Extreme Control of Heat Conduction in High-Aspect-Ratio Nanomaterial Assemblies

High aspect-ratio nanomaterials, such as nanotubes, fibrils, and monolayers, when assembled into films and composites, enable large controllability of heat conduction, leading to features like ultra-low, tunable, or directional thermal conductivities. Over the past several years, in the Thermal Energy Engineering Lab (TEEL) at the University of Tokyo, in collaboration with other researchers, we have explored various nanomaterial assemblies using carbon nanotubes (CNTs), cellulose nanofibrils (CNFs), and 2D materials. For CNTs and CNFs, forming aerogels can reduce thermal conductivity to levels below that of air. On the other hand, when these nanomaterials are aligned and assembled into dense filaments or films, thermal conductivity significantly increases. This is expected for CNTs, which are known to exhibit thermal conductivity comparable to or even higher than metals. Achieving this with CNFs, however, is non-trivial, given their inherently disordered nature. So far, we have achieved thermal conductivities of 10 W/mK in sub-ten-micron-diameter filaments and 4 W/mK in films, both in the alignment direction, while retaining mechanical flexibility. The thermal conductivity and its anisotropy can be tuned by adjusting the degree of alignment. For materials like CNFs, which are surface-active, water content also plays a significant role in determining heat conduction. Furthermore, compositing different nanomaterials enhances heat conductivity control via hetero-interfaces. A particularly striking example is found in 2D heterostructures, where the hetero-interface can be embedded at the true nanoscale. Here, phonon transport is significantly hindered not only by interface scattering but also by the modulation of phonon states. By designing optimized aperiodic 2D heterostructures, we can achieve low thermal conductivities, on the order of tens of mW/mK. The key mechanism behind this effect is the suppression of phonon tunneling. In this talk, I will discuss our approaches to material fabrication, thermal measurements, numerical simulations, optimization, and modeling of various nanomaterial assemblies, as well as the potential applications we are targeting.