Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 213-227). / The structure of metallic glass controls its mechanical properties; this structure can be altered by thermomechanical processing. This manuscript presents a model for this structural evolution of metallic glass under thermal and mechanical stimuli. The foundation of this model is a potential energy landscape; this consists of three pieces: a function for the energy of any given stable state, a density of states function across the landscape, and a model for the energetic barriers between stable states. All three of these pieces are parameterized in terms of the configurational potential energy of the glass, which is split into isochoric and dilatative degrees of freedom. Under a thermal or mechanical stimulus, the glass traverses the potential energy landscape by way of isotropic relaxation or excitation events, and by shear transformations. The rates of these events are calculated using transition state theory. This model is first implemented in homogeneous form, treating the glass nanostructure as a statistical distribution; this implementation, while devoid of spatial detail, is nonetheless able to fit many of the experimental results on homogeneous flow previously in the literature. The second implementation of the model is in a mesoscale discrete shear transformation zone dynamics framework; this couples the model's rate equations to discrete points in a finite element model under realistic thermomechanical loading, and propagates the effects of local events via static elasticity. Emphasis is placed on efficient computer implementation of the new model's physics, improving on the previous state of the art with stiffness matrix factor caching and geometric multigrid methods. These numerical improvements produce a 200x speedup over previous algorithms, enable rapid simulations of glass with evolving elastic properties, and facilitate the first-ever metallic glass simulations of physical nanomechanical experiments with matching length and time scales. / by Thomas James Hardin. / 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 133-139). / Shape memory ceramics rely on martensitic transformations which are similar to those found in metallic shape memory materials, but ceramics offer several advantages over metals such as higher operating temperatures and larger transformation stresses. However, polycrystalline shape memory ceramics have shown poor cycling performance which limits their use in practical applications. This is due to the inherent physical constraints of the grains that create stress concentrations and eventually leads to intergranular fracture. Here it is proposed that single crystalline and oligocrystalline ceramics-made with a single grain or very few grains-will avoid the physical constraints found in polycrystalline materials that lead to intergranular fracture and result in shape memory ceramics with enhanced cycling performance. Zirconia was chosen for study because it has the necessary martensitic transformations and has shown limited shape memory properties when in the bulk polycrystalline form. Focused ion beam milling was used to make single crystal and oligocrystal pillars of varying diameter that were compression tested using a nanomechanical testing platform to determine the mechanical properties. This work showed that removing grain constraints in micron-scale shape memory zirconia prevented cracking and fracture. It also enhanced the number of achievable repeatable cycles from five in bulk materials to at least hundreds in small structures. The transition from single- to oligo- to poly-crystal was explored and it was found that fracture is more likely in polycrystals and that the transformation stresses increase as pillar diameter is increased, the opposite of what is observed in shape memory metals. This phenomenon is attributed to the higher stiffness of ceramics making the stored elastic energy more important. The effect of crystal orientation was investigated to aid in design and optimization. Orientation maps were produced for fracture behavior, elastic modulus, transformation stress, and transformation strain. Finally four different scale-up architectures were proposed and implemented - powders, wires, foams, and thin films - and each demonstrated shape memory properties thereby paving the way for deployment in practical applications. / by Alan Lai. / Ph. D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2002. / Includes bibliographical references (leaves 46-48). / Ferromagnetic shape-memory alloys (FSMAs) are a recently-developed class of active materials which show large extensional strains when a magnetic field is applied. Shear strains of 6% have been observed at room temperature in martensitic Ni-Mn-Ga single crystals. The strain effect in Ni-Mn-Ga FSMAs is a result of twin boundary motion in the martensite phase, and can be induced by either field or stress. Most Ni-Mn-Ga FSMA research so far has focused on actuation in single crystals. However, the mechanical loss inherent to twin boundary motion also makes this material attractive for energy absorption/vibration damping applications. This thesis describes the preliminary investigation of FSMA/polymer composites for eventual use in vibration damping, and should serve as a stepping stone toward further studies. The research moved through four stages: suction-casting polycrystalline Ni-Mn-Ga alloys