Tunable Permittivity Research Articles

Ultralow Loss and High Tunability in a Non‐perovskite Relaxor Ferroelectric

AbstractDielectric ceramics are fundamental for electronic systems, including energy storages, microwave applications, ultrasonics, and sensors. Relaxor ferroelectrics show superb performance among dielectrics due to their high efficiency and energy density by the nature of nanodomains. Here, a novel non‐perovskite relaxor ferroelectric, Bi6Ti5WO22, with ultralow loss, ≈10−3, highly tunable permittivity, ≈2200 at room temperature with 40% tunability and the superparaelectric region at room temperature is presented. The actual crystal structure and the nanodomains of Bi6Ti5WO22 are demonstrat Various‐temperature neutron powder diffraction and in situ high‐resolution transmission‐electron‐microscopy illustrate the twinning effect, subtle structure change and micro‐strain in the material influenced by temperature, manifesting the actual crystal structure of Bi6Ti5WO22. Compared with dielectric loss of BaTiO3‐based dielectric tunable materials, the loss of Bi6Ti5WO22 is more than an order of magnitude lower, which makes it exhibit a figure of merit (≈240), much higher than that of conventional dielectric tunable materials (< 100), endorse the material great potential for direct applications. The present research offers a strategy for discovering novel relaxor ferroelectrics and a highly desirable material for fabricating energy storage capacitors, microwave dielectrics, and ultrasonics.

Morphological, Dielectric, and Impedance Study of Ag-Coated Lead Oxide–Lignocellulose Composite Sheets for Energy Storage and Tunable Electric Permittivity Applications

Functional materials, in the combination of lignocelluloses, known as natural fibers, with oxide materials, can result in cultivating functional properties such as flexibility, relativity good electrical conduction, good electrical charge storage capacity, and tunable electric permittivity. This study presents the morphological, dielectric, and impedance properties of lignocellulose–lead oxide (LC/PbO2) composite sheets electrodeposited with silver metallic nanoparticles for various time spans. The uncoated samples show a rather simple behavior where the impedance data fit well to the two-system model with different relaxation times. On the other side, the impedance spectra of the electrodeposited sample have varying features, which mainly depend upon the deposition thickness of the Ag particles. The common feature is the drift of conductive species, as seen from the straight-line behavior in the Nyquist plots, which were fitted using a Warburg element in the equivalent circuit model.

Tunability Behavior of (Ba, Ca)(Zr, Ti)O Ceramic Capacitors Powered by Thermally Induced Phase Transitions With Applications to Nonlinear Transmission Lines

Ferroelectric ceramics with low loss dielectric, high permittivity, and nonlinear behavior are essential in capacitive nonlinear transmission lines (NLTLs) for radio frequency (RF) generation. NLTLs have the potential to generate soliton waves for high-power microwave (HPM) applications in mobile defense platforms and satellite communications. In this work, we synthesize, characterize, and investigate the nonlinear behavior of (Ba,Ca)(Zr,Ti)O3 ceramics system to be applied as nonlinear elements in capacitive NLTLs. The ceramics were structurally and microstructural characterized. Anomalies in dielectric and pyroelectric curves access the temperatures of phase transitions. The permittivity tunability was studied as a function of electric field and frequency in temperatures near phase transitions. The phase transitions of materials are identified by temperature where the permittivity and percentage of nonlinear tunability factor (NTF) are higher, which are required to produce RF on NLTLs.

In Situ Formation of CoS2 Hollow Nanoboxes via Ion-Exchange for High-Performance Microwave Absorption.

Hollow nanoboxes structure have raised great attention as microwave absorption materials on account of their ultralow density and large specific area. By introducing an adjustable interior cavity structure, the dielectric loss and microwave absorption performance were affected by the tunable complex permittivity and impedance matching was improved. In our study, hollow CoS2 nanoboxes with designable interspaces were successfully fabricated based on the surfactant-assisted solution method and followed by an in situ ion-exchange process. The structure, elemental compositions and morphology of the products were characterized by XRD, XPS, EDX, SEM and TEM, respectively. In addition, microwave absorption performance and the intrinsic mechanism are investigated in-depth. The paraffin-based composites with 20 wt.% filling contents exhibited superior microwave absorption capacities in view of both maximum reflection loss value (RLmax, −54.48 dB) and effective absorption bandwidth (EAB, below −10 dB, 6.0 GHz), which can be ascribed to unique hollow structure and good impedance matching. With these considerations in mind, this study provides a reference for the construction of high-performance microwave absorbers with unique hollow structure.

Heterogeneous Interfaces of Magnetic CIP and Binary Dielectric Carbon-Based Material Toward High-Efficiency Microwave Attenuation

Carbon-based materials, as lightweight absorbers, have been widely applied in varied fields to counteract the adverse effects of electromagnetic waves. Nevertheless, there remain challenges in coping with the impairment of microwave absorption performance caused by the excessively high permittivity of carbon materials. Herein, carbonyl iron powder (CIP) was added to graphene oxide/carbon black (RGO-CB) composites to adjust the permittivity improving impedance matching and electromagnetic absorption properties. In this work, the RGO provides a three-dimensional structure net, which introduces circularly and variously electromagnetic loss interfaces within the RGO-CB/CIP composites. Owing to the unique architecture and synergy of the three components, the as-prepared absorber exhibits good impedance matching and gratifying microwave absorption properties. It was found that the 1:1 mixture of RGO-CB and CIP in the composite had superb impedance matching at the thicknesses of 2.6 and 2.7 mm to achieve complete X-band microwave absorption. Such excellent performance benefits from the multiple polarization provided by the heterogeneous interfaces and the ideal impedance match attributed to the tunable permeability and permittivity of this work.

Wide-range tunable Ba0.7Sr0.3TiO3 films capacitors on LiNbO3 substrates

Ba0.7Sr0.3TiO3 (BST) films on Y-cut LiNbO3 substrates are investigated to realize wide-range tunable BST capacitors. The performances of the BST films are improved by the optimal magnetron sputtering conditions. It is found that the permittivity tunability of the BST capacitor reaches 300% under an applied voltage of 5 V. Moreover, the film has a high dielectric constant 2700 under an applied voltage of 0 V, and the Q value of the BST capacitor exceeds 3000. The results demonstrate that the BST film on the LiNbO3 substrate has potential as a wide-range tunable capacitor for tunable surface acoustic wave filters.

Self-Healable, Self-Repairable, and Recyclable Electrically Responsive Artificial Muscles.

Elastomers with high dielectric permittivity that self‐heal after electric breakdown and mechanical damage are important in the emerging field of artificial muscles. Here, a one‐step process toward self‐healable, silicone‐based elastomers with large and tunable permittivity is reported. Anionic ring‐opening polymerization of cyanopropyl‐substituted cyclic siloxanes yields elastomers with polar side chains. The equilibrated product is composed of networks, linear chains, and cyclic compounds. The ratio between the components varies with temperature and allows realizing materials with largely different properties. The silanolate end groups remain active, which is the key to self‐healing. Elastomeric behavior is observed at room temperature, while viscous flow dominates at higher temperatures (typically 80 °C). The elasticity is essential for reversible actuation and the thermoreversible softening allows for self‐healing and recycling. The dielectric permittivity can be increased to a maximum value of 18.1 by varying the polar group content. Single‐layer actuators show 3.8% lateral actuation at 5.2 V µm–1 and self‐repair after a breakdown, while damaged ones can be recycled integrally. Stack actuators reach an actuation strain of 5.4 ± 0.2% at electric fields as low as 3.2 V µm–1 and are therefore promising for applications as artificial muscles in soft robotics.

Electrically Tunable Plasmonic Absorber Based on Cu-ITO Subwavelength Grating on SOI at Telecom Wavelength

An electrically controlled optical absorption is numerically proposed in a plasmonic waveguide on silicon-on-insulator (SOI) consisting of copper-indium tin oxide (ITO) based subwavelength grating at 1.55 µm wavelength. The Cu-ITO subwavelength grating in a form of a discontinuous Cu layer filled with ITO together with electrically tunable permittivity of ITO provides us with a tunable absorption and efficient guidance of plasmonic mode. An n-type ITO is used which exhibits a significant change in the carrier concentration with the applied voltage resulting in a change in optical absorption at a telecom wavelength of 1.55 µm. We numerically observe maximum tuning in absorption at a grating period of 600 nm and a duty cycle of 50%. The proposed device shows a smaller effective mode area of Am = 0.01421/µm2 and a plasmonic confinement factor of 34.67%. The device is reported to have an extinction ratio of 10.28 dB for a 100-µm-long device at a low voltage of 6 V.

