Dynamic properties and sensitivity of semiconductor metal-oxide thick-film sensors to various gases in air gaseous medium

Sensors and Actuators B 96 (2003) 413–434

Dynamic properties and sensitivity of semiconductor metal-oxide thick-film sensors to various gases in air gaseous medium V.V. Malyshev∗ , A.V. Pislyakov Russian Research Center “Kurchatov Institute”, Institute of Molecular Physics, 1 Kurchatov Sq., 123182 Moscow, Russia Received 20 April 2003; received in revised form 6 June 2003; accepted 18 June 2003

Abstract A technique has been developed for formation of square pulses of the concentration of the gases under analysis in a measuring chamber with a sensing element. The pulse edge duration is of the order of fractions of a second. The methods of investigation and experimental data processing are discussed. A method of time constant calculation at the level of 90% of the maximum value in a conductivity pulse (σ) of ↑ ↓ semiconductor metal-oxide sensors is graphically illustrated both on the gas concentration increase τ0.9 , and decrease τ0.9 . Investigations have been carried out of dynamic parameters and sensitivity (S = σ/σ0 ) of gas sensors in dry gaseous mixtures of air with methane, hexane, hydrogen, carbon monoxide, ammonia, hydrogen sulphide and ethanol in the temperature range 300–600 ◦ C with a 50 ◦ C step at two values of gas concentrations, i.e., 20 ppm and 1 vol.% for methane, 10 and 2000 ppm for hexane, 100 ppb and 1000 ppm for hydrogen and ethanol, 5 and 98 ppm for carbon monoxide, 10 and 100 ppm for ammonia, and 3 and 30 ppm for hydrogen sulphide. Proceeding from a combination of dynamic parameters, sensitivity and sensor drift in a gas mixture, structures of gas-sensitive layers are defined for sensors most suitable for detection of the gases under analysis in air. Numerical values of parameters, the values of exponent, n, in the formula for sensitivity S − 1 = (σ − σ0 )/σ0 = ACn and sensor sensitivity thresholds have been defined. It has been found, that sensor sensitivity to the gases under analysis in most cases is of extremal character, and its temperature interval is practically independent of the gas concentration value. The time constants of sensors are shown to be of sharply drop-down character with the value stabilization at the minimum level τ0.9 ≈ 1–2 s at temperatures above 500 ◦ C and are independent of the gas concentration either. Besides, it has been established, that covering a chip with a metal cap reduces the sensor quick-action approximately by 2–4 times. The results of measurements and calculations are presented in the graphic and tabulated forms. © 2003 Elsevier B.V. All rights reserved. Keywords: Chemical gas sensors; Sensor; Quick-action; Sensitivity; Time constant; Gas mixture

1. Introduction The parameters that determine essentially the practical applicability of gas sensors are their dynamic properties (quick-action) and sensitivity to a peculiar gas. A large number of publications are devoted to measurements of sensor sensitivity, including thick-film metal-oxide sensors (see, e.g., [1–5]). The authors of this paper have also attended to this challenge. Over only the last 5–7 years we have published no less than 10 papers with the results of experimental studies of sensitivity and concentration dependencies of thick-film sensors to methane, hydrogen, propane, hydrogen sulphide and other gases (see, e.g., [6–9]). The problem of investigation of sensor dynamic characteristics has been given far less attention and much less ∗ Corresponding author. E-mail address: [email protected] (V.V. Malyshev).

0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00579-3

to the procedure of correct determination of this parameter in sensors. In many papers (see, e.g., [10–14]) and advertisements of sensors, the term “quick-action” or “time of response” is, as a rule, taken to mean the starting moment of sensor response to the appearance of gases in atmospheric air, which naturally takes several seconds. Meanwhile, some papers [15,16] cite too high values of ∼2–5 min of time of response. Actually, quick-action and time of response are far from being identical. Many known manufacturers of gas sensors and gas analysers (e.g., Japanese Corporations “Figaro” and “Riken Keiki” [17]) realize it perfectly well, hence, in the “time of response” column they indicate time constant τ 0.9 , equal to the time it takes the signal to reach 90% of the pulse amplitude when the pulse of the concentration of the gas under analysis is applied step-wise to the sensor. Hereinafter, we will assume time constant τ 0.9 as a measure of quick-action. From this definition of quick-action it follows, that the measurement of this magnitude would require

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rather complex gas equipment providing the formation and application of a gas concentration pulse and corresponding detection of the sensor response. Besides, none of the reports or advertisements mentions another, no less important, dynamic parameter, namely, sensor’s capability to restore its readings on gas concentration falling down. This parameter is the same time constant τ 0.9 , with the only difference, that it corresponds to the time span required for the signal to reach 10% of the value of the pulse amplitude in air on similar step-wise reduction of the gas concentration and on a pure air pulse supplied to the sensor. Thus, as to gas sensors, two dynamic parameters should be used that would describe their properties most accurately and reliably, i.e., time constant on the increase of the con↑ centration of the gas under analysis, τ0.9 , and time constant ↓ on the gas concentration decrease, τ0.9 . Our measurements have proven the introduction of a new parameter to be fairly justified, since, in fact, the values of time constants, τ 0.9 , on the gas concentration increase and decrease are often strongly different. The authors have carried out a vast series of investigations of dynamic parameters and sensitivity of thick-film metal-oxide sensors to seven most widespread gases that are of interest from the point of view of ecology and explosion safety, namely, methane, hexane, hydrogen, carbon monoxide, ammonia, hydrogen sulphide and ethanol. A gas-dynamic installation (GDI) currently in operation at the RRC “Kurchatov Institute” [6,9] has been modified to permit simultaneous measurements of dynamic parameters and sensitivity of sensors within rather short time spans. A computer-based electronic technique has been devised for sensor data logging when a square pulse of the concentration of the gas under analysis is supplied to the sensor. A procedure of experimental data processing is discussed that gives a pictorial presentation of sensor quick-action and sensitivity and permits expeditious determination of optimal temperature conditions for sensor operation as applicable to specific service conditions. Besides, we have studied the heating temperature dependence of quick-action and sensitivity of sensors with different structures of gas-sensitive layers in the range from 300 to 600 ◦ C for two strongly different concentrations of the gases under analysis both for an open make of a sensor and for sensors packed in a protective metal casing with an explosion-proof metal grid on the face. At the present stage of investigations, the authors did not pose themselves a problem of an all-round study of the dependencies of sensor parameters on the gas medium humidity, although we touched upon this problem in our earlier reports. Therefore, in order to partly put on a par the sensors, the gases under analysis and the ambient temperature, all the researches were carried out in dry gaseous media with zero humidity. We hope, this paper will be of interest to specialists engaged in practical application of sensors as primary gas concentration converters in gas-control equipment.

2. Object of investigations: a semiconductor gas sensor The report analyses semiconductor gas sensors made by a thick-film technology with screen printing. A sensor design was described and illustrated in [8,9], therefore, we will omit this issue in this paper. Here we will only mention the basic technological processes of forming particular gas-sensitive layers, which were used in investigations of different mixtures of air with the gases under analysis. It is worth mentioning, that the metrological and operation characteristics of sensors are determined both by chemical agents in use and by the process of manufacturing the powders for gas-sensitive layers. Therefore, initial chemical reagents and the process of powder manufacturing were selected mainly proceeding from a necessity of gaining batch-to-batch reproducible results. Sensors with three different structures of the gas-sensitive layer (based on pure SnO2 , on the SnO2 + 3% La2 O3 structure and on the SnO2 +3% Pd structure) were investigated in all the seven gases under analysis. Sensors with gas-sensitive layers of other structures were additionally used for investigations in gas mixtures of air with the same gas (except for hydrogen). The base of all the sensors was tin dioxide, SnO2 . Tin dioxide powders were made by chemical precipitation of tin sulphate from water solutions. After precipitation, the powders of SnO2 ·nH2 O were purified of SO4 2− ions with distilled water by decantation, followed by drying at the temperature of 180 ◦ C and calcination at 600 ◦ C for 2 h. Finally, the SnO2 powders were modified, depending on the designation, with various additions that have proven to be most suitable for detection of the gases under analysis. To produce the SnO2 + 3% Pd composition, metal Pd (3 mass%) was added to the SnO2 powder. Pd was precipitated onto the SnO2 surface from palladium chloride solution with the HCOONa assistance. Next, after chlorine ions had been washed out, the SnO2 :Pd powder was dried at 120 ◦ C and mixed up with organic terpineol- and ethylcellulosebased binder with resulting paste. The as-produced paste was then used for making gas-sensitive layers of the sensors. Ultimately, the paste replicas were dried at 120 ◦ C for 10–15 min and annealed in the conveyor electric furnace (DEK-840) at 750 ◦ C for 15 min. To improve the sensor selectivity to methane, a coating of catalytically active ␥-aluminium oxide was attached to the (SnO2 + 3%Pd)-based gas-sensitive layer of sensors as a filter. The surface of the aluminium oxide was pre-covered with palladium (up to 2.5 mass%) and platinum (up to 2.5 mass%) with the method of impregnation with solutions followed by thermal decomposition. The catalytic layer deposition and annealing processes were similar to the above-described operations of the main gas-sensitive layer formation. In order to produce a SnO2 + 3% La2 O3 structure, the SnO2 powder was impregnated with water solution of

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La(NO3 )3 . Further, the suspension was dried up and tempered at 600 ◦ C for 1 h. In order to produce a SnO2 + 3% La2 O3 + 1% Sb2 O5 structure, SnO2 ·nH2 O, SbCl3 was first introduced into tin sulphate solution and then precipitated. Then lanthanum oxide was added to the as-obtained SnO2 :Sb powder with the above described procedure. This structure was used in gas mixtures of air with carbon monoxide and ethanol. A gas-sensitive layer based on the SnO2 +MnCoO4 structure (in the mass ratio 90:10) was formed in sensors to be used in gas mixtures of air with ammonia. The layer was produced by addition of the synthesized powder of MnCoO4 to the previously obtained powder of SnO2 . Then the mixture of powders was averaged with agate glasses and balls in ethanol in a ball grinder for 1 h. After averaging, the composition was dried up at 120 ◦ C for 2 h and mixed with an organic binder with resulting paste. For production of the SnO2 + 1% CuO structure, which was to be used in gas mixtures of air with hydrogen sulphide, copper oxide was added by means of impregnation of the SnO2 powder with water solution of Cu(NO3 )2 with consequent drying and tempering at 600 ◦ C within 1 h. For manufacturing of heaters, a composite material was used based on ruthenium dioxide and platinum and possessing sufficiently high TCR (≈10−3 and 3 × 10−3 per degree, accordingly). Bonding pads were made of platinum-based conductive paste. As-prepared gas-sensitive layers and other components of the sensors were then sequentially assembled on a substrate made of 100% Al2 O3 ceramics (0.2 mm thick) by the method of stenciled printing and finally were annealed in a pipeline electric furnace in the atmosphere of air. The substrate splitting into chips was made by means of laser scribing. The chip size after scribing was 2.0 mm × 0.4 mm × 0.2 mm. Pin-outs, made of gold wire of Ø30 ␮m or platinum wire of Ø20 ␮m, were then attached to bonding pads of the chips by means of laying and consequent annealing of platinum conducting paste. The opposite ends of electric pin-outs were soldered to the glass panel leads with tin solder. In our investigations, we have made use of two modifications of sensor design. In an open modification, the chip was soldered to the leads of a standard 7-pin radio panel and was not covered with any casing. In a covered modification, the chip was soldered to the leads of a glass panel of Ø10.8 mm, which was tightly fit and hermetically packed in a protective metal casing with a gas-permeable explosion-proof metal grid at the opposite end face.

3. GDI and procedure for determination of dynamic parameters and sensitivity of sensors Papers [6,9] give a brief description of a GDI we used for our study of sensor sensitivity. However, this installation cannot be used directly for measurements of sensor quick-action. For this purpose, we had to modify both the

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GDI and the measurement procedure so as to ensure the following fundamental principles in compliance with the problem to be solved: 1. The installation is to provide the formation of gas concentration square pulses in the measuring chamber equipped with a sensor with the duration of the pulse rise and droop edges no longer than fractions of a second. 2. The volume of the measuring chamber and service line should be as small as possible, and the number of sensors for this reason cannot be more than 1. 3. With the purpose of attaining the maximum objectivity of sensor reading, a differential principle of comparison of its reading in a gas mixture of air with the gas under analysis against its reading in pure diluter gas (air) should be used. It is especially important on taking measurements in the range of microconcentrations from units of ppb to units of ppm for the gases always present in atmospheric air, such as methane (Catm = 2.2 ppm) and hydrogen (Catm = 0.5 ppb). Note that on preparing gas mixtures in the indicated range of concentrations, we may have to do with small additions to atmospheric concentrations of gases, i.e., Cgm = Cgas + Catm , where the total gas concentration in a gas mixture, a calculated value of gas concentration and the gas concentration in a cylinder with air are indicated in consequence. From the above reasoning it follows, that on the zero line of the GDI the air of the same composition should be used as on the gas mixture preparation. It is only under these conditions that we may gain the desirable effect from the admixture gas concentration. The above-mentioned criteria have made the underlying principles for the GDI design, whose schematic diagram is shown in Fig. 1. The installation is based on electron mass-flow controllers (MFCs) with measurement limits 1.0 cm3 /s (q1 ) and 2.5 cm3 /s (q2 , q3 , q4 ), mounted on four identical gas lines. Besides, all the lines are equipped with identical gas pressure stabilizers (GPSs), manometers (M) and electromagnetic stop valves (EMV). The designation of all these devices is to ensure the setting and stabilization of the gas flow-rate in the gas lines of the GDI at a preset level. In the course of our investigations, a gas mixture of air with the gas under analysis was supplied via lines 1 and 2, and lines 3 and 4 were always used for certificated pure air supply. We employed industrial verification gas mixtures of air with the gas under analysis or gas mixtures of air with the gases prepared on home laboratory gas-mixing installation in cylinders under pressure as initial gas mixtures. The calibration gas mixture, formed in the line of mixing gas flows q1 , q2 and q3 , and pure air (q4 ) were supplied to an electromagnetic gas flow switch (EGFS) and then were inlet in a preset sequence into the measuring chamber of volume ≈1 cm3 with one sensor. The concentration of the gases under analysis in the calibration gas mixture was determined by the dilution degree of initial gas mixtures with pure air

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Fig. 1. The principal scheme of GDI for investigations and determinations of dynamic parameters and sensitivity of semiconductor gas sensors to analysable gases into air.

and was calculated by the following formula: C=

C0 (q1 + q2 ) C0 (q1 + q2 ) = q1 + q 2 + q 3 Q

(1)

where q1 , q2 , q3 , q4 are gas flow-rates in lines 1, 2, 3 and 4, Q = q1 + q2 + q3 = q4 is a resultant gas flow, which was always sustained at the level of 2.5 cm3 /s, and C0 is concentration of the gas under analysis in the initial gas mixture. The maximum controllable dilution factor of the initial gas mixture was actually within 25–30. The peculiarity of the procedure applied for research and measurements of sensor parameters consisted in a feasibility of generating single pulses or a sequence of square pulses of concentration of the gas under analysis C(t) in the measuring chamber with a sensor, the amplitude of the pulses being equal to the concentration of the gas in the calibration gas mixture. Under the influence of these pulses, sensor responses were formed in the form of conductivity pulses σ(t), which were displayed on the monitor. It allowed us to receive a pure differential effect (with allowance for criterion 3) from the gas concentration effect on the sensor and simultaneously to estimate its dynamic parameters, i.e., ↑ ↓ time constants τ0.9 and τ0.9 . Gas concentration pulses were formed with the EGFS, the position of which controlled the delivery of pure air or calibration gas mixture into the measuring chamber with the sensor. It simulated the generation of concentration pulses of the gas under analysis. Time constant τ 0.9 was determined graphically from σ(t) plots with regard for a feasibility of the processes of concentration levelling on a change of the gas medium. Since the measuring chamber was placed immediately on the EGFS, the time delay of the gas concentration pulse edge before the pulse en-

tered the measuring chamber was ≤0.01 s at the gas flow of 2.5 cm3 /s, and the time of the gas flow front passing through the measuring chamber with the installed sensor was ≤0.4 s. From this moment on the character of changes of pulses σ(t) was determined exceptionally by the processes proceeding on the surface of the gas-sensitive layer of the sensor, i.e., by its own properties. The investigations were carried out under sensor thermal stabilization conditions, which are characterized by the constancy of heater resistance Rh and electric power Wh applied to the heater. The sensor heating up and measurement of its conductivity were made with a special 8-channel electron device, one of the channels of the device being connected to the sensor under study. The technological process was completely automated and proceeded under the IBM PC software developed for the process. The software on principle allowed us to change the duration of concentration pulses of the gas under analysis over a wide time span of tpulse ≥ 5 s without noticeable distortions of the sensor conductivity pulse and the time between adjacent points ␦t ≥ 0.16 s. In our investigations, the duration of the gas concentration pulse was taken equal to 100 s, and the measurement period equal to 300 s. At the same time it was assumed that 100 s is a sufficiently long period for the dynamics of the sensor (quick-action and sensor drift in a gas mixture) to manifest itself in full measure. The time interval between the adjacent points was taken equal to 1 s. It is self-evident, that before starting direct measurements after a temperature change, the sensor was seasoned for some time to stabilize its readings in pure air. The measurements were taken at different heating temperatures in the range from 300 to 600 ◦ C with a 50 ◦ C step. The choice of this range was by no means random. Below 300 ◦ C practical application of sensors is of no interest because of their very low quick-action, high instability of readings and high resistance of the gas-sensitive layer, while 600 ◦ C is the limiting temperature for the sensor based on tin dioxide to be operable for a long time without noticeable changes of the characteristics of the heater and the gas-sensitive layer. The procedure of taking measurements and processing of the experimental data is illustrated in Fig. 2, which shows a concentration pulse of the gas under analysis, C(t), and corresponding conductivity pulses of the sensor, σ(t). The waveform also shows three possible types of configuration of conductivity impulses, namely, slowly increasing σ 1 (t), rapidly increasing with the signal amplitude of σ 2 (t) and impetuously increasing with a consequent drop-down of indications, which is commonly referred to as “with nose”, σ 3 (t). All the pulses are shown in the “symbol-line” format and visualize the dynamic properties of the sensor both on the gas concentration increase and decrease. The time constant and the edges of the concentration pulse of the gas under analysis were not registered directly and were not measured. However, from the fact, that at temperatures above 500 ◦ C, as seen, e.g., from plots in Figs. 3, 5 and 6, the time constant of the sensor τ0.9 ≈ 0.9 s on the gas concentration

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configuration and its sensitivity to the gas under analysis. For example, on using a sensor in gas-control devices, it is pulse σ 2 (t) that most likely meets this criterion. This pulse features comparatively high quick-action, nearly an absence of sensor drift in a gas mixture and sufficient sensitivity to the gas under analysis. At the same time, for detection of fast varying processes it is better to use pulse σ 3 (t), though this may result in the sensor desensitization (possibly, noticeable) to the gas in question. To sum up, the suggested procedure of measuring and processing the experimental data permits rather prompt (within 1 h) determination of the basic parameters of a sensor (as well as of any other gas-sensitive element) and assessment on its basis of the applicability of the sensor to concrete service conditions.

