Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005. / Includes bibliographical references (p. 59-61). / This thesis presents a novel methodology for simulating solidification using fluid structure interactions coupled with the phase-field method while allowing for topology changes in the solid-liquid interface with non-stationary solids. This methodology is implemented using a finite difference scheme and a semi-implicit integration method. Pure translation and pure rotation cases are demonstrated and a preliminary simulation of applied shear on a particle is presented. These results demonstrate that the model shows promise for understanding the behavior of various systems like semi-solid metals, polymer mixtures, and moving solids undergoing chemical reactions. / by Jorge A. Vieyra Salas. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / Includes bibliographical references (p. 61-62). / Rational design of protein additives has been limited by the understanding of mechanism of protein and additive interaction. In this work we have applied molecular dynamics with all atom potentials in order to study the thermodynamic effect of additives on proteins. The method is based on statistical mechanical model that characterizes the preferential binding of proteins to either water or additives. Extensive study was done on model systems comprising of additives urea, glycerol & arginine hydrochloride and proteins RNaseT1 and hen egg lysozyme. Trajectories in range 10-19 nanoseconds were analyzed in order to validate this method and compared with the experimental results. The method was found to agree with experimental results for the first 2 nanoseconds and the extended runs were studied further to narrow down the cause of deviations. Protein RNaseT1 was found to be very unstable and consequently showed very high deviations in preferential binding for longer runs. Constraining the protein using harmonic potential has resulted in better averages for RNase T1. / (cont.) Lysozyme has been found to be very stable and the calculations are in good agreement with experimental values. Local preferential binding calculations showed the importance of structure as well as sequence in prediction of preferential binding of protein. / by Chetan Shinde. / S.M.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000. / Includes bibliographical references. / Side chain liquid crystal polyurethanes are liquid crystal elastomers with the potential to couple mechanical stress fields with optical changes brought on by liquid crystal alignment. These materials excite interest because they are the first thermoplastic liquid crystal elastomers (LCTPEs) that utilize the physics of microphase segregation rather than chemical crosslinking to achieve elastomeric properties: LCTPEs offer the potential to use traditional plastic processing technologies to effect the liquid crystal monodomain ordering required for mechanooptic applications. This thesis describes the design. synthesis and characterization of two series of side chain liquid crystal po I yurethanes. The first series of polyurethanes are made with traditional hard segments and cyano-biphenyl mesogens pendant on the soft segment. The synthesis of the soft segment macrodiol. a carbinol endcapped hydromethylsiloxane oligomer that can be functionalized with mesogen via a hydrosilylation technique, is effected by a polycondensation approach. These side chain liquid crystalline (LC) siloxane macromonomers are then converted to segmented polyurethanes using traditional urethane chemistry. Structural analysis and characterization of the resulting polymers is provided. along with a comparison study of the thermal and optical behavior of these urethanes as the spacer length is increased from 3 to 8 methylene units. Discussions of the effects of the hard segment mobility on the phase behavior of these LC polyurethanes is offered. Fourier-transform infrared (FT-IR) dichroism experiments performed on the cyano-biphenyl polyurethanes provide an opportunity to study the interplay between polyurethane morphology and liquid crystal ordering as the material is exposed to mechanical deformation. This complex material follows the trend established in the literature for both side chain liquid crystal siloxane homopolymers and segmented polyurethanes. At low strains. the soft segments align with the strain inducing an orientation in the "lone" horn segments. Up to strains of 40%, the LC mesogens align with the strain field and the hard segments in hydrogen bonded domains align perpendicular to the field. At strains above 40%, a rearrangement of the ordering that results in symmetric layers and hard segments aligning parallel to the field is found. A model is proposed to represent these findings. and reflections on the cooperative movement of the different macromolecular components of the polyurethane are offered. Phase modulated dynamic Ff-IR experiments are performed to study the response of these materials to sinusoidal mechanical perturbations in an attempt to further understand the response of the polyurethanes to