JPS6239383B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas sensor that detects oxygen gas concentration in high-temperature combustion gas or exhaust gas as a change in element resistance. This was done for the purpose of stabilizing the material to prevent this.

Generally, in many gas sensors, metal oxides react with changes in oxygen partial pressure in a gas atmosphere.
This method takes advantage of the change in resistance value.

The inventors found that perovskite-type composite oxides are effective as this type of sensor, and among them, LaCoO ₃ , which has an inherently low electrical resistance, has been found to be highly sensitive to temperature changes in resistance, including the external measurement circuit, at high temperatures. It has been attracting attention as a material with low p-type behavior. Another feature is that the catalytic action of this material is effective in quickly achieving chemical equilibrium in combustion exhaust gas.

However, LaCoO ₃ as the basic material or some of its La replaced by alkaline earth metals
It has been found that La _1-x Sr _x CoO ₃ and the like are reduced and decomposed into La ₂ O ₃ and Co when left in a reducing gas-excess atmosphere at high temperatures, that is, in the so-called rich burn region. This reaction progresses reversibly when returned to lean burn, and LaCoO ₃ or La _1-x Sr _x CoO ₃ is synthesized again, making it possible to apply it as a gas sensor that exhibits a very large resistance change. However, during this reversible reaction, as a secondary effect, an increase in contact resistance occurred due to contact deterioration at the electrode portion. This increase in contact resistance was greatly improved by mixing glass containing a Co component into the basic material during firing of the sensor. On the other hand, it has been found that the resistance over time in the rich burn of the gas sensor configured as described above changes in a two-step process, as shown in FIG. 1, for example. In this case, in the range below the first saturation point of the two saturation points in Figure 1,
In other words, it has been found that by using the device in a range where the resistance change is as small as about two digits, the above-mentioned contact deterioration of the electrode portion is completely eliminated. Therefore, it has been found that when used below the first saturation point, it can be used without any problem when used in general usage methods such as gas appliances.

However, even when using gas equipment, etc., the rich burn state may continue for a long period of time, so a sensor that exhibits stable performance even in such cases is desired.

The present invention was made against this background, and is an improvement of the basic material so that it can withstand long-term rich burn treatment.

In addition to the LaCoO ₃ system, the inventors have developed perovskite-type composite oxides using other lanthanum-based rare earth metals for La, or alkaline earth metals for a part of the La.
We also investigated many systems in which Co was replaced with other transition metals. as one of them
LaMnO ₃ was also considered.

Synthesized using La ₂ O ₃ and MnO ₂ as starting materials
LaMnO ₃ has a monoclinic perovskite structure and a lattice constant of 5.471 Å. This LaMnO ₃
The sensor element using only LaCoO ₃ had higher resistance than LaCoO 3, and the resistance change during rich burn and lean burn was small, about 3 times as much. However, even when left in Rich Burn for a long time, the electrode does not deteriorate like LaCoO ₃ .
It was very stable. According to X-ray analysis, this stability is thought to be due to the fact that LaMnO ₃ does not decompose even in rich burn, the perovskite structure remains unchanged, and the lattice expansion is only observed.

The inventors investigated a material that combines the features of LaCoO ₃ with its low resistance and large resistance change, and LaMnO ₃ 's stability under rich burn. This method examines a series of chemical compositions of LaMn _1-x Co _x O ₃ .

LaMn _1-x Co _x O ₃ (o<x<1) is La ₂ O ₃ ,
It is obtained by mixing MnO ₂ and Co ₂ O ₃ raw materials in a predetermined composition ratio and reacting and synthesizing them at 1000℃, but La ₂ O ₃ is excessive in the chemical composition ratio. was determined by X-ray analysis, so each oxide was prepared within a range in which La ₂ O ₃ was not excessive for each composition. After dry stirring the prepared oxide for 20 hours, a 1% aqueous methylcellulose solution was added, and the mixture was sieved and granulated using a 50-mesh screen. Put the granulated powder into a press mold and press 350 to 500
It was molded into a disk shape under a pressure of Kg/cm ² . This disc-shaped molded body was calcined by heating at 1000° C. for 5 hours in an air atmosphere in an alumina boat to synthesize a composite oxide LaMn _1-x C _x O ₃ having the desired composition. disk-shaped
After mechanically crushing LaMn _1-x Co _x O ₃ , it was wet-pulverized using methyl alcohol to reduce the particle size to 0.2.
It was made into a powder of less than μm. Nine types of powders were prepared in this manner.

