JPH07318542A - Ultrasonic gas sensor - Google Patents

Abstract

PURPOSE:To obtain an ultrasonic gas sensor comprising a thin piezoelectric plate equipped with an interdigital electrode provided on one surface of a non- piezoelectric substrate and a sensitive membrane provided on the other surface thereof and measuring a gas, adsorbed by the sensitive membrane, qualitatively or quantitatively. CONSTITUTION:When an electric signal is inputted to an interdigital electrode T, only such electric signals as having a central frequency corresponding to the interdigital electrode T and the frequency in the vicinity thereof are converted into surface acoustic waves which propagate to a thin piezoelectric ceramic plate 1. The surface acoustic wave leaks, as a bulk wave, into an acrylic plate 2 and reflected on the interface between the acrylic plate 2 and the air before being outputted, in the form of an electric signal, from an interdigital electrode R. When an sensitive membrane adsorbs a target gas, electric output signal from the interdigital electrode R is varied. Consequently, the gas detection sensitivity of the liquid is enhanced while reducing the size and weight of the apparatus.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION The present invention provides a piezoelectric thin plate provided with interdigital electrodes on one plate surface F1 of a non-piezoelectric substrate, and a sensitive film on the other plate surface F2 of the non-piezoelectric substrate. The present invention relates to an ultrasonic gas sensor for qualitatively or quantitatively measuring a gas adsorbed on a sensitive film.

2. Description of the Related Art As a conventional odor evaluation method in the fields of food, cosmetics, environmental tests, etc., a sensory test method utilizing human sense of smell and a method by instrumental analysis are mainly mentioned. The sensory test method has a problem of individual difference in sensitivity and a problem that the discriminating ability is changed due to physiological and psychological influences, and thus lacks objectivity and lacks reliability. Examples of the method based on instrumental analysis include a gas chromatographic method and an absorptiometric method. However, these methods have a problem that the measurement result and the sensory quantity do not always match. Other methods include an evaluation method using a crystal oscillator sensor. This utilizes the fact that the frequency changes when gas molecules are adsorbed on a crystal oscillator coated with a gas sensitive film. Since this crystal oscillator sensor has a frequency unique to the crystal oscillator, manufacturing technology, mass production technology,
It has problems in durability, sensitivity, reproducibility, etc. In this way, the conventional odor evaluation methods have problems in lack of objectivity and lack of reliability, inconsistency between measurement results and sensory quantities, sensitivity, reproducibility, manufacturing technology and mass production technology. Was there.

It is an object of the present invention to be compact and lightweight, to have high detection sensitivity, excellent reproducibility, excellent measurement accuracy and high-speed response, easy measurement method, low power consumption drive, and processing. It is to provide an ultrasonic gas sensor excellent in productivity and mass productivity.

An ultrasonic gas sensor according to claim 1 is characterized in that a piezoelectric thin plate having at least two sets of interdigital electrodes T and R is provided on one plate surface F1 of a non-piezoelectric substrate, An ultrasonic gas sensor having a sensitive film on the other plate surface F2 of the piezoelectric substrate, and a gas sensing means for qualifying or quantifying the gas adsorbed on the sensitive film, wherein the thickness of the piezoelectric thin plate is the blind. Is less than or equal to the electrode period length of the electrode T and R, the gas sensing means inputs an electric signal to the interdigital electrode T to excite a surface acoustic wave at the interface between the non-piezoelectric substrate and the piezoelectric thin plate, Means for mode-converting the surface acoustic wave into a bulk wave in the non-piezoelectric substrate, reflecting the bulk wave by the plate surface F2, and outputting the reflected bulk wave as an electric signal from the interdigital transducer R. Including The electrode period lengths of the interdigital transducers T and R are substantially equal to the wavelength of the surface acoustic wave, and the phase velocity of the surface acoustic wave excited at the interface and mode-converted as a bulk wave in the non-piezoelectric substrate is It is characterized in that it is higher than the transverse wave velocity in the non-piezoelectric substrate alone. The ultrasonic gas sensor according to claim 2, wherein the output end of the interdigital transducer R is the interdigital transducer T via an amplifier.
An oscillator using a propagation path of a surface acoustic wave in the non-piezoelectric substrate between the interdigital transducer T and the interdigital transducer R as a delay element. The signal loop is formed by the interdigital transducer T
And the surface acoustic wave propagation path and the interdigital transducer R
And the amplifier. Claim 3
The ultrasonic gas sensor described in [1] is characterized in that the interdigital transducers T and R have an arc shape. The ultrasonic gas sensor according to claim 4, wherein the non-piezoelectric substrate is made of an acrylic plate, the piezoelectric thin plate is made of piezoelectric ceramic, and the polarization axis of the piezoelectric ceramic has a plate surface having interdigital electrodes in the piezoelectric ceramic. And is vertical. Ultrasonic gas sensor according to claim 5, wherein the non-piezoelectric substrate is made of an acrylic plate, and the piezoelectric thin plate LiNbO ₃
It is characterized by being made of another single crystal. An ultrasonic gas sensor according to a sixth aspect of the present invention is characterized in that the sensitive membrane is formed of a phospholipid membrane, a cellulose membrane or another polymer thin film.

