JPS5840138B2 - Kanenseigaskenchisoshi - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses a sintered body mainly composed of alpha-type ferric oxide (α-Fe203) as a base material, and a gamma-type ferric oxide (γ-Fe203) having a spinel crystal structure is formed on the base body. The present invention relates to a combustible gas detection element characterized by having a sintered film mainly composed of -Fe2O3) as a gas sensitive body.

Conventionally used gas detection elements include elements that use platinum as a combustible catalyst and detect the heat of combustion as resistance changes due to increased humidity in platinum wires or other resistors, and elements that detect color changes due to carbon monoxide in palladium salts. 2. Description of the Related Art Elements that use a phototube to detect a combustible gas, and elements that use an N-type oxide semiconductor and utilize its high mobility to detect a combustible gas as a change in resistance are known.

None of these gas sensing elements are perfect, and various practical drawbacks have been pointed out.

For example, the detection element using the platinum catalyst described above has excellent stability but low sensitivity.

Furthermore, a sensing element that uses a change in color tone of palladium salt has the drawback that it is difficult to store the element for a long period of time, and furthermore, it cannot withstand repeated use.

Among the above-mentioned sensing elements, sensing elements using N-type oxide semiconductors have the characteristics of high sensitivity and durability over repeated use, and are said to be formed with a simple structure.

It is attracting attention because of its practical advantages.

Among N-type semiconductors, tin oxide (
Sn02), zinc oxide (ZnO), cadmium oxide (C
dO) etc. are known.

For example, a detection element using S n 02 has high sensitivity, but it cannot be said to completely meet practical requirements in terms of temperature characteristics, changes over time during continuous energization, life span, and the like.

CdO and ZnO have low sensitivity as detection elements, and CdO and ZnO have low sensitivity as detection elements.
d, Zn, etc. are materials whose use should preferably be avoided in order to prevent pollution.

This also includes titanium oxide (T 102 ), aluminum oxide (A1203), tungsten oxide (WO3),
Although molybdenum oxide (Mo03) can be used as a material for gas detection, it has not attracted attention as a practical material.

In addition to this, there is one that uses gamma type ferric oxide (γ-F e 20 a ) that has been discovered relatively recently.

This involves dispersing comma-shaped ferric oxide (γ-Fe2O3) powder, which is used as a magnetic recording medium such as magnetic tape, into a liquid, then coating it on an insulating substrate and baking it at a temperature of about 400℃. , a film is formed.

After forming a pair of electrodes on this film, 250
This is an application of the phenomenon that when the resistance value is measured in a state heated to about .degree. C. to 350.degree. C., the resistance value changes significantly depending on the composition of the atmospheric gas at that time.

The gas sensitivity characteristics of such elements are evaluated by the ratio (RA/RG) of the resistance value when the atmospheric gas is air (RA) and the resistance value when flammable gas is mixed (RG). The larger the value, the more sensitive the sensing element is.

Taking the case of the gamma type ferric oxide (γ-Fe2O3) film mentioned above as an example, when the temperature of the element is maintained at 270°C and the air contains 0.1% by volume of propane gas as a flammable gas. , RA/RG shows an extremely large value of about 80, and reacts sensitively to flammable gases.

Such a high gas sensitivity property is due to the fact that among the many types of ferric oxide (Fe203) such as alpha, beta, gamma, delta, and epsilon types, gamma type ferric oxide (γ-Fe2O3) ) is observed only in

As already mentioned, gamma-type ferric oxide (γ-Fe2
Although the combustible gas detection element using the film of No. 03) has excellent gas sensitivity characteristics, it has drawbacks because it is a film obtained by coating and heating, and improvements are needed.

Among these, the strength of the film itself or the strength of its adhesion to the insulating substrate is extremely weak, which limits the shape of the element, and it is necessary to avoid applying vibration or other mechanical forces to the element as much as possible. There are issues that need to be resolved in order to advance practical application.

Furthermore, due to insufficient sintering, it is not sufficiently stable for boiling tests and long-term storage under humidity conditions, for example, 70° C. and 95%, and this point must also be resolved.

