- Email: [email protected]
Ag composites
Accepted Manuscript
Enhancement of the critical current density in YBCO/Ag composites Bilal A. Malik , Manzoor A. Malik , K. Asokan PII: DOI: Reference:
S0577-9073(16)30134-4 10.1016/j.cjph.2016.10.015 CJPH 127
To appear in:
Chinese Journal of Physics
Received date: Revised date: Accepted date:
1 April 2016 5 October 2016 7 October 2016
Please cite this article as: Bilal A. Malik , Manzoor A. Malik , K. Asokan , Enhancement of the critical current density in YBCO/Ag composites, Chinese Journal of Physics (2016), doi: 10.1016/j.cjph.2016.10.015
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights
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The maximum enhancement in JC (by a factor of 6) is observed for YBCO+15%Ag. The pinning force is enhanced by a factor of 10 for the same composite at the same temperature. TC does not change with Ag addition. However, TC0 is found to be higher for composites. The XRD results show that Ag does not react with YBCO but stays as an adhering material. The SEM measurements show that Ag enhances grain connectivity.
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Enhancement of the critical current density in YBCO/Ag composites Bilal A. Malika, Manzoor A. Malika,*, [email protected], and K. Asokanb a Department of Physics, University of Kashmir, Srinagar- 190006, India b Materials Science Division, Inter University Accelerator Centre, New Delhi -110067, India * Corresponding author: Abstract
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( and wt.%) composite samples have been prepared by the solid state reaction method. The changes in structure are confirmed from the X-ray diffraction analysis and SEM measurements. The critical current density is calculated using Bean’s formula from the magnetization measurement. We find that the addition of silver in YBCO enhances the critical current density ( ) by a factor of nearly six (for 15% Ag) in comparison to pure YBCO. Enhancement of the pinning force ( ) by a factor of ten is also reported. The enhancement in is investigated over a wide range of magnetic fields. These significant changes in and are attributed to the presence of Ag particles as efficient artifical pinning centers in YBCO. Key words
Superconductivity; YBCO; Magnetization; Critical current density; Pinning force. Introduction
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Cuprate superconductors, owing to their relatively high transition temperatures and upper critical magnetic fields, are ideal material for promising applications like energy storage systems, current limiters, magnetic bearings etc. These applications require a high critical current density ( ) even at high applied magnetic field. For maintaining high at higher fields in cuprates, the complex vortex dynamics, poor flux pinning, weak intergrain linking etc. are major limiting factors [1-3]. In cuprates, the perfect diamagnetic state is maintained up to a lower critical field ( ), beyond which the flux begins to penetrate and vortex formation takes place. The situation becomes complex under the application of a transport current for because of the Lorentz force ( ) acting on vortices. These vortices move under the influence of the transport current leading to energy dissipation, with the result that the materials no longer remain in a zero resistance state. The effect gets more pronounced in high temperature superconductors (HTSCs) when operated at relatively high temperatures. Therefore, to maintain a high at higher fields the motion of vortices needs to be arrested.
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In order to arrest the motion of vortices, potential wells must be created in which a vortex has lower energy than in the surroundings. These vortices sit at pinning sites. Neither the Lorentz force nor thermal energy ( ) will be able to move the vortices if the potential wells are very deep. This is achieved by the process of flux pinning. One of the simple methods of achieving flux pining is to introduce inhomogeneities on the order of the coherence length ( ) in the superconducting material. In recent years, the identification of materials which can act as efficient pining centers has generated a lot of interest. Several non-superconducting nano inclusions find their applicability for flux pinning in (YBCO) . The nature of these nano inclusions has been found to be either magnetic [4, 5] or non-magnetic [6] or ferroelectric [7-9]. Efficient flux pining in YBCO has been achieved by chemical
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doping and the introduction of secondary phases [10-14]. Doping of impurities like Gd and Ho have resulted in the enhancement of [10, 11], owing to the high density of stacking faults found to act as effective pinning centers. At lower magnetic field, precipitates like YBaCuO5 [12] and Y2BaCuNbOx [13] have been found to act as pinning centers. The addition of nano particles of Al 2O3 [15], BaZrO3[16] and BaCeO3[17] in YBCO for improving pinning has drawn lot of attention from the scientific community over the last decade. Point defects are believed to be the main source of pinning centers in thermally activated creep regions. The co-action of point defects and planar defects are responsible for flux pinning in soft vortex liquid regions [18]. This paper reports on the effect of silver (Ag) addition in YBCO based on X-ray diffraction, scanning electron micrographs and magnetization measurements. The choice of Ag was motivated by the fact that it acts as a good transport medium among the grains besides providing the necessary pinning. Appreciable changes in the XRD have been found and subsequent enhancement in and were found over the entire range of the magnetic field.
