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A H2S sensor based on Na-β-alumina as solid electrolyte and Na2S as auxiliary electrode
Sensors and Actuators B 43 (1997) 230 – 234
A H2S sensor based on Na-b-alumina as solid electrolyte and Na2S as auxiliary electrode F. Vandecruys *, R. Stephen, F. De Schutter, J. Vangrunderbeek Vlaamse Instelling 6oor Technologisch Onderzoek, Boeretang 200, 2400 Mol, Belgium Accepted 28 April 1997
Abstract A solid-state sensor for H2S was evaluated using (single phase) Na-b-alumina as solid electrolyte and Na2S as auxiliary electrode. These auxiliary electrodes are used to convert the sulphur chemical potential difference across the electrolyte to an equivalent sodium chemical potential difference. Tests of the electrochemical cell: (Pt/(H2S +H2 + Ar)/Na2S//Na-b-alumina// Na2S/(H2S+ H2 +Ar)/Pt) revealed that the H2S concentration variations were followed by the experimental E.M.F. values according to the Nernst law. Nevertheless, a drift of the E.M.F. was observed. Therefore, it was not possible to obtain reproducible E.M.F. values. However, after two weeks the drift disappears and reproducible E.M.F. values were measured. The drift can be explained by the fact that the Na2O activity of the b-alumina is changing during the test, due to the reaction of the H2S gas with b-alumina. This reaction also introduces a second phase, a-alumina, at the interface of the b-alumina and this can explain the fact that the sensor becomes stable after two weeks. From this study, it can be concluded that the E.M.F. drift can be minimised by using a double phase solid electrolyte instead of a single phase solid electrolyte. © 1997 Elsevier Science S.A. Keywords: Solid-state sensor; Na-b-alumina; Na2S; H2S sensor
1. Introduction Several investigators have tried to develop a solid electrolyte which conducts sulphur. Jacob et al. [1] investigated a cell using CaS+ZrO2(CaO) as solid electrolyte. Nagana [2] investigated the ionic conductivity of CaS. Unfortunately, no acceptable material has been identified. An alternative approach is using an auxiliary layer. Taniguchi [3] and Jacob [4] used CaF2 as solid electrolyte and Cu+ Cu2S +CaS or Mo2S +CaS as auxiliary electrodes. The disadvantage of this cell is its slow response to changes in gas composition: 104 s to attain equilibrium E.M.F. at 1200 K. Jacob [5] also investigated a cell using Na-b-alumina in combination with a Na2S-auxiliary electrode. The results of his study are compared with our results. The auxiliary layer (Na2S) used in this study can be formed in situ by the following reaction: 2Na2O.11Al2O3(b-alumina) + S2(g) =2Na2S(s)+O2(g)+ 22a-Al2O3(a-alumina)
(1)
* Corresponding author. Tel.: + 32 14 335925; fax: + 32 14 321185; e-mail: [email protected] 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 1 3 1 - 7
Once this Na2S layer is formed at the beginning of the test, the equilibrium between Na–S–Na2S can be installed: 2Na2S (s)=4Na(l)+S2(g)
(2)
Because the Na2S activity is unity and since the S2 pressure is fixed, the Na activity in Eq. (2) will be fixed at a level depending on the S2 pressure. So, the auxiliary layer converts the sulphur concentration to an equivalent sodium concentration. It is assumed that the E.M.F. of the sensor is determined by the Na/Na + couple at both sides (anode and cathode) of the sensor. The Na + activity in the b-alumina is the same at both sides of the sensor because the Na2O activity in b-alumina [= (Na2O)b ] and also the O2 pressure are fixed so the Na + activity is fixed by the equilibrium: 2(Na2O)b = 4Na + + 2O2 −
(3)
On the other hand, the Na activity, which is related to the sulphur concentration, is fixed by Eq. (2). Therefore, the E.M.F. is given by the equation: C E= EC − EA = RT/F ln a A Na/a Na
(4)
F. Vandecruys et al. / Sensors and Actuators B 43 (1997) 230–234
231
2. Experimental procedure Single phase Na-b-alumina was used to investigate different types of cells: Cell I: Pt/(H2S+ H2 + Ar) /Na2S//Na-b-alumina//Na2S/(H2S+ H2 + Ar)/Pt
(8)
