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Microstructural and superconducting properties of Yb-substituted (Bi,Pb)-2212 superconductor sintered at different temperatures
Journal of Alloys and Compounds 472 (2009) 13–17
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructural and superconducting properties of Yb-substituted (Bi,Pb)-2212 superconductor sintered at different temperatures P.M. Sarun a , S. Vinu a , R. Shabna a , A. Biju b , U. Syamaprasad a,∗ a b
National Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum, Kerala 695019, India M.E.S. College, Nedumkandam, Kerala 685553, India
a r t i c l e
i n f o
Article history: Received 4 April 2008 Received in revised form 24 April 2008 Accepted 29 April 2008 Available online 24 June 2008 PACS: 74.72.Hs 74.25.Qt Keywords: High TC superconductor Flux pinning and creep
a b s t r a c t The influence of sintering temperature on the microstructure and superconducting properties of Bi1.7 Pb0.4 Sr1.8 Yb0.2 Ca1.1 Cu2.1 O8+ı has been investigated. Yb-free (Bi1.7 Pb0.4 Sr2.0 Ca1.1 Cu2.1 O8+ı ) and Ybsubstituted (Bi1.7 Pb0.4 Sr1.8 Yb0.2 Ca1.1 Cu2.1 O8+ı ) superconducting samples are prepared by solid-state synthesis in bulk form. Significant variations in microstructure, critical current density (JC ) and flux pinning properties have been observed for Yb-substituted samples, sintered at different temperatures in the range 846–852 ◦ C. The flux pinning force (FP ) calculated from the field dependant JC values show that the irreversibility lines (ILs) of Yb-substituted samples shift towards higher fields to different extents depending on the sintering temperature. The samples sintered at 846 ◦ C show maximum flux pinning force of 896 kN m−3 and the peak position of FP shifts to higher fields (0.92 T) as against 16 kN m−3 and 0.12 T for the undoped sample sintered at 848 ◦ C. But the self-field JC value of the samples sintered at 846 ◦ C is lower than that of the samples sintered at 852 ◦ C, which show the maximum self-field JC due to improved microstructure. The variation in microstructure followed by very high enhancement of self-field JC , JC –B characteristic and pinning force density due to Yb substitution within a narrow temperature range is of great technological relevance. © 2008 Elsevier B.V. All rights reserved.
1. Introduction (Bi,Pb)-2212 superconductor is one of the most promising high temperature cuprates currently utilized for making tapes and wires for application at high temperatures up to liquid nitrogen temperature. However, their poor performance under magnetic fields, which arises from the weak pinning of flux lines, still prevents their extensive high-field applications at high temperatures. The weak flux pinning of (Bi,Pb)-2212 superconductor is due to their large anisotropy and short coherence length [1]. Therefore, there are two ways to enhance the flux pinning ability and hence the transport properties of (Bi,Pb)-2212 superconductor at high temperatures and high magnetic fields. One method is to reduce the anisotropy of the material [2], which strengthens the interlayer coupling between Cu–O2 layers and increases the flux pinning potential. The other method is to introduce artificial defects, with the size matching the coherence length, as extra pinning centers. It was reported that Pb doping at Bi-site of Bi-2212 crystals significantly improves the caxis conductivity by reducing the anisotropy [3], which enhances
the intrinsic pinning and thereby improves the critical current density (JC ) [4–6]. Also, substitution of rare-earth (RE) ions in the place of Ca/Sr stabilizes the crystal structure of the (Bi,Pb)-2212 system with the introduction of artificial defects as pinning centers. RE substitution also deteriorates the flaky grain morphology of (Bi,Pb)-2212 with increase in RE concentration [7]. However, it is interesting to note that inspite of the deterioration of microstructure, the RE substituted samples give much better JC values and have better pinning capabilities as compared to the RE-free sample. In the present work, we report improved microstructure, enhanced transport JC in self- and applied-fields, and highly improved flux pinning strength of Bi1.7 Pb0.4 Sr1.8 Yb0.2 Ca1.1 Cu2.1 O8+ı superconductor sintered at different temperatures. Yb is chosen as the RE dopant in this work because it has been found that Yb substitution substantially increases TC and JC comparing with other REs [8]. The doping level of Yb is taken as Yb = 0.2 in the stoichiometric level because it has been found to be the optimum concentration for best superconducting properties [7,9]. 2. Experimental
