Y.Y. Luo

Energy dispersive X-ray spectroscopy analysis of YBa2Cu3−xSnxO7−δ superconductors fabricated by melt-texture-growth method

Abstract Making use of the non-zero spatial temperature gradient at certain locations of a common tube furnace, we have fabricated bulk rectangular samples with nominal compositions YBa 2 Cu 3− x Sn x O 7−δ with highly oriented single crystal-like layers. The temperature range of 1028-930°C is believed to be a crucial period during which single crystal-like layers are formed in the MTG process and our report is based on samples prepared with a cooling rate of 5°C/h in the stated range. We have also carried out detailed energy dispersive X-ray spectroscopy analysis along a number of crystal layers, obtaining atomic percentages along these layers. With simple, plausible assumptions, we have found the phase amounts along the crystal layers. The process of analysis is repeated for the loose grains occurring in between the layers. Generally speaking, the amount of Y-123 present in the layers is around 50%-75%. The DC magnetization loops of these samples were also measured giving us very encouraging J c values of about 3.0 × 10 4 A cm −2 .

FTIR Spectra of the M(EDTA)n− Complexes in the Process of Sol-Gel Technique

Sol-gel is one of the effective techniques for preparing YBa2Cu3O7 − δ (YBCO) coatings at relatively low cost. Its process from gel to oxide coatings, however, was not yet fully understood. This is one of the obstacles for achieving YBCO coatings with good superconducting properties. In this paper, we chose EDTA as the complexant to prepare gels with different kinds of M(EDTA)n− complexes, where M is any combination of Y, Ba, and Cu cations, in order to investigate the decomposition process of the gels with the help of Fourier transform infrared (FTIR) spectra. Comparing the FTIR spectra of the gel powders with those of the calcined powders, we found that the decomposed temperatures were different for different M(EDTA)n− complexes. We considered this was the cause of the Ba segregation (as BaCO3) in the coatings at low heating temperature, which in turn blocked the formation of high quality YBa2Cu3O7−δ.

Microdefects and macroscopic characterization of MTG YBa2Cu3−xSnxO7−y samples using Y2Sn2O7

Abstract We have used Y 2 Sn 2 O 7 powders in preparing the Sn-doped YBCO sample materials in order to supply oxygen internally during the crystal growth process. The better samples, which show high proportions of microdefects, allow bulk critical current > 3 × 10 4 A cm −2 . We analyze the microdefects on a scale from 30 μm – 100 A of the melt-textured growth (MTG) samples with our SEM and 200 kV TEM. Clear bright and dark interference lines of thickness ≈ 3 A appear in our micrographs. These quasi-parallel lines, which represent structural features of atomic layers, indicate that many dislocations, twin boundaries and stacking faults appear in the MTG sample. Atomic layers appear to “add in” the Y-123 crystal growth.

Effects of Precursors with Fine BaZrO3 Inclusions on the Growth and Microstructure of Textured YBCO

We prepared precursors with fine BaZrO3 inclusions for melt-texture growth of high-temperature superconductors with nominal composition of Y1.5Ba2Cu3Oy + x BaZrO3 (x = 0, 0.04, 0.08, and 0.1), by the process of liquid-phase powder melt-texture growth (LPMG). We then investigated the effects of the precursors on the texture growth and the microstructure of composite YBa2Cu3O7-δ/Y2BaCuO5 (YBCO). The BaZrO3 particles were about 100 nm in size in the textured YBa2Cu3O7-δ (Y123), observed by the scanning electron microscopy (SEM). Owing to the pushing effects of growing fronts, fine particles were pushed out of and thus gathered in front of the growing fronts or between Y123 grains. The gathered particles in turn blocked the further growth of the Y123 grains. However, BaZrO3 particles seeded Y123 grains, giving rise to multigrain growths. These negative effects of BaZrO3 fine particles exclude themselves as effective flux pinning centers in the textured YBCO.