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Experimental details
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Pure YBCO was prepared by the solid state reaction route. According to the exact stoichiometric ratio Y:Ba:Cu=1:2:3 high purity powders of Y2O3 (99.99%), BaCO3 (99.99%) and CuO (99.99%) precursors were thoroughly mixed. After initial grinding this mixture of powders was calcined at 900⁰C in air for 12h. Subsequent calcinations were done at 915⁰C and 930⁰C for the same duration of time in air with intermediate grinding. In each calcination cycle, a slow cooling rate was maintained as compared to the heating rate, and the samples were re-ground well before the next cycle. Superconductor YBCO+ metallic Ag composite samples were formed by adding Ag in the pre-reacted YBCO powder. A series of polycrystalline composite samples of YBCO+xAg with nominal composition and wt.% were then pressed into square pellets of size 1 cm and sintered at 930⁰C for 16h in the presence of oxygen to achieve the proper density. In order to get the exact oxygen stoichiometry, the process was followed by oxygen annealing at 650⁰C, 550⁰C and 450⁰C for the duration of 12h at each temperature and cooled to room temperature for another 6h. The phase purity of these pure and composite samples were determined by using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation ( ). The diffraction data were collected over the diffraction angle range ⁰ ⁰ with settings of 40 mA current and 40 Kv voltage. Magnetization measurements up to 14 T were carried out with a Quantum Design Physical Property Measurement system having a Vibrating Sample Magnetometer (VSM) attachment.
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Results and discussion
Rietveld fitted XRD patterns of YBCO+xAg composite samples at room temperature are shown in figure 1. The structural analysis was performed using the Rietveld refinement with the help of the FullProof software, which confirms that all the samples crystallize in the orthorhombic phase with space group Pmmm. The appearance of Ag as a secondary phase in YBCO is revealed by the presence of some additional tiny peaks of Ag in the diffraction pattern marked by (*) at ⁰ and ⁰ The lattice parameters of all the samples and other fitted parameters are given in Table 1. It is evident from Table 1 that the lattice parameters as well as the orthorhombicity remain nearly unchanged with the Ag addition, which substantiates earlier results [19, 20] . However, the effect of Ag addition in YBCO is
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evident from the observation of an extra hump in some reflections, in particular, the main peak. Several authors [21-23] have attributed the appearance of a hump in some reflections to an increase in crystallinity due to the bridging of grains. Further, it has been argued in [7] that secondary phase
BaTiO3 particles stay around the grain and might act like a catalyst to improve the structural quality at the grain boundaries. Based on the literature, we have attributed the hump to crystallinity due to bridging of YBCO grains. Hence our XRD results confirm that the addition of Ag
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in YBCO brings significant changes in crystallinity and makes the grains to behave in coherent way. Further, addition of Ag in YBCO effects the distribution of grain orientations (crystallographic texture) in the microstructure which governs the properties of materials. Besides, due to the metallic nature of Ag, it will act more effectively at grain boundaries than dielectric materials like BaZrO3, BaTiO3 etc.
Figure (1): Rietveld fitted XRD pattern of ( and wt.%) composites at room temperature. Secondary phases are denoted by symbol (*). The inset shows the
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appearance of a hump in the main peak with the addition of Ag.
Compound
a(Å)
b(Å)
c(Å)
Vol.( Å3)
χ2
GoF{(b-a)/b} index X 100 YBCO 3.834 3.885 11.680 173.975 1.33 1.153 1.313 YBCO+5%Ag 3.821 3.878 11.674 172.983 1.68 1.297 1.470 YBCO+15%Ag 3.823 3.880 11.683 173.297 2.02 1.420 1.469 YBCO+20%Ag 3.822 3.880 11.676 173.148 2.20 1.488 1.495 Table 1. Lattice parameters, volume, quality of fit (χ2), goodness of fit (GOF) and orthorhombicity.
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Figure 2: SEM images of YBCO+xAg composites: a) x=0%, b) x=5 %, c) x=15% and d) x=20 %.
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The transport properties are highly dependent on the microstructure, therefore, it becomes necessary to analyze the microstructure of YBCO+xAg composites. Figure 2 shows the microstructure changes occurring in YBCO by the addition of Ag. It is observed from the SEM images that pristine YBCO
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is highly porous and of low density. Addition of Ag results in the following changes in the microstructure: (i) The grains observed in the pristine sample are almost circular, while in the composite samples rod like elongated grains have been found. Hence grain boundary area drops drastically, which is important for maintaining high transport in superconductors working in a magnetic environment. (ii) Ag particles on the surface of grains increase the grain connectivity by filling up cracks and voids, and hence the porosity reduces significantly. Interestingly, it is found that the existence of Ag in fractions (chunks) prevents the formation of a continuous metallic layer throughout the sample. This also further supports the XRD results: that Ag does not react with YBCO to form
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a new phase or structure but links as the connecting channels on the grain boundaries that brings significant changes in the transport and mechanical properties of the YBCO superconductor.
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To study the effect of Ag on the transition temperature (TC) of YBCO, the variation of the electrical resistivity as a function of temperature along with its temperature derivative is shown in figure 3. Fig. 3(a) and 3(b)
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Figure 3: Temperature dependence of electrical resistivity and its temperature derivative of YBCO+xAg composites in the presence of a 0 and 12 T magnetic field.
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show the variation of resistivity and its temperature derivative at 0 K. It is clear that all samples show superconductivity around 90 K, but transition broadening decreases by the addition of Ag in YBCO. In the absence of any magnetic field, the appearance of an extra hump in the temperature derivative plot at lower temperature shows that the grains are weakly connected in the case of a pristine sample. However, this hump disappears with the addition of Ag in YBCO. This shows that the addition of Ag in YBCO makes the grains to couple strongly. These effects have been further investigated at 12 T magnetic field (figures 3(c) and 3(d)), where the pristine sample shows a long tailing behavior. This tailing behavior is due to weak grain connectivity, which has been significantly improved by the addition of Ag in YBCO, as the YBCO+xAg composites show less tailing behavior. Further, the onset of the transition temperature (TCon) both in the presence and absence of a magnetic field does not shift by the addition of Ag in YBCO. However the zero resistivity transition temperature (TC0) is found to be higher for composite samples as compared to pristine samples. The detailed magneto resistivity studies of these composites have been discussed in our recent paper [24].
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20M a a 1 3b
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Jc
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Magnetization measurements play an important role in understanding the critical parameters of type II superconductors beyond the lower critical field . The isothermal magnetization measurements M-H up to 14 T were done at 5 K and are shown in figure 4(a). The applied magnetic field is set parallel to the long axis of the measured sample, so as to expose a maximum area and to avoid demagnetization. It is clear from the figure 4(a) that the magnetization has a peak at , beyond which flux penetrates and magnetization begins to decrease gradually. However, the decrease in magnetization is less for Ag composite samples as compared to pristine samples over the entire investigated range of magnetic field. Anderson [25] and Kim et al. [26] treat the vortices in a mixed state of any superconductor as well defined elastic objects which can be pinned due to various interactions, e.g., impurities, stress, point defects, extended defects and secondary phases etc. This confirms our observation, that the secondary phase of Ag in YBCO can pin the moving vortices and impedes the motion of flux lines. This results in irreversibility in magnetization. These defects trap the field, thereby enhancing . The critical current density of the composite samples was calculated from the magnetization width of the M-H loop using Bean’s critical state model [27]:
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where and are the dimensions of the samples in the plane perpendicular to the direction of the applied field (both in cm) and . (in emu/cm3 ) is the difference in the hysteretic magnetization between the curves obtained while increasing and decreasing the magnetic fields. The variation of the critical current density of the composite samples as a function of the applied magnetic field is shown in figure 4(b). The plots clearly suggest that Ag composite samples have substaintially higher values of as campared to pristine over the entire range of magnetic fields. The maxiumum value of in the case of pure YBCO is
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Figure 4 : (a) Isothermal magnetization hysteresis loops of composite samples at 5 K, (b) variation of as a function of applied magnetic field.
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A/cm2 and the same gets enhanced up to A/cm2 for the 15%Ag sample. Thus the addition of 15% Ag enhances by a factor of 6. The enhancement of is more than that reported by other authors [7, 23, 28, 29]. The variation of with Ag wt.% is shown in figure 5(a). As is obivious from the figure, is optimized at 15%Ag in YBCO. Beyond this again decreases due to weak pinning. It has been argued [in 30] that the reason for poor pinning may be either a high defect density or the defects can be of a size larger than the coherence length . For defects to act as effective pinning centers their size must be of the order of [31]. Even defects of size smaller than are poor pins, because their effect is averaged over a large volume.
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Figure 5: (a) Variation of the critical current density with Ag wt.%. (b) Pinning force density of ( and wt.%) composite samples as a function of the applied magnetic field.
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We have also calculated the pinning force density for both pure and Ag composite samples. The variation of as a function of the applied magnetic field at 5 K is shown in figure 5(b). The plot clearly reveals that the pinning force density increases appreciably upon addition of Ag in YBCO over the entire range of the applied magnetic field. In the case of the 15%Ag added sample, the pining force is about 10 times that of the pristine sample. These values of are significantly higher than the values found in YBCO: BaZrO3 composites [23, 32]. This shows that the addition of Ag particles in YBCO apart from increasing inter-grain connectivity (decreasing grain boundary area by staying around outside grains) also act as artificial pinning centers. This results in the enhancement of and dominance of the pinning force over the Lorentz force even at high magnetic field. Conclusion: In this study, we have shown that the addition of Ag particles in YBCO acts as artificial pinning centers. The addition of Ag in YBCO brings about significant changes in the crystallinity,
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Acknowledgement
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and . Both and have been found to increase in Ag composite samples over the whole range of the magnetic field. In the case of YBCO+15%Ag the enhancement of is nearly six times, while the enhancement in is more than ten times. Beyond 15%Ag, both and have been found to decrease but are still higher than in the pristine sample. Therefore, the efficient pinning is restricted up to 15%Ag in YBCO. Based on our XRD, SEM and magnetization measurements, we deduce that the optimal addition of Ag in YBCO brings a significant enhancement in the physical parameters like and thereby making these composite materials attractive for technological applications.
The authors would like to thank the Director of IUAC for financial support through theIUAC–UGC sponsored Project (UFR-52307). Bilal A. Malik, in particular is thankful to Dr. Alok Banerjee, UGCDAE Indore for the use of a VSM facility. References
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[1] S. Ravi. and V. S. Bai, Phys. Rev. B 49 (1994) 13082. [2] G. Blatter et al., Rev. Mod. Phys. 66 (1994) 1125.
[3] D. C. Larbalestier et al., Physica C 185 (1991) 315. [4] K. Ishida, et al., Physica C 179 (1991) 29.
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[5] Y. Zhao, C. H. Chengand, J. S. Wang, Supercond. Sci. Technol. 18 (2005) S43. [6] T. Haugan et al., Nature, 430 (2004) 867.
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[7] A. K. Jha,N. Khare, Physica C 469 (2009) 810. [8] A. Kujur, D. Behera,Thin solid films 520 (2012) 2195.
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[9] A. Kujur,D.Behera, Journal of Magnetism and Magnetic Materials 377 (2015) 34. [10] S. Nariki, N. Sakai, M. Murakami, I. Hirabashi, Physica C 439 (2006) 62.
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[11] Y. Feng, A. K. Pradhan, Y. Zhao, Y. Wu, N. Koshizuka, L. Zhou, Supercond. Sci.Technol. 14 (2001) 224.
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[12] P. J. Kung, M. P. Maley, M. E. McHenry, J. O. Willis, M. Murakami, S. Tanaka, Phys. Rev. B 48 (1993) 13922. [13] M. N. Hasan, M. Kiuchi, E. S. Otabe, T. Matsuhita, M. Muralidhar, Supercond. Sci. Technol. 20 (2007) 345. [14] N. P. Liyanawaduge, A. Kumar, R. Jha, B. S. B. Karunarathne, V. P. S. Awana, P. Sunwong, J of Alloys and Compounds 543 (2012) 135. [15] A. Mellekh, M. Zouaoui, F. B. Azzouz, M. Annabi, M. B. Salem, Solid State Commun. 140 (2006) 318.
