All posts by webmaster

How accurately can we simulate and understand the transformation mechanisms of matter ?

By in Seminar

Fabio Pietrucci, Sorbonne Université, IMPMC, Paris

fabio.pietrucci@sorbonne-universite.fr

Molecular dynamics simulations can complement experiments by providing detailed, atomic- scale information about transition mechanisms between different states of materials, including nanostructures, solids, solutions, biomolecules etc. If interatomic forces are accurately described, in principle, transition states (difficult to capture in experiments due to their short lifetime) can be identified, barriers and rates can be quantitatively estimated. This kind of information can be useful to characterize the behavior of materials in real conditions of temperature and pressure, and to make sense of synthesis or degradation processes.

However, a major hurdle consists in the long characteristic timescale of many transformation processes, exceeding by far what can be simulated today (typically, from nanoseconds to microseconds). I will present some methods developed in my group, that tackle the latter challenge exploiting two strategies. The first consists in applying external forces on some flexible order parameters, specifically designed to capture and accelerate changes in the topology of the atomic network during a transformation. The second consists in directly exploring transition states and mechanisms using “transition path sampling” techniques: the resulting trajectories, projected on an order parameter, can be effectively modeled by Langevin equations, that in turn allow (based on a recently demonstrated variational principle) to optimize in a unified way the order parameter definition, the free-energy landscape and the kinetic rate. I will discuss applications to problems ranging from structural changes in core-shell nanoparticles, to crystal nucleation, to protein-protein interaction.

References

F. Pietrucci, Rev. Phys. 2, 32 (2017).
S. Pipolo, M. Salanne, G. Ferlat, S. Klotz, A.M. Saitta, F. Pietrucci, Phys. Rev. Lett. 119, 245701 (2017). L. Mouaffac, K. Palacio-Rodriguez, F. Pietrucci, J. Chem. Theory Comput. 19, 5701 (2023).

Synthèse par CVD de films de nitrure de bore aux propriétés optimisées pour dispositifs en optoélectronique

By in Phd Defence

Résumé

Dans la famille des matériaux bidimensionnels (2D), le nitrure de bore a été identifié comme un matériau stratégique. Ce semi-conducteur à grand gap (>6eV), atomiquement plan, résistant chimiquement et thermiquement, peut jouer plusieurs rôles dans les hétérostructures de matériaux 2D : substrat de graphène pour préserver la mobilité exceptionnelle de ses porteurs de charge ou couche encapsulante pour protéger d’autres matériaux 2D sensibles à leur environnement ou exalter leurs propriétés. Des démonstrateurs de principe ont été réalisés avec des monocristaux de BN. Les dimensions latérales et l’homogénéité en épaisseur du BN sont limitées par la dimension initiale millimétriques des cristaux et leur mise en oeuvre par exfoliation mécanique. Cette technique est donc difficilement industrialisable. Il est nécessaire de développer des synthèses de films de BN de dimensions, structure et qualité contrôlées pour permettre une montée en échelle. Dans cette thèse en partenariat avec la PME Annealsys, nous avons choisi de développer la synthèse de films de BN sur nickel par dépôt chimique en phase vapeur à basse pression (LPCVD). Dans un premier temps, nous avons transposé sur le bâti de l’équipementier Annealsys le procédé de synthèse de BN sur des substrats de nickel polycristallin à partir de borazine déjà maitrisé par l’équipe. Nous avons confirmé que la morphologie et la qualité du BN dépend de l’orientation cristallographique du nickel sous-jacent et que l’orientation (111) du nickel est la plus favorable pour la synthèse de film continu de BN. Nous avons donc ensuite travaillé avec des substrats monocristallins de Ni(111) /YSZ/Si(111). Nous avons porté une attention particulière à la préparation de ces substrats spécifiques et développé un traitement de stabilisation in-situ dans le bâti de dépôt, compatible avec un procédé industriel. La structure et la qualité des films de BN synthétisés, i.e. épaisseur, rugosité, séquence d’empilement, cristallinité et taille de domaines, ont été caractérisées de l’échelle atomique à l’échelle millimétrique par un panel de techniques de microscopies et spectroscopies (AFM, MEB, Raman, MET. . .). Nous avons mis en place une méthodologie de caractérisation statistique à l’échelle centimétrique, indispensable à la vérification de l’homogénéité des films de BN, prérequis pour la fabrication de dispositifs performants. Nous avons fait varier des paramètres de synthèse clés tels que la quantité de gaz précurseur ou l’épaisseur du substrat de nickel et étudié leur impact sur les films de BN. Les résultats sont discutés d’un point de vue mécanisme de croissance.

