JP5061287B2 - Gas sensor and hydrogen concentration detection system - Google Patents

Description

The present invention relates to a catalytic combustion type gas sensor using heat generation in a catalytic reaction between a gas and a catalyst medium.

Conventionally, a gas sensor for detecting a gas corresponding to a use has been put into practical use. In particular, a gas sensor that detects flammable gas is positioned as an important sensor for ensuring safety in use.
In recent years, fuel cells have been developed for automobiles and households in order to reduce the environmental load. Since a fuel cell uses hydrogen as a fuel, detecting a gas leak of hydrogen gas is an important matter when using the fuel cell safely. This gas leak is detected by a hydrogen sensor,
A hydrogen sensor that responds only to hydrogen gas and quantitatively detects a hydrogen concentration of about 0.05 to 5% is required.

Currently, a contact combustion type sensor or the like is known as a gas sensor for detecting a gas such as hydrogen. This contact combustion type sensor uses heat generated by the contact combustion phenomenon on the surface of a substance having a gas detection function, and detects a change in electrical resistance from this temperature change as a voltage from a bridge circuit paired with a fixed resistance. Is.
For example, as shown in Patent Document 1 as an example of a contact combustion type hydrogen sensor, a hydrogen reaction catalyst layer as a catalyst medium and a thin film thermistor for detecting the heat generation temperature of the hydrogen reaction catalyst layer are stacked via an insulating layer. A bridge circuit is configured using elements, and the hydrogen concentration can be detected by detecting the bridge voltage.

As another hydrogen sensor, as shown in Patent Document 2, in order to detect molecules in a gas, in a transversal SAW filter, a comb-like IDT (Interdigital Tran
There is known a gas sensor in which a gas adsorber is provided between an excitation electrode and a reception electrode each consisting of a sducer) electrode.
This gas sensor has a surface acoustic wave (SAW) excited by an excitation electrode.
ave) passes through a gas adsorbent whose weight has been changed by gas adsorption, and the propagation speed of the surface acoustic wave changes, and this is detected as a frequency change by the receiving electrode to detect the gas concentration.

JP 2005-98742 A JP-A-8-68781

However, a conventional gas sensor such as a catalytic combustion type hydrogen sensor has a problem that the gas concentration cannot be detected with high sensitivity because a change in voltage detected from a bridge circuit or the like is minute. In addition, since a gas sensor using a transversal SAW filter has a bandwidth of the SAW filter, a slight frequency fluctuation cannot be detected, and there is a problem in gas concentration detection sensitivity.
The present invention has been made to solve the above problems, and an object of the present invention is to provide a gas sensor capable of detecting a gas concentration with high sensitivity.

In order to solve the above problems, a gas sensor of the present invention includes a piezoelectric substrate, an IDT electrode provided on the piezoelectric substrate, a gas reaction catalyst film provided on the piezoelectric substrate and generating heat in response to gas. An oscillation circuit for exciting the IDT electrode, wherein the gas reaction catalyst film is provided on a propagation path of a surface acoustic wave excited by the IDT electrode, a quartz substrate is used as the piezoelectric substrate, The X axis is the X axis, the mechanical axis is the Y axis, and the optical axis is the Z axis. The crystal substrate is a plane including the X axis and the Z axis, the X axis is the rotation axis, and the crystal A plane formed by rotating θ ° from the + Z axis of the crystal to the −Y axis of the crystal is defined as a main surface, and the IDT electrode has the surface acoustic wave parallel to the X axis. The angle θ is 33 or more and 53. And it satisfies the below.

According to this configuration, the gas contacts the gas reaction catalyst film formed on the piezoelectric substrate to generate heat, and this heat increases the temperature of the piezoelectric substrate. Since the piezoelectric substrate has a characteristic that the sound velocity of the wave propagating through the piezoelectric substrate changes depending on the temperature, the speed at which the surface acoustic wave propagates, that is, the frequency changes as the temperature of the piezoelectric substrate increases. By detecting a change in frequency, it is possible to detect the presence of gas or the concentration of gas.
The gas sensor of the present invention uses a resonator-type surface acoustic wave device, and does not have a bandwidth like a SAW filter and can detect slight frequency fluctuations. Can be detected.
In addition, quartz has good workability and is suitable for mass production, and has an advantage that the temperature sensitivity coefficient of the quartz substrate can be adjusted by appropriately selecting the cut angle (cutting orientation) of the main surface of the quartz. In other words, by appropriately selecting the cut angle of the main surface of the crystal, it is possible to obtain a good sensitivity to temperature, and thus it is possible to provide a gas sensor that can detect the gas concentration with high sensitivity and is excellent in mass productivity.
In addition, when θ satisfies 33 or more and 53 or less, a gas sensor having an absolute value of a temperature sensitivity coefficient of 30 to 60 ppm / ° C. can be configured, heat generation with respect to gas concentration can be detected efficiently, and a gas sensor excellent in sensitivity can be obtained. Can be provided.