∗ Corresponding author. Tel.: +91 4712515373; fax: +91 4712491712. E-mail address: [email protected] (U. Syamaprasad). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.04.103
Bi1.7 Pb0.4 Sr2−x Ybx Ca1.1 Cu2.1 O8+ı [where x = 0.0 and 0.2] superconductor is prepared by conventional solid-state method using high purity chemicals (Aldrich >99.9%) such as Bi2 O3 , SrCO3 , CaCO3 , CuO, PbO and Yb2 O3 . The ingredients were accurately weighed, mixed and ground using an agate motor and pes-
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P.M. Sarun et al. / Journal of Alloys and Compounds 472 (2009) 13–17
tle. The samples were subjected to a three-stage calcination process in air at 800 ◦ C/15 h + 820 ◦ C/40 h + 840 ◦ C/60 h and intermediate grinding was done between each stage of calcination. After calcination the samples were pelletized using a cylindrical die under a pressure of 600 MPa. The pellets of Yb-free sample (0Yb848) were heat treated at 848 ◦ C, the optimized sintering temperature for Yb-free sample, for 120 h (60 h + 60 h), with one intermediate pressing under the same stress. The pellets of Yb-substituted samples were grouped into four sets and hereafter named as 2Yb846, 2Yb848, 2Yb850 and 2Yb852 for the samples heat treated at 846 ◦ C, 848 ◦ C, 850 ◦ C and 852 ◦ C, respectively, for 120 h (60 h + 60 h), with one intermediate pressing under the same stress. Phase analysis of the samples was done using XRD (Philips X’pert Pro) equipped with an X’celerator and a monochromator at the diffracted beam side. The microstructural and elemental analyses of the samples were done using SEM (JEOL JSM 5600 LV) and EDS, respectively. For electrical measurements the samples were cut into rectangular bar of dimensions 12 mm × 3 mm × 1 mm. The transition temperature (TC ) of the samples was determined by the DC fourprobe method in a bath cryostat. A temperature controller (Lakeshore L340) was used for the accurate control and monitoring of temperature of the samples. The transport critical currents of the samples in self- and applied-fields were measured at 64 K using the four-probe method with the standard criterion of 1 V/cm. The JC –B characteristic was studied with field ranging from 0.0 T to 1.2 T.
3. Results and discussion Fig. 1 shows the XRD patterns of pellets after the last stage heat treatment which reveals that all the samples contain only (Bi,Pb)-2212 phase and no peaks of any secondary phase are observed at this stage. This indicates that all the reactant phases are converted into (Bi,Pb)-2212 and the substituted Yb ions enter into the crystal lattice of (Bi,Pb)-2212. Also Fig. 1 shows that as sintering temperature of Yb-substituted samples are increased, the peak height corresponding to the 0 0 l planes also increases. This shows that the texturing increases with increasing sintering temperature. This figure also shows that all the peaks shift towards higher angle with Yb substitution. The peakshift indicates that c-axis length of the Yb-substituted samples decreases as compared to Yb-free samples. This decreased c-axis length also shows that the substituted Yb enters into the crystal structure. In high temperature cuprate superconductors, when a trivalent cation RE replaces a divalent cation Sr2+ , the charge neutrality is established by incorporating additional oxygen in the Bi–O layers of the structure. As a result the net positive charge in the Bi–O planes reduces and hence the repulsion between them is reduced. This results in the contraction of Bi–O layers and causes the reduction of the c-axis length [3].
Fig. 1. XRD patterns of the Yb-free and Yb-substituted pellets after last stage heat treatment.
Fig. 2 shows the SEM images of the fractured surfaces of Yb-free sample sintered at 848 ◦ C and Yb-substituted samples sintered at 846 ◦ C, 848 ◦ C, 850 ◦ C and 852 ◦ C. The grain morphology of the Ybfree sample (0Yb848) shows clear and flaky grains with layered growth typical of (Bi,Pb)-2212, whereas for Yb-substituted samples 2Yb846, 2Yb848, 2Yb850 and 2Yb852 a systematic change in microstructure with respect to the sintering temperature is observed. The characteristic flaky morphology of pure (Bi,Pb)-2212 grains transforms into rounded grains with reduced texture in 2Yb846. Similarly, as the sintering temperature increases from 846 ◦ C to 852 ◦ C, the flaky morphology and better texture reappear with gradual improvement in microstructure with respect to increase in temperature. The result shows that the optimum sintering temperature for obtaining the best microstructure for the Yb-substituted superconductor sample is 852 ◦ C or more. This improved microstructure plays an important role to increase the critical current density of the system. The EDX spectra of the Ybfree (0Yb848) and Yb-substituted (Bi,Pb)-2212 (2Yb852) grains are shown in Fig. 3. The presence of Yb is detected in the grains of Yb-substituted (Bi, Pb)-2212 superconductor. This is a clear evidence that the Yb atoms are successfully substituted into the crystal structure of (Bi,Pb)-2212. Temperature dependence of resistivity shows that the Ybsubstituted samples have much higher TC value compared to the Yb-free sample (Table 1). It is found that all the Yb-substituted samples show identical TC of 94.6 K, irrespective of the sintering temperature while the TC measured for the Yb-free sample is only 80.1 K. In the case of self-field JC the sample sintered at 846 ◦ C show a JC value of 1354 A/cm2 at 64 K. The JC value increases with increase of sintering temperature (Table 1) and finally the sample sintered at 852 ◦ C shows maximum self-field JC (2551 A/cm2 at 64 K). The enormous enhancement of JC in bulk sample is obviously due to the improvement in microstructure of the sample. The Yb-free sample processed at its optimum temperature show a JC value of only 117 A/cm2 at 64 K. The reported JC values for undoped Bi-2212 bulk samples are in the range 100–1000 A/cm2 at 77 K [8,10]. The field dependence of normalized JC [JC (B)/JC (0)] is shown in Fig. 4. The self-field JC of different samples are given in the inset of the figure. The JC –B characteristics of the Yb-substituted samples are found to be much better than that of the Yb-free sample. That is, the deterioration of JC due to the magnetic field is significantly reduced as a result of Yb substitution. This shows that the substitution of Yb at the Sr site enhances the flux pinning properties of (Bi,Pb)-2212. In the case of Yb-substituted samples, the sample sintered at 846 ◦ C gives the best JC –B characteristics but its self-field JC value is low (1354 A/cm2 ). As the sintering temperature increases, the JC –B performance of Yb-substituted samples gradually decreases and the sample sintered at 852 ◦ C shows the least JC –B performance among the Yb-substituted samples, but it shows maximum self-field JC (2551 A/cm2 ) as mentioned earlier. The result can be understood from the microstructure of the samples (Fig. 2). In sample 2Yb846, the grains are smaller, rounded-like and less textured and hence the pinning due to the grain boundary is more effective, leads to the best JC –B performance. However, more grain boundary weak-links in the sample reduces the selffield JC . In 2Yb852, the flaky grains are highly textured due to the optimum sintering temperature, which leads to less pinning and higher self-field JC . However, the ratio JC (B)/JC (0) of 2Yb852 is still much higher than that of 0Yb848 (Yb-free) by a factor of 6.7 at 0.28 T. Further, it is also observed that all the Yb-substituted samples show a slower degradation in JC than that of 0Yb848 in the presence of increasing magnetic field. This shows the enhanced flux pinning properties of Yb-substituted samples. The flux pinning properties of superconductors are investigated by the determi-
P.M. Sarun et al. / Journal of Alloys and Compounds 472 (2009) 13–17
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Fig. 2. SEM micrograph of the fractured surface of Yb-free and Yb-substituted sample sintered at different temperatures.
nation of pinning force density (Fp = JC × B) [11]. The normalized pinning force density (Fp /Fpmax ) as a function of applied field is shown in Fig. 5 and the inset shows corresponding volume pinning force density Fp of the Yb-free and Yb-substituted samples. It is seen that the maximum value of FP is shifted to much higher field values for Yb-substituted sample. For example the Yb-substituted samples 2Yb846 and 2Yb848, the FP values are 896 kN/m3 and 794 kN/m3 as against 16 kN/m3 for Yb-free sample (Table 1). Similarly for 2Yb846 and 2Yb848 the peak positions of Fp /Fpmax appear beyond 0.92 T and 0.80 T, respectively, as against 0.12 T for 0Yb848. This indicates that the irreversibility line (IL) of Yb-substituted samples shift towards much higher fields compared to the Yb-free sample. This confirms that the flux pinning strength of (Bi,Pb)-2212 significantly increases with Yb substitution at the Sr site.
Table 1 Self-field JC and FPmax values of Yb-free and Yb-substituted samples obtained at 64 K
Fig. 3. EDX patterns of Yb-free and Yb-substituted sample sintered at 848 ◦ C and 852 ◦ C.
