Improvement in hydrogen sorption properties of Mg by reactive mechanical grinding with Cr2O3, Al2O3 and CeO2

We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H₂ (reactive mechanical grinding) with oxides Cr₂O₃, Al₂O₃ and CeO₂. The hydriding rates of Mg are reportedly controlled by the diffusion of hydrogen through a growing Mg hydride layer. The added oxides can help pulverization of Mg during mechanical grinding. A part of Mg is transformed into MgH₂ during reactive mechanical grinding. The Mg+10wt.%Cr₂O₃ powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO₂ and Mg+10wt.%Al₂O₃. The Mg+10wt.%Cr₂O₃ powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. Mg+10wt.%Cr₂O₃ absorbs 5.87wt.% H at 573 K, 11 bar H₂ during 60 min at the first cycle. The Mg+10wt.%Cr₂O₃ powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO₂ and Mg+10wt.%Al₂O₃. It desorbs 4.44 wt.% H at 573 K, 0.5 bar H₂ during 60 min at the first cycle. All the samples absorb and desorb less hydrogen at the fifth cycle than at the first cycle. It is considered that this results from the agglomeration of the particles during hydriding–dehydriding cycling. The average particle sizes of the as-milled and cycled powders increase in the order of Mg+10wt.%Cr₂O₃, Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. The quantities of hydrogen absorbed or desorbed for 1 h for the first and fifth cycles decrease in the order of Mg+10wt.%Cr₂O₃, Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. The quantities of absorbed or desorbed hydrogen increase as the average particle sizes decrease. As the particle size decreases, the diffusion distance shortens. This leads to the larger hydriding and dehydriding rates. The Cr₂O₃ in the Mg+10wt.%Cr₂O₃ powder is reduced after hydriding–dehydriding cycling. The much larger chemical affinity of Mg than Cr for oxygen leads to a reduction of Cr₂O₃ after cycling.     

Hydrogen storage properties of Mg-based mixtures elaborated by reactive mechanical grinding

The hydrogen sorption properties of Mg + 10 wt% WO₃ and Mg + 5 wt% Cr₂O₃ mixtures made by reactive (under hydrogen) mechanical grinding were studied and compared with those of elemental Mg subjected to a similar preparation procedure. It was observed that both oxides have an important catalytic effect on hydrogen absorption and desorption. Moreover, in the case of Cr₂O₃ addition, both milling speed (i.e, milling energy and milling mode) and ball to powder weight ratio influence drastically the hydrogen sorption kinetics.     

Behavior of Rigid Footings on Gravel under Axial Load and Moment

A laboratory testing program was conducted to study the settlement and rotation response of rigid square footings under combined axial load and moment. A total of 17 tests were performed in which the size of the footing, footing embedment, axial load, and load eccentricity were changed. The test soil consisted of a fine and well-graded gravel contained in a box with dimensions: 1.52×1.52?m (5×5?ft) cross section and 0.91?m (3?ft) deep. The soil was compacted in layers 150?mm (6?in.) thick to an approximate relative density of 84%. In each test, the axial load, moment, settlement at the center of the footing, and footing rotation were measured. Concentrically loaded footings with different sizes exhibited a similar behavior in terms of the applied stress-normalized settlement (settlement divided by size of footing) response. The analytical model proposed was based on such normalized response as an input, and it was calibrated to account for the change in soil stiffness with confinement. The formulation captures the inherent nonlinear deformations of the soil with load and the coupled nature of settlements and rotations of footings under axial load and moment. The model was tested by comparing calculated values with laboratory measurements from tests not included in its calibration. The comparisons showed a satisfactory agreement between calculations and measurements, bringing confidence in the analytical formulation proposed and the methodology used.     

In situ X-ray diffraction under H2 of the pseudo-AB2 compounds: YNi3.5Al0.5Mg

The hydrogenation and dehydrogenation behaviours of the YNi_3.5Al_0.5Mg compound were studied by in situ X-ray diffraction under hydrogen pressure and at room temperature. The changes of (i) the lattice parameters, (ii) the crystallite size and (iii) the lattice strain during the sorption process (i.e. along the PC isotherms) were studied. These results indicate that the crystallite size decreases by a factor of 2. The micro deformations increase at first and then tend to almost zero at the end of the sorption cycle. This behaviour is explained in terms of co-existence of the metal (i.e. α$\alpha$ phase) and metal hydride (i.e. β$\beta$ phase) phases. The change in crystallinity is consistent with the hydrogen induced amorphisation process existing in a lot of AB₂ compounds. No anisotropic effects can be highlighted on this pseudo-AB₂ compounds in contrary with what could be observed in AB₅ compounds.     

