【作者】 刘海燕；

【导师】 魏作君；

【作者基本信息】 浙江大学 ， 工程（专业学位）， 2020， 硕士

【摘要】 当今世界经济社会高速发展,对电力网络也提出更高要求,远距离大容量输电已经成为一种趋势,也因此需要具有更高耐电性能的高压电缆。高压电缆的绝缘层对其耐压等级具有重要影响,目前应用最为广泛的绝缘材料主要为交联聚乙烯。交联聚乙烯具有良好的绝缘性能、较强的热稳定性及机械稳定性,但其热固性特点使其不可回收利用,这一缺点也促使人们增加对热塑性材料低密度聚乙烯(LDPE)和高密度聚乙烯(HDPE)的研究,以期代替热固性交联聚乙烯材料。对于绝缘材料使用过程常见的电树枝和空间电荷等问题,引入纳米粒子、聚合物共混、添加电压稳定剂和化学接枝等方法是目前较为普遍的手段,其中又以加入电压稳定剂最为普遍,包含离域结构芳香环和极性基团的分子是目前较为认可的电压稳定剂。在聚合物中加入电压稳定剂具有溶解度受限和易迁移的缺点,因此人们试图将电压稳定剂接枝到聚乙烯分子链上进行改善。本文首先通过改良Hummers法制备氧化石墨烯,并通过气相沉积法制备掺氮石墨烯、掺硼石墨烯及硼氮共掺杂石墨烯,进一步通过溶液共混法将0.2 wt%掺杂石墨烯加入聚乙烯混合材料中(90%LDPE+10%HDPE,下文将此称为LH),制备共混材料。对加入掺杂石墨烯的共混材料进行电性能测试:在电树枝测试中,三种掺杂石墨烯的加入不同程度地提高了材料起树电压,其中硼氮共掺杂石墨烯提高程度最大,为17%;掺氮石墨烯、掺硼石墨烯分别提高起树电压7.8%和10.7%。在空间电荷测试中,三种掺杂石墨烯在抑制聚乙烯材料内部空间电荷积累,改善材料内部空间电荷分布方面都有一定作用效果。在直流电导率测试中,三种掺杂石墨烯的加入都较大程度提高了材料的电导率,但相比较而言,硼氮共掺杂石墨烯对电导率的提高程度最小。筛选七种含有不同给电子基团和吸电子基团的芳香性分子作为电压稳定剂,通过涂覆浸渍方法将1 wt%的上述分子加入LH中,进行了同样的电性能测试。结果发现,在电树枝实验中,3-氨基苯甲酸表现最佳,加入后可提高材料起树电压50%;其他五种同时连接给吸电子基团的分子加入LH后,将材料的起树电压提高了 8%-43%,而只含吸电子基团的4-氰基苯硼酸的加入则导致起树电压降低了 5%。进一步通过高斯模拟对七种分子能隙差Eg进行计算,并与起树电压进行线性拟合,发现较好的负相关性,即Eg越小,对抑制起树越有利。在空间电荷实验中,3-氨基苯甲酸和3-氨基苯硼酸表现较优,均表现出明显的空间电荷抑制效果。在直流电导率实验中,3-氨基苯甲酸仍表现较优,在303 K、313K和323 K测试条件下,其均起到降低材料电导率作用,同时3-氨基苯硼酸也可在303K和313 K下降低电导率。将上述性能最佳的3-氨基苯甲酸通过溶液法接枝到聚乙烯分子链上,采用自由基引发接枝机理。通过FTIR表征证明接枝成功,在接枝样品中出现直接共混样品中不具备的C-N(烷基碳)特征峰和N-H(仲氨基)特征峰。通过DSC和TGA表征发现,此条件下接枝操作对材料热性能影响很小,基本可以忽略。对于电性能测试,相比LH,接枝样品可提高起树电压29%,对比发现其对电树枝虽也具有较明显的抑制作用但不如涂覆浸渍法样品,推测此结果是由于非常有限的接枝率。在空间电荷抑制方面,接枝样品未表现出明显作用,认为有限的接枝率使分子在聚合物内部形成的陷阱数量也十分有限,达不到空间电荷积累抑制的阈值。而在电导率方面,在较高温323 K和333K下,接枝样品都表现出明显降低电导率作用,推测原因是极性分子3-氨基苯甲酸本身就具有一定的导电性,较低的接枝率反而有利于其直流电导率性能。综合以上结果认为电压稳定剂对三项测试的作用机理存在相似性,从分子结构特征分析,认为分子的芳香环上连有能力匹配的吸电子基团和给电子基团会使其在提高绝缘材料电性能方面表现更优异。抑制电树枝方面,具有该结构的分子如同一个微型电场,吸电子基团和供电子基团如同两极,可以对捕获的高能电子进行更彻底的能量缓冲;在空间电荷积累抑制和直流电导率作用方面,分子中的芳香环可以充当载流子的结合区域,吸电子基团和给电子基团可以分别充当电子和空穴陷阱,因此可以对载流子实现更好的捕获。更多还原

