Negative Dielectric Material Research Articles

Carbon dots induced homeotropic alignment in a negative dielectric nematic liquid crystal material

Abstract Recently, doping guest materials such as quantum dots (QDs) into liquid crystals (LCs) has been of great interest since their addition substantially enhances the properties of LC and opens new avenues for scientific advancement. Here, we report the induction of homeotropic alignment in cells without alignment layers of the negative dielectric nematic liquid crystal, N-(4-Methoxybenzylidene)-4-butylaniline (MBBA) by doping with carbon dots (CDs ~2.8±0.72 nm). The CDs-MBBA composites (CDs concentration: 0.002, 0.01, 0.03, 0.1 and 0.3 wt%) were investigated using optical polarising microscopy, electro-optical and dielectric techniques. Polarizing optical micrographs and voltage dependent optical transmission revealed the induced homeotropic alignment for all the composites under investigation. Interestingly, the least concentrated sample, 0.002 wt% exhibited partial homeotropic alignment. However, due to light leakage, the optical transmission value below threshold voltage was relatively higher than the rest of the composites. MBBA is a negative dielectric material, hence the application of a voltage across the cell was able to switch the alignment from a dark to a bright state for all composites. However, above a certain voltage (> threshold voltage), the bright state produced some instabilities. The value of dielectric permittivity was observed to decrease with increasing concentration, confirming the effect of CDs in producing homeotropic alignment in MBBA. Measurements as a function of temperature were conducted to examine the thermal stability of the induced alignment. The alignment was found to be stable throughout the nematic phase of MBBA. The induction of such alignment without conventional alignment (i.e., rubbing of polyimides) technique can be helpful in addressing the evolving display demands by making liquid crystal displays (LCDs) and other display devices cost effective.

Negative permittivity at kHz regulated by length-diameter ratio of short carbon fibers in phenolic resin composites

Negative dielectric materials show bright application prospects in the electromagnetic protection, energy storage and radio transmission fields. However, high negative permittivity and expensive manufacturing costs limit their applications in above fields. Here, phenolic resin composites containing short carbon fibers (SCF) with different length-diameter ratios were prepared by mechanical mixing and dry pressing. The effects of length-diameter ratio and mass fraction of SCF on conductivity, permittivity and impedance were investigated. The AC conduction mechanism changed from jump conduction to metal-like conduction due to the increased carrier concentration resulting from improved connectivity of the 3D conductive network. In addition, a weak negative permittivity was obtained in SCF429-10/PF composite. At SCF of 20 wt%, when the length-diameter ratio increased from 114 to 429, the negative permittivity mechanism changed from Lorentz & Drude to Drude model, indicating that the plasma oscillation of delocalized electrons was the main cause of negative permittivity. The relationship between reactance and permittivity were discussed by impedance and equivalent circuit analysis. This work provided a new strategy for obtaining weak negative permittivity, which exhibited potential applications in the fields of electromagnetic protection, high dielectric capacitors, and perfect absorption.

A Triboelectric Nanogenerator for Energy Harvesting from Transformers’ Vibrations

Transformers can produce gases dissolved in oil that can cause damage to their structures, and preventing failures caused by these gases is a goal to be reached. There is a demand for wireless sensors to monitor those gases. Alongside its development, there is a growing interest in new energy sources enabling these technologies. Triboelectric nanogenerators can gather energy from the environment, such as mechanical energy from vibrations, and convert it into electricity from the contact of two dielectric materials. In this work, the authors propose the study of a low-cost and straightforward triboelectric nanogenerator (TENG) based on ZnO nanorods as a positive dielectric material, with PDMS:GO composites at different concentrations as the negative dielectric material. All the studies were carried out in a wide frequency range varying from 45 to 250 Hz. Additionally, an analysis of the addition of a steel spring into the TENG to improve the device’s generating output is shown. A power density of 246 mV m−2 and 4 V of the output voltage was obtained using a PDMS:GO 4% (w/w) composite and a steel spring. A correlation between the “mass-spring” system and the better performance of the triboelectric device is presented. Further, vibration frequencies in several external points of the transformer walls and the device’s performance in these frequencies are shown, and the results gathered from this data are discussed.

