Regulation Of Heat Conduction Of Low-dimensional Materials By Phonon Resonance Structures

For semiconductor materials,phonons are the main carrier of heat,which affects the heat transport dominated by phonons.It can be considered from two aspects of particle properties and wave properties of phonons.The structural design of low thermal conductivity materials is of great significance for the application of thermoelectric materials and heat insulation materials.In this paper,Si nanowires as the research object,using the phonon resonance effect caused by the surface branching structure of nanowires to reduce the thermal conductivity of materials,mainly carried out the following two parts of the study:1.A ring is introduced on the surface of Si nanowire as a phonon resonator to block phonon transmission.To increase the resonance effect,the influence of the height and width of the ring for the thermal conductivity of Si nanowires was studied by using equilibrium molecular dynamics(EMD)method.The research shows that the main factor affecting the thermal conductivity is the height of the ring.The thermal conductivity of Si nanowires decreases with the increase of the height of the ring.When the height reaches 2nm,the thermal conductivity of Si nanowires basically does not decrease,and the ring can reduce the thermal conductivity by up to 61.9%.The change of ring width has little effect on the thermal conductivity of Si nanowires.The phonon band flattening phenomenon in the phonon dispersion relationship confirms that the ring does cause phonon resonance effect,and the slope of phonon dispersion curve decreases,resulting in the phonon group velocity decrease.The variation of the lowest resonance frequency shows that the effect of ring height on the thermal conductivity of Si nanowires is limited by the phonon standing wave length in the ring.The normalized thermal conductivity distribution of phonon contribution shows that the introduction of the ring can reduce the phonon contribution to thermal conductivity,and the increase of the ring height can further enhance this effect.The research results provide some theoretical references for the structural design of thermoelectric materials and thermal insulation materials.2.For the traditional phonon resonance structure,the contact volume with the substrate is increased to enhance the resonance effect.In extreme cases,the resonance structure will be connected and the resonance effect will be weakened.Based on this limitation,we designed a new thread resonance structure on the surface of Si nanowires to further enhance the resonance effect and reduce the thermal conductivity of the Si nanowires to a greater extent.The influence of thread period on the thermal conductivity of Si nanowires was studied by non-equilibrium molecular dynamics method(NEMD).It has been shown that increasing the period of the thread can reduce the thermal conductivity and phonon transmission function of the Si nanowires.When the period of the thread is 6P(Wrap a nanowire of a certain length six times),the maximum reduction of the thermal conductivity can reach 36%,while the maximum reduction of the traditional resonant structure Nanopillar is 15%,and the maximum reduction of the Nanowall is 12%.The phonon dispersion relationship and the mean group velocity confirm the existence of phonon resonance effect,and the phonon standing wave can be generated in the thread,which resonates with the phonon in the main nanowire and blocks the heat conduction.The phonon transmission function is further indicated that the resonance effect caused by the ring mainly inhibits the low frequency(<4THz)phonon transmission.The normalized distribution of phonon modes indicates that the resonance effect mainly occurs near the resonance frequency.By comparing the effect of the surface thread structure with the traditional phonon resonance structures such as Nanopillar and Nanowall on the thermal conductivity,it is found that under the same conditions,the screw thread has a stronger inhibitory effect on the thermal conductivity of nanowires.Threaded phonon resonance structure has the following advantages:(1)The contact volume between the screw thread and the substrate material is larger,which can provide more resonant phonons and thus produce stronger resonance effect;(2)Solve the problem that the resonance effect is weakened due to the connectivity of the resonance structure.