Dielectric Gradient Research Articles

Ultrahigh Temperature Lead-Free Film Capacitors via Strain and Dielectric Constant Double Gradient Design.

With the development of miniaturization, lightweight and integration of electronic devices, the demand for high-temperature dielectric capacitors is becoming urgent. Nevertheless, the breakdown strength and polarization are deteriorated at high temperatures due to the thermal energy assisting the electron transport and impeding the dipole alignment. Here, a structure of capacitor with double gradients of dielectric constant gradient and strain gradient is designed to achieve high breakdown strength, high working temperature, and high energy storage density simultaneously. It is found that the designed structure of BaHf0.17 Ti0.83 O3 /1mol% SiO2 doped BaZr0.35 Ti0.65 O3 /0.85BaTiO3 -0.15Bi(Mg0.5 Zr0.5 )O3 exhibits excellent energy storage performance. The energy storage density of 127.3 J cm-3 with an energy storage efficiency of 79.6% is realized in the up-sequence multilayer with period N= 2 at room temperature. Moreover, when the working temperature varies from -100 to 200 °C, the energy storage density of the N= 4 capacitor keeps stably at 84.62 J cm-3 with an energy storage efficiency 78.42% at 6.86 MV cm-1 . All these properties promise great potential applications of the designed multilayer capacitors with the double gradients in harsh environments, and the design principle can be applicable to other systems to boost working temperature.

Optical Trapping Separation of Chiral Nanoparticles by Subwavelength Slot Waveguides.

Enantiomer separation opens great opportunities to develop the technologies of pharmaceutics, chemicals, and biomedicine, but faces daunting challenges. Here, we discover a considerable chiral-dependent trapping force to separate nanometer-scale enantiomers in a new silicon-based waveguide platform. The electromagnetic chirality gradient of strongly confined evanescent fields can be largely enhanced by the counterpropagating slot waveguides so that the resulting chiral gradient forces can shift the trapping equilibrium positions of dielectric gradient forces. Especially, there exists a transitional width for the slot waveguides to exchange the trapping equilibrium positions between two opposite enantiomers. Our thoroughly numerical investigations demonstrate that the chiral-separable slot waveguides here can offer high efficiency and feasibility of separating chiral nanoparticles, and may pave a route toward new on-chip chiral optical tweezers or optofluidic transport systems for large-scale chiral separation.

Multimaterial 3-D Printed Compressed Luneburg Lens for mm-Wave Beam Steering

Conformal, steerable lens antennas are of particular interest for millimeter-wave (mm-wave) antenna designers, as they enable low cost solutions for applications such as 5G mobile communications, radio-wave imaging, and satellite communications. Recent advances in additive manufacturing technology have opened up new possibilities for realizing graded-dielectric electromagnetic devices. In this letter, a compressed Luneburg lens fabricated from multimaterial 3-D printing is presented. Such a device has a steep dielectric gradient and cannot be easily realized using an effective medium approach that has become typical of 3-D printed graded-index devices. Instead, five different dielectric filaments were used to print the lens with a 100% filament fill-factor. The lens is excited by a WR-10 open-ended waveguide probe across the 75–110 GHz band, and achieves a bore sight gain of 22 dBi, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 3 dBi scan angle of 25 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> at 84 GHz.

Algorithm for optical characterization of dielectric gradient index nanofilm by surface plasmon resonance spectroscopy

A numerical algorithm for determining the refractive index profiles of gradient nanofilms is proposed. A physical justification for the necessity of using spectroscopic measurements in surface plasmon resonance sensing in addition to angular measurements for the unambiguous reconstruction of the shape of the gradient refractive index profile is given. The proposed approach can be effective for nanofilms made of dielectric materials transparent in the IR and THz range.

