【作者】 宋红涛；

【导师】 党智敏；

【作者基本信息】 北京化工大学 ， 材料科学与工程， 2011， 博士

【摘要】 高压直流电缆线路因其输送容量大、造价低、损耗小、可靠性高、寿命长等特点而应用越来越广泛。随着电气工程和电子技术的快速发展,柔性导电聚合物具有广阔的应用前景。在一定的应用方面,高介电常数和高电导率的多功能聚合物基复合材料在越来越受到广泛的关注。其中,聚乙烯(PE)是高压直流输配电领域最好的绝缘材料之一。作为高压直流电缆的屏蔽层,往往使用高介电常数和高电导率的聚乙烯基复合材料来抑制空间电荷的形成。在会形成大量空间电荷的绝缘层,高电导率可以消散电缆屏蔽层积聚的空间电荷。另外,诱发的反电场能够使得电缆内导体表面的电场降低。因此,高介电常数和高电导率的内屏蔽层是保证高压直流电缆安全运行的关键因素之一。本论文基于电缆用聚乙烯材料的绝缘特性,提出利用纳米技术及合适的工艺制备LDPE/A12O3、LDPE/AlN及LDPE/MWCNT复合材料,就其综合介电性能开展研究。通过对不同电场下材料中空间电荷及其对电介质材料中电场分布影响规律的研究,掌握影响复合材料的空间电荷产生和分布的主要因素,通过对纳米复合材料的介电性能和微观结构综合分析,研究复合材料绝缘结构和半导体屏蔽层结构,主要工作如下：通过熔融共混法制备不同填料含量LDPE/Nano-Al2O3、LDPE/Nano-AIN复合材料,然后采用不同冷却工艺将其热压成型。分别研究了不同纳米Al203和纳米A1N含量对复合材料结晶、介电性能、击穿场强和空间电荷特性的影响,同时通过扫描电镜(SEM)分析A1203和A1N在LDPE基体中的分散状态以及用差示扫描量热法(DSC), X射线衍射(XRD)研究复合材料的热稳定性和结晶性能。结果表明,冷却工艺影响复合材料的热稳定性,同时基体中均匀分布的纳米A1203和A1N的引入对复合材料的结晶性和电性能产生了较大的影响。随着A1203和A1N含量的增加,复合材料的介电性能提高,击穿场强有所下降,空间电荷的集聚变化明显。利用电声脉冲法(Pulsed Electroacoustic Method, PEA)测量技术对不同填料含量的LDPE/Nano-Al2O3、LDPE/Nano-AIN复合材料空间电荷特性进行对比分析,主要结论如下：纯LDPE薄膜中出现了双偶极注入现象,注入电荷量随着外加电压的升高而增加,纳米A12O3、AlN掺杂的LDPE/ A12O3、LDPE/AlN复合材料的空间电荷特性与纯LDPE薄膜明显不同。异号电荷的出现表明离子化的产生；纳米粒子的引入可能阻碍了电荷的注入,注入电荷的相互作用、离子化和纳米对电荷注入的影响在形成电荷包的时候,这三者之间存在一个相互竞争的过程。含有0.5%Al2O3共混物抑制空间电荷效果最好,电荷的迁移率最高。纳米A1N粒子的加入对聚乙烯材料的空间电荷没有抑制作用。通过熔融共混法制备不同填料含量LDPE/WMCNT复合材料,分别研究了在不同温度、频率下多壁碳纳米管WMCNT含量对复合材料介电性能、电导率和的影响,同时通过扫描电镜(SEM)分析WMCNT在LDPE基体中的分散状态以及用差示扫描量热法(DSC)研究WMCNT含量对复合材料的结晶性能的影响。结果表明,LDPE/MWCNT复合材料的介电常数与电导率随着碳纳米管含量的增加而增大,在碳纳米管体积分数接近0.081时,复合材料的介电常数与电导率得到大幅度提高。这些结果可以很好的通过渗阈理论进行解释。渗流阈值相对较高是可能是因为碳纳米管在熔融共混的过程中由于很强的机械剪切力很容易被破坏。碳纳米管体积分数为0.08的LDPE/MWCNT复合材料具有高的介电常数和电导率,可以满足于高压直流电缆屏蔽层的应用需要。更多还原

