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Semiconductor method of determining oxygen microconcentrations in different gases
Sensors and Actuators, BI (1990) 210-214
210
Semiconductor Method of Detemining Oxygen Microconcentrations in Different Gases E. YE. GUTMAN
and I. A. MYASNIKOV
Karpov Physico&emical
hzsriture, 10 Obukha ul., Moscow
103064 (U.S.S.R.)
Abstract The ability metal oxides to their electric conductivity reversibly on variations of the partial pressure of oxygen has been utilized for the development of a sensor for analysis of oxygen in different gases. Data are considered on the dependence of electric conductivity on oxygen concentration, which is essential for selection of an optimal metal oxide. Various detection principles based on the adsorption and absorption of oxygen; various types of sensors using oxidized foil, thin films and sintered powder columns; and various methods (equilibrium, kinetic, and continuous) for measuring oxygen concentrations are proposed. Studies have been made of the main characteristics of the sensors in detecting oxygen in hydrogen, nitrogen, ethylene and inert gases. Various designs of gas-analyser sensors have been developed, with due consideration of their operation under conditions of variable pressures and temperatures of gases and with the aim of minimizing the preparation and response times. The results of industrial tests of the sensors are presented. Introduction In a number of branches of science and engineering (electronics and semiconductors, chemistry, metallurgy, laser engineering, biology, etc.), it is often required to measure microconcentrations of oxygen in different gases. The existing methods (chemical, magnetic, electrochemical, chromatographic, calorimetric) have some or other particular associated drawbacks; first, with their working characteristics, such as insufficient sensitivity, high inertness and high cost or complexity, as well as with some difficulties of automation, remote control and multipoint measurements. Most of these problems are eliminated by using semiconductors, mainly metal oxides, because of their high sensitivity and chemical stability in interaction with oxygen and other
gases. Notwithstanding the available positive experience in the development of semiconductor oxygen sensors [ 11,detailed studies in this field are needed, since detection is influenced by a large number of factors. The present work prepares the ground for the optimal selection of a metal oxide, sensor design and methods for measuring oxygen concentrations. Specific cases of oxygen detection in different gases have been studied. Various designs of gas-analyser sensors and the results of their tests in process gases are discussed.
Experimental Sensors with different electrical conductivities have been used: sintered polycrystalline ZnO and TiOz films 0.1 - 10 pm
211
sensor signals were essentially the equilibrium a = Aaa,/a’ and kinetic (daldt),,, values of electric conductivity. Here a,, and a are the initial and equilibrium values respectively, Aa = a0 - a is a change, and (da/d?), _ o is the initial rate of variation of the electric conductivity. The values of a and da/dt are the measures of oxygen adsorption by the surface of a sensor [2], and are proportional to the oxygen concentration in the gaseous phase surrounding the sensor. The design and operating principle of gasanalyser sensors are given later in the paper.
Results and Discussion
I. Effect of Oxygen Adsorption on Electric Conductivity of Metal Oxides and Oxygen Sensors The dependence of the equilibrium electric conductivity a on the oxygen concentration PO2 is usually described by the relationship a = kP”,,
(1)
where x is a constant for a given interval of temperatures and concentrations and characterizes the sensitivity of a sensor to oxygen. Relationship (1) can be obtained analytically under certain assumptions on the nature of disorder of the lattice of a given metal oxide and the mechanism of interaction of the oxide with oxygen [3,4]. In eqn. (l), x is negative for electronic semiconductors and positive for hole semiconductors. Therefore, it is expedient to use some n-type specimens at low values of Pq2 and p-type specimens at high values of PO*. Besides, according to Meyer’s rule on the dependence of the activation energy of conductance on the concentration of charge carriers [5], with a decrease of PO*, the temperature coefficient of electric conductivity decreases for n-type specimens and increases for those of ptype. Sensors based on SnO,, CdO, V20s, ZrOz, B&O3 and MnO, have turned out to be unsuitable for detection of microconcentrations PO = lo-‘10-6% by volume in nitrogen at PN2= ! atm and T = 573 K, because of their low sensitivity (x = 0.1). Sensors based on Nb205, MOO,, WO, and PbO possess a sufliciently high sensitivity, but their use involves certain difficulties in view of either low electric conductivity (Nb,OS, PbO) or substantial inertness ( W03, MOO,) or both. From the standpoint of the requirements set forth for sensors to measure microconcentrations of oxygen, the most suitable materials are ZnO and TiOz. Comparison of the data on electric conductivity of the above-mentioned sintered polycrystalline specimens of ZnO and TiO, with the data
