Propane/butane semiconductor gas sensor with low power consumption

Transcription

1 Sensors and Actuators B 44 (1997) Propane/butane semiconductor gas sensor with low power consumption Jong H. Kim a, *, Jae S. Sung a, Young M. Son a, A.A. Vasiliev b, V.V. Malyshev b, E.A. Koltypin b, A.V. Eryshkin b, D.Yu. Godovski b, A.V. Pisyakov b, S.S. Yakimov b a Korea Institute of Energy Research, P.O. Box 5, Daeduck Science Town, Taejon, South Korea b Asdfd , RRC Kurchato Institute, Moscow, Russia Received 7 July 1997; accepted 8 July 1997 Abstract The semiconductor gas sensors which have a reduced power consumption as low as 100 mw were developed for the application to portable gas detection devices. Sensors with a base material of tin dioxide and 4 wt.% of Pd dopant were fabricated by means of thick film technology. Both the composition parameters of the sensing layer and the heater were optimized in order to sense propane/butane in air with the threshold level of 100 ppm. The original method of tin dioxide preparation for the sensing layer and for the thick film heater being stable at temperatures up to 600 C was developed Elsevier Science S.A. Keywords: Semiconductor; Sensor; Power consumption 1. Introduction Nowadays the semiconductor gas sensors are broadly applied to the atmosphere monitoring system, the toxic or explosive gases detection system, the chemical processing facilities, the intelligent buildings with environmental control functions and so on. One of the most broadly utilized semiconductor gas sensors is the tin oxide based sensor, which is commercialized as a worldfamous product by Figaro, Japan. The Figaro sensor consists of the platinum coil heater placed inside the miniature ceramic tube. On the external surface of the tube, tin dioxide sensing layer is placed along with the contact pads contacting thin connection wires. Even though this structure is very reliable and shows a good stability of the sensor properties 1, its disadvantage is a high power consumption from 400 mw to 1 W due to heating of the massive ceramic tube. This level of power consumption has a limit for the sensors to be adopted in a battery operation portable device. * Corresponding author. Tel.: ; fax: ; kjhy@sun330.kier.re.kr 1 Specification of Figaro TGS 813 and TGS 842 Taguchi gas sensors. Through the cooperative research between Institute of Molecular Physics (RRC Kurchatov Institute, Russia) and Korea Institute of Energy Research (KIER, Korea), lower power consumption semiconductor gas sensors of thick film type were aimed to develop. As a target the power consumption of the sensors for propane/butane in air is as low as 100 mw. This level of power consumption seems to be good enough for the battery operation portable devices which is able to run for about 10 h without recharging the battery. 2. Technology for low power consumption sensor As the basic technology for the fabrication of sensors the thick film processing was applied. Advantages of the thick film technology are: the possibility of large scale mass production (one can obtain several thousands sensor chips from one substratum) and the possibly automation of all steps of the sensor fabrication. The other distinctive advantage of the thick film technology is the stability characteristics of printed thick film layers. Several steps were undertaken in order to decrease the power consumption and optimize the construction of semiconductor gas sensors /97/$ Elsevier Science S.A. All rights reserved. PII S (97)

