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Thermodynamics of nickel orthosilicate
M-1727 J. Chem. Thermodynamics 1984, 16, 1103-l 105
Thermodynamics
of nickel
orthosilicate
G. R6G Institute of Materials Engineering, Academy of Mining and Metallurgy,
30059 Krakbw,
Poland
and G. BORCHARDT Institute of General Metallurgy, Technical University of Clausthal, 3392 Clausthal-Zellerfeld.
F.R.G.
(Received 10 May 1984) The standard Gibbs free energy, enthalpy, and entropy of formation of nickel orthosilicate have been determined with the help of a solid-electrolyte galvanic cell involving Ni’+ exchanged p-alumina electrolyte. The results have been compared with those obtained by other authors.
1. Introduction Nickel orthosilicate, NizSiO,, belongs to the olivines. The olivine-group minerals are one of the more important constituents of igneous rocks and are believed to be a dominant phase in the upper mantle of the earth. Accurate values for their thermodynamic properties are thus necessary for the quantitative solution of both geochemical and geophysical problems. In our previous works thermodynamic quantities for magnesium,“*‘) cobalt,(‘T3) calcium,(4) and iro#) olivines have been determined. The thermodynamic quantities for nickel olivine existing in the literature are rather scarce and incomplete. The present study was initiated so as to provide thermodynamic quantities for Ni,Si04 over the widest possible temperature range with the help of a solid-state galvanic cell. Nickel sodium p”-alumina was used as a solid electrolyte. The divalent ion-exchanged p- and p”-aluminas are a new family of solid electrolytes being good divalent ion conductors.@’ Even a small concentration of some divalent cation was found to determine the conduction process in /3-alumina.‘7) Partially ion-exchanged p-aluminas were applied successfully in reversible galvanic cells.“* *) The solidelectrolyte cell employed in the study of Ni,Si04 was of the type: PtJO,(NiOl(Ni,
Na)-P”-aluminalNi,Si0,
+ SiO,jO,(Pt.
(1)
2. Experimental All the materials used were of reagent grade. Exchange of the Na+ ions in l3”-alumina with the Ni’+ was carried out as follows. The p”-alumina powder was 0021-9614/84/121103+03
rSO2.00/0
0 1984 Academic Press Inc. (London) Limited
1104
G. ROG
AND
G. BORCHARDT
synthesized by heating the appropriate mixture of carefully dried y-Al,O,, Na,CO,, and MgCO,. Batches with nominal mass fractions: 0.092 of Na,O, 0.041 of MgO, and 0.867 of Al,O, were used. The mixture was first calcined at 1520 K for 8 h. After milling, the powder was pressed into pellets 12 mm in diameter and 3 mm thick at 390 MPa pressure, and sintered in a closed alumina vessel at 1870 K. The temperature of the furnace was allowed to rise from room temperature to 1870 K over a period of 35 min. The temperature of 1870 K was held for 1 h. Then the pellets were kept in the same furnace at 1720 K for 6 h. The resulting samples had more than 95 per cent of the theoretical density and the composition: 0 17 . The B”-alumina pellets were immersed in molten (O.SNaCl Na 1.80Mg0.62Al10.32 f O.SNiCl,) at 1060 K and held there for 12 h. After exchange, the pellets were treated with anhydrous ethanol to remove surplus chlorides. The fraction of Ni’+ ion-exchange amounted to (0.22 kO.02). As a result of chemical analysis, the Ni-j3”-alumina electrolyte can be described by the formula Ni o.zoNal.40Mgo.62Allo.32~17. Half-cells were prepared by pressing pellets 3 mm thick and 12 mm in diameter from nickel oxide or from a mixture of silica and nickel orthosilicate. Ni,SiO, was prepared by a “gelling” method allowing its formation at the temperature 1520 K.‘9’ The half-cells and electrolyte pellets were assembled in a simple spring-loaded alumina holder. The cell was placed in the furnace and heated to the measurement temperature. The e.m.f values were monitored with a digital voltmeter (Unitra 1321, internal resistance 10” Q). The apparatus was described previously.o0) Five independent series of e.m.f. measurements were performed in the temperature range 990 to 1350 K. The time taken for the cell to attain equilibrium did not exceed 3.5 h.
