JPS5879149A - Gas detection element - Google Patents

Abstract

PURPOSE:To make it possible to measure with high sensitivity and selectively gases of low temperature such as NH3, Cl, H2S, and so on by making the specific elecricity conductivity of a metal oxide semiconductor larger in the area near the surface than in the internal area and making the thickness of the area near the surface smaller than a specified value. CONSTITUTION:The thickness of an area 4 near the surface of a metal oxide semiconductor 3 such as SnO2, ZnO, MgFe2O4, etc. is made below 500mu, and the specific electricity conductivity of the area 4 is made larger than that of the semiconductor 3. For example, the semiconductor 3 is constituted with SnO2- Cr2O3 (100:0.5 weight ratio), and a layer 4 of SnO2-Sb2O3 (100:0.8 weight ratio) that has the specific electricity conductivity larger than that of the layer 3 is painted on the layer 3. Or the sintering temperature is controlled to make the specific electricity conductivity of the layer 4 larger than that of the layer 3. Platimum electrodes 1 and 2 are provided on this semiconductor element, and they also serve as a heater. The element becomes a durable detection element that detects selectively with high-sensitivity noxious gases such as CO, NH3, Cl, H2S, etc.

Description

Detailed Description of the Invention The present invention relates to the improvement of gas detection elements, and in particular to the improvement of elements that selectively detect gases such as Co, NH3, C12, H2S, etc. This is effective for composition detection, etc.

Gas detection elements currently in use can be broadly classified into two types. One of them is to fully utilize the catalytic combustion of combustible gas using an oxidation catalyst, and the combustion heat k is detected as a change in the resistance value of a width-measuring resistor. The other method utilizes changes in the resistance value of a metal oxide semiconductor due to adsorption and desorption of gas, and is widely used due to its high reliability.

Attempts have also been made to selectively detect gases such as co, NH3, Cl2, H2S, etc. using elements that utilize changes in the resistance value of metal oxide semiconductors. Sensitivity to these gases can be improved by lowering the heating temperature of the element. However, there is a limit to lowering the heating temperature of the element. This is because problems such as deterioration of humidity characteristics, decrease in response speed, and deterioration of characteristics over time may occur.

Such a metal oxide 31' conductor is also an oxidation catalyst for combustible gases. Gases that are relatively susceptible to oxidation, such as CO and NH3, are removed during diffusion into the metal oxide semiconductor. On the other hand, interfering gases such as hydrogen and ethanol are less susceptible to oxidation. It is already known from Japanese Unexamined Patent Publication No. 155292/1983 that metal oxide semiconductors function as filters for gases.

Considering the filter effect, if the thickness of the metal oxide semiconductor is made small, gases such as CO and NH3 should be selectively detected. For this purpose, there is no other way than to support the semiconductor film using a base material such as alumina. However, the coefficient of thermal expansion of a semiconductor and that of a substrate generally do not match. And the element has its on/off throat and vc
Exposure to thermal shock. As a result, the semiconductor may peel off from the substrate or crack. This tendency becomes more pronounced as the semiconductor film becomes thinner. Since the thermal expansion coefficients of the semiconductor and the substrate do not match, it is impossible to make the semiconductor film thin enough to obtain selectivity for gases such as CO.

The overall object of the present invention is to provide an element that can advantageously detect low-temperature gases such as CO, NH3C12, I2S, etc., using a metal oxide semiconductor having a large thickness.

The gas-sensitive element of the present invention has at least one pair of electrical connections to a gas-sensitive metal oxide semiconductor to increase the piezoelectric conductivity of the metal oxide semiconductor ostensibly in the vicinity of the surface.
The hole is made larger than the internal head, and the thickness of the area near the surface is 500/7 or less.

As the metal oxide semiconductor, those whose resistance value changes due to adsorption and desorption of gas are widely used, and 5n02.
TiO2, In203Fe203. ZnO+CoO+N
iO, MgFe2O4, etc. can be used. Metal oxide semiconductors contain various additives, such as noble metal catalysts to improve sensitivity, binders such as alumina and amorphous silica to prevent cracks and improve mechanical strength, and metal oxide semiconductors. You can also add it.

