Influence of nano-SiO2 dispersion on the direct current dielectric properties of SiO2/LDPE nanocomposite
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摘要: SiO2/低密度聚乙烯(LDPE)复合材料的介电性能与纳米SiO2在LDPE基体中的分散性密切相关。为研究室温下拉伸处理对纳米SiO2颗粒在LDPE基体中分散性的作用机制，本文选取7 nm粒径的疏水型纳米二氧化硅(SiO2)与LDPE熔融共混制备SiO2/LDPE纳米复合材料。将制备好的纳米复合材料经过三次拉伸处理，利用扫描电子显微镜(SEM)、差示扫描量热仪(DSC)表征纳米粒子的分散性以及复合材料的结晶度，利用热刺激电流法(TSC)测试分析复合材料的陷阱能级和陷阱密度。通过对纳米复合材料的空间电荷，电导电流，直流击穿强度进行实验测试，研究了拉伸对纳米粒子分散性的影响及其所导致的直流介电性能的改变。结果表明室温下拉伸有助于纳米粒子的分散，使纳米SiO2粒子的团聚尺寸从200 nm左右缩减到100 nm左右；但拉伸会破坏LDPE的结晶结构，劣化其性能；通过掺杂纳米SiO2引入深陷阱能级可以改善LDPE的直流介电性能。经过拉伸的SiO2/LDPE的空间电荷积累得到抑制，电导电流的机制发生改变。通过对电导电流的数据进行拟合处理，发现拉伸后的SiO2/LDPE的电导以离子跳跃电导为主，其跳跃距离减少到1.98 nm左右。SiO2/LDPE相比LDPE直流击穿强度提高约43%，室温拉伸处理后SiO2/LDPE击穿强度降低的主要原因是拉伸过程导致的LDPE基体结构缺陷。Abstract: The dielectric properties of SiO2/low-density polyethylene (LDPE) composites are closely related to the dispersion of nano-SiO2 in LDPE matrix. In order to study the mechanism of tensile treatment on the dispersion of nano-silica particles in the LDPE matrix at room temperature, a hydrophobic nano-silica (SiO2) with a particle size of 7 nm was selected to be fused and blended with LDPE to prepare SiO2/LDPE nanocomposites. The prepared nanocomposites were stretched three times, and the dispersion of nanoparticles and the crystallinity of the composites were characterized by scanning electron microscope (SEM) and differential scanning calorimeter (DSC), The trap energy levels and trap densities of the composites were analyzed by thermally stimulated galvanometry (TSC). The effects of stretching on the dispersion of nanoparticles and the resulting changes in DC dielectric properties were investigated by experimentally testing the space charge, electrical conductivity, and DC breakdown strength of the nanocomposites. The results show that stretching at room temperature helps the dispersion of nanoparticles and Reduce the agglomeration size of nano-SiO2 particles from about 200 nm to about 100 nm. However, stretching will destroy the crystal structure of LDPE and deteriorate its properties; The DC dielectric properties of LDPE can be improved by introducing deep trap levels by doping nano-SiO2.The space charge accumulation of the stretched SiO2/LDPE is suppressed, and the mechanism of conducting current is changed. By fitting the conductance current data, it is found that the conductance of the stretched SiO2/LDPE is dominated by ion hopping conductance, and its hopping distance is reduced to about 1.98 nm. Compared with LDPE, the DC breakdown field strength of SiO2/LDPE is improved by about 43%, The main reason for the decrease of the breakdown strength of SiO2/LDPE after stretching at room temperature is the structural defects of the LDPE matrix caused by the stretching process.
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Key words:
- SiO2/LDPE /
- room temperature stretching /
- dispersion /
- space charge /
- conductance /
- breakdown strength
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图 1 试样拉伸处理过程示意图
Figure 1. Schematic diagram of the tensile treatment process of the sample
图 2 低密度聚乙烯(LDPE)及其纳米复合介质结晶形态图
Figure 2. Crystal morphology of low-density polyethylene (LDPE) and its nanocomposite medium
图 4 不同极化时间下LDPE与SiO2/LDPE复合材料的空间电荷分布
Figure 4. Space charge distribution of LDPE and SiO2/LDPE composites under different polarization times
图 5 不同短路时间下LDPE与SiO2/LDPE复合材料空间电荷分布
Figure 5. Space charge distribution of LDPE and SiO2/LDPE composites under different short-circuit times
图 6 试样内部空间电荷的平均体电荷密度
Figure 6. Average volume charge density of space charge inside the sample
图 8 LDPE及其纳米复合介质的电导特性
Figure 8. Conductivity characteristics of LDPE and its nanocomposite media
图 9 复合材料高场电导的离子跳跃电导拟合曲线
Figure 9. Ion jump conductance fitting curve of composite material high field conductance
图 11 SiO2/LDPE纳米复合介质直流电场击穿场强Weibull分布图
Figure 11. SiO2/LDPE nanocomposite dielectric DC electric field breakdown field strength Weibull distribution
表 1 LDPE及其复合材料的DSC数据
Table 1. DSC data of LDPE and its composites
Sample Tc /℃ Tm /℃ ΔHm /(J∙g−1) Wc /% LDPE 114.3 92.17 116.53 37.19 Stretch LDPE 110.5 95.5 116.38 39.64 SiO2/LDPE 111.2 95.5 111.86 38.53 Stretch SiO2/LDPE 110.5 96.17 113..05 38.94 Notes：Tc and Tm are the crystallization peak temperature and melting peak temperature; ΔHm and Wc are the enthalpy of fusion and crystallinity of each sample. 
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表 2 各试样的电导斜率和过渡阈值电场
Table 2. Conductivity slope and transition threshold electric field of each sample
Sample β E(kV/mm) jΩ jt EΩ−t LDPE 0.53 4.45 9.52 Stretch LDPE 0.18 4.24 5.74 SiO2/LDPE 0.22 2.57 21.11 Stretch SiO2/LDPE 1.05 Notes：β and E are the slope and electric field strength；jΩ，jt and EΩ−t are the slope of the conductance current density curve and the threshold EΩ−t of the transition electric field.
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表 3 各试样的特征击穿场强和形状参数
Table 3. The characteristic breakdown field strength and shape parameters of each sample
Sample E0/(kV·mm−1) β LDPE 387.1 12.67 Stretch LDPE 354.7 11.72 SiO2/LDPE 554.4 15.05 Stretch SiO2/LDPE 491.0 7.634 Notes：E0 and β are the characteristic breakdown field strength and shape parameters, respectively
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