JP7203663B2 - gas sensor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a gas sensor that utilizes changes in the electrical characteristics of a gas detection part due to contact with a target gas under specific conditions.

A gas sensor whose electrical characteristics change in a manner corresponding to the target gas upon contact with the target gas, a heater portion that heats the gas sensor, a heating control section that controls heating by the heater portion, and the electrical characteristics of the gas sensor. A gas detection device is known that has a gas detection unit that measures and detects a target gas. In such a gas detection device, the heater portion is controlled by the heating control section to heat the gas sensor to an appropriate temperature according to the type of target gas, and the electrical characteristics (electric resistance value, voltage value) of the gas sensor are measured. etc.). Specifically, for each of a plurality of target gases, after heating to a specific measurement temperature, the electrical characteristics such as the electrical resistance value after a specific time has passed are stored in advance in association with the target gas, and the temperatures are stored in order. Measure the electrical properties when changed, and detect the target gas that matches the measured value for temperature.

The gas sensor includes a gas detection section provided on a semiconductor substrate, a heater section that heats the gas detection section, and a catalyst section (gas selective combustion section) that covers the gas detection section. Here, the catalyst portion is an alumina sintered material supporting 7% palladium as a catalyst.

JP 2011-27752 A

However, when detecting low concentrations of volatile organic compounds (VOCs) such as acetone as the target gas, there are cases where sufficient sensitivity cannot be obtained with conventional gas sensors. This is thought to be because volatile organic compounds are more likely to burn in the catalyst part when a catalyst part carrying a high concentration of precious metal catalyst is used, making it difficult to detect with the gas detection part. I have come to know.

An object of the present invention is to detect acetone as a low-concentration volatile organic compound as a target gas with high sensitivity.

In order to achieve the above object, a gas sensor according to one embodiment of the present invention comprises a gas detection part whose electrical characteristics change upon contact with acetone, which is a target gas, when heated to a predetermined measurement temperature; A heater unit that heats a gas detection unit and a catalyst unit that covers the gas detection unit, and the catalyst unit supports a combination of one or more of palladium, iridium, platinum, and rhodium as a catalyst metal. and the total content of the catalyst metals in the catalyst portion is 0.1% by mass or more and 3% by mass or less.

With such a configuration, the combustion of acetone , which is the target gas, is suppressed in the catalyst section, the detection sensitivity for acetone , which is the target gas, is improved, and it becomes possible to detect a low-concentration target gas.

Moreover, it is preferable that platinum and iridium are supported as the catalyst metal.

With such a configuration, when detecting acetone as the target gas, the detection sensitivity of acetone is efficiently improved, and low-concentration acetone can be detected.

Further, it is preferable that the carrier of the catalyst part is a transition metal oxide.

With such a configuration, the detection sensitivity of the target gas is efficiently improved, and it becomes possible to detect a low-concentration target gas.

BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic which shows the outline|summary of a gas detection apparatus. It is a figure explaining the detection sensitivity of acetone in this embodiment. 4 is a diagram for explaining the detection sensitivity of acetone in Comparative Example 1. FIG. FIG. 10 is a diagram illustrating the detection sensitivity of acetone in Comparative Example 2; FIG. 10 is a diagram for explaining the detection sensitivity of acetone in Comparative Example 3;

A gas detection device 100 according to this embodiment will be described with reference to FIG. The gas detection device 100 has a sensor element 20 (an example of a gas sensor), a heating control section 12, a gas detection section 13 (an example of a control section), and a temperature detection section 14 (an example of an environment measurement section). The sensor element 20 has at least a gas detection section 10 , a catalyst section 11 and a heater section 6 .

The gas detection device 100 heats the gas detection unit 10 to an appropriate temperature according to the type of the target gas by energizing the heater unit 6 with the heating control unit 12 , thereby electrically heating the gas detection unit 10 . Target gases containing low concentrations of volatile organic compounds (VOCs) are detected based on their characteristics (electrical resistance, voltage, etc.). A volatile organic compound contained in the target gas is acetone . Moreover, the concentration of the volatile organic compound in the target gas is 0.1 ppm or more and 2.0 ppm or less . The following description is based on an example embodiment in which acetone is detected as the target gas. First, a specific configuration of the sensor element 20 will be described.

(sensor element)
The sensor element 20 is supported by the silicon substrate 1 and constitutes a diaphragm. The sensor element 20 has a support portion 5 , an insulating portion 7 , a gas detection portion 10 and a catalyst portion 11 . A support portion 5 is formed on the silicon substrate 1 , and a heater portion 6 is formed on the support portion 5 . The insulating portion 7 is formed on the support portion 5 so as to cover the entire heater portion 6 . A pair of joint portions 8 are formed on the insulating portion 7 , and an electrode portion 9 is formed on the joint portions 8 . A gas detection part 10 is formed between a pair of electrode parts 9 on the insulating part 7 . A catalyst portion 11 is formed on the insulating portion 7 so as to cover the gas detection portion 10 . The sensor element 20 may have a bridge structure, and the heater section 6 may also be used as an electrode.

The support portion 5 is formed by laminating a thermal oxide film 2, a Si ₃ N ₄ film 3, and a SiO ₂ film 4 in this order. The heater section 6 generates heat when energized, and heats the gas detection section 10 and the catalyst section 11 .

The gas detection unit 10 is a semiconductor layer containing metal oxide as a main component. For example, the gas detection unit 10 is a mixture containing tin oxide (SnO ₂ ) as a main component. The gas detection unit 10 changes its electrical resistance value upon contact with the target gas. The gas detector 10 may be a thin film with a thickness of about 0.2 to 1.6 μm, or a film (thick film) with a thickness exceeding 1.6 μm.

The catalyst section 11 is heated by the heater section 6 to a high temperature, and burns the active (combustible) gas at that temperature. Furthermore, at that temperature, an inert combustible gas, which is the target gas, is permeated and diffused to reach the gas detection unit 10 . Accordingly, the detection accuracy of combustible gas is enhanced by appropriately controlling the heating temperature. In other words, when detecting acetone , which is a combustible gas, the catalyst unit 11 burns a reducing gas (non-target gas) such as hydrogen gas or alcohol gas other than the target gas as much as possible to detect the gas detection unit. 10, and even if a non-target gas reaches the gas detection device 100, the gas detection device 100 utilizes the difference in the electrical characteristics during heating due to the difference in the temperature dependence of the reaction (combustion) characteristics in the gas detection unit 10. has the function of giving gas selectivity to Therefore, the gas detection unit 10 can efficiently detect acetone as a combustible gas that is a target gas that has permeated and diffused through the catalyst unit 11 .

The catalyst part 11 is configured by supporting a catalyst metal on a carrier whose main component is a metal oxide. Specifically, metal oxides carrying catalyst metals are bonded to each other via a binder.

When detecting acetone as a combustible gas, the catalyst metal has a low oxidation removal ability for the target gas when heated to an appropriate heating temperature, and is an interference gas that can cause false detection when detecting the target gas. A metal is used as a catalyst having a high oxidation removal ability against (reducing gases such as ethanol and H2 (hydrogen) and other miscellaneous gases) . When detecting acetone as a combustible gas, palladium (Pd), platinum (Pt), rhodium (Rh), iridium (Ir), etc. can be used as catalyst metals. A material containing at least one of iridium and rhodium is used.

As the carrier for supporting the catalyst metal, transition metal oxides and the like are used, such as alumina _{( Al2O3} ) _, zirconium oxide ₍ ZrO2), yttrium oxide ( _Y2O3 ), and cerium oxide ₍ CeO2). , lanthanum oxide (La ₂ O ₃ ), titanium oxide (TiO ₂ ), hafnium oxide (HfO ₂ ), niobium oxide (Nb ₂ O ₅ ), or tantalum oxide (Ta ₂ O ₅ ) are used.

