The influence of mechanical milling parameters on hydrogen desorption from Mgh2-Wo3 composites
Introduction
One of the most popular methods used in the synthesis of nanostructured and amorphous materials is mechanical milling which represents the way toward green chemistry. Mechanical milling attracts the attention due to economic price, simple handling and no need for a solvent. During mechanical milling a material undergoes to the reduction of grain size, increase in number of grain boundaries and change of surface area [1], [2], [3]. As a complex process, mechanochemical synthesis involves optimization of a numerous parameters such as: type of mill (high-energy and low-energy mills), properties of milling container (material, size and shape), milling speed, type and size of milling balls, milling time, ball-to-powder weight ratio (BPR), vial's fill level, temperature and atmosphere (air, nitrogen, argon, etc.) that have an impact on the final product [4]. Among them, milling speed, milling time and BPR (that is an energy input) play an important role on the efficiency of the milling process. Nevertheless, one has to be aware of the fact that various difficulties can arise during milling such as: contamination of the sample by the material of the milling tools (the most common contamination is with Fe and WC [5]), increase of the temperature of the vial, and in some cases the balls could be pinned to the walls of the vial making no impact force on the powder [6]. The BPR ratio is proved to be one of the most important variables during mechanical milling. Different BPR ranging from 1:1 [7], [8] up to 240:1 has been used [9], although the ratio 10:1 is used in the most of the experiments [10], [11], [12]. It is expected that higher BPR ratio will increase the rate of particle size reduction rate [6], [13], [14]. However, one should be aware of the fact that high BPR in combination with prolonged milling time leads to cold welding of particles. It also leads to pronounced contamination with Fe caused by the collision of milling balls and vial walls, as we have mentioned.
Mechanical milling under diverse conditions (different type of mills [15], [16], [17], BPR [18], [19], [20], different milling atmospheres [21], [22], [23]) is widely used as a method to improve sorption kinetics of various hydrides [24], [25], [26], [27], [28] and mostly magnesium hydride MgH2 [29], [30], [31], [32], [33]. Regarding MgH2, the most common way to improve sorption kinetics is mechanical milling with the addition of transition metals – TM (Nb, V, Ti, Mn, Fe, Co, Ni, Cu, Zn) [34], [35], [36], [37] or transition metal oxides –TMO (Nb2O5, Cr2O3, V2O5, VO2, TiO2, WO3, MoO2, Mn2O3) [38], [39], [40], [41], [42], [43]. Zaluska et al. reported that some of the effects of milling that improve kinetics are particle and grain size reduction, as well as the increase of quantity of defects [44]. Mechanical milling of the MgH2 with TM or TMO introduces structural defects, increases specific surface area and the number of active sorption sites. Additives can act as efficient milling reagent for MgH2 and can improve the hydrogen sorption by reducing particle size and desorption temperature [32]. The TM or TMO chemisorbs hydrogen and transfers it to the Mg matrix [45], [46]. The interface between the Mg and TM acts as an active nucleation site for the hydride phase [47], [48].
This work deals with the influence of different milling conditions and different additive concentrations on the desorption properties of MgH2 –WO3 milling blends. Due to the fact that tungsten is a multivalent metal, it is expected that the addition of the oxide WO3 might produce an improvement in hydrogen sorption, similar to those obtained by VO2 or TiO2 addition [43], [49].
Section snippets
Material and methods
MgH2-WO3 composites were prepared by mechanical milling of the as-received MgH2 powder (Alfa Aesar, 98% purity) with the addition of 5, 10 and 15 wt% of WO3 (Koch Light Labs, 99.9% purity). 10 wt% is commonly used quantities for additives in magnesium-based composites although amounts can go up to 50 wt% [26], [34], [37], [40], [41], [46]. With the addition of 10 wt% of additive the capacity of MgH2 remains above satisfactory 5 wt% of hydrogen. Mechanical milling was performed in two mills SPEX
Results
XRD profiles of composites with different quantity of WO3 (5, 10 and 15 wt%) obtained by mechanosynthesis in SPEX 5100 and SPEX 8000 M are presented in Fig. 1 and Fig. 2, respectively. Reflections corresponding to tetragonal β-MgH2, metallic Mg and WO3 are present in all samples. The broadening of typical peaks corresponding to β-MgH2 tetragonal structure is observed in all composites indicating the reduction in crystallite size and accumulation of mechanical strain. Debye-Scherrer formula [37]
Conclusions
This paper deals with the influence of different milling conditions on MgH2-WO3 milling bland obtained by two SPEX mills (5100 and 8000 M). It has been shown by SEM analysis that even though the energy input is different, the dispersion of additive particles is uniform in both cases. On the other hand, the mechanical milling in SPEX mill 5100 leads to a bimodal particle size distribution, while milling in SPEX 8000 M gives polymodal distribution. DSC results show that better reduction of
Acknowledgment
This research was financially supported by The Ministry of Education, Science and Technology of the Republic of Serbia under grant III 45012. This work is also supported by the Environmental Protection and Energy Efficiency Fund of the Republic of Croatia and the Croatian Science Foundation, project no. PKP-2016-06-4480.
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