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Influences of Yb substitution on the intergrain connections and flux pinning properties of Bi-2212 superconductors
Physica C 511 (2015) 26–32
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Influences of Yb substitution on the intergrain connections and flux pinning properties of Bi-2212 superconductors Shengnan Zhang a,⇑, Chengshan Li a, Qingbin Hao a, Tianni Lu a,b, Pingxiang Zhang a a b
Superconducting Materials Research Center, Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China College of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 13 August 2014 Received in revised form 2 February 2015 Accepted 4 February 2015 Available online 14 February 2015 Keywords: High temperature superconductor Doping Intergrain connection Critical current density
a b s t r a c t Polycrystalline bulks Bi2Sr2xYbxCaCu2.0O8+d (Bi-2212) with Yb doping content of x = 0, 0.01, 0.02 and 0.05) were fabricated by solid state sintering process. Bi-2212 precursor powders were synthesized by modified co-precipitation process, and Yb2O3 powders were added into the precursor powders during the calcination process as dopants. The influences of Yb substitution on the lattice parameter, microstructure and related superconducting properties were systematically investigated. The amorphous components within the grain boundaries were observed with HRTEM, which contributed to the formation of weak links. Therefore, both the decreasing number of porosity and better crystallized grain boundary structure after Yb doping obviously enhanced the intergrain connections. Meanwhile, doping of Yb ions into Bi-2212 matrix also contributed to the enhancement of point pining, thus lead to the improvement of in-field critical current density. Based on the enhancements of both intergrain connections and flux pinning properties, improvement of critical current density was obtained with the optimal doping content of x = 0.02. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Since the first discovery of Bi2Sr2CaCu2Ox (Bi-2212), it is considered to be one of the most promising high-temperature superconductors (HTS) due to its excellent properties under lowtemperature, high magnetic field conditions, such as the high irreversibility field beyond 100 T [1] and high critical current density exceeding 266 A mm2 up to 45 T [2]. As the only HTS so far, which can be made into round wires with isotropic cross sections, it can greatly simplify the winding process for cables and magnets fabrication. Therefore, it exhibites great potentials for the applications as insert coils in high field magnets and Rutherford cables for accelerators [3–9]. Recently, the maximum total magnetic field of 33.8 T was successfully achieved under the background field of 31.2 T with Bi-2212 insert coils, which proved the effectiveness of Bi-2212 in practical applications. However, there are two crucial factors which still limit the transport properties of Bi-2212 superconductors. One is the intergrain weak links due to the low extent of texture [10–13], high porosity [14,15] and/or grain boundaries with secondary phases or amorphous layers [16,17]. The other is the weak flux pinning,
⇑ Corresponding author. Tel.: +86 29 86231079; fax: +86 29 86224487. E-mail address: [email protected] (S. Zhang). http://dx.doi.org/10.1016/j.physc.2015.02.004 0921-4534/Ó 2015 Elsevier B.V. All rights reserved.
which can be attributed to its intrinsic lattice structure [18]. Therefore, optimization processes are required in order to obtain Bi-2212 superconductors with highly orientated microstructure, high density, clean grain boundaries and strong flux pinning properties to enhance the current carrying capacity of Bi-2212 for practical applications. Based on the previous studies, substitution may provide various effects on the optimization of HTS materials by tuning the lattice structure, microstructure, thermal dynamic properties, and electric and/or magnetic properties. Pb substitution at Bi site of Bi-2212 is considered to be the most successful example [19–22]. The introduction of Pb could decrease the lattice parameter c of Bi-2212, which not only reduced the anisotropic behavior of superconducting properties, but also introduced effective flux pinning centers to enhance the in-field current capacity. Therefore, the superconducting critical parameters were effectively optimized. Rare earth (RE) elements doping on the Sr/Ca site of Bi-2212 were also studied [10,23–31]. All the reported RE ions, such as Gd3+, Ho3+, Yb3+ and Eu3+ could work as effective pinning centers in the BSCCO system, thus obviously improved the critical current density. Besides, by tuning the thermodynamic properties of Bi-2212, the B2O3 doping resulted in the faster growth and better alignment of the Bi-2223 grains, which also improved the critical current density. Therefore, it is interesting to study the doping effects of other elements on Bi-2212 for further improvement of superconducting properties.
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In this study, Yb was chosen as dopant to substitute Sr ions in Bi-2212 matrix. It was found that after proper amount of Yb doping, bulks with better orientation and higher density were prepared, which lead to the formation of well crystallized grain boundaries. The influences of Yb ions on the related superconducting properties including weak link behavior and critical current density were systematically investigated. 2. Experimental details Bi2Sr2xCaCu2.0O8+d (x = 0, 0.01, 0.02, and 0.05) precursor powders were prepared by modified co-precipitation process [24] with starting materials of Bi2O3, SrCO3, CaCO3, and CuO (>99.9%). Yb2O3 (>99.99%) powders with the atomic ratio of 0, 0.01, 0.02 and 0.05 corresponding to the x value were added before the calcination process. Single phased Yb doped Bi-2212 precursor powders were obtained after a series of calcination processes in air at 800 °C/12 h, 820 °C/20 h, and 850 °C/20 h with intermediate grinding. Silver crucibles were adopted for the calcination in order to avoid the contamination of powders. The precursor powders were then densely packed into a stainless steel die and cold pressed into pellets with the diameter of Ø30 mm, and thickness of 1.5 mm. The bulks were then sintered at 865 °C for 24 h in ambient atmosphere. The density of bulks was measured using the standard Archimedes method. Polycrystalline X-ray diffraction (XRD) patterns on both precursor powders and bulks were taken on an X-ray diffraction (XRD, Bruker D8 Advance) with Cu-Ka radiation (k = 0.1542 nm). The texture degrees of (0 0 l) peaks were mostly used to assess the quality of textural structures, which were calculated as following,
P I00l F 00l ¼ P 100% I
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3. Results The X-ray diffraction patterns of precursor powders after calcination are shown in Fig. 1(a). The major phase can be indexed into orthorhombic Bi-2212 structures with no detectable secondary phase. The X-ray diffraction patterns of the sintered bulks are plotted in Fig. 1(b). After sintering, the major phase is still Bi-2212 in all these bulks. Diffraction peak of the secondary phase is undetectable in the sintered bulks. And the diffraction peak of Yb2O3 can be observed only on the x = 0.05 sample. Besides, textural structures can also be noticed with the obvious increasing intensity of (0 0 l) peaks, comparing with Fig. 1(a). As shown in Fig. 2(a) and (b), the SEM images of fractured surface of the x = 0 and 0.02 bulk are obtained. Misoriented grains with small average grain size can be observed on the Yb free bulk. While after Yb doping, the plate-like grains with larger radius and similar thickness are obtained. And the texture with the plate surface perpendicular to the pressing direction is formed. Generally speaking, there are usually some secondary phases appeared, including Bi-2201 (Bi2Sr2CuO6+d) and alkali earth cuprates (AEC, (Ca, Sr)mCunOz) in the sintered Bi-2212 bulks. The BSE images of x = 0 and 0.02 bulks are shown in Fig. 2(c) and (d), respectively. By combining the EDX analyses, phase distribution can be distinguished in the backscattering images. The gray background represents the Bi-2212 major phase, white particles are Bi-2201 and black dots are AEC phases and pores. By combining the BSE images and secondary electron images, the AEC phase and pores can further be distinguished and most of the black dots are determined to be pores. The area ratio of Bi-2201 phase in Yb-free and x = 0.02 bulks are 4.8% and 2.2%, respectively. And
ð1Þ
where I00l represents for the intensity of (0 0 l) diffraction peaks of P Bi-2212, I is the total diffraction intensity of the pattern. Transmission electron microscopy (TEM) samples were prepared by a grinding dimpling, and ion milling (Gatan PIPS). The high-resolution transmission electron microscopy (HRTEM) was performed on a JEOL JEM-2010 microscope. The scanning electron microscopy (SEM) of fracture surface and backscattering electron (BSE) images were obtained with JEOL JSM-6700F. The compositional analysis was taken by Inca-X-Stream energy-dispersive X-ray spectroscopy (EDX). The AC susceptibility was measured by the Superconducting Quantum Interference Device (SQUID, MPMS-XL-7) with the AC magnetic field of 0.1, 0.5, 1.0 and 1.5 Oe and frequency of 333 Hz. Meanwhile, SQUID was also used to measure the magnetic susceptibility with the background temperature of 4.2 K. The same specimen was used for both the AC susceptibility and magnetization measurements. The specimens were cut from the pellet with the dimension of 2 2 1.5 mm, with the 2 2 surface parallel to the pellet surface. During the measurements, specimen was put into a capsule with the 2 2 surface perpendicular to the magnetic field direction and fixed with low-temperature glue to avoid the movements of specimen in magnetic field or during cooling. After that, Bean model [32] was adopted for the calculation of the critical current density (Jc) as shown below,
Jc ¼
20DM bð1 b=3aÞ
ðb < aÞ
ð2Þ
where a and b are the length and the width of the specimen respectively, which are both perpendicular to the applied magnetic field, and DM is the difference in magnetization between the magnetization value with increasing field M+ and decreasing field M at same magnetic field.