Samples
TC (K)
Self-field JC (A/cm2 )
FPmax × 103 (N m−3 )
Field at which FPmax occurs (T)
0Yb848 2Yb846 2Yb848 2Yb850 2Yb852
80.1 94.6 94.6 94.6 94.6
117 1354 1621 2037 2551
16 896 794 699 644
0.12 0.92 0.80 0.68 0.60
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P.M. Sarun et al. / Journal of Alloys and Compounds 472 (2009) 13–17
Fig. 4. Variation of normalized JC (B) as a function of applied magnetic field (inset: variation of self-field JC value of the samples with sintering temperature).
In the case of Bi-2212 the flux lines undergo a cross-over from 3D flux lines to 2D pancake vortices at higher fields and temperatures. The 2D pancake vortices are mainly confined in the Cu–O2 layers [12]. Also Pb doping in the system improves the coupling between the Cu–O2 layers by reducing the anisotropy and increasing the c-axis conductivity [2]. Substitution of each Yb atom at the Sr site of (Bi,Pb)-2212 supplies an additional electron to the system and reduces hole density. This shifts the ‘over-doped’ system to ‘optimally doped’ system, as far as the hole concentration is concerned, which enhances both the TC as well as JC of (Bi,Pb)-2212 system. Since the Yb content in all the doped samples are identical, their TC observed are also identical. However, the JC value depends on the microstructure also. From the SEM images, it is
clear that 2Yb852 has the best microstructure with respect to the grain texturing as compared to all the Yb-substituted samples. This is the reason for the enormous increase of JC value of the sample 2Yb852. The formation of rounded grains and decrease of texturing reduce the self-field JC of samples 2Yb846, 2Yb848 and 2Yb850. Also the results clearly demonstrate that the substitution of Yb in (Bi,Pb)-2212 superconductor significantly enhances its flux pinning strength at a relatively high temperature of 64 K. The Yb substitution in Sr site shifts the system from over-doped to optimally doped condition. The crystal defects created due to the Pb doping are mainly confined in the Bi layer, but the defects produced by Yb substitution are mainly in the Sr layer. The vortices are confined in the Cu–O2 layers, which are closer to Sr layer than the Bi layers.
Fig. 5. Variation of normalized pinning force density as a function of magnetic field (inset: variation of pinning force density FP as a function of magnetic field).
P.M. Sarun et al. / Journal of Alloys and Compounds 472 (2009) 13–17
Thus, the strongly coupled vortices due to Pb doping in Bi-layer are effectively pinned by the defects in the Sr layer which is the main reason for the enhanced flux pinning and the unusually high JC –B performance of the Yb-substituted (Bi,Pb)-2212 superconductor. 4. Conclusion Significant variations in microstructure, critical current density and flux pinning properties are observed in Bi1.7 Pb0.4 Sr1.8 Yb0.2 Ca1.1 Cu2.1 O8+ı superconductor sintered at different temperatures 846 ◦ C, 848 ◦ C, 850 ◦ C and 852 ◦ C. The results indicate that the properties including microstructural variation are highly temperature-sensitive. The sample 2Yb846 shows the best JC –B characteristics and least JC while 2Yb852 shows the least JC –B characteristics and best JC . Based on the pinning force scaling analysis and microstructural observations, it is concluded that sintering the Yb-substituted samples at an optimum temperature improves texture and reduces the defects. This explains the JC and JC –B performance of the system in the temperature range 846–852 ◦ C. The enhancement of critical current density and flux pinning properties are discussed based on the changes in chemical as well as electronic inhomogeneities due to substitution of Yb at Sr sites.
