Isotropic behavior of critical current for MgB2 ex situ tapes with 5 wt.% carbon addition

Physica C 483 (2012) 222–224

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Isotropic behavior of critical current for MgB2 ex situ tapes with 5 wt.% carbon addition Anna Kario a,c,⇑, Vadim Grinenko a, Alex Kauffmann a, Wolfgang Häßler a, Christian Rodig a, Pavol Kovácˇ b, Tibor Melišek b, Bernhard Holzapfel a a b c

Institute for Metallic Materials, IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany Institute of Electrical Engineering, SAS, Dúbravska cesta 9, 841 04 Bratislava, Slovakia Karlsruhe Institute of Technology, Institute for Technical Physics, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

a r t i c l e

i n f o

Article history: Received 10 May 2012 Received in revised form 20 July 2012 Accepted 30 July 2012 Available online 21 September 2012 Keywords: MgB2 ex situ C doping Anisotropy Tapes

a b s t r a c t Using high energy milled a standard commercially available ex situ MgB2 with 5% C addition was obtained tapes with excellent critical current density value Jc  3.5  104 A/cm2 at 9 T and 4.2 K. The Jc is independent on the direction of magnetic field respect to a tape plane which makes these superconductors very perspective for applications in high magnetic fields. It was found that weak crystalline texture and low upper critical field anisotropy are responsible for this magnetically isotropic Jc. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The isotropic MgB2 conductors with excellent critical current in high magnetic fields are needed for technical applications [1]. To enhance a critical current density (Jc), carbon additions have been found as most successful [2–5]. Moreover the C doping enhances upper critical fields (Hc2) decreases its anisotropy c [6,7] and reduces powder reactivity [8]. In situ approach is usually used to obtain doped MgB2 since it is easier to introduce carbon into the MgB2 structure during the Mg + 2B reaction in comparison to adding carbon to the formed MgB2 (ex situ approach). On the other hand in situ approach leads to a low material density in the conductor core due to MgB2 volume reduction after synthesis. This inevitably results in a reduction of the engineering Jc. Therefore, the higher core density can be achieved in the case of ex situ MgB2. Deformation by rolling to tape shape is more efficient for material density in comparison to only drawing, which is used for wire production. Unfortunately rolling leads to texture and significant Jc anisotropy [9,10]. This anisotropy has been explained by material texture induced during rolling as well as intrinsic anisotropy c of MgB2. Recently it was shown that the Jc anisotropy of the rolled tapes can be significantly reduced after a high energy milling ⇑ Corresponding author at: Institute for Metallic Materials, IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany. E-mail address: [email protected] (A. Kario). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2012.07.013

of the laboratory made ex situ precursor MgB2 powder with C or SiC [11,12]. However in these works were not given a physical explanation of this effect. There are several open questions: (i) if during milling C can incorporate to the crystal structure; (ii) if milling helps to reduce the resulting structure of the rolled tapes; (iii) if it is possible to achieved similar results with a standard commercially available MgB2 powder or purity of the material plays important role in resulting SC properties. The last point is a crucially important for a real commercial application of the MgB2 tapes. Therefore, in this work commercially available ex situ precursor MgB2 powder is used and rolling is applied as a final deformation to increase density of the material. Moreover, successful C doping is achieved to further improve the ex situ MgB2 properties. 2. Preparation and experimental methods The precursor powders were prepared as in [13]. Tapes with those powders were heat treated at Ar atmosphere at 800 °C/1 h and 850 °C/1 h. The phase composition was examined by XRD (X’Pert PW 3040, Philips, Co-radiation). Using the Rietveld refinement, lattice parameters for all the powders after milling are calculated to examine the substitution of carbon in the lattice (shown in Table 1). For qualitative texture analysis an X-ray powder diffractometer in Bragg–Brentano geometry with Co-radiation (Philips X’Pert MPD) was used. The non-textured reference sample was prepared using the mounted and metallographically ground and

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A. Kario et al. / Physica C 483 (2012) 222–224 Table 1 Lattice parameters, phase composition, C content in the lattice and critical temperature. a (Å)

c (Å)

MgB2

MgO (wt.%)

x in

Powder p20 h p20 h5C

3.081 3.076

3.530 3.535

95 100

1(4 Mg) –

0.010 0.023

Tapes t20h t20h800 t20h850 t20h5C t20h5C800 t20h5C850

3.080 3.081 3.080 3.071 3.076 3.075

3.528 3.529 3.528 3.533 3.532 3.531

100 94 81 100 94 90

– 6 12(7MgB4+x) – 6 10

0.014 0.011 0.015 0.035 0.024 0.027

Tc

23 31 33 17 27 28

polished initial MgB2 powder. The rolled tapes were prepared and H–2H scans were measured on the top sections of the tapes. The data are normalized to the {1 0 1 1}-reflection. The temperature and magnetic field dependent resistance was measured using a physical property measurement system Quantum Design. The Tcon, Tc and Tcoff were determined using the 90%, 50% and 10% criterion of the normal state resistance at superconducting (SC) transition. To calculate anisotropy factor c = Hc2||ab/Hc2||c percolation model proposed by Eisterer et al. was used [14] where Hc2||a is the upper critical field along and Hc2||c perpendicular to the ab plane, respectively. Transport Jc of the tapes were determined by the standard four probe method using the 1 lV/cm electric field criterion on 5 cm long sample.

Fig. 1. Comparison of H–2H scans of the initial MgB2 powder and top sections of the rolled tapes.

3. Results In Table 1 lattice parameters, phase composition and amount of carbon in the structure are shown. Two powders for comparison are prepared: 20 h milled (p20h) and 20 h milled with 5 wt.% carbon addition (p20h5C). Using those powders tapes are deformed: 20 h milled (t20h) and 20 h milled with 5 wt.%C (t20h5C). Those tapes are heat treated at 800 °C/1 h (t20h800, t20h5C800) and 850 °C/1 h (t20h850, t20h5C850). First during high energy milling of ex situ powder with carbon and second conductor rolling, this element is introduced in the structure, as it was concluded from a reduction of the a-lattice parameter. Beside reduced lattice parameter a, the C doped tapes have lower Tc (Table 1), higher @ lHc2/@T and lower c (see below Fig. 2) compared to the tapes prepared without carbon additions [comment 1]. It is known that ex situ powder after high energy milling is highly reactive, decomposing to Mg and B, where Mg reacting to MgO and B to higher borides [8]. Additions, carbon reduce reactivity, what can be observed by increased MgB2 content (Table 1). In the Fig. 1 texture measurements for MgB2 powder, and two tapes: t20h800 and t20h5C800 are shown. For those measurements grinded longitudinal section of the tape was used. Because of that on scans reflexes from tape matrix are visible. There are only slight deviations between the intensities of the non-textured powder sample and the rolled tapes which can be mainly summarized by weakly decreased intensity of the {1 0 1 0}-reflections and corresponding increased intensity of the {0 0 0 2}-reflections in the rolled tapes. This corresponds to an increased number of {0 0 0 2}-planes within the rolling plane of the tapes. Thus, we can conclude that no significant texture in both tapes was observed, in agreement with previous work [10]. This can be related to the small grain size caused by high energy milling. To estimate the effect of the C doping on the c values of our tapes the magnetic field dependence of the transition width (DT)

Fig. 2. Transition width versus magnetic field of the rolled tapes. Inset shows temperature dependence of Hc2.

(see Fig. 2) of the C doped and undoped tapes has been analyzed using the percolation model proposed by Eisterer et al. [14]. of DT ¼ T on c  T c ¼ DT 0 þ

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðc2  1Þp2c þ 1  1 ð@ l0 H90% c2 =@TÞ

