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Superconducting properties of sodium doped (Pb,Cd)-1212 type compounds
Physica C 339 (2000) 137±142
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Superconducting properties of sodium doped (Pb,Cd)-1212 type compounds T.S. Sampath Kumar a,b,1, B. Latha a,* a b
Department of Nuclear Physics and Materials Science Center, University of Madras, Guindy Campus, Chennai 600 025, India Department of Metallurgical Engineering and Regional Sophisticated Instrumentation Center, Indian Institute of Technology, Chennai 600 036, India Received 10 November 1999; accepted 4 May 2000
Abstract The eect of sodium doping (3±8 mol%) on the structural, thermal and superconducting properties of (Pb0:5 , Cd0:5 )Sr2 (Y0:6 ,Ca0:4 )Cu2 O7ÿd has been studied. For 5 mol% NaNO3 doping, relatively pure superconducting phase was formed without SrCuO2 and Sr5 Pb3 CuO12 impurities as observed in both 3% and 8% doped as well as undoped superconductors. The cell parameters and cell volumes are also found to be minimum for the 5% doped superconductor, but were slightly larger than for the undoped superconductor. Highest Tc of 55 K was also observed for 5% doped sodium containing sample and the Tc enhancement was 15 K above the sodium free superconductor. The eect of calcium content on the Tc enhancement by 5% sodium addition was investigated for (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd (x 0:3, 0.5) superconductors. The sodium free superconductors show only onset of superconductivity (Tc;onset ), which increases from 23 to 49 K with increasing calcium content. The resistivity behaviour before Tc changes from semiconducting to metallic nature. Addition of sodium leads to the observation of zero resistance (Tc;zero ) at 25 K only for x 0:5 sample, which is however lower than the Tc;zero of 30 K exhibited by the superconductor with x 0:4 calcium content. The lowering of melting point and denser microstructure with larger grains were observed upon 5% sodium doping on (Pb0:5 ,Cd0:5 )Sr2 (Y0:6 ,Ca0:4 )Cu2 O7ÿd superconductor. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: A7460M; 74.62.Bf Keywords: (Pb,Cd)-1212; Superconductivity; Sodium; Resistivity; Oxygen annealing
1. Introduction Sodium nitrate (NaNO3 ), widely used in the glass industry as an oxidizer, has high oxygen potential [1] and hence has been tried as an addi* Corresponding author. Tel.: +91-44-2351-444; fax: +91-44235-2870. E-mail addresses: [email protected] (T.S. Sampath Kumar), [email protected], [email protected] (B. Latha). 1 Also Corresponding author. Tel.: +91-44-445-8607; fax: +91-44-235-2545/0509.
tive in improving the properties of ceramic superconductors. For example, the critical temperature of YBa2 Cu3 O7ÿd (123) superconductor sintered in air with 4.5 wt.% NaNO3 has been found to be almost the same as that of pure 123 superconductor sintered in oxygen [2]. Further, NaNO3 and its decomposition products NaNO2 or Na2 O are liquids and induce liquid phase sintering at the selected sintering temperature of 123 at 890°C resulting in a denser structure with larger grains. These eects increase the critical current density
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with increasing NaNO3 content upto 4.5 wt.