聚合物绝缘材料表面陷阱与电致发光现象研究

聚合物材料老化前会出现电致发光现象，其紫外分量可能导致材料的劣化。因此电致发光的研究可为分析材料老化和击穿机理提供依据。该文首先基于等温电流衰减理论，设计了材料表面陷阱参数测量装置，实验中采用多针电极对低密度聚乙烯(polyethylene，LDPE)、聚丙烯(polypropylene，PP)和聚四氟乙烯(polytetrafluoroetylene，PTFE)3种材料进行表面充电，记录表面电位衰减并按照等温电流衰减理论计算得到材料表面陷阱参数。结果显示，LDPE的陷阱密度较PTFE和PP大，LDPE和PTFE的电子空穴陷阱分布成对称结构，PP的电子陷阱密度明显大于空穴陷阱密度，这与材料的分子结构的对称性有密切关系。同时基于单光子计数模块设计了微弱发光数据采集系统，对上述材料在真空中交流电压下的电致发光特性进行了测量和分析。材料的陷阱密度对其电致发光特性具有重要的影响，材料的陷阱密度越高，发光越弱，反之则越强。     

直流电压下SF_6中环氧复合绝缘的表面电荷积聚与衰减特性

为了研究直流电压下聚合物绝缘材料表面电荷的积聚和衰减特性,选择环氧复合绝缘材料为实验试品,采用对称平面压接电极,施加不同幅值的直流电压,利用静电探头测量了大气压SF_6气体中不同充电时间下绝缘材料的表面电位分布。实验结果表明:在直流电压作用下,环氧复合绝缘表面会发生同极性电荷积聚,电极中心线上表面电位呈钟形分布,表面电位的峰值和半峰值宽度随电压幅值增加不断增加。在相同幅值电压下,低幅值时(10 kV),正极性下表面电荷密度较负极性下高;高幅值时(10 kV),负极性下表面电荷密度较正极性下高。随充电时间增加,电极中心线表面电位仍然呈钟形分布,和外施电压同极性的表面电荷密度较高且分布较为集中。表面电位的衰减分为两个阶段,符合双指数函数规律,初期衰减快,后期衰减慢,正电荷衰减速度大于负电荷,电压幅值越高,起始电位衰减越快,衰减完全所需要的时间越长。结合实验结果,分析认为表面电荷主要通过表面电导衰减。     

聚合物绝缘材料直流闪络特性及表面形貌实验研究

研究绝缘材料沿面闪络特性及闪络过程中材料表面形貌的变化,可以为提高绝缘材料沿面耐压性能及材料表面改性处理提供参考。文中采用正极性直流电压源,对大气环境下聚四氟乙烯(PTFE)和聚甲基丙烯酸甲酯(PMMA)的沿面闪络特性及闪络前后表面形貌进行了实验研究。结果表明:绝缘材料直流闪络电压近似服从韦伯分布,可借助韦伯分布统计方法求出任意概率下的闪络电压。采用三维显微镜及原子力显微镜观测了闪络前后材料的表面形貌,发现材料表面的放电痕迹均在阴极附近更为严重,并且闪络后材料表面出现连续状沟壑,表面起伏变大、粗糙度变大。文中分析表明,绝缘材料的沿面闪络电压受电场畸变程度及材料表面形貌特性的影响,可为提高聚合物绝缘材料的沿面耐压性能提供一定的参考。     

直流高压下阀头绝缘材料电荷特性试验研究

基于平行平板电极,对直流高压下充氧阀头绝缘材料表面电位进行测量,研究了不同试验条件(电压作用时间、电压幅值、电压极性)对阀头绝缘材料表面电位的影响,并对比不同材质阀头绝缘材料电荷积累特性,为特殊环境下绝缘材料的选取和优化、高压环境下绝缘材料表面电场的建模和计算提供了参考。     

直流高压下阀头绝缘材料电荷特性试验研究

基于平行平板电极,对直流高压下充氧阀头绝缘材料表面电位进行测量,研究了不同试验条件(电压作用时间、电压幅值、电压极性)对阀头绝缘材料表面电位的影响,并对比不同材质阀头绝缘材料电荷积累特性,为特殊环境下绝缘材料的选取和优化、高压环境下绝缘材料表面电场的建模和计算提供了参考。     

表面电荷对典型聚合物绝缘材料直流闪络电压的影响

积聚在聚合物绝缘材料上的表面电荷是导致电场畸变、诱发沿面闪络的重要原因。研究表面电荷对聚合物绝缘材料沿面闪络电压的影响,对于保障电气设备的绝缘安全具有重要意义。选取聚四氟乙烯、环氧树脂和硅橡胶为试样,通过针–板电极向试样表面注入电荷,采用静电电位计测量表面电荷密度,分析表面电荷积聚、衰减特性。测量有、无表面电荷及电荷衰减过程中不同时刻的直流闪络电压,计算表面电荷对闪络电压的静态和动态影响指数Lindex和Sindex,评估表面电荷对闪络电压的影响。结果表明:充电完成时表面电荷对各材料闪络电压的影响程度为:环氧树脂硅橡胶聚四氟乙烯;表面电荷衰减期间其对各材料闪络电压的影响程度为:硅橡胶环氧树脂聚四氟乙烯。     

交流电压下聚合物材料表面的电荷分布特性

采用Pockels电光效应原理,对大气条件交流电压下聚合物绝缘材料的表面电荷分布进行了研究。实验以聚苯二甲酸乙二醇酯(polyethylene terephthalate,PET)和聚偏氟乙烯(polyvinylidene fluoride,PVDF)为样品,在针板电极上施加幅值5kV、频率20Hz的正弦交流电压,施加电压分别从正半波开始和从负半波开始,采用高速CCD摄像机纪录1个周期内2种聚合物材料在沿面放电过程中表面电荷分布的变化。实验结果表明,材料表面电荷分布的极性效应非常明显,负电荷分布半径要小于正电荷分布。材料特性也显著影响电荷分布,随着施加电压的升高,PET表面正流注通道逐步延伸,呈弯曲状分支分布,而PVDF表面正流注通道的形成明显要快,并呈现规则的轮辐状分布。分析认为,这与不同材料的禁带宽度、表面层电离能、介电常数和厚度等的差异都有很大关系。     

高电场下ZnS:Cu 复合涂层的电致发光机理EI北大核心CSCD

输变电设备安装运行过程中不可避免产生各种绝缘缺陷,将电致发光材料引入绝缘系统,能够直观显示高电场区,进而实现绝缘缺陷的自检测。该文针对ZnS:Cu复合涂层在高电场下的电致发光机制进行研究,利用棒–板结构电极在变压器油与空气中的不同特点,通过发光分布、放电电流与表面电荷等特性的观测,分别考察在无放电和有放电情况下的电致发光特征。结果表明,ZnS:Cu复合涂层的电致发光来源于无放电时的电荷极化发光与高电场下的气体放电诱导发光,其中电荷极化发光分布与电场强度密切相关;放电起始后,气体放电沉积在涂层表面的正负电荷向ZnS:Cu迁移形成激子,进而以辐射跃迁的方式产生发光。实验获得了ZnS:Cu复合涂层的基本发光机制,对在电场分布定性表征与绝缘缺陷自诊断中的应用具有指导意义。     

真空冲击电压下氧化铝陶瓷沿面闪络现象的发展过程

真空沿面闪络的发生与发展的过程是电气与电子领域众多学者长期以来所关注的问题,这一复杂的放电现象至今还不能完全从机理上给予说明.本文采用电气、光学联合检测的方法,利用增强型电荷耦合器件(ICCD)对真空冲击电压下氧化铝陶瓷沿面闪络现象的动态发展过程进行了研究,考察了在试样表面溅射金属薄膜电极对闪络发光现象的影响.从闪络的电压、电流和发光波形看出,闪络发生时的电流和发光峰值出现时间一致,并滞后于电压截断点.从ICCD图像可以发现,在试样表面发生完全闪络之前其表面已经出现预闪络发光通道,而沿面闪络发光的最强阶段出现在闪络发生之后的几十纳秒,此时电流出现最大值,能够发现在电极的边缘位置发光最为强烈;溅射电极之后试样的沿面闪络电压有所降低,闪络发生时的发光强度提高,在闪络发生时电极之间的材料内部可能出现体内的复合过程.     

低温温度梯度下环氧玻璃纤维材料表面电荷积聚行为及其对沿面闪络特性的影响

超导能源管道终端由于其特殊的运行环境,终端用绝缘材料在承受高电场的同时,也承受着近百摄氏度的低温温度梯度。为此基于超导电缆终端常用绝缘材料——环氧玻璃纤维材料,研究了低温温度梯度下绝缘材料表面电荷积聚特性及其对沿面闪络特性的影响机制。首先测量了环氧玻璃纤维材料在不同温度以及不同温度梯度下的沿面闪络特性;随后利用仿真软件建立了绝缘材料气-固界面的电荷迁移模型,分析了不同温度以及不同温度梯度对材料表面电荷积聚特性和表面电场分布特性的影响;最后结合试验结果与仿真结果,提出了低温温度梯度对绝缘材料沿面闪络的影响机制。试验结果表明：当温差ΔT=100K时,绝缘材料表面的局部放电起始电压与闪络强度分别为无温度梯度时的74.0%和75.9%。而仿真结果显示,低温温度梯度下绝缘材料表面电荷积聚现象明显,表面最高电场强度可达到不存在温度梯度时的10倍左右。因此,温度梯度下材料表面电荷积聚以及电场的畸变被认为是造成绝缘材料沿面绝缘强度下降的重要原因。该研究有助于理解低温温度梯度下材料表面电荷积聚特性及其对电场分布和闪络电压的影响机制,对低温下绝缘材料的绝缘特性研究以及绝缘优化设计具有重要意义。     

