JPS6313640B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to polyvinylidene fluoride (hereinafter referred to as PVDF).
Freshwater, seawater, the human body, etc., constructed using piezoelectric organic materials such as The present invention relates to an ultrasonic transducer used in the face of

It has been known that polymer-based piezoelectric materials can be used as ultrasonic transducers by appropriately molding them and polarizing them as necessary, but this method is largely dominated by various drawbacks such as insufficient piezoelectricity. It has been widely used and is underutilized despite its advantages such as its malleability and good acoustic impedance matching. For example, the piezoelectric constant d ₃₃ in those 33 directions is 1/5 to 1/5 of that of conventionally known and general-purpose piezoelectric ceramics (such as PZT) for this purpose.
1/10 times is the upper limit that can be practically obtained. However, it is known that broadband ultrasonic coupling with very good impulse response can be obtained by being prepared for some electrical insertion loss.

Efforts have been made to improve the performance of such polymer-based piezoelectric materials from the aspects of the material itself, its composition, molding techniques, polarization techniques, etc., but essentially the piezoelectric constant is a fraction of that of PZT. It was considered difficult, and therefore, it was only applied to a limited range as a single-plate or single-film two-terminal device.

1 and 2 show a conventional ultrasonic transducer as a two-terminal device with a single membrane structure. The transducer shown in Fig. 1 has thin film electrodes 2 and 3 formed by vapor-depositing gold, aluminum, etc. on both sides of PVDF 1 whose thickness is approximately 1/2λ (λ is the wavelength of the ultrasonic wave). , which is immersed in water and its thickness vibration is combined with the water to transmit and receive ultrasonic waves to and from the target object. In addition, the transducer shown in Figure 2 uses PVDF with a film thickness of approximately 1/4λ.
A similar thin film electrode 2 and a brass plate 4 are placed on the opposite surface of 1.
(approximately 0.1 to 5 mm thick) is bonded to the thin film electrode 2.
It emits ultrasonic waves to a target object through water. Note that the back side of the brass board 4 is left in the air. However, in any of these transducers, it is difficult to increase the film thickness of PVDF1, so it is difficult to obtain a transducer with a low resonant frequency (o) (approximately 1 to 10 MHz) suitable for medical use due to the structure. difficult. In addition, in the transducer shown in Fig. 2, the PVDF film itself is held in the thickness direction by a cantilever structure and 1/4λ resonance is caused, or the thickness including the brass plate 4 is reduced to 1/4.
Since it can be operated as 2λ, it is much more advantageous in terms of strength than the transducer shown in Figure 1, and even lower o can be obtained, but its overall sensitivity (or bidirectional insertion loss) is It is considerably worse than conventional transducers constructed with PZT (e.g. -10 to -
Therefore, there was a problem that it could only be applied at close range, that is, to extremely limited target objects where ultrasonic energy has low attenuation, such as the body surface or mammary glands.

In view of these points, it is an object of the present invention to achieve overall sensitivity comparable to conventional ceramic piezoelectric elements by laminating a plurality of PVDF single films having electrodes, and to achieve an overall sensitivity comparable to that of conventional ceramic piezoelectric elements. An object of the present invention is to provide an ultrasonic transducer that does not lose its broadband characteristics and good impulse response.

The present invention will be explained in detail below using the drawings. FIG. 3 is an explanatory main part configuration diagram showing an embodiment of the piezoelectric element membrane portion of the ultrasonic transducer according to the present invention. In Figure 3, 10 (10 ₁ to 10 _o )
is a PVDF membrane having electrodes 2 and 3 on both sides, and a propagation medium 20 suitable for the propagation of ultrasonic waves, such as water, a medium with acoustic properties almost equivalent to water, or silicone rubber, can be used. In Figure 3, silicone rubber 20 is interposed between each film and PVDF is used.
Single membranes are laminated. By holding both ends of these PVDF single films with holding frames 31 and 32, good flatness is maintained, and a drive circuit to be described later can be connected to the electrodes of each PVDF film. An acoustic lens 40 is attached to the open end of the laminated PVDF film 10 on the surface side in order to obtain a desired directivity of the ultrasonic beam emitted from or arriving there. As this acoustic lens, for example, a silicon rubber plate shaped like a convex lens or an acrylic resin plate shaped like a concave lens can be used. However, even when such an acoustic lens is not used, the aperture end can achieve sufficiently good acoustic coupling. In that case, this open end is roughly equivalent to one planar vibrator of equal size. On the other hand, a sound-absorbing packing material 50 is attached to the back side of the laminated PVDF film 10 to realize an acoustically non-reflective termination. This packing material 50 is made by mixing appropriate amounts of a lightweight body such as a hollow glass balloon and heavy metal powder or powder with a heavy specific gravity such as alumina or silica sand in appropriate amounts to emphasize the acoustic loss. Silicone rubber or the like made to have an impedance that closely matches the average acoustic impedance can be used.

