JP2023132213A - Gas sensor and method for detecting gas - Google Patents

Abstract

To provide a gas sensor and a method for detecting gas that can identify the type of a combustible gas.SOLUTION: The gas sensor includes: a sealed container having an inflow port and a discharge port for a sample gas; a heater arranged in the sealed gas; a detector arranged in the sealed container, the detector having a reference element and a detection element; and an external circuit arranged outside the sealed container. The external circuit has an operation processing unit for specifying the shape of a response waveform from the detector to the sample gas.SELECTED DRAWING: Figure 5B

Description

The present disclosure relates to a gas sensor and a gas detection method for calculating the concentration of combustible gas.

Patent Document 1 discloses a gas sensor capable of detecting multiple types of combustible gases. Specifically, the gas sensor of Patent Document 1 has three gas detection sections arranged in series along a gas flow path, and each of the three gas detection sections detects a specific combustible gas. Contains a catalyst for combustion. During operation of the gas sensor, each gas detection section is set to a different operating temperature, and three types of combustible gases contained in the mixed gas are sequentially catalytically burned in each gas detection section. Patent Document 1 discloses that H ₂ , CO, and CH ₄ can be detected by the above mechanism.

However, the gas sensor of Patent Document 1 requires a gas detection section whose operating temperature and catalyst are adjusted for each of the plurality of combustible gases to be detected (in other words, multiple gas detection sections are required). It is large-scale and difficult to downsize. Furthermore, the combustion temperature of the combustible gas also changes depending on the component ratio of the sample gas, the gas flow rate, the pressure, and the like. Therefore, with the gas sensor of Patent Document 1, it is difficult to distinguish between two or more types of combustible gases that burn in the same temperature range, and there is a possibility that an error may occur in detecting the concentration of a specific combustible gas. Therefore, there is a need to develop a gas sensor and a gas detection method that can identify the type of combustible gas in a simpler manner.

Patent No. 4375336

The present disclosure has been made in view of the above circumstances, and an object thereof is to provide a gas sensor and a gas detection method that can identify the type of combustible gas.

In order to achieve the above object, the gas sensor according to the present disclosure includes:
a closed container having an inlet and an outlet for sample gas;
a heater installed in the sealed container;
a detection unit disposed inside the airtight container and including a reference element and a detection element sensitive to gas concentration;
an external circuit installed outside the airtight container;
The external circuit includes an arithmetic processing section that specifies the shape of a response waveform of the detection section to the sample gas.

A gas sensor having the above characteristics has high gas selectivity. That is, the gas sensor according to the present disclosure can identify the type of combustible gas contained in the sample gas, and can specify the concentration of the identified combustible gas. In particular, gas types can be identified using one set of detection units (a combination of a reference element and a detection element) without installing a plurality of detection units.

Preferably, the arithmetic processing unit determines overshoot or undershoot of the response waveform.

Preferably, the arithmetic processing unit identifies a CO response waveform having an undershoot.

Preferably, the heater heats the sample gas introduced into the closed container to 100° C. or higher.

Preferably, an inner wall cross section of the closed container in a plane perpendicular to the inflow direction of the sample gas is circular.

Preferably, the sensing element includes a catalyst portion and a thermistor film. Alternatively, it is preferable that the sensing element has a catalyst portion and a platinum resistor. Further, it is preferable that the catalyst portion of the sensing element is a porous body supporting 1 wt % or more of a noble metal based on the total amount of the catalyst portion.

The gas detection method according to the present disclosure includes:
The sample gas introduced into the sealed container is heated to a predetermined temperature by a heater installed in the sealed container,
identifying a response waveform to the sample gas from the output difference between a detection element and a reference element installed in the sealed container;
identifying the type of gas contained in the sample gas from the shape of the response waveform;
The concentration of the gas contained in the sample gas is calculated from the magnitude of the output signal in the response waveform.

With the gas detection method having the above characteristics, the type of combustible gas contained in the sample gas can be identified with high accuracy, and the concentration of the identified combustible gas can be specified. In particular, gas types can be identified using a simple device having one set of detectors (a combination of a reference element and a detector element) without using a large-scale device having multiple detectors.

Preferably, in the gas detection method, it is determined whether a rising portion in the response waveform is an overshoot or an undershoot, and the type of gas contained in the sample gas is identified based on the determination result.

Preferably, in the gas detection method, it is determined whether or not the response waveform has an undershoot, and based on the determination result, it is determined whether or not the sample gas contains CO gas.

Preferably, the predetermined temperature is 100°C or higher.

Preferably, the sample gas introduced into the sealed container is heated to a plurality of temperatures in a range of 100° C. or more and 400° C. or less,
identifying the response waveform to the sample gas for each of a plurality of temperatures;
The type of gas contained in the sample gas is identified from the shape of the response waveform at each temperature.

FIG. 1 is a conceptual diagram showing a gas sensor 2 according to an embodiment of the present disclosure. FIG. 2A is a schematic cross-sectional view showing an example of a closed container. FIG. 2B is a schematic cross-sectional view taken along line IIB-IIB shown in FIG. 2A. FIG. 3A is a schematic diagram showing a cross section of a main part of the sensing element. FIG. 3B is a schematic diagram showing a cross section of a main part of the reference element. FIG. 4A is a perspective view showing a modification of the closed container. FIG. 4B is a cross-sectional view taken along line IVB-IB in FIG. 4A. FIG. 5A is a schematic diagram showing an example of a method for analyzing a response waveform (200° C.). FIG. 5B is a schematic diagram showing an example of a method for analyzing a response waveform (350° C.). FIG. 6 is a graph showing the combustion temperature range of combustible gas.

Hereinafter, the present disclosure will be described in detail based on embodiments shown in the drawings.

