Reduction of titanium dioxide in a nonequilibrium hydrogen plasma
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I.
INTRODUCTION
A plasma, in its most basic definition, is simply a highly ionized gas. The high degree of ionization is produced and maintained by applying an electromagnetic field (either AC or DC) to the gas. Plasmas with a degree of ionization approaching unity (i.e., all gas molecules are ionized) are referred to as equilibrium or thermal plasma. Thermal plasmas are characterized by high gas pressures (p 5 760 torr or greater) and high gas temperatures (T . 3000 K). These conditions of temperature and pressure allow the plasma to approach thermal equilibrium (thermal equilibrium in a plasma refers to equilibrium between all the species found in the plasma, i.e., free electrons, neutral species, ion, etc.). As such, equilibrium plasmas are used in industry as high-enthalpy sources for the thermal decomposition of ores, and as heat sources in smelting.[1] Plasmas with a degree of ionization much less than one are termed nonequilibrium or ‘‘cold’’ plasmas. These plasmas are characterized by low gas pressures (typically p , 1 torr) and low molecular temperatures. The term cold results from the fact that the temperature of the gas molecules in the plasma is significantly less than that of the plasma’s free electrons; a typical nonequilibrium plasma has a molecular temperature less than 1300 K, whereas the electron temperature is roughly 10,000 K.[2] Since the temperature of a solid in contact with a cold plasma will be determined by the molecular temperature, nonequilibrium plasmas are used to generate chemically active species in a variety of industrial processes. The two forms of plasmas discussed previously reflect the high- and low-pressure extremes found in plasma proDANIEL E. BULLARD, Senior Research Engineer, is with the U.S. Steel Research Center, Monroeville, PA 15146. DAVID C. LYNCH, Professor and Associate Department Head, is with the Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85721. Manuscript submitted September 26, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
cessing. Between these two extremes is a transition region that is of interest in this investigation. ‘‘Transition’’ plasmas are characterized by high electron temperatures, moderate molecular temperatures, a moderate degree of ionization, and intermediate pressures. Because of the difference in the molecular and electron temperatures, these plasmas are considered nonequilibrium. At the pressures used in this work, 6 to 46 torr, the temperatures of the molecular species will be significantly lower than those observed in thermal plasmas; however, the electron temperatures are still large enough that substantial amounts of chemically reactive species can be formed.[3,4] These plasmas offer an interesting opportunity for use in chemical processing by providing significant concentrations of chemically reactive species at low to moderate temperatures. One of the more useful reactive species that can be formed in a plasma is radicals. A radical is nothing more than an atom or group of atoms with neutral ch
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