JPS5853737B2 - 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, a gas sensing element using γ-F e 203 as a gas sensitive material is being developed.

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-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.

This phase transition from γ phase to α phase is irreversible,
Once γ-Fe2O3 has been transferred to α-Fe 20B, 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.

What is the temperature at which γ-Fe2O3 undergoes a phase transition to α-F C203?

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.
Upon oxidation, γ-Fe 203 is obtained by heating at temperatures in the range of 00°C.

The 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-Fe203 is reduced to Fe3O4, and this is heated at a high temperature, for example, 900°C, in an inert atmosphere.
γ-Fe 203 can also be obtained by roasting at a temperature of 100° C., further heating at a temperature within the range of L00 to 700° C., and oxidizing it.

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

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

γ-F6203 not only undergoes a phase transition to α-Fe203 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 γF 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 improve it in some way.

Methods for producing γ-F e 203 can be roughly divided into the following two methods.

One of them is a method using dehydration of γ-FeOOH,
Another method is to oxidize Fe3O4.

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 Fe0CI crystals with the same crystal structure in water. .

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,
Alternatively, there is a method of heating Fe C204 or FeCO3 in steam or nitrogen.

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 that have been transformed into 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-
Fe203. and at least one selected from the oxide group consisting of CaO, SrO, and BaO, each containing 80 to
A sintered body containing 99.8 mol strips and a composition ratio of 20 to 0.2 mol rice 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 flammable 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 1 mol, 2 mol and 0.015 mol of Fe C12e FeCl a and 5rC12, respectively, were accurately weighed out and dissolved in 11 pure water.

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

This causes the following reaction.

FeCl + 2 FeCl s+ 0゜015S
rC12+8.03NaOH→Fe5S rO, 015
04, o15 + 8.03N aCl +4.015
I (20) According to the above reaction formula, 8.03 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 way, a constant pH value may be maintained at all times by supplementing the consumed amount of NaOH at the same time as the iron salt mixed solution is added dropwise.

After the dropping of the iron salt bath solution was completed, the solution was filtered and the precipitate was washed by decantation.

When the CI concentration in the washing solution was below 5XIO-5M, the washing was stopped, filtered, and the resulting material was dried in a dryer at a temperature of 5o-too<0>C for 4-10 hours.

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

Through this oxidation treatment, γ-F e2 modified with Sr
I was able to get 03.

Chemical analysis of this γ-Fe2O3 revealed that SrO was 0.
．． It contained 89 moles φ.

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

Furthermore, the phase transition temperature from γ-Fe20a to α-Fe203 determined by differential thermal analysis was 680°C.

The Sr-modified modified-Fe 203 obtained as described above was finely ground, and an organic binder was added thereto to form a paste.

On the other hand, gold paste for baking was printed on the main surface of an alumina porcelain plate measuring 51 m x 5 mm x 0.5 mm in the form of a comb with an interval of 5 mm, and baked at a temperature of 800° C. to form electrodes in advance.

On the electrode baking surface of this alumina porcelain plate, Sr metamorphosed Fe203 was further applied to a thickness of 20 μm.

This was heated, and the temperature was gradually increased while being careful not to cause cracks, and the temperature was maintained at 3509C 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 Sr modified γFe2O3 film that had been baked, and the entire body was surrounded by a 100 mesh stainless steel wire mesh, and a combustible gas detection element was installed. completed.

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

In the figure, 1 is an alumina magnetic plate, 2 is a film-like Sr metamorphosis-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 1
．． It was 65MΩ.

When this is placed in air containing 1 volume φ of propane gas, its resistance value is 28. It was OKΩ.

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 100 hours.

Thereafter, the platinum heating element 4 is energized again to generate γ-Fe 20
The resistance value of the three-gas sensitive coating 2 was measured in air while being maintained at a temperature of 300°C, and was found to be 1.83 MΩ.

In air containing 1 volume φ of propane gas, the resistance value was 27.5 KQ.

Example 2 Follow the same procedure as Example 1, changing the amount of 5rC12 added,
Various samples were prepared.

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 SrO and resistance (RG), and the relationship between SrO amount and resistance (RG).
The relationship between rOr abundance and sensitivity (RA/RG) is shown.

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 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 element is once heated to a temperature of 400°C.
The resistance value characteristics after holding for o hours are shown.

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

Looking at the gas sensitivity (curves I and IV), S
The amount of rOr in the γ-Fe 20s sintered body is from 0.2 to
It can be seen that when 20 mol φ is included, there is a significant improvement.

This effect of addition to γ-Fe2O3 is similar to that of CaO
or Bad, or 2 of SrO, CaO and BaO
Even when more than one species was added in combination, almost the same tendency was observed.

The experimental results are summarized in the table below. Example 3 1.9 mol B of Fe504 powder with an average particle size of 0.1 μm
0.1 mole of aCO3 was weighed out, water was added, 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 at a temperature of 950° 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 l pairs 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 body made of a bulk sintered body mainly composed of γ-Fe 203.

