| With the rapid development of semiconductor technology and micromachiningtechnology, the feature sizes of the devices have entered into the nanoscale.Nanodevices have become the pursuit of new devices around the world due to theirnovel physical properties and wide prospect of application. However, the small size ofnanodevices makes their dissipative power density multiply, large amounts of heatgenerated per unit time. Therefore, heat dissipation becomes very important in thedesign and operation of these devices. According to the Landauer transport theory andscattering matrix calculation method, the presented thesis investigates thermaltransport properties of one-dimensional nanomaterials at low temperatures, and somemeaningful results are obtained.First of all, we investigate the phonon transport and heat transport characteristicsby using the elastic continuum model when the P wave is illuminated through thedouble T nanostructure in the mixed mode (P and SV mode), and we also compare theheat transport of the SH wave with that of the mode P wave. The results show that theinfluence of the height of T-shaped structure on the phonon transport is maximum, butthe influence of the coupling length is minimum.The minimum reduced frequencystimulating the conversion of SV mode is ω/Δ=0.70, which is not related to the sizeof the nanometer structure parameters. With the lower frequency, the phonontransmission coefficient is mainly decided by the P mode, comparing to the higherreduced frequency, which is decided by SV wave or SV and P wave.Under the limit ofthe temperature to zero, the thermal conductivity uncorrelated to nano cavitygeometry parameters tended to the universal quantization valueπ2k2BT/3h.Whetherit’s incident P wave and SH wave We found that the small changes of the geometricparameters can induce different variation on the thermal conductivity, and the effectof the stub height of SH wave is the strongest. In addtion, the mode conversion playsan important role on the thermal conductivity, especially, the coupling of two stubswith mixing mode is larger than that with single SH mode at the higher temperatures.Next, we study the modulating function on the phonon of the phonon cavity ofwide-narrow structure. On the one hand, the modulating action of cavity has beeninvestigated when the two heat reservoir is fixed. The results playes that to thetransmission coefficient sharply decrease, particularly in the range of the high frequency,owing to the introduction of the cavity; and we only observed theresonance transmission phenomenon in0mode, what is more, the location and densityof the resonant transmission peak depends on the frequency of incidence phononand the structure parameters l; the longer the horizontal width of the cavity, thestronger the modulus conversion, and the more complex the transmission spectrum aswell.By researching the heat transport, it’s found that the mode conversion is verysmall when the the temperature tends to zero, and the zero mode of P wave plays andecisive role on thermal conductivity. However, the mode conversion has a greatcontribution to the thermal conductivity in the higher temperature. Furthermore, thepositions of the phonon cavity lying in the two heat reservoir also have a great impacton the thermal conduction.When the temperature is lower, the transport curve issymmetrical about the center of the two heat reservoir, but emerges the irregularoscillation pattern relatived to the higher temperature. On the other hand, we alsostudy the effects on the thermal transport of the quantum wire by the more uniformdistribution of phonon cavity. It turns out that both the phonon transmission spectrumand thermal transport spectrum have the similar characteristics, and with theincreasing number of the phonon cavity, the similar characteristics are more obvious.At the same time, the periodic and quasi periodic change rule of phonon transmissionspectrum is influenced by the width and height and number of phonon cavity, and thelength of phonon coupling. So we could adjust the phonon transmission coefficientand transport by changing the gemetric parameters and number of the uniformdistribution of phonon cavity and.And then, we discuss the transmission coefficient and characteristics of phononin a symmetric or an asymmetric Y-branching quantum nanostructures at lowtemperatures. The results show that the zero mode phonon’s separation phenomenonis obvious in comparison, when the phonon through the two types of Y-branchingsemiconductor nanostructures. Nevertheless, the1mode’s separation phenomenononly shows in certain areas, and some other areas appear the phenomenon ofalternating, which is different from the SH mode.By considering the relation between the transmission coefficient and heattransport coefficient of phonon and the symmetry coefficient(s) of the symmetricY-branching structure, We found that when the symmetry coefficient is a constant, theheat distribution phenomenon is more remarkablely with the adding of thetemperature comparing with the constant temperature, it decreases with the increasingof the symmetry coefficient. Beyond that we also observed the resonance transmission and the noninteger quantized thermal conductivity.We known that both the symmetricand the asymmetric Y-branching structure could be treated as a mode splitter or heatshunt.Finally, we also deliberat the influence of the material properties to the phonontransport and the heat transport in asymmetric Y branch structure. The results provethat it is remain exist the mold separation and shunt phenomenon, but the moldseparation phenomenon of the1model phonon disappeared in the GGGI materialstructure.Additionly, the phonon thermal transport is sensitive to the mass density andthe elastic constants of materials. I hope that these results provide a good theoreticalguidance to design the nano composite material thermal device for us. |
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