Improvement of Upper Critical Field and Critical Current Density in Single Walled CNT Doped ${\hbox{MgB}}_{2}/{\hbox{Fe}}$ Wires

We evaluated the doping effect of single walled carbon nanotubes (SWCNT) on the phase formation, microstructure, critical current density (J_c), and upper critical field (H_c2) of MgB₂ superconductor. All samples were sintered at temperatures ranging from 650 to 1000degC for 30 min. A typical high sintering temperature (900degC) sample was composed of well-consolidated MgB₂ grain of 100 to 200 nm and nanosized particles, resulting in enhancement of J_c and H_c2. This indicates that lattice defects and microstructural change have occurred as a result of carbon (C) coming from SWCNT. On the other hand, a sample sintered at low temperature (700degC) contained relatively large grains. Specifically, SWCNT doped MgB₂ wire sintered at 900degC exhibited excellent J_c > 10⁴ Acm^-2 up to 8 T at 4.2 K. This result indicates that flux pinning for sample produced at a high sintering temperature are enhanced by SWCNT doping. SWCNT is one of the promising C sources for MgB₂ superconductor with excellent J_c.

I. Introduction

With a high critical temperature of 39 K, MgB₂ superconductor is one of the most promising materials for applications [1]. However, the current density of un-doped MgB₂ is sharply depressed due to poor flux pinning as the magnetic field is increased. From the applications point of view, the effect of carbon (C) doping on the upper critical field and of the MgB₂ is critical. Recently, many groups have studied C containing compounds and composites, such as SiC, B₄C, and carbon nanotubes (CNTs), finding that they improved the and of MgB₂ [2]–[6]. A record high of 29 T and of 37 T have been achieved for nano-SiC doped wire at 4.2 K. However, SiC doped MgB₂ has a poor thermal and electrical properties. On the other hand, CNTs have excellent thermal, electrical, and mechanical properties [7]. For example, CNTs can carry current densities up to 10⁹ to 10¹⁰Acm⁻² and remain stable for extended periods of time. The CNT addition may improve the connectivity between grains. The thermal conductivity for CNTs is about 3000 Wm⁻¹K⁻¹ and could benefit the heat dissipation and thermal stability of the MgB₂ wires. In addition, CNTs have axial strength and stiffness, approaching that for ideal C fiber. If aligned, CNTs could improve the mechanical properties of MgB₂-CNT composite wire. What is worth noting is that CNT can be used as a C source. In the literature, differences in the reactivity of CNT are attributed to the number or size of CNT open end [8]. In general, as the single walled CNT (SWCNT) have a much smaller diameter compared to multi walled CNTs (MWCNTs), a much larger number of open ends can be introduced in SWCNTs. In our study, therefore, we evaluated the doping effect of SWCNT on MgB₂ superconductor. Table I Specifications of SWCNT

	SWCNT
Purity	>90%
Outer diameter [nm]	1–2
Length	5–15
Impurity components	Amorphous C, Mg, Co, Mo, SiO2

Fig. 1.

High resolution transmission electron microscope (HRTEM) image of SWCNTs. The SWCNTs were assembled into bundles. The diameter of each SWCNT ranged from 0.9 to 1.8 nm.