Fabrication of buffer layers and seed layers on biaxially textured Ni tapes for YBCO superconducting wire

Fabrication of buffer layers and seed layers on biaxially textured Ni tapes for YBCO superconducting wire

Physica C 357±360 (2001) 938±941 www.elsevier.com/locate/physc Fabrication of bu€er layers and seed layers on biaxially textured Ni tapes for YBCO s...

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Physica C 357±360 (2001) 938±941

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Fabrication of bu€er layers and seed layers on biaxially textured Ni tapes for YBCO superconducting wire H. Kurosaki a,*, T. Yuasa a, T. Maeda a, Y. Yamada a, S.B. Kim a, T. Watanabe b, K. Wada b, I. Hirabayashi a a

Superconductivity Research Laboratory, ISTEC, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan b The Furukawa Electric Co., Ltd., 500 Kiyotaki, Nikko 321-1493, Japan Received 16 October 2000; accepted 9 November 2000

Abstract We have deposited Y2 O3 bu€er layers by metal-organic chemical vapor deposition (MOCVD) on textured Ni tapes coated with biaxially aligned NiO(1 0 0) layers grown by surface oxidation epitaxy method. X-ray di€raction measurements showed that Y2 O3 grew cube-on-cube on NiO layer, i.e. it has also biaxial orientation. With these Y2 O3 layers, in-plane aligned YBa2 Cu3 O7 d (YBCO) (0 0 1) thick ®lms (3 lm in thickness) have been successfully grown by liquid phase epitaxy (LPE) method on the textured Ni tapes. YBCO seed layers, required in LPE process, had been also deposited by MOCVD. The LPE-grown YBCO thick ®lms exhibit superconductivity transition temperature of Tc; onset ˆ 90 K and Tc; zero ˆ 65 K by four-probe transport measurement. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.76.Bz; 81.15.Gh; 81.15.Lm; 85.25.Kx Keywords: Superconducting wire; Metal-organic chemical vapor deposition; Liquid phase epitaxy; Bu€er layer; YBCO; Y2 O3

1. Introduction In application of YBCO ®lm to high current superconducting wire, biaxially textured Ni(1 0 0) tape is a promising substrate [1]. However, biaxially aligned bu€er layers are requisite to obtain a high quality YBa2 Cu3 O7 d (YBCO) layer on this tape. A novel approach to forming such bu€er layer on the long Ni tape is surface oxidation epitaxy (SOE) method [2], in which the textured Ni tapes are coated with biaxially aligned NiO(1 0 0)

*

Corresponding author. Fax: +81-52-871-4090. E-mail address: [email protected] (H. Kurosaki).

layers by oxidation of their own surface. Since this process does not employ any vacuum apparatus or complex devices for long substrate, it is suitable for practical production of long YBCO superconducting wire. Liquid phase epitaxy (LPE) method o€ers attractive features such as high growth rate, ability to grow thick, high quality ®lm, and simpli®ed apparatus (no use of vacuum apparatus) for mass production of YBCO ®lm. Recently, the growth temperature has been lowered from 1000°C to 920°C by addition of ¯uorine to BaO±CuO ¯ux, and application of LPE method to fabrication of YBCO wire with metallic tape has gained practicality [3]. Trials to grow YBCO ®lms on Ag-based

0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 4 4 7 - 6

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tapes by this process have been already started with Ag-saturated ¯ux [4]. From the point of view described above, an attempt has been made to grow YBCO layers by LPE method on the NiO/Ni tapes fabricated by SOE method [2]. YBCO seed layers, which are necessary for epitaxial growth of YBCO ®lm in LPE method, have been deposited direct on SOEgrown NiO layers by pulsed laser deposition. Although YBCO(0 0 1) thick ®lms have been obtained, in-plain alignments of these ®lms were poor and superconductivity transition has not been observed. To improve the in-plane alignment and crystallinity of LPE-grown YBCO ®lm on the NiO/Ni tape, certain bu€er layers are required. In this study, we deposited Y2 O3 bu€er layer on the NiO/Ni tapes, becauseY2 O3 has small lattice mismatch with YBCO and is resistant to BaO± CuO ¯ux. Deposition of the Y2 O3 bu€er layer was carried out with metal-organic chemical vapor deposition (MOCVD). This process is suitable for long substrate owing to its high growth rate and capability to coat large area. Furthermore, it has already revealed that MOCVD-grown Y2 O3 bu€er layers biaxially align YBCO grains on MgO single crystal, whose lattice constant is close to that of NiO [5]. Then YBCO thick ®lms were deposited on the Y2 O3 /NiO/Ni structures by LPE method using YBCO seed layers that are also grown by MOCVD. 2. Experimental Deposition for Y2 O3 bu€er and YBCO seed layers were carried out in a conventional cold-wall MOCVD system. The precursors employed were Y(TMHD)3 -4tBPNO, Ba(DPM)2 -2tetraene, and Cu(TMHPD)2 . They were maintained at certain temperatures higher than each melting point to stabilize their vaporization rates, and their vapors were transported into a reactor by pure Ar carrier gas. The biaxially aligned NiO/Ni tape was cut into 1-cm-length pieces and clipped on a ceramic heater. Fabrication of NiO/Ni tape by SOE method is described elsewhere [2]. In the case of Y2 O3 deposition, Y precursor and pure O2 gas was introduced into the reactor and the tapes were

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heated to 600±750°C at a system total pressure of 2 Torr with an O2 partial pressure of 0.3 Torr. In the case of YBCO deposition, Y, Ba, Cu precursors and O2 gas were simultaneously introduced into the reactor and the tapes were heated to 820°C at a system total pressure of 3 Torr with an O2 partial pressure of 1.3 Torr. Deposition of YBCO thick ®lms by LPE method was performed with Ag-saturated BaO±CuO±BaF2 ¯ux. The mixing molar ratio of the ¯ux was BaO:CuO:BaF2 ˆ 34:5:62.5:3. YBCO was put on the bottom of an Y2 O3 crucible as Y source. The growth temperature was 930°C. The LPE-grown YBCO ®lms were annealed at 450°C for 24 h in O2 ¯ow atmosphere to make them superconductor. The obtained ®lms were examined by h±2h and /-scan X-ray di€raction (XRD) and scanning electron microscopy (SEM). The superconductivity transition temperature (Tc ) was measured by four-probe transport measurement. 3. Results and discussion  thickness were obtained Y2 O3 ®lms of 1000 A on NiO/Ni tapes by 20-min-MOCVD. XRD h±2h scan patterns of Y2 O3 /NiO/Ni structure consist of Y2 O3 (4 0 0), Y2 O3 (2 2 2), and substrate re¯ections. Y2 O3 ®lms have to be (1 0 0)-orientated to obtain biaxially oriented YBCO(0 0 1) layers. Fig. 1 shows substrate temperature dependence of orientation degree of Y2 O3 ®lm de®ned by I…4 0 0†=fI…4 0 0† ‡ I…2 2 2†g, where I…4 0 0† and I…2 2 2† denote XRD peak height of Y2 O3 (4 0 0) and Y2 O3 (2 2 2) in h±2h scan. It was found that orientation degree is 75% at substrate temperature of 650°C, i.e. nearly (1 0 0)-orientated Y2 O3 can be obtained at this temperature. Fig. 2a shows a XRD /-scan pattern of the SOE-grown NiO(1 0 0) layer through the NiO(2 2 0) re¯ection, and Fig. 2b shows that of MOCVD-grown Y2 O3 (1 0 0) ®lm through the Y2 O3 (2 2 2) re¯ection. Both Fig. 2a and b consist of four equivalent peaks, indicating good in-plane alignment of the NiO and Y2 O3 layer. In addition, Fig. 2a and b illustrates that Y2 O3 ®lms grow cube-on-cube on NiO layers.  thickness were YBCO seed layers of 5000 A obtained by 30-min-MOCVD on the biaxially

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Fig. 1. Substrate temperature dependence of orientation degree of Y2 O3 .

aligned Y2 O3 bu€er layers. XRD h±2h scans for the YBCO/Y2 O3 /NiO/Ni structures showed the YBCO ®lms to be (0 0 1) orientated. Fig. 2c is a XRD /-scan pattern of a seed layer through YBCO(1 0 3) re¯ection. Most of the YBCO grains was aligned with YBCO[1 0 0]kY2 O3 [0 1 1]. However, re¯ection peaks of YBCO ®lm (Fig. 2c) are broader than those of Y2 O3 (Fig. 2b). This may be caused by the incomplete orientation of Y2 O3 bu€er layer (orientation degree ˆ 75%). In the case of YBCO/Y2 O3 /MgO structure, it has been revealed that lower orientated Y2 O3 bu€er layer makes in-plane alignment of YBCO degrade [5]. To make sure the e€ect of Y2 O3 bu€er layer for inplane alignment of YBCO ®lm on SOE-grown NiO layer, samples without Y2 O3 bu€er layer were fabricated. YBCO ®lms grown direct on NiO/Ni tapes by MOCVD were c-axis oriented, but their in-plane alignment was poor as shown in Fig. 2e. YBCO thick ®lms of 3 lm thickness were obtained by 2 min deposition of LPE method on YBCO (seed)/Y2 O3 /NiO/Ni structures. XRD h±2h scans showed that LPE-grown YBCO ®lms were also (0 0 1)-orientated. XRD /-scan pattern of this ®lm through YBCO(1 0 3) is shown in Fig. 2d. Biaxially aligned YBCO thick ®lm was successfully obtained by LPE method. Surface morphology of an LPE-grown YBCO ®lm is shown in Fig. 3.

Fig. 2. XRD /-scan patterns of (a) SOE-grown NiO layer, (b) MOCVD-grownY2 O3 bu€er layer, (c) MOCVD-grown YBCO seed layer, (d) LPE-grown YBCO thick ®lm, (e) MOCVDgrown YBCO ®lm without Y2 O3 bu€er layer.

Fig. 3. SEM image of an LPE-grown YBCO thick ®lm.

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Large square grains of YBCO, peculiar to LPE growth, are seen and no cracks were detected by SEM observation. Four-probe transport measurement reveals a Tc; onset of 90 K and a Tc; zero of 65 K: although this is not sucient, superconductivity transition of the LPE-grown YBCO ®lm on Ni tape was observed for the ®rst time. In the case where Ag-based substrates are used, the ¯ux intrudes into the interface between YBCO layer and substrate [4]. Y2 O3 bu€er layers obtained here would prevent the intrusion and protect the NiO layer. 4. Conclusion Biaxially aligned Y2 O3 (1 0 0) bu€er layers have been deposited by MOCVD on textured Ni tapes coated with biaxially aligned NiO(1 0 0) layers grown by SOE method. With these bu€er layers, in-plane oriented YBCO(0 0 1) thick ®lms (3 lm in thickness) have been successfully obtained by LPE method on Ni tapes. They exhibit Tc; onset ˆ 90 K and Tc; zero ˆ 65 K by four-probe transport measurement; this suggests the possibility of using LPE method for fabrication of YBCO thick ®lms on textured Ni tapes toward superconducting wire.

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Acknowledgements This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications.

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