Browsing by Author "Williams, John, advisor"

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Open Access
A calcium aluminate electride hollow cathode
(Colorado State University. Libraries, 2014) Rand, Lauren Paula, author; Williams, John, advisor; Reynolds, Melissa, committee member; Sampath, Walajabad, committee member; Yalin, Azer, committee member
The development and testing of a hollow cathode utilizing C12A7 (12CaO.Al2O3) electride as an insert are presented. Hollow cathodes are an integral part of electric propulsion thrusters on satellites and ground-based plasma sources for materials engineering. The power efficiency and durability of these components are critical, especially when used in flight applications. A low work function material internal to the cathode supplies the electrons needed to create the cathode plasma. Current state-of-the- art insert materials are either susceptible to poisoning or need to be heated to temperatures that result in a shortened cathode lifetime. C12A7 electride is a ceramic in which electrons contained in sub-nanometer sized lattice cages act as a conductive medium. Due to its unique atomic structure and large size, C12A7 electride has a predicted work function much lower than traditional insert materials. A novel, one-step fabrication process was developed that produced an amorphous form of C12A7 electride that had a measured work function 0.76 eV. A single electride hollow cathode was operated on xenon for over 60 hours over a two-month period that included 20 restarts and 11 chamber vent pump-down sequences with no sign of degradation, and on iodine for over 20 hours with no apparent reactivity issues. The operations of cathodes with three different orifice sizes were compared, and their effects on the interior cathode plasma modeled in a zero- dimensional phenomenological model.
Open Access
A non-invasive Hall current distribution measurement system for Hall effect thrusters
(Colorado State University. Libraries, 2015) Mullins, Carl Raymond, author; Williams, John, advisor; Shipman, Patrick, committee member; Yalin, Azer, committee member
A direct, accurate method to measure thrust produced by a Hall Effect thruster on orbit does not currently exist. The ability to calculate produced thrust will enable timely and precise maneuvering of spacecraft—a capability particularly important to satellite formation flying. The means to determine thrust directly is achievable by remotely measuring the magnetic field of the thruster and solving the inverse magnetostatic problem for the Hall current density distribution. For this thesis, the magnetic field was measured by employing an array of eight tunneling magnetoresistive (TMR) sensors capable of milligauss sensitivity when placed in a high background field. The array was positioned outside the channel of a 1.5 kW Colorado State University Hall thruster equipped with a center-mounted electride cathode. In this location, the static magnetic field is approximately 30 Gauss, which is within the linear operating range of the TMR sensors. Furthermore, the induced field at this distance is greater than tens of milligauss, which is within the sensitivity range of the TMR sensors. Due to the nature of the inverse problem, the induced-field measurements do not provide the Hall current density by a simple inversion; however, a Tikhonov regularization of the induced field along with a non-negativity constraint and a zero boundary condition provides current density distributions. Our system measures the sensor outputs at 2 MHz allowing the determination of the Hall current density distribution as a function of time. These data are shown in contour plots in sequential frames. The measured ratios between the average Hall current and the discharge current ranged from 0.1 to 10 over a range of operating conditions from 1.3 kW to 2.2 kW. The temporal inverse solution at 2.0 kW exhibited a breathing mode of 37 kHz, which was in agreement with temporal measurements of the discharge current.
Open Access
A numerical model for the determination of biomass ignition from a hotspot
(Colorado State University. Libraries, 2015) McArdle, Patrick, author; Williams, John, advisor; Gao, Xinfeng, committee member; Shipman, Patrick, committee member
The determination of biomass ignition from an inert spherical hotspot using a fourth-order finite-volume method is presented. The transient ignition-combustion system is modeled by two coupled reaction-diffusion equations. One equation governs the heating characteristics of the biomass while the other governs the mass loss of the biomass. The combustion assumes a one-step, 1st-order Arrhenius reaction. This work is motivated and funded by the Department of Defense Legacy Program to create a munition specific fire danger rating system. Improving fire danger rating systems on military lands would minimize the economic and environmental impact of soldiers training on protected habitats. A better understanding of these ignition characteristics would also improve current fire spread models. Our result shows that given the ignition criteria derived from a simplified non-dimensional model and specifying critical values found by Gol'dshleger et al., an ignition probability can be established by varying the biomass properties based on moisture content. Following the procedure developed in this thesis, the computed ignition probabilities correlate well with experimental ignition data that was obtained at the Center for Environmental Management of Military Lands. Moreover, numerically solving the coupled reaction-diffusion system provides additional insight into more realistic ignition criteria involving mass loss. The numerical solution suggests more sources of heat loss, in addition to convection, must be considered for a more realistic ignition model.
