JPS61147147A - Waste gas sensor - Google Patents

Abstract

PURPOSE:To increase the oxygen sensitivity of a compd. using as the title sensor material and to improve the sensing precision by adding the amorphous or the unvitreous gel of at least one member consisting of SiO2 and GeO2, etc. to perovskite compd. CONSTITUTION:In the structure of a gas sensing pieces 8 of a sensor, a pair of nobel metallic electrodes 16, 18 are buried in a sintered body 14 of ATiO3-delta perovskite compd. (A shows an element of at least one member of a group consisting of Ca, Sr, Ba, Ra and delta shows a nonstoichiometric parameter.) and the whole parts are coated with a protective coating layer 20. The change of waste gas composition is sensed by a resistance value of ATiO3-delta compd. The oxygen sensitivity is improved by adding the amorphous or unvitreous gel of 1-20mol% at least one member of a group consisting of SiO2, GeO2, HfO2, ZrO2 and ThO2 per 1mol ATiO3-delta to ATiO3-delta compd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an exhaust gas sensor that uses a change in the resistance value of a perovskite compound, and particularly relates to improving its oxygen sensitivity.

The exhaust gas sensor of the present invention is used to control the air-fuel ratio of automobile engines, boilers, heating furnaces, etc., and to prevent incomplete combustion in stoves.

[Prior art]

JP-A-56-54340 discloses 5rTio8. discloses an exhaust gas sensor using According to it, 5ETjO
a-a is an n-type metal oxide semiconductor, and the resistance becomes high as the air-fuel ratio λ increases both in the region where ^1 and in the region where λ>1. Next, the resistance value of the semiconductor (
Rs) is rearranged as nR5= and Po2, and the value of n is defined as the oxygen gradient.

According to JP-A-56-54840, the absolute value of the oxygen gradient in 8rTi08° is approximately equal to the oxygen gradient in P-type semiconductor LaC1003°, which is approximately 0.11 at 700°C.

Moreover, Japanese Patent Application Laid-Open No. 56-54840 discloses 8rTi03. Sintering agent: OaO64%, B20826%, SiO□10%
discloses the use of a high melting point glass.

[Problem of invention]

The problem of this invention is to use 8r as an exhaust gas sensor material.
TiO8, and caTio3. The aim is to improve the oxygen sensitivity of compounds such as.

[Structure of the invention]

In the exhaust gas sensor of this invention, perovskite compound A
Ti03=3, here A is Ca, Br,
Ba.

Changes in exhaust gas composition are detected from the resistance value of at least one member of the group consisting of Ra, where δ represents a non-stoichiometric parameter.

Compound Ar1o8. contains 1 to 20 mol% of SiO2, Ge09. HfO2,zrO2゜Th
At least a member of the group consisting of O2 is added to improve oxygen sensitivity. The 5io2, GeO2, etc. to be added are limited to amorphous and non-vitreous materials.

caTio3. The resistance values of and 5rrio8- exhibit a peculiar behavior with respect to changes in the air-fuel ratio λ. Lean burn area (
In the region (λ〉1), the resistance value decreases as the number of people increases, and P
Shows form. On the other hand, when λ is changed from 1 or more to less than 1, the change in resistance value is slight, and a critical change in resistance value near the equivalence point cannot be obtained. Purely academically, this means that caTio3. and 8rTi08- suggests that it is a semiconductor that is neither P-type nor N-type near the equivalence point, or that the response to atmospheric changes near the equivalence point is unmeasurably slow.

caTio3. Ya5rrio3. The oxygen gradient in the lean burn region is, for example, about 0.21 at 700'C.

On the other hand, LaCOO8,-, 8rFe08. (
7) The teno-oxygen gradient at 700°C is approximately 0.11 and 5 rrio
It is about 172 of 3B. In this way, caTi o and 5rrio3.
There is a-δ isomerism.