with suitable target compositions, processing the polycrystalline material into powder, fabricating FSMA/polymer composites, and preliminary characterization of the stress-strain behavior of these composites. Suction casting produced three polycrystalline alloys which all showed significant variance from the target compositions as well as a high degree of compositional inhomogeneity. From the composition, structural analysis, and magnetic characterization, it was determined that Alloy 1 was martensite at room temperature, while Alloys 2 and 3 were austenitic. Alloy 3 showed a martensite transition temperature around 10° C. While only one of the three alloys shows a majority of martensite at room temperature, it is likely that the powders made from the polycrystalline material cover a wide range of compositions, so results concerning the structure of the powders reflect only the major phase present. The powders were mixed with urethane polymer at powder volume fractions of 10%, 20%, and 30%, followed by curing in a 4.2 kOe field to align the particles. Scanning electron microscopy and magnetic characterization confirmed the alignment of particles into chains within the polymer matrix. The dynamic stress-strain behavior of composites was characterized for low stresses at various frequencies. The static stress-strain behavior of the composites under compressive loading to stresses of 10 MPa was also characterized. In the FSMA/polymer composite samples, the first compressive loading test gave a stress/strain curve which is linear with one modulus up to a threshold stress. Beyond this threshold stress, the curve is linear but with a smaller modulus. Successive compressive stress-strain curves exhibited a linear stress/strain relationship with a modulus value between those of the two regions in the first test, with some hysteresis in the stress-strain response present between the first and second tests. A urethane sample and a urethane/20% volume fraction Al powder composite, by comparison, showed only linear stress-strain behavior with no significant changes between the first and second compressive loading tests. It is likely that the observed stress-strain behavior of the FSMA composites derives from stress-induced twin boundary motion in the martensite phase present. / by Robin H.C. Ivester. / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 127-146). / Superparamagnetic microbeads (SPBs) have been widely used to capture and manipulate biological entities in a fluid environment. Chip-based magnetic actuation provides a means to transport SPBs in lab-on-a-chip technologies. This is usually accomplished using the stray magnetic field from patterned magnetic micro structures or domain walls in magnetic nanowires. Recently, many studies have focused on sub-micron sized antidot array of magnetic materials because non-magnetic holes affect the micromagnetic properties of film. In this work, a method is presented for directed transport of SPBs on magnetic antidot patterned substrates by applying a rotating elliptical magnetic field. We find a critical frequency for transport beyond which the bead dynamics transition from stepwise locomotion to local oscillation. We also find that the out-of-plane (Hoop) and in-plane (Hip) field magnitudes play crucial roles in triggering bead movements. Namely, we find threshold values in Hoop and Hip that depend on bead size which can be used to independently and remotely to address specific bead populations in a multi-bead mixture. In addition, these behaviors are explained in terms of the dynamic potential energy landscapes computed from micromagnetic simulations of the substrate magnetization configuration. Furthermore, we show that large-area magnetic patterns suitable for particle transport and sorting can be fabricated through a self-assembly lithography technique, which provides a simple, cost-effective means to integrate magnetic actuation into microfluidic systems. Finally, we observed the transport of bead motion on antidot arrays of multilayered structures with perpendicular magnetic anisotropy (PMA), and found that the dynamics of SPBs on a PMA substrate are much faster than on a substrate with in-plane magnetic anisotropy (IMA). Our findings provide new insights into the enhanced transport of SPBs using PMA substrates and offer flexibility in device applications using the transportation or sorting of magnetic particles. / by Minae Ouk. / Ph. D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2003. / Includes bibliographical references (leaves 81-85). / by Rachel Sharp. / S.B.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000. / Vita. / Includes bibliographical references (leaves 224-234). / The field of nanotechnology has received burgeoning interest in recent years as the characteristic dimensions for many applications (such as integrated circuits and magnetic storage media) become smaller and smaller. In this work, block copolymers are harnessed in order to produce both porous and relief nanostructures. The interest in using these materials is due to the unique morphologies that block copolymers form and the fact that these nanostructures do so by self assembly. With careful selection of the relative volume fraction and phases, nanostructures with highly ordered and complex pore structures with a vast range of different symmetries can be produced; structures that are not attainable by more conventional processing