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Gas sensitivity of semiconductor Fe2O3-based thick-film sensors to CH4, H2, NH3
434
Sensors and Actuators B, 18-19 (1994) 434-436
Gas sensitivity of semiconductor Fe&based CH,, Hz, and NH3
thick-film sensors to
V.V. Malyshev, A.V. Eryshkin, E.A. Koltypin, A.E. Varfolomeev and A.A. Vasiliev Russian ScientijTcCentre ‘KurchatovInstitute’,123182 Moscow (Russian Federation)
Abstract Gas sensitivity of Fe,O,-based thick-film sensors in gas mixtures of air with CM,, H, and NHs has been analysed. Dependences of relative variation of the sensor conductivity on the heater power and the concentration of the gas under analysis have been derived. Optimal thermal conditions ensuring the best gas sensitivity have been determined for each of the gases. The estimated time constant of a sensor is at 90%.
IllhWIllCtiOD
Semiconductor gas sensors based on metal oxides are widely used as transducers in the measurement of concentrations of gases of various types. At the same time of unfading interest is a search for new structures ensuring high gas sensitivity combined with good manufacturability. Traditionally, the most widely spread materials for gas-sensitive layers are SnO, and ZnO. Less attention has been paid to other metal oxides (Fe,O,, Ti02, etc.). This paper reports on the experimental results on the sensitivity of thick-film sensors based on the Fe,O, suspension with 20% of SnOz to CH4, Hz and NH, within concentration ranges of 0.1 to 11 vol.%, 0.1 to 4 vol.%, and 0.001 to 15 vol.%, respectively. The sensors have been manufactured in our laboratory with simple and accessible processes [l].
The sensor was fabricated on a porcelain tube of 1.5 mm in diameter and 4 mm in length (Fig. 1). Inside the tube is a heater made of nickel wire (50 pm in
Fig. 1. Design of the aoalysed sensors based on the FelOs suspension with 20% of SnOZ.
0925-40051’94/$7.00 Q 1994Elsevier Sequoia. All rights reserved SSDI 0925-4005(93)01031-X
diameter) and the outer surface of the tube is covered with a thick film of semiconducting Fe,O, paste with 20% SnO,. Ni-Cr alloy wires (50 pm in diameter) were bonded to a gas-sensitive layer with the Pd-Ag alloy paste. The semiconductor resistance at room temperature was l@ to 10’ R and that of the heater was 3 to 5 &I. Electrical contacts were made of the Pd-Ag alloy paste. The heater performed the function of a temperature-sensitive element in the electronic circuit of a sensor thermal regulator. The power applied to the heater was 0.3 to 1.3 W. The procedure of analysis and measurement of the sensor parameters is illustrated in Fig. 2. The gas dynamic installation consists of two identical gas lines, one of which transports metrologically certified air to gas flow switch 2 (line 1) and the other delivers the certified testing gas mixture (line 2). To eliminate the pressure breaks and the thermoanemometric effect which may affect the sensor output the flows of air and gas mixtures were balanced and stabilized to flow rate Q and pressure P. For this purpose gas-flow forming elements and control and measuring devices for flow parameters (see blocks 1 and 3 in Fig. 2) were installed in the gas lines. Automatic gas dynamic installations which we used for our investigation are described in detail in ref. 2. The distinguishing features of the procedure applied consist in the creation of a sequence of ‘quasi-rectangular’concentration pulses C(r) of the gas under analysis (the amplitude of which is equal to the concentration of the initial gas mixture) in measuring chamber 4 containing the gas sensor and in the registration of the sensor output in the form of conductivity impulses u(t) on chart strip 7 or a display. It allowed us to pick out the net effect of the admixture action and at the same
435
Results and discussions The sensor sensitivity to CI& Hz and NH, as a function of the heater power is plotted in Fig. 3. The plots manifest a clearly expressed extremal character inherent in metal oxide sensors. The best gas sensitivity is achieved at power levels of 0.48 W for NH,, 0.9 W for H2 and 1.05 W for CI-L+ The dependences of the gas sensitivity on the gas concentration within &14 vol.% are plotted in Fig. 4 and in the concentration range up to 1 vol.% in Fig. 5. The plots correspond to the heating conditions close to or equal to the optimal value. It is significant that practically no saturation of gas sensitivity has been observed for Hz and CH, up to concentrations equal to or exceeding the lower limit of explosiveness. It was only for NH, that the saturation of the concentration dependence has been observed at C> 8 vol.% (- 0.5 of the lower limit of explosiveness).
Air
Fig. 2. Key diagram of the gas dynamic installation procedure of measuring sensor parameters.
and the
time to determine the dynamic parameters of the sensors, The concentration impulses were formed with a two-position electromagnetic switch of gas flow 2 [2]. Figure 2 also shows the procedure of the experimental data processing and sensor parameters determination. The change in the sensor conductivity has been taken as a measure of gas sensitivity AU= o-u,, or A&,, where (Tand u, are the sensor conductivities in a gas mixture and in pure air, respectively. The time constants of concentration increase T&, and decrease T& were determined using the a(t) graph with allowance for the non-rectangular character of concentration impulse C(t). The duration of the front edge, tr,, was below 0.8 s for the gas-flow rate of 10 l/h, which is considerably less than the r0.9 values. The investigation was performed under thermal stabilization of the sensor heater. If there exists a temperature gradient on the porcelain tube (assessed as several tens degrees) the determination of the gas sensitivity layer temperature turns out troublesome. Hence, we have chosen an easily measurable technical parameter, power applied to the heater, as a criterion of the sensor heating degree, rather than a physical parameter (temperature). For temperature estimations one may consider that at heater power of 1 W temperature is 450 to 470 “C. For comparison of research results obtained for different gas mixtures the magnitude of the gas flows of air and gas mixtures were always taken constant and equal to 10 l/h. In order to avoid the moisture effect on the sensor readings we used well-dried gases with relative humidity 20%.
L 0
I
I
0.2
0.s
0.6
0,s 4.0
f.2
I.4 P (WI
Fig. 3. Gas sensitivity of sensors to CH,, Hz and NH3 as a function of the heater power.
7 6
2 1
0
246
6
ID
ie
I4 C%vol
Fig. 4. Gas sensitivity of sensors to CX&,Hz and NH, as a function of the gas concentration within 0 to 15 vol.%.
436
Conclusions
0
2020 4000 mo uoooIOMIO Cppn
Fig. 5. Gas of the gas
sensitivityof sensors to CH,, H2 and NHs as a function concentrationin the range below 1 vol.%.
As it follows from Fig. 5, the sensors have good sensitivity to NH, and H2. Our estimates of the sensor sensitivity threshold give us the values of up to 5 ppm for NHs, 50 ppm for Hz and 500 ppm for CH, (at Aol q=O.l). It can be stated that sensors possess good selectivity to NH3 in respect to CH+ The selectivity effect of NH, to H, and H, to CH, is expressed much weaker. Thus, for comparable concentrations (C=2 vol.%) at W= 0.48 W, the gas sensitivity to NH3 exceeds that to CH, by more than 10’ and is more than twice as high as the sensitivity to H,. The estimated time constant of the sensors, T~.~,has been found to be 10 s for CH, and H, and 20 s for NH,.
A gas dynamic test bed forming concentration pulses of the gas to be analysed has been used for investigation of the gas sensitivity of thick-film sensors based on the Fe,O, suspension with 20% of SnO, to CI&, H, and NHJ. The results are: 1. The best gas sensitivity of the sensors is attained at the heater power levels of 0.48 W for NH,, 0.9 W for Hz and 1.05 W for CH,. 2. Dependences of the gas sensitivity (relative variation of the sensor conductivity), A&,, to CH.,, Hz and NH, on the gas concentration have been derived in the concentration ranges of 0.1 to 11 vol.%, 0.1 to 4 vol.%, and 0.003 to 15 vol.%, respectively. The estimated sensitivity threshold of the sensors at h~/u~=O.l has been found to be 500 ppm for CH,, 50 ppm for Hz, and 5 ppm for NH+ 3. The estimated time constant of the sensors, T,,~, has turned out to be 10 s for CH, and H, and 20 s for NH,.
References 1 AA. Vasilievand M.A. Polykarpov, Senrors and Actuators B, 7 (1992) 626-G’. 2 V.V. Malyshev, D. Yu. Godovskii, A.V. Eryshkin, E.A. Koltypin, A.A. Vasiliev, AS. Razumov and A.E. Vartolomeev, Pnx 6th ht. Metmlogy Congress, LiUe, France, Oct. 19-21, 1993.
