Optical and X-ray emission spectroscopy of high-power laser-induced dielectric breakdown in molecular gases and their mixtures

Large-scale plasma was created in molecular gases (CO, CO2, N2, H2O) and their mixtures by high-power laser-induced dielectric breakdown (LIDB). Compositions of the mixtures used are those suggested for the early earth's atmosphere of neutral and/or mildly reducing character. Time-integrated optical spectra emitted from the laser spark have been measured and analyzed. The spectra of the plasma generated in the CO-containing mixtures are dominated by emission of both C2 and CN radicals. A vibrational temperature of approximately 10(4) K was determined according to an intensity distribution in a vibronic structure of the CN (B2Sigma(+)u-X2Sigma(+)g) violet band. For comparison, the NH3-CH4-H2-H2O mixture has been irradiated as a model of the strongly reducing version of the early earth's atmosphere. In this mixture, excited CN seems to be significantly less abundant than C2. The LIDB experiments were in the molecular gases carried out not only in the static cell but also using a large, double stream pulse jet (gas puff target) placed in the vacuum interaction chamber. The obtained soft X-ray emission spectra indicate the presence of highly charged atomic ions in the hot core of high-power laser sparks.     

Spark discharge assisted laser induced breakdown spectroscopy

Laser induced breakdown spectroscopy is combined with a spark discharge to operate in a laser triggered spark discharge mode. This spark discharge laser induced breakdown spectroscopy (SD-LIBS) is evaluated for Al and Cu targets in air under atmospheric pressure. Significant enhancement in the measured line intensities and the signal-to-background ratios, which depend on the spark discharge voltage and the laser fluence, is observed in spark discharge laser induced breakdown spectroscopy when compared to laser induced breakdown spectroscopy alone for similar laser conditions. The measured line intensities increase with the applied voltage for both targets, and the ratio of the measured line intensity using spark discharge laser induced breakdown spectroscopy to that using laser induced breakdown spectroscopy is found to increase as the laser fluence is decreased. For Al II 358.56, such intensity enhancement ratio increases from 50 to 400 as the laser fluence is decreased from 48 to 4 J/cm² at an applied voltage of 3.5 kV. Thus, spark discharge laser induced breakdown spectroscopy allows for using laser pulses with relatively low energy to ablate the studied material, causing less ablation, and hence less damage to its surface. Moreover, applying spark discharge laser induced breakdown spectroscopy gives up to 6-fold enhancement in the S / B ratio, compared to those obtained with laser induced breakdown spectroscopy for the investigated spectral emission lines.     

Effects of equivalence and fuel ratios on combustion characteristics of an RCCI engine fueled with methane/n-heptane blend

Rising fuel costs and efforts for reducing greenhouse gases have led researchers to propose optimized models of combustion which have high efficiency and low emissions. Reactivity controlled compression ignition (RCCI) engines are attractive due to their high efficiency and low NO_x and soot emissions over a wide range of operating conditions. In this study, methane and n-heptane are used as low and high reactive fuels, respectively, to create suitable fuel stratification within the cylinder. Modeling is carried out by AVL FIRE coupled with a chemical kinetics solver to investigate the effects of fuel ratio, initial temperature and equivalence ratio on the combustion performance and emission characteristics. Methane/n-heptane ratios are varied according to the energy ratio of each fuel while total input energy and total equivalence ratios are fixed. By increasing methane energy ratio from 65% to 85% in the constant intake temperature and pressure, the mixture Octane number increases, which would lead to an increase in ignition delay up to 5 crank angles. As a result, IMEP would be enhanced and also NO_x emission decreases because of lower combustion temperature. By increasing intake temperature, the maximum in-cylinder pressure, heat release rate and NO_x emission would increase significantly while soot emission decreases, and also ringing intensity increases up to 10%. On the other hand, increasing intake temperature reduces volumetric efficiency; as a result, IMEP is reduced by 11%. Also by increasing equivalence ratio from 0.35 to 0.55 in a constant energy ratio, noticeable growth in the maximum amount of pressure and temperature could be achieved; consequently, NO_x emission would increase significantly, IMEP increases by 43%, and ISFC decreases by 30%. The results indicate that these parameters have significant effects on the heavy-duty RCCI engine performance and emissions.

     

通用小型汽油机催化器应用研究

简要介绍了中国通用小型汽油机行业的现状及美国EPA对于通用小型汽油机的排放要求,并结合国内通用小型汽油机的排放统计现状,指出了催化器在通用小型汽油机行业应用的技术可能性,并针对通用小型汽油机的排放特点,指出了催化器应用于通用小型汽油机需注意的问题.     

