THE COMBINED INFLUENCE OF SiC AND RARE-EARTH OXIDES DOPING ON SUPERCONDUCTING PROPERTIES OF MgB 2 WIRES

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1 THE COMBINED INFLUENCE OF SiC AND RARE-EARTH OXIDES DOPING ON SUPERCONDUCTING PROPERTIES OF MgB 2 WIRES Hui Fang, Brenden Wiggins, and Gan Liang Department of Physics, Sam Houston State University, Huntsville, TX ABSTRACT Ti-sheathed MgB 2 wires doped with different amount of nanosized rare-earth oxide (Yb 2 O 3, Gd 2 O 3, and Dy 2 O 3 ) and/or nanosized SiC were investigated. X-ray diffraction patterns suggested the existence of Mg 2 Si phase due to the SiC addition, while no any phase related to rare-earth oxide could be detected. Strong enhancement of in-field current carrying capability was observed on 2.5 wt.% Yb 2 O 3 doped sample annealed at 800 o C. Dual doping with rare-earth oxide and SiC does not enhance J C due to the negative effect caused by SiC addition. INTRODUCTION Magnesium diboride, MgB 2 superconductor has been considered as a promising candidate for large-scale applications such as superconducting magnets at 20 K due to its relative high critical temperature in comparison with low-temperature superconductors and low raw material and process cost in comparison with high-temperature superconductors [1-4]. Currently, powder-in-tube (PIT) method is widely employed to fabricate long length MgB 2 superconducting wires/tapes for magnet winding [5-7]. In PIT process, reacted (ex situ) or non-reacted (in situ) Mg and B powders are packed into a metallic tube, followed by cold work, such as swaging, drawing, rolling, to make long length wires/tapes. The chosen metallic sheath has to be low-cost, chemically compatible with MgB 2 compound, and adequate for mechanical strength, which leaves Fe, Ni, Ti, and stainless steel (SS) ideal sheath metals [8]. Compared with the commercially available low-temperature superconductors, the main obstacle of MgB 2 for its practical applications in the presence of magnetic field is its lack of flux pinning [9]. Various approaches including chemical doping and irradiation have been employed to generate flux pinning centers such as nano-scale secondary non-superconducting phase, lattice defects, etc. Among all explored methods, adding nano-scale dopants has been proved a very effective way for introducing pinning centers into MgB 2 [10]. In the past seven years, a great deal of research effort has been made in searching for proper dopants and more than thirty different doping species have been investigated. These dopants include metal elements, borides, nitrides, oxides, carbon, carbides, organic compounds and other materials [11-14]. Currently, the most effective dopants are carbon and carbon compounds. More recently, nano-scale rare-earth oxides such as Dy 2 O 3, Y 2 O 3, Eu 2 O 3, Ho 2 O 3, Pr 6 O 11, have been emerging as effective dopants to enhance current carrying capability of MgB 2 in the presence of external magnetic field [15-19]. These nanosized rare-earth oxides react with B to form their respective borides without any significant substitution at the Mg/B site [20].

