Determination of standard gibbs energies of formation of Cr 2 N and CrN
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mation of Cr2N and CrN determined by the present work are more negative than the values in the JANAF Tables by about 14 and 5 kJ/mol, respectively, in the measured temperature range.
I. INTRODUCTION
IT
is necessary to know the thermodynamic properties of various compounds relevant to the refining reactions in the steelmaking process to control the impurities in steel. The standard Gibbs energy is one of the most important thermodynamic properties that determine the stabilities of compounds and the directions in which various chemical reactions proceed. However, many of the data are generally obtained by calorimetry at comparatively lower temperatures, and the direct measurements of the standard Gibbs energies at higher temperatures are relatively limited. The standard Gibbs energies of formation of alkaline earth carbides,[1] oxides,[2] and calcium nitride[3] were previously determined by one of the authors; it is found that the values described in some of the literature include certain errors. Chromium is widely used as one of the alloying elements for stainless steels, etc.; the formation of chromium nitride is often found in the chrome steel because the thermodynamic affinity of chromium with nitrogen is stronger than that of iron. Accordingly, in the present study, the standard Gibbs energies of formation of Cr2N and CrN are investigated at higher temperatures using both a chemical equilibration technique and thermogravimetry and differential thermal analysis (TG-DTA). The standard Gibbs energy of formation of Cr2N is determined as follows. The reaction of formation of Cr2N is expressed by
HIDEKI ONO-NAKAZATO, Research Associate, KENJI TAGUCHI, Graduate Student, and TATEO USUI, Professor, are with the Department of Materials Science and Processing, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan. KEISHI TAMURA, formerly Graduate Student, Department of Materials Science and Processing, Graduate School of Engineering, Osaka University, is with Hitachi Ltd., Kanagawa 244-0817, Japan. YUUJI TOMATSU, formerly Undergraduate Student, Department of Materials Science and Processing, Osaka University, is with Nissho Iwai Corporation, Osaka 541-0042, Japan. Manuscript submitted March 26, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
2Cr (s) ⫹
1 N2 (g) ⫽ Cr2N (s) 2
[1]
The value of the standard Gibbs energy, ⌬G⬚, for Eq. [1] can be derived by measuring ⌬G⬚ for Eq. [2] through equilibrating chromium in liquid silver with solid Cr2N at a controlled nitrogen pressure. 2Cr (in Ag) ⫹
1 N2 (g) ⫽ Cr2N (s) 2
[2]
The ⌬G⬚ for Eq. [2] can be represented by ⌬G⬚(2) ⫽ RT ln
a2Cr ⭈ PN1/22 aCr2N
[3]
where aCr and aCr2N denote the activity of chromium and Cr2N, respectively, relative to pure solids; PN2 the nitrogen partial pressure (atm); R (⫽8.314) the gas constant(J/mol ⭈ K); and T the temperature (K). Assuming that the solubility of silver in solid Cr2N is negligible, Eq. [3] is represented by Eq. [4] or [5] under the present experimental condition of PN2 ⫽ 1(atm). ⌬G⬚(2) ⫽ 2RT ln aCr ⫽ 2RT ln (␥⬚
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