Standard gibbs energies of formation of the carbides of manganese by emf measurements
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I.
INTRODUCTION
M A N G A N E S E is one of the most common alloying elements in steel. Complex carbides involving manganese are often found to occur in alloy steels. A knowledge of the thermodynamic properties of the carbides of manganese would be of great value in the understanding of the more complex carbide systems. A number of experimental measurements of the thermodynamic properties of manganese carbides are found in literature. These are listed in Table I along with the experimental techniques adopted. It is seen that among the various carbides of manganese, Mn7C 3 has been studied most extensively. II-71 A comparison of the standard Gibbs energies of formation of this carbide, AG~,In7C3, reported from the various experimental data reveals that the results are somewhat consistent with each other, as can be seen in Table II. On the other hand, the same is not true in the case of the other carbides of manganese. For example, AH~In23C6 has been variously reported as - 3 8 4 . 9 kJ tool-l, Ill - 8 2 . 8 kJ mol-1, t8J and - 2 4 5 . 5 kJ mo1-1.19]The present work aims at obtaining a set of self-consistent thermodynamic data for the carbides in the Mn-C system under well-defined experimental conditions.
DU SICHEN, Graduate Student, S. SEETHARAMAN, Docent and Senior Lecturer, and L.-I. STAFFANSSON, Professor, are with the Department of Theoretical Metallurgy, The Royal Institute of Technology, S-100 44 Stockholm, Sweden. Manuscript submitted January 25, 1988.
Table I. Reference Moattar and Andersontll Tanaka et al. t21 McCabe and HudsonI31 Gokcen and Fujishiro/4j Butler et al. tsj Benz[6] Eremenko et al. [71 Alekseev and SchwartsmanlSJ Dawson and Sale191
METALLURGICALTRANSACTIONSB
Solid-state galvanic cell technique with C a F 2 a s the solid electrolyte was employed as the experimental method in the present work. The galvanic cell can be represented as ( - ) M n , MnF 2, C a F 2 / / C a F 2 / / C a F 2, MnF2, 'Mn-C'( + ) where 'Mn-C' stands for appropriate manganese-carbon alloys corresponding to the various two-phase fields in the MnC system. The phase diagram available in the literature rl~ has been reproduced in Figure 1. CaF 2 is a pure fluoride ion conductor under the experimental conditions. Mn-MnF2 was used as the reference electrode in all the experiments.
II.
EXPERIMENTAL
A. Materials
Pure Mn powder (99.9 pct, E. Merck, W. Germany) having a particle size less than 150/zm and specpure graphite powder of pelletable quality from Johnson Matthey Ltd., U.K. were used in the preparation of the Mn-C alloys. Single crystals of CaF 2 (15 mm in diameter and 5 mm in thickness) which were used as electrolytes were supplied by Harshaw Chemicals, Holland. CaF 2 powder (suprapure, E. Merck, W. Germany) and MnF 2 powder (Ventron Alpha produkte, W. Germany) were used in the preparation of the electrodes. Both CaF 2 and MnF 2 powders were dried at 673 K in vacuum (2 • 10-5 bar) for 12 hours before use. The argon gas used in the present work was supplied by AGA Special Gas, Stockholm. The gas was passed through col
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