2 Despite extensive research on searching doping species, little attention has been paid to dual or even triple dopants, which may bring breakthrough towards the possible high field applications of MgB 2. In this research, we investigated the effects on Ti-sheathed MgB 2 wires doped with several nanosized rare-earth oxides Yb 2 O 3, Gd 2 O 3, and Dy 2 O 3. We also reported the effect on Ti-sheathed MgB 2 wires dual doped with nanosized rare-earth oxides and SiC. Our selection of titanium as sheath material was based on the following facts: (1) the performance of Ti-sheathed wires is comparable to or even better than iron-sheathed wires [21]; (2) the magnetic measurement is much easier on Ti-sheathed wire than ferromagnetic metal sheathed wire. EXPERIMENTAL The powder-in-tube method was used to fabricate nanosized rear-earth oxides and SiC doped Ti-sheathed MgB 2 wires. The titanium tube used in this study had an outer diameter of 6 mm and a wall thickness of 1 mm. One end of a 10 cm long titanium tube was sealed by crimping. Then desired powder was filled into the tube gradually to ensure the tight pack. The remaining end of the tube was crimped and sealed afterwards. To make the desired powder, commercially available Mg powder (Alfa-Aesar, nominally 99.8% pure, -325 mesh), B powder (Alfa-Aesar, nominally 99.99%, amorphous phase, -325 mesh), Yb 2 O 3 powder (MTI, nominally 99.9%, 50 nm), Gd 2 O 3 powder (MTI, nominally 99.9%, nm), Dy 2 O 3 powder (MTI, nominally 99.9%, 40 nm), and SiC powder (Nanostructured & Amorphous Materials Inc., nominally 99%, amorphous phase, 15 nm) were stoichiometrically mixed. The powder mixture was then mechanically milled by a Spex-8000 high-energy ball mill for 100 minutes. Tungsten carbide balls and vial were used as milling medium and the mass ratio of ball to powder was 20:1. The entire filling procedure was carried out in a high purity argon atmosphere. The powder-filled tube was rolled to a wire with a square cross-sectional area of about 1 mm by 1mm. A motorized groove rolling mill with 17 different grooves of sizes from 6 mm to 1 mm was used for the wire rolling. As-rolled wires were cut into 4 inch long segments and heat treated at desired temperatures for 30 minutes. A high purity argon gas flow was maintained throughout the heat treatment process to avoid the oxidation of titanium sheath. The XRD patterns on the core materials of the wires were obtained by using a Rigaku D/Max Ultima II diffraction machine with Cu K radiation. To prepare XRD sample, core material of the wires was removed from the titanium sheath carefully using a scissor and a blade, followed by grinding. The temperature dependent magnetization, M (T), was measured in zerofield-cooled (ZFC) mode by using a Magnetic Property Measurement System (MPMS) from Quantum Design. The applied field for M (T) measurement is 20 Oe. The hysteresis loops of magnetization, M (H), up to 5 Tesla were measured using the same MPMS. The cross-sectional area of the MgB 2 wire for MPMS measurement was about 1 mm by 1 mm and the MgB 2 cores had cross-sectional area of about 0.55 mm by 0.55 mm. The magnetic Critical current density of the samples was calculated with the formula J c = 20 M/[a(1-a/3b)] from Bean critical state model [22], where M is the difference between the upper and lower branches of the hysteresis loops with unit emu/cm 3.

3 RESULTS AND DISCUSSIONS MgB 2 wires doped with Yb 2 O 3 and SiC Figure 1 shows the XRD patterns of the core materials of Ti-sheathed MgB 2 wires doped with various amount of Yb 2 O 3 and/or SiC and annealed at 800 o C. To our surprise, no any phase related to Yb 2 O 3 could be observed on XRD patterns, which we believe is due to very small size or very small doping amount of Yb 2 O 3. Meanwhile, the XRD patterns also reveal that with the increase of annealing temperature, the relative amount of MgO increases. For SiC doped wires, three different phases were identified: MgO, Mg 2 Si, and the main phase MgB 2 for which the Miller indices are labeled in the figure. The MgO impurity phase was formed due to the trapped oxygen in the raw powder, and the formation of Mg 2 Si should be due to the reaction between Mg and SiC. With the increase of SiC amount, Mg 2 Si amount increases as well. Figure 1. The XRD patterns of pristine and various amount Yb 2 O 3 and/or SiC doped MgB 2 wires annealed at 800 o C. Figure 2 shows the temperature dependent dc magnetization of Ti-sheathed MgB 2 wires doped with various amount of Yb 2 O 3 and/or SiC and annealed at 800 o C. Addition of Yb 2 O 3 only has slight effect on T C of MgB 2 wire, while addition of SiC impacts T C dramatically. 2.5 wt.% SiC doping lowers T C from 36.2 K to 35.4 K, while 5 wt.% SiC drops T C to about 33.6 K. This can be explained by the existence of Mg 2 Si phase observed on XRD patterns. T C of co-doped 2.5 wt.% Yb 2 O 3 and 2.5 wt.% SiC is 35.2 K, only slightly lower than that of sample doped with SiC alone, which proved small effect on T C brought by Yb 2 O 3 doping.

