Mechanical Engineering
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Item TRITIATED NITROXIDE FOR BETAVOLTAIC CELL NUCLEAR BATTERY: 3D BETA FLUX MODELING, SYNTHESIS, STABILITY ANALYSIS, AND COATING TECHNIQUES(2019) Russo, John A; Bigio, David; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Beta (β-) radioisotope energy sources, such as tritium (3H), have shown significant potential in satisfying the needs of a sensor-driven world. The limitations of current β- sources include: (i) low beta-flux power, (ii) intrinsic isotope leakage (instability) and (iii) beta self-absorption. The figure of merit is the β--flux power (dPβ/dS), where an optimal portion of incident beta particles penetrates the semiconductor depletion region. The goal of this research was to identify a compound to contain a beta emitter that can permit beta-flux power of at least 733nW/cm2 from one side, where it can be used for both planar and high aspect ratio microstructure technology (HARMST) transducers. Nitroxides were chosen because of previous demonstrated deuteration, ease of synthesis, diversity of structure, and pliability. A 6-membered nitroxide was prepared and tritiated with a specific activity of 103Ci/g. The product was stable after 256 days with only 2% tritium loss. A betavoltaic (βV) cell nuclear battery prototype was demonstrated with the tritiation of a 5-membered nitroxide stable in liquid and solid form and a specific activity (Am) of 635Ci/g, the highest recorded Am for an organic compound. A dPβ/dS of 51 nW/cm2 and 102 nW/cm2 were generated when dispensed on βV (4H-SiC) and PV (InGaP) cells. Improvements to increase the dPβ/dS closer to the theoretical limit were identified and demonstrated such as dispensing with different solvents to reduce evaporation time, and increasing solute (nitroxide) concentration in the dispensed solution. A βV cell nuclear battery model was developed, producing a blueprint on what nitroxide characteristics are required to maximize the dPβ/dS and electrical power density (P_(e,vol)). The percent error and percent difference were less than 6% compared to experimental data and other models. For the planar coupling configuration, increasing Am while increasing mass density increases P_(e,vol). Increasing the surface area interaction with the radioisotope and transducer increases the volume radioactivity (Vm), but does not always generate a higher β- source efficiency nor P_(e,vol) compared to the planar coupling configuration. The rectangular pillar array produced the highest 4.54 mW/cm3 of at the highest Vm where HARMST feature width and gaps are proportionally minimized at 100 nm wide.Item Leveraging Porous Silicon Carbide to Create Simultaneously Low Stiffness and High Frequency AFM Microcantilevers(2014) Barkley, Sarice; Solares, Santiago; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many operative modes of the atomic force microscope (AFM) are optimized by using cantilever probes that have both a low force constant and a high resonance frequency. Due to fabrication limitations, however, this ideal cannot be achieved without resorting to sizes incompatible with standard AFM instrumentation. This project proposes that cantilevers made from electrochemically etched porous silicon carbide (SiC) enjoy reduced force constants without significantly sacrificing frequency or size. The study includes prototype fabrication, as well as parametric experiments on the etching recipe and suggestions to improve the process. Analysis of the mechanical properties of the prototypes proves that introducing porosity to the structure greatly reduces the force constant (porous k = 0.27 bulk k) while only slightly reducing the resonance frequency (porous f0 = 0.86 bulk f0).Item Failure Mechanisms in Wideband Semiconductor Power Devices(2006-06-05) Zhang, Xiaohu; Bernstein, Joseph; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Silicon carbide (SiC), as one of the wide bandgap semiconductors, is a promising material for next-generation power devices due to its high critical electric field, high thermal conductivity, and high saturated electron drift velocity properties. Extensive studies have been focused their electrical characterizations. Failure mechanisms of SiC devices, however, have not been fully explored. In this work the failure mechanisms of SiC power devices, including Schottky diodes, power MOSFETs and IGBTs, are investigated. The characteristics of SiC Schottky diodes have been investigated and simulated based on the drift-diffusion model. Interface state degradation has been identified as the mechanism responsible for the non-catastrophic failure happened in Schottky diode. Experimental and simulation results are provided to support this conclusion. Single-event burnout (SEB) and single-event gate rupture (SEGR) failure mechanisms have been investigated for SiC power MOSFETS in details in this work since power MOSFETs have been used in very critical applications. The features of SiC power MOSFET SEB and SEGR failures have been simulated successfully and compared to those of Si power MOSFETs. The much better robustness of SiC power MOSFES against SEB failures has been demonstrated by the simulation results. At last the latch-up failure mechanism has been investigated for SiC IGBTs. Compared to Si IGBTs, the results show that SiC IGBTs have a stronger capability against the latch-up failure. The design and application guideline for SiC power devices can be made base on the results obtained in this work.