Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006. / Includes bibliographical references (p. 104-106). / An experimental study was performed to evaluate the utility of electrospun carbon nanofiber supports for sputtered platinum catalyst in proton exchange membrane fuel cells. The performance of the sputtered nanofiber supports was similar to that of sputtered commercial gas diffusion layers in single cell fuel cell tests. However, sputtered platinum electrodes performed significantly worse than commercial thin film electrodes due to high activation and concentration voltage losses. Cyclic voltammetry and rotating disc electrode experiments were performed in order to evaluate the influence of platinum loading and particle size on the electrochemical active area and oxygen reduction performance of the sputtered platinum. Active area per weight catalyst decreased with sputtering time, and the oxygen reduction activity slightly increases with increasing sputtering time. Both of these effects are thought to be due to increasing platinum particle size as sputtering time is increased. / by Simcha Lev Singer. / S.M.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 27-28). / Both ground source heat pumps operating on electricity and micro-combined heat and power systems operating on fossil fuels offer potential for the reduction of green house gas emissions in comparison to the conventional approaches for providing heating, air conditioning and electric power to residential homes. Factors that may impact the relative merits are actual system operating efficiencies, regional primary energy sources for electric power generation, actual space conditioning and electric demands as well as regional climate factors. The purpose of this study is to make a consistent, realistic comparison of these greenhouse gas reduction strategies as applied to typical single-family residential homes across the United States. The study identifies both the regional variations and specific magnitudes of reductions that could be expected with these technologies when implemented within the current energy infrastructure. These comparisons are achieved by identifying the performance characteristics of both technologies, developing typical application scenarios and collecting important regional data associated with electric power production and climate variations. The results show that indeed regional variations exist in the relative merits of micro-CHP systems and ground source heat pumps on reducing the carbon emissions for households. Specific results are sensitive to the assumptions made regarding the carbon production characteristics of incremental increases or decreases of electrical demand on the local electricity utility grid. / by Brittany Guyer / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 133-146). / Climate change is among the most critical challenges that are facing the human race in the 21st century. One of the major factors that leads to climate change is the increasing consumption of fossil fuels, driven by industrialization and economic growth at an unprecedented pace. For a secure and sustainable future of energy and the environment, new clean and efficient energy technologies are in urgent need. Thermoelectric materials are a group of materials that can directly convert heat into electricity. Being solid state, clean, reliable and without moving parts, thermoelectric energy conversion holds great promise as a candidate technology to harvest energy from thermal sources, such as the sun and terrestrial heat sources, as well as improve the efficiency of existing energy systems by recycling the inevitable waste heat. The bottleneck that prevents large-scale deployment of thermoelectric modules so far, however, is the relatively low efficiency and high cost. A good thermoelectric material needs to conduct electricity well and conduct heat poorly to attain high efficiency. Remarkable progress has been made in the past decade to decouple the charge and heat transport and thus improve the material performance. Most of the progress has been based on a more detailed understanding of the transport and interaction of fundamental energy carriers, such as electrons and phonons in most semiconductors, and magnons in magnetic materials. These understandings have been achieved through the development of both first-principles simulations and experimental spectroscopic tools, in particular for phonon transport and phonon-phonon interaction, which have enabled calculations and measurements at the single-phonon-mode level. Information gained from these studies formed the foundation of the successful engineering efforts of designing nanostructured thermoelectric materials. Although the nanostructuring approach has been able to reduce the thermal conductivity of thermoelectric materials down to proximity of the amorphous limit, it has been realized by the community that further improvement of thermoelectric materials requires breakthroughs in boosting the electrical transport properties, including the electrical conductivity and the Seebeck coefficient. Despite several existing strategies, a prerequisite for systematic improvement is, again, insight into the transport and interaction of fundamental carriers, particularly involving electrons, at the single-mode level. This insight has largely remained lacking in terms of electrons, both on the simulation side and on the experimental side. This thesis aims to develop both simulation and experimental tools to study nanoscale electron, phonon and magnon transport and their interactions, with a particular emphasis on understanding the electron-phonon interaction at the single-mode level. This is among the most important forms of carrier interactions and determines the intrinsic electron transport properties of most conductors. Regarding phonon transport, we applied first-principles lattice dynamics to study phonon-phonon interaction and lattice thermal conductivity in a strongly-correlated thermoelectric compound FeSb 2. On electronphonon interactions, we studied from first-principles the intrinsic electrical transport properties of phosphorene, which are limited by electron-phonon interactions, analyzed its anisotropy and evaluated its potential as a thermoelectric material; we studied how free carriers can in turn scatter phonons through the electron-phonon interaction and reduce the lattice thermal conductivity; to verify this finding, we designed an ultrafast photoacoustic spectroscopic technique to directly detect the damping of a single phonon mode due to electron-phonon interaction. On phonon-magnon interactions, we applied the coupled Boltzmann equation to analyze coupled phonon-magnon diffusion and proposed a novel magnon cooling effect. These fundamental discoveries can potentially lead to new design principles for more efficient thermoelectric materials in the future. / by Bolin Liao. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages [111]-115). / The design process can be seen as a complex, ambiguous, ill-defined problem with no clearly correct answer. At the same time, the early stages of the design process carry importance with regard to design outcomes, sometimes with far reaching consequences. With the proliferation of computer modelling tools, designers are moving away from traditional design tools such as sketching, and begin designing in CAD earlier than before. This thesis focuses on the early stages of the design process, and on how selected design tools - sketching, foam prototyping, and computer modelling - influence the design outcomes of an individual designer in the early conceptual phases of the process. Through the use of controlled design experiments with experienced design practitioners, this thesis seeks to examine how different design tools impact the design outcomes. Analysis of video and audio recordings, interviews, and talkaloud protocols are used to gain insights, and investigate how different tools impact the design outcomes and decision making of individual designers in the early stages of the design process. As an example, does a designer who creates foam models - thereby receiving tactile feedback as they are creating the model - consider ergonomics more than a designer working in CAD? Results suggest clear differences in quantity and quality of concepts depending on which design tool was used, as well as between designers themselves, highlighting the importance of using an appropriate design process and set of tools in the early conceptual stages of a design task. / by Anders Häggman. / Ph. D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1992. / Includes bibliographical references. / by Jane E. Boon. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1987. / Bibliography: leaves 37-38. / by Richard Davis Webb, Jr. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004. / Includes bibliographical references (p. 178-182). / Vibration and noise are an ever-present problem in the majority of mechanical systems, from consumer products to precision manufacturing systems. But most approaches for vibration suppression are expensive and invasive, so only a small subset of the techniques developed in research labs are widely used. In this thesis, we present a novel wave-based damping approach for the suppression of vibration in machines and structures. Our studies show that significant broad-band damping can be attained with little added mass via dynamic interaction between a structure and a low-density, low-wave-speed medium (such as a foam or powder). This damping phenomenon has great promise for many applications because it is robust (that is, not tuned), does not introduce significant creep into a structure, can accommodate large strains, and can be realized using materials that are light weight, low cost, durable, insensitive to temperature, and easy to package. We report on several experiments in which flexural and longitudinal vibration are attenuated using this approach. Experiments on flexural vibration of structures filled with low-density powder show that high damping is obtained (with loss factors as high as 12 percent for a powder fill whose mass is 2.3 percent of that of the beam) over a broad frequency range. Somewhat surprisingly, the response is found to be linear over a wide range of amplitudes. We propose that the powder can be modeled as a fluid in which pressure waves can propagate and find that such a model matches the experiments well. These findings suggest that any moderately lossy medium in which the speed of wave propagation is sufficiently low can be used to obtain similar responses. / (cont.) We find that low-density foams coupled to structures exhibit com- parable attenuations over a somewhat broader frequency range, and that the responses can be accurately predicted if dilatation and shear waves are included in the model. We develop simplified models for these phenomena, and thence obtain guidelines for design of structures incorporating low-wave-speed media. The approach is compared to other damping techniques, and applications to belt- driven positioning systems and precision flexure assemblies are described. . / by Kripa K. Varanasi. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005. / Includes bibliographical references. / A study was performed to determine possible factors that contribute to successful implementation of new technologies in developing nations. Engineers and other inventors have devoted great effort to Appropriate Technology design over the last two decades, but few comprehensive case studies currently exist examining factors that lead to technology success. Existing studies of appropriate technology were summarized and a quantitative model was created to tabulate the data. Factors of local maintenance, local production, and local need of a technology were found to be the most important to sustainable technology implementation. The model was then tested with a current Appropriate Technology project to examine the relevance of its results. Overall, the model proved applicable, though furthers studies are suggested to refine the model. / by Edward F. Hsieh. / S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006. / Includes bibliographical references (p. 18). / Biologically active materials providing a range of applications from tissue engineering to microdevices have begun to revolutionize biomedical science. New chemistries, however, must be developed for functionalization of these materials with each different molecule. This paper explores a technique for developing multi-functional, biologically active hydrogels utilizing the high streptavidin-biotin binding affinity. Streptavidin was conjugated to acryl-PEG-N-hydroxysuccinimide, a commercially available molecule that allows chemical binding to poly(ethylene glycol) (PEG) diacrylate and dextran acrylate hydrogels. Such gels were made by photocrosslinking solutions of APN and streptavidin conjugated at various molar ratios, along with a gelling polymer under an ultraviolet (UV) lamp. Acryl group conjugation was confirmed through high performance liquid chromatography (HPLC) and mass spectrometry. Protein binding was assayed through the use of rhodamine-labeled streptavidin and fluorescent microscopy. Gels were incubated overnight in solution to determine diffusion. After 7 days, PEG showed no diffusion while dextran acrylate demonstrated 100% protein loss. / by Elizabeth Hempel. / S.B.

Thesis (S.M. in Mechanical Engineering and Naval Architecture and Marine Engineering)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008. / 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. 62-63). / The gas turbine exhaust uptakes in the HALIFAX class frigates of the Canadian Navy have experienced thermally-induced fatigue cracking since soon after the commissioning of these ships. The uptake structure is heavily stiffened in order to meet shock resistance requirements. Unfortunately, the result has been that thermal expansion of the uptake shell is constrained, thus every flash-up and shut-down of a gas turbine results in a fatigue cycle of its uptake with extremely high stresses. Among the methods proposed to address the problem is the structural redesign of the uptakes within the constraints of the original mounting arrangements. Any such redesign would be required to reduce thermal stresses while still meeting the shock resistance requirements. This work presents the redesign of the uptakes such that they continue to meet shock requirements while incorporating design aspects, developed in the literature, which are anticipated to reduce thermal stresses. The original intention was to use the modal-based design response spectrum method to assess shock resistance. However, due to excessive stresses in the original model and in all subsequent modifications using this method, the less-rigorous base acceleration method was primarily used. / by Simon A. Summers. / S.M.in Mechanical Engineering and Naval Architecture and Marine Engineering