JP2754648B2 - Ultrasonic transducer and acoustic imaging device using the transducer - Google Patents

Description

The present invention relates to an ultrasonic transducer having a piezoelectric ceramic thin plate / fused quartz structure and an acoustic imaging apparatus using the same.

(Prior Art) In recent years, the importance of acoustic imaging has been increasing in various fields including medical diagnosis and destructive inspection. A transducer that controls the function of transmitting and receiving ultrasonic waves is the basis of realizing acoustic imaging. A thickness vibration transducer is generally used as a means for radiating a bulk wave into water. In this vibration mode, since the frequency is determined by the thickness of the piezoelectric body, there is a restriction in increasing the frequency. On the other hand, an elastic wave such as a Rayleigh wave or a Lamb wave becomes a so-called leaky wave that propagates while radiating a part of the energy as a bulk wave in the liquid when the liquid is loaded on the propagation medium. For such mode conversion from a leaky wave to a bulk wave, an interdigital transducer (hereinafter abbreviated as IDT) is effective. The use of this type of transducer has the advantage that it is possible to utilize the potentially advanced technology cultivated in the field of surface acoustic waves, and it is relatively easy to increase the frequency as compared with the use of the thickness vibration mode. From such a viewpoint, a basic study has been made on a substrate material in a single crystal or ceramic state as an interdigital transducer for a leaky surface wave, and the possibility of application to acoustic imaging, a sensor, and the like has been reported.

In general, a leaky Rayleigh wave propagating on a substrate that is sufficiently thick compared to its wavelength has no velocity dispersion, and uses a single mode as a transducer. A leaky Lamb wave propagating through a thin substrate having a thickness equal to or less than the wavelength has a plurality of modes having velocity dispersion. In this case, since both surfaces of the substrate can be used, the IDT can be configured without directly contacting water, which is advantageous in application.

(Problems to be Solved by the Invention) However, in the field of acoustic imaging, there is a tendency to increase the frequency to improve the resolution, and when a leaky Lamb wave transducer is used for this purpose, the thickness of the substrate must be reduced. Therefore, problems including strength arise.

The present invention has been made to solve this problem, and it is an object of the present invention to provide a leaky surface acoustic wave transducer having a piezoelectric ceramic thin plate and a fused quartz structure, which is to be applied to an acoustic imaging apparatus.

Means for Solving the Problems In order to achieve the above object, the present invention provides a piezoelectric substrate, an input and receiving interdigital transducer arranged on the piezoelectric substrate, and an input and receiving interdigital transducer. A pulse is applied to a leaky surface acoustic wave transducer consisting of a composite structure consisting of a fused quartz that covers the interdigital transducer, and an input interdigital transducer that is in contact with the liquid and is arranged on the piezoelectric substrate and covered with the fused quartz. Excitation applies a surface acoustic wave to the piezoelectric substrate from the intersecting width of the electrode fingers of the interdigital transducer, modulates the surface acoustic wave, radiates it into the liquid as a bulk wave, and is reflected from the subject. The bulk wave propagated again through the liquid layer is converted to a delayed electric signal by a wave-receiving IDT arranged on a piezoelectric substrate and covered with molten quartz. An acoustic imaging apparatus comprising a transducer unit issuing Ri.

(Operation) According to the present invention having the above-described configuration, first, a pulse is applied to an input interdigital transducer arranged on a piezoelectric substrate and covered with molten quartz. Then, a leaky surface wave is excited on the piezoelectric substrate from the portion of the intersecting width of the electrode fingers of the interdigital transducer, and the leaky surface wave is mode-converted and radiated into the liquid as a bulk wave. Then, the bulk wave reflected from the subject and propagated through the liquid layer again is extracted as a delayed electric signal by a wave-receiving IDT arranged on a piezoelectric substrate and covered with a fused quartz. At this time, by relatively scanning the subject, an acoustic image of the subject is obtained based on the delayed electric signal obtained from the transducer section.

