Scientists of LEM, LP2N (France), Polytechnique Montréal, Université de Montréal (Canada) have succeeded in encapsulating organic dye molecules inside a boron nitride nanotube. This encapsulation protects efficiently organic dye molecules against degradations inherent to their surrounding conditions and improves the fluorescence over a time scale longer by 104 with respect to that of free dyes.
More details on the of CNRS INP website.
Speaker : Dr. Lisa Belkacemi-Rebrab (Centre des Matériaux, MINES ParisTech)
Date and Location: Friday 16/10/20 14h00, LEM meeting room (E2.01.20), Châtillon.
Originally from Belgium near Mons, René Caudron did most of his studies there, before joining Onera in 1964 where he was part of the small group that, around Paul Costa, set up the ONERA Solid State Physics Laboratory created at the initiative of Raimond Castaing. He spent his entire career at ONERA in the Materials Department and then at the LEM. He was one of the essential members of the laboratory, an extraordinary engineer-physicist, originally by many aspects of its national and international reputation.
At a time when almost all experiments were set up “at home”, René participated in all the research “manips” of LEM during the first twenty-five years of its existence, which were devoted to the study of the electronic structure of transition compounds: carbides, nitrides, hydrides, borides. These were low-temperature experiments, the highlight of which was undoubtedly its specific low-temperature heat measuring device, one of the most efficient at the time. He thus contributed to validating the models developed at Orsay and Strasbourg on diluted alloys. This was his thesis work.
He took part in all the experimental studies of the laboratory, specialising for a long period in the study of spin glasses, before moving on to the study of chemical effects in alloys. On this occasion, he built his famous G4.4 diffuse scattering spectrometer, installed on the CEA’s Orphée nuclear reactor in Saclay, which he was in charge of until his retirement in 2003, and which was also among the most efficient in the world.
An outstanding physicist and experimentalist, René Caudron made a deep impression on his colleagues, interns and doctoral students, all of whom testify to having met in him an extraordinary researcher and, above all, a man of conviction of unspeakable kindness and modesty.
A multiscale modeling methodology involving discrete dislocation dynamics (DDD) and crystal plasticity finite element method (CPFEM) is used to study the physical origin and to simulate the grain size effect in FCC polycrystalline plasticity. This model is based on the dislocation density storage–recovery framework, expanded on the scale of slip systems. DDD simulations are used to establish a constitutive law incorporating the main dislocation mechanisms controlling strain hardening in monotonically deformed FCC polycrystals. This is achieved by calculating key quantities controlling the accumulation of the forest dislocation density within the grains and the polarized dislocation density at the grain boundaries during plastic deformation. The model is then integrated into the CPFEM at the polycrystalline aggregate scale to compute short- and long-range internal stresses within the grains. These simulations quantitatively reproduce the deformation curves of FCC polycrystals as a function of grain size. Because of its predictive ability to reproduce the Hall-Petch law, the proposed framework has a great potential for further applications.
Speaker: Maoyuan Jiang
Date and Location: Monday 09/03/20 14h00, LEM meeting room (E2.01.20), Châtillon.
Diffraction Contrast Tomography (DCT) is a near-field X-ray diffraction technique for the inspection of ductile materials at the micron scale. It has traditionally been used for the study of undeformed polycrystalline materials with grain sizes of a few tenths of microns. It uses a box-sized monochromatic X-ray beam, which allows it to scan large regions of millimeter sized sample (with up to thousands of grains) in a relatively short time.
Recent work has introduced sub-grain orientation reconstruction (6D-DCT), which has made DCT a viable tool for the reconstruction of slightly deformed materials.
Topo-tomography (TT) is also a near-field X-ray diffraction technique, which, on the other hand, allows to focus on a single grain with a high-resolution detector and to obtain sub-micron level shape information.
In this talk, we will first present how the data is acquired and reconstructed in modern DCT and TT acquisitions. Then, we will present their 6D and 5D extensions (respectively) for the reconstruction of sub-grain level orientation information. Finally, we will discuss future applications, including the combined use of DCT and TT data in a single 6D reconstruction for the investigation of slip bands formation at the onset of deformation.
Speaker: Dr Nicola Viganò
Date and Location: Friday 21/02/20, 14h00 LEM meeting room (E2.01.20), Châtillon.
Speaker: Dr. Moustafa Benyoucef
Date and Location: Tuesday 10/12/19, 10h30 LEM meeting room (E2.01.20), Châtillon.
Fig. 1 (top) Experimental reconstruction of the u111 displacement field on a 250 nm Pt NP (bottom) u111 displacement field obtained by energy minimization of a simulated Pt NP (right) εxx, εyy and εzz components of the strain tensor derived from the simulation
Physical properties at small length scale deviate strongly from the bulk counterpart, typically below the micrometer. For instance, mechanical strength increases with reducing size, large residual strain due to processing are present in nanostructures. Thus a better understanding of the physical properties in relationship with the microstructure is needed for nanoscale materials. Because of its good spatial resolution (~ 10 nm) and excellent sensitivity to atomic displacements and local strain [1,2], Bragg coherent diffraction imaging (BCDI) has emerged in the past two decades as a powerful tool to probe the structure and local displacement field inside nanoscale objects [3]. When combined with in situ mechanical loading, BCDI is particularly relevant for the study of defect nucleations in isolated nanoparticles [4] or to investigate intragranular deformation mechanisms in polycrystalline thin films [5].
Nowadays, the length scales that are accessible by BCDI and that can be simulated by Molecular Dynamics (MD) simulation are almost converging. The coupling between the two methods is therefore particularly relevant and allows to get a detailed picture of the deformation mechanisms in nanostructures at the atomic scale. This coupled approach has been used to study the surface relaxation of metallic nanoparticles (Au, Pt). An excellent quantitative agreement is obtained between the component of the displacement field measured experimentally and calculated by energy minimization (Molecular Statics) (Fig. 1). With this approach, the measurement of only one Bragg reflection is required to derive the 3D displacement field and the six independent components of the strain tensor from the simulation [6]. The two techniques can also be combined to identify defect structures nucleated during in situ mechanical loading [4,5] and to interpret the evolution of the strain field in nanoparticle catalysts during gas reaction [7,8]..
[1] Watari, M. et al. Nature Materials 10, 862–866 (2011).
[2] Labat, S. et al. ACS Nano 9, 9210–9216 (2015).
[3] Robinson, I. & Harder, R. Nat Mater 8, 291–298 (2009).
[4] Dupraz, M. et al. Nano Lett. 17(11) (2017).
[5] Cherukara, M. et al. Nat. Comm. 9 (2018).
[6] Dupraz et al. to be submitted (2019)
[7] Kim, D. et al. Nat. Comm. 9, 3422 (2018).
[8] Dupraz, M. et al. in preparation
Speaker: Dr. Maxime Dupraz
Date and Location: Monday 25/11/19, 14h00 LEM meeting room (E2.01.20), Châtillon.
Georges Saada was born on August 10, 1932 in Sfax, Tunisia. He left Tunisia at the age of seventeen, after high school and obtaining his baccalaureate. In Paris, he prepared the Grandes Ecoles at the Louis-le-Grand and Buffon high schools. He joined the École Polytechnique in 1952 and then prepared a thesis in metal physics. He also graduated at the École Nationale Supérieure des Télécommunications.
After military service, his career began in the army, which he left in 1960. Indeed his skills led him to scientific research. After five years at the Institut de Recherche de la Sidérurgie, as a research engineer, he chose to focus his career on university teaching. First a lecturer at the University of Lille, he participated in 1969 in the creation of the University of Paris XIII Villetaneuse where he became a professor in 1971. From 1973 to 1981, he was at the head of the Laboratoire des Propriétés Mécaniques et Thermodynamiques des Matériaux.
In 1981, he was appointed Head of Mission for Higher Education at the Ministry of National Education, Alain Savary.
He returned to the University of Paris XIII and joined the Laboratoire d’Etude des Microstructures in 1990.
Georges Saada played a pioneering role in the field of plasticity of materials. His work has had a major impact on the development of this discipline, with seminal contributions to the understanding of the physical mechanisms that cause deformation of metal alloys. His work has been recognized by the award of the Grande Médaille de la Société Française de Métallurgie et de Matériaux in 2008.