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Thermodynamic stability of Li3NbO4 by emf measurements using a novel composite electrolyte
SOLID
STATE ELSEVIER
IONICS
Solid State Ionics 93 ( 1997) 341-346
Thermodynamic
stability of Li,NbO, by emf measurements novel composite electrolyte
using a
R. Subasri, O.M. Sreedharan” Metallurgy
Division,
Indira
Gmdhi
Centre
for Atomic
Kesearch.
Kalpakkam,
%mil
h’adu 603 102, India
Received IS July 1996: accepted I.5 October 1996
Abstract The thermodynamic stability of Li,NbO, relative to LiNbO, and Liz0 was determined by oxygen potential measurements in the temperature rang 1009-l 115 K using a novel composite electrolyte made of alpha + beta alumina. The functioning of the composite electrolyte was independently assessed by emf measurements with well established low oxygen potential electrodes such as Nb/NbO, Ta/Ta,O, and AIIAI,O,. Kqwwdst
Lithium niobates; Stability; Emf: Gibbs energy
1. Introduction The ternary compounds in the Li-Nb-0 system are candidate tritium-breeding blanket materials in fusion reactors [I]. In addition, LiNbO, and LiTaO, find applications as wave-guides in opto-electronic devices [2]. The phase-diagram for the pseudo-binary system Li,O-Nb,O, was first compiled by Roth et al. [3]. Four ternary compounds with the stoichiometries LiNbO,, Li,NbO 4’ LiNb,O 8 and were reported to be present in the LiPb,,O,, system. There are also ternary compounds in this system where Nb exists in the oxidation state less than + 5. Kumada et al. reported the synthesis of LiNbO, and its potential application as lithium intercalating cathode materials in secondary batteries 141. Recently Gesselbracht et al. reported the exist*Corresponding author. Ol67-2738/97/$17X0 01997 Hlsevier Science B.V. All rights reserved PII SOl67-2738(96)00539-5
ence of superconductivity in Li,NbO, (0.45 5x I 0.5) with a T, of about 5 K 151. Despite the growing interest in the ternary lithium niobium oxides, very little work has so far been reported on the thermodynamic characterisation of these compounds. Azad et al. reported some DTA studies on the formation of AMO, compounds (A = Li, Na, K and M = Nb, Ta) from A,CO, and M,O, 161. Fortunately, the thermodynamic properties of, at least, LiNbO, were compiled by Hesselmann et al. which were based on calorimetric data 171. Thus LiNbO, could serve as the reference material for the thermodynamic characterisation of other ternary compounds in the Li-Nl0 system. The results of the galvanic cell studies employed for the determination of standard Gibbs energy of formation of Li,NbO,, i.e. AC) (Li,NbO,) are presented in this paper and the high temperature stability of this oxide relative to LiNbO, is also discussed.