Therefore, according to the present invention, it is possible to provide an acoustic imaging apparatus that can be expected to be applied in the field of nondestructive inspection and enables beam steering with a very simple mechanism.

(Example) In the following, the behavior of a leaky surface wave propagating at a solid-liquid interface and the analysis results of the operation characteristics of the IDT are shown, and a specific transducer design and reflection C mode using this leaky surface wave transducer are shown. The configuration of the acoustic imaging apparatus and the acoustic imaging results will be described.

Also, when designing a leaky surface wave IDT, it is necessary to clarify the excited surface wave velocity and conditions for efficiently emitting bulk waves into water. Then, water /
The propagation characteristics of leaky surface waves and the operation characteristics of an IDT in a piezoelectric ceramic / fused quartz structure will be described based on numerical calculations.

FIG. 1 is a configuration diagram showing an acoustic imaging apparatus of the present invention.
FIG. 2 shows the distribution of the displacement of the underwater bulk wave radiation and the leaky surface wave by the IDT. The k in Fig. 2 leaky surface wave wave number, d is the piezoelectric ceramic substrate thickness, 2p the IDT electrode period length, U _1, U ₃ is a x ₁ and x ₃ the direction of displacement of a leaky surface wave . Incidentally, the polarization axis of the piezoelectric substrate in the thickness direction (x ₃ direction). The application of the rf pulse electric signal to the IDT excites a leaky surface wave with a wavelength of 2p, but its displacement is more concentrated in the piezoelectric thin plate than in the fused quartz. This wave while traveling along the substrate in the x ₁ direction, is mode converted wave number matching conditions, is emitted in an oblique direction with longitudinal waves form in the water.

To analyze the propagation characteristics in this layered structure, Farnell's method was extended to the following points. First, since the leaky wave is a wave that attenuates while emitting a part of energy into water, the surface wave velocity v is treated in the form of a complex number. Here, the wave number in the propagation direction is proportional to 1 / v, and the imaginary component of v is related to the attenuation constant of the leaky surface wave. Secondly, the continuity of displacement, stress and electric flux density at the solid-liquid interface was also considered. The thickness of the fused quartz is treated as semi-infinite. Under these conditions, the velocity v that satisfies the electrical and mechanical boundary conditions at the two interfaces was obtained by a numerical substitution method. Piezoelectric substrate is piezoelectric ceramic NEPEC
-6 (made by Tohoku Metal).

The numerical calculation result of the velocity dispersion curve of the leaky surface wave is the wave number k
This is shown in FIG. 3 in the form of a function of the product of the thickness and the thickness d of the piezoelectric substrate. The numbers in the figure correspond to the mode orders. Opening and short-circuiting indicate that the surface of the piezoelectric ceramic is electrically open or short-circuited, and specifically corresponds to whether or not there is a metal deposition film. As kd increases, leaky surface waves appear in the order of 0th, 1st, 2nd and 3rd modes. The 0th-order mode decreases relatively rapidly in the range of kd = 0 to 1.0, and when kd becomes sufficiently large, water / NEPEC-
Asymptotic to the speed for the six structures. The circles in FIG. 3 indicate the experimental results for verifying the validity of the numerical calculation results. The thickness of the substrate is 220μm, 360μm, 480μm, 640μm,
This is an experimental result in an element configuration in which two pairs of IDTs each having a logarithm of 10 are provided, and is obtained from the product of the electrode period length of the IDT and the center frequency. It can be seen that the theoretical value and the experimental value agree relatively well.

FIG. 4 shows that the distance L at which the leaky surface wave attenuates to 1 / e is the wavelength λ.
And is expressed in the form of a kd function. L / λ
Is smaller, the mode conversion from leaky surface waves to underwater bulk waves is performed over a short distance, and a wavefront conforming to the shape of the IDT is formed in the water. Is desirably small. The value of L / λ in the zero-order mode is smaller than that in the higher-order mode, and is substantially constant regardless of the value of kd. On the other hand, in the case of the higher-order mode, it can be said that the characteristic is that the value of L / λ is large and the dependence on kd is large.

