Correlation between morphology and transport properties of composite films: Charge transport in composites

Metal/dielectric composite films consisting of metal objects located in dielectric matrix are investigated by computer simulation. The complete computer experiment is devoted to the study of correlation between structural properties and electrical characteristics of composite films. In the present analysis transport properties of films are calculated near the metal-dielectric transition when the basic mechanism of charge transport is the tunnel effect. The conductivity of composite film is disseminated into individual percolation paths influenced by object arrangements in the composite film.     

Improved Dielectric Breakdown Strength of Covalently-Bonded Interface Polymer–Particle Nanocomposites

Interfacial covalent bonding is an effective approach to increase the electrical resistance of a polymer–particle composite to charge flow and dielectric breakdown. A bifunctional tether reagent bonded to an inorganic oxide particle surface assists with particle dispersion within a thermosetting epoxy polymer matrix but then also reacts covalently with the polymer matrix. Bonding the particle surface to the polymer matrix resulted in a composite that maintained the pure polymer glass transition temperature, compared to modified or unmodified particle dispersions that lacked covalent bonding to the polymer matrix, which depressed the polymer glass transition to lower temperatures. The added interfacial control, directly bonding the particle to the polymer matrix, appears to prevent conductive percolation across particle surfaces that results in a reduced Maxwell–Wagner relaxation of the polymer–particle composite and a reduced sensitivity to a dielectric breakdown event. The inclusion of 5 vol% particles of higher permittivity produces a composite of enhanced dielectric constant and, with surface modification to permit surface cross-linking into the polymer, a polymer–particle composite with a Weibull E ₀ dielectric breakdown strength of 25% greater than that of the pure polymer resulted. The estimated energy density for the cross-linked interface composite was improved 260% compared to the polymer alone, 560% better than a polymer–particle composite synthesized using bare particles, and 80% better than a polymer–particle composite utilizing bare particles with a dispersant.     

以能带理论诠释直流聚乙烯绝缘中空间电荷的形成和抑制机理

高压直流塑料交联聚乙烯电缆的研发难点是消除其中的空间电荷效应. 目前, 国内外学者普遍通过添加纳米粒子在聚乙烯体内形成深陷阱捕获电荷的机理来抑制电荷积聚, 但此抑制机理违背了电场的基本理论. 以能带理论全面阐述聚合物介质中空间电荷的形成和抑制机理, 从一级陷阱模型出发, 应用电荷入陷和脱陷动力方程, 推导了聚合物介质中空间电荷的形成过程. 在聚合物介质中引入深陷阱后, 介质Fermi能级位移, 电极与介质之间界面接触由Ohm接触转变为阻塞接触. 考虑到无定形相中大量的陷阱密度, 电荷耗尽区宽度小于100 Å, 电极与介质之间的界面对电子变得透明, 形成中性接触, 在电压作用下, 这种聚乙烯介质中不再可能形成空间电荷. 最后, 在纯聚乙烯和纳米改性后含有深陷阱的聚乙烯两种试样上, 分别测量了电导与电场强度的关系和空间电荷分布曲线, 实验结果符合理论推导. 关键词： 直流绝缘 能带理论 空间电荷 抑制机理     

Preparation,characterization and application of Nano CdS doped with alum composite electrolyte

A new composite electrolyte has been developed for electrochemical application and studied in details. The system contains Alum doped with nanosize cadmium sulphide (CdS) particles in the desire ratio. The structural studies were carried out by using X-ray diffraction (XRD) as well as infrared spectroscopy (IR) which affirms the composite nature of the system. The electrical properties, including ion transport studies and complex impedance spectra confirm the ionic nature of sample as well as enhancement in ionic conductivity by CdS doping. The temperature dependence measurement confirms the Arrhenius nature of sample, which is commonly observed in the ionic composite system. The dielectric constant varies with temperature, and this data is used to calculate the number of charge carrier (n/n_o) contributing to conductivity and fits well with emf variation. A cell was fabricated by sandwiching the composite between graphite and stainless steel electrodes, which shows an emf of 7 mV.     

Triboelectric charging and dielectric properties of pharmaceutically relevant mixtures

Binary mixtures of pharmaceutically relevant powders were investigated using dielectric spectroscopy over a frequency range of 10⁻³ to 300 kHz. Two different binary mixtures were studied as a function of concentration; pseudoephedrine hydrochloride in dicalcium phosphate dihydrate and acetaminophen in microcrystalline cellulose, respectively. Dielectric properties obtained from measurements of these systems are reported and found to follow a trend similar to the observed triboelectric behavior after low-shear tumble blending. Powder samples for charge measurement were mixed using a stainless steel blender and dispensed directly into a Faraday pail. For the two binary mixtures studied, low-frequency conductivity calculated from the imaginary part of the complex permittivity (or loss factor) was observed to be sensitive to water content. Furthermore, the unanticipated trends previously reported in the measured specific charge after blending were observed to correspond with the surface charge density calculated from the capacitance of the composite material. The implications of moisture and the physical and chemical properties of these dielectric mixtures are also discussed with supporting results.     

