Thermodynamic investigation of the solid and liquid AuSb alloys

Intermetallics 2 (1994) 285-288

Thermodynamic investigation of the solid and liquid Au-Sb alloys Pascal Anres, Hel~ne Bros & Robert Castanet* Centre de Thermodynamique et de Microcalorim~trie du CNRS, 26 rue du 141eme R.I.A., F-13003, Marseille, France

(Received 18 November 1993; accepted 17 January 1994) The heat content of solid and liquid AuSb2 compound was measured from 298 K to T (375-963 K) on heating (drop method) with the help of a Tian-Calvet calorimeter. The heat capacity of the liquid compound as well as its enthalpy of fusion were deduced. The enthalpy of the liquid decreases strongly when temperature increases between the melting point and 831 K. The enthalpy of formation of the Au-Sb melts was also determined by direct reaction calorimetry at 916 K with respect to concentration. The enthalpy of mixing is weakly negative in the whole range of concentration (h~in : -3-47 kJ/mol at Xau = 0-775) in agreement with the results of B6ja at 923 K. Our data disagree with the much more negative earlier data of Kameda et al. and of Hino et al. by emf and vapor pressure measurements. Finally, the liquid/Au(cr) phase boundary determined at 916 K from the break in the hf (XAu) curve agrees well with the phase diagram calculated by Okamoto and Massalski but not with their experimental results. Key" words." calorimetry, enthalpy, enthalpy of mixing, heat capacity, gold,

antimony. 1 INTRODUCTION

A u - S b liquid and the enthalpy of the solid and liquid AuSb 2 compound.

According to O k a m o t o and Massalski's critical assessment, ~ liquid A u - S b alloys belong to the class o f melts whose thermodynamic properties show a very strong temperature dependence. However, there are very large disagreements between the results o f previous measurements of the enthalpy of mixing since it shows a minimum of - 3 . 2 kJ/mol at 923 K according to B6ja z but - 1 8 . 5 kJ/mol at 1023 K from O k a m o t o and Massalski. Moreover, the critical assessment of these last authors leads to strongly negative Cp~-data which correspond to very unusual b e h a v i o u r ) Moreover, the melting point (or the peritectic temperature) o f the single c o m p o u n d (AuSb2) is low (733 K) when compared to those of the pure components. Then, from its phase diagram, the expected behaviour of the Au-Sb melts should be very close to that o f the G e - T e , 4 A u - P b s and A u - T e 6'7 liquids, the thermodynamic properties o f which we investigated previously, although the enthalpy of mixing o f A u - S b appears to a little more negative. Hence, as in the case of A u - T e melts, we carried out measurements of the enthalpy o f mixing of the

2 CALORIMETRIC METHODS The apparatus employed was a very high-temperature (T < 1800 K) Tian Calvet calorimeter. The drop methods used (direct reaction calorimetry and enthalpimetry) have already been described. ~ The molar heat content variation of AuSb 2 from 298 K to T (375-963 K) was deduced from the heat effects corresponding to drops of solid AuSbz samples (~30 rag) in an empty graphite crucible placed in the cell of the calorimeter. The measurements were repeated about ten times at each temperature. The samples were synthesized by melting together pure components in suitable proportions. They were annealed for two days just below the melting temperature (725 K) and slowly cooled to room temperature. Then we verified their structural state by X-ray diffraction. The diffractogram obtained agrees well with the A S T M files. These samples were used for the heat-content determinations. The integral enthalpy of formation of the A u - S b melts, h f, was deduced from the heat

* To whom correspondence should be addressed. 285

Intermetallics 0966-9795/94/$7.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain

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P. Anres, H. Bros, R. Castanet

effects corresponding to successive additions of small quantities of Au (about 20-50 mg according to the concentration range) at TO (near 298 K) into the liquid bath placed in a graphite crucible at the bottom of the calorimetric cell at temperature T. Before the first addition of gold, the melt was pure antimony (about 1 g). The thermal effects correspond to the reaction

50

f

o

E

t

¢: @

25

I ITfus

o

I

¢1 o Z

nAu(cr,T0) + mAuxSby (1,7) (n + m)Auxm xSby4y (1, 7)

0 300

Each heat effect corresponding to successive additions of gold leads to the enthalpy of formation of the alloy against composition with respect to pure liquid components at T taking into account the enthalpy change of gold from To to T and its enthalpy of fusion (neglecting any temperature dependence) deduced from Hultgren et al. 9 The pure components used were purchased from Koch-Light and had metallic impurities less than 10-3 mass %. The calibration of the calorimeter was performed by adding some small pieces of a-alumina (U.S. National Institute of Standards and Technology) the enthalpy change of which from To to T is well known. 1°

