JPS5853739B2 - 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 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. , it is necessary that the operating point be extremely stable.

Therefore, an element that detects force by utilizing changes in the resistance of a semiconductor must have a resistance that changes quickly due to gas, and even after the resistance value reaches the set value, the resistance does not change gradually. It is not allowed to go.

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

However, γ-Fe20s 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 7' 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-F e 203 undergoes a phase transition to α-F e 203 varies depending on the production conditions, but is approximately 40
It is within the temperature range of 0 to 330°C.

For example, by precipitation method magnetite) (Fe304
) is made into fine particles of 150
Oxidation by heating at temperatures in the range ˜400′C provides γ-F 6203.

The γ-F e 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.

Additionally, α-Fe 203 is reduced to Fe3O4, which is then heated at a high temperature, e.g. 900℃, in an inert atmosphere.
γ-Fe2O3 can also be obtained by roasting at a temperature of 0.degree. C. and further heating at a temperature in the range of ioo to 700.degree.

γ-Fe2O3 obtained by this method has a relatively high transition temperature to αFe2O3.

As mentioned above, in the γ-Fe203 combustible gas detection element, the gas sensitive part must be kept at a very painful temperature due to sensitivity, response speed, etc.

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 phase transition temperature.

Therefore, it cannot be said that the phase transition temperature of γ-F e 203 is high enough to use it as it is in a 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 raise it even higher.

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

One of them is a method by dehydration of γ-Fe00H,
Another method is the oxidation of Fe s 04.

Methods for producing γ-Fe00H include a method of slowly oxidizing ferrous salt, a method of slowly oxidizing Fe(OH)2, or a method of heating Fe0CI crystals with the same crystal structure in water. be.

Methods for producing Fe3O4 include a method of reducing α-Fe203 or α-Fe00H with hydrogen or the like, a method of co-precipitating a ferrous salt and a ferric salt in an alkaline solution,
Or F e 20s or F e COs etc.
There are methods such as combustion in water vapor or nitrogen.

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 80
~99.8 mol% of γ-F6203 with A as an additive
A sintered body containing at least one selected from the three oxide groups of l2O3, Ga2O3 and In2O3 at a ratio of 0.2 to 20 mol% is used as a gas sensitive body, and a pair of electrodes for measuring electrical resistance is attached to this. A combustible gas detection element is characterized in that it detects a combustible gas by applying a heater for heating and changing the electrical resistance value of the gas sensitive body due to a change in the concentration of the flammable gas. .

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

Example l FeCl2. FeCl3 and A I (N03)s
1 mol, 2 mol, and 0.05 mol of each were accurately weighed out, and these were dissolved in 11 pure water.

This mixed solution was slowly dropped into a solution of 16 moles of NaOH dissolved in 11 moles of pure water.

This resulted in the following reaction.

FeCl2+ 2 FeCl3+0.05 AI (N
O3)3+8.15NaOH-+F e3Alo, o5
04,075+8.15NaCl+4.075H
20 According to the above reaction formula, 8.15 mol of NaOH is required, but an excess of NaOH was added in order to reduce fluctuations in the hydrogen ion concentration (pH) of the solution.

In addition to adding excess NaOH in this manner, the pH value may always be maintained at a constant value by supplementing the consumed amount of NaOH at the same time as the iron salt mixed bath liquid is added dropwise.

After the addition of the iron salt solution was completed, the filtered precipitate of this solution was washed by decantation.

When the CI concentration in the washing solution was below 5×10 −5 M, washing was stopped, filtered, and the obtained material was dried in a drier at a temperature of 80-100° C. for 4-10 hours.

Grind the dried material in a mortar and then heat the powder to 300-400°C.
Oxidation treatment was carried out by heating at a temperature of 1 to 3 hours.

Through this oxidation treatment, γ-Fe2O modified with AI
I was able to get 3.

Chemical analysis of this γ-Fe2O3 revealed that A 120
3 was contained in an amount of 1.62 mol%.

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

Furthermore, from γFe2O3 obtained by differential thermal analysis, α
-The phase transition temperature to Fe203 increased by 6° C. when not added.

The U-modified γ-Fe203 obtained by I 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 porcelain plate with dimensions of 5iiX 5iiX 0.5mm, a baking gold paste was applied to
The electrodes were preformed by printing a comb shape with a spacing of , and baking it at a temperature of 800°C.

On the electrode-baked surface of this alumina porcelain plate, AI modified Fe203 was further applied to a door 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.

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 Al modified Fe2O3 film, and the whole was surrounded by a 100-mesh stainless steel wire mesh to form a combustible gas detection element. completed.

