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The application of solid electrolytes to the thermodynamic study of some alkaline earths silicates
THE APPLICATION OF SOLID ELECTROLYTES TO THE THERMODYNAMIC STUDY OF SOME ALKALINE EARTHS SILICATES GRZEGORZ R&z Institute of Materials Engineering, School of Mining and Metallurgy, 30-059 Krakbw, Poland
(Received 20 September 1976) Abstract-Solid electrolytes based on stabilized zirconia (or thoria) cannot be used to the construction of the solid galvanic cells for the study of thermodynamic properties of the alkaline earths silicates. They become a partially electronic conductors at low chemical potentials of oxygen, as those in the coexisting alkaline earths silicate phases. It was attempted to use as the solid electrolyte a mixture of small amounts of alkaline earth fluoride in lhe corresponding silicale. Then, the solid galvanic cells consisting of such electrolytes were applied to the determination of the standard Gibbs free energies of formation of some magnesium, strontium and barium silicates.
Stabilized zirconia cannot be used as a solid electrolyte in the galvanic cells applied to the study of the thermodynamics of the alkaline earths silicates, because it becomes an electronic conductor in the oxygen pressure range corresponding to the equilibria in the Me(FSi02 systems (Me0 = MgO, CaO, SrO or BaO). Therefore, some fluorides, exhibiting pure ionic conductivity, have been used as the solid electrolytes in the study of the thermodynamic of some oxide systems having very low values of the equilibrium oxygen pressures [i]. However, the attempts to apply the fluorides as the solid electrolytes in the silicate systems investigated in the present work remain unsuccessful because of the low melting temperatures of fluorides,, their volatility, and high chemical activity with respect to silica. In order to avoid the difliculties mentioned above, the new type of the solid electrolyte was prepared by the introduction of a small amount of fluoride MgFZ, SrF, [2] or CaF,, BaF, [3] into the corresponding silicate phase formed in the investigated system. The electrolytes obtained, and the conditions of their preparation are given in Table 1.
The pure silicates of magnesium_ strontium and barium have been found to be insulators. The considerable ionic conductivity appears, however, after the introduction of the small amounts of respective fluoride into the nonconducting silicate. This increase of the conductivity is probably due to the fluoride ions introduced into the certain positions of the open crystal lattice of the silicate. The solid electrolytes obtained have high melting temperatures, considerable mechanical resistance and are practically inert with respect to the silicate phases present in the galvanic cell. In order to test the electrolytes obtained, the emu values of the following galvanic cells were measured: PtlMgSiO,,
Mg,Si0,1/Mg,Si04 + MgF, llMg2Si04,MgOPt
(11
PtlSiO,, MgSiO,(IMgSiO, + MgF,IIMgSiO,, Mg,SiO,lPt (2) and standard Gibbs free energies of formation of MgSiO,, and Mg,SiO, from oxides MgO and SiO, were calculated [Z]. The values obtained for temperatures of 1200, 1300 and 140OK are given in Table
Table 1. The preparation conditions for the silicate solid electrolytes. where Y is the weight percent of component, T, and t, the temperature and time of preheating, and T,, rs the temperature and trme of sintering respectively (Me denotes Mg, Sr or Ba)
wt. % NO.
1
2 3 4 5 6 7 8 9 10
The sample
Mg$30~, MgF, MgSiO,,
MgFZ Sr,SiO,, SrF, SrzSiO,, SrF2 SrSiO,, SrF, Ba,SiO,, BaF2 Ba2Si04, BaF2 BaSiO,, 3aF2 Ba,Si,O,, BaF,
BaSi,O,, BaF,
Me0
SiO,
MeF,
Tp,K
tp,h
T,, K
t,, mm.
