Dramatic effects of chlorine addition on expanding

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Dramatic effects of chlorine addition on expanding synthesis conditions for fluorine-free metal–organic decomposition YBa2Cu3O y films To cite this article: Takanori Motoki et al 2017 Appl. Phys. Express 10 023102

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Applied Physics Express 10, 023102 (2017) https://doi.org/10.7567/APEX.10.023102

Dramatic effects of chlorine addition on expanding synthesis conditions for fluorine-free metal–organic decomposition YBa2Cu3Oy films Takanori Motoki1,4*, Shuhei Ikeda1, Genki Honda2, Tatsuoki Nagaishi2, Shin-ichi Nakamura3, and Jun-ichi Shimoyama1,4 1

Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara 252-5258, Japan Sumitomo Electric Industries, Ltd., Osaka 554-0024, Japan 3 Center for Instrumental Analysis, Aoyama Gakuin University, Sagamihara 252-5258, Japan 4 JST-ALCA, Chiyoda, Tokyo 102-0076, Japan 2

*E-mail: [email protected] Received October 19, 2016; accepted December 13, 2016; published online January 6, 2017 The synthesis conditions of fluorine-free metal–organic decomposition (FF-MOD)-processed YBa2Cu3Oy (YBCO) films on buffered metallic substrates have been systematically investigated. Chlorine addition to the starting solution was found to be quite effective for expanding the synthesis conditions of highly c-axis-oriented YBCO films. YBCO films showing a high critical current, >100 A/cm (77 K, >0 T), were successfully obtained by sintering at 740 °C, which is >50 °C lower than the typical sintering temperature for FF-MOD-processed YBCO films. This strongly indicated that chlorine addition is promising for the development of long and homogeneous YBCO tapes even by sintering at a low temperature of >740 °C. © 2017 The Japan Society of Applied Physics

EBa2Cu3Oy (REBCO, RE: rare-earth element)-coated conductors have been enthusiastically developed since the first successful studies on the synthesis of biaxially oriented buffer layers by the ion-beam-assisted deposition (IBAD) method.1) Among the various methods of fabricating REBCO layers on metallic substrates, metal– organic decomposition (MOD) is a cost-effective method of preparing REBCO tapes because neither a vacuum system nor a high-power laser device is required. In particular, fluorinefree MOD (FF-MOD) has been developed as a promising method because homogeneously textured REBCO crystals form directly through simple reactions.2–13) For practical application, it is necessary to prepare textured REBCO films on buffered metallic substrates with a large length. In this case, a low sintering temperature and a short sintering time are preferable to prevent undesirable phenomena, namely, reactions between REBCO crystals and the surface of buffer layers, generation of cracks owing to different thermal expansion coefficients, and atomic diffusion among the buffer and REBCO layers. These phenomena always deteriorate the transport properties of REBCO-coated conductors.14–20) However, most of the studies on FF-MOD-processed REBCO films have been performed using single crystalline substrates, such as SrTiO3 and LaAlO3, and sintering temperatures of ∼800 °C are adopted, while the optimal sintering temperature depends on the partial oxygen pressure (PO2). In our previous studies on FF-MOD-processed YBCO films grown on SrTiO3(100) substrates,10,11) it was revealed that chlorine (Cl) addition to the starting solution, which resulted in the generation of the oxychloride Ba2Cu3O4Cl2 (Ba2342), promoted the growth of biaxially aligned YBCO crystals owing to the good lattice matching between YBCO and Ba2342. The Cl-added YBCO films showed an enhanced critical current density (Jc) with high reproducibility without decreasing the critical temperature (Tc). In this study, the effects of Cl addition on the synthesis conditions and critical current (Ic) properties of FF-MOD-processed YBCO films on low-cost Ni=Cu clad substrates were systematically investigated. Although our studies were performed for small pieces, the used substrates were cut from long tapes. Propionate-based solutions with nominal compositions of Y : Ba : Cu : Cl ¼ 1 : 2 þ 2x : 3 þ 3x : 2x ðx ¼ 0; 0:05Þ

R

were carefully prepared. The total cation concentration of the solution was controlled to be 1 mol=L and Cl was added by mixing HCl (6 mol=L) to the solution. Green films were formed by the spin-coating of the solution on buffered clad substrates [CeO2=Y-stabilized-ZrO2 (YSZ)=Y2O3=Ni=Cu] with typical dimensions of 10 mm × 10 mm × 0.2 mmt, which were cut from long tapes. Spin-coating and calcination at ∼500 °C under flowing oxygen atmosphere were repeated three times. Then, the green films were heated in a tube furnace under O2=Ar gas flow with PO2 = 3 or 10 Pa, where the sintering temperature (Ts) and sintering time (ts) were systematically changed. Finally, the films were annealed at 450 °C for 12 h under oxygen flow and rapidly cooled to room temperature to control the oxygen composition of YBCO. The typical thickness of the prepared YBCO films was ∼0.5 µm. Details of the synthesis processes can be found elsewhere.10,11) Constituent phases and the crystallinity of YBCO were investigated by the surface X-ray diffraction (XRD) method with θ–2θ scanning. The Ic values (A=cm) of the prepared films were evaluated by an inductive method at 77.3 K without an external field. The cross-sectional microstructures of the films and electron diffraction patterns were observed by transmission electron microscopy (TEM) with an acceleration voltage of 400 kV. Firstly, Cl-free and Cl-added (i.e., x = 0 and x = 0.05, respectively) YBCO films were prepared under various Ts values ranging from 760 to 840 °C, while other synthesis conditions were kept unchanged (PO2 = 10 Pa, ts = 1 h). Figures 1(a) and 1(b) show the surface XRD patterns of Clfree and Cl-added YBCO films, respectively. For comparison, an XRD pattern of a buffered substrate after the firing process (840 °C, 1 h) is also shown in Fig. 1(a). In both Cl-free and Cl-added YBCO films with Ts = 840 °C, no diffraction peaks of YBCO crystals were detected, while impurity phases, such as YCuO2 and BaCeO3, were confirmed. This suggests that the reactions among the surface of the buffer layers, CeO2, and (Y,Ba,Cu)-oxide occurred at high Ts. For YBCO films with low Ts (below 820 °C), the generation of these impurities was relatively suppressed, and strong peaks due to c-axis-aligned YBCO were observed for both Cl-free and Cl-added films. For the Cl-free films, however, the intensities of YBCO(00l) reflections were low for samples with Ts ≤ 780 °C. Therefore,

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(b) Fig. 1. Surface XRD patterns of (a) Cl-free YBCO films and a buffered substrate and (b) Cl-added YBCO films sintered at various temperatures. The green and red dotted lines represent the 2θ angles of YBCO(00l) and Ba2342(00l) peaks, respectively.

the optimal Ts range for Cl-free YBCO films was narrow under PO2 = 10 Pa and ts = 1 h. On the other hand, strong YBCO(00l) peaks were observed for Cl-added films even for a sample with Ts = 760 °C. Note that (00l) peaks of Ba 2342 were also observed for Cl-added films, as shown in Fig. 1(b). This indicated that Ba2342 assisted the grain growth of c-axisaligned YBCO at low Ts. On the basis of these results, various sintering conditions, Ts and ts, under fixed PO2 = 10 Pa were investigated for both Cl-free and Cl-added films. Ic (A=cm) values at 77 K, ∼0 T are summarized in Figs. 2(a) and 2(b). Spheres and X-marks represent Ic > 0 and Ic = 0, respectively, and Ic values are shown just below the corresponding spheres. In some cases, several films were sintered under the same conditions to confirm reproducibility. It was revealed that the reproducibility of the Ic values was not high for Cl-free YBCO films under some sintering conditions, while Cl-added films sintered under the same conditions showed similar Ic values. For example, four Cl-free YBCO films sintered under PO2 = 10 Pa, Ts = 800 °C, and ts = 30 min showed Ic values of 0, 0, 54, and 55 A=cm, whereas four Cl-added YBCO films sintered under the same conditions exhibited Ic values of 50, 58, 58, and 60 A=cm. In Fig. 2, the average Ic values are indicated. Corresponding to the changes in (00l) peak intensities in XRD diffraction patterns shown in Fig. 1, relatively high Ic values were observed only for films sintered at ∼800 °C for 1 h when Cl was not added. Although YBCO(00l) peaks with high intensities were detected for the Cl-free YBCO film with Ts = 820 °C, the observed Ic value was almost zero, which could be explained by the generation of BaCeO3. The generation of BaCeO3 has been reported for both trifluoroacetate-MOD and FF-MOD-processed REBCO films prepared on a CeO2 layer.2,7,9,18–20) According to these studies, the quality of the film gradually degrades with increasing amount of BaCeO3 mainly due to the change in the cation composition of the superconducting layer deviating from the 123 stoichiometry, resulting in the generation of secondary phases. Scanning electron microscopy (SEM) observation and compositional analysis by energy-dispersive

Fig. 2. Summarized relationship between synthesis conditions and Ic (77 K, ∼0 T) values of prepared (a) Cl-free and (b) Cl-added YBCO films under PO2 = 10 Pa. The spheres and X-marks in the figures indicate the prepared films showing Ic > 0 and Ic = 0, respectively. Ic values (A=cm) are shown by the red letters below the corresponding spheres.

X-ray spectrometry (EDS) performed for the Cl-free YBCO film with Ts = 820 °C revealed that a large number of Y–Cu–O precipitates of ∼10 µm size were dispersed in the film corresponding to the generation of BaCeO3, while no cracks were found in the film. Therefore, the formation of Y–Cu–O precipitates is considered to be one of the major reasons for the observed poor critical current properties. On the other hand, the optimal sintering conditions resulting in high-Ic films were largely expanded by the addition of Cl. Highly textured YBCO films showing Ic > 50 A=cm were obtained by sintering only for 1 min at 800 °C or lowtemperature sintering at 760 °C for 30–60 min. It is suggested that textured YBCO can be prepared under lower Ts or shorter ts by Cl addition compared with the conventional Cl-free films. It is important to note that three Cl-added YBCO films with Ts = 820 °C and ts = 1 h exhibited Ic values of 37, 57, and 80 A=cm, while two Cl-free films prepared under the same conditions did not show finite Ic values. Although the reason for these results is unclear at present, Cl addition possibly contributes to the maintenance of a current path in the films sintered at 820 °C. The relatively poor reproducibility of Ic for Cl-added films with high Ts is considered to be due to the difference in reaction level between (Y,Ba,Cu)-oxide and CeO2 layers. Subsequently, the optimization of sintering conditions in a low-PO2 region (PO2 = 3 Pa) was attempted. The Ic values

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Fig. 4. (a) Cross-sectional TEM image of the Cl-added YBCO film prepared under PO2 = 3 Pa, Ts = 740 °C, and ts = 18 h, and (b) SAED pattern observed for a region including three buffer layers (CeO2, YSZ, and Y2O3) and YBCO.

Fig. 3. Relationship between synthesis conditions and Ic (77 K, ∼0 T) values of prepared (a) Cl-free and (b) Cl-added YBCO films under PO2 = 3 Pa.

of Cl-free and Cl-added YBCO films are summarized in Figs. 3(a) and 3(b), respectively. It is generally known as the Hammond–Bormann line, that is, the appropriate Ts to obtain textured films tends to drop as PO2 decreases.21) It was revealed for Cl-added YBCO films that the optimal Ts markedly shifted toward low temperature, while the optimal Ts and ts under PO2 = 3 Pa were almost unchanged for Cl-free ones. In such a low-PO2 region, it is not effective to prepare films under relatively high Ts > 780 °C especially for Cladded films where the decomposition of Ba2342 oxychloride would take place. Rather, low-temperature and long-time sintering is considered to be effective under PO2 = 3 Pa. Textured YBCO films showing high Ic > 80 A=cm were obtained by sintering the films at 740–750 °C for 12–18 h, which is ∼50 °C lower than the typical sintering temperature adopted in the conventional FF-MOD process. Figure 4(a) shows a cross-sectional TEM image of the Cladded YBCO film prepared under PO2 = 3 Pa, Ts = 740 °C, and ts = 18 h, showing the highest Ic of ∼100 A=cm. A flat and homogeneous YBCO film was observed with rectangular Ba2342 precipitates mainly at the interface of YBCO and CeO2 layers. Contrasted vertical stripes with an interval of ∼100 nm, corresponding to YBCO twins, were confirmed to be continuous from the bottom to the surface, which can be supporting evidence for the biaxial orientation of the YBCO film. This trend of the twin configuration is quite similar to our previous study on highly textured YBCO films prepared on SrTiO3(100) single crystals.10) A selected area electron

diffraction (SAED) pattern including three buffer layers and YBCO is shown in Fig. 4(b). Although the surface of the Ni substrate was oxidized during sintering processes, as shown in Fig. 4(a), three buffer layers and YBCO were textured and coherently piled up. In conclusion, Cl-free and Cl-added YBCO films were prepared on buffered metallic substrates under various synthesis conditions by the FF-MOD method. It was found that undesirable reactions between (Y,Ba,Cu)-oxide and buffer layers occurred at high Ts ∼ 840 °C, which suppressed the formation of YBCO. For Cl-free films, the sintering condition range to form highly textured YBCO was very narrow. On the other hand, textured YBCO films were found to form under a wide range of sintering temperatures when Cl was added. Furthermore, high Ic up to ∼100 A=cm at 77 K was recorded from a film with Ts as low as 740 °C, which is ∼50 °C lower than the typical Ts for FF-MOD-processed YBCO films. Cl addition enabled the formation of flat and homogeneous YBCO films under low Ts where undesirable reactions, such as the generation of BaCeO3, were suppressed. Therefore, Cl addition is very promising for fabricating long tapes especially by sintering at low temperature. Acknowledgments This work was partially supported by the ALCA project of the Japan Science and Technology Agency (JST) and by the JSPS Grant-in-Aid for Research Activity Start-up, Grant Number 16H07161.

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