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Jc-B-T surface and irreversibility field in CNT doped MgB2 tapes and wires

Electrical resistivity and critical current density of undoped and doped with short multiwalled carbon nanotubes (SMW_CNT) Fe-sheathed wires and tapes annealed at 900°C and 1000°C respectively, have been investigated in the temperature range 2-40K and in magnetic field B ~ 16T. The amount of dopand was chosen to correspond to stoichiometric formula MgB1.8C0.2. Doping via partial substitution of boron with carbon in MgB2 decreases the superconducting transition temperature Tc, but increases the low temperature upper critical field Hc2, due to alloying effects in two gap MgB2 superconductor [1] and these effects are more pronounced in sample annealed at higher temperature. The actual amount of substitution of boron with carbon was deduced from the decrease of Tc upon doping. At the same time, the inhomogeneous doping, nanoprecipitates formed during annealing and the remnants of SMW-CNT enhance the pinning of vortices in doped samples at lower temperatures (T<25 K) and higher fields. This shows up in the enhanced effective pinning potential Uo(B), irreversibility field Hirr(T), critical current density Jc(B, T) and the volume pinning force Fp=JcB at elevated fields. In particular, for tape annealed at 900°C Jc( 12T, 10K)=3500 A/cm2 which equals the best result obtained at T=5K by CNT doping [2] and Jc(5K) of this sample was already at 8T ten times larger than that of the undoped wire. Wide temperature and field range employed in present investigation enables detailed study of Jc-B-T surfaces as well as to obtain the scaling laws for Uo(B), Jc(B, T) and Hirr(T). Somewhat different enhancement of Hirr and Jc(B) in doped wire and tape is attributed to better alignment of CNTs in the latter case [3]. References: [1] A. Gurevich, Phys. Rev. B67, 184515 (2003) [2] J. H. Kim et al., Appl. Phys. Lett. 89, 122510 (2006) [3] P. Kovač et al., Supercond. Sci. Technol. 20, 105 (2007) ; S.X.Dou et al, Adv. Mater. 18(6), 785 (2006) 335

MgB2; superconductivity; Jc-B-T surfaces

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