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Aviv Amirav - One of the best experts on this subject based on the ideXlab platform.

  • gas chromatography mass spectrometry with supersonic molecular beams
    Journal of Mass Spectrometry, 2008
    Co-Authors: Aviv Amirav, Alexander Gordin, Marina Poliak, Alexander B Fialkov
    Abstract:

    A new approach for liquid chromatography mass spectrometry (LC-MS) is described, based on achieving soft thermal vaporization followed by supersonic expansion and direct sample compound Ionization, while in a supersonic molecular beam (SMB). The soft molecular vaporization step utilizes spray formation that is continued by fast thermal vaporization inside a channel supersonic nozzle, followed by ultrafast supercooling in a supersonic expansion. The short time (several microseconds) spent by the vaporized compound in the heated nozzle prior to its expansion cooling may result in incomplete vibrational equilibrium and thus reduced degree of dissociation. In addition, even if vibrational equilibrium at the nozzle temperature is obtained, the sample compounds have significantly reduced time for their dissociation, which is thus further minimized (kinetic consideration). As soon as the molecules expand and form a SMB, they are supercooled and any further dissociation is avoided. While in the SMB, the sample molecules can be ionized either by electron Ionization as described in this paper or by hyperthermal Surface Ionization. The major goal of this method is to obtain high quality library searchable electron Ionization mass spectra, for a broad range of thermally labile compounds, with higher sensitivity than that achievable by particle beam LC-MS. The soft thermal vaporization nozzle is described and mass spectral results with corticosterone are demonstrated. The potential advantageous features of this new method are discussed.

  • liquid chromatography mass spectrometry with supersonic molecular beams
    Journal of the American Society for Mass Spectrometry, 2000
    Co-Authors: Aviv Amirav, Ori Granot
    Abstract:

    A new approach for liquid chromatography mass spectrometry (LC-MS) is described, based on achieving soft thermal vaporization followed by supersonic expansion and direct sample compound Ionization, while in a supersonic molecular beam (SMB). The soft molecular vaporization step utilizes spray formation that is continued by fast thermal vaporization inside a channel supersonic nozzle, followed by ultrafast supercooling in a supersonic expansion. The short time (several microseconds) spent by the vaporized compound in the heated nozzle prior to its expansion cooling may result in incomplete vibrational equilibrium and thus reduced degree of dissociation. In addition, even if vibrational equilibrium at the nozzle temperature is obtained, the sample compounds have significantly reduced time for their dissociation, which is thus further minimized (kinetic consideration). As soon as the molecules expand and form a SMB, they are supercooled and any further dissociation is avoided. While in the SMB, the sample molecules can be ionized either by electron Ionization as described in this paper or by hyperthermal Surface Ionization. The major goal of this method is to obtain high quality library searchable electron Ionization mass spectra, for a broad range of thermally labile compounds, with higher sensitivity than that achievable by particle beam LC-MS. The soft thermal vaporization nozzle is described and mass spectral results with corticosterone are demonstrated. The potential advantageous features of this new method are discussed.

  • Electron impact mass spectrometry of alkanes in supersonic molecular beams
    Journal of the American Society for Mass Spectrometry, 1995
    Co-Authors: Shai Dagan, Aviv Amirav
    Abstract:

    The electron impact mass spectrometry of straight chain alkanes C_8H_18-C_40H_82, squalane, methylstearate, 1-chlorohexadecane, 1-bromohexadecane, and dioctylphthalate was studied by sampling them with supersonic molecular beams. A fly-through Brink-type electron impact ion source was used, utilizing a vacuum background ion filtration technique based on differences between the kinetic energy of the supersonic beam species and that of thermal molecules. The 70-eV electron impact mass spectra of all the alkanes were characterized by a pronounced or dominant molecular weight peak together with all the fragment ions normally exhibited by the standard thermal 70-eV EI mass spectra. In contrast, the NIST library of most of these molecules did not show any molecular weight peak. By eliminating tile intramolecular thermal vibrational energy we gained control over the degree of molecular ion fragmentation by the electron energy. At an electron energy of 18 eV the molecular ion dissociation was further reduced considerably, with only a small absolute reduction in the peak height by less than a factor of 2. The effect of vibrational cooling increased with the molecular size and number of atoms. Pronounced differences were observed between the mass spectra of the straight chain triacontane and its branched isomer squalane. Similar mass spectra of octacosane (C_28H_58) achieved with 70-eV EI in a supersonic molecular beam were obtained with a magnetic sector mass spectrometer by using an electron energy of 14 eV and an ion source temperature of 150 °C. However, this ion source temperature precluded the gas chromatography-mass spectrometry (GC-MS) of octacosane. The GC-MS of alkanes was studied with an ion trap gas chromatograph-mass spectrometer at an ion source temperature of 230 °C. Thermal peak tailing was observed for C_20H_42 and heavier alkanes, whereas for C_28H_58 and heavier alkanes the severe peak tailing made quantitative GC-MS impractical. In contrast, no peak tailing existed even with C_40H_82 for GC-MS in supersonic molecular beams. The minimum detected amount of eicosane (C_20, H_42) was shown to be 60 fg. This was demonstrated by using single ion monitoring with the quadrupole mass analyzer tuned to the molecular weight peak of 282 u. The coupling of electron impact mass spectrometry in supersonic molecular beams with hyperthermal Surface Ionization and a fast GC-MS inlet is briefly discussed.

  • fast high temperature and thermolabile gc ms in supersonic molecular beams
    International Journal of Mass Spectrometry and Ion Processes, 1994
    Co-Authors: Shai Dagan, Aviv Amirav
    Abstract:

    Abstract This work describes and evaluates the coupling of a fast gas chromatograph (GC) based on a short column and high carrier gas flow rate to a supersonic molecular beam mass spectrometer (MS). A 50 cm long megabore column serves for fast GC separation and connects the injector to the supersonic nozzle source. Sampling is achieved with a conventional syringe based splitless sample injection. The injector contains no septum and is open to the atmosphere. The linear velocity of the carrier gas is controlled by a by-pass (make-up) gas flow introduced after the column and prior to the supersonic nozzle. The supersonic expansion serves as a jet separator and the skimmed supersonic molecular beam (SMB) is highly enriched with the heavier organic molecules. The supersonic molecular beam constituents are ionized either by electron impact (EI) or hyperthermal Surface Ionization (HSI) and mass analyzed. A 1 s fast GC—MS of four aromatic molecules in methanol is demonstrated and some fundamental aspects of fast GC—MS with time limit constraints are outlined. The flow control (programming) of the speed of analysis is shown and the analysis of thermolabile and relatively non-volatile molecules is demonstrated and discussed. The tail-free, fast GC—MS of several mixtures is shown and peak tailing of caffeine is compared with that of conventional GC—MS. The improvement of the peak shapes with the SMB—MS is analyzed with the respect to the elimination of thermal vacuum chamber background. The extrapolated minimum detected amount was about 400 ag of anthracence- d 10 , with an elution time which was shorter than 2s. Repetitive injections could be performed within less than 10 s. The fast GC—MS in SMB seems to be ideal for fast target compound analysis even in real world, complex mixtures. The few seconds GC—MS separation and quantification of lead (as tetraethyllead) in gasoline, caffeine in coffee, and codeine in a drug is demonstrated. Controlled HSI selectivity is demonstrated in the range of 10 1 to 10 4 anthracene/decane which helped to simplify the selective analysis of aromatic molecules in gasoline. The contribution of SMB to the operation of the fast GC—MS is summarized and the compatibility with conventional GC having a megabore column is shown. Splitless injections of 100 μL sample solutions for trace level concentration detection is also presented (with a conventional GC).

