JPS5853736B2 - 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-F e203 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, utilizing this property, development of a gas detection element using γ-Fe203 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 slow, and as the temperature approaches 400°C, the response speed is faster, 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-Fe 20s 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.

This phase transition from γ phase to α phase is irreversible,
Once γ-Fe2O3 has been transferred to aFe2O3, it is not easy to transfer it back to γ-Fe203.

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 varies depending on the manufacturing conditions, but is approximately 400-630°C.
Within the temperature range of ℃.

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-Fe20a.

γFe 203 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-Fe20s is reduced to Fe304, which is then heated at a high temperature, for example, 900°C, in an inert atmosphere.
γ-Fe 203 can also be obtained by roasting, further heating at a temperature within the range of 100 to 700°C, and oxidizing.

γ-Fe2O3 obtained by this method has a relatively high transition temperature to α-Fe203.

As mentioned above, in the γ-Fe203 combustible gas detection element, the gas sensitive portion must be maintained at a fairly high temperature due to sensitivity, response speed, etc.

γFe2O3 not only undergoes a phase transition to α-Fe 203 at high temperatures, but also gradually undergoes a phase transition if left for a long time even at a temperature considerably lower than the phase transition temperature.

Therefore, it cannot be said that the phase transition temperature of γFe2O3 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-Fe 20s can be roughly divided into the following two methods.

One of them is a method using dehydration of γ-FeOOH,
Another method is the oxidation of Fe s04.

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. .

As a method for producing Fe3O4, α-Fe 203
Alternatively, a method of reducing α-Fe00H with hydrogen or the like, a method of co-precipitating a ferrous salt and a ferric salt in an alkaline solution, a method of heating FeC204 or FeCOs, etc. in steam or nitrogen, etc. There is.

γ-F e 203 produced by such a method has a phase transition temperature of 630° C. at most, 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-
Fe2O3, and at least one selected from the oxide group consisting of L120, Na2O, and 20, respectively.
A sintered body containing a composition ratio of 0 to 99.9 moles φ, lO to o, and t moles is used as a gas sensitive body, and a pair of electrodes for measuring electrical resistance and a heater for heating are attached to it to make it combustible. This method is characterized in that a combustible gas is detected by using the change in the electrical resistance value of the gas sensitive body due to a change in gas concentration.

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

Example 1 1.9 mol of Fe 304 powder with an average particle size of 0.1 pm and 0.1 mol of Na2COa 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 cooling the sintered body, the temperature was gradually raised and maintained at a temperature of 400°C in an oxidizing atmosphere, and γ-Fe2O
A sintered body containing 3 as a 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.

This entire element is covered with a stainless steel wire mesh, and the platinum heating element 4 is energized, and the γ-Fe203 sintered body 1 is
The temperature was maintained at 50'C.

At this time, the resistance value of the combustible gas detection element in air was 190Ω.

When this was placed in air containing 0.5 volume φ of Inobu Kun, the resistance value was 7.6 KQ, 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 too long.

Then, when the γFe2O3 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 volume φ of isobutane, the resistance values were 203Ω and 8.4Ω, respectively. It was Ω.

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 Na2CO3 added.

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

FIG. 2 shows the relationship between the amount of Na2O and the resistance (RG), and the relationship between the amount of Na20 and the sensitivity (RA/RG), respectively.

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

In the figure, curve (■) indicates the initial resistance value characteristic of the element in air containing flammable gas.

Curve ■ shows the initial sensitivity characteristics.

In addition, the curve (■) shows that the element is once heated to a temperature of 400°C.
It shows the resistance value characteristics 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 Na2O increases, the percentage deterioration due to high temperature storage becomes smaller and the properties become more stable.

And if we look at the gas sensitivity (curve n = IV
), the amount of NazO in the γFe2O3 sintered body is o, t-
It can be seen that there is a significant improvement when the amount of mol.

The effect of addition to γ-Fe 203 is that L1
Even when two or more of Na2Op Lt20, Tumor 01, Na2Op Lt20, and Na2Op Lt20 were added in combination, almost the same tendency was observed.

The experimental results are summarized in the table below. Example 3 1 mol, 2 mol and 0.015 mol of Fe C12t Fe C13 and LiCl were accurately weighed out, respectively, and dissolved in 12 pure waters.

This mixed bath solution was slowly dropped into a solution in which 116 moles of NH and C were dissolved in 1 liter of pure water.

This causes the following reaction.

According to the above reaction formula, about 8 mol of NH4OH is required, but an excess of NH4OH was added in order to reduce fluctuations in the hydrogen ion concentration (pH) of the bath solution.

In addition to adding excess NH4OH like this,
A constant pH value may be maintained at all times by supplementing the consumed amount of NH4OH simultaneously with the dropping of the iron salt mixed bath solution.

After the addition of the iron salt bath solution was completed, the solution was filtered to obtain a precipitate, which was dried using a drier at a temperature of 80 to 100°C for 4 to 10 hours.

After crushing the dried mango in a mortar, the powder is heated to 300-400℃.
Oxidation treatment was carried out by heating at a temperature of 1 to 3 hours.

Through this oxidation treatment, r -Fe modified with Li
I was able to get 20 seconds.

Chemical analysis of this γ-Fe 20a revealed that L1□O
It contained 0.43 mole φ.

Further, when the presence of α-Fe203 was investigated by X-ray powder diffraction, the presence of αFe203 could not be recognized.

Furthermore, by differential thermal analysis, αFe was extracted from r-Fe203.
When the phase transition temperature to 2O3 was investigated, it was found to be 30° C. higher than when Li was not included.

The Li modified Fe203 obtained by straining as described above was finely ground, and an organic binder was added thereto to form a paste.

