Theory Analysis And Lattice Boltzmann Numerical Simulation Of Micro-/Nano-scale Heat Transfer

Ultrafast pulse-laser heating technology has been widely applied to material science, nanotechnology and so on. The law of nano-scale heat transfer in laser heating process has still to be explored, which is the main constraint on the further development of the ultrafast pulse-laser heating technology. The sizes of many microelectronic devices have reached nanometer scales. In order to design their cooling system, we have to investigate the principle of the micro-/nano-scale heat transfer. The experimental, theoretical and numerical results show that the classical Fourier’s law which implies the infinite transport speed of the heat is not applicable to micro-/nano-scale heat conductions. Therefore, the study of micro-/nano-scale heat conduction is of great scientific significance and practical value. In this thesis, based on the Cattaneo-Vernotte (CV) heat conduction model, the dual-phase-lagging (DPL) heat conduction model and the improved CV model which implies size effect, the heat conduction of thin film with nano-scale thickness induced by the ultrafast laser heating is systematically investigated. The lattice Boltzmann method is also employed to simulate the ultrafast laser heating problems and the heat conduction caused by nano-scale hotspots.Firstly, based on the Fourier law and the CV model, the heat conductions in gold films with nano-scale thickness induced by an ultrafast laser are investigated, and the comparison between the two models is also made. It is found that for the CV model the heat propagates in a wave form which is different from the diffusion propagation given by the classical Fourier law. The thermal wave velocities of the CV model are also determined when the Knudsen number is known. We show that the large Knudsen number always leads to the high temperature under the adiabatic boundary conditions when the system reaches its steady state.Then based on the DPL heat conduction model, the heat conductions in gold films with nano-scale thickness exposed to an ultrafast laser heating is studied. It is found that the large ratio of the phase lag of temperature gradient to the phase lag of heat flux may reduce the maximum temperature occurring at the boundary heated by laser and shorten the time for the system to reach its steady state. Based on the DPL heat conduction model the necessary condition for the occurrence of the thermal wave is established. The results show that the thermal wave disappearing when the phase lag of temperature gradient is greater that the phase lag of heat flux, which is consistent with the calculation result and Tang’s result. We also investigate the influence of the surface accommodation coefficient on the temperature profiles in the thin film under the slip boundary condition. It is found that the large value of surface accommodation coefficient may decrease the temperature at the boundary heated by the laser. Furthermore, we find that the large Knudsen number may lead to high heat flux when the system reaches its steady state.In many micro/nano-scale heat conductions the thermal conductivity is dependent on the characteristic length of the system, which is called "size effect" Based on the improved CV model which reflects the size effect, the heat conductions in a gold film with nano-scale thickness induced by an ultrafast laser is investigated in the present thesis. It is found that the thermal wave peak obtained by the improved CV model always occurs at the heated boundary, which is much different to the result obtained by the CV model. The velocities of thermal waves predicted by the two models are also different. The big or small of these two velocities depends on whether the Knudsen number is greater than 1.1027. The difference between the improved CV model and the DPL model is quite significant, especially for the high Knudsen number or the large phase lag of temperature gradient in the DPL model.The lattice Botlzmann method is utilized to simulate the ultrafast laser heating problems of thin silicon films with nano-scale thickness. The results show that the energy transport induced by the ultrafast laser heating of thin films behaves as a thermal wave in the transitional regime, and the large Knudsen number leads a high maximum energy density. The classical Fourier law cannot capture such a kind of wave-like phenomena and severely underestimates the energy level. When two waves induced by the ultra-fast laser heating on two surfaces of a thin film with nano-scale thickness meet each other in the inner region of the film, their interaction may cause significant temperature rise there. Although the hyperbolic heat conduction model can also show the wave-like behavior, it would underestimate the peak value of energy when two thermal waves meet each other in the inner region. Furthermore, by controlling the heating delay time between the two boundaries of the thin film we can change the position where the energy density achieves the highest value, which would be useful for ultra-fast laser heating technology.Finally, the LBM is employed to study the heat conductions induced by nano-scale hotspots in the thin silicon film of Silicon-On-Insulator (SOI) transistor. It is found that the energy transports in a wave-like way in the transitional regime, and the boundary conditions have a significant impact on the energy level in the film.