JP3312781B2 - Gas concentration measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel method for measuring gas concentration using a solid electrolyte type gas sensor element. More specifically, in a gas concentration measurement method for a long period of time using the gas sensor, an error in the measurement value is extremely small, and the gas concentration measurement method can significantly improve the reliability over time.

[0002]

2. Description of the Related Art Conventionally, measurement of inorganic gases such as carbon dioxide, nitrogen oxides and sulfur oxides in the atmosphere has been carried out in various fields and used for combustion control and environmental measurement.
Various analytical methods such as an infrared absorption method, an ultraviolet absorption method, and a chemiluminescence method have been mainly employed for the measurement of such an inorganic gas. However, it has been pointed out that the apparatus is large, expensive, and requires maintenance. Was. For these devices, a solid electrolyte type gas sensor element utilizing a change in electromotive force of a solid electrolyte has been developed as a small, simple and inexpensive gas sensor element.

As the solid electrolyte type gas sensor element,
One of a pair of electrodes formed through the solid electrolyte layer,
A gas sensor element having a structure in which an auxiliary electrode layer made of a metal salt having a dissociation equilibrium between the mobile ion species of the solid electrolyte layer and the gas to be measured has been proposed. For example, JP
No. 0-256043 discloses β-Al ₂ O ₃ or Na _{1 + x} Zr ₂ which is a sodium ion conductor as a solid electrolyte layer.
Si _x P _{3 over} x O ₁₂ (where x is from 0.4 to 2.8, typically N
ASICON), an auxiliary electrode layer made of a metal salt of the same or a different element as the mobile ions of the solid electrolyte layer is provided on one of the electrodes provided via the solid electrolyte layer to form a working electrode. A formed solid electrolyte type gas sensor element has been proposed.

When the gas sensor element having the above configuration is placed in an atmosphere containing the gas to be measured, an electromotive force according to the Nernst equation shown in the following (1) is generated between the two electrodes,
The concentration of the gas to be measured can be measured from this electromotive force.

EMF = C−RT / nF · ln [G] (1) where EMF is an electromotive force of the solid electrolyte type gas sensor element, C is a constant, R is a gas constant, T is a temperature of the gas sensor element, and n is a temperature of the gas sensor element. The reaction order, F is the Faraday constant, and [G] is the concentration of the gas to be measured in the atmosphere.

The electromotive force in the above-mentioned solid electrolyte type gas sensor element is generally measured by amplifying it with an amplifier represented by an operational amplifier.

[0007]

When the concentration of the gas to be measured is measured using the solid electrolyte type gas sensor element, the gas sensor element is electrically connected to a voltage measuring circuit. Therefore, in such a circuit, a direct current always flows in a fixed direction through the gas sensor element.

However, when the solid electrolyte type gas sensor is used for a long period of time, a phenomenon occurs in which an electromotive force (hereinafter, referred to as a reference electromotive force) as a reference for measurement gradually increases or decreases. as a result,
There is a problem that the error of the measured value of the concentration of the gas to be measured increases with time.

[0009]

Means for Solving the Problems The present inventors have developed a measuring method capable of suppressing the above-mentioned change in the reference electromotive force of such a solid electrolyte gas sensor element and maintaining the accuracy of the concentration of the gas to be measured for a long period of time. I did research to do it.

As a result, in a solid electrolyte type gas sensor element in which the mobile ion and the ion species of the gas to be measured are different due to the use of the auxiliary electrode material, the mobile ion moves in the solid electrolyte by the electric current and moves to one electrode side. It has been found that the accumulation causes the electromotive force of the gas sensor element to change. Based on such findings, as a result of further research, the amount of current flowing through the solid electrolyte was determined by intermittently connecting the solid electrolyte gas sensor and the amplifier and intermittently connecting the current flowing through the solid electrolyte. In addition, when the solid electrolyte type gas sensor and the amplifier are not electrically connected, the changed electromotive force can be returned to the initial state to some extent. As a result, it has been found that the change in the reference electromotive force of the solid electrolyte gas sensor can be reduced, and that the gas to be measured can be measured and measured with high accuracy over a long period of time, and the present invention has been proposed.

That is, according to the present invention, a solid electrolyte type gas sensor element having a working electrode formed by providing an auxiliary electrode substance in at least one electrode layer is connected to a voltage measuring circuit, and the sensor corresponding to the concentration of the gas to be measured is provided. upon measuring the electromotive force of the device intermittently performs the connection between the solid electrolyte type gas sensor and a voltage measurement circuit, have <br/> line measurement of the electromotive force at the time of the connection, and at the time of non-connection solid electrolyte Type gas sensor element
And the polarity opposite to the electromotive force generated by the solid electrolyte gas sensor element.
This is a method for measuring gas concentration, wherein no voltage is applied .