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Acknowledgements P.M. Sarun acknowledges Council of Scientific and Industrial Research (CSIR), India, for Senior Research Fellowship, S. Vinu acknowledges University Grant Commission (UGC), India, for Junior Research Fellowship and R. Shabna acknowledges Kerala State Council for Science, Technology and Environment (KSCSTE), Kerala, India, for Junior Research Fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
M.P. Maley, J. Appl. Phys. 70 (1991) 6189. S. Nomura, H. Yoshino, K. Ando, Supercond. Sci. Technol. 6 (1993) 858. B. Zhao, W.H. Song, J.J. Du, Y.P. Sun, Physica C 386 (2003) 60. I. Chong, Z. Hiroi, M. Izumi, J. Shimoyama, Y. Nakayama, K. Kishio, T. Terashima, Y. Bando, Y. Takano, Science 276 (1997) 770. K.K. Uprety, J. Horvat, X.L. Wang, M. Ionescu, H.K. Liu, S.X. Dou, Supercond. Sci. Technol. 14 (2001) 479. S. Wu, J. Schwartz, G.W. Raban, Physica C 246 (1995) 297. A. Biju, R. Aloysius, U. Syamaprasad, Mater. Lett. 61 (2007) 648. A. Biju, K. Vinod, P.M. Sarun, U. Syamaprasad, Appl. Phys. Lett. 90 (2007) 072505. A. Biju, R.P. Aloysius, U. Syamaprasad, Physica C 440 (2006) 52. B.A. Marinkovic, S.K. Xia, E.T. Serra, F. Rizzo, Mater. Chem. Phys. 91 (2005) 301. M.R. Koblischka, M. Murakami, Supercond. Sci. Technol. 13 (2000) 738. L.S. Uspenskaya, A.B. Kulakov, A.L. Rakhmanov, Phys. Rev. B 68 (2003) 104506.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructural and superconducting properties of Yb-substituted (Bi,Pb)-2212 superconductor sintered at different temperatures P.M. Sarun a , S. Vinu a , R. Shabna a , A. Biju b , U. Syamaprasad a,∗ a b
National Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum, Kerala 695019, India M.E.S. College, Nedumkandam, Kerala 685553, India
a r t i c l e
i n f o
Article history: Received 4 April 2008 Received in revised form 24 April 2008 Accepted 29 April 2008 Available online 24 June 2008 PACS: 74.72.Hs 74.25.Qt Keywords: High TC superconductor Flux pinning and creep
a b s t r a c t The influence of sintering temperature on the microstructure and superconducting properties of Bi1.7 Pb0.4 Sr1.8 Yb0.2 Ca1.1 Cu2.1 O8+ı has been investigated. Yb-free (Bi1.7 Pb0.4 Sr2.0 Ca1.1 Cu2.1 O8+ı ) and Ybsubstituted (Bi1.7 Pb0.4 Sr1.8 Yb0.2 Ca1.1 Cu2.1 O8+ı ) superconducting samples are prepared by solid-state synthesis in bulk form. Significant variations in microstructure, critical current density (JC ) and flux pinning properties have been observed for Yb-substituted samples, sintered at different temperatures in the range 846–852 ◦ C. The flux pinning force (FP ) calculated from the field dependant JC values show that the irreversibility lines (ILs) of Yb-substituted samples shift towards higher fields to different extents depending on the sintering temperature. The samples sintered at 846 ◦ C show maximum flux pinning force of 896 kN m−3 and the peak position of FP shifts to higher fields (0.92 T) as against 16 kN m−3 and 0.12 T for the undoped sample sintered at 848 ◦ C. But the self-field JC value of the samples sintered at 846 ◦ C is lower than that of the samples sintered at 852 ◦ C, which show the maximum self-field JC due to improved microstructure. The variation in microstructure followed by very high enhancement of self-field JC , JC –B characteristic and pinning force density due to Yb substitution within a narrow temperature range is of great technological relevance. © 2008 Elsevier B.V. All rights reserved.
1. Introduction (Bi,Pb)-2212 superconductor is one of the most promising high temperature cuprates currently utilized for making tapes and wires for application at high temperatures up to liquid nitrogen temperature. However, their poor performance under magnetic fields, which arises from the weak pinning of flux lines, still prevents their extensive high-field applications at high temperatures. The weak flux pinning of (Bi,Pb)-2212 superconductor is due to their large anisotropy and short coherence length [1]. Therefore, there are two ways to enhance the flux pinning ability and hence the transport properties of (Bi,Pb)-2212 superconductor at high temperatures and high magnetic fields. One method is to reduce the anisotropy of the material [2], which strengthens the interlayer coupling between Cu–O2 layers and increases the flux pinning potential. The other method is to introduce artificial defects, with the size matching the coherence length, as extra pinning centers. It was reported that Pb doping at Bi-site of Bi-2212 crystals significantly improves the caxis conductivity by reducing the anisotropy [3], which enhances
