JPS5853735B2 - Kanenseigaskenchisoshi - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a combustible gas detection element, and particularly to a combustible gas detection element that uses a sintered body containing γ-Fe2O3 with a spinel type crystal structure as a main component phase as a gas sensitive body. .

BACKGROUND ART In recent years, with the spread of gas appliances, accidents caused by gas have been occurring frequently, and various measures are being considered to prevent accidents.

One example is a device that detects gas leaks and issues an alarm.

The present invention aims to provide an element for detecting this gas leak.

γ-Fe2O3 is an n-type oxide semiconductor, and has the property that its electrical resistance rapidly decreases when it comes into contact with a reducing gas at high temperature.

Currently, by utilizing this property, development of a gas detection element using γ-Fe2O3 as a gas sensitive material is underway.

This γ-Fe2O3 exhibits good gas sensitivity in the temperature range of 250 to 400°C.

Sensitivity and response speed show opposite trends with temperature, 2
At 50 to 300°C, the sensitivity is good, but the response speed is a little slow7<, and as the temperature approaches 400°C, the response speed is fast, but the sensitivity is slightly lower.

Therefore, it is desirable to use this type of material at temperatures around 350°C.

By the way, elements for detecting gas leaks are now required to operate in a fairly narrow gas concentration range in order to prevent explosions and to prevent malfunctions. , a very stable operating point is required.

Therefore, an element that detects gas using the resistance change of a semiconductor must have a resistance that changes quickly depending on the gas, and even after the resistance value reaches the set value, the resistance value must change gradually. It is not allowed.

In order to respond quickly to gas, the gas-sensitive part must operate stably at a relatively high temperature.

However, γ-Fe2O3 is an unstable phase at high temperatures, and if left at high temperatures for a long time, it undergoes a phase transition to α-Fe203, which is stable even at high temperatures. The phase transition to is irreversible,
Once γ-Fe2O3 has been transferred to α-Fe203, it is not easy to transfer it back to γ-Fe2O3.

α-Fe203 has extremely low sensitivity to combustible gases and also has high electrical resistance.

Therefore, when a phase transition occurs from γ-Fe2O3 to α-Fe203, Fe2O3 loses its function as a gas sensitive material.

The temperature at which γ-Fe2O3 undergoes a phase transition to α-Fe203 is
Although it varies depending on the manufacturing conditions, it is approximately 400 to 630℃.
within the temperature range.

For example, magnetite (Fe304) is produced by precipitation method.
Make fine particles of 150 to 4 in the air.
Oxidation by heating at temperatures in the range of 00°C yields γ-Fe2O3.

The γ-F e203 produced in this manner has a low phase transition temperature, and in particular, the lower the alkali concentration during precipitation during magnetite production, the lower the temperature.

In addition, α-Fe203 is reduced to Fe,04, which is roasted at a high temperature, for example, 900°C, in an inert atmosphere, and further heated at a temperature in the range of 100 to 700°C to oxidize. Also, γ-Fe2O3 can be obtained.

γ-Fe203 obtained by this method has a relatively high transition temperature to α-Fe203.

As mentioned above, the γ-F e 20 s combustible gas detection element requires the gas-sensitive portion to be maintained at a fairly high temperature due to sensitivity, response speed, and other considerations.

γ-Fe2O3 not only undergoes a phase transition to α-Fe203 at high temperatures, but also undergoes a gradual phase transition if left for a long time even at a temperature considerably lower than the four-phase inverter.

Therefore, it cannot be said that the phase transition temperature of γ-Fe2O3 is sufficiently high to use it as it is in the gas-sensitive part.

Since the gas-sensitive part is used at a fairly high temperature, it is required that the gas-sensitivity remains stable even if it is left at the operating temperature for a long time. We have to improve it in some way.

Methods for producing γ-Fe2O3 can be broadly classified into the following two methods.

One of them is a method using dehydration of γ-FeOOH,
The other one is Fe50. This method involves oxidation of

Methods for producing γ-FeOOH include a method of slowly oxidizing ferrous salt, a method of slowly oxidizing Fe(OH)2, or a method of heating Fe0C1 crystals with the same crystal structure in water. be.

