Enhancement of critical current density in glycine-doped MgB2 bulks

Materials Chemistry and Physics 136 (2012) 778e782

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Enhancement of critical current density in glycine-doped MgB2 bulks Qi Cai, Zongqing Ma, Yongchang Liu*, Liming Yu Tianjin Key Lab of Composite and Functional Materials, School of Materials Science & Engineering, Tianjin University, Tianjin 300072, PR China

h i g h l i g h t s < Carbon doping can be realized by glycine doping. < Enhancement of critical current density over the entire field was obtained in 5 wt% glycine-doped sample. < Glycine played a role in both grain refinement and introducing pinning effect.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 April 2012 Received in revised form 18 July 2012 Accepted 28 July 2012

Samples of MgB2 þ x wt% glycine (with x ¼ 0, 2, 5 and 8) were sintered at 800  C for 30 min. Obvious enhancement of critical current density (Jc) over the entire field was obtained in 5 wt% glycine-doped sample, indicating that grain connectivity and pinning effect were both improved. The improvement of Jc was attributed to the pinning effect by C substitution, MgO nano-particles, and relatively smaller MgB2 grains. Besides, 2 wt% glycine could only improve Jc at low field, while excessive glycine (above 5 wt%) would lead to unfavorable Jc performance. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Superconductors Sintering Electron microscopy Superconductivity

1. Introduction Intermetallic compound MgB2 has attracted worldwide attention because of its superconductivity at 39 K as well as the potential for practical applications [1]. Numerous achievements have been made on this topic during the last ten years. However, the pressing issue remains that the critical current density (Jc) in pure MgB2 decreases with increasing applied magnetic field as a result of weak flux pinning [2]. A range of methods such as doping with metallic elements or carbon-based compounds have been carried out in an attempt to introduce flux pinning nucleus, and thus improve Jc over the entire applied field [3,4]. Of these trials, the carbon-containing compound was considered to be a group of effective dopants, owing to the C atom’s replacement of B in the MgB2 lattice [5,6]. Lattice distortion, as the necessary consequence of C substitution, is the main reason for Jc improvement. Here, carbon substitution can be confirmed by the shrinkage of the lattice parameter a with increasing C-doping level [5]. With respect to the option of carbon source and the enhancement of superconducting properties, a lot of efforts have been made. Yamamoto et al. [6] showed that C could

* Corresponding author. Tel./fax: þ86 22 87401873. E-mail address: [email protected] (Y. Liu). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.07.056

replace B by using B4C as the source. This substitution led to significant enhancement of Jc at high field. In addition, SiC was widely accepted as the most exciting dopant to improve Jc performance at high field. Corresponding results have been reported by a large variety of researchers [7,8]. The optimal content of SiC and mechanism of improving Jc were illustrated respectively. However, it was found to have some negative effects on Jc in the low field region [9], due mainly to the worse grain connectivity. Afterward, Zhou et al. [10] demonstrated the favored performance of sugardoped MgB2 sample. The Jc results exhibited the same excellence as that of nano-scale SiC-doped sample at high field. Moreover, an additional benefit on the performance at low fields was provided. Recent studies have also addressed the effects of doping with polymer, polymer metallic complex, or organic rare-earth salt on superconducting properties of MgB2 [11e13]. The interesting possibilities of C substitution and flux pinning were revealed after the addition of these organic compounds. Amino acid is a kind of carbon-containing organic compound, having a structure with amidogen and carboxyl. Glycine (abbr. Gly) with the chemical formula of C2H5NO2 is the simplest one in structure, and it will decompose at high temperature [14]. CO2 is one of the main decomposition products in deoxygenated atmosphere below 400  C [14], which is lower than the starting temperature of MgB2 in solidesolid reaction stage [15]. Gaseous

Q. Cai et al. / Materials Chemistry and Physics 136 (2012) 778e782

products would be firstly adsorbed on the surface of Mg or B particles, which showed physisorption at low coverage. Then they would further affect the process of phase formation. For example, dissociative C would appear from the reaction of 2Mg(s) þ CO2(g) ¼ 2MgO(s) þ C(s) [16] at relatively high temperature. The simple substance C could enter MgB2 lattice and realize the effect of doping with carbon-containing compound. In this work, glycine was introduced into MgB2 system in an attempt to realize doping with amino acid. Samples with glycine doping were prepared by solid-state sintering method. Phase composition, microstructure, and the effects of glycine content were investigated. Furthermore, a study on C-doping level, superconducting transition temperature (Tc), and Jc was conducted. 2. Experimental details Amorphous B powders (99% purity, 25 mm in size), Mg powders (99.5% purity, 100 mm in size), Gly powders (99% purity) were mixed in a molar ratio of MgB2 þ x wt% Gly (with x ¼ 2, 5 and 8). The mixture was ground thoroughly in an agate mortar and then pressed into cylindrical pellets (F5  1.5 mm) under a pressure of 5 MPa. All the doped samples were sintered in a differential thermal analysis machine (Netzsch DSC 404C) at 800  C for 0.5 h under flowing high-purity Ar gas at a heating rate of 10  C min1. After that, phase composition and microstructure were detected by means of X-ray diffraction (XRD, Rigaku D/max 2500 CuKa), scanning electron microscopy (SEM, Hitachi S-4800), and transmission electron microscope (TEM, JEM-100CX II), respectively. Superconducting properties were measured by the physical property measurement system (Quantum Design PPMS-9) after the samples were cut into a slab of size about 4  2  1 mm3. The corresponding Jc values were calculated from the width of magnetization hysteresis loops based on the Bean model [17].

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3. Results and discussions Fig. 1(a) shows the X-ray diffraction patterns of MgB2 þ x wt% Gly samples (with x ¼ 0, 2, 5, 8). Pure MgB2 was employed to serve as the reference sample of x ¼ 0. Apparently, MgB2 were the main phase in all the samples, and residual Mg phase was found in the patterns of Gly-doped ones, especially in 8 wt% Gly-doped sample. Based on X-ray diffraction results, further analyses were made as follows. Fig. 1(b) focused on the region from about 48 to 63 , i.e. the enlargement of Fig. 1(a). It was clear that (110) diffraction peaks shifted in the direction of high angle while (002) peak stayed at the same position. This indicated that there occurred some substitutions of C for B in the MgB2 crystal structure, which could lead to lattice distortion. The pinning effects of these defects were beneficial to Jc performance. To estimate the level of C substitution, y in the formula of Mg(B1yCy)2, can be estimated as:

c y ¼ 7:5D a

(1)

where D(c/a) is the change in c/a compared to a pure sample [18]. The values of c, a, and y were listed in Table 1, where c and a values of pure MgB2 [19] were also given. It was found that the highest Cdoping level was obtained in 5 wt% Gly-doped sample; this sample would present the most favored Jc performance in theory. However, a decrease in C-doping level was observed when Gly content increased to 8 wt%. It was considered to be related to excessive glycine while no characteristic peaks of C phase were observed in XRD patterns. Since the reactants of reaction 2Mg(s) þ CO2(g) ¼ 2MgO(s) þ C(s) that produced dissociative C contained solid Mg and gaseous CO2, the reaction (connection of Mg and CO2) must be affected by the other gaseous products that could also be adsorbed on the surface of Mg or B particles.

Fig. 1. (a) X-ray diffraction patterns, (b) the (002) and (110) Bragg reflections, (c) average grain size along the normal direction of (001), (d) Mg content in impurities, for the MgB2 þ x wt% Gly samples (with x ¼ 0, 2, 5, and 8).

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Table 1 Comparison of lattice parameters, C level, Tc, Jc, and Hirr for MgB2 þ x wt% Gly samples with x ¼ 0, 2, 5, and 8. Samples

Pure MgB2

MgB2 þ 2 wt% MgB2 þ 5 wt% MgB2 þ 8 wt% Gly Gly Gly

a-axis (Å) c-axis (Å) y in Mg(B1yCy)2 Tc (K) Jc at 0 T (105 A cm2) Jc at 3 T (103 A cm2) Hirr (T)

