JP3050696B2 - Oxidizing gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a sensor for oxidizing gas such as NOx and ozone.

[0002]

2. Description of the Related Art The inventors have proposed an oxidizing gas sensor using a thick or thin WO3 film (Japanese Patent Laid-Open No. 4-65,662).
issue). This sensor detects NOx and ozone, for example, detects NOx in diesel exhaust gas, and uses it for controlling the introduction of outside air into the cabin of an automobile. Ozone is detected and used for controlling an ozone deodorizer. Or also N from the burner
Ox is detected and used for combustion control.

[0003] Next, the inventor proposed to add an additive such as Re, S, Pb, and Ru in order to improve the stability over time of the WO3 sensor or to improve the NOx sensitivity and response speed. No. 2-269768, 1990
(Application on 0/8).

[0004] However, these oxidizing gas sensors have extremely high resistance, and have a resistance value of just under 1 MΩ even in air. The resistance of these sensors increases when they come in contact with NOx or ozone.
It reaches about MΩ. Therefore, it is difficult to measure the resistance value itself. Next, an oxidizing gas sensor using WO3 shows almost no sensitivity to ethanol, isobutane, CO and the like. However, it shows a slight sensitivity to hydrogen. For this reason, there is hydrogen as an interfering gas, and there is a problem in that the detection accuracy is reduced due to hydrogen coexisting with NOx in the exhaust gas or hydrogen generated together with ozone in the ozone generator.

Another prior art is disclosed in Japanese Patent Application Laid-Open No.
No. 69,546 proposes to add 40 to 5% by weight of WO3 to 60 to 95% by weight of In2 O3. The purpose of adding WO3 here is to use low-resistance In2O3.
The purpose of the present invention is to increase the sensitivity to a flammable gas such as isobutane or to improve the concentration dependency to a flammable gas for other purposes. The measurement temperature here is 450 ° C.

[0006]

The object of the present invention is (1) low resistance, (2)
An object of the present invention is to provide a sensor for an oxidizing gas which is less hindered by hydrogen.

[0007]

An oxidizing gas sensor according to the present invention is a WO sensor.
3 to 50% by weight and a mixture of 5 to 50% by weight of In2 O3 as a main component. Here In2O3
The role of (1) is to reduce the resistance value of WO3 to make it easier to handle, and (2) to reduce the hydrogen sensitivity and to reduce the hydrogen coexisting with NOx and ozone in diesel exhaust gas and gas from an ozone generator. To reduce the impact.

The decrease in sensitivity to NOx and ozone due to the addition of In2O3 is caused by the fact that the In2O3 content is 30% by weight (WO3
(0% by weight), it is extremely small, and is almost equivalent to a gas sensor without addition of In2O3. In2O3 content of 40
% By weight (WO3 60% by weight), NOx and ozone sensitivity are slightly reduced, and the content of In2O3 is reduced to 50% by weight.
Then, NOx and ozone sensitivity are greatly reduced. Further, the content of In2O3 is reduced to 60% by weight (WO3 40% by weight).
Then, the sensitivity is significantly reduced. from these things,
The composition of the main component of the sensor is as follows.
n2O3 5 to 50% by weight, preferably WO3 90 to 50% by weight.
55% by weight, 10 to 45% by weight of In2O3, more preferably 80 to 55% by weight of WO3, 20 to 45% by weight of In2O3
% By weight, most preferably 80 to 65% by weight of WO3, In
2O3 20 to 35% by weight.

WO3 has low sensitivity to flammable gas, and only flammable gas has a problem with hydrogen sensitivity. For example, sensitivity to ethanol and isobutane is originally low. Hydrogen sensitivity poses a problem as an effect of hydrogen coexisting in exhaust gas when, for example, NOx in diesel exhaust gas is detected and outside air introduction control of an automobile is performed. Similarly, there is a problem in measuring the ozone concentration and controlling the ozone generator. In2O3
Does not reduce the sensitivity to hydrogen and does not increase the sensitivity to gases such as ethanol.

WO3 can be used as either a thick film or a thin film. However, it is preferable to use a thick film having extremely high resistance, low resistance and little influence of accumulation of poisoning substances. For example, when used for outside air introduction control of an automobile, a thick film, which is relatively less affected by poisons such as carbon accumulated in the sensor, is preferable. Even when a WO3 thick film is used, the resistance value in air (for example, 300 [deg.] C.) reaches just under 1 M [Omega]. Touching the oxidizing gas further increases the resistance value of the sensor, making it difficult to measure the sensor resistance. On the other hand, when In2O3 is added, the resistance value of the sensor decreases to about 1/10,
It becomes easy to measure the resistance value in Ox or ozone. In order to reduce the resistance value, the weight ratio of WO3 to In2O3 is 95: 5 to 50:50, preferably 80:20 to 5:50.
0:50.

