J. Heberlein

Thermal plasma waste treatment

Plasma waste treatment has over the past decade become a more prominent technology because of the increasing problems with waste disposal and because of the realization of opportunities to generate valuable co-products. Plasma vitrification of hazardous slags has been a commercial technology for several years, and volume reduction of hazardous wastes using plasma processes is increasingly being used. Plasma gasification of wastes with low negative values has attracted interest as a source of energy and spawned process developments for treatment of even municipal solid wastes. Numerous technologies and approaches exist for plasma treatment of wastes. This review summarizes the approaches that have been developed, presents some of the basic physical principles, provides details of some specific processes and considers the advantages and disadvantages of thermal plasmas in waste treatment applications.

Superhard silicon nanospheres

Abstract Successful deposition and mechanical probing of nearly spherical, defect-free silicon nanospheres has been accomplished. The results show silicon at this length scale to be up to four times harder than bulk silicon. Detailed measurements of plasticity evolution and the corresponding hardening response in normally brittle silicon is possible in these small volumes. Based upon a proposed length scale related to the size of nanospheres in the 20– 50 nm radii range, a prediction of observed hardnesses in the range of 20– 50 GPa is made. The ramifications of this to computational materials science studies on identical volumes are discussed.

Arc instabilities in a plasma spray torch

The control over coating quality in plasma spraying is partly dependent on the arc and jet instabilities of the plasma torch. Different forms of instabilities have been observed with different effects on the coating quality. We report on an investigation of these instabilities based on high-speed end-on observation of the arc. The framing rate of 40,500 frames per second has allowed the visualization of the anode attachment movement and the determination of the thickness of the cold-gas boundary layer surrounding the arc. The images have been synchronized with voltage traces. Data have been obtained for a range of arc currents, and mass flow rates for different gas injectors and for anodes displaying different amounts of wear. The analysis of the data has led to quantitative correlations between the cold-gas boundary layer thickness and the instability mode for the range of operating parameters. The arc instabilities can be seen to enhance the plasma jet instabilities and the cold-gas entrainment. These results are particularly useful for guiding plasma torch design and operation in minimizing the influence of plasma jet instabilities on coating properties.

Radial structure of a low-frequency atmospheric-pressure glow discharge in helium

The spatial structure of a low-frequency atmospheric-pressure glow discharge was studied experimentally. The radial current distribution and discharge light emission were simultaneously measured at different phases during the ac voltage cycle. The glow discharge is formed by a radially propagating ionization wave. We also observed discharge regimes with several current pulses per half cycle corresponding to the successive, spatially separated breakdowns.

Nanoparticle formation using a plasma expansion process

We describe a process in which nanosize particles with u narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements. by calorimetric energy balances. and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nmt. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 not or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the experiments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiated by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.

Non-equilibrium modelling of arc plasma torches

A two-temperature thermal non-equilibrium model is developed and applied to the three-dimensional and time-dependent simulation of the flow inside a DC arc plasma torch. A detailed comparison of the results of the non-equilibrium model with those of an equilibrium model is presented. The fluid and electromagnetic equations in both models are approximated numerically in a fully-coupled approach by a variational multi-scale finite element method. In contrast to the equilibrium model, the non-equilibrium model did not need a separate reattachment model to produce an arc reattachment process and to limit the magnitude of the total voltage drop and arc length. The non-equilibrium results show large non-equilibrium regions in the plasma - cold-flow interaction region and close to the anode surface. Marked differences in the arc dynamics, especially in the arc reattachment process, and in the magnitudes of the total voltage drop and outlet temperatures and velocities between the models are observed. The non-equilibrium results show improved agreement with experimental observations.

Effect of an axial magnetic field upon the development of the vacuum arc between opening electric currents (currents read contacts)

In one series of experiments, a constant axial magnetic field B/sub ax/ was produced in the gap between opening Bruce-profile, chromium-copper contacts. Peak arc currents up to 40 kA at 50 Hz were studied with B/sub ax/ u to 112 mT. For each peak current, a critical field B/sub crit/ was determined above which anode spots did not form. For B/sub ax/ greater than B/sub crit/, the time from contact parting to the formation of a diffuse vacuum arc was linearly dependent on separation current. In a second series, the contacts were designed to generate a self B/sub ax/ when AC current up to 123 kA peak flowed. This field was approximately proportional to the current waveform through the contacts, and it was always higher than the B/sub crit/ predicted for the instantaneous current. The time from contact parting to the formation of the diffuse arc was consistent with that observed in the first experimental series. Once the diffuse arc had formed, the contacts showed no distributed melting for currents up to 81 kA, but gross melting occurred at 102-123 kA. The formation of diffuse arcs is discussed, and the anode melting is explained. >

Effects of current limitation through the dielectric in atmospheric pressure glows in helium

The influence of the dielectric barrier on the discharge regime of a uniform atmospheric pressure glow discharge is studied through fast, time-resolved imaging of the discharge optical emission and by a one-dimensional fluid model. The experiments show that the discharge regime can be adjusted over a wide range from a glow-like regime with a pronounced Faraday dark space and positive column to a Townsend-like discharge regime in which those features are absent. The determining factor for the discharge regime is the current limitation through the dielectric. Results of the one-dimensional fluid model confirm this observation. The fluid model also indicates that metastable helium atoms generated during a discharge pulse contribute significantly to the pre-ionization of the gas before the next breakdown through Penning ionization of nitrogen impurities.

Focused nanoparticle-beam deposition of patterned microstructures

A method was developed for fabricating nanocrystalline microstructures. This method involves synthesizing nanoparticles in a thermal plasma expanded through a nozzle, and then focusing the nanoparticles to a collimated beam by means of aerodynamic lenses. High-aspect-ratio structures of silicon carbide and titanium were deposited on stationary substrates, and lines and two-dimensional patterns were deposited on translated substrates. Linewidths equalled approximately 50 μm. This approach allows the use of much larger nozzles than in previously developed micronozzle methods, and also allows size selection of the particles that are deposited.

New approaches in thermal plasma technology

Thermal plasmas offer unique advantages for materials processing, such as high fluxes of heat and of reactant species. Recent developments have concentrated on improving control of these fluxes across the boundaries surrounding the thermal plasma. Secondary discharges (hybrid plasma generators) and pulse modulation of the plasma have been some of the approaches for this end. The use of such methods is described for selected applications. Plasma characterization through advanced models and diagnostics are concentrating on description of plasma instabilities and various nonequilibrium conditions. Understanding of these effects will allow their use for enhanced processing methods.