Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Predicting the conditions in which a compound adopts a metastable structure when it crystallizes out of solution is an unsolved and fundamental problem in materials synthesis, and one which if understood and harnessed, could enable the rational design of synthesis pathways towards or away from metastable structures. Although metastable phases are ubiquitous in both nature and technology, only a heuristic understanding of their underlying thermodynamics and formation mechanisms exists. In this thesis, we aim to address two important outstanding questions regarding the fundamental nature of metastability: Which metastable phases can form? Under which conditions will they form? We will employ a variety of computational and theoretical approaches to elucidate quantitative insights to these two questions. To better predict which metastable materials can be made, we first seek to understand the metastable materials that have been made. We data-mine the Materials Project, a high-throughput database of DFT-calculated energetics of ICSD structures, to explicitly quantify the energy scale of metastability for 29,902 inorganic crystalline phases. We reveal the influence of chemistry and composition on the accessible range of crystalline metastability, and identify motifs characteristic of highly metastable compounds. We further assert that not all low-energy metastable materials can necessarily be made, and argue for a concept of "remnant metastability" - that observable metastable phases are remnants of thermodynamic conditions where they were once the lowest free-energy phase. Recently, exciting thermochemistry experiments have demonstrated that for many compounds, as metastability of a phase increases, its surface energy decreases. This effect is significant enough to trigger a reversal of relative phase stability in nanoparticles. Because nucleation and growth starts at the nanoscale, we hypothesize that the direct precipitation of metastable phases during crystallization may be 'remnant metastability' of size-dependent nanoscale phase stability. We develop algorithms for the automated, efficient, and high-throughput calculation of surface energies via DFT. We combine these algorithms with prior theoretical frameworks to predict solid-aqueous equilibria, enabling the calculation of nucleation barriers of competing polymorphs as a function of solution chemistry, thereby predicting the solution conditions governing polymorph selection. We apply this approach to resolve the long-standing 'Calcite-Aragonite Problem' - the observation that calcium carbonate precipitates as the metastable aragonite polymorph in marine environments, rather than the stable phase calcite - which is of tremendous relevance to biomineralization, carbon sequestration, paleogeochemistry, and the vulnerability of marine life to ocean acidification. We identify a direct relationship between the calcite surface energy and solution Mg/Ca ion concentrations, showing that the calcite nucleation barrier surpasses that of metastable aragonite in solutions with Mg/Ca ratios consistent with modern seawater, allowing aragonite to dominate the kinetics of nucleation. Our ability to quantify how solution parameters distinguish between polymorphs marks an important step towards the ab initio prediction of materials synthesis pathways in solution. / by Wenhao Sun. / Ph. D.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004. / Includes bibliographical references (leaves 74-75). / (cont.) cost between these two processes was related mainly to their powder scrap rates, 15 % for the Pressing-TLI and 80% for the 3DP-TLI. The high scrap rate value of the 3DP process originates from the fact that powder is sieved before printing, eliminating the coarse and very fine particles. A possible option to decrease this value is to recycle or sell the extra powder, which will reduce the fabrication cost significantly. The model also shows that the main cost for both processes is the powder cost. TLI technical parameters such as heating and cooling rates were included in the model in order to predict the cost behavior when those are manipulated. Because the powder cost dominates the total fabrication cost, variations in the heating and cooling rates do not significantly affect the cost. / Tool steels are iron-based alloys that are melted and processed to develop characteristics useful in the working and shaping of other metals. Tools for such processes must withstand high loads without breaking and without undergoing excessive wear or deformation. Fabrication of direct tool steel parts with complex geometry is possible using Transient Liquid-Phase Infiltration (TLI) in conjunction with Three-Dimensional Printing (3DP). Tool steel parts can also be manufactured using TLI in combination with Cold Powder Methods such as Uniaxial Pressing. Both approaches produce a final part of homogenous composition without significant dimensional change, offering advantages over-traditional infiltration and full-density sintering [1]. Now that the expertise in the TLI has been developed in the MIT laboratories, an economic evaluation represents a complementary action for introducing TLI in the commercial market of Rapid Prototyping and Powder Metallurgy. A process-based cost model was developed to describe and measure the performance of the 3DP-TLI and Pressing-TLI combined processes. Operating conditions such as cycle time, material cost, labor cost, production volume and financial parameters were introduced into the model in order to calculate a total fabrication cost per part. Different charts showing cost behaviors and their relations with production volume, batch size, effectiveness in the powder utilization, and weight of the part are presented. The results show that the optimum point in the cost-production volume curve was located at 13,000 parts per year with a fabrication cost of $19.90 per part, for the Pressing-TLI case, and $61.73 per part for the 3DP-TLI alternative (based on a one-half pound D2 tool steel part). The difference in cost / by Juan Alfredo Barradas Martinez. / M.Eng.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010. / Includes bibliographical references (p. 151-158). / The use of compound semiconductor heterostructures to create electron confinement has enabled the highest frequency and lowest noise semiconductor electronics in existence. Modem technology uses two-dimensional electron gasses and there is considerable interest to explore one-dimensional electron confinement. This thesis develops the materials science toolkit needed to fabricate, characterize, and control the compositional, structural and electronic properties of core-shell GaAs/AlGaAs nanowires towards studying quasi-one-dimensional confinement and developing high mobility electronics First, nanowire growth kinetics were studied to optimize nanowire morphology. Variations in nanowire diameter were eliminated by understanding the role Ga adatom diffusion on sidewall deposition and vertical growth was enabled by understanding the importance of Ga and As mass-transport to nanowire nucleation. These results demonstrate that arrays of vertically-aligned GaAs nanowires can be produced. Then, the deposition of epitaxial AlGaAs shells on GaAs nanowires was demonstrated. By reducing the nanowire aerial density the stability of the nanowire geometry was maintained. A variety of analytical electron microscopy techniques confirmed the shell deposition to be uniform, epitaxial, defect-free, and nearly atomic sharp. These results demonstrate that core-shell nanowires possess a core-shell interface free of many of the imperfections that lithographically-defined nanowires possess. Finally, the adverse effect of the Au seed nanoparticle during n-type doping was identified and n-type doping was achieved via the removal of the Au nanoparticle prior to doping. A combination of energy dispersive X-ray spectroscopy, current-voltage, capacitance-voltage, and Kelvin probe force microscopy demonstrated that if the Au seed nanoparticle is present during the shell deposition, Au diffuses from the seed nanoparticle and creates a rectifying IV behavior. A process was presented to remove the Au nanoparticle prior to shell deposition and was shown to produce uniform n-type doping. The conductivity of GaAs/n-GaAs nanowires was calculated as a function of donor concentration and geometric factors taking into account the effects of Fermi level pinning. The control demonstrated over all of these parameters is sufficient enough for core-shell nanowires to be considered candidates for high mobility electronics. / by Michael Joseph Tambe. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from PDF version of thesis. Vita. / Includes bibliographical references (p. 163-174). / Semiconductor nanostructures exhibit distinct properties by virtue of nano-scale dimensionality, allowing for investigations of fundamental physics and the improvement of optoelectronic devices. Nanoscale morphological variations can drastically affect overall nanostructure properties because the investigation of nanostructure assemblies convolves nanoscale fluctuations to produce an averaged result. The investigation of individual nanostructures is thus paramount to a comprehensive analysis of nanomaterials. This thesis focuses on the study of individual GaAs, AlGaAs, and ZnO nanostructures to understand the influence of morphology on properties at the nanoscale. First, the diameter-dependent exciton-phonon coupling strengths of individual GaAs and AlGaAs nanowires were investigated by resonant micro-Raman spectroscopy near their direct bandgaps. The one-dimensional nanowire architecture was found to affect exciton lifetimes through an increase in surface state population relative to volume, resulting in Fröhlich coupling strengths stronger than any previously observed. Next, ZnO nanowire growth kinetics and