As a means for changing the piezoelectric conductivity of the metal oxide 21 near the surface and inside the conductor, valence control, control of the number of carriers by controlling the firing conditions of the semiconductor, etc. may be used. Good luck with valence control! Examples of increasing the electrical conductivity of 14100 include the addition of 5b203 to 5n02, T + 02 to Fe2O3 + Al to ZnO2.
2O3, I-i20 to CoO, Li to Nio
There are 20 additions, etc. For the opposite example, C to 5n02
Addition of r2O3 to ZnO I20. Cr to NiO
2O3's? There are A additions, etc. The piezoelectric conductivity is controlled by controlling the firing conditions as follows.

The higher the firing conditions for a semiconductor, the fewer defects in its crystal and the lower the electrical conductivity. By utilizing this property and selecting the firing temperature, the electrical conductivity of the semiconductor can be controlled. Then, using the above method, the piezoelectric conductivity near the surface of the metal oxide semiconductor and inside the metal oxide semiconductor are made different. Specifically, the following methods may be used: (1) Use a material with high piezoelectric conductivity near the surface, (2) Use a material with low piezoelectric conductivity inside, (2) Use both in combination.

Although not particularly limited, when used in 5n02, it is desirable to avoid adding tin chloride to already fired SnO2. 5nCI! This is because either the piezoelectric conductivity of 5n02 increases due to the addition of 5nC12, or the sintering of the added 5nC12 is insufficient, resulting in low stagnation over time.

A wide variety of known shapes can be used for the shape of the gas detection element, and there is no necessity to reduce the thickness of the metal oxide semiconductor. Existing shapes, such as those shown in FIGS. 1 and 2, can be used by cutting them.

The reason why the part of the metal oxide semiconductor where the piezoelectric conductivity should be higher than the inside is set to 500 ti or less from the surface is as follows.
This is because the sensitivity to CO and NH3 decreases if the temperature exceeds this range. When this part is formed by vapor deposition etc., the thickness is 1
It may be about μ.

The operation of the gas detection element of this invention can be considered as follows.

The electrical conductivity of the gas detection element is determined by the metal oxide semiconductor near the surface. And near the surface,
Gases such as CO and NH3+H2S are also not affected by the oxidation activity of semiconductors. The electrical conductivity inside the semiconductor, which is affected by filter activity, has little effect on the output of the device. Therefore, by increasing the electrical conductivity near the surface of the metal oxide semiconductor from the inside, gases such as CO and NI3 can be detected with high sensitivity.

In the following, I will explain and explain the example of Tanimi-Gan.

[citrus materials]

Neutralize with 5nC14 aqueous solution ya=NHs, S n (
OH) 4 precipitate denominator. After washing the precipitate with water, heat at 300°C for 2
Perform the first stage and firing of the time. 5n02ke2 after 1st Akatsuki
Next firing (600°C x 2 hours) L2 and pulverization. This 5n
02 is hereinafter referred to as 5nO2 (a). All of the following materials shall be pulverized after secondary firing.
I will omit explanation 1 to that effect.

5l) 203 of 5n02100 parts by weight π08 parts by weight after the 1st stage and dawn was added and baked at 600°C for 2 hours.

In this firing process, antimony is oxidized with V measurement, and 5
Valence control of n02 is performed. by SnO
The It electrical conductivity of 2 is increased approximately 50 times that of 5n02(A). This is shown as k S n 02 S b203.

Sn 02100 weight i1'c Mi'n after primary firing
A Crc13 aqueous solution (C+; 0.5 weight and 11 parts with 5Q3 exchange) is added and baked at 600°C for 2 hours. This reduces the specific electrical conductivity by about 11/1000 of 5 nO2 (A). This is designated as S n 02-Cr 203 .

ZnCj? The aqueous solution of Zn(01()2) is neutralized with NaOH water and washed with water repeatedly to obtain a precipitate of Zn(01()2. This precipitate is subjected to primary firing for 2 hours at 300°C in the atmosphere, and then exposed to the atmosphere. The product is heated for 2 hours at 500° C. for secondary firing.

