According to our results the optimum concentration for Fe dopant is 0.5 wt.% while
in the case of Co and Ni it changes to 1 wt.%. The corresponding kNB values are: 2.54 * 10-4, 2.24 * 10-4 and 2.19 * 10-4 s -1. Dopant level increases up to optimum concentration positively affected by