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Ag superconducting composite materials
PHYSICA Physica C 253 (1995) 89-96
ELSEVIER
Elastic anomalies in Bi-Pb-2223/Ag superconducting composite materials R.J. Topare b, K. Ganesh a, N.K. Sahuji b, S.S. Shah b, p. Venugopal Reddy a,* a Department of Physics, College of Science, Osmania University, Hyderabad, 500 007, India b Department of Physics, Dr. B.A. Marathwada University, Aurangabad, 431 004, India
Received 30 January 1995; revised manuscript received 3 May 1995
Abstract
A series of high-Tc superconducting samples having the compositional formula Bil.7Pb0.3Sr2Ca2CU3Oloq-x% Ag20 [x varies between 0-6 in the interval of 1] have been prepared by the solid-state reaction method. After usual characterisation by XRD, resistivity and scanning electron microscopy, bulk density porosity studies etc., the ultrasonic measurements were carried out by the pulse transmission technique in the temperature range of 80-300 K. It has been observed that two samples viz., Bi-2223-0 and Bi-2223-1 are found to exhibit an elastic softening in the temperature region 300 to 210 K, while all the seven samples exhibited a velocity maximum between 125 to 190 K. Qualitative explanations for both the phenomena of elastic softening and the velocity maxima are given on the basis of the microstructure and ordering readjustment of the oxygen atoms among the Cu-O planes of the samples.
1. I n t r o d u c t i o n
It was generally recognised that the phenomenon of lattice/structural instabilities above Te is a common feature among A-15 compounds (Nb3Sn, V3Si, Nb3Ge etc.), generally known as the conventional high-T~ superconductors. In fact, it has been observed that there is a close relationship between the high value of T~ and the lattice instabilities among A-15 type of superconductors [1]. Immediately after the discovery of ceramic superconductors, many investigators started looking for a similar type of correlation between the high value of T~ and instabilities if any. As the lattice instabilities are manifest well in the elasticity studies, a number o f investigations
* Corresponding author.
were undertaken with a view to know whether the correlation between the high value of Tc and Structural instabilities is still valid among the ceramic superconductors. Even eight years after the discovery of high-T~ superconductors, there are several reports both in favour and against the phenomenon of lattice instabilities among this new class of materials. During the last five years, a few papers published from this laboratory also [2-4] consistently indicate the presence of lattice instabilities among ceramic superconductors. However, none o f the published works so far give any direct or indirect evidence for a direct relationship between these two parameters. With a view to shed some more light on this particular aspect and generalise the observed results, the authors of the present investigation have undertaken yet another investigation on another set of samples viz., B i - 2 2 2 3 / A g composites. In fact when A g 2 0 is
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R.J. Topare et aL / Physica C 253 (1995) 89-96 10-
added, it is expected to remove the microstructural defects of the ceramic superconductors. As such, the influence of A g 2 0 on the lattice instabilities is another aspect of the present investigation. In view of this, the ultrasonic-velocity measurements of the Bi2 2 2 3 / A g composites, have been undertaken both as a function of temperature and composition and the results are presented here.
82 2. Experimental Samples with the nominal composition Bil. 7Pbo.sSr2Ca2Cu3010 + x % A g 2 0 ( x = 0 - 6 in the interval of 1) were prepared by the well-known solidstate reaction method using highly pure (99.999%) Bi20 s, PbO, SrCO 3, CaCO 3, CuO and Ag20. The powders after thorough mixing and grinding were calcined at 800°C for 24 h followed by two more similar heat treatments at 810 and 840°C for 24 h each. The calcined powders after pressing into discs of 10 m m in diameter and 2 to 3 m m in thickness were sintered at 850°C for 200 h. With the aim to determine the superconductivity transition temperature (T¢), the DC resistance measurements were carried out over a temperature range of 80-300 K, by using the standard four-probe method. Later, the X-ray diffraction studies were undertaken with C u K c~ radiation in the scanning range 20 = 4 - 6 0 °. The surface morphology of a solid material is an important surface property and is very much useful in understanding its defect structure, grain size, voids etc.; and as such it was also studied by scanning electron microscopy. Bulk densities of solids in general are essential to evaluate their elastic moduli and hence they were also determined
0
1O 0
, 150
200
i
250
300
Temperature (K)
Fig. 1. Resistance vs. temperature plot of Bil.7 Pbo.3Sr2Ca2Cu 3Olo.
by an immersion method. Finally, the ultrasonic velocities were measured over a temperature range 90-300 K by the pulse transmission technique, the details of which are given elsewhere [5].