Open Access
Antibacterial effects of sputter deposited silver-doped hydroxyapatite thin films
(Colorado State University. Libraries, 2011) Trujillo, Nathan Anthony, author; Popat, Ketul, advisor; Williams, John, advisor; Reynolds, Melissa, committee member; Crans, Debbie, committee member
Over recent years, researchers have studied innovative ways to increase the lifespan of orthopedic implants in order to meet the soaring demand of hip and knee replacements. Since many of these implants fail as a result of loosening, wear, and inflammation caused by repeated loading on the joints, coatings such as hydroxyapatite (HAp) on titanium with a unique topography have been shown to improve the interface between the implant and the natural tissue. Other serious problems with long-term or ideally permanent implants are bacterial colonization. It is important to prevent initial bacterial colonization as existing colonies have potential to become encased in an extracellular matrix polymer (biofilm) which is resistant to antibacterial agents. The following work considers the potential of etching using plasma based ion implantation and ion beam sputter deposition to produce hydroxyapatite thin films on etched titanium doped with silver as an antibacterial component. Plasma-based ion implantation was used to examine the effects of pre-etching on plain titanium. Topographical changes to the titanium samples were examined and compared via scanning electron microscopy. It was determined that plasma-based ion implantation at -700eV could etch titanium to produce similar topography as ion beam etching in a shorter processing time. Hydroxyapatite and silver-doped hydroxyapatite thin films were then sputter deposited on titanium substrates etched at -700eV. For silver-doped films, two concentrations of silver (~0.5wt% and ~1.5wt%) were used. Silver concentrations in the film were determined using energy dispersive x-ray spectroscopy. Film thicknesses were determined by measuring the surface profile using contact profilometry. Staphylococcus epidermidis (SE) and Pseudomonas aeruginosa (PA) adhesion studies were performed on plain titanium, titanium coated with hydroxyapatite, titanium coated with ~0.5 wt% silver-doped hydroxyapatite, and titanium coated with ~1.5wt% silver-doped hydroxyapatite. It was discovered during the study that the films were delaminating from the samples thus killing bacteria in suspension. Release studies performed in addition to adhesion confirmed that the silver-doped films prevented SE and PA bacterial growth in suspension. To prevent delamination, the films were annealed by heat treatment in air at a temperature of 600°C. X-ray diffraction confirmed the presence of a crystalline hydroxyapatite phase on each sample type. Films were immersed in PBS at 37°C and remained in incubation for four weeks to determine there was no delamination or silver leaching.
Open Access
Development of a sensory substitution API
(Colorado State University. Libraries, 2018) Martinez, Marco, author; Williams, John, advisor; Stone-Roy, Leslie, advisor; Alciatore, David, committee member; McConnell, Ross, committee member
Sensory substitution – or the practice of mapping information from one sensory modality to another – has been shown to be a viable technique for non-invasive sensory replacement and augmentation. With the rise in popularity, ubiquity, and capability of mobile devices and wearable electronics, sensory substitution research has seen a resurgence in recent years. Due to the standard features of mobile/wearable electronics such as Bluetooth, multicore processing, and audio recording, these devices can be used to drive sensory substitution systems. Therefore, there exists a need for a flexible, extensible software package capable of performing the required real-time data processing for sensory substitution, on modern mobile devices. The primary contribution of this thesis is the development and release of an Open Source Application Programming Interface (API) capable of managing an audio stream from the source of sound to a sensory stimulus interface on the body. The API (named Tactile Waves) is written in the Java programming language and packaged as both a Java library (JAR) and Android library (AAR). The development and design of the library is presented, and its primary functions are explained. Implementation details for each primary function are discussed. Performance evaluation of all processing routines is performed to ensure real-time capability, and the results are summarized. Finally, future improvements to the library and additional applications of sensory substitution are proposed.
Open Access
Development of plasma cleaning and plasma enhanced close space sublimation hardware for improving CdS/CdTe solar cells
(Colorado State University. Libraries, 2012) Swanson, Drew, author; Williams, John, advisor; Sampath, W. S., advisor; Sites, James, committee member
A scalable photovoltaic manufacturing process that employs a heated pocket deposition technique has been developed at Colorado State University. It allows for the economical manufacturing of single-junction thin-film CdTe solar cells with efficiencies over 13%. New techniques that further increase cell efficiency and reduce production expenses are required to make solar energy more affordable. To address this need a hollow-cathode plasma source was added to the load-lock region of the CSU single-vacuum in-line CdTe-cell fabrication system. This plasma source is used to clean the transparent-conductive-oxide layer of the cell prior to the deposition of the CdS and CdTe layers. Plasma cleaning enables a reduction in CdS thickness by approximately 20 nm, while maintaining an improved cell voltage. Cell current was improved and cell efficiency was increased by 1.5%. Maps generated by scanning white-light interferometry, electroluminescence, and light-beam-induced current all show uniformity improvement with plasma cleaning treatment. To further increase cell efficiency a hollow-cathode plasma-enhanced close space sublimation (PECSS) source was utilized to modify the CdS window layer material as it was being deposited. This was done by integrating PECSS into the CSU inline CdS/CdTe-cell fabricating system and by sublimating the CdS semiconductor material through a plasma discharge. To date oxygenated CdS (CdS:O) cells have been grown by sublimating CdS through a PECSS source operated on oxygen. Data are presented showing that PECSS CdS:O films have increased the band gap of the window layer therefore reducing absorption loss, increasing cell current, and improving efficiency by 1.2%.
Open Access
Evaluation of electrical tongue stimulation for communication of audio information to the brain
(Colorado State University. Libraries, 2016) Moritz, Joel Adrian, Jr., author; Williams, John, advisor; Stone-Roy, Leslie, advisor; Alciatore, David, committee member; Malcolm, Matt, committee member
Non reparative solutions to damaged or impaired sensory systems have proven highly effective in many applications but are generally underutilized. For auditory disorders, traditional reparative solutions such as hearing aids and implant technology are limited in their ability to treat neurological causes of hearing loss. A method to provide auditory information to a user via the lingual nerve is proposed. The number of mechanoreceptors in the tongue exceeds the number of inner hair cells in the cochlea and the dynamic range of neurons in both systems is comparable suggesting that the achievable throughput of information in the lingual nerve is comparable to that of the auditory nerve. This supports the feasibility of transmitting audio information to the brain via the lingual nerve. Using techniques implemented in similar successful technology, the achievable throughput of the dorsal surface of the tongue using existing stimulation methods without additional innovation was estimated to be as high as 1,800 bits per second for an experienced user, in the same range required by many audio codecs used for spoken language. To make a more accurate estimation of achievable throughput, devices were developed to stimulate the tongue electrically, and an experiment to map the sensitivity of the tongue to a form of electrotactile stimulus was performed. For the population tested, discrimination ability of the tongue varied greatly. For all participants estimates for the immediately achievable throughput for the surface of the tongue was sufficient to communicate basic phonetic information to the participant. The estimated throughput for an experienced user was estimated to be as high as 1,400 bits per second. Lingual sensitivity maps were generated that will allow researchers and developers to manufacture electrode arrays that can reliably stimulate lingual nerve endings in a discriminatory manner. In another study we tested the feasibility of sending audio information to a person via the tongue. Preliminary data are presented on participants in a learning study that were able to discern stimuli generated from recorded voices, supporting our hypothesis on immediately achievable throughputs.
Open Access
Experimental evaluation of a standalone hollow cathode apparatus with a magnetic field
(Colorado State University. Libraries, 2024) Ku, Emily X., author; Williams, John, advisor; Dumitrache, Ciprian, committee member; Thornton, Christopher, committee member
Testing hollow cathode assemblies independently from their use in Hall or gridded ion thrusters offers advantages such as reduced test facility size, lower power requirements, and improved diagnostic access. Standalone tests can reveal important cathode characteristics like ignition time, keeper ignition voltage, tip temperature, and current capability. Replicating the plasma phenomena that occur when a cathode operates within a thruster is challenging but essential, as these phenomena can generate energetic ions that erode cathode and keeper surfaces, limiting thruster lifespan. The primary challenge is to accurately emulate thruster conditions in standalone tests and verify this emulation through comparison with cathode-thruster operations. This thesis presents data on a standalone hollow cathode operated with magnetic fields that emulate those in electric propulsion devices, testing it both without an applied magnetic field and with permanent and solenoidal magnetic fields. Measurements of keeper, anode, and cathode-to-ground voltages were conducted over a range of anode currents and flow rates. At certain conditions, the plasma discharge transitioned to a less stable mode known as plume mode, with higher flow rates shifting this transition to higher anode currents. Introducing a magnetic field decreased the anode current at which this voltage shift occurred. Important findings in this work include: (1) Repeat tests with no magnetic field show that the transition behavior was different from one test to another, indicating that transition behavior may be affected by minute changes in cathode apparatus, or there are significant uncertainties associated with the transition and (2) Significant hysteresis in plume mode transition was observed when increasing and then decreasing anode current. These two findings along with the deleterious effects of the magnetic field have important implications on cathodes operating with Hall thrusters, which often exhibit large, rapid oscillations in discharge current.
Open Access
Feasibility assessment of magnetic sensors for measurement of Hall current induced changes to the static magnetic field nearby a Hall thruster
(Colorado State University. Libraries, 2013) Morozko, Zoe, author; Williams, John, advisor; Stansloski, Mitchell, committee member; Thornton, Christopher, committee member
A Hall thruster is an electric propulsion device that produces thrust electrostatically by accelerating propellant to velocities 5 to 10 times higher than is achievable using conventional chemical thrusters. This is accomplished through the application of static, crossed electric and magnetic fields that are concentrated in a region close to the exit plane of the thruster. During operation an azimuthal plasma-electron current develops in the region where the electric and magnetic fields are concentrated. This embedded plasma current is referred to as the Hall current. The thrust produced from accelerating the propellant is transferred to a satellite or spacecraft through interaction between the Hall current and the magnetic coils used to produce the static magnetic field within the thruster. The Hall current can be calculated and the thrust can be determined in real time by measuring the magnetic field produced by the Hall current using sensors located external to the thruster. This work investigates the feasibility of placing magnetic sensors in the regions close to the exit of the thruster to measure the external magnetic field and correlate it to the Hall current. A finite element magnetic solver was used to identify several locations outside of the thrust plume and near the pole piece where the magnetic field magnitude changes by several Gauss in a background field level of ~50 Gauss. Magnetic sensors based on the giant magnetoresistive effect were identified as acceptable with regard to sensitivity, and measurements made with these sensors in a simulated high background magnetic field environment demonstrated that changes of 0.5 Gauss could be easily measured. This work also presents the development of a thrust stand that will be useful in future work to demonstrate the overall concept. Special focus was directed to the design of the data acquisition system and in-vacuum calibration system used to make measurements with the thrust stand.
Open Access
High efficiency thermoelectric devices fabricated using quantum well confinement techniques
(Colorado State University. Libraries, 2011) Jurgensmeyer, Austin Lee, author; Williams, John, advisor; Bradley, Thomas, advisor; Evangelista, Paul, committee member
Experimental results are presented of thermoelectric materials, specifically two-dimensional quantum well confinement structures, formed by ion beam sputter deposition methods. Applications of these thermoelectric devices include nearly any system that generates heat including waste heat. The targeted applications of this research include harvesting of waste heat from stand-alone generator systems and automobiles. Thermoelectric generator modules based on an in-plane orientation of nano-scale, thin-film, superlattices have demonstrated very high performance and are appropriate for a wide range of waste heat recovery applications. In this project, the first, fast, ion-beam-based deposition process was developed for producing Si/SiC (n-type) and B4C/B9C (p-type) superlattices. The deposition process uses low-cost powder targets, a simplified substrate holder with embedded heater, a QCM deposition rate monitor, and stepper-motor-controlled masks. Deposition times for individual layers are shown to be significantly shorter than those achieved in magnetron-based systems. As an example of the speed of the process, a 10-nm thick Si layer can be deposited in as little as 20 sec while a SiC layer can be deposited in less than 100 sec. Electrical resistivities, thermal conductivities and Seebeck coefficients are reported for the deposited films as well as their respective non-dimensional figures of merit (zT). Figures of merit (zT) approaching 20 at modest temperatures of ~600 K were observed. These measurements are made in-plane where enhanced Seebeck values and reduced electrical resistivities have also been reported in the literature. A method for directly measuring thermal conductivity in the plane of the superlattice is described that uses MEMs-based SiN cantilevers. Results are presented for various deposition variables, including film thickness, temperature, deposition energy, and material. Scanning white light interferometry (SWLI) and scanning electron microscopy (SEM) were used to characterize film thickness. In addition to the experimental effort, an analysis was performed to predict the performance of a thermoelectric module fabricated with the superlattice films deposited on ceramic substrates. Thermal efficiencies approaching 15% are predicted for modest cold and hot side temperatures. Thermal conduction through the substrate was found to be the largest factor limiting the performance of the modeled thermoelectric modules.
