V. Antal

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.

Thermal stability of NdBCO/YBCO/MgO thin film seeds

Thermal stability of the Nd1+x Ba2−x Cu3O7−δ (Nd-123 or NdBCO) thin films deposited on MgO substrate, with YBa2Cu3O7−δ (Y-123 or YBCO) buffer layer (NdBCO/YBCO/MgO thin film), has been experimentally studied in order to determine the optimal film thickness acting as seed for bulk YBCO growth. YBCO bulk superconductors with Y2BaCuO5 (Y-211) and CeO2 addition were prepared by the top seeded melt growth process in a chamber furnace using NdBCO/YBCO/MgO thin film seeds of different thicknesses (200–700 nm with 20 nm YBCO buffer layer) and different maximum temperatures, T max. The maximum temperatures varied in the range of 1040 °C–1125 °C. The highest thermal stability 1118 °C was observed in the case of NdBCO/YBCO/MgO thin film of 300 nm thickness. These results are corroborated with differential scanning calorimetry and high temperature x-ray diffraction measurements, as well as microstructure observations.

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.

Microstructure and Superconducting Properties of YBCO Bulk Superconductors With RE Substitutions

The influence of RE (RE = Yb, Sm) substitutions on the microstructure and superconducting properties of Y-Ba-Cu-O (YBCO) bulk superconductors has been studied for different RE concentrations and their combinations. Precursor powders YBa₂Cu₃O_7-δ (Y-123), Y₂O₃ and CeO₂ were enriched with different amounts of Yb₂O₃ and (or) Sm₂O₃ powders with the aim to increase critical current density, i.e., Jc, in self-field as well as in higher magnetic fields, by introducing additional pinning centers. YBCO bulk superconductors with RE₂O₃ addition were prepared by the optimized top-seeded melt-growth process in the form of single grains. Microstructure analysis revealed that RE₂O₃ addition leads to a higher amount of smaller Y₂BaCuO₅ (Y-211) particles, which is related to high critical current density (J_c ~ 7 × 10⁴ A/cm²) of the samples with RE₂O₃ addition in low magnetic fields. In this paper, the effect of RE₂O₃ addition on Y-211 particle size, critical temperature T_c, and critical current density J_c is reported.

The Influence of Sintering Conditions on Microstructure of La-Ca-Sr-Mn-O Magnetocaloric Ceramics

A La0.67Ca0.33-xSrxMnO3 (x=0.33; 0.03; 0) perovskite magnetocaloric powders were prepared by solid state synthesis in air. Phase transformations were characterized by thermal analyses and the phase composition was confirmed by X-ray powder diffraction. Pressed pellets were sintered at different temperatures. Microstructure analyses were done by polarized light microscopy and scanning electron microscopy. The influence of sintering conditions on porosity, grain size and content of secondary phases is shown.

YBCO Bulk Superconductors with Sm Addition

The influence of Sm addition on the microstructure and superconducting properties of Y-Ba-Cu-O (YBCO) bulk superconductors has been studied. Precursor powders YBa2Cu3O7-δ (Y-123), Y2O3 and CeO2 were enriched with different amounts of SmBa2Cu3Oy (Sm-123) or Sm2O3 powders with the aim to increase critical current density, Jc,by introducing additional pinning centers. YBCO bulk superconductors with SmBa2Cu3Oy (Y123-Sm) or Sm2O3 (Y123-SmO) powder addition were prepared by the optimized top seeded melt growth process in the form of single grains. Microstructure analysis revealed that Sm2O3 addition leads to a higher amount of smaller Y2BaCuO5 (Y-211) particles, what is related to high critical current densities (Jc ~ 7 x 104 A/cm2) of the YBCO samples with Sm2O3 addition in low magnetic fields. The effect of Sm addition in the form of SmBa2Cu3Oy as well as Sm2O3 powder on Y2BaCuO5 particle size, critical temperature, Tc, and critical current density, Jc, is reported.

Growth Parameters in Y123-Y211-CeO 2 System

Conditions for epitaxial growth of single-grain YBCO bulk by TSMG process were tested in the YBa₂Cu₃O₇- Y₂BaCuO₅ system with CeO₂ addition. The obtained results define the temperature window of isothermal single-grain growth. The measurements of crystal growth show that the growth rate decreases with the distance from the seed. Observed behavior is explained by decreasing peritectic temperature in the melted rest of the sample due to the segregation of CuO, which appears in the system as a result of the reaction of the Y123 phase with CeO₂.

Influence of BaCeO3 and BaO2 Addition on the Microstructure of Y-123/Y-211 Bulk Superconductors

YBCO (YBa2Cu3O7/ Y2BaCuO5) bulk single-grain superconductors were prepared by Top-Seeded Melt-Growth (TSMG) process. CeO2, BaCeO3 and BaO2 powders were added to nominal composition Y1.5Ba2Cu3Oy (Y-123) with the aim to refine Y2BaCuO5 (Y-211) secondary particles, which provide pining. Added powders were refined by friction milling. The system behaviour was characterized by thermal analyses. Microstructure of prepared samples was studied by polarized light microscopy and the influence of additions is described.

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.