JPS61155745A - Exhaust gas sensor - Google Patents

Abstract

PURPOSE:To reduce detection errors due to coexistence of a combustible gas yet to react and oxygen with a high sensitivity to oxygen, by combining ASnO3-delta of N-type metal oxide semiconductor and BTiO3-delta of P-type metal oxide semiconductor. CONSTITUTION:An N-type gas detection piece 8 and a P-type gas detection piece 10 are provided in a recess between a substrate 2 and a ceramics tube 4 through a threshold section 12. Here, the N-type gas detection piece 8 is composed of an N-type perovskite compound such as BaSnO3, RaSnO3 and Ba0.7Ra0.3SnO3 to which a small amount of precious metal medium and a gel such as SiO2 are added. The precious metal catalyst is provided to suppress the sensitivity to a combustible gas to achieve a balance with the oxygen sensitivity and employs precious metal such as Pt, Ir, R, Os and Rh and a mixture thereof. The P-type gas detection piece 10 composed of a P-type perovskite compound such as SrTiO2, CaTiO3, Sr0.7Ca0.3TiO3 connected to a pair of precious metal electrodes. BTiO3 is used as porous sintered body to which 10-600mug/g of precious metal catalyst is added.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] This invention relates to the improvement of an exhaust gas sensor that combines an n-type metal oxide semiconductor and a p-type metal oxide semiconductor, and is used for controlling the air-fuel ratio of engines and boilers, and for use in stoves. It is suitable for preventing incomplete combustion of.

[Prior art]

Japanese Patent Publication No. 57-37824 discloses an exhaust gas sensor that combines an n-type metal oxide semiconductor and a p-type metal oxide semiconductor. The feature of this technology is that the combination of two semiconductors compensates for the temperature dependence of the sensor and doubles the detection signal for the air-fuel ratio (λ).

These requirements for exhaust gas sensors are: (1) High sensitivity to oxygen; (2) Suppression of sensitivity to combustible gas and balance with oxygen sensitivity; thereby reducing unreacted flammable gas. There are two points: detection errors due to residuals are suppressed.

[Problem of invention]

An object of the present invention is to provide an exhaust gas sensor that is highly sensitive to oxygen and has a small detection error due to the coexistence of unreacted combustible gas and oxygen.

[Structure of the invention]

The exhaust gas sensor of the present invention includes: (1) an n-type metal oxide semiconductor ASnO3-δ, where A represents at least an element of the group consisting of Ba and Ra, and δ represents a non-stoichiometric parameter; (2) P-type metal oxide semiconductor BTiOB-δ, where K
B is at least a member of the group consisting of Sr and Ca, and δ represents a non-stoichiometric parameter.

[Notation]

In the following, compounds are presented without the non-stoichiometric parameter δ. In addition, as a concept indicating oxygen sensitivity, oxygen gradient (n
) is introduced and defined as R8=KlIP02°, (R8: resistance value of semiconductor).

〔Example〕

(N) Structure of an exhaust gas sensor The structure of an exhaust gas sensor will be explained with reference to Figures 1 and 2. In the figure, (2) is a 6-hole tube base made of alumina, and at its tip is a ceramic tube (4) with a built-in heater. ) is the installation. This ceramic tube (4) is equipped with a membrane heater (6) made of tungsten, platinum, etc. inside, and the N-type gas detection piece (8) and P-type gas detection piece QO are kept at a constant temperature. The heater is for heating the membrane to a temperature of 10.degree.

An n-type gas detection piece 8 and a p-type gas detection piece aO are provided in the recessed part between the base body (2) and the ceramic tube (4) with the threshold part (2) interposed therebetween.