  • electron impact and hyperthermal Surface Ionization mass spectrometry in supersonic molecular beams
    Journal of Mass Spectrometry, 1991
    Co-Authors: Aviv Amirav
    Abstract:

    Supersonic molecular beams (SMB) are characterized by unidirectional motion in space, hyperthermal kinetic energy of up to 30 eV, supercooling of the molecular vibrational−rotational degrees of freedom, heavy species focusing (jet separation) and atmospheric-pressure sample inlet. These features can help to improve the performance of electron impact (EI) mass spectrometry (MS) and allow hyperthermal Surface Ionization (HSI) mass spectrometry in SMB. The following aspects of EI and HSI mass spectrometry in SMB are discussed and demonstrated: (a) background filtration, (b) enhancement of the molecular ion peak (in EI), (c) enhanced control over the degree of ion fragmentation, (d) isotopic and elemental information, (e) functional group information in both EI and HSI, (f) isomer and structural information, (g) atmospheric-pressure sample introduction, (h) mechanisms of HSI, (i) HSI of metal atoms and (j) coupling gas chromatography with EI and HSI mass spectrometry. The potential sensitivity of both EI and HSI mass spectrometry in SMB is evaluated, and the superior sensitivity of HSI as an ion source is demonstrated and compared with EI and thermal Surface Ionization. The coupling and construction of EI and HSI ion sources in a supersonic molecular beam−mass spectrometer apparatus is discussed and possible future prospects are outlined

Igor V Adamovich - One of the best experts on this subject based on the ideXlab platform.

  • Surface Ionization wave propagation in the nanosecond pulsed Surface dielectric barrier discharge the influence of dielectric material and pulse repetition rate
    Plasma Sources Science and Technology, 2020
    Co-Authors: Bangdou Huang, Igor V Adamovich, Cheng Zhang, Yuri Akishev, Tao Shao
    Abstract:

    In this work, the propagation of the Surface Ionization wave (SIW) in the nanosecond pulsed Surface dielectric barrier discharge with different dielectric materials and pulse repetition rates is investigated. The current waveforms at different locations along the route of the SIW propagation are obtained, based on a specially designed ground strip array geometry. The temporal evolution and spatial distribution of the electric field during the SIW propagation are measured by using the electric field induced second harmonic (EFISH) generation method. The distribution of the residual Surface potential after the discharge is mapped with a Kelvin electrostatic probe, which verifies both the existence of the residual electric field and its opposite direction to that during the SIW propagation. It is found that with the dielectric material on which the Surface charges decay faster, there are the well-pronounced primary and secondary SIWs with a higher velocity on the voltage rising edge and both the peak current and the peak electric field are also higher, with a less spatial attenuation along the SIW propagation route. It is demonstrated that the residual Surface charges with the same polarity as the high-voltage pulse suppress the development of the Surface Ionization wave.

  • electric field distribution in a Surface plasma flow actuator powered by ns discharge pulse trains
    Plasma Sources Science and Technology, 2018
    Co-Authors: Simeni M Simeni, Kraig Frederickson, Yong Tang, Igor V Adamovich
    Abstract:

    Electric field vector components in a nanosecond pulse, Surface dialectric barrier discharge plasma actuator are measured by picosecond second harmonic generation, for positive, negative, and alternating polarity pulse trains. Plasma images show that in the same polarity train, the positive polarity discharge develops as two consecutive Surface Ionization waves, while the negative polarity discharge propagates as a single diffuse Ionization wave. In the alternating polarity train, both positive and negative polarity discharge plasmas become strongly filamentary. In all pulse trains, the measurement results demonstrate a significant electric field offset before the discharge pulse, due to the Surface charge accumulation during previous discharges pulses. This demonstrates that charge accumulation is a significant factor affecting the electric field in the discharge, even at very low pulse repetition rates. Peak electric field measured in the alternating polarity pulse train is lower compared to that in same polarity trains. However, the coupled pulse energy in the alternating polarity train is much higher, by a factor of 3–4, most likely due to the neutralization of the Surface charge accumulated on the dielectric during the previous, opposite polarity pulses. This suggests that plasma Surface actuators powered by alternating polarity pulse trains may generate higher amplitude thermal perturbations, producing a stronger effect on the flow field. The present results show that the time scale for the electric field reduction in the plasma after breakdown is fairly long, several tens of ns, including the conditions when the discharge develops as a diffuse Ionization wave. This suggests that a considerable fraction of the energy is coupled to the plasma at a relatively low reduced electric field, several tens of Townsend. At these conditions, the discharge energy fraction thermalized as rapid heating would remain fairly low, thus limiting the effect on the flow caused by the high-amplitude localized thermal perturbations.