2 J.H. Kim et al. / Sensors and Actuators B 44 (1997) (1) The use of two-side construction (see Fig. 1) which allows miniaturization of the sensor chip. The final dimensions of the sensor chip were as small as 1.5 mm 0.3 mm 0.15 mm. The thickness of the alumina substratum was decreased up to 0.1 m. (2) For the preparation of tin oxide powder, an oxalate method with an original preparation process [1] was followed. After the precipitation the powder was dried at 500 C for 2 h. Then, an averaged particle size was observed as about 0.1 m by SEM pictures as shown in Fig. 2(a) and the specific surface area was measured as approximately 18 m 2 g 1 by BET method. (3) To increase the sensitivity, some preliminary investigations on doping of catalytic novel metals were conducted for various combinations of mixing ratio and a few groups of Pd and Pt particle sizes [4]. But in this paper all sensors were restricted to 4 wt.% of Pd doped sensing material. After doping, by means of an organic solution of HCOONa, an averaged cluster diameter was increased about two times compared with the undoped tin oxide powders as shown in Fig. 2(b). The paste for screen printing of the sensing layer mixed with the Pd doped tin oxide powder (60 wt.%) and the glass binder (40 wt.%) composed of B 2 O 3, SiO 2 and some metal oxides was used. (4) The composition of the resistive material for thick film heater was developed [2]. The material is based on 65 wt.% of ruthenium dioxide and 35 wt.% of the glass binder. As shown in Fig. 3(a) and (b), it was observed as about 0.3 m of an averaged cluster size before drying the ruthenium oxide and about 0.1 m of particle size after two steps of drying at 400 C for 2 h and then 600 C for 2hinorder to prevent recrystallization. The prepared heater material is stable up to temperatures of 600 C. The other advantage of the heater is the possibility to change its resistance by varying the ratio of ruthenium dioxide to the glass Fig. 1. Structure of developed sensor. 1, Alumina substratum; 2, sensing layer; 3, contact pads; 4, connecting wires; 5, disjunction layer; 6, heater; 7, contact pads. Fig. 2. SEM pictures of tin oxide: (a) dried at 500 C and (b) doped with 4 wt.% Pd. binder. Thus, the heater parameters can be adjusted in order to provide the necessary temperature at the requested voltage (i.e. 3 V). (5) The construction of the sensor was optimized in order to decrease heat losses by heat conduction, radiation and convection. A computer simulation of heat transfer for the sensor was conducted by the computational scheme of the finite elements method [4]. From the results of simulations, the materials and dimensions for contacting wires, contacting pads, the substratum could be optimized to minimize heat losses in the sensor. Based on the general thick film printing technology, all fabrication procedures including photomask preparation, screen printing, drying, firing, laser cutting of the substratum to separate printed chips and wire contacting by 20 m diameter of platinum wire were conducted.

3 454 J.H. Kim et al. / Sensors and Actuators B 44 (1997) Experimental Sensors were tested in the gas-dynamic mixing installation [3] as shown in Fig. 4. The gas-dynamic installation consists of four similar gas feeding lines. Initial air propane gas mixture with the concentration of 1.0 vol.% arrives at a two-position electromagnetic switch of gas flows (ESGF) via line 1. Air carbon monoxide gas mixture with a concentration of 0.1 vol.% or, in the other case, air methane gas mixture with concentration of 0.25 vol.% arrive at ESGF via line 2. Pure air comes via lines 3 and 4. All lines consist of similar pressure stabilizers (PS), manometers (M), mass flow controller (MFC) and electromagnetic valves (EV1 4). Stirring of the initial gas mixtures and air takes place immediately prior to ESGF. It leads to the formation of the diluted gas mixture with a concentration of components to be a value of C. Depending on the position of ESGF, the flows of air and gas mixture may arrive either at the chamber, containing a sensor, or at the vast. Fig. 4. Schematics of gas-dynamics installation. Concentration of test gas (propane) and admixture (carbon monoxide or methane) in the multicomponent gas mixtures in the chamber is determined from expression: C i =C 0i [q i /(q 1 +q 2 +q 3 )]=C 0i (q i /Q) where C 0i is a gas concentration of initial i-gas mixture, q i is a flow rate of this gas mixture in line i (i=1, 2), q 3 and Q are air flow rates in lines 3 and 4, respectively. The degree of dilution of initial gas mixture, K i =Q/q i, the accuracy of test gas concentrations and the range of test gas concentration are determined by the limits of measurement of the MFC. The MFCs can change the flow rate with minimum steps of 1.1% with respect to a maximum flow rate. The flow rate errors at a given Fig. 3. SEM pictures of ruthenium dioxide powder: (a) before drying and (b) after drying. Fig. 5. Conductance variations on heater power.

