JPH052097B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] This invention relates to a perovskite compound ASnO ₃ −δ
The present invention relates to the improvement of exhaust gas sensors using an exhaust gas sensor, and particularly to the suppression of temperature dependence of output in a reducing atmosphere.
The exhaust gas sensor of the present invention is effective in controlling the air-fuel ratio of automobile engines, boilers, etc., and preventing incomplete combustion in stoves, etc. [Prior art] Japanese Patent Application No. 59-63900 by the same inventor discloses perovskite compounds BaSnO ₃ -δ, SrSnO ₃ -δ,
An exhaust gas sensor using CaSnO ₃ -δ etc. is disclosed. Here, alkaline earth elements such as Ba and Sr can be replaced with lanthanide elements and the like. The basic advantages of this exhaust gas sensor are (1) resistance to high temperature reducing atmospheres, (2) high sensitivity to oxygen, and (3) relatively good balance between flammable gas sensitivity and oxygen sensitivity. . Also, the peculiarity of this exhaust gas sensor is that (1) perovskite compounds such as LaCoO ₃ and SrFeO ₃ are P-type metal oxide semiconductors, whereas (2) they are closer to n-type metal oxide semiconductors than P-type. The point that shows the behavior is located at . In other words, BaSnO ₃ -δ and RaSnO ₃ -δ are completely n-type, while CaSnO ₃ -δ and SrSnO ₃ -δ are strong n-type near the equivalence point (air-fuel ratio λ is 1), and are in the lean burn region (λ>1). ) has a weak P type. For details on these points, we quote Japanese Patent Application No. 1983-63900. Controlling the temperature dependence of output is a fundamental issue for exhaust gas sensors (for example, Japanese Patent Publication No. 1745-46 of 1982).
Here, if the temperature dependence of either the reducing agent (λ<1) or the oxidizing agent (λ>1) can be made sufficiently small, the equivalence point can be accurately detected. Further, even in the lean burn region, effects such as being able to easily detect the temperature of the sensor or easily compensating the sensor output can be obtained. [Problem to be solved by the invention] The problem to be solved by this invention is to improve the reduction side (λ
<1) The purpose is to suppress the temperature dependence of the output. [Structure of the Invention] The exhaust gas sensor of the present invention is characterized in that a perovskite compound A ₁ -xLaxSnO ₃ -δ is used as a gas-sensitive material. Here, A represents at least an element of the group consisting of Ca, Sr, Ba, and Ra, and δ represents a non-stoichiometric parameter. x is greater than 0 and 2 mol% or less, for example 30 mol ppm to 2 mol%, preferably 100
~5000 mol ppm, more preferably 100-2000 mol ppm. Substitution with a small amount of La significantly reduces the temperature coefficient of resistance in a reducing atmosphere (λ<1) without impairing oxygen sensitivity or sensitivity near the equivalence point.
Replacement with excessive amounts of La impairs sensitivity to atmospheric changes. Instead of La, other lanthanide elements,
For example, if Ce is used, the temperature coefficient of resistance in a reducing atmosphere cannot be sufficiently suppressed unless it is replaced by about 10 mol%. [Example] An example showing the best mode will be described below, but the present invention is not limited thereto. In particular, in the following, various external additional elements are added to indicate the plague mode, but these are not intended to limit the invention itself. Note that the following notation will be used: (1) The non-stoichiometric parameter δ will be omitted, and (2) the amount of substitution of La, Ge, etc. will be expressed in units of mol ppm and mol %. In FIG. 5, 2 is a heat-resistant insulating base made of alumina or the like, 4 is a metal housing for inserting the sensor into the exhaust pipe of an automobile engine, and 6 is a heater tube that also serves as a protective cover for the gas detection piece 8. heater tube 6
is made of ceramics, and has a heater pattern 10 of platinum, tungsten, etc. buried inside. 8 is a gas detection piece, perovskite compound A ₁
−xLaxSnO ₃ is used as the δ gas sensitive material. here A
is a member or multiple member element of the group consisting of Ca, Sr, Ba, and Ra. The substitution ratio x of La should be greater than 0 and less than 2%, for example, 30ppm to 2%.
%, more preferably 100 to 5000 ppm, most preferably 100 to 2000 ppm. The gas detection piece 8 includes
Add a precious metal catalyst to suppress combustible gas sensitivity and balance oxygen sensitivity in the lean burn region. The amount of catalyst is 10 μg to 2 mg per 1 g of A ₁ −xLaxSnO ₃ in terms of metal, including Pt, Pt−Rh, Pd, Rh, Ir,
Ru, Os, Re (rhenium), etc. are used. Other additives to the gas detection piece 8 include amorphous materials such as SiO ₂ .
Add a non-vitreous gel as an oxygen sensitizer. In addition to SiO ₂ , ZrO ₂ , GeO ₂ , HfO ₂ can be used as a sensitizer.
