Plasma system

The substrate may be heated to unacceptable levels by the high temperature of the gases and cooling is usually necessary. Temperature control and substrate cooling remain a problem in arc-plasma systems. However, deposition is rapid and efficient, high... 

The substrate temperature should be kept between 800 and 1000°C and cooling may be necessary. Gas composition and other deposition parameters are similar to those used in a microwave-plasma system. Deposition rate is low, reported as 0.5 to 1 im/h. 

A thermal plasma system has been developed for the decomposition of methane. A schematic diagram of the experimental apparatus is shown in Fig. 1. The system consists primarily of D.C. plasma torch, plasma reactor and filter assembly. Plasma was discharged between a tungsten cathode and a copper anode using N2 gas. All the experiments were carried out at atmospheric pressure at 6 kW input electric power and N2 flow rate of 10 to 12 1/min. The feed gas (CH4) flow rates were varied from 3 to 15 1/min depending on the operating conditions, shown in Table. 1. 

Direct thermal decomposition of methane was carried out, using a thermal plasma system which is an environmentally favorable process. For comparison, thermodynamic equilibrium compositions were calculated by software program for the steam reforming and thermal decomposition. In case of thermal decomposition, high purity of the hydrogen and solidified carbon can be achieved without any contaminant. 

A plasma process is characterized by many parameters, and their interrelations are very complex. It is of paramount importance to understand, at least to a first approximation, how the plasma parameters have to be adjusted when the geometrical dimensions of the plasma system are enlarged. Especially of use in scaling up systems are scaling laws, as formulated by Goedheeret al. [148, 149] (see also Section 1.3.2.2). 

Figure 2. Plasma density (particles/m ) against energy for various plasma systems.

Several types of nonthermal plasma systems have been reported in the literature for reforming of hydrocarbons to hydrogen-rich gas ... 

The plasma decomposition process is applicable to any hydrocarbon fuel, from methane to heavy hydrocarbons. Similar to oxidative plasma reforming, plasma decomposition processes fall into two major categories thermal and nonthermal plasma systems. 

Table 2.5. Plasma system components, and operating conditions [225]... 

Fig. 1. Typical a.c. plasma systems used for hydrogenation of semiconductor samples. A. In this aparatus, hydrogen is pumped through the quartz tube (Q) and a plasma excited by inductive coupling of 13.56 MHz r.f. power with a copper coil (c2). The sample rests on a graphite block (b) that is heated by 440 KHz power coupled by a second coil (cl). A pyrometer (P) measures the sample temperature. B. In this system, a high frequency oscillator is used for plasma excitation while the sample is heated in a tube furnace (Pearton et al., 1987).

Fio. 1. Schematic diagram of a remote hydrogen plasma system. 

Additional considerations in the design and operation of a remote hydrogen plasma system include the following (1) convective heat transfer between the downstream gas and the specimen can introduce a significant difference between the heater temperature and the specimen surface temperature (this effect is generally not as severe as for direct immersion in... 

Fig. 7. Depth profiles of deuterium in n-type (P-doped) silicon after deuteration in a remote plasma system at 150°C (a) entire profile after a 120 min deuteration and (b) near-surface profiles after different durations of deuteration. Also shown is the uniform P concentration.

Essential to the identification of H-induced defects in silicon was the use of a remote hydrogen plasma system as described in Section 1.2. The alternative of direct immersion in a plasma introduces charged-particle bombardment and possible photochemical effects that can obscure the purely chemical consequences of hydrogen migrating into silicon. While the evidence presented below strongly argues for the existence of H-induced defects, many issues remain to be resolved. 

In both cases, the samples from contaminated sites were rinsed with a solvent to obtain an extract of contaminated transformer oil. The effects of biological degradation were investigated by using a commercial mixture of microorganisms and pure strain under aerobic and anaerobic condition. In the thermal method, a laboratory plasma system was used to decompose the contaminated transformer oil by a direct injection of the oil extract into the plasma system or by melting the extract samples with power plant fly ash in the plasma reactor. For the contaminated transformer oil both methods showed a destruction efficiency of 99.99% and the products of destruction were environmentally friendly. 

The plasma process on an industrial scale is a quite sophisticated and know-how packed technological system. The target applications of the thermal plasma systems presently are ... 

Worldwide, there are numerous plasma system designs for treatment of all types of wastes. Economical considerations limit their commercial applications to the most profitable actions. Presently they commercially operate in Switzerland and Germany for low level nuclear waste vitrification, in France and the USA for asbestos waste vitrification, in the USA and Australia for hazardous waste treatment, in Japan and France for municipal fly ash vitrification. The most of installations is working in Japan because there 70% of municipal waste is incinerated and the ash can not be used as landfill. EU Regulations banning the disposal to landfill of toxic and hazardous wastes after year 2002 may cause wider use of plasma waste destruction technology in Europe. 

Baumann H, Heumann KG. 1987. Analysis of organobromine compounds and HBr in motorcar exhaust gases with a GC/microwave plasma system. Fresenius Zanal Chem 327 186-192. 

Because of the series capacitor and/or the dielectric coating of the electrodes, the negative potentials established on the two electrodes in a plasma system may not be the same. For instance, the ratio of the voltages on the electrodes has been shown to be dependent upon the relative electrode areas (77). The (theoretical) dependence is given by Equation 1, where V is the voltage and A is the area (7d). 

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