R. K. Williams

Recent progress in the fabrication of high-Jc tapes by epitaxial deposition of YBCO on RABiTS

Abstract Progress made in the fabrication of rolling assisted biaxially textured substrates (RABiTS) and epitaxial deposition or formation of HTS on such substrates is reported. Significant progress has been made in understanding the role of meso-scale defects such as grain boundaries on long-range current flow of HTS conductors made using the RABiTS approach. Both experimental and theoretical calculations suggest that in well-textured samples these commonly present defects do not provide an intrinsic barrier to current flow in long-length conductors. Significant progress has also been made in the reel-to-reel deposition of oxide buffer layers and in the fabrication of long-length superconductors using the ex situ BaF 2 technique. Finally, non-magnetic, mechanically strengthened, biaxially textured metal templates have been fabricated with high quality oxide buffer layers. Epitaxial formation of YBCO on such substrates yields critical current densities over 1 MA/cm 2 at 77 K, 0 T.

Grain‐boundary compositions in YBa2Cu3O7−x from Auger electron spectroscopy of fracture surfaces

Comparison of Auger electron spectra obtained from intergranular and transgranular areas exposed on fracture surfaces of sintered YBa2Cu3O7−x specimens indicates that most grain‐boundary surfaces are deficient in oxygen and rich in copper compared to the bulk. The thickness of this region of altered composition is estimated to be in the range of 15–50 A. No evidence of segregation of impurities to grain boundaries was seen. The results suggest that the grain‐boundary layer is nonsuperconducting and a likely contributor to the problem of low critical current densities in these materials. It is believed that carbon at grain boundaries which has been reported results from incomplete calcination in material which is slightly off stoichiometry.

Cube-textured nickel substrates for high-temperature superconductors

The biaxial textures created in metals by rolling and annealing make them useful substrates for the growth of long lengths of biaxially textured material. The growth of overlayers such as high-temperature superconductors requires flat substrates with a single, sharp texture. A sharp cube texture is produced in high-purity Ni by rolling and annealing. We report the effect of rolling reduction and annealing conditions on the sharpness of the cube texture, the incidence of other orientations, the grain size and the surface topography. A combination of high reduction and high-temperature annealing in a reducing atmosphere leads to >99% cube texture, with a mosaic of about the rolling direction, about the transverse direction, and about the normal direction.

Epitaxial growth of La2Zr2O7 thin films on rolled Ni-substrates by sol-gel process for high Tc superconducting tapes

Abstract A solution process was used to grow epitaxial La 2 Zr 2 O 7 (LZO) buffer layers on roll-textured Ni (100) substrates to produce YBa 2 Cu 3 O 7− δ (YBCO)-coated conductors. The LZO precursor solution was prepared by an all alkoxide sol–gel route using mixed metal methoxyethoxides in 2-methoxyethanol. The partially hydrolyzed solution was either spin-coated or dip-coated onto the textured Ni substrates. The amorphous thin film was then heat treated at 1150°C under (96%)Ar/(4%)H 2 atmosphere for 1 h. X-ray diffraction (XRD) of the buffer layer indicated a strong c -axis orientation on the Ni (100) substrate. The LZO (222) pole figure revealed a single cube-on-cube texture. SEM images of the LZO buffer layer showed a dense microstructure without cracks. The YBCO deposited on the sol–gel LZO-buffered Ni substrates with sputtered YSZ and CeO 2 top layers had a critical current density of 480,000 A/cm 2 at 77 K and self-field.

Strengthened, biaxially textured Ni substrate with small alloying additions for coated conductor applications

Abstract Fabrication of a biaxially textured, strengthened Ni substrate with small alloying additions of W and Fe is reported. The substrates have significantly improved mechanical properties compared to 99.99% Ni and surface characteristics which are similar to that of 99.99% Ni substrates. High quality oxide buffer layers can be deposited on these substrates without the need for any additional surface modification steps. Grain boundary misorientation distributions obtained from the substrate show a predominant fraction of low-angle grain boundaries. A high critical current density, J c , of 1.9 MA/cm 2 at 77 K, self-field is demonstrated on this substrate using a multilayer configuration of YBCO/CeO 2 /YSZ/Y 2 O 3 / Ni–3at.%W–1.7at.%Fe. This translates to a I c /width of 59 A/cm at 77 K and self-field. J c at 0.5 T is reduced by only 21% indicating strongly-linked grain boundaries in the YBCO film on this substrate.

Epitaxial growth of BaZrO3 films on single crystal oxide substrates using sol-gel alkoxide precursors

Abstract Epitaxial BaZrO 3 (barium zirconium oxide) films were grown on single crystal substrates. A BaZrO 3 precursor solution was prepared by sol-gel synthesis using an all-alkoxide route. The barium precursors were prepared by reacting barium metal with 2-methoxyethanol, and zirconium precursors were prepared by exchanging ligands between zirconium n-propoxide and 2-methoxyethanol. The resulting BaZrO 3 precursor solution was partially hydrolyzed and spin-coated on sapphire (100), SrTiO 3 (strontium titanium oxide) (100), and LaAlO 3 (lanthanum aluminum oxide) (100) substrates. The films were post-annealed in oxygen at 800 °C for 2 min in a rapid thermal annealer. The coating and the annealing procedures were repeated three times to obtain the desired thickness, 300 nm. X-ray diffraction studies showed the presence of a single (100) cube texture for BaZrO 3 films on SrTiO 3 and LaAlO 3 substrates. The BaZrO 3 films grown on sapphire had a random texture. The BaZrO 3 films grown on SrTiO 3 substrates had a sharp texture compared to that on LaAlO 3 substrates. This may be due to the relatively smaller lattice mismatch between SrTiO 3 and BaZrO 3 .

Alternative Buffer Architectures for High Critical Current Density YBCO Superconducting Deposits on Rolling Assisted Biaxially-Textured Substrates

Although high critical current density (Jc) YBCO can be deposited on rolling assisted biaxially-textured substrates (RABiTS) with the YSZ/CeO2/Ni architecture, improvement in uniformity is needed due to the presence of two-component YBCO epitaxy and cracking in the CeO2 buffer. We have determined that Yb2O3 is an excellent buffer material that provides a single-component YBCO epitaxy. In addition, crack-free epitaxial Y2O3 can be consistently deposited onto textured Ni substrates. High quality YBCO films have been deposited, and Jc as high as 1.8 ×106 A/cm2 at 77 K has been obtained on this Yb2O3/Y2O3/Ni alternative RABiTS architecture.

SOL-GEL SYNTHESIS OF LAALO3; EPITAXIAL GROWTH OF LAALO3 THIN FILMS ON SRTIO3 (100)

A LaAlO{sub 3} precursor solution was prepared via an all alkoxide sol-gel route. The solution of lanthanum methoxyethoxide and aluminum methoxyethoxide in 2-methoxyethanol was prepared via ligand exchange starting from lanthanum isopropoxide and aluminum sec-butoxide and was used to make both LaAlO{sub 3} powders and films. Complete hydrolysis of the solution formed a gel that yielded well-crystallized LaAlO{sub 3} powders when fired in air at 800{degree}C. A partially hydrolyzed solution was spun-cast on SrTiO{sub 3} (100) single crystal substrates. Epitaxial films of LaAlO{sub 3} were subsequently formed during pyrolysis in O{sub 2} at 800{degree}C in a rapid thermal annealing furnace for a total of 8 min. The films were strongly c-axis oriented, verified by x-ray rocking curve results from the (003) plane with full-width at half-maximum (FWHM)=0.87{degree}, and had good in-plane texture shown by a {phi} scan of the (202) plane with FWHM=1.07{degree}. {copyright} {ital 1997 Materials Research Society.}

Effects of composition and processing on the superconductivity of La1+zBa2-zCu3Oy

Abstract The effects of composition, processing time ( t ), temperature ( T ) and O 2 partial pressure ( p O 2 ) on the superconducting transition temperature, T c , were investigated for La 1+ z Ba 2- z Cu 3 O y . Tallon and Flowers prediction of T c ≈ 100 K [Physica C 204 (1993) 237] at z = 0 could not be attained. Instead, the T c maximum of 93–94K is similar to that of the yttria analog. As z was increased, the T c values increased gradually by a few kelvin until z = 0.2, which is at the orthorhombic to tetragonal transition, and then decreased more abruptly, with no superconductivity for z ≥0.5. Crucial steps in the process at all z values include the highest temperature anneal (1065°C) in 0.1 MPa O 2 , reduction of the oxygen content at 950°C and cooling to 300–400°C, both at 100–1000 Pa O 2 , and reoxygenation at 0.1 MPa O 2 during slow cooling (5–20°C/min) to T c decreased fairly monotonically with decreasing y at z = 0.08, and a La excess of 0.2 and Cu nonstoichiometry on both sides of Cu/(La+Ba) = 1 generally depressed T c .

Oxidation induced decomposition of YBa2Cu3O7−x

At low oxygen potentials, YBa2Cu3O7−x decomposes by chemical reactions involving reduction. The results of this study show that the compound also decomposes at higher oxygen potentials. The initial decomposition products were found to be Y2BaCuO5 and a Ba‐Cu oxide phase. The Ba‐Cu phase was found to be a peroxide‐type and the decomposition can be described by a chemical reaction involving oxidation: 4(YBa2Cu3O7−x)+(1/2 − 3/2δ +2x)O2⇄2Y2BaCuO5+3Ba2Cu3O6−δ+CuO. At 800 °C the equilibrium pressure for this reaction is slightly <1 bar, and the enthalpy change is ∼80‐kJ/mol Y123. This result is not consistent with the conclusions drawn from some other studies of the stability of the superconducting Y‐Ba‐Cu‐O compounds, and the differences are discussed.