JP2001141693A - Solid electrolyte type nitrogen oxide gas sensor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid electrolyte type nitrogen oxide gas sensor in which an electromotive force and a current value hardly change even after the sensor is left without being heated, in a high humidity ambience or a high moisture concentration ambience such as a condensed ambience or the like, and a change of the electromotive force and the current value can be reduced even when a long time has passed after an operation start and can be held within 2.0% as compared with at the operation start. SOLUTION: The solid electrolyte type nitrogen oxide gas sensor element has a pair of electrode layers formed to a surface of a solid electrolyte layer including at least one selected from Li2Si2O5, Li2TiSiO5, LiLaSi04 and LiLa9(Si04)6O2. The solid electrolyte type nitrogen oxide gas sensor element has a pair of electrode layers formed to a surface of a solid electrolyte layer particularly including at least one selected from (a) Li2Si2O5, Li2TiSiO5, LiLaSiO4 and LiLa9(SiO4)6O2 and at least one selected from (b) titanium oxide, silica, zirconia and zirconium silicate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor element for measuring the concentration of nitrogen oxide gas in an atmosphere, which is incorporated in a combustion control facility or an environmental measuring facility. More specifically, the present invention relates to a solid oxide nitrogen oxide gas sensor element having improved stability in a high humidity environment and improved stability over time.

[0002]

2. Description of the Related Art In recent years, there has been an increasing interest in environmental issues, and a sensor for measuring and controlling the concentration of nitrogen oxide gas released into the atmosphere has attracted attention. Among such sensors, a solid electrolyte type nitrogen oxide gas sensor element utilizing changes in the electromotive force and current value of the solid electrolyte is one of the most compact and compact.
Because of its simplicity and low cost, its practical application is eagerly desired.

[0003] Such a solid electrolyte type nitrogen oxide gas sensor element has a structure in which a surface of a solid electrolyte layer, which is an ion conductor, is provided.
It has a structure in which a pair of electrode layers are formed. These sensor elements usually operate in a state where they are heated to a constant temperature of 100 ° C. to 500 ° C., and are used in an atmosphere containing a nitrogen oxide gas.

As a solid electrolyte layer of such a nitrogen oxide gas sensor, NASICON (Na _{1 + A} Zr ₂₎ is generally used.
_A cationic conductor such as Si _A P _3-A O ₁₂ , where 0 ≦ A ≦ 3) and β-Al ₂ O ₃ is used. Further, as a method of detecting the nitrogen oxide gas concentration, an electromotive force detection method, a current detection method, and the like are known.

[0005] The solid oxide nitrogen oxide gas sensor having the above structure can accurately measure the concentration of nitrogen oxide gas contained in the atmosphere, and can be made small and inexpensive.
It is accepted as a highly versatile sensor element.

[0006]

However, a nitrogen oxide gas sensor using a solid electrolyte, which has been developed so far, has a problem in that the operation is stopped and the operation is stopped in a high humidity atmosphere or a dew atmosphere. When left in an atmosphere with a high moisture concentration, when the operation is started again, even if the nitrogen oxide gas concentration is the same, the measured electromotive force and current value are lower than the values before leaving. There was a problem that it was significantly reduced.

Further, even if the device is operated while being maintained at a constant heating temperature, there is a problem that the above-mentioned electromotive force and current value continue to decrease over time, for example, about 20% after 1000 hours. Was.

[0008] These problems are factors that hinder the practical use of the solid oxide nitrogen oxide gas sensor. Therefore, even if the device is left unheated for a long time in an atmosphere having a high moisture concentration such as high humidity or dew condensation, the measured electromotive force and current value do not change. There has been a demand for the development of a solid oxide nitrogen oxide gas sensor in which the electromotive force and the current value are the same as those at the start of operation.

[0009]

Means for Solving the Problems The present inventors have intensively studied to develop a solid electrolyte type nitrogen oxide gas sensor having the above characteristics. As a result, Li ₂ Si ₂ O ₅ , Li ₂ TiSiO ₅ , LiLaSi
By using a material containing at least one selected from O ₄ and LiLa ₉ (SiO ₄ ) ₆ O ₂ , even if it is left unheated in an atmosphere having a high moisture concentration such as high humidity or dew, The electromotive force and the current value are almost the same as before leaving, and the change of the electromotive force and the current value is extremely small even after 1000 hours from the start of the operation, and may be within 2.0% compared to the time of the start of the operation. It has been found that a solid electrolyte type nitrogen oxide gas sensor that can be obtained is obtained, and the present invention has been proposed.

That is, the present invention relates to Li ₂ Si ₂ O ₅ , Li ₂ T
iSiO ₅ , LiLaSiO ₄ , and LiLa ₉ (Si
A solid electrolyte type nitrogen oxide gas sensor element comprising a pair of electrode layers formed on the surface of a solid electrolyte layer containing at least one selected from O ₄ ) ₆ O ₂ .
(A) Li ₂ Si ₂ O ₅ , Li ₂ TiSiO ₅ , LiLaS
On the surface of a solid electrolyte layer containing at least one selected from iO ₄ and LiLa ₉ (SiO ₄ ) ₆ O ₂ and (b) at least one selected from titanium oxide, silica, zirconia and zirconium silicate, This is a solid electrolyte type nitrogen oxide gas sensor element formed with a pair of electrode layers.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of the nitrogen oxide gas sensor element of the present invention will be described below in detail.

The gas to be measured by the solid oxide nitrogen oxide gas sensor element of the present invention is a nitrogen oxide gas. Known nitrogen oxide gas can be measured without any particular limitation, but usually includes nitrogen monoxide gas, nitrogen dioxide gas, and a mixed gas thereof.

