Frequency Independent Behavior Research Articles

Polypropylene/basalt thick film composites: structural, mechanical and dielectric properties

AbstractIn this work, polypropylene/volcanic basalt rock (PP/VBR) thick film composites with different VBR powder mass ratio varying from 0.5 wt.% to 20.0 wt.% were prepared by using the hot press technique. The effects of VBR powder doping on mechanical, structural and dielectric properties of PP were investigated by stress-strain measurements, Fourier transform infrared analysis, thermal gravimetric analysis, scanning electron microscopy and dielectric spectroscopy methods. The highest tensile strength, percentage strain and energy at break were achieved for 0.5 wt.% VBR powder doped PP composite. According to the stress-percentage strain curves of the samples, it was observed that 0.5 wt.% VBR powder doping increases the mechanical performance of PP polymer. In addition, regardless of the doping concentration level of basalt powder, the real part of complex dielectric function (ε′) of all PP composites display approximately frequency independent behavior between 100 Hz and 1 MHz. On the other hand, 0.5 wt.% VBR powder doped PP composite has also the lowest dielectric constant at the vicinity of 2.7 between 100 Hz and 1 MHz. The composite also has considerably low dielectric loss which has a crucial importance for technological applications. For these reasons, PP/0.5 wt.% VBR composite with the highest tensile strength can be considered as a suitable candidate for microelectronic devices. Furthermore, the alternative current conductivity mechanism was determined as nearly constant loss due to approximately constant dielectric loss between 10 Hz and 1 MHz.

Finite frequency Seebeck coefficient of metals: A memory function approach

We study the dynamical thermoelectric transport in metals subjected to the electron-impurity and the electron-phonon interactions using the memory function formalism. We introduce a generalized Drude form for the Seebeck coefficient in terms of thermoelectric memory function and calculate the latter in various temperature and frequency limits. In the zero frequency and high temperature limit, we find that our results are consistent with the experimental findings and with the traditional Boltzmann equation approach. In the low temperature limit, we find that the Seebeck coefficient is quadratic in temperature. In the finite frequency regime, we report new results: In the electron-phonon interaction case, we find that the Seebeck coefficient shows frequency independent behavior both in the high frequency regime (ω⪢ωD, where ωD is the Debye frequency) and in the low frequency regime (ω⪡ωD), whereas in the intermediate frequencies, it is a monotonically increasing function of frequency. In the case of the electron-impurity interaction, first it decays and then after passing through a minimum it increases with the increase in frequency and saturates at high frequencies.

Copper Cobalt Magnetic Ceramic Materials Characterization at Terahertz Frequencies

This paper presents the complex index of refraction and the complex permittivity of a magnetic ceramic material made of copper (Cu), cobalt (Co) and iron oxides. The index of refraction and the extinction coefficient of the CuCo-ferrite exhibit an almost frequency independent behavior and were averaged to n=3.62±0.05 and k=0.06±0.02. The corresponding complex permittivity was e’=13.12±0.35 for the real part and e”=0.46±0.15 for the imaginary part. The absorption coefficient and the transmittance of the CuCo-ferrite were also determined. The absorption coefficient exhibits a dip at ~0.35 THz, which corresponds to a peak in transmittance at this particular frequency. The impact of the observations on the potential realization of novel THz electronic devices is discussed.

Synthesis of a NbO Type Homochiral Cu(II) Metal-Organic Framework: Ferroelectric Behavior and Heterogeneous Catalysis of Three-Component Coupling and Pechmann Reactions.

