R.I. Tomov

Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3

Transport properties of aliovalent-ion-doped BiFeO3 (BFO) thin films have been studied in order to identify the cause of high leakage currents. Doping of 2at.% Ti4+ ions increased the dc resistivity by more than three orders of magnitude. In contrast, doping of 2+ ions such as Ni2+ reduced the dc resistivity by two orders of magnitude. Current–voltage (I–V) characteristics indicated that the main conduction mechanism for pure and Ni2+ doped BFO was space charge limited, which was associated with the free-carriers trapped by the oxygen vacancies, whereas in the Ti4+ doped BFO, field-assisted ionic conduction was dominant.

YBa2Cu3O7−δ coated conductor deposited onto non-magnetic ternary alloy NiCrW RABiTS tape by in situ pulsed laser deposition

Abstract Pulsed laser deposition of YBa 2 Cu 3 O 7− δ (YBCO)/buffer (Y 2 O 3 , YSZ, CeO 2 ) heterostructures have been performed in situ onto recently developed non-magnetic oxygenation resistant NiCrW tape. The influence of the critical processing parameters on texture development are investigated and the issues involved in NiO formation and relation to the substrate surface quality are discussed. The roles of Ni poisoning YBCO as well as local cation disorder are considered as possible current limiting factors. X-ray diffraction has been used for macro-texture evaluation. Both buffers and YBCO layers show good biaxial alignment with ω and ϕ scans having best YBCO FWHM values of 4.0° and 6.5° respectively. A comparison is made with results achieved on industrial Ni 50 Fe 50 tape. The film morphology has been characterized using atomic force microscopy and scanning electron microscopy. The cation disorder has been studied by Raman spectroscopy. Critical temperatures of 90 K (Δ T c =5 K) have been measured. Direct transport as well as magnetic measurements shows the critical current density J c is 0.2 MA/cm 2 in self-field at liquid nitrogen temperatures.

Hysteresis losses in YBCO coated conductors on textured metallic substrates

Hysteresis losses of YBCO coated conductors on two different textured metallic substrates - (NiFe and NiCrW)- were measured. The measurements were performed in a dc SQUID magnetometer at 5 K in applied magnetic fields up to 6 T. An YBCO layer of the sample on NiCrW substrate was cut to form 2 filaments and measured again in a perpendicular magnetic field as well as in a field at an angle of 45/spl deg/ with respect to the tape face. In a perpendicular magnetic field the hysteresis losses of the 2 filaments were a factor of about 0.6 lower than the losses of the original single filament. The influence on the magnetization of dividing a monocore tape into 2 and 4 filaments was numerically modeled using the critical state model. For full penetration and a perpendicular magnetic field the magnetization decreases proportionally with increasing number of filaments. Hysteresis losses in metallic substrates were substantially lower than the total losses of the composite tapes.

Investigation of the growth and stability of (1 0 0)[0 0 1] NiO films grown by thermal oxidation of textured (1 0 0)[0 0 1] Ni tapes for coated conductor applications during oxygen exposure from 700 to 1400 °C

Abstract Thin ( nucleation rather than competitive grain growth controls the texture in this case. Nevertheless, the removal of the native oxide layer is crucial. Further oxidation of these templates in the temperature range from 800 to 1400 °C produced single crystal-like NiO films with a thickness up to ~70 μm and an improved cube texture. Comparison with the literature data indicates that volume diffusion of Ni contributes predominantly to the NiO growth over the whole temperature range. These NiO films can act as a good oxidation barrier at lower temperatures (~800 °C) for coated conductor applications.

Texture and microstructure analysis of epitaxial oxide layers prepared on textured Ni–12wt%Cr tapes

Oxide layers for the preparation of YBa2Cu3O7−x coated conductors were grown on highly textured Ni–12wt%Cr tapes in pure oxygen using surface oxidation epitaxy at temperatures between 1000°C and 1300°C. Microstructural investigations revealed a layered oxide structure. The upper layer consists mainly of dense cube textured NiO. This is followed by a porous layer containing NiO and NiCr2O4 particles. A detailed texture analysis showed a cube-on-cube relationship of the NiCr2O4 spinel to the metal substrate. Untextured Cr2O3 particles in a nickel matrix were found in a third layer arising from internal oxidation of the alloy. A high surface roughness and mechanical instability of the oxide were observed, depending on oxidation temperature and film thickness. However, mechanically stable oxide layers have been prepared using an additional annealing step in a protective atmosphere. Additionally, mechanical polishing or a second buffer layer, which grows with a higher smoothness, may be applied to reduce the surface roughness for coated conductor applications.

A Comparative Study on the Conductive Properties of Coated and Printed Silver Layers on a Paper Substrate

The industrial sector of flexible printed electronics has shown a dynamic growth in the last decades. Therefore, demand for new inks, coatings and printing methods leading to improved performances of the electronic components at room temperature is also increasing. Here, we present a study on the conductive properties of silver layers obtained by different coating and printing methods. The results obtained proved that drop-on-demand inkjet printing of water-based inks containing micron-sized silver flakes with narrow-size distribution is a feasible method for in situ fabrication of conductive silver coatings that does not require additional heat treatment. A rigorous optimization Taguchi experiment was carried out considering the major process parameters. This experiment showed that the printing pressure was the dominant factor defining the quality of the printed coatings and tracks.

Inkjet printing of multiple Ce0.8Gd0.2O2 buffer layers on a Ni-5%W substrate

The successful inkjet printing of multiple cerium gadolinium oxide (Ce0.8Gd0.2O2) layers on highly textured Ni-5%W is reported using a stable ink, developing a solid-liquid interface comparable with that arising from dip coating. Two different approaches were used for the deposition of CGO layers using a 16-nozzle piezoelectric drop-on-demand print head. Two overlapping square arrays of droplets with constant volume and spacing were printed, with and without an intermediate CGO crystallization. The shortest possible heat treatment of the deposited layers was applied, potentially suitable for continuous large scale production. The results from X-ray diffraction show that the single phase Ce0.8Gd0.2O2 was obtained in all cases, but only the approach with intermediate CGO crystallization can produce a highly textured buffer layer. Optical micrographs and atomic force microscopy (AFM) also indicate the good quality of deposited films after heat treatment.

YBCO/Nd2CuO4/NiO/Ni coated conductors fabricated by liquid phase epitaxy based techniques

Liquid phase epitaxy (LPE) has a great potential in producing low-cost, high-current, coated conductors due to its fast growth rate in excess of 1 µm min−1 and the capability of growing thick films up to 10 µm without degrading the structural perfection or Jc. The main problem for LPE is the chemical reaction between the films and substrates at elevated growth temperatures. Former efforts have been focused on the reduction of growth temperatures. This has been proved to be unsuccessful due to the limited degree of temperature reduction; reducing the growth temperature also reduces the solubility of YBCO in the liquid, making the growth even more difficult. An alternative solution to this problem is to search for a special buffer which is particularly resistant to the attack of the high-temperature liquid. We have recently developed a new buffer, Nd2CuO4, which was very stable in the cuprate solution at temperatures around 950 °C and therefore extremely useful for LPE. Initial growth of YBCO on Nd2CuO4 buffered, surface oxidized Ni substrates showed a sharp superconducting transition at 90 K and transport Jc over 105 A cm−2 (77 K). Although YBCO could be grown on Nd2CuO4 by LPE without any other intermediate seed layer, growth of Nd2CuO4 on both sides of the NiO/Ni substrate with 100% coverage was not easy and had a low success rate. Complete coverage of Nd2CuO4 buffer on the NiO/Ni substrates was the critical step to avoid Ni contamination and achieve a high Jc.

Pulsed laser deposition of epitaxial YBCO/oxide multilayers onto textured metallic substrates for coated conductor applications

The development of a viable HTS coated conductor technology requires the deposition of biaxially aligned Y1Ba2Cu3CuO7-y (YBCO) layers onto flexible metallic substrates. Using pulsed laser deposition (PLD) YBCO/CeO2/YSZ heterostructures have been deposited onto biaxially textured Ni and Ni-alloy substrates. The influence of the critical processing parameters on the texture is investigated and some of the issues involved in the deposition of heterostructures are discussed. The texture in the layers has been characterized using X-ray Diffraction (XRD). The as deposited architecture shows YBCO layers with biaxial alignment with XRD (omega) and (phi) scans having best FWHM values of 3.5 degrees and 11 degrees respectively. The growth of the layers has also been studied using Atomic Force Microscopy. Tc at 90 K with sharp transition ((Delta) equals 2K) have been measured.

Irreversible properties of coated conductors deposited by PLD on textured technical substrates

Results on the angular and the field dependence of the critical transport current density as well as on the temperature dependence of the irreversibility fields in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// coated conductors are presented. The films were deposited by pulsed laser deposition on (Y/sub 2/O/sub 3//YSZ/CeO/sub 2/) triple buffer structures. Either textured magnetic Ni-Fe alloys or recently developed nonmagnetic Ni-Cr-W alloys with good biaxial texture (8.4/spl deg/ and 9.4/spl deg/ for /spl omega/ and /spl phi/ scans) were used as substrates. The influence of artificial pinning centers, introduced by fast neutron irradiation, on the irreversible properties of the YBCO films is discussed.