JPH05223769A - Nitride oxide gas detector element - Google Patents

Abstract

PURPOSE:To obtain an NOX gas detector element showing high sensitivity and selectivity stably for a long period against low concentration NOX gas. CONSTITUTION:On an insulated base plate 1, Au (gold) and Sn (tin) are evaporated after heat treatment so that Au weight is lower than 7 times of Sn weight. Then by heat treating in the air, a gas sensing body 2 consisting mainly of SnO2 (tint oxide) is formed, a pair of comb shape Au electrodes 3 are formed on the gas sensing body 2 and NOX gas sensor element is produced.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a relative low concentration of NO _X (nitrogen oxide) gas, to NO _X gas detecting element which exhibits excellent sensitivity and selectivity stably for a long period of time.

[0002]

2. Description of the Related Art NO _X gas, which is mainly composed of NO (nitrogen monoxide) gas and NO ₂ (nitrogen dioxide) gas, is discharged from internal combustion engines of automobiles, airplanes, etc., heating equipment at home, etc. It is a gas with a high risk of having a great impact on the human body, living things, living environment, etc. due to its compounding, adsorption, and reaction with other gases. Conventionally, a chemiluminescence method, an infrared absorption method, an electrolysis method, etc. have been used for the measurement of this NO _x gas, but these methods have a problem that a large-scale device is required and maintenance is difficult. .

Therefore, in order to solve the above problem, NO
Studies have been made various for small maintenance-free of the NO _X gas sensing element including an oxide semiconductor in contact with the _X gas changes in resistance. For example, Japanese Patent Laid-Open No. 61-93945 discloses Sn containing Fe (iron) on a substrate.
A gas-sensitive thin film formed mainly of O ₂ polycrystal is disclosed in Japanese Patent Application Laid-Open No. 1-150849, which has an oxygen defect with a non-stoichiometric parameter (δ) of 0.01 <δ <0.5. And oxides containing titanium atoms are introduced.

[0004]

However, when looking at the detected temperature in the above-mentioned conventional NO _x gas detecting element, it is 300 to 400 in JP-A-61-93945.
C., 300 to 700 in JP-A-1-150849
Since the temperature is considerably high, the heater requires a large amount of electric power, and there is a risk of combustible gas being ignited, a sensing element that operates at a lower temperature is desired. Further, when looking at the gas sensitivity, it is 1 in JP-A-61-93945.
2 or less for 00 ppm NO gas, JP-A-1-150
In Japanese Patent Publication No. 849, a value as low as about 5 is shown, and higher sensitivity is desired for incorporation in a simpler circuit. The object of the present invention is to solve the above-mentioned conventional drawbacks.
00 ° C. is also operable in the following relatively low temperature to provide a NO _X gas sensing element showing selectivity and high sensitivity to low concentrations of the NO _X gas stably for a long period of time.

[0005]

In order to achieve the above object, the NO _x gas detecting element of the present invention comprises an insulating substrate, SnO ₂ provided on the substrate as a main component, and 700 parts by weight with respect to 100 parts by weight of Sn. It is composed of a gas sensitive body containing Au (gold) of a certain amount or less, and a pair of electrodes provided on the gas sensitive body.

As the gas sensitive material used in the present invention, one containing SnO ₂ as a main component and 700 parts by weight or less of Au with respect to 100 parts by weight of Sn is used. It is formed by directly forming an oxide semiconductor by a sputtering method or the like, or by forming Sn in the same film shape and then oxidizing it. When the content of Au exceeds 700 parts by weight with respect to 100 parts by weight of Sn, S
Since nO ₂ cannot maintain semiconductivity, NO _x gas cannot be detected substantially. Further, it is desirable that the film thickness of the gas sensitive body is 100 Å or more, and if it is less than 100 Å, the resistance value of the element becomes too high and sufficient sensitivity cannot be obtained. Further, Au contained in the gas sensitive body
Is in the form of finely divided particles having a particle size of 10 μm or less. If such particulate Au is not contained at all, the sensitivity becomes low, which is not preferable.

The substrate used in the present invention is
For example, a heat resistant and insulating substrate such as a ceramic substrate made of Al ₂ O ₃ (aluminum oxide) or SiO ₂ (silicon oxide) is used.

The electrodes used in the present invention include:
For example, Au, Pt (platinum), or the like is used, and is formed in contact with and facing the gas sensitive body by a screen printing method, a sputtering method, an evaporation method, or the like.

The NO _x gas detecting element of the present invention may be provided with a heating element to keep the temperature of the element at a predetermined temperature. A self-temperature control type (PTC) heater may be used as the heating element.

[0010]

According to the present invention, NO _X gas detection that can operate even at a relatively low temperature of 300 ° C. or less and stably exhibits high gas sensitivity and gas selectivity for a long time even with a low concentration of NO _X gas. An element can be obtained.

