Browsing by Author "Williams, John D., committee member"

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Open Access
Applications and advanced sintering techniques of functionally graded ZnO-based thermoelectric material
(Colorado State University. Libraries, 2017) Cramer, Corson Lester, author; Ma, Kaka, advisor; James, Susan P., advisor; Williams, John D., committee member; Sampath, Walajabad, committee member; Neilson, Jamie R., committee member
Thermoelectric generator (TEG) materials provide a unique solid-state energy conversion from heat to electricity. Nanostructured TEGs experiencing transient thermal loads at medium to high-temperatures are susceptible to degradation due to thermal stress cracking, which subsequently causes decreased lifetime. Previous efforts to prevent the thermal degradation have led to the following approaches: geometric pinning, compositional gradients, and segmentation of different materials. In the present research, functionally graded zinc oxide (ZnO) materials with graded grain size distribution were fabricated using a water sintering strategy via spark plasma sintering (SPS) with a thermal gradient in combination with modified tooling and strategic mechanical load schedules. Samples with homogeneous grain size distribution were also fabricated as a baseline for comparison. The primary objective of the work is to investigate the correlation between the processing conditions, formation of graded microstructure, and the resultant thermoelectric (TE) output performance and lifetime of the ZnO materials. The fundamental understanding of this correlation will contribute to future design of TEG materials using the approach of graded microstructure. The hypothesis is as follows: in a TEG material with graded grain size distribution, one side that consists of coarse (micron-sized) grains is exposed to the heat source. This coarse-grained side of the material can mitigate thermal stress cracking by spreading the heat more quickly during transient heating and thus provide improved thermal stability. The other side of the TEG material consists of fine grains (submicron-sized) and still exhibits high efficiency. In the current study, both continuously graded ZnO materials and a five-layer discretely graded ZnO material were fabricated. Microstructural characterization shows that the grain size gradient of the continuously graded materials across a 10-mm thickness goes from submicron scale (average size ~ 180 nm) to micron scale (~1.2 μm). The thermoelectric properties of the baseline ZnO materials with uniform grain sizes were measured. Using the data obtained from those samples with uniform grain sizes, the peak efficiencies of the continuously graded materials and the five-layer graded materials were simulated and compared to the experimentally measured values. The lifetime of the ZnO samples was evaluated from the electrical resistance at the cycling temperature. The results of the final efficiencies suggest that the thermoelectrical performance of the ZnO materials benefit from the grain size gradation. In addition, the sintering behavior of the continuously graded ZnO system is investigated and compared to that of the isothermally sintered samples to establish a predictive model of the microstructure (density-grain size-time relation). A discrepancy is observed between the prediction of the continuously graded materials and the experimental results. This discrepancy is attributed to a stress shielding that develops during sintering due to differential sintering from the temperature gradient. The stress shielding occurs when denser, and thus stiffer material develops adjacent to less dense and less stiff material causing the stress to vary because the stress is not evenly distributed. The stress shielding effect during sintering is further investigated through theoretical sintering equations. Using the viscoelastic analogy in sintering, the stress to be added to the sample during sintering in a thermal gradient is quantified to compensate the discrepancy from the samples sintered isothermally based on an average strain rate difference.
Open Access
Cavity enhanced Thomson scattering for plasma diagnostics
(Colorado State University. Libraries, 2019) Friss, Adam J., author; Yalin, Azer P., advisor; Marchese, Anthony J., committee member; Polk, James E., committee member; Williams, John D., committee member; Yost, Dylan C., committee member
Measurements of electron number density (nₑ) and electron energy distribution function (EEDF) are of great importance to the study of weakly ionized plasmas, such as those used in laser preionization, semiconductor processing and fabrication, electric propulsion devices, and atmospheric pressure plasmas. Currently, these parameters can be measured by physical probes, e.g. Langmuir probes, or with the use of non-intrusive Laser Thomson Scattering (LTS). While physical probe measurements have been an indispensable tool of the plasma physics community, they affect plasma source operation and result in unwanted plasma perturbation. LTS measurements are appealing due to the non-perturbing nature of the technique, but suffer from low signal levels and optical interference, making application to low-density plasma systems very challenging. This dissertation describes the development of a novel cavity enhanced Thomson scattering (CETS) diagnostic that enables sensitive, non-perturbing measurements of plasma properties. The technique is based upon frequency locking a high-power, narrow-linewidth continuous wave (CW) laser source to a high-finesse optical cavity to build-up intra-cavity power to a level where it can serve as an interrogation laser source. In this way, intra-cavity powers as high as ~12 kW have been generated from a ~5 W laser source and sensitive measurements on a plasma source and gas samples placed within the optical cavity were performed. Despite the CETS technique being widely applicable to a variety of plasma sources, this work focused on the measurement of electric propulsion devices, such as hollow cathodes and Hall effect thrusters. These devices are used as in-space propulsion systems on satellites and scientific probes and may be used as the primary in-space propulsion systems for exploration of the Moon, Mars, and beyond. This work describes the development of the CETS diagnostic including the cavity locking approach, creation of a gas and plasma scattering model, and the development of both a low- and high-power experimental instrument. CETS is demonstrated by performing rotational Raman and Rayleigh scattering measurements on a variety of gases and by performing Thomson scattering measurements in the plume of a hollow cathode. The cathode measurement campaign was conducted over a range of operating conditions, and electron densities and temperatures in the range of ~10¹² cm⁻³ and ~3 eV were measured. Finally, a mobile fiber coupled version of the CETS setup designed for use in large vacuum facilities is presented, and Thomson scattering measurements made with the mobile instrument in the plume of a hollow cathode are discussed.
