JPH11344458A - Hydrogen gas detection element and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a hydrogen gas detection element hardly deteriorated in its sensitivity even if it is exposed to hydrogen of high concentration. SOLUTION: In a gas detection element wherein a sensitive layer 2 comprising a semiconductor layer 2 of a semiconductor based on indium oxide is provided so as to cover a noble metal wire 1, 1-5 atm % of oxide of a metal selected from lanthanum, calcium, balium, gallium, aluminum, zirconium, yttrium and scandium is added and a dense silica membrane 3 is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen gas sensing element, and more particularly to a semiconductor type gas sensing element having a sensitive layer formed of a semiconductor containing indium oxide semiconductor as a main component and covering a noble metal wire. And its manufacturing method. Such hydrogen gas detection elements are mainly used in chemical factories that use hydrogen gas as a reducing agent, carrier gas, fuel, etc.
It is used for monitoring gas leaks in semiconductor manufacturing plants, hydrogen fuel cells for electric vehicles, engine systems, and the like.

[0002]

2. Description of the Related Art Heretofore, as this type of hydrogen gas detecting element, a gas detecting element provided with a sensitive layer formed of a semiconductor mainly composed of a tin oxide semiconductor over a noble metal wire is known. Attempts have been made to develop a hydrogen gas detecting element in which at least one lanthanum-based metal oxide is added to the sensitive layer to improve the selectivity of hydrogen gas at low concentrations.

[0003]

It has been found that the above-mentioned conventional hydrogen gas detecting element detects high-concentration hydrogen gas of 100 ppm or less with high sensitivity, and is used for initial detection of hydrogen gas leakage. It is expected to be effective. This is because hydrogen gas has a property that it has a small molecular radius and easily leaks from an extremely small pinhole, etc. In addition, the lower limit of hydrogen gas explosion is as low as 4% (Vol), and the explosive gas concentration region is low. Because of its large size, early warning of gas explosion is required. However, such a hydrogen gas detecting element has a drawback that once it is exposed to a high concentration of hydrogen gas, the sensitivity is deteriorated.
Specifically, the above-described hydrogen gas detection element is an excellent sensor that works with high sensitivity and high selectivity at a hydrogen gas concentration of 100 ppm or less, but the sensitivity decreases when exposed to hydrogen gas of 500 ppm or more. An experimental result of deterioration has been obtained.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a hydrogen gas detecting element which has high humidity stability and is hardly deteriorated in sensitivity even when exposed to a high concentration of hydrogen in view of the above-mentioned drawbacks.

[0005]

In order to achieve this object, a feature of the hydrogen gas detecting element of the present invention is to provide a sensitive layer formed of a semiconductor containing indium oxide as a main component so as to cover a noble metal wire. A gas sensing element, wherein the sensitive layer has lanthanum, calcium, barium, gallium,
An oxide of at least one metal selected from aluminum, zirconium, yttrium and scandium is added in an amount of 1 to 5 atm% (atomic%) and a dense silica thin film is formed. A gas sensing element provided with a sensitive layer formed of a semiconductor containing indium oxide as a main component, wherein the sensitive layer includes lanthanum, calcium, barium, gallium,
An oxide of at least one metal selected from aluminum, zirconium, yttrium and scandium may be added in an amount of 1 to 5 atm%, and a hydrogen-selective permeable silica thin film may be formed in the sensitive layer. A gas sensing element provided with a sensitive layer formed of a semiconductor containing indium oxide as a main component, wherein the sensitive layer is selected from lanthanum, calcium, barium, gallium, aluminum, zirconium, yttrium, and scandium. Preferably, at least one or more metal oxides are added at 1 to 5 atm%, and a silica thin film is formed on the sensitive layer by chemical vapor deposition. Further, the sensitive layer is a sintered body of indium oxide to which 1 to 5 atm% of an oxide of at least one metal selected from lanthanum, calcium, barium, gallium, aluminum, zirconium, yttrium and scandium is added. A current is passed through the noble metal wire in the hexamethyldisiloxane gas at 350 ° C. to 550 ° C. (temperature of the sensitive portion).
It is preferably formed by a chemical vapor deposition process under the conditions of 25 minutes to 25 hours. In addition, the characteristic configuration of the method for manufacturing a hydrogen gas detecting element for achieving the above object is as follows. An indium oxide paste containing 1 to 5 atm% of a substance is applied, and the indium oxide is sintered by energizing the noble metal wire, and the sintered body of the indium oxide is mixed with the noble metal wire in hexamethyldisiloxane gas. And a current is passed through the noble metal wire,
A chemical vapor deposition process is performed at 25 to 25 hours at a temperature of from 550 to 550 ° C to form a silica thin film.

