Showing papers on "Permittivity published in 2021"

Lotus Leaf-Derived Gradient Hierarchical Porous C/MoS2 Morphology Genetic Composites with Wideband and Tunable Electromagnetic Absorption Performance.

Abstract: Inspired by the nature, lotus leaf-derived gradient hierarchical porous C/MoS2 morphology genetic composites (GHPCM) were successfully fabricated through an in situ strategy. The biological microstructure of lotus leaf was well preserved after treatment. Different pores with gradient pore sizes ranging from 300 to 5 μm were hierarchically distributed in the composites. In addition, the surface states of lotus leaf resulted in the Janus-like morphologies of MoS2. The GHPCM exhibit excellent electromagnetic wave absorption performance, with the minimum reflection loss of − 50.1 dB at a thickness of 2.4 mm and the maximum effective bandwidth of 6.0 GHz at a thickness of 2.2 mm. The outstanding performance could be attributed to the synergy of conductive loss, polarization loss, and impedance matching. In particularly, we provided a brand-new dielectric sum-quotient model to analyze the electromagnetic performance of the non-magnetic material system. It suggests that the specific sum and quotient of permittivity are the key to keep reflection loss below − 10 dB within a certain frequency range. Furthermore, based on the concept of material genetic engineering, the dielectric constant could be taken into account to seek for suitable materials with designable electromagnetic absorption performance.

Precise regulation of weakly negative permittivity in CaCu3Ti4O12 metacomposites by synergistic effects of carbon nanotubes and grapheme

Abstract: Precise control of value and dispersion characteristics of negative permittivity is still an unsolved problem in the practical application of random metamaterials. In this work, the ternary percolation nanocomposites (CNTs-GR-CCTO) were prepared via a low-temperature sintering process by using graphene (GR), carbon nanotubes (CNTs), and copper calcium titanate (CCTO). As the total carbon contents increased, the three-dimensional carbon network was formed, and negative permittivity is realized, which exhibits the Lorentz-type and Drude-type dispersion behaviors. The synergistic effect between GR and CNTs was studied in detail. While the GR sheets separate the CNTs, the CNTs also act as wires to connect the GR sheets. Controlled conductive paths were created by changing the ratio of CNTs to GR, achieving precise regulation of the negative permittivity, which is further certified by the equivalent circuit analysis. We provide a novel strategy for the precise regulation of negative permittivity of carbon-matrix metamaterials, which will greatly facilitate applications of metamaterials with negative permittivity. Precise regulation of weakly negative permittivity is achieved in CaCu3Ti4O12 metacomposites by synergistic effects of carbon nanotubes and graphene.

Engineering defects in 2D g-C3N4 for wideband, efficient electromagnetic absorption at elevated temperature

Abstract: Metal-free 2D nanomaterials such as graphitic carbon nitride (g-C3N4) nanosheets have attracted enormous attention due to their ultralow mass density, excellent chemical stability, high specific surface area, unique electronic structure and permittivity. However, the electromagnetic (EM) wave absorption performance of g-C3N4 cannot satisfy the requirements for addressing the ever-increasing occurrence of EM pollution. Herein, we demonstrate that the creation of pores in g-C3N4 nanosheets, combined with subsequent doping with phosphorus (P) and sulphur (S) atoms, give rise to a continuous frequency dispersive behaviour along with an enhanced conductive loss capability. As a result, the S/P-doped nanoporous g-C3N4 exhibit an efficient EM absorption over a wide frequency region (e.g., 6.0 GHz of >90% of absorption effectiveness at a sample thickness of 1.8 mm) at elevated temperatures (e.g., >4.0 GHz of >90% of absorption effectiveness at a thickness of 1.2 mm at 150 °C). Overall, our results reported in this work unmask new principles by which metal-free 2D nanomaterials can be modified to enable a significant enhancement in their EM absorption performance.

Local negative permittivity and topological phase transition in polar skyrmions

Abstract: Topological solitons such as magnetic skyrmions have drawn attention as stable quasi-particle-like objects. The recent discovery of polar vortices and skyrmions in ferroelectric oxide superlattices has opened up new vistas to explore topology, emergent phenomena and approaches for manipulating such features with electric fields. Using macroscopic dielectric measurements, coupled with direct scanning convergent beam electron diffraction imaging on the atomic scale, theoretical phase-field simulations and second-principles calculations, we demonstrate that polar skyrmions in (PbTiO3)n/(SrTiO3)n superlattices are distinguished by a sheath of negative permittivity at the periphery of each skyrmion. This enhances the effective dielectric permittivity compared with the individual SrTiO3 and PbTiO3 layers. Moreover, the response of these topologically protected structures to electric field and temperature shows a reversible phase transition from the skyrmion state to a trivial uniform ferroelectric state, accompanied by large tunability of the dielectric permittivity. Pulsed switching measurements show a time-dependent evolution and recovery of the skyrmion state (and macroscopic dielectric response). The interrelationship between topological and dielectric properties presents an opportunity to simultaneously manipulate both by a single, and easily controlled, stimulus, the applied electric field.

