Rolling-assisted Biaxially Textured Substrates Research Articles

Comprehensive study of epitaxial (Nd,Eu,Gd)Ba2Cu3O7−δ films grown on textured templates

We report the successful epitaxial growth and comprehensive study of structural and superconducting properties of (Nd,Eu,Gd)Ba2Cu3O7−δ films with a thickness of up to 1.2 μm prepared by on-axis pulsed laser deposition on SrTiO3 (STO) single crystals as well as on ion-beam-assisted deposition (IBAD) and rolling-assisted biaxially textured substrate (RABiTS)-based metal templates. X-ray diffraction demonstrates the epitaxial quality of the grown films on both single crystalline substrates and metal-based templates. In addition, no significant changes in surface morphology were found with increasing thickness for films on all types of substrates. On all substrates, a T c value of about 89 K was observed, however, significant inhomogeneities indicating small superconducting volume were found for all films on STO. In general, the best transport characteristics over a wide range of magnetic fields and temperatures were observed for thicker films on IBAD-MgO and RABiTS templates, respectively. The J c values at 77 K for the films on IBAD-MgO templates are comparable to YBCO films, but inferior in the low temperature region and high magnetic fields, whereas the films on RABiTS and especially on STO showed much lower J c over the entire temperature and magnetic field range. A maximum pinning force of about 7 GN·m−3 at 65 K was found for the 900 nm thick films on IBAD-MgO template. In addition, the pinning mechanism seems to depend on both temperature and type of substrate. In general, no correlation between local microstructure and transport characteristics was revealed for NEG films on both metal templates.

Effect of Silver Doping on the Superconducting and Structural Properties of YBCO Films Grown by PLD on Different Templates.

We report the local structural and superconducting properties of undoped and Ag-doped YBa2Cu3O6+x (YBCO) films with a thickness of up to 1 µm prepared by pulsed laser deposition on SrTiO3 (STO) single crystals and on ion-beam-assisted deposition (IBAD) and rolling-assisted biaxially textured substrate (RABiTS)-based metal templates. X-ray diffraction demonstrates the high crystalline quality of the films on both single crystalline substrates and metal-based templates, respectively. Although there was only a slight decrease in Tc of up to 1.5 K for the Ag-doped YBCO films on all substrates, we found significant changes in their transport characteristics. The effect of the silver doping mainly depended on the concentration of silver, the type of substrate, and the temperature and magnetic field. In general, the greatest improvement in Jc over a wide range of magnetic fields and temperatures was observed for the 5%Ag-doped YBCO films on STO substrates, showing a significant increase compared to undoped films. Furthermore, a slight Jc improvement was observed for the 2%Ag-doped YBCO films on the RABiTS templates at temperatures below 65 K, whereas Jc decreased for the Ag-doped films on IBAD-MgO-based templates compared to undoped YBCO films. Using detailed electron microscopy studies, small changes in the local microstructure of the Ag-doped YBCO films were revealed; however, no clear correlation was found with the transport properties of the films.

Effect of Annealing Temperature on Cube Texture Formation in Ni7W/Ni12W/Ni7W Compound Substrate

This study is to fabricate a composite substrate (Ni7W/Ni12W/Ni7W) with weak magnetization, strong reinforcement, and improved cube texture via Rolling Assisted Biaxially Textured Substrates (RABiTS) route. The formation of cube texture with reduction in twin boundaries due to recrystallization annealing was investigated thoroughly. The optimized recrystallization annealing process revealed 97.4% of cube texture and 83.9% of small angle grain boundaries at an angle of (<10°). Higher W content and presence of defects (holes, dislocations, and grain boundaries) within inner layer of the substrate restricted the formation of cube texture along the normal direction (ND), as compared to rolling direction (RD). Furthermore, significant improvement in mechanical strength (σ0.2 = 205 ± 2 MPa) and saturation magnetization (2.6 emu/g) was found for Ni7W/Ni12W/Ni7W as compared to Ni7.5W substrates.

