Nanometric dielectrics

It is suggested that a major field of study in the future development of dielectrics will concern their properties when relatively few molecules are involved. Such smallness arises naturally at interfaces of nanometric thickness and will occur also when dielectrics are employed in the nano-technical devices of the future. It already occurs in living systems where the dielectric and conductive properties of biomaterials are vital in sustaining activity. The transverse and lateral properties of interfaces, including the effects of molecular ordering, are considered and it is suggested that the advent of scanning tunneling and atomic force microscopies provides a significant opportunity for nanometric dielectric studies. An important feature, suggested for future exploitation, is the cross-coupling in interfaces of force fields arising from electrical, mechanical, chemical and entropic potential gradients. Application of these concepts to biology and to the behavior of polymer gels which may lead to development of muscle-like actuators and transducers are considered. Finally, attention is drawn to the likely role of nanometric interfacial processes in the initiation of electrical breakdown in insulating materials     

Dielectric properties of epoxy nanocomposites

The dielectric properties of epoxy nanocomposites with insulating nano-fillers, viz., TiO₂, ZnO and AI₂O₃ were investigated at low filler concentrations by weight. Epoxy nanocomposite samples with a good dispersion of nanoparticles in the epoxy matrix were prepared and experiments were performed to measure the dielectric permittivity and tan delta (400 Hz-1 MHz), dc volume resistivity and ac dielectric strength. At very low nanoparticle loadings, results demonstrate some interesting dielectric behaviors for nanocomposites and some of the electrical properties are found to be unique and advantageous for use in several existing and potential electrical systems. The nanocomposite dielectric properties are analyzed in detail with respect to different experimental parameters like frequency (for permittivity/tan delta), filler size, filler concentration and filler permittivity. In addition, epoxy microcomposites for the same systems were synthesized and their dielectric properties were compared to the results already obtained for nanocomposites. The interesting dielectric characteristics for epoxy based nanodielectric systems are attributed to the large volume fraction of interfaces in the bulk of the material and the ensuing interactions between the charged nanoparticle surface and the epoxy chains.     

Local properties at interfaces in nanodielectrics: An ab initio computational study

First-principles computational methodologies are presented to study the impact of surfaces and interfaces on the dielectric and electronic properties of emerging technologically important systems over length scales of the order of inter-atomic distances. The variation of dielectric constant across Si-SiO₂, Si-HfO₂ and SiO₂-polymer interfaces has been correlated to interfacial chemical bonding environments, using the theory of the local dielectric permittivity. The local electronic structure variation across Si-HfO₂ and SiO₂-polymer interfaces, including band bending, band offsets and the creation of interfacial trap states have been investigated using a layer-decomposed density of states analysis. These computational methods form the groundwork for a more thorough analysis of the impact of surfaces, interfaces, and atomic level defects on dielectric and electronic properties of a wide variety of nano-structured systems.     

Simulation of heterogeneous nanodielectrics using the local field method

The dipole-dipole interaction is significantly responsible for the dielectric behaviour of matter. In the past, several analytic models have been developed to describe these dielectric properties. With increasing performance of personal computers, bigger and bigger systems can be simulated numerically on a microscopic scale by calculating the local electric fields at each dipole site. In doing so and applying the method of image dipoles in order to consider the electrodes we can easily calculate the local fields, polarization and effective susceptibility for a system of nanometric size with dimensions smaller than 100 /spl Aring/ which consists of more than one sort of atoms, e. g. binary mixtures, or different lattice constants. With this method all depolarization fields within the sample are regarded inherently. The different phases can be arranged differently so that we can investigate the influence of the granularity of the mixture. Defects and inclusions can also be easily included. We have studied for example the impact of an air-filled sphere in a dielectric on the effective susceptibility and simulated local fields for a pyramidal inclusion. For a given plane in the three-dimensional sample we can determine the local fields at (grain) boundaries.     

Effect of physical aging on dielectric, thermal and mechanicalproperties of cast-epoxy insulators

Samples of cast bisphenol-A and cycloaliphatic epoxy insulation were quenched from a temperature above T_g (170°C) to room temperature. Variations in dielectric, thermal and mechanical properties were examined during subsequent sub-T_g annealing (physical aging). It was found that the impact strength of the epoxy insulation decreased considerably with physical aging. Moisture intake and subsequent dielectric loss at room temperature were also found to decrease with physical aging. Glass transition temperature and excess enthalpy measured by DSC demonstrated significant increase with increased sub-T_g annealing. On the other hand, dielectric properties showed different changing patterns during sub-T_g annealing in different epoxy systems. It is concluded that epoxy insulating materials should be properly annealed before assessing their dielectric and other physical properties because of the possibility of unknown rapid cooling histories     

聚合物纳米复合电介质的界面性能研究进展

聚合物纳米电介质以其优异的性能而受到广泛关注,其中聚合物基体与纳米粒子间的界面作用机理成为研究热点。为此,综合国内外研究成果,论述了聚合物纳米电介质中界面区域的重要性,并从界面化学结构、物理结构模型、机械（力学）性能、热力学性能等方面强调了界面区域在电气绝缘性能中的作用机理。界面是纳米填充物与聚合基体之间的纳米级过渡区域,由于其独特的形成机理,其具有与聚合物基体和纳米填充物不一致的理化性质。界面在复合材料中占有主导地位,其微观结构及性能将直接影响复合材料的宏观性能。     

Dielectric performance of interfaces in premolded cable joints

Great progress has been made in developing high-quality insulating materials, in maintaining consistent manufacturing processes, and in designing cable joints for easy installation. However, little attention has been given to the interfaces in cable joints. Greases are generally used as a lubricant for installation of premolded cable joints (excluding heat-shrink and cold-shrink splices), and are therefore always found at these interfaces. Analysis of field-aged cable joints has revealed two common problems related to greases: dry and sticky interfaces causing great difficulties in power disconnection, and interfacial discharges (or surface tracking) leading to dielectric breakdown. The focus of this study is on the dielectric performance of interfaces in cable joints. Measurements of breakdown voltage and pressure were carried out at interfaces of both new and field-aged cable joints. The effect of surface conditions was also investigated. Results show that the dielectric performance at interfaces is intimately related to the surface conditions, and a distinct long-term behavior is found for each type of interface in cable joints. The significance of the results and their implications are discussed     

Environmental effects on cracking and delamination of dielectric films

The effects of temperature and moisture on the adhesive and cohesive strength of dielectric materials and the interfaces that are composed of those materials commonly found in thin-film interconnect structures are reviewed. Debond growth rate versus debond driving energy curves (V-G curves) were collected over a range of environmental conditions for both dielectrics and dielectric/metal interfaces. Both are found to exhibit characteristics consistent with stress corrosion cracking mechanisms found in the bulk glass literature. The mechanisms identified in both systems are explained in terms of the salient chemical reactions occurring at the debond tip.     

Dielectric mixtures: electrical properties and modeling

A review of the current state of understanding of dielectric mixture properties, and approaches to use numerical calculations for their modeling are presented. It is shown that interfacial polarization can yield different non-Debye dielectric responses depending on the properties of the constituents, their concentrations and geometrical arrangements. Future challenges on the subject are also discussed.     

Modeling of dielectric relaxation spectra of polymers in thecondensed phase

Discusses models and their possible implication in the interpretation of the relaxation spectra of polymers, particularly involving the concept of order/disorder systems. Furthermore, some examples of the application of the Dissado-Hill model have been provided in this work to explain quasi-dc (QDC) conduction processes, which have been observed in crosslinked polyethylene (XLPE) and low density polyethylene (LDPE), showing an ageing behavior with continuing use with an ac field in a humid environment. It is also shown in this paper how a measurement of dielectric relaxation properties on a wide frequency range may be used as a tool to investigate the ageing of polymeric power distribution cables with an ac field in a humid environment. Equivalent electrical analog circuits of dielectric relaxation spectra are also shown     

Computer modeling of interaction of gas discharge plasma with solid dielectric barriers

A computer model describing charge transfer in a system consisting of two parallel-plate metallic electrodes covered with solid dielectric barriers immersed in gas medium is proposed. The material of the barriers is supposed to be a non-ideal insulator whose properties correspond to polyethylene and air is considered as a gas phase. The model is based on continuity equations for fluxes of charge carriers and accounts for their drift and diffusion and also for different sources of their generation and losses in different media. The continuity equations are coupled with Poisson's equation for computing electric fields affected by temporal and spatial variations of space charges in the system. Results of the computer simulations are obtained for the case when the applied field in the gas exceeds its breakdown threshold, i.e. charge transfer in the gas phase takes place in the form of an electrical discharge (electron avalanche and streamer). Evolution of generated discharge plasma is analyzed taking into account conditions on gas-solid interfaces and in the bulk of the solid dielectric barriers.     

