- Email: [email protected]
Detection of water droplets on exhaust gas sensors
Sensors and Actuators B 148 (2010) 624–629
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Short communication
Detection of water droplets on exhaust gas sensors D. Schönauer, R. Moos ∗ Bayreuth Engine Research Center, Functional Materials, University of Bayreuth, Universitätsstr. 30, D-95440 Bayreuth, Germany
a r t i c l e
i n f o
Article history: Received 12 March 2010 Received in revised form 7 May 2010 Accepted 22 May 2010 Available online 31 May 2010 Keywords: Lambda probe On board diagnosis (OBD) Cold start Exhaust gas aftertreatment
a b s t r a c t In order to avoid water droplet-borne stress on planar ceramic exhaust gas sensor elements, it is suggested to apply interdigital electrodes (IDE) on the sensor elements. As soon as water droplets impinge the sensor element, the capacitance of the IDE increases. Not before the water is evaporated, the capacitance reaches its initial value, indicating that the danger of droplet-borne stress is over. During engine cold start, this signal can be used to initiate sensor heating period. Initial experiments that demonstrate the feasibility of this concept are described in this study. The influence of droplet sizes on the capacitance of interdigital electrodes is investigated and a quadratic correlation between water droplet diameter and capacitance is found out and is explained qualitatively by a physics-based model. Transient experiments investigating the behavior during droplet evaporation as well as real engine cold start experiments conclude the paper. The engine cold start tests prove that the start point and the end point of water condensation on a ceramic exhaust gas sensor element can be detected. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In order to minimize automotive vehicle emissions, catalysts and exhaust gas sensors are integrated in the exhaust system. Both elements have to be heated quickly to working temperature [1,2], since otherwise the vehicles do not meet the stringent emission legislation [3]. The sensor element of a typical novel planar exhaust gas sensor like a lambda probe [4], an NOx sensor [5], or an ammonia sensor [6] is made of yttria stabilized zirconia ceramic. Due to its brittleness, thermal shocks have to be avoided. Although the ceramic sensor elements are protected by metallic sensor housings consisting of protection tubes with bores for gas inflow [7,8], it is a serious issue that the elements are prone to be damaged by condensed water or water droplets (water splash) during engine cold start. Condensation occurs on the exhaust tube wall, in the sensor housing, and on the sensor element itself. If condensed water impinges the heated ceramic sensor element, resulting thermal stress may lead to failures. Novel sensors are heated to operation temperature within only five seconds after engine start [3]. It is aimed to reduce this time interval even more [7]. Therefore, the danger of damaging sensors by water condensation becomes increasingly serious. To minimize these effects, even protective elements are discussed [9]. A novel approach to detect water condensation on a ceramic exhaust gas sensor element is suggested in this work. On one side of a typical planar zirconia-based sensor as described for instance in
∗ Corresponding author. Tel.: +49 921 55 7401. E-mail address: [email protected] (R. Moos). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.05.060
refs. [3,4], an insulating layer and an interdigital electrode structure (IDE) are applied. As soon as water reaches the sensor element, the electrical impedance of the IDE changes, indicating water droplets covering the sensor. Not before the exhaust reaches a temperature at which all water droplets are evaporated from the IDE, the sensor heater power is turned on or it is increased to bring the sensor element as fast as possible to its operation temperature. This article describes initial experiments to demonstrate the feasibility of this concept. The influence of droplet sizes on the capacitance of interdigital electrodes is described and a correlation between both will be shown in this paper. Moreover, a small physics-based model to explain this correlation will be developed. Transient experiments investigating the behavior during droplet evaporation as well as real engine cold start experiments conclude the paper. 2. Experimental Gold interdigitated electrodes were screen-printed (thickness tIDE ≈ 8 m) on alumina substrates (96% Al2 O3 in thick-film quality, tsub = 635 m, permittivity εr,sub ≈ 9.8 ± 10%, resistivity at room temperature ≈ 1013 cm [10]) with 100 m lines (l) and 100 m spaces (s) as sketched in Fig. 1. Since l = s, the metallization ratio as defined in ref. [11] is 1. The IDE covered an electrode area of AIDE = L × B = 5.76 mm × 4.41 mm = 25.4 mm2 . Alumina was selected as substrate material for two reasons. Firstly, because in the eventual application, one might either print an alumina insulation layer on the zirconia substrate—a technique which is already in use to electrically insulate the platinum heater
D. Schönauer, R. Moos / Sensors and Actuators B 148 (2010) 624–629
625
Fig. 1. Sketch of the sensor set up and the meaning of the letter symbols.
from the zirconia sensor part [8]. Secondly, since in some embodiments the sensor consists of a composite of high-temperature co-fired alumina and YSZ layers, serving as heater substrate or encompassing the sensor function, respectively [12]. The complex impedance Zmeasured of our IDE structure was measured using a Novocontrol alpha analyzer in the frequency range from 100 Hz to 1 MHz. Single droplets with a defined volume >0.5 l were applied onto the IDE structure at room temperature. The droplets of deionized water were applied by an Eppendorf reference pipette with a maximal volume of 10 l. The tolerance of the droplet volume is around ±2.5%. The water droplet diameter, ddrop , was determined under the light-optical microscope. The cable capacitance Ccable was determined to be 1.2 pF. A sensor was cut right in front of the interdigital electrodes (as sketched by the dotted line L1 in Fig. 1) and the capacitance Cfeed of the sensor contact feeds in the feed region was determined (Cfeed = 1.3 pF). The admittances of cable and feeds, Ycable = j2fCcable and Yfeed = j2fCfeed were calculated for each single frequency f and subtracted from the measured admittance, Y = Ymeasured − Yfeed − Ycable ,
(1)
wherein for the measured admittance Ymeasured = 1/Zmeasured holds. The corrected impedance Z is the reciprocal value of Y, i.e. Z = 1/Y. During the water evaporation period, the evaporation process was observed and photographed for some sensors under the lightoptical microscope with simultaneously acquiring appropriate impedance spectra. Finally, dynamic experiments were conducted in a gasoline engine test bench (dynamometer). Therefore, a sensor element with a more exhaust gas sensor-like geometry (i.e., AIDE ≈ 35 mm2 ) was mounted into a metallic sensor housing without a protection tube and was placed between two bricks of a three way catalyst. An engine cold start at 0 ◦ C was realized in a climate chamber. The sensor temperature was measured by an integrated thermocouple on the backside of the IDE structure. In addition to the thick-film devices as described above, some photolithographically processed thin-film gold IDE structures (l = s = 20 m) on 99% alumina obtained from an earlier project were also investigated. Details of the preparation can be found in ref. [15]. Due to the poor long-term stability of thin-films in the tail pipe, these devices were not intended for application in the engine exhaust but to prove the physical model.
