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Raman spectroscopy is employed to study both the strain and the carbon substitution level in SiC-doped MgB2 bulk samples. Raman spectroscopy was demonstrated to be a better method to distinguish the individual influences of strain and carbon than standard X-ray diffraction. It is found that the lattice parameter correlation method for C content determination is invalid for highly strained samples. Our result also provides an alternative explanation for lattice variation in non-carbon-doped MgB2, which is basically due to lattice strain. [ABSTRACT FROM AUTHOR]

Abstract: In the past 5 years, various kinds of doping of MgB2, including single elements (metal and non-metal), silicates, various carbon sources, and other compounds have been investigated and reported. Most nanoparticle doping leads to improvement of critical current density, J c(H), and performance, but some types show a negative effect. In this paper, the effect of carbon doping on J c and the upper critical field, H c2, of MgB2 is reviewed. Carbon substitution effects make two distinguishable contributions to the enhancement of J c field performance: increase of H c2 and improvement of flux pinning, both because carbon substitutes for boron in the MgB2 lattice. Among all the carbon sources so far, nano-SiC has been confirmed to be the most effective dopant to enhance the J c in magnetic fields and H c2. An irreversibility field, H irr, of 10T has been achieved with nano-SiC doping at 20K, exceeding H irr of NbTi at 4.2K. Besides that, H c2 of carbon alloyed MgB2 film has reached the value of 71T. The significant enhancement in J c(H) and H c2 via carbon substitution has provided great potential for practical applications of MgB2. The dual reaction model proposed by the authors’ group provides a comprehensive understanding of the mechanism of enhancement in J c and H c2 by chemical doping. Further improvement in self-field J c performance while maintaining the already achieved in-field performance remains as a major challenge in the development of MgB2. [Copyright &y& Elsevier]

Abstract: We studied the effects of MgB2 with embedded multiwall carbon nanotubes (MWCNTs) on the crystallinity, lattice parameters, critical current density (J c), upper critical field (H c2), irreversibility field (H irr), and microstructure of MgB2. Fe sheathed un-doped and MWCNT doped MgB2 wires were fabricated by the powder-in-tube (PIT) method and sintered at the high sintering temperatures of 900°C. We observed that for the MWCNT doped sample high temperature sintering resulted in depressed crystallinity, shrinkage of the a-lattice parameter, higher J c up to 12T, and lower critical temperature (T c) values. Specifically, MWCNT doped samples sintered at 900°C exhibited excellent J c, ∼104 Acm−2 up to 9T at 4.2K. This can be explained by lattice distortion and poor crystallinity due to carbon (C) substitution from the MWCNT. [Copyright &y& Elsevier]

The effect of doping MgB[sub 2] with carbon nanotubes on transition temperature, lattice parameters, critical current density and flux pinning was studied for MgB[sub 2-x]C[sub x] with x=0, 0.05, 0.1, 0.2, and 0.3. The carbon substitution for B was found to enhance J[sub c] in magnetic fields but depress T[sub c]. The depression of T[sub c], which is caused by the carbon substitution for B, increases with an increasing doping level, sintering temperature, and duration. By controlling the extent of the substitution and addition of carbon nanotubes we can achieve the optimal improvement on critical current density and flux pinning in magnetic fields while maintaining the minimum reduction in T[sub c]. Under these conditions, J[sub c] was enhanced by two orders of magnitude at 8 T and 5 K and 7 T and 10 K. J[sub c] was more than 10 000 A/cm2 at 20 K and 4 T and 5 K and 8.5 T, respectively. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]