Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (p. 50-51). / by Sandra K. Joung. / M.S.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 1996. / Vita. Cataloged from PDF version of thesis. / Includes bibliographical references (pages 144-153). / Components made of high-performance ceramics or superalloys are subject to strict requirements with regard to their geometric and dimensional accuracy. The surface finish and edge zone characteristics have a large effect on the component's performance. These requirements can not be met directly by the sintering process used in the manufacture of ceramic materials or traditional casting of superalloys. Grinding is both technically and economically the number one choice when one has to consider machining these materials. Metal Single Layer (MSL) grinding technology provides an alternative way to make use of the superabrasives, diamond and CBN, in grinding these materials. One of the primary challenges in MSL grinding technology is to develop suitable active braze alloy(s) which can bond the superabrasive grits. Ticusil (Ag-Cu eutectic+4.5 wt% Ti) and 70Cu-21Sn- 9Ti (wt%) are two of the currently used active braze alloys. The primary failure mode of these two MSL wheels in the grinding test is transverse fracture and debonding of the diamond grits. The high applied load is responsible for transverse fracture of the diamond grit, and the intermetallic phase existing at the interface between the diamond and the braze alloy is one of the causes of the debonding of the diamond grits. Also, a finite element analysis shows that most of the residual thermal stresses and the thermal mismatch strains are localized at the diamond/braze alloy interface. This results in potential weakness of this area. Moreover, the inherent defects, such as voids, and the brittle intermetallics in the interface can cause crack initiation and propagation. Both deteriorate the life of the grinding wheel. The failure of the braze alloy can be divided into two categories. If the grinding process is very abrasive, such as green concrete grinding, the wear resistance of the braze dominates the fracture of the braze alloy. On the other hand, failure of the braze alloy can also result from cracks at the interface. In such a case, the fatigue resistance of the braze alloy plays an important role in determining the wheel's life. The wear resistance of the braze alloy can be improved by introducing suitable hard particles. It was found that a braze alloy of 77Cu-23Sn-12.5Ti-7.5Zr-10TiC-0.2C (by weight) exhibits excellent performance in a wear test (a ten fold improvement), which is further confirmed in the grinding test (a two fold increase in life). The fatigue resistance of the active braze alloy can be modified by either reducing the volume fraction of the brittle intermetallic phase in the braze and/or enhancing the ductility of the braze alloy matrix. A ductile active braze alloy can be achieved by combining the two-layer structure and two step brazing process. To aid dissolution and diffusion of the Cu atoms into the Cu/Sn/Ti braze alloy, a lower volume fraction of the intermetallic phase and higher ductile matrix of the braze can be achieved. Both have beneficial effects in modifying the ductility of the active braze alloy, and make removal of the braze alloy from the substrate by acid etching easier. / by Ren-Kae Shiue. / Ph. D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (p. 63-66). / by Shuba Balasubramanian. / M.S.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Includes bibliographical references (leaves 51-55). / Hard drive industry is facing scaling challenge for areal density to be further increased. This is due to the triangular conflictions among thermal stability (superparamagnetic effect), single-to-noise ratio and writability of the recording media. One of the most promising methods to overcome this constraint is the patterned magnetic media technology. Although it is facing many challenges, the large potential gains in density offered by patterned media make it one of the possible milestones on the horizon for future of the disk drives industry. One of the biggest challenges for patterned media is to realize its mass fabrication provided reduced cost per bit. The basic fabrication approach is to use lithography to pattern the magnetic materials on the platter. However, patterned media requires well-ordered nanoarrays with dimensions less than 25 nm, which challenges the state-of-art lithography technologies. This M. Eng. project focuses on evaluations of the technologies and fabrication schemes potential for patterned media from various aspects like technical barriers, cost and intellectual properties. Technologies including E-beam lithography, nanoimprint lithography, templated diblock copolymer self-assembly and self-assembled magnetic nanoparticles are discussed. Cost modeling was done to prove the enormous gain in revenue for the proposed fabrication scheme. It is proposed that the fabrication scheme of templated diblock copolymer for making the master stamp for nanoimprint followed by nanoimprint lithography for mass production has the largest potential for patterned media. However, more R & D is needed for templated self-assembly of diblock copolymer before it is ready for this application. / (cont.) E-beam lithography which is a mature technology can also be a choice for making the stamp followed by mass production enabled by nanoimprint lithography, without a significant loss of gain in revenue for ultra-high-density media fabrications. Although the cost of a master stamp fabricated by E-beam is estimated to be 50 times more than for templated self-assembly of diblock copolymer lithography. / by Lei Du. / M.Eng.

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.