JP2544144B2 - Gas sensor and manufacturing method thereof - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a gas sensor using a change in resistance value of a metal oxide semiconductor and a method for manufacturing the gas sensor. The invention particularly relates to improving the temperature dependence of gas sensors.

[Prior Art] As a problem of a metal oxide semiconductor gas sensor, it is known that the influence of ambient humidity is large. That is, the resistance value of the gas sensor changes in accordance with the fluctuation of the ambient humidity, especially the absolute humidity, and reduces the gas detection accuracy. These are well known.

The humidity dependency of the sensor is particularly remarkable when the heating temperature of the sensor is made relatively low and a gas such as CO is detected. This is because the heating temperature of the sensor is low and the influence of humidity becomes particularly large.

Here, the related prior art is shown. JP-A-50-56,2
No. 96 discloses a CO sensor in which 1 wt% or less of chlorine is added to SnO ₂ . According to this publication, the addition of chlorine enhances the sensitivity to CO, but the effect on humidity dependence has not been examined.

[Problem of the Invention] An object of the present invention is to suppress humidity dependency of a gas sensor.

[Structure of the Invention] In the present invention, fluorine in an amount not less than the amount of impurities is added to the metal oxide semiconductor of the gas sensor. Note that it is rare to use a fluoride as a starting material for a metal oxide semiconductor, and the impurity concentration of fluorine in the metal oxide semiconductor is extremely low.

The timing and form of addition of fluorine are arbitrary, but they are preferably added after the preparation of the metal oxide semiconductor. When added to a metal oxide semiconductor before adjustment, for example, a sol of metal hydroxide, fluorine will be lost in the subsequent firing step.

Fluorine is preferably added as a simple substance of fluorine or a nonmetallic compound of fluorine. As is well known, the characteristics of metal oxide semiconductors are sensitive to trace amounts of metal impurities.
Therefore, when fluorine is added as a metal compound, the influence of metal impurities added at the same time becomes a problem. Of course, it is known to add a noble metal catalyst such as Pd, Pt, Ir, Ru, Au to the metal oxide semiconductor of the gas sensor. The addition of the noble metal catalyst is effective for improving the response characteristics of the sensor and adjusting the relative sensitivity. Therefore, fluorine may be added at the same time as these noble metal catalysts. It is also known to add a time-dependent inhibitor such as vanadium V or rhenium Re to the metal oxide semiconductor. If you add something like this,
Fluorine may be added at the same time as these.

The effect of fluorine lies in suppressing the humidity dependency of the gas sensor. This effect occurs for various gases such as CO, hydrogen, ethanol, isobutane and methane. The inventor estimated the mechanism for improving the humidity dependence by fluorine as follows. That is, the added fluorine replaces the surface hydroxyl group of the metal oxide semiconductor. The main cause of humidity dependency of the sensor is the surface hydroxyl group, and when the surface hydroxyl group decreases, the humidity dependency also decreases.

When fluorine is added, the sensitivity to CO is also improved and the dependence of sensor output on CO concentration is also improved. Fluorine also improves the response characteristics of the sensor at low temperatures, especially the response characteristics to CO. Therefore, fluorine is particularly excellent in improving the detection performance for CO.

The preferable addition amount of fluorine is 0.5 to 100 mgr in terms of fluorine atom per ₁ gr of SnO ₂ , and the more preferable addition amount is 2
~ 60 mgr.

The type and preparation conditions of the metal oxide semiconductor used, and the shape and structure of the gas sensor are arbitrary. Examples will be described below by taking the case of using SnO ₂ as the metal oxide semiconductor as an example.

[Example] Terminology In the examples, the amounts of additives are shown by the following terms. The amount of addition is shown in terms of atoms or simple substance, and addition of 10 mgr per 1 gr of SnO ₂ is 1%. For example, addition of 1% of fluorine means that 10 mg of fluorine in terms of atoms was added to 1 gr of the metal oxide semiconductor.

