Huaran Liu

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.

Effect of CeO2 cap layers on the preparation of YBCO films on rolling-assisted biaxially textured substrates by pulsed laser deposition

YBCO films were grown on CeO2/YSZ/CeO2 buffered rolling-assisted biaxially textured substrates (RABiTS) by pulsed lased deposition (PLD). CeO2 cap layer was deposited prior to YBCO growth. CeO2 cap layers of different thickness were prepared to evaluate the thickness dependence of the YBCO films. The microstructure and surface morphology of the films were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the thickness of the cap layer can remarkably affect the preparation of the subsequent YBCO layer. The possible mechanisms responsible for the dependence of the structure and the preparation of the YBCO films on the thickness of the CeO2 cap layers are discussed.

Growth orientation and surface morphology of CeO2 films for high-Jc YBCO films on biaxially textured Ni tape by PLD

Epitaxial cerium oxide buffer layers were deposited on biaxially-textured (001) Ni tape using reel-to-reel pulsed laser deposition in a vacuum chamber. Relationship between microstructure and deposition parameters was systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). We found the optimal deposition parameters for CeO2 buffer layer which were quite suitable for preparing high-Jc YBCO films. The relationship between CeO2 layer thickness and crack formation has also been discussed.