As the solid electrolyte type gas sensor element used in the present invention, a solid electrolyte type gas sensor element having a known structure in which an auxiliary electrode substance is present in at least one electrode layer to constitute a working electrode is used without any particular limitation.

FIG. 1 shows a typical structure. That is, it has a pair of electrode layers 7 and 9 with the solid electrolyte layer 5 interposed therebetween, the auxiliary electrode substance 8 is present on one of the electrode layers 7 to constitute a working electrode, and the other electrode layer 9 is used as a reference electrode This is an embodiment configured as such. In the figure, reference numerals 2 and 3 denote a heater for heating the solid electrolyte type gas sensor element and an alumina substrate, respectively, and reference numeral 1 denotes a power supply for the heater. Reference numeral 6 denotes a platinum wire constituting a lead for taking out the output of the solid electrolyte gas sensor element.

The mode of the solid electrolyte type gas sensor element used in the present invention is not limited to the mode in which the layers are provided in the vertical direction as shown in FIG. 1, but the layers are arranged in parallel on the same plane. A known structure such as a formed thin film type can be adopted, and what kind of structure is adopted may be appropriately selected according to the use of the gas sensor element.

In the solid electrolyte type gas sensor according to the present invention, as the solid electrolyte material constituting the solid electrolyte layer, a material having ion conductivity and being solid under the use conditions is generally used. For example, NASICON, β-Al having ion conductivity of sodium, lithium, etc.
₂ O ₃ , β-Ga ₂ O ₃ , Li _{16 -2x} Zn _x (GeO ₄ )
₄ (where x is 0 ≦ x <8 and is generally called LIICON), Li ₄ GeO ₄ —Li ₃ VO _{4 and the} like are preferably used.

As a method for producing the solid electrolyte layer, a conventionally known method can be employed without any limitation. Generally, a method of sintering powder after molding, a method of kneading the powder with a binder and a solvent to form a paste, then producing a green sheet by a doctor blade method and finally sintering, or a thin film forming method such as a sputtering method For example, a method of forming a substrate on a substrate is preferably used.

In the solid electrolyte type gas sensor of the present invention, the electrode layer present via the solid electrolyte layer is not particularly limited as long as it is made of a substance having electron conductivity. For example, specific examples of the substance having electron conductivity include noble metals such as platinum, gold, silver, palladium, and rhodium and oxides thereof, and a general formula La _1-x Sr _x BO ₃
(Where B represents an element such as Co, Cu, Fe, Ni, etc., x
Is a number of 0.01 to 0.5), and a conductive perovskite-type oxide represented by, for example, a composite composition obtained by mixing the above-mentioned noble metal and metal oxide. Among them, especially platinum, gold,
Precious metals such as silver, palladium and rhodium are preferably used, and more preferably platinum and gold. The electrode layer is
Each is composed of the same or different kinds of the above-mentioned electron conductive materials.

In the solid electrolyte type gas sensor according to the present invention, the working electrode is constituted by allowing an auxiliary electrode substance to be present in the electrode layer. As the auxiliary electrode substance, any substance having a dissociation equilibrium with the gas to be measured can be used without limitation. In particular, alkali metals such as Na, Li, and K and carbonates of alkaline earth metals such as Ca, Mg, and Ba; Bi, Ag, La, Cu
Carbonate or alkali metal such as Na, Li, K or C
Sulfates and nitrates of alkaline earth metals such as a, Mg and Ba are suitably selected according to the gas to be measured.

Further, the embodiment in which the working electrode is formed by allowing the auxiliary electrode substance to exist in the electrode layer is a mode in which the working electrode is formed by laminating the auxiliary electrode substance on the electrode layer as shown in FIG. An embodiment in which the working electrode is formed by mixing the constituent material and the auxiliary electrode material is employed without any particular limitation.

Further, the auxiliary electrode substance is generally present only on the working electrode side, and the gas sensor having such a structure does not require a reference gas on the reference electrode side when detecting gas, so that it is small and has a simple structure. It has the characteristic that it is. Of course, a structure in which the auxiliary electrode material is present in both electrode layers is also adopted, but in this case, the reference electrode is brought into contact with the atmosphere in which the working electrode is to be measured, and the reference electrode is brought into contact with a certain reference gas atmosphere. It is necessary to measure the concentration of the measurement gas.

