JP7137280B2 - Freon gas sensor - Google Patents

Description

The present invention relates to a chlorofluorocarbon gas sensor equipped with a semiconductor gas detection element for detecting chlorofluorocarbon gas.

Conventionally, as a gas sensor having a sintered body gas sensing part, a catalytic combustion gas sensor, a semiconductor gas sensor, a solid electrolyte gas sensor, and the like are known.

For example, as a semiconductor type gas sensor, a freon gas sensor for detecting freon gas is known (see, for example, Patent Document 1).

When detecting chlorofluorocarbon gas, there is a demand for a chlorofluorocarbon gas sensor that not only has high sensitivity, but also can accurately detect a wide range of concentrations from low to high concentrations (for example, 100 ppm to 10000 ppm). In particular, the conventional gas sensor corresponding to freon gas does not behave well in a high concentration range.

JP-A-2000-111507

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a chlorofluorocarbon gas sensor equipped with a semiconductor gas detection element capable of accurately detecting chlorofluorocarbon gas over a wide concentration range.

The problems to be solved by the present invention are as described above, and the means for solving the problems will now be described.

That is _, the flon gas sensor of the present invention is a flon gas sensor equipped with a semiconductor type gas detection element, comprising: a noble metal wire; and a heating section for heating the gas sensing section.

Moreover, the Freon gas sensor of the present invention is provided with a protective layer formed on the outer peripheral side of the gas sensing portion.

Further, in the chlorofluorocarbon sensor of the present invention, the protective layer comprises a first protective layer formed on the gas sensitive portion side and a second protective layer formed on the outer peripheral side of the first protective layer, The first protective layer is thinner and denser than the second protective layer.

Further, in the Freon gas sensor of the present invention, the Ce content is 0.05 wt % or more and 0.60 wt % or less.

Further, in the Freon gas sensor of the present invention, the content of WO ₃ is 0.5 wt % or more and 5 wt % or less.

Further, in the Freon gas sensor of the present invention, the Mo content is 0.075 wt % or more and 3 wt % or less.

Further, in the Freon gas sensor of the present invention, the first protective layer has a thickness of 10 to 200 nm.

Further, in the Freon gas sensor of the present invention, the second protective layer has a thickness of 0.02 to 0.40 mm.

Further, in the Freon gas sensor of the present invention, the first protective layer contains aluminum oxide as a main component.

Further, in the Freon gas sensor of the present invention, the second protective layer contains aluminum oxide as a main component.

ADVANTAGE OF THE INVENTION According to this invention, the Freon gas sensor which can detect Freon gas accurately in a wide concentration range is realizable.

1 is a schematic diagram showing a freon gas sensor having a bridge circuit according to one embodiment of the present invention; FIG. FIG. 2 is a schematic diagram showing the configuration of a semiconductor gas detection element; 4 is a graph showing the relationship between detected gas concentration and gas sensitivity; The graph which expands and shows the Example in FIG. Graph showing gas sensitivity characteristics with respect to R32 (1000 ppm) when contents of Ce, WO ₃ and Mo are changed. Graph showing gas sensitivity characteristics for R32 (5000 ppm) when the contents of Ce, WO ₃ and Mo are varied. A graph showing gas sensitivity characteristics for R32 (10000 ppm) when the contents of Ce, WO ₃ and Mo are varied. 2 is a graph showing the CFC sensitivity ratio to R32, where (a) is a graph when the gas concentration of R32 is 1000 pm (5000 ppm sensitivity=100), and (b) is a graph when the gas concentration of R32 is 10000 pm (5000 ppm sensitivity=100). A graph comparing the response time of each Freon gas sensor to R32 (8000 ppm). A graph comparing the gas concentration dependence (n=3) of each Freon gas sensor. Graph normalized based on FIG. 10 with R32 gas concentration 2000 ppm=1. 4 is a graph showing the temperature/humidity dependence of the Freon gas sensor according to the example.

Next, a chlorofluorocarbon gas sensor 100 having a semiconductor type gas detection element Rs, which is an embodiment of the present invention, will be described with reference to the drawings.

