Dielectric Loss Processes Research Articles

ZnO-decorated SiC@C hybrids with strong electromagnetic absorption.

A novel strategy is provided to improve the absorption of SiC nanomaterials through surface carbonization of SiC nanowires and hydrolysis. SiC@C-ZnO composites were synthesized with different dosages of ZnNO3·6H2O. Composition, microstructure, and electromagnetic properties of the composites were characterized and analyzed. Results from TEM and XRD show that crystalline ZnO particles adhere to the surface of amorphous carbon, and the ZnO content increases as a function of a dosage of ZnNO3·6H2O. The as-prepared SiC@C-ZnO hybrids exhibit effective electromagnetic absorption, which is related to a synergy effect of different dielectric loss processes. The minimum reflection loss reached -65.4 dB at 11 GHz at a sample thickness of 3.1 mm, while the effective absorption bandwidth (EAB) reached 7 GHz at a sample thickness of 2.56 mm. Furthermore, the EAB of the samples can also cover the whole X band and Ku band at small sample thicknesses (2.09-3.47 mm). The excellent properties of the materials suggest great prospect as electromagnetic absorbers.

Flexible TiN/Co@Carbon nanofiber mats for high-performance electromagnetic interference shielding and Joule heating applications

Carbon nanofibers (CNFs) are self-standing, lightweight and stable materials that naturally suitable for electromagnetic interference (EMI) shielding and Joule heating applications. However, intrinsic low conductivity severely restricts the operational performance of CNFs. Current methods that can improve the conductivity of CNFs inevitably sacrifice their flexibility for exchange. This work develops a nitride forming metal and magnetic metal codope method of nitrogen-doped CNFs by using electrospinning and annealing, and obtains highly conductive and flexible CNFs embedded with TiN and Co nanoparticles (TCCNFs). Multiple characterizations and electrical and mechanical tests are carried out to reveal that the strong Ti–N covalent bond can enhance the mechanical robustness and the Co nanoparticles contribute to improve the conductivity of CNFs. As a result, a multilayer porous TCCNF mat exhibits an ultrahigh EMI shielding effectiveness of 93 dB in the 8.2–12.4 GHz frequency range due to the combined effects of multi-reflection, dielectric polarization relaxation, and conducting and magnetic loss processes. Additionally, the TCCNF mat-based heater shows the highest Joule heating efficiency of 1508 °C W−1 cm2 among the flexible conductive materials and a superfast Joule heating (cooling) rate of 28 (31) °C/s. The TCCNF mat-based heater is able to reduce the restoring time of Ti3C2Tx MXene humidity sensor by about 78%. The TCCNF mats should also be potential electrode materials for wearable electronics.

Microwave-driven heterogeneous catalysis for activation of dinitrogen to ammonia under atmospheric pressure

This paper presents an innovative approach to producing energy-dense, carbon–neutral liquid ammonia as a means for carrying energy. This approach synergistically integrates microwave reaction chemistry with novel heterogeneous catalysis that decouples dinitrogen activation from high-temperature and high-pressure reactions, altering reaction pathways and increasing ammonia formation rate. Results presented here demonstrate that ammonia synthesis can be conducted at 280 ℃ and ambient pressure to achieve ~1 mmol ammonia/g cat/h over supported ruthenium catalyst systems utilizing microwave irradiation. It is further shown that adding promoter ions such as potassium, cerium, and barium significantly improves the ammonia production rate over undoped ruthenium-based catalysts. This effect could be attributed to enhanced dielectric loss processes that lead to stronger microwave absorption by the catalyst. Measurement of the equilibrium constant under microwave conditions showed a higher ammonia yield than under thermal equilibrium conditions for both the iron- and ruthenium-based catalysts. Finally, this study also illustrates the advantages of using a variable-frequency microwave reactor for ambient-pressure ammonia synthesis. Mechanistically, investigators believed that the oscillating electric fields of the radiation can couple with adsorbed nitrogen on the surface and accelerate its dissociation. Since dinitrogen dissociation on the surface is rate limiting, this effectively accelerates the reaction. Overall, the study provides an in-depth analysis of the parameters affecting the use of microwaves in catalyzed ammonia synthesis. This process is fundamentally different from the commercial Haber–Bosch process and provides an alternative method of ammonia synthesis for certain applications, as it is tolerant to intermittent supplies of renewable energy, therefore effectively operating at variable rates of production.

