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Research was performed on the development of relationships between the microstructure of nickel and yttria-stabilized zirconia (YSZ) coatings and the processing parameters used for their deposition by atmospheric plasma spraying (APS). Research was also performed on the development of relationships between the microstructure of plasma sprayed Ni-YSZ coatings and the electrochemical performance of metal-supported solid oxide fuel cells (SOFCs) incorporating these coatings as anodes. Three APS processes were used to deposit Ni-YSZ coatings: dry-powder plasma spraying (DPPS), suspension plasma spraying (SPS), and solution precursor plasma spraying (SPPS). These processes differ in the form of the feedstock injected into the plasma. The composition of the Ni-YSZ coatings deposited with each spray process could be controlled through adjustment of the plasma gas composition and stand-off distance, as well as adjustment of feedstock properties including agglomerate size fraction for DPPS, NiO particle size and suspension feed rate in SPS, and the enthalpy of decomposition of the precursors used in SPPS. The porosity of the Ni-YSZ coatings could be controlled through the addition of a sacrificial pore forming material to each feedstock, with coating porosities up to approximately 35% being achieved for each coating type. Metal-supported SOFCs were fabricated to each have anodes deposited with a different plasma spray process, where all anodes had nominally identical composition. The microstructures obtained for each anode type were distinctly different. SPPS led to the most uniform mixing of the smallest Ni and YSZ particles. These anodes most resembled typical structures from anodes fabricated using conventional methods. It was found that the polarization resistance, Rp, associated with the high frequency (> 1 kHz) range of the impedance spectrum correlated to the three phase boundary length (TPBL) density of each anode, with lower Rp values corresponding to higher TPBL densities. It was also found that the Knudsen diffusion coefficient and effective ordinary diffusion coefficient of the porous anodes correlated with the Rp associated with the low frequency (< 1 kHz) range of the impedance spectrum. Therefore, the impedance spectrum can be used to compare microstructural differences among plasma sprayed Ni-YSZ anodes.

Research was performed on the development of relationships between the microstructure of nickel and yttria-stabilized zirconia (YSZ) coatings and the processing parameters used for their deposition by atmospheric plasma spraying (APS). Research was also performed on the development of relationships between the microstructure of plasma sprayed Ni-YSZ coatings and the electrochemical performance of metal-supported solid oxide fuel cells (SOFCs) incorporating these coatings as anodes. Three APS processes were used to deposit Ni-YSZ coatings: dry-powder plasma spraying (DPPS), suspension plasma spraying (SPS), and solution precursor plasma spraying (SPPS). These processes differ in the form of the feedstock injected into the plasma. The composition of the Ni-YSZ coatings deposited with each spray process could be controlled through adjustment of the plasma gas composition and stand-off distance, as well as adjustment of feedstock properties including agglomerate size fraction for DPPS, NiO particle size and suspension feed rate in SPS, and the enthalpy of decomposition of the precursors used in SPPS. The porosity of the Ni-YSZ coatings could be controlled through the addition of a sacrificial pore forming material to each feedstock, with coating porosities up to approximately 35% being achieved for each coating type. Metal-supported SOFCs were fabricated to each have anodes deposited with a different plasma spray process, where all anodes had nominally identical composition. The microstructures obtained for each anode type were distinctly different. SPPS led to the most uniform mixing of the smallest Ni and YSZ particles. These anodes most resembled typical structures from anodes fabricated using conventional methods. It was found that the polarization resistance, Rp, associated with the high frequency (> 1 kHz) range of the impedance spectrum correlated to the three phase boundary length (TPBL) density of each anode, with lower Rp values corresponding to higher TPBL densities. It was also found that the Knudsen diffusion coefficient and effective ordinary diffusion coefficient of the porous anodes correlated with the Rp associated with the low frequency (< 1 kHz) range of the impedance spectrum. Therefore, the impedance spectrum can be used to compare microstructural differences among plasma sprayed Ni-YSZ anodes.

