Bernhard Holzapfel

An all chemical solution deposition approach for the growth of highly textured CeO2 cap layers on La2Zr2O7-buffered long lengths of biaxially textured Ni–W substrates for YBCO-coated conductors

A reel-to-reel, dip coating process has been developed to continuously deposit epitaxial La2Zr2O7 (LZO) and CeO2 on 5 m long cube-textured {100} (001)Ni tapes. Recent results for La2Zr2O7 and CeO2 buffer layers deposited on long lengths of Ni substrate for the realization of YBa2Cu3O7−x (YBCO)-coated conductors are presented. The major achievement is the development of a new all chemical solution deposition (CSD) process leading to the formation of highly textured buffer layers at moderate annealing temperatures. Reproducible highly textured, dense and crack-free LZO buffer layers and CeO2 cap layers were obtained for annealing temperatures as low as 900 °C in a reducing atmosphere (Ar–5 at.%-H2). The thickness of the LZO buffer layers was determined to be (200 ± 10) nm per single coating; prepared cerium oxide layers showed a thickness of 60 nm ± 10 nm. Pulsed laser deposition (PLD) was used to grow YBCO films on these substrates. A Tc 0 of T = 90.5 K and ΔTc = 1.4 K was obtained on PLD-YBCO/CSD-CeO2 /CSD-LZO/Ni–5 at.% W, which shows the outstanding features of this new buffer layer architecture processed by CSD. The large layer thickness combined with low annealing temperatures is the main advantage of this new process for low-cost buffer layer deposition on Ni-RABiTS (rolling-assisted biaxially textured substrates).

Structure-related optical properties of laser-deposited BaxSr1−xTiO3 thin films grown on MgO (001) substrates

Abstract Thin films of Ba x Sr 1− x TiO 3 (BST), with 0≤ x ≤1, were grown on MgO (001) substrates by off-axis laser deposition at elevated substrate temperatures. The influence of the growth temperature and the film composition on the crystalline and morphological properties (film structure, grain size and shape, surface roughness), as well as on the refractive index n ( λ =550 nm) and the optical band gap energy, were studied by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and UV–vis spectral reflectometry. The dispersion of the refractive index was analyzed in terms of the parameters E o and E d of the single oscillator dispersion model of Wemple and DiDomenico. The films with x 3 films seems to be reduced and the surface roughness is increased. A relative decrease in the sticking coefficient of the ad-molecules in the temperature range 730–840 °C was found for Ba 0.65 Sr 0.35 TiO 3 .

Touching the properties of NbTi by carbon doped tapes with mechanically alloyed MgB2

Combining mechanical alloying and powder-in-tube processing is a promising way to obtain tapes with excellent properties. Taking advantage of the properties of nanocrystalline precursor powders, it was possible to obtain Jc values of 104A∕cm2 at 12.1T and 4.2K. Evidential substitution of carbon into the MgB2 changed the electron scattering and therefore raised the Bc2 up to 12T at 10K. Systematic investigation on the influence of the heat treatment showed that, although an interfering Fe2B reaction layer was formed, an excellent Jc of 104A∕cm2 at 14.3T and 4.2K was achieved.

Chemical solution deposition using ink-jet printing for YBCO coated conductors

This paper reports the successful application of ink-jet printing to the deposition of both continuous coatings and multi-filamentary structures of YBCO. Stable inks have been prepared using both the established TFA-MOD route and novel fluorine-free precursors with appropriate rheological properties for ink-jet printing. Continuous and well textured coatings with lengths exceeding 100?m and a thickness of 0.5??m have been deposited by electromagnetic ink-jet printing from TFA precursors on LZO-buffered Ni?W substrates and samples have achieved a Jc around 1.5?MA?cm?2 (self-field, 77?K). On single crystal substrates, continuous coatings and multi-filamentary structures have been deposited using piezoelectric ink-jet printing both from TFA-?and water-based precursors, achieving Jc values up to 3?MA?cm?2.

Ink-jet printing of YBa2Cu3O7 superconducting coatings and patterns from aqueous solutions

The objective of this paper is the development of ink-jet processing as a new technique for chemical solution deposition of YBCO coatings and patterns. Our research is mainly focused on the investigation and determination of the rheological parameters towards the printability of water-based inks in order to produce continuous YBCO coatings or multi-filamentary patterns on SrTiO3 substrates. A 0.185 mol L−1YBCO ink with a viscosity of 4.77 mPa s and a surface tension of 67.9 mN m−1, resulting in a ratio Re/We1/2 of 7.37, is developed. Its printing behaviour is further verified using a camera with strobed illumination to quantify the droplet velocity and volume. After optimization of the deposition parameters, a 350 nm thick YBCO coating showing preferential c-axis orientation could be grown on SrTiO3. This layer exhibits a critical current of 0.67 MA cm−2 at 77 K in self-field. Finally, the shape and dimensions of printed YBCO tracks were determined using optical microscopy and non-contact profilometry, showing 200 nm thick and 200 μm wide tracks.

Epitaxial Growth of Superconducting Ba(Fe1-xCox)2As2 Thin Films on Technical Ion Beam Assisted Deposition MgO Substrates

The biaxially textured growth of superconducting Co-doped BaFe2As2 (Ba-122) thin films has been realized on ion beam assisted deposition (IBAD) MgO coated conductor templates by employing an iron buffer architecture. The iron pnictide coated conductor showed a superconducting transition temperature of 21.5 K, which is slightly lower than that of Co-doped Ba-122 films on single crystalline MgO substrates. A self-field critical current density of over 105 Acm-2 has already been achieved even at 8 K. The current experiment highlights the potential of possible coated conductor applications of the iron-based superconductors.

Low-field magnetoresistance of La0.7Sr0.3MnO3 thin films with gradually changed texture

A gradual change of the grain orientation with the substrate temperature (TS) during deposition has been observed for La0.7Sr0.3MnO3 films on Y-stabilized ZrO2(100) substrates. Epitaxial growth has been found around TS≈800 °C. Both the low-field and the high-field magnetoresistance (MR) observed for the ferromagnetic state of these polycrystalline colossal MR manganite films were found to systematically depend on the degree of texture. The experimental results suggest a crucial role of a highly resistive region at the grain boundaries for this MR effect.

Intrinsic and extrinsic properties of epitaxial Nd2Fe14B films

Epitaxial Nd2Fe14B films with the c axis (easy magnetization axis) perpendicular to the film plane were prepared on a Ta(110) buffer on single crystalline Al2O3(0001) substrates using pulsed laser deposition. Due to the epitaxial film growth an almost perfect alignment of all magnetic moments perpendicular to the film plane is achieved. Coercivities up to 2 T are obtained for Nd-rich films where the Nd2Fe14B grains are magnetically decoupled. An analysis of the coercivity mechanism shows that the switching mechanism is nucleation dominated and the high coercivity is achieved by avoiding nucleation within the grains. Intrinsic properties like the spin reorientation temperature of 135 K and spin reorientation angle of 30° at 4.2 K of these films are in very good agreement to single crystal data.

Versatile fluoride substrates for Fe-based superconducting thin films

We demonstrate the growth of Co-doped BaFe2As2 (Ba-122) thin films on CaF2 (001), SrF2 (001), and BaF2 (001) single crystal substrates using pulsed laser deposition. All films are grown epitaxially despite of a large misfit of −10.6% for BaF2 substrate. For all films, a reaction layer is formed at the interface confirmed by X-ray diffraction and for the films grown on CaF2 and BaF2 additionally by transmission electron microscopy. The superconducting transition temperature of the film on CaF2 is around 27 K, whereas the corresponding values of the films on SrF2 and BaF2 are around 22 K and 21 K, respectively. The Ba-122 on CaF2 shows almost identical crystalline quality and superconducting properties as films on Fe-buffered MgO.

Oxypnictide SmFeAs(O,F) superconductor: a candidate for high–field magnet applications

The recently discovered oxypnictide superconductor SmFeAs(O,F) is the most attractive material among the Fe-based superconductors due to its highest transition temperature of 56 K and potential for high-field performance. In order to exploit this new material for superconducting applications, the knowledge and understanding of its electro-magnetic properties are needed. Recent success in fabricating epitaxial SmFeAs(O,F) thin films opens a great opportunity to explore their transport properties. Here we report on a high critical current density of over 105 A/cm2 at 45 T and 4.2 K for both main field orientations, feature favourable for high-field magnet applications. Additionally, by investigating the pinning properties, we observed a dimensional crossover between the superconducting coherence length and the FeAs interlayer distance at 30–40 K, indicative of a possible intrinsic Josephson junction in SmFeAs(O,F) at low temperatures that can be employed in electronics applications such as a terahertz radiation source and a superconducting Qubit.