3 RESULTS

500

700

900

1100

Temperature (K)

Fig. 1. Variations per mole of atoms in the heat content of AuSb2 with temperature from 298 K to T.

suits at 713 and 723 K clearly show a prefusion phenomenon not already observed. The enthalpy jump at 733 K leads to a value of the enthalpy of fusion (18.64 kJ per mole of atoms, taking into account the prefusion phenomenon) close to that determined by Itagaki 12 (18.62 k J/tool). The results obtained in the liquid state exhibit a strong temperature dependence as shown in Fig. 1. From 733 to 831 K they can be fitted to the following equation: H(T) - H (298 K) = - 88-02 + 0-262 T - 0.138 10-3 T 2

3.1 Enthalpimetry The values of the enthalpy of AuSb 2 from 375 to 963 K are given in Table 1 and shown in Fig. 1. In the solid state (T < 733 K), our data agree well with the values calculated from the Cp-data of Kelley 1~ and Itagaki. 12 Since it was not the purpose of this work to determine the thermodynamic properties of AuSb 2 in the solid state, we measured its enthalpy only at six temperatures. We did not derive its heat capacity. However, the re-

where H is in kJ/mol and T in K. Above 831 K, the heat capacity can be considered as nearly temperature-independent (26.7 J/ (K mol)). The heat capacity of AuSb2 in the liquid state is shown in Fig. 2 as a function of temperature, according to the present work and to 80 ITfu,

Table 1. Variations of the heat content of AuSb 2 from 298K to T per mole of atoms

60 o

T(K)

375 490 613 683 713 723 733 741 749 754 761

H(T) - H(298 K) (kJ/mol)

T(K)

2.06 4.93 8.24 10-48 11-74 12.81 29.79 30.71 31.05 31.25 31.55

767 790 811 831 850 871 903 917 933 963

H(T) - H(298 K) (kJ/mol) 31-95 32.61 34.19 34.49 34.25 36.00 35.57 35.88 37-60 36.98

E .-j

40

8 20

0

700

i

i

800

900

1000

Temperature (K)

Fig. 2. Heat capacity of the AuSb 2 melts with respect to temperature. Full line: present work; dashed line: according to ItagakiJ 2

287

Thermodynamic investigation of solid and liquid A u-Sb alloys Table 2. Molar integral enthalpy of formation of the Au-Sb melts at 916 K referred to both pure liquid components E

XAu

-h f

XA~

-h f

(kJ/mol)

(kJ/mol) E

Series 1

0.101 0.131 0.165 0.206 0.245 0.286 0.329 0.379 0.431 0.487 0.558 0.620 0.765 0.818 0-869 0.933 0-972

Series 2

0.62 0-91 1.03 l- 10 1.35 1-62 1.98 2-23 2.67 2.76 3.1 l 3.27 3.43 4-42 6-53 9.25 11.13

0.044 0-087 0.143 0.206 0.259 0.313 0.378 0.425 0.468 0-508 0-580

"6

0.17 0.44 0-68 1.21 1.65 1.78 2-12 2.43 2.52 2.70 2.45 Series 3

0.627 0-704 0-777 0.835 0.900 0.940 0-963

2.74 2.92 3-19 5.03 7-93 9-74 10.84

Itagaki. ~2 The temperature-independent results of Itagaki from 733 to 873 K match the mean value of our results in the same temperature range. Such a decrease in the heat capacity of a metallic melt has already been observed in the liquid state by Komarek and coworkers on the Cd-Sb ~3'14 and on the Cu-Sb alloys. ~5 We observed the same behaviour in the Ge0 ~5Te0.854and A u T e J liquid alloys. 3.2 Enthalpy of formation of the melt The results of the measurements of the enthalpy of formation of the Au-Sb liquid alloys at 916 K are listed in Table 2 and shown in Fig. 3 versus the mole fraction of Au (XAu). The heat content of gold used to change the reference state from Au(cr, 298 K) to Au(1,916 K) was taken from the compilation of Hultgren et al. 9 As can be seen from Fig. 3, our results at 916 K agree well with those of B6ja 2 at 923 K. They can be fitted to the following equation:

:E

\ -12 ! 0.00

0.25

0.50

0.75

1.00

Mole fraction of Au

Fig. 3. Molar integral enthalpy of formation of the Au-Sb melts referred to both liquid components. Full line and open circles: present work (916 K) (the straight line in the Au-rich part corresponds to the liquid-Au(s) two-phase domain;) thin line: supercooled liquid (extrapolated); dotted line: from Bdja 2 (923 K).