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[16] C. Xu, A. Hu, M. Ichihara, N. Sakai, I. Hirabayashi, M. Izumi, Physica C 460 (2007) 1341. [17] C. J. Kim, N. Quadir, A. Mahmood, Y.H. Han, T.H. Sung, Physica C 463 (2007) 344. [18] T. Yang, Z.H. Wang, H. Zhang, J. Fang, Y. Nie, L. Qiu, S.Y. Ding, Physica C 384 (2003) 130. [19] T. Nishio, T. Y. Itoh, Y. F. Ogasawara, M. Suganuma, Y. Yamada, U. Mizutani, J. Mater. Sci. 24 (1989) 3228.
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[20] J. H. Miller, S. L. Holder, J. D. Hunn, G. N. Holder, Appl. Phys. Lett. 54 (1989) 256. [21] A. Mohanta and D. Behera , J. Supercond. Nov. Magn. 23 (2010) 275. [22] A. Mohanta and D. Bhera, Physica C 470 (2010) 295. [23] B. A. Malik, M. A. Malik and K. Asokan AIP ADVANCES 6 (2016) 045317
[24] B. A. Malik, M. A. Malik and K. Asokan, Current Applied Physics 16 (2016) 1270.
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[25] P. W. Anderson, Phys. Rev. Lett. 9 (1962) 309. [26] Y. B. Kim, C. F. Hempstead, A.R. Strnad, Phys. Rev. 131 (1963) 2486. [27] C. P. Bean Phys. Rev. Lett.8 (1962) 250.
[28] H. Huhtinen, V. P. S. Awana, A. Gupta, H. Kishan, R. Laiho and A. V. Narlikar, Supercond. Sci. Technol. 20 (2007) S159. [29] I. Dhingra, B. K. Das and S. C. Kashyap, Bull Matr. Sci 17 (1994) 153
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[30] L. Civale, Supercond. Sci. Technol. 10 (1997) A11.
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[31] A. M. Campbell and J. E. Evetts ,Adv. Phys. 21 (1972) 199.
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[32] A. K. Jha and N. Khare, Journal of Magnetism and Magnetic Materials 322 (2010) 2653.
Enhancement of the critical current density in YBCO/Ag composites Bilal A. Malik , Manzoor A. Malik , K. Asokan PII: DOI: Reference:
S0577-9073(16)30134-4 10.1016/j.cjph.2016.10.015 CJPH 127
To appear in:
Chinese Journal of Physics
Received date: Revised date: Accepted date:
1 April 2016 5 October 2016 7 October 2016
Please cite this article as: Bilal A. Malik , Manzoor A. Malik , K. Asokan , Enhancement of the critical current density in YBCO/Ag composites, Chinese Journal of Physics (2016), doi: 10.1016/j.cjph.2016.10.015
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights
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The maximum enhancement in JC (by a factor of 6) is observed for YBCO+15%Ag. The pinning force is enhanced by a factor of 10 for the same composite at the same temperature. TC does not change with Ag addition. However, TC0 is found to be higher for composites. The XRD results show that Ag does not react with YBCO but stays as an adhering material. The SEM measurements show that Ag enhances grain connectivity.
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Enhancement of the critical current density in YBCO/Ag composites Bilal A. Malika, Manzoor A. Malika,*, [email protected], and K. Asokanb a Department of Physics, University of Kashmir, Srinagar- 190006, India b Materials Science Division, Inter University Accelerator Centre, New Delhi -110067, India * Corresponding author: Abstract
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( and wt.%) composite samples have been prepared by the solid state reaction method. The changes in structure are confirmed from the X-ray diffraction analysis and SEM measurements. The critical current density is calculated using Bean’s formula from the magnetization measurement. We find that the addition of silver in YBCO enhances the critical current density ( ) by a factor of nearly six (for 15% Ag) in comparison to pure YBCO. Enhancement of the pinning force ( ) by a factor of ten is also reported. The enhancement in is investigated over a wide range of magnetic fields. These significant changes in and are attributed to the presence of Ag particles as efficient artifical pinning centers in YBCO. Key words
Superconductivity; YBCO; Magnetization; Critical current density; Pinning force. Introduction
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Cuprate superconductors, owing to their relatively high transition temperatures and upper critical magnetic fields, are ideal material for promising applications like energy storage systems, current limiters, magnetic bearings etc. These applications require a high critical current density ( ) even at high applied magnetic field. For maintaining high at higher fields in cuprates, the complex vortex dynamics, poor flux pinning, weak intergrain linking etc. are major limiting factors [1-3]. In cuprates, the perfect diamagnetic state is maintained up to a lower critical field ( ), beyond which the flux begins to penetrate and vortex formation takes place. The situation becomes complex under the application of a transport current for because of the Lorentz force ( ) acting on vortices. These vortices move under the influence of the transport current leading to energy dissipation, with the result that the materials no longer remain in a zero resistance state. The effect gets more pronounced in high temperature superconductors (HTSCs) when operated at relatively high temperatures. Therefore, to maintain a high at higher fields the motion of vortices needs to be arrested.
