Effect of heat treatment temperature on superconducting performance of B4C added MgB2/Nb conductors

A previously observed enhancing effect of addition of 10 wt.% of B4C on critical current density (Jc) dependence on magnetic flux density (B) in MgB2 superconductors is examined here in more detail. Nb sheathed in situ MgB2 monofilamentary Powder-in-Tube (PIT) samples are prepared with the 10 wt.% addition of boron carbide. The samples are heat treated at different temperatures, namely at 650, 700, 750, and 800 degree C for 30 min. The phases and lattice parameters of the samples are studied using X-ray Diffraction (XRD). The amount of B4C does not decrease from the initial amount with increasing heat treatment temperature (Tht) but instead increases. This indicates that some of the Mg from the precursor reacts with the Nb sheath. Magnetic Jc(B) characteristics are obtained at several temperatures in external B in a Vibrating Sample Magnetometer (VSM). The magnetic and transport Jc(B) results disagree with each other which can be explained by unsuitability of Nb as the sheath with higher Tht. From these results, also pinning force densities (Fp) are calculated. No effect on pinning mechanism of B4C added samples is observed by the heat treatment conditions. Resistance dependencies on temperature are also measured in a Physical Properties Measurement System (PPMS). Mg deficiency with high Tht is supported by these measurements.