Dielectric Constant Tunability Research Articles

Structures and properties of LiBaF3 microwave dielectric ceramics for ULTCC applications

Perovskite structural LiBaF3 ceramics with excellent microwave dielectric properties were synthesized using traditional solid-state method. By regulating the sintering temperatures from 600 °C to 675 °C, LiBaF3 ceramics exhibit tunable dielectric constant (εr) from 12.42 to 13.72, adjustable Q × f value from 55000 GHz to 73000 GHz, and regulable temperature coefficient of resonance frequency (τf) from −36.78 ppm/°C to −34.68 ppm/°C. The properties are strongly associated with the density, and can be further optimized by 0.5 wt%TiO2 addition (εr = 14.43, Q × f = 56000 GHz, τf = -1.26 ppm/°C). The LiBaF3 ceramics also exhibit excellent compatibility with Ag and Al electrodes. Furthermore, relationships between multiscale structures and microwave dielectric properties of the ceramics were revealed. Oxidation behaviors between LiBaF3 and O2 would result in the lattice shrinkage, the introduction of defect level, the decrease of bandgap and density, and then deteriorating the microwave dielectric properties. All the findings make the LiBaF3 ceramic a promising candidate for ULTCC application, and may facilitate the development of other novel microwave dielectric materials.

Lightweight polypropylene/BaTiO3 composite foams with tunable dielectric constant

Lightweight dielectric foam materials with tunable dielectric constant of 2–1 have broad application prospects in the field of Luneburg lens. Expandable polystyrene (EPS) foam is commonly used for fabricating discrete dielectric layers of Luneburg lens, while its temperature resistance and mechanical properties are poor. EPS foam with dielectric constant of >1.5 is also difficult to prepare because of its low expansion ratio. In this work, choosing polypropylene (PP) with better temperature resistance and mechanical properties as polymer matrix, PP/BaTiO3 composite foams are fabricated by subcritical CO2 batch foaming and steam-chest molding. Through the dual regulation of expansion ratio and mold-opening distance, the dielectric constant of PP/BaTiO3 foams can be tunable from ∼1.19 to ∼1.91, with the density increasing from ∼0.14 to ∼0.59 g/cm3. Compared with EPS foams, PP/BaTiO3 foams have lower density under the similar dielectric constant, indicating the weight reduction of polymer dielectric foam materials by introducing high dielectric fillers.

Novel relaxor ferroelectric BTWO nanofillers for improving the energy storage performance of polymer-based dielectric composites

The fast growth of electronic gadgets and power systems has increased the demand for high energy-storage polymer-based film capacitors, However, because of the relatively low dielectric constant (εr), the discharged energy density (Ud) is severely limited, so increasing the εr of nanocomposites is an effective way to increase Ud. In this paper, Bi6Ti5WO22 (BTWO), a novel non-perovskite relaxation ferroelectric ceramic, is selected as nanofillers due to its ultra-low loss and high tunable dielectric constant, and BTWO/P(VDF-HFP) nanocomposites are prepared by compounding with ferroelectric polymers. BTWO nanoparticles' high εr (∼2200) and low loss (∼0.001) increase not only the breakdown strength (Eb) and dielectric properties of nanocomposites, but also their insulating and mechanical characteristics. It is worth noting that when the 5 vol% BTWO nanofillers are loaded, the εr of the nanocomposite is 11.3 and the highest Eb is up to 575 MV·m−1, and the optimal Ud and efficiency (η), with 22.6 J·cm−3 and 71.3 %, are attained at the same moment. In this work, a new relaxation ferroelectric ceramic BTWO is selected as the filler, which provides a new strategic route for the preparation of energy storage polymer-based dielectric.

A facile and large-scale route to prepare nitrogen/oxygen (N/O) co-doped two-dimensional carbon nanomesh with excellent microwave absorption properties

Two-dimensional porous carbon materials have been considered good candidates for developing lightweight microwave absorbers because of their low density and tunable dielectric constant. However, large-scale synthesis, with precise control of the microstructure, by a simple method, remains is still a great challenging. Herein, a two-dimensional N/O co-doped carbon nanomesh (NOCN) was prepared via large-scale route by simple carbonization of analogue polyurea (PU) nanosheet consisted of p-phenyldiisocyanate and urea, and the graphitization degree, porous structure, sheet size and the heteroatom doping content could be easily adjusted by controlling carbonization temperature. Thus, the electromagnetic parameter and the corresponding microwave absorption can be regulated. When the carbonization temperature was 900 °C (NOCN-900), the obtained sample exhibited the best microwave absorption performance, and the reflection loss (RL) value was −54.2 dB with an effective absorption bandwidth (EAB) of 7.44 GHz at a thickness of 2.3 under fill loading of only 5 wt%. The facile and large-scale synthesis route combined with excellent performance makes NOCN-900 to be a great promising candidate for practical application.

Novel accordion-like structure of SiC/C composites for enhanced electromagnetic wave absorption

Two-dimensional (2D) composites have been identified as a rising star in the exploration of efficient electromagnetic wave (EMW) absorbers due to their structural designability and tunable dielectric constants. However, how to address the high filling rate of absorbers with 2D microstructures and the easy stacking problem between layers remain difficult and painful points. Inspired by the adjustable spacing of expanded graphite layers, it is rationally proposed to prepare unique and novel 2D accordion like SiC/C composites by microwave-assisted method and carbothermal reduction strategy. The microstructural modification and multicomponent modulation are achieved by regulating the carbothermal reduction temperature. The results confirm that the presence of plentiful inhomogeneous interfaces in 2D accordion like SiC/C composites, the enhanced dipole polarization, the improved conductive loss due to the unique 2D accordion like structure endow carriers with more paths, and the optimized impedance matching due to the porous structure synergistically result in superior EMW absorption properties. 2D accordion like SiC/C-2 exhibits a minimum reflection loss of −54.52 dB at 8.72 GHz with an ultra-low fill rate of 3%, and the effective absorption range covers 11.40 GHz (7.60–18 GHz) at a total thickness of 1.20 mm (1.23–2.43 mm), which is 71.25% of the whole range of frequencies. Additionally, radar cross section (RCS) simulations verify that 2D accordion like SiC/C-2 can dissipate more EMW in a real environment. This work provides a reference for the rational construction of novel high-performance 2D structural absorbers.

Synthesis and characterization of Cu/YIG nanoparticles- Terahertz material

Copper/Yttrium Iron Garnet (Cu/YIG) nanoparticles (NP) were prepared using the in situ method to understand how the amount of copper affects their structural, electrical, and magnetic properties. Structural analysis using XRD determined that all samples have a single cubic crystal structure with the general formula A3B5O12. At room temperature, the complex permittivity (ϵ), complex permeability (μ), and S-parameter were measured as a function of frequency. When there is a high amount of copper, the dielectric loss is dominated by conduction and polarization. The results suggest that conduction occurs through the grain boundaries in these samples. The complex permeability measurements at room temperature indicate that the permeability decreases with increasing amounts of copper. This may be due to the difference in magnetic moment between the copper and iron ions. The Cu/YIG nanocomposites with a tunable negative dielectric constant have great potential in electromagnetic damping, shielding, and antenna fields.

Atomically Thin Metal-Dielectric Heterostructures by Atomic Layer Deposition.

