Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Includes bibliographical references (leaves 87-89). / by David C. Grundy. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1985. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Bibliography: leaves 104-107. / by Robert Gordon Dixon, Jr. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2003. / Includes bibliographical references (leaves 145-155). / A continuum thermodynamic model is developed for the treatment of interfaces in materials. This phase-field model includes energetic contributions from chemical, structural and electrostatic effects. A small parameter set is introduced in the model. These parameters should be adjusted based on the relation of equilibrated phase-field structures to atomistic simulations and experimental observations. To compare the continuum fields to discrete data sets, coarse-graining methods are proposed. Two methods of measuring local atomic structure are developed, one based on Voronoi tessellations and the other based on bond-angle distribution functions. A coarse-graining method based on volume averaging over Voronoi tessellations is employed to continuize both structure and composition information. These fields give insight into the local atomic environment that is necessary for devising continuum models. The stability of intergranular films in CaO-doped Si₃N₄ is examined. A modified associate model that incorporates bulk-thermodynamic Gibbs free energy functions and treats non-bulk equilibrium compositions is developed. Kinetic equations are derived to solve for the equilibrium fields. While the simulations do not reach equilibrium, some trends can be observed. Locally charged domains are kinetically stable near the film. Non-uniform calcium distributions in the doped films indicate that calcium is preferentially located near the film-grain interface. The structural width of the film can be tuned by altering appropriate model parameters. / (cont.) Future avenues of research suggested by this thesis include: developing algorithms for measuring local structure and orientation in multi-component systems and devising atomistic simulations of intergranular pockets that couple experimental information with interatomic potentials. / by Catherine M. Bishop. / Ph.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 (pages 188-191). / Generally, thin films are away from equilibrium in the as-deposited state and will agglomerate, or dewet, to form islands given sufficient atomic mobility. This dewetting process can occur in the solid state, well below the films' melting temperature. Dewetting initiates from film-substrate- vapor three-phase boundaries, which can form naturally via natural hole formation in the continuous film or be intentionally made via pre-patterning of the film. These boundaries, or film edges, then retract via capillarity-driven mass transport. As the edges retract, many phenomena occur, making the dewetting morphology complex. Dewetting of thin films has traditionally been considered undesirable in the microelectronics industry, but more recent studies utilized dewetting in many applications, including sensors, solar cells, fuel cells, and catalysts for nanowire and nanotube growth. Solid-state dewetting of single crystal films leads to a very regular morphology, due to anisotropy in surface energy and surface self-diffusivity. When such films are templated by pre patterning, regular dewetting patterns, much smaller than the original templates, can be made. Thus, templated dewetting can be a potential method for generating complex nanostructures with sub-lithographic length scales. However, controlling such dewetting patterns necessitates in depth quantitative understanding of anisotropic solid-state dewetting. The research described in this thesis focused on detailed analysis of individual dewetting phenomenologies, using single crystal Ni films as a model system. The rate of capillarity-driven edge retraction was found to depend strongly on the in-plane orientation of the edge and the ambient condition. It was determined that the surface structure of the top facet under different ambient conditions are closely linked to the differences in the edge retraction rates. As the edge retracts, a thickening rim forms due to mass accumulation at the edge. Also, a valley can form ahead of the rim under certain conditions, and this valley can pinch off the rim, resulting in wireline structures. This pinch-off phenomenon was also found to be strongly dependent on the inplane orientation of the retracting edge. It was determined that pinch-off occurs more easily when the additional surface energy cost associated with initiation of valley formation is smaller. The retracting rim itself can also become unstable and can form finger-like structures similar to the cellular structures that often form during solidification. It was found that mass is rejected from the finger tip, so that the rim at the tip maintains a constant height and the fingered front moves at a constant rate. A steady-state model for