Coated Conductor Applications Research Articles

Growth of Thick MgB2 Films by Impinging‐Jet Hybrid Physical‐Chemical Vapor Deposition

Thick MgB2 films are grown using a novel impinging-jet hybrid physical-chemical vapor deposition process. An increased amount of the boron source gas generates high growth rates. Superconducting properties of the thick films are comparable to previous results from other processes, which indicate that this is a promising new process for MgB2 deposition for coated conductor applications, such as wires and tapes for MRI magnets.

Technique for developing highly strengthened and biaxially textured composite substrates for coated superconductor tapes

The development of reinforced and biaxially textured Ni5W/Ni9W/Ni5W composite substrate for coated conductor applications is presented. The configuration of this substrate is a thin, sharp cubic textured Ni5W outer layer on a Ni9W alloy core that provides the mechanical strength while decreasing the magnetization of the whole substrate. The key technique is to use thin Ag and Cu foils between initial Ni5W and Ni9W alloy pieces to bond them together, followed by cold-rolling and recrystallization. The obtained composite substrate has a very sharp cubic texture on the Ni5W outer layer and its yield strength approaches 200MPa, thus being 50MPa higher than commercially used Ni5W substrates. The saturation magnetization of this composite substrate at 77K was reduced by 75% and 40% compared with pure Ni and Ni5W substrates, respectively. The present results show that this new substrate is suitable for the epitaxial growth of the CeO2 buffer layers.

Study of Compositional and Artificial Pinning Effects on Superconductivity of Pulsed-Laser Deposited Gd-123 Films for Coated Conductor Applications

Critical temperature (Tc), critical current density (Jc) and flux pinning characteristics of superconducting films are crucially influenced by their chemical composition. We studied the effect of small composition variations on superconductivity of GdBa2Cu3O7-x (Gd-123) films grown by pulsed laser deposition (PLD) technique. Several pathways to control the composition of Gd-123 films were studied: variation of clearance between edge of laser plume and substrate surface, utilization of off-stoichiometric target compositions and addition of GdZrO3. Positive effect of Ba-deficiency on superconductivity of Gd-123 films was shown. Comparative study of Gd-123 and Y-123 films on CeO2 capped Gd2Zr2O7 (GZO) buffered Hastelloy tapes confirmed the advantage of Gd-based compositions for application in magnetic fields and their high potential for fabrication of long-length coated conductors.

Enhancement of dislocation density in YBa2Cu3Oy thin films prepared by low temperature growth technique

In order to achieve further lower substrate temperature (Ts) of YBa2Cu3Oy (YBCO) thin films for coated conductor application, we developed low temperature growth (LTG) II technique. In the LTG II technique, a c-axis oriented (Yb1−xNdx)Ba2Cu3Oy thin layer was deposited on MgO substrate as seed layer at low Ts and then YBCO layer was fabricated on the seed layer at the same Ts. As a result, we achieved that the LTG II-YBCO thin films showed complete c-axis orientation at lower Ts than that for obtaining c-axis oriented PLD-YBCO thin films by 15°C, and showed a high Jc. We discussed the orientation mechanism of the LTG II-YBCO thin film using a simple nucleation model.

Development of cube textured Ni–5at.%W alloy substrates for YBCO coated conductor application using a powder metallurgy process

In this paper, Ni–5at.%W alloy substrate for YBCO coated conductor was fabricated by a dry powder metallurgy process including powder compaction, cold isostatic pressing (CIP), cold rolling and annealing for recrystallization. Ni and W powders were ball-milled at this process for various times of 10, 30, 50 and 100h in argon atmosphere. The rod-like Ni–W alloy compacts were sintered at 1150°C for 1h in 96%Ar–4%H2 atmosphere. The sintered rods were cold rolling into thin tape of 70–90μm thickness with 5% reduction at each path. The Ni–W alloy tapes were annealed at 800–1200°C in an atmosphere of 96%Ar–4%H2 mixing gas for the development of cube texture. The tape with the best properties of low surface roughness, small grain size and strong cube texture was obtained at the condition annealed at 1200°C using ball-milled powder for 30min. The W addition to Ni improved the mechanical properties by solid solution hardening and inhibited grain growth for annealing heat treatment. The tapes were characterized by X-ray pole-figure, optical microscopy (OM), scanning electron microscopy (SEM) and scanning probe microscopy (SPM).

