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Solid oxide fuel cells (or SOFC) are a class of solid-state electrochemical conversion devices that produce electricity directly from oxidizing a fuel gas. They consist in an anode-cathode duet separated by a solid electrolyte, i.e. an oxide conducting material. The anode is fed with hydrogen or other fuels whereas the cathode is in contact with air, meaning oxygen. Overall, a SOFC operates thanks to the combined action of two external stimuli: a gaseous environment and temperature.Owing to the recent advances in in situ and operando transmission electron microscopy (TEM), we have set up an experiment to operate a SOFC inside an environmental TEM to identify how the device microstructure determines its electrical properties.An elementary anode-electrolyte-cathode sandwich was prepared by Focused Ion Beam (FIB), and mounted on a heating and biasing microelectromechanical (MEMS) based specimen holder (DENSsolutions) inserted a FEI Titan ETEM.Our sample is made as a standard SOFC: the cathode is based on strontium-doped lanthanum manganite (LSM), the electrolyte is yttria-stabilized zirconia (YSZ) and the anode is a NiO cermet; both electrodes were co-sintered with YSZ. NiO was first reduced to Ni, leaving pores in the structure due to the volume loss and hence enabling the penetration of the fuel to the triple phase junctions Ni/YSZ/porosity at the anode side. For practical reasons, we have used a single chamber configuration where the anode and cathode were exposed simultaneously to the oxidant and reducing gases. Thanks to a difference in the catalytic activity between the electrodes, O2 should reduce at the cathode, while H2 should oxidize at the anode, thus leading to a voltage difference between the two terminals and detected by the biasing system of the holder.The reduction of NiO was then first performed under a forming gas N2:H2 in the ratio 20:1 under 15 mbar up to 750°C (N2 was constantly used as a mixing vector gas for safety reasons due to the need of mixing O2 and H2 in the single-chamber configuration). The O2 to H2 ratio was then increased to trigger the operation of the cell. A small quantity of oxygen was introduced into the microscope up to about 16 mbar at 600°C, a temperature sufficient to promote the dissociation of O2 molecules in the ETEM.At this point, the variation of current was correlated to the evolution of the gas fuel composition and the anode microstructure. The latter was followed by conventional and high resolution imaging, diffraction work and EELS (Electron Energy-Loss Spectroscopy).The system was then cycled several times by decreasing and re-increasing the O2 concentration in the gas flow and correlations between microstructure, gas composition, and cell voltage were positively established, as it will be discussed at the conference. Results were further confirmed by macroscopic ex situ tests in an oven using the same materials [1].The operation of a SOFC in a single chamber configuration was demonstrated thanks to operando ETEM. Nevertheless, the pressure and flow limits with this instrument have restricted its operation to a transient working state. This operando experiment opens numerous perspectives to investigate possible failure pathways affecting SOFCs, like poisoning of active sites or coarsening of the Ni catalyst [2].[1] Q. Jeangros, M. Bugnet T. Epicier, C. Frantz, S. Diethelm, D. Montinaro, E. Tyukalova, G. Pivak, J. Van herle, A. Hessler-Wyser, M. Duchamp, under review (submitted 2021/10).[2] The authors acknowledge the French microscopy network METSA (www.metsa.fr) for funding and CLYM (Consortium Lyon-St-Etienne de Microscopie, www.clym.fr) for the ETEM access. The FIB preparation was performed at the Facilities for Analysis, Characterization, Testing and Simulations (FACTS, Nanyang Technological University NTU). Additional support was provided by the INSTANT project (France-Singapore MERLION program 2019-2020) and the start-up grant M4081924 at NTU.