Figure 4 is an enlarged view of a part of such a laminated PVDF film.Each PVDF film including the electrode maintains good flatness, its thickness is t, and adjacent films are well parallel to each other. Indicate that the interval is d. Each array arranged at intervals like this
The PVDF film needs to be electrically coupled (transmission/reception) with an appropriate delay between the films so as to form traveling wave coupling. FIG. 5 is a configuration diagram showing an embodiment of a delay line capable of transmitting and receiving ultrasonic waves by driving the PVDF film 10, and includes an LC delay line consisting of an inductance L and a capacitance C, and an input/output termination resistor. It is composed of Z ₁ and Z _p . Here, each PVDF film and each tap of the LC delay line are connected with a predetermined delay time relationship in the order of arrangement. That is, when transmitting waves, the closer the PVDF membrane is to the opening end in the sound wave emission direction, the slower the drive is, and when receiving waves, the signal received by the PVDF membrane closer to the opening end is delayed for a longer time before being driven to the entire electrical terminal T. appears in In these processes, the electric signal being transmitted or received traveling through the delay line is always combined with the ultrasonic energy propagating through the array structure formed by each PVDF film and the intermediate medium. The cadences are roughly matched, so this configuration provides electrical and acoustic energy conversion (coupling) according to the principle of traveling wave coupling.

Here, the delay time Δto for one section is approximately Δto=d/if the sound velocity in the silicone rubber 20 is Co.
It is necessary to select Co. In addition,
As long as the PVDF film thickness is sufficiently thinner than d, there is no practical problem in substituting the sound velocity in water (1.5 km/s) for Co. On the other hand, if a material acoustically equivalent to a PVDF membrane or a PVDF membrane itself without polarization is used as the intermediate medium,
The advantage arises that multiple reflections of ultrasound waves within the array can be prevented. However, in that case, acoustic impedance matching with the target area is required at the open end, which becomes more complicated. As is well known, such an impedance matching layer is realized by a quarter-wave plate or the like having an impedance that is the geometric mean of the impedance of water, which is the target region, and the average impedance of this array structure.
It is placed in the position of lens 40 in FIG. However, since such a matching layer is defined in wavelength units, it is not possible to ensure broadband performance over many octaves. Therefore, as the most preferred embodiment, the average acoustic impedance of the array structure in the sound ray direction is approximately equivalent to that of water (PVDF
If the film is sufficiently thinner than the thickness of the intermediate medium or the inter-film spacing d, it is preferable that they are approximately equivalent.

Further, each interval d does not necessarily have to be constant, and may be different as long as the electrical delay between each film is adjusted appropriately. Also, PVDF
If the number n of membranes satisfies n·k〓1 (k is the electrical/mechanical coupling coefficient per membrane), large coupling can be obtained without sacrificing the frequency bandwidth. The coupling coefficient k is determined by piezoelectric ceramics (e.g.
PZT) can easily achieve a value of about 0.5 to 0.7.
For PVDF membrane, it is weak at about 0.1 to 0.15. Therefore, if the deficiency in k is compensated for by the number of sheets n, and n·k=1, a bonding strength commensurate with that of a PZT veneer can be obtained.

The frequency characteristic (or impulse response) when a plurality of PVDF single films are combined is the product of the element determined by the frequency characteristic of the unit membrane and the element determined by the arrangement. Therefore, as long as the periodicity of the array and the electrical delay between each film match up to the maximum operating frequency where the wavelength λ becomes t = 1/2λ, a roughly flat broadband frequency characteristic can be obtained. It will be done. However, even if n·k≫1, a larger gain cannot be obtained than in the case of approximately n·k〓1. Therefore, it is preferable to set it to n·k1. In the case of PVDF, the equivalent k is about 0.1 to 0.15, so n
≒7 to 10 is optimal.