In view of these points, the present invention has developed gamma-type ferric oxide (γ
In order to take full advantage of the excellent gas sensitivity properties of -F e 203), as a result of various experiments, we have changed from the conventional method of coating a film on a ceramic substrate to alpha-type oxidation film. By forming a thin layer mainly composed of gamma-type ferric oxide (γ-Fe2O3) sintered film on top of a sintered body mainly composed of diiron (α-Fe203), it is possible to overcome the drawbacks of the past. This solved the problem of strength and stability against humidity.

A detailed explanation will be given below based on examples.

[Example 1] 3% by weight of water was added to triiron tetroxide (Fe3O4) powder with an average particle size of 0.1 micron, and the powder was thoroughly ground.
After mixing, the mixture was compression molded into a square plate (13×13×2its3).

Thereafter, this molded body was sintered in vacuum at 700° C. for 1 hour.

After cooling the triiron tetroxide (Fe204) sintered body thus obtained, the temperature was gradually raised in air to 800°C,
An alpha-type ferric oxide (α-Fe 203 ) sintered body was obtained.

Next, acicular triiron tetroxide (Fe
304) Add water to the powder, thoroughly pulverize and mix with a ball, apply to one side of the alpha-type ferric oxide (α-Fe203) sintered body to a thickness of 20 microns, and 750 μm in vacuum.
C. for 1 hour to sinter the triiron tetroxide (Fe304) film.

After cooling this, the temperature was gradually raised to 350° C. in air and oxidized to form a gamma type ferric oxide (γ-Fe2O3) sintered film.

A comb-shaped gold electrode was provided on the sintered film thus obtained by vapor deposition.

FIG. 1 is a perspective view of the combustible gas detection element thus obtained.

In the figure, 1 is a substrate made of alpha-type ferric oxide (α-Fe203) sintered body, and 2 is gamma-type ferric oxide (γ-Fe203).
3 is an electrode, and 4 is a lead wire.

In the device obtained under the above conditions, the adhesion strength of the comma-type ferric oxide (γ-F e 203) sintered film to the alpha-type ferric oxide (α-F e 203 ) sintered substrate was as low as that of the sintered film. A cellophane tape having a width of 5iIIIB was adhered thereon, and the measurement was performed by pulling the tape perpendicularly to the substrate surface.

As a result, when a gamma type ferric oxide sintered film was formed directly on a commercially available alumina substrate, the strength was approximately 50g15111
1, and the γ-Fe203 sintered film was peeled off along with the tape.

On the other hand, in the example sample, 140 g7511N
Only the tape was peeled off by the force of , and there was no peeling of γ-Fe2O3.
It was confirmed that the adhesive strength of No. 3 was sufficiently strong.

In other words, the adhesive strength of the γ-Fe2O3 film in this example is 1
50g/51'11 or more, whereas gamma type ferric oxide (γ-Fe2O) for commercially available alumina substrates
3) Adhesive strength of the film: 50.915 It can be seen that this is much stronger than 1.

The resistance value of this element at room temperature was 12.5 MΩ.

Next, in order to measure the gas sensitivity characteristics, the device was held in a measurement container, and the temperature was gradually raised and maintained at 300°C.
IA? If you measure the resistance value by flowing air at a flow rate of /min,
RA=240Ω.

When the resistance value of the element stabilized, the atmospheric gas was changed from air to a mixed gas with air containing 0.1% by volume of propane gas and flowed at the same flow rate of 11/min. After about 10 seconds, BQ = 4. ．． It decreased to OKΩ and reached equilibrium in a vague state.

This is approximately 60 when expressed as gas sensitivity RA/RG.

When this element was taken out from the measurement container and RA and RG were measured in the same manner one day, one week, and one month later, the amount of change over time was within ±5%.

In addition, these elements were placed in air at 300°C and at 300°C.
1000 in air containing 0.1% by volume of propane gas.
When the resistance was held for 0 hours and the influence it had on the resistance values RA and RG was investigated, it was confirmed that the amount of change was within ±5% in both cases, which was a good result.

Furthermore, even if the device was poured into pure water and boiled for 1 hour, no change was observed in the gas sensitivity characteristics, and even if it was left in 70°C and 95% humidity for 10,000 hours, no change in the characteristics was observed. There wasn't.