As shown in FIG. 1, the freon gas sensor 100 according to the present embodiment is configured by incorporating a semiconductor type gas detection element Rs and fixed resistors R0, R1, and R2 into a bridge circuit. The bridge circuit is constantly or intermittently energized by the power supply E, and the semiconductor type gas detection element Rs reaches a temperature suitable for detection. Further, the resistance value of the semiconductor type gas detection element Rs changes when the gas to be detected is adsorbed. Therefore, in the CFC sensor 100 according to the present embodiment, the change in the resistance value of the semiconductor gas detection element Rs is taken out as a deviation voltage, and this is used as the sensor output V to measure the concentration of the gas to be detected in the air. be able to.

As shown in FIG. 2, the semiconductor gas detection element Rs of this embodiment includes a coil-shaped noble metal wire 1 which is an example of a noble metal wire serving as a detection electrode, a gas sensing portion 2 covering the noble metal wire 1, a gas A coil-shaped heater 3 as a heating portion for heating the sensing portion 2 and a protective layer 4 formed on the outer peripheral side of the gas sensing portion 2 are provided. The gas sensitive part 2 is provided so as to freely come into contact with the gas to be detected, and is formed in a substantially spherical shape.
The noble metal wire 2 also serves as the coil-shaped heater 3, and these are integrally constructed.

As materials for the noble metal wire 1 and the coil heater 3, for example, platinum or platinum added with rhodium or the like can be appropriately selected and used, but the materials are not limited to these. Further, the wire diameter, coil diameter, number of turns, etc. of the coiled heater 3 are not limited to those shown in the drawings, and may be appropriately set as required.

The gas sensing part 2 is formed using a metal oxide semiconductor containing, for example, a metal oxide such as tin oxide as a main component and to which Ce (cerium), WO ₃ (tungsten oxide), and Mo (molybdenum) are added. can be done. The gas sensitive part 2 can be arbitrarily selected according to the type of gas to be detected.
It should be noted that the diameter of the gas sensing portion 2 is preferably 0.30 to 0.60 mm.

The Ce content is preferably 0.05 wt % or more and 0.60 wt % or less. More preferably, it is 0.07 wt% or more and 0.20 wt% or less. By setting the Ce content within the above range, the CFC sensitivity ratio between the low concentration range (for example, 0 to 1000 ppm) and the high concentration range (for example, 10000 ppm or more) of the CFC gas to be detected is reduced (gas The linearity of the sensitivity curve is excellent), and it is possible to accurately detect Freon gas in a wide concentration range.

For example, when the Ce content is zero, the gas sensitivity is high in the low concentration range of freon gas, but the change in sensitivity is small in the high concentration range, making it unsuitable for use.
Further, when the Ce content is 1.0 wt %, the gas sensitivity is too low in the low CFC concentration range, and the difference in sensitivity is large in the high CFC concentration range. In the case of combustible gases other than freon gas, the gas sensitivity ratio may be further deteriorated.

The content of WO3 is preferably 0.5 wt% or more and ₅ wt% or less.
If the WO3 content is less than 0.05 wt% _, sufficient sensitivity to Freon gas cannot be obtained. On the other hand, when the content of WO3 exceeds ₅ wt%, the sensitivity to freon gas decreases.
In addition, as in the case of Ce, by setting the content of WO ₃ within the above range, the low concentration range (for example, 0 to 1000 ppm) and the high concentration range (for example, 10000 ppm or more) of Freon gas, which is the gas to be detected The chlorofluorocarbon gas sensitivity ratio is small (the linearity of the gas sensitivity curve is excellent), and chlorofluorocarbon gas can be detected with high accuracy in a wide concentration range.

The Mo content is preferably 0.075 wt % or more and 3 wt % or less.
When the Mo content is less than 0.075 wt%, sufficient sensitivity to Freon gas cannot be obtained. On the other hand, if the Mo content exceeds 3 wt %, the sensitivity to Freon gas decreases and the selectivity of the sensor to Freon gas deteriorates.
In addition, by including Mo in the gas sensitive portion 2, fluctuations in the Air value due to fluctuations in temperature and humidity can be suppressed.
In addition, as in the case of Ce, by setting the Mo content within the above range, a low concentration range (for example, 0 to 1000 ppm) and a high concentration range (for example, 10000 ppm or more) of Freon gas, which is the gas to be detected. CFC gas sensitivity ratio becomes smaller (the linearity of the gas sensitivity curve is excellent), and CFC gas can be detected with high accuracy in a wide concentration range.