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In order to arrest the motion of vortices, potential wells must be created in which a vortex has lower energy than in the surroundings. These vortices sit at pinning sites. Neither the Lorentz force nor thermal energy ( ) will be able to move the vortices if the potential wells are very deep. This is achieved by the process of flux pinning. One of the simple methods of achieving flux pining is to introduce inhomogeneities on the order of the coherence length ( ) in the superconducting material. In recent years, the identification of materials which can act as efficient pining centers has generated a lot of interest. Several non-superconducting nano inclusions find their applicability for flux pinning in (YBCO) . The nature of these nano inclusions has been found to be either magnetic [4, 5] or non-magnetic [6] or ferroelectric [7-9]. Efficient flux pining in YBCO has been achieved by chemical
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doping and the introduction of secondary phases [10-14]. Doping of impurities like Gd and Ho have resulted in the enhancement of [10, 11], owing to the high density of stacking faults found to act as effective pinning centers. At lower magnetic field, precipitates like YBaCuO5 [12] and Y2BaCuNbOx [13] have been found to act as pinning centers. The addition of nano particles of Al 2O3 [15], BaZrO3[16] and BaCeO3[17] in YBCO for improving pinning has drawn lot of attention from the scientific community over the last decade. Point defects are believed to be the main source of pinning centers in thermally activated creep regions. The co-action of point defects and planar defects are responsible for flux pinning in soft vortex liquid regions [18]. This paper reports on the effect of silver (Ag) addition in YBCO based on X-ray diffraction, scanning electron micrographs and magnetization measurements. The choice of Ag was motivated by the fact that it acts as a good transport medium among the grains besides providing the necessary pinning. Appreciable changes in the XRD have been found and subsequent enhancement in and were found over the entire range of the magnetic field.
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Experimental details
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Pure YBCO was prepared by the solid state reaction route. According to the exact stoichiometric ratio Y:Ba:Cu=1:2:3 high purity powders of Y2O3 (99.99%), BaCO3 (99.99%) and CuO (99.99%) precursors were thoroughly mixed. After initial grinding this mixture of powders was calcined at 900⁰C in air for 12h. Subsequent calcinations were done at 915⁰C and 930⁰C for the same duration of time in air with intermediate grinding. In each calcination cycle, a slow cooling rate was maintained as compared to the heating rate, and the samples were re-ground well before the next cycle. Superconductor YBCO+ metallic Ag composite samples were formed by adding Ag in the pre-reacted YBCO powder. A series of polycrystalline composite samples of YBCO+xAg with nominal composition and wt.% were then pressed into square pellets of size 1 cm and sintered at 930⁰C for 16h in the presence of oxygen to achieve the proper density. In order to get the exact oxygen stoichiometry, the process was followed by oxygen annealing at 650⁰C, 550⁰C and 450⁰C for the duration of 12h at each temperature and cooled to room temperature for another 6h. The phase purity of these pure and composite samples were determined by using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation ( ). The diffraction data were collected over the diffraction angle range ⁰ ⁰ with settings of 40 mA current and 40 Kv voltage. Magnetization measurements up to 14 T were carried out with a Quantum Design Physical Property Measurement system having a Vibrating Sample Magnetometer (VSM) attachment.
