1.School of Resources and Environment, University of Electronic Science and Technology of China, Chengdu 610054, China 2.Key Laboratory for Physical Electronics and Devices of the Ministry of Education, Department of Electronic Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11805030).
Received Date:29 October 2018
Accepted Date:05 December 2018
Available Online:01 March 2019
Published Online:05 March 2019
Abstract:The charging characteristics and microscopic mechanism of space electrons irradiated polymers are the basis for the study and protection of spacecraft polymer charging and discharging characteristics. Monte Carlo method is used to simulate the scattering process of space electrons, and the fast secondary electron model simulates the generation of secondary electrons. The finite difference method is used to solve the charge transport process of charge continuity equation, current density equation and Poisson equation. The capture process realizes the transmission process of space electrons through the equation based on the Poole-French effect. Based on the electronic scattering/transport synchronization model and combined with the geostationary earth orbit electronic spectrum distribution theoretical formula of the French National Aeronautics and Space Research Agency (ONERA) and the ground experimental method of the agency (SIRNE), a scattering model based on the electron spectrum distribution in geosynchronous orbit is established. The numerical simulation of the charging process of space electrons irradiated polymers is carried out. The space charge density, space potential, electric field distribution and the space potential of polymer sample under the irradiation of single- and multi-energy electrons in space environment are obtained. The relationship among charging characteristics, microscopic parameters and surface potential of the sample is clarified. The surface potential characteristics of space electrons irradiated polymer are consistent with the experimental results. The single energy charge potential and strength are higher than those of multi-energy electrons. When the charging reaches a steady state, the electron mobility is smaller (less than 10–11 cm2·V–1·s–1), and the absolute value of the space potential is significantly enhanced with the decrease of the electron mobility. When the composite rate is large (greater than 10–14 cm3·s–1), the absolute value of the spatial potential increases with recombination rate increasing. The study of the charging characteristics of space electrons is not comprehensive because only the mode of single-energy electron irradiation is taken into consideration. The research results are of great scientific significance and practical value for revealing the charging characteristics and microscopic mechanism of space electrons irradiated polymer and improving the research level of spacecraft charge and discharge fault mechanism. Keywords:space radiation/ polyimide/ charging characteristics/ numerical simulation
其中Ne和Nh分别为电子俘获密度和空穴俘获密度, Se和Sh分别为电子俘获截面和空穴俘获截面. 通过C++编程的并行计算实现以上的充电过程, 优化各种参数, 诸如计算时间步长和有限差分网格等, 计算一个样品参数(例如电子迁移率为10–11 cm2·V–1·s–1)条件下的充电过程达到稳态所需要的时间为6 h. -->
3.1.数值模拟结果与实验数据的比较
图2所示为数值模拟结果与实验数据的比较[32]. 其中, 实验中聚酰亚胺样品的厚度为25 ${\text{μ}}\rm m$. 通过数值模拟, 电子迁移率为10–11 cm2·V–1·s–1, 俘获密度为1014 cm–3时, 数值模拟与实验数据结果相吻合. 文中的空间电子分布主要在10—400 keV之间, 二次电子产额小于1, 空间电子辐照聚合物形成负充电, 内部空间电荷分布总体为负, 随着空间电子的辐照样品内部沉积的电子数量逐渐增多, 图2得到的表面电位逐渐降低. 随着空间电子进一步辐照聚合物样品, 样品内部使得电子向下输运的电场逐渐增强, 更多的电子向样品底部运动, 经过一定的辐照时间到达样品底部形成样品电流. 需要注意的是, 聚合物样品的平衡机制会随样品厚度和入射电子能量的不同而有所差异. 图 2 空间电子辐照聚合物表面电位数值模拟和实验数据的比较[32] Figure2. Numerical simulation and experimental data comparison of surface potential of space electron irradiation polymer[32].
23.2.空间电子散射形成的局部等离子体分布 -->
3.2.空间电子散射形成的局部等离子体分布
图3给出了电子入射能量为10, 20和30 keV, 电子数目为1000个的初始散射电子密度分布图. 散射形成的电子空穴分布形态相似. 由散射电子密度分布可以看出, 能量越高电子密度分布越平缓, 峰值位置越靠近样品底部, 峰值越小. 当入射电子能量较高时, 电子在样品中能量损失到零所需的步数变多, 运动的距离也会变长. 入射电子能量较高时, 散射类型更多的是弹性散射, 弹性散射不会产生二次电子和空穴, 所以电子密度会较低. 值得注意的是, 文中选择的入射电子能量均为大于10 keV的电子, 样品表面的正电荷分布几乎消失, 主要原因为发生非弹性散射的位置距离样品表面较远, 生成二次电子能量较低而无法运动到样品表面并逸出, 所以不会出现正电荷密度分布. 图 3 空间电子散射 (a)散射电子密度分布; (b)散射空穴密度分布 Figure3. Space electron scattering: (a) Scattering electron density distribution; (b) scattering hole density distribution.
23.3.空间电荷密度、空间电位和空间电场的分布 -->
3.3.空间电荷密度、空间电位和空间电场的分布
空间电荷分布是影响充电过程的主要因素. 已知空间电荷分布能够确定空间电位和电场分布. 电子入射到聚合物样品后, 首先与样品发生的是快速的散射过程, 形成一定的散射分布. 图 4 空间电子辐照聚合物空间电荷和电子密度分布 (a)空间电荷; (b)电子密度 Figure4. Space charge and electron density distribution of irradiated polymers: (a) Space charge; (b) electron density.