JPS6214047A - Exhaust gas sensor - Google Patents

Abstract

PURPOSE:To enable the use of a sensor under a severe atmosphere with a higher durability of an electrode, by connecting a Pt electrode with ZrO2 precipitated in a grain boundary to a metal oxide semiconductor containing Sn element to form a sensor. CONSTITUTION:For example, equal mole of BaCO3 and SnO2 are mixed to be made to react in the air at a specified temperature for a specified time to obtain a perovskite compound BaSnO3, which is crushed and a Pt electrode with ZrO2 added thereto is buried thereinto to be formed into a sensor chip 8. The chip thus obtained is heated and sintered in the air at a specified temperature for a specified time. Then, the sensor chip 8 is housed into a recess 6 provided at the end of an insulation substrate 4 made of alumina or the like. Pt electrodes 10 and 12 in which ZrO2 is precipitated on a grain boundary are housed into grooves 14 and 16 provided on the substrate 4 and the ends thereof are connected to base metal external leads 18 and 20. Then, an alumina sheet 22 is stuck on the substrate 4 leaving the perimeter of the chip 8 to protect the electrodes 10 and 11 being separated from the atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an improvement in an exhaust gas sensor that utilizes a change in the resistance value of a metal oxide semiconductor, and particularly to an improvement in the 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, etc.

[Prior Art] The problem that Pt electrodes of exhaust gas sensors are corroded by high-temperature reducing atmospheres has been pointed out for a long time. For example, U, S, P 4237.722 states that the cause of corrosion is the reaction between carbon in exhaust gas and Pt, and proposes using a Pt-Rh alloy as a highly durable electrode.

Further tests by the inventors have shown that the Pt-Rh alloy has Ti
It was confirmed that Ot had excellent durability when used as a metal oxide semiconductor (Table 1). However, 5nO
The situation is different for compounds containing Sn elements such as t and B a S n O1. When Pt-Rh alloy electrodes are used with these compounds, the electrodes are corroded by the high temperature reducing atmosphere. A Pt-Rh alloy electrode is effective for T i Ot, but not for B a S n Os or Snow. Elemental analysis of the corroded electrode revealed that S was present in the electrode.
It was found that n was dissolved in solid solution. The cause of corrosion is not the reaction with carbon, but the formation of an alloy between Pt and Sn.

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

[Structure of the Invention] The exhaust gas sensor of the present invention is characterized by a combination of a metal oxide semiconductor containing the Sn element and a PT electrode in which ZrOt is deposited at the grain boundaries.

Such metal oxide semiconductors include the above-mentioned 5nOy and B.
In addition to aSnO3, for example, Ca5nOs, 5rSnO,
, or mixtures of these and other metal oxide semiconductors such as Tidy, and furthermore, compounds in which a part of the Sn element in these compounds is replaced with Ti or the like.

The electrode may be made of Pt as a main component, with ZrOt precipitated at its grain boundaries, and Rh or Au as a third component.
It is also possible to add the following.

[Example] Production of exhaust gas sensor Equimolar amounts of B a COs, 5rCOs, CaCO5 and Snow were mixed and reacted in air at 1200°C for 4 hours to form perovskite compounds Ba5nOs and SrSnO.
3. Obtain Ca5nOa. After grinding the obtained compound,
Commercially available Z r O! Pt electrode (diameter 70μ) doped with
is embedded and molded into the sensor chip shown in FIGS. 1 and 2. The molded chip is heated to 1300° C. for 4 hours in air to perform sintering. In the following, a Pt electrode in which Zr0t is precipitated at grain boundaries will be referred to as a pt-ZrCh electrode.

Using the sintered chip, the exhaust gas sensor (2) shown in FIGS. 1 and 2 is assembled. In the figure, (4) is an insulating substrate made of alumina, etc., and a recess (6) is provided at the end of the insulating substrate.
The sensor chip (8) is housed in the sensor chip (8). Chip (
The electrodes (lO) and (12) of 8) are housed in the grooves (14) and (16) provided in the substrate (4), and their ends are connected to the base metal external leads (1, 8) and (20). Connecting. Next, a thin alumina plate (22) is bonded to the substrate (4), leaving the area around the chip (8), to protect the electrodes (10) and (!2) by shielding them from the atmosphere.

It goes without saying that the structure of the gas sensor (2) may be any other suitable structure.

As another example, P was added to 5nO1 calcined at 1200°C.
A similar gas sensor (2) is obtained by connecting a Zr0t electrode and sintering at 1300°C.

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

(a) The Pt Zr0t electrodes (10) and (12) were replaced with a Pt-rth alloy electrode (Rh40wt
% or 13wt%) (BaSnOs, S
now).

(b) P with 1.0 wt% of T i Ot precipitated
L (diameter 80μ) as an electrode (Sift).

(c) Pt electrode doped with 5 wt% Au (diameter 7
0 μ, CaS no,).

(d) Pt-Rh alloy (Rh40vt%) with Ti1
t electrode.

Note that in these comparative examples, the manufacturing conditions for the sensor (2) are the same as in the first example.

