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Selective oxidation of cube textured Ni and Ni–Cr substrate for the formation of cube textured NiO as a component buffer layer for REBa2Cu3O7+x (REBCO) coated conductors
Physica C 372–376 (2002) 831–834 www.elsevier.com/locate/physc
Selective oxidation of cube textured Ni and Ni–Cr substrate for the formation of cube textured NiO as a component buffer layer for REBa2Cu3O7þx (REBCO) coated conductors Z. Lockman a
a,*
, W. Goldacker b, R. Nast b, B. deBoer c, J.L. MacManus-Driscoll a
Department of Materials, Royal School of Mines, Imperial College London, Prince Consort Road, London SW7 2BP, UK b Forschungszentrum Karlsruhe, Technik und Umwelt, Germany c Institut f€ur Festk€orper- und Werkstofforschung, Dresden, Germany
Abstract Thermal oxidation of cube textured, pure Ni and Ni–Cr tapes was undertaken under different oxidation conditions to form cube textured NiO for the use as a first component of buffer layer for the coated conductor. Cube textured NiO was formed on pure Ni after oxidising for more than 130 min in O2 at 1250 °C. The oxide thickness was >30 lm. Much shorter oxidation times (20–40 min, NiO thickness of 5 lm) and lower temperature (1050 °C) were required to form a similar texture on Ni–Cr foils. In addition, NiO formed on Ni–13%Cr was more highly textured than Ni–10%Cr. A Cr2 O3 inner layer and NiO outer layer was formed on the Ni–Cr alloys. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Cube textured Ni and Ni–Cr; RABiTS; NiO buffer; Oxidation
1. Introduction The surface oxidation of polycrystalline and single crystal Ni and Ni–Cr has been widely studied due to the importance of NiO as a protective layer on Ni alloys for high temperature applications. Highly cube textured NiO on textured Ni alloys is an important first layer of multicomponent buffers for REBCO coated conductors.
*
Corresponding author. Tel.: +44-207-594-6803; fax: +44207-584-3194. E-mail address: [email protected] (Z. Lockman).
On single crystal (1 0 0)Ni, (1 0 0)NiO is always the major orientation formed, supporting the epitaxial relationship between substrate and oxide [1]. However, together with fh 0 0gNiO, are fh 1 1gNiO orientations. For a dilute Ni–Cr alloy (with Cr content in the region of 10–13%) cube textured NiO is the fast growing oxide leaving the Cr2 O3 as internal film. NiCr2 O4 precipitates are also formed at the interface of the NiO/Cr2 O3 or Cr2 O3 /Ni–Cr. Internal oxides affect the smoothness of the final oxide. In this work, temperature and time of oxidation, of both Ni and Ni–Cr were altered in order to achieve highly cube textured NiO.
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 9 2 0 - 6
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Z. Lockman et al. / Physica C 372–376 (2002) 831–834
2. Experiments
3. Results
Pure Ni and Ni–Cr alloys were cold rolled down to a thickness of around 100 lm. The recrystallisation process was completed in an inert atmosphere to obtain a cube texture [2,3]. The foils were then chemically polished using a 0.5% HF solution for a few seconds to remove any random airformed oxide. All specimens were oxidised in a horizontal furnace with gas flow control and monitoring. Pure Ni tapes were oxidised at different temperatures from 1000–1300 °C in oxygen while the Ni–Cr tapes were oxidised in air at 1050 °C. The oxidised specimens were examined with aid of optical and scanning electron microscope. Texture measurements of the oxide were done using the X-ray diffraction including h–2h and pole figure analyses.
3.1. Pure nickel: oxidation at 1000–1300 °C in O2
Fig. 1. (a) I(2 0 0):I(1 1 1)NiO versus oxidation temperature. Inset: (1 1 1)NiO pole figure for the cube textured NiO. (b) I(2 0 0):I(1 1 1)NiO versus oxidation time. Inset, thickness versus oxidation time for NiO formed at 1250 °C in oxygen.
Highly textured fh 0 0gNiO forms if the tape is oxidised at 1240–1260 °C (Fig. 1(a)). At <1240 °C, all the low index phases: (1 1 1), (1 0 0), (3 1 1) and (2 2 0)NiO were detected by XRD. At 1240–1250 °C (1 0 0)NiO strongly predominates. Fig. 1(b) shows that at the optimum temperature, 1250 °C,
Fig. 2. (a) Cross-section of NiO on pure Ni formed at 1250 °C (3H, O2 ). (b) NiO microstructure showing surface square cube textured grains.
Z. Lockman et al. / Physica C 372–376 (2002) 831–834
the oxide texture improves with time and thickness (Fig. 1(b) inset). The cross-section (Fig. 2(a)) of NiO formed at 1250 °C consists of compact NiO. Fig. 2(b) shows surface cube NiO grains above the compact grains below. The formation of these equiaxed cube grains corresponds to the sharp improvement in texture [4].
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dation. This layer prevents Ni cations outward diffusion and NiO growth effectively stops. The improvement in texture appears to coincide with a
3.2. Ni–10%Cr and Ni–13%Cr: oxidation at 1050 °C in air NiO on Ni–Cr is mainly cube textured. I(2 0 0):I(1 1 1)NiO on Ni–13%Cr is shown in Fig. 3(a). Strong texture is observed at short oxidation times (oxidation for 2–5 min) due to the epitaxial relationship of the oxide and the substrate. Longer oxidation times leads to more (1 1 1) growth, although (1 0 0) largely predominates. A continuous Cr2 O3 layer forms after only few minutes of oxi-
Fig. 4. Cross-section of NiO on Ni–Cr (formed at 1050 °C, >90 min).
Fig. 3. (a) I(2 0 0):I(1 1 1)NiO on Ni–13%Cr at different oxidation time. (b) (III)NiO PF for Ni–10%Cr oxidised at 1050 °C in air for 40 min. (c) Microstructure of NiO on Ni–10%Cr after 40 min oxidation.
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smoother NiO surface possibly by selective growth of grains with (1 0 0) orientation. Even though oxide formed after 10–40 min does not exhibit strong cube textured NiO, the oxide tends to adhere more to the substrate. Pole figure of the best oxide is shown in Fig. 3(b) (oxidation time 40 min). The single in plane orientation is formed. The microstructure of the oxide formed after 40 min oxidation is shown in Fig. 3(c). Flat wellconnected grains with roughness (rms) of 200–400 nm were formed. Fig. 4 shows a cross-section of scale formed after 90 min of oxidation. We see that while the surface of NiO is <10 lm thick, sub layer oxides are 10 lm thick or more. The surface roughness has increased to >600 nm.
4. Conclusion Very highly textured (1 0 0)NiO forms on RABiTS pure Ni after oxidation at 1250 °C for 3 h (thickness 30 lm). More promising for applica-
tions is the thin (5 lm) highly biaxially textured NiO on Ni–10% and 13%Cr formed at 1050 °C after 20–40 min with rms surface roughness of 200–400 nm. Acknowledgements Materials Department Imperial College London, UK and Universiti Sains Malaysia are thanked for financial support. References [1] N.N. Khoi et al., J. Electrochem. Soc. 122 (11) (1975) 1495– 1503. [2] W. Goldacker, R. Nast, et al., International Workshop of Superconductivity (ISTEC & MRS), Honolulu, Hawaii, USA, 2001. [3] B. deBoer, N. Reger, L. Fernandezey, et al., Physica C 351 (1) (2001) 1–84, 38–41. [4] Z. Lockman, W. Goldacker, R. Nast, B. deBoer, J.L. MacManus-Driscoll, Trans. Appl. Supercond. IEEE 11 (1) (2001) 3325–3328.
Selective oxidation of cube textured Ni and Ni–Cr substrate for the formation of cube textured NiO as a component buffer layer for REBa2Cu3O7þx (REBCO) coated conductors Z. Lockman a
a,*
, W. Goldacker b, R. Nast b, B. deBoer c, J.L. MacManus-Driscoll a
Department of Materials, Royal School of Mines, Imperial College London, Prince Consort Road, London SW7 2BP, UK b Forschungszentrum Karlsruhe, Technik und Umwelt, Germany c Institut f€ur Festk€orper- und Werkstofforschung, Dresden, Germany
Abstract Thermal oxidation of cube textured, pure Ni and Ni–Cr tapes was undertaken under different oxidation conditions to form cube textured NiO for the use as a first component of buffer layer for the coated conductor. Cube textured NiO was formed on pure Ni after oxidising for more than 130 min in O2 at 1250 °C. The oxide thickness was >30 lm. Much shorter oxidation times (20–40 min, NiO thickness of 5 lm) and lower temperature (1050 °C) were required to form a similar texture on Ni–Cr foils. In addition, NiO formed on Ni–13%Cr was more highly textured than Ni–10%Cr. A Cr2 O3 inner layer and NiO outer layer was formed on the Ni–Cr alloys. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Cube textured Ni and Ni–Cr; RABiTS; NiO buffer; Oxidation
1. Introduction The surface oxidation of polycrystalline and single crystal Ni and Ni–Cr has been widely studied due to the importance of NiO as a protective layer on Ni alloys for high temperature applications. Highly cube textured NiO on textured Ni alloys is an important first layer of multicomponent buffers for REBCO coated conductors.
*
Corresponding author. Tel.: +44-207-594-6803; fax: +44207-584-3194. E-mail address: [email protected] (Z. Lockman).
On single crystal (1 0 0)Ni, (1 0 0)NiO is always the major orientation formed, supporting the epitaxial relationship between substrate and oxide [1]. However, together with fh 0 0gNiO, are fh 1 1gNiO orientations. For a dilute Ni–Cr alloy (with Cr content in the region of 10–13%) cube textured NiO is the fast growing oxide leaving the Cr2 O3 as internal film. NiCr2 O4 precipitates are also formed at the interface of the NiO/Cr2 O3 or Cr2 O3 /Ni–Cr. Internal oxides affect the smoothness of the final oxide. In this work, temperature and time of oxidation, of both Ni and Ni–Cr were altered in order to achieve highly cube textured NiO.
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 9 2 0 - 6
832
Z. Lockman et al. / Physica C 372–376 (2002) 831–834
2. Experiments
3. Results
Pure Ni and Ni–Cr alloys were cold rolled down to a thickness of around 100 lm. The recrystallisation process was completed in an inert atmosphere to obtain a cube texture [2,3]. The foils were then chemically polished using a 0.5% HF solution for a few seconds to remove any random airformed oxide. All specimens were oxidised in a horizontal furnace with gas flow control and monitoring. Pure Ni tapes were oxidised at different temperatures from 1000–1300 °C in oxygen while the Ni–Cr tapes were oxidised in air at 1050 °C. The oxidised specimens were examined with aid of optical and scanning electron microscope. Texture measurements of the oxide were done using the X-ray diffraction including h–2h and pole figure analyses.
3.1. Pure nickel: oxidation at 1000–1300 °C in O2
Fig. 1. (a) I(2 0 0):I(1 1 1)NiO versus oxidation temperature. Inset: (1 1 1)NiO pole figure for the cube textured NiO. (b) I(2 0 0):I(1 1 1)NiO versus oxidation time. Inset, thickness versus oxidation time for NiO formed at 1250 °C in oxygen.
Highly textured fh 0 0gNiO forms if the tape is oxidised at 1240–1260 °C (Fig. 1(a)). At <1240 °C, all the low index phases: (1 1 1), (1 0 0), (3 1 1) and (2 2 0)NiO were detected by XRD. At 1240–1250 °C (1 0 0)NiO strongly predominates. Fig. 1(b) shows that at the optimum temperature, 1250 °C,
Fig. 2. (a) Cross-section of NiO on pure Ni formed at 1250 °C (3H, O2 ). (b) NiO microstructure showing surface square cube textured grains.
Z. Lockman et al. / Physica C 372–376 (2002) 831–834
the oxide texture improves with time and thickness (Fig. 1(b) inset). The cross-section (Fig. 2(a)) of NiO formed at 1250 °C consists of compact NiO. Fig. 2(b) shows surface cube NiO grains above the compact grains below. The formation of these equiaxed cube grains corresponds to the sharp improvement in texture [4].
833
dation. This layer prevents Ni cations outward diffusion and NiO growth effectively stops. The improvement in texture appears to coincide with a
3.2. Ni–10%Cr and Ni–13%Cr: oxidation at 1050 °C in air NiO on Ni–Cr is mainly cube textured. I(2 0 0):I(1 1 1)NiO on Ni–13%Cr is shown in Fig. 3(a). Strong texture is observed at short oxidation times (oxidation for 2–5 min) due to the epitaxial relationship of the oxide and the substrate. Longer oxidation times leads to more (1 1 1) growth, although (1 0 0) largely predominates. A continuous Cr2 O3 layer forms after only few minutes of oxi-
Fig. 4. Cross-section of NiO on Ni–Cr (formed at 1050 °C, >90 min).
Fig. 3. (a) I(2 0 0):I(1 1 1)NiO on Ni–13%Cr at different oxidation time. (b) (III)NiO PF for Ni–10%Cr oxidised at 1050 °C in air for 40 min. (c) Microstructure of NiO on Ni–10%Cr after 40 min oxidation.
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smoother NiO surface possibly by selective growth of grains with (1 0 0) orientation. Even though oxide formed after 10–40 min does not exhibit strong cube textured NiO, the oxide tends to adhere more to the substrate. Pole figure of the best oxide is shown in Fig. 3(b) (oxidation time 40 min). The single in plane orientation is formed. The microstructure of the oxide formed after 40 min oxidation is shown in Fig. 3(c). Flat wellconnected grains with roughness (rms) of 200–400 nm were formed. Fig. 4 shows a cross-section of scale formed after 90 min of oxidation. We see that while the surface of NiO is <10 lm thick, sub layer oxides are 10 lm thick or more. The surface roughness has increased to >600 nm.
4. Conclusion Very highly textured (1 0 0)NiO forms on RABiTS pure Ni after oxidation at 1250 °C for 3 h (thickness 30 lm). More promising for applica-
tions is the thin (5 lm) highly biaxially textured NiO on Ni–10% and 13%Cr formed at 1050 °C after 20–40 min with rms surface roughness of 200–400 nm. Acknowledgements Materials Department Imperial College London, UK and Universiti Sains Malaysia are thanked for financial support. References [1] N.N. Khoi et al., J. Electrochem. Soc. 122 (11) (1975) 1495– 1503. [2] W. Goldacker, R. Nast, et al., International Workshop of Superconductivity (ISTEC & MRS), Honolulu, Hawaii, USA, 2001. [3] B. deBoer, N. Reger, L. Fernandezey, et al., Physica C 351 (1) (2001) 1–84, 38–41. [4] Z. Lockman, W. Goldacker, R. Nast, B. deBoer, J.L. MacManus-Driscoll, Trans. Appl. Supercond. IEEE 11 (1) (2001) 3325–3328.