The repetitive microwave discharge as a catalyst for a chemical reaction

Results are presented from studies on plasma catalysis of the decomposition of methane into hydrogen and carbon in a repetitive microwave discharge. The dependence of the propagation velocity of a plasma channel on the sort of gas is determined; from this dependence, a preliminary conclusion can be drawn about both the mechanism for the development of the discharge and the ion composition of the discharge plasma. The measurements of the electron temperature in the discharge show that the rate at which active particles are produced is high enough to explain the acceleration of a chemical reaction by chain processes with the participation of these particles.     

Modeling of argon discharge characteristics of planar-type surface wave plasmas in an electron fluid model

The planar-type surface wave plasma (SWP) device permits the generation of high-density and uniform processing plasmas via 2.45-GHz microwave power without the application of an external magnetic field. In the present study, the discharge characteristics in the SWP device were analyzed using a two-dimensional numerical simulation code, and the results were compared with experimental observations. The simulation code is based on the finite-difference time-domain (FDTD) method for the microwave field and on the electron fluid model for the argon discharge plasma. Experimental measurements were performed, and they showed that the surface-wave discharge at a filling pressure of 10-100 mtorr has characteristic electron-density distributions that have a peak at approximately 2 cm from the surface. This characteristic of the electron density profiles, as well as the electron temperature profiles in the plasma, is reproduced by the simulation code, albeit with some discrepancies. In order to reduce the effects of these discrepancies, intentional changes in the electron heat conductivity were introduced, and the adiabatic assumption was found to result in a reasonable electron temperature profile. The effects of the alumina window thickness were also investigated in the simulation.     

CF4/Ar微波放电与CS2反应的可见化学发光研究

利用交叉分子束-化学发光研究方法,研究了气相条件下不同反应压力时CF4／Ar微波放电的产物与二硫化碳CS的交叉反应在可见区(300～900nm)的化学发光,利用密度泛函B3LYP和二阶微扰MP2理论分别在6-311 G(3df,2pd)和6-311 G(d,p)基组下计算了可能的反应产物电子基态的构型和振动频率,将得到的光谱标识为F2S2电子激发态的发射谱,通过对光谱和产物的分析,认为微波放电得到的亚稳态的Ar原子与CS发生碰撞反应使CS解离为S原子和CS,解离得到的S原子与微波放电产生的F原子发生多步反应得到了F2S2的激发态。     

Behaviour of fully ionized seed plasma excited by microwave

Nonequilibrium plasmas with cesium metal vapor ionization in helium and argon gases at moderate pressures are excited with microwave power. The structures and behaviour of the seeded plasmas are experimentally examined, particularly under the condition of Full seed (cesium atoms) ionization. By cesium seeding, the minimum power sustaining the plasma is reduced markedly, and both a broad plasma observed in pure helium and unsteady filament-like plasmas in pure argon change to the steady and broad plasma locating close to the inner surface of a discharge tube, it is revealed from the electron temperature measurements that the plasma can be in the regime of full seed ionization for suitable microwave powers, where the electron density is kept almost constant. The thickness of the fully ionized seed (FIS) plasma decreases with increasing the mole fraction of cesium vapor, and is almost independent of noble gas pressure. The thickness almost coincides with the skin depth determined from the electrical conductivity almost uniform in the FIS plasma. These facts suggest that the FIS plasma will be easily produced and maintained as long as the microwave power is consumed to the electron heating     

Measurement of gas solutions in small samples of intermetallic alloys

This article describes a measuring system that was proposed so as to enable measurement of the content of dissolved gas in samples, whose shape is determined by the specific requirements of the simultaneous measurements and cannot be modified to satisfy the requirements on solubility measurements. The apparatus — a dynamic UHV vacuum system — consists of a measuring chamber fitted with a heater based on electron bombardment, which permits a change in the sample temperature according to the selected schedule. The design of the heater permits reduction of additional heating in the actual measuring chamber, so that the increase in pressure caused by the action of the heater can be neglected. The measuring part of the system permits recording of changes in the overall pressure and partial pressures of selected gases in the measuring chamber. The lowest detectable amount of dissolved gas is less than 10⁻⁵ Pa m³. The results of measurement of the solubility of hydrogen in Ti and Fe aluminides in samples that are simultaneously used to measure the electrical conductivity are given as an example of the suitability of the apparatus for such measurements. This work is a part of research plan MSM 0021620834 financed by the Ministry of Education, Youth and Sports of the Czech Republic.     