342
R. Subasri. O.M. Sreedharan
I Solid State tonics 93 (1997) 341-346
2. Experimental (TMI) < High purity (total metal impurities 0.01%) were used as starting substances. Out of these, Nb, Nb,O,, Ta, Ta,O, and a-Al,O, are from Johnson Matthey (UK) while Li,CO, and @alumina are from Purex Laboratories (India) and Polyceram (USA) respectively. The Nb/NbO electrode was made by mixing Nb and Nb,O, in 8:l mole ratio followed by compaction of this mixture in the form of disks with dimensions 10 mm diameter and 2-3 mm thickness at a pressure of 100 MPa. These disks were sintered at 1273 K for 24 h in evacuated and sealed silica ampoules. This procedure of compaction, encapsulation and sintering was repeated l-2 times till the phase analysis by X-ray powder diffraction (XRD) showed the reaction to be complete within its 5 mass% limit of detection of impurity phases. An intimate mixture of o-Al,O, and p-alumina powder in a weight ratio 1:20 was made with a vibratory ball mill using a recrystallised alumina ball. After mixing, disks of dimensions 10 mm diameter and 2-3 mm thickness were made by using l-3% polyvinyl acetate as binder. These disks were initially heated in air to facilitate incineration of the binder. Subsequently, the pellets were sintered in air at 1873 K for a few hours. An SEM picture of the polished pellet showed the uniform dispersion of ol-Al,O, grains among grains of p-alumnina. The sintered pellets were found to be of density better than 95% theoretical computed for the bi-phasic compacts. The other test electrodes Ta/Ta,O, and Al/Al,O, were made by mixing the metals and their oxides in the ratio 1O:l and 8:1, respectively. The procedure for compaction and sintering was the same as that described for Nb/NbO. Ultra high purity helium was used as the cover gas for an open-cell stacked pellet assembly [8]. A calibrated Pt-10% Rh/Pt thermocouple was used for temperature measurements with the hot junction located in the vicinity of the cell head. The galvanic cell was located at the uniform temperature zone (UTZ) of a bifilar wound tubular furnace in order to minimise thermo-electric voltages and AC pick-ups. Null emf measurements (within 21 mV) were made on a symmetric cell with two Nb/NbO as the electrodes and the biphasic (cw+ @alumina) disk as the compo-
site electrolyte (CE). The reproducibility on temperature cycling, micro-polarization and insignificant variation in emf with minor variation in ratio of co-existing phases were used as tests for reversibility. For the 3 phase electrode, the ternary oxide Li,NbO, was prepared by heating a mixture of Li,CO, and Nb,O, in mole ratio 3:l and heating in air at 1173 K for 24 h with intermediate grinding. The electrode was made by mixing Li,O, Li,NbO, and Nb followed by compacting into disks of 10 mm diameter and 2-3 mm thickness at 100 MPa pressure. Moisture or CO, absorbed by the electrodes were readily eliminated by outgassing and evacuating at high temperature. The following galvanic cells were studied in the present investigation. Pt, Nb, NbO/CE/Ta,O,, Pt, Al, Al,O, /CE/NbO, Pt, Nb, Li,NbO,,
Ta, Pt,
(I)
Nb, Pt,
(II)
Li,O/CE/NbO,
Nb, Pt.
(III)
3. Results The emf results shown in Figs. l-3
from the galvanic cells I to III could be fitted into the following
.. ExPERlMENML RUNS
32-
Fig. 1. Temperature dependence of emf of galvanic NbO/~+@hnina/Ta,O,, Ta, Pt.
cell Pt, Nb,
R. Subasri, O.M. Sreedharan
I Solid State Ionics 93 (1997) 341-346
343
Em?.3 (mV) = 739.54 - 0.411T
(1009-1145
K). (3)
4. Discussion
T(K) Fig. 2. Temperature dependence of emf of galvanic Al,O, /CY+ P-alumina/NbO, Nb, Pt.
cell Pt, Al,
The lower electrolytic domain boundary (LEDB) in the log PO, -1 IT space for ionic conduction of 99% or better for the calcia-stabilized zirconia (CSZ) and yttria-doped thoria (YDT) were graphically represented in many excellent reviews [9- 1 l] along with the lines for the equilibrium PO, of some well defined reference electrodes. From these graphs it could be seen that the plot of log PO, against 1 /T for Nb/NbO and Ta/Ta,O, are much below the LEDB of CSZ but just above that for YDT. However, those for Al/Al,O, and Nh co-existing with its mixed oxide should be below the LEDB of YDT. Pratt identified LEDB of B-alumina to be coinciding with the log PO, line of Al/Al,O,. Thus for making oxygen potential measurements in the vicinity of those of Nb/NbO, the biphasic (01+ B-alumina) composite electrolyte should be resorted to for extending the LEDB beyond that for YDT. Prior to the reporting of results on the AGF of L&NbO, for the first time, it would be imperative to establish the reliability of the functioning of the CE for very low oxygen potential determinations. The verification of the Nernstian behaviour of CE for a few reference electrodes was reported elsewhere [ 12,131. In order to process the emf Eq. (l), the half cell reactions and the over-all cell reaction for the galvanic cell I may be represented as follows for the passage of 10 faraday of electricity At anode: 5Nb + 50-‘(CE)
T IKI Fig. 3. Temperature
dependence
of emf of galvanic
-+ 5Nb0 + lOe-(Pt).