Open Access
CdTe alloys and their application for increasing solar cell performance
(Colorado State University. Libraries, 2016) Swanson, Drew E., author; Sampath, W. S., advisor; Sites, James R., committee member; Williams, John D., committee member; Popat, Ketul, committee member
Cadmium Telluride (CdTe) thin film solar is the largest manufactured solar cell technology in the United States and is responsible for one of the lowest costs of utility scale solar electricity at a purchase agreement of $0.0387/kWh. However, this cost could be further reduced by increasing the cell efficiency. To bridge the gap between the high efficiency technology and low cost manufacturing, a research and development tool and process was built and tested. This fully automated single vacuum PV manufacturing tool utilizes multiple inline close space sublimation (CSS) sources with automated substrate control. This maintains the proven scalability of the CSS technology and CSS source design but with the added versatility of independent substrate motion. This combination of a scalable deposition technology with increased cell fabrication flexibility has allowed for high efficiency cells to be manufactured and studied. The record efficiency of CdTe solar cells is lower than fundamental limitations due to a significant deficit in voltage. It has been modeled that there are two potential methods of decreasing this voltage deficiency. The first method is the incorporation of a high band gap film at the back contact to induce a conduction-band barrier that can reduce recombination by reflecting electrons from the back surface. The addition of a Cd1-xMgxTe (CMT) layer at the back of a CdTe solar cell should induce this desired offset and reflect both photoelectrons and forward-current electrons away from the rear surface. Higher collection of photoelectrons will increase the cells current and the reduction of forward current will increase the cells voltage. To have the optimal effect, CdTe must have reasonable carrier lifetimes and be fully depleted. To achieve this experimentally, CdTe layers have been grown sufficiently thin to help produce a fully depleted cell. A variety of measurements including performance curves, transmission electron microscopy, x-ray photoelectron spectroscopy, and energy-dispersive x-ray spectroscopy were performed to characterize these cells. Voltage improvements on the order of 50 mV are presented at a thin (1 μm) CdTe absorber condition. However an overall reduction in fill factor (FF) is seen, with a strong reduction in FF as the magnesium incorporation is increased. Detailed material characterization shows the formation of oxides at the back of CdMgTe during the passivation process. A CdTe capping layer is added to reduce oxidation and help maintain the uniformity of the CdMgTe layer. A tellurium back contact is also added in place of a carbon paint back contact, reducing the impact of the valance band offset (VBO) from the CMT. With the addition of the capping layer and tellurium back contact a consistent 50 mV increase is seen with improved FF. However this voltage increase is well below modeled Voc increases of 150 mV. CMT double hetero-structures are manufactured and analyzed to estimate the interface recombination at the CdTe/CMT interface. The CdTe/CMT interface is approximated at 2*105 cm s-1 and modeling is referenced predicting significant reduction in performance based on this interface quality. To improve interface quality by removing the need for a vacuum break, the deposition hardware is incorporated into the primary deposition system. Second, CdTe has a somewhat higher band gap than optimal for single-junction terrestrial solar-cell power generation. A reduction in the band gap could therefore result in an overall improvement in performance. To reduce the band gap, selenium was alloyed with CdTe using a novel co-sublimation extension of the close-space-sublimation process. Co-sublimated layers of CdSeTe with various selenium concentrations were characterized for optical absorption and atomic concentrations, as well as to track changes in their morphology and crystallinity. The lower band-gap CdSeTe films were then incorporated into the front of CdTe cells. This two-layer band-gap structure demonstrated higher current collection and increased quantum efficiency at longer wavelengths. Material characterization shows the diffusion of selenium through the CdTe during passivation resulting in improved in lifetime and a reduced voltage deficit at lower band gaps.
