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Graphene oxide (GO)-based nanocomposites have received a great attention due to their emerging applications. Here, we thoroughly examined the structural, electronic and surface properties of $$\hbox {GO}/\hbox {TiO}_{2}$$ nanotubes nanocomposite. The nanocomposite is prepared by simple impregnation of $$\hbox {TiO}_{2}$$ nanotubes (HNT400) with GO dispersion. GO is elaborated by an improved Hummer’s method, while HNT400 is obtained using alkaline hydrothermal treatment of $$\hbox {TiO}_{2}$$ P25, followed by calcination at $$400^{\circ }\hbox {C}$$ . XRD and Raman analyses show that GO nanosheets do not change the structural properties of $$\hbox {TiO}_{2}$$ nanotubes. TEM analysis confirms the formation of GO nanosheets assembled to $$\hbox {TiO}_{2}$$ nanotubes. XPS and EPR results confirm the electron transfer between GO and $$\hbox {TiO}_{2}$$ nanotubes. PL analysis reveals that GO inhibits the recombination of photogenerated electron–hole pairs in the nanocomposite. The ac conductivity measurements suggest the presence of grain and grain boundary effects in GO/HNT400.

WOS:000516579400003; Dielectric elastomers are considered to be promising materials for applications in energy harvesting, haptic interface or biomedical actuation, thanks to their high energy density and attractive efficiency. Nevertheless, the performance of dielectric elastomer transducers (DETs) are affected by losses, especially losses induced by leakage current. This work reports an extensive investigation into the stability of the current-time characteristics of VHB4910 dielectric elastomers, currently in use for such applications. It should be noted that conductive pastes that act as compliant electrodes play an important role in the behavior of the current. The Schottky current is thus disturbed at the electrode - elastomer interface and an increase in current is observed over time when the device is subjected to electrical voltages over long periods of time. Pre-stress, commonly applied for application using VHB4910, influences also significantly the electrical response. Thus, for a same stretching of the material, asymmetric stretching is preferred over symmetric stretching in order to obtain a lower leakage current. This difference in behavior is explained by the fact that the polymer chains are stretched in preferential directions which will then lead to specific responses of both the dipoles and other conduction mechanisms such as hopping. Finally, a simple analytical model is proposed to report the current behavior in these materials according to the nature of the electrodes and the pre-stretching applied.

We report on the negative capacitance effect in TiO2 films and its relation to charge trapping. We demonstrate that by increasing temperature, it is possible to suppress this effect. This phenomenon is studied for other high-k materials through a simple protocol which allows to control capacitance change with frequency. Indeed, the application of a bias stress increases the charge trapping. This compensates the polarization, and the capacitance reaches negative values. Results are correlated to the density functional theory within the Drude formalism. The work concerns the ability to control the NC for the field effect transistors applications.

In this study, the dielectric properties of metal-oxide-metal capacitors based on Tantalum Titanium Oxide (TiTaO) thin films deposited by reactive magnetron sputtering on aluminum bottom electrode are investigated. The structure of the films was characterized by Atomic Force Microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The dielectric properties of TiTaO thin films were studied by complex impedance spectroscopy over a wide frequency range (10 -2 - to 10 5 Hz) and temperatures in -50 °C to 325 °C range. The contributions of different phases, phases’ boundaries and conductivity effect were highlighted by Cole – Cole diagram (ɛ” versus ɛ’). Two relaxation processes have been identified in the electric modulus plot. A first relaxation process appears at low temperature with activation energy of 0.37 eV and it is related to the motion of Ti 4 + (Skanavi’s model). A second relaxation process at high temperature is related to Maxwell-Wagner-Sillars relaxation with activation energy of 0.41 eV.

