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Stability of solid electrolyte based thick-film CO2 sensors
Microelectronics Reliability 49 (2009) 614–620
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Stability of solid electrolyte based thick-film CO2 sensors C. Belda a,*, M. Fritsch a, C. Feller a, D. Westphal b, G. Jung c a b c
Fraunhofer Institute for Ceramic Technologies and Systems, IKTS Dresden, Winterbergstr. 28, 01277 Dresden, Germany ZIROX Sensoren and Elektronik GmbH, Am Koppelberg 21, 17489 Greifwald, Germany ACEOS GmbH Dresden, Tatzberg 47, 01307 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 6 November 2008 Received in revised form 15 January 2009 Available online 31 March 2009
a b s t r a c t Long-term stability is one of the major issues in the ongoing development of solid electrolyte based, screenprinted potentiometric CO2 sensors. Though improvements of the sensor have been reported by several research groups, mostly due to changes in material combinations, little is referred to the influence of basic technological aspects in relation to the observable degradation effects. In view of this aspect potentiometric thick-film CO2 sensors based on the electrochemical cell: Au, O2, CO2 |Ma2CO3–MeCO3 (Au)||Na-b/b00 – Al2O3||Na2Si2O5 SiO2 (Au)|O2, (CO2), Au {Ma = Na or Li; Me = Ba or Ca} were investigated regarding the long-term stability at improved operating conditions. As previous works at the IKTS in Dresden have shown that the microstructure of the Na-beta-Aluminate (NBA) and the sensor preheating conditions have considerable influence on the stability, now different carbonate materials where evaluated regarding the influence on stability at improved operating conditions. So far, satisfying performance over more than 2000 h was demonstrated independent of the carbonate material used in the miniaturized CO2 sensor. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Fraunhofer Institute for Ceramic Technologies and Systems showed that the long-term stability of sensors defined by:
Since miniaturized, low cost CO2 sensors facilitate a variety of possible economical applications, solid electrolyte based potentiometric CO2 sensors have been subject of research and development for more than 20 years now [1–3]. However, until today only bulk sensors, based on pressed electrolyte pellets and a quite sophisticated design, are commercially available (e.g. [4]). As these sensors demonstrate that the solid electrolyte based CO2 sensor can show proper performance and sensor stability for several years, further improvements of the production costs are envisaged to open up new markets. According to this demand, further miniaturization and a simple design of these sensors using inexpensive highthroughput deposition techniques such as screen-printing is required. As a result, the development and investigation of the screen-printed potentiometric CO2 sensors is a contemporary issue in this field of research [5,6]. A main problem in developing a miniaturized printed CO2 sensor is the long-term stability. Numerous approaches have been made to increase the stability and enhance the overall sensor performance of the potentiometric CO2 sensors by utilizing a variety of different electrode and electrolyte materials [7]. Though many promising material combinations were found we assume that a more crucial factor in improving the sensor stability is connected to basic technological aspects such as preheating parameter, electrolyte properties and interface conditions. Recent works at the
Au;O2 ; CO2 ; jNa2 CO3 ðAuÞkNa-b=b00 —Al2 O3 kNa2 Si2 O5 SiO2 ðAuÞjO2 ;
* Corresponding author. E-mail address: [email protected] (C. Belda). 0026-2714/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2009.02.014
ðCO2 Þ; Au can be significantly improved due to changes in basic operation parameters. The reactions at the electrodes with the gaseous components, leading to the overall cell potential of this sensor also referred to as electromotive force (EMF), can be described by Eqs. (1) and (2) for the sensing and reference electrode, respectively,
1 Na2 CO3 2Naþ þ 2e þ O2 þ CO2 2 1 þ 2SiO2 þ 2Na þ 2e þ O2 Na2 Si2 O5 2
ð1Þ ð2Þ
The electrochemical junction between both half cell reactions is accomplished by the Na-beta-Aluminate (NBA) electrolyte, which is known to show good performance in solid state CO2 sensors [8–10]. Due to its high sodium ion conductivity at elevated temperatures a fast equilibration of the sodium ion activity at both electrolyte/electrode interfaces is achieved. Therefore, the overall cell reaction can be described by (3), giving no dependency of the oxygen partial pressure.
Na2 CO3 þ 2SiO2 Na2 Si2 O5 þ CO2
ð3Þ
The cell potential (E) of the potentiometric sensor and the relation between the EMF and the CO2 partial pressure pCO2 follows from the resulting Nernst equation (4). Whereas the activity of the ideal gases is described by their partial pressure and those of pure solids is 1.
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C. Belda et al. / Microelectronics Reliability 49 (2009) 614–620
E ¼ E0
RT lnðpCO2 Þ 2F
ð4Þ
In Eq. (4), E0 stands for the standard cell potential, R is the universal gas constant, F the Faraday constant and T the absolute temperature. Though the partial pressures of the gaseous components generally have to be normalized to the overall atmospheric pressure, our studies of this sensor showed a negligible influence under laboratory conditions. Also taking into account a pressure correction would prove impractical for most applications. During operation in different fields of application the CO2 sensors might become exposed to more or less harsh conditions as for example a wide range of relative humidity (RH) varying from <10% to >90% RH or even considerable amounts of SOX and NOX. Since especially humidity is known to influence the performance of the Na2CO3 built sensor and research demonstrated that the sensing performance can be significantly improved by mixing the Na2CO3 sensing material with an alkaline earth carbonate of lower water solubility [3], a change in carbonate material is logical. Given that both the alkali and alkaline earth carbonates act as anion conductor of their corresponding cation at elevated temperatures and analogous dissociation reactions as in (1) can be proposed, resulting in a change of the cation activity with a change in CO2 partial pressure, Na2CO3 can be substituted by these carbonate materials. In this case it is presumed that the electrolyte still provides the electrochemical junction between the two electrodes due to the formation of sodium containing mixed carbonate phases at the interface of the sensing electrode. A lot of research groups have published satisfying results for the mixed alkali and alkaline earth carbonate sensing electrodes with respect to the sensor performance, cross-sensitivity and influence of water vapor [7]. Anyway, since stability is a key factor for the applications of miniaturized thick-film CO2 sensor, long-term stability under controlled operating conditions has to be established with these materials prior to the investigation of further characteristics. In this study, long-term stability of planar type solid electrolyte based thick-film CO2 sensors, featuring an improved NBA electrolyte and different kinds of carbonate sensing electrodes were investigated. Sensing electrodes (SE) consisted of mixtures from Na2CO3 or Li2CO3 with BaCO3 or CaCO3, and Au, whereas the reference electrodes (RE) composed of a Na2Si2O5–Au mixture. Both electrodes were applied to the electrolyte by screen-printing of a prepared thick-film paste. Stability of the different sensor modifications was tested by conditioning the sensors under virtual operating conditions, further referred to as preheating, while measuring sensor characteristics and electrolyte resistance as a function of storage time.
2. Experimental 2.1. Preparation of the sensors The NBA electrolyte was fabricated in a solid state reaction with the supporting Al2O3 substrate by applying a ball milled and dried mixture of Na2CO3, MgCO3 and Al2O3 to the substrate and tempering above 1200 °C for several hours. Resulting NBA layers possessed a compact or porous structure depending on the processing parameters. The resulting NBA layers were characterized by X-ray diffraction and energy dispersive X-ray spectroscopy at the cross section after their fabrication. It is assumed that in comparison to CO2 sensors built on a dense electrolyte, a porous electrolyte structure might be of advantage enhancing the electrode adhesion and enlarging the triple phase boundary necessary to achieve a fast EMF equilibration. Na2CO3–Au electrode, the mixed alkali and alkaline earth carbonate–Au sensing electrodes and the Na2Si2O5–Au reference
Table 1 Composition of the sensing and reference electrode of the tested sensor modifications. #
SE
RE
A B C D
Na2CO3 (Au) Na2CO3–BaCO3 (Au) Li2CO3–CaCO3 (Au) Li2CO3–BaCO3 (Au)
Na2Si2O5 Na2Si2O5 Na2Si2O5 Na2Si2O5
(Au) (Au) (Au) (Au)
Fig. 1. Schematic sketch of the investigated solid electrolyte sensor, with SE the sensing and RE the reference electrode, (a) supporting substrate, (b) NBA electrolyte layer, (c) Au electrodes and (d) inert barrier.
electrodes were prepared by dispersing the powdered mixtures into an organic binder, forming a screen printable paste and subsequently printing and sintering them onto the electrolyte coated substrates. Resulting sensor modifications are listed in Table 1. In addition an inert barrier was printed between the electrodes to make sure that there is no significant mass or current transport across the surface of the electrolyte. The electrodes were connected using Au wire and an Au paste, while debindering and sintering the electrode contacts directly inside the testing chamber prior to the electrochemical characterization. The accomplished sensor cells featured electrodes with an area of about 4 mm2 each and a thickness of up to 100 lm. A schematic sketch of the investigated sensor cell is given in Fig. 1. 2.2. Electrochemical characterization Sensor characteristics were measured using a testing chamber, consisting of a ceramic sensor-mounting inside a quartz-glass tube placed into a split tube laboratory furnace (HTM Reetz GmbH, Germany), provided with a gas inlet and outlet. The sensors were heated externally through the furnace during measurement. The EMF was recorded in different CO2 concentrations ranging from 1000 ppm up to 200,000 ppm, using a Keithley 2000 digital multimeter (Keithley Instruments Inc., USA) with an input resistance of >10 GX and adapted data collection software. CO2 concentrations were adjusted using different mass flow controllers (MKS instruments GmbH, Germany) varying the flows of dry synthetic air and purified CO2 (P99.995%) through the quartz tube while keeping the overall flow rate constant at 5000 cm3/h. The characteristic electrolyte resistance of the sensor cell was measured by electrochemical impedance spectroscopy using a Zahner IM6e electrochemical workstation (ZAHNER-elektrik Gmbh & CoKG, Germany). The impedance spectra were recorded in a frequency range of 0.1 Hz to 1 MHz and subsequently fitted with the simplified equivalent circuit given in Fig. 2. 2.3. Thermal conditioning of the sensors Long-term stability of the manufactured sensors was investigated by conditioning the sensors in a laboratory muffle furnace (Linn High Therm GmbH, Germany) at different temperatures in ambient air, prior and after the electrochemical characterization.
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C. Belda et al. / Microelectronics Reliability 49 (2009) 614–620
Fig. 2. Fitted equivalent circuit, with (1) polarization resistance, (2) electrolyte resistance, (3) diffusion resistance, (4) double layer capacity and (5) cell lead inductance.
3. Results 3.1. Electrolyte stability The prepared NBA layers were characterized as a mixture of Nab and Na-b00 -alumina by X-ray diffraction. Initial impedance spectroscopy of the prepared electrolytes also gave a good agreement
of the recorded impedance spectra with the fitted equivalent circuit and both, the porous and the dense NBA layer, showed sufficient conductivity after the preparation (see Fig. 3, upper panel). However, the electrode/electrolyte arrangement was not suitable to determine the specific conductivity, so electrolytes were qualified comparing the overall electrolyte resistance R, characteristic to the specific cell layout. The electrolyte resistance of the sensor cells built on the porous NBA layer significantly increases during the conditioning of the sensors at 600 °C for about 500 h, shown in Fig. 4. The initial electrolyte resistance of the sensors built with a porous layer was slightly higher (see Fig. 4) according to the limited conduction paths through the porous NBA structure, but still provided an acceptable sensor function after fabrication as shown in Fig. 5. The fast degradation of the porous material can be attributed to reactions of the NBA electrolyte with atmospheric CO2 and H2O which causes irreversible changes in the material composition
Fig. 3. Impedance spectra of a porous NBA layer, prior to conditioning (top) and after conditioning at 600 °C for 500 h and occurring sensor degradation.
Fig. 4. Characteristic electrolyte resistance of sensor cells with porous or compact electrolyte layer, before and after conditioning for 500 h at 600 °C.
C. Belda et al. / Microelectronics Reliability 49 (2009) 614–620
617
Fig. 5. Transient sensor response to different CO2 concentrations at 600 °C after fabrication, for a sensor with (I) dense and (II) porous electrolyte and Na2CO3 sensing electrode.
[11] associated with a significant increase of the characteristic electrolyte resistance. Along with the increase of the resistance a change in the impedance spectra was observed (see Fig. 3, lower panel), described by an additional RC element in the equivalent circuit. This additional resistance is assumed to refer to an increase in resistance at the grain boundaries. A change in impedance spectra of the dense electrolyte was not observed. As the electrolyte degradation was only observed at sensors built with a porous NBA layer and also occurred at changed preheating conditions, resulting in a slow EMF formation and an ongoing drift at different CO2 concentrations demonstrated in Fig. 6 (for
modification A), further investigations of the long-term stability focused on sensors built on a dense electrolyte layer. 3.2. Sensor stability In preliminary investigations of the sensor modification A it was found that the stability of the printed CO2 sensors shows a considerable dependency to the preheating conditions. For instance the sensors with the dense electrolyte, depicted in Fig. 6, also showed a degradation effect after about 600 h at 600 °C. This degradation was not accompanied by any changes in electrolyte resistance or impedance spectra, but resulted in a significant loss of sensitivity
Fig. 6. Transient sensor response to different CO2 concentrations, after 500 h at 600 °C, for a sensor with (I) dense and (II) porous electrolyte.
