Solid state synthesis and X-ray diffraction characterization of Pu 3+(1-2x)Pu4+xCa2+xPO4

In the framework of the 1991 French law concerning nuclear waste management, several studies have been carried out in order to elaborate crystalline matrices for specific immobilization of the radionuclides. In the case of high level and long-lived minor actinides (Np, Am and Cm), which are high level and long-lived radioactive elements, monazite, a light rare earth (Re) orthophosphate with general formula Re3+PO4 (with Re = La to Gd), has been proposed as a host matrix, thanks to its high resistance to self irradiation and its low solubility. Monazite crystallizes in the monoclinic space group P21/n. In this structure, trivalent cations (Re3+) could be substituted by an equivalent amount of bivalent (A2+) and tetravalent (B4+) cations, allowing the simultaneous incorporation of Am3+, Cm3+ and Np4+. According to Podor's work1, the limit of a tetravalent element incorporation in monazite is related to its size in the ninefold coordination (RIX)

Solid-State Synthesis of Monazite-type Compounds Containing Tetravalent Elements

International audienceOn the basis of optimized grinding/heating cycles developed for several phosphate-based ceramics, the preparation of brabantite and then monazite/brabantite solid solutions loaded with tetravalent thorium, uranium, and cerium (as a plutonium surrogate) was examined versus the heating temperature. The chemical reactions and transformations occurring when heating the initial mixtures of AnO2/CeO2, CaHPO4·2H2O (or CaO), and NH4H2PO4 were identified through X-ray diffraction (XRD) and thermogravimetric/differential thermal analysis experiments. The incorporation of thorium, which presents only one stabilized oxidation state, occurs at 1100 °C. At this temperature, all the thorium−brabantite samples appear to be pure and single phase as suggested by XRD, electron probe microanalyses, and μ-Raman spectroscopy. By the same method, tetravalent uranium can be also stabilized in uranium−brabantite, i.e., Ca0.5U0.5PO4, after heating at 1200 °C. Both brabantites, Ca0.5Th0.5PO4 and Ca0.5U0.5PO4, begin to decompose when increasing the temperature to 1400 and 1300 °C, respectively, leading to a mixture of CaO and AnO2 by the volatilization of P4O10. In contrast to the cases of thorium and uranium, cerium(IV) is not stabilized during the heating treatment at high temperature. Indeed, the formation of Ca0.5Ce0.5PO4 appears impossible, due to the partial reduction of cerium(IV) into cerium(III) above 840 °C. Consequently, the systems always appear polyphase, with compositions of CeIII1-2xCeIVxCaxPO4 and Ca2P2O7. The same conclusion can be also given when discussing the incorporation of cerium(IV) into La1-2xCeIIIx-yCeIVyCay(PO4)1-x+y. This incomplete incorporation of cerium(IV) confirms the results obtained when trying to stabilize tetravalent plutonium in Ca0.5PuIV0.5PO4 samples

Structural study of polymorphism and thermal behavior of CaZr(PO4)2

International audienceThe crystal structure of CaZr(PO4)2 has been revised by ab initio Rietveld analysis of X-ray powder diffraction data. At room temperature, CaZr(PO4)2 crystallizes in the orthorhombic space group Pna21 (Z = 4). Differential thermal analysis suggests a reversible second order transition at 1000°C confirmed by high temperature XRD analysis that brings out the existence of a high temperature form, very similar to the room temperature one, but more symmetrical (Pnma, Z = 4). Analysis of the crystal parameters evolution during heating reveals that CaZr(PO4)2 exhibits a quite low thermal expansion coefficient of 6.11 10-6K-1. This value stems from a combination of several mechanisms, including Coulombic repulsion and bridging oxygen rocking motion

Synthèse par voie solide et frittage de céramiques à structure monazite (application au conditionnement des actinides mineurs)

Dans le cadre de la loi de 1991 concernant la gestion des déchets nucléaires en France, plusieurs études sont menées pour mettre au point des matrices cristallines spécifiques de conditionnement. La monazite, un orthophosphate de terre rare (TR3+) de formule TR3+PO4, est un minéral naturel contenant très souvent des quantités non négligeables de thorium et d uranium. Les qualités de résistance de ce matériau vis-à-vis des irradiations et de l altération aqueuse en font un candidat pour le conditionnement spécifique des actinides mineurs (Np, Am et Cm). Il est important désormais de vérifier la conservation de ces propriétés sur des matériaux de synthèse, ce qui sous-entend de parfaitement maîtriser toutes les étapes de l élaboration de pièces de monazite, de la synthèse des poudres jusqu à l élaboration par frittage de pastilles à microstructure contrôlée. C est dans ce cadre-là que s intègre le travail présenté dans ce document. La première partie de la thèse traite de l étude par voie solide de la synthèse de TR3+PO4 (TR = La3+ à Gd3+, Pu3+ et Am3+). Les réactions intervenant lors de la calcination des réactifs sont décrites dans le cas de monazites à un seul ou à plusieurs cations, permettant d établir un protocole de synthèse. L incorporation de cations tétravalents (Ce4+, U4+ et Pu4+) dans la structure monazite a également été étudiée. La deuxième partie traite de l élaboration de pièces de monazite à densité et microstructure contrôlées ainsi de leurs propriétés mécaniques et thermiques associées. L étude du broyage et du frittage, peu abordée jusqu alors, est présentée. Les résultats expérimentaux sont confrontés à des modèles théoriques afin d en déduire les mécanismes de densification et de grossissement de grains. Par la compréhension des différents phénomènes physico-chimiques se produisant lors des différentes étapes d élaboration (synthèse, broyage, frittage), ce travail a permis la mise au point d un protocole de fabrication de pièces de monazite TR3+PO4 à microstructure contrôlée. Les relations qui existent entre les différentes étapes du processus d élaboration du matériau ont pu être mises en évidence.In the framework of the French law of 1991 concerning the nuclear waste management, several studies are undertaken to develop specific crystalline conditioning matrices. Monazite, a rare earth (TR3+) orthophosphate with a general formula TR3+PO4, is a natural mineral containing significant amount of thorium and uranium. Monazite has been proposed as a host matrix for the minor actinides (Np, Am and Cm) specific conditioning, thanks to its high resistance to self irradiation and its low solubility. Its is now of prime importance to check the conservation of these properties on synthesized materials, which implies to master all the stages of the elaboration process, from the powder synthesis to the sintering of controlled microstructure pellets. This work can be divided into two main parts: The first part deals with the synthesis by high temperature solid state route of TR3+PO4 powders (with TR3+ = La3+ to Gd3+, Pu3+ and Am3+). The chemical reactions occurring during the firing of starting reagents are described in the case of monazite with only one or several cations. From these results, a protocol of synthesis is described. The incorporation of tetravalent cations (Ce4+, U4+, Pu4+) in the monazite structure was also studied. The second part of the present work deals with the elaboration of controlled density and microstructure monazite pellets and their related mechanical and thermal properties. The study of crushing and sintering is presented. For the first time, experimental results are confronted with theoretical models in order to deduce the densification and grain growth mechanisms. By the comprehension of the various physicochemical phenomena occurring during the various stages of the monazite pellets elaboration process (powder synthesis, crushing, sintering ), this work allowed the development of a protocol of elaboration of controlled microstructure monazite TR3+PO4 pellets. The determination of some mechanical and thermal properties could thus be carried out. The relations between the various stages of the material development process could be highlighted.LIMOGES-BU Sciences (870852109) / SudocSudocFranceF

Nanostructured Y2O3 ceramics elaborated by Spark Plasma Sintering of nanopowder synthesized by PEG assisted combustion method: The influence of precursor morphological characteristics

Dense yttria ceramics were prepared by Spark Plasma Sintering of a nanopowder synthesized using a PEG assisted combustion method. Densification occurs between 800 degrees C and 900 degrees C without any additive. This corresponds to one of the lowest sintering temperature found in the literature for Y2O3. Because of a significant release of organic species, the Y2O3 precursors obtained by this synthesis route contains macropores that have a negative impact on the final microstructure. We show that the emergence of these macropores can be minimized by decreasing the annealing temperature used for the precursor powder (in a temperature range of 300-650 degrees C) as opposed to the usual 800 degrees C. Finally, a precursor annealed at 650 degrees C allows us to obtain fully dense ceramics, with a very fine and homogeneous microstructure (and a grain size around 300 nm). Vickers microhardness and fracture toughness were measured and discussed in relation to the microstructure of the ceramics

The role of the milling step on the sintering behaviour of monazite powder LaPO4. Densification and resulting microstructure.

International audienceSintering behaviour of monazite powder was investigated as a function of the powder milling conditions. To this aim, two techniques were used: attrition-milling and mixer-milling. We show that it is of prime importance to control all the milling parameters in order to obtain controlled microstructure and to avoid abnormal grain growth or residual large porosity

Reducing ultra-low thermal expansion of β-Zr2O(PO4)2 by substitutions?

International audienc

The role of the milling step on the sintering behaviour of monazite powder LaPO4. Densification and resulting microstructure.

International audienceSintering behaviour of monazite powder was investigated as a function of the powder milling conditions. To this aim, two techniques were used: attrition-milling and mixer-milling. We show that it is of prime importance to control all the milling parameters in order to obtain controlled microstructure and to avoid abnormal grain growth or residual large porosity

Densification and grain growth during solid state sintering of LaPO4

International audienceThis work is devoted to the kinetic study of densification and grain growth of LaPO4 ceramics. By sintering at a temperature close to 1500 degrees C, densification rate can reach up to 98% of the theoretical density and grain growth can be controlled in the range 0.6-4 mu m. Isothermal shrinkage measurements carried out by dilatometry revealed that densification occurs by lattice diffusion from the grain boundary to the neck. The activation energy for densification (E-D) is evaluated as 480 +/- 4 kJ mol(-1). Grain growth is governed by lattice diffusion controlled pore drag and the activation energy (E-G) is found to be 603 +/- 2 kJ mol(-1). The pore mobility is so low that grain growth only occurs for almost fully dense materials

Structural and piezoelectric properties evolution induced by cobalt doping and cobalt/niobium co-doping in BaTiO3

International audienceLead–zirconate–titanate PbZr(1−x)TixO3 (PZT), materials for piezoelectric applications are the dominant piezoceramics since their properties are high. However PZTs need to be substituted by safer materials due to health and environmental problems of lead. In this work, the crystal structure modifications of BaTiO3 induced by cobalt doping and niobium/cobalt co-doping were investigated in the range 0.1–25 at%. Low Co and Co/Nb doping level significantly improve piezoelectric properties of BaTiO3. The piezoelectric constant, obtained for BaTiO3 doped with 0.075 at% of Cobalt (d33=200 pC/N), is twice of that measured for pure BaTiO3. At high doping concentration, the structure of BaTi1−x CoxO3 and BaTi1−x CoxNbxO3 change from tetragonal to hexagonal while BaTi1−x CoxNb2xO3 remains tetragonal and gradually turns into cubic. It suggests that the ionic charge defect is balanced by oxygen vacancies formation when Ti4+ are substituted by Co2+/Co3+