3. Results and discussion 3.1. Electrical resistance versus temperature
The electrical-resistance measurement of all the samples is carried out in the temperature range of 80-300 K and the plots of all the samples are found to show a single-step superconducting transition with T~ fluctuating around 105 K. The resistance versus temperature plot of the sample Bi-2223-0 sample is shown in Fig. 1. The room-temperature resistance value R300, Tc onset, Tc(o) and AT for all the samples are given in Table 1. As expected, the R300 is almost
Table 1 Room-temperature data of BI-2223/Ag composites Compositional formula Ri i.TPb0.3Sr2Ca2Cu3Olo Bit.7Pbo.3Sr2Ca2CU3Olo Bil.vPbo.3Sr2Ca2Cu3010 BiL7Pb0.3Sr2Ca2Cu3O10 Bil.7Pb0.3Sr2Ca2Cu3Oio Bil.7Pbo.3Sr2Ca2Cu3O10 Bil.TPb0.3Sr2Ca2Cu3Olo
+ + + + + +
I% 2% 3% 4% 5% 6%
Ag20 Ag20 Ag20 Ag20 AgzO Ag20
Sample code
R3o0 (m~)
Tconset (K)
Tc(o) (K)
AT (K)
Bi-2223-0 Bi-2223-1 Bi-2223-2 Bi-2223-3 Bi-2223-4 Bi-2223-5 Bi-2223-6
10.2 5.3 5.3 7.4 4.8 6.6 3.7
113 109 107 109 106 106 109
106 106 104 106 104 104 106
2 3 3 3 2 2 3
R.J. Topare et a l . / Physica C 253 (1995) 89-96
found to decrease with increasing concentration of Ag. 3.2. X-ray diffraction
It has been observed from the XRD patterns of the samples that all of them are having a major orthorhombic phase and a close examination of the patterns reveals that an additional peak representing silver is found to appear by the side of the peak with a [hkl] value of [1 1 11]. Using the d spacing values, the lattice parameters and hence the X-ray densities have been computed and are included in Table 2. The volume fraction of the high-T~ phase present in all the samples of the present investigation has been estimated using the XRD data and relation [6] I00 lo¢a)/( Ioo lo(n) + Ioo 8(L)),
(1)
where Ioo 10¢H) and I008¢L) are the intensities of reflection pertaining to the high- and the low-T~ phases, respectively. It has been found that the volume fraction of high-T~ phase in all the samples is about 80%. 3.3. Scanning electron microscopic studies
The scanning electron micrographs of all the samples are shown in Figs. 2(a-g). It can be seen from the figures that the grains of the samples are randomly oriented with reasonable size of more than 5 p~m. It has also been observed that the size of the grains and hence the voids are found to decrease with increasing silver concentration. In fact, this observation is in conformity with the experimentally obtained porosity values.
91
3.4. Elastic moduli at room temperature
The experimental values of longitudinal (V t) and shear wave velocities ( ~ ) of Bi-2223/Ag composites along with those of Young's (E), and rigidity moduli (G) are given in Table 2. A continuous increase in the values of both the moduli and Tc and a gradual decrease in the R300 values with increasing Ag20 concentration clearly indicate that the mechanical strength of the samples is increasing continuously. The observed behaviour is quite possible because silver oxide is expected to precipitate along the grain boundaries, improving the weak links between the grains. 3.5. Correction to zero porosity
In general, the high-T~ superconducting samples prepared by the solid-state reaction method are highly porous. In fact. the porosities of the samples of the present investigation are found to vary from 23 to 40% and as such the measured elastic moduli do not have much significance unless they are corrected to zero porosity. Therefore, the elasticity parameters have been corrected to zero porosity to obtain the intrinsic material values using the Ledbetter and Datta formula [7]. For this purpose, it has been assumed that the sample possesses mainly randomly distributed spherical pores and that the voids contain zero elastic resistance to both dilation and shear. The equations employed are Go=(1/2AI)[-A
2 + ( A 2 2 - 4 A 1 A 3 ) 1/2]
(2)
and (3)
B o = 4 G OB / 4 ( 1 - C) G O - 3 CB,
Table 2 Elastic moduli at room temperature Sample code
Vt (m/s)
Vs (m/s)
C (%)
p (10 3 k g / m m 3)
Pm (10 3 k g / m m 3)
G (GPa)
E (GPa)
Bi-2223-0 Bi-2223-1 Bi-2223-2 Bi-2223-3 Bi-2223-4 Bi-2223-5 Bi-2223-6
2776 2520 2688 2777 2898 3699 3763
1162 1211 1274 1327 1406 1452 1459
40.4 38.9 38.3 33.7 25.9 23.3 24.2
3.73 3.82 3.86 4.15 4.64 4.80 4.74
6.27 6.26 6.27 6.26 6.23 6.24 6.23
5.0 5.6 6.2 7.3 9.2 10.1 10.1
14.0 15.1 16.9 19.7 24.7 28.5 28.5
92
R.J. Topare et al. / Physica C 253 (1995) 89-96
Fig. 2. Scanning electron micrograph of Bi 17Pbo 3Sr2Ca2CuaOlo + x% Ag20; (a) x = 0, (b) x = 1, (c) x = 2, (d) x = 3, (e) x = 4, (f) x = 5 , (g) x = 6 .
