Papers by Keyword: Hydrogen Storage Materials

Abstract: The aim of this work is to investigate the influence of some processes variables on the microstructure and hydrogen absorption kinetics of MgH₂ - X wt.% TiFe composites. Samples were synthesized by high-energy ball milling in a planetary (X = 40, 50, 60) and shaker mill (X = 40) under high-purity argon atmosphere. Commercial MgH₂ instead of Mg powder was used in order to reduce adherence on the vial and balls. TiFe powder was previously produced by ball milling a mixture of TiH₂ and Fe powders followed by a reaction synthesis at 600oC. Milled composites samples were characterized by XRD and SEM analysis. Milling time was preliminary investigated (X = 40) in the planetary ball mill (6 to 36h). TiFe particle size reduction was shown to be difficult since they are surrounded by MgH2 matrix. Strong particle reduction was obtained by using a shaker mill only for 2 hours and adding cyclohexane as process control agent. No reaction between MgH₂ and TiFe compound was observed in any milled sample. Hydrogen absorption kinetics measurements of the as-milled samples were conducted on an Sieverts' type apparatus at room temperature after hydrogen desorption at 350oC under vacuum. The best hydrogen kinetics (3 wt% at the first hour) was attained by the planetary milled sample (36 h). Higher hydrogen capacity was observed for the sample milled in the shaker mill (4.0 wt.%), but only after 13h.

Abstract: The intermetallic alloy LaNi_3.8AlMn_0.2 and its deuteride LaNi_3.8AlMn_0.2D_3.2 were studied by neutron powder diffraction. The experimental results show that the crystal structure of LaNi_3.8AlMn_0.2 is CaCu₅ type with the hexagonal P6/mmm space group, the substituted Al atoms occupy 2c and 3g sites, while Mn atoms are only located on the 3g sites. For the corresponding deuteride LaNi_3.8AlMn_0.2D_3.2, the P6/mmm space group gives the best refinement, but D atoms enter two interstitial sites 6m and 12n.

Abstract: TiFe compound was produced by high-energy ball milling of TiH₂ and Fe powders, followed by heating under vacuum. TiH₂ was used instead of Ti in order to avoid the strong particles adhesion to grinding balls and vial walls. Mixtures of TiH₂ and Fe powders were dry-milled in a planetary mill for times ranging from 5 to 40 hours. The amount of sample, number and diameter of the balls were kept constant in all experiments. After milling, samples were heated under dynamic high-vacuum for the synthesis reaction. As-milled and heat-treated materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA). The mean crystallite sizes and microstrains were determined by XRD line profile analysis using the Warren-Averbach method. As-milled materials presented only Fe and TiH₂ phases. Nanostructured TiFe compound was formed after heat treatment. TiH₂ was effective for providing low adherence of the powders during milling.

Abstract: The hydrogen storage properties of Li₄BN₃H₁₀ doped by Cl anion are investigated by using first-principles method based on density functional theory. According to the calculated results of formation enthalpy and substitution enthalpy, Cl^- doping may result in the substitution of H by Cl^- in the hydride lattice and accordingly, a favorable thermodynamics modification. The electronic structure analysis indicates that the main peak of H-1s moves close to Fermi level when substituting H^- by Cl^-. The stability of hydrogen in the doped hydride is lowered compared with that in the hydride without doping, which improves the hydrogen desorption properties of the hydride.

Abstract: The structural, electronic and thermodynamics stability properties of the new hydrogen storage materials Mg₁₂Ni_6-xCr_x (x=0, 1) are analyzed based on the density functional theory (DFT). The enthalpy of hydride formation of Mg-Ni alloys is-65.07kJ/mol (H₂). For a small range of Cr doping, the enthalpy of hydride formation of the Mg-Ni-Cr system is-54.99 kJ/mol (H₂), widely accepted as considerable value for applications. The thermodynamics properties of Mg₁₂Ni_6-xCr_x H₂(x=0, 1) are expected to be improved by addition of Cr elements. The results will play an important role in practical research field.

Abstract: Mg, in the form of MgH_2, is one kinds of materials widely used as hydrogen storage materials. Absorption and desorption properties of hydrogen which comes from metal hydride depend on materials itself, addition of elements, as well as manufacturing method. In this research, Mg as hydrogen storage were prepared by mechanical alloying with Ni, Cu, and Al as element addition and variation milling time for 10, 20 and 30 hours. Some morphological analyses (XRD, SEM) were done to observe phase transformation. Absorption and desorption properties characterization were employed by DSC and hydrogenation tests. The improvement in milling time decreased particle size, therefore enhanced wt% of absorbed hydrogen and decrease onset desorption temperature. However, the excessive of agglomeration and cold welding on mechanical alloying process resulted in bigger particle size. Alloying elements, Al and Cu, served as catalyst, while Ni acted as alloying which reacted with hydrogen. Mg10wt%Al with 20 hours milling time at hydrogenation temperature 250°C, 3 atm pressure, and 1 hour holding time resulted in the highest weight percent of H₂ (0.38%wt). However, Mg10wt%Al with 30 hours milling time had the lowest onset temperature, 341.49°C

Abstract: Magnesium hydride is considered as one of the most interesting alternatives for the reversible storage of hydrogen. In this study,The feasibility and reaction process of different materials（Mg and pretreated Al powders(mainly composed of Al and Ni)）were discussed and investigated by thermodynamic calculation. The effects of material proportioning, mill time, mill energy density were also investigated by XRD analysis and TG-DSC technique, and analyzed the essence mechanism. The results show that nano-sized MgH2 and Mg(AlH4)2 could be directly synthesized by pure Mg and pretreated Al powder, The product composite hydrides release 2.7wt% H2 through multi-step decompositions, of which the starting endothermic peaks are as low as 265°C.

Abstract: According to ¹H NMR study in fcc hydrides of disordered Ti-V alloys hydrogen atoms are distributed over both octahedral and tetrahedral sites. The occupancy of octahedral sites increases with the vanadium concentration. In hydrides of ternary Ti-V-Cr alloys hydrogen atoms occupy tetrahedral sites only.

Abstract: Various calcium borohydride samples were investigated by means of combined measurements of thermogravimetry and mass spectrometry, and anelastic spectroscopy. On heating, the release of 2-5% tetrahydrofuran (THF) is detected in all the samples at temperatures below ~480 K, even in those which were previously thermally treated, according to procedures known from the literature, in order to remove the solvent. Dehydrogenation takes place above 480 K. Above room temperature the temperature dependence of the Young modulus of Ca (BH₄)₂ clearly monitors the release of THF and two irreversible structural phase transitions: from the α to the α’ phase around 460 K and from the α’ to the β phase, nearly completely evolved around 590 K. Moreover, the coefficient of elastic energy dissipation presents two dynamic processes below room temperature; a peak around 120 K characterized by an activation energy of 0.20 eV and a pre-exponential factor typical of atom-cluster relaxations, that we attributed to the dynamics of THF molecules retained in the borohydride lattice, and a peak around 200 K, possibly due to the relaxation of H vacancies.