wo3 thick-film gas sensors

Sensors and Actuators B 46 (1998) 8–14 WO 3 thick-film gas sensors A.A. Tomchenko *, V.V. Khatko, I.L. Emelianov Laboratory of Electronic Engineering Materials, Physical Technical Institute, Academy of Sciences of Belarus, Zhodinskaya 4 , 220141 Minsk, Belarus Received 6 May 1997; received in revised form 1 September 1997; accepted 3 September 1997 Abstract Tungsten trioxide is a promising material for NO x gas sensors. This paper describes the preparation of tungsten trioxide thick films by the use of the traditional screen-printing technology. We present the characterization of both their structural properties by means of XRD measurements and the film morphology by the SEM microscope studies, and the electrical response of the films of various thicknesses to NO. The NO-sensing properties of WO 3 thick films doped with Bi 2 O 3 are also investigated in this paper. The thick films show a good sensitivity to NO over a wide temperature range and much better results are obtained by the doped films. It is demonstrated that a bottom thick-film layer of about 15 mm thick determines the sensing characteristics of the WO 3 -based sensors. We also present the fabrication of planar-type WO 3 -based thick-film sensors for on-line monitoring of NO concentrations. The sensor with the sensing film doped with Bi 2 O 3 shows a good sensitivity to low NO concentrations (2–300 ppm) at 300°C. © 1998 Elsevier Science S.A. All rights reserved. Keywords : NO sensors; Tungsten trioxide; Thick-film sensors; Screen printing 1. Introduction The aim of this work is to describe the behaviour of thick-film NO sensors obtained by screen-printing tung- sten trioxide-based pastes on alumina substrates. Gas sensors based on semiconducting oxides and manufactured by the screen-printing method have cer- tain advantages with respect to other types of gas sensors. To detect small concentrations of a reactive gas in air, the surface reactions are much more relevant than the bulk changes, so that the specific surface of gas sensitive elements must be as high as possible. The screen-printing technology is adequate for satisfying such a primary requirement. In addition to high sensi- tivity, thick-film sensors have high stability combined with light simple construction at low cost. Nitrogen oxides generated by combustion facilities are dangerous to health. Their maximum emissions are limited in most countries. The limiting values for the emissions are generally being reduced nowadays. This means increased demands on the monitoring tech- niques. In other words, there is an increasing need to detect low concentrations of air pollutants like NO x which frequently occur in everyday life. At present, a lot of oxide semiconductors are used in the development of gas sensors; however, only a few of them are sensitive to NO x [1– 5]. WO 3 exhibits the most promise for employment in NO sensors. In 1991, Akiyama et al. [6] found high sensitivity of tungsten trioxide sintered films to NO x and developed Taguchi sensor-like selective gas sensors for low concentrations of NO x . Sberveglieri et al. [7] have recently observed a good and reversible electrical NO x response of WO 3 thin films prepared by r.f. sputtering on alumina sub- strate. The results of the NO-sensing properties of WO 3 thick films of various thicknesses are reported in this paper. The thick films were prepared by the use of the traditional screen-printing technology commonly em- ployed for thick-film hybrid circuits production. The fabrication of planar-type WO 3 -based thick-film sensors for on-line monitoring of NO concentrations in atmo- sphere and in steam power plant exhausts is also de- scribed in this paper. * Corresponding author. Tel.: + 375 172 685612; fax: +375 172 637693; e-mail: khatko@cz.itmo.by 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S09 2 5 -4 005(97)003 1 5 - 8 A.A. Tomchenko et al. / Sensors and Actuators B 46 (1998) 8 –14 9 Table 1 Composition and thickness of the thick-film elements manufactured for the study Element code Bi 2 O 3 and glass frit added Thickness (mm) 15noA1 25noA2 A3 no 35 no 45A4 B1 yes 15 yes 25B2 Fig. 1. SEM micrographs of the surface of the WO 3 thick films: (a) A2; (b) B2. 2. Experimental Thick-film sensors were prepared by printing and firing thick-film pastes. Alumina substrates of 0.6 mm thick with a RuO 2 heater printed on the back side were used. Screen-printing pastes were prepared by mixing the WO 3 powder with an organic vehicle in a weight ratio of 4:1. The organic vehicle was based on lanoline. A maximum burn-out temperature of the vehicle was approximately 500°C. Some sensors were manufactured using a paste with 2.5 wt.% Bi 2 O 3 powder additive as activator and 3.5 wt.