In this thesis, we report the various aspects of fabricating a Cold Atmospheric Plasma system, which can be used for Plasma Enhanced Chemical Vapour Deposition. The greatest advantage of this system is its vacuum free operation, which provides a cost e effctive alternative over conventional high vacuum systems. We have designed a reactor geometry for such a plasma system, in which, the contamination due to ambient air is kept at a minimum value using a low flow of Ar (500 sccm). Towards this end, we have modeled and simulated the flow pattern of Ar gas entering the reactor geometry and have studied its e effectiveness in removing air from the plasma zone. We have fabricated such a geometry and studied the contamination at different flow rates of Ar by observing the plasma optical emission. Further, the aspect of lamentation in atmospheric pressure plasma has been studied and we have identified a few process parameters which can convert a filamentary discharge to a diffused glow. Subsequently, a complete system was developed, including an in-house built high voltage power supply, to generate a plasma with low contamination and less number of laments. We have also carried out plasma diagnostics, specifically to estimate the Electron Energy Distribution Function (EEDF) of the plasma, by analysing the radiation emitted from an Ar plasma, acquired using an Optical Emission Spectroscope. The peaks in the spectrum were curve flatted with Voigt pro les and their widths and intensities were mapped to the electron number density and the EEDF of the plasma, using the mathematical models for Stark broadening and Corona population respectively. An optimization routine based on Nelder-Mead simplex algorithm was run to estimate the optimal values of these plasma parameters that produced a good match between the simulated spectrum and the experimentally acquired one. This analysis estimated that the value of electron number density in our plasma was in the range 0:82 1017 cm 3 to 3:56 1017 cm 3 and the electron temperature was in the range 0.36 eV -0.39 eV . It also predicted that the EEDF closely approximated a Maxwellian distribution. As a proof of concept, the fabricated reactor was used to deposit thin films of Polyacetylene over microscopic cover glass slides by polymerizing Acetylene gas in the cold plasma. Deposition rates as high as 1 m=min, were obtained during thin lm deposition of the polymer. The polymeric structure of the lm was studied using NMR and FT-IR. XPS measurement revealed 5% O2 inclusion in the samples. XRD showed no distinguishable peak, indicating the amorphous state of the films. The surface morphology investigated using SEM revealed highly porous broid kind of structures, which appeared to be agglomeration of particles with sizes in the order of few micrometers. P-type Polyacetylene lms were fabricated by doping them with 5.3% by atomic concentration of I2 vapours. The UV-Visible spectroscopy study revealed a bandgap of 2.05 eV for undoped and 1.49 eV for the doped Polyacetylene samples. The lms exhibited an increase in conductivity by two orders of magnitude; from 3:6 10 13 1cm 1 to 3:5 10 11 1cm 1 for un-doped and doped Polyacetylene samples respectively.

Experimental studies into the use of cold atmospheric plasmas for inactivating foodborne pathogens are presented in this thesis. Eliminating the possibility that treatment delivered by a plasma to a population or assemblage of micro-organisms is unevenly distributed is an essential pre-requisite to attempting to interpret inactivation kinetics with a view to elucidating mechanisms of inactivation. A filtration method of depositing cells evenly on the surface of a membrane without cell stacking was developed and used throughout the work described here. Two atmospheric plasma systems were evaluated and each brought about microbial inactivation in a distinct way. A pulsed radio frequency plasma jet operated at 3.47 MHz caused gross morphological changes to L. innocua whereas a low frequency air mesh plasma system operated at a frequency of 24 kHz led to the inactivation of these bacteria without inducing observable structural changes. Changing the operating parameters of the plasma jet system had a significant effect on the composition of the reactive plasma species generated as revealed by changes to the mode of inactivation of bacteria. In addition to inactivating bacteria, the pulsed plasma jet was shown to be highly effective in degrading and removing amyloid aggregates from the surface of mica coupons. Amyloids have widely been used as a non-infectious model for prions, and the results obtained here show potential for the application of gas plasma technology for removing prions from abiotic surfaces in medical and other applications. It has widely been assumed that bacterial envelopes are the principal sites at which reactive plasma species bring about damage to cells. However, changing the composition of the bacterial membranes of E. coli and Listeria innocua by cultivating them at widely different temperatures to induce changes proved not to result in enhanced inactivation. Flow cytometry was also used to provide additional insights into possible mechanisms of inactivation. The following fluorescent dyes were used either singly or in combination; SYTO 13, DiBAC4(3), cFDA and PI. The results obtained with the dyes DiBAC4(3) and PI showed that Gram positive bacteria became depolarised prior to the bacterial membrane becoming compromised, possibly suggesting that the inactivating plasma species are affecting membrane proteins responsible for maintaining the bacterial charge. Differences between the fluorescent dye staining of Gram negative and Gram positive species were obtained using SYTO13 and PI demonstrating that the different membrane structures affect their interaction with the plasma. In additional studies, the air mesh plasma was used to treat multi-drug resistant strains of Methicillin resistant Staphylococcus aureus (MRSA) in an attempt to reverse antibiotic resistance. MRSA PM 64 was shown to reverse its antibiotic resistance to Oxacillin, Kanamycin and Trimethoprim. Culturing the bacteria in a nutrient limited media led to increased resistance towards plasma treatment and maintenance of their high levels of antibiotic resistance.