The protective layer 4 is composed of a first protective layer 41 formed on the gas sensing section 2 side and a second protective layer 42 formed on the outer peripheral side of the first protective layer 41 . The first protective layer 41 is thinner and denser than the second protective layer 42 .

The first protective layer 41 is provided on the outer peripheral surface of the gas sensing section 2 . The first protective layer 41 is a coating layer containing amorphous aluminum oxide (Al ₂ O ₃ ). The thickness and specific surface area of the first protective layer 41 can selectively and reliably adsorb organic silicone gas, making the CFC sensor 100 resistant to silicone poisoning (improved silicone durability). The thickness of the first protective layer 41 is not particularly limited as long as it is a thickness that does not easily cause cracking or peeling of the gas and does not easily retain fluorocarbon gas, which is the gas to be detected. However, the thickness of the first protective layer 41 is preferably set to about 10 to 200 nm. , more preferably 20 to 90 nm.

The first protective layer 41 preferably accounts for 0.02 wt % or more and 0.6 wt % or less with respect to the weight of the gas sensing section 2 .
If the first protective layer 41 is less than 0.02 wt % with respect to the weight of the gas sensing section 2 , the durability of the silicone as a protective layer cannot be ensured, and the first protective layer 41 does not cover the weight of the gas sensing section 2 . If it exceeds 0.6 wt %, the densification progresses too much, so that chlorofluorocarbon gas cannot enter the sensing part 4, and the sensitivity decreases when detecting high-concentration chlorofluorocarbon gas.

The first protective layer 41 is denser than the second protective layer 42 , that is, the second protective layer 42 has a porous structure with larger gaps than the first protective layer 41 . By setting the thickness and specific surface area of the first protective layer 41 as described above, the organosilicon gas can be selectively and reliably adsorbed, and the CFC gas sensor 100 can be made resistant to silicone poisoning. .

The first protective layer 41 is mainly composed of amorphous aluminum oxide, and may contain at least one of α- and γ-aluminum oxides having a crystal structure as other components. Alternatively, a metal catalyst such as a palladium (Pd) catalyst may be supported on the surface of amorphous aluminum oxide.

Amorphous aluminum oxide in the present invention does not have a crystal structure like α- and γ-aluminum oxide, and its particles are polymerized to form a filamentous body that is three-dimensionally entangled to form feathers. Say what makes up a state.

The second protective layer 42 is provided on the outer peripheral surface of the first protective layer 41 . The second protective layer 42 is a coating layer made of a sintered metal oxide containing aluminum oxide (alumina), silica-alumina, or the like as a main component, and is less dense than the first protective layer 41 . Since the second protective layer 42 is provided on the outer peripheral surface of the first protective layer 41, the synergistic effect of the first protective layer 41 and the second protective layer 42 is such that the organic silicone gas enters the gas sensing section 2. can be prevented, and the Freon gas sensor 100 can be made resistant to silicone poisoning. The thickness of the second protective layer 42 is preferably set to 0.02 to 0.40 mm, more preferably 0.05 to 0.15 nm.

Gases to be detected include methane gas, liquefied petroleum gas (LPG), hydrogen, carbon monoxide, hydrogen sulfide, chlorofluorocarbons, ammonia, other combustible gases, and toxic gases, in addition to chlorofluorocarbons. is not limited to The chlorofluorocarbon sensor 100 of the present embodiment is characterized by mainly detecting chlorofluorocarbon gas, but it is possible to detect the above gases in addition to chlorofluorocarbon gas. For example, the freon gas sensor 100 can detect isobutane gas, etc., and when the freon gas sensor 100 is periodically calibrated, isobutane gas with a known concentration, which is easily available, is used as a calibration gas instead of freon gas. be able to.

