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Browsing by Author "Neilson, James, advisor"

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    Atmospheric processing of chemical compounds and direct measurements of particle loss by dry and wet deposition
    (Colorado State University. Libraries, 2019) Emerson, Ethan Walker, author; Farmer, Delphine, advisor; Neilson, James, advisor; Ravishankara, A. R. Ravi, committee member; Borch, Thomas, committee member; Barisas, George, committee member; Jathar, Shantanu, committee member
    Anthropogenic pollutants, like NOâ‚“ and black carbon (BC), are ubiquitous in the atmosphere and impact human health and the climate. Understanding the atmospheric fate of such pollutants is critical in understanding their impact. This work focuses on understanding the loss of two key pollutants: the chemical termination of gas phase NO and NOâ‚‚ (NOâ‚“) and the deposition of refractory black carbon (rBC) particles. Additionally, because the tools to analyze particle fluxes and coated rBC are lacking, this work describes the development of software to analyze particle fluxes and estimate the thickness of organic coatings on rBC. Removal of aerosols from the atmosphere occurs via wet and dry deposition. Black carbon (BC) is one form of aerosol that impacts atmospheric temperature, cloud formation and properties, the albedo of snow and ice surfaces, and the timing of snowmelt. Parameterization of BC dry deposition is particularly limited due to the lack of available instrumentation for measuring the process, and thus there is a lack of observational datasets with which to evaluate existing models. We present observations of dry and wet deposition rates of size-resolved coated rBC and total aerosol number by eddy covariance technique using a single particle soot photometer (SP2; Droplet Measurement Technologies Inc.) and ultra high sensitivity aerosol spectrometer (UHSAS; Droplet Measurement Technologies Inc.) from the remote Southern Great Plains ARM Climate Research facility in north-central Oklahoma. Using these data, we show that (1) wet deposition dominates the removal of rBC from the atmosphere, (2) dry deposition measurements agree with sophisticated deposition parameterizations, and (3) a simple parameterization adequately describes size-resolved deposition. We assess the implications of this parameterization in GEOS-Chem. Size-resolved deposition schemes, such as those used in current chemical transport models use schemes that have not been compared to recent measurements. Using aggregated deposition velocities from literature observations and those collected by our group, we show that the current scheme used in chemical transport models does not accurately describe observed deposition velocities. Highly sophisticated leaf level models can accurately describe the aggregated observations, but they are ill-suited to global chemical transport models. We present a simple scheme that reasonably describes size-resolved particle deposition in a simple sectional scheme that includes atmospheric parameters. The result of this update is substantial changes in particle concentrations across the globe and these impact cloud condensation nuclei, the direct and indirect effects, and PM2.5 concentrations. NOâ‚“ is a key pollutant that propagates atmospheric chemistry through the coupled HOâ‚“-NOâ‚“ cycle. Trace gas measurements from the 2015 spring and summer SONGNEX campaign conducted at the Boulder Atmospheric Observatory (BAO) in Northern Front Range Metropolitan Area of Colorado (NFRMA) are characteristic of environment impacted by oil and natural gas, agricultural operations, traffic, biogenic, and urban sources. Using a previously published PMF analysis of volatile organic compounds, we show the impact of a changing atmospheric composition due to emissions from anthropogenic sources on NOx sinks and the implications of HOâ‚“-NOâ‚“ propagation through box modelling. These results indicate that the NFRMA is sensitive to NOâ‚“ and VOC mixing ratios during spring, summer, and smoke-impacted periods.
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    Compositional tuning, crystal growth, and magnetic properties of iron phosphate oxide
    (Colorado State University. Libraries, 2017) Tarne, Michael, author; Neilson, James, advisor; Shores, Matthew, committee member; Ross, Kate, committee member
    Iron phosphate oxide, Fe3PO4O3, is a crystalline solid featuring magnetic Fe3+ ions on a complex lattice composed of closely-spaced triangles. Previous work from our research group on this compound has proposed a helical magnetic structure below T = 163 K attributed to J1-J2 competing interactions between nearest-neighbor and next-nearest-neighbor iron atoms. This was based on neutron powder diffraction featuring unique broad, flat-topped magnetic reflections due to needle-like magnetic domains. In order to confirm the magnetic structure and origins of frustration, this thesis will expand upon the research focused on this compound. The first chapter focuses on single crystal growth of Fe3PO4O3. While neutron powder diffraction provides insight to the magnetic structure, powder and domain averaging obfuscate a conclusive structure for Fe3PO4O3 and single crystal neutron scattering is necessary. Due to the incongruency of melting, single crystal growth has proven challenging. A number of techniques including flux growth, slow cooling, and optical floating zone growth were attempted and success has been achieved via heterogenous chemical vapor transport from FePO4 using ZrCl4 as a transport agent. These crystals are of sufficient size for single crystal measurements on modern neutron diffractometers. Dilution of the magnetic sublattice in frustrated magnets can also provide insight into the nature of competing spin interactions. Dilution of the Fe3+ lattice in Fe3PO4O3 is accomplished by substituting non-magnetic Ga3+ to form the solid solution series Fe3-xGaxPO4O3 with x = 0, 0.012, 0.06, 0.25, 0.5, 1.0, 1.5. The magnetic susceptibility and neutron powder diffraction data of these compounds are presented. A dramatic decrease of the both the helical pitch length and the domain size is observed with increasing x; for x > 0.5, the compounds lack long range magnetic order. The phases that do exhibit magnetic order show a decrease in helical pitch with increasing x as determined from the magnitude of the magnetic propagation vector. This trend can be qualitatively reproduced by increasing the ratio of J2/J1 in the Heisenberg model. Intriguingly, the domain size extracted from peak broadening of the magnetic reflections is nearly equal to the pitch length for each value of x, which suggests that the two qualities are linked in this unusual antiferromagnet. The last chapter focuses on the oxyfluoride Fe3PO_7-xFx. Through fluorination using low-temperature chimie douce reactions with polytetrafluoroethylene, the magnetic properties show changes in the magnetic susceptibility, isothermal magnetization, and neutron powder diffraction. The magnetic susceptibility shows a peak near T = 13 K and a zero field cooled/field cooled splitting at T = 78 K. The broad, flat-topped magnetic reflections in the powder neutron diffraction exhibit a decrease in width and increase in intensity. The changes in the neutron powder diffraction suggest an increase in correlation length in the ab plane of the fluorinated compound. Iron phosphate oxide is a unique lattice showing a rich magnetic phase diagram in both the gallium-substituted and fluorinated species. While mean-field interactions are sufficient to describe interactions in the solid solution series Fe{3-xGaxPO4O3, the additional magnetic transitions in Fe3PO7-xFx suggest a more complicated set of interactions.
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    Enabling and understanding low-temperature kinetic pathways in solid-state metathesis reactions
    (Colorado State University. Libraries, 2020) Todd, Paul Kendrick, author; Neilson, James, advisor; Finke, Richard, committee member; Prieto, Amy, committee member; Henry, Chuck, committee member; Ma, Kaka, committee member
    For the kinetic pathway to influence the outcome of a solid-state reaction, diffusion barriers must be lowered or circumvented through low-temperature chemistry. Traditional ceramic synthesis use high temperatures to overcome diffusion, yet they result in the thermodynamically stable product. If the desired product lies higher in energy, they are unattainable at such temperatures. Extrinsic parameters, like pressure, can be used to change the stability of products (kinetic trapping), yet require extreme conditions. Another strategy involves kinetically controlling the energy barriers of the reaction to select for a given product. Here, we use solid-state metathesis reactions to understand and control kinetic pathways in the formation of complex oxides and binary metal sulfides. Through simple changes to precursor composition, three unique polymorphs of yttrium manganese oxide are synthesized, two of which are metastable phases. Using in situ diagnostics, the reaction pathways are characterized to identity intermediates and the temperature regimes at which they react. Using this information we identify why different polymorphs form using different precursors. Additionally, small functional organosilicon molecules are shown to catalyze the formation of iron(II) sulfide using metathesis reactions. Here we show that the Si-O functional group stabilizes intermediate species along the pathway to avoid forming more stable intermediates. The result is higher yields of FeS2 at lower temperatures and times. The included chapters will hopefully better inform future solid-state chemists when exploring new composition spaces and reaction pathways.
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    Kinetic control of solid state metathesis reactions
    (Colorado State University. Libraries, 2017) Martinolich, Andrew J., author; Neilson, James, advisor; Prieto, Amy, committee member; Krummel, Amber, committee member; Shores, Matthew, committee member; de la Venta, Jose, committee member
    The control of solid state reaction pathways will enable the design and discovery of new functional inorganic materials. A range of synthetic approaches have been used to shift solid state chemistry away from thermodynamic control, in which the most energetically favorable product forms, towards a regime of kinetic control, so that metastable materials can be controllably produced. This work focuses on the use of solid state metathesis in the preparation of transition metal sulfides and selenides, and understanding the reaction pathways through which these reactions proceed. Through a range of structural probes combined with thermal analysis techniques, the reaction pathways are identified. The challenge of changing the pathway is then tackled, aiming to maximize mixing in the reaction mixtures to overcome the classical diffusion limitations in solids at low temperatures. Changing the reaction pathway promotes the formation of metastable intermediates and products, highlighted by the formation of the superconducting cubic polymorph of CuSe2. Future work is suggested, surrounding the idea of maximizing diffusion and mixing at low temperatures. Understanding the properties of reactants, intermediates, and products to direct the reaction pathway is paramount in controlling the pathways through which reactions occur. This will progress the field of synthetic solid state chemistry towards the ability to design materials and reactions that are not limited by thermodynamics, in turn yielding the discovery of a range of new, functional compounds.