Fe(Se,Te) coated conductors deposited on simple rolling-assisted biaxially textured substrate templates

In this paper we study the feasibility of Fe(Se, Te) coated conductors on a simple rolling-assisted biaxially textured substrate (RABiTS) template. Starting from commercially available Ni–5 at% W tapes which show an out-of-plane orientation of about 7° and an in-plane orientation around 5°, we realized a RABiTS template for Fe(Se, Te) coated conductor fabrication by depositing CeO2 thin films on the metallic tape. The oxide buffer layers, deposited via pulsed laser ablation, exhibit an out-of-plane and an in-plane orientation suitable for Fe(Se, Te) deposition and act as a chemical barrier against Ni diffusion. Fe(Se, Te) thin films deposited on such a simple template show a superconducting transition at 16 K and very high upper critical field values with a ΔTc,0 of only 3 K in 18 T. The transport critical current reaches values of 1.7 × 105 A cm−2 at 4.2 K in self-field and is reduced by less than one order of magnitude up to 18 T, remaining isotropic with respect to the magnetic field direction.

Effect of different deformation and annealing procedures on non-magnetic textured Cu60Ni40 alloy substrates

In this work, a series of specimens was prepared by the casting method. Sharp cube-textured substrates were processed by heavy cold rolling and recrystallization annealing (i.e., the rolling-assisted biaxially textured substrates (RABiTS) method). Both the rolling and the recrystallization texture in the alloy tapes were investigated by X-ray diffraction and electron back-scatter diffraction, respectively. The results showed that a strong copper-type deformation texture was obtained in the heavy cold-rolled substrate. In addition, the recrystallization annealing process was found to be very important for the texture transition in the Cu–Ni alloy substrates. The cube texture content in the Cu60Ni40 alloy substrates reached 99.7% (≤10°) after optimization of the cold-rolling procedure and the recrystallizing heat-treatment process, whereas the content of low-angle grain boundaries (from 2° to 10° misorientation) in the substrate reached 95.1%.

Observation of zero resistance in as-electrodeposited FeSe

Superconducting FeSe films were electrochemically deposited on rolling-assisted biaxially textured substrate (RABiTS) tape. We observed zero resistivity in the as-electrodeposited FeSe film prepared on the RABiTS when the appropriate voltage was applied while it was dipped into the solution. When the RABiTS tape was dipped in the solution without applying voltage, a thin Se film was deposited on the substrate. The compositional ratio of the FeSe film got closer to the stoichiometric ratio with decreasing the dipping time before applying voltage.

Development and characterization of biaxially-textured lanthanum zirconate film grown on cold-rolled Ni-W substrate by chemical solution deposition

High temperature superconducting tapes based on cuprate compounds are nowadays available on long lengths by several companies. Because high superconducting properties are strongly influenced by the crystallographic orientation of the superconducting film, and because the appropriate crystallographic orientation is obtained through epitaxial film growth, the development of an oriented template is of crucial technological importance. In the rolling assisted biaxially textured substrate (RABiTS) technique, a strong cube texture is obtained by cold rolling fcc metal tape and high temperature annealing, followed by the deposition of one or more buffer layer by either physical vapor deposition (PVD) or chemical solution deposition (CSD). Using the latter approach, it has been recently shown that it is possible to combine substrate recrystallization and buffer layer growth in a single step if the onset of recrystallization occurs at a temperature lower than buffer layer nucleation. In the present contribution, a detailed investigation of biaxially-oriented La2Zr2O7 (LZO) buffer layer grown on cold-rolled Ni-W substrate by combined EBSD, AFM and TEM analyses is reported. It is shown that it is possible to obtain highly {001}〈110〉-textured LZO film directly on cold-rolled Ni-W substrate. Although some small polycrystalline regions appear, this approach leads to a strong suppression of grain boundary grooving typically occurring in RABiT-type substrates. Further, it is shown that, notwithstanding the microstructure of the metal substrate continues to evolve during the conversion heat treatment, the obtained LZO film is well adherent, without cracks or delamination.

Effect of the Bending Strain on the Inhomogeneity of Critical Current for YBCO Coated Conductors

The inhomogeneity of critical current in long superconducting tape is one of the factors affecting the performance of the superconducting magnet. We investigate the effect of the bending strain on the inhomogeneity of critical current for the YBCO-coated conductor by a Hall sensor array system. Two kinds of the commercial YBCO-coated conductor tapes, which are prepared by rolling-assisted biaxially textured substrate (RABiTS) and ion-beam-assisted deposition (IBAD), are used for measurement. The bending strain is applied to the coated conductor tapes, and the distribution of the perpendicular component of the remanent field in the direction along the tape length after bending is measured by a Hall sensor array. The local critical current of the tape is calculated through the method of calibration and the distribution of the critical current in the long tape is described by the Weibull function. The result shows that the inhomogeneity of critical current in the long tape is significantly increased when bending strain exceeds an irreversible strain. The change of the superconducting layer structure is believed to be the cause of the critical current inhomogeneity growth.