FIG. 1 shows the structure of this combustible gas detection element.

In the figure, 1 is an alumina porcelain plate, 2 is a film-like Al metamorphosed Fe203 gas sensitive body, 3 is a comb-shaped gold electrode, and 4 is a comb-shaped gold electrode.
is a platinum heating element, 5 and 6 are lead wires, and are connected to electrodes 3 and 6, respectively.
It is connected to a platinum heating element 4.

Electricity was applied to the platinum heating element 4, and the γ-Fe203 gas sensitive body 2 was maintained at a temperature of 300°C.

At this time, the resistance value between the electrodes 3 in the air is ■5
．． It was 5MΩ.

When this was placed in air containing 1% by volume of propane gas, its resistance value was 1.2 MΩ.

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 4 was cut off, and the platinum heating element 4 was left in an electric furnace maintained at a temperature of 400'C for 1000 hours.

After that, the platinum heating element 4I is energized again to generate γ-F e
The resistance value of the 203 gas-sensitive injection film 2 was measured in air while being maintained at a temperature of 300° C., and it was found to be 13.4 MΩ.

In air containing propane gas of % by volume, the resistance value is 1. It was IMΩ.

Example 2 Various samples were prepared in the same manner as in Example 1, but with different amounts of At(NO3)3 added.

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

Figure 2 shows the relationship between Al2O3 content and resistance (RG),
and the relationship between At 203 content and (RA/RG), respectively.

Note that RG 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 I shows the initial resistance value characteristic 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 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 Al2O3 content increases, the deterioration of characteristics due to high temperature storage becomes smaller and the properties become more stable.

Looking at the gas sensitivity (curves ■, ■), Al
The 2O3 content is 0.1 to 2 in the γ-Fe203 sintered body.
It can be seen that when the content is 0 mol%, there is a significant improvement.

This effect of addition to γ-Fe2O3 is due to Ga2
O3 or In2O3, or A l 203 , Ga
Even when a combination of two or more of 2O3 and In2O3 was added, almost the same tendency was observed.

The experimental results are summarized in the table below.

Example 3 1 mol of Fe 304 powder with an average particle size of 0.1μ,
0.05 mol of In(OH)3 and 0.05 mol of Ga(OH)3.
05 mol was weighed out, water was added thereto, and the mixture was 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 in a nitrogen stream at a temperature of 850°C.

After cooling the sintered body, the temperature was gradually raised and maintained at a temperature of 400°C in an oxidizing atmosphere, and γ-F e 2
A sintered body containing 03 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. 3 is a perspective view showing an example of the structure of the combustible gas detection element manufactured as described above.

In the figure, numeral 11 is a combustible gas sensitive material made of a bulk sintered body mainly composed of γ-Fe2O3.

12 is a pair of comb-shaped electrodes, 13 is an inorganic adhesive, 14 is a platinum heating element, and 15 and 16 are lead wires connected to the comb-shaped electrodes 12 and the platinum resistor 14, respectively.

This entire element was covered with a stainless steel wire mesh, and the platinum heating element 14 was energized, and the γ-Fe203 sintered body 11
was heated and maintained at a temperature of 350°C.

At this time, the resistance value of the combustible gas detection element in air was 57KQ.

When this was placed in air containing 0.5% by volume of isobutane, the resistance value was 4. IKΩ, and its resistance value changed significantly 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 γFe2O3 sintered body 11 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 values were 63Ω and 5.2Ω, respectively. It was Ω.

Example 4 1 mol and 0.01 mol of FeCI□ and GaCl3 were weighed, respectively, and dissolved in 1 liter of pure water.

Separately, 1.2 mol of (I'JH4)2C204 was weighed out and dissolved in 11 pure water.

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

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

When the CI concentration was below 5×10 −5 M, the washing was stopped and filtered.

The resulting material was dried.

If cleaning is prolonged, ferrous iron will oxidize and become ferric iron, which will dissolve and turn the supernatant liquid orange, so care must be taken when cleaning.

After drying, air was shut off and thermal decomposition was carried out at a temperature of 400'C for 3 hours in a nitrogen stream saturated with water vapor.

This was cooled with air shut off to obtain Fe3O4 containing Ga2O3.

Next, this Fe3O4 was placed in the air at a concentration of 100 to 15
Slow oxidation was performed at a temperature of 0°C to obtain γ-Fe2O3 modified with Ga.

Then, a combustible gas detection element was manufactured using the same procedure as in Example 1.

The gas-sensitive film of this combustible gas detection element was coated at a temperature of 30
When heated to 0℃ and measured the resistance value in air, it was 3.
．． It was 2MΩ.

