S. Pavan Kumar Naik

Limited infiltration due to reactive sintering of nano-Sm2O3 with preforms—its effect on (Y, Sm)Ba2Cu3O7−δ superconductors

Preform optimized infiltration growth process (POIGP) is employed to fabricate mixed rare earth (Y, Sm)Ba2Cu3O7−δ superconductors of high performance. POIGP creates wide range of pinning centres and also enables nano-particle additions to stable Y2BaCuO5 (Y-211) preforms. Nano-Sm2O3 particles (30–50 nm) were introduced in 0, 10, 20 and 30 wt.%, homogenously and without agglomeration, into the Y-211 preforms and the corresponding composites YBCO-Ar, YSm-10, YSm-20 and YSm-30 are fabricated. Though all the samples were processed in argon atmosphere to suppress the formation of solid solutions with Sm at Ba site, the effort was successful only in the YSm-20 sample, as seen from sharp diamagnetic transition and XRD studies. Magnetic property measurements were carried out on samples which showed strong (00l) texture, but were not necessarily single crystalline since no seed was used in the experiments. The calculated critical current densities (J c) are thus reflective of the values in the basal plane, in the most favourable direction. The final composites revealed YSm-20 to exhibit better J c with substantial flux pinning up to 9 T, at 77 K. Detailed examination of microstructures and field dependence of J c reveal that the addition of nano-particles is not always rewarding and can lead to reactive sintering of Y-211 and limit the extent of liquid phase inflow into preforms. This not only can lead to incomplete reaction leaving larger 211 particles in the matrix, but can also shift the composition of liquid phase such that it promotes the formation of undesired solid solutions in IG processed REBCO as seen in YSm-30. The effect of various factors like open porosity in the preform, the ease of liquid phase infiltration and the liquid phase composition on the residual 211 size in the composites, suppression of RE/Ba solid solutions, and the resultant J c versus H curves is discussed.

Relevance of Nanosized Defects to Hardness and Current Density of YBCO Superconducting Composites

In this paper, we compare YBa2Cu3O7-δ samples fabricated by the infiltration growth process: the chosen samples have microstructures that are considered ideal for supporting high current densities; they have small spherical Y2BaCuO5 precipitates uniformly distributed in their superconductor matrices. However, they differ dramatically in their critical current density .Ic versus H curves at high magnetic fields. It is expected from theory that flux pinning at high magnetic fields would be modified by defects of nanometric dimensions occurring in the superconductor. We have investigated the superconductor matrices of the samples using a nanoindenter, with the objective of probing differences in nanometric volumes within them. The measurements show highly enhanced nanohardness and elastic modulus in the superconductor matrices of samples that support high current densities at high magnetic fields; such an enhancement is absent in a sample whose current density, although starting at a high value at zero magnetic field, falls rapidly with increasing field. The enhanced nanohardness in samples showing enhanced current density at high magnetic fields has been attributed to nanometer-sized defects associated with twinning present in the superconductor matrix.

THE EFFECT OF CeO2 NANOPARTICLE DOPING ON REFINEMENT OF Y-211 PARTICLES IN POIG PROCESSED YBCO COMPOSITES

Doping of non-interacting nanoparticles in YBCO superconductors in order to get fine microstructure defects has been studied. Y-211 inclusion, itself has large potential to create structural defects without affecting Y-123 matrix phase. Nanoparticles of grain refining agent like CeO2/Pt/PtO2 are interesting dopants, especially CeO2 for being cheaper than Pt. POIG process, as is one of the advance processing techniques to get uniform distribution of fine Y-211 particles of the order of 1-2 μm, throughout the YBCO composites. Finer Y-211 particles can drastically increase the pinning centers. Introduction of fine sized CeO2 as grain refining agent to reduce the size of Y-211 particles would create additional pinning centers. In the present work, the effect of CeO2 on Y-211size and distribution in POIG processed YBCO composite was studied. It was observed that with increase in CeO2 content Y-211 particle size decreases below 1μm and even less than 0.5μm for 10 weight percent CeO2 doping. The mechanism for refinement appears to be the formation of BaCeO3 on the surface of Y-211 during liquid phase infiltration that leads to splitting of Y-211 into smaller particles.