Showing papers on "Dielectric published in 2012"
TL;DR: In this paper, the authors focus on the important role and challenges of high-k polymer-matrix composites (PMC) in new technologies and discuss potential applications of highk PMC.
1,412 citations
TL;DR: In this paper, high performance p-type field effect transistors based on single layered (thickness, 0.7 nm) WSe2 as the active channel with chemically doped source/drain contacts and high-kappa gate dielectrics were reported.
Abstract: We report high performance p-type field-effect transistors based on single layered (thickness, ~0.7 nm) WSe2 as the active channel with chemically doped source/drain contacts and high-{\kappa} gate dielectrics. The top-gated monolayer transistors exhibit a high effective hole mobility of ~250 cm2/Vs, perfect subthreshold swing of ~60 mV/dec, and ION/IOFF of >10^6 at room temperature. Special attention is given to lowering the contact resistance for hole injection by using high work function Pd contacts along with degenerate surface doping of the contacts by patterned NO2 chemisorption on WSe2. The results here present a promising material system and device architecture for p-type monolayer transistors with excellent characteristics.
1,176 citations
TL;DR: Due to high permittivity, the magnetic dipole resonance is observed in the visible spectral range for Si nanoparticles with diameters of ∼200 nm, thereby opening a way to the realization of isotropic optical metamaterials with strong magnetic responses in thevisible region.
Abstract: Strong resonant light scattering by individual spherical Si nanoparticles is experimentally demonstrated, revealing pronounced resonances associated with the excitation of magnetic and electric modes in these nanoparticles. It is shown that the low-frequency resonance corresponds to the magnetic dipole excitation. Due to high permittivity, the magnetic dipole resonance is observed in the visible spectral range for Si nanoparticles with diameters of ∼200 nm, thereby opening a way to the realization of isotropic optical metamaterials with strong magnetic responses in the visible region.
932 citations
TL;DR: The term nonlinear optics (NLO) was coined to describe the nonlinear relationship between dielectric polarization P and electric field E in optical media to develop materials with the ability to alter the frequency of light, to amplify light signal, and to modulate light intensity or phase factors.
Abstract: The term nonlinear optics (NLO) was coined to describe the nonlinear relationship between dielectric polarization P and electric field E in optical media. NLO is a cornerstone of the emerging field of photonics, in which photons instead of electrons are used for signal transmission and processing. The vision of photonic signal transmission, processing, and storage has attracted a great deal of attention from both the engineering and the scientific communities because of its great impact in many of the existing and future information technologies. The first step toward realization of these revolutionary technologies is to develop tools to manipulate photons. For example, it is desirable to develop materials with the ability to alter the frequency of light, to amplify light signal, and to modulate light intensity or phase factors. NLO phenomena can be the key to achieving these important functions. One of the most common NLO behaviors is second-harmonic generation (SHG), in which a NLO material mediates the “adding-up” of two photons to form a new one with twice the frequency. The SHGphenomenonwas first demonstrated by Franken et al. in 1961. In their pioneering work, a laser beam with a wavelength of 694.2 nm was irradiated through a quartz crystal and an output ultraviolet radiation with a wavelength of 347.1 nm (double frequency) was detected. After this discovery, numerous nonlinear optical phenomena have been studied and a number of NLO-active materials have been developed. Second-harmonic generation can be quantitatively described by the second-order nonlinear optical susceptibility χ, a third-rank tensor with 27 components. The tensor elements are related to each other tomeet the requirements of both inherent and structural symmetries, which greatly reduces the number of independent components of the susceptibility tensor. Only crystals in noncentrosymmetric crystal classes can have nonvanishing χ. Moreover, for material crystallizing in the noncentrosymmetric 422, 622, and 432 crystal classes, the second-order NLO response might also vanish due to structural symmetry as well as Kleinman’s symmetry. Many inorganic compounds crystallize in noncentrosymmetric space groups and have been found to be SHG active. Some important examples are potassium dihydrogen phosphate (KDP = KH2PO4), lithium niobate (LiNbO3), and barium sodium niobate (Ba2NaNb5O15). 7 New inorganic compounds have been explored for NLO applications including but not limited to metal borates 12 and metal oxides. Recent structural studies on the inorganic systems have led to a better understanding of crystal growth/packing, paving the way for potentially manipulating their crystallization tendency to form noncentrosymmetric structures. Since the 1970s molecular NLO materials, including organic, organometallic, and inorganic complexes, have been of increasing interest to synthetic chemists. 19 The existing library of organic compounds was first screened, and the urea crystal has become a SHG standard because of its high SHG efficiency and usual availability. In a microscopic view, the second-order NLO susceptibility χ is related to the first hyperpolarizability β of a molecule. According to the classical two-level model, β is enhanced by a large transition moment and a large dipole moment difference between the ground and the charge transfer excited state. A donor acceptor type of molecule often possesses both a large transition moment and a large excited state dipole moment. As a result, most of the organic SHG chromophors belong to this category. However, most of the molecules with large β values also possess a large dipole moment, which induces formation of centrosymmetric assemblies of molecules due to dipole dipole interactions. One of the methods to avoid the centrosymmetric alignment of molecular dipoles is to trap them inside the channels of asymmetric porous host structures. 28 Other methods include formation of poled polymers in which the required asymmetry is imposed by the external electric field 35 and the Langmuir Blodgett (LB)
888 citations
TL;DR: Ferroferric oxide (Fe(3)O(4)) was selected as the phase in multiwalled carbon nanotube (MWCNT)-based composites for enhancing magnetic properties to obtain improved electromagnetic attenuation and exhibited enhanced magnetic properties coupled with increased dielectric properties.
Abstract: Light-weight nanocomposites filled with carbon nanotubes (CNTs) are developed for their significant potentials in electromagnetic shielding and attenuation for wide applications in electronics, communication devices, and specific parts in aircrafts and vehicles. Specifically, the introduction of a second phase into/onto CNTs for achieving CNT-based heterostructures has been widely pursued due to the enhancement in either dielectric loss or magnetic loss. In this work, ferroferric oxide (Fe(3)O(4)) was selected as the phase in multiwalled carbon nanotube (MWCNT)-based composites for enhancing magnetic properties to obtain improved electromagnetic attenuation. A direct comparison between the two-phase heterostructures (Fe(3)O(4)/MWCNTs) and polyaniline (PANI) coated Fe(3)O(4)/MWCNTs, namely, three-phase heterostructures (PANI/Fe(3)O(4)/MWCNTs), was made to investigate the interface influences of Fe(3)O(4) and PANI on the complex permittivity and permeability separately. Compared to PANI/Fe(3)O(4)/MWCNTs, Fe(3)O(4)/MWCNTs exhibited enhanced magnetic properties coupled with increased dielectric properties. Interfaces between MWCNTs and heterostructures were found to play a role in the corresponding properties. The evaluation of microwave absorption of their wax composites was carried out, and the comparison between Fe(3)O(4)/MWCNTs and PANI/Fe(3)O(4)/MWCNTs with respect to highly efficient microwave absorption and effective absorption bandwidth was discussed.
798 citations
TL;DR: In this paper, a perspective and future development of relaxor-PbTiO3 (PT) piezoelectric materials are given. And the physical origins and unique loss characteristics in relaxorPT crystals are discussed with respect to their crystal structure.
Abstract: Ferroelectrics are essential components in a wide range of applications, including ultrasonic transducers, sensors, and actuators. In the single crystal form, relaxor-PbTiO3 (PT) piezoelectric materials have been extensively studied due to their ultrahigh piezoelectric and electromechanical properties. In this article, a perspective and future development of relaxor-PT crystals are given. Initially, various techniques for the growth of relaxor-PT crystals are reviewed, with crystals up to 100 mm in diameter and 200 mm in length being readily achievable using the Bridgman technique. Second, the characterizations of dielectric and electromechanical properties are surveyed. Boundary conditions, including temperature, electric field, and stress, are discussed in relation to device limitations. Third, the physical origins of the high piezoelectric properties and unique loss characteristics in relaxor-PT crystals are discussed with respect to their crystal structure, phase, engineered domain configuration, macr...
746 citations
TL;DR: In this paper, the authors fabricate MoS2 field effect transistors on both SiO2 and polymethyl methacrylate (PMMA) dielectrics and measure charge carrier mobility in a four-probe configuration.
Abstract: We fabricate MoS2 field effect transistors on both SiO2 and polymethyl methacrylate (PMMA) dielectrics and measure charge carrier mobility in a four-probe configuration. For multilayer MoS2 on SiO2, the mobility is 30-60 cm2/Vs, relatively independent of thickness (15-90 nm), and most devices exhibit unipolar n-type behavior. In contrast, multilayer MoS2 on PMMA shows mobility increasing with thickness, up to 470 cm2/Vs (electrons) and 480 cm2/Vs (holes) at thickness ~50 nm. The dependence of the mobility on thickness points to a long-range dielectric effect of the bulk MoS2 in increasing mobility.
613 citations
TL;DR: In this paper, a structural investigation of the electrically characterized capacitors by grazing incidence X-ray diffraction is presented in order to gain further insight on the potential origin of ferroelectricity.
Abstract: Incipient ferroelectricity is known to occur in perovskites such as SrTiO3, KTaO3, and CaTiO3. For the first time it is shown that the intensively researched HfO2 thin films (16 nm) also possess ferroelectric properties when aluminium is incorporated into the host lattice. Polarization measurements on Al:HfO2 based metal–insulator–metal capacitors show an antiferroelectric-to-ferroelectric phase transition depending on annealing conditions and aluminium content. Structural investigation of the electrically characterized capacitors by grazing incidence X-ray diffraction is presented in order to gain further insight on the potential origin of ferroelectricity. The non-centrosymmetry of the elementary cell, which is essential for ferroelectricity, is assumed to originate from an orthorhombic phase of space group Pbc21 stabilized for low Al doping in HfO2. The ferroelectric properties of the modified HfO2 thin films yield high potential for various ferroelectric, piezoelectric, and pyroelectric applications. Furthermore, due to the extensive knowledge accumulated by various research groups regarding the HfO2 dielectric, an immediate relevance of ferroelectric hafnium oxide thin films is anticipated by the authors.
608 citations
TL;DR: A benchmark for force fields is devised in order to test the ability of existing force fields to reproduce some key properties of organic liquids, namely, the density, enthalpy of vaporization, the surface tension, the heat capacity at constant volume and pressure, the isothermal compressibility, the volumetric expansion coefficient, and the static dielectric constant.
Abstract: The chemical composition of small organic molecules is often very similar to amino acid side chains or the bases in nucleic acids, and hence there is no a priori reason why a molecular mechanics force field could not describe both organic liquids and biomolecules with a single parameter set. Here, we devise a benchmark for force fields in order to test the ability of existing force fields to reproduce some key properties of organic liquids, namely, the density, enthalpy of vaporization, the surface tension, the heat capacity at constant volume and pressure, the isothermal compressibility, the volumetric expansion coefficient, and the static dielectric constant. Well over 1200 experimental measurements were used for comparison to the simulations of 146 organic liquids. Novel polynomial interpolations of the dielectric constant (32 molecules), heat capacity at constant pressure (three molecules), and the isothermal compressibility (53 molecules) as a function of the temperature have been made, based on expe...
602 citations
TL;DR: Experimental evidence is presented that a single low-loss dielectric subwavelength sphere of moderate refractive index radiates fields identical to those from equal amplitude crossed electric and magnetic dipoles, and indistinguishable from those of ideal magnetodielectric spheres, and these Kerker scattering conditions only depend on a/λ.
Abstract: Magnetodielectric small spheres present unusual electromagnetic scattering features, theoretically predicted a few decades ago. However, achieving such behaviour has remained elusive, due to the non-magnetic character of natural optical materials or the difficulty in obtaining low-loss highly permeable magnetic materials in the gigahertz regime. Here we present unambiguous experimental evidence that a single low-loss dielectric subwavelength sphere of moderate refractive index (n=4 like some semiconductors at near-infrared) radiates fields identical to those from equal amplitude crossed electric and magnetic dipoles, and indistinguishable from those of ideal magnetodielectric spheres. The measured scattering radiation patterns and degree of linear polarization (3–9 GHz/33–100 mm range) show that, by appropriately tuning the a/λ ratio, zero-backward (‘Huygens’ source) or almost zero-forward (‘Huygens’ reflector) radiated power can be obtained. These Kerker scattering conditions only depend on a/λ. Our results open new technological challenges from nano- and micro-photonics to science and engineering of antennas, metamaterials and electromagnetic devices. The absence of forward or backward scattered radiation by magnetodielectric spheres was predicted decades ago, yet direct measurements have remained elusive. Geffrin et al. present unambiguous evidence of such scattering effects in the gigahertz range for a sub-wavelength dielectric sphere.
539 citations
TL;DR: In this paper, orientational polarization in polar polymers can be utilized for high energy density and low loss dielectrics, which can be used for next-generation dielectric capacitors for pulsed power and power conditioning applications.
Abstract: The state-of-the-art polymer dielectrics have been limited to nonpolar polymers with relatively low energy density but ultralow dielectric losses for the past decades. With the fast development of power electronics in pulsed power and power conditioning applications, there is a need for next-generation dielectric capacitors in areas of high energy density/low loss and/or high temperature/low loss polymer dielectrics. Given limitations in further enhancing atomic and electronic polarizations for polymers, this Perspective focuses on a fundamental question: Can orientational polarization in polar polymers be utilized for high energy density and low loss dielectrics? Existing experimental and theoretical results have suggested the following perspectives. For amorphous polar polymers, high energy density and low loss can be achieved below their glass transition temperatures. For liquid crystalline side-chain polymers, dipole mobility is so high that they saturate at relatively low electric fields, and only li...
TL;DR: Control of light at the nanoscale is demanding for future successful on-chip integration and most optical nanoantennas consist of plasmonic nanoparticles due to their ability to capture and concentrate visible light at subwavelength dimensions.
Abstract: We study in detail a novel type of optical nanoantennas made of high-permittivity low-loss dielectric particles. In addition to the electric resonances, the dielectric particles exhibit very strong magnetic resonances at the nanoscale, that can be employed in the Yagi-Uda geometry for creating highly efficient optical nanoantennas. By comparing plasmonic and dielectric nanoantennas, we demonstrate that all-dielectric nanoantennas may exhibit better radiation efficiency also allowing more compact design.
TL;DR: The synthesis of large-area h-BN film is reported using atmospheric pressure chemical vapor deposition on a copper foil, followed by Cu etching and transfer to a target substrate, and the mobility of the CVD graphene device remains the same before and after device integration.
Abstract: Hexagonal boron nitride (h-BN) is a promising material as a dielectric layer or substrate for two-dimensional electronic devices. In this work, we report the synthesis of large-area h-BN film using atmospheric pressure chemical vapor deposition on a copper foil, followed by Cu etching and transfer to a target substrate. The growth rate of h-BN film at a constant temperature is strongly affected by the concentration of borazine as a precursor and the ambient gas condition such as the ratio of hydrogen and nitrogen. h-BN films with different thicknesses can be achieved by controlling the growth time or tuning the growth conditions. Transmission electron microscope characterization reveals that these h-BN films are polycrystalline, and the c-axis of the crystallites points to different directions. The stoichiometry ratio of boron and nitrogen is close to 1:1, obtained by electron energy loss spectroscopy. The dielectric constant of h-BN film obtained by parallel capacitance measurements (25 μm2 large areas) ...
TL;DR: For SERS from carbon nanoparticles, it is found that the dielectric screening introduced by the nanoparticle can drastically redistribute the field enhancement associated with the Fano resonance and lead to a significantly modified SERS response compared to what would be anticipated from the bare nanocluster.
Abstract: While the far field properties of Fano resonances are well-known, clusters of plasmonic nanoparticles also possess Fano resonances with unique and spatially complex near field properties. Here we examine the near field properties of individual Fano resonant plasmonic clusters using surface-enhanced Raman scattering (SERS) both from molecules distributed randomly on the structure and from dielectric nanoparticles deposited at specific locations within the cluster. Cluster size, geometry, and interparticle spacing all modify the near field properties of the Fano resonance. For molecules, the spatially dependent SERS response obtained from near field calculations correlates well with the relative SERS intensities observed for individual clusters and for specific Stokes modes of a para-mercaptoaniline adsorbate. In all cases, the largest SERS enhancement is found when both the excitation and the Stokes shifted wavelengths overlap the Fano resonances. In contrast, for SERS from carbon nanoparticles we find that the dielectric screening introduced by the nanoparticle can drastically redistribute the field enhancement associated with the Fano resonance and lead to a significantly modified SERS response compared to what would be anticipated from the bare nanocluster.
TL;DR: In this article, the maximum reflection loss reached −45.1 dB with a thickness of the absorber of only 2.5 mm, and the Debye relaxation processes in graphene/polyaniline nanorod arrays are improved compared to polyanilines nanorods.
Abstract: In the paper, we find that graphene has a strong dielectric loss, but exhibits very weak attenuation properties to electromagnetic waves due to its high conductivity. As polyaniline nanorods are perpendicularly grown on the surface of graphene by an in situ polymerization process, the electromagnetic absorption properties of the nanocomposite are significantly enhanced. The maximum reflection loss reaches −45.1 dB with a thickness of the absorber of only 2.5 mm. Theoretical simulation in terms of the Cole–Cole dispersion law shows that the Debye relaxation processes in graphene/polyaniline nanorod arrays are improved compared to polyaniline nanorods. The enhanced electromagnetic absorption properties are attributed to the unique structural characteristics and the charge transfer between graphene and polyaniline nanorods. Our results demonstrate that the deposition of other dielectric nanostructures on the surface of graphene sheets is an efficient way to fabricate lightweight materials for strong electromagnetic wave absorbents.
TL;DR: Detailed Cu 2p, Cu LVV, O 1s and O KLL spectra are reported which are in good agreement with previous results and are mainly due to modifications of the 3d and O 2p electron configurations.
Abstract: The electronic and optical properties of Cu, CuO and Cu(2)O were studied by x-ray photoelectron spectroscopy (XPS) and reflection electron energy-loss spectroscopy (REELS). We report detailed Cu 2p, Cu LVV, O 1s and O KLL spectra which are in good agreement with previous results. REELS spectra, recorded for primary energies in the range from 150 to 2000 eV, were corrected for multiple inelastically scattered electrons to determine the effective inelastic scattering cross section. The dielectric functions and optical properties were determined by comparing the experimental inelastic electron scattering cross section with a simulated cross section calculated within the semi-classical dielectric response model in which the only input is Im(-1/e) by using the QUEELS-e(k,ω)-REELS software package. By Kramers-Kronig transformation of the determined Im(-1/e), the real and imaginary parts (e(1) and e(2)) of the dielectric function, and the refractive index n and extinction coefficient k were determined for Cu, CuO, and Cu(2)O in the 0-100 eV energy range. Observed differences between Cu, CuO and Cu(2)O are mainly due to modifications of the 3d and O 2p electron configurations.
TL;DR: This work demonstrates, for the first time, an all-dielectric metamaterial composite in the midinfrared based on micron-sized, high-index tellurium dielectric resonators, and provides evidence of optical magnetism.
Abstract: We demonstrate, for the first time, an all-dielectric metamaterial composite in the midinfrared based on micron-sized, high-index tellurium dielectric resonators. Dielectric resonators are desirable compared to conventional metallodielectric metamaterials at optical frequencies as they are largely angular invariant, free of Ohmic loss, and easily integrated into three-dimensional volumes. Measurements and simulation provide evidence of optical magnetism, which could be used for infrared magnetic mirrors, hard or soft surfaces, and subwavelength cavities.
TL;DR: It is shown that owing to the coupling between magnetization and ferro electric polarization at the interface between the electrode and barrier of a multiferroic tunnel junction, the spin polarization of the tunnelling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization ofThe barrier.
Abstract: Spin-polarized transport in ferromagnetic tunnel junctions, characterized by tunnel magnetoresistance, has already been proven to have great potential for application in the field of spintronics and in magnetic random access memories. Until recently, in such a junction the insulating barrier played only a passive role, namely to facilitate electron tunnelling between the ferromagnetic electrodes. However, new possibilities emerged when ferroelectric materials were used for the insulating barrier, as these possess a permanent dielectric polarization switchable between two stable states. Adding to the two different magnetization alignments of the electrode, four non-volatile states are therefore possible in such multiferroic tunnel junctions. Here, we show that owing to the coupling between magnetization and ferroelectric polarization at the interface between the electrode and barrier of a multiferroic tunnel junction, the spin polarization of the tunnelling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization of the barrier. Selecting the spin direction of the tunnelling electrons by short electric pulses in the nanosecond range rather than by an applied magnetic field enables new possibilities for spin control in spintronic devices.
TL;DR: A free-standing polymer electrolyte called an ion gel is employed in both organic and inorganic thin-film transistors as a high capacitance gate dielectric.
Abstract: A free-standing polymer electrolyte called an ion gel is employed in both organic and inorganic thin-film transistors as a high capacitance gate dielectric. To prepare a transistor, the free-standing ion gel is simply laid over a semiconductor channel and a side-gate electrode, which is possible because of the gel's high mechanical strength.
TL;DR: This work establishes a unique way to obtain colossal magnetodielectricity, independent of any striction effects, by engineering the asymmetric hopping contribution to the dielectric constant via the tuning of the relative-spin orientations between neighboring magnetic ions in a transition-metal oxide system.
Abstract: We report magnetic, dielectric, and magnetodielectric responses of the pure monoclinic bulk phase of partially disordered La2NiMnO6, exhibiting a spectrum of unusual properties and establish that this compound is an intrinsically multiglass system with a large magnetodielectric coupling (8%-20%) over a wide range of temperatures (150-300 K). Specifically, our results establish a unique way to obtain colossal magnetodielectricity, independent of any striction effects, by engineering the asymmetric hopping contribution to the dielectric constant via the tuning of the relative-spin orientations between neighboring magnetic ions in a transition-metal oxide system. We discuss the role of antisite (Ni-Mn) disorder in emergence of these unusual properties.
TL;DR: Nanophotonics aims to find reliable solutions to enhance the absorption of light in thin films, and engineering the absorbing material at the nanoscale indeed leads to interferences that can significantly increase light absorption.
Abstract: Elaborating reliable and versatile strategies for efficient light coupling between free space and thin films is of crucial importance for new technologies in energy efficiency. Nanostructured materials have opened unprecedented opportunities for light management, notably in thin-film solar cells. Efficient coherent light trapping has been accomplished through the careful design of plasmonic nanoparticles and gratings, resonant dielectric particles and photonic crystals. Alternative approaches have used randomly textured surfaces as strong light diffusers to benefit from their broadband and wide-angle properties. Here, we propose a new strategy for photon management in thin films that combines both advantages of an efficient trapping due to coherent optical effects and broadband/wide-angle properties due to disorder. Our approach consists of the excitation of electromagnetic modes formed by multiple light scattering and wave interference in two-dimensional random media. We show, by numerical calculations, that the spectral and angular responses of thin films containing disordered photonic patterns are intimately related to the in-plane light transport process and can be tuned through structural correlations. Our findings, which are applicable to all waves, are particularly suited for improving the absorption efficiency of thin-film solar cells and can provide a new approach for high-extraction-efficiency light-emitting diodes.
TL;DR: In this article, the effects of filler shape, surface modification and polarity of polymer matrix on the microstructure, dielectric constants and breakdown strength of polymer composites are investigated in detail.
Abstract: Flexible polymer composite films are prepared by a solution cast method with polar polyvinylidene fluoride (PVDF) or non-polar epoxy as the polymer matrix. BaTiO3 nanoparticles and BaTiO3 nanofibers with large aspect ratio are used as dielectric fillers after surface modification by polydopamine. The effects of filler shape, surface modification and polarity of polymer matrix on the microstructure, dielectric constants and breakdown strength of polymer composites are investigated in detail. Surface modification by polydopamine improves the compatibility between BaTiO3 and polymer as well as passivating the surfaces of BaTiO3. At the same volume fraction, composites filled with BaTiO3 nanofibers exhibit greater dielectric constants than the composites filled with BaTiO3 nanoparticles. The polydopamine layers on BaTiO3 nanofibers give rise to stronger interfaces between the fillers and polymer matrices. Improved breakdown strengths are achieved in both composites. This work may provide a general strategy for flexible polymer nanocomposites with greatly enhanced dielectric constants and breakdown strength.
TL;DR: In this paper, X-ray diffraction patterns and transmission electron microscopy images confirmed the formation of PANI coating and incorporation of BaTiO3 or Fe3O4 nanoparticles.
Abstract: Composite absorbers based on conducting fabrics possessing moderate conductivity and dielectric/magnetic properties were prepared by in situ incorporation of nanoparticles of BaTiO3 (15–25 nm) or Fe3O4 (25–40 nm) within coated poly(aniline) (PANI) matrix. The X-ray diffraction patterns and transmission electron microscopy images confirmed the formation of PANI coating and incorporation of BaTiO3 or Fe3O4 nanoparticles. Scanning electron microscopy images show formation of thick and uniform coating of PANI over individual fibers and in interweave regions. The dielectric studies show that incorporation of BaTiO3 lead to enhancement of dielectric properties of PANI whereas magnetization measurements revealed that incorporation of Fe3O4 resulted in noticeable improvement in magnetic properties with saturation magnetization of 17.9 emu/g. The Ku-band (12.4–18.0 GHz) shielding studies revealed that pure PANI-coated fabric show total shielding effectiveness (SET) of −15.3 dB which enhanced to −16.8 and −19.4 dB ...
TL;DR: A survey of the key points for the design and development of textile antennas, from the choice of the textile materials to the framing of the antenna is presented.
Abstract: In the broad context of Wireless Body Sensor Networks for healthcare and pervasive applications, the design of wearable antennas offers the possibility of ubiquitous monitoring, communication and energy harvesting and storage. Specific requirements for wearable antennas are a planar structure and flexible construction materials. Several properties of the materials influence the behaviour of the antenna. For instance, the bandwidth and the efficiency of a planar microstrip antenna are mainly determined by the permittivity and the thickness of the substrate. The use of textiles in wearable antennas requires the characterization of their properties. Specific electrical conductive textiles are available on the market and have been successfully used. Ordinary textile fabrics have been used as substrates. However, little information can be found on the electromagnetic properties of regular textiles. Therefore this paper is mainly focused on the analysis of the dielectric properties of normal fabrics. In general, textiles present a very low dielectric constant that reduces the surface wave losses and increases the impedance bandwidth of the antenna. However, textile materials are constantly exchanging water molecules with the surroundings, which affects their electromagnetic properties. In addition, textile fabrics are porous, anisotropic and compressible materials whose thickness and density might change with low pressures. Therefore it is important to know how these characteristics influence the behaviour of the antenna in order to minimize unwanted effects. This paper presents a survey of the key points for the design and development of textile antennas, from the choice of the textile materials to the framing of the antenna. An analysis of the textile materials that have been used is also presented.
TL;DR: This Account describes how magnetic fields can guide the assembly of superparamagnetic colloidal building blocks into periodically arranged particle arrays and how the photonic properties of the resulting structures can be reversibly tuned by manipulating the external magnetic fields.
Abstract: Responsive photonic structures can respond to external stimuli by transmitting optical signals. Because of their important technological applications such as color signage and displays, biological and chemical sensors, security devices, ink and paints, military camouflage, and various optoelectronic devices, researchers have focused on developing these functional materials. Conventionally, self-assembled colloidal crystals containing periodically arranged dielectric materials have served as the predominant starting frameworks. Stimulus-responsive materials are incorporated into the periodic structures either as the initial building blocks or as the surrounding matrix so that the photonic properties can be tuned. Although researchers have proposed various versions of responsive photonic structures, the low efficiency of fabrication through self-assembly, narrow tunability, slow responses to the external stimuli, incomplete reversibility, and the challenge of integrating them into existing photonic devices ...
TL;DR: In this article, BaTiO3 nanofibers with a large aspect ratio prepared via electrospinning and modified by dopamine were used as dielectric fillers in polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE)-based nanocomposites.
Abstract: BaTiO3 nanofibers with a large aspect ratio prepared via electrospinning and modified by dopamine were used as dielectric fillers in poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE)-based nanocomposites. Highly flexible polymer nanocomposite films were fabricated via a simple solution-cast method. Enhanced dielectric permittivities were obtained at a low volume fraction of BaTiO3 nanofibers. The breakdown strength of the polymer nanocomposites was also improved, which is favorable for enhanced ferroelectric properties in the nanocomposites. Pr ∼9.1 μC cm−2 was achieved in the nanocomposites with 10.8 vol% BaTiO3 nanofibers. The improved breakdown strength and enhanced ferroelectric properties are attributed to the combined effect of the surface modification by dopamine, the large aspect ratio of the BaTiO3 nanofibers and the improved crystallinity of the polymer nanocomposites induced by the BaTiO3 nanofibers.
TL;DR: In this paper, a comprehensive definition of relaxors is proposed: relaxors are defined as ferroelectrics in which the maximum in the temperature dependence of static susceptibility occurs within the temperature range of dielectric relaxation, but does not coincide with the temperature of singularity of relaxation time or soft mode frequency.
Abstract: In this review the dielectric properties of relaxor ferroelectrics are discussed and compared with the properties of normal dielectrics and ferroelectrics. We try to draw a general picture of dielectric relaxation starting from a textbook review of the underlying concepts and pay attention to common behavior of relaxors rather than to the features observed in specific materials. We hope that this general approach is beneficial to those physicists, chemists, material scientists and device engineers who deal with relaxors. Based on the analysis of dielectric properties, a comprehensive definition of relaxors is proposed: relaxors are defined as ferroelectrics in which the maximum in the temperature dependence of static susceptibility occurs within the temperature range of dielectric relaxation, but does not coincide with the temperature of singularity of relaxation time or soft mode frequency.
TL;DR: Higher dielectric constant and lower loss factor were simultaneously achieved in rGO-PVA/PVDF nanocomposites at a frequency range lower than 1 × 10³ Hz, which provides a potential design strategy based on graphene interface engineering, which would lead to higher-performance flexible dielectrics materials.
Abstract: In this work, two series of nanocomposites of poly(vinylidene fluoride) (PVDF) incorporated with reduced graphene oxide (rGO) and poly(vinyl alcohol)-modified rGO (rGO-PVA) were fabricated using solution-cast method and their dielectric properties were carefully characterized. Infrared spectroscopy and atom force microscope analysis indicated that PVA chains were successfully grafted onto graphene through ester linkage. The PVA functionalization of graphene surface can not only prevent the agglomeration of original rGO but also enhance the interaction between PVDF and rGO-PVA. Strong hydrogen bonds and charge transfer effect between rGO-PVA and PVDF were determined by infrared and Raman spectroscopies. The dielectric properties of rGO-PVA/PVDF and rGO/PVDF nanocomposites were investigated in a frequency range from 10² Hz to 10⁷ Hz. Both composite systems exhibited an insulator-to-conductor percolating transition as the increase of the filler content. The percolation thresholds were estimated to be 2.24 vol % for rGO-PVA/PVDF composites and 0.61 vol % for rGO/PVDF composites, respectively. Near the percolation threshold, the dielectric permittivity of the nanocomposites was significantly promoted, which can be well explained by interfacial polarization effect and microcapacitor model. Compared to rGO/PVDF composites, higher dielectric constant and lower loss factor were simultaneously achieved in rGO-PVA/PVDF nanocomposites at a frequency range lower than 1 × 10³ Hz. This work provides a potential design strategy based on graphene interface engineering, which would lead to higher-performance flexible dielectric materials.
Patent•
08 Jun 2012TL;DR: In this paper, a dielectric antenna is divided into a first region and a second region, and the second region could have a bending portion, and a conductor covers a surface of the second regions of the unit to form a waveguide structure.
Abstract: A dielectric antenna includes at least one dielectric unit. Each dielectric unit is separated into a first region and a second region, and the second region could have a bending portion. A conductor covers a surface of the second region of the dielectric unit to form a waveguide structure. The waveguide structure has a first endpoint connected to the first region and a second endpoint serving as a signal feeding terminal for feeding or receiving signals.
TL;DR: It is shown that non-toxic ferroelectrics like BaTiO3 with dense patterns of charged domain walls are expected to have strongly enhanced piezoelectric properties, thus suggesting a new route to high-performance, lead-free ferro electrics.
Abstract: While commonly used piezoelectric materials contain lead, non-hazardous, high-performance piezoelectrics are yet to be discovered. Charged domain walls in ferroelectrics are considered inactive with regards to the piezoelectric response and, therefore, are largely ignored in this search. Here we demonstrate a mechanism that leads to a strong enhancement of the dielectric and piezoelectric properties in ferroelectrics with increasing density of charged domain walls. We show that an incomplete compensation of bound polarization charge at these walls creates a stable built-in depolarizing field across each domain leading to increased electromechanical response. Our model clarifies a long-standing unexplained effect of domain wall density on macroscopic properties of domain-engineered ferroelectrics. We show that non-toxic ferroelectrics like BaTiO(3) with dense patterns of charged domain walls are expected to have strongly enhanced piezoelectric properties, thus suggesting a new route to high-performance, lead-free ferroelectrics.