K. Y. Tan

Reaction method control of impurity scattering in C-doped MgB2: proving the role of defects besides C substitution level

In this study, Si and C were incorporated into polycrystalline MgB 2 via in situ reaction of Mg and B with either SiC or with separate Si and C (Si+C). The electrical transport and magnetic properties of the two series of samples were compared. The corrected resistivity at 40 K, ρA(40 K), is higher for the samples reacted with SiC regardless of the carbon (C) substitution level, indicating larger intragrain scattering because of the simultaneous reaction between Mg and SiC and carbon substitution during the formation of MgB2. In addition, because of the cleaner reaction route for the samples reacted with SiC, the calculated active area that carries current, AF, is twice that of the (Si+C) samples. On the other hand, the upper critical field, Hc2, was similar for both sets of samples despite their different C substitution levels, which proves the importance of defect scattering in addition to C substitution level. Hence, the form of the precursor reactants is critical for tuning the form of Hc2(T).

Optimizing C Incorporation Into Magnesium Diboride

In this work, either SiC or separate Si and C (Si+C) powders of up to 10 weight percentage were in situ reacted with Mg and B (molar ratio of 1:2) and the superconducting properties compared. The latter shows a smaller a -axis lattice cell parameter as compared to the same level of SiC additions, indicating a higher level of C substitution. In those samples, a larger increase in the c -axis is also noticeable and a more severe degradation in the superconducting transition temperature is observed. However, reaction with SiC causes a greater broadening in the transition width. At both 5 K and 20 K, for samples reacted with SiC (up to 5 wt.%) a stronger improvement in the magnitude of critical current density, Jc, is obtained. On the other hand, samples reacted with (Si+C) show a weaker Jc dependence on field at 5 K because of higher C substitution. At 20 K, the Jc decreases more rapidly with field for the same level of SiC additions. Hence, for high temperature (20 K) and low field ( <;5 T) applications, reaction with SiC is preferred for obtaining higher Jc(H).

X-Ray Powder Diffraction Study on the MgB2 Superconductor Reacted with Nano-SiC: The Effects of Sintering Temperature

SiC added MgB2 polycrystalline samples were synthesized at low (650°C) and high (850°C) temperatures in order to study the sintering effect on the phase formation and superconducting properties. The MgB2 bulks with additions of 0wt%, 1wt%, 3wt% and 5wt% SiC were studied with powder X-ray diffraction technique. We observed that MgB2 remained as the primary phase for both sintering temperatures in all samples with the presence of MgO and Mg2Si as the main impurities. Some diffraction peaks associated with unreacted SiC is also noticeable. The relative intensity of the Mg2Si peaks was found to decrease in samples sintered at higher temperature. Temperature dependent magnetic moment measurements showed that the superconducting transition temperature, Tc decreases as the SiC addition level increases while lower sintering temperature degrades Tc to a greater extent. The changes in the physical properties is discussed based on the results of phase formation, full width half maximum (FWHM), lattice parameter and crystallite size.