JPH0417376B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an exhaust gas sensor that uses the fact that the resistance value of a metal oxide semiconductor changes depending on the composition of exhaust gas, and a method for manufacturing the same.

Exhaust gas sensors are used, for example, to control the air-fuel ratio of automobile engines and boilers, or to prevent incomplete combustion in heating appliances such as stoves. For example, in the case of a stove, incomplete combustion is prevented by constantly maintaining the air-fuel ratio (λ) at 1 or more. In the case of automobile engines, λ is controlled to about 1.0±0.01,
Suppresses the generation of CO, hydrocarbons (HC), or NOx. For other car engines and boilers,
Setting λ to around 1.1 not only prevents the generation of toxic substances such as CO, HC, or NOx, but also saves fuel.

[Prior art]

SnO ₂ has favorable properties as a material for exhaust gas sensors. _TiO2 , not _SnO2 , is attracting attention as a sensor material. However, _SnO2 is
It has lower resistance than TiO ₂ and can simplify ancillary circuits.
The resistance of TiO ₂ often reaches over 100 MΩ, but it is difficult to accurately measure such high resistance values. Second, the temperature coefficient of resistance of _SnO2 is smaller than that of _TiO2 . Therefore, errors caused by fluctuations in the heating temperature of the sensor are small. Furthermore, exhaust gas sensors using SnO ₂ have a lower operating temperature than those using TiO ₂ . Therefore, the power consumption of the sensor heater can be reduced.

The problems with SnO ₂ as an exhaust gas sensor material are:
It has low durability against high-temperature reducing atmospheres.
By controlling the degree of SnO ₂ crystal growth, durability against high-temperature reducing atmospheres can be improved.
No. 180936). For example, the sensor shown in Japanese Patent Application Laid-Open No. 58-180936 can withstand an atmosphere of λ=0.9 at 900°C. But for more stringent conditions, e.g. 900
Under conditions where λ is approximately 0.83 at °C, SnO ₂ is reduced,
The resistance value decreases irreversibly. Therefore, controlling crystal growth alone is insufficient.

Another problem with SnO ₂ is that it is highly sensitive to flammable gases such as CO and HC. Even when λ is larger than 1, the exhaust gas contains about 1000 ppm of flammable gas. The concentration of combustible gas varies depending on the condition of each engine and is not constant. Here, there is no problem as long as the metal oxide semiconductor's sensitivity to oxygen and sensitivity to combustible gas are balanced. However, in the case of SnO ₂ , the sensitivity to combustible gas is much greater than the sensitivity to oxygen, so its resistance value is dominated by the concentration of unburned combustible gas in the exhaust gas and does not reflect λ correctly.

SnO ₂ also has the disadvantage of slow response to changes in λ.

Regarding the SnO _2- based exhaust gas sensor,
No. 53-55099 discloses that adding about 0.2 mol% MgO to SnO ₂ can improve the sensitivity to oxygen.

It is known to use perovskite compounds as exhaust gas sensor materials. For example, special public service in 1977-
No. 19837 discloses using SrFeO ₃ -δ as an oxygen sensor. Ninomiya et al. also investigated the sensitivity of various perovskite compounds to flammable gases.
It describes the use of this characteristic in exhaust gas sensors (National Technical Report Vol. 29, No. 3, P475-483, 1983).

[Problem of invention]

The object of the present invention is to provide a new exhaust gas sensor material that improves its durability against high-temperature reducing atmospheres, suppresses sensitivity to unburned combustible gas, and prevents deterioration in detection accuracy due to unburned gas. The objective is to further improve the response to changes in the atmosphere.

Next, an object of the manufacturing method of the present invention is to provide a method of manufacturing an exhaust gas sensor material that satisfies the above-mentioned problems.

By the way, in order to reduce detection errors due to temperature fluctuations of the exhaust gas sensor, it is necessary to provide a heater to keep the temperature of the exhaust gas sensor constant. Furthermore, it is difficult to completely eliminate the sensitivity of metal oxide semiconductors to flammable gases. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an exhaust gas sensor that is equipped with a heater that is less likely to be corroded by high-temperature exhaust gas, and further removes unreacted combustible gas from the exhaust gas in advance.

[Structure of the invention]

The exhaust gas sensor of the present invention comprises a compound ASnO ₃ −δ
(Here, A represents at least a member of the group consisting of Ba, Sr, and Ca, and δ represents a non-stoichiometric parameter.)
It is characterized in that at least one pair of electrodes is connected to a gas sensitive body containing. Note that the compound ASnO ₃
Regarding -δ, the A component and the Sn component may each be replaced by other metal elements at a rate of 20 mol% or less. The most preferred here is the compound
This is a case where BaSnO ₃ -δ is used as ASnO ₃ -δ. BaSnO ₃ -δ has high sensitivity to changes in λ, especially to changes in oxygen partial pressure, low resistance, and quick response to changes in atmosphere. Another feature of BaSnO ₃ -δ is that its sensitivity to oxygen increases as the crystal grows.

