Ge-rich Ge-Sb-Te (GGST) alloys are of high interest for industrial production of complementary metal-oxide-semiconductor integrated phase change random access memories (PCRAM) of high performance. BEOL-integrated GGST-based PCRAM constitute a relevant solution for low-power on-chip non-volatile memory production and in-memory computing, addressing new challenges of automotive and artificial intelligence applications for example. During cycling, in the low-resistive SET state, the crystallized GGST alloy is mainly made of nano-grains of two phases: diamond Ge and a ternary rock-salt (RS) Ge-Sb-Te (GST) phase. The RS-GST phase is metastable and generally assumed to exhibit the stoichiometry Ge2Sb2Te5 corresponding to the stable hexagonal Ge2Sb2Te5 compound. However, recent works suggest that this metastable RS-GST phase is not stoichiometric and can accept higher Ge contents, meaning that the Ge content of RS-GST could vary depending on the Ge excess level in the GGST alloy compared to the Ge2Sb2Te5 reference stoichiometry. Consequently, the performance of GGST-based PCRAMs could vary with Ge composition of GGST alloy, suggesting the existence of an ideal GGST alloy composition showing a good compromise between crystallization temperature, programming current density, resistance, threshold voltage drift… In the present work, the structure and composition of crystallized GGST films elaborated through Ge, Sb, and Te co-sputtering were studied by X-ray diffraction, high resolution transmission electron microscopy, and atom probe tomography. The metastable RS-GST phase crystallized in GGST films exhibiting a Ge excess between 23% and 42% is found to be able to incorporate a large amount of Ge, of about 50 at% and sometimes beyond.

An experimental study was conducted to investigate the effects of altering fresh (U,Pu)O2 MOX of high alpha-specific activity under alkaline conditions (pH = 12), in the presence of 2 mmol.L-1 silicate ions and in anoxic environment. This test was conducted in conjunction with analogous experiments using (U,Ce)O2 simulant materials of homogeneous and heterogeneous microstructure. The results obtained indicated that the oxidative dissolution induced by alpha-radiolysis of water in contact with (U,Pu)O2 MOX was suppressed under these alteration conditions. The uranium concentration determined at equilibrium was found to be fully comparable to those obtained with the (U,Ce)O2 model materials. This observation suggested that the alteration mechanism occurring at the solid/ solution interface was independent of the alpha-activity of the material and highlighted the analogy of the two types of materials in the tested conditions. Post-alteration characterizations of the material surface using Raman spectroscopy, TEM/EDS and SEM were conducted. The results obtained supported the assumption that adsorption of silicate ions onto reactive sites in the material was responsible for the inhibition of uranium oxidation. This study revealed the important role played by silicate ions in reducing uranium release in alkaline conditions and under alpha radiolysis of water. Considering that uranium is a tracer of the MOX matrix alteration, these results had positive implications regarding the long-term management of spent fuel in deep disposal especially in case of contact with cementitious water.

This study investigates the dissolution kinetics of synthetic basaltic glass under circum-neutral to basic conditions (pH20 degrees C from 6 to 10) and temperatures of 30, 60 and 90 degrees C, with a particular focus on the roles of dissolved oxygen (O2(aq)) and silica (SiO2(aq)) concentrations. Surface retreats were measured using vertical scanning interferometry, and the thickness of alteration layers (amorphous silica-rich surface layer, referred to as ASSL) using X-ray reflectivity and transmission electron microscopy performed on focused ion beam-milled lamellae. As expected, the dissolution rate of basaltic glass increases with increasing pH from neutral to basic conditions. A modest influence of O2(aq) concentrations was observed, attributed to Fe(II) oxidation and the associated formation of a passivating Fe(III)-Si-rich surface layer. Most strikingly, the dissolution rate was found to decrease exponentially with increasing SiO2(aq) concentrations, which is inconsistent with the transition state theory. Instead, this behavior is consistent with a mechanism governed by classical nucleation theory in the studied conditions, resulting in the following overall dissolution rate law: r = () k0.10n.pHT.exp Ea.exp k1 k2, with r being the dissolution rate (in mol/m2/s), k0 = 552 mol/ RT |ln[SiO2(aq) ]/KTeq | m2/s, n = 0.35, Ea = 84 kJ/mol, k1 = 0.40, k2 = 3.87, pHT is the pH value at the considered temperature T, R the gas constant, and KTeq the solubility constant of amorphous silica at the considered temperature. Taken together, these findings provide new insights into the coupled effects of pH, O2(aq), and SiO2(aq) on basaltic glass reactivity, offering a refined kinetic framework for modeling glass weathering in natural and engineered environments.

