JP3929164B2 - Gas sensing element and production method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gas detection element and a production technique thereof.
[0002]
[Prior art]
Gas detectors are installed in household gas alarms, portable gas detectors, industrial stationary gas detection alarms, etc., and in various situations in social life, from general households to high-pressure gas production facilities and semiconductor factories nationwide. It plays a part in ensuring the safety of people in Japan.
Here, in order to improve the gas sensitivity characteristics corresponding to each application and to improve the durability of the gas detection element, the metal oxide semiconductor layer, which is the gas detection part of the gas detection element, is covered to provide catalytic function and protection. Many gas detection elements having other layers having functions and the like are produced.
[0003]
The gas detection element has been produced by the following method, for example. That is, first, a metal oxide semiconductor layer is formed by sintering a metal oxide semiconductor. Next, the target catalyst or the like is made into fine particles and made into a paste form together with a binder, which is formed in a layer form on the surface of the metal oxide semiconductor layer, which is a gas sensitive part, by coating or printing. Thereafter, the paste is dried, the temperature is raised, and sintering is performed again, so that a catalyst layer or a protective layer coating layer is formed on the surface of the metal oxide semiconductor layer.
That is, the gas detection element obtained by the above method has a structure in which the metal oxide semiconductor layer, which is a gas sensitive portion, is relatively densely covered and hardened by the coating layer obtained by sintering fine particles.
[0004]
[Problems to be solved by the invention]
However, the above-described production method of the gas detection element has the following problems.
That is, when forming a coating layer on the surface of the metal oxide semiconductor layer, the catalyst particles and the like themselves usually do not have an adhesive force. Therefore, the adhesive force or adhesive force can be increased by preparing a paste or adding a binder. It was necessary to use it in the attached state.
In the above case, when the particle size of the catalyst particles is large, it is difficult to prepare a paste having a homogeneous property. In order to prevent such uneven application of the paste from occurring, there has been a situation that the catalyst particles and the like must be made fine.
[0005]
Furthermore, it took time and effort to sinter again for the paste to form the coating layer. Therefore, when a material that cannot be sintered unless it is heated to a high temperature is selected as the catalyst, the metal oxide semiconductor layer, which is a gas sensitive part, cannot be sintered unless it is heated to a high temperature. There is also a problem that the semiconductor layer is exposed to an unnecessarily high temperature, which may cause cracks and decrease gas-sensitive performance.
[0006]
Further, the structure of the conventional gas detection element has a problem that the gas permeability is easily lowered because the metal oxide semiconductor layer which is a gas sensitive portion is covered and hardened with a relatively dense coating layer. Moreover, since it is such a relatively dense coating layer, there has been a problem that it is liable to crack during sintering or heat cycle operation.
[0007]
Accordingly, an object of the present invention is to obtain a gas sensing element with good performance stability and a method for easily producing the gas sensing element in view of the above-mentioned drawbacks.
[0008]
[Means for Solving the Problems]
In order to achieve this object, the gas sensing element production method of the present invention comprises applying a paste-like metal oxide semiconductor to a noble metal wire, and the metal or metal oxide is in an undried state. Coarse grains including at least one are adhered and covered, and the metal oxide semiconductor is sintered together with the coarse grains to form a coarse layer on the surface of the metal oxide semiconductor layer. The coarse particles forming the coarse particle layer include at least one selected from the group consisting of platinum, palladium, gold, rhodium, silica, silica alumina, zeolite, alumina, tin oxide, zinc oxide, and indium oxide. If it is, it is suitable. Furthermore, it is more convenient if the coarse particles have a particle size of 0.5 μm to 50 μm.
[0009]
[Action]
In the present invention, a noble metal wire is coated with a paste-like metal oxide semiconductor, and the metal oxide semiconductor is in an undried state, and is coated with coarse particles containing at least one of metal or metal oxide, The metal oxide semiconductor is sintered together with the coarse particles at the sintering temperature of the metal oxide semiconductor to form a coarse layer on the surface of the metal oxide semiconductor layer.
Here, since the coarse particles adhere to the metal oxide semiconductor due to the adhesive force of the dispersion medium (water or organic solvent) of the paste-like metal oxide semiconductor, the coarse particles adhere to the metal oxide semiconductor without a binder. I was able to.
Since the binder is not required, the coarse particles do not need to be prepared in a paste form, and even the coarse particles having a large particle size can be attached to the metal oxide semiconductor. When the coarse particle layer is formed on the metal oxide semiconductor layer by the coarse particles having a large particle diameter, the gas permeability of the gas detection element can be improved, so that the gas responsiveness of the gas detection element can be improved. It became possible to make it better.
[0010]
Moreover, once the coarse particles adhere to the paste-like metal oxide semiconductor, the coarse particles themselves do not have an adhesive force, so that the coarse particles do not adhere to the adhered coarse particles. Accordingly, the coarse particles adhere to the surface of the metal oxide semiconductor with a uniform thickness equal to the diameter thereof. By sintering the metal oxide semiconductor together with the coarse particles, a gas detection element in which a coarse layer having a uniform thickness was provided on the surface of the metal oxide semiconductor layer was obtained.