Fig. 2. Impedance data of our IDE structure when covered with droplets of different diameters as indicated in (a) and partially in (b) (same symbols in (a) and (b)): (a) plot in the complex plane, (b) Nyquist plot. In addition, data of the droplet-free IDE are plotted, (c) the inset denotes an appropriate equivalent circuit.
3. Results and discussion 3.1. Static experiments Impedance spectroscopy results are plotted either in the complex plane or as a Nyquist plot vs frequency for different droplet sizes (Fig. 2a and b). From the shape of the spectra in Fig. 2a, an equivalent circuit comprising an R||C-element and an unknown complex serial element Zs as depicted in Fig. 2c can be deduced. Since the conductance of the alumina substrate can be neglected due to its high resistivity, R is assumed to represent the resistance of the droplet, whereas the capacitance C comprises the capacitances of the substrate, the surrounding air, and the droplet. The serial element Zs is rather attributed to electrode/water interfacial effects than to intrinsic alumina or water properties. In the Nyquist representation, one can allocate the appearance of Zs , R, and C to distinct frequency ranges (see Fig. 2b). Since Zs affects the impedance only at low frequencies, it makes sense to evaluate the impedance plots at frequencies f > 1 kHz, where the influence of Zs can be neglected. When the IDE is covered with water droplets, one may assume that both R and C depend on the degree of coverage with water. R is determined at 4.5 kHz (a frequency range in which R determines the behavior), whereas C is evaluated at 878 kHz (at that frequency the behavior is almost capacitive), from the absolute value and the phase angle of the impedance, assuming an R||C equivalent circuit. Both frequency points are indicated in Fig. 2b.
626
D. Schönauer, R. Moos / Sensors and Actuators B 148 (2010) 624–629
Let us assume that the sensor is a capacitor with the capacitance C. C0 is its capacitance if no droplet covers the IDE structures. C splits up into two parallel capacitors, a lower part, Cbottom , which is determined by the substrate, and an upper part, Ctop , which is given by the capacitances of air and droplet, Cair and Cdrop , respectively. C = Cbottom +Ctop
(2)
Since the substrate is much thicker than the electrode dimensions, i.e. tsub l, s, one may assume for the calculation of Cbottom that only a negligible part of the electric field is outside of the substrate. Hence, Cbottom remains unaffected from ambient influences, i.e. Cbottom = C0,bottom . Methods to calculate the capacitances of planar IDE structures as a function of the geometrical parameters tsub , l, s, and, tIDE are well described in the literature (see ref. [11] or [14] and the literature cited therein). In general, one has to solve elliptic integrals. A more handy method is suggested here for a first order approximation. For that purpose, it is sufficient to define a geometry factor, F, allowing to ascribe the capacitance calculation to the well-known parallel-plate capacitor geometry. Please note that in general F is a function of dsub , l, and s but we will see later that for our approximation we only have to assume that F is constant within one sample. For the bottom side this leads to Cbottom = ε0 × εr,sub × F
(3a)
with ε0 being the dielectric constant of the vacuum. For the uncovered droplet-free IDE surface one obtains C0,top = Cair = ε0 × εr,air × F
(3b)
If a relative fraction, a, of the IDE structure is covered with water droplets, Eq. (3b) has to be expanded: Fig. 3. (a) Capacitance C and resistance R for different droplet diameters as determined by light-optical microscopy, (b) Capacitance change C vs square of the droplet diameter. The dotted line obeys Eq. (9) with a capacitance of the droplet-free IDE structure of C0 = 7.1 pF.
the permittivity of air εr,H2 O ≈ 1 , droplet size ddrop and capacitance C increase in unison. The relation C(ddrop ) appears to be quadratic. This point will be discussed later. If one evaluates much more data taken at different days, one finds that the overall behavior of the resistance when the IDE is droplet-covered is always the same, but the absolute resistance values are not fully reproducible. This is ascribed to the non-constant quality of the utilized deionized water as well as to the non-constant droplet temperature; both affecting the water conductivity. In strong contrast to that, data points of the capacitance C can be found within a small scatter band of ±3 pF. It is noteworthy to say that for the capacitance of the uncovered interdigital electrode structure a value of C0 = 7.1 pF was measured (see Fig. 3a), which agrees quite well with FEM calculations for the same geometry with an electrode height of 7 m (6.1 pF in ref [13]).
The aim of the calculation is on the one hand to understand the behavior analytically and on the other hand to find a function that correlates the capacitance values with the droplet sizes. It would be helpful, if no fit factor were required.
C0 = ε0 εr,air +εr,sub F
(4)
By combining Eq. (2) with Eq. (3b) and Eq. (3c), one can derive the total capacitance C of the droplet-covered IDE structure
C = Cbottom + Ctop = ε0 εr,sub F + ε0 (1 − a) εr,air + aεr,water F
= ε0 εr,sub + (1 − a) εr,air + aεr,water F
(5)
If one measures the uncovered capacitance C0 , one can calculate the geometry factor F from Eq. (4) using the known values of εr,air and εr,sub . Combining F with Eq. (5), one obtains the droplet-covered capacitance, which depends besides the known permittivities only on the droplet-covered fraction a.