Improvement of hydriding and dehydriding rates of Mg via addition of transition elements Ni, Fe, and Ti

An Mg-10wt%Ni-5wt%Fe-5wt%Ti sample was prepared by mechanical grinding under H₂ (reactive mechanical grinding) using a planetary ball mill. The phases and their weight percentages were analyzed with the Full Proof program from the XRD patterns of the Mg-10Ni-5Fe-5Ti samples after reactive mechanical grinding and after dehydriding at the seventh cycle. The Mg-10Ni-5Fe-5Ti sample after reactive mechanical grinding contained Mg, TiH₂, MgH₂, and Ni phases, and the sample dehydrided at the seventh cycle contained Mg, TiH₂, MgO, Mg₂Ni, and Fe phases. The prepared Mg-10Ni-5Fe-5Ti sample had an effective hydrogen-storage capacity larger than 5 wt%H. The activated Mg-10Ni-5Fe-5Ti sample absorbed 5.31 and 5.51 wt%H for 5 and 60 min, respectively, at 573 K under 12 bar H₂ and desorbed 1.58, 3.64, and 5.18 wt%H for 10, 30, and 60 min, respectively, at 573 K under 1.0 bar H₂. The effects of reactive mechanical grinding, hydriding-dehydriding cycling, and addition of transition elements Ni, Fe, and Ti were discussed.     

Low Temperature Alcoholic Fermentations in High Sugar Concentration Grape Musts

ABSTRACT: Low temperature fermentations can increase the quality of wine produced from some aromatic grape varieties. However, these fermentations in musts with a high sugar content may have limitations. The fermentation kinetics and the composition of the resulting wines of 5 different musts have been compared. Maximum fermentation rates correlated well with the available nitrogen. The correlation between the available nitrogen and fermentation rates improved if selected amino acids are corrected for the total sugar content. We propose this new nitrogen parameter as a way of predicting fermentation rates. Must fermentations at low temperatures yielded wines with lower levels of acetic acid, acetaldehyde and ethyl acetate, which can be considered to be positive.     

Reactive mechanical grinding of magnesium in hydrogen and the effects of additives

The use of reactive mechanical grinding (MG under H₂) of magnesium powder improves the hydrogen sorption properties. The hydrogenation of Mg starts in situ during the milling process, thus circumventing the activation procedure that is generally required for Mg. The effects of the addition of various elements or compounds have been studied. The hydriding is determined to be a two-step process: nucleation and diffusion. A direct relationship exists between the nucleation duration and the specific surface area of the magnesium powder. A critical milling time exists up to which the diffusion process is improved and above which no more improvement is observed (the maximum internal stress in the powder is also reached at this critical time). The diffusion is controlled by the number of crystallites per particle that can be reduced by increasing the milling time up to 10 hr. The addition of Co (catalyst), YNi (hydrogen pump), or oxides (abrasive elements and nucleation centers) leads to an improvement of the hydrogen sorption properties (but a strong dependence upon the milling time is reported). Finally, the sorption properties of these mixtures are comparable with those reported for MgH₂-metal mixtures.     

Reactivity of complex hydrides Mg2FeH6, Mg2CoH5 and Mg2NiH4 with lithium ion: Far from equilibrium electrochemically driven conversion reactions

The electrochemical reaction of lithium ion with Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄ complex hydrides prepared by reactive grinding is studied here. Plateaus at an average potential of 0.25 V, 0.24 V and 0.27 V corresponding to discharge capacities of 6.6, 5.5 and 3.6 Li can be achieved respectively for Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄. From in situ X-ray diffraction (XRD) characterizations of complex hydride based electrodes, dehydrogenation leads to a decrease of the intensities of the diffraction peaks suggesting a strong loss of crystallinity since formation of Mg and M (M = Fe, Co, Ni) peaks is not observed. ⁵⁷Fe Mössbauer spectroscopy confirms the formation of nanoscale Fe or an amorphous Mg–Fe alloy during the decomposition of Mg₂FeH_6. Interestingly, lattice parameter variations suggest phase transitions in the Mg₂NiH₄ system involving the formation of low hydrogen content hydride Mg₂NiH, while an increase of lattice parameters of Mg₂CoH₅ hydride could be attributed to the formation of a Mg₂CoH₅Li_x solid solution compound up to x = 1.     

Production of hydrogen from magnesium hydrides hydrolysis

Magnesium hydride could be considered as a good candidate for the hydrolysis reaction because it can be produced at a relatively low cost. However, this reaction is incomplete and very slow because of the formation of a magnesium hydroxide layer on the surface of MgH₂ particles. In order to overcome this problem, various treatments such as ball milling with or w/o additives, addition of acids, ultrasounds and increase of temperature, have been tried. Different characterization methods such as XRD, BET, particle size, SEM, etc. have been used to explain the effects of the treatments cited above on the improvement of the kinetics and the yield of the MgH₂ hydrolysis reaction.