 

Candidate:
Laure Tailpied

Jury:
Pr. Luc Imhoff – Université de Bourogne- Rapporteur
Dr. Laëticia MARTY – Université Grenoble Alpes – Rapporteur
Dr. Berangère Toury – Université Lyon 1 – Examinatrice
Pr. Franck Vidal – Sorbonne Université – Examinateur
Dr. Jean-Manuel Decams – Annealsys – Invité
Dr. Amandine Andrieux-Ledier – ONERA – Encadrante
Dr. Annick Loiseau – ONERA – Directrice de thèse

Mercredi 25 avril 2023 à 14h00
Salle Contensou, ONERA, 29 Avenue de la Division Leclerc,92320, Chatillôn

Cristallographie des bicouches homophases désorientées par rotation-translation

By in Seminar

Denis Gratias et Marianne Quiquandon
CNRS-UMR 8247 IRCP, Chimie-ParisTech PSL, Paris

On se propose de discuter la symétrie résultant de la superposition de deux couches monoatomiques cristallines identiques désorientées l’une par rapport à l’autre d’une rotation-translation (α|τ).
Un réseau de coïncidence apparaît —défini par le groupe intersection des groupes de translation des réseaux des monocouches— pour un ensemble dense dénombrable de valeurs de la rotation α, qu’on discutera en toute généralité pour les quatre types de réseaux bidimensionnels, oblique, rectangle, carré et hexagonal. Ces valeurs singulières d’angle α associées aux normes σ des vecteurs unitaires du réseau de coïncidence se répartissent dans le plan (α, σ) selon des branches indexées par des suites de Farey et dont on discutera les propriétés.
Pour une rotation donnée, les symétries spatiales de ces bicouches se répartissent en un
petit nombre seulement de groupes selon la valeur de la translation τ. Ainsi les bicouches de
graphène à réseau de coïncidence ne peuvent présenter que 6 types de groupes d’espace quelles que soient la rotation a de coïncidence et la translation τ.
Dans le cas générique d’absence de réseau de coïncidence, la bicouche présente une
symétrie quasipériodique de rang 4 au plus qu’on peut décrire par une méthode de coupe à partir d’un espace de dimension 4. On montrera l’importance fondamentale du réseau-0 (0-lattice) pour décrire les symétries des figures de moiré de ces édifices.

Virtual material design

By in Seminar

Maxime Moreaud

IFPEN, Solaize

Since 2017, IFPEN has fully entered the race for accelerated design of new materials with models creating links between synthesis and effective properties. Its AI and materials teams propose new tools for the numerical generation and characterization of materials microstructures.  
This approach realistically considers the microstructure to capture morphological and topological details at scales of interest. Numerical models link to synthesis or processing parameters, and estimate textural and usage properties. In this talk, we will discuss the general ideas of this approach, examples of multi-scale microstructures, and some recent work on numerical textural characterizations such as tortuosity and deep learning accelerated physisorption simulation.

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Rydberg atoms: a versatile tool for quantum technologies

By in Seminar

Sylvain Schwartz

Laboratoire QTECH (ONERA)

Rydberg atoms are by definition atoms which have been excited to a state with a large principal quantum number, resulting in exaggerated properties such as a large atomic size, a long lifetime compared to other excited states and large matrix elements for the dipole operator. In practice, dipole-dipole interactions between Rydberg atoms are at the heart of quantum simulations, where they are used to create entangled atomic states. But the large dipole of Rydberg atoms can also result in a strong coupling with external electromagnetic fields, making these atoms good candidates to be used as very sensitive probes of electromagnetic environment in the GHz to THz range. I this talk, I will give a brief overview of the state of the art of quantum simulation and quantum metrology with Rydberg atoms, and present the ongoing project that we have in the QTech lab at ONERA about quantum metrology with cold Rydberg atoms trapped in optical potentials. Possible applications include electromagnetic intelligence, THz imaging and scientific applications such as the calibration of black-body shifts in state-of-the-art optical clocks (in collaboration with SYRTE and laboratoire Aimé Cotton).

