Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999. / Includes bibliographical references. / Lithium manganese oxides have attracted wide attention as low-cost, nontoxic intercalation cathode materials for rechargeable lithium batteries. In this work, the stability of these compounds during synthesis and in use has been studied in several respects. (1) Phase stability of LiMnO2 polymorphs has been determined under the high temperature synthesis conditions. Effects of temperature, oxygen partial pressure, and dopant (Al) content on the phase stability have been discussed based on a possible stability mechanism. (2) The mechanism of improved cycling stability of electrochemically transformed spinel compared to conventional spinel has been identified. Atomic rearrangement from the ordered rocksalt to spinel type cation ordering results in an antiphase nanodomain structure, which becomes a ferroelastic domain structure during the cubic ---> tetragonal Jahn-Teller transformation, and thereby accommodates the transformation strains. (3) Al-doped spinels exhibit much improved capacity stability at elevated temperatures compared to undoped spinels. This effect has been discussed with respect to proposed mechanisms of Mn dissolution and capacity loss. (4) Magnetic properties are critically influenced by phase stability, cation ordering, and Mn valence in lithium manganese oxides. In the paramagnetic temperature regime, it has been observed that antiferromagnetic interactions between the Mn ions are strongest in the orthorhombic phase among LiMnO2 polymorphs having the average Mn valence of 3+, while decreasing Mn valence strengthens the antiferromagnetic interactions in LixMn2O4 spinel. At temperatures below the paramagnetic temperature regime, spin-glass behavior is observed in both LixMn2O4 and monoclinic LiMnO2 compounds, which is attributed to geometrical frustration due to structure ( cation ordering) and magnetic disorder due to a disordered distribution of Mn valence. As spin-glass behavior is commonly observed in both well-crystallized, conventional spinel and highly disordered, transformed spinel, magnetic characterization cannot easily be used to distinguish the two different spinels. / by Young Il-Jang. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014. / 220 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 122-127). / Bismuth Telluride and its solid solutions are currently front running thermoelectric materials because of their high figure of merit. When processed via mechanical alloying to obtain nanocrystalline structures, their efficiency is increased dramatically, due to enhanced phonon scattering at grain boundaries. However, the excess free energy of these interfaces renders them inherently susceptible to grain growth, therefore there is a need for materials with enhanced thermal stability. Despite this, little is known about the relevant processing science of these materials with respect to mechanical alloying and powder consolidation. This shortcoming is addressed here via systematic study of the processing-structure relationships that govern these processing operations. Firstly, during mechanical alloying, the primary mechanism of mixing between elemental constituents is revealed, as well as the limitations to subsequent grain refinement. The resultant behaviour is unique in the literature on mechanical alloying, due to the unusual thermal and thermodynamic properties of the compound and its elements, rendering deformation-induced heating effects especially prevalent. Next, during sintering operations of the powders, the kinetics of grain growth and porosity evolution were studied. By quantifying these processes, a thermal budget map for the nanocrystalline compound is constructed, to allow predictive powder and guidance of both processing and device operation at elevated temperatures. Finally, based on the improved understanding in processing science and thermal stability of these materials, a new class of thermally stable composites is engineered, with improved thermal stability, and hence enhanced thermoelectric properties. / by Samuel A. Humphry-Baker. / Ph. D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 37-39). / Local heat delivered by magnetic nanoparticles (MNPs) selectively attached to their target proteins can be used to manipulate and break up toxic or obstructive aggregates. We applied this magnetic hyperthermia treatment to the amyloid beta (A[beta]) peptide, which unnaturally folds and self-assembles forming amyloid fibrils and insoluble plaques characteristic of amyloidgenic diseases such as Alzheimer's disease. We demonstrate remote disaggregation of A[beta] aggregates using heat dissipated by ferrite MNPs in the presence of an alternating magnetic field (AMF). Specific targeting was achieved by MNP functionalization with a targeting peptide sequence that binds a hydrophobic domain of A[beta]. AMF parameters and MNP composition and size were tailored to maximize hysteretic power losses. Transmission electron microscopy image analysis and thioflavin T fluorescence spectroscopy were used to characterize the morphology and size distribution of aggregates before and after AMF stimulus. We found that the AMF stimulus is effective at destabilizing A[beta] deposits and causing a reduction in aggregate size. This targeting scheme has potential as a therapy for amyloidosis and as a minimally invasive tool for analyzing and controlling protein aggregation. / by Colleen Loynachan. / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 127-136). / The functional versatility and endurable self-healing capacity of soft materials in nature is found to originate from the dynamic supramolecular scaffolds assembled via reversible interactions. To mimic this strategy, extensive efforts have been made to design polymer networks with transient crosslinks, which lays the foundation for synthetic self-healing hydrogels. Towards the development of stronger and faster self-healing hydrogels, understanding and controlling the gel network dynamics is of critical importance, since it provides design principles for key properties such as dynamic mechanics and self-healing performance. For this purpose, a universal strategy independent of exact crosslinking chemistry would be regulating the polymer material's dynamic behavior by optimal network design, yet current understanding of the relationship between network structure and macroscopic dynamic mechanics is still limited, and implementation of complex network structure has always been challenging. In this thesis, we show how the dynamic mechanical properties in a hydrogel can be controlled by rational design of polymer network structures. Using mussel-inspired reversible catechol coordination chemistry, we developed a nanocomposite hydrogel network (NP gel) with hierarchical assembly of polymer chains on iron oxide (Fe3O4) nanoparticles as network crosslinks. With NP gel as a model system, we first investigated its unique dynamic mechanics in comparison with traditional permanent and dynamic gels, and discovered a general approach to manipulate the network dynamics by controlling the crosslink structural functionality. Then we further explored the underlying relationship between polymer network structure and two key parameters in relaxation mechanics, which elucidated universal approaches for designing relaxation patterns in supramolecular transient gel network. Finally, by utilizing these design principles, we designed a hybrid gel network using two crosslinking structures with distinct relaxation timescales. By simply adjusting the ratio of two crosslinks, we can precisely tune the material's dynamic mechanics from a viscoelastic fluid to a rigid solid. Such controllability in dynamic mechanics enabled performance optimization towards mechanically rigid and fast self-healing hydrogel materials. / by Qiaochu Li. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1993. / Includes bibliographical references (leaves 183-190). / by Omar Salman Abdul-Hamid. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1997. / Includes bibliographical references (leaves 187-196). / by Libby K. Louie. / Ph.D.

