Plasma arc-jet

A common DC glow discharge PACVD system may be varied by operating the plasma at higher pressures and powers at which a DC arc discharge between the electrodes can be produced. In 1988, Kurihara et al.t l first reported the use of the DC plasma arc-jet CVD method, in which... 

Gas-phase activation above the deposition surface is essential for achieving appreciable diamond growth rates. The various CVD methods differ primarily in the way they produce gas-phase activation. The most abundant carbon-containing gaseous species present in most activated systems are methyl radicals and acetylene molecules which are also considered to be predominant growth precursors for diamond, almost independent of the deposition methods used. However, in systems that dissociate a significant fraction of H2, such as DC plasma arc-jet CVD, carbon atoms, aside from acetylene, are also abundant in the gas phase. 

One of the critical factors affecting growth rates is the gas-phase temperature which can be reached in a CVD method. A comparison of the various CVD methods to each other demonstratesl that typical linear growth rates correlate positively with estimated gas-phase temperatures (Fig. 1), approaching 1 mm h in atmospheric pressure plasma arc-jet CVD with temperatures around 6000 to 7000 K. The partial pressures of various gas species in typical CVD processes have been calculated as a function of... 

Plasma arc-jet and oxyacetylene torch ablation tests are much more convenient for operations and with much lower cost, which therefore are often used for primary evaluations of materials about their thermal shock resistance and ablation resistance properties . The difference between these two methods is their gaseous composition and flow speed. A plasma of air may exhibit ultra high temperatures above 5000°C and high velocities of 2 Mach but with ionized air atmosphere. On the other hand, the generally used oxyacetylene torch is of combustion gaseous products of carbon mono/dioxide, water vapor, OH and active hydrocarbon species with gas velocity less than 1 Mach and temperatures above 3000°C. A HVOF torch exhibits... 

Argon, helium, and their mixtures with other gases are used as the working fluids in plasma arc devices for producing plasma jets with temperatures in excess of 50,000 K. These devices are used for cutting metals and for spray coating of refractory alloys and ceramics (qv) (see Plasma technology). 

The sudden expansion of the gases, as they are heated in the arc plasma, causes the formation of a high-speed arc jet so that the atomic hydrogen and the reactive carbon species are transported almost instantly to the deposition surface and the chances of hydrogen recombination and of vapor-phase reactions are minimized. 

A number of other deposition methods have been used for growing diamond, with varying degrees of success. These include oxyacetylene welding torches, arc jets and plasma torches, laser ablation and hquid phase crystallisation, but none of these yet reahstically compete with the hot filament or microwave systems for reliability and reproducibility. 

Apparatus. Plasma Jet. A section of the plasma arc gun is shown in Figure 1. A direct current arc was struck between the thoriated tungsten rod, which acted as the cathode, and the copper tube anode. Both electrodes were... 

Plasma Arc Welding. In the transferred-arc mode of the PAW process, shown in Figure lc, the arc is between a nonconsumable electrode and the base metal, in a manner similar to the GTAW process. The unique feature is the flow of inert gas around the electrode and through a restricted orifice, which constricts the arc to form a plasma jet. A second, outer stream of shielding gas protects the molten metal from atmospheric contamination. In the nontransferred arc mode of the PAW process, the arc is between the electrode and the constricting orifice. This mode is used for plasma spraying... 

Experiments105 have been performed with an uncooled rapid transit probe in a nitrogen arc jet in which methane was transformed into HCN and C2H2. Locations of chemical reactions were determined and compared with mass spectra of the quenched plasma gas recorded at the same place. The maximum yields of HCN (1.1 mole%) and C2H2 (0.5 mole%) were found at a distance of 25 mm along the axis of the jet, but at temperatures of 5500 and 2500 K, respectively. 

A modification of the arc discharge method is reahzed in the so-called DC-arc jet (plasma jet). In this case, the electrodes are arranged in a way to form a sort of nozzle for the reactant gases. The cathode encloses the anode in a certain distance, and the gas mixture is led through the resultant gap. It partly decomposes between the electrodes before it hits the cooled substrate where the diamond film is deposited then (Figure 6.16). In this manner, an accurate control of the deposition zone is achieved, yet the results are highly dependent on the nozzle geometry and on a very constant reactant flow. 

A non-linear wall-stabilized non-transferred arc is shown in Fig. 4 8. It consists of a cylindrical hollow cathode and coaxial hollow anode located in a water-cooled chamber and separated by an insulator. Gas flow blows the arc column out of the anode opening to heat a downstream material, which is supposed to be treated. In contrast to transferred arcs, the treated material is not supposed to operate as an anode. Magnetic 7x5 forces cause the arc roots to rotate around electrodes (Fig. 4-48), which provides longer electrode lifetime. The generation of electrons on the cathode is provided in this case by field emission. An axisymmetric version of the non-transferred arc, usually referred to as the plasma torch or the arc jet, is illustrated in Fig. 4-49. The arc is generated in a conical gap in the anode and pushed out of this opening by gas flow. The heated gas flow forms a very-high-temperature arc jet, sometimes at supersonic velocities. 

Deposition from the Vapor Phase Submicron (3-SiC can be continuously produced by decomposing gaseous or volatile compounds of silicon and carbon in inert or reducing atmospheres, at temperatures above 1400 °C [83]. The particle size and morphology will depend considerably on the reaction temperature and on the composition of the gas phase. A wide variety of reactants, and several methods of heating (e.g., dc arc jet plasma [84], high-frequency plasma [85], laser [86, 87] and thermal radiation [88]) are possible. 

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