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ABy (A = rare earth or Mg, B = transition metal, 2 < y < 5)-based alloys with varying stoichiometries are being studied in recent years for hydrogen storage and as negative electrode materials for Ni-MH batteries. Their structure can be described as the periodic stacking along the c crystallographic axis of two fundamental layers, [AB5] and [A2B4] (labeled as C and L, respectively), and often exhibits stacking defects (loss of periodicity locally). The layer type and the stacking sequence play an important role in the hydrogenation and electrochemical properties. The volume mismatch between the two subunits [AB5] and [A2B4] is the main cause of the multiplateau behavior and the irreversibility of the binary RNiy (R = rare earths) compounds. The multielement compounds with [A2B4] and [AB5] subunits that are similar in size tend to show better stability. In this paper, the defect structure of four different materials exhibiting different types of stacking faults is analyzed by utilizing a combination of high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and X-ray diffraction (XRD) data. The analysis of the X-ray diffraction patterns is done using the FAULTS software, which allows one to determine the nature of the stacking faults and to quantify them. The quantitative description of the different case studies has brought unprecedented insight into rationalizing and correlating the differences between the different structural subunits and hydrogen storage properties., This work has been supported by the French ANR (Agence Nationale de la Recherche) program PROGELEC under the contract MALHYCE ANR-2011-PRGE-006 01. SAFT and CNRS are also acknowledged for the financial and technical support of the Ph.D. thesis of V.C.

International audience; Chemically architectured alloys are a new concept of microstructure in which two phases are separated by a 3D network of fluctuations of composition, which is called interphase, and which induces a strengthening. Chemically architectured alloys were processed by spark plasma sintering of a mixture of pure Ni and CoCrFeMnNi high entropy alloy with varying conditions. They were characterized by scanning electron microscopy coupled with energy dispersive spectroscopy and electron backscattered diffraction, microhardness and compression tests. It was shown that the width of the interphase can be decreased by decreasing the sintering temperature and increasing the applied pressure. The strengthening effect of the interphase increases when its width decreases and its volume fraction increases. The microstructure of the chemically architectured alloys can be finely controlled by the processing parameters which will permit to maximize the strengthening. Chemical architecturation is thus an efficient and tunable strengthening mechanism.

The phase stability, electronic structure, magnetic and mechanical properties of ANiy compounds, A = {Y, Sm, Gd}, y = {2, 3, 72, 195, 4, 5}, and A2Ni7H8 hydrides have been investigated by systematic first-principles calculations. Supported by a discussion to understand the role of A and y parameters, the results are compared with published and new experimental data regarding the heat of formation of some compounds and their hydrides. The main results can be summarized as follows: (i) ANi3 and A2Ni7 are clearly stable by the density functional theory calculation, whereas the stability of ANiy for y>72 is not highlighted distinctively, in agreement with the difficulty to identify a single phase by experimental synthesis for some compounds such as Y5Ni19; (ii) ANiy compounds are found more rigid for A = Y and may explained the difficulty to observe Y5Ni19 in comparison with Sm5Ni19 and Gd5Ni19; and (iii) the calculated and measured heats of formation of A2Ni7H8 hydrides are found in good agreement, around −2...

The skutterudite materials belonging to the CoSb3 family are widely studied for thermoelectric applications. They are typically used under vacuum, but applications under oxidizing environments are increasingly considered. The addition of ytterbium is known to enhance the thermoelectric properties of CoSb3, but the corresponding impact on the oxidation behavior of the skutterudite material is barely explored. For that purpose, the present research describes the oxidation behavior of Yb0.2Co4Sb12 under a flow of air at 800 K for 15, 50, 100, and 1000 hours. The oxidation treatment induces the growth of a surface layer made of three oxides in various amounts as a function of the oxidation time. The spinel oxide CoSb2O4/CoO·Sb2O3 and CoSb2O6 are observed from 15 hours of oxidation, whereas Sb2O4 is formed only from 100 hours of treatment. To assess the impact of ytterbium, the oxidation behavior of Yb0.2Co4Sb12 is compared to that of CoSb3 oxidized in the same experimental conditions. The results show that the low amount of ytterbium promotes the oxidation reactions of the skutterudite material. Nonetheless, the impact on the degradation of the material remains acceptable to use Yb0.2Co4Sb12 for thermoelectric applications under oxidizing environments.

Sm2Fe17 compounds are high-performance permanent magnets. Cobalt substitution allows us to further improve their magnetic properties. Depending on the thermal treatment, cobalt-substituted compounds can be synthesized either in the TbCu7 (disordered) or in the Th2Zn17 (ordered) structure type. Rietveld refinement of the number of transition metal dumbbells replacing rare-earth atoms from synchrotron powder diffraction data shows that the TbCu7 disordered structure has the same composition as the ordered one (a transition metal-to-rare earth ratio of 8.5). Then, cobalt site occupancies have been determined in both structures using synchrotron resonant (anomalous) diffraction. Cobalt is found to be absent from the dumbbell sites. The diffraction results are confirmed by Mossbauer spectroscopy.

