0000000000133638
AUTHOR
D. Aymes
Continuous hydrothermal synthesis of nanometric BaZrO3 in supercritical water
Abstract Nanocrystalline barium zirconate (BaZrO 3 ) was synthesized using a hydrothermal synthesis process working in supercritical conditions and in a continuous way. By this method, we succeeded in the continuous and rapid production of nanopowders. As a preliminary work three barium precursors have been investigated: barium hydroxide (Ba(OH) 2 ), barium acetate (Ba(CH 3 COO) 2 ) and barium nitrate (Ba(NO 3 ) 2 ). Two of them (Ba(CH 3 COO) 2 and Ba(NO 3 ) 2 ) led to the pure perovskite phase. Then an experimental design has been conducted in order to determine the influence of the experimental parameters on the crystallinity and the grain size of the final product.
Spark plasma sintering of cobalt ferrite nanopowders prepared by coprecipitation and hydrothermal synthesis.
International audience; Cobalt ferrite exhibits a high coercivity at room temperature and a strong magnetic anisotropy compared to the other spinel ferrites and, consequently appears as an interesting material for permanent magnets and high-density recording. The magnetic properties depend also on the crystallite size. In order to keep the powder properties in a bulk material, dense nanostructured cobalt ferrite has to be sintered. A field activated sintering process like spark plasma sintering (SPS) may be promising for such challenge. The present paper deals with: (i) the preparation of cobalt ferrite by two methods: coprecipitation and hydrothermal synthesis in supercritical water; (ii) …
Far infrared absorption by acoustic phonons in titanium dioxide nanopowders
We report spectral features of far infrared electromagnetic radiation absorption in anatase TiO2 nanopowders which we attribute to absorption by acoustic phonon modes of nanoparticles. The frequency of peak excess absorption above the background level corresponds to the predicted frequency of the dipolar acoustic phonon from continuum elastic theory. The intensity of the absorption cannot be accounted for in a continuum elastic dielectric description of the nanoparticle material. Quantum mechanical scale dependent effects must be considered. The absorption cross section is estimated from a simple mechanical phenomenological model. The results are in plausible agreement with the absorption b…
Continuous hydrothermal synthesis of inorganic nanopowders in supercritical water: towards a better control of the process
International audience; A hydrothermal synthesis process working in supercritical conditions (T > 374 °C, P > 22 MPa) and in a continuous mode has been developed for inorganic nanopowder synthesis. This paper presents a review of the past 5 years of research conducted on this process. Numerous materials (oxides: ZrO2, TiO2, Fe2O3..., ferrites: Fe2CoO4..., or BaZrO3) were obtained with specific features. Some technical issues have been solved, that are presented here. Heat transfer was studied, leading to a more efficient design of the reactor. Future developments have been examined through process engineering, in which our group is engaged, especially through CFD modelling.
Experimental set up for determining the temperature-oxygen partial pressure conditions during synthesis of spinel oxide nanoparticles
Abstract Nanometric spinel oxide powders, Fe 3− x M x O 4 , where M is a transition element, have been synthetized by soft chemistry. This method generally leads to a non-stoichiometric phase, Fe 3− x M x O 4+δ where δ is the deviation from stoichiometry so that further annealing at low temperatures around 450°C and low oxygen partial pressure around 10 −25 Pa given by N 2 /H 2 /H 2 O gas mixtures is required: this enables a stoichiometric compound to be obtained and a nanometric size to be maintained. The complete set up consisting of a gas mixer, a thermogravimetric apparatus and a preparative furnace is described. Some results concerning the conditions of temperature and oxygen partial p…
Inelastic neutron scattering due to acoustic vibrations confined in nanoparticles: theory and experiment
The inelastic scattering of neutrons by nanoparticles due to acoustic vibrational modes (energy below 10 meV) confined in nanoparticles is calculated using the Zemach-Glauber formalism. Such vibrational modes are commonly observed by light scattering techniques (Brillouin or low-frequency Raman scattering). We also report high resolution inelastic neutron scattering measurements for anatase TiO2 nanoparticles in a loose powder. Factors enabling the observation of such vibrations are discussed. These include a narrow nanoparticle size distribution which minimizes inhomogeneous broadening of the spectrum and the presence of hydrogen atoms oscillating with the nanoparticle surfaces which enhan…
Hydrothermal synthesis of nanostructured inorganic powders by a continuous process under supercritical conditions
Abstract In this study, using a prototype of hydrothermal synthesis in subcritical and supercritical water working in a continuous way, nanometric ceramic precursors with perfectly defined composition are produced: spinel ferrites such as Fe2CoO4, TiO2 with anatase structure and also perovskite structures such as BaZrO3. The as-prepared powders are fully characterized by complementary experiments: X-ray diffraction, electron microscopies, EDX spectrometry, surface area measurement, etc. Thus, particles size, morphology, aggregation state, crystal structure, composition are investigated. Moreover, magnetic properties of the ferrites products are studied. The powders obtained are pure phases …
ChemInform Abstract: Continuous Hydrothermal Synthesis of Nanometric BaZrO3in Supercritical Water.
