JPS6128933B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas-sensitive element, and particularly to a high-sensitivity gas-sensitive element suitable for detecting low-concentration Freon gas.

Gas-sensitive elements are known that utilize the fact that the specific resistance of a metal oxide semiconductor surface changes when a gas comes into contact with the metal oxide semiconductor surface. For example, when a reducing gas comes into contact with ZnO, SnO ₂ , Fe ₂ O ₃ , etc., which exhibit N-type semiconductivity, the resistance value decreases, and in metal oxide semiconductors, which exhibit P-type semiconductivity, the increase and decrease in specific resistance is the opposite. become. Therefore, a gas sensitive body is constructed from such a metal oxide semiconductor, and a pair of electrodes are attached to the gas sensitive body to detect a change in resistance value.

The selectivity and reactivity of metal oxide semiconductors to various gases as described above are determined by the semiconductor surface temperature, the state of the surface electron level, the porosity of the gas sensitive material, the size of pores, etc., but in general, only metal oxide semiconductors However, the sensitivity is low and the selectivity is not good. Therefore, attempts have been made to improve the gas detection sensitivity by adding noble metals such as Pt and Pd as catalysts to metal oxide semiconductors, or by providing the catalyst layer on the surface of the gas sensitive body. By using these noble metal catalysts, the detection sensitivity for reducing gases such as CO and isobutane is improved to some extent, and in particular, those using ZnO as the metal oxide semiconductor can detect these gases up to about 1000 ppm. However, the detection ability for so-called freon gas is so poor that it cannot be put to practical use, and furthermore, it cannot be used at all to detect low concentration freon gas of 100 ppm or less.

Therefore, the present inventors investigated the metal oxide semiconductor as the gas sensitive material and the catalyst used as a gas sensitive element for Freon detection, and found that ZnO-
Sb ₂ O ₃ based oxide semiconductor and vanadium or vanadium-molybdenum as SiO ₂ , A ₂ O ₃ or SiO ₂ -A
The present invention was completed by discovering that a combination of catalysts supported on ₂ O ₃ is sensitive to Freon gas.

That is, an object of the present invention is to provide a highly sensitive gas-sensitive element for detecting Freon gas, and the gas-sensitive element of the present invention has the following features: The gas sensitive material contains 99.5 to 80% by weight of ZnO.
It is composed of a metal oxide semiconductor consisting of 0.5 to 20% by weight of Sb ₂ O ₃ , and the surface of the gas sensitive body is coated with A ₂ O ₃ , SiO ₂ or
It is characterized in that a layer of a catalyst made of vanadium or vanadium-molybdenum supported on a carrier made of SiO ₂ -A ₂ O ₃ is provided.

To further explain the catalyst of the present invention, when the component supported on the carrier is vanadium, the amount of vanadium supported is preferably 0.1 to 50% by weight, more preferably 0.5 to 15% by weight, based on the carrier. In addition, when the supported component is vanadium-molybdenum, the amount of vanadium supported is 0.1 to 50% relative to the carrier.
It is preferable that the weight % is more preferably 0.5 to 15 weight %, and the amount of molybdenum supported is 0.05 to 0.5, more preferably 0.1 to 0.2, expressed as a g atomic ratio to vanadium. With such a supported amount, particularly excellent catalytic action is obtained, and the gas-sensitive element becomes even more sensitive. In the catalyst used in the present invention, A ₂ O ₃ , SiO ₂ , or SiO ₂ -A selected as a carrier
The combination of ₂ O ₃ and the V and V-Mo supported thereon is a major factor in improving the sensitivity, and the amount of the supported amount makes it possible to obtain an even more excellent gas-sensitive element.

In addition, in the present invention, the metal oxide semiconductor used as the gas sensitive material is 99.5 to 80% by weight of ZnO.
and 0.5 to 20% by weight of Sb ₂ O ₃ . If the proportion of Sb ₂ O ₃ is less than 0.5% by weight, variations in the characteristics of the gas-sensitive elements manufactured will become noticeable and the sensitivity will decrease, while if it exceeds 20% by weight, the sensitivity will decrease and the gas-sensitive elements will deteriorate. Each of these is undesirable because the resistance value becomes too large to be used in practical use.

According to the gas-sensitive element of the present invention, freon gas, that is, halogenated hydrocarbon gas such as CHCF ₂ , which was difficult to detect with conventional gas-sensitive elements,
_CHF3 , _CHC2F , _CC2F2 , _{CC3F , CCF3}
It is possible to detect such substances with high sensitivity even at low concentrations of 100 ppm or less. Therefore, it can be effectively used for detecting Freon gas leaking from minute holes in piping.

1 and 2 show one embodiment of the present invention, and FIG. 1 is a cross-sectional view of a cylindrical element,
FIG. 2 is a perspective view of the element mounted on the pin foot. A detailed description will be given below with reference to the drawings.

This gas-sensitive element includes a pair of electrodes 2 provided on the outer peripheral surface of a cylindrical insulating base 1, a layer 3 made of a metal oxide semiconductor so as to cover the insulating base 1 and the electrodes 2, and a layer 3 made of a metal oxide semiconductor. The layer functions as a gas sensitizer. Further, a catalyst layer 4 is provided on the surface of the metal oxide semiconductor layer 3. These metal oxide semiconductor layer 3 and catalyst layer 4 are in a porous state. This gas-sensitive element is used by being attached to a pin foot as shown in FIG. In the figure, 5 represents an electrode lead wire, 6 represents an insulating plate, and 7 represents a heater.
The heater is provided to increase the sensitivity of the element by heating the gas sensitive body to around 450°C.

The gas-sensitive element of the present invention can be manufactured, for example, as follows.

