Ann. ocaip. Hyg. Vol. 18, pp. 63-68. Pergamon Press 1973. Printed in Oremt Britain
SOLID STATE DETECTORS FOR CARBON MONOXIDE* J. G. FIRTH, A. JONES and T. A. JONES
Abstract—The mechanism by which a gas can produce an electrical conductivity change in a semiconductor is outlined. The resulting advantages and disadvantages in the use of metal oxide semiconductors in gas detection devices for carbon monoxide are described and ways of reducing the disadvantages are discussed.
INTRODUCTION RESEARCH on solid state sensors for the detection of carbon monoxide is being carried out in many countries at the present time including Britain, America and Japan. Because the threshold limit value of carbon monoxide is 50 ppm, in air, most interest centres on the measurement of concentration ranges of 0-100 or 0-200 ppm and devices using the electrical resistance changes produced in semiconducting metal oxides by adsorption of carbon monoxide have a great deal of promise for measurement in these ranges. Such devices have a number of properties which make them attractive for use in the detection of gases in general, the most obvious of which are:
(1) they can be small and rugged and hence easily used in field equipment; (2) they only need to be connected to a supply of electrical power to produce a signal which is a function of the gas concentration; (3) they lend themselves to the production methods developed in the electronics industries and hence should be cheap and easy to manufacture. The object of this paper is to outline the mechanism by which these devices produce a signal in a gas and the practical advantages and disadvantages which result from this. SIGNAL MECHANISM
It has been known for many years (GARNER, 1955) that adsorption of a gas on the surface of a semiconductor will produce a measurable change in the electrical conductivity of the solid. Such effects have been often used in studies of adsorption processes on solids, studies of active species on surfaces and studies of the mechanism of catalytic reactions (GOODWIN and MARK, 1972). Chemisorption of any gas on the surface of a solid results in the formation of a bond which changes the electron distribution in the solid surface which in turn can result in a change in the electrical conductivity of the solid. The simplest case is the adsorption of the gas to produce charged species on the solid. The formation of such •Crown Copyright 1974. 63
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
Safety m Mines Research Establishment, Red Hill, Sheffield
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J. G. FIRTH, A. JONES and T. A. JONES
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
species must result in the donation or abstraction of electrons from the solid depending on the sign of the charge on the adsorbed species. This electron transfer can produce a sufficient change in the concentration of electrons or holes which cause electrical conductivity in the solid to produce a measurable change in conductivity. The most widely used solids in this type of sensor are the oxides of the transition metals and heavy metals such as tin. These oxides are semiconductors because of the ability of the metal to exist in different oxidation states. All these oxides are nonstoichiometric and contain a slight excess (n-type semiconductor) or deficiency (p-type semiconductor) of metal ions. In order to preserve electrostatic neutrality, the excess metal in n-type semiconductors is present as ions with a lower charge than that of the parent metal ion in the lattice, e.g. Sn2+ in SnO2. Such ions can be represented as having localised energy levels just below that of the conduction band of the oxide and these localised electrons can be thermally excited into the conduction band of the oxide and cause it to have electrical conductivity. In the process, of course, Sn2+ is converted to Sn*+. Similarly, in p-type semiconductors, in order to preserve electrostatic neutrality, some of the metal ions are present as ions with a higher charge than that of the parent ion of the lattice, e.g. Ni3+ in NiO. These ions can be represented as having localised energy levels just above the top valence band of the oxide, and electrons can be thermally excited from the valence band into these levels leaving positive holes in the valence band which cause the oxide to have electrical conductivity. The simplest mechanism by which adsorption of a gas in an ionic form can produce a change in the conductivity of an oxide is by changing the concentration of those metal ions in the oxide lattice which have a charge different to that of the main metal ion in the lattice. Thus adsorption of neutral oxygen molecules on the surface