4. Experimental

Fig. 2. Illustration of method of determination of quick-action (time constant τ 0.9 ) and sensitivity of semiconductor metal-oxide gas sensors.

increase and decrease, we may conclude, that the time constant of a gas pulse, which (according to physical laws) should always be lower than the identical parameter of a sensor, actually makes fractions of a second, which meets the above-mentioned criterion 1. It gives us good grounds to state, that concentration pulses of the gas under analysis are ↑ in fact nearly rectangular. Time constant τ0.9 was calculated in seconds by the number of points on the σ(t) plot from the beginning of the gas concentration pulse till the value of the signal reached 90% of the maximum of the sensor conduc↓ tivity pulse. Parameter τ0.9 was calculated in a similar way from the end of the sensor conductivity pulse to the value of 10% of the sensor reading in pure air. With the σ(t) plots we may also determine sensor sensitivity S, equal to the ratio of the maximum conductivity value in pulse σ(t) to the value of conductivity in pure air σ 0 before start of gas mixture, i.e., S = σmax /σ0 . The sensor sensitivity after the gas pulse, S∗ , is to be defined as a ratio of conductivity at the pulse end σ(t) to sensor conductivity in pure air in the steady state σ0∗ (one may use the minimum value over 300-s measurements for S∗ evaluation), i.e., S ∗ = σ/σ0∗ . This value coincides, by definition, with sensitivity S for two configurations of conductivity pulses σ 1 (t) and σ 2 (t). In case of the σ 3 (t) pulse, magnitude S∗ is noticeably smaller than S, and the difference S = S − S ∗ depends on the sensor drift in the gas mixture. Henceforth, we will disregard magnitude S∗ because of its indeterminacy. The optimal temperature regime of sensor operation, most suitable for specific service conditions, can be easily and visually determined from its conductivity pulse

In accordance with the above-described procedure, investigations have been performed of dynamic parameters and sensitivity of sensors with different gas-sensitive layer compositions to methane, hexane, hydrogen, carbon monoxide, ammonia, hydrogen sulphide and ethanol. Pure SnO2 -based sensors and those with additions of 3% La2 O3 and 3% Pd have been studied with all the seven gases. Besides, for some gases, we tried different structures of gas-sensitive layers. With the purpose of gaining the most objective data, we employed the same samples of sensors in all the studies. Before taking measurements, the samples were subjected to long-term blowing-off in pure air with each of the gases till the indications were recovered (on necessity) at practically the same level. The generally accepted opinion is that the sensor quickaction grows with the gas concentration increase; hence, it is low in the range of the gas microconcentrations. To check the validity of this statement, we have purposefully taken measurements in gas mixtures at two drastically distinguished values of concentrations, i.e., 20 ppm and 1 vol.% for methane (500-fold difference of concentration); 10 and 2000 ppm for hexane (200-fold difference); 100 ppb and 1000 ppm for hydrogen and ethanol (10,000-fold difference); 5 and 98 ppm for carbon monoxide (nearly 20-fold difference); 10 and 100 ppm for ammonia (10-fold difference); and 3 and 30 ppm for hydrogen sulphide (10-fold difference). As initial gas mixtures for our studies with methane and hydrogen in the high-concentration range, and with carbon monoxide, we made use of commercial verifying gas mixtures in cylinders under pressure of ≈100 atm, e.g., air mixed with methane with concentration of 2.06 vol.% (potential preparation accuracy 2% rel.), air mixed with hydrogen with concentration of 0.51 vol.% (6% rel.) and air mixed with carbon monoxide with concentration of 98 ppm (4% rel.) and 11 ppm (20% rel.). The initial gas mixtures of air with hydrogen and methane in the range of microconcentrations and mixtures with hexane, ammonia, hydrogen

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sulphide and ethanol were prepared on a home laboratory gas-mixing installation by the method of direct mixing of gas components in pressurized cylinders. The preparation process was implemented in several steps (usually in 1–3 steps, depending on the gas concentration). In experiments with methane and hydrogen, use was made of the above-said verifying gas mixtures. In experiments with the rest of the gases, we utilized pure gases. When preparing gas mixtures, pure air was used (verifying zero gas) at all the stages of dilution. This gas, according to criterion 3, was supplied to lines 3 and 4 of the GDI. Calculations of gas concentration at the n-the stage of dilution were made with the formula: Cn−1 Pi,n−1 Cn = P
n

(2)

where n is a serial number of the stage of dilution, Pi,n−1 is partial pressure of a gas mixture with gas concentration Cn−1 , produced at the previous stage of dilution, and P
n is total pressure of a gas mixture at the last n-stage of dilution. Gas mixtures of air with methane (with concentration 30.3 ppm) and air with hydrogen (220 ppb) were prepared for investigations in the range of microconcentrations of methane and hydrogen. The aggregate instrumental error of concentration in the above gas mixtures in view of an error of initial gas mixtures did not exceed 5 and 14% rel., accordingly. For our investigations the following gas mixtures were prepared: with concentrations 12 and 2500 ppm for experiments with hexane, with concentrations 11 and 120 ppm for ammonia, with concentrations 4 and 40 ppm for hydrogen sulphide and with concentrations 120 ppb and 1100 ppm for ethanol. The aggregate instrumental error of concentration in indicated gas mixtures was within 4% rel., and only a mixture of ethanol with air in concentration 120 ppb, prepared in three stage, gave an error of 6% rel. From the measurement results at two concentration values we evaluated exponent n in the known dependence of sensitivity on concentration and the sensitivity threshold of sensors σ − σ0 ≈ Cn or S − 1 ≈ Cn (3) σ0 The minimum change of sensor sensitivity for evaluation of the sensitivity threshold was taken 10% rel. This method of evaluation of parameter n, which is in fact a three point method including a zero value, with regard for a large distinction of gas concentration values, is expected to give the most objective information. The experimental data are presented for each gas under analysis in the form of conductivity pulse waveforms only for one gas-sensitive layer (which is the best from our viewpoint) at two above-mentioned concentration values. At the same time, the dependencies of dynamic parameters and sensitivity on the heating temperature are presented for all the types of layers under study for the same values of gas concentrations. Such data representation will allow us to avoid overburdening of our report with excessive infor-

mation, on the one hand, and on the other hand, will permit an immediate estimate and comparison of the characteristics of various gas-sensitive layers in each of the gases under analysis. The results of exponent n calculations in formula (3) and of the sensitivity threshold are reported on in separate Section 4.8 for all of the seven gases and for the sensors most suitable for their detection. 4.1. Sensor dynamic parameters and sensitivity to methane In the course of our investigations of gas mixtures of air with methane, we employed sensors with four types of the gas-sensitive layer, namely: based on pure SnO2 , of SnO2 + 3% La2 O3 composition, of SnO2 + 3% Pd composition, and of SnO2 + 3% Pd composition with a catalytic Al2 O3 layer. The sensors with gas-sensitive layer of structure 1 and 2 had an open design, sensors of structure 3 were made both in open and covered modifications, sensors with catalytic layer structure 4 were made only in the covered modification. Fig. 3 shows the conductivity pulse waveforms for the best (from our point of view) of the sensors of the fourth structural type, and Figs. 4–6 plot the dependencies of sensor sensitivity to methane, S, and of time constants,

Fig. 3. The conductivity pulse waveforms of sensor based on structure the SnO2 + 3% Pd with catalytic layer from Al2 O3 under influence of pulses of methane concentration 20 ppm and 1 vol.% into air depending on temperatures of heating.

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Fig. 4. The sensitivity of sensors with various gas-sensitivity layers to methane at its concentration 20 ppm and 1 vol.% into air depending on temperatures of heating. ↑

↓

τ0.9 and τ0.9 , on heating temperature for all the investigated types of a gas-sensitive layer. Waveforms and all the dependencies are given for two values of methane concentration in air, i.e., 20 ppm and 1 vol.%. As seen from plots in Figs. 4–6, the sensitivity to methane is of clearly extremal character, and the dynamic parameters

419

Fig. 5. The time constant of sensors with various gas-sensitivity layers at increase of methane concentration 20 ppm and 1 vol.% into air depending on temperatures of heating.

manifest a sharply drooping behaviour with stabilization at the minimum level at temperatures above 450–500 ◦ C, irrespective of the methane concentration. The maximum sensitivity values and the minimum values of dynamic parameters are summarized in Table 1 with indication of heating temperature.

