Dynamic dielectric properties of an isotactic polypropylene matrix grafted with maleic anhydride (CA 100) and then crosslinked by polyether amine molecules and reinforced with different weight percentages of graphite nanoplatelets (GNPs), KNG180, were studied for the first time and compared to those obtained by DMA (Dynamic Mechanical Analysis). The main objective of this work was to investigate the reinforcement effect of GNPs focusing on the GNPs/matrix interfacial adhesion using dynamic dielectric relaxation spectroscopy in the frequency range from 0.1 Hz to 1 MHz and temperature range from 20 to 140 °C. The obtained interfacial polarization increments $${\Delta \varepsilon }_{\mathrm{MWS}}$$ from MWS (Maxwell Wagners Sillars) relaxation showed a threshold value of 3% in weight of KNG180. This analysis suggests that interfacial compatibility between matrix and fillers in the case of nanocomposite KNG180 3 wt% is higher than those of other nanocomposites. A new plasma treatment was used to modify graphite nano-fillers to produce different types of nanocomposites. The 5 wt% plasma treated graphite nanocomposite shows a good dispersion of the nano-fillers but also a high value of $${\Delta \varepsilon }_{\mathrm{MWS}}$$ , which is an indication of high graphite/graphite interaction. This evolution could show that this material can be close to the formation of an electrical percolation network.

This work deals with the dielectric properties of silane treated pineapple leaf fiber and kenaf fiber reinforced phenolic hybrid composites. The aim of the present paper is to investigate the effect of silane treatment on the pineapple leaf fiber–kenaf fiber/matrix interfacial adhesion using the dielectric relaxation spectroscopy in the frequency range from 0.1 Hz to 1 MHz and temperature range from 50 to 180℃. Our hybrid composites were fabricated by hand lay-up method at 50% total fiber loading. All the results obtained were discussed in terms of dynamic molecular and interfacial process. Two interfacial polarizations identified as the Maxwell–Wagner–Sillars effect are observed. We note that silane treatment improved the interfacial adhesion between pineapple leaf fiber/kenaf fiber and phenolic resin and it will help to develop high performance kenaf fiber/pineapple leaf fiber reinforced polymer composites for industrial applications. In fact, as known, the silane treatment developed hydrophobic nature in pineapple leaf fiber and kenaf fiber which is very positive for fiber/matrix compatibility.

Dielectric measurements were performed on Poly(lactic acid) (PLA) polymer, Poly(lactic acid)/Poly(butylene succinate) (PLA/PBS) blend matrix and its green composite reinforced with Jute fibers in temperature range from 20 to 140 °C and frequency range from 10−1 Hz to 1 MHz. Thermal analyses based on differential scanning calorimetry technique have evidenced different thermal transitions of the semi-crystalline polymer occurred for PLA one. Addition of PBS into PLA polymer enhanced its crystallinity. However, the presence of Jute fibers in the green composite material lowered such effect and increased the PLA polymer glass transition temperature, Tg. These could be attributed to the PBS contribution into the Jute fibers/blend matrix interactions. Accordingly, dielectric analyses have revealed the presence of three relaxation processes for PLA polymer. These were identified to the α mode relaxation, the dc conductivity effect and the Maxwell–Wagner–Sillars (MWS) interfacial polarization effect. The latter effect was attributed to the accumulation of charges at the crystalline phase/amorphous phase interfaces of PLA polymer. Two additional dielectric relaxations appeared for PLA/PBS blend matrix. These were associated with β relaxation and interfacial polarization effect originating from PLA polymer and the semi-crystalline character of PBS polymer, respectively. The incorporation of Jute fibers into the PLA/PBS blend matrix gave rise to other dielectric relaxations associated with the water dipoles and interfacial (Jute fibers/blend matrix) polarization effects. Analysis of dielectric relaxations at high temperature above the glass transition temperature using the Havriliak–Negami model allowed probing Jute fibers/blend matrix interactions. Adhesion mechanism occurred in the studied green composite was determined by vibrational analysis based on FT-IR technique.

