Mechanical Engineering & Applied Mechanics Doctoral Work

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Aerosol-Based Ultrafine Material Deposition for Microelectronics
(North Dakota State University, 2012) Hoey, Justin Michael
Aerosol-based direct-write refers to the additive process of printing CAD/CAM features from an apparatus which creates a liquid or solid aerosol beam. Direct-write technologies are poised to become useful tools in the microelectronics industry for rapid prototyping of components such as interconnects, sensors and thin film transistors (TFTs), with new applications for aerosol direct-write being rapidly conceived. This research aims to review direct-write technologies, with an emphasis on aerosol based systems. The different currently available state-of-the-art systems such as Aerosol Jet™ CAB-DW™, MCS and aerodynamic lenses are described. A review and analysis of the physics behind the fluid-particle interactions including Stokes and Saffman force, experimental observations and how a full understanding of theory and experiments can lead to new technology such as nozzle designs are presented. Finally, the applications of aerosol direct-write for microelectronics are discussed in detail including the printing of RFID antennas, contacts and active material for TFTs, the top metallization layer for solar cells, and interconnects for circuitry.
The Physico-Chemical Investigation of Interfacial Properties in Natural Fiber/Vinyl Ester Biocomposites
(North Dakota State University, 2012) Huo, Shanshan
Bast fibers are one of most widely used types of cellulosic natural fibers. Flax fibers, a specific type of bast fiber, have historically been used as reinforcements in composites because they offer competitive advantages, including environmental and economic benefits, over mineral-based reinforcing materials. However, the poor interfacial properties due to the hydrophilicity of flax fibers and the hydrophobicity of most polymer matrices reduce the mechanical performance of flax thermoset composites. On the other hand, the structure of flax fiber is more complex than synthetic fibers, which causes most of traditional mechanical tests from the transverse direction to evaluate the interfacial properties of flax composites are not valid. In this study, the physical and chemical properties of flax fibers, vinyl ester resin and their composites are investigated. A comprehensive understanding of flax fiber, vinyl ester systems and their composites has been established. Surface modifications to the flax fiber and chemical manipulations on vinyl ester systems have been studied to improve the interfacial properties of flax/vinyl ester biocomposites. A new chemical manipulation method for vinyl ester system has been invented. The specific interlaminar shear strength of alkaline treated flax/VE with 1.5% AR shows approximately 149% increase than untreated flax/VE composites. NaOH/Ethanol treated flax/VE with AR shows 33% higher in specific flexural modulus and 73% better in specific flexural strength than untreated flax/VE composites. In addition, AR modified alkaline treated flax composites performs approximately 75% better in specific tensile modulus and 201% higher in specific tensile strength than untreated flax/VE composites. Flax/VE composite with high elastic modulus, which is higher than their theoretically predicted elastic modulus, was achieved. The effects of thermal properties of flax fibers and vinyl ester resin systems on the interfacial properties of their biocomposites were also studied. The theory of modifying the thermal properties of flax and vinyl ester to improve the interfacial adhesion has been proved by the study of the thermal residual stresses in their composites by XRD techniques.
High-Performance Simulations for Atmospheric Pressure Plasma Reactor
(North Dakota State University, 2012) Chugunov, Svyatoslav
Plasma-assisted processing and deposition of materials is an important component of modern industrial applications, with plasma reactors sharing 30% to 40% of manufacturing steps in microelectronics production [1]. Development of new flexible electronics increases demands for efficient high-throughput deposition methods and roll-to-roll processing of materials. The current work represents an attempt of practical design and numerical modeling of a plasma enhanced chemical vapor deposition system. The system utilizes plasma at standard pressure and temperature to activate a chemical precursor for protective coatings. A specially designed linear plasma head, that consists of two parallel plates with electrodes placed in the parallel arrangement, is used to resolve clogging issues of currently available commercial plasma heads, as well as to increase the flow-rate of the processed chemicals and to enhance the uniformity of the deposition. A test system is build and discussed in this work. In order to improve operating conditions of the setup and quality of the deposited material, we perform numerical modeling of the plasma system. The theoretical and numerical models presented in this work comprehensively describe plasma generation, recombination, and advection in a channel of arbitrary geometry. Number density of plasma species, their energy content, electric field, and rate parameters are accurately calculated and analyzed in this work. Some interesting engineering outcomes are discussed with a connection to the proposed setup. The numerical model is implemented with the help of high-performance parallel technique and evaluated at a cluster for parallel calculations. A typical performance increase, calculation speed-up, parallel fraction of the code and overall efficiency of the parallel implementation are discussed in details.
