Thermoelastic Damping In Laminated Composite Microrods Resonators With Circular Cross-section

Micro-resonator is one of the most important component of Micro-Electro-Mechanical System / Nano-Electro-Mechanical System(MEMS/NEMS).Micro-resonator is usually applied in sensors,communication devices,energy harvesting equipment and other miniature machineries.The minimization of damping of the structure of the micro-resonator is the key to improve the reliability,increase the options of frequency and optimize the energy converting of micro-resonators.Existing methods to eliminate external damping(including air damping,squeeze-film damping and supporting damping)of micro-resonators are mainly optimizing the structure,reducing the surface roughness,and reducing the working pressure,thermal annealing and changing the boundary conditions.These methods make structure damping controlled by thermoelastic damping.However,as an inherent damping,thermoelastic damping are inevitable in micro-resonators.Thermoelastic damping is the key to influence the quality of microresonators,so the research of thermoelastic damping is basic in kinetic analysis of MEMS.With the fast development of MEMS,various micro-resonators with complicated structures are applied widely.This thesis aims to study two and three-layer micro rods.The mechanism and model of thermoelastic damping of these structures are proposed to provide theoretical basis of optimizing lengthways vibration of micro-rod resonators.The main content of this thesis are as follows.First,double-layer micro-rods bearing axial load are studied.The field function of temperature is obtained by generalized orthogonal function method.According to the theoretical framework established by Bishop and Kinra,analytical model of the thermoelastic damping of double-layer micro-rods are solved.The validity of the model is proved by Green function.Then the comparison between model constructed and the results calculated by ANSYS is made.The convergence of the model,the fitting of analytical model and finite element model are discussed in this thesis.The limitation of this model and the influence of metal coating,external excitation force,the thickness and length of micro-rods to thermoelastic damping are also studied.Researches show that thermoelastic damping of double-layer Si micro-rods bearing axial load increase first and then decrease with the increase of the thickness of metal coating.The length of micro-rods has little influence on thermoelastic damping when the length of micro-rods is much larger than radius.Then,the thermoelastic damping of triple-layer micro-rods bearing axial load are studied.The field function of temperature is obtained by generalized orthogonal function method.On this basis,the analytical model of thermoelastic damping of triple-layer micro-rods is established.The comparison between triple-layer micro-rods and double-layer micro-rods and single-layer micro-rods are made to prove the validity of the model.The convergence of the model was studied on MATLAB.The comparison of the analytical model and finite element model are made.The limitation of this model and the influence of metal coating,external excitation force and the thickness of micro-rods to thermoelastic damping are also discussed in this thesis.Research show that if thickness ratio of every layer stays stable,the peak value of thermoelastic damping of triple-layer micro cylinder rods remains unchanged while the whole thickness varies.But the frequency of reaching the peak value decreases when the whole thickness increases.The performance of micro-resonators would be promoted by changing the whole thickness of triple-layer micro cylinder resonators to adapt the frequency of reaching the peak value of thermoelastic damping.