Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1997. / Includes bibliographical references. / by Darren Thoams Castro. / Sc.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references. / Block copolymers microphase separate to form periodic patterns with period of a few nm and above without the need for lithographic guidance. These self-assembled nanostructures have a variety of bulk geometries (alternating lamellae, gyroids, cylinder or sphere arrays, tiling patterns, core-shell structures) depending on the molecular architecture of the polymer and the volume fraction of its blocks. And in thin films, surface interaction and commensurability effect influence the self-assembly and result in more diverse morphologies including hexagonal-packed perforated lamellae, square array of holes. The progress of self-assembly can be tracked in situ using Grazing Incidence Small Angle X-ray Scattering, and the annealed morphology can be revealed in 3D using TEM tomography. Moreover, non-bulk morphologies can be produced, the ordering of the microdomains can be improved and their locations directed using various templates and processing strategies. The blocks can themselves constitute a functional material, such as a photonic crystal, or they can be used as a mask to pattern other functional materials, functionalized directly by various chemical approaches, or used as a scaffold to assemble nanoparticles or other nanostructures. Block copolymers therefore offer tremendous flexibility in creating nanostructured materials with a range of applications in microelectronics, photovoltaics, filtration membranes and other devices. / by Wubin Bai. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 183-204). / Energy storage, especially through electrochemical mechanisms such as batteries, is crucial for sustaining the ever-increasing energy needs of the future in a fossil-free manner. While the current industrial workhorse, lithium ion batteries, has shown tremendous improvements in energy and power-densities, via both materials selection and engineering advancements, the lithium ion technology is approaching the fundamental limits of what more can be achieved. Multi-valent (MV) chemistry, that pairs an energy-dense MV metal anode (such as Mg) with a high voltage cathode has the potential to surpass the energy densities achieved by current Li-ion batteries, along with improved safety and lower costs. However, moving into newer chemistries leads to newer challenges, such as developing cathodes that can reversibly intercalate Mg at high voltages, high rates and high capacities, apart from designing electrolytes that remain stable against both the electrodes. In this thesis, I focus on the challenge of MV cathode design and I explore the thermodynamic and kinetic properties of candidate oxide cathode materials for MV batteries, including polymorphs of V₂O₅, spinel-Mn₂O₄ and layered-Mg₂Mo₃O₈, using first-principles based methods. The undercurrent of the thesis is to obtain design principles that will aid in both optimization of existing cathodes and in the identification of new candidate materials. Utilizing a diverse set of tools, I benchmark the calculated properties, including average voltage curves, lattice parameters, cation-anion decorations in structures and activation barriers for Mg diffusion, to experimental observations, where possible. Finally, this thesis should serve as a guide for other computational-theorists and experimentalists, in the search for an energy-dense MV cathode that will in turn aid in the realization of a high energy density MV battery. / by Sai Gautam Gopalakrishnan. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged student-submitted from PDF version of thesis. / Includes bibliographical references (pages 143-157). / Photoelectrochemical (PEC) water splitting has been suggested as a promising techinique for large-scale hydrogen fuel production. In particular, spontaneous photocatalytic overall water splitting on self-standing particles in water without external driving potential has been highlighted as a clean and economical energy generation method for the future. Among various photocatalytic materials, some cobalt-based materials including CoP, Co₂P, Co(OH)₂, CoO, have attained major interest because they exhibit improved catalytic activity for hydrogen evolution in the form of nanoparticles, unlike most cobalt-based materials which have been assessed as water oxidizing catalysts in the past decade. CoO nanoparticles have been observed to photocatalytically split water into H₂ and O₂ at room temperature without an externally applied potential or co-catalyst, with high photo-catalytic efficiency (solar-to-hydrogen efficiency of ~5%) which hits the record among single-material self-standing photocatalysts. The photocatalytic activity of CoO nanoparticles was experimentally shown to stem from the optimal conduction and valence band edge positions (Ec and Ev) relative to water reduction and oxidation potential levels (H+/H₂ and H₂O/O₂), such that the Ec and EV span the water redox potentials. The overall water splitting is not expected from CoO micropowder or bulk CoO because they have band edges far below the H+/H2 level, which are not optimal for overall water splitting. However, the origin of the shift in the band edges due to decrease in particle size (from bulk or micropowder to nanoparticle) was unknown. Moreover, the mechanism by which H₂ and O₂ simultaneously and spontaneously evolve on the nanoparticles, as well as how the CoO nanoparticles could exhibit a high photocatalytic efficiency even without a co-catalyst or an external driving potential have