Integrated microwave structures using an advanced thick-film technology

Compositional modulation and annealing treatment in BaTiO3 to simultaneously achieve colossal permittivity, low dielectric loss, and high thermal stability

Inserting insulating barriers into conductive particle channels: A new paradigm for fabricating polymer composites with high dielectric permittivity and low dielectric loss

A facile method to increase the charge storage capability of polymer nanocomposites

Abstract: Lead-free Sr0.6Ca0.4TiO3 (SCT) ceramic was prepared by the solid state reaction route. X-Ray diffraction technique showed the phase purity and identified the orthorhombic perovskite structure of the material. Scanning Electronic Microscopy observation evidenced homogeneous morphology and dense microstructure for the ceramic. The dielectric and conductivity properties of the sample were studied using complex impedance measurement technique in a wide range of frequencies and temperatures: from 100 Hz to 1.8 GHz and from 25°C to 550°C. The ceramic exhibits a stable dielectric permittivity and low dielectric losses in frequency and temperature up to 200°C. This is very interesting in view of developing high-quality lead-free ceramic capacitors for applications requiring high temperatures; for example, in cars. The increase in dielectric permittivity for temperatures higher than 200°C may be related to oxygen vacancies that are heat-activated in the material. Dielectric losses show the existence of a dielectric relaxation at low temperatures and low frequencies. Conductivity measurement investigated at high temperatures show on one hand high AC conductivity values attributed to the high temperature jumping process and on the other hand two electrical conductivity mechanisms above 400° C in the material. Keywords: Strontium calcium titanate, Ceramic, Structure, Dielectric properties, Conductivity.

The suitability of nanoclay reinforcement for improvement of structural and electrical properties of thermosetting epoxy systems at 10 GHz is investigated via a resonant technique. The potential of nanoclay reinforcement to improve mechanical properties and mitigate moisture diffusion in polymer materials has been well-documented in recent years. Further, evidence has shown that the presence of moisture in polymer systems has a profoundly deleterious effect on relative permittivity and loss tangent of the material. This is particularly important for construction or coating of radar protecting structures (radome), in which low relative permittivity and loss tangent are critical to radar transparency. Therefore, the addition of nanoclay reinforcement to polymer composites used in radome applications may prove a viable method for dielectric and structural performance improvement and moisture absorption minimization. The relative permittivity and loss tangent of two epoxy resin systems are evaluated as a function of organoclay weight percentage using a split-post dielectric resonator operating at an X-band frequency. Nanoclay content up to 5% by weight is investigated for both systems. The addition of nanoclay did not have a significant effect on the relative permittivity of the material, contributing only up to a 1% decrease (improvement) compared to the neat epoxy. The material loss tangent, however, exhibited a consistent downward trend, with a nearly 13% decrease recorded for the nanoclay content of 5% by weight in the most extreme case. Based on these results, the addition of nanoclay to polymer composite materials used in radome applications has no detrimental effect on the dielectric properties of the material, and as such may prove to be a viable option for improving radome performance and longevity.

In this paper, NBCTO/epoxy composite films with different contents of NBCTO were prepared by solution mixing and casting method. The structure, morphology, thermostability and dielectric properties of the composite films were studied by X-ray diffraction, scanning electron microscopy, thermo-gravimetric analysis and broadband dielectric spectrometer. The composite film showed a homogeneous structure and better thermostability, and the dielectric permittivity was almost independent of the frequency range from 10 to 106 Hz at room temperature. The dielectric permittivity and dielectric loss of NBCTO/epoxy composite film increased with increasing in NBCTO content, and the dielectric permittivity was as high as 37.10 whereas the dielectric loss was only 0.019 at 1 kHz when the NBCTO volume fraction reached 50 vol%. Theoretical analysis indicated that the dielectric permittivity calculated by the EMT model was close to the experimental data. The results showed that NBCTO/epoxy composite might have potential applications in the electronic devices.

Polypropylene/carbon nanotube nano/microcellular structures with high dielectric permittivity, low dielectric loss, and low percolation threshold

ABSTRACTComposite materials with a high permittivity (high‐k) and low dielectric loss represent an important research direction for the rapid development of modern electronic. This article is about high‐k composite with low dielectric loss (dielectric constant is approximately 11, and dielectric loss is only 0.02 at 1 MHz and about 50 wt % of filler) based on a polytetrafluorethylene (PTFE) compounded with priderite (K1.46Ti7.2Fe0.8O16). The dielectric permittivity about ε' ≈ 103 and the dielectric loss of tgδ ≈ 2 have been found for filler content about 50 wt % (30 vol %) and, respectively, ε' ≈ 11 and tgδ ≈ 0.02 for 1 MHz. To produce filler, amorphous potassium polytitanate was synthesized by molten salt method, modified in aqueous solution of iron sulfate, crystallized at 700 °C and further treated in the aqueous dispersion of PTFE. The obtained product was pressured, dried and investigated by X‐ray diffraction and scanning electron microscopy. Dielectric properties of the composite with different ceramic filler content (1–90 wt %) were studied using impedance spectroscopy in the frequency range of 10−2 to 106 Hz. The influence of frequency on electric conductivity, permittivity, and dielectric losses was analyzed taking into account the experimental data on porosity, apparent density obtained for the composites. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48762.

Surface modification of core‐shell structured Al (Al@Al2O3) nanoparticles was performed using γ‐(Aminopropyl)‐triethoxysilane (APS) and dopamine (DA), respectively, and the microstructures, dielectric properties and thermal conductivities of the Al/poly(vinylidene fluoride) (PVDF) nanocomposites were investigated. Both DA and APS enhance the interfacial bonding strength between the fillers and the matrix, leading to homogeneous dispersion of Al nanoparticles in PVDF matrix. Compared with raw Al nanoparticles, surface‐treated Al/PVDF exhibit much higher dielectric permittivity due to the enhanced interfacial interactions between the two components, whereas, the dielectric loss and electric conductivity of the nanocomposites still remain at rather low levels owing to the insulating alumina shell preventing effectively core Al from direct contact. The dynamic dielectric properties results reveal that dielectric constant and loss increase with temperature due to the gradually enhanced mobility of molecular chain segments of PVDF for the raw Al/PVDF and treated Al/PVDF nanocomposites. Additionally, the PVDF nanocomposites with Al treated with APS and DA show enhanced thermal conductivities compared with raw Al/PVDF under the same filler loading because of reduced thermal interfacial resistance promoting phonon transfer across the interfaces. POLYM. ENG. SCI., 59:103–111, 2019. © 2018 Society of Plastics Engineers

A photo-dielectric effect (i.e., a change in dielectric permittivity due to photo-irradiation) has been demonstrated in LaAl0.99Zn0.01O3-δ. Photo-irradiation with an incident energy of 3.4 eV was found to enhance the dielectric permittivity in LaAl0.99Zn0.01O3-δ over a wide frequency range from 100 Hz to 1 MHz. The change in dielectric permittivity in the high-frequency region hardly depended on frequency and was not accompanied by an increase in dielectric loss, indicating an intrinsic photo-dielectric effect in LaAl0.99Zn0.01O3-δ that is not due to photo-conduction. The dependence of the photo-dielectric effect on incident energy suggests the existence of deep in-gap states introduced by Zn substitution. The mechanism of the photo-dielectric effect in LaAl0.99Zn0.01O3-δ relates to the dielectric response of the photo-excited electrons trapped in the deep in-gap states, which work as effective polar displacements under an applied electric field. These findings are expected to contribute to the development of photo-capacitors that enable the remote control of the dielectric response via photo-irradiation.

Polycrystalline alumina with extremely low microwave dielectric loss is reported with properties analogous to a theoretical ensemble of randomly oriented, single crystal sapphire grains. By avoiding deleterious impurities and by careful control of microstructure, we show that grain boundaries in aluminum oxide have only a limited influence on the dielectric loss. A method of measuring the electric permittivity and loss tangent of low-loss microwave ceramic dielectrics is reported. The electrical parameters such as relative permittivity and loss tangent are extracted using the radial mode matching technique. The measured values for ultralow loss polycrystalline aluminum oxide agree well with theoretical values modelled on an ensemble of randomly oriented anisotropic single crystal sapphire grains.

The use of nematic liquid crystal (LC) mixtures for microwave frequency applications presents a fundamental drawback: many of these mixtures have not been properly characterized at these frequencies, and researchers do not have an a priori clear idea of which behavior they can expect. This work is focused on developing a new procedure for the extraction of the main parameters of a nematic liquid crystal: dielectric permittivity and loss tangent at 11 GHz under different polarization voltages; splay elastic constant K11, which allows calculation of the threshold voltage (Vth); and rotational viscosity γ11, which allows calculating the response time of any arbitrary device. These properties will be calculated by using a resonator-based method, which is implemented with a new topology of substrate integrated transmission line. The LC molecules should be rotated (polarized) by applying an electric field in order to extract the characteristic parameters; thus, the transmission line needs to have two conductors and low electric losses in order to preserve the integrity of the measurements. This method was applied to a well-known liquid crystal mixture (GT3-23002 from MERCK) obtaining the permittivity and loss tangent versus bias voltage curves, the splay elastic constant, and the rotational viscosity of the mixture. The results validate the viability of the proposed method.

This paper reports the dielectric permittivities of several low loss ceramics of A(B11/2B21/2)O3 mixed oxide perovskites, Ba(Mg1/3Ta2/3)O3 (BMT), Sr(Al1/2Nb1/2)O3 (SAN), Sr(Al1/2Ta1/2)O3 (SAT), Sr(Ga1/2Ta1/2)O3 (SGT), and their modified compositions 0.7Sr(Al1/2Nb1/2)O3-0.3NdGaO3 (SAN-NG) and 0.7Sr(Al1/2Ta1/2)O3-0.3NdGaO3 (SAT-NG), measured at microwave frequencies. Measurements of the frequency-dependent dielectric constant and loss tangent are reported. Their loss tangent values are on the order of 10− 4 to 10− 5, which can not be evaluated by the traditional reflection/transmission spectra or impedance measurement techniques. A parallel-plate dielectric resonance method was adopted for dielectric measurements for such low loss materials. The frequency spectra of dielectric constants and loss tangents of were investigated by the well known classical oscillator model.

ABSTRACTPoly(vinylidene fluoride) (PVDF)/polymethyl methacrylate (PMMA)/carbon nanotubes (CNTs) composites with improved dielectric and electromagnetic shielding effectiveness properties are prepared by one‐step melt mixing. The microcosmic appearance and rheological behavior of PVDF/PMMA/CNTs composites illustrate that the CNTs formed well dispersion in the composite system. As a result, the dielectric permittivity of the PVDF/PMMA/CNTs composites increases with the increasing CNTs content. Meanwhile the dielectric loss is effectively restrained by obstructing the formation of conductive path due to the addition of PMMA. Moreover, beta polymorph is obtained in the PVDF/PMMA/CNTs composites, which also makes contribution to improving the dielectric properties of the composites. Therefore, with the addition of 3.5 vol% CNTs, the PVDF/PMMA/CNTs (FAC3.5) composite exhibits higher dielectric permittivity (13,686) and lower dielectric loss (0.625) than the PVDF/CNTs composite (9348 and 0.748, respectively) at frequency of 104 Hz. Additionally, the FAC3.5 sample displays an outstanding total electromagnetic shielding effectiveness reaching 17 dB, which is six times that of pure PVDF and exceeds the critical 15 dB benchmark required for commercial applications. It suggests a novel strategy for preparing dielectric composites with both lower dielectric loss and high electromagnetic shielding effectiveness through precise modulation of the loading content, dispersion optimization, and interfacial engineering of nanoparticles.

Polymer dielectric composites with high dielectric permittivity and low dielectric loss play an increasing important role for modern electronic and electric industry. In this study, polyvinylidene fluoride (PVDF) incorporated with expanded graphite (EG) or reduced graphene oxide (rGO) was prepared by solution casting and the following hot pressing. The structure, morphology, and properties of the obtained products were characterized. In comparison with EG, rGO exhibits well dispersion and strong interaction with PVDF matrix. With the increasing content of conductive fillers, the dielectric permittivity of both PVDF/EG and PVDF/rGO composites shows an increasing trend whereas the dielectric loss keeps at a low level. The dielectric permittivity of PVDF filled with 4 wt% EG and 8 wt% rGO reaches 420 and 38 at 100 Hz, respectively. Moreover, the introduction of EG and rGO improves the mechanical strength and the thermal stability of PVDF matrix.