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Thermodynamics of formation of high calcium niobates from e.m.f. measurements
Journal of Alloys and Compounds, 179 (1992) L25-L27 JAL lO1
L25
Letter
Thermodynamics of formation of high calcium niobates from e.m.f, m e a s u r e m e n t s S. Raghavan Department of Metallurgical Engineering, Indian Institute of Technology, Madras O00 036 (India)
(Received August 3, 1991) 1. I n t r o d u c t i o n
The phase diagram [1] of the system CaO-Nb205 shows the existence of three compounds, namely CaO.Nb20~, 2CaO-Nb205 and 3CaO.NbeO~. The Gibbs energy of formation of m onoc a l c i um niobate has been determined [2] in the t e m p e r a t u r e range 1 2 4 5 - 1 3 0 0 K using a CaF2 solid electrolyte galvanic cell. F rom the data so obtained and from e.m.f, measurements using similar galvanic cells in the pr e s ent work, the Gibbs energies of formation of 2CaO-Nb205 and 3CaO-Nb205 from the c o m p o n e n t oxides have been determined in the t e m p e r a t u r e range 1 2 2 5 - 1 2 8 7 K. Calcium fluoride was used as a solid electrolyte for the determination of Gibbs energies of formation of c o m p o u n d s o f calcium oxide by the high t e m p e r a t u r e galvanic cell technique [3-5]. It is possible to determine the Gibbs energies of formation of 2CaO. Nb205 and 3CaO. Nb205 by setting up the following cells incorporating CaFe as the solid electrolyte. 1.1. C e l l A
02(g), Pt ICaO, CaF2 IJCaF2 IICaO- Nb205, 2CaO-Nb205, CaF2 [Pt, O2(g) The reaction at the CaOICaF2 interface is CaO + 2 F - --- CaF2 + -~O2(g) + 2 e The reaction at the CaF21CaO.Nb20~, 2CaO.Nb2Os, CaF2 interface is CaF2 + ~ O2(g) + CaO-Nb20~ + 2 e - = 2CaO. Nb20~ + 2 F Therefore the net cell reaction is
(1)
CaO + CaO. Nb20~ = 2CaO- Nb205 The Gibbs energy change of reaction (1) is given by AG o = - 2 F E where E is the e.m.f, of the cell and F, the faraday (in SI units).
0925-8388/92/$5.00
© 1992 - Elsevier Sequoia. All rights reserved
L26 1.2. Cell B
O2(g), PtlCaO, CaF2 IICaF2 II2CaO. Nb20~, 3CaO-Nb2Os, CaF2 IPt, O2(g) The cell reaction is CaO + 2CaO. Nb205 = 3CaO. Nb205
(2)
Using the Gibbs energy of formation of CaO.Nb205 determined in previous work [2], the Gibbs energies of formation of 2CaO-Nb205 and 3CaO-Nb20~ from the component oxides can be calculated from the Gibbs energy changes involved in reactions (1) and (2) respectively.
2. Materials Calcium fluoride, for use as the solid electrolyte and as an additive to improve the conductivity of the electrodes, was prepared from reagent grade calcium carbonate and hydrofluoric acid by the method of Taylor and Schmalzried [3]. The electrolyte pellet was sintered in an atmosphere of CO2-free dry oxygen gas at 1323 K for 4 h. The dicalcium and tricalcium niobates were prepared from dried reagent grade CaCOa and Nb20~ by mixing in stoichiometric proportions and sintering the oxide mixture pellets at 1623 K for 72 h with intermittent grinding. The formation of the niobates was confirmed by X-ray diffraction. The electrode pellets were prepared by mixing the appropriate compounds, prepared earlier, in roughly equimolar proportions. About 15 wt.% of calcium fluoride was then added to the mixture and the mixed powder pressed into a cylindrical pellet at 235 MPa and finally sintered at 1273 K in dry CO2-free oxygen gas before use.
