Significantly enhanced critical current density in nano-MgB₂ grains rapidly formed at low temperature with homogeneous carbon doping

High performance MgB₂ bulks using carbon-coated amorphous boron as a boron precursor were fabricated by Cu-activated sintering at low temperature (600 °C, below the Mg melting point). Dense nano-MgB₂ grains with a high level of homogeneous carbon doping were formed in these MgB₂ samples. This type of microstructure can provide a stronger flux pinning force, together with depressed volatility and oxidation of Mg owing to the low-temperature Cu-activated sintering, leading to a significant improvement of critical current density (J_c) in the as-prepared samples. In particular, the value of J_c for the carbon-coated (Mg_1.1B₂)Cu_0.05 sample prepared here is even above 1 × 10⁵ A cm⁻² at 20 K, 2 T. The results herein suggest that the combination of low-temperature Cu-activated sintering and employment of carbon-coated amorphous boron as a precursor could be a promising technique for the industrial production of practical MgB₂ bulks or wires with excellent J_c, as the carbon-coated amorphous boron powder can be produced commercially at low cost, while the addition of Cu is very convenient and inexpensive. (paper)