有源等离子体在开放大气环境下的反应扩散过程研究

利用一维模型用数值模拟的方法,研究了低气压开放环境下大气等离子体在存在等离子体源的情况下的反应扩散过程.得到了考虑化学反应、扩散以及漂移共同作用下的大气等离子体的主要成分随等离子体注入流量的变化规律及一定流量情况下主要带电成分随时空的演化规律. 将数值模拟结果与一个近似解析公式相衔接,估计达到稳态时维持一定电子密度所需要的等离子体流量,可据此进一步估计所需功率. 关键词： 大气等离子体 等离子体源 数值模拟 反应扩散过程     

含氧等离子体中臭氧形成过程数值模拟

利用数值模拟程序对不同条件下氧等离子体和大气等离子体中臭氧的形成过程进行数值模拟. 结果发现：臭氧的形成与游离态氧原子密切相关;臭氧的最大数密度与初始电子数密度有关;臭氧最大数密度持续的时间与脉冲放电大气等离子体中放电间隔时间选取相关,最佳的放电间隔是游离态氧原子达到最大的时间. 关键词： 等离子体 数值模拟 臭氧产生     

氩等离子体射流在空气环境中冲击平板层流流动与传热的数值模拟

本文采用组合扩散系数方法处理不同气体组分之间的扩散，对氩等离子体的流射入空气环境并撞击平板时的层流流动和传热进行了数值模拟．这种新的处理混合气体中质量扩散的方法有助于更准确地描述等离子体条件下的组分扩散与能量输运。文中给出了射流中速度、温度及氩质量分数的分布情况，以及基板处热流密度分布的若干典型的数值模拟结果．     

大面积表面波等离子体的特性研究

介绍一种矩形反应腔体结构的表面波等离子体源，通过对垫层介质及天线结构的优化设计，产生了大面积均匀的等离子体.采用静电探针测量了等离子体参数(密度，电子温度)在不同运行条件(气压，功率)下的空间分布；采用扩散模型，数值模拟了等离子体密度在垂直方向上的空间分布，并比较好地解释了实验测量结果. 关键词： 表面波等离子体 狭缝天线 数值模拟 探针测量     

再入飞行器等离子体预测与气体组分相关性

对一类大钝头再入飞行器,理论分析电子密度预测与气体组分的相关性,使用经验证的数值模拟方法研究化学反应模型对等离子体预测的影响.研究发现:马赫数是影响等离子体预测与气体组分相关性的重要参数,马赫数越高,不同气体组分模型所得电子密度差异越大.气体组分模型对等离子体预测的影响在驻点强压缩区域和身部位置基本一致.对于大钝头再入飞行器,高度H=60 km,马赫数大于23时应该采用11组分化学反应模型.     

热化学模型对高超声速磁流体控制数值模拟影响分析

针对等离子体流场的模拟准确性问题及其对高超声速磁流体控制的影响,通过数值求解三维非平衡Navier-Stokes流场控制方程和Maxwell电磁场控制方程,建立了三维低磁雷诺数磁流体数值模拟方法及程序,分析了不同空气组分化学反应模型和壁面有限催化效率等因素对高超声速磁流体控制的影响.研究表明:不同空气组分化学反应模型对高超声速磁流体流场结构、气动力/热特性控制的影响不容忽视;对于本文计算条件,Park化学反应模型在组分模型一致性、等离子体模拟准确性等方面具有一定优势;磁控热防护效果,受壁面有限催化复合系数影响较大,两者呈非线性关系,不同表面区域差异较大;磁场对磁阻力伞及其磁阻力特性影响,受壁面催化效应的影响相对较小.     

几种典型化学物质的电离层释放效应研究

建立了一个包括中性释放气体扩散、离子化学反应及等离子体扩散等过程的化学物质电离层释放三维动力学模型,基于该模型构建了化学物质释放数值模拟软件平台,对H₂O,CO₂,H₂和SF₆等几种典型化学物质的电离层释放进行了数值模拟,给出了这些物质在电离层释放后产生的扰动特性和释放区域主要粒子的时空变化规律. 关键词： 典型化学物质释放 三维动力学模型 电离层扰动效应     

大气等离子体中负氧离子产生和演化过程数值模拟

文章利用数值模拟程序对不同条件下大气等离子体中负氧离子的形成及演化过程进行数值模拟.结果发现:负氧离子数密度衰减过程中出现拐点,进入一个较为平稳的状态;平稳期负氧离子数密度与初始电子数密度近似成线性关系;水分含量对负氧离子的演化过程具有重要影响,水分含量越高,负氧离子或其团簇数密度平稳期后的衰减越缓慢. 关键词： 等离子体 数值模拟 负氧离子     

