JP2019191123A - Chlorofluorocarbon gas sensor - Google Patents

Abstract

To provide a chlorofluorocarbon gas sensor comprising a semiconductor type gas detection element which can accurately detect chlorofluorocarbon gas over a wide range of concentration.SOLUTION: A chlorofluorocarbon gas sensor comprising a semiconductor type gas detection element Rs comprises: a noble metal wire 1; a gas sensitive part 2 which covers the noble metal wire 1 and is formed by using a metal oxide semiconductor having tin oxide as a main component and Ce, WO, and Mo as additives; and a coil-shaped heater 3 for heating the gas sensitive part 2. In addition, it is provided with a protection layer 4 formed on an outer circumference of the gas sensitive part 2.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a chlorofluorocarbon gas sensor including a semiconductor type gas detection element that detects chlorofluorocarbon gas.

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

  For example, a CFC gas sensor that detects CFC gas is known as a semiconductor gas sensor (see, for example, Patent Document 1).

  When detecting chlorofluorocarbon gas, not only high sensitivity but also a chlorofluorocarbon gas sensor capable of detecting with high accuracy in a wide concentration range (for example, 100 ppm to 10000 ppm) from low concentration to high concentration is desired. In particular, a conventional gas sensor compatible with chlorofluorocarbon gas does not show a good behavior in a high concentration region.

JP 2000-111507 A

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a Freon gas sensor including a semiconductor type gas detection element capable of accurately detecting Freon gas in a wide concentration range.

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

That is, the CFC gas sensor according to the present invention is a CFC gas sensor having a semiconductor type gas detection element. The metal oxide semiconductor includes a noble metal wire, the noble metal wire, and tin, oxide, and Ce, WO ₃ , and Mo added thereto. A gas sensitive part formed by using and a heating part for heating the gas sensitive part.

  The CFC gas sensor of the present invention is provided with a protective layer formed on the outer peripheral side of the gas sensitive part.

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

  In the CFC gas sensor of the present invention, the Ce content is 0.05 wt% or more and 0.60 wt% or less.

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

  In the chlorofluorocarbon gas sensor of the present invention, the Mo content is 0.075 wt% or more and 3 wt% or less.

  In the chlorofluorocarbon gas sensor of the present invention, the thickness of the first protective layer is 10 to 200 nm.

  In the CFC gas sensor of the present invention, the thickness of the second protective layer is 0.02 to 0.40 mm.

  In the Freon gas sensor of the present invention, the first protective layer is mainly composed of aluminum oxide.

  In the Freon gas sensor of the present invention, the second protective layer is mainly composed of aluminum oxide.

  According to the present invention, it is possible to realize a Freon gas sensor capable of accurately detecting Freon gas in a wide concentration range.

Schematic which shows the Freon gas sensor which has a bridge circuit which concerns on one Embodiment of this invention. The schematic diagram which shows the structure of a semiconductor gas detection element. The graph which shows the relationship between to-be-detected gas concentration and gas sensitivity. The graph which expands and shows the Example in FIG. Ce, in case of changing each content of _WO 3, Mo, graph showing the sensitivity characteristics with respect to R32 (1000 ppm). Ce, in case of changing each content of _WO 3, Mo, graph showing the sensitivity characteristics with respect to R32 (5000 ppm). Ce, in case of changing each content of _WO 3, Mo, graph showing the sensitivity characteristics with respect to R32 (10000 ppm). It is a graph which shows the Freon gas sensitivity ratio with respect to R32, (a) is a graph with the gas concentration of R32 being 1000 pm (5000 ppm sensitivity = 100), (b) is a graph with the gas concentration of R32 being 10,000 pm (5000 ppm sensitivity = 100). The graph which compared the response time of each freon gas sensor with respect to R32 (8000 ppm). The graph which compared the gas concentration dependence (n = 3) of each Freon gas sensor. 11 is a graph normalized based on FIG. 10 with a gas concentration of R32 of 2000 ppm = 1. The graph which shows the temperature and humidity dependence of the Freon gas sensor which concerns on an Example.

