Phonon engineering in thermal materials with nano-carbon dopants

Abstract

The unique geometric and thermal properties of carbon nanoparticles (NPs)—including nanotubes, graphene, and nanodiamonds—have led to their use as additives in many composite material systems. In this review, we investigate the mechanisms behind the altered thermal conductivity (κ) of thermoelectric (TE) and other thermal materials that have been composited with carbon NPs. We provide a comprehensive overview and analysis of the relevant theoretical and applied literature, including a detailed review of the available thermal conductivity data across five common classes of TE materials (Bi2Te3 variants, skutterudites, metal–oxide, SnSe, Cu2Se) in combination with carbon additives, including graphene, nanotubes, carbon black, carbon fiber, and C60. We argue that the effectiveness of carbon NPs in reducing κ in TE composites generally arises due to a combination of the presence of the carbon NP interfaces and significant changes in the microstructure of the host material due to compositing, such as suppressed grain growth and the introduction of pores, dislocations, and strain. Carbon NPs themselves are effective phonon scatterers in TE composites due to a significant mismatch between their high-frequency phonon distribution and the lower-frequency phonon distribution of the host material. While carbon NP doping has proven itself as an effective way to increase the performance of TE materials, there is still a significant amount of work to do to precisely understand the fundamental thermal transport mechanisms at play. Rigorous material characterization of nanocomposites and spectroscopic studies of the precise lattice dynamics will greatly aid the development of a fully quantitative, self-consistent model for the thermal conductivity of carbon nanocomposites.