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The electrical and thermal properties of sodium sulfate mixed with lanthanum sulfate and aluminum oxide
Solid State Ionies 20 (1986) 153-157 North-Holland, Amsterdam
THE ELECTRICAL AND THERMAL PROPERTIES OF SODIUM SULFATE
MIXED WITH LANTHANUM SULFATE AND ALUMINUM OXIDE Nobuhito IMANAKA, Yasuo YAMAGUCHI, Gin-ya ADACHI *, Jiro SHIOKAWA Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka 2-l, Suita Osaka 565, Japan and
I-Iideki YOSHIOKA Chemistry Division, Industrial Research Institute of Hyogo Prefecture, Yukihira 3.1.12, Suma, Kobe, Hyogo 654, Japan Received 29 July 1985 Accepted for publication 11 November 1985
Sodium sulfate mixed with lanthanum sulfate and aluminum oxide was pzepaxed and its phases and electrical properties were investigated. The Na2'SO4-La2 (SO,0a -AI2Oa sample maintains a Na2SO4-I-similarphase and exhibits higher electrical conductivity compared with unmixed sodium sulfate. Electromotive force (EMF) measurements were carried out by constructing the sulfur oxides concentration cell. The measured EMF was in good agreement with the calculated EMF, in the inlet SO2 gas concentration from 100 ppm to 1%.
1. Introduction
The deterioration of the environment by acid rain, which results from the absorption of sulfur oxides and nitrogen oxides, has been becoming a serious problem in recent years. The regulation of the SOx and NOx in exhausted gas is an urgent issue. The sulfur dioxide gas detection utilizing alkali metal sulfates [1-12] (M = Li, Na and K) as a solid electrolyte has been extensively investigated. However, phase transition occurs in the alkali metal sulfates and results in the permeation of ambient gases through cracks in the electrolyte. Lithium sulfate is considerably hygroscopic. Furthermore, a heat of transformation for Li2SO 4 is the highest among those for alkali metal sulfates. This means that the phase transition of this sulfate is difficult to suppress. The electrical conductivity of potassium sulfate is the lowest in these three alkali metal sulfates and does not seem to be a good candidate for the electrolyte of a gas cell. * To whom all coxrespondenee should be addressed.
Sodium sulfate also exhibits a phase transformation from Na2SO4-I (a high temperature phase) to Na2SO4-III (a low temperature phase) [13,14] at approximately 513 K. The electrical conductivity of the sodium sulfate is relatively low compared with other sodium cationic conductors such as/3-alumina [15] and NASICON [16,17]. Many efforts [18-23] to dope mono-, di-, or trivalent cations have been focussed on to enhance the conductivity. In our earlier paper [10], the phase transition was suppressed by doping sodium vanadate and rare earth sulfates (Ln = Pr and Y) simultaneously into sodium sulfate. However, the Na2SO4-NaVO3-Ln2(SO4) 3 (Ln = Pr and Y) electrolyte was unable to apply at a temperature higher than about 723 K. In our previous investigation [24], rare earth sulfates (Ln = Y and Gd) and silicon dioxide were mixed in order to increase the electrical conductivity and to prevent the electrolyte from becoming ductile, respectively. The stabilization of Na2SO4-I-similar phase at room temperature was achieved by this mixing. In this study, aluminum oxide was mixed instead
154
N. Imanaka et al./NazS04 mixed with La2(S04) a and A1203
of silicon dioxide so as to obtain harder and further heat-endurable electrolyte than the silicon dioxide mixed one. As a rare earth sulfate, lanthanum sulfate, whose oxide is relatively inexpensive in the rare earth family, was employed.