As the binder for binding the carrier, fine powders of metal oxides such as zirconium oxide, fine silica powder, silica sol, magnesia and the like are used. Alumina fine powder or alumina sol can also be used as long as it does not impede the function of the catalyst portion 11 as long as it is used in a very small amount as a binder. Moreover, each of the above-described catalyst metal, metal oxide as a carrier, and binder may be used alone, or two or more thereof may be used in combination.

The amount of the catalytic metal contained in the catalyst part 11 is preferably 3% by mass or less with respect to the total mass of the catalytic metal and the carrier, for example, 1% by mass or more and 3% by mass or less, or 2% by mass or more and 3% by mass or less. , 0.1% by mass or more and 3% by mass or less, more preferably 0.1% by mass or more and 1% by mass or less. In this case, it is preferable to use platinum and iridium as catalyst metals.

(heating controller)
The heating control unit 12 controls the energization of the heater unit 6, and includes a heating operation in which the heater unit 6 is energized to heat the gas detection unit 10, and a non-heating operation in which the heater unit 6 is not energized (the gas detection unit 10 is heated). non-heating operation to stop). Further, the heating control section 12 can heat the heater section 6 to an arbitrary set temperature by controlling the voltage or current applied to the heater section 6 .

Specifically, the heating control unit 12 receives power supply from a power source such as a battery (not shown) and energizes the heater unit 6 of the sensor element 20 to heat the sensor element 20 . The heating temperature, that is, the temperature reached by the gas detection unit 10 and the catalyst unit 11 is controlled by changing the voltage applied to the heater unit 6, for example.

(Gas detector)
The gas detection unit 13 measures the electrical characteristics of the gas detection unit 10 to detect the target gas. For example, the gas detection unit 13 measures the electrical resistance value (electrical characteristics) between the pair of electrode units 9 to measure the resistance value of the gas detection unit 10, and the presence of the target gas can be determined from the change in the resistance value. Detect the concentration of the target gas.

The temperature detection unit 14 detects the gas detection unit 10, or the temperature of the gas detection unit 10 and its surroundings (hereinafter collectively referred to as the ambient temperature). Specifically, the temperature detection unit 14 is a temperature sensor such as a thermistor. Also, by measuring the resistance value of the heater section 6, the temperature of the heater section 6 (substantially equivalent to the temperature of the gas detection section 10) can be detected. The temperature detected by the temperature detector 14 is sent to the heating controller 12 .

(Detection of target gas)
The operation of the gas detection device 100 configured as described above when detecting a target gas will now be described. In the gas detection device described below, a heating operation for detecting acetone is performed.

In the acetone detection heating operation, the heating control unit 12 controls energization of the heater unit 6 based on the ambient temperature to cause the heater unit 6 to generate heat, and the gas detection unit 10 and the catalyst unit 11 are set at the measured temperature of acetone. This is the operation of heating to the temperature for acetone detection. The heating operation for detecting acetone can detect acetone and heat and remove substances adsorbed on the surface of the gas detection unit 10 (moisture, dirt, etc.). The acetone detection temperature is a predetermined temperature between 150°C and 350°C. For example, the acetone detection temperature is 300° C., at which the presence and concentration of acetone is detected.

Specifically, the heating control unit 12 controls the energization of the heater unit 6, sets the gas detection unit 10 and the catalyst unit 11 to the acetone detection temperature, and maintains the temperature for 0.05 to 0.5 seconds. Hold. During this time, the gas detector 13 measures the resistance value of the gas detector 10 and detects the presence and concentration of acetone from the value. In addition, this operation burns hydrogen, carbon monoxide, etc., removes substances adsorbed on the surface of the gas detection unit 10, such as moisture, etc., and cleans the surface. Suppressed.

