Mechanical Engineering
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Item Phonon Modeling in Nano- and Micro- scale Crystalline Systems(2018) VanGessel, Francis; Chung, Peter; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Submicrometer phonon systems are becoming increasingly relevant in modern day technology. Phonon mechanisms are notably relevant in a number of solid-state devices including lasers, LEDs, transistors, and thermoelectrics. Proliferation of these devices has been driven by advancements in silicon-on-insulator technology. These advancements have allowed for the manufacture of devices with complex nanostructures and dimensions deep in the sub-microscale regime. However, accompanying improvements in the manufacture and design of novel crystalline systems is the requirement for accurate computational approaches for phonon modeling in nanostructured, anisotropic, and complex materials. The phonon Boltzmann transport equation is uniquely well suited to modeling energy transfer at the nano- and micro- meter length scales and is therefore an excellent candidate for this simulation task. However, current Boltzmann modeling approaches utilize a range of assumptions and simplifications that restrict their validity to isotropic, nominally one or two dimensional, or compositionally simple systems. In this dissertation we present an original finite volume-based methodology for the solution of the three dimensional full Brillouin zone phonon Boltzmann transport equation. This methodology allows for separate real and reciprocal space discretization. By taking a sampling of vibrational modes throughout the first Brillouin zone our methodology captures three unique sources of phonon anisotropy. We investigate the effect of phonon anisotropy in a fin field effect transistor, calculating the effect that incorporating various sources of anisotropy has on the resultant temperature fields. In a second study, we consider phonon flow through silicon nanowires with a modified boundary geometry. The three-dimensional flow fields are calculated and thermal transport below the Casimir limit is observed. Reduction in thermal conductivity is a result of maximizing the phonon backscatter that occurs in our phononic system. The backscatter serves to create regions of highly misaligned phonon flux. In addition, our silicon nanowire geometry has properties analogous with a high-pass phonon filter. In the final study we apply our Boltzmann transport methodology to the simulation of phonon transport in the energetic material, RDX. We study phonon transport in the vicinity of a material hotspot, the location at which chemistry initiates in the material. By applying Boltzmann modeling, applied for the first time to this material, we gain valuable insights into the interplay between thermal transport and phonon modes linked with initiation.Item ADVANCEMENT OF MOIRÉ INTERFEROMETRY FOR RATE-DEPENDENT MATERIAL BEHAVIOR AND MICROMECHANICAL DEFORMATIONS(2018) Wu, Bulong; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Moiré interferometry is an optical technique to map full field in-plane deformations with extremely high resolution and signal to noise ratio. The technique is advanced and implemented to study the rate-dependent thermo-mechanical behavior of Sn-based Pb-free solder alloys and micromechanical deformations. In Part I, the mechanical/optical configuration of moiré interferometry for real-time observation of thermal deformations is enhanced to provide measurement capabilities required for the analyses. Two most notable advancements are (1) development of a conduction-based thermal chamber for a wide range of ramp rates with accurate temperature control, and (2) implementation of microscope objectives in the imaging system to observe a microscopic field of view. The advanced system is implemented to analyze the anisotropic behavior of Sn-based Pb-free solder alloys. A novel copper-steel specimen frame is developed to apply a controlled loading to single-grain solder joints. After measuring the grain orientation by electron backscatter diffraction (EBSD), detailed in-situ deformation evolutions and accumulated deformations of solder alloys are documented during a thermal cycle of -40 °C to 125 °C. The results quantify grain orientation-dependent deformations that can lead more accurate anisotropic constitutive properties of Sn-based Pb-free solder alloys. In Part II, an advanced immersion microscopic moiré interferometry system based on an achromatic configuration is developed and implemented for higher displacement sensitivity and spatial resolution. In order to achieve the desired displacement resolution, a high frequency grating (2500 lines/mm) is fabricated on a silicon substrate using lithography first. The square profile is subsequently modified by reactive-ion etching so that it can be used to produce a specimen grating by replication. Secondly, the algorithm of the optical/digital fringe multiplication method is improved to further enhance the measurement resolution of the immersion microscopic moiré interferometry. The system and the noise-free grating are used to analyze thermal deformations of micro-solder bumps. With the basic contour interval of 200 nm, the displacement resolution of 25 nm is achieved with the multiplication factor of 8.Item Fully Anisotropic Solution of the Three Dimensional Boltzmann Transport Equation(2016) VanGessel, Francis; Chung, Peter W.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The development of accurate modeling techniques for nanoscale thermal transport is an active area of research. Modern day nanoscale devices have length scales of tens of nanometers and are prone to overheating, which reduces device performance and lifetime. Therefore, accurate temperature profiles are needed to predict the reliability of nanoscale devices. The majority of models that appear in the literature obtain temperature profiles through the solution of the Boltzmann transport equation (BTE). These models often make simplifying assumptions about the nature of the quantized energy carriers (phonons). Additionally, most previous work has focused on simulation of planar two dimensional structures. This thesis presents a method which captures the full anisotropy of the Brillouin zone within a three dimensional solution to the BTE. The anisotropy of the Brillouin zone is captured by solving the BTE for all vibrational modes allowed by the Born Von-Karman boundary conditions.Item Characterization, Modeling, and Optimization of Polymer Composite Pin Fins(2005-08-24) Bahadur, Raj; Bar Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Increasing electronic product manufacturing volumes and cooling requirements necessitate the use of new materials and innovative techniques to meet the thermal management challenges and to contribute towards sustainable development in the electronic industry. Thermally conductive polymer composites, using high thermal conductivity fillers such as carbon fibers, are becoming commercially available and provide favorable attributes for electronic heat sinks, such as low density and fabrication energy requirements. These polymer composites are inherently anisotropic but can be designed to provide high thermal conductivity values in particular directions to address application-specific thermal requirements. This Thesis presents a systematic approach to the characterization, analysis, design, and optimization of orthotropic polymer composite fins used in electronic heat sinks. Morphological characterization and thermal conductivity measurements of thermally conductive Poly-Phenylene Sulphide composites are used to determine the significant directional thermal conductivity in such composites. An axisymmetric orthotropic thermal conductivity pin fin equation is derived to study the orthotropic thermal conductivity effects on pin fin heat transfer rate and temperature distribution. FEM simulation and water cooled experiments, focusing on the radial temperature variations in single pin fins, are used to validate the analytical model. Theoretical models, CFD modeling, and experiments are used to characterize the thermal performance of heat sinks, fabricated of PPS composite pin fins, in air natural convection and forced convection modes. Simplified solutions, for the orthotropic fin heat transfer rate that are easy to use and can be easily implemented in a heat sink design and optimization scheme, are presented.