等离子体中活性粒子分析及化学动力学机理

利用发射光谱测量技术分析了介质阻挡放电等离子体激励空气产生的主要活性粒子,利用零维等离子体动力学模型模拟了甲烷/空气中放电阶段主要活性粒子的演化规律,并通过敏感性与化学路径分析研究了O原子影响甲烷点火过程的化学动力学机理。研究表明：空气中介质阻挡放电等离子体主要产生N2和O2的激发态粒子,激发态粒子的数密度随着电压的增加而增大;激发态粒子经过一系列物理化学反应最终转化成若干自由基,其中O原子的摩尔分数最大;O原子缩短甲烷点火延迟时间一个量级,原因在于添加O原子后甲基（CH3）的氧化途径由自点火过程中的经O2直接氧化为CH3O和CH2O转变为经HO2和O原子氧化为CH3O和CH2O,由于后者的基元反应速率快,因而明显缩短了点火延迟时间。     

介质阻挡放电等离子体的化学动力学效应研究

为了进一步揭示等离子体强化甲烷点火过程的化学动力学机理,设计和搭建了介质阻挡放电等离子体激励空气的实验系统,实验测量了在空气中介质阻挡放电等离子体激励器产生的发射光谱,利用光谱技术分析了等离子体激励空气产生的若干活性粒子,给出了零维均质点火模型以及敏感性分析和化学路径分析的计算方法,模拟了不同初始温度下NO和O₃对甲烷点火延迟时间的影响,并分析了活性粒子NO和O₃强化甲烷点火的化学动力学过程。研究表明：介质阻挡放电等离子体激励空气主要产生N₂和O₂的若干激发态粒子,并最终转化成存活时间较长的活性粒子NO_x和O₃,等离子体对甲烷点火过程的影响可以简化成活性粒子NO_x和O₃对甲烷点火过程的影响;CH₃的氧化速率决定了甲烷点火过程的快慢,在自点火过程中CH₃的氧化路径是反应式R155和R156,初始温度较低时R155和R156的反应速率慢,所以甲烷的点火延迟时间长;NO缩短点火延迟时间是由于CH₃的氧化路径由自点火过程中的反应式R155和R156改为反应式R327 CH₃O₂+NO=CH₃O+NO₂和R328 CH₃+NO₂=CH₃O+NO;O₃强化甲烷点火过程同样是由于O₃改变CH₃的氧化路径,从化学动力学上缩短点火延迟时间。     

大直径射流的放电特性及其等离子体参数和活性粒子分布的光谱诊断

大气压等离子体射流因其产生的等离子体羽富含活性粒子而在废水净化、元素探测、材料处理等方面具有良好的应用前景.通常等离子体羽的直径较小,限制了其工作效率.针对于此,利用交流电压激励大气压氩气等离子体射流,产生了直径约为14 m m的大尺度均匀等离子体羽.采用发射光谱法对电子密度和氧原子浓度随不同实验参数的变化关系进行了研...     

等离子体化学气相沉积氮化镓薄膜中的N2/TMG等离子体发射光谱分析

对电子回旋共振低温等离子体化学气相沉积工艺(ECR-PECVD)沉积GaN薄膜过程中的氮气和三甲基镓有机金属气体(TMG)混合气体等离子体发射光谱进行分析。结果表明TMG 在等离子体自加热条件下就发生离解,N₂/TMG混合气体ECR等离子体主要以Ga气体粒子和亚稳态氮分子为主。发射光谱特征谱线随微波功率变化分析表明：当微波功率高于400 W时,亚稳态氮分子浓度随着微波功率的增加而增加,而高激发态氮分子以及氮分子离子浓度却随着减少。     

同轴介质阻挡放电中活性粒子的光谱检测

同轴介质阻挡放电具有广泛的应用前景。采用光学方法,利用水电极介质阻挡放电装置,对空气同轴介质阻挡放电特性及活性粒子光谱强度进行了研究。光谱测量结果发现放电发射谱中存在777.5和844.6nm的氧原子谱线,表明等离子体中产生了高化学活性的氧原子。测量了放电中氧原子谱线强度随物理参数(外加电压、流量和含量)的变化关系。结果发现氧原子谱线强度随着外加电压的增加而增强;随着流量或氩气含量(空气中混入少量氩气)的增加,谱线强度先增大后减小,当流量为30L.min-1或者氩气含量为16.7%时氧原子谱线强度达到最大值。     

直流电弧等离子体点火器化学效应研究

为研究直流电弧等离子体点火器的化学效应,测量了点火器喷出的等离子体射流的发射光谱,分析得到等离子体射流与常压环境空气相互作用产生的粒子种类,通过烟气分析仪定量测量了距离点火器出口8cm处的NO和CO含量,并研究了点火器弧电流、工作介质流量对NO和CO体积分数的影响。实验结果表明:等离子体点火器喷出的等离子体射流与环境空气相互作用可产生活性粒子H,O和N,带电粒子O2+,N2+和激发态粒子N2(A3),N2(B3),N2(C3)和O2(b1)等;增大弧电流、工作介质流量,等离子体射流头部附近NO和CO体积分数增大。     

