Dielectric Transport Research Articles

Quantifying Dielectric Material Charge Trapping and De-Trapping Ability Via Ultra-Fast Charge Self-Injection Technique.

Recently, utilizing the air breakdown effect in the charge excitation strategy proves as an efficient charge injection technique to increase the surface charge density of dielectric polymers for triboelectric nanogenerators (TENGs). However, quantitative characterization of the ability of dielectric polymers to trap reverse charges and the effect on the startup time of secondary self-charge excitation (SSCE) are essential for extensive applications. Here, an ultra-fast charge self-injection technique based on a self-charge excitation strategy is proposed, and a standard method to quantify the charge trapping and de-trapping abilities of 23 traditional tribo-materials is introduced. Further, the relationship among the distribution of dielectric intrinsic deep, shallow trap states, and transportation of trapped charges is systematically analyzed in this article. It shows that the de-trapping rate of charges directly determines the reactivation and failure of SSCE. Last, independent of TENG contact efficiency, an ultra-high charge density of 2.67 mC m-2 and an ultra-fast startup time of SSCE are obtained using a 15µm poly(vinylidene fluoride-trifluoroethylene) film, breaking the historical record for material modification. As a standard for material selection, this work quantifies the charge trapping and de-trapping ability of the triboelectric dielectric series and provides insights for understanding the charge transport in dielectrics.

Reduced-order model to predict thermal conductivity of dimensionally confined materials

Predicting nanoscale thermal transport in dielectrics requires models, such as the Boltzmann transport equation (BTE), that account for phonon boundary scattering in structures with complex geometries. Although the BTE has been validated against several key experiments, its computational expense limits its applicability. Here, we demonstrate the use of an analytic reduced-order model for predicting the thermal conductivity in dimensionally confined materials, i.e., monolithic and porous thin films, and rectangular and cylindrical nanowires. The approach uses the recently developed “Ballistic Correction Model,” which accounts for materials' full distribution of phonon mean-free-paths. The model is validated against BTE simulations for a selection of base materials, obtaining excellent agreement. By furnishing a precise yet easy-to-use prediction of thermal transport in nanostructures, our work strives to accelerate the identification of materials for energy-conversion and thermal-management applications.

Inhibition conduction loss for distinct improvement of energy storage density over a broad temperature range in polyetherimide-based composite films

The exploration of high-energy-density polymer dielectrics that can operate stably under high temperatures is of great significance in order to meet the growing needs for modern high-power electronics. However, the drastically deteriorated capacitive performances brought by the inevitable conduction loss at elevated temperature grievously hinder their application in extreme circumstances. Herein, a class of sandwich-structured dielectric nanocomposites capitalizing on binary wide-bandgap nanofillers is reported. The synergy of wide bandgap Al2O3 nanoparticles and hydroxyl-modified BNNSs can build higher interfacial barrier to efficiently depress the thermionic charge injection and carrier transport in dielectrics, yielding a substantially inhibited electrical conduction of the composites under elevated temperature. Indeed, the resulting sandwich-structured polymer composites deliver first-class-level discharged energy density (Ud) of ∼2.23 J/cm3 with efficiency (η) > 90% even under the ultra-high temperature of 200 °C, which is by far the current free-standing high-temperature dielectric polymer and polymer-based composites reported to date. Together with remarkable dielectric stability over a wide temperature and frequency range and outstanding cyclability, this contribution puts forward an efficient approach to groping for scalable high-temperature dielectric polymer nanocomposites for advanced electrical energy storage.

