JPH03134553A - Go2 gas sensor using solid electrolyte - Google Patents

Abstract

PURPOSE:To obtain the stable characteristic of electromotive force for a long time by connecting the exposed surface of a piece of solid sodium carbonate and the exposed surface of a piece of solid electrolyte and, remarkably reducing the generation of a drift with time. CONSTITUTION:A piece 11 of Na2Co3 which is a pressurized molding or sintered body of Na2Co3 is used. A metallic electrode 12 and a lead 13 are provided at one side of the Na2Co3 piece 11, thereby forming an anode. On the other hand, a metallic electrode 15 and a lead 16 are provided at one side of a sodium ion conductive solid electrolyte 14 such as NASICON etc., thereby forming a cathode. The ion conductivity between the piece 11 and electrolyte 14 is secured by connecting the exposed surfaces of the piece 11 and electrolyte 14. Accordingly, a drift of the electromotive force with time is remarkably reduced and therefore the stable characteristic of the electromotive force can be obtained for a long time.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention provides a method for measuring CO2 gas concentration in a gas phase.
2.Regarding a gas sensor. In particular, the present invention relates to the improvement of a CO2 gas sensor that combines a sodium ion conductive solid electrolyte such as NASICON and sodium carbonate.

Such a CO2 gas sensor is useful as a means for controlling CO2 concentration in, for example, a plant factory.

[Conventional technology]

We have previously proposed a CO2 sensor in which a battery is constructed by combining a sodium ion conductive solid electrolyte and sodium carbonate, and the electromotive force of the battery measures CO26 degrees in the gas phase (Tosl+io MARUY).
AMA et al,, 5solid 5tate Io
nics.

23.107-112 (1987)), the configuration and principle of the conventional CO2 sensor will be explained below. Note that, as the sodium ion conductive solid electrolyte, for example, NASICON (Na 5upper 1onlc
Na++x Zr2P
3-X SIX 012 (0≦X≦3), Ha-
Known materials such as β-alumina can be used.

FIG. 6(A) shows the conventional CO2 sensor described above. In the figure, 1 is a NASICON sintered body. At both ends of the NASICON sintered body 1, porous Au electrodes 2a and 2b are formed by applying and baking Au paste.

Leads 3a and 3b are connected to each Au electrode 2a and 2b, respectively. Further, a Na2CO3 layer 4 is formed on the surface of one porous Au electrode 2b by applying a saturated aqueous solution of Na2CO3 and drying it.

FIG. 6(B) is an enlarged view of the portion surrounded by O in FIG. 6(A). As shown in the figure, the porous Au electrode 2b is composed of a large number of Au particles 2...
The three layers 4 penetrate into the gaps between the Au particles 2....

With the above configuration, a battery is formed in which the Au electrode 2a is used as an anode and the Au electrode 2b is used as a cathode, as shown by the following formula.

CO2,02, Au/Naz CO3// NASIC
ON/Au, 02 At the anode and cathode of this battery, electrode reactions shown by the following equations occur, respectively.

Anode〉 Na2CO3−+2Na” + 28− +co□+l
/202 cathode>2Na" +2 e -+1/202- Na2O Therefore, the overall cell reaction is expressed by the following equation.

Na2CO3→CO2+ Na2O Furthermore, according to Nernst's equation, the electromotive force E of the above battery is given by the following equations (1) and (2).

E=C-R” 1 n (aNa2oPcoz)
−■F C−Ikami(ΔG°CO2+ΔG°Nm2o−Δ”Na2
C03) ``'■F However, R: Gas constant T: Absolute temperature F: Faraday constant a Na2O: Activity of Na2O in NASICON PCO2: Partial pressure of CO2 in the gas phase ΔG0.: Standard production freedom of substance f The unknown quantities on the right side of the energy formula ■ are a N520 and P.2
There are two. If a Na2O is constant, P. .

2 is a function of E, and therefore P CO2 can be determined by setting E to DI. Therefore, P.C.O.

Determine E when flowing various gases with known pl,
When we back-calculated the value of a Na2O from the measured value, we found that a
Na□. It was found that P does not depend on CO2 and always takes a constant value. Regarding this, 5olidSt
ate 1onlcs, 23.107-112 (198
7).

Therefore, in an atmosphere where P CO2 is unknown, the above conventional CO
P CO2 in the atmosphere can be detected by placing two sensors and setting the electromotive force E to 11111.

Note that other sodium ion conductive solid electrolytes can be used instead of NASICON in the above. That is, as is clear from the above, the NAS
The only condition required for ICON is that a Na2O is constant. As long as this condition is satisfied, Na-β
- Sodium ion conductive solid electrolytes other than NASICON, such as alumina, etc. may be used.

[Problems to be Solved by the Invention] The inventors conducted various studies on the above-mentioned conventional CO2 sensor and found the following problems.

