A. Goyal

Irradiation-free, columnar defects comprised of self-assembled nanodots and nanorods resulting in strongly enhanced flux-pinning in YBa2Cu3O7−δ films

The development of biaxially textured, second-generation, high-temperature superconducting (HTS) wires is expected to enable most large-scale applications of HTS materials, in particular electric-power applications. For many potential applications, high critical currents in applied magnetic fields are required. It is well known that columnar defects generated by irradiating high-temperature superconducting materials with heavy ions significantly enhance the in-field critical current density. Hence, for over a decade scientists world-wide have sought means to produce such columnar defects in HTS materials without the expense and complexity of ionizing radiation. Using a simple and practically scalable technique, we have succeeded in producing long, nearly continuous vortex pins along the c-axis in YBa2Cu3O7?? (YBCO), in the form of self-assembled stacks of BaZrO3 (BZO) nanodots and nanorods. The nanodots and nanorods have a diameter of ~2?3?nm and an areal density (matching field) of 8?10?T for 2?vol.% incorporation of BaZrO3. In addition, four misfit dislocations around each nanodot or nanorod are aligned and act as extended columnar defects. YBCO films with such defects exhibit significantly enhanced pinning with less sensitivity to magnetic fields H. In particular, at intermediate field values, the current density, Jc, varies as Jc~H??, with ?~0.3 rather than the usual values 0.5?0.65. Similar results were also obtained for CaZrO3 (CZO) and YSZ incorporation in the form of nanodots and nanorods within YBCO, indicating the broad applicability of the developed process. The process could also be used to incorporate self-assembled nanodots and nanorods within matrices of other materials for different applications, such as magnetic materials.

Comparative Study of Thickness Dependence of Critical Current Density of Yba 2 Cu 3 O 7–δ on (100) SrTiO 3 and on Rolling-assisted Biaxially Textured Substrates

We investigated the dependence of critical current density ( J c ) on thickness of Yba 2 Cu 3 O 7−δ (YBCO) films grown by pulsed laser deposition on (100) SrTiO 3 (STO) and on rolling-assisted biaxially textured substrates (RABiTS). The thickness of YBCO films varied from 0.19 to 3 μm. The highest J c s of 5.3 and 2.6 MA/cm 2 at 77 K, self-field were obtained for 0.19-μm YBCO films on STO and RABiTS, respectively. J c was found to decrease exponentially with YBCO thickness on both substrates. However, the results suggest different mechanisms are responsible for the J c reduction in the two cases. On STO, growth of a -axis grains within c -axis films and broadening of the in-plane texture were observed in thick films. On RABiTS, degradation in cube texture as well as development of a porous surface morphology were found to correlate with film thickness.

High Critical Current Density YBa2Cu3O x Tapes Using the RABiTs Approach

Progress in the fabrication of epitaxial, high-Jc, biaxially aligned YBCO thick films on Rolling-assisted biaxially textured substrates (RABiTs) is reported. RABiT substrates comprise a biaxially textured metal substrate with epitaxial oxide buffer layers suitable for growth of superconductors. Oxide buffer layers have been deposited using three techniques: laser ablation, electron-beam evaporation, and sputtering. Epitaxial YBCO films grown using laser ablation on such substrates have critical current densities approaching 3 × 106 A/cm2 at 77 K in zero field and have field dependences similar to epitaxial films on single crystal ceramic substrates. Critical current densities in excess of 0.2 MA/cm2 have been obtained on stronger, nonmagnetic substrates. In addition, samples with Je of 12.5 kA/cm2 at 77 K have been fabricated. The highest strain tolerence obtained so far is 0.7% in compression and 0.25% in tension. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high-Jc wire capable of carrying high currents in high magnetic fields and at elevated temperatures.

Thickness dependence of microstructure and critical current density of Yba2Cu3O7−δ on rolling-assisted biaxially textured substrates

The change in microstructure associated with the decrease in critical current density ( J c ) of Yba 2 Cu 3 O 7−δ (YBCO) films with increasing thickness was examined. Samples of pulse laser deposited YBCO films varying in thickness from 0.19 to 3.0 μm on rolling-assisted biaxially textured substrates with an architecture of CeO 2 /YSZ/CeO 2 /Ni were prepared by tripod polishing for cross-sectional electron microscopy. More randomly oriented grains in the upper portion of the YBCO film surface were observed with increasing film thickness, resulting in less cube texture. In addition, increases in mismatch across the boundaries of the c -axis grains with increasing time during deposition, along with the development of BaCeO 3 and Y 2 BaCuO 5 phases at the YBCO/CeO 2 interface, contributed to the degradation of film properties. Surface outgrowths of the YBCO film were examined as well as the defect structures and second-phase formations within the films.

Fabrication of high-critical current density Yba 2 Cu 3 O 7−δ films using a fluorine-free sol gel approach

Superconducting YBa2Cu3O7− (YBCO) films with critical transition temperature Tc(0) of 90 K were fabricated via a fluorine-free, metal trimethylacetate based sol-gel route. Precursor films were spin-coated onto single-crystal (001) LaAlO3 (LAO) and (001) SrTiO3 (STO) substrates. Optimization of the burnout process resulted in films with excellent out-of-plane and in-plane texture. Transport critical current densities, Jc, of 1.6 and 1.1 MA/cm were obtained at 77 K in self-field on films grown on LAO and STO substrates, respectively. This is the first demonstration of high-Jc YBCO film fabrication using a fluorine-free, ex-situ process.

