【作者】 黄兴溢；

【导师】 江平开；

【作者基本信息】 上海交通大学 ， 材料学， 2008， 博士

【摘要】 目前,我国以及全球电力的输送正朝高压、超高压方向发展。经济以及城市化的发展对电缆的需求量逐年增加。作为电力输送系统的一部分,电缆终端处的电场非常集中,击穿容易发生。电缆终端击穿失效是导致供电事故的主要因素之一。除了电缆终端以外,大功率发电机线棒、户外绝缘套管等的应力控制也是电力设备中非常值得关注的关键问题。具有高介电常数的材料可将电缆终端半导电屏蔽剥离处的高电场强度降至安全范围内,将高电位移向电缆末端,而不是集中在电缆屏蔽切断处附近,从而使电缆终端外绝缘电场分布趋于发散、均匀,确保电路输送安全。聚乙烯具有高的介电强度,非常低的电导,是用于生产电线电缆的优质绝缘材料。因聚乙烯在高频下具有非常低的介电损耗并具有优异的力学性能,更是最合适的高频电缆用绝缘材料。然而,聚乙烯的介电常数很低,在电缆终端处的应用受到了限制。为了拓展聚乙烯在电缆终端处的应用,必须增加它的介电常数。添加金属颗粒、导电纤维以及碳纳米管等是增加聚合物介电常数的重要手段。聚合物/导体复合材料是典型的逾渗体系,复合材料的介电常数反比于填充物的实际填充分数与临界填充分数(逾渗阈值)之差。因此,要得到高的介电常数就必须使得填充物的分数接近临界值而又不能高于临界值。如果填充分数合适,可以得到非常高的介电常数。然而,具有逾渗行为的复合材料的介电性能对材料的组成非常敏感,组成的轻微变化就会引起材料性能的很大变化,比如复合材料会在很小的浓度范围内发生由绝缘体到导体的转变,使材料失去作为电介质的价值,这给材料的生产以及材料性能指标的重现性带来了很大的挑战。在本文中,选择了一种自钝化的纳米铝为原材料,制备了具有较高介电常数的聚乙烯纳米铝复合材料。铝是一种自钝化金属,纳米颗粒经钝化后表面形成一层几纳米厚的氧化膜,这层氧化膜一方面可以阻止金属核的进一步氧化,还可以阻止颗粒之间形成类似金属的导电通路,保证材料在较高的填充体积下保持绝缘性能;纳米铝的其它优点是具有较低的密度,利于复合材料的加工,并且铝对于聚乙烯没有降解作用。本工作主要研究了以下几个基本问题:(1)聚乙烯纳米铝复合材料的介电性能与纳米铝含量以及施加交流电场频率的关系;(2)聚乙烯纳米铝复合材料的介电增强机理;(3)纳米铝的表面化学特征对复合材料微结构以及电学性质的影响;(4)聚乙烯纳米铝复合材料的微结构、电学性质以及流变性质之间的关联;(5)影响聚乙烯纳米铝复合材料的介电强度的因素以及提高介电强度的方法。(1)聚乙烯纳米铝复合材料的介电性能与纳米铝含量以及施加交流电场频率的关系。聚乙烯纳米铝复合材料的介电性能(介电常数与介电损耗)不仅与纳米颗粒的含量有关,还与测试的频率有关。大的介电行为的差别主要出现在低频区。在纳米颗粒含量较低的情况下,复合材料基本上没有表现出介电弥散现象,这是因为纳米颗粒的尺寸很小,少量纳米颗粒与聚合物基体间形成的界面可以被忽略。随着纳米颗粒含量增加,纳米颗粒与聚合物基体之间的有效界面面积增加,纳米颗粒之间的介质厚度变薄,造成了明显的界面空间电荷极化。当纳米颗粒的浓度超过24wt%时,低频区的介电参数(介电常数与介电损耗)随着纳米颗粒的增加而减少,这个现象可能是由三方面的原因造成的。一是溶剂残留或纳米颗粒的不完善堆积造成了微孔洞的存在;二是纳米颗粒在高浓度时出现了严重的团聚造成纳米颗粒与聚合物之间有效界面面积的减小;三是纳米颗粒的团聚造成等效氧化壳层厚度的增加。根据对复合材料介电参数的研究,发现聚乙烯纳米铝复合材料并没有表现出常规的聚合物金属复合材料所普遍拥有的介电逾渗现象。(2)聚乙烯纳米铝复合材料的介电增强机理、模型。将复合材料看作是由聚乙烯基体、大量金属核以及自钝化氧化层组成的等效电路,解释了聚乙烯纳米铝复合材料的介电常数与频率以及颗粒含量的关系。(3)纳米铝的表面化学对复合材料微结构以及电学性质的影响。纳米铝颗粒经过辛基硅烷表面处理以后,纳米颗粒在聚乙烯基体中的分散的到了显著改善,复合材料的介电性能得到了提高。根据逾渗理论,表面改性的和未改性的纳米颗粒复合材料的逾渗阈值均低于理论值,临界指数均高于根据理论计算的普适值。但纳米颗粒经过表面处理以后,复合材料具有较高的逾渗阈值与临界指数。聚乙烯纳米铝复合材料具有低于理论值的逾渗阈值是由于纳米颗粒的小尺寸以及彼此容易团聚的特性造成的。复合材料具有高与普适值的临界指数是因为纳米颗粒之间的接触电阻存在一定的分布造成的。逾渗阈值以及临界指数的差别均与纳米颗粒的表面处理以及表面处理引起的颗粒在聚合物基体中的分散改善有关。研究结果说明,纳米颗粒的表面处理使人为的控制纳米复合材料的介电性能变为可能。(4)聚乙烯纳米铝复合材料的微结构、电学性质以及流变性质之间的关联。研究显示,纳米复合材料的微结构、导电以及流变性质之间存在着很强的关联。纳米铝颗粒表面处理改性以后,复合材料具有高的流变逾渗阈值。纳米复合材料的电逾渗阈值要低于形成导电通路所需要的纳米颗粒的浓度,这是因为形成流变逾渗所要求的纳米颗粒团簇之间的距离要大于形成导电通路所要求的纳米颗粒之间的距离。(5)影响聚乙烯纳米铝复合材料的介电强度的因素及提高介电强度的方法。聚合物金属复合材料的介电强度是由金属颗粒的分布及分散情况决定的,与颗粒本身的尺寸没有必然的关系。颗粒的浓度超过14vol%,只有表面处理的纳米颗粒聚乙烯复合材料还保持一定的介电强度,微米复合材料、采用相容剂的纳米复合材料以及表面未处理的纳米颗粒复合材料都失去了介电强度。这一结果说明,通过表面化学处理,改善金属颗粒的分散以及提高颗粒与聚合物基体的相容性是提高聚合物金属复合材料的介电强度的有效手段。本论文的主要创新之处:1.通常的聚合物金属复合材料在高金属含量的情况下是导电材料。本文选择了一种自钝化的纳米金属铝颗粒作为填充剂,制备了具有较高介电常数的聚合物纳米复合材料,这种填充剂使得复合材料在高填充的情况下仍保持电介质材料的特性,拓展了聚合物金属复合材料的应用。2.在较高纳米铝含量的情况下,制备的聚乙烯复合材料仍保持较好的力学性能和介电强度。更多还原

【Abstract】 The rapid development of economy around the world, especially in our country brings out the rapid increase of the demand of electric power, which leads the development of electric transmission and distribution systems to be directed forward the trends of high voltage (HV) or extra high voltage (EHV). And also, the quantitive and qualitive demands for power cables become larger and larger with the rapid urbanization process and with the operation condition growing severer and severer. Cable terminations are considered to occupy an important part of the electric transmission and distribution systems and also to be one of the weakest parts of electric transmission and distribution cable networks. At cable terminations, especially for medium and high voltage cables, the electric field has both radial and tangential components because of their complex configurations, which may be intensive