Cell II: Pt/Fe2O3 − 3Na2O.5Fe2O3 − O2 /Na-b-alumina/Fe2O3 − 3Na2O.5Fe2O3 − O2/Pt
Fig. 1. Experimental set-up of cell I (Na reference can be a ternary system or a reference H2S+ H2 gas)
The Na activity in Eq. (4) can be replaced by the sulphur pressure from Eq. (2): A E= RT/4F ln P C S2/P S2
(5)
The partial pressure of sulphur is dependent on the ratio of H2S to H2, by virtue of the reaction: H2(g)+1/2S2(g)= H2S(g)
(6)
Therefore, the E.M.F. can be expressed in terms of the ratios of H2S to H2: E = RT/4F ln (PH2S/PH2)C/(PH2S/PH2)A
(7)
(9)
In both cells the surfaces of the Na-b-alumina were coated with platinum paste. A gas tight bonding between an a-alumina support tube and b-alumina closed end tube sample was obtained with a glass paste providing two separate compartments: a reference side (inside) and a measuring side (outside). Fig. 1 gives a schedule of the experimental set-up. The Fe2O3 –3Na2O.5Fe2O3 powder [6] in cell II was prepared by mixing anhydrous Na2CO3 with Fe2O3 in the mol ratio 5/9. The mixture was calcined at 850°C for 48 hours and then at 700°C for 12 h. The different gas compositions were obtained by mixing high-purity H2S (10%) + Ar with H2 (4%)+ Ar. The cell was heated by a resistance furnace and the temperature was measured at the inside and the outside of the cells. The potential difference between reference and measuring electrode were measured with a high input impedance voltmeter (FLUKE Hydra).
Fig. 2. E.M.F. value of Cell I and H2S/H2 ratio in function of time
232
F. Vandecruys et al. / Sensors and Actuators B 43 (1997) 230–234
has an influence on the composition of the b-alumina leading to the drift. From the phase diagram of b-alumina (Fig. 3), it is clear that, due to Eq. (1), the composition of b-alumina is changing in the direction of the double phase (a+ b)-alumina. Therefore, after a certain period a second phase, a-alumina, will be introduced at the surface of the b-alumina. In this two-components/two-phase system, the Na2O activity is fixed if the temperature and pressure are constant. In an a-alumina and b-alumina solid electrolyte, the Na2O activity is fixed by the following reaction [8–11]: Na2O+ 11Al2O3(a)= Na2O.11Al2O3(b)
Fig. 3. Phase diagram of b-alumina (2Bb= b-alumina; 3Bb= b¦-alumina)
3. Results and discussion In Fig. 2 the experimental E.M.F. values for cell I are shown as a function of time (the E.M.F. value was measured every minute) at a constant temperature of 600°C. In addition, the H2S/H2 ratio at the measuring electrode is also given as a function of time. It shows that the cell reacts in the intended direction upon changes to the H2S/H2 ratio. However, it also shows that the E.M.F. is not stable and that there is not a reproducible E.M.F. value in function of time. This drift becomes more clear if the sensor is exposed to a constant H2S/H2 ratio. We then expect a constant E.M.F. value but in practise a drifting E.M.F. value was measured. This drift can be explained by the fact that the Na2O activity in the b-alumina is changing due to Eq. (1). Single phase b-alumina (Fig. 3: phase diagram of balumina [7]) is a two-component/one-phase system, therefore the Na2O activity is fixed if the temperature, pressure and composition are constant. Due to Eq. (1), the composition is changing, resulting in the change of the Na2O activity which leads to the change of the Na + activity (Eq. (3)). Therefore the Na/Na + couple, which in essence determines the E.M.F., is also changing, leading to the observed drift. The reaction (Eq. (1)) of H2S with Na-b-alumina results in the formation of Na2S and a-alumina. Because the activity of Na2S was already unity, there will be no influence on the equilibrium between H2S and Na2S (Eq. (2)). However, as mentioned above, Eq. (1)
(10)