Fluid Mechanics of Micro Cold Spray Direct Write Process
(North Dakota State University, 2012) Bhattacharya, Sourin
Cold spray, also known as the gas dynamic spray process, was first discovered in the 1980s while doing high speed two phase wind tunnel experiments. The principle underlying this process is that if a metal particle is accelerated to a velocity above a certain critical velocity, upon impact on a substrate the particle and substrate will undergo rapid plastic deformation and form a “splat”. This process is currently being used for coatings applications. In this process, metal particles of diameter 5 μm to 50 μm are accelerated to a very high velocity (>500 m/s) and are deposited on substrates. Based on principles similar to cold spray process, we have developed a novel direct write process known as the Micro Cold Spray Direct Write (MCS-DW) process. Initial results from our experimental study have shown that conductive patterns of copper, tin and aluminum can be printed on flexible and rigid substrates using this process. The smallest feature size that can be printed using this process is 50 μm. In order to improve the deposition efficiency of the MCS-DW process, numerical studies were carried out to simulate the flow of aerosol particles through different nozzle geometries. It was found that a convergent capillary nozzle with a linear converging section of length 19 mm and a straight capillary of length 14 mm can be used to accelerate and focus silver particles of diameter 2 μm. Copper particles of diameter 3 μm can accelerate to their critical velocity by using a longer straight section of length 30 mm.
Fluid Dynamics of Material Micro-Deposition: Capillary-Based Droplet Deposition and Aerosol-Based Direct-Write
(North Dakota State University, 2012) Lutfurakhmanov, Artur
With rapid development of the direct-write technology, in addition to requirement of non-destructive printing, there is a need for non-expensive, robust, and simplified techniques of micro/nano fabrication. This dissertation proposes a new technique of non-invasive lithography called Capillary-Based Droplet Deposition and suggests improvements to existing Aerosol-Jet Direct-Write method that leads to deposition of thinner lines. A hollow capillary filled with liquid is a dispensing tool employed for the Capillary-Based Droplet Deposition method. Due to pressure applied from one side of the capillary, a liquid meniscus is formed at the opposite side of the capillary. After the meniscus touches the substrate, a liquid bridge between the capillary and substrate is formed. The capillary retraction causes the bridge rupturing and liquid droplet deposition. In the first part of this dissertation, the Capillary-Based Deposition method is considered both theoretically and experimentally. From bridge modeling, it is found that the droplet size is dependent on pressure applied, inner radius and wall thickness of the capillary, and liquid-capillary and liquid-substrate equilibrium contact angles. Three deposition scenarios are identified showing that minimum deposited droplet size is about 15% of the capillary inner diameter. Modeling results are verified in experiments with different water-glycerol solutions used as test liquid and with capillaries of wide range of inner diameters. The second part of the dissertation is devoted to theoretical investigation of the Aerosol-Jet Direct-Write method where few micron width lines are created from aerosol droplets that move along with the gas flowing through a converging micro-nozzle. Gas velocity and density profiles inside and outside of the nozzle are obtained from iv ANSYS/CFX simulation. Aerosol droplet trajectories and velocity components are calculated using all forces acting on the particles in the flow. Comparing all forces, it is found that only Stokes and Saffman forces are relevant for simulation of the gas-particle interaction. Original 1D equation for Saffman force is extended to two dimensional gas flows. For some parameter ranges, Saffman force is found to be negligibly small. Based on simulation results, two nozzle designs are proposed in order to collimate aerosol particles with diameters of 1.5-5.0 microns toward the nozzle centerline.