The Ni-MH alkaline batteries are made of a nickel hydroxide cathode and a metal hydride anode. The first generation of LaNi5-type anodes absorbs 6 hydrogens per mole but corrodes in basic medium. Chemistry allows to overcome this limitation by forming super-stoichiometric multi-substituted alloys giving capacities of 300 mAhg-1 over hundreds of cycles. Today, new alloys are developed by crystallographic intergrowths, they give capacities 30% higher by substituting rare earths by lighter Mg. Ni-MH batteries thus keep their full place in stationary and mobile applications thanks to their reliability, compactness, safety, and low cost., Les batteries alcalines Ni-MH sont faites d'une cathode d'hydroxyde de nickel et d'une anode d'hydrure métallique. La première génération d'anodes de type LaNi 5 absorbe six atomes d'hydrogène par mole d'alliage mais se corrode en milieu basique. La chimie permet de pallier cette limite en formant des alliages multisubstitués sur-stoechiométriques donnant des capacités de 300 mAhg-1 sur des centaines de cycles. Aujourd'hui, de nouveaux alliages sont développés par intercroissances cristallographiques, qui présentent des capacités supérieures de 30 % en substituant les terres rares par du magnésium plus léger. Les batteries Ni-MH gardent ainsi leur place dans les applications stationnaires et mobiles grâce à leur fiabilité, leur compacité, leur sécurité et leur faible coût. Mots-clés Hydrures métalliques, structures d'intercroissances, hydroxyde de nickel, batteries alcalines, stockage électrochimique, énergie.

International audience; A2B7 compounds (A = rare earth or Mg, B = transition metal) are widely studied as active materials for negative electrode in Ni-MH batteries. By playing on the substitution rate of both A and B elements, it is possible to prepare various compositions. This strategy will help to improve the properties, for example get a higher reversible hydrogen capacity and a well-adapted hydrogen sorption pressure. Indeed, A can be almost all light rare earths (La to Gd), yttrium and alkaline earth metals (Mg or Ca), whereas B can contain various late transition metals (Mn to Ni). To understand the effects of composition on the physicochemical properties of La2Ni7-based compound, various pseudo-binary systems have been investigated: Gd2-xLaxNi7 (x = 0, 0.6, 1, 1.5), Sm2-xLaxNi7 (x = 0, 0.5, 1, 1.5), Y2-xLaxNi7 (x = 0, 0.4, 0.5, 0.6, 0.8, 1, 1.5, 1.75) and A0.5La1.1Mg0.4Ni7 (A = Sm, Gd and Y). To determine their crystallographic properties, X-ray diffraction analysis was performed followed by Rietveld refinement. Thermodynamic properties regarding reversible hydrogen sorption were investigated at room temperature. Quaternary compounds present drastically improved sorption properties in the frame of practical energy storage applications with reversible capacity equivalent to 400 mAh/g for the compound La1.1Sm0.5Mg0.4Ni7.

International audience; The chemically architectured alloys were proposed as a new concept of microstructure, with a multiscale architecturation. They are composed of a solid-solution at the atomic scale, grain boundaries and composition gradients at the micronic scale and finally a 3D network of gradients at the mesoscale. Composition gradients are between two phases with the same crystalline structure but different compositions. As a first application of this concept, powders of pure Ni and CoCrFeMnNi high entropy alloy were successfully densified by Spark Plasma Sintering. Then the microstructure and mechanical properties were characterized by scanning electron microscope coupled with energy dispersive spectroscopy and electron backscattered diffraction, X-ray diffraction, nanoindentation and compression tests. The obtained chemically architectured alloys present a homogeneous distribution of the two phases with a 10 µm wide chemical gradient in between. By comparison with reference non architectured materials, the chemical architecturation induces a 35 % increase of the yield strength. Thus the concept of chemically architectured metallic alloys and composition gradients was proven as very promising: processing of this complex microstructure is possible and it induces a significant mechanical strengthening.

Intermetallic compounds are key materials for energy transition as they form reversible hydrides that can be used for solid state hydrogen storage or as anodes in batteries. ABy compounds (A = Rare Earth (RE); B = transition metal; 2 < y < 5) are good candidates to fulfill the required properties for practical applications. They can be described as stacking of AB5 and AB2 sub-units along the c crystallographic axis. The latter sub-unit brings a larger capacity, while the former one provides a better cycling stability. However, ABy binaries do not show good enough properties for applications. Upon hydrogenation, they exhibit multiplateau behavior and poor reversibility, attributed to H induced amorphization. These drawbacks can be overcome by chemical substitutions on the A and/or the B sites leading to stabilized reversible hydrides. The present work focuses on the pseudo-binary Sm2MnxNi7-x system (0 0.3 leading to larger and flatter isotherm curves, allowing for reversible capacity >1.4 wt %. Regarding corrosion, the binary compound corrodes in alkaline medium to form rare earth hydroxide and nanoporous nickel. As for the Mn-substituted compounds, a new corrosion product is formed in addition to those above mentioned, as manganese initiates a sacrificial anode mechanism taking place at the early corrosion stage., Comment: Special Issue : Hydrogen Storage Properties of Materials

