Michael Schindl

Preparation and characterization of (100) cube textured Ag substrates for in-plane oriented HTS tapes

A systematic study of texturing mechanisms in pure Ag is presented, the goal being to obtain a cube textured {100}001 tape that can be used as a substrate for superconducting coated conductors. It is shown that a sharp cube texture can be produced after pre-heated deformation and recrystallization. Optimal parameters are detailed. 99.99% pure Ag powder is used as starting material. Fine grained (15 µm) Ag ingots are pre-heated at 100 °C before standard cold rolling. The effect of pre-heating on deformation texture is to change the main texture from the brass orientation {110}112 to the Goss {110}001 one. The presence of a copper-type texture with relatively high values of the ODF (oriented distribution function) after the 100 °C pre-heated deformation can be understood as a slight increase of the stacking fault energy of Ag during the pre-heated deformation. The optimal annealing is achieved at 700 °C for 30 min in a primary vacuum. This leads to a sharp cube texture. The FWHM values of the three x-ray pole figures are, at no more than 10°, the smallest ever reported for cube textured Ag tapes. The formation of cube texture is interpreted in terms of orientation nucleation and orientation growth theories that play a common role during the texturing process. The stability of cube texture at high temperature up to 900 °C indicates that the tapes are suitable for epitaxial growth of superconducting oxides.

Stable (110) textured Ag ribbons for biaxially aligned YBa2Cu3O7-d coated tapes

Texture development in pure Ag was investigated in order to obtain {110}uvw textured Ag ribbons that can be used as a substrate for YBa2Cu3O7 superconducting coated tapes without any buffer layer. The starting material was 99.95% pure commercial Ag foils. A 20% deformation reduction was used at each step of the cold rolling process after which an optimal annealing was achieved at 800 °C for 4 h in a primary vacuum. This process leads to large-grain ribbons with the {110}112 orientation. A sharp and pure texture is obtained with a FWHM smaller than 6° in the three x-ray pole figures that were used to calculate the orientation distribution function. Crystallographic orientation maps achieved by electron back-scattering diffraction show a misorientation from the normal direction of less than 4°. A twinning mechanism is used to explain the formation of the {110}112 texture. The stability of the {110}112 texture is confirmed by further annealings up to 900 °C, a practical temperature for coated tapes preparation. Lengths up to 25 cm with a uniform {110}112 texture have been obtained.

Epitaxial growth of biaxially oriented YBCO films on silver

YBCO films were deposited on (100), (110) and (111) oriented silver single crystals and { 100} 100, { 110} 211 and { 012} 100 biaxially textured Ag substrates by pulsed laser deposition. It is shown that the (100) and (110) orientated single crystals and the { 110} biaxially textured Ag tape are all suitable for the deposition of YBCO thin films with c-axis in-plane alignment. The Jc of YBCO film deposited on { 110} 211 biaxially textured Ag foil is 7×105A cm-2 at 77 K, 0 T. A scheme for the regular growth of YBCO on silver was put forward.

Electrodeposition of biaxially aligned Tl-based superconductors on Ag tapes

An alternative preparation method of Tl(1212) and Tl(1223) biaxially aligned tapes on Ag was developed: electrodeposition. This method is based on the co-deposition of the metals by an electrochemical process followed by a high temperature heat treatment in order to form the superconducting phase. Nitrates of the metals are dissolved in DMSO and the deposition on conducting substrates is performed in a standard three electrodes cell. The thermal treatment is performed in a high isostatic pressure furnace (1 bar O/sub 2//50 bar He) in order to prevent Tl evaporation. The main advantage of this low cost technique is that it is very fast, (/spl sim/1 /spl mu/m/min) and can be easily scaled up to produce long length tapes in a continuous process. The desired stoichiometry is obtained by varying iteratively the concentration of nitrates in the starting solution. Moreover, the reaction temperature and time of the thermal treatment are drastically reduced with respect to the powder synthesis due to the amorphous form of the precursor. Excellent biaxial alignment is observed on both {110} Ag single crystal and textured Ag tapes, while epitaxial growth is observed on SrTiO/sub 3/ single crystal. T/sub c/ of 118 K and 60 K were measured resistively for Tl(1223) and Tl(1212) samples, respectively.

Reinforced {110} textured Ag/Ni composite ribbons for biaxially oriented YBCO coated tapes

New, mechanically reinforced, {110} textured Ag composite ribbons were fabricated as possible inexpensive substrates for coated conductors without any buffer layer. The key part of this work was the use of a thin but textured Ag outlayer on an AgMg, Ni, or Ni alloys core that ensures the mechanical reinforcement while decreasing the amount of Ag. A thin Cu (50 µm) foil was intercalated between rectangular Ag and AgMg, Ni, or Ni alloys pieces to obtain a tough bond. After sintering, the ingots sustained a cold plastic deformation up to thicknesses of 300, 100 and 50 µm without any intermediate annealing. The prepared composite ribbons were then recrystallized to obtain the {110} texture in the top Ag layer. For the Ag/AgMg ribbons, a further annealing in flowing oxygen was performed to reinforce the ribbons by an internal oxidation process in the AgMg layer without changing the {110}011 texture in the Ag outerlayer. A unique and stable {110}011 texture was obtained in Ag/Ni composite ribbons after annealing at 800 °C. The highest degree of texture was observed in 300 µm thick Ag/Ni ribbon, x-ray orientation distribution function and electro backscattered diffraction (EBSD) analysis of the top Ag layer in this ribbon revealed a sharp {110}011 texture with an in-plane misorientation angle of 10-15°. Meanwhile, a promising {110}011 texture was also found in 50 µm thick ribbons with 25° in-plane misorientation angle. In the last case, a substrate with no more than 16 µm of the textured Ag layer was produced by this new technique, which decreased the total amount of Ag by 40% compared with pure Ag ribbons of the same thickness.

Preparation of {110} textured Ag ribbons for biaxially aligned superconducting tapes

The aim of our work is to produce biaxially aligned HTS conductors by epitaxial growth on textured Ag. This paper will focus on the preparation of long length textured Ag ribbons. After cold rolling and annealing in vacuum at 800/spl deg/C, different {110} directions appear in the recrystallized pure Ag ribbons. The {110} texture is obtained after cold rolling by adding a 500/spl deg/C step before the annealing at 800/spl deg/C. If the Ag ingots are annealed at 600/spl deg/C in vacuum before the cold rolling, the 800/spl deg/C annealing leads to a sharp {110} texture. The quality of the texture is studied by X-ray and electronic diffraction. Three X-ray pole figures are used to calculate the ODF function, a SEM is used to obtain orientation maps from EBDP. The surface roughness is studied by AFM. We have produced lengths of 25 cm with a sharp {110} texture (FWHM=7/spl deg/ ODF peak). The characteristics of the two {110} and {110} orientation are presented in detail. It is shown that those {110} ribbons are suitable for epitaxial growth of superconducting cuprates. The Ag textured ribbons are used as substrates for different superconducting systems such as Tl-1223 and Y123.

Material developments in Ag/Bi,Pb(2223), Ag/RE(123) and Ag/Tl(1223) tapes: texture in HTS

Abstract High energy X-ray diffraction was used to compare Bi,Pb(2223) tapes prepared by the standard rolling technique and from a periodic pressing deformation process. Periodic pressing increases the critical current, although low energy X-rays on the tapes surface show a poorer texture. From transmission measurements, no significant effect of the different deformation process is detected on the texture of the whole ceramic core. The key toward high critical currents in Bi,Pb(2223) tapes has to be found in the thermodynamics of the phase formation process rather than in further improvement of texture. To get an enhanced critical current density in the Y123 and Tl2223 systems, a biaxial texture is mandatory. In Ag/RE(123) systems, there is a commensurability between the Ag surface and the superconducting cell as shown form sputtering experiments on oriented Ag single crystals. To be of industrial interest, long lengths of textured Ag ribbons have to be produced and fast deposition techniques are needed. We have shown that it is possible to control the texture formation in Ag ribbons. X-rays and EBSD analysis of highly textured {1 1 0}〈0 1 1〉 is reported. Electrodeposition of Tl(1223) and spray pyrolysis of RE(123) are presented as two fast and cheap techniques for biaxially aligned coated tapes preparation on textured Ag ribbons. Pure Tl(1223) is formed from the simultaneous deposition of all metals dissolved in DMSO followed by a phase formation process under high pressure (50 bar He/1 bar O 2 ). Pure and biaxially aligned Y123 is produced from an aqueous nitrate solution in a one step process at 900°C.

Development of YBa2Cu3O7−x superconducting films on (100) SrTiO3 and (110) Ag substrates by chemical spray pyrolysis

YBa2Cu3O7−x films have been deposited using spray pyrolysis on (100) SrTiO3, (110) Ag single crystals and {110} textured Ag tapes. An extended and systematic study on the optimization of composition, carrying gas flow, deposition temperature as well as annealing time was performed. It was found that these parameters are strongly correlated and affect the morphology, orientation and superconducting properties of the YBCO films. For the Y-123 film deposited on Ag single crystals, the Tc onset was at 90 K and a transport Jc value of around 104 A cm−2 (at 77 K) was observed. On SrTiO3 substrates, an in-plane orientation FWHM value of 0.70° was measured for the (102) pole figure. As-grown films already exhibit Jc values of 104 A cm−2, thus indicating that the correct structure is already formed during deposition. It was found that the temperature gradient in the pyrolysis zone played a fundamental role during the deposition. After modification of this heat gradient by adding a metallic susceptor, a noticeable improvement in the properties of the films was obtained. The films exhibited a Tc onset of 91 K (ΔTc = 1.5 K), while the Jc value at 77 K in self-field approached 105 A cm−2. According to our analysis, we find a 40 × 40 μm area in our best film where the critical current density reaches 5 × 106 A cm−2.

Spray pyrolysis of YBa/sub 2/Cu/sub 3/O/sub 7-x/ coated conductors on (100) SrTiO/sub 3/ substrate

YBa/sub 2/Cu/sub 3/O/sub 7-x/ films have been deposited using spray pyrolysis on (100) SrTiO/sub 3/ substrate. An extended study on the optimization of the composition, carrying gas flow, deposition temperature as well as annealing time was performed. It was found that these parameters are strongly correlated and affect the morphology, orientation and superconducting properties of the YBCO films. As grown films already exhibit J/sub c/ values of 10/sup 4/ A/cm/sup 2/, thus indicating that the correct structure is already formed during deposition. It was found that the temperature gradient in the pyrolysis zone played a fundamental role during the deposition. After modification of this heat gradient by adding a metallic susceptor, a noticeable improvement in the properties of the films was obtained. An in-plane orientation FWHM value of 0.70/spl deg/ was measured from the (102) pole figure. The films exhibited a T/sub c/ onset of 91 K (/spl Delta/T/sub c/=1.5 K), while the J/sub c/ value at 77 K in self-field approached 10/sup 5/ A/cm/sup 2/. Based on an assumed typical intragranular critical current density, we found a 40/spl times/40 /spl mu/m area in our best film where the assumed critical current density is well above 10/sup 6/ A/cm/sup 2/.

Spray Pyrolysis of

YBa/sub 2/Cu/sub 3/O/sub 7-x/ films have been deposited using spray pyrolysis on (100) SrTiO/sub 3/ substrate. An extended study on the optimization of the composition, carrying gas flow, deposition temperature as well as annealing time was performed. It was found that these parameters are strongly correlated and affect the morphology, orientation and superconducting properties of the YBCO films. As grown films already exhibit J/sub c/ values of 10/sup 4/ A/cm/sup 2/, thus indicating that the correct structure is already formed during deposition. It was found that the temperature gradient in the pyrolysis zone played a fundamental role during the deposition. After modification of this heat gradient by adding a metallic susceptor, a noticeable improvement in the properties of the films was obtained. An in-plane orientation FWHM value of 0.70/spl deg/ was measured from the (102) pole figure. The films exhibited a T/sub c/ onset of 91 K (/spl Delta/T/sub c/=1.5 K), while the J/sub c/ value at 77 K in self-field approached 10/sup 5/ A/cm/sup 2/. Based on an assumed typical intragranular critical current density, we found a 40/spl times/40 /spl mu/m area in our best film where the assumed critical current density is well above 10/sup 6/ A/cm/sup 2/.