Table 1 Sensor dynamic parameters and sensitivity to methane Sensor

Parameter

C = 20 ppm

C = 1 vol.%

SnO2 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

2.2 at T = 520 ◦ C 1 at T ≥ 450 ◦ C 1 at T ≥ 500 ◦ C

16 at T = 470 ◦ C 0.9–1 at T ≥ 450 ◦ C 0.9–1 at T ≥ 450 ◦ C

SnO2 + 3% La2 O3 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

2.2 at T = 400 ◦ C 0.9–1 at T ≥ 500 ◦ C 1–3 at T ≥ 500 ◦ C

26 at T = 410 ◦ C 0.9–1 at T ≥ 400 ◦ C 0.9–1 at T ≥ 500 ◦ C

SnO2 + 3% Pd open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

2.2 at T = 400 ◦ C 0.9–1 at T ≥ 500 ◦ C 1–3 at T ≥ 500 ◦ C

26 at T = 410 ◦ C 0.9–1 at T ≥ 400 ◦ C 0.9–1 at T ≥ 500 ◦ C

SnO2 + 3% Pd + Al2 O3 (catalyst) covered modification

Smax τ0.9 ↑ s τ0.9 ↓ s

2.1 at T = 370 ◦ C 2–4 at T ≥ 550 ◦ C 4 at T ≥ 550 ◦ C

53 at T = 350 ◦ C 1–1.5 at T ≥ 500 ◦ C 3 at T ≥ 400 ◦ C

SnO2 + 3% Pd covered modification

Smax τ0.9 ↑ s τ0.9 ↓ s

1.7 at T = 440 ◦ C 3–5 at T ≥ 550 ◦ C 6–7 at T ≥ 550 ◦ C

14.5 at T = 410 ◦ C 1–1.5 at T ≥ 500 ◦ C 3–4 at T ≥ 450 ◦ C

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of 1.8–2 at C = 20 ppm and 20–30 at C = 1 vol.% and, what is of great significance, it has the minimal sensor drift on detecting both high concentrations and microconcentrations. These conclusions are in a good agreement with the results of Ref. [8], where other factors and arguments were used for determination of the optimum regime. 4.2. Sensor dynamic parameters and sensitivity to hexane

Fig. 6. The time constant of sensors with various gas-sensitivity layers at decrease of methane concentration 20 ppm and 1 vol.% into air depending on temperatures of heating.

From the data adduced it follows, that the maximum sensitivity to methane is inherent in sensors based on the SnO2 + 3% Pd structure with a catalytic Al2 O3 layer, while the temperature range of the maximum is practically independent of the methane concentration in air. All the sensors under study demonstrate higher quick-action on the methane concentration rise than on its decrease. The quick-action proper, in contradiction to earlier suppositions, is virtually independent of the methane concentration. This statement holds true over the temperature range 500–550 ◦ C. As the temperature goes down, the sensor quick-action is observed to decrease rather sharply, especially on detection of microconcentrations. It is important to note, that chip covering results in the increase of dynamic parameters by approximately 2–3 times, and it produces the most pronounced effect on the concentration fall-down. The service characteristics of sensors are determined by a combination of parameters with regard for an sensor drift in a gas mixture of air with methane rather than by individual parameters, which manifests itself clearly on the waveforms in Fig. 3. Proceeding from this circumstance, we may state, that the best sensor of those under study is a sensor based on structure SnO2 + 3% Pd with a catalytic Al2 O3 layer operating in the temperature range 450–500 ◦ C. This sensor, made in a covered modification, has quick-action of 2–4 s on both edges of the gas concentration pulse, sensitivity at the level

In the course of our investigations in gas mixtures of air with hexane, we employed sensors with four types of the gas-sensitive layer, namely: based on pure SnO2 , of SnO2 + 3% La2 O3 composition, of SnO2 + 3% Pd composition, and of SnO2 + 3% Pd composition with a catalytic Al2 O3 layer. The sensors with a gas-sensitive layer of structures 1–3 were made in an open modification and sensors with a catalytic layer were made only in the covered modification. Fig. 7 shows the conductivity pulse waveforms for the best (from our point of view) of the sensors of structural type SnO2 + 3% La2 O3 , and Figs. 8–10 plot the dependencies of sensor sensitivity to hexane, S, and of time ↑ ↓ constants, τ0.9 and τ0.9 , on heating temperature for all the investigated types of a gas-sensitive layer. Waveforms and all the dependencies are given for two values of hexane concentration in air, i.e., 10 and 2000 ppm. As seen from plots in Fig. 8, the sensitivity to hexane for three compositions is of weakly expressed extremal

Fig. 7. The conductivity pulse waveforms of sensor based on the structure of SnO2 + 3% La2 O3 under influence of pulses of hexane concentration 10 and 2000 ppm into air depending on temperatures of heating.

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Fig. 8. The sensitivity of sensors with various gas-sensitivity layers to hexane at its concentrations 10 and 2000 ppm into air depending on temperatures of heating.

Fig. 9. The time constant of sensors with various gas-sensitivity layers at increase of hexane concentrations 10 and 2000 ppm into air depending on temperatures of heating.

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Fig. 10. The time constant of sensors with various gas-sensitivity layers at decrease of hexane concentrations 10 and 2000 ppm into air depending on temperatures of heating.

character, and for structure SnO2 + 3% Pd it is of drooping character. Notice should be taken of the fact that at temperatures above 500 ◦ C a sensor based on structure SnO2 + 3% Pd (without a catalytic layer) practically ceases to respond to hexane. The dynamic parameters of the sensors have a sharply drooping behaviour on the temperature increase (except for a SnO2 + 3% Pd-based sensor) with stabilization at the minimum level at temperatures above 500 ◦ C, irrespective of the hexane concentration (Figs. 9 and 10). The values of the maximum sensitivity (if available) and the minimum values of dynamic parameters are summarized in Table 2. From the data of Table 2 it follows, that the maximum sensitivity to hexane is inherent in sensors based on pure SnO2 and on the SnO2 + 3% La2 O3 structure, while the temperature range of the maximum is practically independent of the hexane concentration in air. It is worth noting, that all the sensors under study (except for the sensor based on structure SnO2 +3% La2 O3 ) manifested the quick-action reduction on the average by 2–4 times as the hexane concentration changed from 10 to 2000 ppm. This situation is diametrically opposite to the tendencies in methane–air gas mixtures [8]. This statement holds true over the temperature range 500–550 ◦ C. As the temperature goes down, the sensor quick-action is observed to decrease rather sharply for sensors based on pure SnO2 and on the SnO2 + 3% La2 O3 structure, especially on detection of high concentration.

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Table 2 Sensor dynamic parameters and sensitivity to hexane Sensor

Parameter

C = 10 ppm

C = 2000 ppm

SnO2 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

2.7 at T = 6–7 at T ≥ 550 ◦ C ≤1 at T ≥ 450 ◦ C

27 at T = 450–500 ◦ C 1–2 at T ≥ 550 ◦ C ≈2 at T ≥ 550 ◦ C

SnO2 + 3% La2 O3 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

3.2 at T = 500–550 ◦ C ≤1 at T ≥ 500 ◦ C 1–1.5 at T ≥ 550 ◦ C

17.4 at T = 550 ◦ C ≤1 at T ≥ 550 ◦ C 3–5 at T ≥ 500 ◦ C

SnO2 + 3% Pd open modification

S τ0.9 ↑ s τ0.9 ↓ s

2.5 (300 ◦ C) → 1.2 (450 ◦ C) 7–8 at T ≥ 350 ◦ C 4–5 at T ≥ 350 ◦ C

Smax = 35 at T = 350 ◦ C 1–1.5 at T ≥ 350 ◦ C 2.5–3.5 at T ≥ 350 ◦ C

SnO2 + 3% Pd + Al2 O3 (catalyst) covered modification

S τ0.9 ↑ s τ0.9 ↓ s

1.9 (300 ◦ C) → 1.2 (600 ◦ C) 3–5 at T ≥ 500 ◦ C 6–10 at T ≥ 500 ◦ C

11.5 (400 ◦ C) → 2.2 (600 ◦ C) 1–2 at T ≥ 550 ◦ C 6–8 at T ≥ 400 ◦ C

With all the sensor parameters taken together with regard for an sensor drift in a gas mixture of air with hexane (Fig. 7), we may state, that the best sensors of those under study are the sensors based on pure SnO2 and on structure SnO2 +3% La2 O3 operating in the temperature range 400–550 ◦ C. However, the preference should be given to the SnO2 + 3% La2 O3 -based sensor, since it features an essentially higher quick-action. This sensor has fairly high sensitivity to hexane at both values of concentration, and high sensitivity in the microconcentration range; it has the highest quick-action of <1 s on the concentration increase and of 1–5 s on the concentration fall-down. Finally, and this is of great significance, it has the minimal sensor drift on detecting both high concentrations and microconcentrations. The sensor sensitivity at 500 ◦ C makes ≈3.2 at 10 ppm and ≈17.1 at 2000 ppm.