4 J.H. Kim et al. / Sensors and Actuators B 44 (1997) level of flow rate under the maximum is about 0.4% of the given flow rate. An accuracy of the propane concentration in the initial gas mixture prepared is 3 4% with respect to the concentration of 1.0 vol.%. Thus, increasing the dilution ratio, the total error of the propane concentration will increase from about 2% at C=0.9C 0 to about 16% at C=0.1C 0. An important condition for gas sensors operation and device calibration is constancy and equality of gas flow rates, incoming to the ESGF. To meet this condition, it is necessary to stabilize the rate of air flow in line 3 at the level of q 3 =Q (q 1 +q 2 ). Test sensors were put into the measuring chamber. In this chamber it is possible to monitor four or eight sensors simultaneously. Signals of the sensors were measured for the square-like sequences of concentrations. Air, passing through the chamber between impulses, was used as a standard in the determination of the sensitivity parameters of gas sensors. As a criterion of sensitivity the ratio ( 0 )/ 0 was used, where 0 is the conductance in the standard air, and is the conductance in the gas mixture. The duration of concentration impulses was selected in such a way to reach the equilibrium signal, corresponding to the test value of concentration. In this study the duration of impulsive impurity supply was set up for 150 s. The time interval between concentration impulses was 250 s, enabling the desorption of the impurity gas from the sensor surface. Since the pure air and gas mixtures always pass the same way into the humidifier before entering the chamber, their humidities are always similar. Gas pressure in the humidifier, placed practically on the exit of installation, is always near the atmospheric pressure. This enables, in principle, to reach a high degree of humidifying up to maximum level of RH%. The conductivity of sensing layers for each sensor was measured by an on-line data acquisition and control system composed of an IBM compatible PC, DC power supply, Hewlett-Packard Multimeter (Model 3457A) and Hewlett-Packard Switch Control Unit (Model 3488A). The power consumption of each sensor was also obtained by measuring the voltage and the current supplied to each heater. The duration of concentration step and the blow down step could be arbitrary selectable in the menu screen of the data acquisition and control program. In order to find optimal heater power, at first, resistance variations for different heater powers from 80 to 160 mw were measured at the test conditions as 1.0 vol.% of propane concentration, 50 RH% of gas mixture and 6lh 1 of the flow rate. The results are plotted as shown in Fig. 5. The plot shows that the maximum sensitivity occurs at 145 mw. At 100 mw the sensitivity around 14 is rather lower than the maximum. However, it is notable that the sensitivity around 14 is still high enough to determine a cut-off level of sensor signal to actuate portable devices. Also, for more reliable portable devices in practical utilization, more stable sensing layer characteristics at an operating heater power is prior to the maximum sensitivity value. As shown in Fig. 5, the most stable conductance ( ) variations under the given concentration of the gas mixture occurs near 100 mw of heater power since the derivative of the conductance is zero. Furthermore, it is well known that the maximum gas sensitivity of gas sensors to methane is observed at the temperature which is higher that the work temperature for propane. Therefore, it can be considered that the optimal work regime of the sensor with the maximum stability is about 100 mw of heater power. 4. Results and discussion 4.1. Optimal heater power Fig. 6. (a) Sensitivity variations on propane concentrations up to 0.1 vol.%; (b) Sensitivity variations on propane concentrations up to 1.02 vol.%.

