In many important materials processing operations, such as welding and plasma processing, a solid or a liquid metal is exposed to a plasma. In such systems, the activity of a species, such as nitrogen, in the plasma cannot be estimated from a straightforward application of the established thermochemical principles of gas-metal reactions. Currently, there is no established method for the determination of activity of a species in a plasma. In this article, a method for determination of nitrogen activity in plasmas is presented. High-purity tantalum and niobium samples were isothermally exposed to a well-characterized helium-nitrogen plasma, and the corresponding nitrogen solubility in each metal was determined by the vacuum fusion method. In each case, the activity of nitrogen in the plasma with respect to a standard state of 1 atm nitrogen pressure was calculated from the nitrogen solubility data and its value was demonstrated to be about an order of magnitude higher than the partial pressure of nitrogen in the source gas used to generate the plasma. The solubility of a species in a condensed phase exposed to a plasma can be used to determine its activity in the plasma.
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