- Email: [email protected]
Solubility of Ag in YBa2Cu3O6+y and its effect on superconducting properties
PHYSICA
Physica C 178 ( 1991 ) 64-70 North-Holland
Solubility of Ag in YBa2Cu306 +.v and its effect on superconducting
properties A n u p K. G a n g o p a d h y a y a n d T.O. M a s o n Northwestern University, Department of Materials Sctence& Engineering. Mc('ormick School olt:'ngineering and Apphed Science, Evanston, IL 60208, USA
Received 10 April 1991 Revised manuscript received 13 May 1991
In order to determine the solid solubility of Ag in YBa2Cu306+v and its effect on superconducting properties, a series oi" YBa2Cu3_ ~AgxO6+v samples (x_< 0.75 ) have been prepared and characterized by X-ray diffraction, optical microscopy, and electron microprobe analysis. The central results arc that Ag goes into solid solution up to about x=0.06 and that initially (up to x=0.035 ) superconducting properties are improved - T,. goes up by 1.1 K, intragrain critical current density is marginally improved and Meissner effect is considerably enhanced. However, at higher x all the superconducting propcrties are considerably degraded. The implication of these results on the usc of Ag as a cladding material for 123 or in a composite arc discussed.
I. Introduction Since the discovery o f high t e m p e r a t u r e oxide superconductors, Ag has been extensively used either as a d o p a n t or as an a d d i t i v e ( c o m p o s i t e s ) in YBa2Cu306+.~ (123) to i m p r o v e its superconducting properties. On the other hand, due to its relatively low reactivity with 123 at the usual processing temperatures and fast diffusivity o f oxygen, Ag is commonly e m p l o y e d as a cladding material for shaping 123 in the form o f wires, tapes, etc. Several beneficial effects o f Ag-123 composites such as lower normal state resistivity [ 1-3 ], lower contact resistivity, modest transport critical current density (J,.) enhancement [1,2,5], i m p r o v e d mechanical strength [ 6 ], and better corrosion resistance against moisture and CO2 [7], have been cited. However, c o m p a r a tively fewer studies have been m a d e using Ag as a d o p a n t and quite often the published results are in a p p a r e n t contradiction. F o r example, K a o et al. [ 8 ] reported an e n h a n c e m e n t in Jc up to about 20 at.% substitution o f Cu by Ag without any change in the superconducting critical t e m p e r a t u r e T,.. In contrast, M a t s u m o t o et al. [9] observed a modest 2.5 K Tc e n h a n c e m e n t in (YBa2Cu3Oy)o.7Ago.3 and slightly lower in YBa2Cu3_,AgxO,, system for x = 0 . 3 with
some enhancement o f J~. However, in these studies [8,9] no direct a t t e m p t has been m a d e to establish the a m o u n t o f Ag that really goes into solid solution. Therefore, at present it is not at all clear as to what extent the change in the observed superconducting properties represents intrinsic behavior (effect o f doping) vis-a-vis changes in the intergranular regions. Additionally, due to the wide scale use of Ag as cladding material in the fabrication o f 123 superconductors, it is extremely i m p o r t a n t to know the reactivity and solid solubility o f Ag in 123. To shed some light on these issues, we have carefully prepared several polycrystalline specimens of starting c o m p o s i t i o n s YBa2Cuy_,Ag,-O6+, ( x < 0 . 7 5 ) and characterized them by X-ray p o w d e r diffraction, optical microscopy and quantitative wavelength dispersive electron probe micro-analysis ( E P M A ) . Various properties such as diamagnetic onset t e m p e r a t u r e (T,.), Meissner effect (field cooling), and intragranular J,. at 5 K have been measured as a function o f increasing doping ( x ) . F r o m these data the solubility limit o f Ag in 123 could be clearly established and the influence o f doping on the intrinsic superconducting properties determined.
0921-4534/91/$ 03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved
A.K. Gangopadhyay, T.O. Mason/Solubility of Ag in YBa2Cu306+y 2. Experimental
The samples were prepared by solid state reaction of high purity (99.9%) Y203, BaCO3,CuO and Ag20 using two different routes. In the first method, appropriate compounds of Y, Ba, Cu and Ag were thoroughly hand mixed in the desired ratio and calcined at 880°C (32 h) in air with one intermediate grinding. The product was reground and recalcined at 900°C (72 h) in flowing O2 with three intermediate grindings and furnace cooled at 0.3°C/min to room temperature. Finally, they were pelletized and sintered at 940°C (4 h) in flowing 02 and reoxygenated at 450°C (24 h). In the second method, Ag was added after the first calcination of YBa2Cu3_xO6+y at 880°C (32 h) in air. Subsequent heat treatments were identical to method 1. Surprisingly, samples prepared by the second method were of higher phase purity and showed superior superconducting properties. Although the reason for this difference is not clear yet, it is possible that the 123 phase can tolerate small amount of Cu deficiency which can in turn facilitate the incorporation of Ag into the solid solution in the second method. Whatever the reason, results presented here are for the samples prepared by the second method. The sintered pellets were characterized by X-ray powder diffraction using Cu Kct radiation. Samples for optical microscopy and electron microprobe analysis (JEOL superprobe 733 ) were polished down to 0.3 ~tm using kerosene as a lubricant. For quantitative analysis, large grained stoichiometric 123 was used as a standard for Y, Ba and Cu, whereas AgBiS was used for Ag. A 15 keV electron beam with 1 ~tm spot size excited the characteristic X-rays. Using "ZAF" corrections the concentration of Y, Ba, Cu and Ag were determined in the samples under study. No attempt was made to determine the 02 content of the present samples. Magnetic properties of the normal and superconducting states were measured by a superconducting quantum interference device magnetometer (SQUID, Quantum Design Inc.) equipped with a 5.5 T superconducting magnet.