R.J. Topare et al. / Physica C 253 (1995) 89-96 Table 3 Elastic moduli corrected to zero porosity
Sample code
GO (GPa)
Eo (GPa)
Tm (K)
Bi-2223 -0 Bi-2223-1 B i-2223-2 Bi-2223-3 Bi-2223-4 Bi-2223-5 Bi-2223-6
10.7 11.9 13.1 13.8 14.9 15.3 15.6
32.2 34.1 37.6 39.5 41.8 45.3 46.1
125 130 160 145 160 170 190
where A~ = 8(1 - C ) / 3 , A z = (3 - 2 C ) B - [(8/3) + 4 C ) G and A 3 = - 3 ( 1 + C)BG; C, p and P0 are the void fraction, the macroscopic and X-ray mass densities, respectively, while B, G and E represent the experimental values of bulk, rigidity and Young's moduli, respectively, and B 0, G O and E 0 are the respective elastic moduli of a non-porous material. The elastic moduli corrected to zero porosity are given in Table 3. It can be seen from the table that the E 0 and G O values of the Bi-2223-0 sample are low when compared with those reported by others. This may be due to the fact that the sample is having 20% of Bi-2212 phase also. In general it is known that the low-T~ Bi-2212 phase always exhibits low elastic moduli when compared with high-Tc Bi-2223 phase [8]. 3.6. Interpretation of elastic data in terms of strength of the materials Following Wooster's work [9], the experimental data obtained in the present study may be interpreted in terms of the binding forces between various ions. As both the elastic moduli are increasing continuously with increasing A g 2 0 concentration, it may be presumably assumed that the binding forces between various ions of the samples might be increasing continuously. This must have resulted due to the fact that the silver precipitates on the grain boundary, improving the weak links between the grains.
93
the Bi-2223-0 and Bi-2223-1 sample after an initial decrease is found to increase continuously with decreasing temperature, while that of the remaining ones [Figs. 4 and 5] is found to increase continuously right from 300 K onwards. Thereafter, in all the cases, the velocity reaches a maximum value at a particular temperature, hereafter designated as Tm. On further decrease of temperature, however, the velocity is found to decrease continuously. The values of Tm for all the samples are noted and are given in Table 3. Except in the case of the third sample, Tm is found to increase continuously with increasing AgaO concentration and the observation clearly indicates that AgzO addition might be responsible for shifting the velocity maxima towards the high-temperature side. A close examination of the velocity versus temperature plots indicates that but for the occurrence of two distinct and different phenomena, the velocity is found to increase continuously with decreasing temperature and as such it may be concluded that the general elastic behaviour of the samples of the present investigation is analogous to that of a number of oxide materials [10,11]. The two anomalous phenomena observed are (1) a decrease of velocity with decreasing temperature in the case of Bi-2223-0 and Bi-2223-1 samples in the temperature range 300-210 K and (2) The exhibition of a velocity maximum at a particular temperature (Tm) by all the samples.
4000-
~
3000 .O~---0
E ~2000 (i)
>
3. 7. Elastic behaoiour at low temperatures 1000
The plots of the longitudinal velocity versus temperature of all the samples are shown in Figs. 3 to 5. It can be seen from Fig. 3, that the velocity of both
go
1¢o
~g6 . . . . . . . ~ki . . . . . . . Temperature (K)
~6d'
Fig. 3. Longitudinal velocity vs. temperature of Bi 1.7Pbo.3Sr2Ca2 Cu3010 + x% Ag20; x = 0, x = 1.
R.J. Topare et al. / Physica C 253 (1995) 89-96
94 5500-
~ v
4500
E
83500 (1> >
~¢
2500
90
,~
X-- 3
140
~
190
2
Temperoture
240
290
(K)
Fig. 4. Temperature variation of the longitudinal velocity of Bil.TPbo.3Sr2Ca2Cu3Olo + x % A g 2 0 ; x = 2, x = 3, x = 5.
Now the occurrence of these two anomalies can be explained as follows.
3. 7.1. Decrease of velocity with decreasing temperature A close examination of the literature on the elastic behaviour of high-To materials indicates that elastic anomalies especially in the vicinity of room temperature are very common. For example, Tritt et al. [12] reported a decrease in Young's modulus by a factor of 3 to 4 in Bi-2212 whiskers in the temperature
5500
%-
"E'45oov xr-ll
0 -~50o >
4
2500
O0
........ 14d i .......
1~d . . . . . . . ~k; . . . . . . . Temperature (K)
2]d '
Fig. 5. Plots o f the longitudinal velocity vs. t e m p e r a t u r e o f B i l . 7 P b o . 3 S r 2 C a 2 C u 3 O l o + x % A g 2 0 ; x = 4, x = 6.