High-k Oxide Field-Plated Vertical (001) β-Ga 2 O 3 Schottky Barrier Diode With Baliga’s Figure of Merit Over 1 GW/cm 2

Abstract: We report a vertical (001) $\beta $ -Ga2O3 field-plated (FP) Schottky barrier diode (SBD) with a novel extreme permittivity dielectric field oxide. A thin drift layer of $1.7~\mu {m}$ was used to enable a punch-through (PT) field profile and very low differential specific on-resistance ( $\text{R}_{\text {on-sp}}$ ) of 0.32 $\text{m}\Omega $ -cm2. The extreme permittivity field plate oxide facilitated the lateral spread of the electric field profile beyond the field plate edge and enabled a breakdown voltage ( ${V}_{\textit {br}}$ ) of 687 V. The edge termination efficiency increases from 13.2% for non-field plated structure to 61% for high permittivity field plate structure. The surface breakdown electric field was extracted to be 5.45 MV/cm at the center of the anode region using TCAD simulations. The high permittivity field plated SBD demonstrated a record high Baliga’s figure of merit (BFOM) of 1.47 GW/cm2 showing the potential of Ga2O3 power devices for multi-kilovolt class applications.

Temperature stable Sm(Nb1−xVx)O4 (0.0 ≤ x ≤ 0.9) microwave dielectric ceramics with ultra-low dielectric loss for dielectric resonator antenna applications

Abstract: Herein, the structure and dielectric properties of Sm(Nb1−xVx)O4 (SNV-x) (0.0 ≤ x ≤ 0.9) ceramics were studied by crystal structure refinement, Raman, transmission electron microscope (TEM), far-infrared/THz reflectivity spectroscopy and microwave dielectric tests. Three kinds of ultra-low dielectric loss and temperature-stable Sm(Nb1−xVx)O4 (0.2 ≤ x ≤ 0.4) ceramics with permittivities of 18.01–16.89, Q × f values of 97 800–75 200 GHz (@∼8.6 GHz), and TCF of −5.6 (x = 0.2), to +2.3 ppm °C−1 (x = 0.3), then −6.3 (x = 0.4) ppm °C−1 were synthesized in this system. It was found that V5+ substitution can reliably induce the phase transition of monoclinic fergusonite (M-fergusonite, I2/a) to tetragonal zircon phase (T-zircon, I41/amd) (x ≈ 0.3), while effectively reducing the phase transition temperature. TEM shows that there were two different orientation domain structures in the M-fergusonite phase, and the widths of the two domain structures get closer with an increase in B-site substitution. Moreover, the variations in permittivity (er), quality factor (Q × f), and the temperature coefficient of resonance frequency (TCF) were strongly related to the crystal distortion and phase transition. Notably, a rectangular dielectric resonator antenna (RDRA) was fabricated with an Sm(Nb0.8V0.2)O4 (SNV-0.2) specimen. The antenna resonated at 27.04 GHz and had a bandwidth of ∼820 MHz (S11 < −10 dB). This system is a good candidate for 5G and future millimeter-wave applications.

Significantly enhanced high permittivity and negative permittivity in Ag/Al2O3/3D-BaTiO3/epoxy metacomposites with unique hierarchical heterogeneous microstructures

Abstract: High dielectric permittivity materials are widely employed in various electronic devices. To satisfy the ongoing miniaturization of electronic devices, materials with further enhanced dielectric permittivities are strongly desired. In this work, a novel design of epoxy composites based on Ag/Al2O3/3D-BaTiO3 foams with hierarchical heterogeneous microstructures are prepared. It is found that, the spatial distribution of the Ag particles can be easily controlled via adjusting the Ag+/Al3+ mole ratios, yielding highly tailorable dielectric properties. When the Ag+/Al3+ mole ratios are low, the Ag particles are well isolated by surrounding Al2O3, yielding the formation of numerous equivalent micro-capacitors and substantially enhanced dielectric permittivity. Moreover, the dielectric permittivities of the composites increase with higher Ag+/Al3+ mole ratios. Consequently, a high dielectric permittivity of 160 @10 kHz, which is about 35 times that of the epoxy matrix, is achieved in the composite with a Ag+/Al3+ mole ratio of 1.8. Meanwhile, a low tangent of about 0.062 is maintained. As the Ag+/Al3+ mole ratio increases, the Ag particles become interconnected, forming Ag networks. Consequently, a plasma-like negative phenomenon which should be attributed to the plasma oscillation of free electrons in the percolative Ag networks, is observed. This work offers an effective route to design polymer composites with tailorable high permittivity and negative permittivity.

