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The Faraday effect is a magneto-optical (MO) phenomenon that causes the plane of linearly polarized light to rotate when passing through a medium subjected to a parallel magnetic field. Informed by the established quantum mechanical model developed by Buckingham and Stephens, we sought to identify molecules that would exhibit large MO responses. Magnetic circular dichroism studies of ferrocenium in the 1970s revealed its potential as an MO material; however, it has not been evaluated in the context of Faraday rotation and thin-film optical applications. Herein, we report near-infrared (NIR) Faraday rotation in thin films of decamethylferrocenium/poly(methyl methacrylate) composites with maximum Verdet constants of -3.45 × 104 deg T-1 m-1 at 810 nm (absorbance = 0.09) and -1.44 × 104 deg T-1 m-1 at 870 nm (absorbance = 0.01). These polymer-metallocene thin films deliver larger Verdet constants than commercially used NIR inorganic Faraday rotators and are facile and inexpensive to produce. The temperature dependence and distinct lineshape of the MO responses observed in decamethylferrocenium radical cations, decamethylmanganocene, and chromocene are in accordance with the quantum mechanical model. The observation of a strong C-term Faraday rotation in solid-state organometallic materials provides the groundwork for the development of high-performance metallocene-based Faraday rotators.

As an alternative to inorganic fillers based composites which present high density, an epoxy based thermosetting (TS) phase was in situ created in a polypropylene (PP) matrix and compatibilized by ...

International audience; During high-electron-mobility transistor elaboration process, a thermal treatment of In0.2Al0.8N (InAlN) barrier layer is performed in order to improve electrical performances. We showed previously that In0.2Al0.8N/GaN heterostructures, annealed at 850°C under O2 partial pressure, present a specific in-depth organization. Angle-resolved X-ray photoelectron spectroscopy is a powerful tool to precisely determine the spatial localization and relative position of the different interfaces, from InAlN until buried GaN layer. The proposed in-depth model of the stack evidences (1) an Al-rich surface oxide with embedded N2 molecules, (2) an interlayer of InAlN

International audience; An accurate monitoring of axial and radial thermal strains of specimens of the Callovo–Oxfordian claystone, heated under constant confining stress conditions close to that prevailing in the Bure Underground Research Laboratory at 490 m depth, evidenced a particular behavioural feature. The effect of the transverse isotropy of the claystone on thermal strains was not only confirmed, but it was also shown that the thermo-elasto-plastic overall volumetric response was mainly due to the axial response, in which some influence of the adsorbed water is suspected. Conversely, the reversible radial response observed along a temperature cycle could be modelled through a simple homogenisation method that confirmed that it was governed by the thermo-elastic expansion/contraction response of the constitutive minerals. Hence, the simultaneous occurrence of elastic thermal radial strains and elasto-plastic thermal axial strains was evidenced.

International audience; Like bricks of compacted bentonite/sand mixtures, mixtures made up of pellets and powder of bentonite are considered as a possible material to make the sealing plugs used to fill up galleries and ensure long-term watertightness in deep radioactive waste disposal. Pellets/bentonite mixtures have a low permeability, high swelling capacity, good radionuclide retention capability and operational advantages in terms of placement in the galleries. Following a previous in-depth characterisation of bentonite pellets/powder mixture conducted by the same group, an investigation of the water-retention properties and microstructure changes of a bentonite pellet subjected to wetting/drying cycles under free swelling conditions was carried out by means of mercury intrusion porosimetry (MIP) and X-ray microtomography. A complete description of the changes in water content, void ratio and degree of saturation of the pellet was provided. Data showed that the free swelling of the pellet is due to the combined effect of both crack propagation at the macro scale, and the swelling of bentonite grains, governed by hydration mechanisms along the smectite faces at the nano scale. Significant development of a crack network is observed between 38 and 9 MPa. For suctions below 9 MPa, there is a significant decrease of the platelet thickness and an increase in the disorder of the platelet assembly, resulting in the average MIP entrance pore radius increasing to 0·4 μm within the expanded bentonite grains.

International audience; Three solid solutions of Tetragonal Tungsten Bronze structure containing Li were prepared by solid state route starting from the parent compound Ba2NdFeNb4O15. These solid solutions involve Li insertion compensated by cationic removal. All cationic sites were involved to investigate their influence on the dielectric transitions observed in Ba2NdFeNb4O15, i.e. from relaxor (R) to ferroelectric (FE) and finally to paraelectric (PE). All solid solutions display the same pattern, which is an evolution from a relaxor to ferroelectric crossover, leading to a single transition (relaxor to paraelectric). This evolution is accompanied by a sudden increase of the ordering temperature (Tm). A focus on structure refinements shows the slight differences induced by the insertion of Li and points towards the role of anionic framework as responsible for the dielectric properties of these materials. This study completes the work performed on other cationic substitutions performed in the title material, and represents a new example of Li substituted TTB materials.

International audience; The determination of the transversely isotropic poroelastic parameters and the permeability of the Opalinus Clay, a possible host rock for deep radioactive waste disposal in Switzerland, is necessary for better predicting the hydromechanical response around the galleries. Transient experiments with fast undrained loading followed by a drainage phase, established for testing very low permeability claystones like the Opalinus Clay, were carried out on a sample equipped with strain gauges. The data obtained with this procedure showed that the values of a given poroelastic parameter, obtained from different loading paths, are reasonably close to each other, indicating that measurements are reliable and repeatable. An average intrinsic permeability perpendicular to bedding of 1.68 × 10–21 m2 was back-calculated through two consolidation phases, with similar values after loading and unloading. Some parameters, such as drained and undrained bulk moduli, exhibit a strong anisotropy. Conversely, some other parameters, like the Biot coefficients parallel and perpendicular to bedding, are close to each other. The values of these parameters were compared with those from other studies, enlarging the existing database of the hydromechanical properties of Opalinus Clay.

