JPS6213620B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas sensing element, particularly to a combustible gas sensing element using a gamma type ferric oxide (γ-Fe ₂ O ₃ ) sintered body having a spinel crystal structure as a gas sensitive body. It is something.

Conventionally used combustible gas detection elements include methods that use an oxidation catalyst, such as platinum black, to detect the temperature rise due to the heat of combustion of combustible gas with a resistor in contact with the oxidation catalyst, and methods that use an N-type oxide semiconductor. , a method of detecting a change in electrical conductivity of an oxide due to oxidation-reduction due to contact with a combustible gas, a method of detecting a change in contact resistance of powder depending on the atmosphere, etc. are known.

In the method using an oxidation catalyst, temperature changes are detected as changes in electrical resistance using a platinum resistor or thermistor, but the sensitivity is poor because the temperature coefficient is small. It also requires strict temperature control. A method using powder contact resistance is a method of detecting a change in contact electrical resistance between powder particles, but it is inferior in stability because the contact state of the particles easily changes. N
The method using a type oxide semiconductor has a large change in electrical resistance of the semiconductor with respect to gas concentration and is highly sensitive. Therefore, there is no need for strict temperature control in response to temperature changes as in the method using an oxidation catalyst. This also simplifies the structure of the element.

As an N-type oxide semiconductor gas sensing element material,
SnO ₂ , ZnO, CdO, TiO ₂ , Fe ₂ O ₂ , WO ₃ , MoO ₃
etc. are known. Although sensing elements using SnO ₂ have high sensitivity, they do not completely meet practical requirements in terms of temperature characteristics, changes over time, lifespan, etc. ZnO and CdO have low sensitivity and lack practical use. TiO ₂ , Fe ₂ O ₃ , WO ₃ , MoO ₃ and the like have almost no sensitivity and have not been used as gas detection materials to date.

Among the above three methods, it is also possible to produce an element that detects gas by using powdered N-type semiconductor material and changing its contact resistance. This can be achieved, for example, by applying SnO ₂ powder between the electrodes, and the synergistic effect of SnO ₂ 's highly sensitive electrical resistance change to combustible gas and the powder's contact resistance change results in even more sensitivity. A highly combustible gas detection element can be constructed. However, the contact state of powder particles tends to change, resulting in poor stability.
SnO ₂ itself is not sufficient as a gas detection element due to deterioration of its sensitivity to combustible gases due to changes in its temperature characteristics.

The present invention was achieved by discovering remarkable gas-sensitive characteristics in γ-Fe ₂ O ₃ , which has a spinel crystal structure, which is one of the crystal phases of ferric _oxide . By using a sintered body of O ₃ as a gas sensitive body, a gas sensing element having high sensitivity and stable gas sensitive characteristics is provided.

Next, a concrete explanation will be given with reference to examples.

[Example 1] As shown in Figures 1a and b, a silver paste for baking (when the sintering temperature is high, gold, platinum, silver-palladium etc.) was printed in a comb-shaped pattern with intervals of 0.5 mm and baked at a temperature of 800°C. α-Fe ₂ O ₃ on this baked electrode 2
(particle size: 0.1 μm) was kneaded into a paste with a 2% aqueous solution of polyvinyl alcohol and applied to a thickness of 0.1 mm. After drying this at 60℃ for 3 hours and at 100℃ for 1 hour, being careful not to create any cracks,
Sintering was carried out at 700-1300°C for 1-10 hours in air or oxygen. When the temperature is high, α-Fe ₂ O ₃ sintered in this way becomes more semiconductive and has a black metallic luster in appearance. When measuring electrical resistance, it is 1 to 10 ¹²
There is a wide range depending on Ω and the degree of semiconducting.
All of these were confirmed to be α-Fe ₂ O ₃ by X-ray diffraction.

This was reduced at 200-450°C under a flow of hydrogen gas containing saturated steam for 10-60 minutes. By this operation, α
- All Fe ₂ O ₃ changed to Fe ₃ O ₄ regardless of whether it was converted into a semiconductor or not. If water vapor is absent or in a small amount, FeO will be generated at the same time, so care must be taken in the reduction process. The same goes for higher temperatures.

Next, the Fe ₃ O ₄ element is heat-treated in air at 150 to 400°C to form a γ-Fe ₂ O ₃ sintered body 3.
I got it. Note that high temperature or rapid oxidation (such as oxidation under high oxygen pressure or coexistence of an oxidizing agent) will promote the formation of α-Fe ₂ O ₃ , so in the above heat treatment process, the temperature should not be lower than 400 °C. You need to heat it up gradually.

