Browsing by Author "Feser, Joseph P."
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Item Convective flow through polymer electrolyte fuel cells(University of Delaware, 2005) Feser, Joseph P.Hydrogen powered PEM fuel cells have three primary loss mechanisms: activation over-potential, ohmic overpotential, and the mass-transport limited over-potential. It is suggested that convection in the form of channel bypass can be used to increase reactant concentrations in the catalyst layer which will improve reaction kinetics. Further, if convection can be made the dominant mechanism for gas transport, the diffusion-limited mass-transport overpotential can be reduced or removed. In order to determine under what conditions this can take place, an analytic model was developed for convective flow within a single serpentine channel configuration. The model shows that the channel length and in-plane permeability of the gas diffusion layers are most important factors. ☐ Particle Image Velocimetry was used to observe the velocity fields in representative test sections of an interdigitated and a serpentine fuel cell. Using ex-situ methods, it was shown that it is possible observe secondary flows with primary-to-secondary velocity ratios approaching 100-to-1. Channel bypass was observed in both configurations. Local variation in permeability appears to cause local variation in velocity fields in the channel. ☐ A radial permeability experiment designed and fabricated to characterize and differentiate in-plane permeability of three gas diffusion layers manufactured by different techniques. It was shown that experiments can use either a wetting liquid or a gas of known viscosity as the host fluid and reach identical conclusions. However, flowrates' dependence on pressure is different for gases and liquids and must be recognized when large pressure differentials are present.Item Thermal transport across metal silicide-silicon interfaces: An experimental comparison between epitaxial and nonepitaxial interfaces(American Physical Society, 2017-02-22) Ye, Ning; Feser, Joseph P.; Sadasivam, Sridhar; Fisher, Timothy S.; Wang, Tianshi; Ni, Chaoying; Janotti, Anderson; Ning Ye, Joseph P. Feser, Sridhar Sadasivam, Timothy S. Fisher, Tianshi Wang, Chaoying Ni, and Anderson Janotti; Ye, Ning; Feser, Joseph P.; Wang, Tianshi; Ni, Chaoying; Janotti, AndersonSilicides are used extensively in nano- and microdevices due to their low electrical resistivity, low contact resistance to silicon, and their process compatibility. In this work, the thermal interface conductance of TiSi2, CoSi2, NiSi, and PtSi are studied using time-domain thermoreflectance. Exploiting the fact that most silicides formed on Si(111) substrates grow epitaxially, while most silicides on Si(100) do not, we study the effect of epitaxy, and show that for a wide variety of interfaces there is no dependence of interface conductance on the detailed structure of the interface. In particular, there is no difference in the thermal interface conductance between epitaxial and nonepitaxial silicide/silicon interfaces, nor between epitaxial interfaces with different interface orientations.While these silicide-based interfaces yield the highest reported interface conductances of any known interface with silicon, none of the interfaces studied are found to operate close to the phonon radiation limit, indicating that phonon transmission coefficients are nonunity in all cases and yet remain insensitive to interfacial structure. In the case of CoSi2, a comparison ismade with detailed computational models using (1) full-dispersion diffuse mismatch modeling (DMM) including the effect of near-interfacial strain, and (2) an atomistic Green’ function (AGF) approach that integrates near-interface changes in the interatomic force constants obtained through density functional perturbation theory. Above 100 K, the AGF approach significantly underpredicts interface conductance suggesting that energy transport does not occur purely by coherent transmission of phonons, even for epitaxial interfaces. The full-dispersion DMM closely predicts the experimentally observed interface conductances for CoSi2, NiSi, and TiSi2 interfaces, while it remains an open question whether inelastic scattering, cross-interfacial electron-phonon coupling, or other mechanisms could also account for the high-temperature behavior. The effect of degenerate semiconductor dopant concentration onmetal-semiconductor thermal interface conductance was also investigated with the result that we have found no dependencies of the thermal interface conductances up to (n or p type) ≈1 × 1019 cm−3, indicating that there is no significant direct electronic transport and no transport effects that depend on long-range metal-semiconductor band alignment.