As shown in FIG. 1, the gas sensor 2 according to the present embodiment includes a sealed container 10, a detection section 20 disposed in the internal space of the sealed container 10, and an external circuit 50.

The closed container 10 is a container for isolating and sealing the sample gas from the outside world, and has an inlet 12 and an exhaust port 14 for the sample gas, and a heater 4 . When the gas sensor 2 is in operation, a sample gas is introduced through the inlet 12, and the sample gas filled in the sealed container 10 is heated to a predetermined temperature by the heater 4. That is, the closed container 10 is a furnace body that burns the combustible gas to be measured.

More specifically, the closed container 10 can be configured with a chamber 10a having a structure as shown in FIGS. 2A and 2B. The chamber 10a has a hollow cylindrical shape, and includes a pair of end walls 11 facing each other in the X-axis direction, and a side wall 13 connecting the pair of end walls 11 along the X-axis direction. Inside the closed container 10, there is an internal space 15 covered by the inner wall surface of the end wall 11 and the inner wall surface 13a of the side wall 13, and this internal space 15 is filled with a sample gas. The capacity of the internal space 15 is not particularly limited, and is preferably, for example, 1 cm ³ to 10 cm ³ .

Further, the sample gas inlet 12 is arranged at the center of one end wall 11. On the other hand, the exhaust port 14 is arranged at the center of the other end wall 11 facing the end wall 11 having the inflow port 12 . That is, the sample gas is introduced into the chamber 10a from one end in the X-axis direction, and a flow path for the sample gas is formed along the X-axis direction. Note that in FIGS. 2A and 2B, the X axis, Y axis, and Z axis are substantially perpendicular to each other. Electromagnetic valves may be installed at the inflow port 12 and the exhaust port.

The shape of the chamber 10a is not limited to the hollow cylindrical shape shown in FIGS. 2A and 2B, and the chamber 10a may have a hollow prismatic shape. However, in a cross section perpendicular to the sample gas inflow direction (X-axis direction) shown in FIG. 2B, it is preferable that the cross-sectional shape of the internal space 15 is circular. That is, in a cross section perpendicular to the inflow direction (X-axis direction) of the sample gas, the cross section (outer wall cross section) of the outer wall surface 13b of the side wall 13 is not particularly limited, and may be a quadrangle or a polygon; The cross section of the inner wall surface 13a (inner wall cross section) is preferably circular. By making the inner wall cross section of the closed container 10 circular in this way, the sample gas can be more uniformly diffused inside the closed container 10, and the thermal uniformity inside the closed container 10 can be improved.

It is preferable that the chamber 10a is made of a material having high heat resistance. Examples of highly heat-resistant materials include ceramic materials such as alumina, zirconia, silica, magnesia, boron nitride, and firebrick. The ceramic material constituting the chamber 10a may exist in the form of alumina fibers, silica glass, ceramic fibers, ceramic wool, or the like. The chamber 10a may be made of a single material, or may be made of a plurality of materials. For example, the end wall 11 and side wall 13 of the chamber 10a may each have a multilayer structure in which a plurality of materials are stacked along the thickness direction of the wall.

In the chamber 10a shown in FIGS. 2A and 2B, the heater 4 is arranged on the inner wall surface 13a of the side wall 13, and a meandering heater wiring pattern is formed along the inner wall surface 13a. The arrangement and wiring pattern of the heater 4 in the chamber 10a are not particularly limited, and may be set so as to sufficiently heat the internal space 15 of the chamber 10a. For example, the heater 4 may be placed on the outer wall surface 13b of the side wall 13, or may be embedded inside the side wall 13. The wiring pattern of the heater 4 may be a plurality of I-shaped patterns. Further, the heater 4 may be formed on each end wall 11 in a spiral pattern. The material of the heater 4 in the chamber 10a is not particularly limited, and may include metal materials such as nichrome, tungsten, molybdenum, platinum, copper, kanthal, iron, austinite stainless steel, high nickel, silicon carbide, lanthanum chromite, carbon, dichloromethane, etc. Non-metallic materials such as molybdenum silicide can be used.

A detection unit 20 is arranged in the internal space 15 of the chamber 10a. It is preferable that the detection unit 20 is disposed approximately at the center of the internal space 15. For example, in FIGS. 2A and 2B, the detection unit 20 is located at the center in the X-axis direction and at the center in the Z-axis direction.

Further, the chamber 10a may have a wiring outlet 16 for pulling out various wirings 24 such as the end wiring of the heater 4 and the wiring of the detection unit 20 (see FIG. 2B). Preferably, the wiring outlet 16 has a known sealing structure to prevent the sample gas introduced into the internal space 15 from leaking.

Note that the sealed container 10 is not limited to the embodiment shown in FIGS. 2A and 2B. For example, the sealed container 10 may be a package 10b that can be surface mounted on a circuit board or the like as shown in FIGS. 4A and 4B. There may be. This package 10b is preferably made of a ceramic material such as alumina, silica, zirconia, boron nitride, or the like.

The package 10b has an upper end wall 11a and a lower end wall 11b that face each other in the Z-axis direction, and a side wall 13 that connects the upper end wall 11a and the lower end wall 11b. The inflow port 12 is formed approximately at the center of the upper end wall 11a in the XY plane, and the exhaust port 14 is formed approximately at the center of the lower end wall 11b in the XY plane. That is, in the package 10b, the sample gas is introduced from one end in the Z-axis direction, and a flow path for the sample gas is formed along the Z-axis direction. The capacity of the internal space 15 in the package 10b is not particularly limited. In the package 10b, the capacity of the internal space 15 can be made smaller than that of the chamber 10a, and is preferably, for example, 0.01 cm ³ to 1 cm ³ .

As shown in FIG. 4A, the package 10b has a circular planar shape when viewed from the Z-axis direction, but the planar shape of the package 10b is not particularly limited, and may be a quadrilateral or a polygon. Good too. However, similarly to the chamber 10a, it is preferable that the cross section of the inner wall of the package 10b perpendicular to the sample gas inflow direction (Z-axis direction) is circular. That is, although the outer shape of the package 10b may be a rectangular parallelepiped shape or a prismatic shape, it is preferable that the cross-sectional shape of the internal space 15 perpendicular to the sample gas inflow direction (Z-axis direction) is circular.

In the package 10b shown in FIG. 4B, the heater 4 is formed in a spiral wiring pattern on the inner wall surface of the upper end wall 11a. Note that the heater 4 may be formed not only on the inner wall surface of the upper end wall 11a but also on the inner wall surface of the lower end wall 11b. When forming the heater 4 on the lower end wall 11b, after forming the heater 4 on the surface (inner wall surface) of the lower end wall 11b, an insulating layer is formed so as to cover the heater 4 on the lower end wall 11b, and then an insulating layer is formed on the insulating layer. What is necessary is just to form the detection part 20. Further, in the case of a small-capacity sealed container such as the package 10b, the heater 4 may be formed on the outer wall surface, and the wiring pattern of the heater 4 may be a meandering pattern or the like. The material of the heater 4 in the package 10b is not particularly limited. For example, the heater 4 of the package 10b may be formed by forming a metal thin film containing molybdenum, platinum, tungsten, tantalum, nichrome, palladium, or iridium using a known film forming method, and patterning the metal thin film. I can do it.

Further, in the package 10b, it is preferable that the detection section 20 is arranged approximately at the center of the internal space 15 on the XY plane so that the detection section 20 can easily come into contact with the sample gas introduced from the inlet 12. . The wiring of the detection unit 20 and the end wiring of the heater 4 can be drawn out to the outer wall surface of the package 10b via a through-hole electrode penetrating the upper end wall 11a or the lower end wall 11b. In this case, the hole of the through-hole electrode may be filled with a filler such as an inorganic adhesive to ensure the airtightness of the internal space 15.

The detection unit 20 disposed inside the airtight container 10 as described above includes a detection element 21 and a reference element 22. The detection element 21 is an element that is sensitive to combustible gas and whose output changes depending on the concentration of the combustible gas. It is preferable that the detection element 21 is a catalytic combustion type sensor element. On the other hand, the reference element 22 is an element that is not sensitive to combustible gas and detects heat that does not change with respect to the concentration of combustible gas. That is, although the reference element 22 cannot detect the combustion heat of the combustible gas, the output changes depending on the environmental temperature of the internal space 15 heated by the heater 4. More specifically, it is preferable that the sensing element 21 and the reference element 22 have structures as shown in FIGS. 3A and 3B, respectively.

FIG. 3A is a schematic cross-sectional view showing a film stack 21a, which is a main part of the sensing element 21. As shown in FIG. This membrane stack section 21a has a sensitive membrane 211 and a catalyst section 213. The sensitive film 211 can be made of a material whose physical properties such as resistance value change depending on temperature changes, and is preferably a platinum film or a thermistor film. Examples of constituent materials for the thermistor film include composite metal oxides, amorphous silicon, polysilicon, germanium, and the like.

When the sensitive film 211 is composed of a thermistor film, it is preferable that a pair of electrode films 212 in contact with the sensitive film 211 are laminated in the film laminated portion 21a, as shown in FIG. 3A. The electrode film 212 is an electrode for reading the resistance value of the thermistor film, and is preferably made of a conductive material with a high melting point. Specifically, the electrode film 212 is made of molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or any of the above elements. It can be composed of an alloy containing one or more of these, and is preferably composed of platinum. When the sensitive film 211 is made of a platinum film, the electrode film 212 may not be formed, and a continuous wiring pattern may be formed from the platinum film.

The catalyst portion 213 is preferably a porous carrier material supporting a catalyst material. As the carrier material, for example, oxide materials such as aluminum oxide (gamma alumina, etc.), silicon oxide, cerium oxide, zirconium oxide, yttrium oxide, titanium oxide, and tin oxide can be used. As catalyst materials, noble metals such as platinum (Pt), gold (Au), palladium (Pd), ruthenium (Ru), rhodium (Rh), and iridium (Ir), or rare earth element oxides, bismuth oxides, etc. Metal oxides can be used, preferably noble metals. When a noble metal is supported as a catalyst, the content of the noble metal in the catalyst portion 213 is preferably 1 wt% or more, and more preferably 5 wt% or more and 20 wt% or less, based on the total amount of the catalyst portion 213.

In the film stacking section 21a, it is preferable that an insulating film 215 is stacked so as to cover the upper surface and/or lower surface of the sensitive film 211. The insulating film 215 has a role of protecting the sensitive film 211 from being exposed. The material of the insulating film 215 is not particularly limited, and may be silicon oxide, silicon nitride, or the like. Further, a heater resistive film may be laminated on the film laminated portion 21a. When the closed container 10 is a chamber 10a as shown in FIGS. 2A and 2B, if the capacity of the internal space 15 is large, the detection element 21 may be insufficiently heated. In such a case, it is preferable that the sensing element 21 be heated to the same temperature as the set temperature of the internal space 15 by the heater resistance film laminated on the film laminated portion 21a.

The sensitive film 211, the electrode film 212, each insulating film 215, and the heater resistive film included in the film stack section 21a can be formed by various film forming methods such as the CVD method and the sputtering method, and may also be formed by a photoetching method or a sputtering method. It may be processed into a predetermined pattern by a lift-off method or the like. For the catalyst part 213, a raw material paste is applied onto the membrane stacking part 21a (on the insulating film 215 covering the top surface of the sensitive film 211) by screen printing or dispensing using a dispenser, and then heat-treated at a predetermined temperature. By doing so, it can be formed. Further, the film stack 21a may be present in direct contact with the surface of the substrate, or may be supported by a plurality of beams on a cavity formed in the substrate. . That is, the sensing element 21 may have an air bridge structure.