2. Experimental
Pt net
quart;
quartz tube(A}
quartz rod(B)
2.1. Materials Sodium sulfate (purity: 99.99%) and aluminum oxide (purity: 99.98%) were purchased from Wake Pure Chemical Industries Ltd. Lantanum oxide (purity: 99.99%) was bought from Shiga Rare Metal Industries Ltd. Lanthanum sulfate was prepared by adding a concentrated sulfuric acid into the lanthanum oxide. Sodium sulfate (Na2SO4), lanthanum sulfate (La 2 (SO4)3) , and aluminum oxide (A1203) were preheated in a porcelain crucible for dehydration before weighing. The appropriate amount of Na2SO 4 , La2(SO4)3, and A1203 was mixed thoroughly in an agate mortar. The mixture of Na2SO4-La2(SO4)3A1203 was melted at 1473 K in a platinum crucible in air atmosphere, and then quenched in air. The resultant was ground (<74 pan) and made into pellets in a hydrostatic pressure (2.65 × 10 s Pa). The pellets were sintered at 1073 K for 1 h in air and then quenched in air. Platinum powder was sputtered on both center surfaces (5 × 10 -3 m in diameter) of the electrolyte (1.3 X 10 -2 m in diameter).
Pt lead
Fig. 1. The apparatus for the EMF measurements.
(from 30 ppm to 1%) was regulated with Standard Gas Generator (SGGU-711SD) from Standard Technology Co. The constant SO 2 and 02 gas mixture (about 3%) was flowed in the outer quartz tube (C) as a reference. The platinum net was inserted in the tube (A) so as to accelerate the oxidation from SO 2 to SO 3 . As the electrode, platinum net was applied in order to obtain the good contact between the ambient gas and the electrolyte.
3. Results and diseusalon 3.1. Phases and thermal properties
2.2. Measurements Phases and thermal properties were measured by X-ray diffractometer (Rigaku Rotatlex) and by DTA-TG instrument (Rigaku Thermoflex). Electrical conductivity measurements were conducted by the complex impedance method with a vector impedance meter 4800 A from Hewlett Packard Company. The electromotive force (EMF) was measured with Takeda Riken Digital Multimeter TR-6841. The apparatus for the EMF measurements is illustrated in fig. 1. The sample was fixed between quartz tube (A) and quartz rod (B). The ringed glass packing was adapted for the purpose of completely separating test gas from reference gas. The test SO 2 and 0 2 gas mixture was introduced from the tube (A). The test SO 2 gas concentration
X-ray and DTA results for Na2SO4-La2(SO4)3A120 3 are tabulated in table 1. All samples show a new phase, A, which is analogous to the Na2SO4-I phase, together with aluminum oxide phase. This Na2SO4-I phase is effective for Na + cationic conduction. Lanthanum aluminate (LaAIO3) coexists except for the sample no. 7. Furthermore, unknown phase, B, exists in the samples from no. 4 to 7. In the DTA measurements, all samples exhibited an endothermal peak at 593 or 603 K. This means that a phase transition occurs. However, the peaks are appreciably small. The doping of La2(SO4) 3 and A1203 into sodium sulfate considerably contributes to the suppression of the phase transformation.
N. Imanaka et al./Na2S04 mixed with La 2 (S04)3 and A1203
155
Table 1 The phases and thexmal properties of Na2 S04 -La2 (S04)3 - A I 2 0 3 .
Sample no.
Na2SO4 (mol%)
La2 (SO4)3 (mol%)
A1203 (mol%)
Phases
1 2 3 4 5 6 7
55.0 52.1 50.0 48.0 44.8 42.4 40.1
5.0 7.8 10.1 12.0 15.3 18.1 19.8
40.0 40.1 39.9 40.0 39.9 39.5 40.1
A + A120 a A + AI20 a A + AI20 a A + A12Oa A + Al2Oa A + A1203 A + A1203
DTA peak (K) + LaAIO3 (s) + LaAIOa + LaAIOa + LaAIO3 + B(s) + LaAIOa + B (s) + LaAIO3 + B +B
593 603 603 603 603 603 593
* A: Na2SO4-I-similax phase; B: unknown phase; (s): small amount.
3.2. Electrical conductivity measurements The plots of log(aT) versus 1/T for the samples nos. 1-3 are shown in fig. 2. A deflection in the crT versus lIT curves exists in these samples. This kneel temperature is approximately 603 K, which is almost consistent with the DTA results. All samples show higher electrical conductivity than pure sodium sulfate. The mixing of La2(804) 3 into sodium sulfate makes more effective cation vacancies for ionic migration because of the electroneutralization. Fig. 3 presents the electrical conductivity results
for the samples nos. 4-7. The bend in the curve also occurs in no. 4. This deflection temperature at about 603 K is identical to the temperature at DTA peak. The curves for the Na2SO4-La 2 (SO4)3-A1203, of which La2(SO4) 3 has been mixed more than 15.3 mol%, show almost straight in the o1"versus lIT relation. The highest aT value was obtained in the sample no. 5. The cation vacancies begin to make dusters which do not contribute to the cation conduction by the mixing of La2(SO4) 3 more than 18.1 mol%. From these results, the most appropriate sample
r/'c 2
7O0 •
i
!
