Synthesis of MgB2 by exposure of polycrystalline boron to magnesium vapor

Polycrystalline boron in lump form was reacted with magnesium vapor inside steel tubes at 900 C to produce MgB₂ using four to five times the stoichiometric requirement of Mg. The boron lumps were encased inside Ta foil folded over to form a boat to isolate them from the liquid Mg. These materials were sealed inside steel tubes by arc welding. Reactions were allowed to proceed for times ranging from 2.5 to 837 hours. Upon opening a tube after a 42-hour reaction time a gray powder was removed which X-ray diffraction indicated was MgB₂. SEM examination of the surfaces of the powder particles revealed a dense layer of 1-2 micron diameter crystallites which appear to be small plates with hexagonal symmetry. Metallographic examination of sections cut through the particles indicated the presence of a significant volume fraction of unreacted boron. The reaction appears to have proceeded initially along the grain boundaries in the polycrystalline lumps breaking them up into particles corresponding to the grain size. Magnetic susceptibility measurements on the powder and resistivity measurements on a cold pressed pellet show a sharp transition at 39 K despite the presence of unreacted boron. Even after reaction times up to 837 hours there was still a significant amount of unreacted boron.