Structural stability of sodium borohydride (NaBH4) during controlled mechanical milling

The complex hydride NaBH4 was subjected to controlled mechanical milling (CMM) under various milling modes in the magneto-mill Uni-Ball-Mill 5. X-ray diffraction was employed to study the variation of lattice parameter and crystallite (grain) size within the powder particles with milling time up to 200 h. It is found that the lattice parameter of the compound varies only modestly during prolonged milling (max. ∼0.15% after 50 h) and its rate of increase is only slightly faster under high-energy impact (IMP2) mode as opposed to low-energy shearing (LES) mode. Essentially there is no formation of true nanostructure in the powder particles and the average crystallite (grain) size remains on the order of a few tens of nanometers. A bimodal or even multimodal crystallite size distribution within powder particles of NaBH4 is observed after CMM. The results clearly demonstrate a high structural stability of the compound under heavy deformation conditions imposed by milling.