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Mechanical tensile promotion and superconducting properties of highly Pb-content HTc-BPSCCO superconductor
Materials Science and Engineering B103 (2003) 71 /76 www.elsevier.com/locate/mseb
Mechanical tensile promotion and superconducting properties of highly Pb-content HTc-BPSCCO superconductor Morsy M.A. Sekkina, Khaled M. Elsabawy * HTc-Ceramic Superconductors Unit, Chem. Department, Faculty of Science, Tanta University, Tanta, Egypt Received 21 November 2002; accepted 22 April 2003
Abstract The samples of the general formula (Bi0.8Pb0.2x )2Sr2Ca2Cu3O10, where 0.05/x 5/0.3 mol% were prepared by the conventional high temperature solid state reaction technique. The extra lead (x ) was added as very fine powder of pure Pb-metal with particle size 5/50 mm. The superconducting measurements for HTc-2223-phase proved that, best Tc /109 K is for the sample with x /0.2 (Pbcontent /0.4 mol%) while the lowest Tc /98 K is for the sample with x /0.3 mol%. The evaluated crystalline lattice structure of the prepared samples mainly belongs to the superconductive tetragonal phase (2223), besides secondary (2201) and (2212) phases in minor. The c -axis lattice parameter exhibits length elongation as x (Pb-content) increases from x /0.1 to 0.3. The maximum mechanical tensile strength values at 295 K were promoted and found to be 37.5, 38.6 and 41.4 MPa as Pb-content increases from x /0.1 to 0.3, respectively. # 2003 Elsevier B.V. All rights reserved. Keywords: BPSCCO-superconductors; Oxides; X-ray diffraction; Mechanical properties
1. Introduction HTcs-ceramics are generally recognized to have poor mechanical properties, such as low stiffness, strength and toughness and for these defects in mechanical properties many authors investigated the effect of some additions such as Al2O3, Ag-metal and Ag2O during the processing to improve such these defects and to increase range of applications [1 /5]. Superconducting properties of Bi2Sr2CaCu2Ox (BSCCO) depend on oxygen content [6 /11]. The superconducting transition temperature Tc decreases for x ]/ 8.18 and is dependent on annealing temperatures and cooling rates [12]. Oxygen vacancies have also been suggested as major source of flux pinning in BSCCO [13]. The kinetics of oxygen motion and the formation and migration of oxygen defects in BSCCO can be studied by measurement of the oxygen-tracer diffusion * Corresponding author. Present address: C/o Jansen Gp., Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D70569 Stuttgart, Germany. Fax: /49-711-689-1010. E-mail addresses: [email protected], [email protected] (K.M. Elsabawy). 0921-5107/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5107(03)00143-0
parameters; these provide not only input to theoretical point-defect models, but can also be useful in developing fabrication techniques. The pseudo-tetragonal 85 K BSCCO superconductor Bi2Sr2CaCu2Ox , or 2:2:1:2 consists of one Ca atom symmetrically located between the layer sequences Cu /O, Sr/O and Bi /O each layer parallel to the ab plane [14]. Because the structure is pesudo-tetragonal, diffusion in BSCCO can be described by two diffusion coefficients, one in the ab plane and one parallel to the c direction. X-ray analysis [15] indicates that the cations and anions in BSCCO undergo incommensurate modulations, these modulations have been attributed to the presence of extra oxygen ions in the Bi /O layer [16]. The presence of the extra oxygen ions in the Bi/O layer suggests that diffusion in the ab plane occurs via an interstitial mechanism. Diffusion in the c direction, on the other hand, involves crossing Sr/ O, Cu /O and Ca-planes, on which there are no low energy sites for oxygen interstitials and on which the concentration of vacant oxygen sites is low. Diffusion in the ab plane could thus be much faster than along the cdirection. The Ag-sheathed multifilamentary types of (Pb/Bi)2Sr2CaCu2Ox (Bi-2223) are an important development towards the practical use of high Tc-super-
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conductors for power application. These types show an improved mechanical stability and strain tolerance as compared with mono-filamentary types [17]. For the development of Bi-2223 multi-filamentary tapes in long lengths it is very important to control the defects introduced into the filaments which may alter or even impede the current flow [18]. Primo et al. [19] have been investigated the fast synthesis of single-phased 110 K bismuth superconductor by freeze-drying technique of acetic precursors and the kinetic role of calcium and copper oxide deducing that, formation of bismuth/lead oxoacetate intermediate and excess of Ca2CuO3 are the two main factors driving the fast synthesis of the 110 K HTc-BPSCCO-superconductor. Xi et al. [20] studied the effect of Bi/Pb ratio and annealing temperature on the HTc-BPSCCO system and they reported that, the optimum ratio of Bi/Pb is 1.8:0.3 and opimum annealing temperature is in between 845 and 855 8C. The major goal of the present article is to investigate the effects of new variations of Pb-content using pure Pb-metal as ceramic softener with fixed Bi ratio /0.8 mol% on the mechanical, structural, electrical, thermal and superconducting parameters of BPSCCO. 