Showing papers on "Dielectric published in 2016"

Recent Progress on Ferroelectric Polymer-Based Nanocomposites for High Energy Density Capacitors: Synthesis, Dielectric Properties, and Future Aspects.

Abstract: Dielectric polymer nanocomposites are rapidly emerging as novel materials for a number of advanced engineering applications. In this Review, we present a comprehensive review of the use of ferroelectric polymers, especially PVDF and PVDF-based copolymers/blends as potential components in dielectric nanocomposite materials for high energy density capacitor applications. Various parameters like dielectric constant, dielectric loss, breakdown strength, energy density, and flexibility of the polymer nanocomposites have been thoroughly investigated. Fillers with different shapes have been found to cause significant variation in the physical and electrical properties. Generally, one-dimensional and two-dimensional nanofillers with large aspect ratios provide enhanced flexibility versus zero-dimensional fillers. Surface modification of nanomaterials as well as polymers adds flavor to the dielectric properties of the resulting nanocomposites. Nowadays, three-phase nanocomposites with either combination of fillers...

Giant Energy Density and Improved Discharge Efficiency of Solution-Processed Polymer Nanocomposites for Dielectric Energy Storage.

Abstract: Large-aspect-ratio composite nanofibers with interior hierarchical interfaces are employed to break the adverse coupling of electric displacement and breakdown strength in flexible poly(vinylidene fluoride-hexafluoropropylene) nanocomposite films, a small loading of 3 vol% BaTiO3@TiO2 nanofibers gives rise to the highestenergy density (≈31.2 J cm(-3)) ever achieved in polymer nanocomposites dielectrics.

Facile Synthesis of Fe3O4/GCs Composites and Their Enhanced Microwave Absorption Properties

Abstract: Graphene has good stability and adjustable dielectric properties along with tunable morphologies, and hence can be used to design novel and high-performance functional materials. Here, we have reported a facile synthesis method of nanoscale Fe3O4/graphene capsules (GCs) composites using the combination of catalytic chemical vapor deposition (CCVD) and hydrothermal process. The resulting composite has the advantage of unique morphology that offers better synergism among the Fe3O4 particles as well as particles and GCs. The microwave-absorbing characteristics of developed composites were investigated through experimentally measured electromagnetic properties and simulation studies based on the transmission line theory, explained on the basis of eddy current, natural and exchange resonance, as well as dielectric relaxation processes. The composites bear minimum RL value of −32 dB at 8.76 GHz along with the absorption bandwidth range from 5.4 to 17 GHz for RL lower than −10 dB. The better performance of the c...

The origin of ultrahigh piezoelectricity in relaxor-ferroelectric solid solution crystals

Abstract: The discovery of ultrahigh piezoelectricity in relaxor-ferroelectric solid solution single crystals is a breakthrough in ferroelectric materials. A key signature of relaxor-ferroelectric solid solutions is the existence of polar nanoregions, a nanoscale inhomogeneity, that coexist with normal ferroelectric domains. Despite two decades of extensive studies, the contribution of polar nanoregions to the underlying piezoelectric properties of relaxor ferroelectrics has yet to be established. Here we quantitatively characterize the contribution of polar nanoregions to the dielectric/piezoelectric responses of relaxor-ferroelectric crystals using a combination of cryogenic experiments and phase-field simulations. The contribution of polar nanoregions to the room-temperature dielectric and piezoelectric properties is in the range of 50-80%. A mesoscale mechanism is proposed to reveal the origin of the high piezoelectricity in relaxor ferroelectrics, where the polar nanoregions aligned in a ferroelectric matrix can facilitate polarization rotation. This mechanism emphasizes the critical role of local structure on the macroscopic properties of ferroelectric materials.

