M. Di Michiel

European Synchrotron Radiation Facility, Grenoble, Rhône-Alpes, France

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Publications (147)391.55 Total impact

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    ABSTRACT: Evolution of the inner microstructure of uniaxially pressed copper powders was investigated by in situ X-ray microtomography. Experiments were carried out at the European Synchrotron in Grenoble, France. Sintering was performed under reducing atmosphere at 1050 °C. Qualitative and quantitative information from the 3D images acquired along the whole sintering cycle were analyzed. From that, macro- and microstructural features of the evolving microstructure were obtained. We found that large pores can be created during sintering and then they can grow during the whole thermal cycle by reducing the initial relative density of the sample by 10 %. The effect of the pore shape and the heterogeneous distribution of the pore volume fraction inside the sample are responsible for the behavior observed during sintering. The heterogeneous deformation measured is controlled by the evolution of the porosity.
    Journal of Materials Science 06/2014; 49(12). · 2.16 Impact Factor
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    ABSTRACT: The lattice parameter changes in three types of Nb3Sn superconducting wires during uniaxial stress–strain measurements at 4.2 K have been measured by high-energy synchrotron x-ray diffraction. The nearly-stress-free Nb3Sn lattice parameter has been determined using extracted filaments, and the elastic strain in the axial and transverse wire directions in the different wire phases has been calculated. The mechanical properties of the PIT and RRP wire are mainly determined by the properties of Nb3Sn and unreacted Nb. This is in contrast to the bronze route wire, where the matrix can carry substantial loads. In straight wires the axial Nb3Sn pre-strain is strongest in the bronze route wire, its value being smaller in the PIT and RRP wires. A strong reduction of the non-Cu elastic modulus of about 30% is observed during cool-down from ambient temperature to 4.2 K. The Nb3Sn Poisson ratio at 4.2 K measured in the untwisted bronze route wire is 0.35. The present study also shows that the process route has a strong influence on the Nb3Sn texture.
    Superconductor Science and Technology 03/2014; 27(4):044021. · 2.76 Impact Factor
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    ABSTRACT: The texture of Nb3Sn in recent multifilamentary composite wires has been studied by neutron diffraction, synchrotron x-ray diffraction and electron backscatter diffraction. In powder-in-tube (PIT) type superconductors the Nb precursor filaments exhibit a strong <110> fiber texture as a consequence of the severe cold drawing process, and a <110> texture is also observed in the Nb3Sn. In the Nb-Ta precursor of the restacked rod process (RRP) strand there is an additional texture component, and in both Ta-alloyed and Ti-alloyed RRP type conductors the Nb3Sn grains grow with a preferential <100> orientation.
    01/2014; 27(2).
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    ABSTRACT: High-brilliance and high-energy polychromatic X-ray diffraction (XRD) has been used for time-resolved studies of rapid solidification microstructure evolution in situ during simulated spot welding in austenitic steels. Weld pools of 5.0 mm diameter and 0.6–1.5 mm depth were formed at the steel plate surfaces by radiation heating from halogen lamps. Solidification was initiated by powering off the lamps, and completed within 1.5 s while measuring diffraction from the solidifying grains of the pool at a 1 kHz frame rate. The data contain time-resolved information on individual grain growth and overall solid fraction evolution, and furthermore reveal prominent individual and collective motion of grains during early stages of solidification, presumably caused by convective currents in the pool. Ultrafast polychromatic high-energy XRD is novel in studies of microstructure evolution during welding. The experimental technique could quite readily be used for similar studies in real welding.
    Acta Materialia. 01/2014; 68:159–168.
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    ABSTRACT: In-situ synchrotron x-ray experiments have been used to follow the evolution of the diffraction peaks for crystalline dendrites embedded in a bulk metallic glass matrix subjected to a compressive loading-unloading cycle. We observe irreversible diffraction-peak splitting even though the load does not go beyond half of the bulk yield strength. The chemical analysis coupled with the transmission electron microscopy mapping suggests that the observed peak splitting originates from the chemical heterogeneity between the core (major peak) and the stiffer shell (minor peak) of the dendrites. A molecular dynamics model has been developed to compare the hkl-dependent microyielding of the bulk metallic-glass matrix composite. The complementary diffraction measurements and the simulation results suggest that the interface, as Maxwell damper, between the amorphous matrix and the (211) crystalline planes relax under prolonged load that causes a delay in the reload curve which ultimately catches up with the original path.
    Scientific Reports 01/2014; 4:4394. · 2.93 Impact Factor
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    ABSTRACT: An emerging theme of modern composites and devices is the coupling of nanostructural properties of materials with their targeted arrangement at the microscale. Of the imaging techniques developed that provide insight into such designer materials and devices, those based on diffraction are particularly useful. However, to date, these have been heavily restrictive, providing information only on materials that exhibit high crystallographic ordering. Here we describe a method that uses a combination of X-ray atomic pair distribution function analysis and computed tomography to overcome this limitation. It allows the structure of nanocrystalline and amorphous materials to be identified, quantified and mapped. We demonstrate the method with a phantom object and subsequently apply it to resolving, in situ, the physicochemical states of a heterogeneous catalyst system. The method may have potential impact across a range of disciplines from materials science, biomaterials, geology, environmental science, palaeontology and cultural heritage to health.
    Nature Communications 09/2013; 4:2536. · 10.02 Impact Factor
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    ABSTRACT: We demonstrate that Si planar parabolic lens with long focus distance can collimate high energy X-rays with microradian precision. A divergent beam from a wiggler at the ESRF ID15 beamline is reduced from 15 microradian down to 1 microradian. We propose a new imaging technique for direct measurements of beam divergence. It is shown experimentally that precision better than 1 mrad may be really achieved. Measurements are done at the energy range 60 - 90 keV. Contribution of source size and diffraction phenomena to the precision and ultimate sensitivity of the developed technique is discussed.
    Proc SPIE 07/2013;
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    ABSTRACT: Different solutions have been proposed over the years to optimize control of the temperature and atmosphere over a catalyst in order to reach an ideal reactor behavior. Here, a new innovative solution which aims to minimize temperature gradients along the catalyst bed is demonstrated. This was attained by focusing the infrared radiation generated from the heating elements onto the catalyst bed with the aid of an aluminium shield. This method yields a ~0.13 K mm-1 axial temperature gradient ranging from 960 to 1173 K. With the selection of appropriate capillaries, pressures of 20 bar (2 MPa) can be attained.
    Journal of Applied Crystallography 07/2013; 46(5):1523. · 3.34 Impact Factor
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    ABSTRACT: X-ray microtomography is a fruitful technique to characterize damage during high-temperature deformation. Usually, such characterizations are carried out in post mortem conditions. In the present study, three-dimensional (3-D) damage characterization was performed in in situ conditions, namely directly during high-temperature deformation tests. Continuous 3-D damage characterization was successfully performed during high-temperature deformation of a magnesium alloy. Nucleation and growth of cavities were studied and a particular attention was also given to the possibility of coalescence between cavities.
    Scripta Materialia 07/2013; 69(1):85–88. · 2.82 Impact Factor
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    ABSTRACT: The internal crystalline structure of a human molar tooth has been non-destructively imaged in cross section using X-ray diffraction computed tomography. Diffraction signals from high energy X-rays which have large attenuation lengths for hard biomaterials have been collected in a transmission geometry. Coupling this with a computed tomography data acquisition and mathematically reconstructing their spatial origins, diffraction patterns from every voxel within the tooth can be obtained. Using this method we have observed the spatial variations of some key material parameters including nanocrystallite size, organic content, lattice parameters, crystallographic preferred orientation and degree of orientation. We have also made the link between the spatial variations of the unit cell lattice parameters and the chemical make-up of the tooth. In addition, we have determined how the onset of tooth decay occurs through clear amorphisation of the hydroxyapatite crystal, and we have been able to map the extent of decay within the tooth. The described method has strong prospects for non-destructive probing of mineralised biomaterials.
