Browsing by Subject "Plasma"

Now showing 1 - 12 of 12
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    Atmospheric Pressure Non-Thermal Plasma: A Tool for Inactivating Airborne Pathogens
    (2019-12) Schiappacasse, Charles
    Pathogens spread by airborne transmission represent a persistent threat to economic stability and human/animal health. These pathogens are particularly prevalent in the agricultural sector, especially in animal rearing facilities. However, the agricultural industry currently lacks an efficient and cost effective means of controlling airborne pathogens. The present study explored the possibility of developing a new type of antimicrobial air treatment system based on non-thermal plasma technology. The study consisted of an initial laboratory testing stage followed by a pilot scale study performed in a local poultry rearing facility. During the laboratory testing stage two prototype non-thermal plasma reactors were developed and challenged in a closed air circulation system with artificially aerosolized Newcastle Disease Virus and avian influenza virus. The results indicated that both viruses could be rapidly inactivated below the limit of detection after sub second exposure to non-thermal plasma. Specifically, Newcastle Disease Virus was completely inactivated after 7.7x10-3 seconds of direct plasma treatment with a specific energy input of 171 J/L of air. Although the high virus inactivation effects are believed to predominately be attributable to direct non-thermal plasma exposure, preliminary experiments revealed that liquid-based virus collection strategies (e.g. use of an SKC BioSampler) were susceptible to liquid-based inactivation of collected viruses via indirect non-thermal plasma exposure. Attempts were made to circumvent the issue of liquid-based inactivation by employing a gelatin-filter based virus collection strategy. However, due to the high ozone emissions (80ppm) of the non-thermal plasma reactors, surface-based inactivation effects of viruses collected on the filters could not be ruled out as a contributing mechanism to the high virus inactivation rates. Due to biosafety concerns, flow rates >28LPM were not tested and the upper limit of non-thermal plasma’s virus inactivation efficiency was not determined. Additionally, virus samples taken from the 6-jet collision nebulizer, used to aerosolize viruses in this study, revealed that nebulization stress did not contribute to virus inactivation. Finally, the effects of relative humidity on virus losses within the closed air circulation system were explored. Findings showed a strong correlation (R2>0.99) between increasing relative humidity and decreasing airborne virus concentrations. This relationship is thought to predominately be due to humid aerosols experiencing greater condensation losses within the closed air circulation system when compared with less humid aerosols. However, the small volume of condensed solution visualized within the system was not sufficient to account for all of the viruses lost between the nebulizer and sampling port (gelatin filters). Correcting for adhesion losses and possible low gelatin filter sampling efficiencies did not completely account for non-plasma treated viral losses. Therefore, it is possible that the viruses used in the present study experience a decrease in infectivity with increasing humidity levels. The second stage of this study involved the design and fabrication of a pilot scale non-thermal plasma air treatment system and challenging it with ambient aerobic bacteria at a local turkey barn. Technical complications with a commercial high voltage power supply resulted in the pilot scale system operating at approximately 10% of its specified input power resulting in a specific energy input of only ~19.4 J/L. As a result, the pilot system was not able to reduce airborne bacteria concentrations relative to air samples that did not receive plasma treatment. Overall, the findings from the present study indicate that non-thermal plasma based air treatment technologies can be an effective tool for controlling airborne pathogens. However, the successful industrial implementation of this technology will require appropriate power supplies and methods to mitigate ozone emissions.
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    Atomic-scale investigations of multiwall carbon nanotube growth.