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 electrode 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-F e20a sintered body 1
1 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 259Ω.

When this was placed in air containing more than 0.5 volume of isobutane, the resistance value was 19.8Ω, 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 γFe 20a sintered body 11 was kept at a temperature of 350° C., the resistance values were measured in air and in air containing 0.5 volume of isobutane.
They were 275Ω and 23.3KQ, respectively.

Example 4 1 mol and 0.01 mol of F e C12 and CaCl2 were weighed, respectively, and dissolved in 1 liter of pure water.

Separately, 1.2 mol of (NH4)2C204 was weighed out and dissolved in 12 pure water.

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

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

When the C1 concentration was below 5×10 M, washing was stopped and the solution was filtered.

The resulting material was dried.

If the washing is prolonged, the ferrous iron will oxidize and become ferric iron, which will dissolve and the supernatant liquid will be colored orange.

At this time, since only the Fe component is extracted, a reducing agent such as ascorbic acid is added to prevent oxidation of ferrous iron.

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 is cooled while blocking air, and F containing CaO is
e3O4 was obtained.

Next, this Fe3O4 is 100 to 15 in air.
Ca-modified γ-Fe2O3 was obtained by slow oxidation at a temperature of 0°C.

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'C and measured the resistance value in air,
It was 1.8MQ.

Furthermore, 0. ■The resistance value in air containing propane gas with a capacity of ★ was 120Ω.

Next, using an electric furnace, heat at a temperature of 400°C.

After heating for a period of time, the resistance value was measured in the same manner as described above.

As a result, it is 1.47M in the air! Q, and in air containing propane gas with a capacity of 041 Ω, it was 117Ω.

Example 5 pec12 and 5rC12 at 1 mol and 0 mol, respectively
．． 0.12 mol was accurately weighed, dissolved in 0.51 mol of pure water, and this was dropped into 1 liter of 4N-NaOH solution.

Next, add 0.5 air! It was fed at a rate of l/min for oxidation.

The obtained precipitate was washed, dried, and converted into Sr-FeOO
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 was 2.3 MQ, and the value in air containing 0.4 volumes of propane gas was 52 KQ.

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 too long.

Then, the same measurement as before was performed, and the resistance value was 3.4 MQ in air, and 61 ohms in air containing 0.4 capacity of propane gas.

As described above, γ-Fe2O3, which contains 0.2 to 20 moles φ of at least one selected from the oxide group of CaO, SrO and BaO, has excellent gas sensitivity characteristics and is resistant to high temperature storage. Its characteristics are extremely stable.

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 in humidity, and applying voltage in humidity.

These results were the same for both a film-like sintered body made into a paste with an organic binder and baked, and a bulk-formed sintered body compressed without an organic binder. This is thought to be due to the effect of adding SrO and BaO.

The deterioration of the γ-Fe2O3-based gas sensitive material as a sensing element is mainly due to heat, but it can be roughly divided into (1
There are two types of deterioration: (2) an increase in the resistance value of the sintered body in a gas-containing atmosphere and (2) a decrease in gas sensitivity characteristics (rate of change in resistance due to gas), and these two types of deterioration often proceed simultaneously.

Therefore, these two are considered to be related to each other, and γ-Fe
This can be explained by the phase transition of 2O3.

On the other hand, the above-mentioned deterioration is accelerated when combined with high-humidity soaking, boiling, etc., but this is easier to understand if you consider that humidity directly or indirectly affects the above-mentioned phase transition. .

In that case, there is a high possibility that the additive that improves the heat resistance of γ-Fe20s 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.

In addition, 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 Caf, f, or the like.

The starting materials are not limited to the compounds shown in the examples, but ultimately γ-Fe 203, CaO*
Any material may be used as long as it forms a sintered body containing at least one of S r O and BaO.

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.

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 Fe304 to γ-Fe2O3, and the response time and recovery time become longer.

In this case, the metamorphosed Fe s04 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-F e 20a will precipitate.

For mass production, it is best 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, modified γF e2
It is desirable that the powder No. 03 has a particle size of 0.11 tm 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 device according to the present invention is made of γ-Fe.
2O3, and at least one selected from the oxide group consisting of CaO, SrO, and BaO, each containing 80 to 99%
．． A sintered body containing a composition ratio of 8 moles and 20 to 0.2 moles φ is used as a gas sensitive body, and it is provided with l pairs of electrodes for measuring electrical resistance and a heater for heating, and the concentration of combustible gas is determined. The combustible gas is detected by using the change in the electrical resistance value of the gas sensitive body due to the change.

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 using only 20 g of γ-Fe, and the response is significantly improved.

The resistance change of the element is small even with changes in outside temperature.
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.

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 the drawing]

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] 1 r Fe2O3+ and CaO and SrO and Ba
The gas sensitive body is a sintered body containing at least one kind selected from the oxide group consisting of A pair of electrodes for measurement and a heater for heating are provided, and the flammable 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 flammable gas. Flammable gas detection element.