Skip to Main Content
An alternative mechanism for improvement of Jc has been reported by the author's group in SiC-MgB2 composite which was made by pre-mixing SiC and B, followed by Mg diffusion and reaction. In contrast to the common practice of improving the Jc, and Hc2 of MgB2 through chemical substitution, it shows only a small decrease in the critical temperature, Tc, and little increase in resistivity, ρ. The further analysis indicated that, there is no SiC decomposition and C substitution, the enhancement of properties is induced by thermal strain caused by the different thermal expansion coefficients (α) of the MgB2 and SiC phases for SiC-MgB2 composite. The thermal strain in the MgB2 phase was demonstrated with x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. By taking advantage of residual thermal strains, we are able to design a composite with only a small decrease in Tc, and little increase in ρ, but a significant improvement in Jc and Hc2. The strain engineering was applied to the grapheme doping to MgB2 where graphene has low to negative thermal expansion coefficient. It was found that the graphene doping at even 1% level achieved the optimally Jc(H) performance (1×104 A/cm2 at 5 K, 8 T), compared to the level for other carbon containing dopants at 5-10% level. The upper critical field has been enhanced to 13 T at 20 K for the optimal doping level. Another unique feature for grapheme doping is the very low resistivity, good grain connectivity in low field range and following δT pinning rather than δl for near all doped MgB2. The Raman studies show that the active E2g mode was split into two parts: the softened mode corresponding to tensile strain and the hardened mode attributed to the carbon substitut- on effect.