聚酰亚胺电导率随温度和电场强度的变化规律

介质深层充电对航天器安全运行构成了重大威胁.以聚酰亚胺为代表的此类聚合物绝缘介质的电导率受温度影响显著,又因为充电过程中局部产生强电场(10~7V/m量级),因此,其电导率模型需要综合考虑温度和强电场的影响,这对介质深层充电的仿真评估意义重大.已有的两类模型,不是低温区间不适用,就是没有充分考虑强电场的影响.基于跳跃电导理论,本文分析对比了现有电导率模型,提出了适用于较宽温度范围且合理考虑强电场增强效应的电导率新模型,并采用某型聚酰亚胺电导率测试数据做出验证.此外,为了提高新模型在强电场下的低温适用范围,尝试对强电场因子中的温度做变换,取得了满意的效果.参数敏感度分析表明新模型在电导率拟合与外推方面具有参数少、适用性强的优势.

Study on temperature and electric field dependence of conductivity in polyimide

The deep dielectric charging (DDC) imposes a potential threat on spacecrafts. On the one hand, this kind of polymer insulator dielectric, represented by polyimide, is significantly dependent on temperature; on the other hand, during the charging process the high electric field (at the level of 10⁷ V/m) will enhance the conductivity of the dielectric. Therefore, in order to make a precise assessment of DDC by computer simulation, the conductivity model should take into account the temperature and electric field dependences. In this field, two conductivity models are usually adopted for DDC simulation. One of them is proposed by Adamec. It puts emphasis on the enhanced conductivity due to high electric field, while its temperature dependence is based on the famous Arrhenius formula. Adamec model can make good performance versus electric field, but it is inappropriate in low temperatures. Another model combines the thermally assistant hopping conductivity and the variable-range hopping conductivity together, so it shows advantage in the temperature dependence, which is named as TAH & VRH model. Although this model also can include the influence from electric fields, the effectiveness is not so good as that of Adamec model. In order to combine the advantages of these two models, i.e. the Adamec model and TAH & VRH model, a new conductivity model is proposed with fewer parameters than those in TAH & VRH. It is derived by replacing the Arrhenius formula in Adamec model with a simplified temperature model referred to as TAH & VRH model. This formulation enables the new model to deal with a wider temperature range and keep the good performance versus high electric fields. The proposed model is verified partly by the measured data of a kind of polyimide. Satisfactory agreement is obtained in data fitting by using the new model, where the temperature dependence is better than that of Adamec model. In addition, to overcome the unreasonable increase in conductivity in low temperature and high electric field, a useful technique is proposed. By temperature mapping in the electric field correlated factors namely the carrier concentration and mobility enhancement factor, this technique can extend the feasible temperature range to a lower limit. This is done according to the assumption that the carrier concentration is small at low temperatures, and consequently the electric field influence should not be large. At high temperatures or in low electric fields, the temperature mapping is of little effect. Finally, analysis of the model's sensitivity versus several parameters is provided, demonstrating the advantage of applicability of the new model with fewer parameters.