Hereafter are the summaries of the short and long projects now finished and selected in the frame of the 2013 Call of LabEx EMC3:
Metrology through numerical holography and electronic tomography of nano-objects
Axis 3 : Transdisciplinary research – GPM/CRISMAT/CORIA
The tomographic atom probe (TAP) is a device providing 3D-chemical pictures based on the positioning of clear identified atoms of a material. The knowledge drawn from these characterisations is strategically important for the development of functional and structural new materials used in industries such as transport, energy production and storage, light emission devices.
However, the TAP suffers a limitation linked to the physical process dynamics leading to data collection. With this device, collected ions come from the surface of the sample (pencil tip with a nano curve) whose forms define the ion paths. The evaporation dynamics is very fluctuating and changes the surface morphology when different areas appear on it. This leads to important evaporation, incomplete reconstruction or a damaged spatial resolution.
In order to fix this phenomenon, several groups included GPM in Rouen, developed correlative microscopic approaches by combining TEM (Transmission Electron Microscopy) and TAP. However, this approach is complicated (transfers of nanosamples) and hardly replicable (sample oxidation, irradiation). In order to control TAP reconstructions, it is currently essential to determine in-situ, while collecting, the morphologic changes at the surface of the sample. In the NanoHolo project, the method planned lies on in-line holography.
It is a special imaging method already used in TEM. It consists in forming a punctual electronic source thanks to a corrector of spherical aberrations. Interferences between the reference electronic wave (source) and the diffused wave through the nano-object (in this case a TAP tip) shape an electronic hologram it is possible to register with a CCD camera. From those holograms, sample pictures have been reconstructed by adapting existing reconstruction algorithms.
However, the ultimate goal which was to model the 3D-profile of a nano-object by in-line holography could not be reached. The source filling the specification note of the atom probe was not available and the first approach planned on the project could not work. The second approach is based on the development of new numerical reconstruction model. It might be more advanced and adapted to electrons but nothing proves the success of this new approach.
Schematic view of the process leading to the creation of an in-line hologramme in STEM
Leader : Williams LEFEBVRE (GPM)
Study of the functional properties of a nanocrystalline stainless steel synthetized by flash sintering
Axis 1: Materials for Energy – GPM/CRISMAT
The objective of the CMAIN project (2014-2015) was to elaborate a stainless steel AISI 316L with grain size ranging in the nanometer range and to study its mechanical properties and corrosion resistance by comparing it to a traditional stainless steel with conventional grain size. Structures lightening is currently at stake in the sector of the transports with the CO2 reduction policy and to decrease the cost of the raw materials.
The originality of this project stands in the use of flash sintering of metal parts to develop samples of 316L stainless steel. This process allows to study the influence of the grain size in the nanometric range on the functional properties of the material.
As planned, the decrease of the grain size from 3000 nm to 400nm increased from about 50% the flexibility of a material. However it hardens the process of plastic deformation (stamping, ironing, folding) of stainless steel. It then tends to prove that synthetised nanocrystalline stainless steel have better mechanical properties while preserving a good corrosion resistance. The used strategy within the project turned out to be relevant.
The results obtained during CMAIN on the functional properties are encouraging. Regarding the mechanical properties, data collection from macroscopic tests allows to compare the behaviour of nanocrystalline samples against conventional stainless steel. Even if the influence of oxides on the mechanical behaviour is hard to quantify, it seems like 316L stainless steel with grain size in the nanometer range does not behave the same with an initial strong stretching.
Thus, the synthesis through flash sintering offers various perspectives. At first, the synthesis of a large metal part nanometric grain materials with high mecanical resistance properties allows the characterisation of macroscopic mechanical properties. It is then possible to model the mechanical behaviour of this kind of material to better predict their behaviour and durability during the life of the material. Besides, to remedy the loss in versability of the material, flash sintering make possible the synthesis of controlled-microstructure material with a strong contrast on grain size. The population of nanometric-sized grains can increase the mechanical resistance whereas greater-sized grains can increase the versability.
In the wake of the results of this project, a thesis has been funded. CMAIN also opened new perspectives: other projects have emerged on the crushing process under Argon and allowed both GPM and CRISMAT to bring new progresses and skills on powder metallurgy.
