JP2645568B2 - Halogenated hydrocarbon gas sensor - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a halogenated hydrocarbon gas sensor using a change in resistance of a metal oxide semiconductor.

[Prior Art] It has been pointed out that halogenated hydrocarbons such as freon destroy the upper ozone layer in the atmosphere, resulting in environmental destruction. The main halogenated hydrocarbon is freon,
About 40% of Freon consumption is Freon 113 (CClF2, CC12F)
Occupied by. Other major freons are Freon 12 (CC12F2, consumption about 25%), Freon 11 (CC
13F, consumption about 20%), Freon 22 (CHClF2 consumption about 15
%). Of these freons, Freon 22 is a substance that has relatively little effect on the environment, and Freon 11 and Freon 12 have Freon 123 and Freon 134 that have little effect on the environment.
Alternative substances such as a have been developed. However, no substitute has been developed for Freon 113, which accounts for the largest consumption.

In order to prevent environmental destruction due to freon, a sensor for detecting the leak is required. In particular, this sensor has high consumption and must be used in the future.
113 is required to have high sensitivity.

The inventors measured the Freon sensitivity of the metal oxide semiconductor gas sensor. The results show that (1) the sensitivity of the gas sensor to freon is inadequate, (2) especially important freon.
The sensitivity to 113 was poor.

Here, the related prior art is shown. Tokiko Sho 61-6
No. 0,381 points out a sensitizing effect of a sulfate ion on a SnO2 gas sensor. According to this publication, sulfate ions are indiscriminately improved in sensitivity to various combustible gases such as methane, isobutane, and hydrogen, and are considered to be particularly preferable for sensors for gas leak alarms.

[Problems of the Invention] An object of the present invention is to improve the sensitivity of a gas sensor to halogenated hydrocarbons, particularly to improve the sensitivity to CFC 113.

[Structure of the Invention] In the present invention, a sulfate ion is added to a metal oxide semiconductor. The addition amount is 0.1 to 10 mgr SO4 ^2- / gr metal oxide semiconductor, more preferably 1 to 100 mgr SO4 ^2- / gr metal oxide semiconductor, and still more preferably 1 to 70 mgr SO4 ^2- / gr metal oxide semiconductor. I do.

Although the details of the mechanism of sensitization of the added sulfate ion are unknown, it can be assumed that the sensitization mechanism mainly exists on the surface of the metal oxide semiconductor and increases the sensitivity to the halogenated hydrocarbon in relation to the acidity. The sensitizing effect on the halogenated hydrocarbon is specific to sulfate ions. For example, phosphorus, an element adjacent to sulfur, does not show such an effect.

Next, the effect of the addition of sulfate ions appears from about 0.1 mgr SO 4 ²⁻ / gr metal oxide semiconductor and increases with the addition amount. There is a region where the effect of addition of sulfate ions increases sharply.
The effect sharply increases in the range of ~ 10 mgr SO4 ^2- / gr metal oxide semiconductor. When the addition amount is more than this, the addition amount dependency decreases. There is no particular upper limit on the amount of sulfate ions to be added, but it is generally not preferable to add an excessive amount of the additive.
The upper limit was SO4 ²⁻ / gr metal oxide semiconductor.

As already indicated, the effect of sulfate ions is related to their acidity and independent of the type of metal oxide semiconductor. Therefore, the type of the metal oxide semiconductor is arbitrary. For example, a halogenated hydrocarbon gas sensor may be obtained by adding sulfate ions to ZnO, In2O3, or the like.

Hereinafter, an example will be described using a SnO2 gas sensor as an example. The addition amount of sulfate ion indicates the addition amount per 1 gr of the metal oxide semiconductor in mgr units.