mechanical fields. In dynamic Ff-IR. the viscoelastic reorientation of various segments of the macromolecule can be monitored as a function of applied strain. Evidence is presented for the two types of hard segments: those involved in hydrogen bonding within hard domains, and those found in "lone" hard segments in the soft matrix. Evidence is also presented for two types of mesogens: those found in smectic layers. and those not involved in smectic ordering postulated to be located at the hard domain interface. The hard domains and the smectic layers have strong viscous components to their mechanical response. The "free" mesogens and the "lone" hard segments, on the other hand, exhibit a more elastic response. Once again. evidence for the mechanical coupling of the hard segments and the LC mesogens is of~·ered. In an attempt to improve the mechanical properties of the LC segmented polyurethanes. and to explore piezoelectric and electro-mechanical behaviors. new LC segmented polyurethanes are designed with higher overall molecular weight. longer soft segments. higher degrees of microphase segregation. and smectic C* mesogens. Vinylmethylsiloxane macrodiols are anionically synthesized using a diifunctional intitiator. and then polymerized to form segmented polyurethanes by condensation reactions with diisocyanates and chain extenders. The degree of functionalization of the soft segment. and the length of the hard segment are varied in attempts to decrease the degree of mechanical coupling between the hard segment and the mesogens. It is found that polyurethanes with 50% functionalized soft segments provide phase segregated morphologies. form cohesive polyurethane films. and show decoupling in the thermal phase behavior of the LC soft segment and the hard segment. The study of side chain liquid crystal polyurethanes described in this thesis provides a fundamental understanding of the properties of this new class of materials. The interdependance of the hard segment and the mesogen on the thermal and mechanical responses of these materials is a key finding. The development of LC polyurethanes in which this cooperative interaction is muted leads to materials with great potential for mechano-optic applications. The findings reported here should be helpful in testing the piezoelectric and mechano-responsive behaviors of these promising materials. / by Bindu R. Nair. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Page 214 blank. / Includes bibliographical references. / Silicon based light sources, especially laser devices, are the key components required to achieve a complete integrated silicon photonics system. However, the fundamental physical limitation of the silicon material as light emitter and the limited understanding of tli~ excitation mechanism of Er in dielectric media by optical and electrical pumping methods impedes the progress of the research activities in this area. Silicon rich nitride (SRN) has been investigated as a strong candidate for silicon based laser devices. SRN has many advantages over other Si-based materials systems. These advantages include a high electrical injection level at low voltages, a low annealing temperature for Si nanocluster (Si-nc) formation and a large refractive index for strong optical confinement. Strong light emission from localized states in Si-nc embedded in SRN was demonstrated with a PLQE (Photoluminescence Quantum Efficiency) of 7%. This effect was confirmed through several experiments and first principle calculations. Thue Morse aperiodic structures were fabricated with light emitting SRN and SiO2 materials, for the first time. Through the resonance phenomena achieved using this approach an emission enhancement of a factor of 6 was demonstrated experimentally. A sequential annealing technique was investigated to enhance the light emission from the Si-nc based light emitter. Electrical injection was greatly improved with annealing treatments of SRN based devices. In particular, bipolar electrical injection into SRN led to electroluminescence which was comparable to photoluminescence in peak shape and spectral position. Er doped SRN (Er:SRN) was fabricated through a co-sputter technique to achieve light emission at the wavelength of 1.54 [mu]m. / (cont.) Energy transfer from SRN td Er was confirmed and shown to have a strong dependence on Si content. Si racetrack resonator structures with a low loss value of 2.5 dB/cm were fabricated through a Local Oxide (LOCOS) process and coupled with an Er:SRN layer to investigate gain behavior. Electrical injection properties into the Er:SRN layer were investigated and the electroluminescent device was fabricated. A detailed discussion on optical and electrical excitation of Er is provided to clarify the difference of the Er excitation mechanisms. A comparison of key simulation parameters used within the two level equations for optical and electrical excitation of Er atoms is provided to explain how the parameters contribute to each excitation mechanism. The most significant differences between the parameters and excitation mechanisms are also explained. Finally a summary of important factors to achieve a silicon based laser is provided and discussed for future investigation based on the experimental data and the investigation presented in this work. / by Jae Hyung Yi. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 118-122). / Thermoelectrics study the direct inter-conversion between heat flow and electrical power, which has a wide range of applications including power generation and refrigeration. The performance of thermoelectricity generation and the refrigeration is characterized by a dimensionless number called the Figure-of-Merit (ZT), defined as ZT = [sigma]-S 2T / [kappa], where a is the electrical conductivity, K is the thermal conductivity, S is the Seebeck coefficient, and T is the absolute temperature. Before 1993, the upper-limit of ZT was barely 1. After the efforts of more than twenty years, the upper-limit of ZT has been pushed up to ~2. However, for the thermoelectric technology to be commercially attractive, the value of ZT and the cost of production have to be further improved. Most of the ZT enhancing strategies that have been proposed since 1993 involve the changing and the controlling of the dimension of materials systems, the scattering mechanism(s) of carriers, the shape of the electronic band structure and the density of states, and the magnitude of the band gap. As further research is carried out, it is found that these strategies do not always work to enhance ZT. Even for a working materials system, the improvement margin of increasing ZT can be small. The balancing between [sigma] and S 2 / [kappa] has significantly limited the improvement margin for our ZT enhancing goal. Therefore, we have two problems to explore: (1) how can we deal with the strong correlation between [sigma] and S2 / [kappa] , when trying to enhance ZT, and (2) how can we make the above mentioned strategies more convergent as we change the dimension of materials systems, the scattering mechanism(s) of carriers, the shape of electronic band structure, and the magnitude of the band gap? This thesis aims to explore the solutions to these two major problems at the research frontier of thermoelectric ZT enhancement. The first problem is discussed by providing a new framework of pseudo-ZTs, where the electronic contribution (zte) and the lattice contribution (ztL) to the overall ZT can be treated in a relatively separate manner. The second problem is discussed under this new framework of pseudo-ZTs, through four subsections: (i) scattering and system dimension; (ii) band structure; (iii) density of states; (iv) band gap. The one-to-one correspondence relation between the carrier scattering mechanism(s) and the maximum Seebeck coefficient is further studied. A new tool for scattering mechanism(s) inference and for the Seebeck coefficient enhancement is provided. For the band structure and the band gap part, advanced band engineering methods are provided to study nanostructured narrow-gap materials, the Dirac cone materials, and the anisotropic materials, which are historically found to be good thermoelectric materials. To further demonstrate the newly developed theories, this thesis has also illustrated the application of these models in some specific materials systems, including the graphene system, the transition metal dichalcogenides monolayer materials systems, the Bi1 -xSbx alloys system, the In1.xGaxN alloys system, and the (BiiySby) 2(S1_xTex)3 alloys system. / by Shuang Tang. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (leaves 146-152). / by Bo Zheng. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2002. / Includes bibliographical references. / Dendrimers are three-dimensional, hyperbranched macromolecules that possess a uniform size and shape. Most dendrimers are spherical in shape; however, the shape of the dendrimer can be adjusting using the number and the position of the branching groups in the core. For example, dendritic rods have been prepared by assembling a dendron around each repeat unit of a linear polymer core, and hybrid-linear dendritic diblock copolymers have been prepared by attaching a dendron to the end functional group of a linear polymer. This linear block in the diblock copolymers also adds physical integrity and an assembly mechanism for arrangement of the polymer. Nonetheless, no one has combined the unique shape of the dendritic rod with that of the linear dendritic diblock copolymer. The objective of this research was to prepare a linear-dendritic rod diblock copolymer, and to examine its assembly behavior in solution, at the air/water interface, and in the bulk. These polymers consisted of a linear poly(ethylene oxide)-poly(ethylene imine) diblock copolymer around which poly(amido amine) branches were divergently synthesized. The dendritic branches were terminated with amine and ester groups, as well as alkyl chains of various lengths in order to "tune" the amphiphilic nature of the polymer. / (cont.) A fundamental change in the assembly behavior of the polymers was observed at generation 4.0 (eight end-groups). In