Ignition of Quiescent Lean Propane–Air Mixtures at High Pressure by Nanosecond Repetitively Pulsed Discharges

We present an experimental study of lean mixture ignition by nanosecond repetitively pulsed (NRP) discharges. The plasma is created in a lean propane/air mixture at pressure up to 10 bar and equivalence ratio 0.7, premixed in a constant volume vessel. We characterize the initial spark radius, the ignition kernel development and the flame propagation as a function of pressure (up to 10 bar) and the pulse energy (1–6 mJ per pulse). Comparisons with a conventional igniter show that better results are obtained with NRP discharges in terms of flame propagation speed, in particular at high pressure, due to the increased wrinkling of the flame front that is induced by NRP discharges.     

Comparison of laser induced breakdown spectroscopy and spark induced breakdown spectroscopy for determination of mercury in soils

Mercury is a toxic element found throughout the environment. Elevated concentrations of mercury in soils are quite hazardous to plants growing in these soils and also the runoff of soils to nearby water bodies contaminates the water, endangering the flora and fauna of that region. This makes continuous monitoring of mercury very essential. This work compares two potential spectroscopic methods (laser induced breakdown spectroscopy (LIBS) and spark induced breakdown spectroscopy (SIBS)) at their optimum experimental conditions for mercury monitoring. For LIBS, pellets were prepared from soil samples of known concentration for generating a calibration curve while for SIBS, soil samples of known concentration were used in the powder form. The limits of detection (LODs) of Hg in soil were calculated from the Hg calibration curves. The LOD for mercury in soil calculated using LIBS and SIBS is 483 ppm and 20 ppm, respectively. The detection range for LIBS and SIBS is discussed.     

Emission spectra of nitrous oxide supported acetylene flames at atmospheric pressure

The emission spectra of a premixed flame of acetylene supported by nitrous oxide have been recorded under different fuel-gas mixture conditions. The emission spectra in these flames of a series of metals, for which it is difficult to obtain a significant population of ground state atoms for atomic absorption spectroscopy in more conventional flames, have also been studied. The red secondary zone which is present in the fuel-rich flames shows emission attributable to long-lived CN and NH species which form a strongly reducing atmosphere to inhibit refractory oxide formation from elements such as molybdenum, titanium and aluminium introduced into the flame. An attempt has also been made to explain some of the reactions which may occur between the flame species above the primary reaction zone.     

Optical Diagnostics of the Effect of Oxygen on Bi-Level Contact Etch

The effect of oxygen flow rate on bi-level contact etch was studied by observing uv-visible emission from the plasma, during CHF₃/CO/O₂ etching of di-electric layers consisting of SiO₂ and SiN_x. The emission intensity of CN at 387 nm drifted progressively from wafer to wafer during plasma etch. Such a phenomenon became more obvious when using low or high oxygen flow rate, whereas for intermediate flow rates, no significant drift of emission intensity was observed. The critical dimension (CD) bias of each wafer showed a strong correlation with CN emission intensity. Possible mechanisms for such an intensity drift phenomenon are proposed. The drift of emission intensity indicates that the contribution of chamber wall polymers in wafer etching is non-negligible. The CN emission intensity is an indication of the magnitude of etching rate. Our results suggest that the variation of plasma emission intensity might be used as an index for in-line monitoring of CD bias fluctuation.     

三维直接数值模拟部分预混发动机内的燃烧过程

采用三维直接数值模拟方法研究了一个类似于部分预混燃烧(PPC)发动机条件下高辛烷值燃料PRF70的着火过程。文章采用了简化的PRF化学动力学机理,包含33个组分和38步基元反应。计算中根据发动机的几何尺寸和真实运行工况加入了气缸内压缩/膨胀的效果,并考虑了燃料的两次喷射,其中第一次喷射形成了较均匀的混合气,第二次燃料喷射增加了混合物分层。研究发现,PPC的燃烧过程非常复杂,是均质压燃、预混燃烧和扩散燃烧三种主要燃烧模式的结合。在两次燃料喷射之间的区域为近化学计量比燃烧,是氮氧化物的生成区;而在化学计量比(φ)大于2的区域,混合不充分聚集了大量未燃碳氢和CO。文章使用Marching cube算法捕捉了三维火焰锋面随时间的变化。最后,使用反应锋面上高斯曲率(k_g)与平均曲率(k_m)的联合概率密度函数(PDF)以及平均曲率随时间变化的概率密度函数,揭示了球形火焰锋面和马鞍形火焰锋面的存在,前者占主要地位,并且随着燃烧的进行,负曲率增加,主要是因为中心的燃料浓区在逐渐消耗。     