3. Results and discussion The reaction of cell (1) may be written 2NiO + SiOZ = Ni,Si04. The standard molar Gibbs free energy of formation by means of the relation: ArGk(Ni,Si04)
(2)
A,Gg(Ni,SiO,)
was calculated
= -4FE,
(3) where E is the e.m.f. and F is the Faraday constant. The e.m.f. was found to vary linearly with temperature: E/mV = (41.3+2.1)-(0.023+0.003)(T/K). The ArGk as a function of temperature and (4): A,Gk/(kJ.mol-‘)
was calculated by combining
= -(15.94*0.82)+(8.9*
1.2) x 10m3(T/K).
(4) equations (3) (5)
In table 1 the ArGi values calculated for several temperatures in the range studied are listed together with those by Taylor and Schmalzried.” l) However, in table 2 the enthalpy A,H: and entropy A,Sk values at 970 K calculated from equation (5) are
THERMODYNAMICS
OF Ni,SiO,
1105
TABLE 1. The standard molar Gibbs free energy of formation ArGk of Ni,SiO, (from oxides) at different temperatures T/K:
Reference This work 11
910
1070
7.31
6.42 6.69
1170
1270
1370
4.64 5.02
3.75 4.18
-A,G”,/(kJ.mol-‘) 7.53”
5.52 5.86
’ Extrapolated value.
TABLE 2. The standard molar enthalpy and the standard molar entropy of formation of Ni,SiO, (from oxides) at 970 K _.____
Reference This work
- Ar HL(970 K)/(kJ mol - r) 15.94
9
13.93
11
-
-A&(970
K)/(J.K-’
.mol-‘)
8.9 8.4
compared with previous values. (9, “) As seen from both tables and from equation (5) the results compared differ in the error limits from one another. Equation (5) makes it possible to estimate the temperature of spontaneous decomposition of Ni,Si04. Ni,SiO, becomes unstable relative to NiO and SiOz above 1790 K at atmospheric pressure. The value obtained agrees well with that resulting from the phase diagram of (NiO + Si0,).(12) The authors gratefully acknowledge financial support to Research Project No. 03.10 (Polish Academy of Sciences). REFERENCES 1. Rag, G.; Langanke, B.; Borchardt, G.; Schmalzried, H. J. Chem. Thermodynamics 1974, 6, 1113. 2. Rag, G.; Borchardt, G. J. Electrochem. Sot. 1984, 131, 380. 3. Koziowska-Rag, A.; Rag, G. Pol. J. Chem. 1979, 53, 2083. 4. Rag, G.; Kozlowska-Rag, A. J. Chem. Thermodynamics 1983, 15, 107. 5. Rag, G. Kozibski, S. J. Chem. Thermodynamics l!X3, 15, 111. 6. Farrington, C. C.; Dunn, B. Solid State Ionics 1982, 7, 267. 7. Ni, J.; Tsai, Y.; Whitmore, D. H. Solid State Ionics 1981, 5, 199. 8. Komm, T. Z.; Tret’yakov, Yu. D.; Kaul, A. R. Zh. Phys. Khim. 1976, 50, 2115. 9. Navrotsky, A. J. Inorg. Nucl. Chem. 1971, 33, 4035. 10. Rag, G. Sci. Bull. Acad. Min. Metail. Krakbw, Ceram. 1976, 32, 7. 11. Taylor, R. W.; Schmalzried, H. J. Phys. Chem. VW, 68,2444. 12. Toropov, N. A.; Barsakovski, V. P.; Lapin, V. V.; Kurtseva, N. N. Phase diagrams of silicate sysfems. Vol. 1. Nauka: Moscow. 1969, p. 124.
Thermodynamics
of nickel
orthosilicate
G. R6G Institute of Materials Engineering, Academy of Mining and Metallurgy,
30059 Krakbw,
Poland
and G. BORCHARDT Institute of General Metallurgy, Technical University of Clausthal, 3392 Clausthal-Zellerfeld.
F.R.G.
(Received 10 May 1984) The standard Gibbs free energy, enthalpy, and entropy of formation of nickel orthosilicate have been determined with the help of a solid-electrolyte galvanic cell involving Ni’+ exchanged p-alumina electrolyte. The results have been compared with those obtained by other authors.
1. Introduction Nickel orthosilicate, NizSiO,, belongs to the olivines. The olivine-group minerals are one of the more important constituents of igneous rocks and are believed to be a dominant phase in the upper mantle of the earth. Accurate values for their thermodynamic properties are thus necessary for the quantitative solution of both geochemical and geophysical problems. In our previous works thermodynamic quantities for magnesium,“*‘) cobalt,(‘T3) calcium,(4) and iro#) olivines have been determined. The thermodynamic quantities for nickel olivine existing in the literature are rather scarce and incomplete. The present study was initiated so as to provide thermodynamic quantities for Ni,Si04 over the widest possible temperature range with the help of a solid-state galvanic cell. Nickel sodium p”-alumina was used as a solid electrolyte. The divalent ion-exchanged p- and p”-aluminas are a new family of solid electrolytes being good divalent ion conductors.@’ Even a small concentration of some divalent cation was found to determine the conduction process in /3-alumina.‘7) Partially ion-exchanged p-aluminas were applied successfully in reversible galvanic cells.“* *) The solidelectrolyte cell employed in the study of Ni,Si04 was of the type: PtJO,(NiOl(Ni,
Na)-P”-aluminalNi,Si0,
+ SiO,jO,(Pt.
(1)
2. Experimental All the materials used were of reagent grade. Exchange of the Na+ ions in l3”-alumina with the Ni’+ was carried out as follows. The p”-alumina powder was 0021-9614/84/121103+03
rSO2.00/0
0 1984 Academic Press Inc. (London) Limited
1104
G. ROG
AND
G. BORCHARDT
synthesized by heating the appropriate mixture of carefully dried y-Al,O,, Na,CO,, and MgCO,. Batches with nominal mass fractions: 0.092 of Na,O, 0.041 of MgO, and 0.867 of Al,O, were used. The mixture was first calcined at 1520 K for 8 h. After milling, the powder was pressed into pellets 12 mm in diameter and 3 mm thick at 390 MPa pressure, and sintered in a closed alumina vessel at 1870 K. The temperature of the furnace was allowed to rise from room temperature to 1870 K over a period of 35 min. The temperature of 1870 K was held for 1 h. Then the pellets were kept in the same furnace at 1720 K for 6 h. The resulting samples had more than 95 per cent of the theoretical density and the composition: 0 17 . The B”-alumina pellets were immersed in molten (O.SNaCl Na 1.80Mg0.62Al10.32 f O.SNiCl,) at 1060 K and held there for 12 h. After exchange, the pellets were treated with anhydrous ethanol to remove surplus chlorides. The fraction of Ni’+ ion-exchange amounted to (0.22 kO.02). As a result of chemical analysis, the Ni-j3”-alumina electrolyte can be described by the formula Ni o.zoNal.40Mgo.62Allo.32~17. Half-cells were prepared by pressing pellets 3 mm thick and 12 mm in diameter from nickel oxide or from a mixture of silica and nickel orthosilicate. Ni,SiO, was prepared by a “gelling” method allowing its formation at the temperature 1520 K.‘9’ The half-cells and electrolyte pellets were assembled in a simple spring-loaded alumina holder. The cell was placed in the furnace and heated to the measurement temperature. The e.m.f values were monitored with a digital voltmeter (Unitra 1321, internal resistance 10” Q). The apparatus was described previously.o0) Five independent series of e.m.f. measurements were performed in the temperature range 990 to 1350 K. The time taken for the cell to attain equilibrium did not exceed 3.5 h.
