The processing and properties of single grain Y–Ba–Cu–O fabricated from graded precursor powders

Abstract

The preparation of single grain, Y-Ba-Cu-O (YBCO) bulk superconductors by top-seeded melt-growth (TSMG) usually involves precursor powders that contain a uniform distribution of the constituent YBa 2 Cu 3 O 7−δ (Y-123) and Y 2 BaCuO 5 (Y-211) phase compounds. However, it has been observed that the concentration of Y-211 particles in the fully melt processed superconducting bulk increases significantly with distance from the seed, which results in a degradation of superconducting properties towards the edge and bottom of the sample. Here we investigate the effect of preparing bulk YBCO superconductors by TSMG using spatially graded Y-211/Y-123 precursor powder. The graded precursor bulks were prepared with a maximum composition of 40 wt% Y-211 in the vicinity of the seed, which decreased to 30 wt% and then 20 wt% towards the bottom and edge of the green body. Standard samples were melt processed from precursor powders containing 30 wt% Y-211 to enable comparison. The field trapping ability, T c and J c , of three graded and two standard samples were investigated and compared statistically. The distribution of Y-211 particles along different growth directions of the samples was analysed, and any crystallographic misorientation was investigated. The observed distribution of Y-211 particles in YBCO is explained qualitatively by trapping/pushing theory, and its correlation with the superconducting properties of the melt processed bulk samples has been analysed. Finally, the practical feasibility of the graded technique is evaluated.

Effect of Y-211 particle size on the growth of single grain Y–Ba–Cu–O bulk superconductors

“…These sample types are referred as "NN", "NT", "TN" and "TT", where the first and second letters represent the manufacturers of the Y-123 and Y-211 precursor powders, respectively. All powder mixtures consisted, by weight, of 70 wt% Y-123 and 30 wt% Y-211 and an excess of 1.0 wt% cerium oxide (purity 99.9%, supplied by Alfa Aesar), CeO 2 , which inhibits the coarsening of Y-211 particles during melt processing and limits their size in the fully processed sample microstructure [20][21][22][23][24].…”

“…Compositional measurements were performed over cross-sections located approximately 1 mm under the site of the seed, and then in regular, horizontal steps towards the edge of the sample. The observed variation in atomic % of the Y:Ba ratio was used to estimate the variation of the proportion of Y-211 particles by weight as a function of sample position, which can be justified based on the chemical content of the phases present in the peritectic reaction [20]. The aim of this analysis was to determine the compositional changes that occur during crystal growth in samples without sub-grains and for a variety of single grain sizes.…”

“…The variation of Y-211 in the sample (estimated from the variation of the ratio of Y:Ba atomic % [20]) indicates that the amount of Y-211 within the single grain increases with increasing distance from the seed. This trend in distribution of Y-211 particles is consistent with particle pushing/trapping theory for YBCO, in which Y-211 inclusions are modelled as inactive particles in the melt [4].…”

Processing and Properties of Bar-Shaped Single-Seeded and Multi-Seeded YBCO Bulk Superconductors by a Top-Seeded Melt Growth Technique

“…The growth rate of the Y-123 phase is of the order of 0.2-0.4 mm/h [12,13], which limits practically the ultimate size of the final single grain. Additionally, it has been reported previously that large samples exhibit increasing crystallographic misorientation of the Y-123 phase with increasing distance from the seed [14]. As a result, it is not easy to fabricate (RE)BCO single grains reliably in sizes of diameter larger than ∼ 40 mm.…”

“…This is an unavoidable effect, which occurs due to particle pushing effects associated with the YBa 2 Cu 3 O 7−x (Y-123) growth front. Recently, composition grading techniques were employed to minimize inhomogeneities in the Y-211 concentration throughout the volume of the sample, thereby achieving greater uniformity in its superconducting properties [9,10,14].…”