(4a)
+ 2Ta + 50-‘(CE).
(4b)
cell III.
At cathode: least-squares expressions over the temperature indicated within parentheses.
ranges Ta,O, Over-all
ErkO.5 (mV) = 115 - 0.071T
Et, k-0.07 (mV) = 820 - 0.164T
(970-1120
(820-860
K),
K),
(1)
(2)
+ lOe-(Pt)
cell reaction:
5Nb + Ta,O,
-+ 5Nb0 + 2Ta.
Thus for computation
of standard
(4) Gibbs
energy
344
R. Subasri. O.M. Sreedharan I Solid Statelonics 93 (1997) 341-346
change AG: for reaction, the Nernst equation
Eq. (4), one makes use of
AG; = - 1OFE; = SAGF(Nb0)
- AGi(Ta,O,).
(5)
Since AGi(Nb0) data are quite reliably known (as given in Eq. (6)) from doped thoria-based measurements (instead of relying on the questionable ZrO,based measurements alone), the corresponding data for Ta,O, could be derived and is given in Eq. (7). AGF(Nb0)
kJmol_’
= - 415.29 + 0.086T
(800-1200
K),
AGF plot using CE (present studies) and that assessed by Hesselmann. However the emf results from the cells using CSZ can be seen to show considerable deviation from the other sets of data. Making use of the free-energy functions listed in the literature, a value of -205427 kJ mol-’ could be computed for the standard enthalpy of formation for Ta,O, from a third law analysis. This AH& value is in excellent agreement with -2053.7 kJ mol-’ from Hesselmann et al. [7]. Since Nb/NbO was used as reference for cell II as in the case of cell I, the AGF(Al,O,) could be computed in the same manner, i.e. AG;(Al,O,)
= - 6FE,, + 6AG;(NbO).
(8)
(6) AGF(Ta,O,)tO.S
kJ mol-’
= - 1965.5 + 0.363T. (7)
For an effective assessment of the performance of CE, a comparison of AGF(Ta,O,) derived from the emf results on cell I with those reported in the literature (based on emf results using conventional CSZ (Matsushita) and YDT (Worrell) is warranted [14,15]. Such a comparison is made in Fig. 4 which includes the data from the compilation by Hesselmann et al. [7]. There is very good agreement in the
-1470 -lJmwBE
L s 3 \
-1520
0" N G -cr -1570
a
-1620
970
1020
1070
1120
T(K)
Fig. 4. Free energy
of formation
of Ta,O,.
1170
Substituting Eq. (2) and Eq. (6) in Eq. (8), the standard Gibbs energy of formation for Al,O, was computed to be AGF(A1,0,)+0.5
kJ mol-’
= - 1708.03 + 0.339T. (9)
It was found that the cell did not function reversibly in the liquidus range of Al in contrast to the observation made by Kumar and Fray [ 121. In addition, the lower temperature emf study was probably limited by higher impedance leading to a narrow temperature interval (of about 50 degrees) of validity of Eq. (9). Hence when the temperature interval of Gibbs energy of measurement is narrow, not much reliability could be associated with the derived enthalpy and entropy data from such emf measurements according to Kubaschewski [ 161. However the values of AGF as such within this range of 820-870 K are quite consistent with literature data [7,12,17]. For instance at a mean temperature of 840 K the AGi(Al,O,) derived from this work yields a value of - 1422.4 kJ mol-’ which agrees well (+-5 kJmol_‘) with those of” 7 1412.7, - 1408.7 and - 1409.7 kJ mol-’ reported by Hesselmann, Kumar et al. and Choudury [7,12;17]. It can be seen that the composite electrolyte could be employed for very low oxygen potential measurements extending to PO, -1 IT regions below the LEDB of YDT. The application of this electrolyte for the determination of AGF(Li,NbO,) is discussed in the following section.
