost carbon-based material, are composed of parallel carbon layers. Carbon atoms within the graphite layers connect to each other to form six-member rings through strong covalent bonds, while the parallel carbon layers are joined together by weak van der Waals force. Such a structure makes it possible to intercalate some small molecules into the interlayer space of graphite. In the present study, natural graphite flakes were first converted to graphite intercalation compounds (GICs), through intercalation and chemical oxidation in the presence of concentrated H2SO4 andHNO3.When heated at high temperatures, because of the volatilization of the mixed acid, the GICs could be expanded up to a few hundred times along the direction perpendicular to the carbon-layer plane of the intercalated graphite, so that expanded graphite (EG), which is a worm-like material (see the scanning electron microscopy (SEM) image, shown in Fig. 1a), could be obtained. It is also important to note that some functional groups could be introduced to the graphite during the preparation of the GICs and EG. After ultrasonic treatment, the graphite worms were fragmented into exfoliated graphite nanoplates with diameters of 0.5–25mm and thicknesses of 20–60 nm (see S3 in Supporting Information), as shown by the SEM image in Figure 1b. C O M M U N IC A TI O N www.advmat.de

High‐Density Carrier Accumulation in ZnO Field‐Effect Transistors Gated by Electric Double Layers of Ionic Liquids

Abstract: Very recently, electric-field-induced superconductivity in an insulator was realized by tuning charge carrier to a high density level (1 × 1014 cm−2). To increase the maximum attainable carrier density for electrostatic tuning of electronic states in semiconductor field-effect transistors is a hot issue but a big challenge. Here, ultrahigh density carrier accumulation is reported, in particular at low temperature, in a ZnO field-effect transistor gated by electric double layers of ionic liquid (IL). This transistor, called an electric double layer transistor (EDLT), is found to exhibit very high transconductance and an ultrahigh carrier density in a fast, reversible, and reproducible manner. The room temperature capacitance of EDLTs is found to be as large as 34 µF cm−2, deduced from Hall-effect measurements, and is mainly responsible for the carrier density modulation in a very wide range. Importantly, the IL dielectric, with a supercooling property, is found to have charge-accumulation capability even at low temperatures, reaching an ultrahigh carrier density of 8×1014 cm−2 at 220 K and maintaining a density of 5.5×1014 cm−2 at 1.8 K. This high carrier density of EDLTs is of great importance not only in practical device applications but also in fundamental research; for example, in the search for novel electronic phenomena, such as superconductivity, in oxide systems.

Optimization of Dielectric Barrier Discharge Plasma Actuators for Active Aerodynamic Flow Control

Abstract: This paper presents the results of a parametric experimental investigation aimed at optimizing the body force produced by single dielectric barrier discharge plasma actuators used for aerodynamic flow control. A primary goal of the study is the improvement of actuator authority for flow control applications at higher Reynolds number than previously possible. The study examines the effects of dielectric material and thickness, applied voltage amplitude and frequency, voltage waveform, exposed electrode geometry, covered electrode width, and multiple actuator arrays. The metric used to evaluate the performance of the actuator in each case is the measured actuator-induced thrust which is proportional to the total body force. It is demonstrated that actuators constructed with thick dielectric material of low dielectric constant produce a body force that is an order of magnitude larger than that obtained by the Kapton-based actuators used in many previous plasma flow control studies. These actuators allow operation at much higher applied voltages without the formation of discrete streamers which lead to body force saturation.