    (2010-06) Behr, Michael John
    The fundamental processes of carbon nanotube (CNT) growth by plasma-enhanced chemical vapor deposition (PECVD) were investigated using a suite of characterization techniques, including attenuated total-reflection Fourier transform infrared spectroscopy (ATR-FTIR), optical emission spectroscopy (OES), Raman spectroscopy, convergent-beam electron diffraction (CBED), high-resolution transmission and scanning-transmission electron microscopy (TEM, STEM), energy dispersive x-ray spectroscopy, and electron energy-loss spectroscopy (EELS). It is found that hydrogen plays a critical role in determining the final CNT structure through controlling catalyst crystal phase and morphology. At low hydrogen concentrations in the plasma iron catalysts are converted to Fe3C, from which high-quality CNTs grow; however, catalyst particles remain as pure iron when hydrogen is in abundance, and produce highly defective CNTs with large diameters. The initially faceted and equiaxed catalyst nanocrystals are deformed by the surrounding CNT structure during growth. Although catalyst particles are single crystalline, they exhibit combinations of small-angle (~1-3 degree) rotations, twists, and bends along their axial length between adjacent locations. Fe3C catalyst nanoparticles that are located inside the base of well-graphitized CNTs of similar structure and diameter do not exhibit a preferred orientation relative to the nanotube axis, indicating that the graphene nanotube walls are not necessarily produced in an epitaxial process directly from Fe3C faces. Chemical processes occurring at the catalyst-CNT interface during growth were inferred by measuring, ex situ, changes in atomic bonding at an atomic scale with EELS. The observed variation in carbon concentration through the base of catalyst crystals reveals that carbon from the gas phase decomposes on Fe3C, near where the CNT walls terminate at the catalyst base. An amorphous carbon-rich layer at the catalyst base provides the source for CNT growth. These results suggest that what is required for CNT growth is a graphene seed and a source of decomposed carbon. Hydrogen atoms also interact with the graphene walls of CNTs. When the flux of H atoms is high, the continuous cylindrical nanotube walls are etched nonuniformly. Etch pits form at defective sites along the CNT, from which etching proceeds rapidly. It is determined that H etching occurs preferentially at graphene edges.
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    Gene and protein expression in canine follicular thyroid carcinoma.
    (2011-08) Metivier, Kristy Stacy
    The major goal of this study was to investigate the molecular characteristics of canine follicular thyroid carcinoma (FTC). This is a rapidly growing and highly aggressive tumor in dogs, and many patients present with evidence of invasion or metastasis. Some smaller independent studies have attempted to evaluate the role of single molecules such as p53 and thyroid transcription factor-1 in tumor development, often with inconclusive results. In the present study, a genome-wide approach was employed to achieve the first objective of determining the gene expression profile of FTC compared to normal thyroid tissue. Microarray analysis was performed in a pilot study using five FTC tissues and four normal thyroid gland tissues, and this showed 489 transcripts as differentially expressed between the two groups; 242 were down-regulated and 247 were up-regulated. Some important biological functions that were affected include regulation of cell shape, cell adhesion, regulation of MAP kinase activity, angiogenesis, and regulation of cell migration. Osteopontin was a gene of interest as tumors consistently expressed it at high levels while it was expressed at low levels in all of the healthy samples. One of its up-stream regulators, VEGFA, was also differentially expressed but with a smaller fold change. The expression of osteopontin was validated by quantitative PCR using three groups: non-invasive FTC (tumors with capsular invasion only), invasive FTC (tumors with capsular and vascular invasion), and normal thyroid tissue. Both non-invasive FTC and invasive FTC had higher osteopontin gene expression than normal thyroid tissue but the two tumor groups were not different from each other. The second objective was to determine the protein expression of osteopontin and VEGFA in the same cases using semi-quantitative scoring of tissues stained by immunohistochemistry. The results were similar, with non-invasive and invasive FTC having higher osteopontin protein expression than normal thyroid tissue, but showing no difference from each other. With respect to VEGFA, there was no difference in gene or protein expression among the three groups. The final objective was to determine the plasma concentration of VEGFA and osteopontin in dogs diagnosed with FTC compared to clinically healthy dogs using a commercially available canine-specific ELISA. In this case, both VEGFA and osteopontin had higher plasma concentrations in dogs with FTC compared to healthy dogs. A small number of FTC cases were also measured two weeks after surgical removal of the tumor. Some cases showed a post-surgical decrease in VEGFA and osteopontin while others either remained the same or increased; however, the sample size for this comparison was small. The consistent expression of osteopontin in both tissues and blood suggest that it is a promising marker for identification of canine FTC. As in human studies of osteopontin in aggressive carcinomas, it is also possible to investigate it as a means of monitoring response to therapy, recurrence, and clinical outcome.
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    Hydrodynamics of strongly coupled non-conformal fluids from gauge/gravity duality.
