Effects of Si and C doping on the superconducting properties of mgb2

C and Si powders of different sizes were doped into MgB2 separately or together. Samples were made by a solid-state reaction method. It was found that the C doping had a strong negative effect on critical temperature (T,) while the Si doping did not significantly depress T-c. All of these doping materials increased J(c) at higher fields when the doping particles were of nanometer sizes. At 20 K and 4 T, J(c) of the sample doped with nano-Si achieved 10(4) A/cm(2), which is 2 orders of magnitude higher than that of coarse Si doped MgB2. J(c) of the sample doped with nano-carbon is one order of magnitude higher than for the sample doped with coarse carbon particles.