Plenary talk - strain engineering for improvement in Jc and Hirr and Hc2 in MgB2

An alternative mechanism for improvement of J_c 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 J_c, and H_c2 of MgB₂ through chemical substitution, it shows only a small decrease in the critical temperature, T_c, 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 T_c, and little increase in ρ, but a significant improvement in J_c and H_c2. 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 J_c(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 E_2g mode was split into two parts: the softened mode corresponding to tensile strain and the hardened mode attributed to the carbon substitut- on effect.