can be used, and the amount added is 20 to 200 mmol per 1 mole of A ₁ -xLaxSnO ₃ . Oxygen sensitivity increases by 10-20% with the addition of enhancers. Note that it is not necessary to add a sensitizer or a catalyst, and the action of La is independent of whether or not these things are added. A ₁ −xLaxSnO ₃ has the property of undergoing a solid phase reaction with Al ₂ O ₃ at high temperatures and decomposing into a composite oxide of alkaline earth and aluminum and SnO ₂ . In order to suppress solid phase reactions, the gas sensing piece 8 is coated with a porous protective film 12 . For example, the material is
SiO ₂ colloidal gel, or spinel (MgAl ₂ O ₄ ),
Mullite (Al ₆ Si ₂ O ₁₃ ) may be used as long as it does not react with A ₁ −xLaxSnO ₃ . The thickness of the coating is, for example, 40-100μ. The gas detection piece 8 includes a pair of noble metal electrodes 14,1.
External leads 18 and 20 are connected via 6. The gas detection piece 8 is manufactured, for example, as follows. A predetermined amount of lanthanum carbonate is added to BaCO ₃ , SrCO _3, etc., and the total amount and equimolar amount of SnO ₂ are mixed. Mixture in air for 4 hours at 1100-1400℃
Calcined at °C, the perovskite compound A ₁ −
Obtain xLaxSnO ₃ . After grinding the product, colloidal silica,
Colloids of GeO ₂ , ZrO ₂ , HfO ₂ and ThO ₂ are mixed and molded into the shape of the gas detection piece 8 . Then, it is fired in air at 1200-1500°C for 4 hours. The firing temperature is, for example, the same as or 100°C higher than the calcination temperature. The existing form of SiO ₂ etc. after firing is an amorphous, non-vitreous gel, with a broad peak in X-ray diffraction (the average crystallite diameter determined from the half width is 10 to 60 Å).
degree). Instead of colloidal silica, silica may be added in the form of xerogel or the like. After firing, the detection piece 8 is impregnated with an aqueous solution of a noble metal salt, dried, and then thermally decomposed at, for example, 950°C.
The method of adding the noble metal catalyst itself is well known. Finally, a protective film 12 is provided by thermal spraying or sintering after coating, and the structure shown in FIG. 5 is assembled to form a sensor. The characteristics of the sensor are shown below. Figures 1 to 3 show the resistance values in exhaust gas at air-fuel ratios λ of 1.01 and 0.99 for BaSnO ₃ -based materials calcined at 1200°C and fired at 1300°C. The additives were 0.1 mg of Pt per 1 g of perovskite compound and 50 mmol of SiO ₂ colloid per 1 mole of the compound. From Figure 1, by replacing 1000 ppm of La, λ=
The temperature coefficient of resistance at 0.99 is drastically reduced, and λ=
It can be seen that the ratio of the resistance value (Rs) between 1.01 and λ=0.99 does not change much. For example, if we detect whether the low resistance value (Rs) of the sensor is 300Ω or more,
It is possible to reliably detect whether λ>1 or λ<1. This applies to applications where the detection target is λ>1 or λ<1, such as control at the equivalence point of an automobile engine,
This is important for detecting incomplete combustion. Next, when using it in a lean burn region, for example, the ratio of the resistance value in the lean region to the resistance value in the rich region monotonically decreases with temperature, so the temperature of the sensor is known from this ratio and the temperature is controlled by controlling the heater 10. It can also be set constant. In FIG. 2, when comparing the one with 1 mol % substitution and the one with 10 mol % substitution, it can be seen that the sensitivity to the atmosphere is drastically reduced by 10 mol % substitution. In addition, λ = 0.99 for both the 1 mol % substituted and 10 mol % substituted ones.
The resistance values were almost the same. In Fig. 3, when examining the case where Ba is replaced with 10 mol% Ce, it can be seen that the effect of Ge is extremely small compared to La. Note that even when Nd or Yb was used instead of Ce, only a small effect was obtained. Another feature of the substitution with La is that if the amount of substitution is appropriate, the oxygen sensitivity does not decrease much. In order to indicate oxygen sensitivity, we introduce the concept of oxygen gradient, and define the resistance value (Rs) of the sensor as the n value when Rs=K·PO ₂ ⁿ . In principle, n is measured using an _N2 balance system with an oxygen concentration of 1 to 10.
This is done by changing it to %. FIG. 4 shows the (n) values for the same firing conditions and additive materials as in FIG. 1. The measured temperature is
Existing at 700℃. With La substitution of 3000 ppm or less, the oxygen gradient n hardly decreases. These results are shown more generally in Table 1. In the table, the oxygen gradient of CaSnO ₃ and SrSnO ₃ is negative because
This is because P-type property occurs when λ>1. Also λ<
When changing from 1 to λ>1, the resistance value increases and exhibits n-type property.