Surface exciton modes for plane and spherical semiconductor-metal interfaces

An approximate method is developed for investigating the nature of interface exciton modes in a composite spatially dispersive medium. The method is general enough to be applicable to any composite system, in which each component is described by an arbitrary bulk dielectric functionε(q, ω). It is based on the extension of the usual electrostatic-image method of solving the Poisson’s equation, in the presence of an external point charge in the system. We have applied our general method to a composite system of a finite metal slab surrounded by a semiconductor on one side and the vacuum on the other side. Similarly, we have also considered the case of a metallic sphere of radiusR, surrounded by a semiconductor, with a spherical interface between them. With assumed spatially dispersive model dielectric functions for the bulk metal and the bulk semiconductor, the nature of the electron-electron interaction and the interface exciton modes in the metallic region are obtained in both the cases. For the relevant size of the metal large compared to the atomic dimensions over which the bulk dielectric functions are non-local due to the spatial dispersion, it is shown that one can obtain the interface exciton modes by first defining new effective dielectric functions for each of the media making the particular interface, and then using the usual expression which determines the modes in the non-dispersive case.     

Study on the Failure Factors of Composite Insulation in High-Voltage Storage Capacitors

High-voltage storage capacitors, which have composite insulations of polypropylene film and paper impregnated with oil, are working under charge–discharge mode. The insulations are not only exposed to electrical stress but also to thermal stress. Failure factors of composite insulation in capacitors are analyzed from three aspects: partial discharge (PD), space charge, and thermal factors. In order to investigate the possible influences on the dielectric properties of materials in capacitors, aging tests were conducted on the capacitors. Some parameters were detected in aged capacitors compared with nonaged ones. Experimental results reveal that PD is the key factor leading to the breakdown of the capacitor, space charge is the direct factor that induces the breakdown, and thermal factor just accelerates the degradation process.      

Tuneability amplification factor and loss of high-contrast composites

The problem of tuneability of nonlinear ferroelectric–dielectric composite materials is addressed. Attention is concentrated on the analysis of the tuneability amplification factor, K, of a composite material, which is introduced as the ratio of the effective tuneability of a composite material to the tuneability of its ferroelectric (tuneable) component. Previously, ferroelectric–dielectric composite materials have been designed with an effective tuneability amplification factor slightly greater than 1 (i.e. 1.1–1.4) [A.K. Tagantsev et al., J. Electroceramics 11 (2003) p.5; A.G. Kolpakov et al., J. Electroceramics 18 (2007) p.129]. It is demonstrated that the tuneability amplification factor can take values significantly greater than 1. Numerical prototypes (structural designs integrated with finite element method tools) of microstructures are presented with K in the range 3 to 30. The effective tuneability of nonlinear composite materials strongly depends on the microgeometry and microtopology of the material and, in general, cannot be described in terms of volume fraction of components of composite material. In the designs presented, the increased tuneability is due to concentration of the high electric field in narrow regions with carefully selected geometry. The problem of loss in ferroelectric–dielectric composite materials is discussed. In the general case, the loss tangent stays between that of the components of the composite. For high-contrast ferroelectric–dielectric composite materials, the effective loss tangent is practically equal to the loss tangent of the ferroelectric.     

Detail electrical and dielectric studies on carbon-fly ash composite

A new solid-state composite was prepared by using fly ash as host matrix and carbon shoots of camphor as dispersoid, which are abundantly available and cheap. The composite was characterized by using different techniques. For conductivity and dielectric measurements, we used complex impedance spectroscopy. Maximum conductivity was obtained at 4% of carbon in fly ash. The dielectric measurements affirm conductivity pattern. X-ray diffraction and Fourier transform infrared spectroscopy (FTIR) was conducted which shows composite nature of sample. Total ionic transference number test reveals that conductivity is mainly due to ions.     

Electrostatic coupling with interfacial free charge in ferroelectric-paraelectric bilayers and superlattices

Based on a thermodynamic model, we studied the interfacial free charge effect on the polarization, spatial inversion symmetry, and dielectric response in ferroelectric-paraelectric bilayer. The broken spatial inversion symmetry and imprint due to free charge are not found. Effect of free charge on the dielectric constant is not monotonic. Contradiction between the equilibrium polarization and non-linear field-induced dielectric polarization in paraelectric layer is found. Our study reveals that treating coupling strength as an “input” parameter taken dependent of the polarization is inappropriate to describe electrostatic coupling with interfacial free charge.     

Space charge behavior and its modified electric field in the cross-linked polyethylene under applied voltage DC and different temperatures

In this paper, space charge behavior in dielectric material and the capacitive charge at an electrode-dielectric interface, at room temperature under an applied electric field, has been investigated. This was done for a single sample and for a combination of the sample and a non-stressed sample using the pulsed electro-acoustic (PEA) method. A negative charge injected at the dielectric interface under an electric field (polarization) and high temperatures was focused on. It was found that negative charge injection takes place under all the test conditions, and this charge can be affected by the electric field and the temperature.     

