Claude Estournès

Stabilisation of active nickel catalysts in partial oxidation of methane to synthesis gas by iron addition

Abstract Mixed LaNixFe(1−x)O3 perovskite oxides (0≤x≤1) have been prepared by a sol–gel related method, characterised by X-ray diffraction (XRD), specific surface area measurements, transmission electron microscopy (TEM) coupled to an energy dispersive X-ray spectrometer (EDS). These systems are the precursors of highly efficient catalysts in partial oxidation of methane to synthesis gas. Studies on the state of these systems after test show the stabilisation of active nickel by increasing the amount of iron. These systems permit to control the reversible migration of nickel from the structure to the surface. The best mixed perovskite for the partial oxidation of methane is LaNi0.3Fe0.7O3.

Sintering and densification of nanocrystalline ceramic oxide powders: a review

Abstract Observation of the unconventional properties and material behaviour expected in the nanometre grain size range necessitates the fabrication of fully dense bulk nanostructured ceramics. This is achieved by the application of ceramic nanoparticles and suitable densification conditions, both for the green and sintered compacts. Various sintering and densification strategies were adopted, including pressureless sintering, hot pressing, hot isostatic pressing, microwave sintering, sinter forging, and spark plasma sintering. The theoretical aspects and characteristics of these processing techniques, in conjunction with densification mechanisms in the nanocrystalline oxides, were discussed. Spherical nanoparticles with narrow size distribution are crucial to obtain homogeneous density and low pore-to-particle-size ratio in the green compacts, and to preserve the nanograin size at full densification. High applied pressure is beneficial via the densification mechanisms of nanoparticle rearrangement and sliding, plastic deformation, and pore shrinkage. Low temperature mass transport by surface diffusion during the spark plasma sintering of nanoparticles can lead to rapid densification kinetics with negligible grain growth.

Low-losses, highly tunable Ba0.6Sr0.4TiO3/MgO composite

Spark plasma sintering (SPS) is an efficient tool to obtain highly densified ferroelectric-dielectric ceramic composites with clean interfaces and tunable properties. Dielectric MgO and ferroelectric Ba0.6Sr0.4TiO3 (BST) were combined in two-dimensional multilayer and three-dimensional random powders design. Their unmodified BST Curie temperature proves the suppression of interdiffusion while dielectric losses are below 0.5% and the tunability is 40% at room temperature. The composites and pure BST with similar densities (>95%) were obtained, owing reliable comparison of their dielectric properties. Such SPS ceramics can be used as experimental input for simulation and are potential candidates for high frequency applications.

Influence of the preparation conditions on the synthesis of high surface area SiC for use as a heterogeneous catalyst support

The influence of different parameters (temperature, duration and SiO source) on the synthesis of silicon carbide SiC according to the gas-solid reaction between SiO vapors and activated charcoal was investigated. The material obtained retained the general shape of the activated charcoal, which is an advantage because of the difficulty in post shaping SiC, due to the high strength of the material. High temperature (>1250 °C) and long reaction duration led to a high C* → SiC conversion but with a relatively low surface area (20–25 m2 · g−1) due to sintering via the surface diffusion phenomenon. The combination of a lower reaction temperature (1200 °C), longer reaction duration (15 h) and high (Si + SiO2)/C* weight ratio allowed SiC to be obtained with a surface area of around 50 m2 · g−1, which can be used as a support material for heterogeneous catalysis.

Nickel nanoparticles in silica gel: Preparation and magnetic properties

Abstract The reduction of nickel in silica gels was measured as a function of nickel content within the temperature range 600–900°C. This leads to the formation of metallic nickel nanoparticles homogeneously dispersed in this amorphous matrix. Furthermore, a Transmission Electron Microscopy (TEM) study has revealed that the size of these particles is distributed following a log-normal law. Moreover, this study has shown that the particles size is nearly independent of the nickel content while its distribution becomes wider when it increases. Magnetic measurements have shown, whatever nickel concentration and treatment temperature are, a superparamagnetic behavior. Finally, magnetization simulations, as a function of applied magnetic field, have been performed considering Langevin function in order to calculate a theoretical particles-size distribution and to compare it with the experimental one.

NMR of microporous compounds: From in situ reactions to solid paving

Abstract Alumino- and gallo-phosphate materials present a large variety of structures. Among them, two new families the ULM and the MIL provide a large class of microporous compounds. A systematic study has been undertaken for understanding the main steps of solids formation. As these materials form under hydrothermal conditions, we developed in-situ NMR devices and methods for following pH and concentration of species during crystal formation. Coordination state of aluminum evolution during synthesis suggests a general mechanism for the structural building unit formation. This mechanism is analyzed by quantitative evolution of liquid and solid phases in situ and ex situ. The dynamics of the different stages is therefore followed and the main steps of phase dissolution and crystallization are sequenced. Nucleation, which is the most delicate step of evolution to characterize, is shown to witness unexpected chemical bond formation not seen in steps before. These chemical bonds formed during nucleation and crystal growth are demonstrated by comparing building agents in solution and crystal structure.

Synthesis of CoFe2O4 nanowire in carbon nanotubes. A new use of the confinement effect.

Cobalt ferrite nanowires with an average diameter of 50 nm and lengths up to several micrometers were synthesized inside carbon nanotubes under mild reaction conditions using the confinement effect provided by the carbon tubular template.

Microstructural investigation and magnetic properties of CoFe2O4 nanowires synthesized inside carbon nanotubes

Cobalt ferrite nanowires with an average diameter of 50 nm and lengths up to several micrometers were synthesized inside multi-walled carbon nanotubes under mild reaction conditions, i.e. 100°C and atmospheric pressure, using an aqueous nitrate precursor salt filling the tubes. The concept of a confinement effect inside carbon nanotubes has been advanced to explain the formation of CoFe2O4 under such mild reaction conditions. The formation of caps near the tube tips at the beginning of the nitrate decomposition meant that each nanotube was considered as a closed nanoreactor, in which the reaction conditions could be very different to the macroscopic conditions outside the tube. A post-synthesis treatment under inert atmosphere allowed the growth of CoFe2O4 particles, from a disordered hair-like dendritic structure at 100°C to highly crystallized domains at higher temperatures. A material with high coercivity at room temperature for small particles of about 25 nm in diameter was obtained by submitting the CoFe2O4 nanowires after calcination in air at 100°C to an argon treatment at 550°C for 2 h.

Controlling internal barrier in low loss BaTiO3 supercapacitors

Supercapacitor behavior has been reported in a number of oxides including reduced BaTiO3 ferroelectric ceramics. These so-called giant properties are however not easily controlled. We show here that the continuous coating of individual BaTiO3 grains by a silica shell in combination with spark plasma sintering is a way to process bulk composites having supercapacitor features with low dielectric losses and temperature stability. The silica shell acts both as an oxidation barrier during the processing and as a dielectric barrier in the final composite.

Preparation of transparent oxyapatite ceramics by combined use of freeze-drying and spark-plasma sintering

Lanthanum silicate oxyapatites, ion-conducting materials presenting a strong aversion against densification, have been obtained in the form of dense transparent ceramics, by combining the beneficial use of freeze-drying and spark plasma sintering methods.