Fabrication of the cube textured NiO buffer layer by line-focused infrared heating for coated conductor application

Epitaxial growth of NiO on the bi-axially textured Ni–3at.%W (Ni–3W) substrate as seed layer for coated conductor were studied. The bi-axially textured NiO was formed on the Ni–3W tapes using a line-focused infrared heater by oxidizing the surface of the substrate at 800–950°C for 15–120s in oxygen atmosphere. The thickness of the NiO layer could be controlled by changing heat-treatment, which was estimated as approximately 200–500nm in the cross-sectional SEM micrographs of the NiO/Ni template. This thickness is enough to block the diffusion of the Ni in the substrate to the superconducting layer. The samples showed strong texture development of NiO layer. The sample oxidized at 900°C with the tape transferring speed of 30mm/h exhibited ω-scan full width at half maximum (FWHM) values for Ni–3W(200) and NiO(200) were 3.97°, and 3.67°, and φ-scan FWHM values for Ni–3W(111) and NiO(111) were 9.58°, and 8.79°, respectively. Also, the (111) pole-figure of the NiO buffer layer showed the good symmetry of the four peaks, securing the epitaxial growth of the buffer layers on the NiO layer. Also NiO layer exhibited root-mean-square roughness value of 39nm by AFM (10×10μm) investigation.

Highly reinforced and cube textured Ni alloy composite substrates by a hybrid route

A Ni-based tri-layer composite substrate designed as a substitute for traditional substrate architecture, was obtained by sintering together two Ni–5 at.%W layers on both sides of a metallurgically prepared powder Ni–12 at.%W inner layer. An advanced sparking plasma sintering technology was used, followed by cold rolling and annealing. The composite substrates obtained have a very sharp cube texture on the Ni–5 at.%W outer layer and the yield strength exceeds 320 MPa, while the magnetization behaviour was dramatically reduced. These properties render these tri-layer substrates suitable for the epitaxial growth of buffer layers for coated conductor applications.

Flux-pinning characteristics as a function of density of columnar defects comprised of self-assembled nanodots and nanorods in epitaxial YBa 2Cu 3O 7− δ films for coated conductor applications

Pure YBa 2Cu 3O 7− δ (YBCO) and YBCO films with extended columnar defects comprised of self-assembled columns of nanodots and nanorods of BaZrO 3 (BZO), CaZrO 3 (CZO) and 8 mol.% Y 2O 3 stabilized ZrO 2 (YSZ) were prepared by pulsed laser deposition on RABiTS templates of configuration CeO 2/YSZ/Y 2O 3/Ni–W. Effects on increasing the size and density of columnar defects by varying the total volume fraction BZO, CZO or YSZ incorporations within the YBCO film are reported. BZO volume fraction was varied between 0% and 8%, CZO between 0% and 2% and YSZ 0–2%. The critical transition temperature ( T c) was found to linearly decrease with increase in vol.% of BZO, CZO and YSZ, with the steepest decline for CZO additions. Nevertheless, YBCO films with these columnar defects exhibit significantly reduced J c dependence on applied field. At 77 K, best properties are obtained for 1–2 vol.% BZO incorporation in the form of columns of self-assembled nanodots and nanorods in fields of 0.1–1.5 T.

Effect of processing variables on cube texture formation in powder metallurgically prepared Ni and Ni–W alloy tapes for use as substrates for coated conductor applications

Development of textures have been studied in powder metallurgically prepared pure Ni and Ni–5 at.% W alloy after heavy cold rolling (∼95% reduction in thickness) and annealing for use as substrates for coated superconductor applications. The cold rolling textures of the two materials do not show any significant difference. However, Ni–5 at.% W alloy is found to develop a much sharper cube texture after annealing which indicate the beneficial role of W in increasing the cube volume fraction in Ni. W increases the volume fraction of the cube component by decreasing the volume fraction of the rolling direction (RD)-rotated cube grains. Partial recrystallization studies clearly indicate that in contrast to pure Ni sharp cube oriented grains form in Ni–5 at.% W alloy right from the beginning of recrystallization. A near 100% cube texture is obtained in Ni–5 at.% W alloy by an optimized two stage rolling (TSR) procedure followed by annealing at 1350 °C.