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C. Belda et al. / Microelectronics Reliability 49 (2009) 614–620
at lower CO2 concentrations. This behavior is well known for this kind of sensor and is assumed to be caused by a loss of Na2CO3 at the triple phase boundaries, limiting the obtainable maximum cell potential at low CO2 concentrations. In this study sensors have been exposed to different preheating conditions prior and after the electrochemical characterization showing that the stability can be increased by simply changing the preheating parameter and optimizing the operating conditions. In this way satisfying long-term stability of over 2500 h could be achieved for the sensor, as can be seen from the EMF vs. log pCO2 characteristic in Fig. 7. Whereas the initial changes of the standard cell potential during the first 700 h are considered to be related to activation processes in the electrodes and might be shortened by thermal cycling. Furthermore, we observed equally good stability for the sensor modifications featuring alternative carbonate sensing materials at
these improved conditions as shown in Figs. 8 and 9 from the transient sensor response to different CO2 concentrations and the resulting sensor characteristic, respectively. Besides high sensitivity, good linearity and low hysteresis over more than 2000 h, a very stable cell potential at a constant CO2 concentration is achieved, as it can be seen from the cell potential at 1000 ppm CO2 for the different modifications in Fig. 10, before and after the characterization of the transient response behavior depict in Fig. 9. With the presented sensors demonstrating good stability over more than 2000 h at operation temperature, further investigations regarding later degradation effects due to water vapor or other contaminants can be conducted. The changes of preheating conditions approved to have a major influence on the sensors long-term stability prior to the material utilized in the sensing electrode.
Fig. 7. EMF vs. log pCO2 relation of sensor modification A at optimized preheating conditions before and after 700 h and 2500 h, respectively.
Fig. 8. Transient sensor response to different CO2 concentrations, after >2000 h at optimized preheating conditions, for sensor modifications A–D (see Table 1).
C. Belda et al. / Microelectronics Reliability 49 (2009) 614–620
619
Fig. 9. EMF vs. log pCO2 relation of the sensor modifications A–D after >2000 h at optimized preheating conditions.
Fig. 10. EMF over time at 1000 ppm CO2 before and after the transient changes in CO2 concentration, for the sensor modifications A–D.
4. Conclusion
Acknowledgement
In this work, different sensing materials and electrolyte preparations were investigated regarding the stability of a solid electrolyte based, screen-printed potentiometric CO2 sensors. It was observed that the electrolyte properties, such as morphology, heavily influences the overall sensor performance leading to an electrolyte degradation with an increasing open porosity, due to an increased electrolyte surface which enables chemical interaction with atmospheric H2O and CO2. In addition we were able to demonstrate the important role of technological aspects such as preheating and operation conditions which strongly influence the overall sensor stability. At improved thermal conditions the investigated sensors showed more than 2000 h stability, good sensitivity and stable cell potential independent of the sensing material.
The financial support of AiF (Grant-No. KF 0103 304 RK 5) is kindly acknowledged. References [1] Weppner W. Solid-state electrochemical gas sensors. Sensors Actuat 1987;12:107–19. [2] Hotzel G, Weppner W. Potentiometric gas sensors based on fast solid electrolytes. Sensors Actuat 1987;12:449–53. [3] Miura N, Yao S, Shimizu Y, Yamazoe N. High-performance solid-electrolyte carbon dioxide sensor with a binary carbonate electrode. Sensors Actuat B: Chem 1992;9:165–70. [4] [15.07.08]. [5] Kida T, Miyachi Y, Shimanoe K, Yamazoe N. NASICON thick film-based CO2 sensor prepared by a sol–gel method. Sensors Actuat B: Chem 2001;80:28–32.
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[6] Sahner K, Schulz A, Kita J, Merkle R, Maier J, Moos R. CO2 selective potentiometric sensor in thick film technology. Sensors 2008;8:4774–85. [7] Fergus JW. A review of electrolyte and electrode materials for high temperature electrochemical CO2 and SO2 gas sensors. Sensors Actuat B: Chem 2008;134:1034–41. [8] Feller C, Kretzschmar C, Westphal D. CO2 solid electrolyte sensor with screen printing. In: Proceedings of the 48th international scientific colloquium 2003, Ilmenau, Germany, September 22–25, 2003.