mechanisms were found to evolve by altering precursor concentrations. The cathodoluminescence of nanowires grown by reaction-limited kinetics were quenched at the nanowire tips, likely due to point defects associated with the high Zn supersaturation required for reaction-limited growth. Further, cathodoluminescence was quenched in the vicinity of Au nanoparticles, which were found on nanowire sidewalls due to the transition in growth mechanism, caused by excited electron transfer from the ZnO conduction band to the Au Fermi level. Finally, ZnO nanowalls were grown by significantly increasing precursor flux and diffusion lengths over that of the ZnO nanowire growth. Nanowall growth began with the Au-assisted nucleation of nanowires, whose growth kinetics was a combination of Gibbs- Thomson-limited and diffusion-limited, followed by the domination of non-assisted film growth to form nanowalls. Nanoscale morphological variations, such as thickness variations and the presence of dislocations and Au nanoparticles, were directly correlated with nanoscale variations in optical properties. These investigations prove unequivocally that nanoscale morphological variations have profound consequences on optical properties on the nanoscale. Studies of individual nano-objects are therefore prerequisite to fully understanding, and eventually employing, these promising nanostructures. / by Megan Marie Brewster. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999. / Includes bibliographical references. / by Martin Lawrence Panchula. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Vita. / Includes bibliographical references (leaves 51-52). / by Zain Hasham. / M.S.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1986. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE / Bibliography: leaves 185-199. / by Norman J. Marchand. / Sc.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1997. / Includes bibliographical references (p. 107-111). / by Christopher William San Marchi. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (leaves 108-110). / by Todd S. Stefanik. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004. / Includes bibliographical references (p. 106-107). / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Three Dimensional Printing (3DP) is a process that enables the fabrication of geometrically complex parts directly from computer-aided design (CAD) models. The success of 3DP as an alternative manufacturing technology to bulk machining of materials for complex parts has been demonstrated. By proof of concept, 3DP has demonstrated the ability to create parts with Local Control of the Composition (LCC). LCC allows tailoring the material properties in regions of a part for functional purposes. In this work, LCC was studied and demonstrated by fabricating bimetallic bars consisting of two layers of Fe-Ni alloys with different composition and, hence, different thermal expansion properties; the coefficient of thermal expansion (CTE) of Fe-Ni system is sensitive to its composition. Two types of the binder/dopant slurries were made for making the LCC bars. One type consisted of dispersions of Fe₂O₃ particles in water, and the other consisted of dispersion of NiO in water. The LCC bars were successfully made by printing the Fe₂O₃/NiO slurries into Fe-30Ni base powders. After heat treatment to impart strength to the printed bars, the bars were successfully retrieved from unbound powders. The bars, then, were annealed at 1400 ⁰C for 2 hours for sintering and homogenization. The final composition of the base powders were changed accordingly. In the layers on which an Fe₂O₃ slurry was printed, the Fe composition of the layers increased on average to 72wt%. Similarly, the Ni composition of the Ni-enriched layers of the bars increased on average to 33wt%. The densification and local homogenization resulting from reduction and sintering treatments were not satisfactory. / (cont.) The major problem was presumably caused by the oxide residues. The presence of the oxide powders was evident from the microprobe measurement. The oxide residues caused the local compositions to be inhomogeneous. As a result, the compositional profiles showed considerable scatter. Moreover, the residues impeded the sintering rate of the bars; the sintering densities of the bars were as small as 78% of the theoretical density. The resulting bimetallic bars did exhibit bending deflection on uniform heating. However, the bending deflections were much smaller than expected. Evidently, the compositional profiles of the bars critically influence their thermal bending properties. The scatter in the compositional profiles resulted in local variations of CTE in the bars, which degraded the thermal bending properties. A linear elastic model that allows prediction of the deflection as a function of composition profile shows good agreement with the observed deflections in the bimetallic bars with LCC. / by Ratchatee Techapiesancharoenkij. / S.M.