Broadband high-efficiency dielectric metasurfaces for the visible spectrum

Abstract: Metasurfaces are planar optical elements that hold promise for overcoming the limitations of refractive and conventional diffractive optics. Original dielectric metasurfaces are limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. Thus, it is critical that new materials and nanofabrication techniques be developed to extend dielectric metasurfaces across the visible spectrum and to enable applications such as high numerical aperture lenses, color holograms, and wearable optics. Here, we demonstrate high performance dielectric metasurfaces in the form of holograms for red, green, and blue wavelengths with record absolute efficiency (>78%). We use atomic layer deposition of amorphous titanium dioxide with surface roughness less than 1 nm and negligible optical loss. We use a process for fabricating dielectric metasurfaces that allows us to produce anisotropic, subwavelength-spaced dielectric nanostructures with shape birefringence. This process is capable of realizing any high-efficiency metasurface optical element, e.g., metalenses and axicons.

Porous CNTs/Co Composite Derived from Zeolitic Imidazolate Framework: A Lightweight, Ultrathin, and Highly Efficient Electromagnetic Wave Absorber

Abstract: Porous carbon nanotubes/cobalt nanoparticles (CNTs/Co) composite with dodecahedron morphology was synthesized by in situ pyrolysis of the Co-based zeolitic imidazolate framework in a reducing atmosphere. The morphology and microstructure of the composite can be well tuned by controlling the pyrolysis conditions. At lower pyrolysis temperature, the CNTs/Co composite is composed of well-dispersed Co nanoparticles and short CNT clusters with low graphitic degree. The increase of pyrolysis temperature/time promotes the growth and graphitization of CNTs and leads to the aggregation of Co nanoparticles. The optimized CNTs/Co composite exhibits strong dielectric and magnetic losses as well as a good impedance matching property. Interestingly, the CNTs/Co composite displays extremely strong electromagnetic wave absorption with a maximum reflection loss of −60.4 dB. More importantly, the matching thickness of the absorber is as thin as 1.81 mm, and the filler loading of composite in the matrix is only 20 wt %. The...

Resonantly Enhanced Second-Harmonic Generation Using III-V Semiconductor All-Dielectric Metasurfaces.

Abstract: Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced nonlinear optical processes due to their larger mode volumes. Here, we present, for the first time, resonantly enhanced second-harmonic generation (SHG) using gallium arsenide (GaAs) based dielectric metasurfaces. Using arrays of cylindrical resonators we observe SHG enhancement factors as large as 10(4) relative to unpatterned GaAs. At the magnetic dipole resonance, we measure an absolute nonlinear conversion efficiency of ∼2 × 10(-5) with ∼3.4 GW/cm(2) pump intensity. The polarization properties of the SHG reveal that both bulk and surface nonlinearities play important roles in the observed nonlinear process.

Multiwavelength polarization-insensitive lenses based on dielectric metasurfaces with meta-molecules

Abstract: Metasurfaces are nano-structured devices composed of arrays of subwavelength scatterers (or meta-atoms) that manipulate the wavefront, polarization, or intensity of light. Like other diffractive optical devices, metasurfaces suffer from significant chromatic aberrations that limit their bandwidth. Here, we present a method for designing multiwavelength metasurfaces using unit cells with multiple meta-atoms, or meta-molecules. Transmissive lenses with efficiencies as high as 72% and numerical apertures as high as 0.46 simultaneously operating at 915 nm and 1550 nm are demonstrated. With proper scaling, these devices can be used in applications where operation at distinct known wavelengths is required, like various fluorescence microscopy techniques.

1D/2D Carbon Nanomaterial‐Polymer Dielectric Composites with High Permittivity for Power Energy Storage Applications

Abstract: With the development of flexible electronic devices and large-scale energy storage technologies, functional polymer-matrix nanocomposites with high permittivity (high-k) are attracting more attention due to their ease of processing, flexibility, and low cost. The percolation effect is often used to explain the high-k characteristic of polymer composites when the conducting functional fillers are dispersed into polymers, which gives the polymer composite excellent flexibility due to the very low loading of fillers. Carbon nanotubes (CNTs) and graphene nanosheets (GNs), as one-dimensional (1D) and two-dimensional (2D) carbon nanomaterials respectively, have great potential for realizing flexible high-k dielectric nanocomposites. They are becoming more attractive for many fields, owing to their unique and excellent advantages. The progress in dielectric fields by using 1D/2D carbon nanomaterials as functional fillers in polymer composites is introduced, and the methods and mechanisms for improving dielectric properties, breakdown strength and energy storage density of their dielectric nanocomposites are examined. Achieving a uniform dispersion state of carbon nanomaterials and preventing the development of conductive networks in their polymer composites are the two main issues that still need to be solved in dielectric fields for power energy storage. Recent findings, current problems, and future perspectives are summarized.

Highly Sensitive, Flexible, and Wearable Pressure Sensor Based on a Giant Piezocapacitive Effect of Three-Dimensional Microporous Elastomeric Dielectric Layer.

Abstract: We report a flexible and wearable pressure sensor based on the giant piezocapacitive effect of a three-dimensional (3-D) microporous dielectric elastomer, which is capable of highly sensitive and stable pressure sensing over a large tactile pressure range. Due to the presence of micropores within the elastomeric dielectric layer, our piezocapacitive pressure sensor is highly deformable by even very small amounts of pressure, leading to a dramatic increase in its sensitivity. Moreover, the gradual closure of micropores under compression increases the effective dielectric constant, thereby further enhancing the sensitivity of the sensor. The 3-D microporous dielectric layer with serially stacked springs of elastomer bridges can cover a much wider pressure range than those of previously reported micro-/nanostructured sensing materials. We also investigate the applicability of our sensor to wearable pressure-sensing devices as an electronic pressure-sensing skin in robotic fingers as well as a bandage-type pr...

Small magnetic nanoparticles decorating reduced graphene oxides to tune the electromagnetic attenuation capacity

Abstract: High efficiency and lightweight are key factors for microwave absorption materials. Searching for the above necessary features is still a great challenge. Herein, we deposited small magnetic nickel ferrite nanoparticles on reduced graphene oxide nanosheets uniformly (NiFe2O4/r-GO) using a facile one-pot hydrothermal method with free of chemical agents, and investigated their permittivity, permeability and microwave absorption. Notably, we find an effective strategy for tuning microwave attenuation by the synergistic effect of dielectric and magnetic loss, which originates from inducing multiple relaxations and multiple resonances. The best impedance matching of NiFe2O4/r-GO was sought out. The minimum reflection loss (RL) can reach −42 dB with a broad bandwidth (RL ≤ −10 dB) of 5.3 GHz. Meanwhile, the multiple regions endow the absorbers with selectivity for efficient absorption. Our results demonstrate that the as-prepared NiFe2O4/r-GO is a promising candidate for application in communication devices, high speed processors, information security, electronic countermeasures and electromagnetic interference shielding.

Magnetic graphene@PANI@porous TiO2 ternary composites for high-performance electromagnetic wave absorption

Abstract: A two-step strategy combining in situ polymerization and a hydrothermal process has been developed for coupling polyaniline (PANI) with porous TiO2 anchored on magnetic graphene. The microstructure and morphology of magnetic graphene@PANI@porous TiO2 were characterized by FETEM, FESEM, XRD, XPS and VSM in detail. The results indicated that magnetic graphene@PANI was completely covered by porous TiO2 with random orientations and the saturation magnetization value of the composite was 19.2 emu g−1. PANI was used to decrease the absorber thickness, while the porous TiO2 with a large surface area was designed to enhance the interaction between the electromagnetic (EM) wave and the absorber through multiple reflections, thus enhancing EM wave absorption properties. As an EM wave absorber, the maximum reflection loss of the composite was up to −45.4 dB due to the better normalized characteristic impedance (close to 1) at a thickness of only 1.5 mm and the absorption bandwidths exceeding −10 dB were 11.5 GHz when the thickness ranged from 1 to 3.5 mm. The excellent EM wave absorption performance was ascribed to the combined contribution from the enhanced dielectric relaxation processes, the unique porous nanostructures, the quarter-wave length matching model and the well-matched normalized characteristic impedance. Consequently, it is believed that the composite could be used as an excellent EM wave absorption material and the two-step strategy offered an effective way to design a high-performance EM wave absorber with a relatively thin thickness.

Invited article: Broadband highly-efficient dielectric metadevices for polarization control

Abstract: Metadevices based on dielectric nanostructured surfaces with both electric and magnetic Mie-type resonances have resulted in the best efficiency to date for functional flat optics with only one disadvantage: a narrow operational bandwidth Here we experimentally demonstrate broadband transparent all-dielectric metasurfaces for highly efficient polarization manipulation We utilize the generalized Huygens principle, with a superposition of the scattering contributions from several electric and magnetic multipolar modes of the constituent meta-atoms, to achieve destructive interference in reflection over a large spectral bandwidth By employing this novel concept, we demonstrate reflectionless (∼90% transmission) half-wave plates, quarter-wave plates, and vector beam q-plates that can operate across multiple telecom bands with ∼99% polarization conversion efficiency

Electromagnetic Property and Tunable Microwave Absorption of 3D Nets from Nickel Chains at Elevated Temperature

Abstract: We fabricated the nickel chains by a facile wet chemical method. The morphology of nickel chains were tailored by adjusting the amount of PVP during the synthesis process. Both the complex permittivity and permeability of the three-dimensional (3D) nets constructed by nickel chains present strong dependences on temperature in the frequency range of 8.2–12.4 GHz and temperature range of 323–573 K. The peaks in imaginary component of permittivity and permeability mainly derive from interfacial polarizations and resonances, devoting to dielectric and magnetic loss, respectively. The effect from both dielectric and magnetism contribute to enhancing the microwave absorption. The maximum absorption value of the 3D nickel chain nets is approximately −50 dB at 8.8 GHz and 373 K with a thickness of 1.8 mm, and the bandwidth less than −10 dB almost covers the whole investigated frequency band. These are encouraging findings, which provide the potential advantages of magnetic transition metal-based materials for mic...

Negative capacitance in multidomain ferroelectric superlattices

Abstract: The stability of spontaneous electrical polarization in ferroelectrics is fundamental to many of their current applications, which range from the simple electric cigarette lighter to non-volatile random access memories1. Research on nanoscale ferroelectrics reveals that their behaviour is profoundly different from that in bulk ferroelectrics, which could lead to new phenomena with potential for future devices2, 3, 4. As ferroelectrics become thinner, maintaining a stable polarization becomes increasingly challenging. On the other hand, intentionally destabilizing this polarization can cause the effective electric permittivity of a ferroelectric to become negative5, enabling it to behave as a negative capacitance when integrated in a heterostructure. Negative capacitance has been proposed as a way of overcoming fundamental limitations on the power consumption of field-effect transistors6. However, experimental demonstrations of this phenomenon remain contentious7. The prevalent interpretations based on homogeneous polarization models are difficult to reconcile with the expected strong tendency for domain formation8, 9, but the effect of domains on negative capacitance has received little attention5, 10, 11, 12. Here we report negative capacitance in a model system of multidomain ferroelectric–dielectric superlattices across a wide range of temperatures, in both the ferroelectric and paraelectric phases. Using a phenomenological model, we show that domain-wall motion not only gives rise to negative permittivity, but can also enhance, rather than limit, its temperature range. Our first-principles-based atomistic simulations provide detailed microscopic insight into the origin of this phenomenon, identifying the dominant contribution of near-interface layers and paving the way for its future exploitation.

Low contrast dielectric metasurface optics

Abstract: We demonstrate low contrast dielectric metasurface optical elements for operation at visible frequencies. Our devices show transmission efficiencies as high as 90% and focal spots on the order of the design wavelength.

Structural Changes Underlying Field-Cycling Phenomena in Ferroelectric HfO2 Thin Films

Abstract: Since 2011, ferroelectric HfO2 has attracted growing interest in both fundamental and application oriented groups. In this material, noteworthy wake-up and fatigue effects alter the shape of the polarization hysteresis loop during field cycling. Such changes are problematic for application of HfO2 to ferroelectric memories, which require stable polarization hystereses. Herein, electrical and structural techniques are implemented to unveil how cyclic switching changes nanoscale film structure, which modifies the polarization hysteresis. Impedance spectroscopy and scanning transmission electron microscopy identify regions with different dielectric and conductive properties in films at different cycling stages, enabling development of a structural model to explain the wake-up and fatigue phenomena. The wake-up regime arises due to changes in bulk and interfacial structuring: the bulk undergoes a phase transformation from monoclinic to orthorhombic grains, and the interfaces show changes in and diminishment of a nonuniform, defect rich, tetragonal HfO2 layer near the electrodes. The evolution of these aspects of structuring contributes to the increase in Pr and the opening of the constricted P–V hysteresis that are known to occur with wake-up. The onset of the fatigue regime is correlated to an increasing concentration of bulk defects, which are proposed to pin domain walls.