Therefore, as long as a- and b-alumina are present in both phases, Eq. (1) has no influence on the Na2O activity and the E.M.F. value will be stable. Fig. 4 gives the E.M.F. values and the H2S/H2 ratios in function of time of the H2S sensor after two weeks testing in a H2S/H2 atmosphere. From this figure it is clear that there is a reproducible (not drifting) E.M.F. value. For a single phase b-alumina a liquid phase sintering procedure is sufficient to obtain b-alumina which contains no open porosity. However, for liquid phase sintering in the (a+ b) phase field the liquid is only obtained above 2000°C (Fig. 3). Therefore, it is difficult to make a dense double phase (a+ b)-alumina solid electrolyte. An alternative argument to this hypothesis is the evaluation of an O2 sensor based on single phase Na-balumina. There is no interaction between O2 and b-alumina and therefore the E.M.F. value should be stable. An oxygen sensor based on b-alumina can be obtained by using a ternary system [6] such as Fe2O3 – 3Na2O.5Fe2O3 powder open to air. The Fe2O3 –3Na2O.5Fe2O3 powder fixes the Na2O activity in the powder by the reaction: 5Fe2O3 + 3Na2O= 3Na2O.5Fe2O3
(11)
The Na activity in this powder is fixed at a level depending on the O2 pressure by the reaction (similar to Eq. (2)): 2(Na2O)Fe – Na powder = 4Na(l)+ O2(g)
(12)
So if the O2 pressure is changed, the Na activity is changing as well, resulting in another E.M.F. value. Fig. 5 indicates that changes in O2 pressure were followed by the E.M.F. value and that there was almost no drift observed. Only a small deviation of 2.5 mV over a period of 14 hours was observed which can be attributed to the interaction of Fe2O3 with Na2O in b-alumina or to the decomposition of b-alumina in an oxygen potential gradient [12]. From the investigation of Jacob [5], which focused on the stability of the Na2S auxiliary layer, it was concluded that the sensor could not be used in an oxygen or oxygen-bearing atmosphere (e.g. humidity:
F. Vandecruys et al. / Sensors and Actuators B 43 (1997) 230–234
233
Fig. 4. E.M.F. value of Cell I after two weeks testing and H2S/H2 ratio in function of time
PH2O/PH2 : PO2) because the reverse of Eq. (1) will take place in which the a-alumina and Na2S is consumed to generate b-alumina. Another observation was the existence of a low sulphur-pressure boundary determined by the loss of sodium from the auxiliary electrode. The high sulphur pressure boundary is characterised by the formation of Na2S2 − x. Therefore, the sensor can only be used to a limited temperature and pressure. Moreover, from this study it became clear that the Na2O activity in b-alumina can be fixed by the in situ formation of the (a + b) phase in the solid electrolyte, resulting in a stable (no drift) E.M.F. value.
4. Conclusions The evaluation of cell I in which a single phase b-alumina was used, revealed that the E.M.F. value was not stable. The drift can be explained by the fact that the Na2O-activity of the b-alumina is changing during the test, due to the reaction of the H2S gas with b-alumina. This reaction also introduces a second phase, a-alumina, at the interface of the b-alumina and this can explain the fact that the sensor becomes stable after two weeks. Therefore, it can be concluded that the E.M.F. drift can be minimised by using a double phase
Fig. 5. E.M.F. value of Cell II and O2 changes in function of time
234
F. Vandecruys et al. / Sensors and Actuators B 43 (1997) 230–234
solid electrolyte instead of a single phase solid electrolyte. There is no interaction between O2 and b-alumina. Therefore, a O2 sensor based on single phase b-alumina (cell II) was evaluated and revealed that there was almost no drift observed. Only a small deviation of 2.5 mV over a period of 14 hours was observed which can be attributed to the interaction of Fe2O3 with Na2O in b-alumina.
[2] [3]
[4]
[5] [6]
Acknowledgements The authors wish to acknowledge the funding of this project by the European Union in the framework of JOULE II-extension. The technical and academic staff of both the departments of Energy and Materials are thanked for their help throughout this work. Last but not least we would like to thank Prof. Dr R.V. Kumar (University of Cambridge) and Prof. Dr R. Gijbels (University of Antwerp) for helpful discussions.