Corona discharge as a temperature probe of atmospheric air microwave plasma jet

We developed and tested a new method for temperature measurements of near-LTE air plasmas at atmospheric pressure. This method is specifically suitable for plasmas at relatively low gas temperature (800–1700 K) with no appropriate radiation for direct spectroscopic temperature measurements. Corona discharge producing cold non-equilibrium plasma is employed as a source of excitation and is placed into the microwave plasma jet. The gas temperature of the microwave plasma jet is determined as the rotational temperature of N₂? produced in the corona discharge. The corona probe temperature measurement was tested by the use of a thermocouple. We found a fairly good agreement between the two methods after correcting the thermocouple measured temperatures for radiative losses. The corona probe method can be generally applied to determine the temperature of the near-LTE plasmas and contrary to the thermocouple it can be used for higher plasma temperatures and is not affected by radiative losses and problems of interaction with the microwave plasma and electromagnetic fields.     

微波液相放电等离子体发射光谱研究

液相放电能够产生各种活性物质,其中羟基自由基(OH),氢自由基(H)被认为是引发液相化学反应的主要活性物种,但由于其活性强寿命短的特点,测量比较困难,由于缺少标准样品,定量测量更为困难。用光学方法测量自由基是一种直接测量方法,其特点是瞬时在线测量,能立即获得数据,进行时间和空间分布测量。为了研究微波水中放电产生的自由基特性,利用发射光谱诊断技术对微波水中放电产生的活性物质进行了在线检测,考察了微波功率、反应器内部压强对OH自由基相对光谱强度的影响,并观测了等离子体中OH自由基强度的空间分布;同时,估算了微波液相等离子体中的电子激发温度。实验结果表明,微波水中放电可以产生大量的OH,H,O自由基,其中OH自由基的相对光谱强度最强,并随微波功率的增加呈现明显上升的趋势,随反应器内部压强的增大而迅速减弱;以OH为主的自由基主要产生于电极尖端附近。微波液相等离子体的电子激发温度约为0.33×104 K。     

分子发射光谱法测量微波等离子体气体温度

通过OH自由基A2Σ+→X2Π_r电子带系分子发射光谱测温法,实现了对氩气、氮气、空气三种大气压微波等离子体气体温度的测量。探究了不同微波功率、不同气体流量下气体温度的变化规律,测量了氮气、空气微波等离子体羽流的轴向温度分布。实验结果表明,不同工作条件下微波等离子体核心温度普遍超过2 000 K,空气微波等离子体可超过6 000 K;同样工作条件下三种微波等离子体气体温度满足：T_Ar<T_N2<T_Air;气体温度总体上随微波功率增加而小幅增加,随气体流量下降而小幅降低;氮气与空气等离子体羽流温度沿轴向迅速降低。为验证分子发射光谱测温法的准确性,以热电偶测温作为比对,对温度较低的介质阻挡放电氩气等离子体进行了温度测量,实验表明,分子发射光谱法与热电偶所测结果十分接近。     

Photoenhanced deposition of silicon oxide thin films using a novel windowless internal nitrogen discharge lamp