International audience; To describe the water retention and transfer properties of an unsaturated soil over the whole range of matric suction, it is necessary to account for both capillary and adsorption phenomena. Existing models combine well‐known empirical functions for capillary water at lower suctions and more physically based ones for adsorptive water at higher suctions. To determine their full set of parameters, they however require different optimization procedures, among which those coming from capillary models are empirical. In this context, the main objective of this work is to develop a simple and robust physically based model of the water retention and transfer properties of unsaturated soils valid from saturation to oven dryness. To do so, new capillary‐based water retention and hydraulic conductivity functions founded on the fractal approach have been derived from the pore size distribution, by means of the Young‐Laplace law and Mualem's model. To describe adsorption phenomena, these functions are combined with those used in the Peters‐Iden‐Durner (PID) model, providing a model along the full range of suctions, with less parameters than the existing models. Our work also shows that some parameters are directly determined from the experimental grain size distribution data (the fractal dimension), or from the water retention data (air entry suction and residual water content), leaving only two parameters to be optimized. The model was successfully validated with respect to published experimental data from 10 different coarse, sandy, and clayey soils.

In the framework of a deep geological radioactive waste disposal in France, the hydromechanical properties of the designated host rock, the Callovo-Oxfordian claystone (COx), are investigated in laboratory tests. Experiments presented in this study are carried out to determine several coefficients required within a transversely isotropic material model. They include isotropic compression tests, pore pressure tests, and deviatoric loading tests parallel and perpendicular to the bedding plane. We emphasize the adapted experimental devices and testing procedures, necessary to detect small strains under high pressures, on a material, which is sensitive to water and has a very low permeability. In particular, we discovered a significant decrease of elastic stiffness with decreasing effective stress, which was observed to be reversible. In both isotropic and deviatoric tests, a notable anisotropic strain response was found. The Young modulus parallel to bedding was about 1.8 times higher than the one perpendicular to the bedding plane. A notably low Poisson ratio perpendicular to the bedding plane with values between 0.1 and 0.2 was evidenced. While the anisotropy of the back-calculated Biot coefficient was found to be low, a significant anisotropy of the Skempton coefficient was computed. The performed experiments provide an overdetermined set of material parameters at different stress levels. Using all determined parameters in a least square error regression scheme, seven independent elastic coefficients and their effective stress dependency are characterized. Parameters measured under isotropic loading are well represented by this set of coefficients, while the poroelastic framework with isotropic stress dependency is not sufficient to describe laboratory findings from triaxial loading., accepted manuscript

This contribution deals with a Deep Level Transient Spectroscopy study of emission and capture processes in innovative HEMT structures based on InAlGaN/GaN, grown on N+ doped SiC substrates with a SI epitaxial SiC layer and AlGaN back-barrier. Three different gate types (Schottky, FAT FET and GTLM) were examined. The attention was focused on identified deep energy levels and possible origins compared to various measurement conditions and the gate types. Arrhenius evaluations confirmed traces of Ga-vacancies, dislocations, self-donors associated with the Ga interstitial impurities. Measured DLTS spectra validated an excellent quality of the growth process.

This work deals with the average temperature model of two AlGaN/GaN HEMTs exhibiting similar thermal but different electrical properties. Output I-V characteristics dependent on isothermal and thermal device properties allow comparison of output conductance and current temperature sensitivity of both devices. Nevertheless thermal gradient and dissipated power density variation in the active device area give the model some restrictions. However more input thermal parameters bring availability to perform more accurate calculations.

International audience; The behaviour of clayey soils is strongly correlated to their microstructure and evolution thereof. Microstructural investigations have contributed to understanding soil behaviour and have supported the development of multi-scale coupled models. One of the most accessible methods to characterise soil microstructure is mercury intrusion porosimetry, which provides a pore-size distribution (PSD) ranging from few nanometers to several hundreds of micrometers. PSDs can be used to compute micro and macro strains or simply to estimate, in aggregate microstructures, the void ratios associated with macropores and micropores. However, in both cases, a boundary has to be set to separate the different pore populations. This paper discusses some criteria that have been proposed in the literature to separate the pore populations. The discussion is illustrated with extensive micro structural data obtained for Maryland clay. The paper highlights the effect of initial conditions and boundary conditions on the delimiting diameter given by some of the criteria. It is also shown that using different criteria will yield different values of delimiting pore size, with a risk of obtaining unrealistic estimates of micro and macro void ratios and strains. Finally, it is suggested to account for known soil behaviour to interpret microstructural data.

