Rolling-assisted Biaxially Textured Substrates Research Articles

Investigation of Texture Formation in Ni-7at.%W Alloy Substrates by Spark Plasma Sintering Technique

A sharp cube textured alloy substrates with high strength and low magnetism are required in coated conductors' applications. In this work, the cube textured Ni-7at.%W substrates, starting from advanced powder metallurgy ingots followed by Rolling Assisted Biaxially Textured Substrate (RABiTS) method, have been prepared. The fabrication processes of Ni-7at.%W tapes with sharp cube texture by spark plasma sintering(SPS) routes were described. By Electron Back Scattering Diffraction (EBSD) analysis, the percentage of the cube texture component in Ni-7at.%W tape surface is found to reach 99.4% within a misorientation angle smaller than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> . Moreover, the formation mechanism of the cube texture in the Ni-7at.%W tape was also investigated by EBSD. Particular attention was focused on the difference of the texture components along the thickness of the partially recrystallized samples.

The Development of Second Generation HTS Wire at American Superconductor

Development of the second generation (2G) YBCO high temperature superconducting wire has progressed rapidly and its performance is approaching, and in some areas exceeding, that of first generation (1G) HTS wire. American Superconductor's approach to the low-cost manufacturing of 2G wire is based on a wide-strip (4 cm) process using a metal organic deposition (MOD) process for the YBCO layer and the RABiTS (rolling assisted biaxially textured substrate) process for the template. In addition, the wide-strip RABiTS/MOD-YBCO process provides the flexibility to engineer practical 2G HTS wires with architectures and properties tailored for specific applications and operating conditions through slitting to custom widths and laminating with custom metallic stabilizers. This paper will review the status of the 2G manufacturing scale up at AMSC and describe the properties and architecture of the 2G wire being developed and tested for various applications including in cables, coils and fault current limiters. Performance of 100 meter class, 4 mm wide wires at 77 K, self-field has reached 100 A (250 A/cm-width) with single-coat YBCO and 140 A (350 A/cm-width) with double-coat YBCO. A 5 cm inner diameter coil fabricated from the latter wire achieved 1.5 T at 64 K, confirming the capability of the wire for coil applications.

AC loss study of antiparallel connected YBCO coated conductors

Some applications of high temperature superconducting conductors require a non-inductivewinding, which may be constructed from antiparallel connected YBCO (yttrium bariumcopper oxide) tapes. In the case of AC applications, this antiparallel winding changes theAC losses from that of an isolated conductor. This study focuses on the effect of the spatialseparation and misalignment between conductors on their AC loss behavior for YBCOconductors on both rolling assisted biaxially textured substrate (RABiTS) and ion beamassisted deposition templates in an effort to fully understand the behavior of theseconductors in real world applications. For RABiTS samples, the study was carried out forall three possible configurations (the so-called back-to-back, front-to-front and same-wayconfigurations) to clarify the effect of the ferromagnetic substrate on the ACloss behavior in these conductor configurations. Numerical simulations were alsoemployed in some cases to compare with and elucidate experimental observations.

Fabrication of YBa2Cu3O7 thin film on cube-textured Cu tape

We achieved a biaxially oriented YBa2Cu3O7 film on a {100} ⟨001⟩ textured Cu tape with Jc of 1.9×106 A cm−2 at 77 K. A Ni overlayer was deposited by electroplating technique on the {100} ⟨001⟩ textured Cu tape to improve oxidation resistance of Cu substrate, and then YBa2Cu3O7 was deposited on the CeO2/YSZ/CeO2 buffered Ni-electroplated Cu tape. We confirmed that the Ni-electroplated Cu tape lost its ferromagnetism after 700 °C at 1 h heat treatment. Our rolling assisted biaxially textured substrate (RABiTS) type coated conductor can be used for ac applications and is thought to be more cost feasible than the standard RABiTS architecture using Ni-alloy tapes.

