Showing papers on "Dielectric loss published in 2017"

Lightweight and efficient microwave absorbing materials based on walnut shell-derived nano-porous carbon

Abstract: Lightweight microwave absorbing materials have drawn tremendous attention. Herein, nano-porous biomass carbon materials have been prepared by carbonization with a subsequent potassium hydroxide activation of walnut shells and the microwave absorption properties have also been investigated. The obtained samples have large specific surface areas with numerous micropores and nanopores. The sample activated at 600 °C with a specific surface area of 736.2 m2 g-1 exhibits the most enhanced microwave absorption performance. It has the maximum reflection loss of -42.4 dB at 8.88 GHz and the effective absorption bandwidth (reflection loss below -10 dB) is 1.76 GHz (from 8.08 GHz to 9.84 GHz), corresponding to a thickness of 2 mm. Additionally, the effective absorption bandwidth can reach 2.24 GHz (from 10.48 GHz to 12.72 GHz) when the absorber thickness is 1.5 mm. Three-dimensional porous architecture, interfacial polarization relaxation loss, and the dipolar relaxation loss make a great contribution to the excellent microwave absorption performance. In contrast, the non-activated sample with lower specific surface area (435.3 m2 g-1) has poor microwave absorption performance due to a poor dielectric loss capacity. This comparison highlights the role of micropores and nanopores in improving the dielectric loss property of porous carbon materials. To sum up, porous biomass carbon has great potential to become lightweight microwave absorbers. Moreover, KOH is an efficient activation agent in the fabrication of carbonaceous materials.

Dielectric thermally conductive boron nitride/polyimide composites with outstanding thermal stabilities via in-situ polymerization-electrospinning-hot press method

Abstract: Micrometer boron nitride/polyamide acid ( m BN/PAA) compound was firstly fabricated via in -situ polymerization, performed to obtain the m BN/PAA electrospun fibers by electrospinning technology, finally to fabricate the dielectric thermally conductive m BN/polyimide ( m BN/PI) composites via thermal-imidization followed by hot press method. The obtained m BN/PI composite with 30 wt% m BN presents relatively highly thermally conductive coefficient ( λ ), excellent dielectric constant ( e ) & dielectric loss tangent (tan δ ), and extremely outstanding thermal stability, λ of 0.696 W/m K, e of 3.77, tan δ of 0.007, T Heat-resistance index ( T HRI ) of 279 °C and glass transition temperature ( T g ) of 240 °C, which presents a great potential for packaging in integration and miniaturization of microelectronic devices.

Improved thermal conductivity and dielectric properties of hBN/PTFE composites via surface treatment by silane coupling agent

Abstract: To achieve polymer-based composites for electronic packaging with low dielectric constant, low dielectric loss tangent and high thermal conductivity, silane coupling agent KH550 modified hexagonal boron nitride (hBN) platelets were introduced into PTFE matrix via a cold pressing and sintering method. The effect of surface treatment on the morphology, thermal conductivity and dielectric properties of the composites was investigated. The results revealed that after surface treatment, the interfacial adhension between hBN platelets and PTFE matrix was improved and the in-plane orientation degree of hBN platelets in PTFE matrix decreased, which effectively improvd the thermal conductivity of the composites. The thermal conductivity of hBN-KH550/PTFE composite with 30 vol% filler content is 0.722 W/mK, which is 2.7 folds of pure PTFE. Moreover, the enhanced interfacial adhension and reduced surface hydrophilicity of hBN platelets significantly decreased the interfacial polarization, resulting in not only lower dielectric constant and dielectric loss tangent but also weaker frequency-dependence.

50th Anniversary Perspective: Dielectric Phenomena in Polymers and Multilayered Dielectric Films

Abstract: High dielectric constant and low dielectric loss are desirable electrical properties for next-generation polymer dielectrics that show promise for applications in pulsed power, power electronics, and printable electronics. Unfortunately, the dielectric constant of polymers is often limited to 2–5, much lower than that of inorganic dielectrics, because of the nature of hydrocarbon covalent bonds for electronic and atomic polarizations. It is essential to understand the fundamental physics of different types of polarization and the associated loss mechanisms in polymers. In this Perspective, we discuss the characteristics of each polarization and explain how to enhance the polarization using rational molecular designs without causing significant dielectric losses. Among various approaches for high dielectric constant and low loss polymers, the multilayer film technology is of particular interest because a multilayer film is a unique one-dimensional system with tailored material choices, layer thicknesses, a...

