Effect of addition of planetary milled Gd-211 on the microstructures and superconducting properties of air-processed single grain Gd-Ba-Cu-O/Ag bulk superconductors

摘要

Planetary milling technique has been a very promising way to obtain bulk superconductors with very high critical current density, J_c, albeit a detail characterisation of milled secondary phase precursor powders in particular has not been reported to date. Hence we report systematic studies of the effect of addition of planetary milled Gd_2BaCuO_5 (Gd-211) on the final microstructures and superconducting properties of air-processed Gd-Ba-Cu-O/Ag bulk samples. Average size of Gd-211 precursor particles, which were planetary milled with 1.0 mm ZrO_2 beads, has been observed to decrease significantly from 1.03 μm to 0.52 μm with increasing milling duration. Besides the size distribution of milled Gd-211 was narrow compared to that of the reference powder. A small amount of Zr was detected, however, in the milled Gd-211 powder by the inductively coupled plasma-optical emission spectroscopy (ICP-OES) and its content was increased with increasing milling period, which led to an inhomogeneous bulk microstructure. Significantly, the average size of Gd-211 particles milled for 45 min has been observed to decrease from 0.73 μm to 0.48 μm without severe contamination of Zr when the diameter of the beads were reduced from 1.0 mm to 0.3 mm. Trapped magnetic field of single grain Gd-Ba-Cu-O/Ag bulk sample with 32 mm in diameter prepared from almost Zr free Gd-211 fine particles recorded over 1.5 T at 77 K, which was almost 1.3 times greater than that of the reference sample. Nevertheless the repulsive force of both bulk samples showed around 57 N at a gap of zero between the sample surface and SmCo_5 permanent magnet.