Electromechanical coupling coefficient k ² from the electrical energy to the IDT represents the conversion efficiency of the leaky surface wave is determined from the following equation commonly known.

k ² = 2 | v _o −v _s | v _o (1) Here, v _o and v _s are surface wave velocities in an electrically open state and a short-circuit state. The mode conversion efficiency C for the surface wave power is converted into water bulk waves while the surface wave is one wavelength propagates is defined from the ratio of the real component of the velocity v _r and imaginary components v _i as follows.

_{C = 1-exp (4πv i} / v r) ... (2) FIG. 5 is a diagram showing a kd-dependent calculation result of the mode conversion efficiency. It can be seen that the zero-order mode has a mode conversion efficiency that is three times or more that of the other modes.

The total effective conversion efficiency η as a transducer in which the electric energy applied to the IDT is converted into surface waves and further converted into underwater bulk waves is obtained from the product of k ² and C. 6 and 7 are diagrams showing calculation results of effective conversion efficiencies for two types of transducers in a case where there is no counter electrode at the interface with water and a case where there is a counter electrode.
Although the zero-order mode has a large mode conversion efficiency, the effective conversion efficiency is not so large. This is because the difference between the speed in the electrically open state and the speed in the short-circuit state is small, as is clear from FIG. 3 and equation (1).
This is because a large electromechanical coupling coefficient cannot be obtained. Further, the value of kd that takes the peak value of the 0th-order mode differs depending on the presence or absence of the counter electrode. When kd is used in the range of 0 to 1.0, it can be said that the provision of the counter electrode is more advantageous.

FIG. 8 is a diagram showing the measurement results of the band-pass characteristics of the insertion loss between the input and output electrodes when the water is loaded and when it is not, with respect to the transducer having an electrode cycle length of 480 μm. This result corresponds to the case of kd = 3.8 in FIG. The greater the difference in insertion loss at each frequency, the greater the energy radiated into water. Observing the input and output waveforms at frequencies of 6.2MHz and 8.3MHz, which have large effective conversion efficiencies, show that in air, the signal output after propagating on the substrate becomes very small due to contact with water, By arranging the plate at an appropriate position, a large reflected signal is received, and it is confirmed that the submerged bulk wave is efficiently radiated. These results correspond well to the calculation results in FIG. 7, and it is understood that the results of the numerical analysis are appropriate.

FIG. 1 shows a leaky surface acoustic wave transducer for constituting an acoustic imaging apparatus and a propagation path of an ultrasonic wave. When an RF pulse is applied to the input arc-shaped IDT, a leaky surface wave is excited on the piezoelectric ceramic substrate from the portion of the intersection width of the IDT electrode finger. Here, the transducer part is in contact with water, but the excited surface wave is immediately mode-converted,
Emitted into water as bulk waves. In this case, the radiation angle θ _w into the water is determined by the leakage surface wave velocity v and the underwater bulk wave velocity v
_{From w} is given by:

θ _w = sin ^-1 (v _w / v) (3) This bulk wave is reflected at the elastic discontinuity on the surface or inside of the subject, propagates through the water layer again, and It can be extracted as a delayed electric signal in the arc-shaped IDT.

The IDT here is provided between the piezoelectric ceramic and the fused quartz. This is to protect the electrode fingers of the IDT from water, stabilize the operation as a transducer, and to bring out characteristics that are superior to those provided at the interface with water in terms of effective conversion efficiency. . Note that, by making the IDT into an arc shape, it is possible to focus the ultrasonic beam in the liquid despite the flat device configuration.

The design of the electrode period length of the IDT is based on the above numerical calculation results. That is, in order to draw out the characteristics of the layered structure as a leaky surface wave, the selection conditions were to operate in a plurality of modes, to have relatively large speed dispersion, and to obtain conversion efficiency at which signal processing could be performed. Specifically kd
= 3.2 to satisfy the condition of = 3.2. Transducers for imaging systems include:
Focusing the bulk wave beam in water and increasing the spatial resolution is necessary to obtain a clear acoustic image,
The IDT has an arc shape, and the IDT is divided into four parts. Two sets of the IDTs function as the transmitting IDTs, and the transducers do not require a circulator. Table 1 shows the specifications of a specific transducer manufactured for acoustic imaging.

