YBCO Grains Research Articles

Time relaxation of residual resistance of HTSC-based composites

Magnetoresistance curves R( H) and time relaxation of the residual resistance of bulk YBCO + CuO composites have been studied. These composites represent the network of Josephson junctions, where CuO acts as a barrier between YBCO grains. It has been shown that hysteresis of the R( H) dependences is related to the flux trapping within the YBCO crystallites. This flux induces magnetic field in the intergrain media. Time relaxation of the residual resistance is caused by relaxation of the magnetization of YBCO grains. The time dependence of the residual resistance obeys a logarithmic law, which describes the Anderson-like relaxation processes in the YBCO grains.

Crossover from S–I–S to S–F–S junctions in composites Y 3/4Lu 1/4Ba 2Cu 3O 7 + Y 3(Al 1− xFe x) 5O 12

The transport properties of the two phases composites based on Y 3/4Lu 1/4Ba 2Cu 3O 7 (YBCO) and Y 3(Al 1− x Fe x ) 5O 12 ( x = 0.0 … 1.0) have been studied to investigate the effect of magnetic ordering in garnet on superconducting properties of the composites. The composites represent network of Josephson junctions where the non-superconducting ingredient (garnet) acts as barriers separating YBCO grains. It is found that magneto-active ions (Fe) in garnet are responsible for an additional suppression of Josephson coupling in the composites. At some critical concentration of Fe atoms full suppression of super current is observed in a temperature range from T C of YBCO grains down to the characteristic temperature T m. This behavior is attributed to the interaction of super current carriers with spins of Fe atoms located in the barriers. The crossover from S–I–S (S – superconductor, I – insulator) to S–F–S (F – ferrimagnet) junctions is observed in the composites Y 3/4Lu 1/4Ba 2Cu 3O 7 + Y 3(Al 1− x Fe x ) 5O 12 with x increase.

(110) facets and dislocation structure of low-angle grain boundaries in YBa2Cu3O7−δ and Y0.7Ca0.3Ba2Cu3O7−δ thin film bicrystals

The facet and dislocation structure of 5° and 7° [001]-tilt grain boundaries of YBa2Cu3O7−δ (YBCO) and Y0.7Ca0.3Ba2Cu3O7−δ (YCaBCO) thin film bicrystals were studied. A 24° [001]-tilt YBCO grain boundary was also examined to contrast with the low angle grain boundary faceting behavior. All the low-angle grain boundaries exhibit strong faceting along (100)/(010) and (110) and possess both straight symmetric segments containing equally spaced [100] unit dislocations and step asymmetric segments composed of (110) and (100)/(010) facets. Grain boundaries with a higher degree of meander acquired up to 40% (110) facets. The atomic structure of (110) facets was revealed by the atomic resolution Z-contrast imaging. The (110) facets are dissociated for both the YBCO and YCaBCO grain boundaries. We also found the Ca-doped (110) facets to be more extended along the grain boundary plane, consistent with our earlier finding of a dissociated dislocation core in Ca-doped (100) facets. These 5° and 7° misorientations that we studied are just in the range at which YBCO grain boundaries start to become obstacles to current flow. The above results will be helpful for understanding the current transport across YBCO low-angle grain boundaries.

On-Line Characterization of YBCO Coated Conductors Using Raman Spectroscopy Methods

The use of Raman spectroscopy for on-line monitoring of the production of superconducting YBa2Cu3O6+X (YBCO) thin films on long-length metal tapes coated with textured buffer layers is reported for the first time. A methodology is described for obtaining Raman spectra of YBCO on moving tape exiting a metal-organic-chemical-vapor-deposition (MOCVD) enclosure. After baseline correction, the spectra recorded in this way show the expected phonons of the specific YBCO crystal orientation required for high supercurrent transport, as well as phonons of non-superconducting second-phase impurities when present. It is also possible to distinguish YBCO films that are properly textured from films having domains of misoriented YBCO grains. An investigation of the need for focus control on moving tape indicated that focusing of the laser on the surface of the highly reflective YBCO films exiting the MOCVD enclosure tends to produce aberrant photon bursts that swamp the Raman spectrum. These photon bursts are very likely a consequence of optical speckle effects induced by a combination of surface roughness, crystallographic texture, and/or local strain within the small grain microstructure of the YBCO film. Maintaining a slightly out-of-focus condition provides the best signal-to-noise ratio in terms of the obtained Raman spectra. In addition to examining moving tape at the post-MOCVD stage, Raman spectra of the film surface can also be recorded after the oxygen anneal performed to bring the YBCO to the optimum superconducting state. Consideration is given to data processing methods that could be adapted to the on-line Raman spectra to allow the tagging of out-of-specification tape segments and, at a more advanced level, feedback control to the MOCVD process.

