Investigation Of Microscale Heat Transfer Mechanism During Femtosecond Lasers Interactions With Nanomaterial

Micro-and nano-technology has been developing rapidly since 1980 s.The traditional hydrodynamics and heat transfer are challenged with the smaller size of devices and the shorter time of processing.A new subdiscipline,micro-scale heat transfer,is being established to study the mechanism of flow and heat transfer in the micro-scale region.Femtosecond laser processing is one of the applications of micro-scale heat transfer.Comparing with the traditional processing,femtosecond laser possesses many advantages,such as “Non-thermal” processing,ultrafast melting and resolidification,the ability of processing in the transparent material.In addition,femtosecond laser has already been used to many applications,such as material science,bio-science,medicine,aeronautics and astronautics.However,despite of lots of advantages and brilliant application prospects,it is quite hard to precisely control the femtosecond laser processing.Furthermore,the micro-scale heat transfer involves a huge number of theories and methodologies of modern physics.It is crucial for the development of the theoretical science to find the appropriate clues in the sea of information and to establish a stream of the knowledge structure system.In this thesis,we investigate the femtosecond laser processing from the perspectives of macro-heat transfer,meso-heat transfer,micro-heat transfer and micro-scale irradiation.The advancements of the above theories are introduced.The numerical methods from these theories are established,developed and proposed.The work is as follow:(1)Macro-Heat TransferThe femtosecond laser processing is described by macro-theory.The two leading models based on the continuous medium assumption,parabolic two step(PTS)model and dual hyperbolic two step(DHTS)model,are compared detailedly including the temperatures below the melting point,heat wave effect and melting and resolidification.It can be found that the results predicted by DHTS model coincide with the experimental data better than PTS do.In addition,DHTS model can describe the superposition and reflection of heat wave.A new interfacial energy equation based on DHTS model is derived.(2)Meso-Heat TransferIn this part,the femtosecond laser processing is described by meso-theory.The derivations of Boltzmann transport equation(BTE)and lattice Boltzmann method(LBM)are introduced.We applied the classical LBM to the laser processing and proposed a two-step Boltzmann equation with an electron-lattice collision operator.It can be found that when the pulse width is longer than the electron relaxation time,the model can predict the experimental data.Furthermore,we derived a new collision operator and proposed an expanded lattice Boltzmann method(ELBM).The results predicted by the ELBM agree with the analytical solutions quite well.(3)Micro-Heat TransferWe use the micro-theory to describe the melting behavior of nano-material.Molecular dynamics method is adopted to simulate the bulk gold and nano-gold film.For the bulk gold simulation,it can be found that the internal energy will increase sharply at 1300 K,which agrees with the experimental measurement(the melting point from the experiment is 1336 K).When the temperature is lower than 1200 K,the lattice structure of gold atoms still remains.However,when the temperature increases up to 1300 K,distribution of gold atoms is disorder.For the nano-gold film,when the thickness of gold film decreases,the internal energy will increase and the melting point of gold film will decrease.For the 1.2-nm gold film,the melting point is about 900 K.(4)Micro-IrradiationThe micro-irradiation effect is investigated during the laser sintering.The influence of scattering on the sintering is also studied.A distribution function is defined.A Monte Carlo method based on Mie scattering theory is proposed.For the blackbody particle(non-scattering effect),the distribution of laser source possesses three peaks and the light intensity near the bottom tends to zero.Also,we derive an approximate analytical solution.If we consider the scattering effect,the light intensity near the bottom is greater than that of blackbody particle.The research in this thesis provides some theoretical guidance for the femtosecond laser processing,and gives some help in the development of micro-scale heat transfer theory.Finally,it should be noted that our present research is only at the introductory level.Its potential is tremendous.We still need to make more efforts.Also we appreciate it if those who are dedicated to micro-scale heat transfer can provide us with more guidance and help.