V. Boffa

Properties of in-situ laser-pulsed deposited YBCO thin films on MgO with SrTiO3 buffer layer

Abstract Epitaxial YBa 2 Cu 3 O 7− y films were grown by pulsed-laser ablation on SrTiO 3 buffered (100) MgO substrate without any pre- or postannealing of the substrate or STO buffer layer. The role of the SrTiO 3 layer is to provide a better lattice mach to the YBCO film and to prevent the interaction between YBCO and MgO. The as-obtained YBCO films exhibit excellent structural and superconducting properties with T c (R = 0) = 90 K , R(300) R(100) = 2.8, Δ T = 0.8 K , J c ≈ 10 6 A/cm 2 (at 77 K and zero magnetic field) and a full width half maximum of the (005) YBCO peak of about 0.18°. These characteristics are very close to those of the YBCO film deposited on SrTiO 3 single crystal.

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.

Phase stability for the in situ growth of Nd1+xBa2−xCu3Oy films using pulsed-laser deposition

We have determined the stability line in the 1/T−log[P(O2)] phase space for the synthesis of Nd1+xBa2−xCu3Oy (NdBCO) films. A systematic study of Tc, Jc, and ρ(T) dependence on oxygen partial pressure and temperature for the deposition of thin NdBCO films grown by pulsed-laser deposition was performed. The conditions for optimal NdBCO film growth were determined by varying oxygen partial pressure from 0.02 to 400 mTorr, and substrate temperature between 730 and 800 °C. The results show that the best NdBCO films are obtained at oxygen pressures in the range of 0.2–1.2 mTorr, depending on the substrate temperature. This is more than two orders-of-magnitude lower than the correspondent oxygen pressure appropriate for YBa2Cu3O7−δ film growth.

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.

Pulsed laser deposition of high critical current density YBa2Cu3O7−x/CeO2/Ni–W architecture for coated conductors applications

Epitaxial YBa2Cu3O7−x (YBCO) films with critical current densities exceeding 1 MA cm−2 (at 77 K in self magnetic field) were epitaxially grown on biaxially textured Ni 5 at% W (Ni–W) substrate using only a CeO2 buffer layer. Both YBCO and CeO2 films were deposited in situ by pulsed laser ablation. The oxidation behaviour of the Ni–W substrate enables the epitaxial growth of a CeO2 oxide buffer layer without using the forming gas (Ar 4%H2). SEM investigations of YBCO and CeO2 films reveal a dense microstructure without cracks or porosity. X-ray pole figures show a cube-on-cube epitaxy with a [100]YBCO//[110]CeO2//[100]Ni–W epitaxial relationship. The YBCO superconducting film has a good out-of-plane and in-plane texture with a full-width-half-maximum of 5.8° and 6.6°, respectively. No severe degradation of JC with film thickness, up to 1 μm thick films, was observed. The use of a single buffer layer on Ni–W substrates represents an important simplification in the scaling-up for long tape fabrication.

Self-field effects on critical current values in Tl-1223 tapes

Abstract We have measured the properties of a large number of Tl-1223 tapes as part of a programme to optimise the fabrication procedures. During this work we have noticed that the highest I c values are not proportionally dependent on the cross-sectional geometry of the tapes. There seems to be a limiting I c value of around 10–13 A in our samples (and those of many other groups). We have performed a series of experiments on self-field effects in Tl-1223 tapes and compared the results with calculated field distributions in tape-shaped conductors in different configurations. We conclude that the I c are self-field limited in these tapes when the currents being carried are around 10 A.

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.

High-quality surface YBCO thin films prepared by off-axis pulsed laser deposition technique

Abstract Epitaxial YBCO superconducting thin films have been grown in-situ by XeCl excimer laser ablation using an off-axis deposition geometry. This PLD geometry allows to obtain films with an excellent surface morphology without droplets or other particulates. The as-obtained YBCO films exhibit good structural and superconducting properties with T c( R = 0) =89.5 K, R (300) / R (100) =2.8, ΔT = 1.3 K, J c = 2 × 10 6 A/cm 2 (at 77 K and zero magnetic field) and full width half maximum of the rocking curve of the (005) YBCO peak of about 0.16°. These structural and electrical properties are very close to those of the YBCO films deposited by the conventional PLD on-axis geometry.

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.