In the sensing element 21 having the membrane laminated part 21a, when flammable gas heated to a predetermined temperature comes into contact with the element surface, the flammable gas combines with oxygen and burns due to the catalytic action of the catalyst part 213. Combustion heat generated by combustion of the combustible gas is transmitted to the sensitive membrane 211, and the resistance value of the sensitive membrane 211 changes. In the gas sensor 2, the resistance value of the sensitive film 211 included in the sensing element 21 is output to the external circuit 50. Alternatively, the resistance value of the sensitive film 211 may be converted into a current value, voltage value, etc. and output.

FIG. 3B is a schematic cross-sectional view showing the film stack section 22a, which is the main part of the reference element 22. As shown in FIG. 3B, the membrane stack 22a of the reference element 22 can have the same structure as the membrane stack 21a of the sensing element 21, except that it does not include the catalyst section 213. Specifically, the resistive film 221 included in the film stack 22a corresponds to the sensitive film 211 in the sensing element 21, and the resistive film 221 is preferably a platinum film or a thermistor film. The material of the resistive film 221 may be different from that of the sensitive film 211, but is preferably the same material as the sensitive film 211. Further, it is preferable that the electrode film 212 and the insulating film 215 included in the film stack portion 22a are also made of the same material as the sensing element 21.

Note that in the film stack section 22a of the reference element 22, instead of forming the catalyst section 213, a dummy section may be formed on the insulating film 215 that covers the upper surface of the resistive film 211. It is preferable that the dummy part is made of a material that does not have catalytic activity against combustible gas and has a heat capacity comparable to that of the catalyst part 213. Specifically, the dummy portion is preferably made of an oxide material such as aluminum oxide, silicon oxide, cerium oxide, zirconium oxide, yttrium oxide, titanium oxide, or tin oxide that does not contain a catalyst material. Further, it is preferable that the volume of the dummy part is approximately the same as the volume of the catalyst part 213.

Since the reference element 22 having the membrane stack part 22a does not have the catalyst part 213, even if flammable gas heated to a predetermined temperature contacts the reference element 22, no combustion reaction of the flammable gas occurs. That is, in the reference element 22, the resistance value of the resistive film 221 of the film stack 22a changes depending on the temperature of the internal space 15 heated by the heater 4 (that is, the temperature of the sample gas filled in the internal space 15). . In the gas sensor 2, the resistance value of the resistive film 221 included in the reference element 22 is output to the external circuit 50. Alternatively, the resistance value of the resistive film 221 may be converted into a current value, voltage value, etc. and output.

In addition, in the detection part 20, in order to prevent the combustion heat of the flammable gas generated on the surface of the detection element 21 (the surface of the catalyst part 213) from being transmitted to the reference element 22, a Preferably, a reference element 22 is arranged.

In the gas sensor 2, a wire drawn out from the detection section 20 is connected to an external circuit 50 installed outside the closed container 10. The external circuit 50 has at least an arithmetic processing section 53, and may also include a differential amplifier 51, an AD converter 52, and the like. The differential amplifier 51 is a circuit that compares the output of the detection element 21 and the output of the reference element 22 and amplifies the difference. The amplified signal output from the differential amplifier 51 is input to the AD converter 52. The AD converter 52 digitally converts the supplied amplified signal and supplies the converted output signal to the arithmetic processing section 53.

The arithmetic processing unit 53 may include a memory, an MPU, and the like necessary for predetermined arithmetic processing. The arithmetic processing unit 53 receives the output signal of the detection unit 20, identifies the type of combustible gas, and calculates the concentration of the identified combustible gas based on a response waveform analysis method described later.

When the sensing element 21 and the reference element 22 include a heater resistive film, the external circuit 50 may include a control IC for controlling the output of the heater resistive film. Further, the external circuit 50 may include a voltage follower, a DA converter, etc. in addition to the above.

Furthermore, the gas sensor 2 may be provided with a heater control section 40, as shown in FIG. The heater control unit 40 is a mechanism for controlling the output of the heater 4 and adjusting the temperature of the internal space 15 of the closed container 10. The heater control unit 40 may include a control circuit that performs PID control, a heater power supply circuit, and the like. In controlling the output of the heater 4, the circuit of the heater control section 40 may be configured such that the output signal of the reference element 22 is supplied to the heater control section 40. Alternatively, a thermocouple may be installed in the internal space 15 of the closed container 10 and a signal from the thermocouple may be supplied to the heater control unit 40.

Furthermore, the gas sensor 2 may be provided with a gas introduction mechanism 30. The gas introduction mechanism 30 is a mechanism that continuously or intermittently sends sample gas toward the internal space 15 of the closed container 10. A simple configuration of the gas introduction mechanism 30 includes a compressor or a gas pump. The gas introduction mechanism 30 also includes a regulator that controls the pressure of the sample gas, a mass flow controller that controls the flow rate of the sample gas, a control circuit that controls the electromagnetic valves arranged at the inlet and exhaust ports, and a control circuit that controls the purge gas. An introduction mechanism and the like may also be included. When sending out the sample gas in intermittent operation, opening and closing of the solenoid valve may be controlled by the control circuit of the gas introduction mechanism 30. Further, when the sample gas is diluted with a carrier gas such as air or an inert gas and introduced into the closed container 10, the gas introduction mechanism 30 may include a gas mixer.

Next, a method of measuring a sample gas containing a combustible gas using the gas sensor 2 according to this embodiment will be described. Examples of flammable gases include CO (carbon monoxide), H ₂ (hydrogen), CH ₄ (methane), etc., and the sample gas contains at least one combustible gas, and contains two or more flammable gases. May contain gas.

