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Tl- and (Hg,Re)-1223 oxide films by spray pyrolysis for practical applications
Physica C 372–376 (2002) 876–879 www.elsevier.com/locate/physc
Tl- and (Hg,Re)-1223 oxide films by spray pyrolysis for practical applications S. Phok a, Ph. Galez a, J.L. Jorda a,*, Z. Supardi b, D. De Barros P. Odier c, A. Sin b,1, F. Weiss b
b,c
,
a
LAIMAN, BP 806, F-74016 Annecy-Le-Vieux-Cedex, France LMGP, INPG, F-38402 St. Martin d‘H eres Cedex, France Laboratoire de Cristallographie, CNRS, BP 166, F-38042 Grenoble Cedex, France b
c
Abstract We report here on promising Tl- and (Hg,Re)-1223 films prepared using in part ultrasonic spray pyrolysis. Tl- and Hg-free precursors are first deposited by the aforementioned technique from a nitrate solution on either MgO or LaAlO3 substrates heated at 800 °C. The second step consists in a thallination or a mercuration performed in a closed system. The resulting films, 0.35–2 lm in thickness, exhibit a good c-axis orientation with Dx < 1° as well as an in-plane texture. Both Tl- and (Hg,Re)-1223 films are superconducting with Tc ¼ 110 K and 134 K, respectively. Jc values deduced either from a screening method (7:5 105 A cm2 at 77 K) or from magnetisation measurements (1:6 106 A cm2 at 77 K) are strong indications of promising transport properties. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Tl- and (Hg,Re)-1223; Coated conductors; Spray pyrolysis
1. Introduction A intense research activity is actually devoted to the preparation of tapes or coated conductors for practical applications, however most studies concern YBCO, while TlBa2 Ca2 Cu3 Oz (Tl1223), Tc ¼ 115 K, and (Hg,Re)-1223, Tc ¼ 134 K, are potential candidates for high current applications. Among the various methods used for deposition, spray pyrolysis is very attractive for its low cost and
easily scalable characteristics. Very high performances have been obtained recently [1] for (Tl,Pb)1223 on Ag substrates in a two steps process. We report here on the preparation of the precursor ‘‘Ba2 Ca2 Cu3 Ox ’’ (with 0.25 Re added in the case of mercury based films) by spray pyrolysis at high temperature and their subsequent thallination or mercuration to form thick superconducting films.
2. Precursor deposition and analysis *
Corresponding author. Tel.: +33-4-50092280; fax: +33-450276535. E-mail address: [email protected] (J.L. Jorda). 1 Present address: Pirelli labs., Viales Sarca 222, 20126 Milano, Italy.
Precursor layers with composition Ba:Ca:Cu ¼ 2:2:3 are first deposited by spray pyrolysis. The starting solution is a mixture of Ba, Ca and Cu nitrates prepared by dissolving BaCO3 , CaCO3 ,
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 8 8 6 - 9
S. Phok et al. / Physica C 372–376 (2002) 876–879
CuO and ReO2 in concentrated nitric acid (65%). Based on recent studies in the (Hg,Re)Ba2 Ca2 Cu3 Ox system [2,3], for which a Rex Ba2 Ca2 Cu3 Oy precursor is used, the cation ratio has been changed in order to adjust the deposit composition. The spray is produced from the solution by ultrasonic waves generated at 775 kHz by a piezoelectric ceramic. It is led to a MgO or LaAlO3 substrate, heated at 800 °C, with an argon flow (3 l/min). The deposition time is set at 5 min. The as-deposited precursor films are highly homogeneous and show an amorphous-like X-ray diffraction pattern. Crystallisation and grain growth are observed after annealing at 850 °C in an oxygen atmosphere. In absence of Re, the equilibrium involves the BaCuO2 and Ca2 CuO3 phases as expected, from the BaO–CaO–CuO phase equilibrium diagram [4]. These phase relations are significantly modified when Re is added [2].
877
Fig. 1. Backscattered electronic micrograph of a thallined film on LaAlO3 substrate with composition Tl-1223.
3. Formation and characterisation of the Tl-1223 films Following the procedure established in previous works for bulk samples [4–6], the as-deposited precursor films are thallinated at 900 °C for 3 h in a sealed quartz tube using a pellet of nominal composition Tl-1223, about 300 mg in weight, as a thallium source. The pellet is kept in contact with the precursor film during the high temperature run. Both pellet and film are wrapped in gold foil. Under these conditions, the Tl-1223 phase is found to grow as platelet-like grains (Fig. 1) with an overall thickness ranging from 0.35 to 2 lm. Best samples have been prepared on LaAlO3 substrates. The well-defined grain boundaries, revealed in Fig. 1, are clear evidence that the formation of the superconducting Tl-1223 phase does not imply a liquid phase, and is mainly controlled by solid–gas reactions. The X-ray diffraction patterns confirm the purity of the films and show a pronounced preferred orientation with the mean c-axis being perpendicular to the substrate (Fig. 2). The angular dispersion measured on the (0 0 6) reflection (inset of Fig. 2) is Dx ¼ 0:6° which points out a good orientation of the plates.
Fig. 2. h=2h X-ray diffraction spectrum and rocking curve (insert) of a Tl-1223 film on LaAlO3 substrate.
The samples are superconducting with a critical temperature Tc of about 113 K (Fig. 3) and a transition width of 2 K. At 77 K, in zero field, the critical current measured by a screening method is as high as 7:5 105 A cm2 (inset of Fig. 3). 4. Formation of the superconducting (Hg,Re)-1223 phase The mercuration is performed ex-situ at 860 °C during 5 h with a low cooling rate sequence
878
S. Phok et al. / Physica C 372–376 (2002) 876–879
Fig. 3. Measurement of ac susceptibility v and critical current Jc (insert) of a Tl-1223 film on LaAlO3 substrate.
(10 °C/h) as described elsewhere [3]. MgO substrates have been used in these experiments. The crystallization proceeds during the mercuration which involves a gas–solid controlling step. Platelike crystals grow (10–2 lm) while the thickness of the film decreases. X-ray pole figures (Fig. 4) indicate a strong c-axis orientation (Dx ¼ 0:7° for the (0 0 6) reflection by rocking curve) and a good degree of epitaxy for the thinnest films (300 nm). However the grains have low degree of connectivity and some impurities (e.g. CaHgO2 ) remain. The films are superconducting at 134 K with a rather sharp transition (5 K) and evidences of a weak link behaviour at the foot of the resistivity curve are visible [7]. Intergrain coupling is low, giving a critical current density smaller than 103 A cm2 at 77 K (self-field), but dc magnetisation evidences Jc intragrain values reaching 1:6 106 A cm2 at 77 K.
5. Conclusions The results which are reported here show the possibility to prepare high Tc thick films of nonsubstituted Tl-1223 and (Hg,Re)-1223, by a high temperature spray pyrolysis process involving nitrate precursors. The crystallisation of the phase is performed ex situ and results in bi-axial texturation in both cases. The critical temperature of the films are comparable to those of bulk samples and evidence is given for high critical currents. Improvements of orientation and connectivity is still under work but, in the present state of the art, this simple chemical deposition method of thin and thick films by spray pyrolysis appears to be very promising for coated conductors synthesis.
Acknowledgements
Fig. 4. X-ray pole figure of (0 0 6) (a) and (1 0 2) (b) Bragg reflection. The figure is plotted with iso-intensity lines from >90% to 20% (dashed lines) by steps of 10%.
This work was financially supported by the GROWTH project: SUPERTEXT (BRPR-CT-97O556) and a grant from the Region Rhone-Alpes. The authors thanks Dr. L. Ortega for pole figure measurements and Dr. A. Sulpice for magnetisation measurements.
S. Phok et al. / Physica C 372–376 (2002) 876–879
References [1] W. Li et al., Supercond. Sci. Technol. 12 (1999) L1–L4. [2] A. Sin et al., Physica C 341 (2000) 399. [3] A. Sin et al., Supercond. Sci. Technol. 13 (2000) 617.
879
[4] Th. Hopfinger, Ph.D. thesis, Universite de Savoie, France, 1999. [5] J.L. Jorda et al., J. Supercond. 11 (1998) 87. [6] Ph. Galez et al., Supercond. Sci. Technol. 14 (2001) 583. [7] A. Sin et al., Thin Solid films 388 (2001) 251.