This is a high stability that cannot be found in film elements obtained by directly coating and baking γ-Fe203 on a ceramic substrate such as alumina or forsterite, and subjecting it to heat treatment.

Furthermore, a temperature cycle test was conducted for 10 cycles between room temperature (15 to 30°C) and 300°C, and it was confirmed that no abnormalities such as cracking or electrode peeling occurred.

Also, no abnormality was found in the appearance or gas sensitivity characteristics in the vibration test applied to ordinary electronic device parts.

[Example 2] Commercially available triiron tetroxide (Fe30
4) Compression mold the powder into an appropriate size and heat it in a nitrogen stream for 9
After heating to 00°C, a Fe 304 sintered body is made, and after cooling, the temperature is gradually raised to 800°C to form an alpha-type ferric oxide (α-pe 2 o 3 ) sintered body. Obtained.

Next, this alpha-type ferric oxide (α-Fe 20s
) The sintered body is machined to have an outer diameter of 2 mm and an inner diameter of 1.51 mm.
11. Length 61n1! An L cylinder was cut out.

Then, electrodes made of platinum ribbons having a width of 0.21111 mm were fixed to both ends of this cylindrical body.

On the other hand, triiron tetroxide (Fe30) with an average particle size of 0.2 microns
4) Put the powder together with pure water in a ball mill, grind it to make a triiron tetroxide (Fe s 04 ) mixture, and place it on the outside of the cylinder of the alpha-type ferric oxide (α-Fe203) sintered body. A triiron tetroxide (Fe304) film having a thickness of 20 square meters was formed by blowing with compressed air.

Then, it was heated to 850°C in a nitrogen stream to give Fe50. It was made into a sintered film.

The cylinder on which the triiron tetroxide (FesO+) film was formed was then gradually heated to 350°C and oxidized in an oxidizing atmosphere, forming a comma-shaped ferric oxide on the cylinder of α-Fe203 sintered body. A (γ-Fe2O3) sintered film was formed.

Next, create a resistance wire for heating by spirally winding a platinum wire with a thickness of 0.1 mm, insert it into the cylindrical body, and fix both ends of the resistance wire to both ends of the tube with adhesive to make it flammable. A gas detection element was obtained.

FIG. 2 shows the cross-sectional structure of the combustible gas detection element of this embodiment.

In the figure, 11 is a base made of an alpha-type ferric oxide (α-Fe203) sintered body, 12 is an electrode, 13 is a gamma-type ferric oxide (γ-Fe2O3) gas sensitive body, 14 is a heating element, 1
5 is an inorganic adhesive for supporting the heating element.

When the resistance value of these elements was measured at room temperature, it was 25 MΩ.

Next, in order to measure the gas sensitivity characteristics, the element was placed in a measurement container, and a current was applied to the heating element to gradually raise the temperature.
After maintaining the power at 100 watts, air was flowed at a flow rate of IA/min and the resistance value RA was measured, and the resistance value RA was 520 Ω.

When the resistance value of the element stabilized, the atmospheric gas was changed from air to air containing 0.1% by volume of isobutane gas, and when the same flow rate was 11/min, the resistance value of the element reached almost equilibrium after about 10 seconds. , Ω is shown at RG=6.

This corresponds to a sensitivity of R, A/RG=65.

The electrical stability and mechanical durability of these elements were examined in the same manner as in Example 1, and it was confirmed that they exhibited sufficient stability.

[Example 3] A molded body of triiron tetroxide (Fe3s4) was made in the same manner as in Example 2, sintered at 900°C in an argon gas stream, cooled, and then gradually heated to 400°C1 in air. The mixture was oxidized to obtain a sintered body of comma-shaped ferric oxide (γ-Fe2O3).

Next, this comma-shaped ferric oxide (γ-Fe2O3) sintered body was machined to have an outer diameter of 2n, an inner diameter of 1.5mm, and a length of 6mm.
The same cylindrical body of mm was cut out.

Next, platinum ribbon electrodes with a width of 0.2 tttx were fixed to both ends of this cylinder.

On the other hand, in the same manner as in Example 2, a triferric iron (F e 304 ) mixed solution was prepared, and placed on the outside of the cylindrical body of the gamma type ferric oxide (γ-Fe2O3) sintered body.
Spray with compressed air to make a 20 micron thick iron tetrahydride (
Fes 04) film was formed.

