[en] Working with the (Environmental) Scanning Electron Microscope (ESEM) at high temperature has been a long lasting goal for many researchers and remains to this day a difficult technique to operate efficiently. This article describes the properties of several different furnaces developed over the past 40 years, focusing specifically on the operation of the high temperature stage associated with the ESEM. Guidelines and advices for correct use of these systems are provided regarding the experience of the authors. A focus on sintering studies illustrates the main difficulties to implement this experimental technique. (authors)

[en] Understanding the dissolution mechanism at the solid/solution interface of Nd₂Zr₂O₇ pyrochlore is a critical step to evaluate its suitability to immobilize radionuclides. In this aim, the chemical durability of Nd₂Zr₂O₇ pyrochlore sintered samples in 4 M HCl was examined by several complementary techniques. Microstructural and structural changes were evidenced at the solid/solution interface. Environmental Scanning Electron Microscopy (ESEM), Atomic Force Microscopy (AFM) and X-Ray Reflectivity (XRR) showed that the dissolution occurs heterogeneously at the surface with a complex interplay of several dissolution rate contributors. The most important process observed is the chemo-mechanical corrosion of the pyrochlore as a consequence of the grain boundaries dissolution. The evolution of the crystal structure at the solid/solution interface was also followed by Grazing Incidence X-Ray Diffraction (GI- XRD). It was inferred to the dissolution mechanism as a consequence of Nd and Zr occupancy of non-equivalent cationic positions in the pyrochlore structure. Regarding the chemical durability of the pyrochlore as a conditioning matrix for plutonium, minor actinides or fission products, this work demonstrates the impact of the microstructure. By increasing the grain size, and thus by decreasing the density of grains boundaries, it would be possible to limit the detachment of the grains and thus the subsequent microstructural crumbling. (authors)

[en] The microstructural evolution of several Ce_1_-_xLn_xO_2_-_x_/_2 (Ln = Nd, Er, x = 0.28; 0.59 and 0.76) sintered pellets was studied during their dissolution in 4 M HNO_3 at 90 C or 60 C. Environmental Scanning Electron Microscopy (ESEM) experiments were developed to follow the changes in the sample microstructure and their consequences on the dissolution evolution by monitoring a constant zone of the material throughout the alteration process. The Ln(III) content was the parameter affecting most of the dissolution kinetics of such solid solutions. From a microstructural point of view, grain boundaries dissolve preferentially during the first dissolution step due to their relative weakness compared to the well-crystallized bulk material. Crystal defects, namely screw and edge dislocations, constitute the second kind of preferential dissolution zone and induce the formation and growth of corrosion pits, mainly of pyramidal shape on the whole surface. Both microstructural parameters are responsible for significant and non-linear increase of the reactive surface developed at the solid/liquid interface and thus important modifications of the topology. On sintered samples and even more clearly on the powdered Ce_0_._7_2Nd_0_._2_8O_1_._8_6 sample, the formation of a gelatinous layer resulting from processes occurring in the back-end of the dissolution process (e.g. local saturation of the leaching solution), was evidenced through ESEM observations. This gel is also presumably responsible for the progressive decrease of the normalized dissolution rate. From these observations, it appears that the study of material dissolution kinetics can clearly benefit from the monitoring of the microstructure evolution and particularly that of the reactive surface area during dissolution. (authors)

[en] We have developed two integrated thermocouple (TC) crucible systems that allow precise measurement of sample temperature when using a furnace associated with an environmental scanning electron microscope (ESEM). Sample temperatures measured with these systems are precise (±5C) and reliable. The TC crucible systems allow working with solids and liquids (silicate melts or ionic liquids), independent of the gas composition and pressure. These sample holder designs will allow end users to perform experiments at high temperature in the ESEM chamber with high precision control of the sample temperature. (authors)

[en] Several CeO₂-based mixed oxides with general composition Ce_1-xLn_xO_2-x/2 (for 0 ≤ x ≤ 1 and Ln = La, Nd, Sm, Eu, Gd, Dy, Er, or Yb) were prepared using an initial oxalic precipitation leading to a homogeneous distribution of cations in the oxides. After characterization of the Ce/Nd oxalate precursors and then thermal conversion to oxides at T 1000 degrees C, investigation of the crystalline structure of these oxides was carried out by XRD μ-Raman spectroscopy. Typical fluorite Fm(3-bar)m structure was obtained for relatively low Ln(III) contents, while a cubic Ia(3-bar) superstructure was evidenced above x ≅ to 0.4. Moreover, since Nd₂O₃ does not crystallize with the Ia(3-bar)-type structure, two-phase systems composed with additional hexagonal Nd₂O₃ were obtained for x(Nd) ≥ 0.73 in the Ce_1-xNdO_2-x/2 series. The effect of heat treatment temperature on these limits was explored through μ-Raman spectroscopy, which allowed determining the presence of small amounts of the different crystal structures observed. In addition, the variation of the Ce_1-xLn_xO_2-x/2 unit cell parameter was found to follow a quadratic relation as a result of the combination between increasing cationic radius, modifications of cation coordination, and decreasing O-O repulsion caused by oxygen vacancies. (authors)