【Abstract】 High voltage direct current (HVDC) cables are attracting more and more interesting for its low loss, long life, high reliability and so on. With the rapid development of electrical engineering and electronic technology, a demand for flexible conducting polymer composites is attracting a broad attention. In some applications, functional polymer-based composites with high dielectric permittivity and high electrical conductivity have a potential ability to meet some special needs. For instance, polyethylene (PE) is one of the excellent insulators in the field of high voltage direct current (HVDC) transmission.A functional shield layer often consisting of PE-based composite with high dielectric permittivity and electrical conductivity is important in the HVDC cable field because this type of shield layer can prevent the formation of space charges. Because of the space charges produced in the shield layer, they may also be released due to the high conductivity of the shield layer. In addition, the induced opposite electrical field can also force the high electric field at irregular surface of conductors in the cables to decrease. Therefore, the suitable inner shield layer materials with high dielectric permittivities and electrical conductivities are key factors to sustain the safe running of HVDC cables.The present paper takes LDPE as an excellent insulation. A use nanotechnology and the appropriate method of preparation were proposed to synthesize LDPE/A12O3, LDPE/A1N and LDPE/MWCNT nanocomposites. The dielectric properties and the performance enhancement were studied under the special electric field. The influence of space charge formation and distribution on the electric field and the migration and accumulation in the dielectrics were also studied. The insulating structures and the novel shield layer were developed by controlling the dielectric properties and microstructures of materials. The main conclusions are as follows.LDPE/A12O3 and LDPE/A1N nanocomposites were prepared by using melting method and subsequently molded via hot pressing methods with different cooling processes. The influences of nano-Al2O3 and nano-AIN concentration on crystallization, the dielectric properties, breakdown strength and space charge distribution of the composites were explored. The LDPE/nano-Al2O3 composites were also studied by Scanning electron microscopy (SEM), Differential Scanning Calorimeter (DSC) and X-ray diffraction (XRD). Results show that the cooling process influences thermal stability of the composites and the well-dispersed Nano-materials in the matrix have taken great effect on the crystalline and electrical properties of the composites. With the increase of Al2O3 and A1N fillers, the dielectric properties of the composites are improved, breakdown strength decreased and the accumulation of space charge changed significantly.There is increasing evidence that degradation of polymer insulation under high electric stress is associated with space charge formation. In fact, space charge accumulation in DC regime is the main reason that still limits the use of polymers as insulation for high voltage DC power cables. The space charge dynamics in LDPE/A12O3 and LDPE/A1N nanocomposites have been investigated by using the Pulsed Electroacoustic Method (PEA) technique. Bipolar charge injection has taken place in pure LDPE film. The amount of space charge injected increases with the duration of applied voltage. The space charge characteristics in nanocomposites containing the nano-Al2O3 or nano-AIN particles are very different from the pure LDPE film. Heterocharges dominate the distribution indicating ionization occurrence. The addition of the nano-Al2O3 or nano-A1N particles into LDPE matrix may also hinder charge injection process. Test results showed that that the 0.5 wt% nano-Al2O3 added to LDPE greatly decreases space charge accumulation and increases space charge mobility. While, nano-AIN blends can’t improve space chare accumulation and mobility.LDPE/MWCNT nanocomposites were prepared by using melting method and subsequently molded via hot pressing methods with different cooling processes. The influences of MWCNT concentration on crystallization, the dielectric properties, and conductivity of the composites were explored. The LDPE/MWCNT composites were also studied by Scanning electron microscopy (SEM), Differential Scanning Calorimeter (DSC). Enhanced dielectric-permittivity and conductivity are observed in the LDPE/MWCNT nanocomposites. Conductivity and dielectric permittivity of the LDPE/MWCNT nanocomposites first increase gently with the increase of MWCNT concentration, and they increase significantly as the MWCNT volume fraction was near 0.081. The results are explained well by employing the percolation theory. A relative high percolation threshold (fc=0.081) is discovered because the MWCNT is broken easily during the melting mixture process due to the strong mechanical agitation. The LDPE/MWCNT nanocomposite at fMWCNT=0.08 displays the high dielectric permittivity and the high conductivity, which would satisfy the need as a potential shield layer materials in the HVDC field.更多还原