Improvement of critical current density in MgB2 superconductors by Zr doping at ambient pressure

We present the superconducting properties and phase compositions of Mg_1-xZr_xB₂ bulk samples fabricated by a solid-state reaction at ambient pressure. It is found that a small amount of Zr atoms may be introduced into the lattice of MgB₂, while the majority of them forms ZrB₂ phase. The Mg_0.9Zr_0.1B₂ sample shows the highest J_C of 2.1×10⁶A/cm² in 0.56 T at 5 K and 1.83×10⁶A/cm² in self-field at 20 K, higher irreversibility field and larger upper critical field in MgB₂ bulk samples. The combination of good grain connection, the reduction of grain size and small ZrB₂ particles in the sample may be responsible for the significant enhancement of J_C in Zr-doped samples. This technique has a great potential to prepare high performance MgB₂ bulk samples and wires on an industrial scale.