International audience; To better understand results of SEALEX in situ tests carried out at Tournemire Underground Research Laboratory, the hydromechanical behavior of a pellet/powder MX80 bentonite mixtures prepared at a dry density of 1.49 Mg/m3 were investigated by means of microfocus X-ray computed tomography (μ-CT) observations and laboratory small scale infiltration tests. Radial and axial swelling pressures as well as relative humidity were monitored while wetting. Two configurations were considered: for the first, a pellet/powder mixture was prepared following a specific protocol to minimize initial structural heterogeneity; the second one was specially designed to study a strong heterogeneous mixture distribution. μ-CT observations performed on the two samples during hydration revealed an apparently homogeneous sample for the first mixture after 100 days of hydration. For the second specimen, several voids were still observed after 40 days of hydration. A comparison was made between the in situ and mock-up tests. It was observed that the evolutions of radial and axial swelling pressures depend on the initial heterogeneous distribution of the mixture. This heterogeneity is due to the different dry density values at the vicinity of the different sensors. The final values of axial swelling pressures were different for both configurations for the same global dry density.; Pour mieux comprendre les résultats des tests in situ SEALEX effectués au laboratoire de recherche souterrain de Tournemire, le comportement hydromécanique de mélanges de bentonite MX80 pastille/poudre préparés à une densité sèche de 1,49 Mg/m3 a été étudié au moyen d’observations de microtomographie aux rayons X (μ-CT) et des tests d’infiltration à petite échelle en laboratoire. Les pressions de gonflement radiales et axiales ainsi que l’humidité relative ont été contrôlées pendant le mouillage. Deux configurations ont été envisagées : pour la première, un mélange pastille–poudre a été préparé selon un protocole spécifique afin de minimiser l’hétérogénéité structurelle initiale ; le second a été spécialement conçu pour étudier une forte distribution de mélanges hétérogènes. Les observations de μ-CT effectuées sur les deux échantillons au cours de l’hydratation ont révélé un échantillon apparemment homogène pour le premier mélange après 100 jours d’hydratation. Pour le second échantillon, plusieurs vides ont encore été observés après 40 jours d’hydratation. Une comparaison a été faite entre les tests in situ et maquette. Il a été observé que les évolutions des pressions de gonflement radiales et axiales dépendent de la distribution hétérogène initiale du mélange. Cette hétérogénéité est due aux différentes valeurs de densité sèche à proximité des différents capteurs. Les valeurs finales des pressions de gonflement axiales étaient différentes pour les deux configurations pour la même densité sèche globale.

The aim of this paper is to investigate the long-term hydro-mechanical behaviour of a highly heterogeneous MX80 bentonite pellet/powder mixture (80/20 in dry mass), which is one of the candidate sealing materials in deep geological repositories. In spite of the operational advantages related to the use of the mixture, structural heterogeneities resulting from the installation process constitute a matter of concern and require special approaches to describe adequately the material behaviour during hydration. In this study, a double structure formulation taking into account the initial structural heterogeneity of the material as well as damage to pellets upon wetting is proposed. The formulation is applied to the modelling of a 1/10 mock-up of Sealex large-scale tests. Hydraulic and mechanical parameters are determined from an extensive laboratory programme carried out on a single pellet of bentonite and on the pellet/powder mixture. To ensure an adequate analysis of the test, the initial heterogeneous structural distribution of the material is determined by image analysis of microfocus X-ray computed tomography observations. The model allows the anisotropic swelling behaviour of the mixture to be satisfactorily reproduced when accounting for the spatial variation of the material's initial porosity. In particular, the long-term (decade) hydro-mechanical behaviour of the mixture can be well described. Detailed analysis of the modelling results demonstrates the existence of dry density gradients at the long term and their influence on swelling pressure anisotropy.

The impact of varying the GaN channel layer thickness ( ${t}_{\text {ch}}$ ) in InAlGaN/AlN/GaN HEMTs with C-doped AlGaN back barriers is investigated. ${t}_{\text {ch}}$ was 50, 100, and 150 nm, and the gate length of the fabricated HEMTs ranged from 50 to 200 nm. It is found that short-channel effects (SCEs) are significantly mitigated with a small ${t}_{\text {ch}}$ . For HEMTs with a gate length of 50 nm, the drain-induced barrier lowering changes from 40 to 93 mV/V as ${t}_{\text {ch}}$ is increased from 50 to 150 nm. On the other hand, it is shown that dispersive effects are more severe for a smaller ${t}_{\text {ch}}$ , as demonstrated by a sixfold increase in the dynamic ON-resistance for ${t}_{\text {ch}} = 50$ nm compared to ${t}_{\text {ch}} = 150$ nm. The tradeoff between dispersion and SCEs is reflected in large-signal measurements at 30 GHz. The 50-nm channel, mainly limited by dispersion, exhibits an output power of 3.5 W/mm. The thicker channels reach a maximum of around 5 W/mm, but for different gate lengths due to the difference in severity of the SCEs. This paper elucidates the interplay between SCEs and dispersion related to ${t}_{\text {ch}}$ , its consequences for the large-signal performance and for the limitation in downscaling of the gate length.

On-wafer short term step-stress tests were carried out to evaluate InAlGaN/GaN HEMT devices. Three types of transistor were studied, each one having a specific two dielectric layer passivation. The results of these tests demonstrate that the upper layer of the passivation has a strong impact on the ageing of the devices. When the upper layer is an Al2O3 Atomic Layer Deposited one, the transistors show a better stability of their electrical parameters than those passivated with the other stacks.

International audience; Measuring and modelling the permeability of tight rocks remains a challenging task. In addition to the traditional sources of errors that affect more permeable formations (e.g. sample selection, non-representative specimens, disturbance introduced during sample acquisition and preparation), tight rocks can be particularly prone to solid–fluid interactions and thus more sensitive to the methods, procedures and techniques used to measure permeability. To address this problem, it is desirable to collect, for a single material, measurements obtained by different methods and pore fluids. For that purpose, a benchmarking exercise involving 24 laboratories was organized for measuring and modelling the permeability of a single low-permeability material, the Grimsel granodiorite. The objectives of the benchmark were: (i) to compare the results for a given method, (ii) to compare the results between different methods, (iii) to analyse the accuracy of each method, (iv) to study the influence of experimental conditions (especially the nature of pore fluid), (v) to discuss the relevance of indirect methods and models and finally (vi) to suggest good practice for low-permeability measurements. To complement the data set of permeability measurements presented in a companion paper, we focus here on (i) quantitative analysis of microstructures and pore size distribution, (ii) permeability modelling and (iii) complementary measurements of permeability anisotropy and poroelastic parameters. Broad ion beam—scanning electron microscopy, micro-computerized tomography, mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) methods were used to characterize the microstructures and provided the input parameters for permeability modelling. Several models were used: (i) basic statistical models, (ii) 3-D pore network and effective medium models, (iii) percolation model using MICP data and (iv) free-fluid model using NMR data. The models were generally successful in predicting the actual range of measured permeability. Statistical models overestimate the permeability because they do not adequately account for the heterogeneity of the crack network. Pore network and effective medium models provide additional constraints on crack parameters such as aspect ratio, aperture, density and connectivity. MICP and advanced microscopy techniques are very useful tools providing important input data for permeability estimation. Permeability measured—orthogonal to foliation is lower that—parallel to foliation. Combining the experimental and modelling results provide a unique and rich data set.

