Dojun Youm

Comparative study on the crack formations in the CeO2 buffer layers for YBCO films on textured Ni tapes and Pt tapes

Abstract The crack formations in the CeO 2 buffer layers on textured Ni tapes and Pt tapes, which were used for the substrates of the high T c superconducting YBCO films, were comparatively studied. Since these two metallic tapes, which were textured by rolling and recrystallization, have very different properties of thermal expansion coefficients, lattice matching conditions, and chemical reactivities with the buffer layers, the comparisons of their buffer layer growth features were suggestive. Both Ni and Pt showed no salient reactive diffusion in the interface with the buffer layers. Numerous cracks were formed in the CeO 2 buffer layer on Ni, while very few cracks were formed in the buffer layer on Pt. These results indicated it was unlikely that the effect of the thermal expansion coefficient differences of the films and the substrate caused the cracks. The CeO 2 film deposited by the sputtering method on the thin template CeO 2 film predeposited on Ni by e-beam evaporation, showed very few cracks. Detailed SEM observations indicated a possible relation between the cause of the crack formations and the lattice mismatching which seemed to distort the lattices and to produce the lattice defects.

Ultra-High Performance, High-Temperature Superconducting Wires via Cost-effective, Scalable, Co-evaporation Process

Long-length, high-temperature superconducting (HTS) wires capable of carrying high critical current, Ic, are required for a wide range of applications. Here, we report extremely high performance HTS wires based on 5 μm thick SmBa2Cu3O7 − δ (SmBCO) single layer films on textured metallic templates. SmBCO layer wires over 20 meters long were deposited by a cost-effective, scalable co-evaporation process using a batch-type drum in a dual chamber. All deposition parameters influencing the composition, phase, and texture of the films were optimized via a unique combinatorial method that is broadly applicable for co-evaporation of other promising complex materials containing several cations. Thick SmBCO layers deposited under optimized conditions exhibit excellent cube-on-cube epitaxy. Such excellent structural epitaxy over the entire thickness results in exceptionally high Ic performance, with average Ic over 1,000 A/cm-width for the entire 22 meter long wire and maximum Ic over 1,500 A/cm-width for a short 12 cm long tape. The Ic values reported in this work are the highest values ever reported from any lengths of cuprate-based HTS wire or conductor.

Fabrication of Sm1Ba2Cu3O7 coated conductors using the co-evaporation method

We fabricated a coated conductor 7.5 m long and 9 mm wide using a biaxially textured Ni tape. The standard buffer layers, CeO2/Y SZ/CeO2, were used. A thick Sm1Ba2Cu3O7 (SBCO) film was deposited using the co-evaporation method, followed by in situ deposition of an Ag protection layer. The whole length of SBCO film was deposited in a single batch process, which took 7 h. We checked the distribution of critical currents (Ic) by I–V measurements in all the sections comprising the tape. Ic at 77 K was 90–130 A in most parts of the whole length.

Growth of textured c-axis normal Y1Ba2Cu3O7−δ thin films on yttria-stabilized zirconia substrates with crystalline axes tilted with respect to the surfaces

Abstract Well textured c -axis normal Y 1 Ba 2 Cu 3 O 7−δ (YBCO) thin films on yttria-stabilized zirconia(YSZ) substrates whose crystalline axes are tilted by 45°, 30°, 20°, 10°, 0° (untilted) with respect to the surfaces have been fabricated using dc-sputtering. CeO 2 buffer layers of the same thickness, 15 A, were deposited on all substrates prior to YBCO deposition. For untilted crystalline YSZ substrate, an oxygen ion milling on its surface is necessary. Microscopic effects of the ion milling were investigated by X-ray photoelectron spectroscopy and atomic force microscope. The crystalline qualities of the YBCO films were investigated by X-ray diffraction 2 θ and Ф-scan measurements. For all films, the measured zero resistance temperatures ( T c s) and critical current densities ( J c s) were 84–85 K and above 10 6 A/cm 2 at 77 K, respectively.

Comparative studies on the growth conditions of CeO2 and Y2O3 buffer layers on NiW tapes

We comparatively studied the growth conditions of Y2O3 films and CeO2 films on biaxially textured Ni and NiW tapes. We found that the windows of both substrate temperatures and oxygen partial pressures for the proper growth of CeO2 films were wider than those of Y2O3 films. XRD data showed that the qualities of in-plane and out-of-plane textures of the two types of films were almost similar. The SEM observations indicated that the surface of the CeO2 films was smoother than those of Y2O3 films. The inferiority of CeO2 films to Y2O3 films was crack formation. However, we found that if YSZ over-layers were grown on the CeO2 films using dc-reactive sputtering, the cracks were not observed on the surface of the YSZ films anymore.