[Operation and effect] A technique of depositing a silica thin film on a sensitive layer mainly composed of a tin oxide semiconductor, zinc oxide or the like is known (see Japanese Patent Application Laid-Open No. 56-168542).
By forming such a thin film densely, a technique for restricting the sensitive layer from contacting with a gas other than hydrogen gas and improving the selectivity of hydrogen gas has been put into practical use (Japanese Patent Publication No. 61-1986).
-31422 gazette). However, with respect to a sensitive layer having a specific composition, although a technique for forming a dense thin film is known, in order to form a dense thin film, the properties of a base material on which the thin film is to be formed, etc. The properties may be greatly affected, and various studies are required depending on the type of the sensitive layer. It is difficult to obtain a silica thin film that selectively transmits only hydrogen gas. Is also hard to predict. However, the present inventors have recently focused on the function of the silica thin film and are selected from lanthanum, calcium, barium, gallium, aluminum, zirconium, yttrium and scandium which are considered to exhibit durability against exposure to hydrogen gas. As a result of intensive studies using a gas sensing element having a sensitive layer containing indium oxide as a main component to which at least one or more metal oxides are added in an amount of 1 to 5 atm%, hydrogen selection using an indium oxide semiconductor as a main material was performed. When a dense silica thin film is formed on a sensitive layer having a property, a new finding has been obtained that the gas detection element exhibits the above-described hydrogen gas selectivity.

That is, such a hydrogen gas detecting element is
The high-performance sensing element further improves the hydrogen gas selectivity of the sensitive layer and also has improved durability against high-concentration hydrogen gas. Here, when the silica thin film is formed so as to selectively transmit hydrogen gas, the selectivity at the time of detecting hydrogen gas can be increased, which is effective and formed by a chemical vapor deposition process. Then, it is possible to further effectively prevent deterioration due to the high concentration gas. Further, the silica thin film is formed by passing a current through the noble metal wire in a hexamethyldisiloxane gas and performing a chemical vapor deposition process at 350 ° C. to 550 ° C. for 25 minutes to 25 hours. If so, any of the characteristics can be simultaneously improved, so that a gas sensing element having extremely excellent stability can be provided.

At this time, as a method of manufacturing such a hydrogen gas detecting element, an indium oxide semiconductor paste is applied to a noble metal wire, and the indium oxide semiconductor is sintered by energizing the noble metal wire, and the indium oxide semiconductor is formed. The sintered body is introduced into the hexamethyldisiloxane gas together with the noble metal wire, a current is applied to the noble metal wire, and a chemical vapor deposition treatment is performed at 350 ° C. to 550 ° C. for 25 minutes to 25 hours to have a silica thin film. If formed into a hot wire type semiconductor gas sensing element, the noble metal wire can be used as a Joule heat supply source when performing a chemical vapor deposition process, and a hydrogen gas sensing element can be easily obtained, and such a form can be obtained. Is characterized in that its diameter can be reduced. Body type gas sensing element is at the same time has a function as a heater noble metal wire and functions as an electrode,
When the sensitive layer is formed with a small diameter, when Joule heat is supplied to the sensitive layer, the surface temperature is easily controlled to be substantially constant, which helps to obtain a uniform silica film. In addition, since the structure is simple, there is an advantage that the handling is easy and the production cost can be set low.

