Research On Heat Transfer Process Of Nano-scale Silicon Thin Films Under The Influence Of Phonon Dispersion Relation

Micro and nano thin films have been widely used in chip,medicine and industry for their special structure and function.Exploring the heat transfer mechanism of micro and nano thin films has become a hot topic due to the significant differences in thermal transfer properties between micro and nano materials and bulk materials.When the size of the object is very small,or the thermal action time is very short,the traditional Fourier law does not apply,and the ultrafine laser with the picosecond effect on the nanometer scale of the film will undoubtedly produce quite complex energy transfer phenomena.Based on gray lattice Boltzmann model and dispersive lattice Boltzmann model,the thesis studies how the energy transfer of ultrafast laser irradiation in the direction and longitudinal direction of the film thickness and compares the results with those of one-dimensional model and other two-dimensional models.Based on the constructed physical model of nano-silicon thin film irradiated by ultrafast laser,combined with the derived lattice Boltzmann governing equation,the energy transfer in the thin film during and after laser heating is analyzed under the premise of grey hypothesis.The results show that the energy density increases rapidly at the position where the film is heated during the heating stage.After the heating,the energy is transferred in the form of waves along the direction of the film thickness and along the longitudinal direction.Affected by the grey hypothesis,the speed of phonons in the film is unique,so the wave crest has only one place,and the wave crest also moves within the film along with the transmission of phonons.Compared with the one-dimensional model,it is found that the energy density simulated by the two-dimensional model is significantly smaller along the thickness direction of the film heating position,and this gap will gradually increase with time,which indicates that the longitudinal energy transfer of the film cannot be ignored.The dispersion model considers the dispersion effect of phonons and divides phonons into four branches: transverse acoustic branch,longitudinal acoustic branch,transverse optical branch and longitudinal optical branch.The simulation results show that the acoustic phonon and the optical phonon show different characteristics.The acoustic phonon has strong transfer property,while the optical phonon has strong heat capacity although the transfer speed is slower.When the left side of the film is illuminated by a laser,compared with other models,the energy density curve along the film thickness simulated by the dispersion model always presents a phenomenon of higher on the left and lower on the right,and after the end of laser irradiation,the energy on the left side of the film begins to decline and the energy on the right side begins to rise until it gradually reaches the equilibrium state.The results of the dispersive model are superimposed by the results of different phonon branches,which are also reflected in the energy density curve.