JPH1056692A - Ultrasonic wave transducer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To propagate a surface acoustic wave to a non-piezoelectric plate efficiently and to convert the surface acoustic wave propagated to the non- piezoelectric plate into an electric signal efficiently in a short response time. SOLUTION: In the case of using an interdigital electrode 1 for an input electrode, when an electric signal is given to the interdigital electrode 1, a surface acoustic wave is excited in the vicinity of the surface of a piezoelectric ceramic plate 2 on which the interdigital electrode 1 is provided and propagated to a non-piezoelectric plate 3. In the case of using the interdigital electrode 1 for an output electrode, the surface acoustic wave propagated to the non- piezoelectric plate 3 is propagated in the vicinity of the surface of the piezoelectric ceramic plate 2 on which the interdigital electrode 1 is placed and converted into an electric signal and outputted from the interdigital electrode 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic transducer including a piezoelectric plate having at least one set of interdigital electrodes and a non-piezoelectric plate.

2. Description of the Related Art As a conventional method of exciting ultrasonic waves to a non-piezoelectric plate, there are a method of indirectly exciting a wedge-shaped transducer using a bulk wave oscillator, a method of directly exciting a piezoelectric thin-film transducer, and the like. No.
The wedge-shaped transducer is used for non-destructive inspection or the like by ultrasonic waves, but is used only in a relatively low frequency region due to the problem of machining accuracy of the wedge angle. Piezoelectric thin film transducers are a method in which a piezoelectric thin film such as ZnO is vapor-deposited on a substrate, and ultrasonic waves are excited by an interdigital electrode. Since the interdigital electrode exhibits various transmission characteristics, it is used as a high-frequency device. It is limited to belts and has problems in workability and mass productivity. As described above, the conventional method has problems in machining accuracy, workability, mass productivity, and the like, and the use frequency range is also limited. Therefore, an ultrasonic transducer that solves these problems will be described.
Has been filed by the present inventor in Japanese Patent Application No. 4-218335. This ultrasonic transducer has an ultrasonic device comprising a piezoelectric thin plate and an interdigital electrode provided on a non-piezoelectric plate, and can not only excite a surface acoustic wave near the surface of the non-piezoelectric plate, but also The surface acoustic wave excited near the surface of the non-piezoelectric plate can be converted into an electric signal. However, when the surface acoustic wave is excited near the surface of the non-piezoelectric plate, the ratio of the surface acoustic wave leaking into the non-piezoelectric plate is large in this ultrasonic transducer. Not only is there a way to support the non-piezoelectric plate, but also devised to suppress unnecessary signals. Further, the efficiency of converting a surface acoustic wave excited near the surface of the non-piezoelectric plate into an electric signal is poor.

The conventional ultrasonic transducer has problems not only in machining accuracy, workability and mass productivity, but also in power consumption, support method, generation of unnecessary signals, and the like. was there. An object of the present invention is to provide an ultrasonic transducer which is excellent in workability and mass productivity, can be easily supported, can efficiently transmit a surface acoustic wave to a non-piezoelectric plate with low power consumption. It is another object of the present invention to provide an ultrasonic transducer capable of efficiently converting a surface acoustic wave propagating on a non-piezoelectric plate into an electric signal with a short response time.

According to a first aspect of the present invention, there is provided an ultrasonic transducer comprising a non-piezoelectric plate provided on a piezoelectric plate having a set of interdigital electrodes. The IDT is provided on one plate surface of the piezoelectric plate, and the lower end surface of the non-piezoelectric plate is fixed to the one plate surface of the piezoelectric plate via the IDT, and the IDT is provided. Is supplied with an electric signal having a frequency substantially corresponding to the electrode period length p of the IDT,
A surface acoustic wave having a wavelength substantially equal to the electrode period length p is excited near the surface of the one plate surface of the piezoelectric plate, and the surface acoustic wave is propagated to an upper end surface of the non-piezoelectric plate. The surface acoustic wave propagated to the plate is a 0th-order mode and a first-order or higher-order mode wave. The phase velocity of the 0th-order mode surface acoustic wave is equal to that of the piezoelectric plate alone in an electrically short-circuit state. It is almost equal to the speed of the propagating Rayleigh wave,
The phase velocity of the surface acoustic wave in the higher order mode is approximately equal to the velocity of the Rayleigh wave propagating to the piezoelectric plate alone in an electrically open state, and the thickness of the piezoelectric plate is equal to the electrode period length p. The thickness d of the non-piezoelectric plate is smaller than the electrode period length p, and the phase velocity of the surface acoustic wave propagating to the non-piezoelectric plate alone is equal to or greater than the elastic surface Smaller than the phase velocity of the wave. 3. The ultrasonic transducer according to claim 2, wherein the ultrasonic transducer comprises a non-piezoelectric plate provided on a piezoelectric plate having a set of interdigital electrodes, wherein the interdigital electrode is one of the piezoelectric plates. A lower end surface of the non-piezoelectric plate is fixed to the one plate surface of the piezoelectric plate via the interdigital electrode, and the interdigital electrode is an upper end surface of the non-piezoelectric plate. Converts the surface acoustic wave propagated in the vicinity of the surface of the one plate surface of the piezoelectric plate into an electric signal having a frequency substantially corresponding to the electrode period length p of the IDT, and outputs the electric signal. The surface acoustic wave propagated near the surface of the
A first-order or higher-order mode wave, the wavelength of which is substantially equal to the electrode period length p, and the phase velocity of the zero-order mode surface acoustic wave is the piezoelectric plate alone in an electrically short-circuited state. And the phase velocity of the surface acoustic wave of the first or higher order mode is substantially equal to the velocity of the Rayleigh wave propagating to the piezoelectric plate alone in an electrically open state. The thickness of the piezoelectric plate is about three times or more the electrode period length p, the thickness d of the non-piezoelectric plate is smaller than the electrode period length p, and the phase of the surface acoustic wave propagating to the non-piezoelectric plate alone The velocity is smaller than the phase velocity of the surface acoustic wave propagating to the piezoelectric plate alone. In the ultrasonic transducer according to a third aspect, the other plate surface of the piezoelectric plate is supported by a support substrate. In the ultrasonic transducer according to a fourth aspect, the piezoelectric plate is made of a piezoelectric ceramic, and a direction of a polarization axis of the piezoelectric ceramic is parallel to a thickness direction of the piezoelectric ceramic.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An ultrasonic transducer according to the present invention has a structure in which a non-piezoelectric plate is provided on a piezoelectric plate having a pair of interdigital electrodes. The IDT is provided on one plate surface of the piezoelectric plate, and the lower end surface of the non-piezoelectric plate is fixed on the plate surface of the piezoelectric plate on which the IDT is provided. The interdigital electrode is used for input, and the electrode period length p of the interdigital electrode
By adopting a structure in which an electric signal having a frequency substantially corresponding to that of the interdigital transducer is input to the interdigital transducer, the electrode cycle length p of the interdigital transducer near the surface of the plate surface on which the interdigital transducer is provided is provided. A surface acoustic wave having substantially the same wavelength is excited, and the surface acoustic wave can be propagated to the upper end surface of the non-piezoelectric plate. At this time, 0 is propagated to the non-piezoelectric plate.
