A high-accuracy, calibration-free technique for measuring the electrical conductivity of molten oxides
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I.
INTRODUCTION
AS part of a larger study of the physical chemistry of molten oxides for use as electrolytes in prospective hightemperature electrochemical processes such as electrolytic steelmaking,[1,2] the electrical conductivities of multicomponent molten oxides were investigated. In order to support the electrolysis of iron oxide to produce pure liquid iron and oxygen gas, for example, a melt must (1) be an ionic conductor; (2) have an acceptable value of electrical conductivity, i.e., one that satisfies the thermal needs of the electrolysis cell while sustaining economically viable metal production rates; (3) be free of elements more noble than iron—such elements would codeposit with iron and contaminate the metal product; (4) have a low vapor pressure; (5) be less dense than liquid iron—this ensures good separation of metal product from electrolyte; (6) have a high solubility for iron oxide; (7) be environmentally benign, and (8) be low cost. Melts based on SiO2 are promising as candidate electrolytes, as many silicates are known to meet the last six criteria. The first two criteria, those concerning the electrical properties of the melt, are the most difficult to assess due to a general lack of data and conflicting information.[15–24] Obtaining reliable data, then, became the focus of the present study. In response to the lack of a fully satisfactory technique for making high-accuracy measurements in the melts of interest, a new technique was developed, the co-
SUSAN L. SCHIEFELBEIN, Senior Scientist, Science and Technology, is with Corning, Inc., Corning, NY 14831. DONALD R. SADOWAY, Professor of Materials Chemistry, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4307. Manuscript submitted July 3, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
axial cylinders technique.[3,4] The purpose of this article is to demonstrate the applicability of the new technique to the measurement of the electrical properties of oxide melts. II.
REVIEW OF AVAILABLE TECHNIQUES
For the measurement of the electrical conductivity of a liquid, the literature reports 11 techniques based on eight electrode designs. In the opinion of the authors, none is fully capable of supplying high-accuracy data in molten oxide systems. Thus, although the electrical conductivities of various molten oxides have been measured in the past, it is the opinion of the authors that these data are, by and large, inaccurate. For a complete review of these techniques along with an explanation of their shortcomings, the reader is directed to another document.[5] Very briefly, what sets high-accuracy techniques apart from low-accuracy techniques is that high-accuracy techniques establish well-defined current paths, i.e., current paths whose functional dependence upon the electrical properties of (1) the liquid under investigation, (2) the electrodes, and (3) the container is well characterized. There were seven low-accuracy techniques based on five electrode designs: (1) two wire, (
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