That is, conventional CO. If the sensor is operated for a long time, the fifth
As shown by the curve X in the figure, the electromotive force E of the sensor gradually changes even though the P CO2 in the atmosphere is constant. This phenomenon is called drift, and it is a major obstacle to practical application because it prevents accurate ipJ determination of CO2 concentration.

In view of the above circumstances, an object of the present invention is to obtain stable electromotive force characteristics over a long period of time by improving the conventional CO□ sensor and significantly reducing the occurrence of drift over time.

[Means to solve the problem]

The above problem can be solved by infiltrating the Na2CO3 layer into the gap between the porous Au electrodes to form an anode and
Instead of forming a contact between SICON and the Na2CO3 layer, this is achieved by adopting the following configuration.

That is, as shown in FIG. 1, a Na2CO piece 11 made of a press-molded body of Na2CO or a sintered body thereof is used, and a metal electrode 12 and a lead 13 are provided on one side of the Na2CO piece 11 to form an anode. On the other hand, the cathode is formed by providing a metal electrode 15 and a lead 16 on one side of a sodium ion conductive solid electrolyte piece 14 such as NASICON, as in the conventional case. Then, as shown in the figure, Na2
By bringing the exposed surfaces of the CO3 piece 11 and the solid electrolyte piece 14 into contact, ionic conductivity between them is ensured.

In the present invention, as Na2CO311, materials having suitable strength for processing, such as pressure-molded products and sintered products thereof, can be used.

On the other hand, as the sodium ion conductive solid electrolyte 14, known ones such as NASICON and Na-β-alumina can be used. These are, for example, So I
-gel method, mechanical mill preparation method, and other known methods. For the production of NASICON, Nai PO4・12 +120 and Zr5j
04 as raw material! : You may use the simple method.

Both the Na2CO3 piece 11 and the solid electrolyte piece 14 may be cut into appropriate sizes from the block-shaped mass produced as described above.

The metal electrode 12 is made of Au, and the metal electrode 15 is made of Au.
Alternatively, it is preferable to use PT, and it is preferable to use a porous electrode so that the electrochemical reaction with the gas at the anode and cathode can be sufficiently carried out. Such a porous electrode can be formed by applying the metal paste and firing it.

In order to bring the Na2CO3 piece 11 provided with the metal electrode 12 and the solid electrolyte piece 14 provided with the metal electrode 15 into contact as shown in FIG. 1, and to generate sufficiently stable ionic conductivity between the two, It is desirable to press the two together at a pressure within a range that does not destroy either.

Further, in order to increase the contact area, it is desirable to polish the contact surfaces between the two.

The sensor element constructed as described above can be heated to a temperature of preferably 500'C by combining with a suitable heating device.
By operating within a temperature range from above to below the melting point of Na2COi, stable output corresponding to the carbon dioxide concentration can be obtained.

[Action and effect of the invention]

As will be explained in detail in the examples below, with the above configuration,
The drift of the electromotive force E over time, which was a problem with conventional CO□ sensors, was significantly reduced, and stable electromotive force characteristics could be obtained over a long period of time. From this fact, it can be considered that the drift that has been a problem in the past is due to the following causes.

As shown in FIG. 6(B), the part of the Na2C03 layer 4 of the conventional CO2 gas sensor that is effective as an anode enters the gap between the porous Au electrodes 2a and becomes NASICO.
This is only the part that is in contact with N1. When the sensor is operated, a kind of reaction layer is formed near the contact interface between the two due to a chemical reaction. At this time, in the conventional CO2 gas sensor, since the effective portion of the Na2CO3 layer 4 itself is extremely thin, this reaction layer has a large influence and changes the electromotive force characteristics. This seems to be the primary cause of drift.

In addition, in the conventional CO□ gas sensor, in addition to the bond between the Na2CO3 layer 4 and NASICON 1, the Au particles 2 and N
A junction with ASICONI is formed. The reaction layer is also formed near the interface between the two on the NASICON I side. Therefore, in addition to the target batteries mentioned above,
A second battery using this reaction layer and the Au particles 2 as anodes is formed in parallel.

Therefore, when a conventional CO2 gas sensor is operated for a long period of time, an electromotive force is generated by this second battery. This seems to be the second cause of drift.

The present invention eliminates the above two causes of drift by using independently formed Na2 C03 pieces 11 and mechanically mating the NASICON pieces 14 by approximately 14 degrees. That is, in the present invention, Na2co constituting the anode
Since the thickness of the piece 11 is large, the influence of the reaction layer can be kept small. In addition, the Au electrode 1 on the anode side
2 has no contact with the NASICON piece 14, so Na
z The boundary between CCh piece 11 and NASICON piece 14 is about 1
Even if the fourth reaction layer is formed, no electromotive force is generated by the second battery.