Sensors and Actuators B, 18-19 (1994) 434-436
Gas sensitivity of semiconductor Fe&based CH,, Hz, and NH3
thick-film sensors to
V.V. Malyshev, A.V. Eryshkin, E.A. Koltypin, A.E. Varfolomeev and A.A. Vasiliev Russian ScientijTcCentre ‘KurchatovInstitute’,123182 Moscow (Russian Federation)
Abstract Gas sensitivity of Fe,O,-based thick-film sensors in gas mixtures of air with CM,, H, and NHs has been analysed. Dependences of relative variation of the sensor conductivity on the heater power and the concentration of the gas under analysis have been derived. Optimal thermal conditions ensuring the best gas sensitivity have been determined for each of the gases. The estimated time constant of a sensor is at 90%.
IllhWIllCtiOD
Semiconductor gas sensors based on metal oxides are widely used as transducers in the measurement of concentrations of gases of various types. At the same time of unfading interest is a search for new structures ensuring high gas sensitivity combined with good manufacturability. Traditionally, the most widely spread materials for gas-sensitive layers are SnO, and ZnO. Less attention has been paid to other metal oxides (Fe,O,, Ti02, etc.). This paper reports on the experimental results on the sensitivity of thick-film sensors based on the Fe,O, suspension with 20% of SnOz to CH4, Hz and NH, within concentration ranges of 0.1 to 11 vol.%, 0.1 to 4 vol.%, and 0.001 to 15 vol.%, respectively. The sensors have been manufactured in our laboratory with simple and accessible processes [l].
The sensor was fabricated on a porcelain tube of 1.5 mm in diameter and 4 mm in length (Fig. 1). Inside the tube is a heater made of nickel wire (50 pm in
Fig. 1. Design of the aoalysed sensors based on the FelOs suspension with 20% of SnOZ.
0925-40051’94/$7.00 Q 1994Elsevier Sequoia. All rights reserved SSDI 0925-4005(93)01031-X
diameter) and the outer surface of the tube is covered with a thick film of semiconducting Fe,O, paste with 20% SnO,. Ni-Cr alloy wires (50 pm in diameter) were bonded to a gas-sensitive layer with the Pd-Ag alloy paste. The semiconductor resistance at room temperature was l@ to 10’ R and that of the heater was 3 to 5 &I. Electrical contacts were made of the Pd-Ag alloy paste. The heater performed the function of a temperature-sensitive element in the electronic circuit of a sensor thermal regulator. The power applied to the heater was 0.3 to 1.3 W. The procedure of analysis and measurement of the sensor parameters is illustrated in Fig. 2. The gas dynamic installation consists of two identical gas lines, one of which transports metrologically certified air to gas flow switch 2 (line 1) and the other delivers the certified testing gas mixture (line 2). To eliminate the pressure breaks and the thermoanemometric effect which may affect the sensor output the flows of air and gas mixtures were balanced and stabilized to flow rate Q and pressure P. For this purpose gas-flow forming elements and control and measuring devices for flow parameters (see blocks 1 and 3 in Fig. 2) were installed in the gas lines. Automatic gas dynamic installations which we used for our investigation are described in detail in ref. 2. The distinguishing features of the procedure applied consist in the creation of a sequence of ‘quasi-rectangular’concentration pulses C(r) of the gas under analysis (the amplitude of which is equal to the concentration of the initial gas mixture) in measuring chamber 4 containing the gas sensor and in the registration of the sensor output in the form of conductivity impulses u(t) on chart strip 7 or a display. It allowed us to pick out the net effect of the admixture action and at the same
435
Results and discussions The sensor sensitivity to CI& Hz and NH, as a function of the heater power is plotted in Fig. 3. The plots manifest a clearly expressed extremal character inherent in metal oxide sensors. The best gas sensitivity is achieved at power levels of 0.48 W for NH,, 0.9 W for H2 and 1.05 W for CI-L+ The dependences of the gas sensitivity on the gas concentration within &14 vol.% are plotted in Fig. 4 and in the concentration range up to 1 vol.% in Fig. 5. The plots correspond to the heating conditions close to or equal to the optimal value. It is significant that practically no saturation of gas sensitivity has been observed for Hz and CH, up to concentrations equal to or exceeding the lower limit of explosiveness. It was only for NH, that the saturation of the concentration dependence has been observed at C> 8 vol.% (- 0.5 of the lower limit of explosiveness).