Our new approach consists of applying the Gaussian spatial distribution function to study the trapping process bipolar electronic charge transport at dielectric-dielectric as well as dielectric electrode interfaces. Indeed, the application of the Gaussian spatial distribution function, to the traps spatial density distribution as well as to the corresponding trapping coefficients, has not been investigated so far for bipolar transport modeling in insulating and dielectric materials. In fact, this approach takes into account the non-uniformity of the trap density spatial distribution, which constitutes the main contribution that presents a more realistic aspect taking into account the existence of the inhomogeneities in the dielectric materials. Several numerical resolution techniques have been applied to the theoretical equations formulated with associated assumptions and physical conditions. The obtained results show several physical phenomena, such as charge accumulation and Maxwell-Wagner behavior. In this paper, to validate our approach, we have focused our investigations on the accumulation phenomenon of trapped charges. Our results are in good agreement with those obtained by the pulsed electro-acoustic experimental technique, for the example, epoxy-polyethylene interfaces.

A polyester polymer matrix filled with olive-pomace grains was investigated using impedance spectroscopy in the frequency range 100 Hz to 1 MHz and temperatures from 300 to 360 K. Two relaxations processes were identified. One could be attributed to the α relaxation associated with the glass transition and observed in the neat matrix, while the second was attributed to the accumulation of charges at the pomace grains/polyester interfaces. The relaxation parameters were evaluated from isothermal dielectric spectra by applying the empirical Havriliak-Negami function.

The dielectric properties of sugar palm fiber (SPF) reinforced-phenolic (PF) composites have been studied in terms of bonding between fiber and matrix. The paper aims to investigate the effect of alkaline treatment and sea water treatment on SPF composite using the dielectric relaxation spectroscopy in the frequency range from 0.1 Hz to 0.1 MHz and temperature range from 80 degrees C to 200 degrees C. The results were discussed in terms of dynamic molecular and interfacial process. Our analysis suggests that interfacial adhesion in the case of alkaline treated composite is higher than those of untreated and sea water treated composites. (C) 2018 Elsevier B.V. All rights reserved.

The study of secondary electron emission (SEE) yield as a function of the kinetic energy of the incident primary electron beam and its evolution with charge accumulation inside insulators is a source of valuable information (even though an indirect one) on charge transport and trapping phenomena. We will show that this evolution is essentially due, in plane geometry conditions (achieved using a defocused electron beam), to the electric field effect (due to the accumulation of trapped charges in the bulk) in the escape zone of secondary electrons and not to modifications of trapping cross sections, which only have side effects. We propose an analytical model including the main basic phenomena underlying the space charge dynamics. It will be observed that such a model makes it possible to reproduce both qualitatively and quantitatively the measurement of SEE evolution as well as to provide helpful indications concerning charge transport (more precisely, the ratios between the mobility and diffusion coefficient with the thermal velocity of the charge carrier).

In this work, nanocomposite polymer films based polyvinyl alcohol with different weight ratios of nanoparticles TiO2 were prepared by the casting method. Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR) and dielectric spectroscopy of all compositions have been investigated. The SEM analysis showed that TiO2 nanoparticles were well dispersed in the PVA matrix. However, below 30 wt% aggregates were observed. Differential Scanning Calorimetry evaluated the degree of crystallinity and transition temperature. Infrared spectroscopy evidenced the presence of interactions between the PVA and TiO2. Impedance spectroscopy measurements of all samples were studied as functions of temperature and frequency. Dielectric permittivity and loss tangent reveal seven relaxation processes ascribed to electrode polarization, conduction phenomena, Grain-Boundary-Relaxation, MWS relaxation, and (alpha(a), alpha(c) and beta) relaxations. The activation energy values E-a associated with DC conductivity confirm the protonic type of the conductivity and present a lower value for the PVA with 5% amount of TiO2. In addition, the temperature dependence of the exponent power s denoted that conduction in the sample bulk follows the CBH model. (C) 2018 Elsevier B.V. All rights reserved.

