Carbon Nanotubes Addition Effects on MgB2 Superconducting Properties

Abstract

Since the discovery of superconductivity at 39 K in MgB2 (Nagamatsu et al., 2001), considerable progress has been made in the understanding of the fundamental properties and the development of commercial applications of this material. The strong potential for technological uses of MgB2 is due to a unique combination of characteristics, such as a high transition temperature Tc ~ 39K, chemical simplicity, lightweight and low cost of the raw materials (Buzea et al., 2001). In addition, the absence of weak-link behavior at grain boundaries in polycrystalline samples (Larbalestier et al., 2001) allows the use of simple Powder in Tube (PIT) methods to fabricate wires and tapes (Flükiger et al., 2003). One of the most important issues for MgB2 magnet applications is the simultaneous enhancement of its critical current density (Jc) and the upper critical field (Hc2). Thus, on one hand, the pinning force may be improved by the incorporation of defects (nano particle doping, chemical substitutions, etc.). On the other hand, the doping level affects the intraband scattering coefficients and the diffusivity of the two bands of this peculiar superconductor, and these changes may cause a significant Hc2 variation. Carbon or C-compounds additions have been very successful to improve Jc and/or Hc2, and the effects of carbon doping on superconductivity in MgB2 has been extensively studied. The Jc-H performance can be greatly improved by adding different carbon sources, such as carbon doped MgB2 filaments (Wilke et al., 2004, 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), B4C (Wilke et al., 2005b; Ueda et al., 2005; Yamamoto et al., 2005a, 2005b), carbon nanohorn (Ban et al., 2005) and, particularly, carbon nanotubes (Dou et al., 2003; Yeoh et al., 2004, 2005, 2006; Serquis et al., 2007; Serrano et al., 2008, 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 MgB2 bulk and wire samples, based on the known literature data and our own results.