Research of Size- and Shape-Dependent Thermodynamic Properties of the Actual Melting Process of Nanoparticles C

Melting phase transitions of nanoparticles are often involved in the preparations, research studies, and applications of nanomaterials. However, because of the changing melting temperature of nanoparticles during the melting process, the current relations of melting thermodynamic properties fail to accurately describe their actual melting behaviors. In this study, accurate thermodynamic relations between integral melting enthalpy and entropy and the size of nanoparticles with different shapes (sphere, rod, wire, and regular polyhedrons) were derived for the first time through designing a thermochemical cycle. In the experiment, Ag nanospheres, nanowires, and nanocubes with different sizes were prepared by chemical reduction methods, and differential scanning calorimetry was employed to determine the melting temperature, the melting enthalpy and the melting entropy. The experimental results agree with the theoretical predictions, indicating that the melting thermodynamic properties decrease with the particle size decrease and present linear variations with the inverse particle size within the experimental size range. Moreover, the melting enthalpy and entropy of nanoparticles in identical equivalent diameters take the same sequence as that of melting temperature as Tₒ(wire) > Tₒ(sphere) > Tₒ(cube). The derived relations of melting thermodynamic properties can quantitatively describe the actual melting behaviors of nanoparticles, and the findings herein provide us a comprehensive understanding of the melting thermodynamic properties of nanomaterials in the whole melting process.