The influence of Nd–Ba–Cu–Mg–O generic seed crystal composition on Tc of seeded, bulk (RE)–Ba–Cu–O grains

A Mg-doped Nd–Ba–Cu–O generic seed crystal has been developed for the fabrication of any type of rare earth (RE) based (RE)–Ba–Cu–O single grain superconductor by the top seed melt growth (TSMG) technique. Melt processed crystals of Nd–Ba–Cu–O enriched with various amounts of MgO exhibit increased melting points compared to other (RE)–Ba–Cu–O systems, whilst retaining a similar crystal structure. The Mg-doped Nd–Ba–Cu–O phase is stable with Ba–Cu–O liquid at elevated temperature and, as a result, can be used to seed the growth of any (RE)–Ba–Cu–O single grain. Key seed properties have been investigated as a function of MgO content, including Tc, the distribution of MgO inclusions within the YBa2Cu3O7−δ (Y-123) matrix, melting point and composition. MgO doping results usually in a decrease in Tc of the parent (RE)BCO compound. It is particularly important, therefore, to investigate fully any variation of Tc associated with the diffusion of Mg into a grain grown from a Mg-containing seed. We report the superconducting and microstructural properties of YBCO, SmBCO and GdBCO single grains fabricated using various Nd–Ba–Cu–Mg–O generic seed crystals with a wide range of MgO content. Finally, we report the optimum composition of Nd–Ba–Cu–O generic seed crystal from a study of the variation of Nd–Ba–Cu–O melting temperature with concentration of Mg in the precursor seed composition.

NanoSIMS analysis of Ca doping at a grain boundary in a superconducting YBCO Ca-123/123 bicrystal

Calcium doping is well known to increase the grain boundary critical current, Jcgb, in polycrystalline Y1Ba2Cu3O7-δ, (YBCO). High resolution Secondary Ion Mass Spectroscopy (nanoSIMS) analysis has been used to map the distribution of Ca+ ions in three dimensions in an YBCO grain boundary grown on a 24° SrTiO3 bicrystal. High resolution SIMS imaging has produced Ca, Sr, and Ti elemental maps at 20 nm intervals from the YBCO surface. From the variation of Ca concentration with depth a Ca grain boundary diffusion coefficient can be estimated. Moreover, localised ‘hot spots’ of Ca at the YBCO grain boundary have been observed, perhaps at regions with a high dislocation density. There is also some diffusion of Sr and Ti ions from the substrate along the boundary. This is an important observation because it has been proposed that these ions are electrically active.

Properties of Mg-doped Nd-Ba-Cu-O generic seed crystals for the top seeded melt growth of (RE)-Ba-Cu-O bulk superconductors

We have recently developed a new generic seed crystal that has been used successfully to fabricate any oriented, single grain (RE)-Ba-Cu-O bulk superconductor by a cold seeding technique. In this paper we report the chemical, structural and microstructural properties of these seed crystals, including the variation of melting point, crystallographic parameters and volume fraction of Mg-rich inclusions in the Nd1 + xBa2-x(Cu1-yMgy)3Oz matrix as a function of externally added MgO content. The influence of Mg-doping on the superconducting transition temperatures of YBCO grains fabricated using these seeds is investigated. Finally, an optimum MgO content of the generic seed that effectively controls the orientation of the seeded grain without compromising its superconducting properties is suggested from the many seed crystals fabricated with a wide range of Mg-rich addition.

Current-step-like structure in a YBCO grain boundary Josephson junction coupled to a gigahertz signal

We demonstrate current-step-like structures observed in a YBa2Cu3O7−δ (YBCO) grain boundary Josephson junction (GBJJ) coupled to a gigahertz (GHz) signal. It is well known that a microwave-current-driven Josephson junction does not exhibit clear Shapiro steps under a low frequency condition of f << IcRn/ϕ0, where f, Ic, Rn, and ϕ0 are the microwave frequency, the critical current of the junction, the normal resistance of the junction, and a flux quantum, respectively. Instead of Shapiro steps, however, we often observe an anomalous upturn in the current-voltage curve of YBCO GBJJ's coupled to a GHz signal. We refer to the upturn as a current-step-like structure. Numerical analysis using an rf-field-driven two-junction superconducting quantum interference device (SQUID) model indicates that the envelope of small Shapiro steps, including the effects of microwave-induced motion of flux quanta, appears as a current-step-like structure.