% [3]. The orthorhombic to tetragonal phase transition has been found to increase by about 80°C for (YBa2 Cu3 )0:74 Na0:26 O7ÿd material due to the ability of bearing more oxygen atoms in the lattice [3]. Addition of 4.5 mol% NaNO3 during sintering of the Y1ÿx Zrx Ba2 Cu3 O7ÿd x 0:025 and 0:5 in air decreases the sintering temperature to 850°C and improves the thermal stability without altering the Tc [4]. Sodium doping also helps in reducing lattice instabilities of Bi2 Sr2 Ca0:7 Na0:3 Cu2 Oy superconductor [5]. Recently, sodium doping in Hg0:9 Na0:1 Ba2 Ca2 Cu3 Oy has increased the Tc by more than 20 K using low temperature oxygen annealing at 300°C [6,7]. Above all, the bene®cial eects of NaNO3 additive include a higher amount of superconducting phase and an enhanced Meissner eect as well as an increase in the critical current density. Materials with the 1212-type structure are believed to have potential of becoming 90 or 110 K class superconductors owing to their close structural relationship with YBa2 Cu3 O7ÿd and TlSr2 CaCu2 O7ÿd superconductors [8]. The chemical ¯exibility of the 1212-type structure also provides a means to tailor new superconducting materials with improved ¯ux pinning property [9]. Among the Pb-based 1212 superconductors, the (Pb0:5 ,Cd0:5 )Sr2 (Y0:6 ,Ca0:4 )Cu2 O7ÿd relatively has the highest Tc , but the percentage of the sample exhibiting bulk superconductivity is small in comparison with the magnetization values observed for an ideal superconductor [10]. Addition of silver has been found to increase the bulk superconductivity of (Pb,Cd)-1212 materials [11,12]. Interestingly, appreciable Tc enhancement was also observed in (Pb,M)-1212 superconductors (M Cd, Mg and Cu) [11±14]. Hence, an attempt is made to investigate the structural and superconducting properties of sodium doped (Pb,Cd)-1212 materials so as to provide additional ¯exibility in processing using cheaper additives. 2. Experimental The samples were synthesized by the conventional solid state reaction method using high pu-
rity PbO, SrCO3 , Y2 O3 , CaCO3 , CdO and CuO (Cerac, 99.99%) materials. They were weighed corresponding to the stoichiometric composition of (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 and mixed in an agate mortar. The mixture was calcined at 850°C for 16 h in air. The product was ground, pressed into 10 mm diameter pellets and sintered at 930°C for 10 h in air. The samples were then reground with or without the addition of 3, 5 and 8 mol% NaNO3 powder and pressed into pellets of 8 mm in diameter and 2 mm thick under a pressure of 5 tons. The pellets were sintered at 850°C for 12 h in ¯owing oxygen atmosphere and furnace cooled. The powders from bulk samples were examined with a high resolution Guinier X-ray powder diffractometer using monochromatic CuKa radiation (Seifert, XRD 3000). The Xwith k 1:540598 A ray diraction patterns (XRD) were recorded in steps of 0.01° with one second counting time in each step. The dc resistance of the sample was measured by the standard four probe technique with a computerized system of nanovoltmeter (Keithley, model 181), programmable current source (Keithley, model 220) and cryocontroller (Lake Shore Cryotronics, model DRC-93C) coupled to a closed cycle helium refrigerator (Janis model 22C). The contacts to the sample for electrical measurements were made by attaching ®ne copper leads with a conductive silver paste on the surface of the sintered pellet. The applied current was 10 mA. The dierential thermal analysis (DTA) was carried out in a ¯owing oxygen atmosphere with the heating rate of 10°C/min (Perkin Elmer-PC 7 system). The freshly fractured surface of the samples was examined using a scanning electron microscope (SEM) (Jeol, JSM 840).
3. Results and discussion The XRD patterns of undoped and sodium doped (3±8 mol%) (Pb0:5 Cd0:5 )Sr2 (Y0:6 Ca0:4 )Cu2 O7ÿd superconductors are shown in Fig.1. The diractograms of all the samples show nearly identical patterns corresponding to that of the
T.S. Sampath Kumar, B. Latha / Physica C 339 (2000) 137±142
Fig. 1. Powder XRD patterns of pure and NaNO3 doped (Pb0:5 Cd0:5 )Sr2 (Y0:6 Ca0:4 )Cu2 O7ÿd superconductors: (ÔÕ) SrCuO2 , (ÔÕ) Sr5 Pb3 CuO12 .
(Pb,Cd)-1212 superconductor [15], but with SrCuO2 impurity (shown by ÔÕ). Relatively pure
139
superconducting phase was obtained for the 5% doped sample. The Sr5 Pb3 CuO12 impurity phase (shown by ÔÕ) was also observed for the 3% and 8% doped superconductors. The majority of the re¯ections in all the diractograms were indexed on the basis of a tetragonal unit cell and the cell parameters were determined by least square analysis of minimum 20 diraction peaks. The estimated precession error for a- and c-axis are 0.002 respectively. The cell parameters of and 0.007 A, the doped superconductors are relatively larger than those of the undoped superconductor as listed in Table 1 and do not show any systematic variation with the amount of sodium doping. The variation of resistance with temperature of the sodium free (Pb0:5 Cd0:5 )Sr2 (Y0:6 Ca0:4 )Cu2 O7ÿd superconductors and 3±8% sodium containing samples are shown in Fig. 2. All the samples exhibit superconducting behaviour and the value of Tc are listed in Table 1. By comparing the results, it is obvious that sodium doping increases Tc upto 5 mol%. The temperature corresponding to the onset of superconductivity (Tc;onset ) shows a marginal increase of about 5 K upon doping while the temperature at which resistance tends to zero (Tc;zero ) exhibits an enhancement of about 15 K for 5 mol% NaNO3 addition. This is the ®rst time that such an enhancement of Tc upon sodium addition in (Pb,Cd)-1212 superconductor has been observed. With 8% doping, the Tc;zero value decreases but higher than that of the undoped superconductor. Interestingly, the normal state resistivity has been found to decrease with increasing amount of doping. But the highest Tc found for 5% doping probably indicate a shift
Table1 Compositions and properties of samples in the (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5/NaNO3 System Ca content and NaNO3 (mol%)
Cell parameters a (A)
c (A)
3 v (A)
T c;onset
T c;zero
Melting point (°C)
x 0:3 x 0:3&5 x 0:4 x 0:4&3 x 0:4&5 x 0:4&8 x 0:5 x 0:5&5
3.82 3.82 3.81 3.82 3.82 3.82 3.82 3.81
11.98 11.97 11.96 12.00 11.97 12.01 11.99 11.97
175 175 174.1 175 174.6 175.6 174.7 174.1
22 25 48 49 55 45 50 52
± ± 16 ± 30 25 ± 25
1058 1055 1068 ± 1065 ± 1063 1061
Transition temperature (K)
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Fig. 2. Temperature dependence of resistance for pure and NaNO3 doped (Pb0:5 Cd0:5 )Sr2 (Y0:6 Ca0:4 )Cu2 O7ÿd superconductors.
from an under-doped state to an optimally doped state with the maximum Tc possible for the system. Addition of silver has also been found to enhance the superconducting transition temperature of (Pb,M)-1212 superconductors (M Cu or Cd) and the Tc enhancement increases with calcium content of the samples [12]. Hence, the eect of 5 mol% NaNO3 addition on the calcium content x 0:3; 0:5 of (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd superconductors have also been investigated. Fig. 3 shows the temperature dependence of electrical resistance of sodium free and sodium containing
Fig. 3. Temperature dependence of resistance for pure and 5 mol% NaNO3 doped (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd x 0:3 and 0:5 superconductors.
superconductors with x 0:3 and 0.5 calcium content. The undoped superconductors show only Tc;onset , which increases from 23 to 49 K with increasing calcium content from x 0:3 to 0.5, respectively. The resistivity behaviour before Tc is attained also changes from semiconducting to metallic nature as the calcium content increases [12]. The addition of sodium leads to the observation of zero resistance only in the composite with x 0:5. This composite also exhibits an enhancement of Tc;onset of about 5 K and the metallic nature of resistivity behaviour before superconducting. But the normal state resistivity of the sodium doped samples, which is less than that of the undoped samples with calcium content up to x 0:4 changes trend for x 0:5 sample. The highest Tc;onset of 57 K has been observed for the composite with x 0:4. So, the present study suggests the optimum value of the calcium content in the (Pb,Cd)-1212 superconductor to be around x 0:4 also as has reported earlier [12]. The DTA measurements were conducted at temperatures below 1200°C. The DTA traces of the samples without and with 5 mol% NaNO3 addition for x 0:4 are shown in Fig. 4. Both the traces exhibit a single endothermic peak corresponding to the melting point of the sample. For the sodium free sample, the onset temperature for melting is 1068°C, while the melting point of the
Fig. 4. DTA traces of pure and 5 mol% NaNO3 added (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 superconductors.
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Fig. 5. SEM of pure and 5 mol% sodium doped (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 superconductors: (a) x 0:3, (b) as (a) NaNO3 , (c) x 0:4, (d) as (c) NaNO3 , (e) x 0:5 and (f) as (e) NaNO3 .
sodium containing sample is 1065°C. The DTA curve for the sodium doped sample with x 0:3
show the melting point at 1055°C, which is less than that of the corresponding undoped sample.
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Although the DTA curve for the doped sample is less than that of the undoped sample with x 0:5 calcium content, the DTA trace show two endopeaks which indicate an additional phase event besides melting. The large amount of calcium substitution expected to change the phase diagram signi®cantly. So, within the calcium solubility limit x 6 0:4, the sodium doping decreases the melting point of the (Pb,Cd)-1212 superconductors. The microstructural changes upon NaNO3 addition on the (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 superconductor were investigated by using SEM and the micrographs are displayed in Fig. 5. Pure (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd sample showed a granular structure, which is typical of a sintered ceramic material with relatively broad distribution of grain sizes. The grain size range of 4±8 lm determined for undoped (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 samples were found to increases with increasing calcium concentration. The 5 mol% sodium doped (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 samples exhibit grain sizes of 5±12 lm, which are larger than those for pure (Pb,Cd)-1212 ceramics. Addition of NaNO3 to (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )-Cu2 O7ÿd superconductors seems to cause grain growth from liquid phase sintering induced by NaNO2 , Na2 O or their mixtures. 4. Conclusions By the addition of sodium nitrate and optimization of calcium content, the superconducting properties of (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 can be signi®cantly enhanced. An appreciable rise in Tc from 48 to 55 K was observed upon 5 mol% NaNO3 addition on (Pb0:5 Cd0:5 )Sr2 (Y0:6 Ca0:4 )Cu2 O7ÿd superconductor. Furthermore, the addition also leads to the formation of relatively pure superconducting phase without SrCuO2 and Sr5 Pb3 CuO12 impurity phases. Lowering of the melting point and enlargement of grain size takes place on sodium addition. Also, the present study suggests the optimum calcium
value of the (Pb,Cd)-1212 superconductor to be around x 0:4.
Acknowledgements This work was partially supported by COSIST, SAP and Superconductivity Programmes of UGC, Government of India. One of the authors (BL) acknowledge CSIR, New Delhi, Government of India, for ®nancial assistance in the form of Senior Research Fellowship. The author gratefully acknowledges Prof. G. Shanmugam for his help in the preparation of the manuscript.
References [1] I.-W. Chen, S. Keating, C.Y. Keating, X. Wu, J. Xu, P.E. Reyes-Morel, T.Y. Tien, Adv. Ceram. Mater. 2 (1987) 457. [2] E.H. Lee, J.Y. Jeong, H. Song, S.-Y. Yoon, Jpn. J. Appl. Phys. 30 (1991) L1092. [3] C.H. Chen, B. Kumar, J. Am. Ceram. Soc. 77 (1994) 1025. [4] T.S. Sampath Kumar, N.S. Hari, T. Thirumal, M. Venkatesan, Trans. Ind. Inst. Met. 48 (1995) 217. [5] M. Chandra Sekhar, B. Gopala Krishna, R. Ravinder Reddy, P. Venugopal Reddy, S.V. Suryanarayana, Supercond. Sci. Technol. (1996) 29. [6] J.Z. Wu, S.H. Yoo, T. Aytug, A.A. Gapud, B.W. Kang, S. Wu, W. Zhou, J. Supercond. 11 (1998) 169. [7] T. Aytug, A.A. Gapud, S.H. Yoo, B.W. Kang, S.D. Gapud, J.Z. Wu, Physica C 313 (1999) 121. [8] R.S. Liu, P.P. Edwards, Y.T. Huang, S.F. Wu, P.T. Wu, J. Solid State Chem. 86 (1990) 334. [9] T. Kano, T. Doi, A. Soeta, T. Yuasa, N. Inoue, K. Aihare, S. Matsude, Appl. Phys. Lett. 59 (1991) 3186. [10] J.Y. Lee, J.S. Swinnea, H. Stein®nk, J. Mater. Res. 4 (1989) 763. [11] J.M. Parberry, T.P. Beales, Chin. J. Phys. 31 (1993) 1221. [12] B. Latha, K. Ravichandran, D. Ravichandran, T. Nagarajan, T.S. Sampath Kumar, Supercond. Sci. Technol. 11 (1998) 1367. [13] H.B. Liu, D.E. Morris, X.P. Jiang, A.P.B. Sinha, Physica C 222 (1994) 119. [14] H.B. Liu, D.E. Morris, A.P.B. Sinha, Physica C 204 (1994) 119. [15] T.P. Beales, C. Dineen, W.G. Freeman, S.R. Hall, M.R. Harrison, M. Jacobson, S.J. Zammattlo, Supercond. Sci. Technol. 5 (1992) 47.
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Superconducting properties of sodium doped (Pb,Cd)-1212 type compounds T.S. Sampath Kumar a,b,1, B. Latha a,* a b
Department of Nuclear Physics and Materials Science Center, University of Madras, Guindy Campus, Chennai 600 025, India Department of Metallurgical Engineering and Regional Sophisticated Instrumentation Center, Indian Institute of Technology, Chennai 600 036, India Received 10 November 1999; accepted 4 May 2000
Abstract The eect of sodium doping (3±8 mol%) on the structural, thermal and superconducting properties of (Pb0:5 , Cd0:5 )Sr2 (Y0:6 ,Ca0:4 )Cu2 O7ÿd has been studied. For 5 mol% NaNO3 doping, relatively pure superconducting phase was formed without SrCuO2 and Sr5 Pb3 CuO12 impurities as observed in both 3% and 8% doped as well as undoped superconductors. The cell parameters and cell volumes are also found to be minimum for the 5% doped superconductor, but were slightly larger than for the undoped superconductor. Highest Tc of 55 K was also observed for 5% doped sodium containing sample and the Tc enhancement was 15 K above the sodium free superconductor. The eect of calcium content on the Tc enhancement by 5% sodium addition was investigated for (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd (x 0:3, 0.5) superconductors. The sodium free superconductors show only onset of superconductivity (Tc;onset ), which increases from 23 to 49 K with increasing calcium content. The resistivity behaviour before Tc changes from semiconducting to metallic nature. Addition of sodium leads to the observation of zero resistance (Tc;zero ) at 25 K only for x 0:5 sample, which is however lower than the Tc;zero of 30 K exhibited by the superconductor with x 0:4 calcium content. The lowering of melting point and denser microstructure with larger grains were observed upon 5% sodium doping on (Pb0:5 ,Cd0:5 )Sr2 (Y0:6 ,Ca0:4 )Cu2 O7ÿd superconductor. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: A7460M; 74.62.Bf Keywords: (Pb,Cd)-1212; Superconductivity; Sodium; Resistivity; Oxygen annealing
1. Introduction Sodium nitrate (NaNO3 ), widely used in the glass industry as an oxidizer, has high oxygen potential [1] and hence has been tried as an addi* Corresponding author. Tel.: +91-44-2351-444; fax: +91-44235-2870. E-mail addresses: [email protected] (T.S. Sampath Kumar), [email protected], [email protected] (B. Latha). 1 Also Corresponding author. Tel.: +91-44-445-8607; fax: +91-44-235-2545/0509.