Functional Groups Assisted Tunable Dielectric Permittivity of Guest-Free Zn-Based Coordination Polymers for Gate Dielectrics.

The dielectric properties of coordination polymers has been a topic of recent interest, but the role of different functional groups on the dielectric properties of these polymers has not yet been fully addressed. Herein, the effects of electron-donating (R=NH2 ) and electron-withdrawing (R=NO2 ) groups on the dielectric behavior of such materials were investigated for two thermally stable and guest-free Zn-based coordination polymers, [Zn(L1 )(L2 )]n (1) and [Zn(L1 )(L3 )]n (2) [L1 =2-(2-pyridyl) benzimidazole (Pbim), L2 =5-aminoisophthalate (Aip), and L3 =5-nitroisophthalate (Nip)]. The results of dielectric studies of 1 revealed that it possesses a high dielectric constant (κ=65.5 at 1 kHz), while compound 2 displayed an even higher dielectric constant (κ=110.3 at 1 kHz). The electron donating and withdrawing effects of the NH2 and NO2 substituents induce changes in the polarity of the polymers, which is due to the inductive effect from the aryl ring for both NO2 and NH2 . Theoretical results from density functional theory (DFT) calculations, which also support the experimental findings, show that both compounds have a distinct electronic behavior with diverse wide bandgaps. The significance of the current work is to provide information about the structure-dielectric property relationships. So, this study promises to pave the way for further research on the effects of different functional groups on coordination polymers on their dielectric properties.

Two‐Dimensional C/MoS2‐Functionalized Ti3C2Tx Nanosheets for Achieving Strong Electromagnetic Wave Absorption

AbstractThe fabrication of ultra‐high performance absorbers with low thickness, light weight, hierarchical structure, and strong microwave absorption (MA) capability in a wide frequency range is a critical objective of the scientific community. The unique two‐dimensional lamellar and sandwich‐like structure of MoS2 (S‐Mo‐S) in combination with Ti3C2Tx offers exceptional metallic features, an enriched surface area and a hierarchical structure; hence, it is an appealing alternative to high‐efficiency microwave absorbers. Herein, two‐dimensional C/MoS2‐functionalized Ti3C2Tx hierarchical structures with superior attenuation capabilities (dielectric losses) are creatively designed and fabricated, and a minimal reflection loss (RLmin) of −42.7 dB is achieved at a thickness of only 1.5 mm. These excellent microwave absorption properties are attributed to better impedance matching, which is due to the tunable complex permittivity; intensive interfacial polarization, which stems from the numerous interfaces of multiple components; multiple scattering, which originates from the presence of plentiful two‐dimensional nanosheets; and ohmic loss, which arises from the formation of conductivity networks. The interconnected sandwich‐like and layered structure is proven to be an effective strategy for enriched microwave absorption by functionalized Ti3C2Tx nanocomposites.

Graphene optical modulators using bound states in the continuum

Graphene-based optical modulators have been widely investigated due to the high mobility and tunable permittivity of graphene. However, achieving a high modulation depth with a low insertion loss is challenging owing to low graphene-light interaction. To date, only waveguide-type modulators have been extensively studied to improve light-graphene interaction, and few free-space type modulators have been demonstrated in the optical communication wavelength range. In this study, we propose two graphene-based optical free-space type modulators in a simple silicon photonic crystal structure that supports bound states in the continuum. The designed modulator with an ultra-high quality factor from the bound states in the continuum achieves a high modulation depth (MD = 0.9972) and low insertion loss (IL = 0.0034) with a small Fermi level change at the optical communication wavelength. In addition, the proposed modulators support outstanding modulation performance in the normal chemical vapor deposition (CVD) graphene (mobility = 0.5 m2/Vs). We believe the scheme may pave the way for graphene-based optical active devices.

Low-loss NiZnCo ferrite composites with tunable magneto-dielectric performances for high-frequency applications

A series of Ni0.5Zn0.3Co0.2Fe2O4/epoxy magnetodielectric composites were synthesized through solvent-assisted method and demonstrate tunable permeability and permittivity, ultralow losses and high refractive index over a broad frequency region of the VHF and UHF bands. Experimental results reveal that the permeability μ' decreases from ~4.2 to ~3.5 while the permittivity ε' was improved from ~5.0 to ~6.1 with an increase in epoxy weight fraction from 1 to 20 wt%. These variations are shown to be intimately related to the microstructures in accordance with effective medium theory and magnetic circuit model. Correspondingly, the characteristic impedance of the magnetodielectric composite medium is varied from ~0.75 to ~0.90 while maintaining a refractive index around 4.5. Typical magnetic loss as tan δμ and dielectric loss as tan δε at 0.5 GHz are as low as in the order of 10−2 and 10−3, respectively. These magnetodielectric materials provide excellent radio frequency results that allowing for the realization of the high-performance miniaturized electronic devices, systems, and platforms.

Optical modulator based on a silicon-ITO grating embedded rib structure with a tunable group delay.

An optical modulator based on an engineered silicon-indium tin oxide (Si-ITO) structure is proposed with a tunable group delay. A large group delay is reported by slowing down the light in a Si-ITO grating embedded rib structure. Optical modulation and a tunable group delay are realized by utilizing the electrically tunable permittivity of ITO in the engineered waveguide. The extinction ratio over 8 dB for a 10 µm long device and the modulation efficiency around 12 V-µm are reported for a wide wavelength from 1530 to 1570 nm. The resulting modulation efficiency and the extinction ratio show a significant improvement as compared to conventional modulators based on rib waveguides. We also report around 82 psec electrical tuning in the group delay for a wide wavelength range. This concept is promising in view of realizing tunable delay lines, along with slow light modulators with a reduced device footprint and low energy dissipation.

Quad-polarization reconfigurable holographic artificial impedance surface based on liquid crystal at terahertz band

A novel quad-polarization reconfigurable holographic artificial impedance surface that exploits the tunable dielectric permittivity of nematic liquid crystal is proposed. The arc-shaped strip is introduced and serves as the fundamental element, and further combined with holographic modulation, electromagnetic radiation at the terahertz band is realized. By partitioning the circular surface into several sectors, studies show that just two impedance values for each sector are enough to reconfigure among different polarizations. The presented demonstrator has a radius of 4.3λ 0, where λ 0 is the wavelength in free space at 1.25 THz, and is excited by a monopole. It can be dynamically reconfigured among vertical linear polarization, horizontal linear polarization, right-hand circular polarization, and left-hand circular polarization at the boresight.

Optical Modulation via Coupling of Distributed Semiconductor Heterojunctions in a Si -ITO-Based Subwavelength Grating

A mechanism of optical intensity modulation is proposed by utilizing the electro-optic coupling in distributed semiconductor heterojunctions of p-type silicon ($\mathrm{Si}$) and n-type indium tin oxide (ITO) in the form of the subwavelength grating in a rib waveguide. The coupled multiple semiconductor heterojunctions of $\mathrm{Si}$-ITO are made to exhibit efficient optical intensity modulation via electrically tunable permittivity of ITO. The subwavelength grating is a nanophotonic element that not only provides a way to couple multiple heterojunctions, but it also gives rise to efficient optical (fiber to chip) coupling at a wavelength of 1550 nm. Lateral coupling of distributed heterojunctions via depleted charge density distributed along vertical and horizontal directions enable the device to show a high extinction ratio of 24 dB. Also, electrical tuning of the coupling efficiency for an 80-\textmu{}m long device is reported, which exhibits the multifunctional nature of the proposed nanophotonic device. The proposed modulation scheme, with a modulation efficiency of 0.34 V/mm and energy consumption of 36 pJ, may open pathways for energy-efficient compact devices and circuits for large-scale optoelectronic integration. The proposed mechanism of optical modulation takes advantage of distributed semiconductor heterojunctions and enables electrically tunable inherent optical coupling with a nanophotonic element called a subwavelength grating, which further improves the modulation performance compared with a conventional $\mathrm{Si}$-ITO heterojunction.

Electromagnetic properties of multiphase composites containing barium strontium titanate and nickel zinc ferrite inclusions from 1-4 GHz

The growing importance of nonlinear transmission lines for pulsed power and high power microwave devices in defense and industrial applications motivates material development for system optimization. This study investigates composites manufactured with various volume loadings of both nonlinear magnetic (nickel zinc ferrite) and dielectric (barium strontium titanate) inclusions in a silicone matrix. Increasing the volume fraction of barium strontium titanate (BST) and nickel zinc ferrite (NZF) increased the permittivity with a stronger dependence on BST volume fraction. Increasing NZF volume fraction increased the magnetic permeability, while changing BST volume fraction had no effect. The DC dielectric breakdown voltage decreased exponentially with increased NZF volume fraction. Adding as little as 5% BST to an NZF composite more than doubled the breakdown threshold compared to a composite containing NZF alone. For example, adding 10% BST to a 15% NZF composite increased the breakdown strength by over 800%. The combination of tunability of permittivity and permeability by managing BST and NZF volume fractions with the increased dielectric breakdown strength by introducing BST make this a promising approach for designing high power nonlinear transmission lines with input pulses of hundreds of kilovolts.