In the oxidizing gas sensor of the present invention, WO
The main component may be a mixture of In 3 and In 2 O 3. In addition to this, Re, S, Pb, and Ru may be used to suppress changes over time.
And the like may be added.

[0012]

EXAMPLE Ammonium tungstate was heated to a maximum temperature of 50.
It was pyrolyzed at 0 ° C for 1 hour to obtain WO3. Next, InCl3
Is neutralized with aqueous ammonia, and the obtained precipitate is dried and calcined at a maximum temperature of 500 ° C. for 1 hour in air.
n2O3. In2O3 and WO3 were mixed at a predetermined weight ratio, pulverized in a ball mill, kneaded with an organic solvent, and printed on an alumina substrate. After printing, it was baked at 700 ° C. for about 30 minutes to obtain a gas sensor mainly containing a mixture of WO 3 and In 2 O 3. A pair of electrodes were printed on the alumina substrate so that the resistance value of a mixture of WO3 and In2O3 could be measured, and a heater was provided so that the sensor could be heated to about 300C. The shape of the printed mixture film of WO3 and In2O3 is a rectangular shape of 0.6 mm x 1.2 mm, and the film thickness is about 10 m.

An aqueous solution of Re nitrate is impregnated into a film of a mixture of WO3 and In2O3 after firing, and dried at 650.degree.
After heat treatment for a minute, the oxide of Re was carried on WO3 or In2O3. The amount of Re added is about 1 atm% in atomic ratio to the total amount of tungsten atoms and In atoms. The addition of Re is for improving the stability over time of the sensor, and may not be particularly added, and S, Pb, Ru, or the like may be added instead of Re.

Although a thick film sensor of a printing type is shown here, a thin film of a mixture of WO 3 and In 2 O 3 may be used, and the shape is arbitrary. It may be a type sensor.

The obtained gas sensor of a mixture of WO 3 and In 2 O 3 was heated to 300 ° C. for one week to remove the drift of the initial characteristics, and then the sensitivity to NOx, hydrogen, ozone and the like was measured. The measurement was performed by arranging various kinds of gas sensors having different compositions in a test tank and injecting gas in a batch system. Since it is difficult to accurately measure the concentrations of NOx and ozone, the concentrations of NOx and ozone are approximate values. NO
In order to prevent the influence of concentration fluctuations of x and ozone, various types of gas sensors were simultaneously placed in the test tank, and the sensitivity was measured at the same time. As a result, although the concentrations of NOx and ozone were not accurate, all the sensors were exposed to the same concentration of NOx and ozone, and the resistance values were measured.

1 to 4 show the sensitivity to about 1 ppm of NO2, about 2 ppm of NO, and 1000 ppm of hydrogen. The measurement temperature is 300 ° C. FIG. 1 shows the characteristics of a gas sensor whose main components are 70% by weight of WO3 and 30% by weight of In2O3, and FIG. 2 shows the characteristics of 60% by weight of WO3 and 40% by weight of In2O3.
5 shows the characteristics of a gas sensor whose main component is a mixture of weight%. FIG. 3 shows the characteristics of a gas sensor containing 50% by weight of WO3 and 50% by weight of In2 O3, and FIG. 4 shows the characteristics of a gas sensor containing no In2 O3. In each gas sensor, about 1 atm% of Re is added in an atomic ratio to the total of tungsten atoms and In atoms. The added Re is an inhibitor of change with time, and has little effect on the sensitivity and the resistance value of the sensor.

As apparent from FIGS. 1 to 4, In2O3
The resistance value of the sensor decreases due to the addition of
The resistance value in m decreases from about several tens MΩ in WO3 alone to about 1 MΩ. As a result, it is possible to accurately measure the resistance value of the sensor. In addition, plain WO3
Has a hydrogen sensitivity as shown in FIG. This causes a decrease in detection accuracy due to hydrogen in exhaust gas when detecting NOx in diesel exhaust gas. Hydrogen sensitivity is
It is also a problem when measuring the NOx concentration in the exhaust gas from the boiler. As apparent from FIGS. 1-3, In2O3
Hydrogen sensitivity is extremely low in a gas sensor with
Decrease in detection accuracy due to coexisting hydrogen is not a problem. Note that all of the sensors have extremely low sensitivity to ethanol, isobutane, and CO, and do not affect the detection accuracy.

Table 1 shows changes in sensor characteristics due to the addition of In2O3. Note that Sample 1 and Sample 7 in the table are comparative examples, and the manufacturing conditions of the sensor are as described above, and the sensor is common to those in FIGS.