C = C0
1+a
εr,water − εr,air εr,air + εr,sub
(6)
The relative fraction of the IDE coverage a is defined as the ratio 2 and the IDE area AIDE = between the droplet area Adrop = 1/4ddrop L × B. a=
3.2. Model calculation
(3c)
Combining Eq. (2), Eq. (3a), and Eq. (3b), one obtains for the total capacitance of the uncovered sensor
Fig. 3a shows that with increasing droplet diameter, ddrop , the capacitance C increases (left axis), whereas the resistance R decreases (right axes). At a first glance, this behavior can be understood qualitatively: the non-negligible conductivity of the water leads to a decreasing resistance droplet size. with an increasing Since the permittivity of water εr,H2 O ≈ 80.3 is much higher than
Ctop = Cair + Cdrop = ε0 (1 − a)εr,air + aεr,water F
Adrop AIDE
=
1 2 ddrop 4
L×B
(7)
Now, a quadratic relation between the capacitance C and the droplet diameter appears: C = C0 + C0
1 4
L×B
×
εr,water − εr,air 2 × ddrop εr,air + εr,sub
(8)
D. Schönauer, R. Moos / Sensors and Actuators B 148 (2010) 624–629
627
Fig. 4. Light-optical microscope of a droplet during evaporation. a) freshly impinged, (b) after 300 s, (c) after 490 s, and (d) after 600 s.
With the constants and the geometrical data from above, one obtains for the capacitance change C:
C = C − C0 = 0.226 × C0
ddrop mm
2
(9)
The good agreement between Eq. (9) and the measured data are shown in a representation of the capacitance change vs the square of the droplet diameter in Fig. 3b. The dotted line was calculated with Eq. (9) and the measured value of C0 = 7.1 pF. The triangles show the measured C values for single droplets with varying diameters. These data are taken from Fig. 3a. Some additional measurements with several droplets on the IDE surface were conducted. The open circles in Fig. 3b depict the results of a measurement with two or four single droplets, respectively. From the diameter of each single droplet, the area was calculated and summarized to obtain a total droplet area, Atotal . From Atotal , the effective diameter deff was calculated:
deff =
Atotal × 4
occurs. The question arises, how the evaporation process affects the capacitance change C. Fig. 4 shows light-optical microscope pictures of the same droplet directly after impinging, after 300 s, after 490 s, and after 600 s, respectively. Since the droplet was illuminated from one side, the diminishing shadow indicates that during evaporation the droplet looses height. This is in contrast to its diameter that remains constant. If the assumption of an almost field-free droplet tip is correct, which means that the droplet height plays only a negligible role for the capacitance increase, one should expect an almost constant capacitance as long as the droplet adheres on the IDE. Not before the droplet is in its final stage and its height becomes smaller than the electrode geometry and/or its diameter becomes remarkably smaller, a capacitance decrease should occur. That is exactly what can be observed from Fig. 5. Here, the transient behavior of C measured at a frequency of 300 kHz for four different droplets during evaporation from a thick film IDE (details see above) is plotted. At t0 , water drops are applied
(10)
Fig. 3b illustrates that the model is also applicable for multiple droplets covering the IDE structure. The obtained capacitance changes agree very well with the model curve. The good agreement between measurement and model was confirmed with thin-film IDE structures with l = s = 20 m. Since in this case the capacitance of the droplet-free sensor is C0 = 40 pF, the sensitivity could be improved by almost a factor of 5.7. Like for the thick-film IDE, the determined capacitance changes obey Eq. (9) within an error of maximum 3 pF. 3.3. Transient behavior The above-derived good agreement is due to the fact that the permittivity of water is very large and that the droplet height is higher than the dimension of lines and spaces of the IDE structure. Therefore, all field lines remain on the bottom of the droplet, very close to the IDE. In the tip of the droplet, almost no electrical field
Fig. 5. Capacitance change of a droplet-covered IDE during evaporation. The droplet diameter increased from A to D. For the meaning of t0 , tA , tB , tC , and tD see text.
628
D. Schönauer, R. Moos / Sensors and Actuators B 148 (2010) 624–629
Fig. 6. Capacitance change of an IDE-covered sensor element during cold start of an internal combustion engine. The sensor temperature is shown on the right axis.
