Tolga Aytug

Electrical properties of epoxy resin based nano-composites

We investigate the electrical properties of composite materials prepared as nano- and sub-micron-scale metal-oxide particles embedded in a commercial resin. The filler particles are barium titanate and calcium copper titanate. The physical and structural characteristics of the constituents and the fabricated composites are reported. Electrical characterization of the composite samples is performed using time- and frequency-domain dielectric spectroscopy techniques. The electrical breakdown strength of samples with nano- and sub-micron-sized particles have better electrical insulation properties than the unfilled resin.

MOD approach for the growth of epitaxial CeO2 buffer layers on biaxially textured Ni-W substrates for YBCO coated conductors

We have grown epitaxial CeO2 buffer layers on biaxially textured Ni–W substrates for YBCO coated conductors using a newly developed metal organic decomposition (MOD) approach. Precursor solution of 0.25 M concentration was spin coated on short samples of Ni–3 at%W (Ni–W) substrates and heat-treated at 1100 °C in a gas mixture of Ar–4%H2 for 15 min. Detailed x-ray studies indicate that CeO2 films have good out-of-plane and in-plane textures with full-width-half-maximum values of 5.8° and 7.5°, respectively. High temperature in situ XRD studies show that the nucleation of CeO2 films starts at 600 °C and the growth completes within 5 min when heated at 1100 °C. SEM and AFM investigations of CeO2 films reveal a fairly dense microstructure without cracks and porosity. Highly textured YSZ barrier layers and CeO2 cap layers were deposited on MOD CeO2-buffered Ni–W substrates using rf-magnetron sputtering. Pulsed laser deposition (PLD) was used to grow YBCO films on these substrates. A critical current, Jc, of about 1.5 MA cm−2 at 77 K and self-field was obtained on YBCO (PLD)/CeO2 (sputtered)/YSZ (sputtered)/CeO2 (spin-coated)/Ni–W.

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.

Superhydrophobic materials and coatings: a review.

Over the past few years, the scientific community, as well as the worlds coatings industry has seen the introduction of oxide/polymer-based superhydrophobic surfaces and coatings with exceptional water repellency. Online videos have caught the publics imagination by showing people walking through mud puddles without getting their tennis shoes wet or muddy, and water literally flying off coated surfaces. This article attempts to explain the basics of this behavior and to discuss and explain the latest superhydrophobic technological breakthroughs. Since superhydrophobic surfaces and coatings can fundamentally change how water interacts with surfaces, and the fact that earth is a water world, it can legitimately be said that this technology has the potential to literally change the world.

Enhanced flux pinning by BaZrO3 and (Gd,Y)2O3 nanostructures in metal organic chemical vapor deposited GdYBCO high temperature superconductor tapes

We have formed BaZrO3 nanocolumns and (Gd,Y)2O3 nanoprecipitates in reel-to-reel metal organic chemical vapor deposition (MOCVD) processed (Gd,Y)Ba2Cu3O7−x coated conductors and increased the critical currents (Ic) of the conductors in applied magnetic fields to remarkable levels. A (Gd,Y)Ba2Cu3O7−x tape of 1 m in length with 6.5% Zr-additions and 30% composition rich in both Gd and Y showed Ic values of 813 A/cm width at (self-field, 77 K) and above 186 A/cm width at (1 T, 77 K). The strongly enhanced flux pinning over a wide range of magnetic field orientations can be attributed to the bidirectionally aligned defect structures of BaZrO3 and (Gd,Y)2O3 created by optimized MOCVD conditions.

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.

Enhancement of flux pinning and critical currents in YBa2Cu3O7−δ films by nanoscale iridium pretreatment of substrate surfaces

We have acquired positive results in a controlled study to investigate the effects of substrate surface modification on the growth-induced flux-pinning nanostructures in YBa2Cu3O7−δ (YBCO) films. Nanoscale iridium (Ir) particles were applied to single-crystal SrTiO3 substrate surfaces using dc-magnetron sputtering. Superconducting properties of YBCO films grown on the Ir-modified substrates, measured by transport and magneto-optical imaging, have shown substantial improvement in the critical current densities (Jc) at 77 K over those on untreated, control substrates. Results also show a nearly uniform enhancement of Jc over all orientations of magnetic field. Present results are found to be consistent with cross-sectional transmission electron microscopy investigations. Ultimately, the objective of this approach is to produce enhancements in the properties of coated conductors by a simple pretreatment of the substrate surface.

Superconducting MgB2 films via precursor postprocessing approach

Superconducting MgB{sub 2} films with T{sub c}=38.6K were prepared using a precursor-deposition, ex situ postprocessing approach. Precursor films of boron, {similar_to}0.5 {mu}m thick, were deposited onto Al{sub 2}O{sub 3} (102) substrates by electron-beam evaporation; a postanneal at 890{degree}C in the presence of bulk MgB{sub 2} and Mg metal produced highly crystalline MgB{sub 2} films. X-ray diffraction indicated that the films exhibit some degree of c-axis alignment, but are randomly oriented in plane. Transport current measurements of the superconducting properties show high values of the critical current density and yield an irreversibility line that exceeds that determined by magnetic measurements on bulk polycrystalline materials. {copyright} 2001 American Institute of Physics.

Enhanced flux pinning in MOCVD-YBCO films through Zr additions: systematic feasibility studies

Systematic effects of Zr additions on the structural and flux pinning properties of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (YBCO) films deposited by metal-organic chemical vapor deposition (MOCVD) have been investigated. Detailed characterization, conducted by coordinated transport, x-ray diffraction, scanning and transmission electron microscopy analyses, and imaging Raman microscopy have revealed trends in the resulting property/performance correlations of these films with respect to varying mole percentages (mol%) of added Zr. For compositions {le} 7.5 mol%, Zr additions lead to improved in-field critical current density, as well as extra correlated pinning along the c-axis direction of the YBCO films via the formation of columnar, self-assembled stacks of BaZrO{sub 3} nanodots.

High critical current density in epitaxial HgBa2CaCu2OX thin films

High quality superconducting HgBa2CaCu2Ox (Hg-1212) thin films have been reproducibly fabricated using cation-exchange method. The thin films have pure Hg-1212 phase and have smooth surface morphology. The superconducting transition temperatures of these films are in the range of 120–124 K. The critical current density Jc is up to 3.2×106 A/cm2 at 77 K and drops only by a factor of 2 at 100 K and self field. At 110 K, a Jc of 7.8×105 A/cm2 has been obtained. X-ray diffraction pole figures show that these films are epitaxially grown on LaAlO3(001) substrates, which is consistent with a χmin of 19% obtained using Rutherford backscattering/channeling analysis.