gas sensor

Abstract

This invention provides a gas sensor that uses platinum components in its filter while suppressing the effects of toxic substances, including alcohol. [Solution] The gas sensor 1 comprises a sensor element F1 for outputting a signal corresponding to the concentration of the gas to be detected, which is coated with an oxidation catalyst on a heat-generating resistor; a casing 11 that covers the outer circumference of the sensor element F1 and has an opening 11b formed in the top plate 11a; and a filter section 12 provided between the opening 11b of the casing 11 and the sensor element F1, which adsorbs the gas that the sensor element F1 is to be poisoned by. The filter section 12 has a first filter section 12a made of activated carbon or a ceramic support bearing platinum, and a second filter section 12b made of type A silica gel provided on the opening 11b side of the first filter section 12a.

Description

Technical Field

[0001] The present invention relates to a gas sensor.

Background Art

[0002] Conventionally, a gas sensor has been proposed that includes, as a filter for a gas sensor, an adsorption part that adsorbs a gas to be poisoned of a sensor element and a catalyst part made of platinum or the like that adsorbs a gas to be poisoned of the sensor element in a temperature and humidity environment different from that of the adsorption part (see, for example, Patent Document 1). In addition, a filter for a sensor using silica gel has also been proposed (see, for example, Patent Document 2).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] As a result of intensive studies, the inventors of the present case have found that alcohol affects sensor deterioration. Here, since the platinum component has an effect of adsorbing poisoning substances such as siloxane, sulfur, and chlorine, it is appropriate for use in a filter. However, when the platinum component is exposed to a large amount of alcohol, it loses the adsorption power for the above substances (siloxane, sulfur, chlorine, etc.).

[0005] The present invention has been made to solve such problems, and an object thereof is to provide a gas sensor capable of suppressing the influence of poisoning substances containing alcohol while using a platinum component in a filter.

Means for Solving the Problems

[0006] The gas sensor according to the present invention comprises a sensor element for outputting a signal corresponding to the concentration of a target gas, wherein a catalyst is coated onto a heat-generating resistance element; a casing covering the outer circumference of the sensor element and having an opening formed in a part thereof; and a filter section provided between the opening of the casing and the sensor element for adsorbing the gas to be poisoned by the sensor element, wherein the filter section has a first filter section made of activated carbon or a ceramic support bearing platinum, and a second filter section made of type A silica gel provided on the opening side of the first filter section. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a gas sensor that can suppress the effects of toxic substances, including alcohol, while using platinum as a filter. [Brief explanation of the drawing]

[0008] [Figure 1] This figure shows a gas detection device including a partial configuration of a gas sensor according to an embodiment of the present invention. [Figure 2] This is a cross-sectional view of the gas sensor according to this embodiment. [Figure 3] This is a cross-sectional view showing an example of a sensor element. [Figure 4] This is a cross-sectional view of a gas sensor according to the first reference example. [Figure 5] This is a cross-sectional view of a gas sensor according to the second reference example. [Figure 6] This graph shows the progression of alarm concentrations in poisoning tests. [Modes for carrying out the invention]

[0009] The present invention will be described below in accordance with preferred embodiments. It should be noted that the present invention is not limited to the embodiments shown below, and can be modified as appropriate without departing from the spirit of the invention. Furthermore, in the embodiments shown below, some illustrations and descriptions of certain components are omitted. It goes without saying that, regarding the details of the omitted technologies, publicly known or well-known technologies are applied as appropriate, to the extent that they do not contradict the content described below.

[0010] Figure 1 is a diagram showing a gas detection device including a partial configuration of a gas sensor according to an embodiment of the present invention. As shown in Figure 1, the gas detection device GD according to this embodiment is a so-called catalytic combustion type gas sensor and has a sensor element F1. This sensor element F1 is arranged on the first side E1 that constitutes a bridge circuit. A reference element F2 is arranged on the second side E2 which is connected in series with the first side E1. A first fixed resistor R1 and a second fixed resistor R2 are arranged on the third and fourth sides E3 and E4 which are connected in parallel with the first and second sides E1 and E2, respectively.