High-Energy Density Capacitors Utilizing 0.7 BaTiO3–0.3 BiScO3 Ceramics

Abstract: A high, temperature-stable dielectric constant ( ~ 1000 from 0° to 300°C) coupled with a high electrical resistivity (~10 12 Ω · cm at 250°C) make 0.7 BaTiO 3 -0.3 BiScO 3 ceramics an attractive candidate for high-energy density capacitors operating at elevated temperatures. Single dielectric layer capacitors were prepared to confirm the feasibility of BaTiO 3 -BiScO 3 for this application. It was found that an energy density of about 6.1 J/cm 3 at a field of 73 kV/mm could be achieved at room temperature, which is superior to typical commercial X7R capacitors. Moreover, the high-energy density values were retained to 300°C. This suggests that BaTiO 3 -BiScO 3 ceramics have some advantages compared with conventional capacitor materials for high-temperature energy storage, and with further improvements in microstructure and composition, could provide realistic solutions for power electronic capacitors.

Substrates Matter: Influence of an Adjacent Dielectric on an Individual Plasmonic Nanoparticle

Abstract: Studying the plasmonic properties of metallic nanoparticles at the individual nanostructure level is critical to our understanding of nanoscale metallic systems. Here we show how the presence of a nearby dielectric substrate modifies the energies of the plasmon modes of a metallic nanoparticle. The adjacent dielectric lifts the degeneracy of the dipole plasmon modes oriented parallel and perpendicular to the substrate, introducing a significant energy splitting that depends strongly on the permittivity of the substrate. This energy splitting can easily be misinterpreted as an anomalously broadened plasmon line shape for excitation of an individual nanoparticle with unpolarized light.

Dielectric and Piezoelectric Properties in Mn‐Modified (1−x)BiFeO3–xBaTiO3 Ceramics

Abstract: In the current work, the bulk (1−x)BiFeO3–xBaTiO3 system has been studied as a potential lead-free piezoelectric material. Barium titanate (BaTiO3) in solid solution with bismuth ferrite (BiFeO3) is observed to stabilize the perovskite structure and improve switching behavior. Samples with various content of BaTiO3 were prepared via solid-state route, and pure perovskite phase was confirmed by X-ray diffraction. Modification of the BaTiO3–BiFeO3 material with Mn improved DC resistivity by one to five orders of magnitude (7.6 × 1012 vs. 2.7 × 107Ω·m for 25 mol% BaTiO3 at room temperature) and polarization hysteresis measurements indicated “hard” ferroelectric behavior with the highest strain response at 33 mol% BaTiO3. Finally, low-field piezoelectric d33 coefficient of 116 pC/N and ferroelectric transition temperature above 450°C are reported for 25 mol% BaTiO3 composition.

Colossal dielectric constants in transition-metal oxides

Abstract: Many transition-metal oxides show very large (“colossal”) magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energy-storage devices In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is today’s most investigated material with colossal dielectric constant Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2−xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant

Electrocaloric Materials for Solid‐State Refrigeration

Abstract: The electrocaloric effect (ECE) in dielectric materials has great potential in realizing solid-state cooling devices with compact size and high efficiency, which are highly desirable for a broad range of applications. This paper presents the general considerations for dielectric materials to achieve large ECE and reviews the experimental efforts investigating ECE in various polar dielectrics. For practical cooling devices, an ECE material must possess a large isothermal entropy change besides a large adiabatic temperature change. We show that polar dielectrics operated at temperatures near order―disorder transition have potential to achieve large ECE due to the possibility of large change in polarization induced by electric field and large entropy change associated with the polarization change. We further show that indeed the ferroelectric poly(vinylidene fluoride-trifluoroethylene)-based polymers display a large ECE, i.e., an isothermal entropy change of more than 55 J (kgK) ―1 and an adiabatic temperature change of more than 12 °C, at temperatures above the order-disorder transition.

Utilization of a Buffered Dielectric to Achieve High Field-Effect Carrier Mobility in Graphene Transistors

Abstract: We utilize an organic polymer buffer layer between graphene and conventional gate dielectrics in top-gated graphene transistors. Unlike other insulators, this dielectric stack does not significantly degrade carrier mobility, allowing for high field-effect mobilities to be retained in top-gate operation. This is demonstrated in both two-point and four-point analysis and in the high-frequency operation of a graphene transistor. Temperature dependence of the carrier mobility suggests that phonons are the dominant scatterers in these devices.

Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics

Abstract: Stoichiometric and nonstoichiometric (Bi05Na05)TiO3 (BNT) ceramics were prepared by a conventional ceramic fabrication process This study revealed that the high conductivity of BNT ceramics is associated with Bi vaporization during sintering An x-ray study revealed that a tetragonal phase exists in the temperature range between 330 and 480 °C in BNT ceramic as well as BNT single crystals In addition, the depolarization temperature Td, rhombohedral-tetragonal phase transition temperature TR-T, and the temperature Tm of the maximum dielectric constant were determined to be 187, approximately 300, and 325 °C, respectively, from the temperature dependences of dielectric properties using unpoled and poled specimens The piezoelectric properties of all vibration modes and the temperature dependences of the piezoelectric properties were measured using fully poled BNT ceramics It was also revealed that BNT ceramics exhibit three thermal depoling processes at Td, between Td and TR-T, and between TR-T and Tm

Magnetic−Plasmonic Core−Shell Nanoparticles

Abstract: Nanoparticles composed of magnetic cores with continuous Au shell layers simultaneously possess both magnetic and plasmonic properties. Faceted and tetracubic nanocrystals consisting of wustite with magnetite-rich corners and edges retain magnetic properties when coated with a Au shell layer, with the composite nanostructures showing ferrimagnetic behavior. The plasmonic properties are profoundly influenced by the high dielectric constant of the mixed iron oxide nanocrystalline core. A comprehensive theoretical analysis that examines the geometric plasmon tunability over a range of core permittivities enables us to identify the dielectric properties of the mixed oxide magnetic core directly from the plasmonic behavior of the core−shell nanoparticle.

Stacked dielectric elastomer actuator for tensile force transmission

Abstract: This paper presents a novel approach for active structures driven by soft dielectric electro-active polymers (EAPs), which can perform contractive displacements at external tensile load. The active structure is composed of an array of equal segments, where the dielectric films are arranged in a pile-up configuration. The proposed active structure has the capability of exhibiting uniaxial contractive deformations, while being exposed to external tensile forces. The serial arrangement of active segments has one contracting degree of freedom in the thickness direction of the dielectric EAP film layers. Due to the envisaged tension force transmission capability, special attention is paid to the electrode design which is of paramount importance with regard to functionality of the actuator. A compliant electrode system with anisotropic deformation properties is presented based on nano scale carbon powder. In experiments, the free deformation as well as the contractive motion under external tensile loading of several actuator configurations with different setups is characterized. These involve the study of various sizes and numbers of stacked film layers as well as different electrode designs.

High dielectric loss and its monotonic dependence of conducting-dominated multiwalled carbon nanotubes/silica nanocomposite on temperature ranging from 373 to 873 K in X-band

Abstract: The dielectric properties of multiwalled carbon nanotubes/silica (MWNTs/SiO2) nanocomposite with 10 wt % MWNTs are investigated in the temperature range of 373–873 K at frequencies between 8.2 and 12.4 GHz (X-band). MWNTs/SiO2 exhibits a high dielectric loss and a positive temperature coefficient (PTC) of dielectric effect that complex permittivity increases monotonically with increasing temperature. The PTC effect on the dielectric constant is ascribed to the decreased relaxation time of interface charge polarization, and the PTC effect on the dielectric loss is mainly attributed to the increasing electrical conductivity. The loss tangent strongly supports the dominating contribution of conductance to the dielectric loss.

Dual nonlinear dielectric resonance and nesting microwave absorption peaks of hollow cobalt nanochains composites with negative permeability

Abstract: The permittivity and permeability behaviors of the hollow cobalt nanochains composites have been investigated in 2–18 GHz. The permittivity presents two dielectric resonance peaks at about 12.3 and 14.5 GHz, respectively, which mainly results from the cooperative consequence of the hollow structure and the one-dimensional structure of the as-synthesized Co nanochains. The negative permeability behavior within 12.3–18 GHz is attributed to radiation of the magnetic energy according to the as-established equivalent circuit model. Two strong absorption peaks of the composites nest at the resonance frequencies due to the effect of the dual nonlinear dielectric resonance and the negative permeability behavior.