A la suite des propositions de la commission européenne, visant à concrétiser les objectifs de réduction des émissions de dioxyde de carbone (CO2) des voitures. Pour atteindre cet objectif, les constructeurs automobiles doivent réduire le poids de la voiture. Ainsi l'équipementier FAURECIA, fabricant de sièges de voiture désire remplacer les structures métalliques par des structures hybrides plastique-métal (PMH). Et en plus, il désire également utiliser un matériau composite en remplacement du métal pour diminuer le poids et utiliser la technique du plasma atmosphérique pour améliore le force d'adhérence à l'interface de pièce structure hybride.C'est dans ce contexte que nous avons étudié des effets de plasma traitement sur l'adhérence dans la structure hybride pour proposer la meilleure condition de traitement. L'objectif de ce travail était de caractériser l'effet de plasma traitement par détermination des paramètres dans le processus de traitement telle que la vitesse de balayage, la distance entre le substrat et la torche plasma et le nombre de passages de la torche, puis de prédire l'initiation du délaminage sous chargement quasi-statiques dans l'éprouvette de simple recouvrement par l'utilisation le critère de la rupture. Un autre objectif était d'étudier les modèle micromécanique pour évaluer la fiabilité de leur. Et ils seront appliqués pour prévoir les comportements mécaniques de matériau thermoplastique renforcé par fibre de verre court. Afin d'atteindre les objectifs présenté, les plusieurs essais telles que l'essai de traction, l'essai simple recouvrement et l'essai de l'ARCAN-Mines sont été réalisé. En parallèle, les techniques de l'émission acoustique (EA), du rayonnement infrarouge (RI) et de la microscopie optique ont été utilisées pour suivre les mécanismes de la rupture de l'éprouvette étudiée. En enfin, la méthode des éléments finis a été utilisé pour simuler les essais et pour permettre de vérifier la fiabilité du critère de rupture. / Following the proposals of the European Commission, to achieve the goals of emission reduction of carbon dioxide (CO2) from cars. To achieve this objective, automakers must reduce the weight of the car. Thus the supplier Faurecia, manufacturer of car seats desires to replace metal structures by structure plastic-metal hybrid (PMH). And they desire also to use a composite material to replace metal in order to reduce weight. Moreover in order to improve the adhesion strength at the interface piece hybrid structure,Atmospheric plasma technique was used.In this context, we studied the effects of plasma treatment on term of adhesion in the hybrid structure in order to provide the best condition of treatment. The objective of this study was to characterize the effect of plasma treatment by determination at the parameters in the process such as the scanning speed, the distance between the substrate and the plasma torch and the number of passes of the torch, then to predict the initiation of delamination under quasi-static loading test in specimen of single lap shear by using the criterion of rupture. The other objective was to study the micromechanical model to assess the reliability of them. And they will be applied to predict the mechanical behavior of Short Glass Fiber reinforced thermoplastic. In order to achieve the objectives presented, the several tests such as tensile test, single lap shear test and ARCAN-Mines test have been executed. In parallel, techniques acoustic emission (AE), infrared radiation (IR) and optical microscopy were used in order to follow the failure mechanisms of the specimen studied. Finally, the finite element method was used to simulate the tests and allow to verify the reliability of the failure criterion.