Hydrodynamic heat transport in dielectric crystals in the collective limit and the drifting/driftless velocity conundrum

We apply a recently developed method for solving the linearized phonon Boltzmann equation to study the hydrodynamic thermal transport in dielectrics in the collective limit, i.e., when normal collisions dominate resistive ones. The method recovers Guyer and Krumhansl results for a single Debye branch and extends them to general dispersion relations and branches. Specifically, we obtain explicit microscopic expressions for the phonon distribution and for the transport coefficients in this limit. We find that the phonon distribution differs from the commonly used displaced distribution in two terms: one accounting for viscous flow and another one which allows us to solve a long-standing issue on drifting and driftless second-sound velocities. Thus, the new method allows us to generalize previous results and fill some gaps on fundamental aspects of the collective limit through a simple mathematical formalism. We compare the hydrodynamic framework with previous models and discuss its limitations.

Quantum master equation approach to heat transport in dielectrics and semiconductors

We report on the derivation of the heat transport equation for nonmetals using a quantum Markovian master equation in Lindblad form. We first establish the equations of motion describing the time variation of the on-site energy of atoms in a one dimensional periodic chain that is coupled to a heat reservoir. In the continuum limit, the Fourier law of heat conduction naturally emerges, and the heat conductivity is explicitly obtained. It is found that the effect of the heat reservoir on the lattice is described by a heat source density that depends on the diffusion coefficients of the atoms. We show that the Markovian dynamics is equivalent to the long wavelength approximation for phonons, which is typical for the case of elastic solids. The high temperature limit is shown to reproduce the classical heat conduction equation.

Charge transport mechanism in La:HfO2

Currently, it is generally accepted that the charge transport in dielectrics is limited by the Coulomb trap ionization in a strong electric field (Frenkel effect). In the present work, the charge transport mechanism in La:HfO2 was experimentally studied, and four theoretical conductivity models—the Frenkel effect of Coulomb trap ionization, Hill-Adachi model of overlapping Coulomb potentials, Makram–Ebeid and Lannoo model of multiphonon isolated trap ionization, and Nasyrov-Gritsenko phonon-assisted tunneling between traps—were quantitatively analyzed. It was shown that the charge transport mechanism in La: HfO2 is qualitatively described by the Frenkel effect, but the Frenkel effect predicts an abnormally low trap concentration value and a large high-frequency dielectric constant value, which is not consistent with the experiment. The charge transport in La:HfO2 is quantitatively described by the model of phonon-assisted tunneling between neighboring traps.

Charge transport mechanism in dielectrics: drift and diffusion of trapped charge carriers

In this study, we developed a continuum theory of the charge transport in dielectrics by trapped electrons and holes, which takes into account two separate contributions of the current of trapped charge carriers: the drift part and the diffusion one. It was shown that drift current is mostly dominant in the bulk, while the diffusion one reaches significant values near contacts. A comparison with other theoretical models and experiments shows a good agreement. The model can be extended to two- and three-dimensional systems. The developed model, formulated in partial differential equations, can be numerically implemented in the finite element method code.

Use of Sobol indexes for efficient parameter estimation in a charge transport model

This article reports on a parameter sensitivity analysis on a mathematical model for the charge transport in dielectrics, using the Sobol sensitivity method. The main point of the work is to perform a sensitivity analysis under the variation of the impacting experimental factors such as the temperature, the applied electric field, and the charging time. Useful indications were obtained on which variables contribute the most to the variability of the output and which variables can be neglected. A quantitative assistance is derived from these results. Indeed, this study helps in revealing the experimental conditions where the model becomes sensitive to particular variables. Then, the sensitivity analysis results are used to determine the best starting points for optimization process in order to precisely and rapidly estimate each model parameter.

Multiphonon trap ionization transport in nonstoichiometric SiNx

It is generally accepted that the charge transport in dielectrics is governed by coulombic trap ionization due to a barrier lowering in high electric fields (Frenkel effect). In this paper, the charge transport mechanism in Si3N4 and nonstoichiometric silicon rich SiNx is experimentally studied and quantitatively analyzed with five theoretical models: Frenkel model of Coulomb traps ionization, Hill-Adachi model of overlapping Coulomb traps, Shklovskii-Efros percolation model, Makram-Ebeid and Lannoo model of multiphonon isolated traps ionization and Nasyrov-Gritsenko model of phonon-assisted electron tunneling between nearby traps. It is shown that the charge transport in Si3N4 and SiNx is qualitatively described by Frenkel effect, but Frenkel effect predicts an enormously low attempt to escape factor value. The charge transport at traps energies Wt = 1.6 eV and Wopt = 3.2 eV in Si3N4 and SiNx can be described by an increase in traps concentration in the framework of Makram-Ebeid and Lannoo model and Nasyrov-Gritsenko model. The Makram-Ebeid and Lannoo model quantitatively describes the charge transport in Si3N4 and SiNx with low silicon enrichment. The charge transport in nonstoichiometric SiNx with high silicon enrichment is well explained by Nasyrov-Gritsenko model.

Hole-alleviated trap transport in dielectrics

At present, the nature of ionic conductivity in dielectrics remains unclear. It is believed that the ionic transport is due to local Joule heating. In this paper, another ionic conductivity mechanism is proposed and considered. Taking into account the two-band conductivity, the diffusion and drift of traps in a dielectric are studied after the holes are captured on them. It is assumed that capturing holes onto traps leads to a significant decrease in their activation energy, which, in turn, leads to an increase in ionic conductivity in dielectrics. Considering the drift and diffusion of traps with captured holes, it was possible to describe the high conductivity in thin Si3N4 films.

Simulation of Cycle-to-Cycle Instabilities in SiOx- Based ReRAM Devices Using a Self-Correlated Process with Long-Term Variation

Cycle-to-cycle (C2C) current variability occurring in ReRAM devices is not only a stochastic feature inherent to electron transport in low-dimensional conducting structures but also a consequence of the measurement protocol used to characterize the device evolution during resistance switching. In such latest case, C2C changes depend on the particular arrangement of the ions or vacancies that form the conducting filament spanning the dielectric film. In this letter, a discrete first-order autoregressive model AR(1) with long-term variation is used to represent both the random and the “deterministic” behaviors of the high resistance state current. Simulation of C2C instabilities in SiO x is carried out through the quantum point-contact model for filamentary electron transport in dielectrics with fluctuating confinement potential barrier height. Simplicity is of utmost importance, since the proposed approach is aimed for circuit simulation environments in which complex and time-consuming computations need to be avoided.

Charge transport in dielectrics by tunneling between traps

The theory of charge transport in dielectrics by tunneling between traps is developed. In contrast to the Frenkel model, traps in silicon nitride are characterized by two energies, optical and thermal ones, and ionization occurs by the multiphonon mechanism. The theory predicts that tunneling between such traps is thermally stimulated: the half-difference of the optical and thermal energies plays the role of the activation energy. This theory successfully explains the experimental current-voltage characteristics of silicon-enriched silicon nitride. Such silicon nitride contains a large number of traps whose nature is associated with excess silicon. Charge transport in this material occurs by tunneling between adjacent traps.

Charge transport in dielectrics via tunneling between traps

A theory for charge transport in dielectrics via tunneling between traps was developed. Unlike in the Frenkel model, in the present theory the traps are characterized with the thermal and optical ionization energies, and ionization proceeds by the multi-phonon mechanism. The theory predicts the tunneling between such traps to be a thermally stimulated process whose activation energy equals half to the difference between the optical and thermal ionization energies. The theory provides an adequate description to the experimental current-voltage characteristics of silicon-rich silicon nitride films. Such films have a high density of traps originating from the excess silicon present in the material, with charge transport proceeding as a charge carriers tunnel between closely spaced traps.

Imaging deep trap distributions by low vacuum scanning electron microscopy

The distribution of deep traps in a bulk dielectric (Al2O3) is imaged by low vacuum scanning electron microscopy (LVSEM). The image contrast corresponds to spatial variations in radiation-induced, field-enhanced conductivity. A methodology is presented for identification of such contrast, the behavior of which is explained by a model of charge generation and transport in dielectrics imaged by LVSEM. The technique presented is applicable to studies of charge traps in dielectrics, device failure modes, and contrast mechanisms in electron microscopy.