Relaxor ferroelectric 0.9BaTiO3–0.1Bi(Zn0.5Zr0.5)O3 ceramic capacitors with high energy density and temperature stable energy storage properties

Abstract: A relaxor ferroelectric ceramic for high energy storage applications based on 0.9BaTiO3–0.1Bi(Zn0.5Zr0.5)O3 (0.9BT–0.1BZZ) was successfully fabricated via a conventional solid-state method. The sintered samples have a perovskite structure with a pseudocubic phase, showing a moderate dielectric constant (500–2000), low dielectric loss (tan δ < 0.15) and highly diffusive and dispersive relaxor-like behavior. The weak dielectric nonlinearity exhibits a dielectric constant change of ∼10% as the bias electric field increases from 0 kV cm−1 to 40 kV cm−1. Extra slim polarization–electric field loops accompanying the slow decrease of breakdown strength from 266.5 kV cm−1 to 217.7 kV cm−1 are observed in a measured temperature range of 30–150 °C. A maximum energy density of 2.46 J cm−3 was obtained at the electric field of 264 kV cm−1 close to the breakdown strength at ambient temperature. Temperature stability of both energy density and energy efficiency exists in a wide temperature range, which makes BT–BZZ ceramics promising candidates for high power electric applications.

Facile preparation of in situ coated Ti3C2Tx/Ni0.5Zn0.5Fe2O4 composites and their electromagnetic performance

Abstract: A novel family of Ti3C2Tx/ferrite composites with high reflection loss was developed using a facile in situ co-precipitation method. The as-synthesized Ti3C2Tx/ferrite composite with a 5 wt% Ti3C2Tx MXenes loading exhibited high reflection loss (−42.5 dB) at 13.5 GHz. The effective absorption bandwidth of the 5 wt% Ti3C2Tx/Ni0.5Zn0.5Fe2O4 composite reached ∼3 GHz (12–15 GHz) in the K-band. The incorporation of Ti3C2Tx MXenes improved the electromagnetic impedance of the Ti3C2Tx/Ni0.5Zn0.5Fe2O4 composite resulting from the enhanced electrical conductivity. The potential electromagnetic wave absorption mechanisms were revealed, which may contain magnetic loss, dielectric loss, conductivity loss, multiple reflections, and scattering. The technique is facile, fast, scalable, and favorable for the commercialization of this composite. This study provides a potential way to develop EM wave absorbing materials for a large family of MXenes/ferrite composites.

Dielectric relaxation and Maxwell-Wagner interface polarization in Nb2O5 doped 0.65BiFeO3–0.35BaTiO3 ceramics

Abstract: Electrical characterizations of Nb2O5 doped 0.65BiFeO3–0.35BaTiO3 (0.65BF–0.35BT) ceramic were carried out over broad temperature and frequency ranges through dielectric spectroscopy, impedance spectroscopy, and ac conductivity measurements. The dielectric constant and loss tangent are drastically reduced with introducing Nb2O5 into the 0.65BF–0.35BT system. Two dielectric anomalies are detected in the temperature regions of 100 °C ≤ T ≤ 280 °C and 350 °C ≤ T ≤ 480 °C, and the Curie temperature (TC) was confirmed in higher temperature region. A dielectric relaxation with large dielectric constants was detected near the TC. This dielectric relaxation becomes even stronger with the gradual increase in the Nb2O5 content. Impedance spectroscopy results clearly show the contributions of grains and grain boundaries in the frequency range of 100 Hz ≤ f ≤ 1 MHz, and the relaxation processes for grains and grain boundaries are non-Debye-type. The grain boundaries are more resistive than that of the grains, revealing the inhomogeneity in samples. The experimental results are well fitted based on a Maxwell-Wagner (MW) interfacial polarization model below 100 kHz, and the MW interfacial polarization effect becomes more and more obvious with the increase in the Nb2O5 content. The increase in dielectric constant is possibly related to space charge polarization, which is caused by charges accumulated at the interface between the grain and grain boundaries. Frequency dependence of the ac conductivity confirms the MW interfacial polarization effect below 100 kHz.

Hexagonal boron nitride/polymethyl-vinyl siloxane rubber dielectric thermally conductive composites with ideal thermal stabilities

Abstract: Hexagonal boron nitride/polymethyl-vinyl siloxane rubber (hBN/VMQ) dielectric thermally conductive composites were fabricated via kneading followed by hot compression method. The thermally conductive coefficient (λ), thermal diffusion coefficient (α), dielectric constant (e) and dielectric loss tangent (tan δ) values were all increased with the increasing addition of hBN fillers. When the volume fraction of hBN fillers was 40 vol%, the corresponding λ and α was 1.110 W/m K and 1.174 mm2/s, 6 and 9 times than that of pure VMQ matrix, respectively. The corresponding e and tan δ was 3.51 and 0.0054, respectively. Furthermore, the tensile strength and THeat-resistance index (THRI) values were both maximum with 20 vol% hBN fillers, tensile strength of 3.31 MPa, 12 times than that of pure VMQ matrix (0.28 MPa), and THRI of 253.8 °C. The obtained hBN/VMQ composites present great potential for packaging in continuous integration and miniaturization of microelectronic devices.

Fe3O4@Carbon@Polyaniline Trilaminar Core–Shell Composites as Superior Microwave Absorber in Shielding of Electromagnetic Pollution

Abstract: The present work reports fabrication of trilaminar core–shell composites of Fe3O4@C@PANI as efficient lightweight electromagnetic wave absorber by facile hydrothermal method and subsequent high-temperature calcination followed by its encapsulation through oxidative polymerization of aniline. The prepared composite structure was characterized by FTIR, XRD, XPS, TEM, HRTEM, and SQUID. The measurement of reflection loss, complex permittivity, complex permeability, and total shielding efficiency of the composites has been carried out in the frequency range of 2–8 GHz. Our findings showed lowest reflection loss (∼33 dB) in composite comprised of Fe3O4@C:aniline (1:9 wt/wt) corresponding to shielding efficiency predominantly due to absorption (∼47 dB) than reflection (∼15 dB). Such high value of shielding efficiency could be ascribed to the presence of dual interfaces and dielectric–magnetic integration in Fe3O4@C@PANI. In all probability, higher dielectric loss through interface polarization and relaxation eff...

Investigation on the broadband electromagnetic wave absorption properties and mechanism of Co3O4-nanosheets/reduced-graphene-oxide composite

Abstract: A cobaltosic-oxide-nanosheets/reduced-graphene-oxide composite (CoNSs@RGO) was successfully prepared as a light-weight broadband electromagnetic wave absorber. The effects of the sample thickness and amount of composite added to paraffin samples on the absorption properties were thoroughly investigated. Due to the nanosheet-like structure of Co3O4, the surface-to-volume ratio of the wave absorption material was very high, resulting in a large enhancement in the absorption properties. The maximum refection loss of the CoNSs@RGO composite was–45.15 dB for a thickness of 3.6 mm, and the best absorption bandwidth with a reflection loss below–10 dB was 7.14 GHz with a thickness of 2.9 mm. In addition, the peaks of microwave absorption shifted towards the low frequency region with increasing thickness of the absorbing coatings. The mechanism of electromagnetic wave absorption was attributed to impedance matching of CoNSs@RGO as well as the dielectric relaxation and polarization of RGO. Compared to previously reported absorbing materials, CoNSs@RGO showed better performance as a lightweight and highly efficient absorbing material for application in the high frequency band.

Electromagnetic wave absorption properties of a carbon nanotube modified by a tetrapyridinoporphyrazine interface layer

Abstract: Thinner absorbents with high dielectric loss usually cannot meet the requirement of impedance match, and multi-layer absorbents with excellent performance usually cannot be thin. Thus, it is a challenge to balance strong dielectric loss and impedance matching. An impedance matching interface layer can provide abundant interfaces, which are highly desirable for enhancing electromagnetic absorbing capability and decreasing surface reflection. In this study, cobalt tetrapyridinoporphyrazine (CoTAP) was assembled on the surface of multi-walled carbon nanotubes (MWCNTs) as a shell via a coordination bond; this produced a heterostructure and enhanced interfacial polarization loss at the hetero-interface. The impedance matching characteristic of the CoTAP–CNT hybrid can be optimized by the CoTAP shell with an intermediate conductivity. Contact resistance between CNTs can be increased via insulation owing to the CoTAP shell, which decreases surface EM reflection. When the CNT content of the CoTAP–CNTs hybrid is 30 wt% and the thickness of the absorber is 2.1 mm, the minimum value of the reflection coefficient and the corresponding frequency are −54.7 dB and 9.8 GHz, respectively. The combination of CNTs and the intermediate dielectric loss CoTAP in a core–shell hybrid can overcome the contradiction of strong dielectric loss and impedance matching of traditional materials; this can be considered as an effective route for designing high-performance EM absorbing materials.