The dissertation is split into two parts. The first part will be focused on changes in material properties found at the nanoscale, as miscibility and electronic structure can change significantly with size. The formation of classically-immiscible bimetallic nanoparticles (BNPs) becomes favorable at the nanoscale and novel catalytic properties can emerge from the bimetallic alloying. The formation of alloyed and non-alloyed BNPs is achieved through pulse laser ablation (PLA) and a significant increase in catalytic activity is observed for both. Recently discovered, the increased activity in the non-alloyed BNPs, deemed multicomponent photocatalysis, is examined and the proposed mechanism discussed. The second part of the talk will focus on thermal barrier coatings (TBCs), which are advanced, multi-layered coatings used to protect materials in high temperature environments. MCrAlY (M=Ni, Co) bond coats deposited via atmospheric plasma spray (APS) are intrinsically rough and initially the roughness provides a high surface area platform for the mechanical interlocking of the yttria stabilized zirconia (YSZ) top coat, which provides the bulk of the thermal insulation. After high temperature exposure, a protective oxide scale forms at the top coat/bond coat interface however the convex asperities of the bond coat can grow non-α-Al2O3 type oxides that can be detrimental for coating lifetime. A surface modification technique that removes the asperities while leaving intact the concavities is used to examine the role that roughness distribution has on 1100°C APS coating lifetime. Lastly, recent work validating a modelling strategy for evaluating 900°C TBC lifetimes, which can typically surpass 25 kh, is presented. Differences in coating-substrate interdiffusion behavior over 5-20 kh of 900°C exposure are discussed and reproduced with Thermo- Calc/DICTRA for three superalloys (1483, 247, X4) deposited with high velocity oxy fuel (HVOF) NiCoCrAlY coatings.

Le dioxyde de carbone, principal gaz à effet de serre lié aux activités humaines, est considéré comme l’un des gaz les plus problématiques pour notre environnement ces dernières années, principalement à cause du réchauffement climatique qu’il engendre. C’est pour cette raison que l’augmentation de sa teneur dans l’atmosphère nous concerne tous quant aux conséquences futures pour notre planète. Afin de limiter l’émission de CO2, sa conversion en composés à valeur ajoutée présente un grand intérêt et est possible notamment via des procédés plasmas. Plus particulièrement, les décharges à barrière diélectrique (DBD) sont utilisées depuis quelques années pour générer des plasmas froids opérant à pression atmosphérique, principalement pour des applications en traitement de surface, mais également pour le traitement d’effluents gazeux.Lors de cette thèse, nous nous sommes focalisés sur le processus de dissociation du CO2 en CO et O2 via un réacteur DBD à flux continu et avons analysé sa conversion et son efficacité énergétique via différentes études. Celles-ci ont été réalisées grâce à plusieurs méthodes de diagnostic, comme par exemple la spectrométrie de masse utilisée pour déterminer la conversion et l’efficacité du processus, la spectroscopie d’émission optique, l’oscilloscope pour une caractérisation électrique, etc. afin d’avoir une meilleure compréhension du comportement des décharges CO2.Dans un premier temps, nous avons réalisé une étude détaillée d’un plasma CO2 pur où nous avons fait varier différents paramètres, tels que le temps de résidence, la fréquence, la puissance, la pulsation de la haute tension et l’épaisseur et la nature du diélectrique. Le CO2 donne lieu généralement à une décharge filamentaire, consistant en de nombreuses microdécharges réparties au niveau de la zone du plasma. Celles-ci constituent la principale source de réactivité dans une DBD. Un aperçu détaillé de l’aspect physique de ces microdécharges a été réalisé grâce à la caractérisation électrique, permettant de mieux comprendre les propriétés électriques de la décharge et des microdécharges. En effet, nous avons pu déterminer l’importance de la tension présente au niveau du plasma, de l’intensité du courant plasma, du nombre de microdécharges et de leur temps de vie sur l’efficacité du processus de dissociation de CO2.Ensuite, nous avons conclu ce travail avec des études combinant le CO2 en phase plasma avec de l’eau ou du méthane afin de produire des molécules à valeur ajoutée telles que les syngas (CO et H2), mais aussi des hydrocarbures (C2H6, C2H4, C2H2 et CH2O) dans le cas de l’ajout du méthane. A travers ces études, nous avons obtenu une meilleure connaissance de la chimie et de la physique qui ont lieu dans ce type de plasma. / Carbon dioxide appears as one of the most problematic gases for the environment, mostly because it is responsible for global warming. This is why its increasing concentration into the atmosphere, mainly due to anthropogenic activities, is a real concern for planet Earth. In order to prevent the release of large amounts of CO2, its conversion into value-added products is of great interest. In this context, plasma-based treatments using dielectric barrier discharges (DBDs) are nowadays more and more used for the conversion of this gas. In this thesis, we investigated the CO2 splitting process into CO and O2 via a flowing cylindrical DBD and we studied its conversion and energy efficiency by means