Regina Ciancio

Mechanical fragility and tri-dimensional crack structure in NdBaCuO top seeded and multi-seeded melt-textured samples

In large TS-MTG Nd123 single-domain samples, the main lifetime limiting problem is the appearing of macroscopic fissures in the bulk, that evolve into cracks and lead to sample destruction. To analyze this phenomenon we grew single-seeded pellets and multi-seeded bars, and characterized them by XRD, PLOM, SEM and EDS. We measured inductively their superconducting properties Tc and Jc. Fissures appear to belong to micro-fissures families, parallel to the crystallographic planes. We correlate their origin to local tetra-ortho structural transition, due to spontaneous oxygenation during growth process, and their evolution toward cracks to elastic relaxation of Nd422-rich quenched liquid deposits present in the growing front concavities. MS-TSG bars growth dynamics appear to differ from pellets one. In fact, with appropriate inter-seed spacing, the absence of quenched liquid deposits near the boundaries joining adjacent domains was observed. This fact reduces mechanical fragility perpendicularly to boundaries, but does not affect the fragility along the same. Further studies are in progress on this aspect.

Thermal characterization of GdSr 2 RuCu 2 O y -based mixtures in the GdSr 2 RuO 6 –CuO pseudobinary system

Fundamental to investigating the coexistence of superconductivity and magnetic ordering in GdSr2RuCu2Oy (Gd1212) is the fabrication of macroscopic crystallographically oriented samples. To achieve this, we need to identify the conditions in which Gd1212 incongruent melting reaction, producing solid GdSr2RuO6 (Gd1210) in Cu-rich liquid, dominates over decomposition due to RuO2 sublimation. Only in these conditions is it possible, indeed, to grow oriented samples by any melting and resolidifying technique. Moreover, the optimal mixture of Gd1212 and its melting products has to be identified to perform melt-textured or flux-flow growth. By means of thermogravimetric and differential thermal analysis (TG–DTA) we analyzed, up to 1200 °C in air, several mixtures of Gd1212 with Gd1210 or CuO, performing a scan of the entire CuO–Gd1212–Gd1210 coexistence line of the Gd–Sr–Ru–Cu–O phase diagram. Gd1212 melting temperatures have shown a certain dependence on composition, decreasing about 30 °C for high Gd1210 content samples and about 20 °C for high CuO content samples with respect to pure Gd1212. A huge undercooling was observed for resolidification processes that were revealed to start at temperatures around 990 °C. Measurements proved that in this range of temperatures in air, no mass is lost at the end of each melting process, so that they are not affected by RuO2 sublimation. A strong dependence on the atmosphere has also been observed for pure Gd1212, melting temperature decreasing by more than 80 °C from 100% O2 atmosphere to He atmosphere (less than 10% O2). Measurements revealed that high O2 partial pressures favor RuO2 sublimation, whereas low oxygen atmospheres prevent resolidification.

Morphological and structural study on GdSr/sub 2/RuCu/sub 2/O/sub 8/ melt-textured samples

Strongly anisotropic magnetic and superconducting properties have been recently observed to coexist in Gd1212 (GdSr/sub 2/RuCu/sub 2/O/sub 8/). Magnetic ordering appears to be due to Ru planes while Cu planes are correlated to the superconducting properties. The unavailability of single crystals makes relevant the fabrication of oriented samples of macroscopic size by melt texturing techniques. Then we have grown MTG samples starting from 1 cm diameter pellets. Gd1210 (GdSr/sub 2/RuO/sub 6/) is added, analogously to Y211 in YBCO MTG processes. Preliminary studies have been performed on Gd1212 powders to explore the range of temperatures and the reactions involved. XRD analysis shows that a certain degree of orientation is present in our samples. By Polarized Light Optical Microscopy we have observed large oriented domains of almost millimeter size. Larger domains are expected by optimization of growth parameters. Images show even a homogeneous distribution of Gd1210 precipitates with a mean size of a few micrometers. SEM and EDS analyses have also been performed, confirming optical observations. Resistivity measurements show a 12 K wide superconductive transition with T/sub c/ onset at about 40 K. Optimization of successive annealing treatments are expected to reduce this width.