Parylene is a generic name indicating a family of polymers with the basic chemical structure of poly-p-xylylene. Parylene N and Parylene C are the most popular for applications. Curiously, Parylene D (poly( dichloro-p-xylylene), (C8H6Cl2)) was forgotten for applications. This report is the consequence of a later availability of a commercial dimer of Parylene D and also to the recent advent of fluorinated Parylenes allowing extending applications at higher temperatures. In our work, from a dielectric analysis, we present the potentialities of Parylene D for applications particularly interesting for integration in organic field-effect transistors. Dielectric and electrical properties, macromolecular structures, and dynamics interaction with electric field as a function of frequency and temperature are studied in 5.8 μm thick Parylene D grown by chemical vapor deposition. More exactly, the dielectric permittivity, the dissipation factor, the electrical conductivity, and the electric modulus of Parylene D were investigated in a wide temperature and frequency ranges from -140 to +350 °C and from 0.1 Hz to 1 MHz, respectively. According to the temperature dependence of the dielectric permittivity, Parylene D has two different dielectric responses. It is retained as a nonpolar material at very low temperature (like Parylene N) and as a polar material at high temperature (like parylene C). The dissipation factor shows the manifestation of two relaxations mechanisms: γ and β at very low and high temperatures, respectively. The γ relaxation is assigned to the local motions of the C-H end of the chains when the cryogenic temperature range is approached. A broad peak in tan δ is assigned to the β relaxation. It corresponds to rotational motion of some polar C-Cl groups. For temperature above 260 °C a mechanism of Maxwell-Wagner-Sillars polarization at the amorphous/crystalline interfaces was identified with two activation energies of Ea1 = 2.12 eV and Ea2 = 3.8 eV. Moreover, the conductivity and the dielectric permittivity relaxation processes have been discussed in terms of nearly constant loss (NCL) and universal dynamic regime (UDR). Finally, ionic conduction and electrode polarization effects are identified at very high temperatures and their physical origins are discussed.

In this Letter, we evaluate the so-called forming process of virgin HfO2 cells under ac and dc stress by capacitance–voltage measurements. It is found that the ac nonlinearity is higher than the dc one. This is related to the dispersive nature of hopping conduction according to the electrode polarization model. Under short stress times, both capacitance and current are unaffected. However, after longer stress times, no recovery is observed. Consequently, the current increases and a plateau appears in the capacitance. The observed degradation is consistent with the RRAMs-forming process. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thin and transparent Parylene D (-CH 2 -C 6 H 2 Cl 2 -CH 2 -) films have been prepared by chemical vapor deposition (CVD). The impact of the crystallinity transformations and the thermal stability were identified in Parylene D based on the dielectric spectroscopy (DS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Changes of (ac)-conductivity with temperature (from 240°C to 340°C) and frequency (from 10−1 Hz to 106 Hz) are discussed. Good insulating properties of Parylene D were obtained until operating at high temperatures. Moreover, its dielectric properties were inserted between those of Parylene N and Parylene C making Parylene D interesting to replace the latter for specific applications. However, DSC measurements of these samples show after multiple melting-crystallization cycles the appearance of an instability endothermic peak when the crystallization of the sample is completed. This peak increases and shifts towards lower temperatures as the sample is further thermally treated. At this point, the thermal instability of Parylene D is associated to a crystallographic structure change and not due to a chemical degradation.

Amorphous barium titanate films (a-BTO) were grown by the RF magnetron sputtering technique at low temperature and under various oxygen/argon mixing ratios (OMR). Their dielectric properties (dielectric constant, loss and ac conductivity in the 0.1–105 Hz range), dc leakage currents, and thermally stimulated currents were systematically studied as a function of OMR in the sputtering gas (0% to 20 %). We demonstrate that the presence of oxygen plays a significant role in improving the dielectric properties of a-BTO thin films. The oxygen incorporation successfully suppressed some electrical defects. The bulk conductivity and leakage currents are reduced although a small decrease of dielectric constant has been observed. Films prepared at higher oxygen ratio (20%) have a small leakage current density (3.6 × 10− 7 A/m2) and lower tangent loss (2 × 10− 3).

Conduction mechanisms in BaTiO3 films deposited at low temperatures on Cu-electrodes have been investigated in transient and steady regimes as a function of temperatures and electric field. This work aims to identify possible defects which govern the leakage current. Electrical measurements reveal that Space Charge Limited Current (SCLC) constitutes the main leakage mechanism in both the transient and the steady regimes. Based on the theory of SCLC, two types of defects can be detected. At higher temperatures, oxygen vacancies constitute the main defects which migrate across the film to generate an ionic leakage current. Diffusion of these defects is thermally activated with an activation energy around 1 eV. Moreover, at lower temperatures the J -E measurements reveals the presence of a discrete set of shallow traps at 0.45 eV below the conduction band with an effective density of 4 × 1022 m–3. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dielectric elastomers such as 3M VHB4910 acrylate film have been widely used for electromechanical energy conversion such as actuators, sensors and generators, due to their lightweight, high efficiency, low cost and high energy density. Mechanical and electric properties of such materials have been deeply investigated according to various parameters (temperature, frequency, pre-stress, nature of the compliant electrodes…). Models integrating analytic laws deduced from experiments increase their accuracy. Nevertheless, leakage current and electrical breakdown reduce the efficiency and the lifetime of devices made with these polymers. These two major phenomena are not deeply investigated in the literature. Thus, this paper describes the current-voltage characteristics of acrylate 3M VHB4910 and investigates the stability of the current under high electric field (kV) for various temperatures (from 20°C to 80°C) and over short (300 s) and long (12h) periods. Experimental results show that, with gold electrodes at ambient temperature, the current decreases with time to a stable value corresponding to the conduction current. This decrease occurs during 6 hours, whereas in the literature values of current at short time (less than 1 hour) are generally reported. This decrease can be explained by relaxations mechanisms in the polymer. Schottky emission and Poole-Frenkel emission are both evaluated to explain the leakage current. It emerges from this study that the Schottky effect constitutes the main mechanism of electric current in the 3M VHB4910. For high temperatures, the steady state is reached quickly. To end, first results on the leakage current changes for pre-stretch VHB4910 complete this study.

