Marcelle Gaune-Escard

Thermal and conductometric studies of NdBr3 and NdBr3-LiBr binary system

The heat capacity of solid NdBr3 was measured by Differential Scanning Calorimetry in the temperature range from 300 K up to the melting temperature. The heat capacity of liquid NdBr3 was also determined. These results were least-squares fitted to a temperature polynome. The melting enthalpy of NdBr3 was measured separately. DSC was used also to study phase equilibrium in the NdBr3-LiBr system. The results obtained provided a basis for constructing the phase diagram of the system under investigation. It represents a typical example of simple eutectic system. The eutectic composition, x(NdBr3)=0.278, was obtained from the Tamman construction. This eutectic mixture melts at 678 K. The electrical conductivity of NdBr3-LiBr liquid mixtures and of pure components was measured down to temperatures below solidification. Reflectance spectra of the pure components and their solid mixtures (after homogenisation in the liquid state) with different composition were recorded in order to confirm the reliability of the constructed phase diagram.

Phase diagram and electrical conductivity of TbBr3-NaBr binary system

Several experimental techniques were used to characterise the physicochemical properties of the TbBr3-NaBr system. The phase diagram determined by DSC, exhibits an eutectic and a Na3TbBr6 stoichiometric compound that decomposes peritectically (759 K) shortly after a solid-solid phase transition (745 K). The eutectic composition, x(TbBr3)=39.5 mol%, was obtained from the Tamman method. This mixture melts at 699 K. With the corresponding enthalpy of about 16.1 kJ mol-1. Diffuse reflectance spectra of the pure components and their solid mixtures (after homogenisation in the liquid state) confirmed the existence of new phase exhibiting its own spectral characteristics, which may be possibly related to the formation of Na3TbBr6 in this system. Additionally, the electrical conductivity of TbBr3-NaBr liquid mixtures was measured down to temperatures below solidification over the whole composition range.

Electrical conductivity of molten binary NdBr3 - alkali bromide mixtures

Electrical conductivity of liquid binary NdBr3 - alkali metal bromide mixtures was measured as a function of temperature over the whole composition range. Prior to these measurements, NdBr3 and alkali bromides were reinvestigated: a new assessment of literature data was made because of the discrepancy with reference values on NdBr3, LiBr and CsBr. The classical Arrhenius equation describes well our electrical conductivity data for mixtures. These results are discussed in terms of complex formation in the melts.

Electrochemical and Thermodynamic Properties of EuCl3 and EuCl2 in an Equimolar NaCl-KCl Melt

Abstract The electroreduction of EuCl3 in an equimolar NaCl-KCl mixture was studied at 973 -1123 K by different electrochemical methods. This reduction of Eu(III) in NaCl-KCl melt occurs via two successive reversible steps involving transfer of one and two electrons. The diffusion coefficients of Eu(III) and Eu(II) were measured by linear sweep voltammetry and chronopotentiometry. The values found by these methods are in a good agreement. The standard rate constants for the redox reaction Eu(III) + e~ Eu(II) were calculated from cyclic voltammetry data. The sluggish kinetics of this reaction is discussed in terms of substantial rearrangement of the europium coordination sphere. Special attention was paid to the choice of working electrodes for the study of electrode reactions. The formal standard potentials E*E u (^ /Ba, E* eU (hr/eu and the formal redox potentials ^*Eu(ni)/Eu(ii) were determined from open-circuit potentiometry and linear sweep voltammetry data. The free Gibbs energy changes for the reaction EuCl3(so1) EuCl2(SOi.) + 1/2 Cl2(g.) and the equilibrium constants of the metal-salt reaction 2Eu(III) + Eu 3 Eu(II) were calculated. The thermodynamics of the formation of dilute solutions of europium di-and trichloride in an equimolar NaCl-KCl melt were determined. It was shown that electrochemical transient techniques give the possibility of the determination of the relative partial molar mixing enthalpy of europium trichloride and dichloride in NaCl-KCl melt.