Utilization of a Buffered Dielectric to Achieve High Field-Effect Carrier Mobility in Graphene Transistors

Abstract: We utilize an organic polymer buffer layer between graphene and conventional gate dielectrics in top-gated graphene transistors. Unlike other insulators, this dielectric stack does not significantly degrade carrier mobility, allowing for high field-effect mobilities to be retained in top-gate operation. This is demonstrated in both two-point and four-point analysis, and in the high-frequency operation of a graphene transistor. Temperature dependence of the carrier mobility suggests that phonons are the dominant scatterers in these devices.

Ion Gel-Gated Polymer Thin-Film Transistors: Operating Mechanism and Characterization of Gate Dielectric Capacitance, Switching Speed, and Stability

Abstract: We report comprehensive characterization of electrolyte-gated polymer thin-film transistors (TFTs) incorporating solution processable polymer semiconductors and high capacitance “ion gel” gate dielectrics. The ion gel dielectrics comprise self-assembled networks of triblock copolymers such as poly(styrene-b-methylmethacrylate-b-styrene) [PS-PMMA-PS] that are swollen with ionic liquids, e.g., (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). The capacitance of the gels is exceptionally large (>10 μF/cm2 at 10 Hz), which is derived from the high concentration of mobile ions and facilitates operation of ion gel-gated organic TFTs (GEL-OTFTs) at very low voltages (< 2.5 V). Gate-induced hole densities in GEL-OTFTs employing different polythiophene semiconductors in the channel are on the order of 1014 carriers/cm2, with associated saturation hole mobilities that are also remarkably large, ∼1 cm2/(V s), likely because of the large gate-induced carrier densities. Examination of the ...

Weakly Coupled Relaxor Behavior of BaTiO3–BiScO3 Ceramics

Abstract: The structural and dielectric properties of (1−x)BaTiO3–xBiScO3 (x=0–0.5) ceramics were investigated to acquire a better understanding of the binary system, including determination of the symmetry of the phases, the associated dielectric properties, and the differences in the roles of Bi2O3 and BiScO3 substitutions in a BaTiO3 solid solution. The solubility limit for BiScO3 into the BaTiO3 perovskite structure was determined to be about x=0.4. A systematic structural change from the ferroelectric tetragonal phase to a pseudo-cubic one was observed at about x=0.05–0.075 at room temperature. Dielectric measurements revealed a gradual change from proper ferroelectric behavior in pure BaTiO3 to highly diffusive and dispersive relaxor-like characteristics from 10 to 40 mol% BiScO3. Several of the compositions showed high relative permittivities with low-temperature coefficients of capacitance over a wide range of temperature. Quantification of the relaxation behavior was obtained through the Vogel–Fulcher model, which yielded an activation energy of 0.2–0.3 eV. The attempt characteristic frequency was 1013 Hz and the freezing temperature, Tf, ranged from −177° to −93°C as a function of composition. The high coercive fields, low remanent polarization, and high activation energies suggest that in the BiScO3–BaTiO3 solid solutions, the polarization in nanopolar regions is weakly coupled from region to region, limiting the ability to obtain long-range dipole ordering in these relaxors under field-cooled conditions.

Universal transduction scheme for nanomechanical systems based on dielectric forces

Abstract: Any polarizable body placed in an inhomogeneous electric field experiences a dielectric force. This phenomenon is well known from the macroscopic world: a water jet is deflected when approached by a charged object. This fundamental mechanism is exploited in a variety of contexts-for example, trapping microscopic particles in an optical tweezer, where the trapping force is controlled via the intensity of a laser beam, or dielectrophoresis, where electric fields are used to manipulate particles in liquids. Here we extend the underlying concept to the rapidly evolving field of nanoelectromechanical systems (NEMS). A broad range of possible applications are anticipated for these systems, but drive and detection schemes for nanomechanical motion still need to be optimized. Our approach is based on the application of dielectric gradient forces for the controlled and local transduction of NEMS. Using a set of on-chip electrodes to create an electric field gradient, we polarize a dielectric resonator and subject it to an attractive force that can be modulated at high frequencies. This universal actuation scheme is efficient, broadband and scalable. It also separates the driving scheme from the driven mechanical element, allowing for arbitrary polarizable materials and thus potentially ultralow dissipation NEMS. In addition, it enables simple voltage tuning of the mechanical resonance over a wide frequency range, because the dielectric force depends strongly on the resonator-electrode separation. We use the modulation of the resonance frequency to demonstrate parametric actuation. Moreover, we reverse the actuation principle to realize dielectric detection, thus allowing universal transduction of NEMS. We expect this combination to be useful both in the study of fundamental principles and in applications such as signal processing and sensing.

Origin of electric dipoles formed at high-k/SiO2 interface

Abstract: A model for the physical origin of the dipole formed at high-k/SiO2 interface is proposed. In our model, an areal density difference of oxygen atoms at high-k/SiO2 interface is considered as an intrinsic origin of the dipole formation. The oxygen movement from higher-oxygen-density side to a lower-oxygen-density one will determine the direction of interface dipole. The bonding energy relaxation at the interface explains why the oxygen density difference is the driving force of the oxygen movement. Our model enables the prediction of the dipole directions for candidate gate dielectrics, including those so far not reported.

Coax core insulator waveguide

Abstract: A communication device consistent with certain implementations has a coaxial cable having length and first and second ends. The coaxial cable further has a central conductor, a dielectric insulator surrounding the central conductor, and an electric shield conductor surrounding the dielectric insulator. The dielectric insulator serves as a dielectric waveguide having a characteristic impedance Z at an operating frequency range. A termination for electrical energy coupled into or out of the dielectric insulator at approximately the characteristic impedance Z at the operating frequency range to utilize the dielectric insulator as a waveguide for transmission of signals along the length of the coaxial cable, and wherein the center conductor is further used to communicate an electrical signal between the first and second ends. This abstract is not to be considered limiting, since other embodiments may deviate from the features described in this abstract.

Physically and Mineralogically Based Spectroscopic Dielectric Model for Moist Soils

Abstract: In this paper, the error of dielectric predictions for moist soils was estimated, regarding the semiempirical mixing dielectric model (SMDM) developed by Dobson , which is a universally recognized one, and the generalized refractive mixing dielectric model (GRMDM) recently elaborated by Mironov The analysis is based on the measured dielectric data presented in by Curtis and the papers of Dobson These data cover a broad variety of grain-size distributions observed in 15 soils and the frequency range from 45 MHz to 26.5 GHz, with the temperature being from 20 degC to 22 degC. The SMDM was found to deliver predictions with substantially larger error for the soils, whose dielectric data were not used for its development, while the GRMDM ensured dielectric predictions for all the soils analyzed with as small error as the SMDM did in the case of the soils that it was based on. To secure the same convenience in application of the GRMDM, which the SMDM possesses, the spectroscopic parameters of that model were correlated with the clay percentages of the respective soils. As a result, a new mineralogy-based dielectric model was developed. For the moist soils other than those whose dielectric data were used for its development, this model was shown to demonstrate noticeably smaller error of dielectric predictions, with clay percentage being the only input parameter, as compared with the error observed in the case of the SMDM.

