Browsing by Author "Feng, Dawei"

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Thermodynamic Measurements on Alloys and Compounds in Ag-Au-Se and Ag-Pd systems by the Electromotive Force Method
(2014) Feng, Dawei
School of Chemical Technology | Doctoral dissertation (article-based)
Gold and silver chalcogenides are significant minerals and major carriers of precious metals, and silver palladium alloy is one of the most important silver alloys with various industrial applications. The Ag-Au-Se ternary system and the Ag-Pd binary system have been investigated by the electromotive force (EMF) method in this study. For the Ag-Au-Se ternary system, the numerical values of the standard thermodynamic functions of the compounds Ag2Se (naumannite), AuSe, and Ag3AuSe2 (fischesserite) have been determined by the EMF method in a solid-state galvanic cell with superionic conductor RbAg4I5 and AgI as solid state electrolyte. The compounds Ag2Se, AuSe, and Ag3AuSe2 were synthesized from pure elements in evacuated quartz glass ampoules and then examined to be homogenous by Scanning Electron Microscope (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS). According to the experimental data on the EMF versus temperature, the analytical equations were obtained for the polymorphic forms of the compounds Ag2Se, AuSe, and Ag3AuSe2. The temperature-dependent relationships of the Gibbs energy of formation on the compounds Ag2Se, AuSe and Ag3AuSe2 in their polymorphic forms and the standard thermodynamic functions of the compounds within the temperature range were also obtained. For the Ag-Pd binary system, thermodynamic measurements of silver-palladium binary alloys have been performed over the temperature range 450–750 K by the EMF method with the superionic conductor AgI as the solid electrolyte. The activity and partial molar Gibbs energy of silver were obtained for Ag-Pd alloys over the whole composition range and the thermodynamic properties of palladium were calculated using the Gibbs-Duhem equation. The results show that chemical activities of silver exhibit negative deviations over most of the composition range, and the activities of palladium are characterized by both negative and positive deviations from the ideal Raoultian behavior. The results also show the minimum integral enthalpy of mixing on Ag-Pd alloys is located at 60 at.% Ag. The thermodynamic properties of the Ag-Au-Se ternary system and the Ag-Pd binary system obtained are significant for the production and recycling of precious metals. Moreover, the EMF method has been improved by advanced cell design, gas purification system, as well as temperature measurement, which can be used as an experimental tool for thermodynamic measurement for other alloys and compounds.