[0011] Sensor element F1 detects the target gases (methane gas (CH4), propane gas (C3H8), and isobutane (C4H8). 10 An oxidation catalyst (see reference numeral 22 in Figure 3, described later) for burning the target gas is coated onto the sensor element F1. Sensor element F1 burns the target gas on the oxidation catalyst, and this combustion heat causes a change in the resistance value of the heating resistor. Reference element F2 is coated with a material that prevents the target gas from undergoing catalytic combustion.

[0012] In this type of catalytic combustion gas sensor, when the sensor element F1 is exposed to the target gas, the resistance value of the sensor element F1 changes according to the concentration of the target gas. On the other hand, the resistance value of the reference element F2 remains almost unchanged. Therefore, a voltage signal is output based on the resistance difference between the two elements F1 and F2. This voltage signal becomes the concentration signal of the target gas, and after passing through an amplifier or the like, it is input to a microcontroller or the like, where the concentration of the target gas is determined.

[0013] Note that the gas detection device GD also includes a variable resistor VR for adjusting the power supply E of the sensor element F1 and the balance of the bridge circuit.

[0014] FIG. 2 is a cross-sectional view of the gas sensor according to the present embodiment. The gas sensor 1 includes a sensor element F1 and a filter 10 for the gas sensor.

[0015] FIG. 3 is a cross-sectional view showing an example of the sensor element F1. As shown in FIG. 3, the sensor element F1 includes a heating resistor 21 formed by patterning a platinum (Pt) wire, and an oxidation catalyst (catalyst) 22 coated on the heating resistor 21 via a protective film. The sensor element F1 is formed on a thin film diaphragm D straddling a recess 24 formed from the upper surface of a substrate 23. Connection pad portions are integrally formed at both ends of the heating resistor 21, and these pad portions are bonded to lead terminals 26 passing through a base 25 shown in FIG. 2 with wires 27.

[0016] Note that the oxidation catalyst 22 is composed of, for example, a noble metal simple substance such as palladium or platinum, or a ceramic carrier supporting at least one of palladium and platinum (a few wt% to several tens of wt%).

[0017] Referring to FIG. 2, the filter 10 for the gas sensor includes a casing 11, a filter portion 12, and a support member 13. The casing 11 covers the outer periphery of the sensor element F1 together with the base 25 on which the sensor element F1 is mounted, and an opening 11b is formed in a top plate (a part) 11a facing the base 25. The opening 11b is formed by, for example, a circular opening with a diameter of φ1.0 mm. It is preferable that a reference element F2 is also mounted on the base 25. That is, in the present embodiment, it is preferable that the casing 11 covers not only the sensor element F1 but also the reference element F2.

[0018] The filter unit 12 is provided between the opening 11b of the casing 11 and the sensor element F1, and adsorbs poisoning substances such as siloxane, sulfur, and chlorine that are targets of poisoning of the sensor element F1. Such a filter unit 12 is configured to include a first filter unit 12a and a second filter unit 12b.

[0019] The first filter unit 12a is composed of activated carbon carrying platinum. This first filter unit 12a has the effect of adsorbing various poisoning substances such as siloxane, sulfur, and chlorine. In addition, the first filter unit 12a can adsorb sulfur components even in high humidity by carrying a platinum component. Note that the first filter unit 12a may be a ceramic carrier carrying platinum.

[0020] The second filter unit 12b is composed of type A silica gel. Type A silica gel has a smaller pore diameter than type B silica gel, has excellent moisture absorption ability in low humidity, and has less moisture release. This type A silica gel has a large surface area and innumerable silanol groups on the surface, so the adsorption ability by silanol groups is higher than the adsorption ability by capillary action into the gaps between fine particles. On the other hand, type B silica gel has a small specific surface area, and the adsorption by the capillary action acts preferentially rather than the adsorption by silanol groups. Such type A silica gel has the action of selectively adsorbing various polar molecules by the chemical adsorption force of the silanol groups on the surface. Therefore, type A silica gel can preferentially adsorb substances with high polarity such as alcohol (for example, ethanol). In addition, type A silica gel also has a small amount of adsorption and desorption of the adsorbed substances due to changes in temperature and humidity.

