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Sr-doping for promoted high-Tc BPSCCO superconductors
Physica C 377 (2002) 254–259 www.elsevier.com/locate/physc
Sr-doping for promoted high-Tc BPSCCO superconductors Morsy M.A. Sekkina, Khaled M. Elsabawy
*
Faculty of Science, Department of Chemistry, HTc-Ceramic Superconductors Unit, Tanta University, Tanta, Egypt Received 8 December 2001; received in revised form 15 January 2002; accepted 16 January 2002
Abstract Nearly single phase superconducting samples of the general formula (Bi/Pb)2 (Sr2x Cax Þ2 Cu3 O10 were prepared by the usual high temperature solid state reaction technique and ceramic procedures. The prepared sample with x ¼ 0:6, with optimally nominal oxygen content (over doped) shows Tc of 107 K. The increase in Tc is also accompanied by an increase in the room temperature c-axis length. The crystal structure of the sample was determined by Cu-Ka X-ray and c ¼ diffractometry. This crystal belongs to a tetragonal system and has a space group of I4/mmm with a ¼ 3:6732 A . The TGA and DTA thermal analyses on the green samples included steps of thermal decomposition of 34:3176 A CuCO3 (300–400 °C), CaCO3 (660–710 °C), SrCO3 (710–800 °C) and finally to the high temperature solid state formation of superconductive phase at 820 °C. Finally a proposed crystalline structure symbol was put forward. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Sr-doped BPSCCO; Diffractometry; Optimally; Thermal analyses
1. Introduction There are three principal superconducting phases in the Bi–Sr–Ca–Cu–O (BSCCO) system, which can be described by the formula Bi2 Sr2 Can1 Cun O2nþ4 . The nominal superconducting transition temperatures, Tc s, are 10, 90 and 110 K for n ¼ 1(2201), 2(2212) and 3(2223), respectively [1,2]. These pseudo-tetragonal superconductors differ from each other by having 1, 2 or 3 Cu–O
*
Corresponding author. Address: Max Plank Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany. Fax: +49-0711-689-1010. E-mail addresses: [email protected], [email protected] (K.M. Elsabawy).
planes and 0, 1 or 2 Ca-planes with an increase in the c-axis lattice parameter from 24.6 to 30.6 to . Transmission electron microscopy indi37.1 A cates that the cations and anions in BSCCO undergo incommensurate modulations. It is generally agreed that the incommensurate lattice is related to the oxygen content, but it is still not clear whether the modulations can be explained by inserting extra oxygen ions in the Bi–O layers [3, 4]. Recently, the new superconducting system, SrCan1 Cun O2nþy F2þd (n ¼ 1, 2 and 3), has been discovered. The superconducting transition temperatures (Tc s) are 46 K for n ¼ 1 phase, 99 K for the n ¼ 2 phase and 110 K for n ¼ 3 phase, respectively [5–7]. It has been established that excess interstitial fluorine as well as oxygen atoms in the new system inject carriers into the CuO2 sheets,
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and lead them to becoming superconducting as in the case of La2 CuO4 Fd superconducting [8,9] system. Furthermore, in the pseudotetragonal system, it was shown that the excess interstitial fluorine also plays an important dominant structural role, rather than merely being an electronic dopant. In order to extensively study the physical as well as structural properties of the fluorine-based superconducting system, they [8,9] have prepared a series of compounds Sr2 Ndx Ca1x Cu2 O5þy Fd (0 6 x 6 1) under pressure [10]. Superconductivity was observed in the range (0:5 6 x 6 1), and the highest Tc is around 85 K for the samples with composition of Sr2 Nd0:2 Ca0:8 Cu2 O5þy F1þy , x ¼ 0:2. Onyskiewicz et al. [11] studied the X-ray diffraction (XRD), ESR, IR, DR(UV-Vis.) as well as resistivity and magnetic susceptibility data of single phase superconducting perovskite YBa2 Cu3 O7 and EuBa2 Cu3 O7 on the differential thermal analysis (DTA) and thermogravimetric analysis (TGA) curves. They observed endothermal effects corresponding to polymorphous transition on the BaCO3 and a pronounced endothermic peak which appears above 917 in the range (917–958 °C). This effect was accompanied by a mass loss well marked on TGA and DTA curves, according to the results of studies carried out on the samples obtained in temperatures above this endothermic effect. This peak corresponds not only to BaCO3 decomposition but also to the solid state reactions in M–Ba– Cu–O system. Xi et al. [12] 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 optimum annealing temperature is in between 845 and 855 °C. The aim of this article is to evaluate the optimum ratio of Bi/Pb and its effect on the electrical, structural and thermal properties of BPSCCO high-Tc superconductors.
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ðSr1:2 Ca0:8 Þ2 Cu3 O10 ; ðBi0:5 Pb0:5 Þ2 ðSr1:4 Ca0:6 Þ2 Cu3 O10 and ðBi0:5 Pb0:5 Þ2 ðSr1:6 Ca0:4 Þ2 Cu3 O10 were prepared by the conventional solid state reaction method and sintering procedure using the approprtiate amounts of Bi2 O3 , PbO, SrCO3 , CaCO3 and CuCO3 each of purity >99%. The mixtures were calcined at 800 °C under compressed O2 atmosphere for 20 h then reground and pressed into pellets (thickness 0.2 cm and diameter 1.2 cm). Sintering were carried out under oxygen stream at (780–840 °C) for 40 h. The temperature was slowly cooled down (20 °C/h) till 500 °C 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 test was thoroughly applied as a preliminary test for the achievement of superconductive phase and hence superconductivity. 2.2. Structural measurements The XRD measurements were carried out at room temperature on the ground samples using Cu-Ka radiation source and a computerized Shimadzu (Japan) diffractometer with two theta scan technique at the Institute of International Superconducting Technology Center (ISTEC), Tokyo, Japan. 2.3. Superconducting measurements The DC-electrical resistivity of the prepared materials were undertaken as a function of temperature using the modified four-probe technique and the temperature was recorded in the cryogenic temperature zone down to 30 K using liquid helium refrigerator at the Institute of Superconducting Technology Center (ISTEC), Tokyo, Japan. 2.4. Thermal analyses measurements
2. Experiments 2.1. Samples preparation The pure BPSCCO (Bi0:5 Pb0:5 Þ2 Sr2 Ca2 Cu3 O10 and samples of variant Sr/Ca content; ðBi0:5 Pb0:5 Þ2 -
The TGA and DTA thermal analyses measurements were carried out on the green mixtures of the prepared samples using a computerized Shimadzu (Japan) TGA/DTA analyzer and Al2 O3 as reference for DTA measurements.
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3. Results and discussion 3.1. Thermogravimetric analysis and diffrential thermal analysis measurements The TGA and DTA analyses were carried out on the green mixtures of the prepared high-Tc ceramic BPSCCO superconductors. From TGA/ DTA curves (see Fig. 1(a)–(d)) the TGA analysis can be divided into four fundamental steps. The first step occupies the region from room temperature till 230 °C for which the weight loss occurred is attributable to the humidity of samples and partial decomposition of CuCO3 . The second region lies from 230–400 °C at which cupric carbonate CuCO3 decomposed completely into CuO plus CO2 . The third region of temperature from 400– 660 °C at which the weight loss occurred attribut-
Fig. 1. The TGA and DTA curves for starting powder materials corresponding to: (a) (Bi0:5 Pb0:5 Þ2 Sr2 Ca2 Cu3 O10 , (b) (Bi0:5 Pb0:5 Þ2 (Sr1:2 Ca0:8 Þ2 Cu3 O10 , (c) (Bi0:5 Pb0:5 Þ2 (Sr1:4 Ca0:6 Þ2 Cu3 O10 , (d) (Bi0:5 Pb0:5 Þ2 (Sr1:6 Ca6:4 Þ2 Cu3 O10 HTc -ceramic superconductors.
able to the partial decomposition of both CaCO3 and SrCO3 respectively incorporated with high temperature solid state reaction in the system. The fourth step occupying the range 660–820 °C is assigned to complete decomposition of both CaCO3 and SrCO3 , partial sublimation of pb and finally the formation of the superconductive tetragonal phase at 820 °C. These results were in full agreement with Onyszkiewicz et al. [11], deducing that, on DTA curves besides endothermal effects corresponding not only to polymorphous transition on BaCO3 [13] but also to the solid state reactions in the M–Ba– Cu–O system. 3.2. Structural measurements Fig. 2(a)–(d) displays the X-ray diffraction patterns of pure BPSCCO (Bi0:5 Pb0:5 Þ2 Sr2 Ca2 Cu3 O10 and variant Sr/Ca content; ðBi0:5 Pb0:5 Þ2 ðSr1:2 Ca0:8 Þ2 Cu3 O10 ; ðBi0:5 Pb0:5 Þ2 ðSr1:4 Ca0:6 Þ2 Cu3 O10 ;ðBi0:5 Pb0:5 Þ2 ðSr1:6 Ca0:4 Þ2 Cu3 O10 : Analysis of the corresponding 2h values and the interplanar ) were carried out by pdp (X-ray spacings d (A powder diffraction data analysis program) using computer in Japan, indicated that, the X-ray crystalline structure belongs to mainly a single phase 2223 which is tetragonal crystal form a ¼ b 6¼ c. The unit cell dimensions were calculated using the parameters of the most intense X-ray reflection peaks and were found to be a ¼ b ¼ and c ¼ 34:3176 A for the pure BPSCCO 3:6732 A (see Table 1). It is clear that c-axis has length elongation while a-axis has length compression by increasing Sr-content due to ionic radius of Sr-ion higher than that of Ca-ion such that, Sr2þ ¼ 118 pm while Ca2þ ¼ 100 pm. From Fig. 2(a)–(d) it is clear that, only peak intensity characteristic to the HTc -2223-phase at 2h value 4:8–5 was observed referring to structure quality of 2223-phase, this is in full agreement with results reported by Xi et al. [12]. Koyama et al. [14,15] reported that, the formation of 2223-phase for PBSCCO system is a function of annealing temperature applied during thermal cycle of preparation, and they found that the best annealing temperature for the formation of the HTc -2223-phase was achieved in between
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affect cation diffusion. The incommensurate modulations in the Bi–O planes would affect Bi-diffusion. In addition, there is believe for little exchange between the Y and Ba ions in the 123 system, whereas it is generally accepted that Ca ion can substitute for Sr in the BSCCO structures [20,21]. Finally, it is very difficult to form Y or Ba vacancies in 123-system, but it has been shown that the BSCCO compositions can be alkaline-earth deficient relative to the ideal phases [22]. Furthermore Chen et al. [16] investigated Sr-diffusion in the BSCCO system, as a function of a number of Cu-layers, temperature and oxygen partial pressure and they deduced that increasing of oxygen partial pressure enhances the formation of HTc 2223 superconductive phase. Finally a proposed crystalline structure symbol was put forward.
Fig. 2. The obtained room temperature XRD patterns for the prepared: (a) (Bi0:5 Pb0:5 )2 Sr2 Ca2 Cu3 O10 , (b) (Bi0:5 Pb0:5 )2 (Sr1:2 Ca0:8 )2 Cu3 O10 , (c) (Bi0:5 Pb0:5 )2 (Sr1:4 Ca0:6 )2 Cu3 O10 , (d) (Bi0:5 Pb0:5 )2 (Sr1:6 Ca6:4 )2 Cu3 O10 HTc -ceramic superconductors.
Table 1 The calculated lattice parameters for the prepared samples ) ) Material a ¼ b (A c (A (Bi0:5 Pb0:5 )2 Sr2 Ca2 Cu3 O10 (Bi0:5 Pb0:5 )2 (Sr1:2 Ca0:8 )2 Cu3 O10 (Bi0:5 Pb0:5 )2 (Sr1:4 Ca0:6 )2 Cu3 O10 (Bi0:5 Pb0:5 )2 (Sr1:6 Ca0:4 )2 Cu3 O10
3.6732 3.5943 3.5631 3.5467
34.3176 34.6194 34.6432 34.6627
825 and 865 °C and increasing annealing temperature decrease the ratio of HTc -phase formed. Chen et al. [16–18] studied the diffusion of Y, Ba, Sr and Cu in Y–Ba–Cu–O system and Bi–Sr– Ca–Cu–O system, the results indicated that the two cations that occupy the A-site, Y and Ba, diffuse much more slowly than Cu, which occupies B-site. Yttrium diffuses more slowly than does Ba. All cations diffuse more slowly than oxygen [19]. There are several important differences between YBCO and BSCCO superconductors that could
3.3. Superconducting measurements The DC-electrical resistivity versus cryogenic temperatures were carried out using the four probe terminal technique. Thus, Fig. 3(a)–(d) shows the temperature dependence of electrical resistivity of the prepared pure BPSCCO (Bi0:5 Pb0:5 )2 Sr2 Ca2 Cu3 O10 and those of variant Sr/Ca content; (Bi0:5 Pb0:5 Þ2 (Sr1:2 Ca0:8 )2 Cu3 O10 , (Bi0:5 Pb0:5 )2 (Sr1:4 Ca0:6 )2 Cu3 O10 and (Bi0:5 Pb0:5 )2 (Sr1:6 Ca0:4 )2 Cu3 O10 respectively. It is clear that the critical Tc -offsets of the variant Sr/Ca superconductors samples are 103, 107 and 102 K, respectively. While the Tc -offset ¼ 101 K for the pure BPSCCO superconductor, and
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Fig. 3. The variation of DC-electrical resistivity as a function of absolute temperature for: (a) (Bi0:5 Pb0:5 )2 Sr2 Ca2 Cu3 O10 , (b) (Bi0:5 Pb0:5 )2 (Sr1:2 Ca0:8 )2 Cu3 O10 , (c) (Bi0:5 Pb0:5 )2 (Sr1:4 Ca0:6 )2 Cu3 O10 , (d) (Bi0:5 Pb0:5 )2 (Sr1:6 Ca6:4 )2 Cu3 O10 HTc -ceramic superconductors.
their Tc -onsets are 110, 120 and 109–111 K, respectively. While Tc -onset for the pure BPSCCO superconductor is 106 K. These data indicate that the best superconducting sample is that with Sr/Ca content ¼ (Sr1:4 Ca0:6 )2 which enhances the formation of the superconductive tetragonal phase. For samples a, b and c we report an interesting observation of a sharp peak (anomaly) in the range of 10% of the normal resistivity [23] lying just above the superconducting transition in the abplane. These crystals are of very high degree of crystallinity and structure quality. Although the presence of this anomaly, they have very sharp transition, and this is due to the presence of small oxygen content inhomogeneities. References [1] M. Tarascon, W.R. Mckinnon, P. Barboux, D.M. Hwang, B.G. Bagley, L.H. Greene, B.W. Hull, Y. Lepage, Phys. Rev. B 38 (1988) 8885.
[2] W. Zandbergen, W.A. Broen, F.C. Mijhoff, G. Van Tendeloo, S. Amelinckx, Physica C 156 (1988) 325. [3] V. Petricek, Y. Gao, P. Lee, P. Coppens, Phys. Rev. B 42 (1990) 387. [4] Y. Shen, D. Richards, D.G. Hinks, A.W. Mitchell, Appl. Phys. Lett. 59 (1991) 1559. [5] M. Al-Mamouri, P.P. Edwards, C. Greaves, M. Slaski, Nature 369 (1994) 282. [6] T. Kawashima, Y. Matsui, E. Takayama-Muromachi, Physica C 254 (1995) 131. [7] P.R. Slater, P.P. Edwards, C. Greaves, I. Gameson, M.G. Francesconi, J.P. Hodges, M. Al-Mamouri, M. Slaski, Physica C 241 (1995) 151. [8] A.C.W.P. James, S.M. Zahurak, D.W. Murphy, Nature 338 (1989) 240. [9] B.M. Tissue, K.M. Cirillo, J.C. Wright, M. Daeumling, D.C. Larbalestier, Solid State Commun. 65 (1988) 51. [10] T. Isobe, J.Q. Li, Y. Matsui, F. Izumi, Y. Kanke, E. Takayama-Mouromachi, Physica C 269 (1996) 5. [11] J. Onyszkiewicz, M. Koralewski, P. Czarecki, R. Micnas, S. Robaszkieicz, Physica C 147B (1987) 166–174. [12] Z. Xi, C. Ji, Z. Lain, J. Supercond. 421 (3) (1990). [13] G. Liptay, in: Akade-miai Kiado (Ed.), Atlas of Thermodynamical Curves, Budapest, 1976.
M.M.A. Sekkina, K.M. Elsabawy / Physica C 377 (2002) 254–259 [14] S. Koyama, U. Endo, T. Kawai, Jpn. J. Appl. Phys. 27 (1988) L186. [15] T. Hatano, K. Aota, S. Zkeda, K. Nakamura, K. Ogawo, Jpn. J. Appl. Phys. 27 (1988) L2055. [16] N. Chen, S.J. Rothman, J.L. Routbort, K.C. Goretta, J. Matter. Res. 7 (1992) 2308. [17] D. Gupta, R.B. Laibowitz, J.A. Lacey, Phys. Rev. Lett. 64 (1990) 2675. [18] J.L. Routbort, S.J. Rothman, N. Chen, J.N. Mundy, J.E. Baker, Phys. Rev. B 43 (1991) 5489.
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[19] S.J. Rothman, J.L. Routbort, J.E. Baker, Phys. Rev. B 40 (1989) 8852. [20] B. Hong, J. Hahn, T.O. Mason, J. Am. Ceram. Soc. 73 (1990) 1965. [21] T.G. Holesinger, D.J. Miller, L.S. Chumbley, M.J. Kramer, K.W. Dennis, Physica C 202 (1992) 109. [22] B. Hong, T.O. Mason, J. Am. Ceram. Soc. 74 (1999) 1045. [23] J. Mosqueira, A. Pomar, J. Maza, F. Vidal, J. Appl. Phys. 76 (3) (1994) 1943–1945.
Sr-doping for promoted high-Tc BPSCCO superconductors Morsy M.A. Sekkina, Khaled M. Elsabawy
*
Faculty of Science, Department of Chemistry, HTc-Ceramic Superconductors Unit, Tanta University, Tanta, Egypt Received 8 December 2001; received in revised form 15 January 2002; accepted 16 January 2002
Abstract Nearly single phase superconducting samples of the general formula (Bi/Pb)2 (Sr2x Cax Þ2 Cu3 O10 were prepared by the usual high temperature solid state reaction technique and ceramic procedures. The prepared sample with x ¼ 0:6, with optimally nominal oxygen content (over doped) shows Tc of 107 K. The increase in Tc is also accompanied by an increase in the room temperature c-axis length. The crystal structure of the sample was determined by Cu-Ka X-ray and c ¼ diffractometry. This crystal belongs to a tetragonal system and has a space group of I4/mmm with a ¼ 3:6732 A . The TGA and DTA thermal analyses on the green samples included steps of thermal decomposition of 34:3176 A CuCO3 (300–400 °C), CaCO3 (660–710 °C), SrCO3 (710–800 °C) and finally to the high temperature solid state formation of superconductive phase at 820 °C. Finally a proposed crystalline structure symbol was put forward. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Sr-doped BPSCCO; Diffractometry; Optimally; Thermal analyses
1. Introduction There are three principal superconducting phases in the Bi–Sr–Ca–Cu–O (BSCCO) system, which can be described by the formula Bi2 Sr2 Can1 Cun O2nþ4 . The nominal superconducting transition temperatures, Tc s, are 10, 90 and 110 K for n ¼ 1(2201), 2(2212) and 3(2223), respectively [1,2]. These pseudo-tetragonal superconductors differ from each other by having 1, 2 or 3 Cu–O
*
Corresponding author. Address: Max Plank Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany. Fax: +49-0711-689-1010. E-mail addresses: [email protected], [email protected] (K.M. Elsabawy).