Next, such a gas sensitizer material is a compound
ASnO ₃ −δ, or by heating the compound ASnO ₃
It can be obtained by calcining a substance that is converted to -δ in a non-reducing atmosphere. The preferred firing temperature range is
The temperature is 1000-2000℃ and the firing time is 30 minutes to 20 hours.

In order to use the above gas sensitive body as an exhaust gas sensor, for example, a pair of electrodes is connected to the gas sensitive body,
The electrode is housed in the hole using a heat-resistant insulating two-hole tube base, and the gas sensing element is surrounded by a heater with a conductive pattern embedded inside the heat-resistant insulating material, leaving an exhaust gas inlet to introduce the exhaust gas. An oxidation catalyst layer may be provided in the mouth. Here, by using a conductive pattern as the heat generating part of the heater, the heat generating part can be manufactured by printing, vapor deposition, or the like. By embedding the heat generating part inside the heat-resistant insulating material, the heat generating part can be protected from corrosion by exhaust gas. Furthermore, by providing an oxidation catalyst layer at the exhaust gas inlet, unreacted combustible gas in the exhaust gas can be removed. A preferable oxidation catalyst layer is a honeycomb supporting an oxidation catalyst such as Pt, Pd, or Rh. Furthermore, the compound
ASnO ₃ -δ has the property of reacting with alumina, which is often used as a heat-resistant insulating material for substrates, heaters, etc., and decomposing it. In order to prevent the solid phase reaction, an intermediate layer may be provided between the gas sensitive body and the heater, substrate, etc. to reduce the chemical potential of the aluminum component. Preferred materials for the intermediate layer are spinel (MgAl ₂ O ₄ ) and mullite (Al ₆ Si ₂
O ₁₃ ) and cordierite (Mg ₂ Al ₄ Si ₅ O ₁₈ ). Below, preferred embodiments of the invention are described.

[Adjustment of gas sensitive body]

(A) Preparation of BaSnO ₃ BaCO 3 with purity of 99.99% or more and BaSnO ₃ with purity of 99.99% or more
SnO ₂ (surface area 50 m ² /g) is kneaded using an equimolar amount of water. The reason why BaCO ₃ is used as a starting material here is that it contains little crystal water and adsorbed water, and the amount added can be easily controlled. Other than the problem that it becomes difficult to control the amount added, other substances such as BaO, Ba
(OH) ₂ , BaCl ₂ , BaSO ₄ , or metallic barium may be used as the barium raw material. The reason why SnO ₂ is used is to similarly facilitate the control of the amount added, and instead of SnO ₂ , SnCl ₄ , SnSO ₄ , Sn(OH) ₄ , etc. may be used.

The mixture is then calcined in air at 500-1200°C for 1 hour. Calcination temperature: 500℃, 800℃, 1000℃
Four types of temperature were used: ℃ and 1200℃. When calcined at 500℃, the product is a mixture of SnO ₂ and BaCO ₃ ;
BaSnO ₃ −δ is not generated. At 800°C, almost all of it is converted to BaSnO ₃ -δ, and at 1000°C or higher, it is completely converted to BaBnO ₃ -δ. The calcination atmosphere only needs to be non-reducing, such as an inert atmosphere such as 100% N ₂ or O ₂
A strong oxidizing atmosphere such as 100% may be used.

The calcined product is wet-milled (using water as a dispersion medium) in a ball mill for 30 minutes.

The pulverized material was press-molded into a 3 mm diameter molded material with a pair of 100 μ diameter Pt-Rh alloy electrodes embedded in it.
It is molded into a cylindrical pellet with a height of 3 mm and fired in air to obtain a gas sensitive material. Firing temperature is 1000~2000
℃, and the heating time is 4 hours. Here, the firing temperatures are 1100℃, 1200℃, 1270℃, 1300℃, 1400℃.
We specifically investigated six types: ℃ and 1500℃.
For samples calcined at 1200°C, the firing temperature range was set to 1300°C or higher. During the firing process, BaSnO ₃
-δ crystals grow, providing durability against high-temperature atmospheres during use (usually below 950°C). Also, during this process, the BaSnO ₃ -δ pellets are sintered, increasing their mechanical strength. The firing atmosphere only needs to be non-reducing; for example, an inert atmosphere such as 100% N ₂ or 100% CO ₂ or a strong oxidizing atmosphere such as 100% O ₂ may be used. In the sample where the calcination temperature was 500℃, BaCO ₃ and SnO ₂ reacted during the calcination process.
BaSnO ₃ −δ is generated.