Strain characteristics of critical current in RABiTS/Y-Ba-Cu-O coated conductors

The strain dependence of critical current I c of a commercial RABiTS (Rolling-Assisted Biaxially Textured Substrates) YBCO coated conductor tape with Cu stabilizing layers on both surfaces has been studied by two kinds of bending apparatus and a tensile apparatus at 77 K and 293 K. According to the voltage measurements using multi-taps in the longitudinal direction, local I c degradation was also observed. In the continuous bending-tension test at 77 K, in which YBCO layer was subjected to the tensile strain, I c began to decrease rapidly beyond the strain of 0.6% after initial moderate degradation. In the continuous bending-compression tests, the I c began to decrease rapidly beyond the strain of 0.4%. The strain for the beginning of degradation of I c obtained by tensile tests at 77 K was almost the same as that by the bending-tension tests. Bending test at RT using FRP holders showed I c degradation not so different from that by the continuous bending tests at 77 K. In all tests, the I c initially degraded locally irrespective of the position in the specimens. Furthermore, I c degradation by repeated bending was also studied.

On narrowing coated conductor film: the emergence of granularity-induced field hysteresisof transport critical current

The critical current density Jc in polycrystalline or granular superconducting material is known to be hysteretic with applied fieldH due to the focusing of the field within the boundary between adjacent grains. This is of concernin the so-called coated conductors, wherein a superconducting film is grown on a granular,but textured, surface of a metal substrate. While previous work has mainly been onJc determined using induced or magnetization currents, the present work utilizes transport currentvia an applied potential in strip geometry. It is observed that the effect is not as pronouncedusing transport current, probably due to a large difference in criterion voltage between the twotypes of measurements. However, when the films are narrowed by patterning into 200, 100, or80 µm, the hysteresis is clearly seen, because of the forcing of percolation across higher-angle grainboundaries. This effect is manifested in ex situ films grown on ion-beam-assisted-deposited(IBAD) yttria stabilized zirconia (YSZ) substrate and those grown on rolling-assistedbiaxially textured substrates (RABiTS) which have grains that are 100–1000 times larger.The hysteresis is more pronounced for the latter, which is more likely to have a weakgrain boundary spanning the width of the microbridge. This is also of concern toapplications in which coated conductors will be striated in order to reduce theAC losses.

Pd layer on cube-textured substrates for MOD-TFA and PLD YBCO coatedconductors

Pd films were deposited on rolling assisted biaxially textured substrate(RABiTS) Ni–5 at.% W in order to exploit the Pd effect of the texturesharpening with respect to that of the substrate, for the development ofYBa2Cu3O7−x (YBCO)coated conductors. The Pd sharpening effect was relevant in the out-of-plane direction where the reductionfor the ω-scans’ full width at half maximum (FWHM) ranged from 55 to 65%,depending on the substrate roughness. The obtained minimum valuesof the FWHM in the transverse rolling direction of the (002) Pdω-scan and in the(111) Pd ϕ-scanwere of about 2.5° and 5°,respectively. The CeO2/YSZ/CeO2 (YSZ is Y2O3-stabilised ZrO2) heterostructure of the buffer layer was developed by pulsed laser deposition(PLD). In order to transfer the sharp orientation of the Pd film, both the seedCeO2 layer and the YSZ layer were deposited at low temperatures(450 °C), lowenough to avoid Pd/Ni–W interdiffusion. The YBCO, films deposited by both PLD and metal-organicdeposition (MOD) using metal trifluoroacetate acid (TFA), exhibited rolling direction (005)ω-scan and the (113)ϕ-scan FWHMvalues of about 2° and 5°, respectively. In spite of the complete interdiffusion between Ni and Pd duringthe YBCO film deposition, the coated conductors exhibit good adherence, aswell as a smooth and crack-free surface. A zero-resistance critical temperature(TC0) of 90.8 K for the MOD-TFA YBCO films and critical current-density(JC) upto 2.2 MA cm−2 at 77 K and self-field for PLD YBCO films have been obtained.

