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International audience; Doping of transition metals within a semiconductor quantum dot (QD) has a high impact on the optical and magnetic properties of the QD. In this study, we report the synthesis of Mn 2+-doped Ag−In−Ga−Zn−S (Mn:AIGZS) QDs via thermolysis of a dithiocarbamate complex of Ag + , In 3+ , Ga 3+ , and Zn 2+ and of Mn(stearate) 2 in oleylamine. The influence of the Mn 2+ loading on the photoluminescence (PL) and magnetic properties of the dots are investigated. Mn:AIGZS QDs exhibit a diameter of ca. 2 nm, a high PL quantum yield (up to 41.3% for a 2.5% doping in Mn 2+), and robust photo-and colloidal stabilities. The optical properties of Mn:AIGZS QDs are preserved upon transfer into water using the glutathione tetramethylammonium ligand. At the same time, Mn:AIGZS QDs exhibit high relaxivity (r 1 = 0.15 mM −1 s −1 and r 2 = 0.57 mM −1 s −1 at 298 K and 2.34 T), which shows their potential applicability for bimodal PL/magnetic resonance imaging (MRI) probes.

International audience; Magnetic Resonance Imaging is used here to study four phenomena: two-phase water/oil flow in porous media, rheology of fiber suspensions, natural convection in complex fluids and water diffusion in polymer membranes. These studies required the use or development of appropriate devices to maintain controlled external conditions in terms of mechanical stress, temperature or humidity. This article focuses on the instrumentation and methods used to perform MRI measurements on these systems.; L'Imagerie par Résonance Magnétique est utilisée ici pour l'étude de quatre phénomènes : l'écoulement diphasique eau/huile dans des milieux poreux modèles, le cisaillement de suspensions de fibres, la convection dans des fluides complexes et la diffusion de l'eau dans des membranes polymères. Ces études ont nécessité l'utilisation ou le développement de dispositifs adaptés, permettant de maintenir des conditions externes contrôlées en termes de contrainte mécanique, température ou hygrométrie. Cet article met l'accent sur l'instrumentation et les méthodes mises en oeuvre pour mener à bien des mesures d'IRM sur ces systèmes.

Natural convection in a melting material (hexadecane) is studied experimentally using Magnetic Resonance Imaging. This imaging technique allows to discriminate solid and liquid phases and to measure velocity vectors in the volume sample, even through/in opaque phases. Within our experimental conditions, the convection occurs above R a ≈ 1430 . The thermo-convective flow affects the melting interface leading to higher liquid height in uprising flow and lower liquid height in downward flow regions. Our experiments show that (i) the transient evolution of the mean liquid height h ¯ is enhanced with convection since h ¯ ∝ t 0.8 ( h ¯ ∝ t 1 2 − 3 β with β = 1 / 4 ), while h ¯ ∝ t 0.5 in the conductive regime similarly to the Stefan’s problem; (ii) the steady averaged liquid height h ¯ increases 4 times larger in the convective regime than in the conductive regime. In our experiments the range of A, ratio between the solid and liquid heights, is such as A > 1.5 . Within this range of values we obtain convective pattern under the form of hexagons / polygons. As the melting boundary grows, we observe a decrease in the polygon number. The polygonal pattern is characterized by a constant dimensionless wavelength, i.e. λ / h ¯ ≈ 2 , and thus a constant wavenumber around 3.1 similarly to the primary bifurcation in the classical Rayleigh–Benard Convection (RBC). Heat transfer is evaluated via the Nusselt number N u . Close to the onset of convection and above, our results highlight a scaling law N u ∝ R a β with β = 1 / 4 similarly to the classical RBC. The kinetic energy E c is also evaluated via velocity measurements leading to E c / E 0 ∝ N u 4 and therefore to R e ∝ R a 2 β ∝ R a 1 / 2 . Within the frame of our experiments, all scaling laws are converging to β = 1 / 4 . They reflect clearly an increase in the convective intensity from both thermal and dynamical points of view.

A coil system generating a vertical radio-frequency (rf) field gradient (B

International audience; Three complementary Kraft lignins and a Soda lignin were characterized by NMR (31P, 13C, and HSQC) and GPC. Their theoretical yield in monomers (TMY) assuming a complete cleavage of all β-O-4 linkages was calculated based on these analyses. The most recalcitrant Kraft lignin according to the TMY criteria was depolymerized in supercritical ethanol (250 °C, H2, 110 bar) with Pt/C, Ni/C, and Ru/C catalysts. All catalysts present an important effect on monomers (analyzed by GC/MS), oligomers (by UV fluorescence), and char yields. They promote cracking reactions and the stabilization of the broken bonds by H-transfers. The hydrogenation of the side chains of the monomers is also promoted to notably produce propyl-guaiacol. We show that an oligomeric pool prevails for all catalysts. Then, the effect of lignin structure on the depolymerization mechanism was studied by comparing the 4 lignins with the least precious catalyst (Ni/C). The S/G ratio of monomers is well related to the S/G ratio of lignins. Under our conditions, the β-O-4 content and the TMY do not control the yields in monomers. They are not relevant indicators for the depolymerization of these recalcitrant technical lignins. Future work should focus on novel catalysts and processes to improve the selective conversion of C–C bonds if monomers remain the targeted product.

