Linfei Liu

Fast Growth Processes of Buffer Layers for YBCO Coated Conductors on Biaxially-Textured Ni Tapes

Epitaxial growth of CeO2 buffer layers on biaxially textured (001) Ni tapes was studied using reel-to-reel pulsed laser deposition. Relationship between microstructure and deposition parameters was systematically studied in order to develop reliable long tape coating processes. Prior to buffer layer deposition, Ni tapes were in situ annealed in forming gas (97% argon + 3% hydrogen) under various pressures and tape speeds to accelerate reel-to-reel annealing processes. It was found that orientation and texture of CeO2 buffer layers were sensitive to deposition parameters. X-ray diffraction analyses showed that CeO2 buffer layers had pure (001) orientation at forming gas pressure of 5 mTorr and 700degC. Under optimized deposition conditions, highly (001) oriented CeO2 buffer layers have been achieved at a high tape speed of 20-50 m/h. In-lane texture of CeO2 buffer layers was 7 degree which was comparable to the value of Ni tapes.

Microstructure Analysis of High-Quality Buffer Layers on Textured NiW Tapes for YBCO Coated Conductors

Epitaxial CeO2 seed layer and multiple CeO2/YSZ/CeO2 buffer layers were in-situ deposited in a reel to reel pulsed laser deposition chamber on rolling assisted biaxially textured NiW tapes. Epitaxial relationship and surface morphology of each layer were systematically studied by x-ray diffraction (XRD), high resolution scanning electron microscope (SEM) and atomic force microscope (AFM). The in-plane texture of CeO2 cap layer is 6-7 degrees, which was comparable to NiW substrate. During deposition parameter optimization, it was observed that the orientations of YBCO layers were related with individual NiW grains. Although c-axis orientation was the dominant orientation of YBCO films under optimum deposition conditions, a small amount of a-axis orientation was observed in YBCO films. These sun-micro scale a-axis orientated YBCO grains were not uniformly distributed in YBCO films, but only located inside a small percentage of NiW grains. SEM observation showed that grain size of buffer layers was sensitive to deposition parameters. The influence of buffer layer grain size and surface roughness on the epitaxial quality of YBCO layers was investigated.

Influence of Buffer Layer Surface Morphology on YBCO Critical Current Density Deposited on NiW Tapes

YBCO thin films were grown on NiW tapes under continuous moving deposition process. All of YBCO layer, buffer layer, and cap layer were deposited in a compact reel-to-reel pulsed laser deposition system. Since high critical current density is the most important and effective factor to improve performance/price ratio of coated conductor for large scale applications, we focused our research work on enhancing critical current density of YBCO layers on NiW substrates. It was found that superconducting transport properties of YBCO layers were dependent on not only in-plane texture but also surface morphology of buffer layers, especially surface structure and large particles along grain boundary in NiW substrates. High quality YBCO layers with >; 4.0 × 106 A/cm2 (at 77 K, in zero magnetic field) were fabricated on cap layers with nano-scale surface roughness.

Effect of Deposition Temperature on the Epitaxial Growth of YBCO Thin Films on RABiTS Substrates by Pulsed Laser Deposition Method

Epitaxial YBCO thin films were grown on CeO2/YSZ/CeO2 buffered RABiTS substrates by PLD method and the effects of deposition temperature on their microstructure and the critical current were studied. YBCO thin films were prepared with substrate temperature from 700 to 820°C. The YBCO grown at below 740°C showed mixed a-axis and c-axis orientation, and the film grown at higher temperature showed high c-axis orientation. The (001) preferred orientation of the YBCO films was improved with increasing the temperature. While the critical current of the YBCO thin film increased firstly with increasing substrate temperature and had a maximum value at 770°C. The Ic of the YBCO film with 0.2 ¿m thickness was 66 A (Jc = 3.3 MA/cm2) at 77 K and 0 T external field.

