Thickness-Driven Structural Transition and Its Impact on Thermoelectric and Phonon Transport in Single-Walled Carbon Nanotube Films

Single-walled carbon nanotube (SWCNT) films are promising materials for thermoelectric power generation; however, the dependence of their transport properties on their thickness remains insufficiently understood. This study examined the relationship between the transport properties and the internal structure of SWCNT films with thicknesses ranging from 28 to 193 µm. The structural, mechanical, thermoelectric, and phonon transport properties exhibited a discontinuous dependence on the film thickness. Films up to 72 µm in thickness formed a uniform, dense network that maximized electrical conductivity, whereas films exceeding 97 µm exhibited a coarse and densely layered morphology. This coarse-dense structure increased the contact resistance between SWCNT bundle layers, leading to a reduction in electrical conductivity. Additionally, the increased number of layered interfaces increased phonon scattering, which decreased thermal conductivity and phonon mean free path. These findings provide insights into phonon transport in SWCNT films and have implications for SWCNT-based thermoelectric generator design and optimization.