THERMODYNAMIC STUDY OF OXIDE SYSTEMS BY KNUDSEN-EFFUSION COUPLED WITHMASS-SPECTROMETRY

Knowledge of the thermodynamic properties of liquid silicates or alumi nosilicates is of fundamental interest in fields as various as geochem istry or steelmaking. In the first case, it is of importance to gather a coherent set of the thermodynamic quantities steering the condition s of formation of the different geological layers; in the second case, to know the equilibrium conditions to optimize the metallurgical proc ess. 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 eff usion method with mass spectrometry which allows simultaneously the qu alitative and quantitative analysis of the vapour phase and then gives access to the partial quantifies, yielded experimental informations o n silicate melts these last years. Let us notice that the great number of constituants which form the natural systems make impossible an exh austive study. Then it is necessary to valid estimation and modelizati on process of these quantifies on simple systems which can be used as references. This paper gathers a set of data obtained by this method a pplied to the following systems : (Na2O-K2O)-Al2O3-6SiO(2), (Na2O-K2O) -SiO2, (Na2O-K2O)-2SiO(2), Na2O-Cs2O-SiO2 and BaO-SrO. The proposed me thod is based on the study of the equilibria between the condensed and the vapour phases. For example, if binary mixtures of alkali oxides a re heated in a Knudsen-cell made of Pt, the vapour species in equilibr ium are M (Na, K, Cs) and O-2,. At lower temperature, 700 K, the measu rements show the existence in small quantities of M2O+ which decompose s following : [[M2O]] = 2[M] + 1/2 [O-2] The equilibrium constant of t he reaction is written as : K(T) = NF>O2</INF><SUP>1/2</SUP>P<INF>M</I NF><SUP>2</SUP> )/a<INF>M2O </INF>where P and a are respectively the p artial pressures and the activity. This method does not require the pr essure calibration of the apparatus since the ratio of the ionic inten sities is measured. In case of a binary system, if exists a relationsh ip between the ratio of the ionic intensities I-i(+)/I-j(+) of the com ponents of the alloy for a molar fraction x(i) and the activity coeffi cient gamma(i). The study of the variation of this ratio versus temper ature leads to the partial molar enthalpy of formation of each constit uent, and examples are given in this study, showing the validity of th is method compared to dissolution calorimetry methods in some cases. I n 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 ion ic currents measurements. However, it is possible to solve the ternary system by recording the intensifies of a single pair of ions if the m easurements are carried out along paths of constant mole fraction of t he third or non-volatile compound. The results obtained by this method are presented both with reference to quasibinaries or to the oxide co nstituents. It can be seen that the thermodynamic behaviour is charact erized by small negative deviations compared to ideality and a symmetr ical variation versus the molar fraction of the Gibbs free energies an d enthalpies of formation of the silicate melts. This corresponds to a regular solutions behaviour.