Person: Pharr, Matt Mathews
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Pharr
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Matt Mathews
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Pharr, Matt Mathews
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Publication Diffusion, Deformation, and Damage in Lithium-Ion Batteries and Microelectronics(2014-06-06) Pharr, Matt Mathews; Suo, Zhigang; Vlassak, Joost J.; Suo, Zhigang; Vlassak, Joost; Rice, James; Ramanathan, ShriramThis 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.Publication Variation of stress with charging rate due to strain-rate sensitivity of silicon electrodes of Li-ion batteries(Elsevier BV, 2014) Pharr, Matt Mathews; Suo, Zhigang; Vlassak, JoostSilicon is a promising anode material for lithium-ion batteries due to its enormous theoretical energy density. Fracture during electrochemical cycling has limited the practical viability of silicon electrodes, but recent studies indicate that fracture can be prevented by taking advantage of lithiation-induced plasticity. In this paper, we provide experimental insight into the nature of plasticity in amorphous LixSi thin films. To do so, we vary the rate of lithiation of amorphous silicon thin films and simultaneously measure stresses. An increase in the rate of lithiation results in a corresponding increase in the flow stress. These observations indicate that rate-sensitive plasticity occurs in a-LixSi electrodes at room temperature and at charging rates typically used in lithium-ion batteries. Using a simple mechanical model, we extract material parameters from our experiments, finding a good fit to a power law relationship between the plastic strain rate and the stress. These observations provide insight into the unusual ability of a-LixSi to flow plastically, but fracture in a brittle manner. Moreover, the results have direct ramifications concerning the rate-capabilities of silicon electrodes: faster charging rates (i.e., strain rates) result in larger stresses and hence larger driving forces for fracture.Publication Measurements of the Fracture Energy of Lithiated Silicon Electrodes of Li-Ion Batteries(American Chemical Society (ACS), 2013) Pharr, Matt Mathews; Suo, Zhigang; Vlassak, JoostWe have measured the fracture energy of lithiated silicon thin-film electrodes as a function of lithium concentration. To this end, we have constructed an electrochemical cell capable of testing multiple thin-film electrodes in parallel. The stress in the electrodes is measured during electrochemical cycling by the substrate curvature technique. The electrodes are disconnected one by one after delithiating to various states of charge, that is, to various concentrations of lithium. The electrodes are then examined by optical microscopy to determine when cracks first form. All of the observed cracks appear brittle in nature. By determining the condition for crack initiation, the fracture energy is calculated using an analysis from fracture mechanics. In the same set of experiments, the fracture energy at a second state of charge (at small concentrations of lithium) is measured by determining the maximum value of the stress during delithiation. The fracture energy was determined to be \(\Gamma = 8.5 ± 4.3\) \(J/m^2\) at small concentrations of lithium (∼Li_{0.7}Si) and have bounds of \(\Gamma = 5.4 ± 2.2\) \(J/m^2\) to \(\Gamma = 6.9 ± 1.9\) \(J/m^2\) at larger concentrations of lithium (∼Li_{2.8}Si). These values indicate that the fracture energy of lithiated silicon is similar to that of pure silicon and is essentially independent of the conncentration of lithium. Thus, lithiated silicon demonstrates a unique ability to flow plastically and fracture in a brittle manner.