电子束产生大尺度等离子体过程的数值模拟研究

建立了一个四组分一维混合模型，对电子束注入大气产生大尺度等离子体的过程进行了数值模拟.结果表明了能量为140keV、流强为50mA/cm²的注入电子束，可以产生线度为0.5m，密度为10¹²cm^-3量级的大气环境下等离子体.电子束所伴随的空间电荷效应由于等离子体的产生会很快消失，不影响后续的等离子体产生过程.电子束注入流强主要影响产生等离子体的密度，而电子束能量则同时影响其空间线度和密度. 关键词： 电子束 碰撞 电离     

等离子体电极普克尔盒电光开关单脉冲过程数值模拟

采用流体模型对等离子体电极普克尔盒(PEPC)电光开关单脉冲过程进行了数值模拟分析.模型包括带电粒子连续性方程、动量守恒方程、电子平均能量方程及空间电位泊松方程.分别采用隐式指数差分格式，超松弛迭代法(SOR)和经典四阶龙格-库塔法(R-K)对带电粒子连续性方程，泊松方程和电子平均能量方程进行数值求解.模拟分析了PEPC单脉冲过程中的带电粒子浓度、电子温度、空间电场、PEPC的放电电流、晶体两侧电压和开关效率的时间演化特性.模型得出了PEPC中气体放电等离子体的微观物理过程与PEPC宏观参量的关系，对设计 关键词： 等离子体电极普克尔盒 电光开关 数值模拟 气体放电     

Characterization of electronegative plasmas

The characterization of the plasma state is of great interest in industrial applications based on plasma enhanced chemical vapour deposition (CVD) processes. We have performed experiments on a capacitively coupled radio frequency discharges in air and SF₆. The use of gases containing electronegative components, such as oxygen or fluorine, leads to quite peculiar discharges, due to the presence of negative ions which affects the transport properties of such a plasma. Plasma parameters have been measured by means of movable electrostatic Langmuir probes. The implementation of a suitable numerical model of gas-phase chemistry and transport phenomena allow us to predict the amount of negative ions. In particular we show that the ion to electron density ratio strongly depends on the diffusion process and on ion recombination rates. Thus measuring it leads to a better understanding of ion diffusion and in particular of the ambipolar electric field.     

Spatial correlations in nonequilibrium systems: The effect of diffusion

We show how the ideas of the fluctuation-dissipation theory can be used to calculate spatial correlations in interacting systems away from equilibrium. The only spatially dependent dissipative process considered is diffusion, and spatial correlations are generated by the nonlocal spatial dependence of the chemical potential. The results are the lowest order in a hierarchy of generalized hydrodynamic type calculations applicable to nonequilibrium systems. We derive equations for the density correlation functions at stationary state supported by diffusive fluxes and show that they have the local equilibrium form. The static correlation function is obtained from the fluctuation-dissipation theorem, which we show to be equivalent to the Ornstein-Zernike integral equation. At equilibrium we demonstrate that the dynamical structure factor obtained by these methods includes the expected wave-vector dependent diffusion constant. Finally we derive a necessary and sufficient condition for local equilibrium to obtain at a stationary state and show by explicit calculation that chemical processes can give rise to significant nonequilibrium correlations.     

A lattice Boltzmann model for coupled diffusion

Diffusion coupling between different chemical components can have significant effects on the distribution of chemical species and can affect the physico-chemical properties of their supporting medium. The coupling can arise from local electric charge conservation for ions or from bound components forming compounds. We present a new lattice Boltzmann model to account for the diffusive coupling between different chemical species. In this model each coupling is added as an extra relaxation term in the collision operator. The model is tested on a simple diffusion problem with two coupled components and is in excellent agreement with the results obtained through a finite difference method. Our model is observed to be numerically very stable and unconditional stability is shown for a class of diffusion matrices. We further develop the model to account for advection and show an example of application to flow in porous media in two dimensions and an example of convection due to salinity differences. We show that our model with advection loses the unconditional stability, but offers a straight-forward approach to complicated two-dimensional advection and coupled diffusion problems.     