Dependencies of microstructure and stress on the thickness of GdBa2Cu3O7 − δ thin films fabricated by RF sputtering

GdBa2Cu3O7 − δ (GdBCO) films with different thicknesses from 200 to 2,100 nm are deposited on CeO2/yttria-stabilized zirconia (YSZ)/CeO2-buffered Ni-W substrates by radio-frequency magnetron sputtering. Both the X-ray diffraction and scanning electron microscopy analyses reveal that the a-axis grains appear at the upper layers of the films when the thickness reaches to 1,030 nm. The X-ray photoelectron spectroscopy measurement implies that the oxygen content is insufficient in upper layers beyond 1,030 nm for a thicker film. The Williamson-Hall method is used to observe the variation of film stress with increasing thickness of our films. It is found that the highest residual stresses exist in the thinnest film, while the lowest residual stresses exist in the 1,030-nm-thick film. With further increasing film thickness, the film residual stresses increase again. However, the critical current (Ic) of the GdBCO film first shows a nearly linear increase and then shows a more slowly enhancing to a final stagnation as film thickness increases from 200 to 1,030 nm and then to 2,100 nm. It is concluded that the roughness and stress are not the main reasons which cause the slow or no increase in Ic. Also, the thickness dependency of GdBa2Cu3O7 − δ films on the Ic is attributed to three main factors: a-axis grains, gaps between a-axis grains, and oxygen deficiency for the upper layers of a thick film.

Development of Long REBCO Coated Conductors by PLD-REBCO/Sputter-CeO2/IBAD-MgO at SJTU and SSTC

Through collaboration with SJTU, a pilot REBCO coated conductor fabrication line has been set up at the Shanghai Superconductor Technology Corporation Ltd., in 2013. To reduce the cost of coated conductor tapes, simplified tape architecture was developed, i.e., PLD-REBCO/Sputter-CeO2/IBAD-MgO on metal tapes with amorphous oxide barrier layer. High-quality long REBCO coated conductors have been successfully fabricated by pulsed laser deposition (PLD) plus magnetron sputter and ion-beam-assisted deposition (IBAD) processes. Under optimized conditions, the IBAD-MgO layers showed pure (001) orientation and excellent in-plane texture. AFM observation showed MgO layer had very smooth surface. The RMS is less than 1 nm. On the textured MgO layer, sputter deposited single CeO2 cap layer showed pure (001) orientation and excellent in-plane texture of 47-6°. A reel-to-reel PLD process with high deposition rate was already scaled up to 100-m/h tape speed. One-micrometer-thick REBCO films had a high critical current density of over 3.0 MA/cm2 (at 77 K, self-field). The critical current density of 2 μm thick films is still more than 2.5 MA/cm2. Hundred-meterlong coated conductor tapes with over 500 A/cm performance have been routinely fabricated. Currently, the process optimization for kilometer-long coated conductor tapes is underway.

Fabrication and characterization of La2Zr2O7 films on different buffer architectures for YBa2Cu3O7−δ coated conductors by RF magnetron sputtering

La2Zr2O7 (LZO) films were grown on different buffer architectures by radio frequency magnetron sputtering for the large-scale application of YBa2Cu3O7−x (YBCO)-coated conductors. The three different buffer architectures were cerium oxide (CeO2), yttria-stabilized zirconia (YSZ)/CeO2, and CeO2/YSZ/CeO2. The microstructure and surface morphology of the LZO film were studied by X-ray diffraction, optical microscopy, field emission scanning electron microscopy, and atomic force microscopy. The LZO films prepared on the CeO2, YSZ/CeO2, and CeO2/YSZ/CeO2 buffer architectures were preferentially c-axis-oriented and highly textured. The in-plane texture of LZO film on CeO2 single-buffer architecture was ∆ φ = 5.5° and the out-of-plane texture was ∆ ω = 3.4°. All the LZO films had very smooth surfaces, but LZO films grown on YSZ/CeO2 and CeO2/YSZ/CeO2 buffer architectures had cracks. The highly textured LZO film grown on CeO2-seed buffered NiW tape was suitable for the epitaxial growth of YBCO film with high currents.

