Tail flame

The reactants ate fed into the tail flame of a d-c nitrogen plasma. The reaction occurs rapidly at temperatures around 1500°C and the HCl reacts with excess ammonia to form ammonium chloride. Similar reactions have been carried out using furnaces, lasers, and r-f plasmas (34) as the source of heat. Other routes using titanium tetrachloride starting material include... 

A hybrid plasma reactor was developed for the synthesis of fine ceramic powders (59). The reactant SiCl4 was injected into a DC-arc plasma jet and decomposed completely in a hybrid plasma an RF-plasma superimposed on the DC-arc plasma. The reaction with the second reactant NH3 and/or CH4 gas, which was injected into the tail flame of the plasma, formed Si3N4 UFPs and/or Si3N4 + SiC mixed UFPs, with structures that were amorphous. 

Arcs can be considered as gaseous resistance heaters and offer temperatures up to 50,000°K. The sustained temperatures realizable from electric arcs can be divided into three general regions according to the current density of the conducting path. The lower temperatures (up to 4000°K. and a current density of 60 amp./cm.2) make the anode material incandescent, but as the current density is increased beyond a critical level the voltage drop shifts suddenly from a uniform drop between cathode and anode to a drop concentrated at the anode surface (for a dc arc—at both electrodes for an ac arc). The transition from the conventional to a high intensity arc is marked by pronounced increases in brilliance and temperature the arc path becomes distorted, and a jet of plasma, called the tail flame, issues from the rapidly... 

Fia. 11. Tail flame from high-intensity electric arc (FI). 

Salinger (44) reported the successful conversion of methyltrichloro-silanes to silicon carbide in a 50-kW RF plasma torch. The liquid methyltrichlorosilanes were fed to the tail flame of various plasmas and the solid products were recovered in an acid-resistant bag filter. Up to 85% recovery of theoretical solid product was reported, which was subsequently heated at 500°C to remove elemental carbon. Under the best condition (20-25% vol. hydrogen in argon plasma at 36 kW), up to 70%o conversion to j3-SiC was obtained with ca. 10% conversion to amorphous SiC. Salinger suggested that the good crystallinity of the )3-SiC so obtained meant that the reaction occurred in a gas temperature range in which )3-SiC was the stable crystalline form (i.e., < 2300°C). 

The first reported heat treatment of a solid in an RF plasma was carried out by Reed (43). He injected powders at high velocity through an atmospheric pressure argon plasma and deposited molten powder on a crystal boule in the plasma tail flame. A sapphire polycrystal, some... 

Because a polymer-forming luminous gas phase such as the tail-flame portion of an inductively coupled radio frequency glow discharge behaves as a fluid, the deposition mechanism can be investigated by examining the influence of the fluid mechanical aspects of luminous gas phase on the deposition rate of polymer. 

Wavelength must be stable, constant, free from electronic noise and frequent atmospheric temperature change. These are important parameters to be optimised so as to obtain the maximum signal for that element. The alkali and alkaline metals are usually observed in the tail flame while elements such as Cr, Nb, Ta and Be are observed in the body of the plasma. A compromise set of conditions may necessitate the analysis of several elements. 

With the so-called current-free or transferred plasma, the observation zone is situated outside the current-carrying zone. A source such as this can e.g. be realized by the use of a supplementary gas flow directed perpendicular to the direction of the arc current and by the observation zone being in the tail-flame. In this observation zone no current is flowing. This type of plasma reacts significantly on cooling as no power can be delivered to compensate for temperature drops. Therefore, it is fairly insensitive to the addition of easily ionized elements. They do not cause a temperature drop but only shift the ionization equilibrium and give rise to ambipolar diffusion, as discussed previously. 

Fig. 97. Three-electrode plasma jet. (a) electrodes (b) tail flame (c) cathode block (d) plasma column (e, f) argon + anode block front (Reprinted with permission from Ref. [369].)...

In down-wash configuration, the flame is established in the wake of the burner tube. A recirculation vortex in the wake of a burner tube appears as a flame sheet. When R is further reduced, the flame tip is severely deflected by the crossflow. A small recirculation bubble was observed by Huang and Chang [16] atR = 0.04. For a value of R between 0.1 and 1, the impact of a cross-flow stream is dominant. An axisymmetric tail flame forms downstream of the recirculation vortex and the flame widens. This structure is characterized by several features such as flickering and bifurcation. In jet-dominated mode, the recirculation vortex disappears and only the tail part remains attached to the burner. The transition from crossflow-dominated to jet-dominated conditions occurs from 1 = 1 to 3. For R>3, the effect of crossflow becomes negligible the jet fluid mechanics dictate the flame characteristics. For R > 10, the flame detaches from the burner and stabilizes above the exit plane of the burner tip. Depending upon the jet exit velocity and burner diameter, the flame is either attached to the burner tip or stabilizes as a lifted flame until it blows out. 

It is found that the presence of reactants such as metal halides in the plasma causes problems with stability in the case of R.F. device and c rrosion in electrode plasmas. However, the extreme plasma temperatures are not necessary and satisfactory results are obtained by injecting reactants into the plasma tail flame as demonstrated by Audsley and Bayliss for the production of SiOj by reaction of SiCl4 and oxygen using an RF plasma... 

Munz, R. J. The decomposition of molyMenum disulphide in an induction plasma tail flame, Ph. D. dissertation, McGill Univ. 1974... 

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