Daniel Caurant

Synthesis by a soft chemistry route and characterization of LiNixCo1−xO2 (0 ≤ x ≤ 1) cathode materials

LiNixCo1−xO2 cathode materials have been prepared using a soft chemistry route for 0 ≤ x ≤ 1. Co-precipitation of hydroxides is followed by drying and heating at temperatures between 700 and 800 °C for a short time (2 to 5 h) under oxygen flow. Due to the high reactivity of the co-precipitates, preparation time is shorter than with powders mixing methods generally used. A DTA-TGA study has been performed for each co-precipitate. LiNixCo1−xO2 compounds show good crystallinity, but lithium deficiency appears for nickel-rich samples. LiNi0.8Co0.2O2 shows the best electrochemical charge-discharge behavior among all the nickel-cobalt compounds studied. This has been attributed to a low lithium deficiency and to a high nickel content.

Crystallization study of (TiO2, ZrO2)-rich SiO2-Al2O3-CaO glasses Part I Preparation and characterization of zirconolite-based glass-ceramics

Nuclear power reactors generate long-lived radionuclides such as minor actinides (Np, Am, Cm) which are mainly responsible for the long term radiotoxicity of high level nuclear wastes obtained after reprocessing of nuclear spent fuel. Specific highly durable matrices such as glass-ceramics appear as good candidates for the immobilization of minor actinides. This work concerns the synthesis and the characterization of zirconolite (CaZrTi2O7) based glass-ceramics prepared by controlled devitrification of (TiO2, ZrO2)-rich SiO2-Al2O3-CaO parent glasses for which neodymium was selected to simulate the radioactive trivalent minor actinides. The present study reports the effect of increasing TiO2, ZrO2 and CaO amounts in glass composition on the structure and the composition of the zirconolite crystals (formed as the only crystalline phase in the bulk of the glass), on their nucleation rate I(Z) and on the volume proportion of crystalline phase V of the glass-ceramics. It appears that I(Z) and V strongly increase when the parent glass composition changes. Neodymium electron spin resonance (ESR) shows that the total amount of Nd3+ ions incorporated in the zirconolite phase increases with TiO2, ZrO2 and CaO amounts in parent glass composition.

Crystallization study of (TiO2, ZrO2)-rich SiO2-Al2O3-CaO glasses Part II Surface and internal crystallization processes investigated by differential thermal analysis (DTA)

Controlled crystallization of (TiO2-ZrO2)-rich calcium aluminosilicate glasses led to zirconolite in the bulk, and titanite and anorthite on the surface. Such glass-ceramics can be envisaged for minor actinides immobilization. In this study, the crystallization of three glass compositions with increasing TiO2, ZrO2 and CaO amounts was followed by differential thermal analysis (DTA). The effect of glass particle size and of heating rate on DTA curves was studied in order to investigate nucleation mechanisms and to extract the corresponding crystal growth activation energies Ec for the different crystalline phases. Exothermic effects associated with the crystallization of a phase having a defect-fluorite structure in the bulk and its consecutive transformation into zirconolite were only detected for the highly TiO2, ZrO2 and CaO enriched glasses due to their higher crystallization rate. Using an Avrami constant n = 3 and a dimensionality of crystal growth m = 3, the activation energy of defect-fluorite crystal growth was found to be Ec = 440 kJ · mol−1 (modified Kissinger method). Titanite and anorthite grow only from glass surface with activation energies of respectively 493 and 405 kJ · mol−1 (n = m = 1, Kissinger method). DTA study of melt crystallization during cooling showed that baddeleyite (ZrO2) crystals firstly crystallize but become unstable versus zirconolite for higher undercooling.