Screening and many-body effects in two-dimensional crystals: Monolayer MoS 2

Abstract: We present a systematic study of the variables affecting the electronic and optical properties of two-dimensional (2D) crystals within ab initio $GW$ and $GW$ plus Bethe-Salpeter equation ($GW$-BSE) calculations. As a prototypical 2D transition metal dichalcogenide material, we focus our study on monolayer ${\mathrm{MoS}}_{2}$. We find that the reported variations in $GW$-BSE results in the literature for monolayer ${\mathrm{MoS}}_{2}$ and related systems arise from different treatments of the long-range Coulomb interaction in supercell calculations and convergence of $k$-grid sampling and cutoffs for various quantities such as the dielectric screening. In particular, the quasi-2D nature of the system gives rise to fast spatial variations in the screening environment, which are computationally challenging to resolve. We also show that common numerical treatments to remove the divergence in the Coulomb interaction can shift the exciton continuum leading to false convergence with respect to $k$-point sampling. Our findings apply to $GW$-BSE calculations on any low-dimensional semiconductors.

Sandwich-structured polymer nanocomposites with high energy density and great charge–discharge efficiency at elevated temperatures

Abstract: The demand for a new generation of high-temperature dielectric materials toward capacitive energy storage has been driven by the rise of high-power applications such as electric vehicles, aircraft, and pulsed power systems where the power electronics are exposed to elevated temperatures. Polymer dielectrics are characterized by being lightweight, and their scalability, mechanical flexibility, high dielectric strength, and great reliability, but they are limited to relatively low operating temperatures. The existing polymer nanocomposite-based dielectrics with a limited energy density at high temperatures also present a major barrier to achieving significant reductions in size and weight of energy devices. Here we report the sandwich structures as an efficient route to high-temperature dielectric polymer nanocomposites that simultaneously possess high dielectric constant and low dielectric loss. In contrast to the conventional single-layer configuration, the rationally designed sandwich-structured polymer nanocomposites are capable of integrating the complementary properties of spatially organized multicomponents in a synergistic fashion to raise dielectric constant, and subsequently greatly improve discharged energy densities while retaining low loss and high charge-discharge efficiency at elevated temperatures. At 150 °C and 200 MV m(-1), an operating condition toward electric vehicle applications, the sandwich-structured polymer nanocomposites outperform the state-of-the-art polymer-based dielectrics in terms of energy density, power density, charge-discharge efficiency, and cyclability. The excellent dielectric and capacitive properties of the polymer nanocomposites may pave a way for widespread applications in modern electronics and power modules where harsh operating conditions are present.

Near-field dielectric scattering promotes optical absorption by platinum nanoparticles

Abstract: A technique to enhance optical absorption by platinum nanoparticles is exploited for driving photocatalytic redox reactions.

Mechanically Tunable Dielectric Resonator Metasurfaces at Visible Frequencies

Abstract: Devices that manipulate light represent the future of information processing. Flat optics and structures with subwavelength periodic features (metasurfaces) provide compact and efficient solutions. The key bottleneck is efficiency, and replacing metallic resonators with dielectric resonators has been shown to significantly enhance performance. To extend the functionalities of dielectric metasurfaces to real-world optical applications, the ability to tune their properties becomes important. In this article, we present a mechanically tunable all-dielectric metasurface. This is composed of an array of dielectric resonators embedded in an elastomeric matrix. The optical response of the structure under a uniaxial strain is analyzed by mechanical–electromagnetic co-simulations. It is experimentally demonstrated that the metasurface exhibits remarkable resonance shifts. Analysis using a Lagrangian model reveals that strain modulates the near-field mutual interaction between resonant dielectric elements. The abil...

