Critical current densities and n -values of MgB 2 strands over a wide range of temperatures and fields

Transport measurements of critical current density, J ct , in monocore powder-in-tube MgB 2 strands have been carried out at temperatures, T , of from 4.2 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) MgB 2 , 2% carbon doped MgB 2 , and 3% carbon doped MgB 2 . Magnetization loops ( M – B ) were also measured, and magnetic critical current density, J cm , values extracted from them. The transport, J ct ( B ) and magnetic, J cm ( B ), critical current densities were compared. Also studied was the influence of doping on the resistively measured irreversibility field, B irr , and upper critical field, B c2 . Critical current densities, J ct , and n -values were extracted from transport measurements and were found to be universally related (for all B and T ) according to in which m = 0.52 ± 0.11. Likewise n was found to be related to B according to n ∝ 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 ) dependences 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 J ct ( B ) measurements made at one arbitrarily chosen temperature.