Thermal Radicals Research Articles

The Influence of Ozone on the Flame Propagation Behavior of n-Heptane/air and i-Octane/air Premixed Flame in an Engine-Relevant Environment

ABSTRACT The promotional effect of ozone on lean premixed spherical flame propagation in an engine-relevant, high-temperature, high-pressure environment was investigated using an optically accessible rapid compression and expansion machine (RCEM). The laminar burning velocity, burned gas Markstein length, and initial flame development time based on mass fraction burned (MFB), were examined for n-heptane/air and i-octane/air premixture (φ = 0.55) with 15 [ppm] ozone, through self-emission observation of a propagating flame with a high-speed camera and combustion pressure analysis. The results showed that ozone reduced the initial flame development time and increased the laminar burning velocity for n-heptane/air premixture under temperature conditions in which the low-temperature oxidation (LTO) reaction was more clearly pronounced; but ozone did not enhance either the initial flame development or the laminar burning velocity in the case of i-octane/air premixture, where the LTO reaction was weak owing to the fuel molecular structure, even under the same temperature conditions. These results suggest that the enhancement of the LTO reaction by ozone, and the subsequent generation of thermal energy and CO radicals through an H2O2 loop reaction in the pre-flame region, were responsible for the increase in the laminar burning velocity. Furthermore, ozone increased the burned gas Markstein length, flame thickness, and Zeldovich number of the n-heptane/air mixture under the temperature conditions in which the laminar burning velocity was enhanced by ozone. This suggests that the chemical reaction in the reaction zone was more active in response to the increase in flame temperature.

Поверхностная функционализация квазидвумерного MoS2 в азотной и кислородной плазме

The paper deals with the experimental and theoretical study of the interaction of quasi-two-dimensional MoS2 films with nitrogen and oxygen plasmas in order to reveal the main mechanisms of their surface functionalization and to analyze its effects on the structure and properties of the films. Diagnostics of samples before and after treatment with radicals and ions was performed ex situ by Raman spectroscopy, spectroscopic ellipsometry, atomic force microscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The results of the experimental study showed that nitrogen and oxygen plasma treatment of ultrathin MoS2 films leads to the modification of the near-surface layers of the samples due to the removal of sulfur and the incorporation of incident atoms into forming vacancies. Nitrogen and oxygen ions with even low (less than 30 eV) energy causes partial removal of upper layers of the films leading to their partial destruction, while remote plasma (mainly by thermal radicals) guarantees more ‘soft’ treatment of films. The obtained spectroscopic data also indicated different effects of radicals (thermal O and N atoms) on the MoS2 surface. To analyze these effects, the computer modelling with density functional theory method was carried out. Its results enables to reveal the main mechanisms of the surface functionalization of MoS2 monolayers and to explain the experimental data.

Inerting effect of carbon dioxide on confined hydrogen explosion

Taking maximum flame propagation velocity, maximum explosion pressure, maximum rate of pressure rise and time-average of rising pressure impulse as index, this paper is aimed at evaluating the inerting effects of carbon dioxide on confined hydrogen explosion by varying initial pressure, carbon dioxide addition and equivalence ratio. The results indicated that under enhancing hydrodynamic instability, the stronger flame destabilization occurs with the increase of initial pressure. At Φ = 0.8 and Φ = 1.0, the destabilization effect of thermodiffusive instability continues to increase with the increase of carbon dioxide addition. At all equivalence ratios, the destabilization effect of hydrodynamic instability decreases monotonously with the increase of carbon dioxide addition. All of maximum flame propagation velocity, maximum explosion pressure, maximum rate of pressure rise and time-average of rising pressure impulse reach the peak value at Φ = 1.5, and decrease significantly with increasing carbon dioxide addition. The inerting effect of carbon dioxide could be attributed to the reduction of thermal diffusivity, flame temperature and active radicals. The chemical effect of carbon dioxide reaches the peak value at Φ = 1.0. With the increase of carbon dioxide addition, the chemical effect continues to decrease at Φ = 0.8 and Φ = 1.0, and increase monotonously at Φ = 2.5.

High density H2 and He plasmas: Can they be used to treat graphene?