【table】

[Table] Table 2 shows the effects of noble metal catalysts and oxygen sensitizers. The effect of the catalyst is to balance the sensitivity of combustible gases such as CO, H ₂ and C ₃ H ₆ with the sensitivity to oxygen, and to eliminate errors caused by the coexistence of unreacted combustible gases in the lean burn region. In a system containing 4.6% _O2 at 700℃
Increase the CO concentration from 1000ppm to 10000ppm and find the ratio of resistance values. Due to this change, the _O2 concentration decreases by 0.45% and the ratio decreases in the _BaSnO3 system by back calculation from the oxygen gradient.
When it reaches 1.02, a balance of sensitivity is obtained.
The CO sensitivity is greatly suppressed by adding 20wtppm of Pt, and sufficiently suppressed by 100wtppm. Addition of excessive amount of Pt impairs the responsiveness of the sensor. At 900°C, λ is switched between 1.01 and 0.99 in a 2-second cycle of 1 second each and a 6-second cycle of 3 seconds each. Here, if sensitivity is defined as the ratio of resistance values between λ=1.01 and 0.99, the greater the sensitivity in each cycle and the smaller the difference due to change in cycle, the better the response. 10 ⁴ ppm Pt
addition impairs responsiveness. The effect of catalyst is Pt
Even if Rh, Ir, Ru, Os, etc. are substituted, CO can be replaced with H ₂ or C ₃ H ₆
Even if the temperature at which the response was measured was changed to another temperature, the result was almost the same. Moreover, the _same results could be obtained by using RaSnO ₃ -based, CaSnO ₃ -based, or SrSnO _{3 -based products instead of BaSnO 3} -based ones. The oxygen gradient is independent of La and catalyst, and ranges from 3 to 20
0.03 to 0.03 in BaSnO ₃ system by addition of mol% SiO ₂
Improved by 0.04. GeO _{2 ,} ZrO ₂ , HfO ₂ instead of SiO ₂
The same result could be obtained using . Note that the above explanation does not exclude substituting the A element or Sn element of A ₁ -xLaxSnO ₃ -δ with another element, and also that A ₁ -xLaxSnO ₃ -δ
This does not preclude its use in combination with other additives, such as SnO ₂ .

【table】

〔Effect of the invention〕

According to the present invention, the temperature coefficient of resistance of the exhaust gas sensor in a reducing atmosphere can be reduced.

[Brief explanation of the drawing]

1, 2, and 4 are characteristic diagrams of the exhaust gas sensor of the embodiment, FIG. 3 is a characteristic diagram of the conventional exhaust gas sensor, and FIG. 5 is a longitudinal sectional view of the exhaust gas sensor of the embodiment. .

Claims (1)

[Claims] 1 At least one pair of electrodes is connected to a sintered body containing a perovskite compound A ₁ -xLaxSnO ₃ -δ as a gas-sensitive material, and where A is selected from Ca, Sr, Ba, and Ra. δ is a non-stoichiometric parameter, and 0<x≦0.02,
Exhaust gas sensor. 2. The exhaust gas sensor according to claim 1, characterized in that x is 30 mol ppm or more and 2 mol % or less. 3. The exhaust gas sensor according to claim 2, characterized in that x is 100 mol ppm or more and 0.5 mol % or less. 4. The exhaust gas sensor according to claim 3, characterized in that x is 100 mol ppm or more and 0.2 mol % or less.