It is considered that the above-mentioned effects are obtained for the following reasons. When a thin film of silica is formed on the sensitive layer, as shown in FIG. 21 (schematic diagram), silica crystals close the pores of the sensitive layer, which is usually porous, in accordance with the properties of the sintered surface of indium oxide. Or a thin film having an extremely fine porous structure is formed on the outer surface of the sensitive layer to form a dense layer having the function of a molecular sieve. A gas such as ethanol having a large molecular size cannot pass through the dense layer of silica, and only hydrogen gas mainly passes through the silica thin film to reach the sensitive layer.
The hydrogen gas that reaches the sensitive layer contacts the indium oxide,
It reacts with the surface adsorbed oxygen to generate water molecules and free electrons. The free electrons generated at this time are measured as a hydrogen detection output, but the generated water molecules are transmitted to the sensitive layer and released to the outside. Then, the adsorbed oxygen becomes insufficient. Then, on the sensitive layer side of the silica thin film, since the oxygen gas permeation rate of the silica thin film is slower than that of the hydrogen gas, the gas tends to be delayed, and the gaseous oxygen concentration also decreases. Gaseous oxygen is less likely to adversely affect the reaction,
Even with a small amount of hydrogen gas, free electrons are reliably generated, so that reliable gas detection can be performed. Also, the oxidation number of indium is known to be stable only in trivalent state, and it is considered that it is unlikely to exist stably in a reduced state. Since the oxidation number easily returns to the original value, it is considered that the behavior is stable. Furthermore, lattice oxygen of indium oxide is known to have high ionicity, and surface adsorbed oxygen is thermally stable and has low oxidizing activity, so it is considered to be advantageous for strong reducing power of hydrogen. .

As a result, while having high sensitivity to hydrogen gas, there is little deterioration even when exposed to high concentration hydrogen gas of 1000 ppm or more, and the humidity dependency is small. Also provided a hydrogen gas detection element that is hardly poisoned. This has made it possible to detect gas with high reliability over a wide concentration range.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. Commercially available indium hydroxide (In (O
H) The fine powder of ₃ ) is fired using an electric furnace to obtain indium oxide powder. This indium oxide is further pulverized to form a fine powder, formed into a paste using a dispersion medium such as 1.3-butanediol, applied in a spherical shape covering the noble metal wire 1, and after drying, a current is passed through the noble metal wire 1. Then, sintering was performed in the air to obtain a hot-wire type semiconductor gas detection element including only the sensitive layer 2. Lantern, calcium, barium, gallium,
A solution of a salt of at least one metal selected from aluminum, zirconium, yttrium and scandium is impregnated, dried and calcined to allow the sensitive layer 2 to carry various metals in the form of oxides. The hot-wire semiconductor gas detection element thus produced is subjected to, for example, a saturated vapor pressure of hexamethyldisiloxane (HMDS) (about 9 Vol at 35 ° C.).
%) In the environment. The heating is adjusted so that current flows through the noble metal wire 1 to generate Joule heat so that the entire sensitive layer 2 becomes higher than the decomposition temperature of hexamethyldisiloxane. Then, the hexamethyldisiloxane in the atmosphere is thermally decomposed to form a dense silica thin film 3 on the surface of the sensitive layer 2, which is used as a hydrogen gas detecting element (see FIG. 1).

This hydrogen gas detecting element was incorporated in a bridge circuit shown in FIG. 2 and used as a gas detecting device. At this time, the sensor output (output) is obtained by the following equation.