In the ultrasonic gas sensor of the present invention, the piezoelectric thin plate provided with the interdigital electrode is provided on one plate surface F1 of the non-piezoelectric substrate,
It has a simple structure in which a sensitive film is provided on the other plate surface F2 of the non-piezoelectric substrate. By adopting a structure in which an electric signal is input to the interdigital transducer T, it is possible to excite a surface acoustic wave having a velocity V _{S on} the piezoelectric thin plate. Moreover, by adopting the structure in which the piezoelectric thin plate is fixed on the plate surface F1 of the non-piezoelectric substrate, it is possible to perform mode conversion in such a manner that this surface acoustic wave leaks to the non-piezoelectric substrate as a bulk wave. The phase velocity of the surface acoustic wave leaked at this time is higher than the velocity V _AT of the transverse wave in the single non-piezoelectric substrate. In other words, satisfies the relationship of V _S of the surface acoustic wave excited on the piezoelectric sheet is greater than V _{A T} is leaked to the non-piezoelectric substrate. Among surface acoustic waves excited on the piezoelectric thin plate, those having V _S smaller than V _AT are not leaked to the non-piezoelectric substrate. In this way, among the surface acoustic waves excited on the piezoelectric thin plate, the wave whose phase velocity is larger than the velocity V _AT of the transverse wave in the single non-piezoelectric substrate and smaller than the velocity V _{AL of the} longitudinal wave is almost the velocity V _AT. It is efficiently converted into waves having the same velocity and leaks to the non-piezoelectric substrate. Further, among the surface acoustic waves excited on the piezoelectric thin plate, a wave having a phase velocity larger than the velocity V _AL of the longitudinal wave in the single non-piezoelectric substrate is effective for a wave having a velocity almost equal to the velocity V _AT or the velocity V _AL. Well converted and leaked to non-piezoelectric substrate. By adopting a structure in which the bulk wave leaked to the non-piezoelectric substrate is reflected by the plate surface F2, the reflected bulk wave is converted into a surface acoustic wave again at the interface between the non-piezoelectric substrate and the piezoelectric thin plate, and the piezoelectric thin plate. And can be output as an electric signal from the interdigital transducer R. When the ultrasonic gas sensor of the present invention is used, the plate surface F of the non-piezoelectric substrate
Since the sensitive film fixed to 2 has gas sensitivity to a specific gas, when the sensitive film adsorbs a target gas,
The propagation velocity of ultrasonic waves changes. Therefore, the interdigital transducer R
A phase difference in propagation delay is generated in the electric signal output at. That is, the fact that the sensitive film has adsorbed the gas and the amount of the adsorbed gas can be understood as the phase difference between the electric signals output to the interdigital transducer R. As the sensitive membrane, for example, a phospholipid membrane or a cellulose membrane is used. In this way, the ultrasonic gas sensor of the present invention can not only qualify the gas but also measure the adsorption amount of the gas. Of course, by replacing the liquid, it is possible to perform qualitative and quantitative determination for another type of gas. By using the arc-shaped interdigital electrodes as the interdigital electrodes T and R, firstly, the surface acoustic wave excited by the piezoelectric thin plate by inputting an electric signal to the interdigital electrode T is applied to the plate surface F2 of the non-piezoelectric substrate. When the light is reflected by, the reflection points on the