JP2003042999A - Nitrogen oxide sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitrogen oxide sensor comprising slightly water-soluble structure, which is usable even in an atmosphere, especially containing water vapor, and is usable in practical way. SOLUTION: An oxide ion conductor 2, constituted of stabilized zirconia and a detection electrode 3 constituted from a mixed nitrate of europium oxynitrate and lithium nitrate are formed, so as to sandwich a solid electrolyte 1, constituted from a complex having a composition of Mg1+ XZr4 P6 O24+ X+Zr2 O(PO4 )2 which conducts magnesium ions. A current-collecting gold net 4 and a gold wire 6 serving as an electrode terminal are provided on the upper surface of the solid electrolyte 1. A current-collecting platinum net 5 and a platinum wire 7 as an electrode terminal are provided on the under surface of the oxide ion conductor 2.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a nitrogen oxide sensor.

[0002]

2. Description of the Related Art Nitrogen oxide is a harmful gas that coexists in exhaust gas and is one of the main causative substances of acid deposition, and causes serious damage not only to humans but also to the environment. The most effective method of suppressing the emission of nitrogen oxide gas into the atmosphere is to promptly find the leakage site of nitrogen oxide gas and prevent further leakage of nitrogen oxide gas. For this purpose, it is necessary to install a nitrogen oxide sensor at each site around the flue gas and always detect the nitrogen oxide concentration in the atmosphere.

A compact and inexpensive nitrogen oxide sensor is required to accurately measure and monitor the nitrogen oxide concentration at each site around the flue gas. Conventionally, various nitrogen oxide sensors using a solid electrolyte have been proposed because they have a quick response and high selectivity. However, since such a solid electrolyte is water-soluble, it has not been possible to mount the nitrogen oxide sensor made of the solid electrolyte and put it into practical use, particularly in an atmosphere such as flue gas containing water vapor.

Further, the sensing electrode forming the nitrogen oxide sensor together with the solid electrolyte could not be made of a material having sufficient poor water solubility. I could not provide.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a nitrogen oxide sensor which is composed of a structure having poor water solubility and can be used especially in an atmosphere containing water vapor and can be put to practical use. To aim.

[0006]

[Means for Solving the Problems] In order to achieve the above object,
The present invention, magnesium ions, aluminum ions,
The present invention relates to a nitrogen oxide sensor including a solid electrolyte that conducts rare earth ions, zirconium ions, or hafnium ions, an oxide ion conductor provided so as to sandwich the solid electrolyte, and a detection electrode containing nitrate ions.

The present inventors have found that magnesium ions, aluminum ions, rare earth ions, zirconium ions,
We have also succeeded in developing a new solid electrolyte that conducts hafnium ions. Then, various physical properties of these solid electrolytes were investigated and examined, and it was found that they exhibited extremely high water solubility. Furthermore, they have found that substances containing nitrate ions also show extremely high water solubility.

Therefore, it has been conceived to use the above-mentioned solid electrolyte as a structure of a nitrogen oxide sensor and use the above-mentioned substance containing nitrate ions as a detection electrode. And
It has been found that an oxide ion conductor serving as a receptor for the cation conductor of the solid electrolyte is provided adjacent to the solid electrolyte to function as an actual nitrogen oxide sensor.

Since oxide ion conductors generally exhibit high poor water solubility, no problems occur even when used in a water vapor-containing atmosphere.

The nitrogen oxide sensor of the present invention as described above is
It is composed of a poorly water-soluble structure and does not cause any inconvenience even when used in an atmosphere containing water vapor.
Therefore, the nitrogen oxide concentration can be actually detected by arranging it at each site around the flue gas. That is, according to the present invention, a practical nitrogen oxide sensor can be provided.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below in detail based on the embodiments of the invention. FIG. 1 is a configuration diagram showing an example of a nitrogen oxygen substance sensor of the present invention. The nitrogen oxide sensor 10 shown in FIG. 1 includes a solid electrolyte 1 and the solid electrolyte 1.
The oxide ion conductor 2 is provided on the lower surface of the solid electrolyte 1, and the detection electrode 3 is provided on the upper surface of the solid electrolyte 1. In addition,
The solid electrolyte 1 and the oxide ion conductor 2 are bonded and fixed with an inorganic adhesive (not shown).

Further, a gold net 4 is provided on the detection electrode 3, and a platinum net 5 is provided on the oxide ion conductor 2. Further, a gold wire 6 and a platinum wire 7 are provided via the gold net 4 and the platinum net 5, respectively.

