Emf of the cell was measured as a function of oxygen concentration in liquid cobalt at 1798, 1873 and 1973 K. Least-mean squares regression analysis of the ...
Bd. 76 (1985) H. 10
Thermodynamics of Co-O Solutions 709 ----------------------------------------
------------------------------------~---
Thermodynamic Properties and Phase Boundaries of CO-O Solutions K. Thomas Jacob and Jnan P. Hajra (Department of Metallurgy, Indian Institute of SCience, Bangalore 560012, India) The thermodynamic properties of liquid unsaturated Co-O solutions have been determined by electrochemical meas urements using (Y203)Th02 as solid electrolyte. The cell can be represented as, Pt, Mo0 2 + Mo I (Y20 3)Th0 2 1Oco, W, Pt. Emf of the cell was measured as a function of oxygen concentration in liquid cobalt at 1798, 1873 and 1973 K. Least-mean squares regression analysis of the experimental data gives for the free energy of solution of diatomic oxygen in liquid cobalt LJG8(co) = -84935 - 7.61 T (± 400) JIg-atom and self interaction parameter for oxygen = - 97240lT + 40.52 (± 1) where the standard state for oxygen is an infinitely dilute solution in which the activity is equal to at.%. The present data are discussed in comparison with those reported in the literature and the phase diagram for the Co-O system.
[8
Thermodynamische Eigenschaften und Phasengrenzen von Co-O-Losungen Die thermodynamischen Eigenschaften von flUssigen, ungesattigten Co-O-Lbsungen wurden durch elektrochemische Messungen bestimmt, und zwar unter Verwendung eines (Y20 3)Th0 2 -Festelektrolyten in der elektrochemischen Kette Pt, Mo0 2 + Mo I (Y20 3)Th0 2 1 OCo, W, Pt. Die EMK des Elements wurde als Funktion der Sauerstoffkonzentration in flUssi gem Kobalt bei 1798, 1873 und 1973 K gemessen. Die Regressionsanalyse der Versuchsdaten durch die Methode der kleinsten Quadrate ergab fUr die freie Energie der Lbsung des molekularen Sauerstoffs in flUssigem Kobalt AG8(co) = - 84935 - 7,61 T (± 400) JIg-Atom und fUr den Wechselwirkungsparametervon Sauerstoff = - 97240lT + 40,52 (± 1), wobei der Bezugszustand von Sauerstoff eine unendlich verdUnnte Lbsung ist, in der die Aktivitat gleich dem At.-% ist. Die vorliegenden Daten werden den in der Literatur berichteten gegenUbergestellt und anhand des berechneten Zustandsdiagramms des Co-O-Systems diskutiert.
[8
1
Introduction
2
Thermodynamic properties and phase boundaries of Co-O solutions in both solid and liquid state are not well estab lished. Early investigations of Seybolt and Mathewson 1) were directed towards solid solubility of oxygen in cobalt at temperatures between 873 and 1773 K. Asanti and Kohl meyer 2) studied the solubility of oxygen in liquid cobalt at temperatures between 1823 and 1973 K and found that liquid cobalt dissolves upto a maximum of 11 at.% oxygen. The cobalt-rich eutectic composition1)2) was reported to be between 0.77 and 0.91 at. % of oxygen at 1724 K. A com plete phase diagram of the Co-O system is not yet avail able. Hansen 3) gives a schematic phase diagram for Co rich compositions. Thermodynamic behaviour of dilute solutions of oxygen in molten cobalt has been investigated by several researchers 4) to 12). Many of them4) to 7)9) 10) have studied the reaction, (1 )
where Oeo stands for oxygen dissolved in cobalt. Fischer and Ackermann B) employed a solid state electrochemical cell with (CaO)Zr02 as the electrolyte for determining Gibbs' energy of solution of oxygen in liquid cobalt. Fischer and Janke 11 ) have made similar measurements on liquid Co-Cu-O alloys. Because of differences in the reported results, a recent review by Chang and Chang12) has identi fied the need for further experiments. Electrochemical measurements using (Y203)Th02 as electrolyte have been carried out in the present study as part of a larger pro gramme of research on thermodynamic properties of interstitial solutes in metals and alloys. A calculated phase diagram for Co-rich compositions based on these results is also presented along with data available in the litera ture4)7) 12).
Experimental Aspects
Emf measurements were made using an apparatus similar to that described earlier13)14). The oxygen potentials over liquid Co-O alloys, contained in Th0 2 crucibles, were measured with a solid state cell, which can be represented as, Pt, Mo+Mo02/(Y203)Th02/0co, W, Pt.
(I)
Preliminary experiments with (CaO)Zr02 solid electrolyte indicated an increasing tendency for reaction between the solid electrolyte and liquid cobalt with oxygen concentra tion. The outer surface of the electrolyte tube appeared to be destabilized after approximately 3 h exposure to liquid cobalt. The emf of the cells with (CaO)Zr0 2 was slightly time dependent. These problems were greatly reduced when (Y20 3)Th0 2 was used as an electrolyte. Cobalt metal used in the experiments was 99.999% pure. The tungsten lead 2 mm in diameter was sheathed in a ceramic tube. The W-pt contact was at the same temperature as the liquid metal, thus avoiding the need for corrections to the measured emfforthermoelectric effect. The reference electrode consisting of an equimolar mixture of Mo + Mo0 2 was chosen to minimize the emf of the cells. The reference electrode was placed inside the (Y20 3)Th0 2 tube, which contained 7.5 mole % Y20 3. The tungsten lead was dipped into liquid cobalt only intermittently for meas urements to avoid significant dissolution. Attempts to find other inert materials for electrical contact to liquid cobalt were unsuccessful. Lanthanum chromite, which showed some promise, was found to introduce chromium into the liquid metal. Since interaction between chromium and oxygen is expected to be strong, the use of lanthanum chromite can lead to systematic errors. The solid electro lyte tube was also withdrawn and held just above the liquid
710
Z. Metallkde.
K Thomas Jacob and Jnan P. Hajra
metal during heating and cooling periods and during addi tion of cobalt or oxygen to the melt. The emf of the cell was measured as a function of oxygen concentration in liquid cobalt at 1798, 1873 and 1973 K. The oxygen concentration was varied by adding small amounts of cobalt monoxide wrapped in a foil of cobalt, a Co-O master alloy or oxygen free cobalt. Steady emfs were obtained after each addition in approximately 10 min. Reproducibility of the emf was better than ± 1 mY. The cells were found to be stable and reversible at each temperature. The emf was insensitive to the flow rate of the purified argon over the melt. Small cur rents (50 pA) were passed through the solid electrolyte for 200 s in both directions and the emf response after each oxygen titration was recorded. The emfs were found to return to the steady state value before coulometric titra tion in approximately 600 s, thus establishing reversibility of the cell. Oxygen concentration of the melt correspond ing to each emf was determined by suction sampling and subsequent analysis. Sampling was done with a silica tube, flushed with argon gas. Silica tube with the sample was quenched in liquid nitrogen. Oxygen analysis was done by an inert gas fusion technique in which the dissolved oxy gen is reacted with a carbon crucible to produce carbon monoxide. A Leco TC 236 unit was used for the analysis. The samples taken towards the end of a run were also ana lysed for tungsten. The maximum tungsten content was 0.14 at.%. When larger amounts of tungsten (upto 0.4 at.%) were intentionally dissolved in liquid cobalt at fixed oxygen concentration, the emf and therefore the activity of oxy gen was unaltered.