However, conventionally, it was thought that drift was caused not by the above-mentioned cause on the anode side but due to another cause on the cathode side. That is, according to the conventional thinking, as the battery reaction described above progresses, the NASICON on the cathode side
Na of NASICON 1 at the interface between 1 and Au electrode 2b
It was thought that the cause was a change in 2O activity.

Therefore, the present invention is not limited by the conventional way of thinking;
By making improvements from a completely different perspective, the conventional C
This should be evaluated as having solved the problem in O□ gas sensors.

〔Example〕

FIG. 2 is a perspective view showing a CO2 gas sensor according to an embodiment of the present invention. In the same figure, 21 is 7■×4II
It is a Na2Co3 piece cut out to the dimensions of IIX211I11. A porous Au electrode 22 is formed on one end surface along the longitudinal direction of this Na2CO3 piece 21, and the electrode has a
A u lead wire 23 is connected. - direction, 24 is 7 m
This is a NASICON piece cut out to a size of wX 4 mtsX 2 tits. A porous Au electrode 25 is also formed on one end surface along the longitudinal direction of the NASICON piece 24, and an Au lead wire 26 is connected to the electrode. N
The surfaces of the a2CO piece 21 and the NASICON piece 24 were polished with No. 1200 paper.

The Na2COi piece 21 and the NASICON piece 24 are stacked one on top of the other so that the respective porous Au electrodes 22 and 25 are located on opposite sides like a single claw 4. Furthermore, as shown in the figure, both are stacked with their positions shifted in the longitudinal direction. As a result, the Au electrode 22 of the Na2CO3 piece 21 becomes
Prevents the contact claw 4 from forming on the SICON piece 24, and also prevents the NASI
The Au electrode 25 of the CON piece is Na2co.

This prevents contact with the piece 21.

The Naz 003 piece 21 and the NASICON piece 24 are sandwiched between two alumina brakes (27, 28), and are pressure-welded by tightening with heat-resistant bolts (29, 30).

With the configuration of the above embodiment, the preferable conditions described above are realized. That is, the NASICON piece 24 and the single claw 4
Since the contact surfaces of the CO3 pieces 21 are polished and the two are pressed together via the alumina plates 27 and 28 and the heat-resistant bolts 29 and 30, good ionic conductivity can be achieved between the two. Moreover, since the Aug electrodes 22 and 25 are porous, electrode reactions can be efficiently caused at the anode and cathode. In addition, NASI
By stacking the CON piece 24 and the Na2CO3 piece 21 with their positions shifted as described above, a situation where the two are short-circuited via the Au electrode is prevented.

Next, an example of manufacturing a CO2 gas sensor according to the above embodiment,
The tests conducted to evaluate its characteristics will be explained.

Production Example> Creation of Na2COi Piece 21 Na2CO3 as a raw material was dried in advance at 200''C for 1 hour, and then ground in a planetary agate mill for about 5 minutes.

By applying a pressure of about 3 t/ca+2 to this pulverized raw material, it was pressure-molded into a shape with a diameter of 20 rAm and a thickness of about 3II11. This molded product was placed on an alumina board and fired at 740° C. for 5 hours in an air atmosphere to obtain a sintered body.

From the sintered body obtained above, the length is 7 cm, the width is 4 cm, and the height is 2 m.
A piece 21 of Na2CO of m was cut out. After roughening this with 800 grit sandpaper, use 120 grit sandpaper.
Finish polishing was performed using No. 0, and the dirt on the surface was wiped off.

Preparation of NASICON Piece 24 Single Claw 4 PO4' 12 +120 and Zr5I04 were weighed at a molar ratio of 1:2, and ground in an engineering plastic mortar for about 5 minutes. This mixture was placed in an alumina crucible, dried at 200° C. for 6 hours, and then ground for about 5 minutes using a planetary agate mill. By applying a pressure of approximately 3t/C112 to this raw material powder, a diameter of 2
It was press-molded into a shape with a thickness of about 5I1m and a thickness of about 5I1m. This molded product was placed on a PT receipt and exposed to air for 11 days.
It was calcined at 47°C for 50 hours.

This calcined body was crushed again in an agate mortar, and then crushed in a planetary agate mill for about 5 minutes. Approximately 3 tons to the obtained powder
A pressure of /c 112 or more was applied to form the product into a shape with a diameter of 201111% and a thickness of about 31%. The obtained molded product was placed on a PT receipt and heated at 1247°C for 1
By firing for 2 hours, a sintered block of NASICON was obtained.

From the sintered body obtained above, a NASICON piece 24 with a length of 71111%, a width of 4mm, and a height of 2ma+ was roughly ground with a sand vapor number 800, and then finished polished using a sandpaper number 1200. did. Furthermore, ultrasonic cleaning was performed for 1 minute using distilled water.

Formation of electrodes 22.25 Naz 003 piece 21 and NASIC made above
Au lead wires 23 and 26 are connected to 4 parts between each of the ON pieces 24.
After winding, Au paste was applied so that the winding was in contact with the single lead wire. Next, by sintering at 700° C. or higher, a porous Au electrode 22.25 was formed.

Assembly of the sensor The above Na2CO forming the porous Au electrode 22.25
, piece 21 and NASICON piece 24 were overlapped as shown in the space between the pieces 4. This is alumina plate 27.
28 and heat-resistant bolts 29.30, the CO2 gas sensor of the embodiment shown in FIG. 2 was assembled by tightening the Na2co piece 21 and the NASICON piece 24 as tightly as possible within the range between the broken pieces.

Test method As shown in Fig. 3, a tubular electric furnace 31 having an isothermal zone wider than the dimensions of the sensor of the above embodiment was used, and a quartz tube with a diameter of 20+ oa+ was placed in the center of the electric furnace 31. Furnace core tube 3
Passed 2. Inside this furnace core tube 32, C of the above embodiment is placed.
An O2 gas sensor 20 was placed. The gas sensor 20 is N
It is desirable to operate the ASICON at a temperature at which its ionic conductivity is expressed and a stable electrode reaction occurs (a temperature between about 500°C and below the melting point of Na2C (h). Therefore, the temperature of the sensor 20 is set to about 620°C. The heating temperature of the electric furnace 31 was set.The temperature of the sensor 20 was measured with a thermocouple 33 placed near the sensor 20.
The power unit 35 is a temperature controller.

Further, in order to keep the CO2 concentration near the sensor 20 constant, a standard gas with a known CO2 concentration (500 ppm to about 20%) was flowed into the furnace core tube 32 from the cylinder 34 through the flow meter 37. Standard gas flow rate is 200cc/ll
It was set to 1n.

The electromotive force of the CO2 gas sensor 20 is processed by a data logger 28 and a personal computer 29 and displayed on a display 40.

Test results■ By changing the CO2 concentration of the standard gas, the CO2 of the electromotive force E
The dependence on concentration was investigated. The results are shown in FIG. As shown in the figure, in the CO□ sensor 20 of the above embodiment,
The electromotive force E has good linearity with respect to log(Pco2). From this, it can be seen that the sensor 20 of the above embodiment functions as a good CO2 sensor.

(2) The CO2 concentration of the standard gas was fixed at 972 ppm, and the stability of the electromotive force E over time was tested. The results are shown in curve A in FIG.

For comparison, a similar test was also conducted on the conventional CO2 sensor shown in FIG. The results are shown as curve X in FIG.

As shown in the figure, the CO2 sensor of the above embodiment has significantly smaller drift than conventional sensors and has extremely stable electromotive force characteristics.

More specifically, based on the measured value of the electromotive force E, CO
2. Calculate the activity of Na2O using the time of switching the concentration as a standard,
Using this value, C after 900ks (approximately 10EI) has elapsed.
When calculating the O□ concentration, the calculated value for the conventional sensor is 0.172%. Since the true 002a degree is 0.992%, the relative error in this case reaches -82.7%. On the other hand, in the case of the sensor of the above example, the CO2 concentration calculated as above is 0.981%,
The relative error is only -3.1%. This result clearly shows that the output drift in the CO2 sensor of the above example has been significantly improved.

[Brief explanation of the drawing]

Fig. 1 is a diagram conceptually showing the configuration of a CO2 gas sensor according to the present invention, Fig. 2 is a perspective view showing a preferred embodiment of the CO2 gas sensor according to the present invention, and Fig. 3 is a diagram showing a configuration for testing the output characteristics of the CO2 gas sensor. FIGS. 4 and 5 are diagrams showing the output characteristics of the CO2 gas sensor according to the embodiment shown in FIG. 2, and FIG. 6 is a sectional view showing the conventional CO2 gas sensor.

Claims (4)

[Claims] (1) A CO_2 gas sensor consisting of a battery that combines an anode part and a cathode part that are each formed independently, and is operated under predetermined temperature conditions, and is constructed by forming an electrode on a part of the surface of a solid sodium carbonate piece. and a cathode part configured by forming an electrode on a part of the surface of a sodium ion conductive solid electrolyte piece, the exposed surface of the solid sodium carbonate piece and the exposed surface of the solid electrolyte piece. A CO_2 gas sensor characterized by being in contact with. (2) C according to claim 1, wherein the electrode is a porous electrode.
O_2 gas sensor. (3) The CO_2 gas sensor according to claim 1 or 2, wherein the contact surfaces of the solid sodium carbonate piece and the solid electrolyte piece are polished, and both are mechanically tightened and fixed. (4) The CO_2 gas sensor according to any one of claims 1 to 3, wherein the solid electrolyte piece is a NASICON piece.