Phase Diagram of Nanoparticles of Palladium-Platinum Alloys

Abstract

Abstract Melting and crystallization temperatures of mesoscopic metal particles have been calculated within the framework of thermodynamics of surface phenomena and the electron-statistical method, for which it is necessary to take into account the dimensional dependences of the surface and interphase energies. All estimates were carried out in the model of heterogeneous melting in the presence of a skin layer. Phase diagrams have been constructed for binary Pd-Pt nanoparticles with solid core linear dimensions of 5 and 8 nm and a skin layer thickness of 2 nm. The surface and interfacial energies, as well as their temperature and size dependences, were estimated by a modified electronic statistical method. A comparative analysis of the results obtained using different expressions for the size dependence of the specific heat of fusion of nanoparticles is carried out. It was found that the shape of the diagram has significant differences from the phase diagram for the bulk phases of the metals under consideration. The main reason for the difference is the melting-crystallization hysteresis and the appearance of a smooth maximum on the liquidus and solidus lines. The maximum on the lines of melting and crystallization temperatures can be explained by a more pronounced size dependence of the interfacial characteristics of platinum. In accordance with the criterion that determines the more surface-active component, it can be concluded that with a change in the melting point ratios of pure components and lower values of the surface energy of palladium, a core (Pd) - shell (Pt) structure will form in nanoparticles.