When the nitrogen oxide sensor 10 is placed in a predetermined nitrogen oxide-containing atmosphere, the detection electrode 3 detects the nitrogen oxide and the cation conductor moves in the solid electrolyte 1. As a result, an ion driving force is generated in the thickness direction of the solid electrolyte 1. The ionic current flowing through each part of the solid electrolyte 1 is collected as a generated current by the gold net 4 and the platinum net 5, and a predetermined amount of current is measured via the gold wire 6 and the platinum wire 7.

Since the electromotive force between the gold wire 6 and the platinum wire 7 is proportional to the detected concentration of nitrogen oxide, the nitrogen oxide in the nitrogen oxide-containing atmosphere is measured by measuring the current (electromotive force). The concentration can be detected.

The cation conductor that has moved in the solid electrolyte 1 is bound to the oxide ion that has been conducted in the oxide ion conductor 2 at the interface 8 between the solid electrolyte 1 and the oxide ion conductor 2 and is stable. Exists as a simple oxide.

The solid electrolyte 1 is required to conduct magnesium ions, aluminum ions, rare earth ions, zirconium ions, or hafnium ions according to the present invention. As a solid electrolyte that conducts magnesium ions, a composite having a composition of Mg _{1 + X} Zr ₄ P ₆ O _{24+ X} + Zr ₂ O (PO ₄ ) ₂ or Mg _1-2Y (Zr _1-Y Nb _Y ) ₄ P ₆ O ₂₄ (0 ≦ Y <1 /
It is preferable that the solid solution of 2) is composed of a solid solution having a Nasicon type or β-iron sulfate type crystal structure.

The composite and the solid solution are, for example, MgHP
O_Four・ 3H₂O, ZrO (NO₃) ・ 2H₂O and NH _FourH₂PO_FourPowder of different composition
Mix powders in a prescribed ratio and bake if necessary
More mixed powder is obtained, and this mixed powder is made into a predetermined shape and size.
After being molded into a
To make. The solid solution is, for example, MgHPO._Four・ 3H₂O,
ZrO (NO₃) ・ 2H₂O, Nb₂O_FiveAnd NH_FourH₂PO_FourPowder of composition
By mixing at a predetermined ratio and firing if necessary
A mixed powder is obtained, and this mixed powder is given a predetermined shape and size, for example,
For example, it is formed into pellets and then baked under specified conditions.
It is made by making.

The solid electrolyte that conducts aluminum ions has a composition of Al _1/3 Zr ₂ (PO ₄ ) _{3 and is} composed of a compound having a crystal structure of Nasicon type or β-iron sulfate type. Is preferred. Similarly, the solid electrolyte that conducts rare earth ions has a composition of R _1/3 Zr ₂ (PO ₄ ) ₃ (R: rare earth ions) and is a compound having a Nasicon-type or β-iron sulfate-type crystal structure. It is preferably composed of

These are powders having a composition of Al (OH) ₃ or Sc ₂ O ₃ and powders having a composition of ZrO (NO ₃ ) · 2H ₂ O mixed at a predetermined ratio and fired if necessary. Thus, a mixed powder is obtained, and the mixed powder is formed into a predetermined shape and size, for example, pellets, and then fired under predetermined conditions.

Further, the zirconium ion conducting solid electrolyte preferably has a composition of ZrNb (PO ₄ ) ₃ and is composed of a compound having a Nasicon type or β-iron sulfate type crystal structure. Similarly, the solid electrolyte that conducts hafnium ions preferably has a composition of HfNb (PO ₄ ) ₃ and is composed of a compound having a Nasicon type or β-iron sulfate type crystal structure.

These are powders having a composition of ZrO ₂ or Hf (SO ₄ ) ₂ and powders having a composition of Nb ₂ O ₅ mixed at a predetermined ratio,
A mixed powder is obtained by firing if necessary, and the mixed powder is formed into a predetermined shape and size, for example, pellets, and then fired under predetermined conditions.

The Nasicon type crystal structure means a crystal structure having a three-dimensional tunnel and designed so that a specific ion species can easily move in the crystal. The origin of the name is sodium ion (Na
⁺ ) As a mobile ion species Na _{1 + X} Zr ₂ P _8-X Si _X O
₁₂ (X≈2): NASICON). Further, the β-iron sulfate type crystal structure exhibits a structure similar to the above-mentioned NASICON type crystal structure, and similarly has a three-dimensional tunnel in which a specific ionic species can be easily moved.