Further, the gas sensor of the present invention, it is preferable the gas reaction catalyst film is found provided on front Kiden播路between the alternate finger electrodes in the IDT electrode.

According to this configuration, since the gas reaction catalyst film is formed between the interdigitated electrodes of the IDT electrode that excites the surface acoustic wave, the heat generated by the gas reaction catalyst film is immediately applied to the piezoelectric substrate on the surface acoustic wave propagation path. It is transmitted. Therefore, the speed at which the surface acoustic wave excited by the IDT electrode propagates can be changed in a short time, and a gas sensor having excellent responsiveness and sensitivity can be provided.

In the gas sensor of the present invention, it is preferable that the gas reaction catalyst film is formed on an insulating film covering the IDT electrode and on the surface acoustic wave propagation path.
The gas sensor of the present invention preferably includes an insulating film that covers the IDT electrode, and the gas reaction catalyst film is provided on the insulating film and on the propagation path.

According to this configuration, since the gas reaction catalyst film is provided via the insulating film, it is possible to prevent a short circuit between the IDT electrode and the gas reaction catalyst film and to dispose the gas reaction catalyst film in a wider area. is there. From this, the heat generated by the gas can be sufficiently transmitted to the surface of the piezoelectric substrate,
It is possible to provide a gas sensor having excellent responsiveness and sensitivity without taking time to increase the temperature of the surface of the piezoelectric substrate.

In the gas sensor of the present invention, the gas reaction catalyst film is preferably formed of a material selected from platinum or palladium.
In the gas sensor of the present invention, it is desirable that platinum or palladium is used for the gas reaction catalyst film.

According to this configuration, by using platinum or palladium as the gas reaction catalyst membrane, a gas reaction catalyst membrane having high hydrogen selectivity and hardly responding to other flammable gases not containing hydrogen can be configured. .

In the gas sensor of the present invention, the surface acoustic wave excited from the IDT electrode is preferably a Rayleigh wave or an STW wave.

According to this configuration, the Rayleigh wave and the STW wave are concentrated in an area of about several μm on the substrate surface, and the temperature change of the substrate surface portion appears as a change in the frequency of the surface acoustic wave without taking time. . Therefore, by using a Rayleigh wave or STW wave as the surface acoustic wave, it is possible to provide a gas sensor with good responsiveness in detecting gas such as hydrogen.
The hydrogen concentration detection system of the present invention includes a piezoelectric substrate, an IDT electrode provided on the piezoelectric substrate, a gas reaction catalyst film provided on the piezoelectric substrate and generating heat in response to a gas containing hydrogen , An oscillation circuit for exciting the IDT electrode, wherein the gas reaction catalyst film is provided on a propagation path of the surface acoustic wave excited by the IDT electrode, a quartz substrate is used as the piezoelectric substrate, When the axis is the X-axis, the mechanical axis is the Y-axis, and the optical axis is the Z-axis, the crystal substrate is a plane including the X-axis and the Z-axis, the X-axis is the rotation axis, and + Z of the crystal The IDT electrode has a plane that is rotated by θ ° with the direction of rotation from the axis to the −Y axis of the crystal as positive, and the surface acoustic wave propagates parallel to the X axis. The θ is 33 or more and 53 or less. Characterized in that the plus.

