JPH06100562B2 - 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 used. 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, SrFeO ₃ and LaNiO ₃ , LaC
The situation is different for perovskite compounds such as oO ₃ . 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 perovskite compounds such as SrFeO ₃ .

Elemental analysis of the corroded electrode revealed that elements such as Fe, Ni, and Co dissolved in the electrode and formed an alloy with Pt.
The cause of corrosion is not the reaction with carbon but the formation of alloys with Fe, Ni, and Co. Regarding the alloy formation mechanism, the inventor presumed as follows. When a compound such as SrFeO ₃ , LaNiO ₃ or LaCoO ₃ is exposed to a reducing atmosphere at a high temperature, Fe, Ni and Co elements in the compound are partially reduced to generate a low valence element. By the way, these elements originally have the property of easily forming an alloy with Pt, and the reduced elements diffuse into Pt to form an alloy such as Pt-Fe. And these alloys are fragile and are cut by stress such as thermal shock,
It loses its function as an electrode. On the other hand, TiO ₂ is a stable oxide,
Since it is difficult to undergo reduction, a Ti element having a low valence and a metallic Ti element are not generated, and the Pt-Rh alloy electrode does not corrode.

Challenge [challenge INVENTION This invention is for SrFeO ₃ or LaNiO _3, LaCoO perovskite compounds such as ₃ is to provide a stable electrode.

[Structure of the Invention] The exhaust gas sensor of the present invention uses a perovskite compound containing at least one metal element of the group consisting of Fe, Ni, and Co as a metal oxide semiconductor, and the electrode has Zr at a grain boundary.
It is characterized by using Pt in which O ₂ is projected.

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 perovskite compounds include, for example, SrFeO ₃ , La
There are NiO ₃ , LaCoO ₃ , and LaNi _1- x CoxO ₃ , and any perovskite compound containing any element of Fe, Ni, and Co may be used.

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.

[Example] Manufacture of exhaust gas sensor Equimolar amounts of SrCO ₃ and Fe ₂ O ₃ were mixed and left in air for 4 hours 12
The reaction is performed at 00 ° C. to obtain a perovskite compound SrFeO ₃ .
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. Molded chips in air for 4 hours 13
Sintering is performed by heating to 00 ° C. In the following, the Pt electrode was out folding the ZrO ₂ in the grain boundary, and Pt-ZrO ₂ electrode. ZrO ₂
The 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, LaNiO ₃ is obtained from equivalent amounts of lanthanum oxalate and NiO, and the same gas sensor (2) is obtained. Similarly, a gas sensor (2) using LaCoO ₃ is obtained from lanthanum oxalate and CoO.

As a comparative example, the Pt-ZrO ₂ electrodes (10) and (12) were set to 80 μm in diameter.
The same gas sensor (2) is manufactured except that the Pt-Rh alloy electrode (Pt60wt%, Rh40wt%) of (3) is replaced. It is known that the corrosion resistance of Pt-Rh alloy increases with the amount of Rh added.

As a comparative example similar to Pt with ZrO ₂ extruded, 1.0 wt% T
Pt (diameter 80 μ) in which iO ₂ is projected is used as the electrode of SrFeO ₃ .

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

The above-mentioned Pt-Rh alloy electrode was connected to TiO ₂ (rutile phase) calcined at 1200 ° C and sintered at 1300 ° C to obtain a sensor (2) of another comparative example.

Semiconductor characteristics SrFeO ₃ , LaNiO ₃ , and LaCoO ₃ are all metal oxide semiconductors made of p-type perovskite compounds, and the resistance value decreases with the equivalence ratio (λ). Characteristics of the exhaust gas sensor are already known of these compounds, in JP-B 49-19,837 is attached to SrFeO _3, it is attached to the LaNiO ₃ JP 58-
No. 16,124 and LaCoO ₃ are disclosed in JP-A-57-204,445. The characteristics of these compounds are insensitive to substitution of constituent elements, and La and Sr are
e, Co, and Ni can be used by substituting with other transition metal element or the like. And the problem of electrode corrosion is Fe, Co, Ni
It occurs when any of the above elements is included, and the use of the Pt-ZrO ₂ electrode is effective even when substitution is performed.

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. In this example, ZrO ₂ was added at 0.6 wt%. Pt containing ZrO ₂ is known from JP-B-46-29243, JP-B-54-3803, and JP-A-46-2655. Since the added ZrO ₂ cannot be dissolved in Pt as a solid solution, it exists in a dispersed state in Pt and tends to break out to the crystal grain boundaries. This is because ZrO ₂ cannot be solid-dissolved in Pt, so that the crystal grain boundary becomes a stable existence position. Then, ZrO ₂ is broken into grain boundaries in the Pt electrode by wire drawing of the Pt material to the electrode wire and sintering at 1300 ° C. after embedding the Pt electrode shown in the section of manufacturing the exhaust gas sensor described above. Put out. The projected ZrO ₂ blocks the entry of dissimilar metals into the Pt electrode along the grain boundaries and prevents corrosion. As the Pt-ZrO ₂ electrode, an alloy of Pt and Rh or 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 SrFeO 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, although the wire diameter was small, none of them was broken. When a broken Pt-Rh alloy was inspected, it was found that Sn diffused inside the electrode and alloyed. On the other hand, in the Pt-ZrO ₂ electrode, Fe is only a solid solution at a low concentration on the surface of the electrode, and diffusion into the crystal is due to the diffusion of Zr at the grain boundary.
It was blocked because of O ₂ .

The attached to the same test in Fig. 4 shows the result of the LaNiO _3.
Samples 1 to 6 use the above Pt-Rh alloy, and sample 7
12 is obtained by using the Pt-ZrO _2. The durability of the electrode is increased by the use of Pt-ZrO _2, is common.

Figure 5 shows the results for LaCoO ₃ . Pt of samples 7-12
-ZrO ₂ and Pt-Rh of Samples 1 to 6 differ greatly in durability.

The 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 an 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 to 5 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-46-2655 (JP, A) JP-A-55-124059 (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 a pair of electrodes is connected to a metal oxide semiconductor whose resistance value changes with gas, wherein the metal oxide semiconductor contains at least one metal element of the group consisting of Fe, Ni, and Co. It is a perovskite compound contained, the main component of the electrode is Pt, and Zr
An exhaust gas sensor characterized in that O ₂ is emitted.