JP2517291B2 - Gas sensor - Google Patents

Description

Description: FIELD OF APPLICATION OF THE INVENTION The present invention relates to a gas sensor using a change in the resistance value of SnO ₂ .

[Prior Art] Problems in detecting a gas such as CO with a SnO ₂ gas sensor are as follows.

(1) Sensor characteristics are unstable over time.

(2) The influence of ambient humidity, especially absolute humidity, is large.

(3) Response speed to gas is low.

These are because the sensor sensitivity to CO increases at relatively low temperatures, so the heating temperature of the sensor must be kept low, and because SnO ₂ with high activity is used, materials that are unstable over time must be used. Related to things. The aging of the sensor and the dependency on humidity reduce the detection accuracy of CO and the like.

Here, the related prior art is shown. JP-A-50-56,29
No. 6 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 we have not examined the change over time and the dependence on humidity.

Further, JP-A-60-100,752-755, especially 60-100,753, discloses adding vanadium to SnO ₂ . It is shown that vanadium is added after the sensor is sintered by, for example, an impregnation method, and that the addition of vanadium can suppress the change with time of the sensor. Next, JP-A-62-9
No. 0530 shows that addition of Re to SnO ₂ can suppress the change with time.

[Problem to be Solved by the Invention] The problem to be solved by the present invention is to suppress the temporal change and humidity dependency of a gas sensor in detecting gas such as CO.

[Structure of the Invention] In the gas sensor of the present invention, fluorine and vanadium in an amount not less than the amount of impurities are added to SnO ₂ . The preferred addition amount is
In terms of atoms per SnO ₂ 1gr, fluorine is 0.5 to 100 mgr and vanadium is 0.3 to 10 mgr. More preferable addition amount of fluorine is ₂ to 60 mgr and vanadium per ₁ gr of SnO _2.
0.8 to 10 mgr, and the most preferable range of addition amount is SnO ₂
2 to 50 mgr fluorine and 0.8 to 5 mg vanadium per 1 gr
r.

The first effect of fluorine lies in suppressing the humidity dependence of the sensor on gases such as CO, H ₂ and ethanol. The second effect of fluorine is to increase the sensitivity to CO. Since the addition of fluorine hardly changes the sensitivity to gases such as hydrogen and ethanol, the relative sensitivity to CO is also improved. Third of fluorine
The effect of is to increase the dependence of sensor output on CO concentration. The fourth effect of fluorine is to improve the response characteristics of the sensor to gases such as CO. However, fluorine has the drawback of increasing the heating temperature dependence of the sensor output.

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 SnO ₂ . 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.

The effect of vanadium lies mainly in suppressing the aging of the sensor. Further, vanadium suppresses the heating temperature dependence of the sensor and suppresses the detection error due to the fluctuation of the heating temperature. In this way, the disadvantage of fluorine, that is, the increase in heating temperature dependence is compensated by the addition of vanadium. However, vanadium reduces the relative sensitivity to CO and the concentration characteristics of the sensor output to CO, and weakens the effect of fluorine on the humidity dependence.

The effects obtained by combining all of these are the suppression of aging of the sensor and the dependence on humidity, and the improvement of response characteristics.

The inventor examined using rhenium instead of vanadium. However, at the temperature suitable for CO detection, rhenium did not sufficiently suppress the change over time.

The amount added to SnO ₂ is shown by the following nomenclature. 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, 1% addition of vanadium means that 10 mgr of vanadium was added to SnO ₂ 1gr in terms of metal, and it corresponds to addition of 13.1 mgr in terms of V ₂ O ₅ .

[Example] Adjustment of gas sensor An aqueous solution of SnCl ₄ was neutralized with ammonia to obtain a 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 crushed with a ball mill for 4 hours, an aqueous solution of HF was added to the bulkiness of the SnO ₂ powder, and the mixture was left for 1 hour.
After that, the powder was dried at 200 ° 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 examples, the amount of fluorine added is 0.1%, 0.25%, 0.5.
%, 2%, 3%. 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 increase the gas temperature 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.

Then, SnO ₂ was pulverized again in a ball mill for 4 hours,
It was applied on the surface of an insulating pipe, impregnated with a silica sol binder, and sintered at 550 ° C for 10 hours 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. The film thickness of SnO ₂ is 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 known as the applicant's gas sensor "TGS711". A binder such as silica sol may not be used.

For the gas sensor after sintering, a solution of a vanadium compound (here, an aqueous solution of vanadium succinate) was dropped,
Vanadium was supported. Dry the loaded sensor,
It was heated to 450 ° C. for 1 hour to thermally decompose the vanadium compound into V ₂ O ₅ or the like. There is a concentration distribution in vanadium concentration,
High concentration is distributed on the sensor surface and low concentration is distributed inside the sensor. However, it is difficult to accurately measure the concentration distribution of vanadium, and the amount of vanadium added to the total amount of SnO ₂ is shown here. The addition amount of vanadium is 0.05%, 0.1
% And 0.2%. The effect of vanadium on the stabilization of aging characteristics increases with the addition amount. On the other hand, adverse effects such as deterioration of humidity dependence and reduction of CO sensitivity are not significantly different even if the amount of vanadium added is 0.1% or 0.2%. Therefore, the preferred range of vanadium addition is 0.03 to 1%,
More preferably 0.08 to 1%, still more preferably 0.08 to 0.5
%.