500
tOO
i
i
300 !
!
700
200 i
I
r/'c 5OO 4OO 3OO
2O0
I
! -
-3
-4
I
I
1.0
1.5
2.0
kKIr
Fig. 2. Temperature dependences of electrical conductivities for Na2 SO4 -La2 (SO4)a -AI2 O3. (=) Na2 SO4 :La2 (SO4) s : AI203 = 55.0:5.0:40.0 (no. 1);(-) Na2SO4:La2(SO4) 3 : AI20a = 52.1:7.8:40.1 (no. 2) ; (e) Na2 SO4 :La2 (SO4)a : AI2Os = 50.0:10.1:39.9 (no. 3);( )Na2SO4.
1.0
1.5
kKIT
2.0
Fig. 3. Temperature dependencesof electrical conductJvities for Na2 SO4-La2(SO4)3-AI203. (o) Na2SO4 :La2(SO4) 3 : A1203 = 48.0:12.0:40.0 (no. 4); (e) Na2 SO4:La2(SO4)3 : AI=O3 = 44.8:15.3:39.9 (no. 5);(") Na=SO4:La2(SO4)3: AI2Oa = 42.4:18.1:39.5 (no. 6); (A) Na2SO4 :La2(SO4)3 : AI=O3 =40.1:19.8:40.1 (no. 7);( ) Na2SO4.
156
N. Imanaka et al./Na2SO 4 mixed with La2(SO4) 3 and AI203
for the solid electrolyte for sulfur dioxide gas detector is found to be the sample no. 5. 3.3. E M F measurements
I00
The variation of the EMF for the Na2SO 4 La2(SO4)3-AI203 (no. 5) at 973 K is shown in fig. 4. The inlet SO 2 gas concentration was varied from 30 ppm (log(Pso2)in = -4.52) to l%(log(Pso2)in = -2.0), while the reference inlet SO 2 gas content was fixed at approximately 3% by sulfur dioxide and oxygen gas mixing. The measured EMF was in good agreement with the calculated EMF, in the inlet SO 2 gas concentration from 100 ppm (log(Pso2)in = - 4 . 0 ) to 1%. The difference between the measured and calculated EMF was almost 40 mV at 30 ppm. This may be attributed to the fact that the gas permeation occurs through the microcraeks which have been resulted from the phase transition. In this method of SO 2 gas detection, both test and reference gas compartments should be f'flied with a constant SO 2 gas concentration by the control of the SO 2 and 02 gas flow volume. In order to make the apparatus more compact, that is, to eliminate the reference gas regulation, it was attempted to replace the reference gas with the reference solid electrode, which generated a certain gas pressure of SO 2 . Furthermore, the oxygen gas for di-
300
> 200 E
IOC
i
,
-40
•
•
-3.0 -2.0 I.og(pr~in
Fig. 4. The variation of the EMF for Na 2 SO4-Lax (SO4)3A12Oa (44.8: 15.3:39.9) solid electrolyte at 973 K. ( .) calculated EMF (1).
E
-100 -z,.O - 3 0
-20
t og (pso2)~ Fig. 5. The variation of the EMF fo~ Na 2 SO4 -La2 (SO4)a -
A1203 (44.8:15.3:39.9) solid electrolyte with the solid reference electrode method (NiSO4 + NiO) at 973 K. ( ) calculated EMF (7).