A chiral tetracarboxylic acid ligand, H4L, incorporating the (S)-(+)-2-methylpiperazine moiety in its middle, solvothermally forms a homochiral Cu(II) framework, {[Cu2(L)(H2O)2]·(4DMF)(4H2O)}n (LCu). It forms a non-interpenetrated structure consisting of [Cu2(COO)4] paddle-wheel secondary bonding units (SBUs) with NbO topology. Interestingly, the framework LCu exhibits excellent ferroelectric properties. It shows a remnant polarization (Pr) of ∼3.5 μC cm-2 and a coercive field (Ec) of ∼12 kV cm-1 with a distinct electric hysteresis loop. Dielectric studies of LCu reveal almost frequency-independent behavior with a dielectric constant (εr) of ∼42 and a low dielectric loss (tan δ) of ∼0.04 up to 106 Hz, for potential use in high-frequency applications. In addition, activated framework LCu' having uncoordinated metal sites acts as an efficient heterogeneous catalyst in the three-component coupling of amines, aldehydes, and alkynes, as well as in Pechmann reactions of phenols with β-ketoesters.

Studies on the dielectric behavior and AC conductivity of Zn2+ doped α-PbO nanoparticles synthesized by precipitation method

We reported the structural, dielectrical properties and AC conductivity of the chemically synthesized Zinc doped α-PbO nanoparticles after it was calcined at 240 °C in air. X-ray diffraction analysis was used to find the tetragonal structure in the prepared samples which belongs to α phase of PbO. Dielectric properties were measured in the temperature range of 25–250 °C and the frequency range of 1 Hz–7MHz using Broad band dielectric spectroscopy. Dielectric constant shows the frequency independent behavior at high temperature. Loss tangent decreases with increase in frequency which reveals the suitability in high frequency devices. Zn2+ doping enhances the conductivity of PbO due to the mobility of charge carriers by substitutional defects. Variations in the electrical and dielectrical behavior with respect to frequency, temperature and composition of Zn2+ doping on PbO nanoparticles were discussed in detail.

Synthesis and physical characterization of γ-Fe2O3 and (α+γ)-Fe2O3 nanoparticles

Magnetic nanoparticles were synthesized at different hydrothermal temperatures (HT; 100, 130, 160 and 190 °C) by using a facile hydrothermal route combined with a subsequent calcination process. The calcined materials were analyzed for phase, microstructure, and magnetic and dielectric properties through different characterization techniques. The structural analyses revealed that the material prepared at a HT of 100 °C and sequentially calcined at 300 °C for 3 h showed a high degree of the maghemite structure. On the other hand calcined materials showed a small additional peak belonging to the hematite structure. FESEM micrographs of the materials calcined at HT, of 100 °C and 190 °C showed spherical-like nanoparticles with diameters in range 30 - 54 nm. Materials prepared at a HT of 160 °C followed by calcination at 300 °C for 3 h exhibited the highest saturation magnetization, Ms = 67 emu/g, with a lower coercivity; all materials were in a single domain state. A high dielectric constant (105.54) was observed for the calcined material that had been prepared at a HT of 130 °C. The dielectric properties of synthesized materials showed an almost frequency-independent behavior.

Structural, electrical, magnetic and magnetoelectric properties of (1−y) [Ba0.6−xCaxSr0.4Zr0.25Ti0.75O3] + (y) [(Li0.5Fe0.5)0.4Ni0.18Cu0.12Zn0.3Fe2O4] composites