Two‐dimensional Ti3C2Tx/poly(vinylidene fluoride) metacomposites with weakly negative permittivity

AbstractIn recent years, random metacomposites with tunable negative permittivity have aroused widespread attention because of their distinctive properties and broad potential prospects. It is indicated that an enormously negative permittivity with a large value is not conducive to performance optimizing and impedance matching. In this article, the 2D Mxene Ti3C2Tx was selected as the conductive phase instead of metal and carbon materials to fabricate Ti3C2Tx/poly(vinylidene fluoride) metacomposites. The results of dielectric properties indicated that the conductive Ti3C2Tx networks were formed by Ti3C2Tx contacting with each other near the percolation threshold (ƒc). The as‐obtained metacomposites containing 75 wt% Ti3C2Tx generated percolating phenomenon. Additionally, when the fraction of Ti3C2Tx was below ƒc, the frequency dispersion behaviors of conductivity were in accordance with the Jonscher's power law, which proved that conducting mechanism was dependent on hopping conduction. While above ƒc, the conductivity conformed to the Drude model and Lorentz model, which demonstrated a conductive behavior. Furthermore, the real part of permittivity transformed to negative value at 75 wt%, which can be explained as the existence of the low‐frequency plasma oscillation and electric‐dipole‐induced resonance. The dielectric loss of the metacomposite was also analyzed by the imaginary permittivity spectra. Small negative permittivity values in the range of −550 to −400 were observed, owing to the low free carrier concentration in composites. The appearance of weakly negative permittivity opened up a new direction in negative dielectric materials by 2D layered Ti3C2Tx nanomaterials.

Cloaking Due to Anomalous Localized Resonance in Plasmonic Structures of Confocal Ellipses

If a core of dielectric material is coated by a plasmonic structure of negative dielectric material with nonzero loss parameter, then anomalous localized resonance may occur as the loss parameter tends to zero and the source outside the structure can be cloaked. In this paper we consider cloaking by anomalous localized resonance (CALR) on structures where the core and the shell are confocal ellipses. For such structures we compute the critical elliptic radii such that, for any source inside it, CALR takes place and, for any source outside it, CALR does not take place. The method of this paper uses the spectral analysis of the Neumann--Poincare-type operator associated with two interfaces (the boundaries of the core and the shell) which was developed in [H. Ammari, G. Ciraolo, H. Kang, H. Lee, and G. W. Milton, Arch. Ration. Mech. Anal., 208 (2013), pp. 667--692].

Anomalous localized resonance using a folded geometry in three dimensions

If a body of dielectric material is coated by a plasmonic structure of negative dielectric material with non-zero loss parameter, then cloaking by anomalous localized resonance (CALR) may occur as the loss parameter tends to zero. If the coated structure is circular (two dimensions) and the dielectric constant of the shell is a negative constant (with loss parameter), then CALR occurs, and if the coated structure is spherical (three dimensions), then CALR does not occur. The aim of this paper is to show that CALR takes place if the spherical coated structure has a specially designed anisotropic dielectric tensor. The anisotropic dielectric tensor is designed by unfolding a folded geometry.

Combining radiationless interference with evanescent field amplification

The conventional approach for radiationless interference exploits the interference of evanescent components for the purpose of deep-subwavelength focusing and image formation. As a result, deep subwavelength feature size is achieved at the price of severe exponential decay of the field strength. We propose to overcome the limitation of the conventional approach by combining radiationless interference with evanescent field amplification as provided by the surface polaritons at the interface between positive- and negative-dielectric materials. Our approach removes the exponential decay and, moreover, allows a much wider range of wave vectors, including both propagating and evanescent field components, to participate in the image-formation process.