Observation of liquids within a transmission electron microscope (TEM) has long been impossible owing to basic thermodynamic limitations due to the need for a high vacuum, typically 10-5 mbar or better, within the column of the instrument, making it impossible to maintain a liquid state at room temperature. The development of dedicated MEMS-based sealed liquid cells made possible the observation of liquids in general, and water in particular, within the transmission electron microscope but he presence of electron-transparent membranes used to seal such cells seriously hinders the capability of controlling relative humidity (RH) and to perform water condensation from a humid atmosphere. The environmental TEM, where a partial pressure can be controlled and maintained in the pole-pieces gap (where the sample holder tip containing the sample is inserted), enables observations under gas without any sealing membranes. In this “open-cell” configuration, observing water layers is possible under a partial pressure of a few mbar when the temperature is cooled down close to the dew point in order to insure a thermodynamic equilibrium between the solid, gas and liquid states; liquid water can effectively be stabilized in a temperature and pressure range of typically -5 to 10°C and 5 to 15 mbar, respectively. These experimental conditions are achievable with “open-cell” cooling sample-holders within the environmental TEM allowing, through the control of RH, direct visualization of hygroscopic growth, deliquescence and efflorescence of model and real aerosols. Such studies may eventually lead to a better understanding of the hygroscopic behaviour of atmospheric aerosols which are known to act as cloud condensation nuclei.NaCl is among the most widespread aerosols found above oceans; we have thus used NaCl nanoparticles as relevant model aerosols in this preliminary approach. These nanoparticles were obtained by vaporizing a 1mM salt solution onto classical holey (or lacey) carbon TEM grids. We then used a Gatan liquid-nitrogen (LN2) cryo-holder to cool down the specimen to around 0°C within an objective lens aberration-corrected Titan ETEM G2 80-300 kV (FEI/TFS) under variable water pressure (up to 10 mbar); we could efficiently adjust the temperature by mixing LN2 with an adequate volume of ethanol. The microscope is equipped with a SDD XMaxN EDX spectrometer (Oxford Insts.) and a OneView camera (Gatan Insts.) capable of acquiring 4kx4k images at 25fps.Considering the high voltage at which experiments were performed (80 and 300kV), our first concern was to ensure that observations could be performed with controlled electrons doses low enough to avoid noticeable (ideally any) irradiation damage of NaCl. This could be achieved using and an electron flux of ≈ 20 e-.Å-2.s-1 and total doses < 4000 e-.Å-2. Then, we performed water condensation/evaporation cycles to follow the evolution of NaCl cubes under different RH environments and after dissolution, their recrystallisation and redispersion of NaCl on the carbon-film surface (see Figure).The authors acknowledge the ANR (Project PRC WATEM; ANR-20-CE42-0008) for financial support and the Consortium Lyon–St-Etienne de Microscopie (CLYM) for access to the Ly-EtTEM.Observation of liquids within a transmission electron microscope (TEM) has long been impossible owing to basic thermodynamic limitations due to the need for a high vacuum, typically 10-5 mbar or better, within the column of the instrument, making it impossible to maintain a liquid state at room temperature. The development of dedicated MEMS-based sealed liquid cells made possible the observation of liquids in general, and water in particular, within the transmission electron microscope but he presence of electron-transparent membranes used to seal such cells seriously hinders the capability of controlling relative humidity (RH) and to perform water condensation from a humid atmosphere. The environmental TEM, where a partial pressure can be controlled and maintained in the pole-pieces gap (where the sample holder tip containing the sample is inserted), enables observations under gas without any sealing membranes. In this “open-cell” configuration, observing water layers is possible under a partial pressure of a few mbar when the temperature is cooled down close to the dew point in order to insure a thermodynamic equilibrium between the solid, gas and liquid states; liquid water can effectively be stabilized in a temperature and pressure range of typically -5 to 10°C and 5 to 15 mbar, respectively. These experimental conditions are achievable with “open-cell” cooling sample-holders within the environmental TEM allowing, through the control of RH, direct visualization of hygroscopic growth, deliquescence and efflorescence of model and real aerosols. Such studies may eventually lead to a better understanding of the hygroscopic behaviour of atmospheric aerosols which are known to act as cloud condensation nuclei.NaCl is among the most widespread aerosols found above oceans; we have thus used NaCl nanoparticles as relevant model aerosols in this preliminary approach. These nanoparticles were obtained by vaporizing a 1mM salt solution onto classical holey (or lacey) carbon TEM grids. We then used a Gatan liquid-nitrogen (LN2) cryo-holder to cool down the specimen to around 0°C within an objective lens aberration-corrected Titan ETEM G2 80-300 kV (FEI/TFS) under variable water pressure (up to 10 mbar); we could efficiently adjust the temperature by mixing LN2 with an adequate volume of ethanol. The microscope is equipped with a SDD XMaxN EDX spectrometer (Oxford Insts.) and a OneView camera (Gatan Insts.) capable of acquiring 4kx4k images at 25fps.Considering the high voltage at which experiments were performed (80 and 300kV), our first concern was to ensure that observations could be performed with controlled electrons doses low enough to avoid noticeable (ideally any) irradiation damage of NaCl. This could be achieved using and an electron flux of ≈ 20 e-.Å-2.s-1 and total doses < 4000 e-.Å-2. Then, we performed water condensation/evaporation cycles to follow the evolution of NaCl cubes under different RH environments and after dissolution, their recrystallisation and redispersion of NaCl on the carbon-film surface (see Figure).The authors acknowledge the ANR (Project PRC WATEM; ANR-20-CE42-0008) for financial support and the Consortium Lyon–St-Etienne de Microscopie (CLYM) for access to the Ly-EtTEM.

