Abstract
The engineering of fine Y2Ba1Cu1O5 (Y-211) inclusions of average particle size 1−2μm within the continuous, superconducting YBa2Cu3O7−δ (Y-123) phase matrix of single-grain, bulk high temperature Y–Ba–Cu–O (YBCO) superconductors is fundamental to achieving high critical current density in these materials. However, bulk samples fabricated using fine Y-211 precursor powders generally suffer from incomplete growth of the Y-123 phase compared to samples fabricated with coarser Y-211 particles of average particle size >5μm in the precursor powder. In this study, the effects of Y-211 particle size on processing parameters during growth of large single YBCO grains are reported. Chemical analysis using EDX of cross-sections of single YBCO grains indicates that the loss of liquid phase during melt processing is greater when finer Y-211 precursor powder is employed, which accounts for the observed growth limitations and low sample yield. Specific modifications to the single grain growth process have been made based on the results of this investigation to reduce liquid loss and to enable the use of fine Y-211 precursor powder without compromising complete single grain growth or yield.
Highlights
Bulk high temperature superconductors (HTS) can support trapped fields that are much greater than the maximum field generated by a permanent magnet
A deficiency in liquid phase, determines the maximum grain size, since the depletion of this phase ends the peritectic recombination reaction and terminates the crystal growth process. These results suggest that samples containing finer Y-211 precursor powders experience more severe problems with liquid phase deficiency under the same conditions as samples containing precursor powder containing a larger Y-211 particle size
We have confirmed that the use of fine Y-211 precursor powders improves the superconducting properties of bulk YBCO samples fabricated by top seeded melt growth, as well as leading to faster crystal growth
Summary
Bulk high temperature superconductors (HTS) can support trapped fields that are much greater than the maximum field generated by a permanent magnet. To achieve good superconducting properties, the resultant bulk samples must consist of a single, well-oriented grain since the presence of grain boundaries interrupts the flow of super current [8,9,10,11,12] and limit the field trapping ability of the sample This is achieved by the use of a small seed crystal of a material of similar crystal structure to the Y-123 phase and a higher melting temperature (such as a Sm–Ba–Cu–O or NdBCO single grain). Improving the success rate of single grain growth rate is important for the production of bulk samples in general, for the reduction of wastage, for the development of a batch process and for reduced fabrication costs, all of which are essential for the production of bulk single grain superconductors for practical applications
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