Super electro-optic modulation in bulk KTN:Cu based on electric-field-enhanced permittivity

The electric-field-enhanced effect of permittivity can improve the performance of electro-optic modulators and deflectors. A theoretical model of super electro-optic modulation based on the field-enhanced effect of the permittivity was proposed. Results showed that a strong field-enhanced effect can greatly reduce the half-wave voltage and increase the modulation depth as a result of increased relative dielectric permittivity and permittivity gradient to the electric field. For bulk paraelectric KTN:Cu near the Curie temperature, we found a novel phenomenon that the response of relative dielectric permittivity to the bias electric field was closely related to the frequency, including attenuation, invariance, and enhancement. We effectively selected the frequencies corresponding to the strong field-enhanced effect by measuring the dielectric-frequency spectrum under the bias voltage. At these frequencies, a phase retardation of π was achieved through 2Vpp AC modulation voltage, indicating that the half-wave voltage was reduced by one order of magnitude.

Analytical model for the spatiotemporal permittivity of uncured-composite devices in an AC electric field

Electric-field grading by dielectric permittivity gradient devices is an effective way of enhancing the insulation performance. In situ electric-field-driven assembly is an advanced method for the fabrication of insulating devices with adaptive permittivity gradients; however, there is no theoretical guidance for its use in design. We develop an analytical model for the spatiotemporal permittivity of an uncured-composite device in an AC electric field and investigate the coupling effects between the in situ assisted electric field and rod-like filler self-assembly in three devices: a pin-flat insulator, a basin insulator, and a silicone-gel-insulated gate bipolar transistor. Our model is based on optical images and dielectric permittivity monitoring, thus avoiding complicated electrodynamic calculations. The electric-field uniformity follows a U-shaped curve with assisted-voltage application time. We also find a combination of experimental parameters that constitutes an optimal tradeoff between internal and surface electric-field uniformities. This work establishes a theoretical design framework to optimize the performance (e.g. flashover voltage and breakdown strength) of a composite device.

Polarization-controlled efficient and unidirectional surface plasmon polariton excitation enabled by metagratings in a generalized Kretschmann configuration.

In this paper, we propose a generalized Kretschmann configuration that employs a metagrating to replace the prism, realizing polarization-controlled efficient and unidirectional surface plasmon polariton (SPP) excitation. This dielectric phase gradient metagrating on the top surface of a silica substrate is designed to deflect incident light, which subsequently launches SPP wave by means of momentum matching on the metal film coated on the bottom surface. A series of metagratings is designed to enable the SPP excitation by circularly or linearly polarized incident light. The flexibility and tunability of this design to efficiently control SPPs show potential to find wide applications in diverse integrated optics and SPP devices.

One-dimensional terahertz dielectric gradient metasurface for broadband spoof surface plasmon polaritons couplers.

At present, most of the gradient metasurfaces used to construct surface plasmon polaritons (SPPs)/spoof SPPs (SSPs) couplers are usually compact metal antennas working under reflection and transmission. In reflection mode, meta-couplers link propagating waves and surface waves (SWs), and SWs will undergo significant scattering before coupling to an Eigen SPP in the target system. In transmission mode, metal meta-couplers will encounter complex multilayer designing at the microwave/terahertz region and metal absorption loss at optical frequencies. In this Letter, to the best of our knowledge, a novel design using dielectric gradient metasurfaces instead of metal metasurface couplers is proposed to excite broadband SSPs on the metal groove array. We demonstrate that the well-designed phase dielectric gradient metasurface converts the normal incident terahertz wave to the predetermined angle in the dielectric substrate and then excites the broadband SSPs with the transmission coupling between the dielectric meta-coupler and SSPs surface. This research may open up new avenues in simple and broadband plane dielectric meta-couplers for SSPs in ultra-thin and compact functional devices for versatile applications.

Analytical model for the transient permittivity of uncured TiO 2 whisker/liquid silicone rubber composites under an AC electric field

The electric field grading of dielectric permittivity gradient devices is an effective way of enhancing their insulation properties. The in-situ electric field-driven assembly is an advanced method for the fabrication of insulation devices with adaptive permittivity gradients, however, there is no theoretical guidance for design. In this work, an analytical model with a time constant is developed to determine the transient permittivity of uncured composites under an applied AC electric field. This model is based on optical image and dielectric permittivity monitoring, which avoids the direct processing of complex electrodynamics. For a composite with given components, the increased filler content and electric field strength can accelerate the transient process. Compared with the finite element method (FEM) based on differential equations, this statistical model is simple but efficient, and can be applied to any low-viscosity uncured composites, which may contain multiple fillers. More importantly, when a voltage is applied to an uncured composite insulating device, the proposed model can be used to analyse the spatiotemporal permittivity characteristics of this device and optimise its permittivity gradient for electric field grading.