When the water inventory evaporates during a hypothetical loss of coolant accident in the core of a pressurized water reactor, the fuel rods temperature increases substantially, resulting in the clad ballooning and the formation of blocked sub-channels. During the reflooding phase, where water is injected into the core and a steam-droplets flow is created above the water level, the presence of ballooned zone forces the steam flow to redistribute towards intact sub-channels, which degrades the cooling of ballooned clads within blocked sub-channels. Looking to better understand this flow redistribution process, this study presents experimental results of magnetic resonance velocimetry measurements of three-component velocity fields in several 49-element ballooned bundles to evaluate geometric effects (blockage ratio, length and coplanarity) on the flow dynamics, as well as the flow rate effect (Reynolds number from 1,936 to 9,599). The flow redistribution occurred in the transition zone upstream of the balloon. Moreover, the transverse velocities at this location are higher for higher blockage ratios and they can reach nearly the same magnitude of the axial velocity. The amount of deviated flow is approximately equal to the sub-channel’s blockage ratio and is virtually insensitive to the blockage length or the flow rate. Furthermore, the flow’s axial velocity reduces up- and downstream of blocked sub-channels and this reduction is more substantial as the blockage ratio increases. Results with non-coplanar balloons showed that a less intense flow redistribution takes place; however, a downstream balloon can affect the flow redistribution dynamics caused by an upstream ballooned zone. Finally, for all the tested bundles and tested flow rate ranges, the flow was remarkably homogenized by the downstream mixing spacer grid, restricting the clad ballooning effects only to the portion between the two spacer grids where the ballooned zone is located regardless of the balloon geometry and the flow rate. The present results may be useful as well for other research areas involving flow blockages in heat exchangers.

Titanium chemistry in aqueous acidic media has been extensively investigated over the last decades. Hydrolyzed species such as Ti(OH)

International audience; The quantification of the bound and free water within spruce wood and the water and solid matrix interactions are still unknown by the researchers. Many specimens were firstly studied by Nuclear Magnetic Resonance function of the relative humidity. For each test, the bound and free water were calculated. After that, spruce wood specimens were studied by X-Ray Diffraction, function of the relative humidity. For each test, the crystalline and amorphous phases were quantified. This paper presents innovative results concerning the bound and free water uptake within wooden materials, and the water and wooden solid matrix interactions.

During a loss of coolant accident (LOCA), blocked sub-channels may appear due to the swelling of the fuel rods’ cladding, which results in flow redistribution during the reflooding phase. For this reason, special attention has been paid to the effect of fuel rods ballooning on the thermal-hydraulics in LOCA conditions. Due to the practically impossible physical or optical access to blocked sub-channels, no experiment so far has performed precise three-component velocity field measurements in the presence of ballooned regions. In this study, we used magnetic resonance velocimetry (MRV) to obtain three-component velocity fields of water flow within two 7 × 7 fuel rods bundles built mainly in plastic, one regular and one containing sixteen ballooned fuel rods with 90% blockage ratio and 240 mm blockage length. We present herein results with 50 lpm water flow rate, which corresponds to a Reynolds number of 1936. With the regular bundle, the performance of spacer grids’ mixing vanes to homogenize the flow was notable. With the ballooned bundle, we observed transverse velocities upstream of the ballooned zone that are as intense as the bulk mean velocity. Furthermore, there are substantial decreases in the axial velocity within blocked sub-channels up- and downstream of the ballooned zone, reaching near-zero and even negative values downstream, indicating flow recirculation. Although the flow is highly affected by the ballooned zone, the mixing spacer grid placed downstream remarkably homogenized the flow and effects of the flow redistribution disappeared. Finally, with the present ballooned bundle configuration, about 90% of the flow that should pass through blocked sub-channel deviates towards less resistant regions, which suggests a predominant geometric effect on the flow redistribution.

This study describes experimental research on two-phase flow displacement using Magnetic Resonance Imaging (MRI) techniques. The overall purpose of this investigation is to determine kinetics process of phase trapping during (water-oil) two-phase flow, the front deformation and the phases saturation propagation along a vertical model. In these water flooding experiments, the porous medium model consists of packed beads of polystyrene (0.4 mm < dp < 0.6 mm) or sand grains (0.02 mm < dp < 0.50 mm). In order to conduct high accuracy experiments, a Nuclear Magnetic Resonance (NMR) spectrometer operating at 14 T (corresponding to a 600 MHz 1H resonance) equipped with an imaging device was used. With this equipment we can measure and visualize the two-phase flow in a vertical model of porous medium under ambient conditions. The obtained results have shown that the oil saturation profile is strongly influenced by the material properties such as the phase wetting, the sample porosity and permeability as well as the injection rate. The influence of flow velocity on the residual oil saturation was also studied. The experimental results allow an essential understanding of immiscible fluid displacement in two different types of porous medium that differ from each other mainly by the effects of wettability.

