Abstract
The wetting of boron with liquid magnesium is a critical factor in the synthesis of MgB2 bulk superconductors by the infiltration and growth (IG) process. Poor wetting characteristics can therefore result potentially in non-uniform infiltration, formation of defects in the final sample structure and poor structural homogeneity throughout the bulk material. Here we report the fabrication of near-net-shaped MgB2 bulk superconductors by a modified precursor infiltration and growth (MPIG) technique. A homogeneous bulk microstructure has subsequently been achieved via the uniform infiltration of Mg liquid by enriching pre-reacted MgB2 powder within the green precursor pellet as a wetting enhancer, leading to relatively little variation in superconducting properties across the entire bulk sample. Almost identical values of trapped magnetic field of 2.12 T have been measured at 5 K at both the top and bottom surfaces of a sample fabricated by the MPIG process, confirming the uniformity of the bulk microstructure. A maximum trapped field of 3 T has been measured at 5 K at the centre of a stack of two bulk MgB2 samples fabricated using this technique. A steady rise in trapped field was observed for this material with decreasing temperature down to 5 K without the occurrence of flux avalanches and with a relatively low field decay rate (1.5%/d). These properties are attributed to the presence of a fine distribution of residual Mg within the bulk microstructure generated by the MPIG processing technique.
Highlights
The ability of a type II bulk superconductor to trap magnetic field is a direct indication of the presence of strong flux pinning, and high critical current density, Jc, within the sample microstructure
Near-net shaped MgB2 bulk superconductors have been fabricated by a modified precursor infiltration and growth (MPIG) method, incorporating MgB2 powder in the precursor as wetting enhancer to facilitate in-flux of Mg, leading to a more uniform infiltration process
Almost identical value of trapped magnetic flux was measured at the top and bottom surfaces of a single bulk sample suggest that MgB2 bulk samples fabricated by MPIG method are homogeneous
Summary
The ability of a type II bulk superconductor to trap magnetic field is a direct indication of the presence of strong flux pinning, and high critical current density, Jc, within the sample microstructure. That the presence of residual Mg, especially in the form of continuous channels, may have an adverse effect on the trapped field performance of MgB2 bulk superconductors fabricated by an IG process. The resulting spatial variation in superconducting properties of bulk samples fabricated by this technique and the trapped fields achieved are reported.
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