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. 
G Z Li, Y Yang, M A Susner, M D Sumption, and E W Collings
cond-mat.supr-con