Effects of Boron Purity, Mg Stoichiometry and Carbon Substitution on Properties of Polycrystalline MgB2

By synthesizing MgB2 using boron of different nominal purity we found values of the residual resistivity ratio (RRR = R(300K)/R(42K)) from 4 to 20, which covers almost all values found in literature. To obtain high values of RRR, high purity reagents are necessary. With the isotopically pure boron we obtained the highest RRR ∼ 20 for the stoichiometric compound. We also investigated Mgx 11B2 samples with 0.8 < x < 1.2. For the range Mg0.8 11B2 up to Mg1.2 11B2 we found average values of RRR between 14 and 24. For smaller variations in stoichiometry (x = 1±0.1) RRR = 18±3. All of our data point to the conclusion that high RRR (∼ 20) and low ρ0 (≤ 0.4μΩcm) are intrinsic material properties associated with high purity MgB2. In addition we have performed initial work on optimizing the formation of carbon doped MgB2 via the use of B4C. Nearly single phase material can be formed by reaction of nominal Mg(B0.8C0.2)2 for 24 hours at 1200 ◦C. The Tc for this composition is between 21.9K and 22.7K (depending on criterion).