1. 1 National University of Science and Technology POLITEHNICA Bucharest , Bucharest, Romania Academy of Romanian Scientists , Ilfov 3, 050044 Bucharest , Romania
2. 2 National University of Science and Technology POLITEHNICA Bucharest , Bucharest, Romania Academy of Romanian Scientists , Ilfov 3, 050044 Bucharest , Romania
3. 3 National University of Science and Technology POLITEHNICA Bucharest , Bucharest, Romania Academy of Romanian Scientists , Ilfov 3, 050044 Bucharest , Romania
4. 4 National University of Science and Technology POLITEHNICA Bucharest , Bucharest , Romania