Atomic scale modeling of the plasticity of body-centered cubic transition metals

By in Seminar

Baptiste Bienvenu, Chu Chun Fu and Emmanuel Clouet

Université Paris-Saclay, CEA, Service de Recherches de Métallurgie Physique, 91191 Gif-sur-Yvette

At low temperature, plasticity of body-centered cubic (BCC) transition metals is governed by the glide in compact {110} planes of screw dislocations with a ½<111> Burgers vector, experiencing a high friction with the crystal lattice. The aim of this work is to build laws to predict the plastic flow stress based on atomic scale modeling of the core properties and mobility of these dislocations (using ab initio calculations and molecular dynamics), allowing to link them to macroscopic mechanical properties (yield stress, slip system activity).
In this context, a special care is given to the case of chromium (Cr), the only BCC transition metal having a structure close to antiferromagnetism, a spin-density wave, below ambient temperature. To qualify the impact of magnetism on the plasticity of Cr, ab initio calculations at zero temperature were coupled to Monte Carlo simulations at finite temperature. This allowed to conclude that magnetism has only a marginal influence, except at very low temperature where the ½<111> Burgers vector of these dislocations generates magnetic faults given that it does not respect the magnetic order of Cr.
In the following, a systematic study across all seven BCC transition metals (vanadium, niobium, tantalum, chromium, molybdenum, tungsten and iron) helped develop a yield criterion reproducing the experimental features of the so-called “non-Schmid” effects, characteristic of these metals at low temperature. However, some effects cannot be captured by this criterion, accounting for the motion of isolated dislocations only. This is for instance the case of anomalous slip, observed in all BCC transition metals except iron, and characterized by slip activity of ½<111> dislocations in low-stressed {110} planes. Through in situ observations in a transmission electron microscope performed by Daniel Caillard (CEMES-CNRS, Toulouse), coupled with atomistic simulations, a new mechanism explaining this phenomenon in all BCC metals has been evidenced, based on the high mobility of multi-junctions. Finally, the mobility of dislocations with a <100> Burgers vector, most often observed as junctions between ½<111> dislocations but rarely considered as possible slip systems, is studied using atomistic simulations. It was evidenced that, even if the mobility of <100> screw dislocations is competitive with the conventional ½<111> in {110} planes, <100> dislocations are locked at low temperature along a mixed orientation requiring a very high stress to start moving, thus explaining their low slip activity.

Ladislas Kubin passed away

By in News

It is with great sadness that we learned of the death of our colleague Ladislas Kubin, which occurred on October 18, 2022.

A graduate of the Ecole Centrale Paris in 1966, Ladislas Kubin carried out his thesis work at the Solid State Physics Laboratory of the University of Orsay. Recruited at the CNRS in 1968, he spent his entire research career there, first at the Electronic Optics Laboratory in Toulouse, then at the Physical Metallurgy Laboratory in Poitiers and finally at the Laboratory for Microstructural Investigations (LEM), a joint ONERA-CNRS research unit, which he joined from its creation in 1988 and where he continued his career until his emeritus in 2008.

Ladislas Kubin was an outstanding physicist and has profoundly marked the field of plasticity physics through experimental and theoretical research dealing with the individual and collective behavior of dislocations in order to better understand the deformation mechanisms of crystalline metals and alloys. On the experimental level, in the first part of his career, he endeavored to develop means of in situ study of the individual behavior of dislocations by transmission electron microscopy. This work then led him to develop precursor models for the prediction of dynamic aging phenomena in alloys such as the Portevin-Le Chatelier effect, then at the turn of the 1990s to initiate an original approach to mesoscopic simulation of the mechanisms of plastic deformation of metals. He was thus one of the founders of the French school of dislocation dynamics and one of the first to establish the link between their behavior and the plastic response of metallic materials.