This paper reviews the production of silicon oxide thin films using a novel photoenhanced deposition technique incorporating a windowless nitrogen discharge lamp contained within the deposition vessel. This process has the potential to produce better semiconductor/insulator interfaces than those produced using conventional RF glow discharge techniques, since there is no damage to the growing film surface from energetic ions produced in the glow discharge plasma. The use of an internal lamp obviates the need for a window between the lamp and reaction chamber and thus overcomes the problems of attenuation of short wavelength ultraviolet radiation by the window. All the ultraviolet output of the lamp can now directly couple into the reaction gases, so the need for sensitising agents is also removed. Thin films of silicon oxide have been deposited onto single crystal silicon wafers from nitrous oxide-silane gas mixtures. The physical properties are comparable to those of high quality insulator films deposited by plasma enhanced techniques. The results of electrical measurements indicate that this material is of sufficiently high quality to be used as a low temperature deposited gate dielectric for thin film devices. To confirm this, thin film transistors (TFTs) have been fabricated using the material (and an active layer of amorphous silicon deposited in the same system) and characteristics of these TFTs are also presented.     

Method of determining the sodium diffusion coefficient in inert gases

A method of measuring the sodium diffusion coefficient in neon and argon is presented. It is based on recording the concentration change of sodium in a sodium discharge as it diffuses along a discharge tube. A diagram of the apparatus is given and the technique of measurements is described. It is pointed out that the proposed method can also be used for measuring the diffusion coefficient of other alkaline metals in inert gases and their mixtures.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 57–62, August, 1987.     

Characterization of PECVD boron carbonitride layers

BCN films on silicon substrates were deposited with two different PECVD techniques. A microwave plasma with RF-bias enhancement (MW-PECVD) and a direct current glow discharge plasma system (GD-PECVD) was used with N-trimethylborazine (TMB) and triethylamine borane (TEAB) as precursors and with benzene as an additional carbon source. Argon and nitrogen were used as plasma gases. Substrate temperature, substrate bias and gas composition were varied. ERDA (elastic recoil detection analysis) measurements yield information on the layer composition regarding the concentrations of the elements boron, carbon, nitrogen and hydrogen. Depth profiles are also available. The hydrogen content in the produced BCN layers strongly depends on the substrate temperature and increases up to 35 at.%. Depth profiles show a homogeneous distribution of the elements B, C, N and H over the entire layer thickness. Further, the layers were examined regarding their structure (FTIR spectroscopy) and their mechanical properties (nanoindentation measurements).     

Complex Calculations of High-Pressure Discharge Irradiation Characteristics

A wide range of problems related to the determination of the high-pressure irradiation parameters in inert gases and vapors of metals is considered. A new method for calculating equilibrium states of the computed thermodynamic systems is described and its applications to the determination of the plasma composition are disclosed. The method for computing basic discharge characteristics including the determination of material functions of the plasma producing media along with the optical absorption factors' a temperature field in a discharge stabilized by a quartz shell with regard to irradiation energy transfer carried out by bright-line and continous spectra, spectral distributions for the radiant fluxes and discharge irradiation efficiency is presented. Results of multiple calculations to obtain said characteristics in krypton, xenon, vapors of sodium, potassium, rubidium and their mixtures with inert gases are given. Calculated data is a satisfactory agreement with the results of experiments published by various authors.     

Spectral characteristics of a dense gas-metal reflection discharge plasma

Elemental composition of the plasma, which mostly includes working substances introduced into the discharge through puffing of hydrogen and sputtering of titanium, is determined by the spectrometric method. The analysis of the spectrum of microwave signal fluctuations shows that the characteristic spectrum peaks correspond to ion cyclotron frequencies of the gases and metal vapors forming the atmosphere of the discharge. The characteristic peaks of the optical and microwave discharge fluctuation spectra are found to coincide.     

The problem of the transmission of energy from an electromagnetic field into a high-frequency discharge plasma

On the basis of a consideration of the interaction between the electromagnetic field and the plasma we determine the power evolved in a discharge as a function of the various parameters of the field, the nature of the gas, and the discharge temperature.From a consideration of the thermodynamical properties we determine the power drawn off by heated gases of the plasma burner as a function of the temperature of these gases and the consumption.Finally, from a consideration of the energy balance we determine the resulting temperature and power of the discharge as a function of the above-mentioned parameters. The results of calculations coincide with the experimentally measured values of the power and discharge temperature.In conclusion the authors would like to express their thanks to Professor S. A. Medvedev for his assistance in discussing the results of this study.     

