PhD positions offers

The current energy context reveals the need for deep changes in all energy-intensive sectors. These transitions affect the choice of energies and materials, particularly in the field of transport. For aeronautical applications, composites - in particular new materials with a thermoplastic matrix - are widely spread to lighten structures. To guarantee the integrity of structures, materials must be tested in accident situations, especially subjected to a flame. This accidental configuration generates multiphysical phenomena that are very difficult to represent: the heterogeneous material with an intrinsic complex geometry is subjected to thermal or chemical aggression, which modifies its structure and its thermo-physical properties. This attack has a huge impact on its mechanical properties, but mechanical stresses also influence the distribution of heat transfer.

The present PhD is thus part of the scientific transdisciplinary topic of the thermo-mechanical behavior of composites subjected to thermal attack, which has been studied for a dozen years in the research team Mechanics of Materials of the GPM, in collaboration with CORIA, a laboratory specialized in heat transfer, fluid mechanics and combustion. The latest works highlighted the importance of porosity in the mechanisms occurring during a thermal attack. Porosities are generated within the matrix by its degradation (pyrolysis) and by debonding due to internal stresses, in particular expansion stresses induced by strong thermal gradients. These pockets of vacuum or gas, which can connect to each other, will drastically modify the distribution of mechanical stresses and heat transfer within the material. It is therefore essential to be able to represent these porosities in terms of quantity and morphologic distributions.

Among the means of characterization available in the research team, X-ray tomography permits to statistically quantify these porosities depending on the conditions. The distribution of porosities will not only be studied as a function of time, but will also be considered in relation to thermo-physical properties such as thermal diffusivity to verify mixing laws.

In order to represent the application and the real geometry of a fiber/matrix network, these variations in porosity in the matrix will be analyzed to explain its role in matrix/yarn debonding, or even intra-yarn debonding. This analysis of thermo-mechanical couplings will lead to the definition of thermo-mechanical behavioral laws. Finally, these laws will be implemented in FE simulations developed in the laboratory to help better representation of the phenomena.

The doctoral student will hence develop methodologies to conduct in situ X-ray tomography to identify the kinetics of porosity formation/growth/coalescence. This prominent mechanism, greatly influences heat transfer and stress distribution within the material, and acts at several scales. The multi-scale and multiphysics effects of porosity are thus the key factors to be studied to understand the thermo-mechanical coupling of composite materials exposed to fire.

Beginning: October 2024

Contacts: benoit.vieille@insa-rouen.fr, tanguy.davin@insa-rouen.fr

Keywords: composite materials, thermo-mechanical analysis, multi-scales approach, porosity, XR tomography

Beginning of the project: as soon as possible, ideally September 2024

Context

Metallic alloys are essential materials at all levels for the production, storage or transport of hydrogen. The aim of this project is to deepen fundamental knowledge of the interactions at atomic scales between hydrogen and crystalline defects or carbides in steels. This is an important issue, since it is linked to hydrogen embrittlement, a phenomenon with major industrial consequences. Several phenomenological models already exist which are based on hypothesis related to these hydrogen/defect interactions. There is however a lack of knowledge especially in complex or dynamic configurations when other solutes are present (such as carbon in solid solution) or when dislocations interact with carbides that have potentially trapped H atoms.

Objectives

In this PhD project, we propose an original approach based on model microstructures obtained by heat treatment of steels to obtain different configurations (martensite, tempered martensite, ferrite with nanoscaled carbides). These materials will be electrolytically charged with hydrogen, and the trapping sites identified by TDS and atom probe tomography. Particular attention will be paid to the competition between carbon and hydrogen atoms for segregation on dislocations and grain boundaries. This information will be correlated with simulation undertaken by the other partners of the “Hystyle ANR Project”. To study dynamic effects, microstructures will be aged in-situ by transmission electron microscopy under hydrogen atmosphere. Finally, correlation with embrittlement mechanisms will be undertaken via micromechanical tests (in-situ compression or bending of micro-samples) enabling individual structural entities (grain boundaries, for example).

Profile required

- Engineering degree and/or a Master degree.
- Strong motivation for experimental research on advanced techniques.
- Solid background in materials science and physical metallurgy (phase transformations, crystalline defects and relationships between microstructures and mechanical properties).
- Prior experience in the field of microstructural characterization, mechanical properties and study of structure/properties relationships in a metallic alloys would be highly appreciated.
- Good written and oral communication skills, and fluency in English.

Host laboratories

The PhD project will be based at the Groupe de Physique des Matériaux UMR CNRS 6634 laboratory, located at the University of Rouen Normandy. https://gpm.univ-rouen.fr/

Part of the PhD work will be carried out at the “Laboratoire des Sciences de l'Ingénieur pour l'Environnement (LaSIE)” - UMR CNRS 7356 located at the University of La Rochelle, where various visits are planned. https://lasie.univ-larochelle.fr/Presentation

As part of the "Hydrogen in Steels - A transition scales problem - HYSTYLE" project, scientific exchanges will also take place with other laboratories of the consortium.

