Low temperature plasma-catalytic NOx synthesis in a packed DBD reactor: effect of support materials and supported active metal oxides

The direct synthesis of NOx from N2 and O2 by non-thermal plasma at an atmospheric pressure and low temperature is presented, which is considered to be an attractive option for replacement of the Haber-Bosch process. In this study, the direct synthesis of NOx was studied by packing different catalyst support materials in a dielectric barrier discharge (DBD) reactor. The support materials and their particle sizes both had a significant effect on the concentration of NOx. This is attributed to different surface areas, relative dielectric constants and particles shapes. The nitrogen could be fixed at substantially lowered temperatures by employing non-thermal plasma-catalytic DBD reactor, which can be used as an alternative technology for low temperature synthesis. The γ-Al2O3 with smallest particles size of 250–160 μm, gave the highest concentration of NOx and the lowest specific energy consumption of all the tested materials and particle sizes. The NOx concentration of 5700 ppm was reached at the highest residence time of 0.4 s and an N2/O2 feed ratio of 1 was found to be the most optimum for NOx production. In order to intensify the NOx production in plasma, a series of metal oxide catalysts supported on γ-Al2O3 were tested in a packed DBD reactor. A 5% WO3/γ-Al2O3 catalyst increased the NOx concentration further by about 10% compared to γ-Al2O3, while oxidation catalysts such as Co3O4 and PbO provided a minor (∼5%) improvement. These data suggest that oxygen activation plays a minor role in plasma catalytic nitrogen fixation under the studied conditions with the main role ascribed to the generation of microdischarges on sharp edges of large-surface area plasma catalysts. However, when the loading of active metal oxides was increased to 10%, NO selectivity decreased, suggesting possibility of thermal oxidation of NO to NO2 through reaction with surface oxygen species