the intrinsic pinning and thereby improves the critical current density (JC ) [4–6]. Also, substitution of rare-earth (RE) ions in the place of Ca/Sr stabilizes the crystal structure of the (Bi,Pb)-2212 system with the introduction of artificial defects as pinning centers. RE substitution also deteriorates the flaky grain morphology of (Bi,Pb)-2212 with increase in RE concentration [7]. However, it is interesting to note that inspite of the deterioration of microstructure, the RE substituted samples give much better JC values and have better pinning capabilities as compared to the RE-free sample. In the present work, we report improved microstructure, enhanced transport JC in self- and applied-fields, and highly improved flux pinning strength of Bi1.7 Pb0.4 Sr1.8 Yb0.2 Ca1.1 Cu2.1 O8+ı superconductor sintered at different temperatures. Yb is chosen as the RE dopant in this work because it has been found that Yb substitution substantially increases TC and JC comparing with other REs [8]. The doping level of Yb is taken as Yb = 0.2 in the stoichiometric level because it has been found to be the optimum concentration for best superconducting properties [7,9]. 2. Experimental
∗ Corresponding author. Tel.: +91 4712515373; fax: +91 4712491712. E-mail address: [email protected] (U. Syamaprasad). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.04.103
Bi1.7 Pb0.4 Sr2−x Ybx Ca1.1 Cu2.1 O8+ı [where x = 0.0 and 0.2] superconductor is prepared by conventional solid-state method using high purity chemicals (Aldrich >99.9%) such as Bi2 O3 , SrCO3 , CaCO3 , CuO, PbO and Yb2 O3 . The ingredients were accurately weighed, mixed and ground using an agate motor and pes-
14
P.M. Sarun et al. / Journal of Alloys and Compounds 472 (2009) 13–17
tle. The samples were subjected to a three-stage calcination process in air at 800 ◦ C/15 h + 820 ◦ C/40 h + 840 ◦ C/60 h and intermediate grinding was done between each stage of calcination. After calcination the samples were pelletized using a cylindrical die under a pressure of 600 MPa. The pellets of Yb-free sample (0Yb848) were heat treated at 848 ◦ C, the optimized sintering temperature for Yb-free sample, for 120 h (60 h + 60 h), with one intermediate pressing under the same stress. The pellets of Yb-substituted samples were grouped into four sets and hereafter named as 2Yb846, 2Yb848, 2Yb850 and 2Yb852 for the samples heat treated at 846 ◦ C, 848 ◦ C, 850 ◦ C and 852 ◦ C, respectively, for 120 h (60 h + 60 h), with one intermediate pressing under the same stress. Phase analysis of the samples was done using XRD (Philips X’pert Pro) equipped with an X’celerator and a monochromator at the diffracted beam side. The microstructural and elemental analyses of the samples were done using SEM (JEOL JSM 5600 LV) and EDS, respectively. For electrical measurements the samples were cut into rectangular bar of dimensions 12 mm × 3 mm × 1 mm. The transition temperature (TC ) of the samples was determined by the DC fourprobe method in a bath cryostat. A temperature controller (Lakeshore L340) was used for the accurate control and monitoring of temperature of the samples. The transport critical currents of the samples in self- and applied-fields were measured at 64 K using the four-probe method with the standard criterion of 1 V/cm. The JC –B characteristic was studied with field ranging from 0.0 T to 1.2 T.
3. Results and discussion Fig. 1 shows the XRD patterns of pellets after the last stage heat treatment which reveals that all the samples contain only (Bi,Pb)-2212 phase and no peaks of any secondary phase are observed at this stage. This indicates that all the reactant phases are converted into (Bi,Pb)-2212 and the substituted Yb ions enter into the crystal lattice of (Bi,Pb)-2212. Also Fig. 1 shows that as sintering temperature of Yb-substituted samples are increased, the peak height corresponding to the 0 0 l planes also increases. This shows that the texturing increases with increasing sintering temperature. This figure also shows that all the peaks shift towards higher angle with Yb substitution. The peakshift indicates that c-axis length of the Yb-substituted samples decreases as compared to Yb-free samples. This decreased c-axis length also shows that the substituted Yb enters into the crystal structure. In high temperature cuprate superconductors, when a trivalent cation RE replaces a divalent cation Sr2+ , the charge neutrality is established by incorporating additional oxygen in the Bi–O layers of the structure. As a result the net positive charge in the Bi–O planes reduces and hence the repulsion between them is reduced. This results in the contraction of Bi–O layers and causes the reduction of the c-axis length [3].
Fig. 1. XRD patterns of the Yb-free and Yb-substituted pellets after last stage heat treatment.