Here, the significance of each step will be explained. Firing is
Promote crystal growth of BaSnO ₃ −δ, heat during use,
In particular, it provides durability against high-temperature reducing atmospheres.
The formation temperature of BaSnO ₃ -δ is about 800°C, and the firing temperature is much higher than the formation temperature of BaSnO ₃ -δ.
Therefore, the firing effect is not affected by the starting material;
It is determined by the firing temperature and time. The firing temperature needs to be 1000°C or higher and 2000°C or lower, preferably 1200°C to 1800°C, more preferably 1260°C to 1800°C. The maximum operating temperature of an exhaust gas sensor is thought to be around 950℃, so
It is necessary to fire at 1000°C or higher, more preferably at 1200°C or higher. Note: 1000
The specific surface area of the product fired at 1200°C is approximately 20 m ² /g and the average crystallite diameter is approximately 0.04 μ; the specific surface area of the product fired at 1200 °C is approximately 2.5 m ² /g and the average crystallite diameter is 0.3 μ. It is. By the way, the sensitivity of BaSnO ₃ -δ to oxygen increases dramatically by setting the firing temperature to 1260°C or higher. From this, the specific surface area of BaSnO ₃ -δ should be 1.5 m ² /g or less, and the average crystallite diameter should be
It is preferable to set it to 0.5μ or more. The upper limit of firing temperature is that it is difficult to obtain a high firing temperature;
This is determined by the fact that the gas sensitive material becomes denser and its response speed to changes in the atmosphere decreases. From these points, the firing temperature is 2000°C (50μ as the average crystallite diameter of BaSnO ₃ -δ), preferably 1800°C,
(20 μ as the average crystallite diameter of BaSnO ₃ −δ), and the following. From the viewpoint of ease of process control, the firing time is preferably 30 minutes to 20 hours, more preferably 1 to 10 hours.

The purpose of calcination is to generate BaSnO ₃ -δ in advance, to reduce shrinkage during firing, and to increase the strength of the gas sensitive body after firing, but it has no essential meaning.

Note that the porosity of the gas sensitive material of the example was 15 to 40%. Further, regarding the connection of the electrodes, the electrodes may be connected to the gas sensitive body after firing.

Figure 1 shows the X-ray diffraction diagram of a sample whose calcination temperature was 800°C and firing temperature was 1400°C. The peak in the diffraction diagram corresponds to BaSnO ₃ −δ, and unreacted BaO and
There are no peaks corresponding to BaCO ₃ or SnO ₂ .
This compound, BaSnO ₃ −δ, is estimated to be a perovskite compound (National
Below of Standard Darts Monograph 25
Volume Section 3, 11P 1964). Next, an electron micrograph (10,000x) of the same sample is shown in Figure 2.
Furthermore, although the calcination temperature was set at 800°C, the relationship between the calcination temperature, specific surface area, and average crystallite diameter is shown in FIG.

(B) Adjustment of SrSnO ₃ −δ SrCO ₃ is mixed with an equimolar amount of SnO ₂ and BaSnO ₃ −
A gas sensitive material is obtained by the same process as for adjusting δ.
The product is a perovskite compound SrSnO ₃ −
δ, and the manufacturing conditions and manufacturing precautions are the same as BaSnO ₃ −δ. The degree of crystal growth of the resulting SrSnO ₃ −δ, such as the specific surface area and average crystallite diameter, depends on the BaSnO ₃
It is almost the same as the case of −δ.

(C) Adjustment of CaSnO ₃ −δ Mix equimolar amounts of CaCO ₃ and SnO ₂ and make BaSnO ₃
A gas sensitive material is obtained by the same process as for adjusting -δ. The product is a perovskite type compound
The manufacturing conditions and manufacturing precautions for CaSnO ₃ −δ are as follows:
It is the same as the case of BaSnO ₃ −δ. In addition, the degree of crystal growth is BaSnO ₃ −δ if the firing conditions are the same.
The specific surface area is approximately 1/4, and the average crystallite diameter is approximately 4
It will be doubled.

(D) Adjustment of SnO ₂ Except for using only SnO ₂ without using BaCO ₃
By the same process as the adjustment of BaSnO ₃ −δ,
A gas sensitizer consisting of SnO ₂ is obtained.