lution was substituted for air so as to be of more practical utilization. Fig. 5 indicates the EMF results for the Na2SO 4 La 2 ( S O 4 ) 3 - A I 2 0 3 w i t h solid reference electrode method (NiSO 4 + NiO) at 973 K. The reference SO 2 gas concentration was maintained at approximately 2800 ppm because both SO 3 gas, which resulted from the equilibrium between NiSO 4 and NiO, and 0 2 gas content, which was controlled by air circulation, were kept constant. The EMF coincides with the calculation, in the inlet SO 2 gas concentration between 100 ppm and 1%. The measured EMF was about 30 mV lower than the calculated EMF at 30 ppm. This is analogous to the case of fig. 4. This result also refers to the gas penetration through the microcracks in the electrolyte. In conclusion, sodium sulfate mixed with lantha. num sulfate and aluminum oxide contains Na2SO4-Isimilar phase and exhibits 30 times larger conductivity compared with unmixed sodium sulfate at 973 K. The phase transition was suppressed except for a very small one at 603 K. The EMF characteristics for the Na2SO4-La 2 (SO4) 3-A120 3 solid electrolyte were fairly in good accordance with the calculated EMF both the reference SO 2 and 0 2 gas concentration and the reference solid electrode (NiSO 4 + NiO) method, in the inlet SO 2 gas concentration between 100 ppm and 1%.
N. Imanaka et al./Na2SO 4 mixed with La2(S04) a and A l 2 0 a
References [1] K.T. Jacob and D.B. Rao, J. Electrochem. Soc. 126 (1979) 1842. [2] W.L. WorreU, in: Proc. Internat. Meeting on Chemical Sensors, Fukuoka, 1983, ed. T. Seiyama et al. (Kodansha, Tokyo and Elsevier, Amsterdam, 1983) . p. 332. [3] W.L. Worrell and Q.G. Liu, J. Electroanal. Chem. 168 (1984) 355. [4] M. Gauthier and A. Chamberland, J. Electrochem. Soc. 124 (1977) 1579. [5 ] M. Gauthiex, A. Chamberland, A. B61anger and M. Poirier, J. Electrochem. Soc. 124 (1977) 1584. [6] M. Gauthier, R. Bellemare and A. B61anger, J. Electrochem. Soc. 128 (1981) 371. [7] M. Gauthier and C.W. Bale, Metall. Trans. 14B (1983) 117. [8] N. Imanaka, G. Adachi and J. Shiokawa, Chem. Letters (1983) 287. [9] N. Imanaka, G. Adachi and J. Shiokawa, Denki Kagaku 51 (1983) 93. [10] N. Imanaka, G. Adachi and J. Shiokawa, Bull. Chem. Soc. Japan. 57 (1984) 687. [11] N. Imanaka, G. Adachi and J. Shiokawa, in: Ptoc. International Meeting on Chemical Sensors, Fukuoka, 1983, ed. T. Seiyama et al. (Kodansha, Tokyo and Elsevier, Amsterdam, 1983) p. 348.
157
[12] A. B61anger, M. Gauthier and D. Fauteux, J. Electrochem. Soc. 131 (1984) 579. [13] E.L. Kreidl and I. Simon, Nature 181 (1958) 1529. [14] Y. Saito, K. KobayashJ and T. Muruyama, Thermochim. Acta 53 (1982) 289. [15] M.S. Whittingham and R.A. Huggins, J. Chem. Phys. 54 (1971) 414. [16] H.Y-P. Hong, Mater. Res. Bull. 11 (1976) 173. [17] J.B. Goodenough, H.Y-P. Hon$ and J.A. Kafalas, Mater. Res. Bull. 11 (1976) 203. [ 18] H.H. H6fer, W. Eysel and U. yon Alpen, J. Solid State Chem. 36 (1981) 365. [19] H.H. H6fer, W. Eysel and U. yon Alpen, Mater. Res. Bull. 13 (1978) 265. [20] R.M. Murray and E.A. Secco, Can. J. Chem. 56 (1978) 2616. [21 ] K.L. Keester, W. Eysel and Th. Hahn, Acta CrystaUogr. (Sect. A) 31 (1975) $79. [22] H.H. H6fer, U. yon Alpen and W. Eysel, Acta CrystaUogr. (Sect. A) 34 (1978) $358. [23] N. Imanaka, G. Adachi and J. Shiokawa, Can. J. Chem. 61 (1983) 1557. [24] N. lmanaka, Y. Yamaguchi, G. Adachi and J. Shiokawa, Bull. Chem. Soc. Japan. 58 (1985) 5.
