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Plasma decomposition reactions are used for various gas phase chemical processes including the decomposition of ammonia. In this work we show that pure ammonia can be effectively decomposed at atmospheric pressure and ambient temperature using a packed-bed plasma reactor moderated with BaTiO3 ferroelectric pellets without catalyst. The decomposition rate and energy efficiency of this ferroelectric barrier discharge reactor have been monitored as a function of applied voltage (up to a maximum value of 2.5 kV) and flow rate. For each operating condition reaction efficiencies have been correlated with the parameters defining the electrical response of the reactor. It is found that plasma current and volume inside the reactor and hence the energy efficiency of the process and the decomposition rate vary with the applied voltage and the flow of ammonia (a maximum decomposition rate of 14% and an energy efficiency of 150 LH2/kWh has been determined under optimized operation conditions). The role of back reactions (i.e. N2 + 3H2 → 2NH3) in decreasing reactor performance is another key effect affecting the overall efficiency for the ammonia decomposition. The possibilities of ferroelectric barrier discharge reactors to induce the decomposition of ammonia and the importance of keeping the operating temperature below the Curie temperature of the ferroelectric material are highlighted.

Plasma decomposition reactions are used for various gas phase chemical processes including the decomposition of ammonia. In this work we show that pure ammonia can be effectively decomposed at atmospheric pressure and ambient temperature using a packed-bed plasma reactor moderated with BaTiO3 ferroelectric pellets without catalyst. The decomposition rate and energy efficiency of this ferroelectric barrier discharge reactor have been monitored as a function of applied voltage (up to a maximum value of 2.5 kV) and flow rate. For each operating condition reaction efficiencies have been correlated with the parameters defining the electrical response of the reactor. It is found that plasma current and volume inside the reactor and hence the energy efficiency of the process and the decomposition rate vary with the applied voltage and the flow of ammonia (a maximum decomposition rate of 14% and an energy efficiency of 150 LH2/kWh has been determined under optimized operation conditions). The role of back reactions (i.e. N2 + 3H2 → 2NH3) in decreasing reactor performance is another key effect affecting the overall efficiency for the ammonia decomposition. The possibilities of ferroelectric barrier discharge reactors to induce the decomposition of ammonia and the importance of keeping the operating temperature below the Curie temperature of the ferroelectric material are highlighted.

Dielectric barrier discharges (DBDs) typically operate in the filamentary regime and thus exhibit great spatial and temporal non-uniformity. In order to optimize DBDs for various applications, such as in plasma catalysis, more fundamental insight is needed. Here, we consider how the millions of microdischarges, characteristic for a DBD, influence individual gas molecules. We use a Monte Carlo approach to determine the number of microdischarges to which a single molecule would be exposed, by means of particle tracing simulations through a full-scale packed bed DBD reactor, as well as an empty DBD reactor. We find that the fraction of microdischarges to which the molecules are exposed can be approximated as the microdischarge volume over the entire reactor gas volume. The use of this concept provides good agreement between a plasma-catalytic kinetics model and experiments for plasma-catalytic NH3 synthesis. We also show that the concept of the fraction of microdischarges indicates the efficiency by which the plasma power is transferred to the gas molecules. This generalised concept is also applicable for other spatially and temporally non-uniform plasmas., SCOPUS: ar.j, info:eu-repo/semantics/published

