HE Jinliang, PENG Simin, ZHOU Yao, YANG Yang, HU Jun
State Key Laboratory of Control and Simulation of Power System and Generation Equipment(Department of Electrical Engineering, Tsinghua University), Haidian District, Beijing 100084, China
The development of HVDC transmission technology in China puts forward higher requests to the insulating properties of dielectric materials, polymer nanodielectric materials have drawn extensive attention for their excellent properties, and the interface has become a hot topic. The interface is the nanoscale transition region between nano-filler and polymer matrix, due to its special formation mechanism, the interface has different properties from the polymer matrix and nano-filler. Meanwhile, because the interface plays a leading role in the composite materials, its microstructure and properties will directly influence the macroscopic properties of the nanocomposite. This paper reviewed the introduced formation mechanism, structure and model of the interface, discussed its influence on aggregation structure of polymer and the trap theory and analyzed the mechanism of interface in the electrical properties of nanocomposites. Finally the study on local dielectric properties of the interface by using electrostatic force microscopy (EFM) was discussed.
KEY WORDS :nanocomposite;interface;aggregation structure;trap effect;charge transport;electrostatic force microscope (EFM);
T. J. Lewis[14,22]和J. K. Nelson[18,23]等认为界面区存在介电双层(electric double layer)结构。复合材料中的纳米颗粒在电场作用下,表面聚集电荷(设为正电荷),而异号电荷(设为负电荷)由于极化作用将在纳米颗粒外部积聚起来,形成屏蔽层,聚合物基体中的带电粒子,在库仑力作用下发生迁移扩散,在纳米颗粒周围形成介电双层,其结构如图3所 示[22]。介电双层的电导率一般比聚合物基体要大得多[20],当复合材料中相邻颗粒的介电双层发生重叠时,重叠的部分在外部电场作用下会形成导电的通路,从而改变材料内部的电荷输运机制,进而影响复合材料的老化及击穿等性能。
图3
介电双层模型示意图
Fig. 3
Scematic illustration of electric double layer
