Thesis advisor: Dunwei Wang / Electrochemical CO2 reduction reaction (CRR) provides a solution to both the increasing global demand of energy by forming valuable chemical products for fuel production, and global warming by reducing the amount of CO2 in the environment. To efficiently reduce CO2, we sought to understand the reaction mechanism using a polycrystalline Au electrode and the super concentrated LiTFSI solution (WiS) as the electrolyte. By varying both the electrolytic potential and the concentration of WiS, we investigated the factors determining product selectivity and found that reaction kinetics and mass transport together direct the selectivity towards CO. We probed the rate limiting step (RLS) of CO2 reduction by observing the variation of product distribution with water availability in solution, and discovered that the RLS was likely to involve only a single electron transfer to form COO*–. Lastly, we proposed that in WiS, H2O were the dominant proton sources for both CO2 reduction and H2 evolution reactions. In 21m WiS, the competing hydrogen evolution reaction was kinetically inhibited, so CO production was favored with a selectivity of 90% at a potential as early as -0.4V vs RHE. This study demonstrated the great potential of WiS as a platform for studying multi-proton, multi-electron transfer reactions. / Thesis (BS) — Boston College, 2018. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Scholar of the College. / Discipline: Chemistry.

The thesis deals with the design of a suitable technology for serial production of a pin. The component material is structural steel 11,320 5R. Due to the series production and material savings, the molding technology was chosen to be cold. For production, the CM 4-5 ECO HATEBUR process press was designed with a nominal force of 1 700 kN. Progressive tool and drawing documentation of tools for the final operation were also designed. The tool load was verified by manual calculation, and the finite element method was calculated to heat the formed blank due to molding.

Two experimental methods, optical limiting and degenerate four-wave mixing, are employed to study the nonlinear optical properties of various novel fullerenes structures. Optical limiting refers to decreased transmittance of a material with increased incident light intensity. Detailed measurements of the wavelength-dependence of fullerene optical limiters have illustrated several key features of reverse saturable absorption. Most important among these is the requirement of weak but non-negligible ground state absorption. We have shown that the optical limiting performance of C₆₀ can be extended into the near infrared range by appropriate modifications of the structure such as higher cage fullerenes or derivatization of the basic C₆₀ molecule. The higher cage fullerene C₇₆ shows improved optical limiting behavior compared to C₆₀, for wavelengths higher than 650 nm, but becomes a weak limiter in the 800 nm range. C₈₄, even at high concentrations in [alpha]-chloronaphthalene, does not reach the good performance of C₆₀, but instead shows weak optical limiting in the 800 nm range. We also demonstrate that by attaching various groups to the C₆₀ molecule, we can extend the optical limiting performance in the near infrared regime. The C₆₀ derivatives studied, (C₆₀ cyclic ketone, C₆₀ secondary amine, C₆₀CHC₆H₄CO₂H, and C₆₀C₄H₄(CH₃)CH₂O₂C(CH₂)CO₂H), have a similar characteristic: the attached groups cause a symmetry-breaking of the C₆₀ sphere and, therefore, there are new allowed transitions that appear as absorption features up to 750 nm. The optical limiting measurements show that these materials, even for low input energies, have an exceptionally strong optical limiting response in the 640 to 750 nm spectral region. For wavelengths higher than 800 nm, however, they become transparent and no optical limiting is observed. Excited state absorption cross-sections obtained from analysis of the optical limiting data reveal that the C₆₀ derivatives have a maximum triplet-triplet absorption cross-section at 700 nm, which is shifted from the 750 nm value for the C₆₀ molecule. For the first time, optical limiting measurements are performed on five separate C₈₄ isomers. These intriguing results show that the optical limiting behavior is strongly dependent on the cage symmetry. It is also found that the most abundant isomer does not have the strongest optical limiting performance, but is in fact one of the weaker optical limiters of the isomers isolated so far. The endohedral metallofullerenes are a unique class of fullerene materials and consist of one or more metal atoms encapsulated inside the buckyball cage. An important characteristic of these materials is the charge-transfer from the dopant atoms to the fullerene cage, which has a high electron affinity. The charge-transfer is similar to the optical excitation in a material, but although the electrons are placed in the lowest unoccupied molecular orbital (LUMO), there are no holes produced in the highest occupied molecular orbital (HOMO). This is an important analogy, since it has been previously shown that optical excitation enhances the nonlinear optical properties of a material. The nonresonant degenerate four-wave mixing experiments performed on the endohedral metallofullerene Er₂@C₈₂, at 1064 nm, show that the third order nonlinear susceptibility value is increased by orders of magnitude relative to the empty cage fullerenes, thus, confirming the charge-transfer process from the encapsulated atoms to the fullerene cage. We obtain a value [gamma]_xyyx⁽³⁾( &#173 [omega]; [omega], [omega], &#173 [omega])= &#173 8.65 &#215 10⁻³² esu for the molecular second order hyperpolarizability, which is almost three orders of magnitude larger than the values reported in literature for an empty cage fullerene. / Ph. D.

No description available.