3. E x p e r i m e n t a l details The assembly used in the present work was similar to that described earlier [6l. The performance of the polycrystalline CaF2 as a solid electrolyte was checked by operating the cell O2(g), PtICaO, CaF2 IICaF2 IlMgO, MgF2 IPt, O2(g) in the temperature range 1 1 7 3 - 1 2 7 3 K. The e.m.f, values obtained in the present work for the above cell agreed with those reported by Pizzini and Morlotti [7]. The temperature of the cell was measured with a Pt/Pt-13%Rh thermocouple welded to the platinum foil in contact with the reference electrode of the cell. The platinum wire of the couple also served as the lead wire for measuring the cell e.m.f. The temperature of the furnace was controlled by another thermocouple to within _ 3 K. The cell e.m.f, and the thermal e.m.f, were measured using a Keithley 617 digital electrometer with an input impedance of about 1014 ohms. The reversible e.m.f.s of the cells were measured in the temperature range 1225-1287 K. The e.m.f.s remained
L27 c o n s t a n t for several h o u r s o n c e equilibrium was attained a n d c o u l d be r e p r o d u c e d on t e m p e r a t u r e cycling. The time of equilibrium was a b o u t 4 h.
4. R e s u l t s
and discussion
The m e a n values of the e x p e r i m e n t a l e.m.f, of cell A and cell B were f o u n d to be 314_+ 2 mV a n d 163 + 1.5 mV r e s p e c t i v e l y in the t e m p e r a t u r e r a n g e 1 2 2 5 - 1 2 8 7 K. A cell with p u r e CaO on both sides g a v e an e.m.f. close to zero. C h a n g e s in gas flow rate did n o t affect the cell e.m.f. The e.m.f, v a l u e s c o u l d be r e p r o d u c e d , even after an initial polarization of the cell, by p a s s i n g an external current. There was an u n c e r t a i n t y o f + 3 K in the t e m p e r a t u r e m e a s u r e m e n t but this is n o t likely to have i n t r o d u c e d a n y serious error, since the variation o f e.m.f, with t e m p e r a t u r e was very small. The e.m.f, of 3 1 4 ± 2 mV at a m e a n t e m p e r a t u r e of 1256 K for cell A is directly related to the s t a n d a r d Gibbs e n e r g y c h a n g e of - 6 0 . 6 1 ( _ + 0 . 5 ) kJ m o l - 1 for r e a c t i o n (1) at 1 2 5 6 K. This value is c o m b i n e d with the s t a n d a r d Gibbs e n e r g y o f f o r m a t i o n o f C a O . N b 2 0 5 f r o m the c o m p o n e n t o x i d e s det e r m i n e d f r o m p r e v i o u s w o r k [2] to give AG °-- -- 0 . 0 3 3 4 5 T - 75.82(_+ 1.0) LI t o o l - 1 in the t e m p e r a t u r e r a n g e 1 2 4 5 - 1 3 0 3 K. The Gibbs e n e r g y o f f o r m a t i o n so o b t a i n e d is -178.44(__+ 1.5) kJ mo1-1 at 1256 K for the r e a c t i o n 2CaO + Nb205 = 2 C a O . Nb20~ This value is c o m b i n e d with the s t a n d a r d Gibbs e n e r g y c h a n g e of - 3 1 . 5 ( __ 0.3) kJ tool -1 f o r r e a c t i o n (2) at 1256 K o b t a i n e d f r o m the e.m.f, o f 163_+1.5 mV f o r cell B at a m e a n t e m p e r a t u r e o f 1 2 5 6 K. The Gibbs e n e r g y o f f o r m a t i o n so o b t a i n e d is - 2 0 9 . 9 4 ( ± 1.6) kJ m o l - i at 1256 K for the r e a c t i o n 3CaO + Nb20~ = 3 C a O . Nb2Oa
References 1 2 3 4 5
M. Ibrahim, N. F. H. Bright and J. F. Rowland, J. Am. Ceram. Soc., 45 (1962) 329. S. Raghavan, Trans. I n d i a n Inst. Met., 44 (1991) 285. R. W. Taylor and H. Schmalzried, J. Phys. Chem., 68 (1964) 2444. Su-I1 Pyun, J. A m . Ceram. Sot., 57 (1984) 594. Yu. Ya. Skolis, V. A. Levitskii, L. N. Lykova and T. A. Kalinina, J. S o l i d S t a t e Chem., 38 (1981) 10. 6 D. V. Vecher and A. A. Vecher, Zh. Fiz. K h i m . , 41 (1967) 2916. 7 S. Pizzini and R. Morlotti, J. Chem. Soc., F a r a d a y Trans., 68 (1972) 1601.
