JP2001174433A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a stable output without generating an unstable compound on the interface of a cationic conductive solid electrolyte in a gas sensor using the solid electrolyte. SOLUTION: The gas sensor 1 comprises an auxiliary electrode 2, the cationic conductive solid electrolyte 3 containing a trivalent cation as a conduction seed, an oxide ion conductive solid electrolyte 4 containing an oxide ion as a conduction seed, and a reference electrode 5. The electrode 3 is disposed under the electrode 2, and the electrolyte 4 is disposed under the electrode 3. Thus, the sensor 1 detects a predetermined gas. In the sensor 1, the electrode 2 is made of a metallic acid salt containing anion of the gas to be detected, and the electrode 3 is made of a single crystal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a gas sensor using a cation-conductive solid electrolyte.

[0002]

2. Description of the Related Art In order to efficiently reduce the total emission of carbon dioxide, which is a major cause of global warming, it is most preferable to perform desulfurization and denitration simultaneously with decarbonation before releasing the exhaust gas into the atmosphere. It is effective. For that purpose, it is most effective to monitor and control the emission amount of each generation source. Therefore, it is essential to develop a small, lightweight, and inexpensive sensor.

As a small and inexpensive sensor, a flammable gas sensor using a semiconductor or the like has already been put into practical use as an alarm device, but is inevitably affected by a coexisting gas and has a low selectivity.

When mounting a carbon dioxide sensor, a combustion system is mainly used, and the measurement of carbon dioxide in the presence of various gases is a prerequisite. High selectivity is also required because exhaust gases coexist.

In the combustion system, the oxygen concentration in the atmosphere constantly fluctuates, and it is necessary to construct a sensing system that is not affected by the oxygen concentration at all.

[0006] Among various materials, a solid electrolyte has a feature that only one ion can move in a solid, and since mobile ions are limited in principle, high gas selectivity can be expected.

As a solid electrolyte for a carbon dioxide sensor,
Candidate carbonates include carbonate ions, in which the ions are mobile or the cations migrate instead. However, carbonate ions are so bulky that they cannot move in solid form as ions. Further, carbonate has a problem in its own chemical stability under severe environment.

On the other hand, instead of carbonate ions, by conducting alkali metal ions which can easily move in a solid, the same phenomenon as the conduction of carbonate ions occurs, and carbon dioxide gas can be detected. can do.
In this case, since the movable ion is an alkali metal ion and the detection gas is a carbon dioxide gas, it is necessary to use an auxiliary electrode combining these two kinds of cations and anions. Furthermore, since the measurement atmosphere is in the above-mentioned various atmospheres, the alkali metal carbonate itself is required to be stable in the various atmospheres.

[0009] Among the many alkali metal carbonates, lithium carbonate is the least soluble and satisfies this requirement. Therefore,
To date, the present inventors have realized carbon dioxide gas sensing using lithium ions as movable alkali metal ions and lithium carbonate as auxiliary electrodes, and have only one ion that is characteristic of solid electrolytes. By taking advantage of the features described above, a solid electrolyte that conducts oxide ions in the form of oxygen ions is further combined with the above-mentioned solid electrolyte to construct a sensor with excellent selectivity that is completely unaffected by changes in oxygen concentration in the atmosphere. Have been successful.

Specifically, a solid electrolyte that conducts the above-mentioned monovalent alkali metal ions and a solid electrolyte that conducts divalent oxide ions (here, stabilized zirconia) are used. By using these combinations, the stabilized zirconia covers the alkali metal oxide generated at the interface, so that the effects of various miscellaneous gases from the surroundings can be completely prevented, leading to stability of the output and, consequently, maintenance. A small free-type sensor has been realized (Japanese Patent Laid-Open No. Hei 4-229847).

[0011]

However, the present inventor has
A detailed study of such a sensor revealed that lithium oxide was formed at the interface between the solid electrolytes and that this compound was extremely unstable in the atmosphere, causing fluctuations in the sensor output and causing cracks in the element and damage to the sensor. It was found that there were major drawbacks such as instantaneous breakage.

It is an object of the present invention to provide a gas sensor which shows a stable output without generating an unstable compound at the interface of the solid electrolyte.

[0013]

SUMMARY OF THE INVENTION The present invention provides an auxiliary electrode,
A cation conductive solid electrolyte using a trivalent cation as a conductive species, an oxide ion conductive solid electrolyte using an oxide ion as a conductive species, and a reference electrode, wherein the cation conductive solid electrolyte is an auxiliary electrode. Is located under
The oxide ion conductive solid electrolyte is disposed below the cation conductive solid electrolyte, a gas sensor for detecting a predetermined gas, wherein the auxiliary electrode is made of a metal salt containing an anion of the gas, A gas sensor, wherein the cation conductive solid electrolyte is made of a single crystal.

According to the present invention, a CO ₂ gas sensor using a combination of a trivalent A13 ⁺ ion conductor single crystal and stabilized zirconia as an oxide ion conductor and using lithium carbonate as a reference electrode can quickly and accurately produce CO ₂ gas. Was detected, and it was revealed that a reversible response was stably exhibited.

The present inventors have studied solid electrolytes that conduct various trivalent cations in order to obtain a gas sensor that exhibits stable output.

As a result, the present inventors have found that a gas sensor using a polycrystalline solid electrolyte that conducts trivalent cations has problems not only in long-term durability but also in initial stability. In the case of the gas sensor using the polycrystalline solid electrolyte, it was found that the electromotive force value dropped about 50 mV after several days.

Based on such knowledge, the present inventors have studied the stability of the gas sensor in more detail. As a result, it was found that Li ₂ CO ₃ used as a reference electrode was a grain boundary in A1 ₂ (WO ₄ ) ₃ polycrystal. It was clarified that it was spreading through.

Based on this finding, the present inventor believes that the use of a single crystal having no grain boundaries will increase the stability of the sensor, and has attempted to use a single crystal solid electrolyte as the solid electrolyte for the sensor. Was. Heretofore, no attempt has been made to fabricate a gas sensor using a single crystal that conducts trivalent cations.

The present inventor has been developing Sc ₂ (W
It has been clarified that a group of tungstates having an O ₄ ) _3- type structure conducts trivalent ions. In addition, this Sc ₂
(WO ₄ ) A series of tungstate single crystals having a _3- type structure has been successfully grown for several years.

Therefore, the present inventor has developed a recently developed Sc ₂
(WO ₄ ) A solid electrolyte that uses a completely new trivalent ion having a _three- type structure as a conductive species. Among them, the mobile ionic species are narrowed down to aluminum ions that exist in large quantities on the earth.
A single-crystal body whose conduction can be controlled on a two-dimensional conductive surface was selected as a form thereof, and a gas sensor was produced by combining this single-crystal solid electrolyte and a solid electrolyte using oxide ions as a conductive species.

As a result, the inventor of the present invention has found that such a gas sensor is a thermodynamically stable oxide (aluminum oxide,
That is, it was found that alumina) can be intentionally generated at the interface between the two solid electrolytes, and the property that the activity of the oxide is stable can be effectively used.

The inventor of the present invention has also proposed a hybrid (composite) type in which a single crystal solid electrolyte of such a gas sensor is used in combination with an oxide ion conductor solid electrolyte, so that a change in oxygen concentration in the measurement atmosphere can be prevented. It has become possible to realize a simple sensor that is not affected at all, and it has become possible to measure in any atmosphere, and has found out that it has excellent initial characteristics and long-term stability, and has completed the present invention.

In the gas sensor of the present invention, on the auxiliary electrode side,
A single crystal as a cation conductive solid electrolyte having a trivalent cation as a conductive species is arranged. In such a single crystal, there is no grain boundary unlike a polycrystal, and the movable cations of the auxiliary electrode do not diffuse through the grain boundary of the polycrystal, so that the output of the gas sensor is extremely stably maintained. Is done.

As described above, according to the present invention, a single-crystal cation conductive solid electrolyte using a trivalent cation as a conductive species and an oxide ion conductive solid electrolyte using an oxide ion as a conductive species are combined. By adopting the configuration of the gas sensor, a thermodynamically stable oxide is intentionally generated at the interface between the solid electrolytes and the characteristic that the activity of the oxide is stable is effectively used, and the output of the gas sensor is reduced. Remarkably stabilizes.

Further, according to the present invention, the output can be maintained very stably, the maintenance-free type can be used, and the control of the atmosphere on the reference electrode side is not required at all, and the gas can be accurately measured in any atmosphere. It can be designed to be compact and compact, can be installed correctly at each gas emission source, and can realize a new and inexpensive chip-type practical gas sensor that is versatile and can be used for industrial exhaust gas. It is highly expected that the technology will be put to practical use in the near future in the field of exhaust gas control.

[0026]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view of a CO ₂ gas sensor cell according to an example of the present invention.

As shown in FIG. 1, a gas sensor 1 according to an embodiment of the present invention includes an auxiliary electrode 2, a cation-conductive solid electrolyte 3 that conducts trivalent cations, and an oxide ion-conductive solid that conducts oxide ions. An electrolyte 4 and a reference electrode 5 are provided. Conductive wires 6 and 7 made of a platinum wire or the like are connected to the surface 2 a of the auxiliary electrode 2 and the surface 4 a of the oxide ion conductive solid electrolyte 4. The surface 2 of the auxiliary electrode 2
A is provided with a platinum net 8, and an inorganic adhesive layer 9 is formed around the sensor cell 1.