We present a zero-dimensional plasma kinetics model, including both surface and gas phase kinetics, to determine the role of vibrationally excited states in plasma-catalytic ammonia synthesis. We defined a new method to systematically capture the conditions of dielectric barrier discharges (DBDs), including those found in packed bed DBDs. We included the spatial and temporal nature of such discharges by special consideration of the number of micro-discharges in the model. We introduce a parameter that assigns only a part of the plasma power to the micro-discharges, to scale the model conditions from filamentary to uniform plasma. Because of the spatial and temporal behavior of the micro-discharges, not all micro-discharges occurring in the plasma reactor during a certain gas residence time are affecting the molecules. The fraction of power considered in the model ranges from 0.005%, for filamentary plasma, to 100%, for uniform plasma. If vibrational excitation is included in the plasma chemistry, these different conditions, however, yield an ammonia density that is only varying within one order of magnitude. At only 0.05% of the power put into the uniform plasma component, a model neglecting vibrational excitation clearly does not result in adequate amounts of ammonia. Thus, our new model, which accounts for the concept in which not all the power is deposited by the micro-discharges, but some part may also be distributed in between them, suggests that vibrational kinetic processes are really important in (packed bed) DBDs. Indeed, vibrational excitation takes place in both the uniform plasma between the micro-discharges and in the strong micro-discharges, and is responsible for an increased N2 dissociation rate. This is shown here for plasma-catalytic ammonia synthesis, but might also be valid for other gas conversion applications., SCOPUS: ar.j, info:eu-repo/semantics/published

We propose a mechanism to explain many features of the multi-filament dielectric barrier discharge: while part of the charge deposited during previous discharge cycles is immobile on the dielectric over time periods of seconds, the larger fraction of the deposited charge must be mobile on time-scales of hundreds of ns. For alumina, we estimate that a sheet resistance of 3 MΩ sq-1 is consistent with the multi-filament discharge; an increase in conductivity of at least 12 orders of magnitude. The existence of this type of plasma-induced surface conductivity could prove relevant in modeling a wide range of plasma devices, in addition to DBD.

Presented at the 2012 AFOSR Space Propulsion and Power Program Review held 10-13 September in Arlington, VA. U.S. Government or Federal Rights License

Atmospheric or higher pressure nonequilibrium or cold plasmas are susceptible to instabilities and the lifetime of the discharge depends various parameters such as the gas (attaching/non-attaching, atomic/molecular, pure gas/gas mixture), the applied external electric field (DC, pulsed, or AC), and the magnitude of the discharge current. Various approaches have been pursued to stabilize atmospheric pressure discharges by extending the lifetime of the discharge to hundreds of microseconds. The most promising stabilization approaches are based on the recognition that arc formation in high-pressure plasmas can be avoided and stable high-pressure plasmas can be generated and maintained when the plasmas are spatially constricted to dimensions of tens to hundreds of microns. These types of discharges have come to be known as microdischarges or microplasmas. This project conducted basic research in microplasmas at atmospheric or higher pressures. A further and very important objective of this project was to address the major national concern of attracting young people to science and science-related careers., The original document contains color images.