  • electric field measurements in a nanosecond pulse discharge in atmospheric air
    Journal of Physics D, 2017
    Co-Authors: Marien Simeni Simeni, Kraig Frederickson, Walter R Lempert, Cheng Zhang, Benjamin M Goldberg, Igor V Adamovich
    Abstract:

    The paper presents the results of temporally and spatially resolved electric field measurements in a nanosecond pulse discharge in atmospheric air, sustained between a razor edge high-voltage electrode and a plane grounded electrode covered by a thin dielectric plate. The electric field is measured by picosecond four-wave mixing in a collinear phase-matching geometry, with time resolution of approximately 2 ns, using an absolute calibration provided by measurements of a known electrostatic electric field. The results demonstrate electric field offset on the discharge center plane before the discharge pulse due to Surface charge accumulation on the dielectric from the weaker, opposite polarity pre-pulse. During the discharge pulse, the electric field follows the applied voltage until 'forward' breakdown occurs, after which the field in the plasma is significantly reduced due to charge separation. When the applied voltage is reduced, the field in the plasma reverses direction and increases again, until the weak 'reverse' breakdown occurs, producing a secondary transient reduction in the electric field. After the pulse, the field is gradually reduced on a microsecond time scale, likely due to residual Surface charge neutralization by transport of opposite polarity charges from the plasma. Spatially resolved electric field measurements show that the discharge develops as a Surface Ionization wave. Significant Surface charge accumulation on the dielectric Surface is detected near the end of the discharge pulse. Spatially resolved measurements of electric field vector components demonstrate that the vertical electric field in the Surface Ionization wave peaks ahead of the horizontal electric field. Behind the wave, the vertical field remains low, near the detection limit, while the horizontal field is gradually reduced to near the detection limit at the discharge center plane. These results are consistent with time-resolved measurements of electric field components, which also indicate that vertical electric field reverses direction after the Ionization wave.

  • Surface charge dynamics and oh and h number density distributions in near Surface nanosecond pulse discharges at a liquid vapor interface
    Journal of Physics D, 2015
    Co-Authors: Caroline Winters, Walter R Lempert, Vitaly Petrishchev, Zhiyao Yin, Igor V Adamovich
    Abstract:

    The present work provides insight into Surface charge dynamics and kinetics of radical species reactions in nanosecond pulse discharges sustained at a liquid-vapor interface, above a distilled water Surface. The near-Surface plasma is sustained using two different discharge configurations, a Surface Ionization wave discharge between two exposed metal electrodes and a double dielectric barrier discharge. At low discharge pulse repetition rates (~100 Hz), residual Surface charge deposition after the discharge pulse is a minor effect. At high pulse repetition rates (~10 kHz), significant negative Surface charge accumulation over multiple discharge pulses is detected, both during alternating polarity and negative polarity pulse trains. Laser induced fluorescence (LIF) and two-photon absorption LIF (TALIF) line imaging are used for in situ measurements of spatial distributions of absolute OH and H atom number densities in near-Surface, repetitive nanosecond pulse discharge plasmas. Both in a Surface Ionization wave discharge and in a double dielectric barrier discharge, peak measured H atom number density, [H] is much higher compared to peak OH number density, due to more rapid OH decay in the afterglow between the discharge pulses. Higher OH number density was measured near the regions with higher plasma emission intensity. Both OH and especially H atoms diffuse out of the Surface Ionization wave plasma volume, up to several mm from the liquid Surface. Kinetic modeling calculations using a quasi-zero-dimensional H2O vapor / Ar plasma model are in qualitative agreement with the experimental data. The results demonstrate the experimental capability of in situ radical species number density distribution measurements in liquid-vapor interface plasmas, in a simple canonical geometry that lends itself to the validation of kinetic models.