Evolution of dielectric loss-dominated electromagnetic patterns in magnetic absorbers for enhanced microwave absorption performances

Abstract: As the growing criterion of electromagnetic wave (EMW) absorption materials, micro/nano-scale magnetic materials are drawing more and more attention for their unique features compared to bulky absorbers. Generally, the complex permeability of micro/nano-scale magnetic absorbers varies in a relatively narrow range, whatever for the storage of magnetic energy or the dissipation of magnetic energy. If so, how the small variation of permeability affects the ultimate performances is still unclear. Here, a strategy of electromagnetic parameters regulation for the magnetic materials is applied to understand the loss contribution in micro/nanoscale magnetic absorbers. After analyzing the evolution of electromagnetic maps of ten ferrosoferric oxide samples, it can be found that the dissipation contribution of permeability for magnetic materials is weaker than that of permittivity, in spite of its significant role in determining the impedance matching characteristics. In summary, this work systematically explores the loss contribution in micro/nano-magnetic absorbers for the first time, which is of great importance in designing and optimizing the microwave absorption properties of magnetic absorbers.

Magnetic, dielectric and structural properties of spinel ferrites synthesized by sol-gel method

Abstract: Sol-gel procedure was taken into consideration to produce ferrite series with chemical formula MnYbyFe2-yO4 with y = 0.00 to 0.10 (step = 0.025). Structural, electrical and magnetic aspects of ytterbium (Yb) switched Mn-ferrites were the main focus. Every sample exhibited cubic phase spinel assembly. Exchange of ytterbium with iron in the system produced enlargement in coercivity while abridgment in remanence and saturation magnetization. Involvement of Yb ion in the ferrite assembly led to reduction of both real and imaginary constituents of permittivity. The ytterbium introduction in the lattice enhanced the value of impedance. The examination of hysteresis loop was done under the range of -2K to +2KOe. Temperature dependent resistivity, activation energy and drift mobility were carried out by two probe method. The characterization outcomes recommended that the compounds are appropriate for microwave absorption usages.

Exploration of crystal structure, magnetic and dielectric properties of titanium-barium hexaferrites

Abstract: The BaFe12-xTixO19 hexaferrites up to x = 3.00 were extended. XRD patterns were Rietveld fitted for P63/mmc (no. 194) space group and the unit cell parameters were defined. The a parameter and V volume of unit cell change non-monotonically with × while the c parameter has linear behavior. The Ti4+ cations substitute the Fe3+ cations in the 2a, 4fVI and 12 k octahedral positions. This is confirmed by the Mossbauer spectroscopy. Ms spontaneous magnetization was determined with the law of approach to saturation from field magnetization at 5 K and 300 K. The e/ real part of the permittivity increases constantly with increasing temperature and decreases with frequency. The main objective of this study is an interpretation of the magnetic and dielectric properties of the titanium-barium hexaferrites which has performed in frame of breakdown of Fe3+ - O2– - Fe3+/2+(Ti4+) indirect superexchange interactions taking into account the positions occupation.

β-(Al 0.18 Ga 0.82 ) 2 O 3 /Ga 2 O 3 Double Heterojunction Transistor With Average Field of 5.5 MV/cm

Abstract: Maintaining high average fields between the gate and drain is imperative in achieving near theoretical performance in ultra-wide band gap semiconductors like $\beta $ -Ga2O3. In this letter we report on a field management strategy to reduce the peak electric field at the drain side corner of the gate by using a composite dielectric layer consisting of an extreme permittivity dielectric like BaTiO3 and a low- $\kappa $ dielectric like SiO2 overlapped over the gate electrode. Using this strategy in $\beta $ -(Al0.18Ga0.82)2O3/ Ga2O3 double heterojunction field effect transistor, we achieved a record average breakdown field of 5.5 MV/cm at a gate-drain spacing of $1.15~\mu \text{m}$ along with an improved power figure of merit of 408 MW/cm2. The reported works shows the effectiveness of integrating extreme dielectric materials with ultra-wide band gap semiconductors in significantly improving breakdown performance.