Fig. 1. (a) X ray diffraction patterns of Yb doped Bi-2212 (a) precursor powders and (b) sintered bulks.
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Fig. 2. Typical SEM images of fractural surfaces and back scattering images of (a and c) x = 0 and (b and d) x = 0.02 bulk with the pressing direction of bulks marked in the images correspondingly.
the change of phase content should be attributed to the influence of Yb on the thermal dynamic properties of Bi-2212 bulks as discussed below. HRTEM images of x = 0 and 0.02 bulks are obtained to analyze the microstructure change with Yb doping. During the observation, obvious differences are noticed. And the most important one is about the structure of grain boundaries. In the Yb free bulk, amorphous components with the thickness of over 3 nm can be observed at many grain boundaries, as shown in Fig. 3(a). While in Yb doped samples, most of grain boundaries are well crystallized, with the typical image shown in Fig. 3(b). The AC susceptibility is widely used as a nondestructive method for the determination and characterization of the intergrain component in the polycrystalline high temperature superconductors. In particular, the imaginary component of the AC susceptibility has been widely used to probe the nature of weak links in polycrystalline superconductors [33–36]. As shown in Fig. 4(a) and (b), the AC susceptibility of x = 0 and 0.02 bulks are measured under different AC field. It can be observed that for both samples, the real part of AC susceptibility, v0 , shows two drops as the temperature decreased. The first drop at Tc is due to the transition within grains, which keeps constant with the change of AC field, Hac. By comparing two images, the increase of Tc can be noticed from 81 K to 84 K after Yb doping. And the second drop at peak temperature, Tp is due to the occurrence of the superconducting coupling between grains, which is much sharper for Yb doped sample than that of Yb free sample. While on each of the imaginary part curve, v00 , a peak which is a measure of the dissipation in the sample, is observed correspondingly. With the increase of Hac, the peak of v00 shifts towards lower temperature and broadened. After Yb doping, the peak appears at higher temperatures, which suggests that the inter-granular coupling between grains in Bi-2212 sample is improved by doping. The critical current densities of Yb doped Bi-2212 bulks are calculated based on the magnetization measurement with the magnetic field of 0–6 T at 4.2 K. As shown in Fig. 5, with the increase
Fig. 3. HRTEM images of Yb doped Bi-2212 bulks with doping contents of (a) x = 0, and (b) x = 0.02; Typical grain boundary with amorphous component can be observed in (a), grain boundary which is well crystallized was shown in (b) with small misalignment angle of 1.6°.
of Yb doping content, the self-field Jc increases to the maximum value, then decreases. And the in-field Jc under low magnetic field also increases correspondingly. An obvious increase of critical
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S. Zhang et al. / Physica C 511 (2015) 26–32
Fig. 6. Zoomed in area of the diffraction peaks of (0 0 2 0), (1 3 9) and (0 4 0).
Fig. 4. Temperature dependence of the AC susceptibility for Bi-2212 bulks with Yb content of (a) x = 0, (b) x = 0.02 measured in the variation of AC field, Hac = 0.1, 0.5, 1.0, and 1.5 Oe at the frequency of 333 Hz.
current density, Jc, for 30% is obtained on x = 0.02 Yb doped sample at zero field, comparing with the Yb free bulk. 4. Discussion The substitution of Yb can be revealed by the analyses of both lattice parameter and critical temperature. Firstly, by zooming in the diffraction peaks of (0 0 2 0) and (0 4 0), as shown in Fig. 6, the
shift of diffraction peaks towards higher degrees can be clearly observed, which implies the decrease of lattice parameters. After the refinements with FullprofÒ, the lattice parameters are obtained and listed in Table 1. It is noticed that the lattice parameter a and b are almost the same within the studied doping range, suggesting a near tetragonal lattice structure. Both the lattice parameter a (b) and c decreases with increasing Yb content. Since the ion radii of Yb3+ is 0.86 Å, smaller than that of Sr2+ (1.12 Å), the substitution of Yb3+ on Sr site could lead to the shrink of lattice [10]. Besides, the higher valence of Yb3+ than Sr2+ can also cause the increase of oxygen content, which is also a reason for the decrease of lattice parameter c [37]. Secondly, the critical temperatures of x = 0 and 0.02 bulks obtained with AC susceptibility measurement imply the change of carrier concentration, which also can be related to the non-equivalent substitution of Yb3+ at Sr site. Since holes are the main carriers in Bi-2212 systems [38], the substitution of Yb3+ at Sr site can lead to the increase of carrier concentration. And the samples are tuned closer to the optimal doping region. Therefore, both these changes indicate that the Yb3+ ions have successfully entered into the Bi-2212 matrix. 4.1. Intergrain connection Textural structure is a crucial factor for the intergrain connections of all cuprates superconductors. The textural structure is analyzed based on the estimation of texture degree with Eq. (1) as plotted in Fig. 7 and listed in Table 1. The texture degree increases with Yb doping from 50.6% (x = 0) to 58.4% (x = 0.02). It is deduced that the preferred orientation of (0 0 l) is formed in the bulks after sintering, considering the texture degree of precursor powders is only 18.2%. Meanwhile, the full-width at half maximum (FWHM) values of diffraction peaks are also measured on the XRD patterns in Fig. 1(b) and listed in Table 1. The decrease of FWHM from 0.119° to 0.097° suggests an enhancement of crystallization with the increase of Yb Table 1 Microstructural and physical parameter of Yb doped Bi-2212 bulks.
Fig. 5. Critical current density of Bi-2212 bulks with different Yb contents in the magnetic field of 0–6 T at 4.2 K.
x value
0
0.01
0.02
0.05
Lattice parameter, a (Å) Lattice parameter, b (Å) Lattice parameter, c (Å) Texture degree (%) Density (g cm3) FWHM (deg.) Peak temperature, Tp (K) (@ Hac = 0.5 Oe)
5.411(2) 5.408(2) 30.847(2) 50.6(3) 4.53 0.119 60
5.408(8) 5.405(9) 30.814(1) 51.9(6) 4.66 0.107 –
5.408(0) 5.404(8) 30.808(8) 58.4(1) 4.83 0.097 63
5.406(8) 5.403(2) 30.783(9) 52.5(8) 4.69 0.114 –
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Fig. 7. Dependence of texture degree on the x values and on the FWHM values shown in the inset.
content from 0 to 0.02. Based on the SEM observation, the average grain radius of x = 0 and 0.02 bulks is estimated to be 0.98 lm and 1.17 lm, respectively. The 20% increase of average grain size is consistent with the FWHM analysis. And the increase of grain radius can contribute to the enhancement of textural structure, as shown in the inset of Fig. 7. On the other hand, the densities of all these bulks are measured and listed in Table 1. The maximum density is also obtained at the Yb doping content x = 0.02, with the increase of 6.6% comparing with the Yb free bulk, which suggests the decrease of porosity. By comparing the two BSE images in Fig. 2, obvious decrease of porosity is also noticed correspondingly. There are usually several crucial factors which can influence the sintering density of bulks, such as sintering temperature, pressure, and addition of lubricant or binder systems. In this case it is deduced that the sintering properties of Bi-2212 bulks are obviously enhanced with the addition of refractory additive (Yb2O3) due to overall decrease of the melting temperature under the same sintering process. The decrease of porosity and enhancement of textural structure can lead to the increasing contact areas of Bi-2212 grains. Based on our previous studies, the substitution of Yb at Sr site can improve the crystallization of Bi-2212 crystals [39]. Therefore, during the sintering process, more grain boundaries with highly crystallized structures as observed with HRTEM in Fig. 3(b) are formed. And the blocking or scattering effect of grain boundaries on the carriers is weakened. And the increase of grain size reduces the number of grain boundaries, which also weakens the scattering effect of grain boundaries to the carriers. It is concluded that the Bi-2212 bulks with higher density, higher textural structure and larger grains can be obtained with proper amount of Yb doping. The intergrain connections can be qualitatively analyzed with the AC susceptibility data. By comparing the AC susceptibility of x = 0 and 0.02 bulks under the same Hac field, the shift of peaks on the imaginary part curve, v00 of Yb doped sample towards higher temperature is observed. As shown in Fig. 8 the peak temperatures, Tp of the doped sample are all higher than that of Yb free sample correspondingly, which confirms the enhancement of intergrain connections due to the microstructure improvement as discussed above.
Fig. 8. AC field dependence of peak temperature, Tp, with the linear fitting shown as dashed lines.
intra-granular pinning force can be estimated with Eq. (3), as follow.
1
2 l0 Tp ¼ Tc 1 Hac 8Rg ag ð0Þ
! ð3Þ
where, the values of Tc and Tp are taken from Fig. 3 (b) and (c), Rg is the average grain radius, which is measured on the SEM images as discussed above, ag(0) is the intra-granular pinning force density for Abrikosov vortices. In order to compare the pinning force density of these samples, we assume the ag(0) of Yb free sample to be a0 g(0), and the obtain ag(0) of x = 0.02 sample is 1.3a0 g(0). The 30% increase indicates that the intra-granular pinning force is enhanced by Yb doping. It is well known that lattice defects generated by the doping of foreign ions with proper size can work as effective flux pinning center. Therefore, it indicates that proper amount of Yb doping can enhance the intra-granular pinning force density for Abrikosov vortices effectively. In order to further investigate the flux pinning mechanism variation of these samples, the pinning force densities of Bi-2212 bulks doped with different Yb content are calculated with the equation Fp = Jc B [41]. The scale plots of normalized pinning force density (f = Fp/Fpmax) as a function of reduced magnetic fields (b = B/Birr) are plotted in Fig. 9. All the Fp–B curves could be well fitted to the Dew-Hughes equation as:
4.2. Flux pinning properties The flux pinning properties can also be analyzed with AC susceptibility measurement. By fitting the Tp–Hac curves as shown in Fig. 8 with the linear dependence from Müller model [40], the
Fig. 9. Reduced Flux pinning force of Bi-2212 bulks with different Yb contents as a function of the reduced magnetic field at 4.2 K. The fitting of Fp–B curve of x = 0.01 bulk is presented as an example in the inset. The black dots are measured data and red line is the fitting curve. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
S. Zhang et al. / Physica C 511 (2015) 26–32 Table 2 Values of pinning parameters for Bi-2212 bulks with Yb content of x = 0–0.05 obtained with Dew-Hughes model. x value
0
0.01
0.02
0.05
p q Birr (T) A, 106 Nm3
0.64 0.94 6.2 712.4
0.69 1.22 7.2 866.0
0.78 1.82 7.3 1281.2
0.72 0.98 6.5 779.5
p
q
F p ¼ AðB=Birr Þ ð1 B=Birr Þ
ð4Þ
where A is a numerical parameter independent of the applied field, Birr is the irreversible field, p and q are parameters describing the actual pinning mechanism. The fitting parameters for different samples are given in Table 2. For the Yb free bulk, the values of p and q are 0.64 and 0.94, respectively. It is found that with increasing amount of Yb doping, the values of p and q increase to p = 0.78 and q = 1.82 when x = 0.02. Based on the Dew-Hughes model, the flux pinning mechanism can be recognized as volume pinning of Dj type for p = 1 and q = 1, and normal point pinning for p = 1 and q = 2, respectively. Therefore, the change of main flux pinning mechanism from the volume pinning of Dj type to the normal point pinning mechanism can be deduced [42]. On the other hand, according to the Dew-Hughes model, the peak position of Fp appears at a reduced field value of h = 0.5 for volume pinning of Dj type and h = 0.33 for point pinning mechanism. As shown in Fig. 9, the peak position shifts from h = 0.39 for Yb free bulk to h = 0.35 and 0.30 with