François Cansell

Supercritical fluid processing: a new route for materials synthesis

Supercritical fluids exhibit a range of unusual properties that can be exploited for new reactions which are qualitatively different from those involving classical solid state chemistry. After giving a brief introduction to these fluids we describe their use in inorganic chemistry and related fields. We then present two examples concerning different areas of solid state chemistry: (i) the formation of novel inorganic nanoparticles; (ii) the preparation of new open-structure oxy(hydroxy)fluorides, thus showing the advantages of this supercritical fluid processing that can be seen as an alternative method to regular solution chemistry or solid-gas reactions.

Single-step synthesis of well-crystallized and pure barium titanate nanoparticles in supercritical fluids

Single-step synthesis of ultra-fine barium titanate powder with a crystallinity as high as 90% and without barium carbonate contamination has been successfully performed under supercritical conditions using a continuous-flow reactor in the temperature range 150–380 °C at 16 MPa. To synthesize this bimetallic oxide, alkoxides, ethanol and water were used. The influence of the synthesis parameters on the BaTiO3 powder characteristics was investigated. The results show that the water to alkoxide precursor ratio, the reactor temperature and the Ba:Ti molar ratio of alkoxide precursor play a major role in the crystallization of pure and well-crystallized BaTiO3 nanoparticles. The continuous mode of operation without post-treatments for powder washing, drying or crystallization increase the industrial interest.

Attempt to explain the changes in solvation of polystyrene in supercritical CO2/ethanol mixtures using infrared and Raman spectroscopy

Abstract The polystyrene//CO 2 /ethanol system has been investigated in the supercritical domain using infrared absorption (FTIR) and Raman scattering spectroscopies. The pressure and temperature were fixed in the range 20–35 MPa at 423 K for CO 2 /ethanol mixtures with proportion in weight from (70/30) to (58/42). These conditions are those used in our previous study concerning the fractionation of this macromolecule. A surprising finding reported before was that the CO 2 /ethanol mixture made of two non-solvents of PS at ambient conditions is able to dissolve PS chains under high temperature and pressure. In the present article, this question will be addressed at a microscopic level from a discussion of the intermolecular interactions between the polymer and the medium and between the CO 2 and ethanol components provided by the analysis of the spectra.

Organic low molecular weight aerogel formedin supercritical fluids

Aerogels of 2,3-didecyloxyanthracene (DDOA) are prepared nfrom supercritical fluids in two different ways. In the first, the gel is nformed by mixing DDOA and ethanol, and dried by flushing with supercritical nCO2. This process produces a white, light and friable aerogel (density nof 6×10−3xa0gxa0cm−3), constituted nof fibers with diameters ranging from 100 to 200xa0nm. In the second, nDDOA is solubilized in supercritical CO2 to give a gel which is ndried under the same experimental conditions. The resulting very low density naerogel (2×10−3xa0gxa0cm−3), nhas a specific surface equal to 10xa0m2xa0g−1, nand constitutes a network of long fibers with diameters varying from 100 to n200xa0nm.

Continuous supercritical synthesis and dielectric behaviour of the whole BST solid solution

In this study we show that pure and well crystallized nanoparticles of Ba(x)Sr(1-x)TiO(3) (BST) can be synthesized over the entire range of composition through the hydrolysis and further crystallization of alkoxide precursors under supercritical conditions. To our knowledge, this is the first time that the whole ferroelectric solid solution has been produced in a continuous way, using the same experimental conditions. The composition of the powder can be easily controlled by adjusting the feed solution composition. The powders consist of soft-aggregated monocrystalline nanoparticles with an average particle size ranging from approximately 20 to 40 nm. Ferroelectric ceramics with accurately adjustable Curie temperature (100-390 K) can thus be obtained by sintering.

The ferroelectric transition temperature as an intrinsic probe for sintered nanocrystalline BaTiO3 synthesized under supercritical conditions

We have investigated the sintering behaviour of cubic BaTiO3 nanoparticles obtained under supercritical conditions. The ferroelectric properties of samples sintered between 950 and 1300 °C were measured from room temperature up to 200 °C and in the frequency range 100 Hz–10 kHz. The influence of the initial powder characteristics on the microstructure and dielectric properties of ceramics have been investigated. The ferroelectric transition temperature was used as an intrinsic probe for the ceramic characterization and was then found to be dependent on the Ba:Ti ratio of the initial powder. Understanding the role of the key parameters at different scales (bulk and nanoscale composition, tetragonal distortion, microstructure) on the permittivity, losses and transition temperature would lead to a possible modulation of ferroelectric properties.

Experimental set-up for electrochemical measurements in hydrothermal sub- and supercritical oxidation: polarization curves, determination of corrosion rates and evaluation of the degradability of reactors during hydrothermal treatments of aqueous wastes

Abstract Electrochemical data concerning metallic materials submitted to aqueous media at high pressures and high temperatures, which are close to the critical point, are scarce. This is essentially due to the difficulties encountered during the setting-up of an appropriate system of analysis. This paper, in the peculiar context of organic wastes supercritical water oxidation, proposes, for both sub and supercritical conditions, some technical solutions to define a cell able to give access to quantitative electrochemical information. For special media (pH≥1.7 and chloride ions content≈1 g/l), using a specific device consisted of a TA6V electrochemical cell, ensuring lasting and chemically inert electrical isolation and water tightness, along with a hydrogen dynamic electrode (HDE), it is possible to obtain reliable ‘current–potential’ curves allowing a quantitative exploitation, particularly for corrosion rates determination.

Anodic Behavior Modeling of the C-276 Alloy in Chlorinated Media Simulating the Hydrothermal Conditions of Organic Waste Treatment

Experimental current-potential curves corrected from the contribution due to the medium oxidation are used to propose a dissolution-passivation model of the nickel base alloy C-276. This model is controlled by the major presence of nickel and shows a reaction chain mechanism with a bifurcation and two competitive determinant branches for the setup of the protection of the alloy. Considering a Langmuir-type adsorption, we formulate a current-potential law which leads, via a mathematical optimization calculation using Nelders simplex, to the eight kinetics parameters involved in the five considered elementary steps. The respective influence of the different branches can be determined. It is used in association with the scanning electron microscopy profiles obtained at fixed potentials, to evaluate the limited reliability of the C-276 alloy as a plant reactor material for waste treatment under hydrothermal conditions, either sub- or supercritical oxidation. This evaluation relies on the comparison with the titanium T60 whose anodic behavior and performances have already been published. The determination of the kinetic parameters also underlines the role played by the oxygen source (H 2 O 2 ) on the mechanism, a role partly hidden by strict observation of the current-potential experimental curves.