Optical studies of surface discharge under dc voltage in vacuum

Utilizing a high-sensitivity photomultiplier tube (PMT) and an intensified charge coupled device (ICCD) camera, the light emission from the surface of polytetrafluoroethylene (PTFE) and alumina ceramic samples was investigated in vacuum under a stepped dc voltage in order to study the initiation and development of surface discharge. Different from the classical electrode contact by butting electrodes on a sample directly, to achieve a fine contact between electrodes and the surface of a sample, the metalized electrodes were employed by sputtering two additional gold contacts on each sample in advance. The two types of materials showed quite different optical characteristics. It was suggested that there were several processes possibly related to the initiation mechanism of the surface discharge. Besides the field electron emission from the cathode triple junction (CTJ) for the classical electrode contact, there were charge injection and transport in the surface states of dielectrics, and charge trapping and detrapping phenomena. Electroluminescence (EL) emission was emitted due to the radiative recombination of electrons and holes injected. These processes were strongly dependent on the electrode contact manner and the surface states of insulating materials.     

Influence of frequency, electrode material and superimposed dc onac electroluminescence in polymer films

Summarizes new results on the influence of some experimental parameters on ac electroluminescence (EL) of polymers used as insulating materials in electrical engineering. Electroluminescence under a uniform ac field is investigated in low density polyethylene (PE) and polyethylene naphthalate (PEN) films with semi-transparent electrodes on both sides. Particular emphasis is given to polyethylene. The dependence of EL on the voltage frequency is investigated in the range of 10^-2 Hz to 10⁺² Hz with the help of phase-resolved measurements. The influence of the electrode/polymer contact is investigated by changing the electrode material (gold, silver, and calcium). The effect of a superimposed dc bias on the ac voltage is also reported     

Time-resolved space charge and electroluminescence measurements in polyethylene under ac stress

Time-resolved space charge (SC) and electroluminescence (EL) measurements are carried out on polyethylene films stressed under ac voltage at industrial frequency to probe injection, trapping and recombination of charge. SC is measured by pulsed electro-acoustic (PEA) technique at each zero voltage cross-over point of the ac voltage, thereby getting rid of the capacitive charge. EL is detected using photon counting techniques with a time resolution down to 50 /spl mu/s. Both SC and EL exhibit a threshold response as a function of the applied voltage. Numerical values of these thresholds are in good correspondence for SC and EL measurements in every tested material. Observations are consistent with a model of bipolar injection with an unbalanced situation in positive and negative charge behavior. It is also shown that different polyethylene materials behave differently relative to the field above which the charge is detected, providing thereby a way to compare the space charge behavior of those materials under ac field.     

Relation between electroluminescence and degradation in XLPE

In order to clarify the relationship between electroluminescence (EL) and degradation in crosslinked polyethylene (XLPE), the EL measurement was carried out using three kinds of samples, block samples with two different electrode systems, and film samples. The spectral measurements revealed that the UV component of EL is below the detectable level of ~0.1 c/s (count per second), in contrast to partial discharge (PD) light from microvoids or the tiny electrical tree which includes a UV component of ≳2 c/s. Secondly, aging tests using XLPE block samples with needle-plane electrodes revealed that electrical degradation can take place even when the applied voltage is below the EL detection voltage. The above experimental results suggest that EL is not the origin of electrical degradation, i.e. photodegradation by the UV component of EL is not the dominant mechanism of electrical degradation     

Electroluminescence from surface layer of insulating polymer underac voltage application

Electroluminescence from a surface layer of polytetrafluoroethylene (PTFE) has been investigated using a photomultiplier tube and an intensified charge-coupled device (ICCD) camera under ac or half rectified ac (HRAC) electric fields tangential to the polymer surface. The electroluminescence (EL) characteristics from the polymer surface layer showed a similarity to those from PTFE and other polymer films subjected to ac or HRAC electric fields in the normal direction to the surface. ICCD imaging of EL indicated a large area of EL activity where the EL intensity was maximum at the electrode edge and EL was still observable 2 mm from the electrode. The injection and transport of electrons and holes via surface states in the surface layer and the subsequent electron-hole recombination give rise to EL, strongly depending on the spatial distribution of the surface states. The injection length of charges can be estimated to be at least 2 mm from the electrode in the tangential direction to the surface     

