The Physical Mechanism Investigation Of Energy Transport And Its Modulation In Nanoscale Materials

With the increase of transistor density in integrated circuit and decrease of characteristic size in micro-electro-mechanical system,heat dissipation problem becomes more and more important,especially nowadays the transistor size is 7 nm.That is because the power density is as high as several hundreds W/cm²,if the energy cannot be dissipated to the external environment timely,the temperature of device will increase and the lifetime and stability will decrease.Modern semiconductor industry is based on the material Si.For the bulk crystalline Si,the thermal conductivity at room temperature is 150 W/m-K,more than 70%of which is contributed from phonons with mean free math larger than 100 nm.However,the transistor size typically is smaller than 100 nm.Since the heat transport in bulk and nanoscale size are different,it is necessary and significant to investigate the heat transport in nanoscale materials.The size decrease has two effects on the heat transport,one is on the thermal conductivity due to the Casimir size effect and the other is on the interfacial thermal conductance due to the large surface vol?me ratio.This dissertation applies atomic Green's function method to investigate the heat transport at the interface and applies molecular dynamics simulation and first-principles calculation to investigate the heat transport in low-dimensional materials.The purpose of investigating the heat transport is for controlling the heat transport,thus through first-principles method this dissertation raises and investigates a new mechanism thermal switch based on solid-state phase transition ferroelectric materials.Apart from the control of heat transport,this dissertation also investigates the electrocaloric cooling based on ferroelectric materials and analyzes the entropy change during the cooling process.For the heat transport at interface,this dissertation applies atomic Green's function method to investigate the pressure,contact type and contact area effects on the phonon transmission at interface and interfacial thermal resistance.The multi-layer graphene is adopted to explore the pressure effects and the results indicate that cross-plane 10 GPa pressure can increase the phonon transmission significantly and reduce the interfacial?cross-plane?thermal resistance by4 times.Compared with the cross-plane pressure,the in-plane pressure has negligible effects on the interfacial thermal resistance.For the contact type effects,the parallel carbon nanotubes is adopted and the results show that with the overlap increasing,the interfacial thermal resistance of offset parallel carbon nanotubes decreases due to more atoms involved in the interface thermal transport.At the same overlap,nested parallel carbon nanotubes have much less interfacial thermal resistance than that of offset parallel ones.Through the analysis of phonon transmission spectr?m,there are two ways to decrease the interfacial thermal resistance.One is using the same chiral carbon nanotubes and the other is designing the contact type which can allow more atoms to involve the interfacial thermal transport.For the contact area effects,the van der Waals interface composed of graphene sheets is adopted and the results indicate that as the contact area increases from several atoms to several square nanometers,the interfacial thermal resistance per unit area decreases instead of a constant,which is caused by the increase of phonon transmission per unit area.By calculating the coupled atom pairs at the interface,it is found that more atoms are involved in the interfacial thermal transport with the contact area increasing,which predicts the converged interfacial thermal resistance per unit area at¹0 nm²contact area.Apart from the heat transport at interface,this dissertation applies molecular dynamics simulation and first-principles calculation methods to investigate the heat transport in low-dimensional materials.For the two-dimensional van der Waals layered materials such as multi-layer graphene or graphite,simulation results indicate that in the transient state,the heat transport is dominated by the elastic constant along the corresponding direction,while in the steady state,in-plane phonon modes dominate the heat transport along all directions except those very close to the cross-plane direction.For the quasi-one-dimensional van der Waals nanowires Ta2Pd3Se8,thermal bridge measurement shows that at room temperature the ballistic phonon transport can persist 13?m.The iso-energy surface indicates that this long ballistic transport is caused by the highly focused longitudinal phonons.The first-principles calculation predicts that with the cross area increasing,the nanowire thermal conductivity first decreases and then increases.Furthermore,the thermal conductivity of single nanowire is larger than that of bulk.All the prediction is consistent with the recent experimental results.After a deep understanding of heat transport,this dissertation applies first-principles method to investigate the control of heat transport.For the first time this dissertation raises a new mechanism thermal switch based on ferroelectric material BaTiO₃ order-disorder phase transition.The calculation results indicate that compared with the disordered structure,the thermal conductivity of ordered structure is 3.9 times higher.The external electric field can transfer the disordered structure to the ordered structure and increase the thermal conductivity of ordered structure by 2.4 times at most.Therefore,combing the above two effects,the thermal conductivity of BaTiO₃ under external electric field can change 9.4 times at most.At the same time,this dissertation raises two ways to improve the switching ratio of ferroelectric materials based solid-state thermal switch.One is that the structure should be single domain under electric field and the other is that the phase transition should be order-disorder.This dissertation also investigates the electrocaloric cooling based on ferroelectric materials PbTiO₃ and PbZr_0.5Ti_0.5O₃ by first-principles method.The results show that for PbTiO₃,the sign of vibrational entropy change is dependent on the direction of electric field.In the Ericsson cycle,if the negative and positive electrocaloric cooling effects are combined,the cooling energy density increases 3.2 times.For the PbZr_0.5Ti_0.5O₃,electric field is applied along the[001],[011]and[111]respectively?the polarization direction is along[001]?.The results indicates electric field along[001]can cause the largest electrocaloric cofficient.For both PbTiO₃ and PbZr_0.5Ti_0.5O₃,the vibrational entropy change is close to the experimental data,indicating that the vibrational entropy change is an important part in the total entropy change during the isothermal process.Finally,this dissertation applies 3?method to measure the thermal conductivity of SrTiO₃and DyScO₃.Meanwhile,the measurement sensitivity and error is analyzed.