Hereinafter, an embodiment of a gas sensor embodying the present invention will be described with reference to the drawings, taking a hydrogen sensor as an example.
(First embodiment)

FIG. 1 is a configuration diagram showing a configuration of a hydrogen reaction detecting element according to the hydrogen sensor of the present embodiment, FIG. 1 (a) is a schematic plan view, and FIG. 1 (b) is an AA disconnection in FIG. 1 (a). FIG.
The hydrogen reaction detection element 3 includes an IDT electrode 13, a reflector 15, and a hydrogen reaction catalyst film 14 on the surface of a quartz crystal substrate 11 made of a piezoelectric material. As described above, the hydrogen sensor of this embodiment uses a 1-port resonator type surface acoustic wave device.
The IDT electrode 13 is configured such that a number of interdigital electrodes 12a and 12b made of aluminum (Al) or an aluminum alloy are alternately arranged to excite surface acoustic waves.
Although various types of surface acoustic waves can be used as the excited surface acoustic waves, Rayleigh waves and STW waves (a type of SH waves) in which the elastic waves are concentrated on the substrate surface are suitable. Further, between the crossed finger electrodes 12a and 12b at substantially the center of the IDT electrode 13, platinum (P
The hydrogen reaction catalyst film 14 made of t) or palladium (Pd) is formed in the propagation path of the surface acoustic wave excited by the IDT electrode 13.
When the pitch of the interdigital electrodes 12a and 12b of the IDT electrode 13 is P, the wavelength λ of the surface acoustic wave is expressed as λ = 2P. When the thickness of the IDT electrode 13 is H, the normalized electrode thickness H / λ is set to H / λ = 0.08.

A reflector 15 made of aluminum (Al) or aluminum alloy having a lattice structure is disposed on both sides of the IDT electrode 13 in the direction in which the surface acoustic wave propagates.
Further, the cross finger electrode 12a of the IDT electrode 13 is connected to the input side terminal 16, and the cross finger electrode 1 is connected.
2 b is connected to the output side terminal 17. The input side terminal 16 and the output side terminal 17 are
It is connected to an oscillation circuit (not shown) that excites the IDT electrode 13. As described above, the hydrogen sensor is configured by including the hydrogen reaction detecting element 3 using the resonator-type surface acoustic wave device and the oscillation circuit.
The IDT electrode 13, the reflector 15, and the hydrogen reaction catalyst film 14 are formed using conventional methods such as vacuum deposition, sputtering, and photolithography.

Here, the cut angle (cutting orientation) of the quartz substrate 11 will be described in detail. Quartz has a frequency-temperature characteristic that depends on the cut angle, and a desired temperature sensitivity coefficient can be obtained by appropriately selecting the cut angle.
FIG. 2 is an explanatory view for explaining the cut angle of the quartz crystal substrate used in this embodiment.
Of the three crystal axes that are orthogonal to each other, the electrical axis is the X axis, the mechanical axis is the Y axis, and the optical axis is the Z axis.
As shown in FIG. 2, a quartz substrate 1 is used which is obtained by rotating a Y plate whose principal surface is a plane formed by an X axis and a Z axis by rotating θ ° counterclockwise around the X axis. The IDT electrode 2 is formed so that the surface acoustic wave propagates in a direction parallel to the X axis. When the counterclockwise rotation about the X axis is displayed as a positive sign, the cut angle θ = 33 to 53 (°) in this embodiment.
A cut angle is used.

Next, how the cut angle is set as described above will be described.
Since the hydrogen sensor of the present invention is a contact combustion type, it uses heat generated when hydrogen gas contacts the hydrogen reaction catalyst film. Since the amount of heat generated by this heat generation is very small, it is necessary to efficiently detect a temperature change, and a quartz substrate having a high temperature dependency in frequency is desired.
The temperature dependence of the natural frequency in a quartz substrate is related to the change due to thermal expansion of the quartz substrate, the temperature dependence of the elastic constant, and the density change due to thermal expansion. did.

Furthermore, the following points were considered in selecting the crystal cut angle. First, when the detection sensitivity of the hydrogen concentration is 0.1% or less, the temperature change when detecting the hydrogen concentration of 5% is 2 to 3
The temperature change is 0.04 to 0.06 ° C. per hydrogen concentration of 0.1%. Here, the frequency detection limit is about 1 ppm, and the temperature sensitivity (temperature sensitivity coefficient) of the quartz substrate is 30 pp.
If m / ° C. or more, a frequency change of 1.2 ppm or more per hydrogen concentration of 0.1% can be obtained. Here, a value representing a frequency change by a frequency deviation at a temperature change of 1 ° C. is called a temperature sensitivity coefficient (ppm / ° C.).
On the other hand, increasing the temperature sensitivity coefficient improves the detection sensitivity of the hydrogen concentration. However, if the temperature sensitivity coefficient is large, the fluctuation of the frequency increases and the handling becomes difficult. Assuming that the operating temperature range of the hydrogen sensor is 50 to 220 ° C., a temperature sensitivity coefficient of 60 ppm / ° C. at which a frequency variation of about 10,000 ppm is obtained with a temperature change of 170 ° C. is considered appropriate.
From this, it can be said that selecting a crystal cut angle at which the absolute value of the temperature sensitivity coefficient is 30 to 60 ppm / ° C. is suitable for a crystal substrate used in a hydrogen sensor.