[9] Hong HS, Kim JW, Jung SJ, Park CO. Thick film planar CO2 sensors based on Na b-alumina solid electrolyte. J Electroceram 2005;15:151–7. [10] Näfe H, Aldinger F. CO2 sensor based on a solid state oxygen concentration cell. Sensors Actuat B: Chem 2000;69:46–50. [11] Bates JB, Dohy D, Anderson RL. Reaction of polycrystalline Na b0 0 -alumina with CO2 and H2O and the formation of hydroxyl groups. J Mater Sci 1985;20:3219–29.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Stability of solid electrolyte based thick-film CO2 sensors C. Belda a,*, M. Fritsch a, C. Feller a, D. Westphal b, G. Jung c a b c
Fraunhofer Institute for Ceramic Technologies and Systems, IKTS Dresden, Winterbergstr. 28, 01277 Dresden, Germany ZIROX Sensoren and Elektronik GmbH, Am Koppelberg 21, 17489 Greifwald, Germany ACEOS GmbH Dresden, Tatzberg 47, 01307 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 6 November 2008 Received in revised form 15 January 2009 Available online 31 March 2009
a b s t r a c t Long-term stability is one of the major issues in the ongoing development of solid electrolyte based, screenprinted potentiometric CO2 sensors. Though improvements of the sensor have been reported by several research groups, mostly due to changes in material combinations, little is referred to the influence of basic technological aspects in relation to the observable degradation effects. In view of this aspect potentiometric thick-film CO2 sensors based on the electrochemical cell: Au, O2, CO2 |Ma2CO3–MeCO3 (Au)||Na-b/b00 – Al2O3||Na2Si2O5 SiO2 (Au)|O2, (CO2), Au {Ma = Na or Li; Me = Ba or Ca} were investigated regarding the long-term stability at improved operating conditions. As previous works at the IKTS in Dresden have shown that the microstructure of the Na-beta-Aluminate (NBA) and the sensor preheating conditions have considerable influence on the stability, now different carbonate materials where evaluated regarding the influence on stability at improved operating conditions. So far, satisfying performance over more than 2000 h was demonstrated independent of the carbonate material used in the miniaturized CO2 sensor. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Fraunhofer Institute for Ceramic Technologies and Systems showed that the long-term stability of sensors defined by:
Since miniaturized, low cost CO2 sensors facilitate a variety of possible economical applications, solid electrolyte based potentiometric CO2 sensors have been subject of research and development for more than 20 years now [1–3]. However, until today only bulk sensors, based on pressed electrolyte pellets and a quite sophisticated design, are commercially available (e.g. [4]). As these sensors demonstrate that the solid electrolyte based CO2 sensor can show proper performance and sensor stability for several years, further improvements of the production costs are envisaged to open up new markets. According to this demand, further miniaturization and a simple design of these sensors using inexpensive highthroughput deposition techniques such as screen-printing is required. As a result, the development and investigation of the screen-printed potentiometric CO2 sensors is a contemporary issue in this field of research [5,6]. A main problem in developing a miniaturized printed CO2 sensor is the long-term stability. Numerous approaches have been made to increase the stability and enhance the overall sensor performance of the potentiometric CO2 sensors by utilizing a variety of different electrode and electrolyte materials [7]. Though many promising material combinations were found we assume that a more crucial factor in improving the sensor stability is connected to basic technological aspects such as preheating parameter, electrolyte properties and interface conditions. Recent works at the
Au;O2 ; CO2 ; jNa2 CO3 ðAuÞkNa-b=b00 —Al2 O3 kNa2 Si2 O5 SiO2 ðAuÞjO2 ;
* Corresponding author. E-mail address: [email protected] (C. Belda). 0026-2714/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2009.02.014
ðCO2 Þ; Au can be significantly improved due to changes in basic operation parameters. The reactions at the electrodes with the gaseous components, leading to the overall cell potential of this sensor also referred to as electromotive force (EMF), can be described by Eqs. (1) and (2) for the sensing and reference electrode, respectively,
1 Na2 CO3 2Naþ þ 2e þ O2 þ CO2 2 1 þ 2SiO2 þ 2Na þ 2e þ O2 Na2 Si2 O5 2
ð1Þ ð2Þ
The electrochemical junction between both half cell reactions is accomplished by the Na-beta-Aluminate (NBA) electrolyte, which is known to show good performance in solid state CO2 sensors [8–10]. Due to its high sodium ion conductivity at elevated temperatures a fast equilibration of the sodium ion activity at both electrolyte/electrode interfaces is achieved. Therefore, the overall cell reaction can be described by (3), giving no dependency of the oxygen partial pressure.