Efficient closed cells specimen holders for Liquid Cell Transmission Electron Microscopy (TEM) has led to many advances over the last two decades but still suffer limitations due to their sealing membranes which hamper comfortable tomography approach, optimized chemical analysis and signal-to-noise ratio. ‘Windowless’ approaches in a dedicated Environmental TEM can not only overcome these limitations but provide opportunities to study water (liquid) condensation-evaporation experiments as required for example to study the water uptake of aerosols in the context of climate and atmospheric chemistry [1]. We present here a development and operationalization of such a windowless, Wet ETEM method using a Peltier based MEMS chip compatible with a DENSsolutions Wildfire holder. The environmental conditions for the evaporation and condensation cycles require a pressure of 6-15 mbar and a temperature within a 0-11oC typical range. As for Environmental Scanning EM (ESEM) [2], these conditions can be achieved using a FEI-Titan ETEM instrument installed at CLYM Lyon, where a gas pressure of up to about 20 mbar can be maintained around a cooled specimen in the pole pieces gap. Adequate temperatures were obtained by placing at the tip of the Wildfire holder a Peltier stage which can produce a temperature difference of ~40 K between the hot and the cold junctions. A ‘micro’ Peltier stage with adequate dimensions for being safely introduced in the ETEM was then connected to a custom printed circuit board (PCB) compatible with the DENSsolutions holder. The sample is deposited on a classical TEM carbon grid mounted in direct contact with the cold side of the Peltier device insuring its efficient cooling.Initial experiments have been conducted on NaCl crystals used as model aerosols [1]. Owing to the fast cooling when a current is applied to the Peltier, a very efficient and easily controlled condensation was observed with a vapor pressure typically around 7 mbar in the column. These preliminary results will be discussed in comparison to similar experiments conducted with a classical cryo-holder (Elsa model from Gatan) cooled down by a liquid nitrogen reservoir as previously performed [1,3]. As a first statement it appears that a much better and faster stability, resulting in a much better knowledge of the exact relative humidity (RH), is obtained with the proposed solution. These results offer new perspectives for the study of the deliquescence or efflorescence of aerosols through condensation and evaporation cycles which can then be easily performed and reproduced [4]. [1] M.E. Wise, G. Biskos, S.T. Martin, L.M. Russell and P.R. Buseck. Phase Transitions of Single Salt Particles Studied Using a Transmission Electron Microscope with an Environmental Cell. Aerosol Science and Technology, 39:9 (2005) 849-856, DOI: 10.1080/02786820500295263. [2] A. Bogner, G. Thollet, D. Basset, P. H. Jouneau, and C. Gauthier. Wet STEM: A new development in environmental SEM for imaging nano-objects included in a liquid phase. Ultramicroscopy, 104 3 (2005) 290-301. DOI: 10.1016/j.ultramic.2005.05.005.[3] B. Levin, D. Haiber, Q. Liu and P.A. Crozier. An Open-Cell Environmental Transmission Electron Microscopy Technique for In Situ Characterization of Samples in Aqueous Liquid Solutions. Microscopy and Microanalysis, 26(1) (2020) 134-138. DOI: 10.1017/S1431927619015320.[4] This work is support by the French National Research Agency under the WATEM program ANR-20-CE42-0008 Thanks are due to CLYM (www.clym.fr) for the access to the ETEM.

SSCI-VIDE+ATARI:MEME+FCA:EEH:CLH:LMA:TEP; International audience; Observing liquids in a Transmission Electron Microscope (TEM) has long been impossible owing to basic thermodynamic limitations due to the need for a high vacuum, typically 10-5 mbar or better, within the column of the instrument, making it impossible to maintain a liquid state at room temperature. With the development of dedicated sealed liquid cells mounted on specific specimen holders, Liquid Cell TEM (LCTEM) has become possible about a decade ago, opening a huge range of possible applications in the field of biology, crystal growth or electrochemistry [1]. In parallel, Environmental TEM (ETEM) was also developed [2]; here a partial pressure can be maintained in the pole-pieces gap where the tip of the sample holder, including the sample itself, is inserted, allowing to perform observations under gas without any sealing membranes as needed with the close-cell technology. Such an ‘open-cell’ approach was also developed in Scanning EM (ESEM, e.g. [3]). With these dedicated ETEM or ESEM configurations, observing liquid such as water layers is possible under a partial pressure of a few mbar if the temperature is cooled down close to the dew point in order to insure a thermodynamic equilibrium between the solid, gas and liquid states: for water, the liquid state can effectively been stabilized in a temperature and pressure range of typically 0 to 11°C and 6 to 15 mbar respectively [4], which are conditions easily accessible in ETEM and ESEM. While LCTEM in a close-cell permits to reach atmospheric pressure, thus allowing to observe water at room temperature, it has the drawback of its advantage: the presence of top and bottom sealing membranes makes it very difficult to perform water condensation from a humid atmosphere and to control water vapor states. Such experiments are possible in ‘open-cell’ ESEM [5] and ETEM [6] and enhance our understanding of the hygroscopic behavior of atmospheric aerosol particles that are known to act as cloud condensation nuclei [7]. Hygroscopic growth, deliquescence and efflorescence of model and real) atmospheric nanoparticles can be directly visualized by these techniques. The present contribution aims at establishing conditions under which aerosols can be adequately observed in a Titan ETEM (FEI/TFS). We use a Gatan liquid-nitrogen (LN2) cryo-holder to cool down the specimen around 0°C. We adjust the temperature by mixing LN2 with a controlled quantity of ethanol. For the purpose of this preliminary investigation, we use NaCl nanoparticles, obtained by vaporizing a salt solution onto a classical holey carbon TEM grid, as a model aerosols. Observations were performed at 300 kV under a humid atmosphere generated by pumping a small sealed water reservoir connected to one of the input lines of the ETEM, the pumping being insured by the molecular turbopumps of its vacuum system. The presence of water (vapor) was controlled by the residual gas analyzer equipping the microscope and by Electron Energy-Loss Spectroscopy (EELS). In a first step, and considering the high voltage at which experiments were performed, a control of the electron flux and dose was realized using different illumination settings in order to define safe imaging conditions avoiding noticeable irradiation damage of the nanocrystals (Fig. 1). Then, both water condensation and evaporation have been performed to follow the evolution of NaCl cubes (Fig. 2). Results will be discussed in terms of relationships between percentage of relative humidity and water uptake of the NaCl particles as a function of T and P [8]. References:[1] FM Ross (Ed.), Liquid Cell Electron Microscopy (Advances in Microscopy and Microanalysis), Cambridge University Press, Cambridge (2017), 524 p.[2] TW Hansen, J.B. Wagner (Ed.), Controlled Atmosphere TEM, Springer, New York, (2016), 332 p.[3] A Bogner et al. Micron, 38 (2007) 390. [4] DJ Stokes. Principles and Practice of Variable Pressure/Environmental Scanning Electron Microscopy (VP-ESEM), (2008), John Wiley & Sons Ltd., 221 p.[5] RC Hoffman et al. J. Aerosol Science, 35 7 (2004) 869. [6] ME Wise et al. Aerosol Science & Technology, 39:9 (2005) 849; C Cassidy et al. Plos One, 12 11 (2017) e0186899; BDA Levin et al. Microscopy and Microanal. (2020) 1.[7] U Lohmann et al. An Introduction to Clouds: From the Microscale to Climate, Cambridge University Press, Cambridge, UK (2016), 391 p.[8] The authors acknowledge the Consortium Lyon – St-Etienne de Microscopie (CLYM, www.clym.fr), the Centre Technologique des Microstructures (http://microscopies.univ-lyon1.fr/) for practical assistance and the French National Research Agency (ANR, www.anr.fr) for supporting project ANR-20-CE42-0008.