In nowadays magnetic confinement fusion devices, plasma facing components are armoured with refractory high Z metals like tungsten (W). Despite W high melting point, moderate neutron activation, low tritium retention and erosion, its thermomechanical properties make it prone to recrystallization, cracking and/or melting under thermal shocks.Carbon was discarded as plasma-facing material in fusion for its high level of erosion and tritium retention. However, carbon as Chemical Vapor Deposition (CVD) diamond has low retention and outstanding performances under thermal shocks because of its high thermal conductivity. Unfortunately, it is comparable to graphite for erosion. For this reason, tungsten-diamond (W-diamond) test samples made of a ~mm thick CVD diamond substrate for a fast distribution of heat with a ~10 µm W surface coating for protection against erosion were manufactured. According to modelling, W-diamond is expected to have a thermal behaviour similar to pure CVD diamond.The objective of this paper is to explore the domain of viability of the metal-diamond interface under thermal shocks of a new armor material not as limiting as bulk W for present day machines with low irradiation levels like WEST, W7X and JT60-SA. Various configurations were considered: types and thickness of diamond substrates, pure W coatings and a molybdenum interlayer. Different levels of thermal shocks were also performed with a laser on these samples:● up to 104 cycles of 1 ms at 1-3 GW.m-2 (~32-95 MW.m-2.s0.5) to simulate fast transients like edge-localized modes or disruptions,● up to 20 cycles of 1 s at 100 MW.m-2 to simulate divertor reattachment.Post-mortem analysis showed pristine W coatings and metal-diamond interfaces for all samples after slow transients at 100 MW.m-2 and after fast transients up to 1.7-2 GW.m-2. Bulk ITER-grade W samples tested in the same conditions showed cracked and melted surfaces at the lowest thermal loads.

The dissolution of International Simple Glass (ISG) was investigated at 90 degrees C and alkaline conditions with various concentrations of dissolved Si and Ca to unravel the combined effects of those elements on ISG reactivity. Experiments were conducted over durations ranging from 20 days to 3 months. Through morphological, structural, and chemical characterizations, the glass dissolution rate was proven to be strongly correlated with the activity of dissolved silica in the solution. While dissolved calcium did not significantly impact the dissolution rate, precipitation of calcium silicate hydrates (CSH) during the experiments enhanced ISG dissolution rate, though to a modest extent. The 3-months experiments highlighted the strong correlation between the dissolution mechanism and the evolution of the nature of secondary phases in saturated solution. During the first 20 days and at high Si and Ca concentrations, CSH precipitated and aggregated, without preventing the passivating impact of the gel layer at the surface of the glass: the dissolution was controlled by diffusion. Then, a resumption of dissolution occurred between 19 days and 76 days, corresponding to the CSH growth, and a possible mechanistic switch to a hydrolysis-controlled reaction rate. Finally, in some experiments, a drop in pH due to carbonate precipitation was observed along with a decrease in the dissolution rate, falling back in a diffusion-limited regime. Overall, this study shows that at 90 degrees C, pH = 10 and concentrations of SiO2(aq) exceeding 50 % of saturation with respect to amorphous silica, irrespective of Ca concentration but in presence of CO2(aq), ISG exhibits a very good chemical durability.

Transmission electron microscopy (TEM), X-ray diffraction (XRD) and positron annihilation spectroscopy (PAS) characterizations were combined on polycrystalline UO2 disks, implanted with low energy xenon ions, to probe the microstructural transformations due to irradiation by taking advantage of their different sensitivities to radiation damage. While XRD provides information on elastic strain (and thus free swelling) and microstrains, PAS is sensitive to vacancy defects and TEM allows the direct observation of extended defects (dislocations and vacancy objects). These data were gathered in order to understand mechanisms involved in the damage build up in UO2. At a very low damage level, an increase of the free swelling is highlighted along with first vacancy defects, and then with interstitials dislocation loops. At a higher damage level, dislocation lines appear through loop interactions, inducing a small free swelling relaxation associated with an increase of microstrains. The results of this extensive experimental study are then discussed in the light of literature data.

In this study, the influence of the working atmosphere on the sinterability and chemical durability of Nd-doped UO2 mixed oxides was investigated. To this end, the starting powder was first prepared by a hydroxide coprecipitation route, resulting in a nano-sized granulometry combined with a high specific surface area. The powders were then converted to oxides by heating and sintered in pellet form at 1600 degrees C under an argon or reducing (Ar-4 %H2) atmosphere. The use of argon or reducing atmosphere resulted in very different densification pathways and final microstructures. The reducing sintering atmosphere hindered the uranium (IV) oxidation that could occur at high temperature, leading to the formation of U3O8, as was the case when working under argon atmosphere. Regarding the microstructure of the sintered pellets, the use of an argon sintering atmosphere resulted in an average grain size ten times larger than that of a reducing sintering atmosphere, while macroscopic properties such as relative density, porosity and homogeneity of cation distribution at the pellet scale remained the same. Nevertheless, a slight local enrichment of Nd at the grain boundaries was observed for the pellet sintered under Ar-4 %H2. In a second step, the study of the chemical durability of these sintered samples showed a significant influence of the sintering atmosphere on the dissolution kinetics and mechanism. These differences could be related to the microstructural properties of the pellets, i.e. the average grain size and the occurrence of grain boundaries. The cation distribution in the pellets could also influence their chemical durability, such as local Nd enrichment, the formation of defects in the oxygen sublattice and the presence of a different fraction of U(V) depending on the sintering atmosphere, as shown by HERFD-XANES measurements. The use of reducing or argon sintering atmospheres could even direct the charge compensation mechanisms that occur into the solid, thereby simultaneously affecting the sinterability and chemical durability of the samples.

