JP2529546B2 - Gas detector - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detector for detecting a gas component or its concentration,
In particular, it relates to a gas detector using a gas-sensitive transition metal oxide.

[0002]

2. Description of the Related Art Conventionally, an oxygen gas detector has been used as a gas detector for detecting the presence or concentration of gas.
Combustible gas detectors have been put to practical use. Among these, there is one using a gas-sensitive metal oxide having a characteristic that its electric resistance changes when it comes into contact with gas. For example, oxides of transition metal elements such as TiO ₂ , CoO, and NiO can be used as the oxygen gas detector .

The transition metal oxides exemplified here are non-stoichiometric compounds. Then, the amount of charge carriers (holes, electrons) in this non-stoichiometric compound changes depending on the partial pressure of oxygen gas in the surroundings. Therefore, the conductivity changes according to the partial pressure of oxygen gas in the surroundings.

[0004]

By the way, the above oxygen gas is used.
Among the detectors , for example, in a titania sensor, an ionic current (from cathode to anode
Due to the current due to the movement of oxygen ions O ²⁻ ) , the anode side is in a state of lacking Ti ⁴⁺ and is in an excess of O ^{2−, which} causes lattice defects (acids originally present in titania).
Oxygen atoms enter the elementary defects (holes) and fill the oxygen defects.
As a result, oxygen defects are reduced. That is, the anode
Side titania is the cathode side titania due to external DC voltage.
By the transfer of O ^2- from A , it is electrically oxidized and
The electrical conductivity decreases. For this reason, use an oxygen gas detector.
While using, the resistance between the anode and the titania gas sensitive layer
Rises and the resistance value of the gas sensitive layer becomes different from the initial value.
Will deteriorate.

The present invention has been made as a result of investigating the problems of the above-mentioned conventional techniques, and an object of the present invention is to provide a gas detector which is less deteriorated due to aging.

[0006]

The present invention adopts the following means in order to solve the above problems. That is, the present invention includes a pair of electrodes consisting of an anode and a cathode, and a porous material which covers the electrodes and contains a gas-sensitive transition metal oxide, and whose electric resistance changes according to the gas component and / or the gas concentration. preparedness and a gas sensitive layer, and the gas-sensitive layer near the anode, to the gist of the gas detector, characterized in that the addition of an electron donor additive containing a valence of greater element from the transition metal It

Here, the above-mentioned electrode is not particularly limited as long as it is a heat-resistant conductor, but in general, one containing tungsten, molybdenum, gold or platinum group as a main component is used. As the gas-sensitive metal oxide used in the gas-sensitive layer, the substance may be selected according to the gas component to be detected, but as the commonly used substances, TiO ₂ , SnO
₂ , transition metal oxides such as CoO, ZnO, Nb ₂ O ₅ , Cr ₂ O ₃ , and NiO are included. In the present invention, any one or a combination of two or more of these may be used. Good.

Further , as an electron donor additive, for example,
For example, in an oxygen gas detector mainly composed of titania, Ti
Elements such as Ta and Nb, which have high valence, and their oxides
No. In order to clearly show the present invention, the coupling
As an expression equivalent to, the expression of valence rather than valence is
Since it is appropriate, it is expressed as valence.

Here, the electron donor additive is an excess of
It means a substance that supplies electrons, and improves electrical conductivity.
For this, the donor additive is added. In particular,
Larger valence than the transition metal oxide that constitutes the gas-sensitive layer
An element such as Ti (valence 4; therefore Ti ion is T
i ( ⁴⁺ ), Ta (valence 5; therefore Ta ion)
Is Ta ⁵⁺ ) or Nb (valence 5; therefore Nb ion is Nb
⁵⁺ ) is added, the number of surplus electrons in the crystal lattice is increased.
Means a means of increasing. For example for Titania
When Ta with high valence is added, Ta is TiO 2.
It enters into the crystal lattice of ₂ and participates in bonding, but tetravalent T
Since Ta is pentavalent as compared with i,
Azukara not free electrons (= excess of electrons e ^-) with one of the
It will be. Thus, for example, Ta is the surplus electron supply side.
Since it is defined as (donor), Ta is an electron donor addition
Become an agent.

The gas detector of the present invention can be produced, for example, by providing a gas sensitive layer or the like on a ceramic substrate by a hybrid technique such as a thick film technique. Alternatively, it may be formed into a disk type, a bead type or the like used in a thermistor or the like without using the thick film technique or the like.

Further, a heating element may be provided in the vicinity of the gas sensitive layer for the purpose of reducing the fluctuation of the temperature characteristic of the gas detector during the measurement. Then, a part of this heating element and one electrode of the gas detector may be connected to apply a voltage to the gas sensitive layer to reduce the number of terminals and simplify the measurement circuit.

