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

Abstract

This work describes in detail the simultaneous enhancement of the upper critical field ͑H c2 ͒ and the critical current density ͑J c ͒ of MgB 2 bulk samples doped with nano-SiC particles, as well as single-walled and double-walled ͑dw͒ carbon nanotubes ͑CNTs͒. The magnetization properties were examined in a superconducting quantum interference device magnetometer, and four-probe transport measurements were performed using a 50 T pulsed magnet to determine H c2 ͑T͒. We found that the J c enhancement is similar in all doped samples at 5 K but nano-SiC addition is more effective to improve the flux pinning in the high temperature range ͑T ജ 20 K͒; this improvement cannot solely be attributed to the C incorporation to the lattice but also to the presence of other types of defects ͑i.e., several kinds of nanoinclusions͒. CNTs produce a better C incorporation that is more effective to enhance H c2 ͓i.e., dwCNT-doped samples reached a record H c2 ͑0͒ϳ44 T value for bulk MgB 2 ͔. All the H c2 ͑T͒ curves obtained for different types of doping can be successfully described using a model for a two-gap superconductor in the dirty limit.

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 .…”

“…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 . 7,8 However, it is not clear if this two effects are connected. Recent works have studied the activation energy in dirty MgB 2 samples, but the results are not conclusive.…”

“…8 The powders were ground inside a glove box and pressed under $500 MPa into small pellets with dimensions of 6 mm in diameter and $4 mm in thickness, wrapped together with extra 20 at % Mg turnings (99.98% Puratronic) in Ta foil and then placed in an alumina crucible inside a tube furnace in flowing Ar=H 2 at 900 C for 30 min.…”

“…As can be observed (with exception of the lower temperatures data in the pure sample at l 0 H ¼ 1 T) pinning energies obtained from both methods are very similar, and this validates the fitting procedures, including the estimation of J c0 ðTÞ from Eq. (8).…”

Carbon Nanotubes Addition Effects on MgB2 Superconducting Properties

“…Carbon or C-compounds additions have been very successful to improve J c and/or H c2 , and the effects of carbon doping on superconductivity in MgB 2 has been extensively studied. The J c -H performance can be greatly improved by adding different carbon sources, such as carbon doped MgB 2 filaments (Wilke et al, 2004(Wilke et al, , 2005a, nanocarbon (Soltanian et al, 2003;Ma et al, 2006;Yeoh et al, 2006;Häβler et al, 2008), amorphous carbon (Senkowicz et al, 2005), diamond (Cheng et al, 2003), B 4 C (Wilke et al, 2005b;Ueda et al, 2005;Yamamoto et al, 2005aYamamoto et al, , 2005b, carbon nanohorn (Ban et al, 2005) and, particularly, carbon nanotubes Yeoh et al, 2004Yeoh et al, , 2005Yeoh et al, , 2006Serquis et al, 2007;Serrano et al, 2008Serrano et al, , 2009); Shekhar, 2007;Vajpayee et al, 2010;etc). In this chapter we present a review of recent developments in the study of the effect of carbon nanotubes (CNT) on the superconducting properties of MgB 2 bulk and wire samples, based on the known literature data and our own results.…”