solution, the hydrodynamic and viscometric radii were found to increase to a much greater extent than expected for the generation 4.0 and 4.5 polymers, consistent with a breakdown of the spherical approximation as the dendritic block extended into a rod-like shape. Similarly, at the air/water interface, the dendritic block of generation 4.0-alkyl terminated polymers all adopted a horizontal rod configuration, while the dendritic block of the lower generation polymers took on a random coil configuration, whose shape depended on the length of the terminal alkyl groups as well as the generation number of the dendritic block. Finally, in the bulk, direct observation of the generation 4.0-dodecyl terminated polymer with TEM indicated that the polymer was adopting a rod- or worm-like conformation, while the lower generation polymers only exhibited traditional diblock copolymer or polymer brush behavior. / by Catherine Marie Bambenek Santini. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / Includes bibliographical references (leaves 98-100). / Internal surgical intervention necessitates the intentional wounding of tissue. In certain clinical procedures, the desired wound healing response requires the use of closure techniques, such as suturing or stapling of disjoined tissues. Risk factors associated with these techniques are largely attributed to the discrete nature of the mechanical forces arising in the tissues. Adhesive sealants can mitigate risk by imparting a continuous stress distribution to tissues upon closure, as opposed to destructive stress concentrations. A novel class of dextran:polyethylene glycol hydrogels are a potential alternative to the limited selection of available adhesive sealants. Multiple compositional variations are available for both the dextran and polyethylene glycol components, making a wide range of clinically relevant material properties achievable. Key material properties determining sealant efficacy include hydration and degradation in an aqueous medium, elastic modulus, adhesion strength to tissue, and biocompatibility. Relationships between these pertinent properties and available compositional variations are determined for dextran:polyethylene glycol materials. / (cont.) Gravimetric, mechanical and biological testing reveal the following compositional determinants of material properties in dextran:polyethylene glycol copolymers: constituent molecular complexity dictates material hydration and degradation, solid content dictates elastic modulus, available aldehyde groups dictate adhesion strength, and material solid content and reactive group ratio dictate induced cell proliferation and cytotoxicity. Knowledge of these property determinants facilitates development of an optimal dextran:polyethylene glycol material in a small bowel resection model for adhesive sealants, and furthers the understanding of these complex copolymers for other sealant applications. Generalization of the identified property determinants to other material classes provides a vehicle for advancement of adhesive sealant technologies. / by Tarek Shazly. / S.M.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 35). / We examined the feasibility of using the resonant frequency of magnetic bead-domain wall (DW) couples in a host fluid to measure particle size. Nickel-Iron (Permalloy) rings, made using electron beam lithography, served as the tracks for nucleating and moving DWs, and Invitrogen Dynabeads M-270 magnetic beads were used for the experiment. Tween-20 surfactant in solution and SiO2 capping layers for the structures were used to overcome substrate-bead interaction and maintain bead mobility. The resonant frequency of 40 bead-DW couples was measured and found to lie in a range between 18.3 and 42.7 Hz with a median of 31.1 Hz. In addition, sets of resonance experiments were performed to examine the dependence of the resonant frequency on driving amplitude, DW type, and position on the permalloy (Py) ring. The resonant frequency populations of beads bound to head-head and tail-tail DWs overlapped, but each DW type seemed to be centered around a different frequency. Examining different positions on a ring showed that a large contribution to the spread in resonant frequencies may come from DW pinning due to structural defects or remanent surfacebead interaction. Finally, the resonant frequency is independent of the driving amplitude, a finding which supports the linear spring model for DW-bead interaction. We conclude that resonance measurements made with optical methods reliably distinguish particles of different hydrodynamic radius. This work has also helped identify and address some of the obstacles to improve the reliability of these resonance measurements as indicators of particle size. By demonstrating this detection capability, we can proceed with the development of spin-valve -based resonance devices suitable for clinical applications. / by Daniel Mauricio Montana Fernandez. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1997. / Vita. / Includes bibliographical references. / by Harold D. Ackler. / Ph.D.