Li 1s core exciton in LiH studied by x-ray Raman scattering spectroscopy

The Li 1s core excitation spectra in LiH was studied by means of x-ray Raman scattering (XRS) spectroscopy in a wide range of momentum transfers q. The analysis of the near-edge region of the measured spectra in combination with q-dependent ab initio calculations of XRS spectra based on the Bethe-Salpeter equation (BSE) reveals that the prominent peak at the excitation onset arises from two main contributions, namely a pre-edge peak associated to a p-type core exciton and strong transitions to empty states near the bottom of the conduction band, which is in contrast to previous experimental studies that attributed that feature to a single excitonic peak. The p-like angular symmetry of the core exciton is supported by BSE calculations of the relative contributions to the XRS spectra from monopole and dipole transitions and by the observed decrease of its normalised intensity for increasing momentum transfers. Higher energy spectral features in the measured XRS spectra are well reproduced by BSE, as well as by real-space multiple-scattering calculations.Peer reviewe

Li K-shell electron excitation at low momentum transfer by inelastic X ray scattering in LiH

Ponencia presentada en la 99º Reunión Nacional de la Asociación de Física Argentina, 2014Se presenta un estudio del espectro de excitaciones de electrones de coraza K del Li en LiH mediante dispersión inelástica de rayos x medido en alta resolución. La contribución de los electrones de coraza se extrae del espectro medido y su análisis se realiza por medio de simulaciones ab initio. La estructura fina próxima al borde de absorción en el espectro medido se interpreta en términos de los canales de excitación habilitados, para el valor del momento transferido al sistema, y la densidad de estados proyectada en simetría. Los efectos del hueco de coraza generado en el proceso de dispersión son apreciables. La contribución de transiciones dipolares predomina en el espectro medido. Se observa un corrimiento químico de 2.2eV del borde K del Li a energías mayores respecto del Li metálico.A study of the excitation spectrum of core electrons of Li in LiH using high resolution inelastic x-ray scattering is presented. The core electron contribution is extracted from the whole excitation spectrum and the analysis is accomplished by means of ab initiosimulations. The near edge fine structure of the spectrum is interpreted in terms of the excitation channels allowed for the momentum transfer value, and the symmetry projected density of states. Core-hole effects were found to be appreciable. The main contribution to the spectrum is associated to dipole transitions to empty p-states. A Li K edge chemical shift of 2.2eVto higher energies is observed relative to that of metallic Li.Fil: Paredes Mellone, Oscar Ariel. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Ceppi, Sergio Andrés. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Ceppi, Sergio Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Arneodo Larochette, P. P. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina.Fil: Stutz, Guillermo Eduardo. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Stutz, Guillermo Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Física Atómica, Molecular y Química (física de átomos y moléculas incluyendo colisión, interacción con radiación, resonancia magnética, Moessbauer Efecto.

Effect of Fe additive on the hydrogenation-dehydrogenation properties of 2LiH + MgB2/2LiBH4 + MgH2 system