2. Experiments 2.1. Sample preparation The parent BPSCCO (Bi0.8Pb0.2)2Sr2Ca2Cu3O10 and its variant Bi/Pb content family members; (Bi0.8Pb0.3)2Sr2Ca2Cu3O10, (Bi0.8Pb0.4)2Sr2Ca2Cu3O10, and (Bi0.8Pb0.5)2Sr2Ca2Cu3O10 were prepared by the conventional solid state reaction method and sintering procedure using the appropriate amounts of Bi2O3, PbO, SrCO3, CaCO3 and CuCO3 each of highly pure chemical grade purity /99.9%. The mixtures were calcined at 800 8C under a compressed O2 atmosphere for 20 h then the extra lead (x ) was added as very fine powder of pure Pb-metal with particle size 5/50 mm after that the mixture reground and pressed into pellets (thickness 2 cm and diameter 1.2 cm). Sintering were carried out under oxygen stream at (780 /830 8C) for 60 h. The temperature was slowly cooled down (20 8C h 1) till 500 8C and annealed there for 20 h under oxygen stream, then the furnace is cooled slowly down to room temperature. Finally the materials are kept in vacuum desiccator over silica gel dryer. A levitation preliminary superconductivity test was thoroughly applied for the achievement of superconductive phase and hence superconductivity. 2.2. Structural measurements The X-ray diffraction (XRD) measurements were carried out at room temperature on the fine ground
samples using Cu-Ka radiation source, Ni-filter and a computerized Shimadzu (Japan) diffractometer. 2.3. Superconducting measurements The cryogenic DC-electrical resistivity of the prepared materials were undertaken as a function of temperature using the four-probe technique and the temperature was recorded in the cryogenic temperature zone down to 30 K using liquid helium refrigerator. 2.4. Thermal analysis measurements The TGA thermal analysis measurement was carried out on the green mixtures of the prepared samples to select the best sintering temperature using a computerized Shimadzu (Japan) TGA analyzer. 2.5. Mechanical measurements Samples were cut from the bulk by certain dimensions 3 /3 /5 mm3, to make as possible the longitudinal loading direction of the specimens parallel to a- or bdirection and perpendicular to the c -direction of the bulk. Tensile tests were carried out at room temperature 295 K and details of loading was described by Tomita and Murakami [21], the displacement speed of the acuator of testing machine was in between 0.10 and 0.15 mm min 1. and observation of fracture morphology of the samples was carried out using SEM. 2.6. Microstructure measurements The morphological features and bulk grain size calculations were carried out through different sectors in the investigated samples using high resolution SEM (Philips-USA).
3. Results and discussion 3.1. Structural measurements Fig. 1a /d: displays the X-ray powder diffractometry patterns of the parent BPSCCO (Bi0.8Pb0.2)2Sr2Ca2Cu3O10 and variant Bi/Pb content family members; (Bi0.8Pb0.3)2Sr2Ca2Cu3O10, (Bi0.8Pb0.4)2Sr2Ca2Cu3O10 and (Bi0.8Pb0.5)2(Sr2Ca2Cu3O10. The analysis of the corresponding 2u values and the interplanar spacings ˚ ) were carried out by pdp (X-ray powder diffraction d (A data analysis program) using computer, indicated that, the X-ray crystalline structure belongs mainly to a HTc2223-phase which is a tetragonal crystal form. This is beside the existence of 2212 and 2201 phases in minor as shown in Fig. 1a /d. The unit cell dimensions were
M.M.A. Sekkina, K.M. Elsabawy / Materials Science and Engineering B103 (2003) 71 /76
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calculated using parameters of the most intense X-ray reflection peaks for HTc-2223-phase and found to be ˚ and c /37.6732 A ˚ for the parent a /b /3.8176 A BPSCCO (Table 1). From Table 1 it is clear that the c -axis exhibits length elongation (see Fig. 1e) while either a or b axis exhibits length compression (shrinkage) by increasing Pb-content (x /0.1 0/0.3) due to atomic radius of Pb-ion is greater than that of Bi-ion. These results are in full agreement with Hong et al. [22], they have been reported that, c-axis of 2223-HTcBPSCCO is increased as Sr-content increased at the expense of Ca-content which is achieved in our article, c -axis increases as Pb-content increased. Table 1 The calculated lattice parameters for the prepared samples Materials
˚) a /b (A
˚) c (A
(Bi0.8Pb0.2)2Sr2Ca2Cu3O10 (Bi0.8Pb0.3)2Sr2Ca2Cu3O10 (Bi0.8Pb0.4)2Sr2Ca2Cu3O10 (Bi0.8Pb0.5)2(Sr2Ca2Cu3O10
3.81769/(0.03) 3.85439/(0.03) 3.83319/(0.03) 3.80679/(0.03)
37.67329/(0.03) 37.71949/(0.03) 37.73329/(0.03) 37.78279/(0.03)
3.2. Thermogravimetric measurement The TGA analysis was carried out on the green mixtures of the prepared HTc-ceramic BPSCCO superconductor. From TGA curves (see Fig. 2a/d), the TGA analysis can be divided into four fundamental steps as the following; the first step that occupies the region from room temperature untill 230 8C for which the weight loss occurred is attributable to the humidity content of samples and partial decomposition of CuCO3. The second region from 230 up to 400 8C is due to exothermic complete decomposition of CuCO3 into
Fig. 1. The obtained room temperature X-ray: diffraction patterns for the prepared: (a) (Bi0.8Pb0.2)2Sr2Ca2Cu3O10; (b) (Bi0.8Pb0.3)2Sr2Ca2Cu3O10; (c) (Bi0.8Pb0.4)2Sr2Ca2Cu3O10; (d) (Bi0.8Pb0.5)2Sr2Ca2Cu3O10. HTc-ceramic superconductors. (e) The variation of c -axis vs. Pbcontent ratio.
Fig. 2. The TGA curves for starting powder materials corresponding to: (a) (Bi0.8Pb0.2)2Sr2Ca2Cu3O10; (b) (Bi0.8Pb0.3)2Sr2Ca2Cu3O10; (c) (Bi0.8Pb0.4)2Sr2Ca2Cu3O10; (d) (Bi0.8Pb0.5)2Sr2Ca2Cu3O10. HTc-ceramic superconductors.
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Fig. 3. The variation of DC-electrical resistivity as a function of Absolute temperature for: (a) (Bi0.8Pb0.2)2Sr2Ca2Cu3O10; (b) (Bi0.8Pb0.3)2Sr2Ca2Cu3O10; (c) (Bi0.8Pb0.4)2Sr2Ca2Cu3O10; (d) (Bi0.8Pb0.5)2Sr2Ca2Cu3O10. HTc-ceramic superconductors.
CuO plus CO2. The third region of exothermal decomposition in the range 400 /660 8C in junction with weight loss in the TGA curve is attributable to three contributers, the first and the second are the partial thermal decomposition reactions of both CaCO3 and SrCO3, respectively, and the third reaction is due to initial solid state reaction between mixed oxides forming precursor of new phase (phase transition). And the four step occupying the range between 660 and 820 8C at which, complete decomposition of both CaCO3 and SrCO3 passing through solid state reaction among oxide mixture. For sample (d) with x /0.3 mol% highest content of Pb. One can notify an extra weight loss peak at /700 8C attributable to partial sublimation of Pb [23] and finally the formation of the pseudo-tetraganal superconductive phase indothermally at 790 8C depending on chemical constitutions present. 3.3. Superconducting measurements The cryogenic DC-electrical resistivity versus cryogenic temperatures was carried out using the four probe terminal technique. Thus Fig. 3a /d shows the temperature dependence of electrical resistivity of the prepared
Fig. 4. (A) Average of mechanical tensile strength for the samples: (a) (Bi0.8Pb0.2)2Sr2Ca2Cu3O10; (b) (Bi0.8Pb0.3)2Sr2Ca2Cu3O10; (c) (Bi0.8Pb0.4)2Sr2Ca2Cu3O10; (d) (Bi0.8Pb0.5)2Sr2Ca2Cu3O10. HTc-ceramic superconductors. (B) The variation of fracture stress as a function of Pb-content.
the parent BPSCCO, (Bi0.8Pb0.2)2Sr2Ca2Cu3O10 and variant Bi/Pb content; (Bi0.8Pb0.3)2Sr2Ca2Cu3O10, (Bi0.8Pb0.4)2Sr2Ca2Cu3O10, and (Bi0.8Pb0.5)2Sr2Ca2Cu3O10, respectively. It is clear that, the critical Tcoffsets of the variant Bi/Pb superconductor samples are
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106, 109 and 98 K, respectively, while the Tc-offset / 101 K for the parent BPSCCO superconductor, and their Tc-onsets are 113, 115 and 113 K, respectively, while Tc-onset for the parent BPSCCO superconductor is 110 K. These data indicate that the best superconducting sample is that with Bi/Pb content / (Bi0.8Pb0.4)2 with x /0.2 mol% which enhances the formation of the superconductive tetragonal phase. Sastry and West [24] have confirmed that, the Tc of 2223-phase is less sensitive, than the 2212 and 2201 phases to both composition and heat treatment conditions, which is fully consistent with our results. 3.4. Mechanical tensile and microstructure (SEM) measurements The tensile strength as a function of displacement of the actuator (stroke mm) at the fracture at 295 K were
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carried out see Fig. 4a, it is clear that, the maximum average of tensile strength increases as Pb-content increase and the maximum tensile average 41.4 MPa is for the sample with Pb-content /0.5 mol%, which means increasing of Pb-content plays an important role in the liquid phase (Pb-rich phase) during the reaction conversion of 2212 0/2223 and moreover extra lead particles (x -values) dispersed regularlly in the matrix of BPSCCO making it more ductile material and consequently the stroke increases from x /0.1 mol% (Stroke /0.2 mm) to x /0.3 mol% (Stroke /0.29 mm) and these results are in full agreement with those reported by [25,26]. From Figs. 1e and 4b; one can indicates that, the solubility limits of Pb reach /0.45 mol% which is considered higher than that reported by Ikeda et al. [27,28], they reported that the solubility limits of Pb, xPb /0.35 /0.4 mol%. In our speculations this enhancement
Fig. 5. The SEM micrograph through ab-plane for the materials corresponding to: (a) (Bi0.8Pb0.2)2Sr2Ca2Cu3O10; (b) (Bi0.8Pb0.3)2Sr2Ca2Cu3O10; (c) (Bi0.8Pb0.4)2Sr2Ca2Cu3O10; (d) (Bi0.8Pb0.5)2Sr2Ca2Cu3O10. HTc-ceramic superconductors.