Imaging breakdown spots in SiO/sub 2/ films and MOS devices with a conductive atomic force microscope

Conductive atomic force microscopy (C-AFM) was used to study the dielectric breakdown of SiO/sub 2/ layers at a nanometric scale. First, bare oxide regions were stressed and broken down using the tip as the metal electrode of a MOS structure. The results show that the initial breakdown is electrically propagated to neighbor regions, affecting their dielectric strength. Moreover, the area affected by the initial breakdown depends on the breakdown hardness. In particular, it is shown that this area is smaller when the current through the structure is limited during the experiments. The effect of the current limitation is analyzed in detail. Based on the results, a qualitative picture of the breakdown process is presented, which accounts for this effect. Finally, for the first time, the breakdown spots in standard MOS devices (with poly-Si gate) are electrically imaged with C-AFM. The areas of the observed spots are in agreement with those obtained on bare oxides.     

多胶粉云母带胶粘剂的结构与性能分析

本文对大型高压发电机主绝缘用F级粉云母带的粘合剂进行了研究,对其中几种典型的固化体系,如环氧树脂-桐油酸酐-双马来酰亚胺体系、环氧树脂-羧酸盐(苯甲酸铅)体系以及此二者的混合体系的介电性能、力学性能和耐热性能进行了分析,并对有关体系的固化反应作了探讨.研究结果认为体系中引入双马来酰亚胺可提高热变形温度、热态强度和介电性能,双马来酰亚胺在固化中主要是自聚而不是和环氧基共聚.环氧-羧酸盐体系虽然热变形温度和热态强度较高,但高温下介电性能差,耐热老化性能也不好.但如严格控制铅盐的用量,把上述两种体系结合起来还是值得重视的.     

Understanding the strength of epoxy-polyimide interfaces for flip-chip packages

Polyimides are commonly used as organic passivation layers for microelectronic devices due to their unique combination of low dielectric constant, high thermal stability, and excellent mechanical properties. Polyimides are well known to have poor adhesion to epoxy resins. Many surface treatment methods have been developed to increase epoxy-polyimide adhesion. These include various ion beam, plasma treatment, or chemical treatment methods. The goal of this research is to understand the strength of epoxy-polyimide interfaces by studying the effect of polyimide chemical structure on epoxy-polyimide adhesion. The four polyimides chosen in this study are commonly used in the microelectronics industry: poly (pyromellitic dianhydride-oxydianiline [PMDA-ODA], poly (3,3/spl acute/,4,4/spl acute/-biphenyltetracarboxylic dianhydride-phenylene diamine [BPDA-PDA], poly (hexafluoroisopropylidene-diphthalic anhydride-oxydianiline) [6FDA-ODA], and 5(6) -Amino-1-(4-aminophenyl)-1,3,3, trimethylindanbenzophenonetetacarboxylic dianhydride copolymer [BTDA-DAPI]. The adhesive strengths between an epoxy resin and these four polyimides were characterized using interfacial fracture mechanics and the critical interfacial strain energy release rates ranged from 20 to 179 J/m/sup 2/ depending on the particular polyimide used. The loci of failure for these epoxy-polyimide interfaces were analyzed using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR), and found to be at the interphase region for all four interfaces. It is interesting that the strength of the interfaces appears to be related to the predicted interfacial widths from solubility parameter theory.     

Multiscale modeling of screening effects on conductivity of graphene in weakly bonded graphene-dielectric heterostructures

Graphene is often surrounded by different dielectric materials when integrated into realistic devices. The absence of dangling bonds allows graphene to bond weakly via the van der Waals interaction with the adjacent material surfaces and to retain its peculiar linear band structure. In such weakly bonded systems, however, the electronic properties of graphene are affected by the dielectric screening due to the long-range Coulomb interaction with the surrounding materials. Including the surrounding materials in the first principles density functional theory (DFT) calculations is computationally very demanding due to the large supercell size required to model heterogeneous interfaces. Here, we employ a multiscale approach combining DFT and the classical image-potential model to investigate the effects of screening from the surrounding materials (hBN, SiC, SiO₂, Al₂O₃, and HfO₂) on the dielectric function and charged impurity scattering limited conductivity of graphene. In this approach, the graphene layer is modeled using DFT and the screening from the surrounding materials is incorporated by introducing an effective dielectric function. The dielectric function and conductivity of graphene calculated using the simplified two-band Dirac model are compared with DFT calculations. The two-band Dirac model is found to significantly overestimate the dielectric screening and charged impurity scattering limited conductivity of graphene. The multiscale approach presented here can also be used to study screening effects in weakly bonded heterostructures of other emerging two-dimensional materials such as metal dichalcogenides.     