Contact persons and application

Xavier SAUVAGE, xavier.sauvage@univ-rouen.fr 

Abdelali OUDRISS, abdelali.oudriss@univ-lr.fr 

Xavier Feaugas, xavier.feaugas@univ-lr.fr

Multiferroic (MF) materials are materials that possess spontaneous magnetic order (i.e. local magnetization) and electric polarization (i.e. in the absence of magnetic field and electric field) below a temperature called the transition temperature. These materials have significant potential for technological applications in the field of very high-density information storage and spin electronics. In the latter case, the challenge consists of controlling/manipulating the local magnetization of the material using an electric field, which would allow very significant energy savings. However, we are still far from the ideal MF material for which it would be possible to control the local magnetization with a weak electric field at room temperature and which would have a large electric polarization that could easily be “reversed”. This project therefore concerns the study, by numerical simulations, of existing MF materials with the aim of better understanding and confirming/disproving/completing the experimental results as well as the study of new potentially MF materials (doped materials for example). This work will be based on realistic modeling based on ab initio calculations using density functional theory and on Monte Carlo simulations. The properties studied will be magnetic order at low temperatures, phase transitions, manipulation of local magnetization using an electric field and manipulation of electric polarization using a magnetic field. This work should make it possible to highlight conditions for improving the properties of MF materials, in particular increasing their transition temperature for future applications at room temperature.

Skills required : the candidate should have good skills in magnetism in solids, in statistical thermodynamic and should be attracted to numerical simulations.

Financial support : grant from the University of Rouen - Normandy (about 1400 euros /month).

PhD supervisor : Pr. D. Ledue (denis.ledue@univ-rouen.fr)
 

The objective of this thesis is to study trap phenomena in the latest generations of Gallium Nitride (GaN)-based High Electron Mobility Transistors (HEMTs). Various methods for characterizing trap behavior in GaN HEMTs exist, such as Deep Level Transient Spectroscopy (DLTS) for capacitance and current (C-DLTS and I-DLTS), Athermal Direct Current Transient Spectroscopy (A-DCTS), Deep Level Optical Spectroscopy (DLOS), Gate-Lag (GL) and Drain-Lag (DL) transient measurements, the study of transconductance frequency dispersion, S-parameter measurements, and low-frequency noise measurements. The temporal and thermal ranges explored by these techniques are sometimes different, highlighting the complementarity of these measurements.

The candidate must have training in electronics, with knowledge of RF power transistors. Knowledge in the field of materials (semiconductors, solid state physics) and physical simulation will be appreciated.

Contacts : Pascal Dherbécourt, 02 32 95 51 57. Niemat Moultif, 02 32 95 50 78.
Interested candidates should send a cover letter and their CV to : pascal.dherbecourt@univ-rouen.fr ; niemat.moultif0@univ-rouen.fr; mohamed.masmoudi@univ-rouen.fr .

A solid can be in a state of zero stress at the macroscopic scale, and have non-zero stresses at its local scale. Metallic materials can be affected by these states, particularly if phase transformations are involved in the manufacturing process. The interactions between heat, metallurgy and mechanics are indeed the source of the development of these stresses, as is well known in the fields of welding, casting or additive manufacturing. As these internal stresses are generally present in an undesired and poorly controlled way, they are referred to as residual stresses. They are added to those due to external mechanical loading and can therefore contribute to premature damage to a material: reduced fatigue life, acceleration of stress-dependent physical mechanisms (diffusion of chemical species, phase transformation, oxidation).

Various techniques have therefore been developed to control these residual stress states. On the macroscopic scale of a part, they consist in locally removing an element of material, for example by drilling, and measuring the deformations due to stress relaxation. This measurement can be carried out using Digital Image Correlation (DIC), providing a deformation field and opening up the possibility of characterizing a multiaxial field. In this multiaxial context, the process of moving from measured deformations to residual stresses is based on Finite Element Analysis (FEA) reproducing the material removal operation.

This method can be transposed to the micrometer scale of the part: by integrating a Focus Ion Beam (FIB) probe into a Scanning Electron Microscope (SEM), it is possible to perform a material removal operation on a scale of a few micrometers, while capturing the image of the material as it is being machined. Having previously marked the material with ~10 nm markers, the Digital Image Correlation (DIC) method can be used to track the displacements of the markers and deduce a deformation field. Given the characteristic dimensions (plot, trench, markers), the stresses involved (~100 MPa) and the high stiffnesses, the accuracy required for a usable displacement measurement is of the order of 1 nm.

Ensuring the conditions for sufficiently accurate field measurements is the first challenge of this thesis. The first objective is therefore to develop the SEM-FIB-CIN analysis protocol based on samples with simple, known residual stress conditions: uniaxial stress of approximately known value (XRD method), grain size large enough to guarantee homogeneous mechanical fields. The second objective is to extend the method to a context of heterogeneous (intrinsically multiaxial) microstructure and mechanical fields. To be able to take into account the presence in a close vicinity of a grain other than the one treated, or even of another material phase, it is then necessary to take this vicinity into account in the finite element analyses. This context is that of steels produced by additive manufacturing, or that of an austenitic-ferritic steel. Both are highly heterogeneous, with characteristic grain-to-grain or phase-to-phase lengths of the order of a few μm; both can feature stress states of the order of 300-400 MPa, and for both, detailed knowledge of these states represents a major industrial challenge, given the cutting-edge applications targeted.

Contact : Fabrice Barbe, fabrice.barbe@insa-rouen.fr