500 ◦ C

of the hydrogen concentration. The values of the maximum sensitivity and the minimum values of dynamic parameters are summarized in Table 3 with indication of the heating temperature. From the data of Table 3 it follows that the maximum sensitivity to hydrogen is inherent in sensors based on the SnO2 + 3% Pd structure in the low-temperature range and in sensors based on the SnO2 + 3% La2 O3 structure in the high-temperature range. This phenomenon manifests itself mainly under high concentration of hydrogen, while in

4.3. Sensor dynamic parameters and sensitivity to hydrogen In the course of our investigations of gas mixtures of air with hydrogen, we employed sensors with three types of the gas-sensitive layer, namely: based on pure SnO2 , of SnO2 +3% La2 O3 composition and of SnO2 +3% Pd composition. The sensors with a gas-sensitive layer of structures 1 and 2 were made in an open modification and sensors with a gas-sensitive layer of structure 3 were made both in the open and covered modifications. Fig. 11 shows the conductivity pulse waveforms for the best (from our point of view) of the sensors of structural type SnO2 +3% La2 O3 , and Figs. 12–14 plot the dependencies of sensor sensitivity to hydrogen, S, ↑ ↓ and of time constants, τ0.9 and τ0.9 , on heating temperature for all the investigated types of a gas-sensitive layer. Waveforms and all the dependencies are given for two values of hydrogen concentration in air, i.e., 100 ppb and 1000 ppm. From the plots in Figs. 12–14 it can be seen that the sensitivity to hydrogen is of clearly extremal character at 1000 ppm, and the dynamic parameters of the sensors have a sharply drooping behaviour with stabilization at the minimum level at temperatures above 450–500 ◦ C, irrespective

Fig. 11. The conductivity pulse waveforms of sensor based on structure SnO2 + 3% La2 O3 under influence of pulses of hydrogen concentrations 100 ppb and 1000 ppm into air depending on temperatures of heating.

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8

SnO2- open SnO2+ 3% La2O3 - open SnO2+ 3% Pd - open SnO2+ 3% Pd - casing

H2

7

150

125

100

6 sensitivity to hydrogen (S =σ/σ 0 )

423

75

1000 ppm

5

50 4 25 3 0 2 -25 1

100 ppb -50 300

350

400

450

500

550

600

Fig. 13. The time constant of sensors with various gas-sensitivity layers at increase of hydrogen concentrations 100 ppb and 1000 ppm into air depending on temperatures of heating.

0

temperature of sensors ( C) Fig. 12. The sensitivity of sensors with various gas-sensitivity layers to hydrogen at its concentrations 100 ppb and 1000 ppm into air depending on temperatures of heating.

the range of microconcentrations, the sensitivity goes down monotonically with the temperature growth. In the range of microconcentrations, all the sensors under study manifest higher quick-action on the hydrogen concentration rise than on its falling down. At the same time, on higher concentrations the quick-action on both edges of the hydrogen

concentration pulse is equal and has the minimum possible value of τ0.9 = 0.9–1 s, irrespective of the open or covered modification of the chip. The quick-action proper is virtually independent of the hydrogen concentration, except for pure SnO2 -based sensors, as is the case with methane. This statement holds true over the temperature range 500–550 ◦ C. As the temperature goes down, the sensor quick-action is observed to decrease rather sharply, especially on detection of microconcentrations. It is important to note, that chip covering results in the increase of dynamic parameters in the

Table 3 Sensor dynamic parameters and sensitivity to hydrogen Sensor

Parameter

C = 100 ppb

C = 1000 ppm

SnO2 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

1.6 at T = 3–5 at T ≥ 500 ◦ C 9–10 at T ≥ 500 ◦ C

50 at T = 370 ◦ C 0.9–1 at T ≥ 400 ◦ C 0.9–1 at T ≥ 400 ◦ C

SnO2 + 3% La2 O3 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

2.2 at T = 400 ◦ C 0.9–1 at T ≥ 500 ◦ C 0.9–1 at T ≥ 450 ◦ C

62 at T = 430 ◦ C 0.9–1 at T ≥ 450 ◦ C 0.9–1 at T ≥ 400 ◦ C

SnO2 + 3% Pd open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

1.7 at T = 450 ◦ Ca 0.9–1 at T ≥ 450 ◦ C 1–3 at T ≥ 500 ◦ C

140 at T = 350 ◦ C 0.9–1 at T ≥ 400 ◦ C 0.9–1 at T ≥ 400 ◦ C

SnO2 + 3% Pd covered modification

Smax τ0.9 ↑ s τ0.9 ↓ s

1.7 at T = 450 ◦ Ca 2–5 at T ≥ 500 ◦ C 9–12 at T ≥ 500 ◦ C

70 at T = 350 ◦ C 0.9–1 at T ≥ 400 ◦ C 0.9–1 at T ≥ 400 ◦ C

a

The value is not at the maximum, but at specified temperature.

370 ◦ Ca

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Fig. 14. The time constant of sensors with various gas-sensitivity layers at decrease of hydrogen concentrations 100 ppb and 1000 ppm into air depending on temperatures of heating.

region of high hydrogen concentrations, and on detection of microconcentrations, they increase by approximately 2–3 times, this effect being most pronounced on the concentration decrease. With all the sensor parameters taken together with regard for an sensor drift in a gas mixture of air with hydrogen (Fig. 11), we may state that the best sensor of those under study is the sensor based on structure SnO2 + 3% La2 O3 operating in the temperature range 400–550 ◦ C. This sensor, made in the open modification, has equally high quick-action of τ0.9 = 0.9–1 s on both edges of the gas concentration pulse in the high-concentration range and in the microconcentration range. The sensor sensitivity is ≈1.6 at C = 100 ppb and 50–60 at C = 1000 ppm. 4.4. Sensor dynamic parameters and sensitivity to carbon monoxide In the course of our investigations of gas mixtures of air with carbon monoxide (CO), we employed sensors with four types of the gas-sensitive layer, namely: based on pure SnO2 , of SnO2 + 3% La2 O3 composition, of SnO2 + 3% Pd composition and of SnO2 + 3% La2 O3 + 1% Sb2 O5 composition. All the sensors were in the open modification. Fig. 15 shows the conductivity pulse waveforms for the best (from our point of view) of the sensors of structural type SnO2 + 3% La2 O3 + 1% Sb2 O5 , and Figs. 16–18 plot the

Fig. 15. The conductivity pulse waveforms of sensor based on structure SnO2 + 3% La2 O3 + 1% Sb2 O5 under influence of pulses of carbon monoxide concentrations 5 and 98 ppm into air depending on temperatures of heating.

dependencies of sensor sensitivity to carbon monoxide, S, ↑ ↓ and of time constants, τ0.9 and τ0.9 , on heating temperature for all the investigated types of a gas-sensitive layer. Waveforms and all the dependencies are given for two values of carbon monoxide concentration in air, i.e., 5 and 98 ppm. From the plots in Figs. 16–18 it can be seen that the sensitivity to CO is of clearly extremal character for sensors with La and Sb admixtures, and the dynamic parameters of the sensors have a sharply drooping behaviour with stabilization at temperatures above 450–500 ◦ C, irrespective of the carbon monoxide concentration. The values of the maximum sensitivity and the minimum values of dynamic parameters are summarized in Table 4 with indication of the heating temperature. From the data of Table 4 it follows that the maximum sensitivity to CO is inherent in sensors based on pure SnO2 in the range of moderate temperatures and in sensors based on the SnO2 + 3% La2 O3 and SnO2 + 3% La2 O3 + 1% Sb2 O5 structures in the high-temperature range. This phenomenon manifests itself both under high concentration of CO and in the range of microconcentrations. All the sensors under study manifest high quick-action of τ0.9 = 0.9–1 s on both edges of the CO concentration pulse, irrespective of the gas concentration. This statement holds true at temperatures above 500 ◦ C. As the temperature goes down, the

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Fig. 16. The sensitivity of sensors with various gas-sensitivity layers to carbon monoxide at its concentrations 5 and 98 ppm into air depending on temperatures of heating.