Before measuring the sample gas, purge gas is supplied to the internal space 15 of the closed container 10. As the purge gas, air or an inert gas such as Ar gas may be used. With the internal space 15 filled with purge gas, the internal space 15 is heated to a constant measurement temperature by the heater 4. The measurement temperature is preferably 100°C or higher, more preferably 100°C or higher and 400°C or lower, and even more preferably 200°C or higher and 400°C or lower. As described above, the sample gas is introduced into the internal space 15 by the gas introduction mechanism 30 while the internal temperature of the closed container 10 is kept constant at the intended measurement temperature. The introduced sample gas is heated to a predetermined measurement temperature by the heater 4 within the sealed container 10 at a constant temperature.

Note that in the above description, the sample gas is introduced after the internal space 15 of the closed container 10 is heated, but the internal space 15 may be heated after the sample gas is introduced into the internal space 15. That is, the sample gas may be continuously introduced into the internal space 15 by the gas introduction mechanism 30, and in this state, the heater 4 may be turned on to heat the internal space 15 filled with the sample gas to the measurement temperature. .

When the sample gas is heated to the measurement temperature in the internal space 15, a combustion reaction of the combustible gas contained in the sample gas occurs on the surface of the detection element 21. The sensitive film 211 of the detection element 21 detects the combustion heat of the combustible gas, and the resistance value changes depending on the combustion heat. On the other hand, no combustion reaction occurs on the surface of the reference element 22, and the resistance film 221 of the reference element 22 outputs a resistance value according to the temperature of the internal space 15 (ie, the temperature of the sample gas). During measurement, the sample gas is kept at a constant temperature by the heater 4, so the resistance value of the resistive film 221 in the reference element 22 hardly changes, and the output of the reference element 22 remains almost constant. .

For example, the external circuit 50 reads the resistance value of the sensitive film 211 and the resistance value of the resistive film 221, and calculates the difference between them as the output signal of the detection unit 20. This output signal is supplied to the arithmetic processing unit 53, and the arithmetic processing unit 53 calculates the response waveform of the detection unit 20 to the sample gas based on the output signal (that is, the output difference between the detection element 21 and the reference element 22). Identify. Examples of response waveforms are shown in FIGS. 5A and 5B. Note that the output signal of the detection unit 20 is calculated based on the difference in resistance value, the difference in current value, the difference in voltage value, or the difference in calorific value between the detection element 21 and the reference element 22, and the output signal is The unit depends on the output method of each element.

FIG. 5A is a graph showing an example of the response waveform of each combustible gas when the flammable gas is introduced into the closed container 10 set at 200°C. In FIG. 5A, the solid line is the response waveform of CO, the dotted line is the response waveform of _H2 , and the dashed line is the response waveform of _CH4 . Further, the horizontal axis of the graph indicates the elapsed time of measurement (unit: min or sec), and the vertical axis of the graph indicates the magnitude of the output signal. In other words, the response waveform is a graph representing a change over time in the output signal of the detection unit 20 from the start of the combustion reaction of the combustible gas until the end of the measurement. T0 in FIG. 5A is the time when the sample gas is introduced into the heated closed container 10, and T3 is the time when the gas introduced into the internal space 15 is switched from the sample gas to a purge gas that does not contain flammable gas. be.

CH ₄ does not burn at 200°C even in the presence of a catalyst. Therefore, as shown in FIG. 5A, a CH ₄ response waveform cannot be obtained at a measurement temperature of 200°C. On the other hand, CO and H ₂ are burned at 200° C. due to the catalytic action of the catalyst section 213, and response waveforms as shown in FIG. 5A are obtained for each. The output value when CO is combusted at 200°C (the difference in output between the detection element 21 and the reference element 22) is comparable to the output value when H ₂ is combusted at 200°C. In other words, there is almost no difference between S2 _CO and S2 _H2 shown in FIG. 5A, and it is difficult to distinguish between CO and H ₂ just by comparing S2 _CO and S2 _H2 . On the other hand, the response waveform of CO and the response waveform of H ₂ have different shapes, and the arithmetic processing unit 53 identifies the type of combustible gas contained in the sample gas from this difference in shape.

More specifically, in the gas sensor 2 of this embodiment, the response waveform to the combustible gas is a square wave (rectangular wave). In particular, distortion or disturbance may occur in the rising portion of the response waveform (ie, the waveform immediately after the start of the combustion reaction). The distortion or disturbance of this waveform differs depending on the type of combustible gas, the measured temperature, etc. At a measurement temperature of 200° C., the response waveform of CO has an undershoot, and the response waveform of H ₂ has an overshoot.

Undershoot means "waveform distortion" in which the rising portion of a square wave is below the steady-state value of the waveform. On the other hand, overshoot means "waveform disturbance" in which the rising portion of the square wave protrudes upward from the steady value of the waveform. Note that the steady value of the waveform means the output value when the output signal becomes stable after a predetermined period of time has elapsed from the start time T0 of the combustion reaction. In the response waveform shown in FIG. 5A, the output signal value at time B can be regarded as a steady value.

In order to determine whether the response waveform is undershoot or overshoot, the following analysis may be performed, for example. First, the time immediately after the response waveform rises is set as time A, and the time when the change in the output signal in the response waveform becomes stable is set as time B. It is assumed that at time A, the sample gas introduced into the closed container 10 has reached the intended measurement temperature. The time required for the response waveform to rise and the time required for the output signal to stabilize vary depending on the capacity of the internal space 15. Therefore, the above-mentioned time points A and B may be appropriately set in consideration of the capacity of the internal space 15 and the response waveform actually obtained. For example, when the capacity of the internal space 15 is 1 cm ³ to 10 cm ³ , time A is the time when the elapsed time T1 from T0 is 5 seconds to 15 seconds, and time B is the time when the elapsed time T2 from T0 is 30 seconds to 50 seconds. It can be a point in time.

