I (I9
Journal of the Less-Common Metals. 163 ( 1990) 109- 1I3
STANDARD
GIBBS
FREE ENERGY
OF FORMATION
OF ZrOS
SUI ZHI-TONG, XIAO XING-YI, HUANG KE-QIN and WANG CHANG-ZHEN Metallurgical I’hysicochemistry Division, Northeast University
ofTechnology.Shenyang
(China)
(Received December 14, I9891
Summary
The standard molar Gibbs free energy of formation of ZrOS has been determined in the temperature range 824- 1246 K using a solid electrolyte galvanic cell of the type Pt]ZrOS, ZrO,]ZrO,(Y,O,)]Ni, NiO]Pt The measured e.m.f. could be represented (E/mV)*
by
1..5= 1591.06-0.9251T
Using this equation and the reported standard molar Gibbs free energy of formation of Cu$, NiO and ZrO,, the standard molar Gibbs free energy of formation of ZrOS was calculated and is given by A,G(ZrOS, s)/(J moll’)f30000=
-689040-44.14Tlog
T+ 103.88T
1. Introduction
The chemistry of rare earth oxysulphides is well known [l-5] and they have extensive technological applications, but for the transition metals oxysulphides were not known until recently [6-S]. Therefore it is necessary to study the thermodynamic properties of these compounds. In the present study, the Gibbs free energy changes of formation of zirconium oxysulphide have been measured by using a galvanic cell technique with yttriastabilized zirconia (YSZ) solid electrolyte having an Ni/NiO reference electrode. The data are combined with the standard Gibbs free energy of formation of CL@, NiO and ZrO, to obtain the standard Gibbs free energy of formation of ZrOS from elements in their standard state.
2. Experimental
details
2.1. Materials ZrS, was prepared from powder Zr (99.85%, 400 mesh) and sublimated sulphur (99.9%) in an evacuated quartz ampoule. Considering the loss of sulphur 0 Elsevier Sequoia/Printed
in The Netherlands
110
in process, the molar ratio of zirconium to sulphur was taken as 1: 2.5. After heating in a resistance furnace at 900 “C for 5 days, a dark red product was formed and excess sulphur was removed by CS, extraction for 20 h. ZrOS was synthesized by using pellets of a mixture of ZrO, with equimolar amounts of ZrS, in an evacuated quartz ampoule at 1000 “C for 1 week. The formation of ZrOS in the samples was confirmed by X-ray diffraction. 2.2. Cell assembly The experimental arrangement is shown in Fig. 1. A mixture of equimolar ZrO, and ZrOS was packed into a YSZ tube, 5 cm long and 5 mm outer diameter. A separate crucible of 3 mm outer diameter, containing a copper and Cu,S mixture in molar ratio 1:2, was placed inside the YSZ tube. The mixture of copper and CL@ was chosen since the sulphur vapour pressure over the ZrOS, ZrO, electrode was fixed by it. The YSZ tube was covered with an alumina disk. A closed atmosphere for the dissociation reaction was provided by cement between the YSZ tube and the disk. The reference electrode was a mixture of nickel and NiO in the molar ratio 9 : 1. A platinum wire served as the electrode lead for the electrode. 2.3. Experimental procedure The cell assembly was placed in a vertical resistance furnace with Fe-Cr-Al wire double winding. The temperature in the oven zone was controlled to * 1 K using a DWK 702 temperature controller. The reference electrode side of the YSZ tube was placed in an atmosphere of 99.99% purity argon which was purified using silica gel, P20, and by using magnesium chips at 520-540 “C. The e.m.f. of the cell Pt 1ZrOS, ZrO, 1ZrO,( Y203) 1Ni, NiO 1Pt
Fig. 1. A schematic diagram of the arrangement of the cell: 1, platinum leads; 2, cement; 3, disk; 4, A&O, crucible; 5, quartz crucible; 6, Cu. Cu,S (1: 2); 7, YSZ tube; 8, ZrOS, ZrO,; 9, Ni, NiO; 10, A&O, powder.
