Elimination of oxygenation cracks in top-seeded melt-growth YBCO superconductors by high pressure oxygenation

Abstract

As-grown YBa 2 Cu 3 O 7 bulk superconductors have low oxygen content, YBa 2 Cu 3 O 6.3 , are not superconducting, and must be oxygenated to form YBa 2 Cu 3 O 7 . During standard oxygenation at 400 • C the shortening of crystal lattice parameters causes intensive cracking. The created cracks allow oxygen penetration into the bulk and cause the oxygenation time to be technologically acceptable but reduce significantly the superconducting properties. Here we show that it is possible to eliminate the formation of oxygenation cracks and to reach a critical current density 2.5 times higher than in bulk oxygenated in a standard way. The oxygenated crack-free samples were obtained by high pressure oxygenation with progressively increasing oxygen partial pressure.

Critical Current Density and Microstructure of NdBa2Cu3O7Single Crystal

“…Measurement of these parameters on the micrographs from the etched {001} surface estimated mean values: l = 5.2 µm and d = 9.8 µm. Then, l/d = 0.53 which is the value close to what was measured in TSMG YBCO bulk superconductor [8]. This value points out that intrinsic value of critical current density should be three times higher than the volume critical current density determined estimated from magnetisation measurements.…”

The influence of post-growth thermal treatments on the critical current density of TSMG YBCO bulk superconductors

“…At the same time, the oxygen gap between the surface oxygenated layer and the core tetragonal phase must be kept lower than the critical one for crack formation. Oxygenation at 750 • C and the pressure from 160 bars of small samples cut from the TSMG YBCO bulk confirmed the possibility of eliminating oxygenation cracks and the expected significant increasing of J c [9,10] (figure 4). As the critical oxygen gap is lower for a/b-crack formation than for a/c-crack formation and the a/bmicrocracking is influenced by the size of 211 particles, we may obtain six different cracking microstructures depending on the oxygenation parameters and the size of 211 particles with related reduced effective sample cross-section as expressed in figure 5.…”

“…The critical current density at 77 K in the BSS sample YBa 2 (Cu 1−x Ag x )O 7 (x = 0.005) which is higher after high pressure oxygenation than after standard oxygenation[9].…”

Time Dependent Changes in Ag Doped YBCO Superconductors

“…The explanation of this behaviour was given by detailed observation [7] which revealed inhomogeneous distribution of Ag in the single-grain bulk samples caused by the low Ag partition coefficient at solidification. Significant improvement of pinning properties of Ag-doped YBCO bulk superconductors can be reached by HPO [8] (Fig. 2b) which eliminates the formation of oxygenation cracks and increases the effective cross-section of the samples [9].…”