Next, the output signal value S1 at time A and the output signal value S2 at time B are specified, and their difference "S2-S1" is calculated. If 0<(S2-S1) is satisfied, it can be determined that the response waveform is an undershoot. In fact, it can be confirmed that the CO response waveform shown in FIG. 5A satisfies 0<(S2 _CO -S1 _CO ) and is an undershoot. On the other hand, if (S2-S1)<0 is satisfied, it can be determined that the response waveform is an overshoot. The H ₂ response waveform shown in FIG. 5A satisfies (S2 _H2 −S1 _H2 )<0, and it can be confirmed that there is an overshoot. Note that if (S2-S1)=0.00, it is determined that no distortion or disturbance of the waveform has occurred.

Note that the concentration of combustible gas contained in the sample gas can be calculated based on the magnitude of the output signal in the response waveform. That is, the concentration of combustible gas is calculated from the output signal value at the time when the output becomes stable (time point B). In the gas sensor 2, the arithmetic processing unit 53 analyzes the shape of the response waveform described above and calculates the gas concentration.

FIG. 5B is a graph showing an example of the response waveform of each combustible gas when the flammable gas is introduced into the closed container 10 set at 350°C. When the measured temperature is 350° C., CO, H ₂ , and CH ₄ all burn on the catalyst section 213, and a response waveform as shown in FIG. 5B is obtained. At a measurement temperature of 350°C, S2 _CO , S2 _H2 , and S2 _CH4 all have similar values, and it is difficult to identify the type of combustible gas just by detecting the output value of the detection unit 20. . As shown in FIG. 5B, the shape of the response waveform differs depending on the type of combustible gas, and the gas type is identified based on this difference in shape. Specifically, CO has an undershoot response waveform even at 350° C., and H ₂ and CH ₄ have an overshoot response waveform.

In CO measurement, the response waveform always has an undershoot in the measurement temperature range of 100°C to 400°C. The undershoot is a response waveform specific to CO, and the presence or absence of the undershoot allows identification of CO contained in the sample gas. That is, when a sample gas whose components are unknown is introduced into the closed container 10 and the above measurement is performed, if the arithmetic processing unit 53 identifies an undershoot response waveform, it can be determined that CO is present in the sample gas. . Then, the concentration of CO can be calculated from the magnitude of the undershoot response waveform. In this way, the gas sensor 2 can selectively detect CO and specify the concentration of CO by determining the presence or absence of undershoot.

Both H ₂ and CH ₄ have overshoot response waveforms, but have different combustible temperature ranges. When an unknown sample is measured with the gas sensor 2, if an overshoot response waveform is obtained in the low temperature range of 100° C. to 200° C., it can be determined that H ₂ is present in the sample gas. On the other hand, if no output is obtained in the low temperature range but an overshoot response waveform is obtained in the high temperature range of over 200°C, it can be determined that CH ₄ is present in the sample gas. In this way, the gas sensor 2 performs measurements while changing the heating temperature (measurement temperature) of the closed container 10, and by identifying the shape of the response waveform obtained at each temperature, H ₂ present in the sample gas can be measured. Alternatively, CH ₄ can be identified, and the concentration of H ₂ and CH ₄ can be calculated based on the determination result. In addition, when carrying out measurements in a plurality of temperature zones, it is preferable to purge the internal space 15 with air or inert gas when changing the set temperature in the closed container 10.

The reason why the response waveform to CO specifically undershoots is not necessarily clear, but it is thought to be related to the bond dissociation energy of the combustible gas. Unlike other combustible gases, CO has a triple bond of C and O, so the bond dissociation energy of CO is higher than that of other combustible gases. In the process of burning CO and producing CO ₂ , a lot of energy is required to dissociate the triple bond, so it is thought that an undershoot appears in the response waveform to CO.

(Summary of embodiments)
The gas sensor 2 according to the present embodiment includes a closed container 10 having an inlet 12 and an exhaust port 14 for sample gas, a heater 4 installed in the closed container 10, and an inside (internal space 15) of the closed container 10. The detection unit 20 includes a detection element 21 and a reference element 22 arranged therein, and an external circuit 50 installed outside the closed container 10. The external circuit 50 includes an arithmetic processing section 53 that specifies the shape of the response waveform of the detection section 20 to the sample gas.

In the gas sensor 2 having the above characteristics, the sample gas introduced into the closed container 10 is heated to a predetermined measurement temperature by the heater 4, and the response waveform to the sample gas is specified from the output difference between the detection element 21 and the reference element 22. Then, the type of combustible gas contained in the sample gas is specified from the shape of the response waveform, and the concentration of the specified combustible gas is calculated from the output signal value. In particular, the arithmetic processing unit 53 serves as a mechanism for determining overshoot or undershoot of the response waveform.

The gas sensor 2 can identify with high accuracy the type of combustible gas contained in an unknown sample or mixed gas, and can reduce errors when detecting gas concentration. In particular, in the gas sensor 2, gas types can be identified using one set of detection units 20 without installing a plurality of detection units 20.

In addition, in the gas sensor 2, the heater 4 installed in the sealed container 10 heats the sealed internal space 15 to a constant temperature, so the reactivity between the catalyst part 213 and the combustible gas can be increased, resulting in high accuracy. Detection of combustible gas is possible. In particular, by making the inner wall section of the closed container 10 perpendicular to the inflow direction of the sample gas circular, the thermal uniformity of the internal space 15 can be further improved, and the accuracy of gas detection can be further improved.

