Critical current densities and<i>n</i>-values of MgB₂strands over a wide range of temperatures and fields

Abstract

Transport measurements of critical current density, Jct, in monocore\npowder-in-tube MgB2 strands have been carried out at temperatures, T, of from\n4.2 K to 40 K, and in transverse fields, B, of up to 14 T. Processing methods\nused were conventional continuous-tube-forming-filling (CTFF) and\ninternal-magnesium-diffusion (IMD). Strands with several powder compositions\nwere measured, including binary (undoped) MgB2, 2% carbon doped MgB2, and 3%\ncarbon doped MgB2. Magnetization loops (M-B) were also measured, and magnetic\ncritical current density, Jcm, values extracted from them. The transport,\nJct(B) and magnetic, Jcm(B), critical current densities were compared. Also\nstudied was the influence of doping on the resistively measured irreversibility\nfield, Birr and upper critical field Bc2. Critical current densities, Jct, and\nn-values were extracted from transport measurements and were found to be\nuniversally related (for all B and T) according to n \\propto Jctm in which m =\n0.52 \\pm 0.11. Likewise n was found to be related to B according to n \\propto\nB-p with a T-dependent p in the range of about 0.08~0.21. Further analysis of\nthe field (B) and temperature (T) dependencies of n-value resulted in an\nexpression that enabled n(B,T), for all B and T, to be estimated for a given\nstrand based on the results of transport Jct(B) measurements made at one\narbitrarily chosen temperature.\n