Open Access
Characterization of scandium oxide thin films for use in interference coatings for high-power lasers operating in the near-infrared
(Colorado State University. Libraries, 2010) Krous, Erik M., author; Menoni, Carmen S., advisor; Marconi, Mario C., committee member; Williams, John D., committee member
The work presented in this thesis aims to investigate scandium oxide (scandia), deposited using dual ion beam sputtering, as a high-index material for interference coatings to be implemented in high-power lasers. Ion beam sputtered scandia coatings have the potential to allow for the power scaling of high-power lasers operating in the near-infrared. Ion beam sputtering is the technique currently used by many commercial companies to produce low-loss, high-damage-threshold coatings required by lasers operating with high fluences. The development of scandia, and other thin film materials, requires the reduction of defects in the material through modification of growth processes and post deposition treatment. Material defects give rise to absorption of laser light and laser induced damage initiation sites. The growth parameter investigated in this work is the oxygen partial pressure in the deposition chamber during the reactive sputtering process of a metal Sc target to form Sc2O3. The film properties are sensitive to the oxygen partial pressure. At 2 μTorr oxygen partial pressure, the films are metallic and highly absorbing with an absorption, at λ = 1.064 μm, of > 104 ppm. The absorption decreases to 10 ppm at 5 μTorr oxygen partial pressure and at 38 μTorr, the absorption reaches a value of 35 ppm. This, along with the increase in absorption near the optical band edge, suggests an increase in shallow-type defect concentrations for increasing oxygen partial pressures. The observed defects contain unpaired electrons, as assessed by electron paramagnetic measurements, that have a paramagnetic absorption signal with principle g-values [gxx, gyy, gzz] = [2.018, 2.019, 2.058]. Generally, the concentration of the paramagnetic species increased with increasing oxygen partial pressure. These spin defects are possibly O2̅ interstitials in the deposited films. These defects contribute to an approximately 40% increase in the film stress observed in x-ray diffraction measurements and measurements of stress-induced fused silica substrate curvature.
Open Access
Combining fundamental studies with advanced characterization for analyzing nitric oxide polymer systems
(Colorado State University. Libraries, 2014) Joslin, Jessica Marie, author; Reynolds, Melissa M., advisor; Ladanyi, Branka M., committee member; Krummel, Amber T., committee member; Ackerson, Christopher J., committee member; Williams, John D., committee member
Nitric oxide (NO) releasing materials have been investigated over the past couple of decades as potential biomaterials. A multitude of NO releasing platforms have been reported with different NO release properties indicating use in various bioapplications. Despite the positive implications associated with these materials, the field is currently limited by a couple of major issues. First, the reservoirs of NO stored in current systems do not allow the prolonged and controllable release necessary for long-term usage. Additionally, the field has experienced a lack of complete characterization of both the NO loading and release processes associated with these systems. To develop NO releasing platforms with enhanced NO reservoirs and controllable NO release profiles, fundamental studies are required to probe the physical processes that occur in these polymers. To enhance NO reservoirs in polymer systems, it is critical to probe the efficiency of the NO loading process. Systematic studies are presented where the efficiency and nature of S-nitrosothiol NO donor formation is investigated in a polymer environment. The nitrosating agent and polymer presence have a significant impact on the kinetics of S-nitrosation. Also, due to the versatile nature of nitrosation, NO byproducts that form competitively with S-nitrosothiols are characterized. By tuning the polymer functional groups, competitive nitrosation products can be eliminated and NO recoveries enhanced. Another critical obstacle towards understanding NO materials involves probing NO donor behavior in conjunction with NO release. For model polymers containing covalently linked S-nitrosothiol moieties, spectroscopy is coupled to direct NO detection to fully characterize the NO loading and release stages. Decomposition of the S-nitrosothiol moiety is directly correlated to NO release. S-nitrosothiol blended films are also investigated to determine the spatial distribution of NO release, which is critical to ensure a localized NO effect at the material surface. Finally, NO releasing polymer substrates are exposed to water plasma processing conditions. Surface wettability is significantly enhanced, while the NO release kinetics are maintained, suggesting that these materials can withstand processing towards tunable surface properties. Overall, fundamental and systematic studies of model NO releasing materials are presented that have not been formerly considered. Only by characterizing these materials completely can this class of biomaterial be better understood towards the ability to control the therapeutic and surface properties.
Open Access
Continuous-wave cavity ring down spectroscopy sensor for Hall thruster erosion measurement
(Colorado State University. Libraries, 2011) Tao, Lei, author; Yalin, Azer P., advisor; Marchese, Anthony J., committee member; Menoni, Carmen S., committee member; Williams, John D., committee member
Hall thruster and other Electric propulsion (EP) devices have become appealing alternatives to traditional chemical propulsion thrusters for space applications due to this high specific impulse (Isp), which allows high fuel efficiency. However, the uncertainty of the lifetime for Hall thruster hinders its development in future applications requiring a long operational time (several thousands of hours). Sputter erosion of boron nitride (BN) acceleration channel wall is principal lifetime limitation for Hall thrusters. The sputtered particles can redeposit causing a critical contamination effect. There is an urgent need for improved experimental tools to understand the BN sputter erosion process and lifetime assessment for Hall thrusters. The present research applies continuous wave cavity ring down Spectroscopy (CW-CRDS) as a diagnostic tools to study the sputter erosion process for Hall thrusters. Two CW-CRDS erosion sensors have been developed for in situ monitoring of sputtered manganese (Mn) and BN. As a stepping stone towards BN detection, a Mn erosion sensor was first developed. This sensor is based upon detection of Mn atoms via an absorption line from ground state at a wavelength of 403.076 nm. Measurements of sputtered Mn atom number density and its hyperfine structure are presented. Additionally, end-point detection has been done for a multilayer target, which can be potentially applied to the industrial sputtering systems. The same system has also been applied for detecting eroded atoms from the acceleration channel wall in an anode layer type Hall thruster. The results show the validity of the CW-CRDS erosion sensor for Hall thruster lifetime estimation. A BN erosion sensor has also been developed for the detection of sputtered boron atoms from Hall thrusters by probing atomic absorption lines of boron (250 nm) with CW-CRDS. A photonic crystal fiber was used to couple the ultraviolet laser light to the cavity within the vacuum chamber. The experimental detection limits and signal-to-noise values show potential for Hall thruster BN erosion studies. Finally, the velocity distributions of sputtered boron atoms at different ion energies were measured with laser induced fluorescence (LIF). These velocity distribution are necessary for interpretation of signals from the BN erosion sensor.