固态高压脉冲形成线研究

 为了实现高压脉冲形成线小型化,开展了平板固态脉冲形成线研究。实验研究了两种作为储能介质的固态材料的高压体击穿特性、沿面闪络特性和频率响应特性,一种是特种复合材料,其相对介电常数在50～250之间,另一种是功能陶瓷,其相对介电常数在200～1 000之间。在此基础上,研制了两种平板固态脉冲形成线。特种复合介质固态脉冲形成线试验电压为3 kV左右,输出电压脉冲半高全宽（FWHM）可以达到58 ns;功能陶瓷介质固态脉冲形成线的试验充电电压超过120 kV,输出电压脉冲半高全宽为92 ns。     

Dielectric and electro-optic properties of 6CHBT nematic liquid crystals and silver nanoparticles composites

We have prepared the composite of a room temperature nematic liquid crystalline material namely 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT) and silver nanoparticles (AgNPs). Thermodynamic, electro-optic and dielectric properties of the prepared composite have been investigated. From these investigations, parameters such as transition temperature, dielectric anisotropy, dielectric permittivity and splay elastic constant have been determined for the pristine as well as composite systems. Threshold switching voltage of the pure and composite samples has also been determined from transmission voltage curve. The effects of dispersion of AgNPs on dielectric and electro-optic parameters of LC-NPs composite have been discussed in terms of existing standard models.     

Charge relaxation in conducting dielectric films with shallow and deep traps

A nonsteady-state boundary-value problem of the relaxation of a charge injected into a dielectric film is solved analytically with allowance for the film conductivity and capture of charge carriers by traps having a finite emptying rate. The one-and two-zone modes of charge relaxation are considered. The obtained general expressions reduce to earlier published formulas derived in particular cases. Numerical calculations and an analysis of the experimental data available in the literature on the electret state of oxide films deposited on metal substrates confirm the applicability of the proposed model of dielectric relaxation.     

束缚电荷特性的实验演示

电介质表面在极化过程中产生束缚电荷,束缚电荷与自由电荷有明显的区别,通过一系列的实验,演示了束缚电荷既不能离开电介质,也不能在电介质中自由移动,也就是“束缚”的特性。     

不同电介质结构下介质阻挡放电特性研究

设计制作了单面有氧化铟锡(ITO)导电介质层的双玻璃介质层的介质阻挡放电装置,研究了其放电特性,并将其与双玻璃介质层和单玻璃介质层的介质阻挡放电进行了比较.从电荷输运的角度分析,上述三种装置分别实现了电荷的二维、零维和三维输运.采用两种不同的双玻璃介质层装置,获得了单个稳定的放电丝.与无ITO导电层的双玻璃结构得到的单个放电丝相比,单面有导电ITO介质的双玻璃结构中,单放电丝呈"T"字型,其光晕是前者光晕的2倍,其放电电流大于前者电流,其放电时间间隔长短交替现象更为明显,且存在强度大小交替的现象.分析表明,壁电荷输运及二次电子发射的不同导致了不同电介质结构放电特性的不同. 关键词： 介质阻挡放电 壁电荷 二次电子发射     

High dielectric properties of polymer-based ternary blend films using fillers’ synergy between asymmetrical Na-bearing OH–C60 and insulative diamond

Polymer/highly-conductive carbon composites are used as dielectrics. However, a high dielectric loss is induced by leakage current. In this study, ternary polymer composites with hydroxyfullerene and diamond were fabricated. Ternary composites exhibited more promising dielectric traits compared with polymer/hydroxyfullerene composites. A high dielectric constant was achieved using polar hydroxyfullerene. A significantly reduced dielectric loss was achieved owing to insulative diamond. Polymer/hydroxyfullerene interaction and branching of leakage current were studied. The best ternary composite showed a dielectric constant of ∼26 and dielectric loss of ∼0.21 at 20 Hz. This work may enable the large-scale fabrication of advanced dielectrics.     

Electric Charging in Dielectric Barrier Discharges with Asymmetric Gamma‐Coefficients

A dielectric barrier discharge with different electrode surfaces is investigated and the dynamic evolution of surface charge on the dielectric surface is measured optically. It is found that the amount of surface charge after the positive and the negative half‐cycle are not equal, i.e. a bias charge emerges. To understand this phenomenon, the transfered charge per half‐cycle is estimated from the Paschen curve and the shape of the driving voltage waveform. It turns out that the charge bias is necessary to compensate for the asymmetry of the discharge conditions due to the different surfaces and hence is required for a balanced charge transfer in the steady state (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of charged device model (CDM) ESD in MEMS

The effect of charge injection due to CDM ESD in capacitive MEMS structures is analyzed. The results show that as feature size is reduced, ESD injected charge produces a change for the stiction effect which is inversely proportional to the square of the plate area and a change in the dielectric layer breakdown which is inversely proportional to the plate area. An electric field model is developed to examine charge and voltage modes in MEMS. A charge injection test method is proposed to determine the susceptibility of MEMS to CDM ESD.