enough to cause the surface discharge or flashover and finally cause the failure of them. Besides cable terminations, the stress control in bushings and the coils of motors and generators have attracted much attention of the investigators. The use of high-dielectric-constant materials has been strongly recommended in order to equalize the inhomogeneous distribution of electric field in the electric power apparatuses.Polyethylene is known to be one of the most widely used polymeric insulating materials. The high dielectric strength, very low electrical conductivity, low dielectric loss at high frequencies and remarkable mechanical properties allow it to be used as an outstanding insulation in wire and cable applications, particularly in high frequency cables. However, its low dielectric constant makes it impossible for PE to be used for electrical stress control in cable terminations. One effective method for expanding its application to cable terminations is to increase its dielectric constant.1111The common approach to increase the dielectric constant of a polymer, e.g. PE, is known to adopt metal particles, conductive fibers and carbon tubes. Polymer/conductor composites are classical percolation systems, in which the permittivity of a composite showing the percolation behavior is inversely proportional to the difference between the real filling volume fraction of fillers and the critical filling volume fraction (the threshold value of percolation). Hence, if high values of permittivity are needed for composite materials, the filling volume fraction of fillers should be similar to the critical value but not higher than it; if an appropriate value of the filling volume fraction is selected, then very high permittivity value of the composite material can be realized. However, the dielectric properties of the composites having the percolation characteristic are known to be quite sensitive to the constitution of the material; a little change in the constitution can produce significant changes in the performances of composites. A very low value of percolation threshold gives challenges to production of a composite and reproducibility of performance indices of the material.1111In our work, we have chosen aluminum (Al) nanoparticles as fillers. Al is a self-passivation metal, where the self-passivated oxide layer forms a thin insulating boundary around the surface of its metallic core, allowing the polyethylene/Al composites as a percolation system to have a high dielectric constant. In addition, Aluminum not only is low in density, but also has not any catalysis and degradation effects on polyethylene in contrast to copper, which leads polyethylene/Al nanocomposites to be a kind of the promising materials for application in cable terminations.In our work, we mainly focused on the following five scientific topics: (1) the filler concentration and frequency dependences of dielectric behaviors of PE/Al nanocomposites; (2) the mechanism/model for the higher dielectric constant of the PE/Al nanocomposites; (3) the influence of the chemistry characteristics of Al nanoparticle surface on the microstructure, electrical properties and rheological behaviors of PE/Al nanocomposites; (4) the correlation between rheological, electrical, and microstructure characteristics of PE/Al nanocomposites; (5) the factors influencing the dielectric strength of PE/Al nanocomposites and some methods used to improve the dielectric strength of the nanocompsites.