    Acta biomaterialia 06/2013; · 5.09 Impact Factor
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    ABSTRACT: X-ray absorption spectroscopy and X-ray diffraction are suitable probes of the chemical state of a catalyst under working conditions but are limited to bulk information. Here we show in two case studies related to hydrothermal aging and chemical modification of model automotive catalysts that enhanced detailed information of structural changes can be obtained when the two methods are combined with a concentration modulated excitation (cME) approach and phase sensitive detection (PSD). The catalysts are subject to a modulation experiment consisting of the periodic variation of the gas feed composition to the catalyst and the time-resolved data are additionally treated by PSD. In the case of a 2 wt% Rh/Al2O3 catalyst, a very small fraction (ca. 2%) of Rh remaining exposed at the alumina surface after hydrothermal aging at 1273 K can be detected by PSD. This Rh is sensitive to the red-ox oscillations of the experiment and is likely responsible for the observed catalytic activity and selectivity during NO reduction by CO. In the case of a 1.6 wt% Pd/Al2O3-Ce1-xZrxO2 catalyst, preliminary results of cME-XRD demonstrate that access to the kinetics of the whole material at work can be obtained. Both the red-ox processes involving the oxygen storage support and the Pd component can be followed with great precision. PSD enables the differentiation between Pd deposited on Al2O3 or on Ce1-xZrxO2. Modification of the catalyst by phosphorous clearly induces loss of the structural dynamics required for oxygen storage capacity that is provided by the Ce(4+)/Ce(3+) pair. The two case studies demonstrate that detailed kinetics of subtle changes can be uncovered by the combination of in situ X-ray absorption and high energy diffraction methods with PSD.
    Physical Chemistry Chemical Physics 05/2013; · 3.83 Impact Factor
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    ABSTRACT: The methanol to gasoline process over the zeolite catalyst ZSM-5 in a lab-sized reactor bed (4 mm diameter) has been studied in operando with high energy synchrotron X-ray diffraction. The fast z-scan method was used, scanning the reactor repeatedly and at speed through the X-ray beam. The X-ray diffraction data were processed using high throughput parametric Rietveld refinement to obtain real structural parameters. The diffraction data show only very subtle changes during the process and this allows us to demonstrate the combination of very large data volumes with parametric Rietveld methods to study weak features of the data. The different possible data treatment methodologies are discussed in detail and their effects on the results obtained are demonstrated. The trends in unit cell volume, zeolite channel occupancy and crystallite strain indicate that more or larger reaction intermediates are present close to the reactor outlet.
    Physical Chemistry Chemical Physics 04/2013; · 3.83 Impact Factor
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    ABSTRACT: The behaviour of the Pd phase in three way catalysts (TWCs) having a Ce-Zr oxide as a promoter component has been examined under CO/NO cycling conditions. In order to extract information on the role of the noble metal and the metal-promoter interface, two catalysts in, respectively, the presence or the absence of the promoter and having a similar Pd particle size were analyzed by a synchronous multitechnique approach using X-ray absorption (XAS) or high energy X-ray diffraction (HXRD) in combination with diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and mass spectrometry (MS). From a structural point of view it is shown that the promoter oxide strongly influences Pd behaviour, limiting morphological (size/shape) and structural (formation of carbide/oxide phases) modifications occurring under dynamic conditions in the absence of the promoter. Activity measurements show that these phenomena have important chemical consequences in terms of CO/NO pollutant metal coverage(s), the occurrence of NO and/or CO dissociation steps as well as the subsequent transformation of the pollutants into CO2 and N2.
    Physical Chemistry Chemical Physics 04/2013; · 3.83 Impact Factor
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    ABSTRACT: The freezing of colloidal suspensions is encountered in many natural and engineering processes such as the freezing of soils, food engineering and cryobiology. It can also be used as a bio-inspired, versatile and environmentally friendly processing route for porous materials and composites. Yet, it is still a puzzling phenomenon with many unexplained features, owing to the complexity of the system and the space and time scales at which the process should be investigated. This study demonstrates the interest in fast X-ray computed tomography for providing time-lapse, three-dimensional, in situ imaging of ice crystal growth in a colloidal silica suspension. The experimental measurements show that the local increase in colloid concentration does not affect the growth kinetics of the crystals until the colloidal particles become closely packed. For particles much smaller than ice crystals, the concentrated colloidal suspension is equivalent to a simple liquid phase with higher viscosity and a freezing point determined by the concentration of colloidal particles.