[7] [8] [9]
[10] [11]
References [12] [1] K.T. Jacob, M. Iwase, Y. Waseda, Sulphur potential measure-
.
ments with a two-phase sulphide-oxide electrolyte, J. Appl. Electrochem. 12 (1982) 55 – 67. K. Nagana, Metal. Trans. 5 (1974) 899 – 903. M. Taniguchi, M. Wakihara, J. Nii, Sulfur gas sensor using a calcium fluoride silid elctrolyte, J. Electrochem. Soc. 135 (1) (1988) 217 – 221. K.T. Jacob, D.B. Rao, G. Nelson, Some studies on a solid-state sulfur probe for coal gasification systems, J. Electrochem. Soc. 125 (5) (1978) 753 – 762. K.T. Jacob, M. Iwase, Y. Waseda, a sensor for H2S or S2 based on b-alumina, Adv. Ceram. Mater. 1 (3) (1986) 264 – 270. L. Zhang, D.J. Fray, J.C. Deheyrer, F. de Schutter, Reference electrodes of simple galvanic cells for developing sodium sensors for use in molten aluminum, Metallur. Mater. Trans. 27B (1996) 794 – 800. J.D. Hodge, Kinetics of the b¦-to-b transformation in the system Na2O – Al2O3, J. Am. Ceram. Soc. 66 (3) (1983) 166 – 169. M. Itoh, Measurements of Na2O activity in b-alumina by EMF method, Trans. Jap. Inst. Metals 26 (5) (1985) 353 – 361. N.S. Chodhury, Thermodynamical stability of b- and b¦-alumina elctrolytes in Na/S cells, J. Electrochem. Soc. 133 (2) (1986) 425 – 431. R.V. Kumar, D.A.R. Kay, Thermodynamic stabilities of some b and b¦ aluminas, Metal. Trans. 16B (1985) 295 – 301. F.A. Elrefaie, W.W. Smeltzer, Thermodynamic properties of the ionic conducting phases of Na-Al – O system between 800 and 1200 K, Solid-state Ionics 12 (1984) 517 – 524. K.T. Jacob, Decomposition of b-alumina in an oxygen potential gradient, J. Appl. Electrochem. 13 (1983) 469 – 472.
A H2S sensor based on Na-b-alumina as solid electrolyte and Na2S as auxiliary electrode F. Vandecruys *, R. Stephen, F. De Schutter, J. Vangrunderbeek Vlaamse Instelling 6oor Technologisch Onderzoek, Boeretang 200, 2400 Mol, Belgium Accepted 28 April 1997
Abstract A solid-state sensor for H2S was evaluated using (single phase) Na-b-alumina as solid electrolyte and Na2S as auxiliary electrode. These auxiliary electrodes are used to convert the sulphur chemical potential difference across the electrolyte to an equivalent sodium chemical potential difference. Tests of the electrochemical cell: (Pt/(H2S +H2 + Ar)/Na2S//Na-b-alumina// Na2S/(H2S+ H2 +Ar)/Pt) revealed that the H2S concentration variations were followed by the experimental E.M.F. values according to the Nernst law. Nevertheless, a drift of the E.M.F. was observed. Therefore, it was not possible to obtain reproducible E.M.F. values. However, after two weeks the drift disappears and reproducible E.M.F. values were measured. The drift can be explained by the fact that the Na2O activity of the b-alumina is changing during the test, due to the reaction of the H2S gas with b-alumina. This reaction also introduces a second phase, a-alumina, at the interface of the b-alumina and this can explain the fact that the sensor becomes stable after two weeks. From this study, it can be concluded that the E.M.F. drift can be minimised by using a double phase solid electrolyte instead of a single phase solid electrolyte. © 1997 Elsevier Science S.A. Keywords: Solid-state sensor; Na-b-alumina; Na2S; H2S sensor