Hexagonal La

Materials with Cu12-xNixSb4-yBiyS13-zSez (0 ≤ x ≤ 1, 0 ≤ y ≤ 0.4, 0 ≤ z ≤ 3) compositions were prepared by melting the elements under vacuum. All samples were studied in the as-cast form, and parts of them were characterized after either a spark plasma sintering or a melt spinning/glass-crystallization procedure. Powder X-ray diffraction shows that all materials are mainly constituted by tetrahedrite, except in the case of the Cu12Sb4S10Se3 composition treated by SPS that is constituted by skinnerite, famatinite and tetrahedrite. Electrical conductivity and Seebeck coefficient measurements point to a large effect of the preparation technique and experimental conditions on the electrical transport properties. The as-cast samples have low Seebeck coefficients (α300K ≤ 35 μV/K), and are not suitable for thermoelectric applications. The spark plasma sintering procedure did not significantly change the Seebeck coefficient values and can degrade the electrical conductivity, particularly if skinnerite and famatinite are formed. Materials treated by melt spinning/glass-crystallization show higher Seebeck coefficients and often lower electrical resistivity when compared with the as-cast samples, indicating this method as a suitable technique for quick preparation of good thermoelectric tetrahedrites

International audience; Hydrides have been used since a long time for solid-state hydrogen storage and electrochemical nickel-metal hydride batteries. Besides these applications, growing attention has been devoted to their development as anode materials, as well as solid electrolytes for Li-ion and other ion batteries. Herein, we review and summarize the recent advances of hydrides as negative electrodes for Ni-MH and A-ion batteries (A = Li, Na), and as electrolyte for all solid-state batteries (ASSB). Metallic hydrides such as intergrowth compounds are highlighted as the best compromise up to now for Ni-MH. Regarding anodes of Li-ion batteries, MgH2, especially its combination with TiH2, provides very promising results. Complex hydrides such as Li-borohydride and related closo-borates and monovalent carborate boron clusters appear to be very attractive as solid electrolytes for Li-based ASSB, whereas closo-hydroborate sodium salts and closo-carboborates are investigated for Na- and Mg-ASSB. Finally, further research directions are foreseen for hydrides in electrochemical applications.

The influence of the synthesis and sintering methods on the structural, microstructural, stoichiometry and thus the magnetocaloric properties of the La0.7Ca0.3MnO3 compound has been thoroughly studied. Soft-chemistry (sol-gel and polyol) synthesis and spark plasma sintering (SPS) procedures, used to obtain nanostructured materials, were compared to the traditional ceramic ones. This work evidences that SPS treatment not only reduces the grain size but also leads to oxygen sub-stoichiometry. This entails a lower Mn4+ content in the manganite, and probably heterogeneity in the Mn4+ grain distribution, which have consequences on the magnetic and magnetocaloric properties: a lower maximum entropy variation which is compensated by a larger transition temperature range. The sample prepared by combining polyol soft-chemistry synthesis and SPS sintering displays a higher maximum relative cooling power value, equal to 70% of that of gadolinium, the most effective material for domestic magnetic refrigeration.

The high temperature oxidation behavior of two highly alloyed refractory austenitic steels is studied at 1000 °C under air. Both short-term oxidation kinetics and long-term oxidation products are investigated. Structural, elemental and morphological analyses are coupled to characterize the oxide phases’ formation. Chromia and spinel are the main phases for both samples although they suffer important descaling after 4 weeks oxidation, but alloying elements are responsible for the formation of many other oxides. Microstructures are compared and discussed. New Raman spectra are reported for manganese chromite spinels. Their assignments are provided and discussed at the light of the proposed cationic distribution.

Tetrahedrites are widespread minerals with general formula Cu10M2Sb4S13 (M = Cu, Mn, Fe, Co, Ni, Zn). Their thermoelectric properties can be tuned through proper doping and reach zT values as high as 1, being considered promising low-cost thermoelectric materials. However, for practical application in thermoelectric devices, it is necessary to establish their ability to operate for long periods under working temperatures and atmospheres. We present herein studies of oxidation in air of Cu12Sb3.9Bi0.1S10Se3 tetrahedrite at four different temperatures between 230°C and 375°C, together with preliminary corrosion studies in aggressive NaCl electrolyte. Surface oxidation already occurs at the lower studied temperatures, but a strong decrease of the oxidation rate is observed for materials treated at intermediate temperature (275°C), where a continuous surface layer of Cu2−xS forms, pointing to a protective effect of this layer that could be applied in devices operating at such temperatures. For the material treated at higher temperatures (350°C and 375°C), no tetrahedrite phases were seen after 1500 h, which can be related to the (tetrahedrite + chalcostibite + antimony → skinnerite) reaction that occurs above 280°C. Corrosion studies indicated that increasing the oxidation temperature unfortunately leads to a decrease of the corrosion resistance of tetrahedrite-based phases.

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Hexagonal La$_2$Ni$_7$ and rhombohedral Y$_2$Ni$_7$ are weak itinerant antiferromagnet (wAFM) and ferromagnet (wFM), respectively. The crystal structure and magnetic properties of $A_2B_7$ intermetallic compounds ($A$ = La, Y, $B$ = Ni) have been investigated combining X-ray powder diffraction and magnetic measurements. The La$_{2-x}$Y$_x$Ni$_7$ intermetallic compounds with $0 \leq x \leq 1$ crystallize in the Ce$_{2}$Ni$_7$-type hexagonal structure with Y preferentially located in the [$AB_2$] units. The compounds with larger Y content ($1.2 \leq x < 2$) crystallize in both hexagonal and rhombohedral (Gd$_2$Co$_7$-type) structures with a progressive substitution of Y for La in the $A$ sites belonging to the [$AB_5$] units. Y$_2$Ni$_7$ crystallizes in the rhombohedral structure only. The average cell volume decreases linearly versus Y content, whereas the c/a ratio presents a minimum at $x = 1$ due to geometric constrains. The magnetic properties are strongly dependent on the structure type and the Y content. La$_{2}$Ni$_7$ displays a complex metamagnetic behavior with split AFM peaks. Compounds with x = 0.25 and 0.5 display a wAFM ground state and two metamagnetic transitions, the first one towards an intermediate wAFM state and the second one towards a FM state.T$_N$ and the second critical field increase with the Y content, indicating a stabilization of the AFM state. LaYNi$_7$, which is as the boundary between the two structure types, presents a very wFM state at low field and an AFM state as the applied field increases. All the compounds with $x > 1$ and containing a rhombohedral phase are wFM with $T_C$ = 53(2) K. In addition to the experimental studies, first principles calculations using spin polarization have been performed to interpret the evolution of both structural phase stability and magnetic ordering for $0 \leq x < 2$., 26 pages (7 for supplementary material), 4 tables, 9 main figures and 8 figures in supplementary material

Ferromagnetic La0.7Ca0.3−xBaxMnO3 (x

Sustainable development is the way to ensure the Human progress within the bounds of the ecological possible. In this context thermoelectric systems can play an important role. However, the price of most high-performance thermoelectric devices is high, mainly due to the use of expensive elements, which raised the interest for cheap thermoelectric materials. It is also clear that the production of competitive thermoelectric devices critically depends on other factors, like the manufacturing costs, and that the materials fabrication simplicity, reproducibility, and use of easy scale-up processes will also play a fundamental role.Tetrahedrites, with generic formula Cu10M2Sb4S13 (M = Cu, Mn, Fe, Co, Ni, Zn), are world spread sulfosalt minerals that crystallize in the cubic Cu12Sb4S13-type structure. They are environment friendly materials mainly formed by non-expensive elements. Recently, mineral based and synthetic tetrahedrites were considered as promising thermoelectric materials, with zT ~ 1 at T ~ 700 K. Though, tetrahedrite melts incongruently, the preparation of appropriate tetrahedrite samples usually requiring long-term annealing procedures. Here we present a set of systematic studies on the use of tetrahedrites as low-cost and sustainable thermoelectric materials. The raw material prices evaluation, the feasibility of rapid, scalable, cheap tetrahedrite preparation and their long term stability and resistance to oxidation under working conditions are highlighted.

A Cu 12 Sb 3.6 Bi 0.4 S 10 Se 3 glass was prepared by melt spinning and crystallized by heat treatments at selected temperatures, the total preparation procedure taking less than one day. The sample characterization by powder X-ray diffraction, scanning and transmission electron microscopy complemented with EDS indicate the formation of compact materials, with a tetrahedrite relative weight fraction higher than 90% when treated at temperatures close to the crystallization peaks (∼200° C). Selenium enters the tetrahedrite structure, while bismuth precipitates in submicron and nanosized spherical shape phases depleted in copper and enriched in antimony, sulfur and selenium (when compared with the matrix composition). The characterization of electrical transport properties (electrical resistivity and Seebeck coefficient) indicate a behavior similar to that obtained by other methods on Cu 12 Sb 4 S 13 , with a maximum power factor of ∼400 μW/K 2 m at room temperature.

International audience; Powder metallurgy is a promising processing path to produce high entropy alloys (HEA) with improved mechanical properties. According to this, a bulk CoCrFeMnNi alloy was milled with a wide range of conditions. It was shown that a powder which is micronic, approximately spherical and with nanometric crystallites could be produced by a cryo-milling which was followed by a short duration planetary milling. Next, this powder was fully densified by spark plasma sintering. According to X-ray diffraction, the single phase of the bulk alloy remains stable during both milling and sintering. However, carbides and oxides precipitate during sintering, as shown by scanning electron microscopy coupled with energy dispersive spectroscopy. Electron backscattered diffraction evidences that those precipitates limit the growth of grains. By nanoindentation measurements, it was shown that preparing a CoCrFeMnNi HEA by milling and sintering significantly increases the hardness compared to conventional processing by melting and casting. Moreover, the different strengthening contributions were calculated and analyzed. It revealed that grains have a strengthening contribution as described by the Hall & Petch law, contrary to crystallites.