Plasma-surface interactions in all-metal-wall tokamaks

Gas seeding is often used in tokamaks to reduce the power load onto the divertor target plates. Nitrogen is the preferred seeding species because of its favourable radiative properties as well as its apparent beneficial effect on plasma confinement. However, nitrogen molecules are chemically reactive with hydrogen and its isotopes to form stable tritiated ammonia. The last studies and observations show that with a 5% conversion into ammonia, 0.2 g of associated tritium could be trapped per pulse in the future ITER D-T operation. The formation of large quantities of tritiated NH3 has consequences for several aspects of the ITER operation and maintenance. In particular, cryopump would need more frequent regeneration that would limit ITER operational cycle. Since NH3 is a polar molecule, it can be easily adsorbed on metallic surfaces and in particular on ITER first-wall material beryllium (Be), divertor material tungsten (W) and on the vacuum vessel and pipework made of stainless steel (SS). The in-vessel T inventory in ITER is limited to 1 kg for safety reasons and the formation and sticking of large quantities of tritiated ammonia could contribute to the overall inventory while the recovery of T from ND2T is still an open issue. This thesis seeks to develop a fundamental understanding of the ammonia formation process, to explore possible ways to decrease it and to investigate the formed ammonia interaction with materials inside the tokamak. This study will, therefore, address the following questions: 1- Where and how does the ammonia formation occur i.e: in the plasma and/or on the surface and how does it depend on the surface material? 2- What is the impact of specific parameters from the fusion reactor environment, in particular temperature and presence of other gases, on the ammonia formation and is there possible ways to decrease the formed quantity? 3- How much of the ammonia produced quantity can stick on fusion device relevant materials (tungsten, stainless steel, beryllium and boron) and is ammonia adsorbed as a molecule or does it undergo dissociative adsorption? According to these objectives, the three chapters following the theory chapter 1 dealing with the basics of nuclear fusion will be the following: Chapter 2 begins by laying out the theoretical dimensions of plasma-assisted catalysis and dominant reactions pathways for ammonia synthesis. The experimental part includes a detailed description of a newly built setup and the experimental procedure developed for the ammonia production study. The effect of the presence of tungsten and stainless steel surfaces on ammonia production for different nitrogen-hydrogen plasma composition will then be presented. The last part of this chapter gives fundamental information about the nature of the reactive processes occurring during RF plasma-assisted ammonia synthesis will be presented based on RGA results and surface chemistry analysis carried out using X-ray photoelectron spectroscopy (XPS). Chapter 3 is concerned with the effect of two parameters on the ammonia production from nitrogen-hydrogen plasma including the sample surface temperature and He or Ar addition. The results will be shown for temperature values ranging from room temperature (RT) to 1270 K, a value that the ITER tungsten divertor can reach in the active areas where the plasma impact occurs. On the other hand, the main drive of the noble gas admixture effect is to determine a process that reduces the ammonia production in the nuclear device without modifying the positive effects of nitrogen seeding on plasma performance. While He will be present as an intrinsic plasma impurity in ITER, Ar gas was identified as the best candidate for the simultaneous enhancement of core and divertor radiation in the case where elevated main chamber radiation is desired as well. In the chapter 4, ammonia sticking on different fusion-relevant materials will be presented. In particular, the interaction of NH3 molecules with W, SS and Be surfaces using QMB and XPS techniques will be shown. In addition to these materials used for the tokamak components, boron and gold surfaces will also be investigated as well. The former element is largely used in tokamaks to decrease the oxygen (O) content (by boronization) while the latter can be used as a reference for the QMB technique. NH3 adsorption/desorption study will be presented by examining the effect of both pressure and surface material on sticking along with an XPS study to analyze the residual NH3 molecules sticking on the surface. The last two chapters of this dissertation include further material related topics for the fusion research namely metallic first mirror plasma cleaning and tokamak wall reflection measurements and simulation. The reliability of optical diagnostic systems is a key element for successful ITER operations. Particular attention has to be given to first mirrors, involved in almost 40 diagnostics, and whose reflectivity might change due to erosion and deposition from the first wall. Mirror surface recovery techniques will be required and in situ plasma cleaning is considered as the most promising solution. Chapter 5 presents the findings on the removal of relevant ITER contaminants namely beryllium deposits with deuterium gas. The use of deuterium is of crucial interest for ITER and the fusion community as it possesses a unique set of advantage in regard to cleaning: (i) effective on beryllium, (ii) harmless for the mirror material and (iii) fully compatible with other ITER systems (Neutral Beam Injection, cryo-pumping . . . ). Mirrors with a Be deposited films, as well as mirrors exposed in JET-ILW heavily contaminated with beryllium, are employed in this study and two sputtering regimes, at low and high deuterium energy are studied. The performances of next step fusion facilities, such as ITER, will strongly depend on the ability to monitor and protect the vessel walls from excessive heat loads coming from the plasma power deposition. Infra-red (IR) thermography systems are commonly used in tokamaks to fulfill such requirements by providing thermal images of the plasma facing components (PFCs) under plasma exposure. However, with the introduction of all-metal walls in fusion devices, the significant contribution of reflected flux in the collected flux by the cameras will affect the interpretation of IR measurements, leading to inaccurate PFC temperature estimation. This could lead to excessive interruptions of the plasma shots and limitations on scenario development towards high performances. The development of a photonic simulation taking into account the contribution of the reflected flux in the collected flux for surface temperature evaluation is, therefore, essential to discriminate the parasitic light-reflection to other thermal events with a real risk for the machine protection. Furthermore, in order to get accurate results, this simulation will certainly have to be based on experimental data set for different PFC materials. Chapter 6 start by describing a photonic simulation carried out using the ray tracing software SPEOS taking into account the multiple inter-reflections of the ray in the vacuum vessel. In the following experimental section, a new redesigned Basel University Laboratory Goniometer (BULGO) will be presented with a description of the apparatus, the measurement and calibration procedure, and the assessment of the accuracy of the device. Directional and total emissivity are also deduced by indirect measurements. The results are presented for tungsten samples at different roughness. Tungsten is the material chosen for the most critical component in tokamak (divertor) and which is exposed to highest heat loads and for which the roughness can be changed during experimental campaign (erosion/deposition phenomenon). The relation between reflectance/emittance and roughness will also be discussed in this chapter. Experimental results are then used as input of photonic simulation and the resulting IR synthetic image is compared with the experimental image of WEST tokamak. In the conclusion, a brief summary of the findings and areas for further research will be presented

Plasma catalysis using low melting point metals.