Fig. 17. The time constant of sensors with various gas-sensitivity layers at increase of carbon monoxide concentrations 5 and 98 ppm into air depending on temperatures of heating.

sensor quick-action is observed to decrease rather sharply, especially on detection of microconcentrations. With all the sensor parameters taken together with regard for an sensor drift in a gas mixture of air with carbon monoxide (Fig. 15), we may state, that the best sensor of those under study is the sensor based on structure SnO2 +3% La2 O3 +1% Sb2 O5 operating in the temperature range 450–550 ◦ C. This sensor has a slightly lower quick-action at the level of ∼1–2 s on both edges of the gas concentration pulse as compared to the sensors of other types, but it has a much smaller sen-

sor drift. The sensitivity of the sensor makes 1.7–1.8 at C = 5 ppm and 5–6.5 at C = 98 ppm. 4.5. Sensor dynamic parameters and sensitivity to ammonia In the course of our investigations of gas mixtures of air with ammonia (NH3 ), we employed sensors with four types of the gas-sensitive layer, namely: based on pure SnO2 , of SnO2 + 3% La2 O3 composition, of SnO2 + 3% Pd

Table 4 Sensor dynamic parameters and sensitivity to carbon monoxide Sensor

Parameter

C = 5 ppm

C = 98 ppm 360 ◦ C

SnO2 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

2.1 at T = 0.9–1 at T ≥ 450 ◦ C 1–2 at T ≥ 450 ◦ C

12 at T = 360 ◦ C 0.9–1 at T ≥ 450 ◦ C 0.9–1 at T ≥ 500 ◦ C

SnO2 + 3% La2 O3 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

2.1 at T = 400 ◦ C 0.9–1 at T ≥ 450 ◦ C 0.9–1 at T ≥ 500 ◦ C

7 at T = 400 ◦ C 0.9–1 at T ≥ 500 ◦ C 0.9–1 at T ≥ 500 ◦ C

SnO2 + 3% Pd open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

1.8 at T = 450 ◦ C 2–4 at T ≥ 500 ◦ C 1–2 at T ≥ 550 ◦ C

6.5 at T = 440 ◦ C 1–2 at T ≥ 450 ◦ C 1–2 at T ≥ 450 ◦ C

SnO2 + 3% La2 O3 + 1% Sb2 O5 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

1.8 at T = 450 ◦ C 2–4 at T ≥ 500 ◦ C 1–2 at T ≥ 550 ◦ C

6.5 at T = 440 ◦ C 1–2 at T ≥ 450 ◦ C 1–2 at T ≥ 450 ◦ C

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Fig. 18. The time constant of sensors with various gas-sensitivity layers at decrease of carbon monoxide concentrations 5 and 98 ppm into air depending on temperatures of heating.

composition and of SnO2 + MnCoO4 (90:10) composition. All the sensors were made in the open modification. Fig. 19 shows the conductivity pulse waveforms for the best (from our point of view) of the sensors of structural type SnO2 + MnCoO4 (90:10), and Figs. 20–22 plot the dependencies of sensor sensitivity to ammonia, ↑ ↓ S, and of time constants, τ0.9 and τ0.9 , on heating temperature for all the investigated types of a gas-sensitive layer. Waveforms and all the dependencies are given for two values of ammonia concentration in air, i.e., 10 and 100 ppm. As seen from plots in Fig. 20, the sensitivity to ammonia is of extremal character for almost all of the sensors, especially pronounced at C = 100 ppm. At C = 10 ppm, the zone of the maximum extends and shifts to the lower temperature range. Only the SnO2 + 3% Pd-based sensors manifested a monotonous falling down of sensitivity with the temperature rise. The dynamic parameter behaviour patterns as a function of temperature were the same as for other gases (Figs. 21 and 22). On the leading edge of the gas pulse, quick-action is observed to decrease with the temperature drop although this tendency is less explicit. On the contrary, as the gas concentration diminishes, the time constant increases drastically with the temperature drop, so that at temperatures below 400–450 ◦ C this parameter cannot be calculated under experimental conditions. At temperatures above 500 ◦ C, the dynamic parameters reach

Fig. 19. The conductivity pulse waveforms of sensor based on SnO2 + MnCoO4 (90:10) under the influence of pulses of ammonia concentrations 10 and 100 ppm into air depending on temperatures of heating.

their minimum stable value irrespective of the ammonia concentration. The maximum values of sensitivity and its values at the extreme points of the range as well as the minimum values of dynamic parameters are summarized in Table 5. From the data of Table 5 it follows that the maximum sensitivity to NH3 is inherent in sensors based on pure SnO2 in the temperature range 400–450 ◦ C and in sensors based on the SnO2 + 3% La2 O3 composition at temperatures above 500 ◦ C (for C = 10 ppm). The best quick-action was manifested by the sensors on the basis of structures SnO2 + 3% La2 O3 and SnO2 + MnCoO4 (90:10). The time constant of these sensors is at the level of τ0.9 = 0.9–3 s (at T > 500 ◦ C) on both edges of the NH3 concentration pulse irrespective of the gas concentration. With all the sensor parameters taken together with regard for an sensor drift in a gas mixture of air with NH3 (Fig. 19), we may state that the best sensor of those under study is the sensor based on structure SnO2 + MnCoO4 (90:10) operating in the temperature range 500–550 ◦ C. This sensor have high quick-action (τ0.9 = 0.9–3 s) on both edges of the gas concentration pulse, good sensitivity to ammonia and much smaller sensor drift as compared to the sensors with the gas-sensitive layer of other types. The sensitivity of this device is 2.0 at C = 10 ppm and 5.6 at C = 100 ppm.

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Table 5 Sensor dynamic parameters and sensitivity to ammonia Sensor

Parameter

C = 10 ppm 400 ◦ C

C = 100 ppm

SnO2 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

4.5 at T = 1–1.5 at T ≥ 500 ◦ C 4–5 at T = 450–550 ◦ C

8.3 at T = 450 ◦ C 1–2 at T ≥ 450 ◦ C 1–3 at T ≥ 500 ◦ C

SnO2 + 3% La2 O3 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

2.1 at T = 400 ◦ C 1–2 at T ≥ 450 ◦ C 3–6 at T ≥ 500 ◦ C

5.5 at T = 550 ◦ C 0.9–1 at T ≥ 500 ◦ C 0.9–1 at T ≥ 500 ◦ C

SnO2 + 3% Pd open modification

S τ0.9 ↑ s τ0.9 ↓ s

Smax = 5.9 at T = 350 ◦ C 0.9–2 at T ≥ 450 ◦ C 8–11 at T ≥ 350 ◦ C

14 (350 ◦ C) → 1.3 (600 ◦ C) 1–2 at T ≥ 400 ◦ C 1–2 at T ≥ 450 ◦ C

SnO2 + MnCoO4 open modification

Smax τ0.9 ↑ s τ0.9 ↓ s

2.3 at T = 400 ◦ C 1–2 at T ≥ 500 ◦ C 2.5–4 at T ≥ 500 ◦ C

5.9 at T = 450 ◦ C ≤ 1 at T ≥ 550 ◦ C 1–2 at T ≥ 500 ◦ C

In the course of our investigations of gas mixtures of air with hydrogen sulphide (H2 S), we employed sensors with four types of the gas-sensitive layer, namely: based on pure SnO2 , of SnO2 + 3% La2 O3 composition, of SnO2 + 3% Pd composition and of SnO2 + 1% CuO composition. The latter type of the sensors was made in the covered modification. Fig. 23 shows the conductivity pulse waveforms for

the best (from our point of view) of the sensors of structural type SnO2 + 3% La2 O3 , and Figs. 24–26 plot the dependencies of sensor sensitivity to hydrogen sulphide, S, and of ↑ ↓ time constants, τ0.9 and τ0.9 , on heating temperature for all the investigated types of a gas-sensitive layer. Waveforms and all the dependencies are given for two values of H2 S concentration in air, i.e., 3 and 30 ppm. It should be stressed out, that the investigations of gas mixtures of air with hydrogen sulphide have appeared the most laborious in comparison with the rest of the gases. We

Fig. 20. The sensitivity of sensors with various gas-sensitivity layers to ammonia at its concentrations 10 and 100 ppm into air depending on temperatures of heating.

Fig. 21. The time constant of sensors with various gas-sensitivity layers at increase of ammonia concentrations 10 and 100 ppm into air depending on temperatures of heating.

4.6. Sensor dynamic parameters and sensitivity to hydrogen sulphide

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Fig. 22. The time constant of sensors with various gas-sensitivity layers at decrease of ammonia concentrations 10 and 100 ppm into air depending on temperatures of heating.

Fig. 23. The conductivity pulse waveforms of sensor based on structure SnO2 + 3% La2 O3 under influence of pulses of hydrogen sulphide concentrations 3 and 30 ppm into air depending on temperatures of heating.

Fig. 24. The sensitivity of sensors with various gas-sensitivity layers to hydrogen sulphide at its concentrations 3 and 30 ppm into air depending on temperatures of heating.

had to subject the sensors to long-term training at different gas concentrations and heating temperatures [9], before we succeeded in gaining an admissible sensor drift and, accordingly, reliable measurements of sensitivity and time constants of the sensors, although not over the complete temperature range, but only at temperatures >400–450 ◦ C. At lower temperatures, the numerical values of parameters are of the benchmark character because of noticeable sensor drift. The values of sensor sensitivity to hydrogen sulphide are plotted in Fig. 24. As seen from the plot, the sensitivity of the sensors (except for sensors based on pure SnO2 and on the SnO2 + 3% La2 O3 composition) manifests a drooping behaviour on the temperature increase. The dynamic parameters of the sensors, similarly to those of other gases, have a sharply drooping pattern with stabilization at the minimum level at temperatures above 500–550 ◦ C, irrespective of the hydrogen sulphide concentration (Figs. 25 and 26). The one exception is SnO2 + 1% CuO-based sensors, where no stabilization of parameters has been observed. The values of the maximum sensitivity, or its values in extreme points of the range, and the minimum values of dynamic parameters are summarized in Table 6. From the data of Table 6 it follows that the maximum sensitivity to hydrogen sulphide is inherent in sensors based on SnO2 +3% La2 O3 and SnO2 +1% CuO compositions at temperatures above 500 ◦ C, irrespective of the gas concentration.

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Table 6 Sensor dynamic parameters and sensitivity to hydrogen sulphide Sensor

Parameter

C = 3 ppm (400 ◦ C)

C = 30 ppm (600 ◦ C)

2.5 (500 ◦ C) → 1.6 (600 ◦ C) 2–4 at T ≥ 550 ◦ C 3–4 at T ≥ 550 ◦ C

SnO2 open modification

S τ0.9 ↑ s τ0.9 ↓ s

2.2 → 1.3 2–3 at T ≥ 500 ◦ C 2–6 at T ≥ 450 ◦ C

SnO2 + 3% La2 O3 open modification

S τ0.9 ↑ s τ0.9 ↓ s

Smax = 2.0 at T = 450 ◦ C 1.5–4 at T ≥ 500 ◦ C 1.5–5 at T ≥ 550 ◦ C

3.5 (450 ◦ C) → 2.1 (600 ◦ C) 1–4 at T ≥ 500 ◦ C 2–5 at T ≥ 500 ◦ C

SnO2 + 3% Pd open modification

S τ0.9 ↑ s τ0.9 ↓ s

2.1 (350 ◦ C) → 1.2 (600 ◦ C) 2–5 at T ≥ 450 ◦ C 3–5 at T ≥ 450 ◦ C

2.3 (400 ◦ C) → 1.2 (600 ◦ C) 2–3.5 at T ≥ 450 ◦ C 3–4.5 at T ≥ 450 ◦ C

SnO2 + 1% CuO covered modification

S τ0.9 ↑ s τ0.9 ↓ s

1.7 (500 ◦ C) → 1.6 (600 ◦ C) 10–18 at T ≥ 550 ◦ C ≈30 at T ≥ 500 ◦ C

3.1 (500 ◦ C) → 2.2 (600 ◦ C) 9–11 at T ≥ 550 ◦ C 9–25 at T ≥ 550 ◦ C

All the sensors under study (except for structure SnO2 + 1% CuO) possess approximately identical quick-action at the level of τ0.9 = 1.5–5 s on both edges of the H2 S concentration pulse irrespective of the gas concentration. It is valid for temperatures above 500–600 ◦ C, while an abrupt slowdown of quick-action is observable on the lowering of temperature. With all the sensor parameters taken together with regard for an sensor drift in a gas mixture of air with H2 S (Fig. 23), we may state that the best sensor of those under study is

the sensor based on structure SnO2 + 3% La2 O3 operating at temperatures above 500 ◦ C. This sensor has sufficiently high quick-action (τ0.9 = 1.5–4 s) on both edges of the gas concentration pulse on high concentrations and on microconcentrations and its sensitivity to hydrogen sulphide (at T = 500 ◦ C) makes S ≈ 1.8 at C = 3 ppm and 3.0 at C = 30 ppm. The sensor based on SnO2 + 1% CuO features the least sensor drift and nearly the same sensitivity to hydrogen sulphide, but, as follows from Table 6, its quick-action is by 5–7 times worse.

Fig. 25. The time constant of sensors with various gas-sensitivity layers at increase of hydrogen sulphide concentrations 3 and 30 ppm into air depending on temperatures of heating.

Fig. 26. The time constant of sensors with various gas-sensitivity layers at decrease of hydrogen sulphide concentrations 3 and 30 ppm into air depending on temperatures of heating.

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Fig. 27. The conductivity pulse waveforms of sensor based on pure SnO2 under influence of pulses of ethanol concentrations 100 ppb and 1000 ppm into air depending on temperatures of heating.

4.7. Sensor dynamic parameters and sensitivity to ethanol In the course of our investigations of gas mixtures of air with ethanol, we have made use of sensors with four types of the gas-sensitive layer, namely: based on pure SnO2 , of SnO2 + 3% La2 O3 composition, of SnO2 + 3% Pd composition and of SnO2 + 3% La2 O3 + 1% Sb2 O5 composition. All the sensors were made in the open modification. Fig. 27 shows the conductivity pulse waveforms for the best (from our point of view) of the sensors on the basis of pure SnO2 , and Figs. 28–30 plot the dependencies of sensor sensitivity ↑ ↓ to ethanol, S, and of time constants, τ0.9 and τ0.9 , on heating temperature for all the investigated types of a gas-sensitive layer. Waveforms and all the dependencies are given for two values of ethanol concentration in air, i.e., 100 ppb and 1000 ppm. From the plots in Fig. 28 it can be seen that the sensitivity to ethanol of the sensors under study (except for a sensor based on structure SnO2 + 3% Pd) is of clearly extremal character over the temperature range 350–400 ◦ C at C = 1000 ppm, while at C = 100 ppb the temperature dependence of sensitivity has a drooping character for all the sensors. A sharp increase of quick-action has been observed at the temperature increase for all the sensors, irrespective of the ethanol concentration, followed by stabilization of their

Fig. 28. The sensitivity of sensors with various gas-sensitivity layers to ethanol at its concentration 100 ppb and 1000 ppm into air depending on temperatures of heating.

dynamic parameters at the minimum at temperatures above 500 ◦ C (Figs. 29 and 30). The values of the maximum sensitivity, or its values in extreme points of the range, and the minimum values of dynamic parameters are summarized in Table 7. From the data of Table 7 it follows that the maximum sensitivity to ethanol is inherent in sensors based on SnO2 +3% La2 O3 and SnO2 + 3% La2 O3 + 1% Sb2 O5 compositions with retention of the tendency irrespective of the gas concentration over the whole temperature range under analysis. All the sensors under study possess high quick-action at the level of τ0.9 = 0.9–2 s (at T > 500 ◦ C) on both edges of the ethanol concentration pulse irrespective of the gas concentration. A rather abrupt slowdown of quick-action has been observed on the lowering of temperature, especially at temperatures below 400–450 ◦ C. With all the sensor parameters taken together with regard for an sensor drift in a gas mixture of air with ethanol (Fig. 27), we may state that the best sensor of those under study is the sensor based on pure SnO2 operating at temperatures above 500 ◦ C, although it has lower sensitivity to ethanol in comparison with other sensors. This sensor has equally high quick-action (τ0.9 =0.9–1 s) on both edges of the gas pulse in the microconcentration and high-concentration ranges, and its sensitivity at 500 ◦ C makes S ≈ 1.7 at C = 100 ppb and S ≈ 13.2 at C = 1000 ppm.

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431

Table 7 Sensor dynamic parameters and sensitivity to ethanol Sensor

Parameter

C = 100 ppb (350 ◦ C)

C = 1000 ppm (600 ◦ C)

85 (350 ◦ C) → 5.7 (600 ◦ C) 1–1.5 at T ≥ 500 ◦ C 2 at T ≥ 400 ◦ C

SnO2 open modification

S τ0.9 ↑ s τ0.9 ↓ s

3.2 → 1.3 0.9–1 at T ≥ 500 ◦ C 1–2 at T ≥ 500 ◦ C

SnO2 + 3% La2 O3 open modification

S τ0.9 ↑ s τ0.9 ↓ s

10 (350 ◦ C) → 1.7 (600 ◦ C) 0.9–1.5 at T ≥ 500 ◦ C 2–4 at T ≥ 450 ◦ C

Smax = 103 at T = 450 ◦ C 0.9–1 at T ≥ 550 ◦ C 2 at T ≥ 450 ◦ C

SnO2 + 3% Pd open modification

S τ0.9 ↑ s τ0.9 ↓ s

2.6 (350 ◦ C) → 1.1 (600 ◦ C) 0.9–1.5 at T ≥ 450 ◦ C 0.9–2 at T ≥ 500 ◦ C

100 (350 ◦ C) → 2.1 (600 ◦ C) 0.9–1.5 at T ≥ 400 ◦ C 2 at T ≥ 350 ◦ C

SnO2 + 3% La2 O3 + 1% Sb2 O5 open modification

S τ0.9 ↑ s τ0.9 ↓ s

5.0 (350 ◦ C) → 1.5 (600 ◦ C) 2–3 at T ≥ 500 ◦ C 3–6 at T ≥ 500 ◦ C

Smax = 200 at T = 350 ◦ C 0.9–1.5 at T ≥ 500 ◦ C 2–3 at T ≥ 400 ◦ C

4.8. Formula for sensor sensitivity and sensitivity threshold Measurements taken at two strongly different concentration magnitudes of the gas under analysis provide a convenient tool for determination of the exponent n value in the formula for sensitivity (3) S−1=

σ − σ0 = ACn σ0

Fig. 29. The time constant of sensors with various gas-sensitivity layers at increase of ethanol concentration 100 ppb and 1000 ppm into air depending on temperatures of heating.

where A is a factor of proportionality. Knowing exponent n means, in essence, knowing the concentration dependence of sensors, which would allow one to calculate their readings for any gas concentration over the whole range under analysis. Besides, determination of the sensitivity threshold of sensors in this case will be more reliable. Such calculations were made for all the seven gases under study and for the sensors with the best parameters for their detection at the optimal heating temperature of 500 ◦ C. The

Fig. 30. The time constant of sensors with various gas-sensitivity layers at decrease of ethanol concentration 100 ppb and 1000 ppm into air depending on temperatures of heating.

