Author(s):
1. Dimitrina Koeva, Technical University – Sofia, Branch Sliven, Bulgaria, Bulgaria
2. Ralena Kutkarska, Technical University – Sofia, Branch Sliven, Bulgaria, Bulgaria
3. Kristiyan Valchev, Technical University – Sofia, Branch Sliven, Bulgaria, Bulgaria
4. Svilen Rachev, Technical University of Gabrovo, Department of Physics, Chemistry and Ecology 4 Hadzhy Dimitar str., Bulgaria

Abstract