MICROSCOPIE+MEME+EEH:FCA:LBU:TEP; International audience; Supported catalysts are generally composed of, at least, one nanometric active phase deposited and/or synthesized at the surface of a convenient nanometer/micrometer powder support. As such these are rather complex systems with many variable parameters: dispersion on the support, size distribution, morphology, structure, defects, variable chemical phases, variable exposed facets of the active phases, interaction with the support, etc. It is thus quite cumbersome to correlate a catalytic behavior with specific physicochemical properties. The synthesis of nanometric particles (NPs) with well controlled characteristics is thus crucial to better understand the behavior of the active phase during reaction and to better correlate its behavior with its catalytic performance. The di-block copolymer approach is well suited to obtain well controlled (size, ordered arrays) supported NPs which can be used as supported metallic catalysts on both flat (model catalysts) and powder (realistic catalysts) supports [1]. In this approach, an amphiphilic di-block copolymer dissolved in toluene yields a system of inverse micelles. We can then metallize the core of these micelles by introducing metal salts. Such systems can then be deposited on flat surfaces by dip or spin coating for instance. We have already extended this method to the synthesis of bimetallic model catalysts [2]. However, many questions are still open concerning, for instance, the formation of unique NPs at the core from the pre-formed bimetallic seeds (Fig. 1b-d) or the stability of the arrays of bimetallic NPs (Fig. 1a) with increasing temperature in presence of reactive gases.In order to gain insight on such issues we have used an Environmental TEM (Titan ETEM G2 from FEI/ThermoFisher Scientific) operated at 300kV under gas pressures up to 20 mbar. To vary the temperature, we have used a WildFire support holder with dedicated heating microchips (DENS Solutions) capable of reaching temperatures up to 1300°C. On top of the latter an Au-rich AuPd core-metallized PS-b-P2VP micellar solution was deposited by spin-coating. Special care was taken to minimize/prevent any influence of the electron beam on the observed events; for instance, a new area was systematically used before each increasing step in temperature (Fig. 2). Temperature treatment under oxygen is crucial both to achieve the formation of unique NPs on the micellar core and to eliminate the copolymer. We could see that the seeds within the micellar cores begin to sinter at around 350°C (consistent with the temperature at which the polymers are decomposed and gasified) and the unique NPs are obtained at around 500°C (Fig. 2). The network is remarkably stable above 500°C and up to 800/900°C (Figure 3); around 1000°C, when the temperature approaches the melting temperature of Au (1064°C), the NPs begin to decompose (Fig. 3). These observations clearly open the possibility of using these structures under rather extreme catalytic conditions (and thus widens the range of catalytic applications); the stability of the arrays appears to be (only) dependent on the intrinsic properties of the metals used [3].References:[1] B Roldan Cuenya, Accounts of Chemical Research 46 (2013) 1682.[2] E Ehret et al, Nanoscale 7 (2015) 13239.[3] The authors acknowledge the French Microscopy and Atom probe network (METSA) and the Consortium Lyon – St-Etienne de Microscopie (CLYM) for supporting this work.