The aim of this work is to study the dielectric properties of thin films of tungsten trioxide. WO3 is an interesting material in the field of gas sensors. Here, we investigate the dependence of the electrical conductivity with temperature as well as the effect of the pressure on the thermal activation energy. For this purpose, we have used Impedance Spectroscopy (IS) to study the dielectric properties of RF sputtered WO3 thin films. Particularly, we have emphasized the effect of oxygen pressure on the surface conductivity.

Capacitance nonlinearities were studied in atomic layer deposited HfO2 films using two types of signals: a pure ac voltage of large magnitude (ac nonlinearities) and a small ac voltage superimposed to a large dc voltage (dc nonlinearities). In theory, ac and dc nonlinearities should be of the same order of magnitude. However, in practice, ac nonlinearities are found to be an order of magnitude higher than dc nonlinearities. Besides capacitance nonlinearities, hopping conduction is studied using low-frequency impedance measurements and is discussed through the correlated barrier hopping model. The link between hopping and nonlinearity is established. The ac nonlinearities are ascribed to the polarization of isolated defect pairs, while dc nonlinearities are attributed to electrode polarization which originates from defect percolation paths. Both the ac and dc capacitance nonlinearities display an exponential variation with voltage, which results from field-induced lowering of the hopping barrier energy.

aLaboratory for Materials, Organization and Properties (LMOP), Campus Universitaire-El Manar, 2092 Tunis, Tunisia bMicroelectronics Technology Laboratory (LTM), Joseph Fourier University (UJF), National Center for Scientific Research (CNRS), CEA-LETI Minatec Campus, 38054 Grenoble Cedex 9, France cGrenoble Electrical Engineering Laboratory (G2ELab), Joseph Fourier University (UJF), National Center for Scientific Research (CNRS), 38042 Grenoble Cedex 9, France

The electrical conduction mechanisms of semicrystalline thermoplastic parylene C (-H(2)C-C(6)H(3)Cl-CH(2)-)(n) thin films were studied in large temperature and frequency regions. The alternative current (AC) electrical conduction in parylene C is governed by two processes which can be ascribed to a hopping transport mechanism: correlated barrier hopping (CBH) model at low [77-155 K] and high [473-533 K] temperature and the small polaron tunneling mechanism (SPTM) from 193 to 413 K within the framework of the universal law of dielectric response. The conduction mechanism is explained with the help of Elliot's theory, and the Elliot's parameters are determined. From frequency- and temperature-conductivity characteristics, the activation energy is found to be 1.27 eV for direct current (DC) conduction interpreted in terms of ionic conduction mechanism. The power law dependence of AC conductivity is interpreted in terms of electron hopping with a density N(E(F)) (~10(18) eV cm(-3)) over a 0.023-0.03 eV high barrier across a distance of 1.46-1.54 Å.

Dielectric measurements have been performed on ion beam sputtering (IBS) barium strontium titanate Ba0.7Sr0.3TiO3 thin films at annealing temperatures 470 and 700 °C using impedance spectroscopy. The effect of the annealing temperature upon the electrical properties of the films is also investigated using capacitance–voltage techniques. Increasing annealing temperature suggested the increases of density and grain size, whereas the density of the trapped oxygen vacancy may be decreasing with increasing annealing temperature. The barrier height (E a) of the oxygen vacancy decreases with increasing annealing temperature. The C–V characteristics were investigated in relation to the annealing temperature to identify the anomalous capacitance in the MIM configuration films. Among all measurement temperatures, it was observed that the data fit well by the “LGD” model. The interfacial effect and its dependence of morphology structure have been studied, and the results are discussed.

