掺杂含量对环氧纳米复合电介质陷阱与空间电荷的影响

环氧纳米复合电介质具有抑制空间电荷积聚、高电阻率、高击穿强度等优异性能,对直流电力设备的发展具有重要的作用.但纳米粒子含量对纳米复合电介质陷阱、电导率和空间电荷的影响机理尚不清楚.本文在纳米复合电介质交互区结构模型的基础上提出了计算交互区浅陷阱和深陷阱密度的方法,得到了浅陷阱和深陷阱密度随纳米粒子含量的变化关系.随着纳米粒子含量的增加,浅陷阱密度逐渐增大,深陷阱先增加然后由于交互区重叠的影响而逐渐减少.研究了纳米粒子含量对浅陷阱控制载流子迁移率的影响,发现随着纳米粒子的增多,浅陷阱大幅增多,浅陷阱之间的平均间距迅速减小,导致载流子更容易在浅陷阱间跳跃迁移,浅陷阱控制载流子迁移率增大.建立了纳米复合电介质的电荷输运模型,采用电荷输运模型计算研究了环氧/二氧化钛纳米复合电介质的空间电荷分布、电场分布和电导率特性.发现在纳米粒子添加量较小时,交互区的深陷阱对电导的影响起主导作用;纳米粒子添加量进一步增加,浅陷阱对电导的影响将起到主要作用.     

Experiment and simulation of space charge suppression in LDPE/MgO nanocomposite under external DC electric field

In this paper, space charge dynamics under DC electric field of −100 kV/mm in low-density polyethylene (LDPE) and its nanocomposite containing a small amount of MgO nanoparticles were measured using an improved pulsed electro-acoustic (PEA) system. Unlike negative packet-like space charge accumulating in LDPE films, no remarkable space charge was observed in LDPE/MgO nanocomposite films, which indicated that the introduction of MgO nanoparticles played a key role on the space charge suppression. Different with current qualitative models, this paper describes space charge suppression on the basis of simulation using the bipolar charge transport model, which featured bipolar charges injection, transport, trapping, recombination, and extraction process. It was shown from the simulation that trap depth, trap concentration, local electric field and charge injection barrier height were all significant factors on the space charge suppression process. A deeper trap depth in LDPE/MgO nanocomposites made it easier for traps to capture mobile carriers. And a larger trap concentration effectively slowed down the whole carrier movement although there seemed a trap concentration threshold less than 30 Cm⁻³, above which this effect became slight. In addition, both the high permittivity of LDPE/MgO nanocomposites and low local electric field in the vicinity of cathode led to a larger injection barrier height based on the Schottky injection law, which would tremendously block the charge injection. At last, the suppression mechanism of space charge formation in the LDPE/MgO nanocomposites is presented.     

聚合物材料空间电荷陷阱模型及参数

采用电声脉冲测量技术研究了直流电场下低密度聚乙烯材料的电荷入陷和脱陷特征. 发现在不同电场周期下样品的电荷衰减呈现不同的特征, 为此提出了一个简单的基于两陷阱水平的入陷和脱陷模型, 并计算了相应的参数, 如陷阱能级和密度. 确定了不含任何添加剂的低密度聚乙烯样品中存在的两种水平的陷阱能级分别为: 较浅陷阱能级0.77–0.81 eV 对应的浅陷阱电荷密度为(1.168–1.553)× 10¹⁹ m^-3; 较深陷阱能级0.96–1.01 eV 对应的深陷阱电荷密度为(1.194–4.615)× 10¹⁸ m^-3. 最后初步验证了材料的深陷阱能级和对应的深陷阱电荷密度随老化而增加, 可考虑将模型中的两能级陷阱参数作为老化诊断特征参量. 关键词： 聚合物 空间电荷 陷阱 老化     

Nonlinear Dependence of Response Rateon Intensity in Doped KNSBN Crystals

l.IntroductionThephotorefractiveresponserateisacritica1issuebecauseitdirectlydeterminestheprocessingsPeedofphotorefractivedevices[1]-TherelationshipbetweentheresPonserateandthelightintensitycanprovideusefuIinformationaboutthedefectlevels,theprocessofchargeCarriersexcited,transportedaMdtrapPed["'].Conventionalsing]elevelmod-e1s[4jpredictthatthephotorefractiveresponserateislinearlydePendentonthelightinten-sity.However,experimentaldatashowedthatthephotorefractiveresponserateofsomecrystalsvarie…     

Surface electroluminescence phenomena correlated with trapping parameters of insulating polymers

Electroluminescence (EL) phenomena are closely linked to the space charge and degradation in insulating polymers, and dominated by the luminescence and trap centers. EL emission has been promising in defining the onset of electrical aging and in the investigation of dissipation mechanisms. Generally, polymeric degradation reveals the increment of the density of luminescence and trap centers, so a fundamental study is proposed to correlate the EL emission of insulating polymers and their trapping parameters. A sensitive photon counting system is constructed to detect the weak EL. The time- and phase-resolved EL characteristics from different polymers (LDPE, PP and PTFE) are investigated with a planar electrode configuration under stepped ac voltage in vacuum. In succession, each sample is charged with exposing to multi-needle corona discharge, and then its surface potential decay is continuously recorded at a constant temperature. Based on the isothermal relaxation current theory, the energy level and density of both electron and hole trap distribution in the surface layer of each polymer is obtained. It is preliminarily concluded that EL phenomena are strongly affected by the trap properties, and for different polymers, its EL intensity is in direct contrast to its surface trap density, and this can be qualitatively explained by the trapping and detrapping sequence of charge carriers in trap centers with different energy level.     

聚乙烯空间电荷包行为的形成机理与仿真方法研究

聚乙烯中的空间电荷包行为是空间电荷的一种特殊的输运行为.研究表明,空间电荷包行为由于受材料本身特性、外加电场大小以及环境温度等的影响,导致其产生过程及传输特性上存在较大差异,这些因素给空间电荷包行为产生机理研究带来了较大困难.通过对电荷的电极注入过程、载流子的体内迁移规律及空间电荷与体内陷阱的相互作用机制进行分析,探讨了不同外加电场及不同深度陷阱能级对电荷包行为造成的相关影响,在此基础上建立物理模型来描述电荷包的产生和迁移过程.模型中提出了在高于阈值电场时,载流子迁移速度与电场关系存在负微分迁移率的假设.基于此模型对空间包行为的模拟结果与实验结果取得较好的一致. 关键词： 空间电荷包 数值模拟 负微分迁移率     

基于类耿氏效应的低密度聚乙烯中空间电荷包行为的模拟仿真

低密度聚乙烯材料中的空间电荷包现象通常会引起严重电场畸变而影响其击穿特性. 本文借鉴半导体中的耿氏效应的负微分迁移率机理来描述电荷包的形成机理,并结合载流子的注入条件及体内陷阱对电荷迁移的影响等因素,对文献中报道的两类外加场强不同且迁移趋势各异的空间电荷包行为进行了模拟仿真,模拟的电荷包大小随电场变化规律,电荷包迁移速率随时间变化规律等与相应实验结果符合.模拟结果表明,产生耿氏效应的负微分迁移率是造成电荷包非弥散传输的主要原因,其与材料电极注入情况及体内陷阱态的共同作用导致了空间电荷包行为迁移的多样性. 关键词： 空间电荷包 耿氏效应 模拟仿真 负微分迁移率     

迁移率对单层有机发光器件中电场与载流子分布的影响

有机发光器件的宏观特性与有机层中的电场和载流子浓度分布密切相关。建立的有机电致发光器件模型是由两个金属电极中间夹一层有机发光薄膜材料组成的单层器件,金属与有机发光层之间为欧姆接触。模型以载流子运动的扩散-漂移理论为基础,利用数值方法研究了有机发光层中双极载流子注入时的电势、电场、载流子浓度和复合密度分布。分析结果表明:当两种载流子的迁移率相同时,电场强度、载流子浓度、复合密度的分布呈对称形式。而当电子和空穴的迁移率μn和μp相差比较大时,高迁移率的载流子不仅仅分布在注入端附近而且还有一小部分能够传输到另一端,而低迁移率的载流子只分布在其注入端附近;当μn、μp的大小相差不大时,载流子传输情况就介于两者之间。当μn/μp的比值变化时,电场强度的极大值向载流子迁移率小的注入端偏移。     

Correlation between the anisotropy of hopconduction magnetoresistance and reciprocal band masses in Ge

Near 70 kG the d.c. magnetoresistance of phonon-assisted hopping of charge carriers between shallow impurities is found to have an anistropy depending on the reciprocal effective mass calculated from the L₁ conduction band minima in the case of n-type Ge and from the heavy hole component of the Γ′₂₅ valence band maximum in case of p-type Ge. These correlations are discussed and it is suggested that they are connected with the symmetry of the impurity wave functions and that the magnetic field tends to remove the light hole component of an acceptor wave function.     

