JPH053893B2 - - Google Patents

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 useful for controlling the air-fuel ratio of engines and boilers, preventing incomplete combustion in stoves, etc. It is suitable for [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 by combining two semiconductors, it is possible to compensate for the temperature dependence of the sensor and double the detection signal for the air-fuel ratio (λ). These requirements for exhaust gas sensors are: (1) to be highly sensitive to oxygen; (2) to suppress sensitivity to flammable gases and balance it with oxygen sensitivity; There are two points: Detection errors due to residual gas are suppressed. [Problem of the 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 has the following features: (1) 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 is a non-stoichiometric amount. (2) BTiO ₃ -δ of a p-type metal oxide semiconductor, where B is at least a member of the group consisting of Sr and Ca, and δ is a non-stoichiometric parameter. It is characterized by a combination of . [Notation] In the following, compounds are represented without the non-stoichiometric parameter δ. Further, as a concept indicating oxygen sensitivity, an oxygen gradient (n) is introduced and defined as Rs=K·Po ₂ ⁿ , (Rs: resistance value of semiconductor). [Example] (A) Structure of exhaust gas sensor The structure of the exhaust gas sensor will be explained with reference to FIG. 1 and FIG. 2. In the figure, 2 is alumina 6
It is a hole tube base, and a ceramic tube 4 with a built-in heater is attached to its tip. This ceramic tube 4 is equipped with a film heater 6 made of tungsten, platinum, etc. inside, and has an n-type gas detection piece 8 and a
This is for heating the shaped gas detection piece 10 to a constant temperature. Note that various types of heaters can be used in addition to the membrane heater 6 shown in the figure. An n-type gas detection piece 8 is inserted into the recess between the base 2 and the ceramic tube 4 via the threshold 12.
and a p-type gas detection piece 10 are provided. Here, the n-type gas detection piece 8 is made of BaSnO ₃ ,
It is made by adding a small amount of a noble metal catalyst and a gel such as SiO ₂ to an n-type perovskite compound such as RaSnO ₃ or Ba _0.7 Ra _0.3 SnO ₃ . Precious metal catalysts are used to suppress sensitivity to combustible gases and achieve a balance with oxygen sensitivity, and include Pt, Ir, Ru,
Precious metals such as Os and Rh or mixtures thereof are used. The appropriate amount of addition is 10 μg to 5 mg per 1 g of AnSnO ₃ in terms of metal. Gels such as SiO ₂ are used as oxygen sensitizers, and SiO ₂ , GeO ₂ ,
Amorphous, non-vitreous gels of ZrO ₂ and HfO ₂ are used. Note that amorphous herein means that the half width determined by X-ray diffraction is 60 Å or less. The amount of SiO ₂ etc. added is 1 to 30 per mole of ASnO ₃
Mol% is preferred. It is not necessary to add a noble metal catalyst, SiO ₂ or the like. Another problem with the n-type gas detection piece 8 is that
AnSnO ₃ reacts with basic 2 alumina etc.,
The purpose is to prevent decomposition into AAl ₂ O ₄ and SnO ₂ . Therefore, the area around the n-type gas detection piece 8 is
Cover with a substance that does not react with AnSnO ₃ . As the coating material, mullite, spinel, cordierite, or the aforementioned gel such as SiO ₂ or GeO ₂ is used. To explain the structure of the n-type gas detection piece 8 in more detail with reference to FIG. 3, 14 is a porous sintered body of _ASnO3 , 16 and 18 are noble metal electrodes, and 20 is a thickness.
It is a protective film of mullite of about 100μ. The p-type gas detection piece 10 is made of SrTiO ₃ , CaTiO ₃ ,
A pair of noble metal electrodes (not shown) are connected to a p-type perovskite compound such as Sr _0.7 Ra _0.3 TiO ₃ . BTiO ₃ is used as a porous sintered body, and 10 to 600 μg/g of noble metal catalyst is added.
The significance and composition standards of the catalyst are the same as for the n-type, and the amount added is 10 to 10% in terms of metal per 1g _{of BTiO3} .
600μg is preferred. Of course, it is not necessary to add a catalyst. As is well known, perovskite compounds are substances that are insensitive to substitution, such as A element, Sn element,
About 10 mol% of the B element and the Ti element may be replaced with other elements. Furthermore, ASnO ₃ and BTiO ₃ may be used in combination with other compounds within a range where their resistance value becomes dominant. Returning to FIGS. 1 and 2, 22 is a metal fitting for attaching the sensor to an exhaust pipe of an automobile engine, a combustion chamber of a stove, a boiler, or the like.
Further, 24 and 26 are lead pins connected to the membrane heater 6, and 28 and 30 are lead pins 32 and 34 connected to the n-type gas detection piece 8, and the lead pins 32 and 34 are connected to the p-type gas detection piece 10.
It is a lead pin connected to. (B) Ancillary circuit An example of ancillary circuit is shown in Fig. 4. Load resistors R ₁ and R ₂ are connected to the gas detection pieces 8 and 10 to form a bridge circuit, and a power source E _B is connected. In addition, the voltage applied to each load resistor R ₁ and R ₂ is applied to the amplifier A1 and A2.
Take out via. Comparing ASnO ₃ and BTiO ₃ here, the resistance value at 700℃ in the lean region is ASnO ₃ .
Just over 10KΩ, with BTiO ₃ it's just under 100KΩ. Also, the ratio of resistance values at 500℃ and 900℃ in the lean region is
ASnO ₃ has a strength of over 100 times, and BTiO ₃ has a strength of just under 1000 times.
Therefore, the values of resistors R ₁ and R ₂ and the amplifiers A1 and A2
Adjust the gain and match the resistance value. In addition, the output from the n-type gas detection piece 8 is 1.5
It is connected to an exponentiation circuit M1 of the order of magnitude, and the temperature coefficient is also matched. The exponentiation circuit M1 may be omitted. The outputs of the exponentiation circuit M1 and the amplifier A2 are input to the division circuit D1, and the temperature-compensated output is applied to the control circuit 40 to control 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 exponentiation circuit M1 and amplifier A2 are applied to multiplier circuit M2 to obtain a signal that depends only on temperature. The width of the output pulse of the oscillation circuit 42 is varied by the output of the multiplier circuit M2 in the voltage-pulse width modulation circuit 44, and the on-time of the switching transistor 46 is varied. In this way, the power applied from the power source E _B ' to the membrane heater 6 is varied depending on the temperature of the exhaust gas sensor, and the heating temperature is kept constant. (C) Manufacture of gas detection pieces 8 and 10 Alkaline earth carbonate and SnO ₂ or TiO ₂ are mixed and calcined at 1200°C to form ASnO ₃ or BTiO ₃ .
After pulverizing these substances, they are fired at 1300°C to form gas detection pieces 8 and 10. The preferred deformation range is
The calcination conditions are not important as the calcination conditions for ASnO ₃ are 1200-1500°C. Regarding BTiO ₃ , 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 preferably impregnated after calcination and supported by thermal decomposition at about 900 to 1000°C. Further, the protective film 20 is preferably provided after firing by thermal spraying, sintering after coating, or the like. As a comparative example, La ₂ (CO ₃ ) ₃ and CoO
LaCoO ₃ (p-type) calcined at 1200℃ and fired at 1300℃
was used. It was also calcined at 1200℃ and fired at 1300℃.
TiO ₂ (n-type) was used as another comparative example. All of these comparative samples contained 100 μg/g.
Added Pt. (D) Characteristics near the equivalence point (λ=1) A combination of ASnO ₃ and BTiO ₃ also has
The combination of TiO ₂ and LaCoO ₃ also has almost the same characteristics near the equivalence point. (E) Oxygen Sensitivity Table 1 shows the oxygen gradient of each gas detection piece 8, 10. The oxygen gradient of _ASnO3 is about 50% more than that of _TiO2
Largely, the enzyme gradient of BTiO ₃ is about twice that of LaCoO ₃ . Next, BaSnO ₃ , RaSnO ₃ , CaTiO ₃ and
They are mutually equivalent to SrTiO ₃ . Furthermore, Ba _0.7
The properties of Ra _0.3 SnO ₃ are BaSnO ₃ , Sr _0.7 Ca _0.3 TiO ₃
The properties of _SrTiO3 are very similar. Also, the addition of Pt had no effect on the oxygen gradient. Table 2 shows the effect of adding SiO ₂ etc. to ASnO ₃ . Oxygen gradient is 0.03-0.04 due to addition of SiO2 _etc.
Improved.

【table】

[Table] Figure 5 shows the oxygen sensitivity of an example in which BaSnO ₃ and SrTiO ₃ are combined and a comparative example in which TiO ₂ and LaCoO ₃ are combined. The measurement was carried out in _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 10 (normalized to 1 at an oxygen partial pressure of 1%) is shown as the output. (F) Sensitivity to combustible gas Detection errors in the lean burn region arise from an imbalance between sensitivity to unreacted combustible gas and oxygen sensitivity. Table 3 shows the sensitivity of flammable gases using CO as a representative of CO and H ₂ and propylene as a representative of hydrocarbons. For samples 9 and 10, changes in the ratio of the resistance values of the n-type gas detection piece 8 and the p-type gas detection piece 10 are shown.

〔Effect of the invention〕

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

[Brief explanation of the drawing]

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. Figure 4 is a block diagram of the auxiliary circuit, Figure 5~
FIG. 6 is a characteristic diagram of the exhaust gas sensor of the example. 2...Substrate, 4...Ceramics, 6...Membrane heater, 8...N-type gas detection piece, 10...P-type gas detection piece.

Claims (1)

[Scope of 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 The physical 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, (where B is at least a member of the group consisting of Sr and Ca, and δ represents a non-stoichiometric parameter),
An exhaust gas sensor characterized by: