JP3295325B2 - Method for producing gas-sensitive body for carbon dioxide gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an alkali metal ion conductive material for detecting carbon dioxide gas in an atmosphere in various fields such as combustion control, plant growth management, fermentation management, environmental monitoring, and medical diagnosis. Solid electrolyte (hereinafter referred to as
(Also referred to as a solid electrolyte). In particular, the present invention relates to a method for manufacturing a gas-sensitive body for a carbon dioxide gas sensor in which a decrease in output and sensitivity due to long-time use in a high-temperature and high-humidity atmosphere is prevented, and stability over time is improved.

[0002]

2. Description of the Related Art Conventionally, carbon dioxide concentration has been measured in various fields. Infrared absorption or glass electrode systems have been mainly used for measuring these carbon dioxide concentrations.However, it has been pointed out that the devices using these methods are large and require maintenance even in normal measurement. I was

[0003] For these devices, there has been proposed a carbon dioxide gas sensor which utilizes a change in electromotive force of a solid electrolyte which is a gas-sensitive substance, which is small, simple and inexpensive.

The carbon dioxide gas sensor is composed of a solid electrolyte layer which is a gas-sensitive substance, and a working electrode layer and a reference electrode layer accompanying the solid electrolyte layer. However, the carbon dioxide gas sensor using the above-mentioned solid electrolyte as a gas-sensitive substance has a problem that the sensitivity is reduced due to the influence of water vapor generated in the atmospheric gas or water generated by dew condensation and the long-term reliability is lacking. Was.

[0005] As a conventional method for manufacturing a carbon dioxide gas sensor, the bipolar layer and a solid electrolyte, such as NASICON (NA), are used.
Also called SICON. ), A method of inserting a zirconia-yttria mixture (hereinafter referred to as YSZ), which is a ceramic, has been adopted. For example, Japanese Patent Application Laid-Open
4553 and JP-A-4-230842 propose a method of printing a reference electrode layer on a YSZ piece and pressing the YSZ piece against a NASICON pellet.

[0006]

However, in the step of pressing, it is not only difficult to install the apparatus, but also the operation is not easy. In addition, the adhesiveness between the pressed NASICON pellets and the YSZ pieces is poor, and they are easily peeled off. Furthermore, although it has an effect on water vapor in the operating atmosphere and water generated by its condensation, the carbon dioxide gas sensor obtained by such a method cannot prevent moisture for a long time in a high-temperature and high-humidity atmosphere, and the output electromotive force decreases. However, it was difficult to prevent this.

Therefore, it can be easily produced from a solid electrolyte,
It has been desired to develop a gas-sensitive body for a carbon dioxide gas sensor having a stable sensitivity without a change in output electromotive force for a long time even in a high-temperature and high-humidity atmosphere.

[0008]

Means for Solving the Problems The present inventor has made intensive studies to solve such problems. As a result, when a compact made of a solid electrolyte (hereinafter, referred to as a solid electrolyte pellet) is embedded and sintered with ceramic powder, a ceramic layer is firmly formed from the surface of the pellet to the inside thereof, so that the ceramic layer is formed in a high-temperature and high-humidity atmosphere. In addition, it has been found that the resistance to water vapor and water for a long time (hereinafter referred to as water resistance) has been improved, and it has been found that the above-mentioned problem can be solved, and the present invention has been proposed. Furthermore, after embedding and sintering the solid electrolyte pellets with ceramic powder, by performing a polishing treatment, the adhesion between the working electrode layer and the reference electrode layer is further improved due to the anchor effect of the solid electrolyte pellet surface. I found something.

That is, the present invention is a method for producing a gas-sensitive body for a carbon dioxide gas sensor, which comprises sintering a compact formed of an alkali metal ion-conductive solid electrolyte in a ceramic powder.

Another aspect of the present invention is a method for producing a gas-sensitive body for a carbon dioxide gas sensor, which further comprises polishing after sintering.

Still another aspect of the present invention is a carbon dioxide gas sensor in which a gas-sensitive body for a carbon dioxide gas sensor manufactured by any one of the above manufacturing methods is interposed between a working electrode layer and a reference electrode layer.

The alkali metal ion conductive solid electrolyte used in the present invention can be used without any particular limitation from known materials. For example, Na1 + xZr2SixP3-xO12 having cation conductivity of sodium, lithium, etc. (where 0 ≦ x ≦ 3
(Hereinafter referred to as NASICON), β-Al2O3,
β-Ga2O3, Li16-2yZny (CeO4) 4 (provided that 0 ≦
y ≦ 8, hereinafter referred to as a lithicon. ), Or ZrO2, CeO2, etc., to which Y2O3, MgO, CaO, etc., which have an anionic conductivity such as oxygen, can be used without limitation. Among them, NASICON, .beta.-Al
2O3, lithicon, and Y2O3-stabilized ZrO2 are particularly suitable as gas-sensitive substances. Although the thickness of the solid electrolyte is not particularly limited, it is generally adopted in the range of 0, 2 to 1, and 0 mm, and a chip having a size of about 1 to 5 mm on a side is preferable for work.

In the present invention, the formation of the solid electrolyte powder into a compact has the object of improving workability and providing a shape which is easily sintered.

As the molded article used in the present invention, a molded article obtained by a known method can be used without any particular limitation. For example, a method of uniaxially molding a solid electrolyte powder,
A method in which the solid electrolyte powder is kneaded with a binder and a solvent to form a paste, and then a green sheet is prepared by a doctor blade method or the like, dried, divided, and the like is suitably used.

The ceramic powder used in the present invention is necessary for forming a layer of the ceramic component on the surface of the solid electrolyte. The presence of the ceramic layer is effective not only in preventing water resistance but also in preventing burning.

Known materials can be used as the ceramic powder without any particular limitation. For example, YSZ powder, alumina powder, zirconia powder and the like can be mentioned. In particular, YSZ powder which has a water resistance effect and does not seize even at the sintering temperature with the solid electrolyte is preferable.

The YSZ powder is a mixed powder of zirconia and yttria. Zirconia contributes to preventing water resistance.
Yttria easily forms a film on the surface of the solid electrolyte,
Do not allow moisture to penetrate inside. As the YSZ powder, the composition ratio of zirconia and yttria may be any ratio, but in particular, zirconia to yttria is preferably 70 to 99 to 30 to 1 mol%.

In the present invention, it is necessary to embed a sintered body made of a solid electrolyte in ceramic powder and sinter it. In particular, it is preferable to embed the entire surface of the molded body with ceramic powder. By embedding, a ceramic layer is formed on the surface of the solid electrolyte. Further, as compared with a method of forming a ceramic layer by pressure welding, sputtering, or the like, the method can be performed easily and simply, which is advantageous.

As the embedding method, a known embedding method can be used without any particular limitation. For example, an embedding method in which a compact is sandwiched between ceramic powders and an embedding method in which the compact is pushed into ceramic powder are preferably used.

In the present invention, the ceramic powder component used for embedding on the surface of the solid electrolyte can form a ceramic layer by sintering. Also, the solid strength is improved.

The sintering method employed in the present invention may be any known method without any particular limitation. For example, a method of raising the temperature to a sintering temperature in an electric furnace and sintering at the sintering temperature for several hours is suitably used. The sintering temperature is not particularly limited by the type of the solid electrolyte, but is usually 600 ° C to 1 ° C.
A temperature range of 800 ° C. is employed. For example, for NASICON, the temperature is preferably 1100 ° C. or higher.

By the above method, a gas-sensitive body for a carbon dioxide gas sensor having improved workability, adhesion to the working electrode layer and the reference electrode layer, and improved water resistance can be manufactured. After polishing, the workability and the adhesion to the working electrode layer and the reference electrode layer can be further improved. Further, the polishing has an effect of removing unnecessary ceramic lumps present on the surface of the solid electrolyte, and further strengthening the ceramic layer to improve water resistance.

The polishing method is not particularly limited, and a known polishing method can be employed. Specifically, a polishing method using a polishing plate, a polishing method using light or water, and the like, among them, a polishing method using a ball mill, workability is good, because it is easy, further, Unnecessary ceramic lumps present on the surface of the solid electrolyte are removed, and the ceramic layer, which has an effect on water resistance, is not damaged.Because the beads in the pot are dispersed, the ceramic layer is made denser and more resistant to water. It is particularly preferable because it has the effect of improving the properties.

The polishing method using a ball mill is not particularly limited. For example, there are methods such as a planetary ball mill, a stirring ball mill and a vibrating ball mill, but a planetary ball mill in which the movement of beads in a pot is large and high in efficiency is preferable. Further, the planetary ball mill has advantages in that it has better workability and can be easily used as compared with the above-mentioned ball mill method.

The material of the beads used in the ball mill is
Any material such as ceramics, metal, and plastic may be used, but it is preferable to use ceramics, particularly YSZ balls, in order to form a dense ceramics layer without concern about contamination. Its particle size is
Although not particularly limited, in the case of a YSZ ball, the size is preferably equal to or less than the size of the solid electrolyte molded body.

The gas-sensitive body for a carbon dioxide gas sensor obtained by the method of the present invention is suitably used as a material constituting a sensor for detecting carbon dioxide. Such a sensor is basically constituted by a working electrode layer, a reference electrode layer, and a gas-sensitive material interposed between these working electrode layers. As the working electrode layer and the reference electrode layer, known materials for producing a gas sensor can be used without particular limitation. Examples of the material constituting both electrode layers include noble metals such as gold, platinum, and silver and oxides thereof, and among them, preferred are noble metals, and particularly preferred are high safety and good workability. Gold and platinum.

In the working electrode layer, in addition to the above materials, a metal carbonate is usually used as an auxiliary electrode material. The metal carbonate can be used without any limitation as long as it has a dissociation equilibrium with carbon dioxide. Particularly, carbonates of alkali metals such as Na, Li, and K are preferable.

The method for forming the working electrode layer and the reference electrode layer is not particularly limited. For example, the reference electrode layer pastes the electrode material with terpineol or the like,
It can be formed by applying to one surface of a solid electrolyte, drying, and firing. The working electrode layer is formed by forming an electrode layer in the same manner as the reference electrode layer, and then impregnating the electrode layer with an aqueous solution of an auxiliary electrode material. After that, the method of baking is general.

[0029]

According to the production method of the present invention, even in a high-temperature and high-humidity atmosphere, the output electromotive force does not change for a long time, and a gas-sensitive substance for a carbon dioxide gas sensor having a stable sensitivity can be supplied by a simple method.

[0030]

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

Example 1 A special grade sodium phosphate decahydrate and a special grade zirconium silicate were mixed with ethanol in a ball mill, dried, and dried.
C. for 6 hours, then added polyvinyl alcohol as a binder to the powder pulverized by a grinder, and uniaxially formed a NASICON powder granulated by a spray drier to obtain a molded product. It was embedded in a special grade YSZ powder and sintered at 1100 to 1220 ° C. for 6 hours. Place the sintered NASICON pellets and YSZ beads in a 250 ml zirconia pot, and use a planetary ball mill for 550.
Spin at 0 rpm / min for 30 minutes.

The NASICON pellet was subjected to ultrasonic cleaning in an ethanol solution for 10 minutes, and then dried at 100 ° C. for 20 minutes to obtain a pellet-shaped gas-sensitive material for a carbon dioxide gas sensor.

A commercially available high-purity gold paste mixed with 25% by weight of high-purity lithium carbonate and formed into a paste was screen-printed on one side of the gas-sensitive material pellet for a carbon dioxide gas sensor, dried at 120 ° C. for 20 minutes, and worked. An extreme layer was formed. The commercially available gold paste was screen-printed on the other side of the NASICON pellets and dried at 120 ° C. for 20 minutes to form a reference electrode layer.

Further, a commercially available platinum paste was screen-printed on an alumina substrate in a corrugated form, dried at 100 ° C. for 1 hour, and then baked at 1000 ° C. for 1 hour. And 120
C. for 20 minutes, and calcined at 650.degree. C. for 1 hour to produce a carbon dioxide gas sensor.

As an evaluation method, a discriminative evaluation as to whether the gas-sensitive material for a carbon dioxide gas sensor has sufficient strength as a pellet and can be manufactured as a sensor, a sensitivity evaluation to carbon dioxide, and an effect of water resistance are confirmed. For the evaluation, three methods of dew condensation test evaluation were used.

The sensor production evaluation was evaluated in three steps of ○, Δ, and ×, where ○ indicates that the sensor was easily manufactured, Δ indicates that the sensor was troublesome at the time of manufacturing the sensor, but the sensor was manufactured.
× indicates that it was impossible to produce a sensor.

The sensitivity was evaluated using a carbon dioxide concentration of 300 ppm.
The value obtained by subtracting the electromotive force of the sensor in 1000 ppm from the electromotive force of the sensor was used as the sensitivity, and those with the value of 15 mV or more were marked with ○, those with less than 15 mV were marked with x, and It was evaluated on a scale. The condensation test is -10
° C, 0% R.C. H. And 20 ° C., 100% R. H. The carbon dioxide gas sensor produced is left in a constant temperature and humidity chamber in which the carbon dioxide gas sensor is repeated once every three hours, and each atmosphere is experienced once, and one cycle is defined as one cycle. The power was measured and evaluated. In addition, the electromotive force was shown as a value in a 300 ppm atmosphere which is almost equal to the concentration of carbon dioxide in the atmosphere. The sensitivity was shown as a value obtained by subtracting the sensor electromotive force at 1000 ppm from the sensor electromotive force at carbon dioxide concentration of 300 ppm. The change in the electromotive force and the change in sensitivity are indicated by the difference between the electromotive force and the sensitivity before and after the elapse of 15 cycles, and the smaller the value, the better. The production conditions are shown in Table 1 and the results are shown in Table 2.

Example 2 Nasicon pellets embedded and sintered in the ceramic powder obtained in Example 1 were rubbed against an alumina plate one by one with a finger and polished. To produce a carbon dioxide gas sensor. The same evaluation as in Example 1 was performed for the obtained carbon dioxide gas sensor. The production conditions are shown in Table 1 and the results are shown in Table 2.

Examples 3, 5, 6 A gas-sensitive material for a carbon dioxide gas sensor was obtained in the same manner as in Example 1 except that the sintering conditions and the ceramic powder used for embedding were as shown in Table 1. A carbon dioxide gas sensor was fabricated and evaluated. The production conditions are shown in Table 1 and the results are shown in Table 2.

Example 4 A gas-sensitive material for a carbon dioxide gas sensor was obtained in the same manner as in Example 2 except that the ceramic powder used for embedding was as shown in Table 1, and a carbon dioxide gas sensor was produced.
The same evaluation as in Example 1 was performed for the obtained carbon dioxide gas sensor. The manufacturing conditions are shown in Table 1 and the results are shown in Table 2.
Shown in Example 7 Instead of the NASICON powder of Example 1, spray-dried zirconia powder was uniaxially molded, embedded in YSZ powder on an alumina substrate, and pellets sintered at 1300 ° C. for 6 hours were used. A carbon dioxide gas sensor was produced by the same operation as in Example 1, and the same evaluation as in Example 1 was performed. The production conditions are shown in Table 1 and the results are shown in Table 2.

Example 8 A carbon dioxide gas sensor was produced in the same manner as in Example 1 without polishing the NASICON pellets embedded and sintered in the ceramic powder obtained in Example 1. The same evaluation as in Example 1 was performed for the obtained carbon dioxide gas sensor. The production conditions are shown in Table 1 and the results are shown in Table 2. Comparative Examples 1 and 2 On the other hand, after spray-dried NASICON powder was uniaxially formed, it was sintered at 1100 to 1220 ° C. for 6 hours without embedding.
Using the pellets that had not been polished, a carbon dioxide gas sensor was manufactured in the same manner as in Example 1 below.

About the obtained carbon dioxide gas sensor,
Evaluation was performed in the same manner as in Example 1. The manufacturing conditions are shown in Table 1,
The results are shown in Table 2.

Comparative Examples 3 and 4 1100 to 1220 ° C. without embedding as in Comparative Examples 1 and 2.
Then, the pellets sintered for 6 hours were polished by the same ball mill as in Example 1. Hereinafter, a carbon dioxide gas sensor was manufactured using the pellets in the same manner as in Example 1.

With respect to the obtained carbon dioxide gas sensor,
The same evaluation as in Example 1 was performed. The production conditions are shown in Table 1 and the results are shown in Table 2.

Comparative Examples 5 and 6 Instead of the NASICON powder of Comparative Example 1, a spray-dried zirconia powder was used. The pellet was sintered at 1300 ° C. without embedding, and a pellet which was not polished was used as Comparative Example 5. A pellet subjected to the same ball mill polishing treatment as in Example 1 was used as Comparative Example 6. Using the pellets of Comparative Examples 5 and 6, a carbon dioxide gas sensor was produced in the same operation as in Example 1, and the same evaluation as in Example 1 was performed. The production conditions are shown in Table 1 and the results are shown in Table 2.

[0046]

[Table 1]

[0047]

[Table 2]

1100 without embedding as in Comparative Examples 2 and 4
Although pellets sintered at ℃ can be used to make sensors, sufficient sensitivity has not been obtained. With respect to the other comparative examples in which embedding was not performed, it was impossible to manufacture the sensor because the pellet was warped. On the other hand, in the pellets embedded and sintered in the examples, the sensor can be easily manufactured, and sufficient sensitivity is obtained. Regarding water resistance,
Good results were obtained with a slight decrease in sensitivity.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-230842 (JP, A) JP-A-3-134553 (JP, A) JP-A-7-63726 (JP, A) JP-A 8- 327592 (JP, A) JP-A-10-97811 (JP, A) JP-A-8-5606 (JP, A) JP-A-6-265520 (JP, A) JP-A-9-229902 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01N 27/416 G01N 27/406

Claims (3)

(57) [Claims] 1. A method of manufacturing a gas-sensitive body for a carbon dioxide gas sensor, comprising embedding a molded body made of an alkali metal ion-conductive solid electrolyte in ceramic powder and sintering the molded body. 2. The method for producing a gas-sensitive body for a carbon dioxide gas sensor according to claim 1, wherein a polishing treatment is performed after sintering. 3. A carbon dioxide gas sensor characterized in that the gas-sensitive body for a carbon dioxide gas sensor obtained by the production method according to claim 1 or 2 is interposed between a working electrode layer and a reference electrode layer. .