Temperature measurements in a decaying laser-induced plasma in air at elevated pressures

This article discusses two measurement techniques for temperature determination of laser-induced plasmas in a gas at pressures relevant for combustion engines. Plasmas induced by laser breakdown in air at initial pressures ranging from 0.3 MPa to 2.5 MPa are investigated using optical spectroscopy. Results for 0.8 MPa, 1.2 MPa and 1.6 MPa are reported here. Due to the elevated pressure, a significant contribution from continuum radiation is apparent. The first temperature measurement technique relies on the interpretation of the continuum emission. The second technique is based on the line emissions from different elements and ionization stages in the plasma and is implemented with the multi-element Saha-Boltzmann plot method. The methodology may be applicable for temperature measurements under various conditions, e.g., for plasmas in high pressure gas environments such as in industrial applications of laser-induced breakdown spectroscopy or for plasma sources for illumination purposes. We investigate optimizations of laser-induced spark ignition. The energy released in the laser-induced plasma is determined based on temperature measurements.     

Analysis of explosive and other organic residues by laser induced breakdown spectroscopy

With the aim of realizing a compact instrument for detection of energetic materials at trace levels, laser induced breakdown spectroscopy was applied on residues from nine explosives in air surroundings. Different potentially interfering organic materials were also analyzed. The residues were not uniformly distributed on an aluminum support and single-shot discrimination was attempted. For a single residue type, large shot-to-shot fluctuations of the line intensity ratios characteristic for organic samples were observed, which made material classification difficult. It was found that both atomic and molecular emission intensities, as well as their ratios, are strongly affected by an amount of the ablated support material, which mainly determines the plasma temperature. With respect to the spectra from the clean support, emission intensities of atomic oxygen and nitrogen are always reduced in the presence of an organic material, even if its molecules contain these elements. This was attributed to chemical reactions in a plasma containing carbon or its fragments. Hydrogen atomic emission depends strongly on the local humidity above the sampled point and its line intensity shows shot to shot variations up to 50%, also on a homogeneous sample. It is argued that shock waves generated by previous spatially and/or temporally close laser pulses blow away a relatively heavy water aerosol, which later diffuses slowly back towards the sampled point. C₂ and CN exhibit a peak emission behavior with atomic Al emission, and their variable ratio indicates an existence of different formation or removal mechanisms from the plasma, depending on the plasma parameters and on the composition of the organic residue. On the basis of these observations, an attempt is made to establish a suitable procedure for data analysis and to determine the optimal experimental conditions, which would allow for discrimination of explosives from other, potentially interfering, residues.     

A Better Understanding of the Very Low-Pressure Plasma Polymerization of Aniline by Optical Emission Spectroscopy Analysis

In the wavelength range from 200 up to 1000 nm, optical emission from electronically excited fragments (CN, CH, NH, C₂, H₂, N₂, N₂⁺, and H-Balmer) is detected when aniline plasmas are generated in a multi-dipolar microwave reactor. The optical emission spectrum monitored in very low-pressure conditions (~?1 mTorr) shows important characteristics. The H_α, H_β and CN species are the most radiating systems and according to the NH/N₂ intensity ratio, two different operating regimes are observed suggesting a change in the reaction pathways. These two regimes are correlated to changes of the plasma characteristics (electron temperature and density) deduced from Langmuir probe measurements. The plasma thermodynamic state is quantified by implementing numerical simulation codes for synthetic spectra calculations. The rovibrational temperatures (T_r, T_v) are determined for some neutral species (CN, CH). The obtained values of T_r and T_v show the non-local thermodynamic equilibrium of the vibrational and rotational states (T_r?<?T_v). Moreover, the very low pressure aniline-based plasmas deviate substantially from the Boltzmann distribution. Correlation between the optical emission data and the solid phase analysis allows proving that the in situ characterization of the plasma phase is an important predictive tool of physico-chemical properties of the film. From these correlated data, we deduce preponderant chemical reaction pathways which help to better understand the plasma generation. Relative contributions of the dehydrogenation of C–H and N–H groups are established in order to deduce the leading initiation reaction for H-Balmer line formation.     