Characteristics of an argon rf plasma: Active discharge and laminar sonic flow region

The electron density, the excitation and the gas temperatures have been obtained from optical spectrometric, microwave interferometric and thermocouple measurements in an argon rf plasma. The investigation was carried out in an active discharge and a channel region characterized by sonic laminar flow. Optical measurements of the excitation temperature were made in the 4000–5000 Å range for discharge pressures of ∼ 63–644 torr and input rf power of 42–60 kW. Excitation temperatures in the discharge ranged between ∼ 7000–10,000 K. Electron densities measured by optical and microwave techniques showed good agreement. Thermocouple measurements in the channel and N₂ rotational spectra from traces of N₂ injected in the argon, as well as gas dynamic considerations, indicated that the gas temperature in the discharge and the channel regions were 2900–4400K and 1900–3300K, respectively. These values were substantially lower than the excitation temperatures corresponding to these regions, indicating that the plasma was in a two-temperature state in both regions. Standard tests for local thermal equilibrium (LTE) showed that the first excited level of argon constituted the bottleneck level.     

Ionization-overheating instability threshold in the electrodeless microwave discharge plasma

Experiments with a pulsed freely localized electrodeless microwave discharge in air under a pressure corresponding to the ascending branch of the pressure dependence of the breakdown field are described. The discharge is initiated in a focused quasi-optical electromagnetic beam. The minimal threshold values of the electromagnetic field (for which a brighter plasma channel extended along the field is formed in the initially spatially homogeneous diffuse discharge plasma) are determined by varying air pressure for several fixed values of this field in the focal region of the beam. In accordance with the prevailing theory, this phenomenon is interpreted as the result of evolution of ionization-overheating instability in the microwave discharge plasma.     

微波液相放电乙醇制氢发射光谱研究

氢能作为一种高热值、无污染的清洁能源日渐受到国内外专家学者的追捧。微波液相放电技术在醇类中制氢具有光明的研究前景,为氢能的研究开发开辟了一条新的途径。通过对乙醇制氢发射光谱分析,有利于分析微波液相放电醇类制氢机理的探讨,为进一步改进微波液相放电制氢技术奠定基础。本文采用2.45 GHz频率微波在液相醇类中放电实现了微波液相等离子体制氢,并借助发射光谱仪对微波液相放电乙醇制氢光谱特性进行了研究。研究结果显示：微波液相放电乙醇制氢过程中,能产生大量的H,O,OH,CH,C₂等活性粒子;乙醇放电光谱中OH自由基、H自由基和O自由基的光谱强度要远大于纯水中OH自由基、H自由基和O自由基的光谱强度;高能粒子打开水分子中的O—H键,脱氢制氢的过程较乙醇分子难度要大,因此在微波乙醇放电制氢过程中,氢气的来源主要是乙醇分子的脱氢重组,水分解产氢的贡献度较低;在外界压力与温度一定的条件下,OH,H,O自由基的发射光谱强度随着功率的增加显著增强,而CH和C₂活性粒子发射光谱强度则出现减弱趋势,这表明较大的微波功率不仅使产生的高能粒子的能量增加,同时高能粒子的密度也有所增加,导致较多的CH和C₂基团被充分碰撞打开。     

Spatial structures of continuous microwave discharge

The paper deals with the results of investigations of spatial structures of continuous microwave discharge in a quasi-optical resonator. The results are given of experimental observations and easurements of the parameters of plasma in discharges of different forms, and the reasons are analyzed for the formation of spatial discharge structures. It is demonstrated that, as a result of the plasma-resonance amplification of the field, the discharge makes a transition to the contracted state with a size that is much less than the microwave-frequency wavelength and with an electron concentration in excess of the critical. It is found that the stratification of the contracted state across the electric field vector, which arises in some gases, is caused by the development of thermoelectric-current instability that was not previously observed in microwave discharges.