B0

ð1Þ

where DT0 is the SC transition width at zero magnetic field which supposed is field independent, pc is a value of the percolation threshold, and B0 is an external magnetic field. The derivative is T-dependent and defined from experimental data shown in the inset Fig. 2. Here we neglect the texture in the rolled tapes since it is very weak (see above). In our analysis for simplicity we supposed that c is T-independent. It is a good approximation in the case of high C doping level [6,17] where c is nearly constant at temperatures 0.7–0.9 Tc. For clean undoped samples c is temperature dependent [6,17] and varies between 3 and 5 in the experimentally measured T range. We expect that our reference samples are slightly doped by C because of preparation method (see text). Therefore, we expect that these samples have the reduced anisotropy with weaker temperature dependence. The width DT0 is the sample dependent value and related with variation of the superconducting properties of the MgB2 grains. Thus, the large DT0 = 2 K of the C doped samples compared to

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achieved in the case of in situ tapes [9,20,21] but in contrast to them our ex situ C doped tapes have magnetically isotropic Jc which is important in the case of practical application of MgB2. 4. Conclusions

Fig. 3. Critical current anisotropy for tapes in parallel and perpendicular magnetic field.

DT0 = 0.8 K of the undoped samples (see Fig. 2) we relate with nonhomogeneous C distribution within the MgB2 grains. It is expected that weak links between the SC grains leads to additional broadening at low magnetic fields Fig. 2. We suppose that the weak inter-grain currents are suppressed by the field above B0 > 2 T and DT is defined by the grains properties. Therefore, pc value is a field independent above B0 > 2 T and only slightly enhanced compared to the theoretically expected value pc0  0.16 for the irregular lattice [15,16] since the weak links contribution is small in the case of the MgB2 (majority of the inter-grain links do not limit the SC currents [16–18]). Thus, we adopted pc in the range of 0.2–0.3 in accord with [14]. Finally, using DT0 and c like fitting parameters (see Fig. 2) we obtained for the C doped MgB2 ct20h5C800 = 1.8(2) which should be compared with ct20h800 = 2.8(5) for the undoped samples. The obtained value of ct20h800 is less than expected in the case of clean MgB2 [6,19] which is related with minor C admixture (see Table 1) and small crystal size. We attributed slightly reduced a-lattice parameter of the tapes prepared without extra carbon additions to a small amount of carbon incorporated to MgB2 from WC vials during the high energy milling of the powder. In turn the ct20h5C800 value of the C doped tapes corresponds to previously reported anisotropy parameters for 6–10% C doped samples [6,19]. This C doping level is even higher than determined from a-lattice parameter (Table 1). Therefore, obtained c values additionally confirms successful C doping of our ex situ tapes. In Fig. 3, Jc for different samples in parallel and perpendicular to the tape surface magnetic field are shown. The low texture of our rolled undoped MgB2 tapes results in Jc which is only weakly dependent on the magnetic field direction compared to usually reported data in the case of rolled tapes [9,10]. The C doping additionally reduces Jc anisotropy respect to the field direction due to c reduction and strongly enhances Jc in high magnetic fields which is explained by the higher olHc2/@T values of these tapes (inset Fig. 2). We found that the heat treatment temperatures influence Jc values more in the case of C doped samples. In both cases higher values are for samples heat treated at 800 °C/1 h. To our knowledge the C doped tapes heat treated at 800 °C/1 h have the highest Jc ever reported for ex situ MgB2 conductors for magnetic fields above 6 T [13]. Moreover, obtained Jc is comparable with the best Jc values

