Etude physico-chimique du role du milieu dans l'endommagement des cylindres de laminoirs a chaud : application a la formulation d'une emulsion de laminage

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Élaboration, caractérisation et propriétés de stockage d'hydrogène électrochimique des alliages (Mg2Ni1 xMnx (x = 0, 0.125, 0.25, 0.375) et Mg2 xAlxNi (x = 0, 0.25) + 5 wt.% MWCNTs préparés par mécanosynthèse)

L utilisation des combustibles fossiles (énergies non renouvelables) est responsable de l augmentation de la concentration en gaz à effet de serre dans l atmosphère. Parmi les solutions de remplacement envisagées, l hydrogène apparaît comme le vecteur énergétique le plus séduisant. Son stockage dans des intermétalliques permet d obtenir des capacités massiques et volumiques (e.g. 140 g/L) supérieures à celles obtenues en voie liquide ou sous pression (respectivement 71 et 40 g/L). Dans les accumulateurs Nickel-Métal Hydrure (Ni-MH), l électrode négative est constituée d un composé intermétallique qui absorbe l hydrogène de façon réversible dans des conditions normales de pression et de température. Ce travail de thèse vise d une part, à synthétiser les alliages Mg2Ni1-xMnx (x =0, 0.125, 0.25, 0.375) et les alliages Mg2-xAlxNi (x = 0, 0.25) avec ou sans nanotubes de carbone (MWCNTs) par mécanosynthèse et d autre part, d étudier les effets des substitutions/additions sur la composition et la microstructure des alliages Mg2Ni afin d améliorer leurs propriétés de stockage d hydrogène.Les résultats obtenus montrent que les capacités de décharge des alliages Mg2Ni1-xMnx(x = 0, 0.125, 0.25, 0.375) augmentent avec le temps de broyage. Pour l alliage Mg2Ni0.625Mn0.375 broyé durant 48 h, nous avons mis en évidence la formation d une nouvelle phase Mg3MnNi2 qui est relativement stable. Par conséquent, Mg3MnNi2 est capable d améliorer de manière significative la stabilité des cycles tout en maintenant une capacité de décharge relativement élevée.Les résultats obtenus par la théorie de la fonctionnelle de la densité (DFT) en utilisant le programme CASTEP montrent d une part, que les paramètres de maille et les coordinations atomiques sont en parfait accord avec les résultats expérimentaux. D autre part, que la stabilité des phases décroit graduellement selon l ordre suivant : Mg2Ni sans aucune substitution >Mg3MnNi2 > Mg2Ni avec substitution par Mn.L addition de nanotubes de carbone et de Al ont des effets synergétiques sur la capacité de stockage d hydrogène électrochimique dans le cas des alliages Mg2-xAlxNi (x = 0, 0.25) + 5 wt.% MWCNTs.The use of fossil fuels (non-renewable energy) is responsible for increasing the concentration of greenhouse gases in the atmosphere. Among the considered alternatives, hydrogen is seen as the most attractive energy vector. The storage in intermetallics makes it possible to obtain mass and volume capacities (e.g. 140 g/L) higher than those obtained by liquid form or under pressure (respectively 71 and 40 g/L). The negative electrode of Nickel-Metal Hydride (NiMH) batteries, is constituted by an intermetallic compound which is able to reversibly absorb hydrogen under normal conditions. In this work, on the one hand, Mg2Ni1-xMnx(x=0, 0.125, 0.25, 0.375) and Mg2-xAlxNi (x = 0, 0.25) electrode alloys with and without multiwalled carbon nanotubes (MWCNTs) have been prepared by Mechanical Alloying. On the other hand, influence of the partial elements substitution on the microstructure and electrochemical hydrogen storage properties of Mg2Ni-type alloy has been studied.The results show that the discharge capacities of Mg2Ni1-xMnx (x =0, 0.125, 0.25, 0.375) alloys increase with the prolongation of milling time. The new phase Mg3MnNi2 is formed only when x=0.375 after 48 h of milling. Mg3MnNi2 phase is relatively stable during charge/discharge cycles and therefore can significantly enhance the cycle stability under simultaneously maintaining a high discharge capacity.Based on the calculated results of first principles, the lattice parameters and atomic coordinates are in good agreement with the experimental results and the stability of phases gradually decreases along the sequence pure Mg2Ni phase > Mg3MnNi2 phase > Mn-substitution doped Mg2Ni phase.When Al and MWCNTs are added simultaneously, the highest discharge capacity is obtained for Mg1.75Al0.25Ni-MWCNTs composite, which implies that MWCNTs and Al have synergistic effects on electrochemical hydrogen storage capacity of milled alloys.BELFORT-UTBM-SEVENANS (900942101) / SudocSudocFranceF