Adjustment of Gas Sensor SnCl ₄ aqueous solution was neutralized with ammonia to obtain stannic acid sol. Water was added to the sol and the mixture was centrifuged and washed until silver ions could not be detected with a silver nitrate test paper. The sol after washing is dried and pyrolyzed in air at 600 ° C for 1 hour.
₂

The obtained SnO ₂ was ground with a ball mill for 4 hours, an aqueous solution of HF was added to the SnO ₂ powder, and the mixture was left for 1 hour. Then 200 ℃
The powder was dried at 450 ° C. and further heated in air at 450 ° C. for 1 hour to support fluorine on SnO ₂ . Fluorine mainly exists as fluorine ions on the surface of the SnO ₂ particles. In the embodiment, the amount of fluorine added is 0.1%, 0.25%, 0.5%, 2%, 3
% Of 5 In the following, the amount of fluorine added is 0.5% unless otherwise specified.

The SnO ₂ after the addition of fluorine was pulverized again in a ball mill for 4 hours, and a noble metal catalyst was added to enhance gas sensitivity at low temperature. The noble metal catalyst may not be added. Here, 0.5% Pd was used as a catalyst, SnO ₂ was impregnated with a Pd aqua regia solution, and after pyrolyzed for 1 hour at 600 ° C. Pd is Au, P
It can be changed to any noble metal such as t, Rh, Ir and Re.

After the addition of the noble metal catalyst, the material was ground in a ball mill for 4 hours, then coated on the surface of an insulating pipe, impregnated with a silica sol binder, and sintered at 550 ° C. for 10 minutes to obtain a gas sensor. The shape of the sensor is such that a pair of gold electrodes are printed on the surface of an insulating pipe and a SnO ₂ film is provided. Note that SnO ₂
Has a film thickness of about several tens of μm. A coil-shaped heater was placed inside the pipe so that the sensor could be heated. The shape of this sensor is the gas sensor “TGS71” of the applicant.
It is well known as "1". It is not necessary to use a binder such as silica sol.

A supplement will be added regarding the conditions for adding fluorine. The inventor dipped the dried stannic acid sol in a 2 wt% concentration HF aqueous solution, baked it at 600 ° C. for 1 hour, and made SnO ₂ . Obtained SnO ₂
No fluorine could be detected by elemental analysis of. Therefore, fluorine is added when SnO ₂ is obtained. Regarding the points other than this, the timing and method of addition of fluorine are arbitrary.
For example, fluorine may be added after sintering the sensor, or may be dissolved in an organic solvent instead of the water solvent. Fluorine may be added, for example, by exposing SnO ₂ powder to an HF gas stream.

Fluorine was added as HF, but other addition forms may be used. A preferred addition form is HF or an ammonium salt of HF,
A non-metal compound of fluorine such as an HF salt of an amine compound, or a fluorine salt of a noble metal catalyst when a noble metal catalyst is added. When a salt of fluorine and a metal other than this is added, the effect of the added metal becomes a problem. Therefore, such an addition form is not preferable. For example, SnO ₂ with Bi (N
When impregnated with an aqueous solution of O ₃ ) ₃ or Pb (NO ₃ ) ₂ and added about 3 mol% of Bi or Pb, the humidity dependency was significantly deteriorated.
This is an example in which no fluorine is added, but if fluorine is added as BiF ₃ or PbF ₂ etc., the humidity dependence of the sensor will be worse than in the case of adding HF.

After the sensor was completed, the fluorine content was measured by elemental analysis. Table 1 shows the fluorine content. In the table, the aqueous solution concentration indicates the concentration of the HF aqueous solution used for the addition, the F ⁻ concentration indicates the HF concentration as the concentration of F ⁻ ions, and the fluorine content indicates the amount of fluorine in SnO ₂ . Note that the fluorine content is 0.5 wt% with a 1% HF aqueous solution when all of the added HF is converted to SnO _2.
%. Further, the fluorine content at the time of baking at 450 ° C. after the HF treatment was 0.7% when the HF aqueous solution concentration was 2 wt%.
Furthermore, at 210 ° C., fluorine in SnO ₂ is stable, and it is considered that the fluorine content does not change with time even if a sensor is used.

