Mechano-chemical activation synthesis (MCAS) of disordered Mg(BH4)2 using NaBH4

Mechano-chemical activation synthesis (MCAS) of a powder mixture ratio 2NaBH4 and MgCl2 was carried out by ball milling in a magneto-mill from 1 to 100 h duration with the objective of initiating a metathesis reaction 2NaBH4 + MgCl2 → Mg(BH4)2 + 2NaCl. It is found from X-ray diffraction (XRD) that the reaction has been only partially completed resulting in the formation of NaCl and a (Na,Mg)BH4 solid solution. The solutionizing effect of Mg becomes more intense as milling time increases. No XRD peaks of crystalline Mg(BH4)2 are observed. Although the result of XRD analysis in the present work does not indicate the presence of a crystalline Mg(BH4)2 hydride, the similarity of thermal events in the DSC test of the milled powder mixtures to those of crystalline Mg(BH4)2 [D.S. Stasinevich, G.A. Egorenko, Russ. J. Inorg. Chem. 13 (1968) 341-343; V.N. Konoplev, V.M. Bakulina, Izv. Akad. Nauk SSSR 1 (1971) 159-161] indicates the possibility of presence of a “disordered” Mg(BH4)2 in the in the milled powder mixture, as specifically proposed by Nakamori et al. [Y. Nakamori, K. Miwa, A. Ninomiya, H. Li, N. Ohba, S.-I. Towata, A. Züttel, S.-I. Orimo, Phys. Rev. B74 (2006) 045126; Y. Nakamori, H.-W. Li, K. Kikuchi, M. Aoki, K. Miwa, S. Towata, S. Orimo, J. Alloys Compd., in press, doi:10.1016/j.jallcom.2007.03.144]. It is hypothesized that the presence of (Na,Mg)BH4 solid solution in the MCAS process inhibits the formation of crystalline Mg(BH4)2. Regardless of the nature of the synthesized hydride phase the milled powders desorb hydrogen within the 275-420 °C temperature range and the total amount of hydrogen desorbed gradually decreases with increasing milling time reaching ∼2.1 wt.% for the powder milled for 100 h. However, regardless of the milling time the total amount of hydrogen desorbed within 275-420 °C is always less than the theoretical hydrogen capacity in the initial mixture of 2NaBH4 and MgCl2 (∼4.7 wt.%).