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Preparation of YBaCuO high transition temperature superconductors using BaO2 and BaCO3
Mat. Res. B u l l . , Vol. 23, p p . 1469-1477, 1988. P r i n t e d in the USA. 0025-5408/88 $3.00 + .00 C o p y r i g h t (c) 1988 Pergamon P r e s s plc.
PREPARATION
OF Y-Ba-Cu-O HIGH TRANSITION TEMPERATURE CONDUCTORS USING BaO 2 AND BaCO 3
M. Leskeli a, C.H. Mueller,
SUPER-
J.K. Truman and P.H. Holloway
Department
of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 aon sabbatical leave from Department of Chemistry, University of Turku, SF-20500 Turku, Finland ( R e c e i v e d J u l y 13, 1988; Communicated b y A. Wold)
ABSTRACT The preparation of superconducting YBa2Cu307_ x in static air and flowing He atmosphere from BaCO 3 and BaO 2 mixed with Y203 and CuO has been studied. In static air BaO 2 reacted at slightly lower temperature than BaCO 3 and different mechanisms were observed for the formation reactions. Using BaO2, the orthorhombic phase is readily formed, while high temperatures or prolonged heating times are needed to obtain the orthorhombic phase with BaCO 3. In flowing helium atmosphere, both starting materials led to products which contained metallic Cu and binary oxide compounds, with no ternary oxides being detected. MATERIALS
INDEX:
yttrium,
barium,
copper,
oxides,
peroxides
Introduction Originally the rare earth-alkaline earth-copper oxides have been prepared by a solid state synthesis of individual oxides or some relative easily decomposing compound like carbonate (i). Bednorz and Muller, however, used the oxalate precipitation method in their experiment to get better mixing of the ions before calcining to oxide (2). Today, superconducting orthorhombic YBa2Cu307_ x (123) powder is most often made using Y203, BaCO 3 and CuO powders as starting materials (3,4). The weighed powders are thoroughly mixed in a mill and the mixture is calcined in an open crucible. The powders are not pelletized prior to calcining due to the large molar volume change between starting materials and products. Different crucible materials have been employed and according to Engler (5) alumina, 1469
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Pt and Au best withstand the conditions required for preparation. For Ba, the BaCO 3 starts to decompose and react with Y and Cu oxides at 900-950 °C, and long firing times are needed to complete the reaction. The times used are 12-24 hours and 950 °C is a critical temperature because just above it the 123 phase starts to decompose to BaCuO2, CuO and Y2BaCuO 5 (211) (6). However, carbonate does not always completely decompose during calcining. Furthermore, at the reaction temperatures, particles are sintering and grain growth occurs. Thus remilling and recalcining are needed, with conditions similar to the first calcining. The final step in the processing is the slow cooling in flowing 02 to increase the oxygen content of the compound which is critical for the formation of the oxygen-deficient orthorhombic phase and superconducting properties (7-9). This step can be carried out in connection with the second calcining. Because of the high decomposition temperature of BaCO3, alternative Ba source materials have been studied for the solid state synthesis. BaO, Ba(OH) 2 and BaO 2 have been used, and the products obtained have been purer and denser than superconductors synthesized from BaCO 3 (10-15). Higher critical temperature and current are also claimed for the products prepared from BaO 2 than for those made from BaCO 3 (12). However, the details of preparation using BaO 2 were incomplete. An alternative preparation method for YBa2Cu307_ x is to use solutions and coprecipitation prior to calcination. This is a good way to improve the homogeneous mixing of ions on the atomic scale. Coprecipitation of Y, Ba and Cu by oxalate (16-18) or carbonate (19,20) from nitrate or acetate solution has been accomplished. The calcination temperature with oxalates is lower than used in the solid state reaction described above, but the cabonate precipitation needs the same annealing procedure used in the solid state reaction. Complexation of Y, Ba and Cu with citrate is another possible solution-based route for the synthesis of the 123 phase (21,22). A third method employed in the preparation of 123 is sol gel processes. Metal hydoxides and alkoxides have been used as starting materials (23,24). The advantages of sol gel processing are the low temperature required and the