FIG. 3 shows the θ ° when the quartz substrate described in FIG. 2 is rotated by θ ° with the X axis as the rotation axis.
And a temperature sensitivity coefficient. Here, the graph shows the relationship between θ and the temperature sensitivity coefficient when the operating temperature is 100 ° C. and the normalized electrode thickness of the IDT electrode is H / λ = 0.08.
From this graph, when a cut angle θ having an absolute value of the temperature sensitivity coefficient of 30 to 60 ppm / ° C. is selected, θ = 33 to 53 (°).

In the hydrogen sensor configured as described above, the hydrogen reaction detecting element 3 has, for example, a cut angle θ =
In the case of 33 (°), a frequency temperature characteristic as shown in FIG. 4 is shown. According to this graph, it can be seen that the frequency of the hydrogen reaction detecting element 3 tends to decrease as the temperature of the quartz substrate 11 increases.
Returning to FIG. 1, the hydrogen sensor including the hydrogen reaction detecting element 3 having such characteristics generates heat when a gas containing hydrogen contacts the hydrogen reaction catalyst film 14, and the heat is transmitted to the crystal substrate 11, and the crystal The temperature of the substrate 11 rises. The IDT electrode 13 excited by the oscillation circuit oscillates at a frequency corresponding to this temperature. For example, the hydrogen concentration can be detected from the difference (frequency shift) between the detected frequency and the frequency excited in the absence of hydrogen.
FIG. 5 is a graph showing the correlation between the frequency shift of the surface acoustic wave and the hydrogen concentration, and the cut angle θ =
This is a case of 33 (°) (temperature sensitivity coefficient: 30 ppm / ° C.) and an operating temperature of 100 ° C. Thus, the frequency shift corresponds to the hydrogen concentration, and the hydrogen concentration can be detected from the frequency shift.

In such a hydrogen reaction detecting element 3, when a predetermined high frequency signal voltage is applied between the input side terminal 16 and the output side terminal 17, a surface acoustic wave having the same frequency as the input signal is excited on the surface of the quartz substrate 11. . The surface acoustic wave propagates to both sides of the IDT electrode 13, is reflected by the left and right reflectors 15 and returns toward the center of the IDT electrode 13. As a result, the surface acoustic wave is confined between the reflectors 15, and the standing wave Will occur.
When the hydrogen reaction catalyst film 14 comes into contact with hydrogen, the hydrogen reaction catalyst film 14 generates heat, and the crystal substrate 11
Temperature rises. Since the quartz substrate 11 has a characteristic that the sound velocity of the wave propagating through the quartz substrate changes depending on the temperature, the velocity at which the surface acoustic wave propagates as the temperature of the quartz substrate 11 increases,
That is, the frequency changes. By detecting this change in frequency, the presence of hydrogen or the concentration of hydrogen can be detected.

The conventional transversal surface acoustic wave device has a frequency characteristic with a certain bandwidth with respect to the center frequency, whereas the resonator type surface acoustic wave device has a narrow center frequency bandwidth, Has a high Q value. From this, it is possible to reduce noise and improve the sensitivity by using a resonator type surface acoustic wave device rather than using a transversal type surface acoustic wave device.
Then, by using platinum or palladium as the hydrogen reaction catalyst film 14, a gas reaction catalyst film having high hydrogen selectivity and hardly responding to other combustible gas not containing hydrogen can be formed.