Internationally recognized specialist, Ladislas Kubin was editor for the journal Acta Materialia and the author of the book “Dislocations, mesoscale simulations and plastic flow”. He received bronze and silver medals from the CNRS and the Gay-Lussac Humboldt prize.

Ladislas has trained many doctoral and post-doctoral students and interacted with many researchers in France and abroad. His knowledge in the areas of modeling and experiments was extensive and sought after.

A great physicist and valued colleague has passed away. His memory and his teaching remain.

His funeral will take place on Tuesday October 25, 2022 at 2:30 p.m. at the Saint Charles church in Biarritz (France).

Our thoughts go out to his family and loved ones to whom we extend our sincere condolences.

The LEM

Data-driven models of atomic simulations in discrete and continuous state spaces

By in Seminar

Thomas Swinburne
CINaM, Marseille

Building models for the plasticity, thermodynamics and kinetics of metals is challenging as subtle aspects of atomic cohesion must be faithfully reproduced, and predictions often require averaging over large, complex configuration ensembles. I will discuss how the energy landscapes of atomic systems can be rapidly explored at scale and “coarse-grained” when the dynamics are thermally activated thus thus scale separated[1,2] and how data-driven techniques, typically used to regress energies for modern cohesive models, can be used to capture a much wider range of properties such as defect entropics[3] or dislocation properties. When the dynamics do not have a clear timescale separation, coarse graining is much more challenging. I will discuss how a data-driven approach can provide a solution, producing efficient surrogate models which can predict the evolution of nanoparticle ensembles and the yielding of complex microstructures, offering new perspectives for multiscale modelling approaches[4].

[1]  TD Swinburne and D Perez, NPJ Comp. Mat 2020, MSMSE 2022
[2]  TD Swinburne and DJ Wales JCTC 2020, 2022
[3]  C Lapointe et al. PRMat 2020
[4]  TD Swinburne, In Prep.

Optical properties of black phosphorus: from bulk crystal to atomic layers

By in Phd Defence

Abstract

Black phosphorus is a small gap semiconductor (about 0.3 eV) that has recently joined the family of two-dimensional materials. Due to its modulable band gap from mid-infrared to visible depending on the thickness, its strong anisotropy in the atomic plane as well as the high mobility of charge carriers it is promised to a high application potential in the field of optoelectronics. The objective of this thesis was to study the optical properties of the black phosphorus crystal and its atomic layers.


After a description of the different instrumental developments realized during this thesis, the methods of
fabrication of the samples are discussed. Two points have to be mastered: The elaboration of thin layers and their protection from environmental conditions to avoid their oxidation. In a first part, several methods known as “Top-Down”(mechanical exfoliation, gold assisted exfoliation, ion etching) are compared on the basis of the quality, the size, the thickness of the obtained samples as well as the ease of the operating mode execution. In a second part, two methods of thin film protection are presented: alumina passivation (by ALD or aluminum evaporation) and encapsulation of BP flakes into hBN flakes (hBN / BP / hBN heterostructures).


The strong anisotropy of black phosphorus makes the identification of the orientation of the crystallographic axes a key point in the study of the material. For this purpose, a procedure has been proposed using polarized Raman spectroscopy. It has been confronted and validated by different experimental means (TEM observations, EBSD) and theoretical means (modeling of the Raman intensity in thin films). The vibrational properties have also been studied as a function of the number of atomic layers. Several effects have been noticed at high (> 100 cm-1) and low (< 100 cm-1) frequencies and are attributed to dimensionality reduction and resonance phenomena. Thanks to the peculiar excitation conditions used in this study, a large number of modes related to inter-plane vibrations are for the first time identified and have been shown to be accurate indicators of crystallite thickness. The photoluminescence of the bulk crystal is for the first time studied at room and cryogenic temperatures.

Several band-edge emission components have been identified as excitonic, including a fine line due to the free exciton. The analysis of their behavior as a function of temperature as well as a calculation of the binding energy of the free exciton taking into account the anisotropy of the medium have made it possible to establish a new reference value for the black phosphorus gap at 0.287 eV at 2 K. The photoluminescence study of the exfoliated crystals revealed the disappearance of the fine line of luminescence in favor of a wide band. This change is attributed to the density of defects introduced by the mechanical exfoliation as evidenced by a broadening of Raman bands. The photoluminescence band was followed as a function of the thickness of the exfoliated layers down to 8 atomic layers. Below a threshold thickness evaluated at 25 nm, a shift of the band towards high energies is highlighted, and is very well described by a quantum confinement model. No significant difference is observed between the alumina passivated and hBN encapsulated samples, which indicates that the dielectric effects are not predominant in the thickness range studied.