Epitaxial Growth of High-J c NdBa2Cu3O7−δ Films on RABiTS by Pulsed Laser Deposition

NdBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (NdBCO) films were grown on rolling-assisted biaxially textured substrates (RABiTS) via pulsed laser deposition. c-Axis-oriented epitaxial NdBCO films with high performance were obtained under optimal deposition conditions. Transmission electron microscopy analysis shows that the NdBCO film grown on RABiTS has a clear interface with a CeO{sub 2} cap layer and a nearly perfect lattice structure. The NdBCO film exhibits higher {Tc} of 93.7 K and better in-field J c in magnetic fields and at all field orientations, compared to pure YBCO films.

Transport AC losses in YBCO coated conductors

Transport AC loss measurements have been made on YBCO-coated conductors prepared ontwo different substrate templates—RABiTS (rolling-assisted biaxially textured substrate) andIBAD (ion-beam-assisted deposition). RABiTS samples show higher losses compared withthe theoretical values obtained from the critical state model, with constant critical currentdensity, at currents lower than the critical current. An origin of this extra AC loss wasdemonstrated experimentally by comparison of the AC loss of two samples with differentI–V curves. Despitea difference in I–V curves and in the critical currents, their measured losses, as well as the normalizedlosses, were practically the same. However, the functional dependence of the losseswas affected by the ferromagnetic substrate. An influence of the presence of aferromagnetic substrate on transport AC losses in YBCO film was calculatednumerically by the finite element method. The presence of a ferromagnetic substrateincreases transport AC losses in YBCO films depending on its relative magneticpermeability. The two loss contributions—transport AC loss in YBCO films andferromagnetic loss in the substrate—cannot be considered as mutually independent.

EPITAXIAL GROWTH OF BUFFER LAYERS AND YBCO FOR COATED CONDUCTOR

An Y 2 O 3 film, as seed layer, was directly deposited on the meter long length rolling assisted biaxially textured substrate (RABiTS) NiW tapes by the reactive magnetron sputtering process. Yttria stabilized zirconia (YSZ) and CeO 2 films were further carried out as the barrier and cap layers, and YBCO layer was prepared by magnetron sputtering technique as well. X-ray diffraction was employed to characterize the texture of film. The results showed that on NiW tapes highly textured films were obtained.

The incorporation of nanoscale columnar defects comprised of self-assembledBaZrO3 nanodots to improve the flux pinning and critical current density ofNdBa2Cu3O7−δ films grown on RABiTS

We report significant enhancement in the flux-pinning of epitaxialNdBa2Cu3O7−δ (NdBCO) films on rolling-assisted biaxially textured substrates (RABiTS) via theincorporation of a two-dimensional array of columnar defects comprised of self-assembledBaZrO3 (BZO)nanodots. Pure NdBCO films and NdBCO films incorporating BZO nanodots, both with the same thicknessof 0.6 µm, were epitaxially grown on RABiTS by pulsed laser deposition. A verydense and homogeneous distribution of BZO nanodot columns withtypical intercolumn spacing of around 15–20 nm oriented along thec-axis of the filmwas observed in the NdBCO+BZO film. Compared to the pure NBCO film on RABiTS, theNdBCO+BZO film on RABiTS has an in-field critical current density(Jc) of a factor of more than 2 in 0.1–1.5 T and a smaller power-law exponentα∼0.21 in the fieldregime where Jc∼H−α.

Reversible tensile strain dependence of the critical current in YBCO coated conductor tapes

The tensile strain dependence of the critical current, Ic, in two kinds of commercially available YBCO coated conductors (CC) and another one fabricated at KERI, which were processed by using the rolling-assisted biaxially textured substrates (RABiTS) and the ion-beam assisted deposition (IBAD) techniques, have been investigated. The mechanical properties including the Young’s modulus and 0.2% offset yield stress at 77K have been derived. The IBAD-processed samples showed a higher yield stress than the RABiTS/MOD processed one. Also, the irreversible strain/stress limits of Ic have been investigated at 77K and self-field. The influence of stabilizer on the Ic degradation behavior in YBCO CC tapes with IBAD substrates was discussed. The irreversible strain limits defined by unloading could be explained well by the variation in the n-value-strain relation.