R. Subasri,
O.M. Sreedharan
1 Solid State tonics 93 (1997) 341-346
5. Studies on Li,NbO,
to be consistent with the observation phase equilibrium studies.
According to the pseudo-binary phase diagram of L&O-FJb,O, reported by Roth et al. [3], Li,NbO, is almost a line compound that can co-exist with either L&O on the Li rich side or with LiNbO,. No mutual solubility between Li,O and Li,NbO, was shown in this diagram even upto 1600 K. Phase equilibrium studies in inert atmosphere showed that metallic Nb co-existed with Li,O and Li,NbO, thereby making it possible to fix the oxygen potentials in this 3 phase mixture as in cell III. The overall galvanic cell reaction for cell III is given by 5Nb0 + 3Li,O + 2Li,NbO,
+ 3Nb
(9a)
for the passage of 10 faraday of electricity. The AGi(Li,NbO,) could be calculated by substituting Eqs. (3) and (6) and AGi(Li,O) from the literature into the following expression derived from Eq. (9) AGfO(Li,NbO,)
= 1/2[AGz
- 3AGF(Li,O)
- SAG;(NbO)].
345
made by the
6. Conclusion From the foregoing discussion it can be seen that the composite a+ p-alumina is promising to be useful for very low oxygen potential measurements and hence can be used to generate Gibbs energy data on very stable compounds like Li,NbO,.
Acknowledgments The authors are very grateful to the authorities of IGCAR for the keen interest and constant encouragement provided during the work. One of the authors namely R. Subasri is also thankful to the CSIR, New Delhi for the financial support provided.
(10)
Thus
References
AGF(Li,NhO,)+-0.1
kJ mol-’
= - 2288.79 + 0.611T.
(11)
The above equation is valid only for the temperature range 1009-1140 K. The slope of Eq. (11) is quite high to be associated with the negative of the standard entropy of formation. However, at a mean temperature of 1073 K, the ;GF(Li,NbO,) is - 1634 kJ mol-’ as per Eq. (11). In order to test the correctness of Gibbs energy values, the AG: of the following reaction could be computed at this temperature from Gibbs energy data on L&O and LiNbO,: L&O + LiNbO, + Li,NbO,, AG: = AGF(Li,NbO,) - AGF(Li,O).
(12)
- AGF(LiNb0,) (13)
Substituting values of -455 kJ mol-’ and - 1044 kJ mol-’ for Gibbs energy data on Li,O and LiNbO, respectively, the AG: for Eq. (12) could be computed to be - 135 kJ mol- ‘. This value is thus found
M. Yasumoto and S. Tanaka, Adv. in: r11 M. Yamawaki, Ceramics, eds. G.W. Hollenberg and I.J. Hastings (27 Columbus Am. Cer. Sot. Inc., 1990) (370) 147. 121M.M. Leonberger and F.J. Abouelleil, J. Am. Cer. Sot. 72(8) (1989) 1311. [31 R.S. Roth, H.S. Parker, W.S. Brower and J.L. Waring, in: Fast lon Transport in Solids, Solid State Batteries and Devices, ed. W. Go01 (North-Holland, Amsterdam, 1973) p. 227. [41 N. Kumada, M. Suguru, M. Fumio and K. Nobukazu, J. Solid State Chem. 73 (1988) 33. [51 M.J. Gesselbracht, T.J. Richardson and A.M. Stacy, Nature 345 (1990) 325. [61 A.M. Azad and O.M. Sreedharan, 5th Natl. Symp. Thermal Analysis (IIT, Kharagpur, 1985). and K. Hesselmann, Therr71 0. Knacke, 0. Kubaschewski mochemical Properties on Inorganic Substances, Vol. 2 (Springer, Berlin, 199 1). and M.D. Kar181 O.M. Sreedharan, M.S. Chandrasekharaiah khanavala, High Temp. Sci. 9 (1977) 109. [91 J.N. Pratt, Metall. Trans. 21A (1990) 1223. UOI C.B. Choudary, H.S. Maiti and E.C. Subba Rao, in: Solid Electrolytes and their Applications, ed. E.C. Subba Rao (Plenum Press, 1980). Proc. Symp. on High Temp. Chem. 1111 O.M. Sreedharan, (Bombay, 1982) p. 109. U21 R.V. Kumar and D.J. Fray, Solid State Ionics 70/71 (1994) 588.