On the influence of morphology and chemical defects on charge transport dynamics in polyethylene: thermal ageing and concentration gradient

A blend composed of a high-density polyethylene and a low-density polyethylene was subjected to thermo-oxidative ageing at 160 °C, under conditions where oxygen diffusion effects lead to a non-uniform distribution of chemical defects throughout the specimens. The existence of highly and lightly oxidised regions was demonstrated by Fourier transform infrared spectroscopy and confocal Raman microprobe spectroscopy and the effect of these on the dielectric behaviour of the system was investigated. Dielectric spectroscopy revealed a broad loss peak at frequencies above 1 Hz that increases in strength with increasing ageing time, which we associate with the motion of polar chain segments within the polymer. For ageing times below 3 h, samples exhibited reduced conductivity under a constant electric field compared with the initial unaged system; above 3 h, the conductivity was found to increase monotonically with ageing time and corresponded to the appearance of an additional low frequency (<1 Hz) dielectric loss process and marked migration of space charge into the bulk away from the relevant physical electrode. This change in behaviour was found to occur abruptly and the final distribution of space charge was found to correlate well with the distribution of ageing inducing chemical defects within the system. From these observations, the effects of chemical defects on charge transport dynamics are discussed.

A new strategy for the preparation of silver/epoxy/poly(vinylidene fluoride) dielectric composites with a multi‐interface structure for suppressed dielectric loss

Dielectric materials with low dielectric loss are highly desirable for future energy storage and management. Relatively high dielectric loss and complex fabrication process with high cost are current major factors limiting percolative dielectric composites for applications. In this study, low dielectric loss has been obtained using multi‐interface silver/epoxy/poly(vinylidene fluoride) (Ag/epoxy/PVDF) percolative composites by simple thermal curing and mechanical crushing, coupled with melt mixing and hot pressing. Isolated Ag/epoxy islands were formed in the PVDF matrix. The dielectric permittivity of the composite near the percolation threshold reached 132 whereas the loss tangent was around 0.1 at 100 Hz. This multi‐interface structure played an important role in suppressing the dielectric loss and achieving the relatively high dielectric permittivity in these composites. Our results suggest that the simple and low‐cost fabrication of the Ag/epoxy/PVDF is a promising approach for the development of low‐loss percolative dielectric composites. POLYM. COMPOS., 39:E2606–E2610, 2018. © 2018 Society of Plastics Engineers

Structural, magnetic, and dielectric studies of the Aurivillius compounds SrBi5Ti4MnO18 and SrBi5Ti4Mn0.5Co0.5O18

We have successfully synthesized the Aurivillius compounds SrBi5Ti4MnO18 and SrBi5Ti4Mn0.5Co0.5O18 using a modified Pechini method. Both samples have an orthorhombic structure with the space group B2cb. The valence state of Mn is suggested to be +3 and the doped Co ions exist in the form of Co2+ and Co3+ based on the results of x-ray photoelectron spectroscopy. The sample SrBi5Ti4MnO18 exhibits a dominant paramagnetic state with the existence of superparamagnetic state as evidenced by the electron paramagnetic resonance results, whereas SrBi5Ti4Mn0.5Co0.5O18 undergoes a ferrimagnetic transition at 161 K originating from the antiferromagnetic coupling of Co-based and Mn-based sublattices, and a ferromagnetic transition at 45 K arising from the Mn3+-O-Co3+ (low spin) interaction. The sample SrBi5Ti4Mn0.5Co0.5O18 exhibits two dielectric anomalies. One corresponds to a relaxor-like dielectric relaxation which follows the Vogel-Fulcher function and the other dielectric relaxation obeys the Arrhenius law arising from the collective motion of oxygen vacancies. In addition, the sample SrBi5Ti4Mn0.5Co0.5O18 exhibits a magnetodielectric effect caused by the Maxwell-Wagner effect because of the conductivity of the sample. This is demonstrated by the fact that the activation energy in dielectric loss process is close to that for dc conductivity and the magnetodielectric effect is sensitive to the measured frequency.

Dilectric and thermal properties of PEO doped with cadimium chloride salt

The main aim of this research is to investigate the effect of salt concentration on the dielectric properties (AC (σ AC), permittivity (ɛ′), dielectric loss (σ″), and dielectric relaxation process) and melting behavior of polyethylene oxide (PEO)/CdCl2 complexes. The dielectric study was carried out over a frequency range 102–335 kHz and a temperature range 25–45°C. The AC conductivity, permittivity and dielectric loss of the PEO/CdCl2 complexes increase with increasing salt concentration and temperature. Also, it was found that the addition of CdCl2 salt to PEO host reduced the melting temperature of PEO host. Dielectric results reveal that the relaxation process of these complexes is due to viscoelastic relaxation or non-Debye relaxation at room temperature. Additionally, it was found that relaxation behavior remained viscoelastic at different temperatures and salt concentrations.

Some general remarks on the dielectric loss mechanisms in mineral oils

The mechanisms of conduction and dielectric loss pertaining to electrical insulating oils, are examined with specific reference to the conduction and dielectric loss processes involving ionic and macroscopic particle charge carriers. Specific examples are presented; one of the most serious types of conduction by particles is that due to semiconductive copper sulfide particles, which are formed in a reaction between copper conductors or copper particles and the corrosive sulfur components contained in the oils.

Dielectric relaxation studies in transition metal ions doped tellurite glasses

Vanadotellurite and vanado-cobalt tellurite glasses were investigated for dielectric properties in the frequency range 50 Hz to 5 MHz and temperature range 300–500 K. Hunt's model has been invoked to analyze the data and the results indicated that the relaxation process has a local character and can be described by hops between each pair of sites. Activation energies for dielectric loss process were determined and they were in close agreement with dc activation energies. Barton, Nakijama and Namikawa's (BNN) linear relation between ac and dc conductivities has been verified in both the glass systems.

A comparison of relaxation processes in structurally related van der Waals glass formers: The role of internal degrees of freedom

Depolarized dynamic light scattering (DLS), dielectric relaxation (DS), and deuterium NMR studies of fragile van der Waals glass forming liquids phenylphthalein-dimethylether (PDE) and cresolphthalein-dimethylether (KDE) are presented. In PDE a new dielectric loss process was found, which can be attributed to the 180 degrees flip of the phenyl rings. The previous finding that the distribution of the structural relaxation times measured for PDE and KDE by DS is substantially narrower than that measured by DLS is explained by partial decoupling of the dynamics of the dipole moment from the structural relaxation of the sample. The dynamics of PDE and KDE is compared with the previous studies of two other structurally similar liquids: 1,1'-di(4-methoxy-5-methylphenyl)cyclohexane (BMMPC) and 1,1'-bis(p-methoxyphenyl)cyclohexane (BMPC) in order to relate dynamical features with the chemical structure of the material. The evidence for the intramolecular character of the secondary relaxations observed in BMPC and PDE is presented.

Dielectric properties of BaTi 1− xSn xO 3 solid solution

The rentgenographic and dielectric properties of BaTi 1− x Sn x O 3 solid solutions were measured and analyzed as a function of Sn concentration x in room temperature. The mechanism of dielectric loss and electric polarization, involving correlation between microstructural features and dielectric ones, observed as macroscopic properties was postulated and discussed. Two dielectric loss processes were isolated for frequencies values which are Sn concentration dependent.

Electroactive properties of flexible piezoelectric composites

A flexible piezoelectric composite with 0-3 connectivity, made from Lead Zirconate Titanate (PZT) powder and vegetable-based polyurethane (PU), was doped with small amount of semiconductor powder. As a result a composite with 0-0-3 connectivity was obtained. The nature of absorption and steady state electrical conduction and the dielectric behaviour have been studied for this ceramic/polymer composite. The dielectric loss processes of the composite were observed to be dominated by those the polymer. Adding a semiconductor phase in the composite the electrical conductivity can be controlled and a continuous electric flux path could be created between the PZT grains. This composite may be poled at low voltage and in shorter time compared with composites without a conductive phase.

Is the Maxwell–Sillars–Wagner model reliable for describing the dielectric properties of a core–shell particle–epoxy system?

Dielectric measurements have been performed on neat acrylic core–shell particles, and core–shell modified-epoxy networks in order to assess the accuracy of the Maxwell–Wagner–Sillars (MWS) model in describing the dielectric properties of heterogeneous polymeric systems. Measured and predicted interfacial polarizations are compared. The temperature dependence of the frequency of maximum interfacial dielectric loss process is well described by the MWS model and from the temperature dependence of the conductivity of the two phases. However, the measured and predicted absolute values differ and a distribution in the interfacial relaxation time is observed. This is attributed to the effect of a gradient in concentration across the interface between the acrylic particles and the epoxy matrix. It is thus concluded that the nature and properties of interphases can strongly influence the dielectrical properties and interfacial polarization processes in polymer blends, and that theoretical models which do not take into account details of interfacial charge transfer might not be reliable in describing the morphology of heterogeneous polymeric systems.