As a method for producing Fe3o4, α-Fe20
3. Alternatively, a method of reducing α-FeOOH with hydrogen etc., a method of co-precipitating a ferrous salt and a ferric salt in an alkaline solution, or a method of heating FeC2O4 or FeCOs etc. in steam or nitrogen There are ways to do this.

γ-Fe2O3 produced by such a method has a phase transition temperature of at most 630°C, as described above.

One possible way to increase this phase transition temperature is to introduce a different element.

The present invention was completed as a result of various research studies regarding elements suitable for metamorphosis of γ-Fe2O3 as a combustible gas sensitive material and the composition of an element.

That is, the combustible gas detection element according to the present invention has γ-
Fe20a, and at least one kind selected from the oxide group consisting of WO3 and MoO3, respectively, at 60 to 9
A sintered body containing a composition ratio of 9.9 mol% and 40 to 0.1 mol% is used as a gas sensitive body, and a pair of electrodes for measuring electrical resistance and a heater for heating are provided to the sintered body to detect combustible gas. The combustible gas is detected by using the change in the electrical resistance value of the gas sensitive body due to the change in the concentration of the gas.

Hereinafter, the device according to the present invention will be described in detail based on Examples.

[Example 1] 0.9 mol of Fe3O4 powder and 0.1 mol of WO3 having an average particle size of 0.1 μm were weighed out, water was added thereto, and they were thoroughly ground and mixed.

The mixture was vacuum dried at room temperature and then compression molded into a square shape.

The molded body was sintered at a temperature of 750° C. in a nitrogen stream.

After the sintered body was cooled, the temperature was gradually raised and maintained at a temperature of 400°C in an oxidizing atmosphere to form γ-Fe2O3.
A sintered body containing as the main component was obtained.

Gold was deposited on one of the main surfaces of the sintered body thus produced to form a pair of comb-shaped electrodes.

Then, a platinum heating element was attached to the other main surface with an inorganic adhesive to form a combustible gas detection element.

FIG. 1 is a perspective view showing an example of the structure of a combustible gas detection element manufactured as described above.

In the figure, 1 is a combustible gas sensitive body made of a bulk sintered body mainly composed of γ-Fe2O3.

2 is a pair of comb-shaped electrodes, 3 is an inorganic adhesive, 4 is a platinum heating element, and 5 and 6 are lead wires connected to the comb-shaped electrode 2 and the platinum resistor 4, respectively.

The entire device was covered with a stainless steel wire mesh, and the platinum heating element 4 was energized to heat and maintain the γ-F C20s sintered body 1 at a temperature of 300°C.

At this time, the resistance value of the combustible gas detection element in air was 75. It was IKΩ.

When this was placed in air containing 0.5 volume □ of isobutane, the resistance value was 2.28Ω, and the resistance value changed greatly due to the presence of flammable gas.

Next, this element was left in an electric furnace maintained at a temperature of 400° C. for 1000 hours.

Then, when the γ-Fe203 sintered body 1 was kept at a temperature of 350°C and the resistance values were measured in air and in air containing 0.5% by volume of isobutane, the resistance was 81.3Ω. , 2.43Ω.

This shows that the gas sensitivity remains stable over a very long period of time.

[Example 2] Various samples were prepared in the same manner as in Example 1, but with different amounts of WO3 added.

The characteristics of each of these samples were measured under the same conditions as in Example 1.

Figure 2 shows the relationship between the amount of WO3 and the resistance (Ro), and the relationship between the amount of WO3 and the resistance (Ro).
The relationship between the amount of O3 and the sensitivity (RA, /Ro) is shown.

Note that Ro is the value in air containing flammable gas, and R
A is the value in air that does not contain it.

In the figure, curve II shows the initial resistance value characteristics of the element in air containing flammable gas.

Curve ■ shows the initial sensitivity characteristics.

In addition, curve (■) shows that the device is heated to 400°C for 1000°C.
It shows the characteristics of resistance value after holding for a certain period of time.

Similarly, the curve ■ shows the sensitivity characteristics.