V = −E ｛rs / (rs + r0) −r1 /
(R1 + r2)｝ Here, each variable is as follows. V: sensor output E: bridge voltage rs: resistance of the hot-wire semiconductor gas detection element Rs r0: resistance of the fixed resistance R0 r1: resistance of the fixed resistance R1 r2: resistance of the fixed resistance R2

The sensitivity was determined as the difference between the output in the air coexisting with the detection gas and the output in the clean air. When the sensitivity is expressed as the relative sensitivity, the ratio is defined as a ratio where the sensitivity output under a specific condition is set to 1 and the sensitivity under another condition is indicated.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. Example 1 Commercially available indium hydroxide (In (OH) ₃ )
(Purity 99.99atm, manufactured by Kojundo Chemical Laboratory Co., Ltd.)
%) In an electric furnace for 4 hours at 600 ° C. to obtain indium oxide powder. This indium oxide is further pulverized to a fine powder, made into a paste using 1.3-butanediol (dispersion medium), and covered with a platinum wire coil (wire diameter 20 μm) as a noble metal wire, and a spherical shape having a diameter of 0.50 mm. After drying, a current was passed through the platinum wire coil and sintering was performed at 600 ° C. for 1 hour in air to obtain a hot-wire type semiconductor gas detection element. This hot wire type semiconductor gas sensing element is impregnated with an aqueous solution of lanthanum nitrate, dried and fired, and 1 atm with respect to the indium oxide.
m%. Further, a silica thin film was deposited on the hot-wire type semiconductor gas sensing element under the following conditions.

HMDS vapor deposition treatment Processing temperature: 550 ° C. Processing time: 25 minutes HMDS vapor pressure: 9 Vol% (35 ° C.) Sensitivity measurement voltage: 2.3 V (10Ω)

The sensitivity characteristics of the gas detection element thus prepared were examined, and the change in gas sensitivity when the gas detection element was exposed to a high concentration of hydrogen gas was examined. The result was as shown in FIG.

In other words, it can be seen that a gas detection element which does not easily cause a change in sensitivity even when exposed to high concentration hydrogen is obtained.

Comparative Example 1 Tin oxide (Sn
A hot-wire type semiconductor gas sensing element having a sensitive layer mainly composed of O ₂ ) semiconductor was obtained. The hot wire type semiconductor gas sensing element is impregnated with an aqueous solution of cerium nitrate, dried and fired to add 2 atm% to the tin oxide. Further, a silica thin film was vapor-deposited on the hot-wire semiconductor gas detection element under the following conditions.

Processing temperature: 550 ° C. Processing time: 25 minutes HMDS vapor pressure: 9 Vol% (35 ° C.) Sensitivity measurement voltage: 2.3 V (10Ω)

The sensitivity characteristics of the gas detection element thus prepared were examined, and changes in gas sensitivity when the gas detection element was exposed to a high concentration of hydrogen gas were examined. The recovery of gas sensitivity after exposure was also investigated. As a result, FIG.
It became like. The hydrogen gas exposure condition was 3% hydrogen gas for 10 minutes, and the degree of recovery of the gas sensitivity was examined by the gas sensitivity after maintaining the energized state under a normal environment for one day after the exposure test.

That is, it can be seen that the gas sensing element having the sensitive layer containing tin oxide as a main component has a greatly reduced hydrogen sensitivity after being exposed to a high concentration of hydrogen gas. It can also be seen that the sensitivity has not recovered.

COMPARATIVE EXAMPLE 2 In the same manner, a hot-wire type semiconductor gas sensing element was formed without adding the above-mentioned additive to a sensitive layer containing an indium oxide semiconductor as a main component. The change in gas sensitivity when exposed to hydrogen gas was examined. The recovery of gas sensitivity after exposure was also investigated. As a result, the result was as shown in FIG. still,
The hydrogen gas exposure conditions were 3% hydrogen gas for 10 minutes, and the degree of recovery of the gas sensitivity was determined by the gas sensitivity after maintaining an energized state under a normal environment for two days after the exposure test.

That is, it can be seen that the hydrogen sensitivity is greatly reduced after exposure to a high concentration of hydrogen gas. However, it can be seen that the gas sensing element having a sensitive layer containing an indium oxide semiconductor as a main component is superior in recovering sensitivity as compared with a gas detection element containing a tin oxide semiconductor as a main component.

Example 2 The dependence of the sensor output by chemical vapor deposition on the amount of lanthanum added to the sensitive layer was examined, and the results are as shown in FIG. The conditions for the vapor deposition are the same as in the first embodiment.