1. Introduction Several investigators have tried to develop a solid electrolyte which conducts sulphur. Jacob et al. [1] investigated a cell using CaS+ZrO2(CaO) as solid electrolyte. Nagana [2] investigated the ionic conductivity of CaS. Unfortunately, no acceptable material has been identified. An alternative approach is using an auxiliary layer. Taniguchi [3] and Jacob [4] used CaF2 as solid electrolyte and Cu+ Cu2S +CaS or Mo2S +CaS as auxiliary electrodes. The disadvantage of this cell is its slow response to changes in gas composition: 104 s to attain equilibrium E.M.F. at 1200 K. Jacob [5] also investigated a cell using Na-b-alumina in combination with a Na2S-auxiliary electrode. The results of his study are compared with our results. The auxiliary layer (Na2S) used in this study can be formed in situ by the following reaction: 2Na2O.11Al2O3(b-alumina) + S2(g) =2Na2S(s)+O2(g)+ 22a-Al2O3(a-alumina)
(1)
* Corresponding author. Tel.: + 32 14 335925; fax: + 32 14 321185; e-mail: [email protected] 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 1 3 1 - 7
Once this Na2S layer is formed at the beginning of the test, the equilibrium between Na–S–Na2S can be installed: 2Na2S (s)=4Na(l)+S2(g)
(2)
Because the Na2S activity is unity and since the S2 pressure is fixed, the Na activity in Eq. (2) will be fixed at a level depending on the S2 pressure. So, the auxiliary layer converts the sulphur concentration to an equivalent sodium concentration. It is assumed that the E.M.F. of the sensor is determined by the Na/Na + couple at both sides (anode and cathode) of the sensor. The Na + activity in the b-alumina is the same at both sides of the sensor because the Na2O activity in b-alumina [= (Na2O)b ] and also the O2 pressure are fixed so the Na + activity is fixed by the equilibrium: 2(Na2O)b = 4Na + + 2O2 −
(3)
On the other hand, the Na activity, which is related to the sulphur concentration, is fixed by Eq. (2). Therefore, the E.M.F. is given by the equation: C E= EC − EA = RT/F ln a A Na/a Na
(4)
F. Vandecruys et al. / Sensors and Actuators B 43 (1997) 230–234
231
2. Experimental procedure Single phase Na-b-alumina was used to investigate different types of cells: Cell I: Pt/(H2S+ H2 + Ar) /Na2S//Na-b-alumina//Na2S/(H2S+ H2 + Ar)/Pt
(8)
Cell II: Pt/Fe2O3 − 3Na2O.5Fe2O3 − O2 /Na-b-alumina/Fe2O3 − 3Na2O.5Fe2O3 − O2/Pt
Fig. 1. Experimental set-up of cell I (Na reference can be a ternary system or a reference H2S+ H2 gas)
The Na activity in Eq. (4) can be replaced by the sulphur pressure from Eq. (2): A E= RT/4F ln P C S2/P S2
(5)
The partial pressure of sulphur is dependent on the ratio of H2S to H2, by virtue of the reaction: H2(g)+1/2S2(g)= H2S(g)
(6)
Therefore, the E.M.F. can be expressed in terms of the ratios of H2S to H2: E = RT/4F ln (PH2S/PH2)C/(PH2S/PH2)A
(7)
(9)
In both cells the surfaces of the Na-b-alumina were coated with platinum paste. A gas tight bonding between an a-alumina support tube and b-alumina closed end tube sample was obtained with a glass paste providing two separate compartments: a reference side (inside) and a measuring side (outside). Fig. 1 gives a schedule of the experimental set-up. The Fe2O3 –3Na2O.5Fe2O3 powder [6] in cell II was prepared by mixing anhydrous Na2CO3 with Fe2O3 in the mol ratio 5/9. The mixture was calcined at 850°C for 48 hours and then at 700°C for 12 h. The different gas compositions were obtained by mixing high-purity H2S (10%) + Ar with H2 (4%)+ Ar. The cell was heated by a resistance furnace and the temperature was measured at the inside and the outside of the cells. The potential difference between reference and measuring electrode were measured with a high input impedance voltmeter (FLUKE Hydra).