65
3. Results and discussion
3.1. X-ray powder diffraction For x < 0.18, all the lines in the X-ray powder diffraction pattern could be accounted for by the orthorhombic 123 phase. However, for x>0.18, BaCuO2 (011 ), Y2BaCuO5 (211 ) and metallic Ag were detected as impurity phases in growing amount. No other phase such as CuO, BaCO3 or Y2Cu205 could be detected up to the highest value of x ( = 0 . 7 5 ) studied in the present investigation. Since we are replacing Cu by Ag, as long as Ag goes into the solid solution, no impurity phase should be expected. As soon as the solubility limit is exceeded, the situation becomes similar to that of Cu-deficient 123 composition. A look at the triangular region bounded by 123-211-011 in the phase diagram of Y ~.50-CuO-BaO [ 10] suggests that deficiency in Cu should give rise to 211 and 011 as impurity phases and the amount of impurity phases will be determined by the deviation from stoichiometry of Cu, unless the phase diagram is significantly modified in the presence of Ag. Therefore, the presence of only 211 and 011 as impurity phases at higher x would imply that the solubility limit of Ag in 123 has been exceeded. Thus, only from the X-ray data the solubility limit appears to be around x = 0.18. As will be shown in the next sections, this is an overestimate caused by the relatively lower X-ray scattering factors of 211 and 011 phases compared to 123 [ 11 ] and due to some additional complications.
3.2. Optical microscopy Optical microscopy on the polished section of different samples revealed some interesting features. For x > 0.09, Ag could be distinctly observed as metallic precipitates which increase in amount with increasing x. This would indicate that the solubility of Ag in 123 is x<0.09. However, as will be shown in the next section, this along with the X-ray data provide an overestimate. As shown in fig. 1, addition of Ag has some interesting effects on the average grain size determined by the linear intercept method. Although the grain size passes through a maximum for x=0.06, the overall change in grain size with composition is min-
66
A.K. Gangopadhyay. "ll O. Mason/SoluhtliO' q#..l~ in }'Ba ?( 'u ~(),, ~ ,
imal. This result is essentially different from the anomalous grain growth observed in Ag-123 composites by several workers [ 12,13 ]. As mentioned in the previous section, since above the solubility limit impurity phases should start appearing, sintering and grain growth processes can be adversely affected by the presence of these phases at the grain boundaries. Such a decrease in grain size has been reported in the earlier studies [14,15] on Cu-deficient 123 superconductors. On the other hand, Ag solubility in 123 can be expected to influence the grain boundary mobility and hence the grain growth characteristics. This could account for the initial increase in grain size with composition in fig. 1. 3.3. E l e c t r o n
microprobe
analysis
In order to determine the actual compositions of 123 in the samples, quantitative wavelength dispersive electron probe microanalysis (EPMA) was performed. A large number of grains of different morphologies were selected at random for this purpose. The cation concentrations (Y, Ba, Cu and Ag) were determined using appropriate "ZAF" corrections. For x < 0.06, we were unable to detect the presence of any impurity phases in the doped material. Only for higher values o f x were 211,011 and metallic Ag impurities detected. Y, Ba and Cu were found to be in the ratio of 1 : 2 : 3 as expected in the 123 phase. Deviation from these ratios never exceeded by more than 3% which is within the statistical uncertainties.
I
89
'
'
~'a'o ~:o;_x,X, gxO6i,
'
'
't
-0.1
L
* x =0. ~:31
X
T (K) '
NomLne,
o'
I
Ag
'
'
Cone
b . b d
.
(X)
Fig. 1. Electronprobe microanalysis (EPMA) Agcontents in 123 and grain size vs. dopingin YBa2Cu3_rAg~O6+v.
The Ag content inside the YBa2Cu~ ,Ag,.O~,+,.grains was determined by taking data on at least 20 different grains in each sample. In fig. 1 are plotted the analyzed Ag content against the nominal concentration .v. The analyzed concentration increases approximately linearly with nominal .r and then saturates around the value of x=0.06. Therefore, we conclude that x=0.06 is the solubility limit of Ag in 123. The disturbing feature of the EPMA results is that the analyzed concentration is always much smaller than the nominal one and the slope of the line is much smaller (0.2) than the expected value of 1. One obvious explanation would be an error in the microprobe analysis resulting from the use of an improper standard for Ag. To check this, we have used two different kinds of standards, namely, a Cu-rich phase (2.9 at.% Ag in Cu) in a Cu-Ag eutectic alloy and the compound AgBiS. Both the standards yield nearl) the same results. A second possibility is that equilibrium had not yet been achieved, i.e. Ag had not been fully incorporated into the 123. Measurements on a different batch of samples reacted for a longer time (up to 5 days) were not statistically different than the earlier time results. Furthermore, the relatively small error bars in fig. 1 (and small grain-tograin variation) argue against inhomogeneous mixing of Ag in the microstructure which might lead to kinetic problems and point-to-point variations in 123 Ag content. Therefore, although unexpected, this discrepancy between measured (123) and nominal Ag content does not appear to be an experimental artifact. The central questions are, why even below the solubility limit all the Ag is not incorporated into the solid solution and where the remaining Ag is in the microstructure. As mentioned earlier, for .r> 0.06, we did observe the presence of metallic Ag both in optical microscopy as well as in EPMA and the presence of211 and 011 phases in the microprobe. Since the starting composition is Cu-deficient, if all the Ag is not incorporated into the solid solution, 211 and 011 phases would naturally occur as impurity phases along with metallic Ag. It is relevant to mention that we observed considerable amount of Ag ( ~ 3 at.%) inside the 011 grains: whereas no appreciable (at least an order of magnitude lower than in 01 l phase) Ag signal could be detected in the 211 grains. Therefore,
A.K. Gangopadhyay, T.O. Mason/Solubility of Ag in YBa2Cu306+y
even below the solubility limit only part of the Ag goes into solid solution in 123; the rest occurs as metallic Ag and some goes into the 011 phase. Transmission electron microscopy studies of an x = 0 . 6 sample confirmed the presence of Ag as a separate phase. We therefore conclude that Ag is present as a distinct phase at all compositions studied. (The presence of second phases at virtually all compositions may account for the negligible overall grain size versus composition dependence in fig. 1.) A few other interesting observations deserve mentioning. When the composition of the Ag phase was analyzed, substantial amount of Cu (1 to 3.5 at.%) was always detected. This signal cannot be from the background since the corresponding signals for Y and Ba were always an order of magnitude lower. This is consistent with a recent observation [ 16] that AgO eutectic liquid forms above 931°C and dissolves Cu from 123 leading to the decomposition of 123 into 211 phase. This result has some important implications on the use of Ag in high concentrations in composites or in the use of Ag as cladding material. It has been observed by several workers [ 1,2,5,6 ] that the transport critical current density is substantially reduced at higher concentrations of Ag in Ag-123 composites. Although several explanations have been put forward, it seems quite likely that among other things, the leaching action of Cu by Ag-O eutectic liquid can be very detrimental to transport critical current density. 3.4. Superconducting properties
In fig. 2 are shown both the diamagnetic onset temperature and the temperature at which 10% of the Meissner signal is attained in a field cooling experiment in a field of about 7 0 e on various samples. The inset shows the onset of diamagnetism on an enlarged temperature scale. The most important observation is a small (1.1 K) but distinct enhancement of T~ due to Ag doping. It should be emphasized that Tc in the undoped system, even under optimal 02 loading, never reaches such a high value of 93.8 K. Therefore, we ascribe the Tc enhancement to the doping effect. We recall that such enhancement in Tc was reported earlier [9]. However, it never exceeded 93 K and since the superconducting transitions were rather broad, it was not very apparent that
~0.06
67
........................