That is, the one subjected to the vapor deposition treatment was compared with the one not subjected to the vapor deposition treatment to have ethanol 2000 times.
It can be seen that the hydrogen sensitivity to ppm is high and the gas selectivity is excellent.

The dependence of the sensitivity on the hydrogen gas concentration when the amount of added lanthanum was varied was examined.
It was as shown in FIG. Referring to FIGS. 3 and 6, it was found that if the added amount of lanthanum is 1 atm% or more, a sufficient effect is exhibited, and even if the added amount is increased, the gas selectivity is not significantly affected. In addition, since the phenomenon that the gas response speed decreases as the addition amount is observed, it was also found that the addition amount is desirably 5 atm% or less.

Example 3 Similarly, the sensitivity characteristics were examined by variously changing the added metal oxide.
It looked like 6. In other words, it can be seen that the durability against exposure to high-concentration hydrogen was similarly improved by using calcium, barium, gallium, aluminum, zirconium, yttrium, and scandium instead of lanthanum.

Example 4 Similarly, the detection characteristics of various types of gases for a hot-wire semiconductor gas detection element having a sensitive layer containing indium oxide semiconductor to which lanthanum oxide was added as a main component were examined, as shown in FIG. Became. That is, it is understood that high hydrogen selectivity is obtained.

[Embodiment 5] The sensitivity output of hydrogen gas sensitivity (see FIG. 15) in the above-described hot-wire type semiconductor gas detection element was 500 ppm, 1000 ppm, and 2000 ppm.
When re-expressed on the basis of, as shown in FIGS.
A relatively linear graph can be obtained. From this,
It can be seen that a hydrogen-selective gas detection element that is easy to set and is not easily affected by the interference gas can be obtained.

[Another Embodiment] Another embodiment will be described below. In the above embodiment, commercially available indium hydroxide was sintered to obtain indium oxide for preparing the paste, but a precipitate of indium hydroxide was obtained from an aqueous solution of indium salt such as indium chloride, and washed with water. Drying,
Indium oxide may be obtained by sintering.
(Co-precipitation method) In the above embodiment, HMDS was used to form a silica thin film, but halosilane (Six _x H _4-x ),
Alkyl silane (R _x SiH _4-x ), alkyl halosilane (R _x SiX _4-x ), silyl alkoxide ((RO) _x
Si (OH) _4-x ) (where X is a halogen, R is an alkyl group, x is an integer of 1 to 4, and a mixture of a plurality of types may be used for both X and R). Compounds can also be used. In such a case, the conditions are considered to be different from those of HMDS, but a dense silica thin film is expected to be obtained under conditions similar to those described above.
In the present invention, the noble metal wire is not limited to the platinum wire coil, and other noble metals such as an alloy of platinum and palladium may be used.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional perspective view of a hydrogen gas detecting element.

FIG. 2 is a circuit diagram of a gas detection device.

FIG. 3 is a gas sensing element of the present invention (additive lanthanum, 1a
(tm%) is a graph showing durability against exposure to high concentration hydrogen.

FIG. 4 is a graph showing the durability of a gas detection element of a comparative example to exposure to high-concentration hydrogen.

FIG. 5 is a graph showing the durability of a gas detection element of a comparative example to exposure to high-concentration hydrogen.

FIG. 6 is a graph showing the dependence of gas detection sensitivity on the amount of lanthanum added.

FIG. 7 shows a gas detection element (lanthanum additive, 2a) of the present invention.
(tm%) is a graph showing durability against exposure to high concentration hydrogen.

FIG. 8 shows a gas detection element of the present invention (additive lanthanum, 4a
(tm%) is a graph showing durability against exposure to high concentration hydrogen.

FIG. 9 shows a gas sensing element of the present invention (additive lanthanum, 6a
(tm%) is a graph showing durability against exposure to high concentration hydrogen.

FIG. 10 shows a gas sensing element (additive calcium,
Graph showing durability against exposure to high concentration hydrogen (2 atm%).