<p> Three dimensional fiber reinforced foam cores (3DFRFC) can have improved mechanical properties under specific strain rates and fiber volumes. This study explored different manufacturing techniques for the 3DFRFC and tested the specimens at dynamic loading rates of 69&ndash;10³ s^&ndash;1. Flexural bend test showed that glass fibers made the samples stronger yet more brittle while quasi-static compression tests showed a decrease in performance with 3DFRFC. High strain impact tests validated previously published studies by showing an 18&ndash;20% reduction in the maximum force experienced by the fiber reinforced core and its ability to dissipate the impact force in the foam core sandwich panel. The results show potential for the cost-effective manufacturing method used in this study to produce an improved composite foam core sandwich panel for armored applications where high strain rates are present and reduce the overall weight of vehicles while maintaining the desired strength performance.</p><p>

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / Includes bibliographical references (leaves 63-65). / Thin film component properties are critical design elements in almost all industries. These films are particularly important in the performance of micro- and nano-electromechanical systems (MEMS and NEMS). Residual stress in thin film components is often treated as an unavoidable side effect of processing steps and the degree of residual stress can drastically affect the performance and properties of the final product. While high levels of residual stress are often detrimental to performance, control of the stress and stress gradients can also be used to enhance performance and even generate new capabilities. The work presented in this thesis examines the role of island coalescence in the development of structure and stress in thin films. The primary methods of investigation are molecular dynamics (MD) and finite element analysis (FEA). The semi-empirical MD calculations show that coalescence is a very rapid process for unconstrained spheres and for hemispheres allowed to slide on a frictionless substrate. Particle rotations are commonly observed during the coalescence calculations. The extent of neck formation between 2 particles is consistent with continuum models even down to length scales which would normally be outside the range in which the models might be expected to be applicable. The MD calculations also show that internal island defects may be induced by the island coalescence process, but only for a particular range of island sizes. We present an energetic model for the existence of such a size range and have located experimental evidence in the literature for such defects. Our FEA work extends an earlier study on the effects of contact angle on island coalescence. Our FEA study of islands with greater than 90 degree contact angle coalescence shows that neck formation occurs very similarly to the free sphere coalescence case. We conclude that MD and FEA calculations are useful tools in analyzing the island coalescence process and can provide mechanistic insight beyond what is available from the more general continuum models. / by Andrew Rikio Takahashi. / S.M.

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. / Includes bibliographical references (p. 167-184). / T cells are required by their immunological roles to recirculate in the body and migrate to tissue sites, a journey that exposes them to distorting forces and physical obstacles that hinder their movement. Therefore, they must possess appropriate deformability to accommodate and adapt to these mechanical stimuli to migrate unimpeded. Since T cells alter their physical properties and migration routes upon activation, they may possess dissimilar mechanical properties as a result of this process. This hypothesis was tested using the techniques micropipette aspiration and atomic force microscopy, which allow the investigation of the elastic and viscous responses of single T cells. It was discovered that the activation process reduced T cell stiffness by more than three folds, a finding that agrees with the motility gain observed in activated T cells. The same testing procedure was applied to Wiskott-Aldrich syndrome protein (WASp)-deficient T cells that exhibit abnormal morphology and impaired chemotaxis. The stiffness of the diseased cells in the naive state was 1.5 times less than that of the non-diseased cells, a result that may be due to the disrupted polymerization and cross-linking of the actin cytoskeleton in the absence of WASp, a regulator of actin growth and organization. Furthermore, the viscous response of the diseased cells in the activated state was found to be impaired. Chemokines were found to dramatically reduce the stiffness of naive T cells that were induced to migrate. These findings suggest that WASp plays an important role in maintaining cell mechanical property and facilitating T cell extravasation by tailoring the cells' deformability. At the molecular level, activation of T cells is triggered by the binding of their surface receptors to antigens, a mechanism that is also key in T cell development. In both cases, the bond strength, conventionally measured by the affinity (KD) or the dissociation rate (koff) of the interacting pair, dictates the biological outcome. Since a few weak interactions may nudge a sub-threshold signal over the threshold strength, and observing that the current methods for measuring KD and koff lack the resolution to detect very weak bonds, this work explored the possibility of utilizing dynamic force spectroscopy (DFS) to study very weak binding strengths. Preliminary results confirm this capability. / by Irene Yin-Ting Chang. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (p. 191-204). / by Gerardo Damián Garbulsky. / Ph.D.