Mechanical Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/2263
Browse
9 results
Search Results
Item Burning Emulations of Condensed Phase Fuels Aboard The International Space Station(2022) Dehghani, Parham; Sunderland, Peter B; Quintiere, James G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Little is known about the fire hazards of solids and liquids in microgravity. Ground-based tests are too short to overcome ignition transients and testing dozens of condensed fuels in orbit is prohibitively expensive. Burning rate emulation is one way to address this gap. It involves emulating condensed fuels with gases using a porous burner with embedded heat flux gages. This is a study of microgravity burning rate emulation aboard the International Space Station. The burner had porous round surfaces with a diameter of 25 mm. The fuel mixture was gaseous ethylene, and it was diluted with various amounts of nitrogen. The resulting heats of combustion were 15 – 47.2 kJ/g. The flow rate, oxygen concentration in the ambient, and pressure were varied. Heat flux to the burner was measured with two embedded heat flux gages and a slug calorimeter. The effective heat of gasification was determined from the heat flux divided by the fuel flow rate. Radiometers provided the radiative loss fractions. A dimensional analysis based on radiation theory yielded a relationship for radiative loss fraction. RADCAL, a narrow-band radiation model, yielded flame emissivities from the product concentrations, temperature, flame length, and pressure. Previously published analytical solutions to these flames allowed prediction of flame heights and radius, and when combined with the radiation empirical relationship led to corrections of total heat release rate from the flames due to radiative loss. Average convective and radiative heat flux were obtained from the analytical solution and a model based on the geometrical view factor of the burner surface with respect to the flame sheet, that was used to calculate the heat of gasification. All flames burning in 21% by volume oxygen self-extinguished within 40 s. However, steady flames were observed at 26.5, 34, and 40% oxygen. The analytical solution was used to quantify flame steadiness just before extinction. The steadiest flames reached more than 94% of their steady-state heat fluxes and heights. A flammability map as a plot of the heat of gasification versus heat of combustion was developed based on the measurement and theory for nominal ambient oxygen mole fractions of 0.265, 0.34, and 0.4.Item ENHANCED DIFFUSIOOSMOSIS AND THERMOOSMOSIS IN POLYELECTROLYTE-BRUSH-FUNCTIONALIZED NANOCHANNELS(2018) Maheedhara, Raja; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One of the holy grails of nanofluidic systems is to ensure significant flow rates without applying a large pressure gradient. This has motivated researchers to study different mechanisms of liquid transport in nanochannels involving physical effects that exploit the large surface-to-volume ratio of such nanochannels. This thesis will focus on two highly efficient non-pressure-driven flow mechanisms in nanochannels functionalized by grafting the inner walls of nanochannels with end-charged polyelectrolyte (PE) brushes. We study two mechanisms to achieve flow augmentation: (i) ionic diffusioosmosis (IDO), triggered by the application of an external concentration gradient, and (ii) ionic thermoosmosis (ITO), triggered by a temperature gradient. We find a non-intuitive scenario where the flow in nanochannels can be significantly augmented by grafting the nanochannels with PE brushes. Given the difficulty in attaining a desirable flow strength in nanochannels, we anticipate that this thesis will serve as an important milestone in the area of nanofluidics.Item The Effect of Package Geometry on Moisture Driven Degradation of Polymer Aluminum Capacitors(2016) Bevensee, Helmut Manfred; Azarian, Michael H.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polymer aluminum electrolytic capacitors were introduced to provide an alternative to liquid electrolytic capacitors. Polymer electrolytic capacitor electric parameters of capacitance and ESR are less temperature dependent than those of liquid aluminum electrolytic capacitors. Furthermore, the electrical conductivity of the polymer used in these capacitors (poly-3,4ethylenedioxithiophene) is orders of magnitude higher than the electrolytes used in liquid aluminum electrolytic capacitors, resulting in capacitors with much lower equivalent series resistance which are suitable for use in high ripple-current applications. The presence of the moisture-sensitive polymer PEDOT introduces concerns on the reliability of polymer aluminum capacitors in high humidity conditions. Highly accelerated stress testing (or HAST) (110ºC, 85% relative humidity) of polymer aluminum capacitors in which the parts were subjected to unbiased HAST conditions for 700 hours was done to understand the design factors that contribute to the susceptibility to degradation of a polymer aluminum electrolytic capacitor exposed to HAST conditions. A large scale study involving capacitors of different electrical ratings (2.5V – 16V, 100µF – 470 µF), mounting types (surface-mount and through-hole) and manufacturers (6 different manufacturers) was done to determine a relationship between package geometry and reliability in high temperature-humidity conditions. A Geometry-Based HAST test in which the part selection limited variations between capacitor samples to geometric differences only was done to analyze the effect of package geometry on humidity-driven degradation more closely. Raman spectroscopy, x-ray imaging, environmental scanning electron microscopy, and destructive analysis of the capacitors after HAST exposure was done to determine the failure mechanisms of polymer aluminum capacitors under high temperature-humidity