Correlated enhancement ofH_c2andJ_cin carbon nanotube doped MgB₂

Abstract

The use of MgB 2 in superconducting applications still awaits for the development of a MgB 2 -based material where both current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB 2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, J c is in the range 2.5-10%at depending on field and temperature. Record values of the upper critical field, H c2 (4K) = 41.9 T (with extrapolated H c2 (0) ≈ 44.4 T) are reached in a bulk sample with 10%at DWCNT content. The measured H c2 vs T in all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich et al.

SiC and carbon nanotube distinctive effects on the superconducting properties of bulk MgB2

“…30 The H c2 ͑0͒ of dwCNT is close to the maximum H c2 value as a function of x. 11 A decrease in H c2 ͑0͒ was also observed for a larger x value ͓see Fig. 2͑b͔͒, in agreement with very recently reported data.…”

“…8 as described elsewhere. 11 The upward curvature, characteristic of the presence of two gaps, 9,10 is apparent in these H c2 ͑T͒ data. The inset displays the H c2 ͑0͒ extrapolations and the fit parameters 1 / D , 1 / D as function of x.…”

“…The synthesis temperatures were the optimal ones for each kind of doping. 5,11 In both cases ͑SiC and CNT͒ part of carbon dissolves into the MgB 2 structure during the fabrication process, 13,19 and the shift in the a-lattice parameter, obtained from x-ray diffraction, can be used as a measure of the actual amount of C ͑x͒ in the Mg͑B 1−x C x ͒ 2 structure. 22 The synthesis temperature, lattice parameters, and x values obtained from fitting the single crystal data of Kazakov et al 25 and the neutron diffraction data of Avdeev et al 27 are listed in Table I.…”

“…The pinning force may be improved by the incorporation of defects ͑nanopar-ticle doping, chemical substitutions, etc.͒. [1][2][3][4][5][6] On the other hand, the doping level affects the interband scattering coefficients ⌫ , ⌫ and the diffusivity of each band D , D , as predicted by Gurevich et al, [7][8][9][10][11] and these changes may cause a significant H c2 variation. Earlier attempts succeeded in raising the intragrain J c through the incorporation of Mg͑B 1−x O x ͒ 2 , 1 SiC, 2,12,13 Al, 3 Dy 2 O 3 , 4 nano-C, 5 and carbon nanotubes, 6,12,[14][15][16] which produce a decrease in the critical temperature ͑T c ͒ by chemical substitution and probably lattice strain.…”

Carbon nanotubes effects on the relaxation properties and critical current densities of MgB2 superconductor

“…3 In the last years, several groups have achieved a good improvement of the transport properties. In bulk polycrystalline samples, good results for increasing J c have been achieved by chemical doping or by the inclusion of nonsuperconducting phases, in particular with carbon nanotubes (CNT) inclusions, [4][5][6][7][8][9] with diameters close to the MgB 2 coherence length n. On the other hand, theoretical results predict and experimental studies confirm that the presence of two superconducting gaps produces significant H c2 enhancement in the dirty limit. 10 In a previous publication we have reported results in samples with double wall CNTs additions showing both effects: an important increase in the critical current density and the upper critical field H c2 .…”