Determination of standard gibbs energies of formation of CaC 2 , SrC 2 , and BaC 2
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I.
INTRODUCTION
IT is well known that CaO and BaO are excellent components of refining fluxes due to their highly basic nature. Their accurate standard Gibbs energies of formation are needed for evaluating their refining properties. Previously, we evaluated the standard Gibbs energy of formation of CaO m and its accuracy depending on the standard Gibbs energy of formation of CaC2. The standard Gibbs energies of formation of MC2 (M: Ca, Sr, and Ba) by various investigators are not in accord, as shown in Section III. For this reason, the purpose of the present work was to obtain reliable data for MC2 and to establish the standard Gibbs energies of formation of MO. The reaction of formation of MC2 is expressed by Eq. [1], M (1) + 2C (s) = MC2 (s) [11 The value of the standard Gibbs energy, A G ~ , for Eq. [1] can be derived by measuring AG ~ for Eq. [2] by equilibrating solid iron with liquid M and solid MC2. M (1) + 2C (in 7 iron) = MC~ (s)
[2]
Assuming that the solubilities of carbon and Fe both in liquid M and in solid MC2 are negligible, AG ~ for Eq. [2] is represented by Eq. [3]. AG ~ = 2 R T l n a c [31 where ac is the carbon activity relative to pure graphite. The value of carbon activity in ? iron covering the whole austenite temperature range has been measured by Ban-ya e t a l . t21 using CO2/CO equilibration of Fe-C alloys and is represented by Eq. [4]. log ac -
3770 T
+ 2.72 log T
3860Yc Yc - 10.525 + - + l o g T 1-Yc
[4]
HIDEKI ONO, AKIO KOBAYASHI, Graduate Students, FUMITAKA TSUKIHASHI, Associate Professor, and NOBUO SANO, Professor, are with Department of Metallurgy, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan. METALLURGICAL TRANSACTIONS B
where Yc is the atom ratio n c / n F e . Accordingly, the value of AG ~ for Eq. [1] can be derived from the carbon concentration of ~, iron which is measured at temperatures ranging from 1223 to 1673 K. Chipman t31 proposed a similar equation to describe the activity of carbon based on the data by previous investigators. However, Eq. [4] was chosen in the present study because it is based on the more recent measurement by the same author and covers a wider temperature range.
II.
EXPERIMENTAL
The apparatus consisted of a SiC electric resistance furnace which is connected to a PID controller with a Pt/6 pct Rh-Pt/30 pct Rh thermocouple. Temperature was controlled within -+2 K over a length of 10 cm in the furnace from 1223 to 1673 K. A mullite tube (50-ram OD, 42-ram ID, 630-mm long) was used as a furnace tube. MC2 was synthesized by mixing metallic M (99 pct purity) and graphite powders in a graphite crucible (26-mm OD, 22-mm ID, 4 5 - m m long) tightly covered with a lid for 24 hours at 1273 K in an argon atmosphere. The synthesized compound was confirmed to be MC2 by X-ray diffraction analysis. An iron thin sheet (5 • 50 • 0.5 m m , 99.99 pct purity) as the sample was equilibrated with MC~ weighing about 1 g and M (99 pct purity) weighing about 5 g in an iron crucible (20-ram OD, 15-mm ID, 62-mm long) in an argon atmosphere. A lid was screwed in
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