of an. oxide to form two oxygen ions results in the transfer of four electrons from the solid to the adsorbed gas where they are localised. On n-type oxides such as zinc oxide these four electrons will be released by a change of two Zn° ions to two Zn2+ ions with a resultant decrease in conductivity. On a p-type oxide such as nickel oxide, these four electrons would be released by a change of four Ni 2+ ions to four Ni3* ions with a resultant increase in conductivity. Carbon monoxide can adsorb on an oxide surface either as a neutral molecule or as a positively charged ion (CO+) depending on the temperature and chemical nature of the metal oxide. In the latter type of adsorption an electron will be donated to the solid resulting in an increase in conductivity in an n-type oxide or a decrease in conductivity in a p-type oxide. However, if adsorbed oxygen ions are present on the surface both adsorbed ions and molecules of carbon monoxide will react with oxygen ions to form ultimately carbon dioxide which will be desorbed. The overall reaction is the removal of adsorbed oxygen ions in a neutral form and hence the electrons trapped on these ions will be donated back to the oxide lattice to become re-associated with the impurity centres. Hence adsorption of carbon monoxide will result in an increase in conductivity on a n-type semiconductor. In a carbon monoxide-oxygen mixture, oxygen will re-adsorb on the surface so that at any concentration of carbon monoxide, an equilibrium concentration of adsorbed oxide ions and carbon monoxide is present on the surface of the oxide and hence the conductivity will be determined by the gas phase concentration of gases. However, since in general, oxygen is strongly adsorbed on most oxides large changes
Solid state detectors for carbon monoxide
65
in gas phase oxygen concentration (1-50 per cent v/v) above a certain minimum concentration produce only small changes in the conductivity of the oxide. Carbon monoxide, however, is generally weakly adsorbed so that relatively large changes in the conductivity will be produced by changing the gas phase concentration of carbon monoxide. ADVANTAGES AND DISADVANTAGES
TABLE 1. CONDUCTION IN STANNIC OXIDE AT 200°C
Gas Air Hydrogen Methane Propylene Butane Carbon monoxide Carbon dioxide Hydrogen sulphide Sulphur dioxide
Concentration (v/v) 2% 2% 2% 2% 2% lOOppm 5% 20ppm lOppm
Conductance
(n-1)
1 x 10-* 27-5 x 10-^ 211 x 10-* 28-3 x 10-* 28-3 x 10-* 6-8 x 10-* 1 x 10-* 18-5 x 10-* 22-5 x 10-^
The rate at which the equilibrium conductivity in a particular gas mixture is reached will depend on the rate of adsorption of the different gases on the oxide surface. Similarly the rate at which the conductivity returns to its original value will depend on the rate of desorption of the gas from the surface. For some gases, these can be slow processes, e.g. on stannic oxide at 200°C the adsorption of methane and sulphur dioxide occurs in a few seconds but, although the readsorption of oxygen is rapid so that the return on removal of methane takes place in a few seconds, the desorption of sulphur dioxide is slow and can take up to 10 min. Hence it is possible for devices based on metal oxides to have a slow response to the gas, i.e. some minutes and a slow return after exposure to the gas. However, since both adsorption, surface
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
The concentration of electrons or holes, causing conduction in many metal oxides at temperatures up to 700°C, is sufficiently low for adsorption of a gas at low concentration to produce a large change in the conductivity of the oxide. Thus devices using metal oxides can produce large signals in low concentrations of many gases, e.g. exposure of stannic oxide at 200°C to lOOppm of CO can double the conductivity, Since the degree of adsorption of a gas is determined by the temperature and chemical nature of the solid, particularly the metal ions present, the large range of metal oxides which are semiconducting make it possible to detect and measure a large range of gases. However, it is obvious .that any gas which can adsorb on a metal oxide surface at a given temperature and cause electron transfer with the solid or can interact with adsorbed oxide ions will produce a conductivity change in the oxide. Thus in a mixture of gases, the observed conductivity change will be that produced by electron transfer from a number of gases in the mixture. This intrinsic lack of specificity is illustrated for stannic oxide at 200°C in Table 1. It is the major disadvantage of semiconductor devices and a large fraction of the research effort in this field is devoted to the improvement of specificity for particular applications.