for specimens based on single-crystalline films and sub-surface layers of single crystals under the conditions of oxygen adsorption has shown that all these specimens have essentially the same quantitative relationships between electric conductivity and oxygen concentration. This result is obviously associated with the absence of pronounced bridges between individual crystallites of the specimens as a result of the occurrence of liquid-like coalescence, in which the electric conductivity changes because of the lack of free current carriers rather than due to the appearance of new intercrystalline barriers [6]. These sintered polycrystalline specimens have turned out to be the most suitable as oxygen sensors, because of their higher sensitivity, ease of manufacture and reliability in operation. A study of the kinetics of electric conduction in adsorption of oxygen on metal oxides is essential for the development of a mathematical model of the sensor operation, which would make it possible to calculate oxygen concentrations in gases with a high accuracy and high speed by the electric conductivity data. The existing kinetic models take into consideration the charging of the surface [7’J, recharging of biographical surface states [8], formation of different forms of adsorbed oxygen [9], etc. The model that corresponds fully to the experimental data assumes non-dissociative adsorption of oxygen on a metal oxide at moderate temperatures with the use of interstitial atoms of the metal (centres of impurity conduction) as adsorption centres [4]. The model mentioned discriminates between the alternative models and describes the kinetics of electric conduction in a wide range of temperatures and oxygen concentrations. 2. Types and Principles of Detection Various methods of oxygen detection have been developed: gaseous, liquid and vapour [ lo]. The liquid and vapour versions are promising for measuring oxygen concentrations in various liquids at 296 K. The latter version requires no intermixing and can detect oxygen in cases when the oxygen concentration in vapours is many times that in the liquid. The gaseous version is considered here in more detail; it is simpler and has been studied more thoroughly, but, because of the relatively high activation energy of adsorption-desorption processes, requires heating of the sensor to a high temperature (above 473 K). The gaseous version suggests two main principles of detection, which are based on the phenomena of oxygen absorption and adsorption. The operation of absorption (high-temperature) sensors is based on establishing an equilibrium between the chemisorbed oxygen and the oxygen of the gaseous phase at moderate temperatures. In that
212
tion sensors. Adsorption sensors introduce no changes into the composition of the gaseous phase and make it possible to determine microconcentrations of oxygen in quite different gases, including reducing ones, without the risk of their dissociation or substantial interaction with oxygen.
CT,
0
1273
773
TK
Fig. 1. Dependence of equilibrium electric conductivity (u) of the tibn of a ZnO sensor on temperature at PO, = 3 x 10e3 Torr.
case, the phase composition of the oxides is not changed [ 1l- 131. Isobars of equilibrium conductivity of ZnO (Fig. 1) and TiOz (not shown in the Figure), which have two maxima and are associated with the occurrence of several competing processes in oxygen adsorption (so that some of these processes increase the sensor signal and others decrease it), make it possible to establish the optimal temperatures of detection of absorption and adsorption sensors [ 11, 121. As has been found for both types of sensor, the dependence of the equilibrium electric conductivity on oxygen concentration satisfies eqn. (1). It has been found that in the temperature range 973- 1373 K, x = -0.25 for all specimens; in the temperature range 573-773 K, x = -0.5 for ZnO and TiOz films and x = -0.02 for oxidized T&foil (in the latter case, the diffusion of oxygen inside a specimen is not high and shunting from the surface through the volume of foil takes place, unlike the case with films) (Fig. 2). Oxidized foil specimens are more suitable for absorption sensors, whereas films and porous columns of metal oxides are more suitable as specimens for adsorp-
(ai
(2
24
I#? @I
z4
3. Methods of Quantitative Measurement Regarding the determination of oxygen microconcentrations in gases, two methods of operation of adsorption sensors have been studied: the equilibrium method, in which the sensor is present continuously in the medium being analysed; and the kinetic method, in which a pulse of the gas being analysed is fed to a sensor placed in a flow of gas purified from oxygen. Both methods have been investigated for detection of oxygen by ZnO sensors in different gases and the main characteristics have been measured. For instance, for an Oz/Hz system, the range of measured concentrations is 1 x 10-6 to 1 x lo-‘% by volume; the response time is 3-5 minutes for the equilibrium method and 10 s for the kinetic method; the reversibility is practically complete; the reproducibility is equal to l-10% and the accuracy is lo-20% for both methods; permissible temperature variations are equal to 5 “C; and the service life of continuously operating sensors at a specified accuracy of measurement is not less than a year. It has been found that there is a linear relationship between the equilibrium and kinetic characteristics of electric conductivity of sensors and the concentrations of oxygen in gases (Figs. 3 and 4). An example of calibration of a ZnO sensor for use in the kinetic method is shown in Fig. 5. The time interval between measurements is determined by the time of relaxation of the electric conductivity to its initial value or, in other words, by the desorption time of oxygen from the surface of ZnO. In such conditions, as experiment shows, the main sensor characteristics are better reproduced over a longer
3’fgpg
Fig. 2. Dependence of electric conductivity of TiOz specimens on oxygen pressure: (a) 773 K, (b) 1173 K; I, film; 2, foil.
