http://hdl.handle.net/1721.1/7846 Search the Channel search http://dspace.mit.edu/simple-search http://hdl.handle.net/1721.1/40940 Title: Converting sugarcane waste into charcoal for Haiti <br/> <br/>Authors: Toussaint, Etienne Clement <br/> <br/>Description: Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.; Includes bibliographical references (leaves 46-47). http://hdl.handle.net/1721.1/35626 Title: Modeling the deformation and failure behavior of FCC and HCP nanocrystalline materials <br/> <br/>Authors: Wei, Yujie, Ph. D. Massachusetts Institute of Technology <br/> <br/>Abstract: As foreseen by Richard Feynman in his famous talk titled There's Plenty of Room at the Bottom in 1959, scientists nowadays are miniaturizing structures in materials to achieve better performance as concerned in technical applications. Reducing grain sizes in polycrystalline materials into the range of less than 100nm, for example, could achieve extraordinary high strength in these so called nanocrystalline (nc) materials. The reduced grain size gives rise to new deformation mechanisms in nc materials. It is now widely accepted that there is a strong interplay between dislocation-based deformation in the crystalline grain interiors and the inelastic deformation mechanisms operative in the grain-boundary regions. Grain-boundary regions play an increasingly significant role as the grain size decreases below the 100nm level. In this dissertation, constitutive models have been developed to investigate the deformation mechanisms of nc materials, with focus on modeling grain-boundary decohesion in nc materials. Two micromechanical models have been developed to capture the deformation in grain boundaries in nc materials.; (cont.) To the end, a phenomenological constitutive model has been developed for powder-processed nc materials, where the plastic flow could be pressure-dependent, plastically-dilatant, and non-normal. To model the deformation in grain boundaries, we made the assumption that the mechanical behavior of a grain boundary is governed by the interaction of two coupled surfaces of neighboring grains. Mechanical response due to the relative sliding and separation between the two coupled surfaces is represented by traction-separation laws. An isothermal, rate-independent elastic-plastic interface model has been developed, which accounts for both reversible elastic, as well as irreversible inelastic separation-sliding deformations at the interface prior to failure. The interface model, which represents the deformation in grain boundaries, is coupled with an isothermal, rate-independent crystal-plasticity model for grain interiors, and to study the deformation and fracture response of nc nickel. Secondly, motivated by the fact that the ultimate size limit for nc materials would be bulk amorphous materials, we treated the non-equilibrium, more or less disordered grain boundaries in nc material as amorphous layers with finite thickness of [approx.] 1nm.; (cont.) A viscoplastic amorphous constitutive model is applied to capture the deformation in grain-boundary regions. The model has been applied to predict the mechanical behavior of nc nickel with different grain sizes and subject to tensile deformation at different strain-rates. Many nanocrystalline materials are made by powder-consolidation; and their macroscopic response is reminiscent of cohesive granular materials. Based on the deformation behavior observed on powder consolidated nc materials from different groups, we have formulated a model for the response of pressure-sensitive and plastically-dilatant elastic-viscoplastic powder consolidated nc materials; where the plastic flow could be pressure-dependent, plastically dilatant, and non-normal. <br/> <br/>Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.; Includes bibliographical references (p. 175-188). http://hdl.handle.net/1721.1/35625 Title: In-situ backplane inspection of fiber optic ferrules <br/> <br/>Authors: Wilson, Andrew Kirk, 1977- <br/> <br/>Abstract: The next generation of supercomputers, routers, and switches are envisioned to have hundreds and thousands of optical interconnects among components. An optical interconnect attains a bandwidth-distance product as high as 90 GHz.km, about 200 times higher than can be attained by a copper interconnect. But defects (such as dust or scratches) as small as 1 micron on the connector endfaces can seriously degrade performance. Therefore, for every mate and de-mate, optical connectors must be inspected to ensure high performance data transmission capabilities. The tedious and time consuming task of manually inspecting each connector is one of the barriers to adoption of optics in the backplanes of large card-based machines. This thesis provides a framework and method for in-situ automatic inspection of backplane optical connectors. We develop an inspection system that fits into the envelope of a single daughter card, moves a custom microscope objective in three degrees of freedom to image the connector endfaces, and detects and classifies defects with major diameter of one micron or larger.; The inspection machine mounts to the backplane in the same manner as a daughter card, and positions the microscope with better than 0.2 micron resolution and 15 micron repeatability in three degrees of freedom. Despite tight packaging constraints, the ultra-long working distance custom microscope objective attains 1 micron Rayleigh resolution via deconvolution. Several images taken at different exposures and focus settings are fused to extend the imaging sensor's limited dynamic range and depth of field. A set of machine-vision algorithms are developed to process the resulting image and detect and classify the fiber core, cladding and their defects. <br/> <br/>Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.; Includes bibliographical references (p. 193-200). http://hdl.handle.net/1721.1/35620 Title: Nanoscale heat conduction with applications in nanoelectronics and thermoelectrics <br/> <br/>Authors: Yang, Ronggui, Ph. D. Massachusetts Institute of Technology <br/> <br/>Abstract: When the device or structure characteristic length scales are comparable to the mean free path and wavelength of energy carriers (electrons, photons, phonons, and molecules) or the time of interest is on the same order as the carrier relaxation time, conventional heat transfer theory is no longer valid. Tremendous progress has been made in the past two decades to understand and characterize heat transfer in nanostructures. However most work in the last decade has focused on heat transfer in simple nanostructures, such as thin films, superlattices and nanowires. In reality, there is a demand to study transport process in complex nanostructures for engineering applications, such as heat transfer in nanoelectronic devices and the thermal conductivity in nanocomposites which consists of nanowires or nanoparticles embedded in a matrix material. Another class of problems which are rich in physics and might be explored for better design of both nanoelectronic devices and energy conversion materials and devices are coupled electron and phonon transport. Experimentally, most past work has been focused on thermal conductivity characterization of various nanostructures and very little has been done on the fundamental transport properties of energy carriers.; (cont.) This thesis work contributes to the following aspects of heat transfer, nanoelectronics, and thermoelectrics. 1) Simulation tools are developed for transient phonon transport in multidimensional nanostructures and used to predict the size effect on the temperature rise surrounding a nanoscale heat source, which mimics the heating issue in nano-MOSFETs. 2) Semiconductor nanocomposites are proposed for highly efficient thermoelectric materials development where low thermal conductivity is a blessing for efficiency enhancement. Both the deterministic solution and Monte Carlo simulation of the phonon Boltzmann equation are established to study the size effect on the thermal conductivity of nanocomposites where nanoparticles and nanowires are embedded in a host material. 3) Explored the possibility of creating nonequilibrium conditions between electrons and phonons in thermoelectric materials using high energy flux coupling to electrons through surface plasmons, and thus to develop highly efficient thermoelectric devices.; (cont.) 4) Established a sub-pico second optical pump-probe measurement system where a femtosecond laser is employed and explored the possibility of extracting phonon reflectivity at interfaces and the phonon relaxation time in a material, which are the two most fundamental phonon properties for nanoscale energy transport from the pump-probe measurements. <br/> <br/>Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.; Includes bibliographical references.