F. Fabbri

Design of an Industrially Feasible Twisted-Stack HTS Cable-in-Conduit Conductor for Fusion Application

Taking advantage of the large experience of the ENEA Superconducting Laboratory in the manufacture and characterization of large scale superconducting systems, a project was launched, aimed at using High Temperature Superconductor (HTS) 2G wires for the manufacture of a cable-in-conduit conductor (CICC). In particular, the main aim was the definition of a conductor design fully compatible with existing cabling technologies, to be promptly transferred to an industrial scale production. The considered layout is based on 150 HTS tapes, arranged as five layered structures of twisted tapes wound on a helically slotted core with external round jacket. All manufacturing steps (slotted core production, tape stacking and insertion into the ducts, external wrapping and jacketing) are fulfilled by using equipments and technologies available at TRATOS Cavi S.p.A. These CICCs are intended for operation using forced flow of Helium. A 2D local thermal model has been developed for the optimization of the cooling configuration. This conductor is designed to target 20 kA at 4.2 K and 15 T (or, alternatively, higher temperature, in self-field and LN2 cooling) corresponding to a Je of about 55 A/mm2. The production of a short dummy sample revealed that the exploited industrial production process is very promising for the development of HTS CICC.

Laser-ablation deposition of CeO2 thin films on biaxially textured nickel substrates

Abstract Ceria buffer layers were deposited by pulsed laser ablation on (100)[001] cube-textured Ni substrate for the fabrication of high-current Y–Ba–Cu–O tapes for power applications. The films were grown both on Pd-buffered Ni and on pure Ni substrates. The CeO 2 film deposited directly on Ni was not fully epitaxially grown as it had a polycrystalline component. On the contrary, the θ –2 θ X-ray diffraction pattern of Ni–Pd–CeO 2 architecture contained only the (h00) peaks of Pd, Ni and CeO 2 , indicating good epitaxy of Pd and CeO 2 on Ni, and the pole figure showed (100)[001] texture for Pd film and Ni substrate, and (100)[011] for CeO 2 film. The surface of the as-obtained CeO 2 films was smooth and continuous, hence appropriate for further YBCO film deposition.

Biaxial texturing of Ni alloy substrates for YBCO coated conductors

Abstract Biaxial cube texturing and surface quality of the metallic substrate are key parameters for high J c coated conductors. Experiments have demonstrated that the sharp cube texture can be developed up to 11 at.% V, 12 at.% Cr and 5 at.% W alloys; the respective cube textured area fractions result to be 96%, 97% and 99%. For all the studied alloys, FWHM values of ω - and φ -scans are in the range of 5–6° and 6–8°, respectively. Microstructural analyses reveal smooth surfaces with grooves localized at the grain boundaries of twinned cube textured and misoriented grains.

Development of biaxially aligned buffer layers on Ni and Ni-based alloy substrates for YBCO tapes fabrication

This paper presents the development of the CeO/sub 2/ buffer layers on both the non magnetic Ni-V and the Pd-buffered Ni substrates by pulsed laser ablation (PLD). The CeO/sub 2/ films show strong in-plane and out-of-plane orientations. The SEM studies showed that the CeO/sub 2/ films are both smooth and continuous. The magnetic and structural properties of the new Ni-V substrate are also presented. The results demonstrate that Ni-V is an alternative substrate for the fabrication of textured YBCO conductors.

Epitaxial growth of YBa2Cu3O7-δ on Ni89V11 non-magnetic biaxially textured substrate using NiO as buffer layer

The superconducting YBa2Cu3O7-δ/CeO2/NiO multilayer structure was grown in situ by pulsed-laser deposition on biaxially textured Ni89V11 non-magnetic alloy. The role of vanadium is to decrease the Curie temperature of Ni and to favour the formation of oriented NiO. The (00l)NiO buffer layer has been formed by the controlled oxidation of the Ni-V substrate under 10 mTorr oxygen pressure and at 700 °C. The critical current density of 0.6 MA cm-2, at 77 K and zero magnetic field, was obtained for 0.7 µm thick YBa2Cu3O7-δ films.

Surface morphology of pulsed laser deposited YBa2Cu3O7-δ and NdBa2Cu3O7-δ thin films on SrTiO3 substrates

A morphological study of on-axis NdBa2Cu3O7-δ (NBCO) and on-axis and off-axis YBa2Cu3O7-δ (YBCO) pulsed laser deposited films is presented. YBCO films deposited in on-axis geometry exhibit a high droplet density of about 107 cm-2 and three-dimensional features probably related to stoichiometric outgrowths. YBCO off-axis films present a surface with very low droplet density (about 105 cm-2) and the density of other defects like holes and stoichiometric outgrowths is less than 103 cm-2. In spite of a higher droplet density with respect to off-axis YBCO (about 106 cm-2), NBCO films have a surface without other defects. Moreover, the NBCO film morphology is almost insensitive to variation in the deposition conditions. Atomic force microscopy analysis revealed that YBCO off-axis deposited films grow in a spiral growth mode, while NBCO films exhibit a texture, probably due to an initial two-dimensional growth. The YBCO off-axis and NBCO films have a mean roughness in a 10×10 µm2 area of about 7.5 and 1.45 nm, respectively.