Enhanced Flux-Pinning in Dy-Doped, MOD YBCO Films on RABiTS

Significant enhancements in flux-pinning were obtained for Dy-doped, YBCO films via a metalorganic deposition (MOD) process on rolling-assisted biaxially textured substrates (RABiTS). It has been reported previously that incorporation of excess rare-earth ions during the MOD process, results in improvement of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for H//c, however, a decrease in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for H//ab is observed. We report here that by altering the processing conditions the reduction in the magnitude of the current peak for H//ab can be minimized while significantly enhancing the random pinning at all field orientations. The result is a YBCO film with significantly reduced anisotropy compared to the typical YBCO films prepared by the MOD process. This is accomplished by incorporating both a high density of stacking faults and (Dy, Y) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanoparticles which result in the strong pinning for H//ab and a broad pinning peak for H//c respectively.

High Critical Current YBCO Films Prepared by an MOD Process on RABiTS Templates

The metal organic deposition (MOD) of YBCO high temperature superconducting films on RABiTS (rolling assisted biaxially textured substrates) templates has been developed at American Superconductor as a low-cost, scalable manufacturing process for the commercialization of the second generation (2G) HTS wire. The MOD process is based on the deposition of a triflu-oroacetate (TFA) based metal organic precursor film which is converted, in an ex-situ process, to the superconducting YBCO film. A major goal of the development has been achieving high critical currents. This paper reports the preparation and characterization of MOD-YBCO films with critical currents exceeding 500 A/cm-w (77 K, self-field) using a scaleable thick film approach on RABiTS templates. The high critical current films were obtained through optimization of the precursor composition, nucleation and growth conditions. The through-thickness dependence of the critical current density of MOD Alms as a function of film thickness and a correlation of the through-thickness transport properties and microstructure of the thick MOD/RABiTS samples is reported.

${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7}$ Coated Conductor Grown by Hybrid Liquid Phase Epitaxy

The hybrid liquid phase epitaxy (HLPE) approach has the potential to be a high rate-low cost process. This deposition process is used to grow epitaxial YBa2Cu3O7 (YBCO) films in the presence of a thin (100 to 500 nm) liquid BaO-CuO flux layer. Despite the presence of the molten flux, YBCO growth is found to be feasible on technical substrates because of the limited dissolution of the buffer layer in the thin flux layer. The high atom mobility at the growing interface under the liquid flux enables high growth rates (demonstrated to be greater than 10 nmldrs-1 in a wide temperature regime and predicted to be up to ~5 nmldrs-1). We report results on HLPE YBCO films grown on (100) strontium titanate and textured technical substrates such as rolling-assisted biaxially textured substrates (RABiTS) and ion-beam-assisted-deposited (IBAD) MgO. High values of the critical temperature above 90 K and critical current ~400 A per cm width for 3 mum thick films on single crystal substrates and promising high values on technical substrates are obtained. Furthermore, there is potential for making thicker films of the same quality since film density is maintained with thickness. Angular-dependent transport critical current as a function of applied field for these films, as well as microstructural measurements by transmission electron microscopy are also reported. The angular and field dependence of critical current density are similar to standard PLD (pulsed laser deposition) films. Although the films are of high epitaxial quality they contain yttria precipitates which are possible sources of flux pinning centers.

Progress in AMSC scale-up of second generation HTS wire

American Superconductor has successfully scaled up its low-cost, high volume second generation (2G) HTS wire process into pre-pilot scale production, with performance approaching first generation (1G) HTS wire. AMSC’s manufacturing approach is based on RABiTSTM/MOD wide strip technology, with metal organic deposition (MOD) process for the YBCO layer and the Rolling Assisted Biaxially Textured Substrate (RABiTS) process for the template. In this paper, we review the status of the 2G manufacturing scale up at AMSC and describe the properties and architecture of the 2G wire being manufactured and developed for various applications.