[0019]

[Table 1] Characteristic sample of WO3 / In2O3 sensor WO3 / In2O3 Resistance value (MΩ) NO2 sensitivity NO sensitivity Hydrogen sensitivity No. (weight ratio) (in NO2ppm) (about 1ppm) (about 2ppm) (1000ppm) 1 100 / ... 25 5 44 2 2 90/10 10 5 40 1.6 3 80/20 5 5 40 1.5 4 70/30 2.5 5 44 1.15 60/40 1.56 30 1.2 6 50 / 50 0.35 5 1.2 7 40/60 0.12 3 1.3 * Sensitivity is indicated by the ratio of the resistance value to that in the air. For NOx, the increase in the resistance value is the sensitivity, and for H2, the resistance is the resistance. The decrease in the value was regarded as the sensitivity. * The measurement temperature was 300 ° C., and all carried about 1% of Re in atomic ratio with (W + In).

As is apparent from Table 1, the resistance value decreases with the amount of In2O3 added. In the sensor of Sample 3 to which 20% by weight of In2O3 was added, the resistance value in NO2 ppm was 1%.
0 MΩ or less, which is a sufficiently practical range. For this reason, the lower limit of the amount of In2O3 added is 95% by weight of WO3, and 5% by weight of In2O3, preferably 90% by weight of WO3.
10% by weight of In2O3, more preferably WO3
The content was 20% by weight of In2O3 with respect to 80% by weight. Then N
The sensitivity to O2 is not significantly affected by the amount of In2O3 added, and is up to 50% by weight of In2O3. Almost constant. However, in Sample 7, to which 60% by weight of In2O3 was added, the NO2 sensitivity was drastically reduced. On the other hand, NO sensitivity is In2O
It is almost constant up to 30% by weight, and begins to decrease at 40% by weight of In2O3, and sharply decreases at 50% by weight of In2O3. From this, the upper limit of the amount of In2O3 added is WO3.
50% by weight or more and 50% by weight or less of In2O3, preferably 55% by weight or more of WO3 and 45% by weight or less of In2O3, more preferably 65% by weight or more of WO3 and In2O3.
O3 is 35% by weight or less. Hydrogen sensitivity decreases with the addition of In2O3, and has little effect on the In2O3 content.

Although the characteristics are shown here for NOx, which is a representative of the oxidizing gas, almost the same characteristics were obtained for ozone. The sensitivity of the sensor to ozone is about 100
The maximum was reached at about ° C, but the response speed gradually decreased when the sensor was constantly kept at 100 ° C. Therefore, the temperature of the sensor is changed in a cycle of one minute, and heat cleaning is performed at 300 ° C. for 20 seconds.
Seconds characteristics were measured. When the ozone sensitivity is shown for the sensors in Table 1, the sensitivity to 0.1 ppm of ozone in the range of Sample 1 to Sample 5 is almost constant and about 10, and the ozone sensitivity of 0.1 ppm is 7 in the sensor of Sample 6. In the sensor of Sample 7, the ozone sensitivity was 5.

The temperature dependence of the sensor will be described. NOx
The sensitivity increased at the low temperature side, but the response performance decreased at the low temperature side. As the operating temperature was increased, the NOx sensitivity decreased and the hydrogen sensitivity increased. The optimal operating temperature taking these factors into account is 280 ° C. to 300 ° C., and the data at 300 ° C. is shown in FIGS.

[0023]

According to the oxidizing gas sensor of the present invention,
(1) Lowering the resistance of the sensor to facilitate the measurement of the resistance, and (2) reducing the hydrogen sensitivity to reduce NOx
Interference with hydrogen coexisting with the like can be reduced.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram of an oxidizing gas sensor according to an embodiment mainly containing a mixture of 70% by weight of WO3 and 30% by weight of In2O3.

FIG. 2 is a characteristic diagram of an oxidizing gas sensor according to an embodiment mainly containing a mixture of WO3 60% by weight and In2 O3 40% by weight.

FIG. 3 is a characteristic diagram of an oxidizing gas sensor according to an embodiment mainly including a mixture of 50% by weight of WO3 and 50% by weight of In2O3.

[FIG. 4] WO3 as a main component, without adding In2O3.
Characteristic diagram of conventional oxidizing gas sensor

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Hidemi Murayama 1-3-5 Senba Nishi, Minoh City Inside Figaro Giken Co., Ltd. (56) References JP-A-59-192950 (JP, A) (58) Survey Field (Int.Cl. ⁷ , DB name) G01N 27/12 JICST file (JOIS)

Claims (1)

(57) [Claims] (1) 95 to 50% by weight of WO3 and In2 O3
An oxidizing gas sensor mainly containing a mixture of about 50% by weight.