    (2009-08) Springer, Greggory Todd
    The subject of relativistic hydrodynamics is explored using the tools of gauge/gravity duality. A brief literature review of AdS/CFT and gauge/gravity duality is presented first. This is followed by a pedagogical introduction to the use of these methods in determining hydrodynamic dispersion relations, w(q), of perturbations in a strongly coupled fluid. Shear and sound mode perturbations are examined in a special class of gravity duals: those where the matter supporting the metric is scalar in nature. Analytical solutions (to order q4 and q3 respectively) for the shear and sound mode dispersion relations are presented for a subset of these backgrounds. The work presented here is based on previous publications by the same author [1], [2], and [3], though some previously unpublished results are also included. In particular, the subleading term in the shear mode dispersion relation is analyzed using the AdS/CFT correspondence without any reference to the black hole membrane paradigm.
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    Investigation of Chlorinated Silicon Nanoparticles and In situ Analysis of the Size Distribution of Plasma Produced Particles
    (2017-10) Johnson, Jasmine
    Chlorine terminated silicon nanoparticles are produced from a non equilibrium plasma setup. The reactive chlorinated surface is exploited to functionalize the particles using Grignard chemistry and to stabilize the particles in solvent. By functionalizing the particles using Grignard chemistry, the particles form an optically clear solution in diethyl ether that is stable for four months. Chlorine surface coverage was successfully modulated by changing the precursor flowrate during synthesis. The solubility of as produced particles in methyl ethyl ketone was found to be related to the amount of precursor used during synthesis. A low pressure differential mobility analyzer (LPDMA) was installed downstream of a silicon nanocrystal plasma reactor in order to take in situ measurements of particle size distributions. Roughly equal amounts of positive and negative particles are observed. Size distributions of particles measured by the LPDMA are found to be much broader than those observed in TEM. This broadening is believed to be due to in flight coagulation. Inserting a mesh downstream of the plasma reduced the broadness of the distributions.
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    Mechanisms for Nanoparticle Synthesis and Charging in Nonthermal Plasmas
    (2016-08) Le Picard, Romain
    Nanoparticle formation, charging, and transport in plasmas have been extensively studied due to increasing interest. In the semiconductor industry, dust particles are considered as defects and are therefore unwanted as they can damage electronic devices during plasma etching or chemical vapor deposition. Potential applications are emerging, including biomedicine or photovoltaics, and require unique particle size and material properties. With the advancement of new technologies, along with a better understanding of particle formation, it is possible to experimentally tailor particle properties as small as 1 nm in diameter. The aim of this thesis is to contribute to the understanding of the mechanisms underlying the formation of nanoparticles in plasmas. A two-dimensional model is developed to self-consistently examine nanoparticle formation, growth, charging, and transport in low-pressure, capacitively-coupled RF flowing plasmas. The experimental set-up modeled is a narrow quartz tube in which a gas mixture of argon-helium-silane flows. The silane dissociation is mostly produced by electron impact due to highly energetic electrons. The nanoparticle cloud is coupled to the plasma. The spatial evolution of the particle size distribution and charge distribution is presented. We show that nanoparticles are mostly negatively charged and pushed along the centerline of the discharge due the ambipolar electric field. However, particles are not trapped in the axial direction, which allows nanoparticles to grow as they flow through the tube. The model predicts the possibility of producing crystalline nanoparticles due to exothermic reactions (e.g., electron-ion recombination and hydrogen reactions) on nanoparticle surfaces. The charging of nanoparticles in plasmas plays a significant role in their growth mechanisms and transport. In a typical parallel plate plasma system, nanoparticles get negatively charged due to collisions with electrons and are trapped at the center of the discharge due to the ambipolar electric field. At small nanoparticle sizes (< 10 nm), the number of electrons that can coexist on a single particle is limited, referred to as particle charge limit. We studied the effect of particle charge limits on charge distributions in low-pressure nonthermal plasmas, by developing an analytical expression for the charge distribution and comparing it with a stochastic charging model. Particle charging plays a significant role in particle transport since charged particles respond to the electric field. Under typical plasma-enhanced chemical vapor deposition conditions, a certain fraction of particles can be neutral or positive and escape the plasma and deposit on the wafer. To better understand and control particle deposition, we studied the effects of ion collisionality with the background neutral gas, electron emission processes, electronegativity, and charge limit on charge distributions. Tailoring particle size and flux to a substrate is possible while using a pulsed plasma. Because particles are mostly negatively charged, they can be accelerated to the substrate when applying a positive DC bias when the plasma is OFF. Silicon particle formation, growth, and transport are discussed for the case of a parallel-plate capacitively-coupled RF pulsed plasma in hydrogen-silane. The transport of such particles in the afterglow is discussed, based on preliminary work exploring system behavior during the first cycle of such a pulsed plasma.