tive in improving the properties of ceramic superconductors. For example, the critical temperature of YBa2 Cu3 O7ÿd (123) superconductor sintered in air with 4.5 wt.% NaNO3 has been found to be almost the same as that of pure 123 superconductor sintered in oxygen [2]. Further, NaNO3 and its decomposition products NaNO2 or Na2 O are liquids and induce liquid phase sintering at the selected sintering temperature of 123 at 890°C resulting in a denser structure with larger grains. These eects increase the critical current density
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with increasing NaNO3 content upto 4.5 wt.% [3]. The orthorhombic to tetragonal phase transition has been found to increase by about 80°C for (YBa2 Cu3 )0:74 Na0:26 O7ÿd material due to the ability of bearing more oxygen atoms in the lattice [3]. Addition of 4.5 mol% NaNO3 during sintering of the Y1ÿx Zrx Ba2 Cu3 O7ÿd x 0:025 and 0:5 in air decreases the sintering temperature to 850°C and improves the thermal stability without altering the Tc [4]. Sodium doping also helps in reducing lattice instabilities of Bi2 Sr2 Ca0:7 Na0:3 Cu2 Oy superconductor [5]. Recently, sodium doping in Hg0:9 Na0:1 Ba2 Ca2 Cu3 Oy has increased the Tc by more than 20 K using low temperature oxygen annealing at 300°C [6,7]. Above all, the bene®cial eects of NaNO3 additive include a higher amount of superconducting phase and an enhanced Meissner eect as well as an increase in the critical current density. Materials with the 1212-type structure are believed to have potential of becoming 90 or 110 K class superconductors owing to their close structural relationship with YBa2 Cu3 O7ÿd and TlSr2 CaCu2 O7ÿd superconductors [8]. The chemical ¯exibility of the 1212-type structure also provides a means to tailor new superconducting materials with improved ¯ux pinning property [9]. Among the Pb-based 1212 superconductors, the (Pb0:5 ,Cd0:5 )Sr2 (Y0:6 ,Ca0:4 )Cu2 O7ÿd relatively has the highest Tc , but the percentage of the sample exhibiting bulk superconductivity is small in comparison with the magnetization values observed for an ideal superconductor [10]. Addition of silver has been found to increase the bulk superconductivity of (Pb,Cd)-1212 materials [11,12]. Interestingly, appreciable Tc enhancement was also observed in (Pb,M)-1212 superconductors (M Cd, Mg and Cu) [11±14]. Hence, an attempt is made to investigate the structural and superconducting properties of sodium doped (Pb,Cd)-1212 materials so as to provide additional ¯exibility in processing using cheaper additives. 2. Experimental The samples were synthesized by the conventional solid state reaction method using high pu-
rity PbO, SrCO3 , Y2 O3 , CaCO3 , CdO and CuO (Cerac, 99.99%) materials. They were weighed corresponding to the stoichiometric composition of (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 and mixed in an agate mortar. The mixture was calcined at 850°C for 16 h in air. The product was ground, pressed into 10 mm diameter pellets and sintered at 930°C for 10 h in air. The samples were then reground with or without the addition of 3, 5 and 8 mol% NaNO3 powder and pressed into pellets of 8 mm in diameter and 2 mm thick under a pressure of 5 tons. The pellets were sintered at 850°C for 12 h in ¯owing oxygen atmosphere and furnace cooled. The powders from bulk samples were examined with a high resolution Guinier X-ray powder diffractometer using monochromatic CuKa radiation (Seifert, XRD 3000). The Xwith k 1:540598 A ray diraction patterns (XRD) were recorded in steps of 0.01° with one second counting time in each step. The dc resistance of the sample was measured by the standard four probe technique with a computerized system of nanovoltmeter (Keithley, model 181), programmable current source (Keithley, model 220) and cryocontroller (Lake Shore Cryotronics, model DRC-93C) coupled to a closed cycle helium refrigerator (Janis model 22C). The contacts to the sample for electrical measurements were made by attaching ®ne copper leads with a conductive silver paste on the surface of the sintered pellet. The applied current was 10 mA. The dierential thermal analysis (DTA) was carried out in a ¯owing oxygen atmosphere with the heating rate of 10°C/min (Perkin Elmer-PC 7 system). The freshly fractured surface of the samples was examined using a scanning electron microscope (SEM) (Jeol, JSM 840).