Ova doktorska disertacija bavi se proučavanjem fenomena Plazma Elektrolitičke Oksidacije (PEO) ventilnih metala (Al, Mg, Ti, Та) i oksidnih slojeva koji se dobijaju u elektrolitima koji sadrže volfram, pre svega u 12-volframsilicijumskoj kiselini. Primenom metoda optičke emisione spektroskopije i analize procesa u realnom vremenu, omogućeno je određivanje: temperature, elektronske gustine plazme, dimenzionalne distribucije i aktivne površine mikro pražnjenja. Pokazano je da PEO procesi na svim ispitivanim metalima pokazuju tendenciju da se broj mikro pražnjenja smanjuje sa vremenom, a da površine poprečnih preseka mikro pražnjenja rastu. Sa porastom vremena PEO procesa broj kanala za pražnjenje se smanjuje, dok njihov dijametar raste i dolazi do povećanja hrapavosti oksidnih površina. Procenat aktivne površine istovremeno izložen događajima mikro pražnjenja takođe raste u početku PEO procesa, dostiže svoj maksimum, a potom polako opada. Sve spektralne linije detektovane u optičko emisionim spektrima u toku PEO procesa pripadaju ventilnim metalima i jonskim i atomskim elementima prisutnim u elektrolitu. Pored jasno izraženih atomskih i jonskih linija, u optičko emisionom spektru dobijenom tokom PEO procesa na magnezijumu, jasno je vidljiva i vibraciona traka MgO molekula, koja odgovara elektronskom prelazu sa pobuđenog B1Σ+ nivoa na osnovni elektronski nivo X1Σ+. Na sličan način, u spektru dobijenom tokom PEO procesa na aluminijumu opaža se jaka vibraciona traka AlO sistema, koja odgovara elektronskom prelazu sa pobuđenog B2Σ+ nivoa na osnovni elektronski nivo X2Σ+. U slučaju PEO procesa na aluminijumu i magnezijumu, koristeći spektroskopske podatke dobijene za AlO, OH i MgO trake, uz primenu kvantno-mehaničkih proračuna, određene su temperature plazme. Iz vibracionih traka MgO molekula procenjeno je da su temperature plazme (11000 ± 2000) K, dok su u slučaju podataka iz OH trake dobijene nešto niže vrednosti temperature (3500 ± 500) K. Iz analiza vibracione trake Al, This doctoral dissertation is focused on investigation of a relatively new, highvoltage anodization process, called Plasma Electrolytic Oxidation (PEO) on valve metals (such as Al, Mg, Ti and Ta) and their oxide coatings obtained in tungsten containing heteropoly acids. Very useful data concerning temperature, dimensional distribution of the microdischarges, their spatial density, and electron density were obtained using optical emission spectroscopy and real time imaging of PEO events. It is shown that the size of microdischarges becomes larger while their number decreases with increased time of PEO. Spatial density of microdischarges is the highest in the early stage of the PEO process and then slowly decreases. The percentage of oxide coatings’ area simultaneously covered by active discharge sites decreases slowly with extended PEO time. All atomic and ionic lines detected in optical emission spectra originate either from electrolyte or from the metal substrate. During the PEO process on magnesium, wide transition band was also recorded corresponding to the v′−v″=0 band sequence of the B1Σ+ – X1Σ+ emission transition of MgO. During the PEO process on aluminum, visible band of B2Σ+ - X2Σ+ emission of AlO system was also detected. By applying spectroscopic, quantum-chemical and thermodynamical methods to analyze spectra in the range of those bands it was possible to determine the temperature of the microdischarges. In the case of PEO process on magnesium, calculated temperatures are about 11000 K ± 2000 K, while analysis of OH bands in the same sysstem yields sligtly lower value of 3500 K ± 500 K. Analysis of AlO band system gave temperature of 8000 K ± 2000 K. The analysis of hydrogen Balmer Hβ line shape in the case of PEO of tantalum and titanium, indicates the presence of two types of discharges during PEO process and the electron number densities of Ne ≈ 0.8×1015 cm−3 and Ne ≈ 2.1×1016 cm−3 are determined. These two electronic concentrations are similar to tho

Arrays as large as 250,000 Si microcavity plasma devices have been fabricated and tested extensively. Exciting a phosphor with VUV emission from these arrays yields values of the luminous efficacy > 7 lumens/W, considerably higher than that available fro conventional plasma displays. A new microplasma technology based on nanopourous A12O3 grown on Al has been developed, and arrays with emitting areas > 20 cm2 have been demonstrated. Successfully sealed in plastic sheets, or with a thin glass or quartz window, these arrays are quite inexpensive and manufacturable by roll-to-roll processing. Shaping of the electric field within a microplasma has been achieved by engineering the plasma/dielectric interface.

Devices with micrometer and submicrometer gaps can face a serious challenge due to electrical breakdown during manufacturing, handling, and operation. Therefore, it is necessary to be aware of the breakdown voltage at different gaps. Since the Paschens law is not valid for gaps smaller than several micrometers, modified Paschen curve should be used to predict breakdown voltage for microdevices. One of the possible mechanisms responsible for the reduction of the maximum operation voltage at small gaps is the field emission (FE). In this paper, particle-in-cell/Monte Carlo collision simulations, including the ejection of electrons from the cathode due to a high electric field, have been carried out to estimate the significance of the FE effect on the breakdown voltage in microgaps.