On the other hand, on the main surface of an alumina magnetic plate with dimensions of 5 mm x 5 mm x 0.5 mm, gold paste for baking was printed in the form of a comb with intervals of 0.5 mm, and baked at a temperature of 800°C. An electrode was formed in advance.

On the electrode baking surface of this alumina magnetic plate, 20s of L1 modified γ-Fe was further applied to a thickness of 20 μm.

This was heated, the temperature was gradually raised to avoid cracks, the temperature was maintained at 350°C for 2 hours, and then cooled.

A platinum heating element was brought into contact with the other main surface of the alumina porcelain plate so as not to come into contact with the baked Li metamorphic-F e 20 s film, and the whole was surrounded by a 100-mesh stainless steel wire mesh to prevent combustible heating. Completed a 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 L
13 is a comb-shaped gold electrode, 14 is a platinum heating element, and 15 and 16 are lead wires connected to the electrode 13 and the 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 3008C.

At this time, the resistance value between the electrodes 13 in the air is 1
．． It was 5MΩ.

When this was placed in air containing 1 volume of propane gas, its resistance was 55Ω.

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 to generate 1-Fe2O.
3 maintains the gas-sensitive film 12 at a temperature of 300°C,
When the resistance value was measured in air, it was 1.6 MO.

In air containing 1 volume of propane gas, the resistance value was 67Ω.

Example 4 1 mol, 0.01 mol, and 0.01 mol of F e C12-KCl and LiCl were weighed, respectively, and dissolved in 1 liter of pure water.

Separately, 1.2 mol of (NH4)2C204 was weighed out and dissolved in 1 liter of pure water.

The above iron salt mixed solution was added to this (NH4)2C204 solution while stirring.

Stirring for 5-10 minutes produced a yellow precipitate, which was washed by decantation.

The precipitate was then dried and further pyrolyzed in a stream of nitrogen at a temperature of 400°C saturated with water vapor for 3 hours.

After pyrolysis, cool to room temperature while blocking air to extract K and L.
Fe3O4 containing i was obtained.

Next, this Fe3O4 is placed in the air at 100 to 150
By slow oxidation at a temperature of °C, γ-Fe2O3 modified with K and Li was obtained.

Then, a combustible gas detection element was manufactured using the same procedure as in Example 3.

The gas-sensitive film of this combustible gas detection element was coated at a temperature of 30
When heated to 0°C and measured the resistance value in air,
It was 1.6MQ.

Furthermore, 0. The resistance value in air containing propane gas per 1 volume was 180Ω.

Next, after heating in an electric furnace at a temperature of 400° C. for 1000 hours, the resistance value was measured in the same manner as described above.

As a result, it was 1.8 MΩ in air, and 215 KQ in air containing propane gas with a capacity range of 0.1.

As described above, L120-Na2O and at least one selected from the group of 20 oxides are added in an amount of 0.1-10
γ-F e203 containing mole φ has excellent gas sensitivity characteristics and has extremely stable characteristics 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 properties.

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.

Furthermore, since the operating temperature can be increased, the time required for the resistance value to recover after gas sensitivity can be shortened to one-third to one-fifth of that of a material that does not contain Li or the like.

The starting materials are not limited to the compounds shown in the examples, but ultimately γ-Fe2O3, Li20
Any material may be used as long as it forms a sintered body containing at least one of the following.

The atmosphere during high temperature sintering as in Example 1.2 was
The atmosphere is not limited to nitrogen, and may be a non-oxidizing atmosphere such as an inert gas such as argon, carbon dioxide gas, or an inert gas containing a small amount of hydrogen, or a vacuum.

The firing temperature for producing the bulk γ-Fe203 sintered body is recommended to be within the range of 500 to 1200°C.

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

Furthermore, if it exceeds 1200'c, grain growth becomes significant, making it difficult to oxidize Fe 304 to γ-Fe2O3, and the response time and recovery time become longer.

In this case, the modified Fe 304 is oxidized to produce γ-
The oxidation temperature when obtaining Fe 203 is desirably 700°C or less.

When the temperature exceeds 700°C, a large amount of α-F e 203 will precipitate.

For mass production, it is best to gradually increase the temperature from a relatively low temperature of 00 to 200° C. If such an oxidation treatment is performed, cracks will not occur in the sintered body.

When forming a film-like sintered body as in Examples 3 and 4,
The powder of modified γ-Fe203 is 0.1. It is desirable that the particle size be less than 1 cm.

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-Fe 20s has a small particle size, so it tends to become superphosphoric acid, and the sensitivity to combustible gas deteriorates.

As explained above, the device according to the present invention has γFe2
O3, and at least one selected from the group of oxides consisting of Li2O, Na2O, and 20% each.
A sintered body containing a composition ratio of 9.9 moles φ, lO~o, and t moles was used as a gas sensitive body, and a pair of electrodes for measuring electrical resistance and a heater for heating were attached to it to detect the combustible gas. This method is characterized in that a combustible gas is detected by using the change in the electrical resistance value of the gas sensitive body due to a change in concentration.

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-Fe2O3, 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 addition, in the present invention, even if the α-Fe203 component or the like is contained in the sintered body to some extent, its essential properties will not be lost.

Further, in order to further improve the characteristics or obtain properties more suitable for the purpose, it is also possible to further add and contain other components.

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] 1 7" Sintering containing at least one selected from the oxide group consisting of Fe2O3, Li2O, Na2O, and 20 in a composition ratio of 90 to 99.9 mol φ and lO to 0.1 mol, respectively. The body is used as a gas sensitive body, and a pair of electrodes for measuring electrical resistance and a heater for heating are provided on this gas sensitive body, so that the electrical resistance value of the gas sensitive body changes due to a change in the concentration of a combustible gas. It is characterized by detecting combustible gas using