International audience; Nd-YAG QS laser cleaning of soiled stone at 1064 nm can sometimes result in a more yellow appearance compared to other cleaning techniques. Especially in France, this yellowing effect is still considered as a major aesthetic issue by the architects and conservators. One explanation states that the yellowing is linked to the formation of iron-rich nanophase(s) through the laser beam interaction with black crusts that would re-deposit on the cleaned substrate after irradiation. To characterize these nanophases, a model crust containing hematite was elaborated and laser irradiated using a Nd-YAG QS laser. The color of the sample shifted instantaneously from red to a bright yellow and numerous particles were ablated in a visible smoke. Transmission electron microscopy (TEM) was used to examine the morphology and the crystallinity of the neo-formed compounds, both on the surface of the samples and in the ablated materials. In addition, an investigation of the chemical and structural properties of the nanophases was conducted by X-ray dispersive energy (EDX) and electron energy loss (EELS) spectroscopies. It was found that both the surface of the sample and the ablated materials are covered by crystallized nano-spheres and nano-residues, all containing iron and oxygen, sometimes along with calcium and sulfur. In particular an interfacial area containing the four elements was evidenced between some nanostructures and the substrate. Magnetite Fe3O4 was also identified at the nanoscale. This study demonstrates that the laser yellowing of a model crust is linked to the presence of iron-rich nanophases including CaxFeySzOδ nanostructures and magnetite Fe3O4 at the surface after irradiation.

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International audience; Short range order of As40-xCuxTe60 (x = 0, 10, 20, 25, 30) glasses was studied by neutron- and X-ray diffraction, combined with extended X-ray absorption fine structure (EXAFS) measurements at the K-edges of all components. Large-scale structural models were generated by fitting the experimental datasets simultaneously in the framework of the reverse Monte Carlo simulation technique. These simulations revealed that As and Te atoms bind to about 3 and 2 As/Te neighbors, respectively, both of which possess Cu neighbors. The CuTe bond length is 2.57 ± 0.02 Å while the CuAs distance is as high as 2.86 ± 0.04 Å. The results further showed that besides As and Te, Cu atoms also bind to Cu. The total coordination number of Cu is significantly higher than 4 for the compositions x = 25 and 30.

Materials based on Cu12Sb4S13 tetrahedrites have been seen in recent years as promising materials for thermoelectric applications. However, the effect of small amounts of additional elements (used to tune the electrical transport properties) on the formation of the phases was not investigated. In this work we present such study, by means of powder X-ray diffraction, scanning electron microscopy complemented with energy-dispersive spectroscopy, and differential scanning calorimetry, using the Taguchi method to design the experiments. The effect of Ni, Bi and Se (i) in the volume percentage of tetrahedrite, (ii) in the temperature at which the Class III (tetrahedrite + chalcostibite + antimony → skinnerite) reaction occurs and (iii) in the final melting temperature of Cu12−xNixSb4−yBiyS13−zSez materials rapidly cooled from 950 °C was investigated. Se was observed to have a strong positive influence on the formation of tetrahedrite, while Ni and Bi were seen to promote the decrease of its volume percentage. A decrease of the Class III reaction and final melting temperatures was also observed after the introduction of Se, with Ni inducing the increase of the reaction and the decrease of the melting temperatures and Bi having only minor effects. The analysis of the microstructures indicate that high Ni concentrations lead to the first solidification of the NiS phase, while in the other compositions the (tetrahedrite/skinnerite) phase or mixtures of phases is first formed.

International audience; Recrystallization of amorphous compounds can lead to the stabilization of metastable crystalline phases, which offers an interesting way to unveil novel binary or ternary compounds and control the transport properties of the obtained glass ceramics. Here, we report on a systematic study of the Cu–As–Te glassy system and show that under specific synthesis conditions using the spark-plasma-sintering technique, the α-As2Te3 and β-As2Te3 binary phases and the previously unreported AsTe3 phase can be selectively crystallized within an amorphous matrix. The microstructures and transport properties of three different glass ceramics, each of them containing one of these phases with roughly the same crystalline fraction (∼30% in volume), were investigated in detail by means of X-ray diffraction, scanning electron microscopy, neutron thermodiffraction, Raman scattering (experimental and lattice-dynamics calculations), and transport-property measurements. The physical properties of the glass ceramics are compared with those of both the parent glasses and the pure crystalline phases that could be successfully synthesized. SEM images coupled with Raman spectroscopy evidence a “coast-to-island” or dendriticlike microstructure with microsized crystallites. The presence of the crystallized phase results in a significant decrease in the electrical resistivity while maintaining the thermal conductivity to low values. This study demonstrates that new compounds with interesting transport properties can be obtained by recrystallization, which in turn provides a tuning parameter for the transport properties of the parent glasses.