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of solubility limits is due to two reasons 1st, the selected particle size of Pb-metal 5/50 mm increase the compatibility of Pb in the matrix of BSCCO and the 2nd is addition of extra lead (x ) as metal as starting material decreases the temperature of formation liquid phase than that of lead oxide and consequently increase the chance of reaction between 2212-phase and Pb-richphase to produce HTc-2223-phase [29]. Azzouz et al. [30] investigated the effect of lead oxide additions up to (20%)on the final stage of processing BPSCCO tap and they confirmed that PbO play an important role in the step of liquid phase formation and considered as catalytic role as Ca/Pb reservoir that finally react with 2212-phase to form HTc-2223 superconducting phase. Fig. 5a/d; show the SE-micrograph of investigated samples. The calculation of average grain size were carried out and found to be in between 0.4 and 1.25 mm. The results obtained from SEM mapping and EDX analyses supported the results of X-ray specially in the view of multi-composition phases. Some of Pb-metal was identified between grains in the bulk of sample (d) which have higher Pb-content /0.5 mol% (x /0.3 mol%). 3.5. Conclusion The samples of the general formula (Bi0.8Pb0.2x )2Sr2Ca2Cu3O10, were prepared by the conventional high temperature solid state reaction technique. The extra lead (x ) addition which added as very fine powder of pure Pb-metal with particle size 5/50 mm exhibits remarkable promotion in the mechanical tensile properties and processing of PBSCCO system. The maximum average of tensile strength increases as Pb-content increase and was found to be 41.4 MPa for the sample with maximum Pb-content /0.5 mol% (x / 0.3). The extra lead (x ) in the investigated range 0.0 5/ x 5/0.3 mol% has a small effect in both superconducting and microstructure properties of PBSCCO system.
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[3] N. Alford, T.W. Button, J.D. Birchall, Supercond. Sci. Technol. 3 (1990) 1 /7. [4] F.G. Chan, M. Cankurtaran, G.A. Saunders, D.P. Almond, P.J. Ford, Supercond. Sci. Technol. 413 (1991) 20. [5] A. Miyase, Y. Yuan, M.S. Wong, J. Schon, S.S. Wang, Supercond. Sci. Technol. 8 (1995) 626. [6] H. Niu, N. Fukushima, K. Ando, Jpn. J. Appl. Phys. 27 (1988) L1442. [7] D.E. Morris, C.T. Hultgren, A.M. Markelz, J.Y.T. Wei, N.G. Asmar, J.H. Nickel, Phys. Rev. B39 (1989) 6612. [8] J. Zhao, M.S. Seehra, Physica C 159 (1989) 639. [9] M.R. Presland, J.L. Tallon, R.G. Buckely, R.S. Liu, N. Flower, Physica C 176 (1990) 95. [10] H.C.I. Kao, W.L. Chen, T.P. Wei, J.C. Lui, C.M. Wang, Physca C 177 (1993) 376. [11] G. Miehe, T. Vogt, H. Fuess, M. Wilhelm, Physica C 171 (1990) 339 /343. [12] J.M. Tarascon, W.R. McKinnon, P. Barboux, D.M. Hwang, B.G. Bagley, L.H. Greene, G.W. Hull, Y. Lepage, N. Stoffel, M. Giround, Phys. Rev. B38 (1988) 885. [13] E.M. Chudnovsky, Phys. Rev. Lett. 65 (1990) 3060. [14] R.M. Hazen, C.T. Prewitt, R.G. Angel, N.L. Ross, L.W. Finger, C.G. Hadidiacos, D.R. Veblen, P.H. Hor, R.L. Meng, C.W. Chu, Phys. Rev. Lett. 60 (1988) 1174. [15] H.W. Zandbergen, W.A. Groen, F.C. Mijhoff, G. Van Tendeloo, S. Amelinkx, Physica C 156 (1988) 325. [16] V. Petrick, Y. Gao, P. Lee, P. Coppens, Phys. Rev. B42 (1990) 387. [17] Z. Han, P. Skov-Hansen, T. Ferltof, Supercond. Sci. Technol. 10 (1997) 371. [18] U. Welp, D.O. Gunter, G.W. Crabtree, W. Zhong, U. Balash, P. Haldar, V.K. Vlasko Vlasov, V.I. Nikitenico, Nature (London) 276 (1996) 44. [19] V. Primo, F. Sapina, M. Sanchis, R. Ibanez, A. Beltran, D. Beltran, Solid State Ionics 63 /65 (1993) 872 /882. [20] X. Zhengping, J. Chunlin, Z. Lain, J. Supercond. (3) (1990) 421 / 423. [21] M. Tomita, M. Murakami, Supercond. Sci. Technol. 13 (2000) 722. [22] B. Hong, T.O. Mason, J. Am. Ceram. Soc. 74 (5) (1991) 145 /152. [23] D. Pandey, A.K. Singh, P. Srivastava, A.P. Singh, S. Inbanathan, G. Singh, Physica C 241 (1995) 279 /291. [24] P.V.S. Sastry, A.R. West, Physica C 235 /240 (1994) 503 /504. [25] M. Tomita, M. Murakami, Supercond. Sci. Technol. 15 (2001) 932. [26] L.N. Wu, C. Hsu, C. Lee, Jpn. J. Appl. Phys. 37 (1998) L438. [27] Y. Ikeda, M. Takano, Z. Hiroi, K. Oda, J. Takada, Y. Miura, Y. Takeda, O. Ymamoto, H. Mazaki, Jpn. J. Appl. Phys. 27 (1998) L2067. [28] Y. Ikeda, H. Ito, S. Shimomura, Z. Hiroi, Y. Bando, J. Takada, K. Oda, Y. Miura, Y. Takeda, T. Takada, Physica C 190 (1991) 18. [29] Y.S. Sung, E.E. Hellstrom, Physica C 252 (1995) 155. [30] F.B. Azzouz, A. Chiragui, B. Yangui, M. Ben Salem, Physica C 356 (2001) 83.
