Gerard Picard

Physical principles and miniaturization of spark assisted chemical engraving (SACE)

Keywords: SACE ; glass micro-machining ; microengineering ; robotics ; [SACE] ; Micro-factory Reference LSRO-CONF-2006-028View record in Web of Science Record created on 2006-06-06, modified on 2017-05-10

Determination of the solubility product of plutonium sesquioxide in the NaCl+CaCl2 eutectic and calculation of a potential–pO2− diagram

Reactions between plutonium trichloride and oxide ions were studied in the fused NaCl+CaCl2 equimolar mixture at 823 K, by potentiometry with an yttria-stabilised zirconia membrane electrode. Titration curve demonstrated the existence of a precipitated plutonium oxide (Pu2O3) with dissociation constant 10−17.5 (molality scale). The combination of this data with the standard potentials allowed to set up the potential–pO2− diagram of plutonium which summarises the properties of plutonium species in the melt.

Molten salt route for ZnSe high-temperature electrosynthesis

Electrosynthesis appears as one of the best methods to prepare advanced materials under the form of coatings or thin layers. The advantages of such a process are economic (low investment for an easy composition control and large area deposition) and ecologic (no product loss). Yet, aqueous electrolysis exhibits two serious drawbacks: the low crystallinity of the electrodeposited material (due to the low working temperature) and the hydrogen discharge (preventing electrodeposition of very electronegative elements as Ga). In this context, using high-temperature molten salts as electrolytes is a promising way to elaborate materials with tailored structures and properties. This method was successfully applied to the electrodeposition of the semiconductor ZnSe which is a good candidate for window layers in thin film solar cells. Electrochemical investigations performed in molten CaCl2–NaCl mixture at 550°C are reported. ZnSe was electrodeposited on glass sheets covered with SnO2. Optimized melt composition and potential led to yellow, transparent and adherent thin films containing up to 90% well crystallized ZnSe with a ratio Zn/Se close to 1 and a 1-mm grain size. The energy band gap measured is 2.5 eV.

Thermodynamic Study of Corrosion of Iron in NaNO3 ‐ NaNO2 Mixtures

The influence of oxoacidity on the corrosion of iron in molten sodium nitrate‐nitrite mixtures has been demonstrated by means of potential‐pO2− diagram. Oxoacidobasic titrations of iron(III) have been carried out by means of potentiometric measurements with an yttria‐stabilized zirconia membrane electrode, indicator of pO2−. They have shown the formation of three different species of iron(III) and have allowed the determination of experimental oxoacidity constants in the temperature range from 693 to 773 K. These results and the thermochemical data of the literature have been used to establish the potential‐pO2− diagram relative to iron. This diagram permits one to forecast that three different compounds may be formed, according to the pO2−, when iron is in contact with the salt: or in strongly basic media, or in moderately basic media, in acidic media. A good correlation exists between our predictions and the corrosion tests reported in the literature.

Density-functional-based molecular-dynamics simulations of molten salts

The physicochemical properties of two molten salts, namely, KCl and NaCl, have been studied with a molecular-dynamics approach using a density-functional-based tight-binding (DFTB) model. The obtained results have been compared with a number of previously reported simulations, carried out on smaller systems and using classical force-field techniques. A good agreement has been found for both structural parameters and macroscopic properties, such as self-diffusion coefficients. Furthermore, our DFTB results are very close to the available experimental data. From a more general point of view, our results demonstrate the applicability of DFTB as an efficient tool in the modeling of melts. At the same time, the quality of the obtained results supports the use of this as a reliable alternative to the more expensive ab initio dynamics approaches, if accurate parameters are provided.

Investigation of the corrosion and passivation processes of magnesium in the molten 2HF-KF mixture by AC impedance spectroscopy

Metallic magnesium is chemically corroded by the 2HF-KF mixture at 80{degrees}C, with the formation of a passivating microporous magnesium fluoride layer as revealed by electron scanning micrograph and cyclic voltammetry. A more compact layer can be obtained electrochemically. This paper reports on the corrosion mechanism studied by ac impedance measurements. The reaction pathway involves oxidation, desorption, diffusion, and presipitation steps. Kinetic parameters (charge and mass transfer) have been determined by a computerized simulation method.

Reactions of Formation and Stability of Iron (II) and (III) Oxides in LiCl ‐ KCl Eutectic Melt at 470°C

The study of the different oxides of iron (II) and iron (III) was carried out potentiometrically in LiCl-KCl eutectic at 470/sup 0/C by means of an yttria-stabilized zirconia electrode indicator of the oxide ion activity and an iron electrode indicator of the ferrous ion activity. These measurements were complemented by the determination of cyclic voltammograms and by x-ray diffraction and infrared spectrophotometrY analysis of certain compounds formed. The main results obtained are the following: strong oxidizing power (oxidation of Cl-/sup 1/ ions into Cl/sub 2/) and high oxoacidity (great stability of ferric oxide) of Fe/sup 3/ ions; possible redissolution of Fe/sub 2/O/sup 3/ in the oxide ion rich media by the formation of ferrate (III) ion; FeO/sub 2/-*Fe/sup 2 +/ ions react with O/sup 2/- ions to to form ferrous oxide, which is probably stabilized in the form of a solid solution FeO-LiFeO/sub 2/ of composition Fe/sub 1-y/Li/sub y/O. Magnetic oxide, Fe/sub 3/O/sub 4/, is stable and can be obtained by the action of oxide ions on a mixture of iron (II) and iron (III). This work is pertinent to the construction of rechargeable and high performance batteries. 34 refs.

A theoretical investigation of gadolinium (III) solvation in molten salts

The solvation of lanthanides [here Gd(III)] in molten LiCl and KCl has been studied using a classical solvation approach, based on clusters of increasing size. In particular, density-functional calculations have been carried out on charged and neutral clusters, containing up to 35 chlorine halide molecules. A number of properties have been then evaluated and analyzed, including structural, vibrational, and thermochemical data. Special attention has also been devoted to the analysis of the local structure of the solvent surrounding the Gd3+ cation, a problem deeply investigated by experimentalists. Our results show that the charged clusters are not suitable to model the first solvation shell in such species, since their structures are strongly affected by the large electrostatic contribution. In contrast, more reliable simulations are obtained using the neutral clusters. In the latter, the coordination of Gd(III) in molten salts is computed to be 8 or 6, according to the salt LiCl or KCl. Furthermore, a good agreement is found with the experimental structural data and Raman spectra. Finally, preliminary results of potential interest for the estimation of solvation thermodynamics, a key parameter for exploiting molten salt chemistry, are reported for neutral clusters.

Dismutation of divalent americium induced by the addition of fluoride anion to a LiCl–KCl eutectic at 743 K

The three oxidation states of americium (III), (II) and (0) are stable in a molten LiCl–KCl eutectic at 743 K. Addition of fluoride anion to the melt led to the disproportionation of Am2+ into Am3+ and Am. The effect of fluoride additions was monitored with potentiometric and cyclic voltammetry methods and the relative cumulative formation constants of the Am(III) fluoro-complexes determined.

Electrochemical investigations in molten potassium disulphate at 430°C: I. Determination of the acidity range

Abstract Molten potassium disulphate is a solvent in which acidity, defined as pSO4=−log m(SO42−) can be modified. A method for measuring this acidity was developed from potentiometric and voltammetric studies of the solvent reduction and allowed the determination (at 430°C) of the solvent autodissociation constant (Ki,p/atm mol kg−1=10−3.2). The solubilities of potassium sulphate, sulphur dioxide and sulphur trioxide were also determined (resp. 0.25, 10−3 and 9×10−2 mol kg−1).