【作者】 田付强；

【导师】 雷清泉； 王毅；

【作者基本信息】 北京交通大学 ， 电力系统及其自动化， 2012， 博士

【摘要】 纳米复合改性是提高电力设备绝缘材料电性能的有效途径,聚合物基无机纳米复合电介质的研究具有十分重要的工程意义和理论价值。陷阱对聚合物电导、老化和击穿等电性能有显著影响,对聚合物及其纳米复合物陷阱特性及陷阱对电性能影响的研究,有助于揭示聚合物基纳米复合电介质电性能的机理。本论文以低密度聚乙烯(LDPE)为聚合物基体材料,重点研究了ZnO、ZnxMg1-xO(x=0.5)、 ZnO+MMT(montmorillonite)掺杂及纳米颗粒表面偶联剂处理对LDPE陷阱特性和电性能的影响,并以陷阱特性作为切入点,研究了纳米电介质电性能的微观和介观机理。论文主要工作如下：(1)采用熔融共混法制备了不同掺杂量的LDPE/ZnO、LDPE/ZnxMg1-xO、 LDPE/ZnO+MMT纳米复合电介质,并采用扫描电镜、红外光谱、X射线衍射、紫外-可见吸收光谱等技术手段对其进行了结构表征,研究了不同种类纳米颗粒在LDPE基体中的分散性和其与基体的相容性、纳米颗粒与基体的界面物理化学作用、纳米掺杂引起的LDPE结晶度及微晶尺寸变化和纳米复合物的紫外-可见光吸收特性,并研究了纳米颗粒偶联剂处理对上述特性的影响。研究表明,复合物中ZnO和ZnxMg1-xO纳米颗粒的粒径基本上都在50-200nm之间；偶联剂与纳米颗粒之间产生了化学键合作用,并且降低了纳米颗粒团聚物的尺度；表面处理的ZnO纳米颗粒与MMT混合掺杂增大了MMT的层间距,促进了二者在LDPE中的分散,ZnO与MMT片层形成了一定的交织结构；ZnO和ZnxMg1-xO掺杂提高了LDPE的结晶度；LDPE/ZnO和LDPE/ZnxMg1-xO纳米复合物对250-400nm之间的紫外光有很强的吸收作用。(2)针对传统的热刺激电流方法在定量研究聚合物陷阱特性方面存在的缺陷,提出了适用于研究能级连续分布的聚合物陷阱特性的改进的等温放电电流和热刺激电流分析方法,通过实验分析证实了该方法的有效性,并以此得到了LDPE及其纳米复合物的陷阱特性。研究得出,纳米掺杂能够使LDPE的陷阱能级密度增加2-6倍,陷阱深度基本不变；陷阱均来自于界面区高分子链的空间构象缺陷,其物理本质是空腔；陷阱能级密度的变化与纳米颗粒本身的性质和形状、掺杂量、偶联剂处理有密切关系。(3)研究了纳米掺杂引起LDPE陷阱能级密度变化的微观机制,提出并证实了纳米掺杂改性的界面陷阱机理理论模型。研究得出,纳米复合物中陷阱能级密度的增加源于两个方面：一是纳米颗粒促进了聚乙烯的异相成核过程,提高了成核速率而降低了球晶生长速率,从而减小了球晶尺寸,生成了大量均匀的小球晶,在晶区与非晶区之间产生了更多的界面,该界面能够形成大量空腔陷阱；另一方面纳米颗粒与聚合物之间的界面区由于分子可动性、形态结构、自由体积等不同于聚合物基体而存在大量空腔陷阱。这两种界面产生的陷阱效应是导致纳米复合物中陷阱能级密度增加的主要原因。(4)研究了不同纳米掺杂对LDPE电性能特别是电荷输运特性的影响,并基于界面陷阱理论给出了纳米复合物电导降低、击穿场强和耐电树枝老化寿命增加的物理机制。研究表明,通过ZnO、ZnxMg1-xO和ZnO+MMT掺杂,大幅度提高了LDPE的电性能。其中,纳米复合物的载流子迁移率是LDPE的1/4～1/20,体积电阻率是LDPE的2-20倍,高场电导电流最小是LDPE的1/8～1/2；积累的同极性和异极性空间电荷密度和电荷量最小是LDPE的1/8～1/3,偶联剂处理有助于空间电荷的抑制；耐电树枝老化寿命最大是LDPE的30倍,ZnO纳米颗粒与MMT片层混合掺杂的纳米复合物的耐电树枝老化能力最好,其次分别是ZnxMg1-xO和ZnO掺杂；纳米复合物的击穿场强最大比LDPE提高了10%～20%。研究表明纳米掺杂的界面陷阱效应及其本身特性是引起纳米复合物电性能改善的关键因素。(5)为了深入理解纳米复合物中的空间电荷行为及其机理,通过对电极电荷注入和输运过程中界面陷阱调制作用的实验和理论分析,提出了LDPE/ZnO纳米复合电介质中空间电荷抑制的陷阱机理理论模型。研究得出,纳米颗粒掺杂的界面陷阱效应有效降低了注入电荷积累的深度,促进了注入电荷在电极界面附近的入陷,降低了界面电场,弱化了界面附近的杂质电离,从而抑制了电极界面附近异极性空间电荷的形成；另一方面,界面电场的降低导致电极注入电荷的减少,体电导能够及时将注入的少量电荷导出,因而抑制了材料体内同极性空间电荷的积累。这对高压和超高压直流电缆绝缘材料的开发具有指导意义。更多还原

