Droplets motion through modulation of surface energy gradient
Droplet motion through electro-wetting is nowadays largely studied and used in several systems and applications. In order to be useful, this technique needs an electrical field to monitor the droplet. For this post-doctoral fellowship, the main objective is to define an alternative method to the using of the electro-wetting technique in order to generate a droplet motion. The elaboration of surfaces with energy gradients conceived by thin film deposition or by laser ablation will be realized inside this study. The main difficulty is related to the patterns realization in order to obtain the appropriate hydrophilic/hydrophobic resolution. Apart from these “classical” techniques, an innovative method will be studied here by using switchable molecules. These molecules could modify the contact angle between a surface and a droplet, when acting on the potential of hydrogen (pH) or the wall temperature. For all the defined surfaces, the post-doctoral fellow will also analyze the coupling effect between the surface energy gradient and a thermal energy gradient on the droplet motion dynamics.
Porous layer integration for advanced temporary substrates
Double transfer of thin single crystalline processed layer can be very interesting for all technologies that require front and back side engineering of the silicon active film. With the increase of the electronical system complexity, this alternative technology can offer new opportunities to miniaturize the semiconductor devices. To fulfill such requirements, a recent alternative technology was developed at the CEA-LETI, based on the use of porous silicon substrates [1]. This new technology will be of a great interest for 3D integration, back-side imager but also MEMS or photovoltaics applications. This technology should now be validated at a larger scale, and we need to focus on all involved mechanisms such as the porous silicon layer rupture.
In a first place, the applicant should comprehend the specification of porous materials in thin film configuration, including elaboration steps and distinctive properties of transferred porous layers. Subsequently he/she may need to interact with Leti’s technological experts to determine process improvements to be implemented to reach pre-established specifications of desired prototypes. In order to evaluate and recommend appropriate materials and equipments, he/she may need to extensively focus on the behaviour of porous material under specific stress conditions such as chemical, thermal or mechanical solicitations. The purpose is to ensure compliance to Smart Cut or Smart stacking technologies that involve amongst others processes molecular bonding technology.
Later, the effort should be focused on the development of a specific technology to induce the mechanical separation inside the buried porous silicon layer. One line of approach would be to trigger the mechanical separation by ultrasound solicitation. Understanding the mechanisms of the splitting and characterising the resulting structures are part of the expected work to be completed in this project.
[1] A-S.Stragier et al., JECS,158 (5) H595-H599 (2011)
Modelling of interstitial cluster evolution in bcc metals after helium implantation
Under irradiation, structural materials inside nuclear reactors undergo changes in mechanical properties, which result from the formation of point defect clusters, such as cavities (clusters of vacancies) and interstitial dislocation loops (clusters of self-interstitial atoms). Understanding the formation processes of such clusters is thus of prime importance. Recently, three-dimensional interstitial clusters, known as C15 clusters, have been shown theoretically to be highly stable in iron [1]. In order to detect such clusters experimentally, an idea is to make them grow, as shown for dislocation loops after helium implantation [2].
This approach will be carried out experimentally in various bcc metals in the framework of the ANR project EPigRAPH, in collaboration with Chimie ParisTech, GEMaC and LPS.
In this project, the following modelling tasks will be performed by the postdoc:
- DFT calculations will be done to obtain the energetic properties of point defects and point defect clusters in the bcc metals envisaged in the project.
- These data will then be used to parameterize a kinetic model based on cluster dynamics [3]. This formalism is particularly well adapted to simulate the evolution of point defect clusters over long physical times.
The modelling work will be performed in close collaboration with another postdoc working on the experimental part.
[1] M. C. Marinica, F. Willaime, J.-P. Crocombette, Phys. Rev. Lett. 108 (2012) 025501
[2] S. Moll, T. Jourdan, H. Lefaix-Jeuland, Phys. Rev. Lett. 111 (2013) 015503
[3] T. Jourdan, G. Bencteux, G. Adjanor, J. Nucl. Mater. 444 (2014) 298
Automatic driving of a finite element software based upon a domain decomposition strategy. Application to ultrasonic non-destructive testing.
One the most important field of activity at the DISC (Department of Imaging and Simulation for Control) of CEA - LIST is to provide a comprehensive set of tools for modeling and simulation for Non-Destructive Testing (NDT). These tools are gathered within the computational platform CIVA. Most of the ultrasound models -- elaborated by the LSMA (research laboratory for Simulation and Modeling in Acoustics) -- are based upon semi-analytical methods. Although very efficient, these methods suffer from a loss of precision as soon as some critical phenomena (e.g. head waves or caustics) or some particular features of the material (e.g. flaws or heterogeneities ) appear in the control experiment. To circumvent these limitations, one of the field of research in the LSMA is to build coupling schemes between semi-analytical and numerical methods. Following this strategy, a computational software based upon high-order finite elements combined with domain decomposition strategies is developped in order to address 3D configurations. The work proposed here focuses on increasing the complexity of the configurations reachable within this coupling strategy. A typical example being the fluid-structure interaction in the case of flaws reaching the bottom of the material to control.