CaCl2+ KCl + NaCl molten-salt mixtures. Experimental and estimated enthalpies of mixing

The CaCl2+ KCl + NaCl molten-salt mixture is a candidate system for thermal-energy storage. Estimates of the amount of energy which can be stored by such a system requires knowledge of the enthalpies of formation of the liquid mixtures. We have therefore performed direct measurements of the enthalpy of mixing at 1083 K for the three limiting binary systems and the ternary mixture. However, estimates of the ternary enthalpy of mixing were made from those referring to the binary limiting systems using either Kolhers equation or the results derived from the surrounded-ion model, which was extended in the present work to ternary asymmetrical systems of the kind AX2+ BX + CX.

Electrical Conductivity of Melts Containing Rare-Earth Halides. I. MCl-NdCl3 (M = Li, Na, K, Rb, Cs)

The electrical conductivity of molten MCl-NdCl3 (M = Li, Na, Rb and Cs) has been measured from the liquidus temperature up to ~ 1180 K. The measurements were performed in usual U-shaped capillary quartz cells with platinum electrodes. The molar conductivity (Λ) has been computed by using literature data on the densities of the binary systems. In all cases, when the temperature range exceeds about 100 K, the plot lnΛ vs. 1/T is not a straight line. The activation energy of the conductivity does not remain constant but reduces with increasing temperature. In the specific and molar conductivity isotherms strong deviations from additivity are noted. The results are discussed in terms of octahedral local coordination of Nd3+ over the entire concentration range.

Unravelling the Internal Complexities of Molten Salts

Abstract Much experimental and theoretical effort has gone into revealing the internal complexities of molten salts for the past two decades. In this paper we shall show how neutron diffraction and computer simulation techniques have helped in gaining a better understanding of these systems at the microscopic level. Firstly, a short review on the structure of molten halide systems as revealed by these techniques will be presented. Complementarity of using X-rays with neutrons and, some recent results on the structure of molten DyCl3 obtained by combining neutron and X-ray diffraction with molecular dynamic simulations will be discussed. Neutron diffraction isotopic substitution techniques have played an important role in elucidating the interatomic structure of a diversity of molten salts. Pair distribution functions (PDFs), determined for a number of 1:1 and 2 :1 halide melts, provided theorists with a critical test of their model potentials. It is now clear that for 1:1 molten systems theoretical models based on Fumi-Tosi potentials can adequately describe many of the structural features. Nevertheless, the chal-lenge is two fold: (i) to determine real interatomic potentials for 2:1 and 3 :1 molten systems capable of reproducing not only the microscopic structural details obtained at the partial PDF, gαβ (r) level, but also their macroscopic behaviour and, (ii) to characterise the structure in binary molten salt mixtures.

Molybdenum Electrometallurgical Processes in Ionic Melts

Data concerning electrochemistry of molybdenum in melts are considered and generalized. Data about mechanisms of metal electroreduction and electrodeposition in form of different cathode deposits are discussed and systematized. The state of art in these areas is analyzed, and prospectives are discussed.

Phase Diagram and Electrical Conductivity of CeBr3-KBr

This paper continues our research program on lanthanide halide-alkali metal halide systems. Differential scanning calorimetry (DSC) was used to investigate the phase equilibria of the CeBr3-KBr system. This system is characterized by the two congruently melting compounds K3CeBr6 and K2CeBr5 and the three eutectics located at the CeBr3mole fractions 0.193 (837 K), 0.295 (855 K) and 0.555 (766 K). K3CeBr6 forms at 775 K and melts congruently at 879 K with the related enthalpies 54.5 and 41.7 kJ mol−1, respectively. K2CeBr5 melts congruently at 874 K with the enthalpy 82.4 kJ mol−1. The electrical conductivity was measured of all CeBr3-KBr mixtures and of the pure components down to temperatures below solidification. The experimental determinations were conducted over the entire composition range in steps of about 10 mol%. The specific electrical conductivity decrease with increasing CeBr3 concentration, with significantly larger conductivity changes in the potassium bromide-rich region. The results are discussed in terms of possible complex formation.

Enthalpy of Mixing of the PrCl3-LiCl and NdCl3-LiCl Molten Salt Systems

The molar enthalpies of mixing (ΔmixHm) of liquid PrCl3-LiCl and NdCl3-LiCl mixtures have been measured at 1044 K. For both systems these enthalpies are negative in the whole composition range, with a minimum at xPrCl3 and xNdCl3 ≈ 0.4. The results are compared with existing mixing enthalpy data on lanthanide chloride-alkali metal chloride systems and discussed in terms of complex formation in the melts.