We will supplement the conditions for adding fluorine and vanadium. 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 . Fluorine could not be detected by elemental analysis of the obtained SnO ₂ . 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, or a vanadium fluorine compound. 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, for comparison, an aqueous solution of a nitrate of Bi or Pb was dropped into SnO ₂ and thermally decomposed to add about 3 mol% of these metals. When Bi or Pb was added, the humidity dependence of the sensor deteriorated significantly. Therefore, it is preferable to add fluorine as a non-metal compound or a simple substance, or as a fluorine compound of a noble metal catalyst or a fluorine compound of vanadium.

The addition method and the addition timing of vanadium are arbitrary. However, vanadium is preferably added after sintering the sensor. In this case, vanadium is added mainly by the impregnation method or by exposing the sensor to the vapor of the vanadium compound. This is because if vanadium is added after the sensor is sintered and the vanadium is biased to the surface of the sensor, the effect of suppressing the change over time is greater, and the humidity dependence is reduced, the relative sensitivity is lowered, and the gas concentration characteristics are lowered. This is because the adverse effects of the above are small.

The addition amount of vanadium was determined from the amount of the dropped solution. The fluorine content was determined by elemental analysis after the sensor was completed. Table 1 shows the fluorine content. In the table, the aqueous solution concentration shows the concentration of the HF aqueous solution used for the addition, and the fluorine content shows the amount of fluorine in SnO ₂ . If it should be noted all of fluorine in the HF aqueous solution used has moved to SnO _2, fluorine content in the SnO ₂ is HF concentration of 0.5% 1 wt%. After HF treatment, 45
At the stage of firing at 0 ° C, the fluorine content was 0.7% when a 2 wt% HF aqueous solution was used.

Table 1 Fluorine content HF aqueous solution concentration (wt%) Fluorine content (%) 0.4 0.1 1 0.25 2 0.5 10 2 23 3 Gas sensor characteristics As a general rule, the sensor characteristics are evaluated by setting the heating temperature of the sensor to 210 ° C, and 4 each Results are shown as the average of ~ 8 sensors. All measurements were performed in air.

FIG. 1 shows the results of the acceleration test with respect to changes with time. In this test, the sensor temperature was raised to 280 ° C,
By accelerating the change over time, the vertical axis in the figure shows the change in resistance value in CO 100 ppm, and is displayed based on the resistance value Ro of the sensor in CO 100 ppm after 5 days of manufacture. In the example in which 0.2% vanadium and 0.5% fluorine were added to SnO ₂ , the change with time is small. The change with time is large in the case of adding only 0.5% of fluorine without adding vanadium or the comparative example in which neither vanadium nor fluorine is added. From the figure, it is possible to suppress the change with time of the sensor by adding vanadium,
It can be seen that fluorine does not contribute to the suppression of change over time. Table 2 shows the characteristics over time when the sensor was heated to 210 ° C.

Table 2 Aging characteristics sample R / Ro (CO) R / Ro (EtOH) No addition of V and F * 0.50 0.82 F 0.5% * 0.42 0.68 V 0.05% F 0.5% 0.57 0.96 V 0.1% F 0.5% 0.62 0.98 V 0.2 % F 0.5% 0.75 0.92 V 0.2% F 0% * 0.88 0.84 Re1% * F 0% * 0.60 * * indicates comparative example, R / Ro is after production for 100ppm gas
The resistance value after 76 days based on the resistance value on the 13th day. Rhenium was added by dropping a solution of rhenium nitrate instead of the vanadium solution.

In the table, a larger amount of rhenium is added than vanadium, but the contribution to the improvement of the characteristics over time is small. In the following, the heating temperature of the sensor is set to 210 ° C in principle, and the characteristics after 13 days of production are described.

2 and 3 show the humidity dependence of the sensor for CO. The horizontal axis represents the CO concentration in ppm, and the vertical axis represents the ratio R / Ro between the sensor resistance value R under each condition and the sensor resistance value Ro in CO 100 ppm at 20 ° C. and 65% R.H. Humidity conditions are -10 ℃ R.H100%, 20 ℃ R.H65%, 50 ℃ R.H65%.
The person 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.

FIG. 2 shows the result in a comparative example in which neither fluorine nor vanadium was added, and FIG. 3 shows the result when 0.5% fluorine and 0.2% vanadium were added. The humidity dependency is small in the example, and the humidity dependency is large in the comparative example.

 Table 3 shows the humidity dependence of the sensor for CO.

