1 Introduction

Owing to its large current capacity and high irreversible field, the second generation high-temperature superconducting (HTS) YBa2Cu3O7−x (YBCO) tape, also called YBCO-coated conductor (CC), is of wide prospection to be used in electric power field, such as current transmission cables, transformers, motors, and electromagnets [1, 2]. The YBCO CC is composed of flexible metal substrate, buffer layer, YBCO layer, and protective layer from bottom to top. The YBCO CC with high performance requires the YBCO layer should be highly textured [3]. Up to now, there are following two mainstream routes to obtain textures for YBCO CC: the rolling-assisted biaxially textured substrate (RABiTS) and the ion beam-assisted deposition (IBAD) texturing [4, 5]. Compared to RABiTS, IBAD technique is cost-effective and the finally obtained YBCC CC is of higher performance, even though its structure is more complex [6, 7].

Nowadays, the typical templates applying IBAD technique are the following: LaMnO3/homo-epi MgO/IBAD-MgO/Y2O3/Al2O3/Hastelloy tape and PLD-CeO2/IBAD-MgO/Y2O3/Al2O3/Hastelloy tape [6, 8]. In order to get superior template for high-performance YBCO CC, smooth surface with average surface roughness of ∼ 1 nm is necessary through several methods for planarizing Hastelloy tape, such as electrochemical or mechanical polishing, unfortunately, which is cost-expensive and environmentally harmful [9, 10]. Moreover, the Y2O3 and Al2O3 usually are deposited by the vacuum coating technique, which would somewhat increase the fabricating cost. An alternative method, solution deposition planarization (SDP) process using multiple chemical solution coatings of Y2O3 with a total thickness of approximately 1 μm [10] is developed not only to planarize the tape but also to provide the effective barrier layer replacing of amorphous Y2O3 and Al2O3.

Considering its easy composition adjustment, relatively low vacuum requirement and high throughput for the deposition of YBCO films as well as its mature application in semiconductor industry [6], the metal-organic chemical vapor deposition (MOCVD) has become one of the main techniques to fabricate YBCO CCs. Unfortunately, the MOCVD growth of multi-element oxide thin films is difficult to control due to the reciprocal effect among the temperature, compositions, and impurity phases.

In this paper, MOCVD was used to fabricate YBCO films on simple, cost-effective SDP-based templates of LaMnO3 /homo-epi MgO/IBAD-MgO/SDP-Y2O3/Hastelloy tape, and the effects of substrate temperature on the evolvement of the microstructures and superconducting properties of YBCO films were investigated.

2 Experimental Details

In the experiments, a custom-built MOCVD system was applied to depositing YBCO films on home-made IBAD-MgO templates of LaMnO3/homo-epi MgO/IBAD-MgO/SDP-Y2O3/Hastelloy tape [11, 12]. The metal-organic solids of Y(tmhd)3, Ba(tmhd)2, and Cu(tmhd)2 (tmhd: 2,2,6,6,-tetramethyl-3,5-heptanedionate) were dissolved into tetrahydrofuran (THF) by the concentration of 0.063, 0.133, and 0.163 mol/L, respectively, to form a mixed liquid precursor. Such liquid precursor was evaporated to gas, which then was transported by the carrier of argon and mixed with the reactive gas of oxygen and N2O. Finally, the mixed gas showered to the preheated IBAD-MgO templates to form YBCO films. The preset temperature of IBAD-MgO templates were 750 to 850 ∘C by the interval of 20 ∘C and the corresponding deposition rate was ∼ 600 nm/min. Finally, the as-formed YBCO films were annealed at 500 ∘C for 40 min in the atmosphere of 1 bar O2 to be transferred into superconducting phase. More details about the deposition could be found elsewhere [11].

The texture of the as-fabricated YBCO films was examined by X-ray diffraction (XRD, Bede D1 system). The surface microstructure was characterized by scanning electron microscope (SEM, JEOL7500F). The film thickness was obtained by step profiler (Veeco Dektak 150). And the critical current density J c at 77 K and self-field was measured through nondestructive induction method by Leipzig J c-scan system [13].

3 Results and Discussion

The as-deposited YBCO films were 1.2- μm-thick and the corresponding XRD 𝜃-2 𝜃 scans were showed in Fig. 1. At the lower substrate temperature (T s) of 750 ∘C, there are clear peaks of YBCO(00l) and (h00), which means the a-axis- and c-axis-oriented YBCO grains coexisted in the film. Meanwhile, the peak intensity of YBCO (200) is relatively strong and comparable to that of YBCO(006), indicating that there are considerable volume of the a-axis-oriented grains in the film. As T s increases to 770 ∘C, the peak intensity of YBCO(200) decreases a lot, while the peak intensity of YBCO(00l) becomes stronger, revealing that the growth mode of YBCO film is gradually transferring from the partially a-axis-oriented growth to the fully c-axis-oriented growth as T s increases. When T s is in the range from 790 to 850 ∘C, there are only (00l) peaks of YBCO, indicating that the YBCO films fabricated at such T s are completely c-axis oriented. However, the intensity of (00l) peaks become stronger at T sof 790 ∼ 810 ∘C, then weaker at T sof 810 ∼ 850 ∘C. Around T s of 810 ∘C, the YBCO(00l) peaks are most sharp and intense, implying that the YBCO film crystallizes best and is of the best c-axis epitaxial growth. As T sincreases from 810 to 850 ∘C, the intensities of (00l) peaks decline dramatically, attributed to the deteriorated texture and nonstoichiometric composition, which is proved by the presence of impurity phase of CuYO2.

Fig. 1
figure 1

The XRD 𝜃-2 𝜃 scans of YBCO films deposited at substrate temperature of 750, 770, 790, 810, 830, and 850 ∘C. The asterisk and number symbols represent peaks of Hastelloy metal and YBCO(103), respectively

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It is worth noting that the peaks of Y2O3(004) appear in the 𝜃-2 𝜃 patterns of all the YBCO films and becomes stronger as T s increases. This diffraction peak of Y2O3(004), although the work to identify the origin is in progress, is likely from the Y2O3 grains precipitated during the YBCO film deposition or the crystallization of the amorphous SDP-Y2O3 because of the high deposition temperature of MOCVD-YBCO film. Then, on the one hand, the Y2O3 precipitates are helpful in improving the flux pinning [14, 15]. On the other hand, however, the crystallization of the amorphous SDP-Y2O3 may lead to the formation of grain boundaries which are the main diffusion passage of oxygen elements and transition metal elements [16], thus may result in the oxidization of Hastelloy tape and the diffusion of metal elements into YBCO layer, ultimately deteriorating the mechanical and superconducting properties of the YBCO. Replacing Y2O3 with Y2O3-Al2O3 composite [17] can be a way to avoid the crystallization of the amorphous SDP-Y2O3.

To further demonstrate the evolvement of the film orientation and crystallization with T s increasing, XRD χ-scan [18] has been performed on YBCO(102). The χ-scan patterns were depicted in Fig. 2, in which peaks located around 33.4 ∘ and 56.6 ∘ represented the a-axis- and c-axis-oriented YBCO grains, respectively. It is apparent that the peak around 33.4 ∘ declines and then disappears with T s increasing, while the peak around 56.6 ∘ rises first and then drops. To give a quantitative study on the content of the a-axis component, the value I a=I a (102)/(I c(102) +I a(102)), where I a(102) and I c(102) are the integrated intensities of peaks around 33.4 ∘ and 56.6 ∘, respectively, is used. I avalues at 750 and 770 ∘C are 58.1 and 14.2 %, respectively, and are zero above 790 ∘C, which means that the YBCO film transfers its growth mode from the a-axis dominated to the c-axis dominated as T s increases from 750 to 850 ∘C, which is consistent with that in Fig. 1.

Fig. 2
figure 2

The XRD χ−scans on (102) planes of YBCO films deposited at substrate temperature of 750, 770, 790, 810, 830, and 850 ∘C

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XRD ω-scan and ϕ-scan have been performed on YBCO(005) and YBCO(103), respectively, to examine the out-of-plane and in-plane textures. The full width at half maximum (FWHM, Δω or Δϕ) values determined by the ω-scans and ϕ-scans were plotted in Fig. 3, and the smaller FWHM value corresponds to the better texture. Figure 3 shows that the textures of YBCO film are strongly affected by T s, especially the out-of-plane textures. At T s= 810 ∘C, the Δω of 1.4 ∘ and Δϕ of 3.0 ∘ are the smallest, corresponding to the best out-of-plane and in-plane texture, which is comparable to those reported in ref. [17]. The inset in Fig. 3 is the pole figure of YBCO(103) and shows that there are four 90 ∘ spaced peaks, which implies that the YBCO prepared at 810 ∘C was single-domain epitaxial without misoriented grains. As T s is further away from 810 ∘C, the FWHM value becomes larger, meaning that the texture of YBCO film became worse.

