The phase composition and microstructure evolution of pure and carbon-doped MgB_(2)bulks exposed to hydrochloric acid(pH=2)for different time were investigated by X-ray diffraction(XRD)and Scanning electron microscopy(SEM)systematically.XRD.results show the decomposition of pure and carbon-doped MgB_(2)bulks exposed to acid is rapid decay with exposure time.The decomposition reaction will finish completely after 5 h exposure and the main solid products are B(OH)_(3)and MgCl_(2)(6H_(2)0).The SEM results demonstrate that the reaction among pure and carbon-doped MgB_(2)as well as HC1 occurs at grain boundaries.But the grain connection of carbon-doped MgB_(2)is better than that of pure MgB_(2)after acid exposure.The T_(c)of MgB_(2)decreases after 10 min acid exposure,but T_(c)is almost unchanged in carbon-doped MgB_(2)after the same exposure.This result indicates that substitution of C for boron in MgB_(2)can improve of the resistance to the corrosion of acid.
The superconducting properties of polycrystalline Sr0.6K0.4Fe2As2 were strongly influenced by Ag doping(Supercond.Sci.Technol.23(2010) 025027).Ag addition is mainly dominated by silver diffusing,so the annealing process is one of the essential factors to achieve high quality Ag doped Sr0.6K0.4Fe2As2.In this paper,the optimal annealing conditions were studied for Ag doped Sr0.6K0.4Fe2As2 bulks prepared by a one-step solid reaction method.It is found that the annealing temperature has a strong influence on the superconducting properties,especially on the critical current density Jc.As a result,higher heat treatment temperature(~900℃) is helpful in diffusing Ag and reducing the impurity phase gathered together to improve the grain connectivity.In contrast,low-temperature sintering is counterproductive for Ag doped samples.These results clearly suggest that annealing at ~900℃ is necessary for obtaining high Jc Ag-doped samples.
The crystal structure and the superconductivity for samples Mg（B1-xCx）2 （0〈 x 〈0.09） prepared by a hybrid microwave synthesis have been investigated. The starting material B10C is also obtained by using the microwave method. The carbon can distribute uniformly in the Mg（B1-xCx）2 samples because boron and carbon are mixed on an atomic scale in the staring material B10C. The dependences of both lattice parameters and superconducting transition temperature Tc on carbon content accord with those reported in the literature. The upper critical field He2 at 20 K can be enhanced from about 4.3 T for x = 0 to 10 T for x = 0.05. The critical current density Jc of Mg（B0.95 C0.05）2 is 1.05×10^4 A/cm^2 at 20 K and 1 T.