Dielectric Relaxation in Small Confining Geometries

ABSTRACTBroadband dielectric spectroscopy (10−2 Hz – 109 Hz) is employed to study the molecular dynamics of the glassforming liquid salol (phenyl salicylate) being contained in (dielectric inactive) porous glasses with pore sizes of about 2.5 nm, 5.0 nm and 7.5 nm. Besides the bulk relaxation (I) of salol, two further dielectric loss processes are observed due to the geometrical constraint of the inner surfaces of the pores: the “interfacial relaxation” (II) and a Maxwell-Wagner polarization (III). The “interfacial relaxation” is assigned to the restricted dynamics of the molecules close to the inner surface of the pores. It shows a strong pore-size dependence for pores &lt; 5 nm both in relaxation rate and dielectric strength.

Collective and molecular dynamics in low molar mass and polymeric ferroelectric liquid crystals

Abstract By use of a novel sample preparation technique, it is possible to measure with one sample cell the collective and molecular dynamics (10−1 Hz-109 Hz) of macroscopically (in bookshelf geometry) oriented ferroelectric liquid crystals. Below 106 Hz, two ferroelectric modes, Goldstone- and soft-mode, are observed, being assigned to fluctuations of the phase and amplitude of the helical superstructure respectively. Between 106 and 109 Hz, one dielectric loss process exists, the β-relaxation which originates from the librational motion (hindered rotation) of the mesogen around its molecular long axis. This process does not split or broaden at the non-ferroelectric-ferroelectric phase transition and it has an Arrhenius type temperature dependence. In comparing a racemic mixture with a chiral sample, it performs a similar frequency and temperature dependence. The experimental findings for the β-relaxation are in qualitative contrast to the predictions of the (generalized) Landau expansion of the free ene...

Collective and molecular dynamics in ferroelectric liquid crystals: from low molar to polymeric and elastomeric systems

Broadband dielectric spectroscopy delivers in the frequency range from 10 Hz to 1010 Hz two collective dielectric loss processes (soft and Goldstone modes) and one molecular relaxation (β-relaxation). The soft mode and Goldstone mode are assigned to the fluctuation of the amplitude and the phase of the helical superstructure. The β-relaxation corresponds to the libration (hindered rotation) of the mesogene around its long molecular axis. At the SmA–SmC* phase transition this process does not split or broaden, and the temperature dependence of its relaxation rate does not show any deviation from an Arrhenius-like behavior. Its dielectric strength does not decline at the SmA–SmC* phase transition. These experimental findings are in contrast to the common explanation of the origin of the saturation polarization (“induced spontaneous polarization”), which is based on the existence of a “free” rotation inside the SmA phase and its strong hindrance in the ferroelectric SmC* phase. Furthermore, the high frequency results require a reformulation for the (generalized) Landau theory as applied to the SmA–SmC* phase transition. In comparing low molar mass and polymeric (elastomeric) FLC, the collective and molecular dynamics are qualitatively similar, independent of the molecular architecture (e.g. side-chain, combined main- and side-chain or crosslinked systems).

A reconstruction of charge movement during the action potential in frog skeletal muscle