% glass binder, which contained Si, Al, Zn, Ti and Pb as its main components. The compo- sitions and thicknesses of the thick-film sensing ele- ments manufactured for the study are listed in Table 1. The films thicker than 15 mm were prepared by repeated printing of the respective pastes. The second layer and subsequent layers were printed onto the previously printed film once that was dried in air at 150°C for 10 min. Ag– Pd paste was used as electrode material. First, the electrodes were printed and fired at 850°C. The sensing layer of 8 mm× 5 mm in size was printed and then fired at 680°C for 10 min in flowing dry air. The printed WO 3 thick films sintered at lower temperatures were loose and had inadequate adhesion to the sub- strate. On the other hand, an annealing at temperatures above 700°C destroyed the films as a result of the intensive chemical interaction of WO 3 with the elec- trode material and the substrates. It was noted that at elevated annealing temperatures the chemical durability of the films of set B was superior to the films without frit. The size of the grains of the fired films ranged from 100 to 750 nm; Fig. 1(a) and (b) refers to the A2 and B2 samples, respectively. One can see from the figures that the thick films with the frit and the activator had smaller grains than the films without the additives. To get reproducible and comparable data, each sample was exposed at 400°C for 100 h in dry air before use. X-ray diffraction measurements were performed by ‘DRON-3’ powder diffractometer with Cu Kh radiation in Bragg-Brentano parafocusing geometry. For electrical measurements, the samples were set in a measurement chamber where their d.c. conductivity was measured at various temperatures (from room tem- perature up to 350°C), and surrounding atmospheres (dry air and 2 –300 ppm NO diluted in air, at a flow rate of1lmin −1 ). The measuring set-up and method employed have been described in detail elsewhere [8]. A.A. Tomchenko et al. / Sensors and Actuators B 46 (1998) 8 –14 10 3. Results and discussion The XRD powder dash-pattern of the fired WO 3 thick film on alumina substrate (A2 sample) is shown in Fig. 2 where the reflections from the substrate are indicated with asterisks. All of the peaks can be as- signed to the triclinic WO 3 phase or to the monoclinic WO 3 phase (and to the Al 2 O 3 substrate). The diffrac- tion patterns of the WO 3 triclinic phase by JCPDS (card no. 20-1323) and WO 3 monoclinic phase by JCPDS (card no. 24-747) are shown, respectively, on the top and on the bottom of the figure, for compari- son. The other films of set A had an analogous phase structure as well as the films of set B. The triclinic phase dominated in the films. This phase is stable from about −40 to about +17°C, but it is probable that heat treatment of the films under the conditions out- lined above could stabilize it in a different temperature range. Fig. 3 shows representative heating curves for the thick-film sensors. The curves have been obtained by heating in atmospheric air. The heating rate was 3 K min −1 . The forms of the curves for different sensors differ only slightly. It can be seen that the film conduc- tance increased by about four orders of magnitude between 20 and 400°C. The behaviour of the conduc- Fig. 3. Conductance of WO 3 thick films as a function of inverse temperature measured in air at a heating rate of3Kmin −1 . Fig. 2. XRD powder dash-diffractogram of WO 3 thick film printed onto alumina (A2 film) with the peaks from Al 2 O 3 indicated with asterisks. tance in the range 250–325°C clearly demonstrated a strong slope change or even a small decrease in conduc- tance. It probably implies the phase transition from monoclinic WO 3 phase to orthorhombic one. As would be expected, the phenomenon showed up more pro- nouncedly with an increase in the film thickness and for the films of set A without the additives with respect to the films of set B with activators. The sensors began to show NO-activity at about 100°C. Fig. 4 shows the gas-sensing properties of the films A1 and B1 when the gas flow was changed between air and 163 ppm NO in air at different temper- atures. The conductivity of the films decreased upon NO introduction, thus showing an n-type semiconduc- tor behaviour. At 150°C, the conductance of the ele- ments decreased sharply upon gas exposure but did not return to the original level when air flow was restored. The conductances of both films A1 and B1 were recov- ered to about 88% of their original levels. Furthermore, the desorption process at this temperature was very slow and recovery times were more than 25 min. At 200°C, NO desorption was faster than at 150°C, but the gas sensitivity, expressed as G a /G g where G a is the conductance in air and G g is the conductance in 163 ppm NO, was lower. On further increasing the temper- ature of the elements, the tendency was retained. So, A.A. Tomchenko et al. / Sensors and Actuators B 46 (1998) 8 –14 11 Fig. 4. Response behaviour of the conductance of the WO 3 thick films at different temperatures: (a) A1; (b) B1. Gas flow was changed from air to 163 ppm NO in air and back to air. the sensitivity of A1 film changed from 33 at 150°C to 2.44 at 300°C and its recovery time changed from 28 to 3.4 min, respectively. In contrast, the 90% response time of the sensors increased somewhat with increasing temperature. The response times of A1 film were 3 min at 150°C and 4.4 min at 250°C, and for B1 film they were 1 and 3.3 min, respectively. Obviously, the re- sponse time increase resulted from an increase of the contribution of diffusion processes to the general change of thick film conductance upon NO exposure at elevated temperatures, namely, of the reactive gas diffu- sion into oxide bulk (in the grains). Since the bulk processes are much slower than surface reactions the overall response time of the sensors tended to increase with the temperature. Fig. 5 shows the gas-sensing properties of the A2 and B2 films, measured at the same temperatures. The overall behaviour of the elements was not so good as that of the A1 and B1 films up to 300°C. So, at 150°C, NO sensitivities of A2 and B2 films were 6.2 and 27.6, respectively. At 250°C they were 2.5 and 5, respectively. As in the case of 15 mm thick films, the sensitivities of A2 and B2 films decreased, their response times in- creased slightly and their recovery times decreased with increasing operation temperature. For example, the re- sponse times of B2 were 1.95 min at 150°C and 2.4 min at 300°C, the recovery times being 13.6 min and 7 min, respectively. At 300°C, the sensitivities of the A2 and B2 films were 2.1 and 2.9, respectively. On further increasing the film thickness, the sensitivi- ties of the elements of set A as well as their response and recovery times remained nearly unchanged and were approximately those of the 25 mm thick films at respective temperatures. For the A3 and A4 films the sensitivities were 8.1 and 6.8 at 150°C, and 2.6 and 2.4 at 300°C, respectively. In other words, an increase in the thickness of the films over about 20 mm brought about insignificant changes in the sensor characteristics. On the basis of obtained results it may be concluded that the sensitivity of the sensors is determined by the properties of the bottom thick-film layer of about 20 mm thick located between contacts. Fifteen micrometres A.A. Tomchenko et al. / Sensors and Actuators B 46 (1998) 8 –14 12 Fig. 5. Response behaviour of the conductance of the WO 3 thick films at different temperatures: (a) A2; (b) B2. Gas flow was changed from air to 163 ppm NO in air and back to air. thick porous films (A1, B1) had the highest sensitivity. As the thickness of the thick film increases, the density of its lower layer increases significantly and the area of the surfaces opened for the interaction with NO in this layer decreases sharply. A change in conductance of the film is caused by the gas penetration through pores of the upper layers into the bottom layer. The sensitivity of the sensor decreases with increase in film thickness and becomes minimum when the density of the layer between contacts reaches its maximum peak. Since the maximum density is usually obtained after printing of the second layer, the sensitivity of the sensor elements thicker than 20 mm was approximately constant. Considering the influence of the film composition, much better results were obtained by the films of set B. The thick films with Bi 2 O 3 as an activator showed a higher sensitivity and shorter response and recovery times as compared to films of the set A. It is possible that the difference in microstructure mentioned above is responsible for the difference in the characteristics be- tween the sets. Xu et al. [9] have recently shown with SnO 2 -based gas sensors a direct relationship between thick-film crystallite size, metal oxide additives and sensing properties of the sensors. It is likely that there is an analogous relationship for the WO 3 -based gas sen- sors. However, this assumption needs experimental ver- ification. Since sensors for exhaust gas monitoring generally operate in high-temperature surroundings (] 300°C), the gas sensing possibilities of the sensors have been studied more extensively at 300°C by using the B2 film as an example sensor. The sensor response to 2 ppm NO is shown in Fig. 6. It was quite fast and reversible after gas removal. The correlation between NO sensitiv- ity of the sensor and NO concentration at 300°C is shown in Fig. 7. Fig. 8 shows the transient response of the sensor to changes in NO concentration. The sensor responded sensitively to changes in NO concentration over the whole concentration range tested (2–300 ppm). Despite the fact that the sensitivity of B2 sensor as well as of the other sensors of sets A and B at 300°C was lower than that at 150–250°C, it remained suffi- A.A. Tomchenko et al. / Sensors and Actuators B 46 (1998) 8 –14 13 ciently high for detection changes in NO concentration within indicated concentration limits. The exhausts of combustion facilities usually include CO which may interfere with the detection of NO. As a preliminary test, B2 sensor was exposed to CO at 200– 300°C. The responses of the sensor to 1000 ppm CO were less than 1% and thereby CO has insignificant effect on NO sensitivity of the WO 3 -based sensors. 