Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005. / Includes bibliographical references (p. 57-59). / This study explores the possibility of extending the Ph.D. work of Yan Xu on copper-tin alloys (University of Pennsylvania, 1999) to model stress accelerated grain boundary oxidation (SAGBO) in INCOLOY alloy 908. The steady state model involves the embrittlement along the grain boundary due to oxygen diffusion with the concentration gradient and the stress field ahead of the crack tip as the driving forces. As oxygen forms brittle phases with the segregates in the grain boundary, it reduces the cohesive strength of the grain boundary and causes intergranular cracking in the material. The extensions to the original model include (1) dependence of oxygen concentration at crack tips on oxygen partial pressure and (2) a new creep law specific to nickel-based superalloys. While the steady state model correctly indicates temperature as one of three leading factors in SAGBO, it fails to capture the effects of the two remaining factors: applied loading and oxygen partial pressure. / by Chaiyod Soontrapa. / S.M.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005. / Includes bibliographical references (leaves 46-47). / Photovoltaic cells based on organic semiconducting materials have the potential to compete with the more mature crystalline and thin film based photovoltaic technologies in the future primarily due to the expectation of significantly reduced manufacturing costs. Stabilized power conversion efficiencies of organic photovoltaics are still well below those of crystalline Si photovoltaics, however a continuous, high throughput, roll-to-roll manufacturing process involving low temperature deposition or printing techniques is expected to partially account for their reduced efficiency and boost their commercial attractiveness. In addition, organic photovoltaics are flexible, light weight, and not fragile which makes them particularly suitable for transportation and portable electronics applications. Four organic photovoltaic technologies as well as the advantages and setbacks of each are described including Graetzel (wet) cells, blended photovoltaics, asymmetric tandem cells with hybrid planar-mixed molecular heterojunctions, and external antenna photovoltaics. A variety of start-up companies in various stages of commercialization of these technologies as well as the intellectual property related to these technologies is also discussed. / (cont.) A simplified cost model is presented to quantitatively estimate the possible cost reductions that continuous roll-to-roll production could entail for three different scenarios. Finally, a discussion of potential business strategies for licensing and commercializing organic photovoltaics is presented. / by Jonathan Barr. / M.Eng.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014. / 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 110-121). / Nature provides many examples of flexible armor systems which may serve as a source of inspiration for materials scientists and engineers. This thesis explores multiscale material and morphological design principles of the shells of chitons (Mollusca: Polyplacophora). The chiton shell consists of eight plates encircled by a structure known as a girdle, which is often covered by scales. The shell provides protection while permitting the flexibility needed to conform to rough substrata, as well as to roll defensively into ball-like conformation to cover its soft ventral side. In typical flat conformations, X-ray micro-computed tomography revealed that the shape and imbrication of the plates results in an overall continuous curvature and constant armor thickness. However, in defensive postures, vulnerable regions exist between the plates due to decreases in plate overlap. In the peripheral scale armor, gradients in the size and overlap of the scales control local levels of flexibility and protection. Scale armor prototypes inspired by the girdle scales were fabricated via multi-material 3D printing. Bending tests demonstrated that the stiffness of the bio-inspired scale armor is highly anisotropic. Remarkably, in certain species, a visual system is integrated within the shell plates. The system contains hundreds of lens eyes, which were found to be capable to forming images. Ray-trace simulations of individual eyes determined that they have a resolution of ~9°, which is consistent with prior behavioral experiments. Unlike the protein-based lenses of most animal eyes, the lenses of chitons, like their shells, are principally composed of aragonite. Chitons are able to tailor the local shape, crystallography, and interfaces of aragonite to achieve a multifunctional armor. However, the integration of lens eyes was found to locally decrease penetration resistance, suggesting a materials-level trade-off between protection and sensation. / by Matthew James Connors. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / Includes bibliographical references (p. 193-199). / Computational methods, including density functional theory and the cluster expansion formalism, are used to study materials for hydrogen storage. The storage of molecular hydrogen in the metal-organic framework with formula unit Zn40(02C-C6H6-COD3 is considered. It is predicted that hydrogen adsorbs at five sites near the metal-oxide cluster, in good agreement with recent experimental data. It is also shown that the metal-oxide cluster affects the electronic structure of the organic linker, qualitatively affecting the way in which hydrogen binds to the linker. Lithium imide (Li2NH), a material present in several systems being considered for atomic hydrogen storage, is extensively investigated. A variation of the cluster expansion formalism that accounts for continuous bond orientations is developed to search for the ground state structure of this material, and a structure with a calculated energy lower than any known is found. Two additional discrete cluster expansions are used to predict that the experimentally observed phase of lithium imide is metastable at temperatures below approximately 200 K and stabilized primarily by vibrational entropy at higher temperatures. A new structure for this low-temperature phase that agrees well with experimental data is proposed. A method to improve the predictive power of cluster expansions through the application of statistical learning theory is developed, as