Influence of the seed distance on the microstructure and the superconducting properties of grain boundaries in a multi-seeded melt growth monolith

The influence of the seed distance (d(s)) on the properties of grain boundaries (GBs) has been studied in order to obtain a precipitate-free GB. On the top of one monolith, two distances between two seeds were chosen, i.e. 4 and 11 mm. For d(s) similar or equal to 11 mm, some copper-rich phases and large pores can be observed throughout the GB. In contrast, for d(s) similar or equal to 4 mm, neither residual copper-rich phases nor pores are observed within the GB. The growth direction for the two samples seems to be parallel or nearly parallel to the GB. It reveals that the absence of misorientation between the two domains cannot prevent the formation of a residual copper phase layer throughout the GB; only a shorter seed distance will do this. However, element-mapping measurements revealed the occurrence of macrosegregation of the Y-211 particles on top of the monolith and centred on the GB. This tends to vanish towards the bottom of the sample and only a thin layer remains. Magneto-optical measurements performed on the sample with d(s) similar or equal to 4 mm, revealed that the critical current density through the GB differs according to the growth directions.