Mechanical tensile promotion and superconducting properties of highly Pb-content HTc-BPSCCO superconductor Morsy M.A. Sekkina, Khaled M. Elsabawy * HTc-Ceramic Superconductors Unit, Chem. Department, Faculty of Science, Tanta University, Tanta, Egypt Received 21 November 2002; accepted 22 April 2003
Abstract The samples of the general formula (Bi0.8Pb0.2x )2Sr2Ca2Cu3O10, where 0.05/x 5/0.3 mol% were prepared by the conventional high temperature solid state reaction technique. The extra lead (x ) was added as very fine powder of pure Pb-metal with particle size 5/50 mm. The superconducting measurements for HTc-2223-phase proved that, best Tc /109 K is for the sample with x /0.2 (Pbcontent /0.4 mol%) while the lowest Tc /98 K is for the sample with x /0.3 mol%. The evaluated crystalline lattice structure of the prepared samples mainly belongs to the superconductive tetragonal phase (2223), besides secondary (2201) and (2212) phases in minor. The c -axis lattice parameter exhibits length elongation as x (Pb-content) increases from x /0.1 to 0.3. The maximum mechanical tensile strength values at 295 K were promoted and found to be 37.5, 38.6 and 41.4 MPa as Pb-content increases from x /0.1 to 0.3, respectively. # 2003 Elsevier B.V. All rights reserved. Keywords: BPSCCO-superconductors; Oxides; X-ray diffraction; Mechanical properties
1. Introduction HTcs-ceramics are generally recognized to have poor mechanical properties, such as low stiffness, strength and toughness and for these defects in mechanical properties many authors investigated the effect of some additions such as Al2O3, Ag-metal and Ag2O during the processing to improve such these defects and to increase range of applications [1 /5]. Superconducting properties of Bi2Sr2CaCu2Ox (BSCCO) depend on oxygen content [6 /11]. The superconducting transition temperature Tc decreases for x ]/ 8.18 and is dependent on annealing temperatures and cooling rates [12]. Oxygen vacancies have also been suggested as major source of flux pinning in BSCCO [13]. The kinetics of oxygen motion and the formation and migration of oxygen defects in BSCCO can be studied by measurement of the oxygen-tracer diffusion * Corresponding author. Present address: C/o Jansen Gp., Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D70569 Stuttgart, Germany. Fax: /49-711-689-1010. E-mail addresses: [email protected], [email protected] (K.M. Elsabawy). 0921-5107/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5107(03)00143-0
parameters; these provide not only input to theoretical point-defect models, but can also be useful in developing fabrication techniques. The pseudo-tetragonal 85 K BSCCO superconductor Bi2Sr2CaCu2Ox , or 2:2:1:2 consists of one Ca atom symmetrically located between the layer sequences Cu /O, Sr/O and Bi /O each layer parallel to the ab plane [14]. Because the structure is pesudo-tetragonal, diffusion in BSCCO can be described by two diffusion coefficients, one in the ab plane and one parallel to the c direction. X-ray analysis [15] indicates that the cations and anions in BSCCO undergo incommensurate modulations, these modulations have been attributed to the presence of extra oxygen ions in the Bi /O layer [16]. The presence of the extra oxygen ions in the Bi/O layer suggests that diffusion in the ab plane occurs via an interstitial mechanism. Diffusion in the c direction, on the other hand, involves crossing Sr/ O, Cu /O and Ca-planes, on which there are no low energy sites for oxygen interstitials and on which the concentration of vacant oxygen sites is low. Diffusion in the ab plane could thus be much faster than along the cdirection. The Ag-sheathed multifilamentary types of (Pb/Bi)2Sr2CaCu2Ox (Bi-2223) are an important development towards the practical use of high Tc-super-