Abstract Nanocrystalline barium zirconate (BaZrO 3 ) was synthesized using a hydrothermal synthesis process working in supercritical conditions and in a continuous way. By this method, we succeeded in the continuous and rapid production of nanopowders. As a preliminary work three barium precursors have been investigated: barium hydroxide (Ba(OH) 2 ), barium acetate (Ba(CH 3 COO) 2 ) and barium nitrate (Ba(NO 3 ) 2 ). Two of them (Ba(CH 3 COO) 2 and Ba(NO 3 ) 2 ) led to the pure perovskite phase. Then an experimental design has been conducted in order to determine the influence of the experimental parameters on the crystallinity and the grain size of the final product.
Characteristics of LiFePO4 obtained through a one step continuous hydrothermal synthesis process working in supercritical water
International audience; The olivine-like material LiFePO4 was prepared via a continuous hydrothermal synthesis process working from subcritical to supercritical water conditions. The influence of some processing parameters–temperature and reaction time–was investigated in terms of material purity, grain size and morphology. Supercritical conditions were found to be attractive to synthesize in one step a well-crystallized material without impurities. The primary particles size was in the nanometric range. They showed a natural tendency to form micron size agglomerates, which were supposed to be the cause of the limited capacity, as demonstrated through a cross study using laser particle size…
Structural evolution of LiOH: evidence of a solid–solid transformation toward Li2O close to the melting temperature
Abstract The structural evolution of LiOH has been studied between 10 K and 1670 K using a combination of neutron and X-ray diffraction and calorimetric measurements. The room temperature tetragonal phase of LiOH has been observed down to 10 K. Above the room temperature a dehydration of solid LiOH into solid Li2O is observed at a temperature and speed which strongly changes with the thermal history and the partial pressure of water vapour. Depending on these conditions the transformation of LiOH in to Li2O before the fusion temperature can be complete, partial or suppressed. In this latter case, as previously reported in the literature, a first order structural phase transition of LiOH is …
A comparative study of methanol carbonation on unsupported SnO2 and ZrO2
International audience; The aim of this work was to explore the catalytic properties of SnO2 in the coupling of methanol with carbon dioxide to afford dimethyl carbonate. SnO2 nanopowders were produced by hydrolysis of tin tetra-tert-butoxide dissolved in n-butanol. The samples were much more active than a commercial one due to their higher surface areas. In addition, they exhibited excellent recyclability. However, comparison with ZrO2, prepared and tested under the same experimental conditions, showed that zirconia-based catalysts were more selective and are, among the heterogeneous catalysts already reported, still the more selective. SnO2 also catalyzed the formation of dimethyl ether l…
High magnification SEM observations for two types of granularity in a high burnup PWR fuel rim
Abstract Rim microstructure of an UO2 pellet irradiated up to 61 GWd/tU, observed with high magnification scanning electron microscopy, presented two types of subgrains: polyhedral and round subgrains. Round subgrains were also observed on the surface of fabrication pores along the pellet radius. These round-shaped subgrains seem to be associated to free surface rearrangement. This new type of round subgrains was proved to be independent of the rim effect. Observations of the fuel structure in the vicinity of rim area evidenced two types of defects specific to rim effect: planar defects and small intergranular pores. Hypothesis is made that the accumulation of planar defects could generate …
Synthèse de poudres nanométriques de titanate de strontium par émulsion stabilisée mécaniquement: maîtrise et prédiction de la taille des particules
Resume La synthese de poudres nanometriques de titanate de strontium a partir d'une emulsion eau dans alcool stabilisee par agitation mecanique a ete etudiee avec l'objectif de maitriser a la lois la taille et la stœchiometrie des poudres obtenues. La taille des particules a ete ajustee par l'intermediaire de deux parametres du milieu reactionnel: la fraction volumique de la phase aqueuse et la concentration en precurseurs. Une relation entre la taille des gouttelettes de l'emulsion et la taille des particules des poudres de SrTiO3 obtenues apres precipitation a ainsi ete mise en evidence. Elle s'explique a partir d'un modele qui implique, entre autres hypotheses, que la vitesse de germinat…