[0012]

[Operation] a) First, the meaning of the donor will be described. acid
Metal oxides such as titanium oxide (titania; TiO ₂ ) are electrically conductive.
Gas conduction occurs because the metal atom in the metal oxide is the oxygen source.
It is believed that it is not completely connected to the child.

For example, when a titanium atom is replaced with two oxygen atoms,
If all are bonded, the titanium atom is essentially a metal atom
All four of the free electrons we had were used to bond with oxygen.
Therefore, an acid composed only of such titanium atoms
Titanium nitride resistors conduct very little electricity.

However, assuming that the titanium atom is one oxygen atom,
If they are not bonded, two free electrons remain, so
These electrons will contribute to electrical conduction. Real acid
Titanium oxide (uses all four free electrons to bond with oxygen.
Titanium oxide (used) and two not used for bonding
Left free electrons of) titanium oxide mixed in a certain proportion
It is believed that each of them is covalently bonded to form a lattice.
it seems to do.

That is, titanium oxide contains titanium atoms
Due to incomplete bonds, there are free electrons that are not included in the bonds.
I do. In this way, for atoms with free electrons
This is called a donor. In normal titanium oxide, titanium
Atoms exist as donors,
Causing electrical conduction. In this case, the grid is completely shaped
Since it is not formed, the place where the oxygen atom is missing has a lattice defect.
It is called a pit (oxygen defect).

However, a direct current is passed through the titanium oxide.
Then, due to the ionic current, oxygen ions from the cathode to the anode
As it is transported, oxygen becomes excessive near the anode. result
As a result, oxygen atoms enter the originally existing lattice defects.
However, the titanium atom fills the lattice defects, so the titanium atom is a donor.
Is lost (that is, free electrons are lost), and the electric conduction is bad.
Change over time.

B) Therefore, in the present invention, as a countermeasure,
Instead of the titanium atom,
Oxidize a large number of atoms (ie, high valence atoms) around the anode
Consider fitting it into a titanium lattice. For example, from titanium
Tantalum (Ta), which has a high valence, is used instead of titanium.
When fitted into the lattice, it becomes a lattice with free electrons.

That is, the valence of tantalum is 5.
And, for one lattice defect, one free electron is more than titanium
Will occur more frequently. This is due to the excess oxygen
Suppresses the change in electrical conductivity over time even if lattice defects disappear.
Work. That is, oxygen ions are carried by direct current,
Assuming that lattice defects have disappeared due to excess oxygen
However, one free electron still remains.

That is, the present invention is based on the above-mentioned principle.
Therefore, it is possible to effectively suppress the change in electrical conductivity with time.
Wear. As described above, in the present invention, the gas-sensitive layer around the anode is formed.
Addition of electron donor additives (which supply excess electrons)
Therefore, due to the bias between the anode and cathode,
Flow, for example, Ti ⁴⁺ deficiency and O ² − excess on the anode side.
And oxygen atoms enter the oxygen deficiency and the oxygen deficiency is reduced.
Even a little, there are excess electrons around the anode.
Is maintained. Therefore, long-term gas detector
Even if oxygen deficiency is gradually reduced by use, this excess
The electrons reduce the electrical conductivity around the anode so much.
Not, therefore, change with time of the inter-electrode resistance of the gas sensitive layer is Ru is reduced.

[0020]

As described above, according to the gas detector of the present invention, deterioration due to use can be reduced and stable characteristics can be obtained. That is, according to the present invention, around the anode
In by adding an electron donor additive, Ru can suppress the deterioration with increasing near the anode resistance.

[0021]

Embodiments of the present invention will be described with reference to the drawings. Note that the scale of each drawing is different for the sake of explanation. First, an embodiment of the present invention will be described with reference to FIG . The present embodiment is an oxygen gas detector 10 using TiO ₂ as a gas sensitive layer.

As shown in the partially cutaway perspective view of FIG.
On the ceramic substrate 12, the terminals 13a, 13b, 13
The electrode pattern 16 such as the detection electrodes 16a and 16b and the thermal resistance electrode 16e connected to the platinum lead wires 14a, 14b and 14e by e is formed, and the ceramic substrate 1 and the ceramic substrate 1 are further formed on the electrode pattern 16.
The first ceramic laminate 18 integrated with the second ceramic laminate 18 and the second ceramic laminate 19 are laminated on the first ceramic laminate 18 respectively. The first ceramic laminated plate 18 is formed with anode side and cathode side lower window portions 20a and 20b divided by a partition portion 18a, and further,
The second ceramic laminated plate 19 has two lower window parts 20.
a, 20 b and Uemado portion 20c which together form the window portion 20 is formed.

