For many decades, fiber reinforced polymers (FRPs) have been extensively utilized in load-bearing structures. Their formability and superior in-plane mechanical properties have made them a viable replacement for conventional structural materials.  A major drawback to FRPs is their weak interlaminar properties (e.g., interlaminar fracture toughness). The need for lightweight multifunctional structures has become vital for many applications and hence alleviating the out-of-plane mechanical (i.e., quasi-static, vibration, and impact) and electrical properties of FRPs while retaining minimal weight is the subject of many ongoing studies. The primary objective of this dissertation is to investigate the fundamental processes for developing hybrid, multifunctional composites based on surface grown carbon nanotubes (CNTs) on carbon fibers' yarns. This study embraces the development of a novel low temperature synthesis technique to grow CNTs on virtually any substrate. The developed method graphitic structures by design (GSD) offers the opportunity to place CNTs in advantageous areas of the composite (e.g., at the ply interface) where conventional fiber architectures are inadequate. The relatively low temperature of the GSD (i.e. 550 C) suppresses the undesired damage to the substrate fibers. GSD carries the advantage of growing uniform and almost aligned CNTs at pre-designated locations and thus eliminates the agglomeration and dispersion problems associated with incorporating CNTs in polymeric composites. The temperature regime utilized in GSD is less than those utilized by other synthesis techniques such as catalytic chemical vapor deposition (CCVD) where growing CNTs requires temperature not less than 700 °C. It is of great importance to comprehend the reasons for and against using the methods involving mixing of the CNTs directly with the polymer matrix, to either fabricate nanocomposites or three-phase FRPs. Hence, chapter 2 is devoted to the characterization of CNTs-epoxy nanocomposites at different thermo-mechanical environments via the nanoindentation technique. Improvements in hardness and stiffness of the CNTs-reinforced epoxy are reported. Long duration (45 mins) nanocreep tests were conducted to study the viscoelastic behavior of the CNT-nanocomposites. Finally, the energy absorption of these nanocomposites is measured via novel nanoimpact testing module. Chapter 3 elucidates a study on the fabrication and characterization of a three phase CNT-epoxy system reinforced with woven carbon fibers. Tensile test, high velocity impact (~100 ms⁻¹), and dynamic mechanical analysis (DMA) were employed to examine the response of the hybrid composite and compare it with the reference CFRP with no CNTs. Quasi-static shear punch tests (QSSPTs) were also performed to determine the toughening and damage mechanisms of both the CNTs-modified and the reference CFRP composites during transverse impact loading. The synthesis of CNTs at 550 C via GSD is the focus of chapter 4. The GSD technique was adjusted to grow Palladium-catalyzed carbon filaments over carbon fibers. However, these filaments were revealed to be amorphous (turbostratic) carbon.  Plasma sputtering was utilized to sputter nickel nano-films on the surface of the substrate carbon fibers. These films were later fragmented into nano-sized nickel islands from which CNTs were grown utilizing the GSD technique.  The structure and morphology of the CNTs are evaluated and compared to CNTs grown via catalytic chemical vapor deposition (CCVD) over the same carbon fibers. Chapter 5 embodies the mechanical characterization of composites based on carbon fibers with various surface treatments including, but not limited to, surface grown CNTs. Fibers with and without sizing were subjected to different treatments such as  heat treatment similar to those encountered during the GSD process, growing CNTs on fabrics via GSD and CCVD techniques, sputtering of the fibers with a thin thermal shield film of SiO₂ prior to CNT growth, selective growth of CNTs following checkerboard patterns, etc. The effects of the various surface treatments (at the ply interfaces) on the on-axis and off-axis tensile properties of the corresponding composites are discussed in this chapter. In addition, the DMA and impact resistance of the hybrid CNT-CFRP composites are measured and compared to the values obtained for the reference CFRP samples. While the GSD grown CNTs accounted for only 0.05 wt% of the composites, the results of this chapter contrasts the advantages of the GSD technique over other methods that incorporate CNTs into a CFRP (i.e. direct growth via CCVD and mixing of CNTs with the matrix). Understanding the behavior of the thin CFRPs under impact loadings and the ability to model their response under ballistic impact is essential for designing CFRP structures.  A precise simulation of impact phenomenon should account for progressive damage and strain rate dependent behavior of the CFRPs. In chapter 6, a novel procedure to calibrate the state-of-the-art MAT162 material model of the LS-DYNA finite element simulation package is proposed. Quasi-static tensile, compression, through thickness tension, and in-plane Isopescu shear tests along with quasi-static shear punch tests (QSSPTs) employing flat cylindrical and spherical punches were performed on the composite samples to find 28 input parameters of MAT162. Finally, the capability of this material model to simulate a transverse ballistic impact of a spherical impactor with the thin 5-layers CFRP is demonstrated. It is hypothesized that the high electrical conductivities of CNTs will span the multifunctionality of the hybrid composites by facilitating electromagnetic interference (EMI) shielding. Chapter 6 is devoted to characterizing the electrical properties of hybrid CNT-fiberglass FRPs modified via GSD method. Using a slightly modified version of the GSD, denser and longer CNTs were grown on fiberglass fabrics.  The EMI shielding performance of the composites based on these fabrics was shown to be superior to that for reference composites based on fiberglass and epoxy. To better apprehend the effect of the surface grown CNTs on the electrical properties of the resulting composites, the electrical resistivities of the hybrid and the reference composites were measured along different directions and some interesting results are highlighted herein. The work outlined in this dissertation will enable significant advancement in protection methods against different hazards including impact, vibrations and EMI events. / Ph. D.

The National Science Foundation has announced a plan to establish a Deep Underground Science and Engineering Laboratory (DUSEL) for interdisciplinary research in physics, geosciences, biosciences and engineering. The proposed laboratory will extend to a depth of about 2200 meters and will consist of research facilities for long term study. To date, eight sites in North America have been proposed to host DUSEL. One of these sites, known as Kimballton, is located near Butt Mountain in Giles County in southwestern Virginia. Two seismic lines were acquired along the top of Butt Mountain in June of 2004 to support the ongoing integrated site characterization effort by the Kimballton Science Team. Both lines, approximately 3 km in length, are standard multifold seismic reflection data aimed at imaging faults, thrust sheets, and repeated sections of Paleozoic rocks in the vicinity of the proposed Kimballton site. Crooked line geometry, irregular geophone spacing, ground roll, and poor impedance contrasts between juxtapositioned rock units were challenges in processing the data. Non-standard processing techniques included the use of travel time tomography to accurately constrain near surface velocities, the use of 2D median filters to remove ground roll, and stacking only offsets exceeding 500 m. Interpretation of seismic data supports a triplicated stratigraphic section caused by the stacking of the the St. Clair and Narrows thrust sheets. The St. Clair and Narrows faults are interpreted as shear zones within ductile units of the Martinsburg Formation. 3D travel time tomography was used to build a near surface velocity model of Lines 1 and 2 for the purposes of imaging near surface structure and constraining the extent of topographic lineaments, which are interpreted as bedrock joint systems. Interpretation of the velocity models suggests that the broadly folded strata of the Butt Mountain synclinorium dip gently to the east along the hinge surface. The surface extrapolation of the Lookout Rock fault and the intersection of topographic lineaments with the seismic lines are expressed as low velocity zones that extend to depths of 150 m. This may be related to accelerated weathering along jointed rock surfaces. Results of this study have already been incorporated into the NSF proposal submitted by the Kimballton Science Team (http://www.phys.vt.edu/~kimballton/s2p/b2.pdf). / Master of Science

Cystoisospora canis is an Apicomplexan parasite of the small intestine of dogs. C. canis produces monozoic tissue cysts (MZT) that are similar to the polyzoic tissue cysts (PZT) of Toxoplasma gondii, a parasite of medical and veterinary importance, which can reactivate and cause toxoplasmic encephalitis. We hypothesized that C. canis is similar biologically and genetically enough to T. gondii to be a novel model for studying tissue cyst biology. We examined the pathogenesis of C. canis in beagles and quantified the oocysts shed. We found this isolate had similar infection patterns to other C. canis isolates studied. We were able to superinfect beagles that came with natural infections of Cystoisospora ohioensis-like oocysts indicating that little protection against C. canis infection occurred in these beagles. The C. canis oocysts collected were purified and used for future studies. We demonstrated in vitro that C. canis could infect 8 mammalian cell lines and produce MZT. The MZT were able to persist in cell culture for at least 60 days. We were able to induce reactivation of MZT treated with bile-trypsin solution. In molecular studies, we characterized C. canis genetically using ITS1 and CO1 to build phylogenetic trees and found C. canis was most similar to C. ohioensis-like with ITS1 and more similar to T. gondii than any other coccidia using ITS1 and CO1. We identified genes and proteins involved with virulence, cyst wall structure, and immune evasion of T. gondii and examined the DNA of C. canis for orthologs. C. canis had orthologs with 8 of 20 T. gondii genes examined. Monoclonal and polyclonal antibody and lectin studies demonstrated similar tissue cyst wall proteins on C. canis MZT and T. gondii PZT. Our findings in vitro and using genetic characterization of C. canis indicated the presence of similar genes and proteins, and its close phylogenetic location with T. gondii demonstrate that C. canis may serve as a model to examine tissue cyst biology. The system we described provides a simple model to produce tissue cysts and to study host factors that cause reactivation of tissue cysts. / Ph. D.

Increased operations of aircraft, both commercial and military, in hostile desert environments have increased risks of micro-sized particle ingestion into engines. The probability of increased sand and dust ingestion results in increased life cycle costs, in addition to increased potential for performance loss. Thus, abilities to accurately characterize inlet sand would be useful for engine diagnostics and prognostic evaluation. Previous characterization studies were based on particle measurements performed a posteriori. Thus, there exists a need for in situ quantification of ingested particles. The work presented in this thesis describes initial developments of a line-of-sight optical technique to characterize ingested particles at concentrations similar to those experienced by aircraft in brownout conditions using light extinction with the end goal of producing an onboard aircraft diagnostic sensor. By measuring the extinct light intensity in presence of particles over range of concentrations, a relationship between diameters, concentration and light extinction was used for characterization. The particle size distribution was assumed log-normal and size range of interest 1-10 μm. To validate the technique, particle characterization in both static and flow based tests were performed on polystyrene latex spheres of sizes 1.32 μm, 3.9 μm, 5.1 μm, and 7 μm in mono-disperse and poly-disperse mixtures. Results from the static experiments were obtained with a maximum relative error of 11%. Concentrations from the static experiments were obtained with a maximum relative error of 18%. Mono-dispersed and poly-dispersed particle samples were sized in a flow setup, with a maximum relative error of 12% and 10% respectively across all diameter samples tested. Uncertainty in measurements were quantified, with results indicating a maximum error of 17% in diameters due to sources of variability and showed that shorter wavelength lasers provide lower errors in concentration measurements, compared to longer wavelengths. For real time, on-board measurements, where path lengths traveled by light are much larger than distances traveled in initial proof of concept experimental setups, requirements would be to install sensitive detectors and powerful lasers to prevent operation near noise floors of detectors. Vibration effects from the engine can be mitigated by using larger area collection optics to ensure that the transmitted light falls on active detector areas. Results shown in this thesis point towards validity of the light extinction technique to provide real time characterization of ingested particles, and will serve as an impetus to carry out further research using this technique to characterize particles entering aircraft engine inlets. / Master of Science

Realization of low-resistance electrical contacts on both sides of a solar cell is essential for obtaining the best possible performance. A key component of a solar cell is a metal contact on the illuminated side of the cell which should efficiently collect carriers. These contacts can be formed using an opaque metal grid/finger pattern. The metal electrode may be used alone or in combination with a broad-area transparent conductive film. This work aims at investigating the impact of the electrical contacting scheme employed in InGaN/GaN Schottky barrier solar cells on their performance. InGaN is a III-V compound semiconductor and has a tunable direct band-gap (0.7 eV to 3.4 eV) which spans most of the solar spectrum; this fact, along with other beneficial material properties, motivates the study of InGaN photovoltaic devices. A number of groups have recently investigated InGaN-based homo-junction and hetero-junction p-i-n solar cells. However, very few groups have worked on InGaN Schottky solar cells. Compared to p-n junctions, Schottky barrier solar cells are cheaper to grow and fabricate; they are also expected to improve the spectral response because of near surface depletion regions in the shorter wavelength regions. In this particular work on InGaN based solar cells, a Schottky diode structure was used to avoid the issue of highly resistive p-type InGaN. In this study, platinum (Pt) is used to form a Schottky barrier with an InGaN/GaN absorber region. Electrical and optical properties of platinum films are investigated as a function of their thickness. InGaN/GaN Schottky solar cells with platinum as the transparent conductive film are reported and their performance is evaluated as a function of the metal thickness. / Master of Science

Anterior cervical discectomy and fusion (ACDF) is one of the most common cervical spine surgery procedures performed worldwide. ACDF utilizes autologous bone graft (autograft) from the iliac crest to induce fusion between neighboring vertebrae following the procedure. The iliac crest is widely considered the gold-standard autograft for ACDF procedures due to its osteoinductive, osteoconductive, and osteointegrative properties. However, harvesting from a second surgical site, as seen with iliac crest autograft, is commonly associated with short- and long-term complications. To mitigate iliac crest harvest site complications, a novel autograft location must be identified. The adjacent cervical vertebral body has been identified as a potential alternative donor site to the iliac crest. Previous studies have shown that this novel autograft site does not biomechanically compromise the vertebral body harvest site and has demonstrated clinically successful fusion rates comparable to those of the iliac crest. Despite prior successful fusion, a direct morphological and mechanical comparison between autograft from the adjacent cervical vertebra and iliac crest has not been thoroughly investigated. The primary goal of this thesis was to morphologically and mechanically compare the cervical spine and iliac crest. It was hypothesized that the cervical spine and iliac crest would not significantly vary in their morphological properties; however, due to daily physiological loading at each graft location, it was hypothesized that the two graft locations would differ mechanically. A clinical model utilizing iliac crest and cervical vertebral bone from human donors was characterized at the meso- and microscale to quantify morphological properties and collagen organization using micro-computed tomography (microCT) and second-harmonic generation (SHG) imaging modalities, respectively. A pre-clinical large animal model was used to characterize the mechanical and material properties of lumbar spine tissue, under similar physiological loading as the cervical spine, relative to the iliac crest through uniaxial compression testing. No significant difference was identified in the morphological and collagen organization properties in tissue from a human clinical cohort; however, directionality and anatomical location significantly impacted the mechanical and material properties in a bovine comparative anatomy model. Here, trabecular bone from the lumbar vertebra was found to be transversely isotropic whereas iliac crest trabecular bone was nearly isotropic; thus, directionality and anatomical location should be considered and quantified when selecting autograft tissue for future ACDF surgeries. Further characterization of the mechanical properties of cervical vertebral tissue and determination of correlations between directionality, anatomical location, and morphology through microCT and compression testing should be completed before adopting the cervical vertebra as the gold standard autograft for ACDF procedures. / Master of Science / Anterior cervical discectomy and fusion (ACDF) is a common upper spine surgery that helps to stabilize the spine by fusing two or more vertebrae together. To achieve this fusion, surgeons often use bone grafts taken from the patient's own hip, specifically the iliac crest. While this method is effective, it can lead to complications at the hip bone harvest site. To avoid these complications, researchers are exploring the possibility of using bone from a nearby vertebra in the upper spine as an alternative graft source. Early studies suggest that using bone from the upper spine does not weaken the spine and achieves similar success rates in fusion as the hip bone. However, a detailed comparison between both graft sites has not been thoroughly investigated until now. The main goal of this thesis was to compare the bone from the upper spine and the hip in terms of structure and strength. It was expected that the two types of bone would be similar in structure but different in strength due to difference forces they experience in the body. The research involved examining human bone samples from both the upper spine and hip using advanced imaging techniques to analyze their structure and collagen organization. Additionally, a large animal comparative model was used to test the strength and material properties of bone from the lower spine and hip, which experience similar forces as the human upper spine and hip. The findings showed no significant difference in the structure and collagen organization of the human bone samples. However, in the animal model, the strength and material properties of the bone significantly varied depending on the direction and location. Bone from the lower spine was found to be significantly stronger in one direction in comparison to two other directions in the lower spine and all three directions in the hip. These results suggest that when choosing bone for fusion in ACDF surgeries, it is important to consider the direction and location of the graft. Further research is needed to fully understand the mechanical properties of upper spine bone and to confirm its suitability as a standard graft for ACDF procedures.

