M Bououdina

Mechanical alloying and electronic simulations of (MgH2+M) systems (M=Al, Ti, Fe, Ni, Cu and Nb) for hydrogen storage

Abstract Mg-based alloys are promising candidates for hydrogen storage applications. Here, mechanical alloying (MA) was used to process powder mixtures of MgH2 with 8 mol % M (M=Al, Ti, Fe, Ni, Cu and Nb) in order to modify hydrogen storage properties of the Mg hydride. Electronic simulations of the systems were carried out to clarify the mechanisms of the alloy effects. X-ray diffraction (XRD) of the milled samples revealed the formation of new phases: a bcc solid solution phase for the (MgH2+Nb) mixture; TiH2 phase for the (MgH2+Ti); and MgCu2 phase for the (MgH2+Cu). For all the mixtures, a high-pressure phase, γ-MgH2, was also identified after mechanical alloying. Further qualitative and quantitative phase analyses were carried out using the Rietveld method. Scanning electron microscopy (SEM) of the milled powder clearly showed substantial particle size reduction after milling. Dehydrogenation at 300°C under vacuum shows that the (MgH2+Ni) mixture gives the highest level of hydrogen desorption and the most rapid kinetics, followed by MgH2 with Al, Fe, Nb, Ti and Cu. Theoretical predictions show that the (MgH2+Cu) system is the most unstable, followed by (MgH2+Ni), (MgH2+Fe), (MgH2+Al), (MgH2+Nb), (MgH2+Ti). The predicted alloying effects on the stability of MgH2 generally agree with the experimentally observed change in the hydrogen desorption capacity. The differences were discussed in the text.

Comparative study of mechanical alloying of (Mg+Al) and (Mg+Al+Ni) mixtures for hydrogen storage

Abstract Mg is a desirable hydrogen-storage material of high capacity, but suffers from low kinetics and is difficult to activate. Addition of appropriate alloying elements, such as Al and Ni, and a fine microstructure can ease the problems. In order to increase the solubility of Al into Mg and to refine the powder structure, a (Mg+10 at.% Al) mixture was investigated by mechanical alloying. It was noted that the amount of elemental Al decreases with increasing milling time. After 20 h of milling, an h.c.p. (Mg, Al) solid solution was obtained with a volume contraction of 1.2%. An energy dispersive X-ray (EDX) analysis confirmed its formation and showed its chemical composition as (Mg+12 at.% Al). When 10 at.% Ni was further added to the (Mg, Al) solid solution as a catalytic element, a new Mg 12 Al 17 phase was formed, in addition to a modified (Mg, Al, Ni) solution. When the (Mg+10 at.% Al) mixture milled for 2 h was annealed at 400°C for 2 h, all the Al dissolved into Mg and formed a single (Mg, Al) solid solution. When the (Mg+10 at.% Al+10 at.% Ni) mixture was sintered at the same condition, a new AlNi phase was formed, in addition to the presence of a (Mg, Al, Ni) solid solution.

Structural stability of mechanically alloyed (Mg+10Nb) and (MgH2+10Nb) powder mixtures

In order to improve the hydrogen storage characteristics of magnesium, both chemical alloying by Nb and mechanical alloying (MA) of (Mg+10 wt.%Nb) and (MgH2+10 wt.%Nb) powder mixtures were investigated, with particular attention paid to their structural stability. Extensive powder refinement was noted for both compositions within 20 h of milling at 250 rpm. Even nano-sized particles were generated in the hydride mixture. XRD and Rietveld analyses show the formation of a bcc phase in each case. The amount of the bcc phase increases with increasing milling time to the detriment of Nb. For the (Mg+10 wt.%Nb) mixture, it is confirmed that the newly formed phase is a bcc-(Nb,Mg) solid solution, with an extended solubility of Nb in Mg. However, for the (MgH2+10 wt.%Nb) powder mixture, the new bcc phase can be a Nb hydride (NbHx, x<1.0), or a bcc-(Nb,Mg) solid solution, or a (Nb,Mg)Hx solid solution, or even a mixture of the three.

In situ X-ray diffraction study of hydrogen-induced phase decomposition in LaMg12 and La2Mg17

Abstract In situ X-ray diffraction (XRD) shows that LaMg 12 and La 2 Mg 17 are thermally stable under argon upon heating to 330°C, while under hydrogen they decompose to LaH 3 and MgH 2 at about 290 and 270°C, respectively. The decomposition is believed to result from the large difference in enthalpy between the parent compounds and the resultants. Prior to the decomposition, the lattice parameters of LaMg 12 and La 2 Mg 17 do not change, indicating that hydrogen does not dissolve into them to form solid solution of LaMg 12 H x and La 2 Mg 17 H x . The enhanced mobility of La and/or Mg was observed in the presence of hydrogen, which might be related to the formation of the copious vacancy.