Recent Progress on Ultrathin Metalenses for Flat Optics.

SummaryAs technology advances, electrical devices such as smartphones have become more and more compact, leading to a demand for the continuous miniaturization of optical components. Metalenses, ultrathin flat optical elements composed of metasurfaces consisting of arrays of subwavelength optical antennas, provide a method of meeting those requirements. Moreover, metalenses have many other distinctive advantages including aberration correction, active tunability, and semi-transparency, compared to their conventional refractive and diffractive counterparts. Therefore, over the last decade, great effort has been focused on developing metalenses to investigate and broaden the capabilities of metalenses for integration into future applications. Here, we discuss recent progress on metalenses including their basic design principles and notable characteristics such as aberration correction, tunability, and multifunctionality.

Gradient dielectric constant sandwich-structured BaTiO3/PMMA nanocomposites with strengthened energy density and ultralow-energy loss

High energy storage density with low-energy loss polymer films are essential for high-performance electric devices. To avoid the high-energy loss of utilizing nonlinear polymer materials, a sandwich nanostructure comprising a linear polymer poly(methyl methacrylate) (PMMA) matrix embedded with a high dielectric constant BaTiO3 (BT) interlayer and poly(vinylidene fluoride) (PVDF) binder was constructed using a solution casting strategy. This structural design takes advantage of each component in the composite. The good dispersion of BT particles in the binder, which was incorporated between PMMA, enabled a high dielectric constant and fewer defects. Additionally, the excellent film formation ability of the PVDF binder guarantees the uniform thickness and stable structure of the BT mid-layer, and good miscibility between PVDF and PMMA enhanced the interaction between each layer. Interestingly, since the dielectric constant of PVDF was between BT fillers and PMMA, a dielectric gradient distribution mitigated the local electric field concentration, as proven by the simulation results. Consequently, a low-loss linear PMMA composite film exhibited satisfying breakdown strength and excellent discharged energy density, which were 25% and 460% higher than those of pristine PMMA, respectively.

Refractive Index Sensing Utilizing Tunable Polarization Conversion Efficiency With Dielectric Metasurface

Conventional metal metasurfaces imparting abrupt local phase profiles suffer from strong inherent loss and limited polarizability, leading to low polarization conversion efficiency of electromagnetic fields and insensitive responses with changes of external environment. Here, by adopting lossless silicon nanopillars to constitute a dielectric gradient metasurface, an incident circularly polarized light can be partly converted to a transmitted light of opposite helicity. And the polarization conversion efficiency varies sensitively with the refractive index of surrounding media. Simulation shows that the intensities between 0th and 1st diffraction orders change oppositely with the liquid refractive index, the proposed dual-channel (0th and 1st diffraction orders) average sensitivity reaches 16.134 dB/RIU at a wavelength of 1550 nm. We also analyzed the biosensing characteristics of an ultrathin molecular layer of 30-nm-thick coated on the silicon nanopillars surface. The refractive index sensing utilizing dielectric metasurface that we reported has excellent performance, which has potential applications environmental monitoring, medicine, biochemical sensing, and optical regulations.

Effects of dielectric gradients‐mediated ions partitioning on the electrophoresis of composite soft particles: An analytical theory

AbstractIn this work, we report original analytical expressions defining the electrophoretic mobility of composite soft particles comprising an inner core and a surrounding polymer shell with differentiated permeabilities to ions from aqueous background electrolyte and to fluid flow developed under applied DC field conditions. The existence of dielectric permittivity gradients operational at the core/shell and shell/solution interfaces is accounted for within the Debye–Hückel approximation and flat plate configuration valid in the thin double layer regime. The proposed electrophoretic mobility expressions, applicable to weakly to moderately charged particles with size well exceeding the Debye layer thickness, involve the relevant parameters describing the particle core/shell structure and the electrohydrodynamic features of the core and shell particle components. It is shown that the analytical expressions reported so far in literature for the mobility of hard (impermeable) or porous particles correspond to asymptotic limits of the more generic results detailed here. The impacts of dielectric‐mediated effects of ions partitioning between bulk solution and particle body on the electrophoretic response are further discussed. The obtained expressions pave the way for a refined quantitative, analytical interpretation of electrophoretic mobility data collected on soft (nano)particles (e.g., functionalized dendrimers and multilayered polyelectrolytic particles) or biological cells (e.g., viruses) for which the classical hard core‐soft shell representation is not appropriate.

Ni(OH)2 as an novel shell layer material for core-shell dielectric filler based on barium titanate and their dielectric polymer composites in P(VDF-HFP) matrix

Dielectric capacitors with high dielectric constant and energy density are of great importance for electronic devices. Barium titanate(BaTiO3) have been widely explored for polymer-based dielectric composites owning to its ultrahigh dielectric constant and commercial availability. However, due to the large difference of dielectric constant between BaTiO3 and polymer matrix, the interface becomes fragile under electric field, which is not conducive for the improvement in breakdown strength and energy storage density. Herein, a flower like barium titanate/nickel hydroxide(BTO/Ni(OH)2) core-shell structured filler has been fabricated to provide a novel approach to modify the interface between BTO and P(VDF-HFP) matrix for dielectric polymer composites. Ni(OH)2 nanoflakes have grown in-situ on the surface of hydroxylated BaTiO3 particles to form the flower like core-shell assemblies via hydrothermal reaction. Then, these synthetic BTO/Ni(OH)2 particles with specific flower-like core-shell structure can be uniformly dispersed in P(VDF-HFP) without any additional surface modification due to their surface –OH groups. Meanwhile, Ni(OH)2 shell triggered beta phase of P(VDF-HFP) near core-shell filler based on the interfacial interaction between Ni(OH)2 and P(VDF-HFP). Thus, the intensity of the local electric field can be gradually reduced due to the formation of a dielectric constant gradient. Besides, these Ni(OH)2 nanoflakes shell are able to cut down the local electric field and prevent the growth of electrical trees on the surface of BaTiO3 during breakdown process. When the composite filler content is 10 wt%, the dielectric constant and breakdown strength increases to 14.8 and 330 kV/mm from 11 to 260 kV/mm, respectively. And the calculated energy density is 7.13 J/cm3, which is 1.5 times that of 10 wt% BTO/P(VDF-HFP) composite film. This study provides a novel shell material Ni(OH)2 for the preparation of BTO core-shell structure filler, which has good compatibility with polymer substrates.

Study of the dielectric gradient values of reinforced concrete sample by considering radar measurements in an inverse problem

ABSTRACT The goal of this study is to develop a tool that will evaluate the moisture content gradient in the depth of reinforced concretes in the context of building safety. The monitoring of ageing phenomena, such as the corrosion of steel reinforcements, depends on the amount of water and on its gradient as they are directly related to the dielectric values of the concrete. In this paper, to obtain these values, we propose to solve an inverse problem for reinforced concrete. First, an experimental device based on radar measurements is proposed with its 3D numerical model. Next, we describe an inverse process to determine a dielectric profile according to the depth and validate it on measured data obtained for control samples.

Beam Tilting Approaches Based on Phase Gradient Surface for mmWave Antennas

This article presents four different approaches utilized in beam tilting that are based on the phase gradient (phase shifting) surfaces. Each approach is individually analyzed and then compared to other techniques. Beam tilting is obtained through various techniques by transforming the phase distribution of Fabry–Perot cavity antenna (FPCA) in the near-field region by means of inexpensive and passive surfaces including wedge-shaped dielectric lens (WSDL), discrete multilevel grating dielectric (DMGD), printed gradient surface (PGS), and perforated dielectric gradient surface (PDGS). To enhance efficiency in the millimeter-wave (MMW) band, the FPCA is fed through printed ridge gap waveguide. In addition, considerable enhancement in radiation performance of the antenna at 60 GHz is acquired by a partially reflective surface (PRS), including a gain enhancement from 6.5 to 22 dB. To validate the suggested methodology and performance of the designs, several prototypes are chosen for fabricating. A satisfactory agreement between the numerical results and experimental ones is achieved. The proposed designs are also applicable for MMW frequencies especially the narrower band communication system.