o
5
5
c
~
i4 2°
e4
3co O
N0- 1~2
2
>,,
--
- .... .....
Gretn AQ
SLze Conc.
e
(,_
(D 1 >
<
NomLnml
Ag
conc,(X)
Fig. 2. Diamagnetic onset and 10% Meissner signal values of T¢ in YBa2Cua_xAgxO6+ v.
the enhancement was due to doping. Moreover, the maximum in Tc was observed for x=0.3, which is well above the solubility limit of Ag in 123. Therefore, the knowledge of the Ag content along with the good quality of the materials in the present study, prove unambiguously that Ag doping enhances To. However, the increase is not monotonic. After reaching a plateau for 0.06 < x < 0 . 1 8 , Tc drops for higher x even before the solubility limit is reached. This behavior is somewhat reminiscent of the well known [ 17 ] Tc versus hole concentration curves of various high-To oxide superconductors. A recent high pressure study on 123 [ 18] has also shown a maximum in T~ as a function of pressure. Drawing a parallel from these studies [ 17,18 ] it seems that doping with Ag increases the hole concentration which in turn enhances To. This seems logical since Ag is normally monovalent and addition of Ag should increase the hole concentration, unless the oxygen content is reduced. Additional supporting data, especially oxygen content, are required to prove the validity of these conjectures. Whatever the mechanism, we would like to point out that Ag and Au [ 19 ] are the only two known dopants which can enhance Tc in 123. Another interesting effect of Ag addition is the initial increase of the Meissner volume fraction (up to x = 0.12) followed by a decrease at higher x as shown in fig. 3. It should, however, be remembered that the Meissner volume fraction in high-T~ oxide superconductors not only depend on the actual superconducting volume fraction, but also on the flux pinning
68
A.K. Gangopadhyay, 7~O. Mason /So/ubtlit ~' ol,.lg in YBae('u ¢0,, ~,
8. I
-0.1
.......... e. es . . . . .
x = O . {36
. . . . .
x
=0.09
. . . . . x=e. le
,, ~ ....
-8,7
., ~
""
ie
,,,)g,, 5b'" TEMPERATURE
( K)
Fig. 3. Meissner effect (field cooling) in 7 0 e magnetic field for various compositions (.v) ofYBa2Cu> ,Ag,O6+,..
properties of the material and on the average grain size (the ratio of the average grain size and the London penetration depth). Even in single crystals and at small applied magnetic fields, flux expulsion is never complete ( < 100%). It has been suggested [ 20 ] that because of the existence o f the so called "'irreversibility line" below itc, in a field cooling experiment, flux can be expelled only down to the temperature of the "irreversibility line" for the given field. Thus, if the "irreversibility line" is shifted towards lower temperatures (due to weakening of the flux pinning) as a result of doping, Meissner volume fraction would be enhanced. But then, one should also observe a decrease in the intragrain critical current density. As will be shown in below, this is surely not the case and hence the above explanation does not hold. Also the variation of grain size from sample to sample is not so dramatic as to account for the change in the Meissner signal. However, in granular materials, especially at low magnetic fields, one should also consider the flux pinning by the intergranular regions. Our preliminary TEM studies suggest that even below the solubility limit, some Ag occurs as a grain boundary phase. This, in turn, can change the pinning behavior of the grain boundaries. Therefore, the enhanced Meissner signal may not be an intrinsic (intragrain) property and may reflect a decrease in the pinning energy o f the intergrain regions. This idea is supported by a recent measurement [ 21 ] o f the intergranular pinning energy from
flux creep measurements in low fields in AgYBa2Cu30~,+, composites. It clearly shows a decrease in the intergranular pinning energy due to Ag addition. However, it is important to point out that the above result is not at all inconsistent with the observed [1,2,5] initial increase in transport (intergrain) critical current density in YBa2Cu306+,. superconductor due to Ag addition. Since transport J,: is determined by the intergranular weak-links, the observed J~, enhancement is more likely related to improved weak-links (possibly caused by cleaner grain boundaries [2,22] ) rather than any change in the grain boundary pinning behavior. In contrast, the decrease in the Meissner signal at higher x could be a result of poor superconducting properties as is evident from the diminishing sharpness of the transition as well as decreasing T,.. In fig. 4, we show the intragrain ( intrinsic ) critical current density (Jc) at 5 K for several samples as a function of applied magnetic field. J¢. was calculated from the Bean's formula [23] J~.=3OAM/d, where AM is the irreversible magnetization and "'d" is the diameter of the particles. AM was determined from the hysteresis curve and the average grain size was taken for "d". Some interesting points are apparent from fig. 4. Up to x = 0 . 1 2 , J~ remains nearly unchanged in the doped system; if at all, over this composition range, there is a slight enhancement over the
@
7
. . . . . . . . . . . .
, ....
, . . . . . . . . .
, .....
" - , ~ -" -" :.: = g , ..... ..... ::~:: .....
gO
x=0.06 x=g. 12 x=0.18 x =g. 24
E O
\ <5
,%
e.e
.....
....
....
s'.e
....