3.08489 [19] 3.52107 [19] N/A 39.00 [1] 1.77

3.08235 3.52315 0.012 37.15 2.86

3.08220 3.52884 0.025 36.15 3.07

3.07970 3.52245 0.018 35.65 0.60

5.31

4.56

8.03

0.14

4.33

4.41

5.00

3.03

Pinning effect of grain boundaries is another factor in ameliorating Jc performance besides the substitution of C for B. HalleWilliamson analysis was employed to calculate the average grain size from X-ray diffraction results, and the equation could be written as:

l

Bcosq ¼ 0:9 þ h sinq d

(2)

where d is the crystallite size, l, the wavelength of the X-radiation used, B, the peak width at half the maximum intensity, q, the Bragg angle, and h, the average micro-strain [20]. To calculate the average grain size along the normal direction of {001}, a straight line fitted by least square method was obtained with the slope as h and the intercept as 0.9l/d when Bcos q was plotted against sin q. After substituting l with 1.54056 Å, the obtained d results were plotted in Fig. 1(c). It was seen that the average grain size was less than 100 nm (within the application range of the formula) in this direction, and a monotonous decrease with the increase of glycine

content was noted. All the values were consistent with SEM observations (see Fig. 2 (a)e(c)). Most of the crystals were found to have hexagonal plate shape with typical edge angles and flat surfaces. The smooth surfaces and sharp edges indicated that these grains were of high quality. The three images displayed a similar phenomenon that general grain size (thickness) along the normal direction of (001) was less than 100 nm and decreased with the increasing glycine content. It is well known that the smaller the grain size is, the stronger the pinning effects of grain boundaries exert [21]. Hence, the above results implied a possibility for enhancement of Jc in 5 wt% and 8 wt% Gly-doped samples. Apart from this, FWHM of the highest (101) characteristic peaks of MgB2 were calculated from the XRD patterns shown in Fig. 1(a), as 0.467, 0.465 and 0.497 for MgB2 þ x wt% Gly samples with x ¼ 2, 5 and 8, respectively. The values were suddenly increased when the content was 8%, indicating imperfect crystallinity as well as small grains [8]. It was also in agreement with the SEM image (see Fig. 2(c)) in which some incomplete or adherent MgB2 phase associated with the superconducting properties was shown. Unfortunately, glycine was also seemed to be a barrier to the generation of MgB2, which could be inferred from the increasing Mg content in all the possible impurities (see Fig. 1(d)). As glycine content went up, the content of Mg in impurities, including Mg and MgO, increased. Actually, there was an improvement in the Jc performance of in situ MgB2 by excess Mg addition that was useful to increase the grain connectivity [22]. Hence, the increase in Jc at low field could be attributed to the residual Mg phase. Furthermore, the influence of MgO phase was discussed as follows. The morphology of impurities was seen on the MgB2 matrix in Fig. 2(a)e(c). The size of these particles was in accordance with those in TEM images (see Fig. 2(d)). Black secondary phase particles were distributed homogenously on matrix in general, but nanoparticle agglomeration could also be found. In order to identify these impurities, selected area electron diffraction (SAED) results were

Fig. 2. SEM images of MgB2 þ x wt% Gly samples with (a) x ¼ 2; (b) x ¼ 5; and (c) x ¼ 8. (d) TEM images of MgB2 þ 2 wt% Gly sample, the inset shows SAED results of MgB2 and MgO phase.

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presented in the insets of Fig. 2(d). So the particles observed in both SEM and TEM were confirmed to be MgO. In general MgeB system, MgO was most likely to generate from 2Mg(s) þ O2 ¼ 2MgO(s) [23], a small quantity of O2 source participating in the reaction existed in Ar atmosphere. However, most MgO phase was thought to generate from the reaction of 2Mg þ CO2 ¼ 2MgO þ C in Gly-doped samples, resulting in higher content of MgO (see Fig.1(d)). Vapor pressure was affected by decomposed gases of glycine, and MgO particles couldn’t grow into whiskers or bulks [24]. Therefore, the large amount of MgO acted as effective pinning centers in this system for the observed size is most 20 nm, about three times larger than the coherence length of MgB2. At the same time, it was found that MgO content decreased after excessive glycine doping, which indicated that glycine played a role in preventing Mg from generating not only MgB2, but also MgO. It should also result from the other products of glycine whose generating temperature (about 282  C) is only a little lower than that of CO2 (about 302  C) [14]. A large amount of gaseous products decomposed from excessive glycine prevented the reaction 2Mg þ CO2 ¼ 2MgO þ C from being proceeded to some extent, so the pinning effect by MgO were suppressed. It could be easily speculated that 8 wt% Gly-doped sample would exhibit less favored properties than 5 wt% Gly-doped sample into which pinning effects were successfully introduced by relatively higher MgO content. Fig. 3(a) represents the measured temperature-dependent magnetic moment for the prepared MgB2 þ x wt% Gly samples with x ¼ 0, 2, 5, and 8. Tc of pure MgB2 is 39 K [1], corresponding to