Magnetic properties of nanocrystalline Fe-10%Ni alloy obtained by planetary ball mills

International audiencePlanetary ball mill PM 400 from Retsch (with different milling times for X = 400 rpm, x = 800 rpm) and P4 vario ball mill from Fritsch (with different milling conditions (X/x), X and x being the disc and the vial rotation speeds, respectively) are used for obtaining nanocrystalline Fe-10wt% Ni. The structure and magnetic properties are studied by using X-ray diffraction, SEM and hysteresis measurements, respectively. The bcc-Fe(Ni) phase formation is identified by X-ray diffraction. The higher the shock power and the higher milling time are, the larger the bcc lattice parameter and the lower the grain size. The highest value of the coercivity is 1600 A/m for Fe-10 wt.%Ni (with shock mode (424 rpm/100 rpm) after 36 h of milling), while the lowest value is 189 A/m for (400 rpm/800 rpm) after 72 h of milling. The milling performed in the friction mode has been found to lead the formation of alloys exhibiting a soft magnetic behavior for nanocrystalline Fe-10%Ni

First principles investigation of the substitutional doping of Mn in Mg2Ni phase and the electronic structure of Mg3MnNi2 phase

International audienceThe substitutional doping of Mn in Mg2Ni phase and the electronic structure of Mg3MnNi2 phase have been investigated by first principles density functional theory calculations. The calculation of enthalpy of formation shows that among the four different lattice sites of Mg(6f), Mg(6i), Ni(3b) and Ni(3d) in Mg2Ni unit cell, the most preferable site of substitution of Mn in Mg2Ni lattice has been confirmed to be Mg(6i) lattice site. The constructed Mg9Mn3Mg(6i)Ni6 structure by replacing 3 Mg atoms at Mg(6i) lattice sites with 3 Mn atoms in the Mg2Ni unit cell is less stable. In contrast, the cubic Mg3MnNi2 phase that has the same composition as that of Mg9Mn3Mg(6i)Ni6 structure possesses good stability. Analysis of density of states (DOS) indicates that there is a strong hybridization between Mg s, Mg p and Ni d electrons, which is dominant in controlling the structural stability of pure and Mn-doped Mg2Ni phases. The Mn-substitution in Mg2Ni unit cell weakens the interaction between Mg s, Mg p and Ni d electrons, especially for Mg9Mn3Mg(6i)Ni6 phase. The cubic Mg3MnNi2 phase possesses a strong hybridization between Mn and Mg, Ni atomic orbits under simultaneously retaining the strong bonding among Mg s, Mg p and Ni d electrons. Based on the calculated results, the stability of phases gradually decreases along the sequence pure Mg2Ni phase &gt Mg3MnNi2 phase &gt Mn-substitution doped Mg2Ni phase

Intermetallic Compounds Synthesized by Mechanical Alloying for Solid-State Hydrogen Storage: A Review

International audienceHydrogen energy is a very attractive option in dealing with the existing energy crisis. For the development of a hydrogen energy economy, hydrogen storage technology must be improved to over the storage limitations. Compared with traditional hydrogen storage technology, the prospect of hydrogen storage materials is broader. Among all types of hydrogen storage materials, solid hydrogen storage materials are most promising and have the most safety security. Solid hydrogen storage materials include high surface area physical adsorption materials and interstitial and non-interstitial hydrides. Among them, interstitial hydrides, also called intermetallic hydrides, are hydrides formed by transition metals or their alloys. The main alloy types are A2B, AB, AB2, AB3, A2B7, AB5, and BCC. A is a hydride that easily forms metal (such as Ti, V, Zr, and Y), while B is a non-hydride forming metal (such as Cr, Mn, and Fe). The development of intermetallic compounds as hydrogen storage materials is very attractive because their volumetric capacity is much higher (80–160 kgH2m−3) than the gaseous storage method and the liquid storage method in a cryogenic tank (40 and 71 kgH2m−3). Additionally, for hydrogen absorption and desorption reactions, the environmental requirements are lower than that of physical adsorption materials (ultra-low temperature) and the simplicity of the procedure is higher than that of non-interstitial hydrogen storage materials (multiple steps and a complex catalyst). In addition, there are abundant raw materials and diverse ingredients. For the synthesis and optimization of intermetallic compounds, in addition to traditional melting methods, mechanical alloying is a very important synthesis method, which has a unique synthesis mechanism and advantages. This review focuses on the application of mechanical alloying methods in the field of solid hydrogen storage materials

First principles investigation of the substitutional doping of rare-earth elements and Co in La4MgNi19 phase

