Correlation between microstructure and superconducting properties of MgB2 bulk samples with Mg addition and Mg/hBN co-additions

The microstructure of polycrystalline MgB2 has a strong influence on the current carrying ability, with grain boundaries and non-superconducting nanoparticles acting as good flux pinning centres which improve the local (intrinsic) critical current density (J (c)) of the material, whereas porosity and poor connectivity between grains or particles adversely affect macroscopic current transport. Previous studies have found that hexagonal boron nitride (hBN) doping improves intrinsic J (c) by introducing nanoscale flux pinning centres, and Mg doping improves extrinsic J (c) by liquid-assisted sintering. Here we investigate the effect of co-doping with 5 wt.% Mg and 1 wt.% hBN with the aim of combining the improved intrinsic and extrinsic properties in bulk MgB2 samples fabricated using field assisted sintering. Additionally, the influence of ball milling and processing temperatures on MgB2 samples with only Mg additions is reported. By correlating microstructure with superconducting properties, we show that the presence of Mg liquid during processing of Mg-doped samples accelerates the reaction between BN and MgB2, forming an impurity phase, MgNB9, the presence of which is detrimental to superconducting performance. Nevertheless, we have achieved a considerable improvement in performance of samples doped only with Mg by increasing the sintering temperature.

Automated summary

The microstructure of polycrystalline MgB2 significantly affects its current carrying ability, with grain boundaries and non-superconducting nanoparticles acting as good flux pinning centers. Previous studies have shown that hBN doping improves intrinsic critical current density (Jc) and Mg doping improves extrinsic Jc. In this study, the co-doping effect of 5 wt.% Mg and 1 wt.% hBN on bulk MgB2 samples fabricated using field assisted sintering is investigated. Furthermore, the influence of ball milling and processing temperatures on Mg-doped samples is also reported. The presence of Mg liquid during processing accelerates the reaction between BN and MgB2, forming an impurity phase, MgNB9, which adversely affects superconducting performance. However, a significant improvement in performance is achieved by increasing the sintering temperature in samples doped only with Mg.