T. Tanaka[24]等在化学、电学和形态学理论的基础上提出了多核模型(multi-core model),如图4[24]所示,假设复合材料中的颗粒均为球形,即每一个直径十几纳米的颗粒均被相同数量级尺寸的界面层包围,然后与外部的聚合物体相连。界面区从内到外被划分成键合层(bonded layer)、束缚层(bound layer)和松散层(loose layer)。其中键合层主要是纳米颗粒表面的一些基团与聚合物分子链之间的化学键,束缚层由球晶、片晶等有序结构组成,主要是深陷阱区,松散层是由一些无定形结构构成,有大量浅陷阱存在。纳米颗粒可能带电荷,因而自由载流子会在界面区建立厚度在几十纳米左右的Gouy- Chapman扩散层,形成介电双层,与界面的三层结构在空间上相互重叠,对复合材料的介电性能产生重要影响。Tanaka等运用该模型阐释了聚合物纳米复合材料所表现出来的耐电晕放电、耐局部放电腐蚀、耐电树等部分介电性能的相关机理。
图4
多核模型示意图
Fig. 4
Scematic illustration of the multi-core model
但是该模型不能定量描述,且在实验中尚未观察到界面的三层结构。
T. J. Lewis基于介电双层理论提出的模型和T. Tanaka提出的多核模型在一定程度上很好地解释了聚合物纳米复合材料的一些特性,对界面的研究具有重要意义,但这两个模型都还不能很好地解释复合材料介质损耗、击穿场强等实验的结果,具有局限性。此外,两种模型都把界面解释为较明显的过渡层,具有纳米级规则的微观结构。但实际上界面区是由化学键合作用和范德华力形成,在空间上应该是边界较模糊的区域,没有规则的形态。因此,想要建立起更为普适的界面区微观模型还有待进一步深入的研究。
田付强[31]对低密度聚乙烯(low density polyethylene,LDPE)和掺杂ZnO纳米颗粒的样品进行了电晕刻蚀,去除LDPE表面无定形区,并在原子力显微镜(atomic force microscope,AFM)下观察形貌,结果发现比起LDPE,纳米复合材料中ZnO纳米颗粒作为异相成核剂,球晶尺寸明显减少,数量显著增加。X. Huang[32]等利用酸性溶液腐蚀LDPE/SiO2材料脆断面的无定形区,发现LDPE中存在由片晶或片晶束堆积形成的球晶结构,球晶直径为10μm左右;而在LDPE/SiO2纳米复合物中球晶破碎,结构极为不完整,如图8所示[32]。S. Peng[33]等同样观察到MgO纳米颗粒掺杂使得LDPE/MgO复合材料球晶尺寸减小,数量增加。C. M. Chan[34]等人的研究也表明,纳米颗粒的掺杂使得聚丙烯(polypropylene,PP)体系的球晶尺寸减小,数量 增加。
图8
LDPE和LDPE/SiO2纳米复合材料结晶形貌的SEM照片
Fig. 8
SEM iamges of crystalline morphology of LDPE and LDPE/SiO2
T. Takada[7]等对LDPE/MgO纳米复合材料陷阱特性进行了研究,指出纳米掺杂可引入1.5~5eV较深的陷阱能级。K. S. Shah[42]等的试验结果表明,纳米掺杂含量越高,偶联剂使用量越大,引入的深陷阱越多,使得材料的电导率越低,电阻率越高。Y. Takai[43]指出,聚合物的无定形区由于结构无序而形成的缺陷(l~2eV)、结晶区缺陷以及化学杂质产生的缺陷(2~5eV)俘获电荷后不易通过热激发被释放,而界面区和相界面的空腔陷阱能级较低(约1eV)。M. Meunier[44-45]等对PE内部陷阱深度的仿真表明,物理构象缺陷引入的陷阱能级一般都低于0.3eV,而化学缺陷引入的陷阱能级可能大于1eV。
Y. Cao[47]等研究了PI/无机纳米复合材料的电导特性,发现2%纳米掺杂后材料的电导率比未掺杂和微米掺杂的都要小,Cao通过TSC测试发现纳米掺杂产生了深陷阱,降低了载流子迁移率,从而致使电导率减小。
R. C. Smith[48]等利用电声脉冲法(pulsed electro- acoustic,PEA)研究了XLPE进行微米和纳米掺杂后的空间电荷特性,发现微米复合物在阴阳两极前面都出现了异极性空间电荷,而纳米复合物阴阳两极前面都出现了同极性空间电荷。Smith认为微米掺杂XLPE的载流子迁移率可能很高,两电极注入的电子和空穴在测量时已经迁移到了对面电极,从而在两电极附近呈现异极性空间电荷。而纳米掺杂后,引入的大量陷阱使得载流子自由程变短,迁移率降低,测量时注入电荷仍然停留在注入电极附近,从而呈现为同极性空间电荷。
J. K. Nelson[49]等研究了环氧树脂(epoxy)/TiO2的微米和纳米复合材料的介电常数,发现在高频时,微米掺杂的复合物介电常数比基体聚合物大,而纳米掺杂后的介电常数比基体要小。Nelson认为微米掺杂导致界面发生Maxwell-Wagner极化,使得其介电常数增大,而纳米颗粒与聚合物分子之间存在着较强的相互作用,可能阻碍界面区极性高分子链段或侧基的转向。P. Murugaraj[50]等研究了PI/Al2O3和PI/SiO2体系,计算得到的界面区介电常数也远高于纳米颗粒和聚合物基体的介电常数,而实验也发现复合材料的介电常数明显高于无机颗粒和聚合物基体。说明复合材料介电常数增大很可能是由于界面区形成了高极化率的偶极子,从而增大了界面区的介电常数。这些偶极子本质上可能是偶联剂与纳米颗粒表面反应形成的共价键或氢键。