pure gold (-12-55 according to Hultgren et al,). The phase boundary deduced from this work agrees well with calculations for the Au-rich liquidus by Okamoto and Massalski ~ (Fig. 4), but differs from their assessed experimental phase diagram based mainly on the investigations of Vogel. ~6 However, a more recent calculation of the phase diagram by Okamoto Massalski ~7 is in better agreement with the experimental results. Our results confirm the weak negative values of the enthalpy of mixing of the Au-Sb melts. They differ strongly from those of Kameda et al. zs (973-1073 K) and Hino et al. 19 (1073-1473 K) obtained from indirect methods (emf measurements and vapour-pressure measurements with the transportation method, respectively). Moreover, their results assessed by Okamoto and Massalski ~ lead to very unusual negative values of the excess heat

1250

h~ ®

k-

1000

750

h f = XAu(1 -- XAu)( -- 6.806 + 1.817XAu-- 22.41X2A.)

where h f is in kJ/mol, which leads to a minimum of -3.47 kJ/mol at XA, = 0-69. For Xau > 0'775, the Au-Sb alloys correspond to a two-phase domain and the data can be fitted according to a linear dependence with respect to concentration. We found h f = -12.36 kJ/mol at XA~ = 1, which is very close to the enthalpy of crystallization of

500 0.00

~ 0.25

~ 0.50

0.75

~ 1.00

Mole fraction of Au

Fig. 4. Phase diagram of the Au Sb system. Dashed lines: experimental phase boundaries critically assessed by Okamoto and Massalski; ] full lines: calculated by Okamoto and Massalski; ] square point: from the break in the h r curve at 916 K (present work).

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P. Anres, H. Bros, R. Castanet

capacity which is inconsistent with the thermodynamic behaviour o f the A u S b 2 melts.

4 CONCLUSION Out heat content measurements on liquid AuSb2 show clearly that Cp xs decreases when the temperature increases from the melting point to 831 K (Hultgren's assessed heat capacity o f pure liquid gold and tellurium, are independent of temperature). The exact value of Ce xs cannot be calculated since the heat capacity o f supercooled pure gold cannot be evaluated with precision at a temperature so far from its melting point. It can be concluded from this work that the thermodynamic properties of the A u - S b melts deviate more from ideality than those of the A u - T e we investigated recently] They show the behaviour of a weakly associated liquid, since their enthalpy of mixing is weakly negative in the whole range of concentration and the heat content of AuSb2 decreases just above the melting point.

REFERENCES 1. Okamoto, H. & Massalski, T. B., Bull. Alloy Phase Diagram, 5 (1984) 166.

2. B6ja, R., Thesis, Univ. Aix-Marseille, 1969. 3. Bergman, C. & Komarek, K. L., Calphad, 8 (1984) 283. 4. Castanet, R. & Bergman, C., Phys. Chem. Liquids, 14 (1985) 219. 5. Michel, M.-L. & Castanet, R., J. Alloys & Compounds, 185 (1992) 241. 6. Bergman, C. & Castanet, R., Ber. Bunsenges. phys. Chem., 81 (1977) 1001. 7. Anres, P., Bros, H., Coulet, A. & Castanet, R., Phys. Chem. Liquids, 28 (1994) 63. 8. Kang, T. & Castanet, R., J. Less-Comm. Metals, 51 (1977) 125. 9. Hultgren, R., Desai, P. D., Hawkins, D. T., Gleiser, M., Kelley, K. K. & Wagman, D. D., in Selected Values of the Thermodynamic Properties of the Elements, Amer. Soc. Met., Metals Park, Ohio, USA, 1973 pp. 47, 443. 10. Certificate of Standard Reference Material 720, Synthetic Sapphire, U.S. Dept. of Commerce, Nat. Bur. Stand., Washington DC, USA. 11. Kelley, K. K., in Critical Evaluation of High Temperature Heat Capacities of Inorganic Compounds, U.S. Bureau of Mines, no. 476, 1949. 12. Itagaki, K., J. Jpn Inst. Met., 40 (1976) 1038. 13. Geltken, R., Komerek, K. L. & Miller, E., Trans. Met. Soc. AIME, 239 (1967) 1151. 14. Schick, G. & Komarek, K. L., Z. Metallkde., 65 (1974) 112. 15. Hayer, E., Komarek, K. L. & Castanet, R., Z. Metallkde., 68 (1977) 688. 16. Vogel, R., Z. Anorg. Chem., 50 (1906) 145. 17. Okamoto, H. & Massalski, T. B., in Phase Diagrams of Binary GoM Alloys, Metals Park, Ohio, USA, 1987. 18. Kameda, K., Azakami, T. & Kameda, M., J. Jpn Inst. Met., 38 (1974) 434. 19. Hino, M., Azakami, T. & Kameda, M., J. Jpn. Inst. Met., 75 (1975) 1175.