Sm-doped YBCO Coated Conductor on the IBAD MgO Template Fabricated by MOD Process

AbstractMetal organic deposition (MOD) is one of the attractive processes for coated conductor applications because it is a non-vacuum cost-effective process. Many research groups have successfully fabricated high-JC YBCO coated conductor on the RABiTS and IBAD-YSZ or GZO template with the MOD process. Reports on MOD-processed YBCO coated conductors prepared on the IBAD-MgO template, however, are hardly found. The precursor solution was coated on the CeO2 capped IBAD-MgO template using the slot-die coating method, calcined at a temperature of 550oC, and fired at high temperatures for 2 h 30 min in a reduced oxygen atmosphere. Optimal processing resulted in YBCO coated conductors exhibiting JC value of 0.75 MA/cm2 at 77 K in self-field. The JC values of YBCO coated conductors were found very sensitive to their microstructure, critical temperature, and in-plane texture.

Self-field reduces critical current density in thick YBCO layers

The engineering current density in YBCO coated conductor applications can be improved in two ways. Either the critical current density should be improved or the superconducting films made thicker. Unfortunately, it has often been observed that the average critical current density decreases when the thickness of films increases. Suggested reasons for this behaviour include e.g. two dimensional pinning properties, microcracks and imperfect crystallographic alignment. However, it is often forgotten that the self-field effect unavoidably reduces the critical current density when the thickness of YBCO films increases and thereby total current rises. In this paper, the influence of self-field on the average critical current density is studied computationally as a function of film thickness. The situation is also scrutinized at different external magnetic fields in order to find ways to distinguish self-field effects from problems related to the manufacturing process. For this purpose, critical current measurements in external field perpendicular to the film surface are proposed.

Cube textured tapes for use in YBa2Cu3O7–δ-coated conductor applications

Abstract YBa2Cu3O7 is a high-temperature superconductor. Due to its complex lattice structure, belonging to the layered perovskite family, a strong biaxial texture of the grains is necessary to achieve technically relevant currents. In this paper, coarse-grained Ni, Ni-4at.% W, and electrolytic copper (E-Cu) obtained from swaged rods, and ultra-fine grained E-Cu produced by equal channel angular pressing, were thermo-mechanically processed. The development of the primary recrystallisation of the cube texture was studied and compared to determine the suitability of these materials as lattice-matched templates for coated conductor applications. Electron backscatter diffraction was performed to measure the texture and microstructure at various deformation levels and annealing temperatures. The texture sharpness formed in E-Cu with a coarse-grained starting microstructure proved superior, and it correlates with an anomaly observed in the temperature sensitivity of the flow stress in tensile tests, arising from crystal rotations.

Numerical analysis of the influence of buffer layer thickness on the residual stresses in YBCO/La 2Zr 2O 7/Ni superconducting materials

The present paper addresses a numerical investigation of the influence of buffer layer thickness on the residual stress in YBCO/La 2Zr 2O 7/Ni architectured materials under cryogenic conditions by using classical lamination theory (CLT) and finite element method (FEM) for coated conductor applications. YBCO/La 2Zr 2O 7 multilayer films were fabricated on Ni tape substrate using reel-to-reel sol–gel and pulse laser deposition (PLD) systems. The microstructural evolution of high temperature superconducting YBCO film and buffer layers with La 2Zr 2O 7 configuration grown on textured Ni tape substrates was investigated by using a scanning electron microscope (SEM). Thermal stress analysis of YBCO/La 2Zr 2O 7/Ni multilayer sample was performed by using CLT in the temperature range of 298–175 K in liquid helium media. The YBCO/La 2Zr 2O 7/Ni sample strip was solved by using FEM for linear or nonlinear cases in the temperature range of 298–3 K in liquid helium media. SEM observations revealed that crack-free, pinhole-free, continuous superconducting film and buffer layer were obtained by sol–gel and PLD systems. In addition to microstructural observations, it was found that the largest compressive stresses and failure occur in La 2Zr 2O 7 buffer layer due to its smallest thermal expansion coefficient. The thickness of La 2Zr 2O 7 buffer layer affects the failure. The stress component of σ x is the smallest in Ni tape substrate due to its largest thickness.