Fiber-reinforced concrete (FRC) is increasingly used in various civil engineering applications. Compared to conventional reinforced concrete (rebars or welded mesh), the FRC has very high ductility in the post-cracking phase and allows more distributed cracks with a smaller opening. The FRC post-cracking behaviour depends on the fiber type (properties of fibers) used and the fiber content. Therefore, the characterization of fiber-reinforced concrete material behaviour under tension is important. In this study, the post-peak behaviour of the FRC using polypropylene macro-synthetic fiber (SikaFiber Force-60 brand) was characterized. According to the European test recommendation (EN 14651, 2005), notched 3‐point bending tests were conducted on 36 prismatic notched beams (dimensions of 600 mm × 150 mm × 150 mm) to identify the stress/crack opening law of FRC. The influence of the fiber dosage was evaluated with six fiber volume fractions of 0.33%, 0.49%, 0.66%, 0.82%, 0.99% and 1.26% corresponding to dosages of 3.0 kg/m3, 4.5 kg/m3, 6.0 kg/m3, 7.5 kg/m3, 9.0 kg/m3 and 11.5 kg/m3 respectively. It was found that the increase of the fiber dosage has no significant influence on the ultimate bending strength, but a substantial increase in the residual tensile flexural stresses was found. A numerical model using the inverse method was also used to reproduce the stress/crack opening behaviour of FRC obtained in the experimental tests. A new approach was proposed to define the post-cracking diagram adapted for the macro-synthetic FRC used. Fib Model Code 2020 approach was also conducted and compared with. The Model Code requirements for FRC’s use in structural applications have also been adapted to the case of macro-synthetic FRC following Hillerborg's concept of fracture energy and a new proposal was made.

National audience; In environmental Transmission Electron Microscopy (ETEM), nanomaterials can be studied under dynamic conditions in a gaseous or liquid environment. An important challenge is to track their evolution in 3D. Intuitively, the acquisition of the tomographic tilt series must be completed in a very short time to prevent any morphological modification due to irradiation and/or dynamic change of the object during its rotation for allowing a relevant volume reconstruction.Recently we have demonstrated than Bright Field (BF) tomography can be performed in a few minutes in an environmental microscope (FEI-TITAN ETEM, 80-300 kV) [1,2]. With the help of fast recording (CMOS or direct electron) cameras capable of acquiring tens or hundreds fps in at least a (2k)² format, acquiring a tilt series during a continuous rotation of the sample without any stop can be completed in only a few seconds. Typical examples on nano-catalysts or electron beam sensitive materials will be reported of tomography acquisitions performed over a tilt amplitude of 140° in short times, down to 5-6 seconds under environmental, e.g. temperature and gas conditions (figure 1). However, several remaining problems must be addressed, like diffraction contrast, blur effects, low SNR. One of the most severe limitations is the imperfect rotation of the goniometer, which lead to systematic drifts of the sample up to the frequent exit of the projection out of the field of view. We will describe a robust solution consisting in a fast computer-routine compatible with a TITAN microscope equipped with a Gatan Oneview camera using the ‘in-situ’ option (figure 2).