Furthermore, in the gas sensor 2, a specific undershoot appears in the response waveform to CO. By determining the presence or absence of undershoot in the arithmetic processing unit 53, it is possible to selectively detect CO. In other words, the gas sensor 2 can be more suitably used as a CO sensor specialized for selective detection of CO. However, the use of the gas sensor 2 is not limited to the detection of CO, but can also detect _H2 , _CH4 , and the like. When identifying combustible gases that exhibit overshoot, such as H ₂ and CH ₄ , it is necessary to measure unknown samples or mixed gases in multiple temperature zones and analyze the shape of the response waveform identified in each temperature zone. is preferred.

Although the embodiments of the present disclosure have been described above, the present invention is not limited to the embodiments described above, and can be variously modified within the scope of the gist of the present disclosure.

Hereinafter, the present disclosure will be described in further detail based on specific examples.

(Comparative example)
In the comparative example, CO gas, H ₂ gas, and CH ₄ gas were detected using a conventional catalytic combustion gas sensor. As a comparative example of a catalytic combustion type gas sensor, a sensor element was used in which a Pt heater resistance film, a thermistor film made of MnNiCo-based oxide, and a catalyst part made of alumina supporting Pt were laminated. The content of Pt in the catalyst part was 15 wt%. Then, the sensor element was heated to 100°C, 150°C, 200°C, 250°C, 300°C, 350°C, and 400°C using a heater resistance film laminated inside the element. A flammable gas was brought into contact with the surface of the element heated to each temperature, and the change in resistance of the thermistor film at that time was read as an output.

The evaluation results of the above comparative example are shown in FIG. In Figure 6, the results shown by the black circle plot and the solid line are the resistance changes when CO gas is catalytically burned, and the square plots and the dotted lines are the resistance changes when _H2 gas is catalytically burned. The result shown by the triangular plot and the dashed line is the resistance change when CH ₄ is catalytically burned.

As shown in FIG. 6, the CO combustion temperature range and the H ₂ combustion temperature range were both 100°C to 400°C. In particular, in the range of 200° C. to 400° C., the amount of resistance change due to CO and the amount of resistance change due to H ₂ were comparable. Regarding CH ₄ , although no combustion reaction occurred in the temperature range below 200°C, CH ₄ catalytically burned in the range _of 250°C to 400°C, and CO, H ₂ , etc. The combustion temperature range of other combustible gases partially overlapped. In particular, at 350° C. to 400° C., the amount of resistance change was approximately the same for CO, H ₂ and CH ₄ .

From the above results, it was found that it is difficult to distinguish between CO, H ₂ and CH ₄ using a conventional catalytic combustion gas sensor that only reads resistance changes. In order to identify the type of combustible gas using a conventional catalytic combustion gas sensor, it was found that it was necessary to prepare multiple pairs of sensor elements with different operating temperatures and catalyst types, as shown in previous literature. .

(Example 1)
In Example 1, the gas sensor 2 having the configuration shown in FIG. 1 was manufactured using the chamber 10a shown in FIGS. 2A and 2B. The capacity of the internal space 15 in the chamber 10a was 3.5 cm ³ . In addition, the sensitive film 211 in the sensing element 21 is a thermistor film made of MnNiCo-based oxide, and the catalyst part 213 is made of γ-Al ₂ O ₃ supporting Pt, and the content of Pt in the catalyst part 213 is 15 wt%. did. The reference element 22 had the same configuration as the detection element 21 described above, except that the catalyst portion 213 was not formed.

(Example 2)
In Example 2, the gas sensor 2 having the configuration shown in FIG. 1 was manufactured using the package 10b shown in FIGS. 4A and 4B. The capacity of the internal space 15 in the package 10b was 1 cm ³ . In addition, the sensitive film 211 in the sensing element 21 is a thermistor film made of MnNiCo-based oxide, and the catalyst part 213 is made of γ-Al ₂ O ₃ supporting Pt, and the content of Pt in the catalyst part 213 is 15 wt%. did. The reference element 22 had the same configuration as the detection element 21 described above, except that the catalyst portion 213 was not formed.

In each of the gas sensors of Examples 1 and 2, the internal space 15 was heated to 200° C. by the heater 4, and a sample gas containing CO, H ₂ or CH ₄ was introduced into the heated internal space 15. When the sample gas was introduced, the resistance values of the detection element 21 and the reference element 22 were read as outputs, and a response waveform of the detection unit 20 at 200° C. was obtained based on the output difference. The above measurements were also carried out at 350°C. That is, the heating temperature of the internal space 15 by the heater 4 was set to 350°C, and the response waveform of the detection unit 20 at 350°C was obtained. Then, the analysis shown in FIGS. 5A and 5B was performed on each of the obtained response waveforms.

Specifically, the time point 10 seconds after the introduction of the sample gas is defined as time point A, and the time point 30 seconds after the introduction of the sample gas is defined as time point B, and the output signal value S1 at time point A and the output signal at time point B are The value S2 was calculated. Then, S2-S1 was calculated, and the shape of the response waveform was determined based on the calculation result. Table 1 shows the results of measuring sample gas using the gas sensor of Example 1, and Table 2 shows the results of measuring sample gas using the gas sensor of Example 2.

As shown in Table 1, when the measurement temperature was 200° C., CO gas and H ₂ gas were combusted, but CH ₄ gas was not combusted. Further, at a measurement temperature of 200° C., the output signal value at time B was approximately the same (2.86 to 2.88) when CO gas was introduced and when H ₂ gas was introduced. However, the response waveform when CO gas was introduced and the response waveform when H ₂ gas was introduced were different in shape. Specifically, when the gas sensor of Example 1 was used, at a measurement temperature of 200° C., the response waveform to CO was an undershoot, and the response waveform to H ₂ was an overshoot.

Moreover, as shown in Table 1, when the measurement temperature was 350° C., CO gas, H ₂ gas, and CH ₄ gas were each combusted. At a measurement temperature of 350° C., the output signal values S2 _CO , S2 _H2 and S2 _CH4 at time B were all at the same level (3.1 to 3.5). However, the shape of the response waveform differed depending on the type of combustible gas, and the response waveform for CO was an undershoot, and the response waveform for H ₂ and CH ₄ was an overshoot.

From the above results, the gas sensor of Example 1 can selectively detect CO by determining the presence or absence of undershoot in the response waveform even when an unknown sample or mixed gas is introduced into the internal space 15. I found out that it is possible. In other words, it was found that the gas sensor of Example 1 was able to calculate the concentration of CO gas by distinguishing it from other combustible gases, regardless of the measurement temperature.

As shown in Table 2, the same results as in Example 1 using the chamber 10a were obtained in Example 2 using the package 10b.

Using each of the gas sensors of Example 1 and Example 2, an experiment was conducted to detect CO, H ₂ , and CH ₄ while changing the measurement temperature. Specifically, the internal space 15 was heated to 100°C, 150°C, 200°C, 250°C, 300°C, 350°C, and 400°C, and response waveforms at each temperature were obtained. Then, the shape of the response waveform was determined using the same method as the analysis performed in Tables 1 and 2. In both Example 1 and Example 2, the determination results shown in Table 3 were obtained.

As shown in Table 3, when a sample gas containing CO was introduced, an undershoot response waveform was obtained in all temperature ranges from 100°C to 400°C. Undershoot was confirmed only in the response waveform to CO. In other words, if an undershoot response waveform is identified at a measurement temperature of 100°C to 400°C, it can be confirmed that CO is contained in the sample gas, and based on the analysis results of the response waveform, it is possible to confirm that CO is present in the sample gas. It was found that the concentration could be selectively detected.

Furthermore, when a sample gas containing H ₂ was introduced, an overshoot response waveform occurred in all temperature ranges from 100°C to 400°C. When a sample gas containing CH ₄ was introduced, no output was obtained below 200°C, and an overshoot response waveform was obtained in the range of 250°C to 400°C. In other words, if an overshoot response waveform is identified at a measurement temperature of 200° C. or lower, it can be determined that H ₂ is contained in the sample gas. On the other hand, if no response waveform is obtained at a measurement temperature of 200℃ or lower, and an overshoot response waveform is identified at a measurement temperature of 250℃ or higher, it can be determined that CH ₄ is contained in the sample gas. . From the results in Tables 1 to 3, for H ₂ gas and CH ₄ gas that exhibit overshoot response waveforms, the response waveforms in the low temperature range below 200°C and the response waveforms in the high temperature range exceeding 200°C are as follows: Through analysis, it was found that the concentration of H ₂ gas or CH ₄ gas could be calculated by distinguishing it from other combustible gases.

From the above analysis results, the type of combustible gas (CO, H ₂ or CH ₄ ) contained in the sample gas can be identified by determining whether the response waveform is overshoot or undershoot. , it was demonstrated that the concentration of the identified combustible gas could be determined.

2... Gas sensor 10... Airtight container 10a... Chamber 10b... Package 11... End wall 12... Inflow port 13... Side wall 13a... Inner wall surface 13b... Outer wall surface 14... Exhaust port 15... Introduction space 4... Heater 20... Detection section 21... Detection element
21a... Film laminated part of sensing element
211... Sensitive membrane
212... Electrode film
213... Catalyst section
215... Insulating film 22... Reference element
22a... Film stack part of reference element
221... Resistive film 30... Gas introduction mechanism 40... Heater control unit 50... External circuit 51... Differential amplifier 52... AD converter 53... Arithmetic processing unit

Claims (13)

a closed container having an inlet and an outlet for sample gas;
a heater installed in the sealed container;
a detection unit disposed inside the airtight container and including a reference element and a detection element sensitive to gas concentration;
an external circuit installed outside the airtight container;
A gas sensor in which the external circuit includes an arithmetic processing unit that specifies the shape of a response waveform of the detection unit to the sample gas. The gas sensor according to claim 1, wherein the arithmetic processing section determines overshoot or undershoot of the response waveform. The gas sensor according to claim 1, wherein the arithmetic processing section specifies a CO response waveform having an undershoot. The gas sensor according to any one of claims 1 to 3, wherein the heater heats the sample gas introduced into the closed container to 100° C. or higher. The gas sensor according to any one of claims 1 to 4, wherein an inner wall cross section of the closed container in a plane perpendicular to the inflow direction of the sample gas is circular. The gas sensor according to any one of claims 1 to 5, wherein the detection element includes a catalyst portion and a thermistor film. The gas sensor according to any one of claims 1 to 5, wherein the detection element includes a catalyst portion and a platinum resistor. 8. The gas sensor according to claim 6, wherein the catalyst section is a porous body supporting 1 wt% or more of noble metal based on the total amount of the catalyst section. The sample gas introduced into the sealed container is heated to a predetermined temperature by a heater installed in the sealed container,
identifying a response waveform to the sample gas from the output difference between a detection element and a reference element installed in the sealed container;
identifying the type of gas contained in the sample gas from the shape of the response waveform;
A gas detection method that calculates the concentration of the gas contained in the sample gas from the magnitude of an output signal in the response waveform. 10. The gas detection method according to claim 9, wherein it is determined whether a rising portion in the response waveform is an overshoot or an undershoot, and the type of gas contained in the sample gas is identified based on the determination result. 10. The gas detection method according to claim 9, wherein it is determined whether or not the response waveform has an undershoot, and it is determined from the determination result whether or not CO gas is contained in the sample gas. The gas detection method according to any one of claims 9 to 11, wherein the predetermined temperature is 100°C or higher. Heating the sample gas introduced into the sealed container to a plurality of temperatures in a range of 100 ° C. or more and 400 ° C. or less,
identifying the response waveform to the sample gas for each of a plurality of temperatures;
11. The gas detection method according to claim 9, wherein the type of gas contained in the sample gas is identified from the shape of the response waveform at each temperature.

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