Specifications of Leaky Surface Wave Transducer Created for Acoustic Imaging Table 1 Fig. 9 shows the measurement results of the frequency dependence of the radiation angle of bulk waves into water. This result corresponds well with the value calculated from the velocity dispersion curve in FIG. This transducer has a center frequency of 9.60MHz, 12.7MHz, 16.2MHz,
It can operate in two modes, and can operate within a certain range near the center frequency of each mode. Within the frequency band corresponding to each mode, the emission angle of the bulk wave changes little by little in a form corresponding to the change in velocity along the dispersion curve with the deviation from the center frequency,
This can be used as a fine-tuning function. The operation mode is switched by changing the frequency greatly. This means that the emission angle of the bulk wave is changed relatively largely, and can be treated as a sparse adjustment function.

FIG. 10 is a block diagram showing an acoustic imaging apparatus of the present embodiment. The reflected delay signal from the sample obtained by the receiving IDT is
The signal is extracted in the form of a DC signal corresponding to a change in amplitude by signal processing. After being captured by a computer (CPU) as a 12-bit digital signal by an A / D converter, the data is processed and output to a CRT or printer (PRN). In the gate circuit section, an analog switch driven by the gate signal removes unnecessary signals included in the received signal and extracts only the desired reflected signal, but by reducing the gate width, the resolution in the depth direction is reduced. Can be improved. When the stage is mechanically scanned two-dimensionally in the X and Y directions by a computer-controlled stepping motor under the above procedure, a C-mode acoustic image is obtained.

 Next, experimental results of the present embodiment will be described.

A 4 mm thick Pyrex glass 7740 (borosilicate glass) manufactured by Corning Co., Ltd. was prepared as a sample for an imaging experiment. Heat was applied to one side of the glass with a gas burner to cause spot-like distortion. FIGS. 11 (a), (b), and (c) show the imaging results when the distance between the transducer and the sample is set to 5 mm and the operating frequency of the transducer is changed to 12.0 MHz, 13.0 MHz, and 14.0 MHz. This is a bird's-eye view of the reflection C-mode image.
Here, one scale in the figure corresponds to 1 mm.

It is clear that the reflection point of the beam is on the opposite side of the surface to which heat has been applied, and that some distortion has reached the back surface. Each acoustic image has a subtle difference, and it can be inferred that the focal position changes in the depth direction. These three acoustic images correspond to the results in the first mode, but the operating frequency is
When the frequency changes from 12.0 MHz to 14.0 MHz, the position of the focal point moves closer to the surface of the sample. Specifically, it means that the focal point in water changes by about 800 μm. This is considered to lead to a change in the position of the reflection point of about 500 μm in the Pyrex glass sample. A significant feature of this method is that by changing the driving frequency in this way, an acoustic image in which the position of the reflection point is delicately controlled can be obtained without changing the distance between the transducer and the sample.

Fig. 12 shows the imaging results when the transducer is operated at the center frequency of each of the 0th, 1st and 2nd modes with the separation distance between the transducer and the sample, that is, the thickness of the water layer fixed at 7.5 mm. It is. In this case, the focal point of the zero-order mode is set so as to be located almost on the surface of the sample. 1
The focal point of the next mode is estimated to be about 1.5 mm from the surface of the Pyrex glass, and the secondary mode is estimated to be around 3.5 mm. FIG. 12 (a) is an acoustic image in the 0th mode near the surface heated by the burner. A large circular distortion exists in a range of about 10 mm in diameter, and the intensity of the reflected signal is large at the center portion, but the degree of the distortion is smaller at the peripheral portion. This indicates that the elastic state is different between the heated central portion and the peripheral portion. FIG. 12 (b) is an acoustic image in the first and second modes corresponding to the state near the center of the sample, and FIG. 12 (c) is the state near the opposite surface of the sample. It can be seen that the degree of distortion decreases as the depth increases. By changing the operation mode by changing the frequency greatly, while keeping the water layer at a constant value,
It is clear that a wide range of acoustic images from the front surface to the back surface of the sample can be obtained.