In the present embodiment, Freon gas is a general term for compounds obtained by substituting fluorine or chlorine for part or all of the hydrogen in hydrocarbons such as methane and ethane. In addition, Freon gas in the present embodiment is used as an alternative refrigerant as an example of its application. Fluorocarbon refrigerants include, for example, hydrofluorocarbons and fluorocarbons. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are alternatives to specific Freon (five types of Freon 11, Freon 12, Freon 113, Freon 114, and Freon 115). Hydrofluorocarbons include difluoromethane (R32), pentafluoroethane (R125), 1,1,1,2-tetrafluoroethane (R134a), 1,1,2-trifluoroethane (R143), 1,1, 1-trifluoroethane (R143a), 1,1-difluoroethane (R152a), monofluoroethane (R161). Fluorocarbons include octafluorocyclobutane (C318). Among these, 1,1,1,2-tetrafluoroethane (R134a), 1,1,1-trifluoroethane (R143a), in particular, have boiling points close to conventional dichlorodifluoromethane (R12). Therefore, it is preferable as an alternative refrigerant.
An example of the above hydrochlorofluorocarbons includes chlorodifluoromethane (R22), which is not used as an alternative refrigerant.
As will be described later, R404A is a quasi-azeotrope mixture refrigerant composed of three components, R125, R-134a and R-143a. R407C is a non-azeotropic mixed refrigerant composed of three components, R32, R125 and R134a. R410A is a quasi-azeotropic mixture refrigerant composed of two components, R32 and R125.

[Gas sensor manufacturing method]
Next, a method for manufacturing the semiconductor type gas detection element Rs will be described.
First, the coiled heater 3 of the noble metal wire 1 is coated with a paste of a metal oxide semiconductor mixed with an organic solvent (binder) such as ethylene glycol to form a ball, and the temperature is 550 to 650° C. (near 650° C.). to form the gas sensing part 2 . At this time, for example, when the wire diameter of the noble metal wire 1 is about 10 to 50 μm, the coil diameter of the coil heater 3 is about 100 to 500 μm, and the number of turns of the coil is 6 to 20, the gas sensitive part 2 has a sphere diameter of approximately 200-1000 μm.
In this embodiment, a platinum wire with a wire diameter of 20 μm is used, the coil diameter is 200 μm, and the number of turns of the coil is 9 or 12.

Next, the milky white sol solution containing "Al ₂ O ₃ .XH ₂ O (X>1)" is diluted with water. Droplets of the diluted sol solution are dropped on the outer peripheral surface of the gas sensing part 2, and after the outer peripheral surface of the gas sensing part 2 is covered and dried, it is fired at 550° C. to 650° C. (around 650° C.) to obtain an amorphous shape. A first protective layer 41 of Al ₂ O ₃ is formed.

Next, a metal oxide such as alumina and an organic solvent (binder) such as ethylene glycol are mixed to form a paste. The semiconductor-type gas sensing element Rs can be manufactured by forming a sintered body by sintering the precious metal wire 1 by self-heating after depositing it so as to have a diameter.

[Example: Fabrication of gas detection element]
The coiled heater 3 of the platinum wire is embedded in tin oxide and formed into a spherical shape. Then, the gas sensitive part 2 was formed by firing at about 650° C. for 2 hours.

Next, as described above, a milky-white sol solution containing "Al ₂ O ₃ .XH ₂ O (X>1)" is diluted with water, and droplets of this diluted sol solution are applied to the gas sensing section 2 described above. It was dried by dropping it on the outer peripheral surface of the. Finally, the first protective layer 41 was formed by firing at around 650°C. The concentration of the diluted sol solution is adjusted so that the first protective layer 41 has a concentration of 0.02 wt % or more and 0.6 wt % or less with respect to the weight of the gas sensing section 2 .

As the gas detecting element in this example, the first protective layer 41 having a film thickness of 30 nm was manufactured.

On the surface of the first protective layer 41 of the semiconductor gas detection element Rs, filaments formed by polymerizing amorphous aluminum oxide particles are three-dimensionally entangled to form feathers.