346
R. Subasn’, O.M. Sreedharan
I Solid State Ionics 93 (1997) 341-346
[13] R. Subasri, O.M. Sreedharan and N.P. Bbat, Sixth Natl. Convention of Electrochemists (Karaikudi, 1995). [14] Y. Matsushita and K. Goto, in: Thermodynamics Proc. Symp. IAEA (Vienna, 1996) p. 111.
[15] W.L. WorreIl, Thermodynamics, Vol. I, Proc. Symp. IAEA (Vienna, 1966) 131. [16] 0. Kubaschewski, High Temp. High Press. 4 (1972) 1. [17] N.S. Choudury, J. Electrochem. Sot. 120 (12) (1973) 1663.
STATE ELSEVIER
IONICS
Solid State Ionics 93 ( 1997) 341-346
Thermodynamic
stability of Li,NbO, by emf measurements novel composite electrolyte
using a
R. Subasri, O.M. Sreedharan” Metallurgy
Division,
Indira
Gmdhi
Centre
for Atomic
Kesearch.
Kalpakkam,
%mil
h’adu 603 102, India
Received IS July 1996: accepted I.5 October 1996
Abstract The thermodynamic stability of Li,NbO, relative to LiNbO, and Liz0 was determined by oxygen potential measurements in the temperature rang 1009-l 115 K using a novel composite electrolyte made of alpha + beta alumina. The functioning of the composite electrolyte was independently assessed by emf measurements with well established low oxygen potential electrodes such as Nb/NbO, Ta/Ta,O, and AIIAI,O,. Kqwwdst
Lithium niobates; Stability; Emf: Gibbs energy
1. Introduction The ternary compounds in the Li-Nb-0 system are candidate tritium-breeding blanket materials in fusion reactors [I]. In addition, LiNbO, and LiTaO, find applications as wave-guides in opto-electronic devices [2]. The phase-diagram for the pseudo-binary system Li,O-Nb,O, was first compiled by Roth et al. [3]. Four ternary compounds with the stoichiometries LiNbO,, Li,NbO 4’ LiNb,O 8 and were reported to be present in the LiPb,,O,, system. There are also ternary compounds in this system where Nb exists in the oxidation state less than + 5. Kumada et al. reported the synthesis of LiNbO, and its potential application as lithium intercalating cathode materials in secondary batteries 141. Recently Gesselbracht et al. reported the exist*Corresponding author. Ol67-2738/97/$17X0 01997 Hlsevier Science B.V. All rights reserved PII SOl67-2738(96)00539-5
ence of superconductivity in Li,NbO, (0.45 5x I 0.5) with a T, of about 5 K 151. Despite the growing interest in the ternary lithium niobium oxides, very little work has so far been reported on the thermodynamic characterisation of these compounds. Azad et al. reported some DTA studies on the formation of AMO, compounds (A = Li, Na, K and M = Nb, Ta) from A,CO, and M,O, 161. Fortunately, the thermodynamic properties of, at least, LiNbO, were compiled by Hesselmann et al. which were based on calorimetric data 171. Thus LiNbO, could serve as the reference material for the thermodynamic characterisation of other ternary compounds in the Li-Nl0 system. The results of the galvanic cell studies employed for the determination of standard Gibbs energy of formation of Li,NbO,, i.e. AC) (Li,NbO,) are presented in this paper and the high temperature stability of this oxide relative to LiNbO, is also discussed.