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sample with x ¼ 0. Tc values of the other samples were measured by plotting tangents and the results were listed in Table 1. Here Tc was found to be decreasing with the increasing amount of Gly, mainly owing to the lattice distortion caused by the substitution of C for B as well as the presence of non-superconducting phases. The measured JceH characteristics of all the samples at 20 K were illustrated in Fig. 3(b). Compared to pure MgB2, Jc performance under low field showed conspicuous enhancement in 2 and 5 wt% Gly-doped samples. It implies that the grain connectivity was optimized after Gly doping for the observed MgO were not aggregated at the grain boundaries and the residual Mg phase. Furthermore, Jc values of 5 wt % Gly-doped sample also exhibited an improvement under high field except at the narrow region around 4 T. It results from the strong C substitution effects, small grains, and pinning effects of MgO nanoparticles. Jc values of 8 wt% Gly-doped MgB2 sample showed a severe decline. On the one hand, it resulted from the imperfect crystallinity confirmed before. On the other hand, more gaseous products generated from the decomposition of glycine would lead to much more holes after escaping out of the matrix. The remaining holes could also be seen in the middle position of Fig. 2(c), which deteriorated the properties despite the positive effects of small size grains, C substitution, and MgO pinning centers. Finally, Irreversible magnetic field Hirr was determined by the magnetic field (H) where Jc ¼ 100 A cm2 [25], the values were also listed in Table 1. The same trend as Jc performance could be found. It is also improved by C-doping effects and pinning of nano-scale MgO impurities in 5 wt% Gly-doped sample. From the above discussions, it was believed that Gly-doped samples were affected by strong flux pinning effects of lattice defects, grain boundaries, and nanometer-scale impurities. And accordingly, excellent superconducting properties were obtained. In order to confirm the improved flux pinning behavior after Gly doping, the field dependence of normalized flux pinning force (Fp/ Fp,max) for pure MgB2 and Gly-doped MgB2 samples were shown in Fig. 4. The relation between flux pinning force and Jc could be described by [26,27]

Fp ¼ m0 Jc ðHÞH

(3)

where m0 is the magnetic permeability in vacuum. Fp is proportional to hn(1  h)m, with h ¼ H/Hirr, and the values of n and m depend on the different flux pinning mechanisms [28]. It was evident that the Fp/Fp,max for all the doped samples were effective in the small-bundle region in comparison with pure MgB2, while

Fig. 3. (a) Temperature-dependent magnetic moments for MgB2 þ x wt% Gly samples with x ¼ 0 [1], 2, 5, and 8. (b) Measured JceH characteristics of MgB2 þ x wt% Gly samples with x ¼ 0, 2, 5, and 8 at 20 K.

Fig. 4. Comparisons of the h (h ¼ H/Hirr) dependence of Fp/Fp,max for MgB2 þ x wt% Gly samples with x ¼ 0, 2, 5, and 8.

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uncompetitive in the single-vortex region. This phenomenon is in agreement with the Jc performance shown in Fig. 2(a). Jc values for 5 wt% Gly-doped MgB2 sample drop slowly with the applied field, since the Fp/Fp,max value is the most effective in the small-bundle region, mainly due to the high MgO impurity content of the samples. 8 wt% Gly-doped MgB2 sample also subjected to large pinning force, so the deterioration of Jc should be attributed to the imperfect crystallinity and the remaining holes. 4. Conclusions In conclusion, effects of glycine content on superconducting properties of MgB2 bulks were investigated, especially on critical current density. Glycine played a role in both grain refinement and introducing pinning effect through lattice distortion by C substitution and nano-scale MgO impurities. Meanwhile, generation of MgB2 was also restrained by Gly doping to some degree. Conspicuous enhancement of critical current density over the entire field was obtained by doping with 5 wt% glycine, which indicated that grain connectivity and pinning effects were both successfully improved by glycine doping. However, doping with 2 wt% glycine only improved Jc at low field, and excessive glycine demonstrated unfavorable effects on Jc performance. Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Grant No. 51077099), Program for New Century Excellent Talents in University and Seed Foundation of Tianjin University for grant and financial support. References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zentani, J. Akimitsu, Nature 410 (2001) 63.

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