Next, a metal oxide such as alumina and an organic solvent (binder) such as ethylene glycol are mixed to form a paste. After being adhered so as to have a diameter, the noble metal wire 1 is self-heated to be sintered to form a second protective layer 42 as a sintered body, thereby producing a semiconductor type gas sensing element Rs.

In addition, as a comparative example, a semiconductor type gas detection element (Comparative Example 1) having a configuration in which the first protective layer 41 is not provided in the semiconductor type gas detection element Rs, and the protective layer 4 (the first protective layer 41, the second protective layer 42) was not provided (Comparative Example 2).

[Gas sensor evaluation]
3 and 4 show the relationship between the gas concentration of a predetermined gas and gas sensitivity.
Next, gas sensitivity characteristics for various chlorofluorocarbons and combustible gases were examined using the chlorofluorocarbon gas sensors provided with the semiconductor type gas detection elements according to the above examples and comparative examples 1 and 2. R32, R407C, R134a, R404A, R410A, and R22 were selected as fluorocarbons to be tested (upper part in FIG. 3). Methane, isobutane, hydrogen, carbon monoxide, and ethanol were selected as combustible gases to be tested (lower portion in FIG. 3. In FIG. 3, R32 is also added for comparison). 3 and 4 show the relationship between gas concentration and gas sensitivity for these gases. According to the example shown in FIG. 4, in the fluorocarbon sensor according to the example, the fluorocarbon gas sensitivity ratio between the low concentration region (100 ppm) and the high concentration region (5000 ppm or more) of chlorofluorocarbon gas is small (the linearity of the gas sensitivity curve is Excellent), and it was found that Freon gas can be detected in a wide concentration range.
Further, as shown in FIGS. 3 and 4, the chlorofluorocarbon gas sensor according to the embodiment can detect various chlorofluorocarbon gases and various combustible gases at a detection gas concentration of 100 to 10000 ppm.

FIG. 9 shows a graph comparing the response time of each of the Freon gas sensors according to Examples and Comparative Examples 1 and 2 for R32 (8000 ppm).
As shown in FIG. 9 , when the CFC sensors according to the example and Comparative Examples 1 and 2 were subjected to a response time test by applying R32 a plurality of times, the CFC sensor according to Comparative Example 1 had a slow response speed. While the sensitivity of the Freon gas sensor according to Example 2 gradually decreased, these drawbacks were not found in the Freon gas sensor according to Example.
Note that FIG. 10 shows the gas concentration dependence (n=3) of R32 for the Freon gas sensors according to the example and comparative examples 1 and 2. As shown in FIG. According to FIG. 10, the freon gas sensor according to the example has less gas concentration dependence at a gas concentration of 1000 to 2000 ppm compared to the freon gas sensors according to comparative examples 1 and 2, and the detection sensitivity in a predetermined gas concentration range. is well-balanced Freon gas sensor.
The graph of FIG. 11 is obtained by standardizing the graph of FIG. 10 assuming that the gas concentration of R32 is 2000 ppm=1.

[Influence of Ce, WO ₃ and Mo content on sensitivity]
In the semiconductor type gas detection element Rs according to the above-described example, the contents of Ce, WO ₃ , and Mo in the gas sensing part 2 were changed to prepare Freon gas sensors, and each concentration of R32 (1000 ppm, 5000 ppm, ) was evaluated (see FIGS. 5, 6, and 7).

According to FIG. 6, the sensitivity to R32 tends to decrease as the Ce content increases. Mo has a sensitivity peak at a content of 0.15 wt %. When the Mo content is high (for example, 0.75 wt% or more), the sensitivity peak due to the difference in the _WO3 content disappears. A sensitivity peak exists at a content of ₃ to 5 wt %.