342
R. Subasri. O.M. Sreedharan
I Solid State tonics 93 (1997) 341-346
2. Experimental (TMI) < High purity (total metal impurities 0.01%) were used as starting substances. Out of these, Nb, Nb,O,, Ta, Ta,O, and a-Al,O, are from Johnson Matthey (UK) while Li,CO, and @alumina are from Purex Laboratories (India) and Polyceram (USA) respectively. The Nb/NbO electrode was made by mixing Nb and Nb,O, in 8:l mole ratio followed by compaction of this mixture in the form of disks with dimensions 10 mm diameter and 2-3 mm thickness at a pressure of 100 MPa. These disks were sintered at 1273 K for 24 h in evacuated and sealed silica ampoules. This procedure of compaction, encapsulation and sintering was repeated l-2 times till the phase analysis by X-ray powder diffraction (XRD) showed the reaction to be complete within its 5 mass% limit of detection of impurity phases. An intimate mixture of o-Al,O, and p-alumina powder in a weight ratio 1:20 was made with a vibratory ball mill using a recrystallised alumina ball. After mixing, disks of dimensions 10 mm diameter and 2-3 mm thickness were made by using l-3% polyvinyl acetate as binder. These disks were initially heated in air to facilitate incineration of the binder. Subsequently, the pellets were sintered in air at 1873 K for a few hours. An SEM picture of the polished pellet showed the uniform dispersion of ol-Al,O, grains among grains of p-alumnina. The sintered pellets were found to be of density better than 95% theoretical computed for the bi-phasic compacts. The other test electrodes Ta/Ta,O, and Al/Al,O, were made by mixing the metals and their oxides in the ratio 1O:l and 8:1, respectively. The procedure for compaction and sintering was the same as that described for Nb/NbO. Ultra high purity helium was used as the cover gas for an open-cell stacked pellet assembly [8]. A calibrated Pt-10% Rh/Pt thermocouple was used for temperature measurements with the hot junction located in the vicinity of the cell head. The galvanic cell was located at the uniform temperature zone (UTZ) of a bifilar wound tubular furnace in order to minimise thermo-electric voltages and AC pick-ups. Null emf measurements (within 21 mV) were made on a symmetric cell with two Nb/NbO as the electrodes and the biphasic (cw+ @alumina) disk as the compo-
site electrolyte (CE). The reproducibility on temperature cycling, micro-polarization and insignificant variation in emf with minor variation in ratio of co-existing phases were used as tests for reversibility. For the 3 phase electrode, the ternary oxide Li,NbO, was prepared by heating a mixture of Li,CO, and Nb,O, in mole ratio 3:l and heating in air at 1173 K for 24 h with intermediate grinding. The electrode was made by mixing Li,O, Li,NbO, and Nb followed by compacting into disks of 10 mm diameter and 2-3 mm thickness at 100 MPa pressure. Moisture or CO, absorbed by the electrodes were readily eliminated by outgassing and evacuating at high temperature. The following galvanic cells were studied in the present investigation. Pt, Nb, NbO/CE/Ta,O,, Pt, Al, Al,O, /CE/NbO, Pt, Nb, Li,NbO,,
Ta, Pt,
(I)
Nb, Pt,
(II)
Li,O/CE/NbO,
Nb, Pt.
(III)
3. Results The emf results shown in Figs. l-3
from the galvanic cells I to III could be fitted into the following
.. ExPERlMENML RUNS
32-
Fig. 1. Temperature dependence of emf of galvanic NbO/~+@hnina/Ta,O,, Ta, Pt.
cell Pt, Nb,
R. Subasri, O.M. Sreedharan
I Solid State Ionics 93 (1997) 341-346
343
Em?.3 (mV) = 739.54 - 0.411T
(1009-1145
K). (3)
4. Discussion
T(K) Fig. 2. Temperature dependence of emf of galvanic Al,O, /CY+ P-alumina/NbO, Nb, Pt.