Medium density fiberboard (MDF) production uses thermomechanically refined fiber processed under shear with high pressure steam. The industry evaluates fiber quality with visual and tactile inspection, emphasizing fiber dimensions, morphology, and bulk density. Considering wood reactivity, the hypothesis is that a variety of chemical and physical changes must occur that are not apparent in visual/tactile inspection. An industry/university cooperation, this work studies effects of refining energy (adjusted by refiner-plate gap) on fiber: size, porosity, surface area, surface and bulk chemistry, fiber crystallinity and rheology, and fiber interaction with amino resins. The intention is to reveal novel aspects of fiber quality that might impact MDF properties or process control efficiency, specific to a single industrial facility. In cooperation with a North American MDF Douglas fir plant, two refining energies were used to produce resin and additive-free fibers. Refining reduced fiber dimensions and increased bulk density, more so at the highest energy. Thermoporosimetry showed increases in sub-micron scale porosity, greatest at the highest energy. Mercury intrusion porosimetry (MIP) revealed porosity changes on a higher dimensional scale. Brunauer-Emmett-Teller gas adsorption and MIP showed that refining increased specific surface area, more so at the highest energy. Inverse gas chromatography showed that the lowest refining energy produced surfaces dominated by lignin and/or extractives. The highest energy produced more fiber damage, revealing higher energy active sites. A novel rheological method was devised to study fiber compaction and densification; it did not distinguish fiber types, but valuable aspects of mechano-sorption and densification were observed. Refining caused substantial polysaccharide degradation, and other degradative effects that sometimes correlated with higher refining energy. Lignin acidolysis was detected using nitrobenzene oxidation, conductometric titration of free phenols, and formaldehyde determination. Formaldehyde was generated via the C2 lignin acidolysis pathway, but C3 cleavage was the dominant lignin reaction. Observations suggested that in-line formaldehyde monitoring might be useful for process control during biomass processing. According to rheological and thermogravimetric analysis, lignin acidolysis was not accompanied by repolymerization and crosslinking. Lignin repolymerization must have been prevented by the reaction of benzyl cations with non-lignin nucleophiles. This raises consideration of additives that compete for lignin benzyl cations, perhaps to promote lignin crosslinking and/or augment the lignin network with structures that impart useful properties. Fiber/amino resin interactions were studied with differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All fiber types, refined and unrefined, caused only a slight increase in melamine-urea-formaldehyde (MUF) resin reactivity. Generally, all fiber types decreased the enthalpy of MUF cure, suggesting fiber absorption of small reactive species. But DSC did not reveal any dependency on fiber refining energy. According to XRD, all fiber types reduced crystallinity in cured MUF, more so with refined fiber, but independent of refining energy. The crystallinity in cured urea-formaldehyde resin was studied with one fiber type (highest refining energy); it caused a crystallinity decrease that was cure temperature dependent. This suggests that resin crystallinity could vary through the thickness of an MDF panel. / PHD / Medium density fiberboard (MDF) is a wood-based composite which is widely used for making kitchen cabinets and furniture. In the process of making MDF, wood particles are softened under steam pressure and under high temperature and pressure, inside a refiner, mechanically cut into wood fibers. Wood fibers are then mixed with adhesive and additives then hot-pressed and form the final board. In the MDF industry, wood fiber quality has significant effect on final board properties and is evaluated based on visual and tactile inspections. The research hypothesis is that, during the refining, a variety of chemical and physical changes must occur that are not apparent in visual/tactile inspection. An industry/university cooperation, this work studies effects of refining energy (adjusted by refiner-plate gap) on fiber: size, porosity, surface area, surface and bulk chemistry, fiber crystallinity and rheology, and fiber interaction with adhesive. The intention is to reveal novel aspects of fiber quality that might impact MDF properties or process control efficiency, specific to a single industrial facility. It was found that refining had significant effect on wood fiber properties: increased surface area, porosity, and changed the surface energy; and also on wood fiber chemistry: significant degradation in wood fiber main chemical components: poly saccharides and lignin. These changes also had effect on fiber/adhesive interaction. Therefore the hypothesis was confirmed that MDF fiber quality must involve more than a simple visual/tactile evaluation and the effect of refining can be detected on other fiber quality aspects. However more research needs to be conducted to test and find feasible new methods for fiber quality evaluation.