Ball-milling of Mg2Ni under hydrogen

Abstract The intermetallic compound Mg 2 Ni is milled under hydrogen in a high-energy planetary mill. The resulting material is a mixture of heavily deformed Mg 2 Ni, low-temperature Mg 2 NiH 4 , and, possibly, high-temperature Mg 2 NiH 4 structures. The relative amount of each phase depends on the initial hydrogen pressure in the milling vial.

Phase components and hydriding properties of the sintered Mg-xwt.% LaNi5 (X=20-50) composites

Abstract Powder mixtures of magnesium and LaNi 5 with nominal composition Mg– x wt.% LaNi 5 ( x =20–50) were pressed into pellets and sintered at 973 K for 1 h. After sintering, it was found that when x =20–40 the major phases were pure Mg, Mg 2 Ni and La 2 Mg 17 , including small amounts of MgO. Further increasing x to 50, Mg 2 Ni and La 2 Mg 17 remained the major phases but pure magnesium disappeared. In contrast to previously published results, the LaMg 12 phase was not found in the above composition range. The phase abundance as a function of composition was quantitatively determined by Rietveld analysis. It was also found that changing the sintering temperature from 723 to 973 K did not change the resultants of the sintered Mg–50 wt.% LaNi 5 composite as well as their relative amounts. All samples were easily activated at 623 K, and their pressure–composition isotherms of hydrogenation were measured at 573 K.

Metal organic chemical vapour deposition (MOCVD) of bone mineral like carbonated hydroxyapatite coatings

For the first time, the MOCVD technique has been used to deposit carbonated hydroxyapatite onto Ti6AL4V substrates using volatile monomeric (liquid) complexes [Ca(beta-diketonate)(2)(L)] and P(OEt)(3).

Structural and thermodynamic properties of the pseudo-binary TiCr2-xVx compounds with 0.0≤x≤1.2

Abstract The TiCr 2− x V x compounds with 0.0≤ x ≤1.2 series have been synthesised and characterised by X-ray powder diffraction. X-Ray qualitative and quantitative phase analysis has been carried out on the as-cast alloys using the Rietveld method. The refinements of the structure shows that the materials crystallise either in the hexagonal or in the cubic Laves phase type for low V contents. For x >0.6, the system is found of b.c.c.-type structure only. The pressure–composition–temperature ( P – C – T ) isotherms measured at 298 K show that the as-cast alloys absorb large amounts of hydrogen, from 4 to 5.2 H/f.u. The P – C – T diagrams reveal also the presence of a relatively flat plateau, and a large hysterisis effect, and correspondingly the hydride cannot be completely dehydrogenated.

Improved kinetics by the multiphase alloys prepared from Laves phases and LaNi5

Abstract We propose a new method to prepare hydrogen absorbing alloys from single phase intermetallic ones. The alloys, (100− x ) Zr 0.5 Ti 0.5 Cr 1.5 Ni 0.5 + x LaNi 5 ( x =5–20 mass%) as prepared by this method showed dramatic enhancement of the hydrogenation properties. The Rietveld analysis on XRD data revealed that pure Cr precipitated from the Laves phase and its relative amount was proportional to the nominal LaNi 5 amount. The PCT curves measured at 313 K showed a large hydrogen capacity, 1.4 wt%.

The investigation of the Zr1-yTiy(Cr1-xNix)2-H2 system 0.0≤y≤1.0 and 0.0≤x≤1.0 phase composition analysis and thermodynamic properties

Abstract The Zr 1− y Ti y (Cr 1− x Ni x ) 2 compounds were synthesized and characterized by means of X-ray powder diffraction. From the Rietveld analysis carried out on the alloys, we determined the phase abundancy and refined the crystal structure. The system crystallized with hexagonal C14-type structure for nickel content less than x =1.0 with some amount of additional phases (ZrNi, Zr 9 Ni 11 , Zr 7 Ni 10 , TiNi) and pure metals (Ni and Cr). For higher nickel concentrations, the system crystallized in the cubic C15-type structure. The amount of additional phases increased with increasing nickel and titanium content. The Pressure–Composition–Temperature measurements using a volumetric method were carried out at 313 K on some selected alloys. The single phase systems were found to absorb a large amount of hydrogen ( H/M ≤1.8 wt.%) but had very poor kinetics. The multiphase systems were found to behave as a single phase (presence of single plateau), have a relatively good hydrogen capacity ( H/M ≤1.4 wt.%) and exhibit faster kinetics. A linear correlation between the heat of formation of the Zr 1− y Ti y (Cr 1− x Ni x ) 2 hydrides estimated from the band structure model and the unit cell volume of the alloys determined from the X-ray Rietveld analysis was obtained. It shows a decrease of hydride stability with increasing unit cell volume.