Structural and calorimetric studies of magnesium-rich Mg-Pd alloys

Abstract

Despite the fact that the affinities of both palladium and magnesium with hydrogen have been studied for many years, information about magnesium-palladium alloys is still incomplete. Although palladium is an extremely expensive metal and magnesium palladium alloys will likely never applied as solid-state hydrogen storage materials, the interaction of these alloys with hydrogen may be useful for broadening knowledge or finding applications as hydrogen splitters, catalysts, and hydrogen sensors. In this work, two types of calorimetric techniques were applied to measure the thermodynamic properties of magnesium-rich alloys from the Mg-Pd system (containing 97.7; 85.4; 80.6; 79.9; 72.3; 70.7; 64.5 at% of Mg respectively). The solution calorimetric method was used to determine the heat of solution of liquid magnesium and palladium in liquid tin. Additionally, the standard enthalpies of formation of six alloys corresponding to the intermetallic phases of the Mg-rich region were measured. These alloys were prepared from pure Mg and Pd, which were melted in a glove box filled with high purity argon, with a very low concentration of impurities. All the obtained alloys were structurally analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM/EDS) methods. Moreover, DSC studies were used to determine the transformation temperatures in the Mg-rich region. The values of the standard formation enthalpy of the studied alloys were determined to be −27.3 ± 0.8 kJ/mol at., −33.4 ± 0.9 kJ/mol at., −35.2 ± 1.4 kJ/mol at., −44.2 ± 0.9 kJ/mol at., −46.0 ± 0.7 kJ/mol at., and −54.3 ± 2.3 kJ/mol at. These values were compared with the data calculated using the Miedema model, as well as with existing literature data for the Mg6Pd and Mg5Pd2 phases.