THE ELECTRICAL AND THERMAL PROPERTIES OF SODIUM SULFATE
MIXED WITH LANTHANUM SULFATE AND ALUMINUM OXIDE Nobuhito IMANAKA, Yasuo YAMAGUCHI, Gin-ya ADACHI *, Jiro SHIOKAWA Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka 2-l, Suita Osaka 565, Japan and
I-Iideki YOSHIOKA Chemistry Division, Industrial Research Institute of Hyogo Prefecture, Yukihira 3.1.12, Suma, Kobe, Hyogo 654, Japan Received 29 July 1985 Accepted for publication 11 November 1985
Sodium sulfate mixed with lanthanum sulfate and aluminum oxide was pzepaxed and its phases and electrical properties were investigated. The Na2'SO4-La2 (SO,0a -AI2Oa sample maintains a Na2SO4-I-similarphase and exhibits higher electrical conductivity compared with unmixed sodium sulfate. Electromotive force (EMF) measurements were carried out by constructing the sulfur oxides concentration cell. The measured EMF was in good agreement with the calculated EMF, in the inlet SO2 gas concentration from 100 ppm to 1%.
1. Introduction
The deterioration of the environment by acid rain, which results from the absorption of sulfur oxides and nitrogen oxides, has been becoming a serious problem in recent years. The regulation of the SOx and NOx in exhausted gas is an urgent issue. The sulfur dioxide gas detection utilizing alkali metal sulfates [1-12] (M = Li, Na and K) as a solid electrolyte has been extensively investigated. However, phase transition occurs in the alkali metal sulfates and results in the permeation of ambient gases through cracks in the electrolyte. Lithium sulfate is considerably hygroscopic. Furthermore, a heat of transformation for Li2SO 4 is the highest among those for alkali metal sulfates. This means that the phase transition of this sulfate is difficult to suppress. The electrical conductivity of potassium sulfate is the lowest in these three alkali metal sulfates and does not seem to be a good candidate for the electrolyte of a gas cell. * To whom all coxrespondenee should be addressed.
Sodium sulfate also exhibits a phase transformation from Na2SO4-I (a high temperature phase) to Na2SO4-III (a low temperature phase) [13,14] at approximately 513 K. The electrical conductivity of the sodium sulfate is relatively low compared with other sodium cationic conductors such as/3-alumina [15] and NASICON [16,17]. Many efforts [18-23] to dope mono-, di-, or trivalent cations have been focussed on to enhance the conductivity. In our earlier paper [10], the phase transition was suppressed by doping sodium vanadate and rare earth sulfates (Ln = Pr and Y) simultaneously into sodium sulfate. However, the Na2SO4-NaVO3-Ln2(SO4) 3 (Ln = Pr and Y) electrolyte was unable to apply at a temperature higher than about 723 K. In our previous investigation [24], rare earth sulfates (Ln = Y and Gd) and silicon dioxide were mixed in order to increase the electrical conductivity and to prevent the electrolyte from becoming ductile, respectively. The stabilization of Na2SO4-I-similar phase at room temperature was achieved by this mixing. In this study, aluminum oxide was mixed instead
154
N. Imanaka et al./NazS04 mixed with La2(S04) a and A1203
of silicon dioxide so as to obtain harder and further heat-endurable electrolyte than the silicon dioxide mixed one. As a rare earth sulfate, lanthanum sulfate, whose oxide is relatively inexpensive in the rare earth family, was employed.
2. Experimental
Pt net
quart;
quartz tube(A}
quartz rod(B)
2.1. Materials Sodium sulfate (purity: 99.99%) and aluminum oxide (purity: 99.98%) were purchased from Wake Pure Chemical Industries Ltd. Lantanum oxide (purity: 99.99%) was bought from Shiga Rare Metal Industries Ltd. Lanthanum sulfate was prepared by adding a concentrated sulfuric acid into the lanthanum oxide. Sodium sulfate (Na2SO4), lanthanum sulfate (La 2 (SO4)3) , and aluminum oxide (A1203) were preheated in a porcelain crucible for dehydration before weighing. The appropriate amount of Na2SO 4 , La2(SO4)3, and A1203 was mixed thoroughly in an agate mortar. The mixture of Na2SO4-La2(SO4)3A1203 was melted at 1473 K in a platinum crucible in air atmosphere, and then quenched in air. The resultant was ground (<74 pan) and made into pellets in a hydrostatic pressure (2.65 × 10 s Pa). The pellets were sintered at 1073 K for 1 h in air and then quenched in air. Platinum powder was sputtered on both center surfaces (5 × 10 -3 m in diameter) of the electrolyte (1.3 X 10 -2 m in diameter).
