Superconducting YBa2Cu3Ox thin layers by solid state
diffusion
Citation for published version (APA):
Severin, J. W., With, de, G., Baller, T. S., & Veen, van, G. N. A. (1988). Superconducting YBa2Cu3Ox thin
layers by solid state diffusion. Materials Research Bulletin, 23(5), 707-717.
https://doi.org/10.1016/0025-5408(88)90036-0
DOI:
10.1016/0025-5408(88)90036-0
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Published: 01/01/1988
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Mat. Res. Bull., Vol. 23, pp. 707-717, 1988.
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SUPERCONDUCTING YBa2Cu30 x THIN LAYERS BY SOLID STATE DIFFUSION
J.W. Severin, G. de With *, T.S. Baller and G.N.A. van Veen Philips Research Laboratories
POB 80000, 5600 JA Eindhoven, the Netherlands
* also affiliated with Eindhoven University of Technology
(Received F e b r u a r y 4, 1988; Communicated b y G. Blasse)
ABSTRACT
A new method for the formation of YBa2Cu30 x superconducting thin layers is described. Between a substrate and a deposited thin layer or b e t w e e n two deposited layers on a substrate, an orthorhombic YBa2Cu30 x layer is formed by a solid state diffusion reaction. Two versions of this method are reported. The first one starts with a BaCuO 2 layer on a hot-pressed Y2Cu205 substrate. The second one starts with an Y2Ba407.CO2 layer on a sintered CuO substrate. A number of the former samples were provided with a capping layer of Y2Cu205. Most of the layers were deposited by laser ablation. The reaction temperature and time were varied between 825 and 900°C and b e t w e e n 0.5 and 6 hour respectively. The best layer upto now was made using a Y2Cu205 substrate with a BaCuO 2 layer and a capping. The resistance versus temperature of this layer shows a T c (50 % drop) of 83 K and a 10-90 % width of 15 K. The resulting layers were characterized by scanning electron microscopy and energy dispersive X-ray analysis. In the layers no preferential
orientation of the 123-compound, formed in the reaction zone, has been detected by X-ray diffraction. Suggestions for further optimization are made.
MATERIALS INDEX: Superconductors, yttrium, barium, copper
Introduction
The recently discovered high-T c superconductors (I), more in particular YBa2Cu30 x (henceforth called 123), are prepared in bulk and as thin films by a wide variety of methods. Thin-layer technology is important for the possible application of 123-material in electronic devices. Thin 123-1ayers can be made
708 J . W . SEVERIN, et al. Vol. 23, No. 5
by e.g. plasma-spraying, laser ablation and sputtering (2,3,4). Much effort is being spend on the optimization of the composition and the structure of 123- layers in order to improve the transition temperature, T c and the critical current, I c. Because of the high reactivity of the 123-compound towards most other compounds at elevated temperatures, only few substrate materials have been found suitable yet. Examination of the phase diagram (5) led us to a new method for the fabrication of superconducting thin 123-1ayers. This
method is based on the formation of a superconducting thin layer by a solid state diffusion reaction at the interface between two non-superconducting compounds. These compounds are found in the phase diagram at opposite ends of a straight line through the 123-composition, as indicated in fig. i.
In this paper two reactions, leading to superconducting thin layers are described. CuO + Y2Ba407.CO 2 and Y2Cu205 + BaCuO 2 react, if mixed as powders in the appropriate ratio's, to single phase orthorhombic 123-powder. Therefore these compounds were chosen for carrying out the diffusion reactions. The advantage of using CuO and BaCuO 2 in these reactions can be explained using the phase diagram. In this diagram CuO and BaCuO 2 are linked with the 123- composition by a tie-line. This means that the 123-1ayer which is formed at the interface between the two starting phases, is stabilized at one side because it does not react any further with either CuO or BaCuO2, depending on which particular reaction is used. Of course, this only happens if not all CuO or BaCuO 2 has b e e n converted. Because CuO is used as substrate material this cannot completely be converted. Supply of BaCuO2, however, takes place from a deposited thin layer, so care must be taken that at least some BaCuO 2 is left behind after the thermal treatment.
The choice for CuO and Y2Cu205 as the substrate materials was based on practical considerations. It was found that the complementary compounds
Y2Ba407.CO 2 and BaCuO 2 are chemically unstable in ambient atmosphere. Moreover BaCuO 2 showed little thermal stability and Y2Ba407.CO2 could not be sintered easily.
Experimental Procedure and Results
...
Y2Cu205 , BaCuO 2 and Y2Ba407.C02 were prepared by standard solid state reactions starting with Y203 , CuO and BaCO3, resulting in single phase compounds as determined by X-ray powder diffraction (XRD).
Y2Cu205 / BaCuO 2 Samples
...
Y2Cu205 substrates were hot-pressed at 925°C and 0.5 kbar in air for 0.5 hour, resulting in a relative density of 95 to I00 %. A BaCuO 2 target was sintered at 800°C for 4 hours in air. On the ground and polished substrates 3 pm thick layers were deposited by laser ablation from the target at a
substrate temperature of 400°C. Details are given in (3). The resulting layers had a black shiny surface when they were taken out of the vacuum chamber, but obtained a green, granular appearance after 8 few minutes exposure to air. The samples were annealed at a few different temperatures for various times in 02 atmosphere. A heating rate of 100°C/h and a cooling rate of 50°C/h were used
Vol. 23, No. 5
YBa2Cu30 x
709
BaO B%Y207 BaY~O 4 Y01. 5
FIG. 1
Phase d i a g r a m taken from ref. 6, in w h i c h the relevant compositions are indicated. In ref. 5 Y2Ba407 is shown to be Y 2 B a 4 0 7 . C O 2 . M o r e o v e r some other differences are present b e t w e e n the p h a s e - d i a g r a m s of refs. 5 and 6. C u O 125 - 1 0 0 ?5 5O 9 5 0 0 D [2 5O [3 D [2 [3 D 0 D D I00 150 2 o 250 T
(~;)
300 FIG. 2R-T curve of the Y2Cu205 / BaCuO 2 sample without capping after annealing for 4 h at 850°C in 02 (sample 0). Note the high resistance above T c.
710 J.W. SEVERIN, et al. Vol. 23, No. 5
Before onneo]~n 9
A
After o~eal~n 9
B
Loyer surface
. . .
Y~Cu205 copper 9 layer
I
. . .
Layer surface
By EDAX:
. . . 1 ~ m
123-C5~
]ayer
Y, B o , [u . . . Bo.Cu
BoCuO 2 ieyer
i
Bulk Yz,Cu20
s swBetrate
II
3
~
Bo, Cu
Ba, Cg . . . 0.5 ~m ] 2 3 - i n t e r f a c e ]oyer Y. Ba0 Cu 5 u b s t m a t e B u l k ] C u FIG. 3A. Schematic of the deposition sequence from the capped
Y2Cu205 / BaCuO 2 sample.
B. Schematic of the element distribition after heat treatment.
Before
After
annealing
annealing
FIG. 4
SEM picture of Y2Cu205 / BaCuO 2 diffusion-layer (sample i).
The light top layer is an orthorhombic 123-1ayer, formed from the original capping layer. Below the cracked boundary a thin 123-1ayer is present which is formed on the original substrate surface.
Vol. 23, No. 5
YBa2Cu30 x
711
throughout the experiments. In order to establish the best a n n e a l i n g
temperature for these samples, reactions were q u a l i t a t i v e l y followed by high- temperature XRD in flowing 02 . The following observations were made.
Initially, at room temperature, no BaCuO 2 was detected on the substrate after laser ablation. Instead an u n i d e n t i f i e d amorphous phase was present. Between 400 and 850°C BaCO 3 reflections appeared in the XRD pattern. The colour change d e s c r i b e d above should be due to a reaction of the amorphous layer with CO 2 from the air. At 850°C BaCO 3 d i s s o c i a t e d and s u b s e q u e n t l y reacted to form BaCuO 2. The BaCuO 2 reacted with the Y2Cu205 substrate in a few minutes to form the 123-compound. W h e n the temperature was raised to 900°C, the 1 2 3 - c o m p o u n d d e c o m p o s e d and Y2BaCuOs, h e n c e f o r t h called 211, and CuO were formed. A p p a r e n t l y the optimum annealing temperature of these samples is about 850°C. It should be n o t e d that the accuracy in the temperature d e t e r m i n a t i o n is limited, at least plus or minus 25°C.
Samples were annealed for two hours at 850°C. The resistance of such samples was m e a s u r e d as a function of temperature by a standard d.c. four- probe technique. A b r o a d superconductive transition was observed. R e p e a t i n g the a n n e a l i n g gave b e t t e r results. In fig. 2 the resulting resistance versus temperature (R-T) curve is shown. A T c of 85 K (50 % resistance drop) was n o t e d but the resistance above T c remained relatively high. The X R D - p a t t e r n of this sample showed the presence of more than four phases, in addition to w e a k reflections o r i g i n a t i n g from the 123-compound.
In order to avoid the reactions described above of the a s - d e p o s i t e d BaCuO 2 layer w i t h CO 2 or H20 from the air, an Y2Cu205 capping of about i um thickness was applied, on top of the BaCuO 2 layer (fig. 3a). This capping was also d e p o s i t e d b y laser ablation. Six capped Y2Cu205 / BaCuO 2 samples were h e a t - t r e a t e d as s u m m a r i z e d in Table i. Now most of the XRD patterns showed 1 2 3 - c o m p o u n d as the m a i n phase. Table I also gives q u a l i t a t i v e l y the phase c o m p o s i t i o n of the layers, as d e t e r m i n e d by XRD. Remarkable is the low intensity of the 123-reflections for sample nr 5. Furthermore, a decrease of the 2 1 1 - r e f l e c t i o n intensities is observed for the sample numbers 3, i and 5, w h i c h have b e e n annealed for an increasingly longer period, 0.5, 2 and 6 hours at 850°C, respectively. The m a i n features of the R-T curves are also given in Table I.
In fig. 4 a SEM fractograph of a sample with capping is shown, to be c o m p a r e d w i t h the schematic of fig. 3b. This sample has b e e n a n n e a l e d for two hours at 850°C in 02 . The presence of Y, Ba and Cu as d e t e r m i n e d by energy- dispersive analysis of X-rays, EDAX, is indicated. As shown in this figure two layers in w h i c h Y, Ba and Cu are detected, are present. On this photo also cracks through the layers can be observed. These cracks can be formed by a m i s m a t c h in thermal expansion coefficients and phase transformations in the layers.
In fig. 5 the R-T curve of sample nr i, with the highest Tc(0 ) is shown. In a d d i t i o n two R-T curves of sample nr 5 are shown. The first R-T curve of this sample was m e a s u r e d in the usual way and then the sample surface was scratched and another R-T curve was measured. A considerable change in electrical b e h a v i o u r of this sample is observed as shown in figs. 6 and 7. This will be e x p l a i n e d in the next section.
712 J . W . S E V E R I N , e t a l . V o l . 2 3 , No. 5 I0
8
6 4 o 2 [] 0 ... ~--~ 0 50 100T
FIG. 5 [] D LI [] 150 2 0 0 250 3 0 0(K)
R-T curve of the Y2Cu205 / BaCuO 2 sample with capping after annealing for 2 h at 850°C in 02 (sample i). Note the low resistance above T c. 120
n~
1 0 0 8 0 - 6 0 - 402O
[3 D D 0 0 D O D I ~ 3 ~ X D D D I I I I ! 0 5 0 100 150 2 0 0 2 5 0 3 0 0T
(K)
FIG. 6R-T curve of the Y2Cu205 / BaCuO 2 sample with capping after annealing for 6 h at 850°C in 02 contacted without scratching
(sample 5).
Vol° 23, No. 5
Y B a 2 C u 3 0 x
713
TABLE 1
123-Layers from Y2Cu205 / BaCuO 2 Diffusion Couples
Sample Heat tr. X - R a y R-T characteristics
Nr. Temp. Time 123 211 BaCuO 2 R(RT) T(90%) T(0)
(°C) (h) (ohm) (K) (K) ... 0 * 850 2 * 2 + + 120 90 55 i0 825 2 +- 30 @ @ 3 850 0.5 + + 300000 x x i 850 2 + +- +- i0 93 75 5 850 6 + 60 93 #
after s c r a t c h i n g the sample surface 15 93 #
9 875 2 + + 55 91 60
13 900 2 + + 40 91 65
RT indicates room temperature * sample w i t h o u t capping
# indicates a non-zero resistance b e l o w T c down to 4 K @ indicates no transition
+ and - are qualitative indications for X-ray reflection intensities + indicates strong reflections
indicates w e a k or absent x no R-T curve c o u l d be m e a s u r e d
CuO / Y 2 B a 4 0 7 . C 0 2 Samples
...
CuO substrates were sintered at 900°C for i0 hours in air, resulting in a relative density of 90 to 95 %. By p l a s m a - s p r a y i n g Y 2 B a 4 0 7 . C O 2 powder, layers were d e p o s i t e d on the ground and p o l i s h e d substrates. The layers h a d a
thickness of 8 /mm, assuming a relative density of 70 %. Initially cream- coloured, the layers turned white after a few minutes exposure to air. A n XRD p a t t e r n of this white powder showed the presence of BaC03, among other u n k n o w n phases. The a s - d e p o s i t e d layers, however, have not b e e n analyzed. A similar d i f f u s i o n r e a c t i o n as described for the Y2Cu205 / BaCuO 2 samples was carried out by a n n e a l i n g for 6 hours at 850°C in flowing oxygen. A n R-T curve of this sample is shown in fig. 8. In this figure a superconductive transition with an onset temperature at 90 K and zero resistivity at 60 K is shown.
The r e s u l t i n g layers were about 8 ~ m thick and h a d a rough surface. In order to o b t a i n thinner and smoother layers, a number of CuO substrates were c o v e r e d b y laser ablation from an Y 2 B a 4 0 7 . C O 2 target. This target was sintered at II00°C for 12 hours in air. No capping h a d b e e n applied, although these layers also a p p e a r e d not to be stable towards air. A series of these samples was h e a t - t r e a t e d in the same way as described for the Y 2 C u 2 0 5 samples, see Table 2. A l t h o u g h the onset temperature was again 90 K, zero resistivity was o b t a i n e d only at about 45 K.
714 J . W . S E V E R I N , e t a l . V o l . 23, N o . 5 20 ne 15- I0- [] o o o o 0 0 2fO 4'0 6 0 I 810 100 , 120 140
r (K)
FIG. 7R-T curve of the Y2Cu205 / BaCuO 2 sample with capping after annealing for 6 h at 850°C in 02 scratched before contacting
(sample 5, scratch). 18 O 0 ~ ~ 15 12 9 o 6 [] 3 o 0 . . . " ~ 0 50 100
T
[] 0 0 150 2 0 0 250 3 0 0(K)
FIG. 8R-T curve of the CuO / Y2Ba407.CO2 sample prepared by plasma- spraying and without capping after annealing for 4 h at 850°C in 02 (sample 7).
Vol. 23, No. 5
YBa 2Cu30 x
715TABLE 2
123-Layers from CuO / Y2Ba407.CO2 Diffusion Couples
Sample Heat tr. Nr. Temp. Time (°C) (h) ... Ii 825 2 +- 4 850 0.5 +- 2 850 2 +- 6 850 6 +- 7* 850 6 + 8 875 2 +- 12 900 2 +- X-ray R-T characteristics 123 211 BaCuO 2 R(RT) T(90%) T(O) (ohm) (K) (K) ... ... .... +- - 30 90 i0 2000 x x 200 90 # 14 90 45 16 90 65 50 90 i0 300 x x
RT indicates room temperature * layer deposited by plasma-spraying # indicates a non-zero resistance below T c
+ and - are qualitative indications for X-ray reflection intensities x no R-T curve could be measured
Discussion
In this section first the layers on Y2Cu205 substrates and then those on CuO substrates are discussed.
In capped Y2Cu205 / BaCuO 2 samples after annealing, two regions in which Y, Ba and Cu prevail have been detected by EDAX, see fig. 4. In the samples without capping, only one such layer can be formed at the interface between the deposited layer and the substrate. This is illustrated by the schematical representation of the layer geometry before and after annealing in figures 3a and 3b. In the XRD patterns a clear difference is observed between the samples without an Y2Cu205 capping on top of the BaCuO 2 layer on the one hand and those with such a capping on the other hand. From the samples without capping only very weak 123-1ines, originating from the underlying 123-interface, were detected. In contrast strong 123-1ines apparently originating from the 123-top layer, appeared in the XRD patterns of the capped samples. In none of the layers any preferential orientation was detected by XRD.
Annealing for two hours at 850=C appeared to be an optimal treatment for the capped Y2Cu205 / BaCuO 2 samples so far. In this case a relatively sharp superconductive transition is obtained, see fig. 5 (sample I). Increasing the 123-interface layer thickness by applying a longer annealing treatment (sample 5) resulted in much lower 123 X-ray intensities, higher resistivity above T c and a non-zero resistance below T e (fig. 6). Apparently the top layer
disappeared. After scratching the surface of this sample, a lower room temperature resistance and a lower resistance below T c was measured, see fig. 7. ~rhese differences in resistivity are ascribed to a contribution of the
716 J . W . S E V E R I N , et al. Vol. 23, No. 5
123-interface layer. This layer could only be contacted after scratching through the 123-top layer and the BaCuO 2 layer, see fig. 3b. The decrease in 211 content of the layers with longer annealing time at 850°C indicates that this compound acts as an intermediate phase in the reaction of Y2Cu205 with BaCuO 2 to the 123-compound. Though, together with 211, CuO should have been present it was not detected. This means that the CuO content was below the detection limit of the X-ray equipment.
The SEM fractograph shows that the thickness of the top layer in which Y, Ba, and Cu prevail, is about I ~m. The interface layer is even thinner.
Consequently I ~m is about the maximum thickness of the 123-1ayer in these samples. A few preliminary conclusions on the diffusion processes that take place, can be drawn. First consider the formation of the 123-interface layer. Above the cracked boundary no Y was detected by EDAX. Only Ba and Cu were found in that region. Just below the boundary not only Y and Cu, but also Ba was detected. If this cracked boundary corresponds to the original substrate surface, this means that Y2Cu205 does not diffuse into the BaCuO 2 layer. BaCuO2, however, penetrates the Y2Cu205 substrate. If this assumption is valid, then the diffusion only proceeds one-way. A higher mobility of the
BaCuO 2 phase with respect to the Y2Cu205 phase is expected for two reasons.
First, the Y2Cu205 phase, which was prepared at about II00°C and sintered to a high relative density at 925°C, finds itself in a stable state while the
BaCuO 2 phase, which was deposited as an amorphous phase, is less stable.
Second, the BaCuO 2 phase, in contrast to the Y2Cu205 phase, is close to its melting point when the diffusion is carried out at 850°C. Next consider the 123-toplayer. Here the diffusion process is less clear. The starting situation is different: probably reaction and diffusion proceed faster because of the higher reactivity of the amorphous state of the laser ablated Y2Cu205 capping phase. This can explain why a thicker layer results.
At the moment it is not yet possible to make a fair comparison between the two types of layers that have been made. Probably the results,
particularly for the layers on CuO, largely depend on the degree of chemical degradation of the layers before annealing. This causes discontinuities in the resulting 123-1ayers. Y2Cu205 / BaCuO 2 samples on which a capping had been applied showed better superconducting properties. In addition to chemical degradation also inhomogeneity in deposited layer thickness was observed.
For further optimization of the layers, a number of suggestions can be made. In order to prevent chemical degradation of the reactive, amorphous layers by reaction with water or carbon dioxide from the air an experimental setup for transfer of the samples from the vacuum chamber directly into the furnace is necessary. This setup is being realized at the moment. Furthermore the influence of layer thickness, homogeneity in layer thickness and substrate surface quality has not yet been examined. The influence of other heat
treatments and the application of other gas atmospheres could also be investigated. Other deposition techniques could be tried as well. We choose for laser-ablation because it is quick, easy to operate and flexible. Finally, chemical compositions can be changed. It is not necessary to start with single phase substrates or layers. Phase mixtures can be used as well.
Vol. 23,
No.5
YBa2Cu3Ox
717
Conclusions
It is shown that it is possible to form superconducting thin 123-1ayers by a solid state diffusion reaction between two non-superconducting starting phases. For the CuO / Y2Ba407.CO 2 samples the best results showed an onset of
the transition at about 90 K but a considerable tail in the resistance- temperature curve is observed. Zero resistivity was measured at 60 K. At present the best result is obtained by using a Y2Cu205 substrate covered with a BaCuO 2 layer and provided with an Y2Cu205 capping. Annealing for 2 h at 850 C in 02 results in a Tc midpoint of 83 K and a transition width of about 15 K. The Y2Cu205 / BaCuO 2 couple therefore appears to be a promising candidate. The fact that for the Y2Cu205 / BaCuO 2 samples with a capping of Y2Cu205 , a superconducting layer on top of the BaCuO 2 layer was found, indicates that it is worth considering to make a superconducting 123-1ayer on other
polycrystalline substrates by depositing a sequence of layers and applying a heat treatment.
Acknowledgements
Many thanks are due to mr. H.A.M van Hal for the preparation of the compounds, dr. J.W.C. de Vries, dr. G.M. Stollman and dr. H.M. van Noort for the electrical measurements, to mr. P. Joosten for the plasma-spraying and mr. H. Smoorenburg and mr. C. Langereis for the X-ray diffraction measurements and to mr. C. Geenen for SEM and EDAX analyses.
i. 2. 3. 4. 5. References
J.C. Bednorz and K.A. Mueller, Z.Phys.B., 64, 189, (1986).
M.K. Wu, J.R.Ashburn, C.J. Torng, P.H. Hor, R.C. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys.Rev. Lett. 58, 908, (1987). See further:
Adv. Ceramo Mater., vol. 2, nr. 3b (1987). Jap. J. Appl. Phys., vol 26, nr. 4 (1987).
W.T.Elam, J.P.Kirkland, R.A. Neiser, E.F. Skelton, S. Sampath, H. Herman, Adv. Ceram.Mat. 2, 411, (1987).
T.S. Baller, G.N.A. van Veen and H.A.M. van Hal,
submitted to Appl. Phys.
B. Dam, H.A.M. van Hal and C. Langereis, submitted to
Europhysics Letters.
D.M. de Leeuw, C.A.H.A. Mutsaers, C. Langereis (to be published).