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Results and discussion
Rietveld fitted XRD patterns of YBCO+xAg composite samples at room temperature are shown in figure 1. The structural analysis was performed using the Rietveld refinement with the help of the FullProof software, which confirms that all the samples crystallize in the orthorhombic phase with space group Pmmm. The appearance of Ag as a secondary phase in YBCO is revealed by the presence of some additional tiny peaks of Ag in the diffraction pattern marked by (*) at ⁰ and ⁰ The lattice parameters of all the samples and other fitted parameters are given in Table 1. It is evident from Table 1 that the lattice parameters as well as the orthorhombicity remain nearly unchanged with the Ag addition, which substantiates earlier results [19, 20] . However, the effect of Ag addition in YBCO is
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evident from the observation of an extra hump in some reflections, in particular, the main peak. Several authors [21-23] have attributed the appearance of a hump in some reflections to an increase in crystallinity due to the bridging of grains. Further, it has been argued in [7] that secondary phase
BaTiO3 particles stay around the grain and might act like a catalyst to improve the structural quality at the grain boundaries. Based on the literature, we have attributed the hump to crystallinity due to bridging of YBCO grains. Hence our XRD results confirm that the addition of Ag
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in YBCO brings significant changes in crystallinity and makes the grains to behave in coherent way. Further, addition of Ag in YBCO effects the distribution of grain orientations (crystallographic texture) in the microstructure which governs the properties of materials. Besides, due to the metallic nature of Ag, it will act more effectively at grain boundaries than dielectric materials like BaZrO3, BaTiO3 etc.
Figure (1): Rietveld fitted XRD pattern of ( and wt.%) composites at room temperature. Secondary phases are denoted by symbol (*). The inset shows the
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appearance of a hump in the main peak with the addition of Ag.
Compound
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c(Å)
Vol.( Å3)
χ2
GoF{(b-a)/b} index X 100 YBCO 3.834 3.885 11.680 173.975 1.33 1.153 1.313 YBCO+5%Ag 3.821 3.878 11.674 172.983 1.68 1.297 1.470 YBCO+15%Ag 3.823 3.880 11.683 173.297 2.02 1.420 1.469 YBCO+20%Ag 3.822 3.880 11.676 173.148 2.20 1.488 1.495 Table 1. Lattice parameters, volume, quality of fit (χ2), goodness of fit (GOF) and orthorhombicity.
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Figure 2: SEM images of YBCO+xAg composites: a) x=0%, b) x=5 %, c) x=15% and d) x=20 %.
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The transport properties are highly dependent on the microstructure, therefore, it becomes necessary to analyze the microstructure of YBCO+xAg composites. Figure 2 shows the microstructure changes occurring in YBCO by the addition of Ag. It is observed from the SEM images that pristine YBCO
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is highly porous and of low density. Addition of Ag results in the following changes in the microstructure: (i) The grains observed in the pristine sample are almost circular, while in the composite samples rod like elongated grains have been found. Hence grain boundary area drops drastically, which is important for maintaining high transport in superconductors working in a magnetic environment. (ii) Ag particles on the surface of grains increase the grain connectivity by filling up cracks and voids, and hence the porosity reduces significantly. Interestingly, it is found that the existence of Ag in fractions (chunks) prevents the formation of a continuous metallic layer throughout the sample. This also further supports the XRD results: that Ag does not react with YBCO to form
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a new phase or structure but links as the connecting channels on the grain boundaries that brings significant changes in the transport and mechanical properties of the YBCO superconductor.
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To study the effect of Ag on the transition temperature (TC) of YBCO, the variation of the electrical resistivity as a function of temperature along with its temperature derivative is shown in figure 3. Fig. 3(a) and 3(b)
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Figure 3: Temperature dependence of electrical resistivity and its temperature derivative of YBCO+xAg composites in the presence of a 0 and 12 T magnetic field.
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show the variation of resistivity and its temperature derivative at 0 K. It is clear that all samples show superconductivity around 90 K, but transition broadening decreases by the addition of Ag in YBCO. In the absence of any magnetic field, the appearance of an extra hump in the temperature derivative plot at lower temperature shows that the grains are weakly connected in the case of a pristine sample. However, this hump disappears with the addition of Ag in YBCO. This shows that the addition of Ag in YBCO makes the grains to couple strongly. These effects have been further investigated at 12 T magnetic field (figures 3(c) and 3(d)), where the pristine sample shows a long tailing behavior. This tailing behavior is due to weak grain connectivity, which has been significantly improved by the addition of Ag in YBCO, as the YBCO+xAg composites show less tailing behavior. Further, the onset of the transition temperature (TCon) both in the presence and absence of a magnetic field does not shift by the addition of Ag in YBCO. However the zero resistivity transition temperature (TC0) is found to be higher for composite samples as compared to pristine samples. The detailed magneto resistivity studies of these composites have been discussed in our recent paper [24].
