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Melt processing of alkali element doped Bi2Sr2CaCu2O8
Physica C 172 ( 1 9 9 0 ) 2 9 5 - 3 0 3 North-Holland
Melt processing of alkali element doped Bi2Sr2CaCu208 S.X. Dou, H . K . Liu, W.M. Wu, W.X. W a n g and C.C. Sorrell School of Materials Science and Engineering, UniversiO, of New South Wales, PO Box 1, Kensington, NSW 2033, Australia
R. W i n n and N. Savvides CSIRO Division of Applied Physics, PO Box 218, Lindfield, NSW 2070, Australia Received 29 May 1990 Revised manuscript received 12 October 1990
The effect of alkali element doping on the superconducting properties of the B i - S r - C a - C u - O system has been investigated. It was found that a T¢ of 94 K and To of 90 K in alkali element doped Bi2Sr2CaCu208 were achieved through the use of a meltprocessing technique. XRD, SEM and EDS examinations showed that the samples did not contain the high-To phase Bi2Sr2CazCu30~o. it is argued that the mechanism for the enhancement of the Tc through alkali element doping derives from a fluxing action, which provides a fast diffusion path for Ca and Cu to the superconducting phase, resulting in the desirable stoichiometry of Bi2SrzCaCu2Os. On the other hand, the substitution of alkali elements in the superconducting phase may give rise to a reducing effect of the oxygen content, leading to the enhancement of the To. The latter mechanism may be compounded with the fluxing action. Alkali elements were found to be excellent sintering aids for processing Bi-based superconductors since they enhance the 7~ and promote grain growth and grain aligment in the bulk materials.
1. Introduction It has been established that there are three superconducting phases in the B i - S r - C a - C u - O system: Bi2Sr2CuO6 (2201) with a T~ up to 20 K I l l , Bi2Sr2CaCu2Os (2212) with a T~ near 85 K [2-4] and Bi2Sr2Ca2Cu3Oio (2223) with a Tc ncar l l0 K [2,4-6]. Single phase 2212 has been prepared with the 7"~ varying from 70 to 92 K depending on the processing conditions [ 7-9 ]. The Tc of 2212 can be raised to 92 K by quenching from high temperature or treating in a low oxygen partial pressure [9]. Recently, an improvement in the T~ of 2212 by doping with Li has been reported by Kawai and coworkers [8]. They found that, in Li-doped Bi-SrC a - C u - O samples, the sintering temperature was lowered, while the transition temperature was raised and the volume of superconducting phase was greatly increased. However, it is not clear whether Li substituted for Cu owing to the difficulty of precise analysis of Li in the superconducting phase. Further, these authors found that K depressed the T~ significantly, and Na did not affect the T¢.
In the present work, similar results for Li doping were obtained. However, the results for K and Na doping are considerably different in that K and Na were found to have a beneficial effect on the T¢ of 2212. It was found that the 92 K transition was reproducible within a wide range of melting conditions, from 8 l0 to 890°C and a sintering time as short as 5 min at the higher temperatures. The effects of oxygen partial pressure, heat treatment procedure and initial composition on the superconducting properties are discussed. A possible mechanism for the enhancement of T¢ is proposed.
2. Experimental procedure Mixtures of Bi203 (99.9%), SrCO3 (99.5%), A2CO3 (99%) (where A = L i , Na, or K), CaCO3 (99%) and CuO (99.9%) in appropriate proportions were ground with a mortar and pestel and calcined at 720°C for 12 h'and 780°C for 10 h. The calcined powders were then pressed into pellets and sintered or partially melted in Ag boats at 740 to
0921-4534/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
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s.x. Dou et al. / Melt processing af alkali element doped Bi2Sr2CaCu208
890°C for 5 min to 15 h. One set o f samples with the same c o m p o s i t i o n was melt-processed at 830°C in varying o x y g e n / n i t r o g e n a t m o s p h e r e s in order to study the effect o f oxygen partial pressure. Quenching and slow cooling from the sintering t e m p e r a t u r e to room t e m p e r a t u r e were also applied to samples with the same compositions. The electrical resistivity was measured by the standard four-probe DC technique [ 10]. The AC magnetic susceptibility was measured by a mutual inductance m e t h o d at a frequency o f 83 Hz [11]. Microstructurai and c o m p o s i t i o n a l analyses were performed with a J E O L JSM-840 scanning electron microscope ( S E M ) e q u i p p e d with a Link Systems AN 10000 energy dispersive spectrometer ( E D S ) . Xray p o w d e r diffraction patterns were o b t a i n e d with a Philips type P W 1 1 4 0 / 0 0 p o w d e r diffractometer using CuKct radiation and scanning rate o f 2 0 = 1 ° / min. Chemical analyses were carried out with a Labtam International Plasmalab inductively coupled plasma a t o m i c emission spectrometer.
