JPS60201249A - Wide-band ultrasonic wave transducer - Google Patents

Abstract

PURPOSE:To obtain an ultrasonic wave transducer improved in electroacoustic transducing efficiency and wide frequency characteristic transducing efficiency and wide frequency characteristics by a couple of electrode terminals to each piezoelectric oscillator of multilayered structure and weighting the intensity of an electric field applied to each piezoelectric layer. CONSTITUTION:Electrodes 3 are fitted to two piezoelectric plates 1 and 2 which differ in thickness to constitute a piezoelectric oscillator, which is stuck and layered by using an adhesive 4 to constitute the transducer of multilayered structure. This transducer, a sound field medium 5, and a back surface load medium 6 are joined together by using the adhesive 4 and used while coupled acoustically. The intensity of the electric field applied to each piezoelectric layer is weighted by connecting an impendance element 11 between electrode terminals 8' and 10 of the piezoelectric oscillator 2 in parallel and applying an exciting voltage to terminals 12 and 13, or connecting an impedance element 14 to the electrode terminal 10 of the piezoelectric oscillator 2 in series and applying the exciting voltage between the terminals 15 and 16. Consequently, wide-band frequency characteristics are obtained without any decrease in transducing efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a multilayer piezoelectric ultrasonic transducer having broadband frequency characteristics necessary for improving the resolution of an ultrasonic imaging device.

In recent years, ultrasound imaging devices using high-frequency ultrasound and ultra-high-frequency ultrasound have been developed, as seen in ultrasound diagnostic equipment and ultrasound microscopes. These ultrasound imaging devices
It is necessary to efficiently generate extremely short ultrasonic pulses, and therefore research is being conducted on widening the frequency characteristics of piezoelectric ultrasonic transducers and increasing conversion efficiency.

Generally, piezoelectric ultrasonic transducers utilize the resonance of a piezoelectric vibrator, so the frequency characteristics in electroacoustic conversion are narrow, and the conversion efficiency largely depends on the acoustic impedance of the material of the sound field medium serving as the load. As the simplest method to improve electroacoustic conversion characteristics, an electrical matching method has been adopted. That is, you can connect an inductance in series or parallel to the electric power (or output) terminal of the transducer, or
This is a way to connect. However, since the frequency characteristics of the transducer essentially depend on the frequency characteristics of the mechanical vibration system, electrical matching cannot be used alone.

There is a limit to the ability to widen the frequency characteristics of a transducer and increase conversion efficiency. Therefore, as a method to improve the characteristics by changing the structure of the mechanical vibration system, (1) a method of inserting a matching layer with a thickness of 174 wavelengths between the piezoelectric vibrator and the acoustic field medium, (2) a method of piezoelectric vibration How to create a multilayer structure for children is discussed. but,
First, in method (1), it is difficult in many cases to obtain a matching layer material having an impedance that provides matching conditions for the impedances of the piezoelectric vibrator and the acoustic field medium. Next, in method (2), the polarity of piezoelectric polarization is determined for each layer by using a plurality of piezoelectric vibrators of the same thickness with the same type of piezoelectric material, that is, one or two wavelengths, or a plurality of piezoelectric vibrators of different types of piezoelectric materials. A structure that is inverted and pasted together is often used.

The method of applying an electric field to the piezoelectric layer of each piezoelectric vibrator is to connect the electrical terminal to the top surface (
The structure is such that an electric field determined by the thickness and dielectric constant of each piezoelectric layer is applied to each piezoelectric layer. Therefore, in a transducer with a multilayer structure made up of piezoelectric vibrators of the same type of piezoelectric material and the same thickness, at a frequency where the wavelength is 1/2.

The ultrasonic waves generated from each piezoelectric vibrator are additively intensified, resulting in a highly efficient transducer and effectively impedance matching with the sound field medium. However, although efficiency can be increased and insertion loss can be improved, there is a drawback that the two-frequency bandwidth narrows in almost inverse proportion to the number of oscillators. On the other hand, in a transducer with a multilayer structure of piezoelectric vibrators made of different piezoelectric materials, the electric field strength applied to each piezoelectric layer changes, so a transducer with a multilayer structure made of piezoelectric vibrators of the same type of piezoelectric material and the same thickness changes. Although the conversion efficiency is not much improved compared to the
The maximum conversion efficiency is achieved at a frequency of /2.

In any case, there is an unavoidable drawback that one frequency bandwidth becomes narrow in almost inverse proportion to the number of oscillators. These characteristics cannot be expected to be improved even if an electric matching circuit is used together with the electric terminal.

Hikari Koyasha The object of the invention is to have good electro-acoustic conversion efficiency.

Ultrasonic transducer 1- which also has broadband frequency characteristics
The aim is to provide services.