Kinetics of an exciton-trion system in shallow GaAs/AlGaAs quantum wells

The formation of three-particle charged exciton complexes (trions) in shallow GaAs/AlGaAs quantum wells in the temperature range 1.7–15 K has been investigated by luminescence spectroscopy and resonant light scattering. The effect of the photon energy and the intensity of additional above-barrier illumination on the trion formation kinetics has been analyzed. It is established that, upon intrawell excitation, illumination leads to the formation of trions when the light photon energy corresponds to the regions of effective formation of trions in the photoluminescence excitation spectra. It is shown that, with an increase in the illumination level, the trion concentration first increases and then reaches a plateau since the quantum well acquires an electric charge whose field equalizes the electron and hole capture rates.     

Fine structure of the emission spectrum of a two-dimensional electron–hole liquid in SiGe/Si quantum wells

The fine structure of the emission spectrum of a quasi-two-dimensional electron–hole liquid in shallow SiGe/Si quantum wells is observed experimentally. This fine structure is explained by the occurrence of steps in the density of states resulting from the coexistence of light and heavy holes in the liquid phase and by the interaction of charge carriers with charge-density oscillations in the liquid.     

Local electrical overstressing in polymer dielectrics

Using a spherical concentric capacitor as a microspike model, the electric field distribution in a polymer dielectric material with shallow and deep electron traps is investigated. It is established that in the course of field emission from microspikes on the electrode and space charge accumulation on the traps, the values of coefficient of electric overstresses depend on temperature, injection barrier height, and trap concentration and depth in the polymer material. The behavior of this coefficient is characterized as a function of the mean field strength. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 3–9, December, 2008.     

Influence of oxidation on space charge dynamics in DC aged low-density polyethylene

In this study, low-density polyethylene (LDPE) samples were aged under different oxygen concentration (0, 20%, and 50%). The ageing was implemented with an average external DC electric field of 50 kV/mm at 40 °C for 1000 h. It was found that augmentation of charge accumulation and suppression of charge dissipation were enhanced with increasing of oxygen concentration. The capture capability of trap sites got stronger with higher oxygen contents, which could be attributed to oxidation product newly generated during DC ageing. These trap sites acted as trap centers to capture the injected carrier while it was on the way towards the opposite electrode and decrease the apparent mobility. FTIR absorption spectra showed that new elements between 1680 and 1750 cm^?1 corresponding to ketone and aldehyde occurred due to oxidation. The bipolar charge injection model was implemented to simulate space charge transport in aged LDPE samples. It was shown that more carriers were captured and the traps gradually filled up when the oxygen concentration in the ageing test increased. A higher oxygen concentration seemed to result in a deeper average trap depth, which increased the amount of trapped charge because it was not easy to detrap. These simulation result corresponded to the experimental measurement.     

Space charge phenomena in oil-paper insulation materials under high voltage direct current

The wide use of high voltage direct current (HVDC) equipments, such as convert transformer, DC bushing, DC cables, makes the research of oil-paper material insulation extraordinarily important. The space charge effect is the key insulation problem of HVDC equipments. This paper studied the space charge characteristics of oil-paper insulation materials by Pulsed Electro-Acoustic (PEA) method. In this paper, we studied and discussed the relationship between space charge and breakdown in oil-paper. The results show that the space charge comes from ionization at lower field intensity. When the stress is getting higher, injection of homo carriers occurs first at cathode and later at anode. The space charge inside oil-paper sample results in an electrical field distortion up to 40%. The conductivity of oil-paper insulation material is much larger than that of polyethylene, so the space charge dissipates fast when depolarized. The fast dissipation of space charge will play a key role of oil-paper insulating performance in condition of polarity reversal under HVDC. When the applied negative electrical field rose to between 50 kV/mm and 57 kV/mm, the oil-paper material got breakdown in a short time. During this process, an obvious movement of space charge was observed.     

