1. Department of Polymer Science and EngineeringShanghai Key Laboratory of Electrical Insulation and Thermal AgingState Key Laboratory of Metal Matrix CompositesShanghai Jiao Tong University Shanghai 200240 China
2. School of Materials Science and EngineeringGeorgia Institute of Technology Atlanta GA 30332‐0245 USA
3. Collaborative Innovation Center for Nanomaterials & DevicesCollege of PhysicsQingdao University Qingdao 266071 China
4. State Key Laboratory of Power SystemsDepartment of Electrical EngineeringTsinghua University Beijing 100084 China