Air
Fig. 2. Key diagram of the gas dynamic installation procedure of measuring sensor parameters.
and the
time to determine the dynamic parameters of the sensors, The concentration impulses were formed with a two-position electromagnetic switch of gas flow 2 [2]. Figure 2 also shows the procedure of the experimental data processing and sensor parameters determination. The change in the sensor conductivity has been taken as a measure of gas sensitivity AU= o-u,, or A&,, where (Tand u, are the sensor conductivities in a gas mixture and in pure air, respectively. The time constants of concentration increase T&, and decrease T& were determined using the a(t) graph with allowance for the non-rectangular character of concentration impulse C(t). The duration of the front edge, tr,, was below 0.8 s for the gas-flow rate of 10 l/h, which is considerably less than the r0.9 values. The investigation was performed under thermal stabilization of the sensor heater. If there exists a temperature gradient on the porcelain tube (assessed as several tens degrees) the determination of the gas sensitivity layer temperature turns out troublesome. Hence, we have chosen an easily measurable technical parameter, power applied to the heater, as a criterion of the sensor heating degree, rather than a physical parameter (temperature). For temperature estimations one may consider that at heater power of 1 W temperature is 450 to 470 “C. For comparison of research results obtained for different gas mixtures the magnitude of the gas flows of air and gas mixtures were always taken constant and equal to 10 l/h. In order to avoid the moisture effect on the sensor readings we used well-dried gases with relative humidity 20%.
L 0
I
I
0.2
0.s
0.6
0,s 4.0
f.2
I.4 P (WI
Fig. 3. Gas sensitivity of sensors to CH,, Hz and NH3 as a function of the heater power.
7 6
2 1
0
246
6
ID
ie
I4 C%vol
Fig. 4. Gas sensitivity of sensors to CX&,Hz and NH, as a function of the gas concentration within 0 to 15 vol.%.
436
Conclusions
0
2020 4000 mo uoooIOMIO Cppn
Fig. 5. Gas of the gas
sensitivityof sensors to CH,, H2 and NHs as a function concentrationin the range below 1 vol.%.
As it follows from Fig. 5, the sensors have good sensitivity to NH, and H2. Our estimates of the sensor sensitivity threshold give us the values of up to 5 ppm for NHs, 50 ppm for Hz and 500 ppm for CH, (at Aol q=O.l). It can be stated that sensors possess good selectivity to NH3 in respect to CH+ The selectivity effect of NH, to H, and H, to CH, is expressed much weaker. Thus, for comparable concentrations (C=2 vol.%) at W= 0.48 W, the gas sensitivity to NH3 exceeds that to CH, by more than 10’ and is more than twice as high as the sensitivity to H,. The estimated time constant of the sensors, T~.~,has been found to be 10 s for CH, and H, and 20 s for NH,.
A gas dynamic test bed forming concentration pulses of the gas to be analysed has been used for investigation of the gas sensitivity of thick-film sensors based on the Fe,O, suspension with 20% of SnO, to CI&, H, and NHJ. The results are: 1. The best gas sensitivity of the sensors is attained at the heater power levels of 0.48 W for NH,, 0.9 W for Hz and 1.05 W for CH,. 2. Dependences of the gas sensitivity (relative variation of the sensor conductivity), A&,, to CH.,, Hz and NH, on the gas concentration have been derived in the concentration ranges of 0.1 to 11 vol.%, 0.1 to 4 vol.%, and 0.003 to 15 vol.%, respectively. The estimated sensitivity threshold of the sensors at h~/u~=O.l has been found to be 500 ppm for CH,, 50 ppm for Hz, and 5 ppm for NH+ 3. The estimated time constant of the sensors, T,,~, has turned out to be 10 s for CH, and H, and 20 s for NH,.
References 1 AA. Vasilievand M.A. Polykarpov, Senrors and Actuators B, 7 (1992) 626-G’. 2 V.V. Malyshev, D. Yu. Godovskii, A.V. Eryshkin, E.A. Koltypin, A.A. Vasiliev, AS. Razumov and A.E. Vartolomeev, Pnx 6th ht. Metmlogy Congress, LiUe, France, Oct. 19-21, 1993.