  Strain engineering has been used to modify materials properties in
ferroelectric, superconducting, and ferromagnetic thin films. The advantage of
strain engineering is that it can achieve unexpected enhancement in certain
properties, such as an increase in ferroelectric critical temperature, Tc, by
300 to 500K, with a minimum detrimental effect on the intrinsic properties of
the material. The strain engineering has been largely applied to the materials
in thin film form, where the strain is generated as a result of lattice
mismatch between the substrate and component film or between layers in
multilayer structures. Here, we report the observation of residual thermal
stress/strain in dense SiC-MgB2 superconductor composites prepared by a
diffusion method. We demonstrate that the thermal strain caused by the
different thermal expansion coefficients between the MgB2 and SiC phases is
responsible for the significant improvement in the critical current density,
Jc, the irreversibility field, Hirr, and the upper critical field, Hc2, in the
SiC-MgB2 composite where the carbon substitution level is low. In contrast to
the common practice of improving the Jc and Hc2 of MgB2 through chemical
substitution, by taking advantage of residual thermal strains we are able to
design a composite, which shows only a small drop in Tc and little increase in
resistivity, but a significant improvement over the Jc and Hc2 of MgB2. The
present findings open up a new direction for manipulation of materials
properties through strain engineering for materials in various forms.
