Synthesis and hydrogen storage properties of Mg-based alloys
Introduction
Metal hydrides have been the focus of recent intensive research activities on hydrogen storage. Significant progress has been made on sodium alanates, advanced BCC alloys, Mg-based alloys and others [1]. Although suffering from a high thermodynamic stability, magnesium hydride is still attractive as a potential hydrogen storage material because of its high storage capacity and low cost [2], [3]. The improvement of hydrogen sorption kinetics has been achieved by various treatments [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], especially by intensive milling of MgH2 with some transition metal additives. For example, the MgH2–5 at.%V composite desorbs at 473 K (under vacuum) and re-absorbs hydrogen rapidly even at room temperature [27], [28]. Recent activities are directed to low-cost synthesis using reactive milling [29], [30] and oxide catalysts [31].
The high thermodynamic stability of MgH2 remains the major obstacle for applications. Research conducted in the last few decades showed that the plateau pressure of magnesium hydride does not change by forming composites, by surface treatment, by adding catalysts, or by forming a nanostructure unless true magnesium alloys such as Mg2Ni, Mg2Cu, Mg2Al3 are formed [2], [3], [6], [25], [28]. Some reports claimed that the plateau pressure of MgH2 can be changed by forming multiphase alloys, for example, in the Mg–Al–Y, Mg–Li–Ni–Zn [32], Mg–Fe–Ti(Mn) [33], and Mg–Zn–Y [24] systems. The changes in plateau pressures were explained by the complex multiphase nature of the composites [24], [33], however, the real physical reasons behind have not been understood or reported.
Magnesium forms solid solution and compounds with few elements under equilibrium states. Non-equilibrium processing methods, such as mechanical alloying and rapid quenching can produce new magnesium alloys and new structures such as amorphous phases, extended solid solutions, and non-stoichiometric intermetallic compounds which may alter the thermodynamic properties. However, none of these new alloys and structures is stable at elevated temperature, e.g. 300 °C, which is needed for activation of most air exposed Mg-based alloys. The multi-component single phase (usually amorphous phase) transforms to the conventional equilibrium multi-phase composites after activation which results in improved kinetics but no destabilization of the magnesium hydride [34], [35], [36], [37], [38], [39]. If the as-prepared alloy can be hydrogenated in situ without exposition to air (this was the case for Mg/Pd sputtered film [40], novel properties may be obtained.
Based on the chemical nature of MgH2, we can imagine various approaches to destabilize magnesium hydrides such as by: (I) forming Mg intermetallic compounds or complex hydrides; (II) doping MgH2 with other ions, (III) creating stress and strain to modify the Mg lattice and; (IV) forming Mg solid solution. Our recent work shows that Mg(Cd) solid solution has reduced reaction enthalpy with hydrogen compared to that of Mg [41]. In this work, we investigate the hydrogen storage properties of some other binary and multi-component Mg solid solutions made by mechanical alloying.
Section snippets
Experimental
Mg-based alloys were prepared by mechanical alloying of Mg (99.9% pure) with other elements (99.9% pure) in a Spex 8000 ball mill under the protection of argon. A hardened steel crucible and three steel balls of 12.7 mm in diameter were used for milling. The ball to powder weight ratio was 10:1. Vanadium catalyst (2.5 at.%) was added after the Mg solid solution was formed and then ball milled for an additional 20 h in order to distribute the catalyst. In addition, graphite (2 wt.%) was also added
Formation of Mg solid solution
Magnesium forms solid solutions with some elements under equilibrium states. The unit cell volume of Mg can be altered by forming solid solution. Fig. 1 shows the unit cell volume and the maximum terminal solubility of some Mg solid solutions under equilibrium conditions. The unit cell volume was derived from the lattice parameters given in [44]. Clearly some elements such as Cd, In, Li, Al, Zn, Ag, Ga and Sn reduce the Mg lattice parameters by forming solid solution, while Sc and Pb expand the
Conclusions
- (1)
Mechanical alloying of Mg with some elements leads to supersaturated solid solution. However, the Mg supersaturated solid solution is not stable. It decomposes upon thermal treatment or hydrogenation.
- (2)
The hydrogen absorption/desorption kinetics is influenced by the formation of solid solution and the complexity of the alloys. Multi-component alloys result in slow hydrogen sorption kinetics and high hysteresis.
- (3)
Alloying Zn, Al, Ag, Ga, In or Cd with Mg reduces the stability of magnesium hydride.
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