Quantum Thermal Transport Properties Of Graphene Systems With Grain Boundaries

The high requirement of device miniaturization for heat dissipation and heat insulation urges people to study the quantum thermal transport properties of low-dimensional materials.The grain boundaries prevalent in graphene have a significant impact on its thermal conduction properties.Combined with the empirical potential method and phonon nonequilibrium Green’s function method,the quantum thermal transport properties of several graphene systems with grain boundaries are systematically studied in this dissertation.We have extended the fourth nearest neighbor force constant model to a certain extent.On the basis of the fourth neighbor force constant model,the changes of bond length,bond angle and coordination number are considered.And the physical quantities related to bond length,bond angle and coordination number are introduced.The expanded fourth neighbor force constant model can be applied to the graphene system with grain boundaries.The extended fourth neighbor force constant model is also convenient to further explore the quantum thermal transport properties of large-scale transport systems.Using the empirical potential calculation,we found that the degeneracy of the phonon spectra of ideal graphene nanoribbons at X point is related to the parity of the width.Zigzag graphene nanoribbons with odd width N_Z(N_Z-ZGNR)are composed of paired degenerate phonon patterns at X point,while N_Z-ZGNR with even width is mostly non-degenerate.Armchair graphene nanoribbons are the opposite.In the graphene system with grain boundaries,the original ideal"ladder"of the transmission spectrum of the system is destroyed by grain boundary,which reduces the phonon transmission coefficient.We found that the wave vector mismatch is one of the reasons for the decrease of phonon transmission coefficient.In addition,the high localization of phonons at grain boundaries can lead to Fano-like resonance phenomena similar to two-dimensional structures.These studies provide references for further understanding of the thermal transport properties of the graphene system with grain boundaries,and also provide new ideas for future research on low-energy quantum devices.