The pulsed electro-acoustic technique in research on dielectrics for electrical engineering. Today's achievements and perspectives for the future

The pulsed electro-acoustic technique is presented to the Electrical Engineering community where it can find many applications, from the development of improved materials for electrical insulation to the control of electrostatic surface discharge (ESD) phenomena. After a short introduction of the space charge problem, we introduce the technique itself and we show how consistent quantitative information can be extracted from this simple method taking examples of dielectrics under field or charged by an electron beam. Non-polar and polar materials are considered and it is shown that space charge and polarization effects can be separated from the spectra analysis. In a last part, we review new perspectives open by this technique to understand and to model electrical transport in dielectrics

Submicron Heat Transport Model in Silicon Accounting for Phonon Dispersion and Polarization

In recent years, the Boltzmann transport equation (BTE) has begun to be used for predicting thermal transport in dielectrics and semiconductors at the submicron scale. However, most published studies make a gray assumption and do not account for either dispersion or polarization. In this study, we propose a model based on the BTE, accounting for transverse acoustic and longitudinal acoustic phonons as well as optical phonons. This model incorporates realistic phonon dispersion curves for silicon. The interactions among the different phonon branches and different phonon frequencies are considered, and the proposed model satisfies energy conservation. Frequency-dependent relaxation times, obtained from perturbation theory, and accounting for phonon interaction rules, are used. In the present study, the BTE is numerically solved using a structured finite volume approach. For a problem involving a film with two boundaries at different temperatures, the numerical results match the analogous exact solutions from radiative transport literature for various acoustic thicknesses. For the same problem, the transient thermal response in the acoustically thick limit matches results from the solution to the parabolic Fourier diffusion equation. In the acoustically thick limit, the bulk experimental value of thermal conductivity of silicon at different temperatures is recovered from the model. Experimental in-plane thermal conductivity data for silicon thin films over a wide range of temperatures are also matched satisfactorily.

Bernhard Gross and the evolution of modern electret research

During his scientific career, spanning the time from the early 1930's to the present date, Bernhard Gross has contributed to many disciplines of physics and engineering. Some of these fields, such as radiation effects in solids and charge storage and transport in dielectrics, are integral parts of electret research, an area which Gross has promoted, and whose development he has guided, over the past six decades. The present article focusses on the outstanding contributions of Bernhard Gross to electret research and discusses the impact of his work on the evolution of this field.

THE MICROSCOPIC PHENOMENOLOGICAL THEORY OF ANALYSIS FOR THE PROBLEM OF NONEQUILIBRIUM FLUCTUATIONS (Ⅰ)——A NEW THEORY OF EXTENDED IRREVERSIBLE THERMODY-NAMICS AND NONEQUILIBRIUM CORRECTIONS OF THE FLUCTUATION-DISSIPATION EXPRESSIONS FOR THE HEAT FL

This paper provide the microscopic phenomenological theory of analysis for the problem of nonequilibrium fluctuations, i.e. the statistical theory for description of nonequilibrium fluctuations. This theory is based on a generalized nonequilibrium entropy and an extension of the Einstein formula for the probability of the fluctuations. We obtain the second moments of nonequilibrium fluctuation of the specific energy and the heat flux in rigid heat conductors by calculation. This approach leads to nonequilibrium corrections to the conventional fluctuation-dissipation expressions for the heat fluctuations. The corresponding numerical corrections are obtained for phonon heat transport in dielectrics and for electronic heat transport in metals.

Exact solutions for space charge transport in dielectrics with non-uniform charge distributions in open circuit

This paper gives solutions for the space charge motion in dielectrics under open circuit condition in two cases: (a) free space charge with initial density linear in space, and (b) single deep-level trapped charges with initial density linear in space and with the assumption of non-retrapping. An analytical solution is given for the first case; solution of the second case requires numerical computation.