Since graphene and other 2D materials have no bulk, a major issue is their sensitivity to surface contaminations, and the development of cleaning processes is mandatory. High density plasmas are attractive to treat (clean, dope, and pattern) 2D materials, because they are a mature industrial technology adapted to large area wafer. However, in these plasmas, the substrate is bombarded by a high flux of both thermal radicals and reactive ions with typical energy above 10 eV, which can easily damage atomic layer thin materials. We have investigated systematically the interaction of H2 and He inductively coupled plasmas (ICPs) with graphene in industrial reactors. We report a specific issue associated with the use of H2 plasma: they etch the inner part of plasma reactor walls, thus releasing impurities in the plasma, most notably O atoms that etch graphene and Si atoms which stick on it. The presence of parasitic oxygen presumably explains the discrepancies found in the literature regarding the impact of reactive plasmas on graphene damages. To get rid of this issue, we propose to use a fluorinated aluminum chamber. In this case, fluorine atoms which are shown to be harmless to graphene are the only impurity in the plasma. Under such conditions, H2 ICP plasma is shown to clean graphene without damages if the ion energy is kept below about 15 eV.

Advances in modifying lignin structures for largely enhancing high-lignin biomass saccharification

Traditional chemical treatments require high energy and concentrated agents to decrease high-lignin biomass recalcitrance during lignin removal. A low-cost and environment-friendly method via modifying the lignin structure without lignin degradation overcomes lignin barrier in wood-feeding termites. In this study, thermal hydroxyl radicals in the biomimetic system performed in high-lignin cassava stillage residues did not reduce total lignin level but altered the lignin monostructure. The major β-O-4′ remained conserved, whereas hydroxylation and modification of selective intermonomer side-chain linkages occurred in the treated lignin. The treatment can enhance enzymatic saccharification with cellulose conversion of cassava stillage residues at 97%, twice higher than that of the control. In addition, the modified lignin exhibited low invalid adsorption capacity into commercial cellulases, thereby improving the performance of cellulosic feedstock. Modifying the lignin structure, instead of removing lignin, provides new insights for development of novel treatments for high-lignin biomass.

Hydrogen-plasma patterning of multilayer graphene: Mechanisms and modeling

We study the atomic-scale etching mechanisms of multilayer graphene, and the subsequent formation of nanopores, when exposed to downstream hydrogen plasma. Our molecular dynamics simulations based on reactive force-field potential reveal precise energy regimes for the transport of ions through, and selective etching of, individual graphene layers within the multilayer structure. Etching initiates with hydrogenation of the graphene basal plane, followed by localized CC bond breaking which leads to the formation of CH2, and subsequently, unstable CH3 bond configurations. We establish the basal plane and edge etching rates of the individual graphene layers as a function of ion energy, and introduce a micromechanics model to predict the 3D-patterned pore structure at experimental length- and time-scales. Our results demonstrate the development of columnar holes in multilayered graphene, which transition to stepped-edge holes at higher fluence due to cumulative effects of basal-plane etching. The contributions of thermal radicals and dehydrogenation effects on the hole growth process are discussed.

Combustion characteristics of a micro segment platinum tubular reactor with a gap

The present study proposes a conceptual design for a combustion chamber for overcoming critical heat loss and flame instability in a micro-thermophotovoltaic power system (micro-TPV). In the design, a platinum tube is used simultaneously as a catalyst reactor and an emitter. The flame-stabilizing mechanisms of a TPV reactor are numerically investigated regarding the effects of reactor configuration and fuel concentration. The interaction of heterogeneous and homogeneous reactions between the inner and outer chamber of the TPV reactor is also examined. Results indicate that the catalytically induced combustion can be anchored in the gap between two segmented platinum tubes by inheriting thermal energy and radicals from the upstream section of the segmented catalyst. The gap not only provides a low-velocity region for stabilizing flames, but also enables the exchange of fuels and radicals from the inner and outer streams.

What makes a dangling bond a binding site for thermal CH3 radicals? — A combined molecular dynamics and potential energy analysis study on amorphous hydrocarbon films

Identification of dangling bonds on amorphous films is not as straight forward as in the case of crystalline materials. The task is further complicated in the case of amorphous hydrocarbon (a-C:H) films by the existence of a wide variety of atomic arrangements. We present a technique based on potential energy analysis of a-C:H films to identify dangling bonds and physisorption sites. However, molecular dynamics simulations of the sticking of thermal CH3 on a-C:H surfaces show that not all dangling bonds are binding sites for a CH3 radical. Furthermore, the total sticking coefficient of the surface is not solely linked to the number of dangling bonds and can even decrease for the same number of dangling bonds because the carbon atoms that possess a binding site, active carbon atoms, show drastically different reactivity towards CH3. The reactivity of active carbon atoms is decided by (a) their type, which is decided by the bonding partners, (b) their distance from the local surface and (c) the local environment. The reactivity of the active carbon atoms can be largely increased by energetic ion bombardment due to hydrogen depletion and local rearrangement.

Elementary processes of H2 plasma-graphene interaction: A combined molecular dynamics and density functional theory study

Elementary interactions between H atoms and monolayer graphene are investigated using classical molecular dynamics (CMD) and density functional theory (DFT). C-H interatomic potential curves and associated energy barriers are reported depending on the H impact position (top, bridge, hollow, vacancy, or edge sites of graphene nanoribbons). Chemisorption of atomic hydrogen and formation of molecular hydrogen from chemisorbed H states on graphene are examined. The influence of graphene temperature and incident species energy on adsorption, reflection, and penetration mechanisms is also presented. Except for impacts at graphene nanoribbon (GNR) edges or at defect locations, H atoms are shown to experience a repulsive force due to delocalized π-electrons which prevents any species with less than 0.4-0.6 eV to chemisorb on the graphene surface. C-H bond formation requires a local sp2-sp3 rehybridization resulting in structural changes of the graphene sample. Chemisorption sites with deep potential wells and no activation barrier are found on GNR edges, which indicate that H thermal radicals can functionalize GNRs on edges while they cannot do it in the basal plane. The presence of one or more H adsorbates on the graphene surface strongly influences subsequent H adsorption and promotes the formation of energetically favourable H pairs at the para- and ortho-locations. Formation of H2 molecule via Eley-Rideal recombination of hot radicals [1–1.3 eV] with chemisorbed H atoms is observed.

The decomposition of 2-pentyl and 3-pentyl radicals

The isomerization and decomposition reactions of 2-pentyl and 3-pentyl radicals have been studied in a single-pulse shock tube over a temperature range of 973–1121K and pressures of 120–800kPa. The results represent the first direct study of the alkene product branching ratio resulting from the kinetics of the competition between isomerization and beta C–C bond scission for a secondary straight-chain alkyl radical at high temperatures. Such species are representative of intermediates important in the combustion of typical hydrocarbon fuels. In the present work, a small quantity of precursor (∼45μL/L) is used to thermally generate H atoms in the presence of excess (E)-2-pentene, leading to the radicals of interest via addition of H to the double bond. Decomposition of the chemically activated pentyl radicals results in the stable olefin products ethene, propene, and 1-butene, which are detected in postshock gas chromatographic analyses utilizing flame-ionization and mass-spectrometric detection. It is shown that the olefin product ratios can be related to the isomerization and decomposition reactions of the 2-pentyl and 3-pentyl radicals and the results are consistent with the existence of distinct non-overlapping cracking patterns for the two radicals. The data are compared with predictions made on the basis of a model developed from experiments on the decomposition of thermal (i.e. not chemically activated) 1-pentyl radicals. Good agreement is observed. In conjunction with an RRKM/Master Equation analysis, the results for 2-pentyl and 3-pentyl radicals are projected over a wide range of temperatures. In addition, the rate constants for addition of H atoms to the alternate double bond positions of (E)-2-pentene are derived relative to a standard reaction and absolute rate constants for these processes are reported.

Gas-phase fragmentation of oligosaccharides in MALDI laser-enhanced in-source decay induced by thermal hydrogen radicals

Laser-enhanced in-source decay (LEISD) MALDI MS recently proposed for structural analysis of oligosaccharides was used to systematically investigate ISD fragmentation of oligosaccharides, which was found to be mediated by thermal hydrogen radicals from a matrix and underwent a charge-induced process, depending on the nature of the matrix and the structure of an oligosaccharide.