Pt lead
Fig. 1. The apparatus for the EMF measurements.
(from 30 ppm to 1%) was regulated with Standard Gas Generator (SGGU-711SD) from Standard Technology Co. The constant SO 2 and 02 gas mixture (about 3%) was flowed in the outer quartz tube (C) as a reference. The platinum net was inserted in the tube (A) so as to accelerate the oxidation from SO 2 to SO 3 . As the electrode, platinum net was applied in order to obtain the good contact between the ambient gas and the electrolyte.
3. Results and diseusalon 3.1. Phases and thermal properties
2.2. Measurements Phases and thermal properties were measured by X-ray diffractometer (Rigaku Rotatlex) and by DTA-TG instrument (Rigaku Thermoflex). Electrical conductivity measurements were conducted by the complex impedance method with a vector impedance meter 4800 A from Hewlett Packard Company. The electromotive force (EMF) was measured with Takeda Riken Digital Multimeter TR-6841. The apparatus for the EMF measurements is illustrated in fig. 1. The sample was fixed between quartz tube (A) and quartz rod (B). The ringed glass packing was adapted for the purpose of completely separating test gas from reference gas. The test SO 2 and 0 2 gas mixture was introduced from the tube (A). The test SO 2 gas concentration
X-ray and DTA results for Na2SO4-La2(SO4)3A120 3 are tabulated in table 1. All samples show a new phase, A, which is analogous to the Na2SO4-I phase, together with aluminum oxide phase. This Na2SO4-I phase is effective for Na + cationic conduction. Lanthanum aluminate (LaAIO3) coexists except for the sample no. 7. Furthermore, unknown phase, B, exists in the samples from no. 4 to 7. In the DTA measurements, all samples exhibited an endothermal peak at 593 or 603 K. This means that a phase transition occurs. However, the peaks are appreciably small. The doping of La2(SO4) 3 and A1203 into sodium sulfate considerably contributes to the suppression of the phase transformation.
N. Imanaka et al./Na2S04 mixed with La 2 (S04)3 and A1203
155
Table 1 The phases and thexmal properties of Na2 S04 -La2 (S04)3 - A I 2 0 3 .
Sample no.
Na2SO4 (mol%)
La2 (SO4)3 (mol%)
A1203 (mol%)
Phases
1 2 3 4 5 6 7
55.0 52.1 50.0 48.0 44.8 42.4 40.1
5.0 7.8 10.1 12.0 15.3 18.1 19.8
40.0 40.1 39.9 40.0 39.9 39.5 40.1
A + A120 a A + AI20 a A + AI20 a A + A12Oa A + Al2Oa A + A1203 A + A1203
DTA peak (K) + LaAIO3 (s) + LaAIOa + LaAIOa + LaAIO3 + B(s) + LaAIOa + B (s) + LaAIO3 + B +B
593 603 603 603 603 603 593
* A: Na2SO4-I-similax phase; B: unknown phase; (s): small amount.
3.2. Electrical conductivity measurements The plots of log(aT) versus 1/T for the samples nos. 1-3 are shown in fig. 2. A deflection in the crT versus lIT curves exists in these samples. This kneel temperature is approximately 603 K, which is almost consistent with the DTA results. All samples show higher electrical conductivity than pure sodium sulfate. The mixing of La2(804) 3 into sodium sulfate makes more effective cation vacancies for ionic migration because of the electroneutralization. Fig. 3 presents the electrical conductivity results
for the samples nos. 4-7. The bend in the curve also occurs in no. 4. This deflection temperature at about 603 K is identical to the temperature at DTA peak. The curves for the Na2SO4-La 2 (SO4)3-A1203, of which La2(SO4) 3 has been mixed more than 15.3 mol%, show almost straight in the o1"versus lIT relation. The highest aT value was obtained in the sample no. 5. The cation vacancies begin to make dusters which do not contribute to the cation conduction by the mixing of La2(SO4) 3 more than 18.1 mol%. From these results, the most appropriate sample
r/'c 2
7O0 •
i
!