Fig. 3
figure 3

The substrate-temperature dependence of FWHM values of ω-scans on YBCO(005) and ϕ-scans on YBCO(103)

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It is known that during film formation, there are following basic processes: atom adsorption and condensation, diffusion, nucleation, island formation and growth, and coalescence, which are strongly affected by T s. At the lower T s of 750 ∘C, the atoms could not receive enough energy from substrate, yielding that the YBCO film tended to grow along the easy growth orientation of the a-axis and was hybrid of the a-axis and c-axis grains. As T s is increased from 750 to 810 ∘C, the adsorbed atoms could gain more energy from substrate, thereby adjust their position to minimize the free energy. YBCO film texture was improved and the best was achieved at 810 ∘C. When T s further increased from 810 to 850 ∘C, the surface diffusion was so severe that the smaller nuclei could move with a greater distance to coalesce with larger nuclei. The larger nuclei annexed the smaller nuclei and continued to grow up so fast that there remained insufficient time for them to be preferentially oriented. Moreover, the excessive T s made the film barium-poor, demonstrated by the formation of the CuYO2 impurity denoted in Fig. 1. The large lattice mismatch between YBCO and CuYO2 would yield enormous strain in the system. Because of the above facts, the intensities of YBCO(00l) peaks declined and the FWHM values increased, meaning that the film crystallization and textures deteriorated.

The SEM was employed to get a comprehensive picture about the microstructure of the film. The corresponding images are showed in Fig. 4, in which the needle-like phases were the a-axis YBCO grains. Figure 4a shows the morphology of the film formed at 750 ∘C, indicating that the typical a-axis grains cover the whole film surface and pinholes are clearly visible [18]. As T s increases, the numbers of a-axis grains and holes decrease, while the average grain size increases as shown in Fig. 4b–f. It should be noted that bright and dark regions start to appear on the surface at 810 ∘C, implying that the film becomes inhomogeneous in composition as T s is increased beyond 810 ∘C.

Fig. 4
figure 4

SEM images of YBCO films deposited at different substrate temperature: a 750,; 770; c 790,; 810, ; 830; f 850 ∘C

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The J c(77 K, 0 T) values of all films were summarized in Fig. 5. It is obvious that J c increased from 0.2 to 1.8 MA/cm 2 as T s increased from 750 to 810 ∘C. It was reported that J c of the a-axis films was only 2–3 % to that of the c-axis film, and the worse textures mean more large-angle grain boundaries which were harmful to supercurrent transportation [3, 19, 20]. Hence, such J c enhancement was benefited from the above-discussed decrease of the a-axis grains and improvement of texture and continuity of YBCO film. As T s increased to 850 ∘C, the J c declined from 1.8 to 0.4 MA/cm 2, which was attributed to the degraded texture, inhomogeneous composition, and emergence of CuYO2 impurities, which was consistent with the above XRD results. Although the yielded maximum J c of 1.8 MA/cm 2 at present is smaller than those of YBCO films reported in refs. [21–24], yet it is expected to improve the J c by further adjusting the process parameters of MOCVD-YBCO deposition, such as total pressure of the chamber, partial pressure of oxygen and N2O, and atom ratio of the metal elements in precursor.

Fig. 5
figure 5

The relationship between J c(77 K, 0 T) of YBCO films and the substrate temperature

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4 Conclusions

The substrate temperature is important to determine the preferential orientation and textures of YBCO film. We have demonstrated the feasibility of using a simple, cost-effective SDP-based IBAD-MgO template for YBCO-coated conductors, and investigated the evolvement of the microstructures and superconducting properties of YBCO films by tuning the substrate temperature. At lower substrate temperature of 750 ∘C, the film growth was dominated by the a-axis aligned mode, which severely deteriorated the superconducting property. As T s increased from 750 to 810 ∘C, the films transferred their preferential orientation from the a-axis to the c-axis, and the in-plane and out-of-plane textures of the films were remarkably improved. Benefiting from the decrease of the a-axis grains and improvement of the textures, the J c was greatly ameliorated from 0.2 to 1.8 MA/cm 2. Further enhanced temperature would degrade the film texturing and lead to inhomogeneous composition and barium-poor.