Copper nanopowders were sintered using Spark Plasma Sintering (SPS). Special attention was paid to the presence of natural surface oxide and reductive thermal treatments under H 2 atmosphere were carried out to remove it. The sintering mechanisms were investigated by the means of disrupted experiments at various temperatures to reach different shrinkage rates. Optimized experiments were also carried out to reach a maximum densification. The samples were characterized and compared using density measurement, X-ray diffraction, mechanical tests (tension and compression) and analysis of the fracture surfaces with scanning electron microscope. On the one hand, it results that during the SPS treatment, the raw powder undergoes a series of oxide transformations from CuO to Cu 2 O. On the other hand, reductive treatment drastically decreases the sintering temperature, from 650 °C to 260 °C, to reach 90% density without grain growth. Higher mechanical strength, up to 750 MPa, also shows better consolidation.

International audience; Chalcogenide glasses and more importantly their glass-ceramics counterparts have been an interesting but very peculiar class of thermoelectric materials, with inherently low thermal conductivity (

The La0.85Na0.15MnO3 perovskite has been synthesized in polycrystalline form using the Spark Plasma Sintering technique (SPS). The in-situ reaction proceeds in a few minutes at 800 °C under 50 MPa with the very high heating rate of 100 °C min−1. Rietveld refinement of the X-ray diffraction patterns indicates that our sample is of pure single phase rhombohedral structure ( R 3 ¯ c symmetry). Paramagnetic–ferromagnetic phase transition at TC = 310 K is observed for our studied sample. Magnetic entropy change, | Δ S M | , deduced from isothermal magnetization curves, is about 2.42 J kg K−1 in a magnetic applied field change of 5 T. Although this value is modest, the magnetocaloric effect extends over a large temperature range leading to an important value of the relative cooling power (RCP). The RCP value at 5 T reaches 316.6 J kg−1. This value corresponds to about 77% of that observed in pure Gd.

Perovskite-type lanthanum manganite (LaMnO3) was synthesized from Mn2O3 and La2O3 precursor powders using the Spark Plasma Sintering technique (SPS). The in situ reaction proceeds in a few minutes at 1000 °C under 50 MPa with a very high heating rate of 100 °C/min. The Rietveld refinement of the X-ray powder diffraction shows that LaMnO3 sample crystallizes in a perovskite structure of O′-type orthorhombic symmetry with Pbnm space group. After annealing at 1000 °C in air for 10 hrs, the symmetry changes to rhombohedral with R-3c space group. For the as prepared sample, two critical temperature points in the temperature dependence of the ZFC and FC magnetization occurring at 125 K and 145 K are observed. They are suggested to be related to A-type antiferromagnetic Neel temperature and ferromagnetic short range order, respectively. The ferromagnetic component in the annealed sample increases indicating the expected variation in the concentration of Mn3+ and Mn4+ ions, which was attributed to the oxygen stoichiometry effects.

International audience; Intermetallic compounds A 2 B 7 (A = rare earth, B = transition metal) are of interest for Ni‐MH batteries. Indeed they are able to absorb hydrogen reversibly and exhibit good specific capacity in electrochemical route. To understand the effect of rare earth on properties of interest such as thermodynamic, cycling stability and corrosion, we synthesized and studied three compounds: Y 2 Ni 7 , Gd 2 Ni 7 and Sm 2 Ni 7. Using Sieverts' method, we plot pc‐isotherms up to 10 MPa and study hydride stability upon solid‐gas cycling. Electrochemical cycling was also performed, as well as calendar and cycling corrosion study. Corrosion products were characterized by means of X‐ray diffraction, electron diffraction, Raman micro‐spectroscopy and scanning and transmission electron microscopies. Magnetic measurements were also performed to calculate corrosion rates. A corrosion mechanism, based on the nature of corrosion products, is proposed. By combining results from solid‐ gas cycling, electrochemical cycling and corrosion study, we attribute the loss in capacity either to corrosion or loss of crystallinity.

International audience; Bi2Te3-based compounds are a well-known class of outstanding thermoelectric materials. β-As2Te3, another member of this family, exhibits promising thermoelectric properties around 400 K when appropriately doped. Herein, we investigate the high-temperature thermoelectric properties of the β-As2−xBixTe3 solid solution. Powder X-ray diffraction and scanning electron microscopy experiments showed that a solid solution only exists up to x = 0.035. We found that substituting Bi for As has a beneficial influence on the thermopower, which, combined with extremely low thermal conductivity values, results in a maximum ZT value of 0.7 at 423 K for x = 0.017 perpendicular to the pressing direction.

β-As2Te3 belongs to the family of Bi2Te3-based alloys, a well-known class of efficient thermoelectric materials around room temperature. As2Te3 exists in two allotropic configurations: α- and β-As2Te3, of which only the latter crystallizes in the same rhombohedral structure as Bi2Te3. Herein, we report on substitution of Bi for As in the As2−x Bi x Te3 system with x = 0.0, 0.015, 0.025, and 0.035. These samples have been characterized by x-ray diffraction and scanning electron microscopy. The transport properties have been measured at low temperatures (5 K to 300 K) in both directions, parallel and perpendicular to the pressing direction. The results are compared with those obtained in previous study on samples substituted by Sn. Compared with Sn, Bi allows for a clear decrease in electrical resistivity while maintaining the thermal conductivity below 1 W/(m K) over the whole temperature range. As a result, a comparable peak ZT value near 0.2 was obtained at room temperature.