Heterostructures increasingly attracted attention over the past several years to enable various optoelectronic and photonic applications. In this work, atomically thin interfaces of Ir/Al2O3 heterostructures compatible with micro-optoelectronic technologies are reported. Their structural and optical properties were determined by spectroscopic and microscopic techniques (XRR, XPS, HRTEM, spectroscopic ellipsometry, and UV/vis/NIR spectrophotometry). The XRR and HRTEM analyses reveal a layer-by-layer growth mechanism of Ir in atomic scale heterostructures, which is different from the typical island-type growth of metals on dielectrics. Alongside, XPS investigations imply the formation of Ir-O-Al bonding at the interfaces for lower Ir concentrations, in contrast to the nanoparticle core-shell structure formation. Precisely tuning the ratio of the constituents ensures the control of the dispersion profile along with a transition from effective dielectric to metallic heterostructures. The Ir coating thickness was varied ranging from a few angstroms to films of about 7 nm in the heterostructures. The transition has been observed in the structures containing individual Ir coating thicknesses of about 2-4 nm. Following this, we show epsilon-near-zero metamaterials with tunable dielectric constants by precisely varying the composition of such heterostructures. Overall, a comprehensive study on structural and optical properties of the metal-dielectric interfaces of Ir/Al2O3 heterostructures was addressed, indicating an extension of the material portfolio available for novel optical functionalities.

Architecture-inspired N-doped carbon nanotube bridging well-arranged MXene nanosheets toward efficient electromagnetic wave absorption

Multilayered Ti3C2Tx MXene, due to its tunable dielectric constants and structural designability, has been identified as rising star for exploring high-efficiency electromagnetic wave absorbers. However, it is still a huge challenge to address insufficient external contact of multilayered MXene at lower filler loading. Herein, inspired by the layer-pillar-layer architecture, a controllable catalytic-assisted nitrogen-doped carbon nanotubes bridging strategy is rationally proposed on the interlaminations of multilayered MXene units (MXene/TiO2/NCNTs, denoted as MTNC) to enhance dielectric dissipation ability. Through chemical vapor deposition, the NCNTs bridges as pillars not only break the confinement effect of multilayered MXene architecture like the process of seed germination but also effectively optimize impedance matching and increase dielectric constants. Simultaneously, the unique NCNTs-encapsulated magnetic components successfully prevents the agglomerations of CoNi nanoparticles and local mismatching problems, which facilitates the construction of three-dimensional magnetic coupling network. Specifically, the N heteroatoms and formed TiO2 species induce dielectric polarization and corresponding relaxation processes. Driven by unique layer-pillar-layer architecture, optimized impedance matching, and synergistic dielectric-magnetic dissipation capabilities, the resultant absorber delivers outstanding reflection loss value of −55.8 dB and broad absorption bandwidth of 5.88 GHz. This work provides a novel structural design strategy and enlightenment for developing high-performance multilayered MXene-based absorbers.

Boron‐Thioketonates: A New Class of S,O‐Chelated Boranes as Acceptors in Optoelectronic Devices

AbstractDevelopment of new n‐type semiconductors with tunable band gap and dielectric constant has significant implication in dissociating bound charge carrier relevant for demonstrating high performance optoelectronic devices. Boron‐β‐thioketonates (MTDKB), analogues to boron‐β‐diketonates containing a sulfur atom in the framework of β‐diketones were synthesized. Bulk transport measurement exhibited an outstanding bulk electron mobility of ≈0.003 cm2 V−1 s−1, which is among the best values reported till date in these class of semiconducting materials and correspondingly a single junction photo responsivity of upto 6 mA W−1 was obtained. This new family of O,S‐chelated boron compounds exhibited luminescence in the far red/near‐infrared region. The remarkable red shift of 89 nm (fluorescence) observed for 4 a in comparison with analogues boron‐β‐diketonate signifies the importance of sulfur in these molecules. MTDKBs with amine functionality have also been investigated as an ON/OFF fluorescent sensor.

Boron-Thioketonates: A New Class of S,O-Chelated Boranes as Acceptors in Optoelectronic Devices.