3. Results and discussion The XRD patterns of undoped and sodium doped (3±8 mol%) (Pb0:5 Cd0:5 )Sr2 (Y0:6 Ca0:4 )Cu2 O7ÿd superconductors are shown in Fig.1. The diractograms of all the samples show nearly identical patterns corresponding to that of the
T.S. Sampath Kumar, B. Latha / Physica C 339 (2000) 137±142
Fig. 1. Powder XRD patterns of pure and NaNO3 doped (Pb0:5 Cd0:5 )Sr2 (Y0:6 Ca0:4 )Cu2 O7ÿd superconductors: (ÔÕ) SrCuO2 , (ÔÕ) Sr5 Pb3 CuO12 .
(Pb,Cd)-1212 superconductor [15], but with SrCuO2 impurity (shown by ÔÕ). Relatively pure
139
superconducting phase was obtained for the 5% doped sample. The Sr5 Pb3 CuO12 impurity phase (shown by ÔÕ) was also observed for the 3% and 8% doped superconductors. The majority of the re¯ections in all the diractograms were indexed on the basis of a tetragonal unit cell and the cell parameters were determined by least square analysis of minimum 20 diraction peaks. The estimated precession error for a- and c-axis are 0.002 respectively. The cell parameters of and 0.007 A, the doped superconductors are relatively larger than those of the undoped superconductor as listed in Table 1 and do not show any systematic variation with the amount of sodium doping. The variation of resistance with temperature of the sodium free (Pb0:5 Cd0:5 )Sr2 (Y0:6 Ca0:4 )Cu2 O7ÿd superconductors and 3±8% sodium containing samples are shown in Fig. 2. All the samples exhibit superconducting behaviour and the value of Tc are listed in Table 1. By comparing the results, it is obvious that sodium doping increases Tc upto 5 mol%. The temperature corresponding to the onset of superconductivity (Tc;onset ) shows a marginal increase of about 5 K upon doping while the temperature at which resistance tends to zero (Tc;zero ) exhibits an enhancement of about 15 K for 5 mol% NaNO3 addition. This is the ®rst time that such an enhancement of Tc upon sodium addition in (Pb,Cd)-1212 superconductor has been observed. With 8% doping, the Tc;zero value decreases but higher than that of the undoped superconductor. Interestingly, the normal state resistivity has been found to decrease with increasing amount of doping. But the highest Tc found for 5% doping probably indicate a shift
Table1 Compositions and properties of samples in the (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5/NaNO3 System Ca content and NaNO3 (mol%)
Cell parameters a (A)
c (A)
3 v (A)
T c;onset
T c;zero
Melting point (°C)
x 0:3 x 0:3&5 x 0:4 x 0:4&3 x 0:4&5 x 0:4&8 x 0:5 x 0:5&5
3.82 3.82 3.81 3.82 3.82 3.82 3.82 3.81
11.98 11.97 11.96 12.00 11.97 12.01 11.99 11.97
175 175 174.1 175 174.6 175.6 174.7 174.1
22 25 48 49 55 45 50 52
± ± 16 ± 30 25 ± 25
1058 1055 1068 ± 1065 ± 1063 1061
Transition temperature (K)
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Fig. 2. Temperature dependence of resistance for pure and NaNO3 doped (Pb0:5 Cd0:5 )Sr2 (Y0:6 Ca0:4 )Cu2 O7ÿd superconductors.
from an under-doped state to an optimally doped state with the maximum Tc possible for the system. Addition of silver has also been found to enhance the superconducting transition temperature of (Pb,M)-1212 superconductors (M Cu or Cd) and the Tc enhancement increases with calcium content of the samples [12]. Hence, the eect of 5 mol% NaNO3 addition on the calcium content x 0:3; 0:5 of (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd superconductors have also been investigated. Fig. 3 shows the temperature dependence of electrical resistance of sodium free and sodium containing
Fig. 3. Temperature dependence of resistance for pure and 5 mol% NaNO3 doped (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd x 0:3 and 0:5 superconductors.