Variation of the logarithm of pseudo-equilibrium constant for reaction (4) expressed as, K'
=
at.% 0
(6)
P112 02 with oxygen concentration is shown in Fig. 2 at different temperatures. Since the results exhibit a linear relation, variation of In f8with concentration can be expressed by Wagner's17) first order interaction parameter,
o 0 at.% 0 In f o =eo-100
(7)
The interaction parameter eg = cl In f810 Xo and AGoo(co) for the reaction (4) are obtained from the slope and intercept of the lines in Fig. 2. The least-mean squares regression analysis gives,
eg = - 97240 T
+ 40.52
(± 1)
AG8(co) = - 84935 - 7.61 T (± 400) J/g.atom
(8) (9)
The uncertainty limits are evaluated on the basis of errors in temperature measurement, emf, oxygen analysis and the possible effects of small amounts of dissolved tung sten on oxygen activity. The activity coefficient of cobalt in the Co-O liquid alloys can be calculated using Gibbs Duhem relation for dilute solutions of oxygen, In YCo
=
X~ [48~20 -
20.26]
(10)
Equations (8) to (10) may be used for calculation of the phase boundaries in the Co-O system.
3
Results Table 1.
The steady emfs obtained for the liquid Co-O alloys using cell I are shown as a function of composition in Fig. 1 at 1798, 1873 and 1973 K. The results are also summarised in Table 1. Since the transport number of oxygen ions in the electrolyte is close to unity at the temperatures and partial pressures of oxygen established at the electrodes, the emf of the cell is given by,
E = RT In 4F
P0 2 (Co-O) P0 2 (Mo + Mo0 2 )
AG~002
=
-
579900 + 167.36 T Jlmol
T,K
emf, mV
Analysed oxygen at.%
- 222 - 137.6 - 118 - 52.7 8.4 65.3 112
0.0037 0.0110 0.0142 0.0330 0.0731 0.154 0.286
1873
- 295 - 206.5 -160.5 -- 62.2 - 21.4 41.8 95.0 106.4
0.0029 0.0087 0.0154 0.0523 0.0871 0.193 0.381 0.442
1973
- 296.4 - 282.5 - 233.9 - 174.5 -128.9 - 87.3 - 26.9 42.7 82.7
0.0067 0.0079 0.0140 0.0282 0.0483 0.0790 0.162 0.374 0.612
(3)
This equation is in good agreement with recent Janaf data16) for Mo0 2. The free energy of solution of oxygen in liquid cobalt is evaluated using a temperature dependent first order interaction model originally suggested by Wag ner17). For the reaction, 1/2 02(g) = Oco(l)
(4)
the 'free energy change is given by,
AG8(co) = - RT In K4 = - RT In
(at.%11~)f8
+ Mo02/(Y203)Th02/
1798
(2)
where P0 2 (Co-O) is the partial pressure of oxygen in equi librium with Co-O alloy and P02(Mo+Mo0 2 ) is the partial pressure of oxygen at the reference electrode. The latter is obtained from the standard free energy of formation of M00 2 15),
Measured emf of the cell Pt, Mo
.Qco, W, Pt as a function of oxygen composition and temperature.
(5)
P0 2
where standard state for dissolved oxygen is taken as an infinitely dilute solution in liquid cobalt in which activity equals at.% and f8 is the activity coefficient of oxygen, expressing deviations from Sievert's (or Henry's) law.
4
Discussion
The standard Gibbs energy of solution of oxygen in cobalt obtained in the present investigation is compared as a "function of temperature in Fig. 3 with data available in
Thermodynamics of Co-O Solutions
Bd. 76 (1985) H. 10
711
-92
1. This study. 5) 2. Flondls & Chipman •
3. Sakuo & Sana 6) ~ 4. Tankins etal 7.\10l
·2
5. BelaY etal 9) 6. Fischer et at 8)11) 7. Chang & Chang 12) (Evaluation)
c;
00-100
(C)
Averin et at. 4) ... Chang & Chang 12)
: [0-0 (5) +[00(5)\
-23 5.0
\\
: I
I
6.0
+
x
7.0 10-4 , K-1
8.0 _
Fig. 4. Variation of logarithm of partial pressure of oxygen over solid and liquid CO-O solutions, saturated with CoO(s), as function of the reciprocal temperature.