are related algorithms. The Bayesian approach to regularization is used to show that by taking advantage of the prior expectation that cluster expansions are local, the convergence and prediction properties of cluster expansions can be significantly improved. / (cont.) A variety of methods to generate cluster expansions are evaluated on three different binary systems. It is suggested that a good method to generate cluster expansions is to use a prior distribution that penalizes the ECI for larger clusters more and has few parameters. It is shown that the generalized cross-validation score can be an efficient and effective substitute for the leave-one-out cross-validation score when searching for a good set of parameters for the prior distribution. Finally it is shown that the Bayesian approach can also be used to improve the convergence and prediction properties of cluster expansions for surfaces, nanowires, nanoparticles, and certain defects. / by Timothy K. Mueller. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Includes bibliographical references (p. 45). / The application of rippled gold nanoparticles with bi-ligand surface morphology as a catalyst was tested. The hydrolysis of 2,4-dinitrophenyl acetate (DNPA) served as the catalytic reaction being analyzed and the bi-ligand composition used was 16-mercaptohexadecanoic acid to imidazole thiol (MHA to IT). The influence of temperature on catalytic reaction of DNPA with the MHA: IT system was tested for ligand rations of 2:1, 1:2, and 1:1 by monitoring the catalytic system on a UV-VIS spectrometer. Catalytic rate constants were obtained and found to increase with increased temperature. The measured catalytic rate constants were greatest overall for the 1:1 system, followed by the 1:2 system, and lastly the 2:1 system. The activation energy for each ligand-ratio system was measured and found to be 22.17 kJ/mol for the 2:1 system, 14.7 kJ/mol for the 1:2 system, and 26.52 for the 1:1 system. The 2:1 and 1:2 systems followed the trend of lower activation energy values for systems with faster rates; however the 1:1 system did not fit this trend as it resulted in the highest activation energy value as well as the fastest reaction rates. / by Cristina F. Stefanescu. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012. / 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. / Ligand-stabilized nanoparticles (NPs) assembled into long-range ordered arrays, also known as "nanoparticle supracrystals (NPSCs)", are expected to provide a powerful general platform for designing new types of solids. In particular, the NPs are themselves self-assembled structures consisting of a core and a self-assembled monolayer of ligand molecules surrounding it. The self-assembled structure of the NPs themselves determines the structure of the self-assembled supracrystals. Ligands are of special interest in this respect, because it is an important component for the NP system which play a major role in the design of self-assembly of the complex matter and also provide a powerful entry into the supracrystal engineering. The increasing ability to control the way in which ligand molecules associate gives means for the designed generation of supraparticle architectures in the self-assembly. In spite of this, elucidation of how the ligands play a role in affecting the structural behavior of NPSCs remains largely unrevealed. In this thesis, the effect of ligands for the two dimensional (2D) self-assembled NPSCs structure was investigated. The key materials advancement that enables this work is that we have been able to synthesize monodisperse gold NPs of same core size but different ligand molecules. Additionally, a new method for monolayer film processing has been developed to prepare the 2D NPSCs, based on a Langmuir assembly through successive compression cycles. Importantly, as there is little effect exhibited by solvent interactions in the NPs structure obtained from this approach, the corresponding NPs structural variation in this work is truly driven by the different ligand interactions in NPSCs. Specifically, we show that such ligand interactions have direct consequences on the ordering and symmetry of the assembled NPSCs structures. Here, we report on a set of NPSC arrays in which small changes in either the NP ligand environment or the ligand configuration geometry induce significant variations in the order parameters of the crystal. First, we show that the packing organization of a 2D NPSC array of hydrophobic alkanethiol ligands varies with subtle chemical changes in the system, leading to a transition between long-range to short-range (almost glassy) ordered phases. The balance between long and short-range order is driven by small differences in intermolecular interpenetration of the ligand molecules, that can be related to ligand conformational and that can be rigorously the experimentally measured. Second, we show the first 2D NPSC structures to have unique anisotropic symmetry due to the interaction between amphiphilic NP ligand shells. It is understood that the ligand interactions on NPs through their unique molecular configuration of amphiphilic ligands may provide the anisotropic feature in the orientational alignment of NPSC symmetry. / by Jin Young Kim. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 115-121). / AlGaN/GaN high electron mobility transistors (HEMTs) constitute a new generation of transistors with excellent electrical characteristics and great potential to replace silicon technology in the future, especially in high power and high frequency applications. However, the poor long term reliability of these devices is an important bottleneck for their wide market insertion and limits their advanced development. This thesis tackles this problem by focusing on understanding the physics behind various degradation modes and providing new quantitative models to explain these mechanisms. The first part of the thesis, Chapters 2 and 3, reports studies of the origin of permanent structural and electrical degradation in AlGaN/GaN HEMTs. Hydroxyl groups (OH-) from the environment and/or adsorbed water on the III-N surface are found to play an important role in the formation of surface pits during the OFF-state electrical stress. The mechanism of this water-related structural degradation is explained by an electrochemical cell formed at the gate edge where gate metal, the II-N surface and the passivation layer