plate surface F2 can be concentrated in the same spot in a dot shape. That is, the bulk wave leaked to the non-piezoelectric substrate can be concentrated and reflected at the focal point on the plate surface F2 in such a manner that the lens just focuses. Secondly, the reflected wave reflected in this way can be efficiently outputted again from the interdigital transducer R as an electric signal. That is, by providing the focal point on the plate surface F2, it is possible to improve the gas detection sensitivity sensed by the sensitive film. A structure is adopted in which the thickness of the piezoelectric thin plate is set to be equal to or less than the electrode period length of the interdigital electrodes T and R, and the electrode period lengths of the interdigital electrodes T and R are made substantially equal to the wavelength of the surface acoustic wave in the first or higher order mode. By doing
Not only can the degree to which the electrical energy applied to the interdigital transducer T is converted into surface acoustic waves can be increased, but also reflection or the like caused by mismatch of acoustic impedance at the interface between the piezoelectric thin plate and the non-piezoelectric substrate, etc. Can be suppressed. Therefore, effective leakage of surface acoustic waves to the non-piezoelectric substrate can be promoted. The smaller the ratio of the electrode period length of the interdigital electrodes T and R, that is, the thickness d of the piezoelectric thin plate to the wavelength λ of the surface acoustic wave (d / λ), the greater the effect. The ultrasonic gas sensor of the present invention overcomes the weakness associated with reducing the thickness d of the piezoelectric thin plate by fixing the piezoelectric thin plate to the non-piezoelectric substrate. That is, the non-piezoelectric substrate plays an important role in overcoming the weakness of the piezoelectric thin plate. By adopting a structure in which the output end of the interdigital transducer R is connected to the input end of the interdigital transducer T via an amplifier, the surface acoustic wave of the non-piezoelectric substrate from the interdigital transducer T to the interdigital electrode R is generated. It is possible to configure an oscillator using the propagation path as a delay element. The signal loop of the oscillator is composed of the interdigital transducer T, the propagation path of the surface acoustic wave, the interdigital transducer R, and the amplifier. In this way, the circuit configuration is simplified, the size and weight of the device are promoted, and driving with low voltage and low power consumption is possible. By adopting an acrylic plate as the non-piezoelectric substrate, a piezoelectric ceramic as the piezoelectric thin plate, and a structure in which the direction of the polarization axis of the piezoelectric ceramic is perpendicular to the plate surface of the piezoelectric ceramic having the interdigital electrodes. The surface acoustic wave can be efficiently excited in the piezoelectric thin plate, and the surface acoustic wave can be efficiently leaked to the non-piezoelectric substrate. By adopting an acrylic plate as the non-piezoelectric substrate and using LiNbO ₃ or other single crystal as the piezoelectric thin plate, it is possible to efficiently excite surface acoustic waves on the piezoelectric thin plate, and further to transfer the surface acoustic waves to the non-piezoelectric substrate. Can be leaked efficiently. By adopting PVDF or other piezoelectric polymer film as the piezoelectric thin plate, it is possible to efficiently excite the surface acoustic wave on the piezoelectric thin plate in a form capable of supporting higher frequencies, and further to efficiently transmit the surface acoustic wave to the non-piezoelectric substrate. Can leak well.