In situ high temperature optical microscopy study of phase evolution in YBa 2Cu 3O 7− δ films prepared by a fluorine-free sol–gel route

Phase evolution of YBa 2Cu 3O 7− δ (YBCO) thin films prepared by a newly-developed fluorine-free sol–gel method were studied by combined use of in situ high temperature optical microscope observation and X-ray diffraction. The reaction sequences associated with yttrium, barium and copper in this route were determined respectively. It was found that the reactions that take place were strongly dependent on the experimental conditions such as water partial pressure. The formation of YBCO started around 710 °C and continued up to 800 °C for about 15 min. The formation mechanism of YBCO was found to be dependent on the barium containing phases, which were controlled by the experimental conditions. The formation of a- and c-axes oriented YBCO grains was also found to be governed by the processing reactions.

Processing Dependence of Texture, and Critical Properties of YBa2Cu3O7−δ Films on RABiTS Substrates by a Non‐Fluorine MOD Method

YBa2Cu3O7−δ (YBCO or Y123) films on rolling‐assisted biaxially textured substrates (RABiTS) were prepared via a fluorine‐free metallorganic deposition (MOD) through spin coating, burnout, and high temperature anneal. The effects of substrate texture and surface energy of the CeO2 cap layer were investigated. Except for the commonly accepted key factors, such as the textures of substrate and buffer layers, we found some other factors, for example, the deposition temperature of the cap layer, are also critical to the epitaxial growth of Y123 phase. With the CeO2 cap layer deposited at relative high temperature of 700°C, a critical current density, Jc, over 1 MA/cm2 has been demonstrated for the first time on Ni‐RABiTS by a fluorine‐free MOD method. Whereas for samples with CeO2 cap layers deposited at a lower temperature of 600°C, even though XRD data showed a better texture on these buffer layers, texture degradations of YBCO grains under the optimized processing conditions were observed and a lower oxygen partial pressure around 40 ppm was necessary for the epitaxial growth of Y123 phase. As a result, Jc fell to 0.45 MA/cm2 at 77 K. The observed phenomena points to the change of surface energy and reactivity of the CeO2 cap layer with respect to the CeO2 deposition temperature. In this paper, the YBCO phase diagram was also summarized.

Superconducting and normal state transport properties of epitaxial 10 at.% Ca substituted YBCO thin films

The improvement of the critical current density in YBCO coated conductor tapes necessarily concerns the study of grain boundary (GB) properties and the study of different pinning mechanisms involving both GB and the interior of the grain (IG). Recently, it has been proposed that the introduction of Ca substituting Y enhances Jc in YBCO grain boundaries (GB). On the other hand enhanced correlated pinning, as observed in YBCO thin films grown on CeO2 buffered substrates, has been ascribed to elongated defects along the c-axis of the film. In order to evaluate the role of calcium and CeO2 buffer layer on the superconducting transport properties of YBCO films, critical current density measurements as a function of temperature, magnetic field and angle between magnetic field direction and c-axis of the film have been performed on epitaxial thin films of unsubstituted and 10at.% Ca substituted YBCO grown on CeO2 buffered Al2O3 single crystal substrates. Results are compared with data obtained on 10at.% Ca substituted YBCO grown on SrTiO3 single crystal substrate.

YBCO Coated Conductor Applications - A Texture Problem

YBa2Cu3O7-δ ("YBCO") is a high-temperature ceramic superconductor. Due to its complex layered structure, a strong biaxial texture of the YBCO grains is necessary to achieve technically relevant currents. In this paper, we describe the development of flexible metallic tapes with strong cube texture, providing a lattice-matched template for buffer and YBCO coatings in proper orientation. Texture analysis was performed using electron backscatter diffraction (EBSD).

Evidence for Extensive Grain Boundary Meander and Overgrowth of Substrate Grain Boundaries in High Critical Current Density ex Situ YBa2Cu3O7−x Coated Conductors

