Thermal conductivity and phonon transport in carbon graphyne nanotubes: Phonovoltaic energy conversion application

We determine the thermal conductivity of α, β, and γ graphyne nanotubes (GNTs) as well as carbon nanotubes (CNTs) using molecular dynamics simulations by means of green-kubo relation in the temperature range of 50 to 400 K. GNTs, which contain sp and sp2 hybridized carbon atoms forming acetylene bonds, demonstrated considerably lower thermal conductivity compared to classical CNTs with an identical size. Among α, β, and γ-GNTs, γ-GNT demonstrated the highest thermal conductivity at all temperatures. The phonon transport parameters of GNTs were utilized to analyze the thermal conductivity results based on frequency dependent values of phonon relaxation time, mean free path, group velocities, and specific heat capacity. It was observed that carbon triple bonds generate higher energy optical phonons, which increase the phonon-phonon scattering, adversely affecting the thermal conductivity of GNTs relative to CNTs, which lack the triple bonds. Also, reducing the thermal conductivity of GNTs relative to CNTs is the considerably lower acoustic phonon group velocities for the former as well as the lower volumetric heat capacity of GNTs. Optical phonons in α-GNT demonstrate high-energy (0.26 eV) with a high population number, making them more energetic than its electronic direct band gap and significantly more energetic than its thermal energy at room temperature. Therefore, we suggest α-GNT as a candidate for phonovoltaic energy conversion applications. Additionally, we calculated the figure of merit of  γ-GNTs as a function of nanotube length and diameter. Based on the outcome of this part this research, γ-GNTs can be introduced as great candidates for thermoelectric applications.