High critical current density YBa 2 Cu 3 O 7–δ coatings on LaMnO 3 -buffered biaxially textured Cu tapes for coated conductor applications

High critical current density (high-J c ) YBa 2 Cu 3 O 7 - Φ (YBCO) films were obtained by pulsed laser ablation on biaxially textured Cu substrates. To achieve epitaxy of LaMnO 3 (LMO) on Cu, thin epitaxial Ni overlayers were deposited on Cu tapes. The structure comprises the layer sequence of YBCO/LMO/Ni/Cu. For 200-nm-thick YBCO, self-field J c values exceeding 1 x 10 6 A/cm 2 at 77 K were achieved. Characterization of these short prototype conductors revealed good structural and morphological properties. Magnetic analysis suggested that hysteretic loss due to the ferromagnetic Ni overlayer is minimal.

Quantification and control of the sulfur c (2 × 2) superstructure on {100}〈100〉 Ni for optimization of YSZ, CeO 2 , and SrTiO 3 seed layer texture

We investigated the influence of a chemisorbed S template with c(2 × 2) structure on the epitaxial growth of different oxide buffer layers on {100}〈100〉 Ni. The sulfur superstructure spontaneously forms on the Ni surface during the texturing anneal as a consequence of segregation. However, depending on the initial S bulk concentration and/or specific annealing conditions, the S layer can cover less than the entire substrate’s surface. We show that an incomplete c(2 × 2) coverage causes degradation of the seed buffer layer texture as compared to the substrate texture. A simple step consisting of an H2S predeposition anneal can be used to control the superstructure coverage and optimize the seed layer texture.

Low cost, single crystal-like substrates for practical, high efficiency solar cells

It is well established that high efficiency (20%) solar cells can be routinely fabricated using single crystal photovoltaic (PV) materials with low defect densities. Polycrystalline materials with small grain sizes and no crystallographic texture typically result in reduced efficiences. This has been ascribed primarily to the presence of grain boundaries and their effect on recombination processes. Furthermore, lack of crystallographic texture can result in a large variation in dopant concentrations which critically control the electronic properties of the material. Hence in order to reproducibly fabricate high efficiency solar cells a method which results in near single crystal material is desirable. Bulk single crystal growth of PV materials is cumbersome, expensive and difficult to scale up. We present here a possible route to achieve this if epitaxial growth of photovoltaic materials on rolling-assisted-biaxially textured-substrates (RABiTS) can be achieved. The RABiTS process uses well-established, industrially scaleable, thermomechanical processing to produce a biaxially textured or single-crystal-like metal substrate with large grains (50-100 {mu}m). This is followed by epitaxial growth of suitable buffer layers to yield chemically and structurally compatible surfaces for epitaxial growth of device materials. Using the RABiTS process it should be possible to economically fabricate single-crystal-like substrates of desired sizes. Epitaxial growth of photovoltaic devices on such substrates presents a possible route to obtaining low-cost, high performance solar cells.

High in-field critical current densities in epitaxial NdBa2Cu3O7−δ films on RABiTS by pulsed laser deposition

We report the epitaxial growth and superconducting properties for NdBa2Cu3O7−δ (NdBCO) films grown on rolling-assisted, biaxially-textured substrates (RABiTS) by pulsed laser deposition. At the optimum deposition temperature of 760u2009°C, the critical current densities, Jc, at 77xa0K, self-field, of NdBCO films ranging in thickness from 0.13 to 0.25xa0µm were found to be in the range 2.2–3.4xa0MAxa0cm−2, with a lower thickness corresponding to a higher Jc. Compared to epitaxial YBa2Cu3O7−δ (YBCO) films, these NdBCO samples are found to have superior field and angular dependences of Jc. In an applied field of 1xa0T, NdBCO of the same thickness has more than twice the Jc of YBCO films with a significantly enhanced Jc peak for a field applied parallel to the c-axis. These results suggest the presence of an increased density of c-axis correlated pinning centres within NdBCO films compared to YBCO films.

Deposition and characterization of YBa 2 Cu 3 O 7−δ /LaMnO 3 /MgO/TiN heterostructures on Cu metal substrates for development of coated conductors

In this paper a novel buffer layer architecture consisting of LaMnO3/ MgO/TiN is proposed as a suitable structural and chemical template for the epitaxial growth of high-transition temperature (Tc) superconductors on Cu metal surfaces. Using techniques such as high-energy electron diffraction and scanning transmission electron microscopy, we present in situ and ex situ analyses of the buffer-layer and superconductor growth with focus on structural properties of the interfaces formed. While MgO is a good barrier to oxygen diffusion, we find that MgO alone is not a suitable buffer layer due to rapid Cu diffusion. Further, growth of MgO with a single epitaxy can be hindered by the presence of impurities such as S, which form strongly bonded superstructures on the metal surface. With the addition of a TiN layer as a barrier to Cu diffusion, oxide formation is suppressed, interfaces are clean, and a single cube-on-cube epitaxy is observed. While the Cu/TiN and TiN/ MgO interfaces are rough, the MgO and LaMnO3 layers planarize the material, leading to growth of smooth YBa2Cu3O7� (YBCO). Residual strain in the YBCO film is 0.25% or less and does not lead to apparent cracking. The superconducting properties of the samples were investigated by electrical transport and magnetization measurements. For the first time, high critical current density (Jc) values are reported for YBCO films grown on (001) single-crystal and {100}〈100〉 textured Cu surfaces without intervening metal coatings. Jc on single crystal-like substrates is as high as 3.5 MA/cm 2 . Reduced Jc of approximately 1 MA/cm 2 on rolled Cu tapes is limited by damage to the tape surface during the rolling process.