<p> In this thesis, the rate-limiting step is determined in a biofuel cell with a bio-anode, a Nation membrane and a conventional, platinum-based cathode using reference electrode method. It was discovered by surprise that the cathode overpotential dominated the cell overpotential. Na + in the membrane was found to hinder the W transport. The cathode overpotential increased due to the presence of Na + in the membrane and at the cathode. The limited H+ transport causes the increase of the cathode overpotential. H+ transport is the rate-limiting step in our biofuel cell, rather than commonly believed electron transport. Moreover, the cell power output degradation is not due to the conventionally believed depletion of the fuel substrate, inter-penetration of the fuel and oxidizer and the degradation of the biocatalysts, but the limited W transport in our biofuel cell. </p> <p> The existing oxygen reduction mechanism at the cathode was questioned and revised. When Na+ occupies all sulfonate groups in the membrane, only the Na+ from the buffer can pass through the membrane. The oxygen reacts with the water transported with Na+ and electrons to produce OH", which balances with the transported Na+ to keep electroneutrality at the cathode. </p> <p> Tris buffer without Na + was utilized as alternative anolyte in the biofuel cell. It was found that the cell with Tris buffer had a poorer performance in comparison with sodium phosphate buffer due to the increases of the anode and cathode overpotentials. Tris buffer does not constitute a solution to the problem. </p> <p> This work represents a step toward a more complete understanding of the properties of biofuel cells. To improve biofuel cell output, the herein identified H+ transport limitation in Na + contained Nation needs to be overcome. </p> / Thesis / Master of Applied Science (MASc)

<p> We consider a model of the chemostat in which three competitor populations compete for a single, essential, growth-limiting nutrient. As well, the least efficient competitor population also acts as a predator on the most efficient competitor population. Bifurcation methods are used to obtain information about the qualitative behaviour of the model. A complete description of the global stability is given for the case when Lotka-Volterra response functions describe both competitor-nutrient and predator-prey interactions. For certain parameter values, the model predicts coexistence of the three species. The model also shows that the elimination of the predator population or the elimination of a competitor population can cause the system to collapse from three species to one.</p> / Thesis / Master of Science (MSc)

Yes

To test factors limiting regeneration of large-seeded hardwoods in the Upper Coastal Plain of South Carolina, we planted one-year old bare root Quercus alba L. and Cornus florida L. seedlings in pine plantation understories and adjacent clearcuts. We hypothesized that soil moisture, light, and leaf and stem herbivory would have relatively strong effects on growth and survival during the first two years after planting. Next to each seedling, we measured five abiotic factors (soil carbon and moisture, gap light index (GLI), and available nitrogen and phosphorus). We also estimated percent stem and leaf herbivory and harvested seedlings after the second year to measure seedling growth. Survival was not significantly different between clearcut (62.9%) and understory (64.6%) treatments. Biomass growth was significantly greater (P<0.05) in the clearcuts for both species. All abiotic variables measured except soil carbon were greater in the clearcuts (P<0.05). Each seedling growth response was regressed against all variables using forward selection (P=0.15). Soil moisture significantly affected survival in 12.5% and biomass growth in 16.7% of the regressions. Light availability significantly impacted biomass growth in 8.3% of the regressions. Neither leaf nor stem herbivory frequently affected survival, but one or the other or both significantly influenced the biomass growth of seedlings in 41.5% of the regressions. Herbivory had the strongest impact on seedling responses. Neither water nor light was very influential for growth or survival responses. We conclude that hardwood restoration in the uplands of the Upper Coastal Plain is best accomplished by planting in recent clearcuts. / Master of Science

I have conducted detailed experimental and theoretical studies of the nonlinear optical properties of semiconductor materials useful for optical limiting. I have constructed optical limiters utilizing two-photon absorption along with photogenerated carrier defocusing as well as the bound electronic nonlinearity using the semiconducting material ZnSe. I have optimized the focusing geometry to achieve a large dynamic range while maintaining a low limiting energy for the device. The ZnSe monolithic optical limiter has achieved a limiting energy as low as 13 nJ (corresponding to 300W peak power) and a dynamic range as large as 105 at 532 nm using psec pulses. Theoretical analysis showed that the ZnSe device has a broad-band response covering the wavelength range from 550 nm to 800 nm. Moreover, I found that existing theoretical models (e.g. the Auston model and the band-resonant model using Boltzmann statistics) adequately describe the photo-generated carriers refractive nonlinearity in ZnSe. Material nonlinear optical parameters, such as the two-photon absorption coefficient β_2=5.5cm/GW, the refraction per unit carrier density σ_n=-0.8∗10^-21cm^3 and the bound electronic refraction n_2=-4∗10^-11esu, have been measured via time-integrated beam distortion experiments in the near field. A numerical code has been written to simulate the beam distortion in order to extract the previously mentioned material parameters. In addition, I have performed time-resolved distortion measurements that provide an intuitive picture of the carrier generation process via two-photon absorption. I also characterized the optical nonlinearities in a ZnSe Fabry-Perot thin film structure (an interference filter). I concluded that the nonlinear absorption alone in the thin film is insufficient to build an effective optical limiter, as it did not show a net change in refraction using psec pulses. An innovative numerical program was developed to simulate the nonlinear beam propagation inside the Fabry-Perot structure. For comparison, pump-probe experiments were performed using both thin film and bulk ZnSe. The results showed relatively long carrier lifetimes (>300 psec) in both samples. A numerical code was written to fit the pump-probe experimental results. The fitting yielded that carrier lifetimes (recombination through traps), radiative decay rate, two-photon absorption coefficient as well as the free carrier absorption coefficient for ZnSe bulk material.

Travel behavior of Prague suburb population Abstract The hinterland of Prague is formed by the process of suburbanization, which influences the spatial structure of whole region. Changed spatial structure puts greater demand on the mobility of the population. The main aim of this work is the research of travel behavior of the Prague suburb population, who makes his movements primarily by the car. The work focuses on the factors influencing travel behavior, which are discussed in theoretical section. The next section discusses the measures against individual transport. The empirical part presents the results of survey in Prague southeast hinterland. Travel behavior is examined in terms of modal split and trip purpose. Further investagation is focused on citizens' satisfaction with the traffic situation in their municipality and reveals their sensitivity to various measures against car use. Key words: mobility, travel behavior, suburbanization, car use limiting, Prague