Samples for measuring characteristics were prepared as follows.
A slurry is prepared by mixing the powder and the binder described above. Binder is polyvinyl butyral (PVB), dibutyl phthalate (DBP)
A predetermined amount of was dissolved in methyl alcohol was used. The mixing ratio was 40 c.c. of methyl alcohol, 4.5 g of PVB, and 2.9 g of DBP. The slurry was used by adding 1.5 ml of binder to 3 g of powder on average and stirring well. Then attach the diameter to the alumina plate.
Two platinum wires each having a length of 0.2 mm and a length of 12 mm were fixed so that they were parallel to each other, and slurry was applied to one end of the two platinum wires. Platinum was fixed on the alumina plate by applying a methyl cellulose aqueous solution to the alumina plate, placing a platinum wire in parallel thereon, and then drying the aqueous solution. After applying the slurry to a platinum wire and drying it by heating at 200°C, the element was removed from the alumina plate using water and fired in air at 1100°C for 3 hours to obtain an element for measurement.

According to the results of measuring the resistance value at room temperature, the resistance change was not a monotonous change as shown in FIG. 2, but showed a peak at x=0.5. Furthermore, resistance often shows temperature changes, but when x=0.6 or more, the temperature changes at 600°C or more are small. on the other hand,
As shown in FIG. 3, the change in resistance value over time at high temperature (approximately 850° C.) had one saturation point when x≦0.8, and did not show the trend shown in FIG. 1. In other words, it was found that there were no multiple saturation points that would cause significant device deterioration. Furthermore, responsiveness is also significantly improved. This is clearly considered to be the effect of adding Mn.

The ratio of resistance values in lean burn and rich burn, which indicates element sensitivity, increases as the value of x increases,
It looked like Figure 4.

To summarize the above results, when part of the Co in LaCoO ₃ is replaced with Mn, the second resistance increase and saturation characteristics that accompany element deterioration are no longer observed, and the response is significantly improved. However, as the amount of Mn increases, the resistance increases and temperature changes also increase. Additionally, the change in resistance when moving from lean burn to rich burn is also reduced. Therefore, to obtain a sensor with excellent sensitivity, responsiveness, and lifetime, and less temperature dependence, 0.6
It is desirable to keep it within the range of ≦x≦0.8. Furthermore, in designing a sensor that operates not only with gas responsiveness but also with a decrease in temperature, it becomes necessary to use a material with an even smaller value of x.
On the other hand, in a sensor that is used under conditions where there is no temperature change, for example, the element is heated at a constant temperature,
The use of a material with a small value of x, which has good stability even if the resistance change is small, is still necessary for an excellent gas sensor.

As is clear from the above, _{sensitivity , sensitivity ,} and It has become possible to design a large number of sensors for various purposes in terms of response, temperature dependence, and resistance value, and the stability under high temperatures has also been greatly improved.

In addition, in the present invention, since responsiveness is improved by substituting Mn, in a series of materials,
It is also expected to improve properties in terms of catalytic action, and can be applied to three-way catalysts for treating exhaust gas from automobiles, etc.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the time change in resistance during response of a gas sensor using LaCoO ₃ , Figure 2 is a diagram showing the relationship between the composition of the gas sensor according to the present invention and the room temperature resistance value, and Figure 3 is the diagram. A diagram showing an example of response characteristics,
FIG. 4 is a diagram showing the relationship between the composition and the change ratio of resistance due to transition from rich burn to lean burn.

Claims (1)

[Claims] 1 In a gas sensor that detects the oxygen concentration in high-temperature combustion gas as a change in resistance value, _the basic material is a composite oxide mainly composed of LaCoO ₃ .
A gas sensor characterized in that part of Co is replaced with Mn.