An assessment of the reduction potential of hydrogen plasma

Abstract

Hydrogen plasma can be used for deoxidation of functional materials containing reactive metals in both bulk and thin film forms. Since the different species in the plasma are not in thermodynamic equilibrium, application of classical thermodynamics to the analysis of such a system is associated with some difficulties. While global equilibrium approaches have been tried, with and without additional approximations or constraints, there is some ambiguity in the results obtained. Presented in this article is the application of a local equilibrium concept to assess the thermodynamic limit of the reaction of each species present in the gas with oxides or oxgyen dissolved in metals. Each reaction results in a different partial pressure of H₂O. Because of the higher reactivity of the dissociated and ionized species and the larger thermodynamic driving force for reactions involving these species, they act as powerful reducing agents. It is necessary to remove the products of reaction from the plasma to prevent back reaction and gradual approach to global equilibrium. A quantiatiave description using the framework of the Ellingham-Richardson-Jeffes diagrams is presented. Metals discussed include vandium, niobium, tantalum, chromium, manganese, titanium and yttrium.