Natural rubber (NR) isolated from Hevea Brasiliens and reinforced with different amounts of cellulose nanowhiskers (extracted from the rachis of date palm tree) was investigated using differential scanning calorimetry and dielectric spectroscopy in the frequency range 0.1 Hz to 1 MHz, and the temperature range 80 to 200 degrees C. The experimental dielectric data were analysed within the formalisms of dielectric permittivity, complex conductivity and complex modulus. The results were discussed in terms of dc conductivity, interfacial polarization and electrode polarization. The paper aims to investigate especially the electrode polarization observed at low frequency and/or high temperature (LF/HT). It is exhibited that the understanding of the frequency space charge relaxation behaviour trapped close to the material/electrode interface will help one to elucidate the charge dynamics in the quasi static range. So charge carrier concentration and mobility in the sample at high temperature can be evaluated through the electrode polarization modeling of broadband dielectric spectroscopy data. (C) 2017 Elsevier B.V. All rights reserved.

In the present study, the dielectric and mechanical properties of natural rubber (NR) based nanocomposites are investigated. Cellulose nanofillers are used in two forms as reinforcing phase: nanofibrillated cellulose (NFC) and cellulose nanocrystals (CNC). In the dielectric study, different relaxation phenomena are detected: the a dipolar relaxation, the lignin and hemicelluloses relaxation, the water dipoles relaxation, the interfacial polarization and the ionic conduction. For the interfacial polarization, the dielectric strength angle epsilon showed lower values for NFC-filled nanocomposites than CNC-filled samples. It was explained with higher interactions between induced dipoles and lower mobility, assuring a better adhesion between the NR and the NFC. Moreover, in tensile tests, the elastic modulus increases with filling indicating the reinforcement effect of nanofillers. In addition, the NR-NFC nanocomposites display the highest tensile modulus. This result shows the higher compatibility of NFC with the NR matrix, and the ensuing higher filler/matrix adhesion. In dynamic mechanical analysis (DMA), a significant reinforcing effect of NFC was shown. This effect is manifested with the high storage modulus E', suggesting that the interactions between the NR matrix and the NFC fibers were stronger.

In the present work, the dielectric properties of nanocomposites based on poly(styrene-Co-2-ethyl hexylacrylate) copolymer (StHA) and cellulose nanocrystals (CNCs) and γ-methacryloxypropyl triethoxysilane (MPS) as a coupling agent were investigated. Four relaxation processes were detected: the β relaxation, the α relaxation, the electrode polarization and the ionic conduction phenomenon. These two later were identified with the real part ε' and the imaginary part ε" of the dielectric permittivity slopes. Moreover, the electrode polarization was spotted using the first derivative of the real part of the dielectric permittivity. The activation energy EA for the β relaxation, the strength parameter D for the α relaxation and the relaxation strength Δε showed a threshold value of 4% in weight of CNC. Variations were explained with attractive and repulsive cellulose-cellulose and cellulose-copolymer interactions. The electrical study confirmed the dielectric results indicating the higher conductivity for the nanocomposite with 4%w.t of CNC. This work provided additional evidence of the formation of a rigid percolation network via hydrogen bonding for nanocomposites based on CNCs.

Isothermal detrapping of holes after electron irradiation (using a SEM) in high purity amorphous SiO2 is evaluated at different temperatures (in the range 300-663 K) by means of the induced and secondary current measurements. In order to single out the hole detrapping, the specific charging conditions (1 keV defocused electron beam of low density) leading to positive charging are adopted. The thermal detrapping, which stems from a single trap, begins at 523 K and is completed at 663 K. After annealing in air at 973K during 48h, two detrapping stages are revealed: the former is connected with an additional shallow trap, while the latter requires temperatures above 663 K for a complete detrapping. The first order kinetics describes reasonably well the detrapping process. The frequency factors (near 10(10) s(-1)) and the activation energies (about 1.6 eV) deduced from this analysis could be assigned, respectively, to the relaxation connected to detrapping and to the trap energy level of the charged oxygen vacancy. (C) 2015 Elsevier B.V. All rights reserved.

