Darren Verebelyi

Low angle grain boundary transport in YBa2Cu3O7−δ coated conductors

Second generation, high-temperature superconducting wires are based on buffered, metallic tape substrates of near single crystal texture. Strong alignment of adjacent grains was found to be necessary from previous work that suggested large angle, YBa2Cu3O7−δ [001]-tilt boundaries reduce Jc exponentially with increasing misorientation angle (θ). We pursue the low-θ regime by evaluating single grain boundaries (GB) and biaxially aligned polycrystalline films utilizing both the rolling-assisted biaxially textured substrates and ion-beam assisted deposition coated conductor architectures. Analysis concludes that an exponential dependence on Jc is applicable for θ≳4°, where the spacing between the periodic disordered regions along the GB become smaller than a coherence length.

Epitaxial growth of La2Zr2O7 thin films on rolled Ni-substrates by sol-gel process for high Tc superconducting tapes

Abstract A solution process was used to grow epitaxial La 2 Zr 2 O 7 (LZO) buffer layers on roll-textured Ni (100) substrates to produce YBa 2 Cu 3 O 7− δ (YBCO)-coated conductors. The LZO precursor solution was prepared by an all alkoxide sol–gel route using mixed metal methoxyethoxides in 2-methoxyethanol. The partially hydrolyzed solution was either spin-coated or dip-coated onto the textured Ni substrates. The amorphous thin film was then heat treated at 1150°C under (96%)Ar/(4%)H 2 atmosphere for 1 h. X-ray diffraction (XRD) of the buffer layer indicated a strong c -axis orientation on the Ni (100) substrate. The LZO (222) pole figure revealed a single cube-on-cube texture. SEM images of the LZO buffer layer showed a dense microstructure without cracks. The YBCO deposited on the sol–gel LZO-buffered Ni substrates with sputtered YSZ and CeO 2 top layers had a critical current density of 480,000 A/cm 2 at 77 K and self-field.

Investigation of YBCO Coated Conductors for Fault Current Limiter Applications

The switching behavior of second generation (2G) HTS wires with a coated conductor (CC) architecture for application in fault current limiters (FCL) has been studied. Measurements have been performed on short pieces of meter length, as well as on bifilar coils wound from 20 meters of CC. CCs with and without stainless steel stabilizers have been compared regarding the temperature reached after applying a constant voltage per length to the conductor for a certain time (typically 50 ms). Three coils from stainless steel stabilized wires have been connected in series to demonstrate uniform switching of the individual coils in a single phase FCL model with 120 kVA nominal power. The response to transient currents has been investigated by applying currents above the critical current Ic with different amplitudes and measuring the time at which an irreversible voltage increase occurs. All CCs investigated proved well suitable for FCL applications exhibiting the required uniformity and excellent current limiting performance.

BEND STRAIN TOLERANCE OF CRITICAL CURRENTS FOR YBA2CU3O7 FILMS DEPOSITED ON ROLLED-TEXTURED (001)NI

Superconducting YBa2Cu3O7/CeO2/YSZ/CeO2 multilayer structures were grown on rolled-textured (001)Ni using pulsed-laser deposition. Critical current densities greater than 1 MA/cm2 were obtained for relatively thick YBa2Cu3O7 films. The compressive and tensile bend strain tolerance of critical currents for the YBa2Cu3O7 films deposited on these rolling-assisted biaxially textured substrates was also determined. These conductors retained up to 80% of their unstrained critical currents for applied compressive bend diameters as small as 1.5 cm and tensile bend diameters of 3.2 cm. The degradation of Jc is caused by the formation and propagation of transverse cracks. The results also suggest a correlation between bend-strain tolerance for these coated conductors and total oxide layer thickness.

Continuous growth of epitaxial CeO2 buffer layers on rolled Ni tapes by electron beam evaporation

Abstract A reel-to-reel, electron beam evaporation system has been developed to continuously deposit epitaxial CeO 2 and other oxide buffer layers on meter long-lengths of rolled Ni tapes. The deposition system includes two interconnected electron beam evaporation chambers and a chamber in which as-rolled Ni tape is in situ annealed to develop biaxial texture. An integral reel-to-reel system with tension control enables motion of the tape with little or no plastic deformation. When depositing epitaxial oxides on Ni, the formation of unfavorably oriented NiO needs to be avoided. Oxide-free, {100}〈100〉-oriented Ni tapes are prepared by control of the partial pressure of H 2 during Ni annealing. In situ annealed rolled Ni tape over 70 cm long and 1 cm wide has been deposited with an epitaxial CeO 2 buffer layer. Critical current density as high as 700 000 A/cm 2 at 77 K in self-field has been achieved for YBa 2 Cu 3 O 7− x (YBCO) films deposited on short segments of these buffered substrates. X-ray in-plane φ-scans have been performed as a function of length to determine the crystallographic consistency of the epitaxial CeO 2 over length. Results of scanning electron microscopy (SEM) examinations of the CeO 2 buffer layer microstructure are presented.