International audienceRE-Mg-Ni alloys, including AB3, A2B7, A5B19 and AB4 types, havereceived extensive attention due to their excellent hydrogenstorage properties. La4MgNi19 is the one of the outstandingcandidate for hydrogen storage. In this work, the structure, phasestability and electronic structure of the different La-site partialsubstituted by Pr, Sm, Gd, Nd and NI-site substituted by Co havebeen investigated by means of the density functional theory. Thecalculation results show that La4MgNi19 alloy shows the negativeenthalpy of formation, indicating the more stable in thethermodynamic. When the substitution of La occurs, among the twosites La(2c) and La(4f), Pr, Nd, Sm and Gd preferentially occupythe La(4f) site. And the addition of the four doping elements willreduce the stability of the phase. Among them, Pr substitutedLa4MgNi19 has the highest structural stability. When Cosubstituting Ni, a single Co atom occupies the Ni(12k)preferentially among the seven different Ni positions. During thisprocess, the crystal structure will be destabilized. The DOSresults show that the system still puts up the metallic characterafter substitution. Sm(La(4f)) has the maximum valence band width.The stability of four doped alloys from high to low is: Pr, Nd, Sm,Gd, which is consistent with the enthalpy of formation results. Theenthalpy of formation of hydrides shows that, the bounding hydrogencapacity of the system can be obtained as Nb > Sm > Pr >Gd

Advanced study of hydrogen storage by substitutional doping of Mn and Ti in Mg2Ni phase

International audienceThe substitutional doping of Mn and Ti in Mg2Ni phase has been investigated by first principles densityfunctional theory calculations. The calculation of enthalpy of formation shows that among the four different lattice sitesof Mg(6f), Mg(6i), Ni(3b) and Ni(3d) in Mg2Ni unit cell, the most preferable site of substitution of Mn in Mg2Ni latticehas been confirmed to be Mg(6i) lattice site. The most preferable site of Ti substitution in Mg2Ni lattice is Mg(6i) positionand the stability of Ti-doped Mg2Ni decreases with the increase of substitution quantity of Ti for Mg

First principles investigation of scandium-based borohydride NaSc(BH4)(4)

International audienceThe periodic crystal structure of NaSc(BH4)4 has been investigated by first principles density functional theory calculations. The enthalpy of formation and cohesive energy of NaSc(BH4)4 are calculated to be −72.69 kJ/mol H2 and 550.76 kJ/mol H, respectively. Total and partial density of states analysis and Mulliken atomic populations of NaSc(BH4)4, as well as total and difference charge-density analysis indicate that there is a strong hybridization between H s and B s, p electrons in BH4 complex. Sc shows a mixed chemical bonding in the [Sc(BH4)4]− complex, partly ionic and partly covalent. Na makes a bonding with other neighboring atoms by ionic bond

Structural and electrochemical hydrogen storage properties of MgTiNix (x ¼ 0.1, 0.5, 1, 2) alloys prepared by ball milling

International audienceA series of MgeTieNi alloys with different Ni content was prepared by mechanical alloyingusing a planetary high-energy ball mill. The structural transformation was characterizedby XRD and SEM. It indicated that the addition of 10 at% Ni did not hinder the amorphizationof MgTi BCC phase. After 40 h of milling, the alloys of MgTiNi and MgTiNi2 mainlyconsisted of the mixture of TiNi þ MgNi þ Mg2Ni and TiNi þ TiNi3 þ MgNix (x &gt 1),respectively. Their discharge capacities were investigated by electrochemical measurementsat galvanostatic conditions. It was shown that all of the studied samples possessedgood cycling performance. Among them, MgTiNi showed the highest discharge capacity. Inthe ternary alloys with the same atom ratio of Ti and Mg, the number of activation cyclesdecreased with increasing the Ni content

Charcterization and first principle study of ball milled Ti-Ni with Mg doping as hydrogen storage alloy

International audienceMicrostructures, electrochemical properties of Ti–Ni and ternary Ti–Ni–Mg alloy were studied after they had been submitted to high-energy ball milling. Influence of milling time and Mg addition on the microstructures of the mechanically milled Ti–Ni alloys was investigated by XRD, SEM. Cycling performances of the electrodes prepared by the milled powders were measured under galvanostatic conditions. It is found that the binary Ti–Ni alloy undergoes a refinement, dynamical recrystallization and amorphization process. With doping of Mg to the starting Ti–Ni powders, an FCC Ti–Mg structure was detected along with the main TiNi BCC phase. First principle calculation was applied to compare the thermodynamic stabilities of several binary alloys involving Ti, Ni and Mg. It was decided that the final product of milling Mg doped Ti–Ni contains an FCC structured TiMg3 phase, which damages electrochemical performance in general as a result of coating effect on the TiNi phase