| Summary: | <p>The microstructure of polycrystalline MgB<sub>2</sub> 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 (<em>J</em><sub>c</sub>) 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 <em>J</em><sub>c</sub> by introducing nanoscale flux pinning centres, and Mg doping improves extrinsic <em>J</em><sub>c</sub> 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 MgB<sub>2</sub> samples fabricated using field assisted sintering. Additionally, the influence of ball milling and processing temperatures on MgB<sub>2</sub> 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 MgB<sub>2</sub>, forming an impurity phase, MgNB<sub>9</sub>, 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.</p>
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