Characteristics of gas sensor As for the sensor characteristics, the heating temperature of the sensor is evaluated at 210 ° C in principle, and the result is shown as an average value of 4 to 8 sensors. All measurements were performed in air.

FIGS. 1 and 2 show the humidity dependence of the sensor. The horizontal axis is
The CO concentration is expressed in ppm, and the vertical axis shows the resistance value R of the sensor under each condition and the resistance value of the sensor in CO 100ppm at 20 ° C R.H65%.
Shows the ratio R / Ro with Ro. Humidity condition is -10 ℃ R.H10
0%, 20 ℃ R.H65%, 50 ℃ R.H65%. As is well known, the temperature dependence of the sensor is small, and the difference in sensor resistance due to these temperature and humidity conditions is largely due to the humidity dependence of the sensor, particularly the absolute humidity dependence of the sensor.

Figure 1 shows the results when 0.5% of fluorine was added.
The figure shows the results of a comparative example in which no fluorine was added. In the embodiment, the humidity dependency is small, and for example, when CO of 200 ppm is detected, CO can be detected in the range of 60 to 700 ppm even if the humidity changes. On the other hand, when 200 ppm of CO is detected in the comparative example, the actual detected concentration fluctuates within the range of 10 ppm to 2000 ppm due to fluctuations in humidity.

Table 2 shows the humidity dependence of the sensor. A comparative example was examined in which chlorine was added instead of fluorine, but the humidity dependence could not be improved. The action of fluorine is considered to be an inherent property of fluorine, not a general property of halogen.

Table 2 Humidity Dependency (CO) Sample Humidity Dependence * F 0% * 7 F 0.1% 3.5 F 0.25% 3.2 F 0.5% 3.0 F 2% 3.0 F 3% 3.0 Cl0.5% * 7 ** For example, change the C1 0.5% system to fluorine,
A sample of chlorine SnO ₂ supported using HCl solution is shown.Humidity dependence is the ratio of the resistance value at −10 ° C. R.H 100% to the resistance value at 100 ° C. R.H 65% for 100 ppm CO. Indicates. The measurement temperature is 210 ° C.

Addition of fluorine also suppresses humidity dependence on other gases. Since CO is mainly detected at 210 ° C, the results at 300 ° C suitable for detecting hydrogen are shown in Table 3. 1 for humidity dependence
Resistance value at -10 ℃ R.H100% against hydrogen of 000ppm, 5
The ratio with the resistance value at 0 ° C R.H 65% is shown.

Table 3 Humidity Dependence (H ₂ ) Sample Humidity Dependence * F 0% * 3.5 F 0.1% 2.5 F 0.25% 2.2 F 0.5% 2.0 F 2% 2.0 F 3% 2.0 Cl0.5% * 4 * Meaning of data The measurement method is the same as in Table 2, and the measurement temperature is
300 ° C.

The addition of fluorine improves the sensitivity to CO at low temperatures, for example 210 ° C., and the CO concentration dependence of the sensor output. And at this temperature, the sensitivity to gases such as ethanol and hydrogen is
It does not change even if the fluorine is changed. The results are shown in Table 4. The measurement conditions and the like are the same as those in Table 2. In the table, the gas sensitivity to CO indicates the ratio between the resistance value in clean air and the resistance value of CO 100 ppm. Also, the gas sensitivity to ethanol and hydrogen is CO100p.
Shows the ratio between the resistance value in pm and the resistance value in 100 ppm ethanol or hydrogen. The smaller this ratio, the higher the relative sensitivity to CO and the smaller the relative sensitivity to ethanol and hydrogen. In addition, the slope shows the gas concentration dependence of the sensor output, and CO100-300
It was measured in the ppm range. The slope indicates the value of n when LogR = K−n · Log (Pco) K is a constant. Ambient temperature and humidity are all 20 ℃
R.H 65% (same below).