the motion of a fingered front was developed and found to be in agreement with experiments. This model accounts for the lateral rejection of mass to form wire-like structures between the fingers, and shows that the finger spacing is determined by the velocity of the finger tips relative to the diffusivity governing mass rejection. Both pinch-off and the fingering instability result in wire-like structures, and these structures can undergo a Rayleigh-like instability in later stages, in which they break up into smaller particles. The characteristic spacing of the de wetted particles was also found to be strongly dependent on the in-plane crystallographic orientation of the wire-like structures. It was further found that for a fixed cross-sectional area, the total surface energy of the wire-like structure determines the particle spacing. Lastly, it was found that while natural holes that form in single crystal films initially have polygonal shapes, as their growth slows, corners often begin to retract faster than the straight edges, in a way similar to the fingers on a fingered front. A computational model was developed for such a corner instability, and the balance between the thinning of the corner tip due to tip lengthening and the thickening of the tip due to mass accumulation during retraction was found to determine the condition under which the corner retracts at a constant rate. / by Gye Hyun Kim. / Ph. D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999. / Also issued in pages. / Includes bibliographical references (leaves 143-152). / Er-doped silicon (Si:Er) is a promising light emitting material for silicon microphotonics. A study of Si:Er excitation/de-excitation mechanisms and luminescence enchancement is presented in this thesis. A model based on impurity Auger and nonradiative nmltiphonon transitions (NRl\·IPT) is shown to describe the temperature quenching of the photoluminescence (PL) intensity from 4K to 300K This model asserts that the nonradiative Auger process is mainly responsible for the temperature quenching below lOOK, and NRMPT backtransfer process is mainly responsible for the temperature quenching above lOOK. Junction photocufrei1t · spectmscopy (JPCS) measurements confirmed the existence of a backtransfer mechanism that grows with temperature in accordance to the model. In order to circumvent the onset of nonradiative transitions at higher temperatures, spontaneous emission enhancement in nrnltilayer Si/Si02 microcavities was explored as a means to increase the PL intensity. Because multilayer microcavity structures cannot be constructed using single crystal silicon, Er-doped polysilicon (poly-Si:Er) was developed as a light emitting material for these microcavities. The poly-Si:Er material exhibited a luminescence very similar to that of Er in single crystal silicon. By crystallizing poly-Si:Er from amorphous material and performing a post-anneal hydrogenation, a reasonably high PL intensity, which was limited by the excitation power, was attained. Microacavities with poly-Si:Er were fabricated and measured for the first time. Cavity quality factors of -60-300 were measured, and an Er enhancement of -20x was observed. A -lOx enhancement of a small background emission from the polysilicon was also observed. The observed enhancement factors match well with computed enhancement factors derived from electric field intensity distribution within the microcavity structure. Exploratory work in optical gain from Si:Er waveguides and vertically coupled ring resonntors was conducted. A fiber coupling technique for low temperature waveguide transmission experiments was developed for the gain experiments. The transmission spectrum of a 3-cm long waveguide was measured at temperatures down to 125K. Because the temperature could not be lowered without debonding the fiber, a net gain could not be observed in this particular waveguide. The application of stimulated emission in Si:Er devices is analyzed and discussed. / by Thomas Duhwa Chen. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004. / "June 2004." / Includes bibliographical references. / Temporal and spatial control over the delivery of therapeutic compounds is an important, fertile, and rapidly advancing field of study in medicine. This work describes the advancement of a new technology of drug delivery from a benchtop prototype releasing tracer molecules to an implantable device for initial animal studies. The improved MEMS (micro-electro-mechanical systems) device was used for the subcutaneous delivery of both tracer molecules (fluorescein and mannitol) and a chemotherapeutic agent (carmustine) in rats. Both temporal and spatial profiles of the tracer molecules were established; only the temporal kinetics of the carmustine were studied. The MEMS drug delivery device is based on a silicon substrate into which microreservoirs are etched. Each reservoir contains an individual dosage of drug and is independently addressable. The microreservoirs are covered with gold membranes which act as anodes. The application of an