Critical Current Density and n-values of MgB2 Strands over a Wide Range of Temperatures and Fields

Transport measurements of critical current density, Jct, in monocore powder-in-tube MgB2 strands have been carried out at temperatures, T, of from 4.2 K to 40 K, and in transverse fields, B, of up to 14 T. Processing methods used were conventional continuous-tube-forming-filling (CTFF) and internal-magnesium-diffusion (IMD). Strands with several powder compositions were measured, including binary (undoped) MgB2, 2% carbon doped MgB2, and 3% carbon doped MgB2. Magnetization loops (M-B) were also measured, and magnetic critical current density, Jcm, values extracted from them. The transport, Jct(B) and magnetic, Jcm(B), critical current densities were compared. Also studied was the influence of doping on the resistively measured irreversibility field, Birr and upper critical field Bc2. Critical current densities, Jct, and n-values were extracted from transport measurements and were found to be universally related (for all B and T) according to n \propto Jctm in which m = 0.52 \pm 0.11. Likewise n was found to be related to B according to n \propto B-p with a T-dependent p in the range of about 0.08~0.21. Further analysis of the field (B) and temperature (T) dependencies of n-value resulted in an expression that enabled n(B,T), for all B and T, to be estimated for a given strand based on the results of transport Jct(B) measurements made at one arbitrarily chosen temperature.Comment: 34 pages, 7 figure

Drawing induced texture and the evolution of superconductive properties with heat treatment time in powder-in-tube in-situ processed MgB2 strands

Monocore powder-in-tube MgB2 strands were cold-drawn and heat-treated at 600C and 700C for times of up to 71 hours and structure-property relationships examined. Drawing-induced elongation of the Mg particles led, after HT, to a textured macrostructure consisting of elongated polycrystalline MgB2 fibers separated by elongated pores. The superconducting Tc, Jc and Fp were correlated with the macrostructure and grain size. Grain size increased with HT time at both 600C and 700C. Jc and hence Fp decreased monotonically but not linearly with grain size. Overall, it was observed that at 700C, the MgB2 reaction was more or less complete after as little as 30 min; at 600C, full reaction completion did not occur until 71 h. into the HT. Transport, Jct(B) was measured in a perpendicular applied field, and the magnetic critical current densities, Jcm\bot(B) and Jcm{\phi}(B), were measured in perpendicular and parallel (axial) applied fields, respectively. Particularly noticeable was the premature dropoff of Jcm\bot(B) at fields well below the irreversibility field of Jct(B). This effect is attributed to the fibrous macrostructure and its accompanying anisotropic connectivity. Magnetic measurements with the field directed along the strand axis yielded a critical density, Jcm\bot(B), for current flowing transversely to the strand axis that was less than and dropped off more rapidly than Jct(B). In the conventional magnetic measurement, the loop currents that support the magnetization are restricted by the lower of Jct(B) and Jcm{\phi} (B). In the present case the latter, leading to the premature dropoff of the measured Jcm(B) compared to Jct(B) with increasing field. This result is supported by Kramer plots of the Jcm{\phi} (B) and Jct(B) data which lead to an irreversibility field for transverse current that is very much less than the usual transport-measured longitudinal one, Birr,t.Comment: 41 pages, 14 figure