The dielectric spectra of a series of poly(styrene-co-2-ethyl hexylacrylate) copolymer/cellulose nanocrystals nanocomposites were investigated. The filler content of cellulose nanocrystals (CNC) extracted from the rachis of date palm tree was fixed at 4% in weight. The study looks at the effects on dielectric behavior as a result of the γ-methacryloxypropyl triethoxysilane (MPS) addition. The dielectric permittivity ε′ and the loss factor ε″ were measured using a dielectric analyzer in the frequency range of 0.1Hz to 1MHz and between the temperature range of −100°C and 100°C. Several notable changes were observed as MPS concentration increased. Dielectric measurements show a α and a β relaxation process associated with the large-scale segmental motion of the main polymer chain and with the localized mobility of the lateral groups, respectively. It was shown that the α relaxation followed the Vogel–Fulcher–Tammann–Hesse (VFTH) dependency. However, the β relaxation obeyed Arrhenius linear behavior. The activation energy for the β relaxation, the strength parameter D for the α relaxation and the relaxation strength Δε for both relaxation processes variations showed a threshold value of 3% in weight of MPS. At low MPS content, silane is mainly located at the surface of the copolymer and it affects the dipole mobility without preventing the formation of percolation network. At high MPS content, silane is also adsorbed onto the cellulose nanocrystals surface and it may inhibit the formation of percolation network, leading to agglomerations. The electrical study confirms the dielectric results indicating the higher conductivity for the nanocomposite with 3% of MPS treatment. The frequency dependencies of the conductivity give evidence for an ionic hopping transport mechanism.

In this work, the electrical properties of green nanocomposites based on natural rubber (NR) had been explored. Nanocellulose was extracted from the rachis of date palm tree and used as nanofillers in two forms: nanofibrillated cellulose (NFC) and cellulose nanocrystals (CNCs). The resulting samples were characterized by dielectric spectroscopy in a board temperature range (20°C–200°C) and in the frequency range of 0.1Hz to 1MHz. The temperature and frequency dependencies of conductivity give evidence for ion transport mechanism via the occurred agreement of experimental results with the employed hopping model (Random Free-Energy Barrier model). Conductivity was found to increase highly for filled nanocomposite, especially at high CNC content. Favorable interactions between NFC and NR were evidenced and assumed to be partially responsible for the lower conductivity of NFC-filled nanocomposites. The NR–CNC green nanocomposite films had a high potential to be used for electrical applications and they should be a very promising candidate for battery separators.

SummaryThis paper focuses on the effect of sample annealing temperature and crystallographic orientation on the secondary electron yield of MgO during charging by a defocused electron beam irradiation. The experimental results show that there are two regimes during the charging process that are better identified by plotting the logarithm of the secondary electron emission yield, lnσ , as function of the total trapped charge in the material QT. The impact of the annealing temperature and crystallographic orientation on the evolution of lnσ is presented here. The slope of the asymptotic regime of the curve lnσ as function of QT, expressed in cm2 per trapped charge, is probably linked to the elementary cross section of electron–hole recombination, σhole, which controls the trapping evolution in the reach of the stationary flow regime.

In this paper, we have applied our bipolar transport model for studying the charge dynamics in the insulating polyethylene materials, from nano to micro -scales and under dc high applied field. We have also applied the model for studying the electrical breakdown phenomenon for these mentioned scales. The principal results are dedicated to the evolution of the external current in low density polyethylene samples. Under notable dc high applied field, the electrical pre-breakdown phenomenon is indicated by an abrupt increase that occurs during the steady state of the external current.

In this paper, we present a new theoretical and numerical formulation for the electrical and thermal breakdown phenomena, induced by charge packet dynamics, in low-density polyethylene (LDPE) insulating film under dc high applied field. The theoretical physical formulation is composed by the equations of bipolar charge transport as well as by the thermo-electric coupled equation associated for the first time in modeling to the bipolar transport problem. This coupled equation is resolved by the finite-element numerical model. For the first time, all bipolar transport results are obtained under non-uniform temperature distributions in the sample bulk. The principal original results show the occurring of very sudden abrupt increase in local temperature associated to a very sharp increase in external and conduction current densities appearing during the steady state. The coupling between these electrical and thermal instabilities reflects physically the local coupling between electrical conduction and thermal joule effect. The results of non-uniform temperature distributions induced by non-uniform electrical conduction current are also presented for several times. According to our formulation, the strong injection current is the principal factor of the electrical and thermal breakdown of polymer insulating material. This result is shown in this work. Our formulation is also validated experimentally.