techniques such as lithography. In this thesis, we have produced porous and relief ceramic nanostructures from self-assembling (template free) block copolymer precursors using a one-step, room temperature technique. To accomplish this, a silicon containing block copolymer system was used where upon exposure to an oxidation process the material undergoes two steps 1) the selective removal of the hydrocarbon block and 2) the formation of a ceramic from the inorganic containing block, resulting in nanoporous and nanorelief ceramics. These structures have potential to be used at temperatures far above the T 8 of traditional nanoporous or nanorelief polymers. By choosing the appropriate morphologies and parent block copolymers, 30 nanostructured ceramics with interfacial areas of-40 m2/g, masks for one-step lithography with a density of-5 x 1011 dots/cm2 or templates for the next generation of nanomagnets can be produced. In addition to these applications, it is envisioned that these structures can be used as photonic band gap materials, high temperature membranes and low dielectric constant materials. Specifically, the formation of both nanoporous and nanorelief structures from an ABA triblock copolymer system of poly(pentamethyldisilylstyrene) P(PMDSS) with polyisoprene was studied. The focus of this thesis is on the oxidation of the double gyroid and ''inverse" double gyroid morphologies using either ozone/uv and oxygen plasma techniques. By transmission electron microscopy (TEM) and atomic force microscopy (AFM), it is shown that the PI can be preferentially removed by oxidation resulting in a nanoporous material in the case of the double gyroid morphology and a nanorelief material in the case of the inverse double gyroid morphology. Oxidation of the P(PMDSS) homopolymer was also studied chemically using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), Fourier Transform Infra-red Spectroscopy (FTIR), Rutherford Backscattering Spectrometry (RBS) and Forward recoil Spectrometry (FRES) and morphologically by AFM. Through these chemical analysis techniques, it is demonstrated that the ozone + uv and uv only oxidation processes converts thin films of P(PMDSS) to a ceramic, specifically silicon oxycarbide, that is far more stable than the parent homopolymer. / by Vanessa Zee-Haye Chan. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 117-123). / The morphologies of polydimethylsiloxane (PDMS)-containing diblock polymers are investigated as a function of volume fraction, segregation, processing procedure, and temperature. Strongly segregated polyisoprene-PDMS and polystyrene-PDMS diblocks are prepared according to standard procedures in the literature by anionic synthesis in the laboratory of Professor Apostolos Avgeropoulos at the University of Ioannina in Ioannina, Greece and their morphologies are investigated using small angle x-ray scattering and transmission electron microscopy. Good agreement is found between this work and other work on the structures of diblocks containing PDMS with a variety of complementary blocks and between this work and theoretical predictions for the morphologies of diblock polymers. Different processing treatments including casting from solvents with a range of preference for each block and a week-long anneal are tested to determine whether processing has a strong effect on final morphology; it is found that in most cases the morphology displayed after processing is consistent independent of the processing treatment, indicating that the morphologies are in equilibrium and fairly robust to preparation procedure. Finally, selected weakly segregated diblocks were studied at varying temperatures using synchrotron small angle x-ray scattering. The diblock samples appeared to be affected by the prior x-ray dose that the materials had received. With limited prior dose, the materials studied were ordered with little dramatic change in morphology throughout the temperatures investigated; under continual irradiation by a 1.371Å (9.1 keV) beam for half an hour, the samples were damaged. The thesis concludes with a summary and suggestions for future work, including a discussion of experimental and theoretical work on the ways that equilibrium morphologies of block copolymers are perturbed when they are spatially confined to dimensions on the order of several times their repeat period. The small domain sizes achievable with and technological relevance of PDMS-containing diblocks make them ideal for use in microelectronics and information storage which provides a motivation for exploring this topic further. / by Charlotte Stewart-Sloan. / S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 126-145). / Electrochemical energy storage devices are clean and efficient, but their current cost and performance limit their use in many transportation and stationary applications. Lithium-ion batteries are one of the leading candidates for these large applications, however their current use of liquid electrolytes negatively effects their lifetime and safety. Furthermore, the liquid electrolyte's potential stability window, thermal stability, and volatility are of particular concern in these large-scale applications. Solid-state electrolytes are investigated as one of the best solutions to overcome these challenges. However, the ionic conductivity and especially (electro)chemical stability of many solid electrolytes are still problematic. The focus of this thesis is on ionic mobility and stability of solid-state Li-ion conductor and descriptors that correlates with these properties. We first provide a comprehensive review of several important families of Li-ion conductors that have been studied and published in the literature focusing on their and an overview of some descriptors that have been proposed to correlate with the ionic conductivity/activation energy, for instance, the volume of the diffusion pathway, high-frequency dielectric constants and frequencies of low-energy optical phonons. Build upon these previous understandings, we propose a new approach to understand ion mobility and stability against of lithium insertion/removal in ion conductors based on lattice dynamics. By combining inelastic neutron scattering measurements with density function theory computation, greater lithium ion mobility was correlated with decreasing lithium vibration frequency that was quantified using a newly proposed descriptor which we phonon band centers. Known superionic lithium conductors were shown to have not only low lithium phonon center but also low anion phonon band center, which unfortunately reduces stability against electrochemical oxidation. Therefore, the interplay between lattice dynamics and ion mobility and stability highlights the need and opportunities to search for fast lithium ion conductors having low lithium band center but high anion band center which exhibit high ion conductivity and high (electro)chemical stability in lithium ion batteries. We show and discuss that Olivines with low lithium band centers but high anion band centers are particularly promising to explore for lithium ion conductors with high ion conductivity and stability. With this new approach, we were able for the first time to account for the trend in ionic conductivity and electrochemical oxidation stability of lithium ion conductors from one common physical origin, their lattice dynamics. Such findings open new avenues for the discovery of new lithium ion conductors with enhanced conductivity and stability using lattice dynamics. Finally, to study the correlation between the actiation energy and the pre-exponential factor, the ionic conductivity and activation energy of lithium in the Li₃PO₄-Li₃VO₄-Li₄GeO₄ system was systematically investigated as model system. The sharp decrease in activation energy upon Ge substitution in Li₃PO₄ and Li₃VO₄ was attributed to the reduction in the enthalpy of defect formation while the variation in activation energy upon increasing Ge content was rationalized in term of the inductive effect. The series of compound with and without partial lithium occupancy were shown to fall into two distinct lines whose slope was related to the inverse of the energy scale associated with phonon in the systems according to multi-excitation entropy theory and the intercept to the Gibbs free energy of defect formation. Compiled data of pre-exponential factor and activation energy for commonly studied Li-ion conductors shows that this correlation is very general, implying an unfavorable trade-off between high pre-exponential factor and low activation energy needed to achieve high ionic conductivity. / by Sokseiha Muy. / Ph. D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references (p. 75-78). / As silicon photonics enters mainstream technology, we find ourselves in need of methods to seamlessly transfer light between the optical fibers of global scale telecommunications networks and the on-chip waveguides used for signal routing and processing in local computing networks. Connecting these components directly results in high loss from their unequal sizes. Therefore, we employ a coupler, which acts as an intermediary device to reduce loss through mode and index matching, and provide alignment tolerance. This thesis presents a potential fiber-to-waveguide coupler design for use in integrating such networks. A quadratic index stack focuses incident light from a fiber in one plane, while a planar lens and linear taper do likewise in the perpendicular plane. Once the mode is sufficiently compressed, the light then enters and propagates through the waveguide. We performed simulations using the beam propagation method and finite difference time domain, among other modeling techniques, to optimize coupling efficiency and gain an understanding of how varying certain parameters affects coupler performance. The simulation results were then incorporated into a mask layout for fabrication and measurement. / by Trisha M. Montalbo. / S.M.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005. / Includes bibliographical references (p. 50-52). / Grain boundary engineering is the manipulation of low stacking-fault energy, face- centered cubic material microstructures to break the connectivity of the general grain boundary network through the addition of special grain boundaries. Grain boundary engineering processing consists of thermomechanical cycling, i.e. repeated strain and annealing sequences and provides a method of producing more robust polycrystalline materials. This evaluation presents an introduction to the fundamental principles of grain boundary engineering, reviews the processing techniques and relevant intellectual property, analyzes the processing variables and their effect on a manufacturing line, surveys the current market and competition, and provides a preliminary cost analysis. / by Jeffrey A. Zelinski. / M.Eng.