PhD THESIS PRESENTATION
First-principles Study of Phonon Transport in Low-dimensional Structures
Mr Xinjiang WANG
Department of Mechanical and Aerospace Engineering, HKUST
||20 Apr 2017 (Thu)
||Room 2571B, HKUST (2/F., Lift #27/28)
Efficient thermal transport in solids plays a key role in maintaining the efficiency of electronic devices while low thermal conductivity is preferred in other fields such as thermoelectrics. There is strong interest in nano-engineering materials to tune thermal transport properties. Since phonon is the major heat carrier in semiconductors and insulators, exploring the mechanisms of phonon transport and scattering with the shrinking size is of great importance for tuning thermal transport properties for targeted applications through nanoengineering.
The first-principles method can be integrated with lattice dynamics to recapture phonon transport properties, offering high accuracy and transferability. In this thesis, the first-principles-based lattice dynamics approach with both the single-mode approximation and fully iterative solution is developed. To address the issues due to inevitable computational errors, such as the unphysical dispersions of low-dimensional structures, a series of corrections based on symmetry invariance have been adopted. To avoid the arbitrary Gaussian smearing factor widely used in conventional first-principles methods, a double integration method is developed with the construction of a higher dimensional hyperspace. After careful verification, this first-principles approach is systematically applied to 2-D thin film, quasi-1-D complex materials and 1-D chain materials.
With the explicit consideration of phonon depletion induced by phonon confinement and the corresponding variation in interphonon scatterings, the in-plane thermal conductivities of Si thin films with different thicknesses have been predicted and excellent agreements with experimental results are found. The validities of adopting the bulk phonon properties and gray approximation of surface specularity in thin film studies are clarified. It is found that in ultra-thin films the phonon depletion effects are largely offset by the reduction of interphonon scatterings.
In quasi-1-D structures such as crystalline polyethylene (PE) and complex metal organic framework (MOF) crystals, it is found that the complex atom arrangement and van der Waals (vdW) interactions help generate rattling-like behaviors of atoms with large vibrational amplitude that would block phonon transport. It is also found that the perfect alignment of PE chains in bulk PE crystals boosts the thermal conductivity by 2 orders than amorphous PE.
As the dimensionality shrinks, for example in 1-D single-chain PE, a significant increase of axial thermal conductivity is observed compared with bulk PE, due to the diminished inter-chain vdW interactions. Different from many precedent studies, the thermal conductivity of single-chain PE is predicted to converge near room temperature. The convergence is attributed to the indirect thermal resistance contribution from large normal scatterings and anharmonic phonon scatterings due to the coupling between low-frequency bending and twisting phonon modes.
The cross-plane heat transport between graphene and substrates is also a great concern for efficient heat dissipation in graphene-based devices. To estimate the thermal boundary conductance (TBC) at the interface, a model based on the Green’s function is derived to account for different interfacial coupling strength, which is missing in most previous studies. It is found to predict TBC with higher accuracy and is able to explain the differences of TBC with different surface treatments.
(Supervisor: Prof. Baoling HUANG)