Fig. 2. E.M.F. value of Cell I and H2S/H2 ratio in function of time
232
F. Vandecruys et al. / Sensors and Actuators B 43 (1997) 230–234
has an influence on the composition of the b-alumina leading to the drift. From the phase diagram of b-alumina (Fig. 3), it is clear that, due to Eq. (1), the composition of b-alumina is changing in the direction of the double phase (a+ b)-alumina. Therefore, after a certain period a second phase, a-alumina, will be introduced at the surface of the b-alumina. In this two-components/two-phase system, the Na2O activity is fixed if the temperature and pressure are constant. In an a-alumina and b-alumina solid electrolyte, the Na2O activity is fixed by the following reaction [8–11]: Na2O+ 11Al2O3(a)= Na2O.11Al2O3(b)
Fig. 3. Phase diagram of b-alumina (2Bb= b-alumina; 3Bb= b¦-alumina)
3. Results and discussion In Fig. 2 the experimental E.M.F. values for cell I are shown as a function of time (the E.M.F. value was measured every minute) at a constant temperature of 600°C. In addition, the H2S/H2 ratio at the measuring electrode is also given as a function of time. It shows that the cell reacts in the intended direction upon changes to the H2S/H2 ratio. However, it also shows that the E.M.F. is not stable and that there is not a reproducible E.M.F. value in function of time. This drift becomes more clear if the sensor is exposed to a constant H2S/H2 ratio. We then expect a constant E.M.F. value but in practise a drifting E.M.F. value was measured. This drift can be explained by the fact that the Na2O activity in the b-alumina is changing due to Eq. (1). Single phase b-alumina (Fig. 3: phase diagram of balumina [7]) is a two-component/one-phase system, therefore the Na2O activity is fixed if the temperature, pressure and composition are constant. Due to Eq. (1), the composition is changing, resulting in the change of the Na2O activity which leads to the change of the Na + activity (Eq. (3)). Therefore the Na/Na + couple, which in essence determines the E.M.F., is also changing, leading to the observed drift. The reaction (Eq. (1)) of H2S with Na-b-alumina results in the formation of Na2S and a-alumina. Because the activity of Na2S was already unity, there will be no influence on the equilibrium between H2S and Na2S (Eq. (2)). However, as mentioned above, Eq. (1)
(10)
Therefore, as long as a- and b-alumina are present in both phases, Eq. (1) has no influence on the Na2O activity and the E.M.F. value will be stable. Fig. 4 gives the E.M.F. values and the H2S/H2 ratios in function of time of the H2S sensor after two weeks testing in a H2S/H2 atmosphere. From this figure it is clear that there is a reproducible (not drifting) E.M.F. value. For a single phase b-alumina a liquid phase sintering procedure is sufficient to obtain b-alumina which contains no open porosity. However, for liquid phase sintering in the (a+ b) phase field the liquid is only obtained above 2000°C (Fig. 3). Therefore, it is difficult to make a dense double phase (a+ b)-alumina solid electrolyte. An alternative argument to this hypothesis is the evaluation of an O2 sensor based on single phase Na-balumina. There is no interaction between O2 and b-alumina and therefore the E.M.F. value should be stable. An oxygen sensor based on b-alumina can be obtained by using a ternary system [6] such as Fe2O3 – 3Na2O.5Fe2O3 powder open to air. The Fe2O3 –3Na2O.5Fe2O3 powder fixes the Na2O activity in the powder by the reaction: 5Fe2O3 + 3Na2O= 3Na2O.5Fe2O3
(11)
The Na activity in this powder is fixed at a level depending on the O2 pressure by the reaction (similar to Eq. (2)): 2(Na2O)Fe – Na powder = 4Na(l)+ O2(g)
(12)
So if the O2 pressure is changed, the Na activity is changing as well, resulting in another E.M.F. value. Fig. 5 indicates that changes in O2 pressure were followed by the E.M.F. value and that there was almost no drift observed. Only a small deviation of 2.5 mV over a period of 14 hours was observed which can be attributed to the interaction of Fe2O3 with Na2O in b-alumina or to the decomposition of b-alumina in an oxygen potential gradient [12]. From the investigation of Jacob [5], which focused on the stability of the Na2S auxiliary layer, it was concluded that the sensor could not be used in an oxygen or oxygen-bearing atmosphere (e.g. humidity:
F. Vandecruys et al. / Sensors and Actuators B 43 (1997) 230–234
233
Fig. 4. E.M.F. value of Cell I after two weeks testing and H2S/H2 ratio in function of time
PH2O/PH2 : PO2) because the reverse of Eq. (1) will take place in which the a-alumina and Na2S is consumed to generate b-alumina. Another observation was the existence of a low sulphur-pressure boundary determined by the loss of sodium from the auxiliary electrode. The high sulphur pressure boundary is characterised by the formation of Na2S2 − x. Therefore, the sensor can only be used to a limited temperature and pressure. Moreover, from this study it became clear that the Na2O activity in b-alumina can be fixed by the in situ formation of the (a + b) phase in the solid electrolyte, resulting in a stable (no drift) E.M.F. value.
4. Conclusions The evaluation of cell I in which a single phase b-alumina was used, revealed that the E.M.F. value was not stable. The drift can be explained by the fact that the Na2O-activity of the b-alumina is changing during the test, due to the reaction of the H2S gas with b-alumina. This reaction also introduces a second phase, a-alumina, at the interface of the b-alumina and this can explain the fact that the sensor becomes stable after two weeks. Therefore, it can be concluded that the E.M.F. drift can be minimised by using a double phase
Fig. 5. E.M.F. value of Cell II and O2 changes in function of time
234
F. Vandecruys et al. / Sensors and Actuators B 43 (1997) 230–234
solid electrolyte instead of a single phase solid electrolyte. There is no interaction between O2 and b-alumina. Therefore, a O2 sensor based on single phase b-alumina (cell II) was evaluated and revealed that there was almost no drift observed. Only a small deviation of 2.5 mV over a period of 14 hours was observed which can be attributed to the interaction of Fe2O3 with Na2O in b-alumina.
[2] [3]
[4]
[5] [6]
Acknowledgements The authors wish to acknowledge the funding of this project by the European Union in the framework of JOULE II-extension. The technical and academic staff of both the departments of Energy and Materials are thanked for their help throughout this work. Last but not least we would like to thank Prof. Dr R.V. Kumar (University of Cambridge) and Prof. Dr R. Gijbels (University of Antwerp) for helpful discussions.
[7] [8] [9]
[10] [11]
References [12] [1] K.T. Jacob, M. Iwase, Y. Waseda, Sulphur potential measure-
.
ments with a two-phase sulphide-oxide electrolyte, J. Appl. Electrochem. 12 (1982) 55 – 67. K. Nagana, Metal. Trans. 5 (1974) 899 – 903. M. Taniguchi, M. Wakihara, J. Nii, Sulfur gas sensor using a calcium fluoride silid elctrolyte, J. Electrochem. Soc. 135 (1) (1988) 217 – 221. K.T. Jacob, D.B. Rao, G. Nelson, Some studies on a solid-state sulfur probe for coal gasification systems, J. Electrochem. Soc. 125 (5) (1978) 753 – 762. K.T. Jacob, M. Iwase, Y. Waseda, a sensor for H2S or S2 based on b-alumina, Adv. Ceram. Mater. 1 (3) (1986) 264 – 270. L. Zhang, D.J. Fray, J.C. Deheyrer, F. de Schutter, Reference electrodes of simple galvanic cells for developing sodium sensors for use in molten aluminum, Metallur. Mater. Trans. 27B (1996) 794 – 800. J.D. Hodge, Kinetics of the b¦-to-b transformation in the system Na2O – Al2O3, J. Am. Ceram. Soc. 66 (3) (1983) 166 – 169. M. Itoh, Measurements of Na2O activity in b-alumina by EMF method, Trans. Jap. Inst. Metals 26 (5) (1985) 353 – 361. N.S. Chodhury, Thermodynamical stability of b- and b¦-alumina elctrolytes in Na/S cells, J. Electrochem. Soc. 133 (2) (1986) 425 – 431. R.V. Kumar, D.A.R. Kay, Thermodynamic stabilities of some b and b¦ aluminas, Metal. Trans. 16B (1985) 295 – 301. F.A. Elrefaie, W.W. Smeltzer, Thermodynamic properties of the ionic conducting phases of Na-Al – O system between 800 and 1200 K, Solid-state Ionics 12 (1984) 517 – 524. K.T. Jacob, Decomposition of b-alumina in an oxygen potential gradient, J. Appl. Electrochem. 13 (1983) 469 – 472.