Effect of Cu addition in reduction of MgO content for the synthesis of MgB2 through sintering

The minor Cu addition (<3 at%) could apparently reduce the amount of MgO impurity in the in situ sintered MgB2 samples, accompanying with the improvement in the critical transition temperature (Tc) from 36.4 K for the undoped sample to 38.6 K for the samples with 3 at% Cu addition. The decrease of MgO impurity within Cu-doped MgB2 samples can be attributed to the following factors: (i) the formation of local Mg–Cu liquid at about 485 °C wrapped the neighboring Mg particles and thus protected them from the oxidation before the melting point of Mg; (ii) when the sintering temperature increased above the melting point of Mg, the minor Cu addition could also decrease the vapor pressure of Mg liquid and then reduce the volatilization of Mg liquid, which can depress the reaction between gaseous Mg and O2 impurity in the protective Ar gas forming MgO impurity. Our results provide a new route to govern the oxidation of Mg during the in situ sintering of pure or doped MgB2 samples by altering the minor amount of Cu addition.