JPS63317753A - Gas sensor and manufacture thereof - Google Patents

Abstract

PURPOSE:To suppress the humidity dependence of a gas sensor, in a gas sensor, wherein a metal oxide semiconductor whose resistance value is changed with gas is used, by adding fluorine to the metal oxide semiconductor. CONSTITUTION:Fluorine, which is more than the amount of impurities, is added to the metal oxide semiconductor of a gas sensor. The added metal oxide semiconductor is substituted with a surface hydroxide group. Since the main cause of the humidity dependence of the sensor is the surface hydroxide group, the surface hydroxide group is decreased by said substitution, and the humidity dependence is decreased. When the fluorine is added, sensitivity for CO is improved. Concentration dependence of the output of the sensor on CO is improved. The responses of the sensor at low temperature, especially the response to CO, are improved with the fluorine.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] 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 same.

The present invention particularly relates to improving the humidity dependence of gas sensors.

[Prior Art] It is known that a problem with metal oxide semiconductor gas sensors is that they are greatly influenced by ambient humidity.

In other words, the resistance value of the gas sensor depends on the ambient humidity, especially absolute, 9
changes with temperature fluctuations, reducing gas detection accuracy. These things are well known.

The humidity dependence of the sensor is particularly significant when heating the sensor to a relatively low temperature and detecting a gas such as CO. This is because the heating temperature of the sensor is low, so the influence of humidity becomes particularly large.

In addition, related prior art will be shown here. Japanese Unexamined Patent Publication 1986-56
．． No. 296 discloses a CO sensor in which 1 wt% or less of chlorine is added to 5 nOt. According to this publication, the addition of chlorine is said to increase the sensitivity to CO, but the effect on humidity dependence is not studied.

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

[Structure of the Invention] In the present invention, fluorine is added to the metal oxide semiconductor of the gas sensor in an amount greater than the amount of impurities. Note that fluoride is rarely used as a starting material for metal oxide semiconductors, and the impurity concentration of fluorine in metal oxide semiconductors is extremely low.

Although the timing and form of addition of fluorine are arbitrary, it is preferably added after the metal oxide semiconductor is adjusted.

When added to a sol of a metal oxide semiconductor, such as a metal hydroxide, before preparation, fluorine will be lost during the subsequent firing step.

Further, fluorine is preferably added as simple fluorine or a nonmetallic compound of fluorine. As is well known, the properties 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, the metal oxide semiconductor of the gas sensor includes Pd, Pt, Ir, Ru.
It is known to add a noble metal catalyst such as , Au or the like.

Addition of a noble metal catalyst is effective for improving the response characteristics of the sensor, adjusting the relative sensitivity, etc. Therefore, fluorine may be added simultaneously with these noble metal catalysts. It is also known to add a time-dependent deterioration inhibitor such as vanadium V or rhenium Re to the metal oxide semiconductor.

When such substances are added, fluorine may be added at the same time as these substances.

The effect of fluorine is to suppress the humidity dependence of the gas sensor. This effect occurs for various gases such as CO, hydrogen, ethanol, isobutane, and methane. The inventor estimated the humidity-dependent bookkeeping mechanism due to fluorine as follows. That is, the added fluorine replaces the surface hydroxyl groups of the metal oxide semiconductor. The main cause of the humidity dependence of the sensor is the surface hydroxyl groups, and as the surface hydroxyl groups decrease, the humidity dependence decreases.

Adding fluorine also improves the sensitivity to CO and also improves the concentration dependence of the sensor output on CO. 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 preferred amount of fluorine added is 0.5 to 100 Illgr, and the more preferred amount is 2 to 60 mgr in terms of fluorine atoms per SnSn0t1.

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

[Example] Associate woodcutter In the examples, the amounts of additives are indicated using the following nomenclature.

All additive amounts are shown in terms of atoms or simple substances, and S
The addition of lOn+gr per nO, 1gr is 1%.

For example, 1% fluorine addition means metal oxide semiconductor Igr.
This means that 10mgr of fluorine was added in terms of atoms.

Adjustment of gas sensor An aqueous solution of S nC14 was neutralized with ammonia to obtain a sol of stannic acid. Water was added to the sol, centrifuged, and washed until chloride ions could no longer be detected using silver nitrate test paper. The washed sol was dried and thermally decomposed in air at 600° C. for 1 hour to obtain SnO.

The obtained SnO was ground in a ball mill for 4 hours, and an aqueous solution of HF was added to the Snow powder and left for 1 hour. The powder was then dried at 200°C and further heated in air at 450°C for 1 hour to remove fluorine from SnO! It was carried by Fluorine exists mainly as fluorine ions on the surface of Sn0w particles. In the example, the amount of fluorine added was 0.1%,
There were five participants: 0.25%, 0°5%, 2%, and 3%. In the following, unless otherwise specified, the amount of fluorine added is 0.5%.