Magnetoelectric composites having the nominal composition of (1-y) [Ba0.6-xCaxSr0.4Zr0.25Ti0.75O3] (BCSZTO) + (y) [(Li0.5Fe0.5)0.4Ni0.18Cu0.12Zn0.3Fe2O4] (LNCZFO) (x = 0.0, 0.1, 0.2 and y = 0.2, 0.4, 0.6, and 0.8) were synthesized by the solid state reaction method and sintered at 1200 °C for 3 h. The X-ray diffraction analysis confirms the coexistence of tetragonal perovskite BCSZTO and spinel LNCZFO phases with Ba6T17O46 as impurity phase. Microstructural and quantitative elemental of the samples were carried out by Field Emission Scanning Electron Microscopy equipped with Energy Dispersive X-ray Spectroscopy. Low frequency dielectric dispersion is attributed due to Maxwell-Wagner interfacial polarization arising from the interface of the two phases in agreement with Koop's phenomenological theory. Frequency independent behavior of dielectric constant at higher frequencies is attributed due to the inability of electric dipoles to follow the first variation of the alternating applied electric field. The dielectric constant decreases with the increasing Ca2+ concentration which is attributed due to the increased resistivity. Maxima in dielectric loss (tanδ) are appeared when the hopping frequency of electrons between different ionic sites becomes nearly equal to the frequency of the applied field. AC conductivity of the composites follows Jonscher's universal power law and the conduction mechanism is attributed due to small polaron hopping. AC conductivity increases with the increase of Ca2+ concentration. The composite materials are found to exhibit excellent frequency dependence and increases with the ferrite concentration. The permeability decreases with the increase of Ca2+ concentration which is attributed to the smaller grain size and increased magneto crystalline anisotropy. The quality factor increases with the Ca2+ ions substitution on account of the high resistive boundary segregation. The magnitude of magnetoelectric (ME) voltage coefficient increase linearly with the increase of the applied filed may be attributed to the nominally linear gradient of magnetostriction with respect to magnetic field. The magnetoelectric coefficient of the composites increases gradually as the increase in the ferrite content which is attributed due to the increase of the mechanical deformation in the magnetostrictive phase. An optimal magnetoelectric voltage coefficient of ∼1.16 V cm−1 Oe−1 is obtained for x = 80 wt% (C-series composites) at room temperature.

Polyvinyl pyrrolidone/Mg(ClO4)2 solid polymer electrolyte: structural and electrical studies

Polymer electrolytes comprising polyvinyl pyrrolidone (PVP) as host polymer and Mg(ClO4)2 as dopant salt have been prepared by solution casting technique using double-distilled water as solvent. The changes in the structural properties on the incorporation of dopant were investigated by XRD and FTIR analysis. The ionic conductivity and dielectric behavior were explored using AC impedance spectroscopy. The ionic conductivity increases with increasing dopant concentration. The conductivity enhancement with the increasing salt concentration is correlated with the increase in amorphous nature of the electrolytes. The frequency dependence of electrical conductivity obeys the universal Jonscher power law. The electrical modulus representation shows a loss feature in the imaginary component. The distribution of relaxation times was indicated by a deformed arc form of the Argand plot. The relative dielectric constant (er) decreases with increase in frequency in the low frequency region whereas a frequency-independent behavior is observed in the high frequency region. The total ionic transference number studies have confirmed that the mobile charge carriers are ions. Results obtained by cyclic voltammetry on SS/60 mol% PVP/40 mol% Mg(ClO4)2 SPE/SS symmetrical cell show evidence for reversibility.

Pulsed current hard anodizing of heat treated aluminum alloys: Frequency and current amplitude influence

Abstract Pulsed current hard anodizing procedures have been applied to the widely used heat treated aluminum alloys AA2024-T3, AA6082-T6 and AA7075-T6. The influence of frequency and of current amplitude on anodic oxide thickness, hardness, defectiveness, volumetric expansion ratio and on process faradic efficiency has been studied. Higher frequencies generate a decrease in coating electric resistance and in general they are less effective in order to overcome typical critical issues arising in alloys difficult to be anodized. In AA2024-T3 and AA6082-T6 higher frequencies led to slight increase in thickness, decrease in compactness and faradic efficiency while hardness remained almost constant or a bit higher. With higher frequencies in AA2024-T3 defective state significantly got worse, in AA6082-T6 it improved. In AA7075-T6 an almost frequency independent behavior occurred. The highest wave amplitude with a slightly cathodic “off” current (reverse pulse) allowed to obtain greater thickness, hardness and compactness while other current amplitudes did not show significant influence on properties analyzed; a very low, but still anodic, “off” current however induced slight hardness decreases.