Furthermore, quartz has good workability and is suitable for mass production, and has an advantage that the temperature sensitivity coefficient of the quartz substrate can be adjusted by appropriately selecting the cut angle (cutting orientation) of the main surface of the quartz. That is, by appropriately selecting the cut angle of the main surface of the crystal, it is possible to obtain a good sensitivity to temperature, and in turn, a hydrogen sensor that can detect the hydrogen concentration with high sensitivity and is excellent in mass productivity.
Further, as the surface acoustic wave excited from the IDT electrode 13, it is preferable to use a Rayleigh wave or STW wave having a high concentration on the substrate surface. The Rayleigh wave and the STW wave are concentrated in an area of about several μm on the substrate surface, and a temperature change on the substrate surface portion appears as a frequency change without taking time. From this, as a surface acoustic wave, Rayleigh wave or S
By using the TW wave, it is possible to provide a hydrogen sensor with good followability in detecting gas such as hydrogen.
(Modification)

FIG. 6 is a schematic plan view showing the configuration of a modified example in which the arrangement of the hydrogen reaction catalyst film in the hydrogen reaction detecting element is changed.
The hydrogen reaction detecting element 3a of the modification shown in FIG. 6A uses a 1-port resonator type surface acoustic wave device as in the first embodiment, and the interdigital electrodes 22a and 22b are formed on the surface of the quartz substrate 21. Are arranged along the propagation path of the surface acoustic wave, and reflectors 25 are arranged on both sides thereof.
6B uses a two-port resonator type surface acoustic wave device, and an excitation IDT electrode 32 and a reception IDT electrode 33 are formed on the surface of the quartz substrate 31. The hydrogen reaction detection element 3b shown in FIG.
And a hydrogen reaction catalyst film 34 disposed in the propagation path of the surface acoustic wave between both IDT electrodes, and reflectors 35 disposed on both the left and right sides so as to sandwich them.

Also in each of the hydrogen reaction detecting elements 3a and 3b as described above, the hydrogen reaction catalyst film 2
4 and 34 come into contact with hydrogen and generate heat, and the temperature of the quartz substrates 21 and 31 rises. From this, the speed, that is, the frequency of the surface acoustic wave propagating through the quartz substrates 21 and 31 changes. And
By detecting the temperature change as a frequency signal, the presence of hydrogen or the concentration of hydrogen can be detected, and the same effect as in the first embodiment can be obtained.
(Second Embodiment)

Next, a hydrogen sensor in the second embodiment will be described.
This hydrogen sensor is different from the first embodiment in the configuration of the hydrogen reaction detection element, and the configuration of the hydrogen reaction detection element will be described in detail.

7A and 7B are configuration diagrams showing the configuration of the hydrogen reaction detecting element according to the hydrogen sensor of the present embodiment. FIG. 7A is a schematic plan view, and FIG. 7B is a broken line BB in FIG. FIG.
The hydrogen reaction detecting element 4 includes an IDT electrode 43, a reflector 45, and a hydrogen reaction catalyst film 44 on the surface of a quartz crystal substrate 41 that is a piezoelectric material. As described above, the hydrogen sensor of this embodiment uses a 1-port resonator type surface acoustic wave device.
The IDT electrode 43 is configured such that a large number of crossed finger electrodes 42a and 42b made of aluminum (Al) or an aluminum alloy are alternately arranged to excite surface acoustic waves.
Although various types of surface acoustic waves can be used as the excited surface acoustic waves, Rayleigh waves and STW waves (a type of SH waves) in which the elastic waves are concentrated on the substrate surface are suitable. An insulating film 48 made of Al ₂ O ₃ is formed on the surface of the IDT electrode 43 that is a propagation path of the surface acoustic wave, and hydrogen made of platinum (Pt) or palladium (Pd) is formed thereon. A reaction catalyst film 44 is formed. The insulating film 48 made of Al ₂ O ₃ is formed by a technique such as an anodic oxidation method, and may be formed not only on a part of the IDT electrode 43 but also on the entire surface of the IDT electrode 43.

A reflector 45 made of aluminum (Al) or aluminum alloy having a lattice structure is disposed on both sides of the IDT electrode 43 in the direction in which the surface acoustic wave propagates.
Further, the cross finger electrode 42 a of the IDT electrode 43 is connected to the input side terminal 46, and the cross finger electrode 4.
2 b is connected to the output side terminal 47. The input side terminal 46 and the output side terminal 47 are
It is connected to an oscillation circuit (not shown) that excites the IDT electrode 43. As described above, the hydrogen sensor is configured by including the hydrogen reaction detecting element 4 using the resonator type surface acoustic wave device and the oscillation circuit.
The IDT electrode 43, the reflector 45, and the hydrogen reaction catalyst film 44 are formed using conventional methods such as vacuum deposition, sputtering, and photolithography.
Further, the cut angle (cutting direction) of the quartz substrate 41 is the same as in the first embodiment, and the description thereof is omitted. A desired temperature sensitivity coefficient is obtained by appropriately selecting the cut angle.

