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Recently, an improvement in the flux pinning performance ofREBa2Cu3O7−δ(REBCO) bulk was achieved employing high energy ultrasonic irradiation (up to 300 W and 60 min) prior to the infiltration-growth process. Here, we demonstrate that a higher ultrasonic power and shorter duration treatment (450 W for 30 min) of theRE2BaCuO5(RE211,RE= Y, Gd) powder produces individual, nanometer-sized and surface damagedRE211 particles (RE211Ultra). We study the growth of YBCO and GdBCO systems via the top-seeded melt-growth method, with the addition of 30 mol% ofRE211Ultra, which were pre-treated by high-energy ultrasonication and compared with the conventional method. Isothermal growth experiments clarified that the addition ofRE211Ultraparticles enables faster and more effective crystal growth via an improved peritectic reaction due to their size in the nanometer range and presence of sharp edges, which are crucial for growing large, single grain bulks ofREBCO. Microstructural investigations by scanning electron microscopy indicated the presence of two differently sizedRE211 inclusions within the bulk GdBCO and YBCO superconductors. All samples showed an onset of superconductivity at ∼92 K or above. Utilizing the effective growth temperature window and the addition ofRE211Ultra, single grain bulks of GdBCO and YBCO were fabricated by the slow-cooling method. The field dependent critical current density (Jc) of the bulk samples usingRE211Ultrawere found to exhibit superior performance over the standard GdBCO/YBCO samples due to significant changes of sample growth and of their microstructures. Here, we discuss various factors affecting the addition ofRE211Ultraon the development of the microstructure, growth of bulk, single grain materials and further, the superconducting properties of differentREBCO superconductors.

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We present results of the first application of the transmission-EBSD (t-EBSD) technique to bulk, infiltration growth (IG)-processed YBa2Cu3O7-δ (YBCO) superconductors with embedded Y2BaCuO5 (Y-211) nanoparticles. By means of focused ion-beam (FIB) milling, TEM slices were prepared from mechanically polished surfaces of bulk, IG-processed YBCO samples. The required optical transparency was reached by additional polishing the resulting surfaces using the FIB and Ar-ion milling. For t-EBSD, the sample was mounted on a homebuilt sample holder in the SEM, which provides the required inclination for t-EBSD. The improved spatial resolution of t-EBSD enabled the investigation of the small Y-211 particles (diameter of about 60-210 nm) embedded in the superconducting YBCO matrix. The fabricated TEM slices further enable the application of transmission electron microscopy to the same sample sections. These tiny Y-211 particles embedded within the YBCO matrix are, together with their strain fields, directly responsible for the high irreversibility fields due to the effective flux pinning of the IG-processed samples.

Microstructures of melt-textured YBCO samples with embedded nanosized particles of Y2BaCuO5 (Y-211) and Y2Ba4CuMOx (M-2411 with M = U,Zr) are analysed by means of electron backscatter diffraction (EBSD). With the recent developments of the EBSD technique, we can directly measure the crystallographic orientation of the embedded nanoparticles, employing a spatial resolution of about 40 nm. The high image quality of the Kikuchi patterns allows true three-phase (YBCO, Y-211 and M-2411) scans to be performed. The Y-211 particles do not exhibit any preferred orientation, even if their size is considerably reduced, to the 100 nm range. The size reduction reduces the negative influence of the Y-211 particles on the YBCO matrix, however. U-2411 particles, which are formed during the processing stage, do not show any orientation, and with increasing concentration, some texture develops. In contrast to this, embedded Zr-2411 particles are fully oriented in the (001) orientation according to the surrounding superconducting matrix.

The local grain orientations and the distribution of Y2Ba4CuUOx (U-2411) phase are measured within melt-textured YBCO samples by means of electron backscatter diffraction (EBSD). In this work, several samples with varying addition (0.1–0.8 wt%) of depleted uranium oxide (DU) were analysed by means of EBSD. The embedded U-2411 particles were found to have sizes around 200 nm, some large particles being present in the samples with a high DU concentration. Combined EBSD and EDX analysis enabled the identification of the Kikuchi patterns of the U-2411 phase, so that a true three-phase EBSD scan (YBCO, Y2BaCuO5 and U-2411) becomes possible.

By means of automated electron backscatter diffraction (EBSD) analysis, we studied the local orientations of embedded Y2BaCuO5(211) particles within melt-textured YBa2Cu3Ox (YBCO) samples. On both components, we obtained high-quality Kikuchi patterns, thus allowing the automated mapping of the crystal orientations and a two-phase analysis of the samples. Investigations were performed on a variety of melt-textured YBCO samples. In melt-textured YBCO with (001) orientation, we find that the embedded 211 particles do not have any preferred orientation. The EBSD maps also reveal that at certain orientations of the 211 particles the YBCO growth is not altered. From the obtained EBSD mappings, we can conclude that the formation of small 211 particles will not disturb the YBCO matrix growth, whereas the presence of large 211 particles causes severe changes in the YBCO growth, leading to the formation of subgrains. The EBSD results are presented in the form of local orientation maps and local pole figures.

We have studied the microstructures of subgrains in large single-domain RE–Ba–Cu–O bulk superconductors and their effects on superconducting properties. Subgrains with high misorientation angles were seen at the positions of the later stage of melt growth, or the positions where the second-phase particles are large in size and volume. An electron backscattering diffraction technique revealed that the orientation of the second-phase particles affects the orientation of the matrix phase, some of which forms the subgrain boundaries. Electric and magnetic measurements have been performed for several samples having different subgrain geometries and the results of I–V and resistivity measurements indicated that subgrains act as weak links, especially in a high magnetic field region.

We have successfully obtained electron backscatter diffraction (EBSD) Kikuchi patterns on various bulk high-Tc superconductors (polycrystalline KClO3-doped and undoped YBa2Cu3O7−δ (YBCO), single crystalline YBCO and NdBa2Cu3O7−y) and on YBCO and NdBCO thin films. We have established a sample polishing procedure to obtain intense Kikuchi patterns without an additional carbon coating of the ceramic superconductors. We present Kikuchi patterns, pole figures and orientation maps of the samples. On the polycrystalline KClO3-doped YBCO samples we find two major orientations (001) and (100) normal to the sample surface, while the undoped sample does not exhibit any orientation maxima, which is ascribed to the effect of a liquid phase due to the KClO3-additive during the preparation process. Furthermore, we discuss the future possibilities for the application of the EBSD technique for growing larger, bulk melt-textured samples for applications.

The influence of KClO3 addition on the magnetic properties and microstructural parameters (grain size, grain boundaries) of high-temperature superconductors (HTSC) with nominal composition Y(1-0.2x)Ba(2-0.2x)KxCu3Oy (x = 0-0.40) were investigated. The ac complex susceptibility ?ac = ?'-i?'' was recorded as a function of temperature and concentration of KClO3 at various ac magnetic fields up to 2?mT. The data were analysed using the relation h = a(1-Tm/Tc)n, where Tm represents the intergranular peak temperature of the imaginary component ?''. The value of a is found to increase for doped samples, which suggests enhanced flux pinning due to the KClO3 addition. The dc susceptibility as a function of temperature was measured by means of a SQUID magnetometer in field-cooled cooling (FCC) and field-cooled warming (FCW) modes in magnetic fields in the range 10?mT??0Ha?7?T on KClO3-doped and pure YBCO samples. The Meissner curves reveal that the samples are single phase in all applied fields, but the low-temperature behaviour of the samples with KClO3 additions is different from that of pure YBCO.

11p; International audience; The model of thermally activated relaxation developed and successfully tested on high-Tc superconductors [Phys. Rev. B 70 (2004) 0245251-9] was applied to magnetic data of a bulk spark-plasma sintered MgB2 sample to elucidate its magnetic relaxation behavior. MgB2 and the related borides form a superconductor class lying between classical and high-Tc superconductors. In accord with this classification, the relaxation phenomena were found to be about ten times weaker than in cuprates. Vortex pinning analyzed in terms of the field dependence of the pinning force density indicates a combined pinning by normal point-like defects and by grain surfaces. An additional mode of pinning at rather high magnetic fields (of still unknown origin) was observed.