possibility of shapeing the product. The main task in our superconductivity study is the preparation and characterization of sputter deposited thin films (25). In preparation of targets for sputtering the need for Ba sources other than BaCO 3 was found. The aim of this study was to detail the reactivity of BaO 2 and BaCO 3 in preparation of sputter deposition targets. In sputter deposition of thin films it is important to know the target composition and the phases present because each phase may have its own sputter yield for each component. Sputter deposition of Ba or superconducting oxides is difficult due to a negative ion effect, which causes a deviation between the target and film compositions under typical deposition conditions (26). In addition Ba migration in target during sputtering depends on phases present. One way to decrease these effects may be the firing of the target in inert atmosphere (27). Experimental The starting materials Y203, BaO2/BaCO 3 and CuO, purchased
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from Alfa Chemical Company, were reported to be at least 99.9 % pure. A slurry consisting of powders mixed in a molar ratio of ½:2:3 and acetone was shaken for 15 minutes to assure homogeneous mixing of the powders. After drying, the powders were fired in a muffle furnace in static air atmosphere at times of 2 hours and temperatures which varied between 500 and 1000 °C. After firing the samples were cooled at the natural cooling rate of the furnace. The time required for the furnace to cool from 900 °C to 500 °C was approximately one hour. The products were characterized by X-ray diffraction (XRD) employing a Philips powder diffractometer and using Ni-filtered CuK~ radiation. The decomposition of the starting materials were also monitored with a diffractometer equipped with a hot-stage sample holder. The heating program contained ramps of i00 °C, i.e. a fast heating to the set value and measurement was carried out after a stabilization time of 15 min. The hot-stage X-ray diffraction measurements were carried in flowing inert atmosphere (He). Results and Discussion Experiments in static air Y203-BaO2-CuO. In static air atmosphere liquid BaO 2 formed above 400 °C and reacted first with copper oxide. At 600-950 °C, XRD data contained peaks from BaCuO2, CuO and Y203 (Fig. I). The main peak of the quaternary 123 compound (28 = 32.5 ° , (103),(013) and (110) reflections) appeared between 700 and 800 °C, but the amount of 123 phase at 800 °C was not high, contrary to earlier reports (6). At 900 °C the main XRD peak originated from the 123 phase. Calculating in a very crude fashion based on the intensities of the peaks, the amount of 123 phase in products fired 2 hours at 900 °C can be eastimated to be about 50 %. By increasing the temperature to 925 and 950 °C, the amount of 123 phase increased to 75 and 85 %, respectively. If the firing time was prolonged to 20 h at 950 °C, impurity peaks (mainly from BaCuO 2) were hardly detectable. Pure product was also obtained after firing for 2 h at 975 or 1000 °C. When BaO 2 was used as a starting material, the reaction forming the 123 phase in the final stage was roughly ½ Y203 + 2BaCuO 2 + CuO --> YBa2Cu307_ x. This reaction limited the rate of formation because the barium-copper oxide seems to form very fast. The reaction explains why the 211 phase is not formed with BaO 2 as readily as with BaCO 3. The XRD data from samples fired at 900-1000 °C clearly shows the formation of the orthorhombic 123 phase. The difference in the d-spacings of the (103) and (013) planes increased with increased firing temperature: from 0.014 A at 900 °C to 0.035 A at 1000 °C. The same tendency can be seen in the separation of d-values of the (020) and (200) reflections at 28 = 46.5 - 47.5 ° (Fig. i). Y203-BaCO3-CuO. The reflections from the starting materials, particulary BaCO3, dominate the XRD data up to 800 °C (Fig. 2). The appearance of the main peak of the 123 phase in this system is difficult to detect because the reflection from (102) BaCO 3
|
i
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i
i
M. LESKEL~., et al.
I
i
I
C
I
--
'I
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9 7 5 °C
975 °C
900o c
9 0 0 °C
,oo°c II
I
/
8 0 0 °C
*
I
5 0 0 °C
500o C 20
30
40
50
60
2e
FIG. I. XRD patterns for Y203-BaO~-CuO mixture fired in static alr atmosphere for 2 h at 500 °C (peaks are from the starting materials), at 800 °C (Y203 and BaCuO2), at 900 °C (123 and BaCuO 2) and at 975 °C (123).