After fluorine addition, S n Ot was ground again in a ball mill for 4 hours, and a noble metal catalyst was added to increase the gas sensitivity at low temperatures. It is not necessary to add a noble metal catalyst. Here, 0.5% Pd was used as a catalyst, and Snow was impregnated with an aqua regia solution of Pd, and after drying, it was thermally decomposed at 600° C. for 1 hour. Pd is Au, Pt.

It can be changed to any noble metal such as Rh, Ir, or Re.

After adding the noble metal catalyst, the material was ground in a ball mill for 1 hour, then applied to the surface of an insulated pipe, impregnated with a silica sol binder, and sintered at 550° C. for 10 minutes to produce a gas sensor. The shape of the sensor is such that a pair of gold electrodes are printed on the surface of an insulated pipe and a 5nO7 film is provided.

Note that the film thickness of 5nOt is approximately several tens of μm. A coil-shaped heater was also placed inside the pipe to heat the sensor. This sensor configuration is known as the applicant's gas sensor "T G S 711". Note that a binder such as silica sol may not be used.

Please provide additional information regarding the conditions for adding fluorine. The inventor immersed the dried stannic acid sol in an HF aqueous solution with a concentration of 2 wt%, and after drying, baked it at 600° C. for 1 hour to obtain Snow. In elemental analysis of the obtained Snow, fluorine could not be detected. Therefore, fluorine is added at the stage when 5 nOt is obtained. Regarding other points, the timing and method of adding fluorine is up to the discretion of the user. For example, fluorine may be added after sintering the sensor, or it may be dissolved in an organic solvent instead of an aqueous solvent. Fluorine may also be added, for example, by exposing 5nOt powder to a HP air stream.

Fluorine was added as HP, but other forms of addition may be used. Preferred forms of addition include HP, ammonium salts of HF, HF salts of amine compounds, and other nonmetallic fluorine compounds;
Alternatively, when a noble metal catalyst is added, a fluorine salt of the noble metal catalyst is used. When a salt of fluorine and other metals is added, the influence of the added metal becomes a problem. Therefore, such an addition form is not preferable. For example, in Snow,
When an aqueous solution of Bi(N O3) 3 or Pb(NOa)t was impregnated and about 1% of 3nO Bi or Pb was added, the humidity dependence was significantly worsened. This is an example in which no fluorine is added, but if fluorine were added as B i F 3 or PbPe, the humidity dependence of the sensor would be worse than when it was added as HP.

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 I-HF aqueous solution used for addition, and the fluorine content indicates the amount of fluorine in 5 nOe.

Note that the fluorine content becomes 0.5 wt% in a 1% aqueous HF solution when all of the added HF is transferred to 5 nOt. Further, the fluorine content at the time of firing 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 5nOt is stable;
It is believed that the fluorine content does not change over time even with the use of the sensor.

0.4 0.1 1 0.252
0.5 Characteristics of Gas Sensor Sensor characteristics are evaluated assuming that the heating temperature of the sensor is 210° C., and the results are shown as the average value of each 4 to 8 sensors. Furthermore, all measurements were performed in air.

Figures 1 and 2 show the humidity dependence of the sensor.

The horizontal axis represents the COa degree in ppm, and the vertical axis represents the resistance value R of the sensor under each condition and the CC0 at 20°CR, 1165%.
It represents the ratio R/Ro with the resistance value R'o of the sensor in 100pp. Also, the humidity conditions are -10'CR, l-1100
%, 20°CR, H65%, 50°Cr1.065%
There were three parties. As is well known, the temperature dependence of the sensor is slight, and the difference in sensor resistance due to these temperature and humidity conditions is mostly due to the humidity dependence of the sensor, particularly the absolute humidity dependence of the sensor.

Figure 1 shows the results when 0.5% fluorine was added.
The figure shows the results of a comparative example with no fluorine added. In the example, humidity dependence is small; for example, when detecting 200 ppm of CO, CO can be detected in the range of 60 to 700 ppm even if the humidity fluctuates. On the other hand, 200 ppm in the comparative example
When CO is detected, the actual detected concentration varies from 10ppm to about 200ppm due to changes in humidity.

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

F0 type* 7 FO, 1% 3.5 FO, 25% 3.2 1015% 3.0 F2 type 3.0 F3 type 3.0 C1O, 5%* 7 ** indicates a comparative example, The CIo, 5% system is changed to fluorine, and ■I shows a sample in which chlorine is supported on Snow using a CI solution.The humidity dependence is the resistance value at -10℃R,H100% against 100 ppm CO. ,50°CR
, shows the ratio with the resistance value at H65%. The measured temperature is 2
The temperature is 10°C.

Addition of fluorine also suppresses the humidity dependence of other gases.

At 210°C, the main detection is co, so Table 3 shows the results at 300°C, which is suitable for hydrogen detection. Humidity dependence includes -10°CR, H10 for 11000pp of hydrogen.
The ratio of the resistance value at 0% and the resistance value at 50° C.R, H65% is shown.