Investigation of ultrafast dynamics of CdTe quantum dots by femtosecond fluorescence up-conversion spectroscopy

The ultrafast carrier relaxation processes in CdTe quantum dots are investigated by femtosecond fluorescence upconversion spectroscopy.Photo-excited hole relaxing to the edge of the forbidden gap takes a maximal time of ～ 1.6 ps with exciting at 400 nm,depending on the state of the photo-excited hole.The shallow trapped states and deep trap states in the forbidden gap are confirmed for CdTe quantum dots.In addition,Auger relaxation of trapped carriers is observed to occur with a time constant of ～ 5 ps.A schematic model of photodynamics is established based on the results of the spectroscopy studies.Our work demonstrates that femtosecond fluorescence up-conversion spectroscopy is a suitable and effective tool in studying the transportation and conversion dynamics of photon energy in a nanosystem.     

Explanation of Anomalous Diffractive Behavior with New Model in Doped KNSBN Crystals

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Defects and radiation induced electronic processes in magnesium aluminate spinel of different compositions

The time dependence of the formation and decay of irradiation-induced optical absorption centers in magnesium aluminate spinel single crystals of different compositions (MgO - 1.0Al 2 O 3 and MgO - 2.5Al 2 O 3 ) was investigated. The kinetics of accumulation of X-ray irradiation-induced absorption bands is consistent with the mechanism of trap filling with free charge carriers through the conduction band. The model includes Coulomb blocking effects on spatially correlated defects. The observed two-stage decay of absorption bands after termination of X-ray irradiation is explained by electron hole recombination between centers of two different distances and/or different potential barriers. UV-irradiation confirms the existence of charge exchange between complex spatially correlated defects.     

从聚酰亚胺单分子链电荷陷阱特性的改变探讨体材料的沿面放电现象

沿面放电是破坏绝缘系统性能的原因之一.聚酰亚胺常用于高频电力设备的气-固绝缘中,为此利用密度泛函理论,从原子分子层面探讨了在外电场下聚酰亚胺及其受极性基团OH~–影响后的单分子链结构、能级与态密度、静电势、激发态等微观参数对陷阱形成以及沿面放电的影响.结果表明,外电场下,聚酰亚胺分子结构卷曲,偶极矩增加,易于积聚电荷形成空间电荷中心,尤属引入极性基团OH~–后变化较明显;聚酰亚胺分子中,苯环区域形成空穴陷阱,酰亚胺环区域形成电子陷阱,且电子陷阱能级的数量较多,其中空间电荷陷阱深度随外电场的增加逐渐变深;聚酰亚胺分子在引入极性基团OH~–后激发能降低,使得分子内部的电子变得容易被激发;电子与空穴的空间分离度随电场增加而降低,利于空穴与电子的复合而发出光子.     

分子束外延高Mg掺杂GaN的发光特性

采用分子束外延技术在蓝宝石衬底上制备Ｍｇ掺杂的立方相ｐ－ＧａＮ,并对其不同温度下的光致发光光谱进行了研究．实验观察到高Ｍｇ掺杂ＧａＮ中施主受主对发光的反常温度行为．理论分析表明,高Ｍｇ掺杂ＧａＮ中施主受主对的发光受到陷阱与受主间竞争俘获非平衡空穴过程和空穴隧穿输运过程的影响．     

电场和温度对聚合物空间电荷陷阱性能的影响

模拟分子的结构和行为有助于更深刻地分析空间电荷陷阱性能变化的微观机理.利用Materials studio软件建立聚乙烯模型,通过分子链段运动产生的能量和自由体积变化对微观结构和电荷陷阱进行分析.结果表明:温度由298 K逐渐升高至363 K的过程中,聚合物分子热运动加剧导致的滑移扩散现象,使自由体积和陷阱能级在363 K处分别出现1542.07 ~3和0.66 eV的最大值和最小值.然而在Z轴方向施加0.0007 Hartree/Bohr(1 Hartree/Bohr=5.2×10~(11)V/m)电场作用时,由于电致伸缩产生Maxwell应力,使分子链段出现局部有序排列,增大范德瓦耳斯能至-360.18 kcal/mol(1 kcal/mol=4.18 kJ/mol),而自由体积降低了279.77 ~3,导致陷阱能级减小0.45 eV.当363 K和0.0007 Hartree/Bohr联合作用时,聚乙烯的陷阱能级相比同温无电场作用降低0.17 eV.分子模拟结果与实测结果相符.利用分子热运动和电致伸缩效应,初步探讨了材料自由体积和范德瓦耳斯相互作用能变化的微观机理,证实分子链段运动改变了微观结构,从而影响电荷陷阱特性.并且与温度相比,电场作用会使材料产生更低能级的空间电荷陷阱.