In the hydrogen sensor including the hydrogen reaction detecting element 4 having such a configuration, when the hydrogen reaction catalyst film 44 comes into contact with hydrogen, the hydrogen reaction catalyst film 44 generates heat, and the heat is transmitted to the crystal substrate 41 so that the crystal substrate The temperature of 41 rises. Then, as the temperature of the quartz substrate 41 rises, the speed at which the surface acoustic wave propagates, that is, the frequency changes. By detecting a change in frequency, it is possible to detect the presence of hydrogen or the concentration of hydrogen.

As described above, in the hydrogen sensor of this embodiment, since the hydrogen reaction catalyst film 44 is provided via the insulating film 48, a short circuit between the IDT electrode 43 and the hydrogen reaction catalyst film 44 is prevented, and the area is increased. It is possible to arrange the hydrogen reaction catalyst film 44. From this, the heat generated by the gas can be sufficiently transmitted to the surface of the quartz substrate 41, and the temperature rise of the surface of the quartz substrate 41 does not take time, and a hydrogen sensor excellent in responsiveness and sensitivity can be provided.
In addition, since the hydrogen sensor of the embodiment can detect hydrogen by detecting a frequency signal caused by the temperature of the quartz substrate 41, gas can be detected with higher sensitivity than the conventional method of detecting gas by detecting voltage. Can be detected.
The conventional transversal surface acoustic wave device has a frequency characteristic with a certain bandwidth with respect to the center frequency, whereas the resonator type surface acoustic wave device has a narrow center frequency bandwidth, Has a high Q value. From this, it is possible to reduce noise and improve the sensitivity by using a resonator type surface acoustic wave device rather than using a transversal type surface acoustic wave device.
By using platinum or palladium as the hydrogen reaction catalyst film 44, a gas reaction catalyst film having high hydrogen selectivity and hardly responding to other combustible gases not containing hydrogen can be formed.
(Modification)

Next, a modification of the hydrogen sensor in the second embodiment will be described.
This hydrogen sensor differs from the second embodiment in the configuration of the hydrogen reaction detection element, and the configuration of the hydrogen reaction detection element will be described in detail. In addition, about the structure similar to 2nd Embodiment, the same code | symbol is attached | subjected and description is abbreviate | omitted.

FIG. 8 is a configuration diagram showing the configuration of the hydrogen reaction detecting element according to the modified hydrogen sensor.
FIG. 8A is a schematic plan view, and FIG. 8B is a schematic partial cross-sectional view taken along the line CC in FIG.
The hydrogen reaction detecting element 5 includes an IDT electrode 43, a reflector 45, and a hydrogen reaction catalyst film 54 on the surface of a quartz crystal substrate 41 that is a piezoelectric material. As described above, the hydrogen sensor of this modification uses a 1-port resonator type surface acoustic wave device. The IDT electrode 43 has an I
An insulating film 58 made of SiO ₂ is formed so as to cover the DT electrode 43, and platinum (P
A hydrogen reaction catalyst film 54 made of t) or palladium (Pd) is formed.

Further, the cross finger electrode 42 a of the IDT electrode 43 is connected to the input side terminal 46, and the cross finger electrode 4.
2 b is connected to the output side terminal 47. The input side terminal 46 and the output side terminal 47 are
It is connected to an oscillation circuit (not shown) that excites the IDT electrode 43. As described above, the hydrogen sensor is configured by including the hydrogen reaction detecting element 5 using the resonator type surface acoustic wave device and the oscillation circuit.

As described above, since the hydrogen sensor of the present modification is provided with the hydrogen reaction catalyst film 54 via the insulating film 58 as in the second embodiment, the IDT electrode 43 and the hydrogen reaction catalyst film 54 are short-circuited. It is possible to arrange the hydrogen reaction catalyst film 54 in a wider area. From this, the heat generated by the gas can be sufficiently transmitted to the surface of the quartz substrate 41, and the temperature rise of the surface of the quartz substrate 41 does not take time, and a hydrogen sensor excellent in responsiveness and sensitivity can be provided.