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    Nonthemal plasma synthesis of indium phosphide nanocrystals and electrical properties of doped silicon nanocrystal films.
    (2010-02) Gresback, Ryan Gerard
    This thesis is concerning the plasma synthesis of semiconductor nanocrystals (NCs). Two systems of nanocrystals were studied, indium phosphide and doped silicon. A new method of synthesis of InP NCs is presented. It represents a new route for the synthesis of high quality compound semiconductor NCs. Additionally the electronic properties of doped silicon NCs were studied as a function of the doping concentration. Indium phosphide nanocrystals (InP NCs) were synthesized using a nonthermal plasma. The NCs were synthesized using a simple capacitively coupled plasma where the precursors are flowed through a 3/8” quartz tube with two outer ring electrodes. The size of the NCs was primarily controlled through the residence time of the NCs in the plasma. Residence times of 2-10 ms lead to particles with mean sizes between ~2.5-4 nm with size distributions less than 25% of the mean particle size. The mass yield for this system was found to be up to 40 mg/hr. When a ZnS shell was grown around the InP NCs, size-tunable emission from the blue-green to the red was observed. Quantum yields as high as 15% were observed with this synthesis route. This route allows for synthesis of free-standing NCs that can be easily manipulated with colloidal based techniques or included in devices without stabilizing ligands. The electrical conductivity of phosphorus doped Si NCs was studied as a function of the doping concentration. Doped Si NCs with mean sizes of 8-13 nm were spun cast onto a substrate with pre-deposited aluminum electrodes. The spin cast process produces films with zero to several monolayers of NCs. The conductivity of the films varies continuously from 10-11 S/cm for intrinsic NCs to 10-1 S/cm for highly doped NCs. These results indicate that the dopants are electrically active. The interpretation of these results means that the electronic properties of NCs can be tuned in a similar fashion as bulk semiconductors by introducing dopants. The ability to successfully dope NCs can have broad impact on the ability to form semiconductor devices.
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    Nonthermal Plasma Synthesis and Plasmonic Properties of Doped Silicon and Titanium Nitride Nanocrystals
    (2017-08) Schramke, Katelyn
    This work examines the nonthermal plasma synthesis of phosphorus and boron doped silicon and titanium nitride nanocrystals. The localized surface plasmon resonance (LSPR) of these materials was investigated. Titanium nitride has a plasmon resonance in the visible and near infrared, like gold nanorods, making it a viable alternative to gold for biological applications like photothermal therapy treatments. Titanium nitride is less expensive and more temperature stable than gold as well as biocompatible. The nonthermal plasma synthesis route is a continuous production method that eliminates the need for long processing and morphology modification steps required for gold nanorod production. The plasmon resonance, composition, and particle uniformity of the titanium nitride was found to be very dependent on the flow rate of ammonia during synthesis. Doped silicon has a plasmon resonance further into the infrared region and a tunable absorption controlled by the substitutional doping concentration in the nanocrystals. The presence of the plasmon absorption was used as a diagnostic tool to understand dopant behavior in doped silicon. The plasmon behavior supports the hypothesis of a uniform doping profile for phosphorus dopants and surface doping profile for boron dopants in silicon nanocrystals.
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    Penetration of Ar and He RF-driven plasma jets into micrometer sized capillary tubes
    (2018-07) Brahme, Amita
    The penetration and propagation of cold atmospheric pressure plasmas into volumes having sub-millimeter to micrometer sizes with large aspect ratios is required for enabling an effective disinfection of the inside of catheter tubes, tooth cavities, skin pores and enhance plasma catalysis in porous catalysts. As filamentary plasmas have often a characteristic diameter on the same length scale as tubes or pores, the penetration of plasma in these tubes and pores is not a priori obvious and can have a huge effect on the plasma properties. Particularly for medical applications, especially on teeth and skin, it is important that the plasma operates at low voltages and near ambient gas temperatures. This study has a goal to complement existing research in this area that has mainly been focused on pulsed discharges with significant overvoltage. We report on a study using RF driven argon and helium plasma jets with the plasma generation outside the capillary followed by its penetration and propagation inside the capillary. We present the experimental determination of the limitations on the penetration diameter, and the underpinning mechanisms of the plasma propagation and penetration process. Experimental results include time resolved imaging of plasma propagation and penetration in capillaries with different internal diameter and report surface electric field measurements. We found that the time between the plasma jet in first contact with the capillary tube surface and the subsequent penetration into the capillary tube spans several RF cycles due to electric fields at the plasma-tube interface below 4 kV/cm. These low electric fields require Penning ionization and/or stepwise ionization and hence a build up of the metastable and electron density to achieve a locally sufficiently large ionization rate to enable penetration and propagation. Furthermore, it is found that the propagation of the argon jet into the capillary occurs during the positive half cycle of the RF waveform and is very similar to the propagation of the jet in surrounding air.
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    Structural and surface correlations to the optical properties of nonthermal plasma-produced silicon nanoparticles
    (2011-06) Anthony, Rebecca Joy
    Nanomaterials have diverse capabilities to enable new technology and to deepen our understanding of our world, providing exciting prospects for scientists and the public alike in a vast span of uses. In the past decade, however, the potential held by nanotechnology has been reframed in the context of helping to slow global climate change and to alter the ways in which we use our energy to reflect more efficient technology and renewable energy sources. Silicon is a standout material in this new framework: as a nanomaterial, silicon can emit light when exposed to an applied voltage or ultraviolet optical excitation source. Silicon nanocrystals also exhibit size-dependent light emission, due to quantum confinement. This thesis is an exploration of the synthesis and processing parameters that affect the optical performance of silicon nanocrystals produced in a nonthermal plasma reactor. The efficiency of this light emission is sensitive to both synthesis environment and post-synthesis treatment. The work presented here is an attempt to deepen our understanding of the effects of different reactor and treatment parameters on the light emission efficiency from silicon nanoparticles, such that the luminescence behavior of the nanoparticles can be specifically engineered. Being able to fine-tune the structure, surface, and optical characteristics of the silicon nanocrystals is key in maximizing their use in luminescence applications. For all of the experiments described here, a nonthermal plasma flow-through reactor has been used to create the silicon nanoparticles. Silane gas is dissociated in the plasma and fragments come together to form silicon clusters, then grow to create nanoparticles. The nanoparticles were collected from the reactor for further processing, characterization, and experiments. The first discovery in this project was that by adjusting the power to the plasma reactor, the crystallinity of the silicon particles can be tuned: low power results in amorphous silicon nanoparticles, and high power yields crystalline nanoparticles. Even more important, the crystallinity of a nanoparticle ensemble relates directly to the photoluminescence (PL) efficiency, or quantum yield, from the ensemble: crystalline silicon nanoparticle samples, after alkyl functionalization, exhibit PL efficiencies of 40% or greater, while amorphous samples emit light with very poor efficiency (<2%). Additional studies of the plasma reactor revealed the importance of injecting a flow of hydrogen gas into the afterglow of the plasma, which turns out to have dramatic implications for the ultimate PL quantum yields of the nanocrystals. This injection scheme was systematically studied by varying the injected gas and its position. Hydrogen injected directly into the plasma afterglow was found to be vital for achieving high quantum-yield silicon nanocrystals, likely due to a reduction in surface trap states due to additional hydrogen passivation at the nanocrystal surface.Further investigations into the nanocrystal surface and how it relates to PL quantum yield showed that the photoluminescence from silicon nanocrystals is not only dependent on synthesis parameters, but also on processing temperatures and procedures following synthesis. While the highest PL efficiencies are found for silicon nanocrystals capped with alkyl chains, the PL efficiency of a nanocrystal ensemble can also be improved simply by heating the sample to temperatures between 150-200° C. This heating step also leads to a change in the hydride structure at the nanocrystal surface, which appears to be brought about by the effusion of silyl (or disilane) groups. Finally, details of the construction of a silicon-nanocrystal-based LED will be discussed. The LED project is part of a collaboration, and while the majority of device-specific aspects of the project were carried out in the lab of Professor R. Holmes by his Ph.D. student Kai-Yuan Cheng, the processing and alterations made to the nanocrystals used in the LED were all the responsibility of the author. The details of the project and a summary of the results bear discussion here in this thesis, as well as outlining of a novel scheme for deposition of SiNCs for device construction.