In the case of BaTi0 and RaTiO3, complete B
It is a 3-δ type metal oxide semiconductor whose oxygen gradient is − at 700°C.
It will be about 0.11.

When approximately 5 mol% of SiO2 is added to these compounds, the oxygen gradient at 700°C becomes caTio3. Ya5rTi
o8. from 0.21 to 0.23 for BaTi0
and RaTiO3-, the absolute value increases by about 0.02 from -0.11 to 8-δ -0.13, respectively. The upper limit of the oxygen gradient is considered to be 1/4 or 1/6 in absolute value (for example, JP-A-57-179654), and 0.0
An increase of 2 can be said to be significant.

An increase in oxygen gradient is not only obtained with SiO2, but also with GeO2, HfO2, zrO□, and ThO2. However, a similar tetravalent metal oxide, T
iO2 did not improve the oxygen gradient.

The existing form of SiO2, GeO3, etc. is limited to an amorphous and non-vitreous gel. Addition of quartz microcrystals, borosilicate glass, etc. did not improve the oxygen gradient.

The amount of addition of 8102, GeO□, etc. is 1 to 20 mol%, more preferably 4 to 12 mol%, per 1 mol of compound ATiO3. Addition of 0.5 mol% has no effect, 2 mol% has a significant effect, and addition of 5 mol% or more saturates the effect. Adding more than this will result in high resistance of the sensor, and the upper limit of the amount added should be 20 mol%, more preferably 12
expressed as mol%.

〔Example〕

(Preparation of compound) CaCO3, 8rO03, BaCO3, RaCO3 or a mixture thereof was mixed with an equimolar amount of TiO2 and calcined in air at 1100°C-1400°C for 1 hour.

In this process, a perovskite compound ATiO3- is produced. In addition, below, a compound is described except for the non-stoichiometric parameter l.

Separately, alkali metal-free silica colloid was prepared, and part of it contained Sio. 1,000 mol ppm (calculated as NaOH per mol) and 5% NaOH were added. In addition, an alkali metal-free silica xerogel (specific surface area: 150 m/l) was prepared.

An equimolar solution of water and ethanol is added to an anhydrous ethanol solution of Ge(OC2H5)4 with stirring while cooling with water. Ge(OC2H5)4 is hydrolyzed and converted to colloidal GeO3.

zr(S04)2. Hf(SO4)2. Th(SO4)
While heating the aqueous solution in step 2 to 80°C, add a plate and water to perform hydrolysis. The reaction solution is kept at 80°C for 12 hours to obtain colloids such as metazirconic acid (zrO2), HfO2, and Th021.

The calcined ATiO3 is pulverized, the above-mentioned colloids and gels are added thereto, it is molded into the shapes of the gas detection piece and temperature detection piece shown in FIG. 4, and it is baked in air for 1 hour.

ATiO3 is a material with extremely good sinterability, and is used as a sintering source)
When the temperature is 300°C or more higher than the calcination temperature, it becomes completely dense and becomes a thermistor. So 1100
A temperature sensing piece was calcined at 1400°C and sintered at 1400°C. Furthermore, no SiO2 or the like was added to the temperature detection piece.

In order to obtain the porosity required for the gas detection piece, it is necessary, not in the sense of limitation, that the difference between the firing temperature and the calcination temperature be 100°C or less. Alternatively, the following measurements were performed mainly on those fired at 1300°C and those whose calcination temperature and firing temperature were both 1300°C.

The form of 5io2 and the like in the sensor was an amorphous, non-vitreous gel, and when X-ray diffraction was performed, a broad peak was obtained. Even in the case of zrO2, which showed the sharpest peak, the average crystallite diameter determined from the peak was about 6OA. In addition, in SiO colloid with 5 mol% NaOH added, 81
At 0°, it becomes vitrified and loses its oxygen sensitivity.

As a comparative example, a gas detection piece was created using the same process to which the following substances were added. (1) Tio□ (rutile phase).