Lithium reactive hydride composite 2LiBH4 + MgH2 (Li-RHC) has been lately investigated owing to its potential as hydrogen storage medium for mobile applications. However, the main problem associated with this material is its sluggish kinetic behavior. Thus, aiming to improve the kinetic properties, in the present work the effect of the addition of Fe to Li-RHC is investigated. The addition of Fe lowers the starting decomposition temperature of Li-RHC about 30 °C and leads to a considerably faster isothermal dehydrogenation rate during the first hydrogen sorption cycle. Upon hydrogenation, MgH2 and LiBH4 are formed whereas Fe appears not to take part in any reaction. Upon the first dehydrogenation, the formation of nanocrystalline, well distributed FeB reduces the overall hydrogen storage capacity of the system. Throughout cycling, the agglomeration of FeB particles causes a kinetic deterioration. An analysis of the hydrogen kinetic mechanism during cycling shows that the hydrogenation and dehydrogenation behavior is influenced by the activity of FeB as heterogeneous nucleation center for MgB2 and its non-homogenous distribution in the Li-RHC matrix.Fil: Puszkiel, Julián Atilio. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Helmholtz-zentrum Geesthacht - Zentrum Für Material- Und Küstenforschung Gmbh;Fil: Gennari, Fabiana Cristina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Centro Atómico Bariloche; ArgentinaFil: Arneodo Larochette, Pierre Paul. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Centro Atómico Bariloche; ArgentinaFil: Ramallo Lopez, Jose Martin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Vainio, U.. Helmholtz-zentrum Geesthacht - Zentrum Für Material- Und Küstenforschung Gmbh; . Deutsches Elektronen-Synchrotron; AlemaniaFil: Karimi, F.. Helmholtz-zentrum Geesthacht - Zentrum Für Material- Und Küstenforschung Gmbh;Fil: Pranzas, P. K.. Helmholtz-zentrum Geesthacht - Zentrum Für Material- Und Küstenforschung Gmbh;Fil: Troiani, Horacio Esteban. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Centro Atómico Bariloche; ArgentinaFil: Pistidda, C.. Helmholtz-zentrum Geesthacht - Zentrum Für Material- Und Küstenforschung Gmbh;Fil: Jepsen, J.. Helmholtz-zentrum Geesthacht - Zentrum Für Material- Und Küstenforschung Gmbh;Fil: Tolkiehn, M.. Deutsches Elektronen-Synchrotron; AlemaniaFil: Welter, E.. Deutsches Elektronen-Synchrotron; AlemaniaFil: Klassen, T.. Helmholtz-zentrum Geesthacht - Zentrum Für Material- Und Küstenforschung Gmbh;Fil: Bellosta Von Colbe, J.. Helmholtz-zentrum Geesthacht - Zentrum Für Material- Und Küstenforschung Gmbh;Fil: Dornheim, M.. Helmholtz-zentrum Geesthacht - Zentrum Für Material- Und Küstenforschung Gmbh

Stabilisation of the 2H martensite in Cu-Zn-AI single crystals with e/a = 1.53

The 2H martensitic phase can be induced by tensile stressing the L2$_1$; ordered $\beta$ phase, provided the temperature is kept between M$_{\rm s}$ and M$_{\rm s}$+25°C, where M$_{\rm s}$ is the temperature for the spontaneous martensitic transformation $\beta \to 2$H. The resulting microstructure depends strongly on the orientation of the tensile axis : If its direction is close to the line joining the [1 0 0]$_\beta$ and the [1 1 1]$_\beta$ directions in the stereographic projection, a twinned martensite is obtained. For other tensile axis directions, a single variant martensite is induced on loading. When diffusional processes are permitted to occur in the 2H phase, by raising the temperature above room temperature, the retransformation temperature A$_{\rm s}$ is increased by more than 300°C in the latter case. For the twinned martensite the measured increase in As is similar to that reported for the 18R martensite. A possibly reason for these different behaviours will be presented

The Two Way Shape Memory Effect in Stabilized Cu-Zn-Al Single and Polycrystals

The influence of martensite stabilization on the two way shape memory effect (TWME) has been studied in a commercial Cu-Zn-Al alloy. It is shown that the TWME is never complete, in contrast to results for single crystals. It depends on the fraction of martensite that is stabilized, i.e. on the stabilization stress and on the temperature to which the sample is heated after stabilization : The lower the stress, the higher is the efficiency of the TWME, but the smaller is also the total TWME strain. No influence of heating velocity could be detected up to 140 °C

Mechanical properties of martensitic Cu–Zn–Al foams in the pseudoelastic regime

The mechanical properties of martensitic Cu–Zn–Al foams produced through molten metal infiltration of a leachable bed of silica gel were investigated. The novel porous shape memory alloy almost reversibly absorbs compression deformations up to 4%. Intergranular fracture occurs in the material along the test, similar to what is observed in polycrystalline solid samples. Despite its tendency to fracture at localized regions, the material is highly resilient, being able to stand several compression cycles. The Cu–Zn–Al foams showed excellent shape recovery after deformation (95%). This previous fact establishes it as a very promising candidate for interesting applications