International audience; Perfluorosulfonic acid (PFSA) membranes are key components for the operation of Polymer Electrolyte Membrane Fuel Cells (PEMFC). They undergo harsh conditions in terms of chemical environment and mechanical fatigue leading to irreversible degradations, up to their failure and to the Fuel Cell (FC) shutdown in worst cases. Through an ex-situ approach, this study aims to provide new insights about the impact of conjoint mechanical and chemical stresses on the structure and the functional properties of PFSA membranes. In this regard, various aging tests have been carried out using a custom-made device able to combine the application of a mechanical stress (sinusoidal profile) and the exposure to an environment containing free radicals (i.e. Fenton's reagents). After about twenty hours of accelerated aging test the analysis of the aged membranes permitted to demonstrate that the exposure to the combined stress significantly altered the morphology of Nafion ™ XL membrane, with the appearance of bubbles close to the membrane surface and even delamination at the PFSA/reinforcement interface. On the other hand, and rather paradoxically, the water sorption and self-diffusion properties of the aged membranes as well as their cell performances remained satisfactory.

International audience; The water electro-osmotic drag coefficient of a cation _ Kdrag(cation)_ is defined as the number of water molecules which is dragged by a cation when it goes through a membrane at equilibrium state (i.e. in the absence of water concentration gradient within membrane). This coefficient is one of the key elements for modeling the water balance through Nafion membrane in a proton Exchange membrane Fuel cell (PEMFC). In this work, we have applied the method of electrophoresis NMR (ENMR) to determine the coefficient of electro-osmotic drag (Kdrag) within Nafion prepared in different cationic forms (H+, Na+ and K+). Applying, in all cases, the same method for determining Kdrag of Nafion membrane in many cationic forms should be appropriate in order to apprehend the mechanisms of water transport which occur in this membrane. This method consists in measuring the phase of the water NMR signal under the application of an electric current in a diffusion-like experiment, this phase being proportional to the flux of water within Nafion. The results show that the electro-osmotic coefficients of water within Nafion depend on the type of counter-cation and on the water content of Nafion. Due to proton lability, the electro-osmotic drag coefficient of water within Nafion in the acid form (Kdrag(H+)) is the lowest among the three studied cationic forms. In a dehydrated state of Nafion, the Kdrag(H+) is lower than 1, suggesting here that the hopping mechanism is favored. In the case of Nafion in sodium and potassium forms, Kdrag of these cations is always higher than 1. This can be rationalized by the vehicular mechanism and also by a strong interaction between the water solvation of the cation and the cation itself.

Attempts to prepare a supramolecular M4L2 box involving two tetra-bipyridyl calix[4]arene units and four copper(I) cations afforded, in fact, complex 2 for which, in the solid state, X-ray diffraction analysis showed that the stoichiometry is indeed M4L2, with the two calixarene subunits associated in a “hand-to-hand” mode by complexation via two adjacent bipyridine units to two copper(I) cations, and each group of two residual, adjacent, bipyridine arms is complexed to one copper(I) centre. Mass spectrometry, UV spectroscopy and preliminary 1H-NMR analysis are in preliminary accordance with a monocalixarenic right/left handed M2L structure. Nevertheless, some particularities of the NMR spectrum related to Ar-CH2-Ar and O-CH2bpy resonance signals, the possible generation of the M2L species during the mass spectrometry experiment and the lack of sensitivity of the UV-visible titration caused us to proceed to deeper investigations, looking for other associations, notably the M4L2 “hand-to-hand” systems. This involved 1D and 2D-NMR spectroscopies, among them 1H–15N HMBC, for the reconstruction of the molecule, but also the measurement of self-diffusion coefficients by NMR and electrochemistry. The 2D-NMR analysis and NMR self-diffusion evaluation were in accordance with the M2L structure, but the electrochemical self-diffusion experiment indicated a M4L2 structure.

International audience; An extension and a new application of the laser-induced fluorescence (LIF) is developed for supercooled droplet characterization: temperature measurements and phase droplet discrimination (liquid, solid or partially solid). In a first step, fluorescent dyes have to be selected to obtain a solution having a high temperature sensitivity. For that, fluorescent spectra at different temperatures of supercooled water within a cooled cell are analyzed and a couple of two fluorescent dyes have been identified. The water (with both dyes) was calibrated in temperature using a suspended droplet up to a minimum temperature of − 17 °C and a sensitivity of 2.56%/°C was obtained. A thorough study of the fluorescence spectrum allows also using LIF to determine the phase of the droplets. Furthermore, with the help of magnetic resonance imaging, it also possible to estimate the ice fraction in the case of supercooled water contained in a cell. Finally, the LIF technique is tested on supercooled droplets flying in a cold air environment for several injection conditions: droplets’ diameters up to 400 µm and a minimum temperature of − 45 °C. All detected droplets are classified according to their states and, for liquid droplets only, the temperature is obtained. Main results show a decrease of the supercooled droplets when the ambient air temperature decreases. Similarly, the number of liquid droplets increases with the ambient air temperature.

International audience; PFSA-based reinforced membranes are used today as the benchmark material for the electrolyte in PEMFCs. Although greatly improved relatively to their unreinforced version, they still suffer from aging and degradation during fuel cell (FC) operation. In this study we first performed proton NMR to characterize the different water populations in the pristine Nafion XL reinforced membrane. Then we used proton and fluorine NMR, FTIR and sorption measurements in order to qualitatively observe the differences induced in the membrane’s chemical structure and properties by long term FC operation. Proton NMR is seen to be an adapted tool to quickly measure a signature that is correlated to the degradation state while FTIR can serve as a local probe of the chemical structure.