of several diagnostic methods, such as mass spectrometry to determine the conversion and energy efficiency of the process, optical emission spectroscopy for gas temperature measurements, and an oscilloscope for electrical characterization, in order to obtain a better understanding of the CO2 discharge itself.First, we focused on an extensive experimental study of a pure CO2 plasma where different parameters were varied, such as the gas residence time, the operating frequency, the applied power, the pulsation of the AC signal, the thickness and the nature of the dielectric. CO2 discharges typically exhibit a filamentary behavior, consisting of many microdischarges, which act as the main source of reactivity in a DBD. A detailed insight in the physical aspects was achieved by means of an in-depth electrical characterization, allowing more insight in the electrical properties of the discharge and more specifically in the microdischarges, which are spread out throughout the active zone of the plasma. It was found throughout this work that the plasma voltage, which reflects the electric field and thus determines how the charged particles are accelerated, the plasma current, which reflects the electron density, but also the number of microdischarges and their average lifetime, play an important role in the efficiency of the CO2 dissociation process. It was revealed that the microdischarge number is important as it represents the repartition of the locations of reactivity. Indeed, as the microfilaments are more spread out in the same discharge volume, the probability for the CO2 molecules to pass through the reactor and interact with at least one microdischarge filament becomes more important at a larger number of microfilaments.The second part of the thesis was dedicated to discharges combining CO2 and H2O or CH4, both being hydrogen source molecules. The combined CO2/H2O or CO2/CH4 conversion allows forming value-added products like syngas (CO and H2), but also hydrocarbons (C2H6, C2H4, C2H2 and CH2O), at least in the presence of methane. Throughout this study, we tried to obtain a better knowledge of the chemistry and physic behind these conversion processes. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Silicon-based ceramics and ceramic matrix composites (CMCs), such as silicon carbide (SiC) fiber reinforced SiC, are promising candidates for hot section components in next generation turbine engines. Environmental barrier coatings (EBCs) are essential for implementing these components as they insulate and protect the substrate from reaction with water vapor in the engine environment. EBCs are typically deposited via atmospheric plasma spraying (APS) and preparing the component surfaces through cleaning and roughening prior to coating is a vital step to ensure sufficient coating adhesion. The adhesion of a plasma sprayed coating to the underlying component is one of the most important properties as the component will not be protected if the coating is not well adhered. Surface roughening of metallic components via grit blasting is well documented and understood, but much less is known about preparing ceramic and ceramic composite surfaces for thermal spray coating. Silicon coatings are often used as a bond coating between SiC-based components and EBC top layers, but the adhesion strength of plasma sprayed Si on these substrates, Si splat formation and the factors that affect coating formation and adhesion have not been well studied. The effects of automated grit blasting process parameters on surface roughness and material loss of a reaction bonded SiC (rb SiC) composite were evaluated. Surface roughness before and after grit blasting was evaluated with a confocal laser scanning microscope. The differences and advantages of automated grit blasting compared to manual grit blasting were observed. Most notably was the level of control at high nozzle traverse speeds resulting in reduction of material loss and consistency of roughening. At high nozzle traverse speeds, the amount of material loss decreased greatly with a small effect on induced surface roughness. The degree of grit blasting induced roughness and material loss was found to be largely dependent on the nature of the composite matrix and reinforcement, as well as blast nozzle traverse speed. A statistical model was developed to predict the substrate thickness loss and induced average roughness based on nozzle traverse speed and blast pressure for automated grit blasting. Additionally, laser ablation was used to create controlled, regularly patterned surface texture on rb SiC substrates to further investigate the role of texture parameters in Si coating adhesion. Si was plasma sprayed onto rb SiC substrates to deposit both thick coatings to evaluate adhesion strength and single splats to study splat formation. Surface roughness/texture, substrate preheat temperature and mean Si particle size were varied in plasma spray coating experiments to observe their role in coating adhesion strength. Si adhesion strength was found to be related to all three factors and a statistical model was developed to predict adhesion strength based on them. Substrate preheat temperature had a significant effect on both Si adhesion strength and Si splat formation on rb SiC. Single splat formation during plasma spraying of Si on SiC was simulated with software called SimDrop. Simulations of Si droplet impact, spreading