P. Maity[51]等研究了纳米掺杂前后Epoxy的耐电晕老化性能,发现纳米复合材料具有更加优异的耐电晕性能。Marit认为,无机纳米颗粒及其周围的界面相具有较强的耐电晕能力,当老化发生后,大量界面相的存在使得老化损失掉的聚合物相对较少,并且随着老化的进行,越来越多的纳米颗粒聚集到表面,抵抗电晕对内部聚合物的进一步侵蚀。T. Tanaka[15]用多核模型对尼龙(Polyamide,PA)/硅酸盐(silicate)层状纳米复合物的耐电晕机理进行了解释。Tanaka认为Silicate与PA所形成的界面区域的第一层存在离子键合,第二层形成包裹的球晶,而且与纳米颗粒之间存在较强的相互作用,具有较强的耐电晕能力。电晕老化应该从最外侧的第三层逐渐向里发展,而第二层的球晶结构可以抵 挡电晕的进一步侵蚀,从而保护了该区域的聚合物分子。
Y. Yang[52] 等通过研究PI/Al2O3纳米复合材料在不同温度下的电晕老化特性,发现一定浓度(约2%)的纳米掺杂能够提高复合材料在高温环境下的耐电晕寿命,并基于多核模型提出了相界面区域的热稳定化效应(thermal stabilization effect)。Yang 认为,由于不同温度条件下高分子链蠕动特性的不同,聚合物纳米复合材料的相界面在不同温度条件下表现出不同的结构和特性。考虑到界面区松散层内的浅陷阱具有较高的热激发概率,以及低密度松散层内较高的载流子平均自由程,相界面区域在复合材料中形成大量的高电导微区。界面区与高电阻率的聚合物基材形成高-低电导互穿体系,进而缓解局部的空间电荷积聚,同时使复合材料保持较高的电阻率。随着温度的升高,界面区的厚度从10nm左右扩展至30nm甚至更大。对于较高掺杂浓度(5%)的纳米复合材料,高温下扩展后的相界面区域相互重叠形成高电导路径,导致高能载流子不断轰击高分子链,最终造成材料老化和破坏。一定浓度的纳米掺杂在高温环境下能够避免大量的界面区重叠,因而能够表现出优良的耐电晕特性。上述热稳定化效应的机理解释在PEA法空间电荷测试以及TSC测试中得到了一定程度的验证,相界面区域更加精确的温度依赖特性还有待进一步研究。
B. R. Varlow[53]等研究了纳米ZnO掺杂对Epoxy基体电树发展特性的影响,发现少量的纳米ZnO掺杂就可以明显提高基体的电树老化击穿时间。Varlow认为纳米颗粒具有很大的比表面积,颗粒周围产生的微小空洞使得电树分支增多,消耗了电树发展的能量。T. Tanaka[6]认为电树的产生主要是由电荷注入和拉出过程所产生的机械疲劳引起的。纳米掺杂抑制了空间电荷的形成,提高了电树引发电场和延长了电树引发时间。纳米颗粒及其界面区域可能扭曲了树枝发展路径,颗粒的高介电常数使得电树枝向纳米颗粒附近发展,而纳米颗粒本身及其界面区域都有较强的耐放电老化特性,从而阻碍了电树的进一步发展。
静电力显微镜(electrostatic force microscope,EFM)是扫描探针显微镜(scanning probe microscopy,SPM)家族中探测长程力的显微镜[54],通过使用导电探针实现对针尖-样品之间静电相互作用的探测。由于使用的导电探针的针尖半径是纳米级,因而静电力显微镜可以在微米甚至纳米尺度下获得高空间分辨率的样品形貌图像和静电力特征图像。通过对图像进行比较分析,可以得到材料在纳米尺度微区上的电学特性。
T. S. Jespersen[56]等利用静电力探测技术表征PMMA/碳纳米管纳米复合材料的亚表面结构信息及介电特性。M. Labardi[57]等用EFM对聚醋酸乙烯酯(PVAc)/蒙脱土(MMT)体系进行了界面区的微区介电测试,发现界面区的介电峰展宽,直接验证了MMT片层对界面区PVAc分子链运动性的影响。张冬冬[58]等利用EFM系统研究了TiO2纳米颗粒填充的环氧树脂基复合材料界面微区在不同温度下的介电常数和介电损耗特性,发现在羟基修饰的纳米颗粒(OH-TiO2)表面,羟基与基体高分子链间存在较强的相互作用,界面处高分子链运动受阻,玻璃化转变温度升高;而全氟硅烷修饰的纳米颗粒(FTS-TiO2)的界面与环氧树脂的相互作用很弱,基本不影响界面处高分子链的结构构象和玻璃化转变过程。
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