anodic voltage, in an aqueous solution containing chloride ions, electrochemically transforms gold into gold chloride which is readily soluble in water. This device allows the delivery of both solid and liquid drugs of a wide variety of compositions. / (cont.) It is important to study the biocompatibility of the device activation process as well as that of the component materials since the activation of the MEMS drug delivery device depends on an electrochemical reaction. Other researchers have studied the biological response to gold, silicon, silicon dioxide and silicon nitride; however, few studies of the effect of voltage applications, particularly of gold electrochemistry, have ever been performed. The effects of both electrochemical dissolution of a macroscopic gold film electrode and the repeated electrochemical activation of gold MEMS microelectrodes on the immune response and fibrous capsule formation were observed, as well as the effect of long term implantation on gold electrochemistry. / by Rebecca Scheidt Shawgo. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering; and, (S.B.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 48-52). / Microstructural and textural observations have been conducted on synthetic calcite with 20 wt% quartz deformed in simple shear using transmission electron microscopy and selected-area diffraction. The marbles were deformed at 873, 973, and 1073 K at a stress of 305, 222, and 127 MPa, respectively, and a strain rate of 10 -4 s -1. The microstructure, shape-preferred orientation (SPO), grain aspect ratios, lattice-preferred orientation (LPO), dislocation densities, and grain sizes were compared to the results of other studies on similar carbonates deformed in triaxial loading, torsion, and simple shear. Microstructures are consistent with other marbles at similar temperatures and stresses, with the only major difference in grain size. The SPO and aspect ratios differ from the theoretical calculations, but are consistent with other marbles. This SPO and aspect ratio is consistent with grains behaving as high-viscosity particles with low-viscosity boundaries. Loading conditions appear to affect the strain at which recrystallization starts, with evidence for new grains at a strain of 3 in this study, compared to minimum strains of at least 4 for others. Dislocation densities are 3.5 x 10 13 m -2 , 8 x 10 13 m -2, and 1.3 x 1014 m -2 for the samples at 873, 973, and 1073 K, respectively, and when inserted into a paleopiezometer, the predicted stresses are 347, 257, and 156 MPa, respectively, which is in good agreement with the applied conditions. / (cont.) Among the recrystallized grain size paleopiezometers, rotation recrystallization is a much better match to the data than migration recrystallization, which is consistent with the evidence from SPO and aspect ratios for low viscosity boundaries. Overall, some evidence emerges for material strength differing among different loading conditions, likely caused by differences in LPO. Future studies on the effect of loading conditions on strength are recommended, as this study is very small and only serves as a preliminary investigation. / by Rachel V. Zucker. / S.B.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 47-48). / Local coordination and topology of ions determine several important properties of materials, including electronic structure, migration barrier, and diffusivity. In this thesis, we employ the Materials Project Database to investigate the coordination preferences of cations and topology of coordination polyhedra in oxides. We calculate the coordination environment preferences of several common cations in oxides, identifying lithium, sodium, calcium, and magnesium ion's preferred coordination numbers are 4- fold/6-fold, 6-fold, 6-fold, and 6-/4-fold coordination respectively. We also develop a method to quantify the connectivity between two polyhedra and determine whether they are point-sharing, edge-sharing, or face-sharing. We find that 4-fold coordinated lithium polyhedra mainly point-share while the 6-fold coordinated lithium polyhedra connectivites are face-sharing. We then build a tool to identify and insert "empty polyhedra" (i.e. coordination polyhedra which are bounded by ions but contain no central ion) which can help to provide a better descriptor of the structure topology. We also find that most connections with lithium polyhedra are with empty polyhedra and that in a connected set of two lithium polyhedra and one empty polyhedron, the coordination polyhedra tend to be either 6-4-6 or 4-6-4 with the empty polyhedron in the center. Finally, we utilize the database to evaluate Pauling's first and second rules, which are guidelines for current understanding of coordination and topology, and observe that the rules are generally accurate only within a 30% error margin. / by Tina Chen. / S.B.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (p. 50). / by Clifford T. Tanaka. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (leaves 119-124). / by Arvind Sundarrajan. / Ph.D.