Contribution of first‐principles calculations to multinuclear NMR analysis of borosilicate glasses

Boron‐11 and silicon‐29 NMR spectra of xSiO2 (1 − x)B2O3 glasses (x = 0.40, 0.80 and 0.83) have been calculated using a combination of molecular dynamics (MD) simulations with density functional theory (DFT) calculations of NMR parameters. Structure models of 200 atoms have been generated using classical force fields and subsequently relaxed at the PBE‐GGAlevel of DFT theory. The gauge including projector augmented wave (GIPAW) method is then employed for computing the shielding and electric field gradient tensors for each silicon and boron atom. Silicon‐29 MAS and boron‐11 MQMAS NMR spectra of two glasses (x = 0.40 and 0.80) have been acquired and theoretical spectra are found to well agree with the experimental data. For boron‐11, the NMR parameter distributions have been analysed using a Kernel density estimation (KDE) approach which is shown to highlight its main features. Accordingly, a new analytical model that incorporates the observed correlations between the NMR parameters is introduced. It significantly improves the fit of the 11B MQMAS spectra and yields, therefore, more reliable NMR parameter distributions. A new analytical model for a quantitative description of the dependence of the silicon‐29 and boron‐11 isotropic chemical shift upon the bond angles is proposed, which incorporates possibly the effect of SiO2B2O3 intermixing. Combining all the above procedures, we show how distributions of SiOT and BOT (TSi, B) bond angles can be estimated from the distribution of isotropic chemical shift of silicon‐29 and boron‐11, respectively. Copyright

Synthesis, structural characterization and magnetic properties of quasistoichiometric LiNiO2

Abstract A study is made on the conditions and methods of preparation of LiNiO 2 and their influence on lithium deficiency. A soft chemistry route by hydroxide precipitation and a classical ceramic method were used. In order to compensate the lithium deficiency in LiNiO 2 , we used a small excess of lithium precursor (never more than 10 mol%). The different samples were characterized by X-ray powder diffraction, followed by a Rietveld refinement. Optimized synthesis conditions lead to Li 1− x Ni 1+ x O 2 quasistoichiometric materials with a lithium molar deficiency, x , less than 1%, confirmed by Rietveld refinement. Electron spin resonance (ESR) and magnetic measurements are in agreement with these very low x values. A spin-glasslike behavior is observed for the most stoichiometric sample. The susceptibility shows a maximum at 7.5 K on zero-field cooling and is almost constant below 7.5 K on field cooling. No anomaly of the susceptibility is observed at high temperature, indicating a good homogeneity of all samples. A strong influence of x in a narrow composition range (0.004–0.062) on the electrochemical cycling behavior of the corresponding samples is observed.

Electrochemical investigation of the Li insertion–extraction reaction as a function of lithium deficiency in Li1−xNi1+xO2

Abstract The electrochemical characteristics of Li1−xNi1+xO2 samples have been extensively investigated as a function of the lithium deficiency x in the narrow composition range 0.008–0.065. In particular, the electrochemical and structural behaviours of the quasi-stoichiometric compound (x=0.008) have been proved to be strongly different from those exhibited by a sample with x=0.025. The faster kinetics for Li transport combined with structural changes which are minimized in the quasi-ideal compound explain the lack of notable polarization voltage and the good cycle life of the latter cathode material with a stable capacity of 140 A h/kg over 40 cycles at C/10 rate. In addition, interesting and useful correlations (i) between the cathode impedance and the lithium deficiency and (ii) between the evolution of the cathode impedance and the cycling performance have been established.

Development of Zirconolite-based Glass-Ceramics for the Conditioning of Actinides