increasing Yb content to x = 0.01 and 0.02, respectively. Therefore, it implies the enhancement of normal point pinning over volume pinning of Dj type, due to the point defects arising out of the substitution of Yb at Sr site. The deviation of p and q values from theoretical ones should be attributed to the existence of normal surface pinning due to the grain boundaries [24]. With the further increase of Yb to 0.05, the Yb2O3 particles appears with the size far larger than the coherent length of Bi-2212, and the characteristic length of introduced lattice defects also increases. The number of effective pinning centers decreases. Thus the main flux pinning mechanism changes back to the volume pinning of Dj type same as that of the Yb free bulk. It is concluded that with the doping of Yb ions into Bi-2212 matrix, lattice defects can be obtained as effective pinning centers. The main flux pinning mechanism changes and the normal point pinning mechanism becomes the dominant mechanism gradually. The flux pinning under low field is enhanced due to the enhancement of point pinning property. However, since this kind of point pinning introduced by secondary phase is recognized as weak pinning, the critical current density at higher magnetic field cannot be effectively enhanced [43]. Therefore, the increase of self-field Jc can be attributed to the improvement of intergrain connections, and the increase of in-field Jc can be related to the enhancement of flux pinning properties as discussed above. 5. Conclusions Bi-2212 superconducting bulks were prepared with different Yb doping contents. The substitution of Yb enhanced the crystallization properties of Bi-2212, therefore Bi-2212 bulks with higher density, larger grain size and higher orientated microstructures were obtained with proper amount of Yb doping. Improvement of critical current density was obtained, which could be attributed to both the improvement of intergrain connections and the enhancements of flux pinning properties. Meanwhile, Yb doping also caused the change of flux pinning mechanism. The dominant
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flux pinning mechanism became the point pinning of normal phase with the increase of Yb doping content to x = 0.02, due to the introduction of effective pinning centers. The results of this study provide us a new opportunity for further enhancement of the current carrying capacity of Bi-2212 wires and tapes for practical applications. Acknowledgements This research was financially supported by National ‘‘973’’ Project, under contract No. 2011CBA00104, National Natural Science Foundation of China under contract No. 51102198 and 51472206, the International Scientific and Technological Cooperation Projects of China No. S2010GR0518, the National ITER Program of China No. 2013GB110001, and the Program for Innovative Research Team in Shaanxi province No. 2013KCT-07. References [1] U.P. Trociewitz, J. Schwartz, K. Marken, H. Miao, M. Meinesz, B. Czabaj, NHMFL Rep. 13 (2006) 31. [2] H. Miao, K.R. Marken, M. Meinesz, B. Czabaj, S. Hong, IEEE Trans. Appl. Supercond. 15 (2005) 2554. [3] D.C. Larbalestier, J. Jiang, U.P. Trociewitz, F. Kametani, C. Scheuerlein, M. Dalban-Canassy, M. Matras, P. Chen, N.C. Craig, P.J. Lee, E.E. Hellstrom, Nat. Mater. 13 (2014) 375–381. [4] H.M. Weijers, U.P. Trociewitz, W.D. Markiewicz, J. Jiang, D. Myers, E.E. Hellstrom, A. Xu, J. Jaroszynski, P. Noyes, Y. Viouchkov, D.C. Larbalestier, IEEE Trans. Appl. Supercond. 20 (2010) 576–582. [5] T. Kiyoshi, A. Sato, H. Wada, S. Hatashi, M. Shimada, Y. Kawate, IEEE Trans. Appl. Supercond. 9 (1999) 472–477. [6] Z. Melhem, S. Ball, S. Chappell, Phys. Proc. 36 (2012) 805–811. [7] M. Dalban-Canassy, D.A. Myers, U.P. Trociewitz, J. Jiang, E.E. Hellstrom, Y. Viouchkov, D.C. Larbalestier, Supercond. Sci. Technol. 25 (2012) 115015. [8] J. Bock, F. Breuer, C.E. Bruzek, N. Lallouet, M.O. Rikel, H. Walter, in: L. Bottura, M. Buzio, T. Taylor (Eds.), Proc. Workshop on Accelerator Magnet Superconductors, 2004, pp. 149–155. [9] K. Ohsemochi, K. Koyanagi, T. Kurusu, T. Tosaka, K. Tasaki, M. Ono, Y. Ishii, K. Shimada, S. Nomura, K. Kidoguchi, H. Onoda, N. Hirano, S. Nagaya, J. Phys.: Conf. Ser. 43 (2006) 825–828. [10] S. Vinu, P.M. Sarun, A. Biju, R. Shabna, P. Guruswamy, U. Syamaprasad, Supercond. Sci. Technol. 21 (2008) 045001. [11] L.N. Bulaevskii, L.L. Daemen, M.P. Maley, J.Y. Coulter, Phys. Rev. B 48 (1993) 13798. [12] D. Buhl, T. Lang, L.J. Gauckler, Appl. Supercond. 4 (1997) 299–317. [13] S. Stassen, A. Vanderschueren, R. Cloots, A. Rulmont, M. Ausloos, J. Cryst. Growth 166 (1996) 281–285. [14] J.Y. Jiang, H.P. Miao, Y.B. Huang, S. Hong, J.A. Parrell, C. Scheuerlein, M.D. Michiel, A.K. Ghosh, U.P. Trocitwitz, E.E. Hellstrom, D.C. Larbalestier, IEEE Trans. Appl. Supercond. 23 (2013) 6400206. [15] F. Kametani, E.G. Lee, T. Shen, P.J. Lee, J. Jiang, E.E. Hellstrom, D.C. Larbalestier, Supercond. Sci. Technol. 27 (2014) 055004. [16] C.J. Eastell, B.M. Henry, C.G. Morgan, C.R.M. Grovenor, M.J. Goringe, IEEE Trans. Appl. Supercond. 7 (1997) 2083. [17] Y.N. Tsay, Q. Li, Y. Zhu, M. Suenaga, K. Shibutani, I. Shigaki, R. Ogawa, IEEE Trans. Appl. Supercond. 9 (1999) 1662. [18] Y. Nakayama, T. Motohashi, K. Otzschi, J. Shimoyama, K. Kitazawa, K. Kishio, Phys. Rev. B 62 (2000) 1452–1456. [19] X.L. Wang, H.K. Liu, S.X. Dou, J. Horvat, D. Millikon, G. Heine, W. Lang, H.M. Luo, S.Y. Ding, J. Appl. Phys. 89 (2001) 7669. [20] A.L. Crossley, Y.H. Li, A.D. Caplin, J.L. MacManus-Driscoll, Physica C 314 (1999) 12–18. [21] A.B. Kulakov, I.K. Bdikin, S.A. Zver’kov, G.A. Emel’chenko, G. Yang, J.S. Abell, Physica C 371 (2002) 45–51. [22] H.L. Su, F. Vasiliu, P. Majewski, F. Aldinger, Physica C 256 (1996) 345–352. [23] A. Amira, Y. Boudjadja, A. Saoudel, A. Varilci, M. Akdogan, C. Terzioglu, M.F. Mosbah, Physica B 406 (2011) 1022–1027. [24] S. Vinu, P.M. Sarun, R. Shabna, P.M. Aswathy, J.B. Anooja, U. Syamaprasad, Physica B 405 (2010) 4355–4359. [25] R. Shabna, P.M. Sarun, S. Vinu, U. Syamaprasad, J. Alloys Compd. 493 (2010) 11–16. [26] R. Shabna, P.M. Sarun, S. Vinu, U. Syamaprasad, J. Alloys Compd. 48 (2010) 797–801. [27] P.M. Saruna, S. Vinua, R. Shabnaa, A. Bijub, U. Syamaprasad, J. Alloys Compd. 472 (2009) 13–17. [28] S. Vinu, P.M. Sarun, R. Shabna, A. Biju, U. Syamaprasad, Mater. Lett. 62 (2008) 4421–4424. [29] P.M. Sarun, S. Vinu, R. Shabna, A. Biju, U. Syamaprasad, Mater. Lett. 62 (2008) 2725–2728.