Fig. 3. Dependence of equilibrium electric conductivity (u) of the film of a ZnO sensor on the concentration of oxygen in nitrogen at 573 K.
213
Fig. 6. Block diagram of ZnO sensor gas-analyser: 1, 7, closing valves; 2, gas preparation unit; 3, 5, catalytic valves; 4, calibrating oxygen source; 6, ZnO sensor. During analysis of a gas, say, H, for Oa, valve 2 is ‘open’ for Oa and valve 5 operates in the regime ‘closed-‘open’ for 0,. In the calibration of a ZnO sensor for Oa in Ha, valve 2 is ‘closed’ for Oa; on switching on of oxygen source 4, valve 5 operates in the regime ‘open’-‘closed’ for 0,.
ok+
CL-----‘---
km
QIS
c&
Fig. 4. Correlation between equilibrium and kinetic characteristics of electric conductivity of the lihn of a ZnO sensor in the determination of oxygen in hydrogen.
t,s
gases. The instrument has been realized in two versions: one with electromagnetic valves and the other with controlled catalytic valves (Fig. 6). The operating principle of both instruments, for instance, in an 0z/H2 system, consists in feeding a pulse of the gas being analysed to a ZnO sensor which has been present before this in a flow of pure hydrogen. This is achieved by means of electromagnetic valves or a controlled catalytic valve. A system of stabilization of the pressure and temperature of the analysed gas ensures the optimal conditions for operation of the sensor. The time of preparation of the instruments to operation is reduced by preheating the sensor to the working temperature under static conditions in the gaseous atmosphere of the instrument. The application of these sensor-analysers in industry has made it possible to study the dynamics of variation of oxygen concentration in hydrogen (see Fig. 7) and demonstrated the advantages of these instruments, which are associated, first of all, with their low response time and with the possibility of automatic multi-point remote-controlled analysis.
Fig. 5. Calibration of the l&t of a ZnO sensor for application in the kinetic method of determination of oxygen in hydrogen.
exploitation period than for the equilibrium method, when the sensor is constantly in the gas to be analysed. 4. Design and Operation of Gas-analyser Sensors The equilibrium and kinetic methods have been realized in various types of gas-analyser sensors. A gas analyser operating by the equilibrium method is intended for measurements of slowly varying concentrations of oxygen in gases. The principal elements of the instrument are a sensor, a reactor with a catalyst for after purification of the gas from oxygen, a microelectrolyser for calibration of the sensor, a pressure regulator, and a system of valves. The instrument is simple in design and its operating principle is clear without further explanations. The gas analyser operating by the kinetic method is intended for measuring quickly changing concentrations of oxygen in
Fig. 7. Tests of ZnO sensor-analyser in commercial hydrogen: I, calibration; II, analysis; III, repeated calibration. [Or], % by volume: I,5 x 10-4; 2.7 x 10m4; 3,1 x 10e3; 4.5 x 10W3; 5,1 x lo-*.
214
Conclusions Further improvements in the semiconductor method for measuring microconcentrations of oxygen in gases, in our opinion, will be associated with the problem of interchangeability of sensors and the development of a continuous method of measurement on the basis of the corresponding kinetic equation of electric conductivity of sensors. It should be possible to detect oxygen in the concentration range 10m6- lo- *% by volume and in the millisecond time range. References I. Takeuchi, Oxygen sensors, Proc. Id Int. Meet. Chemical Sensors, Bordeaux, France, July 7-10, 1986, pp. 69-78. I. A. Myasnikov, E. V. Bol’shun and E. Ye. Gutman, Mechanism of adsorption of radicals on semiconductors and phenomena of desorption of radicals from hot walls, Kinet. Katal., 4 (1963) 861-871. K. Hauffe, Reaktionen in und an Festen Stoffen, Springer, Berlin, 1955, Bl, p. 415s. I. A. Myasnikov, Electronic phenomena in ZnO during sorption of O,, Zhur. Fir. Khimii, 31 (1957) 1721-1731.