conditions.Item Characterization of Non-linear Polymer Properties to Predict Process Induced Warpage and Residual Stress of Electronic Packages(2016) Sun, Yong; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nonlinear thermo-mechanical properties of advanced polymers are crucial to accurate prediction of the process induced warpage and residual stress of electronics packages. The Fiber Bragg grating (FBG) sensor based method is advanced and implemented to determine temperature and time dependent nonlinear properties. The FBG sensor is embedded in the center of the cylindrical specimen, which deforms together with the specimen. The strains of the specimen at different loading conditions are monitored by the FBG sensor. Two main sources of the warpage are considered: curing induced warpage and coefficient of thermal expansion (CTE) mismatch induced warpage. The effective chemical shrinkage and the equilibrium modulus are needed for the curing induced warpage prediction. Considering various polymeric materials used in microelectronic packages, unique curing setups and procedures are developed for elastomers (extremely low modulus, medium viscosity, room temperature curing), underfill materials (medium modulus, low viscosity, high temperature curing), and epoxy molding compound (EMC: high modulus, high viscosity, high temperature pressure curing), most notably, (1) zero-constraint mold for elastomers; (2) a two-stage curing procedure for underfill materials and (3) an air-cylinder based novel setup for EMC. For the CTE mismatch induced warpage, the temperature dependent CTE and the comprehensive viscoelastic properties are measured. The cured cylindrical specimen with a FBG sensor embedded in the center is further used for viscoelastic property measurements. A uni-axial compressive loading is applied to the specimen to measure the time dependent Young’s modulus. The test is repeated from room temperature to the reflow temperature to capture the time-temperature dependent Young’s modulus. A separate high pressure system is developed for the bulk modulus measurement. The time temperature dependent bulk modulus is measured at the same temperatures as the Young’s modulus. The master curve of the Young’s modulus and bulk modulus of the EMC is created and a single set of the shift factors is determined from the time temperature superposition. The supplementary experiments are conducted to verify the validity of the assumptions associated with the linear viscoelasticity. The measured time-temperature dependent properties are further verified by a shadow moiré and Twyman/Green test.Item Advanced FBG Sensor Technique to Measure Effective Chemical Shrinkage and Modulus Evolutions of Polymers with High Polymerization Exothermic Heat(2012) Kim, Yejin; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An advanced technique based on a fiber Bragg grating (FBG) sensor is proposed to measure the critical mechanical properties of polymeric materials during polymerization: effective chemical shrinkage and modulus evolutions. Based on the existing technique implemented with convection oven and two different specimen configurations using same size of FBG, challenges associated with implementation are identified and solutions are proposed. Challenges include temperature instability due to high exothermic heat generated during polymerization inside polymer substrate, temperature controllability due to limitations of convection oven, and mechanical constrain on polymer and fiber during measurement. The proposed system provides modifications to provide easier implementation with accurate results. Modifications such as fabrication of FBG on special fiber, enhancement of heating system, and optimization of system design are combined to provide a tool for rapid but accurate measurement of polymer properties. The proposed technique significantly improves the ability to characterize the mechanical properties of polymeric materials during polymerization which will enhance the accuracy of predictive modeling for design optimization of a microelectronics product at the conceptual stage of product development.Item Characterization of Physical Properties of Multi-Scale Polymer Composites Under Various Processing Conditions(2012) Lederer, Anne Catherine; Bigio, David I; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There is a great interest in using micro and nano scale ingredients as fillers to create composites with enhanced physical properties. This thesis research explores the improvements these fillers offer with focus on combining both micro and nano ingredients to make multi-scale polymer composites. This investigation reveals the interplay of ingredient mixing, microstructural evolution, and processing conditions and characterizes the improvements of thermal and mechanical properties. This data is used to develop fundamental processing-structure-property relationships of these multi-scale composites across different concentrations of microscale and nanoscale ingredients and processing conditions in order to optimize their development.Item MICROFABRICATION AND MODELLING OF DIELECTRIC ELASTOMER ACTUATORS(2012) Balakrisnan, Bavani; Smela, Elisabeth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dielectric elastomer actuators (DEAs) are a class of polymeric actuators that have gained prominence over the last decade. A DEA is comprised of a polymer sandwiched between two compliant electrodes. When voltage is applied between the two electrodes, electrostatic attraction between the electrodes compresses the elastomer in that direction and stretches it in the other two directions. DEAs produce dimensional changes (strains) up to 300% upon application of an electric field. DEAs have tremendous potential for applications requiring large displacements and have been demonstrated for many macro-scale (centimeter and larger) applications