The method for forming the auxiliary electrode material is not particularly limited. For example, a method of impregnating the electrode layer with an aqueous solution of the auxiliary electrode material, a method of pasting the auxiliary electrode material with terpineol or the like, printing, and then firing are appropriately adopted according to the above-described working electrode.

The other structure of the solid electrolyte type gas sensor according to the present invention is not limited at all, and a known structure is employed without any particular limitation. For example, a solid electrolyte type gas sensor is generally used by being heated to 100 to 800 ° C., and such heating may be performed by radiation from a heat source external to the gas sensor, or an alumina substrate as shown in FIG. A heater 2 which is obtained by screen-printing a platinum paste in a wave form and firing it may be provided on 3, and a DC or AC voltage may be applied to the heater from a power supply 1. The mounting position of the heater is not particularly limited as long as it does not hinder the operation of the gas sensor, such as on the electrode layer 9 constituting the reference electrode.

In heating the gas sensor, it is possible to heat the gas sensor to the operating temperature at least only when the gas sensor is connected to the voltage measuring circuit. In order to return to the initial state, it is necessary to heat the gas sensor to the operating temperature even when the gas sensor is not connected.

Further, when an organic gas such as toluene, ethyl acetate or ethanol coexists in the gas to be measured, a filter for selectively cutting off the organic gas is provided between the working electrode and the gas to be measured. May be provided. As the filter, a known filter having such characteristics can be used without any limitation.

In addition, for example, when measuring the gas concentration in a room and measuring the change in the gas atmosphere in the entire space, such as for air conditioning of a room, the connection between the solid electrolyte type gas sensor element of the present invention and a voltage measurement circuit is performed. In order to prevent a local change in gas concentration from being detected when measuring the gas concentration, a means for delaying gas diffusion between the gas sensor element and the gas to be measured and averaging the gas concentration should be taken. Is preferred. As the means, any known method can be used without any limitation. In general, a mode in which a filter or a fixed volume air reservoir is provided between the solid electrolyte type gas sensor element and the gas to be measured is adopted.

A feature of the present invention resides in that the solid electrolyte gas sensor element and the voltage measuring circuit are intermittently connected, and the electromotive force is measured at the time of the connection.

FIG. 2 shows an example of a circuit for measuring the concentration of a gas to be measured using a conventional solid electrolyte type gas sensor element. That is, the measurement of the concentration of the gas to be measured is generally performed by using an electromotive force generated when the solid electrolyte gas sensor element 10 comes into contact with the gas from an amplifier such as an operational amplifier 11 and a voltmeter 12 as shown in FIG. It has been performed by measuring with a voltage measuring circuit, and converting this into a concentration. That is, the generated electromotive force is measured by the voltmeter 12 after impedance conversion by the operational amplifier 11. At the time of the above-described electromotive force measurement, a high impedance operational amplifier is preferably used, and a constant bias current flows in a constant direction in the operational amplifier. This current appears as a flow of mobile ions in the solid electrolyte. As a result, the activity of the solid electrolyte at the interface of mobile ions changes, so that the reference electromotive force of the solid electrolyte sensor is in a certain direction, that is, increases or decreases. To change. Further, even when an amplifier such as an operational amplifier is not used, a battery is formed by connection to the measurement circuit, and a current also flows through the solid electrolyte gas sensor element connected to the circuit, thereby causing the same phenomenon.

According to the present invention, the connection between the solid electrolyte gas sensor element and the voltage measurement circuit is intermittently performed during a predetermined measurement time, and the electromotive force is measured only during the connection time, so that the measurement can be performed within a predetermined time. The total amount of current flowing through the solid electrolyte can be minimized, and the changed electromotive force can be returned to the initial state to some extent during the non-connection time, minimizing the reference electromotive force in one direction to a minimum. He succeeded.

In the present invention, as a method for intermittently connecting the solid electrolyte gas sensor element and the voltage measuring circuit, a known method can be used without any limitation. For example, a method of intermittently connecting by combining a timer and a switch or a relay is preferably employed.

The interval between the connection time and the non-connection time may be arbitrarily set in consideration of the characteristics of the gas sensor element, the required response speed, the measurement accuracy, and the like, and is not particularly limited. In general, it is desirable to suppress the total amount of current flowing through the solid electrolyte during a certain measurement time, and for that purpose, it is preferable to make the non-connection time longer than the connection time.

However, if the total time of the connection time and the non-connection time is too large, it is expected that a problem will occur in that the response time will be delayed, and an appropriate time may be set for use. In general, it is preferable to intermittently connect the gas sensor element and the measurement circuit so that the connection time becomes 0.05 to 3 seconds via an interruption time of 0.5 to 300 seconds.

An aspect in which the effect of the present invention is particularly remarkably exhibited is when the amount of current flowing through the voltage measuring circuit is small. As such a circuit, an amplifier, particularly a voltage measuring circuit using an operational amplifier, is used. This is preferable because the bias current is small and the amount of current flowing through the solid electrolyte gas sensor element can be reduced.

FIG. 3 shows a typical circuit diagram for implementing the method of the present invention. That is, in the embodiment shown in FIG. 3, the solid electrolyte type gas sensor element 16 and the measurement circuit including the operational amplifier 13 and the voltmeter 15 are electrically connected via the switch including the relay 14 having the timer 17. Things.

According to such a circuit, the connection between the solid electrolyte type gas sensor element 16 and the voltage measuring circuit is intermittently performed (intermittently), and by measuring the electromotive force at the time of such connection, the reference can be maintained for a long time. The measurement can be performed while keeping the rate of change of the electromotive force low.

When measuring the electromotive force, an error may occur in the measured gas concentration due to the influence of external electric noise. In this case, the measurement accuracy is improved by averaging the measured electromotive forces several times. It is possible to improve.

In the measuring method of the present invention, known means are used without any particular limitation as other means. For example, the measurement of an electromotive force by a voltmeter can be performed by combining an ammeter and a resistor and calculating from a measured current value. Further, in calculating the gas concentration from the measured electromotive force, a calibration curve indicating the relationship between the gas concentration and the electromotive force is created in advance, and the gas concentration can be obtained from the electromotive force.

[0037]

According to the gas concentration measuring method of the present invention, the amount of current flowing through the solid electrolyte type gas sensor is intermittently limited, so that the change in the electromotive force of the gas sensor can be suppressed low. Since the electromotive force changed during the period returns in the direction of the initial state, the measurement can be performed reliably without a decrease in the accuracy of the measured value of the gas concentration over a long period of time.

The method for measuring gas concentration according to the present invention is suitable for a field where the interval between measurement points may be large, for example, measurement in a case where the change in gas concentration is relatively slow such as measurement of gas concentration in the air or indoors. .

[0039]

The above-mentioned effects according to the present invention not only suppress the decrease in the accuracy of the time when the connection between the solid electrolyte gas sensor element and the voltage measuring circuit is disconnected, but also reduce the current flow during the disconnection. It is presumed that mobile ions in the solid electrolyte accumulated near the electrodes return to an initial state to some extent due to the action of diffusion or the like due to the flow of.

[0040]

EXAMPLES The present invention will be described specifically with reference to the following examples, but the present invention is not limited to these examples.

Examples 1 and 2 and Comparative Example 1 A solid electrolyte type gas sensor element for measuring carbon dioxide having a structure as shown in FIG. 1 was produced. NA as the solid electrolyte 5
Approximately 5m in diameter obtained by molding and sintering SICON powder
A disc-shaped sintered body having a thickness of about 0.5 mm and a thickness of about 0.5 mm was used. Electrode layers 7 and 9 were formed on both surfaces by screen printing, drying and baking at 700 ° C. a gold paste. On the electrode layer 7, as the auxiliary electrode material 8, lithium carbonate was formed into a paste with terpineol in which 5% by weight of ethyl cellulose was dissolved, and the paste was screen-printed, dried, and then melted at 650 ° C.

The alumina substrate 3 is provided on the electrode layer 9 side.
And a heater 2 formed in a corrugated shape by screen printing with a platinum paste, and joined by an adhesive 4 made of glass. The sensor element was used by applying a DC voltage from a power supply 1 to a heater 2 and heating it to 450 ° C.

The evaluation of the stability over time was performed with the sensor left in the air. In Examples 1 and 2, the sensor 16 was connected as shown in FIG. 3, and the output of the operational amplifier 13 measured by the voltmeter 15 was monitored as the electromotive force of the sensor. Input to the operational amplifier was performed intermittently using the relay 14 linked to the timer 17. The connection and non-connection times were 1 second connection and 59 seconds non-connection in the first embodiment, and 0.2 seconds connection and 20 seconds non-connection in the second embodiment. On the other hand, in the case of Comparative Example 1, the measurement was performed with the sensor 10 continuously connected to the operational amplifier 11 as shown in FIG. As the operational amplifier, a type (LF356, manufactured by National Semiconductor) in which current flows from the carbon dioxide sensor toward the operational amplifier was used. It is generally said that the concentration of carbon dioxide in the atmosphere is about 350 ppm, and it has been confirmed that this concentration hardly changes during the evaluation of stability over time.

Table 1 shows changes in the electromotive force of the sensor when the carbon dioxide concentration was measured over a long period of time by the measuring methods of Examples 1 and 2 and Comparative Example 1. In the measurement method of Comparative Example 1, the electromotive force of the sensor in the atmosphere gradually decreases during long-term measurement, whereas in the intermittent measurement as in Examples 1 and 2, the sensor It was found that the rate of change of the electromotive force in the atmosphere was very slow.

[0045]

[Table 1]

Examples 3 and 4, Comparative Example 2 A solid electrolyte type gas sensor element for measuring nitrogen dioxide having a structure as shown in FIG. 1 was produced. Solid electrolyte 5, electrode 7,
9. An element heating heater similar to that of Example 1 was manufactured by the same method. As the auxiliary electrode material 8, barium nitrate was formed into a paste with terpineol in which 5% by weight of ethylcellulose was dissolved, and the paste was screen-printed, dried, and then melted at 600 ° C.

The evaluation of the stability over time was performed while the sensor was left in an atmosphere of 1 ppm of nitrogen dioxide. Example 3,
In the case of 4, the sensor 16 is connected as shown in FIG.
The output of the operational amplifier 13 measured by the voltmeter 15 was monitored as the electromotive force of the sensor. The input to the operational amplifier was performed intermittently using the relay 14 linked to the timer 17. The connection and disconnection times are set to 1 second connection and 29 seconds non-connection in the third embodiment, and 0.1 second in the fourth embodiment.
Second connection and non-connection for 20 seconds. On the other hand, in the case of Comparative Example 2, the measurement was performed with the sensor 10 continuously connected to the operational amplifier 11 as shown in FIG. As the operational amplifier, a type (LF356, manufactured by National Semiconductor) in which current flows from the nitrogen dioxide sensor toward the operational amplifier was used.

[0048]

[Table 2]

Table 2 shows changes in the electromotive force of the sensor when the nitrogen dioxide concentration was measured for a long period of time by the measurement methods of Examples 3 and 4 and Comparative Example 2. In the measurement method of Comparative Example 2, the electromotive force of the sensor gradually decreases during the long-term measurement, whereas in the intermittent measurement as in Example 2, the change in the electromotive force of the sensor is It turned out to be very slow.

Example 5 and Comparative Example 3 A solid electrolyte gas sensor for measuring carbon dioxide was produced in exactly the same manner as in Example 1, and the stability over time was evaluated in exactly the same manner as in Example 1. In the case of the fifth embodiment, wiring is performed as shown in FIG. 3, and the electromotive force of the sensor is measured under the conditions of 1 second connection and 59 seconds of no connection. Were measured continuously. As the operational amplifier, an LM741C (manufactured by National Semiconductor) having a relatively large bias current and having a current flowing from the carbon dioxide sensor toward the operational amplifier was used.

Table 3 shows changes in the electromotive force of the sensor in the atmosphere when the carbon dioxide concentration was measured over a long period of time by the measurement methods of Example 5 and Comparative Example 3. In the continuous measurement method as in Comparative Example 3, the electromotive force of the sensor in the atmosphere rapidly decreases during long-term measurement, whereas in the intermittent measurement as in Example 5, the sensor is The rate of change of the electromotive force in the atmosphere is very slow. Furthermore, the rate of change of the electromotive force in Example 5 was much smaller than 1/60 of the rate of change in electromotive force in Comparative Example 3, which is expected by simply intermittently measuring.

[0052]

[Table 3]

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a typical solid electrolyte gas sensor.

FIG. 2 is a wiring diagram for a general electromotive force measurement using an operational amplifier.

FIG. 3 is a wiring diagram for measuring an electromotive force using an operational amplifier.

[Explanation of symbols]

 Reference Signs List 1 power supply 2 heater 3 alumina substrate 4 glass 5 solid electrolyte layer 12, 15 voltmeter 6 platinum wire 7, 9 electrode layer 8 auxiliary electrode material 11, 13 operational amplifier 14 relay 10, 16 sensor 17 timer

Claims (1)

(57) [Claims] 1. A solid electrolyte type gas sensor element having a working electrode formed by providing an auxiliary electrode substance in at least one electrode layer is connected to a voltage measuring circuit, and the starting of the sensor element corresponding to the concentration of a gas to be measured is performed. upon measuring the power, intermittently performs the connection between the solid electrolyte type gas sensor element and a voltage measuring circuit, have rows measurement of the electromotive force at the time of the connection, or
When the solid electrolyte type gas sensor element is
The electromotive force generated by the electrolyte gas sensor element
A method for measuring gas concentration, characterized in that no voltage is applied .