20
30
40
50
60
20
FIG. 2 XRD patterns, for. Y ~O3-BaCO. 3 CuO mlxture flred in statlc alr atmosphere for 2 h at 500 "C, at 800 °C (peaks are from the starting materials), at 900 °C (BaC03, Y O and 123) and at 97s
oc
11 37.
overlaps with it. However, after 2 hours at 850 "C some 123 phase is present. Using the intensity calcUlations, the amounts of 123 phase obtained at 900, 925 and 950 °C were somewhat lower (10-15 %) than was obtained with BaO 2. The difference was smaller than general'ly expected. In prolonged firing at 950 "C and short firing at 975 °C mixtures containing BaCO 3 behaved similar to BaO 2 containing mixture in that no phases other than 123 were seen by XRD. However, samples made from BaCO 3 and heated at 1000 °C contained detectable amount of 211 phase. Decomposition of the 123 phase does not account for the appearence of the 211 phase, since no other decomposition products were detected and because the product made from BaO 2 did not decompose at these conditions. The
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211 phase is a very stable compound and is obviously formed directly from Y203 , BaO and CuO at 1000 °C. The reaction mechanism to form the 123 phase changed when the Ba source was changed from BaO 2 to BaCO 3. For BaCO 3, the reaction rate controlling factor was the decomposition of barium carbonate. After its decomposition the three oxide consituents immediately reacted to form the 123 phase. The reaction of oxides seems to be fast. The XRD data from the 123 phases made from BaCO 3 and BaO 2 differs slightly. For BaCO3, tetragonal phase was formed at 900-950 °C when fired 2 hours. The orthorhombic phase was obtained only after prolonged firing at 950 °C, 975 °C or 1000 °C. Although the phase was orthorhombic, the (103) and (013) reflections overlaped strongly and the prescence of a shoulder in the peak was hard to detect. The difference in the d-spacings was less than 0.02 A. The presence of the orthorhomic phase above 950 °C was easily confirmed by the separation of (020) and (200) reflections. The slight difference in the d values and separation of reflections indicated that there was a difference in lattice contants in the samples made via BaCO 3 and BaO 2 route. Further, the differences in lattice constants probably reflects differences in oxygen content (28). This work confirms the earlier observation that the oxygen content of the orthorhombic 123 phase is very sensitive to reaction conditions. It should be kept in mind that our samples were made by a very short firing in a static air atmosphere. Further annealings in flowing oxygen may remove the differences.
Experiments in inert atmosphere Y203-BaO2-CuO. The XRD data from Y203-BaO2-CuO starting mixtures contained small peaks of BaCO3, indicating that the reaction between BaO 2 and ambient CO 2 had already occured at room temperature. Barium peroxide started to melt at 400 °C and at 600 °C no BaO 2 peaks could be seen in XRD. It is interesting to note that peaks of BaCO 3 increased slightly with increasing temperature (Fig. 3). This may indicate the prescence of CO 2 in the purged helium and/or crystallization of BaCO 3 formed during the exposure of BaO 2 to air. In the inert atmosphere CuO reduced to metallic copper. The peaks of Cu (28 = 42.9 and 49.8 ° ) appeared at 600 °C and they became stronger during heating to the maximum of 950 °C. The prescence of copper in the product can also be confirmed from its color. The CuO peaks disappeared from the XRD data at 700 °C. The peaks of Y203 were strong up to 700 °C, after which they weakened and disappeared around 900 °C. During heating in inert atmosphere, a strong peak appeared above 600 °C at 33.1 ° , which may originate from Y2Cu205 (29). BaCuO 2 was formed at around 700 °C. At 900 °C, peaks of Y203 disappeared and the formation of a third binary oxide, viz. Ba2Y205 was detected. At 950 °C and in room temperature quenched samples, barium yttrium oxide Ba2Y205, gave the strongest peaks. This is because a large fraction of CuO had been reduced to the metallic phase. Thus, firing of Y203-BaO2-CuO in inert atmosphere produced Cu and binary oxides. In contrast to the results in ref. 15, where
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M. LESKELA, et al.
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9
800
°c
800°C T, c I:
3
300
20
30
40
FIG.
50
2e
3.
XRD patterns for Y203-BaO2-CuO mixture recorded with a hotstage sample holder in flowing He atmosphere at 300 °C (peaks are from the starting materials), at 800 °C (BaCOn, Y203, Cu, BaCuO2, Y2Cu205 ) and at 900 °C (Cu, Ba2Y205, BaCuO2, Y2Cu205).
°C
20
30
40
50 2e
FIG.