Open Access
Development of a Hall thruster test facility
(Colorado State University. Libraries, 2012) Leach, Randolph W., author; Yalin, Azer P., advisor; Williams, John D., committee member; Menoni, Carmen S., committee member
The present thesis details the development of a Hall thruster test facility for low power (<600 W) thrusters. The facility is based on a vacuum chamber, two standard cryogenic pumps and one modified cryogenic pump. The modified cryogenic pump is outfitted with custom built internal components, which are referred to as a cryosail. Estimation as well as measurement of pumping speeds of the two cryogenic pumps and cryosail were conducted resulting in an overall measured pumping speed of 10,500 L/s for Xenon. The ultimate base pressure of the system was 4x10-8 Torr. A SPT-70 Hall thruster was operated at various conditions and set points to include fine tuning the current to the magnets to find efficient thruster operation. Ion current densities at points downstream of the thruster's exit plane were examined by a Faraday probe. Although operation at nominal thruster operating conditions was not achieved, likely due to a problem with magnetic coils, the thruster operation did allow preliminary measurements by Cavity Ring-Down Spectroscopy of sputtered Boron originating from the thruster channel wall.
Open Access
Development of a model for baffle energy dissipation in liquid fueled rocket engines
(Colorado State University. Libraries, 2010) Miller, Nathan A., author; Kirkpatrick, Allan T., advisor; Duflot, Jeanne, committee member; Williams, John D., committee member
In this thesis the energy dissipation from a combined hub and blade baffle structure in a combustion chamber of a liquid-fueled rocket engine is modeled and computed. An analytical model of the flow stabilization due to the effect of combined radial and hub blades was developed. The rate of energy dissipation of the baffle blades was computed using a corner-flow model that included unsteady flow separation and turbulence effects. For the inviscid portion of the flow field, a solution methodology was formulated using an eigenfunction expansion and a velocity potential matching technique. Parameters such as local velocity, elemental path length, effective viscosity, and local energy dissipation rate were computed as a function of the local angle α for a representative baffle blade, and compared to results predicted by the Baer-Mitchell blade dissipation model. The sensitivity of the model to the overall engine acoustic oscillation mode, blade length, and thickness was also computed and compared to previous results. Additional studies were performed to determine the sensitivity to input parameters such as the dimensionless turbulence coefficient, the location of the potential difference in the generation of the dividing streamline, the number of baffle blades and the size of the central hub. Stability computations of a test engine indicated that when the baffle length is increased, the baffles provide increased stabilization effects. The model predicts greatest dissipation for radial modes with a hub radius at approximately half the chamber's radius.
Open Access
Development of Cd1-xMgxTe thin films for application as an electron reflector in CdS/CdTe solar cells
(Colorado State University. Libraries, 2014) Kobyakov, Pavel S., author; Sampath, W. S., advisor; Sites, James R., committee member; Olsson, N. Anders, committee member; Williams, John D., committee member
Efficiencies of CdS/CdTe photovoltaic cells significantly lag behind their theoretical limit, primarily because open-circuit voltage (VOC) of record efficiency cells (872 mV) is well below what is expected for the CdTe band gap (1.5 eV). A substantial VOC improvement can be achieved through addition of an electron reflector (ER) layer to CdTe devices. The ER layer forms a conduction-band barrier that reflects minority-charge carriers (i.e. electrons in p-type CdTe) away from the back surface. Similar to back-surface fields in c-Si, III-V, and CIGS solar cells, the ER strategy is expected to reduce back-surface recombination and is estimated to increase CdTe VOC by about 200 mV based on numerical simulation. The presented research investigates the addition of a thin layer of wider band gap Cd1-xMgxTe (CMT) to achieve a CdTe ER structure. First, a novel co-sublimation process was developed for deposition of Cd1-xMgxTe thin films that demonstrates excellent experimental capabilities, commercial viability, and improved alloy control over other techniques. Next, the effects of processing on material properties of CMT deposition onto CdS/CdTe structures were investigated. It was discovered that substrate temperature during CMT deposition is a critical parameter for achieving uniform CMT film coverage on polycrystalline CdTe. Furthermore, CMT film growth was found to be epitaxial on CdTe where the CMT films retain the same microstructural features as the underlying CdTe grains. Despite film uniformity, significant Mg loss from the CMT film, oxide formation, and a reduction of the optical band gap was found after CdCl2-based passivation treatments. Preliminary process optimization found that band gap degradation can be minimized by utilizing MgCl2 in addition to CdCl2 as a treatment source material. Finally, development of CdS/CdTe/Cd1-xMgxTe electron reflector devices demonstrated a barrier behavior at high voltage bias and improved voltage when CdTe thickness is held below 1 μm. Additional electro-optical characterization and device modeling was used to understand the source of this device behavior. The results suggest the CdTe/Cd1-xMgxTe interface is likely free of detrimental electronic defects and the barrier behavior comes from a larger than expected valence band offset for the material system. Finally, future work to improve ER device performance is suggested.