3. Results and discussion Table I lists the starting compositions, To, and To for samples treated for varying temperatures, times, and cooling rates. It is seen that Li, Na and K raised the Tc o f 2212 to 90 K under the desirable condi-
tions, contrary to previous results [8], in which only Li raised the T¢ while Na and K depressed the 7"~.. Figure 1 shows normalised resistivity-temperature plots for samples with the initial cation ratio o f B i : S r : C a : C u : L i = 2 . 2 : 1.8: 1.05: 1.45:0.7, which were sintered at 740 and 770+C for 15 h and at 850°C for 1 h. It may be noted that, for samples heat-treated at < 800°C, the T¢ was depressed below that o f the 80 K phase Bi2Sr2CaCueO8 (2212 ), while the Tc was enhanced to 90 K for the partially melted samples processed by a wide range o f heat t r e a t m e n t temperatures from 810 to 890°C. Quenching Li-doped samples from a sintering t e m p e r a t u r e o f 830°C (15 h) to room t e m p e r a t u r e in air resulted in s e m i c o n d u c t o r behaviour, although a small transition between 88 and 100 K can be seen in fig. 2. This may be c o m p a r e d with an unquenched sample treated at the same t e m p e r a t u r e for the same period o f time and cooled at a rate o f 1 0 ° C / h to 790°C and thereafter at 6 0 ° C / h to room temperature; here, the To= 90 K and To= 84 K. Quenching all samples from a sintering, t e m p e r a t u r e o f 740°C to r o o m t e m p e r a t u r e had no effect on the transition. Figure 3 shows r e s i s t i v i t y - t e m p e r a t u r e curves for samples with the cation ratio Bi:Sr:Ca:Cu:A=2.2:l.8:l.05:(2.15-x):x, where A = L i , x = 0 . 7 ; A = N a , x = 0 . 6 ; and A = K , x = 0 . 7 . The heat treatment conditions for these samples were as follows. The Li-doped sample was sintered at
Table 1 Compositions, Tc, and To for alkali element doped samples treated under various conditions. Composition
Sintering
Cooling rate (°C/h)
7++. (K)
1o (K)
Temp. (+C)
Time (h)
830 830 830 830
1 1 1 1
10 10 10 10
1.0 0.21 0.05 N2
86 90 90 92
70 86 86 88
Bi:Sr:Ca:Cu:Na 2.2: 1.8: 1.05:1.85:0.3 2.2: 1.8: 1.05:1.55:0.6
885 885
0.08 0.08
60 60
0.21 0.21
92 94
88 90
Bi:Sr:Ca:Cu:K 2.2: 1.8: 1.05:1.45:0.7
850
10
60
0.21
92
88
890
I
60
0.21
84
80
Bi:Sr:Ca:Cu:Li 2.2:1.8:1.05:1.45:0.7
Undoped Sample
Bi:Sr:Ca:Cu:A 2.2: 1.8:1.05:2.15:0.0
297
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830°C for I h in nitrogen and cooled at a rate of 1 0 ° C / h to 790°C and then at 6 0 ° C / h to room temperature. The Na-doped sample was treated at 885°C for 5 m i n in air and cooled at a rate of 6 0 ° C / h to room temperature. The K-doped sample was treated at 850°C for 10 h and cooled at a rate of 6 0 ° C / h to room temperature. It can be seen that Na raised the T,. to 94 K and 7"()to 90 K, while K and Li raised the T~ the same degree, where the T c = 9 2 K and T o = 8 8 K. The u n d o p e d standard sample, which was partially melted at 8 9 0 ° C for I h, showed a T c = 8 4 K and T o = 8 0 K. These results are further confirmed by the AC magnetic susceptibility measurements, as shown in fig. 4. It is clear that Na, K and Li doping increased the 7~. of the 2212 phase relative to the undoped sample. Figure 5 shows photomicrographs of Li- and Nadoped samples. The superconducting phase consisted of elongated plate-like grains that exhibited a certain degree of orientation owing to the partial melting during sintering. EDS analyses revealed that these samples consisted of multiphase assemblages.
150
200
250
iO0
(K}
Temperolure
(K)
Fig. I. Effect on the transition of sintering temperature for Lidoped samples, sintered at 740°C and 770°C for 15 h and 850°C for 1 h in air.
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Fig. 2. Effect on the transition of cooling rate for Li-doped samples quenched from 830:C in air and slowly cooled from 830°C in air.
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The major phase in the Liodoped samples had an average composition of B i : S r : C a : C u = 2 . 2 : 1.7:1.0:2.0 (grey in fig. 5(a) ), while the minor phases included B i : S r : C a = 3 . 4 8 : 1.2:1.53 (white in fig. 5 ( a ) ) and B i : S r : C a : C u = 2 . 2 : 1 . 3 : 0 . 4 1 : 1 . 0 (light grey in fig. 5(a)). The Na-doped sample (fig. 5 ( b ) ) c o n s i s t e d of a lower amount of impurity phases than in the Lidoped sample. No 2223 phase was detected in either sample. This is further confirmed through EDS analyses on whiskers grown in alkali element doped samples. It was often found that a cavity was formed in the partially melted B i - S r - C a - C u - L i - O samples, where crystal growth occurred (fig. 6). With a cooling rate of I °C/min from 870 to 800°C, a clump of whiskers of the 2212 composition was formed. No 2223 whiskers were detected. Since alkali elements have a + l valence state, then their substitution for any elements in the B i - S r - C a Cu-O system would reduce the amount of oxygen in
Fig. 5. Microstructures of (a) Li-doped and (b) Na-doped sampies. The heat treatment conditions are the same as in fig. 4.
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Fig. 6. Morphology of whiskers in Li-doped samples.
the structure or create holes on the Cu-O planes owing to charge balence requirements. Therefore, if substitution occurs, it would be expected that the dependence of the 7~. upon the oxygen partial pressure
S.X. Dou et aL / Melt processing of alkali element doped BieSr2CaCu20s
during heat treatment will be different from that of the undoped samples. Figure 7 shows the temperature dependence of normalised resistivity for samples with the nominal composition Bi2.2Sr~.sCa~.osCut.asLio.7Oa_.v treated at 830°C for 15 h in flowing N2, 02 and air. It may be seen that the sample treated in N2 showed a transition at 92 K with zero resistance at 88 K, while the oxygen-treated sample showed semiconductor behaviour before the superconducting transition and did not reach zero resistance until 70 K. The effect of oxygen partial pressure on the T~ for Na-doped samples is less pronounced than that for Li-doped samples, as shown in fig. 8. A pellet of Na-doped sample was cut into four pieces, which were then treated at 750°C for 15 h in flowing 02, N2, 0.1 arm 02, and air (0.209 arm 02). It is seen that only the oxygen treatment depressed the T,. by 5 K, which is much less pronounced than in the case of the Lidoped sample. The results for N2, 0.1 atm 02, and air are similar, although the 0.1 arm 02 treatment gave slightly higher Tc than the others. The difference in the oxygen pressure effect between the Li-
299
doped and Na-doped samples may indicate the difference in the extra oxygen content in the two samples. It is possible that Li and Na may substitute on different sites in the superconducting phase. The oxygen treatment may result in an increase in the oxygen content in the Bi-O layers, enlarging the Bi-Bi separation and increasing the resistance in these layers [12,13]. On the other hand, low oxygen partial pressure or pure nitrogen is desirable in order to obtain the optimal oxidation states.