  Next, a CFC gas sensor 100 including a semiconductor type gas detection element Rs according to an embodiment of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the chlorofluorocarbon sensor 100 according to the present embodiment is configured by incorporating a semiconductor gas detection element Rs and fixed resistors R0, R1, and R2 into a bridge circuit. The bridge circuit is energized constantly or intermittently by the power supply E, and the semiconductor gas detection element Rs has a temperature suitable for detection. Further, the resistance value of the semiconductor gas detection element Rs changes when the gas to be detected is adsorbed. For this reason, in the chlorofluorocarbon gas sensor 100 according to the present embodiment, the change in the resistance value of the semiconductor gas detection element Rs is extracted 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 the present embodiment includes a coiled noble metal wire 1 that is an example of a noble metal wire serving as a detection electrode, a gas sensitive portion 2 that covers the noble metal wire 1, and a gas A coiled heater 3 as a heating part for heating the sensitive part 2 and a protective layer 4 formed on the outer peripheral side of the gas sensitive part 2 are provided. The gas sensitive part 2 is provided so as to be in contact with the gas to be detected and is formed in a substantially spherical shape.
Note that the noble metal wire 2 also serves as the coil heater 3 and is configured as an integral unit.

  As a material of the noble metal wire 1 and the coiled heater 3, for example, platinum or a material obtained by adding rhodium or the like to platinum can be appropriately selected and used. However, the material is not limited thereto. Further, the wire diameter, the coil diameter, the number of windings, and the like of the coiled heater 3 are not limited to those shown in the drawings, and may be appropriately set as necessary.

The gas sensitive part 2 is formed using, for example, a metal oxide semiconductor containing a metal oxide such as tin oxide as a main component and adding Ce (cerium), WO ₃ (tungsten oxide), and Mo (molybdenum). Can do. The gas sensitive part 2 can be arbitrarily selected according to the type of gas to be detected.
In addition, it is preferable that the diameter of the gas sensitive part 2 is 0.30-0.60 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 content of Ce within the above range, the freon gas sensitivity ratio between the low concentration region (for example, 0 to 1000 ppm) and the high concentration region (for example, 10,000 ppm or more) of the flon gas that is the gas to be detected becomes small (gas The linearity of the sensitivity curve is excellent), and chlorofluorocarbon gas can be detected accurately in a wide concentration range.

For example, when the content of Ce is zero, the gas sensitivity is high in the low concentration region of the chlorofluorocarbon gas, but the sensitivity change in the high concentration region is small and unsuitable for use.
When the Ce content is 1.0 wt%, the gas sensitivity is too low in the low concentration region of the Freon gas, and the sensitivity difference in the high concentration region of the Freon gas is large. In the case of combustible gas other than chlorofluorocarbon gas, the gas sensitivity ratio may be further deteriorated.

The content of WO ₃ is preferably 0.5 wt% or more and 5 wt% or less.
When the content of WO ₃ is less than 0.05 wt%, sufficient sensitivity to Freon gas cannot be obtained. On the other hand, when the content of WO ₃ exceeds 5 wt%, the sensitivity to Freon gas is lowered.
Further, as in the case of Ce, by setting the content of WO ₃ within the above range, the low concentration region (for example, 0 to 1000 ppm) and the high concentration region (for example, 10,000 ppm or more) of the chlorofluorocarbon gas that is the detected gas. The chlorofluorocarbon gas sensitivity ratio becomes 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.
If the Mo content is less than 0.075 wt%, sufficient sensitivity to Freon gas cannot be obtained. On the other hand, when the Mo content exceeds 3 wt%, the sensitivity to the Freon gas decreases and the Freon gas selectivity of the sensor deteriorates.
Moreover, the fluctuation | variation of the Air value by a temperature / humidity fluctuation | variation can be suppressed by containing Mo in the gas sensitive part 2. FIG.
Similarly to the case of Ce, by setting the Mo content within the above range, a low concentration region (for example, 0 to 1000 ppm) and a high concentration region (for example, 10,000 ppm or more) of the chlorofluorocarbon gas that is the detected gas The freon gas sensitivity ratio becomes smaller (the linearity of the gas sensitivity curve is excellent), and the freon gas can be accurately detected in a wide concentration range.

  The protective layer 4 includes a first protective layer 41 formed on the gas sensitive part 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 a dense layer that is thinner than the second protective layer 42.

The first protective layer 41 is provided on the outer peripheral surface of the gas sensitive part 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 are capable of selectively adsorbing the organic silicone gas, making the Freon gas sensor 100 resistant to silicone poisoning (improving silicone durability), and the first protective layer 41. The thickness of the first protective layer 41 is preferably set to about 10 to 200 nm, although it is not particularly limited as long as it does not easily crack or peel off and the thickness of the fluorocarbon gas to be detected is difficult to stay. More preferably, it is 20-90 nm.