Furthermore, the resistance value in air containing propane gas at volume % was 45Ω.

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

As a result, it was 2.3 MΩ in air, and 46Ω in air containing 1% by volume of propane gas.

Example 5 0.8 mol and 0.2 mol of FeC12 and AlCl3, respectively, were accurately weighed, dissolved in 11 parts of pure water, and this was dropped into 1 l of 5N-NaOH solution.

Next, air was fed in at a rate of 0.517 minutes for oxidation.

The obtained precipitate was washed, dried, and transformed into AI-Fe00
I got H.

This was made into a paste and applied to an alumina porcelain plate on which a gold electrode was formed, to produce a combustible gas detection element in the same manner as in Example 1.

A current was passed through the heating element to maintain the gas sensitive part at 350°C.

At this time, the electrical resistance value of the element in air is 23.4M
Ω, and the value in air containing 0.4 volume □ of propane gas was 4.7 MΩ.

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

Then, the same measurements as above were performed, and the resistance value was 37 MΩ in air and 5.4 MΩ in air containing 0.4 volume □ of propane gas.

As mentioned above, Al2O3, Ga 20 s and I
At least one selected from the oxide group of n2O3 is
．． γ-Fe2O3 containing 1 to 20 mol% has excellent gas sensitivity characteristics and is 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.

Good results were also obtained in tests such as boiling, leaving the mixture in a mixed state, and applying a mixed voltage.

r The deterioration of Fe2O3-based gas sensitive materials as sensing elements is mainly due to heat, but there are two main causes: (
1) Increase in the resistance value of the sintered body in a gas-containing atmosphere and (2)
There are two types of deterioration: gas-sensitive customization (resistance change rate due to gas), and these two types of deterioration often proceed at the same time.

Therefore, these two are considered to be related to each other, and rFe2
This can be explained by the phase transition of O3.

On the other hand, the above-mentioned deterioration is accelerated when combined with a high-humidity chamber or boiling treatment, but this is easy to understand if you consider that humidity directly affects gas, and gas indirectly affects the above-mentioned phase transition. .

In that case, there is a strong possibility that the additive that improves the heat resistance of γ-Fe2O3 also improves the moisture resistance.

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 reduced to one-third to one-fifth of that of a material that does not contain AI or the like.

The starting materials are not limited to the compounds shown in the examples, but ultimately γ-Fe2O3, Al2O3,
At least one of Ga2O3 and I n 203
Any material that forms a sintered body containing seeds may be used.

The atmosphere during sintering as in the examples is not limited to nitrogen, but may also 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 it may be 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 the temperature exceeds 1200° C., grain growth becomes significant, making it difficult to oxidize Fe3O4 to γ-Fe 203, and lengthening the response time and recovery time.

In this case, the modified Fe3O4 is oxidized to produce γ-F
The oxidation temperature when obtaining e2O3 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 making a film-like sintered body, modified γFe2O
It is desirable that the powder of No. 3 has a particle size of 0.1μ 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 particle size of the modified γFe2O3 is small, so it tends to become superphosphoric acid, and the sensitivity to flammable gas deteriorates.

As explained above, the element according to the present invention has an 80 to 9
9.8 mol% of 7'-Fe203 was added with Al as an additive.
A sintered body containing at least one selected from the three oxide groups of 2O3, Ga2O3, and In2O3 at a ratio of 0.2 to 20 mol% is used as a gas sensitive body, and a pair of electrodes for measuring electrical resistance is attached to this sintered body. A combustible gas detection element is characterized in that it detects a combustible gas by applying a heater for heating and changing the electrical resistance value of the gas sensitive body due to a change in the concentration of the flammable 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 e 203, 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, flammable gases include city gas, ethyl alcohol, methyl alcohol,
Various flammable gaseous substances can be mentioned, including hydrogen, acetone, and other common 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. 2... Film-like gas sensitive body, 3... Electrode, 4... Platinum heating element, 11... Bulk gas sensitive body, 12... ...Electrode, 14...
...Platinum heating element.

Claims (1)

[Claims] 180 to 99.8 mol% of γ-Fe2O3, and at least one selected from the three oxide groups of Al2O3, Ga20s, and ■n203 as an additive, in an amount of 0.2 to 2
A sintered body containing 0 mol % is used as a gas sensitive body, and a pair of electrodes for measuring electrical resistance and a heater are attached to the gas sensitive body, and the electrical resistance of the gas sensitive body is measured by changing the concentration of combustible gas. A combustible gas detection element characterized by detecting combustible gas using a change in value.