K. W. Hemawan

Compact microwave re-entrant cavity applicator for plasma-assisted combustion

Advantages of combining an electrical discharge with combustion include a faster process, higher intensities, leaner combustion, pollutant reduction by altering by-products of combustion, improved fuel efficiency by achieving more complete combustion, more reliable ignition of combustion, and combustion across a wider range of pressures, temperatures and mixture stoichiometries. The benefits may also include the operation of combustion processes at extreme limits, such as aerospace applications at high speeds and altitudes.

Microwave plasma-assisted premixed flame combustion

A compact microwave plasma/combustion torch has been operated at atmospheric pressure in both plasma-only and plasma-assisted premixed combustion modes. The torch burns CH4∕O2 mixtures with plasma enhancement that modifies combustion, flame structure, flame size, and flame power density. The microwave energy also extends the fuel-lean burn limits.

Plasma Enhanced Combustion using Microwave Energy Coupling in a Re-entrant Cavity Applicator

An atmospheric compact high-Q microwave applicator is used to couple electromagnetic energy directly into the reaction zone of a premixed laminar methane-oxygen flame for energetic enhancement. At low microwave powers (1 to 5 W), the flame is influenced by an electromagnetic field only. As power is increased, the reactive gases in the flame break down and ionize into a plasma plume with significant increase in the flammability limit. 2-D laser induced fluorescence imaging of hydroxyl radicals (OH) is conducted in the reaction zone over this transition, as well as spectrally resolved flame emission measurements to monitor excited state species and derive rotational temperatures using OH chemiluminescence. Measurements are made for two equivalence ratios (φ = 0.9 and 1.1) and two gas flow rates (60 sccm and 100 sccm). In the electromagnetic field only phase (1 to 5 W), flame stability, excited state species, and temperature slightly increased with power while no significant change in OH radicals was detected. With the onset of a plasma plume, a significant rise in both excited state species and OH radical number density was observed.

High pressure microwave plasma assisted CVD synthesis of diamond

Summary form only given. Microwave plasma assisted CVD (MPACVD) of diamond has received renewed attention during the past several years. The recent experiments by Yan et al. (2002) have demonstrated the synthesis of high quality single crystal diamond with growth rates from 10 to over 100 mum/hr with operating pressures of 110-180 torr. High pressure operation allows higher reaction species concentrations and increase deposition rates. Currently, most commercial microwave plasma deposition reactors are designed for operation up to 120 torr and thus this reactor technology is not optimally designed for operation in the 120-300 torr pressure regime. High pressure operation allows higher reaction species concentrations and increase deposition rates. Hence, the objective of this research is to explore the behavior of microwave discharges between 180-300 torr and to learn how to control and efficiently couple to them to enable high pressure MPACVD diamond synthesis. This investigation explores the performance of a microwave cavity reactor (K. P. Kuo and Jes Asmussen, 1997) that has been modified to operate in the higher pressure regime. Major modification of the reactor include the redesign of the substrate holder, cooling stage and gas flow patterns to enhance the plasma power density, stability and also to enable operation from 180-300 torr. The polycrystalline and single crystal diamond films are synthesized using methane/hydrogen gas mixtures. The microwave input power ranges from 1.5-3 kW, gas chemistry 2-10% CH4/H2, and operating pressure from 180 to 300 torr. The substrate holder is water cooled to maintain the deposition temperature of 950-1250degC. The substrate materials are either a one inch silicon wafer or HPHT single crystal diamond seeds. Experimentally measured deposition rates, plasma power densities, optical emission spectroscopy measurements of the discharge temperature and radical species densities versus operating pressure, input power, and gas chemistry are presented. Measured discharge absorbed power densities range from 160-500 W/cm3 as the operating pressure increases and the diamond growth rate increases with increasing power density, operating pressure and higher methane concentration. Depending on the input experimental conditions, the diamond growth rate ranges from 3-50 mum/hr. Methods of achieving deposition uniformity, plasma stability, and long term operation of the reactor will be presented.

Microwave plasma-assisted diamond synthesis reactor design for large deposition areas at high rates

The Microwave plasma assisted CVD synthesis of diamond was first demonstrated during the early 1980s by Kamo et al [1]. Diamond synthesis was achieved in a small ( 2–4 cm ), tubular reactor where microwave energy was coupled into a quartz tube that was inserted through a waveguide. This reactor produced high radical densities and high quality diamond films and was inexpensive, simple to design, construct and operate. Hence it was utilized by many early diamond researchers to experimentally investigate and understand CVD diamond synthesis. However this reactor type had a number of inherent limitations such as small deposition area and low operating pressure regime. Since these early investigations the significant potential of industrial applications of CVD diamond synthesis has spurred numerous, innovative, microwave plasma reactor designs.

A compact microwave reentrant cavity applicator for plasma-assisted combustion

Advantages of combining an electrical discharge with combustion include a faster process, higher intensities, leaner combustion, pollutant reduction by altering by-products of combustion, improved fuel efficiency by achieving more complete combustion, more reliable ignition of combustion, and combustion across a wider range of pressures, temperatures and mixture stoichiometries. The benefits may also include the operation of combustion processes at extreme limits, such as aerospace applications at high speeds and altitudes.

Plasma-assisted combustion in a miniature microwave plasma torch applicator

Summary form only given. A compact microwave plasma torch has been designed and experimentally evaluated for operation in both plasma-only and plasma-assisted combustion modes. The torch is designed to be light and to operate at atmospheric pressure with a torch discharge size of less than 1 mm in diameter. The potential applications of the torch are for materials synthesis, material cutting and welding, and various surface treatments. The operation of the torch in a combustion mode with hydrocarbon gases burning with oxygen or air is investigated with microwave power applied to modify the combustion process. The objective of this investigation is to quantify the changes in the combustion process as microwave power is applied to create or intensify the discharge. This torch employs an open ended coaxial structure of 12 mm outer diameter with the discharge located at the tip of the center conductor that is 5 mm in diameter. Microwave power at 2.45 GHz is coupled into the torch applicator at power levels of 10s to 100s of watts. The discharge is formed at atmospheric pressure where the feed gas flows through a nozzle hole of 200-500 mum diameter located at the end of the applicator center conductor. Two nozzle configurations studied include a water-cooled brass nozzle and a ceramic nozzle. The microwave plasma torch is experimentally evaluated with a variety of feed gas mixtures including argon, mixtures of argon with hydrogen, argon with nitrogen, methane and oxygen, and selected other hydrocarbon gases mixed with oxygen and air. This torch maintains discharges over a wide range of flows from diffusional flow for gentle surface processing to high velocity flow approaching supersonic velocities. Diagnostic measurements performed include (1) gas temperature measured by optical emission spectroscopy (OES), (2) discharge power densities and, (3) discharge volume and size. These measurements are performed versus absorbed microwave power, gas flow rate and gas mixture composition. By using spatially resolved OES measurements, the temperature profile of the discharge is determined. The discharge size for absorbed microwave powers of 10-30 watts using a plasma-only mode of operation (i.e. argon) is 0.4-0.5 mm in diameter and 2-4 mm long for a nozzle diameter of 0.25 mm. When the torch is operated in a plasma-assisted combustion mode the flame (discharge) volume and intensity increase as the microwave power is applied to the flame. Data quantifying the influence of microwave power on the flame will be presented.

Atmospheric microwave discharges for plasma treatment of fibers

Summary form only given. Many contemporary plasma applications require the treatment of irregular shaped substrates such as filamentary or fiber-like objects. Additionally, for reasons of simplicity, efficiency and low process costs, it is desirable to develop plasma sources and associated processes that can continuously process a moving web or strand of materials at atmospheric pressure; i.e. without a high vacuum environment. Thus the article describes the development and testing of atmospheric plasma sources and associated processes that enable the treatment of individual fibers and/or bundles of fibers. Graphite fibers are chosen as the example substrate material. Specifically, a 2.45 GHz, atmospheric discharge plasma source is described. It is able to continuously treat single or multiple graphite fibers or even treat graphite fiber tows. Small cylindrical discharges are created in gases that flow through a small 1-3 mm diameter quartz tube placed axially inside a tunable, cylindrical microwave applicator. The fibers are subjected to plasma treatment as they are passed through the discharge. The reactor design process involved an initial ANSOFT HFSS numerical analysis of the material/discharge loaded reactor excited with a variety of different electromagnetic modes. Based on this analysis, two modes, the TE/sub 112/ and the TM/sub 102/ modes were experimentally evaluated with input power levels of 20-200 W, and with a variety of gases, such as mixtures of argon, nitrogen, methane, etc. The experiments have demonstrated a controllable, atmospheric plasma source capable of heating and treating fibers from 300-850/spl deg/C without the need of an associated vacuum system. Experimental operating parameters, such as substrate temperatures, plasma densities, discharge power densities, etc versus input power, gas flow rate, and gas mixture are presented.

Miniature microwave plasma torch applicator and its characteristics

Summary form only given. A compact microwave plasma torch has been designed and experimentally evaluated. The torch is designed to be light weight and to operate at atmospheric pressure with a torch discharge size of less than 1 mm in diameter. The potential applications of the torch are for materials synthesis, material cutting and welding, and various surface treatments. This torch employs an open ended coaxial structure of 12 mm outer diameter with the discharge located at the tip of the center conductor that is 5 mm in diameter. The discharge is formed at atmospheric pressure where the feed gas flows through a nozzle hole of 200-500 /spl mu/m in diameter located at the end of the center conductor of the applicator. The microwave plasma torch is experimentally evaluated over a range of input power and a variety of feed gas mixtures including argon, mixtures of argon with hydrogen and selected hydrocarbon gases, nitrogen, and air. This torch operates from 10s to 100s of watts of 2.45 GHz input microwave power and is able to maintain discharges over a wide range of flows from diffusional flow of radicals for gentle surface processing to high velocity flow approaching supersonic velocities. Diagnostic measurements performed include (1) gas temperature measured by optical emission spectroscopy (OES), (2) discharge power densities and, (3) discharge volume and size. These measurements are done versus absorbed microwave power and gas flow rate. The discharge size for absorbed microwave powers of 10-30 watts is about 0.7 mm in diameter and 2-4 mm long for a nozzle diameter of 0.4 mm. By using spatially resolved OES measurements, the temperature profile of the discharge is determined.