Amorphous transition metal-boron ultrafine particles prepared by chemical methods

Ultrafine amorphous transition metal-boron (TM-B, TM = Co, Ni, Fe) powders have been prepared, under identical experimental conditions, by chemical reduction of Co2+, Ni2+, and Fe2+ ions with potassium borohydride in aqueous solution. The chemical compositions of the samples are close to TM60B40. The particles have a spheroidal shape with diameters ranging from 10 to 100 nm. X-ray diffraction and transmission electron microscopy investigations show that the Co-B and Ni-B samples are fully amorphous while in a few particles of the Fe-B sample the presence of α-iron microcrystallites are observed. Crystallization of the samples has been investigated by differential scanning calorimetry. Under anaerobic conditions the Co-B sample evolves toward t-Co2B accompanied by a minor quantity of o-Co3B; in the Ni-B and Fe-B samples, crystallization gives rise to a metallic component plus boride phases. A significant decrease of the boride phases and a corresponding increase of the metallic component are obtained in all samples when the thermal treatment is performed in the presence of traces of oxygen.