It has been generally accepted that YBa2Cu3O7−x (YBCO) films deposited on deformation textured polycrystalline metal tapes result in YBCO grain boundary (GB) networks that essentially replicate the GBs of the underlying substrate. Here we report that for thicker YBCO films produced by a BaF2 ex situ process, this is not true. Using electron backscatter diffraction combined with ion milling, we have been able to map the evolution of the YBCO grain structure and compare it to the underlying template in several coated conductors. For thin (≤0.5 μm) YBCO films deposited on rolling-assisted biaxially textured substrates (RABiTS), the YBCO GBs nearly directly overlap the substrate GBs. For 0.7–1.4 μm YBCO films, the GBs were found to meander along the substrate GBs and along the sample normal, with displacements several times the film thickness. In very thick films (2.5–2.9 μm), the YBCO grains can completely overgrow substrate grains and GBs, resulting in a substantial disconnection of the YBCO and substrate GB networks. Similar behavior is found for BaF2 ex situ YBCO films on ion-beam-assisted deposition-type templates. The ability of the YBCO to overgrow substrate grains and GBs is believed to be due to liquid-phase mediated laminar grain growth. Although the behavior of the YBCO GB networks changes with YBCO film thickness, the samples maintained high critical current density (Jc) values of &gt;2 MA/cm2 for films up to 1.4 μm thick, and up to0.9 MA/cm2 for 2.5–2.9-μm-thick films.

Ca-doped YBa2Cu3O7-δ bicrystal junctions fabricated on asymmetric SrTiO3 substrates

Bicrystal grain–boundary Josephson junctions of Ca–doped YBa2Cu3O7–δ that is Y0.7Ca0.3Ba2Cu3O7–δ were fabricated on three bicrystal SrTiO3 (001) substrates with asymmetric 30°, 40° and 45° orientations. An enhancement of the critical current density in these Ca–doped junctions was observed when compared with normal YBCO grain–boundary junctions with similar angular orientations. The observed increase in the critical current density is large for the junctions fabricated on the asymmetric 30° bicrystal substrate and small or negligible, for those on the asymmetric 45° bicrystal substrate. The critical current was modulated by a magnetic field applied in the plane of the junctions. However, the Fraunhofer pattern observed due to the applied magnetic field deviates from the ideal one.

Transmission electron microscopy studies of YBCO coated conductors prepared by pulsed-laser deposition and multiple-stage chemical vapor deposition processes

Three types of YBCO films were deposited on metal tapes with biaxially textured CeO2/Gd2Zr2O7 multilayer. One YBCO film 4μm in thickness was formed by pulsed-laser deposition (PLD) on the stationary substrate and another film 3μm in thickness by PLD using a reel-to-reel substrate tape transferring system. Ic values of the films were 173A and 293A, respectively A further film 1μm in thickness was fabricated by multiple-stage chemical vapor deposition (MSCVD). The Ic value of the MSCVD-YBCO film was 100A. The microstructures of these YBCO films were characterized using transmission electron microscopy (TEM). In the PLD-YBCO films, the films less than 1μm thick were predominantly composed of c-axis oriented grains which were considered to grow spirally, however, irregular instances of a-axis oriented grains were also found. The a-axis oriented grains size increased with increasing thickness of the YBCO films. In the 4μm-thick PLD-YBCO film with an Ic value of 173A, a-axis oriented grains were considered to nucleate on the CeO2. We found many Y2O3 and cupper oxides at the boundaries between a- and c-axes oriented grains and the orientation relationships between YBCO and Y2O3 were determined to be as follows: (001)YBCO//(001)Y2O3 and (100)YBCO//(110)Y2O3. We also found many gaps and pores between YBCO grains. Nucleation of a-axis oriented grains was completely suppressed on the CeO2 in the 3μm-thick PLD-YBCO film with an Ic value of 293A. In the MSCVD-YBCO film, the YBCO film mostly consisted of c-axis oriented grains, however a-axis oriented grains and polycrystal also formed in the film. As the MSCVD-YBCO film became thicker, the a-axis oriented grains grew large, as same as in the PLD-YBCO films. The growth rate of a-axis oriented grain along the substrate normal was much higher than that of c-axis oriented grains in comparison to the difference in the growth rate between a- and c-axes oriented grains in the PLD-YBCO. In these specimens, the grain growth of the a-axis oriented grains was considered to be one of the most significant causes of degradation of Jc values in the YBCO films.

Comparative study of YBCO film growth in the cold wall type and hot wall type MOCVD processes

We made a comparative study of YBCO film growth using two different types of MOCVD reactors, i.e., the cold wall type reactor and the hot wall type reactor. In the cold wall type reactor the substrate was heated using the heat source (halogen lamps) which was located below the substrate, while in the hot wall type reactor it was using an outside resistance heater which surrounded the reaction chamber. In the cold wall reactor, the YBCO film growth had a good lattice match with the substrates at the initial deposition stage. As the film grew further, many a-axis YBCO grains were grown at the top of the film. This is attributed to poor thermal conduction to the top of the film from the heater due to the low thermal conductivity of the formed YBCO oxide. Compared to the cold wall reactor, the temperature of the hot wall chamber was easy to be controlled uniformly in the entire substrate. It led to the growth of the dense c-axis YBCO film over the entire thickness level of the film.

Ca‐doped Ba2Cu3O7–δ bicrystal junctions fabricated on asymmetric SrTiO3 substrates

AbstractBicrystal grain–boundary Josephson junctions of Ca–doped YBa2Cu3O7–δ that is Y0.7Ca0.3Ba2Cu3O7–δ were fabricated on three bicrystal SrTiO3 (001) substrates with asymmetric 30°, 40° and 45° orientations. An enhancement of the critical current density in these Ca–doped junctions was observed when compared with normal YBCO grain–boundary junctions with similar angular orientations. The observed increase in the critical current density is large for the junctions fabricated on the asymmetric 30° bicrystal substrate and small or negligible, for those on the asymmetric 45° bicrystal substrate. The critical current was modulated by a magnetic field applied in the plane of the junctions. However, the Fraunhofer pattern observed due to the applied magnetic field deviates from the ideal one.

Liquid Phase Enhanced Hybrid MOD Approach for High Performance YBCO Films Development

High performance YBCO films have been realized through MOD approach by hybrid of Ba-TFA with TMAP (trimethylacetates with propionic). High I/sub c/ over 140 A/cm-w and high J/sub c/ up to 2.4 MA/cm/sup 2/ at 77 K for 0.6 /spl mu/m YBCO films on RABiTS have been shown on un-bridged and bridged samples. Texture characterizations performed in XRD four-circle diffractometer confirmed well-aligned YBCO grains with a volume fraction of cubic phase /spl sim/95%. T/sub c/ measurements showed sharp transitions in less than 1 K and zero resistance above 90 K. The formation of YBCO phase follows the mechanism of interface island nucleation and epitaxial growth. The liquid phase in the precursor enhances Y123 epi-growth and accelerates the conversion process.

Grain Boundary Properties of Tl-2212 and Tl-1223 Thin Films

After more than 15 years active research, it is now clear that in YBCO grain boundaries act as severe weak links, and in order to produce YBCO thin films carrying high critical currents it is necessary to grow epitaxial films on appropriate substrates. We asked the question whether grain boundaries in thallium based superconductors cause the same degradation of properties, by adding to the very limited published data on the properties of grain boundaries in Tl-HTS systems. We have measured the properties of a statistically significant number of both 'artificial' grain boundaries on LaAlO/sub 3/, LSAT and SrTiO/sub 3/ bicrystal substrates and 'natural' grain boundaries in films grown on MgO substrates. The J/sub c/ values of the bicrystal films are surprisingly high when compared to values for YBCO bicrystals, and for Tl-2212 were the same for 30/spl deg/ and 24/spl deg/ grain boundaries. The 'natural' grain boundaries showed two distinct type of electrical behavior, both with significantly higher J/sub c/ values when compared to 'artificial' boundaries of similar misorientation. The J/sub c/ values of the natural grain boundaries from the bi-epitaxial films show no decrease in J/sub c/ with misorientation angle. The normalized values of J/sub c/ for these Tl-2212 grain boundaries do not fall on the generic J/sub c///spl theta/ trend line for YBCO boundaries.

Dynamic Range of High Temperature Superconducting Josephson Detecting Systems at Millimeter-Wave Band

YBCO grain boundary Josephson junctions (GBJJ's), with the normal-state resistance (R/sub N/) around 1 /spl Omega/ and I/sub C/R/sub N/ product of less than 2 mV at 45 K, where I/sub C/ is the critical current of the junction, are used to study the dynamic range of high temperature superconducting (HTS) Josephson detecting systems at millimeter-wave band. A compact pulse tube cryocooler (PTC), or a liquid helium cryostat (LHC), or a liquid nitrogen cryostat (LNC) provides the necessary low temperature environment for the operation, while Gunn oscillators together with doubler generate signals at the frequencies of 50, 98, 196 GHz whose amplitude can be adjusted by a series of calibrated attenuators. By changing the junction parameters, operating frequencies, and the cryocooler or cryostat, we have carefully looked at the dynamic range of the system. For a measuring channel bandwidth of 1 Hz, the dynamic range is around 38 dB which is determined basically by the preamplifier. This indicates that the PTC does not introduce excess noise compared with other kinds of cryostats such as LHC's or LNC's. A measuring channel bandwidth up to 1 MHz can be helpful if fast measurements are needed.