等离子体增强化学气相沉积中对尘埃颗粒诊断

采用SiH4,C2H4和Ar在射频容性耦合柱状放电室中产生了尘埃颗粒,利用发射光谱测得射频尘埃等离子体放电室中的一些基本碎片的发射光谱,并给出了这些碎片的光发射强度随着实验条件变化的曲线。随着功率和气压的增加,碎片的光发射强度逐步增加,尤其是随功率增加得更快,这说明功率对硅烷和乙烯的离解作用明显。随着硅烷和乙烯流量的增加,碎片的光发射强度随之下降。利用朗缪尔探针的实验结果得出尘埃密度的变化趋势,给出了尘埃密度随射频功率变化的曲线,其结果与硅烷和乙烯的离解变化趋势基本吻合。     

等离子体增强化学气相沉积中对尘埃颗粒诊断

采用SiH4,C2H4和Ar在射频容性耦合柱状放电室中产生了尘埃颗粒,利用发射光谱测得射频尘埃等离子体放电室中的一些基本碎片的发射光谱,并给出了这些碎片的光发射强度随着实验条件变化的曲线。随着功率和气压的增加,碎片的光发射强度逐步增加,尤其是随功率增加得更快,这说明功率对硅烷和乙烯的离解作用明显。随着硅烷和乙烯流量的增加,碎片的光发射强度随之下降。利用朗缪尔探针的实验结果得出尘埃密度的变化趋势,给出了尘埃密度随射频功率变化的曲线,其结果与硅烷和乙烯的离解变化趋势基本吻合。     

圆偏振强激光场下等离子体动力学方程及输运

将Ｆｏｋｋｅｒ－Ｐｌａｎｃｋ方程转换到电子在激光场中振荡的坐标中，得到了光场作用下的电子动力学方程；给出了圆偏振激光场作用下的电子－离子、电子－电子碰撞项；在不同的场强区域讨论了等离子体热电输运．研究表明，当电子在激光场的颤动速度接近或者大于其热运动速度时，与没有激光场情况相比，等离子体的热电输运减小很多． 关键词：     

离子液体在甲烷等离子体转化中作用机理的光谱研究

在直流等离子体甲烷转化反应体系中分别引入C₆MIMBF₄,C₆MIMCF₃COO,C₆MIMHSO₄三种离子液体,采用光谱在线技术检测反应中活性物种的种类和光谱谱峰相对强度的变化,研究了离子液体在气液等离子体甲烷转化反应中的作用机理。结果表明：向甲烷等离子体体系中引入离子液体可得到稳定的气液界面,并提高了甲烷转化率和C₂烃产物收率,C₆MIMCF₃COO和C₆MIMBF₄有助于提高C₂烃选择性,而C₆MIMHSO₄则导致C₂烃选择性下降。在气液等离子体甲烷放电体系中检测到了C,C₂,C₃,CH,H等活性物种的发射光谱,与未引入离子液体时相比大多数活性物种的谱线强度均有增加。核磁共振研究表明反应后离子液体C₆MIMBF₄结构稳定,可认为离子液体作为液体导电介质提高了等离子体放电强度,促进等离子体区气相反应过程,同时离子液体在气液表面反应过程起催化作用。     

Temporal structure of gas temperature fluctuations and ignition of fine particles

The paper studies ignition of fine particles, i.e., irreversible growth of particle temperature from an exothermal heterogeneous reaction, with the rate approximated with the Arrhenius law. The particles are suspended in gas with fluctuating temperature, and heat transfer from the particle surface occurs according to the Newtonian law. The equations take into account the temporal structure of gas temperature fluctuations. Modern methods of functional analysis were applied for deriving a closed equation for the probability density function for the particle temperature distribution. The gas temperature fluctuations lessen the threshold for the particle ignition in the hot gas as compared with the deterministic variant. The equations for probability density function produce a closed system of conjugate equations for the average temperature and dispersion of particle temperature fluctuations. The results of simulation illustrate the phenomenon of self-speeding drift of particle temperature towards the temperature of ignition startup.     

Synthesizing aluminum particles towards controlling electrostatic discharge ignition sensitivity

Aluminum particles were synthesized with shell thicknesses ranging from 2.7 to 8.3 nm and a constant diameter of 95 nm. These fuel particles were combined with molybdenum trioxide particles and the electrostatic discharge (ESD) sensitivity of the mixture was measured. Results show ignition delay increased as the alumina shell thickness increased. These results correlated with electrical resistivity measurements of the mixture which increased with alumina concentration. A model was developed using COMSOL for ignition of a single Al particle. The ignition delay in the model was consistent with the experimental results suggesting that the primary ESD ignition mechanism is joule heating.     

Experimental investigation of effects of airflows on plasma-assisted combustion actuator characteristics

The effects of the airflow on plasma-assisted combustion actuator(PACA) characteristics are studied in detail. The plasma is characterized electrically, as well as optically with a spectrometer. Our results show that the airflow has an obvious influence on the PACA characteristics. The breakdown voltage and vibrational temperature decrease, while the discharge power increases compared with the stationary airflow. The memory effect of metastable state species and the transportation characteristics of charged particles in microdischarge channel are the dominant causes for the variations of the breakdown voltage and discharge power, respectively, and the vibrational temperature calculated in this work can describe the electron energy of the dielectric barrier discharge plasma in PACA. These results offer new perspectives for the use of PACA in plasma-assisted combustion.     

