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René Caudron passed away

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.


CPFEM simulations of grain size effect in FCC polycrystals: a new approach based on surface GND density

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.

Orientation imaging at the onset of plastic deformation

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.

Coupling Bragg Coherent Diffraction Imaging (BCDI) and Molecular Dynamics to investigate nanostructure

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. (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.

Modelling the mechanical behaviour of polycrystalline materials

Understanding the deformation processes that lead to the failure of polycrystalline structural materials is one of the main challenges in materials science. Significant progress has been made in recent decades, thanks to the development of new experimental characterisation techniques and advanced simulation methods.

However, the localisation of plasticity in slip bands and the propagation of plasticity through a polycrystalline aggregate are not fully understood.

One of the difficulties in modelling the mechanical behaviour of polycrystalline materials is its intrinsically  multi-scale nature. Inelastic deformation mechanisms occur at the dislocation scale (dislocation is a crystalline defect ,vector of plastic deformation) that result in the formation of intra-granular microstructures which, in turn, interact with grain boundaries.

This internship project aims to study the first stages of plastic deformation in policrystalline materials (Cu, Ni) using dislocation dynamics (DD). In particular, the microMegas DD code coupled with a mechanical solver will be used, to correctly handle boundary conditions,  to capture the onset of    plastic deformation localisation and thus model the interaction between a slip band and  grain boundaries. 
This mechanism will be studied in a “model” polycrystalline aggregate, and then, if possible,  extended to a digital twin of a real polycrystal.


Job: Internship (4-6 months)

Academic level : Master degree

Location: LEM, Châtillon

Expertise: : Solid state physics, Materials Science. Interest in theoretical physics and numerical simulations

Contacts: Benoit Devincre

Elaboration and characterization of metallic nanoparticles; analysis of synthesis effects in the growth of carbon nanotubes by CVD

The proposed internship topic is part of a cooperative project on the development of new catalysts (metal or bimetallic nanoparticles) for the growth of carbon nanotubes, aiming at controlling their electronic properties during their synthesis.

The nanoparticles as catalysts will be synthesized, in collaboration with the partnering group of Pr V. Huc at the Institute of Molecular Chemistry and Orsay Materials (ICCMO) by combining surface chemistry and coordination chemistry. We propose to compare the same set of metallic or bimetallic nanoparticles synthesized using very different synthetic pathways and then we will compare the catalytic action of these nanoparticles in the growth of nanotubes.

We will focus on the Ni-Ru catalytic system, identified as a good candidate for achieving the desired electronic character selectivity.

– A first part of the experimental work will be to synthesize the nanoparticles using soft chemistry via the colloidal route (optimization of the synthesis by first reducing the Ru3 + in Ru2 + and then combining it with Ni) and Prussian Blue Analogues at ICMMO. The latter will be calcined under argon in order to obtain carbureted species. We will do the structural study (size distribution, composition, crystalline structure) at LEM. For the latter, we will use a powerful set of investigation techniques (high-resolution transmission electron microscopy, diffraction, energy loss spectroscopy and X-ray spectroscopy) present in the laboratory (LEM).

– A second part of the experimental work will be to grow carbon nanotubes by CVD using the nanoparticles developed as a catalyst (colloidal, by “Prussian blue” and Prussian blue “carbides”) and to characterize their structures (chirality, length, type of adhesion to the nanoparticle) by TEM (imaging and diffraction) and Raman spectroscopy. These three sets of catalysts should be compared to observe the influence of the synthesis route and the influence of the carburized phase on the growth of carbon nanotubes.

Job: Internship (4-6 months)

Academic level : Master degree

Location: LEM, Châtillon

Expertise: Good training in condensed matter physics and chemistry with a major focus on nanoscience and courses on synthesis and characterization.
Strong interest for experiments

Contacts: Armelle Girard, Annick.Loiseau

Chemistry and morphology of nanoalloys for growth catalysis of carbon nanotubes by CVD

The proposed internship topic is part of a cooperative project on the development of new bimetallic catalysts in the solid solution state for the growth of carbon nanotubes, aiming at controlling their electronic properties during their synthesis.

Although several techniques are available for the synthesis of transition metal-based bimetallic catalysts, they generally lead to nanoparticles with a core/shell or janus morphology. Nevertheless, our previous studies have shown that it is possible to synthesize bimetallic particles in solid solution state, that is with no elemental segregation within the nanoparticle by combining surface chemistry and coordination chemistry under particular temperature conditions.