As the current requirements of power devices are moving towards high frequency, high efficiency and high-power density, Silicon-based devices are reaching its limits which are instigating the need to move towards new materials. Gallium Nitride (GaN) has the potential to meet the growing demands due to the wide band-gap nature which leads to various enhanced material properties like, higher operational temperature, smaller dimensions, faster operation and efficient performance. The metal contacts on semiconductors are essential as the interface properties affect the semiconductor performance and device operation. The low resistance ohmic contacts for n-GaN have been well established while most p-GaN devices have still high contact resistivity. Significant work has not been found that focuses on software-based modeling of the device to analyze the contact resistance and implement methods to reduce the contact resistivity. Understanding the interface physics in n-GaN devices using simulations can help in understanding the contacts on p-GaN and eventually reduce its metal contact resistivity. In this work, modeling of the metal-semiconductor interface along with the effect of a heavily doped layer under the metal contact is presented. The extent of reduction in contact resistivity due to different doping and thickness of n++ layer is presented with simulations. These results have been verified by the growth of device based on simulation results and reduction in contact resistivity has been observed. The effect of different TLM pattern along with different annealing conditions is presented in the work. / Master of Science / Technology has become part and parcel of the life of humans which is slowing gearing towards Automation, Internet of Things (IoT). The hardware for this is being provided by semiconductor silicon for a very long time. However, the demand is moving towards smaller size and better performance. Silicon material has reached its limitations in terms of dimension scaling and performance enhancement. A quest for new material has led to Gallium Nitride (GaN) which has the potential to provide enhanced properties like higher operational temperature, smaller dimensions, faster operation and efficient performance. Metal contact on the semiconductor is essential as these contacts provide the external connection. The contact characteristic of the metal-semiconductor interface is evaluated by contact resistance. It is expected that contact has linear IV characteristics (ohmic contact) and low contact resistance to avoid perturbing the semiconductor performance in devices. There are metals which can provide ohmic contacts for n-GaN but they offer low contact resistance only on annealing. These contact characteristics are studied by simulating the metal-semiconductor interface by replicating the thermionic and tunneling effects at the junction by physics-based device modeling. It is essential to reduce the contact resistivity for better interface properties which can be provided by a heavily doped (n⁺⁺) layer under the metal layer. The effect of various doping and thickness of n⁺⁺ layer is presented in this research work. Devices were grown based on simulation results and the extent of reduction in contact resistivity due to the n⁺⁺ layer is documented in this research. This reduction in contact resistivity can aid in a significant reduction in power dissipation in the devices which could lead to efficient device operation.

Wide-bandgap semiconductors have made and are continuing to make a major impact on the power electronics world. The most common commercially available wide-bandgap semiconductors for power electronics applications are SiC and GaN devices. This paper focuses on the newest devices emerging that are made with these wide-bandgap materials. The static and dynamic characterization of six different SiC MOSFETs from different manufacturers are presented. The static characterization consists of the output characteristics, transfer characteristics and device capacitances. High temperature (up to 150 °C) static characterization provides an insight into the dependence of threshold voltage and on-state resistance on temperature. The dynamic characterizations of the devices are conducted by performing the double-pulse test. The switching characteristics are also tested at high temperature, with the presented results putting an emphasis on one of the devices. A comparison of the key characterization results summarizes the performance of the different devices. The characterization of one of the SiC MOSFETs is then continued with a short-circuit failure mode operation test. The device is subjected to non-destructive and destructive pulses to see how the device behaves. The non-destructive tests include a look at the performance under different external gate resistances and drain-source voltages. It is found that as the external gate resistance is increased, the waveforms get noisier. Also, as the drain-source voltage is increased, the maximum short-circuit current level rises. The destructive tests find the amount of time that the device is able to withstand short-circuit operation. At room temperature the device is able to withstand 4.5 μs whereas at 100 °C, the device is able to withstand 4.2 μs. It is found that despite the different conditions that the device is tested at for destructive tests, the energy that they can withstand is similar. This paper also presents the static and dynamic characterization of a 600 V, 2A, normallyoff, vertical gallium-nitride (GaN) transistor. A description of the fabrication process and the setup used to test the device are presented. The fabricated vertical GaN transistor has a threshold voltage of 3.3 V, a breakdown voltage of 600 V, an on-resistance of 880 mΩ, switching speeds up to 97 V/ns, and turn-on and turn-off switching losses of 8.12 µJ and 3.04 µJ, respectively, demonstrating the great potential of this device / MS / A key part in a power electronics circuit is the switch component. Currently, the devices usually used as the switch are made from silicon. As the performance limits of silicon are reached though, wide-bandgap semiconductors are proving to be a promising alternative to silicon semiconductors. These wide-bandgap switches will allow for higher powers, higher efficiency and higher temperature operation. The technology is still novel though and so new devices are still being developed. This paper focuses on showing the performance of the newest devices emerging that are made with these wide-bandgap materials. To demonstrate the performance potential of a switching device, the non-switching and switching behavior need to be tested. These tests are described and the results are shown for both Silicon Carbide (SiC) and Gallium Nitride (GaN) semiconductors which are the most common wide bandgap semiconductors. The failure mode operation of one of the SiC devices is also tested. A common failure in power electronics is a short circuit failure where the switch is turned on for a long amount of time and kept on for too long, eventually leading to the device breaking destructively. To understand the limits and capabilities of these devices in a short circuit failure, non-destructive and destructive tests are explained and demonstrated.

This research provides a thorough flow characterization study to compare the functionality of two types of load-leveling valves that are commonly used for air suspension systems of commercial trucks. The first valve features a simple disk/slot design and is relatively compact for installation. The second type is larger and has a sophisticated, chambered design, which allows for considerably quicker fill and exhaust response times in the transient region. A new approach is introduced to estimate the transient mass flow rate of a load-leveling valve under different suspension pressures, without requiring a mass flow meter. An extensive series of dynamic tests are conducted to characterize and compare the two load-leveling valves. A generic heavy-truck pneumatic suspension, consisting of load-leveling valves, airspring, air tank, and air-hose fittings, is configured for testing. The test setup is used to evaluate the transient performance of each type of load-leveling valve in a typical truck suspension. The flow behavior of the system is validated by the force/pressure responses of the air spring due to various displacement excitations. The experimental results describe the detailed flow behavior of both valves. The flow characterization results can be incorporated as one of the most critical parameters for future model development of pneumatic systems. The tests indicate that the leveling valve with chambered design has a far faster transient flow response than the disk valve, although it is more complicated in its mechanical design and therefore costs more. To take advantage of the design simplicity of the disk valve, while also enabling it to have a faster transient response (compared with the chambered design), it is re-designed with larger flow openings and other elements to match the performance of the chambered valve for transient flow. A comparison of the experimental results and simulations validates that the re-designed rotary disk valve performs nearly the same as the chambered valve, but is simpler and costs less. The study's results are directly applicable to improving the transient dynamics of heavy truck air suspensions by providing a better understanding of how load-leveling valves can be used not only to provide ride-height control, but also to influence the roll and pitch dynamics of heavy trucks. / Master of Science / Heavy trucks are balanced using air suspension systems. These pneumatic controls provide stability when a truck undergoes a turn or other change in movement, including roll and pitch. As a truck experiences these changes, air is supplied or purged from the system to balance the truck. Load leveling valves control this flow of air that provides stability and are considered crucial elements in the overall design of a heavy truck. This study evaluates many different types of valves, namely a "chambered" valve and a "disk" valve. The chambered valve is large and has many parts, resulting in a heavy expense but high performance. The disk valve is a simpler design, making it much cheaper but at the expense of performance. The quality of performance that is evaluated here is the time it takes to fill or purge the air suspension, which is related to the mass flow. These characteristics were experimentally obtained and compared. The results showed the disk valve taking more time and having a lower flow rate, making its performance lower when compared to the chambered valve. The next aspect of this study is the modification of this disk valve design that is commercially available to make its performance comparable to the chambered valve. After a series of experiments, the modified design was verified to perform as well as the chambered valve. Overall, these results are important for the future design of heavy truck load leveling valves and clarify important characteristics to consider when designing them. The results from this study can lead to lower costs for heavy truck companies and a better ride for truck drivers.