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CeO2 and YBCO on cube textured nickel
Physica C 337 Ž2000. 67–70 www.elsevier.nlrlocaterphysc
Epitaxial YSZrCeO 2 and YBCO on cube textured nickel Jian Yang ) , Dongqi Shi, Xiaohua Wang, Ansheng Liu, Guansen Yuan General Research Institute for Nonferrous Metals, Beijing 100088, People’s Republic of China
Abstract The YBa 2 Cu 3 O 7yx ŽYBCO. superconducting films deposited on the polycrystal metallic substrates by using the rolling assisted biaxially textured substrates ŽRABiTS. method is reported in this paper. The sharp cube texture in Ni was produced by cold-rolling and recrystallization. The CeO 2 and yttria-stabilized-zirconia ŽYSZ. films were fabricated by magnetron sputtering technique using plane target. Ar and H 2 were used as sputtering gas while CeO 2 film was deposited. If the pressure of hydrogen is appropriate, NiO can be inhibited while CeO 2 is stable. The full width half maximum ŽFWHM. of the f-scan of CeO 2 Ž220. is 118, showing a good in-plane orientation. Using Ar and O 2 sputtering gas YSZ film was deposited. The FWHM of the f-scan of YSZ Ž220. is 158. The conditions of both CeO 2 and YSZ films grown on Ni substrates are more severe than those on single crystal substrates. YBCO film was deposited on YSZrCeO2rNi by using cylinder target in a dc magnetron sputtering system. The transport Jc Ž77 K, 0 T. was 6 = 10 5 Arcm2. The microstructure of the deposited films was observed by scanning electron microscopy ŽSEM. and Auger electron spectrum ŽAES.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: YBCO film; Metallic substrate; Magnetron sputtering
1. Introduction High temperature superconducting ŽHTS. tapes are promising in the applications of transmission cables, motors, energy storage and accelerator magnets, etc. Significant progress has been made in the development of several prototype devices, based on Bi-based HTS tapes. However, because of intrinsic large electronic anisotropy and associated weak flux pinning, the use of the Bi-based HTS tapes in the liquid nitrogen temperature range will be limited to low magnetic-field applications. Recently, new approaches have been mainly followed to deposit biax-
)
Corresponding author.
ially textured YBa 2 Cu 3 O 7yx ŽYBCO. thick films on metallic tapes w1–4x. These YBCO-based tapes offer prospects for operation both at high temperature and in substantial magnetic fields. It has been shown that the YBCO films deposited directly on metallic substrates exhibit poor superconducting properties w5x. The deposition of buffer layers is suggested to retard the oxidation of the substrate, to prevent diffusion of metal element into the YBCO films, and to reduce the lattice mismatch between the metal and YBCO. In this paper, the CeO 2 , yttria-stabilized-zirconia ŽYSZ. buffer layers and the YBCO film were deposited epitaxially on the cube-textured Ni substrate by magnetron sputtering. The results demonstrate that magnetron sputtering is a viable route for the fabrication of YBCO long length tape with high critical current density Ž Jc ..
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 0 5 7 - 5
68
J. Yang et al.r Physica C 337 (2000) 67–70
2. Experimental The Ž100.w00lx cube-textured Ni substrates were prepared by a standard cold-rolling process Ždeformation) 95%. followed by recrystallization thermal treatment. The thickness of the tapes was 0.1–0.2 mm. After polishing and annealing the Ni tapes, the CeO 2 and YSZ buffer layers were deposited by radio frequency Žrf. magnetron sputtering method using ceria and YSZ targets, respectively. A 10% H 2r90% Ar gas was introduced during the deposition of the CeO 2 films. The substrate temperature was 200– 4008C. The YSZ films were grown at the substrate temperature of 700–8008C. The sputtering atmosphere was the mixture of Ar and O 2 ŽAr:O 2 s 2:1.. The YBCO film depositions were conducted on the YSZrCeO2-buffered Ni substrates in a dc magnetron sputtering system, using a cylinder YBCO target. The inner diameter of the target, the target-to-substrate distance, and the substrate temperature were 100 mm, 70 mm, and 7708C, respectively. The sputtering atmosphere was the same as the growth of the YSZ films. The thickness of the YBCO film was about 200 nm. All the films were analyzed by X-ray u –2 u and f-scans. The substrate texture was determined by pole figure measurements. Electrical properties of the YBCO film were measured by standard fourprobe method with the criterion of 1 mVrcm. The morphology of the deposited films was observed by scanning electron microscopy ŽSEM.. The interdiffusion between the substrate and the deposited films was characterized by Auger electron spectrum ŽAES..