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conductors for power application. These types show an improved mechanical stability and strain tolerance as compared with mono-filamentary types [17]. For the development of Bi-2223 multi-filamentary tapes in long lengths it is very important to control the defects introduced into the filaments which may alter or even impede the current flow [18]. Primo et al. [19] have been investigated the fast synthesis of single-phased 110 K bismuth superconductor by freeze-drying technique of acetic precursors and the kinetic role of calcium and copper oxide deducing that, formation of bismuth/lead oxoacetate intermediate and excess of Ca2CuO3 are the two main factors driving the fast synthesis of the 110 K HTc-BPSCCO-superconductor. Xi et al. [20] studied the effect of Bi/Pb ratio and annealing temperature on the HTc-BPSCCO system and they reported that, the optimum ratio of Bi/Pb is 1.8:0.3 and opimum annealing temperature is in between 845 and 855 8C. The major goal of the present article is to investigate the effects of new variations of Pb-content using pure Pb-metal as ceramic softener with fixed Bi ratio /0.8 mol% on the mechanical, structural, electrical, thermal and superconducting parameters of BPSCCO. 2. Experiments 2.1. Sample preparation The parent BPSCCO (Bi0.8Pb0.2)2Sr2Ca2Cu3O10 and its variant Bi/Pb content family members; (Bi0.8Pb0.3)2Sr2Ca2Cu3O10, (Bi0.8Pb0.4)2Sr2Ca2Cu3O10, and (Bi0.8Pb0.5)2Sr2Ca2Cu3O10 were prepared by the conventional solid state reaction method and sintering procedure using the appropriate amounts of Bi2O3, PbO, SrCO3, CaCO3 and CuCO3 each of highly pure chemical grade purity /99.9%. The mixtures were calcined at 800 8C under a compressed O2 atmosphere for 20 h then the extra lead (x ) was added as very fine powder of pure Pb-metal with particle size 5/50 mm after that the mixture reground and pressed into pellets (thickness 2 cm and diameter 1.2 cm). Sintering were carried out under oxygen stream at (780 /830 8C) for 60 h. The temperature was slowly cooled down (20 8C h 1) till 500 8C and annealed there for 20 h under oxygen stream, then the furnace is cooled slowly down to room temperature. Finally the materials are kept in vacuum desiccator over silica gel dryer. A levitation preliminary superconductivity test was thoroughly applied for the achievement of superconductive phase and hence superconductivity. 2.2. Structural measurements The X-ray diffraction (XRD) measurements were carried out at room temperature on the fine ground
samples using Cu-Ka radiation source, Ni-filter and a computerized Shimadzu (Japan) diffractometer. 2.3. Superconducting measurements The cryogenic DC-electrical resistivity of the prepared materials were undertaken as a function of temperature using the four-probe technique and the temperature was recorded in the cryogenic temperature zone down to 30 K using liquid helium refrigerator. 2.4. Thermal analysis measurements The TGA thermal analysis measurement was carried out on the green mixtures of the prepared samples to select the best sintering temperature using a computerized Shimadzu (Japan) TGA analyzer. 2.5. Mechanical measurements Samples were cut from the bulk by certain dimensions 3 /3 /5 mm3, to make as possible the longitudinal loading direction of the specimens parallel to a- or bdirection and perpendicular to the c -direction of the bulk. Tensile tests were carried out at room temperature 295 K and details of loading was described by Tomita and Murakami [21], the displacement speed of the acuator of testing machine was in between 0.10 and 0.15 mm min 1. and observation of fracture morphology of the samples was carried out using SEM. 2.6. Microstructure measurements The morphological features and bulk grain size calculations were carried out through different sectors in the investigated samples using high resolution SEM (Philips-USA).