In the present invention, the solid electrolyte layer is made of Li
It is important to include at least one selected from the group consisting of ₂ Si ₂ O ₅ , Li ₂ TiSiO ₅ , LiLaSiO ₄ , and LiLa ₉ (SiO ₄ ) ₆ O ₂ (hereinafter, these are also collectively referred to as Li-based composite oxides). It is. By using such a solid electrolyte layer, the nitrogen oxide gas sensor element of the present invention,
The electromotive force and current value when left unheated in a high-humidity environment hardly change compared to before.
Further, after the start of the operation, the change in the electromotive force or the current value can be made extremely small. In the present invention,
In order to exhibit the above effect more remarkably, Li ₂ Ti
It is particularly preferable to use a solid electrolyte layer containing SiO ₅ .

Further, in the present invention, the effect of preventing a change in the electromotive force or the current value after being left unheated in a high-humidity environment can be exhibited more remarkably. By further reducing the change in the current value,
After 1000 hours from the start of operation, it is possible to make it within 0.5% compared to the start of operation.
It is preferable that the solid electrolyte layer further contains at least one selected from titanium oxide, silica, zirconia, and zirconium silicate. In particular, titanium oxide and zirconia are preferable because they exhibit more excellent effects.

The content of at least one selected from the group consisting of titanium oxide, silica, zirconia and zirconium silicate contained in the solid electrolyte layer is not particularly limited, but the solid electrolyte layer can be easily formed, and in a high humidity environment. Is preferably 0.05 to 20 times in molar ratio with respect to the Li-based composite oxide, and more preferably 0.2 to 10 times in molar ratio with respect to the Li-based composite oxide. preferable.

In the present invention, as a method for producing a solid electrolyte layer containing a Li-based composite oxide, a known method is employed without any particular limitation. Typical production methods include a method of mixing and firing a solid material for synthesis of a solid electrolyte and molding and sintering the same, and a method of mixing and mixing the raw material for synthesis of a solid electrolyte and then shaping and sintering without firing. Can be mentioned.

A general example of the above manufacturing method will be described below. Preparation of solid electrolyte synthesis raw material, lithium carbonate,
Selected from silica, titanium oxide, lanthanum carbonate, etc.
A method of mixing synthetic raw materials corresponding to the target Li-based composite oxide using a ball mill or the like, or a lithium alkoxide such as LiOCH _3, a silicon alkoxide such as Si (OC ₂ H ₅ ) ₄ , Ti (On) A metal alkoxide selected from titanium alkoxides such as -C ₄ H ₉ ) ₄ and lanthanum alkoxides such as La (OC ₂ H ₅ ) _{3 according} to the target Li-based composite oxide, and hydrolyzed and dried. In general, such a method is adopted. The mixture of the synthesis raw materials adjusted in this manner is heated at 400 ° C to 1300 ° C.
By firing in a temperature range of ° C., a target Li-based composite oxide can be synthesized.

For example, in the case of Li ₂ Si ₂ O ₅ , a metal alkoxide such as lithium carbonate and silica or LiOCH ₃ and Si (OC ₂ H ₅ ) ₄ is used as a synthesis raw material, and Li 2
And Si are reacted at a molar ratio of Li: Si = 1: 1, whereby the Li ₂ Si ₂ O ₅ can be synthesized.

Similarly, in the case of Li ₂ TiSiO ₅ , lithium carbonate, titanium oxide and silica, or LiOCH ₃ , Ti (OnC ₄ H ₉ ) ₄ and Si (OC
Li ₂ TiSiO ₅ can be synthesized by using a metal alkoxide such as ₂ H ₅ ) _{4 and} reacting Li, Ti and Si in a molar ratio of Li: Ti: Si = 2: 1: 1.

Similarly, LiLaSiO_FourIf
As raw materials, lithium carbonate, lanthanum carbonate and silica,
Or LiOCH_Three, La (OC_TwoH_Five)_ThreeAnd Si (OC_Two
H_Five) _FourLi, La and S
i at a molar ratio of Li: La: Si = 1: 1: 1
By doing so, the LiLaSiO_FourCan be synthesized
Wear.

Furthermore, in the case of LiLa ₉ (SiO ₄ ) ₆ O ₂ , lithium carbonate, lanthanum carbonate and silica, or LiOCH ₃ , La (OC ₂ H ₅ ) ₃ and Si
Using a metal alkoxide such as (OC ₂ H ₅ ) ₄ , Li, L
By reacting a and Si in a molar ratio of Li: La: Si = 1: 9: 6, the LiLa ₉ (SiO ₄ ) ₆ O ₂ can be synthesized.

When only a metal alkoxide is used as a starting material, a solid electrolyte layer can be obtained only by hydrolysis and drying of the metal alkoxide.

Next, as a method of forming the synthesized powdery solid electrolyte into a layered material, a method of forming by a press such as a uniaxial press or an isostatic press, followed by sintering,
A method in which the powder is kneaded with a binder and a solvent to form a paste, which is then formed into a green sheet by a doctor blade method or the like and sintered, the paste is formed as a thick film on a substrate by a screen printing method or the like, and then fired to form a paste on the substrate. In general, a method of baking is used. After molding, 50
By heating and sintering in the temperature range of 0 ° C. to 1500 ° C., Li ₂ Si ₂ O ₅ , Li ₂ TiSiO ₅ , LiLa
SiO _4, and Lila ₉ can be obtained a solid electrolyte layer comprising at least one selected from the (SiO _{4) 6} O _2.