【Abstract】 With the rapid development of economic and social in the world today,higher requirements are imposed on the power network.Long-distance and large-capacity power transmission has also become a trend.Therefore,high-voltage cables with higher electrical resistance are required.The insulation layer of a high-voltage cable has an important effect on the dielectric strength.At present,the most widely used insulation materials are mainly cross-linked polyethylene(XLPE),which has good insulation properties,strong thermal stability and mechanical stability.However,the thermosetting characteristic makes it unrecyclable,prompting people to increase the research on thermoplastic materials such as low density polyethylene(LDPE)and high density polyethylene(HDPE)to replace thermosetting XLPE.For improving the common problems such as electrical treeing and space charge in the process of apply ing insulating materials,introducing nanoparticles,polymer blending,adding voltage stabilizers and chemical grafting are currently effective methods.Among them,the addition of voltage stabilizers is the most common.The most used voltage stabilizer usually contains conjugated structure aromatic ring and polar groups.Adding a voltage stabilizer to a polymer has the disadvantages of limited solubility and easy migration.Hence,researchers have tried to graft the voltage stabilizer onto the polyethylene molecular chains for improvement.In this paper,graphene oxide was first prepared by a modified Hummers method.In addition,nitrogen-doped graphene,boron-doped graphene and boron-nitrogen co-doped graphene were prepared by a vapor deposition method.Next,0.2 wt%of doped graphene was added to the polyethylene blending material(90%LDPE+10%HDPE,which is hereinafter referred to as LH)to prepare composites by a solution blending method.Finally,electrical properties were tested on the composites.For the electrical treeing test,the addition of the three kinds of doped graphene increases the tree initiation voltage(TIV)of the composites to different degrees.Among them,boron-nitrogen co-doped graphene has the greatest increase of 17%.Meanwhile,boron-doped graphene and nitrogen-doped graphene increases the TIV by 7.8%and 10.7%,respectively.In the space charge test,all the three kinds of doped graphene have a certain effect on inhibiting the accumulation of space charge and improving the space charge distribution in the composites.In the DC conductivity test,the addition of three kinds of doped graphene has greatly increased the conductivity of the composites.However,in comparison,boron-nitrogen co-doped graphene has the least increase of the conductivity.Seven kinds of aromatic molecules containing different electron-donating groups and electron-withdrawing groups were screened as voltage stabilizers.1 wt%of the above molecules were added to LH by the diffusion loading method to obtain blending materials.The same electrical performance tests were performed.It is found that 3-aminobenzoic acid performed best in the electrical treeing experiment,which can increase the TIV by about 50%compared with the reference sample LH.The five other molecules simultaneously connected to the electron-donating and electron-withdrawing groups increase the TIV by 8%-43%,respectively.4-cyanophenylboronic acid,which contains only electron-withdrawing groups,reduces the TIV by 5%.Further Gaussian simulations were performed to calculate the energy gaps Eg of the seven molecules.Moreover,a linear fit was made between the TIV and Eg,in which a good negative correlation is established.That is,the smaller the Eg is,the more advantageous it is to suppress tree initiation.In the space charge experiment,3-aminobenzoic acid and 3-aminophenylboronic-acid perform better,and both show significant space charge suppression effect.In the DC conductivity experiment,3-aminobenzoic acid still performs best,and it shows the effect of reducing the material conductivity under the test conditions of 303 K,313 K and 323 K.The best-performing 3-aminobenzoic acid was grafted onto the polyethylene molecular chains by the solution grafting method.FTIR characterization displays the characteristic peaks of C-N(alkyl carbon)and N-H(secondary amino)in the grafted samples,which proves that the grafting operation was successful.According to DSC and TGA characterization,it is observed that the effect of the grafting operation on the thermal properties of the grafting material is very small under this condition and can be even ignored.For the electrical performance test,the grafting sample has a 29% increase of TIV compared with LH,which has an obvious inhibitory effect on the electrical treeing.However,it is not as good as diffusion loading samples,presumably due to the very limited grafting rate.In terms of space charge inhibition,the grafted sample shows little effect.It is supposed that the limited grafting rate causes the number of traps formed in the polymer to be very limited,which cannot reach the threshold of space charge accumulation inhibition.While for electrical conductivity,at higher temperatures of 323 K and 333 K,the grafted samples still shows a significant decrease in electrical conductivity.The reason may be that the polar molecule,3-aminobenzoic acid,has a certain conductivity and the lower grafting rate is in favor of the DC conductivity performance.Based on the above results,it is considered that the voltage stabilizers have similarities in the three test mechanism.From the analysis of molecular structure characteristic,it is believed that the co-existence of matched electron-donating and electron-withdrawing groups in the aromatic molecules is favorable for the insulation properties.With regard to the suppression of electrical treeing,the molecules with this structure act like a miniature electric field,and the electron-withdrawing group and the electron-donating group behave like two poles,which can more thoroughly buffer the energy of the captured high-energy electrons.In terms of the inhibition of space charge accumulation and the decrease of DC conductivity,the aromatic ring in the molecule of the structure can serve as a carrier binding region,and the electron-withdrawing group and the electron-donating group can serve as electron and hole traps,respectively.Therefore,better capture of the carriers can be achieved.更多还原