5 456 J.H. Kim et al. / Sensors and Actuators B 44 (1997) Sensiti ity In order to find the sensitivity threshold and dependency on the concentration of propane, the concentration of propane air mixtures was adjusted from 0.01 to 1.02 vol.%. Gas flow rate was maintained constant and was 3.6 l h 1. The humidity of the gas mixture was controlled about 33 RH%. Measurements of the sensor resistance were carried out in quasi-steady state of the sensors. The total error of the propane concentrations in the gas mixture with respect to the adjusted concentration was from 3 to 4.5% at 1.02 vol.% to 16 17% at 0.01 vol.%. From Fig. 6(a) and (b), the results of measurements obtained from two sensors are shown as the dependency of sensitivity which seems to be a logarithmic variation and the measured threshold of sensitivity is confirmed at 0.01 vol.%. A minimum level of sensitivity threshold for propane in air can be approximately estimated in the level vol.% when the sensitivity, ( 0 )/ 0, is at least 0.2. Sensitivity variation for different humidity in the range from 5 to 95 RH%, under the condition of 0.5 vol.% propane concentration, was obtained as in the Fig. 7. From this results a rapid decrease of sensitivity in the region lower than 30 RH% and a slow decreasing of sensitivity in the region above 30 RH% can be seen Selecti ity on carbon oxide Test conditions for the selectivity on the carbon oxide were given as 7 RH% and 0.5 vol.% of propane air mixture with variation of carbon oxide concentration from 25 to 145 ppm. In Fig. 8, it shows negligible dependence on the concentration of carbon oxide. Fig. 8. Sensitivity variations on carbon mono-oxide concentration at 7 RH%. Therefore, the developed sensor has quite good selectivity characteristics on carbon oxide in the range between 25 and 145 ppm Long term stability Test on the long term stability was performed in a separately composed gas supply line [4]. The total duration of experiments was 1600 h of continuous work. As in Fig. 9 the heater tested under 3 V of supplied voltage was very stable maintaining almost constant resistance at 70. In Fig. 10 the sensing layer tested under 5 V with a shunt resister shows decreasing trend with fluctuations of measured voltage from 40 to 20 mv. That means, sufficient preliminary heat treatments at least for 600 h are necessary to obtain more stabilized sensor signals. Fig. 7. Sensitivity variations on humidity at 0.5 vol.% propane. Fig. 9. Stability of heater resistance under 7 RH% of air.

6 J.H. Kim et al. / Sensors and Actuators B 44 (1997) Fig. 10. Stability of sensing layer under 7 RH% of air Responsibility To investigate the responsibility under an impulsive concentration change, during 150 s 7 RH% and 0.6 vol.% of propane air mixture was supplied in the gas dynamic installation. The results, as shown in Fig. 11, are about 12 s of response time and about 150 s of recovery time in terms of the 90% fall and rising time in the impulsive signal. Thus, it is notable that the sensor has very fast responsibility. Fig. 11. Resistance changes on impulsive gas concentration supplied with 7 RH% and 6000 ppm of propane for 150 s. carbon mono-oxide (from 25 to 145 ppm). 4. The heater shows a very stable resistance during 1600 h of continuous operation. 5. The response time in the 7 RH% and 0.6 vol.% of propane air mixture are about 10 and 150 s of the recovery time in terms of the 90% fall and rise time. However, it was also notable that the sensitivity degradation by humidity should be improved and a longer aging time than 600 h is necessary to obtain more reliable reproducibility of the sensor signal. 5. Conclusions The miniature size of tin oxide gas sensor with 4 wt.% Pd doping was developed for the application to a battery operating portable gas alarm device. To decrease the power consumption, geometric design of sensor structure and preparation processes were optimized. From the characteristic measurements, it was found as: 1. Sensitivity, as the relative change of the sensor conductance with respect to reference standard air, is around 14 at 100 mw of optimal heater power. 2. The minimum level of sensitivity threshold maintaining the sensitivity at 0.2 could be estimated as ppm of propane. 3. The sensor shows quite a good selectivity for the admixtured propane (5000 ppm at 7 RH%) with Acknowledgements The authors wish to acknowledge Yu-Kong, Korea for financial support for this international cooperative R&D project. References [1] Tsniitochmash, Patent AE USSR N C01619/02. [2] A.A. Pisliakov, Gas sensitive thick film sensor, Russian Patent N /25/004710, [3] V. Malyshev, A. Eryshkin, E. Koltypin, Installation for calibration of semiconductor gas sensors, Proc. 6th Metrology Congrl., Lille, France, [4] Jong H. Kim, Jae S. Sung, S. Yakimov, Development of Gas (Butane/Propane) Sensor and Alarm System, KIER-IMP Joint Research Report, Taejon, South Korea,