JPH06100561B2 - Exhaust gas sensor - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an improvement in an exhaust gas sensor utilizing a change in resistance value of a metal oxide semiconductor, and more particularly to an improvement in durability of an electrode. The exhaust gas sensor of the present invention is suitable for detecting the air-fuel ratio of automobile engines, stoves, boilers and the like.

[Prior Art] The problem that the Pt electrode of an exhaust gas sensor is corroded by a high-temperature reducing atmosphere has been pointed out for a long time. For example
USP 4237,722 suggests that the cause of corrosion is the reaction between carbon in the exhaust gas and Pt, and proposes to use a Pt-Rh alloy as a highly durable electrode.

The inventors' additional tests also confirmed that the Pt-Rh alloy has excellent durability when TiO ₂ is used as a metal oxide semiconductor (Table 1). However, SnO ₂ and BaSnO ₃
The situation is different for compounds containing elements. When Pt-Rh alloy electrodes are used for these compounds, the electrodes are corroded by the high temperature reducing atmosphere. The Pt-Rh alloy electrode is effective for TiO ₂ , but not for BaSnO ₃ or SnO ₂ . Elemental analysis of the corroded electrode revealed that Sn was a solid solution in the electrode. The cause of corrosion is not the reaction with carbon,
It is the formation of an alloy of Pt and Sn.

[Problem of the Invention] An object of the present invention is to prevent corrosion of an electrode of an exhaust gas sensor using a Sn-based metal oxide semiconductor.

[Structure of the Invention] The exhaust gas sensor of the present invention is characterized by combining a metal oxide semiconductor containing an Sn element and a Pt electrode in which ZrO ₂ is projected at a crystal grain boundary. The ZrO ₂ content in the electrode is 0.01 to 3.0 wt%.

Pt added with ZrO ₂ is known, and ZrO ₂ is dispersed in Pt immediately after production. Here, when Pt added with ZrO ₂ is drawn into an electrode wire or subjected to heat treatment, ZrO ₂ is extruded at the grain boundary of Pt. This is because ZrO ₂ cannot form a solid solution in the Pt crystal. Then, ZrO ₂ protruding at the grain boundary prevents the dissimilar metal from diffusing into Pt along the grain boundary and prevents corrosion of the Pt electrode.

Such metal oxide semiconductors include SnO ₂ and BaSnO ₃ described above.
Besides, for example, CaSnO ₃ or SrSnO ₃ or these and TiO ₂
And other metal oxide semiconductors, and those in which a part of the Sn element of these compounds is replaced with Ti or the like.

The electrode may be Pt as a main component, and ZrO ₂ may be projected at the crystal grain boundary thereof, and may be the one to which the third component Rh, Au, or the like is added.

[Examples] Production of exhaust gas sensor Mixing equimolar amounts of BaCO ₃ , SrCO ₃ , CaCO ₃ and SnO ₂ and reacting them at 1200 ° C. for 4 hours in air, the perovskite compound Ba
SnO ₃ , SrSnO ₃ and CaSnO ₃ are obtained. After pulverizing the obtained compound, a commercially available Pt electrode (diameter 70 μm) added with ZrO ₂ is embedded and molded into a sensor chip shown in FIGS. 1 and 2. The molded chips are heated in air at 1300 ° C. for 4 hours to be sintered. In the following, the Pt electrode was out folding the ZrO ₂ in the grain boundary, and Pt-ZrO ₂ electrode. The ZrO ₂ -added Pt electrode can be obtained from Tanaka Metal Industry Co., Ltd. or Johnson-Massie Company (London).

The exhaust gas sensor (2) shown in FIGS. 1 and 2 is assembled using the chips after sintering. In the figure, (4) is an insulating substrate made of alumina or the like, and a recessed portion (6) provided at the end thereof.
Accommodates the sensor chip (8). The electrodes (10) and (12) of the chip (8) are housed in the groove portions (14) and (16) provided on the substrate (4), and the ends thereof are used as the external leads (18) and (20) of base metal. Connecting. Next, a thin plate (22) of alumina is attached to the substrate (4) while leaving the periphery of the chip (8), and the electrodes (10) and (12) are shielded from the atmosphere to be protected. It goes without saying that the gas sensor (2) may have any structure more than that.

As another example, a PtZrO ₂ electrode was connected to SnO ₂ that had been calcined at 1200 ° C. and sintered at 1300 ° C., and the same gas sensor (2)
And

The following sensor (2) is prepared as a comparative example.

(A) Pt-ZrO ₂ electrodes (10) and (12) are connected to a Pt-
Rh alloy electrode (Rh 40 wt% or 13 wt%) replaced (Ba
SnO ₃ , SnO ₂ ).

(B) An electrode made of Pt (diameter 80 μ) obtained by protruding 1.0 wt% of TiO ₂ (SnO ₂ ).

(C) 5 wt% Au-added Pt electrode (diameter 70μ, CaSn
O ₃ ).

(D) A Pt-Rh alloy (Rh 40 wt%) was used as a TiO ₂ electrode.

In these comparative columns, the manufacturing conditions of the sensor (2) are the same as those in the first embodiment.

Semiconductor characteristics SnO ₂ is well known as a material for exhaust gas sensors. On the other hand, BaSnO ₃ , SrSnO ₃ and CaSnO ₃ are novel for exhaust gas sensors and are all perovskite compounds. BaSn
O ₃ is n-type and its resistance value increases with the air-fuel ratio, and SrSnO ₃ and Ca
SnO ₃ is a compound in which p-type and n-type are mixed.