Magnetic resonance thermometry (MRT) is an innovative technique which can provide 2D and 3D temperature measurements using magnetic resonance imaging (MRI). Despite the powerful advantages of MRT, this technique is sparcely developed and used in the engineering sciences. In this paper, we investigate the possibility to measure temperatures with MRI in a fluid layer submitted to heat transfer. By imposing a vertical temperature gradient, we study the temperature fields in both conductive and convective regimes. The temperature fields are obtained by measuring the transverse relaxation time $$T_2$$ in glycerol, a Newtonian fluid. The MRT protocol is described in detail and the results are presented. We show that for a conductive regime, temperature measurements are in very good agreement with the theoretical profile. In the convective regime, when comparing the temperature and velocity fields obtained by MRI, we get an excellent agreement in terms of flow structure. Temperature uncertainties are found to be less than $$1 \,^{\circ }\hbox {C}$$ for all our results.

International audience; Pectin is a natural biopolymer extracted mostly from citrus peel, sugar beet and apple pomace. In order to improve its functional properties and then to enlarge the field of its potential applications, functionalization reaction of citrus pectin with ferulic acid (FA)-oxidation products was performed in aqueous medium, at 30 °C and pH 7.5, in the presence of Myceliophthora thermophila laccase as biocatalyst. The conjugation between FA-oxidation products and pectin was confirmed using FTIR, UV–Vis and LC–MS analyses. The obtained results suggested that covalent bonds were between the pectin carboxyl groups and FA-oxidation products between the pectin carboxyl groups and FA-oxidation products. The determination of the total phenolic content showed that the modified pectin contained 5 times more phenols than the native pectin. In view of these results, this enzymatic procedure appears as a promising way to provide new pectin-based polymers that are expected to present new properties of interest.

The understanding of lignin softening and pyrolysis is important for developing lignocellulosic biorefinery in order to produce carbon fibers, polymers additives, green aromatics, or biofuels. Protobind lignin (produced by soda pulping of a wheat straw) was characterized by thermogravimetry, calorimetry (for glass transition temperature and heat of pyrolysis reactions), in situ 1H NMR (for the analysis of the mobility of protons upon lignin thermal conversion), and solution-state 13C and 31P NMR (determination of functional groups in lignin). In situ rheology reveals the real-time viscoelastic behavior of lignin as a function of temperature. Upon heating, lignin undergoes softening, through glass transition overlapped with depolymerization, and is followed by the solidification of the softened material by cross-linking reactions. The lignin residues were quenched within the rheometer at the midpoint temperatures of softening and solidification regions and were further analyzed by elemental analysis, GPC-U...

International audience; 15N NMR spin–lattice relaxation times of N2 in the presence of mesoporous SBA-80 and selected zeolites (Y, OMEGA, MORDENITE, ZSM-5, ZSM-11, AlPO-5, RHO) have been measured at 30 MHz and at temperatures ranging from ambient temperature to about 30 K. The interpretation of these experimental data rests on two relaxation mechanisms: spin-rotation (mainly active when N2 molecules reorient rapidly) and dipolar interaction with surface protons (depending essentially on the N2 translational diffusion mobility). Both mechanisms have to be taken into account for all samples except SBA-80 (for which the interpretation of experimental data requires two different spin-rotation interactions), and RHO (for which two different dipolar interactions are required). For all zeolites investigated here it has been indispensable to define a cutoff temperature below which spin-rotation is no longer active, indicating the quasi-disappearance of the gaseous state to the benefit of a condensed state. This cutoff temperature is 77 K for Y, OMEGA, and possibly MORDENITE, whereas it is higher for those zeolites with pore diameters approaching the dinitrogen molecular length. It is thus concluded that these NMR relaxation measurements are able to reveal the physical state of dinitrogen in the presence of SBA-80 and zeolites possessing very different structures.

International audience; Measuring water self-diffusion in the oriented structure of stretched Nafion represents an effective way of studying the correlations between the stress-induced order and the transport properties of the membrane. In this work, we performed pulsed-field gradient NMR experiments to investigate water self-diffusion in a single membrane under traction. The data were collected with a high signal-to-noise ratio and small draw ratio steps using a specially design NMR probe together with a mini traction apparatus. The results demonstrate that the anisotropy of diffusion is much larger in a membrane under traction than in a stack of membranes that were uniaxiafly stretched at high temperature before the measurements. Arguments based on the mechanical properties of Nafion can explain qualitatively the observed differences by highlighting the important role of both applied stress and temperature in the stress-induced order. We used a simple, but realistic, deformation model to analyze the experimental data by extracting the evolution of the diffusion anisotropy as a function of the orientational order parameter of the water conducting channels. Finally, we made analogies between the self-diffusion of water in the anisotropic structure of stretched Nafion and the Brownian dynamics of elongated particles.

Surface NMR probes are appropriate for the study of water properties in thin films as they offer a high signal to noise ratio. Using a new methodology that combines the use of a surface probe and a mini traction machine we performed water self-diffusion measurements in a unique Nafion membrane under traction. We observe that the diffusion anisotropy, correlated to the imposed order in the membrane, is much higher than in a stack of stretched / released samples hydrated at similar water content. The same approach can be used to study membranes under compression.

International audience; The hydration conditions of the membrane can vary widely over short timescales during the operation of a PEMFC. To better understand the factors limiting water transport through the PEM membrane exposed to different boundary conditions we implemented a 1D NMR imaging method using a surface probe. This methodology offers a high temporal resolution, allowing the detailed monitoring of fast transient phenomena.

International audience; The effect of MEA pinhole size on the aging of single (10 x 10 cm2) PEMFC was investigated. Two MEAs were perforated by two pins of different diameters (0.7 and 1.2 mm), in both cases in the H2 inlet zone. The performance losses and the degradation rates of the MEA components were compared to those exhibited by a healthy, not perforated MEA. Perforation by the smaller pin did not promote immediate cell voltage decay and the hydrogen crossover increased only slightly. Perforation by the larger pin resulted in immediate reduction in cell voltage and one decade increase of the hydrogen crossover. LSV and CV voltammograms performed for determination of the crossover and the electrode active surface were changed abruptly by the larger perforation, which confirmed the existence of an internal short circuit within the MEA. Besides, the perforation affected the time variations of the diffusion and charge transfer resistances.

International audience; Complementary analytical methods have been used to study the effect of potassium on the pyrolysis mechanisms of cellulose and lignocellulosic biomasses. Thermogravimetry, calorimetry, high-temperature (1) H NMR spectroscopy (in situ and real-time analysis of the fluid phase formed during pyrolysis), and water extraction of quenched char followed by size-exclusion chromatography coupled with mass spectrometry have been combined. Potassium impregnated in cellulose suppresses the formation of anhydrosugars, reduces the formation of mobile protons, and gives rise to a mainly exothermic signal. The evolution of mobile protons formed from K-impregnated cellulose has a very similar pattern to the evolution of the mass loss rate. This methodology has been also applied to analyze miscanthus, demineralized miscanthus, miscanthus re-impregnated with potassium after demineralization, raw oak, and Douglas fir. Hydrogen mobility and transfer are of high importance in the mechanisms of biomass pyrolysis.

International audience; An experimental investigation of the Rayleigh–Bénard convection in shear-thinning fluids using MRI technics is presented. The experimental setup consists on a cylindrical cavity defined by a finite aspect ratio A = D/d = 6. Qualitative and quantitative results are provided. Flow structure is determined from velocity mapping for a Newtonian fluid, Glycerol and for shear-thinning fluids, Xanthan gum aqueous solutions with weight concentrations ranging from 0.1% to 0.2%. In the case of the Glycerol and the Xanthan solution at 0.1%, one recovers similar results in terms of criticality with Rac ≈ 1800 and patterns since the convection is characterized by rolls. When the Xanthan concentration is increased, the critical Rayleigh number is not modified, however the onset occurs with hexagonal pattern. Because the critical temperature differences increase with the concentrations due to an increase in viscosity, one can think that hexagonal patterns are due to variations of physical properties with temperature (non Oberbeck–Boussinesq effects). Similarities with some results obtained in the Newtonian case are highlighted. We have observed a transition from hexagonal patterns to rolls by increasing the Rayleigh number. This pattern transition is characterized by a discrepancy in the maximal velocity values. By using shear-thinning fluids, results show an increase in the intensity of convection compared with the Newtonian case.

International audience; The study of the co-diffusion of several gases through a microporous solid and the resulting instantaneous distribution (out of equilibrium) of the adsorbed phases is particularly important in many fields, such as gas separation and heterogeneous catalysis. Classical 1H NMR imaging is a good technique for the visualization of these processes but, since the signal obtained is not specific for each gas, this requires that each experiment be performed several times under identical conditions, and each time with only one incompletely deuterated gas. In contrast we have proposed a new NMR imaging technique (based on the so-called NMR lift) which gives a signal characteristic of each adsorbed gas. It can therefore provide directly, at every moment and at every level of the crystallite bed, the distribution of several gases competing in diffusion and adsorption.

International audience; In this article, we have studied by 1H NMR the behavior of water in Nafion under acid, sodium and potassium forms which arises from different chemical treatments with or without EDTA. Self-diffusion coefficients were measured by PGSTE (Pulsed Gradient STimulated Echo) whereas integration of the proton spectrum provides a quantitative information about water content. Examination of the self-diffusion coefficients of water in Nafion shows clearly that: (i) in the acid form, the use of a chelating agent does not affect translational diffusion of water, (ii) regardless of sample conditioning, water within Nafion in acid form diffuses more rapidly than in sodium and potassium forms, in particular at low water content (at λ = 3.4, D = 5.4 × 10−7 cm2 s−1 vs. 7.2 × 10−8 cm2 s−1), and (iii) for a given water amount, water diffusion is identical in sodium and potassium forms. Point (ii) is due to the presence of oxonium ions within Nafion in acid form, favoring both vehicle and Grotthuss mechanisms. It is also due to a higher number of protons available for diffusion (we find an average value of 1.19 in acid form vs. 0.91 in sodium form). Our results suggest in the case of Nafion in acid form that, at low water content, diffusion occurs by jumps between different sites. Moreover, plotting self-diffusion coefficients vs. relative humidity leads to a clear differentiation between sodium and potassium cations, related to their respective solvation number.