3. Results and discussion Metallic Ni tapes were selected as the substrates for the deposition of YBCO films. The Ž111. pole figure of the Ni substrate shows that after cold-rolling and recrystallization treatments, the Ni substrate with sharp cube texture was obtained. Nevertheless, the substrate surface is fairly rough, which might hinder the epitaxial growth of the buffer layers. To overcome this problem, mechanical polishing and electropolishing were performed on the Ni substrates, respectively. It was found that both methods improved the surface fineness. Concerning the sur-
face brightness, the former method seems better than the latter one. The f-scan and pole figure results showed the microstructure of the polished substrates had no apparent difference compared with the asrolled Ni tape, and yet the CeO 2 film grown on the polished substrate was random oriented, indicating the orientation degree in the surface thin layer of the substrate is very important on the epitaxial growth of the CeO 2 film. The surface misorientation of the Ni substrate induced by mechanical polishing can be removed by proper heat treatment. The CeO 2 and YSZ buffer layers were grown by the rf magnetron sputtering using plane target. In order to suppress the formation of NiO during the CeO 2 film growth, Ar and H 2 were used as the sputtering gas. Hydrogen is effective in reducing NiO while having little effect on the CeO 2 films. The deposition temperature was lowered compared with the growth of CeO 2 film on single crystal substrate. Fig. 1 shows the Ž220. f-scan of the CeO 2 film deposited under the substrate temperature of 3708C and the chamber pressure of 380 mTorr Ž10% H 2r90% Ar.. The full width half maximum ŽFWHM. of the f-scan of the film is 118. Fig. 2 shows the v-scan for the CeO 2 film. The FWHM is 5.18, which indicated a good out-plane alignment. It was suggested that the surface fineness of the Ni substrates and the CeO 2 film thickness would affect the integrity of the CeO 2 film. Rough substrate surface may induce pinhole and even delamination on the CeO 2 film. Microcracks usually generate in thick CeO 2 film due to the thermal expansion coefficient difference between the film and the substrate. To strengthen the effect of buffer layer, the YSZ film was deposited epitaxially on the CeO 2 film. The substrate temperature and the chamber pressure were
Fig. 1. The Ž220. f-scan of the CeO 2 film.
J. Yang et al.r Physica C 337 (2000) 67–70
69
Fig. 2. Rocking curve of CeO 2 Ž200. peak.
7508C and 380 mTorr ŽAr:O 2 s 2:1., respectively. SEM observation shows that pinhole and microcrack cannot be found in the YSZ film. The Ž220. f-scan of the YSZ film is shown in Fig. 3. The FWHM is 158, implying that the YSZ film has a good in-plane orientation. The YBCO films were deposited on the YSZrCeO2rNi substrates. The substrate temperature and the chamber pressure were 7708C and 110 mTorr ŽAr:O 2 s 2:1., respectively. The results of the resistivity measurements presented that Tc0 of the YBCO film was above 85 K. The sample size here was 12 = 7 mm2 , and the thickness of the YBCO film was 200 nm. The bridge with 300 mm in width was obtained by the photolithography. The transport Jc Ž77 K, 0 T. was about 6 = 10 5 Arcm2 . Fig. 4 shows the v-scan of the YBCO film on YSZr CeO 2rNi. The rocking curve width of the YBCO Ž005. peak yields a c-axis alignment of 4.78 FWHM. It can be seen clearly that highly out-of-plane oriented YBCO film was obtained on the YSZrCeO2 buffered Ni substrate. However, it should be noted
Fig. 3. The Ž220. f-scan of the YSZ film.
Fig. 4. Rocking curve of YBCO Ž005. peak.
that there are eight peaks appearing at intervals of 458 Žshown in Fig. 5., indicating the existence of two different grain alignments in the a–b plane, i.e. YBCO w100x q w110x I YSZ w110x I CeO 2 w110x I Ni w100x. The lattice mismatch is 11.6% when the YBCO layer was oriented in-plane with respect to YSZ layer as cubic to cubic, while it is 5.2% when the YBCO layer rotated by 458. Therefore, the two different grain alignments probably existed simultaneously. Certainly, if the deposition conditions are properly controlled, it is possible to reach the single orientation, hence, the improvement of Jc in the YBCO film. Fig. 6 shows the Auger electron depth profile of the YBCOrYSZrCeO2rNi films. The platform of every element in the YBCO layer displays that the
Fig. 5. The Ž103. f-scan of the YBCO film on YSZrCeO2 rNi.
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J. Yang et al.r Physica C 337 (2000) 67–70
Fig. 6. Auger electron depth profile of the YBCOrYSZr CeO 2 rNi.
composition is uniform along depth. The interfaces of YBCOrYSZ, YSZrCeO2 and CeO 2rNi are clear, suggesting that extensive diffusion of Ni through the buffer layer microstructure does not occur. In fact, the compositional sharpness of the interfaces observed indicates at most limited interdiffusion in the buffer and Ni layers.
the transport Jc Ž77 K, 0 T. of 6 = 10 5 Arcm2 was obtained by epitaxial growth on the YSZrCeO 2buffered Ni substrate. The f-scan of YBCO Ž103. has FWHM of 158, and the rocking curve FWHM of the YBCO Ž005. peak is 4.78. It was found that the orientation degree and surface fineness of the substrate were in close relationship with the orientation and surface compactness of the buffer layers, and thus, the Jc value. In order to improve the in-plane orientation and thereby the current transport properties of YBCO film, the conditions of YBCO epitaxial growth on the metal substrate should be strictly controlled.
Acknowledgements This research work was supported by the China National Center for Research and Development of Superconductivity and China National Natural Science Foundation Ž59671052..
4. Conclusion
References
The CeO 2 and YSZ buffer layers were successfully deposited on the cube-textured Ni substrate by the magnetron sputtering method. X-ray diffraction results showed the buffer layers were well c-axis oriented and in-plane aligned. The FWHM of f-scan for CeO 2 Ž220. and YSZ Ž220. are 118 and 158, respectively. AES analysis indicated that the buffer layers effectively prevented the oxidation of Ni substrate surface and interdiffusion between substrate and YBCO film. Highly textured YBCO film with
w1x Y. Iijima, N. Tanabe, O. Kohno, Y. Ikeno, Appl. Phys. Lett. 60 Ž1992. 769. w2x R.P. Reade, P. Berdahl, R.E. Russo, S.M. Garrison, Appl. Phys. Lett. 61 Ž1992. 2231. w3x D.P. Norton, A. Goyal, J.D. Budai, D.K. Christen, D.M. Kroeger, E.D. Specht, Q. He, B. Saffian, M. Paranthaman, C.E. Klabunde, D.F. Lee, F.A. List, Science 274 Ž1996. 755. w4x Q. He, D.K. Christen, R. Feenstra, D.P. Norton, M. Paranthaman, E.D. Specht, D.F. Lee, A. Goyal, D.M. Kroeger, Physica C 314 Ž1999. 105. w5x R.E. Russo, R.P. Reade, J.M. McMillan, B.L. Olsen, J. Appl. Phys. 68 Ž1990. 1354.
Epitaxial YSZrCeO 2 and YBCO on cube textured nickel Jian Yang ) , Dongqi Shi, Xiaohua Wang, Ansheng Liu, Guansen Yuan General Research Institute for Nonferrous Metals, Beijing 100088, People’s Republic of China
Abstract The YBa 2 Cu 3 O 7yx ŽYBCO. superconducting films deposited on the polycrystal metallic substrates by using the rolling assisted biaxially textured substrates ŽRABiTS. method is reported in this paper. The sharp cube texture in Ni was produced by cold-rolling and recrystallization. The CeO 2 and yttria-stabilized-zirconia ŽYSZ. films were fabricated by magnetron sputtering technique using plane target. Ar and H 2 were used as sputtering gas while CeO 2 film was deposited. If the pressure of hydrogen is appropriate, NiO can be inhibited while CeO 2 is stable. The full width half maximum ŽFWHM. of the f-scan of CeO 2 Ž220. is 118, showing a good in-plane orientation. Using Ar and O 2 sputtering gas YSZ film was deposited. The FWHM of the f-scan of YSZ Ž220. is 158. The conditions of both CeO 2 and YSZ films grown on Ni substrates are more severe than those on single crystal substrates. YBCO film was deposited on YSZrCeO2rNi by using cylinder target in a dc magnetron sputtering system. The transport Jc Ž77 K, 0 T. was 6 = 10 5 Arcm2. The microstructure of the deposited films was observed by scanning electron microscopy ŽSEM. and Auger electron spectrum ŽAES.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: YBCO film; Metallic substrate; Magnetron sputtering
1. Introduction High temperature superconducting ŽHTS. tapes are promising in the applications of transmission cables, motors, energy storage and accelerator magnets, etc. Significant progress has been made in the development of several prototype devices, based on Bi-based HTS tapes. However, because of intrinsic large electronic anisotropy and associated weak flux pinning, the use of the Bi-based HTS tapes in the liquid nitrogen temperature range will be limited to low magnetic-field applications. Recently, new approaches have been mainly followed to deposit biax-
)
Corresponding author.
ially textured YBa 2 Cu 3 O 7yx ŽYBCO. thick films on metallic tapes w1–4x. These YBCO-based tapes offer prospects for operation both at high temperature and in substantial magnetic fields. It has been shown that the YBCO films deposited directly on metallic substrates exhibit poor superconducting properties w5x. The deposition of buffer layers is suggested to retard the oxidation of the substrate, to prevent diffusion of metal element into the YBCO films, and to reduce the lattice mismatch between the metal and YBCO. In this paper, the CeO 2 , yttria-stabilized-zirconia ŽYSZ. buffer layers and the YBCO film were deposited epitaxially on the cube-textured Ni substrate by magnetron sputtering. The results demonstrate that magnetron sputtering is a viable route for the fabrication of YBCO long length tape with high critical current density Ž Jc ..
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 0 5 7 - 5
68
J. Yang et al.r Physica C 337 (2000) 67–70
2. Experimental The Ž100.w00lx cube-textured Ni substrates were prepared by a standard cold-rolling process Ždeformation) 95%. followed by recrystallization thermal treatment. The thickness of the tapes was 0.1–0.2 mm. After polishing and annealing the Ni tapes, the CeO 2 and YSZ buffer layers were deposited by radio frequency Žrf. magnetron sputtering method using ceria and YSZ targets, respectively. A 10% H 2r90% Ar gas was introduced during the deposition of the CeO 2 films. The substrate temperature was 200– 4008C. The YSZ films were grown at the substrate temperature of 700–8008C. The sputtering atmosphere was the mixture of Ar and O 2 ŽAr:O 2 s 2:1.. The YBCO film depositions were conducted on the YSZrCeO2-buffered Ni substrates in a dc magnetron sputtering system, using a cylinder YBCO target. The inner diameter of the target, the target-to-substrate distance, and the substrate temperature were 100 mm, 70 mm, and 7708C, respectively. The sputtering atmosphere was the same as the growth of the YSZ films. The thickness of the YBCO film was about 200 nm. All the films were analyzed by X-ray u –2 u and f-scans. The substrate texture was determined by pole figure measurements. Electrical properties of the YBCO film were measured by standard fourprobe method with the criterion of 1 mVrcm. The morphology of the deposited films was observed by scanning electron microscopy ŽSEM.. The interdiffusion between the substrate and the deposited films was characterized by Auger electron spectrum ŽAES..
3. Results and discussion Metallic Ni tapes were selected as the substrates for the deposition of YBCO films. The Ž111. pole figure of the Ni substrate shows that after cold-rolling and recrystallization treatments, the Ni substrate with sharp cube texture was obtained. Nevertheless, the substrate surface is fairly rough, which might hinder the epitaxial growth of the buffer layers. To overcome this problem, mechanical polishing and electropolishing were performed on the Ni substrates, respectively. It was found that both methods improved the surface fineness. Concerning the sur-
face brightness, the former method seems better than the latter one. The f-scan and pole figure results showed the microstructure of the polished substrates had no apparent difference compared with the asrolled Ni tape, and yet the CeO 2 film grown on the polished substrate was random oriented, indicating the orientation degree in the surface thin layer of the substrate is very important on the epitaxial growth of the CeO 2 film. The surface misorientation of the Ni substrate induced by mechanical polishing can be removed by proper heat treatment. The CeO 2 and YSZ buffer layers were grown by the rf magnetron sputtering using plane target. In order to suppress the formation of NiO during the CeO 2 film growth, Ar and H 2 were used as the sputtering gas. Hydrogen is effective in reducing NiO while having little effect on the CeO 2 films. The deposition temperature was lowered compared with the growth of CeO 2 film on single crystal substrate. Fig. 1 shows the Ž220. f-scan of the CeO 2 film deposited under the substrate temperature of 3708C and the chamber pressure of 380 mTorr Ž10% H 2r90% Ar.. The full width half maximum ŽFWHM. of the f-scan of the film is 118. Fig. 2 shows the v-scan for the CeO 2 film. The FWHM is 5.18, which indicated a good out-plane alignment. It was suggested that the surface fineness of the Ni substrates and the CeO 2 film thickness would affect the integrity of the CeO 2 film. Rough substrate surface may induce pinhole and even delamination on the CeO 2 film. Microcracks usually generate in thick CeO 2 film due to the thermal expansion coefficient difference between the film and the substrate. To strengthen the effect of buffer layer, the YSZ film was deposited epitaxially on the CeO 2 film. The substrate temperature and the chamber pressure were
Fig. 1. The Ž220. f-scan of the CeO 2 film.
J. Yang et al.r Physica C 337 (2000) 67–70
69
Fig. 2. Rocking curve of CeO 2 Ž200. peak.
7508C and 380 mTorr ŽAr:O 2 s 2:1., respectively. SEM observation shows that pinhole and microcrack cannot be found in the YSZ film. The Ž220. f-scan of the YSZ film is shown in Fig. 3. The FWHM is 158, implying that the YSZ film has a good in-plane orientation. The YBCO films were deposited on the YSZrCeO2rNi substrates. The substrate temperature and the chamber pressure were 7708C and 110 mTorr ŽAr:O 2 s 2:1., respectively. The results of the resistivity measurements presented that Tc0 of the YBCO film was above 85 K. The sample size here was 12 = 7 mm2 , and the thickness of the YBCO film was 200 nm. The bridge with 300 mm in width was obtained by the photolithography. The transport Jc Ž77 K, 0 T. was about 6 = 10 5 Arcm2 . Fig. 4 shows the v-scan of the YBCO film on YSZr CeO 2rNi. The rocking curve width of the YBCO Ž005. peak yields a c-axis alignment of 4.78 FWHM. It can be seen clearly that highly out-of-plane oriented YBCO film was obtained on the YSZrCeO2 buffered Ni substrate. However, it should be noted
Fig. 3. The Ž220. f-scan of the YSZ film.
Fig. 4. Rocking curve of YBCO Ž005. peak.
that there are eight peaks appearing at intervals of 458 Žshown in Fig. 5., indicating the existence of two different grain alignments in the a–b plane, i.e. YBCO w100x q w110x I YSZ w110x I CeO 2 w110x I Ni w100x. The lattice mismatch is 11.6% when the YBCO layer was oriented in-plane with respect to YSZ layer as cubic to cubic, while it is 5.2% when the YBCO layer rotated by 458. Therefore, the two different grain alignments probably existed simultaneously. Certainly, if the deposition conditions are properly controlled, it is possible to reach the single orientation, hence, the improvement of Jc in the YBCO film. Fig. 6 shows the Auger electron depth profile of the YBCOrYSZrCeO2rNi films. The platform of every element in the YBCO layer displays that the
Fig. 5. The Ž103. f-scan of the YBCO film on YSZrCeO2 rNi.
70
J. Yang et al.r Physica C 337 (2000) 67–70
Fig. 6. Auger electron depth profile of the YBCOrYSZr CeO 2 rNi.
composition is uniform along depth. The interfaces of YBCOrYSZ, YSZrCeO2 and CeO 2rNi are clear, suggesting that extensive diffusion of Ni through the buffer layer microstructure does not occur. In fact, the compositional sharpness of the interfaces observed indicates at most limited interdiffusion in the buffer and Ni layers.
the transport Jc Ž77 K, 0 T. of 6 = 10 5 Arcm2 was obtained by epitaxial growth on the YSZrCeO 2buffered Ni substrate. The f-scan of YBCO Ž103. has FWHM of 158, and the rocking curve FWHM of the YBCO Ž005. peak is 4.78. It was found that the orientation degree and surface fineness of the substrate were in close relationship with the orientation and surface compactness of the buffer layers, and thus, the Jc value. In order to improve the in-plane orientation and thereby the current transport properties of YBCO film, the conditions of YBCO epitaxial growth on the metal substrate should be strictly controlled.
Acknowledgements This research work was supported by the China National Center for Research and Development of Superconductivity and China National Natural Science Foundation Ž59671052..
4. Conclusion
References
The CeO 2 and YSZ buffer layers were successfully deposited on the cube-textured Ni substrate by the magnetron sputtering method. X-ray diffraction results showed the buffer layers were well c-axis oriented and in-plane aligned. The FWHM of f-scan for CeO 2 Ž220. and YSZ Ž220. are 118 and 158, respectively. AES analysis indicated that the buffer layers effectively prevented the oxidation of Ni substrate surface and interdiffusion between substrate and YBCO film. Highly textured YBCO film with
w1x Y. Iijima, N. Tanabe, O. Kohno, Y. Ikeno, Appl. Phys. Lett. 60 Ž1992. 769. w2x R.P. Reade, P. Berdahl, R.E. Russo, S.M. Garrison, Appl. Phys. Lett. 61 Ž1992. 2231. w3x D.P. Norton, A. Goyal, J.D. Budai, D.K. Christen, D.M. Kroeger, E.D. Specht, Q. He, B. Saffian, M. Paranthaman, C.E. Klabunde, D.F. Lee, F.A. List, Science 274 Ž1996. 755. w4x Q. He, D.K. Christen, R. Feenstra, D.P. Norton, M. Paranthaman, E.D. Specht, D.F. Lee, A. Goyal, D.M. Kroeger, Physica C 314 Ž1999. 105. w5x R.E. Russo, R.P. Reade, J.M. McMillan, B.L. Olsen, J. Appl. Phys. 68 Ž1990. 1354.