1 is a sectional view showing an embodiment of an ultrasonic gas sensor of the present invention. In this embodiment, the interdigital electrodes T, R,
It comprises a piezoelectric ceramic thin plate 1, an acrylic plate 2, a sensitive film 3 and an amplifier 4. The interdigital electrodes T and R have an arc shape,
It is made of an aluminum thin film and is provided on the piezoelectric ceramic thin plate 1. Piezoelectric thin plate 1 has a diameter of 15 mm and a thickness of 200 μ
It is made of a disc-shaped TDK 101A material (product name) of m, and is fixed to one plate surface of the acrylic plate 2 by an epoxy resin. The acrylic plate 2 has a diameter of 16 mm and a thickness (T
_A ) It consists of a 1 mm disc. The sensitive film 3 is made of a gas-sensitive phospholipid film, and is provided on the other plate surface of the acrylic plate 2. FIG. 2 is a partial perspective view of the ultrasonic gas sensor of FIG. However, in FIG. 2, the interdigital electrodes T, R,
A piezoelectric ceramic sheet 1 and an acrylic sheet 2 are shown. The interdigital electrodes T and R have the same shape and the electrode period length is 2
At P, it has 10 pairs of electrode fingers, and the opening angle (θ _A ) is 45 °. The inside of the ellipse in FIG. 2 is a partially enlarged view of the interdigital electrode T, and the same applies to the interdigital electrode R. The electrode separation distance (L) between the interdigital electrodes T and R is 10 mm. FIG. 3 is a diagram showing a propagation mode until the surface acoustic wave propagating through the piezoelectric ceramic thin plate 1 is propagated through the acrylic plate 2. However, in FIG. 3, the interdigital electrode T, the piezoelectric ceramic thin plate 1 and the acrylic plate 2 are shown. When an electric signal is input to the interdigital transducer T, only the electric signal having a center frequency corresponding to the interdigital electrode T and a frequency in the vicinity thereof is converted into a surface acoustic wave to move the piezoelectric ceramic thin plate 1 at a velocity V. Propagate in _S. If the velocity V _{S of the} surface acoustic wave is larger than the velocity V _AT of the transverse wave in the acrylic plate 2 alone and smaller than the velocity V _{AL of the} longitudinal wave, the surface acoustic wave has a velocity V _AT.
Is converted into a transverse wave and leaked to the acrylic plate 2. The leak angle θ _AT when the bulk wave leaks from the piezoelectric ceramic thin plate 1 correlates with the ratio of V _AT and V _S (V _AT / V _S ). In addition, the velocity V is changed according to the frequency of the electric signal input to the interdigital transducer T.
_{Since S} changes, it becomes possible to change the leakage angle θ _AT by changing the frequency of the electric signal. FIG. 4 is a diagram showing a propagation form of ultrasonic waves when the velocity V _{S of} surface acoustic waves propagating to the piezoelectric ceramic thin plate 1 is higher than the velocity V _AL of longitudinal waves in the acrylic plate 2 alone. In this case, the surface acoustic wave is converted into a transverse wave having a velocity V _AT and a longitudinal wave having a velocity V _AL and leaked to the acrylic plate 2. Leakage angle θ when transverse wave is leaked from the piezoelectric ceramic thin plate 1 as bulk wave
_AT correlates with the ratio of V _AT and V _S (V _AT / V _S ), and the leakage angle θ _{AL in} the case of a longitudinal wave correlates with the ratio of V _AL and V _S (V _AL / V _S ). Since the speed V _S changes according to the frequency of the electric signal input to the interdigital transducer T, the speed V _S is changed by changing the frequency of the electric signal so that the speed V _S can be changed under the condition (V _AT <V _S <V _AL ), or the condition (V _AL <V _S ) as shown in FIG. 4 can be set. Further, it becomes possible to change the respective leak angles θ _AT and θ _AL . FIG. 5 is a diagram showing the propagation form of ultrasonic waves in the vicinity of the interface between the acrylic plate 2 and air. However, the speed V _S is when the condition of FIG. 3 (V _AT <V _S <V _AL ) is satisfied. Further, the sensitive film 3 is not shown in FIG. When the bulk transverse wave propagating through the acrylic plate 2 reaches the interface, the transverse wave reflectance R _T and the reflection angle θ showing the reflection angle θ _AT.
