Yijie Li

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.

Electrical-Thermal Coupled Finite Element Model of High Temperature Superconductor for Resistive Type Fault Current Limiter

A multi-physics finite element model of high temperature superconductors (HTS) is presented in this article. An electrical model based on a set of Maxwells equations and E-J power law is used to solve the critical state of the superconductor. A heat transfer model is added to the electrical model to calculate the temperature distribution and therefore investigate the Jc (T) dependence of the superconductor. The model is used to study the quench behavior of YBCO-coated conductors for the application of resistive type fault current limiters. Some numerical techniques are applied and assumptions are made to simplify the calculation and improve convergence. An equivalent heat transfer coefficient which is much larger than the normal heat transfer coefficient is applied to the region surrounding the superconductors. This equivalent coefficient represents the drastic heat exchange during the boiling of the liquid nitrogen. The cross-section of YBCO tapes is divided into several sub-domains. The temperature is assumed to be uniform in each sub-domain. This simplification significantly improves the convergence of model and still is able to keep a reasonable level of accuracy. The model is then able to simulate the whole process of YBCO tapes quenching and recovering to superconducting state. The numerical results are compared with the fault current experiments and excellent agreement is obtained.

Test of Maximum Endurable Quenching Voltage of YBCO-Coated Conductors for Resistive Superconducting Fault Current Limiter

Voltage tolerance during quench is one of the most important parameters for YBCO-coated conductors, especially for resistive type superconducting fault current limiter (SFCL) applications. How much voltage can be sustained per unit length of YBCO tape when experiencing fault current indicates the total required amount of the tapes to construct a resistive SFCL. In this paper, a step-down transformer is used to provide the short-current. As the voltage of the secondary side is not continuously variable, we change the voltage per unit length of the YBCO tape by varying the length of the tape. And the effect of the duration of short-current is also considered. A series of test have been performed to determine the maximum voltage drop that each YBCO tape can stand at fault current. The V-t curve has been measured after the tape experiencing successive quenches. Through a large number of experiments we will acquire the maximum endurable quenching voltage per unit length of YBCO tapes when experiencing short-current for different durations. The results presented in this paper would provide useful information for optimizing the design of a resistive SFCL.

AC Loss and Contact Resistance of Resistive Type Fault Current Limiter Using YBCO Coated Conductors

Heat loss is one of the most important issues for resistive type superconducting fault current limiters (SFCL) built from YBaCuO-coated conductors. A newly designed 400 V SFCL module by Smart Grid Center, Shanghai Jiaotong University (SJTU) is tested and modeled in order to get a clear understanding of its characteristics of heat losses. Two main losses, including AC loss of superconducting tapes and the losses due to contact resistance inside the SFCL module are investigated. Experiments based on “electrical method” are set up to measure the AC loss at 77 K with lock-in amplifier conditions. Meanwhile, losses from the contact resistance are calculated by analyzing their V-I curve. To better understand the behavior of the AC loss in SFCL module and verify the accuracy of the experimental results, modeling analysis is performed by means of a 2-dimensional finite element modeling (FEM). Numerical results show that the results are very close to what we get from experimental method. Detailed results will be presented and the heat losses of 10 kV SFCL consisting of about 25 modules in series are going to be discussed.

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.