[Freon gas sensitivity ratio]
In the semiconductor type gas detection element Rs according to the above-described embodiment, the contents of Ce, WO ₃ , and Mo in the gas sensing part 2 are changed to produce a freon gas sensor, and a freon gas (R32) low concentration region (for example, 1000 ppm) and a high concentration region (for example, 10000 ppm) were compared (see FIG. 8). In FIG. 8, with the sensitivity at a gas concentration of 5000 ppm set to 100, (a) shows the sensitivity at a gas concentration of 1000 ppm relative to the sensitivity at a gas concentration of 5000 ppm, and (b) shows the sensitivity at a gas concentration of 10000 ppm relative to the sensitivity at a gas concentration of 5000 ppm. there is

According to FIG. 8, when the Mo content is low (e.g., the content is zero, 0.075 wt%), the gas sensitivity in the low concentration region (e.g., 1000 ppm) of Freon gas (R32), which is the detection gas, is low. However, the sensitivity ratio in the high concentration region (eg, 5000-10000 ppm) is good (accuracy below 1000 ppm is not good). _In addition, the increase in sensitivity due to the addition of WO3 is remarkable. This increase in sensitivity peaks especially when the content of WO ₃ is around 3 wt %. Responsiveness to Freon gas is slow (for example, the Freon gas sensor according to Comparative Example 1).

In addition, as the Mo content increases, the sensitivity in the low concentration range of Freon gas (R32) is too high, so the sensitivity ratio in the high concentration side of Freon gas (R32) is poor. Responsiveness to freon gas is faster than when the Mo content is low.

[Temperature/humidity dependence investigation]
Next, using the gas sensor 100 incorporating the gas detection element Rs according to the example, stability against temperature and humidity fluctuations was confirmed. As shown in FIG. 12, the gas sensitivity characteristics were investigated when the temperature was changed from -20° C. to 40° C. and the humidity was changed from 35% RH to 100% RH. Gases to be detected are four types of R32 of 1000 ppm, 2000 ppm, 5000 ppm and 10000 ppm and air. As a result, the sensor output showed a substantially constant value regardless of temperature and humidity fluctuations. Specifically, in the case of R32, the difference in gas sensitivity to temperature and humidity fluctuations is very small, and even when the CFC sensor is used in any environment, there is little change in sensitivity, and it can be used stably as a CFC sensor. . On the other hand, in the case of air, when the temperature and humidity fluctuate, there is no effect of moisture adsorption (high humidity atmosphere) or moisture desorption (low humidity atmosphere) in the air, and there is almost no output fluctuation. This is due to the fact that the gas sensing part 2 is configured by containing predetermined amounts of Ce, WO ₃ and Mo, and it was confirmed that the gas sensor 100 has extremely high stability against temperature and humidity fluctuations. did it.

INDUSTRIAL APPLICABILITY The present invention can be suitably used for a gas sensor for detecting Freon gas, an inspection device using the same, an alarm device, and the like.

1 noble metal wire 2 gas sensitive part 3 coil heater (heating part)
4 protective layer 41 first protective layer 42 second protective layer 100 freon gas sensor Rs semiconductor gas detection element

Claims (10)

A freon gas sensor equipped with a semiconductor type gas detection element,
precious metal wire;
a gas sensing portion covering the noble metal wire and formed using a metal oxide semiconductor containing tin oxide as a main component and to which Ce _, WO3 and Mo are added;
and a heating portion that heats the gas sensitive portion. 2. A chlorofluorocarbon gas sensor according to claim 1, further comprising a protective layer formed on the outer peripheral side of said gas sensing portion. The protective layer is
a first protective layer formed on the side of the gas sensitive part;
A second protective layer formed on the outer peripheral side of the first protective layer,
3. The Freon gas sensor according to claim 2, wherein the first protective layer is thinner and denser than the second protective layer. The Freon gas sensor according to any one of claims 1 to 3, wherein the Ce content is 0.05 wt% or more and 0.60 wt% or less . The freon gas sensor according to any one of claims 1 to ₄ , wherein the content of said WO3 is 0.5 wt% or more and 5 wt% or less. The Freon gas sensor according to any one of claims 1 to 5, wherein the Mo content is 0.075 wt% or more and 3 wt% or less. 4. The freon gas sensor according to claim 3, wherein the first protective layer has a thickness of 10 to 200 nm. 4. The freon gas sensor according to claim 3, wherein the second protective layer has a thickness of 0.02 to 0.40 mm. 4. The Freon gas sensor according to claim 3, wherein the first protective layer contains aluminum oxide as a main component. 4. The Freon gas sensor according to claim 3, wherein the second protective layer contains aluminum oxide as a main component.

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