8
H (T) Fig. 4. M a g n e t i c ( B e a n ' s m o d e l ) i n t r a g r a i n critical c u r r e n t density at 5 K as a f u n c t i o n o f c o m p o s i t i o n (x) in YBa2Cu3 _ ,AgxO6 + ,,.
A.K. Gangopadhyay, T.O. Mason/Solubility of Ag in YBa2Cu306+y
undoped material. For x > 0.12, Jc drops. Interestingly, the concentration dependence of Jc mimics the behavior of Tc. Also noteworthy is the increasing magnetic field dependence of Jc for x > 0.12 which is consistent with the degradation of the superconducting properties as reflected in the diminishing sharpness of the transition (fig. 3). Thus all the data are mutually consistent in the sense that up to about x=0.12 (analyzed x = 0 . 0 3 5 ) the superconducting properties of the doped system are somewhat improved followed by a degradation at higher x.
4. Conclusions In summary, the present work determines for the first time the solubility of Ag in 123. The Ag solubility is about 2 at.% with respect to Cu ( x = 0 . 0 6 ) and is much smaller than assumed in earlier works [8,9]. As a result of doping, superconducting properties exhibit some changes. In particular, Tc shows about 1.1. K enhancement over that of the undoped material. The intrinsic (intragrain) Jc is marginally enhanced. Since this is accompanied by an increase in the Meissner effect, we suggest that doping somewhat weakens the flux pinning properties of the intergrain regions (not that of the grains). At higher x (analyzed x > 0 . 0 3 5 ) , the intragrain superconducting properties are considerably degraded. Additionally, for higher levels of doping (well above the solubility limit) we observed some Cu dissolved (1 to 3.5 at.%) into the metallic Ag phase. This is consistent with a recent study [ 16 ] where Ag has been observed to selectively leach Cu from 123 resulting in its decomposition. This result along with our study of the doped system suggests that Ag can be used beneficially only for small levels of doping. At higher concentrations, it can considerably degrade both intra- and inter-granular properties. Our observations of variable Ag content in 123 along with the presence of four total phases (Ag, 123, 211, and 011 ) over the nominal composition range 0 < x < 0 . 2 suggests that the phase relationships are more complex than originally thought. Current work is aimed at establishing the multicomponent phase relationships in this system.
69
Acknowledgements This work was supported by the Electric Power Research Institute under grant no. RP7911-8. The authors are grateful to D.L. Johnson of Northwestern and J.P. Singh of Argonne National Laboratory for useful discussions and to V.P. Dravid of Northwestern for assistance with the T.E.M. work.
References [ 1 ] B. Dwir, M. Affronte and D. Pavuna, Appl. Phys. Left. 55 (1989) 399. [2IA.K. Gangopadhyay, Y.T. Chou and H. Jain, Paper 123SIII-90 at the Am. Cer. Soc. Mtg., Dallas, TX, 22-26 April, 1990. [ 3 ] V. Plechacek, V. Landa, Z. Blazek, J. Sneider, Z. Trejbalova and M. Cermak, Physica C 153-155 (1988) 878. [4] S. Jin, M.E. Davis, T,H. Tiefel, R.B. Van Dover, R.C. Sherwood, H.M. O'Bryan, G.W. Kammlott and R.A. Fastnacht, Appl. Phys. Len. 54 (1989) 2605. [ 5 ] N. Irnanka, F. Saito, H, Imai and G. Adachi, Jpn. J. Appl. Phys. 28 (1989) L580. [6] J.P. Singh, H.J. Leu, R. B. Poeppel, E. Van Voorhees, G.T. Goudey, K. Winsley and Donglu Shi, J. Appl. Phys. 66 (1989) 3154. [ 7 ] Chin-An Chang and J.A. Tsai, Appl. Phys. Lett. 53 ( 1988 ) 1976. [8] Y.H. Kao, Y.D. Yao, L.Y. Jang, F. Xu, A. Krol, L.W. Song, C.J. Sher, A. Darovsky, J.C. Phillips, J.J. Simmins and R.L. Snyder, J. Appl. Phys. 67 (1990) 353. [9] Y. Matsumoyo, J. Hombo and Y. Yamaguchi, Mater. Res. Bull 24 (1989) 1231, and references therein. [ 10] R.S. Roth, J.R. Dennis and K.C. Davis, Adv. Ceram. Mater. 2 (1987) 303. [ 11 ] T. ltoh, M. Uzawa and H. Uchikawa, J. Am. Ceram. Soc. 71 (1988) C188, [ 12] S. Baliga and A.L. Jain, Appl. Phys. A49 (1989) 139. [13]T.H, Tiefel, S. Jin, R.C. Sherwood, M.E. Davis, G.W. Kammlott, P.K. Gallagher, D.W. Johnson Jr., R.A. Fastnacht and W.W. Rhodes, Mater. Lett. 7 (1989) 363. [ 14] A.K. Gangopadhyay and H. Jain, Paper 142-SIII-90 at the Amer. Cer. Soc. Mtg., Dallas, TX, 22-26 Apr. 1990. [ 15] D.M. Kroeger, L Mater. (1989) 14. [16] R.E. Loehman, A.P. Tomsia, J.A. Pask and A.H. Carim, Physica C 170 (1990) 1. [17] J.B. Torrance, A. Bezinge, A.I. Nazzal and S.S,P. Parking, Physica C 162-164 (1989) 291. [ 18 ] S. Klotz, W. Reith and J.S. Schilling, Physica C 172 ( 1991 ) 423, [19] M.Z. Cieplak, G. Xiao, C.L. Chien, J.K. Stalick and J.J. Rhyne, Appl. Phys. Lett. 57 (1990) 934.
70
A.K. Gangopadhyay, 7~O. Mason/Solubility ()f,4,~ m YBa2( "u~O~ ,
[20] L. Krusin-Elbaum, A.P. Malozemoff, Y. Yeshurun, D.C. Cronemeyer and F. Holtzberg, Physica C 153-155 ( 1988 ) 1469. [21]J. Jung, M.A.K. Mohamed~ S.C. Cheng and J.P. Franck, Phys. Rev. B 42 (1990) 6181.
[22] Y. Malsumoto, J. Hombo, Y. Yamaguchi. M. Nishida and 4. Chiba, Appl. Phys. Lctl. 56 (1990) 1585. [23] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250.
Physica C 178 ( 1991 ) 64-70 North-Holland
Solubility of Ag in YBa2Cu306 +.v and its effect on superconducting
properties A n u p K. G a n g o p a d h y a y a n d T.O. M a s o n Northwestern University, Department of Materials Sctence& Engineering. Mc('ormick School olt:'ngineering and Apphed Science, Evanston, IL 60208, USA
Received 10 April 1991 Revised manuscript received 13 May 1991
In order to determine the solid solubility of Ag in YBa2Cu306+v and its effect on superconducting properties, a series oi" YBa2Cu3_ ~AgxO6+v samples (x_< 0.75 ) have been prepared and characterized by X-ray diffraction, optical microscopy, and electron microprobe analysis. The central results arc that Ag goes into solid solution up to about x=0.06 and that initially (up to x=0.035 ) superconducting properties are improved - T,. goes up by 1.1 K, intragrain critical current density is marginally improved and Meissner effect is considerably enhanced. However, at higher x all the superconducting propcrties are considerably degraded. The implication of these results on the usc of Ag as a cladding material for 123 or in a composite arc discussed.
I. Introduction Since the discovery o f high t e m p e r a t u r e oxide superconductors, Ag has been extensively used either as a d o p a n t or as an a d d i t i v e ( c o m p o s i t e s ) in YBa2Cu306+.~ (123) to i m p r o v e its superconducting properties. On the other hand, due to its relatively low reactivity with 123 at the usual processing temperatures and fast diffusivity o f oxygen, Ag is commonly e m p l o y e d as a cladding material for shaping 123 in the form o f wires, tapes, etc. Several beneficial effects o f Ag-123 composites such as lower normal state resistivity [ 1-3 ], lower contact resistivity, modest transport critical current density (J,.) enhancement [1,2,5], i m p r o v e d mechanical strength [ 6 ], and better corrosion resistance against moisture and CO2 [7], have been cited. However, c o m p a r a tively fewer studies have been m a d e using Ag as a d o p a n t and quite often the published results are in a p p a r e n t contradiction. F o r example, K a o et al. [ 8 ] reported an e n h a n c e m e n t in Jc up to about 20 at.% substitution o f Cu by Ag without any change in the superconducting critical t e m p e r a t u r e T,.. In contrast, M a t s u m o t o et al. [9] observed a modest 2.5 K Tc e n h a n c e m e n t in (YBa2Cu3Oy)o.7Ago.3 and slightly lower in YBa2Cu3_,AgxO,, system for x = 0 . 3 with
some enhancement o f J~. However, in these studies [8,9] no direct a t t e m p t has been m a d e to establish the a m o u n t o f Ag that really goes into solid solution. Therefore, at present it is not at all clear as to what extent the change in the observed superconducting properties represents intrinsic behavior (effect o f doping) vis-a-vis changes in the intergranular regions. Additionally, due to the wide scale use of Ag as cladding material in the fabrication o f 123 superconductors, it is extremely i m p o r t a n t to know the reactivity and solid solubility o f Ag in 123. To shed some light on these issues, we have carefully prepared several polycrystalline specimens of starting c o m p o s i t i o n s YBa2Cuy_,Ag,-O6+, ( x < 0 . 7 5 ) and characterized them by X-ray p o w d e r diffraction, optical microscopy and quantitative wavelength dispersive electron probe micro-analysis ( E P M A ) . Various properties such as diamagnetic onset t e m p e r a t u r e (T,.), Meissner effect (field cooling), and intragranular J,. at 5 K have been measured as a function o f increasing doping ( x ) . F r o m these data the solubility limit o f Ag in 123 could be clearly established and the influence o f doping on the intrinsic superconducting properties determined.
0921-4534/91/$ 03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved
A.K. Gangopadhyay, T.O. Mason/Solubility of Ag in YBa2Cu306+y 2. Experimental
The samples were prepared by solid state reaction of high purity (99.9%) Y203, BaCO3,CuO and Ag20 using two different routes. In the first method, appropriate compounds of Y, Ba, Cu and Ag were thoroughly hand mixed in the desired ratio and calcined at 880°C (32 h) in air with one intermediate grinding. The product was reground and recalcined at 900°C (72 h) in flowing O2 with three intermediate grindings and furnace cooled at 0.3°C/min to room temperature. Finally, they were pelletized and sintered at 940°C (4 h) in flowing 02 and reoxygenated at 450°C (24 h). In the second method, Ag was added after the first calcination of YBa2Cu3_xO6+y at 880°C (32 h) in air. Subsequent heat treatments were identical to method 1. Surprisingly, samples prepared by the second method were of higher phase purity and showed superior superconducting properties. Although the reason for this difference is not clear yet, it is possible that the 123 phase can tolerate small amount of Cu deficiency which can in turn facilitate the incorporation of Ag into the solid solution in the second method. Whatever the reason, results presented here are for the samples prepared by the second method. The sintered pellets were characterized by X-ray powder diffraction using Cu Kct radiation. Samples for optical microscopy and electron microprobe analysis (JEOL superprobe 733 ) were polished down to 0.3 ~tm using kerosene as a lubricant. For quantitative analysis, large grained stoichiometric 123 was used as a standard for Y, Ba and Cu, whereas AgBiS was used for Ag. A 15 keV electron beam with 1 ~tm spot size excited the characteristic X-rays. Using "ZAF" corrections the concentration of Y, Ba, Cu and Ag were determined in the samples under study. No attempt was made to determine the 02 content of the present samples. Magnetic properties of the normal and superconducting states were measured by a superconducting quantum interference device magnetometer (SQUID, Quantum Design Inc.) equipped with a 5.5 T superconducting magnet.
