Jaeun Yoo

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.

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.

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°.

The deposition characteristics and the structural nature of the deposit in the chemical vapour deposition of boron

Abstract Boron was deposited onto graphite by a chemical vapour deposition (CVD) technique using gaseous mixtures of BCl3 and hydrogen. The CVD of boron is a thermally activated process with an apparent activation energy of 18.8±0.7 kcal mol-1 at low temperatures and high gas flow rates. At high deposition temperatures and low gas flow rates the deposition rate depends on the gas phase mass transfer. Boron is deposited as an amorphous material below 1250 K, and mainly as the β rhombohedral modification above 1250 K. However, a small amount of β tetragonal boron is deposited together with the β rhombohedral boron. The β rhombohedral boron has a strong (104) preferred orientation at 1500 K. However, the preference for the (104) orientation is reduced as the BCl3 fraction or the gas flow rate increases.

Relation between the critical current and the n value of ReBCO thin films: A scaling law for flux pinning of ReBCO thin films

Detailed field and angle dependencies of the critical current and the n value for a SmBCO coated conductor have been measured. It was found that the field dependence of the n value can be fitted by an empirical power law with three parameters including the irreversibility field. We also found that there is a correlation between the critical current and the n value which can be described by the Kramer model including thermal activation. The model fits the field dependence of the empirical critical current data at various angles and temperatures with three fitting parameters, the pinning force maximum, the g factor, and the upper critical field. The upper critical field found from fitting was higher than the irreversibility field, and the angular dependence of the upper critical field is in agreement with the Tinkham model. The pinning force maxima do not show a correlation with the upper critical or the irreversibility fields, which is attributed to the difference in the pinning mechanism with a variation ...

Fabrication of YBCO coated conductors using nickel tapes textured in single crystalline qualities

Ni tapes were textured by taking advantage of their secondary recrystallization. The 18 cm long tapes obtained had textures of single crystalline quality with their [001] axes tilted with respect to the surface and their [010] axes parallel to the rolling direction. YBCO/CeO2/YSZ/CeO2 films grown on the Ni tape had the same crystalline orientations. Magnetic field dependent I-V relations were measured on a 5 cm section of the tape. Jc defined by 1 mV criterion was 1.5×105 A cm-2 at 77 K under zero field and was reduced by ~50% under the applied magnetic field of 5 T.

Scanning Hall probe measurements of field distributions of a coated conductor under applied fields

We measured the field profiles near the surface of a coated conductor (CC) under various applied fields by using the scanning Hall probe method. The field, applied in the normal direction, was increased from zero to 171.5 Oe and then decreased to −58.8 Oe. We could not analyse our data completely by the direct use of Brandts calculation but by a modification with unusual field dependences of the introduced parameters. Since Brandts original calculation was based on homogeneous films, it was not suitable for CCs with coarse granular structures. The modified calculations with appropriate parameters are related to the coarse granular structures. Those parameters, D, Jc, and R, represent the three characteristics of the flux penetration network: the average distance of flux penetrations, the density of critical sheet currents, and the range of meandering of the flux penetration front, respectively. The external field dependences of these parameters were different from those of the classical critical state model.

The effects of a magnetic field dependent critical current on the hysteresis loss of a coated conductor

We calculated the current distribution in a coated conductor (CC) from magnetic induction profiles measured using the scanning Hall probe method. An external magnetic field was applied in the normal direction with respect to the tape surface and was decreased stepwise from Hpeak to −Hpeak. From the hysteretic behavior of the current distributions, the energy loss, QM, was calculated for various values of Hpeak. Comparison of the present results with Brandts theoretical calculations revealed that his formula is still valid if one substitutes the magnetic field dependent critical current density into his equation. The theoretical results obtained were consistent with the experimental QM values obtained.

Comparative Study on the Profiles of Intra-Granular Critical Current Density in SmBCO and YBCO Coated Conductors

We measured the magnetic hysteresis loops (M-H loop) of SmBCO coated conductor (CC) and YBCO-CC by using SQUID-magnetometer. These samples were fabricated by RABiTS method and IBAD method. We found the peak shifts of their M-B loops were qualitatively different. To understand this, we measured the magnetic field profiles, H(x), of SmBCO-CC and YBCO-CC by using scanning Hall probe method and calculated their current profiles, J(x). We found that the contrasting difference of M-B loops of the two types of CC was partly due to the inhomogeneous distributions of field and current, which depend on the sample shapes.

Fabrication of growth of textured YBCO thin film on very thin metallic substrate

Abstract A well textured YBCO thin film was grown on a 8000Athick Pt tape with a STO buffer layer. In order to obtain the tape, a multilayer STO/Pt/STO/BaO was deposited on a YSZ(100) substrate and then the STO/Pt/STO layer was separated from the substrate by solving the BaO layer in water. A YBCO film was deposited on the STO/Pt/STO tape. It was confirmed by XRD 2θ-scan and φ-scan measurements that the single crystalline Pt layer, the single crystalline STO buffer layer, and the well textured c-axis normal YBCO film were grown. The R-T measurement showed T c was 79K.