Dynamic effects of a pre-ablation spark in the orthogonal dual-pulse laser induced breakdown spectroscopy

We have observed dynamic effects of a pre-ablation spark on the signal intensity in the orthogonal dual-pulse laser-induced breakdown spectroscopy. We applied pre-ablation and ablation laser pulses with significantly reduced energy for an aluminum metal in open air. Under this experimental condition, the well-known signal enhancement through the increase in ablated mass was negligible. The Al I and II emissions were investigated by both top-view and spatially-resolved side-view collection modes. In this low laser power regime, dynamic effects of a pre-ablation spark on the signal intensity were clearly revealed. The principal factor of signal enhancement is the increase in temperature. Without the mass removal enhancement, effective rarefaction leads to decrease in the Al I emission intensity and simultaneous increase in the Al II emission intensity. This is attributed to the role of Saha equilibrium. Selective prolongation of emission lifetime only for the enclosed part of the analyte plasma in the rarefied region and other fluid-dynamic effects of a pre-ablation spark have been visualized by wavelength-selected time-space correlation maps of plasma emissions.     

Towards a two-dimensional laser induced breakdown spectroscopy mapping of liquefied petroleum gas and electrolytic oxy-hydrogen flames

Two-dimensional mapping of the laser-induced breakdown spectroscopy (LIBS) signal of chemical species information in liquefied petroleum gas (LPG) and electrolytic oxy-hydrogen (EOH) flames was performed with in situ flame diagnostics. Base LIBS signals averaged from measurements at wavelengths of 320 nm to 350 nm describe the density information of a flame. The CN LIBS signal provides the concentration of fuel, while the H/O signal represents the fuel/air equivalence ratio. Here, we demonstrate the meaningful use of two-dimensional LIBS mappings to provide key combustion information, such as density, fuel concentration, and fuel/air equivalence ratio.     

Vibrational emission analysis of the CN molecules in laser-induced breakdown spectroscopy of organic compounds

Laser-induced breakdown spectroscopy (LIBS) of organic materials is based on the analysis of atomic and ionic emission lines and on a few molecular bands, the most important being the CN violet system and the C₂ Swan system. This paper is focused in molecular emission of LIBS plasmas based on the CN (B²Σ–X²Σ) band, one of the strongest emissions appearing in all carbon materials when analyzed in air atmosphere. An analysis of this band with sufficient spectral resolution provides a great deal of information on the molecule, which has revealed that valuable information can be obtained from the plume chemistry and dynamics affecting the excitation mechanisms of the molecules. The vibrational emission of this molecular band has been investigated to establish the dependence of this emission on the molecular structure of the materials. The paper shows that excitation/emission phenomena of molecular species observed in the plume depend strongly on the time interval selected and on the irradiance deposited on the sample surface. Precise time resolved LIBS measurements are needed for the observation of distinctive CN emission. For the organic compounds studied, larger differences in the behavior of the vibrational emission occur at early stages after plasma ignition. Since molecular emission is generally more complex than that involving atomic emission, local plasma conditions as well as plume chemistry may induce changes in vibrational emission of molecules. As a consequence, alterations in the distribution of the emissions occur in terms of relative intensities, being sensitive to the molecular structure of every single material.     

Diagnosis of lubricating oil by evaluating cyanide and carbon molecular emission lines in laser induced breakdown spectra

To prevent engine failure it is essential to change lubricating oil regularly before it loses its protective properties. It is also necessary to monitor the physical and chemical conditions of the oil to reliably determine the optimum oil-change intervals. The present work focuses on studying evolution of the cyanide (CN) and carbon (C₂) molecular spectral emission lines in the laser induced breakdown spectra of lubricating oil as a function of its consumption. The intensities of these molecular bands have been taken as indicator of engine oil degradation at certain mileage. Furthermore, the percentage of decay of CN and C₂ integral intensity values at the corresponding mileage was calculated in order to relate it to the degree of consumption of the motor oil. Such percentage decay of the CN and C₂ integral intensities have been found to increase gradually with increasing mileage which is accompanied with increasing depletion of engine oil. The results of using LIBS technique in the present measurements proved that it is possible to have a direct, straightforward and easy method for prediction of lubricating oil degree of consumption. This may facilitate scheduling the proper time and/or mileage intervals for changing the oil to avoid any possibility of engine failure.     

Kinetic model of atomic and molecular emissions in laser-induced breakdown spectroscopy of organic compounds

A kinetic model previously developed to predict the relative intensities of atomic emission lines in laser-induced breakdown spectroscopy has been extended to include processes related to CN and C₂ molecular emissions. Simulations with this model were performed to predict the relative excited-state populations. The results from the simulations are compared with experimentally determined excited-state populations from 1,064 nm laser irradiation of organic residues on aluminum foil. The model reasonably predicts the relative intensity of the molecular emissions. Significantly, the model reproduces the vastly different temporal profiles of the atomic and molecular emissions. The latter are found to extend to much longer times after the laser pulse, and this appears to be due to the increasing concentration of the molecules versus time. From the simulations, the important processes affecting the CN and C₂ concentrations are identified.     