Nonlinear Fatigue Damage Accumulation in Aircraft Engine Alloys Multiaxial Loading
(North Dakota State University, 2013) Suman, Sandip Kumar
Fatigue is considered to be one of the most frequent phenomena in the failure of many machine parts. Most of the prior studies on fatigue have been limited to uniaxial loading cases with a primary focus on constant amplitude cycles. A detailed exploration of multiaxial fatigue under constant and variable amplitude loading scenarios for a wide variety of aircraft engine alloys has been performed in this study, and a new methodology for the accurate prediction of fatigue damage is developed. A critical-plane based constant amplitude fatigue damage model has been developed in this study which is simple in comparison to prior models developed by other researchers and reduces the computational effort. The constant amplitude fatigue damage model is further used in the development of a multiaxial variable amplitude damage estimation method, with an emphasis on estimating the damage created by both low cycle fatigue (LCF) and high cycle fatigue (HCF) cycles. A significant increase in overall fatigue damage was observed in the tests with the introduction of HCF cycles in the mission histories. The damage due to the HCF cycles has been found to be much greater than predicted by linear damage accumulation theories, although the degree of interaction between the LCF and HCF cycles was found to be very dependent on the multiaxial load paths. In addition, the HCF cycles did not contribute significantly to the accumulation of damage until a certain amount of “pre-damage” had been caused by the LCF cycles. Separate HCF damage computing approaches have been adopted in this study to accurately compute the damage produced by tensile and shear dominant HCF cycles, and a significant improvement in the accuracy of fatigue life prediction has been achieved using the new methodology.
Flow Accelerated Organic Coating Degradation
(North Dakota State University, 2014) Zhou, Qixin
Applying organic coatings is a common and the most cost effective way to protect metallic objects and structures from corrosion. Water entry into coating-metal interface is usually the main cause for the deterioration of organic coatings, which leads to coating delamination and underfilm corrosion. Recently, flowing fluids over sample surface have received attention due to their capability to accelerate material degradation. A plethora of works has focused on the flow induced metal corrosion, while few studies have investigated the flow accelerated organic coating degradation. Flowing fluids above coating surface affect corrosion by enhancing the water transport and abrading the surface due to fluid shear. Hence, it is of great importance to understand the influence of flowing fluids on the degradation of corrosion protective organic coatings. In this study, a pigmented marine coating and several clear coatings were exposed to the laminar flow and stationary immersion. The laminar flow was pressure driven and confined in a flow channel. A 3.5 wt% sodium chloride solution and pure water was employed as the working fluid with a variety of flow rates. The corrosion protective properties of organic coatings were monitored inline by Electrochemical Impedance Spectroscopy (EIS) measurement. Equivalent circuit models were employed to interpret the EIS spectra. The time evolution of coating resistance and capacitance obtained from the model was studied to demonstrate the coating degradation. Thickness, gloss, and other topography characterizations were conducted to facilitate the assessment of the corrosion. The working fluids were characterized by Fourier Transform Infrared Spectrometer (FTIR) and conductivity measurement. The influence of flow rate, fluid shear, fluid composition, and other effects in the coating degradation were investigated. We conclude that flowing fluid on the coating surface accelerates the transport of water, oxygen, and ions into the coating, as well as promotes the migration of coating materials from the coating into the working fluid, where coatings experience more severe deterioration in their barrier property under flowing conditions. Pure water has shown to be a much more aggressive working fluid than electrolyte solutions. The flowing fluid over the coating surface could be used as an effective acceleration method.
Mechanical and Tribological Properties of Carbon Nanofiber Reinforced High Density Polyethylene
(North Dakota State University, 2014) Xu, Songbo
High density polyethylene (HDPE) is widely used as a bearing material in industrial application because of its low friction and high wear resistance. Reinforcing polymers with the appropriate nanofillers is an effective way to obtain a variety of enhanced material properties. Carbon nanofibers (CNFs) with silane coatings (two thicknesses: 2.8 nm and 46 nm) were added into high density polyethylene (HDPE) to improve the tribological properties of the nanocomposite material. The goal of the present study is to investigate how the mechanical, thermal, and wear behavior of HDPE can be altered by the addition of either pristine or silane treated CNFs at different loading levels (0.5 wt.%, 1 wt.%, and 3 wt.%) and to model the wear of the HDPE/CNF nanocomposites under both dry and lubricated conditions. In this study, the wear and friction tests are performed on a pin-on-disc tribometer under dry, bovine serum, and phosphate buffered saline lubricated conditions. The thermal, mechanical, properties, and biocompatibility of HDPE/CNF nanocomposites are characterized and compared with those of the neat HDPE. The correlations of the wear and factors such as work of fracture, thermal conductivity, and friction force are explored. An energy-based wear model is proposed for the dry sliding condition in which a thermal analysis is derived to trace the friction energy loss in the wear process. A wear model for the lubricated condition is developed with incorporation of elastohydrodynamic lubrication theory and Reye's wear model to predict the long term wear.
Computational Biomechanics of Blast-Induced Traumatic Brain Injury: Role of Loading Directionality, Head Protection, and Blast Flow Mechanics
(North Dakota State University, 2015) Sarvghad-Moghaddam, Hesam
In this dissertation, blast-induced traumatic brain injury (bTBI) is studied with respect to the blast wave directionality, mitigation capability of helmet/faceshield, and blast flow mechanics using finite element (FE) and computational fluid dynamics (CFD) schemes. For the FE study, simulations are performed on a detailed FE head model using LS-DYNA, and CFD simulations are carried out using the ANSYS-CFX to examine the underwash development by analyzing the behavior of blast flow from different directions. The following tasks are conducted. First, the effects of the loading direction on the mechanical response of the head and brain is investigated through impact and blast induced loading on the head. Due to the differences in the shape, function, and tolerance of brain components, the response of the head/brain varies with the direction of the impact and blast waves. In identical situations, the head shows to have lower tolerance to side loading. Second, the inclusion of the faceshield as a potential head protective tool against blast threats is evaluated with respect to blast direction. The helmet-faceshield and helmeted assemblies are shown to be most efficient when the head is exposed to blast from the front and top sides, respectively. Faceshield is observed to be effective only in front blast as it might impose either adverse or no effects in other directions. The shockwaves are seen to form a high pressure region in head-helmet-faceshield gap (underwash effect) which induces elevated pressures on the skull. Third, the underwash effect’s mechanism is investigated through CFD simulations of supersonic shockwave flow around the helmeted head assemblies. CFD results reveals that the backpressure is produced due to the creation of a backflow in the exterior flow on the outgoing interior flow. The bottom and side shockwave directions predict the highest underwash overpressures, respectively. Finally, the ICP and shear stress of the brain is evaluated in case of underwash incidence. FEA results show that underwash overpressure greatly changes with the blast direction. It is concluded that underwash clearly altered the tissue response of the brain as it increases ICP levels at the countercoup site and imparts elevated skull flexure.
Experimental and Micromechanical Analysis of Flax and Glass Reinforced Bio-Based Composites
(North Dakota State University, 2015) Hosseini, Nassibeh
Two different novel high-functional bio-based resins from Methoxylated Sucrose Soyate Polyol (MSSP) and methacrylated epoxidized sucrose soyate (MAESS) were used as matrices for composites. Vinyl ester reinforced with flax fiber and E-glass fiber were also produced as the references to highlight the performance of bio-based composites. An appropriate processing conditions for MSSP and MAESS resins using compression molding was established to fabricate high fiber volume content composites. Mechanical properties of composites were assessed by tensile, flexural, interlaminar shear strength (ILSS), nano-indentation, and impact strength. Scanning Electron Microscopy (SEM) of fractured surfaces of flexural specimens were examined to investigate the fiber-matrix interface behavior. MSSP and MAESS resins reinforced with E-glass fiber performed similarly if not superior to previous bio-based and petroleum-based composites studied Tensile strength and modulus of E-glass reinforced MSSP were higher up to 40% and 75% respectively, compared to existing studies. For flexural strength and modulus 130% and 110% improvements were observed. The tensile strength and modules of MAESS and vinyl ester resins reinforced with E-glass fibers are 532 MPa, 36.79 GPa and 536 MPa, 36.40 GPa, respectively. The impact strength of the composites with MAESS resin reinforced with E-glass fibers was 237 kJ/m2, whereas that of the vinyl ester resin reinforced with same E-glass fiber was 191 kJ/m2. Results of SEM images along with flexural strength, interlaminar shear strength and impact tests revealed better wetting of fibers by matrix, stronger adhesion between fiber and matrix and greater interfacial bonding compared to corresponding E-glass/vinyl ester composites. The composites made from flax fiber with MSSP or MAESS resins achieve similar properties to E-glass/MSSP and E-glass/MAESS in terms of specific mechanical properties. Moreover, flax/MSSP and flax/MAESS composites perform similarly, if not superior to previous bio-based and petroleum based composites studied. A micromechanical model and an analytical approach were also developed to predict the stress relaxation response of the flax/MSSP composite material consisting linear viscoelastic flax fiber and bio-based PU matrix. A good agreement between the micromechanical modeling data and experimental results was observed for the linear viscoelastic response of the bio-based composite.
Optimized Evaluation of Bone Tissue Material Properties by Inverse Finite Element Analysis and Femur Fracture Testing
(North Dakota State University, 2015) Javid, Samad
The main objective of this research is to characterize bone inhomogeneous elastic, yield, and post-yield behaviors, using a computational-experimental approach. The current study uses the force-displacement results of one hundred four cadaveric femora that were previously tested to fracture in a fall on the hip loading configuration. Recorded force-displacement data are used to determine stiffness, yield force, and femoral strength values. Finite element (FE) models of the femora are developed from the quantitative computed tomography scans captured before the fracture tests. A power law, or a sigmoid function, is used to determine the elastic modulus from the ash densities for each case modeled. The models are used for FE simulations that mimic the experiments. Inverse finite element analysis is employed to identify the unknown coefficients in the bone density-elasticity relationships. Optimization algorithms are used to minimize the error function between the experimental and FE estimated results in a large subset of female femora. The results of the obtained relationships show a good agreement with the experimental data. This contributes to a coefficient of determination of 70%, which is higher than those of previously proposed density-elasticity relationships on the same set of femora. The parts of the bones with the densities up to 0.5 g/cm3, play an important role in the deformation of the neck and the head of the femur. While power law and sigmoid function show similar correlation in the prediction of stiffness, distribution of stresses and strains are notably different, showing different response in the yield and post-yield behavior. To simulate the material damage, a power density-yield strain relationship is used as the failure criterion in FE models, assuming a ductile and a brittle material behavior for the bone. The unknown coefficients in the density-yield strain relationship are identified for the ductile and brittle material models. The ductile material model shows a more realistic post-yield behavior iv than the brittle model, but it is computationally expensive and may face convergence issues due to nonlinearities. The brittle material model, on the other hand, estimates the bone strength fairly and, due to its simplicity, it seems more applicable for clinical use.
Brain Tissue Mechanical Characterization and Determination of Brain Response under Confined Blasts Explosions
(North Dakota State University, 2015) Rezaei, Asghar
Mechanical experimental tests including stress relaxation, simple monotonic ramps, and impact loads were performed on porcine brain tissues to investigate the response of the brain under different loading scenarios. Linear viscoelastic models were employed to determine the applicability and limitations of the linear mechanical models in tension. In addition the lowest and highest stress values, which can be possibly applied to the tissue due to change in the strain rates, were investigated using stress relaxation experiments to implicitly address the two levels of strain rates. Porcine brainstem samples were tested in six stress relaxation experimental settings at strain amplitudes ranging from 5% to 30% in compression. The lowest stress was directly measured from long-term responses of stress relaxation experiments when the stress values remained constant. The highest stress level was determined by using the quasi-linear viscoelasticity theory and estimating the instantaneous stress of the samples at six strain amplitudes. It was hypothesized that there is a correlation between the two pure elastic behaviors. The hypothesis was true as a strong linear correlation was found between the two elastic responses. The results showed that the instantaneous stress values were 11 times greater than the long-term stress values, practically similar across all strain amplitudes. In the second part of the thesis, a number of computational studies were conducted using a validated human head model. The head model included major components of human head and underwent different blast scenarios in open and confined spaces. The study investigated the effect of reflections from the walls. The results show that when the head was in the vicinity of the wall, the biomechanical parameters were dramatically increased, especially in the corners. Comparing brain biomechanical parameters in confined, semi-confined, and open spaces under blast loads, the brain sustained greater stress and strain values, with larger duration of the loads, in confined spaces. Also, a primary blast injury (PBI) with a tertiary blast injury (TeBI) in a confined space was compared. The results indicated that the PBI due to the incident shock wave was much more injurious than TeBI due to blunt impact.
Microwave Torrefaction of Natural Fibers for Incorporation into Engineering Thermoplastic Biocomposites
(North Dakota State University, 2015) Vold, Jessica Lynne Lattimer
Little work has been done in the area of engineering thermoplastic biocomposites due to the increased processing temperatures which induce degradation of biomass. Torrefaction has been identified as an effective means of preparing biomass for introduction into engineering thermoplastics such as polyamide 6, however it is an energy and time intensive process. This work looks to microwave induced heating to reduce the required energy costs by 70% over a conventional heating method while producing a more homogeneous higher degree of torrefaction torrefied biomass. The torrefied biomasses were analyzed to understand how time, temperature, and power level affect the yield and thermal stability temperature of the fibers. The effects of the addition of torrefied flax shive, hemp hurd, and sunflower hulls to polyamide 6,6 on mechanical and thermal properties were also studied.
Examination of the Impact of Helmets on the Level of Transferred Loads to the Head Under Ballistic and Blast Loads
(North Dakota State University, 2015) Salimi Jazi, Mehdi
The main causes of human Traumatic Brain Injuries (TBIs) in war zones are ballistic impacts and blast waves. While understanding the mechanism of TBI and the brain injury thresholds are in urgent needs, efficiency of helmets as injury protective is not well-examined. To address these gaps, this study investigates the impact of ballistic helmets and padding systems on the biomechanical responses of the brain under dynamic ballistics and blasts loads. A nonlinear human head-neck finite element modeling procedure has been employed for the analysis. The results are examined against de-facto standard experimental data. The response of the finite element head model (FEHM) in terms of biomechanical parameters of the brain has been examined to measure the influence of padding system materials on the level of the loads transferred to the head. The results show when a bullet hits the front of the helmet vertically, the brain experiences the highest amount of stresses in comparisons with other impact orientations. Also, low stiffness foams cause less amount of load to be transferred to the head, indicating the importance of the mechanical properties of the padding system in helmet design. Parametric studies have also been carried out to examine the efficiency of the helmet under various blast situations and intensities by varying standoff distances and orientation angles of the FEHM. The results indicate that the protected heads experience lower accelerations, and stresses than unprotected heads. In general it was found that the performance of the helmet depends on the extent of the coverage of the head by helmet. To examine the influence of the entire human body in comparison with the only head model, the torso and attached to the head was modeled and the responses of the brain to equivalent loadings were examined. In general for the first few milliseconds of the assault on the head, biomechanical parameters of the brain remain independent of the torso. However, one can iv see the body influence as times goes by. As a conclusion one can rely on the results of the head and neck model to be credible enough for brain injury analysis.
Breast Cancer Screening in Native American Women at an Urban Minnesota Community Clinic
(North Dakota State University, 2015) Vaishnav, Molly Cassandra
The Native American Community Clinic (NACC) in Minneapolis, MN, expressed a need for a breast cancer screening patient education brochure and a policy regarding breast cancer screening, due to the absence of these in their clinic. Native American women have some of the lowest breast cancer screening rates along with the poorest five-year survival rate for breast cancer. Early screening and detection of breast cancer is thought to be the key to survival. The reason for these low breast cancer screening rates among Native American female patients is multifactorial, but lack of knowledge and provider recommendation are two known barriers. The purpose of this practice improvement project was to develop a culturally appropriate breast cancer screening patient education brochure with a policy that outlines use. The healthcare providers, medical director, and the patient advisory group at the NACC evaluated the brochure, and the medical director evaluated the policy. The Plan, Do, Study, Act method was utilized to facilitate the process and address the clinic goals. The project first reviewed current guidelines and literature for breast cancer screening. After one set of guidelines was selected, the brochure was created. The healthcare providers, medical director, and patient advisory group members were then given a qualitative survey. The responses to the survey gave suggestions for revisions, which were made to the brochure. Revisions included things such as using different guidelines, including more information on mammography, and including photographs of Native American women. The policy was then created. The policy included which guidelines to use when offering screening, whom to offer screening to, and guidance on how to properly document breast cancer screening and education after each visit. The medical director was then given a qualitative survey, which inquired about necessary revisions. The medical director suggested only one minor revision (change in wording). Following a total of four meetings and multiple revisions, the educational brochure and policy were approved by the NACC medical director. Future research should focus on expanding culturally appropriate patient education materials in clinical settings, such as the NACC.