In such a gas sensor 1, the cation conductive solid electrolyte 3 is arranged below the auxiliary electrode 2 so as to be in contact with the auxiliary electrode 2. The oxide ion conductive solid electrolyte 4 is disposed below the cation conductive solid electrolyte 3 so as to be in contact with the cation conductive solid electrolyte 3.

In the gas sensor 1, the auxiliary electrode 2 is made of a metal salt containing an anion of the gas to be detected. The cation conductive solid electrolyte 3 is made of a single crystal.
The oxide ion conductive solid electrolyte 4 is in contact with the surface 3 a formed at a predetermined angle with the conduction axis of the trivalent cation of the cation conductive solid electrolyte 3.

As described above, according to the present invention, a stable output of a gas sensor is obtained by using a single crystal having a trivalent cation as a conductive species as a solid electrolyte for a sensor.

Various solid electrolytes can be used as the cation-conductive solid electrolyte according to the present invention through which trivalent cations conduct. For example, scandium tungstate, aluminum tungstate, rare earth tungstate, scandium molybdate, aluminum molybdate, rare earth molybdate, β and β ″ aluminas having trivalent ions as conductive species, niobate, tantalate Salt and the like.

In such a solid electrolyte, aluminum,
A trivalent cation of an element selected from the group consisting of gallium, indium, thallium, antimony, bismuth and rare earth elements is conducted.

Among such solid electrolytes, it is preferable to use aluminum tungstate, scandium tungstate, rare earth tungstate, or the like. These solid electrolytes are stable, and oxides formed from their conductive species, aluminum ion, scandium ion, and rare earth ion, are also stable. Such oxides are a cation conductive solid electrolyte and an oxide ion conductive solid electrolyte. Even if the gas sensor is formed at the interface, the output of the gas sensor is not reduced and the gas sensor is not broken.

Various solid electrolytes can be used for the oxide ion conductive solid electrolyte according to the present invention using oxide ions as a conductive species, as long as the oxide ions are conducted. As an example of such a solid electrolyte, stabilized zirconia can be used.

As the reference electrode according to the present invention, various materials having an appropriate material as an electrode can be used. For such a reference electrode, a platinum net provided on an auxiliary electrode as shown in FIG. 1 is used. be able to.

The gas sensor of the present invention can detect and measure at least one gas selected from the group consisting of carbon oxides, nitrogen oxides, and sulfur oxides.

Examples of such a gas include carbon oxides such as carbon dioxide and carbon monoxide, and nitrogen oxides include dinitrogen monoxide, nitric oxide, dinitrogen trioxide, and nitrogen dioxide. The substances include disulfur monoxide, sulfur monoxide, sulfur dioxide, and the like, and any of these gases can be detected and measured.

For the auxiliary electrode according to the present invention, various metal salts can be used depending on the gas to be detected and measured.

As such a metal salt, at least one kind of salt selected from the group consisting of carbonate, nitrate, nitrite and sulfate can be used.

The metal salt is carbonate when the gas to be detected and measured is carbon oxide, nitrate or nitrite when the gas to be detected and measured is nitrogen oxide, and When the gas to be measured is a sulfur oxide, a sulfate can be used.

[0041]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the drawings based on examples and comparative examples. FIG. 2 is a sensor output response curve of the gas sensor of FIG. 1 when the CO ₂ gas concentration is changed. FIG. 3 shows CO ₂ of the gas sensor of FIG.
It is a figure of a sensor output to logarithm of gas pressure.

Example Stabilized zirconia, which is a kind of oxide ion conductive solid electrolyte, was prepared by mixing ZrO ₂ (purity 99.9%) and Y ₂ O ₃ (purity 99.9%) in a molar ratio of 9: 1. Mix, 1600
It was obtained by baking twice at 12 ° C. for 12 hours.

Al³⁺The conduction direction of the ions is parallel to the b axis
Therefore, as shown in FIG. ³⁺Conduct ions
Single crystal as cation conductive solid electrolyte 3
did. Pellet of lithium methoxide (purity 99.9%)
Is fixed to the surface 3a of this single crystal and diluted with Air.
5% CO₂Approx. 1 at 550 ° C, the operating temperature, in gas
By heating for a time, Li as the auxiliary electrode 2₂C
O₃A phase formed.

1% and 200-2000 ppm CO ₂
The gas concentrations were CO ₂ -Air mixed gas and Ai, respectively.
It was adjusted by mixing r and 1% CO ₂ gas diluted with N ₂ , and the total gas flow was 100 ml / min. The sensor output was monitored with a digital voltmeter.

As shown in FIG. 2, this gas sensor
It responds ideally to changes in O ₂ gas concentration, and this response is quick and continuous, indicating that the gas sensor has performance suitable for practical use. The Nernst equation in this sensor is simply expressed as the following equation. E = C (constant) − (R / nF) Tln (Pco ₂ ) (n = 2.00) (Equation 1)

FIG. 3 shows the sensor output with respect to the logarithm of the CO ₂ gas pressure. It was found that at a CO ₂ gas concentration of ₂₀₀ ppm to 1%, a one-to-one linear relationship was exhibited. From this result, it became clear that this CO ₂ gas sensor can accurately detect CO ₂ gas.

The n value calculated from the plot (●) in FIG. 3 is 2.03, and it can be seen that it is in good agreement with the theoretical value n = 2.00 (Equation 1). The electromotive force value (□) several days later is also in good agreement with the above-mentioned electromotive force value (●). From these results, it can be seen that the combination of the Al ⁺³ ion conductor single crystal and the oxide ion conductor It was found that a stable sensor output could be obtained.

Comparative Example As a cation conductive solid electrolyte, A1 ₂ (WO ₄ ) ₃
When a polycrystalline material was used, the slope in the linear relationship between the logarithm of the CO ₂ gas pressure and the electromotive force value was n = 2.01. However, in the case of this polycrystal, after several days, the electromotive force value becomes
About 50mV change, _Li 2 CO ₃ was used as a reference electrode revealed that the diffuse through the grain boundaries of _A1 2 (WO _{4) 3} polycrystal. This was caused by the presence of grain boundaries in the polycrystal, and it was clarified that the use of a single crystal having no grain boundaries increased the stability of the sensor.

As shown in the examples, trivalent A1³⁺ion
Conductor single crystal and stabilized zircon as oxide ion conductor
And lithium carbonate as a reference electrode
CO ₂The gas sensor quickly and accurately₂Detect gas
And a stable reversible response was found.

[0050]

According to the present invention, there is provided a gas sensor comprising a combination of a single-crystal cation conductive solid electrolyte having a trivalent cation as a conductive species and an oxide ion conductive solid electrolyte having an oxide ion as a conductive species. With this configuration, a thermodynamically stable oxide is intentionally generated at the interface between the two solid electrolytes, and the property that the activity of the oxide is stable is effectively used, and the output of the gas sensor is remarkably stable. Become

[Brief description of the drawings]

FIG. 1 is a sectional view of a CO ₂ gas sensor cell according to an example of the present invention.

FIG. 2 is a sensor output response curve when the CO ₂ gas concentration is changed for the gas sensor of FIG. 1;

FIG. 3 is a diagram showing a sensor output with respect to a logarithm of a CO ₂ gas pressure of the gas sensor of FIG. 1;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Gas sensor 2 Auxiliary electrode 2a Surface of auxiliary electrode 3 Cation conductive solid electrolyte 3a Surface of cation conductive solid electrolyte 4 Oxide ion conductive solid electrolyte 4a Surface of oxide ion conductive solid electrolyte 5 Reference electrode 6,7 Conducting wire 8 Platinum net 9 Inorganic adhesive layer

Claims (5)

[Claims] An auxiliary electrode, a cation-conductive solid electrolyte having a trivalent cation as a conductive species, an oxide ion-conductive solid electrolyte having oxide ions as a conductive species, and a reference electrode; A cation conductive solid electrolyte is disposed under the auxiliary electrode, the oxide ion conductive solid electrolyte is disposed under the cation conductive solid electrolyte, and a gas sensor for detecting a predetermined gas. ,
The gas sensor, wherein the auxiliary electrode is made of a metal salt containing an anion of the gas, and the cation conductive solid electrolyte is made of a single crystal. 2. The gas sensor according to claim 1, wherein the trivalent cation is an ion of an element selected from the group consisting of aluminum, gallium, indium, thallium, antimony, bismuth, and a rare earth element. 3. The gas sensor according to claim 1, wherein the cation conductive solid electrolyte is aluminum tungstate. 4. The gas according to claim 1, wherein the gas is at least one gas selected from the group consisting of carbon oxides, nitrogen oxides, and sulfur oxides. Gas sensor. 5. The metal salt according to claim 1, wherein the metal salt is at least one selected from the group consisting of carbonate, nitrate, nitrite and sulfate. A gas sensor as described.