Diffusion, Deformation, and Damage in Lithium-Ion Batteries and Microelectronics.

다국어 초록 (Multilingual Abstract)

This thesis explores mechanical behavior of microelectronic devices and lithium-ion batteries. We first examine electromigration-induced void formation in solder bumps by constructing a theory that couples electromigration and creep. The theory can predict the critical current density below which voids do not form. Due to the effects of creep, this quantity is found to be independent of the solder size and decrease exponentially with increasing temperature, different from existing theories.
We then investigate the interplay between mass transport, deformation, stress, and fracture in lithium-ion battery electrodes. First, we model fracture of elastic electrodes by combining ideas from diffusion kinetics and fracture mechanics. Next, we examine mechanics of high-capacity lithium-ion batteries, which demonstrate inelastic deformation, by constructing a model that accounts for diffusion and elastic-plastic deformation. These models suggest that fracture is prevented in small and soft electrode materials that are cycled slowly.
To investigate crystalline silicon electrodes, we construct a continuum model of concurrent reaction-controlled kinetics and plasticity. To quantify the kinetics of the lithiation process, we perform electrochemical experiments on crystalline silicon wafers of various orientations. Using the velocities measured in these experiments and our continuum model, we correctly predict anisotropic morphologies and fracture patterns developed in crystalline silicon nanopillars.
We then measure the fracture energy of lithiated silicon, finding it to be similar to that of pure silicon and essentially independent of the lithium concentration. These findings demonstrate that lithiated silicon has a peculiar ability to flow plastically but fracture in a brittle manner. To investigate this interesting combination of properties, we measure stresses in silicon thin films as a function of charging rate. Increasing the rate of lithiation resulted in a corresponding increase in the flow stress, indicating rate-sensitive plasticity.
Microelectronics and lithium-ion batteries are rich in mechanics, requiring considerations from large deformation, plasticity, creep, kinetics, and fracture mechanics. These systems involve an intimate coupling between mechanics and a number of other fields, such as chemical reactions, electric fields, mass transport, and electrochemistry. Thus, it is believed that this thesis will provide general insight into systems that involve coupling between mechanics and other disciplines.