An iterative technique to develop plasma processing, consisting of identifying materials with unique properties, calculating conversion efficiencies, product purity and power from thermodynamic data for the chemical reactions selected to produce it and modeling the process mathematically and physically to determine capital and labor costs is illustrated with production of silicon carbide and titanium diboride in Alcoa's DC plasma facility. The work shows that submicron silicon carbide and titanium diboride containing less than 0.2% oxygen can be produced with plasmas at costs competitive with traditional processes. It has also been reported that aluminum nitride, a material which has the high thermal conductivity needed for high power electronic substrates, can be satisfactorily produced by reacting aluminum powder with nitrogen in an RF plasma system. Future application of plasmas for production of ceramics will depend on discovery of unique properties and cost effective process optimization.

Six different composite microstructures were produced by low pressure plasma deposition of γ/γ'-α Ni-Al-Mo materials. Interlaminate spacing, volume fraction of α Mo, and continuity of the laminate structure were varied. Thermal expansion behavior (25–1250°C), density, and room temperature elastic modulus were correlated with volume fraction of α Mo. Tensile ductility (25–960°C) was correlated with both volume fraction and distribution of the α Mo phase, while yield strength was nearly insensitive to composite structure. Composite strengths were greater than were the unreinforced matrix strengths.

Sprayed partially stabilized zirconia (PSZ) has a non equilibrium microstructure due to the splat quench nature of the spray process and it will undergo phase transformation with time at temperature. This study examines the phases present in sprayed PSZ versus powder history, exposure to temperature, and silica impurity level. Finally, thermal quench behavior is examined versus phase content.

Plasma flow field temperatures are determined in a nontransferred arc plasma using emission spectroscopy. This technique is then utilized to identify thermal decomposition and reduction products produced in the plasma plume when metal oxide particles are injected into the plasma arc. The processed particles are then studied using AAS, SEM, EDS and XRD to characterize the chemical changes that have occurred in the particles. A Fourier transform method is used to study changes in particle morphology.

The visible and UV plasma emissions from a metal vapor arc were studied during vacuum arc remelting of 165 mm diameter Inconel 718 electrodes using emission spectroscopy. Internal energy distributions of a number of emitting species were analyzed using Boltzmann statistics. Analysis of the data yielded distribution “temperatures” ranging from 5,400 K to 8,200 K for the neutral atomic arc species, whereas values of 12,200 K to 17,500 K were obtained for singly ionized species. These values illustrate that the arc is not in a state of local thermodynamic equilibrium. The excited state densities of various arc constituents were measured and it was found that highly volatile alloy trace components, such as Mn, were present in the arc in dispro-portionately high concentrations. These data suggest that ionized evaporates play an important role in carrying current between the electrodes.

Rapid densification of MgO in an rf plasma without sintering aids is demonstrated. The sintered densities of MgO are sensitive to the plasma composition and gas pressure. In general, specimens are sintered to higher densities in Ar/O2 and Ar/H2O plasmas than in pure Ar plasmas. This may be explained by the effects of plasma enthalpy and surface recombination of charged particles and of atomic species. Analyses show that the evolution of the microstructures of the sintered specimens remains similar, regardless of the plasma conditions. No substantial grain growth is observed.

Ultrafine beta-SiC powders are synthesized in a thermal argon plasma jet reactor by a reaction between methane and silicon monoxide. High temperatures (>10000 K) and rapid quench rates (>106 K/s) lead to a high degree of super-saturation of the chemical vapor, resulting in homogeneous nucleation of ultrafine SiC particles. The maximum conversion of SiO to SiC determined by thermogravimetric analysis is 97%. Particle size analyses show a bimodal distribution with the majority of the particles falling in a size range from 2 to 40 nm. Larger particles with sizes larger than 80 nm are also observed. Characteristically-shaped SiC single-crystal platelets are observed throughout this work.

Interesting possibilities exist for the scientific design of materials with optimized properties for a diversity of technological applications. For example, the reduction of severe wear and erosion in critical turbine and engine components requires basic studies of intrinsic strengthening, where loads are uniformly transferred across interphase inter-faces. The acheivement of this requires a developed capability for producing selected morphologies on the (i) macro, (ii) micro, and (iii) nanoscale. This involves using a combination of techniques that include the deposition of one, or more, atomic or molecular species in gaseous environments. Recent discoveries suggest, in fact, that it is feasible to design layers where the chemistry and structure at any depth can be pre-selected. Such a capability offers exciting opportunities for forming ‘graded property’ materials, as required in mechanical component and fiber-optic applications. Here, specific radial distributions of chemical species can be used to achieve optimal properties. Another in-triguing possibility is the formation of composite-structure materials, even on the nanoscale, by simultaneous growth of filaments and matrix, using appropriate precur-sors. Thus, metal and polymer matrices may, in principle, be strengthened by various types of fiber and particle distributions. In this approach the basic concept is the syn-thesis of scientifically designed materials for specific technological applications.

The thermal reduction of ilmenite to produce a high grade titania slag and pig iron byproduct using a d.c. transferred-arc plasma furnace has been investigated. Small-scale (50 kVA) batch tests were conducted on three different ilmenite concentrates of widely differing chemical compositions and geological histories to assess the suitability of this reactor configuration for ilmenite smelting. The effects of reducing agent particle size and excess carbon on slag grade, titanium and iron recoveries, and reducing agent efficiency are discussed. A d.c. plasma furnace appears to be an ideal reactor for ilmenite smelting, although accurate control of the furnace thermal balance is essential if reaction between furnace refractories and the titania slag is to be avoided.

A series of powder feeders, developed during the course of an experimental study involving the synthesis of carbon monoxide from pure carbon and carbon dioxide, have been described in detail. The most efficient and reliable of these powder feeders was capable of continuously producing uniform mixtures of powdered carbon entrained in carbon dioxide gas. This device was also capable of producing a relatively wide range of solid/ gas mixture ratios (from about 0.8 to 3.2 g/l) at carbon dioxide gas flow rates between 75 and 200 1/min. In addition, it was relatively easy to vary and control solid/gas mixture ratios and gas flow rates using this powder feeder. Similar powder feeders may be useful in other thermal plasma synthesis or processing applications as well as in other applications requiring accurately blended mixtures of gases and powdered solids

A 40-kW transferred arc plasma melter has been designed and constructed to test the potential of this type of system for the vitrification of high level nuclear waste. Present technology relies on joule or induction heaters which are limited in operating temperature and are characterized by large liquid holdup and long residence times. The use of a plasma melter will allow the production of higher melting glasses and glass ceramics which are more durable and leach resistant in underground storage. The use of a plasma greatly decreases residence times in the heater and may even reduce the loss of volatiles by control of the melting atmosphere and continuous removal of the product as a solid ingot. Standard borosilicate glass has been melted with continuous withdrawal of product in a series of commissioning runs.

Glow discharges have been operated at power inputs of 25 kW at 50 mb and discharge currents up to 2 A at gas flow rates of up to 0.04 kg/s in a 100 mm diameter cavity 0.5 m long. This has been made possible by the use of multiple anodes and cathodes combined with gas injection at the porous anodes. Power loading in excess of 700 kW/kg/s have been achieved. The power density of the gas (W/m3) and the current at which the glow to arc transition occurs scaled with both the number of electrode pairs and the gas flow rate through the porous anodes over the range of investigation.