Unified Description of the Specific Heat of Ionic Bulk Materials Containing Nanoparticles

The specific heat behavior in bulk nanomaterials (NMs) obtained by adding nanoparticles to pure suspending media has attracted a lot interest in recent years. Controversial results about NMs specific heat (c) have been reported in the literature, where nanoparticles (NPs) of different sizes and materials were suspended in solid and liquid salts at different concentrations and temperatures. However, a unified picture explaining the c enhancements and diminutions by adding NPs to pure salts is still missing. In this work, we present a general theoretical thermostatic model aimed at describing the c behavior in two-component ionic bulk nanomaterials containing NPs. The model, designed to work in the dilute regime, divides the NM in three regions: bulk suspending medium (SM), nanoparticles, and interface regions. It includes the effects of temperature, NP size, and NP concentration (mass fraction), allowing us to calculate c variations with respect to the pure SM and the ideal NM (where NP and SM are assumed to not interact). We then use the model to interpret results of our classical molecular dynamics simulations, which we perform in the solid and liquid phases of NMs representative of three different classes, defined according to the atomic interactions at the interface. The analysis reveals nontrivial and competing effects influencing c, such as system-dependent atomic rearrangements at the interface, vibrations of the NP as a whole and c variations coming from the individual NP and SM specific heats. Our study contributes to the interpretation of past controversial results and helps in designing NMs with improved thermal properties, which is highly relevant for industrial applications in thermal energy storage and renewable energy production.