Browsing by Subject "transport"

Now showing 1 - 6 of 6
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    An Assessment of the Safety and Efficiency of Log Trucks with Increased Weight Limits on Interstate Highways in Minnesota and Wisconsin
    (University of Minnesota, 2023) Carson, Michael T.; Blinn, Charles R.; Timothy, J. O'Hara
    Design standards for the Interstate Highway System in the US are generally higher than those on other roads within most states, making it the safest road system in the US. Federal law prevents states from enforcing vehicle weight limits on interstate highways that deviate from established Federal weight limits or state-specific grandfathered weight limits or exceptions. While state gross vehicle weight (GVW) limits for trucks that haul logs exceed federal interstate highway limits in all major timber-producing states that don’t have grandfathered limits, state-legal weight log trucks are not allowed to travel fully loaded on the interstate Trucks hauling logs at legal state limits must travel on state, county, township and local roads. On these routes trucks pass through towns/cities, school zones and encounter on-coming traffic and intersections. All these encounters increase the risk of an accident. This study compared the relative importance of the transport of raw forest products by trucks to the top five non-timber commodities and the fatality rates of log trucks to other heavy trucks in the lower 48 states, compared available national road damage cost estimates for interstate and non-interstate roads and assessed the impact of relaxing interstate weight limits on hauling distance, travel time, safety, pavement damage and CO2 emissions for hauling timber along three travel corridors in Wisconsin and Minnesota. Logs are an important commodity in many states but generally represent a minor percentage of the tonnage of commodities hauled by trucks. On a per load basis, log trucks have a lower fatality rate than other heavy trucks in 83% of the lower 48 states, including in those states that have higher GVW allowances on the interstate due to grandfathering. Due to the higher design standards, pavement damage costs are lowest on interstate highways as compared to other road types. Allowing state-legal, loaded log trucks access to federal interstate highways would improve the overall safety and efficiency of timber transportation while reducing pavement damage costs and CO2 emissions along the three travel corridors. The safety benefits generally exceeded the efficiency gains. Overall, study findings suggest that allowing state-legal, loaded log trucks to operate on interstate highways would improve the safety and efficiency of timber transportation in Wisconsin and Minnesota.
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    Disorder Effects in Quantum Materials
    (2023-08) Huang, Yi
    Decades of dedicated efforts in controlling disorder in conventional semiconductors have laid the foundation for our modern civilization, based on chips and all kinds of electronic devices. Nowadays, there is a growing interest in the so-called quantum materials whose properties are fundamentally altered by quantum-mechanical effects. Such quantum materials include two-dimensional heterostructures, topological insulators, graphene, superconductors, and many others. The strong interaction between electrons and topology within quantum materials gives rise to rich quantum states and phases such as quantum Hall effects and topological phases. For example, an exciting future application of quantum materials is the topological quantum computer, which is believed to be the most robust way to process quantum information. However, engineering such quantum materials must deal with ubiquitous impurities, which often ruin the delicate quantum-mechanical effects of interest and prevent the topological quantum computation from being realized. My dissertation research focuses on analyzing how the disorder affects the resistivity of different kinds of quantum materials, e.g., topological insulator thin films and wires, non-Hermitian random lasers and photonic lattices, and GaAs/AlGaAs heterostructures. Therefore, my dissertation research on improving the understanding of disorder effects in quantum materials has a broader impact on various fields from fundamental research to material engineering and technology.
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    Electronic Transport in Semiconductor Nanocrystal Thin Films
    (2018-06) Benton, Brian
    Semiconductor nanocrystal (NC) thin films have emerged as intriguing materials for low cost synthesis of electronic devices with size-tunable optical and electronic properties that enable unique control over operating characteristics. However, in order to fully realize the potential of these materials so that they can be effectively integrated into useful devices, greater understanding of the electronic transport properties is needed. In particular, the relationship between film morphology, surface chemistry, and disorder leads to unique challenges in engineering the performance of NC-based devices. The standard measurement techniques and modeling schemes developed for bulk semiconductors are not necessarily well suited for these challenges, so a deeper understanding of how they can be applied to semiconductor NC films and how to properly interpret the results is needed. In this thesis, the electronic conduction in two semiconductor NC material systems was explored. First, ZnO was used as a wide bandgap material that was known to have high native doping levels and electronic conduction that can approach metallic behavior. Atomic layer deposition (ALD) Al2O3 was used to passivate thin films of porous ZnO NCs, which have electronic properties that are extremely sensitive to surface oxidation reactions with ambient water vapor. This property was utilized to systematically control the conductivity of ZnO films by photochemically desorbing surface hydroxyl groups in vacuum and performing subsequent electrical measurements in situ. With this technique, we observed conductance increases of up to 105 and associated changes in transport mechanism between Mott and Efros-Shklovskii variable range hopping regimes. Through this analysis, we were able to determine the role of defect states and NC surface depletion in determining the coupling between NCs. Second, Ge NCs were studied as a narrow bandgap material with large quantum confinement effects leading to bandgap increases of up to 50%. Thermal admittance spectroscopy (TAS) and field-effect transistor (FET) measurements were used together to study charge injection in these films. We observed a change from electron conduction to hole conduction in Ge NC FETs after infilling with ALD Al2O3. The dominant barrier for transport in these FETs was determined to be minority carrier injection to the channel due to NC charging. Contact material was not observed to have any effect on the FET polarity, which, along with large hysteresis observed in I-V and C-V measurements, indicates that the transport properties are largely dominated by trap states.
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    Interactions between Plasma and Liquid Micro-Droplets
    (2021-02) Nayak, Gaurav
    The interaction of plasmas with a liquid phase results into various complex multiphase phenomena that is beneficial for a wide range of applications, such as water treatment, nanoparticle synthesis, material processing, combustion, decontamination, food safety and human health care. These applications have been made possible due to the transferof highly reactive species from the gas phase plasma to the bulk liquid phase. Upon interaction with a liquid phase, atmospheric pressure non-equilibrium plasmas produce numerous chemically reactive species, including ions, radicals, electrons and (V)UV photons. The resulting short-lived highly reactive species can exhibit huge density gradients in the liquid phase as their finite lifetime does not allow them to penetrate into the bulk liquid. This makes the multiphase reactivity transfer highly transport limited. Additionally, the presence of electric field-induced effects, charging, evaporation and heat and mass transfer makes plasma-liquid interaction studies more challenging due to the lack of a detailed understanding of the inter-coupling of these phenomena and their direct impact on the plasma-produced reactive species fluxes to the liquid. To understand and overcome the challenge of transport limitations, a novel plasma-liquid configuration was developed, which involves plasma activation of small dispersed liquid droplets or aerosols. The key advantage of such a configuration is that the large surface-to-volume ratio of these micrometer-sized liquid droplets enhances the reactivity transfer from the gas phase plasma to the liquid phase. Since the droplets are immersed in the plasma, the short-lived reactive species (electrons, ions and radicals) produced in the gas phase plasma due to electron impact processes on average only need to cover smaller length scales to reach and subsequently penetrate the droplet. The foremost goal of the research reported in this thesis is the development of a controlled and well-defined plasma-microdroplet reactor, which is easy to model and experimentally accessible with different diagnostic techniques enabling to obtain quantitative measurements of reactive species densities. Due to the efficient generation of reactive species, a radiofrequency (RF)-driven capacitively coupled diffuse plasma generated in parallel-plate configuration at atmospheric pressure is used in this work. Complete characterization of the RF plasma is helpful for the assessment of the role of different gas-phase reactive species generated by the plasma and their respective fluxes to the liquid microdroplets in transit through the plasma. The reactive species potentially playing a key role in plasma-liquid interactions include electrons, OH radicals, H and O atoms, singlet oxygen, O3, He and Ar metastable atoms and molecules. The absolute densities of electrons and metastable atoms and molecules along with gas and electron temperatures that play a major role in plasma-induced chemistry in He and Ar plasmas were measured using optical emission and broadband absorption spectroscopy. We found that the densities of He and Ar metastables peaked near the sheath edges away from the droplet trajectory, while the densities in the bulk were below their respective detection limits. The Ar and He metastable fluxes to the droplet were found to be 2 orders of magnitude and 5 times smaller than the electron flux to droplet, respectively. Hence, the effect of Ar and He metastable atoms on the droplet chemistry can be considered negligible compared to charged species fluxes. However, these metastable species are an important source of ionization in such low electron density plasmas and play, nonetheless, a major role in the plasma dynamics including ionization processes and the generation of radicals in the gas phase. An understanding of the dynamics of liquid microdroplets in the plasma is important as it not only relays information about the residence time of the droplets in the plasma (i.e. treatment time of the solution by short-lived plasma-produced species) but also droplet charging and the presence of electric fields when entering and exiting the plasma. This residence time dictates the fate of the droplet chemistry and its interactions with the plasma. We characterized the droplet and its trajectory by fast frame imaging in diffuse He glow discharge. From the droplet velocity and acceleration data, the various forces acting on the droplet were evaluated. Using the equilibrium of forces and the droplet charge estimated from an analytical model, the electric field at the plasma edges was determined. We also report on the effect of the initial droplet acceleration during droplet ejection from the piezoelectric nozzle, the plasma composition (He with admixtures of Ar, H2O, H2 and O2), gas flow rate, and the plasma power on the droplet dynamics. Plasmas in or in contact with water have been extensively investigated in the context of plasma-aided decomposition and mineralization of recalcitrant organic pollutants in water for environmental remediation. However, plasma, while being highly effective, sometimes lacks energy efficiency. The water treatment is often due to the transfer of long-lived species into the liquid bulk and secondary reactions. Using the approach of microdroplets treated by plasma, the efficiency of plasma treatment of organics in liquid water microdroplets can be improved by increasing the species fluxes to the droplets. Using detailed plasma diagnostics, droplet characterization and ex situ chemical analysis of the treated droplets, we assessed the relative importance of short-lived radicals, such as O and H atoms, singlet oxygen, solvated electrons and ions, besides OH radicals, responsible for the decomposition of formate, a model organic compound inwater treatment studies, dissolved in droplets. We ascertained the role of OH and O radicals in electronegative plasmas. We also showed for electropositive plasmas that solvated electrons and/or ions injected into the droplet were dominantly responsible for plasma-induced chemistry in the droplet. Results suggested that the charged species lead to the formation of H or OH radicals near the droplet interface via secondary reactions, enabling further decomposition of formate in the droplets. The obtained results allowed us to estimate minimum values of transport properties of O in solution and reaction rates of O radical with formate using a one-dimensional reaction-diffusion model. Gold and silver nanoparticles (AuNP and AgNP) exhibit unique optical, electrical, and thermal properties, and can be synthesized by plasmas in a green and environmentally friendly approach without the use of hazardous chemicals, and without producing harmful byproducts. Previously, researchers have established unprecedented gold ions reduction synthesis rates in liquid droplets treated by an RF plasma to synthesize AuNPs. The reported reduction rates are several orders of magnitude larger than for any other reported electron-initiated methods. The mechanism for the synthesis of nanoparticles using this approach is still largely unknown. Using the known gas-phase plasma properties, absorption spectroscopy, and transmission electron microscopy (TEM), we identified the role of hydrogen peroxide in the reduction of gold ions. On the other hand, the reported results for AgNP formation from Ag ions in the same reactor, suggested that the effect of solvated electrons and H radicals were dominant for the reduction of silver ions. We also demonstrate the possibility to use plasma-droplet interactions for the synthesis of surfactant-free, spherical and crystalline Au and AgNPs without the use of any stabilizer(s) within a few milliseconds. In view of the recent COVID-19 pandemic caused by the airborne transmission of SARS-CoV-2 virus aerosols, many excellent surveillance and control measures are being implemented in conned spaces, where the effect of the virus transmission is the highest. The application of plasma-aided virus disinfection is quite nascent, and the interaction of plasma with the virus aerosols in air streams has not been dealt with in detail. The actual mechanism for the virus inactivation is often ascribed to ozone, however, in short time scales of milliseconds, more reactive species might be needed to obtain an effective inactivation. We report on the use of a dielectric barrier discharge (DBD) for in-flight inactivation of airborne aerosolized porcine reproductive and respiratory syndrome (PRRS) virus. The measurements were performed in a laboratory-scale wind test tunnel. Using infectivity tests and reverse transcriptase quantitative polymerase chain reaction (RT-qPCR), we showed a 3.5 log10 reduction in the viable virus titer during in-flight treatment by the DBD within a few milliseconds. We identified both short-lived species such as OH radicals and singlet oxygen and peroxynitrous acid chemistry at low pH in the virus-laden droplets responsible for the observed inactivation of virus aerosols by plasma. The fundamental understanding of the interactions of plasma and liquid microdroplets, gained in this work, have the potential to increase significantly the efficacy of plasma processes. There is no doubt that improved reactor design and plasma generation informed by our findings will further advance the development of such unique interactions for the novel applications of water treatment, nanoparticle synthesis and virus aerosol inactivation.
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    Modeling of Transport Phenomena in Two-Dimensional Semiconductors
    (2016-12) Liu, Yue
    Recently, transition metal dichalcogenides and black phosphorus (BP) emerged as new 2D semiconductors due to the advantages of moderate energy band gap, high carrier mobility, ultra thin film and high anisotropy. Together with graphene, 2D materials have been utilized in the development of biomedical devices, touch screen and display technologies, and flexible applications such as wearable electronics and IoT devices. They also open up new opportunities in research fields including spintronics, optoelectronics and next generation post-silicon transistor. In this dissertation, we present theoretical modeling for several topics related to 2D materials. Starting with the fundamental tight-binding theory of graphene, we review electronic properties for graphene including massless 2x2 Dirac Hamiltonian and pseudo-spin wave function. Followed by discussion of ballistic transport, a detailed analysis on graphene diffusive transport is provided. Ionized impurity scattering and carrier screening effect is considered in the model. The momentum relaxation time and mobility for graphene is modeled. A non-linear Thomas Fermi screening is introduced to improve the simulation accuracy. Taking the real spin into account, the new Hamiltonian is a 4x4 matrix. An external field perpendicular to the graphene breaks the reflection symmetry and introduces a Rashba spin-orbit interaction, which couples pseudo-spin and real spin. The relevant charge carrier states are no longer spin eigenstates. Rashba interaction is found to be quite small compared to Coulomb impurity scattering. To characterize the spin-polarized electrons tunneling from electrodes and transport in graphene, a spin valve device modeling and magnetoresistance calculation is developed. Black phosphorus possesses excellent properties like other 2D materials for high performance nanoelectronic applications. Moreover, there is a uniquely high in-plane anisotropy in BP due to its puckered crystal structure. To model the anisotropic transport, a model based on the BTE is developed, considering the full anisotropic electronic structure. For zero temperature calculation with ionized impurity limited scattering, anisotropy ratio 3-4 can be obtained from the model. Due to the dominating effect of screening, mobility is found to decrease weakly with increasing temperature. For , a smaller anisotropy ratio of 1.8-3.5 matching experimental measurements indicates that impurity scattering is an important mechanism for black phosphorus.
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    Occurrence and Survival of Zebra Mussel (Dreissena polymorpha) Veliger Larvae in Residual Water Transported by Recreational Watercraft
    (2018-12) Doll, Adam
    Zebra mussels (Dreissena polymorpha) are an aquatic invasive species (AIS) that have spread to many waterbodies in North America and transient recreational watercraft are one of the primary pathways of spread. Best management practices for reducing the risk of spreading AIS include draining all water from watercraft before leaving a water body, but removing all water is impractical. Uncertainly exists about whether zebra mussel larvae (veligers) could reside within the “residual water” that remains after draining and survive overland transport to a new water body. At two Minnesota, USA water bodies (Gull Lake and Lake Minnetonka) from July – August (2016-2017) we collected over 300 samples of residual water from recreational watercraft; compartments included ballast tanks, live wells, sterndrive engines, and others. Roughly half (48%) of these samples contained no veligers and the majority (75%) contained five or fewer. Sterndrive engines and ballast tanks ranked 1st and 2nd for volumes of residual water (median of 4945 and 2650 milliliters, respectively), Ballast tank samples contained the largest median number of veligers per sample (247) and sterndrive engines the highest maximum number of veligers (about 4500 for 2 out of 38 engines sampled). We conducted laboratory experiments on veliger survival in residual water of live wells due to the high frequency of fishing boats moving between water bodies, and ballast tanks given their high likelihood of containing veligers. We exposed live well samples to 20°, 27°, 32°, and 38°C air temperature and ballast tanks to 20° and 32°C. For veligers in live well residual water, we observed greater than or equal to 95% mortality after 5 hours of exposure at all temperatures. These same levels of mortality were reached more slowly in ballast tanks (greater than or equal to 95% mortality at both temperatures achieved at 48 hours). Additional prevention steps should be taken (e.g. using hot water) to reduce the risk of transporting living veligers in residual water.