Determination of standard gibbs energy of formation of Ca 3 N 2
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Communications Determination of Standard Gibbs Energy of Formation of CaaNa
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395
0.183 tvl
[5]
T H I D E K I ONO, F U M I T A K A T S U K I H A S H I , and N O B U O SANO A refining technique to remove tramp elements from steels under strongly reducing conditions has been developed using calcium-based fluxes, such as Ca-CaF2, II,21 in which impurities of IVA or VA group elements form Ca3M2 (M: N, P, As, Sb, Bi) or Ca2M (M: Pb, Sn). Although thermodynamic properties of such calcium compounds are needed for characterizing the efficiency of the process, there are few available data for the standard Gibbs energies of formation of these compounds which are derived by direct measurement. Recently, the standard Gibbs energies of these compounds except Ca3N2 were determined by the authors' group using a chemical equilibration technique, pointing out that some literature data include errors.J3.4.5] As for Ca3N2, the values in the literature are limited at temperatures lower than 1500 K, so that we have measured the standard Gibbs energy of formation of Ca3N2 directly at temperatures ranging from 1523 to 1673 K using a similar technique described subsequently. The principle of determination is as follows. The reaction for formation of Ca3N2 is expressed by Eq. [ 1]: 3Ca (1) + N 2 (g) = Ca3N2 (s)
[1]
In order to determine AG O for Eq. [1], AG O for Eq. [2] was measured by equilibrating CaC2, Ca3N2, and C and N in 3' iron.
[2]
AG o for Eq. [2] is represented by Eq. [3]: AG o = RT (6 In ac - 2 In aN)
[3]
where ac and a~ are the carbon activity relative to pure graphite and the activity of nitrogen in 3' iron relative to dilute solution, respectively. The value of carbon activity for 3' iron covering the whole austenite temperature range was measured by Ban-ya et al. and is represented by Eq. [4]. [61 3770 log ac =
T
+ 2.72 log T - 10.525
3860Yc Yc + - + log T 1-Yc
[4]
HIDEKI ONO, Graduate Student, FUMITAKA TSUKIHASHI, Associate Professor, and NOBUO SANO, Professor, are with Department of Metallurgy, The University of Tokyo, Bunkyo-ku, Tokyo, 113 Japan. Manuscript submitted March 22, 1993. METALLURGICALTRANSACTIONSB
1/2 N2 (g) = N (mass pct in 3' Fe)
[6]
AG O = - 8 6 1 3 + 37.42T (J/mol)
[7]
Ca (1) + 2C (s) = CaC2 (s)
[8]
AGO= - 9 0 100 (___500) - 11.0 (+ 1.1)T [J/moll
[9]
The experimental procedure is as follows. An iron sheet (10 x 25 x 0.5 ram, carbon concentration 0.00 to 0.33 mass pct) was equilibrated with CaC2satd-CaF2 slag weighing 4 g and Ca3N2 (supplied from Johnson Matthey, 99 pct purity) weighing 2 g in an iron crucible (20-mm OD, 15-mm ID, 62-mm height) in an argon atmosphere at temperatures ranging from 1523 to 1673 K. The calcium carbide was synthesized by mixing metallic calcium (99.5 pct purity) and graphite powders in a graphite crucible tightly covered with a lid for 24 hours at 1273 K in an argon atmosphere. For the purpose of avoiding evaporation and oxidation of the contents, a lid was screwed into an iron crucible for sealing. The equilibration time was predetermined to be 20 h
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