Multiple-Cell Microfluidic Dielectric Resonator for Liquid Sensing Applications

Abstract: This paper presents a high-sensitive planar microwave sensor based on the metamaterial technology, integrated with a microfluidic channel to be used for liquid sensing in a variety of biomedical applications. The sensor design comprises three cells of circular complementary split ring resonators (CSRRs) engraved on the ground plane of a dielectric substrate in a cascaded configuration. The sensing cells are excited with a time-varying electric field coupled from a planar microstrip-line (MTL) etched on the bottom side of the substrate. The proposed design of multiple coupled resonators would enhance the electric field intensity over a larger sensing region through the inter-resonator coupling, thus improving the overall sensitivity for detecting liquid samples of high permittivity and dielectric losses. The sensor exhibits a reject (band stop) filter behaviour with multiple resonances in the centimeter-wave band 1 – 6 GHz when loaded symmetrically with liquid samples. The sensor is fabricated on an FR4 dielectric substrate with a biocompatible microfluidic channel aligned appropriately over the sensing area to enable consistent and intact sensing of the liquid samples. The CSSR-sensor is numerically modeled and compared in performance to the single-cell CSRR for detecting small variations in the dielectric properties. The sensitivity, reliability, and repeatability of the proposed sensor are practically demonstrated by the in-lab measurements for different liquid samples using a Vector Network Analyzer (VNA) setup where distinctive resonant features (amplitude and frequency) are extracted from multiple resonance modes in the reflection and transmission responses. A high sensitivity is also demonstrated for monitoring glucose level variations in synthetic blood samples of concentrations (70 – 150 mg/dL) which are clinically-relevant to diabetes conditions. Beside its impressive capability in detecting small dielectric variations, the developed sensor features other favorable attributes including compact size, simple fabrication, affordable cost, non-ionizing nature, and minimal health risk or impact. Such key advantages could potentially promote the proposed sensor for integration with other microwave components in an embedded device for non-invasive monitoring of blood glucose for diabetes.

High-Accuracy Complex Permittivity Characterization of Solid Materials Using Parallel Interdigital Capacitor- Based Planar Microwave Sensor

Abstract: This paper presents a planar microwave sensor for the sensitive and accurate characterization of complex permittivity of solid materials. To realize the proposed sensor, the combination of a parallel interdigital capacitor and a dual wide gap resonator was etched on the ground plane and coupled with a transmission line in the top plane of a printed circuit board. The main advantage of the proposed sensor configuration lies in the generation of a high-intensity coupled resonating electric field suitable for the sensitive measurement of complex permittivity of solid materials. The high-intensity electric field enhances the coupling and field interaction, and thereby produces a high-accuracy permittivity characterization of a solid material exposed to the maximum field of the proposed sensor. Our developed microwave permittivity sensor, which characterized the complex permittivity of several materials by exploiting the measured shifts in resonance frequencies, exhibited a high sensitivity (up to 37% shift in resonance frequency for 20% change in permittivity) at least 1.5 times higher than previously reported microwave permittivity sensors. In addition, the proposed sensor exhibited 99.9% and 99.7% sensing accuracies for real and imaginary parts of permittivity, respectively, and the measurement results revealed an excellent (0.06758) and high sensitivity (67.58 MHz per unit change in real permittivity).

Enhanced energy storage performances of CaTiO3-based ceramic through A-site Sm3+ doping and A-site vacancy

Abstract: Ceramic-based capacitors for energy storage devices require simultaneously high energy density and efficiency. Achieving high electric breakdown field based on linear dielectrics is crucial. Here, A-site Sm3+ doped perovskite Ca1-1.5xSmx□0.5xTiO3 ceramics with introduced A-site vacancies (VA) were prepared. All Ca1-1.5xSmx□0.5xTiO3 ceramics crystallize in an orthorhombic structure, with lattice constants a, b, and c linearly decreased. As a result of Sm3+ dopants and VA, the grain size decreased while the ceramics’ density was improved. The permittivity decreases from 176 (x = 0) to 135 (x = 0.1), but tanδ is effectively constrained (∼10−4). What’s more, the dielectric breakdown strength is significantly improved from 429 kV/cm (x = 0) to 547 kV/cm (x = 0.1) with dielectric linearity is maintained. The optimum energy storage density of 2 J/cm3 (x = 0.02) with ultrahigh energy efficiency of over 93.7 % is achieved, which are superior to many existing linear dielectrics and relaxor ferroelectrics. This work confirms the energy-storage enhancement through chemical modifications and microstructural engineering.