  • electric field vector measurements in a Surface Ionization wave discharge
    Plasma Sources Science and Technology, 2015
    Co-Authors: Benjamin M Goldberg, Igor V Adamovich, Patrick S Bohm, Uwe Czarnetzki, Walter R Lempert
    Abstract:

    This work presents the results of time-resolved electric field vector measurements in a short pulse duration (60 ns full width at half maximum), Surface Ionization wave discharge in hydrogen using a picosecond four-wave mixing technique. Electric field vector components are measured separately, using pump and Stokes beams linearly polarized in the horizontal and vertical planes, and a polarizer placed in front of the infrared detector. The time-resolved electric field vector is measured at three different locations across the discharge gap, and for three different heights above the alumina ceramic dielectric Surface, ~100, 600, and 1100 μm (total of nine different locations). The results show that after breakdown, the discharge develops as an Ionization wave propagating along the dielectric Surface at an average speed of 1 mm ns −1 . The Surface Ionization wave forms near the high voltage electrode, close to the dielectric Surface (~100 μm). The wave front is characterized by significant overshoot of both vertical and horizontal electric field vector components. Behind the wave front, the vertical field component is rapidly reduced. As the wave propagates along the dielectric Surface, it also extends further away from the dielectric Surface, up to ~1 mm near the grounded electrode. The horizontal field component behind the wave front remains quite significant, to sustain the electron current toward the high voltage electrode. After the wave reaches the grounded electrode, the horizontal field component experiences a secondary rise in the quasi-dc discharge, where it sustains the current along the near-Surface plasma sheet. The measurement results indicate presence of a cathode layer formed near the grounded electrode with significant cathode voltage fall, ≈ 3 kV, due to high current density in the discharge. The peak reduced electric field in the Surface Ionization wave is 85– 95 Td, consistent with dc breakdown field estimated from the Paschen curve for hydrogen. The present set of data on electric field distribution in a Surface Ionization wave discharge provides an experimental reference for validation of kinetic models and assessing their predictive capability.

Stanka Jerosimic - One of the best experts on this subject based on the ideXlab platform.

  • theoretical and experimental study of small potassium bromide knbr 0 1 n 2 6 and knbrn 1 0 1 n 3 5 clusters
    Journal of Alloys and Compounds, 2020
    Co-Authors: Marko Mitic, Milan Milovanovic, F M Veljkovic, Aleksandra A Pericgrujic, Suzana Velickovic, Stanka Jerosimic
    Abstract:

    Abstract In the present paper, the results of combined theoretical and experimental investigation of small non-stoichiometric bromine-doped potassium clusters are reported. Potassium-bromide clusters were obtained by the Knudsen cell combined with Surface Ionization in the temperature range of 1000–1600 K, and selected by a magnetic sector mass spectrometer. Positive ions of KnBr (n = 3–6) and KnBrn-1 (n = 3–5) clusters were detected for the first time in one set measurement. In order to reveal the geometrical structure of each type of detected cluster, the randomized search algorithm was employed to survey the (Born-Oppenheimer) potential energy Surface of both the neutral and cationic KnBr(0,1+) (n = 1–6) and KnBrn-1(0,1+) (n=3–5) clusters, followed by Density functional theory geometry optimizations, and many lowest-energy conformational isomers are presented. From the total electronic energies of clusters computed by the ab initio RCCSD(T)/ECP10MDF(K),cc-pVTZ-PP(Br) method at obtained equilibrium nuclear geometries, the following stability parameters of clusters were computed: their relative energies, the adiabatic and vertical Ionization energies, binding energies per atom, and dissociation energies. Both experimental and theoretical results have shown that the title clusters belong to the group of “superalkali” clusters.

Mark J Kushner - One of the best experts on this subject based on the ideXlab platform.