3. Results and discussion 3.1. Structural measurements Fig. 1a /d: displays the X-ray powder diffractometry patterns of the parent BPSCCO (Bi0.8Pb0.2)2Sr2Ca2Cu3O10 and variant Bi/Pb content family members; (Bi0.8Pb0.3)2Sr2Ca2Cu3O10, (Bi0.8Pb0.4)2Sr2Ca2Cu3O10 and (Bi0.8Pb0.5)2(Sr2Ca2Cu3O10. The analysis of the corresponding 2u values and the interplanar spacings ˚ ) were carried out by pdp (X-ray powder diffraction d (A data analysis program) using computer, indicated that, the X-ray crystalline structure belongs mainly to a HTc2223-phase which is a tetragonal crystal form. This is beside the existence of 2212 and 2201 phases in minor as shown in Fig. 1a /d. The unit cell dimensions were
M.M.A. Sekkina, K.M. Elsabawy / Materials Science and Engineering B103 (2003) 71 /76
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calculated using parameters of the most intense X-ray reflection peaks for HTc-2223-phase and found to be ˚ and c /37.6732 A ˚ for the parent a /b /3.8176 A BPSCCO (Table 1). From Table 1 it is clear that the c -axis exhibits length elongation (see Fig. 1e) while either a or b axis exhibits length compression (shrinkage) by increasing Pb-content (x /0.1 0/0.3) due to atomic radius of Pb-ion is greater than that of Bi-ion. These results are in full agreement with Hong et al. [22], they have been reported that, c-axis of 2223-HTcBPSCCO is increased as Sr-content increased at the expense of Ca-content which is achieved in our article, c -axis increases as Pb-content increased. Table 1 The calculated lattice parameters for the prepared samples Materials
˚) a /b (A
˚) c (A
(Bi0.8Pb0.2)2Sr2Ca2Cu3O10 (Bi0.8Pb0.3)2Sr2Ca2Cu3O10 (Bi0.8Pb0.4)2Sr2Ca2Cu3O10 (Bi0.8Pb0.5)2(Sr2Ca2Cu3O10
3.81769/(0.03) 3.85439/(0.03) 3.83319/(0.03) 3.80679/(0.03)
37.67329/(0.03) 37.71949/(0.03) 37.73329/(0.03) 37.78279/(0.03)
3.2. Thermogravimetric measurement The TGA analysis was carried out on the green mixtures of the prepared HTc-ceramic BPSCCO superconductor. From TGA curves (see Fig. 2a/d), the TGA analysis can be divided into four fundamental steps as the following; the first step that occupies the region from room temperature untill 230 8C for which the weight loss occurred is attributable to the humidity content of samples and partial decomposition of CuCO3. The second region from 230 up to 400 8C is due to exothermic complete decomposition of CuCO3 into
Fig. 1. The obtained room temperature X-ray: diffraction patterns for the prepared: (a) (Bi0.8Pb0.2)2Sr2Ca2Cu3O10; (b) (Bi0.8Pb0.3)2Sr2Ca2Cu3O10; (c) (Bi0.8Pb0.4)2Sr2Ca2Cu3O10; (d) (Bi0.8Pb0.5)2Sr2Ca2Cu3O10. HTc-ceramic superconductors. (e) The variation of c -axis vs. Pbcontent ratio.
Fig. 2. The TGA curves for starting powder materials corresponding to: (a) (Bi0.8Pb0.2)2Sr2Ca2Cu3O10; (b) (Bi0.8Pb0.3)2Sr2Ca2Cu3O10; (c) (Bi0.8Pb0.4)2Sr2Ca2Cu3O10; (d) (Bi0.8Pb0.5)2Sr2Ca2Cu3O10. HTc-ceramic superconductors.
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Fig. 3. The variation of DC-electrical resistivity as a function of Absolute temperature for: (a) (Bi0.8Pb0.2)2Sr2Ca2Cu3O10; (b) (Bi0.8Pb0.3)2Sr2Ca2Cu3O10; (c) (Bi0.8Pb0.4)2Sr2Ca2Cu3O10; (d) (Bi0.8Pb0.5)2Sr2Ca2Cu3O10. HTc-ceramic superconductors.
CuO plus CO2. The third region of exothermal decomposition in the range 400 /660 8C in junction with weight loss in the TGA curve is attributable to three contributers, the first and the second are the partial thermal decomposition reactions of both CaCO3 and SrCO3, respectively, and the third reaction is due to initial solid state reaction between mixed oxides forming precursor of new phase (phase transition). And the four step occupying the range between 660 and 820 8C at which, complete decomposition of both CaCO3 and SrCO3 passing through solid state reaction among oxide mixture. For sample (d) with x /0.3 mol% highest content of Pb. One can notify an extra weight loss peak at /700 8C attributable to partial sublimation of Pb [23] and finally the formation of the pseudo-tetraganal superconductive phase indothermally at 790 8C depending on chemical constitutions present. 3.3. Superconducting measurements The cryogenic DC-electrical resistivity versus cryogenic temperatures was carried out using the four probe terminal technique. Thus Fig. 3a /d shows the temperature dependence of electrical resistivity of the prepared
Fig. 4. (A) Average of mechanical tensile strength for the samples: (a) (Bi0.8Pb0.2)2Sr2Ca2Cu3O10; (b) (Bi0.8Pb0.3)2Sr2Ca2Cu3O10; (c) (Bi0.8Pb0.4)2Sr2Ca2Cu3O10; (d) (Bi0.8Pb0.5)2Sr2Ca2Cu3O10. HTc-ceramic superconductors. (B) The variation of fracture stress as a function of Pb-content.
