NITROGEN ACTIVITY DETERMINATION IN PLASMAS

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 p lasma cannot be estiamted from a straightforward application of the es tablished 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 n itrogen activity in plasma is presented. High-purity tantalum and niob ium samples were isothermally exposed to a well-characterized helium-n itrogen plasma, and the coresponding nitrogen solubility in each metal was determined by the vacuum fusion method. In each case, the activit y 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 t han the partial pressure of nitrogen in the source gas used to generat e 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.