JPH053905B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a method for manufacturing a combustible gas detection element (hereinafter abbreviated as element). Specifically, the present invention relates to a method for manufacturing an element mainly composed of SnO ₂ and having high sensitivity, which is unprecedented.

[Conventional technology]

With the spread of city gas, leakage accidents such as fires are increasing, and the number of closed buildings is increasing from the perspective of effective heat utilization, and carbon monoxide (CO) due to incomplete combustion is increasing.
Poisoning accidents are also increasing year by year. Working environment standards are also becoming stricter.

Therefore, it is desired to develop a method that can detect flammable gases such as hydrogen, carbon monoxide, methane, propane, butane, acetone, and alcohol with high sensitivity even in trace amounts.

In particular, there are no good detection methods for CO.
Development was an urgent task.

On the other hand, there are methods that utilize the property of fine metal wires such as platinum that change their resistance when they come into contact with flammable gas (catalytic combustion method), and methods that utilize thin wires of precious metals such as platinum that change their resistance when they come into contact with flammable gases, and methods that utilize thin wires of precious metals such as platinum that change their resistance when they come into contact with flammable gases, and methods that utilize thin wires of precious metals such as platinum that change their resistance when they come into contact with flammable gases (catalytic combustion method), and methods that utilize wires of precious metals such as platinum that change their resistance when they come into contact with flammable gas (catalytic combustion _method ), and methods that utilize wires of precious metals such as platinum that change their resistance when they come into contact with flammable gases (catalytic combustion methods), A method (oxide semiconductor method) that utilizes the property of a metal oxide semiconductor to change its resistance value in response to the concentration of combustible gas is known.

The former is expensive because it uses precious metals and has a complicated circuit. On the other hand, the latter is relatively inexpensive because the device is easy to manufacture and a simple circuit is sufficient, but with the device using conventional materials,
It has not been possible to manufacture one with sufficiently high sensitivity (that is, one with a large change in resistance value), and it is particularly unsatisfactory for (CO), so there are no examples of it being put to practical use.

Therefore, if a highly sensitive element can be obtained using the oxide semiconductor method, the amplifier circuit, false alarm prevention circuit, etc. can be simplified, the detection device can be manufactured at low cost, and it can effectively respond to CO, which is extremely important. preferred.

Conventionally, SnO ₂ is produced by calcining the white precipitate obtained by neutralizing a SnCl ₄ aqueous solution with an alkali such as sodium hydroxide, potassium hydroxide, or ammonia aqueous solution.
It is known to manufacture elements from this. However, this element has the disadvantage of low sensitivity.

Special Publication No. 51-25159 contains tin tetrachloride (SnCl ₄ ).
A method is disclosed in which an element is manufactured using SnO ₂ obtained through a process of "neutralizing" an aqueous solution with an aqueous ammonium nitrate (NH ₄ NO ₃ ) solution.

However, in this method, SnCl ₄ aqueous solution
Even if you try to neutralize by adding NH ₄ NO ₃ aqueous solution,
SnCl ₄ , which is a strong acid salt, is not neutralized by NH ₄ NO ₃ , and therefore it is very difficult to obtain a white precipitate (based on β-stannic acid).

These devices also have particularly low sensitivity to CO.

[Purpose of the invention]

An object of the present invention is to provide a method for easily manufacturing an element that can detect flammable gas with high sensitivity.

Another object of the present invention is to provide a method for easily manufacturing an element that can detect carbon monoxide gas with high sensitivity.

[Disclosure of the invention]

According to the present invention, a precipitated product obtained by neutralizing a tin tetrachloride aqueous solution with an ammonia aqueous solution in the presence of ammonium nitrate is calcined at 400 to 700°C, then formed into an element shape, and then heated at 500 to 700°C. A method of manufacturing a gas sensing element is provided, which comprises sintering.

Next, the present invention will be explained in detail.

The method of the present invention consists of three steps as follows. That is, (a) a step of neutralizing an aqueous tin tetrachloride solution with ammonia in the coexistence of ammonium nitrate to obtain a precipitated product, (b) a step of calcining the precipitated product at 400 to 700°C, and (c) calcination. The powder obtained was molded into an element shape, and 500
This is a process of producing a sensing element by heating and sintering at ~700°C.