such as robots, loudspeakers, and motors. There are potentially many useful applications for micro-scale DEAs (less than millimeter-sized devices with micron-sized actuators) in the fields of micro-robotics, micro-optics, and micro-fluidics. However, miniaturization of DEAs is challenging because many of the materials and DEA fabrication methods used on the macro-scale cannot be adapted for micro-scale fabrication of DEAs. This thesis explores the feasibility of developing fabrication strategies for micro-scale DEAs by adapting micro-electromechanical systems (MEMS) technology. In addition, fabrication protocols for micro-scale DEAs have been developed. The other aspect of this thesis is the design of bending DEAs. Benders are useful because for a given actuation strain, greater deflection can be observed by controlling the stiffnesses and thicknesses of different layers. A general guideline for designing bending DEA configurations such as unimorph, bimorph, and multilayer stacks was developed using a multilayer analytical model. The design optimization is based on the effect of thickness and stiffness of different layers on curvature, blocked force, and work. Complaint electrodes and their design are important for DEAs to enable the elastomer to stretch unrestricted. Thus, design criteria for the fabrication of crenellated electrodes and crenellated elastomers with electrodes were investigated. This guideline enabled design of structures with appropriate axial or bending stiffnesses based on the amplitude, angle, length, and thickness. Simple analytical equations for axial and bending stiffness for crenellated electrodes with different shapes were derived. In addition, numerical simulations of crenellated elastomer with stiff electrode were performedItem Towards the Use of Dielectric Elastomer Actuators as Locomotive Devices for Millimeter-Scale Robots(2012) Pearse, Justin Daminabo; Smela, Elisabeth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dielectric elastomer actuators (DEAs) are electromechanical transducers that are promising for small scale applications. The work presented in this thesis seeks to develop DEAs as an actuation technology that would serve the purpose of ambulating millimeter-scale robots in a robust and predictable manner. To begin, the "planar" DEA configuration was characterized and the performances of various elastomers were investigated. Then, based on the requirements of a proposed robot walking gait, two principles were examined as means of converting in-plane actuation strain to bending actuation. Bending DEAs were fabricated and tested, and a maximum end displacement of 1.5 mm was achieved for a 10 mm long sample. Bending actuator design was optimized by maximizing both speed and payload capabilities. Finally, some challenges facing the design of robots ambulated by DEAs were outlined; of particular note is the DEAs' electrostatic interaction with each other and their surroundings.Item Processing-Structure-Microstructure-Property Relationships in Polymer Nanocomposites(2008-01-31) Kota, Arun Kumar; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The optimal development of polymer nanocomposites using carbon nanotube (CNTs) and carbon nanofiber (CNFs) fillers requires a complete understanding of processing-structure-property relationships. The purpose of this understanding is to determine the optimal approach for processing polymer nanocomposites with engineered microstructures and enhanced material properties. In this research, two processing techniques were investigated: solvent processing and twin screw extrusion. The former is a batch process which employs mixing a polymer solution with a filler suspension using long mixing times and low levels of shear mixing. The latter is a continuous process that mixes polymer melts with solid nanoscale ingredients using high levels of shear mixing for a short mixing time. Previous studies conducted on polymer-CNT/CNF using these processes have focused mainly on processing-microstructure and structure-property relationships using one technique or the other. This research focuses on understanding the processing-property relationships by comparing the structure-property relationships resulting from the two processes. Furthermore, the effect of ingredients and processing parameters within each process on microstructure and structure-property relationships was investigated. The microstructural features, namely, distribution of agglomerates, dispersion, alignment, and aspect ratio of the filler were studied using optical, scanning electron, confocal and transmission electron microscopy, respectively. The composition of the filler was determined using thermogravimetric analysis. The electrical, rheological, thermo-oxidative and mechanical properties of the composites were also investigated. Many significant insights related to processing-structure-property relationships were obtained including: (a) deagglomeration is a critical combination of the magnitude of shear rate and the residence time, (b) the structure-property relationships can be modeled using a new methodology based on the degree of percolation by representing the material as an interpenetrating phase composite, (c) annealing can re-establish interconnectivity and improve electrical properties, (d) the degree of dispersion can be resolved using thermogravimetric analysis, and (e) increasing extrusion speed inhibits thermal decomposition and begins to asymptotically increase strength and stiffness through reduction in aspect ratio and size of agglomerates. Finally, a new combinatorial approach was developed for rapidly determining processing-structure relationships of polymer nanocomposites. This dissertation has broad implications in the processing of high performance and multifunctional polymer nanocomposites, combinatorial materials science, and histopathology.