Zirconolite (CaZrTi 2 O 7 ) based glass-ceramics designed for the specific immobilization of plutonium wastes or minor actinides (Np, Am, Cm) from high level radioactive wastes were investigated. To reach an efficient double containment, actinides must be preferentially located in the crystalline phase, which is homogeneously dispersed in a calcium aluminosilicate residual glass. Several heat treatments (between 950° and 1350°C) of a parent glass belonging to the SiO 2 -Al 2 O 3 -CaO system and containing TiO 2 and ZrO 2 were performed to prepare glass-ceramics. Trivalent minor actinides were simulated introducing Nd 2 O 3 in the glass composition. Electron microscopy, X-ray diffraction (XRD) and thermal analysis have shown that devitrification processes in the bulk and on glass surface are different. They lead to the crystallization of zirconolite in the bulk and to a mixture of titanite (CaTiSiO 5 ) and anorthite (CaAl 2 Si 2 O 8 ) near the surface. For heat treatment temperatures greater than or equal to 1250°C, baddeleyite (m-ZrO 2 ) crystals form at the expense of zirconolite in the bulk of glass-ceramics. XRD indicates that the order in zirconolite Ca/Zr planes increases with heating temperature. At the same time, extended defects density decreases.

Paramagnetic defects induced by electron irradiation in barium hollandite ceramics for caesium storage.

We have studied by electron paramagnetic resonance the mechanism of defect production by electron irradiation in barium hollandite, a material used for immobilization of radioactive caesium. The irradiation conditions were the closest possible to those occurring in Cs storage waste forms. Three paramagnetic defects were observed, independently of the irradiation conditions. A hole centre (H centre) is attributed to a superoxide ion O(2)(-) originating from hole trapping by interstitial oxygen produced by electron irradiation. An electron centre (E(1) centre) is attributed to a Ti(3+) ion adjacent to the resulting oxygen vacancy. Another electron centre (E(2) centre) is attributed to a Ti(3+) ion in a cation site adjacent to an extra Ba(2+) ion in a neighbouring tunnel, originating from barium displacement by elastic collisions. Comparison of the effects of external irradiations by electrons with the β-decay of Cs in storage waste forms is discussed. It is concluded that the latter would be dominated by E(1) and H centres rather than E(2) centres.

Effect of MoO3, Nd2O3, and RuO2 on the crystallization of soda–lime aluminoborosilicate glasses

The effect of adding Nd2O3, MoO3, and RuO2 separately or simultaneously on the crystallization of a soda–lime aluminoborosilicate glass during cooling from the melt or glass heating was studied by DTA, XRD (at room and high temperature), SEM, Raman, and optical absorption. Nd2O3 addition strongly reduces liquid–liquid phase separation and crystallization of calcium and sodium molybdates (CaMoO4 (powellite) and Na2MoO4) in Mo-rich compositions as long as Nd3+ ions remain solubilized in the glassy network. This suggests that (MoO4)2− entities and Nd3+ ions are close to each other in the glass structure (Nd3+ ions would prevent the clustering of molybdate entities). The effect of MoO3 addition in Nd-rich compositions is more complex since an increase of the solubility of Nd2O3 is observed, whereas the nucleation rate of an Nd-rich silicate apatite (Ca2Nd8(SiO4)6O2) in the bulk of the glass increases as soon as molybdates crystallized. The addition of RuO2 has a nucleating effect on apatite crystallization in the bulk but not on molybdates crystallization.

Incommensurate modulations in a hollandite phase Bax(Al, Fe)2xTi8–2xO16 intended for the storage of radioactive wastes: a (3+1) dimension structure determination

Intensive researches on specific treatments of radioactive cesium, occurring in high level nuclear wastes, are in progress in France. Nowadays, a cesium immobilization in host matrices with a high chemical durability seems to be the favourite option. In that perspective, titanium based upon hollandite is a good candidate as a host matrix because of its high cesium incorporation ability, its excellent chemical stability, and its capacity to support charge compensation during the Cs+ into Ba2+ transmutation process. In this study we report the preparation and crystal growth in a BaF2—B2O3 flux melt of the BaxCsyMzTi8–zO16 hollandite phase and its characterization by single crystal X-ray diffraction. An attention was focused on materials with M = Al or Al + Fe and with barium only since single crystals including cesium could not be obtained from the flux method. An incommensurate modulation of the Ba cation site occupancy, with I4/m(00γ)00 as a superspace group, was observed for all the single crystals under study.