4 Figure 2. The temperature dependence dc magnetization of Ti-sheathed MgB 2 wires doped with various amount of Yb 2 O 3 and/or SiC and annealed at 800 o C. Figure 3. The field dependence magnetic critical current density measured at 5 K and 20 K of Tisheathed MgB 2 wires doped with various amount of Yb 2 O 3 and/or SiC and annealed at 800 o C. Figure 3 shows the field dependence of critical current density J c measured at 5 K and 20 K of

5 Ti-sheathed MgB 2 wires doped with various amount of Yb 2 O 3 and/or SiC and annealed at 800 o C. The addition of 2.5 wt.% Yb 2 O 3 enhances the current carrying capability of MgB 2 wires at almost all measured field. When increasing the doping amount of Yb 2 O 3 to 5 wt.%, J C values are slightly higher than that of pristine MgB 2 wires, which we believe is due to the decrease of the net superconducting phase amount per unit volume. The addition of SiC generates the negative effect on J C, which is the same as our previous results [23]. 5 wt.% of SiC addition causes dramatically drop of J C, which is not shown in the figure. Due to the negative effect on J C of SiC addition, the J C of wires dual doped with 2.5 wt.% Yb 2 O 3 and 2.5 wt.% SiC is smaller than that of the pristine MgB 2 wire. MgB 2 wires doped with Gd 2 O 3 and SiC Figure 4 shows the temperature dependent dc magnetization of Ti-sheathed MgB 2 wires doped with various amount of Gd 2 O 3 and/or SiC and annealed at 800 o C. Addition of Gd 2 O 3 also only has slight effect on T C of MgB 2 wire, decreasing T C of pristine MgB 2 wire from 36.4 K to 36.2 K. There is almost no difference on T C of 2.5 wt.% Gd 2 O 3 doped wire and 5 wt.% Gd 2 O 3 doped wire. T C of co-doped 2.5 wt.% Gd 2 O 3 and 2.5 wt.% SiC is 35.4 K. Figure 4. The temperature dependence dc magnetization of Ti-sheathed MgB 2 wires doped with various amount Gd 2 O 3 and/or SiC and annealed at 800 o C. Figure 5 shows the field dependence of critical current density J c measured at 5 K and 20 K of Ti-sheathed MgB 2 wires doped with various amount of Gd 2 O 3 and/or SiC and annealed at 800 o C. The addition of 2.5 wt.% and 5 wt.% Gd 2 O 3 shows almost no influence on the current carrying capability of MgB 2 wires at almost all measured field. The negligible difference on J C of 2.5 wt.% SiC doped wire and 2.5 wt.% SiC and 2.5 wt.% Gd 2 O 3 co-doped wire also indicates

6 the negligible influence brought by Gd 2 O 3 addition. Figure 5. The field dependence magnetic critical current density measured at 5 K and 20 K of Tisheathed MgB 2 wires doped with various amount of Gd 2 O 3 and/or SiC and annealed at 800 o C. MgB 2 wires doped with Dy 2 O 3 and SiC Figure 6 shows the temperature dependent dc magnetization of Ti-sheathed MgB 2 wires doped with various amount of Dy 2 O 3 and/or SiC and annealed at 800 o C. Addition of Dy 2 O 3 decreases the T C of pristine MgB 2 wire from 36.4 K to 36.0 K, while there is almost no difference on T C of 2.5 wt.% Dy 2 O 3 doped wire and 5 wt.% Dy 2 O 3 doped wire. Due to the dramatic decrease of T C on SiC doping, the T C of 2.5 wt.% Dy 2 O 3 and 2.5 wt.% SiC co-doped wire drops to 34.0 K. Therefore, addition of Dy 2 O 3 has negative effect on T C of MgB 2 wire. Figure 7 shows the field dependence of critical current density J c measured at 5 K and 20 K of Ti-sheathed MgB 2 wires doped with various amount of Dy 2 O 3 and/or SiC and annealed at 800 o C. The addition of Dy 2 O 3 shows negative effect on the current carrying capability of MgB 2 wires at almost all measured field, while with the increase of Dy 2 O 3 amount from 2.5 wt.% to 5 wt.%, the J C value of the wire increase slightly, which is still much lower than that of pristine wire. The addition of Dy 2 O 3 shows the comparable negative effect to the addition of SiC on MgB 2 wires. Consequently, the current carrying capability of co-doped wire with 2.5 wt.% Dy 2 O 3 and 2.5 wt.% SiC shows the worst value. The different results we obtained in comparison with that reported by other research groups, we believe, is due to the different process we applied and/or different raw materials we used. More systematically work is needed to clarify the effect on superconducting properties of MgB 2 wires doped with nanosized rare-earth oxides.

7 Figure 6. The temperature dependence dc magnetization of Ti-sheathed MgB 2 wires doped with various amount of Dy 2 O 3 and/or SiC and annealed at 800 o C. Figure 7. The field dependence magnetic critical current density measured at 5 K and 20 K of Tisheathed MgB 2 wires doped with various amount of Dy 2 O 3 and/or SiC and annealed at 800 o C.

8 CONCLUSION In this research, we studied the nanosized rare-earth oxides and nanosized SiC doping effects on superconducting properties of Ti-sheathed MgB 2 wires. The addition of Yb 2 O 3 slightly alters the critical temperature, and enhances the critical current density in the presence of external magnetic field at 2.5 wt.% doping amount. Addition of nanosized Gd 2 O 3 shows only negligible influence on superconducting properties of MgB 2 wires, while addition of nanosized Dy 2 O 3 provides negative effects on both critical temperature and critical current density. The addition of SiC causes the dramatic decrease of T C and J C of MgB 2 wires. Consequently, the T C and J C of dual doped MgB 2 wire with rare-earth oxides and SiC are lower than that of non-doped MgB 2 wires. ACKNOWLEDGEMENT This work was supported by the National Science Foundation under Grant No. CHE , an award from Research Corporation for Science Advancement and Enhancement Grants for Research (EGR) of Sam Houston State University. REFERENCES [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, and J. Akrimitsu, Superconductivity at 39 K in magnesium diboride, Nature, vol. 410, 2001, pp [2] D. C. larbalestier, L. D. Cooley, M. O. Riikel, A. A. Polyanskii, J. Jiang, S. Patnaik, X. Y. Cai, D. M. Feldmann, A. Gurevich, A. A. Squitieri, M. T. Naus, C. B. Eom, E. E. Hellstrom, R. J. Cava, K. A. Regan, N. Rogado, M. A. Hayward, T. He, J. S. Slusky, P. Khalifah, K. Inumaru, and M. Hass, Strongly linked current flow in polycrystalline forms of the superconductor MgB 2, Nature, vol. 410, pp , Mar [3] S. Jin, H. Mavoori, C. Bower, and R. B. van Dover, High critical currents in iron-clad superconducting MgB 2 wires, Nature, vol. 411, 2001, pp [4] H. Fang, S. Padmanabhan, Y. X. Zhou, and K. Salama, "High critical current density in ironclad MgB2 tapes", Appl. Phys. Lett., vol. 82, 2003, pp [5] H. Kumakura, A. Matsumoto, H. fuji, and K.Togano, High transport critical current density obtained for powder-in-tube processed MgB 2 tapes and wires using stainless steel and Cu-Ni tubes, Appl. Phys. Lett. vol. 79, 2001, pp [6] G. Grasso, A. Malagoli, C. Ferdeghini, S. Roncallo, V. Braccini, and A. S. Siri, Large transport currents in unsintered MgB 2 superconducting tapes, Appl. Phys. Lett., vol. 79, 2001, pp [7] H. Suo, C. Beneduce, M. Dhalle, N. Musolino, J. Y. Genoud, and R. Flukiger, Large transport critical currents in dense Fe- and Ni-clad mgb 2 superconducting tapes, Appl. Phys. Lett., vol. 79, 2001, pp [8] A.V. Pan, S. Zhou, and S.X. Dou, Iron-sheath influence on the superconductivity of MgB 2 core in wires and tapes, Superconductor Science and Technology 17, S410 (2004). [9] K. Vinod, R.G. Abhilash Kumar, and U. Syamaprasad, Prospects for MgB 2 superconductors for magnet application, Superconductor Science and Technology 20, pp. R1-R13 (2007).

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