Calculations of the heat capacity of Cu clusters synthesized by condensation from the gas phase

Abstract

On the basis of our former simulations, we conclude that the heat capacity in the case of isolated free clusters can exceed that of a bulk material. It was found that at T=200K the increase in the Cu nanocluster heat capacity (D = 6nm) was only 10%, decreasing with growing nanoparticle proportionally to the reduction in the fraction of surface atoms. Thus, the considerably larger heat capacities of copper nanostructures observed in the experimental works cannot be related to the characteristics of free clusters. In our view, these properties of a nanomaterial can be associated with the degree of agglomeration of its constituent particles, i.e., the interphase boundaries and the increase in the root-mean-square displacements of atoms on the combined surface of the interconnected nanoclusters can have a strong effect. To test the above hypothesis, we took copper clusters of various sizes (4071-15149 atoms) that we produced when simulating the synthesis of Cu nanoparticles. Thus, in our molecular-dynamics experiments using a tight-binding potential at high temperatures, we failed to properly assess the role of the interphase boundaries in calculating the heat capacity of nanoparticles. The reason was the mass diffusion of Cu atoms to impart an energetically more favorable shape and structure to the cluster. At low temperatures, the heat capacity of the clusters exceeded that of the bulk same by a value from 10% to 17%. Consequently, the Cu clusters produced in direct experiments cannot be immediately applied in devices using the thermal energy of such clusters, because their external shape and internal structure are nonideal.