聚合物薄膜电介质中空间电荷的瞬态行为研究

Due to the advantages of high electric breakdown strength, low loss, light weight, the polymer dielectric thin films can be utilized as dielectric materials in electrostatic energy storage capacitor with high energy density to meet with the requirement of minimization and lightweight in modern equipment of electronic and electrical power system. The polymer films often suffer from relative high electric field for the polymer capacitors with high energy density, which may cause space charge accumulation within the films. Besides deforming the applied field, part of the accumulating space charge may be intensively released during the capacitor discharge procedure. Companying with the release of space charge, the energy bound by space charge would be simultaneously released. The released energy may bring about local micro-damage or even strong breakdown. However, the commonly used method such as pressure wave propagation method, pulsed electroacoustic method, the thermal pulse method can be applied for monitoring the space charge behavior changing within short interval less than 1 ms. This project would focus on physical mechanism of the instantaneously intensive discharge of space charge trapped in polypropylene and polyester films by measuring and analyzing the transient discharge current. Combining the transient discharge current method, the thermal pulse method would also be used to observe space charge distribution and evolution before and after the intensive release of the trapped charge, and therefore reveal the mechanism of material deterioration or even breakdown during the intensive release of space charge and finally to obtain the key parameters.

聚合物电介质薄膜由于具有高击穿场强，低损耗，密度小等优点，可用作高储能密度静电储能电容器的介质材料，以满足现代电子，电力系统对设备小型化，轻量化的要求，而高储能密度往往需要使聚合物薄膜工作于较高的工作电场。工作于较高电场下的介质薄膜中容易积累空间电荷，除造成介质内的电场畸变外，部分积累的空间电荷在电容器放电过程也集中快速释放，空间电荷集中逃逸过程伴随的能量释放可能导致介质薄膜的微观损伤甚至击穿。但是现有的常用方法如压力波法，电声脉冲法，热脉冲法等无法用于检测小于毫秒级时间内空间电荷动态变化的过程。本项目拟以电容器用聚合物薄膜如聚丙烯，聚酯等为研究对象，利用测量瞬态放电电流的方法研究聚合物薄膜中空间电荷瞬时释放的物理过程，并结合热脉冲法研究薄膜中空间电荷的积累分布和空间电荷瞬态释放前后的变化规律，从而掌握空间电荷的瞬时释放导致薄膜击穿场强降低和空间电荷击穿的相关参数和机制。

本项目利用介质薄膜的瞬态放电电流的动态特征，结合热脉冲法研究薄膜中空间电荷的积累分布的静态特征，检测分析小于毫秒级时间内空间电荷瞬态行为规律，掌握薄膜中空间电荷的瞬态行为规律与薄膜介电性能的影响。本项目主要就以下方面展开研究工作：1）薄膜介质在直流高电场下电荷的注入与积累行为。双面铝金属化的聚丙烯和聚酰亚胺薄膜在室温下的注入阈值电场分别约为100kV/mm和50kV/mm，呈现为非对称的双极性空间电荷注入，温度越高或电场强度越高，非对称性越明显，最终都表现为负电荷的积累。聚酰亚胺薄膜相对聚丙烯薄膜在相同的实验条件下更容易积累电荷，在100kV/mm电场下，积累电荷造成的电场畸变可达原外施电场的1.5倍。较厚薄膜更容易积累电荷，短路后积累负电荷的平均电荷重心向薄膜样品的近表面移动。2）薄膜介质的瞬态电流特性研究。欠阻尼情况下，极化薄膜的振荡衰减电流呈现非线性特征，振荡准周期随周期数逐渐减小，极化电场越高起始周期时间长度越大，周期时间长度衰减速度在高电场强度明显较快。振荡周期呈现非线性特征的起始电场与空间电荷注入阈值电场一致；基于等效电容量获得的电荷积累密度与热脉冲法的结果一致；基于振荡周期变化特性推算的迁移率为10-8m2•V-1•s-1，大大于文献典型值，是一种电荷陷阱完全被填满后并在短路状态下的电荷行为特征；空间电荷限制电流法在外施高电场下的陷阱能级约为10-17m2•V-1•s-1，与文献经典值一致。以上数据体现，瞬态放电电流法可作为一种简单易行的办法，检验薄膜介质的电荷行为特征。由于浅陷阱电荷的主导作用，多次反复充放电情况下的电流行为特征没有明显变化。3）极化薄膜介质的电荷陷阱能级信息，半定量分析薄膜陷阱能级信息。利用瞬态放电电流特征，发现结晶度稍低的聚丙烯薄膜周期时间长度减小更快，具有更多的浅电荷陷阱；高场下的电导特性表明电荷陷阱的填充规律，高温度下电荷陷阱更容易被填满，空间电荷移动受陷阱影响小，与瞬态放电电流衰减快一致；利用等效电容量的变化推算浅陷阱与深陷阱电荷比例，获取半定量陷阱能级信息。