The anode side and cathode side lower window portions 20a and 20b of the window portion 20 are filled with the anode side and cathode side pastes having the compositions shown in Table 1 to form the anode side and cathode side gas-sensitive lower layers 24a. , 24b are formed, the more the upper window portion 20c, the gas sensitive layer 24c and TiO ₂ paste forming the normal of the TiO ₂ in the gas-sensitive layer is filled is formed. The gas sensitive layer 2 is formed by each of these layers.
4 are formed. In addition, the spherical granulated particles 2 for preventing the separation between the ceramic substrate 12 and the gas sensitive layer 24.
2 is interposed.

Next, will be described with reference to FIG. 2 through 6 a manufacturing process of the oxygen gas detector 10. 2 wt% of alumina, 3 wt% of magnesia, and 5 wt% of sintering aid (silica, calcia, etc.) in a pot mill
Mix for 0 hours. Then, 12% by weight of polyvinyl butyral and 4% by weight of dibutyl phthalate were added to the mixture as an organic binder, and methyl ethyl ketone, toluene and the like were added as a solvent. Further, the mixture is mixed in a pot mill for 15 hours to form a slurry, and the substrate and laminating green sheets 12A, 18A, 19A are formed by a doctor blade method.

The shape of the green sheet is 47.8 mm × 4.0 mm × 0.8 in the green sheet 12A for substrates.
mm ^t , first and second stacking green sheets 18A, 1
It is 47.8 mm × 4.0 mm × 0.28 mm ^t at 9A. Then, the first green sheet for lamination 18A
Forms the two anode-side and cathode-side lower window parts partitioned by the partition part 18a with a size of 3.05 mm × 0.8 mm, and further comprises the upper window part 20c of the second stacking green sheet 19A of 3 It is formed with dimensions of 0.05 mm × 2.0 mm.

Next, platinum black and sponge-like platinum
The green syrup used in the above was blended in a ratio of 2: 1.
10 wt% of the material mixture of butylToー
Solvent, etc., to form an electrode paste.
You. Next, thick film printing using the electrode paste adjusted in
The electrode pattern on the green sheet 12A for the substrate
16 is formed. The electrode pattern 16 has been described above.
Sea urchin, detection electrode patterns 16a and 16b, and
The thermal resistance electrode pattern serving as a heater for heating the spray layer 24.
Terminal 16e and a terminal pattern that serves as terminals for both patterns 16 described above.
The turns 13a, 13b, 13e are formed. (FIG.
(A) (b)) Thereafter, the terminal patterns 13a, 13b, 13e
The platinum lead wires 14a, 14b, 1 having a diameter of 0.2 mm
Connect 4e respectively (Figure 3(A) (b)).

Next, the substrate green sheet 12
The first stacking green sheet 18A on A, and the second
The laminated green sheet 19A is laminated and thermocompression-bonded to form a laminated body. At this time, the green sheet for lamination 18
The detection electrode pattern 16 is provided on the windows 20 of A and 19A.
The tips of a and 16b are exposed. Then, it is made of the same material as the green sheet adjusted in the window 20.
A ceramic substrate 12 which is made integral by dispersing and adhering spherical granulated particles (secondary particles) 22 of 0 to 150 mesh for 2 hours at 1500 ° C. in substantially the same atmosphere as the atmosphere. And ceramic laminate 18,1
9 is formed ( FIGS. 4A and 4B ).

When the spherical granulated particles 22 are dispersed and adhered and fired as described above, the respective particles 22 are transferred to the ceramic substrate 12.
Dispersed on top to form an uneven surface. Next, the window portions 20 of the ceramic laminated plates 18 and 19 are filled with a gas-sensitive metal oxide containing TiO ₂ as a main component. Here, first, the sample No. 1 in Table 1 is used.
An anode side paste as shown in 4 is prepared.

That is, with respect to 100 parts by weight of TiO ₂ powder having an average particle size of 1.2 μm which was calcined at 1200 ° C. for 1 hour in air, Ta having an average particle size of 0.5 μm was used as a donor additive for increasing electron conductivity. 5 parts by weight of ₂ O ₅ was added, 20 parts by weight of platinum black was added as a catalyst, and only 2 parts by weight of 3% by weight of ethyl cellulose was added as a binder. These were added to butycarbitol (2- (2- Butoxyethoxy) ethanol (trade name) and mixed to obtain a viscosity of 300 poise to prepare a TiO ₂ paste. And
This anode side TiO ₂ paste is applied as a 20 to 50 μm thick film on the anode pattern 16a in the anode side lower window portion 20a ( FIGS. 5A and 5B ).

On the other hand, as for the paste on the cathode side, first, Ti on the anode side is prepared.
TiO _{2 having an} average particle size of 3.5 μm larger than that of O ₂ powder
To 100 parts by weight of the powder, 5 parts by weight of ZrO ₂ containing 7 mol% of Y ₂ O ₃ as a sintering inhibitor for suppressing the progress of sintering, and 20 parts by weight of platinum black as a catalyst were added, Hereinafter, the same adjustment as for the anode paste is performed. Then, the cathode-side paste is applied to the cathode-side lower window portion 20b in an amount of 20 to 50 μm.
Apply thick film.