Atomistic determination of flexoelectric properties of crystalline dielectrics

Abstract: Upon application of a uniform strain, internal sublattice shifts within the unit cell of a noncentrosymmetric dielectric crystal result in the appearance of a net dipole moment: a phenomenon well known as piezoelectricity. A macroscopic strain gradient on the other hand can induce polarization in dielectrics of any crystal structure, even those which possess a centrosymmetric lattice. This phenomenon, called flexoelectricity, has both bulk and surface contributions: the strength of the bulk contribution can be characterized by means of a material property tensor called the bulk flexoelectric tensor. Several recent studies suggest that strain-gradient induced polarization may be responsible for a variety of interesting and anomalous electromechanical phenomena in materials including electromechanical coupling effects in nonuniformly strained nanostructures, ``dead layer'' effects in nanocapacitor systems, and ``giant'' piezoelectricity in perovskite nanostructures among others. In this work, adopting a lattice dynamics based microscopic approach we provide estimates of the flexoelectric tensor for certain cubic crystalline ionic salts, perovskite dielectrics, $III\text{\ensuremath{-}}V$ and $II\text{\ensuremath{-}}VI$ semiconductors. We compare our estimates with experimental/theoretical values wherever available and also revisit the validity of an existing empirical scaling relationship for the magnitude of flexoelectric coefficients in terms of material parameters. It is interesting to note that two independent groups report values of flexoelectric properties for perovskite dielectrics that are orders of magnitude apart: Cross and co-workers from Penn State have carried out experimental studies on a variety of materials including barium titanate while Catalan and co-workers from Cambridge used theoretical ab initio techniques as well as experimental techniques to study paraelectric strontium titanate as well as ferroelectric barium titanate and lead titanate. We find that, in the case of perovskite dielectrics, our estimates agree to an order of magnitude with the experimental and theoretical estimates for strontium titanate. For barium titanate however, while our estimates agree to an order of magnitude with existing ab initio calculations, there exists a large discrepancy with experimental estimates. The possible reasons for the observed deviations are discussed.

Dielectric Screening Enhanced Performance in Graphene FET

Abstract: We have studied the transport properties of graphene transistors in different solvents with dielectric constant varying over 2 orders of magnitude. Upon increasing the dielectric constant, the carrier mobility increases up to 3 orders of magnitude and reaches ∼7 × 104 cm2/v·s at the dielectric constant of ∼47. This mobility value changes little in higher dielectric constant solvents, which indicates that we are approaching the intrinsic limit of room temperature mobility in graphene supported on SiO2 substrates. The results are discussed in terms of long-range Coulomb scattering originated from the charged impurities underneath graphene.

Recent advances in the understanding of homogeneous dielectric barrier discharges

Abstract: This paper is a state of the art of the understanding on the physics of homogeneous dielectric barrier discharge at atmospheric pressure. It is based on the analysis of present and previous work about the behavior of these discharges and the conditions to get them. Mechanisms controlling the homogeneity during gas breakdown and discharge development are successively discussed. The breakdown has to be a Townsend one, the ionization has to be slow enough to avoid a large avalanche development. During the breakdown, the discharge homogeneity is related to the ratio of the secondary emission at the cathode (γ coefficient) on the ionization in the gas bulk (α coefficient). Higher is this ratio, higher is the pressure × gas gap product (P.d.) value for which a Townsend breakdown is obtained. Among the phenomena enhancing the secondary emission there is the negative charge of the dielectric on the cathode surface, the trapping of ions in the gas and the existence of excited state having a long lifetime compared to the time between two consecutive discharges. The first phenomenon is always present when the electrodes are covered by a solid dielectric, the second one is related to the formation of a positive column and the third one is specific of the gas. During the discharge development, the homogeneity is mainly controlled by the voltage or the current imposed by the electrical circuit /electrode configuration and to the gas ability to be slowly ionized. Larger is the contribution of a multiple step ionization process like Penning ionization, higher will be the working domain of the discharge. A decrease of the gas voltage during the discharge development is a solution to enhance the contribution of this process. After 20 years of research a lot of mechanisms have been understood however there is still open questions like the nature of the Inhibited homogeneous DBD, surface energy transfers, role of attachment and detachment…