Leader : Clément KELLER (GPM)
Towards new nanostructured materials for permanent magnets
Axis 3: Multidisciplinary research – GPM/CRISMAT
The main goal of NEOMAP project (2014-2015) was to synthesize nanocomposite powders (SrFe12O19/Fe), to obtain permanent magnets with improved magnetic properties. The materials should then combine the properties of hexaferrite magnets, less costly and corrosion-resistant and magnets from rare earths, which are more expensive are less resistant to corrosion but whose magnetic properties are superior.
The numerous syntheses allowed to set up a new process, based on the oxidation of magnetite spheric nanoparticules, enabled the synthesis of nanometric hexaferrite powder with high magnetic properties, favourable to get a magnet.
The results of this research showed that spark plasma sintering preserves the initial structure of the powder while improving its magnetic properties. Besides, the synthesis of iron nanoparticles appears to be a promising way to obtain a magnetic nanocomposite, showing that the synthesis of single or dual phased nanometric powders can be considered. The results of this 1-year project led to 5 communications in international conferences.
Following the funding of LabEx EMC3, a funding as been obtained through ANR and 2 projects have followed with the support of the Institute Carnot ESP (Energy and Propulsion Systems). This shows the interest of the scientific and industrial communities towards the recycling of permanent magnets and the research initiated via LabEx EMC3.
Fig. 1 (a) Hysteresis cycle of a nanocomposite powder SrFe12019/Fe obtained by nanometric Fe synthesis with SrFe12O19, in red, compared with the cycle of a SrFe12O19 powder in blue, (b) dM/dH derivative of the nanocomposite powder demagnetizing curve and (c) compared magnetic properties of the different powders studied.
Leader: Jean-Marie LE BRETON (GPM)
Gas emission in irradiated polymers: study of energy transfers towards irradiation defects
Axis 1: Materials for energy – CIMAP/LCMT
Ionizing radiations modify polymers and are is at the source of new chemical groups called defects. In parallel, small molecules are released from the material in the form of gas. During the storage process of waste composed of polymers contaminated with radionuclides, these gas are subject to explosion (H2), corrosion (HF, HCl) or toxicity risks. It is therefore essential to predict all along the life of the nuclear waste, the ageing level of the materials as well as the gas emitted.
In order to achieve these goals, the TEPRI project (2014-2015) planned to set up polyethylene with controlled concentrations with ketone and carboxylic acid functions. Mass concentrations, reaching 2% in carboxylic acids, have been incorporated via the process of reactive extrusion in presence of maleic acid.
Fig. 1: Evolution of the radiochemical yield of G(H2), according to its effective mass content in carboxylic acids functions in PECOOH obtained after 6 and 10 minutes in the extruder. Irradiated samples under inert atmosphere with doses of 56 and 123 kGy, at room temperature with gamma rays.
The second phase of the project consisted in studying the behaviour of these materials under ionizing radiations. The long term objective being to set up a model predicting gas emissions in parcels according to the ageing level of the materials.
Despite the incorporation of reduced mass fractions, the results obtained during this study are a first of its kind in the world in the study of the specific influence of carboxylic acids on gas emissions in polyethylene subject to ionizing radiations, under an oxidizing atmosphere.
The concentrations in acids obtained are lower than expected. Besides, the early stabilization of gas emission can be observed at low carboxylic acids rates. Besides, an higher decrease of G(H2) can be observed in the irradiated polyethylene with electron beams, in oxidizing atmosphere, with doses allowing to create oxidized defected rates similar to the content of carboxylic acids chemically inserted. These two results indicate that the incorporates groups are not distributed in an homogeneous way within the polymer chain but seem to be organized in the form of oligomers within lateral chains.
The increase of the grafting ratio in the polyethylene without drastic modification of the microstructure remains a significant obstacle. The reduction of some conditions on the materials (chain lengths or quantity of material) is a relevant option. In order to solve this issue, the team decided to study the initiation of the behaviour of the grafting via irradiation in very low doses.
The work started in the frame of the TEPRI project has been pursued, at first, by the analysis and understanding of the results of gamma irradiations, and then, through the study of the behaviour under electron beams and heavy ions. These irradiations allowed to confirm the results already obtained under gamma beams.
The results obtained during the TEPRI project acted as a springboard allowing a cofunding through CNRS and ERDF (2016-2019). These 2 combined fundings are destinated to the purchase of equipment adapted, on the one hand, to the incorporation of ketone functions and, on the other hand, to the improvement of the homogeneous incorporation of carboxylic acids functions with higher rates.
Leader: Yvette Ngono-Ravache (CIMAP)