Direct Measurements of Surface Free Energy of Solid Solutions: Phase Transitions and Complexions

Abstract

Surfaces are key elements of nanomaterials and catalysts. The main thermodynamic properties of surfaces are their chemical composition and energy. To determine the composition of the surface, there are a number of effective methods such as AES, XPS, TEM. Direct measurements of the interfacial free energy of “solid–gas” interface are practically not carried out. This is related to experimental difficulties. We have developed a method for in situ measurements of the surface energy of solid metals and alloys. We conducted the experiments on two-component Cu-based systems in an inert or reducing gas atmosphere. The measurements on Cu [Ag] and Cu [Co] solid solutions show the presence of phase transitions on the surfaces. The isotherms of the surface energy have singularities (the minimum in the case of copper solid solutions with silver and the maximum in the case of solid solutions with cobalt). In both cases, the surface phase transitions lead to the surface miscibility gap: a monolayer (multilayer) formation (Cu–Ag) or formation of nanoscale particles (Cu–Co). In accordance with the bulk phase diagrams, the concentration and temperature of the surface phase transitions correspond to the solid solution in the bulk. Experiments on similar systems (Cu–Fe, Cu–Pb) lead to the conclusion that in all peritectic systems, an increase in the surface energy by adding a component with a higher melting point and surface phase transitions “surface solid solution—two-phase surface with particles” should be expected. In eutectic systems, the component with a lower melting point decreases the surface energy and forms continuous layers.