Dielectric and sintering properties of NaNbO3 ceramic prepared by Pechini method

NaNbO₃ powders and ceramics were prepared by Pechini method. The pure phase NaNbO₃ was obtained at temperature as low as 350 °C, and then they obtained fine powders were used to prepared ceramics through conventional sintering process. The effect of sintering temperature on microstructure evolution and dielectric properties of NaNbO₃ ceramics has been determined. Results shown that the microstructure of NaNbO₃ ceramics consisted of stacked plates, which was related the liquid phase. It was important to note that the stacked plate configuration forming the grain has not been emphasized before, and these grains support adequate wetting characteristics for the liquid phase in order to achieve fully dense microstructure. Above 1195 °C, a number of angular grains with flat interfaces started to appear, and joined to each other with the sintering temperature increase. The effect of grains morphology on dielectric properties of the NaNbO₃ ceramics was also detected.     

Effects of frequency, temperature, compression, and air pressure onthe dielectric properties of a multilayer stack of dry kraft paper

Kraft paper is used extensively in the electricity supply industry for the insulation of HV apparatus, such as transformers, capacitors and cables. Because of the fibrous nature of the paper, many layers are stacked or rolled to obtain the required electric strength. The insulation is thus a combination of cellulose and an impregnating fluid, which may be a liquid, a gas or vacuum. The dielectric properties of such a system depend on the dryness, temperature, frequency and the degree of mechanical compression. Although the effects of moisture, temperature and frequency on the dielectric properties of kraft paper have been studied, very little work has been done on the effect of compressive stress, particularly long-term effects. Results are presented of measurements, in the frequency range 50 Hz to 20 kHz, of the effects of compressive stress and temperature on the real and imaginary parts of the complex relative permittivity of a multilayer stack of dry kraft paper under vacuum. The variations of the dc resistance with compressive stress and temperature also are reported. Results are given of the variation of the dielectric properties of a stack of paper following a step change in either the temperature or the air pressure. The long-term variations are attributed to the slow diffusion of ions through the stack, particularly those held at the interfaces between layers     

Dielectric permittivity simulations of layered composites with rough interfacial surfaces

Finite difference quasi-electrostatic simulations are used to predict the interfacial dielectric permittivity of a rough-surfaced contact zone between two distinct materials in a layered composite. Fractional Brownian surfaces, which have fractal geometry, are used to represent the rough interfaces in a model space. The interfacial simulations are combined with a macroscopic analytic model for planar dielectric layers, which allows the calculation of composite permittivity for a layered composite with an arbitrary ratio of surface roughness-to-layer thickness and arbitrary volumetric filling fractions of the constituents. Examples are given for a ceramic-polymer system, and the effects of alternate ratios of constituent dielectric permittivities and changes in surface fractal character are also explored. Compared to the behavior of composites with perfectly flat interfaces, the rough-surfaced composite exhibits a significantly earlier increase in permittivity as a function of the volumetric filling fraction of the higher permittivity material. The behavior with extremely rough surfaces tends towards the predictions of the effective medium approximation     

Use of WKB approximation for analytical boundary conditions in numerical solution of Schrödinger equation: application to semiconductor–high‐k dielectric interfaces

Numerical methods used to solve 1D Schrödinger's equation in quantum structures, such as Numerov's integration of wavefunction or the shooting method iterative solution of energy levels, require knowledge of two‐point boundary conditions at interfaces. This is especially true when the interfaces are not symmetrical or where exponential decay of wavefunction at asymptotically large distances does not hold. A closed‐form expression for boundary conditions, which is not sensitive to intermediate solutions at interfaces, can minimize possible divergence during iterations and relax simulation grid size and simulation time. In this work, the Wentzel‐Kramers‐Brillouin (WKB) approximation within potential barriers is proposed to analytically calculate the boundary conditions for abrupt interfaces, such as dielectric–semiconductor interface. An analytical expression for the slope at the interface is derived, and the errors are estimated with respect to numerical methods. An application is shown for self‐consistent solution of coupled Poisson–Schrödinger's equations at multi‐layer HfO₂‐SiO₂ dielectric gate stack corresponding to International Technology Roadmap for Semiconductors‐projected 10 nm bulk single‐gate Complementary Metal‐Oxide‐Semiconductor (CMOS) technology node, where wavefunction penetration into the dielectric is of critical importance. Application to dual gate structures with 5 nm fin width and high‐k dielectric with 0.5 nm equivalent oxide thickness is also shown. A quantum mechanical simulator ‘hksim’ based on this principle is posted for public domain usage. Copyright © 2015 John Wiley & Sons, Ltd.