Phd Candidate:
Etienne Carré

Jury:
Christophe TESTELIN – Directeur de recherche, CNRS, Sorbonne Université – Rapporteur
Laëticia MARTY – Chargée de recherche, CNRS,Université Grenoble Alpes – Rapporteur
Bruno MASENELLI – Professeur des universités, INSA Lyon – Examinateur
Aurélie PIERRET – Ingénieure de recherche, CNRS, École Normale Supérieure Paris – Examinatrice
Pierre SENEOR – Professeur des universités, CNRS, Université Paris Saclay – Examinateur
Annick LOISEAU – Directrice de recherche, ONERA, Sorbonne Université – Directrice de thèse
Julien BARJON – Professeur des universités, UVSQ – Directeur de thèse
Ingrid STENGER – Maîtresse de conférences, UVSQ – Encadrante

 

Thursday 23th June 2022, 14h30
Salle Contensou, ONERA, 29 Avenue de la Division Leclerc,92320, Chatillôn

Modelling of the propagation of a short crack in ductile material coupling phase-field method and dislocation dynamics

By in Phd Defence

Abstract

The propagation of short cracks in FCC metals is strongly influenced by microstructures, in particular associated with the linear defects of the crystals, i.e. dislocations.

In this work, a new coupling between two methods at the mesoscale is proposed to investigate the interaction of moving cracks with three-dimensional dislocation microstructures. First, crack propagation is predicted by a phase field model. In this approach, cracks are described by some continuous damage field that evolves so as to minimize the total free energy, including stored elastic energy and surface energy associated with the crack. Second, dislocation microstructures are handled by a Dislocation Dynamics (DD) model that describes plastic deformation by the movement of dislocations under external loading.

To couple both models, the DCM (Discrete-Continuous Model) approach is used, where dislocations are described by continuous fields (eigenstrain or Nye tensor) in an elastic solver. Fast Fourier Transform (FFT) based solvers are used for their computational efficiency. Particular discretization schemes have been adopted to minimize the smoothing of dislocation cores, usually performed in MDC approaches. The different schemes are carefully analyzed with respect to the quality of the predicted fields. In addition, the resulting model is implemented using efficient parallelization solutions.

Thanks to this new coupling, we have been able to study the elastic shielding on crack propagation according to the nature of the slip systems and the dislocations density. We have also been able to investigate phenomena and ingredients rarely accounted for, such as dislocation cross slips close to the crack front or the influence of the number of sources. This mesoscale method constitutes a breakthrough for the thorough analysis of physical mechanisms controlling the early stages of fracture in metallic materials.

Keywords : Crack, Plasticity, Multiphysics modelling, Dislocation Dynamics, Phase Field

Phd Candidate:
Luis Eon

Jury:
Stéphane Berbenni – Directeur de Recherche CNRS, LEM3, Metz – Rapporteur
Samuel Forest –  Directeur de Recherche CNRS, CDM, Evry  – Rapporteur
Véronique Doquet – Directrice de Recherche CNRS, LMS, Palaiseau  – Examinatrice
Lionel Gélébart – Ingénieur-chercheur HdR,  CEA/DEN, Gif-sur-Yvette – Examinateur
Rénald Brenner – Directeur de Recherche CNRS, D’Alembert, Paris – Examinateur
Yoann Guilhem – Maître de conférences, LMPS, Gif-sur-Yvette – Examinateur
Riccardo Gatti – Chargé de Recherche CNRS, LEM, Châtillon – Encadrant de thèse
Benoît Appolaire – Professeur des Universités, IJL, Nancy – Directeur de thèse

 

Tuesday 14th June 2022, 10h00
Salle Contensou, ONERA, 29 Avenue de la Division Leclerc,92320, Chatillôn

2018, web site created by HA & RG.