range 330 to 270 K. These authors explained the anomalous elastic behaviour as due to the presence of stress-induced phase transitions. Similarly, Jacobson et al. [13] also observed that the modulus along the a direction is found to be lower than earlier reported values for this material. The authors speculated that the lattice was soft along a, the direction along which whiskers grow and also believed that the low E value of Bi-2212 whiskers along the a direction when compared with that of pallets must be due to the failure of the whiskers to orient in a specific direction. In the present investigation, as there is no confirmation of the stress-induced phase transitions either directly or indirectly, it is thought that the type of anomalies around room temperature in the case of Bi-2223-0 and Bi-2223-1 may not be due to them. Further, since none of the two samples (Bi-2223-0 and Bi-2223-1) of the present investigation exhibited such low moduli along any particular direction, the elastic softenning observed in the present investigation could be attributed to reasons other than the failure of whiskers/grains to orient in any particular direction. Further examination of the literature indicates that Kim et al. [15] reported an anomalous variation of the ultrasonic velocity in the case of Y-123 samples, while an identical behaviour was also reported by Zhao et al. [16] and Xu et al. [17] in the case of sintered forged materials and also by Lang et al. [18] in the case of coarse-grain Y-123 sintered materials. These authors observed that the phenomenon of softening occurs only when the grains of the sintered materials are grown to a sufficient size and that the softening of the lattice may not be possible in a sample with fine grains ( < 4 i~m) and high density ( > 90%) [19,20]. It was also reported [21,22] that the grain size is the main controlling factor for the elastic softening behaviour of the oxide superconductors. In view of these observations, it may be concluded that the anomalous variation of the longitudinal velocity with decreasing temperature observed among the Bi-2223-0 and Bi-2223-1 samples of the present investigation [Fig. 3] could be attributed to the presence of a large number of coarse grains. The high porosity of the order of 40% with least contact among the grains of these samples, as can be seen from their scanning electron micrographs, may also be responsible for the observed elastic behaviour.
R.J. Topare et aL / Physica C 253 (1995) 89-96
3. 7.2. Velocity maximum Velocity maxima are not totally uncommon to high-T~ superconducting materials, as several researchers reported similar ones earlier [23-27]. For example, Nes et al. [14] reported the existence of an internal friction peak in the case of Bi-2212 single crystal at 145 K and attributed this to the Debye relaxation. It was also observed that the elastic instabilities in solids are generally manifest either by a downward cusp or a step. Cusps are usually observed near phase transitions mainly due to the weakening of certain force constants, such as softening of phonon modes etc., while the velocity maxima are observed near order-disorder transitions resulting from the reorientation or ordering of oxygen ions in the copper-oxygen planes. It was also observed that the lattice instabilities above Tc are a sort of precursors to the superconducting phenomenon. In view of this, it may be speculated that the velocity maxima observed among the samples of the present investigation could be due to the ordering readjustments of the oxygen atoms among the C u - O planes. Further, as the velocity maximum (Tm) of the samples is shifting towards the high-temperature side with increasing Ag20, it may be concluded that A g 2 0 is assisting indirectly the phenomenon of ordering of oxygen ions in the C u - O planes. Thus, it may be concluded that the two types of lattice instabilities, viz. the elastic softening in the temperature range 300-210 K and the occurrence of a maximum in the velocity versus temperature plots have been observed among the samples of the present investigation. Further, although both these types of lattice instabilities were observed not only among the samples of the present investigation but also in a few other cases earlier [2,3], no definite relationship between the high value of T~ and lattice instabilities, could be arrived at. As there is no particular trend to understand the phenomenon, some more studies on a variety of superconducting systems with different grain sizes are needed. Investigations of this effect are under way.
Acknowledgements All the authors thank the Department of Science and Technology, Government of India, for providing
95
funds to carry out research under the National Superconductivity Programme (NSP). The authors also thank K.N. Srivasthava, University of Hyderabad, Hyderabad for providing necessary facilities in taking scanning electron micrographs.
References [1] L.R. Testardi, in: Phys. Acoust., eds. W.P. Mansion and B.N. Thruston Vol. 10 (Academic Press, New York, 1973) p. 193. [2] P. Venugopal Reddy and S. Shekar, Physica C 216 (t993) 116. [3] P. Venugopal Reddy and S. Shekar, Supercond. Sci. Technol. 6 (1993) 785. [4] P. Venugopal Reddy, S. Shekar and V.N. Mulay, Int. J. Mod. Phys., to be published. [5] YN. Ramana and P. Venugopal Reddy, Acoust. Lett. 13 (1989) 83. [6] Y. Hayashi, H. Kogure and Y. Gondo, Jpn. J. Appl. Phys. 28 (1989) L2182. [7] H. Ledbetter and S. Datta, J. Acoust. Soc. Am. 79 (1986) 239. [8] J. Dominec, P. Svoboda, V. Plechacek and C. Laermans, Mod. Phys. Lett. B 6 (1992) 1049. [9] W.A. Wooster, Rep. Prog. Phys. 16 (1952) 62. [10] Y.P. Varshni, Phys. Rev. B 2 (1970) 3952. [11] J.B. Watchman, W.E. Terrif, D.G. Lam and C.S. Apstein, Phys. Rev. 122 (1961) 1754. [12] T.M. Tritt, M. Marone, A.C. Ehrlich, M.J. Skove, D.J. Gillespie, R.L. Jacobsen, G.X. Tessema, J.P. Frank and J. Jung, Phys. Rev. Lett. 68 (1992) 2531. [13] R.L. Jacobsen, T.M. Tritt, A.C. Ehrlich and D.J. Gillespie, Phys. Rev. B 47 (1993) 8312. [14] O.M. Nes, K. Fossheim, N. Motohira and K. Kitazawa, Physica C 185-189 (1991) 1391. [15] T.J. Kim, J. Kowalewski, W. Assumes and W. Grill, Z. Phys. B Cond. Matt. 78 (1990) 207. [16] Z. Zhao et al., Phys. Rev. B 39 (1989) 721. [17] M.F. Xu, H.P. Baun, A. Schentron, B.K. Sharma, M. Levy, K.J. Sun, L.E. Toth, S.A. Wolf and D.V. Cubser, Phys. Rev. B 37 (1988) 3675. [18] M. Lang, T. Lechner, S. Riegel, F. Steglich, G. Weber, T.J. Kim, B. Luthi, H. Rietschel and M. Wilhelm, Z. Phys. B Cond. Matt. 69 (1988) 459. [19] W. Ting and K. Fossheim, Int. J. Mod. Phys. B 8 (1994) 275. [20] S. Ewert, S. Guo, P. Lemmens, F. Steltmach, J. Wynants, G. Alert, D. Bonnenberg, H. Kliem, A. Comberg and H. Passing, Solid State Commun. 64 (1987) 1153. [21] V. Muller and D. Mauner, Phase Trans. 22 (1990) 211. [22] Burenkove Yu A et al. Sov. Phys. Solid State 310 (1988) 1873.