and solidification during plasma spraying on smooth and textured SiC surfaces were used to investigate the effects of relevant process parameters on splat formation. Experimentally observed Si splats on smooth substrates at different temperatures during deposition were matched with simulated splats with the same spraying parameters. A change in thermal contact resistance with changing substrate preheat temperature was confirmed by the simulation results. The role of surface texture parameters for a regularly patterned surface texture in splat formation was demonstrated through simulation. This dissertation investigates methods of roughening and preparing a SiC composite substrate for plasma spray coating, as well as factors which affect the adhesion strength and splat formation of plasma sprayed Si through experiments and simulation. The observations made provide valuable insight for understanding and optimizing the manufacturing processes utilized to deposit strongly adhered coatings onto SiC-based composites. In addition, areas of interest in this field for future study and further investigation are introduced and suggested. / Doctor of Philosophy / Silicon-based ceramics and ceramic matrix composites (CMCs), such as silicon carbide (SiC) fiber reinforced SiC, are promising candidates for hot section components in next generation turbine engines. Environmental barrier coatings (EBCs) are essential for implementing these components as they insulate and protect the substrate from reaction with water vapor in the engine environment. EBCs are typically deposited via atmospheric plasma spraying (APS) and preparing the component surfaces through cleaning and roughening prior to coating is a vital step to ensure sufficient coating adhesion. The adhesion of a plasma sprayed coating to the underlying component is one of the most important properties as the component will not be protected if the coating is not well adhered. Silicon coatings are often used as a bond coating between SiC-based components and EBC top layers, but the adhesion strength of plasma sprayed Si on these substrates, Si splat formation and the factors that affect coating formation and adhesion have not been well studied. This dissertation investigates methods of roughening and preparing a SiC composite substrate for plasma spray coating, as well as factors which affect the adhesion strength and splat formation of plasma sprayed Si through experiments and simulation. The observations made provide valuable insight for understanding and optimizing the manufacturing processes utilized to deposit strongly adhered coatings onto SiC-based composites. In addition, areas of interest in this field for future study and further investigation are introduced and suggested.

Optimization is a decision making process to solve problems in a number of fields including engineering mechanics. Bio-inspired optimization algorithms, including genetic algorithm (GA), have been studied for many years. There is a large literature on applying the GA to mechanics problems. However, disadvantages of the GA include the high computational cost and the inability to get the global optimal solution that can be found by using a honeybee-inspired optimization algorithm, called the New Nest-Site Selection (NeSS). We use the NeSS to find optimal parameters for three mechanics problems by following the three processes: screening, identifying relationships, and optimization. The screening process identifies significant parameters from a set of input parameters of interest. Then, relationships between the significant input parameters and responses are established. Finally, the optimization process searches for an optimal solution to achieve objectives of a problem. For the first two problems, we use the NeSS algorithm in conjunction with a third order shear and normal deformable plate theory (TSNDT), the finite element method (FEM), a one-step stress recovery scheme (SRS) and the Tsai-Wu failure criterion to find the stacking sequence of composite laminates and the topology and materials for doubly curved sandwich shells to maximize the first failure load. It is followed by the progressive failure analysis to determine the ultimate failure load. For the sandwich shell, we use the maximum transverse shear stress criterion for delineating failure of the core, and also study simultaneously maximizing the first failure load and minimizing the mass subject to certain constraints. For composite laminates, it is found that the first failure load for an optimally designed stacking sequence exceeds that for the typical [0°/90°]₅ laminate by about 36%. Moreover, the design for the optimal first failure load need not have the maximum ultimate load. For clamped laminates and sandwich shells, the ultimate load is about 50% higher than the first failure load. However, for simply supported edges the ultimate load is generally only about 10% higher than the first failure load. For the atmospheric spray process, we employ the NeSS algorithm to find optimal values of four process input parameters, namely the argon flow rate, the hydrogen flow rate, the powder feed rate and the current, that result in the desired mean particle temperature and the mean particle velocity when they reach the substrate. These optimal values give the desired mean particle temperature and the mean particle velocity within 5% of their target values. / Ph. D.