Johann Ravaux

Stability and Structural Evolution of CeIV1–xLnIIIxO2–x/2 Solid Solutions: A Coupled μ-Raman/XRD Approach

Several CeO(2)-based mixed oxides with general composition Ce(1-x)Ln(x)O(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 °C, investigation of the crystalline structure of these oxides was carried out by XRD and μ-Raman spectroscopy. Typical fluorite Fm ̅3m structure was obtained for relatively low Ln(III) contents, while a cubic Ia ̅3̅ superstructure was evidenced above x ≈ 0.4. Moreover, since Nd(2)O(3) does not crystallize with the Ia ̅3̅-type structure, two-phase systems composed with additional hexagonal Nd(2)O(3) were obtained for x(Nd) ≥ 0.73 in the Ce(1-x)Nd(x)O(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-x)Ln(x)O(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.

Influence of Crystallization State and Microstructure on the Chemical Durability of Cerium–Neodymium Mixed Oxides

To underline the potential links between the crystallization state and the microstructure of powdered cerium-neodymium oxides and their chemical durability, several Ce(IV)(1-x)Nd(III)(x)O(2-x/2) 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 CeO(2) and Ce(1-x)Nd(x)O(2-x/2) solid solutions (with x = 0.09 and 0.16). For CeO(2), 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.

Environmental SEM monitoring of Ce1−xLnxO2−x/2 mixed-oxide microstructural evolution during dissolution

The microstructural evolution of several Ce1−xLnxO2−x/2 (Ln = Nd, Er, x = 0.28; 0.59 and 0.76) sintered pellets was studied during their dissolution in 4 M HNO3 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 LnIII 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 Ce0.72Nd0.28O1.86 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.

Facile one-pot synthesis of copper hexacyanoferrate nanoparticle functionalised silica monoliths for the selective entrapment of 137Cs

In this study, hexacyanoferrate nanoparticle (NP) functionalised silica monoliths were used to selectively remove Cs+ from aqueous solutions which also contained large concentrations of Na+. Various NP loading levels were examined from 0.2 wt% up to 5.1 wt% producing different 133Cs and 137Cs sorption results.

Development of an Integrated Thermocouple for the Accurate Sample Temperature Measurement During High Temperature Environmental Scanning Electron Microscopy (HT-ESEM) Experiments.

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 (±5°C) 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.

Catalytic dissolution of ceria under mild conditions

The dissolution of ceria is studied through a catalytic reduction process involving platinum nanoparticles in mild conditions at near ambient temperature. The deposition of platinum nanoparticles is made by sonication (Ar, 18 W cm−2, 20 kHz), and further dissolution is studied as a function of different parameters such as stirring, sonication, dissolution media and temperature. The dissolution is evaluated using UV-vis spectrophotometry, ICP-AES, and SEM. The quantitative dissolution of ceria can be performed in HNO3–HCOOH–[N2H5][NO3], HNO3–[N2H5][NO3] or H2SO4–HCOOH mixtures at 40 °C. Nevertheless, it is shown that the combined use of ultrasound with nitric media in the presence of platinum nanoparticles can lead to passivating phenomena resulting in a decrease of the dissolution rate.

How to explain the difficulties in the coffinite synthesis from the study of uranothorite

The preparation of Th(1-x)U(x)SiO(4) uranothorite solid solutions was successfully undertaken under hydrothermal conditions (T = 250 °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)O(2) dioxide and amorphous silica were systematically observed for the highest uranium mole loadings. 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 USiO(4) coffinite were assigned to a kinetic hindering associated with the coffinitization reaction.

Mechanism of RuO2 Crystallization in Borosilicate Glass: An Original in Situ ESEM Approach

Ruthenium, a fission product arising from the reprocessing of spent uranium oxide (UOX) fuel, crystallizes in the form of acicular RuO(2) particles in high-level waste containment glass matrices. These particles are responsible for significant modifications in the physicochemical behavior of the glass in the liquid state, and their formation mechanisms are a subject of investigation. The chemical reactions responsible for the crystallization of RuO(2) particles with acicular or polyhedral shape in simplified radioactive waste containment glass are described. In situ high-temperature environmental scanning electron microscopy (ESEM) is used to follow changes in morphology and composition of the ruthenium compounds formed by reactions at high temperature between a simplified RuO(2)-NaNO(3) precursor and a sodium borosilicate glass (SiO(2)-B(2)O(3)-Na(2)O). The key parameter in the formation of acicular or polyhedral RuO(2) crystals is the chemistry of the ruthenium compound under oxidized conditions (Ru(IV), Ru(V)). The precipitation of needle-shaped RuO(2) crystals in the melt might be associated with the formation of an intermediate Ru compound (Na(3)Ru(V)O(4)) before dissolution in the melt, allowing Ru concentration gradients. The formation of polyhedral crystals is the result of the direct incorporation of RuO(2) crystals in the melt followed by an Ostwald ripening mechanism.

Surface decoration of catanionic vesicles with superparamagnetic iron oxide nanoparticles: a model system for triggered release under moderate temperature conditions

We report the design of new catanionic vesicles decorated with iron oxide nanoparticles, which could be used as a model system to illustrate controlled delivery of small solutes under mild hyperthermia. Efficient release of fluorescent dye rhodamine 6G was observed when samples were exposed to an oscillating magnetic field. Our system provides direct evidence for reversible permeability upon magnetic stimulation.

Alternate dipping preparation of biomimetic apatite layers in the presence of carbonate ions

The classical simulated body fluids method cannot be employed to prepare biomimetic apatites encompassing metallic ions that lead to very stable phosphates. This is the case for heavy metals such as uranium, whose presence in bone mineral after contamination deserves toxicological study. We have demonstrated that existing methods, based on alternate dipping into calcium and phosphate ions solutions, can be adapted to achieve this aim. We have also especially studied the impact of the presence of carbonate ions in the medium as these are necessary to avoid hydrolysis of the contaminating metallic cations. Both the apatite-collagen complex method and a standard chemical (STD) method employing only mineral solutions lead to biomimetic apatites when calcium and carbonate ions are introduced simultaneously. The obtained materials were fully characterized and we established that the STD method tolerates the presence of carbonate ions much better, and this leads to homogeneous samples. Emphasis was set on the repeatability of the method to ensure the relevancy of further work performed on series of samples. Finally, osteoblasts cultured on these samples also proved a similar yield and standard-deviation in their adenosine triphosphate content when compared to commercially available substrates designed to study of such cell cultures.