56.5 39.3 X3.1 76.6 61.8 85.0 82.8 71.0 62.6 55.3
42.2 58.5
1.3 2.2 0.9 1.5 2.1 0.5 1.0 1.2 0.6 1.3
1673 1373 1373 1373 1373 1373 1323 1273 1273 1273
18 12 12 12 12 12 10 IO 10 8
1923-1963 18751923 1723%1813 187>1923 1773-1793 177>1823 1873-1903 1693-1713 1643-1673 1593-1623
60 40 30 30 20 40 40 30 20
16.0
21.9 36.1 11.5 16.2 27.8 36.8 43.4
697
15
698
GRZEGORZ
Table 2. The comparison of the values of standard Gibbs free energies of MgSiO, and Mg,SiO, formation, obtained
R&z Table 3. The standard Gibbs free energies of barium siIi_
cates formation
by different authors (T = 14OOK)
-AG;,,.oo
[Cal/mole] Mg,SiO, MgSiO 3
References
14,610 15,100
7,610 7,360
C61
Author’s data
14.35 1 15,440
6,861 8,400
2, and compared with the literature data
[46]. This comparison shows that the data obtained in the present work are in a reIatively good agreement with those reported by other authors. The values of standard Gibbs free energies of SrSiO,, Sr,SiO_,, and Sr,SiO,-formation have been also given [Z]. More recently, the standard Gibbs free energies of formation of the silicates in the BaO-SiO, system have been measured [3], using the solid electrolytes obtained by the introduction of the small amounts of BaFz into the corresponding barium silicates. The emfmeasurements of the following galvanic cells were carried out: PtlSiO,, BaSi,O, PtlBaSiz05, PtlBa,Si,O,, PtlBaSi@, Pt/Ba$iO,,
/lBaSi,O, -I- BaF,/IBaSiz05,
Ba2Si308(Pt
Ba&Os llBasSi,Os + BaF,(/Ba,Si,O,,
BaSiO,(Pt
(4) (5)
Ba2Si0+1jBa,Si0, f BaF, l(BazSi04, BajSiOSIPt
(6)
0.2
BaOIPt
I 0.6
I 0.4 XSi0
BaSi20s Ba2S1308 BaSiO, Ba,SiO, Ba,SiO,
32,250 58,200 24,260 34,600 37,050
- AGj [cal/rnole] 13OOK
14OOK
36,125 62,320 25,890 36,020 40,125
38,750 66,880 27,210 37,303 43,100
BaSiO,
+ BaSilOS
= Ba&Os;
AC:
(4a)
Ba2Si308
+ Ba2Si04
= 4 BaSiOs;
AC:
(5a)
BaSiO,
+ Ba3Si05
BaO + Ba,SiO,
= 2 Ba,SiO,; = Ba,SiOs;
(7)
AC: AC:
(ha) (7a)
AGi denote here the standard Gibbs free energy changes connected with the reactions (3aH7a). Based on the emf measurements of the cells (3)-(7), and on the AGO values of the reactions (3a)-(7a) calculated from these measurements. the standard Gibbs free energies of BaSi,05, Ba&Os, BaSiO,, Ba,SiO,, and Ba,SiO,-formation from barium oxide and silica were calculated in the following way: BaO + 2 Si02 = BaSi,O,;
AGF
AG;=2AG:+3AG;+AG$+AG:+AG: 2 BaO
+ 3 SiO, = BazSi308;
AC:
AC:,
AC:, = AGY + 2 AG; + AC: + AC: + AC; 2 BaO + SiOp = BazSi04;
AC&
AC& = AG: + 2 AC; + AC; + 2 AG,O+ 2 AC; 3 BaO + Si02
= BaJSi05
(8) @a)
AG: = 3 AC; + 6 AC: + 2’AGp + 2 AC: + AC: BaO + SIOz = BaSlO,;
The measurement were carried out in air, in the temperature range 12C&1400 K. The experimental set-up and the preparation procedure was the analogous to that described in [2]. Reactions, which are responsible for the mnfof the cells f+(7), can be written, as follows: Ba,Si,O, + SiOl = 2 BaSi,O,: AC: (3a)
14 -
12OoK
(3)
BaSiO,//BaSiO, + BaFz IlBaSiO,, BalSiOJPt
Ba,SiOs/jBa3Si0, + BaF,/IBa$iO,,
Compound
; AC:
AG$=AG:+2AG:+AG:+ZAG:+3AG:
(9) (9a)
(10) (loa) (11) (114
(12) Wd
The obtained AGo-values are given in Table 3. Then the values of the molar Gibbs free energies of mixing at temperatures of 1200 and 14OOK as a
I 0.8
J
Z
Fig. 1. The molar Gibbs free energy of mixing as a function of the. mole fraction x of Si02 in barium silicates at 1200 and 1400 K
Study of some alkaline earths silicates
function of the mole fraction x of SO2 for all barium have been calculated. They are presented in Fig. 1, which shows that all obtained data are consistent with the thermodynamic requirements. The attempts to carry out the analogous measurements for CaO-SiO, system were unsuccessful because of the poor mechanical resistance of the sintered solid electrolyte cylinders, obtained from CaF, and the corresponding silicate. This made it impossible to cut the disks used for the construction of the
699 REFERENCES
silicates
1. R. W. Taylor and H. Schmalzried,
galvanic
2. G. R6g, 8. Langanke, G. Borchardt and H. Schmalzried, J. them Thermodynamics 6, 1113 (1974). 3. G. R6g, unpublished data. 4. J. Elliot, M. Gleiser, Tkermockemistry for Steelmaking, Vol. 2, p. 305. Addison-Wesley, Reading Mass. (1963). 5. 0. Kubaschevsky and E. Evans, Merallurgical Thermochemistry. Pergamon Press. Oxford (1958). 6. 0. P. MEedlov-Petrosjan, Tkermodynamik dsr Silikate. VEB, Verlag fir Bauwesen, Berlin (1965).
cells.