Among the different PEMFC components the electrolyte membrane is still the object of many studies as its degradation remains a limiting factor of its durability. Quantities of work have focused on the impact of chemical damages due radical attacks leading to the scission of the polymer chains [1, 2]. Recent studies have pointed out the combined effect of mechanical and chemical stressors on the PEM properties [3] and it was demonstrated that the compression level imposed on the membrane changes the microstructure and accelerates the chemical decomposition of the polymer. A set of experimental techniques are available to characterize the membranes properties after degradation, including EIS (proton conductivity), water sorption, scattering methods (microstructure), IR, Raman and 19F-NMR spectroscopy (chemical structure) or DMA (mechanical behavior). These methods are complementary, and it is usually necessary to combined several of them to obtain an accurate description of the membrane properties. Ultimately, the experimental data are analyzed and correlated to understand the link between the chemical structure, the microstructure and the transport properties. In the present study we investigated the link between the changes observed in the chemical structure and the evolution of the water sorption and transport properties in the composite Nafion XL membrane after degradation in controlled conditions (ex-situ Fenton tests) and after long term fuel cell operation in the field. We used a multi-characterization approach, combining 19F-NMR and IRTF spectroscopy, water sorption and 1H-pulsed field NMR to quantify the changes observed in the chemical structure and the evolution of the water sorption and diffusion. The IR measurements demonstrate a heterogeneous chemical degradation between the anode and the cathode side (Figure 1) while the 19F data show a decrease of the IEC after both ex-situ and in-situ degradation. This loss of IEC, mainly due to side chain scissions, is correlated to an important decrease of the membrane water sorption capacity. Interestingly, the 1H-NMR spectra reveal two proton resonances (Figure 2) attributed to two non-exchangeable water populations, respectively in the two external layers of ionomer and in the ionomer dispersed in the central PTFE layer of the composite membrane. The ratio between the two populations is seen to decrease after degradation because the two external layers are decomposed first. However, the diffusion measurements show that, at a similar water content, the water diffusion coefficient is lower after degradation in the external layers as well as in the central layer, showing that the degradation also impacts water dynamics in the ionomer dispersed in the PTFE central reinforcement. This multi-characterization approach was also developed with the aim to propose an efficient and simple protocol able to quickly characterize and correlate the Nafion XL chemical and transport properties. [1] Ghassemzadeh, L.; Holdcroft, S. Quantifying the Structural Changes of Perfluorosulfonated Acid Ionomer Upon Reaction with Hydroxyl Radicals. J. Am. Chem. Soc. 2013, 135 (22), 8181−8184. [2] Zaton, M.; Rozière, J.; Jones, D.J. Current understanding of chemical degradation mechanisms of perfluorosulfonic acid membranes and their mitigation strategies: a review. Sustainable Energy & Fuels. 2017, 1, 409-438. [3] Kusoglu, A.; Calabrese, M.; Weber, A. Z. Effect of Mechanical Compression on Chemical Degradation of Nafion Membranes. ECS Electrochem. Lett. 2014, 3, F33−F36. Figure 1

Sensitivity permitting, natural abundance 13 C offers a much better spectral resolution than proton NMR. This is due to an important chemical shift range and to proton decoupling conditions which yield one peak per carbon with practically no overlap. However, PGSE experiments, which lead to the diffusion coefficient associated with each peak, have scarcely been employed. In this paper, we present and compare different ways to access this quantity and we have effectively verified that, without any precaution, diffusion coefficients cannot be properly determined from standard procedures. The cause of such a failure is decoupling during the gradient pulses. We have used a very simple remedy which proved to be very successful on a model mixture of three monoterpenes and which appears as being of general applicability. 2 Introduction

Liquid Composite Molding (LCM) regroups a number of well known manufacturing techniques of polymer composites based on resin injection through fibrous reinforcements. LCM processes such as RTM (Resin Transfer Molding) have been increasingly used to manufacture parts for a wide range of industrial applications and were shown to be cost effective in the low to medium range of volume production. To improve the performance of these processes, more scientific knowledge of the impregnation phenomena is required. In LCM processing, injection pressure dominates the impregnation of the fibers. In RTM, the formation of macro/micro-voids entrapped between or within the fiber tows is a function of the capillary pressure, the porosity of the fiber bed and the local fluid velocity. The presence of these voids within the laminate is a common source of micro-cracking that reduce short and long term mechanical properties. In this work, an experimental analysis is carried out on different types of fibrous reinforcements to study the optimal impregnation conditions by relating the content of macro/micro-voids to the local capillary number. Finally, a study demonstrating the impact of voids on mechanical properties of composite laminates is carried out.

International audience; Taking into account the previous experimental results obtained by NMR, new procedures for identifying diffusion coefficients for codiffusing benzene and hexane in intra- and intercrystallite spaces of ZSM 5 zeolite were implemented, using high-speed gradient methods and mathematical diffusion models, as well as the NMR spectra of the adsorbed mass distribution of each component in the zeolite bed. These diffusion coefficients were obtained as a function of time for different positions along the bed. Benzene and hexane concentrations in the inter- and intracrystallite spaces were calculated for every position in the bed and for different adsorption times.