planes and 0, 1 or 2 Ca-planes with an increase in the c-axis lattice parameter from 24.6 to 30.6 to . Transmission electron microscopy indi37.1 A cates that the cations and anions in BSCCO undergo incommensurate modulations. It is generally agreed that the incommensurate lattice is related to the oxygen content, but it is still not clear whether the modulations can be explained by inserting extra oxygen ions in the Bi–O layers [3, 4]. Recently, the new superconducting system, SrCan1 Cun O2nþy F2þd (n ¼ 1, 2 and 3), has been discovered. The superconducting transition temperatures (Tc s) are 46 K for n ¼ 1 phase, 99 K for the n ¼ 2 phase and 110 K for n ¼ 3 phase, respectively [5–7]. It has been established that excess interstitial fluorine as well as oxygen atoms in the new system inject carriers into the CuO2 sheets,
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and lead them to becoming superconducting as in the case of La2 CuO4 Fd superconducting [8,9] system. Furthermore, in the pseudotetragonal system, it was shown that the excess interstitial fluorine also plays an important dominant structural role, rather than merely being an electronic dopant. In order to extensively study the physical as well as structural properties of the fluorine-based superconducting system, they [8,9] have prepared a series of compounds Sr2 Ndx Ca1x Cu2 O5þy Fd (0 6 x 6 1) under pressure [10]. Superconductivity was observed in the range (0:5 6 x 6 1), and the highest Tc is around 85 K for the samples with composition of Sr2 Nd0:2 Ca0:8 Cu2 O5þy F1þy , x ¼ 0:2. Onyskiewicz et al. [11] studied the X-ray diffraction (XRD), ESR, IR, DR(UV-Vis.) as well as resistivity and magnetic susceptibility data of single phase superconducting perovskite YBa2 Cu3 O7 and EuBa2 Cu3 O7 on the differential thermal analysis (DTA) and thermogravimetric analysis (TGA) curves. They observed endothermal effects corresponding to polymorphous transition on the BaCO3 and a pronounced endothermic peak which appears above 917 in the range (917–958 °C). This effect was accompanied by a mass loss well marked on TGA and DTA curves, according to the results of studies carried out on the samples obtained in temperatures above this endothermic effect. This peak corresponds not only to BaCO3 decomposition but also to the solid state reactions in M–Ba– Cu–O system. Xi et al. [12] 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 optimum annealing temperature is in between 845 and 855 °C. The aim of this article is to evaluate the optimum ratio of Bi/Pb and its effect on the electrical, structural and thermal properties of BPSCCO high-Tc superconductors.
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ðSr1:2 Ca0:8 Þ2 Cu3 O10 ; ðBi0:5 Pb0:5 Þ2 ðSr1:4 Ca0:6 Þ2 Cu3 O10 and ðBi0:5 Pb0:5 Þ2 ðSr1:6 Ca0:4 Þ2 Cu3 O10 were prepared by the conventional solid state reaction method and sintering procedure using the approprtiate amounts of Bi2 O3 , PbO, SrCO3 , CaCO3 and CuCO3 each of purity >99%. The mixtures were calcined at 800 °C under compressed O2 atmosphere for 20 h then reground and pressed into pellets (thickness 0.2 cm and diameter 1.2 cm). Sintering were carried out under oxygen stream at (780–840 °C) for 40 h. The temperature was slowly cooled down (20 °C/h) till 500 °C 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 test was thoroughly applied as a preliminary test for the achievement of superconductive phase and hence superconductivity. 2.2. Structural measurements The XRD measurements were carried out at room temperature on the ground samples using Cu-Ka radiation source and a computerized Shimadzu (Japan) diffractometer with two theta scan technique at the Institute of International Superconducting Technology Center (ISTEC), Tokyo, Japan. 2.3. Superconducting measurements The DC-electrical resistivity of the prepared materials were undertaken as a function of temperature using the modified four-probe technique and the temperature was recorded in the cryogenic temperature zone down to 30 K using liquid helium refrigerator at the Institute of Superconducting Technology Center (ISTEC), Tokyo, Japan. 2.4. Thermal analyses measurements
2. Experiments 2.1. Samples preparation The pure BPSCCO (Bi0:5 Pb0:5 Þ2 Sr2 Ca2 Cu3 O10 and samples of variant Sr/Ca content; ðBi0:5 Pb0:5 Þ2 -
The TGA and DTA thermal analyses measurements were carried out on the green mixtures of the prepared samples using a computerized Shimadzu (Japan) TGA/DTA analyzer and Al2 O3 as reference for DTA measurements.
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3. Results and discussion 3.1. Thermogravimetric analysis and diffrential thermal analysis measurements The TGA and DTA analyses were carried out on the green mixtures of the prepared high-Tc ceramic BPSCCO superconductors. From TGA/ DTA curves (see Fig. 1(a)–(d)) the TGA analysis can be divided into four fundamental steps. The first step occupies the region from room temperature till 230 °C for which the weight loss occurred is attributable to the humidity of samples and partial decomposition of CuCO3 . The second region lies from 230–400 °C at which cupric carbonate CuCO3 decomposed completely into CuO plus CO2 . The third region of temperature from 400– 660 °C at which the weight loss occurred attribut-
Fig. 1. The TGA and DTA curves for starting powder materials corresponding to: (a) (Bi0:5 Pb0:5 Þ2 Sr2 Ca2 Cu3 O10 , (b) (Bi0:5 Pb0:5 Þ2 (Sr1:2 Ca0:8 Þ2 Cu3 O10 , (c) (Bi0:5 Pb0:5 Þ2 (Sr1:4 Ca0:6 Þ2 Cu3 O10 , (d) (Bi0:5 Pb0:5 Þ2 (Sr1:6 Ca6:4 Þ2 Cu3 O10 HTc -ceramic superconductors.
able to the partial decomposition of both CaCO3 and SrCO3 respectively incorporated with high temperature solid state reaction in the system. The fourth step occupying the range 660–820 °C is assigned to complete decomposition of both CaCO3 and SrCO3 , partial sublimation of pb and finally the formation of the superconductive tetragonal phase at 820 °C. These results were in full agreement with Onyszkiewicz et al. [11], deducing that, on DTA curves besides endothermal effects corresponding not only to polymorphous transition on BaCO3 [13] but also to the solid state reactions in the M–Ba– Cu–O system. 3.2. Structural measurements Fig. 2(a)–(d) displays the X-ray diffraction patterns of pure BPSCCO (Bi0:5 Pb0:5 Þ2 Sr2 Ca2 Cu3 O10 and variant Sr/Ca content; ðBi0:5 Pb0:5 Þ2 ðSr1:2 Ca0:8 Þ2 Cu3 O10 ; ðBi0:5 Pb0:5 Þ2 ðSr1:4 Ca0:6 Þ2 Cu3 O10 ;ðBi0:5 Pb0:5 Þ2 ðSr1:6 Ca0:4 Þ2 Cu3 O10 : Analysis of the corresponding 2h values and the interplanar ) were carried out by pdp (X-ray spacings d (A powder diffraction data analysis program) using computer in Japan, indicated that, the X-ray crystalline structure belongs to mainly a single phase 2223 which is tetragonal crystal form a ¼ b 6¼ c. The unit cell dimensions were calculated using the parameters of the most intense X-ray reflection peaks and were found to be a ¼ b ¼ and c ¼ 34:3176 A for the pure BPSCCO 3:6732 A (see Table 1). It is clear that c-axis has length elongation while a-axis has length compression by increasing Sr-content due to ionic radius of Sr-ion higher than that of Ca-ion such that, Sr2þ ¼ 118 pm while Ca2þ ¼ 100 pm. From Fig. 2(a)–(d) it is clear that, only peak intensity characteristic to the HTc -2223-phase at 2h value 4:8–5 was observed referring to structure quality of 2223-phase, this is in full agreement with results reported by Xi et al. [12]. Koyama et al. [14,15] reported that, the formation of 2223-phase for PBSCCO system is a function of annealing temperature applied during thermal cycle of preparation, and they found that the best annealing temperature for the formation of the HTc -2223-phase was achieved in between
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affect cation diffusion. The incommensurate modulations in the Bi–O planes would affect Bi-diffusion. In addition, there is believe for little exchange between the Y and Ba ions in the 123 system, whereas it is generally accepted that Ca ion can substitute for Sr in the BSCCO structures [20,21]. Finally, it is very difficult to form Y or Ba vacancies in 123-system, but it has been shown that the BSCCO compositions can be alkaline-earth deficient relative to the ideal phases [22]. Furthermore Chen et al. [16] investigated Sr-diffusion in the BSCCO system, as a function of a number of Cu-layers, temperature and oxygen partial pressure and they deduced that increasing of oxygen partial pressure enhances the formation of HTc 2223 superconductive phase. Finally a proposed crystalline structure symbol was put forward.