Open Access
Low work function, long lifetime filament for electron beam-based, wire-fed metal additive manufacturing
(Colorado State University. Libraries, 2018) Nguyen, Bao Gia, author; Bradley, Thomas, advisor; Williams, John, advisor; de la Venta Granda, Jose, committee member
Tantalum filaments are used in electron beam additive manufacturing to thermionically emit electrons that are used to build near-net shape, metal parts. High operating temperatures are required to emit electrons which consequently limits the lifetime of these filaments. This thesis presents the thermionic emission characteristics of drop-in filament replacements that incorporate barium calcium aluminate cermets. Barium calcium aluminate is a low work function material used with hollow cathodes in electric propulsion devices to provide very long service lifetimes by acting as a moderate temperature, electron source. A marriage of these two technologies may limit downtime and increase the productivity and output of electron beam additive manufacturing. Results of extended runtime tests are presented from configurations that immerse the modified filament in plasma and operate it as a vacuum emitter. The effect of contamination by air and fabrication methods are examined and evaluated based on effective work function and current density measurements. The latter includes formation methods for barium diffusion orifices as well as surface preparation methods for cermets. The experimental data collected were used to validate a predictive model that evaluates emission current densities, in both temperature and space-charge limited conditions, and effective work functions based on the fractional surface coverage of barium over a tantalum substrate.
Open Access
Plasma processing for nanostructured topographies
(Colorado State University. Libraries, 2012) Riedel, Nicholas Alfred, author; Williams, John, advisor; Popat, Ketul, advisor; Radford, Donald, committee member; Reynolds, Melissa, committee member
Plasma and directed ion interactions with materials have been widely observed to create complex surface patterns on a micro- and nano- scale. Generally, these texturizations are byproducts of another intended application (such as a feature formation on a sputtering target) and patterning is considered inconsequential or even detrimental. This work examined the possibility of using these phenomena as primary methods for producing beneficial topographies. Specifically, investigations focused on the use of helium plasma exposure and directed ion etching to create nanostructured surfaces capable of affecting biological interactions with implanted materials. Orthogonal argon ion etching and low energy helium plasma texturization of titanium were considered for use on orthopedic and dental implants as a means of increasing osteoblast activity and bone attachment; and oblique angle etching was evaluated for its use in creating topographies with cell deterrent or anti-thrombogenic properties. In addition, the helium driven evolution of surface features on 6061 aluminum alloy was characterized with respect to ion energy and substrate temperature. These surfaces were then considered for ice phobic applications.
Open Access
Single power supply operation of a Hall thruster
(Colorado State University. Libraries, 2024) Robertson, Zachary K., author; Williams, John, advisor; Fankell, Doug, committee member; Roberts, Jacob, committee member
Interest in operating Hall thrusters with a single power supply, facilitated by heaterless hollow cathodes, has motivated this research. Initial investigations into the Safran PPS 1350 confirmed the potential for this configuration. Building on these findings, modifications were made to a laboratory Hall thruster to enable operation with a single power supply and changes were made to the electrical configuration to promote smooth cathode ignition and decrease the influence of the inductance in the magnetic coils. In addition to operating with a laboratory power supply, CisLunar Industries supplied a prototype anode supply that demonstrated capability of running the laboratory Hall thruster under these conditions without efficiency losses, as verified by thrust stand data. A phenomenological efficiency analysis was performed using a Faraday probe and a retarding potential analyzer which supported these results, while also providing pertinent sub-efficiencies. The study concludes that a single power supply configuration is a viable approach to starting and operating a Hall thruster equipped with a heaterless hollow cathode.
Open Access
Thermoelectric properties of Si/SiC thin-film superlattices grown by ion beam sputtering
(Colorado State University. Libraries, 2015) Cramer, Corson Lester, author; Williams, John, advisor; Sampath, Walajabad, committee member; Neilson, James, committee member
There are many mechanical systems that convert heat to work and processes that utilize heat including power plants, automobiles, and foundries. Most of these systems expel large amounts of waste heat to the environment that goes unused. One way of recovering the waste heat is to use a solid-state energy converter based on thermoelectric processes. Nano-scaled materials are of interest for use in thermoelectric devices because their properties enhance the efficiency over those obtained using bulk materials. Some nano-scaled materials systems being considered are thin-film superlattices that utilize quantum confinement effects. Thin-film, superlattice thermoelectric devices could revolutionize traditional heat-to-work systems and heat-only processes if they are coupled to the systems to recycle a fraction of the waste heat into usable power. The advantage of thermoelectrics over traditional mechanical systems is that they use solid-state processes instead of moving parts and working fluids. As a result, they can be made to be more reliable and require less maintenance. This thesis focuses on the characterization of a thin-film, superlattice (SL) thermoelectric material formed by alternating silicon and silicon carbide layers to form an n-type quantum well. Superlattices of 31 bi-layers of Si/SiC (10 nm each) were deposited on silicon, quartz, and mullite substrates using a high-speed, ion-beam sputter deposition process, and the Seebeck coefficient and electrical resistivity are measured as a function of temperature and used to compare film performance. In addition, SL layer thicknesses of 2 and 5 nm were deposited on mullite to determine the effect layer thickness has on the thermoelectric properties.