66
J. G. FIRTH, A. JONES and T. A. JONES
reaction and desorption on solids are all activated processes, their rates increase as the temperature of the solid increases. Hence response times and recovery times of devices can be reduced by raising their operating temperature. IMPROVEMENT OF SPECIFICITY
f
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100 ppt
j
I" 0
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ppm CH 4
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/
200
600
400 Temperature :
C
FIG. 1. Effect of temperature on conductivity of stannic oxide in given concentrations of carbon monoxide and methane.
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
Carbon monoxide is generally encountered in gas mixtures which contain other components whose concentration can change and hence it is possible for these gases to produce spurious signals on a sensing device. Even where carbon monoxide is encountered only as a mixture in air, varying concentrations of water vapour can be present. Adsorbed water vapour acts as an electron donor on many metal oxides although the detailed mechanism of the process is not fully understood. However, in common with many gases which adsorb without reaction, the size of the effect decreases as the temperature of the oxide increases and it is often possible to operate the oxide at a temperature where the effect is zero. Even where this is not possible, the surfaces of many oxides become saturated above a partial pressure of water of about 5 mm Hg since the adsorption of water follows a normal isotherm. Hence over the normal atmospheric range of concentrations little effect of water vapour is observed. Carbon dioxide is also frequently encountered in gas mixtures containing carbon monoxide. In general, however, carbon dioxide has little or no effect on the conductivity of metal oxides. On a given oxide, the adsorption of a gas will be temperature dependent and different gases will adsorb at different temperatures. In general, the fractional change in conductivity produced by a gas of a given concentration increases as the temperature of the oxide is increased before passing through a maximum and decreasing again. This is illustrated for carbon monoxide and methane at 100 ppm in air on stannic oxide in Fig. 1. It can be seen that if this physical form of stannic oxide is operated
Solid state detectors for carbon monoxide
m
300
400
500
500
Ttflpwature : *C
Fio. 2. Effect of temperature on conductivity in given concentrations of carbon monoxide and methane of (a) uranium oxide + 1 mole% palladium; (b) uranium oxide + 1 mole% palladium+ 2 mole % cerium.
not affected by carbon monoxide, water vapour or methane. The addition of 1 mole % of palladium to the lattice produces a conduction change in lOOppm of CO with a temperature dependence similar to that shown in Fig. 1. However, the conductivity is also affected by 1 per cent methane although to a smaller extent. Further addition of 2 mole % of cerium removes the effect produced by methane. At the present time this oxide system has a long response time of about 3 min and work is in hand to reduce this. Since the adsorption of a gas will follow an isotherm of the Langmuirian or Elovich type, the conductivity change produced will not be linearly related to the concentration. The effect of CO on Pd-doped UO2 is shown in Fig. 3. This type of response curve is typical for metal oxide devices. SENSOR CONSTRUCTION
It is obvious that the basic requirements of a metal oxide sensing element are to enable the oxide to be heated to its operating temperature and to enable its electrical resistance to be measured. Many geometries and constructions are therefore possible. One of the earliest forms is that described by TAGUCHI (1970) in which two parallel
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
below 300°C, it will respond to carbon monoxide and not to an equal concentration of methane. However, for the reasons already indicated, adsorption of carbon monoxide at this temperature on SnO2 is slow so that such a device would have a long response time. By employing the temperature variation of the response of oxides to gases it is often possible to improve specificity by operating the oxide at a fixed chosen temperature. A further way of improving specificity is to incorporate into the oxide ions of another metal which will promote the adsorption of a particular gas (BonretaL, 1970). The gas which is adsorbed onto these ions then interacts with adsorbed oxygen on the oxide in the manner already described. It has been found that adsorption of carbon monoxide is increased by incorporating palladium into an oxide. This is illustrated in Fig. 2 for uranium oxide. The conductivity of uranium oxide itself is
J. G. FIRTH, A. JONES and T. A. JONES
68
20
S 1
5
c
I
0
0 0
/
20
m
60
80
Carbon monoxide concentration : ppm
FIG. 3. The effect of carbon monoxide on the conductivity of palladium-doped uranium oxide.