the parent BPSCCO, (Bi0.8Pb0.2)2Sr2Ca2Cu3O10 and variant Bi/Pb content; (Bi0.8Pb0.3)2Sr2Ca2Cu3O10, (Bi0.8Pb0.4)2Sr2Ca2Cu3O10, and (Bi0.8Pb0.5)2Sr2Ca2Cu3O10, respectively. It is clear that, the critical Tcoffsets of the variant Bi/Pb superconductor samples are
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106, 109 and 98 K, respectively, while the Tc-offset / 101 K for the parent BPSCCO superconductor, and their Tc-onsets are 113, 115 and 113 K, respectively, while Tc-onset for the parent BPSCCO superconductor is 110 K. These data indicate that the best superconducting sample is that with Bi/Pb content / (Bi0.8Pb0.4)2 with x /0.2 mol% which enhances the formation of the superconductive tetragonal phase. Sastry and West [24] have confirmed that, the Tc of 2223-phase is less sensitive, than the 2212 and 2201 phases to both composition and heat treatment conditions, which is fully consistent with our results. 3.4. Mechanical tensile and microstructure (SEM) measurements The tensile strength as a function of displacement of the actuator (stroke mm) at the fracture at 295 K were
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carried out see Fig. 4a, it is clear that, the maximum average of tensile strength increases as Pb-content increase and the maximum tensile average 41.4 MPa is for the sample with Pb-content /0.5 mol%, which means increasing of Pb-content plays an important role in the liquid phase (Pb-rich phase) during the reaction conversion of 2212 0/2223 and moreover extra lead particles (x -values) dispersed regularlly in the matrix of BPSCCO making it more ductile material and consequently the stroke increases from x /0.1 mol% (Stroke /0.2 mm) to x /0.3 mol% (Stroke /0.29 mm) and these results are in full agreement with those reported by [25,26]. From Figs. 1e and 4b; one can indicates that, the solubility limits of Pb reach /0.45 mol% which is considered higher than that reported by Ikeda et al. [27,28], they reported that the solubility limits of Pb, xPb /0.35 /0.4 mol%. In our speculations this enhancement
Fig. 5. The SEM micrograph through ab-plane for the materials corresponding to: (a) (Bi0.8Pb0.2)2Sr2Ca2Cu3O10; (b) (Bi0.8Pb0.3)2Sr2Ca2Cu3O10; (c) (Bi0.8Pb0.4)2Sr2Ca2Cu3O10; (d) (Bi0.8Pb0.5)2Sr2Ca2Cu3O10. HTc-ceramic superconductors.
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of solubility limits is due to two reasons 1st, the selected particle size of Pb-metal 5/50 mm increase the compatibility of Pb in the matrix of BSCCO and the 2nd is addition of extra lead (x ) as metal as starting material decreases the temperature of formation liquid phase than that of lead oxide and consequently increase the chance of reaction between 2212-phase and Pb-richphase to produce HTc-2223-phase [29]. Azzouz et al. [30] investigated the effect of lead oxide additions up to (20%)on the final stage of processing BPSCCO tap and they confirmed that PbO play an important role in the step of liquid phase formation and considered as catalytic role as Ca/Pb reservoir that finally react with 2212-phase to form HTc-2223 superconducting phase. Fig. 5a/d; show the SE-micrograph of investigated samples. The calculation of average grain size were carried out and found to be in between 0.4 and 1.25 mm. The results obtained from SEM mapping and EDX analyses supported the results of X-ray specially in the view of multi-composition phases. Some of Pb-metal was identified between grains in the bulk of sample (d) which have higher Pb-content /0.5 mol% (x /0.3 mol%). 3.5. Conclusion The samples of the general formula (Bi0.8Pb0.2x )2Sr2Ca2Cu3O10, were prepared by the conventional high temperature solid state reaction technique. The extra lead (x ) addition which added as very fine powder of pure Pb-metal with particle size 5/50 mm exhibits remarkable promotion in the mechanical tensile properties and processing of PBSCCO system. The maximum average of tensile strength increases as Pb-content increase and was found to be 41.4 MPa for the sample with maximum Pb-content /0.5 mol% (x / 0.3). The extra lead (x ) in the investigated range 0.0 5/ x 5/0.3 mol% has a small effect in both superconducting and microstructure properties of PBSCCO system.
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