In situ resistance measurements and low temperature synthesis of superconducting MgB2 bulk samples

Superconducting MgB2 bulk samples were simply sintered by heating stoichiometric mixture of Mg and B in vacuum. In situ resistance measurements were performed carefully in order to find the best annealing process. The in situ resistance measurements exhibit a decrease in resistance when the heating temperature is increased. The measurements suggested that the Mg powder melting temperature was below 650°C and the temperature of 800°C was higher than required. A MgB2 bulk sample sintered at 750°C has a superconducting transition temperature of Tc(R=0)=40.3K, higher than that sintered at 680 or 810°C.