Theoretical Investigation Of Quantum Thermal Transport In Carbon Nanotubes

As the power density of integrated circuits increases exponentially, the demand of heat dis-sipation also rises dramatically. Possessing superhigh room temperature thermal conductivity,carbon nanotubes become one of the most potential candidates. In contrast, a low thermal con-ductivity with electronic conductivity intact in thermoelectric devices is desired. And precise con-trol of thermal transport in nanoscale is now essential in many applications. It is an urgent taskto understand the thermal transport at the atomic level. In this thesis, a nonequilibrium Green’sfunction method is used to analyze some common deformations’ efect on thermal conductanceof carbon nanotubes, from phonon transport point of view.In radially compressed carbon nanotubes, two typical behaviors are found when phonontunneling a phonon barrier, with analytical form of transmission as a function of deformed lengthgiven. The understanding of tunneling behavior of phonon will help to choose the proper materialsaccording to the characteristics of their phonon dispersions to tune the thermal coupling betweendiferent materials. Because of the Bose–Einstein distribution of phonon, all activated phononscontribute to the thermal conductance, meanwhile electronic conductance is decided by the bandsnear Fermi level. This weighted average properties of thermal conductance makes it difer fromthe behavior of phonon of certain mode substantially. Thus electronic and thermal conductancereact diferently to external perturbation. The diference of responses is the key to improve figureof merit (ZT coefcient) of the material.In nanoscale systems, the Fourier law of heat current no longer works. Thermal transportnot only relates to the material but also the geometric shape. The strain has strong anisotropicefect on thermal conductance. Compared to axial strain, thermal conductance is untouched byradial compression. Robust linear response to radial compression is found in entire elastic de-formation regime. Maintenance of C–C bond length and bond angle, attributing to the uniquehollow structure of nanotubes, minimizes the efect of radial compression. As a result, radialcompression is just a perturbation to most phonon modes. In axially strained carbon nanotubes,phonons are mainly scattered at the interfaces. Low frequency phonon modes are softened inaxially compressed carbon nanotubes, enhances the ability to conduct heat at deformed part. Butmode mismatch of low frequency phonon modes between thermal leads and strained part is moresevere in axially compressed carbon nanotubes than in axially stretched carbon nanotubes, lead- ing to an asymmetric influence of axial compression and axial tension on thermal conductance incarbon nanotubes.Strain-gradient in bending nanotubes broadens the phonon density of state without modesoftening in low frequency phonon modes. So the transmission of low frequency phonons inbending nanotube is more similar to axial tension scenario. This phenomenon resembles theelectronic band structure in ZnO microwires. When the curvature is small, most low frequencyphonon modes keep oscillation mode in deformed part. Thus these phonons can transport throughbending part with their oscillation mode preserved. As a result, quasi-ballistic transport in bentcarbon nanotubes is found despite lack of translation invariance, when the Bloch theorem nolonger holds. Our result explains the experimental phenomenon of nanotube phonon waveguide.