The dissolution of International Simple Glass (ISG) was investigated at 90 degrees C, elevated concentration of dissolved silica and in the presence of calcium, with a specific emphasis on basic pH conditions. The leaching solution was labelled with Si-29, O-18 and Ca-44 in part of the experiments to elucidate the dissolution mechanisms. Based on the isotopic signatures of the gel layer analyzed using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), it was concluded that oxygen atoms mostly originate from the solution for all investigated conditions, while silicon atoms almost exclusively originate from the glass. A negative correlation was found between the initial concentration of calcium in solution and the gel layer thickness, suggesting either the formation of a passivating (Si, Ca)-rich layer, a catalytic effect of Ca on the gel densification or a combination of both. In addition, the pH-dependence of the diffusion coefficient of B within the gel was found to be stronger in the basic pH range than in the acidic pH range, which was suggested to originate from the change in coordination of B species at pH(90) degrees(C) similar to 8.5. Overall, these results suggest that in a (Ca, Si)-rich solution at basic pH, the durability of ISG is stronger than previously thought, as the diffusion coefficient of B under such conditions are lower than expected based on literature.

Prediction of glass alteration kinetics evolution is key to assessing chemical durability. A mechanistic understanding of the different processes controlling the alteration kinetics will enable the development of robust models. The “gel” that forms on the glass surface because of alteration is known to have “passivation” properties that reduce glass alteration rates. However, open questions remain about the passivation mechanism. Tracing experiments using 10B and 18O and characterization of gel layers aging from 7 days to 27 years, formed at pH90°C 9 and 90 °C, provided some insights into the evolution of the gel properties (maturation) and their influence on alteration kinetics. The probable mechanisms by which the glass alteration kinetics reduces have been examined. It has been suggested that both the reactive diffusion of water and the boron diffusion through the gel layer towards the solution have decreased over time. The potential causes and consequences of this decrease have been reviewed.

Optical fibers containing nanoparticles have recently attracted a great interest for applications such as fiber sensors and fiber lasers. Their development remains limited by a lack of control over nanoparticle characteristics (size, density, etc.). Here, we propose a femtosecond laser structuring process that uses local heating at a micrometer scale to engineer the characteristics of the nanoparticles and hence the light scattering of this type of fiber.

The saturable absorption of 2D Bi2Te3 layers is studied by using the Z-scan technique employing infrared 400 fs laser pulses. Optimization of the nonlinearities has been carried out by measuring the third-order nonlinear susceptibilities as a function of the film thickness. A thorough optimization of the thin film annealing conditions has been performed and is presented. For each thickness, the annealing parameters have been separately investigated. Scanning electron microscopy, X-ray diffraction, and UV-Vis spectrophotometry studies have also been performed on the as-deposited and crystallized 2D layers.

Because he thought the nanoparticles I drew for him were too small, too large, too elongated, I ended up drawing a fiber and telling him that the nanoparticles he wanted were inside, which lit up his face.

Tungsten (W) is a common plasma-facing material in nuclear fusion devices. It readily oxidizes in the presence of oxygen, forming tungsten oxides, particularly WO3, which may modify deuterium retention. This experimental study investigates mechanisms of the evolution of deuterium retention in a thermally stable similar to 100 nm WO3 layer grown by thermal oxidation of a W substrate at 1073 K under low pure oxygen pressure of 7 Pa. 500 eV D-2(+) deuterium implantation and Temperature Programmed Desorption (TPD) were used to explore deuterium trapping and release from the oxide as a function of incident ion fluence and storage time in ultra high vacuum. Upon deuterium irradiation, the formation of a W-rich layer on the surface of the oxide is evidenced with x-ray photoelectron spectroscopy. After successive implantation/TPD cycles corresponding to an accumulated deuterium fluence >10(21) D m(-2), the appearance of an amorphous oxide encapsulated between two W-rich layers is observed with transmission electron microscopy. Following this high fluence deuterium implantation, deuterium retention increases by a factor of 10. The findings provide insight into the behavior of tungsten oxides under deuterium implantation, emphasizing the importance of considering its thermal stability and its structural modifications at the surface of tungsten plasma facing components.

