Browsing by Subject "grain boundary"
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Item Imperfect" Block Polymers: Effects of Dispersity and Morphological Defects on Block Polymer Properties"(2020-07) Xu, HongyunThe chemically distinct segments of block polymers drive the formation of various microphase separated morphologies such as lamellae, cylinders packed on a hexagonal lattice and double gyroid. Previous experimental and computational studies have explored the phase behaviors of model, near-perfect block polymers with narrow molar mass distributions of the constituent blocks. However, recent reports have shown that broad block dispersity notably alters the thermodynamic phase behavior of block polymers and have explored the efficacy of using polymer dispersity to enhance performance in various applications such as nanolithography and thermoplastic elastomers. Lithium salt-doped polyether-based block polymers present an attractive system to combine desirable mechanical properties with high ionic conductivities to enable design of safe, high performance electrolytes in solid-state lithium batteries, while the effect of block dispersity in polymer electrolytes has not been studied. In this thesis, we investigate how Li salt-doped block polymer phase behavior and ion conductivities are affected by increased dispersity in the conductive poly(ethylene oxide) (PEO) domains of poly(styrene-block-ethylene oxide-block-styrene) (bSOS) polymers. We blend a series of bSOS triblock polymers with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and construct the corresponding morphology portraits as a function of Li+ loading using small-angle X-ray scattering analyses. We investigate the shift of the lamellar phase boundaries and dilation of the domain spacing caused by the increased O block dispersity. We observe that bSOS affords higher ionic conductivities than the narrow dispersity diblock control samples, as characterized by electrochemical impedance spectroscopy. We rationalize this observation based on a decreased extent of long-range ordering of the lamellar phase in the salt-doped bSOS that reduces ion diffusion pathway torturosity. In other words, the PEO domain continuity is preserved across morphological defects such as grain boundaries. We further explore the idea of continuity through grain boundaries by successfully fabricating mechanically stable nanoporous materials by etching away the matrix domain from a cylindrical phase. We utilize polystyrene/polylactide and polyisoprene/polylactide block polymers to establish the versatility of this matrix etching method, and find that highly interconnected cylinders are present in both cases. We further assess the continuity and size selectivity of the fibril network composed of cross-linked polyisoprene through a permeation experiment as a proof-of concept for future applications as ultrafiltration membranes. This thesis provides new insights into using the ‘imperfections’ in the block polymer architecture and microphase separated morphologies to realize real-world applications as Li-ion battery electrolytes and separation membranes.Item Influence of Grain Boundaries and their Composition on the Deformation Strength of High-purity, Synthetic Forsterite(2016-06) Dillman, AmandaGrain boundaries are an important feature of the mantle. With recent studies suggesting the majority of the upper mantle deforms by grain boundary sliding (Hirth and Kohlstedt, 2003; Hansen et al., 2013), understanding the role grain boundaries play is key. As grain boundary sliding always requires an accommodation mechanism, directly determining the contribution of grain boundary sliding to total strain on a sample is important for modeling deformation in the mantle. Altering grain boundary composition can change the structure and viscosity of the boundary. Understanding the effects of grain boundary composition is necessary for comparing data sets of different olivine as well as for accurately extrapolating experimental data to represent the mantle. In Chapter 2, uniaxial deformation experiments on high-purity synthetic forsterite at high temperature and ambient pressure are used to characterize the contribution of grain boundary sliding to strain in diffusion creep. Experiments were conducted in a one-atmosphere deformation rig, which allowed the polished surfaces of the samples to be analyzed with atomic force microscopy. The high temperature necessary for deformation enabled a great deal of thermal grooving, which can dramatically alter the topography of an initially polished surface. A methodology was developed to correct for the effect of thermal grooving and determine the amount of grain boundary sliding as a function of grain size and stress. A comparison is also made between two popular methods for determining grain size: the line intercept method and the equivalent area circle method. The line intercept method consistently produces larger grain sizes than the equivalent area circle method. In Chapter 3, triaxial compression experiments on forsterite are used to determine the effect of grain boundary chemistry on deformation strength. High-purity synthetic forsterite was doped with either Ca or Pr and then deformed at high temperature and a confining pressure of 300 MPa. Both impurities made the sample stronger, and the presence of Ca induced abnormal grain growth. This supports the theory that grain boundary composition can have a large effect on deformation strength. The hypothesis that the difference in strength between natural and high-purity synthetic olivines is due to the difference in grain boundary composition is not supported by these results. In Chapter 4, the results of experiments on forsterite with a small amount of melt are detailed. Two methods of adding melt were used. The first involved adding Pr to forsterite in concentrations greater than can dissolve in the grain boundary, which induced melting as well as enhanced grain growth. Even with a grain size over an order of magnitude greater than the melt-free sample, the melt bearing samples were weaker than the melt free samples. The second method involved synthesizing forsterite with a composition in equilibrium with a synthesized anorthitic melt. Samples were created with melt fractions < 0.01 and then deformed at a temperature of 1300°C and a confining pressure of 300 MPa. The drop in viscosity at very small melt fractions predicted by Takei and Holtzman (2009) was observed, although the drop occurred over a shorter change in melt fraction than predicted. This result suggests that, at the onset of melting, the mantle will become significantly weaker. In addition, the presence of as little as 0.1% melt in a high purity, synthetic olivine sample brings its deformation strength into agreement with natural samples. This suggests that deformation experiments on natural samples are never entirely melt free. The results of this study establish the role of grain boundary chemistry on polycrystalline deformation. The presence of large cations in olivine grain boundaries makes diffusion creep slower, which limits the regions of the mantle predicted to deform in diffusion creep and expands the regions predicted to deform in a dislocation accommodated grain boundary sliding or dislocation creep. At the onset of melting, this changes, as the melt would remove the impurities from the grain boundaries. Future studies on different types of impurities will allow the grain boundaries of natural olivines to be more accurately modeled.