65
3. Results and discussion
3.1. X-ray powder diffraction For x < 0.18, all the lines in the X-ray powder diffraction pattern could be accounted for by the orthorhombic 123 phase. However, for x>0.18, BaCuO2 (011 ), Y2BaCuO5 (211 ) and metallic Ag were detected as impurity phases in growing amount. No other phase such as CuO, BaCO3 or Y2Cu205 could be detected up to the highest value of x ( = 0 . 7 5 ) studied in the present investigation. Since we are replacing Cu by Ag, as long as Ag goes into the solid solution, no impurity phase should be expected. As soon as the solubility limit is exceeded, the situation becomes similar to that of Cu-deficient 123 composition. A look at the triangular region bounded by 123-211-011 in the phase diagram of Y ~.50-CuO-BaO [ 10] suggests that deficiency in Cu should give rise to 211 and 011 as impurity phases and the amount of impurity phases will be determined by the deviation from stoichiometry of Cu, unless the phase diagram is significantly modified in the presence of Ag. Therefore, the presence of only 211 and 011 as impurity phases at higher x would imply that the solubility limit of Ag in 123 has been exceeded. Thus, only from the X-ray data the solubility limit appears to be around x = 0.18. As will be shown in the next sections, this is an overestimate caused by the relatively lower X-ray scattering factors of 211 and 011 phases compared to 123 [ 11 ] and due to some additional complications.
3.2. Optical microscopy Optical microscopy on the polished section of different samples revealed some interesting features. For x > 0.09, Ag could be distinctly observed as metallic precipitates which increase in amount with increasing x. This would indicate that the solubility of Ag in 123 is x<0.09. However, as will be shown in the next section, this along with the X-ray data provide an overestimate. As shown in fig. 1, addition of Ag has some interesting effects on the average grain size determined by the linear intercept method. Although the grain size passes through a maximum for x=0.06, the overall change in grain size with composition is min-
66
A.K. Gangopadhyay. "ll O. Mason/SoluhtliO' q#..l~ in }'Ba ?( 'u ~(),, ~ ,
imal. This result is essentially different from the anomalous grain growth observed in Ag-123 composites by several workers [ 12,13 ]. As mentioned in the previous section, since above the solubility limit impurity phases should start appearing, sintering and grain growth processes can be adversely affected by the presence of these phases at the grain boundaries. Such a decrease in grain size has been reported in the earlier studies [14,15] on Cu-deficient 123 superconductors. On the other hand, Ag solubility in 123 can be expected to influence the grain boundary mobility and hence the grain growth characteristics. This could account for the initial increase in grain size with composition in fig. 1. 3.3. E l e c t r o n
microprobe
analysis
In order to determine the actual compositions of 123 in the samples, quantitative wavelength dispersive electron probe microanalysis (EPMA) was performed. A large number of grains of different morphologies were selected at random for this purpose. The cation concentrations (Y, Ba, Cu and Ag) were determined using appropriate "ZAF" corrections. For x < 0.06, we were unable to detect the presence of any impurity phases in the doped material. Only for higher values o f x were 211,011 and metallic Ag impurities detected. Y, Ba and Cu were found to be in the ratio of 1 : 2 : 3 as expected in the 123 phase. Deviation from these ratios never exceeded by more than 3% which is within the statistical uncertainties.
I
89
'
'
~'a'o ~:o;_x,X, gxO6i,
'
'
't
-0.1
L
* x =0. ~:31
X
T (K) '
NomLne,
o'
I
Ag
'
'
Cone
b . b d
.
(X)
Fig. 1. Electronprobe microanalysis (EPMA) Agcontents in 123 and grain size vs. dopingin YBa2Cu3_rAg~O6+v.
The Ag content inside the YBa2Cu~ ,Ag,.O~,+,.grains was determined by taking data on at least 20 different grains in each sample. In fig. 1 are plotted the analyzed Ag content against the nominal concentration .v. The analyzed concentration increases approximately linearly with nominal .r and then saturates around the value of x=0.06. Therefore, we conclude that x=0.06 is the solubility limit of Ag in 123. The disturbing feature of the EPMA results is that the analyzed concentration is always much smaller than the nominal one and the slope of the line is much smaller (0.2) than the expected value of 1. One obvious explanation would be an error in the microprobe analysis resulting from the use of an improper standard for Ag. To check this, we have used two different kinds of standards, namely, a Cu-rich phase (2.9 at.% Ag in Cu) in a Cu-Ag eutectic alloy and the compound AgBiS. Both the standards yield nearl) the same results. A second possibility is that equilibrium had not yet been achieved, i.e. Ag had not been fully incorporated into the 123. Measurements on a different batch of samples reacted for a longer time (up to 5 days) were not statistically different than the earlier time results. Furthermore, the relatively small error bars in fig. 1 (and small grain-tograin variation) argue against inhomogeneous mixing of Ag in the microstructure which might lead to kinetic problems and point-to-point variations in 123 Ag content. Therefore, although unexpected, this discrepancy between measured (123) and nominal Ag content does not appear to be an experimental artifact. The central questions are, why even below the solubility limit all the Ag is not incorporated into the solid solution and where the remaining Ag is in the microstructure. As mentioned earlier, for .r> 0.06, we did observe the presence of metallic Ag both in optical microscopy as well as in EPMA and the presence of211 and 011 phases in the microprobe. Since the starting composition is Cu-deficient, if all the Ag is not incorporated into the solid solution, 211 and 011 phases would naturally occur as impurity phases along with metallic Ag. It is relevant to mention that we observed considerable amount of Ag ( ~ 3 at.%) inside the 011 grains: whereas no appreciable (at least an order of magnitude lower than in 01 l phase) Ag signal could be detected in the 211 grains. Therefore,
A.K. Gangopadhyay, T.O. Mason/Solubility of Ag in YBa2Cu306+y
even below the solubility limit only part of the Ag goes into solid solution in 123; the rest occurs as metallic Ag and some goes into the 011 phase. Transmission electron microscopy studies of an x = 0 . 6 sample confirmed the presence of Ag as a separate phase. We therefore conclude that Ag is present as a distinct phase at all compositions studied. (The presence of second phases at virtually all compositions may account for the negligible overall grain size versus composition dependence in fig. 1.) A few other interesting observations deserve mentioning. When the composition of the Ag phase was analyzed, substantial amount of Cu (1 to 3.5 at.%) was always detected. This signal cannot be from the background since the corresponding signals for Y and Ba were always an order of magnitude lower. This is consistent with a recent observation [ 16] that AgO eutectic liquid forms above 931°C and dissolves Cu from 123 leading to the decomposition of 123 into 211 phase. This result has some important implications on the use of Ag in high concentrations in composites or in the use of Ag as cladding material. It has been observed by several workers [ 1,2,5,6 ] that the transport critical current density is substantially reduced at higher concentrations of Ag in Ag-123 composites. Although several explanations have been put forward, it seems quite likely that among other things, the leaching action of Cu by Ag-O eutectic liquid can be very detrimental to transport critical current density. 3.4. Superconducting properties
In fig. 2 are shown both the diamagnetic onset temperature and the temperature at which 10% of the Meissner signal is attained in a field cooling experiment in a field of about 7 0 e on various samples. The inset shows the onset of diamagnetism on an enlarged temperature scale. The most important observation is a small (1.1 K) but distinct enhancement of T~ due to Ag doping. It should be emphasized that Tc in the undoped system, even under optimal 02 loading, never reaches such a high value of 93.8 K. Therefore, we ascribe the Tc enhancement to the doping effect. We recall that such enhancement in Tc was reported earlier [9]. However, it never exceeded 93 K and since the superconducting transitions were rather broad, it was not very apparent that
~0.06
67
........................
o
5
5
c
~
i4 2°
e4
3co O
N0- 1~2
2
>,,
--
- .... .....
Gretn AQ
SLze Conc.
e
(,_
(D 1 >
<
NomLnml
Ag
conc,(X)
Fig. 2. Diamagnetic onset and 10% Meissner signal values of T¢ in YBa2Cua_xAgxO6+ v.
the enhancement was due to doping. Moreover, the maximum in Tc was observed for x=0.3, which is well above the solubility limit of Ag in 123. Therefore, the knowledge of the Ag content along with the good quality of the materials in the present study, prove unambiguously that Ag doping enhances To. However, the increase is not monotonic. After reaching a plateau for 0.06 < x < 0 . 1 8 , Tc drops for higher x even before the solubility limit is reached. This behavior is somewhat reminiscent of the well known [ 17 ] Tc versus hole concentration curves of various high-To oxide superconductors. A recent high pressure study on 123 [ 18] has also shown a maximum in T~ as a function of pressure. Drawing a parallel from these studies [ 17,18 ] it seems that doping with Ag increases the hole concentration which in turn enhances To. This seems logical since Ag is normally monovalent and addition of Ag should increase the hole concentration, unless the oxygen content is reduced. Additional supporting data, especially oxygen content, are required to prove the validity of these conjectures. Whatever the mechanism, we would like to point out that Ag and Au [ 19 ] are the only two known dopants which can enhance Tc in 123. Another interesting effect of Ag addition is the initial increase of the Meissner volume fraction (up to x = 0.12) followed by a decrease at higher x as shown in fig. 3. It should, however, be remembered that the Meissner volume fraction in high-T~ oxide superconductors not only depend on the actual superconducting volume fraction, but also on the flux pinning
68
A.K. Gangopadhyay, 7~O. Mason /So/ubtlit ~' ol,.lg in YBae('u ¢0,, ~,
8. I
-0.1
.......... e. es . . . . .
x = O . {36
. . . . .
x
=0.09
. . . . . x=e. le
,, ~ ....
-8,7
., ~
""
ie
,,,)g,, 5b'" TEMPERATURE
( K)
Fig. 3. Meissner effect (field cooling) in 7 0 e magnetic field for various compositions (.v) ofYBa2Cu> ,Ag,O6+,..
properties of the material and on the average grain size (the ratio of the average grain size and the London penetration depth). Even in single crystals and at small applied magnetic fields, flux expulsion is never complete ( < 100%). It has been suggested [ 20 ] that because of the existence o f the so called "'irreversibility line" below itc, in a field cooling experiment, flux can be expelled only down to the temperature of the "irreversibility line" for the given field. Thus, if the "irreversibility line" is shifted towards lower temperatures (due to weakening of the flux pinning) as a result of doping, Meissner volume fraction would be enhanced. But then, one should also observe a decrease in the intragrain critical current density. As will be shown in below, this is surely not the case and hence the above explanation does not hold. Also the variation of grain size from sample to sample is not so dramatic as to account for the change in the Meissner signal. However, in granular materials, especially at low magnetic fields, one should also consider the flux pinning by the intergranular regions. Our preliminary TEM studies suggest that even below the solubility limit, some Ag occurs as a grain boundary phase. This, in turn, can change the pinning behavior of the grain boundaries. Therefore, the enhanced Meissner signal may not be an intrinsic (intragrain) property and may reflect a decrease in the pinning energy o f the intergrain regions. This idea is supported by a recent measurement [ 21 ] o f the intergranular pinning energy from
flux creep measurements in low fields in AgYBa2Cu30~,+, composites. It clearly shows a decrease in the intergranular pinning energy due to Ag addition. However, it is important to point out that the above result is not at all inconsistent with the observed [1,2,5] initial increase in transport (intergrain) critical current density in YBa2Cu306+,. superconductor due to Ag addition. Since transport J,: is determined by the intergranular weak-links, the observed J~, enhancement is more likely related to improved weak-links (possibly caused by cleaner grain boundaries [2,22] ) rather than any change in the grain boundary pinning behavior. In contrast, the decrease in the Meissner signal at higher x could be a result of poor superconducting properties as is evident from the diminishing sharpness of the transition as well as decreasing T,.. In fig. 4, we show the intragrain ( intrinsic ) critical current density (Jc) at 5 K for several samples as a function of applied magnetic field. J¢. was calculated from the Bean's formula [23] J~.=3OAM/d, where AM is the irreversible magnetization and "'d" is the diameter of the particles. AM was determined from the hysteresis curve and the average grain size was taken for "d". Some interesting points are apparent from fig. 4. Up to x = 0 . 1 2 , J~ remains nearly unchanged in the doped system; if at all, over this composition range, there is a slight enhancement over the
@
7
. . . . . . . . . . . .
, ....
, . . . . . . . . .
, .....
" - , ~ -" -" :.: = g , ..... ..... ::~:: .....
gO
x=0.06 x=g. 12 x=0.18 x =g. 24
E O
\ <5
,%
e.e
.....
....
....
s'.e
....
