Processing of MgB2 bulk superconductor by infiltration and growth

Abstract

Superconductivity in magnesium diboride (MgB2) was discovered in 2001. The relatively high Tc (39 K), high critical current density, long coherence length (∼6 nm), low raw material cost, lower density and relative ease of fabrication make this material an exciting choice for practical applications. Furthermore, lower anisotropy and strongly linked current flow in untextured polycrystalline samples, unlike its HTS counterparts, has enabled the development of different processing routes to fabricate MgB2 in the form of wires, tapes, thin films and bulks. Conventionally, MgB2 is synthesized by in situ sintering, where elemental Mg and B powders are reacted to produce MgB2. Although the superconducting phase can be obtained with relative ease, the resulting sample is generally only around 50% dense, due to formation of large pores inside sintered bulks arising from the volatility of magnesium and 25% volume contraction in MgB2 phase formation. Although the use of high pressure is effective to promote sintering and subsequent densification, the need to use large pressure vessels represents a significant practical limitation for the development of a practical process and of the achievable dimensions in the final MgB2 sample. As a result, the fabrication of high density, bulk MgB2 remains a challenging processing problem. This study explores the “Infiltration and Growth” (IG) technique, an established processing route for fabrication of dense ceramics/ceramic matrix composites, as a potential solution. Boron powders of varying characteristics were infiltrated with Mg(l) to obtain bulk MgB2 samples. The samples were analysed using techniques such as XRD, SEM and hardness to analyse various phases formed during the process. These samples typically contained MgB2 with minor quantities of Mg. Physical properties of superconducting MgB2, such as Tc, Jc and Hc2, were established. Furthermore, the effective current carrying cross-section was estimated from resistivity measurements using Rowel’s analysis. Continuous Mg channels were major defects in IG processed samples and their presence was found to limit long range current flow. These channels are eliminated by incorporating Mg/AlB2/MgB2 powders in the precursor to facilitate in-flux of Mg, leading to a more uniform infiltration process, thereby enabling fabrication of near-net shaped MgB2 bulk superconductors. Such samples showed an almost identical value of trapped magnetic flux at the top and bottom surfaces, suggesting a high degree of uniformity in MgB2. A careful microstructural analysis of a series of samples indicated that MgB2 phase formation in IG process occurred in three distinct stages: (1) Intermediate boride formation (2) Bulk liquid Mg infiltration and (3) MgB2 layer formation. Due to volume expansion involved in stage 1, cracks formed in the β-Boron particles and propagated radially inwards during stage 3. The growing MgB2 particles sintered simultaneously with the formation of grain boundaries during the process. Much enhanced performance of MgB2 was achieved by virtue of C-doping. Increased Jc was attributed to generation of lattice strains and loss of crystallinity in MgB2 as a result of C-doping. Finally, trapped field measurements were performed on homogeneous C-doped MgB2 bulks. The trapped field obtained (4.13 T) in five stacked of bulks is the highest obtained in MgB2 bulks synthesized under ambient pressure conditions.