This was heated to 850°C in an argon stream to sinter the triiron tetraoxide (Fe a 04 ) film and cause a phase transition of the gamma-type ferric oxide (γ-Fe2O3) substrate.
It was set as alpha-type ferric oxide (α-Fe 203 ).

After heating, it is gradually heated in an oxidizing atmosphere at 350°C to convert triiron tetroxide (FeaO+) into comma-type ferric oxide (γ-
oxidized to Fe2O3).

The resistance value of these elements at room temperature was 25 MΩ.

Next, in order to measure the gas sensitivity characteristics, the element was placed in a measurement container, and a current was passed through the heating element to gradually raise the temperature.1.
After keeping the power at 5 watts, air was flowed at a flow rate of 11/min and the resistance was measured, and the resistance was RA=250Ω.

When the resistance values of these elements became stable, the atmospheric gas was changed from air to a mixed gas with air containing 0.1% by volume of propane gas, and the IA? When flowing at a flow rate of about 10
After a few seconds, the resistance value of the element almost reached equilibrium and showed Ω at RQ=4.

This corresponds to RA/RQ=62 in terms of sensitivity.

The electrical stability and mechanical durability of these elements were examined in the same manner as in the previous examples, and the above examples showed much superior stability compared to the conventional coatings. As shown in Figure 2, a combustible sintered film consisting mainly of comma-type ferric oxide (γ-Fe2O3) is baked onto a sintered substrate mainly consisting of alpha-type ferric oxide (α-Fe203). The gas detection element has excellent gas sensitivity characteristics and has extremely good stability against being left at high temperatures, in air containing flammable gases, being left in mixtures, and boiling.

Furthermore, these elements are stable in temperature cycle tests and vibration tests, and fully demonstrate their characteristics as sintered films.

In the example, an example was shown in which triiron tetroxide (Fe204) powder was used as a starting material, but as is clear from the explanation, alpha-type ferric oxide (α-Fe203) was ultimately used.
) Sintered body and gamma type ferric oxide (γ-Fe2O3
) It goes without saying that any material can be used as long as it can be transferred to the film.

In addition, other additives are added when applying or spraying triiron tetroxide (Fe304) powder and sintering to form a gamma-type ferric oxide (γ-Fe2O3) sintered film. By adding powder or using powder that has been modified with additives in advance, it is possible to facilitate sintering or change the resistance value of the sintered film, providing a wide degree of freedom in improving properties.

Similarly, to produce an alpha-type ferric oxide (α-Fe203) sintered body, other additives are added when triiron tetroxide (Fe304) powder is compression-molded and sintered. Alternatively, by using powder that has been modified with additives in advance, it is possible to facilitate sintering or increase the adhesive strength with the gamma-type ferric oxide (γ-Fe2O3) film formed on top of it. .

The device obtained by the present invention is not only sensitive to propane gas and isobutane as shown in the examples, but also sensitive to flammable gases such as city gas, ethyl alcohol, carbon oxide, hydrogen, acetone, and general hydrocarbons. I feel the same way.

As is clear from the above explanation, the sintered film mainly composed of gamma-type ferric oxide (γ-Fe20s) according to the present invention is used as a gas sensitive body, and this is used as a gas sensitive material.
The combustible gas detection element formed on a sintered body substrate mainly composed of 203) has excellent stability against atmospheric gas, temperature, thermal shock, and mechanical vibration at high temperatures; Its practical value is extremely valuable.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of one embodiment of the combustible gas detection element according to the invention, and FIG. 2 is a sectional view of another embodiment of the combustible gas detection element according to the invention. 1, i i-...α-Fe203 sintered body base, 3°13...γ-Fe203 gas sensitive body.

Claims (1)

[Claims] 1. A sintered body mainly composed of alpha-type ferric oxide (α-Fe203) is used as a base, and a sintered film mainly composed of gamma-type ferric oxide (γ-Fe20a) is provided on this base, Further, a pair of electrodes is formed on the sintered film, and the pair of electrodes detects a change in the electrical resistance value of the sintered film due to contact with the flammable gas, thereby detecting the flammable gas. Gas detection element.