8
H (T) Fig. 4. M a g n e t i c ( B e a n ' s m o d e l ) i n t r a g r a i n critical c u r r e n t density at 5 K as a f u n c t i o n o f c o m p o s i t i o n (x) in YBa2Cu3 _ ,AgxO6 + ,,.
A.K. Gangopadhyay, T.O. Mason/Solubility of Ag in YBa2Cu306+y
undoped material. For x > 0.12, Jc drops. Interestingly, the concentration dependence of Jc mimics the behavior of Tc. Also noteworthy is the increasing magnetic field dependence of Jc for x > 0.12 which is consistent with the degradation of the superconducting properties as reflected in the diminishing sharpness of the transition (fig. 3). Thus all the data are mutually consistent in the sense that up to about x=0.12 (analyzed x = 0 . 0 3 5 ) the superconducting properties of the doped system are somewhat improved followed by a degradation at higher x.
4. Conclusions In summary, the present work determines for the first time the solubility of Ag in 123. The Ag solubility is about 2 at.% with respect to Cu ( x = 0 . 0 6 ) and is much smaller than assumed in earlier works [8,9]. As a result of doping, superconducting properties exhibit some changes. In particular, Tc shows about 1.1. K enhancement over that of the undoped material. The intrinsic (intragrain) Jc is marginally enhanced. Since this is accompanied by an increase in the Meissner effect, we suggest that doping somewhat weakens the flux pinning properties of the intergrain regions (not that of the grains). At higher x (analyzed x > 0 . 0 3 5 ) , the intragrain superconducting properties are considerably degraded. Additionally, for higher levels of doping (well above the solubility limit) we observed some Cu dissolved (1 to 3.5 at.%) into the metallic Ag phase. This is consistent with a recent study [ 16 ] where Ag has been observed to selectively leach Cu from 123 resulting in its decomposition. This result along with our study of the doped system suggests that Ag can be used beneficially only for small levels of doping. At higher concentrations, it can considerably degrade both intra- and inter-granular properties. Our observations of variable Ag content in 123 along with the presence of four total phases (Ag, 123, 211, and 011 ) over the nominal composition range 0 < x < 0 . 2 suggests that the phase relationships are more complex than originally thought. Current work is aimed at establishing the multicomponent phase relationships in this system.
69
Acknowledgements This work was supported by the Electric Power Research Institute under grant no. RP7911-8. The authors are grateful to D.L. Johnson of Northwestern and J.P. Singh of Argonne National Laboratory for useful discussions and to V.P. Dravid of Northwestern for assistance with the T.E.M. work.
References [ 1 ] B. Dwir, M. Affronte and D. Pavuna, Appl. Phys. Left. 55 (1989) 399. [2IA.K. Gangopadhyay, Y.T. Chou and H. Jain, Paper 123SIII-90 at the Am. Cer. Soc. Mtg., Dallas, TX, 22-26 April, 1990. [ 3 ] V. Plechacek, V. Landa, Z. Blazek, J. Sneider, Z. Trejbalova and M. Cermak, Physica C 153-155 (1988) 878. [4] S. Jin, M.E. Davis, T,H. Tiefel, R.B. Van Dover, R.C. Sherwood, H.M. O'Bryan, G.W. Kammlott and R.A. Fastnacht, Appl. Phys. Len. 54 (1989) 2605. [ 5 ] N. Irnanka, F. Saito, H, Imai and G. Adachi, Jpn. J. Appl. Phys. 28 (1989) L580. [6] J.P. Singh, H.J. Leu, R. B. Poeppel, E. Van Voorhees, G.T. Goudey, K. Winsley and Donglu Shi, J. Appl. Phys. 66 (1989) 3154. [ 7 ] Chin-An Chang and J.A. Tsai, Appl. Phys. Lett. 53 ( 1988 ) 1976. [8] Y.H. Kao, Y.D. Yao, L.Y. Jang, F. Xu, A. Krol, L.W. Song, C.J. Sher, A. Darovsky, J.C. Phillips, J.J. Simmins and R.L. Snyder, J. Appl. Phys. 67 (1990) 353. [9] Y. Matsumoyo, J. Hombo and Y. Yamaguchi, Mater. Res. Bull 24 (1989) 1231, and references therein. [ 10] R.S. Roth, J.R. Dennis and K.C. Davis, Adv. Ceram. Mater. 2 (1987) 303. [ 11 ] T. ltoh, M. Uzawa and H. Uchikawa, J. Am. Ceram. Soc. 71 (1988) C188, [ 12] S. Baliga and A.L. Jain, Appl. Phys. A49 (1989) 139. [13]T.H, Tiefel, S. Jin, R.C. Sherwood, M.E. Davis, G.W. Kammlott, P.K. Gallagher, D.W. Johnson Jr., R.A. Fastnacht and W.W. Rhodes, Mater. Lett. 7 (1989) 363. [ 14] A.K. Gangopadhyay and H. Jain, Paper 142-SIII-90 at the Amer. Cer. Soc. Mtg., Dallas, TX, 22-26 Apr. 1990. [ 15] D.M. Kroeger, L Mater. (1989) 14. [16] R.E. Loehman, A.P. Tomsia, J.A. Pask and A.H. Carim, Physica C 170 (1990) 1. [17] J.B. Torrance, A. Bezinge, A.I. Nazzal and S.S,P. Parking, Physica C 162-164 (1989) 291. [ 18 ] S. Klotz, W. Reith and J.S. Schilling, Physica C 172 ( 1991 ) 423, [19] M.Z. Cieplak, G. Xiao, C.L. Chien, J.K. Stalick and J.J. Rhyne, Appl. Phys. Lett. 57 (1990) 934.
70
A.K. Gangopadhyay, 7~O. Mason/Solubility ()f,4,~ m YBa2( "u~O~ ,
[20] L. Krusin-Elbaum, A.P. Malozemoff, Y. Yeshurun, D.C. Cronemeyer and F. Holtzberg, Physica C 153-155 ( 1988 ) 1469. [21]J. Jung, M.A.K. Mohamed~ S.C. Cheng and J.P. Franck, Phys. Rev. B 42 (1990) 6181.
[22] Y. Malsumoto, J. Hombo, Y. Yamaguchi. M. Nishida and 4. Chiba, Appl. Phys. Lctl. 56 (1990) 1585. [23] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250.