2444 (1964).
J. phys. Chem. 68,
(Received 20 September 1976) Abstract-Solid electrolytes based on stabilized zirconia (or thoria) cannot be used to the construction of the solid galvanic cells for the study of thermodynamic properties of the alkaline earths silicates. They become a partially electronic conductors at low chemical potentials of oxygen, as those in the coexisting alkaline earths silicate phases. It was attempted to use as the solid electrolyte a mixture of small amounts of alkaline earth fluoride in lhe corresponding silicale. Then, the solid galvanic cells consisting of such electrolytes were applied to the determination of the standard Gibbs free energies of formation of some magnesium, strontium and barium silicates.
Stabilized zirconia cannot be used as a solid electrolyte in the galvanic cells applied to the study of the thermodynamics of the alkaline earths silicates, because it becomes an electronic conductor in the oxygen pressure range corresponding to the equilibria in the Me(FSi02 systems (Me0 = MgO, CaO, SrO or BaO). Therefore, some fluorides, exhibiting pure ionic conductivity, have been used as the solid electrolytes in the study of the thermodynamic of some oxide systems having very low values of the equilibrium oxygen pressures [i]. However, the attempts to apply the fluorides as the solid electrolytes in the silicate systems investigated in the present work remain unsuccessful because of the low melting temperatures of fluorides,, their volatility, and high chemical activity with respect to silica. In order to avoid the difliculties mentioned above, the new type of the solid electrolyte was prepared by the introduction of a small amount of fluoride MgFZ, SrF, [2] or CaF,, BaF, [3] into the corresponding silicate phase formed in the investigated system. The electrolytes obtained, and the conditions of their preparation are given in Table 1.
The pure silicates of magnesium_ strontium and barium have been found to be insulators. The considerable ionic conductivity appears, however, after the introduction of the small amounts of respective fluoride into the nonconducting silicate. This increase of the conductivity is probably due to the fluoride ions introduced into the certain positions of the open crystal lattice of the silicate. The solid electrolytes obtained have high melting temperatures, considerable mechanical resistance and are practically inert with respect to the silicate phases present in the galvanic cell. In order to test the electrolytes obtained, the emu values of the following galvanic cells were measured: PtlMgSiO,,
Mg,Si0,1/Mg,Si04 + MgF, llMg2Si04,MgOPt
(11
PtlSiO,, MgSiO,(IMgSiO, + MgF,IIMgSiO,, Mg,SiO,lPt (2) and standard Gibbs free energies of formation of MgSiO,, and Mg,SiO, from oxides MgO and SiO, were calculated [Z]. The values obtained for temperatures of 1200, 1300 and 140OK are given in Table
Table 1. The preparation conditions for the silicate solid electrolytes. where Y is the weight percent of component, T, and t, the temperature and time of preheating, and T,, rs the temperature and trme of sintering respectively (Me denotes Mg, Sr or Ba)
wt. % NO.
1
2 3 4 5 6 7 8 9 10
The sample
Mg$30~, MgF, MgSiO,,
MgFZ Sr,SiO,, SrF, SrzSiO,, SrF2 SrSiO,, SrF, Ba,SiO,, BaF2 Ba2Si04, BaF2 BaSiO,, 3aF2 Ba,Si,O,, BaF,
BaSi,O,, BaF,
Me0
SiO,
MeF,
Tp,K
tp,h
T,, K
t,, mm.
