Influence of the borohydride concentration on the composition of the amorphous Fe-B alloy produced by chemical reduction of synthetic, nano-sized iron-oxide particles Part I: Hematite

摘要

Amorphous Fe-B alloys can be prepared at room temperature by reduction with borohydride of iron-oxide particles in suspension. By varying the borohydride concentration, amorphous Fe-B alloys with boron contents between 2 and 13 at. percent have been produced by reduction of synthetic (nano-sized particles) and natural (micro-sized) hematite (alpha-Fe_2O_3) using sodium borohydride (NaBH_4). The results presented in this paper were obtained from a systematic study of the effect of borohydride concentration on the resulting reaction products using a variety of experimental techniques, such as X-ray diffraction, wet chemical analyses, thermal analyses, scanning electron microscopy, transmission Mossbauer spectroscopy (TMS) and integral low-energy electron Mossbauer spectroscopy (ILEEMS). Three distinct NaBH_4 concentrations have been applied. Beside unreacted hematite, amorphous Fe_(1-x)B_x alloys have been identified from the TMS spectra recorded at various temperatures between 15 K and room temperature. The amount of Fe_(1-x)B_x increases strongly with increasing NaBH_4 concentration, and for a given concentration with increasing specific surface area (SSA). Thermal analyses have suggested that for any given reduction condition, the boron contents in the formed amorphous alloy has a bimodal distribution. This is found to be consistent with the finding that the contribution of the Fe_(1-x)B_x phase to the total Mossbauer spectra consists of a superposition of a broad sextet and doublet. ILEEMS has further revealed that especially the surface layers of the hematite grains are affected by the reduction processes.