Experimental and Theoretical Studies of Nanostructured Electrodes for Use in Dye-Sensitized Solar Cells
(North Dakota State University, 2017) Gong, Jiawei
Among various photovoltaic technologies available in the emerging market, dye-sensitized solar cells (DSSCs) are deemed as an effective, competitive solution to the increasing demand for high-efficiency PV devices. To move towards full commercialization, challenges remain in further improvement of device stability as well as reduction of material and manufacturing costs. This study aims at rational synthesis and photovoltaic characterization of two nanostructured electrode materials (i.e. SnO2 nanofibers and activated graphene nanoplatelets) for use as photoanode and counter electrode in dye-sensitized solar cells. The main objective is to explore the favorable charge transport features of SnO2 nanofiber network and simultaneously replace the high-priced conventional electrocatalytic nanomaterials (e.g. Pt nanoparticles) used in existing counter electrode of DSSCs. To achieve this objective, a multiphysics model of electrode kinetics was developed to optimize various design parameters and cell configurations. The porous hollow SnO2 nanofibers were successfully synthesized via a facile route consisting of electrospinning precursor polymer nanofibers, followed by controlled carbonization. The novel SnO2/TiO2 composite photoanode materials carry advantages of SnO2 nanofiber network (e.g. nanostructural continuity, high electron mobility) and TiO2 nanoparticles (e.g. high specific area), and therefore show excellent photovoltaic properties including improved short-circuit current and fill factors. In addition, hydrothermally activated graphene nanoplatelets (aGNP) were used as a catalytic counter electrode material to substitute for conventionally used platinum nanoparticles. Improved catalytic performance of aGNP electrode was achieved through increased surface area and better control of morphology. Dye-sensitized solar cells using these aGNP electrodes had power conversion efficiencies comparable to those using platinum nanoparticles with I-/I3- redox mediators. Moreover, a multiphysics model at the device level was developed to predict the power output characteristics of DSSC using different electrode materials. The developed model was validated by the experimental data acquired from lab-fabricated DSSCs. Further, parametric simulation was conducted to analyze the effect of series resistance, shunt resistance, interfacial overpotential, as well as difference between the conduction band and formal redox potentials on device performance. This model correlates the maximum power output of DSSC devices to various design and operating parameters, and it also provides insight into the working principles of newly designed devices.
Characterization, Long-Term Behavior Evaluation and Thermo-Mechanical Properties of Untreated and Treated Flax Fiber-Reinforced Composites
(North Dakota State University, 2017) Amiri, Ali
In recent years there has been a resurgence of interest in the usage of natural fiber reinforced composites in more advanced structural applications. Consequently, the need for improving their mechanical properties as well as service life and long-term behavior modeling and predictions has arisen. In a step towards further development of these materials, in this study, two newly developed biobased resins, derived from soybean oil, methacrylated epoxidized sucrose soyate and double methacrylated epoxidized sucrose soyate are combined with untreated and alkaline treated flax fiber to produce novel biocomposites. Vinyl ester reinforced with flax fiber is used as control in addition to comparing properties of biobased composites against commercial pultruded composites. Effects of alkaline treatment of flax fiber as well as addition of 1% acrylic resin to vinyl ester and the two mentioned biobased resins on mechanical properties are studied. Properties are evaluated in short-term and also, after being exposed to accelerated weathering (i.e. UV and moisture). Moreover, long-term creep of these novel biobased composites and effect of fiber and matrix treatment on viscoelastic behavior is investigated using Time-temperature superposition (TTS) principle. Based on the results of this study, the TTS provides an accelerated method for evaluation of mechanical properties of biobased composites, and satisfactory master curves are achieved by use of this principle. Also, fiber and matrix treatments were effective in increasing mechanical properties of biobased composites in short-term, and treatments delayed the creep response and slowed the process of creep in composites under study in the steady state region. Overall, results of this study reveal the successful production of biocomposites having properties that meet or exceed those of conventional pultruded members while maintaining high biocontent. Composites using treated flax fiber and newly developed resins showed less degradation in properties after accelerated weather exposure. Procedures and methods developed throughout this study, as well as results presented are essential to further development of these novel materials and utilizing them in more advanced structural applications. Results presented in this dissertation have been published as 5 peer reviewed journal articles, 2 book chapters and have been presented in 6 national and international conferences.
