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Critical current as a function of film thickness for spray pyrolyzed films of TlBa2Ca2Cu3Oy
PHYSlCA ELSEVIER
Physica C 247 (1995) 239-242
Critical current as a function of film thickness for spray pyrolyzed films of T1Ba2Ca2Cu3Oy A. Mogro-Campero *, P.J. Bednarczyk, J.E. Tkaczyk, J.A. DeLuca GE Research and Development Center, Schenectady, NY1230I, USA Received 24 January 1995; revised manuscript received 28 March 1995
Abstract
It is imperative for tape applications to produce thicker films of T1Ba2Ca2Cu3Oy with high critical current density on polycrystalline substrates (a useful figure of merit is the critical current per unit width, with a reasonable goal being 10 A mm-1). We have studied the critical current density at 77 K, 0 T (J~) as a function of film thickness in the range 2 to 10 txm. A key parameter for obtaining a high J~ was found to be the film density; films with relative density > 82% showed much higher values of J¢. Based on average values, J¢ was found to peak at 60 kAcm -2 for samples 2.5 to 5 Ixm thick (corresponding to 3 A mm-1 for 5 txm thick samples), and to decrease for thicker samples, yielding 3 to 3.5 A mm-1 from 5 to 10 ixm. There are individual segments with values as high as 8.4 A m m -1.
I. Introduction
Thick films of superconducting TIBa2Ca2Cu3Oy (henceforth referred to as T1-1223) are promising candidates for practical tapes for applications. The irreversibility line for this material compared to other phases and to other high-temperature superconductors has been reported [1]. A figure of merit for tape applications is the critical current per film width, defined as K c (which is also the product of the critical current density, J~, and the film thickness). If Jc is constant as a function of film thickness, K c will clearly increase for thicker films. In this study, thick films of T1-1223 have been made in the thickness range of 2 to 10 Ixm on polycrystalline yttria-stabilized zirconia (YSZ) sub-
* Corresponding author.
strates. Polycrystalline substrates are clearly required for large-scale applications, although metallic substrates are probably preferable ultimately. There is a large data base in our laboratory from previous work using polycrystalline YSZ as a substrate [2], so that processing methods can be quickly adapted to this study on the thickness dependence of Jc. Jc in this paper is reported at 77 K and zero applied field (77 K, ZF). These conditions are useful for comparison with other reported data and are of interest for power transmission. Magnet applications are usually conceived at lower temperature and higher field. For the thick T1-1223 made in this study, a useful approximation is that Jc(77 K, Z F ) = 2 × J¢(40 K, 2 T) [3]. From a large number of samples reported previously with a film thickness of 3 ~ m [2], the best value of Jc converts to K¢ = 3 A mm -1. We have established an interim goal of 10 A mm -1.
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 9 2 1 - 4 5 3 4 ( 9 5 ) 0 0 2 1 4 - 6
240
A. Mogro-Campero et a L / Physica C 247 (1995) 239-242 120
2. Experimental conditions Films were prepared by spray deposition of metal nitrates, as described in detail elsewhere [2]. Only a brief description, including processing steps relevant to the present study, is presented here. Ag is included at 5 mole% (as defined previously [2]) and the sprayed film is decomposed in air at 650°C to yield the Ca2Ba2Cu307 :Ag precursor. A standard film thickness of 2.5 p,m results from several spray/ decomposition cycles. The number of cycles per unit film thickness was found to affect the final film density; conditions for a high film density were used in the film thickness study (requiring 3 cycles Ixm- 1). After deposition (including 650°C decompositions), a final decomposition step at 845°C in oxygen is carried out. Precursor films on polycrystalline YSZ substrates were thallinated in an oxygen flow-through two-zone furnace, with the sample at a temperature of 858°C. The temperature of the T1203 source (about 730°C) was adjusted to yield a T1 stoichiometry of = 0.8. Thallination times varied from 0.5 to 1 h, depending on the film thickness. The film density was calculated from film weight and volume. The density is given as relative density, i.e., % of T1-1223 density (6.3 gem -3) as taken from unit-cell measurements [4]. The critical current density was measured for each sample on four 1 mm segments of a 2 mm wide bridge with a voltage criterion of 10 ixVcm -1 (dictated by the noise level of the voltmeter used). Based on the observed shape of the I(V) curves, the Jc values we report here are a factor of 1.3 higher than if the more usual voltage criterion of 1 txV cm-1 were used.
....
I''"l''"t""l
....
I ....
I'"
E o 100 I-.-
~
80
z IJ.I a
0
~60 z
0
0
HI w
D O
40
.._1 < O
V- 20
|
$
O ......
I ....
I ....
I ...........
60 65 70 75 80 85 90 95 FILM RELATIVE DENSITY (%) Fig. 1. The critical current density at 77 K as a function of relative film density for 2.5 p,m thick films. Solid circles are for samples made with 2 spray/decomposition cycles p,m- 1, and open circles with 3 cycles p,m-1.
shown in Fig. 1 for 2.5 p,m thick films. Data for two groups of samples are shown. Each data point is the average value for the four segments in each sample. It is clear that the number of spray/decomposition cycles determines the density: low values of density and J~ result from 2 cycles ixm -1, whereas higher values of relative density ( > 82%) and a high J¢ result from 3 cycles I~m-1 (higher values of cycles p,m -1 were tried, but density and J¢ did not continue to increase). The high number of cycles Ixm - 1 needed to obtain high J~ values adds to the complexity of this process, and is of concern for scale-up.
3. Results and discussion
3.2. Critical currents
3.1. Film density
The critical current density as a function of film thickness is shown in Fig. 2. Data points are average values for about six samples; error bars are one standard deviation. The amount of material sprayed for the thinnest films was half of that for 2.5 p~m thick films. The film thickness shown for the thinnest films (2 ~m) was obtained by profilometry. However, microscopic examination with transmitted light reveals a large scale porosity, which is reflected in
Using the procedures outlined above, a wide range of values for J¢ ( = 104 to 105 A c m -2) have been obtained for T1-1223 films of 3 Ixm thickness [2]. Although higher density films seem desirable, a quantitative relationship between density and Jc has been lacking for spray-deposited films. It was found in this study that Jc is a function of film density, as
A. Mogro-Campero et al. / Physica C 247 (1995) 239-242
the low density for these films shown in Fig. 2. For the spray process used, a continuous layer is formed at film thicknesses exceeding 2.5 Ixm, so that thinner films suffer from connectivity and are near the percolation limit. The film relative density for films > 2.5 p~m thick is higher than 80% (Fig. 2); yet Jc decreases for films > 5 p~m thick. In a study of epitaxial YBa2Cu30 7 films, J~ was also found to decrease after a critical thickness (in that case the critical thickness was an order of magnitude smaller, 0.4 pLm) [5]. However, in the epitaxial case the disruption of epitaxial growth as a function of film thickness is governed by the lattice mismatch, which does not carry over to this case of polycrystalline substrates, so a different mechanism is bound to be responsible for the thick T1-1223 films. An elucidation of this mechanism will require a detailed microstructural investigation. For example, X-ray diffraction and transmission electron microscopy were used to find local texture and a colony microstructure related to the high Jc in previous thinner spray-pyrolyzed T1-1223 films [6]. Film composition and microstructure in cross-section as a function of
100
"l'"l'"l"'l'"l"
100
80
60 ~ I-Z UJ
tt rr
80 7
~ ~
60 Z D ILl >
40
40 -J LU tr
-J
o ~_
20
20 _-J u_
,~l,,~l,,,l,,,l,~I,, 2
4
6
8
10
0 12
FILM T H I C K N E S S (Ixm) Fig. 2. The critical current density at 77 K (solid circles) and film relative density (open circles) as a function of film thickness. Average values are shown, with error bars of one standard deviation.
1 o00 ~
241
] I rllllT1
I
i [ i iii
I I I JIlil
J
I i i IL
E o I-Z UJ a
~100 Z
UJ rr n0 ..J < I--
~
10 I
10 FILM T H I C K N E S S
100 (p.m)
Fig. 3. The critical current density at 77 K as a function o f film thickness. Solid points are average values, as in Fig. 2; o p e n circles are m a x i m u m values for e a c h film thickness group.
distance from the substrate would also seem to be desirable. As explained in the introduction, a better figure of merit than Jc for tape applications is the critical current per film width. Fig. 3 shows the results in Fig. 2 with respect to a line of constant critical current per film width (Kc). It is apparent that increasing the film thickness from 5 to 10 I~m does not bring us closer to the 10 A mm-1 line. In fact, from 5 to 10 Ixm, values of K c are in the range 3 to 3.5 A mm-1. However, if one focuses on individual Jc measurements on segments (there are about 24 such segments which make up each average value solid data point shown in Figs. 2 and 3), the maximum J¢ values for each thickness group are shown as open circles. The highest value of K~ (i.e., the closest open circle to the 10 A m m -1 line) is 8.4 A m m -~. We use 8.4 A mm-1 as the highest value of K~ to make comparisons with other work (where single values, and not average values are reported). The highest value from our previous work with 3 I~m thick films is 3 A mm -1 [2], also for the T1-1223 phase on polycrystalline YSZ substrates. A high J¢ of 8 × 10 4 Acm -2 was reported for Tl(Bao.sSro.2)2-
242
A. Mogro-Campero et al. / Physica C 247 (1995) 239-242
C a 2 C u 3 0 9 spray deposited onto Ag [7], but the 1
Ixm thick film yields a low K~ value of 0.8 A mm -1. In another recent report on different film thicknesses of (T1,Pb)(Ba,Sr)ECa2Cu309 on single crystal LaA103, K~ values of 1.5 A m m -1 for a 5 Ixm thick film, 0.81 A m m -1 for a 14 ~m thick film, and 0.55 A mm-1 for a 21 p~m thick film were found [8]. Even though the present study has achieved K~ values close to 10 A m m -1 for 10 ~m thick films on individual segments, it is clear from Figs. 2 and 3 that the drop of Jc for thicker films is undesirable, and work is continuing to find ways to increase the average values of J~ for thicker films.
4. Summary For superconducting tape applications, a high value of the critical current per film width ( K c) is desired. Considerable effort has been expended in improving the values of the critical current density of thick films on polycrystalline substrates. In this study, the film density was found to be related to Jc, and a processing step which controls the density was identified. Further improvement of K~ has been sought by increasing the film thickness. J~ as a function of film thickness was measured for films of T1-1223 in the thickness range of 2 to 10 p,m. J~ was found to peak in the 2.5 to 5 I~m thickness range, and decrease for thicker films, such that K~ remains approximately constant (3 to 3.5 A mm -1) in the film thickness range from 5 to 10 I~m.
The above conclusions are based on average values for many samples of each film thickness. The highest value of Kc from single measurements was 8.4 A m m -1 for a segment of a 10 lxm thick film. This shows that high values of K~ are possible, and encourages continuing work.
Acknowledgements We thank L. Vanier for photolithography work and J. Bray for keeping us informed on the literature and for comments, encouragement and support.
References [1] D.H. Kim, K.E. Gray, R.T. Kampwirth, J.C. Smith, D.S. Richeson, T.J. Marks, J.H. Kang, J. Talvacchio and M. Eddy, Physica C 177 (1991) 431. [2] J.A. DeLuca, P.L. Karas, J.E. Tkaczyk, P.J. Bednarczyk, M.F. Garbauskas, C.L. Briant and D.B. Sorensen, Physica C 205 (1993) 21. [3] J.E. Tkaczyk, J.A. DeLuca, P.L. Karas, P.J. Bednarczyk, M.F. Garbauskas, R.H. Arendt, K.W. Lay and J.S. Moodera, Appl. Phys. Lett. 61 (1992) 610. [4] D.P. Matheis and R.L. Snyder, Powder Diffraction 5 (1990) 8. [5] A. Mogro-Campero and L.G. Turner, J. Supercond. 6 (1993) 37. [6] D.M. Kroeger, A. Goyal, E.D. Specht, Z.L. Wang, J.E. Tkaczyk, J.A. Sutliff and J.A. DeLuca, Appl. Phys. Lett. 64 (1994) 106. [7] T.J. Doi, T. Yuasa, T. Ozawa and K. Higashiyama, Jpn. J. Appl. Phys. 33 (1994) 5692. [8] D.L. Schulz, P.A. Parilla, D.S. Ginley, J.A. Voigt and F.P. Roth, Appl. Phys. Lett. 65 (1994) 2472.
Physica C 247 (1995) 239-242
Critical current as a function of film thickness for spray pyrolyzed films of T1Ba2Ca2Cu3Oy A. Mogro-Campero *, P.J. Bednarczyk, J.E. Tkaczyk, J.A. DeLuca GE Research and Development Center, Schenectady, NY1230I, USA Received 24 January 1995; revised manuscript received 28 March 1995
Abstract
It is imperative for tape applications to produce thicker films of T1Ba2Ca2Cu3Oy with high critical current density on polycrystalline substrates (a useful figure of merit is the critical current per unit width, with a reasonable goal being 10 A mm-1). We have studied the critical current density at 77 K, 0 T (J~) as a function of film thickness in the range 2 to 10 txm. A key parameter for obtaining a high J~ was found to be the film density; films with relative density > 82% showed much higher values of J¢. Based on average values, J¢ was found to peak at 60 kAcm -2 for samples 2.5 to 5 Ixm thick (corresponding to 3 A mm-1 for 5 txm thick samples), and to decrease for thicker samples, yielding 3 to 3.5 A mm-1 from 5 to 10 ixm. There are individual segments with values as high as 8.4 A m m -1.
I. Introduction
Thick films of superconducting TIBa2Ca2Cu3Oy (henceforth referred to as T1-1223) are promising candidates for practical tapes for applications. The irreversibility line for this material compared to other phases and to other high-temperature superconductors has been reported [1]. A figure of merit for tape applications is the critical current per film width, defined as K c (which is also the product of the critical current density, J~, and the film thickness). If Jc is constant as a function of film thickness, K c will clearly increase for thicker films. In this study, thick films of T1-1223 have been made in the thickness range of 2 to 10 Ixm on polycrystalline yttria-stabilized zirconia (YSZ) sub-
* Corresponding author.
strates. Polycrystalline substrates are clearly required for large-scale applications, although metallic substrates are probably preferable ultimately. There is a large data base in our laboratory from previous work using polycrystalline YSZ as a substrate [2], so that processing methods can be quickly adapted to this study on the thickness dependence of Jc. Jc in this paper is reported at 77 K and zero applied field (77 K, ZF). These conditions are useful for comparison with other reported data and are of interest for power transmission. Magnet applications are usually conceived at lower temperature and higher field. For the thick T1-1223 made in this study, a useful approximation is that Jc(77 K, Z F ) = 2 × J¢(40 K, 2 T) [3]. From a large number of samples reported previously with a film thickness of 3 ~ m [2], the best value of Jc converts to K¢ = 3 A mm -1. We have established an interim goal of 10 A mm -1.
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 9 2 1 - 4 5 3 4 ( 9 5 ) 0 0 2 1 4 - 6
240
A. Mogro-Campero et a L / Physica C 247 (1995) 239-242 120
2. Experimental conditions Films were prepared by spray deposition of metal nitrates, as described in detail elsewhere [2]. Only a brief description, including processing steps relevant to the present study, is presented here. Ag is included at 5 mole% (as defined previously [2]) and the sprayed film is decomposed in air at 650°C to yield the Ca2Ba2Cu307 :Ag precursor. A standard film thickness of 2.5 p,m results from several spray/ decomposition cycles. The number of cycles per unit film thickness was found to affect the final film density; conditions for a high film density were used in the film thickness study (requiring 3 cycles Ixm- 1). After deposition (including 650°C decompositions), a final decomposition step at 845°C in oxygen is carried out. Precursor films on polycrystalline YSZ substrates were thallinated in an oxygen flow-through two-zone furnace, with the sample at a temperature of 858°C. The temperature of the T1203 source (about 730°C) was adjusted to yield a T1 stoichiometry of = 0.8. Thallination times varied from 0.5 to 1 h, depending on the film thickness. The film density was calculated from film weight and volume. The density is given as relative density, i.e., % of T1-1223 density (6.3 gem -3) as taken from unit-cell measurements [4]. The critical current density was measured for each sample on four 1 mm segments of a 2 mm wide bridge with a voltage criterion of 10 ixVcm -1 (dictated by the noise level of the voltmeter used). Based on the observed shape of the I(V) curves, the Jc values we report here are a factor of 1.3 higher than if the more usual voltage criterion of 1 txV cm-1 were used.
....
I''"l''"t""l
....
I ....
I'"
E o 100 I-.-
~
80
z IJ.I a
0
~60 z
0
0
HI w
D O
40
.._1 < O
V- 20
|
$
O ......
I ....
I ....
I ...........
60 65 70 75 80 85 90 95 FILM RELATIVE DENSITY (%) Fig. 1. The critical current density at 77 K as a function of relative film density for 2.5 p,m thick films. Solid circles are for samples made with 2 spray/decomposition cycles p,m- 1, and open circles with 3 cycles p,m-1.
shown in Fig. 1 for 2.5 p,m thick films. Data for two groups of samples are shown. Each data point is the average value for the four segments in each sample. It is clear that the number of spray/decomposition cycles determines the density: low values of density and J~ result from 2 cycles ixm -1, whereas higher values of relative density ( > 82%) and a high J¢ result from 3 cycles I~m-1 (higher values of cycles p,m -1 were tried, but density and J¢ did not continue to increase). The high number of cycles Ixm - 1 needed to obtain high J~ values adds to the complexity of this process, and is of concern for scale-up.
3. Results and discussion
3.2. Critical currents
3.1. Film density
The critical current density as a function of film thickness is shown in Fig. 2. Data points are average values for about six samples; error bars are one standard deviation. The amount of material sprayed for the thinnest films was half of that for 2.5 p~m thick films. The film thickness shown for the thinnest films (2 ~m) was obtained by profilometry. However, microscopic examination with transmitted light reveals a large scale porosity, which is reflected in
Using the procedures outlined above, a wide range of values for J¢ ( = 104 to 105 A c m -2) have been obtained for T1-1223 films of 3 Ixm thickness [2]. Although higher density films seem desirable, a quantitative relationship between density and Jc has been lacking for spray-deposited films. It was found in this study that Jc is a function of film density, as
A. Mogro-Campero et al. / Physica C 247 (1995) 239-242
the low density for these films shown in Fig. 2. For the spray process used, a continuous layer is formed at film thicknesses exceeding 2.5 Ixm, so that thinner films suffer from connectivity and are near the percolation limit. The film relative density for films > 2.5 p~m thick is higher than 80% (Fig. 2); yet Jc decreases for films > 5 p~m thick. In a study of epitaxial YBa2Cu30 7 films, J~ was also found to decrease after a critical thickness (in that case the critical thickness was an order of magnitude smaller, 0.4 pLm) [5]. However, in the epitaxial case the disruption of epitaxial growth as a function of film thickness is governed by the lattice mismatch, which does not carry over to this case of polycrystalline substrates, so a different mechanism is bound to be responsible for the thick T1-1223 films. An elucidation of this mechanism will require a detailed microstructural investigation. For example, X-ray diffraction and transmission electron microscopy were used to find local texture and a colony microstructure related to the high Jc in previous thinner spray-pyrolyzed T1-1223 films [6]. Film composition and microstructure in cross-section as a function of
100
"l'"l'"l"'l'"l"
100
80
60 ~ I-Z UJ
tt rr
80 7
~ ~
60 Z D ILl >
40
40 -J LU tr
-J
o ~_
20
20 _-J u_
,~l,,~l,,,l,,,l,~I,, 2
4
6
8
10
0 12
FILM T H I C K N E S S (Ixm) Fig. 2. The critical current density at 77 K (solid circles) and film relative density (open circles) as a function of film thickness. Average values are shown, with error bars of one standard deviation.
1 o00 ~
241
] I rllllT1
I
i [ i iii
I I I JIlil
J
I i i IL
E o I-Z UJ a
~100 Z
UJ rr n0 ..J < I--
~
10 I
10 FILM T H I C K N E S S
100 (p.m)
Fig. 3. The critical current density at 77 K as a function o f film thickness. Solid points are average values, as in Fig. 2; o p e n circles are m a x i m u m values for e a c h film thickness group.
distance from the substrate would also seem to be desirable. As explained in the introduction, a better figure of merit than Jc for tape applications is the critical current per film width. Fig. 3 shows the results in Fig. 2 with respect to a line of constant critical current per film width (Kc). It is apparent that increasing the film thickness from 5 to 10 I~m does not bring us closer to the 10 A mm-1 line. In fact, from 5 to 10 Ixm, values of K c are in the range 3 to 3.5 A mm-1. However, if one focuses on individual Jc measurements on segments (there are about 24 such segments which make up each average value solid data point shown in Figs. 2 and 3), the maximum J¢ values for each thickness group are shown as open circles. The highest value of K~ (i.e., the closest open circle to the 10 A m m -1 line) is 8.4 A m m -~. We use 8.4 A mm-1 as the highest value of K~ to make comparisons with other work (where single values, and not average values are reported). The highest value from our previous work with 3 I~m thick films is 3 A mm -1 [2], also for the T1-1223 phase on polycrystalline YSZ substrates. A high J¢ of 8 × 10 4 Acm -2 was reported for Tl(Bao.sSro.2)2-
242
A. Mogro-Campero et al. / Physica C 247 (1995) 239-242
C a 2 C u 3 0 9 spray deposited onto Ag [7], but the 1
Ixm thick film yields a low K~ value of 0.8 A mm -1. In another recent report on different film thicknesses of (T1,Pb)(Ba,Sr)ECa2Cu309 on single crystal LaA103, K~ values of 1.5 A m m -1 for a 5 Ixm thick film, 0.81 A m m -1 for a 14 ~m thick film, and 0.55 A mm-1 for a 21 p~m thick film were found [8]. Even though the present study has achieved K~ values close to 10 A m m -1 for 10 ~m thick films on individual segments, it is clear from Figs. 2 and 3 that the drop of Jc for thicker films is undesirable, and work is continuing to find ways to increase the average values of J~ for thicker films.
4. Summary For superconducting tape applications, a high value of the critical current per film width ( K c) is desired. Considerable effort has been expended in improving the values of the critical current density of thick films on polycrystalline substrates. In this study, the film density was found to be related to Jc, and a processing step which controls the density was identified. Further improvement of K~ has been sought by increasing the film thickness. J~ as a function of film thickness was measured for films of T1-1223 in the thickness range of 2 to 10 p,m. J~ was found to peak in the 2.5 to 5 I~m thickness range, and decrease for thicker films, such that K~ remains approximately constant (3 to 3.5 A mm -1) in the film thickness range from 5 to 10 I~m.
The above conclusions are based on average values for many samples of each film thickness. The highest value of Kc from single measurements was 8.4 A m m -1 for a segment of a 10 lxm thick film. This shows that high values of K~ are possible, and encourages continuing work.
Acknowledgements We thank L. Vanier for photolithography work and J. Bray for keeping us informed on the literature and for comments, encouragement and support.
References [1] D.H. Kim, K.E. Gray, R.T. Kampwirth, J.C. Smith, D.S. Richeson, T.J. Marks, J.H. Kang, J. Talvacchio and M. Eddy, Physica C 177 (1991) 431. [2] J.A. DeLuca, P.L. Karas, J.E. Tkaczyk, P.J. Bednarczyk, M.F. Garbauskas, C.L. Briant and D.B. Sorensen, Physica C 205 (1993) 21. [3] J.E. Tkaczyk, J.A. DeLuca, P.L. Karas, P.J. Bednarczyk, M.F. Garbauskas, R.H. Arendt, K.W. Lay and J.S. Moodera, Appl. Phys. Lett. 61 (1992) 610. [4] D.P. Matheis and R.L. Snyder, Powder Diffraction 5 (1990) 8. [5] A. Mogro-Campero and L.G. Turner, J. Supercond. 6 (1993) 37. [6] D.M. Kroeger, A. Goyal, E.D. Specht, Z.L. Wang, J.E. Tkaczyk, J.A. Sutliff and J.A. DeLuca, Appl. Phys. Lett. 64 (1994) 106. [7] T.J. Doi, T. Yuasa, T. Ozawa and K. Higashiyama, Jpn. J. Appl. Phys. 33 (1994) 5692. [8] D.L. Schulz, P.A. Parilla, D.S. Ginley, J.A. Voigt and F.P. Roth, Appl. Phys. Lett. 65 (1994) 2472.