Inward unidirectional Cu diffusion into a pellet and the resultant alloying of Cu–Mg during in situ reaction of MgB2 were studied. The reaction between Cu and Mg occurred at temperatures as low as 660C in a flowing Ar atmosphere giving rise to the formation of a MgCu2 layer at the interface between the Cu and the MgB2. The kinetics of Cu alloying in the core sample was much enhanced at 800C and above, as shown by energy dispersive X-ray analysis. Consequently, JC was degraded drastically due to the decrease in the superconducting volume fraction. However, the alloying effect did not lower the TC onset, but broadened the transition. A sharp TC transition and JC (6K, 4T)~2 x 104A/cm2 was achieved when annealing was conducted at the lower temperature of 750C. Direct comparison between the pure and Cu-alloyed samples shows that optimization of the heat treatment and hence JC rests on the competing effects of phase formation and grain connectivity versus MgCu2 formation.

The Influence of Unidirectional Copper Alloying on the Critical Current Density of MgB2. S.K.Chen, B.A.Glowacki: Superconductor Science and Technology, 2006, 19, 116-21