Next, a general example of the above manufacturing method will be described below. The synthesis raw material can be prepared in the same manner as in the production method. In the production method described above, the synthetic raw material is molded without firing, and then heated. As a molding method, a uniaxial press in the same manner as the production method, a method of molding by pressing such as isostatic pressing, a method of kneading the powder with a binder and a solvent to form a green sheet by a doctor blade method or the like, Further, a method of forming the paste as a thick film on a substrate by a screen printing method or the like can be used. Further, when a metal alkoxide is used as a starting material, a method of forming into a sheet when hydrolyzing and drying may be employed. After forming, a solid electrolyte layer containing a desired Li-based composite oxide is obtained by heating in a temperature range of 500 ° C. to 1500 ° C. while also synthesizing and sintering the target Li-based composite oxide. Can be.

In addition to these manufacturing methods, a solid electrolyte layer can be obtained by using a thin film forming technique such as sputtering or vapor deposition with a target of a mixture of synthetic raw materials such as Li ₂ O, silica, titanium oxide, and lanthanum oxide. .

In the present invention, the solid electrolyte layer is provided with the above L
In addition to the i-based composite oxide, at least one selected from titanium oxide, silica, zirconia, and zirconium silicate
In the case where a seed is included, a known method is employed without particular limitation as a method for producing the solid electrolyte layer. As a typical production method, i) at the time of producing a Li-based compound by the same method as described above, the Li-based composite oxide to be produced contains at least one selected from titanium oxide, silica, zirconia, and zirconium silicate. A method of adjusting the composition of the synthesis raw material so that the seeds are mixed; ii) a powder of the Li-based composite oxide and at least one powder selected from titanium oxide, silica, zirconia, and zirconium silicate; The method of mixing and shaping this can be mentioned.

Of the above i), Li_TwoSi_TwoO_FiveAnd oxidation
And the production of a solid electrolyte layer comprising
Li by the same method as_TwoSi_TwoO_FiveManufacturing smell
Thus, for example, the synthesis raw materials lithium carbonate and silica
Mixed or LiOCH_ThreeAnd Si (OC_TwoH_Five)_FourMix with
In addition, titanium oxide or Ti (On-C_FourH ₉)
_FourShould be mixed.

In the above i), the production of the solid electrolyte layer containing Li ₂ Si ₂ O ₅ and silica is similarly performed by mixing lithium carbonate and silica, which are synthesis raw materials, or by mixing LiOCH ₃ and Si (OC ₂ H ₅ ) ₄ , Li and Si react in a molar ratio of Li: Si = 1: 1 to form Li ₂
It is sufficient to mix an excess amount of silica or Si (OC ₂ H ₅ ) ₄ with respect to the mixing ratio of the synthesis raw material that produces Si ₂ O ₅ .

Of the above i), the production of the solid electrolyte layer containing Li ₂ Si ₂ O ₅ and zirconia is also performed by mixing lithium carbonate and silica, or by mixing LiOCH ₃ and Si (OC ₂ H).
₅ ) When mixing with ₄ , zirconia or Z
r (On-C ₄ H ₉ ) ₄ may be mixed.

Of the above i), the production of the solid electrolyte layer containing Li ₂ Si ₂ O ₅ and zirconium silicate is also performed by mixing lithium carbonate and silica, or by mixing LiOCH ₃ and Si
(OC ₂ H ₅ ) ₄ and Li and Si
React at a molar ratio of Li: Si = 1: 1 to produce Li ₂ Si ₂ O
₅ than the blending ratio of the starting materials for synthesis to produce, mix an excess of silica and zirconia or, alternatively an excess amount of Si
(OC ₂ H ₅ ) ₄ and Zr (On-C ₄ H ₉ ) ₄ may be mixed.

Li ₂ TiSiO ₅ , LiLaSiO ₄ , L
The production of the solid electrolyte layer containing any one of iLa ₉ (SiO ₄ ) ₆ O ₂ and at least one selected from titanium oxide, silica, zirconia, and zirconium silicate is also described in the above L.
The composition of the starting material may be adjusted as in the case of i ₂ Si ₂ O ₅ .

Incidentally, titanium oxide, silica, zirconia,
The production of the solid electrolyte layer containing two or more kinds selected from zirconium silicate can be performed by combining the above methods.

Next, as a general example of the production method of the above ii), the following method can be mentioned.
In the method ii), first, the powder of the Li-based composite oxide is mixed with at least one powder selected from titanium oxide, silica, zirconia, and zirconium silicate. The powder of the Li-based composite oxide can be obtained by the same method as described above. Titanium oxide powder, silica powder, zirconia powder,
A commercially available powder can be used as the zirconium silicate powder.

Li-based composite oxide, titanium oxide, silica,
For forming a mixed powder with at least one selected from zirconia and zirconium silicate, a press such as a uniaxial press, an isotropic isostatic press or the like is used in the same manner as in the method of producing the solid electrolyte layer containing the Li-based composite oxide. , A method of kneading a synthetic powder with a binder and a solvent to form a paste and then forming a green sheet by a doctor blade method or the like, and a method of forming the paste as a thick film on a substrate by a screen printing method or the like. You.

After molding, sintering is performed in a temperature range of 400 ° C. to 1500 ° C. to obtain Li ₂ Si ₂ O ₅ , Li ₂ TiSi
O ₅ , LiLaSiO ₄ and LiLa ₉ (SiO ₄ ) ₆ O ₂
At least one selected from the group consisting of titanium oxide, silica,
A solid electrolyte layer containing at least one selected from zirconia and zirconium silicate can be obtained.