In the acetone detection heating operation, the heating control section 12 energizes the heater section 6 based on the ambient temperature detected by the temperature detection section 14 . Specifically, the heating control section 12 controls the heater drive voltage VH applied to the heater section 6 based on the ambient temperature. For example, with an ambient temperature of 20° C. as a reference temperature, heater drive voltage VH is set relatively low when the ambient temperature is high, and heater drive voltage VH is set relatively high when the ambient temperature is low. That is, with the ambient temperature of 20° C. as the reference temperature, control is performed so that the higher the ambient temperature, the lower the power for driving the heater, and the lower the ambient temperature, the higher the power for driving the heater. controlled as

In this embodiment, the heating operation for detecting acetone is repeatedly performed with a resting operation (an operation of stopping the energization of the heater section 6). Note that the time for each operation can be appropriately set/changed, and the pause operation can be omitted.

As described above, by setting the total content of catalyst metals in the catalyst portion 11 to 3% by mass or less, the detection sensitivity for acetone as a combustible gas is improved, and low-concentration combustible gas can be detected. It becomes possible. As a result of intensive research, when a noble metal catalyst is supported at a high concentration, the combustible gas, which is the target gas, is likely to burn in the catalyst unit 11, making it difficult for the gas detection unit 10 to detect the target gas. I came to the knowledge that it was for the sake of

Differences in acetone detection sensitivity depending on the combination of catalytic metals and the total content of one or more combined catalytic metals will be described below with reference to FIGS. 2 to 5. FIG. 2 to 5 are graphs of acetone-containing gas with a concentration of 1 ppm and gas without acetone (acetone concentration of 0 ppm) when catalyst parts 11 having different types of catalyst metals and different total contents of catalyst metals are used. 4 shows the change over time of the electrical resistance value of the gas detection unit 10 when the measurement temperature is held at 300°C. In this case, the carrier of the catalyst part 11 is zirconium oxide (ZrO ₂ ).

First, when 1% by mass of platinum and 2% by mass of iridium are contained as catalyst metals in the catalyst part 11, as shown in FIG. There is a sufficient difference in resistance value change over time, and acetone can be detected from the electrical resistance value at a predetermined elapsed time.

On the other hand, as shown in FIGS. 3 to 5, when 2% by mass of platinum and 2% by mass of iridium are contained as catalyst metals in the catalyst portion 11 (Comparative Example 1: FIG. 3), 3% by mass % of platinum and 2% by mass of iridium (Comparative Example 2: FIG. 4), and 7% by mass of palladium, which is commonly used in the past (Comparative Example 3: FIG. 5), The change over time in the electrical resistance value of the acetone-containing gas with a concentration of 1 ppm and the gas not containing acetone are approximately the same.

As a result of the above, by setting the total content of the catalyst metals in the catalyst part 11 to 3% by mass or less, the detection sensitivity of acetone as the target gas is improved, and even if the measurement temperature is lowered, low-concentration target gas can be detected.

Further, in the above-described embodiments, the configuration in which the target gas is detected by the sensor element made of semiconductor was exemplified, but the target gas can be detected by gas sensors having various configurations.

6 heater section 10 gas detection section 11 catalyst section 12 heating control section 13 gas detection section 14 temperature detection section 20 sensor element (gas sensor)
100 gas detector

Claims (3)

a gas detector whose electrical characteristics change upon contact with the target gas, acetone, when heated to a predetermined measurement temperature;
a heater unit that heats the gas detection unit;
a catalyst unit covering the gas detection unit;
The catalyst part supports a combination of one or more of palladium, iridium, platinum, and rhodium as a catalyst metal, and the total content of the catalyst metal in the catalyst part is 0.1% by mass or more3 A gas sensor that is less than or equal to % by mass. 2. The gas sensor according to claim 1, wherein platinum and iridium are supported as said catalyst metal. 3. The gas sensor according to claim 1, wherein the carrier of said catalyst portion is a transition metal oxide.

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