Fig. 2 shows the SEM images of the fractured surfaces of Yb-free sample sintered at 848 ◦ C and Yb-substituted samples sintered at 846 ◦ C, 848 ◦ C, 850 ◦ C and 852 ◦ C. The grain morphology of the Ybfree sample (0Yb848) shows clear and flaky grains with layered growth typical of (Bi,Pb)-2212, whereas for Yb-substituted samples 2Yb846, 2Yb848, 2Yb850 and 2Yb852 a systematic change in microstructure with respect to the sintering temperature is observed. The characteristic flaky morphology of pure (Bi,Pb)-2212 grains transforms into rounded grains with reduced texture in 2Yb846. Similarly, as the sintering temperature increases from 846 ◦ C to 852 ◦ C, the flaky morphology and better texture reappear with gradual improvement in microstructure with respect to increase in temperature. The result shows that the optimum sintering temperature for obtaining the best microstructure for the Yb-substituted superconductor sample is 852 ◦ C or more. This improved microstructure plays an important role to increase the critical current density of the system. The EDX spectra of the Ybfree (0Yb848) and Yb-substituted (Bi,Pb)-2212 (2Yb852) grains are shown in Fig. 3. The presence of Yb is detected in the grains of Yb-substituted (Bi, Pb)-2212 superconductor. This is a clear evidence that the Yb atoms are successfully substituted into the crystal structure of (Bi,Pb)-2212. Temperature dependence of resistivity shows that the Ybsubstituted samples have much higher TC value compared to the Yb-free sample (Table 1). It is found that all the Yb-substituted samples show identical TC of 94.6 K, irrespective of the sintering temperature while the TC measured for the Yb-free sample is only 80.1 K. In the case of self-field JC the sample sintered at 846 ◦ C show a JC value of 1354 A/cm2 at 64 K. The JC value increases with increase of sintering temperature (Table 1) and finally the sample sintered at 852 ◦ C shows maximum self-field JC (2551 A/cm2 at 64 K). The enormous enhancement of JC in bulk sample is obviously due to the improvement in microstructure of the sample. The Yb-free sample processed at its optimum temperature show a JC value of only 117 A/cm2 at 64 K. The reported JC values for undoped Bi-2212 bulk samples are in the range 100–1000 A/cm2 at 77 K [8,10]. The field dependence of normalized JC [JC (B)/JC (0)] is shown in Fig. 4. The self-field JC of different samples are given in the inset of the figure. The JC –B characteristics of the Yb-substituted samples are found to be much better than that of the Yb-free sample. That is, the deterioration of JC due to the magnetic field is significantly reduced as a result of Yb substitution. This shows that the substitution of Yb at the Sr site enhances the flux pinning properties of (Bi,Pb)-2212. In the case of Yb-substituted samples, the sample sintered at 846 ◦ C gives the best JC –B characteristics but its self-field JC value is low (1354 A/cm2 ). As the sintering temperature increases, the JC –B performance of Yb-substituted samples gradually decreases and the sample sintered at 852 ◦ C shows the least JC –B performance among the Yb-substituted samples, but it shows maximum self-field JC (2551 A/cm2 ) as mentioned earlier. The result can be understood from the microstructure of the samples (Fig. 2). In sample 2Yb846, the grains are smaller, rounded-like and less textured and hence the pinning due to the grain boundary is more effective, leads to the best JC –B performance. However, more grain boundary weak-links in the sample reduces the selffield JC . In 2Yb852, the flaky grains are highly textured due to the optimum sintering temperature, which leads to less pinning and higher self-field JC . However, the ratio JC (B)/JC (0) of 2Yb852 is still much higher than that of 0Yb848 (Yb-free) by a factor of 6.7 at 0.28 T. Further, it is also observed that all the Yb-substituted samples show a slower degradation in JC than that of 0Yb848 in the presence of increasing magnetic field. This shows the enhanced flux pinning properties of Yb-substituted samples. The flux pinning properties of superconductors are investigated by the determi-
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Fig. 2. SEM micrograph of the fractured surface of Yb-free and Yb-substituted sample sintered at different temperatures.
nation of pinning force density (Fp = JC × B) [11]. The normalized pinning force density (Fp /Fpmax ) as a function of applied field is shown in Fig. 5 and the inset shows corresponding volume pinning force density Fp of the Yb-free and Yb-substituted samples. It is seen that the maximum value of FP is shifted to much higher field values for Yb-substituted sample. For example the Yb-substituted samples 2Yb846 and 2Yb848, the FP values are 896 kN/m3 and 794 kN/m3 as against 16 kN/m3 for Yb-free sample (Table 1). Similarly for 2Yb846 and 2Yb848 the peak positions of Fp /Fpmax appear beyond 0.92 T and 0.80 T, respectively, as against 0.12 T for 0Yb848. This indicates that the irreversibility line (IL) of Yb-substituted samples shift towards much higher fields compared to the Yb-free sample. This confirms that the flux pinning strength of (Bi,Pb)-2212 significantly increases with Yb substitution at the Sr site.