Next, an example of a hydrogen concentration detection system using the hydrogen sensor of this embodiment will be described.
FIG. 9 is a schematic configuration diagram showing a configuration of a hydrogen concentration detection system using a hydrogen sensor.
The hydrogen sensor 71 includes a hydrogen reaction detection element 3 and an oscillation circuit 70. A feedback resistor 6 and an inverter 7 are connected in parallel to the hydrogen reaction detecting element 3, a gate capacitor 8 is connected to the gate side, and a drain capacitor 9 is connected to the drain side to oscillate the IDT electrode of the hydrogen reaction detecting element 3. A circuit 70 is configured.
The oscillation circuit 70 of the hydrogen sensor 71 is connected to a rectifier 72, and the rectifier 72 is connected to a frequency counter 73. Further, the frequency counter 73 is connected to the arithmetic unit 74, and the arithmetic unit 7
4 outputs a desired signal.

In the hydrogen concentration detection system having such a configuration, the oscillation circuit 70 outputs the frequency signal of the hydrogen reaction detection element 3 to the rectifier 72. The frequency signal is rectified in the rectifier 72 and output to the frequency counter 73. The frequency is measured by the frequency counter 73 and the calculator 7
4 is output. The computing unit 74, for example, computes the hydrogen concentration from the measured frequency and the reference frequency, and generates and outputs a hydrogen concentration value, an alarm signal when the standard value is exceeded, a solenoid valve control signal, and the like. To do.
Thus, in the hydrogen concentration detection system, the hydrogen concentration can be detected using the frequency signal output from the hydrogen sensor 71, and a control signal or the like is output to various devices based on the obtained hydrogen concentration. be able to.

A part of the above circuit can be integrated into an IC, and the frequency counter 73 and the arithmetic unit 74 can be configured as a one-chip microcomputer. Further, the signal processing can be performed using either analog processing or digital processing.

Note that the hydrogen sensor of the present embodiment may include temperature control means for controlling the temperatures of the quartz substrate and the hydrogen reaction catalyst film. For example, when a hydrogen reaction catalyst film is provided in a portion other than the IDT electrode of the quartz substrate and the hydrogen reaction catalyst film is energized, heat is generated and the temperature of the quartz substrate rises. From this, the temperature of the quartz crystal substrate can be controlled at a temperature higher than the gas atmosphere temperature by appropriately controlling the magnitude of the current and the energization time.
In this way, it is possible to eliminate the influence of water generated when the gas medium temperature changes or the catalyst medium generates heat when it reacts with the gas, and a gas sensor with excellent sensitivity can be provided.

In the hydrogen sensor of this embodiment, a quartz substrate is used as the piezoelectric substrate, but the present invention can also be implemented by configuring the piezoelectric substrate using a piezoelectric material such as lithium tantalate or lithium niobate.
Moreover, although platinum (Pt) and palladium (Pd) are used as the hydrogen reaction catalyst film, the hydrogen storage alloy also has the property of absorbing hydrogen and generating heat. Instead of platinum and palladium, hydrogen is used by using a hydrogen storage alloy. It is also possible to configure a sensor.
Furthermore, since the gas sensor of the present embodiment uses heat generated when water is formed by the oxidation action of hydrogen, not only hydrogen (H ₂ ) is detected, but also silane gas (SiH ₄ containing hydrogen atoms). ), Diborane gas (B ₂ H ₄ ), phosphine (PH ₄ ), ethane gas (CH ₄ )
It can be used for detecting gases such as ethylene gas (C ₂ H ₆ ).