cell Pt, Al,
The lower electrolytic domain boundary (LEDB) in the log PO, -1 IT space for ionic conduction of 99% or better for the calcia-stabilized zirconia (CSZ) and yttria-doped thoria (YDT) were graphically represented in many excellent reviews [9- 1 l] along with the lines for the equilibrium PO, of some well defined reference electrodes. From these graphs it could be seen that the plot of log PO, against 1 /T for Nb/NbO and Ta/Ta,O, are much below the LEDB of CSZ but just above that for YDT. However, those for Al/Al,O, and Nh co-existing with its mixed oxide should be below the LEDB of YDT. Pratt identified LEDB of B-alumina to be coinciding with the log PO, line of Al/Al,O,. Thus for making oxygen potential measurements in the vicinity of those of Nb/NbO, the biphasic (01+ B-alumina) composite electrolyte should be resorted to for extending the LEDB beyond that for YDT. Prior to the reporting of results on the AGF of L&NbO, for the first time, it would be imperative to establish the reliability of the functioning of the CE for very low oxygen potential determinations. The verification of the Nernstian behaviour of CE for a few reference electrodes was reported elsewhere [ 12,131. In order to process the emf Eq. (l), the half cell reactions and the over-all cell reaction for the galvanic cell I may be represented as follows for the passage of 10 faraday of electricity At anode: 5Nb + 50-‘(CE)
T IKI Fig. 3. Temperature
dependence
of emf of galvanic
-+ 5Nb0 + lOe-(Pt).
(4a)
+ 2Ta + 50-‘(CE).
(4b)
cell III.
At cathode: least-squares expressions over the temperature indicated within parentheses.
ranges Ta,O, Over-all
ErkO.5 (mV) = 115 - 0.071T
Et, k-0.07 (mV) = 820 - 0.164T
(970-1120
(820-860
K),
K),
(1)
(2)
+ lOe-(Pt)
cell reaction:
5Nb + Ta,O,
-+ 5Nb0 + 2Ta.
Thus for computation
of standard
(4) Gibbs
energy
344
R. Subasri. O.M. Sreedharan I Solid Statelonics 93 (1997) 341-346
change AG: for reaction, the Nernst equation
Eq. (4), one makes use of
AG; = - 1OFE; = SAGF(Nb0)
- AGi(Ta,O,).
(5)
Since AGi(Nb0) data are quite reliably known (as given in Eq. (6)) from doped thoria-based measurements (instead of relying on the questionable ZrO,based measurements alone), the corresponding data for Ta,O, could be derived and is given in Eq. (7). AGF(Nb0)
kJmol_’
= - 415.29 + 0.086T
(800-1200
K),
AGF plot using CE (present studies) and that assessed by Hesselmann. However the emf results from the cells using CSZ can be seen to show considerable deviation from the other sets of data. Making use of the free-energy functions listed in the literature, a value of -205427 kJ mol-’ could be computed for the standard enthalpy of formation for Ta,O, from a third law analysis. This AH& value is in excellent agreement with -2053.7 kJ mol-’ from Hesselmann et al. [7]. Since Nb/NbO was used as reference for cell II as in the case of cell I, the AGF(Al,O,) could be computed in the same manner, i.e. AG;(Al,O,)
= - 6FE,, + 6AG;(NbO).
(8)
(6) AGF(Ta,O,)tO.S
kJ mol-’
= - 1965.5 + 0.363T. (7)
For an effective assessment of the performance of CE, a comparison of AGF(Ta,O,) derived from the emf results on cell I with those reported in the literature (based on emf results using conventional CSZ (Matsushita) and YDT (Worrell) is warranted [14,15]. Such a comparison is made in Fig. 4 which includes the data from the compilation by Hesselmann et al. [7]. There is very good agreement in the
-1470 -lJmwBE
L s 3 \
-1520
0" N G -cr -1570
a
-1620
970
1020
1070
1120
T(K)
Fig. 4. Free energy
of formation
of Ta,O,.
