Nano-Crystalline Mg(2-x)MnxNi Compounds Synthesized by Mechanical Alloying: Microstructure and Electrochemistry

Mechanical alloying of binary and ternary elemental powder mixtures with the nominal compositions of Mg2Ni and Mg(2-x)MnxNi (x= 0, 0.05, 0.10 and 0.15 at. %) were carried out in a planetary ball mill for various milling times of 5, 10, 15, 20, 30 and 60 h. X-ray diffraction and field emission scanning and transmission electron microscopy were used for the characterization of the milled products. Clusters of Mg2Ni-based nano-crystals were produced after 10 h of milling using the binary powder mixture. However, the formation kinetic of Mg2Ni-based structure was found to increase by increasing Mn content. In addition, Mn was found to decrease Mg2Ni crystallite size during milling; a mean Mg2Ni crystallite size of ~6 nm was achieved by high energy ball milling of the initial ternary powder mixture of Mg1.85 Mn0.15Ni after 60h. The milled product consisted of Mg2Ni-based crystallites surrounded by amorphous regions. Addition of Mn to Mg2Ni resulted in increased electrode discharge capacity of the ternary Mg-Ni-Mn milled product compared to that of the binary Mg2Ni. Moreover, discharge capacity of the milled product was found to increase by increasing milling time. However, this was not followed after 30h of milling possibly due to substantial powder oxidation and formation of MgNi2 phase.