meet. Moreover, the permanent decrease of the drain current is directly linked with the formation of the surface pits, while the permanent increase of the gate current is found to be uncorrelated with the structural degradation. The second part of the thesis, Chapters 4 and 5, identifies water-related redox couples in ambient air as important sources of dynamic on-resistance and drain current collapse in AlGaN/GaN HEMTs. Through in-situ X-ray photoelectron spectroscopy (XPS), direct signature of the water-related species is found at the AlGaN surface at room temperature. It is also found that these species, as well as the current collapse, can be thermally removed above 200 °C in vacuum conditions. An electron trapping mechanism based on H₂O/H₂ and H₂O/O₂ redox couples is proposed to explain the 0.5 eV energy level commonly attributed to surface trapping states. Moreover, the role of silicon nitride passivation in successfully removing current collapse in these devices is explained by blocking the water molecules away from the AlGaN surface. Finally, fluorocarbon, a highly hydrophobic material, is proven to be an excellent passivation to overcome transient degradation mechanisms in AlGaN/GaN HEMTs. / by Feng Gao. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Vita. / Includes bibliographical references (p. 167-170). / by Krassimir P. Nenov. / 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 from student-submitted PDF version of thesis. / Includes bibliographical references (pages 159-165). / Phase transitions are commonly observed in ion storage compounds when being used in rechargeable batteries and thus, the phase behavior of ion storage compounds as electrode active materials has significant impact on battery performance. This thesis aims to understand the interplay between materials structure, phase behavior and battery performance. The effects of operating conditions, especially overpotential and temperature, on phase behavior and battery performance are also investigated. Using olivine-type phosphates (i.e. phospho-olivines) with varying composition and particle size as model system, strain accommodation mechanism within single nanoparticles (Chapter 2 to 3) and mesoscale kinetics of nanoparticle aggregates (Chapter 4 to 5) during electrochemically-induced phase transition have been systematically investigated. In the first part, phospho-olivines with varying transformation strain, from 0 - 3vol% for LiMnyFe1-yPO4 (LMFP, y<0.5), 5vol% for LiFePO4 (LFP), to 17vol% for NaFePO4 (NFP), have been studied using operando Powder X-ray Diffraction (PXD), among other methods. While small transformation strain as in LMFP is accommodated and even avoided by formation of metastable solid solution, large transformation strain as in NFP is mitigated by formation and dissolution of intermediate amorphous phase. This novel mechanism to accommodate large transformation strain may pave the way of utilizing battery materials that deem not working otherwise. In the second part, potentiostatic studies are conducted and a model modified from Avrami model is developed to quantitatively describe phase transformation progresses. The phase transition of LMFP and LFP nanoparticle aggregates is found to follow a nucleation and growth process while the growth is governed by lithium ion diffusion. Based on analysis using the modified Avrami model, more instantaneous nucleation and facile growth tend to occur when transformation strain is small (intermediate Mn content and/or small particle size), overpotential is high and/or temperature is high. And instantaneous nucleation and facile growth improve the rate capability of batteries. The relationship between phase behavior and material structure as well as operating conditions is attributed to: 1) decreasing transformation strain reduces energy barrier for both nucleation and growth; 2) increasing overpotential and temperature boost the electrochemical driving force for phase transition and promote more instantaneous nucleation and facile growth. / by Kai Xiang. / 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 171-187). / To minimize risks associated with climate change, we must rapidly reduce greenhouse gas emissions worldwide by shifting reliance away from fossil fuels. Solar photovoltaic (PV) modules are well suited for reducing emissions; however, manufacturing and capital costs must continue to decline for rapid, worldwide PV adoption. Low-cost and Earth-abundant "thin film" materials offer potential in spurring PV growth, but their development is often hampered by the presence of defects, which degrade solar cell efficiency due to short charge-carrier lifetimes. In this thesis, such defects and their impact on lifetime in early-stage PV materials are investigated, focusing on experimental methods to assess lifetime connected to theoretical concepts about both defects and lifetime measurements themselves. First, time-resolved photoluminescence is performed, and both analytical and numerical modeling are used to determine lifetimes exceeding 1 nanosecond in six materials predicted to be "defect tolerant." Two-photon spectroscopy is then employed to decrease the effect of surface recombination, enabling more representative estimates of "bulk" lifetime. Second, the role of impurities is explored by intentionally contaminating lead halide perovskites with iron. Synchrotron-based X-ray techniques are also utilized to investigate the distribution and charge state of incorporated iron, and perovskite solar cells are found to tolerate approximately 100 times more iron in the feedstock than comparable p-type silicon solar cells. In addition, improved methods for extracting lifetime from solar cell devices are explored. Quantum efficiency measurements are performed and modeled on tin monosulfide solar cells to verify that very short lifetimes (30-100 picoseconds) limit device performance. Furthermore, temperature- and illumination-dependent current-voltage measurements are performed and modeled in iron-contaminated silicon solar cells- and analyzed with the help of a Bayesian inference algorithm-to estimate the defect parameters that directly relate to lifetime. Collectively, these studies serve to provide a more robust framework for assessing and mitigating the presence of defects in early-stage PV materials, streamlining efforts to better optimize their photovoltaic performance. / by Jeremy Roger Poindexter. / Ph. D.