We investigate the compaction dynamics of vibrated granular suspensions using both digital imaging technique and MRI measurements. Starting from initialy loose packings, our experimental data suggest the existence of two stages in the compaction dynamics: a fast stage at short times where a rising compaction front propagates through the granular suspension and a slow stage at large times where the packing compacts slowly and homogeneously. The compaction dynamics in each stage can be well fitted to usual stretched exponential laws with stretching exponents equal to 2 and 0.45, respectively. The transition time between these two stages, τ c , depends on the fluid viscosity, vibration intensity and grain diameter. We show that τ −1 and the velocity of the front decrease roughly linearly with the lubrication Peclet number, Pe lub related to the competition between the lubrication stress induced by vibrations and the granular pressure.

International audience; Creation of highly efficient methods of identification by constructing low-cost analytical solutions to direct and conjugate problems is considered. Explicit expressions are constructed for residual functional gradients to identify the transfer parameters in nanoporous media with the known total mass distribution in the solid and gaseous phases of the adsorbed substances. The results of numerical experiments are presented.

International audience; We address the problem of measuring accurately and as quickly as possible a self-diffusion coefficient by the so-called pulsed gradient stimulated echo experiment or, equivalently, by using a sequence involving radio-frequency field gradients. The discussion deals with the smallest diffusion coefficient which can be measured with a reasonable accuracy, possibly taking into account the values of longitudinal and transverse relaxation times along with the nature of gradients. It is shown theoretically and experimentally that the diffusion coefficient can be measured accurately via only two experiments provided that the ratio of the signal amplitudes lies in the range 0.2–0.6.

International audience; A gradient-based sequence is proposed for efficiently filtering out all quantities except the longitudinal two-spin order, created by csa/dipolar cross-correlation rates. The dipolar interaction is between a proton and a heteronucleus, the csa being generally the one of the heteronucleus. Proton detection is carried out to benefit from maximum sensitivity. The resulting two-dimensional spectrum has the same aspect as an HMBC spectrum, implying the existence of a J coupling between the two considered nuclei, but here cross-peaks indicate the strength of the relevant csa/dipolar cross-correlation rate. The method is especially interesting in the case of medium-sized molecules where a given heteronucleus is subjected to several csa/dipolar cross-correlation effects, along with the corresponding J couplings.

International audience

International audience; Numerous analytical methods are commonly used to reveal the mechanism of biomass pyrolysis: NMR, FTIR, GC/MS, etc. But all these methods are seldom combined under same biomass and heating conditions. Here we propose to combine the main cutting edge methods to understand biomass pyrolysis. Miscanthus pyrolysis is conducted under slow and fast pyrolysis conditions, with a good control of temperature history and quench of the solid. Mass transfer effects are carefully reduced by conducting pyrolysis in fixed bed reactor with a suitable flow rate of carrier gas. Then the following analyses are developed: (i) µGC for gas, (ii) GC*GC(heart-cutting)/MS-FID, SEC-MS and direct infusion high resolution MS for condensable products and intermediate species extracted by solvents from the char, (iii) high resolution 2D solid 1H-13C NMR, quantitative 1D 13C NMR and FTIR for the solid (biomass/biochar). Calorimetry and in-situ 1H NMR analyses [1] are also performed under similar thermal and mass transfer conditions as for high resolution NMR and MS analysis. During the talk, we will show how all these analytical methods are complementary and necessary to propose a generalized mechanism of biomass pyrolysis. [1] Dufour A., Castro-Diaz, M., Brosse N., Bouroukba M., Snape C.E., ChemSusChem, 7, 1258-1265, 2012.

Background: Lactic acidosis is associated with cardiovascular failure. Buffering with sodium bicarbonate is proposed in severe lactic acidosis. Bicarbonate induces carbon dioxide generation and hypocalcemia, both cardiovascular depressant factors. The authors thus investigated the cardiovascular and metabolic effects of an adapted sodium bicarbonate therapy, including prevention of carbon dioxide increase with hyperventilation and ionized calcium decrease with calcium administration. Methods: Lactic acidosis was induced by hemorrhagic shock. Twenty animals were randomized into five groups: (1) standard resuscitation with blood retransfusion and norepinephrine (2) adapted sodium bicarbonate therapy (3) nonadapted sodium bicarbonate therapy (4) standard resuscitation plus calcium administration (5) hyperventilation. Evaluation was focused in vivo on extracellular pH, on intracellular pH estimated by P31 nuclear magnetic resonance and on myocardial contractility by conductance catheter. Aortic rings and mesenteric arteries were isolated and mounted in a myograph, after which arterial contractility was measured. Results: All animals in the hyperventilation group died prematurely and were not included in the statistical analysis. When compared with sham rats, shock induced extracellular (median, 7.13; interquartile range, [0.10] vs. 7.30 [0.01]; P = 0.0007) and intracellular acidosis (7.26 [0.18] vs. 7.05 [0.13]; P = 0.0001), hyperlactatemia (7.30 [0.01] vs. 7.13 [0.10]; P = 0.0008), depressed myocardial elastance (2.87 [1.31] vs. 0.5 [0.53] mmHg/μl; P = 0.0001), and vascular hyporesponsiveness to vasoconstrictors. Compared with nonadapted therapy, adapted bicarbonate therapy normalized extracellular pH (7.03 [0.12] vs. 7.36 [0.04]; P < 0.05), increased intracellular pH to supraphysiological values, improved myocardial elastance (1.68 [0.41] vs. 0.72 [0.44] mmHg/μl; P < 0.05), and improved aortic and mesenteric vasoreactivity. Conclusions: A therapeutic strategy based on alkalinization with sodium bicarbonate along with hyperventilation and calcium administration increases pH and improves cardiovascular function.