The effective C doping of commercially available ex situ MgB2 was achieved and rolled doped and undoped MgB2 tapes were obtained and investigated. We found negligible texture and low Hc2 anisotropy c in comparison to the tapes reported in other works. The c = 1.8(2) and 2.8(5) was estimated from broadening of superconducting transition for C doped and undoped tapes, respectively. The especially low c for C doped tapes leads to high critical current density value of Jc  3.5  104 A/cm2 at 9 T and 4.2 K which is nearly independent of the direction of magnetic field respect to the tape plane. This result for the first time demonstrates that use of a standard commercially available MgB2 allows to achieve the Jc values comparable with the highest one reported in a literature which were obtained on a homemade high purity MgB2 powder. Therefore, the present work is an important step forward to a future application of this superconductor. Acknowledgements The authors want to thank Juliane Scheiter for technical assistance. This work was funded by the EU-FP6 Research Project ‘‘NanoEngineered Superconductors for Power Applications’’ NESPA No. MRTN-CT-2006-035619. References [1] D. Larbalestier, A. Gurevich, M. Feldmann, A. Polyanskii, Nature 414 (2001) 368–377. [2] S.X. Dou, S. Soltanian, J. Horvat, X.L. Wang, S.H. Zhou, M. Ionescu, H.K. Liu, P. Munroe, M. Tomsic, Appl. Phys. Lett. 81 (2002) 3419. [3] H. Kumakura, H. Kitaguchi, A. Matsumoto, H. Hatakeyama, Appl. Phys. Lett. 84 (2004) 3669–3671. [4] M.D. Sumption, M. Bhatia, M. Rindfleisch, M. Tomsic, S. Soltanian, S.X. Dou, E.W. Collings, Appl. Phys. Lett. 86 (2005) 092507. [5] W. Gruner, M. Herrmann, A. Nilsson, H. Hermann, W. Häßler, B. Holzapfel, Supercond. Sci. Technol. 20 (2007) 601–606. [6] M. Angst, S.L. Bud’ko, R.H.T. Wilke, P.C. Canfield, Phys. Rev. B 71 (2005). 144512. [7] R.H.T. Wilke, S.L. Budko, P.C. Canfield, D.K. Finnemore, R.J. Suplinskas, S.T. Hannahs, Physica C 424 (2005) 1–16. [8] A. Kario, R. Nast, W. Haßler, C. Rodig, C. Mickel, W. Goldacker, B. Holzapfel, L. Schultz, Supercond. Sci. Technol. 24 (2011) 075011. 7pp. [9] W. Häßler et al., Supercond. Sci. Technol. 23 (2010) 065011. 6pp. [10] P. Lezza, R. Gladyshevskii, V. Abacherli, R. Flükiger, Supercond. Sci. Technol. 19 (2006) 286–289. [11] V. Braccini, A. Malagoli, A. Tumino, M. Vignolo, C. Bernini, C. Fanciulli, G. Romano, M. Tropeano, A.S. Siri, G. Grasso, IEEE Trans Appl. Supercond. 17 (2007) 2766. [12] A. Malagoli, V. Braccini, M. Tropeano, M. Vignolo, C. Bernini, C. Fanciulli, G. Romano, M. Putti, C. Ferdeghini, E. Mossang, A. Polyanskii, D.C. Larbalestier, J. Appl. Phys. 104 (2008) 10. [13] A. Kario, High Energy Milled ex situ MgB2 for Tapes and Wires, Dissertation, Technical University Dresden, 2011. [14] M. Eisterer, M. Zehetmayer, H.W. Weber, Phys. Rev. 90 (2003) 247002. [15] S.C. van der Marck, Phys. Rev. E 55 (1997) 1514. [16] V. Grinenko, E.P. Krasnoperov, V.A. Stoliarov, A.A. Bush, B.P. Mikhajlov, Solid State Commun. 138 (2006) 461–465. [17] K.H.P. Kim, W.N. Kang, M.-S. Kim, C.U. Jung, H.-J. Kim, E.-M. Choi, M.-S. Park, S.I. Lee, Physica C 370 (2002) 13–16. [18] S.B. Samanta, H. Narayan, A. Gupta, A.V. Narlikar, T. Muranaka, J. Akimitsu, Phys. Rev. B 65 (2002) 92510. [19] M.S. Park, H.S. Lee, J.D. Kim, H.J. Kim, M.H. Jung, Y. Jo, S.I. Lee, J. Phys.: Condens. Matter 19 (2007) 7. 242201. [20] K. Togano, J.M. Hur, A. Matsumoto, H. Kumakura, Supercond. Sci. Technol. 22 (2009) 015003. 5pp. [21] P. Kovácˇ, I. Hušek, E. Dobrocˇka, T. Melíšek, W. Häßler, M. Herrmann, Supercond. Sci. Technol. 21 (2008) 015004. 6pp.