Electrochemical device for purifying hydrogen in a reformed gas
This project aims to establish a new research and development on purification devices for fuel reformers for hydrogen fuel cells. This work is of prime importance for fuel cell systems fed by different sources of hydrogen. Used in "power full" or "range extender" modes, the reformer and gas purification system are elements of the chain that have to be optimized.
Objective is to develop an electrochemical device for purifying the gas from a reformer whose basic principle is similar to that of a PEM electrolyzer. The gases from the reformer undergo a selective electrocatalytic oxidation to separate hydrogen and conventional pollutants directly power a fuel cell.
The project will focus on selection and characterization of catalysts electrocatalytic performance and the achievement of functional prototypes. These developments will assess the economic relevance of the device vis-à-vis other systems and identify areas of research to develop thereafter.
Fabrication and characterization of high thermal conductivity SiCf/SiC composites
SiCf/SiC ceramic matrix composites are foreseen candidates for structure materials and claddings in fast neutron reactor of 4th generation. However, their use may be limited because of their too low thermal conductivity in the operating conditions (< 10 W/mK).
SiCf/SiC ceramic matrix composites are now elaborated by chemical vapour infiltration (CVI). In order to improve their thermal conductivity (reduced porosity), it is planned to develop a hybrid elaboration process combining CVI and liquid routes.
The objective of this study is to determine the conditions of elaboration of a SiC matrix by liquid routes and then to characterize the thermo-mechanical behaviour of the hybrid composites, particularly in relation to CVI references.
Large-area processing and design of functional piezoelectric nanomaterials for flexible sensors and systems
CEA LETI develops innovative highly flexible strain sensors which exploit the piezoelectric properties of self-organized gallium nitride nanowires. The fabrication steps are basically: i) nanowire growth, ii) nanowire assembly, iii) encapsulation, iv) contacting. First demonstrators with small active area (1.5 cm²) have already been achieved using the Langmuir Blodgett (LB) technique for the assembly of nanowires. The present project is concerned with the scaling-up of the assembly process over large surface areas, as well as controlled patterning of nanowire assemblies in 1D and 2D by using an innovative CEA LITEN roll-to-roll technology called Boostream® which has the same functionalities as LB in its basic function.
The aim of the post doc is to develop a new building block for the Boostream® equipment enabling a controlled assembly of wires with a pre-defined design. The candidate will carry out studies to optimize the wire assembly, develop the process of film patterning and fabricate, integrate and characterize GaN nanowire piezoelectric transducers with dimensions of 15x15 cm².
More generally, this post doc will also provide the opportunity to develop a generic knowledge to manipulate micro or nano wires or fibers giving new solutions in various fields such as surface structuration, electronic skin, energy...
In situ analytical device based on the LIBS technique for the characterization of hard environment liquid media
The proposed research project aims at developing an in situ analytical device based on the LIBS technique for the characterization of hard environment liquid media such as high temperature melting materials or highly volatile liquid metals used for development of low carbon energy production. The project involves two CEA teams specialized in LIBS instrumentation, analytical developments and high temperature environments.
At high temperature, the molten metals have a high surface reactivity leading to processes of oxidation, slagging … Non-intrusive analysis of this surface by traditional LIBS tools leads to a non-representative results of the molten metal chemical composition. In this project, a new-patented concept based on a mechanical stirring coupled to the LIBS device is developed in order to have a renewable and stable surface of the liquid metal. The aim is to have an on-line representative composition of the metal during the treatment process. The developed demonstrator will be validated for the analysis of impurities (at ppmw ranges) in liquid silicon (T> 1450 °C) during the purification process and the crystallization one for photovoltaic applications. At the end of the project, recommendations for in-situ analysis of liquid sodium (used as cooling fluid in nuclear reactors) will be given.
Proton conducting interpenetrating polymer networks as new PEMFC membranes
This subject takes place in the frame of the development of proton exchange membrane fuel cells (PEMFC) and the main objective is to increase their performance and durability for operation above 100°C at low relative humidity.
The current standard membranes for use in PEMFC applications remain perfluorosulfonated ionomers such as Nafion® due their good proton conductivity and chemical stability. Nevertheless, their proton conductivity decreases for relative humidity below 70% especially at high temperature because of a too low density of proton conducting groups. This characteristic is a limitation for their use in the working conditions of the requirements for the automotive application. With these polymers, an increase of the proton conducting group density leads to a decrease of mechanical and dimensional stability. Yet, this stability is already quite low and decreases the PEMFC durability. The goal of this subject is to develop new membrane structures based on interpenetrating polymer networks that do not present this antagonism between good mechanical stability and proton conductivity. This strategy which has recently been patented by CEA (patent application number 08 06890) is based on the association of two entangled polymer networks, one sulfonated for proton conductivity and one fluorinated for mechanical and chemical stability.
The applicant will make the membranes and then will characterize their mechanical properties, proton conductivity as well as gas permeability. He will also quantify their performance and durability in a running fuel cell.