superconductors with x 0:3 and 0.5 calcium content. The undoped superconductors show only Tc;onset , which increases from 23 to 49 K with increasing calcium content from x 0:3 to 0.5, respectively. The resistivity behaviour before Tc is attained also changes from semiconducting to metallic nature as the calcium content increases [12]. The addition of sodium leads to the observation of zero resistance only in the composite with x 0:5. This composite also exhibits an enhancement of Tc;onset of about 5 K and the metallic nature of resistivity behaviour before superconducting. But the normal state resistivity of the sodium doped samples, which is less than that of the undoped samples with calcium content up to x 0:4 changes trend for x 0:5 sample. The highest Tc;onset of 57 K has been observed for the composite with x 0:4. So, the present study suggests the optimum value of the calcium content in the (Pb,Cd)-1212 superconductor to be around x 0:4 also as has reported earlier [12]. The DTA measurements were conducted at temperatures below 1200°C. The DTA traces of the samples without and with 5 mol% NaNO3 addition for x 0:4 are shown in Fig. 4. Both the traces exhibit a single endothermic peak corresponding to the melting point of the sample. For the sodium free sample, the onset temperature for melting is 1068°C, while the melting point of the
Fig. 4. DTA traces of pure and 5 mol% NaNO3 added (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 superconductors.
T.S. Sampath Kumar, B. Latha / Physica C 339 (2000) 137±142
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Fig. 5. SEM of pure and 5 mol% sodium doped (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 superconductors: (a) x 0:3, (b) as (a) NaNO3 , (c) x 0:4, (d) as (c) NaNO3 , (e) x 0:5 and (f) as (e) NaNO3 .
sodium containing sample is 1065°C. The DTA curve for the sodium doped sample with x 0:3
show the melting point at 1055°C, which is less than that of the corresponding undoped sample.
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Although the DTA curve for the doped sample is less than that of the undoped sample with x 0:5 calcium content, the DTA trace show two endopeaks which indicate an additional phase event besides melting. The large amount of calcium substitution expected to change the phase diagram signi®cantly. So, within the calcium solubility limit x 6 0:4, the sodium doping decreases the melting point of the (Pb,Cd)-1212 superconductors. The microstructural changes upon NaNO3 addition on the (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 superconductor were investigated by using SEM and the micrographs are displayed in Fig. 5. Pure (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd sample showed a granular structure, which is typical of a sintered ceramic material with relatively broad distribution of grain sizes. The grain size range of 4±8 lm determined for undoped (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 samples were found to increases with increasing calcium concentration. The 5 mol% sodium doped (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 samples exhibit grain sizes of 5±12 lm, which are larger than those for pure (Pb,Cd)-1212 ceramics. Addition of NaNO3 to (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )-Cu2 O7ÿd superconductors seems to cause grain growth from liquid phase sintering induced by NaNO2 , Na2 O or their mixtures. 4. Conclusions By the addition of sodium nitrate and optimization of calcium content, the superconducting properties of (Pb0:5 Cd0:5 )Sr2 (Y1ÿx Cax )Cu2 O7ÿd 0:3 6 x 6 0:5 can be signi®cantly enhanced. An appreciable rise in Tc from 48 to 55 K was observed upon 5 mol% NaNO3 addition on (Pb0:5 Cd0:5 )Sr2 (Y0:6 Ca0:4 )Cu2 O7ÿd superconductor. Furthermore, the addition also leads to the formation of relatively pure superconducting phase without SrCuO2 and Sr5 Pb3 CuO12 impurity phases. Lowering of the melting point and enlargement of grain size takes place on sodium addition. Also, the present study suggests the optimum calcium
value of the (Pb,Cd)-1212 superconductor to be around x 0:4.
Acknowledgements This work was partially supported by COSIST, SAP and Superconductivity Programmes of UGC, Government of India. One of the authors (BL) acknowledge CSIR, New Delhi, Government of India, for ®nancial assistance in the form of Senior Research Fellowship. The author gratefully acknowledges Prof. G. Shanmugam for his help in the preparation of the manuscript.
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