Of these, BaSnO ₃ is the best. Oxygen sensitivity of BaSnO ₃ is higher than the SrSnO ₃ and CaSnO _3. BaSnO ₃ and SnO ₂
The oxygen sensitivities of and are about the same, but BaSnO ₃ is superior in durability against high-temperature reducing atmosphere and small detection error due to unreacted combustible gas in exhaust gas.

For example, when SnO ₂ is exposed to an atmosphere having an equivalence ratio λ of 0.9 to 0.95 at 900 ° C. for a long time (4 to 10 hours), the resistance value decreases irreversibly, but BaSnO ₃ does not cause such a decrease. Next, SnO ₂ has high sensitivity to unreacted components (mainly CO and HC) in exhaust gas, and BaSnO ₃ has low sensitivity. Then, with SnO ₂ , the sensitivity to unreacted components is too high compared to the sensitivity to oxygen, so a change in the amount of unreacted components causes a detection error.

Pt-ZrO ₂ Electrode Pt-ZrO ₂ electrodes (10) and (12) are formed by protruding ZrO ₂ at the Pt crystal grain boundary. The amount of ZrO ₂ added is, for example, 0.01 to 3.
0 wt% is preferable, and if it is 0.01 wt% or more, sufficient corrosion resistance can be obtained, and if it is 3.0 wt% or less, the hardness can be suppressed within a range where processing is easy. Zr
Pt added with O ₂ is Japanese Patent Publication No. 46-29243 and Japanese Patent Publication No. 54-380.
No. 3, JP-A-46-2655 and the like are known. Z added
Since rO ₂ cannot be dissolved in Pt as a solid solution, it exists in Pt in a dispersed state.
It has the property of attempting to break out to the grain boundaries. This is ZrO ₂
This is because it is not possible to form a solid solution in Pt, so that the crystal grain boundary becomes a stable existence position. And the drawing process to the electrode wire of Pt material,
After embedding the Pt electrode shown in the section of manufacturing the exhaust gas sensor
By sintering at 1300 ° C. or the like, ZrO ₂ breaks out to the grain boundary in the Pt electrode. The projected ZrO ₂ blocks the entry of dissimilar metals into the Pt electrode along the grain boundaries and prevents corrosion. In this example, the amount of ZrO ₂ added was set to 0.6 wt%, but 0.3 wt% or 1.0 wt%
The results were comparable. Furthermore, the Pt-ZrO ₂ electrode has Pt and Rh
And an alloy of Pt and Au to which ZrO ₂ is added may be used.

Corrosion Resistance of Electrodes In order to evaluate the durability of the electrodes, the following tests were conducted using 6 gas sensors (2) of each material. To the sensor (2), 20,000 cycles of 90 minutes each at 900 ° C. and 350 ° C. in air for 90 seconds each, totaling 3 minutes, are added. The total time for this cycle is 1000 hours. In the middle of the cycle, the resistance value of the sensor (2) is checked for the presence of disconnection, and after the cycle is completed, the state of the electrodes is inspected. Here, the atmosphere in which λ is 0.9 is an extremely strong reducing atmosphere for the atmosphere in which the gas sensor is placed. 900 ° C again
Corresponds to the maximum operating temperature of the gas sensor. 350 ° C
The temperature cycle between and 900 ℃ gives a large thermal shock to the sensor. Therefore, this test is severe in terms of atmosphere, maximum temperature, and thermal shock.

Figure 3 shows the results for BaSnO ₃ . For samples 1 to 6 using the Pt-Rh alloy (Rn40wt%) with a wire diameter of 80μ,
Despite the large amount and large wire diameter, all are broken. On the other hand, for samples 7 to 12, a wire diameter of 70 μ and a ZrO ₂ content of 0.6 wt% P
In the t-ZrO ₂ alloy, the wire diameter is small, but the wire is not broken. Inspection of the broken Pt-Rh alloy revealed that Sn diffused and alloyed inside the electrode. On the other hand, in the Pt-ZrO ₂ electrode, Sn is only a solid solution at a low concentration on the surface of the electrode, and the diffusion into the crystal is due to the diffusion of Zr at the grain boundary.
It was blocked because of O ₂ .

Figure 4 shows the SnO ₂ results for the same test. Samples 1 to 6 are samples using the above Pt-Rh alloy.
No. 12 uses Pt-ZrO ₂ . The durability of the electrode is increased by the use of Pt-ZrO _2, is common.

The test results are summarized and shown in Table 1.

[Advantages of the Invention] According to the present invention, the durability of the electrode of the exhaust gas sensor can be improved and the exhaust gas sensor can be used in a harsh atmosphere.

[Brief description of drawings]

FIG. 1 is a perspective view of the exhaust gas sensor of the embodiment, FIG. 2 is a plan view showing an exploded state of the exhaust gas sensor of the embodiment, and FIGS. 3 and 4 are characteristic diagrams of the embodiment. In the figure, (2) ... gas sensor, (8) ... sensor chip (10), (12) ... electrode.

 ─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-55-124059 (JP, A) JP-A-46-2655 (JP, A) JP-B-46-29243 (JP, B1) JP-B-54- 3803 (JP, B1)

Claims (1)

[Claims] 1. An exhaust gas sensor in which at least one pair of electrodes is connected to a metal oxide semiconductor whose resistance value changes with gas, wherein the metal oxide semiconductor contains a Sn element, and the main component of the electrode is Pt. And Zr is present in the grain boundary.
O ₂ is deposited and the ZrO ₂ content in the electrode is 0.01 to 3.0 w
An exhaust gas sensor characterized by being t%.