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[23] R.J. Kennedy, W.G. Jenks and L.R. Testardi, Phys. Rev. B 40 (1989) 11313. [24] G. Canneli, R. Cantelli, F. Cordero and F. Trequattrini, Supercond. Sci. Technol. 5 (1992) 247. [25] M. Chankurtaran and G.A. Saunders, Supercond. Sci. Technol. 5 (1992) 529.
[26] J. Toulouse, X.M. Wang and D.J.L. Hong, Phys. Rev. B 38 (1988) 7077. [27] Y.-S. He, S.-H. Lin, J. Xiang, A.-S. He, J.-C. Zhang, F. Chang, C.-D. Wei, Z.-H. Zhen, J.-K. Hu, and Qianlin, J. Phys. Cond. Matt. 2 (1990) 1841.
ELSEVIER
Elastic anomalies in Bi-Pb-2223/Ag superconducting composite materials R.J. Topare b, K. Ganesh a, N.K. Sahuji b, S.S. Shah b, p. Venugopal Reddy a,* a Department of Physics, College of Science, Osmania University, Hyderabad, 500 007, India b Department of Physics, Dr. B.A. Marathwada University, Aurangabad, 431 004, India
Received 30 January 1995; revised manuscript received 3 May 1995
Abstract
A series of high-Tc superconducting samples having the compositional formula Bil.7Pb0.3Sr2Ca2CU3Oloq-x% Ag20 [x varies between 0-6 in the interval of 1] have been prepared by the solid-state reaction method. After usual characterisation by XRD, resistivity and scanning electron microscopy, bulk density porosity studies etc., the ultrasonic measurements were carried out by the pulse transmission technique in the temperature range of 80-300 K. It has been observed that two samples viz., Bi-2223-0 and Bi-2223-1 are found to exhibit an elastic softening in the temperature region 300 to 210 K, while all the seven samples exhibited a velocity maximum between 125 to 190 K. Qualitative explanations for both the phenomena of elastic softening and the velocity maxima are given on the basis of the microstructure and ordering readjustment of the oxygen atoms among the Cu-O planes of the samples.
1. I n t r o d u c t i o n
It was generally recognised that the phenomenon of lattice/structural instabilities above Te is a common feature among A-15 compounds (Nb3Sn, V3Si, Nb3Ge etc.), generally known as the conventional high-T~ superconductors. In fact, it has been observed that there is a close relationship between the high value of T~ and the lattice instabilities among A-15 type of superconductors [1]. Immediately after the discovery of ceramic superconductors, many investigators started looking for a similar type of correlation between the high value of T~ and instabilities if any. As the lattice instabilities are manifest well in the elasticity studies, a number o f investigations
* Corresponding author.
were undertaken with a view to know whether the correlation between the high value of Tc and Structural instabilities is still valid among the ceramic superconductors. Even eight years after the discovery of high-T~ superconductors, there are several reports both in favour and against the phenomenon of lattice instabilities among this new class of materials. During the last five years, a few papers published from this laboratory also [2-4] consistently indicate the presence of lattice instabilities among ceramic superconductors. However, none o f the published works so far give any direct or indirect evidence for a direct relationship between these two parameters. With a view to shed some more light on this particular aspect and generalise the observed results, the authors of the present investigation have undertaken yet another investigation on another set of samples viz., B i - 2 2 2 3 / A g composites. In fact when A g 2 0 is
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI 0921-4534(95)00320-7
90
R.J. Topare et aL / Physica C 253 (1995) 89-96 10-
added, it is expected to remove the microstructural defects of the ceramic superconductors. As such, the influence of A g 2 0 on the lattice instabilities is another aspect of the present investigation. In view of this, the ultrasonic-velocity measurements of the Bi2 2 2 3 / A g composites, have been undertaken both as a function of temperature and composition and the results are presented here.