5 W. Meyer and H. Neidel, Uber die Beaiehungen Zwishen der Energiekonstanten E und der Mengenkonstanten a in der Leitwerts-Temperatur-founel bei Oxydischen Halbleitem. 2. Phys., 38 (1937) 1014-1019. 6 V. Ya. Sukharev and I. A. Myasnikov, Theoretical principles of the method of semiconductor sensors in analysis of active gases, Zh. Fiz. Khim., 60 (1986) 2385-2401; 61 (1987) 302-312, 547-596. I F. F. Wolkenstein, Physical Chemistry of Semiconductor Surfaces, Nauka, Moscow, 1973, p. 399. 8 V. F. Kiselev and 0. V. Krylov, Electronic Phenomena in Adsorption and Catalysis on Semiconductors and Dielectrics, Nat&a, Moscow, 1979, p. 234. 0. V. Krylov and V. F. Kiselev, Adsorption and Catalysis on Transition Metals and Their Oxides, Khimiya, Moscow, 1981, p. 288. I. A. Myasnikov, On peculiarities of adsorption of oxygen molecules and radicals on semiconductor metal oxides in gaseous and liquid media, Zh. Fiz. Khim., 55( 1981) 12781287; 2053-2063. E. Ye. Gutman, I. A. Myasnikov and L. M. Chipurilina, On detection of oxygen by zinc oxide tihns in vacuum and different gases, Zh. Fir. Khim., 50 (1976) 1791-1794. E. Ye. Gutman, I. A. Myasnikov and A. G. Davtyan, Semiconductor oxygen detector on the basis of titanium dioxide for detection in vacuum and inert gases, Zh. Fir. Khim., 53 (1979) 2125-2128. S. R. Morrison, Semiconductor gas sensors, Sensors and Actuators, 2 (1982) 329-341.
210
Semiconductor Method of Detemining Oxygen Microconcentrations in Different Gases E. YE. GUTMAN
and I. A. MYASNIKOV
Karpov Physico&emical
hzsriture, 10 Obukha ul., Moscow
103064 (U.S.S.R.)
Abstract The ability metal oxides to their electric conductivity reversibly on variations of the partial pressure of oxygen has been utilized for the development of a sensor for analysis of oxygen in different gases. Data are considered on the dependence of electric conductivity on oxygen concentration, which is essential for selection of an optimal metal oxide. Various detection principles based on the adsorption and absorption of oxygen; various types of sensors using oxidized foil, thin films and sintered powder columns; and various methods (equilibrium, kinetic, and continuous) for measuring oxygen concentrations are proposed. Studies have been made of the main characteristics of the sensors in detecting oxygen in hydrogen, nitrogen, ethylene and inert gases. Various designs of gas-analyser sensors have been developed, with due consideration of their operation under conditions of variable pressures and temperatures of gases and with the aim of minimizing the preparation and response times. The results of industrial tests of the sensors are presented. Introduction In a number of branches of science and engineering (electronics and semiconductors, chemistry, metallurgy, laser engineering, biology, etc.), it is often required to measure microconcentrations of oxygen in different gases. The existing methods (chemical, magnetic, electrochemical, chromatographic, calorimetric) have some or other particular associated drawbacks; first, with their working characteristics, such as insufficient sensitivity, high inertness and high cost or complexity, as well as with some difficulties of automation, remote control and multipoint measurements. Most of these problems are eliminated by using semiconductors, mainly metal oxides, because of their high sensitivity and chemical stability in interaction with oxygen and other
gases. Notwithstanding the available positive experience in the development of semiconductor oxygen sensors [ 11,detailed studies in this field are needed, since detection is influenced by a large number of factors. The present work prepares the ground for the optimal selection of a metal oxide, sensor design and methods for measuring oxygen concentrations. Specific cases of oxygen detection in different gases have been studied. Various designs of gas-analyser sensors and the results of their tests in process gases are discussed.