In this work, we present the potentialities of the surface potential method to investigate electrostatics charges deposited on epoxy resins used in packaging. Curing and post-mould-curing (PMC), with various durations have been performed on epoxy resins. Negative ions are deposited by corona discharge on the surface of these epoxy resins. Then the flowing of charges as a function of time, namely the surface potential decay (SPD), is measured by a non-contacting electrostatic probe. These measurements were performed in a climatic chamber at different temperatures (30degC, 50degC, 70degC) and a 50% relative humidity rate. Our results have shown a strong influence of the curing and the post-mould-curing on the SPD. Practically the fastest SPD is obtained for the sample having the longest curing. The SPD is strongly slowed down when the samples were subjected to a PMC. We suggest that dipoles in the bulk and traps near the surface influence the surface potential decay. For a longer time of curing, traps appear near the surface and trap ions. Thus induces a fast decrease of SPD. We suggest that the PMC reduces dipoles and is favorable to the filling of the traps. Thus, less traps and dipoles after a PMC explain why the SPD is slower

The dielectric behavior of Pt/SrTiO3/Pt thin films is reported in this paper. STO thin films (20 nm and 50 nm) were deposited by ion-beam-sputtering (IBS). This study was carried out in the temperature ranging from 25degC to 150degC and for frequencies in the range 0.01 Hz-1 MHz. Results showed that the permittivity depends strongly on frequency, temperature and sample thickness. For this latter, the dielectric constant decreases as the film thickness is decreased. This can be explained by an interfacial "dead layer effect" that reduces the effective dielectric constant. Using approximative calculations based on the thickness dependence of the dielectric constant, we have estimated the thickness and the dielectric constant of this 'dead layer'. For a given STO thickness, when the frequency is decreased, we observe an increase in the permittivity. This behavior is more pronounced as temperature increases and for the STO with a thickness of 50 nm. A relaxation peak in the loss tangent in the same frequency range confirms a relaxation process. We suggest interfacial relaxations at the grain boundaries to explain this relaxation. This relaxation differs as a function of the thickness: at a given temperature, the relaxation peak is located at higher frequency and is less important for the thinner STO film. This relaxation explains the important increase in tandelta for the 50 nm STO film. An ionic space charge movement with blocking and accumulation of ions at the electrodes could explain the dielectric responses (permittivity and losses) in the lowest frequencies. To conclude, low frequency dielectric spectroscopy appears as essential tool to understand and optimize high permittivity integrated MIM capacitances

The dynamics of microscopic bubbles have been widely studied in low-viscous insulating liquids. The bubble motion (growth, collapse, rebound, etc.) is governed by inertial forces. In this paper, we analyse the bubble dynamics in viscous liquids (/spl mu/ > 3 mPl), and we observed that the implosion time of the bubble becomes more and more important than that obtained in slightly viscous liquids. We conclude that the effect of viscosity controls the bubble dynamics. Experimentally, we have also observed for the first time the transition of the bubble dynamics from the viscous regime to the inertial one and this transition can be explained by a refined analysis of the Rayleigh-Plesset model of bubble dynamics.

The dynamics of microscopic bubbles have been widely studied in low-viscosity insulating liquids. The bubble motion (growth, collapse, rebound, etc) is governed by inertial forces. In this paper, we analyse the bubble dynamics in viscous liquids (/spl mu//spl ges/3 10/sup -3/P a.s) and conclude that the effect of viscosity controls its dynamics. The implosion time of the bubble becomes more important than those obtained in slightly viscous liquids. Experimentally, we demonstrate the transition of dynamics bubble from the viscous regime to the inertial regime. This transition depends on the initial size of the bubble.

In this paper, we present an experimental study of high field conduction phenomena in liquid argon and liquid nitrogen in a point-plane electrode geometry. These liquids behave qualitatively like saturated hydrocarbons except when they are studied close to their normal boiling point. The generation and dynamics of microscopic bubbles as a function of various parameters (nature of the liquid, injected energy and hydrostatic pressure) are presented and discussed. The possible influence of these phenomena on electrical insulation at cryogenic temperatures is also discussed.

We study capacitance dispersion in hydrogen-doped amorphous barium titanate (a-BaTiO3:H). Negative values of capacitance are observed at very low frequencies (

International audience; Time-dependent transient currents were studied at elevated temperatures (200-220 °C) in amorphous barium titanate thin film capacitors. Current transients display a peak whose time position varies with applied voltage and temperature. The response is analyzed through space charge limited current transient theories. Extracted drift mobilities are in the 10−11-10−12 cm2 V−1 s−1 range and show an activation energy of 1 eV. The phenomena are associated with oxygen vacancies migration in BaTiO3.