These are surface mode waves of the second mode and higher order modes. A structure in which the phase velocity of the zero-order surface acoustic wave is almost equal to the velocity of the Rayleigh wave propagating to the piezoelectric plate in an electrically short-circuit state, and the phase of the first-order or higher-order surface acoustic wave By adopting a structure in which the speed is almost equal to the speed of the Rayleigh wave propagating through the piezoelectric plate in the electrically open state, the electric energy applied from the interdigital transducer is converted into a surface acoustic wave Not only can the degree be increased, but also reflection or the like caused by acoustic impedance mismatch at the interface between the piezoelectric plate and the non-piezoelectric plate can be eliminated. By adopting a structure in which the interdigital electrodes are used for output, the surface acoustic waves propagated from the upper end surface of the non-piezoelectric plate to the vicinity of the surface of the plate on which the interdigital electrodes are provided can be interdigitated. It can be output as an electric signal from the electrode. At this time, the surface acoustic wave is output from the interdigital transducer as an electric signal having a frequency substantially corresponding to the electrode cycle length p of the interdigital transducer. The surface acoustic wave propagated to the piezoelectric plate is a wave of a zero-order mode and a first-order or higher-order mode. The electrode period length p is set so that the wavelength of the surface acoustic wave is substantially equal to the electrode period length p, and the phase velocity of the zero-order mode surface acoustic wave propagates to the piezoelectric plate in an electrically short-circuit state. A structure in which the velocity is almost equal to the Rayleigh wave velocity and the phase velocity of the surface acoustic wave of the first or higher order mode is approximately equal to the velocity of the Rayleigh wave propagating to the piezoelectric plate in an electrically open state. By employing a simple structure, the surface acoustic wave propagated to the piezoelectric plate can be efficiently output as an electric signal from the interdigital transducer. In addition, it is possible to eliminate reflections or the like caused by acoustic impedance mismatch at the interface between the piezoelectric plate and the non-piezoelectric plate. In this way, application to aquatic emission and the like is possible, and detection of an abnormal sound at an ultrasonic level becomes possible.
For example, if a piezoelectric plate provided with arc-shaped interdigital electrodes is provided on a non-piezoelectric plate, it is possible to detect an abnormal sound at an ultrasonic level generated on the non-piezoelectric plate. A structure is adopted in which the thickness of the piezoelectric plate is approximately three times or more the electrode period length p of the interdigital transducer, and the thickness d of the non-piezoelectric plate is smaller than the electrode period length p, and the structure propagates to the non-piezoelectric plate alone. By using a substance whose phase velocity of the surface acoustic wave is smaller than the phase velocity of the surface acoustic wave propagating to the piezoelectric plate alone as the non-piezoelectric plate, if the IDT is used for input, the IDT The surface acoustic wave excited in the vicinity of the surface of the plate surface provided with is efficiently transmitted to the upper end surface of the non-piezoelectric plate without leaking into the inside of the piezoelectric plate. Therefore,
Not only low power consumption driving becomes possible, but also the support of the piezoelectric plate becomes easy. Also, when the interdigital transducer is used for output, the surface acoustic wave on the upper end surface of the non-piezoelectric plate can be efficiently output as an electric signal from the interdigital transducer without leaking into the piezoelectric plate. it can. Therefore, not only low power consumption driving is enabled, but also the support of the piezoelectric plate becomes easy. Adopt piezoelectric ceramic as piezoelectric plate,
By adopting a structure in which the direction of the polarization axis of the piezoelectric ceramic is parallel to the thickness direction, surface acoustic waves can be efficiently excited near the surface of the plate surface on which the interdigital electrodes of the piezoelectric plate are provided. Propagating to the upper end surface of the non-piezoelectric plate, or efficiently propagating the surface acoustic wave of the upper end surface of the non-piezoelectric plate to the vicinity of the surface of the plate surface where the interdigital electrodes of the piezoelectric plate are provided,
It can be output as an electric signal from the interdigital electrode.

1 is a sectional view showing an embodiment of the ultrasonic transducer according to the present invention. This embodiment comprises an interdigital electrode 1, a piezoelectric ceramic plate 2, a non-piezoelectric plate 3, and a support substrate 4. The interdigital electrode 1 is made of an aluminum thin film. The piezoelectric ceramic plate 2 is made of TDK 101A material (product name) having a thickness of 1.5 mm. The non-piezoelectric plate 3 is made of glass or a polymer compound such as a fluororesin or an acrylic resin, and has a thickness of 0.1 mm.
15 mm. An interdigital electrode 1 is provided on a piezoelectric ceramic plate 2, and a non-piezoelectric plate 3 is further provided thereon. When the non-piezoelectric plate 3 is made of glass, the thickness is about 20 μm.
Is fixed on the piezoelectric ceramic plate 2 by the epoxy resin of
When the non-piezoelectric plate 3 is made of fluororesin or acrylic resin, the non-piezoelectric plate 3 is applied directly on the piezoelectric ceramic plate 2.
The lower part of the piezoelectric ceramic plate 2 is fixed to and supported by the support substrate 4. FIG. 2 is a perspective view of the ultrasonic transducer of FIG. However, the non-piezoelectric plate 3 and the support substrate 4 are omitted. The interdigital transducer 1 is of a regular type having ten pairs of electrode fingers. The electrode period length p of the IDT 1 is 4
60 μm. When the interdigital transducer 1 is used for input, when an electric signal is inputted from the interdigital transducer 1, only the electric signal of the center frequency indicated by the interdigital transducer 1 and the electric signal of a frequency in the vicinity thereof among the frequencies of the electric signal are elastic surface. The surface acoustic wave is converted into a wave and propagates near the surface of the piezoelectric ceramic plate 2 on which the interdigital electrode 1 is provided, and the surface acoustic wave is further propagated to the non-piezoelectric plate 3. When the interdigital transducer 1 is used for output, the surface acoustic wave propagating through the non-piezoelectric plate 3 is propagated near the surface of the piezoelectric ceramic plate 2 on which the interdigital transducer 1 is provided. Of the surface acoustic waves propagated to the piezoelectric ceramic plate 2, only the surface acoustic wave having the center frequency indicated by the interdigital electrode 1 and a frequency near the center frequency is converted into an electric signal and output from the interdigital electrode 1. FIG. 3 shows the electromechanical coupling coefficient k ² calculated from the phase velocity difference under two different electrical boundary conditions of the piezoelectric ceramic plate ² and the wave number k of the surface acoustic wave.
FIG. 9 is a characteristic diagram showing a relationship between a product (kd) of a non-piezoelectric plate 3 and a thickness d thereof. However, FIG. 3 shows a characteristic diagram in the case where the non-piezoelectric plate 3 is made of a glass plate, and the velocity of the transverse wave of the surface acoustic wave propagating through the glass plate alone is 2297 m / s and the velocity of the longitudinal wave is 4156 m / s. Is shown. This shear wave speed 2297m
/ S and the longitudinal wave velocity of 4156 m / s are approximately 0.9 times the transverse wave velocity of 2340 m / s and the longitudinal wave velocity of 4390 m / s for the piezoelectric ceramic plate 2 alone. In FIG. 3, it can be seen that the electric energy applied to the interdigital electrode 1 is most efficiently converted to a surface acoustic wave in the 0th mode, and it tends to be harder to convert as the mode becomes higher. The electric energy applied to interdigital transducer 1 is likely to be most converted into a surface acoustic wave of zero-order mode when kd value is about 1.6, about 15.5% of the time k ² is the maximum value Show. It is clear that the k ² value deserves evaluation even when compared with a value of about 5% of a LiNbO ₃ single crystal in a practical range as a piezoelectric substrate for surface acoustic waves. FIG. 4 is a characteristic diagram showing the phase velocity of a surface acoustic wave propagating near the surface of the piezoelectric ceramic plate 2,
It is a figure showing the phase velocity of each mode to kd value. However, FIG. 4 shows a characteristic diagram under the boundary condition that the non-piezoelectric plate 3 is made of a glass plate of the same material as that of FIG. 3 and the surface of the piezoelectric ceramic plate 2 is in an electrically open state. There is a cut-off frequency in higher order modes than the first order. ○ mark,
The kd value at which the electric energy applied to the interdigital electrode 1 is most efficiently converted into a surface acoustic wave of each mode (see FIG. 3)
In the calculated values from, k ² represents the kd value) indicating the maximum value. Phase velocity at 0 mark of 0th mode (about 2170m
/ S) is the Rayleigh wave velocity (21) of the piezoelectric ceramic plate 2 alone when the surface of the piezoelectric ceramic plate 2 is electrically short-circuited.
50 m / s). ○ for higher order modes
The phase velocity at the mark is almost constant (about 2370 m / s)
The Rayleigh wave velocity (2340 m) of the piezoelectric ceramic plate 2 alone when the surface of the piezoelectric ceramic plate 2 alone is in an electrically open state
/ S). Figure 5 is a characteristic diagram showing the k ² value calculated from the phase velocity difference between the two electrical boundary conditions of different piezoelectric ceramic plate 2, the relationship between the kd value. However, FIG. 5 shows a characteristic diagram in the case where the non-piezoelectric plate 3 is made of a glass plate, and the velocity of the transverse wave of the surface acoustic wave propagating through the glass plate alone is 1989 m / s and the velocity of the longitudinal wave is 3598 m / s. Is shown. The transverse wave velocity 1989 m / s and the longitudinal wave velocity 359
The value of 8 m / s is approximately 0.8 times each of the transverse wave velocity 2340 m / s and the longitudinal wave velocity 4390 m / s in the case of the piezoelectric ceramic plate 2 alone. In FIG. 5, it can be seen that the electric energy applied to the interdigital electrode 1 is most efficiently converted to a surface acoustic wave in the 0th mode, and it tends to be harder to convert as the mode becomes higher. The electric energy applied to the interdigital electrode 1 is most easily converted into a surface acoustic wave of the 0th mode when the kd value is about 1.6.
At this time, k ² indicates about 18.5% of the maximum value. FIG. 6 is a characteristic diagram showing the phase velocity of the surface acoustic wave propagating near the surface of the piezoelectric ceramic plate 2, and is a diagram showing the phase velocity of each mode with respect to the kd value. However, FIG. 6 shows a characteristic diagram under the boundary condition that the non-piezoelectric plate 3 is made of a glass plate of the same material as that of FIG. 5 and the surface of the piezoelectric ceramic plate 2 is in an electrically open state. There is a cut-off frequency in higher order modes than the first order. A circle indicates a kd value (a value calculated from FIG. 5) at which electric energy applied to the interdigital electrode 1 is most efficiently converted into a surface acoustic wave in each mode.
k ² indicates the kd value) that indicates the maximum value. The phase velocity (approximately 2095 m / s) of the zero-order mode at the mark “○” is the phase velocity (about 2170 m / s) of the zero-order mode at the sign “○” shown in FIG.
/ S), but approximately equal to the Rayleigh wave velocity (2150 m / s) of the piezoelectric ceramic plate 2 alone when the surface of the piezoelectric ceramic plate 2 is in an electrically short-circuited state. The phase velocities of the higher-order modes of the first and higher-order modes are almost constant (about 230
0 m / s), which is smaller than the phase velocity (approximately 2370 m / s) of the high-order mode of the first order or higher shown in FIG. 4 (about 2370 m / s), but the surface of the piezoelectric ceramic plate 2 alone is in an electrically open state. Rayleigh wave velocity of the piezoelectric ceramic plate 2 alone (2340
m / s). 3 to 6, the surface acoustic wave propagated to the non-piezoelectric plate 3 is a wave of a zero-order mode and a first-order or higher-order mode, and the electric energy applied to the IDT 1 is the elasticity of the zero-order mode. The phase velocity that is most easily converted to a surface wave is the piezoelectric ceramic plate 2 in an electrically short-circuit state.
It is almost equal to the speed of the Rayleigh wave propagating to a single substance. Also,
The phase velocity at which the electric energy applied to the interdigital electrode 1 is most easily converted into a first-order or higher-order surface acoustic wave is the velocity of the Rayleigh wave propagating through the piezoelectric ceramic plate 2 that is electrically open. Is almost equal to Furthermore, the phase velocity of each mode of the surface acoustic wave propagated from the piezoelectric ceramic plate 2 to the non-piezoelectric plate 3 decreases as the velocity of the transverse and longitudinal waves of the surface acoustic wave propagating through the non-piezoelectric plate 3 decreases.

When the interdigital transducer is used for input, surface acoustic waves of the zero-order mode and the first-order or higher-order mode are excited in the vicinity of the surface of the piezoelectric plate on which the interdigital transducer is provided. Then, the surface acoustic wave can be propagated to the upper end surface of the non-piezoelectric plate. A structure in which the phase velocity of the zero-order surface acoustic wave is almost equal to the velocity of the Rayleigh wave propagating to the piezoelectric plate in an electrically short-circuited state, The phase velocity is
By adopting a structure that is almost equal to the speed of the Rayleigh wave propagating to the piezoelectric plate that is electrically open, the degree of conversion of the electric energy applied from the interdigital transducer to a surface acoustic wave is increased. In addition to the above, it is possible to eliminate reflection and the like caused by acoustic impedance mismatch at the interface between the piezoelectric plate and the non-piezoelectric plate. When using interdigital electrodes for output,
Zero-order mode and 1 excited on the upper end surface of the non-piezoelectric plate
A higher order surface acoustic wave can be output as an electric signal from the IDT. A structure in which the phase velocity of the zero-order surface acoustic wave is almost equal to the velocity of the Rayleigh wave propagating to the piezoelectric plate in an electrically short-circuited state, By adopting a structure in which the phase speed is almost equal to the speed of the Rayleigh wave propagating to the piezoelectric plate that is electrically open, the non-piezoelectric plate and the interdigital electrodes of the piezoelectric plate are provided. The surface acoustic wave propagated in the vicinity of the surface of the plate surface can be efficiently output as an electric signal from the IDT. In addition, it is possible to eliminate reflections or the like caused by acoustic impedance mismatch at the interface between the piezoelectric plate and the non-piezoelectric plate. A structure is adopted in which the thickness of the piezoelectric plate is approximately three times or more the electrode period length p of the interdigital transducer, and the thickness d of the non-piezoelectric plate is smaller than the electrode period length p, and the structure propagates to the non-piezoelectric plate alone. By employing a non-piezoelectric plate as a non-piezoelectric plate, a substance whose phase velocity of the surface acoustic wave is smaller than the phase velocity of the surface acoustic wave propagating on the piezoelectric plate alone can reduce the electrical energy applied from the interdigital transducer to the 0th mode and the first order The degree of conversion to a higher-order surface acoustic wave can be increased. At this time, the surface acoustic wave can be efficiently propagated to the upper end surface of the non-piezoelectric plate without leaking into the inside of the piezoelectric plate. Further, the surface acoustic waves of the zero-order mode and the first-order or higher-order modes of the non-piezoelectric plate can be efficiently output as electric signals from the interdigital transducer. Therefore, not only low power consumption driving is enabled, but also the support of the piezoelectric plate becomes easy. When the piezoelectric plate is supported, it can be supported anywhere except in the vicinity of the surface where the surface acoustic wave propagates. By adopting a piezoelectric ceramic as the piezoelectric plate, it is possible to efficiently excite a surface acoustic wave near the surface of the plate surface on which the interdigital electrodes of the piezoelectric plate are provided and to propagate the surface acoustic wave to the upper end surface of the non-piezoelectric plate.
Alternatively, the surface acoustic wave on the upper end surface of the non-piezoelectric plate can be efficiently propagated to the vicinity of the surface of the piezoelectric plate on which the interdigital electrodes are provided, and the electric signals can be output from the interdigital electrodes. When a piezoelectric ceramic is used as the piezoelectric plate, a structure is adopted in which the direction of the polarization axis of the piezoelectric ceramic is parallel to the thickness direction.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an embodiment of an ultrasonic transducer according to the present invention.

FIG. 2 is a perspective view of the ultrasonic transducer of FIG.

FIG. 3 is a characteristic diagram showing a relationship between a k ₂ value calculated from a phase velocity difference under two different electrical boundary conditions of the piezoelectric ceramic plate 2 and a kd value.

FIG. 4 is a characteristic diagram showing a phase velocity of a surface acoustic wave propagating near the surface of the piezoelectric ceramic plate 2;

FIG. 5 is a characteristic diagram showing a relationship between a k ² value calculated from a phase velocity difference under two different electrical boundary conditions of the piezoelectric ceramic plate 2 and a kd value.

FIG. 6 is a characteristic diagram showing a phase velocity of a surface acoustic wave propagating near the surface of the piezoelectric ceramic plate 2;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Interdigital electrode 2 Piezoelectric ceramic plate 3 Non-piezoelectric plate 4 Support substrate

Claims (4)

[Claims] 1. An ultrasonic transducer comprising a non-piezoelectric plate provided on a piezoelectric plate having a pair of interdigital electrodes, wherein the interdigital electrode is provided on one plate surface of the piezoelectric plate. The lower end surface of the non-piezoelectric plate is fixed to the one plate surface of the piezoelectric plate via the interdigital electrode, and the interdigital electrode has a frequency substantially corresponding to the electrode cycle length p of the interdigital electrode. Of the piezoelectric plate, a surface acoustic wave having a wavelength substantially equal to the electrode period length p is excited near the surface of the one plate surface of the piezoelectric plate, and the surface acoustic wave is converted to the non-piezoelectric plate. The surface acoustic wave propagated to the non-piezoelectric plate is a wave of a zero-order mode and a first-order or higher-order mode. The phase velocity of the surface acoustic wave of the zero-order mode is electrically Rayleigh propagating to the piezoelectric plate alone in a short-circuit state Approximately equal to the wave velocity, phase velocity of the surface acoustic wave of the primary or higher order mode,
The speed of the Rayleigh wave propagating to the piezoelectric plate in an electrically open state is substantially equal to the speed of the Rayleigh wave, the thickness of the piezoelectric plate is approximately three times or more the electrode period length p, and the thickness d of the non-piezoelectric plate Is smaller than the electrode period length p, and the phase velocity of the surface acoustic wave propagating to the non-piezoelectric plate alone is
An ultrasonic transducer having a phase velocity smaller than a phase velocity of a surface acoustic wave propagating to the piezoelectric plate alone. 2. An ultrasonic transducer comprising a piezoelectric plate having a pair of interdigital electrodes and a non-piezoelectric plate, wherein the interdigital electrodes are provided on one plate surface of the piezoelectric plate. The lower end surface of the non-piezoelectric plate is fixed to the one plate surface of the piezoelectric plate via the interdigital electrode, and the interdigital electrode is formed of the piezoelectric plate from the upper end surface of the non-piezoelectric plate. The surface acoustic wave propagating near the surface of the one plate surface is converted into an electric signal having a frequency substantially corresponding to the electrode period length p of the interdigital transducer and output, and the vicinity of the surface of the one plate surface of the piezoelectric plate is output. The surface acoustic wave propagated to the surface is a wave of a zero-order mode and a first-order or higher-order mode, the wavelength of which is substantially equal to the electrode period length p. Rayleigh wave propagating through the piezoelectric plate in a short-circuit state Speed substantially equal, the phase velocity of the surface acoustic wave of the primary or higher order mode,
The thickness of the piezoelectric plate is substantially equal to the speed of the Rayleigh wave propagating to the piezoelectric plate in an electrically open state, the thickness of the piezoelectric plate is about three times or more the electrode period length p, and the thickness d of the non-piezoelectric plate is The phase velocity of the surface acoustic wave that is smaller than the electrode period length p and propagates through the non-piezoelectric plate alone is
An ultrasonic transducer having a phase velocity smaller than a phase velocity of a surface acoustic wave propagating to the piezoelectric plate alone. 3. The ultrasonic transducer according to claim 1, wherein the other surface of the piezoelectric plate is supported by a support substrate. 4. The ultrasonic transducer according to claim 1, wherein said piezoelectric plate is made of piezoelectric ceramic, and a direction of a polarization axis of said piezoelectric ceramic is parallel to a thickness direction of said piezoelectric ceramic.