Open Access
High heat flux phase change thermal management of laser diode arrays
(Colorado State University. Libraries, 2016) Bevis, Taylor A., author; Bandhauer, Todd M., advisor; Williams, John D., committee member; De Miranda, Michael A., committee member
Laser diodes are semiconductor devices than convert electrical work into light emitted at a specific wavelength over a small spectral bandwidth at a high intensity. A small array of laser diodes can be fabricated on an internally reflective bar that emits light through one edge. If a large number of edge-emitting bars are packed closely together and arrayed to emit light towards the same target, a very high brightness (i.e., light power per unit area) can be achieved, which is useful for a wide range of applications, including advanced manufacturing, inertial confinement fusion energy, and pumping laser gain media. The principle limit for achieving higher brightness is thermal management. State of the art laser diodes generate heat at fluxes in excess of 1 kW cm-2 on a plane parallel to the light emitting edge. As the laser diode bars are packed closer together, it becomes increasingly difficult to remove the heat generated by the diodes in the diminishing space between neighboring diode bars. In addition, the wavelength of the laser diode changes with temperature, and minimizing the variation in wavelength among diodes in very large arrays is very challenging. Thermal management of these diode arrays using conduction and natural convection is practically impossible, and therefore, some form of forced convective cooling must be utilized. Cooling large arrays of laser diodes using single-phase convection heat transfer has been investigated for more than two decades by multiple investigators. Unfortunately, either large temperature increases or very high flow velocities must be utilized to reject heat to a single phase fluid, and the practical threshold for single phase convective cooling of laser diodes appears to have been reached. In contrast, liquid-vapor phase change heat transport can occur with a negligible increase in temperature and, due to a high enthalpy of vaporization, at comparatively low mass flow rates. However, there have been no prior investigations at the conditions required for high brightness edge emitting laser diode arrays: heat fluxes >1 kW cm-2 and a volumetric heat generation rate >10 kW cm-3. In the current investigation, flow boiling heat transfer at heat fluxes up to 1.1 kW cm-2 was studied in a microchannel heat sink with plurality of very small channels (45 × 200 mm) for a phase change fluid (R134a). The high aspect ratio channels (5:1) were manufactured using MEMS fabrication techniques, which yielded a large heat transfer surface area to volume ratio in the vicinity of the laser diode. To characterize the heat transfer performance, a test facility was constructed that enabled testing over a wide range of fluid properties and operating conditions. Due to the very small geometric features, significant heat spreading was observed, necessitating numerical methods to determine the average heat transfer coefficient from test data. The heat transfer correlations were predicted well (mean absolute error, MAE, of ±38.7%) by the correlation of Bertsch et al. This correlation was modified to account for the effect of fin conduction, in the calculation of average heat flux, which yielded an improved MAE of ±8.1%. The new correlation was then used to investigate a range of potential phase change fluids and an alternative microchannel geometry for the laser diode phase change heat exchanger. Finally, a next generation test section design and operating conditions are proposed which are expected to improve diode array brightness up to 5.3× over the state of the art with R134a. If ammonia is used at the working fluid instead of R134a, the brightness could potentially increase by more than 17× over the state of the art.
Open Access
Hydraulic effects of biofilms on the design and operation of wastewater forcemains
(Colorado State University. Libraries, 2016) Michalos, Christopher T., author; Thornton, Christopher I., advisor; Grigg, Neil S., committee member; Julien, Pierre Y., committee member; Williams, John D., committee member
The impact of biofouling on wastewater forcemains is generally not accounted for in current design practice and little information is available in literature regarding the effect of wastewater biofilms on forcemain hydraulics. In practice, many engineers select a clean water, new pipe roughness factor, to perform hydraulic calculations which may lead to under-sizing wastewater lift station pumps. Forcemains have to cope with a particularly challenging task; they have to ensure that solids contained in the wastewater (sand, gravel, organics) are readily transported along with the wastewater. Forcemain design standards generally recommend a velocity of 2.0 ft/s (0.6 m/s) to prevent deposition of solids and a velocity of 3.5 ft/s (1.1 m/s) to re-suspend solids that may have settled. To further complicate forcemain design and operation; wastewater lift station pumps generally operate intermittently which requires remobilization of any material that may have settled while the pumps remain idle. Therefore, forcemains must be designed to be self-cleaning in order to prevent solids deposition which could cause increased sulfide production leading to corrosion and odor issues; loss of capacity through a reduction of cross sectional area; or even blockage at low points, or at the toe of an adversely sloped pipe leading to costly removal. The goal of this research is to identify short-comings in current forcemain design practice by 1) evaluating the hydraulic effect of biofilms on the absolute roughness (ks) of forcemains; 2) evaluating the hydraulic effect of biofilms on Hazen-Williams C factor; and 3) determine critical velocity required for sediment transport, air clearing, self-cleansing, and optimal diameter of forcemains, which are not identified in forcemain design standards. Operational data were collected and evaluated for 20 municipal wastewater forcemains located in the United States. Data from previous studies, academic research, reports, and published papers were used to supplement and support research findings. A total of 415 data points obtained from 68 forcemain systems ranging from 3- to 66 inches in diameter were evaluated as part of this research. Results of the hydraulic analysis