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Influences of Yb substitution on the intergrain connections and flux pinning properties of Bi-2212 superconductors Shengnan Zhang a,⇑, Chengshan Li a, Qingbin Hao a, Tianni Lu a,b, Pingxiang Zhang a a b
Superconducting Materials Research Center, Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China College of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 13 August 2014 Received in revised form 2 February 2015 Accepted 4 February 2015 Available online 14 February 2015 Keywords: High temperature superconductor Doping Intergrain connection Critical current density
a b s t r a c t Polycrystalline bulks Bi2Sr2xYbxCaCu2.0O8+d (Bi-2212) with Yb doping content of x = 0, 0.01, 0.02 and 0.05) were fabricated by solid state sintering process. Bi-2212 precursor powders were synthesized by modified co-precipitation process, and Yb2O3 powders were added into the precursor powders during the calcination process as dopants. The influences of Yb substitution on the lattice parameter, microstructure and related superconducting properties were systematically investigated. The amorphous components within the grain boundaries were observed with HRTEM, which contributed to the formation of weak links. Therefore, both the decreasing number of porosity and better crystallized grain boundary structure after Yb doping obviously enhanced the intergrain connections. Meanwhile, doping of Yb ions into Bi-2212 matrix also contributed to the enhancement of point pining, thus lead to the improvement of in-field critical current density. Based on the enhancements of both intergrain connections and flux pinning properties, improvement of critical current density was obtained with the optimal doping content of x = 0.02. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Since the first discovery of Bi2Sr2CaCu2Ox (Bi-2212), it is considered to be one of the most promising high-temperature superconductors (HTS) due to its excellent properties under lowtemperature, high magnetic field conditions, such as the high irreversibility field beyond 100 T [1] and high critical current density exceeding 266 A mm2 up to 45 T [2]. As the only HTS so far, which can be made into round wires with isotropic cross sections, it can greatly simplify the winding process for cables and magnets fabrication. Therefore, it exhibites great potentials for the applications as insert coils in high field magnets and Rutherford cables for accelerators [3–9]. Recently, the maximum total magnetic field of 33.8 T was successfully achieved under the background field of 31.2 T with Bi-2212 insert coils, which proved the effectiveness of Bi-2212 in practical applications. However, there are two crucial factors which still limit the transport properties of Bi-2212 superconductors. One is the intergrain weak links due to the low extent of texture [10–13], high porosity [14,15] and/or grain boundaries with secondary phases or amorphous layers [16,17]. The other is the weak flux pinning,
⇑ Corresponding author. Tel.: +86 29 86231079; fax: +86 29 86224487. E-mail address: [email protected] (S. Zhang). http://dx.doi.org/10.1016/j.physc.2015.02.004 0921-4534/Ó 2015 Elsevier B.V. All rights reserved.
which can be attributed to its intrinsic lattice structure [18]. Therefore, optimization processes are required in order to obtain Bi-2212 superconductors with highly orientated microstructure, high density, clean grain boundaries and strong flux pinning properties to enhance the current carrying capacity of Bi-2212 for practical applications. Based on the previous studies, substitution may provide various effects on the optimization of HTS materials by tuning the lattice structure, microstructure, thermal dynamic properties, and electric and/or magnetic properties. Pb substitution at Bi site of Bi-2212 is considered to be the most successful example [19–22]. The introduction of Pb could decrease the lattice parameter c of Bi-2212, which not only reduced the anisotropic behavior of superconducting properties, but also introduced effective flux pinning centers to enhance the in-field current capacity. Therefore, the superconducting critical parameters were effectively optimized. Rare earth (RE) elements doping on the Sr/Ca site of Bi-2212 were also studied [10,23–31]. All the reported RE ions, such as Gd3+, Ho3+, Yb3+ and Eu3+ could work as effective pinning centers in the BSCCO system, thus obviously improved the critical current density. Besides, by tuning the thermodynamic properties of Bi-2212, the B2O3 doping resulted in the faster growth and better alignment of the Bi-2223 grains, which also improved the critical current density. Therefore, it is interesting to study the doping effects of other elements on Bi-2212 for further improvement of superconducting properties.
S. Zhang et al. / Physica C 511 (2015) 26–32
In this study, Yb was chosen as dopant to substitute Sr ions in Bi-2212 matrix. It was found that after proper amount of Yb doping, bulks with better orientation and higher density were prepared, which lead to the formation of well crystallized grain boundaries. The influences of Yb ions on the related superconducting properties including weak link behavior and critical current density were systematically investigated. 2. Experimental details Bi2Sr2xCaCu2.0O8+d (x = 0, 0.01, 0.02, and 0.05) precursor powders were prepared by modified co-precipitation process [24] with starting materials of Bi2O3, SrCO3, CaCO3, and CuO (>99.9%). Yb2O3 (>99.99%) powders with the atomic ratio of 0, 0.01, 0.02 and 0.05 corresponding to the x value were added before the calcination process. Single phased Yb doped Bi-2212 precursor powders were obtained after a series of calcination processes in air at 800 °C/12 h, 820 °C/20 h, and 850 °C/20 h with intermediate grinding. Silver crucibles were adopted for the calcination in order to avoid the contamination of powders. The precursor powders were then densely packed into a stainless steel die and cold pressed into pellets with the diameter of Ø30 mm, and thickness of 1.5 mm. The bulks were then sintered at 865 °C for 24 h in ambient atmosphere. The density of bulks was measured using the standard Archimedes method. Polycrystalline X-ray diffraction (XRD) patterns on both precursor powders and bulks were taken on an X-ray diffraction (XRD, Bruker D8 Advance) with Cu-Ka radiation (k = 0.1542 nm). The texture degrees of (0 0 l) peaks were mostly used to assess the quality of textural structures, which were calculated as following,
P I00l F 00l ¼ P 100% I
27
3. Results The X-ray diffraction patterns of precursor powders after calcination are shown in Fig. 1(a). The major phase can be indexed into orthorhombic Bi-2212 structures with no detectable secondary phase. The X-ray diffraction patterns of the sintered bulks are plotted in Fig. 1(b). After sintering, the major phase is still Bi-2212 in all these bulks. Diffraction peak of the secondary phase is undetectable in the sintered bulks. And the diffraction peak of Yb2O3 can be observed only on the x = 0.05 sample. Besides, textural structures can also be noticed with the obvious increasing intensity of (0 0 l) peaks, comparing with Fig. 1(a). As shown in Fig. 2(a) and (b), the SEM images of fractured surface of the x = 0 and 0.02 bulk are obtained. Misoriented grains with small average grain size can be observed on the Yb free bulk. While after Yb doping, the plate-like grains with larger radius and similar thickness are obtained. And the texture with the plate surface perpendicular to the pressing direction is formed. Generally speaking, there are usually some secondary phases appeared, including Bi-2201 (Bi2Sr2CuO6+d) and alkali earth cuprates (AEC, (Ca, Sr)mCunOz) in the sintered Bi-2212 bulks. The BSE images of x = 0 and 0.02 bulks are shown in Fig. 2(c) and (d), respectively. By combining the EDX analyses, phase distribution can be distinguished in the backscattering images. The gray background represents the Bi-2212 major phase, white particles are Bi-2201 and black dots are AEC phases and pores. By combining the BSE images and secondary electron images, the AEC phase and pores can further be distinguished and most of the black dots are determined to be pores. The area ratio of Bi-2201 phase in Yb-free and x = 0.02 bulks are 4.8% and 2.2%, respectively. And
ð1Þ
where I00l represents for the intensity of (0 0 l) diffraction peaks of P Bi-2212, I is the total diffraction intensity of the pattern. Transmission electron microscopy (TEM) samples were prepared by a grinding dimpling, and ion milling (Gatan PIPS). The high-resolution transmission electron microscopy (HRTEM) was performed on a JEOL JEM-2010 microscope. The scanning electron microscopy (SEM) of fracture surface and backscattering electron (BSE) images were obtained with JEOL JSM-6700F. The compositional analysis was taken by Inca-X-Stream energy-dispersive X-ray spectroscopy (EDX). The AC susceptibility was measured by the Superconducting Quantum Interference Device (SQUID, MPMS-XL-7) with the AC magnetic field of 0.1, 0.5, 1.0 and 1.5 Oe and frequency of 333 Hz. Meanwhile, SQUID was also used to measure the magnetic susceptibility with the background temperature of 4.2 K. The same specimen was used for both the AC susceptibility and magnetization measurements. The specimens were cut from the pellet with the dimension of 2 2 1.5 mm, with the 2 2 surface parallel to the pellet surface. During the measurements, specimen was put into a capsule with the 2 2 surface perpendicular to the magnetic field direction and fixed with low-temperature glue to avoid the movements of specimen in magnetic field or during cooling. After that, Bean model [32] was adopted for the calculation of the critical current density (Jc) as shown below,
Jc ¼
20DM bð1 b=3aÞ
ðb < aÞ
ð2Þ
where a and b are the length and the width of the specimen respectively, which are both perpendicular to the applied magnetic field, and DM is the difference in magnetization between the magnetization value with increasing field M+ and decreasing field M at same magnetic field.
Fig. 1. (a) X ray diffraction patterns of Yb doped Bi-2212 (a) precursor powders and (b) sintered bulks.
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S. Zhang et al. / Physica C 511 (2015) 26–32
Fig. 2. Typical SEM images of fractural surfaces and back scattering images of (a and c) x = 0 and (b and d) x = 0.02 bulk with the pressing direction of bulks marked in the images correspondingly.
the change of phase content should be attributed to the influence of Yb on the thermal dynamic properties of Bi-2212 bulks as discussed below. HRTEM images of x = 0 and 0.02 bulks are obtained to analyze the microstructure change with Yb doping. During the observation, obvious differences are noticed. And the most important one is about the structure of grain boundaries. In the Yb free bulk, amorphous components with the thickness of over 3 nm can be observed at many grain boundaries, as shown in Fig. 3(a). While in Yb doped samples, most of grain boundaries are well crystallized, with the typical image shown in Fig. 3(b). The AC susceptibility is widely used as a nondestructive method for the determination and characterization of the intergrain component in the polycrystalline high temperature superconductors. In particular, the imaginary component of the AC susceptibility has been widely used to probe the nature of weak links in polycrystalline superconductors [33–36]. As shown in Fig. 4(a) and (b), the AC susceptibility of x = 0 and 0.02 bulks are measured under different AC field. It can be observed that for both samples, the real part of AC susceptibility, v0 , shows two drops as the temperature decreased. The first drop at Tc is due to the transition within grains, which keeps constant with the change of AC field, Hac. By comparing two images, the increase of Tc can be noticed from 81 K to 84 K after Yb doping. And the second drop at peak temperature, Tp is due to the occurrence of the superconducting coupling between grains, which is much sharper for Yb doped sample than that of Yb free sample. While on each of the imaginary part curve, v00 , a peak which is a measure of the dissipation in the sample, is observed correspondingly. With the increase of Hac, the peak of v00 shifts towards lower temperature and broadened. After Yb doping, the peak appears at higher temperatures, which suggests that the inter-granular coupling between grains in Bi-2212 sample is improved by doping. The critical current densities of Yb doped Bi-2212 bulks are calculated based on the magnetization measurement with the magnetic field of 0–6 T at 4.2 K. As shown in Fig. 5, with the increase
Fig. 3. HRTEM images of Yb doped Bi-2212 bulks with doping contents of (a) x = 0, and (b) x = 0.02; Typical grain boundary with amorphous component can be observed in (a), grain boundary which is well crystallized was shown in (b) with small misalignment angle of 1.6°.
of Yb doping content, the self-field Jc increases to the maximum value, then decreases. And the in-field Jc under low magnetic field also increases correspondingly. An obvious increase of critical
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Fig. 6. Zoomed in area of the diffraction peaks of (0 0 2 0), (1 3 9) and (0 4 0).
Fig. 4. Temperature dependence of the AC susceptibility for Bi-2212 bulks with Yb content of (a) x = 0, (b) x = 0.02 measured in the variation of AC field, Hac = 0.1, 0.5, 1.0, and 1.5 Oe at the frequency of 333 Hz.
current density, Jc, for 30% is obtained on x = 0.02 Yb doped sample at zero field, comparing with the Yb free bulk. 4. Discussion The substitution of Yb can be revealed by the analyses of both lattice parameter and critical temperature. Firstly, by zooming in the diffraction peaks of (0 0 2 0) and (0 4 0), as shown in Fig. 6, the
shift of diffraction peaks towards higher degrees can be clearly observed, which implies the decrease of lattice parameters. After the refinements with FullprofÒ, the lattice parameters are obtained and listed in Table 1. It is noticed that the lattice parameter a and b are almost the same within the studied doping range, suggesting a near tetragonal lattice structure. Both the lattice parameter a (b) and c decreases with increasing Yb content. Since the ion radii of Yb3+ is 0.86 Å, smaller than that of Sr2+ (1.12 Å), the substitution of Yb3+ on Sr site could lead to the shrink of lattice [10]. Besides, the higher valence of Yb3+ than Sr2+ can also cause the increase of oxygen content, which is also a reason for the decrease of lattice parameter c [37]. Secondly, the critical temperatures of x = 0 and 0.02 bulks obtained with AC susceptibility measurement imply the change of carrier concentration, which also can be related to the non-equivalent substitution of Yb3+ at Sr site. Since holes are the main carriers in Bi-2212 systems [38], the substitution of Yb3+ at Sr site can lead to the increase of carrier concentration. And the samples are tuned closer to the optimal doping region. Therefore, both these changes indicate that the Yb3+ ions have successfully entered into the Bi-2212 matrix. 4.1. Intergrain connection Textural structure is a crucial factor for the intergrain connections of all cuprates superconductors. The textural structure is analyzed based on the estimation of texture degree with Eq. (1) as plotted in Fig. 7 and listed in Table 1. The texture degree increases with Yb doping from 50.6% (x = 0) to 58.4% (x = 0.02). It is deduced that the preferred orientation of (0 0 l) is formed in the bulks after sintering, considering the texture degree of precursor powders is only 18.2%. Meanwhile, the full-width at half maximum (FWHM) values of diffraction peaks are also measured on the XRD patterns in Fig. 1(b) and listed in Table 1. The decrease of FWHM from 0.119° to 0.097° suggests an enhancement of crystallization with the increase of Yb Table 1 Microstructural and physical parameter of Yb doped Bi-2212 bulks.
Fig. 5. Critical current density of Bi-2212 bulks with different Yb contents in the magnetic field of 0–6 T at 4.2 K.
x value
0
0.01
0.02
0.05
Lattice parameter, a (Å) Lattice parameter, b (Å) Lattice parameter, c (Å) Texture degree (%) Density (g cm3) FWHM (deg.) Peak temperature, Tp (K) (@ Hac = 0.5 Oe)
5.411(2) 5.408(2) 30.847(2) 50.6(3) 4.53 0.119 60
5.408(8) 5.405(9) 30.814(1) 51.9(6) 4.66 0.107 –
5.408(0) 5.404(8) 30.808(8) 58.4(1) 4.83 0.097 63
5.406(8) 5.403(2) 30.783(9) 52.5(8) 4.69 0.114 –
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Fig. 7. Dependence of texture degree on the x values and on the FWHM values shown in the inset.
content from 0 to 0.02. Based on the SEM observation, the average grain radius of x = 0 and 0.02 bulks is estimated to be 0.98 lm and 1.17 lm, respectively. The 20% increase of average grain size is consistent with the FWHM analysis. And the increase of grain radius can contribute to the enhancement of textural structure, as shown in the inset of Fig. 7. On the other hand, the densities of all these bulks are measured and listed in Table 1. The maximum density is also obtained at the Yb doping content x = 0.02, with the increase of 6.6% comparing with the Yb free bulk, which suggests the decrease of porosity. By comparing the two BSE images in Fig. 2, obvious decrease of porosity is also noticed correspondingly. There are usually several crucial factors which can influence the sintering density of bulks, such as sintering temperature, pressure, and addition of lubricant or binder systems. In this case it is deduced that the sintering properties of Bi-2212 bulks are obviously enhanced with the addition of refractory additive (Yb2O3) due to overall decrease of the melting temperature under the same sintering process. The decrease of porosity and enhancement of textural structure can lead to the increasing contact areas of Bi-2212 grains. Based on our previous studies, the substitution of Yb at Sr site can improve the crystallization of Bi-2212 crystals [39]. Therefore, during the sintering process, more grain boundaries with highly crystallized structures as observed with HRTEM in Fig. 3(b) are formed. And the blocking or scattering effect of grain boundaries on the carriers is weakened. And the increase of grain size reduces the number of grain boundaries, which also weakens the scattering effect of grain boundaries to the carriers. It is concluded that the Bi-2212 bulks with higher density, higher textural structure and larger grains can be obtained with proper amount of Yb doping. The intergrain connections can be qualitatively analyzed with the AC susceptibility data. By comparing the AC susceptibility of x = 0 and 0.02 bulks under the same Hac field, the shift of peaks on the imaginary part curve, v00 of Yb doped sample towards higher temperature is observed. As shown in Fig. 8 the peak temperatures, Tp of the doped sample are all higher than that of Yb free sample correspondingly, which confirms the enhancement of intergrain connections due to the microstructure improvement as discussed above.
Fig. 8. AC field dependence of peak temperature, Tp, with the linear fitting shown as dashed lines.
intra-granular pinning force can be estimated with Eq. (3), as follow.
1
2 l0 Tp ¼ Tc 1 Hac 8Rg ag ð0Þ
! ð3Þ
where, the values of Tc and Tp are taken from Fig. 3 (b) and (c), Rg is the average grain radius, which is measured on the SEM images as discussed above, ag(0) is the intra-granular pinning force density for Abrikosov vortices. In order to compare the pinning force density of these samples, we assume the ag(0) of Yb free sample to be a0 g(0), and the obtain ag(0) of x = 0.02 sample is 1.3a0 g(0). The 30% increase indicates that the intra-granular pinning force is enhanced by Yb doping. It is well known that lattice defects generated by the doping of foreign ions with proper size can work as effective flux pinning center. Therefore, it indicates that proper amount of Yb doping can enhance the intra-granular pinning force density for Abrikosov vortices effectively. In order to further investigate the flux pinning mechanism variation of these samples, the pinning force densities of Bi-2212 bulks doped with different Yb content are calculated with the equation Fp = Jc B [41]. The scale plots of normalized pinning force density (f = Fp/Fpmax) as a function of reduced magnetic fields (b = B/Birr) are plotted in Fig. 9. All the Fp–B curves could be well fitted to the Dew-Hughes equation as:
4.2. Flux pinning properties The flux pinning properties can also be analyzed with AC susceptibility measurement. By fitting the Tp–Hac curves as shown in Fig. 8 with the linear dependence from Müller model [40], the
Fig. 9. Reduced Flux pinning force of Bi-2212 bulks with different Yb contents as a function of the reduced magnetic field at 4.2 K. The fitting of Fp–B curve of x = 0.01 bulk is presented as an example in the inset. The black dots are measured data and red line is the fitting curve. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
S. Zhang et al. / Physica C 511 (2015) 26–32 Table 2 Values of pinning parameters for Bi-2212 bulks with Yb content of x = 0–0.05 obtained with Dew-Hughes model. x value
0
0.01
0.02
0.05
p q Birr (T) A, 106 Nm3
0.64 0.94 6.2 712.4
0.69 1.22 7.2 866.0
0.78 1.82 7.3 1281.2
0.72 0.98 6.5 779.5
p
q
F p ¼ AðB=Birr Þ ð1 B=Birr Þ
ð4Þ
where A is a numerical parameter independent of the applied field, Birr is the irreversible field, p and q are parameters describing the actual pinning mechanism. The fitting parameters for different samples are given in Table 2. For the Yb free bulk, the values of p and q are 0.64 and 0.94, respectively. It is found that with increasing amount of Yb doping, the values of p and q increase to p = 0.78 and q = 1.82 when x = 0.02. Based on the Dew-Hughes model, the flux pinning mechanism can be recognized as volume pinning of Dj type for p = 1 and q = 1, and normal point pinning for p = 1 and q = 2, respectively. Therefore, the change of main flux pinning mechanism from the volume pinning of Dj type to the normal point pinning mechanism can be deduced [42]. On the other hand, according to the Dew-Hughes model, the peak position of Fp appears at a reduced field value of h = 0.5 for volume pinning of Dj type and h = 0.33 for point pinning mechanism. As shown in Fig. 9, the peak position shifts from h = 0.39 for Yb free bulk to h = 0.35 and 0.30 with increasing Yb content to x = 0.01 and 0.02, respectively. Therefore, it implies the enhancement of normal point pinning over volume pinning of Dj type, due to the point defects arising out of the substitution of Yb at Sr site. The deviation of p and q values from theoretical ones should be attributed to the existence of normal surface pinning due to the grain boundaries [24]. With the further increase of Yb to 0.05, the Yb2O3 particles appears with the size far larger than the coherent length of Bi-2212, and the characteristic length of introduced lattice defects also increases. The number of effective pinning centers decreases. Thus the main flux pinning mechanism changes back to the volume pinning of Dj type same as that of the Yb free bulk. It is concluded that with the doping of Yb ions into Bi-2212 matrix, lattice defects can be obtained as effective pinning centers. The main flux pinning mechanism changes and the normal point pinning mechanism becomes the dominant mechanism gradually. The flux pinning under low field is enhanced due to the enhancement of point pinning property. However, since this kind of point pinning introduced by secondary phase is recognized as weak pinning, the critical current density at higher magnetic field cannot be effectively enhanced [43]. Therefore, the increase of self-field Jc can be attributed to the improvement of intergrain connections, and the increase of in-field Jc can be related to the enhancement of flux pinning properties as discussed above. 5. Conclusions Bi-2212 superconducting bulks were prepared with different Yb doping contents. The substitution of Yb enhanced the crystallization properties of Bi-2212, therefore Bi-2212 bulks with higher density, larger grain size and higher orientated microstructures were obtained with proper amount of Yb doping. Improvement of critical current density was obtained, which could be attributed to both the improvement of intergrain connections and the enhancements of flux pinning properties. Meanwhile, Yb doping also caused the change of flux pinning mechanism. The dominant
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flux pinning mechanism became the point pinning of normal phase with the increase of Yb doping content to x = 0.02, due to the introduction of effective pinning centers. The results of this study provide us a new opportunity for further enhancement of the current carrying capacity of Bi-2212 wires and tapes for practical applications. Acknowledgements This research was financially supported by National ‘‘973’’ Project, under contract No. 2011CBA00104, National Natural Science Foundation of China under contract No. 51102198 and 51472206, the International Scientific and Technological Cooperation Projects of China No. S2010GR0518, the National ITER Program of China No. 2013GB110001, and the Program for Innovative Research Team in Shaanxi province No. 2013KCT-07. References [1] U.P. Trociewitz, J. Schwartz, K. Marken, H. Miao, M. Meinesz, B. Czabaj, NHMFL Rep. 13 (2006) 31. [2] H. Miao, K.R. Marken, M. Meinesz, B. Czabaj, S. Hong, IEEE Trans. Appl. Supercond. 15 (2005) 2554. [3] D.C. Larbalestier, J. Jiang, U.P. Trociewitz, F. Kametani, C. Scheuerlein, M. Dalban-Canassy, M. Matras, P. Chen, N.C. Craig, P.J. Lee, E.E. Hellstrom, Nat. Mater. 13 (2014) 375–381. [4] H.M. Weijers, U.P. Trociewitz, W.D. Markiewicz, J. Jiang, D. Myers, E.E. Hellstrom, A. Xu, J. Jaroszynski, P. Noyes, Y. Viouchkov, D.C. Larbalestier, IEEE Trans. Appl. Supercond. 20 (2010) 576–582. [5] T. Kiyoshi, A. Sato, H. Wada, S. Hatashi, M. Shimada, Y. Kawate, IEEE Trans. Appl. Supercond. 9 (1999) 472–477. [6] Z. Melhem, S. Ball, S. Chappell, Phys. Proc. 36 (2012) 805–811. [7] M. Dalban-Canassy, D.A. Myers, U.P. Trociewitz, J. Jiang, E.E. Hellstrom, Y. Viouchkov, D.C. Larbalestier, Supercond. Sci. Technol. 25 (2012) 115015. [8] J. Bock, F. Breuer, C.E. Bruzek, N. Lallouet, M.O. Rikel, H. Walter, in: L. Bottura, M. Buzio, T. Taylor (Eds.), Proc. Workshop on Accelerator Magnet Superconductors, 2004, pp. 149–155. [9] K. Ohsemochi, K. Koyanagi, T. Kurusu, T. Tosaka, K. Tasaki, M. Ono, Y. Ishii, K. Shimada, S. Nomura, K. Kidoguchi, H. Onoda, N. Hirano, S. Nagaya, J. Phys.: Conf. Ser. 43 (2006) 825–828. [10] S. Vinu, P.M. Sarun, A. Biju, R. Shabna, P. Guruswamy, U. Syamaprasad, Supercond. Sci. Technol. 21 (2008) 045001. [11] L.N. Bulaevskii, L.L. Daemen, M.P. Maley, J.Y. Coulter, Phys. Rev. B 48 (1993) 13798. [12] D. Buhl, T. Lang, L.J. Gauckler, Appl. Supercond. 4 (1997) 299–317. [13] S. Stassen, A. Vanderschueren, R. Cloots, A. Rulmont, M. Ausloos, J. Cryst. Growth 166 (1996) 281–285. [14] J.Y. Jiang, H.P. Miao, Y.B. Huang, S. Hong, J.A. Parrell, C. Scheuerlein, M.D. Michiel, A.K. Ghosh, U.P. Trocitwitz, E.E. Hellstrom, D.C. Larbalestier, IEEE Trans. Appl. Supercond. 23 (2013) 6400206. [15] F. Kametani, E.G. Lee, T. Shen, P.J. Lee, J. Jiang, E.E. Hellstrom, D.C. Larbalestier, Supercond. Sci. Technol. 27 (2014) 055004. [16] C.J. Eastell, B.M. Henry, C.G. Morgan, C.R.M. Grovenor, M.J. Goringe, IEEE Trans. Appl. Supercond. 7 (1997) 2083. [17] Y.N. Tsay, Q. Li, Y. Zhu, M. Suenaga, K. Shibutani, I. Shigaki, R. Ogawa, IEEE Trans. Appl. Supercond. 9 (1999) 1662. [18] Y. Nakayama, T. Motohashi, K. Otzschi, J. Shimoyama, K. Kitazawa, K. Kishio, Phys. Rev. B 62 (2000) 1452–1456. [19] X.L. Wang, H.K. Liu, S.X. Dou, J. Horvat, D. Millikon, G. Heine, W. Lang, H.M. Luo, S.Y. Ding, J. Appl. Phys. 89 (2001) 7669. [20] A.L. Crossley, Y.H. Li, A.D. Caplin, J.L. MacManus-Driscoll, Physica C 314 (1999) 12–18. [21] A.B. Kulakov, I.K. Bdikin, S.A. Zver’kov, G.A. Emel’chenko, G. Yang, J.S. Abell, Physica C 371 (2002) 45–51. [22] H.L. Su, F. Vasiliu, P. Majewski, F. Aldinger, Physica C 256 (1996) 345–352. [23] A. Amira, Y. Boudjadja, A. Saoudel, A. Varilci, M. Akdogan, C. Terzioglu, M.F. Mosbah, Physica B 406 (2011) 1022–1027. [24] S. Vinu, P.M. Sarun, R. Shabna, P.M. Aswathy, J.B. Anooja, U. Syamaprasad, Physica B 405 (2010) 4355–4359. [25] R. Shabna, P.M. Sarun, S. Vinu, U. Syamaprasad, J. Alloys Compd. 493 (2010) 11–16. [26] R. Shabna, P.M. Sarun, S. Vinu, U. Syamaprasad, J. Alloys Compd. 48 (2010) 797–801. [27] P.M. Saruna, S. Vinua, R. Shabnaa, A. Bijub, U. Syamaprasad, J. Alloys Compd. 472 (2009) 13–17. [28] S. Vinu, P.M. Sarun, R. Shabna, A. Biju, U. Syamaprasad, Mater. Lett. 62 (2008) 4421–4424. [29] P.M. Sarun, S. Vinu, R. Shabna, A. Biju, U. Syamaprasad, Mater. Lett. 62 (2008) 2725–2728.
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