Tensile stress effects on critical current densities of coated conductors

Tensile stress dependent critical current densities (Jc) of two types of coated conductors, which were fabricated on a pure Ni tape and a NiCr (13 atomic %) tape respectively, were measured at 77 K. Until the stress reached certain values (Kc), the Jc of each sample was constant, it then began to decrease as the stress was increased further. The Kc for tapes of Ni and NiCr were ~100 MPa and ~270 MPa respectively, which means Kc was enhanced almost three times for the sample fabricated on the NiCr. According to the detailed changes of voltage signals for applied currents under various stresses, the degradation process of each sample could be divided into four different regimes.

Nondestructive Measurement of Critical Current Distribution of SmBCO Coated Conductor Using Hall Probe

The Hall probe measurement system was used to measure the critical current distribution of superconducting coated conductor. The system consists of reel to reel moving apparatus, 7 array Hall probe, a rotary encoder and permanent magnet. The magnetic field profile across the width of superconducting coated conductor using Beans critical state model was calculated. The effect of various parameters of the formulas on the magnetic field distribution and the effect of shape and size of artificial defects, which were formed on the surface of SmBa2Cu3O7-d(SmBCO) coated conductor using laser marking system, on the Hall probe magnetic field signal of the Hall probe measurement system was investigated.

Thick SmBCO/IBAD-MgO Coated Conductor for High Current Carrying Power Applications

Long-length (Sm_1+xB_2-xC₃O_7-y) coated conductor was fabricated using ion beam assisted deposition (IBAD)-MgO template (LaMnO₃/IBAD - MgO/Y₂O₃/Al₂O₃/Hastelloy) by the reactive co-evaporation method for electric power applications. SmBCO superconducting layers with thicknesses from 1 to 3 ¿m were deposited to investigate the effect of SmBCO thickness on the in-plane texture, surface morphology and superconducting properties. In this study, the optimal deposition conditions were examined to make thick SmBCO coated conductor without degradation of critical current density. The oxygen partial pressure and temperature were controlled in the range of 1~20 mTorr and 600 ~ 800°C, respectively. SmBCO/IBAD-MgO coated conductors showed high critical current densities over 2.0 MA/cm² with small amount of degradation by increasing the thickness of superconducting layer up to 3 ¿m. The critical current of 637.5 A/cm-w (J_C = 2.12 MA/cm²) at 77 K in self- field was obtained in 3 ¿m-thick and 1 m long coated conductor. This is the highest critical current reported so far of the coated conductor with SmBCO superconducting layer deposited by co-evaporation.

Textures of CeO2 films grown on biaxially textured Pt0.7Pd0.3 substrates

Abstract We investigated the textures of CeO 2 films grown on biaxially textured Pt 0.7 Pd 0.3 tapes. These might be of interest for developing new substrates of high Tc superconducting coated conductors for small scale applications. Using the rf sputtering method we deposited CeO 2 films and characterized their textures by X-ray diffraction and scanning electron microscopy. We found that the growth of the CeO 2 film was highly influenced by the operating gas as well as the inclination of the substrate during deposition. In the mixed gas of Ar+O 2 , the CeO 2 films grew with polycrystalline textures composed of (111)- and (002)-oriented domains. The CeO 2 films deposited in an atmosphere of Ar, with inclination angles of less than 45°, showed similar polycrystallinity. However, the growth of the CeO 2 films was always well textured in Ar gas with an inclination angle larger than 50°.

Effects of the MgO layer thickness on the crystalline orientation in Y1Ba2Cu3O7−δ/CeO2/MgO/SrTiO3 multilayer growth

Abstract The X-ray diffraction (XRD) - o scan measurements of an Y 1 Ba 2 Cu 3 O 7−δ /CeO 2 /MgO/SrTiO 3 multilayer revealed noticeable changes in the effect of the MgO seed layer on the crystalline orientation of the YBCO/CeO 2 overlayer as the thickness of the MgO layer increased from 20 to 60 A. Such a phenomenon was confirmed to originate from the thickness-dependent characteristics of the MgO seed layer itself. Another interesting property of the MgO seed layer, which was deposited by e-beam evaporation, is that the CeO 2 overlayer did not grow epitaxially on it unless its surface was heat treated in an oxygen atmosphere.