International audience; As2Te3 exists in two crystallographic configurations: alpha- and beta-As2Te3, of which only the latter crystallizes in the same rhombohedral structure-type as Bi2Te3. While beta-As2Te3 shows interesting thermoelectric (TE) properties which can be adjusted through alloying, the transport properties of the monoclinic phase alpha-As2Te3, more thermodynamically stable than the beta-phase at room temperature, has not yet been studied thoroughly. We report here on the samples preparation by powder metallurgy, and on the microstructural characterization of polycrystalline alpha-As2Te3. Preliminary results on the electrical and thermal properties measured between 5 K and 523 K are also reported. Transport properties measurements were performed both along and perpendicular to the pressing direction indicating that the transport properties demonstrate some degree of anisotropy. Remarkably, low thermal conductivity values (below 1 W m-1 K-1 above 300 K) were measured suggesting that this compound may be an interesting platform to design novel TE materials with high efficiency.

LaMnO 3 oxide was successfully synthesized from La(OH) 3 and Mn 2 O 3 precursors (molar ratio La/Mn = 0.6) using the spark plasma sintering technique (SPS). The in-situ reaction proceeds in a few minutes at 900 °C under 50 MPa with the very high heating rate of 100 °C/min. LaMnO 3 powder was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and magnetic measurements. The Rietveld refinement of the X-ray powder diffraction shows that LaMnO 3 sample crystallizes in a perovskite structure O′-type orthorhombic symmetry with Pbnm space group. Our sample undergoes an A-type antiferromagnetic order with Neel temperature equal to 138 K. The magnetic susceptibility shows the Curie–Weiss behavior between 170 and 320 K with the effective magnetic moment μ eff = 4.78 μ B /f.u and the Curie constant θ P = − 20 K (antiferromagentic sign).

International audience; The sintering of the 80GeSe2-20Ga2Se3 chalcogenide glass composition as well as the preparation of the corresponding glass-ceramics has been investigated using pulsed current electrical sintering also known as spark plasma sintering. Amorphous powder has been synthesized by mechanical alloying. Due to the electrical insulating property of this glass composition, powder is heated mostly through the heating of the graphite die. It was found that densification of glass powder occurs through a viscous flow mechanism prior to devitrification. A model has been successfully applied to determine the viscosity and activation energy of the glass. This model has been confirmed by ball penetration technique to determine the glass sample viscosity.

An original and rapid way to build efficient thermo-generator in one step based on n-type and p-type (Bi,Sb) 2 (Te,Se) 3 nano-materials embedded in a polymer matrix is described. n-Type and p-type nano-powders are obtained through a mechanical alloying process leading to particle size of about 50 nm. The flexible thermo-generator is assembled in one step by Pulsed Current Electrical Sintering (PECS) also known as Spark Plasma Sintering (SPS) from a stacking of: (1) p-type powder, (2) special design of an electrical insulating polymer matrix and (3) n-type powder. After polishing the two sides of the thermo-generators in order to reveal series of n-type and p-type thermo-elements, electrical connections of these plots are made. Due to the fast sintering treatment, the nanostructure of thermo-elements is conserved. The full thermoelectric characterizations of such thermo-generator have been investigated leading to a generated output voltage of 51 mV for 17 thermocouples (Δ T ∼ 14 °C).

In different fields of civil engineering and cultural heritage, the corrosion behaviour of century old ferrous artefacts must be accurately evaluated, especially under uncontrolled conditions. This approach requires an understanding of the role played by the several hundred micrometre thick corrosion layers formed on ancient metals. Combining the study of historical mild steels and a new re-corroding experiment using isotopic tracers, we show here that mechanisms controlling corrosion kinetics differ according to the nature and the organisation of various microscopic phases, leading to a decoupling of anodic and cathodic reaction or to a homogeneous corrosion.

In the present work, the authors tried to establish degradation indices for heritage ferrous artefacts, especially those used in ancient buildings and submitted to indoor atmospheric corrosion. The authors focused on the site of the Amiens Cathedral in the north of France. Samples coming from this reference site were carefully characterised in order to identify the different phases constituting the corrosion scale. The scale consists in a matrix of iron oxyhydroxide goethite embedded with several ferrihydrite marblings. Other phases such as lepidocrocite and akaganeite are scarcely present in the external part of the corrosion scale. Moreover, electrochemical measurements on both references and ancient samples enable to define the reduction reactive phases. From the nature of these phases and their localisation, two degradation indices were defined to evaluate rust reactivity. Finally, a curve that links these two factors is proposed as a first step towards a corrosion diagnosis.