In FIG. 1, the solid electrolyte 1 is usually 0.7.
having a thickness of mm to 1.1 mm and a surface area of 50 mm
It has a size of ^{2 to} 100 mm ² . The shape can be set to any shape such as a disk shape or a rectangular flat plate shape as required.

The oxide ion conductor 2 is not particularly limited as long as it can move the oxide ions as described above. Preferably, it is composed of at least one of stabilized zirconia, cerium oxide, bismuth oxide, hafnium oxide, thorium oxide, and lanthanum gallate.

In FIG. 1, the oxide ion conductor 2 is
Has a thickness of usually 0.7Mm～1.0Mm, its surface area has a size of 50 mm ² 100 mm ^2. Also,
The shape of the solid electrolyte 1 can be adjusted to an arbitrary shape.

The sensing electrode 3 needs to contain nitrate ions according to the present invention. Specifically, it is preferably composed of a mixed nitrate of europium oxynitrate and lithium nitrate. Substances containing such nitrate ions,
Since it exhibits extremely high water solubility, the nitrogen oxide sensor 10 including the detection electrode 3 made of the above substance can be installed in an atmosphere containing water vapor and can be put to practical use.

It is also possible to substitute europium in the mixed nitrate constituting the detection electrode 3 with another rare earth element. Further, the detection electrode 3 can be configured by using oxynitrate alone, but as described above, it is preferable to use it as a mixed nitrate including lithium nitrate.

The detection electrode 3 is usually 0.5 mm to 0.8 mm
Of is formed with a thickness, size of the surface area so that it can be placed on the solid electrolyte 1 is set ^to 10mm ² ~30mm 2.

[0029]

EXAMPLES The present invention will be described below based on specific examples.
In this example, a nitrogen oxide sensor 10 as shown in FIG. 1 was produced.

(Production of Nitrogen Oxide Sensor) First, MgHP
Powders having a composition of O ₄ .3H ₂ O, ZrO (NO ₃ ) .2H ₂ O, and NH ₄ H ₂ PO ₄ were mixed so that the molar ratio was 1.4: 4.8: 5.4. After heating at 300 ° C. for 5 hours under flowing dry air,
Similarly, under dry air flow, the powder was fired at 1200 ° C. for 12 hours to prepare a mixed powder. Then, this mixed powder is pelletized, and the obtained pellets are put into a predetermined mold and fired at 1200 ° C. for 12 hours under a flow of dry air to obtain Mg _{1 + X} Zr ₄ P ₆ O _{24 + X} + A rectangular flat plate-like solid electrolyte 1 composed of a composite having a composition of Zr ₂ O (PO ₄ ) ₂ was obtained. The thickness of the solid electrolyte 1 is 1.0 mm, and the surface area is 95 m.
It was m ² .

Then, ZrO ₂ powder and Y ₂ O ₃ powder were mixed at a molar ratio of 9: 1 and calcined in air at 1600 ° C. for 6 hours as one term, and calcined for a total of 3 terms. A rectangular flat plate-shaped oxide ion conductor 2 made of zirconia was produced. The oxide ion conductor 2 had a thickness of 1.0 mm and a surface area of 95 mm ² .

Next, the solid electrolyte obtained as described above
1 and the oxide ion conductor 2 with an adhesive called Sumiceram
It was stuck using. After that, Eu (N
O₃) ・ H ₂O and LiNO₃Apply the mixed paste to 300 ℃.
EuONO by raising the temperature at ₃And LiNO₃A mixture of
The detection electrode 3 made of the above was formed to a thickness of 0.7 mm.

Thereafter, as shown in FIG. 1, the solid electrolyte 1
The gold net 4 and the gold wire 6 were provided on the upper surface of the above, and the platinum net 5 and the platinum wire 7 were provided on the lower surface of the oxide ion conductor 2 to complete the nitrogen oxide sensor 10.

(Evaluation of Nitrogen Oxide Sensor) FIG. 2 is a graph showing a response curve of the nitrogen oxide sensor 10 obtained as described above, and FIG. It is a graph which shows the correlation of electric power value) and nitrogen oxide concentration. The output value is an electromotive force value between the gold wire 6 and the platinum wire 7. As the NO gas, 1% NO gas diluted with nitrogen gas was used, and this was further diluted with oxygen gas and nitrogen gas to obtain a NO gas concentration of 100%.
It was made to be ppm to 500 ppm. The total flow rate is set to 100 ml / min and the oxygen concentration is 20% by volume.
It was controlled so that

From FIG. 2, it is understood that when the NO gas concentration is changed with time, the electromotive force value between the gold wire 6 and the platinum wire 7 also changes following the NO gas concentration change. Further, it can be seen that the response speed defined by 90% of the output change is also within a few minutes, which shows a good response.