Enhanced Energy-Storage Density and High Efficiency of Lead-Free CaTiO3–BiScO3 Linear Dielectric Ceramics

Abstract: A novel lead-free (1 – x)CaTiO3-xBiScO3 linear dielectric ceramic with enhanced energy-storage density was fabricated With the composition of BiScO3 increasing, the dielectric constant of (1 – x)CaTiO3-xBiScO3 ceramics first increased and then decreased after the composition x > 01, while the dielectric loss decreased first and increased For the composition x = 01, the polarization was increased into 1236 μC/cm2, 46 times higher than that of the pure CaTiO3 The energy density of 09CaTiO3-01BiScO3 ceramic was 155 J/cm3 with the energy-storage efficiency of 904% at the breakdown strength of 270 kV/cm, and the power density was 179 MW/cm3 Comparison with other lead-free dielectric ceramics confirmed the superior potential of CaTiO3–BiScO3 ceramics for the design of ceramics capacitors for energy-storage applications First-principles calculations revealed that Sc subsitution of Ti-site induced the atomic displacement of Ti ions in the whole crystal lattice, and lattice expansion was caused by va

Rice husk-based hierarchically porous carbon and magnetic particles composites for highly efficient electromagnetic wave attenuation

Abstract: A pre-modification method and a post-modification method have been developed to fabricate rice husk-based porous carbon and magnetic particles composite absorbers (RHPC/Fe and RHPC/Co). Magnetic Fe and Co particles were selected to incorporate into rice husk-based porous carbon to make full use of the synergistic effect between dielectric loss and magnetic loss. The structure, morphology, magnetic properties and electromagnetic (EM) wave absorption performance of the synthesized absorbers were investigated in detail. Inside the porous structure, the incident EM wave can be attenuated by means of Debye dipolar relaxation-derived dielectric loss and exchange resonance-derived magnetic loss through multiple scattering and absorption. For RHPC/Fe, at a thickness of 1.4 mm, a reflection loss (RL) value of −21.8 dB was achieved with the effective absorption bandwidth (RL ≤ −10 dB) of 5.6 GHz. For RHPC/Co, an RL value of −40.1 dB was obtained with the effective absorption bandwidth (RL ≤ −10 dB) of 2.7 GHz at a thickness of 1.8 mm. The excellent EM wave dissipation ability of RHPC/Fe and RHPC/Co can be attributed to the good EM impedance matching condition, quarter-wavelength cancellation and strong EM wave attenuation inside the absorber. Consequently, considering the EM wave absorption performance, the RHPC/Fe and RHPC/Co synthesized in this work are promising candidates in the field of EM wave attenuation.

Core-shell structured BaTiO3@Al2O3 nanoparticles in polymer composites for dielectric loss suppression and breakdown strength enhancement

Abstract: Design of core–shell nanoarchitectures is powerful approach to obtain advanced high-k polymer nanocomposites. Core–shell nanoparticles with uniform amorphous Al2O3 shell layer encapsulating BaTiO3 (BT) particles were fabricated through an effective, facile and low-cost heterogeneous nucleation method. The dielectric behaviors of the polyvinylidene fluoride (PVDF) nanocomposite films were adjusted by varying BT@Al2O3 nanoparticles loadings. The effects of Al2O3 shell and the arising interfaces on the dielectric and electric properties in comparison with bare BT nanoparticles were studied. Due to the highly insulating Al2O3 shell with proper dielectric constant, the local electric field distribution around interfaces between polar BT nanoparticles and PVDF were greatly modified leading to obvious dielectric loss suppression yet maintaining high dielectric constant. The study provides a solution for obtaining high-k dielectric composite with low loss and high breakdown strength, which is highly desired in high power systems.

Colossal permittivity with ultralow dielectric loss in In+Ta co-doped rutile TiO2

Abstract: Colossal permittivity (CP) materials have many important applications in electronics but their development has generally been hindered due to the difficulty in achieving a relatively low dielectric loss. In this work, we report an In + Ta co-doped TiO2 material system that manifests high dielectric permittivity and low dielectric loss based on the electron-pinned defect-dipole design. The dielectric loss can be reduced down to e.g. 0.002 at 1 kHz, giving high performance, low temperature dependent dielectric properties i.e. er > 104 with tanδ < 0.02 in a broad temperature range of 50–400 K. Density functional theory calculations coupled with the defect analysis uncover that electron-pinned defect dipoles (EPDDs), in the form of highly stable triangle-diamond and/or triangle-linear dopant defect clusters with well-defined relative positions for Ti reduction, are also present in the host material for the CP observed. Such a high-performance dielectric material would thus help for practical applications and points to further discovery of promising new materials of this type.

Colossal permittivity of (Mg + Nb) co-doped TiO2 ceramics with low dielectric loss

Abstract: A high dielectric loss is one of the difficulties that hinder the application of colossal permittivity (CP) materials. Here we report the CP behaviors in ceramics of rutile (Mg + Nb) co-doped TiO2, i.e., (Mg1/3Nb2/3)xTi1−xO2. The room-temperature dielectric properties of the pure homogenous ceramics include a relatively high CP (>104) and an acceptable dielectric loss (<0.1) at frequencies from 102 to 105 Hz in the doping concentration range of 0.5% to 7%. In particular, an excellent low dielectric loss of 0.0083 and a high dielectric permittivity of 3.87 × 104 at 1 kHz were obtained for the 1% doped sample. Moreover, the temperature stability (room temperature ∼ 180 °C) and frequency stability (102–105 Hz) of the CP properties were studied. X-ray photoelectron spectroscopy suggests that the superior CP properties could be explained by the electron-pinned defect-dipole mechanism.

Optical properties of pure and doped PVA:PEO based solid polymer blend electrolytes: two methods for band gap study

Abstract: In this work an innovative experimental method has been proposed to estimate the optical bandgap and determine the types of electronic transitions. Solid polymer blend electrolyte films based on PVA:PEO have been prepared by the well known solution cast technique. It was observed that the absorption increased with increasing aluminum salt concentration and shifted to higher wavelengths. Shifting of absorption edge to lower photon energy indicates a good reactivity between the polymer blends and the aluminum salt which in turn the energy band gap decrement is expected. An increase in refractive index for the doped samples has been observed. The miscibility between the aluminum salt and the polymer blends can be well understood from the linear relationship between the refractive index and the volume fraction of the added salt. The increase of extinction coefficient at high wavelengths was observed. The optical band gap measured from the plots of (αhυ)^x versus photon energy (hυ) was compared to that determined from the optical dielectric loss. From the results of the present work it is understood that in order to avoid the plotting of many figures based on Tauc model, optical dielectric loss must be studied. Further research works are required to satisfy that the optical dielectric loss can be used to estimate the band gap and identify the types of electronic transition. The Urbach energy was found to increase with increasing aluminum salt concentration.

Ultralow dielectric, fluoride-containing cyanate ester resins with improved mechanical properties and high thermal and dimensional stabilities

Abstract: In this contribution, we present a new strategy for the fabrication of modified cyanate ester resins combined with ultralow dielectric properties, improved mechanical properties and high thermal and dimensional stabilities. The fluoride-containing compound 2-((3-(trifluoromethyl)phenoxy)methyl)oxirane (TFMPMO), synthesized from m-(trifluoromethyl)phenol (TFMP) and epichlorohydrin (ECH), was used to modify bisphenol A dicyanate ester (BADCy) resins via copolymerization reaction. The BADCy resin modified with 15 wt% TFMPMO presented ultralow dielectric constant (e, 2.75) and dielectric loss tangent values (tan δ, 6.7 × 10−3), high mechanical properties (impact strength of 15.4 kJ m−2 and flexural strength of 141.0 MPa), and superior thermal and dimensional stability (THeat-resistance index of 206 °C and coefficient of thermal expansion of 6.4 × 10−5), and it possesses great potential application in radomes and antenna systems of aircraft.

Ultrafast Discharge and High-Energy-Density of Polymer Nanocomposites Achieved via Optimizing the Structure Design of Barium Titanates

Abstract: Electrostatic capacitors have been applied into high-power-density pulsed power systems comprising moderate energy density and ultrafast charging/discharging in the order of magnitude with a few milliseconds. In this article, different BaTiO3 nanostructures (e.g., nanoparticles, nanofibers, nanotubes, core–shell structure nanofibers) were synthesized and used to prepare polymer composite films in the poly(vinylidene fluoride) (PVDF) matrix. The effects of BaTiO3 nanostructures on the dielectric constant, dielectric loss, alternating current conductivity, breakdown strength, energy density, and mechanical properties of nanocomposites were investigated systematically. The largest dielectric constant of 17.14 was found in the 4 vol % BaTiO3 nanotube/PVDF nanocomposites. The BaTiO3@Al2O3 nanofiber/PVDF nanocomposites show the highest energy density of 12.37 J cm–3 at 450 MV m–1, ultrafast discharge speed (0.32 μs), and good mechanical properties. This article could open up a convenient and effective way for p...