Development of new n-type semiconductors with tunable band gap and dielectric constant has significant implication in dissociating bound charge carrier relevant for demonstrating high performance optoelectronic devices. Boron-β-thioketonates (MTDKB), analogues to boron-β-diketonates containing a sulfur atom in the framework of β-diketones were synthesized. Bulk transport measurement exhibited an outstanding bulk electron mobility of ≈0.003 cm2 V-1 s-1 , which is among the best values reported till date in these class of semiconducting materials and correspondingly a single junction photo responsivity of upto 6 mA W-1 was obtained. This new family of O,S-chelated boron compounds exhibited luminescence in the far red/near-infrared region. The remarkable red shift of 89 nm (fluorescence) observed for 4 a in comparison with analogues boron-β-diketonate signifies the importance of sulfur in these molecules. MTDKBs with amine functionality have also been investigated as an ON/OFF fluorescent sensor.

A liquid crystal leaky-wave antenna with fixed-frequency beam scanning and open-stop-band suppression

ABSTRACT In this article, a liquid crystal (LC) leaky-wave antenna (LWA) is presented with fixed-frequency beam-scanning property operating in millimetre-wave band. Two rows of periodic-perturbation stubs (PPSs) structure are employed on both sides of a microstrip line to generate radiation. The undesirable open stop-band (OSB) that deteriorates the broadside radiation of the LWA can be avoided, and a satisfactory broadside radiation is realised, by introducing a specific offset between the two rows of the PPSs, which is different from the existing methods and is fully studied in this paper. Furthermore, since LC material with tunable dielectric constant is loaded on the antenna substrate, the phase constant of the LWA can be manipulated by applying a controllable biasing voltage from 0 V to 20 V. Therefore, fixed-frequency beam scanning can be realised. The presented LWA is fabricated and measured, and the antenna performances have been verified by the measured results.

Van der Waals materials as dielectric layers for tailoring the near-field photonic response of surfaces.

Epsilon near-zero photonics and surface polariton nanophotonics have become major fields within optics, leading to unusual and enhanced light-matter interaction. Specific dielectric responses are required in both cases, which can be achieved, e.g., via operation near a material's electronic or phononic resonance. However, this condition restricts operation to a specific, narrow frequency range. It has been shown that using a thin dielectric layer can adjust the dielectric response of a surface and, therefore, the operating frequency for achieving specific photonic excitations. Here, we show that a surface's optical properties can be tuned via the deposition/transference of ultra-thin layered van der Waals (vdW) crystals, the thicknesses of which can easily be adjusted to provide the desired response. In particular, we experimentally and theoretically show that the surface phonon resonance of a silica surface can be tuned by ∼50cm-1 through the simple deposition of nanometer-thick exfoliated flakes of black phosphorus. The surface properties were probed by infrared nanospectroscopy, and results show a close agreement with the theory. The black phosphorus-silica layered structure effectively acts as a surface with a tunable effective dielectric constant that presents an infrared response dependent on the black phosphorus thickness. In contrast, with a lower dielectric constant, hexagonal boron nitride does not significantly tune the silica surface phonon polariton. Our approach also applies to epsilon near-zero surfaces, as theoretically shown, and to polaritonic surfaces operating at other optical ranges.

Tunable dielectric constant of water confined in graphene oxide nanochannels

Dielectric behavior of water in pristine and oxide graphene nanochannels with various separation has been investigated by means of molecular dynamic simulations. The motivation in performing this integrated set of simulations was to provide deep insight into the interaction between the size of the enclosure and the oxidation degree of the membrane sheets on the dielectric properties. It was shown that the dielectric constant of confined water decreased with the reduction of the space of the nanochannel because water molecules in such narrower environments were expected to exhibit a greater degree of spatial and orientational order. With the increase of the oxidation concentration, the influence of the wider interval space on the dielectric constant was greater than that of narrower nanochannls. For the widest channel (d = 1.2 nm), a decrease of dielectric constant was observed with the increase of oxidation concentration on the graphene bilayers, while a non-monotonous tendency of dielectric behavior appeared for the relatively narrow nanochannels (i.e. d = 0.6 nm and d = 0.9 nm). To understand the physical picture behind it, we computated the number of hydrogen bonds for the three nanochannels. Results showed that the number of HBs was the lowest in the 1.2 nm nanochannel, as well as the dynamic stability (corresponding the fastest decay rate), suggesting that water molecules are more unstable in this large nanochannel and then less orderly than in the 0.6 nm and 0.9 nm nanochannels.

Simultaneous Reduction and Polymerization of Graphene Oxide/Styrene Mixtures To Create Polymer Nanocomposites with Tunable Dielectric Constants

Polymer nanocomposites containing carbon nanomaterials such as carbon black, carbon nanotubes, and graphene exhibit exceptional mechanical, thermal, electrical, and gas-barrier properties. Although...

Photothermal effects induced by surface plasmon resonance at graphene/gold nanointerfaces: A multiscale modeling study

Surface plasmon resonance (SPR) biosensors have enormous potential in biological recognitions and biomolecular interactions, especially for the real time measurement of disease diagnosis and drug screening. Extensive efforts have been invested to ameliorate the sensing performances, while the photothermal effects, which are induced by the plasmon resonance, have an obvious impact. However, due to the limitations of experimental approaches, the theoretical mechanisms and specific influences of the SPR sensors with photothermal effects are few researched. Here, a multiscale simulation method is developed to investigate the photothermal effects at graphene/gold (Au) nanointerfaces, and to calculate the quantitative contribution of the photothermal effects towards high reliability SPR sensors in order to elucidate their influence on the sensing performances by means of first-principle calculations and molecular dynamics simulations. Our results indicate that the sensitivity and detection accuracy of graphene/Au SPR sensors can be tailored from 0 K to 600 K, due to the tunable dielectric constants of Au and graphene films through temperature variation. By controlling the its material thickness, interfacial combination and lattice strain, an optimized graphene/Au SPR sensor with higher sensitivity, detection accuracy, and reliability to the temperature rising has been achieved. Such multiscale simulation method, which is capable of seeking both the role and the underlying mechanism of the interfacial phenomena, can serve as an excellent guideline for the performance optimization and commercialized application of SPR sensors in the analytical chemistry and biomedical fields.

Photonic band-gap resonators for high-field/high-frequency EPR of microliter-volume liquid aqueous samples

High-field EPR provides significant advantages for studying structure and dynamics of molecular systems possessing an unpaired electronic spin. However, routine use of high-field EPR in biophysical research, especially for aqueous biological samples, is still facing substantial technical difficulties stemming from high dielectric millimeter wave (mmW) losses associated with non-resonant absorption by water and other polar molecules. The strong absorbance of mmW’s by water also limits the penetration depth to just fractions of mm or even less, thus making fabrication of suitable sample containers rather challenging. Here we describe a radically new line of high Q-factor mmW resonators that are based on forming lattice defects in one-dimensional photonic band-gap (PBG) structures composed of low-loss ceramic discs of λ/4 in thickness and having alternating dielectric constants. A sample (either liquid or solid) is placed within the E = 0 node of the standing mm wave confined within the defect. A resonator prototype has been built and tested at 94.3 GHz. The resonator performance is enhanced by employing ceramic nanoporous membranes as flat sample holders of controllable thickness and tunable effective dielectric constant. The experimental Q-factor of an empty resonator was ≈ 420. The Q-factor decreased slightly to ≈ 370 when loaded with a water-containing nanoporous disc of 50 μm in thickness. The resonator has been tested with a number of liquid biological samples and demonstrated about tenfold gain in concentration sensitivity vs. a high-Q cylindrical TE012-type cavity. Detailed HFSS Ansys simulations have shown that the resonator structure could be further optimized by properly choosing the thickness of the aqueous sample and employing metallized surfaces. The PBG resonator design is readily scalable to higher mmW frequencies and is capable of accommodating significantly larger sample volumes than previously achieved with either Fabry-Perot or cylindrical resonators.

