Terry G. Holesinger

Improved flux pinning in YBa2Cu3O7 with nanorods of the double perovskite Ba2YNbO6

We report significantly enhanced critical current densities (Jc) and flux pinning forces (Fp) in applied magnetic fields for YBa2Cu3O7 (YBCO) films with embedded Ba2YNbO6 (BYNO) nanorods. The films were grown by pulsed laser deposition with a target consisting of YBa2Cu3Oy with five molar per cent additions of BaNbOy and Y2O3. With this composition, deposited films were found to contain a high density of BYNO nanorods that frequently traversed the entire thickness of the film (up to 1? ?m), depending upon the deposition conditions. Enhanced Jc performance occurs primarily for applied field orientations near the c-axis of the YBCO, which is nominally along the growth direction of the BYNO nanorods. The threading nanorod density of one film of the present work was measured by plan-view transmission electron microscopy to be 710?850 nanorods??m ? 2. For approximately 1??m thick films, typical Jc(75.6?K, sf) and values were ~ 4.5?MA?cm ? 2 and 1.3?1.5?MA?cm ? 2, respectively. For a 0.5??m thick film, was achieved, and values of Fp in excess of 30 and 120?GN?m ? 3 were achieved at 75.5?K and 65?K, respectively.

Identification of intrinsic ab-plane pinning in YBa/sub 2/Cu/sub 3/O/sub 7/ thin films and coated conductors

The angular-dependent critical current density J/sub c/ in YBa/sub 2/Cu/sub 3/O/sub 7/ films grown by pulsed laser deposition exhibit a sharp peak for magnetic field orientations near the ab plane, which arises from the combined effects of intrinsic pinning and extended defects parallel to the planes. An analysis of the temperature and field dependence of the height and width of this peak allows us to distinguish both contributions. We find that, in a film on single crystal substrate, the peak at low fields is due primarily to the extended defects, but at high fields it is dominated by intrinsic pinning. We compare these results with those of coated conductors with a larger density of ab-oriented correlated defects. We show a novel effect consisting in an inverse correlation between J/sub c/ and the power law exponent (N) of the I-V curves that only occurs in the intrinsic-pinning dominated regime, and we present an interpretation of its origin.

Solution deposition planarization of long-length flexible substrates

Solution deposition planarization (SDP) is studied for preparing smooth flexible substrates in multimeter lengths. We demonstrate 0.5 nm rms surface roughness starting from unpolished metal tapes and a correlation of substrate roughness with the texture of subsequent ion-beam aligned films. Surface roughness reduction in SDP is modeled via film shrinkage during solution deposition and a residual roughness based on film thickness. Use of solution deposited a-Y2O3 to planarize substrates prior to ion-beam textured MgO growth shows an in-plane texture of MgO down to 4°. Utilizing these templates, we demonstrated superconducting YBa2Cu3Oy coated conductors with critical current densities of 2.8–4.0 MA/cm2 at 75 K.

Improvement of IBAD MgO template layers on metallic substrates for YBCO HTS deposition

We present our results to improve ion beam assisted deposition (IBAD) of magnesia (MgO) templates for subsequent YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) deposition. The substrate surface roughness has a significant effect on the initial nucleation texture of IBAD MgO films. We have found that reduction in our substrate surface roughness to /spl sim/ 3.5 nm has resulted in better in-plane texture for IBAD MgO films than those deposited on rougher metal substrates. We have further improved the IBAD MgO deposition parameters by using an in situ reflected high-energy electron diffraction (RHEED) analysis tool that allows for monitoring of IBAD MgO growth. We have found a direct correlation between the RHEED generated intensity versus time curve and in-plane texture. Utilizing X-ray diffraction analysis, we have been able to determine the optimum deposition parameters to routinely grow films in batch mode that have a phi scan /spl Delta//spl phi/ from 6 - 7/spl deg/. Coupling the improvements in substrate preparation with optimization of IBAD MgO deposition has allowed for both batch and continuous deposition (termed c-IBAD MgO) on metallic substrates that result in superior superconducting properties. We have demonstrated that deposited meter lengths have had phi scan FWHM values from 8 - 9/spl deg/ with /spl plusmn/ 10% uniformity. Additionally, we have been able to widen the processing zone in our system and coat two, one-meter lengths simultaneously while preserving good texture quality (/spl Delta//spl phi//sub ave/ /spl sim/ 8/spl deg/) and uniformity (60 - 80% of tape length within /spl plusmn/ 5% of /spl Delta//spl phi//sub ave/) for both tapes.

Advances in YBCO-coated conductor technology

Processes for producing both the YSZ template film by IBAD and the YBCO superconducting film by PLD at 1 m lengths have achieved Ic values of 122 A and Jc values of 1 MA/cm2. Improvements have been made in all stages of the process. Variations of Ic along the length of the 1-m samples stimulated development of a new in-field Ic measurement capability. The use of MgO as an IBAD template film has made great progress and can potentially decrease the time to produce the template film by more than an order of magnitude. A combination of electrical and microstructural investigations are being made to understand and improve the properties of the YBCO coated

Ion-beam assisted deposition of bi-axially aligned MgO template films for YBCO coated conductors

We report the results of experiments with ion-beam-assisted deposition (IBAD) of MgO using in-situ monitoring with Reflected High-Energy Electron Diffraction (RHEED). Strips of polished Haynes 242 and Inconel 625 nickel-based super-alloys have been used as substrates for these experiments. The in-plane texture of the MgO, as measured by X-ray /spl phi/ scan, resulted in FWHM values between 11 and 15/spl deg/. Using pulsed-laser deposition, the IBAD MgO template films were then overcoated with buffer layer films and a final superconducting film of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//. The best superconducting transport properties measured on these YBCO films were: an Ic (75 K, self-field, 1 cm wide) of 41.6 A, and a narrow-bridge J/sub c/ (1.35 /spl mu/m thick film) of 0.46 MA/cm/sup 2/.