[0021] Note that in this embodiment, type A silica gel has, for example, a specific surface area of 650 m 2The assumption is that the silica gel has a specific surface area of ​​500 m², a specific surface area of ​​500 m², a packing density of 0.70 g / mL, a pH of 4.5, a water content of 2.0%, and a pore diameter of 2.5 nm. However, Type A silica gel is not limited to this, as long as adsorption by silanol groups is the primary function. In the above, the pore diameter is a value obtained by calculation from the specific surface area and pore volume. Note that Type A silica gel is not limited to the above, but can also have a specific surface area of ​​500 m². 2 The pore volume may be 0.80 cc / g or more, and the average pore diameter may be 5 nm or less.

[0022] The support member 13 is a member provided on the opposite side of the opening 11b from the filter section 12, and supports the filter section 12 from that opposite side. This support member 13 has a circular hole 13a formed in the center. Therefore, the gas to be detected reaches the sensor element F1 through the opening 11b, through the filter section 12, and through the hole 13a.

[0023] Here, although the first filter section 12a adsorbs various toxic substances such as siloxanes, sulfur, and chlorine, its adsorption capacity for these substances (siloxanes, sulfur, and chlorine, etc.) decreases when exposed to large amounts of alcohol. Therefore, the gas sensor 1 needs to be protected from poisoning of the sensor element F1 with alcohol.

[0024] Therefore, in the gas sensor filter 10 according to this embodiment, the second filter section 12b is provided on the opening 11b side than the first filter section 12a. As a result, the gas outside the casing 11 first has highly polar substances, including alcohol, removed by the second filter section 12b before reaching the first filter section 12a. Then, various toxic substances such as siloxane, sulfur, and chlorine are removed by the first filter section 12a. Thus, the target gas reaches the sensor element F1 in a state from which toxic substances have been removed, and poisoning is suppressed.

[0025] Figure 4 is a cross-sectional view of a gas sensor according to the first reference example, and Figure 5 is a cross-sectional view of a gas sensor according to the second reference example. In Figures 4 and 5, components similar to those shown in Figure 2 are indicated by the same reference numerals as in Figure 2, and their explanations are omitted.

[0026] As shown in Figure 4, in the gas sensor 1a according to the first reference example, the arrangement of the first filter section 12a and the second filter section 12b in the gas sensor filter 10a is reversed. That is, the first filter section 12a is located on the side of the opening 11b inside the casing 11, and the second filter section 12b is located on the sensor element F1 side of the first filter section 12a.

[0027] Furthermore, as shown in Figure 5, the gas sensor filter 10b in the second reference example gas sensor 1b includes the filter section described in Patent Document 1. Specifically, the gas sensor filter 10b in the second reference example includes a third filter section 12c made of activated carbon, a fourth filter section 12d made of a platinum-based catalyst, and a fifth filter section 12e made of calcium carbonate. The gas sensor filter 10b in the second reference example is arranged in the order of the third filter section 12c, the fourth filter section 12d, the fifth filter section 12e, and the third filter section 12c, starting from the opening 11b side.

[0028] Figure 6 is a graph showing the changes in alarm concentrations during the poisoning test. First, in the poisoning test shown in Figure 6, each gas sensor was exposed to toluene, ethanol, nonane, dichlorobenzene, pinene, or siloxane, either as a single gas or a mixture, at concentrations of 5 to 650 ppm. In addition, the poisoning test was conducted with the opening 11b of the casing 11 having a diameter of φ5.0, instead of the usual φ1.0.

[0029] First, in the example shown in Figure 6, the alarm system, including gas sensors 1, 1a, and 1b, initially issues an alarm at a methane concentration of 3000 ppm. However, if gas sensors 1, 1a, and 1b deteriorate due to the effects of toxic substances, the alarm system will no longer issue an alarm at 3000 ppm of methane. In particular, the alarm system needs to issue an alarm at the upper limit of methane concentration shown in Figure 6 (approximately 12500 ppm), and if it only issues an alarm at methane concentrations exceeding this limit, it cannot be said that the product is functioning properly.

[0030] First, in the second reference example, the alarm device equipped with gas sensor 1b showed an increase in alarm concentration starting after approximately 50 days, triggering an alarm at just under 5000 ppm after 80 days, and triggering an alarm at around 10000 ppm after 100 days.

[0031] In the first reference example, the alarm device having the gas sensor 1a showed a rapid progression of poisoning starting from about 70 days after the poisoning environment described above, exceeding the upper limit before 80 days had passed. Therefore, the alarm device having the gas sensor 1a, as in the first reference example, where the opening 11b side is the first filter section 12a and the sensor element F1 side is the second filter section 12b, is actually more prone to poisoning than the one in the second reference example.

[0032] On the other hand, the alarm device including the gas sensor 1 according to this embodiment did not exceed an alarm concentration of 4000 ppm even after 80 days, and the alarm concentration did not exceed 8000 ppm even after 110 days. Therefore, the alarm device having the gas sensor 1 according to this embodiment showed better results than those of the first and second reference examples.

[0033] In this way, the gas sensor 1 according to this embodiment has a second filter section 12b made of type A silica gel located on the opening 11b side, rather than a first filter section 12a made of activated carbon or ceramic support body supported with platinum. Therefore, highly polar substances such as alcohol are preferentially adsorbed by the second filter section 12b. As a result, the gas that reaches the first filter section 12a is free of alcohol. This makes it difficult for alcohol-derived gas to reach the first filter section 12a, and allows for the adsorption of toxic substances such as siloxanes, sulfur, and chlorine. Thus, it is possible to provide a gas sensor 1 that can suppress the effects of alcohol while using a platinum component in the filter.

[0034] Although the present invention has been described above based on embodiments, the present invention is not limited to the above embodiments, and modifications may be made without departing from the spirit of the invention, and other technologies may be combined as appropriate to the extent possible. Furthermore, the present invention may be combined with publicly known or well-known technologies to the extent possible.

[0035] For example, in the above embodiment, the filter section 12 comprises one first filter section 12a and one second filter section 12b, but it may also include other components (e.g., mesh or filter paper). Furthermore, the gas sensor 1 may further include a second filter section 12b between the first filter section 12a and the support member 13, or it may have other filters or other components in between. Also, the gas sensor 1 may further include other filters or other components between the first filter section 12a and the opening 11b. Furthermore, the gas sensor 1 may further include other filters or other components between the first filter section 12a and the second filter section 12b.

[0036] Furthermore, although the above embodiment assumes that the opening 11b is a circular opening, it may be a non-circular opening such as a polygon. The same applies to the hole 13a of the support member 13. Moreover, the opening 11b and hole 13a are not limited to one opening, but may be multiple openings. [Explanation of Symbols]

[0037] 1: Gas sensor 10: Gas sensor filter 11: Casing 11a: Top panel (partial) 11b: Opening 12: Filter section 12a: First filter section 12b: Second filter section 13: Support member 13a: Hole 21: Heat-generating resistor 22: Oxidation catalyst (catalyst) F1: Sensor element

Claims

[Claim 1] A sensor element comprising a catalyst coated onto a heat-generating resistor, which outputs a signal corresponding to the concentration of the gas to be detected, A casing that covers the outer circumference of the sensor element and has an opening formed in a part of it, The casing comprises a filter section provided between the opening and the sensor element, which adsorbs the gas that the sensor element is to be poisoned by, The filter section comprises a first filter section made of activated carbon or a ceramic support bearing platinum, and a second filter section made of type A silica gel located on the opening side of the first filter section. A gas sensor characterized by the following features.

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