Example Preparation of Sample An aqueous solution of SnC14 was neutralized with ammonia to obtain a stannic acid sol. The sol was washed with water, dried and calcined in air at 800 ° C. for 1 hour to obtain SnO 2. This SnO2 was pulverized, impregnated with an aqueous solution of palladium chloride, and calcined at 700 ° C. for 30 minutes in air to support a Pd catalyst. Loading amount is 10mgr Pd / gr Sn
It is O2 (in terms of metal Pd). The effect of sulfate ion is Pd
Irrespective of the type and presence or absence of the catalyst.

The SnO2 after the addition of the catalyst was pulverized, applied to a printed alumina pipe with a pair of gold electrodes, and sintered at 850 ° C. for 10 minutes. After sintering, a heater coil was inserted into an alumina pipe and fixed to a stem to form a gas sensor. This gas sensor configuration is known as the applicant's gas sensor “TGS813”.

Dilute sulfuric acid was dropped on the gas sensor and absorbed in SnO2. Next, heat treatment was performed in air at 650 ° C. for 30 minutes to support sulfate ions. The form of the sulfur compound that is stable at the operating temperature of the gas sensor is sulfate ion, and the sulfate ion may be added in another form. Sulfate ions may be added, for example, as ammonium sulfate, stannous sulfate, or a sulfite compound.
The sulfate ion may be added to a starting material of SnO2, for example, SnO2 before adding a catalyst. The same adjustment conditions were used for Comparative Examples (without addition of a sulfur compound or dilute phosphoric acid added dropwise instead of diluted sulfuric acid). Also in the comparative example where no sulfur compound was added, heat treatment was performed at 650 ° C. for 30 minutes.

The distribution of sulfate ions after the heat treatment and the abundance in SnO2 were measured. Sulfate ions were almost uniformly present in the SnO2 coating, and almost all of the sulfate ions from dilute sulfuric acid remained in the SnO2. Therefore, in the following, the abundance is shown assuming that all the added diluted sulfuric acid is supported on SnO2 as sulfate ions.

FIG. 1 shows the relationship between the freon sensitivity at 430 ° C. and the amount of sulfate ions added. The measurement atmosphere was air at 20 ° C. and 65% RH, and the results are shown as an average of three sensors (the same applies hereinafter). The gases used were all 1000 vol ppm. The effect of sulfate ions on the resistance in air is small and only slightly reduces the resistance. Sulfate ions are ethanol (EtOH), isopropanol (IPA),
It does not affect the resistance to n-octane. Note that n-octane is a representative component of gasoline, and Freon from a car air conditioner is exemplified as an interfering gas when detecting leakage. Ethanol and isopropanol are shown as examples of frequently used solvent vapor.

Next, the effect of sulfate ion is large for Freon 12 and Freon 113, and small for Freon 11 and Freon 22. In addition, the effect of sulfate ion increases with the amount added, but 1 mgr to 10 mg
The increase is particularly remarkable in the range of rSO4 ²⁻ / gr SnO2. Freon 113
In the case of, the sensitivity equivalent to that of isopropanol can be obtained with the sulfate ion of 0.1 mgr / gr SnO2, the sensitivity sharply increases at 1 mgr / gr SnO2 or more, and approaches the saturation at 10 mgr / gr SnO2 or more.

The sensitizing effect of the sulfate ion is not limited to a specific temperature. FIG. 2 shows the heating temperature dependence of the sensor of 6 mgr SO4 ^2- / gr SnO2, and FIG. 3 shows the heating temperature dependence of the sensor without addition of sulfate ions. The resistance value is shown based on the resistance value in air at 280 ° C. In addition, as freon, each
1000 ppm Freon 113 and Freon 22 were used. In the comparative example, the sensitivity to Freon 113 is small at all temperatures, and in the examples, sensitization to Freon 113 is observed in a wide temperature range from a low temperature to a high temperature. The sensitivity to Freon 22 is slightly higher in the embodiment. There is an area where a particularly high sensitivity is obtained around 70 ° C. for Freon 113. For example, while the sensor is intermittently heat-cleaned at a high temperature (for example, 200 to 500 ° C.), the temperature is around 70 ° C. (for example, room temperature to 150 ° C.). ) May be detected.

