  Bulk samples of MgB2 were prepared with 5, 10, and 15% wt.% Y2O3
nanoparticles added using a simple solid-state reaction route. Transmission
electron microscopy (TEM) showed a fine nanostructure consisting of ~3-5 nm YB4
nanoparticles embedded within MgB2 grains of ~400 nm size. Compared to an
undoped control sample, an improvement in the in-field critical current density
Jc was observed, most notably for 10% doping. At 4.2K, the lower bound Jc value
was ~2x105A.cm-2 at 2T. At 20K, the corresponding value was ~8x104A.cm-2.
Irreversibility fields were ~11.5 T at 4.2K and 5.5T at 20K, compared to ~4T
and ~8T, respectively, for high-pressure synthesized bulk samples.