(particle size: approx. 1μ), (2) quartz powder (particle size: approx. 1μ), (3) borosilicate glass powder (810280wt%, B20818)
wt%, Na204wt%, others are Al2O3 and 20)
, (4) High melting point calcium glass powder (CaO64wt
%.

B20326 wt%, 8t0210 wt%). The amount added was 50 da per ATiO31F/.

As another comparative example, Ti02 calcined at 1200°C (
rutile phase) with 5 mol% of Na-free Si per 1 mol.
A gas detection piece was prepared by adding O2 colloid and firing at 1300°C.

(Protective coating) The surface of the obtained temperature detection piece or gas detection piece is coated with a thickness of 100 mm.
Cover with a microporous ceramic membrane to form a protective coating layer. This material may be the same type as the gel used, spinel (MgA60), mullite (A4
Si201B), cordierite (Mg2AI!4
Any porous ceramic may be used, such as 8150□8).

The combustible gas sensitivity of the compound ATiO3 is originally extremely low, and the detection error due to the coexistence of unreacted combustible gas is small. However, in order to eliminate this slight error, the protective coating layer of the gas detection piece is
q noble metal catalyst, here Pt1, is supported. The preferred range of supported amount is 3 to 5 per 1f of coating layer material.
It is 0q. By supporting the catalyst, unreacted combustible gas in the exhaust gas is removed, and detection errors due to the coexistence of unreacted combustible gas components are prevented.

(Oxygen sensitivity) Figures 1 and 2 show that 5rTio due to the addition of SiO2
shows changes in oxygen sensitivity. In each figure, 5rTio3 is calcined at 1200"C and heated to 1800"C.
It was fired at °C, and SiO3 was added as Na-free colloidal silica. Further, R3 represents the resistance value of the gas detection piece, and n represents the oxygen gradient, which is determined by changing the oxygen concentration from 1 to 10% (the same applies hereinafter).

From Figure 1, the addition of 5.0 mol% SiO2 increases the oxygen gradient by 0.02, the addition of 0.5 mol% has a small effect, but the addition of 2.0 mol% already has a significant effect. I know that it will happen. Furthermore, the resistance of the semiconductor increases as the amount of 5IO2 added increases. Next, the change in oxygen gradient at 600°C and the change at 700°C are very similar. Also 800″C
The results at 700°C were almost the same as those at 700°C, so illustration is omitted.

In other words, the improvement of the oxygen gradient is an effect that occurs over a wide temperature range.

Moving on to Figure 2, the resistance value (R3) of the semiconductor is R5=
It can be seen that this can be expressed by K-Po (ni oxygen gradient).

Table 1 shows a summary of the effect of adding SiO2.

The following can be seen from Table 1.

(1) If sio is replaced with TiO2 or quartz powder,
Oxygen sensitivity is not improved.

(2) In the case of glasses such as borosilicate glass or those made by adding a large amount of Na to SiO2 colloid and vitrifying them, the oxygen sensitivity is rather reduced.

(3) Adding 810□ to rio does not improve oxygen sensitivity.

(4) Improvement of oxygen sensitivity is caused by CaTiO3 and BaTio3.
It can also be obtained for ゜RaTiO3.

Improvement in oxygen gradient can be obtained not only with SiO3 but also with GeO2, ZrO2, fO□, and ThO2 (Table 2). However, the effect obtained is 8i0
~GeO-ZrO>Hf02-zTh02).

(Characteristics near the equivalence point) In Figure 3, the atmosphere is changed to λ = 0.99 with a period of 6 seconds for each 3 seconds.
It shows the change in resistance value when switching between 1.01 and 1.01 exhaust gas. The semiconductor used was calcined at 1200°C13
SrTiO3 calcined at 00°C with 5 mol% Sio
. In addition, there is

In contrast to the strong P-morphism in the lean burn region, λ = 1
The change in resistance value in the vicinity is extremely small l/A0. Qualitatively, this is the same for CaTlo 3, and the same holds true even when the firing conditions are changed.