Effects of Laser Repetition Rate on the Structural and Superconducting Properties of YBCO films Deposited by PLD on NiW Tapes

YBa2Cu3O7?x (YBCO) films were fabricated at various laser repetition rates on CeO2/YSZ/CeO2 buffered Ni-W tapes by pulsed laser deposition (PLD). The dependence of structural and superconducting properties on the laser repetition rates was investigated. The microstructure and surface morphology of YBCO films were characterized by X-ray diffraction and atomic force microscopy. And the critical current (Ic) of YBCO films was measured by the conventional four-probe method. The results show that with increasing laser repetition rate the island density decreased and the island size increased. It was found that the texture and Ic of YBCO films were largely dependent on the laser repetition rate. And furthermore it was observed that the film thickness was not in directly proportional to laser repetition rate. Under optimum experimental condition, high quality YBCO films with critical current Ic above 500 A/cm at 77 K and 0T were obtained.

A comparison of the influence of CeO2 and In doped CeO2 interlayer on the properties of the YGBCO/interlayer/YGBCO tri-layer films deposited by pulsed laser deposition

Abstract High critical current (Ic) has long been proved important to REBa2Cu3O7−δ (REBCO or RE123) superconductor for the practical applications. To achieve high Ic values, the primary way is to increase the thickness of superconducting layer and to enhance high critical current density (Jc). However, Jc drops precipitously with increasing the thickness of the films. This phenomenon is called the thickness effect, which can be effectively inhibited by fabricating superconductor/interlayer/superconductor tri-layer structure. In this study, a series of YGBCO/CeO2/YGBCO and YGBCO/In doped CeO2 (CeO2 + In)/YGBCO tri-layer films with different interlayer thickness were fabricated. The microstructure, surface morphology and superconducting property of these samples were measured. It was found that all the tri-layer films had pure c-axis orientation. When using CeO2 film as an interlayer, the two YGBCO layers in the tri-layer films were almost completely isolated, resulting in the low Ic values, which were equivalent to the Ic of a single YGBCO layer. While using CeO2 + In film as an interlayer, the Ic values of all the YGBCO/CeO2 + In/YGBCO trilayers were almost double the Ic of a single YGBCO layer, and the Ic values of the trilayers firstly slow increased and then decreased with increasing the thickness of CeO2 + In interlayer. Under the optimal experimental parameters, the Ic value was much higher than that of the a double YGBCO layer without interlayer. The thickness effect of superconducting layer can be effectively inhibited by inserting CeO2 + In interlayer with wide thickness range.

The Influence of Substrate Temperature of Seed Layer on the Structure and Superconducting Property of BaHfO3-Doped Y0.5Gd0.5Ba2Cu3O7−δ Film Prepared by Pulsed Laser Deposition

For various practical applications, excellent superconducting properties under magnetic field of REBa2Cu3O7−δ (RE = rare earth elements (REBCO))-coated conductors are required, which can be fabricated by introducing artificial pinning centers, such as second-phase dopant. It is well known that BaMO3 (M = Zr, Sn, or Hf) can form self-organized nanorods in REBCO matrix and improve the critical current density (Jc) under applied magnetic field. However, with increasing dopant level, the critical temperature Tc and Jc at 77 K in self-field decreased. In order to inhibit the degradation of the Jc in self-field, we fabricated 5 mol% BaHfO3 (BHO)-doped Y0.5Gd0.5Ba2Cu3O7−δ (YGBCO) seed layers on CeO2-buffered IBAD-MgO tapes by pulse laser deposition (PLD) at a laser repetition of 10 Hz and different substrate temperature Tseed, and subsequently deposited 5-mol% BHO-doped YGBCO upper layers at 160 Hz and higher substrate temperature. The effect of Tseed on the quality of the 5-mol% BHO-doped YGBCO upper layer was investigated. With increasing Tseed from 710 to 820 ∘C, the Δφ value of YGBCO (103) reflection first decreased and then increased, while the Jc at 77 K increased with increasing Tseed to 790 ∘C and decreased at higher Tseed. The 5-mol% BHO-doped YGBCO layer with a seed layer deposited at 790 ∘C had the highest Jc (77 K, self-field) of 4.0 MA/cm2.