L25
Letter
Thermodynamics of formation of high calcium niobates from e.m.f, m e a s u r e m e n t s S. Raghavan Department of Metallurgical Engineering, Indian Institute of Technology, Madras O00 036 (India)
(Received August 3, 1991) 1. I n t r o d u c t i o n
The phase diagram [1] of the system CaO-Nb205 shows the existence of three compounds, namely CaO.Nb20~, 2CaO-Nb205 and 3CaO.NbeO~. The Gibbs energy of formation of m onoc a l c i um niobate has been determined [2] in the t e m p e r a t u r e range 1 2 4 5 - 1 3 0 0 K using a CaF2 solid electrolyte galvanic cell. F rom the data so obtained and from e.m.f, measurements using similar galvanic cells in the pr e s ent work, the Gibbs energies of formation of 2CaO-Nb205 and 3CaO-Nb205 from the c o m p o n e n t oxides have been determined in the t e m p e r a t u r e range 1 2 2 5 - 1 2 8 7 K. Calcium fluoride was used as a solid electrolyte for the determination of Gibbs energies of formation of c o m p o u n d s o f calcium oxide by the high t e m p e r a t u r e galvanic cell technique [3-5]. It is possible to determine the Gibbs energies of formation of 2CaO. Nb205 and 3CaO. Nb205 by setting up the following cells incorporating CaFe as the solid electrolyte. 1.1. C e l l A
02(g), Pt ICaO, CaF2 IJCaF2 IICaO- Nb205, 2CaO-Nb205, CaF2 [Pt, O2(g) The reaction at the CaOICaF2 interface is CaO + 2 F - --- CaF2 + -~O2(g) + 2 e The reaction at the CaF21CaO.Nb20~, 2CaO.Nb2Os, CaF2 interface is CaF2 + ~ O2(g) + CaO-Nb20~ + 2 e - = 2CaO. Nb20~ + 2 F Therefore the net cell reaction is
(1)
CaO + CaO. Nb20~ = 2CaO- Nb205 The Gibbs energy change of reaction (1) is given by AG o = - 2 F E where E is the e.m.f, of the cell and F, the faraday (in SI units).
0925-8388/92/$5.00
© 1992 - Elsevier Sequoia. All rights reserved
L26 1.2. Cell B
O2(g), PtlCaO, CaF2 IICaF2 II2CaO. Nb20~, 3CaO-Nb2Os, CaF2 IPt, O2(g) The cell reaction is CaO + 2CaO. Nb205 = 3CaO. Nb205
(2)
Using the Gibbs energy of formation of CaO.Nb205 determined in previous work [2], the Gibbs energies of formation of 2CaO-Nb205 and 3CaO-Nb20~ from the component oxides can be calculated from the Gibbs energy changes involved in reactions (1) and (2) respectively.
2. Materials Calcium fluoride, for use as the solid electrolyte and as an additive to improve the conductivity of the electrodes, was prepared from reagent grade calcium carbonate and hydrofluoric acid by the method of Taylor and Schmalzried [3]. The electrolyte pellet was sintered in an atmosphere of CO2-free dry oxygen gas at 1323 K for 4 h. The dicalcium and tricalcium niobates were prepared from dried reagent grade CaCOa and Nb20~ by mixing in stoichiometric proportions and sintering the oxide mixture pellets at 1623 K for 72 h with intermittent grinding. The formation of the niobates was confirmed by X-ray diffraction. The electrode pellets were prepared by mixing the appropriate compounds, prepared earlier, in roughly equimolar proportions. About 15 wt.% of calcium fluoride was then added to the mixture and the mixed powder pressed into a cylindrical pellet at 235 MPa and finally sintered at 1273 K in dry CO2-free oxygen gas before use.
3. E x p e r i m e n t a l details The assembly used in the present work was similar to that described earlier [6l. The performance of the polycrystalline CaF2 as a solid electrolyte was checked by operating the cell O2(g), PtICaO, CaF2 IICaF2 IlMgO, MgF2 IPt, O2(g) in the temperature range 1 1 7 3 - 1 2 7 3 K. The e.m.f, values obtained in the present work for the above cell agreed with those reported by Pizzini and Morlotti [7]. The temperature of the cell was measured with a Pt/Pt-13%Rh thermocouple welded to the platinum foil in contact with the reference electrode of the cell. The platinum wire of the couple also served as the lead wire for measuring the cell e.m.f. The temperature of the furnace was controlled by another thermocouple to within _ 3 K. The cell e.m.f, and the thermal e.m.f, were measured using a Keithley 617 digital electrometer with an input impedance of about 1014 ohms. The reversible e.m.f.s of the cells were measured in the temperature range 1225-1287 K. The e.m.f.s remained
L27 c o n s t a n t for several h o u r s o n c e equilibrium was attained a n d c o u l d be r e p r o d u c e d on t e m p e r a t u r e cycling. The time of equilibrium was a b o u t 4 h.
4. R e s u l t s
and discussion
The m e a n values of the e x p e r i m e n t a l e.m.f, of cell A and cell B were f o u n d to be 314_+ 2 mV a n d 163 + 1.5 mV r e s p e c t i v e l y in the t e m p e r a t u r e r a n g e 1 2 2 5 - 1 2 8 7 K. A cell with p u r e CaO on both sides g a v e an e.m.f. close to zero. C h a n g e s in gas flow rate did n o t affect the cell e.m.f. The e.m.f, v a l u e s c o u l d be r e p r o d u c e d , even after an initial polarization of the cell, by p a s s i n g an external current. There was an u n c e r t a i n t y o f + 3 K in the t e m p e r a t u r e m e a s u r e m e n t but this is n o t likely to have i n t r o d u c e d a n y serious error, since the variation o f e.m.f, with t e m p e r a t u r e was very small. The e.m.f, of 3 1 4 ± 2 mV at a m e a n t e m p e r a t u r e of 1256 K for cell A is directly related to the s t a n d a r d Gibbs e n e r g y c h a n g e of - 6 0 . 6 1 ( _ + 0 . 5 ) kJ m o l - 1 for r e a c t i o n (1) at 1 2 5 6 K. This value is c o m b i n e d with the s t a n d a r d Gibbs e n e r g y o f f o r m a t i o n o f C a O . N b 2 0 5 f r o m the c o m p o n e n t o x i d e s det e r m i n e d f r o m p r e v i o u s w o r k [2] to give AG °-- -- 0 . 0 3 3 4 5 T - 75.82(_+ 1.0) LI t o o l - 1 in the t e m p e r a t u r e r a n g e 1 2 4 5 - 1 3 0 3 K. The Gibbs e n e r g y o f f o r m a t i o n so o b t a i n e d is -178.44(__+ 1.5) kJ mo1-1 at 1256 K for the r e a c t i o n 2CaO + Nb205 = 2 C a O . Nb20~ This value is c o m b i n e d with the s t a n d a r d Gibbs e n e r g y c h a n g e of - 3 1 . 5 ( __ 0.3) kJ tool -1 f o r r e a c t i o n (2) at 1256 K o b t a i n e d f r o m the e.m.f, o f 163_+1.5 mV f o r cell B at a m e a n t e m p e r a t u r e o f 1 2 5 6 K. The Gibbs e n e r g y o f f o r m a t i o n so o b t a i n e d is - 2 0 9 . 9 4 ( ± 1.6) kJ m o l - i at 1256 K for the r e a c t i o n 3CaO + Nb20~ = 3 C a O . Nb2Oa
References 1 2 3 4 5
M. Ibrahim, N. F. H. Bright and J. F. Rowland, J. Am. Ceram. Soc., 45 (1962) 329. S. Raghavan, Trans. I n d i a n Inst. Met., 44 (1991) 285. R. W. Taylor and H. Schmalzried, J. Phys. Chem., 68 (1964) 2444. Su-I1 Pyun, J. A m . Ceram. Sot., 57 (1984) 594. Yu. Ya. Skolis, V. A. Levitskii, L. N. Lykova and T. A. Kalinina, J. S o l i d S t a t e Chem., 38 (1981) 10. 6 D. V. Vecher and A. A. Vecher, Zh. Fiz. K h i m . , 41 (1967) 2916. 7 S. Pizzini and R. Morlotti, J. Chem. Soc., F a r a d a y Trans., 68 (1972) 1601.