Reversible effect of strain on transport critical current in Bi2Sr2CaCu2O8?+?x superconducting wires: a modified descriptive strain model

A reversible strain effect on transport critical current Ic was found in Bi2Sr2CaCu2O8 + x (Bi-2212) high-temperature superconducting round wires. Ic showed unambiguous reversibility at 4 K and 16 T up to an irreversible strain limit of about 0.3 % in longitudinal tension, prompting hope that the Bi-2212 conductor has the potential to sustain mechanical strains generated in high-field magnets. However, Ic was not reversible under longitudinal compression and buckling of Bi-2212 grain colonies was identified as the main reason. A two-component model was proposed, which suggests the presence of mechanically weak and strong Bi-2212 components within the wire filaments. Porosity embedded in the weak component renders it structurally unsupported and, therefore, makes it prone to cracking under strain e. Ic(e) is irreversible in tension if the weak component contributes to the transport critical current but becomes reversible once connectivity of the weak component is broken through strain increase or cycling. A modified descriptive strain model was also developed, which illustrates the effect of strain in the Bi-2212 conductor and supersedes the existing descriptive model. Unlike the latter, the new model suggests that higher pre-compressive strains should improve Ic if buckling of Bi-2212 grains does not occur, and should result in a wider Ic(e) plateau in the applied tensile regime without degradation of the initial Ic. The new model postulates that a reversible strain effect should exist even in the applied compressive strain regime if buckling of Bi-2212 grains could be prevented through elimination of porosity and mechanical reinforcement of the wire.

1000 A cm − 1 in a 2 µm thick YBa2Cu3O7 − x film with BaZrO3 and Y2O3 additions

We report here on significant improvements to the in-field and self-field critical current densities (Jc) of multilayer and single-layer YBa2Cu3O7 − x (YBCO) thick films with BaZrO3 (BZO) and Y2O3 additions. In the former case, the composite film consisted of a five-layer architecture with three YBCO layers and two Y2O3 interlayers in a total thickness of 1.8 µm. The multilayer film produced a Jc (75.6 K, self-field) of 4.3 MA cm − 2 (775 A cm − 1) and a minimum Jc (75.6 K, 1 T) of 1.0 MA cm − 2 (175 A cm − 1) over all field orientations in the maximum Lorentz force configuration. We achieved in a single-layer 2.0 µm thick film a Jc (75.6 K, self-field) of 5.2 MA cm − 2 (1010 A/cm-w) and a minimum Jc (75.6 K, 1 T) of 1.2 MA cm − 2 (234 A/cm-w) in the same measurement configuration. For both kinds of films, scanning transmission electron microscopy and transmission electron microscopy imaging were used to identify a uniformly dispersed second-phase microstructure consisting of short, tilted BZO nanorods and tilted Y2O3 nanoparticle layers. We attribute the enhanced performance of the thick YBCO films to the uniformity of the microstructure and the interaction of two different second-phase materials during film growth.

SiC and carbon nanotube distinctive effects on the superconducting properties of bulk MgB2

This work describes in detail the simultaneous enhancement of the upper critical field (Hc2) and the critical current density (Jc) of MgB2 bulk samples doped with nano-SiC particles, as well as single-walled and double-walled (dw) carbon nanotubes (CNTs). The magnetization properties were examined in a superconducting quantum interference device magnetometer, and four-probe transport measurements were performed using a 50T pulsed magnet to determine Hc2(T). We found that the Jc enhancement is similar in all doped samples at 5K but nano-SiC addition is more effective to improve the flux pinning in the high temperature range (T⩾20K); this improvement cannot solely be attributed to the C incorporation to the lattice but also to the presence of other types of defects (i.e., several kinds of nanoinclusions). CNTs produce a better C incorporation that is more effective to enhance Hc2 [i.e., dwCNT-doped samples reached a record Hc2(0)∼44T value for bulk MgB2]. All the Hc2(T) curves obtained for different types o...

A comparison of buffer layer architectures on continuously processed YBCO coated conductors based on the IBAD YSZ process

The microstructures of continuously processed YBa/sub 2/Cu/sub 3/O/sub y/ (YBCO) coated conductors processed with three different architectures are presented. YBCO films were deposited directly on ion-beam-assisted deposition (IBAD) yttria-stabilized zirconia (YSZ) or on intervening layers of Y/sub 2/O/sub 3/ or CeO/sub 2/. Different interfacial reactions were observed in each case. The volume changes that occur with the interfacial reactions were calculated based on the identified reaction products. The calculated volume changes correlate with the observed microstructures and appear to be an important factor in determining an optimal buffer layer system. The interfacial reactions do not preclude the attainment of high I/sub c/ and J/sub c/ values in these coated conductors.