Fabrication and Superconducting Properties of ${\rm MgB}_{2}$ Doped With Polysiloxane Based Copolymers

We present the fabrication and the superconducting properties of MgB₂ ceramic samples doped with carbon via preceramic polymers: linear and cyclic polysiloxane-co-styrene and linear polysiloxane-co-vinyl-ferrocene. The samples were produced using the spark plasma sintering technique. The use of polysiloxane copolymers was suggested by the appropriate content of carbon, silicon, and oxygen which are necessary to increase the upper critical field and to create pinning centers. The short processing time limits the diffusion length of the chemical elements from the polymer into the superconducting grains. Therefore, in addition to doping with carbon, the rest of the components create pinning centers along the polymeric chain, able to better pin the flux lines. The copolymerization allowed us to obtain both linear and cyclic copolymers as well as to control the content of chemical elements, including the presence of small amounts of iron oxides. The latter are responsible for magnetic pinning.