82 2. Experimental Samples with the nominal composition Bil. 7Pbo.sSr2Ca2Cu3010 + x % A g 2 0 ( x = 0 - 6 in the interval of 1) were prepared by the well-known solidstate reaction method using highly pure (99.999%) Bi20 s, PbO, SrCO 3, CaCO 3, CuO and Ag20. The powders after thorough mixing and grinding were calcined at 800°C for 24 h followed by two more similar heat treatments at 810 and 840°C for 24 h each. The calcined powders after pressing into discs of 10 m m in diameter and 2 to 3 m m in thickness were sintered at 850°C for 200 h. With the aim to determine the superconductivity transition temperature (T¢), the DC resistance measurements were carried out over a temperature range of 80-300 K, by using the standard four-probe method. Later, the X-ray diffraction studies were undertaken with C u K c~ radiation in the scanning range 20 = 4 - 6 0 °. The surface morphology of a solid material is an important surface property and is very much useful in understanding its defect structure, grain size, voids etc.; and as such it was also studied by scanning electron microscopy. Bulk densities of solids in general are essential to evaluate their elastic moduli and hence they were also determined
0
1O 0
, 150
200
i
250
300
Temperature (K)
Fig. 1. Resistance vs. temperature plot of Bil.7 Pbo.3Sr2Ca2Cu 3Olo.
by an immersion method. Finally, the ultrasonic velocities were measured over a temperature range 90-300 K by the pulse transmission technique, the details of which are given elsewhere [5].
3. Results and discussion 3.1. Electrical resistance versus temperature
The electrical-resistance measurement of all the samples is carried out in the temperature range of 80-300 K and the plots of all the samples are found to show a single-step superconducting transition with T~ fluctuating around 105 K. The resistance versus temperature plot of the sample Bi-2223-0 sample is shown in Fig. 1. The room-temperature resistance value R300, Tc onset, Tc(o) and AT for all the samples are given in Table 1. As expected, the R300 is almost
Table 1 Room-temperature data of BI-2223/Ag composites Compositional formula Ri i.TPb0.3Sr2Ca2Cu3Olo Bit.7Pbo.3Sr2Ca2CU3Olo Bil.vPbo.3Sr2Ca2Cu3010 BiL7Pb0.3Sr2Ca2Cu3O10 Bil.7Pb0.3Sr2Ca2Cu3Oio Bil.7Pbo.3Sr2Ca2Cu3O10 Bil.TPb0.3Sr2Ca2Cu3Olo
+ + + + + +
I% 2% 3% 4% 5% 6%
Ag20 Ag20 Ag20 Ag20 AgzO Ag20
Sample code
R3o0 (m~)
Tconset (K)
Tc(o) (K)
AT (K)
Bi-2223-0 Bi-2223-1 Bi-2223-2 Bi-2223-3 Bi-2223-4 Bi-2223-5 Bi-2223-6
10.2 5.3 5.3 7.4 4.8 6.6 3.7
113 109 107 109 106 106 109
106 106 104 106 104 104 106
2 3 3 3 2 2 3
R.J. Topare et a l . / Physica C 253 (1995) 89-96
found to decrease with increasing concentration of Ag. 3.2. X-ray diffraction
It has been observed from the XRD patterns of the samples that all of them are having a major orthorhombic phase and a close examination of the patterns reveals that an additional peak representing silver is found to appear by the side of the peak with a [hkl] value of [1 1 11]. Using the d spacing values, the lattice parameters and hence the X-ray densities have been computed and are included in Table 2. The volume fraction of the high-T~ phase present in all the samples of the present investigation has been estimated using the XRD data and relation [6] I00 lo¢a)/( Ioo lo(n) + Ioo 8(L)),
(1)
where Ioo 10¢H) and I008¢L) are the intensities of reflection pertaining to the high- and the low-T~ phases, respectively. It has been found that the volume fraction of high-T~ phase in all the samples is about 80%. 3.3. Scanning electron microscopic studies
The scanning electron micrographs of all the samples are shown in Figs. 2(a-g). It can be seen from the figures that the grains of the samples are randomly oriented with reasonable size of more than 5 p~m. It has also been observed that the size of the grains and hence the voids are found to decrease with increasing silver concentration. In fact, this observation is in conformity with the experimentally obtained porosity values.
91
3.4. Elastic moduli at room temperature
The experimental values of longitudinal (V t) and shear wave velocities ( ~ ) of Bi-2223/Ag composites along with those of Young's (E), and rigidity moduli (G) are given in Table 2. A continuous increase in the values of both the moduli and Tc and a gradual decrease in the R300 values with increasing Ag20 concentration clearly indicate that the mechanical strength of the samples is increasing continuously. The observed behaviour is quite possible because silver oxide is expected to precipitate along the grain boundaries, improving the weak links between the grains. 3.5. Correction to zero porosity
In general, the high-T~ superconducting samples prepared by the solid-state reaction method are highly porous. In fact. the porosities of the samples of the present investigation are found to vary from 23 to 40% and as such the measured elastic moduli do not have much significance unless they are corrected to zero porosity. Therefore, the elasticity parameters have been corrected to zero porosity to obtain the intrinsic material values using the Ledbetter and Datta formula [7]. For this purpose, it has been assumed that the sample possesses mainly randomly distributed spherical pores and that the voids contain zero elastic resistance to both dilation and shear. The equations employed are Go=(1/2AI)[-A
2 + ( A 2 2 - 4 A 1 A 3 ) 1/2]
(2)
and (3)
B o = 4 G OB / 4 ( 1 - C) G O - 3 CB,
Table 2 Elastic moduli at room temperature Sample code
Vt (m/s)
Vs (m/s)
C (%)
p (10 3 k g / m m 3)
Pm (10 3 k g / m m 3)
G (GPa)
E (GPa)
Bi-2223-0 Bi-2223-1 Bi-2223-2 Bi-2223-3 Bi-2223-4 Bi-2223-5 Bi-2223-6
2776 2520 2688 2777 2898 3699 3763
1162 1211 1274 1327 1406 1452 1459
40.4 38.9 38.3 33.7 25.9 23.3 24.2
3.73 3.82 3.86 4.15 4.64 4.80 4.74
6.27 6.26 6.27 6.26 6.23 6.24 6.23
5.0 5.6 6.2 7.3 9.2 10.1 10.1
14.0 15.1 16.9 19.7 24.7 28.5 28.5
92
R.J. Topare et al. / Physica C 253 (1995) 89-96
Fig. 2. Scanning electron micrograph of Bi 17Pbo 3Sr2Ca2CuaOlo + x% Ag20; (a) x = 0, (b) x = 1, (c) x = 2, (d) x = 3, (e) x = 4, (f) x = 5 , (g) x = 6 .