In addition, even when there is a difference in the coefficient of thermal expansion between the metal oxide semiconductor material and the coarse particle material, each of the coarse particles is integrated with the metal oxide semiconductor layer. By the operation by heat cycle, the coarse grain layer was not cracked or peeled off.
[0011]
【The invention's effect】
Therefore, it was possible to easily produce a gas detection element having high performance stability and to provide a gas detection element excellent in gas responsiveness.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1 (a), the gas detection element of the present invention is formed by covering the noble metal wire 1 with the metal oxide semiconductor layer 2 in a spherical shape, and a coarse particle layer on the outer surface of the metal oxide semiconductor layer 2. The embodiment of the present invention will be described below.
[0013]
As shown in FIG. 2 (a), a platinum wire coil 1 a is used as the noble metal wire 1. Next, the colloidal white precipitate obtained by hydrolyzing a commercially available tin salt was dried to obtain tin oxide as the metal oxide semiconductor. After this tin oxide is crushed into fine particles, it is formed into a paste with water or a dispersion medium of an organic solvent and applied in a spherical shape on the platinum wire coil 1a as shown in FIG.
Then, as shown in FIG. 2C, the coarse particles containing at least one of a metal and a metal oxide are attached to and covered with paste-like tin oxide that has not been dried. As a means for adhering the coarse particles to the paste-like tin oxide, a method such as sprinkling the coarse particles on a paste-like tin oxide or burying the paste-like tin oxide in the coarse particles, etc. Can be used.
In this way, the coarse particles are adhered to the surface of tin oxide as the metal oxide semiconductor by the adhesive force of the paste dispersion medium.
[0014]
The coarse particles adhere to and coat the surface of tin oxide as a metal oxide semiconductor by the adhesive force of the paste, so that the coarse particles are not limited in size, and the coarse particles from 1 μm or less to 50 μm It is possible to coat up to grains. When the coarse particles having a large particle size of 10 μm or more are adhered, the gas permeability can be made particularly good.
Moreover, the kind of the coarse particles to be adhered is not limited to a specific type, and for example, even the coarse particles having no sinterability can be adhered by the adhesive strength of the paste.
By sintering the tin oxide as the metal oxide semiconductor adhered by the coarse particles, the coarse particles are integrated into a layer on the surface of the metal oxide semiconductor, and the metal oxide as a gas sensitive part A gas detection element in which the coarse particle layer 3 is formed on the outer surface of the semiconductor layer 2 is obtained.
Here, since the sintering in the production process of the gas detection element is performed only once at a temperature of 700 ° C. at which the tin oxide as the metal oxide semiconductor is sintered, the coarse particles are exposed to a high temperature to be sintered. Inconveniences such as being apt to occur are less likely to occur, and energy is advantageous and reliability is high.
[0015]
FIG. 1 (b) is an enlarged view of the outer surface of the gas detection element of the present invention and shows the coarse particle layer 3 formed on the outer surface of the metal oxide semiconductor layer 2. The coarse particles have the same thickness as the diameter of the coarse particles and cover the surface of the metal oxide semiconductor layer 2 in a uniform state, and the formation of the coarse particle layer 3 having a uniform thickness is always possible. Therefore, the production reproducibility of the gas detection element obtained by the method of the present application is extremely excellent.
In addition, a carbon monoxide selective hot-wire semiconductor sensor or the like may operate in a pattern that repeats three temperature conditions of high temperature, low temperature, and normal temperature. It has been found that since the coarse particles are integrated with each other, cracking and peeling of the coarse particle layer 3 due to the operation of the gas detection element due to the heat cycle are less likely to occur.
[0016]
【Example】
Embodiments of the present invention are described below with reference to the drawings.
A colloidal white precipitate was obtained by dropping an aqueous ammonia solution into a commercially available mixed aqueous solution of tin tetrachloride and a small amount of antimony pentachloride, followed by hydrolysis. This was washed with distilled water several times and dried with a drier to obtain a gel-like sample.
Next, this was baked at 700 ° C. for 2 hours in an electric furnace to obtain tin oxide as a metal oxide semiconductor doped with 0.1 atm% antimony. Here, antimony is added in order to optimize the resistance value of tin oxide by the valence control method.
Next, this was pulverized by a pulverizer to obtain fine powder, and the fine powder was dispersed with a dispersion medium to obtain paste-like tin oxide.
[0017]
The paste-like tin oxide is applied to the platinum wire coil 1a as the noble metal wire 1 having a diameter of 30 μm.
And the alumina which carry | supported the 0.5 wt% palladium catalyst as the said coarse particle is made to adhere to the paste-form tin oxide which is not dried, and is coat | covered.
Gas detection in which the alumina layer 3a as the coarse particle layer 3 is formed on the surface of the tin oxide layer 2a as the metal oxide semiconductor layer 2 by sintering the tin oxide coated with the alumina. An element is obtained.
