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Plasma Processing and Processing Science
because of the lack of a well-developed engineering base for this process. Only in recent years have basic studies of the arc-wire spray process been initiated for improving both our knowledge and technology base.
Plasma Chemical Vapor Deposition
One of the most promising new developments in thermal plasma processing is the plasma chemical vapor deposition (PCVD) process. As a relatively new film deposition technique, it can deposit high-quality, even epitaxial, films at deposition rates considerably higher than those obtained by competing low-pressure methods. One of the most visible developments of this technology is the deposition of diamond and ceramic films, which is expected to have a strong impact on manufacturing processes. In contrast to the previously discussed coating technologies, PCVD is still in the laboratory stage; that is, present research efforts concentrate on the establishment of the knowledge base for this emerging technology.
In the process of PCVD, a high-energy-density plasma produces high-density vapor-phase precursors for the deposition of relatively thick films. A direct current plasma torch, for example, generates a high-temperature, high-velocity plasma jet that impinges on a cooled substrate. With temperatures close to the torch nozzle exit exceeding 104 K, the precursor material that is injected into the plasma is rapidly vaporized and dissociated and, because of the high velocities of the plasma jet (of the order of 100 m/s), accelerated toward the substrate. In front of the cooled substrate, a boundary layer with steep gradients forms. Such boundary layers in chemically reacting gases attracted strong interest in spaceflight and reentry simulation and have been extensively analyzed in these connections.
As the temperature across the thermal boundary layer drops from the plasma temperature to the substrate temperature, the chemically active species will be rapidly driven across the boundary layer by the extremely steep gradients there. Because of the rapid traverse of the species across the boundary layer, chemical reactions appear to be more or less “frozen,” resulting in a strong chemical nonequilibrium situation in the boundary layer, where the species concentration is determined mainly by diffusion rather than by chemical reactions. Maintaining a high concentration of chemically active precursors across the boundary layer is crucial for achieving high deposition rates.
In spite of impressive progress over the past years, the chemistry in the boundary layer and at the substrate surface during the PCVD process is still poorly understood. Efforts are continuing to establish realistic models for this situation and demonstrate by corresponding experiments their validity.
Plasma Waste Destruction
Destruction of waste, especially of toxic waste, has grown into an increasingly pressing problem. Among various waste destruction processes, thermal plasma waste destruction is considered to be a viable option for certain types of waste.
Thermal plasma reactors offer unique advantages for the destruction of hazardous wastes: (1) the high energy density and temperatures associated with thermal plasmas and the corresponding fast reaction times offer the potential of large throughputs in a small reactor; (2) the high temperatures can also be used to obtain very high quench rates, allowing the attainment of metastable states and nonequilibrium compositions; (3) the high heat fluxes at the reactor boundaries lead to fast attainment of steady-state conditions, allowing rapid start-up and shutdown times compared with other thermal treatment devices such as incinerators; (4) use of electric energy reduces gas flow needs and off-gas treatment requirements and offers control over the chemistry, including the possibility of generating marketable coproducts; and (5) all the characteristics combined allow easy integration into a manufacturing process that