56.5 39.3 X3.1 76.6 61.8 85.0 82.8 71.0 62.6 55.3
42.2 58.5
1.3 2.2 0.9 1.5 2.1 0.5 1.0 1.2 0.6 1.3
1673 1373 1373 1373 1373 1373 1323 1273 1273 1273
18 12 12 12 12 12 10 IO 10 8
1923-1963 18751923 1723%1813 187>1923 1773-1793 177>1823 1873-1903 1693-1713 1643-1673 1593-1623
60 40 30 30 20 40 40 30 20
16.0
21.9 36.1 11.5 16.2 27.8 36.8 43.4
697
15
698
GRZEGORZ
Table 2. The comparison of the values of standard Gibbs free energies of MgSiO, and Mg,SiO, formation, obtained
R&z Table 3. The standard Gibbs free energies of barium siIi_
cates formation
by different authors (T = 14OOK)
-AG;,,.oo
[Cal/mole] Mg,SiO, MgSiO 3
References
14,610 15,100
7,610 7,360
C61
Author’s data
14.35 1 15,440
6,861 8,400
2, and compared with the literature data
[46]. This comparison shows that the data obtained in the present work are in a reIatively good agreement with those reported by other authors. The values of standard Gibbs free energies of SrSiO,, Sr,SiO_,, and Sr,SiO,-formation have been also given [Z]. More recently, the standard Gibbs free energies of formation of the silicates in the BaO-SiO, system have been measured [3], using the solid electrolytes obtained by the introduction of the small amounts of BaFz into the corresponding barium silicates. The emfmeasurements of the following galvanic cells were carried out: PtlSiO,, BaSi,O, PtlBaSiz05, PtlBa,Si,O,, PtlBaSi@, Pt/Ba$iO,,
/lBaSi,O, -I- BaF,/IBaSiz05,
Ba2Si308(Pt
Ba&Os llBasSi,Os + BaF,(/Ba,Si,O,,
BaSiO,(Pt
(4) (5)
Ba2Si0+1jBa,Si0, f BaF, l(BazSi04, BajSiOSIPt
(6)
0.2
BaOIPt
I 0.6
I 0.4 XSi0
BaSi20s Ba2S1308 BaSiO, Ba,SiO, Ba,SiO,
32,250 58,200 24,260 34,600 37,050
- AGj [cal/rnole] 13OOK
14OOK
36,125 62,320 25,890 36,020 40,125
38,750 66,880 27,210 37,303 43,100
BaSiO,
+ BaSilOS
= Ba&Os;
AC:
(4a)
Ba2Si308
+ Ba2Si04
= 4 BaSiOs;
AC:
(5a)
BaSiO,
+ Ba3Si05
BaO + Ba,SiO,
= 2 Ba,SiO,; = Ba,SiOs;
(7)
AC: AC:
(ha) (7a)
AGi denote here the standard Gibbs free energy changes connected with the reactions (3aH7a). Based on the emf measurements of the cells (3)-(7), and on the AGO values of the reactions (3a)-(7a) calculated from these measurements. the standard Gibbs free energies of BaSi,05, Ba&Os, BaSiO,, Ba,SiO,, and Ba,SiO,-formation from barium oxide and silica were calculated in the following way: BaO + 2 Si02 = BaSi,O,;
AGF
AG;=2AG:+3AG;+AG$+AG:+AG: 2 BaO
+ 3 SiO, = BazSi308;
AC:
AC:,
AC:, = AGY + 2 AG; + AC: + AC: + AC; 2 BaO + SiOp = BazSi04;
AC&
AC& = AG: + 2 AC; + AC; + 2 AG,O+ 2 AC; 3 BaO + Si02
= BaJSi05
(8) @a)
AG: = 3 AC; + 6 AC: + 2’AGp + 2 AC: + AC: BaO + SIOz = BaSlO,;
The measurement were carried out in air, in the temperature range 12C&1400 K. The experimental set-up and the preparation procedure was the analogous to that described in [2]. Reactions, which are responsible for the mnfof the cells f+(7), can be written, as follows: Ba,Si,O, + SiOl = 2 BaSi,O,: AC: (3a)
14 -
12OoK
(3)
BaSiO,//BaSiO, + BaFz IlBaSiO,, BalSiOJPt
Ba,SiOs/jBa3Si0, + BaF,/IBa$iO,,
Compound
; AC:
AG$=AG:+2AG:+AG:+ZAG:+3AG:
(9) (9a)
(10) (loa) (11) (114
(12) Wd
The obtained AGo-values are given in Table 3. Then the values of the molar Gibbs free energies of mixing at temperatures of 1200 and 14OOK as a
I 0.8
J
Z
Fig. 1. The molar Gibbs free energy of mixing as a function of the. mole fraction x of Si02 in barium silicates at 1200 and 1400 K
Study of some alkaline earths silicates
function of the mole fraction x of SO2 for all barium have been calculated. They are presented in Fig. 1, which shows that all obtained data are consistent with the thermodynamic requirements. The attempts to carry out the analogous measurements for CaO-SiO, system were unsuccessful because of the poor mechanical resistance of the sintered solid electrolyte cylinders, obtained from CaF, and the corresponding silicate. This made it impossible to cut the disks used for the construction of the
699 REFERENCES
silicates
1. R. W. Taylor and H. Schmalzried,
galvanic
2. G. R6g, 8. Langanke, G. Borchardt and H. Schmalzried, J. them Thermodynamics 6, 1113 (1974). 3. G. R6g, unpublished data. 4. J. Elliot, M. Gleiser, Tkermockemistry for Steelmaking, Vol. 2, p. 305. Addison-Wesley, Reading Mass. (1963). 5. 0. Kubaschevsky and E. Evans, Merallurgical Thermochemistry. Pergamon Press. Oxford (1958). 6. 0. P. MEedlov-Petrosjan, Tkermodynamik dsr Silikate. VEB, Verlag fir Bauwesen, Berlin (1965).
cells.
2444 (1964).
J. phys. Chem. 68,