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Magnetization measurements play an important role in understanding the critical parameters of type II superconductors beyond the lower critical field . The isothermal magnetization measurements M-H up to 14 T were done at 5 K and are shown in figure 4(a). The applied magnetic field is set parallel to the long axis of the measured sample, so as to expose a maximum area and to avoid demagnetization. It is clear from the figure 4(a) that the magnetization has a peak at , beyond which flux penetrates and magnetization begins to decrease gradually. However, the decrease in magnetization is less for Ag composite samples as compared to pristine samples over the entire investigated range of magnetic field. Anderson [25] and Kim et al. [26] treat the vortices in a mixed state of any superconductor as well defined elastic objects which can be pinned due to various interactions, e.g., impurities, stress, point defects, extended defects and secondary phases etc. This confirms our observation, that the secondary phase of Ag in YBCO can pin the moving vortices and impedes the motion of flux lines. This results in irreversibility in magnetization. These defects trap the field, thereby enhancing . The critical current density of the composite samples was calculated from the magnetization width of the M-H loop using Bean’s critical state model [27]:
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where and are the dimensions of the samples in the plane perpendicular to the direction of the applied field (both in cm) and . (in emu/cm3 ) is the difference in the hysteretic magnetization between the curves obtained while increasing and decreasing the magnetic fields. The variation of the critical current density of the composite samples as a function of the applied magnetic field is shown in figure 4(b). The plots clearly suggest that Ag composite samples have substaintially higher values of as campared to pristine over the entire range of magnetic fields. The maxiumum value of in the case of pure YBCO is
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Figure 4 : (a) Isothermal magnetization hysteresis loops of composite samples at 5 K, (b) variation of as a function of applied magnetic field.
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A/cm2 and the same gets enhanced up to A/cm2 for the 15%Ag sample. Thus the addition of 15% Ag enhances by a factor of 6. The enhancement of is more than that reported by other authors [7, 23, 28, 29]. The variation of with Ag wt.% is shown in figure 5(a). As is obivious from the figure, is optimized at 15%Ag in YBCO. Beyond this again decreases due to weak pinning. It has been argued [in 30] that the reason for poor pinning may be either a high defect density or the defects can be of a size larger than the coherence length . For defects to act as effective pinning centers their size must be of the order of [31]. Even defects of size smaller than are poor pins, because their effect is averaged over a large volume.
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Figure 5: (a) Variation of the critical current density with Ag wt.%. (b) Pinning force density of ( and wt.%) composite samples as a function of the applied magnetic field.
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We have also calculated the pinning force density for both pure and Ag composite samples. The variation of as a function of the applied magnetic field at 5 K is shown in figure 5(b). The plot clearly reveals that the pinning force density increases appreciably upon addition of Ag in YBCO over the entire range of the applied magnetic field. In the case of the 15%Ag added sample, the pining force is about 10 times that of the pristine sample. These values of are significantly higher than the values found in YBCO: BaZrO3 composites [23, 32]. This shows that the addition of Ag particles in YBCO apart from increasing inter-grain connectivity (decreasing grain boundary area by staying around outside grains) also act as artificial pinning centers. This results in the enhancement of and dominance of the pinning force over the Lorentz force even at high magnetic field. Conclusion: In this study, we have shown that the addition of Ag particles in YBCO acts as artificial pinning centers. The addition of Ag in YBCO brings about significant changes in the crystallinity,
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Acknowledgement
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and . Both and have been found to increase in Ag composite samples over the whole range of the magnetic field. In the case of YBCO+15%Ag the enhancement of is nearly six times, while the enhancement in is more than ten times. Beyond 15%Ag, both and have been found to decrease but are still higher than in the pristine sample. Therefore, the efficient pinning is restricted up to 15%Ag in YBCO. Based on our XRD, SEM and magnetization measurements, we deduce that the optimal addition of Ag in YBCO brings a significant enhancement in the physical parameters like and thereby making these composite materials attractive for technological applications.
The authors would like to thank the Director of IUAC for financial support through theIUAC–UGC sponsored Project (UFR-52307). Bilal A. Malik, in particular is thankful to Dr. Alok Banerjee, UGCDAE Indore for the use of a VSM facility. References
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