to the IDE and the capacitance increases. The moment when water droplets impinge on the sensor element is evident. As shown in Fig. 3a, the capacitance change depends on the droplet diameter ddrop , which increases from curve A to D. After an almost constant course during evaporation, at ti , the capacitances drop to the initial value, indicating that the sensor element is free of droplets and that all droplets are evaporated. Along with the capacitance changes caused by the droplets, the residence times of the droplets on the IDE, tres,i = ti − t0 , increase. Hence, there is a good correlation between droplet size ddrop , capacitance increase C, and residence time tres,i . The behavior of Fig. 5 offers the chance to determine the water condensation onto an exhaust gas sensor directly during the engine cold start phase. The test was conducted as described in section 2 and the results are shown in Fig. 6. Immediately after engine start at t0 , the capacitance increases and the capacitance change C reaches a maximum at t1 . During this period, condensate forms on the sensor element leading to the huge C increase. If one assumes the above-determined slope of 1.6 pF/mm2 and considers the IDE area of 35 mm2 , the capacitance change of 56 pF agrees with a complete coverage of the IDE structure. After t1 , no more condensate forms. The condensed water starts to evaporate and the sensor signal decreases until it reaches again the initial value at t2 , which means that the sensor element is free of condensate. The temperature sensor signal reflects the same behavior. At t1 , the sensor temperature reaches a plateau, indicating that the dew point is reached. As long as there is water on the sensor (t < t2 ), the temperature shows a plateau. As soon as the sensor is free of water, the sensor temperature increases again. Since t1 and t2 are correlated with the formation of condensate on the sensor element itself, the observed behavior can be used to improve the exhaust gas sensor heating strategy. Compared to the C-signal, the temperature is less valuable for detecting the end of the dew point. By integrating an IDE structure on a planar exhaust gas sensor and evaluating the course of the C-signal, an earlier sensor heat-up start could be realized, without considering safety margins. 4. Conclusion and outlook Since the initial experiments were very successful, it is intended in a next step, to apply an IDE directly on a planar lambda sensor. In addition, it might be helpful, to integrate also an additional resistive temperature sensor to combine the capacitance signal with the course of the temperature during cold start. To increase
the sensor signal and/or to obtain sufficiently high signals also from smaller ceramic sensors, the thick-films could be laser patterned, similar as described in ref. [16]. With this technique, even 20 m lines and spaces can be achieved with an electrode height of several m. Alternatively, photoimageable thick-film pastes [17] can be used. They provide approximately the same resolution. From an application point of view, cold starts under different atmospheric conditions (temperature, humidity, air pressure) are necessary to evaluate, whether the suggested method makes sense to develop such an IDE structure on the sensor to serial ripeness. The following advantages of an application of the IDE structure are attainable. Currently, the lambda sensors are heated up after the temperature of the exhaust pipe wall reaches the dew point temperature. The application of a stand-alone or integrated IDE sensor allows introducing a method that provides an objective criterion to start heating the lambda sensor. It directly detects the coverage of the sensor device with condensate and considers thereby the type of protection tube, the mounting position and the robustness of the sensor element. The start of the heating period can be controlled directly by the IDE sensor signal and therefore in dependence of the sensor coverage with condensate. Additionally, the failure risk of starting the heating period if there is still condensate on the sensor can be analyzed by determining the condensate coverage area. References [1] M.V. Twigg, Progress and future challenges in controlling automotive exhaust gas emissions, Applied Catalysis B: Environmental 70 (2007) 2–15. [2] U.G. Alkemade, B. Schumann, Engines and exhaust after treatment systems for future automotive applications, Solid State Ionics 177 (2006) 2291–2296. [3] J. Riegel, H. Neumann, H.-M. Wiedenmann, Exhaust gas sensors for automotive emission control, Solid State Ionics 152–153 (2002) 783–800. [4] R. Moos, A brief overview on automotive exhaust gas sensors based on electroceramics, International Journal of Applied Ceramic Technology 2 (2005) 401–413. [5] N. Kato, K. Nakagaki, N. Ina, Thick film ZrO2 NOx sensor, SAE paper 960334, 1996. [6] D.Y. Wang, S. Yao, M. Shost, J. Yoo, D. Cabush, D. Racine, R. Cloudt, F. Willems, Ammonia sensor for closed-loop SCR control, SAE paper 2008-01-0919, 2008. [7] T. Wahl, J. Riegel, Breitband-Lambdasonden für die Gemischregelung und On Board-Diagnose, 1, in: CTI-Fachkonferenz Emissionsrelevante Sensorik für Otto- und Dieselfahrzeuge, 14–15 November, München, 2006. [8] T. Baunach, K. Schänzlin, L. Diehl, Sauberes Abgas durch Keramiksensoren, Physik Journal 5 (2006) 33–38. [9] M.R. Busch, P. Hawig, US Patent Specification, US 2006/0032744 A1 (2006). [10] Data sheet Rubalit 708S, CeramTec, Germany. [11] R. Igreja, C.J. Dias, Analytical evaluation of the interdigital electrodes capacitance for a multi-layered structure, Sensors and Actuators A: Physical 112 (2004) 291–301. [12] S. Naito, T. Sugiyama, Y. Nakamura, Development of planar oxygen sensor, SAE paper 2001-01-0228, 2001. [13] G. Hagen, Impedimetrische Gassensoren auf Zeolith-Basis [Impedimetric zeolite-based gas sensors], Doctoral Thesis, University of Bayreuth, 2009. [14] H.E. Endres, S. Drost, Optimization of the geometry of gas-sensitive interdigital capacitors, Sensors and Actuators B: Chemical 4 (1991) 95–98. [15] G. Hagen, A. Dubbe, F. Rettig, A. Jerger, T. Birkhofer, R. Müller, C. Plog, R. Moos, Selective impedance based gas sensors for hydrocarbons using ZSM-5 zeolite films with chromium(III)oxide interface, Sensors and Actuators B: Chemical 119 (2006) 441–448. [16] S. Achmann, G. Hagen, J. Kita, I.M. Malkowsky, C. Kiener, R. Moos, Metal—organic frameworks for sensing applications in the gas phase, Sensors 9 (2009) 1574–1589. [17] J. Wood, C. Sabo, A. London, P. Barnwell, Processing of photoimageable thick film materials, in: Proc. SPIE Vol. 3582, Proceedings of the 1998 International Symposium on Microelectronics, San Diego CA, 1–4 November 1998, 1998, pp. 783–788.
Biographies D. Schönauer received her engineering diploma in materials science in 2005 from the University of Bayreuth, Germany. Since 2006 she is a research assistant and a doctoral student at the Chair of Functional Materials at the University of Bayreuth.
D. Schönauer, R. Moos / Sensors and Actuators B 148 (2010) 624–629 Her research interests are gas sensor applications and exhaust gas aftertreatment systems.
