1 Introduction

Recently, worldwide attention has been paid to the preparation of YBa2Cu3O7-δ (YBCO)-based coated conductors, i.e., the second generation tapes because of their wide-ranging potential in large-scale application such as superconducting maglev trains and superconducting electric cables [1–7]. 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 [8–12]. 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 BaCO3, which accumulates and precipitates at the grain boundaries (GBs) of REBCO and thus significantly reduces the transport critical current density Jc [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 BaCO3 can be well controlled, a kind of novel non-fluorine chemical solution deposition method is a promising and cost-effective 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 [17–19].

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 Jc 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 Jc were developed by generating artificial defects [20–23]. 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 (Jc–H) characteristics of high temperature superconductor bulks [24–26]. Dilute impurity doping to the CuO chain not only enhances flux pinning by lattice deformation but also results in a decrease in critical temperature (Tc) [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 LaAlO3 (LAO) single crystal substrate. In-field Jc of YBCO films was improved by this dilute impurity doping method.

2 Experimental

Fe-doped YBa2Cu3-xFe x O7-δ films were deposited on a LaAlO3 (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/O2 mixture gas at 160–500 °C for 11 h and then fired at 770–800 °C in dry Ar/O2 mixture gas for 1 h. Finally, the samples were annealed in dry O2 gas at 400–450 °C for 1 h. Finally, the obtained YBa2Cu3-xFe x O7-δ-coated films with x of 0–0.005 were about 500 nm in thickness.

A Philips X’Pert MRD diffractometer with Cu-Kα radiation was used to record the θ-2θ 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 Jc value of the YBCO film was determined using Bean critical state model formula.

3 Results and discussion

Figure 1 shows the typical θ-2θ X-ray diffraction patterns of YBa2Cu3-xFe x O7-δ (x = 0, 0.0005, 0.001, 0.002, 0.005) films. As can be seen, only (0 0 l) YBCO reflection peaks were detected, excluding the peaks of LAO single crystal substrate. This indicates a strong c-axis orientation for all films.

Fig. 1
figure 1

Typical X-ray diffraction θ-2θ patterns of YBa2Cu3-xFe x O7-δ (x = 0, 0.0005, 0.001, 0.002, 0.005) films

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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 pure YBCO film, but these are not seen in the Fe-doped films. It can be seen that a very small amount of Fe doping improves the surface connectivity of the YBCO films.

Fig. 2
figure 2

The surface morphology of YBa2Cu3-xFe x O7-δ (x = 0, 0.0005, 0.001, 0.002, 0.005) films. ax = 0, bx = 0.0005, cx = 0.001, dx = 0.002, ex = 0.005

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Figure 3 shows the superconductivity critical temperature of YBa2Cu3-xFe x O7-δ (x = 0, 0.0005, 0.001, 0.002, and 0.005) films. The inset shows the temperature dependence of magnetic moment for YBa2Cu3-xFe x O7-δ films. The pure YBCO sample exhibits a sharp superconducting transition; while for the doped samples, the superconducting transition width broadens, showing that such Fe doping leads to a decrease of superconductivity. The decrease may be due to the direct iron substitution for copper in the Cu–O chain. The substitution affects the oxygen vacancies which play an important role in determining the transport properties of YBCO. It is found that the superconductivity critical temperature of YBa2Cu3-xFe x O7-δ decreases with Fe doping. When the value of x = 0.005 for YBa2Cu3-xFe x O7-δ, the Tc value is 1.2 K smaller than that of pure YBCO film. It exhibits a very small decrease of Tc because of the small amount of Fe doping.

Fig. 3
figure 3

The superconductivity critical temperature of YBa2Cu3-xFe x O7-δ (x = 0, 0.0005, 0.001, 0.002, and 0.005) films. Inset is the temperature dependence of magnetic moment for YBa2Cu3-xFe x O7-δ (x = 0, 0.0005, 0.001, 0.002, and 0.005) films

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Figure 4 shows the magnetic field dependence of Jc for YBa2Cu3-xFe x O7-δ (x = 0, 0.0005, 0.001, 0.002, 0.005) films at different temperatures with the magnetic field parallels to the c-axis. The Jc 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 Jc 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 Jc value is significant. It means that the current carrying capability of YBCO film can be improved by doping with appropriate amounts of Fe.

Fig. 4
figure 4

The magnetic field dependence of critical current densities for YBa2Cu3-xFe x O7-δ (x = 0, 0.0005, 0.001, 0.002, 0.005) films. aT = 30 K, bT = 50 K, cT = 77 K

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4 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 Jc improves at 77 K for doping quantities of x less than 0.005.