Here, the n-type gas detection piece (8) is Ba8n03, Ra
5n03s An n-type perovskite compound such as BaO, 7RaO, 3Sn03, etc., a small amount of noble metal catalyst, and 8
102 and other gels. Precious metal catalysts are used to suppress sensitivity to combustible gases and achieve a balance with oxygen sensitivity, and include Pt, Ir, Ruq Os,
A noble metal such as Rh or a mixture thereof is used. The amount added is 10 μ9 to 51 per gram of ASnCa in terms of metal.
1g is appropriate. Gels such as 5i02 are used as oxygen sensitizers, and gels such as 8i02, GeO2, ZrO2
, an amorphous, non-vitreous gel of HfO2 is used.

Note that the term "amorphous" as used herein means that the half width determined by X-ray diffraction is 60λ or less.

The amount of addition of 5i02 etc. is 1 to 1 mole of Mu8n03.
30 mol% is preferred. It is not necessary to add a noble metal catalyst, 8i02, etc.

Another problem with the n-type gas detection piece (8) is that the compound ASnCa
reacts with basic (2) alumina etc. to form AAe20
The purpose is to prevent decomposition into 4 and S n02 .

Therefore, the area around the n-type gas detection piece (8) was covered with compound ASnO3.
Cover with a substance that does not react with The coating material can be 4lite, spinel, cordierite, or the above-mentioned 5i02.
A gel such as GeO2 or the like is used.

To explain the structure of the n-type gas detection piece (8) in more detail with reference to FIG. 3, α→ is a porous sintered body of A3n03;
aQ, (to) is a noble metal electrode, and (1) is a protective film of berite with a thickness of about 100 μm.

The p-type gas detection piece aG is 8rTi03, CaTiO3,
A pair of noble metal electrodes (not shown) are connected to a p-type perovskite structure such as S ro, r Ca□, 3 T iOB . BTiOB is used as a porous sintered body, and a noble metal catalyst of 10 to 600 μ9/9 is added. The significance and composition standards of the catalyst are the same as in the case of n-type, and the amount added is 1O~6 in terms of metal per 1g of BTiCa.
00 μm is preferable.

Of course, it is not necessary to add a catalyst.

As is well known, a perovskite compound is a substance that is insensitive to substitution, and for example, about 10 mol % of the A element, 8n element, B element, and Ti element may be substituted with other elements. Also AS
nOg and BTiOB may be used in combination with other compounds as long as their resistance values are dominant.

Returning to Figures 1 and 2, υ is a metal fitting for attaching the sensor to the exhaust pipe of an automobile engine or the combustion chamber of a stove, boiler, or the like. Also, (to) and (e) are lead pins connected to the membrane heater (6), (c) and ■ are lead pins connected to the n-type gas detection piece (8), (to) and (b) are p
It can be connected to a type gas detection piece QO or connected to a - doped pin.

ω) Auxiliary circuit An example of ancillary circuit is shown in FIG. 4: a gas detection piece (8).

Connect load resistors (R1) and (R2) to 0Q to form a bridge circuit, and connect the power supply (EB). In addition, the voltage applied to each load resistor (Ri), (R2) is controlled by an amplifier (A1),
Take it out via (A2).

Comparing ASnCa and BTiOB here,
The resistance value at 700°C in the lean region is As noa
For 10, it is a little over Ω, and for BTiOB, it is a little less than 0 for 100. In addition, the ratio of resistance values at 500°C and 900°C in the lean region is more than 100 times for ASnO3 and 10 times for BTiOB.
It is slightly less than 00 times. Therefore, the values of the resistors (R1) and (R2) and the amplifier (AI).

Adjust the gain of (A2) and match the resistance values. In addition, the output from the n-type gas detection piece (8) is connected to the east circuit (Ml) to a power of about 1.5, and the temperature coefficient is also matched. There is no need for the east circuit (Ml).

The outputs of the power east circuit (Ml) and the amplifier (A2) are input to the divider circuit (Dl), and the temperature compensated output is sent to the control circuit (
In addition to (g), it also controls the air-fuel ratio.

On the other hand, in order to keep the temperature of the exhaust gas sensor constant, the duty ratio of the voltage applied to the membrane heater (6) is controlled. The outputs of the power east circuit (Ml) and the amplifier (A2) are transferred to the multiplier circuit (M
In addition to 2), we obtain a signal that depends only on temperature. The width of the output pulse of the oscillation circuit (6) is changed by the output of the multiplier circuit (M2) using a voltage-pulse width modulation circuit (incorporated), and the on-time of the switching transistor (2) is changed. In this way, the power applied from the power supply (EB') to the membrane heater (6) is varied depending on the temperature of the exhaust gas sensor, thereby keeping the heating temperature constant.

Ω Gas detection piece (8), production of α1 Mix alkaline earth carbonate with 5n02 or TiO2,
It is calcined at 1200°C to form ASnO3 and BTiCa. After pulverizing these materials, they are fired at 1300″C to form gas detection pieces (8), Q□.The preferred deformation range is A3.
The calcination conditions are not important as the calcination conditions for n0a are 1200-1500°C. For BTiOB, the calcination temperature is set to 1200 to 1300'C, so that the calcination temperature is the same as or 100'C lower than the calcination temperature.

The oxygen sensitizer is preferably added in the form of a sol or gel after calcination, and the noble metal catalyst is impregnated after calcination to give a
It is preferable to carry out thermal decomposition at about 00°C. Further, the protective film (1) is preferably provided after firing by thermal spraying, sintering after coating, or the like.

As a comparative example, La2(CCa)a and OoO are
LaCo0 calcined at 200°C and fired at 1300°C
3 (p type) was used. Also, calcined at 1200°C for 1300
Ti02 (n-type) fired at °C was used as another comparative example.

In each of these comparative samples, 100μ/g was added.
Added a small amount of Pt.

(6) The combination of characteristics Asn03 near the equivalence point (χ = 1) and BTiOB also
The combination of O2 and L a Co O3 also has approximately the same characteristics near the equivalence point.

(6) Oxygen sensitivity Table 1 shows the oxygen gradient of αQ for each gas detection piece (8).

The oxygen gradient of ASnO3 is about 50% larger than that of TiO2, and the oxygen gradient of BTiOB is about twice that of LaCoO3. Next, BaSnO3, Ra 8 n 03, Ca
TiO3 and 8 rT A03 are mutually equivalent. The characteristics of Naori ao, 7 RaO, 38n03 are Ba
5nCa, 8 ro, 7 Cao, a Tioa
The properties of 8rTiOI are very similar to those of 8rTiOI. Also, the addition of Pt did not affect the oxygen gradient.

Table 2 shows the effect of adding S A02 and the like to A3nO3. The oxygen gradient is 0.03 by adding 8i02 etc.
~0.04 improvement.

Table 1 Oxygen gradient I Ba8nOs 8i0x 5 mol%
0.22 (n)2 Ra8nOs /
/ 0.22(n)3 8rTiO1”=・
-0,21(1))4 0aTiOs...
−・・−0,21φ)5* Tio meaning
++m 0.15 (n)6
*LaCoOs 0.11 (
p)7*8rFeO@-0,1
1(p)8 Ba5nOs+8rTiOs
O,439*Ti01+LacoOB
0.26*1 *marked is a comparative example, *2 In both cases, 100μ Rio Pt was added per ILi, *
Measured at 3700°C while varying the 02 concentration from 1 to 1096 (N! balance).For samples 8 and 9, the ratio of the resistance values of the n-type gas detection piece (8) and the p-type gas detection piece αQ Showing the oxygen gradient.

Table 28i02 etc. addition effect T I Ba5nO100,18 2//8i01 3 0.223
tt tt 5 //4
// // 10 s
5 s z 20 ti
6 // GeO250,2157ti
ZrO25ti 8 tt HfO25//9 Ra8
n01 0゛0.1910s
8i0x 5 0,22km1 Both are 12
Addition of 100μ of Pt per layer, *Measured at 2700°C, details of the measurement method are the same as in Table 1. Figure 5 shows a comparison between an example of combining BaSnO3 and 8rTi03 and a combination of TiO2 and LaCoQ3. Example oxygen sensitivity is shown. The measurement was carried out under N2 balance at 700°C, and the ratio of the resistance values of the n-type gas detection piece (8) and the p-type gas detection piece a0 (normalized to 1 with an oxygen partial pressure of 1%),
is shown as output.

(F) Combustible Gas Sensitivity Detection errors in the lean burn region arise from an imbalance between sensitivity to unreacted combustible gas and oxygen sensitivity.

Table 3 shows the combustible gas sensitivity using CO as a representative of CO and N2 and propylene as a representative of hydrocarbons. Regarding sample 9.10, the change in the ratio of the resistance values of the n-type gas detection piece (8) and the p-type gas detection piece α0 is shown.

1 Ba8nOI Pt 100 0.
98 0B52 Ba8n01
-”・0.5 0.43
Ra8n01 Pt 100 038
0.854 Ra8nOB
O,50,458rTiOB Pt 100
1J)2 1176 8rTi01
1f12 137 CaTi0@
Pt 100 102 128 CaTi
Os 11)2 159
Ba8nO1+ Pt 10G+ 0.
96 0.738rTi01 Pt 10
0 10*TiO2+ *Measured with N2 balance yarn containing 0fi4.6% at 1700℃, *2 Comparative example is marked with *, *3 5 mol% 8i01 is added for Mu8n03, *4100 ppm in CO , the ratio of the resistance value in 14 ppm CO, *5 The ratio of the resistance value in 54 ppm propylene and in 500 ppm propylene.

The results for samples 9 and 10 in Table 3 are shown in FIG.

The vertical axis of the figure shows the output of the sensor between the n-type gas detection piece (8) and the p-type gas detection piece (8).
It is expressed as a ratio of the resistance values of the shaped gas detection piece αO, and the value is based on the value at a combustible gas concentration of 0.1%.

A combination of TiO2 and LaCoO3 has high sensitivity to combustible gases. On the other hand, Ba8nCa and 5r
When combined with TiOs, the sensitivity is low and balanced with the oxygen sensitivity. To illustrate this using propylene as an example, the propylene concentration is 5% in 4.6% 02.
By increasing from 00 to soooppm, the output decreases by 27%. Assuming equilibrium with propylene 02, the 02 concentration changes from 4.4% to 2.3%, and the theoretical value of the output change calculated from the oxygen gradient due to this change is also about 27%.

〔Effect of the invention〕

The present invention provides an exhaust gas sensor that is highly sensitive to oxygen and has a small detection error due to residual unreacted combustible gas.

[Brief explanation of drawings]

FIG. 1 is a perspective view with a partial cutout of an exhaust gas sensor according to an embodiment, FIG. 2 is a longitudinal sectional view thereof, and FIG. 3 is a sectional view of an n-type gas detection piece used in the embodiment. FIG. 4 is a block diagram of the auxiliary circuit, and FIGS. 5 and 6 are characteristic diagrams of the exhaust gas sensor of the embodiment. (2)...Substrate, (4)...Ceramics, (6)
...Membrane heater, (8)...N-type gas detection piece,
QO...p-type gas detection piece. Figure 5 PO2 (/-) Figure 6 0.050.1 0.5 1.0 gas conc (@mu)

Claims (1)

[Claims] (1) In an exhaust gas sensor that combines an n-type gas detection piece using an n-type metal oxide semiconductor and a p-type gas detection piece using a p-type metal oxide semiconductor, the n-type metal oxide semiconductor is ASnO_3_ −_δ, (where A is at least a member of the group consisting of Ba and Ra, and δ represents a non-stoichiometric parameter), and the p-type metal oxide semiconductor is BTiO_3_−_δ, (
An exhaust gas sensor characterized in that B is at least a member of the group consisting of Sn and Ca, and δ represents a non-stoichiometric parameter.