Critical current density of YBa2Cu3O7−δ low-angle grain boundaries in self-field

A study has been perfomed on the superconducting critical current density J{sub c} flowing across low-angle grain boundaries in epitaxial thin films of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}. The materials studied were dual grain boundary rings deposited on SrTiO{sub 3} and containing 2{sup o}, 3{sup o}, 5{sup o}, and 7{sup o} tilt boundaries. The current density in self-field was determined by magnetometric methods at temperatures from 5 K to T{sub c}. We conclude that at the higher temperatures of coated conductor applications, there is limited potential for improving J{sub c} by reducing the grain boundary angle below {approx}3{sup o}.A study has been perfomed on the superconducting critical current density Jc flowing across low-angle grain boundaries in epitaxial thin films of YBa2Cu3O7−δ. The materials studied were dual grain boundary rings deposited on SrTiO3 and containing 2°, 3°, 5°, and 7° tilt boundaries. The current density in self-field was determined by magnetometric methods at temperatures from 5 K to Tc. We conclude that at the higher temperatures of coated conductor applications, there is limited potential for improving Jc by reducing the grain boundary angle below ∼3°.

Phase stability for the in situ growth of Nd1+xBa2−xCu3Oy films using pulsed-laser deposition

We have determined the stability line in the 1/T−log[P(O2)] phase space for the synthesis of Nd1+xBa2−xCu3Oy (NdBCO) films. A systematic study of Tc, Jc, and ρ(T) dependence on oxygen partial pressure and temperature for the deposition of thin NdBCO films grown by pulsed-laser deposition was performed. The conditions for optimal NdBCO film growth were determined by varying oxygen partial pressure from 0.02 to 400 mTorr, and substrate temperature between 730 and 800 °C. The results show that the best NdBCO films are obtained at oxygen pressures in the range of 0.2–1.2 mTorr, depending on the substrate temperature. This is more than two orders-of-magnitude lower than the correspondent oxygen pressure appropriate for YBa2Cu3O7−δ film growth.

Epitaxial growth of gadolinium oxide on roll-textured nickel using a solution growth technique

Chemical solution epitaxy was used to deposit an epitaxial film of Gd 2 O 3 on roll-textured nickel. A 2-methoxyethanol solution of gadolinium methoxyethoxide was used for spin-coating and dip-coating. Films were crystallized using a heat treatment at 1160 °C for 1 h in 4% H 2 /96% Ar. Single-layer films were approximately 600 A in thickness, and thicker films could be produced using multiple coatings. θ/2θ x-ray diffractograms revealed only (0041) reflections, indicating a high degree of out-of-plane texture. A pole-figure about the Gd 2 O 3 (222) reflection indicated a single in-plane epitaxy. Scanning electron microscopy showed that the films were smooth, continuous, and free of pin holes. Atomic force microscopy revealed an average surface roughness of 53 A. Electron diffraction indicated that the misalignment of the majority of the grains in the plane was less than 10°. High-current (0.4 MA/cm 2 ) Yba 2 Cu 3 O 7–δ films were grown on roll-textured nickel substrates using Gd 2 O 3 as the base layer in a three-layer buffer structure.

Growth and superconducting properties of YBa2Cu3O7−δ films on conductive SrRuO3 and LaNiO3 multilayers for coated conductor applications

Conductive multilayers of YBa2Cu3O7−δ/SrRuO3/LaNiO3 (YBCO/SRO/LNO), YBCO/SRO, and YBCO/LNO were grown epitaxially on single-crystal LaAlO3 and SrTiO3 substrates. Property characterizations revealed that the YBCO films on SRO and SRO/LNO buffer structures have excellent structural and superconducting properties. The YBCO/LNO, however, suffers degradation in superconducting transition temperatures and critical current densities, despite a good crystalline structure. The SRO/LNO bilayer showed excellent electrical contact with YBCO. These results have demonstrated the structural and chemical compatibility of SRO/LNO multilayers with YBCO and their potential use as conductive buffer layers for YBCO-based coated conductors as well as for other high-temperature superconducting applications.

Conductive buffer layers and overlayers for the thermal stability of coated conductors

We analyze fundamental issues related to the thermal and electrical stability of a coated conductor during its operation. We address the role of conductive buffer layers in the stability of Ni-based coated conductors, and the effect of a metallic cap layer on the electrical properties of Ni alloy-based superconducting tapes. For the first case we report on the fabrication of a fully conductive RABiTS architecture formed of bilayers of conductive oxides SrRuO/sub 3/ and LaNiO/sub 3/ on textured Ni tapes. For the second case we discuss measurements of current-voltage relations on Ag/YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and Cu/Ag/YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// prototype multilayers on insulating substrates. Limitations on the overall tape structure and properties that are posed by the stability requirement are presented.