Papers by Keyword: Reactive Milling

Abstract: Magnesium-based hydrogen storage materials were prepared by reactive milling of magnesium under hydrogen atmosphere with crystallitic carbon, prepared from anthracite coal, as milling aid. The XRD analysis shows that in the presence of 30 wt.% of crystallitic carbon, the Mg easily hydrided into β-MgH₂ of crystal grain size 29.7 nm and a small amount of γ-MgH₂ after 3 h of milling under 1 MPa H₂. The enthalpy and entropy changes of the hydrogen desorption reaction are 42.7 kJ/mol and 80.7 J/mol K, respectively, calculated by the vant Hoff equation from the p-C-T data in 300-380°C.

Abstract: Magnesium is light, abundant and it can store up to 7.6 wt. % of hydrogen forming MgH₂ and accordingly it is a promising material for hydrogen storage. Processing of Mg-based mixtures by high-energy ball milling (HEBM) can produce materials with high level H-sorption properties. In the present report, we display and compare the effects of different nanocrystalline additives (MgF₂, Fe, NbH_0,89, FeF₃, VF₃) on the formation of MgH₂ by reactive milling. The H-desorption behavior of the as-prepared nanocomposites is also evaluated. A combined catalytic effect is observed due to the synergic action of MgF₂ and Fe (or NbH_0,89) on the hydrogenation rate during processing. The transition metal fluorides promote as well the MgH₂ synthesis. By using more energy-intensive milling conditions and adequate additives in given proportions (e.g. 5 mol. % FeF₃), is shown to be very effective for a full and fast synthesis (4 h) of MgH₂ by reactive milling.

Abstract: Production of nanometric ceramic powders is one of the most recent advances in materials science. However the large scale production of some materials is still a challenge. There are two approaches to the fabrication of nanomaterials that results in powders with distinct characteristics. In high energy milling the particle size is reduced by mechanical forces to achieve nanosized particles. Another technique is reactive milling in which nanometric particles are synthesized by mechanically activated reactions. In this work NbC nanoparticles were produced by high energy milling of commercial NbC and by self-sustained high energy reactive milling of Nb₂O₅-Al-C powder mixture. The NbC particles were desagglomerated for 1h in a planetary mill. The obtained powders were characterized by X-ray diffraction, scanning electron microscopy and laser diffraction. The objective of this study was to compare the efficiency of two employed techniques to determine the most of producing nanoscale NbC.

Abstract: The in-situ Fe based nanocomposite containing Al₂O₃ particle is synthesized by reactive milling of Fe₂O₃-Al-Fe powder mixture in toluene medium followed by consolidation of powders using Spark Plasma Sintering process. Transmission electron microscopy investigation of consolidated Fe-Al₂O₃ nanocomposites has shown heterogenous grain structure of Fe consisting of nano, submicron and micron size grains together with nanometer Al₂O₃ particles. The hardness of Fe-Al₂O₃ nanocomposites consolidated at 800°C is 795 MPa.

Abstract: Magnesium-based hydrogen storage powders were prepared by reactive milling under hydrogen atmosphere. The crystallitic carbon, prepared from anthracite coal by demineralization and carbonization, was used as milling aid and synergic hydrogen storage additive of magnesium. Dispersive powders of particle size about 20 to 60 nm and hydrogen capacity of 4.78 wt.% were prepared from magnesium with 40 wt.% of crystallitic carbon by 3 h of milling under 1 MPa of hydrogen atmosphere. The hydrogen stored in carbon increased with the addition of Al, Mo, Co and Fe. FT-IR showed that the carbon atoms at the edges of crystallitic carbon particles were hydrogenated into C-H during reactive milling with hydrogen. The initial dehydrogenation temperature of hydrogen-storage material 60Mg40C is 275.8 °C, and its dehydrogenation plateau pressure at 300 °C is 0.2 MPa and the length of the plateau is 5.0 wt.% of hydrogen capacity.

Abstract: Magnesium complex hydrides as Mg2FeH6 are interesting phases for hydrogen storage in the solid state, mainly due to its high gravimetric and volumetric densities of H2. However, the synthesis of this hydride is not trivial because the intermetallic phase Mg2Fe does not exist and Mg and Fe are virtually immiscible under equilibrium conditions. In this study, we have systematically studied the influence of the most important processing parameters in reactive milling under hydrogen (RM) for Mg2FeH6 synthesis: milling time, ball-to-powder weight ratio (BPR), hydrogen pressure and type of mill. Low cost 2Mg-Fe mixtures were used as raw materials. An important control of the Mg2FeH6 direct synthesis by RM was attained. In optimized combinations of the processing parameters, very high proportions of the complex hydride could be obtained.

Abstract: Magnesium-based hydrogen-storage materials were prepared by reactive ball-milling under hydrogen atmosphere. It was shown that crystallitic carbon from anthracite carbonization was an effective milling aid for magnesium. Dispersive nano-particles about 20 to 60 nm were prepared from magnesium with 35 wt.% of crystallitic carbon additive by milling for 3 h under 1 MPa of hy-drogen atmosphere. The magnesium hydrided into MgH2 and the crystallitic carbon was endowed with C=CH₂ functional group during milling. The hydrogen-storage materials were used for the hy-drodesulfurization of CS₂ and thiophene, and H₂S and MgS yielded after reaction. To add molybdenum into the hydrogen-storage materials was in favor of the hydrogenation of sulfo-compounds.

Abstract: Hydrogen storage in solid hydrides is the most attractive method of on-board hydrogen storage in fuel cell for cars. Mg metal exhibits a high-storage capacity by weight and has been considered a group of potentially attractive candidates for solid-state hydrogen storage. In this study, mechanochemical synthesis of nanocrystalline Mg-based hydrogen storage composites from various starting materials in specialized hydrogen ball mills has been achieved. The reactive synthesis process and the hydrogen desorption behaviors of the composite hydrides were investigated by X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC). The results show that nano-sized MgH2 and Mg(AlH4)2 could be directly synthesized by pure Mg and pretreated Al powder, as well as Mg-Li-Al alloy powder. Alloying element Li could remarkably promote the synthesis of magnesium alanate, the product composite hydrides releasing 6.2wt% H2 through multi-step decompositions, of which the starting endothermic peaks are as low as 65°C.

Abstract: One possible route for the production of nanometric powders is the reactive high-energy milling. For a variety of systems of highly exothermic reactions, the milling can lead to self-sustaining reactions, with the reaction being observed after an induction or ignition time, which produces a temperature increase in the reactants. In this work, WC powder was obtained by reactive high energy-milling, performed in a SPEX 8000 shaker/mill. During milling the highly exothermic displacement reaction of reduction of the WO3 by Mg was performed in presence of carbon to produce WC and MgO. The material to ball mass ratio was fixed in 4:1 and the ignition time of the reaction was determined. In order to characterize the transformations from reactant powders to reaction products, the milling was stopped at given times before, immediately after and after the reaction; the powders obtained were characterized by X-ray diffraction, scanning electron microscopy and specific surface area. Depending on the amount of carbon, W and the W2C were also observed as reaction products. The complete formation of WC was achieved with addition of an excess of carbon.