聚合物绝缘材料电致发光现象研究的进展

最近几十年来,聚合物的电致发光(EL)现象研究引起了国际研究者的广泛重视,得到了许多宝贵的实验数据和结果。为介绍国内外EL现象的研究现状和进展,给国内欲从事此项研究的人员提供较全面的认识,介绍了研究EL的实验装置和方法;在探讨聚合物EL的物理原理的基础上,分析了不同电压波形下的EL模型,并讨论了EL、空间电荷和聚合物老化之间的关系。研究结果表明EL现象与绝缘材料的老化有着密切的联系,但EL是否是材料老化的起因尚存在着争论。今后需进行更多的研究工作来探讨其关系,并希望提出新的更灵敏、更超前的聚合物绝缘状况的表征参数和检测方法。     

Phase resolved electroluminescence measurements in thin films of low density polyethylene using a charge coupled device camera

Electroluminescence (EL) produced by a commercially available additive free low density polyethylene film has been investigated under a 50 Hz AC electrical stress. The spatial distribution, spectral characteristics and the phase angle relationship of EL with respect to the 50 Hz applied AC voltage were studied using a sensitive Peltier cooled charge-couple device (CCD) camera. The experimental results from several test samples show that the EL can be divided into two different groups according to the EL spatial distribution: (1) homogeneous EL and (2) inhomogeneous EL. At the same applied voltage, the spectra of the homogenous and inhomogeneous EL were found to be significantly different. Homogeneous EL had maximum emission intensity in the red and near infrared (>700 nm), whereas the inhomogeneous emission had maximum intensity at shorter wavelengths (550-600 nm). Phase resolved measurements showed that homogeneous EL always produced equal emission intensity in the positive and negative half-cycles (symmetric distribution), whereas, inhomogeneous EL produced an asymmetric intensity distribution. To explain these results, it is suggested that homogeneous emission is associated with charge injection and bipolar recombination in the surface states on both sides of the sample, whereas inhomogeneous emission is associated with field enhancement at defects on one of the two sides of the film which allow charge to be injected into states more representative of the bulk material.     

Surface flashover characteristics of diamond-like carbon thin filmsin vacuum

Synthetic thin films such as diamond-like carbon (DLC) and polycrystalline diamond thin films can be used to increase surface flashover voltage of dielectric materials used in HV devices operating in vacuum. This work presents experimental results identifying some surface flashover characteristics of diamond-like carbon (DLC) thin films in space vacuum conditions. The DLC samples used in the experiments are produced by a microwave plasma deposition technique. The electrode material was copper, and a dc voltage was applied between the electrodes. Surface flashover voltage characteristics and the breakdown voltage wave forms of DLC thin film samples were determined. The results showed that DLC thin films may not be suitable materials for HV applications as dielectric materials in vacuum when coated onto semiconductor materials. Additionally, these results were compared with the surface flashover characteristics of polycrystalline diamond thin film and quartz samples     

Electroluminescence excitation mechanisms in an epoxy resin underdivergent and uniform field

Electroluminescence (EL) excitation mechanisms have been investigated in epoxy resin under divergent and uniform field situations. Metallic wires embedded in the resin were used to produce field divergence whereas film samples were metallised to obtain a uniform field. EL under divergent field was stimulated by a pulse voltage. Light was emitted on the positive and negative fronts of the pulses when the field exceeded 20 kV/mm at the wire surface, with equal intensity and without polarity dependence. There was evidence of space charge accumulation around the wires in multiple-pulse experiments. Charge injection and extraction occurring at both fronts of the pulse provide the condition for EL excitation. Further excitation of the EL during the plateau of the voltage pulse is prevented by the opposite field of the trapped charge. Field computation with and without space charge supports the proposed interpretation. A dc voltage was used for the uniform field experiments. A continuous level of EL is found at 175 kV/mm. Charging/discharging current measurements and space charge profile analyses using the pulsed electroacoustic (PEA) technique were performed at different fields up to the EL level. Dipolar orientation generates a long lasting transient current that prevents the conduction level being reached within the experimental protocol (one hour poling time). The continuous EL emission is nevertheless associated with a regime where the conduction becomes dominant over the orientational polarization. Polarization and space charge contribute to the PEA charge profiles. Homocharge injection at anode and cathode is seen at 20 kV/mm and a penetration of positive space charge in the bulk is detected above 100 kV/mm, suggesting an excitation of the continuous EL by bipolar charge recombination