Two components of the longitudinal wave reflectance R _L indicating _AL are generated. In this way, the bulk transverse wave propagating through the acrylic plate 2 is partially reflected at the interface as the transverse wave reflectance R _T and the longitudinal wave reflectance R _L. By the way, since it is possible to change the leakage angles θ _AT and θ _AL by changing the frequency of the electric signal input to the interdigital transducer T, the bulk wave reflected at the interface is again regenerated from the interdigital electrode R. It becomes possible to output as an electric signal. At this time, by forming the interdigital electrodes T and R in an arc shape, it is possible to concentrate the reflection points on the interface at the same position so that the lens is in focus. Therefore, the reflected bulk wave can be efficiently output from the interdigital transducer R as an electric signal. In the ultrasonic gas sensor of FIG. 1, when an electric signal is input to the interdigital electrode T, only the electric signal of the center frequency corresponding to the interdigital electrode T and the frequency in the vicinity of the frequency of the electric signal is converted into a surface acoustic wave. Propagating through the piezoelectric ceramic thin plate 1 at a velocity V _S. The velocity V _{S of the} surface acoustic wave is the velocity V of the transverse wave in the acrylic plate 2 alone.
_If it is larger than _{AT and} smaller than the longitudinal wave velocity V _AL ,
This surface acoustic wave is converted into a transverse wave having a velocity V _AT and leaked to the acrylic plate 2. When the velocity V _{S of the} surface acoustic wave is larger than the velocity V _AL of the longitudinal wave in the acrylic plate 2 alone, the surface acoustic wave is converted into the transverse wave of the velocity V _AT and the longitudinal wave of the velocity V _AL to be acryl. It is leaked to board 2. When the velocity V _{S of the} surface acoustic wave is smaller than the velocity V _AT of the transverse wave in the acrylic plate 2 alone, this surface acoustic wave is output as an electric signal from the interdigital transducer R, and this electric signal is output by the amplifier 4. It is amplified and input again from the interdigital transducer T. In this way, it is possible to configure an oscillator using the propagation path of the surface acoustic wave in the acrylic plate 2 between the interdigital electrode T and the interdigital electrode R as a delay element. The signal loop of this oscillator consists of the interdigital transducer T, the propagation path, the interdigital transducer R and the amplifier 4. By configuring such an oscillator, the circuit configuration is simplified, the size and weight of the device are promoted, and driving with low voltage and low power consumption becomes possible. When the sensitive gas 3 adsorbs the target gas when the ultrasonic gas sensor of FIG. 1 is driven, the propagation velocity of the bulk wave changes, so that a phase difference in propagation delay occurs in the electric signal output from the interdigital transducer R. . That is, the fact that the sensitive film 3 has adsorbed the gas and the amount of adsorption thereof are determined by the interdigital electrode R
It can be understood as the phase difference of the electric signal output to. FIG. 6 is a characteristic diagram showing a velocity dispersion curve of a surface acoustic wave propagating in a layered medium composed of a piezoelectric ceramic thin plate 1 and an acrylic plate 2 in the ultrasonic gas sensor of FIG. 1. The frequency f of the surface acoustic wave and the piezoelectric ceramic thin plate 2 are shown in FIG. It is a figure which shows the phase velocity of each mode with respect to the product with thickness d of. However, in the piezoelectric ceramic thin plate 1, both the plate surface of the piezoelectric ceramic thin plate 1 that contacts the acrylic plate 2 (the acrylic side plate surface) and the other plate surface of the piezoelectric ceramic thin plate 1 that contacts the air (air side plate surface) are electrical. The one in the open state was used. In the figure, "open" indicates an open state. In addition, the mark ○ indicates the measured value. Surface acoustic waves have multiple modes. fd value is approximately 0.4
