Use of metastable equilibria for determination of Gibbs energy of solids
- PDF / 611,426 Bytes
- 7 Pages / 593.28 x 841.68 pts Page_size
- 50 Downloads / 164 Views
I. INTRODUCTION In the experimental measurement of Gibbs energies of solids, equilibria involving the stable phases have been traditionally exploited. Generally, Gibbs energy measurements are carried out at elevated temperatures where equilibrium can be attained in reasonable time. At the temperatures of the experiment, metastable phases transform to stable ones. Thus Gibbs energies of interesting metastable phases such as metallic glasses and quasicrystals have eluded experimental determination. In one of the commonly used methods for the study of alloy systems, the alloy (A-B) is equilibrated with a compound of the most reactive component with a nonmetallic element (BX2) and the equilibrium partial pressure of the nonmetallic species (X2) is determined experimentally using gas analysis or emf techniques. The partial molar free energy of the most reactive element is determined from experiment and that of the noble component is derived from the Gibbs-Duhem relationship. Alternatively, the integral property can be obtained by integration. This procedure gives reasonable accuracy for binary systems. However, in higherorder systems a very large number of experimental measurements are required to obtain sufficiently accurate data by integration. In practice, the accuracy of the integral free energies obtained by this method decreases with the number of components in the system. Accurate data on integral free energies are required for computation of phase diagrams. Since it is difficult J. Mater. Res. 3 (4), Jul/Aug 1988
http://journals.cambridge.org
to generate these data from the measurement of the activity of the single component in multicomponent systems, methods must be developed to measure activities of all components. One promising technique that has been developed recently is multiple Knudsen cell-mass spectrometry, where a number of species in equilibrium with an alloy can be monitored simultaneously.1 However, vapor pressure techniques are limited to high temperatures where intersting metastable phases decompose. Over the last two decades solid-state galvanic cells have been widely used for thermodynamic measurements on solids.2 Recently, it has been shown that both homogeneous and heterogeneous doping can enhance the conductivity of the electrolyte by two to three orders of magnitude.3 It is therefore possible to employ solidstate cells at lower temperatures4 where metastable phases can persist for periods longer than that required for experiment. The purpose of this communication is to explore the use of metastable equilibria for determination of Gibbs energies of formation of solids. A binary alloy A-B is equilibrated in separate experiments with fluorides of A and B. An equilibrium involving the alloy and AF 2 is metastable, while a mixture of alloy and BF 2 is stable. If both equilibria can be measured, then activities of both components can be independently obtained. The validity of the measurement can be tested against the Gibbs-Duhem relation. Fluorine potentials corresponding to both stable and meta
Data Loading...