Tl- and (Hg,Re)-1223 oxide films by spray pyrolysis for practical applications S. Phok a, Ph. Galez a, J.L. Jorda a,*, Z. Supardi b, D. De Barros P. Odier c, A. Sin b,1, F. Weiss b
b,c
,
a
LAIMAN, BP 806, F-74016 Annecy-Le-Vieux-Cedex, France LMGP, INPG, F-38402 St. Martin d‘H eres Cedex, France Laboratoire de Cristallographie, CNRS, BP 166, F-38042 Grenoble Cedex, France b
c
Abstract We report here on promising Tl- and (Hg,Re)-1223 films prepared using in part ultrasonic spray pyrolysis. Tl- and Hg-free precursors are first deposited by the aforementioned technique from a nitrate solution on either MgO or LaAlO3 substrates heated at 800 °C. The second step consists in a thallination or a mercuration performed in a closed system. The resulting films, 0.35–2 lm in thickness, exhibit a good c-axis orientation with Dx < 1° as well as an in-plane texture. Both Tl- and (Hg,Re)-1223 films are superconducting with Tc ¼ 110 K and 134 K, respectively. Jc values deduced either from a screening method (7:5 105 A cm2 at 77 K) or from magnetisation measurements (1:6 106 A cm2 at 77 K) are strong indications of promising transport properties. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Tl- and (Hg,Re)-1223; Coated conductors; Spray pyrolysis
1. Introduction A intense research activity is actually devoted to the preparation of tapes or coated conductors for practical applications, however most studies concern YBCO, while TlBa2 Ca2 Cu3 Oz (Tl1223), Tc ¼ 115 K, and (Hg,Re)-1223, Tc ¼ 134 K, are potential candidates for high current applications. Among the various methods used for deposition, spray pyrolysis is very attractive for its low cost and
easily scalable characteristics. Very high performances have been obtained recently [1] for (Tl,Pb)1223 on Ag substrates in a two steps process. We report here on the preparation of the precursor ‘‘Ba2 Ca2 Cu3 Ox ’’ (with 0.25 Re added in the case of mercury based films) by spray pyrolysis at high temperature and their subsequent thallination or mercuration to form thick superconducting films.
2. Precursor deposition and analysis *
Corresponding author. Tel.: +33-4-50092280; fax: +33-450276535. E-mail address: [email protected] (J.L. Jorda). 1 Present address: Pirelli labs., Viales Sarca 222, 20126 Milano, Italy.
Precursor layers with composition Ba:Ca:Cu ¼ 2:2:3 are first deposited by spray pyrolysis. The starting solution is a mixture of Ba, Ca and Cu nitrates prepared by dissolving BaCO3 , CaCO3 ,
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 8 8 6 - 9
S. Phok et al. / Physica C 372–376 (2002) 876–879
CuO and ReO2 in concentrated nitric acid (65%). Based on recent studies in the (Hg,Re)Ba2 Ca2 Cu3 Ox system [2,3], for which a Rex Ba2 Ca2 Cu3 Oy precursor is used, the cation ratio has been changed in order to adjust the deposit composition. The spray is produced from the solution by ultrasonic waves generated at 775 kHz by a piezoelectric ceramic. It is led to a MgO or LaAlO3 substrate, heated at 800 °C, with an argon flow (3 l/min). The deposition time is set at 5 min. The as-deposited precursor films are highly homogeneous and show an amorphous-like X-ray diffraction pattern. Crystallisation and grain growth are observed after annealing at 850 °C in an oxygen atmosphere. In absence of Re, the equilibrium involves the BaCuO2 and Ca2 CuO3 phases as expected, from the BaO–CaO–CuO phase equilibrium diagram [4]. These phase relations are significantly modified when Re is added [2].
877
Fig. 1. Backscattered electronic micrograph of a thallined film on LaAlO3 substrate with composition Tl-1223.
3. Formation and characterisation of the Tl-1223 films Following the procedure established in previous works for bulk samples [4–6], the as-deposited precursor films are thallinated at 900 °C for 3 h in a sealed quartz tube using a pellet of nominal composition Tl-1223, about 300 mg in weight, as a thallium source. The pellet is kept in contact with the precursor film during the high temperature run. Both pellet and film are wrapped in gold foil. Under these conditions, the Tl-1223 phase is found to grow as platelet-like grains (Fig. 1) with an overall thickness ranging from 0.35 to 2 lm. Best samples have been prepared on LaAlO3 substrates. The well-defined grain boundaries, revealed in Fig. 1, are clear evidence that the formation of the superconducting Tl-1223 phase does not imply a liquid phase, and is mainly controlled by solid–gas reactions. The X-ray diffraction patterns confirm the purity of the films and show a pronounced preferred orientation with the mean c-axis being perpendicular to the substrate (Fig. 2). The angular dispersion measured on the (0 0 6) reflection (inset of Fig. 2) is Dx ¼ 0:6° which points out a good orientation of the plates.
Fig. 2. h=2h X-ray diffraction spectrum and rocking curve (insert) of a Tl-1223 film on LaAlO3 substrate.
The samples are superconducting with a critical temperature Tc of about 113 K (Fig. 3) and a transition width of 2 K. At 77 K, in zero field, the critical current measured by a screening method is as high as 7:5 105 A cm2 (inset of Fig. 3). 4. Formation of the superconducting (Hg,Re)-1223 phase The mercuration is performed ex-situ at 860 °C during 5 h with a low cooling rate sequence
878
S. Phok et al. / Physica C 372–376 (2002) 876–879
Fig. 3. Measurement of ac susceptibility v and critical current Jc (insert) of a Tl-1223 film on LaAlO3 substrate.
(10 °C/h) as described elsewhere [3]. MgO substrates have been used in these experiments. The crystallization proceeds during the mercuration which involves a gas–solid controlling step. Platelike crystals grow (10–2 lm) while the thickness of the film decreases. X-ray pole figures (Fig. 4) indicate a strong c-axis orientation (Dx ¼ 0:7° for the (0 0 6) reflection by rocking curve) and a good degree of epitaxy for the thinnest films (300 nm). However the grains have low degree of connectivity and some impurities (e.g. CaHgO2 ) remain. The films are superconducting at 134 K with a rather sharp transition (5 K) and evidences of a weak link behaviour at the foot of the resistivity curve are visible [7]. Intergrain coupling is low, giving a critical current density smaller than 103 A cm2 at 77 K (self-field), but dc magnetisation evidences Jc intragrain values reaching 1:6 106 A cm2 at 77 K.
5. Conclusions The results which are reported here show the possibility to prepare high Tc thick films of nonsubstituted Tl-1223 and (Hg,Re)-1223, by a high temperature spray pyrolysis process involving nitrate precursors. The crystallisation of the phase is performed ex situ and results in bi-axial texturation in both cases. The critical temperature of the films are comparable to those of bulk samples and evidence is given for high critical currents. Improvements of orientation and connectivity is still under work but, in the present state of the art, this simple chemical deposition method of thin and thick films by spray pyrolysis appears to be very promising for coated conductors synthesis.
Acknowledgements
Fig. 4. X-ray pole figure of (0 0 6) (a) and (1 0 2) (b) Bragg reflection. The figure is plotted with iso-intensity lines from >90% to 20% (dashed lines) by steps of 10%.
This work was financially supported by the GROWTH project: SUPERTEXT (BRPR-CT-97O556) and a grant from the Region Rhone-Alpes. The authors thanks Dr. L. Ortega for pole figure measurements and Dr. A. Sulpice for magnetisation measurements.
S. Phok et al. / Physica C 372–376 (2002) 876–879
References [1] W. Li et al., Supercond. Sci. Technol. 12 (1999) L1–L4. [2] A. Sin et al., Physica C 341 (2000) 399. [3] A. Sin et al., Supercond. Sci. Technol. 13 (2000) 617.
879
[4] Th. Hopfinger, Ph.D. thesis, Universite de Savoie, France, 1999. [5] J.L. Jorda et al., J. Supercond. 11 (1998) 87. [6] Ph. Galez et al., Supercond. Sci. Technol. 14 (2001) 583. [7] A. Sin et al., Thin Solid films 388 (2001) 251.