Tunable terahertz phase shifter based on dielectric artificial birefringence grating filled with polymer dispersed liquid crystal

An active terahertz (THz) anisotropic manipulation is based on a structure combined polymer dispersed liquid crystal (PDLC) with sub-wavelength dielectric gradient grating. In this structure, the PDLC works as an adjustable anisotropic material due to the change of the optical axis direction induced by applying a biased electric field, while the dielectric grating serves as an artificial high birefringence material. By using an appropriate design, the THz birefringence of this structure can be enhanced or offset under different biased voltages, and the phase shift curve of this structure becomes flatter than that of the pure PDLC cell due to the dispersion manipulation of the grating. Moreover, the experimental results fit with the simulative designing, demonstrating that the phase shift of the structure can vary from π to 0 near 0.8 THz when the electric field increases from 0 to 80V, and this device realizes the function of polarization conversion as a tunable THz half-wave plate. This work exhibits potential applications in THz functional devices, such as actively controlled phase shifters and polarization convertors combined LC with artificial microstructure.

Electrophysical Properties of Epoxy Composite Materials Filled with Carbon Black Nanopowder

The effect of carbon black (CB) nanopowder on the electrical properties of polymer composite systems based on the epoxy resin is investigated using the method of impedance spectroscopy. It is established that the electrical and dielectric properties of the studied systems significantly depend on the nanofiller content. It is found that electrical conductivity and dielectric constant exhibit percolation behavior when the filler’s content increases. In this case, the electrical conductivity increases exponentially, indicating the formation of filler electrically conductive mesh inside the polymer matrix. A small jump in electrical conductivity when reaching the percolation threshold indicates the formation of indirect contacts between the particles. The value of the percolation threshold of conductivity is 8%. It is shown that the dielectric constant of epoxy nanosystems is almost unchanged in the frequency range of 102–105 Hz. It is related to the structural features of the filler particles, which ensure the existence of a minimal dielectric gradient between the matrix and the filler. It is found that the dielectric constant of the studied systems also shows percolation behavior. The obtained material based on the epoxy matrix is characterized by a high value of dielectric constant, which at a carbon black nanopowder content of 29% is 4680. This material is characterized by relative frequency invariance and a high value of dielectric constant, so it has great potential for practical application.

Mobile Digital TV Induction Array Four Helix Antenna

According to the performance requirements of the mobile digital TV system, the present paper originally combines polygonal four helix antenna, induction array antenna, dielectric constant gradient film, graphene conductive ink and nano silver conductive ink, designed a mobile digital TV inductive array four helix antenna. The result of test indicates that this antenna has completely covered the frequency band of the mobile digital TV system. This antenna has strong corrosion resistance and damage resistance, stable physical and mechanical performance and radiation performance, small size, thin thickness and large working bandwidth, so it has broad application prospects.

Design and studies on gradient index metasurfaces for broadband polarization-independent, subwavelength, and dichroic focusing.

This paper presents designs for and simulation studies on planar gradient index metasurfaces for polarization-independent and dichroic subwavelength focusing for broadband applications. Polarization-independent lenses are designed based on dielectric (Si3N4 and TiO2) gradient nanopillars. Dichroic metalenses are designed based on gradient aluminum nanohelices for helicity-dependent focusing of circularly polarized light. The helical shape is considered due to its sensitivity to circularly polarized light of a specific handedness depending upon the orientation of the helices in the lattice; this may help in 3D imaging. In the designed metalenses, the variation in the spatial dimension (fill factor) is in a gradient manner, which leads to directional bending of electromagnetic waves, and strong coupling between the bent electromagnetic waves leads to subwavelength focusing over the high numerical aperture. The designed metasurface can be materialized through multibeam interference using a combination of n plane beams and a nondiffracting Bessel beam of either zeroth or first order presented through the simulated irradiance profile and a proposed single-step experimental setup. The designed TiO2-based metalens focuses the incident arbitrary or plane polarized light to a spot sized 0.314λ, at a wavelength of 635 nm, that is based on Si3N4, enabling polarization-independent subwavelength focusing over a broad (436-810 nm) wavelength range. Realization of these lenses will enable polarization-independent high-numerical-aperture focusing and super-resolution real-time imaging of biological samples.