1. School of Power and Energy, Northwestern Polytechnical University, Xi’an 710072, P. R. China
2. China Gas Turbine Establishment, Chengdu 610500, P. R. China
3. Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, School of Electronic Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China