Step (a) is a step in which an aqueous SnCl ₄ solution is neutralized with NH ₃ in the coexistence of ammonium nitrate to produce a white precipitate whose main component is β-stannic acid ([H ₂ SnO ₃ ] ₅ ).

Ammonia may be blown into the SnCl ₄ aqueous solution as a gas or may be supplied as a liquid solution, but from the viewpoint of ease of operation, it is preferable to supply it in the form of an aqueous solution.

The concentration of SnCl ₄ aqueous solution used here is 20~40wt
%, and when added as an aqueous solution, the concentration of NH ₃ aqueous solution is 10 to 30 wt%, and the concentration of NH ₄ NO ₃ aqueous solution is,
5 to 30 wt% is preferable.

The NH ₃ aqueous solution and the NH ₄ NO ₃ aqueous solution may be mixed in advance and added, or may be added separately. However, in that case, it is necessary to add the NH ₄ NO ₃ aqueous solution first and then add the NH ₃ aqueous solution.

The pH of the neutralization point is 6.5 to 7.5, more preferably 6.8 to 7.0.

In step (b), the white precipitate produced in step (b) is
This is the process of producing SnO ₂ by calcining.

This calcination temperature is preferably 400 to 700°C.

Below 400°C, the residual rate of SnCl ₄ is high, hygroscopicity is large, and the properties are unstable. Also, if the temperature exceeds 700℃, the active sites disappear, so the normal
The performance is not much different from that of SnO ₂ .

In Special Publication No. 51-25159, the calcination temperature is 500 to 700℃.
However, for the purpose of this invention, 500℃
It is not necessary that the temperature be higher than that, and a sufficiently good element can be made at a temperature of 400°C or higher.

The SnO ₂ produced in step (b) is never pure SnO ₂
Instead, Cl atoms, hydroxyl groups, etc. remain,
Furthermore, the specific surface area, pores, and other surface structures are also specific, and these are thought to give SnO ₂ properties that allow flammable gases to be detected quickly and with high sensitivity.

The step (c) is a step of manufacturing an element. the above
The SnO ₂ made in (b) is molded into an element shape of an appropriate size.

In this case, the shape and size of the element can be arbitrary, but
The preferred size is approximately 1 mm square to 5 mm square.

The device is equipped with a pair of electrodes for measuring changes in current flowing in response to changes in electrical resistance of the element, and a heater for heating the element. One electrode may also serve as a heater. The material of the electrode is Pt or Pd-Ir.
etc. are used.

In order to manufacture the element, (b) SnO ₂ must be supplemented with a liquid binder such as water or an aqueous polyvinyl alcohol solution, or a sintering agent that remains in the molded product after heating and maintains its shape, such as silica sol or glass frit. agent is added. Furthermore, in some cases, inorganic auxiliaries such as alumina, silica alumina, and magnesia are added to maintain the strength of the element and improve response characteristics to changes in gas concentration.

If necessary, platinum (Pt), palladium (Pd),
Metal oxides and halides such as gold (Au) and antimony (Sb) are added.

The preferable addition amounts of these are as follows.

The liquid binder may be used in an amount necessary to enable sufficient mixing and easy device molding.

If the sintering aid is too small, it will be ineffective, and if it is too large, it will cover the SnO ₂ particles and prevent the development of properties, so it is generally 0.01 to 5 wt%, preferably 0.1
~1wt%.

If the amount of inorganic auxiliary agent is too large, it will actually become difficult to mold, so a range of 0 to 10 wt% is preferable.

The catalyst is preferably 0.001 to 1 wt% since it is ineffective if it is too small and expensive if it is too large.

Next, the sintering temperature is preferably 500 to 700°C. 500
If the temperature is less than 0.degree. C., the liquid binder remains or the sintering effect is not sufficient, which is not preferable. Also, if it exceeds 700℃ (b)
The properties obtained in the process are lost.

When carrying out the present invention, it is essential to manufacture a gas detection element through the steps (a), (b), and (c), respectively.

For example, in step (a), omitting the addition of NH ₄ NO ₃ and using the white powder obtained by neutralization, (b) (c)
Even if the device is manufactured through this process, only very poor detection sensitivity for flammable gas can be obtained.

Also, in step (a), instead of _NH3 , NaOH,
Even if an element is manufactured through steps (b) and (c) using the white powder obtained by neutralization using hydroxides of alkali metals and alkaline earth metals such as KOH and CaOH, The detection sensitivity for combustible gases is also extremely poor.

In addition, we obtained commercially available SnO ₂ that omitted steps (a) and (b),
Even if a gas detection element is manufactured by pulverizing it and going through the process (c), it has almost no detection sensitivity for combustible gas.

Now, through the steps (a), (b), and (c), the manufactured gas detection element is used to detect combustible gas at an element temperature in the range of 70 to 500°C.

Combustible gases that can be detected by the sensing element of the present invention include hydrogen, carbon monoxide, methane, ethane, propane, butane, acetone, and ethanol. In particular, when used as a carbon monoxide detection element, it has good selectivity and high sensitivity.

Note that when detecting CO gas, it is often required to separate it from H ₂ , ethyl alcohol, methane, ethane, etc. in the coexistence of these gases for selective detection.

For example, when used as a CO alarm, the ability to separate and detect trace amounts of CO from ethyl alcohol is required.

When used as an industrial detector, CO is converted into _H2
It is necessary to selectively detect hydrocarbons such as carbon dioxide, methane, and propane.

Furthermore, elements used to control combustion in burners and the like based on CO concentration are required to have the ability to selectively detect CO from H ₂ and hydrocarbons.

In such a case, the element temperature may be set at 70 to 150°C. In general, with sensing elements using oxide semiconductors such as SnO ₂ , the higher the element temperature, the higher the detection temperature for flammable gases such as H ₂ , ethyl alcohol, and propane; conversely, the higher the element temperature, the higher the detection temperature. This is because the detection element of the present invention has a property that the detection sensitivity becomes low when the temperature of the element decreases. On the other hand, for CO, surprisingly, there is a peak in the detected temperature in the low element temperature range.

As a result, if the above element temperature is adopted, CO can be detected separately without being affected by other gases.

Furthermore, although the limit for CO is 50 ppm, even such a minute amount can be detected with extremely high sensitivity by the element of the present invention.

In the sensing element of the present invention, the electrical resistance of the element changes depending on the concentration of combustible gas. Therefore, by utilizing this change, a circuit including an element, a power source, and an alarm means or a control circuit is configured to detect combustible gas.

An example of specifically configuring a detection device using the element of the present invention will be shown.

Connect the element, power supply, and fixed resistor in series, and
Load a constant voltage of about 10V. Apply an appropriate voltage to the element heater to keep the element temperature constant. The outputs from both ends of the fixed resistor are taken out and connected to an amplifier to operate alarm means such as a bell or light emitting diode, or to operate a control circuit such as a relay.

The device of the invention is easy to fabricate, inexpensive, and very sensitive. Furthermore, since the circuit can be simplified, the cost of the entire device is low.

In addition, when used as a CO detection element, it can be used as an alarm for incomplete combustion in gas water heaters, gas stoves, gas baths, etc. for household use, and for quantitative measurement of CO in various gases, burners that monitor CO concentration, etc. for industrial use. It can be effectively used for combustion control, etc.

Examples will be described in more detail below, but these are merely illustrative and the technical scope of the present invention is not restricted thereto.

Example 1 200 g of SnCl ₄ and 4 g of aluminum nitrate were added to 1 part of water.
dissolve in

On the other hand, 150 g of NH ₄ NO ₃ and 100 g of NH ₃ per 1 water
Prepare an aqueous solution with dissolved in it and stir until the pH reaches 7.0
Add dropwise while stirring. Separate a large amount of white precipitate,
After washing once with water, calcining at 400℃.

Palladium chloride (PdCl ₂ ) is added to the resulting calcined product.
Add 0.1wt% silica gel, 0.1wt% silica gel, and an appropriate amount of 2% polyvinyl alcohol aqueous solution, stir thoroughly and mix to form a paste.

This paste is applied to an alumina carrier with electrodes at both ends and air-dried overnight.

Afterwards, it is baked at 600℃ for 1.5 hours to make the element.

One of the electrodes at both ends of the carrier also serves as a heater, and a circuit is formed in which this element, a power source, and a fixed resistor are connected in series, and the output is taken out from both ends of the fixed resistor.

CO was detected with the element temperature set at 120°C. The results are the resistance of the element in air (Va) and CO
Displayed as the ratio of resistance (Vg) and rate of change in resistance (Va/Vg) when detected.

The results are shown in Figure 1.

It can be seen that it is possible to sufficiently detect concentrations lower than the CO limit of 50 ppm.

On the other hand, at this element temperature, no H ₂ , propane, or ethanol was detected.

Comparative Example 1 An experiment was carried out in the same manner as in Example 1, except that no NH ₄ NO ₃ was added.

At an element temperature of 120°C, there was no sensitivity to CO at all. Only CO was selectively detected at an element temperature of 200°C, but there was no sensitivity at all at a CO concentration of 50ppm.
At 250 ppm, the resistance change rate was 1.6. Furthermore, at an element temperature of 250°C, it was impossible to selectively detect CO from H ₂ propane CO.

It can be seen that the sensitivity is very low compared to Example 1, and it is impossible to put it into practical use as a CO detection element.

Example 2 200g of SnCl ₄ and 4g of aluminum nitrate in 1 part of water
dissolve in

On the other hand, 100 g of NH ₄ NO ₃ and 150 g of NH ₃ per 1 water
Prepare an aqueous solution containing dissolved and stir until the pH reaches 6.5.
Add dropwise while stirring. Separate a large amount of white precipitate,
After washing once with water, calcining at 500℃.

Palladium chloride (PdCl ₂ ) is added to the resulting calcined product.
Add 0.5wt% glass frit, 0.1wt% glass frit, and an appropriate amount of 2% polyvinyl alcohol aqueous solution, stir thoroughly, and mix to form a paste.

This paste is applied to an alumina support with electrodes at both ends and air-dried overnight.

Afterwards, it is baked at 650℃ for 2 hours to make the element. Thereafter, a circuit was constructed in the same manner as in Example 1, and propane was detected.

The element temperature was set at 350°C.

The results are shown in Figure 2.

Comparative Example 2 Instead of 100g of NH ₄ NO ₃ and 150g of NH ₃ ,
The experiment was conducted in the same manner as in Example 2, except that 50 g of NH ₄ NO ₃ and 176 g of NaOH were used.

The sensitivity to propane at a concentration of 1000 ppm, that is, the resistance change rate was approximately 4, which was much smaller than that of Example 2. Furthermore, this element had severe level fluctuations during measurement and was not suitable for practical use.

Comparative Example 3 200 g of SnCl ₄ and 4 g of ammonium nitrate are dissolved in 1.

An attempt was made to neutralize this with an aqueous solution containing 100 g of NH ₄ NO ₃ per 1 portion of water to obtain a precipitate, but no precipitate was obtained.

[Function and effect of the invention]

The reason why the device of the present invention is highly sensitive to combustible gases is unknown, but when an aqueous SnCl ₄ solution is neutralized with an aqueous solution of NH ₄ NO ₃ and NH ₃ as in the present invention, it becomes very bulky. Whereas a high white precipitate can be obtained, when simply neutralizing with alkali as in the conventional method, a white precipitate with a small bulk is produced.
It is believed that the device made by the method of the present invention has much higher sensitivity than the device made by the conventional method in terms of specific surface area, crystal structure, and chemical components such as Cl and _NH3 radicals. It will be done.

[Industrial Applicability] By using the element produced by the method of the present invention, combustible gases can be detected with extremely high sensitivity. In particular, when used under specific conditions, trace amounts of carbon monoxide can be selectively detected with high sensitivity.

Therefore, it must be said that the industrial applicability of the present invention is extremely large.

[Brief explanation of the drawing]

FIGS. 1 and 2 are graphs showing the relationship between resistance change rate and gas concentration.

Claims (1)

[Claims] 1. A precipitated product obtained by neutralizing an aqueous tin tetrachloride solution with ammonia in the coexistence of ammonium nitrate,
After calcination at 500℃ and then molding into element shape,
A method for manufacturing a gas sensing element, characterized by heating and sintering at ~700°C.