1. Center of Nanomaterials for Renewable Energy, State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
2. Xi’an Key Laboratory of Electrical Equipment Condition Monitoring and Power Supply Security, College of Electrical and Control Engineering, Xi’an University of Science and Technology, Xi’an 710054, P. R. China