Devices with micrometer and submicrometer gaps can face a serious challenge due to electrical breakdown during manufacturing, handling, and operation. Therefore, it is necessary to be aware of the breakdown voltage at different gaps. Since the Paschens law is not valid for gaps smaller than several micrometers, modified Paschen curve should be used to predict breakdown voltage for microdevices. One of the possible mechanisms responsible for the reduction of the maximum operation voltage at small gaps is the field emission (FE). In this paper, particle-in-cell/Monte Carlo collision simulations, including the ejection of electrons from the cathode due to a high electric field, have been carried out to estimate the significance of the FE effect on the breakdown voltage in microgaps.

The focus was to obtain yield measurements in DC microdischarges but there will also be an important modeling component conducted in parallel with the experiments. The existing MHCD model was developed for rare gases, and this will be extended to include a description of the O2 kinetics based on previously published work. A particularly important parameter is the gas temperature and the gas heating source term in the model also needs to be extended to include V-T (vibrational to translation) energy exchange ion heating in the sheath (expected to be the dominant component) is included in our present model). We originally proposed measuring the O2(1D) yield using IR absorption spectroscopy and we still feel that it will eventually be necessary to develop this or another or the more traditional spectroscopic methods (eg CARS or UV absorption). However, during the proof-of-principle phase of this project we propose using an isothermal calorimeter as has been developed previously for measurements of O2(1D) yields by several groups 7.8. We have been in contact with Dr, Ikonnikov and he has kindly provided us with additional information on the set-up of Lodin et a17. This calorimetric technique is, in our opinion the best way to obtain a quick inexpensive and accurate estimate of the O2(1D) yield in a single microdiecharge operating in a DC mode, The experimental set-up is simple a nickel coil is positioned in the gas flow on the downstream side of the microdischarge and the chemical energy in the gas flow is detected as a change in the external power required to maintain the coil at a constant temperature (eg 150 degrees C). This power is reduced when O2(1D) is deactivated at the coil surface. The Russian group7 used a self-balancing bridge circuit to maintain the constant temperature and they calibrated their system by measuring the O2(1D) emission signal before and after the detector. They determined that between 70 and 90% of the O2(1D) reacted with the nickel coil., The original document contains color images.

Results of spatio-temporally resolved optical emission spectroscopy of single filaments in a dielectric barrier discharge in nitrogen containing 3% of oxygen are presented. Results of numerical modelling are discussed, too., Pub. in International Conference on Phenomena in Ionized Gases (26th), v4 p39-40, July 2003. Prepared in cooperation with University of Greifswald, Institute of Physics, Greifswald, Germany; General Physics Institute Moscow, Russia. This article is from ADA421147 International Conference on Phenomena in Ionized Gases (26th) Held in Greifswald, Germany on 15-20 July 2003. Proceedings, Volume 4

Two-dimensional structure of the microdischarge channel radiation in the second positive (0,0) molecular band of nitrogen in dielectric barrier discharge in nitrogen-oxygen mixtures between parallel plane electrodes is investigated by means of numerical modelling. The numerical results are applied for the explanation of experimental results., Pub. in International Conference on Phenomena in Ionized Gases (26th), v4 p41-42, July 2003. Prepared in cooperation with University of Greifswald, Institute of Physics, Greifswald, Germany; Moscow State University, Dept. of Chemistry, Moscow, Russia. This article is from ADA421147 International Conference on Phenomena in Ionized Gases (26th) Held in Greifswald, Germany on 15-20 July 2003. Proceedings, Volume 4

Over the past three years, we have pursued a number of microdischarge fabrication techniques and device designs and the results have been gratifying. Today, we can say that devices now operate reliably at voltages less than 100 V and pressures above one atmosphere. Furthermore, the discovery of the pn junction-excited microdischarge offers a viable approach to integrating microdischarges with electronic components and passive and active electro-optical components. In short, we are becoming more convinced that this technology provides an attractive pathway for integrating optical sources onto a Si electronic chip.