Two-dimensional array self-assembled quantum dot sub-diffraction waveguides with lowloss and low crosstalk

We model and demonstrate the behavior of two-dimensional (2D) self-assembled quantumdot (QD) sub-diffraction waveguides. By pumping the gain-enabled semiconductornanoparticles and introducing a signal light, energy coupling of stimulated photons fromthe QDs enables light transmission along the waveguide. Monte Carlo simulation withrandomized inter-dot separation reveals that the optical gain necessary for unity transfer is3.1 × 107 m−1 for a 2D(2 µm length by 500 nm width)array compared to 11.6 × 107 m−1 for a 1D (2 µm length) given 8 nm diameter quantum dots. The theoretical results are borne out inexperiments on 2D arrays by measurement of negligible crosstalk component with as littleas 200 nm waveguide separation and is indicative of near-field optical coupling behavior.The transmission loss for 500 nm wide structures is determined to be close to3 dB/4 µm, whereas that for100 nm width is 3 dB/2.3 µm. Accordingly, higher pump power and gain would be necessary on the narrower device tocreate similar throughput. Considering existing nanoscale propagation methods, whichcommonly use negative dielectric materials, our waveguide shows an improved losscharacteristic with comparable or smaller dimensions. Thus, the application of QDs tonanophotonic waveguiding represents a promising path towards ultra-high density photonicintegrated circuits.

Reflection and negative refraction of left-handed metamaterials at microwave frequencies

We experimentally investigated the microwave reflection of the negative permeability material composed of periodical arrays of the copper split-ring resonators (SRRs).The negative dielectric materials are composed of periodical arrays of copper wires and left-handed materials(LHMs) in a planar slab waveguide, and the microwave is incident to the surface in different directions.We also investigated the negative refraction through a prism fabricated from the LHMs.The experimental results show that for the negative permeability materials,there is a reflection peak at the resonance frequencies of the SRRs, and the reflectivity of the negative dielectric material is closed to 0dB.A single smaller reflection peak occurs in the reflected curve of the LHMs,the reflectivity of which increases with the incident direction,that is,the reflection increases.The reflection peak has a shift with respect to the transmission peak. It is demonstrated that negative refractive index of -0.796 occurs in the LHMs at the frequency o 800MHz.

Electromagnetics of superconductors

It is shown that a superconductor cannot be simply treated as a low-loss conductor; rather, it should be treated as a negative dielectric material (with a negative dielectric constant). This approach is good only for vanishingly small field application with frequency significantly smaller than gap frequency and temperature not too close to the critical temperature of the superconductor. The electromagnetics of negative dielectric materials are discussed in terms of causality, perturbation technique, surface impedance, time-domain interpretation of current components, and computational electrodynamics. >

ChemInform Abstract: DIELECTRIC BEHAVIOR OF N(P‐OCTYLOXYBENZYLIDENE) P‐TOLUIDINE (OBT) IN NEMATIC MESOPHASE AT RADIO FREQUENCIES

Dielectric permittivity and loss have been measured in the frequency range 0.5–3.5 MHz in nematic and isotropic phases of OBT. Measurements in the nematic region with a magnetic field greater than saturation value acting parallel to the rf electric field exhibit a Debye type dispersion of e∥ with relaxation frequency 5.3 MHz at 71° C and activation energy of 27.0 kcal/mole. The results can be explained on the basis of the molecular rotation about a transverse axis occurring under hindering nematic potential. Measurements also indicate that OBT behaves as a positive dielectric material at low frequencies and a negative dielectric material at higher frequencies.

Dielectric behavior of n (p-octyloxybenzylidene) p-toluidine (OBT) in nematic mesophase at radio frequencies

Dielectric permittivity and loss have been measured in the frequency range 0.5–3.5 MHz in nematic and isotropic phases of OBT. Measurements in the nematic region with a magnetic field greater than saturation value acting parallel to the rf electric field exhibit a Debye type dispersion of ε∥ with relaxation frequency 5.3 MHz at 71° C and activation energy of 27.0 kcal/mole. The results can be explained on the basis of the molecular rotation about a transverse axis occurring under hindering nematic potential. Measurements also indicate that OBT behaves as a positive dielectric material at low frequencies and a negative dielectric material at higher frequencies.