The velocity of the wave in the A ₀ mode when it is less than or equal to MHz · mm is smaller than the transverse wave velocity V _AT of the acrylic plate 2. Such a wave is a surface wave in which the energy of the wave locally propagates near the surface and is not leaked to the acrylic plate 2. Speed is V
There is an imaginary component of velocity in waves of A ₀ mode and other modes larger than _AT , and a part of the energy of the waves leaks into the acrylic plate 2 as a bulk wave. Among the surface acoustic waves of each mode, the waves in the region where the velocity is higher than V _{AT and} lower than V _AL can be effectively leaked into the acrylic plate 2 as bulk transverse waves. Waves in the region where the velocity is higher than V _AL are leaked into the acrylic plate 2 as bulk longitudinal waves and bulk transverse waves. FIG. 7 is a characteristic diagram showing the relationship between the mode conversion efficiency C and the fd value in the ultrasonic gas sensor of FIG. However, the piezoelectric ceramic thin plate 1 used was one in which both the acrylic side plate surface and the other air side plate surface of the piezoelectric ceramic thin plate 1 were electrically open. It can be seen that surface acoustic waves propagating to the piezoelectric ceramic thin plate 1 are efficiently leaked to the acrylic plate 2 as bulk waves in any mode except the A ₀ mode. FIG. 8 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference of the piezoelectric ceramic thin plate 1 under two different electrical boundary conditions and the fd value. However, as the piezoelectric ceramic thin plate 1, each interdigital electrode (IDT) is provided on the air side plate surface of the piezoelectric ceramic thin plate 1, and the acrylic side plate surface is electrically opened. K ^{2 in} A ₀ mode
Is a substantially constant value (k near fd = 2.8 MHz · mm
² = 4%). F ₀ = 1.4MH in S ₀ mode
There is one peak (k ² = 17.5%) near z · mm. This peak is from piezoelectric ceramic thin plate 1 to acrylic plate 2
This is considered to correspond to the surface wave leaked to. A ₁
The A ₂ mode and the A ₂ mode also show good values efficiently. In this way, the surface acoustic wave can be efficiently leaked from the piezoelectric ceramic thin plate 1 to the acrylic plate 2 in any mode except the A ₀ mode, and the most efficient efficiency for the acrylic plate 2 can be obtained by adjusting the fd value. Good leakage can be achieved. Further, the structure in which each interdigital electrode is provided on the air side plate surface of the piezoelectric ceramic thin plate 1 has an advantage that it is easy to manufacture. FIG. 9 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference of the piezoelectric ceramic thin plate 1 under two different electrical boundary conditions and the fd value. However, as the piezoelectric ceramic thin plate 1, one in which each interdigital electrode is provided on the air side plate surface of the piezoelectric ceramic thin plate 1 and the acrylic side plate surface is electrically short-circuited is used. In this embodiment, the plate surface of the piezoelectric ceramic thin plate 1 is coated with a metal thin film to electrically short the plate surface. In the figure, "short" indicates a short circuit state. In FIG. 9 as in FIG. 8, surface acoustic waves can be efficiently leaked from the piezoelectric ceramic thin plate 1 to the acrylic plate 2 in any mode except the A ₀ mode, and the acrylic plate 2 can be adjusted by adjusting the fd value. The most efficient leakage to the can be realized. Further, the structure in which each interdigital electrode is provided on the air side plate surface of the piezoelectric ceramic thin plate 1 has an advantage that it is easy to manufacture.