Y2Ba5(Sn3−y−zCuyPtz)Ox in textured YBCO superconductors

Abstract Ultrafine SnO 2 powder was made with grain size down to 20 nm. Such powder was introduced into the textured growth of Y 1+ x Ba 2 Cu 3 O 7+ δ (YBCO) superconductors. Crystalline inclusions, which were cubic in shape and from micrometer to submicrometer in size, were found trapped inside such grown YBCO superconductors. Determined by energy dispersive spectroscopy (EDS), and further confirmed by X-ray diffraction, the composition of the inclusions was Y 2 Ba 5 (Sn 3− y − z Cu y Pt z )O x . This Sn-based phase belongs to double perovskite structures. They were not trapped uniformly inside the YBa 2 Cu 3 O 7− δ (123), possibly caused by the agglomeration of the SnO 2 powder at the very beginning. Nevertheless, it is possible for fine Sn-based inclusions down to a few hundred nanometers to be trapped inside the 123. The addition of the ultrafine SnO 2 to YBCO textured superconductors without Pt did not seem to affect the size of Y 2 BaCuO 5 (211) particles. For comparison, Y 2 Sn 2 O 7 was also introduced to YBCO textured superconductor. In this case, the so-called φ phase YBa 3 Sn 2− x Cu x O 9− δ was found.

Composition analysis of Ca-doped YBa2Cu3O7−δ/Y2BaCuO5 prepared by the top-seeding melt-texture growth

The Ca-doped YBa2Cu3O7−δ/Y2BaCuO5 bulk-textured superconductors were prepared by using the top-seeding melt-textured growth process. Composition analyses were made for such prepared samples. We found that Ca concentration in the YBa2Cu3O7−δ grains decreased along the growing direction; Ca preferred to substitute Y in the YBa2Cu3O7−δ grains, rather than to stay at the grain boundary. The accumulation of extra CuO at the growth front was considered as one of the causes of the inhomogeneous distribution of Ca.

Investigation of the off-diagonal thermoelectric effect on textured YBa2Cu3O7-δ

Both the off-diagonal Seebeck effect and the off-diagonal Peltier effect were investigated on the same textured sample YBa2Cu3O7−δ and with the use of the same experimental setup. The effectiveness of several kinds of heat-conductive media was studied for the measurement. The flatness of both the sample and the heat-conducting block, and the matching between them were found very important for the reduction of the heat resistance of the interfaces. A reasonable agreement was found between the off-diagonal Seebeck coefficient measured by the off-diagonal Seebeck effect and that by the off-diagonal Peltier effect. The steplike feature in the relation between the off-diagonal Seebeck coefficient and the annealing temperature may imply a nonmonotonous change of the Seebeck coefficient along the c axis (Sc) with oxygen content.

Introduction of the ultrafine MgO to the high Tc melt-texture YBa2Cu3O7-δ

Abstract The MgO ultrafine powder was prepared by the common Sol-Gel process. Determined by the line width at half-height of powder X-ray diffraction, the MgO powder was about 20nm in size on average. A supersonic vibration was introduced to destroy the accumulation of the ultrafine powder and to mix the powder with the YBa2Cu3O7-δ precursor. The melt-process was employed to grow textured samples. From the Scanning Electron Microscope (SEM) graphs, some particles were found in the textured samples, which were about several nanometer in size. The composition of these particles, however, leaves to be identified. The magnetic critical current density (Jcm) at 77K dropped rapidly with the increase of Mg content.

Investigation of the off-diagonal Seebeck effect and Peltier effect on textured YBa2Cu3O7−δ☆

Abstract Both of the off-diagonal Seebeck effect and the off-diagonal Peltier effect were investigated with the use of the same textured sample YBa 2 Cu 3 O 7−δ (Y-123) and the same experimental setup. The reliability of the measurement is studied by applying different heat-conductive mediums. The flatness of both of the sample and the heat-conducting block, and the matching between them are found very important for the reduction of the heat resistance. The annealing effect on the S xz is also studied and found in accordance with that studied by the thermoelectric power.

Fabrication of HgBa2CuOx superconductor under atmospheric pressure

We report a simple and safe technology where rather pure Hg-1201 phase superconducting bulk samples can be fabricated. The pellet was heated at about 700°C for only 30 min in open air under atmospheric pressure. We have found that a dehydration process (heating for 20 h at 250°C) is a crucial procedure for success in forming the Hg-1201 phase.