A novel catalytic route for hydrogenation dehydrogenation of 2LiH MgB2 via in situ formed core shell LixTiO2 nanoparticles

A novel catalytic route for hydrogenation dehydrogenation of 2LiH MgB2 via in situ formed core shell LixTiO2 nanoparticles Aiming to improve the hydrogen storage properties of 2LiH MgB2 Li RHC , the effect of TiO2 addition to Li RHC is investigated. The presence of TiO2 leads to the in situ formation of core shell LixTiO2 nanoparticles during milling and upon heating. These nanoparticles markedly enhance the hydrogen storage properties of Li RHC. Throughout hydrogenation dehydrogenation cycling at 400 C a 1 mol TiO2 doped Li RHC material shows sustainable hydrogen capacity of 10 wt and short hydrogenation and dehydrogenation times of just 25 and 50 minutes, respectively. The in situ formed core shell LixTiO2 nanoparticles confer proper microstructural refinement to the Li RHC, thus preventing the material s agglomeration upon cycling. An analysis of the kinetic mechanisms shows that the presence of the core shell LixTiO2 nanoparticles accelerates the one dimensional interface controlled mechanism during hydrogenation owing to the high Li mobility through the LixTiO2 lattice. Upon dehydrogenation, the in situ formed core shell LixTiO2 nanoparticles do not modify the dehydrogenation thermodynamic properties of the Li RHC itself. A new approach by the combination of two kinetic models evidences that the activation energy of both MgH2 decomposition and MgB2 formation is reduced. These improvements are due to a novel catalytic mechanism via Li source sink reversible reaction

Effect of Fe additive on the hydrogenation-dehydrogenation properties of 2LiH + MgB2/2LiBH4 + MgH2\mathrm{2LiH + MgB_{2}/2LiBH_{4} + MgH_{2}} system

Lithium reactive hydride composite 2LiBH$_{4}$ + MgH$_{2}$ (Li-RHC) has been lately investigated owing to its potential as hydrogen storage medium for mobile applications. However, the main problem associated with this material is its sluggish kinetic behavior. Thus, aiming to improve the kinetic properties, in the present work the effect of the addition of Fe to Li-RHC is investigated. The addition of Fe lowers the starting decomposition temperature of Li-RHC about 30 °C and leads to a considerably faster isothermal dehydrogenation rate during the first hydrogen sorption cycle. Upon hydrogenation, MgH$_{2}$ and LiBH$_{4}$ are formed whereas Fe appears not to take part in any reaction. Upon the first dehydrogenation, the formation of nanocrystalline, well distributed FeB reduces the overall hydrogen storage capacity of the system. Throughout cycling, the agglomeration of FeB particles causes a kinetic deterioration. An analysis of the hydrogen kinetic mechanism during cycling shows that the hydrogenation and dehydrogenation behavior is influenced by the activity of FeB as heterogeneous nucleation center for MgB$_{2}$ and its non-homogenous distribution in the Li-RHC matrix

Becas de Verano 2019 del Instituto Balseiro.

El programa de Becas de Verano del Instituto Balseiro ofrece a estudiantes universitarios avanzados o recientemente egresados de carreras de grado en Ciencias o Ingenierías la posibilidad de realizar una pasantía durante el mes de febrero. El objetivo de la misma es familiarizarse con técnicas experimentales y colaborar en tareas de investigación en laboratorios del Centro Atómico Bariloche. Anualmente, 15 estudiantes son seleccionados y reciben ayuda económica completa. Cada participante elige un único tema de investigación entre una serie de propuestas y es guiado por un grupo de investigadores del Centro Atómico Bariloche. La pasantía finaliza con la entrega de un informe y la presentación de un póster en el que se detallan los resultados obtenidos a lo largo del mes de trabaj