The relationship between rheological behavior and microstructure of nanocomposite based on PA6/NBR/clay

Microstructure, rheological properties and their relationships of PA6/NBR/nano‐clay nanocomposite have been investigated. Scanning electron microscopy (SEM) and transmission electron microscopy imaging techniques were used to study micro‐structure. The nano‐clay dispersion was measured by small angle X‐ray diffraction. Frequency sweep, steady shear, startup shear transient, and startup shear free transient experiments were carried out to study rheological characteristics of nanocomposite. SEM micrograph revealed that nano‐clay decreased the size of rubber droplets to a half in comparison with samples without nano‐clay. Storage modulus of nanocomposite containing 7% wt nano‐clay exhibited frequency independent behavior because of physical network which formed in matrix, in contrast to pure PA6 which showed terminal storage modulus at low frequencies. Shear thinning at high shear rates was observed by addition of rubber to PA6 matrix caused by deformation of rubber droplets. Nano‐clay orientation in shear flow field accelerated non‐Newtonian behavior in steady shear experiment. Transient shear viscosity studies were used for measurement the strength of nano‐clay physical network. It was found that network breakdown is more stress consuming factor in comparison with nano‐particle orientation and hydrodynamic forces. Elongation transient viscosity studies showed that critical Hencky strain for linear–nonlinear viscoelastic transition shifted to higher strains by increasing nano‐clay concentration. POLYM. COMPOS., 39:2403–2410, 2018. © 2016 Society of Plastics Engineers

Structural, morphological and magnetic properties variation of nickel-manganese ferrites with lithium substitution

Mixed ferrites with nominal chemical compositions LixNi0.2Mn0.8−2xFe2+xO4 ranging from x=0 to 0.4 in the steps of 0.1 have been prepared by the auto combustion technique. The X-ray diffraction patterns consist of major cubic spinel LixNi0.2Mn0.8−2xFe2+xO4 phase with minor impurity phases (Fe2O3 and MnO) and with Li substitution phase purity has increased, such that for x=0.4 pure phase spinel structure has been obtained. The lattice parameter has decreased with the increase in Li content obeying Vegard’s law. Both the bulk density and theoretical density have decreased with Li content and with sintering temperature (Ts) up to 1300°C ρB has increased and beyond that it has decreased. Morphological studies have performed by a high resolution optical microscope and observed that average grain size noticeably dependent on Li substitution. The initial permeability (μi′′) has found to decrease with Li substitution. The Curie temperature (TC) has determined from the temperature dependent μi′′ and found to increases with Li content. From the room temperature magnetization measurement, it has observed that all samples are in ferrimagnetic state at room temperature. The number of Bohr magneton has been obtained from the observed saturation magnetization. Dielectric constant, dielectric loss tangent, ac conductivity and complex impedance are studied in the frequency range 20Hz–10MHz. Frequency dependence of dielectric constant in lower frequencies indicates a usual dielectric dispersion due to the Maxwell-Wagner type interfacial polarization. Dielectric loss tangent shows similar behavior like dielectric constant. The complex impedance analysis has been used to study the effect of grain and grain boundary on the electrical properties and with Li content both grain and grain boundary resistance show an increasing trend. The ac conductivity shows frequency independent behavior at the low frequency side and with increasing frequency the conductivity increases.

Design of Substrate Integrated Waveguide (SIW) Fed Log Periodic Microstrip Array Antennas

A substrate integrated waveguide fed microstrip array antenna structure using the log-periodic principle has been designed. The antenna elements and the feeding networks have been designed and simulated using Ansoft HFSS software. Three element and six element log periodic microstrip array antennas have been fabricated and tested. The simulated and experimental results are in good agreement. The bandwidth achieved increases from 5 GHz for a three element antenna to 10.1 GHz for a six element antenna, exhibiting a good frequency independent behavior which is also justified by the radiation patterns and values of gain at different frequencies.

A coatable, light-weight, fast-response nanocomposite sensor for the in situ acquisition of dynamic elastic disturbance: from structural vibration to ultrasonic waves