(E) Reaction between MgCO ₃ and SnO ₂ Mix equimolar amounts of MgO and SnO ₂ and
Heated to 500-1200°C for an hour. What was obtained is
In the mixture of MgO, SnO ₂ and Mg ₂ SnO ₄ , no MgSnO ₃ was produced at any temperature. The results were similar even when the starting materials were changed to MgCO ₃ and SnO ₂ .

In order to understand the effect of MgO addition, MgO 10 mol% and
A mixture of 90 mol% SnO ₂ was prepared, calcined in air at 800°C for 1 hour, and after pulverized, heated at 1400°C in air.
The mixture was fired at ℃ for 1 hour to obtain a gas sensitive material.

(F) Mixing BaSnO ₃ −δ, SrSnO ₃ −δ, CaSnO ₃ −δ with SnO ₂ ,
Adjust the mixture with _SnO2 . The adjustment is performed, for example, by adding an excessive amount of SnO ₂ in the previous adjustment of BaSnO ₃ -δ. In this case, the mixing effect of SnO ₂ occurs when SnO ₂ is contained at 20 mol% or more, (BaSnO ₃
-δ, etc. content is 80 mol % or less), and below this, properties similar to those of plain BaSnO ₃ -δ, etc. can be obtained. The final state of SnO ₂ is preferably one with a specific surface area of 5 to 0.1 m ² /g,
The average crystallite diameter is in the range of 0.2 to 10μ.

(G) Supplementary non-stoichiometric parameter δ in BaSnO ₃ −δ etc.
varies depending on the surrounding atmosphere and temperature, but is usually between -0.1 and 0.5.

Next, the specific surface area referred to in this specification refers to a value measured by the BET method using N ₂ as an adsorbent. The average crystallite diameter means the average value of crystallite diameters.The crystallite diameter of each crystal is determined by determining the length of the long axis and short axis of each crystal from an electron micrograph, and then calculating the arithmetic average of the lengths of the long axis and short axis of each crystal. shall be determined by.

BaSnO ₃ -δ, a mixture of BaSnO ₃ -δ and SnO ₂ , etc. can be used in combination with other substances. A typical example is _TiO2 . TiO ₂ has a high specific resistance, and the characteristics of the gas sensitive material are determined by BaSnO ₃ -δ etc. and do not depend on TiO ₂ . Also, TiO ₂
It does not react with BaSnO ₃ -δ, SrSnO ₃ -δ, CaSnO ₃ -δ, or SnO ₂ and has no adverse effect on properties.

As is well known, the properties of composite oxides are insensitive to substitution of component elements (for example, Japanese Patent Publication No. 1983-1983). The A and Sn elements of the compound ASnO ₃ -δ (where A is at least a member of Ba, Sr, and Ca) are each 20
Other metal elements may be substituted as long as the amount is mol % or less, preferably 10 mol % or less, more preferably 3 mol % or less. The effect of the substitution is to reduce the sensitivity to λ and change the resistance value. Substituting elements for element A include, for example, Mg, Ra, lanthanide elements with atomic numbers of 57 to 71, and actinide elements with atomic numbers of 89 to 103. For Sn element, for example,
There are transition metal elements such as Ga, In, Tl, Ge, Pb, Sb, or Bi.

An unfavorable characteristic of BaSnO ₃ -δ, SrSnO ₃ -δ, and CaSnO ₃ -δ is that it reacts with alumina and forms BaAl ₂ O ₄ etc. at high temperatures, especially at temperatures above 1200°C.
It has the property of decomposing into SnO ₂ .

(H) Nomenclature The nomenclature shown in the table below will be used for the gas sensitive material.

Table (Nomenclature) Alphabet (Material) A; BaSnO ₃ -δ B; SrSnO ₃ -δ C; CaSnO ₃ -δ D; SnO ₂ E; SnO ₂ (90 mol%) + MgO (10 mol%) F; SrSnO ₃ -δ (50 mol%) + SnO ₂ (50 mol%) G; CaSnO ₃ -δ (50 mol%) + SnO ₂ (50 mol%) Most significant digit (calcination temperature) 2nd digit (calcination temperature) 0; 1100℃ 1; 500℃ 1; 1200℃ 2; 800℃ 2; 1270℃ 3; 1000℃ 3; 1300℃ 4; 1200℃ 4; 1400℃ 5; 1500℃ For example, NoA42 is calcined at 800℃ for 1 hour. death,
BaSnO-δ fired at 1400°C for 4 hours is shown.

[Structure of exhaust gas sensor]

The structure of the exhaust gas sensor 10 will be explained based on FIGS. 4A and 4B. Reference numeral 12 denotes a gas sensitive body made of the aforementioned cylindrical pellets, inside which a pair of Pt-
Rh alloy electrodes 14 and 16 are buried. Electrode 1
4 and 16 are expensive, so Fe-Cr is added to their ends.
- Al alloy base metal lead wires 18 and 20 are connected so that the internal resistance of the gas sensitive body 12 can be taken out to the outside.

Next, reference numeral 22 denotes a heat-resistant insulating base made of alumina, which is shaped like a two-hole tube. The exposed portions of the electrodes 14, 16 and the lead wires 18, 20 are housed in the holes of the base body 22, to mechanically support them and to prevent the electrodes 14, 16, etc. from being corroded by exhaust gas. The base 22 also supports the bottom of the gas sensitive body 12 . The material for the base body 22 may be any heat-resistant and insulating material, but alumina is used here because of its durability against thermal shock.

A heater tube 24 is fixed to the outer peripheral surface of the base 22 with an inorganic adhesive (not shown).
A gas sensitive body 12 is placed in a cylindrical recess between the heater tube 24 and the base body 22 to provide protection from high-flux exhaust gas flow, etc., and to provide appropriate heating. The heater tube 24 has a conductive pattern 26 made of tungsten, tin oxide, platinum, ruthenium oxide, etc. buried inside a heat-resistant insulating tube made of alumina or the like. The conductive pattern 26 is a heating element of the heater tube 24, and is centrally arranged around the gas sensitive body 12, and is connected to the heater lead wires 28, 30 on the base side of the heater tube 24. The feature of this heater tube 24 is that the conductive pattern 26
is buried in the tube 24. This protects the conductive pattern 26 from corrosion and mechanical damage caused by exhaust gas. The heater tube 24 is
For example, it is manufactured by printing a conductive pattern 26 made of tungsten or the like on the outer peripheral surface of an alumina tube, coating the surface with alumina to a thickness of about 100 μm, and sintering it to form a dense protective coating layer.

As a method of controlling the heater tube 24, for example, the method disclosed in Japanese Patent Publication No. 56-6504 may be used, and the gas sensitive body 12 is maintained at a temperature of, for example, about 700°C.

A honeycomb 32 supporting an oxidation catalyst is provided at the exhaust gas inlet provided in the heater tube 24 as an example of an oxidation catalyst layer. This honeycomb 32 is used to mechanically protect the gas sensitive body 12 and to remove combustible components from exhaust gas. Here, a honeycomb 32 with a thickness of 1.2 mm and a large number of small holes 34 with a diameter of 150μ is used, and the small holes 34 are made of Pt or
It was used by supporting an oxidation catalyst such as Pd or Rh. Note that as the material of the honeycomb 32, MgAl ₂ O ₄ , which is a spinel compound, was used. Exhaust gas is honeycomb 3
While passing through the small holes 34 in the exhaust gas 2, the unreacted combustible components and oxygen react, and the composition of the exhaust gas approaches a chemical equilibrium value. Due to the oxidation activity of honeycomb 32,
The detection error due to the sensitivity of the gas sensitive body 12 to combustible gas is reduced. Instead of the honeycomb 32, another oxidation catalyst layer, such as one in which γ-Al ₂ O ₃ supports Pt, may be used. However, in this case, a separate cover is provided to protect the gas sensitive body 12 from the exhaust gas flow.

By the way, as mentioned above, compounds such as BaSnO ₃ -δ react with alumina and decompose. Therefore, the reaction activity of alumina is reduced to protect the gas sensitive body 12 from reaction with alumina. here,
As shown in FIG. 4B, the gas sensitive body 12 and the base 2
2 and heater tube 24, spinel compound
A 100μ thick intermediate layer 36 of MgAl ₂ O ₄ is provided. That is, the spinel compound MgAl ₂ O ₄ is more stable than alumina, and the aluminum in spinel is BaSnO ₃ −δ
It does not react with etc. Furthermore, as mentioned above, magnesium has low reactivity with tin. Note that the middle layer 36
The material of the base body 22 and heater tube 24 is
Any material may be used as long as it prevents the reaction between the pellets 12 and the pellets 12, and in addition to MgAl ₂ O ₄ , mullite, Al ₆ Si ₂ O ₁₃ , cordierite, and Mg ₂ Al ₄ Si ₅ O ₁₈ are preferable.
Note that in FIG. 4A, illustration of the intermediate layer 36 is omitted.

A metal fitting 38 for attachment to an exhaust pipe or the like is provided, and an inorganic adhesive 42 is filled through a small hole 40 of the metal fitting 38 to fix the heater tube 24 and the metal fitting 38.

[Measurement method]

The characteristics of each sample were determined by the following method.

(A) Durability The issues with the durability of exhaust gas sensors are:
It is durable to high-temperature reducing atmosphere. Atmosphere λ
= 0.8 for 3 seconds and λ = 0.9 for 1 second, and each sample is exposed to this atmosphere for 10 hours. Note that a mixture of CO and air was used as the combustion gas. The reason why CO is used for combustion is to use the substance that is most easily combusted completely and to minimize unreacted components in the exhaust gas. Also, the heating temperature of each sample was set to 900℃.
℃. This cycle assumes exhaust gas from an automobile engine during high acceleration.

(B) Sensitivity and response performance to changes in λ Using a mixture of CO and air, λ = 1.1 and λ = 0.9
Find the resistance value of each sample in the exhaust gas. Here, the steady value of the resistance value (Rs) to each atmosphere and the resistance value when the atmosphere was switched at 1 second intervals were measured. The steady value indicates sensitivity, and the difference between the steady value and the transient value indicates response performance.

(C) Sensitivity to combustible gases A mixture of CH ₄ and O ₂ is introduced into the exhaust gas with λ=1.1, and the effect of unburned hydrocarbons is investigated. The amount introduced is 1 mol% CH ₄ and O ₂ per 100 mol% exhaust gas.
It is 2.2 mol%. The introduction position of CH ₄ etc. was immediately upstream of the exhaust gas sensor 10, and the exhaust gas sensor 10 without the honeycomb 32 was used.

(D) Sensitivity when λ≧1 In the exhaust gas of a mixture of CH ₄ and air, λ is 0.8
to 1.3, and find the sensitivity to changes in λ. The heating temperature for each sample is 700°C. CH ₄
is a fuel that is difficult to burn completely, and by using it as fuel, unburned components are artificially introduced into the exhaust gas. Note that this test assumed an automobile engine with poor combustion characteristics. This test also uses the exhaust gas sensor 10 with the honeycomb 32 removed.

(E) Oxygen sensitivity Using a mixture of N ₂ and O ₂ , find the sensitivity to changes in oxygen partial pressure.

[SnO ₂ ] As a representative sample using SnO ₂ ,
Electron microscope images of the NoD42 sample before and after the durability test are shown in Figures 5A and B. Electron microscope images change during durability tests, and SnO ₂ is fatally affected by exposure to high-temperature reducing atmospheres. It can be assumed that the change in the electron microscope image is due to crystal growth via reduction of SnO ₂ .

Next, regarding NoD42 used, change in resistance value due to durability test (curves 63 and 64), and change in resistance value in exhaust gas.
The sensitivity to CH ₄ (curve 65) is shown in Figure 6A.

Figure 6B also shows the response characteristics (curve) to changes in λ.
66, 67). The response to the change from the reduction side to the oxygen side is slow, and the difference between the transient value (curve 66) and steady value (curve 61) on the oxidation side is large.

Note that if the durability test is performed with λ fixed at 0.9, the effect on sample NoD42 will be much smaller.
This means that the influence received by the exhaust gas sensor 10 increases significantly as λ decreases.

To investigate the effect of MgO, 90 mol% of _SnO2 and
A sample made from 10 mol% MgO (NoE42) was investigated, and the results were similar to SnO ₂ (NoD42), except for a slightly higher resistance value.

[BaSnO ₃ −δ] λ= of the sample using BaSnO ₃ −δ (NoA42)
Steady-state values of resistance at 1.1 and 0.9 (curves 71 and 72),
The transient values at λ = 1.1 and 0.9 (curves 76 and 77) are
As shown in the figure.

BaSnO ₃ -δ is an N-type semiconductor, that is, the resistance value increases as λ increases, and known perovskite compounds are P-type semiconductors, that is, the resistance value decreases as λ increases. This is in contrast.

Next, BaSnO ₃ −δ has a fast response to changes in the atmosphere. Also, the resistance value and temperature coefficient of resistance are very similar to SnO ₂ .

Note that BaSnO ₃ -δ is hardly affected by the durability test and has extremely low sensitivity to CH ₄ , so its illustration is omitted.

These characteristics are not only obtained with the sample (NoA42) fired at 1400°C, but almost the same results can be obtained within the range of Examples.

[SrSnO ₃ -δ] Figure 8 shows the characteristics of a sample (NoB42) using SrSnO ₃ -δ. SrSnO ₃ -δ is not affected by durability tests and has low sensitivity to CH ₄ . It also responds quickly to changes in λ. It differs from BaSnO ₃ -δ in that it has high resistance and low sensitivity to changes in λ.
Regarding SrSnO ₃ -δ, samples fired at 1100°C or higher gave almost the same results as those fired at 1400°C.

[CaSnO ₃ -δ] Figure 9 shows the characteristics of a sample (NoC42) using CaSnO ₃ -δ. The results are similar to those using SrSnO ₃ −δ, but the sensitivity to changes in λ is even smaller. Note that when fired at 1100°C or higher, almost the same results as those fired at 1400°C were obtained.

[SrSnO ₃ −δ + SnO ₂ ] Resistance value at λ = 1.1 and 0.9 of equimolar mixture of SrSnO ₃ −δ and SnO ₂ (NoF42) (curve 101,
102), influence of durability tests (curves 103, 104), CH ₄
The sensitivity to (curve 105) is shown in Figure 10A.

Reflecting that SrSnO ₃ −δ has a higher resistance than SnO ₂ , its temperature coefficient of resistance and sensitivity to λ are similar to those of SnO ₂ . However, the effects of durability tests and sensitivity to CH ₄ are smaller than those of SnO ₂ alone.

The response performance of the same sample to changes in λ is shown in FIG. 10B. The response speed is also faster than that of SnO ₂ alone.
From these facts, there is a composite effect between SrSnO ₃ -δ and SnO ₂ , and the mixing of SrSnO ₃ -δ improves the durability and response speed to high-temperature reducing atmospheres, and improves the resistance to flammable gases. It can be seen that the sensitivity decreases. The combined effect occurs only in mixtures of SrSnO ₃ -δ 80-40 mol% and SnO ₂ 20-60 mol%,
Other than this, plain SrSnO ₃ −δ or plain SrSnO
It has properties similar to SnO ₂ . That is, SrSnO ₃ −δ20
A mixture of 80 mol% of SnO ₂ and 80 mol% of SrSnO 2 had properties similar to those of SnO ₂ , and a mixture of 90 mol% of SrSnO ₃ -δ and 10 mol% of SnO ₂ had properties similar to SrSnO ₃ -δ. A mixture of 70 mol % SrSnO ₃ -δ and 30 mol % SnO gave similar properties to an equimolar mixture of SrSnO ₃ -δ and SnO ₂ .

[ _CaSnO3 -δ+ _SnO2 ] Equimolar mixture of _CaSnO3 -δ and _SnO2 ,
The characteristics of (NoG42) are shown in Figures 11A and B. The results are comparable to those for the mixture of SrSnO ₃ −δ and SnO ₂ .

[Sensitivity when λ≧1] FIG. 12 shows the relationship between the change in λ and the electrical conductivity for the samples using BaSnO ₃ -δ (NoA42) and SnO ₂ (NoD42).

In the case using BaSnO ₃ −δ (broken line 121),
The electrical conductivity changes significantly after λ=1, and when λ>1
The electrical conductivity gradually decreases with λ.
In the case of using SnO ₂ (broken line 122), the electrical conductivity gradually increases when λ≧1. This increase corresponds to an increase in the hydrocarbon concentration (HC) in the exhaust gas. That is, in the case of using SnO ₂ , the measured value when λ>1 lacks reliability due to sensitivity to unburned combustible gas. If the honeycomb 32 is used here, measurement errors due to combustible gas can be reduced, but the measurement accuracy is insufficient.

Next, a mixture of SrSnO ₃ −δ and SnO ₂ (NoF42)
or a mixture of CaSnO ₃ −δ and SnO ₂ (NoG42)
However, although similar characteristics to BaSnO ₃ -δ (NoA42) can be obtained, the measurement error due to combustible gas is larger than that of BaSnO ₃ -δ.

SrSnO ₃ -δ and CaSnO ₃ -δ exhibit both the characteristics of an N-type semiconductor and the characteristics of a P-type semiconductor with respect to changes in λ. As described above, when changing from λ<1 to λ>1, the resistance value increases and exhibits characteristics as an N-type semiconductor. However, when λ>1, the resistance value decreases little by little as λ increases, exhibiting characteristics as a P-type semiconductor. SrSnO ₃ -δ and CaSnO ₃ -δ are N-type and P-type.
It is an intermediate semiconductor between molds.

The decrease in resistance value when λ>1 is not due to the coexistence of flammable gas. SrSnO ₃ −δ and CaSnO ₃ −δ
Even if the O ₂ partial pressure is increased in a mixture of N ₂ and O ₂ , the resistance value still decreases. However, this decrease is so small that it is difficult to use it as a lean burn sensor.

The most preferable lean burn sensor, i.e., a sensor whose purpose is to control λ at a point of λ>1, is one made of BaSnO ₃ −δ, and the second most preferable ones are BaSnO ₃ −δ, SrSnO ₃ − δ, or a mixture of CaSnO ₃ -δ and SnO ₂ .