Low cost synthesis of high- Tc superconducting films on metallic substrates via ultrasonic spray pyrolysis

Ultrasonic spray pyrolysis technique has been used to deposit both in situ and ex situ high temperature superconducting films (HTSC) of Y 1Ba 2Cu 3O x (YBCO) and Bi 2Sr 2CaCu 2O x (BSCCO) compounds over various substrates. Nitrate precursor solutions are used to deposit films of ∼10 μm thickness. Both low temperature spray with substrate temperature T s < 500 °C and high temperature deposition with T s = 550–900 °C are carried out. Superconducting properties of these films are observed to vary with various parameters such as concentration of spray solution, deposition temperature and nature of substrate and annealing process. Best quality films show T c ( R = 0) of 89 K and J c of ∼4 × 10 4 A/cm 2 at 77 K and ∼ 10 5 A/cm 2 at 20 K. X-ray diffraction pattern reveals that the films are textured along c-axis. Successful attempt has been made to deposit in situ superconducting films over polycrystalline Ag for coated conductor applications. Various deposition and annealing conditions are optimized to control the diffusion of Ag from substrate to film, which otherwise can segregate into the grain boundaries and make the films non-superconducting.

Highlights in R&D for coated conductors in Japan

The current 5-year national project since 2003 for development of coated conductors (CC) using Y-system superconductors has passed for almost a half term and has achieved satisfactory results. In this paper, the current status and the future prospect are reviewed.The group of Fujikura Ltd. and SRL-ISTEC has worked on the long tape with high performance in the PLD-YBCO superconducting tapes on the IBAD-Gd2Zr2O7 buffered substrates. The highest value on the product of Ic×L in the world was marked by the result which were 51,940Am (212m×245A) by the SRL group. Fujikura Ltd. also realized the longest tape of 200m with a reasonable high Ic value of 100A. The values have been steadily improved and the trend is going to be continued, since the large equipments for both IBAD and PLD have been installed, and ready to work on large tapes with a high production rate.In another group, the long tape processing has been developed focusing on lowering the production cost. The extremely high Ic value of 470A was obtained in the film by the TFA-MOD method on CeO2 (PLD)/GZO(IBAD)/hastelloy substrate. In the efforts for the long tape in the process, a 25m long tape with its Ic value of 100A was realized by a continuous reel-to-reel system. Additionally, 100m class long tapes were also obtained by the MOCVD and PLD-HoBCO processes.Both groups are aiming to achieve the final goals of 500m long tapes with the high Ic value of 300A/cm-w by the production rate of 5m/h.Furthermore, the feasibility study for applications using coated conductors has been already started due to the above-mentioned success of long tape production. Several kinds of coils using long coated conductors such as a solenoid and a pancake coils and the spiral shaped conductors for cable applications were firstly made. Reasonable high performances were confirmed in the trials. For the future plans of coated conductor applications, the following power devices using coated conductors have been proposed; (1) power cable, (2) transformer, (3) motor, (4) fault current limiter, (5) cryocooler, and so on.

YBa2Cu3O7-x films prepared by TFA-MOD method for coated conductor application

The epitaxial growth of YBCO films both on (001) SrTiO3 (STO) and Ni-W biaxially textured metallic substrates prepared by metal-organic deposition (MOD) using a trifluoroacetic acid (TFA) solution is reported. The degree of epitaxy of the YBCO films was investigated by X-ray diffraction and scanning electron microscopy (SEM). The as deposited films exhibit good morphological and structural properties. The ω-scan of the YBCO films grown on (001) SrTiO3 single crystal substrate and on Pd/CeO2/YSZ/CeO2 buffered biaxially textured Ni-5at%W (Ni-W) tapes has a full width at half maximum (FWHM) of 0.12° and 3.4°, respectively. The ϕ-scan of (113) peak of YBCO film grown on Ni-W substrate has FWHM of 6.1°. The YBCO/STO film has a zero resistance critical temperature of Tc(R = 0) = 92 K and a critical current density Jc > 2 MA/cm2 at 77 K and in zero magnetic field.

Development of cube textured Ni–5 at.%W alloy substrates for coated conductor application using a melting process

Biaxially textured Ni–5 at.%W substrates have been prepared by cold rolling, followed by three different annealing routes. In this paper, the processes of melting Ni and W metals, flat rolling, various annealing methods are described in detail. The Ni–5 at.%W tapes annealed under either high vacuum or flowing Ar (7% H 2) gas were characterized by X-ray pole figures, ODF, EBSD as well as AFM analysis. The texture analysis indicated that as fabricated tapes have a sharp cube texture formed after annealing at a wide temperature range of 800–1100 °C. The high quality of cube orientation on tapes was obtained after a two-step annealing (TSA), where the percentage of the cube texture component was as high as 93.5% within a misorientation angle smaller than 8° from EBSD analysis. Furthermore, it was also observed that the number of twin boundaries in this tape decreased with respect to that of tapes annealed both in vacuum and one-step gas annealing. From AFM on 1 μm 2 areas, it was concluded that the roughness (RMS) on the tape surface reached 0.98 nm.

Highlights of coated conductor development in Japan

Coated conductors using RE–Ba–Cu–O (RE: rare earth element) superconductors havebeen expected to be used in many electric power applications since they have a high criticalcurrent density at liquid nitrogen temperature even under high magnetic fields. Manyefforts have been made in development of processing for a long coated conductor with highsuperconducting performance. In Japan, the new five-year national project for developmentof coated conductor processing was started in FY2003 and was scheduled until the end ofFY2007. The expected goals in this project are 500 m long tapes with a highIc value of300 A cm−1 W−1 at a productionrate of 5 m h−1 etc. The progress in recent years is remarkable, such as long tapes over 100 m in length with highIc valuesof over 100 A cm−1 W−1. The long tapes have been successfully realized by not only one institution but several.Additionally, a solenoid type magnet and a power cable using YBCO conductors havealready been demonstrated as one of the preliminary results for applications. For the futureplans of coated conductor applications, the following power devices using coated conductorshave been proposed: (1) power cable, (2) transformer, (3) motor, (4) fault current limiter,(5) cryocooler and so on.

Thin biaxially textured MgO and TiN films prepared by ion-beam assisted pulsed laser deposition for coated conductor applications

Thin MgO and TiN films have been grown using ion-beam assisted pulsed laser deposition on amorphous substrates. The texture development was analysed using an in situ reflection high-energy electron diffraction (RHEED). Above 250°C, a sharp cube textured nucleation layer was observed in both materials using an ion beam with an energy of 800eV under an angle of 45° relative to the substrate normal. The cube texture is not stable during further ion-beam assisted growth but could be preserved using homoepitaxial growth at higher temperatures. Resulting films revealed an out-of-plane orientation of about 6° and in-plane orientations of less than 13°. MgO and TiN showed close similarities in the texture development, i.e., the texturing was taken place within the first nanometres during nucleation. The observed texture development can be explained through a combination of overall energy minimization and anisotropic sputter rates. Highly textured YBCO layers were grown on these templates using additional buffer layers.

Microstructural Study of&amp;lt;tex&amp;gt;$rm EuBa_2rm Cu_3rm O_7$&amp;lt;/tex&amp;gt;Films by High Resolution X-ray Diffraction

Recent studies have shown that EuBa/sub 2/Cu/sub 3/O/sub 7/ (Eu123) films exhibit excellent surface morphology, high transition temperature, and interesting field-dependent critical current density, which are important for thin film devices and coated conductor applications. In this paper, we report our detailed studies of the microstructures of the Eu123 films deposited under different conditions. We have used high-resolution x-ray diffraction (HRXRD) to investigate the texture evolution and micro-domains of the films. Reciprocal space maps (RSMs) show that the a-axis growth of Eu123 films on SrTiO/sub 3/ substrates at a given deposition temperature is directly related to the deposition rate. On the other hand, by inserting a DyBa/sub 2/Cu/sub 3/O/sub 7/ (Dy123) seed layer, the a-axis growth can be totally eliminated. HRXRD study further reveals that micro-domains with different c-axis lattice parameters are often formed in the Eu123/Dy123, which may result from different oxygen content.