FIG. 4 shows an example in which sulfate ions of 6mgr SO4 ^2- / gr SnO2 were added and a comparative example in which sulfate ions were not added.
The response waveform to 113 (430 ° C) is shown. The response speeds are almost the same, and the difference is the difference in sensitivity.

6mgr SO4 ^2- / gr Sensor to which sulfate ion of SnO2 is added,
140 ° C, 280 ° C, 42
The results at 0 ° C. are shown in Table 1. The sensitivity in the table is shown by the ratio of the resistance value in air to the resistance value in gas.

As is clear from Table 1, the effect of sulfate ions is limited to sensitization to freon, and the sensitivity to other gases is hardly changed or slightly decreased by sulfate ions.

Second, sensitization to freon is a phenomenon specific to sulfate ions. Using orthophosphoric acid instead of dilute sulfuric acid, 6mgr PO4 ^2-
Table 2 shows the characteristics of the comparative example to which the phosphate ion of / gr SnO2 was added.
Shown in

* For all samples, phosphate or sulfate ions of 6mgr / gr SnO2 were added, and the sensitivity was the ratio of resistance in air to resistance in gas.

As is clear from Table 2, phosphorus adjacent to sulfur has no sensitizing effect on freon, and the freon sensitizing effect is specific to sulfate ions. The inventor screened almost all of the typical chlorine and transition metal elements for elements other than those shown here.
However, only germanium was found to have a sensitizing effect on freon other than sulfate ions. In addition, the effect of germanium was exhibited only at low temperatures, and was inferior to sulfate ions.

In the examples, description has been made on freon, which is a typical example of halogenated hydrocarbon. However, it goes without saying that similar substances to freon, for example, halogenated hydrocarbons such as CF3Br used for digestive agents can also be detected. In this specification, the term "halogenated hydrocarbon" refers to a compound composed of carbon, hydrogen, and halogen.

[Effect of the Invention] In the present invention, the sensor sensitivity to halogenated hydrocarbons is improved. Particularly 1mgr SO4 ^2- / gr the amount of sulfate ions
With a metal oxide semiconductor or more, sensitivity to freon can be sufficiently increased, and the influence of other gases can be avoided.

The sensitizing effect is particularly remarkable for Freon 113, which consumes a large amount and has low sensor sensitivity despite the absence of a substitute substance.

The effect of the added sulfate ions is unique to halogenated hydrocarbons and has little effect on sensitivity to other gases or only slightly reduces sensitivity to other gases.

Further, the sensitizing effect on the halogenated hydrocarbon occurs only in sulfate ions, and no sensitizing effect occurs even when a phosphate ion, which is a similar substance, is added.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing the relationship between the added amount of sulfate ions and the sensitivity of halogenated hydrocarbons in the example. FIG. 2 is a characteristic diagram showing temperature characteristics of the embodiment. FIG. 3 is a characteristic diagram showing a temperature characteristic of a conventional example. FIG. 4 is a characteristic diagram showing a response characteristic to a halogenated hydrocarbon in the example.

Claims (4)

(57) [Claims] 1. A gas sensor using a change in the resistance value of a metal oxide semiconductor, wherein 0.1 to 100 mgr of sulfate ion is added per 1 gr of the metal oxide semiconductor to the metal oxide semiconductor. Hydrocarbon gas sensor. 2. The method according to claim 1, wherein the addition amount of the sulfate ion is determined by using a metal oxide semiconductor.
The halogenated hydrocarbon gas sensor according to claim 1, wherein the amount is 1 to 100 mgr per 1 gr. 3. The halogenated hydrocarbon gas sensor according to claim 1, wherein the metal oxide semiconductor is SnO2. The amount of 4. A sulfate ion, 1~70mgr SO4 ^2- /
The halogenated hydrocarbon gas sensor according to claim 3, wherein the sensor is gr SnO2.