Influence of Mg/B ratio and SiC doping on microstructure and high field transport Jc in MgB2 strands

Improvements in Bc2, Birr, and high field transport Jcs are crucial for many MgB2 applications. As transport Jc is inextricably linked with microstructure, a knowledge of the detailed influence of two of the most practical variables – Mg/B ratios and SiC additions – on microstructure, Bc2/Birr, and high field Jc, is of interest. In this work, the influence of Mg/B ratios on microstructure, Birr, and transport Jc was investigated in MgB2 strands with and without SiC doping. The binary compositions chosen were MgxB2 where x = 0.85, 0.90, 1.0, 1.10, and 1.15 in monofilamentary strands. In general, high Mg molar percentages were seen to increase transport Jc, especially at higher fields. At 8 T, for example, heat treatments at 700 °C for 40 min yielded Jcs at 4.2 K which ranged from 5.8 × 103 A/cm2 for x = 0.85 to 2.73 × 104 A/cm2 at x = 1.15. Microstructures evolved from powdery (B-rich) to web-like (Mg-rich). In a second set of samples, Mg-rich stoichiometries were investigated with and without SiC. High field transport Jcs increased, and the microstructure appeared to densify. Both excess Mg and SiC doping significantly increased high field Jc. The highest transport result was seen for SiC dopant (5 mol%) added to 15% excess Mg strands, which yielded a transport Jc at 4.2 K and 8 T of 5 × 104 A/cm2. Variable temperature analysis was performed on the 15% excess Mg strands with and without SiC in fields of 0–15 T. At temperatures below 20 K, excess Mg + SiC doping performed best, while above 20 K, excess Mg without SiC had better transport properties.