JPS63313048A - Gas sensor - Google Patents

Abstract

PURPOSE:To improve sensitivity to hydrogen sulfide and the deriv. thereof, etc. by adding at least one element among 3 groups consisting of Zn, Pb, Ag, Na, Ba, In, and S to the surface of a metal oxide semiconductor. CONSTITUTION:For example, SnO2, etc., are used for the surface of the metal oxide semiconductor so that the semiconductor of the kind different from the compd. of Pb, etc., to be added is formed. The sensitivity to gases improves if Zn, Pb, Ag, etc., are added to such metal oxide semiconductor. The sensor formed by depositing said additives on the surface of the metal oxide semiconductor is effective. Although the structure of the semiconductor is arbitrary, the semiconductor is preferably formed as a thin film and is made into the thin film having the structure in which the many thin film-like small pieces of the metal oxide semiconductor are separated by cracks. The sensitivity to the hydrogen sulfide and the deriv., thereof, etc., is thus improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a gas sensor that utilizes changes in the resistance value of a metal oxide semiconductor.

[Prior art] Malodorous substances, toxic gases, bad breath in the atmosphere, or Si, H4, PH3, A s H3 used in semiconductor manufacturing
There is a need to detect gases such as: In these cases, the detection target is hydrogen sulfide, hydrogen sulfide derivatives such as methyl mercaptan, or S r H4, PH,, A s Hz
, and amine compounds such as methylamine, ammonia, etc.

However, the sensitivity of metal oxide semiconductor gas sensors to these gases is generally insufficient. These gases are 1 pp
It is necessary to detect at an extremely low concentration of about m. Therefore, the inventor considered using additives to metal oxide semiconductors to increase the sensitivity to these gases.

[Problem of the Invention] An object of the present invention is to improve the sensitivity of gas sensors, particularly for hydrogen sulfide and its derivatives, SiH, AsH3°PH, and other compounds and their derivatives, or NOx and SOx%.
The goal is to improve sensitivity to ammonia derivatives such as N H3 and amine compounds.

[Structure of the Invention] The gas sensor of the present invention is a gas sensor that utilizes a change in resistance value of a metal oxide semiconductor, and includes Zn, Pb, Ag, Na, and B on the surface of the metal oxide semiconductor.
a.

It is characterized by adding at least one element of the group consisting of In and S.

Here, metal oxide semiconductors include, for example, Snow and I11.
! 03, ZnO, Fears, etc., as a semiconductor different from the added compound such as PB. When Zn, Pb, Ag, etc. are added to these metal oxide semiconductors, the sensitivity to various gases increases. This effect is particularly remarkable for gases such as hydrogen sulfide, methyl mercaptan, methyl amine, PH, s, and NOx.

The addition of Zt+'PPb, Ag, etc. improves the sensitivity to gases such as hydrogen sulfide. The types of additives were determined through screening, and it is not clear why these additives are effective. However, the improvement in sensitivity seems to be related to the electronegativity of the metal ions added.

Next, it is effective to support these additives on the surface of the metal oxide semiconductor. For example, even if these compounds are added to the starting materials for metal oxide semiconductors, the sensitivity to hydrogen sulfide etc. will not improve much.
The atomic ratio between these elements and the metal elements in the metal oxide semiconductor is preferably 0.3 to 40 atm%. However, when the amount added is increased, the response speed from the gas to the clean air after the gas disappears decreases. Further, since the effect increases when the addition amount is 1 atm % or more, addition of 1 to 30 atm % is more preferable.

Although the structure of the semiconductor is arbitrary, it is preferably in the form of a thin film,
The thin film has a structure in which a large number of thin film-like pieces of metal oxide semiconductor are separated by cracks. This type of thin film is more sensitive to gases such as hydrogen sulfide and NOx than ordinary thin films in which metal oxide semiconductors are continuously connected, thick films, or sintered bodies of metal oxide semiconductors. In this thin film, the properties near the cracks govern the sensor characteristics, and the reason why high sensitivity to gases such as hydrogen sulfide is obtained seems to be related to the cracks.

Examples will be described below regarding the case where Snow is used, but the present invention is not limited thereto.

[Example] A pair of gold electrodes were printed on an alumina heat-resistant insulated pipe,
A coiled heater was housed inside the pipe. This was used as the base of a gas sensor. Note that this base body is compatible with the applicant's gas sensors “T G S 812” and “T G 5813”.
'', etc., and is well known.

As an organometallic compound of Sn, Sn(OCHs)*(
30w of isobutanol using o(cHt)sNHJ
A t% solution was prepared. The type of organometallic compound is arbitrary; for example, the inventor has proposed 5n(-〇〇〇-C7H+5)t
A similar experiment was conducted with a 23 wt% solution of , and the results were similar. The starting material for the metal oxide semiconductor is arbitrary.

A solution of an organometallic compound of Sn is dropped onto an insulating substrate, and 1
After removing the solvent at 10°C, the organometallic compound was thermally decomposed at 500°C to obtain a Snow film.

This process was repeated 1 to 5 times to complete a thin film.

The thickness of the obtained thin film is about 0.3 to 2 μm.

An isobutanol solution of a methoxy compound such as Zn, Pb, or Ag is added dropwise to the fired thin film and thermally decomposed at 500°C to form S.
It was supported on a n0w film. These additives are supported on SnO as oxides or simple substances. When adding a noble metal catalyst, an aqueous solution of catalyst salt was added dropwise after addition of Zn, Pb, etc., and thermally decomposed at 500°C.

Compounds such as Zn are supported on the surface of SnO,
If added uniformly to the water, the effect will be poor. For the example in which the Zn/Sn atomic ratio was 5 ata+% (Zn5:Snl 00), the results are shown in Table 1 for a comparison example in which the starting materials of Zn and Sn were uniformly mixed and thermally decomposed. . The number of applications was 2 times, the sensor temperature was 210° C., and the results are the average value of 3 sensors each. Measurements were also carried out 21 weeks after the sensor was manufactured.
This was done after keeping the temperature at 0°C. Furthermore, the concentration of gases such as hydrogen sulfide was set to 10 ppm (in principle, the same applies hereinafter).

Table 1* 5nOt single 0.07 0.3 Example
0.04 0.4 Comparative example 0
．． 06 0.4* Gas sensitivity is shown using the resistance value Ro in clean air and the resistance value R in gas. In the example, ZnO was added after the completion of the 5nOt film, and in the comparative example, Sn material and Uniformly mixed with Zn material, applied and thermally decomposed.

The structure of a crack-shaped thin film is shown in FIGS. 1 to 3. Furthermore, as a comparative example, FIG. 4 shows a thin film in which SnO films are continuously connected. All of these thin films are 5nOt. In addition, the thin film of the comparative example was made by contacting the vapor of 5n(CtHs)t to a glass substrate heated to 500°C.
CyHS)4 is thermally decomposed. That is, 5n(Ct
Hs)4 was sublimed, guided to a glass substrate with an air stream, and immediately pyrolyzed on the substrate.

Figures 1 to 3 are at 1000x magnification, Figure 4 is at 500x magnification.
It is 0 times. The glass substrate is visible at the bottom of FIG. 4, and the films are continuously connected.

This is a normal 5nOt film. On the other hand, in FIGS. 1 to 3, there are many cracks in the film. Figure 1 shows the S
Figure 2 shows the result of applying the n-compound solution once, Figure 2 shows the result of applying the solution twice, and Figure 3 shows the result of applying the solution five times. When the coating was applied once, many small pieces were observed in the film, and there were cracks between the small pieces. Note that the base of this small piece is in direct contact with the alumina substrate. Each small piece is connected via an extremely thin Sn0w film at the crack portion or a contact portion between the small pieces, and there is electrical conductivity. When the coating was applied twice (FIG. 2), large pieces of SnO2 were present on the upper surface. These small pieces are almost completely separated by cracks and are considered not to contribute to electrical conductivity. A first layer of S.sub.nO is present below the upper layer, and the lower portion thereof is an alumina substrate. There are also many cracks in the first layer 5nOt film, and the film is separated into many small pieces. However, compared to the case where the coating is applied once, cracks are less noticeable, the connections between the small pieces are strong, and the electrical conductivity is high. In the case where the number of applications was 5 times (Fig. 3), the SnO on the upper layer
The second small piece is even larger, with S separated by a crack on the base.
I can now see small pieces. It is not known whether this Sn0w film is two-layered and the underlying layer in the figure is directly an alumina substrate. Moreover, the SnO2 film in FIG. 3 is 5n in FIG.
It has even lower resistance than the Ot film.

These films may have been grown as follows.

When a solution of an organometallic compound is applied and dried, the liquid film contracts during the drying process and separates into many small pieces. When this is thermally decomposed, the shrunken pieces change into small pieces of metal oxide semiconductor, as shown in FIG. 1, and the cracks caused by the shrinkage remain as they are. When a solution of an organometallic compound is applied again onto this film, small pieces of the top layer are formed as the solution dries. The solution will also penetrate into the cracks and the underlying cracks will be partially filled. In this way, a thin film with cracks remaining in the underlying layer will grow in a multilayered structure.

The characteristics of the sensor change depending on the presence or absence of cracks. Fifth
The figure shows the relationship between the number of applications and gas sensitivity.

Note that the 5nOz film is a simple one. Along with the number of applications,
Resistance is decreased, sensitivity to ethanol is increased, and sensitivity to hydrogen sulfide is decreased. As a comparative example in the figure, the results of the sensor in FIG. 4 are shown.

These results are not due to an increase in film thickness. 6th
The figure shows the results when the number of coatings was set to two and the Sn organometallic compound concentration was varied. Although the concentration of Sn changes by about 6 times and the difference in film thickness is large, the characteristics do not change much. It can be understood that the characteristics shown in FIG. 5 do not indicate the influence of film thickness, but rather indicate changes in relative sensitivity due to crack filling.

The sensitivity to gases such as hydrogen sulfide is improved by using a thin film of metal oxide semiconductor having thin film-like pieces and cracks.

Therefore, we proceeded with the experiment using this thin film. Of course, the semiconductor structure may be other than this.

FIG. 7 shows the sensitivity to 10 ppm hydrogen sulfide at 210° C. for a 5nOt film with 5 atm % additive. Note that the number of times the Sn0w film was applied and baked was two times. The horizontal axis is the electronegativity of the metal ion in the additive compound,
The vertical axis represents gas sensitivity, and the case of plain SnO is shown as a comparative example. Zn, Pb, Ag.

Hydrogen sulfide sensitivity improved in the case of Na, Ba, In, and S. Similar tests were conducted using IntO3 as a semiconductor instead of 5nOt, but compounds such as Zn and Pb are effective.
That was common. Although the figure shows hydrogen sulfide, similar results were obtained for methyl mercaptan, methylamine, and NO2.

5I with 5 atm% additives added to Figures 8 and 9.
With IOf, hydrogen sulfide, methyl mercaptan, ethanol, NO at 21O℃! The gas concentration characteristics are shown below. Note that the S n Ox film was coated and fired twice (the same applies hereinafter).

Addition of Zn, PbSAg, etc. increases sensitivity to gases such as hydrogen sulfide. Since the properties of N Ot, No, So, etc. are similar, and the sensitivity of the sensor is also similar, No. 9 is used as a representative. Furthermore, alcohols such as ethanol and propatool are gases that exhibit the greatest sensitivity at low temperatures, and are also extremely common gases. Therefore, ethanol was used as a representative interfering gas.

The amount of compounds such as Zn to be added is 0.3 to 40a based on the atomic ratio of these metal elements to the metal elements in the metal oxide semiconductor.
tm% is preferred. The response speed of the sensor from gas to clean air decreases with the amount added, but if the amount added is small, the effect will be poor, so addition of 1 to 30 atm% is more preferable. Tables 2 and 3 show the effects of the amounts of Zn and Pb added at 21.0°C.

Table 2 0 0.07 0.5 0.30.3 1 0.05 0.4 0.4 2 0.05 0.4 0.4 5 0.04 0.4 0.60.1 15 0.03 0.2 0.8 35 Q, 01 0.2 0.7 Table 3 0 0.07 G, 5 0.30.30.50.04
0.5 0,4 1 0.02 0.3 0,7 2 0.02 0.3 0.70.1 5 0.006 0.3 0.8 15 0.006 G, 2 0.8 1st O Figure 11 shows that 5 atIII% Zn and Pb
The temperature characteristics for each 10 ppm gas are shown for the Sn0w film added with . The horizontal axis is the sensor temperature, and at any temperature, the sensitivity to gases such as hydrogen sulfide is improved by adding Zn''PPb, etc.

FIG. 12 shows the response performance of the TOPPIll sensor to hydrogen sulfide. Gas sensitivity is displayed on a logarithmic scale on the vertical axis.
The position of the vertical axis is shifted by one digit between the case where 5 atn+% Pb is added and the case where Zn is added. After injecting hydrogen sulfide, remove the hydrogen sulfide at 16 minutes. The response to hydrogen sulfide injection is fast, and the resistance value after injection is stable. However, the response when hydrogen sulfide is removed is slow.

The responses when hydrogen sulfide was removed were Pd, Re, and Pt.

This can be improved by adding a noble metal catalyst such as Rh, Ir, or Au.

Using the symbols ΔR and ΔR' shown in FIG. 12, the response speed during hydrogen sulfide removal is evaluated. Here, ΔR corresponds to the ratio of the resistance value 16 minutes after hydrogen sulfide removal to the resistance value in clean air, and ΔR° corresponds to the sensitivity to hydrogen sulfide. Table 4 shows the results.

All samples were doped with 5 atm% of PbO1 or ZnO, and the sensor temperature was 210°C. Note that the noble metal catalysts were all added using a 0.02 mol/1 solution.

Table 4 Sensor response speed (ΔR/ΔR') S no
t + P bo 0 , 3S n Ot
+ P bO+ P d OO, I 5SnOt+Z
nOo, a Sn○, +ZnO+PdOO, 15 SnOt+ZnO+Re Q, I 5 Table 5 shows the relationship between the amount of Zll, Pb, etc. added and the response speed during hydrogen sulfide removal for a sample without the addition of a noble metal catalyst. The evaluation method was the same as in Table 4, and the sensor temperature was 210°C.

Table 5 Sensor addition 1 response speed (atm%)
(ΔR/ΔR') SnOt Single 0.15Sn
Ot+ZnO20,25 〃5 0.3 〃15 0.3 //, 35 0.5SnOt+
P bo 1 0.25〃5
0.3 〃 15' 0.35 To further improve the response speed after removing the gas, it is better to temporarily heat the sensor to a high temperature and perform heat cleaning. The behavior with respect to heat cleaning is schematically shown in FIG. Assume that hydrogen sulfide is injected at time ■, and hydrogen sulfide is removed at time ■.

Next, when heat cleaning is performed from time ■ to time ■, the sensor resistance R temporarily decreases due to the heat cleaning and then recovers. When the heater power is then returned to its normal value, the sensor resistance also returns to its value in clean air. Note that if heat cleaning is not performed, the sensor resistance changes as shown by the broken line, and it is slow to return to the value in clean air.

FIG. 14 shows an auxiliary circuit that periodically heat-cleans the sensor. In the figure, 2 is a gas sensor, 4 is a metal oxide semiconductor film, 6 is a heater, and 8 is a power source such as a battery. 10 is a switching transistor, 1
2 is a load resistance of the sensor, 14 is a capacitor for noise removal, 16.degree. 18 is a reference resistor for compensating the sensor output corresponding to clean air, and 20 is a differential amplifier. 22
is a timer, which generates a heat cleaning signal for about 30 seconds, for example, to keep the transistor 10 on at all times. In this way, heat cleaning is performed at about 350°C. For the next 30 to 150 seconds, the transistor 10 is intermittently turned on at a frequency of about kHz to raise the sensor temperature to 21
Maintain the detection temperature around 0℃. 24 is an AD converter,
The output of the differential amplifier 20 while the sensor is at the detection temperature is converted into a digital value by the signal from the timer 22, and is displayed on the display 26.

FIG. 15 shows an auxiliary circuit in which heat cleaning is performed manually. In the figure, 28 is an oscillation circuit that operates at about 1 kHz, for example, and turns on the transistor 10 intermittently to maintain the sensor 4 at the detection temperature. After gas is detected, heat cleaning is performed manually, and when the switch 32 is turned on, the monostable multivibrator 30, which opens for about 30 seconds to 1 minute, is turned on, and the transistor I
During that time, O is always turned on to perform heat cleaning.

When the monostable multivibrator 30 returns to the normal state, the transistor IO is driven by the output of the oscillation circuit 28 and returns to the detection state. Furthermore, the multi-by-break 30 is connected to the AD converter 24, and prohibits the operation of the converter 24 during heat cleaning to prevent false detection. Note 3
4.36 is a diode for separating signals between the oscillation circuit 28 and the multi-by-break 30.

Although the embodiments have been described with respect to sensitivity to specific gases such as hydrogen sulfide, this does not limit the application of the gas sensor of the present invention.

[Effects of the Invention] The gas sensor of the present invention has high sensitivity to various gases,
Especially hydrogen sulfide and its derivatives, S! H4, AsHs,
Compounds such as PHs and their derivatives, or NOx and S
High sensitivity to gases such as ammonia derivatives such as OX, NH3, and amine compounds.

[Brief explanation of the drawing]

Figures 1 to 3 are electron micrographs showing the particle structure of the gas sensor of the example, Figure 4 is an electron microscope picture showing the particle structure of the gas sensor of the comparative example, and Figures 5 and 6 are of the conventional example. Characteristic diagrams, FIGS. 7 to 13 are characteristic diagrams of the embodiment, and FIGS. 14 and 15 are circuit diagrams of ancillary circuits.

Claims (2)

[Claims] (1) In a gas sensor that utilizes changes in the resistance value of a metal oxide semiconductor, Zn, Pb, Ag, Na,
A gas sensor characterized in that at least one element of the group consisting of Ba, In, and S is added. (2) The gas sensor according to claim 1, wherein the metal oxide semiconductor is a metal oxide semiconductor thin film consisting of a large number of thin film-like pieces of metal oxide semiconductor separated by cracks. .