FO%*3.5 FO11% 2.5 F025% 2.2 1015% 2.0 F2 type 2,O F3 type 2.0 CIo, 5%* 4 *For data quality and measurement method, see Table 2. Same, measurement temperature is 300℃.

Addition of fluorine improves the sensitivity to CO and the COa degree dependence of the sensor output at low temperatures, for example at 210°C. At this temperature, the sensitivity to gases such as ethanol and hydrogen does not change even when fluorine is added. The results are shown in Table 4.

The measurement conditions, etc. are the same as in Table 2. In the table, gas sensitivity to CO is the resistance value in clean air and GO100ppm.
Expresses the ratio to the resistance value inside. In addition, the gas sensitivity to ethanol and hydrogen is determined by the resistance value in C0I00ppIIl and each to
Expresses the ratio to the resistance value in oppm ethanol or hydrogen.

The smaller this ratio is, the higher the relative sensitivity to CO, and the lower the relative sensitivity to ethanol and hydrogen. The slope also indicates the dependence of the sensor output on gas concentration, with CO100 to 300 pp
It was measured in an interval of m. The slope is LogR=to-n-Lo
g(Pco) K indicates the value of n when it is a constant. The ambient temperature and humidity are all 20° C.R and 65% H (the same applies below).

Table 4 Gas sensitivity* FO%* 7 5 1.50.40F0.
1% 8 4 1.00.45F 0.25
% 9 3 0.80.45F0.5%
10 2 0.9 0.45F2% 10
2 0.7 0.45F 3% CIo, 5%* 10 3 ** indicates a comparative example, the measurement temperature is 210°00 From the table, the addition of fluorine increases the sensitivity to CO and the gas of the sensor output to CO It can be seen that the concentration dependence increases.

FIGS. 3 and 4 show the heating voltage dependence of the sensor resistance. The horizontal axis indicates the voltage applied to the heater of the sensor, and the sensor temperature becomes 210° C. when the voltage is applied for 5×.

The vertical axis indicates the resistance value in each 100 ppm gas based on the resistance value Ro to GOlooppm at a heater voltage of 5V. The temperature and humidity conditions were 20°C and 65%.

FIG. 3 shows the results of a sample with 0.5% fluorine added, and FIG. 4 shows the results of a sample with no fluorine added.

Addition of fluorine increases the heating temperature dependence of sensor resistance.

FIG. 5 shows the response characteristics to CO at a heating temperature of 210°C. The upper part of the figure shows the results of a comparative example in which no fluorine was added, and the lower part shows the results of an example in which 0.5% fluorine was added.

CO was injected at 2 minutes and then removed. The response to CO is faster in the embodiment. The surrounding temperature and grudge level were 20°C and 65%.

Note that although the characteristics of the gas sensor alone are shown here, these are different from the characteristics of an actual gas detection device.

For example, changes in absolute humidity can be separated into changes in saturated vapor pressure due to changes in temperature and changes in relative humidity.

If temperature fluctuations are detected and compensated for using a thermistor or the like, the humidity dependence of the gas detection device can be reduced.

Further, although the results regarding a specific metal oxide semiconductor are shown here, the same applies to the case where other metal oxide semiconductors are used. Furthermore, the structure of the sensor and the use of additives other than fluorine are arbitrary.

[Effect of the invention] In this invention, the humidity dependence of the gas sensor can be suppressed.

[Brief explanation of drawings]

Fig. 1 is a characteristic diagram of the embodiment, Fig. 2 is a characteristic diagram of the 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)

[Claims] (1) A gas sensor using a metal oxide semiconductor whose resistance value changes depending on gas, characterized in that fluorine is added to the metal oxide semiconductor. (2) The gas sensor according to claim 1, wherein the metal oxide semiconductor is SnO_2. (3) The amount of fluorine added to SnO_2 is
0.5 to 100 mg in terms of fluorine atoms per 1 gr
The gas sensor according to claim 2, characterized in that r. (4) The amount of fluorine added to SnO_2 is
0.5 to 50 mgr in terms of fluorine atoms per 1gr
The gas sensor according to claim 3, characterized in that: (5) A method for manufacturing a gas sensor using a metal oxide semiconductor whose resistance value changes depending on gas, characterized by including a step of adding fluorine to the metal oxide semiconductor.
Gas sensor manufacturing method. (6) Manufacturing the gas sensor according to claim 5, characterized in that the fluorine is added to a metal oxide semiconductor as at least one member of the group consisting of simple fluorine and nonmetallic compounds of fluorine. Method. (7) The metal oxide semiconductor is SnO_2, the fluorine is a solution of a nonmetallic compound of fluorine, and the metal oxide semiconductor is SnO_2.
7. The method for manufacturing a gas sensor according to claim 6, wherein the gas sensor is added to. (8) The fluorine solution is an aqueous solution of a nonmetallic compound of fluorine, and the fluorine concentration is determined to be 0 by focusing on the concentration of fluorine atoms.
．． The method for manufacturing a gas sensor according to claim 7, characterized in that the concentration is 2 mol/l to 12 mol/l.