Thermodynamic properties of {(1− x)Si + xFe}(l)

Abstract

A Knudsen-effusion method with a mass-spectrometric analysis of evaporation products has been applied to determine thermodynamic properties of {(1−x)Si + xFe}(1). The temperature and mole-fraction range investigated were: T = 1496 K to 1782 K and x = 0.092 to 0.822. Measurements were carried out with the help of double effusion cells. Activities and partial thermodynamic functions of both components were calculated from the values of pFe and PSi found. Partial characteristics of silicon were also obtained by integration of the Gibbs-Duhem equations with the help of αFe and βFe functions as well as by means of the measured I(Fe+)/I(Si+) ratios. In all cases good agreement between the values found in different ways was observed. This confirms the existence of equilibrium in the effusion cell and proves also the reliability of the thermodynamic information acquired. Asymmetric mole-fraction functions ΔfGm and ΔfHm were obtained for {(1 − x)Si + xFe}(l), the extrema being displaced toward the iron corner. The lowest values of ΔfGm (−34.5 kJ·mol−1) and ΔfHm (−40.2 kJ·mol−1) were found at x = 0.57 and 0.535, respectively (T = 1700 K). An examination of composition functions of a set of physicochemical properties of the melts as well as characteristic shapes of αFe, βFe, and curves of ϖ2GEm/ϖx2 against x made it possible to conclude that an association process occurs in {(1−x)Si + xFe}(l) over a wide mole-fraction range. In accordance with it the thermodynamic properties of {(1−x)Si + xFe}(l) were approximated by the ideal-associated-solution model. Complexes FeSi, Fe2Si, Fe3Si, and FeSi2 were supposed to exist in the liquid solution. It was shown that any other set of associated complexes does not lead to an adequate description. Thermodynamic characteristics of formation of the (iron + silicon) melts obtained in the present investigation agree well with most reliable values of ΔfGm, ΔfHm, and component activities, as well as with the phase diagram of {(1−x)Si + xFe}.