Size-Dominant Tunable Permittivity of Starch-Derived Micron-Carbon Spheres as Lightweight Microwave Absorption Absorbers

Size-Dominant Tunable Permittivity of Starch-Derived Micron-Carbon Spheres as Lightweight Microwave Absorption Absorbers

One-dimensional MnO@N-doped carbon nanotubes as robust dielectric loss electromagnetic wave absorbers

Dielectric loss materials exhibit appealing ability for electromagnetic wave attenuation due to their tunable permittivity, favorable stability and light weight. Herein, MnO@N-doped carbon nanotubes with adjustable carbon layer are synthesized by thermal decomposition of pyrrole on the surface of MnO2 nanotubes for electromagnetic absorption application. The morphology, composition, internal defects, conductivity and electromagnetic parameters of the composites are fully investigated. By using ANSYS software to simulate the electric field distribution in the material under the excitation of alternating electromagnetic field, the polarization behavior is analyzed in depth. The combination of MnO and carbon endows the composites with enhanced dielectric loss capacity and better impedance matching. The MnO@N-doped carbon composites achieve an optimal absorption intensity of −62.8 dB at 2.4 mm and a broad effective absorption bandwidth of 5.4 GHz with the thickness of 2.5 mm. The facile synthesis method and the superb EMW absorptivity make the tubular core-shell MnO@N-doped carbon composites a promising EMW absorber, and we believe this study will provide new strategies for designing novel dielectric EMW absorbers in the future.

Surface treatment of two dimensional MXene for poly(vinylidene fluoride) nanocomposites with tunable dielectric permittivity

Dielectric polymer nanocomposites with high dielectric permittivity and low dielectric loss have great applications in energy storage field. In this study, a series of PVDF/MXene nanocomposites was prepared by solution method. The surface of MXene was functionalized with cetyltrimethylammonium bromide (MXene@CTAB) to achieve better dispersion in poly(vinylidene fluoride) (PVDF) matrix. The results revealed that the MXene@CTAB dispersed uniformly and had a good compatibility with PVDF matrix. In addition, the dynamical rheological test revealed that the viscosity and storage modulus of PVDF nanocomposites increased with the increasing of MXene@CTAB loading. The PVDF/MXene@CTAB nanocomposite containing 7 wt% MXene@CTAB exhibited a dielectric permittivity of 82.1 and a loss factor as low as 0.2 at 1 kHz. The presence of CTAB could serve as an insulating layer to suppress dielectric loss and AC conductivity of PVDF nanocomposites simultaneously. The PVDF/MXene@CTAB nanocomposite with high dielectric permittivity may find potential applications in energy storage capacitors for electronics. Moreover, the addition of MXene@CTAB contributed to the formation of Beta phase of PVDF.

Thermally tunable terahertz omnidirectional photonic bandgap and defect mode in 1D photonic crystals containing moderately doped semiconductor

A new one-dimensional photonic crystal (1DPC) containing moderately doped silicon (m-Si) semiconductor is proposed and its tunable properties are theoretically investigated. The tunable complex permittivity of m-Si with temperature is the central idea that makes the 1DPC thermally tunable. The 1DPC systems such as (m-Si/SiO2)12 and defective (m-Si/SiO2)6D(m-Si/SiO2)6 are examined in the spectral range of 0.4–0.8 THz with varying temperature from 40 to 100 K. Air and m-Si are considered as defect layer (D) in two different 1DPC systems. Our simulation shows that the photonic band gap (PBG) shifts to higher frequency and the omnidirectional PBG gets narrower with increasing temperature. The defect mode in both defective 1DPCs shifts to higher frequency with increasing temperature. The temperature sensitivity of peak transmission is found better in the air defective 1DPC, while that of defect frequency is better in the m-Si defective 1DPC. The angle of incidence and polarization dependency of the defect modes are studied in detail. The present work can be utilized for the development of tunable omnidirectional mirrors and narrow bandpass filters in the terahertz region as well as THz light based thermal sensors.