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

Abstract

Mechanical alloying of binary and ternary elemental powder mixtures with the nominal compositions of Mg₂Ni and Mg_(2-x)Mn_xNi (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 Mg_1.85Mn_0.15Ni 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.