4. Conclusions Tungsten trioxide based thick-film sensors with sensi- tive elements of various thicknesses and compositions were fabricated by the screen-printing method and their sensitivity to NO was evaluated under different operat- ing temperatures. The sensors exhibited good sensitivity to NO over the range of operating temperatures under investigation (100 –300°C). It was showed that the sen- sitivity of the sensors decreased, their response times increased slightly and their recovery times decreased with increasing operation temperature. The sensitivity of the sensors decreased with the increasing thickness of the WO 3 films. It was demonstrated that the sensing properties of the sensors with the sensitive elements thicker than 20 mm were defined by the properties of a bottom layer of the thick film. This layer is situated Fig. 7. Concentration dependence of the NO sensitivity of WO 3 sensor (B2 sensor at 300°C). between electrodes and does not exceed their thickness. Some support was obtained for the conclusion that WO 3 -based sensors with Bi 2 O 3 additive could be used for NO monitoring in a high-temperature combustion environment. It was found an influence of the additive on the microstructure of the WO 3 thick films. The addition of Bi 2 O 3 improves the sensing characteristics of the sensors. A good and reversible response was observed to NO concentrations in the range of 2 –300 Fig. 6. WO 3 sensor (B2) response to 2 ppm NO in air at the operation temperature of 300°C. Fig. 8. Conductance changes of WO 3 sensor caused by changes in NO concentration (B2 sensor at 300°C). A.A. Tomchenko et al. / Sensors and Actuators B 46 (1998) 8 –14 14 ppm by the films containing the additives which makes them promising detectors to low NO concentrations. Acknowledgements This work was partially supported by a grant from the International Soros Science Educational Program (ISSEP). References [1] N. Yamazoe, Chemical sensors R&D in Japan, Sensors and Actuators B 6 (1992) 9–15. [2] J. Huusko, V. Lantto, H. Torvela, TiO 2 thick-film gas sensors and their suitability for NO x monitoring, Sensors and Actuators B 15-16 (1993) 245–248. [3] E. Traversa, S. Matsushima, G. Okada, Y. Sadaoka, Y. Sakai, K. Watanabe, NO 2 sensitive LaFeO 3 thin films prepared by sputtering, Sensors and Actuators B 24–25 (1995) 661–664. [4] G. Sberveglieri, S. Groppelli, P. Nelli, V. Lantto, H. Torvela, P. Romppainen, S. Leppavuori, Response to nitric oxide of thin and thick SnO 2 films containing trivalent additives, Sensors and Actuators B 1 (1990) 79–82. [5] T. Ishihara, K. Shiokawa, K. Eguchi, H. Arai, Selective detec- tion of nitrogen monoxide by the mixed oxide of Cr 2 O 3 –Nb 2 O 5 , Chem. Lett. (1988) 997–1000. [6] M. Akiyama, J. Tamaki, N. Miura, N. Yamazoe, Tungsten oxide-based semiconductor sensor highly sensitive to NO and NO 2 , Chem. Lett. (1991) 1611–1614. [7] G. Sberveglieri, L. Depero, S. Groppeli, P. Nelli, WO 3 sputtered thin films for NO x monitoring, Sensors and Actuators B 26-27 (1995) 89–92. [8] A.A. Tomchenko, V.V. Khatko, I.L. Emelianov, NO thick-film sensors for monitoring of power-and-heating plants exhausts, Izmeritelnaya Technika 1 (1997) 34–36 (in Russian). [9] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Correlation between gas sensitivity and crystallite size in porous SnO 2 -based sensors, Chem. Lett. (1990) 441–444. Biographies Alexey A. Tomchenko was born in 1958 in Minsk, Belarus (former Soviet Union). He received his B.Sc. in Electronic Engineering in 1984 from Minsk Radio En- gineering Institute, Belarus. He joined the staff of the Physical Technical Institute of the Academy of Sciences of Belarus in 1981. From 1981 to 1990 he worked there as an engineer. Since 1990 he has worked as a re- searcher at the Laboratory of Electronic Engineering Materials at the same institute. His research interests are X-ray fluorescence spectroscopy; chemistry, physics and technology of oxide thick films; and thick-film gas sensors. Vyachesla6 V. Khatko was born in 1949 in Minsk, Belarus. He received his M.Sc. in Nuclear Physics in 1971 from Belorussian State University and his Ph.D. in Electronic Engineering in 1985 from the Institute of Electronics, Minsk, Belarus. He joined the staff of the Physical Technical Institute of the Academy of Sciences of Belarus in 1976. Since 1990 he has been the Head of the Laboratory of Electronic Engineering Materials at the same institute. His current research interests include the development and application of semiconductor thick- and thin-film sensors. Emeliano6 Iouri graduated from the Department of Physics, Belorussian State University, in 1986. After graduation, he joined the Physical Technical Institute of the Academy of Sciences of Belarus, Department of Film Materials. He has been working there until now as a research worker. His research area covers the field of Langmuir-Blodgett films with a focus on sensor appli- cations. . . . Yamazoe, Chemical sensors R&D in Japan, Sensors and Actuators B 6 (1992) 9–15. [2] J. Huusko, V. Lantto, H. Torvela, TiO 2 thick-film gas sensors and their. reserved. Keywords : NO sensors; Tungsten trioxide; Thick-film sensors; Screen printing 1. Introduction The aim of this work is to describe the behaviour of thick-film NO sensors