Inspired by an innovative sensing philosophy, a light-weight nanocomposite sensor made of a hybrid of carbon black (CB)/polyvinylidene fluoride (PVDF) has been developed. The nanoscalar architecture and percolation characteristics of the hybrid were optimized in order to fulfil the in situ acquisition of dynamic elastic disturbance from low-frequency vibration to high-frequency ultrasonic waves. Dynamic particulate motion induced by elastic disturbance modulates the infrastructure of the CB conductive network in the sensor, with the introduction of the tunneling effect, leading to dynamic alteration in the piezoresistivity measured by the sensor. Electrical analysis, morphological characterization, and static/dynamic electromechanical response interrogation were implemented to advance our insight into the sensing mechanism of the sensor, and meanwhile facilitate understanding of the optimal percolation threshold. At the optimal threshold (∼6.5 wt%), the sensor exhibits high fidelity, a fast response, and high sensitivity to ultrafast elastic disturbance (in an ultrasonic regime up to 400 kHz), yet with an ultralow magnitude (on the order of micrometers). The performance of the sensor was evaluated against a conventional strain gauge and piezoelectric transducer, showing excellent coincidence, yet a much greater gauge factor and frequency-independent piezoresistive behavior. Coatable on a structure and deployable in a large quantity to form a dense sensor network, this nanocomposite sensor has blazed a trail for implementing in situ sensing for vibration- or ultrasonic-wave-based structural health monitoring, by striking a compromise between ‘sensing cost’ and ‘sensing effectiveness’.

Exploration on the P(VP-co-VAc) copolymer based gel polymer electrolytes doped with quaternary ammonium iodide salt for DSSC applications: Electrochemical behaviors and photovoltaic performances

Gel polymer electrolytes (GPEs) consisting of poly(1-vinylpyrrolidone-co-vinyl acetate) P(VP-co-VAc) with the different tetrapropylammonium iodide (TPAI) salt concentration are prepared. The dielectric and electric dispersion behaviors of the GPEs are studied by dielectric relaxation spectroscopy at room temperature. The dielectric studies imply that dielectric constant (ε′) and dielectric loss (ε″) values decrease with increase in frequency at lower frequency region whereas frequency independent behavior is observed in the high frequency region. The trend of the dispersion part of the electric modulus shows a shift towards higher region of frequency indicating shorter relaxation time with an increase in TPAI concentration. The GPE samples were observed to obey Arrhenius behavior and the highest ionic conductivity obtained is 1.60×10−3Scm−1 at room temperature. The GPE samples are then fabricated into dye sensitized solar cells for photovoltaic studies. Highest efficiency of 3.07% is obtained with short circuit current density of 6.86mAcm−2, open circuit voltage of 727mV and fill factor of 62%.

Stretchable and Highly Conductive Carbon Nanotube-Graphene Hybrid Yarns for Wearable Systems

Carbon Nanotubes (CNTs) have emerged as potential candidates for replacement of conventional metals due to their significant mechanical, electrical, thermal properties and non-oxidizing abilities [1, 2]. The density of CNT composites is about five times lower than copper and around half that of aluminium. Moreover, their thermal conductivity is about ten times that of copper. With the above mentioned distinguishing features, CNTs have been of interest in medical, electronics and antenna applications [3]. CNTs are drawn into yarns by pulling and twisting them from CNT forests. Previously we have presented microwave characterization of CNT yarns [4]. Our results have shown that the CNT yarns exhibits frequency independent resistive behavior and is beneficial for wide-band applications such as ultra-wideband (UWB) and wireless body area networks [4]. Electrical conductivity of a CNT yarn depends on the properties, loading and aspect ratio of the CNTs. It also depends upon the twist angle and the characteristics of the conductive network. By doping or adding materials, such as gold, silver or NiCr, electrical conductivity of CNTs can by varied. In [5], highly conductive carbon nanotube-graphene hybrid yarns are reported. They are obtained by drawing vertically aligned multi-walled carbon nanotubes (MWCNT) into long MWCNT sheets.

Multiferroic properties of hexagonal Ba3Ti2MnO9

Noncentrosymmetric hexagonal Ba3Ti2MnO9 presents multiferroic behavior.

An investigation into the modeling of railway fastening

In this paper, the influence of modeling the fastening with solid railpads on the vertical dynamics of railway tracks with monoblock sleepers is investigated. A 3D finite element (FE) model is presented with four different fastening representations: (1) commonly used spring-damper pair, (2) area covering spring-damper pairs, (3) solid railpad connected to the rail, and (4) solid railpad in frictional contact with the rail and fixed to the support by preloaded springs, which represent the clamps. The response of the four models to hammer excitation is simulated in the time domain, and the calculated response is transformed into the frequency domain to analyze how the models capture the seven main characteristics of tracks with monoblock sleepers. The numerical results show that the model with solid railpads and clamps reproduce the seven characteristics at a maximum frequency difference of 6%, while the conventional model with spring-damper pairs reaches only a 27%. In the improvement of the fit from multiple spring-damper railpad models to solid railpad models, the two key aspects identified are the Poisson׳s effect and the damping of the ballast. Additionally, the railpad type investigated showed a frequency-independent behavior, at least with acceptable error. In view of the close fit, the models with solid railpads can be used for track and fastening design and to derive track parameters to, for instance, study the deterioration of tracks.

Selection of the optimal frequencies of viscous damping formulation in nonlinear time-domain site response analysis

Viscous damping is commonly employed in a nonlinear time-domain site response analysis to capture soil damping at small strains. In contrast to the generally accepted concept of the frequency-independent behavior of soil damping, the viscous damping employed as Rayleigh damping is frequency dependent and can overdamp or underdamp wave propagation. This study revisits the issue of selecting the target value of viscous damping frequencies to minimize the effect of frequency-dependent damping. The proposed criterion considers both the site frequency (SF) and frequency characteristics of input motion (e.g., predominant frequency (PF) or mean frequency (MF)) and is more accurate than the widely used protocol in practice. In the Rayleigh damping, the low optimal frequency can be selected as SF but the high optimal frequency should be selected as the maximum between the PF/MF of the input motion and 5SF.

Dielectric properties of polystyrene based composites filled with core-shell BaTiO3/polystyrene hybrid nanoparticles

In this work, core-shell structured BaTiO 3 /polystyrene nanoparticles (BT-PS) with different thickness of PS shell were synthesized through atom transfer radical polymerization and the influence of their shell thickness on dielectric properties of BT-PS/PS composites was studied. Two types of BT-PS with the PS shell of 3 nm or 12 nm were obtained by controlling the polymerization time. The structure of BT-PS was carefully characterized by infrared spectroscopy, X-ray diffraction, thermal gravimetric analysis, and transmission electron microscope. The results confirmed the successful preparation of core-shell BT-PS nanoparticles. Compared to pristine BT, the core-shell particles can be more homogeneously dispersed into PS matrix. As a result, higher dielectric constant, higher breakdown strength, and lower dielectric loss were achieved in BT-PS/PS composites. Moreover, the dielectric constant of BT-PS/PS composites displayed frequency independent behavior. In addition, the composites filled by BT-PS with 3 nm of PS shell showed better dielectric properties than those filled by BT-PS with 12 nm of PS shell. A maximum energy density as large as 4.24 J/cm 3 was obtained in BT-PS/PS films.

Effect of co-substitution of La/Li on structure, microstructure, dielectric and piezoelectric behavior of Bi0.50Na0.50TiO3 ceramics

Polycrystalline specimens with compositions x = 0.04 and y ≤ 0.10 in (Bi1−xLax)0.5(Na1−yLiy)0.5TiO3 system are synthesized by semi-wet technique. Structure, microstructure, dielectric and piezoelectric properties are investigated in order to observe the effect of co-substitution of La/Li ions on A-sites of BNT ceramic system. Rietveld analysis of X-ray diffraction data for all the specimens reveals single phase formation with rhombohedral structure. Microstructures of these specimens indicate a decrease in grain size with homogeneous grain growth up to y ≤ 0.075 followed by anomalous grain growth for y = 0.100. The temperature dependence of dielectric plots exhibits a broad maximum whose position (Tm) is frequency dependent up to y ≤ 0.075. A switching from frequency dependent to frequency independent dielectric behaviour is observed with increasing Li concentration. All the samples show nearly saturated P–E loop at room temperature. The composition x/y = 0.04/0.025 exhibits improved ferroelectric with Pr = 29.26 µC/cm2 and piezoelectric constant of d33 = 70 pC/N.