Raman structural imaging can extract original information in order to answer new questions raised in the recent studies on iron and low alloy steel corrosion. Up to now, this technique has allowed the extraction of the qualitative distribution of the compounds constituting the corrosion product layers. We propose here a methodology to extract quantitative parameters from Raman hyperspectral maps, executed by a home-developed software: LADIR-CAT. Specifically developed for iron corrosion quantitative component imaging, the approach and program operation are described. The LADIR-CAT is applied on ancient corroded iron samples originating from the Amiens cathedral (France) to establish the description and the composition of the corrosion system through quantitative compound imaging.

This paper gives a review of several years of research on archaeological analogues in order to predict the long term behaviour of the steel canisters or the reinforced concrete structures involved in disposal or interim storage of nuclear wastes in France. This article aims at showing the specific methodology, the complementariness with different other approaches and the complete integration of the research on analogues in the frame of research programs on long term prediction. Archaeological sites on which field measurement can be performed were referenced and described. A significant number of artefacts collected from these sites was selected for study. Detailed chemical and microstructural characterisation of the artefacts were undertaken by a combination of microbeam analytical techniques (μRaman, μXRD, μRaman spectroscopy,…). Hypotheses on the corrosion mechanisms were then tested using specific isotopic markers during re-corroding experiments on analogues. Specific parameters were measured, allowing crucial steps in modelling long-term corrosion of steel.

International audience; Lead chalcogenides PbX (X = Te, Se, S) have been the materials of choice for thermoelectric power generation at mid-range temperatures (500–700 K). Here, we report on a new family of chalcogenides α-As2Te3 that exhibit similar thermoelectric performances near 500 K. Sn doping in p-type polycrystalline α-As2−xSnxTe3 (x ≤ 0.075) provides an efficient control parameter to tune the carrier concentration leading to thermopower values that exceed 300 μV K−1 at 500 K. Combined with the structural complexity of the monoclinic lattice that results in extremely low thermal conductivity (0.55 W m−1 K−1 at 523 K), a peak ZT value of 0.8 is achieved at 523 K for x = 0.05. A single-parabolic band model is found to capture well the variations in the transport properties with the Sn concentration and suggests that higher ZT values could be achieved through band structure engineering. These results surpass those obtained in the sister compounds β-As2−xSnxTe3 and further show that α-As2Te3 based materials are competitive with other chalcogenides for thermoelectric applications at intermediate temperatures.

International audience; β-As2Te3 belongs to the prominent family of Bi2Te3-based materials, which show excellent thermoelectric properties near room temperature. In this study, we report a joint theoretical and experimental investigation of its electronic and thermal properties at low temperatures (5–300 K). These results are complemented by specific heat measurements (1.8–300 K) that provide further experimental evidence of the first order lattice distortion undergone by β-As2Te3 near 190 K. Data taken on cooling and heating across this transition show that the lattice distortion has little influence on the electronic properties and further evidence a weak hysteretic behavior. Although first-principles calculations predict a semiconducting ground state, these measurements show that β-As2Te3 behaves as a degenerate p-type semiconductor with a high carrier concentration of 1020 cm−3 at 300 K likely due to intrinsic defects. Calculations of the vibrational properties indicate that the extremely low lattice thermal conductivity values (0.8 W m−1 K−1 at 300 K) mainly originate from low-energy Te optical modes that limit the energy window of the acoustic branches. This limited ability to transport heat combined with a relatively large band gap suggest that high thermoelectric efficiency could be achieved in this compound when appropriately doped.

International audience; Metastable β-As2Te3 (R3̅m, a = 4.047 Å and c =29.492 Å at 300 K) is isostructural to layered Bi2Te3 and is known for similarly displaying good thermoelectric properties around 400 K. Crystallizing glassy-As2Te3 leads to multiphase samples, while β-As2Te3 could indeed be synthesized with good phase purity (97%) by melt quenching. As expected, β-As2Te3 reconstructively transforms into stable α-As2Te3 (C2/m, a = 14.337 Å, b = 4.015 Å, c = 9.887 Å, and β = 95.06°) at 480 K. This β → α transformation can be seen as the displacement of part of the As atoms from their As2Te3 layers into the van der Waals bonding interspace. Upon cooling, β-As2Te3 displacively transforms in two steps below TS1 = 205−210 K and TS2 = 193−197 K into a new β′ As2Te3 allotrope. These reversible and first-order phase transitions give rise to anomalies in the resistance and in the calorimetry measurements. The new monoclinic β′-As2Te3 crystal structure (P21/m, a = 6.982 Å, b = 16.187 Å, c = 10.232 Å, β = 103.46° at 20 K) was solved from Rietveld refinements of X-ray and neutron powder patterns collected at low temperatures. These analyses showed that the distortion undergone by β-As2Te3 is accompanied by a 4-fold modulation along its b axis. In agreement with our experimental results, electronic structure calculations indicate that all three structures are semiconducting with the α-phase being the most stable one and the β′-phase being more stable than the β-phase. These calculations also confirm the occurrence of a van der Waals interspace between covalently bonded As2Te3 layers in all three structures.