Textured Non-Magnetic Ni-V 10 % Alloy Tape for Epitaxial Growth of High TC Superconductors

The problem of developing non magnetic cube textured metallic substrates other than pure Ni, to be used for epitaxial deposition of Y-123 superconducting film (with an intermediate oxide buffer layer) is being tackled since three years by several laboratories. As reported in this paper, we have developed a new substrate, based on Ni-V 10 wt % alloy, which exhibits a cube texture comparable to Ni, but has a Curie temperature < 4.2 K; in addition, it has a hardness which is more than twice higher and a comparable oxidation resistance.

Cube-textured substrates for YBCO-coated conductors: microstructure evolution and stability

The realization of YBCO-based coated conductors with high critical current density involves the deposition of highly biaxially textured YBCO films. The use of epitaxial growth shifts this requirement to the template used for YBCO deposition. In the rolling-assisted biaxially textured substrate (RABiTS) approach, an appropriately oriented template is provided by a cube-textured substrate. The development of a cube texture is the result of recrystallization occurring in a heavily deformed tape, which is activated by high-temperature annealing. In the case of Ni-based alloys, thermal treatment at temperatures ranging from 900 to 1150??C for at least 30?min is commonly used. The determination of the minimum conditions for thermal treatments in terms of temperature?time involved in the recrystallization process is therefore of practical interest. In this work, Ni?5 at.% W alloy has been studied as a substrate for YBCO-coated conductors. 100??m thick tapes have been obtained through heavy cold rolling, followed by annealing in high vacuum. Different thermal treatments with rates of 20??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 of the substrate microstructure at the relatively high deposition temperature of YBCO films. It is found that the substrate begins recrystallizing at 700??C and that the cube texture is fully developed at temperatures higher than 800??C, while annealing at 900??C for 1?h produces a stable microstructure at the typical YBCO deposition conditions.

Critical current density of YBa2Cu3O7-x films with BaZrO3 inclusions on SrTiO3 and Al2O3 substrates

Recently, many efforts have been dedicated to the development of a reliable technology for the introduction of artificial pinning sites in YBa2Cu3O7-x (YBCO) films with the aim of improving the in-field Jc performances. One of the most effective technique resulted to be the inclusion of BaZrO3 (BZO) second phase embedded in the YBCO films. In this contribution we present Jc measurements on BZO-added YBCO films deposited on SrTiO3 (STO) and CeO2-buffered-Al2O3 (ALO) substrates. Samples were deposited by pulsed laser ablation technique using a composite YBCO + 5mol.% BZO target at the optimum conditions for fully oxygenated c-axis oriented YBCO films. Despite of a slight Tc reduction, BZO addition in YBCO-STO films resulted in an improvement of in-field performances with the appearance of a Jc plateau in the low field region which extends up to about 2.5 Tesla irrespective of the temperature at least in the investigated range (down to 65K). On the other hand, samples deposited on ALO did not exhibit any remarkable difference neither in the Jc value nor in the magnetic field dependences as compared with pure YBCO. The presence of 0° (magnetic field parallel to the c-axis) peaks in the Jc. angular behaviour revealed a c-axis correlated character of the pinning forces in BZO added YBCO films grown on both STO and ALO substrates. X-ray diffraction measurements and AFM investigations were carried out in order to determine the influence of BZO addition on films crystalline quality and microstructure.

The Effects of Surface Contamination on the Biaxially Textured Substrate for YBCO Thick Film Deposition

The epitaxial growth of YBa2Cu3O7-x (YBCO) films on biaxially textured substrates is one of the most promising technique for the fabrication of high current superconducting tapes operating at high temperature. Ni is very attractive as substrate because it easily develops a (100)[001] cubic texture. The low oxidation resistance represents the main drawback of the Ni substrate. In order to better assess the role of oxygen on the Ni substrates, a surface physics technique as Auger spectroscopy has been used. It has allowed to evaluate the amount of impurities for different Ni processing and exposure to the air. The results demonstrate that the surface contamination can be efficiently removed by RF sputtering before buffer layer deposition. This procedure allows to obtain CeO2/Pd/Ni architecture by laser ablation with a good epitaxy both of Pd and CeO2 films. On the contrary, when CeO2 is directly deposited on Ni a low epitaxy is obtained. The Auger analysis confirms that the formation of (111) NiO at the Ni-CeO2 interface hampers the epitaxial growth of the ceria film.