500
tOO
i
i
300 !
!
700
200 i
I
r/'c 5OO 4OO 3OO
2O0
I
! -
-3
-4
I
I
1.0
1.5
2.0
kKIr
Fig. 2. Temperature dependences of electrical conductivities for Na2 SO4 -La2 (SO4)a -AI2 O3. (=) Na2 SO4 :La2 (SO4) s : AI203 = 55.0:5.0:40.0 (no. 1);(-) Na2SO4:La2(SO4) 3 : AI20a = 52.1:7.8:40.1 (no. 2) ; (e) Na2 SO4 :La2 (SO4)a : AI2Os = 50.0:10.1:39.9 (no. 3);( )Na2SO4.
1.0
1.5
kKIT
2.0
Fig. 3. Temperature dependencesof electrical conductJvities for Na2 SO4-La2(SO4)3-AI203. (o) Na2SO4 :La2(SO4) 3 : A1203 = 48.0:12.0:40.0 (no. 4); (e) Na2 SO4:La2(SO4)3 : AI=O3 = 44.8:15.3:39.9 (no. 5);(") Na=SO4:La2(SO4)3: AI2Oa = 42.4:18.1:39.5 (no. 6); (A) Na2SO4 :La2(SO4)3 : AI=O3 =40.1:19.8:40.1 (no. 7);( ) Na2SO4.
156
N. Imanaka et al./Na2SO 4 mixed with La2(SO4) 3 and AI203
for the solid electrolyte for sulfur dioxide gas detector is found to be the sample no. 5. 3.3. E M F measurements
I00
The variation of the EMF for the Na2SO 4 La2(SO4)3-AI203 (no. 5) at 973 K is shown in fig. 4. The inlet SO 2 gas concentration was varied from 30 ppm (log(Pso2)in = -4.52) to l%(log(Pso2)in = -2.0), while the reference inlet SO 2 gas content was fixed at approximately 3% by sulfur dioxide and oxygen gas mixing. The measured EMF was in good agreement with the calculated EMF, in the inlet SO 2 gas concentration from 100 ppm (log(Pso2)in = - 4 . 0 ) to 1%. The difference between the measured and calculated EMF was almost 40 mV at 30 ppm. This may be attributed to the fact that the gas permeation occurs through the microcraeks which have been resulted from the phase transition. In this method of SO 2 gas detection, both test and reference gas compartments should be f'flied with a constant SO 2 gas concentration by the control of the SO 2 and 02 gas flow volume. In order to make the apparatus more compact, that is, to eliminate the reference gas regulation, it was attempted to replace the reference gas with the reference solid electrode, which generated a certain gas pressure of SO 2 . Furthermore, the oxygen gas for di-
300
> 200 E
IOC
i
,
-40
•
•
-3.0 -2.0 I.og(pr~in
Fig. 4. The variation of the EMF for Na 2 SO4-Lax (SO4)3A12Oa (44.8: 15.3:39.9) solid electrolyte at 973 K. ( .) calculated EMF (1).
E
-100 -z,.O - 3 0
-20
t og (pso2)~ Fig. 5. The variation of the EMF fo~ Na 2 SO4 -La2 (SO4)a -
A1203 (44.8:15.3:39.9) solid electrolyte with the solid reference electrode method (NiSO4 + NiO) at 973 K. ( ) calculated EMF (7).