The structure of the hydrogen reaction detection element which concerns on the hydrogen sensor of 1st Embodiment is shown, (a) is a schematic plan view, (b) is a schematic cross section along the AA disconnection of the same figure (a). Explanatory drawing which shows the cut angle of a quartz substrate. The graph which shows the relationship between cut angle (theta) and a temperature sensitivity coefficient. The graph which shows the frequency temperature characteristic of a hydrogen reaction detection element. The graph which shows the correlation of the frequency shift of a hydrogen reaction detection element, and hydrogen concentration. The schematic plan view which shows the structure of the modification which changed arrangement | positioning of the hydrogen reaction catalyst film | membrane in a hydrogen reaction detection element. The structure of the hydrogen reaction detection element which concerns on the hydrogen sensor of 2nd Embodiment is shown, (a) is a schematic plan view, (b) is a schematic cross section along the BB broken line of the same figure (a). The structure of the hydrogen reaction detection element of the modification in the hydrogen sensor of 2nd Embodiment is shown, (a) is a schematic plan view, (b) is a schematic cross section along the CC broken line of the same figure (a). Explanatory drawing which shows an example of the hydrogen concentration detection system using a hydrogen sensor.

Explanation of symbols

3, 4, 5 ... hydrogen reaction detecting element, 11 ... quartz substrate as piezoelectric substrate, 12a, 12b ...
Cross finger electrode, 13 ... IDT electrode, 14 ... Hydrogen reaction catalyst film as gas reaction catalyst film, 15 ...
Reflector, 16 ... input side terminal, 17 ... output side terminal, 41 ... crystal substrate as piezoelectric substrate, 42
a, 42b ... crossed finger electrode, 43 ... IDT electrode, 44 ... hydrogen reaction catalyst film as gas reaction catalyst film, 45 ... reflector, 46 ... input side terminal, 47 ... output side terminal, 48 ... insulating film, 54 ... Hydrogen reaction catalyst film as gas reaction catalyst film, 58 ... insulating film, 70 ... oscillation circuit, 71 ... hydrogen sensor as gas sensor, 72 ... rectifier, 73 ... frequency counter, 74 ... calculator.

Claims (7)

A piezoelectric substrate;
An IDT electrode provided on the piezoelectric substrate;
A gas reaction catalyst film provided on the piezoelectric substrate and generating heat in response to a gas;
An oscillation circuit for exciting the IDT electrode,
The gas reaction catalyst film is provided on a propagation path of a surface acoustic wave excited by the IDT electrode ;
A quartz substrate is used for the piezoelectric substrate,
When the electric axis of the crystal of crystal is the X axis, the mechanical axis is the Y axis, and the optical axis is the Z axis,
The quartz substrate is rotated by θ ° with a plane including the X axis and the Z axis as a rotation axis, and a direction rotating from the + Z axis of the crystal to the −Y axis of the crystal as a positive direction. The main surface is a plane that can be
The IDT electrode is provided so that the surface acoustic wave propagates in parallel to the X axis,
The gas sensor characterized in that the θ satisfies 33 or more and 53 or less . The gas sensor according to claim 1,
The gas sensor according to claim 1, wherein the gas reaction catalyst film is provided on the propagation path between the interdigitated electrodes in the IDT electrode. The gas sensor according to claim 1,
A gas sensor comprising an insulating film covering the IDT electrode, wherein the gas reaction catalyst film is provided on the insulating film and on the propagation path. The gas sensor according to any one of claims 1 to 3,
A gas sensor, wherein platinum or palladium is used for the gas reaction catalyst film. The gas sensor according to any one of claims 1 to 4 ,
The gas sensor according to claim 1, wherein the surface acoustic wave excited from the IDT electrode is an STW wave. The gas sensor according to any one of claims 1 to 4 ,
The gas sensor according to claim 1, wherein the surface acoustic wave excited from the IDT electrode is a Rayleigh wave. A piezoelectric substrate;
An IDT electrode provided on the piezoelectric substrate;
A gas reaction catalyst film provided on the piezoelectric substrate and generating heat in response to a gas containing hydrogen ;
An oscillation circuit for exciting the IDT electrode,
The gas reaction catalyst film is provided on a propagation path of a surface acoustic wave excited by the IDT electrode ;
A quartz substrate is used for the piezoelectric substrate,
When the electric axis of the crystal of crystal is the X axis, the mechanical axis is the Y axis, and the optical axis is the Z axis,
The quartz substrate is rotated by θ ° with a plane including the X axis and the Z axis as a rotation axis, and a direction rotating from the + Z axis of the crystal to the −Y axis of the crystal as a positive direction. The main surface is a plane that can be
The IDT electrode is provided so that the surface acoustic wave propagates in parallel to the X axis,
The hydrogen concentration detection system, wherein θ satisfies 33 or more and 53 or less .

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