[0018]
The gas permeability and gas responsiveness of the gas sensing element in which the tin oxide layer 2a is coated with the alumina layer 3a having a particle diameter of 10 μm will be described in comparison with the gas permeability and gas responsiveness of the conventional gas sensing element. .
FIG. 3 is a response waveform of the conventional gas detection element, and FIG. 4 is a response waveform of the gas detection element of the present invention.
In the conventional gas detection element, a metal oxide semiconductor is first sintered to form a metal oxide semiconductor layer, and then the alumina paste having a particle size of 0.5 μm is combined with a binder to form a metal oxide semiconductor. The catalyst layer is formed by forming a layer on the surface of the layer and then sintering again. The thickness of the catalyst layer was 100 μm.
[0019]
According to FIG. 3, when 1000 ppm of isobutane gas (i-C4H10) was input, the sensor output increased gradually with time and required 2 minutes to saturate. When the gas was shut off, the sensor output gradually decreased with time, and it took 1.5 minutes for the sensor output to become zero again.
This indicates that the conventional gas detection element takes a long time to detect the gas even when the gas is input. Even if the gas is shut off, the gas cannot be immediately removed from the gas sensitive part. Show.
On the other hand, according to FIG. 4, when 1000 ppm of isobutane gas (i-C4H10) was input, the sensor output increased rapidly with time and saturated after 30 seconds. When the gas was shut off, the sensor output decreased rapidly with time, and only 30 seconds were required until the sensor output became zero again. This indicates that the gas detection element of the present invention can immediately detect the gas when the gas is input, and if the gas is shut off, the gas is immediately released from the gas sensitive part. Yes.
That is, the coarse particle layer 3 of the gas detection element of the present invention is excellent in gas permeability.
[0020]
FIG. 5 shows sensitivity characteristics of the conventional gas detection element, and FIG. 6 shows sensitivity characteristics of the gas detection element of the present invention. When FIG. 5 and FIG. 6 are compared, the gas selectivity is the same as the conventional one, and it is shown that the gas selectivity sensitivity does not decrease even when the coarse particle layer 3 is used. That is, the coarse particle layer 3 of the gas detection element of the present invention not only has excellent gas permeability, but also shows that the catalyst performance is sufficiently exhibited as in the conventional one.
[0021]
[Another example]
Another embodiment will be described below.
In the previous embodiment, platinum was used as the material of the noble metal wire 1, but any other material may be used as long as it also serves as a heater for keeping the metal oxide semiconductor at a high temperature and an electrode for detecting a change in electrical conductivity of the semiconductor. Or a palladium-platinum alloy or the like.
Moreover, although the coil is employ | adopted as a shape of the noble metal wire 1, it is not limited to this, The electrode printed on the board | substrate may be sufficient.
Tin oxide is employed as the metal oxide semiconductor, but other materials such as zinc oxide can be employed.
As the coarse particles to be coated, palladium-supported alumina was used, but is not limited thereto, and is not limited to platinum, palladium, gold, rhodium, silica, silica alumina, zeolite, alumina, tin oxide, zinc oxide, indium oxide. It is possible to use metal or metal oxide coarse particles containing at least one selected from the group consisting of:
Moreover, although the gas detection element which coat | covered the coarse particle of a 10 micrometer particle diameter was employ | adopted, the particle diameter of a coarse particle is not limited to this, The said coarse particle of a particle diameter of 0.5 micrometer-50 micrometers is used. Can do.
[0022]
In the description of the appended claims, reference is made to the drawings to refer to the drawings for convenience of comparison with the drawings, but the present invention is not limited to the configuration of the attached drawings by such entry.
[Brief description of the drawings]
FIG. 1 is a partially broken perspective view of a gas detection element of the present invention. FIG. 2 is an explanatory diagram showing a manufacturing process of the gas detection element of the present invention. FIG. 3 is a graph showing a response waveform of the gas detection element of the present invention. 4 is a graph showing the response waveform of the conventional gas detection element. FIG. 5 is a graph showing the sensitivity characteristic of the gas detection element of the present invention. FIG. 6 is a graph showing the sensitivity characteristic of the conventional gas detection element.
1 Noble metal wire 2 Metal oxide semiconductor layer 3 Coarse grain layer

Claims (2)

  A noble metal wire (1) is coated with a paste-like metal oxide semiconductor, and the metal oxide semiconductor is in an undried state and includes at least one of metal and metal oxide, and has a particle size of 0.5 μm to 50 μm A method for producing a gas detection element, wherein coarse particles are deposited and coated, and the metal oxide semiconductor is sintered together with the coarse particles to form a coarse particle layer (3) on the surface of the metal oxide semiconductor layer (2). .   The gas detection element according to claim 1, wherein the coarse particles include at least one selected from the group consisting of platinum, palladium, gold, rhodium, silica, silica alumina, zeolite, alumina, tin oxide, zinc oxide, and indium oxide. Production method.

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