Heat transfer fluid mining represents a thermodynamic perturbation for geothermal reservoirs: The pumping of hot water coupled with the re-injection of colder water at depth favors the dissolution of some rock-forming minerals of the deep reservoir (e.g. feldspars), while promoting the precipitation of secondary phases, resulting in a possible change in the permeability and porosity of the reservoir. Such an impact is even greater when one considers the acid stimulations aimed at increasing the injectivity of the geothermal system. In that respect, no consensus exists in the literature regarding the impact of secondary phases on the dissolution rate of primary phases and therefore, on the sustained modification of pore structure. The present study aimed at shedding new light on these questions. Hydrothermal experiments of K-feldspar alteration were conducted at conditions relevant for the geothermal reservoir of Soultz-sous-Forêts (T = 180 °C, acidic pH domain). Measurements of cation release rates were combined with characterizations of secondary coatings (mineralogy, extent of coverage, thickness and porosity) to determine the reactivity of submillimeter K-feldspar powders with and without secondary precipitates. The formation of µm-thick boehmite coatings on K-feldspar grains was found to result in a modest decrease in its reactivity, which might be better explained by the presence of dissolved Al in the bulk solution. This result was independently confirmed by reactive transport simulations, which revealed that the impact of secondary coatings may become significant only when their thickness exceeds a few tens of microns, or if the dissolution rate of the primary phase is significantly greater (106 times) than that of orthoclase. Taken together, this study offers new constraints on the intricate interplay between dissolution and precipitation reactions, of prime importance for modeling more accurately the impact of mass transfer and porosity generation resulting from fluid circulation in geothermal reservoirs.

MCVD germanosilica glass embedded with YbPO4 crystals were for the first time drawn into optical fibers. So-lution doping was used to embed the crystals in the silica soot prior to the collapsing step. We demonstrate, using scanning/transmission electron microscopes and confocal Raman microscope, that YbPO4 crystals survive not only the MCVD process but also the drawing process despite the high temperature involved (up to 2100 degrees C) during fabrication processes. The fiber contains 100 nm-crystals with the same composition and structure as the as-prepared crystals. During the drawing process, these crystals tend to have a preferential orientation of their c -axis along the drawing direction. These results open a new route to fabricate glass-based composite fibers containing crystalline particles without additional post heat-treatment.

The thickness-dependent saturable absorption behaviour of atomically thin bismuth selenide films has been optimized using 515 nm and 1030 nm laser excitation wavelengths. The studies have been performed by the Z-scan technique employing 400 fs laser pulses emitted by a fiber laser at a repetition rate of 100 Hz. The obtained results allowed the determination of the nonlinear optical parameters as a function of the film thickness. Additional studies have been carried out in order to determine the third-order optical nonlinearities as a function of the annealing temperature. The samples have also been studied by spectrophotometry, electron microscopy and X-ray diffraction, which allowed a deeper knowledge of the optical properties, as well as the crystalline structure of the Bi2Se3 material.

Optical fibers containing nanoparticles were proposed twenty years ago to develop new applications in lasers and amplifiers. By encapsulating luminescent ions in the nanoparticles, new emission properties can then appear. However, the particles must be small in size to limit the optical losses by light scattering. More recently, this type of optical fiber has shown a strong potential for application as sensors. They exploit the light scattering property induced by nanoparticles. Then, the development of these fibers for these different applications depends on our ability to control the characteristics of the nanoparticles in the optical fibers. To reach this goal, we discuss how the drawing step is a crucial step to achieve this goal, i.e. to tune the size and the shape of the nanoparticles in the fibre. Through this approach, the characteristics of the nanoparticles are obtained in the as-drawn fiber, avoiding any additional post heat-treatment which may damage the optical fiber.

Topological insulators, such as the Bi2Se3 material, exhibit significant optical nonlinearities. This work investigates the impact of the pulse duration on the nonlinear optical responses of Bi2Se3 layers. Scanning electron microscopy studies have been performed to reveal the crystalline structure of the samples. The nonlinear optical performance has been investigated for a wide range of pulse durations, from 400 fs to 10 ps, using 1030 nm laser excitation. The nonlinear absorption coefficients recorded in this study range from -1.45 x10-7 m/W to -4.86 x10-7 m/W. The influence of two different mechanisms on optical nonlinearities was observed and discussed. Identical experimental conditions have been employed throughout the studies allowing a direct comparison of the results.

In this work, we performed laser annealing of thin Sb2Te3 films to optimize crystallization time and their nonlinear optical properties. The annealed layers were studied by electron microscopy and UV–Vis spectrophotometry. Their nonlinear optical response was investigated by nonlinear transmission and Z-scan measurements. These studies were performed by a femtosecond laser system providing 400 fs laser pulses at 1030 nm. The results were compared with previous findings based on studies of oven-annealed thin films.