M. Šefčiková

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

As-grown YBa2Cu3O7 bulk superconductors have low oxygen content, YBa2Cu3O6.3, are not superconducting, and must be oxygenated to form YBa2Cu3O7. 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.

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

Oxygenation and thermochemical post-growth treatments of top seeded melt-growth (TSMG) YBCO bulk superconductors can significantly influence critical current density. It is shown that, depending on oxygenation conditions and the size of 211 particles, different reductions of intrinsic critical current density values can be obtained due to the reduction in the sample cross-section caused by the presence of a/b-microcracks induced by 211 particles, and a/b-?and a/c-cracks induced by oxygenation. The possibility of eliminating oxygenation cracks by high pressure oxygenation and consequently significantly increasing the macroscopic critical current density is demonstrated. An effective dopant concentration for chemical pinning is proposed and possible clustering of substitutions in the Y123 lattice by thermochemical treatments is shown.

Y211 particle refinement in YBCO bulk superconductors by CeO 2 addition

The influence of preparation parameters as nominal composition (Y 2 O 3 and CeO 2 additions), particle size of starting Y123 powder, milling and mixing conditions and sintering temperature on the refinement of Y 2 BaCuO 5 (Y211) particles in the single-grain YBa 2 Cu 3 O 7 /Y 2 BaCuO 5 (Y123/Y211) bulk superconductors prepared by top-seeded melt-growth process has been studied. The microstructure was analysed after sintering and melt processing by polarised light microscopy and electron microscopy. It is shown that cerium is active in the Y211 particle refinement when it is solved in the peritectic melt, L p . When cerium is present as barium cerate, it has no refining effect. The addition of CeO 2 and barium cerate formation even leads to a faster growth of Y211 particles at the sintering stage due to intensive melt production. Refinement of Y211 particles in the samples with large Y123 particles is related to the formation of very dense Y211 skeleton which is resistant to melt formation at the sintering stage. Critical current density of these single-grain bulk superconductors is significantly influenced by final microstructure.

The Influence of Y211 Milling Conditions on the Microstructure of Y123/Y211 Bulk Superconductors

The YBCO bulk single-grain superconductors were prepared by Top-Seeded Melt-Growth (TSMG) process. Y2BaCuO5 (Y211) fine powder was added to YBa2Cu3O7-δ (Y123) nominal composition. Powder refinement was characterized by X-ray powder difractometry, laser granulometry and scanning electron microscopy. The system behaviour was characterized by thermal analysis. Microstructure of prepared samples was studied by polarized light microscopy. The influence of Y211 addition the sample microstructures are shown.

Elimination of Oxygenation Cracking in YBCO Bulk Superconductors

The reasons for cracking of YBa2Cu3Oδ/Y2BaCuO5 (Y123/Y211 or YBCO) bulk single-grain superconductors are mechanical stresses, which arise in the sample during its fabrication. Two main sources of stresses appearing during fabrication were identified: the different thermal expansion coefficients of 123 and 211 phases and the dependence of 123 phase lattice parameters on the oxygen stoichiometry. As-grown YBCO bulks have low oxygen content, YBa2Cu3O6.3, are not superconducting, and must be oxygenated to form YBa2Cu3O7. 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 material bulk oxygenated in a standard way. The oxygenated crack-free samples were obtained by high pressure oxygenation with progressively increasing oxygen partial pressure.

Studies of cracking behavior in melt-processed YBCO bulk superconductors

An important phenomenon in bulk superconductors fabricated by top-seeded-melt growth (TSMG) is the formation of cracks due to the inherent brittleness of the YBa2Cu3O7-δ (Y-123) phase matrix. These form during the fabrication of the superconducting monolith and play an important role in the limitation of current flow. However, cracks may also form during cooling cycles of the sample to liquid nitrogen temperatures. In this investigation, macrocracks along the c-direction, in particular were analyzed microscopically before and after cooling. In addition we attempt to resolve the c-axis macrocrack formation pattern using the magnetoscan technique.