Fig. 2. The obtained room temperature XRD patterns for the prepared: (a) (Bi0:5 Pb0:5 )2 Sr2 Ca2 Cu3 O10 , (b) (Bi0:5 Pb0:5 )2 (Sr1:2 Ca0:8 )2 Cu3 O10 , (c) (Bi0:5 Pb0:5 )2 (Sr1:4 Ca0:6 )2 Cu3 O10 , (d) (Bi0:5 Pb0:5 )2 (Sr1:6 Ca6:4 )2 Cu3 O10 HTc -ceramic superconductors.
Table 1 The calculated lattice parameters for the prepared samples ) ) Material a ¼ b (A c (A (Bi0:5 Pb0:5 )2 Sr2 Ca2 Cu3 O10 (Bi0:5 Pb0:5 )2 (Sr1:2 Ca0:8 )2 Cu3 O10 (Bi0:5 Pb0:5 )2 (Sr1:4 Ca0:6 )2 Cu3 O10 (Bi0:5 Pb0:5 )2 (Sr1:6 Ca0:4 )2 Cu3 O10
3.6732 3.5943 3.5631 3.5467
34.3176 34.6194 34.6432 34.6627
825 and 865 °C and increasing annealing temperature decrease the ratio of HTc -phase formed. Chen et al. [16–18] studied the diffusion of Y, Ba, Sr and Cu in Y–Ba–Cu–O system and Bi–Sr– Ca–Cu–O system, the results indicated that the two cations that occupy the A-site, Y and Ba, diffuse much more slowly than Cu, which occupies B-site. Yttrium diffuses more slowly than does Ba. All cations diffuse more slowly than oxygen [19]. There are several important differences between YBCO and BSCCO superconductors that could
3.3. Superconducting measurements The DC-electrical resistivity versus cryogenic temperatures were carried out using the four probe terminal technique. Thus, Fig. 3(a)–(d) shows the temperature dependence of electrical resistivity of the prepared pure BPSCCO (Bi0:5 Pb0:5 )2 Sr2 Ca2 Cu3 O10 and those of variant Sr/Ca content; (Bi0:5 Pb0:5 Þ2 (Sr1:2 Ca0:8 )2 Cu3 O10 , (Bi0:5 Pb0:5 )2 (Sr1:4 Ca0:6 )2 Cu3 O10 and (Bi0:5 Pb0:5 )2 (Sr1:6 Ca0:4 )2 Cu3 O10 respectively. It is clear that the critical Tc -offsets of the variant Sr/Ca superconductors samples are 103, 107 and 102 K, respectively. While the Tc -offset ¼ 101 K for the pure BPSCCO superconductor, and
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Fig. 3. The variation of DC-electrical resistivity as a function of absolute temperature for: (a) (Bi0:5 Pb0:5 )2 Sr2 Ca2 Cu3 O10 , (b) (Bi0:5 Pb0:5 )2 (Sr1:2 Ca0:8 )2 Cu3 O10 , (c) (Bi0:5 Pb0:5 )2 (Sr1:4 Ca0:6 )2 Cu3 O10 , (d) (Bi0:5 Pb0:5 )2 (Sr1:6 Ca6:4 )2 Cu3 O10 HTc -ceramic superconductors.
their Tc -onsets are 110, 120 and 109–111 K, respectively. While Tc -onset for the pure BPSCCO superconductor is 106 K. These data indicate that the best superconducting sample is that with Sr/Ca content ¼ (Sr1:4 Ca0:6 )2 which enhances the formation of the superconductive tetragonal phase. For samples a, b and c we report an interesting observation of a sharp peak (anomaly) in the range of 10% of the normal resistivity [23] lying just above the superconducting transition in the abplane. These crystals are of very high degree of crystallinity and structure quality. Although the presence of this anomaly, they have very sharp transition, and this is due to the presence of small oxygen content inhomogeneities. References [1] M. Tarascon, W.R. Mckinnon, P. Barboux, D.M. Hwang, B.G. Bagley, L.H. Greene, B.W. Hull, Y. Lepage, Phys. Rev. B 38 (1988) 8885.
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