[Oxygen sensitivity]

FIG. 13 shows the relationship between the resistance value and oxygen partial pressure of a sample (NoA42) using BaSnO ₃ -δ. It can be seen that the resistance value is determined by the power of the oxygen concentration.

Here, Rs = K・P ^-m _O2 , (Rs is the resistance value of the exhaust gas sensor 10, Po ₂ is the oxygen partial pressure, m is the power, and K is the constant), and the degree of crystal growth of BaSnO ₃ -δ is The relationship with m is shown in FIG. Specific surface area S is used as an indicator of crystal growth, and m is shown at three temperatures: 500°C, 700°C, and 900°C.

By setting the specific surface area to 1.5 m ² /g or less (average crystallite diameter 0.5 μ or more), m increases significantly. As for BaSnO ₃ -δ, sensitivity to oxygen can be increased by advancing crystal growth.

〔Effect of the invention〕

The exhaust gas sensor of the present invention has excellent durability and responsiveness to high-temperature reducing atmospheres, and has small detection errors due to unreacted combustible gas.

Next, with the manufacturing method of the present invention, the above exhaust gas sensor can be easily manufactured.

Further, in the exhaust gas sensor of the present invention, detection errors due to combustible gas can be further reduced by suppressing corrosion of the heater due to exhaust gas and removing unreacted flammable gas in the exhaust gas in advance.

[Brief explanation of drawings]

Figure 1 is an X-ray diffraction diagram of the gas sensitive body of the example, Figure 2 is an electron micrograph showing the particle structure of the gas sensitive body of the example, and Figure 3 is a comparison with the firing temperature of the gas sensitive body of the example. FIG. 2 is a characteristic diagram showing the relationship between surface area and average crystallite diameter. Fig. 4A is a cross-sectional view of the exhaust gas sensor of the example, and Fig. 4B is its A-A, B-B.
FIG. FIG. 5A is an electron micrograph showing the particle structure of the conventional gas sensitive material before the durability test, and FIG. 5B is an electron micrograph showing the particle structure of the conventional gas sensitive material after the durability test. It is. 6A and 6B are characteristic diagrams of a conventional exhaust gas sensor, and FIGS. 7 to 14 are characteristic diagrams of an example exhaust gas sensor. 12... Gas sensitive body, 22... Substrate, 24...
Heater tube, 26... Conductive pattern, 32...
...Honeycomb, 36...middle layer.

Claims (1)

[Claims] 1 Compound ASnO ₃ -δ (where A is Ba, Sr, Ca
δ represents a non-stoichiometric parameter, A element and Sn
Each element may be substituted with another metal element at a rate of 20 mol% or less. ), an exhaust gas sensor comprising at least one pair of electrodes connected to a gas sensitive body containing: 2. In the exhaust gas sensor according to claim 1, the elements substituted for element A are Mg, Ra, lanthanide elements with atomic number 57 to 71, and atomic number 89.
An exhaust gas sensor characterized in that it is at least a member of the group consisting of ~103 actinide elements. 3. The exhaust gas sensor according to claim 2, characterized in that the element substituted for the Sn element is at least a member of the group consisting of a transition metal element and Ga, In, Tl, Ge, Pb, Sb and Bi. Exhaust gas sensor. 4. The exhaust gas sensor according to claim 3, characterized in that the substitution ratios of element A and element Sn are each 10 mol% or less. 5. The exhaust gas sensor according to claim 4, characterized in that the substitution rates of element A and element Sn are each 3 mol % or less. 6. In the exhaust gas sensor according to claim 1, the specific surface area of the compound ASnO ₃ -δ is 20 to 0.04.
An exhaust gas sensor characterized in that the average crystal diameter is 0.04 to 20μ in m ² /g. 7. In the exhaust gas sensor according to claim 6, element A is Ba, and its specific surface area is 1.5.
〜0.04m ² /g and an average crystallite diameter of 0.5 to 20μ. 8. In a method for manufacturing an exhaust gas sensor in which at least one pair of electrodes is connected to a gas sensitive body, the compound ASnO ₃ -δ, (where A is Ba, Sr, Ca
δ represents a non-stoichiometric parameter, A element and Sn
Each element may be substituted with another metal element at a rate of 20 mol% or less. ), and a substance that is at least one member of the group consisting of a substance that is converted into a compound ASnO ₃ −δ by heating, and is fired at 1000 to 2000°C in a non-reducing atmosphere to form the gas sensitizer. How to manufacture the sensor. 9. The method for manufacturing an exhaust gas sensor according to claim 8, wherein the element A is Ba and the firing temperature is 1260 to 1800°C.