determined that 44% of the systems evaluated were operating at velocities between 2- and 3.5 ft/s and 16% of systems were operating at velocities less than 2 ft/s; indicating that these systems are over designed and do not provide sufficient velocity to re-suspend solids promoting sedimentation. The hydraulic effect of biofilms on forcemain flow resistance was evaluated and determined that ks and C factor varied with forcemain velocity. Calculated values of ks ranged from approximately 35 mm to 0.01 mm, with larger values occurring at velocities less than 1 m/s (3.3 ft/s). The upper range of ks values are orders of magnitude larger than the standard clean water, new pipe ks value found in literature. C factor results ranged from approximately 30 to 150; approximately 60% of forcemain systems evaluated are operating at C factors less than 100, which is much lower than the recommended values of 130 – 150, depending on pipe material. Results suggest that biofilms effect forcemains in a similar manner regardless of pipe diameter, material, or age. Although velocity was determined to be the principle factor affecting ks and C factor; a comparison of the C factor results to ks results show that C factor is dependent upon both velocity and diameter. Equations were developed to estimate ks and C factor and should be utilized along with the Colebrook-White / Darcy-Weisbach and Hazen-Williams equations to estimate the friction headloss for forcemains. The required design velocity for self-cleansing, sediment transport, air clearing, and economical diameter ranges from approximately 4- to 11 ft/s, depending on diameter. Selecting a design velocity between 2 ft/s (0.6 m/s) and 3.5 ft/s (1.1 m/s) may not be appropriate and the minimum design velocity should be selected upon either the self-cleansing velocity or economical pipe sizing. Although each system should be evaluated to determine the correct minimum design velocity based upon the proposed system properties, these results indicate that the minimum forcemain design velocity should be at least 5 ft/s (1.5 m/s).
Open Access
Laser diagnostic method for plasma sheath potential mapping
(Colorado State University. Libraries, 2016) Walsh, Sean P., author; Yalin, Azer P., advisor; Williams, John D., committee member; Rocca, Jorge G., committee member
Electric propulsion systems are gaining popularity in the aerospace field as a viable option for long term positioning and thrusting applications. In particular, Hall thrusters have shown promise as the primary propulsion engine for space probes during interplanetary journeys. However, the interaction between propellant xenon ions and the ceramic channel wall continues to remain a complex issue. The most significant source of power loss in Hall thrusters is due to electron and ion currents through the sheath to the channel wall. A sheath is a region of high electric field that separates a plasma from a wall or surface in contact. Plasma electrons with enough energy to penetrate the sheath may result emission of a secondary electron from the wall. With significant secondary electron emission (SEE), the sheath voltage is reduced and so too is the electron retarding electric field. Therefore, a lower sheath voltage further increases the particle loss to the wall of a Hall thruster and leads to plasma cooling and lower efficiency. To further understand sheath dynamics, laser-induced fluorescence is employed to provide a non-invasive, in situ, and spatially resolved technique for measuring xenon ion velocity. By scanning the laser wavelength over an electronic transition of singly ionized xenon and collecting the resulting fluorescence, one can determine the ion velocity from the Doppler shifted absorption. Knowing the velocity at multiple points in the sheath, it can be converted to a relative electric potential profile which can reveal a lot about the plasma-wall interaction and the severity of SEE. The challenge of adequately measuring sheath potential profiles is optimizing the experiment to maximize the signal-to-noise ratio. A strong signal with low noise, enables high resolution measurements and increases the depth of measurement in the sheath, where the signal strength is lowest. Many improvements were made to reduce the background luminosity, increase the fluorescence intensity and collection efficiency, and optimize the signal processing equipment. Doing so has allowed for a spatial resolution of 60 microns and a maximum depth of measurement of ~2 mm depending on conditions. Sheaths surrounding common Hall thruster ceramics at various plasma conditions were measured in an attempt to determine the effect of SEE and a numerical analysis of the plasma-wall interactions was conducted to further understand the phenomena and compare against obtained data.
Open Access
Laser diagnostic methods for plasma sheath potential mapping and electric field measurement
(Colorado State University. Libraries, 2013) Rath, Jordan L., author; Yalin, Azer P., advisor; Williams, John D., committee member; Rocca, Jorge G., committee member
This thesis presents the development of two laser diagnostic approaches for electric field measurements in plasmas and gases. Hall effect thrusters, and other electric propulsion devices, have limited lifetimes due to the erosion of components by ion bombardment of surfaces. A better understanding of the electric field structure in the plasma sheaths near these surfaces would enable researchers to improve thruster designs for extended lifetime and higher efficiency. The present work includes the development of a laser induced fluorescence technique employing a diode laser at 835 nm to measure spatially resolved xenon ion velocity distribution functions (IVDFs) near plasma-surface interfaces (sheaths), from which electric field and spatially-resolved potentials can be determined. The optical setup and demonstrative measurements in a low-density multi-pole plasma source are presented. Also included in this thesis is development of a cavity-enhanced polarimetry technique for electric field measurements in gases via the optical Kerr effect. The high finesse optical cavity allows sensitive measurement of the electric field induced birefringence, improving upon the detection limits of past work using related multi-pass techniques. Experimental results are presented for carbon dioxide, nitrogen, oxygen and air along with comparisons to model predictions based on published Kerr constants.
