Zhongwei Zhang

Hopping processes explain linear rise in temperature of thermal conductivity in thermoelectric clathrates with off-center guest atoms

Qing Xi, 2, 3 Zhongwei Zhang, 2, 3 Jie Chen, 2, 3 Jun Zhou, 2, 3, ∗ Tsuneyoshi Nakayama, 2, 3, 4, † and Baowen Li ‡ Center for Phononics and Thermal Energy Science, School of Physics Science and Engineering, Tongji University, 200092 Shanghai, P. R. China China-EU Joint Center for Nanophononics, School of Physics Science and Engineering, Tongji University, 200092 Shanghai, P. R. China Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, 200092 Shanghai, P. R. China Hokkaido University, 060-0826 Sapporo, Japan Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, USA (Dated: August 1, 2017)

Randomness-Induced Phonon Localization in Graphene Heat Conduction

Through nonequilibrium molecular dynamics simulations, we report the direct numerical evidence of the coherent phonons participating in thermal transport at room temperature in graphene phononic crystal (GPnC) structure and evaluate their contribution to thermal conductivity based on the two-phonon model. With decreasing period length in GPnC, the transition from the incoherent to coherent phonon transport is clearly observed. When a random perturbation to the positions of holes is introduced in a graphene sheet, the phonon wave-packet simulation reveals the presence of notable localization of coherent phonons, leading to the significant reduction of thermal conductivity and suppressed length dependence. Finally, the effects of period length and temperature on the coherent phonon contribution to thermal conductivity are also discussed. Our work establishes a deep understanding of the coherent phonons transport behavior in periodic phononic structures, which provides effective guidance for engineering thermal transport based on a new path via phonon localization.

Tailoring the Thermal and Mechanical Properties of Graphene Film by Structural Engineering

Due to substantial phonon scattering induced by various structural defects, the in-plane thermal conductivity (K) of graphene films (GFs) is still inferior to the commercial pyrolytic graphite sheet (PGS). Here, the problem is solved by engineering the structures of GFs in the aspects of grain size, film alignment, and thickness, and interlayer binding energy. The maximum K of GFs reaches to 3200 W m-1 K-1 and outperforms PGS by 60%. The superior K of GFs is strongly related to its large and intact grains, which are over four times larger than the best PGS. The large smooth features about 11 µm and good layer alignment of GFs also benefit on reducing phonon scattering induced by wrinkles/defects. In addition, the presence of substantial turbostratic-stacking graphene is found up to 37% in thin GFs. The lacking of order in turbostratic-stacking graphene leads to very weak interlayer binding energy, which can significantly decrease the phonon interfacial scattering. The GFs also demonstrate excellent flexibility and high tensile strength, which is about three times higher than PGS. Therefore, GFs with optimized structures and properties show great potentials in thermal management of form-factor-driven electronics and other high-power-driven systems.

Revisit to the Impacts of Rattlers on Thermal Conductivity of Clathrates

Energy conversion from waste heat to electric power is a promising approach for energy harvest, and the clathrates crystals have received lots of attentions in this field from the concept of ‘phonon-glass and electron-crystal’. However, the thermal transport mechanisms and roles of rattlers have yet been clearly revealed in clathrates. By using iterative solution of Peierls-Boltzmann transport equation and first principle calculations, we have systematically revisited the thermal transport properties of a simple binary representative of clathrates, Ba8Si46. Our results confirm that the suppressed phonon lifetime is responsible for the huge reduction of lattice thermal conductivity (κ_l) in clathrates, in addition to the decrease of phonon group velocity. Furthermore, we clarify that phonon scatterings in a wide frequency range and the resonant characteristic scatterings coexist in clathrates, due to the emergence of hybridized modes introduced by the rattlers. We also elucidate that the hybridized modes dramatically suppress the acoustic phonon contribution to κ_l, leading to the non-negligible relative contribution from optical phonon to thermal transport in clathrates. Moreover, the impacts of the hybridized modes on different scattering channels in the phase space are also discussed. Our study provides fundamental physical insights into the impacts of rattlers on thermal conductivity of clathrates, which is valuable towards the design of efficient thermoelectric materials based on the concept of ‘phonon-glass and electron-crystal’.