Thermodynamics and surface properties of the Ag-Pd system and its nanosized phase diagram: tools for the prediction of a nanosized alloy structure

The CALPHAD (Calculation of Phase Diagrams) approach has been widely recognized in different fields of materials science and engineering. Thermodynamic databases have been mainly used for phase diagram optimisation, but they can be also applied for the calculations of surface properties, such as surface tension and surface segregation. The thermodynamics of solid-liquid phase equilibria in nanosized particle systems has been initially studied analysing the melting phenomena of pure metals and alloys. It has been established that the melting points of some pure metals decrease with decreasing the size of their metallic particles. The large surface / volume ratio in nanosized particle systems has significant effects on their thermodynamic properties and phase relations. In the present work the phase diagram of the Ag-Pd nanosized alloys is evaluated qualitatively from the macroscopic thermodynamic point of view and it is calculated as a function of temperature (T), composition (c), size (d) and taking into account that the phase relations are dependent upon the size of particle and its surface property. Accordingly, in order to have precise data on the Ag-Pd surface properties, necessary for the evaluation of the corresponding nanosized phase diagrams, these properties have been analysed for the liquid and the solid phases in terms of thermodynamic models (variants of the regular solution model) based on a monolayer and multilayer. The results obtained using the thermodynamic approach have been compared to those obtained by atomistic simulations. The latter employ a semiempirical potential that has been recently validated against density-functional calculations [1]. Atomistic simulations are able to shed light on the fine details of nanoparticle structure and chemical ordering pattern. [1] F. R. Negreiros, Z. Kuntová, G. Barcaro, G. Rossi, R. Ferrando, and A. Fortunelli, J. Chem. Phys. 132, 234703 (2010).