As is clear from this, it can be seen that as the amount of WO3 increases, the deterioration of characteristics due to high temperature storage becomes smaller and the properties become more stable.

Looking at the gas sensitivity (curves n, rv), W
It can be seen that when the amount of O3 is contained in the γ-Fe203 sintered body in an amount of 0.1 to 40 mol %, a remarkable improvement is achieved.

The effect of addition to γ-Fe2O3 is that M o
03 alone or added together with WO3 showed almost the same tendency.

The experimental results are summarized in the table below.

[Example 3] Fe C2042M o O(C204) ・3H
20 and (N)T4)2WO4, 1 mol each, 0
．． 1 mol and 0.1 mol were accurately weighed and mixed uniformly in a mortar.

The resulting mixture was heated in a nitrogen stream saturated with water vapor.
After pyrolysis at 400℃ temperature for 3 hours, the powder was
Oxidation was carried out by heating at a temperature of 400° C. for 1 to 3 hours.

Through this oxidation treatment, γ-Fe modified with Mo and W
We were able to obtain 2O3.

This γ-Fe2O3 was analyzed by X-ray powder diffraction into α-Fe2
When the presence of 03 was investigated, the presence of α-Fe203 could not be recognized.

Furthermore, by differential thermal analysis, γ-Fe2O3 to α-F
When the phase and transition temperature to e203 was investigated, it was 650°C.

Mo, W metamorphosed γ-Fe203 obtained as described above
was finely ground and an organic binder was added to make it into a paste.

On the other hand, gold paste for baking was printed on the main surface of an alumina porcelain plate measuring 6 mm x 5 mm x 0.5 mm in the form of a comb with an interval of 0.5 mm, and baked at a temperature of 800° C. to form electrodes in advance.

Further, on the electrode baking surface of this alumina porcelain plate, Mo, W
Modified γ-Fe203 was applied to a thickness of 20 μm.

Heat this, gradually raise the temperature being careful not to create any cracks, hold it at a temperature of 350℃ for 2 hours, and then
Cooled.

Mo completed the baking. A platinum heating element was placed in contact with the other main surface of the alumina porcelain plate so as not to contact the W metamorphosed γ-Fe203 film, and the whole was surrounded by a 100 mesh stainless steel wire mesh to complete the combustible gas detection element. .

FIG. 3 shows the structure of this combustible gas detection element.

In the figure, 11 is an alumina porcelain plate, 12 is a film-like M
o, W modified γ-Fe203 gas sensitive body, 13 is a comb-shaped gold electrode, 14 is a platinum heating element, 15° and 16 are lead wires,
They are connected to an electrode 13 and a platinum heating element 14, respectively.

The platinum heating element 14 is energized, and the γ-Fe203 gas sensitive element 1
2 was maintained at a temperature of 300°C.

The resistance value between the electrodes 13 in the air at this time is 4
It was 51Ω.

When this was placed in air containing 1% by volume of propane gas, its resistance value was 14.9Ω.

It can be seen from this that the resistance value changes significantly due to the presence of combustible gas.

Next, the power supply to the platinum heating element 14 was cut off, and the platinum heating element 14 was left in an electric furnace maintained at a temperature of 400° C. for 1000 hours.

After that, the platinum heating element 14 is energized again, and the γ-Fe20
The three-gas sensitive coating 12 is maintained at a temperature of 300°C,
When the resistance value was measured in air, it was 492Ω.

The resistance value was 16.2 Ω in air containing propane gas of 1% by volume.

As mentioned above, γ-Fe2O containing 0.1 to 40 mol% of at least one of WO3 and MoOs
No. 3 has excellent gas sensitivity characteristics, and its characteristics are extremely stable when left at high temperatures.

Regarding high-temperature storage, in the above example, only the results were described under the condition of leaving the product in the air with no electricity applied, but even if the product was left in an electrically heated state or in an air containing flammable gas. , and had excellent stability of characteristics.

Furthermore, it was stable against temperature cycles and vibrations, and had sufficient characteristics as a bulk or film-like sintered body.

The shape can be either a bulk shape or a film shape depending on the purpose of use and the place of use.