From the table, it can be seen that the addition of fluorine increases the sensitivity to CO and the gas concentration dependence of the sensor output to CO.

3 to 4 show the heating voltage dependence of the sensor resistance. The horizontal axis shows the voltage applied to the heater of the sensor, and the sensor temperature becomes 210 ° C when 5V is applied. The vertical axis represents the resistance value in each 100 ppm gas, based on the resistance value Ro to 100 ppm CO at a heater voltage of 5V. The temperature and humidity conditions are 20 ° C and 65%.

FIG. 3 shows the result of the sample to which 0.5% of fluorine foundation was added, and FIG. 4 shows the result of the sample without addition of fluorine. The addition of fluorine increases the heating temperature dependence of the sensor resistance.

Figure 5 shows the response characteristics to CO at a heating temperature of 210 ° C. The upper part of the figure shows the result of the comparative example in which no fluorine was added, and the lower part shows the result of the example in which 0.5% of fluorine was added. CO was injected at time 2 minutes, and then CO was removed. Example is better
Response to CO is prompt. The ambient temperature and humidity is 20 ℃,
65%.

Although the characteristic of the gas sensor alone is shown here, this is different from the characteristic of the actual gas detection device. For example, changes in absolute humidity are caused by changes in saturated vapor pressure due to changes in temperature,
Can be separated into fluctuations in relative humidity. If temperature fluctuations are detected and compensated by a thermistor or the like, the humidity dependency of the gas detection device is reduced.

In addition, here, the results regarding a specific metal oxide semiconductor are shown, but the same applies when other metal oxide semiconductors are used. Further, the structure of the sensor and the use of additives other than fluorine are optional.

[Advantages of the Invention] In the present invention, the humidity dependency of the gas sensor can be suppressed.

[Brief description of drawings]

1 is a characteristic diagram of the embodiment, FIG. 2 is a characteristic diagram of a conventional example, FIG. 3 is a characteristic diagram of the embodiment, FIG. 4 is a characteristic diagram of the conventional example, and FIG. 5 is a characteristic diagram of the embodiment. .

Claims (8)

(57) [Claims] 1. A gas sensor using a metal oxide semiconductor, the resistance value of which changes with gas, wherein fluorine is added to the metal oxide semiconductor. 2. The gas sensor according to claim 1, wherein the metal oxide semiconductor is SnO2. 3. The gas sensor according to claim 2, wherein the amount of fluorine added to the SnO2 is 0.5 to 100 mgr in terms of fluorine atom per 1 gr of SnO2. 4. The gas sensor according to claim 3, wherein the amount of fluorine added to the SnO2 is set to 0.5 to 50 mgr in terms of fluorine atoms per 1 gr of SnO2. 5. A method of manufacturing a gas sensor using a metal oxide semiconductor, the resistance value of which changes with gas, comprising the step of adding fluorine to the metal oxide semiconductor. 6. The fluorine as the at least one member of the group consisting of fluorine alone and a non-metallic compound of fluorine,
The method for manufacturing a gas sensor according to claim 5, wherein the gas sensor is added to the metal oxide semiconductor. 7. The method of manufacturing a gas sensor according to claim 6, wherein the metal oxide semiconductor is SnO2, and the fluorine is added to SnO2 as a solution of a non-metallic compound of fluorine. 8. The solution of fluorine is an aqueous solution of a non-metallic compound of fluorine, and the concentration of fluorine in the aqueous solution is 0.2 mol / 1 to 12 mol / 1 when converted to the concentration of fluorine atoms. A method of manufacturing a gas sensor according to claim 7.