Development of Biaxially Textured LZO Film on Cold-Rolled Metal Substrate

La2Zr2O7 (LZO) film grown by the chemical solution deposition (CSD) technique is a widely studied buffer layer for YBa2Cu3O7-x film growth on a rolling-assisted biaxially textured (RABiT) substrate. A biaxially textured LZO film is epitaxially grown on a cube-textured metal substrate. In a RABiT substrate, cube texture is obtained as primary recrystallization of heavily cold-rolled fcc metals and is normally developed before CSD film deposition to obtain an oriented film growth. In this paper, we show that a biaxially textured LZO film can be grown by depositing the precursor solution directly on a cold-rolled fcc metal substrate and performing a single heat treatment. In fact, we show that if the recrystallization temperature of the metallic substrate is lower than the nucleation temperature of the film, a biaxially textured film is obtained as a result of transient cube texture transfer. Biaxially textured LZO films grown on cold-rolled substrates are compared with films grown on recrystallized substrates. In particular, the growth of LZO on either pure-Cu- or Ni-based substrate was studied. Compared with the LZO film grown on a cube-textured substrate, LZO grown on a cold-rolled substrate shows similar structural properties and morphology. Moreover, films are well adherent without cracks or delamination, suggesting that grain boundary migration in the metallic substrate has no detrimental influence on the film. In some cases, secondary recrystallization of the substrate occurs, without affecting the quality of the LZO film.

Electrochemical Deposition of FeSe on RABiTS Tapes

FeSe film is successfully fabricated onto Rolling Assisted Biaxially Textured Substrate (RABiTS) tapes by an electrochemical deposition technique. The deposited FeSe films tend to become high crystallinity with a decrease in the applied voltage to -1.0 V, and the compositional ratio of Fe to Se approaches 1:1. The sample deposited at -1.0 V shows a superconducting transition approximately 8.0 K in the magnetic susceptibility.

Temperature dependence of critical current and transport current losses of 4 mm YBCO coated conductors manufactured using nonmagnetic substrate

The temperature dependence of the critical current and AC losses were measured on American Superconductor Corporation's (AMSC) second generation high temperature superconducting (2G HTS) wire produced by Rolling Assisted Biaxially Textured Substrate (RABiTS) and Metal Organic Deposition (MOD) process. Wires manufactured with two types of substrates were characterized. The magnetic substrate with composition Ni5a%W exhibits a magnetic signature and has non-negligible AC losses in AC power applications. A new nonmagnetic substrate with an alloy composition Ni9a%W has been developed by AMSC to address the AC losses in 2G HTS. The data presented show that the performance of the new conductor is identical to the conductor with magnetic substrate in terms of critical current density. The data on AC losses demonstrate the absence of ferromagnetic loss component in the new conductor and significantly reduced AC losses at low to moderate values of I/Ic. The reduced losses will translate into reduced capital costs and lower operating costs of superconducting electrical devices for AC applications.

Fabrication of Ni-5 at. %W Long Tapes with CeO2 Buffer Layer by Reel-to-Reel Method

A 10-m-long homemade textured Ni-5at.%W (Ni5W) long tape with a CeO2 buffer layer has been prepared successfully by means of rolling-assisted biaxially textured substrate (RABiTS) route followed by a chemical solution deposition method in a reel-to-reel manner. Globally, the Ni5W substrate and CeO2 film exhibit high homogeneity in terms of biaxial texture over the tape. The average values of full width at half maximum of in-plane and out-of-plane texture are 7.2° and 6.1° in Ni5W substrate, 7.6° and 6.1° in CeO2 buffer layer, respectively, all of those with a small standard deviation. On a microlevel, the CeO2 film epitaxially grows well on top of the Ni5W tape. A continuous, smooth, and crack-free morphology was observed on the CeO2 film and the fraction of low-angle grain boundaries (≤ 10°) is about 98 %. This process is a potential possibility for producing long-length textured CeO2/Ni5W tapes for coated conductors with a low cost.