R. Moos received the Diploma degree in electrical engineering in 1989 and the Ph.D. degree from the University of Karlsruhe, Karlsruhe, Germany, where he conducted research on defect chemistry of titanates. He joined DaimlerChrysler in 1995 and
629
worked in the serial development of exhaust gas aftertreatment systems. In 1997, he switched over to Daimler-Chrysler Research, Friedrichshafen, Germany. As a team leader gas sensors, he headed several projects in the field of exhaust gas sensing. Since 2001, he is head of the Chair of Functional Materials of the University of Bayreuth. His main research interest are materials, systems and concepts for exhaust gas sensing and exhaust gas aftertreatment.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Short communication
Detection of water droplets on exhaust gas sensors D. Schönauer, R. Moos ∗ Bayreuth Engine Research Center, Functional Materials, University of Bayreuth, Universitätsstr. 30, D-95440 Bayreuth, Germany
a r t i c l e
i n f o
Article history: Received 12 March 2010 Received in revised form 7 May 2010 Accepted 22 May 2010 Available online 31 May 2010 Keywords: Lambda probe On board diagnosis (OBD) Cold start Exhaust gas aftertreatment
a b s t r a c t In order to avoid water droplet-borne stress on planar ceramic exhaust gas sensor elements, it is suggested to apply interdigital electrodes (IDE) on the sensor elements. As soon as water droplets impinge the sensor element, the capacitance of the IDE increases. Not before the water is evaporated, the capacitance reaches its initial value, indicating that the danger of droplet-borne stress is over. During engine cold start, this signal can be used to initiate sensor heating period. Initial experiments that demonstrate the feasibility of this concept are described in this study. The influence of droplet sizes on the capacitance of interdigital electrodes is investigated and a quadratic correlation between water droplet diameter and capacitance is found out and is explained qualitatively by a physics-based model. Transient experiments investigating the behavior during droplet evaporation as well as real engine cold start experiments conclude the paper. The engine cold start tests prove that the start point and the end point of water condensation on a ceramic exhaust gas sensor element can be detected. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In order to minimize automotive vehicle emissions, catalysts and exhaust gas sensors are integrated in the exhaust system. Both elements have to be heated quickly to working temperature [1,2], since otherwise the vehicles do not meet the stringent emission legislation [3]. The sensor element of a typical novel planar exhaust gas sensor like a lambda probe [4], an NOx sensor [5], or an ammonia sensor [6] is made of yttria stabilized zirconia ceramic. Due to its brittleness, thermal shocks have to be avoided. Although the ceramic sensor elements are protected by metallic sensor housings consisting of protection tubes with bores for gas inflow [7,8], it is a serious issue that the elements are prone to be damaged by condensed water or water droplets (water splash) during engine cold start. Condensation occurs on the exhaust tube wall, in the sensor housing, and on the sensor element itself. If condensed water impinges the heated ceramic sensor element, resulting thermal stress may lead to failures. Novel sensors are heated to operation temperature within only five seconds after engine start [3]. It is aimed to reduce this time interval even more [7]. Therefore, the danger of damaging sensors by water condensation becomes increasingly serious. To minimize these effects, even protective elements are discussed [9]. A novel approach to detect water condensation on a ceramic exhaust gas sensor element is suggested in this work. On one side of a typical planar zirconia-based sensor as described for instance in
∗ Corresponding author. Tel.: +49 921 55 7401. E-mail address: [email protected] (R. Moos). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.05.060
refs. [3,4], an insulating layer and an interdigital electrode structure (IDE) are applied. As soon as water reaches the sensor element, the electrical impedance of the IDE changes, indicating water droplets covering the sensor. Not before the exhaust reaches a temperature at which all water droplets are evaporated from the IDE, the sensor heater power is turned on or it is increased to bring the sensor element as fast as possible to its operation temperature. This article describes initial experiments to demonstrate the feasibility of this concept. The influence of droplet sizes on the capacitance of interdigital electrodes is described and a correlation between both will be shown in this paper. Moreover, a small physics-based model to explain this correlation will be developed. Transient experiments investigating the behavior during droplet evaporation as well as real engine cold start experiments conclude the paper. 2. Experimental Gold interdigitated electrodes were screen-printed (thickness tIDE ≈ 8 m) on alumina substrates (96% Al2 O3 in thick-film quality, tsub = 635 m, permittivity εr,sub ≈ 9.8 ± 10%, resistivity at room temperature ≈ 1013 cm [10]) with 100 m lines (l) and 100 m spaces (s) as sketched in Fig. 1. Since l = s, the metallization ratio as defined in ref. [11] is 1. The IDE covered an electrode area of AIDE = L × B = 5.76 mm × 4.41 mm = 25.4 mm2 . Alumina was selected as substrate material for two reasons. Firstly, because in the eventual application, one might either print an alumina insulation layer on the zirconia substrate—a technique which is already in use to electrically insulate the platinum heater
D. Schönauer, R. Moos / Sensors and Actuators B 148 (2010) 624–629
625
Fig. 1. Sketch of the sensor set up and the meaning of the letter symbols.
from the zirconia sensor part [8]. Secondly, since in some embodiments the sensor consists of a composite of high-temperature co-fired alumina and YSZ layers, serving as heater substrate or encompassing the sensor function, respectively [12]. The complex impedance Zmeasured of our IDE structure was measured using a Novocontrol alpha analyzer in the frequency range from 100 Hz to 1 MHz. Single droplets with a defined volume >0.5 l were applied onto the IDE structure at room temperature. The droplets of deionized water were applied by an Eppendorf reference pipette with a maximal volume of 10 l. The tolerance of the droplet volume is around ±2.5%. The water droplet diameter, ddrop , was determined under the light-optical microscope. The cable capacitance Ccable was determined to be 1.2 pF. A sensor was cut right in front of the interdigital electrodes (as sketched by the dotted line L1 in Fig. 1) and the capacitance Cfeed of the sensor contact feeds in the feed region was determined (Cfeed = 1.3 pF). The admittances of cable and feeds, Ycable = j2fCcable and Yfeed = j2fCfeed were calculated for each single frequency f and subtracted from the measured admittance, Y = Ymeasured − Yfeed − Ycable ,
(1)
wherein for the measured admittance Ymeasured = 1/Zmeasured holds. The corrected impedance Z is the reciprocal value of Y, i.e. Z = 1/Y. During the water evaporation period, the evaporation process was observed and photographed for some sensors under the lightoptical microscope with simultaneously acquiring appropriate impedance spectra. Finally, dynamic experiments were conducted in a gasoline engine test bench (dynamometer). Therefore, a sensor element with a more exhaust gas sensor-like geometry (i.e., AIDE ≈ 35 mm2 ) was mounted into a metallic sensor housing without a protection tube and was placed between two bricks of a three way catalyst. An engine cold start at 0 ◦ C was realized in a climate chamber. The sensor temperature was measured by an integrated thermocouple on the backside of the IDE structure. In addition to the thick-film devices as described above, some photolithographically processed thin-film gold IDE structures (l = s = 20 m) on 99% alumina obtained from an earlier project were also investigated. Details of the preparation can be found in ref. [15]. Due to the poor long-term stability of thin-films in the tail pipe, these devices were not intended for application in the engine exhaust but to prove the physical model.
Fig. 2. Impedance data of our IDE structure when covered with droplets of different diameters as indicated in (a) and partially in (b) (same symbols in (a) and (b)): (a) plot in the complex plane, (b) Nyquist plot. In addition, data of the droplet-free IDE are plotted, (c) the inset denotes an appropriate equivalent circuit.
3. Results and discussion 3.1. Static experiments Impedance spectroscopy results are plotted either in the complex plane or as a Nyquist plot vs frequency for different droplet sizes (Fig. 2a and b). From the shape of the spectra in Fig. 2a, an equivalent circuit comprising an R||C-element and an unknown complex serial element Zs as depicted in Fig. 2c can be deduced. Since the conductance of the alumina substrate can be neglected due to its high resistivity, R is assumed to represent the resistance of the droplet, whereas the capacitance C comprises the capacitances of the substrate, the surrounding air, and the droplet. The serial element Zs is rather attributed to electrode/water interfacial effects than to intrinsic alumina or water properties. In the Nyquist representation, one can allocate the appearance of Zs , R, and C to distinct frequency ranges (see Fig. 2b). Since Zs affects the impedance only at low frequencies, it makes sense to evaluate the impedance plots at frequencies f > 1 kHz, where the influence of Zs can be neglected. When the IDE is covered with water droplets, one may assume that both R and C depend on the degree of coverage with water. R is determined at 4.5 kHz (a frequency range in which R determines the behavior), whereas C is evaluated at 878 kHz (at that frequency the behavior is almost capacitive), from the absolute value and the phase angle of the impedance, assuming an R||C equivalent circuit. Both frequency points are indicated in Fig. 2b.
626
D. Schönauer, R. Moos / Sensors and Actuators B 148 (2010) 624–629
Let us assume that the sensor is a capacitor with the capacitance C. C0 is its capacitance if no droplet covers the IDE structures. C splits up into two parallel capacitors, a lower part, Cbottom , which is determined by the substrate, and an upper part, Ctop , which is given by the capacitances of air and droplet, Cair and Cdrop , respectively. C = Cbottom +Ctop
(2)
Since the substrate is much thicker than the electrode dimensions, i.e. tsub l, s, one may assume for the calculation of Cbottom that only a negligible part of the electric field is outside of the substrate. Hence, Cbottom remains unaffected from ambient influences, i.e. Cbottom = C0,bottom . Methods to calculate the capacitances of planar IDE structures as a function of the geometrical parameters tsub , l, s, and, tIDE are well described in the literature (see ref. [11] or [14] and the literature cited therein). In general, one has to solve elliptic integrals. A more handy method is suggested here for a first order approximation. For that purpose, it is sufficient to define a geometry factor, F, allowing to ascribe the capacitance calculation to the well-known parallel-plate capacitor geometry. Please note that in general F is a function of dsub , l, and s but we will see later that for our approximation we only have to assume that F is constant within one sample. For the bottom side this leads to Cbottom = ε0 × εr,sub × F
(3a)
with ε0 being the dielectric constant of the vacuum. For the uncovered droplet-free IDE surface one obtains C0,top = Cair = ε0 × εr,air × F
(3b)
If a relative fraction, a, of the IDE structure is covered with water droplets, Eq. (3b) has to be expanded: Fig. 3. (a) Capacitance C and resistance R for different droplet diameters as determined by light-optical microscopy, (b) Capacitance change C vs square of the droplet diameter. The dotted line obeys Eq. (9) with a capacitance of the droplet-free IDE structure of C0 = 7.1 pF.
the permittivity of air εr,H2 O ≈ 1 , droplet size ddrop and capacitance C increase in unison. The relation C(ddrop ) appears to be quadratic. This point will be discussed later. If one evaluates much more data taken at different days, one finds that the overall behavior of the resistance when the IDE is droplet-covered is always the same, but the absolute resistance values are not fully reproducible. This is ascribed to the non-constant quality of the utilized deionized water as well as to the non-constant droplet temperature; both affecting the water conductivity. In strong contrast to that, data points of the capacitance C can be found within a small scatter band of ±3 pF. It is noteworthy to say that for the capacitance of the uncovered interdigital electrode structure a value of C0 = 7.1 pF was measured (see Fig. 3a), which agrees quite well with FEM calculations for the same geometry with an electrode height of 7 m (6.1 pF in ref [13]).
The aim of the calculation is on the one hand to understand the behavior analytically and on the other hand to find a function that correlates the capacitance values with the droplet sizes. It would be helpful, if no fit factor were required.
C0 = ε0 εr,air +εr,sub F
(4)
By combining Eq. (2) with Eq. (3b) and Eq. (3c), one can derive the total capacitance C of the droplet-covered IDE structure
C = Cbottom + Ctop = ε0 εr,sub F + ε0 (1 − a) εr,air + aεr,water F
= ε0 εr,sub + (1 − a) εr,air + aεr,water F
(5)
If one measures the uncovered capacitance C0 , one can calculate the geometry factor F from Eq. (4) using the known values of εr,air and εr,sub . Combining F with Eq. (5), one obtains the droplet-covered capacitance, which depends besides the known permittivities only on the droplet-covered fraction a.
C = C0
1+a
εr,water − εr,air εr,air + εr,sub
(6)
The relative fraction of the IDE coverage a is defined as the ratio 2 and the IDE area AIDE = between the droplet area Adrop = 1/4ddrop L × B. a=
3.2. Model calculation
(3c)
Combining Eq. (2), Eq. (3a), and Eq. (3b), one obtains for the total capacitance of the uncovered sensor
Fig. 3a shows that with increasing droplet diameter, ddrop , the capacitance C increases (left axis), whereas the resistance R decreases (right axes). At a first glance, this behavior can be understood qualitatively: the non-negligible conductivity of the water leads to a decreasing resistance droplet size. with an increasing Since the permittivity of water εr,H2 O ≈ 80.3 is much higher than
Ctop = Cair + Cdrop = ε0 (1 − a)εr,air + aεr,water F
Adrop AIDE
=
1 2 ddrop 4
L×B
(7)
Now, a quadratic relation between the capacitance C and the droplet diameter appears: C = C0 + C0
1 4
L×B
×
εr,water − εr,air 2 × ddrop εr,air + εr,sub
(8)
D. Schönauer, R. Moos / Sensors and Actuators B 148 (2010) 624–629
627
Fig. 4. Light-optical microscope of a droplet during evaporation. a) freshly impinged, (b) after 300 s, (c) after 490 s, and (d) after 600 s.
With the constants and the geometrical data from above, one obtains for the capacitance change C:
C = C − C0 = 0.226 × C0
ddrop mm
2
(9)
The good agreement between Eq. (9) and the measured data are shown in a representation of the capacitance change vs the square of the droplet diameter in Fig. 3b. The dotted line was calculated with Eq. (9) and the measured value of C0 = 7.1 pF. The triangles show the measured C values for single droplets with varying diameters. These data are taken from Fig. 3a. Some additional measurements with several droplets on the IDE surface were conducted. The open circles in Fig. 3b depict the results of a measurement with two or four single droplets, respectively. From the diameter of each single droplet, the area was calculated and summarized to obtain a total droplet area, Atotal . From Atotal , the effective diameter deff was calculated:
deff =
Atotal × 4
occurs. The question arises, how the evaporation process affects the capacitance change C. Fig. 4 shows light-optical microscope pictures of the same droplet directly after impinging, after 300 s, after 490 s, and after 600 s, respectively. Since the droplet was illuminated from one side, the diminishing shadow indicates that during evaporation the droplet looses height. This is in contrast to its diameter that remains constant. If the assumption of an almost field-free droplet tip is correct, which means that the droplet height plays only a negligible role for the capacitance increase, one should expect an almost constant capacitance as long as the droplet adheres on the IDE. Not before the droplet is in its final stage and its height becomes smaller than the electrode geometry and/or its diameter becomes remarkably smaller, a capacitance decrease should occur. That is exactly what can be observed from Fig. 5. Here, the transient behavior of C measured at a frequency of 300 kHz for four different droplets during evaporation from a thick film IDE (details see above) is plotted. At t0 , water drops are applied
(10)
Fig. 3b illustrates that the model is also applicable for multiple droplets covering the IDE structure. The obtained capacitance changes agree very well with the model curve. The good agreement between measurement and model was confirmed with thin-film IDE structures with l = s = 20 m. Since in this case the capacitance of the droplet-free sensor is C0 = 40 pF, the sensitivity could be improved by almost a factor of 5.7. Like for the thick-film IDE, the determined capacitance changes obey Eq. (9) within an error of maximum 3 pF. 3.3. Transient behavior The above-derived good agreement is due to the fact that the permittivity of water is very large and that the droplet height is higher than the dimension of lines and spaces of the IDE structure. Therefore, all field lines remain on the bottom of the droplet, very close to the IDE. In the tip of the droplet, almost no electrical field
Fig. 5. Capacitance change of a droplet-covered IDE during evaporation. The droplet diameter increased from A to D. For the meaning of t0 , tA , tB , tC , and tD see text.
628
D. Schönauer, R. Moos / Sensors and Actuators B 148 (2010) 624–629
Fig. 6. Capacitance change of an IDE-covered sensor element during cold start of an internal combustion engine. The sensor temperature is shown on the right axis.