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    Studies of Submicrometer Particle Charging, Trapping, and Agglomeration in Steady and Transient Plasmas
    (2022-02) Ono, Toshisato
    The particle dynamics in both steady-state and transient plasmas are of interest for applications in particle synthesis in plasmas, and in mitigation of contamination issues in semiconductor processing. Semiconductor processing plasmas need to be operated in an intermediate pressure regime (5-10 Torr) to achieve a higher manufacturing yield, leading to further particle defect issues by the formation or agglomeration of particles. Also, the presence of particles locally perturbs plasmas which lead to chamber arcing issues. This dissertation covers both experimental and theoretical studies of particle behavior in collisional sheaths in both steady-state and transient plasmas, as well as studies of thin-film synthesis by PECVD. The end results of the studies described in each chapter provide (1) a distinct modeling approach to quantify ion attachment and momentum transfer from ions to particles in a collisional sheath, (2) evidence of material-dependent particle trapping locations in steady-state plasmas, (3) the first direct observation of particle agglomeration facilitated by particle charging and decharging in pulsed plasmas, and (4) experimental and theoretical description of the link between plasma properties and synthesized thin-film properties by PECVD. The dimensionless ion attachment coefficients and dimensionless collection forces on negatively charged particles are calculated using ion trajectory models accounting for an external electric field in a collisional sheath, ion inertia, and finite ion mobility. By considering both ion inertia and finite ion mobility, results apply for ion transport from the fully-collisional regime into a regime of intermediate collisionality. We show that ion motion about a charged particle can be parameterized by the ion Stokes number and dimensionless electric field strength. Increasing the ion Stokes number is found to significantly decrease the dimensionless ion attachment coefficients and ion collection forces. We find the charge level is strongly sensitive to both field strength and pressure in the plasma sheath. Calculations are also used to demonstrate that the ion collection force can be sufficiently strong to trap particles close to both the top and bottom electrodes. Using laser light scattering (LLS), we examined the trapping of submicrometer metal oxide particles of 6 distinct material compositions in the sheath boundary of a CCP plasma in the intermediate pressure range. We find the trapping location shifts slightly farther from the top electrode with increasing material dielectric constant. This suggests that the ion drag force is influenced by particle material properties. Measured trapping locations are also compared to model predictions where particle charge levels and the ion drag force are calculated using expressions based on the ion trajectory calculations. Predicted ion densities required for trapping are a factor of 6-16 higher than calculated at the observed particle trapping locations. In total, our results confirm that submicrometer particle trapping occurs at the upper electrode of CCP reactors. Dispersed submicrometer particles are exposed to drastically varying ion and electron densities during both ignition and extinction of non-thermal plasma. RF pulsing enables examination of particle behavior in a transient afterglow and during plasma reignition. By progressively decreasing the pulsing frequency, agglomerates can be grown to highly non-spherical and millimeter scale agglomerates. We find that the pulsing frequencies which lead to agglomerated silica particles do not lead to equally large agglomerates with barium titanate particles, suggesting discharging rates are material dependent. Pulsed plasma systems can serve to control the extent of agglomeration in particulate systems, creating agglomerates multiple orders of magnitude large in length scale than the base primary particles, and this agglomeration is material dependent.
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    Understanding and Improving Plasma Synthesized Silicon Germanium Films for Thermoelectric Applications
    (2017-10) Mork, Kelsey C
    Laser sintering and Phenyl Acetylene surface functionalization of plasma synthesized silicon germanium nanoparticle films and powders were studied to improve electrical conductivities of these materials. Laser sintering greatly improved the electrical conductivity in thin films with peak values of 70.42 S/cm. Phenyl Acetylene functionalization was successful in both doped and undoped silicon germanium nanoparticles. Further studies should be performed to quantify the electrical conductivity values in bulk, compacted pellets of the functionalized nanoparticles. Finally, research into activation energies of compacted silicon germanium films showed drastic differences between energies obtained when using Seebeck coefficient and energies obtained when using electrical conductivity. This is attributed to traps on the surface of the nanoparticles and the potential barrier between nanoparticles.