TEOS surface modification of CLST ceramic particles for PTFE-based composites

PTFE-based ceramic-polymer dielectric composites have been widely researched in the communication field due to their good processing, wide range frequency and temperature stability and being able to provide tunable dielectric constant in a scale. In order to improve the compatibility between the ceramic fillers and polymer matrix without damage of dielectric properties, surface modifiers with less carbon remain are preferred. In this paper, tetraethylorthosilicate (TEOS) is employed as a surface modifier to improve the compatibility between the (Ca, Li, Sm)TiO3 (CLST) ceramic and PTFE, and the dispersion of the ceramic particles in the matrix. FTIR, XPS and TEM results indicate that TEOS is coated on the ceramic particles successfully and forms a silica coating layer. The surface modification improves the dispersion of particles in PTFE and interface contact between the ceramic fillers and PTFE matrix. These improve the thermal stability and reduce the dielectric loss of the dielectric composites. The CLST/PTFE composite modified by TEOS exhibits a dielectric constant of 6.22 with dielectric loss just 0.0012 at microwave frequencies (around 10 GHz).

Tin-Polyester/Polyimide Hybrid System As Flexible Free- Standing Film with Tunable Dielectric Constant for Energy Storage Application

Hybrid free standing films of tin-polyester with polyimide are prepared by blending process which exhibit tunable dielectric constants. The maximum achievable dielectric constant for hybrid systems have 60% load of tin-polyester and their dielectric values raging from ca. 5.5-6.2, close to that of the tin homopolymers. These values show improvement over polyimide p(BTDA-HDA) having a dielectric constant of ca. 3.5 with minimum frequency dispersion maintaining low dissipation factor. The hybrid systems of 60% p(DMTDMG) with 40% polyimide stand out by their morphological and electrical properties showing a breakdown strength of 464 MV/m, with a maximum energy density of 5.9 J/cm3, slightly higher than the commercial standard BOPP with improved operational temperature ca. 140 oC. Figure 1

Reconfigurable Plasmonic Logic Gates

Reconfigurable one-, two-, and three-bit plasmonic logic gate configurations have been proposed, which work by covering a straight slot waveguide with materials with tunable dielectric constants, such as graphene. By encoding the logic states in the values of dielectric constants as opposed to different waveguides, the plasmon excitation problems are minimized and the simplified logic gate configurations could be easily implemented.

Tunable band alignment and dielectric constant of solution route fabricated Al/HfO2/Si gate stack for CMOS applications

The solution route deposition method will reduce the fabrication cost, and it is compatible with existing Si technology. Here, we systematically investigate the impact of annealing temperature on the electrical and dielectric properties along with the band alignment of HfO2 thin films with silicon. The films were fabricated using the hafnium isopropoxide adduct precursor, which is environment friendly and non-toxic in ambient conditions. We have analyzed the band alignment of HfO2/Si stack by using ultra-violet photoelectron spectroscopic and current-voltage (J-V) plot to understand its impact on electrical transport. The bandgap of HfO2 films estimated from Plasmon energy loss spectra is 5.9 eV. The composition analysis is done with X-ray photoelectron spectroscopy that suggests a good stoichiometric ratio of 1:1.96. The atomic force microscopy studies display a smooth surface with the roughness of 1.4 Å without any cracks in the films. It is found that the current conduction mechanisms and barrier heights at both the interfaces are influenced by the annealing temperature; a temperature of 450 °C results in an optimum performance. Interestingly, the high value of dielectric constant (23) in the amorphous phase is attributed to the existence of cubic like short range order in HfO2 films. Moreover, a low leakage current density of 1.4 × 10−9 A/cm2 at −1 V and 1.48 × 10−8 A/cm2 at +1 V in gate and substrate injection modes is achieved. The obtained defect activation energies of 0.91 eV, 0.87 eV, and 0.93 eV for the films annealed at 350 °C, 450 °C, and 550 °C lay below the conduction band edge of HfO2. These energy levels are ascribed to three and four fold oxygen vacancy related traps. The formation of dipoles at the interface, change in the microstructure, and oxygen migration at the interfacial layer are the possible causes for the observed parametric variations in the metal–insulator–semiconductor structure. The electrical properties can be tuned by utilizing suitable annealing temperatures.