Na2 CO3 þ 2SiO2 Na2 Si2 O5 þ CO2
ð3Þ
The cell potential (E) of the potentiometric sensor and the relation between the EMF and the CO2 partial pressure pCO2 follows from the resulting Nernst equation (4). Whereas the activity of the ideal gases is described by their partial pressure and those of pure solids is 1.
615
C. Belda et al. / Microelectronics Reliability 49 (2009) 614–620
E ¼ E0
RT lnðpCO2 Þ 2F
ð4Þ
In Eq. (4), E0 stands for the standard cell potential, R is the universal gas constant, F the Faraday constant and T the absolute temperature. Though the partial pressures of the gaseous components generally have to be normalized to the overall atmospheric pressure, our studies of this sensor showed a negligible influence under laboratory conditions. Also taking into account a pressure correction would prove impractical for most applications. During operation in different fields of application the CO2 sensors might become exposed to more or less harsh conditions as for example a wide range of relative humidity (RH) varying from <10% to >90% RH or even considerable amounts of SOX and NOX. Since especially humidity is known to influence the performance of the Na2CO3 built sensor and research demonstrated that the sensing performance can be significantly improved by mixing the Na2CO3 sensing material with an alkaline earth carbonate of lower water solubility [3], a change in carbonate material is logical. Given that both the alkali and alkaline earth carbonates act as anion conductor of their corresponding cation at elevated temperatures and analogous dissociation reactions as in (1) can be proposed, resulting in a change of the cation activity with a change in CO2 partial pressure, Na2CO3 can be substituted by these carbonate materials. In this case it is presumed that the electrolyte still provides the electrochemical junction between the two electrodes due to the formation of sodium containing mixed carbonate phases at the interface of the sensing electrode. A lot of research groups have published satisfying results for the mixed alkali and alkaline earth carbonate sensing electrodes with respect to the sensor performance, cross-sensitivity and influence of water vapor [7]. Anyway, since stability is a key factor for the applications of miniaturized thick-film CO2 sensor, long-term stability under controlled operating conditions has to be established with these materials prior to the investigation of further characteristics. In this study, long-term stability of planar type solid electrolyte based thick-film CO2 sensors, featuring an improved NBA electrolyte and different kinds of carbonate sensing electrodes were investigated. Sensing electrodes (SE) consisted of mixtures from Na2CO3 or Li2CO3 with BaCO3 or CaCO3, and Au, whereas the reference electrodes (RE) composed of a Na2Si2O5–Au mixture. Both electrodes were applied to the electrolyte by screen-printing of a prepared thick-film paste. Stability of the different sensor modifications was tested by conditioning the sensors under virtual operating conditions, further referred to as preheating, while measuring sensor characteristics and electrolyte resistance as a function of storage time.
2. Experimental 2.1. Preparation of the sensors The NBA electrolyte was fabricated in a solid state reaction with the supporting Al2O3 substrate by applying a ball milled and dried mixture of Na2CO3, MgCO3 and Al2O3 to the substrate and tempering above 1200 °C for several hours. Resulting NBA layers possessed a compact or porous structure depending on the processing parameters. The resulting NBA layers were characterized by X-ray diffraction and energy dispersive X-ray spectroscopy at the cross section after their fabrication. It is assumed that in comparison to CO2 sensors built on a dense electrolyte, a porous electrolyte structure might be of advantage enhancing the electrode adhesion and enlarging the triple phase boundary necessary to achieve a fast EMF equilibration. Na2CO3–Au electrode, the mixed alkali and alkaline earth carbonate–Au sensing electrodes and the Na2Si2O5–Au reference
Table 1 Composition of the sensing and reference electrode of the tested sensor modifications. #
SE
RE
A B C D
Na2CO3 (Au) Na2CO3–BaCO3 (Au) Li2CO3–CaCO3 (Au) Li2CO3–BaCO3 (Au)
Na2Si2O5 Na2Si2O5 Na2Si2O5 Na2Si2O5
(Au) (Au) (Au) (Au)
Fig. 1. Schematic sketch of the investigated solid electrolyte sensor, with SE the sensing and RE the reference electrode, (a) supporting substrate, (b) NBA electrolyte layer, (c) Au electrodes and (d) inert barrier.
electrodes were prepared by dispersing the powdered mixtures into an organic binder, forming a screen printable paste and subsequently printing and sintering them onto the electrolyte coated substrates. Resulting sensor modifications are listed in Table 1. In addition an inert barrier was printed between the electrodes to make sure that there is no significant mass or current transport across the surface of the electrolyte. The electrodes were connected using Au wire and an Au paste, while debindering and sintering the electrode contacts directly inside the testing chamber prior to the electrochemical characterization. The accomplished sensor cells featured electrodes with an area of about 4 mm2 each and a thickness of up to 100 lm. A schematic sketch of the investigated sensor cell is given in Fig. 1. 2.2. Electrochemical characterization Sensor characteristics were measured using a testing chamber, consisting of a ceramic sensor-mounting inside a quartz-glass tube placed into a split tube laboratory furnace (HTM Reetz GmbH, Germany), provided with a gas inlet and outlet. The sensors were heated externally through the furnace during measurement. The EMF was recorded in different CO2 concentrations ranging from 1000 ppm up to 200,000 ppm, using a Keithley 2000 digital multimeter (Keithley Instruments Inc., USA) with an input resistance of >10 GX and adapted data collection software. CO2 concentrations were adjusted using different mass flow controllers (MKS instruments GmbH, Germany) varying the flows of dry synthetic air and purified CO2 (P99.995%) through the quartz tube while keeping the overall flow rate constant at 5000 cm3/h. The characteristic electrolyte resistance of the sensor cell was measured by electrochemical impedance spectroscopy using a Zahner IM6e electrochemical workstation (ZAHNER-elektrik Gmbh & CoKG, Germany). The impedance spectra were recorded in a frequency range of 0.1 Hz to 1 MHz and subsequently fitted with the simplified equivalent circuit given in Fig. 2. 2.3. Thermal conditioning of the sensors Long-term stability of the manufactured sensors was investigated by conditioning the sensors in a laboratory muffle furnace (Linn High Therm GmbH, Germany) at different temperatures in ambient air, prior and after the electrochemical characterization.
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C. Belda et al. / Microelectronics Reliability 49 (2009) 614–620
Fig. 2. Fitted equivalent circuit, with (1) polarization resistance, (2) electrolyte resistance, (3) diffusion resistance, (4) double layer capacity and (5) cell lead inductance.
3. Results 3.1. Electrolyte stability The prepared NBA layers were characterized as a mixture of Nab and Na-b00 -alumina by X-ray diffraction. Initial impedance spectroscopy of the prepared electrolytes also gave a good agreement
of the recorded impedance spectra with the fitted equivalent circuit and both, the porous and the dense NBA layer, showed sufficient conductivity after the preparation (see Fig. 3, upper panel). However, the electrode/electrolyte arrangement was not suitable to determine the specific conductivity, so electrolytes were qualified comparing the overall electrolyte resistance R, characteristic to the specific cell layout. The electrolyte resistance of the sensor cells built on the porous NBA layer significantly increases during the conditioning of the sensors at 600 °C for about 500 h, shown in Fig. 4. The initial electrolyte resistance of the sensors built with a porous layer was slightly higher (see Fig. 4) according to the limited conduction paths through the porous NBA structure, but still provided an acceptable sensor function after fabrication as shown in Fig. 5. The fast degradation of the porous material can be attributed to reactions of the NBA electrolyte with atmospheric CO2 and H2O which causes irreversible changes in the material composition
Fig. 3. Impedance spectra of a porous NBA layer, prior to conditioning (top) and after conditioning at 600 °C for 500 h and occurring sensor degradation.
Fig. 4. Characteristic electrolyte resistance of sensor cells with porous or compact electrolyte layer, before and after conditioning for 500 h at 600 °C.
C. Belda et al. / Microelectronics Reliability 49 (2009) 614–620
617
Fig. 5. Transient sensor response to different CO2 concentrations at 600 °C after fabrication, for a sensor with (I) dense and (II) porous electrolyte and Na2CO3 sensing electrode.
[11] associated with a significant increase of the characteristic electrolyte resistance. Along with the increase of the resistance a change in the impedance spectra was observed (see Fig. 3, lower panel), described by an additional RC element in the equivalent circuit. This additional resistance is assumed to refer to an increase in resistance at the grain boundaries. A change in impedance spectra of the dense electrolyte was not observed. As the electrolyte degradation was only observed at sensors built with a porous NBA layer and also occurred at changed preheating conditions, resulting in a slow EMF formation and an ongoing drift at different CO2 concentrations demonstrated in Fig. 6 (for
modification A), further investigations of the long-term stability focused on sensors built on a dense electrolyte layer. 3.2. Sensor stability In preliminary investigations of the sensor modification A it was found that the stability of the printed CO2 sensors shows a considerable dependency to the preheating conditions. For instance the sensors with the dense electrolyte, depicted in Fig. 6, also showed a degradation effect after about 600 h at 600 °C. This degradation was not accompanied by any changes in electrolyte resistance or impedance spectra, but resulted in a significant loss of sensitivity
Fig. 6. Transient sensor response to different CO2 concentrations, after 500 h at 600 °C, for a sensor with (I) dense and (II) porous electrolyte.
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at lower CO2 concentrations. This behavior is well known for this kind of sensor and is assumed to be caused by a loss of Na2CO3 at the triple phase boundaries, limiting the obtainable maximum cell potential at low CO2 concentrations. In this study sensors have been exposed to different preheating conditions prior and after the electrochemical characterization showing that the stability can be increased by simply changing the preheating parameter and optimizing the operating conditions. In this way satisfying long-term stability of over 2500 h could be achieved for the sensor, as can be seen from the EMF vs. log pCO2 characteristic in Fig. 7. Whereas the initial changes of the standard cell potential during the first 700 h are considered to be related to activation processes in the electrodes and might be shortened by thermal cycling. Furthermore, we observed equally good stability for the sensor modifications featuring alternative carbonate sensing materials at
these improved conditions as shown in Figs. 8 and 9 from the transient sensor response to different CO2 concentrations and the resulting sensor characteristic, respectively. Besides high sensitivity, good linearity and low hysteresis over more than 2000 h, a very stable cell potential at a constant CO2 concentration is achieved, as it can be seen from the cell potential at 1000 ppm CO2 for the different modifications in Fig. 10, before and after the characterization of the transient response behavior depict in Fig. 9. With the presented sensors demonstrating good stability over more than 2000 h at operation temperature, further investigations regarding later degradation effects due to water vapor or other contaminants can be conducted. The changes of preheating conditions approved to have a major influence on the sensors long-term stability prior to the material utilized in the sensing electrode.
Fig. 7. EMF vs. log pCO2 relation of sensor modification A at optimized preheating conditions before and after 700 h and 2500 h, respectively.
Fig. 8. Transient sensor response to different CO2 concentrations, after >2000 h at optimized preheating conditions, for sensor modifications A–D (see Table 1).
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Fig. 9. EMF vs. log pCO2 relation of the sensor modifications A–D after >2000 h at optimized preheating conditions.
Fig. 10. EMF over time at 1000 ppm CO2 before and after the transient changes in CO2 concentration, for the sensor modifications A–D.
4. Conclusion
Acknowledgement
In this work, different sensing materials and electrolyte preparations were investigated regarding the stability of a solid electrolyte based, screen-printed potentiometric CO2 sensors. It was observed that the electrolyte properties, such as morphology, heavily influences the overall sensor performance leading to an electrolyte degradation with an increasing open porosity, due to an increased electrolyte surface which enables chemical interaction with atmospheric H2O and CO2. In addition we were able to demonstrate the important role of technological aspects such as preheating and operation conditions which strongly influence the overall sensor stability. At improved thermal conditions the investigated sensors showed more than 2000 h stability, good sensitivity and stable cell potential independent of the sensing material.
The financial support of AiF (Grant-No. KF 0103 304 RK 5) is kindly acknowledged. References [1] Weppner W. Solid-state electrochemical gas sensors. Sensors Actuat 1987;12:107–19. [2] Hotzel G, Weppner W. Potentiometric gas sensors based on fast solid electrolytes. Sensors Actuat 1987;12:449–53. [3] Miura N, Yao S, Shimizu Y, Yamazoe N. High-performance solid-electrolyte carbon dioxide sensor with a binary carbonate electrode. Sensors Actuat B: Chem 1992;9:165–70. [4]
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