Study on optimization of YBCO thin film stack for compact NMR magnets

A compact high temperature superconducting (HTS) magnet, which consists of a stack of 500 HTS thin film annuli, was constructed and tested. Each thin film annulus, manufactured by the AMSC using the Rolling Assisted Bi-axially Textured Substrate (RABiTS) method, has a square cross-section of 40mm×40mm with a thickness of 80m. It has a 25-mm center hole created by machining. This paper reports a study on the anisotropic Jc issue due to the rolling procedure of the Ni substrate direction and its impact on field homogeneity. Also, three different stacking methods with rotation angles of 22.5°, 90°, and 0° against the rolling direction have been tested to study their impacts on strength, spatial homogeneity, and temporal stability of trapped fields. Finally, the 500-annulus magnet was tested at 21K under a cryogen-free environment using a GM cryocooler. The spatial field homogeneity and temporal stability were measured at 21K and compared with those obtained in a bath of liquid nitrogen at 77K.

Solderability study of RABiTS-based YBCO coated conductors

The solderability of commercially available YBa2Cu3O7−x (YBCO) coated conductors that were made from Rolling Assisted Biaxially Textured Substrates (RABiTS)-based templates was studied. The coated conductors, also known as second-generation (2G) high temperature superconductor (HTS) wires (in the geometry of flat tapes about 4mm wide), were laminated with copper, brass, or stainless steel strips as stabilizers. To understand the factors that influence their solderability, surface profilometry and scanning electron microscopy were used to characterize the wire surfaces. The solderability of three solders, 52In48Sn, 67Bi33In, and 100In (wt.%), was evaluated using a standard test (IPC/ECA J-STD-002) and with two different commercial fluxes. It was found that the solderability varied with the solder and flux but the three different wires showed similar solderability for a fixed combination of solder and flux. Solder joints of the 2G wires were fabricated using the tools and the procedures recommended by the HTS wire manufacturer. The solder joints were made in a lap-joint geometry and with the superconducting sides of the two wires face-to-face. The electrical resistances of the solder joints were measured at 77K, and the results were analyzed to qualify the soldering materials and evaluate the soldering process. It was concluded that although the selection of soldering materials affected the resistance of a solder joint, the resistivity of the stabilizer was the dominant factor.

An improved FEM model for computing transport AC loss in coils made of RABiTSYBCO coated conductors for electric machines

AC loss can be a significant problem for any applications that utilize or produce an ACcurrent or magnetic field, such as an electric machine. The authors investigate theelectromagnetic properties of high temperature superconductors with a particular focus onthe AC loss in superconducting coils made from YBCO coated conductors for use in anall-superconducting electric machine. This paper presents an improved 2D finite elementmodel for the cross-section of such coils, based on the H formulation. The model is used tocalculate the transport AC loss of a racetrack-shaped coil using constant and magneticfield-dependent critical current densities, and the inclusion and exclusion of a magneticsubstrate, as found in RABiTS (rolling-assisted biaxially textured substrate) YBCO coatedconductors. The coil model is based on the superconducting stator coils used in theUniversity of Cambridge EPEC Superconductivity Group’s all-superconductingpermanent magnet synchronous motor design. To validate the modeling results, thetransport AC loss of a stator coil is measured using an electrical method basedon inductive compensation by means of a variable mutual inductance. Finally,the implications of the findings on the performance of the motor are discussed.

Effect of substrate topography for YBa2 Cu3 O7-x coated conductors on the nucleation mechanism of buffer layer

The surface condition of substrate tape is an important factor to obtain epitaxial buffer layer on biaxially textured Ni tape for YBa2Cu3O7-x coated conductors. We prepare ceria films on Ni single crystal, biaxially textured Ni tape and sulfured Ni substrates by direct current magnetron sputtering. The results show that the ceria films prepared on Ni single crystal and sulfured Ni substrates each have a poor-textured grain structure. However, the ceria film fabricated on rolling assisted biaxially textured substrate (RABiTS) exhibits a good c-axis texture and desirable surface morphology. Reflection high-energy electron diffraction analysis indicates that the c(22) superstructure on the RABiTS Ni surface has a dramatic effect on the heteroepitaxial growth of oxide buffer layer.

The electrical measurement of AC losses in a three-phase tri-axial superconductingcable

In a three-phase tri-axial cable, the magnetic interaction between the phases makes the lossmeasurement by an electromagnetic method very complex. We developed the theoreticalbackground showing that three-phase AC loss measurements by the electromagneticmethod are, in principle, possible. We then implemented this theory in practicalmeasurements on a 3 m long, tri-axial cable fabricated from RABiTS (rolling-assistedbiaxially textured substrate) coated conductor. Initially, the proposed measurementtechnique was implemented in the simpler cases when the three cable phases are180° out ofphase, i.e. (0°, 180°,360°) or(0°,360°,180°) ratherthan (0°, 120°,240°) asin a traditional three-phase system. For these cases, the currents in the phases are either in phase (360° phase difference)or anti-phase (180° phase difference). These are essentially single-phase measurements with only one transportcurrent used as the supply for all three phases. This simplification allowed us to use boththe established and the proposed techniques to measure the total losses of the cable forthese cases. An excellent agreement between the two measurement methods confirmed thevalidation of our proposed measurement technique. The proposed measurement methodwas then employed to measure the total AC losses in a cable when it operates in the truethree-phase mode. We believe that these data represent the first electromagneticthree-phase AC loss measurements.

Examination of through-thickness/through-time phase evolution during an MOD-typeREBCO precursor conversion using Raman microscopy

Through-thickness/through-time composition analyses were made onREBa2Cu3O6 + x (REBCO) films prepared from metal–organic deposited (MOD) precursorson a RABiTS (rolling-assisted biaxially textured substrate) template.Y Dy0.5Ba2Cu3O6 + x films were fabricated either by a single precursor coating or by two separate, equivalentprecursor coatings with an intermediate decomposition. Specimens were quenched atselected times along a heat treatment profile that simulated conditions used byAmerican Superconductor, then examined by Raman microscopy along mechanicallymilled slopes through the REBCO films. Upon reaching the reaction temperatureTR, the precursor initiallytransforms into a (Y, Dy)2Cu2O5, CuO, Cu2O, and RE–Ba–F–O phase mix, and this is followed shortly by substratelevel REBCO formation that propagates upwards through time. Our resultsindicate that (1) the single layer (SL) and double layer (DL) films (both ∼ 1.2 µm thick) react to completion at about the same rate and arrive at a similar final compositionand (2) some residual Ba–Cu–O phases persist near the top of the fully reacted films. Theperformance of the SL film was moderately better than that of the DL film, seemingly dueto better through-thickness REBCO texture as determined using x-ray diffraction.

The comparative study of magnetic properties for YBCO coated conductors based on RABiTS and IBAD templates

The magnetic properties of high Tc superconductor (HTS) YBa2Cu3O7−δ (YBCO) coated conductors (CCs) which are based on RABiTS (rolling-assisted biaxially textured substrate) and IBAD (ion-beam-assisted deposition) templates were investigated. The measurement of isothermal magnetizations M(H) for the prepared YBCO-CCs was carried out at 77K and 100K in magnetic fields up to 3000G. The magnetic properties for the applied magnetic fields parallel to the ab-plane are much smaller than that perpendicular to the ab-plane. Large magnetic property in the YBCO-CC based on the RABiTS template comes from the strong ferromagnetism of the textured Ni–W alloy substrate.

Experimental and finite-element method studies of the effects of ferromagnetic substrate on the total ac loss in a rolling-assisted biaxially textured substrate YBa2Cu3O7 tape exposed to a parallel ac magnetic field

This paper presents a study of the total ac loss characteristics of a rolling-assisted biaxially textured substrate (RABiTS) YBa2Cu3O7 (YBCO) sample exposed to a parallel ac magnetic field. The results have shown that, for a given applied magnetic field and transport current, a RABiTS YBCO tape can generate very different magnitudes of ac loss, depending on whether the transport current and applied field have the same phase or opposite phase. The results of this study are very important for the optimization of the design of a RABiTS YBCO cable because they can suggest an appropriate arrangement of RABiTS tapes in a cable to minimize the cable ac loss. In this study, both experimental and finite-element method simulation approaches were employed. A modeling model that takes the magnetic field dependent permeability and ferromagnetic loss of the substrate into account reproduced well the experimental data for both self-field and total ac losses.