metal coils (length ~ 5 m m and separation ~0-5mm) are embedded in a bead of stannic oxide and silica. One of the coils is used to heat the oxide electrically to its operating temperature and both coils are used as electrodes. Another form of device used the oxide as a thin deposit on the surface of a small glass bead ( ~ l m m dia.) between two wire electrodes sealed into the surface of the bead which contains a heater coil (Borr et al., 1970). The physical form of the oxide itself is also important. Very high surface area materials, although desirable for sensitivity, can undergo sintering which affects the contact resistance between particles and hence can produce a slow drift of signal with time. In order to reduce this effect low surface area sintered materials are used and these are operated well below the Tamman temperature to eliminate diffusion. One method of producing oxide particles of "--0-5 y.m dia. with a narrow size distribution has been described (BOTT et al, 1970). This involves precipitation of the oxide from molten ammonium nitrate at 300°C. Material produced in this way has reproducible electrical and chemical properties. It is therefore possible to improve the specificity of an oxide towards a particular gas by careful choice of the oxide, its operating temperature and the additives incorporated into it. However, at the present time, it is difficult to obtain absolute specificity towards a given gas. It must also be remembered that the above variables are not independent. Thus the addition of other metal ions to a lattice can change the conductivity of the oxide by altering the concentration of " impurity " ions or they themselves can act as "impurity" ions in the conduction process. Other physical processes such as sintering rates can also be affected. Acknowledgement—Contributed by permission of the Director, Safety in Mines Research Establishment, Health and Safety Executive. REFERENCES BOTT, B., FIRTH, J. G., JONES, A. and JONES, T. A. (1970) Brit. Pat. 1374575.
GARNER, W. E. (Editor) (1955) Chemistry of the Solid State. Butterworths, London. GOODWTN, T. A. and MARK, P. (1972) Progress in Surface Science (Edited by DAVISON, S. G.) Vol. 1. Pergamon Press, Oxford. TAOUCHI, N. (1970) Brit. Pat. 1280809.
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
5
SOLID STATE DETECTORS FOR CARBON MONOXIDE* J. G. FIRTH, A. JONES and T. A. JONES
Abstract—The mechanism by which a gas can produce an electrical conductivity change in a semiconductor is outlined. The resulting advantages and disadvantages in the use of metal oxide semiconductors in gas detection devices for carbon monoxide are described and ways of reducing the disadvantages are discussed.
INTRODUCTION RESEARCH on solid state sensors for the detection of carbon monoxide is being carried out in many countries at the present time including Britain, America and Japan. Because the threshold limit value of carbon monoxide is 50 ppm, in air, most interest centres on the measurement of concentration ranges of 0-100 or 0-200 ppm and devices using the electrical resistance changes produced in semiconducting metal oxides by adsorption of carbon monoxide have a great deal of promise for measurement in these ranges. Such devices have a number of properties which make them attractive for use in the detection of gases in general, the most obvious of which are:
(1) they can be small and rugged and hence easily used in field equipment; (2) they only need to be connected to a supply of electrical power to produce a signal which is a function of the gas concentration; (3) they lend themselves to the production methods developed in the electronics industries and hence should be cheap and easy to manufacture. The object of this paper is to outline the mechanism by which these devices produce a signal in a gas and the practical advantages and disadvantages which result from this. SIGNAL MECHANISM
It has been known for many years (GARNER, 1955) that adsorption of a gas on the surface of a semiconductor will produce a measurable change in the electrical conductivity of the solid. Such effects have been often used in studies of adsorption processes on solids, studies of active species on surfaces and studies of the mechanism of catalytic reactions (GOODWIN and MARK, 1972). Chemisorption of any gas on the surface of a solid results in the formation of a bond which changes the electron distribution in the solid surface which in turn can result in a change in the electrical conductivity of the solid. The simplest case is the adsorption of the gas to produce charged species on the solid. The formation of such •Crown Copyright 1974. 63
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
Safety m Mines Research Establishment, Red Hill, Sheffield
64
J. G. FIRTH, A. JONES and T. A. JONES
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
species must result in the donation or abstraction of electrons from the solid depending on the sign of the charge on the adsorbed species. This electron transfer can produce a sufficient change in the concentration of electrons or holes which cause electrical conductivity in the solid to produce a measurable change in conductivity. The most widely used solids in this type of sensor are the oxides of the transition metals and heavy metals such as tin. These oxides are semiconductors because of the ability of the metal to exist in different oxidation states. All these oxides are nonstoichiometric and contain a slight excess (n-type semiconductor) or deficiency (p-type semiconductor) of metal ions. In order to preserve electrostatic neutrality, the excess metal in n-type semiconductors is present as ions with a lower charge than that of the parent metal ion in the lattice, e.g. Sn2+ in SnO2. Such ions can be represented as having localised energy levels just below that of the conduction band of the oxide and these localised electrons can be thermally excited into the conduction band of the oxide and cause it to have electrical conductivity. In the process, of course, Sn2+ is converted to Sn*+. Similarly, in p-type semiconductors, in order to preserve electrostatic neutrality, some of the metal ions are present as ions with a higher charge than that of the parent ion of the lattice, e.g. Ni3+ in NiO. These ions can be represented as having localised energy levels just above the top valence band of the oxide, and electrons can be thermally excited from the valence band into these levels leaving positive holes in the valence band which cause the oxide to have electrical conductivity. The simplest mechanism by which adsorption of a gas in an ionic form can produce a change in the conductivity of an oxide is by changing the concentration of those metal ions in the oxide lattice which have a charge different to that of the main metal ion in the lattice. Thus adsorption of neutral oxygen molecules on the surface of an. oxide to form two oxygen ions results in the transfer of four electrons from the solid to the adsorbed gas where they are localised. On n-type oxides such as zinc oxide these four electrons will be released by a change of two Zn° ions to two Zn2+ ions with a resultant decrease in conductivity. On a p-type oxide such as nickel oxide, these four electrons would be released by a change of four Ni 2+ ions to four Ni3* ions with a resultant increase in conductivity. Carbon monoxide can adsorb on an oxide surface either as a neutral molecule or as a positively charged ion (CO+) depending on the temperature and chemical nature of the metal oxide. In the latter type of adsorption an electron will be donated to the solid resulting in an increase in conductivity in an n-type oxide or a decrease in conductivity in a p-type oxide. However, if adsorbed oxygen ions are present on the surface both adsorbed ions and molecules of carbon monoxide will react with oxygen ions to form ultimately carbon dioxide which will be desorbed. The overall reaction is the removal of adsorbed oxygen ions in a neutral form and hence the electrons trapped on these ions will be donated back to the oxide lattice to become re-associated with the impurity centres. Hence adsorption of carbon monoxide will result in an increase in conductivity on a n-type semiconductor. In a carbon monoxide-oxygen mixture, oxygen will re-adsorb on the surface so that at any concentration of carbon monoxide, an equilibrium concentration of adsorbed oxide ions and carbon monoxide is present on the surface of the oxide and hence the conductivity will be determined by the gas phase concentration of gases. However, since in general, oxygen is strongly adsorbed on most oxides large changes
Solid state detectors for carbon monoxide
65
in gas phase oxygen concentration (1-50 per cent v/v) above a certain minimum concentration produce only small changes in the conductivity of the oxide. Carbon monoxide, however, is generally weakly adsorbed so that relatively large changes in the conductivity will be produced by changing the gas phase concentration of carbon monoxide. ADVANTAGES AND DISADVANTAGES
TABLE 1. CONDUCTION IN STANNIC OXIDE AT 200°C
Gas Air Hydrogen Methane Propylene Butane Carbon monoxide Carbon dioxide Hydrogen sulphide Sulphur dioxide
Concentration (v/v) 2% 2% 2% 2% 2% lOOppm 5% 20ppm lOppm
Conductance
(n-1)
1 x 10-* 27-5 x 10-^ 211 x 10-* 28-3 x 10-* 28-3 x 10-* 6-8 x 10-* 1 x 10-* 18-5 x 10-* 22-5 x 10-^
The rate at which the equilibrium conductivity in a particular gas mixture is reached will depend on the rate of adsorption of the different gases on the oxide surface. Similarly the rate at which the conductivity returns to its original value will depend on the rate of desorption of the gas from the surface. For some gases, these can be slow processes, e.g. on stannic oxide at 200°C the adsorption of methane and sulphur dioxide occurs in a few seconds but, although the readsorption of oxygen is rapid so that the return on removal of methane takes place in a few seconds, the desorption of sulphur dioxide is slow and can take up to 10 min. Hence it is possible for devices based on metal oxides to have a slow response to the gas, i.e. some minutes and a slow return after exposure to the gas. However, since both adsorption, surface
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
The concentration of electrons or holes, causing conduction in many metal oxides at temperatures up to 700°C, is sufficiently low for adsorption of a gas at low concentration to produce a large change in the conductivity of the oxide. Thus devices using metal oxides can produce large signals in low concentrations of many gases, e.g. exposure of stannic oxide at 200°C to lOOppm of CO can double the conductivity, Since the degree of adsorption of a gas is determined by the temperature and chemical nature of the solid, particularly the metal ions present, the large range of metal oxides which are semiconducting make it possible to detect and measure a large range of gases. However, it is obvious .that any gas which can adsorb on a metal oxide surface at a given temperature and cause electron transfer with the solid or can interact with adsorbed oxide ions will produce a conductivity change in the oxide. Thus in a mixture of gases, the observed conductivity change will be that produced by electron transfer from a number of gases in the mixture. This intrinsic lack of specificity is illustrated for stannic oxide at 200°C in Table 1. It is the major disadvantage of semiconductor devices and a large fraction of the research effort in this field is devoted to the improvement of specificity for particular applications.
66
J. G. FIRTH, A. JONES and T. A. JONES
reaction and desorption on solids are all activated processes, their rates increase as the temperature of the solid increases. Hence response times and recovery times of devices can be reduced by raising their operating temperature. IMPROVEMENT OF SPECIFICITY
f
col /
100 ppt
j
I" 0
I '
ppm CH 4
V
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/
200
600
400 Temperature :
C
FIG. 1. Effect of temperature on conductivity of stannic oxide in given concentrations of carbon monoxide and methane.
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
Carbon monoxide is generally encountered in gas mixtures which contain other components whose concentration can change and hence it is possible for these gases to produce spurious signals on a sensing device. Even where carbon monoxide is encountered only as a mixture in air, varying concentrations of water vapour can be present. Adsorbed water vapour acts as an electron donor on many metal oxides although the detailed mechanism of the process is not fully understood. However, in common with many gases which adsorb without reaction, the size of the effect decreases as the temperature of the oxide increases and it is often possible to operate the oxide at a temperature where the effect is zero. Even where this is not possible, the surfaces of many oxides become saturated above a partial pressure of water of about 5 mm Hg since the adsorption of water follows a normal isotherm. Hence over the normal atmospheric range of concentrations little effect of water vapour is observed. Carbon dioxide is also frequently encountered in gas mixtures containing carbon monoxide. In general, however, carbon dioxide has little or no effect on the conductivity of metal oxides. On a given oxide, the adsorption of a gas will be temperature dependent and different gases will adsorb at different temperatures. In general, the fractional change in conductivity produced by a gas of a given concentration increases as the temperature of the oxide is increased before passing through a maximum and decreasing again. This is illustrated for carbon monoxide and methane at 100 ppm in air on stannic oxide in Fig. 1. It can be seen that if this physical form of stannic oxide is operated
Solid state detectors for carbon monoxide
m
300
400
500
500
Ttflpwature : *C
Fio. 2. Effect of temperature on conductivity in given concentrations of carbon monoxide and methane of (a) uranium oxide + 1 mole% palladium; (b) uranium oxide + 1 mole% palladium+ 2 mole % cerium.
not affected by carbon monoxide, water vapour or methane. The addition of 1 mole % of palladium to the lattice produces a conduction change in lOOppm of CO with a temperature dependence similar to that shown in Fig. 1. However, the conductivity is also affected by 1 per cent methane although to a smaller extent. Further addition of 2 mole % of cerium removes the effect produced by methane. At the present time this oxide system has a long response time of about 3 min and work is in hand to reduce this. Since the adsorption of a gas will follow an isotherm of the Langmuirian or Elovich type, the conductivity change produced will not be linearly related to the concentration. The effect of CO on Pd-doped UO2 is shown in Fig. 3. This type of response curve is typical for metal oxide devices. SENSOR CONSTRUCTION
It is obvious that the basic requirements of a metal oxide sensing element are to enable the oxide to be heated to its operating temperature and to enable its electrical resistance to be measured. Many geometries and constructions are therefore possible. One of the earliest forms is that described by TAGUCHI (1970) in which two parallel
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
below 300°C, it will respond to carbon monoxide and not to an equal concentration of methane. However, for the reasons already indicated, adsorption of carbon monoxide at this temperature on SnO2 is slow so that such a device would have a long response time. By employing the temperature variation of the response of oxides to gases it is often possible to improve specificity by operating the oxide at a fixed chosen temperature. A further way of improving specificity is to incorporate into the oxide ions of another metal which will promote the adsorption of a particular gas (BonretaL, 1970). The gas which is adsorbed onto these ions then interacts with adsorbed oxygen on the oxide in the manner already described. It has been found that adsorption of carbon monoxide is increased by incorporating palladium into an oxide. This is illustrated in Fig. 2 for uranium oxide. The conductivity of uranium oxide itself is
J. G. FIRTH, A. JONES and T. A. JONES
68
20
S 1
5
c
I
0
0 0
/
20
m
60
80
Carbon monoxide concentration : ppm
FIG. 3. The effect of carbon monoxide on the conductivity of palladium-doped uranium oxide.
metal coils (length ~ 5 m m and separation ~0-5mm) are embedded in a bead of stannic oxide and silica. One of the coils is used to heat the oxide electrically to its operating temperature and both coils are used as electrodes. Another form of device used the oxide as a thin deposit on the surface of a small glass bead ( ~ l m m dia.) between two wire electrodes sealed into the surface of the bead which contains a heater coil (Borr et al., 1970). The physical form of the oxide itself is also important. Very high surface area materials, although desirable for sensitivity, can undergo sintering which affects the contact resistance between particles and hence can produce a slow drift of signal with time. In order to reduce this effect low surface area sintered materials are used and these are operated well below the Tamman temperature to eliminate diffusion. One method of producing oxide particles of "--0-5 y.m dia. with a narrow size distribution has been described (BOTT et al, 1970). This involves precipitation of the oxide from molten ammonium nitrate at 300°C. Material produced in this way has reproducible electrical and chemical properties. It is therefore possible to improve the specificity of an oxide towards a particular gas by careful choice of the oxide, its operating temperature and the additives incorporated into it. However, at the present time, it is difficult to obtain absolute specificity towards a given gas. It must also be remembered that the above variables are not independent. Thus the addition of other metal ions to a lattice can change the conductivity of the oxide by altering the concentration of " impurity " ions or they themselves can act as "impurity" ions in the conduction process. Other physical processes such as sintering rates can also be affected. Acknowledgement—Contributed by permission of the Director, Safety in Mines Research Establishment, Health and Safety Executive. REFERENCES BOTT, B., FIRTH, J. G., JONES, A. and JONES, T. A. (1970) Brit. Pat. 1374575.
GARNER, W. E. (Editor) (1955) Chemistry of the Solid State. Butterworths, London. GOODWTN, T. A. and MARK, P. (1972) Progress in Surface Science (Edited by DAVISON, S. G.) Vol. 1. Pergamon Press, Oxford. TAOUCHI, N. (1970) Brit. Pat. 1280809.
Downloaded from http://annhyg.oxfordjournals.org/ at University of Bath Library & Learning Centre on June 24, 2015
5