FIG. 3 shows the correlation between the output value of the nitrogen oxide sensor 10 and the nitrogen oxide concentration at 250 ° C., when the concentration is increasing (white square plot) and when the concentration is decreasing (black circle plot). Although there is a slight difference between the two, the logarithmic concentration of nitrogen oxides and the output values of the gold wire 6 and the platinum wire 7
It can be seen that the electromotive force values in between correspond substantially to each other and are in a proportional relationship.

From the above, it was found that the nitrogen oxide sensor 10 manufactured according to the present invention can be used as a practical sensor. Therefore, the nitrogen oxide sensor 10
It can be seen that the nitrogen oxide sensor 10 is a practical nitrogen oxide sensor because it can be used even in an atmosphere containing water vapor and the like because of the poorly water-soluble property of the solid electrolyte 1 which is the constituent of FIG.

The present invention has been described in detail based on the embodiments of the invention with reference to specific examples. However, the present invention is not limited to the above contents and does not depart from the scope of the present invention. All modifications and changes are possible. For example, by appropriately selecting the type of substance forming the detection electrode 3, it is possible to apply to not only NO gas but also other gases.

[0039]

As described above, according to the present invention,
Each component such as a solid electrolyte is composed of a poorly water-soluble substance, and as a result, a nitrogen oxide sensor having a completely new structure is provided. Since this nitrogen oxide sensor exhibits poor water solubility as a whole, it can be stably used for a long period of time even in an environment containing water vapor. Therefore, it can be suitably used as a flue gas sensor, an atmospheric monitor for environmental protection, and the like.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an example of a nitrogen oxygen substance sensor of the present invention.

FIG. 2 is a graph showing an example of a response curve of the nitrogen oxide sensor of the present invention.

FIG. 3 is a graph showing the correlation between the output value (electromotive force value) of the nitrogen oxide sensor of the present invention and the nitrogen oxide concentration.

[Explanation of symbols]

1 Solid electrolyte 2 Oxide ion conductor 3 sensing electrodes 4 money net 5 platinum net 6 gold wire 7 platinum wire 8 interfaces

Claims (8)

[Claims] 1. A solid electrolyte that conducts magnesium ions, aluminum ions, rare earth ions, zirconium ions, or hafnium ions, an oxide ion conductor provided so as to sandwich the solid electrolyte, and a sensing electrode containing nitrate ions. Nitrogen oxide sensor equipped with. 2. The solid electrolyte that conducts magnesium ions is a composite of Mg _{1 + X} Zr ₄ P ₆ O _{2 4 + X} + Zr ₂ O (PO ₄ ) ₂ or Mg _1-2Y (Zr _{1 -Y Nb Y) 4 P 6 O} 24 (0 ≦ Y <1/2)
A solid solution having the following composition, characterized in that it is composed of one having a crystal structure of Nasicon type or β-iron sulfate type,
The nitrogen oxide sensor according to claim 1. 3. The solid electrolyte that conducts aluminum ions has a composition of Al _1/3 Zr ₂ (PO ₄ ) ₃ and is composed of a compound having a Nasicon type or β-iron sulfate type crystal structure. The nitrogen oxide sensor according to claim 1, wherein: 4. The solid electrolyte that conducts rare earth ions has a composition of R _1/3 Zr ₂ (PO ₄ ) ₃ (R: rare earth ions) and has a Nasicon type or β-iron sulfate type crystal structure. The nitrogen oxide sensor according to claim 1, wherein the nitrogen oxide sensor is composed of a compound having. 5. The zirconium ion conducting solid electrolyte has a composition of ZrNb (PO ₄ ) ₃ and is composed of a compound having a Nasicon type or β-iron sulfate type crystal structure. The nitrogen oxide sensor according to claim 1. 6. The solid electrolyte that conducts hafnium ions has a composition of HfNb (PO ₄ ) ₃ and is of the Nasicon type or β-type.
-The nitrogen oxide sensor according to claim 1, which is composed of a compound having an iron sulfate-type crystal structure. 7. The oxide ion conductor is composed of at least one of stabilized zirconia, cerium oxide, bismuth oxide, hafnium oxide, thorium oxide, and lanthanum gallate. 5. The nitrogen oxide sensor according to any one of 1. 8. The nitrogen oxide sensor according to claim 1, wherein the detection electrode is composed of a mixed nitrate of europium oxynitrate and lithium nitrate.