Impact of the Starting Powder Composition on

The superposition of strongly anisotropic magnetic and superconducting properties in GdSr2RuCu2O8 (Gd1212) stimulates interest in the fabrication of oriented samples of macroscopic size by Melt-Texturing Growth (MTG). Up to now, the results obtained are unsatisfactory due to the small size and the random distribution of the oriented domains. In order to achieve MTG samples with higher oriented volume fractions, we prepared pellets adding either GdSr2RuO6 (Gd1210) or CuO to highly pure Gd1212 powders. X-ray Diffraction (XRD) analysis revealed a substantial difference between the two systems showing that only in Gd1212-Gd1210 samples it is possible to achieve a superconducting phase. The analysis of the two samples microstructures carried out by Scanning Electron Microscopy with implemented Energy Dispersive Spectroscopy (EDS) revealed that in the case of the Gd1212-Gd1210 samples a low concentration of Gd1210 and SrRuO3 inclusions is observed in a Gd1212 predominant matrix. However, in the Gd1212-CuO samples, micro-sized precipitates of SrRuO3, islands of Gd2CuO4 are homogeneously distributed throughout a CuO matrix. DTA measurements performed on all the samples confirm the presence of additional solidification reactions in the Gd1212-CuO samples. These features have a profound impact on the transport properties of the two samples. The addition of Gd1210 in highly pure Gd1212 powders has been proved to be a favorable condition to obtain superconducting Gd1212 MTG samples.

{\rm GdSr}_{2}{\rm RuCu}_{2}{\rm O}_{8}

The recent great interest on (Gd1212) due to the coexistence of magnetic order and superconductivity in the same cell, has induced investigation on the whole RE1212 family where other Rare Earth elements (RE) are used. Being RE1212 isostructural to RE123 our interest focuses on Nd, that forms the highest RE123 compound and presents a slightly larger ionic radius than Gd and a lower magnetic moment. Pure Nd1212 is characterized by great structural disorder then, to investigate Gd1212 properties variation, only a partial substitution of Nd ions in Gd sites in the Gd1212 cell may be attempted. The possibility of Gd-Nd substitution and the solid solubility limit have been investigated by the synthesis of powders with a nominal composition. Phases formation has been checked by XRD. Experimental results indicate a limit for Nd solubility in Gd1212 of less than 20%. A corresponding limit of about 2% for Gd solubility in Nd1212 has also been observed. More accurate measurements, in the range are needed to better estimate this limit. Variation of the cell parameters have been observed for both phases, both showing shorter c axes and an orthorhombic structure for 1:1 molar ratio mixtures. Thermal properties have been studied by TG-DTA, revealing a substantial insensibility of Nd-rich phase to the presence of the Gd-rich one in contrast with a visible decrease of Gd-rich phase peritectic melting temperature in dependence of Nd-rich phase addition. Further investigations will be needed to clarify all the processes involved in the melting-resolidification cycle.

Melt-Textured Processes

YBCO-like Nd123 and Gd1212 superconductors in the normal state, in the visible and infrared regime, have been analyzed in this work. Superconductors, particularly high-temperature superconductors (HTSCs), hold indeed a great interest in the highly innovative research field of metamaterials and photonics with important applications. Indeed nanostructured devices suffer from increasing resistivity with frequency in metals, and superconducting materials provide a viable route to approach this problem. Moreover, HTSCs are plasmonic materials at nonzero temperature, and their features, not available in conventional materials, make them very attractive for metamaterials use. In particular, we investigate Nd123, for its high Tc, and Gd1212, for the coexistence, in its unitary cell, of magnetic order and superconductivity. Samples were extracted from top-seeded melt-textured growth bulk pellets and bars. Fourier transform infrared spectroscopy analysis, performed in the far-infrared, shows articulated peak structures with k <; kmin and a plateau for higher k, allowing for Nd123 and Gd1212 pseudogap measurement. Raman spectroscopy and microscopy provided a structural mapping of our Nd123 samples, allowing for distinguishing the superconducting matrix from nonsuperconducting phase inclusions (precipitates) and providing an instrument to discriminate among optical responses of the different compounds.

Gd-Nd Solubility in the (Gd,Nd)-Sr-Ru-Cu-O System

High frequencies (visible and near infrared) applications of metamaterials and plasmonic structures are strongly limited by dissipative losses in structures, due to poor conductivity of most used metals in this frequency range. The use of high temperature superconductors (HTSC) is a possible approach to this problem, being HTSC plasmonic materials at nonzero temperature. Negative dielectic constant and variety of charge carriers (electrons or holes) are further very attractive features for plasmonic applications. Characterization of the high frequency response of these materials is then necessary in order to correctly understand the optical parameters of HTSC. We report on FTIR and ellipsometry measurements on NdBa2Cu3O7-δ (Nd123) and the ruthenocuprate superconductor GdSr2RuCu2O8-δ (Gd1212) in optical and near infrared regime. Among YBCO-like cuprate superconductors, Nd123 presents the highest Tc (96K), and the most interesting magnetic response properties. Even more interesting, in view of use for metamaterial, is Gd1212, whose main characteristic is the coexistence, in the same cell, of superconductivity and magnetic order below Tc: Ru ions intrinsic magnetic moments order themselves below 135K, whereas superconductivity onset is at about 40K, depending on fabrication details. We performed measurements on Melt-Textured bulk samples, which present the best superconducting properties. Results confirm the promising feature of the considered materials; further analyses, also on powders and films, are in progress.