In the present invention, the thickness of the solid electrolyte layer is not particularly limited, but is generally 0.02 mm to
It is adopted from a range of 2.0 mm.

In the solid oxide nitrogen oxide gas sensor element of the present invention, a pair of electrode layers is formed on the surface of the solid electrolyte layer. The pair of electrode layers is not limited to the difference in the detection method of the nitrogen oxide gas as long as it is provided as the detection electrode layer and the counter electrode layer in the nitrogen oxide gas sensor element. As a method for detecting a nitrogen oxide gas, an electromotive force detection method and a current detection method are generally used. In the present invention, the electromotive force detection method is particularly preferable.

The solid electrolyte type nitrogen oxide gas sensor element of the electromotive force detection type includes, on the surface of the solid electrolyte layer, an electron conductive substance and a substance capable of causing an equilibrium reaction with nitrogen oxide gas as an auxiliary electrode substance. A sensing pole layer,
A counter electrode layer containing an electron conductive material is formed, and a heater for heating the element is additionally provided. In such a gas sensor element of the electromotive force detection method, the detection electrode layer is usually called a working electrode layer, and the counter electrode layer is usually called a reference electrode layer.

When the above-described solid electrolyte type nitrogen oxide gas sensor element of the electromotive force detection method is left in an atmosphere containing a nitrogen oxide gas under heating at 100 ° C. to 600 ° C., the detection is performed through the solid electrolyte layer. A certain electromotive force is generated between the pole layer and the counter electrode layer according to the nitrogen oxide gas concentration. When the concentration of the nitrogen oxide gas in the standing atmosphere changes, the dissociation equilibrium reaction between the auxiliary electrode substance contained in the sensing electrode layer and the nitrogen oxide gas proceeds until the equilibrium is reached, and solids near the sensing electrode layer A change occurs in the mobile ion concentration of the electrolyte layer.

Since this change in concentration appears as a change in electromotive force, in the above-described solid electrolyte type nitrogen oxide gas sensor element of the electromotive force detection method, the electromotive force at that time is measured by a voltmeter.
The nitrogen oxide gas concentration can be detected by collating with a calibration curve that shows the correlation between the electromotive force and the nitrogen oxide gas concentration created in advance.

In the solid electrolyte type nitrogen oxide gas sensor element of the electromotive force detection method, the auxiliary electrode material for the detection electrode layer causes an equilibrium reaction with the nitrogen oxide gas in an atmosphere containing the nitrogen oxide gas. A known material can be used without limitation as long as it is a substance that can form the compound. For example, an alkali metal nitrate such as sodium nitrate and lithium nitrate and a mixture thereof, or an alkaline earth metal nitrate such as barium nitrate and magnesium nitrate and a mixture thereof are employed. Therefore, it is preferable to use alkaline earth metal nitrates, particularly barium nitrate and strontium nitrate.

The content of the auxiliary electrode material for the detection electrode layer in the electromotive force detection method is not particularly limited, but is preferably 5 to 80% by weight based on 100% by weight of the total weight of the detection electrode. In particular, it is preferably from 10 to 60% by weight in order to reduce the fluctuation of the electromotive force of the sensor element during continuous use.

The electron conductive material contained in the detection electrode layer of the electromotive force detection method is a substance necessary for outputting an electromotive force of the sensor element, like the electron conductive material contained in the counter electrode layer described later, and is known. Material is used without restriction. For example,
Noble metal elements such as platinum, gold, palladium and silver and alloys thereof, or a mixture of two or more of the above noble metal elements are employed. In particular, platinum, gold and mixtures and alloys thereof are resistant to corrosion. It is preferable because it is excellent.

The content of the electron conductive material contained in the detection electrode layer of the electromotive force detection method is not particularly limited, but is preferably 20 to 95% by weight based on the total weight of the detection electrode of 100% by weight. Particularly, it is preferably 40 to 90% by weight.

The electron conductive substance contained in the counter electrode layer of the electromotive force detection method is a substance necessary for outputting an electromotive force of the sensor element, similarly to the above-mentioned electron conductive substance contained in the detection electrode layer, and is known. Material is used without restriction. For example, noble metal elements such as platinum, gold, palladium, and silver and alloys thereof, or a mixture of two or more of the above noble metal elements are employed. In particular, platinum, gold, and mixtures and alloys thereof are resistant. It is suitable because of its excellent corrosiveness.

In the solid electrolyte type nitrogen oxide gas sensor element, the current sensing type sensor element has a sensing electrode layer containing an electron conductive material on the surface of the solid electrolyte layer, and movable ions as the electron conductive material and the auxiliary electrode material. A counter electrode layer containing a substance that can be released therein is formed, and a heater for heating the element is additionally provided.

The above-described current detection type solid electrolyte type nitrogen oxide gas sensor element is heated at 100 ° C. to 600 ° C.
When left in an atmosphere containing nitrogen oxide gas and a specific voltage is applied or short-circuited between the sensing electrode layer and the counter electrode layer, mobile ions or solids contained in the auxiliary electrode material forming the counter electrode layer The mobile ions in the electrolyte move toward the sensing electrode layer to form nitrate in the sensing electrode layer, and a certain current value corresponding to the nitrogen oxide gas concentration is generated between the electrode layers. Then, when the concentration of the nitrogen oxide gas in the left atmosphere changes, the amount of movement of mobile ions in the solid electrolyte layer changes.

Since the change in the amount of ion movement appears as a change in the current value, the current detection type solid oxide nitrogen oxide gas sensor element measures the current value at that time with an ammeter and prepares it in advance. The nitrogen oxide gas concentration can be detected by collating with a calibration curve indicating the correlation between the current value and the nitrogen oxide gas concentration.