ZnO powder and Sb ₂ O ₃ powder are collected and mixed at a predetermined ratio, an appropriate amount of a binder such as methyl cellulose is added thereto, and the mixture is mixed for a certain period of time using a ball mill or the like to form a paste. Now, the paste prepared above is applied to the outer circumferential surface of the cylindrical insulating substrate 1 on which a pair of electrodes 2 have been formed in advance by baking gold paste, etc., and after drying, the paste is baked at 300 to 1000°C to make the gas sensitive. Form body 3.

Next, the catalyst layer 4 is formed on the surface of the gas sensitive body 3 in the following manner.

First, when using a vanadium-molybdenum catalyst as a catalyst, a standard solution of vanadium and molybdenum is prepared. For example, a vanadium standard solution is prepared by adding water to ammonium metavanadate and adding oxalic acid under heating to obtain a standard solution of a predetermined concentration. In addition, for the molybdenum standard solution, an ammonium molybdate aqueous solution with a predetermined concentration is prepared and used as a standard solution. A vanadium-molybdenum standard solution is prepared by taking a predetermined amount of each of the prepared standard solutions and mixing them. Next, a predetermined amount of, for example, A ₂ O ₃ as a carrier is weighed out, immersed in a predetermined amount of the standard solution, and left overnight. After standing, evaporate to dryness,
The catalyst is obtained by crushing it in a mortar, putting it in a quartz crucible as a powder, and firing it in an electric furnace at 300-600°C.

The catalyst thus prepared is made into a slurry using an appropriate solvent (e.g. water, alcohol, etc.), applied to the surface 3 of the gas sensitive body, dried, and
The gas-sensitive element of the present invention can be obtained by firing at 600°C. In addition, when preparing the catalyst slurry,
Adding an organic binder such as methylcellulose, ethylcellulose, or cellulose acetate in addition to a solvent such as water or alcohol increases the mechanical strength of the formed catalyst layer and improves its impact resistance.

Hereinafter, the present invention will be specifically explained with reference to Examples.

Example 1 A gas-sensitive element having the structure shown in FIGS. 1 and 2 was manufactured. The catalyst layer consists of vanadium on carrier A ₂ O ₃ .
Molybdenum is supported, and the amount of vanadium supported is 10% by weight based on the carrier, and the amount of molybdenum supported is 0.1 in g atomic ratio to vanadium.
The sensitizer was composed of a ZnO--Sb ₂ O ₃ -based oxide semiconductor, but we prepared samples with various Sb ₂ O ₃ proportions in the range of 0.2 to 22% by weight, and each was immersed in air.
Freon-22 (CHC) contained in a concentration of 50 ppm
The sensitivity to F ₂ ) was measured. Here, sensitivity is
Let the resistance value of the element in normal air be R ₀ ,
It is expressed as R ₀ /Rg, where Rg is the resistance value of the element in air containing 50 ppm of Freon 22, and the larger this value, the greater the sensitivity of the element.

For comparison, a gas-sensitive element having _{the same configuration as the above case was manufactured, except that Fe 2 O 3 or} A ₂ O ₃ was added instead of Sb 2 O 3 as an additive to ZnO, and the same process was carried out. Measurements were made. The results are shown graphically in FIG. In the figure, curves a, b, and c are respectively
This is the case for Sb ₂ O ₃ , Fe ₂ O ₃ , and A ₂ O ₃ . In addition, curve d uses a ZnO-Sb ₂ O _3- based oxide semiconductor as a sensitizer, and a vanadium catalyst (supported amount 2.0%).
We have shown the results of similar measurements using .

Further, for each of the above-mentioned devices manufactured by adding Sb ₂ O ₃ , the relationship between the device resistance and the gas sensitive material composition in air at a device temperature of about 450° C. is shown as a graph in FIG. Relative resistance values are expressed assuming that the resistance value when Sb ₂ O ₃ is 1% by weight is 1.

As is clear from Fig. 3, the ZnO-Sb ₂ O ₃ system is excellent as a gas sensitive material, and among them, the ZnO-Sb 2 O 3 system has an Sb ₂ O ₃ content of 0.5 to 20% by weight, and even more so in the range of 0.8 to 10% by weight. Sensitivity is sometimes markedly improved. Furthermore, according to the results shown in FIG. 4, it can be seen that when the content of Sb ₂ O ₃ exceeds 20% by weight, the element resistance increases significantly and it cannot be put into practical use.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing an example of the structure of a gas-sensitive element according to the present invention, FIG. 2 is a perspective view showing an example of a device using the element of the present invention, and FIG. FIG. 4 is a diagram showing the relationship between the sensitive material composition and the element resistance. DESCRIPTION OF SYMBOLS 1... Cylindrical insulating base, 2... Electrode, 3... Gas sensitive body, 4... Catalyst layer.

Claims (1)

[Claims] 1. A gas-sensitive element having a gas-sensitive member equipped with a pair of electrodes, wherein the gas-sensitive member contains 99.5 to 80% by weight of ZnO.
It is composed of a metal oxide semiconductor consisting of 0.5 to 20% by weight of Sb ₂ O ₃ , and the surface of the gas sensitive body is coated with A ₂ O ₃ , SiO ₂ or
1. A gas-sensitive element comprising a catalyst layer made of vanadium or vanadium-molybdenum supported on a carrier made of SiO ₂ -A ₂ O ₃ . 2. The gas-sensitive element according to claim 1, wherein the component supported on the carrier is vanadium, and the amount of vanadium supported is 0.1 to 50% by weight based on the carrier. 3 In the statement of claim 1, the component supported on the carrier is vanadium-molybdenum, and the amount of vanadium supported is 0.1 to 0.1 to
50% by weight, and the amount of supported molybdenum is 0.05 to 0.5 expressed as g atomic ratio to vanadium.