4. Discussion The enhancement of the T¢ through alkali element doping may be attributed to three factors: the presence of the high-To phase (2223), the fluxing action of the alkali elements, and the substitution of the alkali elements in the superconducting phases. In the following, the three possible mechanisms for the T¢ enhancement are discussed.
4.1. Absence of the 2223 phase in alkali-doped samples
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150
200
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Temperature (K) Fig. 7. Effect on the transition of sintering atmosphere for Lidoped samples treated at 830~C for 1 h in oxygen, air and nitrogen.
Since Bi-based materials form a series of superconducting phases, which are often found in an intergrowth pattern, the enhancement of the Tc may be attributed to the presence of small amounts of the 2223 phase. Figure 9 shows the X-ray diffraction patterns for alkali element doped samples. The XRD data indicate that all samples consisted of 2212 as the major phase. The minor phases are 2201 and some impurity phases including SrCaCu204, CuO and (Li, Na, K ) - B i - S r - C a - O . However, no 2223 phase was detected. This is agreement with the Tc and susceptibility measurements, which show a single transition. From the XRD results, it is believed that the enhancement in the Tc for the alkali element doped samples cannot be attributed to the presence of the high-To phase (2223). The absence of the highT~ phase in these samples may be explained as follows. (a) Experience has demonstrated that starting compositions with Ca and Cu content higher than in stoichiometric 2223 are essential for the formation of the high-To phase [7,14-17]. Thus, a starting composition near 2212 or one deficient in Cu rela-
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200
Temperature
(K)
250
500
Fig. 8. Effect on the transition of sintering atmosphere for Na-doped samples treated at 885:C for 5 min and post-annealed at 750~C for 15 h in flowingO2, N2, 0.1 atm 02, and air. rive to 2212 is not favourable for the formation of 2223. (b) It has been well established that the formation of the 2223 phase is a very slow process, in particular for samples without Pb doping. Annealing for more than l0 days is needed to reach a Tc o f 105 K [15]. It is, therefore, unlikely that the high-Tc phase can be formed with a sintering time of only 5 min. 4.2. Effect o f alkali element substitution
Table 1I gives the lattice parameters calculated from X-ray diffraction patterns for undoped and alkali element doped samples. It is seen that the lattice parameters for alkali element doped samples show a slight variation compared with the undoped sample. In particular, the c-axes decrease slightly with increasing dopant levels. This may be attributed to the incorporation of a small amount o f alkali elements in the superconducting phases. Li +, Na + and K ÷ have ionic radii o f 0.059 nm, 0.099 nm and 0.138 nm, respectively [5]. From dimensional considerations, if cation substitution occurs, it is most likely that Li ÷ substitutes for Ca 2÷ (0.062 nm), Na + substitutes for Ca -'÷ (0.1 n m ) and K ÷ substitutes for Sr 2÷ (0.116 nm). Since alkali element ions have a closed electron shell, it is unlikely that they can play an electronic role similar to that of Cu 2÷. However,
they could reduce the oxygen content in the structures because they are monovalent. It has been reported that a Tc of 92 K for the 2212 phase has been achieved by annealing in low oxygen partial pressures or by quenching [9]. The substitution of monovalent ions for divalent ions should have a similar reducing effect similar to that of low oxygen partial pressure treatment. Since Li is the lightest metallic element and it cannot be detected by EDS, it is difficult to determine whether Li is incorporated in the superconducting phase, Matsubara et al. [9] analysed the whiskers grown from Li-doped B i - S r - C a - C u - O and found that the whiskers contained 1-3 at% Li. They attributed the enhancement o f the Tc to the Li substitution. In the present work, EDS analysis of single crystals revealed that Na was incorporated in the superconducting phase. The results listed in table III are average values of 8 individual analyses on a TEM powder sample. K ÷ was also detected in the superconducting phase in the samples sintered for 10-30 h at 850~C, as listed in table III. It is interesting to note that the K content in the samples decreased with increasing sintering times, vanishing in the sample sintered for the longest period of time. although the Tc remained nearly unchanged at 90 K. Since the Tc was unaffected while K volatilised out of the samples, it is unlikely that the incorporation of K in the
S.X. Dou et al. /Melt processing of alkali element doped Bi;~r2CaCu20s I
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At this stage, it can be argued that the element of the Tc by alkali element doping may be attributed to liquid-phase sintering. This argument may be supported by the following factors. (a) Li20 and Bi203 form a eutectic at a temperature as low as 700cC at Li20/Bi203=0.1 [20]. This explains why low sintering temperatures could be used with Li additions. This liquid provides a fast diffusion path for Ca, Sr and Cu, which accelerates the reaction to form the superconductor. This is evident from the fact that exaggerated grain growth and alignment were achieved through liquid-phase sintering within a short period o f time (fig. 5). Na20 and K20 should have the same effect although the phase diagrams for K20-Bi203 and Na20-Bi203 are not available. (b) From table III, it is evident that the major phase (2212) in alkali element doped samples has a relatively high C a / S r ratio, indicating that Ca is enriched during melting due to assisted Ca diffusion through the liquid phase. In undoped samples, Ca is often depleted, resulting in low-To values [7]. The fact that the Tc was enhanced to the same degree by K, Na, and Li doping strongly suggests that the enhancement of the Tc may be attributed mainly to the liquid-phase sintering resulting from alkali metal additions. As K was added, the sintering temperature had to be lowered owing to the formation o f a liquid phase that accelerated the formation of 2212. During prolonged sintering, K was volatilised but its initial presence assisted in the formation o f pure 2212, giving rise to an enhancement in the To. Li and Na may
Table I1 Lattice parameters of the undoped and alkali element doped samples using CuKct radiation and a scanning rate of 2t9= 1°/min. Initial composition
Lattice parameters (nm)
Bi:
Sr:
Ca:
Cu:
A
a
b
c
2.2 2.2 2.2 2.2 2.2 2.2
1.8 1.8 1.8 1.8 1.8 1.8
1.05 1.05 1.05 1.05 1.05 1.05
21.5 1.45 1.85 1.55 1.65 1.45
0.0 0.7Li 0.3Na 0.6Na 0.5K 0.7K
0.541 0.539 0.541 0.538 0.540 0.537
0.541 0.540 0.541 0.545 0.542 0.540
3.086 3.084 3.084 3.080 3.078 3.068
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S.X. Dou et al. /Melt processing qf alkali element doped Bi,Sr eCaCueOs
Table III EDS analyses for alkali element doped Bi-Sr-Ca-Cu-O materials. L t-doped samples Bi: Sr: Ca:
2.16 2.80 2.11 0
1.83 1.80 2.96 1.97
1.04 0.64 1.58 1.1
Cu 1.90 1.38 0 4.87
Ca: 0.97 0.38
Cu: 2.00 0.32
Na 0.31 7.49
(2212) Na-rich phase
(2212) (2201)
Na-doped sample
Bi: 2.20 1.05
Sr: 1.62 1.37
K-doped samples
2.11 2.63
St: 1.73 1.60
Ca: 0.98 1.01
Cu: 1.68 1.22
K 0.88 0.45
(sintered at 850=Cfor10h)
2.26 2.71
2.16 2.07
0.66 0.44
1.67 1.27
0.24 0.27
(sintered at 850°C for 20 h)
2.26 2.91
1.94 1.87
0.95 0.40
1.93 1.36
0.00 0.00
(sintered at 850°C for 35 h)
Bi:
Li is undetectable by EDS analysis. Table IV Results of atomic emission spectroscopy analyses for Li- and Kdoped samples. Initial composition
Li or K content (ppm) Before sintering
After sintering
Bi: 2.20
Sr: 1.80
Ca: 1.05
Cu: 1.45
Li 0.70
42
45
Bi: 2.20
Sr: 1.80
Ca: 1.05
Cu: 1.45
K 0.70
187
2
play the same role as K. However, since Li is stable at higher temperatures, it will r e m a i n in the sample while K volatilises during sintering. Chemical analysis by a t o m i c emission spectroscopy have confirmed that only 1% o f the total K a d d i t i o n was present in the K - d o p e d sample after sintering at 850-'C for 35 h, whereas Li was present in the same a m o u n t in the initial and final compositions, as shown in table IV.
5. Conclusions In s u m m a r y , the e n h a n c e m e n t o f the Tc in alkali element d o p e d B i - S r - C a - C u - O through melt-processing is reported. Values for the 7"~ o f 94 K and To o f 90 K have been achieved in K-, Na- and Li-doped samples. The e n h a n c e m e n t o f the 7~ by doping with alkali elements is o f significant technical i m p o r t a n c e since they can be used as ideal sintering aids, providing the following advantages. ( a ) Enhancement o f the superconducting transition up to 92 K, without need o f treatment in low oxygen partial pressure; ( b ) acceleration o f the f o r m a t i o n o f the superconducting phase through transient liquid-phase sintering: (c) achievement o f grain alignment through melttexture growth; ( d ) p r o m o t i o n of single crystal growth through flux action: (e) no c o n t a m i n a t i o n o f the sample in the case o f K doping owing to K volatalization. It is tentatively concluded that the mechanism for the enhancement o f the T~ is by a fluxing action which provides a fast diffusion path for Ca and Cu to the superconducting phase, leading to the desirable 2212 stoichiometry. Thus, the intrinsic T~ for 2212 phase is as high as that o f YBa2Cu3Ov_x if a proper processing route is used. On the other hand, the substitution o f alkali element in the superconducting phase may give rise to a reducing effect o f the oxygen content, leading to an enhancement of the To. The latter m e c h a n i s m cannot be ruled out completely at the present. More work on structure refinement and the location of the alkali elements in the structure is needed.
Acknowledgements The authors are grateful to Metal Manufactures Ltd., and to the C o m m o n w e a l t h D e p a r t m e n t o f Industry, Technology and C o m m e r c e for financial support.
S.X. Dou et al. / Melt processing of alkali element doped Bi2Sr2CaCu20s References [ I ] C . Michael, M. Hervieu, M.M. Border, A. Grandi, F. Deslandes, J. Provost and B. Raveau Z. Phys. B68 (1987) 421. [2 ] H. Maeda. Y. Tanaka, M. Fukutomi and T. Asano, J. Appl. Phys. 27 (1988) L209. [31 R.M. Hazen, C.T. Prewitt, R.J. Angel, N.L. Ross, L.W. Finger, C.G. Hadidiacos, D.R. Veblen, P.J. Heaney, P.H. Hor, R.L. Meng, Y.Y. Sun, Y.Q. Wang, Y.Y. Xue, Z.J. Huang, L. Gao, J. Bechtold and C.W. Chu, Phys. Rev. Lett. 60(1988 1174. [4]M.A. Subramanian, C.C. Torardi, J.C. Calabrese, J. Gopalakrishnan, K.J. Morrissey, T.R. Askew, R.B. Flippen, U. Chowdhry and A.W. Sleight, Science 239 (1988) 1015. [5] J.M. Tarascon, Y. Le Page, P. Barboux, B.G. Bagley, L.H. Greene, W.R. Mckinnon, G.W. Hull, M. Giroud and D.M. Hwang, Phys. Rev. B37 (1988) 9382. [6] S.A. Sunshine, T. Siegrist, L.F. Schneemeyer, D.W. Murphy. R.J. Cava, B. Batlogg, R.B. Van Dover, R.M. Fleming, S.H. Glarum, S. Nakahara, R. Farrow, J.J. Krajewski, S.M. Zahurak, J.V. Waszczak, J.H. Marshall, P. Marsh, L.W. Rupp and W.F. Peck, Phys. Rev. B38 (1988) 893. [7] S.X. Dou, H.K. Liu, A.J. Bourdillon, N.X. Tan, N. Savvides, J.P. Zhou and C.C. Sorrell, Supercond. Sci. Technol. 1 (1988) 178.
303
[8] T. Kawai, T. Horiuchi, K. Mitsui, K. Ogura, S. Takagi and S. Kawai, Physica C 161 (1989) 561. [9] J.L. Tallon, R.G. Buckley, P.W. Gilberd, M.R. Presland, I.W.M. Brown, M.E. Bowden, L.A. Christian and R. Goguel, Nature 333 (1988) 153. [ 10] S.X. Dou, A.J. Bourdillon, C.C. Sorrell, S.P. Ringer and K.E. Easterling, Appl. Phys. Lett. 51 (1987) 535. [ 11 ] M. Nikolo and R.B. Goldfarb, Phys. Rev. B39 (1989) 6615. [12] S.X. Dou, H.K. Liu, A.J. Bourdillon, N.X. Tan and C.C. Sorrell, Phys. Rev. B40 (1989) 5266. [13] H.W. Zandbergen, W.A Groen, F.C. Mijlhoff, G. Van Tendeloo and S. Amelinckx, Physica C 156 (1988) 325. [14] U. Endo, S. Koyama and T. Kawai, Jpn. J. Appl. Phys. 27 (1988) L1476. [ 15 ] D. Shi, M. Tang, M.S. Boley, M. Hash, K. Vandervoort, H. Claus and Y.N. Lwin, Phys. Rev. B40 ( 1989 ) 2247. [ 16] G. Marbach, S. Stotz, M. Klee and J.W.C. de Vries, Physica C 161 (1989) 111. [ 17 ] T. Wada, N. Suzuki, A. Maeda, S. Uchida, K. Uchinokura and S. Tanaka, Jpn. J. Appl. Phys. 27 (1988) LI031. [18] R.D. Shannon and C.T. Prewitt, Acta Crystallogr. B25 (1969) 925. [19] I. Matsubara, H. Tanigawa, T. Ogura, H. Yamashita, M. Kinoshita and T. Kawai, Physica C 167 (1990) 503. [20] E.M. Levin and R.S. Roth, J. Res. Nat. Bur. Stand. 68A (1964) 198.
Melt processing of alkali element doped Bi2Sr2CaCu208 S.X. Dou, H . K . Liu, W.M. Wu, W.X. W a n g and C.C. Sorrell School of Materials Science and Engineering, UniversiO, of New South Wales, PO Box 1, Kensington, NSW 2033, Australia
R. W i n n and N. Savvides CSIRO Division of Applied Physics, PO Box 218, Lindfield, NSW 2070, Australia Received 29 May 1990 Revised manuscript received 12 October 1990
The effect of alkali element doping on the superconducting properties of the B i - S r - C a - C u - O system has been investigated. It was found that a T¢ of 94 K and To of 90 K in alkali element doped Bi2Sr2CaCu208 were achieved through the use of a meltprocessing technique. XRD, SEM and EDS examinations showed that the samples did not contain the high-To phase Bi2Sr2CazCu30~o. it is argued that the mechanism for the enhancement of the Tc through alkali element doping derives from a fluxing action, which provides a fast diffusion path for Ca and Cu to the superconducting phase, resulting in the desirable stoichiometry of Bi2SrzCaCu2Os. On the other hand, the substitution of alkali elements in the superconducting phase may give rise to a reducing effect of the oxygen content, leading to the enhancement of the To. The latter mechanism may be compounded with the fluxing action. Alkali elements were found to be excellent sintering aids for processing Bi-based superconductors since they enhance the 7~ and promote grain growth and grain aligment in the bulk materials.
1. Introduction It has been established that there are three superconducting phases in the B i - S r - C a - C u - O system: Bi2Sr2CuO6 (2201) with a T~ up to 20 K I l l , Bi2Sr2CaCu2Os (2212) with a T~ near 85 K [2-4] and Bi2Sr2Ca2Cu3Oio (2223) with a Tc ncar l l0 K [2,4-6]. Single phase 2212 has been prepared with the 7"~ varying from 70 to 92 K depending on the processing conditions [ 7-9 ]. The Tc of 2212 can be raised to 92 K by quenching from high temperature or treating in a low oxygen partial pressure [9]. Recently, an improvement in the T~ of 2212 by doping with Li has been reported by Kawai and coworkers [8]. They found that, in Li-doped Bi-SrC a - C u - O samples, the sintering temperature was lowered, while the transition temperature was raised and the volume of superconducting phase was greatly increased. However, it is not clear whether Li substituted for Cu owing to the difficulty of precise analysis of Li in the superconducting phase. Further, these authors found that K depressed the T~ significantly, and Na did not affect the T¢.
In the present work, similar results for Li doping were obtained. However, the results for K and Na doping are considerably different in that K and Na were found to have a beneficial effect on the T¢ of 2212. It was found that the 92 K transition was reproducible within a wide range of melting conditions, from 8 l0 to 890°C and a sintering time as short as 5 min at the higher temperatures. The effects of oxygen partial pressure, heat treatment procedure and initial composition on the superconducting properties are discussed. A possible mechanism for the enhancement of T¢ is proposed.
2. Experimental procedure Mixtures of Bi203 (99.9%), SrCO3 (99.5%), A2CO3 (99%) (where A = L i , Na, or K), CaCO3 (99%) and CuO (99.9%) in appropriate proportions were ground with a mortar and pestel and calcined at 720°C for 12 h'and 780°C for 10 h. The calcined powders were then pressed into pellets and sintered or partially melted in Ag boats at 740 to
0921-4534/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
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s.x. Dou et al. / Melt processing af alkali element doped Bi2Sr2CaCu208
890°C for 5 min to 15 h. One set o f samples with the same c o m p o s i t i o n was melt-processed at 830°C in varying o x y g e n / n i t r o g e n a t m o s p h e r e s in order to study the effect o f oxygen partial pressure. Quenching and slow cooling from the sintering t e m p e r a t u r e to room t e m p e r a t u r e were also applied to samples with the same compositions. The electrical resistivity was measured by the standard four-probe DC technique [ 10]. The AC magnetic susceptibility was measured by a mutual inductance m e t h o d at a frequency o f 83 Hz [11]. Microstructurai and c o m p o s i t i o n a l analyses were performed with a J E O L JSM-840 scanning electron microscope ( S E M ) e q u i p p e d with a Link Systems AN 10000 energy dispersive spectrometer ( E D S ) . Xray p o w d e r diffraction patterns were o b t a i n e d with a Philips type P W 1 1 4 0 / 0 0 p o w d e r diffractometer using CuKct radiation and scanning rate o f 2 0 = 1 ° / min. Chemical analyses were carried out with a Labtam International Plasmalab inductively coupled plasma a t o m i c emission spectrometer.
3. Results and discussion Table I lists the starting compositions, To, and To for samples treated for varying temperatures, times, and cooling rates. It is seen that Li, Na and K raised the Tc o f 2212 to 90 K under the desirable condi-
tions, contrary to previous results [8], in which only Li raised the T¢ while Na and K depressed the 7"~.. Figure 1 shows normalised resistivity-temperature plots for samples with the initial cation ratio o f B i : S r : C a : C u : L i = 2 . 2 : 1.8: 1.05: 1.45:0.7, which were sintered at 740 and 770+C for 15 h and at 850°C for 1 h. It may be noted that, for samples heat-treated at < 800°C, the T¢ was depressed below that o f the 80 K phase Bi2Sr2CaCueO8 (2212 ), while the Tc was enhanced to 90 K for the partially melted samples processed by a wide range o f heat t r e a t m e n t temperatures from 810 to 890°C. Quenching Li-doped samples from a sintering t e m p e r a t u r e o f 830°C (15 h) to room t e m p e r a t u r e in air resulted in s e m i c o n d u c t o r behaviour, although a small transition between 88 and 100 K can be seen in fig. 2. This may be c o m p a r e d with an unquenched sample treated at the same t e m p e r a t u r e for the same period o f time and cooled at a rate o f 1 0 ° C / h to 790°C and thereafter at 6 0 ° C / h to room temperature; here, the To= 90 K and To= 84 K. Quenching all samples from a sintering, t e m p e r a t u r e o f 740°C to r o o m t e m p e r a t u r e had no effect on the transition. Figure 3 shows r e s i s t i v i t y - t e m p e r a t u r e curves for samples with the cation ratio Bi:Sr:Ca:Cu:A=2.2:l.8:l.05:(2.15-x):x, where A = L i , x = 0 . 7 ; A = N a , x = 0 . 6 ; and A = K , x = 0 . 7 . The heat treatment conditions for these samples were as follows. The Li-doped sample was sintered at
Table 1 Compositions, Tc, and To for alkali element doped samples treated under various conditions. Composition
Sintering
Cooling rate (°C/h)
7++. (K)
1o (K)
Temp. (+C)
Time (h)
830 830 830 830
1 1 1 1
10 10 10 10
1.0 0.21 0.05 N2
86 90 90 92
70 86 86 88
Bi:Sr:Ca:Cu:Na 2.2: 1.8: 1.05:1.85:0.3 2.2: 1.8: 1.05:1.55:0.6
885 885
0.08 0.08
60 60
0.21 0.21
92 94
88 90
Bi:Sr:Ca:Cu:K 2.2: 1.8: 1.05:1.45:0.7
850
10
60
0.21
92
88
890
I
60
0.21
84
80
Bi:Sr:Ca:Cu:Li 2.2:1.8:1.05:1.45:0.7
Undoped Sample
Bi:Sr:Ca:Cu:A 2.2: 1.8:1.05:2.15:0.0
297
S.X. Dou et al. / Melt processing of alkali element doped Bi:,Sr2CaCu20s 1
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830°C for I h in nitrogen and cooled at a rate of 1 0 ° C / h to 790°C and then at 6 0 ° C / h to room temperature. The Na-doped sample was treated at 885°C for 5 m i n in air and cooled at a rate of 6 0 ° C / h to room temperature. The K-doped sample was treated at 850°C for 10 h and cooled at a rate of 6 0 ° C / h to room temperature. It can be seen that Na raised the T,. to 94 K and 7"()to 90 K, while K and Li raised the T~ the same degree, where the T c = 9 2 K and T o = 8 8 K. The u n d o p e d standard sample, which was partially melted at 8 9 0 ° C for I h, showed a T c = 8 4 K and T o = 8 0 K. These results are further confirmed by the AC magnetic susceptibility measurements, as shown in fig. 4. It is clear that Na, K and Li doping increased the 7~. of the 2212 phase relative to the undoped sample. Figure 5 shows photomicrographs of Li- and Nadoped samples. The superconducting phase consisted of elongated plate-like grains that exhibited a certain degree of orientation owing to the partial melting during sintering. EDS analyses revealed that these samples consisted of multiphase assemblages.
150
200
250
iO0
(K}
Temperolure
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Fig. I. Effect on the transition of sintering temperature for Lidoped samples, sintered at 740°C and 770°C for 15 h and 850°C for 1 h in air.
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The major phase in the Liodoped samples had an average composition of B i : S r : C a : C u = 2 . 2 : 1.7:1.0:2.0 (grey in fig. 5(a) ), while the minor phases included B i : S r : C a = 3 . 4 8 : 1.2:1.53 (white in fig. 5 ( a ) ) and B i : S r : C a : C u = 2 . 2 : 1 . 3 : 0 . 4 1 : 1 . 0 (light grey in fig. 5(a)). The Na-doped sample (fig. 5 ( b ) ) c o n s i s t e d of a lower amount of impurity phases than in the Lidoped sample. No 2223 phase was detected in either sample. This is further confirmed through EDS analyses on whiskers grown in alkali element doped samples. It was often found that a cavity was formed in the partially melted B i - S r - C a - C u - L i - O samples, where crystal growth occurred (fig. 6). With a cooling rate of I °C/min from 870 to 800°C, a clump of whiskers of the 2212 composition was formed. No 2223 whiskers were detected. Since alkali elements have a + l valence state, then their substitution for any elements in the B i - S r - C a Cu-O system would reduce the amount of oxygen in
Fig. 5. Microstructures of (a) Li-doped and (b) Na-doped sampies. The heat treatment conditions are the same as in fig. 4.
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the structure or create holes on the Cu-O planes owing to charge balence requirements. Therefore, if substitution occurs, it would be expected that the dependence of the 7~. upon the oxygen partial pressure
S.X. Dou et aL / Melt processing of alkali element doped BieSr2CaCu20s
during heat treatment will be different from that of the undoped samples. Figure 7 shows the temperature dependence of normalised resistivity for samples with the nominal composition Bi2.2Sr~.sCa~.osCut.asLio.7Oa_.v treated at 830°C for 15 h in flowing N2, 02 and air. It may be seen that the sample treated in N2 showed a transition at 92 K with zero resistance at 88 K, while the oxygen-treated sample showed semiconductor behaviour before the superconducting transition and did not reach zero resistance until 70 K. The effect of oxygen partial pressure on the T~ for Na-doped samples is less pronounced than that for Li-doped samples, as shown in fig. 8. A pellet of Na-doped sample was cut into four pieces, which were then treated at 750°C for 15 h in flowing 02, N2, 0.1 arm 02, and air (0.209 arm 02). It is seen that only the oxygen treatment depressed the T,. by 5 K, which is much less pronounced than in the case of the Lidoped sample. The results for N2, 0.1 atm 02, and air are similar, although the 0.1 arm 02 treatment gave slightly higher Tc than the others. The difference in the oxygen pressure effect between the Li-
299
doped and Na-doped samples may indicate the difference in the extra oxygen content in the two samples. It is possible that Li and Na may substitute on different sites in the superconducting phase. The oxygen treatment may result in an increase in the oxygen content in the Bi-O layers, enlarging the Bi-Bi separation and increasing the resistance in these layers [12,13]. On the other hand, low oxygen partial pressure or pure nitrogen is desirable in order to obtain the optimal oxidation states.
4. Discussion The enhancement of the T¢ through alkali element doping may be attributed to three factors: the presence of the high-To phase (2223), the fluxing action of the alkali elements, and the substitution of the alkali elements in the superconducting phases. In the following, the three possible mechanisms for the T¢ enhancement are discussed.
4.1. Absence of the 2223 phase in alkali-doped samples
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Since Bi-based materials form a series of superconducting phases, which are often found in an intergrowth pattern, the enhancement of the Tc may be attributed to the presence of small amounts of the 2223 phase. Figure 9 shows the X-ray diffraction patterns for alkali element doped samples. The XRD data indicate that all samples consisted of 2212 as the major phase. The minor phases are 2201 and some impurity phases including SrCaCu204, CuO and (Li, Na, K ) - B i - S r - C a - O . However, no 2223 phase was detected. This is agreement with the Tc and susceptibility measurements, which show a single transition. From the XRD results, it is believed that the enhancement in the Tc for the alkali element doped samples cannot be attributed to the presence of the high-To phase (2223). The absence of the highT~ phase in these samples may be explained as follows. (a) Experience has demonstrated that starting compositions with Ca and Cu content higher than in stoichiometric 2223 are essential for the formation of the high-To phase [7,14-17]. Thus, a starting composition near 2212 or one deficient in Cu rela-
300
S.X. Dou et aL / Melt processingof alkali element doped Bi2Sr2CaCu20~ Ivt
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200
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Fig. 8. Effect on the transition of sintering atmosphere for Na-doped samples treated at 885:C for 5 min and post-annealed at 750~C for 15 h in flowingO2, N2, 0.1 atm 02, and air. rive to 2212 is not favourable for the formation of 2223. (b) It has been well established that the formation of the 2223 phase is a very slow process, in particular for samples without Pb doping. Annealing for more than l0 days is needed to reach a Tc o f 105 K [15]. It is, therefore, unlikely that the high-Tc phase can be formed with a sintering time of only 5 min. 4.2. Effect o f alkali element substitution
Table 1I gives the lattice parameters calculated from X-ray diffraction patterns for undoped and alkali element doped samples. It is seen that the lattice parameters for alkali element doped samples show a slight variation compared with the undoped sample. In particular, the c-axes decrease slightly with increasing dopant levels. This may be attributed to the incorporation of a small amount o f alkali elements in the superconducting phases. Li +, Na + and K ÷ have ionic radii o f 0.059 nm, 0.099 nm and 0.138 nm, respectively [5]. From dimensional considerations, if cation substitution occurs, it is most likely that Li ÷ substitutes for Ca 2÷ (0.062 nm), Na + substitutes for Ca -'÷ (0.1 n m ) and K ÷ substitutes for Sr 2÷ (0.116 nm). Since alkali element ions have a closed electron shell, it is unlikely that they can play an electronic role similar to that of Cu 2÷. However,
they could reduce the oxygen content in the structures because they are monovalent. It has been reported that a Tc of 92 K for the 2212 phase has been achieved by annealing in low oxygen partial pressures or by quenching [9]. The substitution of monovalent ions for divalent ions should have a similar reducing effect similar to that of low oxygen partial pressure treatment. Since Li is the lightest metallic element and it cannot be detected by EDS, it is difficult to determine whether Li is incorporated in the superconducting phase, Matsubara et al. [9] analysed the whiskers grown from Li-doped B i - S r - C a - C u - O and found that the whiskers contained 1-3 at% Li. They attributed the enhancement o f the Tc to the Li substitution. In the present work, EDS analysis of single crystals revealed that Na was incorporated in the superconducting phase. The results listed in table III are average values of 8 individual analyses on a TEM powder sample. K ÷ was also detected in the superconducting phase in the samples sintered for 10-30 h at 850~C, as listed in table III. It is interesting to note that the K content in the samples decreased with increasing sintering times, vanishing in the sample sintered for the longest period of time. although the Tc remained nearly unchanged at 90 K. Since the Tc was unaffected while K volatilised out of the samples, it is unlikely that the incorporation of K in the