FIG. 11 is a graph showing the durability of the gas detection element (zirconium additive, 2 atm%) of the present invention to exposure to high concentration hydrogen.

FIG. 12 is a graph showing the durability of the gas detection element (additive aluminum, 2 atm%) of the present invention to exposure to high concentration hydrogen.

FIG. 13 shows a gas detection element (additive barium,
A graph showing the durability against exposure to high concentration hydrogen (2.5 atm%).

FIG. 14 shows a gas detection element (additive gallium,
Is a graph showing durability against exposure to high-concentration hydrogen (0.5 atm%).

FIG. 15 is a graph showing the durability of the gas detection element (additive yttrium, 4 atm%) of the present invention to exposure to high-concentration hydrogen.

FIG. 16 is a graph showing the durability of the gas sensing element of the present invention (additional scandium, 4 atm%) to exposure to high concentration hydrogen.

FIG. 17 is a graph showing gas selectivity of the gas detection element of the present invention.

FIG. 18 is a graph showing a hydrogen sensitivity curve of the gas detection element of the present invention based on 500 ppm hydrogen gas sensitivity.

FIG. 19 is a graph showing a hydrogen sensitivity curve of the gas detection element of the present invention based on 1000 ppm hydrogen gas sensitivity.

FIG. 20 is a graph showing a hydrogen sensitivity curve of the gas detection element of the present invention based on 2000 ppm hydrogen gas sensitivity.

FIG. 21 is a schematic view illustrating an improvement in gas selectivity by a silica thin film.

[Explanation of symbols]

 1 Noble metal wire 2 Sensitive layer 3 Silica thin film

Claims (5)

[Claims] 1. A gas sensing element provided with a sensitive layer formed of a semiconductor containing indium oxide as a main component over a noble metal wire, wherein the sensitive layer includes lanthanum, calcium, barium, gallium, aluminum, A hydrogen gas detection element which contains at least 1 to 5 atm% of an oxide of at least one metal selected from zirconium, yttrium and scandium and forms a dense silica thin film. 2. A gas sensing element provided with a sensitive layer formed of a semiconductor containing indium oxide as a main component and covering a noble metal wire, wherein the sensitive layer includes lanthanum, calcium, barium, gallium, aluminum, A hydrogen gas detection element comprising an oxide of at least one metal selected from zirconium, yttrium, and scandium in an amount of 1 to 5 atm% and forming a hydrogen selective permeable silica thin film on a sensitive layer. 3. A gas sensing element provided with a sensitive layer formed of a semiconductor containing indium oxide as a main component over a noble metal wire, wherein the sensitive layer includes lanthanum, calcium, barium, gallium, aluminum, A hydrogen gas detection element containing an oxide of at least one metal selected from zirconium, yttrium, and scandium in an amount of 1 to 5 atm%, and a silica thin film formed on a sensitive layer by a chemical vapor deposition process. 4. The sensitive layer, wherein the sensitive layer has a lantern,
A sintered body of indium oxide to which 1 to 5 atm% of an oxide of at least one metal selected from calcium, barium, gallium, aluminum, zirconium, yttrium and scandium is added; An electric current is passed through the noble metal wire in siloxane gas, and the temperature is set at 350 to 550 ° C. for 25 minutes to 25 minutes.
The hydrogen gas detection element according to any one of claims 1 to 3, wherein the hydrogen gas detection element is formed by a chemical vapor deposition process under a condition of time. 5. A precious metal wire comprising lanthanum, calcium,
An indium oxide paste containing 1 to 5 atm% of an oxide of at least one metal selected from barium, gallium, aluminum, zirconium, yttrium, and scandium is applied, and the indium oxide is baked by energizing the noble metal wire. Then, the sintered body of indium oxide is introduced into the hexamethyldisiloxane gas together with the noble metal wire, and a current is applied to the noble metal wire to perform chemical reaction at 350 ° C. to 550 ° C. for 25 minutes to 25 hours. A method for producing a hydrogen gas detection element that forms a silica thin film by performing a vapor deposition process.