  • propagation of atmospheric pressure plasmas through interconnected pores in dielectric materials
    Journal of Applied Physics, 2021
    Co-Authors: Juliusz Kruszelnicki, Mark J Kushner
    Abstract:

    The propagation of atmospheric pressure plasmas (APPs) on and through porous dielectric materials is being investigated for plasma-catalysis and functionalizing biomedical materials for tissue scaffolding and bone regeneration. Such plasma functionalization improves wettability and cell attachment, and so uniformity of the treatment of the pore Surfaces is important. The method of propagation of APPs through porous media is not well characterized. In this paper, we discuss results from a computational investigation of humid air APPs propagating through short fully interconnected pore-chains in a dielectric substrate. The properties of the dielectric and pores (diameter 150 μm) were chosen to resemble bone scaffolding. We found that photoIonization is an important feature in plasma propagation through pore-chains to seed electrons in the following pore in the chain. This seeding of electrons in regions of high electric field allows for the formation of micro-streamers and Surface Ionization waves. This is particularly important when the openings between pores are small. The orientation of the pore-chain with respect to the applied electric field has a significant impact on plasma generation, mode of propagation, and fluences of short-lived, reactive species to the Surfaces of the pores. The uniformity of fluences of charged and short-lived neutral species to the pore Surfaces decreases as the angle of the pore chain deviates from being aligned with the applied electric field. Diffusion within pores improves the uniformity of fluences to pore Surfaces for long-lived species on longer time scales compared to their post-discharge uniformity.

  • formation of Surface Ionization waves in a plasma enhanced packed bed reactor for catalysis applications
    Chemical Engineering Journal, 2020
    Co-Authors: Zakaulislam Mujahid, Juliusz Kruszelnicki, Ahmed Hala, Mark J Kushner
    Abstract:

    Abstract Plasma enhanced packed bed reactors (PE-PBRs) intrinsically have complex geometries which makes it difficult to apply conventional scaling laws. For example, controlling the manner of discharge propagation between the micro-discharges (MDs) that occur between dielectrics in PE-PBRs and Surface Ionization waves (SIWs) that propagate along the dielectrics would aid in selectivity during plasma catalysis. An important parameter in that optimization is the pulse power format. In this work, we investigated the role of applied voltage amplitude and polarity on time resolved dynamics in an atmospheric pressure dielectric barrier discharge (DBD) operating in helium having Surface topology resembling PE-PBRs using phase and space resolved optical emission spectroscopy (PROES). To enable systematic studies, the DBD uses an array of dielectric semi-spheres imaged through a top transparent electrode and imaged between the two electrodes from the side. The results were compared with 2-dimensional modeling. We identified three discharge mechanisms: filamentary micro-discharges (F-MDs) in the volume (the space between the Surface of the dielectrics and the counter electrode), Surface micro-discharges (S-MDs) between the dielectric semi-spheres near their contact points, and SIWs over the curved or flat dielectric Surfaces. At the lowest voltages, only F-MDs are generated, once in each half cycle. At intermediate voltages, SIWs also appear which transform into S-MDs at the contact points. When voltage is further increased, several additional pulses are observed, which generate S-MDs at the contact points or a combination F-MDs and S-MDs. The voltage amplitude determines the frequency of these pulses. Modeling results show variations in plasma density and electron temperature for each of the three mechanisms which, in turn, impact production of reactive species.

  • atmospheric pressure plasma jets onto a reactive water layer over tissue pulse repetition rate as a control mechanism
    Journal of Physics D, 2019
    Co-Authors: Seth Norberg, Eric Johnsen, Guy Parsey, Amanda M Lietz, Mark J Kushner
    Abstract:

    The use of plasma jets to treat tissue in the context of plasma medicine often involves a thin intervening liquid layer on top of the tissue. Plasma activated species first transport through and react in the liquid layer prior to reaching the tissue. Of the many parameters that can be used to control this process, pulse repetition frequency (PRF) stands out. Results from a computational investigation of multiple pulses at varying PRF from an atmospheric pressure plasma jet (APPJ) onto a reactive liquid layer are discussed, and three key trends are made clear. First, a high PRF (short time between pulses) enables the gaseous species produced during the previous pulse to remain in the vicinity of the plasma at the onset of the next pulse, thereby increasing the inventory of (H)N x O y and O3 in the gas phase. These species then solvate into the liquid, water in this case, and produce higher densities of aqueous ozone, nitrate, and peroxynitrite. With a lower PRF, reactants produced on a previous pulse are convected away prior to the next discharge pulse with more spatial separation of reactants both above and within the water. As a result, more of the hydroxyl anion (), ozone anion () and nitric oxide (NOaq) reach the tissue beneath the water. The second trend is that the production of H2O2aq and its fluence to the underlying tissue are relatively independent of the PRF. The precursors for H2O2aq are primarily produced by the Surface Ionization wave (SIW) on the top of the liquid, which then directly solvate into the liquid. Lastly, when the plasma plume touches the liquid, the SIW on the water layer increases the production of all aqueous species compared to configurations where the plasma plume does not touch the liquid. These trends are true for all PRF.

  • time resolved evolution of micro discharges Surface Ionization waves and plasma propagation in a two dimensional packed bed reactor
    Plasma Sources Science and Technology, 2018
    Co-Authors: Kenneth W Engeling, Juliusz Kruszelnicki, Mark J Kushner, John E Foster
    Abstract:

    Plasma packed bed reactors (PBRs) are being investigated for applications ranging from pollution remediation to chemical synthesis, including plasma catalysis. Plasma PBRs typically operate as dielectric barrier discharges where the plasma propagating through the PBR strongly interacts with the dielectric packing media and gas in the interstitial spaces. The nature of plasma propagation through this macroscopically porous-like medium is not well understood. Plasma formation in PBRs is a function of many parameters, including dielectric media composition and Surface morphology, dielectric constant, packing fraction, pressure, and the applied voltage waveform. Imaging the plasma propagation through the complex three-dimensional geometry of the packing media and interstitial spaces that make up the PBR is difficult to experimentally execute. In this regard, a two-dimensional PBR composed of dielectric disks was developed to enable optical imaging of plasma formation and propagation. The mode of plasma propagation and the sensitivity of discharge formation to material dielectric constant, applied voltage, and pressure were experimentally and computationally investigated. We found that higher dielectric constants of the packing material produced more intense, localized filamentary micro-discharges between disks. In general, plasma propagation through the PBR at 1 atm is initiated by localized micro-discharges between adjacent dielectric disks, which in turn give rise to Surface Ionization waves (SIWs) that propagate along the dielectric Surface. At pressures below 1 atm, the discharge was more diffuse regardless of the dielectric media, filling the interstitial space instead of forming SIWs.

Alexis Lycourghiotis - One of the best experts on this subject based on the ideXlab platform.

  • mapping the Surface hydr oxo groups of titanium oxide and its interface with an aqueous solution the state of the art and a new approach
    Advances in Colloid and Interface Science, 2008
    Co-Authors: George D Panagiotou, Kyriakos Bourikas, Christos Kordulis, Theano Petsi, Christos S Garoufalis, Athanassios Tsevis, N Spanos, Alexis Lycourghiotis
    Abstract:

    Abstract In this article the “titanium oxide/electrolyte solution” interface is studied by taking in advantage the recent developments in the field of Surface and Interface Chemistry relevant to this oxide. Ab-initio calculations were performed in the frame of the DFT theory for estimating the charge of the titanium and oxygen atoms exposed on the anatase (1 0 1), (1 0 0), (0 0 1), (1 0 3) f and rutile (1 1 0) crystal faces. These orientations have smaller Surface energy with respect to other ones and thus it is more probable to be the real terminations of the anatase and rutile nanocrystallites in the titania polycrystalline powders. Potentiometric titrations for obtaining “fine structured” titration curves as well as microelectrophoresis and streaming potential measurements have been performed. On the basis of ab-initio calculations, and taking into account the relative contribution of each crystal face to the whole Surface of the nanocrystals involved in the titania aggregates of a suspension, the three most probable Surface Ionization models have been derived. These models and the Music model are then tested in conjunction with the “Stern–Gouy–Chapman” and “Basic Stern” electrostatic models. The finally selected Surface Ionization model (model A) in combination with each one of the two electrostatic models describes very well the protonation/deprotonation behavior of titania. The description is also very good if this model is combined with the Three Plane (TP) model. The application of the “A/(TP)” model allowed mapping the Surface (hydr)oxo-groups [TiO(H) and Ti 2 O(H)] of titania exposed in aqueous solutions. At pH > pzc almost all terminal oxygens [TiO] are non-protonated whereas even at low pH values the non-protonated terminal oxygens predominate. The acid-base behavior of the bridging oxygens [Ti 2 O] is different. Thus, even at pH = 10 the greater portion of them is protonated. The application of the “A/TP” model in conjunction with potentiometric titrations, microelectrophoresis and streaming potential experiments allowed mapping the “titania/electrolyte solution” interface. It was found that the first (second) charged plane is located on the oxygen atoms of the first (second) water overlayer at a distance of 1.7 (3.4) A from the Surface. The region between the Surface and the second plane is the compact layer. The region between the second plane and the shear plane is the stagnant diffuse part of the interface, with an ionic strength dependent width, ranging from 20 (0.01 M) up to 4 A (0.3 M). The region between the shear plane and the bulk solution is the mobile diffuse part, with an ionic strength dependent width, ranging from 10 (0.01 M) up to 2 A (0.3 M). At I  > 0.017 M the mean concentration of the counter ions is higher in the stagnant than in the mobile part of the diffuse layer. For a given I , removal of pH from pzc brings about an increase of the mean concentration in the interfacial region and a displacement of the counter ions from the mobile to the stagnant part of the diffuse layer. The mean concentration of the counter ions in the compact layer is generally lower than the corresponding ones in the stagnant and mobile diffuse layers. The mobility of the counter ions in the stagnant layer decreases as pH draws away from pzc or ionic strength increases.