Next, the paste applied over both the anode and cathode electrodes is prepared. First, 100 parts by weight of TiO ₂ powder having an average particle size of 1.2 μm is used as a catalyst for platinum black 10 Add 1 part by weight of rhodium black and 1 part by weight of rhodium black to prepare a paste in the same manner as the above paste. Then, paste this paste into the upper window part 20 on both layers.
Print 50 to 500 μm thick film on c.

After that, the laminated body that has undergone the above steps is
Bake by leaving it in the air at 1200 ° C for 1 hour (Figure 6
(A) (b)). Anode side and shade used in the above steps
The composition of the pole-side pasteTable 1Change likeOxygen gas detection
vesselMake a sample of. This sampleOxygen gas detectorAssembled in
After standing, the response speed and
Measure endurance. The result of the experimentTable 2Shown in. ○ Response speed test In the exhaust gas of propane gas burnerOxygen gas detectorWhen
And expose the assembled sample. This propane gas bar
The exhaust gas temperature is 350 ° C and the air fuel ratio
Is excessive fuel (hereinafter referred to as rich, air-fuel ratio λ = 0.
9) and fuel shortage (hereinafter referred to as lean, λ = 1.1)
Controlled to change between.

Oxygen gas detection when exhaust gas is rich
The voltage applied to the oxygen gas detector is adjusted so that the output of the detector is 1V and the output when lean is 0V. As the response speed, measure the time when the output of the oxygen gas detector changes from 300 mV to 600 mV when the atmosphere changes from lean to rich, and the time when the output changes from 600 mV to 300 mV when the atmosphere changes from rich to lean. . Durability test A sample assembled as the oxygen gas detector S is attached to an actual vehicle, operated in a predetermined durability pattern, and durability is examined from the change in response speed before and after the operation. That is, oxygen gas
The detector S is a commercially available 2000cc E as shown in FIG.
It is attached to the exhaust pipe Man between the engine Eng of the three-way catalyst vehicle with FI and the three-way catalyst THC. And the control unit U
ni controls the operating state of the engine according to the output of the oxygen gas detector S. The output of the oxygen gas detector S is detected by the circuit shown in FIG. Here, B is a power supply and Rc is a comparison resistance.

The engine Eng is operated for 300 hours in the endurance pattern shown in FIG . The solid line in the figure shows the temperature of the sample, and the broken line shows the temperature of the exhaust gas. The response speeds Tlr and Trl described above are measured before and after this operation, and the change is taken as the result of durability. That is, it is determined that the sample having less change in response speed before and after the operation has more excellent durability. In Table 2 , before operation is described as initial and after operation is described as after durability test. Also, RT should be rich
The resistance value between the electrodes 13a and 13b is shown.

[0035]

[Table 1]

[0036]

[Table 2]

From the above experiment, as shown in Table 2 , the following was found. Sample No. of the example . When Ta ₂ O ₅ or Nb ₂ O ₅ is contained in the gas sensitive layer around the anode as in Nos. 2 to 6 , the resistance RT between the electrodes is almost equal to the value after the durability test from the initial value. No change, sample No. of the comparative example. One
Its effect is Ru remarkable der compared with the test results.

[Brief description of drawings]

FIG. 1 is a partially cutaway perspective view showing an oxygen gas detector according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of manufacturing of an example.

FIG. 3 is an explanatory diagram of manufacturing of an example.

FIG. 4 is an explanatory diagram of manufacturing of the example.

FIG. 5 is an explanatory diagram of manufacturing of the example.

FIG. 6 is an explanatory diagram of manufacturing of the example.

FIG. 7 is a diagram for explaining the procedure of a durability test using an oxygen gas detector in an internal combustion engine.

FIG. 8 is a diagram for explaining the procedure of a durability test in which an oxygen gas detector is used in an internal combustion engine.

[9] Ru durable pattern diagram der of the test.

[Explanation of symbols]

10 ... Oxygen gas detector , 12 ... Ceramic substrate, 16, 16a, 16b ... Electrode pattern (electrode),
18 ... 1st ceramic laminated board, 19 ... 2nd ceramic laminated board, 20 ... Window part, 24 ... Gas sensitive layer

Claims (1)

(57) [Claims] 1. A pair of electrodes consisting of an anode and a cathode, and a porous electrode covering the electrodes and containing a gas-sensitive transition metal oxide, the electric resistance of which changes depending on the gas component and / or the gas concentration. comprising a gas sensing layer, and the gas-sensitive layer near the anode, a gas detector, characterized in that the addition of an electron donor additive containing a valence of greater element from the transition metal.