Analytical and numerical studies on mismatch strain related problems

mismatch-strain;self-assembly;lithium battery

Li, Nan ; Hsia ; K. Jimmy

关键词: mismatch-strain;    self-assembly;    lithium battery;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/90468/LI-THESIS-2016.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This thesis presents the analytical, numerical, and experimental study on mismatch strain related problems.Three problems are analyzed: self-assembling of polymer thin film, failure of nanotube and nanorod silicon anodes in lithium batteries and modified failure criterion for earthquake distribution along the earth depth. The first chapter discusses analytical and experimental results of self-assembling of thin films made from Polydimethysiloxane (PDMS)/SU-8 mixture. Material properties can be changed by ultraviolet (UV) light exposure. Gradients of material properties, swelling ratio and young’s modulus, leads to self-assembling in Toluene solution. An analytical model based on beam theory and principle of minimum potential energy was used to predict the folding directions. In order to have a better control of the folding mechanism, ABAQUS models were developed and experiments were conducted with different UV exposure patterns to calibrate both diameters of patterned polymer rolls and angles of partially patterned polymer after assembling. With results from calibration, more complex structures, such as one-degree-of-freedom origami, were designed and fabricated.The second chapter gives failure analysis of nanotube and nanorod silicon anodes in lithium batteries. Silicon is a promising material for lithium batteries with greater energy density. However, silicon has up to 400% volume dilation after lithiation. Large deformation causes mismatch strain and stress between lithiated silicon and non-lithiated silicon and leads to functional failure eventually, especially under repeated lithiation-delithiation cycles. New structures have been proposed by different researchers to avoid functional failure of silicon anodes. A nanotube structure was proposed and compared with previous nanorod structure.ABAQUS models and analytical models were developed to study stress and strain evolution during lithiation process in both nanorod and nanotube structures. Low- cycle fatigue theory was used to explain failure of both structures. In the last chapter, a modified Mohr-Coulomb failure criterion is developed to explain earthquake frequency distribution along the earth depth. It was found that earthquake happened more frequently around 20 and 600 kilometers. To explain the distribution, a new model based on mismatch strain between crust and mantle was proposed by geologists. In their model, crust and mantle had different volume reduction at different depth of the earth. Stress in crust was caused by the mismatch between crust and mantle and varied along the depth. A failure criterion was required to determine if the fracture happened. A nonlinear Mohr- Coulomb failure criterion was developed. The criterion states that ultimate shear stress of rocks will be increased by raising the hydrostatics pressure. It is promising to explain earthquake distribution along the earth depth with stress distribution data from ABAQUS model.