**5. Acknowledgements**

The author would like to thank Prof. J. Müller and Dr. F. Gromball of Technische Universit.t Hamburg-Harburg in Germany for providing experimental conditions and interesting discussion, and also remember Prof. J. Müller with affection for his human and scientific talents. This research was financially supported by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety under contract #0329571B in collaboration with the Hahn Meitner Institute (HMI), Berlin-Adlershof, Department for Solar Energy Research. The author was financially supported by China Scholarship Council (CSC) and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 78-QP-2011).