[0011]

The present invention will be described in more detail below with reference to the drawings. FIG. 1 is a cross-sectional view (conceptual view) showing an embodiment of the NO _X gas detecting element according to the present invention. FIG. 2 is a plan view showing an example of the comb-shaped Au electrode used in the present invention. In this embodiment, first, as shown in FIG. 1, a quartz substrate having a length, width, and thickness of 13 × 10 × 1 mm is prepared as the insulating substrate 1, and Au is deposited on the substrate in an amount of 70 parts by weight per 100 parts by weight of Sn.
Au and Sn are vacuum-deposited (A
u300Å, Sn500Å) and heat treatment (500 ° C.) for 2 minutes in air to obtain a gas sensitive body 2 having a film thickness of 1000Å. Next, a comb-shaped Au electrode 3 having a thickness of 40 nm as shown in FIG. 2 was formed on the gas sensitive body 2 by a vacuum deposition method. Further, a heating element 4 for keeping the temperature of the element at a predetermined temperature is provided on the back surface of the insulating substrate 1. Figure 3
FIG. 4 shows the surface of the gas sensitive body in the NO _x gas detecting element of the present invention manufactured as described above, as observed with a scanning electron microscope. From this, it can be seen that the Au (light colored portion) contained in the gas sensitive body has a particle size of 10 μm or less and is in the form of finely divided particles.

Here, the NO _x gas detecting element of the present invention is fixed in a closed tank, and NO gas and NO ₂ gas having a concentration of 30 ppm, C ₃ H ₈ (propane) gas having a concentration of 1000 ppm, CO ( Various gases such as carbon monoxide gas, H ₂ (hydrogen) gas, and C ₂ H ₅ OH (ethanol) gas were injected with a syringe and stirred with a fan, and the electric resistance value of the element in various gas atmospheres was measured. The sensitivities to various gases are shown in FIG. The sensitivity is [resistance value in gas (R
_gas ) / resistance value in _air (R _air )]. As is clear from FIG. 4, the element of the present invention is several tens of degrees Celsius to 300
It shows high gas selectivity in a wide range up to around ℃.

FIG. 5 shows the time-dependent characteristics of sensitivity to NO gas and NO ₂ gas having a concentration of 30 ppm when the NO _X gas detecting element according to the present invention is kept at 160 ° C. The device of the present invention stably maintains high sensitivity for a long period of time.

For comparison, an element containing no Au mixed in the above-mentioned embodiment was prepared, and its sensitivity to NO gas and NO ₂ gas having a concentration of 30 ppm was measured. The results are shown in FIG. As is clear from FIG. 6, sufficient sensitivity was not obtained when Au was not mixed. Furthermore,
Vacuum deposition of Au and Sn (Au1500Å, Sn5) so that Au exceeds 700 parts by weight with respect to 100 parts by weight of Sn.
When a device having a gas sensitive body having a film thickness of 2200Å obtained by heat treatment (500 Å) for 2 minutes in air (500 ° C.) was also prepared in the same manner, the contained Au contained particles having a size of more than 10 μm. And showed almost no sensitivity to NO _x gas.

[0015]

As described above, according to the present invention, by providing a gas sensitizer containing SnO ₂ as a main component and 700 parts by weight or less of Au to 100 parts by weight of Sn on an insulating substrate, a temperature of 300 ° C. N even at relatively low temperatures below
O _X long-term high gas sensitivity and gas selectivity for gas it becomes possible to provide a NO _X gas sensing element which can be stably maintained.

[Brief description of drawings]

FIG. 1 is a sectional view (conceptual view) showing an embodiment of a NO _x gas detection element according to the present invention.

FIG. 2 is a plan view showing an example of a comb-shaped Au electrode used in the present invention.

FIG. 3 is a scanning electron micrograph (× 2000) showing a particle structure on the surface of the gas sensitive body in the NO _x gas detecting element according to the present invention.

FIG. 4 is a graph showing the sensitivity characteristics of the NO _x gas detection element according to the present invention to various gases.

5 is a graph showing the time characteristic of the sensitivity to NO _X gas of the NO _X gas detector device according to the present invention.

FIG. 6 is a graph showing the sensitivity characteristic of a device having a gas sensitive body not containing Au to NO _x gas.

[Explanation of symbols]

 1 Insulating substrate 2 Gas sensor 3 Comb type Au electrode 4 Heating element

Claims (3)

[Claims] 1. An insulating substrate and Sn provided on the substrate
Consists of a gas sensor containing O ₂ (tin oxide) as a main component and 700 parts by weight or less of Au (gold) with respect to 100 parts by weight of Sn (tin), and a pair of electrodes provided on the gas sensor. Nitrogen oxide gas detection element. 2. The nitrogen oxide gas detection element according to claim 1, wherein the film thickness of the gas sensor is 100 Å or more. 3. The nitrogen oxide gas detection element according to claim 1, wherein at least a part of Au contained in the gas sensor is in the form of finely divided particles having a particle size of 10 μm or less.