to the IDE and the capacitance increases. The moment when water droplets impinge on the sensor element is evident. As shown in Fig. 3a, the capacitance change depends on the droplet diameter ddrop , which increases from curve A to D. After an almost constant course during evaporation, at ti , the capacitances drop to the initial value, indicating that the sensor element is free of droplets and that all droplets are evaporated. Along with the capacitance changes caused by the droplets, the residence times of the droplets on the IDE, tres,i = ti − t0 , increase. Hence, there is a good correlation between droplet size ddrop , capacitance increase C, and residence time tres,i . The behavior of Fig. 5 offers the chance to determine the water condensation onto an exhaust gas sensor directly during the engine cold start phase. The test was conducted as described in section 2 and the results are shown in Fig. 6. Immediately after engine start at t0 , the capacitance increases and the capacitance change C reaches a maximum at t1 . During this period, condensate forms on the sensor element leading to the huge C increase. If one assumes the above-determined slope of 1.6 pF/mm2 and considers the IDE area of 35 mm2 , the capacitance change of 56 pF agrees with a complete coverage of the IDE structure. After t1 , no more condensate forms. The condensed water starts to evaporate and the sensor signal decreases until it reaches again the initial value at t2 , which means that the sensor element is free of condensate. The temperature sensor signal reflects the same behavior. At t1 , the sensor temperature reaches a plateau, indicating that the dew point is reached. As long as there is water on the sensor (t < t2 ), the temperature shows a plateau. As soon as the sensor is free of water, the sensor temperature increases again. Since t1 and t2 are correlated with the formation of condensate on the sensor element itself, the observed behavior can be used to improve the exhaust gas sensor heating strategy. Compared to the C-signal, the temperature is less valuable for detecting the end of the dew point. By integrating an IDE structure on a planar exhaust gas sensor and evaluating the course of the C-signal, an earlier sensor heat-up start could be realized, without considering safety margins. 4. Conclusion and outlook Since the initial experiments were very successful, it is intended in a next step, to apply an IDE directly on a planar lambda sensor. In addition, it might be helpful, to integrate also an additional resistive temperature sensor to combine the capacitance signal with the course of the temperature during cold start. To increase
the sensor signal and/or to obtain sufficiently high signals also from smaller ceramic sensors, the thick-films could be laser patterned, similar as described in ref. [16]. With this technique, even 20 m lines and spaces can be achieved with an electrode height of several m. Alternatively, photoimageable thick-film pastes [17] can be used. They provide approximately the same resolution. From an application point of view, cold starts under different atmospheric conditions (temperature, humidity, air pressure) are necessary to evaluate, whether the suggested method makes sense to develop such an IDE structure on the sensor to serial ripeness. The following advantages of an application of the IDE structure are attainable. Currently, the lambda sensors are heated up after the temperature of the exhaust pipe wall reaches the dew point temperature. The application of a stand-alone or integrated IDE sensor allows introducing a method that provides an objective criterion to start heating the lambda sensor. It directly detects the coverage of the sensor device with condensate and considers thereby the type of protection tube, the mounting position and the robustness of the sensor element. The start of the heating period can be controlled directly by the IDE sensor signal and therefore in dependence of the sensor coverage with condensate. Additionally, the failure risk of starting the heating period if there is still condensate on the sensor can be analyzed by determining the condensate coverage area. References [1] M.V. Twigg, Progress and future challenges in controlling automotive exhaust gas emissions, Applied Catalysis B: Environmental 70 (2007) 2–15. [2] U.G. Alkemade, B. Schumann, Engines and exhaust after treatment systems for future automotive applications, Solid State Ionics 177 (2006) 2291–2296. [3] J. Riegel, H. Neumann, H.-M. Wiedenmann, Exhaust gas sensors for automotive emission control, Solid State Ionics 152–153 (2002) 783–800. [4] R. Moos, A brief overview on automotive exhaust gas sensors based on electroceramics, International Journal of Applied Ceramic Technology 2 (2005) 401–413. [5] N. Kato, K. Nakagaki, N. Ina, Thick film ZrO2 NOx sensor, SAE paper 960334, 1996. [6] D.Y. Wang, S. Yao, M. Shost, J. Yoo, D. Cabush, D. Racine, R. Cloudt, F. Willems, Ammonia sensor for closed-loop SCR control, SAE paper 2008-01-0919, 2008. [7] T. Wahl, J. Riegel, Breitband-Lambdasonden für die Gemischregelung und On Board-Diagnose, 1, in: CTI-Fachkonferenz Emissionsrelevante Sensorik für Otto- und Dieselfahrzeuge, 14–15 November, München, 2006. [8] T. Baunach, K. Schänzlin, L. Diehl, Sauberes Abgas durch Keramiksensoren, Physik Journal 5 (2006) 33–38. [9] M.R. Busch, P. Hawig, US Patent Specification, US 2006/0032744 A1 (2006). [10] Data sheet Rubalit 708S, CeramTec, Germany. [11] R. Igreja, C.J. Dias, Analytical evaluation of the interdigital electrodes capacitance for a multi-layered structure, Sensors and Actuators A: Physical 112 (2004) 291–301. [12] S. Naito, T. Sugiyama, Y. Nakamura, Development of planar oxygen sensor, SAE paper 2001-01-0228, 2001. [13] G. Hagen, Impedimetrische Gassensoren auf Zeolith-Basis [Impedimetric zeolite-based gas sensors], Doctoral Thesis, University of Bayreuth, 2009. [14] H.E. Endres, S. Drost, Optimization of the geometry of gas-sensitive interdigital capacitors, Sensors and Actuators B: Chemical 4 (1991) 95–98. [15] G. Hagen, A. Dubbe, F. Rettig, A. Jerger, T. Birkhofer, R. Müller, C. Plog, R. Moos, Selective impedance based gas sensors for hydrocarbons using ZSM-5 zeolite films with chromium(III)oxide interface, Sensors and Actuators B: Chemical 119 (2006) 441–448. [16] S. Achmann, G. Hagen, J. Kita, I.M. Malkowsky, C. Kiener, R. Moos, Metal—organic frameworks for sensing applications in the gas phase, Sensors 9 (2009) 1574–1589. [17] J. Wood, C. Sabo, A. London, P. Barnwell, Processing of photoimageable thick film materials, in: Proc. SPIE Vol. 3582, Proceedings of the 1998 International Symposium on Microelectronics, San Diego CA, 1–4 November 1998, 1998, pp. 783–788.
Biographies D. Schönauer received her engineering diploma in materials science in 2005 from the University of Bayreuth, Germany. Since 2006 she is a research assistant and a doctoral student at the Chair of Functional Materials at the University of Bayreuth.
D. Schönauer, R. Moos / Sensors and Actuators B 148 (2010) 624–629 Her research interests are gas sensor applications and exhaust gas aftertreatment systems.
R. Moos received the Diploma degree in electrical engineering in 1989 and the Ph.D. degree from the University of Karlsruhe, Karlsruhe, Germany, where he conducted research on defect chemistry of titanates. He joined DaimlerChrysler in 1995 and
629
worked in the serial development of exhaust gas aftertreatment systems. In 1997, he switched over to Daimler-Chrysler Research, Friedrichshafen, Germany. As a team leader gas sensors, he headed several projects in the field of exhaust gas sensing. Since 2001, he is head of the Chair of Functional Materials of the University of Bayreuth. His main research interest are materials, systems and concepts for exhaust gas sensing and exhaust gas aftertreatment.