Fe-doped epitaxial YBCO films prepared by chemical solution deposition

YBa2Cu3O7-δ (YBCO)-coated conductors have wide-ranging potential in large-scale applications such as superconducting maglev trains and superconducting electric cables, but low current carrying capability restrains the practical application of YBCO-coated conductors at high temperatures and high magnetic fields. It is crucial to develop YBCO-coated conductors with high critical current density. In this paper, epitaxial, dense, smooth, and crack-free Fe-doped YBCO films were prepared on a LaAlO3 single crystal substrate via a fluorine-free polymer-assisted metal organic deposition method. The effects of the dilute Fe doping on microstructure and superconducting character of YBCO films were investigated. The critical temperature for superconducting of the Fe-doped YBCO films decreases slightly. However, the in-field critical current density of YBCO films improves with dilute Fe doping of amounts less than x = 0.005, compared to the pure YBCO film. Therefore, the current carrying capability of YBCO film can improve by doping with appropriate amounts of Fe. This means that dilute Fe doping in YBCO films may be a feasible way to prepare high-performance coated conductors.


Introduction
Recently, worldwide attention has been paid to the preparation of YBa 2 Cu 3 O 7-d (YBCO)-based coated conductors, i.e., the second generation tapes because of their wideranging potential in large-scale application such as superconducting maglev trains and superconducting electric cables [1][2][3][4][5][6][7]. The major approaches for fabricating YBCO-coated conductors include in situ processes like pulse laser deposition (PLD), magnetic sputtering, etc., as well as ex situ wet-chemistry approaches. Chemical solution deposition (CSD) and related techniques provide versatile and cost-effective processing routes with controlled stoichiometry and microstructure [8][9][10][11][12]. Therefore, CSD is a preferred fabrication method. In the early work preparing solution-derived REBCO thin films, the critical current densities were quite limited. It was widely believed that organic-based precursors might lead to the formation of stable BaCO 3 , which accumulates and precipitates at the grain boundaries (GBs) of REBCO and thus significantly reduces the transport critical current density J c [13]. In an effort to solve this problem, a promising approach using trifluoroacetates metal organic deposition was developed to deposit YBCO films [14,15]. However, the process leads to the formation of HF which is hazardous, and the microstructure of YBCO films were relatively porous [16], which limits the current transport properties. Alternatively, if the formation of BaCO 3 can be well controlled, a kind of novel non-fluorine chemical solution deposition method is a promising and costeffective approach to the preparation and application of high performance YBCO films. In this paper, YBCO films were prepared using a non-fluorine chemical solution deposition method which was devised in our laboratory [17][18][19].
The large-scale application of YBCO-coated conductors in superconducting magnets, generators, motors, etc., is prevented because of relatively low current carrying capability with increasing magnetic field and temperature. Therefore, enhancing the J c is crucial for the practical applications of the YBCO-coated conductors at high temperatures and high magnetic fields. Several methods for increasing the density and effectiveness of vortex pinning sites to enhance J c were developed by generating artificial defects [20][21][22][23]. Studies have confirmed that incorporation of impurities such as Co, Fe, Ga, Zn, Ni, etc., to the Cu sites of YBCO can significantly increase the critical current density versus magnetic field (J c -H) characteristics of high temperature superconductor bulks [24][25][26]. Dilute impurity doping to the CuO chain not only enhances flux pinning by lattice deformation but also results in a decrease in critical temperature (T c ) [24]. There are few reports about the doping effects of these dilute impurities in YBCO films [27,28], especially those prepared with fluorine-free metal organic deposition (MOD).
The authors have previously published results of cobalt and zinc doping to the copper sites of YBCO [17]. It is a perspective way to essentially improve the current carrying capability of YBCO film. There is no work of Fe doping of YBCO in their early work. Fe is cheap and easy to obtain, so it is a perspective material for superconductor production. In this study, different dilute quantities of Fe doping of YBCO were introduced into the MOD YBCO films. Flux pinning properties of dilute Fe-doped YBCO films were investigated. The films were deposited by a fluorine-free polymer-assisted metal organic deposition (PA-MOD) method [17]. Epitaxial, dense, smooth, and crack-free YBCO films were prepared on a LaAlO 3 (LAO) single crystal substrate. In-field J c of YBCO films was improved by this dilute impurity doping method.

Experimental
Fe-doped YBa 2 Cu 3-x Fe x O 7-d films were deposited on a LaAlO 3 (LAO) single crystal substrate using a fluorine-free method [17]. The precursor solutions were synthesized by dissolving acetates of iron, in addition to acetates of yttrium, barium, and copper, in propionic acid with x = 0, 0.0005, 0.001, 0.002, and 0.005. Then polyvinyl butyral was added into the solution, which was subjected to continuous stirring to adjust the viscosity in order to obtain the final coating solution. The final cation concentration of the solution is 0.6 M. Then the solution was coated to LAO using a spin coater with a rotation speed of 6,000 r/min and dried at 150-200°C for 5-10 min. The coated samples were fabricated in humid Ar/O 2 mixture gas at 160-500°C for 11 h and then fired at 770-800°C in dry Ar/O 2 mixture gas for 1 h. Finally, the samples were annealed in dry O 2 gas at 400-450°C for 1 h. Finally, the obtained YBa 2 Cu 3-x Fe x O 7-d -coated films with x of 0-0.005 were about 500 nm in thickness.
A Philips X'Pert MRD diffractometer with Cu-Ka radiation was used to record the h-2h X-ray diffraction (XRD) patterns. The microstructure analyses of the YBCO layer were performed using an environmental scanning electron microscope (ESEM). Superconducting transition and magnetic hysteretic loop were measured by Quantum-Design SQUID XL. The J c value of the YBCO film was determined using Bean critical state model formula. The microstructure analyses of the YBCO layer were performed, and the results are shown in Fig. 2. Dense, smooth, and crack-free surface morphologies can be observed in all the films. A few particles floating on the film surface may be attributed to texture degradation with the growth of film during the firing procedure. We can see the particles in the Fe-doped YBCO films less than the pure YBCO films. There are some very small pores in the    Figure 4 shows the magnetic field dependence of J c for YBa 2 Cu 3-x Fe x O 7-d (x = 0, 0.0005, 0.001, 0.002, 0.005) films at different temperatures with the magnetic field parallels to the c-axis. The J c values of Fe-doped YBCO films are not significantly higher than that of pure YBCO films at 30 and 50 K. However, at 77 K, Fe-doped YBCO films possess higher J c values than those of pure YBCO films, except for the sample with x = 0.005. As 77 K is a feasible temperature for application of YBCO superconductors by using liquid nitrogen, this improved J c value is significant. It means that the current carrying capability of YBCO film can be improved by doping with appropriate amounts of Fe.

Conclusion
In this paper, from the investigation of the Fe doping effects on YBCO films, it can be seen that the critical temperature for superconducting decreases slightly, and J c improves at 77 K for doping quantities of x less than 0.005.