FIG. 6 shows another embodiment of the ultrasonic transducer of the present invention. In Fig. 6, 60 is a delay line for transmitting waves, 70 (70 ₁ to 70 _o ) is a transmitting pulse amplifier, 80 (80 ₁ to 80 _o ) is a first stage amplifier for receiving waves, and 90 is a delay line for receiving waves. Show each line. The delayed pulses extracted from the transmission delay line 60 and delayed little by little are individually guided to the PVDF transducers via amplifiers 70 ₁ to 70 _o , respectively. In addition, the received signals in each PVDF film are individually transmitted to first-stage receiving amplifiers 80 ₁ to 80 _o.
The signals are introduced into the wave receiving delay line via the wave receiving delay line, where the signals are combined with an appropriate delay and sent out as a wave receiving signal. In this case, the first stage amplifiers 80 ₁ to 80 _o for wave reception are low noise, high input impedance amplifiers, and these are connected to the membrane transducer 1.
Place it as close as possible to the receiving terminal that is separate from the transmitting terminal of 0 ₁ to 10 _o , and connect the input terminal of each amplifier to this terminal at the shortest possible distance while avoiding an increase in stray capacitance. is desirable, and by doing so, it is possible to realize a conversion ability that is even superior to that of conventional ceramic piezoelectric elements, without impairing the broadband characteristics that characterize the present invention.

In addition, multi-stage delay means for wave transmission (delay line 6
The function 0) can be realized not only by a delay line using an analog circuit, but also by a digital system using a preset counter or a shift register.

As explained above, according to the present invention, polymer piezoelectric elements are stacked and arranged at predetermined intervals,
The polymer-based piezoelectric It is possible to realize an ultrasonic transducer having a transducing ability substantially comparable to that of conventional ceramic piezoelectric elements without reducing the broadband properties and good impulse response inherent to the element.

[Brief explanation of the drawing]

1 and 2 are block diagrams of a conventional PVDF membrane transducer, FIG. 3 is an explanatory block diagram showing an embodiment of a piezoelectric element membrane portion of an ultrasonic transducer according to the present invention, and FIG. 5 is a diagram showing an embodiment of the delay line, and FIG. 6 is a configuration diagram showing another embodiment of the present invention. 2, 3... Electrode, 10 (10 ₁ ~ 10 _o )... PVDF
Membrane, 20... Propagation medium, 31, 32... Holding frame, 40
... Acoustic lens, 50 ... Packing material, 60 ... Delay line for wave transmission, 70 (70 ₁ - 70 _o ) ... Transmission pulse amplifier, 80 (80 ₁ - 80 _o ) ... First stage amplifier for wave reception, 90 ... Reception Daytime line for waves.

Claims (1)

[Claims] 1. A plurality of piezoelectric element films made of polymer-based piezoelectric materials stacked and arranged in parallel to each other via a propagation medium, and each tap coupled to an electrode of these piezoelectric element films. a delay line, and each piezoelectric element film is arranged in relation to the delay time relationship of the delay line so that a traveling wave is coupled with an ultrasonic wave propagating perpendicularly to the piezoelectric element film. Duusa. 2. The ultrasonic transducer according to claim 1, wherein the piezoelectric element film is made thinner than the inter-film spacing. 3. The ultrasonic transducer according to claim 1 or 2, wherein water or a medium having acoustic characteristics substantially equivalent to water is used as the propagation medium. 4. The ultrasonic transducer according to claim 1, wherein a material having approximately the same acoustic impedance as the polymer-based piezoelectric material is used as the propagation medium. 5. The ultrasonic transducer according to claim 1, further comprising an acoustic lens at an opening end on the target area side of the array structure. 6. The ultrasonic transducer according to claim 1, further comprising an acoustic impedance matching layer between the target area and the opening end of the array structure on the target area side. 7. The ultrasonic wave according to claim 1, wherein a backing material for forming an acoustically non-reflective termination is disposed at the end of the arrangement structure opposite to the target area side. transducer.