800
0
0.5
1.5
1.0 O,ot.%
1)
2.0
Fig. 6. The calculated Co-O phase diagram based on the pres ent study and the data available in the literature.
Bd. l6 (1985) H. 10
Thermodynamics of Co-O Solutions
Literature
The equations are, In al
l13
= - 0 569462 + 1890.79 _ 1.5766 X 10 T f2
6
(17)
Co·
In a~o = 1.10291 - 1951.9
(18)
T
where the standard state for Co in both cases is pure liquid, the Cp data for solid and liquid Co and enthalpy of fusion used in the calculation are obtained from Janaf tables 16). As shown in Fig. 5, Eq. (17) and (18) give an eutectic tem perature of 1763 K which is 5 K lower than the melting point of pure cobalt. Calculation of eutectic temperature based on activity of cobalt is likely to be more accurate for Co rich alloys than that based on partial pressure of oxygen. A phase diagram for the CO-O system, based on the thermo dynamic calculations is shown in Fig. 6 along with data reported in literature4)7) 12). Clearly the eutectic tempera ture given in the literature appears to be in error. Impurities in cobalt used in the early study1)2) might have produced a iower eutectic. Direct experimental determination of the liquidus and solidus are needed to define the eutectic more precisely. The solid solubility results also need r~confirmation.
The authors are grateful to the Natural Sciences and Engi neering Research Council of Canada and the Materials and Processes Panel of the Aeronautics Research and Devel opment Board of Government of India for financial support.
1) A V SEYBOLT and C. H. MATHEWSON, Trans AIME 117 (1935) 156. 2) P. ASANTI and E. J. KOHLMEYER, Z. Anorg. Chem. 265 (1951) 90. 3) M. HANSEN and K. ANDERKO, Constitution of Binary Alloys, Second Edition, McGraw-Hili Book Co., New York (1958). 4) V V AVERIN, A. Y POLYAKOY, and A. M. SAMARIN, Izv. Akad. Nauk. SSSR., Otd. Tekhn (8) (1957) 120. 5) T. P. FLORIDIS and J. CHIPMAN, Trans.TMS-AIME 212 (1958) 549. 6) H. SAKAO and K. SANO, J. Japan Inst. Metals, 26 (1962) 30. 7) E. S. TANKINS, N. A. GOCKEN, and G. R. BELTON, Trans. TMS AIME 230 (1964) 820. 8) W A. FISCHER and W ACKERMANN, Arch. EisenhUttenw. 37 (1966) 43. 9) B. F. BELOY, LA. NOVOKHATISKY, and Yu. A. LOBANOV, Izv. Akad. Nauk. SSSR, Metal (3) (1967) 19. 10) E. S. TANKINS and W BECK, Z. Metallkde. 58 (1967) 721. 11) W A. FISCHER and D. JANKE, Z. Metallkde. 62 (19l1) 747 12) T. CHANG and Y A. CHANG, Met. Trans. B 7 (1976) 453. 13) K. T. JACOB and M. IWASE, Z. Metalikde. 73 (1982) 316. 14) K. T. JACOB, J. P. HAJRA, and M. IWASE, Arch. EisenhUUenw. 55 (1984) 421. 15) C. B. ALCOCK and J. C. CHAN, Can. Met. Quart 11 (1972) 559. 16) JANAF Thermodynamical Tables, 1974 Supplement, J. Phys. Chem. Ref. Data 3 (1974) 412. 17) C. WAGNER, Thermodynamics of Alloys, Addison-Wesley, Reading, MA (1952). 18) G. K. SIGWORTH and J. F. ELLlOn, Metal Sci. 8 (1974) 298.
(Eingegangen am 18. Marz 1985)