R.J. Topare et al. / Physica C 253 (1995) 89-96 Table 3 Elastic moduli corrected to zero porosity
Sample code
GO (GPa)
Eo (GPa)
Tm (K)
Bi-2223 -0 Bi-2223-1 B i-2223-2 Bi-2223-3 Bi-2223-4 Bi-2223-5 Bi-2223-6
10.7 11.9 13.1 13.8 14.9 15.3 15.6
32.2 34.1 37.6 39.5 41.8 45.3 46.1
125 130 160 145 160 170 190
where A~ = 8(1 - C ) / 3 , A z = (3 - 2 C ) B - [(8/3) + 4 C ) G and A 3 = - 3 ( 1 + C)BG; C, p and P0 are the void fraction, the macroscopic and X-ray mass densities, respectively, while B, G and E represent the experimental values of bulk, rigidity and Young's moduli, respectively, and B 0, G O and E 0 are the respective elastic moduli of a non-porous material. The elastic moduli corrected to zero porosity are given in Table 3. It can be seen from the table that the E 0 and G O values of the Bi-2223-0 sample are low when compared with those reported by others. This may be due to the fact that the sample is having 20% of Bi-2212 phase also. In general it is known that the low-T~ Bi-2212 phase always exhibits low elastic moduli when compared with high-Tc Bi-2223 phase [8]. 3.6. Interpretation of elastic data in terms of strength of the materials Following Wooster's work [9], the experimental data obtained in the present study may be interpreted in terms of the binding forces between various ions. As both the elastic moduli are increasing continuously with increasing A g 2 0 concentration, it may be presumably assumed that the binding forces between various ions of the samples might be increasing continuously. This must have resulted due to the fact that the silver precipitates on the grain boundary, improving the weak links between the grains.
93
the Bi-2223-0 and Bi-2223-1 sample after an initial decrease is found to increase continuously with decreasing temperature, while that of the remaining ones [Figs. 4 and 5] is found to increase continuously right from 300 K onwards. Thereafter, in all the cases, the velocity reaches a maximum value at a particular temperature, hereafter designated as Tm. On further decrease of temperature, however, the velocity is found to decrease continuously. The values of Tm for all the samples are noted and are given in Table 3. Except in the case of the third sample, Tm is found to increase continuously with increasing AgaO concentration and the observation clearly indicates that AgzO addition might be responsible for shifting the velocity maxima towards the high-temperature side. A close examination of the velocity versus temperature plots indicates that but for the occurrence of two distinct and different phenomena, the velocity is found to increase continuously with decreasing temperature and as such it may be concluded that the general elastic behaviour of the samples of the present investigation is analogous to that of a number of oxide materials [10,11]. The two anomalous phenomena observed are (1) a decrease of velocity with decreasing temperature in the case of Bi-2223-0 and Bi-2223-1 samples in the temperature range 300-210 K and (2) The exhibition of a velocity maximum at a particular temperature (Tm) by all the samples.
4000-
~
3000 .O~---0
E ~2000 (i)
>
3. 7. Elastic behaoiour at low temperatures 1000
The plots of the longitudinal velocity versus temperature of all the samples are shown in Figs. 3 to 5. It can be seen from Fig. 3, that the velocity of both
go
1¢o
~g6 . . . . . . . ~ki . . . . . . . Temperature (K)
~6d'
Fig. 3. Longitudinal velocity vs. temperature of Bi 1.7Pbo.3Sr2Ca2 Cu3010 + x% Ag20; x = 0, x = 1.
R.J. Topare et al. / Physica C 253 (1995) 89-96
94 5500-
~ v
4500
E
83500 (1> >
~¢
2500
90
,~
X-- 3
140
~
190
2
Temperoture
240
290
(K)
Fig. 4. Temperature variation of the longitudinal velocity of Bil.TPbo.3Sr2Ca2Cu3Olo + x % A g 2 0 ; x = 2, x = 3, x = 5.