BaTi0 and RaT s Oa are n-type semiconductors, and when increasing the number of people from 0.99 to 1.01, the resistance value increases from 100 to 10.
It increased about 00 times.

(Structure of sensor) The structure of the sensor will be explained with reference to FIGS. 4 and 5. In the figure, (2) is a six-large tube base made of alumina, and a ceramic tube (4) with a built-in heater is attached to its tip. This ceramic tube (4) is equipped with a membrane heater (6) made of tungsten, platinum, etc. inside, and is used to partially heat the gas detection piece (8) and temperature detection piece 00 to 100 degrees. . Regarding the heater, please refer to the membrane heater (6) shown in the figure.
), various other types can be used.

A gas detection piece (8) and a temperature detection piece α1 are provided in the recess between the base body (2) and the ceramic tube (4) via the threshold portion (2). This gas detection piece (8) was used for the above measurements. On the other hand, the temperature detection piece 00 is a thermistor made by densely sintering the same type of semiconductor as the gas detection piece.

Here, the structure of the gas detection piece (8) will be explained in more detail with reference to FIG. In the sintered body α of the compound ATiO3,
A pair of noble metal electrodes 0Q, α mark are buried, and the whole thickness is 10
It is covered with a protective coating layer of about 0 μm to form a gas detection piece (8).

Note that the temperature detection piece (10) may be constructed in the same manner as the gas detection piece (8) except for the point of denseness.

Returning to FIGS. 4 and 5, (a) is a metal fitting for attaching the sensor to the exhaust pipe of an automobile engine or the combustion chamber of a stove, boiler, or the like. Again (c).

Phosphorus is the lead pin connected to the membrane heater (6), (c), (
g) is the lead pin connected to the gas detection piece (8), Oa,
弼 is a lead bottle connected to the wetness detection piece 00.

(Supplementary note) Generally, the characteristics of perovskite compounds are that they are insensitive to substitution, and for ATiO3, about 10 mol % of the element or Ti element may be substituted with other elements.

[Effects of the Invention] According to the present invention, the oxygen sensitivity of the exhaust gas sensor can be increased and the detection accuracy can be improved.

[Brief explanation of drawings]

Figures 1 to 3 are characteristic diagrams of the exhaust gas sensor of the example, and Figure 4
The figure is a perspective view with a partial cutout of the exhaust gas sensor of the embodiment, FIG. 5 is a longitudinal sectional view thereof, and FIG. 6 is a sectional view of a gas detection piece used in the embodiment. (2)... Base body, (4)... Ceramic tube, (6
)...Membrane heater, (8)...Gas detection piece, α
Q...Temperature detection piece. Figure 1 005 2.0 5.01011 015sio2'r6 Figure 2 “1 02 (”/J Procedural amendment (voluntary) December 28, 1982

Claims (4)

[Claims] (1) Using the change in resistance of the perovskite compound ATiO_3_-_δ, where A is at least a member of the group consisting of Ca, Sr, Ba, and Ra, and δ represents a non-stoichiometric parameter. In the exhaust gas sensor, the compound ATiO_3_-_δ contains 1 to 2 per mole of the compound ATiO_3_-_δ.
0 mol% of SiO_2, GeO_2, ZrO_2, H
An exhaust gas sensor characterized in that an amorphous/non-vitreous gel that is at least a member of the group consisting of fO_2 and ThO_2 is added. (2) In the exhaust gas sensor according to claim 1, the A element of the compound ATiO_3_-_δ is Ca and S.
An exhaust gas sensor characterized by being an element that is at least a member of the group consisting of r. (3) The exhaust gas sensor according to claim 1 or 2, wherein the added gel is SiO_2 gel. (4) The exhaust gas sensor according to claim 2 or 3, wherein the amount of the gel added is 4 to 12 mol % per 1 mol of the compound ATiO_3_-_δ.