In addition, since the operating temperature can be increased, the time for resistance value recovery after gas sensitivity can be shortened to one-third to one-fifth compared to those that do not contain W or MO or both W and MO. I was able to do that.

The starting materials are not limited to the compounds shown in the examples, but ultimately γ-Fe2O3, WO3 and Mo
Any material may be used as long as it forms a sintered body containing at least one kind of O3.

The atmosphere during sintering as in the examples is not limited to nitrogen, but may also include inert gas such as argon,
A non-oxidizing atmosphere such as carbon dioxide gas or an inert gas containing a small amount of hydrogen, or a vacuum may be used.

It is recommended that the firing temperature for producing the bulk γ-Fe203 sintered body be within the range of 500 to 12,000C.

When the sintering temperature is lower than 500° C., sintering becomes insufficient and mechanical strength, water resistance, and moisture resistance decrease.

Furthermore, if the temperature exceeds 1200°C, grain growth becomes significant, making it difficult to oxidize Fe3O4 to γ-Fe2O3, and the response time and recovery time become longer.

In this case, the metamorphosed Fe3 o4 is oxidized to produce γ
The oxidation temperature when obtaining Fe2O3 is desirably 700°C or lower.

If the temperature exceeds 700°C, a large amount of α-Fe203 will precipitate.

For mass production, it is preferable to gradually increase the temperature from a relatively low temperature of 100 to 200°C, and by performing such an oxidation treatment, cracks will not occur in the sintered body.

In addition, when forming a film-like sintered body (here, modified γFe2
It is desirable that the O3 powder has a particle size of 0.1 μm or less.

If the particle size becomes too large, the adhesion to the substrate will deteriorate and it will be easily peeled off.

It is desirable that the sintering temperature does not exceed 500°C.

If it is too high, the modified γ-Fe203 has a small particle size, so it tends to become superphosphoric acid, and the sensitivity to combustible gas deteriorates.

As explained above, the element according to the present invention has 7"-
Fe203, and at least one selected from the oxide group consisting of WO3 and MoO3, each containing 60 to 9
A sintered body containing a composition ratio of 9.9 mol% and 40 to 0.1 mol% is used as a gas sensitive body, and a pair of electrodes for measuring electrical resistance and a heater for heating are provided to the sintered body to detect combustible gas. The combustible gas is detected by using the change in the electrical resistance value of the gas sensitive body due to the change in the concentration of the gas.

This element has excellent sensitivity to combustible gases and stable characteristics, and since it is a sintered body, it is resistant to thermal shock and mechanical vibration.

Furthermore, the response time and recovery time to combustible gas are short, and in particular, the recovery time is greatly shortened compared to the case of only γ-F e20s, and the response is significantly improved.

The resistance change of the element is small even when the outside temperature fluctuates.
It is highly practical.

In the present invention, even if the sintered body contains a certain amount of the α-Fe203 component, its essential properties will not be lost.

In order to further improve the properties or obtain properties more suitable for the intended use, it is also possible to further add and contain other fractions.

In addition to propane and isobutane, combustible gases include various flammable gaseous substances such as city gas, ethyl alcohol, methyl alcohol, hydrogen, acetone, and other general hydrocarbons.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the structure of one embodiment of the combustible gas detection element according to the present invention, FIG. 2 is a diagram showing an example of the relationship between the composition ratio, resistance, and sensitivity of this element, and FIG. FIG. 7 is a perspective view showing the structure of another embodiment. 1... Bulk gas sensitive material, 2...
Electrode, 4... Platinum heating element, 12... Film-like gas sensitive body, 13... Electrode, 14...
...Platinum heating element.

Claims (1)

[Claims] 1r-Fe203, and at least one selected from the oxide group consisting of WO3 and Mo03.
A sintered body containing a composition ratio of 0 to 99.9 mol% and 40 to 0.1 mol% is used as a gas sensitive body.
Combustible gas detection characterized by detecting a combustible gas by providing a pair of electrodes and a heating heater so that the electric resistance value of the gas sensitive body changes due to a change in the concentration of the combustible gas. element.