lution was substituted for air so as to be of more practical utilization. Fig. 5 indicates the EMF results for the Na2SO 4 La 2 ( S O 4 ) 3 - A I 2 0 3 w i t h solid reference electrode method (NiSO 4 + NiO) at 973 K. The reference SO 2 gas concentration was maintained at approximately 2800 ppm because both SO 3 gas, which resulted from the equilibrium between NiSO 4 and NiO, and 0 2 gas content, which was controlled by air circulation, were kept constant. The EMF coincides with the calculation, in the inlet SO 2 gas concentration between 100 ppm and 1%. The measured EMF was about 30 mV lower than the calculated EMF at 30 ppm. This is analogous to the case of fig. 4. This result also refers to the gas penetration through the microcracks in the electrolyte. In conclusion, sodium sulfate mixed with lantha. num sulfate and aluminum oxide contains Na2SO4-Isimilar phase and exhibits 30 times larger conductivity compared with unmixed sodium sulfate at 973 K. The phase transition was suppressed except for a very small one at 603 K. The EMF characteristics for the Na2SO4-La 2 (SO4) 3-A120 3 solid electrolyte were fairly in good accordance with the calculated EMF both the reference SO 2 and 0 2 gas concentration and the reference solid electrode (NiSO 4 + NiO) method, in the inlet SO 2 gas concentration between 100 ppm and 1%.
N. Imanaka et al./Na2SO 4 mixed with La2(S04) a and A l 2 0 a
References [1] K.T. Jacob and D.B. Rao, J. Electrochem. Soc. 126 (1979) 1842. [2] W.L. WorreU, in: Proc. Internat. Meeting on Chemical Sensors, Fukuoka, 1983, ed. T. Seiyama et al. (Kodansha, Tokyo and Elsevier, Amsterdam, 1983) . p. 332. [3] W.L. Worrell and Q.G. Liu, J. Electroanal. Chem. 168 (1984) 355. [4] M. Gauthier and A. Chamberland, J. Electrochem. Soc. 124 (1977) 1579. [5 ] M. Gauthiex, A. Chamberland, A. B61anger and M. Poirier, J. Electrochem. Soc. 124 (1977) 1584. [6] M. Gauthier, R. Bellemare and A. B61anger, J. Electrochem. Soc. 128 (1981) 371. [7] M. Gauthier and C.W. Bale, Metall. Trans. 14B (1983) 117. [8] N. Imanaka, G. Adachi and J. Shiokawa, Chem. Letters (1983) 287. [9] N. Imanaka, G. Adachi and J. Shiokawa, Denki Kagaku 51 (1983) 93. [10] N. Imanaka, G. Adachi and J. Shiokawa, Bull. Chem. Soc. Japan. 57 (1984) 687. [11] N. Imanaka, G. Adachi and J. Shiokawa, in: Ptoc. International Meeting on Chemical Sensors, Fukuoka, 1983, ed. T. Seiyama et al. (Kodansha, Tokyo and Elsevier, Amsterdam, 1983) p. 348.
157
[12] A. B61anger, M. Gauthier and D. Fauteux, J. Electrochem. Soc. 131 (1984) 579. [13] E.L. Kreidl and I. Simon, Nature 181 (1958) 1529. [14] Y. Saito, K. KobayashJ and T. Muruyama, Thermochim. Acta 53 (1982) 289. [15] M.S. Whittingham and R.A. Huggins, J. Chem. Phys. 54 (1971) 414. [16] H.Y-P. Hong, Mater. Res. Bull. 11 (1976) 173. [17] J.B. Goodenough, H.Y-P. Hon$ and J.A. Kafalas, Mater. Res. Bull. 11 (1976) 203. [ 18] H.H. H6fer, W. Eysel and U. yon Alpen, J. Solid State Chem. 36 (1981) 365. [19] H.H. H6fer, W. Eysel and U. yon Alpen, Mater. Res. Bull. 13 (1978) 265. [20] R.M. Murray and E.A. Secco, Can. J. Chem. 56 (1978) 2616. [21 ] K.L. Keester, W. Eysel and Th. Hahn, Acta CrystaUogr. (Sect. A) 31 (1975) $79. [22] H.H. H6fer, U. yon Alpen and W. Eysel, Acta CrystaUogr. (Sect. A) 34 (1978) $358. [23] N. Imanaka, G. Adachi and J. Shiokawa, Can. J. Chem. 61 (1983) 1557. [24] N. lmanaka, Y. Yamaguchi, G. Adachi and J. Shiokawa, Bull. Chem. Soc. Japan. 58 (1985) 5.