The first protective layer 41 is preferably 0.02 wt% or more and 0.6 wt% or less with respect to the weight of the gas sensitive portion 2.
When the 1st protective layer 41 is less than 0.02 wt% with respect to the weight of the gas sensitive part 2, silicone durability cannot be ensured as a protective layer, and the 1st protective layer 41 is the weight of the gas sensitive part 2. On the other hand, if it exceeds 0.6 wt%, the densification proceeds too much, so that the chlorofluorocarbon gas cannot enter the sensitive portion 4, and the sensitivity is lowered when detecting the high concentration chlorofluorocarbon gas.

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

  The first protective layer 41 is mainly composed of amorphous aluminum oxide, and may include at least one of α and γ-aluminum oxide having a crystal structure as other components. Further, 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 feathers are formed by three-dimensionally entwining filaments formed by polymerization of the particles. Say what forms the 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 body of a metal oxide mainly composed of aluminum oxide (alumina), silica alumina or the like, and is not denser than the first protective layer 41. As the second protective layer 42 is provided on the outer peripheral surface of the first protective layer 41, the organic silicone gas enters the gas sensitive part 2 as a synergistic effect of the first protective layer 41 and the second protective layer 42. This makes it possible to make the Freon gas sensor 100 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.

  Examples of the gas to be detected include methane gas, liquefied petroleum gas (LPG), hydrogen, carbon monoxide, hydrogen sulfide, chlorofluorocarbon gas, ammonia, other flammable gases, toxic gases, etc. It is not limited to. The CFC gas sensor 100 of the present embodiment is mainly characterized by detecting CFC gas. However, in addition to CFC gas, it is possible to detect the above gas. For example, the chlorofluorocarbon gas sensor 100 can detect isobutane gas or the like, and at the time of periodic calibration of the chlorofluorocarbon gas sensor 100, isobutane gas having a known concentration that is easily available instead of chlorofluorocarbon gas is used as the calibration gas. be able to.

In addition, the chlorofluorocarbon gas in the present embodiment is a general term for compounds in which part or all of hydrocarbon hydrogen such as methane or ethane is substituted with fluorine or chlorine. Moreover, the chlorofluorocarbon in this embodiment is used as an alternative refrigerant as an example of its use. Examples of the chlorofluorocarbon refrigerants include hydrofluorocarbons and fluorocarbons. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) can be substituted for specific chlorofluorocarbons (Freon 11, Freon 12, Freon 113, Freon 114, and Freon 115). Examples of the hydrofluorocarbon include difluoromethane (R32), pentafluoroethane (R125), 1,1,1,2-tetrafluoroethane (R134a), 1,1,2-trifluoroethane (R143), 1,1, Examples thereof include 1-trifluoroethane (R143a), 1,1-difluoroethane (R152a), and monofluoroethane (R161). An example of the fluorocarbon is octafluorocyclobutane (C318). Among these, 1,1,1,2-tetrafluoroethane (R134a) and 1,1,1-trifluoroethane (R143a) have boiling points close to those of conventional dichlorodifluoromethane (R12). And is preferable as an alternative refrigerant.
Note that chlorodifluoromethane (R22) is included as an example of the hydrochlorofluorocarbon, but this is not used as an alternative refrigerant.
As will be described later, R404A is a pseudo-azeotropic refrigerant mixture composed of three components of R125, R-134a, and R-143a. R407C is a non-azeotropic refrigerant mixture composed of three components of R32, R125, and R134a. R410A is a pseudo-azeotropic refrigerant mixture composed of two components R32 and R125.

[Gas sensor manufacturing method]
Next, a manufacturing method of the semiconductor gas detection element Rs will be described.
First, the coiled heater 3 of the noble metal wire 1 is formed into a spherical shape by applying a paste-like material in which a metal oxide semiconductor is mixed with an organic solvent (binder) such as ethylene glycol, and is 550 to 650 ° C. (around 650 ° C.). The gas sensitive part 2 is formed by baking. At this time, for example, when the diameter of the noble metal wire 1 is approximately 10 to 50 μm, the coil diameter of the coiled heater 3 is 100 to 500 μm, and the number of turns of the coil is 6 to 20 times, the gas sensitive part 2 The sphere diameter is approximately 200 to 1000 μm.
In the present embodiment, a platinum wire having a wire diameter of 20 μm is used, and the coil diameter is 200 μm and the number of coil turns is 9 or 12.

Next, a milky white sol solution containing “Al ₂ O ₃ .XH ₂ O (X> 1)” is diluted with water. A droplet of the diluted sol solution is dropped on the outer peripheral surface of the gas sensitive portion 2, covered with the outer peripheral surface of the gas sensitive portion 2, dried, and then fired at 550 ° C. to 650 ° C. (near 650 ° C.) to be amorphous. 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, and this paste is formed on the outer peripheral surface of the first protective layer 41 with predetermined spheres. After making it adhere so that it may become a diameter, semiconductor type gas detection element Rs is producible by baking by self-heating of noble metal wire 1 and forming as a sintered compact.

[Example: Production of gas detection element]
The coiled heater 3 portion of the platinum wire is embedded in tin oxide and formed into a spherical shape. And the gas sensitive part 2 was formed by baking at about 650 degreeC 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 the diluted sol solution are diluted with the above-described gas sensitive unit 2. It was dripped at the outer peripheral surface of and dried. Finally, the first protective layer 41 was formed by baking at around 650 ° C. Note that the concentration of the diluted sol solution is adjusted so that the first protective layer 41 is 0.02 wt% or more and 0.6 wt% or less with respect to the weight of the gas sensitive portion 2.

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

  On the surface of the first protective layer 41 of the semiconductor gas sensing element Rs, a feather state is formed such that filaments formed by polymerizing amorphous aluminum oxide particles are entangled with each other three-dimensionally.

  Next, a metal oxide such as alumina and an organic solvent (binder) such as ethylene glycol are mixed to form a paste, and this paste is formed on the outer peripheral surface of the first protective layer 41 with predetermined spheres. After making it adhere so that it may become a diameter, it baked by the self-heating of the noble metal wire 1, and formed the 2nd protective layer 42 as a sintered compact, and produced semiconductor type gas detection element Rs.

  Further, as a comparative example, the semiconductor gas detection element (Comparative Example 1) having a configuration in which the first protective layer 41 is not provided in the semiconductor gas detection element Rs, and the protective layer 4 (first protective layer 41, second protective layer). A semiconductor gas detection element (Comparative Example 2) without 42) was produced.

[Gas sensor evaluation]
3 and 4 show the relationship between the gas concentration of the predetermined gas and the gas sensitivity.
Next, gas sensitivity characteristics with respect to various types of chlorofluorocarbon gases and flammable gases were examined using the chlorofluorocarbon gas sensor provided with the semiconductor type gas detection elements according to the above-described examples and comparative examples 1 and 2. R32, R407C, R134a, R404A, R410A, and R22 were selected as the fluorocarbon gases to be tested (upper part in FIG. 3). In addition, methane, isobutane, hydrogen, carbon monoxide, and ethanol were selected as the combustible gases to be tested (lower part in FIG. 3; R32 is also added for comparison in FIG. 3). The relationship between the gas concentration and gas sensitivity for these gases is shown in FIGS. According to the embodiment shown in FIG. 4, in the chlorofluorocarbon gas sensor according to the embodiment, the chlorofluorocarbon gas sensitivity ratio between the low concentration region (100 ppm) and the high concentration region (5000 ppm or more) of the chlorofluorocarbon gas becomes small (the linearity of the gas sensitivity curve is reduced). It was found that CFCs can be detected in a wide concentration range.
As shown in FIGS. 3 and 4, the chlorofluorocarbon gas sensor according to the embodiment can detect various chlorofluorocarbon gases and various flammable gases at a detection gas concentration of 100 to 10,000 ppm.

In FIG. 9, the graph which compared each response time of the Example with respect to R32 (8000 ppm) and the Freon gas sensor which concerns on the comparative examples 1 and 2 is shown.
As shown in FIG. 9, when the response time test was performed by applying R32 to the Freon gas sensors according to the example and Comparative Examples 1 and 2 multiple times, the response speed of the Freon gas sensor according to Comparative Example 1 was slow. The CFC gas sensor according to Example 2 gradually decreased in sensitivity, whereas the CFC gas sensor according to Example 2 did not find these defects.
FIG. 10 shows the gas concentration dependency (n = 3) of R32 by the Freon gas sensors according to the example and the comparative examples 1 and 2. According to FIG. 10, the CFC gas sensor according to the example is less dependent on the gas concentration at a gas concentration of 1000 to 2000 ppm than the CFC gas sensor according to Comparative Example 1 and Comparative Example 2, and the detection sensitivity in a predetermined gas concentration region. This is a CFC gas sensor with a good balance.
Further, the graph of FIG. 11 is standardized using the graph of FIG. 10 as the gas concentration of R32 of 2000 ppm = 1.

[Influence on sensitivity due to Ce, WO ₃ and Mo content]
In the semiconductor-type gas detection element Rs according to the above-described embodiment, a Freon gas sensor is manufactured by changing the contents of Ce, WO ₃ , and Mo in the gas sensitive part 2, and each concentration of R32 (1000 ppm, 5000 ppm, 10000 ppm). ) Was evaluated for gas sensitivity characteristics (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 WO ₃ content disappears. However, when Mo is not contained, and when the content is low, WO _A sensitivity peak exists when the content ratio of ₃ is ₃ to 5 wt%.

[Freon gas sensitivity ratio]
In the semiconductor gas detection element Rs according to the above-described embodiment, a Freon gas sensor is manufactured by changing the respective contents of Ce, WO ₃ , and Mo in the gas sensitive portion 2, and a low concentration region of the Freon gas (R 32) (for example, 1000 ppm) and a high-concentration region (for example, 10000 ppm) were compared for the Freon gas sensitivity ratio (see FIG. 8). In FIG. 8, assuming that the sensitivity at a gas concentration of 5000 ppm is 100, (a) shows the sensitivity at a gas concentration of 1000 ppm with respect to the sensitivity at a gas concentration of 5000 ppm, and (b) shows the gas concentration at 10,000 ppm with respect to the sensitivity at a gas concentration of 5000 ppm. Yes.

According to FIG. 8, when the Mo content is low (for example, when the content is zero and 0.075 wt%), the gas sensitivity in the low concentration region (for example, 1000 ppm) of the fluorocarbon (R32) that is the detection gas is low. However, the sensitivity ratio in the high concentration region (for example, 5000 to 10000 ppm) is good (accuracy below 1000 ppm is not good). In addition, the increase in sensitivity due to the addition of WO ₃ is remarkable. This increase in sensitivity has a peak especially when the content of WO ₃ is around 3 wt%. The response to the chlorofluorocarbon becomes slow (for example, the chlorofluorocarbon sensor according to Comparative Example 1).

  Also, the higher the Mo content, the more sensitive the low concentration region of the Freon gas (R32) is, so the sensitivity ratio on the high concentration side of the Freon gas (R32) is worse. Responsiveness to Freon gas is faster than when the Mo content is low.

[Temperature / humidity dependency survey]
Next, stability against temperature and humidity fluctuations was confirmed using the gas sensor 100 incorporating the gas detection element Rs according to the example. As shown in FIG. 12, the gas sensitivity characteristics when the temperature was varied in the range of −20 ° C. to 40 ° C. and the humidity in the range of 35% RH to 100% RH were investigated. As the gas to be detected, 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 gas sensitivity difference with respect to temperature and humidity fluctuations is very small, and even if the CFC gas sensor is used in any environment, the sensitivity change is small and it can be stably used as a CFC gas sensor. . On the other hand, in the case of Air, when temperature and humidity change, there is no influence of moisture adsorption (high humidity atmosphere) and moisture desorption (low humidity atmosphere) in the air, and there is almost no output fluctuation. This is because the gas sensitive part 2 is configured by containing a predetermined amount of Ce, WO ₃ , and Mo, and it is confirmed that the gas sensor 100 has very high stability against temperature and humidity fluctuations. did it.

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

1 Precious metal wire 2 Gas sensitive part 3 Coiled heater (heating part)
DESCRIPTION OF SYMBOLS 4 Protective layer 41 1st protective layer 42 2nd protective layer 100 Freon gas sensor Rs Semiconductor gas detection element

Claims (10)

In chlorofluorocarbon gas sensors equipped with semiconductor gas detection elements,
Precious metal wire,
A gas sensitive part that is formed by using a metal oxide semiconductor that covers the noble metal wire and contains Ce, WO ₃ , Mo as a main component of tin oxide;
A freon gas sensor comprising: a heating unit that heats the gas sensitive unit.   The Freon gas sensor according to claim 1, further comprising a protective layer formed on an outer peripheral side of the gas sensitive portion. The protective layer is
A first protective layer formed on the gas sensitive part side;
A second protective layer formed on the outer peripheral side of the first protective layer,
The fluorocarbon gas sensor according to claim 2, wherein the first protective layer is a dense layer that is thinner than the second protective layer.   4. The CFC gas sensor according to claim 1, wherein the Ce content is 0.05 wt% or more and 0.60 wt% or less. 5. The Freon gas sensor according to claim 1, wherein a content of the WO ₃ 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.   The CFC gas sensor according to any one of claims 3 to 6, wherein the first protective layer has a thickness of 10 to 200 nm.   The CFC gas sensor according to any one of claims 3 to 7, wherein a thickness of the second protective layer is 0.02 to 0.40 mm.   The CFC gas sensor according to any one of claims 3 to 8, wherein the first protective layer contains aluminum oxide as a main component.   The CFC gas sensor according to any one of claims 3 to 9, wherein the second protective layer contains aluminum oxide as a main component.

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