Nicolas Penin

Dual Lithium Insertion and Conversion Mechanisms in a Titanium-Based Mixed-Anion Nanocomposite

The electrochemical reaction of lithium with a vacancy-containing titanium hydroxyfluoride was studied. On the basis of pair distribution function analysis, NMR, and X-ray photoelectron spectroscopy, we propose that the material undergoes partitioning upon initial discharge to form a nanostructured composite containing crystalline Li(x)TiO(2), surrounded by a Ti(0) and LiF layer. The Ti(0) is reoxidized upon reversible charging to an amorphous TiF(3) phase via a conversion reaction. The crystalline Li(x)TiO(2) is involved in an insertion reaction. The resulting composite electrode, Ti(0)-LiF/Li(x)TiO(2) ⇔ TiF(3)/ Li(y)TiO(2), allows reaction of more than one Li per Ti, providing a route to higher capacities while improving the energy efficiency compared to pure conversion chemistries.

Evidence of non-stoichiometry effects in nanometric manganite perovskites: influence on the magnetic ordering temperature

This work tends to evidence that the significant chemical modifications observed in nanometric manganites are not concentrated at the surface of the particles and play a key role on their magnetic properties, especially for the lowest strontium-doping. La1−xSrxMnO3 solid solution with a 27 nm-average crystallite size was prepared via the Glycine Nitrate Process. The evolutions versus x of the Curie temperature (TC) and saturation magnetization of the nanometric solid solution were interpreted taking into account the Goldschmidt tolerance factor, crystallite size, amount of vacancies and the mixed valency of manganese ions. Two distinct populations were distinguished: (i) for x lower than 0.25, the increase of TC with x could be related to the decreasing amount of cationic vacancies that accommodate the substitution at the A-site of the perovskite framework, simultaneously keeping the content of Mn4+% constant; (ii) for x higher than 0.25, the increase of x led to a decrease of the structural distortion, hence favouring orbital overlap and inducing a TC increase. However, when the amount of Mn4+ exceeded the value of ≈35%, competitive superexchange antiferromagnetic interactions were promoted. As a result, the observed behaviour was a compromise between these two competitive tendencies and led to a quasi-constant TC. For x higher than 0.4, the antiferromagnetic interactions became more important, which induced a TC decrease.

Near the Ferric Pseudobrookite Composition (Fe2TiO5).

Because of a very low thermodynamic stability, obtaining a pure monophasic compound of ferric pseudobrookite is quite difficult to achieve. Indeed, the low reticular energy of this phase leads easily to its decomposition and the occurrence of the secondary phases: hematite (Fe2O3) and/or rutile (TiO2). Samples with global composition Fe2-xTi1+xO5 (x = 0, 0.05, and 0.10) have been synthesized by the Pechini route and, thereafter, thermally treated at different temperatures. The concentrations of Fe2O3 and TiO2 secondary phases were accurately determined and correlated with the target compositions and the synthesis parameters, especially the thermal treatment temperature. As revealed by Mössbauer spectroscopy, all iron ions are at the III+ oxidation state. Thus, the formation of hematite or rutile as a secondary phase may be related to the occurrence of cationic vacancies within the pseudobrookite structure, with the amount of vacancies depending on the annealing temperature. In light of the presented results, it appears unreasonable to propose a fixed binary phase diagram for such a complex system. Furthermore, the occurrence of cationic vacancies induces a coloration change (darkening), preventing any industrial use of this reddish-brown pseudobrookite as a ceramic pigment.

Two-Step Synthesis of VO 2 (M) with TunedCrystallinity

Highly crystallized monoclinic vanadium dioxide, VO2 (M), is successfully synthesized by a two-step thermal treatment: thermolysis of vanadyl ethylene glycolate (VEG) and postannealing of the poorly crystallized VO2 powder. In the first thermolysis step, the decomposition of VEG at 300 °C is investigated by X-ray diffraction and scanning electron microscopy (SEM). A poorly crystallized VO2 powder is obtained at a strict time of 3 min, and it is found that the residual carbon content in the powder played a critical role in the post crystallization of VO2 (M). After postannealing at 500 and 700 °C in an oxygen-free atmosphere, VO2 particles of various morphologies, of which the crystallite size increases with increasing temperature, are observed by SEM and transmission electron microscopy. The weight percent of crystalline VO2, calculated using the Fullprof program, increases from 44% to 79% and 100% after postannealing. The improved crystallinity leads to an improvement in metal-insulator transition behaviors demonstrated by sharper and more intense differential scanning calorimetry peaks. Moreover, V2O3 and V2O5 with novel and particular microstructures are also successfully prepared with a similar two-step method using postannealing treatment under reductive or oxidizing atmospheres, respectively.