Open Access
Optimization of the front contact to minimize short-circuit current losses in CdTe thin-film solar cells
(Colorado State University. Libraries, 2015) Kephart, Jason Michael, author; Sampath, W. S., advisor; Sites, James R., committee member; Williams, John D., committee member; McCamy, James M., committee member
With a growing population and rising standard of living, the world is in need of clean sources of energy at low cost in order to meet both economic and environmental needs. Solar energy is an abundant resource which is fundamentally adequate to meet all human energy needs. Photovoltaics are an attractive way to safely convert this energy to electricity with little to no noise, moving parts, water, or arable land. Currently, thin-film photovoltaic modules based on cadmium telluride are a low-cost solution with multiple GW/year commercial production, but have lower conversion efficiency than the dominant technology, crystalline silicon. Increasing the conversion efficiency of these panels through optimization of the electronic and optical structure of the cell can further lower the cost of these modules. The front contact of the CdTe thin-film solar cell is critical to device efficiency for three important reasons: it must transmit light to the CdTe absorber to be collected, it must form a reasonably passive interface and serve as a growth template for the CdTe, and it must allow electrons to be extracted from the CdTe. The current standard window layer material, cadmium sulfide, has a low bandgap of 2.4 eV which can block over 20% of available light from being converted to mobile charge carriers. Reducing the thickness of this layer or replacing it with a higher-bandgap material can provide a commensurate increase in device efficiency. When the CdS window is made thinner, a degradation in electronic quality of the device is observed with a reduction in open-circuit voltage and fill factor. One commonly used method to enable a thinner optimum CdS thickness is a high-resistance transparent (HRT) layer between the transparent conducting oxide electrode and window layer. The function of this layer has not been fully explained in the literature, and existing hypotheses center on the existence of pinholes in the window layer which are not consistent with observed results. In this work numerous HRT layers were examined beginning with an empirical optimization to create a SnO₂-based HRT which allows significantly reduced CdS thickness while maintaining diode quality. The role of this layer was explored through measurement of band alignment parameters via photoemission. These results suggest a negative correlation of work function to device open-circuit voltage, which implies that non-ideal band alignment at the front interface of CdTe is in large part responsible for the loss of electronic quality. Several scenarios explored through 1-dimensional modeling in the SCAPS program corroborate this theory. A sputter-deposited (Mg,Zn)O layer was tested which allows for complete elimination of the CdS window layer with an increase in open-circuit voltage and near complete transmission of all above-bandgap light. An additional window layer material--sputtered, oxygenated CdS--was explored for its transparency. This material was found only to produce high efficiency devices with an effective buffer layer such as the optimized SnO₂-base HRT. The dependence of chemical, optical, electrical, and device properties on oxygen content was explored, and the stability of these devices was determined to depend largely on the minimization of copper in the device. Both sputter-deposited alloy window layers appeared to have tunable electron affinity which was critical to optimizing band alignment and therefore device efficiency. Several scenarios explored through 1-dimensional modeling in the SCAPS program corroborate this theory. Both window layers allowed an AM1.5G efficiency increase from a baseline of approximately 13% to 16%.
Open Access
Part I: Electroreductive polymerization of nanoscale solid polymer electrolytes for three-dimensional lithium-ion batteries. Part II: Physical characterization and hydrogen sorption kinetics of solution-synthesized magnesium nanoparticles
(Colorado State University. Libraries, 2010) Arthur, Timothy Sean, author; Prieto, Amy L. (Amy Lucia), advisor; Bailey, Travis Slade, committee member; Elliott, Cecil Michael, committee member; Shores, Matthew P., committee member; Williams, John D., committee member
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Open Access
Quantification of shear stress in a meandering native topographic channel using a physical hydraulic model
(Colorado State University. Libraries, 2011) Ursic, Michael E., author; Thornton, Christopher I., advisor; Abt, Steven R., committee member; Williams, John D., committee member
Current guidelines for predicting increases in shear stress in open-channel bends were developed from investigations that were primarily prismatic in cross section. This study provides possible increases in shear stress relative to approach flow conditions resulting from planimetric and topographic geometric features. Boundary shear stress estimates were determined by several methods utilizing acoustic Doppler velocimeter (ADV) and Preston tube data in a physical model of a full meander representing native topographic features found in the Middle Rio Grande. Methods examined include: the law of the wall, Preston tube, turbulent Reynolds stress approximations, and a turbulent kinetic energy (TKE) proportionality constant approach. Results from each method were compared by magnitude and distribution and limitations were noted. Measured boundary shear stresses in the bend were, in some instances, nearly thirteen times the approach shear stress. Relationships were determined for the expected increase that may provide practical application. Measured bend velocities were four times greater than approach velocities and relationships were determined between velocity and bend geometry. Multipliers for shear stress and velocities were determined for one-dimensional model results.
Open Access
Real-time erosion measurements of the HiVHAc and SPT-70 Hall thrusters via cavity ring-down spectroscopy
(Colorado State University. Libraries, 2014) Lee, Brian Christopher, author; Lundeen, Stephen R., advisor; Yalin, Azer P., advisor; Roberts, Jacob L., committee member; Krueger, David A., committee member; Williams, John D., committee member
Electric propulsion has moved to the forefront of in-space propulsion in recent years. By making exceptionally efficient use of propellant, electric propulsion devices have significantly reduced the cost of some missions and enabled others, which had not previously been possible. Among these devices, Hall thrusters have shown particular promise. However, for many thrusters of interest, sputter erosion of the insulating channel remains a problem and continues to limit the thruster lifetime. Diagnostic tools to assess the absolute channel erosion rate rapidly remain limited. This thesis describes the use of ultraviolet cavity ring-down spectroscopy (CRDS) as a real-time diagnostic of sputtered boron atoms in the thruster plume. Cavity ring-down spectroscopy is an ultra-sensitive laser-absorption technique which is particularly apt at measuring trace species number densities in the gas phase. In this work, ground-state atomic boron, which was sputtered from the thruster channel, was measured near 250 nm. The interrogating laser was swept across the exit plane of a Hall thruster, providing spatially-resolved boron number density measurements. Additionally, laser-induced fluorescence was used to measure the velocity of sputtered boron along the thruster axis, which were the first measurements of its kind. The measured boron number density and velocity component together provided a total boron flux from the thruster, and therefore, a channel erosion rate. Channel erosion rates of the NASA HiVHAc and the SPT-70 Hall thrusters were measured using CRDS. Absolute erosion rates and trends with operating condition were investigated. Both thrusters were found to erode at rates proportional to the discharge power, which is consistent with the available literature. Profilometry was also used to measure the channel erosion rate of the SPT-70 thruster and revealed a factor of ~5 disagreement with estimates made by CRDS. Calcium fluoride (CaF2) prism retroreflectors were developed, for the first time, as a means to improve both the bandwidth and finesse of optical cavities in the ultraviolet region. The CRDS technique used in thruster erosion measurements employed multilayer dielectric mirrors, which have relatively poor performance in the ultraviolet region. Calcium fluoride prism retroreflectors show promise to outperform the best available dieletric mirrors at 250 nm as well as provide broadband cavity operation. The design, construction, and characterization of the CaF2 prisms is presented.
Open Access
Servo blower control for powered air purifying respirators
(Colorado State University. Libraries, 2012) Wagner, Nicholas Adam, author; Bradley, Thomas H., advisor; Williams, John D., committee member; Young, Peter M., committee member
Powered air purifying respirators (PAPRs) are a form of respiratory protection that uses a motor-coupled fan to provide filtered air to a user through a positive pressure mask. Three types of PAPR devices have been developed of which breath-responsive PAPRs are the most recent. The benefits of breath-responsive PAPRs have been identified and regulatory performance requirements have been put in place for these devices, however, no devices have been certified by any regulatory agencies. This study proposes a novel conceptual design for a breath-responsive PAPR and describes a dynamic simulation of the characteristics of this new PAPR compared to a constant flow design as exercised by a simulated breathing cycle. Additionally, this study describes a prototype of the breath-responsive concept with experimental evaluation of the prototype against regulatory requirements and conceptual design targets.
Open Access
Unification of large-scale laboratory rainfall erosion testing
(Colorado State University. Libraries, 2014) Robeson, Michael D., author; Thornton, Christopher I., advisor; Abt, Steven R., committee member; Watson, Chester C., committee member; Williams, John D., committee member
Water pollution degrades surface waters making them unsafe for drinking, fishing, swimming, and other activities. The movement of sediment and pollutants carried by sediment over land surfaces and into water bodies is of increasing concern with regards to clean waters, pollution control, and environmental protection. Due to increasing environmental concerns about sediment in water bodies derived from construction sites, along with increasingly stringent United States Environmental Protection Agency (USEPA) regulations, it is imperative to be able to have a uniform means to compute soil loss determined at large-scale laboratory rainfall-induced erosion facilities that can eventually be applied to construction sites. This dissertation utilized bare-soil data from the most commonly-utilized large-scale rainfall testing laboratories in the erosion-control industry to develop a unifying prediction equation that can be utilized to provide a proper foundation for determining design parameters to meet USEPA stabilization requirements. The developed equation was determined to be a function of the following key parameters: rainfall intensity, plot area, duration, slope gradient, median raindrop size, raindrop kinetic energy, percentage of clay in the soil, and compacted soil percentage. The developed equation for the prediction of rainfall-induced soil loss, developed from sixty-eight data points collected for this study, had a coefficient of determination (R2) of 0.88. The prediction equation unifies large-scale laboratory rainfall erosion testing and provides a means to determine the appropriate design parameters for USEPA stabilization requirements.