However, studying the thermodynamic behavior of these bimetallic catalysts often requires the implementation of complicated experimental techniques that often need to be coupled with theoretical approaches, even if the latter are still far from being able to reach such a level of complexity while remaining predictive. In collaboration with the partnering group of numerical simulations at the Institute of Molecular Chemistry and Orsay Materials (ICCMO) (Jérôme Creuze and Fabienne Berthier), we will undertake theoretical study to better characterize the behavior of these nanoparticles, in equilibrium and under ultra-vacuum first. Indeed, it is necessary to know the thermodynamics of nanoalloys as isolated systems before studying the influence of external perturbations. We will also study the kinetics of return to equilibrium in order to determine the stability and the lifetime of metastable configurations that will have been identified during the first step of the study.

The internship will thus identify and quantify the key thermodynamic parameters involved in the distribution of constituents within the bimetallic nanoparticles and understand how the thermodynamic variables influence the equilibrium configuration using these parameters. The candidate will be able to compare his results with experiences when possible.

Job: Internship (4-6 months)

Academic level : Master degree

Location: LEM, Châtillon

Expertise: Good training in condensed matter physics and chemistry with a major focus on nanoscience, thermodynamics. Strong interest for numerical calculations

Contacts: Armelle Girard, Annick.Loiseau


Development of a dynamic Monte Carlo code for atomic diffusion calculations

Diffusion processes in solids are relevant for the kinetics of many microstructural changes that occur during preparation, processing, and heat treatment of materials. Typical examples are nucleation of new phases, diffusive phase transformations, precipitation of a second phase, recrystallisation, high-temperature creep, and thermal oxidation. To reach a deep understanding of diffusion in solids, one needs information on the position of atoms and how they move in solids. The atomic mechanisms of diffusion in crystalline solids are closely connected with defects. Point defects such as vacancies or interstitials are the simplest defects and often mediate diffusion in crystals. Ab initio methods as DFT (Density Functional Theory) can provide fundamental information, such as the stable positions of atoms in a crystal lattice and their jumping rates between two neighour sites. However, it is not trivial to obtain diffusion coefficients from these fundamental properties because, in complex solid crystals, there are usually various point defects (vacancy and interstitial positions) and hence several diffusion paths are possible for the diffusing atom. Analytical solutions of multi-state diffusion problems are generally complex. A good alternative is to resource to Kinetic Monte Carlo (KMC), which is a particular Monte Carlo method used for processes with known rates such as atom migration. It consists of mapping N possible events that can occur from a given state. Each event is defined by jump frequency, displacement and cumulative function of the jump frequency: all these input quantites can be obtained from the DFT calculations.

The main aim of this project is to develop a Kinetic Monte Carlo code for studying diffusion of atoms in crystals. The code will receive as input data the results of DFT calculations, which will be performed with the VASP code. The first applications will address the diffusion of interstitial atoms (B, O, N, C) in Ti-Al alloys. The code will be validated by comparison with the analytical solution of the simplest crystal lattices (as the tetragonal TiAl) and then it will be applied for the study of diffusion in more complex geometries (as the hexagonal Ti3Al).

Electronic properties of nanostructured thin films

The control of the composition and morphology of materials at the nanoscale allowed to disclose novel structural, electronic and chemical properties which are fundamental for many recent technological advances. Amongst nanostructures, 2D materials are a class formed of materials which cryistallise as atomically-thin layers. Since the discovery of graphene in the early 2000s, the family of 2D materials grew larger, with the emergence of new systems alike the hexagonal boron nitride (hBN) or the black phosphorus (BP).

Because of their extreme thinness, 2D materials often display electronic properties sizeably different from those of their bulk equivalent. Moreover, their characteristics are strongly influenced by the interaction with the near surroundings: for instance by modifications of the substrate, or changes of their thickness. Van der Waals heterostructures are based on this principle. They are built by stacking layers of different 2D materials on top of each other, so that several properties are combined in the same system and tuned in a controlled fashion. This allows to engineer specific properties aimed for technological development or fundamental research.

In this context, we will consider heterostructures based on hBN and/or BP layers. In order to study these systems from a theoretical perspective, we will elaborate a mixed approach combining analytical and numerical developments in the tight-binding formalism, with ab-initio simulations. The latter will be done on simple reference systems, with the intent to establish a quantitative basis for the parametrization of tight-binding models. This will make possible the investigation of extended systems like realistic heterostructures. More precisely, the objectif will be that of studying the influence of the environment (substrate, stacking …) on the electronic and optical properties of van der Waals heterostructures based on hBN and BP.

Another specificity of this work will consist on coupling the theoretical study with diverse experimental techniques, namely thanks to our rich collaboration network.

2018, web site created by HA & RG.