Characteristics of semiconductors SnO is well known as a material for exhaust gas sensors.

On the other hand, Ba5nO+, 5rSnOs, Ca5nO
, are new to exhaust gas sensors, and both are perovskite compounds. BaSnO3 is n-type, and its resistance value increases with the air-fuel ratio.
8nO3 is a compound in which p-type and n-type are mixed.

Among these, -RaSn(') has the highest oxygen sensitivity, and the oxygen sensitivity of -BaSnOa is superior to that of S rS no and Ca5nO.

high compared to The oxygen sensitivity of Ba5nOs and SnO is comparable, but Ba5nO is superior in terms of durability to high-temperature reducing atmospheres and small detection errors due to unreacted combustible gas in exhaust gas.

For example, the equivalence ratio λ of 5nOt at 900°C is 0.9 to 0°9.
When exposed to the atmosphere of Ba5nO for a long time (4 to IO hours), the resistance value decreases irreversibly, but this does not occur with Ba5nO. Next, 5nOt has high sensitivity to unreacted components (mainly CO and HC) in exhaust gas, while Ba5no has low sensitivity. In SnO, 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 Zr0t electrode Pt 7. The rot electrodes (10) and (12) are made of Z r Ot precipitated at the grain boundaries of PT. Zr
The amount of CL added is, for example, 0. O1-3. Ovt% is preferable, and by setting it to 0.01 wt% or more, sufficient corrosion resistance can be obtained, and by setting it to 3.0 wt% or less, the hardness can be suppressed to a range that is easy to process. In this example, the amount of ZrO+ added was 06 wt%, but the results were the same even if the amount was 0.3 wt% or 1.0 wt%. Further j”'t ZrO
For the two electrodes, an alloy of Pt and Rh or an alloy of Pt and Au to which Zr0z is added may be used.

Corrosion resistance of the electrode In order to evaluate the durability of the N electrode, gas sensors (2
) were used to conduct the following tests.

The sensor (2) is exposed to an atmosphere of 900°C and a lambda of 09, and an atmosphere of 35
A cycle of 90 seconds each in 0°C air for a total of 3 minutes,
Apply 20,000 cycles. The total time for this cycle is 1000 hours. During the cycle, the sensor (2
) Check for disconnection based on the resistance value, and inspect the condition of the electrodes after the cycle is complete. Here, the atmosphere where λ is 0.9 is an extremely reducing atmosphere in which the gas sensor is placed. Further, 900° C. corresponds to the maximum operating temperature of the gas sensor. Furthermore, the temperature cycle between 350°C and 900°C imposes a large thermal shock on the sensor.

Therefore, this test is not harsh in terms of atmosphere, maximum temperature, or thermal shock.

Figure 3 shows the results for BaSnO3. Sample 1~
Pt-Rh alloy 0?6 wire diameter 80μ? In the wires using 40 wt % of Rh, all wires were disconnected despite having a large amount of Rh and a large wire diameter. On the other hand, samples 7 to 12 had a wire diameter of 70μ, ZrO, fio, and 6 wt% Pt Zrot.
In the case of alloys, there was no wire breakage despite the small wire diameter. When the disconnected pt-Rh alloy was inspected, it was found that Sn had diffused into the electrode and formed an alloy.

On the other hand, in the PtZrOt electrode, Sn was only dissolved in solid solution at a low concentration on the surface of the electrode, and diffusion into the interior of the crystal was prevented by ZrOt at the grain boundaries.

Figure 4 shows the results of the same test for SnO. Samples 1 to 6 used the above Pt-Rh alloy,
Samples 7 to 12 are Pt 7. This uses rOt. It is common that the use of Pt ZrOt increases the durability of the electrode.

The test results are summarized and shown in Table 1.

Table 1 Durability of electrodes Semiconductor Number of disconnections N days until disconnection Alternative amount *1 Cycle *21* Ba5n
(), + 6 6000Pt-Rh
(Rh40) 2* Ba5nOa 6Pt-Rh
(Rhl 3) 3 BaSnO30 PL-Zr0°4* Sn0. 6 4000P
t-[1h (Rh40) 5 Snow OPt Zr
0t 6 5rSn0. O Pt Zr0t 7* CaSnO358000 Pt-Au (Au5wt%) 8 CaSnO30 Pt Zr0t 9* Sn0. 4 1300
0Pt-Trot 10* TiO* 0Pt
-Rh (Rh40) *1 All 6 samples, *2 Average value for only broken wires, * *
The marks are comparative examples.

[Effects of the Invention] According to the present invention, the durability of the electrodes of the exhaust gas sensor can be increased and the exhaust gas sensor can be used in harsh atmospheres.

[Brief explanation of drawings]

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

Claims (1)

[Claims] (1) In an exhaust gas sensor in which at least one pair of electrodes is connected to a metal oxide semiconductor whose resistance value changes depending on gas, the metal oxide semiconductor contains Sn element, and the main component of the electrode is Pt, At the grain boundaries, ZrO_2
An exhaust gas sensor characterized by having: