Solid-state amperometric sensor for the In-situ monitoring of slag composition and transport properties
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I. INTRODUCTION
STABILIZED -zirconia-based oxygen activity sensors are utilized extensively throughout the steelmaking process for better control of deoxidation, continuous casting, and ingot-making processes.[1–5] Oxygen activity measurements are usually taken within the molten metal for a direct reading of the oxygen dissolved within the steel melt. However, the composition of the slag phase should not be ignored, because it has a strong influence on the resulting composition of the final metal product. The relative amounts of transition metal oxides such as FeOx , CrOx , and MnOx are known to set the oxygen activity within an oxide melt, thereby controlling the oxidation and reduction reactions that occur at the slag/ metal interface. Chemical analysis for these transition oxides requires more than 30 minutes, while processes such as ladle refining may be completed in less than 30 minutes. This delay creates difficulties in process control. In order to determine the slag composition in situ, several investigators have explored the use of zirconia probes to estimate the concentration of FeOx within the slag phase. By measuring the opencircuit potential (OCP) across the probe, the oxygen activity of the slag phase can be estimated and then related to the FeOx concentration by assuming slag/metal equilibrium and using knowledge of the slag structure and thermodynamics.[6,7] Unfortunately, such techniques become more difficult if the slag is not in equilibrium with iron or if the slag structure is not well known. Amperometric techniques utilizing zirconia membranes have been successful at elevated temperatures for gas sensors,[8,9] diffusion-coefficient measurements,[10] moltenmetal refining,[11–14] fuel cells,[15] and coulometric titration S.C. BRITTEN, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, is Consultant, Mars and Company, Greenwich, CT 06830. U.B. PAL, Professor, is with the Department of Manufacturing Engineering, Boston University, Boston, MA 02215. Manuscript submitted June 28, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
of oxygen.[16,17] However, surprisingly little work has been done using such measurements on high-temperature moltenoxide systems. The theory behind alternating current (AC) and direct current (DC) electrochemical measurement techniques has been covered extensively by several authors[18,19] for the analysis of aqueous electrolyte solutions. This theory should not change dramatically for different electrolyte systems such as oxide melts. Electrolytic methods have been utilized with some success for determining cationic transport numbers within oxide melts,[20,21] but investigations were limited, as investigators realized that metals could not be produced easily by direct electrolysis of molten oxides. Measurements did demonstrate the diffusive nature of the rate limitation at a metallic cathode for small applied potentials within oxide melts.[22,23] Other measurements at lower temperatures for the analysis of gla
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