Owing to an extremely abundant literature making use of spin relaxation for structural studies, this review is limited to carbon-13 spectroscopy, to small or medium size molecules, to stereochemical and preferably geometrical determinations. The parameter of choice is evidently the Nuclear Overhauser effect (NOE) because it depends exclusively on the dipolar interaction mechanism, thus on 1/r6, where r is the distance between the two interacting spins. However, it depends also on the dynamical features of the system under investigation which must be characterized prior to any attempt for obtaining geometrical or stereochemical information. Therefore, this review is devoted not only to 1H–13C NOE but, more generally, to 13C longitudinal relaxation. After comprehensive theoretical developments, experimental methods presently available will be presented. The latter include the usual gated decoupling experiment and pulse experiments of the HOESY (Heteronuclear Overhauser Effect SpectroscopY) family. These pulse experiments, which imply carbon-13 observation, can be one-dimensional, selective one-dimensional or two-dimensional. The emphasis will be put on the interpretation which is different according to the occurrence or not of extreme narrowing conditions. Along with a literature survey, some selected examples will be presented in detail in order to illustrate the potentiality of the method.

PMID: 23678859; International audience; Through 1H NMR relaxometry techniques (determination of the spin–lattice relaxation time as a function of the NMR measurement frequency), we have investigated, on a molecular scale, the water behavior in Nafion NRE 212 under acid, sodium, and potassium forms, the latter arising from different chemical treatments (with and without EDTA). Quantitatively, it turns out that (i) EDTA removes unwanted cations that may affect water mobility and (ii) the natural countercations (sodium and potassium) also affect water mobility according to their size. In order to go further, we have developed a new methodology that rests on the comparison between samples prepared with H2O and D2O. For the latter, residual protons allow us to exclusively access intermolecular contribution to proton relaxation and, thus, enable us to deduce the intramolecular contribution of proton relaxation in H2O. The analysis of this contribution reveals, for the first time, two types of bound water in Nafion.

The effect of high air relative humidity (RH) cycling (RHC 62%/100%) on the degradation mechanisms of a single (5 x 5?cm2) proton exchange membrane fuel cells was investigated. The cell performance was compared to a cell operated at constant humidification (RHC?=?62%). Runs were conducted over approximately 1,500?h at 0.3?A?cm2. The overall loss in cell performance for the high RH cycling test was 12 mu V?h1 whereas it was at 3 mu V?h1 under constant humidification. Impedance spectroscopy reveals that the ohmic and charge transfer resistances were little modified in both runs. H2 crossover measurement indicated that both high RH cycling and constant RH test did not promote serious effect on gas permeability. The electroactive surface loss for anode and cathode during high air RH cycling was more significant than at constant RH operation. The water uptake determined by 1H nuclear magnetic resonance within the membrane electrode assembly (MEA) after high RH cycling was reduced by 12% in comparison with a fresh MEA. Transmission electron microscopy showed bubbles and pinholes formation in the membrane, catalyst particles agglomeration (also observed by X-ray diffraction), catalyst particles migration in the membrane and thickness reduction of the catalytic layers. Scanning electron microscopy was conducted to observe the changes in morphology of gas diffusion layers after the runs.

International audience; In order to alleviate base-line distortions in nitrogen-14 NQR spectra originating from pulse breakthrough, low power radio-frequency (rf) pulses were applied. It is recalled that the required power is four times lower than that for an equivalent NMR experiment. This is easily explained by the fact that, in NMR, half the amplitude of the rf field is active. Moreover, the selectivity profile (i.e. the peak amplitude as a function of the difference between the carrier frequency and the resonance frequency) exhibits a shape which is, in most cases, more favorable in NQR than in NMR. An appropriate theory has been developed for explaining these experimental observations. It is concluded that low power NQR is perfectly feasible and should even be recommended for most applications, provided that the line-width of the NQR signal is not too large.

Owing to an extremely abundant literature making use of spin relaxation for structural studies, this review is limited to carbon-13 spectroscopy, to small or medium size molecules, to stereochemical and preferably geometrical determinations. The parameter of choice is evidently the Nuclear Overhauser effect (NOE) because it depends exclusively on the dipolar interaction mechanism, thus on 1/r6, where r is the distance between the two interacting spins. However, it depends also on the dynamical features of the system under investigation which must be characterized prior to any attempt for obtaining geometrical or stereochemical information. Therefore, this review is devoted not only to 1H–13C NOE but, more generally, to 13C longitudinal relaxation. After comprehensive theoretical developments, experimental methods presently available will be presented. The latter include the usual gated decoupling experiment and pulse experiments of the HOESY (Heteronuclear Overhauser Effect SpectroscopY) family. These pulse experiments, which imply carbon-13 observation, can be one-dimensional, selective one-dimensional or two-dimensional. The emphasis will be put on the interpretation which is different according to the occurrence or not of extreme narrowing conditions. Along with a literature survey, some selected examples will be presented in detail in order to illustrate the potentiality of the method.