Improving Performance Characteristics of Poly (Lactic Acid) (PLA) Based Nanocomposites by Enhanced Dispersion of Modified Cellulose Nanocrystals (CNCs)
(North Dakota State University, 2018) Shojaeiarani, Jamileh
Poly(lactic acid), (PLA) is a biodegradable and biocompatible polymer which has attracted significant attention as a promising substitute for petroleum-based polymers. To optimize the usage of PLA in a wide range of applications, different methods such as polymer blending and the incorporation of traditional and nanofillers have been extensively explored. Cellulose nanocrystals (CNCs), rod-like nanoparticles with a perfect crystalline structure, are considered as outstanding reinforcing agent owing to the excellent mechanical properties. The optimal characteristics of CNCs as a reinforcing agent in the polymer can be achieved through homogeneous dispersion within the polymeric matrix. However, the strong hydrophilic character of CNCs due to the presence of hydroxyl groups on the surface restricts the uniform dispersion of CNCs in the PLA matrix. In this work, three surface modification treatments along with two different mechanical preparation techniques were employed to improve the dispersion quality of CNCs in the PLA matrix. Polymer adsorption, green esterification, and time-efficient esterification were used as surface modification treatments. Solvent casting and spin-coating method were employed to prepare highly concentrated CNCs masterbatches. Nanocomposites were prepared using melt extrusion, followed by an injection molding process. The morphology of masterbatches indicated better CNCs dispersion through spin-coated thin films, suggesting a high evaporation rate and the effect of centrifugal force and surface tension in the spin-coating process decrease the possibility of CNCs aggregate through the film. Consequently, nanocomposites manufactured using spin-coated masterbatches exhibited higher mechanical strength in comparison with solvent cast ones. In the case of surface modification treatments, the most uniform CNCs dispersion was observed in the nanocomposites reinforced by valeric acid through esterification technique. Higher thermal stability was also achieved through the application of esterification technique. This observation was related to the presence of DMAP on the surface of CNCs which turns into inert materials, prohibiting the thermal degradation. The higher molecular weight and lower molecular number observed in spin-coated samples in comparison with film cast nanocomposites led to the higher damping behavior in spin-coated nanocomposites. This observation indicated the more viscoelastic properties in spin-coated samples owing to the presence of more polymer chain freedom in spin-coated nanocomposites.
Development of an Anti-Corrosion Thermally Sprayed Coating System for Oil and Gas Transmission Pipeline
(North Dakota State University, 2018) Abualigaledari, Sahar
Corrosion, a leading cause of failure in metallic transmission pipelines, significantly impacts the reliability and safety of metallic pipelines. To prevent and mitigate pipeline corrosion, various non-metallic coatings and assessment methods have been implemented with different coating techniques. However, reliable, cost-effective, and environmental friendly corrosion mitigation approaches are yet needed to be achieved. Thermal metallic sprayed coatings have shown to be an effective means for pipeline corrosion prevention in marine environments with low cost, but it is not yet studied for on-shore buried and cased crossing pipelines. In this project, innovative composite self-sensing thermal sprayed coatings are proposed to prevent, monitor, mitigate, and manage pipeline corrosion for on-shore buried metallic transmission pipelines. This project focuses on developing the metallic corrosion resistant coating with thermal spray techniques. The compositions, mechanical properties, corrosion resistance, and effectiveness of composite thermal sprayed coatings have been investigated and studies theoretically, numerically, and experimentally at mechanical engineering department, NDSU, Fargo, ND.
Dynamical Modelling of an Idealized Hemispherical Skull Model with Fluid Pressure Interactions Using Modal Analysis
(North Dakota State University, 2019) Eslaminejad, Ashkan
In this dissertation, a non-invasive intracranial pressure (ICP) monitoring technique is introduced by developing a head dynamic model. The technique is based on modal frequency testing and vibration responses analysis of the skull. To examine and verify this methodology, we conducted vibration tests on a hemispherical shell to stand as a surrogate for human cranium to measure the effect of cerebrospinal fluid (CSF) pressure on human skull dynamic response; we utilized a hammer-impact modal testing methodology on the simulated hemispherical shell to extract its dynamic response characteristics. To be able to examine the CSF-skull dynamics interactions, we measured the skull impulse responses using mechanical tensile tests at different strain rates. The modal analysis by finite elements eigenvalue analysis of the upper cranium skull model was conducted to find the material properties of the skull. Linear elastic, as well as, nonlinear hyperelastic material models were assumed for the skull to find its material parameters. In the simulation of the human head, the cranium was modeled as a closed clamped hemispherical aluminum shell under internal fluid pressure. The interactions of CSF with the simulated cranium were studied and the frequency responses were obtained at different interior pressures. A numerical procedure for dynamic analysis of the systems was developed to measure the modal frequencies of the setup. We examined the changes to the peaks of frequency response under different fluid pressure. The results of modal analyses demonstrate changes in the frequency of bending-wave vibration modes, while longitudinal-wave modes are nominally altered under variable pressure conditions. A single-degree of freedom vibrational model was also developed to fit to the data for the sensitive modes. Linear regression analysis of the results reveals that the dynamic model’s equivalent damping and stiffness parameters are sensitive to fluid pressure variations while the equivalent mass parameter is relatively unaffected. As a result of this study we conclude that variance in CSF pressure has a measurable effect on the dynamic characteristics of the cranium and vice-versa. A calibrating system to connect the dynamic changes of the head can stand as a non-invasive system for ICP changes.

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