In this paper, we have investigated the evolution of the secondary electron emission in the case of pure spinel during electron irradiation, achieved in a scanning electron microscope at room temperature, which is derived from the measurement of the induced and the secondary electron currents. It was observed from the experimental results, that there are two regimes during the charging process: a plateau followed by a linear variation, which are better identified by plotting the logarithm of the secondary electron emission yield ln sigma as function of the total surface density of trapped charges in the material Q(T). For positive charging, E-0 = 1.1 and 5 keV, the slope of the linear part, whose value is of about 10(-10) cm(2)charge(-1), is independent of the primary electron energy. It is interpreted as a microscopic cross section for electron-hole recombination. For negative charging of pure spinel, E-0 = 15 and 30 keV, the slope is associated with an electron trapping cross section close to 10(-14) cm(2)charge(-1), which can be assigned to the microscopic cross section for electron trapping. This trapping cross section is four orders of magnitude lower than the recombination one.Lay Description In this paper, we have investigated the evolution of the secondary electron emission from pure spinel irradiated at four primary energies 1.1, 5, 15 and 30 keV, by using a scanning electron microscope (SEM). The dependence of SEE yield on surface density of trapped charges has led to the evaluation of microscopic cross sections for electron-hole recombination (when the material is positively charged: E-0 = 1.1 and 5 keV) and electron trapping (when the material is negatively charged: E-0 = 15 and 30 keV) in pure spinel. The variation of the semi-logarithmic of the secondary electron emission yield (ln sigma) as function of the surface density of trapped charge Q(T) appears to be a valuable way to characterize the charging properties of an insulator surface. According to the different curves we note that when the material charged positively the slopes are the same P = 1.6 10(-10) cm(2)/charge, whatever the primary beam energy E-0 = 1.1 and 5 keV. When the material charged negatively (E-0 = 15 and 30 keV) the slopes are the same P = 1.6 10(-14) cm(2)/charge. This proves that the slope is completely intrinsic to the material, since it is independent on initial conditions (sigma(abs0): initial traps) and conditions of irradiation ((source): the mean distance between electron-hole pairs created by each entering primary electron and R-p: penetration depth); it is interpreted as proportional to the recombination cross section (sigma(hole)) between free electrons and holes (if E-0 = 1.1 and 5 keV) and proportional to trapping cross section (sigma(T)) (if E-0 = 15 and 30 keV), which, is four orders of magnitude lower than the recombination one.

In the present study, charge transport properties of natural rubber based nanocomposites are investigated. Natural rubber is doped at various doping rates with nanofibrillated cellulose extracted from the rachis of date palm tree. The physical origin of both direct current (DC) and alternating current (AC) conductivity of polymer matrix–cellulose nanocomposites has been investigated. In fact, high activation energy values from DC measurements prove that the conductivity in the present samples is of an ionic nature. In addition, DC conductivity is more thermally activated than AC conductivity suggesting the random free-energy barrier model to explain the hopping conduction mechanism. Moreover, AC conductivity of these composites is frequency and temperature dependent, it generally follows the exponential law σAC ~ ωs. Exponent s lies in the range 0 < s < 1, characterizing an ionic hopping mechanism. Finally, using the Barton–Nakajima–Namikawa (BNN) relationship implies that the same ionic hopping mechanism is responsible for both AC and DC conduction.

The evolution of the secondary electron emission from sapphire and polycrystalline alumina during electron irradiation, achieved in a scanning electron microscope at room temperature, is derived from the measurement of the induced and the secondary electron currents. The semi-logarithmic plot of the secondary electron emission yield versus the surface density of trapped charges displays a plateau followed by a linear variation. For positive charging, the slope of the linear part, whose value is of about 10-9 cm(2), is independent of the primary electron energy, the microstructure and the impurities. It is interpreted as an effective microscopic cross section for electron-hole recombination. For negative charging of sapphire, the slope is associated with an effective electron trapping cross section close to 10-11 cm(2), which can be assigned to the dominant impurity trap. These effective values reflect the multiple interactions leading to the accumulation of charges. The yield corresponding to the plateau is controlled by the initial density of impurity traps. A charge transport and trapping > model, based on simplifying assumptions, confirms qualitatively these inferences. (C) 2014 Elsevier B.V. All rights reserved.