Spectroscopic measurements of excited particles in a N2 gas RF plasma-assisted carbon laser ablation

In this work, we investigated a carbon plasma plume produced by laser ablation of a graphite target in a nitrogen gas environment. The spatial distributions of C and N atoms were measured by time-resolved absorption spectroscopy. The spatial distributions of the relative densities of CN radicals, C₂, and C₃ molecules were measured using time-resolved emission spectroscopy. We determined that nitrogen gas produced an increase in carbon atom and molecule densities in the ablation plume. It was observed that the addition of RF plasma to the plume increased the CN radicals and C atom densities, and decreased the C₂ and C₃ molecule densities. The RF plasma changed the evolution of various fractional species of C, N, CN, C₂, and C₃ in the ablation plume. The chemical reactions with and without RF plasma were explained using the evolution and density of the fractional species of C, N, CN, C₂, and C₃in the plume. PACS 52.38.Mf; 42.62.Fi; 33.20.-t; 81.05.Uw     

Elucidating NO coupling effects on ignition of toluene reference fuels by chemical functional group analysis

Derived cetane number (DCN), Research and Motor Octane Numbers (RON and MON) have been fundamentally analyzed using Quantitative Structure-Property Relationship (QSPR) regression models with key chemical functional groups. Both RON and MON exhibit strong sensitivities to the abundances of (CH₂)_n and benzyl-type groups but lack sensitivity to the CH₃ group, most dominant in real gasolines. Residual and EGR gases contain NO_x known to synergize with fuel autoignition chemistry. Two TRF mixtures having high and low aromatic content but sharing the same RON and MON values were used to evaluate NO_x coupling effects. DCN measurements with NO addition were found to be strongly correlated with the abundance of the (CH₂)_n group. Similar experiments of 200 ppm NO in a Rapid Compression Machine show promotion (inhibition) of ignition for the high (low) aromatic TRF fuel. Kinetic modeling attributes the promotion to the NONO₂ interconversion reactions, NO + HO₂ = NO₂ + OH, CH₃ + NO₂ = CH₃O + NO and NO₂ + H = NO + OH. The inhibitive effect relates specifically to low temperature kinetics and high NO loading conditions, leading to the formation of meta-stable species (e.g. CH₃ + NO₂ (+M) = CH₃NO₂ (+M)) that decelerate the rate of conversion of HO₂ to more reactive OH radicals. The coupling of NO with real gasolines depends on chemical composition and temperature conditions not only encompassed by RON and MON criteria, but by the chemical functional group characteristics. The relevance of this finding to the significance of preferential vaporization of multi-component gasolines on low-speed pre-ignition (LSPI) is discussed. Within the context of chemical functional group distributions of five distillation cuts of a marketed ethanol-free gasoline determined by NMR spectroscopy, the analyses identify considerable variations of key functionalities with fuel distillation properties, indicating chemical kinetic autoignition behaviors that are dependent on preferential vaporization.     

Effective potential method for active particles

ABSTRACT

We investigate the steady-state properties of an active fluid modelled as an assembly of soft repulsive spheres subjected to Gaussian coloured noise. Such a noise captures one of the salient aspects of active particles, namely the persistence of their motion and determines a variety of novel features with respect to familiar passive fluids. We show that within the so-called multidimensional unified coloured noise approximation, recently introduced in the field of active matter, the model can be treated by methods similar to those employed in the study of standard molecular fluids. The system shows a tendency of the particles to aggregate even in the presence of purely repulsive forces because the combined action of coloured noise and interactions enhances the effective friction between nearby particles. We also discuss whether an effective two-body potential approach, which would allow to employ methods similar to those of density functional theory, is appropriate. The limits of such an approximation are discussed.     

Surface-science aspects of plasma-assisted etching

Plasma-assisted etching methods have been used in the manufacture of integrated circuits for more than 10 years and yet the surface-science aspects of this technology are poorly understood. The chemistry must be such that the reactive species generated in the plasma react with the surface being etched to form a volatile product. The chemistry is usually dominated by atoms, molecular radicals and low-energy (20–500 eV) positive ions. In microstructure fabrication, the positive ions are accelerated from the plasma towards the etched surface arriving essentially at normal incidence. Thus, the bottom surface of a very small feature being etched is subjected to both energetic ions and reactive neutral species, whereas the sidewalls of the feature are exposed to reactive neutral species only. The role of the energetic ions is primarily to accelerate the reaction between the neutral species and the etched surface (i.e., accelerate the etch rate), thereby reducing the steady-state top-monolayer coverage of the etching species on the etched surface. On the sidewalls, however, the reacting-species coverage is a saturation coverage. The present understanding of some of the surface-science aspects of this complex environment will be summarized, often using the Si-F system as an example, and some phenomena which are not well understood will be described.