After primary firing, add A of 1 layer to 1 layer of Zn0100 layer I.
, l! Add 203 and bake at 500°C for 2 hours.

As a result, the specific electrical conductivity is approximately 30 of Z n 0 (A).
increase twice. Is this thing fZnOAI'? 203.

Add 1 layer of Li20 to 100 parts by weight of ZnO after primary firing
Add water and heat at 500°C for 2 hours. As a result, the specific electrical conductivity decreases to 1/1000 or less of Zn0 (a). This is designated as 'rZnO-L i20.

After the primary firing, ZnOk was heated in oxygen for 2 hours at 1000 C. The electrical conductivity of this material was Z n 0 (
It is approximately 1/H1 of A). This is shown as total ZnQ[F]).

[Structure of element]

The structure shown in FIGS. 1 and 2 is used. In the figure (
1) and (2) are coil-shaped platinum electrodes, which also serve as heat shields. (3) is the internal part of the metal oxide semiconductor, and its thickness is 2 mm. ) from d, electric +1f (1), about 'l' of (2)
Cover Dk. (4) is the surface part of the metal oxide semiconductor, metal oxide semiconductor (3), 1. Apply it to the knee to a predetermined thickness [7]. Note that instead of applying it, you may use a method such as a spy. The thickness of the metal oxide semiconductor (4) may not be uniform. This is because it is difficult to apply the coating to a uniform thickness, and even if there is unevenness in the thickness, it does not affect the characteristics of the device.

FIG. 3 shows a modified example of the structure of the element. In the figure, (]
1) is the internal part of the metal oxide semiconductor with low electrical conductivity, c is the surface part Q of the metal oxide semiconductor, and l
It is mm. 0■, 04) are thick film electrodes made of platinum, and θ0 is a thick heater made of silver-baladicum or the like. In this case, electric ft
There is no need to consider the deterioration of semiconductor 01 due to (13, (14) or the glass component in heater 0). Semiconductor 01
) has a small electrical conductivity. The characteristics of the device of this structure are similar to those of FIGS. 1 and 2.

[Sensitivity characteristics]

Test 1] As a representative example of a gas, 50 ppml of Co was selected, and the resistance value ratio with that in clean air and the resistance value in CC0501)T) were measured. H2 is selected as a representative example of Suishu gas, and H2 gives a weight of 1, which is the same as C050ppm.
Double (hereinafter referred to as H2 equivalent concentration) was determined.

For measurements, the elements shown in Figures 1 and 2 were used, and the temperature was 20°C1.
Experiments were conducted in 65% RH. The addition degree of the element is 5n02
200°C when using ZnO, 250°C when using ZnO
It was set to °C.

The results are shown in the table.

In the table, 141- of the composition indicates the composition of the surface portion, and the lower column indicates the composition of the inside. The thickness of the surface portion is simply indicated as "thickness". '8I8Those marked with * are useless for comparison.

As explained in 1-1, according to the present invention, a metal oxide semiconductor having a sufficient thickness, Co, N, etc.
H3, C.j? 2. Highly sensitive detection of gases such as H2S11
” I can do it.

[Brief explanation of the drawing]

FIG. 1 is a cutaway view of one child of the embodiment, and FIG. 2 is a sectional view taken along the line A-A' in FIG. 1. FIG. 3 is a sectional view of the eleventh embodiment. (1,), (2), 01, (14)...electrode, (3), (]υ...inner part of metal oxide semiconductor, (4), 0° surface part of metal oxide semiconductor Special W "Applicant: Figaro Giken Co., Ltd. r1 - Representative: Ei Chiba Figure 3, 2

Claims (1)

[Claims] 1. In a gas detection element in which at least one pair of electrodes is connected to a bulk metal oxide 1' conductor whose resistance value changes depending on the gas, the specific electrical conductivity of the metal oxide 21' conductor near the surface is measured. Make the area larger than the inner area,
A gas detection element characterized in that the thickness of the region near the surface is 500μ or less.