There is little consensus in the literature on the processing requirements to produce high quality magnesium diboride (MgB_2) material with low impurity content and high density. We have carried out a range of processing procedures to establish key parameters for ex situ production of dense MgB_2 by examining the effect of modifying the characteristics of the starting powder and the processing parameters during heat-treatment. MgB_2 bulk material was produced from as-purchased and modified powders by pressureless heat-treatment, under Ar gas and pressure-assisted methods (spark plasma sintering or hot pressing) under vacuum or Ar gas. Porosity, impurity content, hardness, grain size and bulk superconducting properties were measured. Our results indicate that densification and applied pressure are strongly correlated, while the effect of temperature - is less significant. The optimum processing environment (inert gas or vacuum) was dependent on the technique used. These results indicate that pressure-assisted heat-treatment is required in order to produce dense bulk MgB_2. A positive correlation between magnetization critical current density and bulk density was observed for magnesium diboride bulks of up to around 90% density. Above this level, other microstructural processes such as grain growth begin to influence the critical current density, suggesting that full elimination of porosity is not necessary to obtain good superconducting performance.
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