Table 1 Self-field JC and FPmax values of Yb-free and Yb-substituted samples obtained at 64 K
Fig. 3. EDX patterns of Yb-free and Yb-substituted sample sintered at 848 ◦ C and 852 ◦ C.
Samples
TC (K)
Self-field JC (A/cm2 )
FPmax × 103 (N m−3 )
Field at which FPmax occurs (T)
0Yb848 2Yb846 2Yb848 2Yb850 2Yb852
80.1 94.6 94.6 94.6 94.6
117 1354 1621 2037 2551
16 896 794 699 644
0.12 0.92 0.80 0.68 0.60
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Fig. 4. Variation of normalized JC (B) as a function of applied magnetic field (inset: variation of self-field JC value of the samples with sintering temperature).
In the case of Bi-2212 the flux lines undergo a cross-over from 3D flux lines to 2D pancake vortices at higher fields and temperatures. The 2D pancake vortices are mainly confined in the Cu–O2 layers [12]. Also Pb doping in the system improves the coupling between the Cu–O2 layers by reducing the anisotropy and increasing the c-axis conductivity [2]. Substitution of each Yb atom at the Sr site of (Bi,Pb)-2212 supplies an additional electron to the system and reduces hole density. This shifts the ‘over-doped’ system to ‘optimally doped’ system, as far as the hole concentration is concerned, which enhances both the TC as well as JC of (Bi,Pb)-2212 system. Since the Yb content in all the doped samples are identical, their TC observed are also identical. However, the JC value depends on the microstructure also. From the SEM images, it is
clear that 2Yb852 has the best microstructure with respect to the grain texturing as compared to all the Yb-substituted samples. This is the reason for the enormous increase of JC value of the sample 2Yb852. The formation of rounded grains and decrease of texturing reduce the self-field JC of samples 2Yb846, 2Yb848 and 2Yb850. Also the results clearly demonstrate that the substitution of Yb in (Bi,Pb)-2212 superconductor significantly enhances its flux pinning strength at a relatively high temperature of 64 K. The Yb substitution in Sr site shifts the system from over-doped to optimally doped condition. The crystal defects created due to the Pb doping are mainly confined in the Bi layer, but the defects produced by Yb substitution are mainly in the Sr layer. The vortices are confined in the Cu–O2 layers, which are closer to Sr layer than the Bi layers.
Fig. 5. Variation of normalized pinning force density as a function of magnetic field (inset: variation of pinning force density FP as a function of magnetic field).
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Thus, the strongly coupled vortices due to Pb doping in Bi-layer are effectively pinned by the defects in the Sr layer which is the main reason for the enhanced flux pinning and the unusually high JC –B performance of the Yb-substituted (Bi,Pb)-2212 superconductor. 4. Conclusion Significant variations in microstructure, critical current density and flux pinning properties are observed in Bi1.7 Pb0.4 Sr1.8 Yb0.2 Ca1.1 Cu2.1 O8+ı superconductor sintered at different temperatures 846 ◦ C, 848 ◦ C, 850 ◦ C and 852 ◦ C. The results indicate that the properties including microstructural variation are highly temperature-sensitive. The sample 2Yb846 shows the best JC –B characteristics and least JC while 2Yb852 shows the least JC –B characteristics and best JC . Based on the pinning force scaling analysis and microstructural observations, it is concluded that sintering the Yb-substituted samples at an optimum temperature improves texture and reduces the defects. This explains the JC and JC –B performance of the system in the temperature range 846–852 ◦ C. The enhancement of critical current density and flux pinning properties are discussed based on the changes in chemical as well as electronic inhomogeneities due to substitution of Yb at Sr sites.
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Acknowledgements P.M. Sarun acknowledges Council of Scientific and Industrial Research (CSIR), India, for Senior Research Fellowship, S. Vinu acknowledges University Grant Commission (UGC), India, for Junior Research Fellowship and R. Shabna acknowledges Kerala State Council for Science, Technology and Environment (KSCSTE), Kerala, India, for Junior Research Fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
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