[en] The preparation of Th_1-xU_xSiO₄ uranothorite solid solutions was successfully undertaken under hydrothermal conditions (T = 250 degrees C). From XRD and EDS characterization, the formation of a complete solid solution between x = 0 (thorite) and x = 0.8 was evidenced. Nevertheless, additional (Th,U)O₂ dioxide and amorphous silica were systematically observed for the highest uranium mole loading. The influence of kinetics parameters was then studied to avoid the formation of such side products. The variation of the synthesis duration allowed us to point out the initial formation of oxide phases then their evolution to a silicate phase through a dissolution/precipitation process close to that already described as coffinitization. Also, the uranium mole loading initially considered was found to significantly influence the kinetics of reaction, as this latter strongly slows down for x ≥ 0.3. Under these conditions, the difficulties frequently reported in the literature for the synthesis of pure USiO₄ coffinite were assigned to a kinetic hindering associated with the coffinitization reaction

[en] The preparation of a synthetic uranothorite with desired formula Th_0.5U_0.5SiO₄ was performed under hydrothermal conditions from a mixture of tetravalent thorium and uranium in hydrochloric solution with sodium metasilicate. The XRD Rietveld analysis revealed that the system obtained was composed by two crystallized phases. The first one corresponded to uranium-depleted Th_0.57U_0.43SiO₄ with a = 7.0571(1) angstrom and c = 6.2998(1) angstrom that confirmed the formation of a solid solution between thorite and coffinite end-members. On the other hand, U-enriched Th_0.21U_0.79O₂ dioxide was also pointed out while the formation of amorphous SiO₂ was stated from elementary analyses and mu-Raman spectroscopy. This latter also led to confirm the formulation of the silicate by rejecting unambiguously the presence of structural hydroxyl groups or water molecules. (authors)

[en] Bone is the main target organ for the storage of several toxic metals, including uranium. But the mode of action of uranium on bones remains poorly understood. To better assess the impact of uranium on bone cells, synthetic biomimetic apatites encompassing a controlled amount of uranium were prepared and analyzed. This study revealed the physicochemical impact of uranium on apatite mineralization: the presence of the metal induces a loss of crystallinity and a lower mineralization rate. The prepared samples were then used as substrates for bone cell culture. Osteoblasts were not sensitive to the presence of uranium in the support, whereas previous results showed a deleterious effect of uranium introduced into a cell culture solution. This work should therefore have some original prospects within the context of toxicological studies concerning the effect of metallic cations on bone cell systems. (authors)

[en] To underline the potential links between the crystallization state and the microstructure of powdered cerium neodymium oxides and their chemical durability, several (Ce_1-xNd_xO_2-x/2) (Nd(IV), Ce(III)) mixed dioxides were prepared in various operating conditions from oxalate precursors and then leached. The powdered samples were first examined through several physicochemical properties (crystallization state and associated crystallite size, reactive surface area, porosity ...). The dependence of the normalized dissolution rates on various parameters (including temperature, nitric acid concentration, crystallization state) was examined for pure CeO₂ and Ce_1-xNd_xO_2-x/2 solid solutions (with x = 0.09 and 0.16). For CeO₂, either the partial order related to the proton activity (n = 0.63) or the activation energy (E_A = 37 kJ . mol^-1) suggested that the dissolution was mainly driven by surface reactions occurring at the solid liquid interface. The chemical durability of the cerium neodymium oxides was also strongly affected by chemical composition. The initial normalized dissolution rates were also found to slightly depend on the crystallization state of the powders, suggesting the role played by the crystal defects in the dissolution mechanisms. On the contrary, the crystallite size had no important effect on the chemical durability. Finally, the normalized dissolution rates measured near the establishment of saturation conditions were less affected, which may be due to the formation of a gelatinous protective layer at the solid/liquid interface. (authors)

[en] The HT-ESEM design and image acquisition conditions to perform in situ experiments are fully described. Two examples of applications in the nuclear glass field are reported. (authors)