    Acta Materialia. 04/2013; 61(6):2077–2086.
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    ABSTRACT: The promoting effect of SO(2) on the activity for methane oxidation over platinum supported on silica, alumina and ceria has been studied using a flow-reactor, in situ infrared spectroscopy and in situ high-energy X-ray diffraction experiments under transient reaction conditions. The catalytic activity is clearly dependent on the support material and its interaction with the noble metal both in the absence and presence of sulfur. On platinum, the competitive reactant adsorption favors oxygen dissociation such that oxygen self-poisoning is observed for Pt/silica and Pt/alumina. Contrarily for Pt/ceria, no oxygen self-poisoning is observed, which seems to be due to additional reaction channels via sites on the platinum-ceria boundary and/or ceria surface considerably far from the Pt crystallites. Addition of sulfur dioxide generally leads to the formation of ad-SO(x) species on the supports with a concomitant removal and/or blockage/rearrangement of surface hydroxyl groups. Thereby, the methane oxidation is inhibited for Pt/silica, enhanced for Pt/alumina and temporarily enhanced followed by inhibition after long-term exposure to sulfur for Pt/ceria. The observations can be explained by competitive oxidation of SO(2) and CH(4) on Pt/silica, formation of new active sites at the noble metal-support interface promoting dissociative adsorption of methane on Pt/alumina, and in the case of Pt/ceria, formation of promoting interfacial surface sulfates followed by formation of deactivating bulk-like sulfate species. Furthermore, it can be excluded that reduction of detrimental high oxygen coverage and/or oxide formation on the platinum particles through SO(2) oxidation is the main cause for the promotional effects observed.
    Physical Chemistry Chemical Physics 02/2013; · 3.83 Impact Factor
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    ABSTRACT: It is well known that longer Bi-2212 conductors have significantly lower critical current density (Jc) than shorter ones, and recently it has become clear that a major cause of this reduction is internal gas pressure generated during heat treatment, which expands the wire diameter and dedensifies the Bi-2212 filaments. Here we report on the length-dependent expansion of 5 to 240 cm lengths of state-of-the-art, commercial Ag alloy-sheathed Bi-2212 wire after full and some partial heat treatments. Detailed image analysis along the wire length shows that the wire diameter increases with distance from the ends, longer samples often showing evident damage and leaks provoked by the internal gas pressure. Comparison of heat treatments carried out just below the melting point and with the usual melt process makes it clear that melting is crucial to developing high internal pressure. The decay of Jc away from the ends is directly correlated to the local wire diameter increase, which decreases the local Bi-2212 filament mass density and lowers Jc, often by well over 50%. It is clear that control of the internal gas pressure is crucial to attaining the full Jc of these very promising round wires and that the very variable properties of Bi-2212 wires are due to the fact that this internal gas pressure has so far not been well controlled.
    Superconductor Science and Technology 02/2013; 26(5). · 2.76 Impact Factor
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    ABSTRACT: Bi-2212 round wire is made by the powder-in-tube technique. An unavoidable property of powder-in-tube conductors is that there is about 30% void space in the as-drawn wire. We have recently shown that the gas present in the as-drawn Bi-2212 wire agglomerates into large bubbles and that they are presently the most deleterious current limiting mechanism. By densifying short 2212 wires before reaction through cold isostatic pressing (CIPping), the void space was almost removed and the gas bubble density was reduced significantly, resulting in a doubled engineering critical current density (JE) of 810 A/mm2 at 5 T, 4.2 K. Here we report on densifying Bi-2212 wire by swaging, which increased JE (4.2 K, 5 T) from 486 A/mm2 for as-drawn wire to 808 A/mm2 for swaged wire. This result further confirms that enhancing the filament packing density is of great importance for making major JE improvement in this round-wire magnet conductor.
    IEEE Transactions on Applied Superconductivity 01/2013; 23(3). · 1.20 Impact Factor
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    ABSTRACT: The features associated with modern synchrotron radiation machines (intense and coherent beams) result in a substantial extension of X-ray imaging capabilities in terms of spatial and temporal resolution, phase contrast and 3D images. This allows crystal growth-related information to be obtained which is not available otherwise. After briefly describing the main synchrotron radiation based imaging techniques of interest, we give original examples illustrating the new capabilities for crystal growth: characterisation of crystals grown for applications, such as ice tri-crystals produced for mechanical deformation studies; SiC; crystalline silicon for solar photovoltaic cells; in situ and in real time studies of quasicrystal growth (AlPdMn); and ultrafast tomography for the study of the growth of dendrites in metallic alloys.RésuméLes caractéristiques des installations modernes de rayonnement synchrotron (faisceaux de rayons X intenses et cohérents) ont amené à une augmentation notable des possibilités des techniques dʼimagerie, tant en ce qui concerne la résolution spatiale et temporelle que le contraste de phase et les images tridimensionelles. Ceci permet dʼobtenir des informations sur la croissance cristalline qui ne peuvent être obtenues autrement. Après une brève description de techniques dʼimagerie aux rayons X au synchrotron, nous donnons des exemples originaux qui illustrent les possibilités nouvelles pour les études de croissance cristalline : caractérisation de cristaux produits pour des applications, tels les tri-cristaux de glace que lʼon fait pousser pour des études de déformation mécanique, SiC, silicium monocristallin pour des cellules solaires photovoltaïques, des études in situ et en temps réel de la croissance de quasicristaux (AlPdMn), et la tomographie ultrarapide pour lʼétude de la croissance de dendrites dans des alliages métalliques.
    Comptes Rendus Physique 01/2013; 14(s 2–3):208–220. · 1.82 Impact Factor
  • ACS Catalysis 01/2013; 3(3):339-347. · 5.27 Impact Factor
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    ABSTRACT: The structural (from local to long range order) and electronic evolution of Fe-doped anatase nanomaterials is evaluated using in situ synchrotron based X-ray total scattering/diffraction and absorption techniques and Raman spectroscopy. These methods are combined with UV-visible spectroscopy and infrared diffuse reflectance (DRIFTS) to maximize the level of structural and morphological information regarding these materials. Fe doping of anatase nanomaterials generates solids having Fe at substitutional positions of the structure, and anion vacancies as charge neutrality entities. These structural modifications lead to materials with decreasing band gap energy as a function of Fe content. Concomitantly, mid-gap electronic states are observed to appear. At the same time, the presence of Fe alters the primary particle shape of the anatase without changing significantly the primary particle size. In contrast with previous results, analysis of the combined results strongly indicates that UV and visible photocatalytic activity are here dominated by morphological rather than structural/ electronic issues.
    Catalysis Science & Technology 01/2013; 3:626-634. · 3.75 Impact Factor

Publication Stats

623 Citations
391.55 Total Impact Points

Institutions

  • 1999–2014
    • European Synchrotron Radiation Facility
      • Division of Experiments
      Grenoble, Rhône-Alpes, France
    • Bologna Center
      Bolonia, Emilia-Romagna, Italy
  • 2012–2013
    • Spanish National Research Council
      • Institute of Catalysis and Petrochemistry
      Madrid, Madrid, Spain
    • Birkbeck, University of London
      Londinium, England, United Kingdom
  • 2011–2013
    • CERN
      Genève, Geneva, Switzerland
    • Vienna University of Technology
      • Institute of Materials Science and Technology
      Vienna, Vienna, Austria
    • University of Geneva
      Genève, Geneva, Switzerland
    • Universiteit Utrecht
      • Division of Inorganic Chemistry and Catalysis
      Utrecht, Provincie Utrecht, Netherlands
  • 2010
    • Université de Poitiers
      Poitiers, Poitou-Charentes, France
  • 2000
    • University of Zurich
      Zürich, Zurich, Switzerland
  • 1998–2000
    • Università degli Studi di Trieste
      • Department of Physics
      Trieste, Friuli Venezia Giulia, Italy
    • Sincrotrone Trieste S.C.p.A.
      Trst, Friuli Venezia Giulia, Italy
  • 1995–1998
    • INFN - Istituto Nazionale di Fisica Nucleare
      Frascati, Latium, Italy