111
was measured in the temperature range 824-1246 K with a Keithely 610 C solid state electrometer (1014Q ) and a high input impedance (lO1lQ ) 192 digital voltmeter. The thermocouple was calibrated using a standard. The values of e.m.f.s were taken when the variation of the e.m.f. was within + 1 mV. In order to ensure equilibrium in the cell, the reversibility of the electrode processes was ascertained by momentarily polarizing for several seconds and it was verified that the cell e.m.f. values returned to their original values. A typical measurement run needed lo-15 h. 3. Results The measured cell e.m.f. values are listed in Table 1 and are also plotted as a function of temperature as shown in Fig. 2. The variation of e.m.f.s with temperature is linear in the temperature range investigated. The equation relating the e.m.f. values of the cell to temperature was obtained by a least-squares analysis and reads as E(mV)=
1591.06-0.9251T(
f 1.5 mV)
(I)
The half-cell reaction on the right-hand side is NiO(s)+2e-+Ni(s)+O*-
(2)
TABLE 1 Measured cell e.m.f. at different temperatures T WI E(V)
;
824 0.823
977 0.699
700 ’
. 2 600 . *: soa
go0
go0
1000 II00
r200 IlOO
T.K Fig. 2. Variation of cell e.m.f. with temperature.
1024 0.644
1132 0.537
1246 0.439
112
and the reaction on the left-hand side is ZrOS(s)+02’+Zr02(s)+
1/2S,(g)+2e-
(3)
with the additional equilibrium 2Cu(s)+ l/2 S,(g)+Cu,S(s)
(3’)
fixing the sulphur potential. Thus the overall reaction may be written as ZrOS(s)+2Cu(s)+NiO(s)‘-ZrO,(s)+Cu,S(s)+Ni(s)
(4)
Therefore the Gibbs free energy change associated with the virtual cell reaction can be calculated using the following relationship: G= - nFE
(5)
where II = 2, the number of electrochemical equivalents in the cell reaction, F is the Faraday constant and E is the measured cell e.m.f. d,G(Jmoll’)&300=
-307080+
178ST
(6)
In order to obtain the standard Gibbs free energy of formation of ZrOS, it is necessary to combine the A,G values given by expression (6) with the standard Gibbs free energy of formation of NiO, Cu,S and ZrO,. The values were taken from Kubaschewski and Alcock [9] A,G(NiO, s) (J mol- ‘) * 8400 = - 234 350 + 85.23 T
(7)
A,G(Cu,S, s) (J mall’) k 4200 = - 142 880 - 26.02 T log T+ 120.25 T
(8)
A,G(ZrO,,s)(Jmol-‘)+
(9)
16700=
- 1087590-
18.12Tlog
Thus the standard Gibbs free energy of formation range was obtained as follows: A,G(ZrOS, s)(Jmol-‘)&
30000=
-689040-44.14T
T+247.36T
of ZrOS in the temperature log T+ 103.88T
(10)
Since literature information is not available on the standard Gibbs free energy of formation of ZrOS, the present results could not be compared with other data. However, as more accurate data on the Gibbs free energy of formation of ZrO, become available, the data obtained in this study may be used to obtain more accurate values for the standard Gibbs free energy of formation of the oxysulphide.
References 1 D. A. R. Kay, R. K. Dwivedi and R. V. Kumer, Proc. Int. Co@ on Rare Earth Development and Applications, Beijing, 1985, Vol. 2, p. 1204. 2 R. Akila, K. T. Jacob and A. K. Shukla, Metall. Trans. B, 18B (1987) 287. 3 R. K. Dwivedi and D. A. R. Kay, Metall. Trans. B, I.58 (1984) 523. 4 R. V. Kumar and D.A. R. Kay, Metall. Trans. B, 16B (1985) 287.
113 5 R. J. Fruehan, Mefull. Trans. B, IOB (1979) 143. 6 V. K. Stocks, G. Eulenberger and H. Hahn, Z. anorg. al/g. Chem., 463 ( 1980) 105. 7 G. A. Eisman and H. Steinfink, J. Solid&ate Chetn., 43( 1982) 225. 8 G. A. Eisman, J. S. Swinnea and H. Steinfink, J. Solid State Chem., 56 (1985) 397. 9 0. Kubaschewski and C. B. Alcock, Metallurgical Thermochemistry, Pergamon, Oxford, 5th edn., 1979.