Lead-free relaxor ferroelectric ceramics with high optical transparency and energy storage ability

Abstract: We prepared highly transparent relaxor ferroelectric ceramics based on (K0.5Na0.5)NbO3 using a pressure-less solid-state sintering method without using hot isostatic pressing and spark plasma sintering. A high energy storage density of 2.48 J cm−3 and high transparency in the visible region (ca. 60% at 0.7 μm) were achieved for the 0.8(K0.5Na0.5)NbO3–0.2Sr(Sc0.5Nb0.5)O3 ceramics with submicron-sized grains (about 0.5 μm). The energy storage density of 2.48 J cm−3 exceeded all previous reports for lead-free bulk ceramics. These results demonstrate that the 0.8(K0.5Na0.5)NbO3–0.2Sr(Sc0.5Nb0.5)O3 ceramics are promising lead-free transparent dielectric materials for use in transparent electronic devices. This study not only opens up a new avenue for the design of lead-free transparent ferroelectric ceramics with a high energy storage density, but also expands the applications of (K0.5Na0.5)NbO3-based ceramics into new areas beyond piezoelectric applications.

Broken Symmetry Dielectric Resonators for High Quality Factor Fano Metasurfaces

Abstract: We present a new approach to dielectric metasurface design that relies on a single resonator per unit cell and produces robust, high quality factor Fano resonances. Our approach utilizes symmetry breaking of highly symmetric resonator geometries, such as cubes, to induce couplings between the otherwise orthogonal resonator modes. In particular, we design perturbations that couple “bright” dipole modes to “dark” dipole modes whose radiative decay is suppressed by local field effects in the array. Our approach is widely scalable from the near-infrared to radio frequencies. We first unravel the Fano resonance behavior through numerical simulations of a germanium resonator-based metasurface that achieves a quality factor of ∼1300 at ∼10.8 μm. Then, we present two experimental demonstrations operating in the near-infrared (∼1 μm): a silicon-based implementation that achieves a quality factor of ∼350; and a gallium arsenide-based structure that achieves a quality factor of ∼600, the highest near-infrared qualit...

Generalized Brewster effect in dielectric metasurfaces

Abstract: Devices with greater freedom are desired in nanophotonics. Here, the authors demonstrate theoretically and experimentally that the generalized Brewster effect can be observed in an all-dielectric metasurface potentially for any angle, wavelength and polarization, due to electric and magnetic dipole interference.

Apparatus and methods for generating non-interfering electromagnetic waves on an insulated transmission medium

Abstract: Aspects of the subject disclosure may include, receiving a plurality of communication signals, and generating, according to the plurality of communication signals, signals that induce a plurality of electromagnetic waves bound at least in part to a dielectric material. Each electromagnetic wave of the plurality of electromagnetic waves conveys at least one communication signal of the plurality of communication signals, and the plurality of electromagnetic waves has a multiplexing configuration that reduces an interference between the plurality of electromagnetic waves. Other embodiments are disclosed.

Launcher and coupling system for guided wave mode cancellation

Abstract: Aspects of the subject disclosure may include, for example, a coupling module that includes a waveguide that guides a first electromagnetic wave conveying data from a transmitting device. A dielectric coupler receives the first electromagnetic wave from the waveguide to form a second electromagnetic wave, and that guides the second electromagnetic wave along the dielectric coupler adjacent to a transmission medium, and wherein the dielectric coupler has a length that supports a cancellation of at least one cancelled wave mode from a coupling of the second electromagnetic wave to the transmission medium.

Multi-feed dielectric antenna system with core selection and methods for use therewith

Abstract: In accordance with one or more embodiments, an antenna system includes a dielectric antenna having a feed-point, wherein the dielectric antenna is a single antenna having a plurality of antenna beam patterns. At least one cable having a plurality of conductorless dielectric cores is coupled to the feed-point of the dielectric antenna, each of the plurality of conductorless dielectric cores corresponding to one of the plurality of antenna beam patterns. A core selector switch couples electromagnetic waves from a source to a selected one of the plurality of conductorless dielectric cores, the selected one of the plurality of conductorless dielectric cores corresponding to a selected one of the plurality of antenna beam patterns.

System and dielectric antenna with non-uniform dielectric

Abstract: Aspects of the subject disclosure may include, for example, a solid dielectric antenna having a non-uniform spatial distribution of relative permittivity.