(1) The filler concentration and frequency dependences of dielectric behaviors of PE/Al nanocomposites: The dielectric properties (dielectric permittivity and loss tangent) of PE/Al nanocomposites are dependent not only on the Al nanofiller concentration, but also on the measuring frequency. The significant differences of the dielectric behaviors are found in the extra-low frequency range of 0.1Hz to 10Hz. In the case of quite low loading levels, agglomerates can be hardly found and any dielectric dispersion can not be observed in the extra-low frequency range, which may be attributed to that the formation of the interfaces between Al nanoparticles and PE matrix can be still neglected because of the small number and extra-small diameter of the nanoparticles. The higher the nanofiller loading level, the larger the total effective area of the interfaces between the polymer matrix and the fillers, and the thinner the insulating spacers separating Al nanoparticles, which results in the significant increment of the interfacial space-charge polarization. The values of dielectric characteristics becomes lower again as the filler loading exceeds 24wt%, which may be ascribed to voiding from imperfect filler packing and solvent evaporation, the decrease of the effective area of the interface between polymer matrix and the nanofillers, the increase of the average thickness of the equivalent oxide shell of any Al nanofiller cluster, and the decrease of the volume fraction of the polymer matrix due to the increase of that of the nanofiller cluster. It is found that the dielectric permittivity of the PE/Al nanocomposites does not show any percolative characteristics which have been reported in classical polymer/conductor composites.(2) The mechanism/model for the higher dielectric constant of the PE/Al nanocomposites: PE/Al nanocomposites were modeled with a general equivalent electric circuit consisting of CPE, CAl, RAl, Co, Ro, Ci and Ri (CPE represents the average capacitance of PE domain between two electrodes; CAl, RAl are the equivalent average capacitances and ac resistances between the Al core and the internal interface of the oxide layer, respectively; Co, Ro are the equivalent average capacitance and ac resistance of the oxide layer, respectively; Ci, Ri are the equivalent average capacitance and ac resistance between nanoparticles or between nanoparticles and the electrodes, respectively). It has been found that the equivalent electric circuit can be used to clearly explain the filler concentration and frequency dependences of dielectric behaviors of PE/Al nanocomposites(3) The influence of the chemistry characteristics of Al nanoparticle surface on the microstructure, electrical properties and rheological behaviors of PE/Al nanocomposites: The surface modification of the nanoparticle has been shown to significantly improve the particle dispersion in the PE matrix. The percolation theory has been used to investigate the effect of the surface modification of Al nanofillers on the electrical conductivity of the nanocomposites. The value of percolation threshold for the PE nanocomposites containing octyl-trimethoxysilane-coated Al nanoparticles is higher than for those loaded with un-treated nanoparticles, while both of these values are lower than the predicted ones. The dc critical exponent values are lower for PE nanocomposites filled with un-treated Al nanoparticles than with octyl-trimethoxysilane-coated