Low cost bifunctional initiators for bidirectional living cationic polymerization of olefins. II. hyperbranched styrene–isobutylene–styrene triblocks with superior combination of properties

Both bifunctional initiators, the new low cost bBCB‐diCl [4,9‐dichloro,2,4,7,9‐tetramethyl‐tricyclo[6.2.0.0³⁶]deca‐1(8),2,6‐triene] and the universally used “hindered” HDCCl [1‐(tert‐butyl)‐3,5‐bis(2‐chloropropan‐2‐yl)benzene] induce the living bidirectional block copolymerization of isobutylene (IB) followed by styrene (St), and produce PSt‐b‐PIB‐b‐PSt (SIBS) triblocks. We discovered that the molecular weights of triblocks kept significantly increasing long after St conversion reached completion during syntheses. Results were explained by the formation of blends consisting of the expected linear SIBS plus hyperbranched SIBS, HB(SIBS)_n. The structure of high molecular weight (>10⁶ g/mol) HB(SIBS)_n was characterized by various techniques, and key properties of SIBS/HB(SIBS)_n blends were investigated. The mechanism of HB(SIBS)_n formation and the synthesis of SIBS/HB(SIBS)_n blends was elucidated. The properties of SIBS/HB(SIBS)_n blends are superior to those of SIBS. Thus, whereas SIBS exhibits ∼25 MPa tensile strength and ∼450% elongation, SIBS/HB(SIBS)_n blends exhibit 25–27 MPa tensile strength and >400% elongation; deformation under constant load of SIBS is ∼12%, whereas that of SIBS/HB(SIBS)_n is <1%; permanent set of SIBS is 1.3% whereas that of SIBS/HB(SIBS)_n is <0.5%. SIBS/HB(SIBS)_n blends also exhibit higher yield, yield strength, and toughness than SIBS. The microstructure/property relationship of HB(SIBS)_n is discussed and the reasons for enhanced properties of SIBS/HB(SIBS)_n blends are analyzed. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 705–713     

Methyl formate oxidation: Speciation data,laminar burning velocities,ignition delay times,and a validated chemical kinetic model

The oxidation of methyl formate (CH₃OCHO) has been studied in three experimental environments over a range of applied combustion relevant conditions:

• 1. A variable‐pressure flow reactor has been used to quantify reactant, major intermediate and product species as a function of residence time at 3 atm and 0.5% fuel concentration for oxygen/fuel stoichiometries of 0.5, 1.0, and 1.5 at 900 K, and for pyrolysis at 975 K.
• 2. Shock tube ignition delays have been determined for CH₃OCHO/O₂/Ar mixtures at pressures of ≈ 2.7, 5.4, and 9.2 atm and temperatures of 1275–1935 K for mixture compositions of 0.5% fuel (at equivalence ratios of 1.0, 2.0, and 0.5) and 2.5% fuel (at an equivalence ratio of 1.0).
• 3. Laminar burning velocities of outwardly propagating spherical CH₃OCHO/air flames have been determined for stoichiometries ranging from 0.8–1.6, at atmospheric pressure using a pressure‐release‐type high‐pressure chamber.

A detailed chemical kinetic model has been constructed, validated against, and used to interpret these experimental data. The kinetic model shows that methyl formate oxidation proceeds through concerted elimination reactions, principally forming methanol and carbon monoxide as well as through bimolecular hydrogen abstraction reactions. The relative importance of elimination versus abstraction was found to depend on the particular environment. In general, methyl formate is consumed exclusively through molecular decomposition in shock tube environments, while at flow reactor and freely propagating premixed flame conditions, there is significant competition between hydrogen abstraction and concerted elimination channels. It is suspected that in diffusion flame configurations the elimination channels contribute more significantly than in premixed environments. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 527–549, 2010     

Space-resolved analysis of trace elements in fresh vegetables using ultraviolet nanosecond laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been applied to analyze trace elements contained in fresh vegetables. A quadrupled Nd:YAG laser is used in the experiments for ablation. Analyzed samples come from local markets and represent frequently consumed vegetables. For a typical root vegetable, such as potato, spectral analysis of the plasma emission reveals more than 400 lines emitted by 27 elements and 2 molecules, C₂ and CN. Among these species, one can find trace as well as ultra-trace elements. A space-resolved analysis of several trace elements with strong emissions is then applied to typical root, stem and fruit vegetables. The results from this study demonstrate the potential of an interesting tool for botanical and agricultural studies as well for food quality/safety and environment pollution assessment and control.