Through the above process, a gas sensitive body made of the sintered body 3 of γ-Fe ₂ O ₃ can be formed between the pair of electrodes 2-2 on the alumina ceramic substrate 1.

In the alumina substrate 1, the γ-Fe ₂ O ₃ sintered body 3
A platinum resistance heating element 4 is pasted with an inorganic adhesive on the side opposite to that on which the alumina substrate 1,
The entire body was covered with a 100-mesh protective wire mesh 5 so as not to come into contact with the γ-Fe ₂ O ₃ sintered body 3 and the heating element 4. Further, a lead wire was taken out from the electrode and connected to a resistance meter, and then a current was passed through the heating element to maintain the temperature at (270°C±1°C).

As a result of measuring electrical resistance in air under these conditions,
(350±5) KΩ, and the result of measurement in air containing 1% by volume of propane gas was (6.2±0.2) KΩ. This measurement was carried out 10 times every 5 days for 100 days and the results are shown in Figure 3. Except for initial fluctuations, there are almost no changes over time. Moreover, the gas sensitivity is more than 50 times higher, indicating that both sensitivity and stability are extremely excellent.

[Example 2] A 1% polyvinyl alcohol aqueous solution was added to α-Fe ₂ O ₃ powder (200 mesh) at a rate of 0.1 cc/g, mixed in an agate mortar, and then 20 mm x 50 mm x 2 mm.
It was pressure molded to the dimensions of . Heat this to 800℃~1000℃3
By firing in an oxygen atmosphere for several hours, a sintered plate 11 of α-Fe ₂ O ₃ as shown in FIG. 2 was produced. The appearance is red and black. When the temperature is high, it becomes a semiconductor and has a black metallic luster in appearance. This has low electrical resistance and is not suitable as a gas sensing element substrate. This α-
A sintered body plate 11 of Fe ₂ O ₃ was cut into a size of 20 mm x 10 mm x 1 mm, and a layer was placed on it in the same manner as in Example 1.
An electrode 12 made of silver or the like was printed and baked into a comb shape. Next, reduction treatment produces Fe ₃ O ₄ , which is then slowly oxidized to γ
- Created Fe ₂ O ₃ layer 13. This process is the same as in Example 1, but the reduction is carried out at a depth of 10 μm from the surface of the sintered body.
The reduction time was adjusted so that the reduction was carried out to the extent possible. This adjustment determines the sensitivity. Reduction time is 350
°C for 10 to 15 minutes is appropriate.

When the γ-Fe ₂ O ₃ layer 13 is thick, the contribution of changes in surface electrical resistance to the electrical resistance of the element becomes smaller, resulting in lower sensitivity. However, the entire sintered body 11 is γ-
Even if Fe ₂ O ₃ is used, the sensitivity will be lower than in the above case, but this will not pose any practical problem. On the other hand, if this layer 13 is too thin, the electrical resistance of the device will be high and it will also lack stability.

The structure of the element was the same as in Example 1, in which a platinum resistance heating element 14 was arranged and surrounded by a wire mesh 15. The results are shown in FIG. 4 under the same measurement conditions. As shown in the figure, as in Example 1, both sensitivity and stability are extremely excellent. Furthermore, this example has very good adhesion between the substrate and γ-Fe ₂ O ₃ and can withstand long-term use.

It goes without saying that by adding additives to γ-Fe ₂ O ₃ , its properties such as sensitivity and stability can be controlled.

As described above, the present invention uses a sintered body of γ-Fe ₂ O ₃ , which exhibits remarkable gas-sensitive characteristics, as a gas-sensitive material, and has extremely excellent characteristics in both sensitivity and stability. Since it can provide a gas detection element, it has great practical value.

[Brief explanation of the drawing]

1A and 1B are a cross-sectional view and a partially cutaway plan view showing one embodiment of a gas detection element according to the present invention, FIG. 2 is a cross-sectional view of another embodiment of the present invention, and FIG.
4 and 4 are diagrams showing changes over time in the characteristics of the gas detection element having the configuration shown in FIGS. 1 and 2, respectively. 1... Alumina porcelain plate, 2... Electrode, 3... γ
-Fe ₂ O ₃ sintered body, 4...Platinum resistance heating element, 5...
Wire mesh, 11...α-Fe ₂ O ₃ sintered body plate, 12... Electrode, 13... γ-Fe ₂ O ₃ layer, 14... Platinum resistance heating element, 15... Wire mesh.

Claims (1)

[Claims] 1. A sintered body containing gamma-type ferric oxide as a main component is used as a gas sensitive body, and a combustible gas is detected by detecting a change in electrical conductivity of the gas sensitive body due to contact with a flammable gas. A gas detection element consisting of: