1. School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao 266580, P. R. China
2. Sulige Exploration and Development Branch Company of North China Petroleum Administration Bureau, Erdos 017300, P. R. China
3. Key Laboratory of New Energy Physics & Materials Science in Universities of Shandong, China University of Petroleum, Qingdao 266580, P. R. China