Optimizing critical current density through composition and microstructure in mechanically alloyed magnesium diboride

Carbon doped MgB2 is an emerging superconductor with potential for operation in the 0-10 Tesla range at 4.2 K or 3-4 T up to ∼25 K. In order to be a viable conductor option, mid to high-field Jc(H) must be improved from Jc(8T,4.2K) ∼ 3x10 4 A/cm2 typical of conductors made by in-situ powder in tube reaction. Jc(H) is controlled by H* (a function of Hc2 and flux pinning), flux pinning itself (largely a function of grain size), and connectivity. Current in-situ wires are limited by a trade-off between connectivity and grain size because aggressive sintering heat treatments grow grains. In order to escape this dilemma, this work examines bulk C-doped MgB2 made by sintering pre-reacted powder. An engineering approach was adopted, studying the effect of processing parameters on our primary metric, Jc(H). In this work we used high energy ball milling to simultaneously do the following: (1) Alloy MgB2 with C, (2) refine grains, (3) break up oxide sintering barriers on particle surfaces, and (4) disperse second phases on a fine scale. By this method we obtained extremely fine 20-30 nm grains even after heat treatment at 1000°C---probably due to dispersed second phases retarding grain growth. Heat treatment optimization revealed a temperature window between 900°C and 1000°C (depending on composition and milling time) which was sufficiently hot for sintering, but did not result in excessive grain growth. In this way (combined with hot isostatic pressing) we were able to repeatedly obtain Jc(8T,4.2K) = 7x10 4 A/cm2 or higher---within a factor of 2 of optimized NbTi. An additional benefit of this work is the discovery that C-solubility in MgB2 is a strong function of T for T < 1150°C, which could open the door for further processing strategies.