【Abstract】 Nanocomposite is an effective way to enhance the electrical properties of insulation materials used in the electrical equipments. The investigation of polymer based inorganic nanocomposite is of great practical significance and theoretical value. Traps have prominent effects on the electrical conduction, ageing and breakdown of polymers. The investigations on the trap characteristics of the polymer and its based nanocomposites and on the role of traps in determining their electrical properties are helpful to reveal the mechanism of electrical properties of polymer based nanocomposites. In this thesis, the low density polyethylene (LDPE) is chosen as the matrix. The main effort is devoted to the investigations on the effects of ZnO, ZnxMg1-xO(x=0.5) and ZnO+MMT (montmorillonite) nanoparticle fillers and their surface treatment on the trap characteristics and electrical properties of LDPE. By taking the trap characteristics as the entry point, the mechanisms corresponding to the electrical properties of nanodielectrics are studied. The main work is shown as follows:(1) LDPE/ZnO, LDPE/ZnxMg1-xO and LDPE/ZnO+MMT nanocomposites are prepared by melt bending method and the physical and chemical structures have been investigated by scanning electron microscope (SEM), infrared spectroscopy (IR), X-ray diffraction (XRD) and ultraviolet-visible absorption spectroscopy (UV-vis). The dispersion of nanoparticles in the LDPE matrix, the compatibility of nanoparticles with the matrix, the physical and chemical action between the nanoparticles and the matrix at the interface, the LDPE crystallinity and the crystallize, the UV-vis characteristics of the nanocomposites and the coupling agent treatment on these properties are investigated. Results show that the diameter of the nanoparticles in the nanocomposites is in the range50-200nm. Coupling agents are chemically bonded to the nanoparticles and have decreased the aggregate size of the nanoparticles. The mixed fillers of ZnO and MMT have increased the layer space of MMT and promoted their dispersion. ZnO and MMT have formed pilotaxitic structures. The ZnO and ZnxMg1-xO fillers have increased the crystallinity of LDPE. The LDPE/ZnO and LDPE/ZnxMg1-xO nanocomposites exhibit strong absorption for the ultraviolet light in the range250-400nm. (2) Considering the drawback of the traditional thermally stimulated current (TSC) method in the quantitative investigation of trap characteristics of polymers, the modified analysis methods of isothermal discharge current (MIDC) and thermally stimulated current suitable for the study of the trap characteristics in polymers with continuous energy levels are proposed. The effectiveness of the methods has been verified by experiments and analysis. The trap characteristics of the nanocomposites are obtained by the proposed methods. Research indicates that the trap level density in LDPE is increased about2-6times by nanofillers and the trap level depths are basically unchanged. The interface traps are cavities essentially and come from the space configuration defects of polymer chains. The change of the trap level density is closely related to the property and shapes of the nanoparticles, nanofiller loadings and treatment by coupling agent.(3) The micro mechanism of the trap level density changes in LDPE by nanofillers are investigate and the theoretical mechanism model of trapping effect at the interface for the property improvement of the nanocomposite is proposed and confirmed. The trap density increase in the nanocomposite is derived from two aspects. Firstly, the nanofillers have promoted the heterogeneous nucleation process of LDPE, leading to the formation of uniformly distributed small spherulites and increase of the interface areas between the crystallites and amorphous domains. These interfaces can generate large amounts of cavity traps. Secondly, the interface areas between the nanoparticles and the LDPE matrix are different in molecular mobility, morphology structures and free volume, in which cavity traps can also be generated. The two kinds of trapping effects at the interface are the main cause for the trap level density increase in the nanocomposite.(4) The effect of different nanofillers on the electrical properties, especially charge transportation characteristics of LDPE is investigated and the physical mechanisms of the decrease in electrical conduction and increase in electrical breakdown strength and electrical treeing life in the nanocomposites are given based on the trapping mechanism at the interface. Results show that the carrier mobility of the nanocomposites is about1/4～1/20times that of LDPE and the volume resistivity is about2to20times that of LDPE. The high field conductivity of the nanocomposites is about1/8to1/2that of LDPE at least. The accumulated heterocharge and homocharge are about1/8to1/3that of LDPE at least. Coupling agent treatment is helpful to space charge inhibition. The electrical treeing-resistant life is30times that of LDPE at most. The nanocomposites filled with ZnO and MMT mixtures have the best treeing resistant ability and then are those filled with ZnxMg1-xO and ZnO fillers. The electrical breakdown strength is enhanced by10%to20%than LDPE at most. It has been concluded that the trapping effect at the interface by nanofillers and the properties of nanofillers are the key factors leading to the electrical property improvement of nanocomposites.(5) In order to go deep into the space charge behavior and its mechanism, the theoretical model of trapping mechanism for space charge inhibition in LDPE/ZnO nanocomposite is proposed via the experimental and theoretical analysis of the modulating action of interface trap on the charge injection process from electrodes and transportation process. Research concludes that the trapping effect at the interface caused by nanofillers has effectively decreased the accumulation depth of the injected charge and is helpful to promote trapping of injected charge at the interface around the electrodes, leading to the reduction of interface electric field. This can weaken the impurity ionization and inhibit the hetero space charge accumulation around the electrodes. On the other hand, the reduction of interface electric field decreases the charge injection from electrodes. The small amount of injected charge can be easily conducted out of the bulk. Thus it can suppress the homo space charge accumulation in the material. This is meaningful for the development of electrical insulation materials for high voltage and extra high voltage cables.更多还原