Thermodynamic study of oxide systems by Knudsen-effusion coupled with mass spectrometry

Knowledge of the thermodynamic properties of liquid silicates or aluminosilicates is of fundamental interest in fields as various as geochemistry or steelmaking. In the first case, it is of importance to gather a coherent set of the thermodynamic quantities steering the conditions of formation of the different geological layers; in the second case, to know the equilibrium conditions to optimize the metallurgical process. The few measurements found in the literature can be explained by the experimental difficulties related to these systems : high melting point, high reactivity... The association of the classical Knudsen effusion method with mass spectrometry which allows simultaneously the qualitative and quantitative analysis of the vapour phase and then gives access to the partial quantifies, yielded experimental informations on silicate melts these last years. Let us notice that the great number of constituants which form the natural systems make impossible an exhaustive study. Then it is necessary to valid estimation and modelization process of these quantifies on simple systems which can be used as references. This paper gathers a set of data obtained by this method applied to the following systems : (Na2O-K2O)-Al2O3-6SiO(2), (Na2O-K2O)-SiO2, (Na2O-K2O)-2SiO(2), Na2O-Cs2O-SiO2 and BaO-SrO. The proposed method is based on the study of the equilibria between the condensed and the vapour phases. For example, if binary mixtures of alkali oxides are heated in a Knudsen-cell made of Pt, the vapour species in equilibrium are M (Na, K, Cs) and O-2,. At lower temperature, 700 K, the measurements show the existence in small quantities of M2O+ which decomposes following : [[M2O]] = 2[M] + 1/2 [O-2] The equilibrium constant of the reaction is written as : K(T) = ((PO2PM2)-P-1/2 )/a(M2O) where P and a are respectively the partial pressures and the activity. This method does not require the pressure calibration of the apparatus since the ratio of the ionic intensities is measured. In case of a binary system, if exists a relationship between the ratio of the ionic intensities I-i(+)/I-j(+) of the components of the alloy for a molar fraction x(i) and the activity coefficient gamma(i). The study of the variation of this ratio versus temperature leads to the partial molar enthalpy of formation of each constituent, and examples are given in this study, showing the validity of this method compared to dissolution calorimetry methods in some cases. In case of a ternary system A-B-C, the method requires that the vapour pressures of all three components be high enough to yield reliable ionic currents measurements. However, it is possible to solve the ternary system by recording the intensifies of a single pair of ions if the measurements are carried out along paths of constant mole fraction of the third or non-volatile compound. The results obtained by this method are presented both with reference to quasibinaries or to the oxide constituents. It can be seen that the thermodynamic behaviour is characterized by small negative deviations compared to ideality and a symmetrical variation versus the molar fraction of the Gibbs free energies and enthalpies of formation of the silicate melts. This corresponds to a regular solutions behaviour.