1170
Substituting Eq. (2) and Eq. (6) in Eq. (8), the standard Gibbs energy of formation for Al,O, was computed to be AGF(A1,0,)+0.5
kJ mol-’
= - 1708.03 + 0.339T. (9)
It was found that the cell did not function reversibly in the liquidus range of Al in contrast to the observation made by Kumar and Fray [ 121. In addition, the lower temperature emf study was probably limited by higher impedance leading to a narrow temperature interval (of about 50 degrees) of validity of Eq. (9). Hence when the temperature interval of Gibbs energy of measurement is narrow, not much reliability could be associated with the derived enthalpy and entropy data from such emf measurements according to Kubaschewski [ 161. However the values of AGF as such within this range of 820-870 K are quite consistent with literature data [7,12,17]. For instance at a mean temperature of 840 K the AGi(Al,O,) derived from this work yields a value of - 1422.4 kJ mol-’ which agrees well (+-5 kJmol_‘) with those of” 7 1412.7, - 1408.7 and - 1409.7 kJ mol-’ reported by Hesselmann, Kumar et al. and Choudury [7,12;17]. It can be seen that the composite electrolyte could be employed for very low oxygen potential measurements extending to PO, -1 IT regions below the LEDB of YDT. The application of this electrolyte for the determination of AGF(Li,NbO,) is discussed in the following section.
R. Subasri,
O.M. Sreedharan
1 Solid State tonics 93 (1997) 341-346
5. Studies on Li,NbO,
to be consistent with the observation phase equilibrium studies.
According to the pseudo-binary phase diagram of L&O-FJb,O, reported by Roth et al. [3], Li,NbO, is almost a line compound that can co-exist with either L&O on the Li rich side or with LiNbO,. No mutual solubility between Li,O and Li,NbO, was shown in this diagram even upto 1600 K. Phase equilibrium studies in inert atmosphere showed that metallic Nb co-existed with Li,O and Li,NbO, thereby making it possible to fix the oxygen potentials in this 3 phase mixture as in cell III. The overall galvanic cell reaction for cell III is given by 5Nb0 + 3Li,O + 2Li,NbO,
+ 3Nb
(9a)
for the passage of 10 faraday of electricity. The AGi(Li,NbO,) could be calculated by substituting Eqs. (3) and (6) and AGi(Li,O) from the literature into the following expression derived from Eq. (9) AGfO(Li,NbO,)
= 1/2[AGz
- 3AGF(Li,O)
- SAG;(NbO)].
345
made by the
6. Conclusion From the foregoing discussion it can be seen that the composite a+ p-alumina is promising to be useful for very low oxygen potential measurements and hence can be used to generate Gibbs energy data on very stable compounds like Li,NbO,.
Acknowledgments The authors are very grateful to the authorities of IGCAR for the keen interest and constant encouragement provided during the work. One of the authors namely R. Subasri is also thankful to the CSIR, New Delhi for the financial support provided.
(10)
Thus
References
AGF(Li,NhO,)+-0.1
kJ mol-’
= - 2288.79 + 0.611T.
(11)
The above equation is valid only for the temperature range 1009-1140 K. The slope of Eq. (11) is quite high to be associated with the negative of the standard entropy of formation. However, at a mean temperature of 1073 K, the ;GF(Li,NbO,) is - 1634 kJ mol-’ as per Eq. (11). In order to test the correctness of Gibbs energy values, the AG: of the following reaction could be computed at this temperature from Gibbs energy data on L&O and LiNbO,: L&O + LiNbO, + Li,NbO,, AG: = AGF(Li,NbO,) - AGF(Li,O).
(12)
- AGF(LiNb0,) (13)
Substituting values of -455 kJ mol-’ and - 1044 kJ mol-’ for Gibbs energy data on Li,O and LiNbO, respectively, the AG: for Eq. (12) could be computed to be - 135 kJ mol- ‘. This value is thus found
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R. Subasn’, O.M. Sreedharan
I Solid State Ionics 93 (1997) 341-346
[13] R. Subasri, O.M. Sreedharan and N.P. Bbat, Sixth Natl. Convention of Electrochemists (Karaikudi, 1995). [14] Y. Matsushita and K. Goto, in: Thermodynamics Proc. Symp. IAEA (Vienna, 1996) p. 111.
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