S.X. Dou et al. /Melt processing of alkali element doped Bi;~r2CaCu20s I
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At this stage, it can be argued that the element of the Tc by alkali element doping may be attributed to liquid-phase sintering. This argument may be supported by the following factors. (a) Li20 and Bi203 form a eutectic at a temperature as low as 700cC at Li20/Bi203=0.1 [20]. This explains why low sintering temperatures could be used with Li additions. This liquid provides a fast diffusion path for Ca, Sr and Cu, which accelerates the reaction to form the superconductor. This is evident from the fact that exaggerated grain growth and alignment were achieved through liquid-phase sintering within a short period o f time (fig. 5). Na20 and K20 should have the same effect although the phase diagrams for K20-Bi203 and Na20-Bi203 are not available. (b) From table III, it is evident that the major phase (2212) in alkali element doped samples has a relatively high C a / S r ratio, indicating that Ca is enriched during melting due to assisted Ca diffusion through the liquid phase. In undoped samples, Ca is often depleted, resulting in low-To values [7]. The fact that the Tc was enhanced to the same degree by K, Na, and Li doping strongly suggests that the enhancement of the Tc may be attributed mainly to the liquid-phase sintering resulting from alkali metal additions. As K was added, the sintering temperature had to be lowered owing to the formation o f a liquid phase that accelerated the formation of 2212. During prolonged sintering, K was volatilised but its initial presence assisted in the formation o f pure 2212, giving rise to an enhancement in the To. Li and Na may
Table I1 Lattice parameters of the undoped and alkali element doped samples using CuKct radiation and a scanning rate of 2t9= 1°/min. Initial composition
Lattice parameters (nm)
Bi:
Sr:
Ca:
Cu:
A
a
b
c
2.2 2.2 2.2 2.2 2.2 2.2
1.8 1.8 1.8 1.8 1.8 1.8
1.05 1.05 1.05 1.05 1.05 1.05
21.5 1.45 1.85 1.55 1.65 1.45
0.0 0.7Li 0.3Na 0.6Na 0.5K 0.7K
0.541 0.539 0.541 0.538 0.540 0.537
0.541 0.540 0.541 0.545 0.542 0.540
3.086 3.084 3.084 3.080 3.078 3.068
302
S.X. Dou et al. /Melt processing qf alkali element doped Bi,Sr eCaCueOs
Table III EDS analyses for alkali element doped Bi-Sr-Ca-Cu-O materials. L t-doped samples Bi: Sr: Ca:
2.16 2.80 2.11 0
1.83 1.80 2.96 1.97
1.04 0.64 1.58 1.1
Cu 1.90 1.38 0 4.87
Ca: 0.97 0.38
Cu: 2.00 0.32
Na 0.31 7.49
(2212) Na-rich phase
(2212) (2201)
Na-doped sample
Bi: 2.20 1.05
Sr: 1.62 1.37
K-doped samples
2.11 2.63
St: 1.73 1.60
Ca: 0.98 1.01
Cu: 1.68 1.22
K 0.88 0.45
(sintered at 850=Cfor10h)
2.26 2.71
2.16 2.07
0.66 0.44
1.67 1.27
0.24 0.27
(sintered at 850°C for 20 h)
2.26 2.91
1.94 1.87
0.95 0.40
1.93 1.36
0.00 0.00
(sintered at 850°C for 35 h)
Bi:
Li is undetectable by EDS analysis. Table IV Results of atomic emission spectroscopy analyses for Li- and Kdoped samples. Initial composition
Li or K content (ppm) Before sintering
After sintering
Bi: 2.20
Sr: 1.80
Ca: 1.05
Cu: 1.45
Li 0.70
42
45
Bi: 2.20
Sr: 1.80
Ca: 1.05
Cu: 1.45
K 0.70
187
2
play the same role as K. However, since Li is stable at higher temperatures, it will r e m a i n in the sample while K volatilises during sintering. Chemical analysis by a t o m i c emission spectroscopy have confirmed that only 1% o f the total K a d d i t i o n was present in the K - d o p e d sample after sintering at 850-'C for 35 h, whereas Li was present in the same a m o u n t in the initial and final compositions, as shown in table IV.
5. Conclusions In s u m m a r y , the e n h a n c e m e n t o f the Tc in alkali element d o p e d B i - S r - C a - C u - O through melt-processing is reported. Values for the 7"~ o f 94 K and To o f 90 K have been achieved in K-, Na- and Li-doped samples. The e n h a n c e m e n t o f the 7~ by doping with alkali elements is o f significant technical i m p o r t a n c e since they can be used as ideal sintering aids, providing the following advantages. ( a ) Enhancement o f the superconducting transition up to 92 K, without need o f treatment in low oxygen partial pressure; ( b ) acceleration o f the f o r m a t i o n o f the superconducting phase through transient liquid-phase sintering: (c) achievement o f grain alignment through melttexture growth; ( d ) p r o m o t i o n of single crystal growth through flux action: (e) no c o n t a m i n a t i o n o f the sample in the case o f K doping owing to K volatalization. It is tentatively concluded that the mechanism for the enhancement o f the T~ is by a fluxing action which provides a fast diffusion path for Ca and Cu to the superconducting phase, leading to the desirable 2212 stoichiometry. Thus, the intrinsic T~ for 2212 phase is as high as that o f YBa2Cu3Ov_x if a proper processing route is used. On the other hand, the substitution o f alkali element in the superconducting phase may give rise to a reducing effect o f the oxygen content, leading to an enhancement of the To. The latter m e c h a n i s m cannot be ruled out completely at the present. More work on structure refinement and the location of the alkali elements in the structure is needed.
Acknowledgements The authors are grateful to Metal Manufactures Ltd., and to the C o m m o n w e a l t h D e p a r t m e n t o f Industry, Technology and C o m m e r c e for financial support.
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