Now the occurrence of these two anomalies can be explained as follows.
3. 7.1. Decrease of velocity with decreasing temperature A close examination of the literature on the elastic behaviour of high-To materials indicates that elastic anomalies especially in the vicinity of room temperature are very common. For example, Tritt et al. [12] reported a decrease in Young's modulus by a factor of 3 to 4 in Bi-2212 whiskers in the temperature
5500
%-
"E'45oov xr-ll
0 -~50o >
4
2500
O0
........ 14d i .......
1~d . . . . . . . ~k; . . . . . . . Temperature (K)
2]d '
Fig. 5. Plots o f the longitudinal velocity vs. t e m p e r a t u r e o f B i l . 7 P b o . 3 S r 2 C a 2 C u 3 O l o + x % A g 2 0 ; x = 4, x = 6.
range 330 to 270 K. These authors explained the anomalous elastic behaviour as due to the presence of stress-induced phase transitions. Similarly, Jacobson et al. [13] also observed that the modulus along the a direction is found to be lower than earlier reported values for this material. The authors speculated that the lattice was soft along a, the direction along which whiskers grow and also believed that the low E value of Bi-2212 whiskers along the a direction when compared with that of pallets must be due to the failure of the whiskers to orient in a specific direction. In the present investigation, as there is no confirmation of the stress-induced phase transitions either directly or indirectly, it is thought that the type of anomalies around room temperature in the case of Bi-2223-0 and Bi-2223-1 may not be due to them. Further, since none of the two samples (Bi-2223-0 and Bi-2223-1) of the present investigation exhibited such low moduli along any particular direction, the elastic softenning observed in the present investigation could be attributed to reasons other than the failure of whiskers/grains to orient in any particular direction. Further examination of the literature indicates that Kim et al. [15] reported an anomalous variation of the ultrasonic velocity in the case of Y-123 samples, while an identical behaviour was also reported by Zhao et al. [16] and Xu et al. [17] in the case of sintered forged materials and also by Lang et al. [18] in the case of coarse-grain Y-123 sintered materials. These authors observed that the phenomenon of softening occurs only when the grains of the sintered materials are grown to a sufficient size and that the softening of the lattice may not be possible in a sample with fine grains ( < 4 i~m) and high density ( > 90%) [19,20]. It was also reported [21,22] that the grain size is the main controlling factor for the elastic softening behaviour of the oxide superconductors. In view of these observations, it may be concluded that the anomalous variation of the longitudinal velocity with decreasing temperature observed among the Bi-2223-0 and Bi-2223-1 samples of the present investigation [Fig. 3] could be attributed to the presence of a large number of coarse grains. The high porosity of the order of 40% with least contact among the grains of these samples, as can be seen from their scanning electron micrographs, may also be responsible for the observed elastic behaviour.
R.J. Topare et aL / Physica C 253 (1995) 89-96
3. 7.2. Velocity maximum Velocity maxima are not totally uncommon to high-T~ superconducting materials, as several researchers reported similar ones earlier [23-27]. For example, Nes et al. [14] reported the existence of an internal friction peak in the case of Bi-2212 single crystal at 145 K and attributed this to the Debye relaxation. It was also observed that the elastic instabilities in solids are generally manifest either by a downward cusp or a step. Cusps are usually observed near phase transitions mainly due to the weakening of certain force constants, such as softening of phonon modes etc., while the velocity maxima are observed near order-disorder transitions resulting from the reorientation or ordering of oxygen ions in the copper-oxygen planes. It was also observed that the lattice instabilities above Tc are a sort of precursors to the superconducting phenomenon. In view of this, it may be speculated that the velocity maxima observed among the samples of the present investigation could be due to the ordering readjustments of the oxygen atoms among the C u - O planes. Further, as the velocity maximum (Tm) of the samples is shifting towards the high-temperature side with increasing Ag20, it may be concluded that A g 2 0 is assisting indirectly the phenomenon of ordering of oxygen ions in the C u - O planes. Thus, it may be concluded that the two types of lattice instabilities, viz. the elastic softening in the temperature range 300-210 K and the occurrence of a maximum in the velocity versus temperature plots have been observed among the samples of the present investigation. Further, although both these types of lattice instabilities were observed not only among the samples of the present investigation but also in a few other cases earlier [2,3], no definite relationship between the high value of T~ and lattice instabilities, could be arrived at. As there is no particular trend to understand the phenomenon, some more studies on a variety of superconducting systems with different grain sizes are needed. Investigations of this effect are under way.
Acknowledgements All the authors thank the Department of Science and Technology, Government of India, for providing
95
funds to carry out research under the National Superconductivity Programme (NSP). The authors also thank K.N. Srivasthava, University of Hyderabad, Hyderabad for providing necessary facilities in taking scanning electron micrographs.
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[23] R.J. Kennedy, W.G. Jenks and L.R. Testardi, Phys. Rev. B 40 (1989) 11313. [24] G. Canneli, R. Cantelli, F. Cordero and F. Trequattrini, Supercond. Sci. Technol. 5 (1992) 247. [25] M. Chankurtaran and G.A. Saunders, Supercond. Sci. Technol. 5 (1992) 529.
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