(Effects of the Invention) As described above, according to the present invention, the operation characteristics at the solid-liquid interface of a transducer having a layered structure in which an IDT is arranged between a fused quartz and a piezoelectric ceramic thin plate are clarified by numerical analysis. In addition to experimentally clarifying the validity of the results, the leakage surface wave velocity, the electromechanical coupling coefficient, the mode conversion rate, and the effective conversion efficiency were calculated by multiplying the product of the wave number of the leakage surface wave and the thickness of the piezoelectric ceramic plate. In the form of a function. A leaky surface wave in this structure has a plurality of modes with velocity dispersion, and there are conditions suitable for a submerged ultrasonic transducer, and it is possible to operate in multiple modes and multiple frequencies. The focal point can be moved in the depth direction by increasing or finely controlling the angle of emission of the bulk wave into the water due to the change.

Based on these analysis results, a specific acoustic imaging transducer was designed and manufactured, and an acoustic imaging apparatus based on the pulse reflection method was constructed. The manufactured transducer has a sparse adjustment function by switching the use mode and a fine adjustment function by frequency change in the same mode. As described above, the transducer according to the present invention can perform beam steering with a very simple mechanism.

In addition, the disadvantage that the mechanical strength is weakened due to the thinning of the piezoelectric substrate due to the increase in frequency can be compensated for by adopting the composite structure. As described above, the acoustic imaging apparatus using the leaky surface wave transducer of the present invention can be expected to be applied in the field of nondestructive inspection.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, and FIG.
Figure 3 shows the distribution of underwater bulk wave radiation and the displacement of leaky surface waves due to turbulence, Figure 3 shows the velocity dispersion curve of leaky surface waves with respect to kd, and Figure 4 shows the relationship between 1 / e attenuation distance and kd. FIG. 5, FIG. 5 shows a calculation result of the kd dependency of the mode conversion efficiency, FIG. 6 shows an analysis result of an effective conversion efficiency of a transducer having no counter electrode, and FIG. 7 has a counter electrode. FIG. 8 is a diagram showing an analysis result of an effective conversion efficiency of the transducer, FIG. 8 is a diagram showing a frequency characteristic of insertion loss between input and output electrodes, FIG. 9 is a diagram showing a radiation angle-frequency characteristic of an underwater bulk wave, and FIG. Is a block diagram showing the acoustic imaging apparatus of this embodiment, and FIGS. 11 and 12 are diagrams showing the imaging results in this embodiment.

Claims (2)

(57) [Claims] 1. A composite structure in which a non-piezoelectric body and a piezoelectric substrate on which input and receiving interdigital transducers are arranged are fitted so as to cover the input and receiving interdigital transducers. A pulse is applied to the input interdigital transducer to excite a leaky surface wave from the portion of the intersecting width of the electrode fingers of the interdigital transducer onto the piezoelectric substrate, and the mode of the leaky surface wave is converted into a bulk wave. An ultrasonic transducer that emits in liquid. 2. A method of applying a pulse to an input interdigital transducer, which is in contact with a liquid and is arranged on a piezoelectric substrate and covered with a fused quartz, to apply a pulse to the interdigital transducer from the intersection width of the electrode fingers of the interdigital transducer. Exciting the leaky surface wave above, mode-converting the leaky surface wave, radiating into the liquid as a bulk wave, and reflecting the bulk wave reflected from the subject and propagating again through the liquid layer, onto the piezoelectric substrate A transducer section arranged and taken out as a delayed electric signal by a wave-shaped interdigital transducer covered with a fused quartz, and relatively scanning the subject, and applying the delayed electric signal obtained from the transducer section to the delayed electric signal. An acoustic imaging apparatus for acquiring an acoustic image of the subject based on the acquired acoustic image.