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Table 8 Exponent n and sensor sensitivity threshold Cmin Gas

Sensor (structure of layer)

C (ppm)

n

Cmin

Methane Hexane Hydrogen Carbon monoxide Ammonia Hydrogen sulphide Ethanol

SnO2 + 3% Pd + Al2 O3 (cat. layer) SnO2 + 3% La2 O3 SnO2 + 3% La2 O3 SnO2 + 3% La2 O3 + 1% Sb2 O5 SnO2 + MnCoO4 (90:10) SnO2 + 3% La2 O3 SnO2

0–20–10000 0–10–2000 0–0.1–1000 0–5–98 0–10–100 0–3–30 0–0.1–1000

0.53 0.38 0.49 0.58 0.65 0.39 0.31

0.4 ppm 3.0 ppb 3.0 ppb 0.2 ppm 0.3 ppm 0.01 ppm 0.2 ppb

calculation results of exponent n and the sensitivity threshold with indication of the minimum and maximum gas concentrations in the C column, which were used for calculation of parameters, are given in Table 8. When calculating the sensitivity threshold, Cmin , the minimum change of sensor sensitivity was assumed to be 10% rel. As seen from the table, exponent n for all the gases and sensors under study differs noticeably from the “generally accepted” value of 1/2. The error of n calculation is determined, mainly, by the aggregate error of preparation of calibration gas mixtures, which makes 4% rel. (see Section 4) (for a mixture with ethanol microconcentrations it runs up to 6% rel.). Generally speaking, the larger is the difference in the values of gas concentrations and higher the ratio of respective sensitivity values, the better is the accuracy. The error proper of sensitivity determination, unambiguously related to conductivity, measurable to the third decimal digit, does not exceed 3% rel. This reasoning, undoubtedly, is valid in case of stable readings of the sensors in a gas concentration pulse, or in other words, in the absence of or with negligibly small sensor drift. Attention should be paid to high sensitivity of the sensors to ethanol, hexane, hydrogen and hydrogen sulphide, which may turn out of high practical applicability. Besides, the indicated gases (except for hydrogen) correlate with the smallest values of exponent n.

5. Conclusions 1. A production process of thick-film semiconductor gas sensors has been developed. Sensor prototypes based on tin dioxide with various additions have been manufactured. The sensors have proved to be highly efficient for detection of methane, hexane, hydrogen, carbon monoxide, ammonia, hydrogen sulphide and ethanol. Physically, the prototypes were designed either as an open chip structure or as a chip packed in a protective metal casing with an explosion-proof metal grid on the face end. 2. A GDI currently in operation at the RSC “Kurchatov Institute” has been updated to ensure the formation of square pulses of concentration of the gas under analysis with the edge duration within fractions of a second in the measuring chamber equipped with a sensor. A procedure

of investigations and experimental data processing is discussed. Graphic illustrations are given of a technique of calculation of a time constant at the level of 90% of the maximum value in a sensor conductivity pulse both ↑ ↓ on the concentration increase τ0.9 and decrease τ0.9 . ↑ 3. Dynamic parameters (quick-action, time of constant τ0.9 ↓ and τ0.9 ) and sensitivity (S = σmax /σ0 ) of gas sensors have been studied in dry gas mixtures of air with methane, hexane, hydrogen, carbon monoxide, ammonia, hydrogen sulphide and ethanol in the temperature range 300–600 ◦ C with a 50 ◦ C step for two different concentrations of gases, namely: 20 ppm and 1 vol.% for methane, 10 and 2000 ppm for hexane, 100 ppb and 1000 ppm for hydrogen and ethanol, 5 and 98 ppm for carbon monoxide, 10 and 100 ppm for ammonia and 3 and 30 ppm for hydrogen sulphide. The numerical values of parameters have been determined. The sensor sensitivity to the gas under analysis has been found to have an extremal behaviour for most devices, with the temperature zone practically independent of the gas concentration. It is shown that the dynamic parameters of sensors exhibit a sharply drooping behaviour with stabilization of the values on the minimum level of ≈0.9–1 s at temperatures above 450–500 ◦ C and they do not depend on the gas concentration either. It has also been found that covering of chips with a metal cap reduces the sensor quick-action approximately by 2–4 times, and this effect is still more pronounced on gas concentration falling down and in the range of microconcentrations. The results of measurements and calculations are presented in the graphic and tabulated forms. 4. All of the sensor structures (gas-sensitive layers) we used in our researches are summarized in Table 9 with indication of the gas media in which they were investigated. Proceeding from a combination of dynamic parameters, sensitivity and sensor drift in a gas mixture, structures of gas-sensitive layers have been defined for sensors most suitable for detection of the gases under analysis in the air. In this table, the number of plus signs corresponds to the sensor serviceability level. 5. It has been established that the optimum regime for all the sensors and gases is the temperature range 500–550 ◦ C, where the dynamic parameters reach their minimum values. The best of the sensors for detection of methane is

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Table 9 List of sensor structures and gas media, where they were investigated Sensors (structure of layers) SnO2 SnO2 SnO2 SnO2 SnO2 SnO2 SnO2

+ 3%La2 O3 + 3%Pd + 3%Pd + Al2 O3 (catalyst) + 1%Sb2 O5 + 3%La2 O3 + MnCoO4 + 1%CuO

CH4

C6 H14

H2

CO

NH3

H2 S

C2 H5 OH

+ + + +

+ + + +

+ + +

+ + +

+ + +

+ + +

+ + +

a sensor based on the SnO2 + 3% Pd structure with an Al2 O3 catalytic layer. This sensor, made in a covered modification, has the least sensor drift, quick-action of 2–4 s on both edges of the gas pulse and sensitivity at the level of 1.8–2 at C = 20 ppm and 20–30 at C = 1 vol.%. For detection of hexane, the best parameters were shown by a sensor based on structure SnO2 + 3% La2 O3 . This sensor, made in an open modification, has the least sensor drift, quick-action better than 1 s on increase of the gas concentration and 1–5 s on its falling down and sensitivity of ≈3.2 at C = 10 ppm and ≈17.1 at C = 2000 ppm. For detection of hydrogen the best sensor again has turned out the one based on structure SnO2 + 3% La2 O3 . The open modification of this sensor has the least sensor drift, quick-action of 0.9–1 s on both edges of a gas pulse and sensitivity of ≈1.6 at C = 100 ppb and 50–60 at C = 1000 ppm. The best performances for detection of carbon monoxide were demonstrated by a SnO2 + 3% La2 O3 + 1% Sb2 O5 -based sensor. This sensor, made in an open modification, has the least sensor drift, quick-action of 1–2 s on both edges of the gas pulse and sensitivity S = 1.7–1.8 at C = 5 ppm and S = 5–6.5 at C = 98 ppm. The best parameters for detection of ammonia are inherent in a sensor based on structure SnO2 +MnCoO4 (90:10). This sensor, made in an open modification, has the least sensor drift, quick-action of ≈1–3 s on both edges of the gas pulse and sensitivity of ≈2.0 at C = 10 ppm and ≈5.6 at C = 100 ppm. A sensor based on structure of SnO2 +3% La2 O3 has appeared the most promising for detection of hydrogen sulphide. This sensor, made in an open modification, has the least sensor drift, quick-action of ≈1.5–5 s on both edges of the gas pulse and sensitivity S ≈ 1.8 at C = 3 ppm and S ≈ 3.0 at C = 30 ppm. Finally, a sensor most suitable for detection of ethanol is a pure SnO2 device. This sensor has the least sensor drift, quick-action of ≈0.9–2 s on both edges of the gas pulse and sensitivity S ≈ 1.7 at C = 100 ppb and S ≈ 13.2 at C = 1000 ppm. 6. The values of exponent n have been defined for all the gases under analysis and the sensors most suitable for their detection at the optimum heating temperature of 500 ◦ C by the formula for sensitivity S − 1 = (σ–σ0 )/σ0 = ACn , where A is a factor of proportionality. Exponent n has proven to differ noticeably from the “generally accepted” value of S equal 1/2 for all the gases and sensors under study. From the calculation

+

+

+ +

results, cited separately in Table 8, sensitivity thresholds have been determined, which turned out equal to 0.4 ppm for methane, 3.0 ppb for hexane, 3.0 ppb for hydrogen, 0.2 ppm for carbon monoxide, 0.3 ppm for ammonia, 0.01 ppm for hydrogen sulphide and 0.2 ppb for ethanol.

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Biographies Valeri V. Malyshev graduated from Physical Engineering Institute of Moscow in 1962. He defended his dissertation “The experimental inves-

tigations of thermodynamic properties of hexafluorides of molybdenum, tungsten and uranium in wide region of state parameters” in 1974. He is specialist in experimental physical chemistry, thermophysics and in chemical gas sensors. His main papers are published in the Soviet journal “Teplophysica vysokikh temperatur (High Temperature Thermophysics)”, in international journal “Sensors and Actuators B” and in the Proceedings of Conferences on Thermophysical Properties of Substances and on Chemical Gas Sensors. He is the author for more than 80 scientific publications. Since 1962 he works in Institute of Molecular Physics of Russian Scientific Center “Kurchatov Institute”, at present he is a chief of laboratory. Alexander V. Pislyakov graduated from Polytechnical Institute of Odessa in 1973. He defended his dissertation “Study and development of resistive composition based on tin dioxide for thick film applications” in 1987. He is specialist in physical chemistry and in technology of thick-film gas sensors. His main papers are published in international journal “Sensors and Actuators B” and Proceedings of Conferences on Transducers and Chemical Gas Sensors. He is author for more than 50 scientific publications. Since 1979 he works in Central Institute of Precision Machinery now as chief of laboratory.