3. Results and discussion The reaction of cell (1) may be written 2NiO + SiOZ = Ni,Si04. The standard molar Gibbs free energy of formation by means of the relation: ArGk(Ni,Si04)
(2)
A,Gg(Ni,SiO,)
was calculated
= -4FE,
(3) where E is the e.m.f. and F is the Faraday constant. The e.m.f. was found to vary linearly with temperature: E/mV = (41.3+2.1)-(0.023+0.003)(T/K). The ArGk as a function of temperature and (4): A,Gk/(kJ.mol-‘)
was calculated by combining
= -(15.94*0.82)+(8.9*
1.2) x 10m3(T/K).
(4) equations (3) (5)
In table 1 the ArGi values calculated for several temperatures in the range studied are listed together with those by Taylor and Schmalzried.” l) However, in table 2 the enthalpy A,H: and entropy A,Sk values at 970 K calculated from equation (5) are
THERMODYNAMICS
OF Ni,SiO,
1105
TABLE 1. The standard molar Gibbs free energy of formation ArGk of Ni,SiO, (from oxides) at different temperatures T/K:
Reference This work 11
910
1070
7.31
6.42 6.69
1170
1270
1370
4.64 5.02
3.75 4.18
-A,G”,/(kJ.mol-‘) 7.53”
5.52 5.86
’ Extrapolated value.
TABLE 2. The standard molar enthalpy and the standard molar entropy of formation of Ni,SiO, (from oxides) at 970 K _.____
Reference This work
- Ar HL(970 K)/(kJ mol - r) 15.94
9
13.93
11
-
-A&(970
K)/(J.K-’
.mol-‘)
8.9 8.4
compared with previous values. (9, “) As seen from both tables and from equation (5) the results compared differ in the error limits from one another. Equation (5) makes it possible to estimate the temperature of spontaneous decomposition of Ni,Si04. Ni,SiO, becomes unstable relative to NiO and SiOz above 1790 K at atmospheric pressure. The value obtained agrees well with that resulting from the phase diagram of (NiO + Si0,).(12) The authors gratefully acknowledge financial support to Research Project No. 03.10 (Polish Academy of Sciences). REFERENCES 1. Rag, G.; Langanke, B.; Borchardt, G.; Schmalzried, H. J. Chem. Thermodynamics 1974, 6, 1113. 2. Rag, G.; Borchardt, G. J. Electrochem. Sot. 1984, 131, 380. 3. Koziowska-Rag, A.; Rag, G. Pol. J. Chem. 1979, 53, 2083. 4. Rag, G.; Kozlowska-Rag, A. J. Chem. Thermodynamics 1983, 15, 107. 5. Rag, G. Kozibski, S. J. Chem. Thermodynamics l!X3, 15, 111. 6. Farrington, C. C.; Dunn, B. Solid State Ionics 1982, 7, 267. 7. Ni, J.; Tsai, Y.; Whitmore, D. H. Solid State Ionics 1981, 5, 199. 8. Komm, T. Z.; Tret’yakov, Yu. D.; Kaul, A. R. Zh. Phys. Khim. 1976, 50, 2115. 9. Navrotsky, A. J. Inorg. Nucl. Chem. 1971, 33, 4035. 10. Rag, G. Sci. Bull. Acad. Min. Metail. Krakbw, Ceram. 1976, 32, 7. 11. Taylor, R. W.; Schmalzried, H. J. Phys. Chem. VW, 68,2444. 12. Toropov, N. A.; Barsakovski, V. P.; Lapin, V. V.; Kurtseva, N. N. Phase diagrams of silicate sysfems. Vol. 1. Nauka: Moscow. 1969, p. 124.