Structural-microstructural characteristics and its correlations with the superconducting properties of in situ PIT-processed MgB₂ tapes with ethyltoluene and SiC powder added

We performed structural-microstructural investigations of pure MgB ₂, ethyltoluene and both ethyltoluene-and SiC-added MgB ₂/Fe tapes. The analysis of the microstructure shows that the grain size for the pure and ethyltoluene-added MgB₂ tape sample is in the range of 10-100nm. However, with the addition of both ethyltoluene and SiC, the grain size decreases to about 5-80nm. The a-axis length of the ethyltoluene-added tape samples is slightly decreased, whereas for both ethyltoluene-and SiC-added samples, the a-axis length is decreased by 0.4% as compared to the pure MgB₂ tape sample, showing the amount of carbon substitution is less in ethyltoluene-added tape samples. The reason for the higher J_c values of the ethyltoluene-added MgB₂ tape sample as compared to the pure MgB₂ tapes is the presence of a lesser amount of the impurity phase, MgO. The large improvement in J_c-B properties for the ethyltoluene-and SiC-added MgB₂ tape sample is attributed to (1)the enhancement of upper critical field, B_c2, by the substitution of carbon for boron, (2)pinning by nanosized (5-20nm) particles of Mg₂Si and other silicides, (3)enhanced grain boundary pinning due to the smaller grain size and (4)the presence of a lesser amount of impurity phase, MgO, as compared to the pure sample.