4
XRD patterns for Y203-BaCO3CuO mixture recorded with a hotstage sample holder in flowing He atmosphere at 300 °C (peaks are from the starting materials), at 800 °C (BaCO3, Y203, Cu, Pt sample holder) and at 900 °C (Y203, Cu, BaCuO2, BaO, Pt).
superconducting 123 phase has been obtained in inert atmosphere, no 123 phase was obtained. Discrepancies between the two sets of data may stem from the different oxygen partial pressures in the inert gases. After heating the products at 950 "C in air for several hours, the 123 phase was obtained. Y203-BaCO3-CuO. Copper oxide behaved in this system in a similar fashion, viz. started to reduce to metallic copper around 600 "C. Peaks from Y203 stayed strong through the whole heating procedure, i.e. stepwlse heating up to 950 °C and rapid cooling to room temperature (Fig. 4). BaCO 3 started to decompose at 800 °C and its peaks fully disappeared at 900 o C. Some amount of BaO was present at 900 and 950 "C as well as in the cooled sample, but most
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Y - B a - C u - O PREPARATION
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of it reacted with copper oxide forming BaCuO 2 at 900 °C. It seems that prolonged firing times are needed for BaO to fully react with Y203 and CuO. Note that in Fig. 4 the strong peaks at 40.1 and 46.5 ° are from the platinum sample holder. In inert atmosphere after the stepwise heating procedure, Cu, Y2Cu205, BaCuO 2, BaO and Y203 were formed. No 123 or 211 phases could be detected. Firing of the product mixture in air at 950 °C for 12 hours produced orthorhombic 123 phase and small amounts of 211 phase and BaCuO 2. Conclusions BaCuO 2 is the first ternary crystalline phase formed in the mixture of Y203, BaO 2 and CuO while heated in static air atmosphere. The formation reaction of superconducting YBa2Cu307_ x occurs above 800 °C between Y~O 3 and BaCuO 2. Pure 123 phase can be obtained only above 950 °C, if the heating time is ~ 2 h. Thus, when preparing targets from Y203,BaO2, CuO mixtures, two phases (Y~O3, BaCuO 2) exist below 800 °C, three phases (Y203, BaCuO2, 123) exlst above 800 °C, and one phase (123) exists above 950 °C. A different reaction mechanism was found when BaCO 3 powders were used. The formation of 123 phase takes place at a slightly higher temperature than in the BaO 2 case and the reaction rate is controlled by the decomposition of BaCO 3. Thus, sputter targets prepared below 950 °C always contain unreacted starting materials. In flowing helium atmosphere, both Ba starting materials led to products which contained metallic Cu and binary oxides. No ternary oxides were detected, in contrast to earlier results. The firing of Y-Ba-Cu oxide targets in inert atmosphere leads to an inhomogeneous products which contains conducting copper and insulating binary oxides.
Acknowledgements The financial aid from Emil Aaltonen Foundation and Jenny and Antti Wihuri Foundation to one of the authors (M.L.) is gratefully acknowledged.
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14. A.F. Hepp, J.R. Gaier, W.H. Phillip, J.D. Warner, R.G. Garlick, J.J. Pouch and P.D. Hambourger, Materials Reasearch Society Symposium Proceedings, Vol. 99, M.B. Brodsky, R.C. Dynes, K. Kitazawa and H.L. Tuller (eds.), Pittsburg 1988, p. 615. 15. A.F. Hepp and J.R. Gaier, Mater.
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18. P.K. Gallagher, H.M. O'Bryan, S.A. Sunshine and D.W. Murphy, Mater. Res. Bull. 22 995 (1987). 19. S-I. Lee, J.P. Golben, Y. Song, X-D. Chen, R.D. McMichael and J.R. Gaines, Chemistry of High-Temperature Superconductors. ACS Symposium Series 351, D.L. Nelson, M.S. Whittingham and T.F. George (eds.), American Chemical Society, Washington 1987, p. 272. 20. A.M. Kini, U. Geiser, H-C.I. Jao, K.D. Carlson, H.H. Wang, M.R. Monaghan and J.M. Williams, Inorg. Chem. 26 1834 (1987).
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21. H.H. Wang, K.D. Carlson, U. Geiser, R.J. Thorn, H-C.I. Kao, M. A. Beno, M.R. Monaghan, T.J. Allen, R.B. Proksch, D.L. Stupka, J.M. Williams, B.K. Flandermeyer and R.B. Poeppel, Inorg. Chem. 26 1474 (1987). 22. C. Chu and A. Dunn, J. Am. Ceram.
Soc. 7_~0 C-375
(1987).
23. J.M. Tarascon, P. Barboux, B.G. Bagley, L.H. Greene, W.R. McKinnon and G.W. Hull, Chemistry of High-Temperature Superconductors. ACS Symposium Series 351, D.L. Nelson, M.S. Whittingham and T.F. George (eds.), American Chemical Society, Washington 1987, p. 198. 24. M.J. Cima, R. Chiu and W.R. Rhine, Materials Research Society Fall Meeting, Final Program and Abstracts. Boston Ma, 1987, p. 23. 25. M. Leskel~, J.K. Truman, C.H. Muller, E. Lambers and P.H. Holloway, American Vacuum Society, 34th National Meeting, Anaheim 1987, Poster. M. Leskel~, J.K. Truman, C.H. Muller and P.H. Holloway, to be published. 26. S.M. Rossnagel and J.J. Cuomo, Thin Film Processing and Characterization of High-Temperature Superconductors. American Vacuum Society Series 3. J.M.E. Harper, R.J. Colton and L.C. Feldman (eds.), American Institute of Physics, New York 1988, p. 106. 27. T.I. Selinder, G. Larsson, U. Helmersson, Sundgren and S. Rudner, Appl. Phys. Lett. 28. T.J. Kistenmacher,
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(1987).
29. R. Jones, M.F. Ashley, A.M. Campbell, P.P. Edwards, M.R. Harrison, A.D. Hibbs, D.A. Jefferson, A.I. Kirkland, T. Thanyasiri and E. Sinn, Chemistry of High-Temperature Superconductors. ACS Symposium Series 351, D.L. Nelson, M.S. Whittingham and T.F. George (eds.), American Chemical Society, Washington 1987, p. A-I.
PREPARATION
OF Y-Ba-Cu-O HIGH TRANSITION TEMPERATURE CONDUCTORS USING BaO 2 AND BaCO 3
M. Leskeli a, C.H. Mueller,
SUPER-
J.K. Truman and P.H. Holloway
Department
of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 aon sabbatical leave from Department of Chemistry, University of Turku, SF-20500 Turku, Finland ( R e c e i v e d J u l y 13, 1988; Communicated b y A. Wold)
ABSTRACT The preparation of superconducting YBa2Cu307_ x in static air and flowing He atmosphere from BaCO 3 and BaO 2 mixed with Y203 and CuO has been studied. In static air BaO 2 reacted at slightly lower temperature than BaCO 3 and different mechanisms were observed for the formation reactions. Using BaO2, the orthorhombic phase is readily formed, while high temperatures or prolonged heating times are needed to obtain the orthorhombic phase with BaCO 3. In flowing helium atmosphere, both starting materials led to products which contained metallic Cu and binary oxide compounds, with no ternary oxides being detected. MATERIALS
INDEX:
yttrium,
barium,
copper,
oxides,
peroxides
Introduction Originally the rare earth-alkaline earth-copper oxides have been prepared by a solid state synthesis of individual oxides or some relative easily decomposing compound like carbonate (i). Bednorz and Muller, however, used the oxalate precipitation method in their experiment to get better mixing of the ions before calcining to oxide (2). Today, superconducting orthorhombic YBa2Cu307_ x (123) powder is most often made using Y203, BaCO 3 and CuO powders as starting materials (3,4). The weighed powders are thoroughly mixed in a mill and the mixture is calcined in an open crucible. The powders are not pelletized prior to calcining due to the large molar volume change between starting materials and products. Different crucible materials have been employed and according to Engler (5) alumina, 1469
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Vol. 23, No. I0
Pt and Au best withstand the conditions required for preparation. For Ba, the BaCO 3 starts to decompose and react with Y and Cu oxides at 900-950 °C, and long firing times are needed to complete the reaction. The times used are 12-24 hours and 950 °C is a critical temperature because just above it the 123 phase starts to decompose to BaCuO2, CuO and Y2BaCuO 5 (211) (6). However, carbonate does not always completely decompose during calcining. Furthermore, at the reaction temperatures, particles are sintering and grain growth occurs. Thus remilling and recalcining are needed, with conditions similar to the first calcining. The final step in the processing is the slow cooling in flowing 02 to increase the oxygen content of the compound which is critical for the formation of the oxygen-deficient orthorhombic phase and superconducting properties (7-9). This step can be carried out in connection with the second calcining. Because of the high decomposition temperature of BaCO3, alternative Ba source materials have been studied for the solid state synthesis. BaO, Ba(OH) 2 and BaO 2 have been used, and the products obtained have been purer and denser than superconductors synthesized from BaCO 3 (10-15). Higher critical temperature and current are also claimed for the products prepared from BaO 2 than for those made from BaCO 3 (12). However, the details of preparation using BaO 2 were incomplete. An alternative preparation method for YBa2Cu307_ x is to use solutions and coprecipitation prior to calcination. This is a good way to improve the homogeneous mixing of ions on the atomic scale. Coprecipitation of Y, Ba and Cu by oxalate (16-18) or carbonate (19,20) from nitrate or acetate solution has been accomplished. The calcination temperature with oxalates is lower than used in the solid state reaction described above, but the cabonate precipitation needs the same annealing procedure used in the solid state reaction. Complexation of Y, Ba and Cu with citrate is another possible solution-based route for the synthesis of the 123 phase (21,22). A third method employed in the preparation of 123 is sol gel processes. Metal hydoxides and alkoxides have been used as starting materials (23,24). The advantages of sol gel processing are the low temperature required and the possibility of shapeing the product. The main task in our superconductivity study is the preparation and characterization of sputter deposited thin films (25). In preparation of targets for sputtering the need for Ba sources other than BaCO 3 was found. The aim of this study was to detail the reactivity of BaO 2 and BaCO 3 in preparation of sputter deposition targets. In sputter deposition of thin films it is important to know the target composition and the phases present because each phase may have its own sputter yield for each component. Sputter deposition of Ba or superconducting oxides is difficult due to a negative ion effect, which causes a deviation between the target and film compositions under typical deposition conditions (26). In addition Ba migration in target during sputtering depends on phases present. One way to decrease these effects may be the firing of the target in inert atmosphere (27). Experimental The starting materials Y203, BaO2/BaCO 3 and CuO, purchased
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from Alfa Chemical Company, were reported to be at least 99.9 % pure. A slurry consisting of powders mixed in a molar ratio of ½:2:3 and acetone was shaken for 15 minutes to assure homogeneous mixing of the powders. After drying, the powders were fired in a muffle furnace in static air atmosphere at times of 2 hours and temperatures which varied between 500 and 1000 °C. After firing the samples were cooled at the natural cooling rate of the furnace. The time required for the furnace to cool from 900 °C to 500 °C was approximately one hour. The products were characterized by X-ray diffraction (XRD) employing a Philips powder diffractometer and using Ni-filtered CuK~ radiation. The decomposition of the starting materials were also monitored with a diffractometer equipped with a hot-stage sample holder. The heating program contained ramps of i00 °C, i.e. a fast heating to the set value and measurement was carried out after a stabilization time of 15 min. The hot-stage X-ray diffraction measurements were carried in flowing inert atmosphere (He). Results and Discussion Experiments in static air Y203-BaO2-CuO. In static air atmosphere liquid BaO 2 formed above 400 °C and reacted first with copper oxide. At 600-950 °C, XRD data contained peaks from BaCuO2, CuO and Y203 (Fig. I). The main peak of the quaternary 123 compound (28 = 32.5 ° , (103),(013) and (110) reflections) appeared between 700 and 800 °C, but the amount of 123 phase at 800 °C was not high, contrary to earlier reports (6). At 900 °C the main XRD peak originated from the 123 phase. Calculating in a very crude fashion based on the intensities of the peaks, the amount of 123 phase in products fired 2 hours at 900 °C can be eastimated to be about 50 %. By increasing the temperature to 925 and 950 °C, the amount of 123 phase increased to 75 and 85 %, respectively. If the firing time was prolonged to 20 h at 950 °C, impurity peaks (mainly from BaCuO 2) were hardly detectable. Pure product was also obtained after firing for 2 h at 975 or 1000 °C. When BaO 2 was used as a starting material, the reaction forming the 123 phase in the final stage was roughly ½ Y203 + 2BaCuO 2 + CuO --> YBa2Cu307_ x. This reaction limited the rate of formation because the barium-copper oxide seems to form very fast. The reaction explains why the 211 phase is not formed with BaO 2 as readily as with BaCO 3. The XRD data from samples fired at 900-1000 °C clearly shows the formation of the orthorhombic 123 phase. The difference in the d-spacings of the (103) and (013) planes increased with increased firing temperature: from 0.014 A at 900 °C to 0.035 A at 1000 °C. The same tendency can be seen in the separation of d-values of the (020) and (200) reflections at 28 = 46.5 - 47.5 ° (Fig. i). Y203-BaCO3-CuO. The reflections from the starting materials, particulary BaCO3, dominate the XRD data up to 800 °C (Fig. 2). The appearance of the main peak of the 123 phase in this system is difficult to detect because the reflection from (102) BaCO 3
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i
I
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975 °C
900o c
9 0 0 °C
,oo°c II
I
/
8 0 0 °C
*
I
5 0 0 °C
500o C 20
30
40
50
60
2e
FIG. I. XRD patterns for Y203-BaO~-CuO mixture fired in static alr atmosphere for 2 h at 500 °C (peaks are from the starting materials), at 800 °C (Y203 and BaCuO2), at 900 °C (123 and BaCuO 2) and at 975 °C (123).
20
30
40
50
60
20
FIG. 2 XRD patterns, for. Y ~O3-BaCO. 3 CuO mlxture flred in statlc alr atmosphere for 2 h at 500 "C, at 800 °C (peaks are from the starting materials), at 900 °C (BaC03, Y O and 123) and at 97s
oc
11 37.
overlaps with it. However, after 2 hours at 850 "C some 123 phase is present. Using the intensity calcUlations, the amounts of 123 phase obtained at 900, 925 and 950 °C were somewhat lower (10-15 %) than was obtained with BaO 2. The difference was smaller than general'ly expected. In prolonged firing at 950 "C and short firing at 975 °C mixtures containing BaCO 3 behaved similar to BaO 2 containing mixture in that no phases other than 123 were seen by XRD. However, samples made from BaCO 3 and heated at 1000 °C contained detectable amount of 211 phase. Decomposition of the 123 phase does not account for the appearence of the 211 phase, since no other decomposition products were detected and because the product made from BaO 2 did not decompose at these conditions. The
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211 phase is a very stable compound and is obviously formed directly from Y203 , BaO and CuO at 1000 °C. The reaction mechanism to form the 123 phase changed when the Ba source was changed from BaO 2 to BaCO 3. For BaCO 3, the reaction rate controlling factor was the decomposition of barium carbonate. After its decomposition the three oxide consituents immediately reacted to form the 123 phase. The reaction of oxides seems to be fast. The XRD data from the 123 phases made from BaCO 3 and BaO 2 differs slightly. For BaCO3, tetragonal phase was formed at 900-950 °C when fired 2 hours. The orthorhombic phase was obtained only after prolonged firing at 950 °C, 975 °C or 1000 °C. Although the phase was orthorhombic, the (103) and (013) reflections overlaped strongly and the prescence of a shoulder in the peak was hard to detect. The difference in the d-spacings was less than 0.02 A. The presence of the orthorhomic phase above 950 °C was easily confirmed by the separation of (020) and (200) reflections. The slight difference in the d values and separation of reflections indicated that there was a difference in lattice contants in the samples made via BaCO 3 and BaO 2 route. Further, the differences in lattice constants probably reflects differences in oxygen content (28). This work confirms the earlier observation that the oxygen content of the orthorhombic 123 phase is very sensitive to reaction conditions. It should be kept in mind that our samples were made by a very short firing in a static air atmosphere. Further annealings in flowing oxygen may remove the differences.
Experiments in inert atmosphere Y203-BaO2-CuO. The XRD data from Y203-BaO2-CuO starting mixtures contained small peaks of BaCO3, indicating that the reaction between BaO 2 and ambient CO 2 had already occured at room temperature. Barium peroxide started to melt at 400 °C and at 600 °C no BaO 2 peaks could be seen in XRD. It is interesting to note that peaks of BaCO 3 increased slightly with increasing temperature (Fig. 3). This may indicate the prescence of CO 2 in the purged helium and/or crystallization of BaCO 3 formed during the exposure of BaO 2 to air. In the inert atmosphere CuO reduced to metallic copper. The peaks of Cu (28 = 42.9 and 49.8 ° ) appeared at 600 °C and they became stronger during heating to the maximum of 950 °C. The prescence of copper in the product can also be confirmed from its color. The CuO peaks disappeared from the XRD data at 700 °C. The peaks of Y203 were strong up to 700 °C, after which they weakened and disappeared around 900 °C. During heating in inert atmosphere, a strong peak appeared above 600 °C at 33.1 ° , which may originate from Y2Cu205 (29). BaCuO 2 was formed at around 700 °C. At 900 °C, peaks of Y203 disappeared and the formation of a third binary oxide, viz. Ba2Y205 was detected. At 950 °C and in room temperature quenched samples, barium yttrium oxide Ba2Y205, gave the strongest peaks. This is because a large fraction of CuO had been reduced to the metallic phase. Thus, firing of Y203-BaO2-CuO in inert atmosphere produced Cu and binary oxides. In contrast to the results in ref. 15, where
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800
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800°C T, c I:
3
300
20
30
40
FIG.
50
2e
3.
XRD patterns for Y203-BaO2-CuO mixture recorded with a hotstage sample holder in flowing He atmosphere at 300 °C (peaks are from the starting materials), at 800 °C (BaCOn, Y203, Cu, BaCuO2, Y2Cu205 ) and at 900 °C (Cu, Ba2Y205, BaCuO2, Y2Cu205).
°C
20
30
40
50 2e
FIG.
4
XRD patterns for Y203-BaCO3CuO mixture recorded with a hotstage sample holder in flowing He atmosphere at 300 °C (peaks are from the starting materials), at 800 °C (BaCO3, Y203, Cu, Pt sample holder) and at 900 °C (Y203, Cu, BaCuO2, BaO, Pt).
superconducting 123 phase has been obtained in inert atmosphere, no 123 phase was obtained. Discrepancies between the two sets of data may stem from the different oxygen partial pressures in the inert gases. After heating the products at 950 "C in air for several hours, the 123 phase was obtained. Y203-BaCO3-CuO. Copper oxide behaved in this system in a similar fashion, viz. started to reduce to metallic copper around 600 "C. Peaks from Y203 stayed strong through the whole heating procedure, i.e. stepwlse heating up to 950 °C and rapid cooling to room temperature (Fig. 4). BaCO 3 started to decompose at 800 °C and its peaks fully disappeared at 900 o C. Some amount of BaO was present at 900 and 950 "C as well as in the cooled sample, but most
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of it reacted with copper oxide forming BaCuO 2 at 900 °C. It seems that prolonged firing times are needed for BaO to fully react with Y203 and CuO. Note that in Fig. 4 the strong peaks at 40.1 and 46.5 ° are from the platinum sample holder. In inert atmosphere after the stepwise heating procedure, Cu, Y2Cu205, BaCuO 2, BaO and Y203 were formed. No 123 or 211 phases could be detected. Firing of the product mixture in air at 950 °C for 12 hours produced orthorhombic 123 phase and small amounts of 211 phase and BaCuO 2. Conclusions BaCuO 2 is the first ternary crystalline phase formed in the mixture of Y203, BaO 2 and CuO while heated in static air atmosphere. The formation reaction of superconducting YBa2Cu307_ x occurs above 800 °C between Y~O 3 and BaCuO 2. Pure 123 phase can be obtained only above 950 °C, if the heating time is ~ 2 h. Thus, when preparing targets from Y203,BaO2, CuO mixtures, two phases (Y~O3, BaCuO 2) exist below 800 °C, three phases (Y203, BaCuO2, 123) exlst above 800 °C, and one phase (123) exists above 950 °C. A different reaction mechanism was found when BaCO 3 powders were used. The formation of 123 phase takes place at a slightly higher temperature than in the BaO 2 case and the reaction rate is controlled by the decomposition of BaCO 3. Thus, sputter targets prepared below 950 °C always contain unreacted starting materials. In flowing helium atmosphere, both Ba starting materials led to products which contained metallic Cu and binary oxides. No ternary oxides were detected, in contrast to earlier results. The firing of Y-Ba-Cu oxide targets in inert atmosphere leads to an inhomogeneous products which contains conducting copper and insulating binary oxides.
Acknowledgements The financial aid from Emil Aaltonen Foundation and Jenny and Antti Wihuri Foundation to one of the authors (M.L.) is gratefully acknowledged.
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