In the current sensing type solid electrolyte type nitrogen oxide gas sensor element, the auxiliary electrode material for the counter electrode layer is a material capable of releasing mobile ions into the solid electrolyte in an atmosphere containing nitrogen oxide gas. If so, known materials can be used without limitation. For example, sodium nitrate,
Alkali metal nitrates such as lithium nitrate and the like, and mixtures thereof are employed, but it is particularly preferable to use sodium nitrate since a large sensitivity can be obtained.

Although the content of the auxiliary electrode material for the counter electrode layer in the current detection method is not particularly limited, the total weight of each electrode is 1%.
It is preferably 5 to 80% by weight based on the total weight of 00% by weight, and particularly preferably 10 to 60% by weight because fluctuation of the electromotive force of the sensor element during continuous use can be reduced.

As the electron conductive material contained in the detection electrode layer and the counter electrode layer of the current detection method, the same materials as those contained in each electrode of the electromotive force detection method can be used.

In the solid oxide nitrogen oxide gas sensor element of the present invention, the structure of each electrode layer may be any known structure. The structure of the detection electrode layer in the electromotive force detection method and the structure of the counter electrode layer in the current detection method include, for example, a structure in which an electron conductive material and an auxiliary electrode material for each electrode layer are layered on the surface of the solid electrolyte layer. A structure in which an auxiliary electrode material for each electrode layer is dispersed and present, a structure in which a part or all of the auxiliary electrode material for each electrode layer formed on the surface of the solid electrolyte layer is covered with an electron conductive material, In particular, a structure in which the auxiliary electrode material for each electrode layer is dispersed in the electron conductive material is preferable because the electrode layer can be easily formed.

As a method for forming each electrode layer, a known method is used without any particular limitation. For example, the above-described electron conductive material, and each auxiliary electrode material for the intended electrode layer alone or after mixing, are kneaded with a solvent and a binder to form a paste, and the paste is solid electrolyte layer by a screen printing method or the like. A method of baking on a surface, a method of forming an electron conductive material, and a method of forming an auxiliary electrode material for a target electrode layer by a thin film forming technique such as sputtering or vapor deposition are suitably employed.

The thickness of each electrode layer is not particularly limited, but is generally adopted in the range of 0.001 to 0.03 mm.

In both the electromotive force detection method and the current detection method, the arrangement of the detection electrode layer and the counter electrode layer with respect to the solid electrolyte layer is such that the detection electrode layer and the counter electrode layer are in contact with the surface of the solid electrolyte layer. There is no particular limitation.
Usually, a structure in which a detection electrode layer is formed on one surface of the solid electrolyte layer and a counter electrode layer is formed on the other surface is, for example, a detection electrode layer is formed on one surface of the solid electrolyte layer. And a structure in which both layers of the counter electrode layer and the counter electrode layer are formed at a certain distance.

[0056]

The solid electrolyte type nitrogen oxide gas cell of the present invention
The sensor element has a solid electrolyte layer of Li_TwoSi_TwoO_Five, Li_TwoT
iSiO_Five, LiLaSiO_Four, And LiLa₉(Si
O_Four)₆O _TwoContaining at least one member selected from the group consisting of
In an atmosphere with high moisture concentration such as high humidity or condensation
Even if left unheated, the electromotive force and current value are almost the same as before
Almost unchanged. In addition, the electromotive force and current
The change can be kept small, and 1000
Within 2.0% after starting operation
It is possible to

In particular, when the solid electrolyte layer further contains at least one selected from titanium oxide, silica, zirconia, and zirconium silicate, the above effect is more remarkably exhibited. For example, the change in the electromotive force or the current value after a lapse of 1000 hours from the start of the operation can be within 0.5%. Such a change is significantly smaller than the above-described general change rate of 20% of the conventional sensor element.

Therefore, the present invention is of great technical significance in that nitrogen oxide gas can be measured reliably over a long period of time under any environment. The solid electrolyte type nitrogen oxide gas sensor element of the present invention having such properties is incorporated into a concentration measurement facility and used effectively, for example, in the measurement of nitrogen oxides in the atmosphere and the analysis of exhaust gas from automobiles and the like.

[0059]

EXAMPLES The present invention will be described specifically with reference to the following examples, but the present invention is not limited to these examples. (1) Water resistance test Immediately after manufacturing the nitrogen oxide gas sensor elements of Examples and Comparative Examples, they were placed in a chamber in which the concentration of nitrogen dioxide gas could be freely controlled, and a DC voltage was applied from a power source to a heater to set the sensor elements to 150. Heated to ° C.

After 24 hours of heating, the temperature of the sensor
With the temperature maintained at 0 ° C., the nitrogen oxide gas concentration in the chamber was set to 10 ppm and 100 ppm, and the electromotive force at each concentration was measured and used as the initial electromotive force.
Further, the difference between the electromotive force value at 10 ppm and the electromotive force value at 100 ppm was determined and used as the initial sensitivity.

After measuring the initial electromotive force and sensitivity, the sensor element was taken out of the chamber, placed in a constant temperature bath maintained at a temperature of 60 ° C. and a humidity of 90%, and left standing for 7 days without heating. did.

After standing, it was taken out of the thermostat, and the electromotive force and the sensitivity were measured in the same manner as the method for measuring the initial electromotive force and the sensitivity, and these were defined as the electromotive force and the sensitivity after the water resistance test.

The difference between the electromotive force after the water resistance test and the initial period and the difference between the sensitivities were obtained, and the changes in the electromotive force and the sensitivity after being left in an atmosphere of dew condensation or high humidity in a non-heated state for a long time were observed. . (2) Temporal stability test Immediately after producing the nitrogen oxide gas sensor elements of Examples and Comparative Examples, a DC voltage was applied from a power source to a heater to heat the sensor elements to 150 ° C.

After heating for 24 hours, the nitrogen dioxide gas concentration becomes 1
The heated element was placed in a chamber maintained at 00 ppm, and the electromotive force was measured. This was used as the initial electromotive force.

After measuring the initial electromotive force, the sensor element was taken out of the chamber and left at room temperature for 1000 hours while being heated to 150 ° C. After the standing, the heated element was placed again in the chamber in which the concentration of nitrogen dioxide gas was kept at 100 ppm, and the electromotive force was measured. This was used as the electromotive force after 1000 hours.

The change in electromotive force 1000 hours after the initial electromotive force was determined by the following equation, and the stability over time was evaluated.

(((Electromotive force after 1000 hours) − (initial electromotive force)) / (initial electromotive force)) × 100 (%) Example 1 Solid Electrolyte Nitrogen Oxide Gas Sensor Element of Electromotive Force Detection Method An element having a cross-sectional structure as shown in FIG. 1 was produced. In this nitrogen oxide gas sensor element, a detection electrode layer 1 is formed on one surface of a solid electrolyte layer 2 and a counter electrode layer 3 is formed on the other surface, and a ceramic plate 4 is provided with an adhesive 5 on the counter electrode layer 3.
Are joined by Further, a heater 6 is formed on the surface of the ceramic plate 4 opposite to the surface to which the counter electrode layer 3 is bonded, and receives power from a power supply 7. Further, lead wires are drawn out from the detection electrode layer 1 and the counter electrode layer 3 and connected to a voltmeter 8 to measure the electromotive force.

As a solid electrolyte powder for forming a solid electrolyte layer, LiOCH ₃ and Si (OC ₂ H ₅ ) ₄ were mixed in a solvent at a molar ratio of 1: 1 as shown in Table 1 to obtain a solid electrolyte powder. After decomposing, it is dried at 100 ° C. and further 800
By sintering at 3 ° C. for 3 hours, Li ₂ Si ₂ O ₅ was obtained. The synthesis of Li ₂ Si ₂ O ₅ was confirmed by X-ray diffraction.

The solid electrolyte layer 2 is made of the above-mentioned Li ₂ Si ₂ O ₅
After the powder was uniaxially formed, the powder was sintered in an air atmosphere at 1000 ° C. for 6 hours to obtain a disk-shaped pellet having a thickness of 0.4 mm.

After sintering, pellets were identified by the Rietveld method using Li ₂ Si ₂ O ₅ obtained from the metal alkoxide as described above as a standard sample. Table 2 shows the composition ratio of the solid electrolyte layer by the Rietveld method.

The detection electrode layer is prepared by kneading 5% by weight of ethyl cellulose in terpineol with gold powder as an electron conductive material and 25% by weight of lithium nitrate in a ratio of 100% by weight of the total weight of the detection electrode to paste. And, this is screen printed on one side of the solid electrolyte layer, dried,
It was formed by firing in the air at 200 ° C. for 30 minutes. Thus, a detection electrode layer having a thickness of 0.015 mm was obtained.

The counter electrode layer is formed by kneading gold powder as an electron conductive material into terpineol in which 5% by weight of ethylcellulose is dissolved to form a paste, and this is the surface of the solid electrolyte layer opposite to the surface on which the sensing electrode layer is formed. Was formed by screen printing, drying, and baking for 30 minutes in the air at 650 ° C. Thus, a counter electrode layer having a thickness of 0.015 mm was obtained.

An alumina substrate, on which a platinum heater formed by a screen printing method using a commercially available platinum paste, is mounted on the counter electrode layer, an adhesive made of glass such that the surface on which the heater is not formed is a bonding surface. Bonded with the agent.

The solid electrolyte type nitrogen oxide gas sensor element of the electromotive force detection method manufactured by the above method was subjected to a water resistance test and a stability test with time. Table 3 shows the results. There was almost no decrease in electromotive force and sensitivity in the water resistance test, and the change with time was within 2%.

Example 2 As a solid electrolyte powder for forming a solid electrolyte layer,
LiOCH ₃ , Ti (On-C ₄ H ₉ ) ₄ , Si (OC
_As shown in Table 1, a solid electrolyte type nitrogen oxide gas sensor element was produced in the same manner as in Example 1, except that ₂ H ₅ ) ₄ was mixed in a solvent at a molar ratio of 2: 1: 1 as shown in Table 1. .

The synthesis of Li ₂ TiSiO ₅ was confirmed by X-ray diffraction.

The solid electrolyte layer 2 is made of the above-mentioned Li ₂ TiSi
After uniaxially molding the O ₅ powder, it was sintered in an air atmosphere at 1100 ° C. for 6 hours to obtain a disk-shaped pellet having a thickness of 0.4 mm.

After sintering, the pellets were identified by the Rietveld method using Li ₂ TiSiO ₅ obtained from the metal alkoxide as described above as a standard sample. Table 2 shows the composition ratio of the solid electrolyte layer by the Rietveld method.

The solid electrolyte type nitrogen oxide gas sensor element manufactured by the above method was subjected to a water resistance test and a stability test with time. Table 3 shows the results. There was almost no decrease in electromotive force and sensitivity in the water resistance test, and the change with time was within 1%.

Example 3 As a solid electrolyte powder for forming a solid electrolyte layer,
LiOCH ₃ , La (OC ₂ H ₅ ) ₃ , Si (OC ₂ H ₅ ) ₄
Was mixed in a solvent at a molar ratio of 1: 1: 1 as shown in Table 1 to produce a solid oxide nitrogen oxide gas sensor element in the same manner as in Example 1.

The synthesis of LiLaSiO ₄ was confirmed by X-ray diffraction.

The solid electrolyte layer 2 is made of the above-mentioned LiLaSiO
_{After the four} powders were uniaxially formed, they were sintered in an air atmosphere at 950 ° C. for 6 hours to obtain disk-shaped pellets having a thickness of 0.4 mm.

After sintering, pellets were identified by the Rietveld method using LiLaSiO ₄ obtained from the metal alkoxide as described above as a standard sample. Table 2 shows the composition ratio of the solid electrolyte layer by the Rietveld method.

The solid electrolyte type nitrogen oxide gas sensor element manufactured by the above method was subjected to a water resistance test and a stability test with time. Table 3 shows the results. There was almost no decrease in electromotive force and sensitivity in the water resistance test, and the change with time was within 2%.

Example 4 As a solid electrolyte powder for forming a solid electrolyte layer,
LiOCH ₃ , La (OC ₂ H ₅ ) ₃ , Si (OC ₂ H ₅ ) ₄
Was mixed in a solvent at a molar ratio of 1: 9: 6 as shown in Table 1 to produce a solid oxide nitrogen oxide gas sensor element in the same manner as in Example 1.

The synthesis of LiLa ₉ (SiO ₄ ) ₆ O ₂ was confirmed by X-ray diffraction.

The solid electrolyte layer 2 is made of the above-mentioned LiLa ₉ (S
After uniaxially molding the iO ₄ ) ₆ O ₂ powder, the powder was sintered in an air atmosphere at 950 ° C. for 6 hours to obtain a disk-shaped pellet having a thickness of 0.4 mm.

After sintering, pellets were identified by the Rietveld method using LiLa ₉ (SiO ₄ ) ₆ O ₂ obtained from the metal alkoxide as described above as a standard sample. Table 2 shows the composition ratio of the solid electrolyte layer by the Rietveld method.

The solid electrolyte type nitrogen oxide gas sensor element manufactured by the above method was subjected to a water resistance test and a stability test with time. Table 3 shows the results. There was almost no decrease in electromotive force and sensitivity in the water resistance test, and the change with time was within 2%.

Examples 5 to 7 A solid electrolyte type nitrogen oxide gas sensor element was prepared in the same manner as in Example 1 except that the starting materials for forming the solid electrolyte layer were of the kind and compounding ratio shown in Table 1. Produced.

It was confirmed by X-ray diffraction that a solid electrolyte powder containing Li ₂ Si ₂ O ₅ and at least one selected from titanium oxide, silica, zirconia and zirconium silicate was synthesized.

The solid electrolyte layer 2 was formed into a disk-shaped pellet by uniaxially molding the raw material powder for the solid electrolyte layer and then sintering at 1000 ° C. in an air atmosphere for 6 hours.

After sintering, Li ₂ Si ₂ O ₅ , TiO ₂ , SiO
₂ , ZrO ₂ and ZrSiO ₄ as standard samples,
The pellet was identified by the Rietveld method. Table 2 shows the composition ratio of the obtained solid electrolyte layer by the Rietveld method.

The water resistance test and the aging stability test were performed on the solid electrolyte type nitrogen oxide gas sensor element manufactured by the above method. Table 3 shows the results. There was almost no decrease in electromotive force and sensitivity due to the water resistance test, and the change with time was within 0.5%.

Examples 8 to 10 A solid electrolyte type nitrogen oxide gas sensor element was prepared in the same manner as in Example 1 except that the starting materials for forming the solid electrolyte layer were of the kind and the mixing ratio shown in Table 1. Produced.

Li ₂ TiSiO ₅ and titanium oxide, silica,
The fact that a solid electrolyte powder containing zirconia and at least one selected from zirconium silicate was synthesized was described by X
Confirmed by line diffraction.

The solid electrolyte layer 2 was formed into a disk-shaped pellet by uniaxially molding the raw material powder for the solid electrolyte layer and then sintering at 1100 ° C. in an air atmosphere for 6 hours.

After sintering, Li ₂ TiSiO ₅ , TiO ₂ , S
Using iO ₂ , ZrO ₂ , and ZrSiO ₄ as standard samples, pellets were identified by the Rietveld method. Table 2 shows the composition ratio of the obtained solid electrolyte layer by the Rietveld method.

The solid electrolyte type nitrogen oxide gas sensor element manufactured by the above method was subjected to a water resistance test and a stability test with time. Table 3 shows the results. There was no decrease in electromotive force and sensitivity in the water resistance test, and almost no change over time occurred.

Examples 11 to 13 A solid electrolyte type nitrogen oxide gas sensor element was prepared in the same manner as in Example 1 except that the starting materials for forming the solid electrolyte layer were of the type and the mixing ratio shown in Table 1. Produced.

LiLaSiO ₄ and titanium oxide, silica,
The fact that a solid electrolyte powder containing zirconia and at least one selected from zirconium silicate was synthesized is described by X
Confirmed by line diffraction.

The solid electrolyte layer 2 was formed into a disk-shaped pellet by uniaxially molding the raw material powder for the solid electrolyte layer and then sintering it at 950 ° C. in an air atmosphere for 6 hours.

After sintering, LiLaSiO ₄ , TiO ₂ , Si
Using O ₂ , ZrO ₂ , and ZrSiO ₄ as standard samples, pellets were identified by the Rietveld method. Table 2 shows the composition ratio of the obtained solid electrolyte layer by the Rietveld method.

The solid electrolyte type nitrogen oxide gas sensor element manufactured by the above method was subjected to a water resistance test and a time stability test. Table 3 shows the results. There was almost no decrease in electromotive force and sensitivity due to the water resistance test, and the change with time was within 0.5%.

Examples 14 and 15 A solid electrolyte type nitrogen oxide gas sensor element was prepared in the same manner as in Example 1 except that the starting materials for forming the solid electrolyte layer were of the type and the mixing ratio shown in Table 1. Produced.

LiLa ₉ (SiO ₄ ) ₆ O ₂ and titanium oxide,
It was confirmed by X-ray diffraction that a solid electrolyte powder containing at least one selected from silica, zirconia, and zirconium silicate was synthesized.

The solid electrolyte layer 2 was formed into a disk-shaped pellet by uniaxially molding the raw material powder for the solid electrolyte layer and then sintering it in an air atmosphere at 950 ° C. for 6 hours.

After sintering, LiLa ₉ (SiO ₄ ) ₆ O ₂ , Ti
Using O ₂ , SiO ₂ , ZrO ₂ , and ZrSiO ₄ as standard samples, pellets were identified by the Rietveld method.
Table 2 shows the composition ratio of the obtained solid electrolyte layer by the Rietveld method.

The solid electrolyte type nitrogen oxide gas sensor element manufactured by the above method was subjected to a water resistance test and a stability test with time. Table 3 shows the results. There was almost no decrease in electromotive force and sensitivity in the water resistance test, and the change with time was within 0.5%.

Comparative Example 1 The same procedure as in Example 1 was carried out except that a commercially available high-purity zirconia powder alone was uniaxially formed as the solid electrolyte layer 2 and then sintered in an air atmosphere at 1250 ° C. for 6 hours to obtain a disk-shaped pellet. A solid electrolyte type nitrogen oxide gas sensor element was produced by the method described above.

For the sensor element manufactured as described above,
A water resistance test was performed, but no electromotive force was generated according to the concentration of the nitrogen oxide gas, and no sensitivity was shown to the nitrogen oxide gas. The results are shown in Table 3.

Comparative Example 2 The same procedure as in Example 1 was carried out except that only commercially available zirconium silicate powder was uniaxially formed as the solid electrolyte layer 2 and then sintered at 1250 ° C. in an air atmosphere for 6 hours to obtain a disk-shaped pellet. A solid electrolyte type nitrogen oxide gas sensor element was fabricated by the method.

For the sensor element manufactured as described above,
A water resistance test was performed, but no electromotive force was generated according to the concentration of the nitrogen oxide gas, and no sensitivity was exhibited for the nitrogen oxide gas. The results are shown in Table 3.

Comparative Example 3 The same procedure as in Example 1 was carried out except that only commercially available Li ₂ SiO ₃ powder was uniaxially formed as the solid electrolyte layer 2 and then sintered in an air atmosphere at 1100 ° C. for 6 hours to obtain a disc-shaped pellet. A solid electrolyte type nitrogen oxide gas sensor element was manufactured in the same manner.

For the sensor element manufactured as described above,
The water resistance test and the measurement of the electromotive force stabilization time were performed, and the results are shown in Table 3. The electromotive force is 130 by the water resistance test.
mV or more. Further, the stabilization time was 12 hours or more.

Comparative Example 4 A solid electrolyte type nitrogen oxide gas sensor element was manufactured in the same manner as in Example 1 except that a solid electrolyte layer made of NASICON was used for the solid electrolyte layer.

The solid electrolyte powder for forming the solid electrolyte layer is made of zirconium silicate and sodium phosphate containing Na ₃
Zr ₂ Si ₂ PO ₁₂ is mixed so that the composition becomes 1100.
It was obtained by baking for 6 hours in an air atmosphere at a temperature of ° C.
The solid electrolyte layer is formed by uniaxially molding the solid electrolyte powder,
Sintering was performed at 200 ° C. in an air atmosphere for 10 hours to obtain disc-shaped pellets.

With respect to the sensor element manufactured as described above,
The water resistance test and the measurement of the electromotive force stabilization time were performed, and the results are shown in Table 3. Electromotive force is 200 by water resistance test
mV or more. Further, the stabilization time was 20 hours or more.

[0119]

[Table 1]

[0120]

[Table 2]

[0121]

[Table 3]

[0122]

[Brief description of the drawings]

FIG. 1 is a sectional view showing a typical embodiment of a nitrogen oxide gas sensor element.

[Explanation of symbols]

 1. 1. Detection electrode layer (working electrode layer) Solid electrolyte layer 3. 3. Counter electrode layer (reference electrode layer) Ceramic plate 5. Adhesive 6. Heater 7. Power supply 8. voltmeter

Claims (2)

[Claims] 1. A method according to claim 1, wherein the first and _second electrodes are Li ₂ Si ₂ O ₅ , Li ₂ TiSiO ₅ , Li
A solid electrolyte type nitrogen oxide gas sensor element comprising a pair of electrode layers formed on the surface of a solid electrolyte layer containing at least one selected from LaSiO ₄ and LiLa ₉ (SiO ₄ ) ₆ O ₂ . 2. (a) Li ₂ Si ₂ O ₅ , Li ₂ TiSi
O ₅ , LiLaSiO ₄ and LiLa ₉ (SiO ₄ ) ₆ O ₂
Solid electrolyte type nitrogen formed by forming a pair of electrode layers on the surface of a solid electrolyte layer containing at least one selected from the group consisting of: (b) titanium oxide, silica, zirconia, and zirconium silicate Oxide gas sensor element.