Effect of hydrogen on the growth of thin hydrogenated amorphous carbon films from thermal energy radicals

Molecular dynamics simulations have been performed to investigate the growth of thin hydrogenated amorphous carbon (a-C:H) films from radical species with thermal energy. It is found that the incorporation of H into the film increases the mass density. The maximum mass density is reached for a H flux of about 10%. The atom density of the films reaches a maximum at a H flux of about 30%. It is shown that these effects are a result of the change in microstructure of the films, including a H-induced sp to sp2 to sp3 shift. These results are important for thin a-C:H film deposition techniques where chemisorption of radical species is the main growth mechanism.

Growth of thin hydrocarbon films and hydrogen production in a cusp plasma device

A plasma device with a cusp magnetic field configuration and operating under steady state condition has been built to investigate surface deposition of hydrocarbon films at low temperature and, simultaneously, the cracking of methane in a plasma environment to hydrogen production. The gas (methane or a methane/argon mixture) has been fed to the plasma source (a cylindrical capacitively coupled rf type). The addition of a static magnetic field has increased the particle confinement time and, consequently, the hydrogen production rate. Here the preliminary results concerning the growth of thin hydrocarbon films and the dissociation of methane in the cusp plasma are presented. In our case, at the line and point cusp, we have a large flux of energetic particles as well as neutral, ions, thermal radicals that participate in the growth process. A fraction of the converted methane has been deposited on the substrates, at the line cusp, as amorphous hydrocarbon (a-C: H) film and amorphous carbon (a-C) film. The surface morphology of the film has been observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) has been used to determine the structure of the film. The results have indicated that the film has a prevalently amorphous structure with a presence of crystalline clusters.

Can plasma experiments unravel microscopic surface processes in thin film growth and erosion? Implications of particle–beam experiments on the understanding of a-C:H growth

Despite the widespread use of low-temperature plasmas for deposition and erosion of thin films and surface modification the underlying microscopic mechanisms are often not well understood, especially for reactive plasmas. Thorough parameter studies and the use of modern in situ real-time diagnostics of the plasmas as well as of the growing film provide valuable results to optimize the processes. However, they generally allow only an indirect and therefore qualitative or even ambiguous conclusion about the actual film growth processes. One reason is the complexity that is inherently connected to such plasma processes. A vast number of particles with different energy and reactivity is interacting simultaneously with the surface, thus hindering to reveal microscopic growth mechanisms or reliable values of universal constants. One approach to isolate microscopic mechanisms is the use of model experiments with quantified particle beams. In this article model experiments for the example of amorphous hydrogenated carbon film (a-C:H) growth will be motivated and described. The discrepancy between different experimental results and various growth models proposed over the years are briefly reviewed. Recent results of the particle–beam experiment during the last years established at the Max-Planck-Institut für Plasmaphysik are presented. It will be shown, how the sticking probability of thermal methyl radicals is influenced by substrate temperature and how the presence of other radicals such as atomic hydrogen alter the sticking of methyl. Furthermore, it will be shown how ions participate in film growth and the consequences for plasma experiments are discussed.

Trehalose synthesis is induced upon exposure of Escherichia coli to cold and is essential for viability at low temperatures.

Trehalose accumulates dramatically in microorganisms during heat shock and osmotic stress and helps protect cells against thermal injury and oxygen radicals. Here we demonstrate an important role of this sugar in cold-adaptation of bacteria. A mutant Escherichia coli strain unable to produce trehalose died much faster than the wild type at 4 degrees C. Transformation of the mutant with the otsA/otsB genes, responsible for trehalose synthesis, restored trehalose content and cell viability at 4 degrees C. After temperature downshift from 37 degrees C to 16 degrees C ("cold shock"), trehalose levels in wild-type cells increased up to 8-fold. Although this accumulation of trehalose did not influence growth at 16 degrees C, it enhanced cell viability when the temperature fell further to 4 degrees C. Before the trehalose build-up, levels of mRNA encoding OtsA/OtsB increased markedly. This induction required the sigma factor, RpoS, but was independent of the major cold-shock protein, CspA. otsA/B mRNA was much more stable at 16 degrees C than at 37 degrees C and contained a "downstream box," characteristic of cold-inducible mRNAs. Thus, otsA/otsB induction and trehalose synthesis are activated during cold shock (as well as during heat shock) and play an important role in resistance of E. coli (and probably other organisms) to low temperatures.

Photodissociation dynamcis of ClO radicals in the range (237≤ λ≤270) nm and at 205 nm and the velocity distribution of O( 1D) atoms

Isotopically selected photofragmentation of thermal ClO radicals was studied in the continuous and structured region of the A 2Π ← X 2Π transition in the wavelength range 237< λ<270 nm employing resonance enhanced multiphoton ionisation (REMPI) detection schemes. Photoexcitation of ClO generated Cl( 2P 3/2, 1/2 ) and O( 1D) atoms with a quantum yield of unity. The translational energy of O( 1D) was determined and quantified as a function of the wavelength of the incident radiation. Some experiments on the kinetic energy deposited in Cl( 2P 3/2, 1/2 ) are also reported. From the decrease of the O( 1D) REMPI signal intensity approaching the transition from continuous to structured absorption, the cut-off excitation wavelength for O( 1D) in thermal ClO radicals was evaluated to be 263.4±0.2 nm. In the structured region of the spectrum oxygen atoms are formed in the triplet ground state.

EPR of C60 thermal/photochemical reactions with polystyrene and polymethyl methacrylate

Thermal and photochemical free radical reaction products of C60 with polymethyl methacrylate (PMMA) and polystyrene (PS) in orthodichlorobenzene solution were detected by EPR (electron paramagnetic resonance). Thermal radicals (<100°C) of C60/PMMA and C60/PS samples gave single line first-derivative EPR spectra withg=2.0029. Ultraviolet photolysis of a C60/PMMA solid phase sample gave two radical species; whereas, photolysis of a C60/PS solid phase sample gave only one free radical. EPR signals were also recorded for UV and thermal C60 reaction with free radical initiator, azobis(isobutyronitrile).

Energy Resolved Mass Spectrometry of the a-Si:D Film Growth Species During DC Magnetron Sputtering

ABSTRACTDouble modulated beam mass spectrometry was used to obtain the first measurements of the identity, relative flux, and energy of film precursors in reactive dc magnetron sputtering. In these experiments, a 2″ diameter silicon target was sputtered in an argon plus deuterium plasma at an argon pressure of 1.5 mTorr. This system produces high quality a-Si:D. Energetic neutral Si and SiD were observed, while energetic neutral SiD2 and SiD3 were absent. The arrival rate of D in the form of SiD increased and then saturated at 19 percent of the total flux as the deuterium pressure was varied from 0 to 5 mTorr. The sputtered Si energy agreed qualitatively with the standard Thompson theory. Energetic ions located at mass 2 and 4 were measured with energies much greater than the sputtered Si. Thermal silane was also detected, raising the possibility that thermal silane radicals in addition to energetic Si and SiD contribute to film growth.

Preparation of labelled 38Cl hydrocarbons by recoil 38Cl atom reaction and detection by radio-gas chromatographic techniques

A radio-gas chromatographic technique had been applied for the quantitative determination of the radioactive labelled 38Cl compounds obtained from the recoil 38Cl atom reaction with C 2H 4. Argon gas was added to the system as neutron flux monitor to calibrate the total radioactivity of 38Cl produced. I 2 and HI were added as a scavenger to remove the thermal radicals produced. The yields of radioactivity of CF 2Cl 38Cl and C 2H 5 38Cl depended on the initial mole fraction ratio of CF 2Cl 2/C 2H 4. The major product found was C 2H 5 38Cl (>90%) when HI was added as scavenger. In the other system CF 3Cl/C 2H 4/I 2 we studied the yields of inorganic and organic compounds labelled with 38Cl. The yield of H 38Cl as a function of the mole ratio of CF 3Cl/C 2H 4 is shown.

Influence of volatile metal carbonyls on the reactions of thermal radicals in the C 4H 10 + recoil T system

The addition of Ni(CO) 4 and Fe(CO) 5 to the C 4H 10 + recoil T system causes remarkable increase of ethylene-T and light hydrocarbons-T yields. The effect of metal carbonyls on products yield variation has been ascribed to an intermediate containing the metal and the radicals produced in the reactions. By the use of inert moderators and pressure effect, which enhance the radical density, the essential role of T• and alkyl radicals as limiting reactants is discussed.