The shear viscosity of asymmetric, binary mixtures, consisting of small and long-chain molecules, was computed by means of reversed non-equilibrium molecular dynamics simulations. The molecules were modelled as flexible chains of tangent spheres that interact through a combination of site–site Lennard-Jones (LJ) 12–6 intermolecular forces. The calculations were performed in order to elucidate the mechanisms responsible for the experimentally observed marked decrease in the viscosity of a fluid consisting of large molecules, on addition of lighter species. The simulations of pure species indicate that the chains exhibit a range of configurational shapes, but are in general quite folded, even in a dilute state, when there are no other chains or monomers present; the average radius of gyration of a particular chain is nearly temperature independent and only weakly dependent on density. Analysis of the behaviour of the chains with the different number of segments indicates that the resulting viscosity is proportional to the square root of their moment of inertia. The simulations carried out on the binary mixture, consisting of a monomer and a 16-segment chain species, indicate that the viscosity decrease can be broadly attributed to density, mixing and structural effects. The decrease in density and mixing effects led to a large decrease in viscosity, primarily dominated by the effect of mixing the two species. The structural changes resulted in an increase in viscosity, as the presence of monomers led to configurational relaxation of hexadecamer and to a localization of monomers in the vicinity of the hexadecamers.

International audience

This paper presents performances achieved with InAlGaN/GaN HEMTs with 0.15 µm gate length on SiC substrate. Technology Computer Aided Design simulations were used to optimize the heterostructure. Special attention was paid to the design of the buffer structure. I-V measurements with DC and pulsed bias voltages were performed. CW measurements at millimeter waves were also carried out and are detailed in the following sections. The technology, optimized for power applications up to 45 GHz, demonstrates a current gain cut-off frequency FTof 70 GHz and a maximum available gain cut-off frequency FMAGof 140 GHz. CW Load-pull power measurements at 30 GHz enable to achieve a maximum PAE of 41% associated with an output power density of 3.5 W/mm when biased at VDS= 20 V. These devices, with an improved buffer structure show, reduced recovery time in pulsed operating conditions. These improved characteristics should have a positive impact for pulsed or modulated signal applications.

A preliminary study of the design of an electrically small plasma-based resonator using a localized surface plasmon resonance (LSPR) above 1 GHz is presented. Such a resonator is intended to be used to develop an electrically small antenna with frequency agility. This resonator consists of a plasma discharge confined in a hemispherical glass shell 3 cm in diameter on a ground plane. From 1 to 1.5 GHz, the plasma electron density required to achieve the LSPR must be between 0. $5\times 10^{11}$ and $1.4\times 10^{11}\mathrm{c}\mathrm{m}^{-3}$. Plasma losses are taken into account in this study and provide information on the required gas pressure. Typically for neon gas, the working pressure must be around 50 mTorr. A practical implementation using a miniature inductively coupled plasma (mICP) source is finally discussed.

The water retention curve and the hydraulic conductivity function of a volcanic coarse granular material used as a substrate in an urban green roof in the Paris area was carried out on a newly developed device, in which low suctions were controlled. In the same cell, a hanging column system was used for controlling smaller suctions (up to 32 kPa) and the axis translation technique for larger suctions (up to 50 kPa). Water exchanges were monitored in connected tubes by using a high accuracy differential pressure transducer. The step changes in suction were also used to determine the hydraulic conductivity function by means of Gardner’s method, accounting for the impedance effects due to the high air entry value ceramic porous disk with Kunze and Kirkham’s method. van Genuchten and Brooks and Corey models were used for the water retention curve, but the hydraulic conductivity functions derived from these expressions appeared to lead to a significant under-estimation, confirming the need of operational and simple device for the experimental determination of the hydraulic conductivity function.

International audience; Based on long term investigations of the microstructure of various geomaterials as 14 different as sensitive low plastic clays from Eastern Canada, unsaturated compacted silt from 15 the Paris area, highly plastic compressible deep marine clay from the Gulf of Guinea, 16 compacted MX80 bentonite and the Callovo-Oxfordian claystone, considered as possible 17 barriers in deep geological radioactive waste disposal, two fundamentals and distinct nano 18 and micro mechanisms governing their macroscopic volume changes have been identified. In 19 low plastic structured clays and dry compacted soils, in which an aggregate microstructure has 20 been evidenced, the decrease in volume under mechanical compression result from the 21 collapse of inter-aggregates pores in an ordered manner, from the larger to the smaller, with 22 no effect on the intra-aggregate porosity. The soil skeleton can hence be modelled as an 23 elastic fragile porous matrix, affected by the ordered collapse of its pores. Things are different 24 in plastic soils, due to the significant reactivity of the montmorillonite minerals to changes in 25 water content that results in significant changes of the initial porous matrix. The ordered 26 adsorption of layers of water molecules in clay platelets with respect to the suction or stress 27 applied, evidenced long time ago in Soil science through X-ray diffraction techniques, appear 28 to be able to help better understanding various phenomena like the compression of plastic 29 compressible soils and the hydration of compacted bentonites. Interestingly, it also applies to 30 understand the effects of smectite minerals in the volume changes behaviour of the Callovo-31 Oxfordian claystone. These two mechanisms at nano and microscopic scales hence provide a 32 better understanding of the macroscopic volume changes of a large variety of natural and 33 compacted clayey soils and rocks. 34 Keywords: clayey soils and rocks; microstructure; montmorillonite; pore size distribution; 35 nano scale; water adsorption. 36 37 2

This article presents the performances obtained on a $0.15 \mu \mathrm{m}$ gate length InAlGaN/GaN HEMT technology on SiC substrate. This technology uses a back-barrier buffer layer to ensure the confinement of electrons in the channel, which minimizes variations of the drain current when the HEMT devices are submitted to DC or RF pulses. Measurements of the drain current recovery time are shown when the devices are submitted to V DS , V GS or microwave RF pulses. A comparison with an AlGaN/GaN HEMT structure designed with an iron doped buffer layer is proposed.

This paper presents the measurement results of a MMIC power amplifiers (PA), based on InAlGaN/GaN HEMT technology, for Ka band applications. The three-stages MMIC is operating within a bandwidth of [25-31] GHz and demonstrate over this bandwidth a saturated output power of 40dBm. Each stage uses 8x50μm gate width HEMTs fabricated with a 0.I5μm gate length on 70μm thick SiC substrate.

International audience; This paper reports on atomic layer etching of several III-N materials such as GaN, AlN, AlGaN, and InAlGaN based on a sequential surface modification by chlorine adsorption followed by a low energy Ar plasma exposure to remove the modified layer using a reactive ion etching system. A study on the influence of several parameters, such as gas flow rates, removal step duration, RIE power and number of cycles on the etch per cycle, and the root-mean-square roughness, is performed. Low etch per cycle from 0.17 to 1.85 nm/cycle, respectively, for AlGaN and GaN and surfaces as smooth as the as-grown samples were obtained. The developed process is intended to be used for normally off GaN-based high electron mobility transistor processing.

Due to its intrinsic properties and the possible lattice match to GaN, InAlN is expected to allow the fabrication of optimal high electron mobility transistors for high power and high frequency applications. However, the crystal quality of InAlN nearly lattice-matched to GaN degrades when the layer thickness is increased, and this is a strong limitation for the fabrication of devices in which thick barriers need to be used. In this work, we have carried out a detailed theoretical investigation of the behavior of indium atoms in the alloy. It is clearly shown that in the presence of nitrogen vacancies, which are common defects in these materials, indium nitride clusters will present excess formation energy up to diameters around 1.4 nm. In parallel, Z-contrast TEM observations close to the InAlN/GaN interface show that 2–5 nm size indium rich areas form and are systematically connected to the vertical degradation channels. This is at variance with published results, which concluded that the observed degradation was exclusively either due to the underlying threading dislocations or due to a characteristic three-dimensional growth mode.

In-situ NMR spin-lattice relaxation measurements were performed on several vapor deposited ices. The measurements, which span more than 6 orders of magnitude in relaxation times, show a complex spin-lattice relaxation pattern that is strongly dependent on the growth conditions of the sample. The relaxation patterns change from multi-timescale relaxation for samples grown at temperatures below the amorphous-crystalline transition temperature to single exponential recovery for samples grown above the transition temperature. The slow-relaxation contribution seen in cold-grown samples exhibits a temperature dependence, and becomes even slower after the sample is annealed at 200K. The fast-relaxation contribution seen in these samples, does not seem to change or disappear even when heating to temperatures where the sample is evaporated. The possibility that the fast relaxation component is linked to the microporous structures in amorphous ice samples is further examined using an environmental electron scanning microscope. The images reveal complex meso-scale microporous structures which maintain their morphology up to their desorption temperatures. These findings, support the possibility that water molecules at pore surfaces might be responsible for the fast-relaxation contribution. Furthermore, the results of this study indicate that the pore-collapse dynamics observed in the past in amorphous ices using other experimental techniques, might be effectively inhibited in samples which are grown by relatively fast vapor deposition.

The objective of this work was to study the development of polypropylene (PP) foams with low density ( We have shown that the branched structure of polymer chains can be defined on the basis of a rheological criterion. It defines the notion of fractal behavior (coupling of relaxation modes), which polymer chains must have for nonlinear behavior (strain hardening). This criterion has been defined by analogy with the sol-gel transition, tan δ = 1. Finally, in terms of modeling and simulation, the plasticizing effect of CO2 was modeled from the experimental rheological behavior of the PP plasticized with synthetic oil. A theoretical power law was derived and was then introduced in the mass balance equations to predict the cell growth during depressurization.

We propose a novel model approach for temperature evaluation in the channel region of a InAlN/AlN/gallium nitride high electron mobility transistor (HEMT) due to self-heating effects. The heat transfer in a HEMT device has been investigated experimentally by the nearby temperature sensor and compared by theoretical models solved by both numerical and analytical methods. The average temperature of the channel area of almost 160 °C for dissipated power of 2 W was determined using the drain-source current variation analysis. The electrical and thermal behavioral numerical model under quasi-static conditions have been used to describe the HEMT device. In contrast, the one-dimensional thermal model for analytical evaluation has been proposed as an alternative approach. Surprisingly, the experimental results verified not only the validity of precise numerical simulation but also the simplified analytical model that makes it a reliable tool even for complex electronic devices.

The purpose of this paper is to combine a classical 1D twin-screw extrusion model with machine learning techniques to obtain accurate predictions of a complex system despite few data. Systems involving reactive polyethylene oligomer dispersed in situ in a polypropylene matrix by reactive twin-screw extrusion are studied for this purpose. The twin-screw extrusion simulation software LUDOVIC is used and machine learning techniques dealing with low data limit are used as a correction of the simulation. This research was funded by the French ANR through the DataBEST project.

International audience; The determination of the water retention curve (WRC) and hydraulic conductivity function (HCF) of a specific volcanic coarse granular material used as a substrate for urban green roofs in Europe was carried out on a newly developed specific device in which low suctions, typical of coarse granular materials, were controlled. Smaller suctions (up to 32 kPa) were imposed by using a hanging column system, and larger suctions (between 32 and 50 kPa) were imposed by using the axis translation technique in the same cell. The changes in suction during the tests were monitored by using a high accuracy differential pressure transducer. They were also used to determine the HCF by means of both Kunze and Kirkham’s and Gardner’s methods. The former technique was used at low suctions (

International audience; The water retention, compressibility and swelling properties of a pellet/powder bentonite mixture were investigated in the laboratory. The water retention capacity was determined under constant-volume condition using a specially designed cell allowing vapor exchange in all directions; suction controlled oedometer tests were carried out for the compressibility investigation; both constant-volume and swell-consolidation methods were applied for the swelling pressure determination. The vapor equilibrium technique was used for suction control. Comparable results were found for the water retention capacity of the pellet/powder mixture determined under constant-volume condition and one single pellet under free swelling condition for suctions higher than 4 MPa, suggesting that the suction was mainly controlled by the micro-pores inside the pellets/powder grains and the swelling of the clay particles was accommodated by the existing macro-pores. On the contrary, at lower suctions, the constant-volume conditions gave rise to a lower water retention capacity for the mixture, indicating the restricted clay particle swelling after the filling-up of the existing macro-pores. Oedometer compression curves revealed a different behavior for low and high suctions: at high suctions, the volume change behavior was governed by the pellets rearrangement combined with possible pellets crushing upon loading. By contrast, at low suctions, the mixture lost its initial granular structure during suction decrease; thus, the volume change behavior became similar to that of compacted bentonite. Compared to compacted bentonite, the pellet/powder mixture showed a lower yield stress. A lower value of swelling pressure was found with the constant-volume method, confirming the limitation of the swell-consolidation method in determining soil swelling pressure.

International audience; Pellet/powder bentonite mixture is one of the candidate materials for sealing plugs in deep geological high-level radioactive waste disposal. This note presents an investigation on the structure changes of this mixture occurring during the saturation process by means of X-ray computed micro-tomography. The test was performed in an infiltration column (60 mm in inner diameter and 120 mm in height). Water was supplied to the two ends of the column and the changes of the sample morphology were observed during a period of 100 days of hydration. Digital Volume Correlation (DVC) technique was used to determine the vertical displacement field of the bentonite powder. A pressure transducer was used to measure the axial swelling pressure during the hydration. The results show that the initial distribution of powder in the inter-pellet pores was not homogeneous; the powder filled almost completely the pores in the zones close to the two ends while air-filled inter-pellet voids were observed in the middle of the column specimen. When water started to infiltrate inside the specimen from both ends, the pellets and the powder grains started to swell (because of the swelling properties of smectite, the principal mineral of bentonite) and filled the voids. That induced at the same time increase of swelling pressure and downward movement of powder grains. The results allowed a better understanding on the hydro-mechanical couplings, at the pellet scale, in the pellet/powder bentonite mixture upon wetting

International audience; Measuring the permeability of tight rocks remains a challenging task. In addition to the traditional sources of errors that affect more permeable formations (e.g. sample selection, non-representative specimens, disturbance introduced during sample acquisition and preparation), tight rocks can be particularly prone to solid–fluid interactions and thus more sensitive to the methods, procedures and techniques used to measure permeability. To address this problem, it is desirable to collect, for a single material, measurements obtained by different methods and pore-fluids. For that purpose a collaborative benchmarking exercise involving 24 laboratories was organized for measuring the permeability of a single low permeability material, the Grimsel granodiorite, at a common effective confining pressure (5 MPa). The objectives of the benchmark were: (i) to compare the results for a given method, (ii) to compare the results between different methods, (iii) to analyze the accuracy of each method, (iv) to study the influence of experimental conditions (especially the nature of pore fluid), (v) to discuss the relevance of indirect methods and models and finally (vi) to suggest good practice for low permeability measurements. In total 39 measurements were collected that allowed us to discuss the influence of (i) pore-fluid, (ii) measurement method, (iii) sample size and (iv) pressure sensitivity. Discarding some outliers from the bulk data set (4 out of 39) an average permeability of 1.11 × 10−18 m² with a standard deviation of 0.57 × 10−18 m² was obtained. The most striking result was the large difference in permeability for gas measurements compared to liquid measurements. Regardless of the method used, gas permeability was higher than liquid permeability by a factor approximately 2 (kgas = 1.28 × 10−18 m² compared to kliquid = 0.65 × 10−18 m²). Possible explanations are that (i) liquid permeability was underestimated due to fluid-rock interactions (ii) gas permeability was overestimated due to insufficient correction for gas slippage and/or (iii) gases and liquids do not probe exactly the same porous networks. The analysis of Knudsen numbers shows that the gas permeability measurements were performed in conditions for which the Klinkenberg correction is sufficient. Smaller samples had a larger scatter of permeability values, suggesting that their volume were below the Representative Elementary Volume. The pressure dependence of permeability was studied by some of the participating teams in the range 1–30 MPa and could be fitted to an exponential law k = ko.exp(–γPeff) with γ = 0.093 MPa−1. Good practice rules for measuring permeability in tight materials are also provided.

International audience; This paper summarizes the findings of various investigations carried out for some time in the research group on unsaturated soils at Ecole des Ponts ParisTech. It focusses in more details on the links that can be made between nano, micro, and meso scale phenomena, and the macroscopic response of various Engineered Barrier Systems made up of bentonite, used in various concepts of radioactive waste disposal at great depth. Meso-scale observations using X-Ray microtomography usefully complete some findings established by using more standard techniques of microstructure investigation, like scanning electron microscopy and mercury intrusion porosimetry. The morphology of sand-bentonite and pellet-bentonite powder mixtures can be better understood, together with their changes during hydration. Also, nano-scale mechanisms that govern the hydration of smectites through the consecutive and ordered placement of layers of water molecules along the faces of the smectite minerals are useful for a better understanding of the macroscopic response of Engineered Barrier Systems submitted to hydration under constrained volume conditions.

The presented study of AIGaNIGaN HEMT structures with an isotope-pure epitaxial SiC layer on N+-doped SiC substrate and with AIGaN back barrier between the AIN nucleation layer and the GaN buffer resulted in the identification of seven hole like and two electron like deep energy levels. The defect distribution in these samples were compared with defect distribution measured on reference AIGaNIGaN HEMT structures with a natural epitaxial SiC layer on N+-doped SiC substrate without AIGaN back barrier. Significant differences were observed. Both samples showed traces of carbon and nitrogen interstitials. The number of these defects allowed us to confirm the significant affect of the AIGaN back barrier and isotope SiC layer on the quality and properties of these samples.

In this work an average channel temperature of power InAIN/GaN HEMT prepared on SiC substrate has been investigated using quasi-static 1-V characterization. The analysis of drain current change depending on source resistance, threshold voltage and saturation velocity drop was utilized and used for temperature versus dissipated power calculations in differential form. The source resistance and threshold voltage were determined experimentally by measurements of output and transfer characteristics at different ambient temperatures whereas saturation velocity relative drop was used from available resources. Obtained results were compared with simulations.

This paper presents Deep Level Transient Fourier Spectroscopy study results of deep energy levels in 10 kV SiC based PIN diodes. Five types of samples with/without isotope-pure epitaxial SiC layers were experimentally analyzed and evaluated. Parameters of 8 defects have been identified. $Z_{1/2}$ centers, known as a lifetime killer were clearly identified in all structures. These results indicate a carbon vacancy origin that can have a connection with the fabrication process.

Recent studies have proposed that adding quinine to water while performing Cherenkov volumetric dosimetry improves the skewed percent depth dose measurement. The aim of this study was to quantify the ability of quinine to convert directional Cherenkov emission to isotropic fluorescence and evaluate its contribution to the total emitted light. Aqueous solutions of quinine were prepared with distilled water at various concentrations (0.01-1.2 g l-1). The solutions were irradiated with photon beams at 6 and 23 MV. The dependence of the light produced as a function of sample concentration was studied using a spectrometer with a fixed integration time. Spectral measurements of the luminescent solution and the blank solution (distilled water only) were taken to deconvolve the Cherenkov and quinine contribution to the overall emission spectrum. Using a CCD camera, intensity profiles were obtained for the blank and the 1.00 g l-1 solutions to compare them with the dose predicted by a treatment planning system. The luminescent intensity of the samples was found to follow a logarithmic trend as a function of the quinine concentration. Based on the spectral deconvolution of the 1.00 g l-1 solution, 52.4% ± 0.7% and 52.7% ± 0.7% of the signal in the visible range results from the quinine emission at 6 and 23 MV, respectively. The remaining fraction of the spectrum is due to the Cherenkov light that has not been converted. The fraction of the Cherenkov emission produced between 250 nm and 380 nm in the water and that was absorbed by the fluorophore reached 24.8% and 9.4% respectively at 6 and 23 MV. X-ray stimulated fluorescence of the quinine was then proven to be the principal cause to the increased total light output compared to the water-only signal. This new information reinforces the direct correlation of the solution intensity to the dose deposition.

International audience; Non-compacted pellet/powder bentonite mixtures are considered as candidate sealing plug materials in geological disposals of radioactive waste. Due to its nature, this mixture is characterized by a heterogeneous porosity network, which is responsible for its complex hydro-mechanical (HM) behaviour. The French Institute of Radioprotection and Nuclear Safety (IRSN) has investigated this mixture within the SEALEX project. Both in situ large-scale and laboratory small-scale experiments were carried out. This paper presents the results of a small-scale mock-up test at 1/10th scale of the in situ experiments, in which the pellet/powder mixture was saturated from both sides (top and bottom of the specimen). Both swelling pressure and relative humidity were monitored at several positions of the specimen. Different responses from the sensors were found, depending on the local porosity as well as the evolution of the hydration front. The HM response of the mixture is strongly conditioned by the initial pellet/powder distribution, which depends on the protocol followed for the specimen preparation. After 800-day hydration, an anisotropy was found between the axial and the radial swelling pressures, due to the presence of larger void at the top of the sample and the friction at the cell wall. The sample was still heterogeneous after 800-day hydration mainly due to the initial heterogeneous porosity distribution, combined with the effect of friction and the non-saturation of the mixture. The evolution of injected water with time revealed that the sample was not full saturated after 800-day hydration.

By utilizing a novel three-layer resist process, InAIN/AIN/GaN T - and T -gate high electron mobility transistors with 0.1 μm gate lengths and below have been demonstrated. This process is based on direct electron-beam lithography with a single exposure step. Furthermore, the effect of different T -gate shapes on RF power performance is reported. A T -gate shift to the source side of the Ohmic contact, results in lower gate-to-drain capacitance and in higher transistor RF transducer gain G t .