  • differential potentiometric titration development of a methodology for determining the point of zero charge of metal hydr oxides by one titration curve
    Environmental Science & Technology, 2005
    Co-Authors: Kyriakos Bourikas, Christos Kordulis, Alexis Lycourghiotis
    Abstract:

    A new methodology is presented, called differential potentiometric titration (DPT), which allows the determination of the point of zero charge (pzc) of metal (hydr)oxides using only one potentiometric curve. By performing extensive simulations of potentiometric titrations for various model (hydr)oxides, we found that an inflection point in a H+cons,surf versus pH potentiometric curve (H+cons,surf:  hydrogen ions consumed on the Surface of the (hydr)oxide) and a peak in the corresponding differential curve, dH+cons,surf/dpH versus pH, appear at a pH equal to the pzc assumed for a model (hydr)oxide. This distinguishable peak appears at the same position irrespective of the Surface Ionization and the interfacial model adopted as well as the assumed ionic strength. It was found that the aforementioned peak also appears in the high-resolution differential potentiometric curves experimentally determined for four oxides (SiO2, TiO2, γ-Al2O3, and MgO) that are widely used in various environmental and other techno...

  • Differential potentiometric titration: Development of a methodology for determining the point of zero charge of metal (hydr)oxides by one titration curve
    Environmental Science and Technology, 2005
    Co-Authors: Kyriakos Bourikas, Christos Kordulis, Alexis Lycourghiotis
    Abstract:

    A new methodology is presented, called differential potentiometric titration (DPT), which allows the determination of the point of zero charge (pzc) of metal (hydr)oxides using only one potentiometric curve. By performing extensive simulations of potentiometric titrations for various model (hydr)oxides, we found that an inflection point in a H+(cons,surf) versus pH potentiometric curve (H+(cons,surf): hydrogen ions consumed on the Surface of the (hydr)oxide) and a peak in the corresponding differential curve, dH+(cons,surf)/dpH versus pH, appear at a pH equal to the pzc assumed for a model (hydr)oxide. This distinguishable peak appears at the same position irrespective of the Surface Ionization and the interfacial model adopted as well as the assumed ionic strength. It was found that the aforementioned peak also appears in the high-resolution differential potentiometric curves experimentally determined for four oxides (SiO2, TiO2, gamma-Al2O3, and MgO) that are widely used in various environmental and other technological applications. The application of DPT to the above-mentioned oxides provided practically the same pzc values as the corresponding ones achieved by using four different techniques as well as the corresponding isoelectric point (iep) values determined by microelectrophoresis. Differences between the pzc and iep values determined using various techniques in the case of MgO were attributed to the increasing dissolution of this oxide as pH decreases and the adsorption of cations (Mg2+, Na+) on the MgO/electrolytic solution interface.