Bulk YBa2Cu3O7 − δ (YBCO)/Ag composites with a homogeneous distribution of fine and spherical particles of metallic silver and Y2BaCuO5 (Y-211) have been fabricated using a preform optimized infiltration growth process (POIGP). The effect of two different methods of introducing silver, one directly into the Y-211 preform and the other into the liquid phase source placed above the Y-211 preform, on the microstructures and current densities is investigated. Our results show that the latter method is far superior to the former. The samples obtained by introducing silver through liquid phases are nearly free from defects like voids, pores, macro-cracks and platelet gaps. Current densities in excess of 103 A cm − 2 up to fields of 2.7 T have been recorded in these samples at 77 K. The problems reported in literature with respect to inhomogeneity in the distribution of both Y-211 and Ag particles in the Y-123 matrix and the associated spatial non-uniformity in the current density are successfully resolved in the present work.

http://iopscience.iop.org/article/10.1088/0953-2048/24/8/085005/pdf

YBCO/Ag composites through a preform optimized infiltration and growth process yield high current densities

It is well known that bulk YBa2Cu3O7−x superconductors can carry transport current in a high magnetic field for many practical engineering applications. In this paper, three-dimensional YBCO parts with flexible shapes, high density and good superconducting properties were first successfully prepared by an extrusion-based additive manufacturing technique involving aqueous and non-aqueous pastes composed of milled precursors. The obtained milled powder can effectively reduce the sintering temperature and simultaneously improve densification of YBCO compared with the unmilled sample. Then, the milled powder dispersed in aqueous and non-aqueous systems was printable to build the YBCO part layer by layer, exhibiting shear thinning behaviors with the viscosity, density and porosity mainly related to the binder type and solid loading. It was found that non-aqueous PVB solution and high solid loading seemed to have a slightly positive effect on density, reaching up to 93% of theoretical density after just sintering at 940 °C for 10 h. To the end, a printed YBCO hollow cylinder with a large trapped magnetic field demonstrated its ability to prolong the levitation time, extending the potential application in flywheel technology. The integration of the extrusion printing technique with milled precursors realizes the possibility of producing complex-shaped and scale-up YBCO with useful electromagnetic properties for a broad range of applications.

Extrusion printing of a designed three-dimensional YBa2Cu3O7−x superconductor with milled precursor powder

Current transport properties of YBaCuO–Ag composites with different silver contents have been studied. The pressure response of the resistance of a single Ag/YBCO junction has been determined. The experiments reveal the absence of a dielectric layer in the junction. It has been found that the proximity effect does not contribute significantly to the conductivity of the Ag–YBCO composite for the different ways of introduction of the metallic inclusions into the YBCO. For the strip-like samples of Ag–YBCO composite with the Ag content of 15% by volume the resistivity ρ was found to be ρ =4.7×10 −9 Ω cm at the density of measuring current of J =3×10 4 A/cm 2 and magnetic field B =1 T.

Contact phenomena in Ag–YBCO composites

Nanoparticles of the type M -2411 (with M  = U, Nb, Zr, Ag, etc.) can be embedded within the superconducting YBCO matrix to provide flux pinning sites. Among all these types of nanoparticles, the Ag-2411 particles are special in the sense that they may provide also metallic Ag particles to be formed within the superconductor. The addition of metallic Ag (or silver oxide) was shown to be a means to create a more stable system by filling cracks and voids. Using the advanced polishing method of the YBCO surfaces, we achieve a high EBSD image quality of the recorded Kikuchi patterns. In this contribution, we perform a four-phase EBSD analysis including YBa 2 Cu 3 O x , Y 2 BaCuO 5 , Ag-2411 and metallic Ag. As a result, we find nanometer-sized Ag particles being distributed within the YBCO matrix, and a reduced amount of Ag-2411 particles as compared to the nominal composition. The Ag-2411 particles are well oriented with the surrounding YBCO matrix as found earlier, but the metallic silver particles as detected by EBSD do not exhibit a preferred orientation.

EBSD analysis of melt-textured YBCO with embedded Ag-2411 nanoparticles

Electron localization induced by grain boundary disorder in the normal state of high-Tc superconductors

The textured and Ag doped YBCO superconductors were studied, in which Ag contents were 0–18wt% DTA method was used to determine the starting melt temperatures of YBCO with different Ag contents The critical current densities of the samples were compared It was found that doped Ag in YBCO matrix formed Ag inclusions with size 10–50 μ m observed by SEM The saturated solubility of Ag in YBCO is about 15wt%

D.W. Soh

Papers

Effects of Ag on preparation and Jc of textured YBCO superconductor