CE08 - Matériaux métalliques et inorganiques et procédés associés

3D Bragg ptychography: Developments for metallic nano-twinned thin films – PtyMet

WP1: Nano-twinned gold thin film preparation and initial characterization.
WP2: Atomistic simulations of the relaxed state of nano-twinned gold films.
WP3: CXRD characterization up to 3D Bragg ptychography of nano-twinned gold films.
WP4: Characterization of the macroscopic electrical and mechanical properties of nano-twinned gold films.

From an experimental point of view, we carried out a synchrotron mission in December 2020. Three tests were carried out, on films of 50 or 150 nm, stretched in [100] or [110] directions. Macroscopic deformation, out-of-plane elastic deformation and pole figures were measured in-situ. The goal was to study the behavior of twins during traction (does their volume increase? Do they reorient themselves? Do they store many defects?), and the influence of these twins on the mechanical behavior of the film. The choice of samples and tests was justified by (separate) studies showing that the direction [110] promotes twinning while the direction [100] favors the gliding of perfect dislocations, and by the observation of twin in materials nano-crystallines having high energy of stacking defects. This data processing is very heavy; the PhD student and I share it. One sample is studied, we start the others in September 2021.

From a simulation point of view, we started by studying the coherent twin boundaries (that is, a {111} plane separates the twin from the parent crystal). We determined the displacement field and the excess volume introduced by the joint, and studied how these interact. We then resumed this study with incoherent twin joints, along {112} type planes. These joints are much more delicate since they dissociate, revealing a crystallographic phase called 9R.
In particular, we have shown that these two types of joints introduce contractions of crystal lattices, that two coherent joints attract each other while two incoherent joints repel each other, and that the interaction energy at ~ 2 nm is about 200 times weaker in the first case than in the second.

The priorities are (i) to use the synchrotron data obtained at the end of last year, (ii) to resume the simulation study with other interaction potentials (which should be fast since the method is established) in order to determine the influence of different parameters (mainly elasticity constants and stacking fault energy). I expect to be able to write an article in early 2022 on simulations. The dissemination of the experimental results has not yet been defined: will we group the results according to the thickness of the film? following the direction of traction? will we add simulations or electron microscopy?

publication:
P. Godard, On the use of the scattering amplitude in coherent X-ray Bragg diffraction imaging, Journal of Applied Crystallography, 54, 797-802 (2021)

international conferences:
P. Godard et al, Mechanical behavior study of 50 nm-thin film of gold single crystal with in situ X-ray pole figure measurements, 31/05/21, European Materials Research Society Spring Meeting

Y.F. Woguem et al, Mechanical properties of gold thin films with nano-twins: interactions between coherent twin boundaries and between incoherent twin boundaries, 13/09/21, European Congress and Exhibition on Advanced Materials and Process

Submission summary

Developing electronic circuits on stretchable substrates would allow renewing numerous industrial sectors, in particular those related to flexible electronics and transports. To this aim, we need to prepare and characterize materials with a low electrical resistivity, a high mechanical strength and significant shaping capacities. Nano-twinned metals have these properties, thanks to numerous and structured interfaces: the twin boundaries. At the Institut P’, we know how to prepare gold nano-twinned single crystals. The project aims at characterizing at the nanoscale these interfaces in thin films, and at anticipating their roles on the mechanical and electrical properties. To reach these goals, we want to develop a microscopy technique applied up to now on model materials: Bragg ptychography. This technique, based on a coherent X-ray beam, should rapidly know significant progress due to the present improvements on synchrotron radiation sources and free electron lasers. Bragg ptychography allows characterizing strain field and crystalline defects at a nanometre scale. It is a three-dimensional and non-invasive method, that does not require any sample preparation, and applicable to films with a thickness between a few nanometres to a few micrometres. I greatly participated to its development on model samples (semi-conductors or bio-crystals) and wish now to enlarge the spectrum to include metals.

Therefore, we want optimizing the deposition technique to prepare single crystalline films with an ideal microstructure in view of the mechanical and electrical properties: nano-twins, with a controlled size and density, and as few as dislocations and impurities as possible. These samples will be characterized with Bragg ptychography, and the results will be confronted to atomistic simulations (molecular dynamics) that will give the intrinsic strain field associated to the desired microstructure. Finally, after a transfer of these films on flexible substrates, tensile tests will be performed, during which we will measure in-situ the electrical resistivity, the stress in the film (thanks to classical X-ray diffraction) and the strain of the substrate.

To sum up, this project aims at giving a push to a method (Bragg ptychography) developed during my post-docs on samples studied in the laboratory where I presently work (metallic thin films on polymeric substrates). It gathers persons belonging to three different teams. Indeed, I asked to experts in various fields to join the project: atomistic simulations (S. Brochard and J. Durinck), mechanical (P.O. Renault) and electrical (S. Hurand) properties at the macro-scale, and physical vapour deposition of thin films (M. Drouet). A transverse group will formalize and make durable these new collaborations. Lastly, this project is seen as a springboard to another project like a PRC or an ERC, during which we want to follow, in-situ and locally, the microstructure evolution of these materials during mechanical tests. We will then have a multi-scale view (atomic scale with molecular dynamics, nanoscopic with Bragg ptychography, macroscopic with classical diffraction and electrical resistivity) of the deformation mechanisms of these promising thin films.

Project coordinator

Monsieur Pierre GODARD (Institut P' : Recherche et Ingénierie en Matériaux, Mécanique et Energétique)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Pprime Institut P' : Recherche et Ingénierie en Matériaux, Mécanique et Energétique

Help of the ANR 229,880 euros
Beginning and duration of the scientific project: January 2020 - 48 Months

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