According to the ultrasonic gas sensor of the present invention, by adopting a structure in which an electric signal is input to the interdigital transducer T, it is possible to excite a surface acoustic wave having a velocity V _{S on} the piezoelectric thin plate. In addition, the piezoelectric thin plate is a plate surface F of the non-piezoelectric substrate
By adopting the structure in which the surface acoustic wave is fixed on the surface 1, the surface acoustic wave can be mode-converted in the form of leaking to the non-piezoelectric substrate as a bulk wave. Of the surface acoustic waves excited on the piezoelectric thin plate, the wave whose phase velocity is higher than the velocity V _AT of the transverse wave in the simple substance of the non-piezoelectric substrate and smaller than the velocity V _{AL of the} longitudinal wave has a velocity almost equal to the velocity V _AT. Are efficiently converted to and leaked to the non-piezoelectric substrate. Further, of the surface acoustic waves excited on the piezoelectric thin plate, a wave having a phase velocity larger than the longitudinal wave velocity V _AL in the single non-piezoelectric substrate is the velocity V _AT or the velocity V.
_It is efficiently converted into a wave having a velocity almost equal to _AL and leaks to the non-piezoelectric substrate. By adopting a structure in which the bulk wave leaked to the non-piezoelectric substrate is reflected by the plate surface F2, the reflected bulk wave is converted into a surface acoustic wave of velocity V _S again at the interface between the non-piezoelectric substrate and the piezoelectric thin plate. Then, it propagates to the piezoelectric thin plate and can be output as an electric signal from the interdigital transducer R. By adopting the structure in which the sensitive film is provided on the plate surface F2 of the non-piezoelectric substrate, the propagation speed of the ultrasonic wave changes when the sensitive film adsorbs the target gas, so that the electric signal output from the interdigital transducer R. Propagation delay phase difference occurs in. That is, the fact that the sensitive film has adsorbed the gas and the amount of the adsorbed gas can be regarded as the phase difference between the electric signals output to the interdigital transducer R. In this way, the ultrasonic gas sensor of the present invention enables qualitative and quantitative determination of gas. By using arcuate interdigital electrodes as the interdigital electrodes T and R, when the bulk wave of the non-piezoelectric substrate is reflected by the plate surface F2, the reflection point on the plate surface F2 is exactly the same as the lens focuses. It can be concentrated in a spot shape. Moreover, the reflected wave reflected in this manner can be efficiently output again from the interdigital transducer R as an electric signal. Therefore, the detection sensitivity of the gas sensed by the sensitive film can be improved.
The thickness of the piezoelectric thin plate is equal to or less than the electrode period length of the interdigital transducer,
By adopting a structure in which the electrode period length of the interdigital transducer is approximately equal to the wavelength of the surface acoustic wave of the first or higher order mode, the electric energy applied to the interdigital electrode T is converted into the surface acoustic wave. Not only can the degree be increased, but reflection and the like caused by acoustic impedance mismatch at the interface between the piezoelectric thin plate and the non-piezoelectric substrate can be suppressed. Therefore, effective leakage of surface acoustic waves to the non-piezoelectric substrate can be promoted. The smaller the electrode period length of the interdigital transducer, that is, the ratio (d / λ) of the thickness d of the piezoelectric thin plate to the wavelength λ of the surface acoustic wave, the greater the effect. By adopting a structure in which the output end of the interdigital transducer R is connected to the input end of the interdigital transducer T via an amplifier, the surface acoustic wave of the non-piezoelectric substrate from the interdigital transducer T to the interdigital electrode R is generated. It is possible to configure an oscillator using the propagation path as a delay element. The signal loop of the oscillator is composed of the interdigital transducer T, the propagation path of the surface acoustic wave, the interdigital transducer R, and the amplifier. In this way
The circuit configuration is simplified, the size and weight of the device are promoted, and driving with low voltage and low power consumption is possible. By adopting an acrylic plate as the non-piezoelectric substrate, a piezoelectric ceramic as the piezoelectric thin plate, and a structure in which the direction of the polarization axis of the piezoelectric ceramic is perpendicular to the plate surface of the piezoelectric ceramic having the interdigital electrodes. The surface acoustic wave can be efficiently excited in the piezoelectric thin plate, and the surface acoustic wave can be efficiently leaked to the non-piezoelectric substrate. Further, by adopting a single crystal such as LiNbO ₃ as the piezoelectric thin plate, it is possible to efficiently excite the surface acoustic wave on the piezoelectric thin plate, and further to efficiently leak the surface acoustic wave to the non-piezoelectric substrate. By adopting PVDF or other piezoelectric polymer film as the piezoelectric thin plate, it is possible to efficiently excite the surface acoustic wave on the piezoelectric thin plate in a form capable of supporting higher frequencies, and further to efficiently transmit the surface acoustic wave to the non-piezoelectric substrate. Can leak well.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of an ultrasonic gas sensor of the present invention.