The transfer of intramembrane charge during an action potential at 4 degrees C was reconstructed for a model representing the electrical properties of frog skeletal muscle by a cylindrical surface membrane and 16 concentric annuli ("shells") of transverse tubular membrane of equal radial thickness. The lumina of the transverse tubules were separated from extracellular fluid by a fixed series resistance. The quantity, geometrical distribution and steady-state and kinetic properties of charge movement components were described by equations incorporating earlier experimental results. Introducing such nonlinear charge into the distributed model for muscle membrane diminished the maximum amplitude of the action potential within the transverse tubules by 2 mV but increased the maximum size of the after-depolarization by 3-5 mV and also its duration. However, these changes were small in comparison to the 135-mV deflection represented by the action potential. They therefore did not justify altering the values of the electrical parameters adopted by Adrian R.H., and L.D. Peachey (1973. J. Physiol. [Lond.]. 235:103-131.) and used in the present calculations. Cable properties significantly affected the time course and extent of charge movement in each shell during action potential propagation into the tubular system. Q beta charge moved relatively rapidly in all annuli, and did so without significant latency (approximately 0.3 ms) after the surface action potential upstroke. Its peak displacement varied between 53 and 58% (the range representing the difference fiber edge/fiber axis) of the total Q beta charge. This was attained at 5.4-7.3 ms after the stimulus, depending on depth within the tubules. In contrast, q gamma moved after a 1.7-2.9 ms latency and achieved a peak displacement of up to 22-34% of available charge. Both charge movement species could be driven by repetitive (47.7 Hz) action potentials without buildup of charge transfer. Such stimulus frequencies would normally cause tetanus. Latencies in q gamma charge movement in response to an action potential were resolved into (a) propagation of tubular depolarization required to gain the "threshold" of q gamma charge (0.8-1.5 ms) and (b) dielectric loss processes. The latter took consistently around 1.5 ms throughout the tubular system. Taken with (c) the earlier reports of a minimal latency in delta [Ca2+] signals attributed to tubulo-cisternal coupling following voltage sensing (approximately 2 ms: Zhu, P.H., I. Parker, and R. Miledi., 1986. Proc. R. Soc. Lond. B. Biol. Sci. 229:39-46.). these times can be reconciled to the latency (~ 4-5 ms) reported between the onset of the surface action potential and that of delta [Ca2+] signals (Vergara, J., and M. Delay. 1986. Proc. R. Soc. Lond. B. Biol. Sci. 229:97-110.). This is consistent with a relationship between the q gamma system and excitation-contraction coupling whether as an independent event (e.g.,Adrian, R.H., and C.L.-H. Huang. 1984. J. Physiol. (Lond.). 353:419-434.) or as an end reaction following earlier (q beta) transfers of charge (e.g., Horowicz, P., and M.F. Schneider. 1981. J. Physiol. (Lond.). 314:565-593.; Melzer, W., M.F. Schneider, B.J. Simon,and G. Szucs. 1986. J. Physiol. (Lond.). 373:481-512.)

Temperature dependence of the interfacial dielectric loss process in glass bead‐filled polyethylene

AbstractThe temperature dependence of the interfacial dielectric loss process in glass bead‐filled high density polyethylene (HDPE) due to an interfacially adsorbed water layer has been studied. The frequency of maximum dielectric loss shifts to lower frequencies with decreasing temperature. This is thought to be due to decreasing mobility of the free charge carriers in the adsorbed water layer, resulting in the reduced conductivity of the layer. The shifting of the loss peaks with temperature can be described with the well‐known Arrhenius relation. The energy of activation, as obtained from this relation, shows a stron dependence on the thickness of the adsorbed layer. For thinner layers a higher activation energy is found, indicating that the charge carriers are increasingly affected by the presence of a layer of bound charges on the glass surface. This thickness dependence of the activation energy offers some possible applications. First, information on the water layer thickness can be extracted from the energy of activation, and second, it becomes possible to study the effect of interfacial modifications such as the application of coupling agents.

On the Kinetics of Interface Processes on Silver Halide Crystals

AbstractIn the present paper the activation energies of the transport of Ag+ ions in AgCl and AgBr crystals are calculated with the help of a quantum theoretical equation involving the frequency of the optical phonons of the crystal lattice, the mean coordination number for the diffusing ion, and the vibrational quantum number. Comparison of the calculated activation energies with values available from the literature, which were obtained by measurements of electrical conductivity, dielectric loss, diffusion and mass transfer processes, shows good agreement.

Dielectric monitoring of water absorption in glass-bead-filled high-density polyethylene

Model composites of spherical glass particles dispersed in a matrix of high-density polyethylene (HDPE) were prepared. Samples of the composite material were, after careful drying, exposed to several relative humidities at room temperature. The dielectric properties and the mass gain of the composite samples were monitored during the water absorption from the environment. The dielectric measurements covered the frequency range from 0.1 Hz to 50 kHz. A theoretical model for the dielectric properties of composites with an interlayer describes the detected dielectric loss processes due to the electrically conducting layer of adsorbed water at the filler-matrix interface. A quantitative relationship between the amount of absorbed water and the frequency of maximum dielectric loss can be established by taking into account the thickness dependence of the conductivity of the interfacial water layer.