remained unanswered. In this work, we use first-principles density functional theory (DFT) calculations to explore thermodynamically stable surface configurations of CoO in an aqueous environment in which photocatalytic water splitting occurs. We also calculate the Ec and Ev of CoO surfaces relative to water redox potentials, showing that the band edge positions are sensitive to surface chemistry which is determined by surface orientation, adsorbates, and stoichiometry, and thus growth conditions and operating environment. In particular, we predict that CoO nanoparticles have fully hydroxylated CoO(111) facets (OH*-CoO(111)), with band edges spanning the water redox potentials, while larger CoO particles (such as CoO micropowders) have a full monolayer of hydrogen on the CoO(111) facets, with a band alignment that favors water oxidation but not water reduction. From these calculations, we demonstrate that explicit inclusion of liquid water is crucial for accurately predicting the band edge positions, and thus photocatalytic behavior of CoO in an aqueous solution. In order to find the origin of the high efficiency and spontaneous overall water splitting without an external bias or a co-catalyst, we also elucidate the mechanisms for charge separation and H₂ and O₂ evolution on CoO nanoparticles under illumination in an aqueous solution. We demonstrate that electrons are driven to CoO(100) facets and holes are driven to OH*-CoO(111) facets as a result of a built-in potential arising from the very different potential levels of the two facets. We show that H₂ evolution preferentially occurs on the CoO(100) facets, while O2 evolves on the OH*-CoO(111) surfaces, based on our new criteria. Importantly, we suggest that the conventional criterion for determining the feasibility of H₂ or O₂ generation from water splitting - i.e., EC < H+/H₂ level or Ev > H₂O/O₂ level - is insufficient. Instead, we suggest that a more appropriate set of criteria is whether the photo-excited electrons and holes have sufficient energy to overcome the kinetic barrier for the H₂ and O₂ evolution reaction, respectively, on the relevant surface facet. This work explains why and how photocatalytic overall water splitting has been observed only on CoO nanoparticles. Our understanding of the overall water splitting mechanism on CoO nanoparticles provides a general explanation of experimentally observed overall water splitting phenomena on a variety of self-standing photocatalysts as well as a new approach for screening novel photocatalytic materials for efficient water splitting and other reactions. / by Kyoung-Won Park. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Includes bibliographical references (p. 139-145). / Most materials produced by Nature and by human beings are heterogeneous. They contain domains of different states, structures, compositions, or material phases. How these different domains are distributed in space, or in other words, how they connect to one another, determines their macroscopic properties to a large degree, making the simple rule-of-mixtures ineffective in most cases. This thesis studies the macroscopic effective diffusion, diffusional creep, and elastic properties of heterogeneous grain boundary networks and composite solids, both theoretically and numerically, and explores the microstructure-property correlations focusing on the effects of microstructural connectivity (topology). We have found that the effects of connectivity can be effectively captured by a percolation threshold, a case-specific volume fraction at which the macroscopic effective property undergoes a critical transition, and a set of critical scaling exponents, which also reflect the universality class that the property belongs to. Using these percolation quantities together with the generalized effective medium theory, we are able to directly predict the effective diffusivity and effective diffusional creep viscosity of heterogeneous grain boundary networks to a fairly accurate degree. Diffusion in composite solids exhibits different percolation threshold and scaling behaviors due to interconnectivity at both edges and corners. Continuum elasticity suffers from this complexity as well, in addition to the complicating factor that each phase is always characterized by several independent elastic constants. These issues are each addressed in detail. In addition to studying all the above properties for a random distribution of grain boundaries or phases, we have also studied the effects of correlations in spatial distributions. / (cont.) This topic is especially important in materials science, because virtually no materials exhibit random phase distributions. We have examined the percolation of effective properties for correlated microstructures spanning between the random distribution and the perfectly periodic distribution. An important result of this work is new understanding about what correlations may be considered small, or inconsequential, to the percolation scaling behavior, and which are large or long-range, and lead to a loss of universality. Finally, a rigorous, and easy-to-use, analytical homogenization method is developed for periodic composite materials. / by Ying Chen. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 115-126). / The exact structure of layered inorganic nanotubes is difficult to determine, but this information is vital to using atomistic calculations to predict nanotube properties. A multi-walled nanotube with a circular cross section will have either a mostly incoherent interface or a large amount of tensile strain to accommodate a coherent interface, but a polygonal cross section could result in a coherent interface with considerably less strain. An energy component model is parameterized with atomistic calculations to compare nanotubes with a circular and polygonal cross section. The model shows that for TiS2 nanotubes with some chiralities the radius at which a polygonal shape becomes energetically favorable is approximately 15 A. Due to the higher strain energy and lower interfacial energy the critical radius for polygonal formation of MoS2 nanotubes is 36 A. Both of these values are below the typical radius of TiS2 and MoS 2 nanotubes seen experimentally, indicating that for certain chiralities polygonal nanotubes should form. We also investigate the potential of inorganic nanotubes as energy storage materials. First principles calculations on curved surfaces and distorted slabs are used to analyze the effect of curvature and stacking on voltage and diffusion properties. The effect is qualitatively and quantitatively dependent on the material and structure. The Li voltage on the surface of TiS2 nanotubes decreases with a decreasing radius whether lithium is inside or outside of the nanotube. On the surface of MoS2, the voltage decreases with decreasing radius when Li is inside the tube, but increases with decreasing radius when Li is outside the tube. / (cont.) The activation barrier for lithium diffusion increases with decreasing radius whether Li is outside or inside the nanotube while the barrier decreases in either case for MoS 2. When the stacking is disordered the lithium voltage and activation barrier between TiS2 layers decreases, although the decrease in voltage is not as large as the decrease in activation barrier because the stable lithium site changes from the octahedral site to the tetrahedral site at some stacking arrangements. / by Kevin Tibbetts. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999. / Includes bibliographical references (leaves 89-90). / by Julien Guiu. / S.M.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, June 2015. / Cataloged from PDF version of thesis. "May 2015." / Includes bibliographical references (pages 32-33). / Lithium-polysulfide flow batteries, which utilize the high solubility of lithium polysulfide in non-aqueous electrolytes to enable flowable electrodes, have high theoretical energy density and low raw materials cost. To achieve greater electrode-level energy density, higher sulfur concentrations are needed. In a given electrolyte system, sulfur charge storage capacity (e.g. mAh/g sulfur) decreases dramatically with increasing sulfur concentration at a fixed C-rate, which corresponds to higher current output in higher concentration system. Understanding the limiting factors that undercut the rate capacity is crucial to enhancing the performance of high energy density systems. In particular, we systematically investigate the ionic conductivity and exchange current density at the electrode surface with lithium polysulfide solutions of varying concentration and in differing solvents which solvent molecules of different sizes. Ionic conductivities are measured using a commercially available conductivity probe, while exchange current densities are measured using both impedance spectroscopy and galvanostatic polarization using glassy carbon working electrodes. The electrolyte solvent is found to dramatically affect the solution ionic conductivity and exchange current density. In the concentration range of interest (1-8 M [S]), the ionic conductivity monotonically decreases with increasing sulfur concentration while exchange current density shows a more complicated response in a given solvent system. Between solvent systems, we observed a five-fold increase in ionic conductivity, and a more than 15-fold enhancement in exchange current density. The conductivity and current density results are used to interpret the rate capability of suspension-based cells using lithium-polysulfide electrolyte and carbon black as the cathode with different solvents. With the improvement in kinetics parameters, we also observed better rate capability in solvent. We also study non-carbonaceous electrode materials to understand how the electrode material can affect exchange current density and thus cell capacity. Indium tin oxide electrode shows lower exchange current density then glassy carbon electrode in preliminary results. / by Menghsuan Sam Pan. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1986. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE / Bibliography: leaves 121-126. / by Joey Lou Mead. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. / Includes bibliographical references (leaves 77-79). / Software was developed for solving Maxwell's equations using the finite-difference time-domain method, and was used to study 2D and 3D dielectric composites. The software was written from the ground up to be fast, extensible, and generalized for solving any finite difference problem. The code supports parallelization, allowing solutions to be obtained quickly using a beowulf cluster. An extension to the basic FDTD plane wave source was derived, allowing for the creation of angled, periodic, unidirectional plane waves on a square grid. 1D photonic crystal stacks were arranged in a square array and it was discovered that sizeable bandgaps for 2D and 3D geometries appear along the principle axes for different polarizations of the structure. Furthermore, bandgaps in different directions and polarizations could be made to overlap for reasonably large frequency ranges. The structure show promise for use as a low-threshold lasing and may be optimized to produce a complete photonic bandgap. / by Daniel A. Cogswell. / S.M.