Graphyne Nanotubes: Materials with Ultralow Phonon Mean Free Path and Strong Optical Phonon Scattering for Thermoelectric Applications

Thermal conductivity and phonon transport properties of graphyne nanotubes (GNTs) and conventional carbon nanotubes (CNTs) are studied using nonequilibrium molecular dynamics simulations. The effect of nanotube length on the thermal conductivity and phonon transport transition from a ballistic to a diffusive regime is investigated. It is found that the thermal conductivity is significantly higher for CNTs in comparison to that of GNTs across the entire ballistic–diffusive transport range. Among GNTs, β- and γ-GNTs demonstrated the lowest and highest thermal conductivities, respectively. In addition, ultralow ballistic to diffusive transition length (4.5–7.6 nm) was observed in GNTs, which was significantly lower compared to CNTs. This behavior is due to the extremely low phonon mean free path (MFP), which is primarily due to a higher phonon scattering rate in a graphyne lattice. In the diffusive regime, the thermal conductivity does not converge at lengths up to 200 nm for both GNTs and CNTs; however, the rate of increase in thermal conductivity of GNTs as a function of nanotube length was considerably lower compared to CNTs. Statistical analysis of umklapp phonon–phonon scattering events indicated that high-energy optical phonon modes which are generated by acetylene bonds in GNTs play a major role in scattering and limiting the MFP of heat carriers, leading to significantly shorter ballistic to diffusive transition length in GNTs. Because of a low rate of electron–phonon scattering, the electron MFP in GNTs was estimated to be extremely large and in a microscale range at room temperature. This, in conjunction with ultralow phonon MFP, provides a pathway toward high thermoelectric figure of merit in long GNTs