International audience; 2D AND FAST 3D IN SITU CALCINATION AND REDUCTION OF Pd NANOCATALYSTS SUPPORTED ON ALUMINA IN ENVIRONMENTAL TRANSMISSION ELECTRON MICROSCOPYThierry Epicier1, Siddardha Koneti1, Lucian Roiban1, Anne-Sophie Gay2, Priscilla Avenier21Université de Lyon, MATEIS, INSA de Lyon, UCB Lyon 1, UMR 5510 CNRS, 69621 Villeurbanne Cedex, France2IFP Energies nouvelles, Rond-point de l'échangeur de Solaize, BP 3, 69360 Solaize, FranceEnvironmental Transmission Electron Microscopy (ETEM) has recently open the way to operando studies of nanomaterials down to the atomic resolution in temperature and under a gaseous atmosphere that may reproduce more or less its real environmental conditions of use for real applications [1]. This is especially relevant in the field of nanoparticles (NPs) used in heterogeneous catalysis. The present work concerns the dynamic evolution of Pd small NPs (1-3 nm in size) supported on delta-Al2O3 platelets during the catalysis preparation process. This system has important applications, for example in petrochemistry or environmental catalysis. The calcination and reduction steps are followed under O2 and H2 up to 500°C in a FEI TITAN-ETEM microscope operated at 300 kV. Nowadays attention is paid to the “identical location” approach in TEM [2], where the same Region of Interest (ROI) is studied ‘before’ and ‘after’ a solicitation (heat treatment, exposure to gas…) performed outside the microscope. Typical results will show that this strategy is implicit in ETEM: the same ROI is naturally tracked at any stage during its dynamic evolution. One of the main key features insuring the catalytic efficiency of NPs is their resistance to sintering and their ability to remain homogeneously distributed during the calcination and reduction processes. The first point has been quantified through size measurements at different stages of the preparation process. The second point requires evaluating the localization of NPs in 3D using Electron Tomography (ET). Conventional ET, either in the Scanning or Bright Field TEM modes, consists in acquiring 2D projections over a large angular range, a time-consuming process hardly compatible with an in situ approach. We have thus developed a fast ET procedure [3] allowing rapid acquisitions of tilt series for volume reconstruction in only 1 or 2 minutes and a few seconds in the best cases [4]. Therefore, several tomograms of the same area are recorded in situ without critical irradiation damage of the sample shortly exposed to the electron beam. Typical examples will be shown including statistical distribution parameters, such as the inter-NPs distances measured in different states [5]. [1] ‘Controlled Atmosphere Transmission Electron Microscopy’, ed. T.W. Hansen, J.B. Wagner, Springer, New York (2016) 332 p.[2] K.J.J. Mayrhofer et al., Chem. Comm. 10 (2008) 1144-1147.[3] L. Roiban et al., J. of Microscopy, 269 (2018) 117-126.[4] H. Banjak et al., Ultramicroscopy, 189 (2018) 109-123.[5] CLYM (www.clym.fr) is acknowledged for access to the ETEM which was financially supported by the CNRS, the Région Rhône-Alpes, the ‘GrandLyon’ and the French Ministry of Research and Higher Education. This work is supported by the French National Research Agency ANR (3DCLEAN project n°15-CE09-0009-01).

International audience

International audience

Atom Probe Tomography (APT), Transmission Electron Microscopy (TEM), and 3D mechanical calculations in complex geometry and anisotropic strain fields were employed to study the role of minor elements in the precipitation process of silicon and chromium nitrides in nitrided Fe&ndash, Si and Fe&ndash, Cr alloys, respectively. In nitrided Fe&ndash, Si alloys, an original sequence of Si3N4 precipitation was highlighted. Al&ndash, N clusters form first and act as nucleation sites for amorphous Si3N4 nitrides. This novel example of particle-simulated nucleation opens a new way to control Si3N4 precipitation in Fe&ndash, Si alloys. In nitrided Fe&ndash, Cr alloys, both the presence of iron in chromium nitrides and excess nitrogen in the ferritic matrix are unquestionably proved. Only a certain part of the so-called excess nitrogen is shown to be explained by the elastic accommodation of the misfit between nitride and the ferritic matrix. The presence of immobile excess nitrogen trapped at interfaces can be highly suspected.

Transparent ceramics are usually prepared with the use of a sintering aid, in the case of YAG mostly silicon in the form of SiO2 or TEOS. However, the presence of Si may lead to the formation of secondary phases or to the deterioration of optical and laser properties due to the presence of color centers. It is therefore important to know and control its content in the sintered material. In this work we analyze the quantity and distribution of silicon in transparent YAG ceramics produced by reaction sintering of a mixture of oxide powders under high vacuum. Silicon is added in the mixture as TEOS or nanometric SiO2 powder. Laser-induced breakdown spectroscopy (LIBS) was used to quantify and compare the silicon content after the various steps of the production process (i.e. in the as-shaped samples, after sintering and after air annealing). A strong decrease of Si concentration was observed after the vacuum sintering. TEM/EDX showed an increase of silicon content at grain boundaries after air annealing steps at elevated temperatures.