ones, while the values for these two cases are much higher than the predicted ones. It can be understood that the low values of percolation threshold of the nanocomposites may be related to the small size of the Al nanoparticles and their agglomeration inside the composites, and the much higher critical exponents of the nanocomposites could be ascribed to the nature of the inter-particle contact. The differences of threshold and critical exponent between the nanocomposites can be closely related to the fact that the surface modification can improve the dispersion of the nanoparticles in the polymer matrix. Our results also indicate that the surface modification makes it possible to easily control the values of dielectric permittivity in the comparatively wide range and also to provide an excellent approach able to considerably reduce the dielectric loss of the nanocomposites.(4) The correlation between rheological, electrical, and microstructure characteristics in PE/Al nanocomposites: A strong correlation between the time and concentration dependences of dc conductivity and the rheological properties has been observed in the different nanocomposite systems. The rheological threshold of the composites is smaller than the percolation threshold of electrical conductivity for both of the nanocomposite systems. The difference in percolation threshold is understood in terms of the smaller particle-particle distance required for electrical conduction as compared to that required to impede polymer mobility.(5) The factors influencing the dielectric strength of PE/Al nanocomposites and some methods used to improve the dielectric strength of the nanocompsites: Combined with the observation results on the microstructure of the nanocomposites, the main factor to determine the dielectric strength of the PE/Al nanocomposites is the particle dispersion properties and the externally-introduced charge carriers may only be the secondary factor. It has been also found that only the Al nanofiller surface-treated with the silane coupling agent makes it possible for the PE/Al nanocomposites to still keep the relatively higher breakdown strength even in the higher Al loading levels above 14vol%, which may be ascribed to the existence of very thin barriers of polymeric material between metallic clusters. The dielectric strength results of the macro- and nanocomposites hint at that the surface modification of the metal nanoparticle is surely necessary for preparing the useful polymer/metal composites with high dielectric permittivity and low dielectric loss but without any significant reduction of their dielectric breakdown strength.The innovations of this dissertation are listed as follows:1. It is known that the polymer/metal composites with high metal loading levels show the typical conductive behaviors. In this paper, a self-passivated Al has been chosen as fillers, and PE/Al nanocomposites were prepared for the development of a high permittivity material. It has been found that the composites with relatively high Al concentration have high dielectric constant while they still keep good electrical properties even in so high filler loadings. On the basis of this finding, we have expanded the applications of polymer/metal composites in this work.2. It has been found that the nanocomposites at high filler concentrations still keep good mechanical properties and breakdown strength, which is very important for practical applications.更多还原