Local epitaxy of YBa2Cu3Ox on polycrystalline Ni measured by x-ray microdiffraction

Polychromatic synchrotron x-ray microdiffraction is used to determine the epitaxy of YBa2Cu3Ox (YBCO) films grown on polycrystalline Y.15Zr.85O1.925/CeO2/Y2O3/Ni95W5(Ni) rolling-assisted, biaxially textured substrates (RABiTS). A novel analysis technique is introduced in which the orientation of mosaic films is measured by using a Hough transform to recognize arcs in Laue microdiffraction patterns that correspond to low-index zone axes. While the overall epitaxy is cube-on-cube, grain-by-grain analysis reveals a systematic misorientation of YBCO with respect to Ni: the YBCO [001] rotates toward the direction of the surface normal. The crystal mosaic (for rotation about the rolling direction) measured by a single diffraction pattern sampling a 0.5-μm2 surface area is 0.7° full width at half-maximum for YBCO grown on Ni grains with a low tilt; for more highly tilted grains, the YBCO patterns can no longer be measured, presumably due to the large mosaic. The YBCO mosaic over the entire area of a Ni grain is ∼2.5° and varies with grain size; the mosaic is smaller for larger grains.

Strong enhancement of flux pinning inYBa2Cu3O7−δ multilayers with columnar defects comprised of self-assembledBaZrO3 nanodots

Multilayer structures comprisingYBa2Cu3O7−δ (YBCO) films with columns of self-assembledBaZrO3 (BZO) nanodotswith interlayers of CeO2 or pure YBa2Cu3O7−δ were grown on rolling-assisted biaxially textured substrates (RABiTSs) usingpulsed laser deposition. A significant enhancement of the critical current density(Jc) was observed for the multilayers compared with a single layer ofYBCO + BZO.Jc variesas Jc∼H−α withα of 0.27 forsingle layer of YBCO + BZO and 0.34 for both multilayered films. Enhancement of pinning in the multilayers isattributed to the presence of columnar defects comprised of self-assembled nanodots ofBZO as well as planar CuO-type stacking defects arising as a result of interfacial reactionsin the multilayers.

Cube-textured substrates for YBCO-coated conductors: microstructure evolution andstability

The realization of YBCO-based coated conductors with high critical currentdensity involves the deposition of highly biaxially textured YBCO films. Theuse of epitaxial growth shifts this requirement to the template used for YBCOdeposition. In the rolling-assisted biaxially textured substrate (RABiTS) approach, anappropriately oriented template is provided by a cube-textured substrate. Thedevelopment of a cube texture is the result of recrystallization occurring in aheavily deformed tape, which is activated by high-temperature annealing. In thecase of Ni-based alloys, thermal treatment at temperatures ranging from 900 to1150 °C for at least 30 min is commonly used. The determination of the minimum conditions forthermal treatments in terms of temperature–time involved in the recrystallization process istherefore of practical interest.In this work, Ni–5 at.% W alloy has been studied as a substrate for YBCO-coated conductors.100 µm thick tapes have been obtained through heavy cold rolling, followed byannealing in high vacuum. Different thermal treatments with rates of20 °C min−1 have been performed in order to study the formation and the evolution of the cube texture.Moreover, the annealing time has been varied in order to inspect the thermal stability ofthe substrate microstructure at the relatively high deposition temperature of YBCO films.It is found that the substrate begins recrystallizing at700 °C and that the cube texture is fully developed at temperatures higher than800 °C, whileannealing at 900 °C for 1 h produces a stable microstructure at the typical YBCO deposition conditions.

High in-field critical current densities in epitaxialNdBa2Cu3O7−δ films on RABiTS by pulsed laser deposition

We report the epitaxial growth and superconducting properties forNdBa2Cu3O7−δ (NdBCO) films grown on rolling-assisted, biaxially-textured substrates (RABiTS)by pulsed laser deposition. At the optimum deposition temperature of760 °C, the criticalcurrent densities, Jc, at 77 K, self-field, of NdBCO films ranging in thickness from 0.13 to0.25 µm were found tobe in the range 2.2–3.4 MA cm−2, with a lower thickness corresponding to a higherJc. Comparedto epitaxial YBa2Cu3O7−δ (YBCO) films, these NdBCO samples are found to have superior field and angular dependences ofJc. In an applied field of 1 T, NdBCO of the same thickness has more than twice theJc of YBCO films with a significantly enhancedJc peak for a fieldapplied parallel to the c-axis. These results suggest the presence of an increased density ofc-axis correlated pinning centres within NdBCO films compared to YBCO films.