Table 3 Humidity Dependence (CO) Sample Humidity Dependence * No addition of V and F * 7 F 0.5% * 3.0 V 0.05% F 0.5% 3.2 V 0.1% F 0.5% 3.4 V 0.2% F 0.5% 3.5 V 0.2 % F 0.1% 4.5 V 0.2% F 0.25% 3.8 V 0.2% F 2% 3.2 V 0.2% F 3% 3.2 V 0.2% F 0% 10 V 0.2% Cl 0.5% * 10 ** indicates a comparative example, The Rel% system shows a sample in which Re is changed to vanadium, the V 0.2% and Cl 0.5% system is changed to fluorine, and a sample in which chlorine is carried on SnO ₂ by using an HCl solution is shown. Is -10 ℃ R.H1 against 100ppm CO
The ratio between the resistance value at 00% and the resistance value at 50 ° C R.H65% is shown. A comparative example in which chlorine was added instead of fluorine was examined, but the humidity dependency could not be improved. The action of fluorine is considered to be an inherent property of fluorine, not a general property of halogen.

Table 4 shows the effect of vanadium and fluorine on the gas sensitivity. The measurement conditions and the like are the same as those in Tables 1 to 3. In the table, the gas sensitivity to CO indicates the ratio between the resistance value in clean air and the resistance value in CO 100 ppm. Also, the gas sensitivity to ethanol and hydrogen is 100p for each resistance value in CO100ppm.
It expresses the ratio of pm to the resistance value in ethanol or hydrogen. The smaller this ratio, the higher the relative sensitivity to CO and the smaller the relative sensitivity to ethanol and hydrogen. The slope shows the gas concentration dependence of the sensor output, and was measured in the interval of CO100-300ppm.
The slope indicates the value of n when LogR = K−n · Log (Pco) K is a constant. Ambient temperature and humidity are all 20 ° C R.H 65% (the same applies below).

From the table, it is clear that the addition of fluorine increases the sensitivity to CO and the gas concentration dependence of the sensor output to CO, but the addition of vanadium weakens this effect.

4 to 6 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. 4 shows the result of the sample without addition of fluorine and vanadium, Fig. 5 shows the result of the sample with addition of 0.5% fluorine without addition of vanadium, and Fig. 6 shows the result of 0.2% vanadium and 0.5% fluorine. The result of the sample which added is shown. The addition of fluorine increases the heating temperature dependence of the sensor resistance, and the addition of vanadium decreases the temperature dependence. These results show that in the examples in which vanadium and fluorine were added,
It shows that the heating temperature dependence of the sensor is small and the detection error due to the fluctuation of the power supply voltage is small.

Fig. 7 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 neither fluorine nor vanadium was added, and the lower part shows the result of the example in which 0.5% fluorine and 0.2% vanadium were added. CO at 2 minutes
And then remove CO. The example responds to CO more quickly. This result is due to fluorine, and vanadium has nothing to do with the response characteristics. The temperature and humidity conditions are 20 ° C and 65%.

The addition of vanadium is preferably performed after SnO ₂ is sintered. This more effectively eliminates sensor aging,
This is because the side effect of vanadium on the humidity dependence is suppressed. Results when added by an impregnation method after sintering of SnO _2, and a result at the time of uniformly added to SnO ₂ before sintering, shown in Table 5.

* Both the on SnO ₂ of 0.5% fluorine, added with 0.2% Banadiumu, the impregnation method was impregnated with Banadiumu dropwise Banadiumu solution after sintering of SnO _2, the uniform addition of a material stage SnO ₂ With vanadium added, vanadium is evenly distributed in SnO ₂ , and the change with time shows the resistance value after 76 days based on the resistance value on the 13th day after production for 100ppm CO, and the humidity dependence is 100ppm CO Against −10 ℃ 100
CO, which indicates the ratio of the resistance value in% to the resistance value in 50% at 65%
Sensitivity represents the ratio of the resistance value in clean air to the resistance value in 100 ppm CO, and the slope is n when LogR = K-n-Log (Pco) K- is a constant.
Reveals a value.

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. Further, in the examples, only a gas sensor having higher sensitivity to ethanol than CO has been obtained. However, if an ethanol removal filter such as activated carbon is attached to the sensor, the sensitivity to ethanol can be suppressed.

Furthermore, although the explanation has been given here on the usage conditions for keeping the sensor temperature constant, in the case of detecting CO, ethanol, etc., a technology is known in which the temperature is applied to the sensor and the gas is detected from the transient phenomenon when the temperature changes. is there. Therefore, the gas may be detected while changing the temperature of the sensor. Further, although a specific sensor structure has been described here, it can be similarly implemented when another structure is used.

[Advantages of the Invention] According to the present invention, it is possible to suppress the aging of the gas sensor and the humidity dependency.

[Brief description of drawings]

FIG. 1 is a characteristic diagram of an embodiment, FIG. 2 is a characteristic diagram of a conventional example, FIG. 3 is a characteristic diagram of an embodiment, FIGS. 4 and 5 are characteristic diagrams of a conventional example, FIG. 6, and FIG. The figure is a characteristic diagram of the example.

Claims (2)

(57) [Claims] 1. A gas sensor using the change in the SnO ₂ in the resistance value, the SnO _2, characterized in that the addition of fluorine and Banadiumu gas sensor. _2. The addition amount of fluorine per SnO ₂ 1gr is 0.5 to 100 mgr in terms of fluorine atom, and the addition amount of vanadium per SnO ₂ 1gr is in the range of 0.3 to 10 mgr in terms of vanadium atom. The gas sensor according to claim 1.