Fast ionic transport in Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiS<Subscript>2</Subscript>

The enthalpy of the subsystem of silver ions in the intercalation compounds Ag _x TiS₂ has been calculated from the electrochemically measured thermodynamic functions of the silver subsystem. The ionic conductivity and the coupled chemical diffusion coefficients for silver in the intercalation compound have been measured. The activation energy for diffusion of silver ions is determined and the obtained value is interpreted from analyzing the concentration dependence of the enthalpy of the ionic subsystem. The conclusion has been drawn that the high diffusion mobility is associated with the competition between the covalent and elastic interactions, which decreases the activation energy for diffusion of ions.     

The calculation of the chemical diffusion coefficient for p-type oxides doped with aliovalent impurities

The definition of the chemical diffusion coefficients, obtained by measunng either changes of weight or of electrical conductivity for a pure oxide, is extended to compounds of the same class containing small concentrations of foreign cations having a valency different from that of the ions of the base oxide For simplicity, the treatment is restricted to oxides of type AO showing p-type semiconductivity and containing defects only in the cation subtattice It is shown that in doped oxides the chemical diffusion coefficients are functions not only of the oxygen activity but also of the concentration of the impurity as well as of the ratio between the gradients of these two vanables The chemical diffusion coefficients corresponding to the exclusive presence of gradients of oxygen activity are considered in particular, but the complications ansing in case of different mobilities of the base ions and of the impurity are also pointed out Numerical calculations of the chemical diffusion coefficients are earned out for doped NiO at 1000°C using a convenient model for the defect structure of this oxide, and showing that the addition of a monovalent impurity should produce a slight decrease of the chemical diffusion coefficients while that of a trivalent impurity should produce a large increase of this parameter Finally the theoretical predictions are compared with the few experimental results available so far and possible reasons for the discrepancies observed in some cases are examined.     

Influence of atomic to molecular conversion on the diffusion of ions

An analytical relation which describes the diffusion of positive ions is derived taking into account the process of conversion of atomic ions into molecular ions. The theoretically obtained results can be used for the interpretation of free diffusion measurements, and also for the derivation of the reaction rate coefficient from these measurements.     

The role of the hydration configuration in electronic processes involving ions in aqueous solution

The nature of small ions in electrolytic (e.g., aqueous) solutions is discussed in terms of the configuration of the adjoining polarized medium, and the role of this configuration in electronic transitions involving one or a pair of ions is treated in a general way. Light absorption by a single ion is shown to depend critically on this configuration; a theory developed previously by the authors is reviewed, and future possibilities of exploring the nature of the configuration by investigation of such spectra are explained. Electronic transitions, such as an “electron transfer” process, during binary collisions of ions are treated, and a general method is developed, based uponDebye's “diffusion” theory for ion collision rates and a nonadiabatic type of potential curve. Rates of chemical processes are found to depend decisively on the interaction of two ions at separations where the simple coulomb law is not valid.     

Water softening by combination of ultrasound and ion exchange

Ion exchange resin used in this work was styrene-divinylbenzene co-polymer with sulfonic acid group as a strong acid cation resin. This resin is particularly well suited for the removal of water hardness. In water treatment, commonly used softening processes are chemical precipitation and ion exchange. In this study, a combination of ultrasound and ion exchange was applied for reducing the hardness of water. The rate of exchange or kinetics of ion exchange is governed by several parameters. Therefore, important variables such as intensity of ultrasound, amount of resin, concentration of ions and contact time were investigated. The experimental data related to the removal of magnesium and calcium ions were fitted properly with Langmuir model. The kinetic of removal for both ions was pseudo-first-order. In point of mechanism, the internal porous and film diffusion were both effective in the process. The capacity of sorption and the velocity of removal were higher in the presence of ultrasound than control method and this is related to the cavitation process.     

Evolution of the density profiles and flows of charged particles during the diffusive decay of an electronegative gas plasma

Different scenarios of the spatiotemporal evolution of the parameters of the diffusive decay of a pulsed electronegative gas plasma in the absence of plasma chemical processes are studied. It is shown that nonlinear diffusion in a plasma with negative ions occurs in several stages. The rate of electron density decay increases with time and, in the beginning of the second stage, almost all the electrons escape from the discharge volume. On the other hand, the ion density profile is smoothed out due to ion-ion ambipolar diffusion and the flow of negative ions toward the wall is absent in the first stage of decay. In the second stage, the main diffusion mode is first established and then the ion-ion (electronless) plasma decays exponentially with a characteristic time determined by ion-ion ambipolar diffusion.