Theoretical Study On Thermal Conductivity Of Nanofilms Heated By Ultrafast Lasers

The thermal action time of ultrafast laser technology is mostly in the picosecond-femtosecond order,which will produce very complex thermal conduction phenomenon in the application process,and the quasi-equilibrium hypothesis in Fourier heat conduction law is no longer valid.Based on the Cattaneo-Vernotte(CV)model,dual-phase-model(DPL)and the size effect of nanostructures,the thesis uses the integral variation method to study the two-dimensional heat conduction problem of ultrafast laser heating nano-films.The results of the improved DPL thermal conductivity model and the DPL thermal conductivity model were compared and analyzed.And the results of one-dimensional thermal conductivity model are compared and analyzed.Based on the two-dimensional physical model of ultrafast laser heated nanofilm,the heat transfer process in the film was studied by using CV model.The results show that the heat inside the film is transferred in the form of wave,and the temperature "bulge" phenomenon appears along the thickness and radial direction of the film,which indicates that the heat transfer in the film is at a finite speed,which is a typical non-Fourier heat transfer phenomenon.Compared with the one-dimensional CV model,it is found that the two-dimensional temperature distribution along the thickness direction is lower than the one-dimensional temperature distribution,and the temperature difference increases with the increase of heating time.This indicates that the heat in the radial direction and the thickness direction in the film interacts with each other,which can not be ignored in the heat transfer process inside the film.The results of the DPL model show that the temperature distribution in the film is relatively uniform after considering the hysteresis effect of temperature gradient.As the hysteresis time of temperature gradient increases gradually,the more uniform and gentle the film temperature distribution is,the shorter the time needed to reach the stable state.However,when the hysteresis time of the temperature gradient is longer than the hysteresis time of the heat flow vector,the heat is no longer transferred in a wavy form,but changes into the form of "overdiffusion".Compared with the one-dimensional DPL results,it is found that the temperature distribution along the thickness of the two-dimensional results is lower than that of the one-dimensional results,and the delay time of the temperature gradient will affect the difference between the two results.When into the nanometer level material size,due to the small size effect to make film internal heat conduction process more complicated,the coefficient of thermal conductivity of the silicon thin film membrane are also given in this thesis the thickness change law of size effect based on the improved DPL model to study the ultrafast laser heating nanometer film two-dimensional heat conduction problem,and has made the comparison and analysis with DPL model results of thermal conductivity.The results of the improved DPL model show that the heat is still transferred in a wavy form,but the fluctuation pattern is no longer obvious,and the peak temperature of the film is always at the disturbed position on the surface.Under the adiabatic boundary condition,the peak temperature increases with the increase of Kn.In addition,in the initial stage,the temperature at the center of the heated surface of the film in the improved DPL results is higher than that in the DPL results,but as time goes by,the temperature decreases more rapidly in the improved DPL results.