Plasma catalysis is emerging as one of the most promising alternatives to carry out several reactions of great environmental importance, from the synthesis of nanomaterials to chemicals of great interest. However, the combined effect of a catalyst and plasma is not clear. For the particular case of 1-D nanomaterials growth, the low temperatures synthesis is still a challenge to overcome for its scalable manufacturing on flexible substrates and thin metal foils. Herein, the use of low-melting-point metal clusters under plasma excitation was investigated to determine the effectiveness in their ability to catalyze the growth of 1-D nanomaterials. Specifically, plasma catalysis using Gallium (Ga) was studied for the growth of silicon nanowires. The synthesis experiments using silane in hydrogen flow over Ga droplets in the presence of plasma excitation yielded tip-led growth of silicon nanowires. In the absence of plasma, Ga droplets did not lead to silicon nanowire growth, indicating the plasma-catalyst synergistic effect when using Ga as catalyst. The resulting nanowires had a 1:1 droplet diameter to nanowire diameter relationship when the droplet diameters were less than 100 nm. From 100 nm to a micron, the ratio increased from 1:1 to 2:1 due to differences with wetting behavior as a function of droplet size. The growth experiments using Ga droplets derived from the reduction of Gallium oxide nanoparticles resulted in silicon nanowires with size distribution similar to that of Gallium oxide nanoparticles. Systematic experiments over 100 ºC – 500 ºC range suggest that the lowest temperature for the synthesis of silicon nanowires using the plasma-gallium system is 200 ºC. A set of experiments using Ga alloys with aluminum and gold was also conducted. The results show that both Ga rich alloys (Ga-Al and Ga-Au) allowed the growth of silicon nanowires at a temperature as low as 200 ºC. This temperature is the lowest reported when using either pure Al or Au. The estimated activation energy barrier for silicon nanowire growth kinetics using Al-Ga alloy (~48.6 kJ/mol) was higher compared to that using either pure Ga or Ga-Au alloy (~34 kJ/mol). The interaction between Ga and hydrogen was measured experimentally by monitoring pressure changes in a Ga packed batch reactor at constant temperature. The decrease of the pressure inside the reactor when the Ga was exposed to plasma indicated the absorption of hydrogen in Ga. The opposite effect is observed when the plasma is turned off suggesting that hydrogen desorbed from Ga. This experimental observation suggests that Ga acts as hydrogen sink in the presence of plasma. The formation of Ga-H species in the Ga surface and in the bulk as intermediate is suggested to be responsible for the dehydrogenation of silyl radicals from the gas phase and subsequently for selective dissolution of silicon into molten Ga. The proposed reaction mechanism is also consistent with the experimentally determined activation barrier for growth kinetics (~34 kJ/mol). In addition, theoretical simulations using VASP (Vienna Ab-initio Simulation Package) were used to study atomic hydrogen – molten Ga interactions. The simulation results suggest significant interaction of atomic hydrogen with molten Ga through formation of Ga-H species on the surface and fast diffusion through bulk Ga while supporting the proposed model to explain the Plasma-Ga synergistic effect. Finally, plasma synthesis of silicon nanotubes using sacrificial zinc oxide nanowire thin film as a template was investigated for lithium ion battery anode applications. The silicon nanotube anode showed high initial discharge capacity during the first cycle of 4600 mAh g−1 and good capacity retention (3600 mAh g−1 after 20 cyles). The silicon nanotubes preserved their morphology after cycling and the observed performance was attributed to the change in phase from nanocrystalline silicon hydrogenated (nc-Si:H) to amorphous silicon hydrogenated (a-Si:H) during lithiation. This dissertation demonstrated the plasma synergism with molten metals during vapor-liquid-solid growth of silicon nanowires. A model based on atomic hydrogen interactions with molten metals under plasma excitation has been proposed and validated through systematic experimental studies involving Ga and its alloys with gold and aluminum and theoretical studies involving first principles computations. Finally, the plasma-Ga system has been used to grow successfully silicon nanowires on various technologically useful substrates at temperatures as low as 200 ºC

Plasma variables and tribological properties of coatings in low pressure (0.1 - 10.0 torr) plasma systems

A detailed treatment is presented of the dialog known as plasma surface interactions (PSI) with respect to the coating process and its tribological behavior. Adsorption, morphological changes, defect formation, sputtering, chemical etching, and secondary electron emission are all discussed as promoting and enhancing the surface chemistry, thus influencing the tribological properties of the deposited flux. Phenomenological correlations of rate of deposition, flux composition, microhardness, and wear with the plasma layer variables give an insight to the formation of chemical bonding between the deposited flux and the substrate surface

Modeling of gallium nitride molecular beam epitaxy growth

III-V nitrides (GaN, InN and AlN) are intensely researched for optoelectronic applications spanning the entire visible spectrum. In spite of realization of commercial devices and advances in processing of materials and devices, the understanding of the processing and epitaxial growth of these materials are incomplete. In this study, a rate equation approach is proposed based on physically sound surface processes to investigate the molecular beam epitaxy growth of GaN using ammonia and ECR plasma source. A surface riding layer of Ga and ammonia or N plasma species are included in the model. The surface riding species are allowed to undergo several physical and chemical processes. Rates of all surface processes are assumed Arrhenius type. The necessary model parameters which are unknown were found by fitting results from simulation to experimental values. (Abstract shortened by UMI.)

Investigating plasma modifications and gas-surface reactions of TiO2-based materials for photoconversion

2012 Fall.Includes bibliographical references.Plasmas offer added flexibility for chemists in creating materials with ideal properties. Normally unreactive precursors can be used to etch, deposit and modify surfaces. Plasma treatments of porous and compact TiO2 substrates were explored as a function of plasma precursor, substrate location in the plasma, applied rf power, and plasma pulsing parameters. Continuous wave O2 plasma treatments were found to reduce carbon content and increase oxygen content in the films. Experiments also reveal that Si was deposited throughout the mesoporous network and by pulsing the plasma, Si content and film damage could be eliminated. Nitrogen doping of TiO2 films (N:TiO2) was accomplished by pulsed plasmas containing a range of nitrogen precursors. N:TiO2 films were anatase-phased with up to 34% nitrogen content. Four different nitrogen binding environments were controlled and characterized. The produced N:TiO2 films displayed various colors and three possible mechanisms to explain the color changes are presented. Both O2 treated and N:TiO2 materials were tested in photocatalytic devices. Preliminary results from photocatalytic activities of plasma treated P25 TiO2 powders showed that nitrogen doping treatments hinder photocatalytic activity under UV light irradiation, but silicon deposition can improve it. N:TiO2 materials were tested in photovoltaic devices to reveal improved short-circuit current densities for some plasma-modified films. To understand the gas-phase and surface chemistry involved in producing the N:TiO2 films, NH and NH2 species in pulsed NH3 plasmas were explored by systematically varying peak plasma power and pulsing duty cycle. Results from these studies using gas phase spectroscopy techniques reveal interconnected trends of gas-phase densities and surface reactions. Gas-phase data from pulsed plasmas with two different types of plasma pulsing reveal diminished or increased densities at short pulses that are explained by plasma pulse initiation and afterglow effects. Overall this work reveals characteristics of the plasma systems explored, knowledge of the resulting materials, and control over plasma etching, deposition, and modification of TiO2 surfaces

Synergistic Interactions of H\u3csub\u3e2\u3c/sub\u3e and N\u3csub\u3e2\u3c/sub\u3e with Molten Gallium in the Presence of Plasma

The present study examines the interaction of hydrogen and nitrogen plasmas with gallium in an effort to gain insights into the mechanisms behind the synergetic effect of plasma and a catalytic metal. Absorption/desorption experiments were performed, accompanied by theoretical-computational calculations. Experiments were carried out in a plasma-enhanced, Ga-packed, batch reactor and entailed monitoring the change in pressure at different temperatures. The results indicated a rapid adsorption/dissolution of the gas into the molten metal when gallium was exposed to plasma, even at a low temperature of 100 °C. The experimental observations, when hydrogen was used, indicate that gallium acts as a hydrogen sink in the presence of plasma. Similar results were obtained with Ga in the presence of nitrogen plasma. In addition, density functional theory calculations suggest a strong interaction between atomic hydrogen and molten gallium. This interaction is described as a high formation of Ga-H species on the surface, fast diffusion inside the metal, and a steady state concentration of the gas in the bulk