In this paper, several methods suitable for real time on-chip temperature measurements of power AlGaN/GaN based high-electron mobility transistor (HEMT) grown on SiC substrate are presented. The measurement of temperature distribution on HEMT surface using Raman spectroscopy is presented. We have deployed a temperature measurement approach utilizing electrical I-V characteristics of the neighboring Schottky diode under different dissipated power of the transistor heat source. These methods are verified by measurements with micro thermistors. The results show that these methods have a potential for HEMT analysis in thermal management. The features and limitations of the proposed methods are discussed. The thermal parameters of materials used in the device are extracted from temperature distribution in the structure with the support of 3-D device thermal simulation. The thermal analysis of the multifinger power HEMT is performed. The effects of the structure design and fabrication processes from semiconductor layers, metallization, and packaging up to cooling solutions are investigated. The analysis of thermal behavior can help during design and optimization of power HEMT.

Scintillation dosimetry has evolved towards utilizing 3D liquid dosimeters to perform quality assurance verification of complex treatment configuration for photon, electron and proton beams. However, most of the fluorophores utilized in these dosimeters are alike and present limitations. This study aims to establish the profile of CdSe colloidal quantum dots (cQDs) that were given the role of the fluorophore in a binary liquid scintillation system. We chose to investigate the cQDs because of their wide absorption spectrum, the tunability of their absorption and emission spectra with respect to their size and composition, and their ability to function as an effective energy transfer intermediate. The scintillation intensity and spectral response of three organic solvent-based liquid cQD dispersions have been investigated upon irradiation with kV and MV photon beams. The solvents used to disperse the cQDs were hexane, toluene and linear alkylbenzene. The scintillation efficiency of the cQD dispersions has proven to be dependent on the nature of the solvent, the alkylbenzene cQD liquid dispersion having the brightest light emission of the three solutions, for an equivalent deposited dose in the scintillator. Its light output was found to reach a tenth of the light intensity of a commercial liquid scintillator, Ultima Gold, irradiated under the same conditions. This cQD dispersion also demonstrated a remarkable energy transfer to the cQDs, only 5% of its intensity being due to Cherenkov light production in the solvent. Overall, these results indicate that the alkylbenzene cQD liquid dispersion could be the best choice for a potential cQD-based liquid scintillator.

This paper presents the simulated results and first small-signal on-wafer measurements of two MMIC power amplifiers (PA), based on InAlGaN/GaN HEMT technology, for Ka band applications. The two three-stages MMIC1 & MMIC2 are operating within a bandwidth of [27.5–31]GHz and [37–39.5]GHz, respectively and demonstrate over these bandwidths a saturated output power of 40dBm and 38dBm, respectively. Each stage use 8×50μm gate width HEMTs fabricated with a 0.15μm gate length on 70μm thick SiC substrate.

This work presents the conformal printing of milli-meter-wave transmission lines up to 67 GHz using inks based on silver nanoparticles. Several coplanar (CPW) lines have been printed over 3D Alumina test structures such as slope, half-sphere and ridge, all of them being fabricated using a ceramic additive manufacturing technology. All of the achieved transmission lines show a return loss better than 10 dB and insertion loss better than 0.8 dB up to 60 GHz. Extensive tests have been done on the performances obtained with the Aerosol Jet Printing (AJP) technology. Minimum features (line width, lines spacing) down to 16.5 μm have been obtained on flat surfaces (24 μm on 3D ceramic objects) while obtaining a high conductivity of 2.1107S/m at 49.2 GHz for the directly printed silver layers.

International audience; In this work, local electrical properties of the crystallographic defects including V‐defects and trenches in ternary alloy AlGaN and quaternary alloys Al(Ga,In)N with different indium concentration are studied by light‐assisted Kelvin probe force microscopy. This surface sensitive technique is used to reveal the role of these defects as deep level electron traps. The evolution of topography of the layers from AlGaN to indium‐containing alloys is also investigated; it highlights the transformation of step‐flow to three‐dimensional growth mode. It is also shown that at the coalescence boundaries of growth hillocks, defects are generated which act as electrically active electron traps.

Recent results obtained using InAlGaN/GaN HEMT technology. Material advantages of this heterostructure are depicted and some technological aspects are described. Low lagging effects and high electric field capability is presented. Power densities up to 10W.mm−1 have been obtained at 30GHz. MMIC power amplifiers are briefly reported.

International audience; Mixed uranium-americium oxides are one of the materials envisaged for Americium Bearing Blankets dedicated to transmutation in fast neutron reactors. Conversion and fabrication processes are currently developed to make those materials in the form of dense and homogeneous oxide ceramic pellets or dense granulates incorporating uranium and americium. Their development points out the need of a simplified and optimized process which could lower hazards linked to dust generation of highly contaminating and irradiating compounds and facilitate material transfer in remote handling operations. This reason motivated the development of innovative “dustless” route such as the Weak Acid Resin route (WAR) which provides the oxide precursors in the form of sub-millimeter-sized microspheres with optimal flowability and limits dust generation during conversion and fabrication steps. This study is thus devoted to the synthesis of mixed uranium-americium oxide microspheres by the WAR process and to the characterization of such precursors. This work also deals with their application to the fabrication of dense or porous pellets and with their potential use as dense spherules to make Sphere-Pac fuel.

Robustness of InAlN/GaN devices under proton radiation is investigated. Several proton fluences ranging from 1 × 10 14 to 4 × 10 14 have been considered on two typologies of devices. Displacement damage is found to be the major responsible of device DC degradation leading to threshold voltage positive shift, ON resistance increase and drain current decrease, in all cases well correlated with proton fluence. Negligible difference is noticed in displacement damage effects measured on different device typologies. Furthermore, device geometry does not influence the impact of proton radiation on main DC parameters, either if gate width or length are considered. Radiation significantly affects trapping properties. A good correlation between the so-called current collapse increase and proton fluence is demonstrated when a high gate drain voltage value is imposed as trapping condition. Moreover radiation enhances the contribution of dynamic ON resistance and transconductance peak variation on current collapse increase.