FIG. 2 is a partial perspective view of the ultrasonic gas sensor of FIG.

FIG. 3 is a diagram showing a propagation form of a surface acoustic wave propagating in a piezoelectric ceramic thin plate 1 until being propagated in an acrylic plate 2.

FIG. 4 is a velocity V of a surface acoustic wave propagating on the piezoelectric ceramic thin plate 1.
_The figure which shows the propagation form of an ultrasonic wave when _S is larger than the velocity _VAL of the longitudinal wave in the acrylic plate 2 simple substance.

FIG. 5 is a diagram showing a propagation form of ultrasonic waves in the vicinity of an interface between the acrylic plate 2 and air.

6 is a characteristic diagram showing a velocity dispersion curve of a surface acoustic wave propagating through a layered medium composed of a piezoelectric ceramic thin plate 1 and an acrylic plate 2 in the ultrasonic gas sensor of FIG.

7 is a characteristic diagram showing the relationship between the mode conversion efficiency C and the fd value in the ultrasonic gas sensor of FIG.

FIG. 8 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference under two different electrical boundary conditions of the piezoelectric ceramic thin plate 1 and the fd value.

FIG. 9 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference of the piezoelectric ceramic thin plate 1 under two different electrical boundary conditions and the fd value.

[Explanation of symbols]

 1 Piezoelectric ceramic thin plate 2 Acrylic plate 3 Sensitive film 4 Amplifier T Interdigital electrode R Interdigital electrode

Claims (6)

[Claims] 1. At least two sets of interdigital electrodes T and R.
A gas sensor for qualitatively or quantitatively determining the gas adsorbed on the sensitive film by providing a piezoelectric thin plate having the above on one plate surface F1 of the non-piezoelectric substrate, and providing a sensitive film on the other plate surface F2 of the non-piezoelectric substrate. An ultrasonic gas sensor including means, wherein the thickness of the piezoelectric thin plate is equal to or less than the electrode cycle length of the interdigital electrodes T and R, and the gas sensing means inputs an electric signal to the interdigital electrode T. A surface acoustic wave is excited at the interface between the non-piezoelectric substrate and the piezoelectric thin plate, the surface acoustic wave is mode-converted into a bulk wave in the non-piezoelectric substrate, and the bulk wave is converted into the plate surface F.
And a means for outputting the reflected bulk wave as an electric signal from the interdigital transducer R, wherein the electrode period lengths of the interdigital transducers T and R are substantially equal to the wavelength of the surface acoustic wave. An ultrasonic gas sensor, wherein a phase velocity of the surface acoustic wave excited on the interface and mode-converted as a bulk wave in the non-piezoelectric substrate is higher than a velocity of a transverse wave in the non-piezoelectric substrate alone. 2. An output end of the interdigital transducer R is connected to an input end of the interdigital transducer T via an amplifier, and the non-piezoelectric element between the interdigital electrode T and the interdigital electrode R is connected. An oscillator using a propagation path of a surface acoustic wave on a substrate as a delay element is configured, and a signal loop of the oscillator includes the interdigital electrode T, the propagation path of the surface acoustic wave, the interdigital electrode R, and The ultrasonic gas sensor according to claim 1, comprising an amplifier. 3. The ultrasonic gas sensor according to claim 1, wherein the interdigital electrodes T and R have an arc shape. 4. The non-piezoelectric substrate is made of an acrylic plate, the piezoelectric thin plate is made of piezoelectric ceramic, and the direction of the polarization axis of the piezoelectric ceramic is perpendicular to the plate surface of the piezoelectric ceramic having the interdigital electrodes. The ultrasonic gas sensor according to claim 1, 2, or 3. 5. The ultrasonic gas sensor according to claim 1, wherein the non-piezoelectric substrate is made of an acrylic plate and the piezoelectric thin plate is made of LiNbO ₃ or another single crystal. 6. The ultrasonic gas sensor according to claim 1, wherein the sensitive membrane is a phospholipid membrane, a cellulose membrane or another polymer thin film.