Experimental Sensors with different electrical conductivities have been used: sintered polycrystalline ZnO and TiOz films 0.1 - 10 pm
211
sensor signals were essentially the equilibrium a = Aaa,/a’ and kinetic (daldt),,, values of electric conductivity. Here a,, and a are the initial and equilibrium values respectively, Aa = a0 - a is a change, and (da/d?), _ o is the initial rate of variation of the electric conductivity. The values of a and da/dt are the measures of oxygen adsorption by the surface of a sensor [2], and are proportional to the oxygen concentration in the gaseous phase surrounding the sensor. The design and operating principle of gasanalyser sensors are given later in the paper.
Results and Discussion
I. Effect of Oxygen Adsorption on Electric Conductivity of Metal Oxides and Oxygen Sensors The dependence of the equilibrium electric conductivity a on the oxygen concentration PO2 is usually described by the relationship a = kP”,,
(1)
where x is a constant for a given interval of temperatures and concentrations and characterizes the sensitivity of a sensor to oxygen. Relationship (1) can be obtained analytically under certain assumptions on the nature of disorder of the lattice of a given metal oxide and the mechanism of interaction of the oxide with oxygen [3,4]. In eqn. (l), x is negative for electronic semiconductors and positive for hole semiconductors. Therefore, it is expedient to use some n-type specimens at low values of Pq2 and p-type specimens at high values of PO*. Besides, according to Meyer’s rule on the dependence of the activation energy of conductance on the concentration of charge carriers [5], with a decrease of PO*, the temperature coefficient of electric conductivity decreases for n-type specimens and increases for those of ptype. Sensors based on SnO,, CdO, V20s, ZrOz, B&O3 and MnO, have turned out to be unsuitable for detection of microconcentrations PO = lo-‘10-6% by volume in nitrogen at PN2= ! atm and T = 573 K, because of their low sensitivity (x = 0.1). Sensors based on Nb205, MOO,, WO, and PbO possess a sufliciently high sensitivity, but their use involves certain difficulties in view of either low electric conductivity (Nb,OS, PbO) or substantial inertness ( W03, MOO,) or both. From the standpoint of the requirements set forth for sensors to measure microconcentrations of oxygen, the most suitable materials are ZnO and TiOz. Comparison of the data on electric conductivity of the above-mentioned sintered polycrystalline specimens of ZnO and TiO, with the data
for specimens based on single-crystalline films and sub-surface layers of single crystals under the conditions of oxygen adsorption has shown that all these specimens have essentially the same quantitative relationships between electric conductivity and oxygen concentration. This result is obviously associated with the absence of pronounced bridges between individual crystallites of the specimens as a result of the occurrence of liquid-like coalescence, in which the electric conductivity changes because of the lack of free current carriers rather than due to the appearance of new intercrystalline barriers [6]. These sintered polycrystalline specimens have turned out to be the most suitable as oxygen sensors, because of their higher sensitivity, ease of manufacture and reliability in operation. A study of the kinetics of electric conduction in adsorption of oxygen on metal oxides is essential for the development of a mathematical model of the sensor operation, which would make it possible to calculate oxygen concentrations in gases with a high accuracy and high speed by the electric conductivity data. The existing kinetic models take into consideration the charging of the surface [7’J, recharging of biographical surface states [8], formation of different forms of adsorbed oxygen [9], etc. The model that corresponds fully to the experimental data assumes non-dissociative adsorption of oxygen on a metal oxide at moderate temperatures with the use of interstitial atoms of the metal (centres of impurity conduction) as adsorption centres [4]. The model mentioned discriminates between the alternative models and describes the kinetics of electric conduction in a wide range of temperatures and oxygen concentrations. 2. Types and Principles of Detection Various methods of oxygen detection have been developed: gaseous, liquid and vapour [ lo]. The liquid and vapour versions are promising for measuring oxygen concentrations in various liquids at 296 K. The latter version requires no intermixing and can detect oxygen in cases when the oxygen concentration in vapours is many times that in the liquid. The gaseous version is considered here in more detail; it is simpler and has been studied more thoroughly, but, because of the relatively high activation energy of adsorption-desorption processes, requires heating of the sensor to a high temperature (above 473 K). The gaseous version suggests two main principles of detection, which are based on the phenomena of oxygen absorption and adsorption. The operation of absorption (high-temperature) sensors is based on establishing an equilibrium between the chemisorbed oxygen and the oxygen of the gaseous phase at moderate temperatures. In that
212
tion sensors. Adsorption sensors introduce no changes into the composition of the gaseous phase and make it possible to determine microconcentrations of oxygen in quite different gases, including reducing ones, without the risk of their dissociation or substantial interaction with oxygen.
CT,
0
1273
773
TK
Fig. 1. Dependence of equilibrium electric conductivity (u) of the tibn of a ZnO sensor on temperature at PO, = 3 x 10e3 Torr.
case, the phase composition of the oxides is not changed [ 1l- 131. Isobars of equilibrium conductivity of ZnO (Fig. 1) and TiOz (not shown in the Figure), which have two maxima and are associated with the occurrence of several competing processes in oxygen adsorption (so that some of these processes increase the sensor signal and others decrease it), make it possible to establish the optimal temperatures of detection of absorption and adsorption sensors [ 11, 121. As has been found for both types of sensor, the dependence of the equilibrium electric conductivity on oxygen concentration satisfies eqn. (1). It has been found that in the temperature range 973- 1373 K, x = -0.25 for all specimens; in the temperature range 573-773 K, x = -0.5 for ZnO and TiOz films and x = -0.02 for oxidized T&foil (in the latter case, the diffusion of oxygen inside a specimen is not high and shunting from the surface through the volume of foil takes place, unlike the case with films) (Fig. 2). Oxidized foil specimens are more suitable for absorption sensors, whereas films and porous columns of metal oxides are more suitable as specimens for adsorp-
(ai
(2
24
I#? @I
z4
3. Methods of Quantitative Measurement Regarding the determination of oxygen microconcentrations in gases, two methods of operation of adsorption sensors have been studied: the equilibrium method, in which the sensor is present continuously in the medium being analysed; and the kinetic method, in which a pulse of the gas being analysed is fed to a sensor placed in a flow of gas purified from oxygen. Both methods have been investigated for detection of oxygen by ZnO sensors in different gases and the main characteristics have been measured. For instance, for an Oz/Hz system, the range of measured concentrations is 1 x 10-6 to 1 x lo-‘% by volume; the response time is 3-5 minutes for the equilibrium method and 10 s for the kinetic method; the reversibility is practically complete; the reproducibility is equal to l-10% and the accuracy is lo-20% for both methods; permissible temperature variations are equal to 5 “C; and the service life of continuously operating sensors at a specified accuracy of measurement is not less than a year. It has been found that there is a linear relationship between the equilibrium and kinetic characteristics of electric conductivity of sensors and the concentrations of oxygen in gases (Figs. 3 and 4). An example of calibration of a ZnO sensor for use in the kinetic method is shown in Fig. 5. The time interval between measurements is determined by the time of relaxation of the electric conductivity to its initial value or, in other words, by the desorption time of oxygen from the surface of ZnO. In such conditions, as experiment shows, the main sensor characteristics are better reproduced over a longer
3’fgpg
Fig. 2. Dependence of electric conductivity of TiOz specimens on oxygen pressure: (a) 773 K, (b) 1173 K; I, film; 2, foil.
Fig. 3. Dependence of equilibrium electric conductivity (u) of the film of a ZnO sensor on the concentration of oxygen in nitrogen at 573 K.
213
Fig. 6. Block diagram of ZnO sensor gas-analyser: 1, 7, closing valves; 2, gas preparation unit; 3, 5, catalytic valves; 4, calibrating oxygen source; 6, ZnO sensor. During analysis of a gas, say, H, for Oa, valve 2 is ‘open’ for Oa and valve 5 operates in the regime ‘closed-‘open’ for 0,. In the calibration of a ZnO sensor for Oa in Ha, valve 2 is ‘closed’ for Oa; on switching on of oxygen source 4, valve 5 operates in the regime ‘open’-‘closed’ for 0,.
ok+
CL-----‘---
km
QIS
c&
Fig. 4. Correlation between equilibrium and kinetic characteristics of electric conductivity of the lihn of a ZnO sensor in the determination of oxygen in hydrogen.
t,s
gases. The instrument has been realized in two versions: one with electromagnetic valves and the other with controlled catalytic valves (Fig. 6). The operating principle of both instruments, for instance, in an 0z/H2 system, consists in feeding a pulse of the gas being analysed to a ZnO sensor which has been present before this in a flow of pure hydrogen. This is achieved by means of electromagnetic valves or a controlled catalytic valve. A system of stabilization of the pressure and temperature of the analysed gas ensures the optimal conditions for operation of the sensor. The time of preparation of the instruments to operation is reduced by preheating the sensor to the working temperature under static conditions in the gaseous atmosphere of the instrument. The application of these sensor-analysers in industry has made it possible to study the dynamics of variation of oxygen concentration in hydrogen (see Fig. 7) and demonstrated the advantages of these instruments, which are associated, first of all, with their low response time and with the possibility of automatic multi-point remote-controlled analysis.
Fig. 5. Calibration of the l&t of a ZnO sensor for application in the kinetic method of determination of oxygen in hydrogen.
exploitation period than for the equilibrium method, when the sensor is constantly in the gas to be analysed. 4. Design and Operation of Gas-analyser Sensors The equilibrium and kinetic methods have been realized in various types of gas-analyser sensors. A gas analyser operating by the equilibrium method is intended for measurements of slowly varying concentrations of oxygen in gases. The principal elements of the instrument are a sensor, a reactor with a catalyst for after purification of the gas from oxygen, a microelectrolyser for calibration of the sensor, a pressure regulator, and a system of valves. The instrument is simple in design and its operating principle is clear without further explanations. The gas analyser operating by the kinetic method is intended for measuring quickly changing concentrations of oxygen in
Fig. 7. Tests of ZnO sensor-analyser in commercial hydrogen: I, calibration; II, analysis; III, repeated calibration. [Or], % by volume: I,5 x 10-4; 2.7 x 10m4; 3,1 x 10e3; 4.5 x 10W3; 5,1 x lo-*.
214
Conclusions Further improvements in the semiconductor method for measuring microconcentrations of oxygen in gases, in our opinion, will be associated with the problem of interchangeability of sensors and the development of a continuous method of measurement on the basis of the corresponding kinetic equation of electric conductivity of sensors. It should be possible to detect oxygen in the concentration range 10m6- lo- *% by volume and in the millisecond time range. References I. Takeuchi, Oxygen sensors, Proc. Id Int. Meet. Chemical Sensors, Bordeaux, France, July 7-10, 1986, pp. 69-78. I. A. Myasnikov, E. V. Bol’shun and E. Ye. Gutman, Mechanism of adsorption of radicals on semiconductors and phenomena of desorption of radicals from hot walls, Kinet. Katal., 4 (1963) 861-871. K. Hauffe, Reaktionen in und an Festen Stoffen, Springer, Berlin, 1955, Bl, p. 415s. I. A. Myasnikov, Electronic phenomena in ZnO during sorption of O,, Zhur. Fir. Khimii, 31 (1957) 1721-1731.
5 W. Meyer and H. Neidel, Uber die Beaiehungen Zwishen der Energiekonstanten E und der Mengenkonstanten a in der Leitwerts-Temperatur-founel bei Oxydischen Halbleitem. 2. Phys., 38 (1937) 1014-1019. 6 V. Ya. Sukharev and I. A. Myasnikov, Theoretical principles of the method of semiconductor sensors in analysis of active gases, Zh. Fiz. Khim., 60 (1986) 2385-2401; 61 (1987) 302-312, 547-596. I F. F. Wolkenstein, Physical Chemistry of Semiconductor Surfaces, Nauka, Moscow, 1973, p. 399. 8 V. F. Kiselev and 0. V. Krylov, Electronic Phenomena in Adsorption and Catalysis on Semiconductors and Dielectrics, Nat&a, Moscow, 1979, p. 234. 0. V. Krylov and V. F. Kiselev, Adsorption and Catalysis on Transition Metals and Their Oxides, Khimiya, Moscow, 1981, p. 288. I. A. Myasnikov, On peculiarities of adsorption of oxygen molecules and radicals on semiconductor metal oxides in gaseous and liquid media, Zh. Fiz. Khim., 55( 1981) 12781287; 2053-2063. E. Ye. Gutman, I. A. Myasnikov and L. M. Chipurilina, On detection of oxygen by zinc oxide tihns in vacuum and different gases, Zh. Fir. Khim., 50 (1976) 1791-1794. E. Ye. Gutman, I. A. Myasnikov and A. G. Davtyan, Semiconductor oxygen detector on the basis of titanium dioxide for detection in vacuum and inert gases, Zh. Fir. Khim., 53 (1979) 2125-2128. S. R. Morrison, Semiconductor gas sensors, Sensors and Actuators, 2 (1982) 329-341.