Structurally, the present invention belongs to a piezoelectric multilayer structure transducer, but each of a plurality of piezoelectric vibrators bonded together in multiple layers, that is, an electrode terminal pair is attached to each piezoelectric layer to apply an electric field independently. Accordingly, a predetermined weighting is applied to the electric field strength applied to each piezoelectric layer. Specifically, a new terminal pair formed by connecting an electrical impedance element such as an inductance or capacitance in series or parallel to one or more electrode terminal pairs is further connected in series with an electrode terminal pair of another piezoelectric vibrator. Alternatively, the piezoelectric vibrators may be connected in parallel, and the wiring may be such that an applied electric field is applied to each piezoelectric vibrator so that the direction of vibration displacement of the piezoelectric vibrator in each layer is alternately reversed for each layer. In this case, for multiple piezoelectric vibrators bonded together in multiple layers, it is not necessary to create a structure in which the polarity of their piezoelectric polarization is reversed for each layer, but the acoustic vibration displacement of the piezoelectric vibrators is It is only necessary to correctly determine the phase of the electric signal applied to the electrode terminal pair in relation to the piezoelectric polarization direction so that the direction is reversed for each layer. This will be explained below with reference to one drawing.

In order to briefly explain the main points of the present invention, a case will be discussed in which the piezoelectric vibrator has a two-layer structure. Figure 1 shows a transducer with a multilayer structure in which two piezoelectric plates (layers) 1 and 2 of different thickness are each attached with an electrode 3 as a piezoelectric vibrator, and are bonded together in layers using an adhesive 4. Sound field medium 5. This shows the case where the back load medium 6 is bonded to the backside load medium 6 using adhesive 4 and acoustically coupled. Note that the piezoelectric material may be the same type of material or different types of materials;
Also, each electrode has an electrode terminal? , 8.9 and 10 are the drawers. In this configuration, an example of how to configure an electric circuit for weighting the electric field applied to each piezoelectric layer is shown in FIGS. 2(A) and (II). Normally, the thickness of the adhesive and the thickness of the electrode are Since they are very thin compared to the thickness of the plate, the influence of the adhesive and electrodes on the acoustic properties is negligible. Therefore, in FIG. 2 and the following drawings, the thickness of the adhesive and the electrode is zero, and the terminals 8 and 9 are at the same potential, and these two terminals are collectively designated as 8'. Although the back loading medium is omitted in Figure 9, it is generally known that broadband characteristics can be obtained by adding a back loading medium to the transducer (this is well known and is not directly relevant to the present invention).

First, FIG. 2(A) shows a piezoelectric vibrator 2 in contact with a sound field medium.
(hereinafter referred to as the second vibrator) electrode terminal pair 8', 1
This is a case where an impedance element 11 such as a coil, a capacitor, or a resistor is connected in parallel between 0 and 0. At this time, the electrode terminal 8' is already connected to the electrode terminal of the piezoelectric vibrator in the next layer as described in the explanation of FIG. 1, so in this case, the impedance element 11 is connected in parallel. The new terminal pair 8' and 10 are the electrode terminal pair 7 of the next piezoelectric vibrator (hereinafter referred to as the second vibrator).
, 8' are connected in series. As a result, the terminal pair 12, 13 obtained here becomes the electrical terminal of the transducer for applying the excitation voltage. Note that if the value of the impedance element 11 is made variable so that it can be adjusted, it is convenient for later adjustment of the characteristics of the manufactured transducer. A new terminal pair is created by connecting impedance element 14 in series with terminal 10, that is, terminal 14 and terminal 8'.

The circuit has a circuit configuration in which the electrode terminal pair 7 and 8 of the first vibrator are connected in parallel, and the terminal pair 15 and 16 serve as the electrical terminals of the transducer to which the excitation voltage is applied.

The reason why the ultrasonic transducer having such a configuration has broadband characteristics and low loss characteristics is explained as follows. First, by appropriately selecting the material of the piezoelectric vibrator and the vibrator thickness ratio, the operation of each piezoelectric vibrator is affected by the values of dielectric constant and electromechanical coupling coefficient in the former case, and by the difference in applied voltage in the latter case. Weighting is done. Next, if an impedance element is added externally to one piezoelectric vibrator (or one or more piezoelectric vibrators in the case of a multilayer structure consisting of two or more layers), the change in the value will affect the piezoelectric vibrator. The magnitude and phase of the applied voltage are adjusted, and therefore the magnitude and phase of the excitation voltage of each piezoelectric vibrator are appropriately adjusted and their operations are weighted. The weighting of the operation due to both of the above effects is superimposed, making it possible to generate and radiate ultrasonic waves exhibiting desired frequency characteristics into the sound field medium. That is, according to the present invention, broadband characteristics can be obtained without greatly reducing conversion efficiency, and by adjusting the impedance added from the outside.

Frequency characteristics can be easily adjusted.

EXAMPLE 1 An embodiment of the present invention will be described below, distinguishing whether the materials of the piezoelectric vibrators used are the same or different.

Figures 3 and 4 show two sheets of lead derconate titanate (P
ZT) This is an example in which the obtained electroacoustic conversion characteristics are shown in a case where the same type of piezoelectric vibrators using piezoelectric vibrators are used. First, Figure 3 shows two P sheets with a thickness ratio of 0.4.
This is the frequency characteristic of the electroacoustic conversion loss, that is, the effective attenuation amount, obtained when ZT vibrators are bonded together with their polarities reversed and bonded to fused quartz (FQ) as a sound field medium. In the figure, the horizontal axis represents the half-wavelength resonance angular frequency ω of the first vibrator. The vertical axis shows the effective attenuation (dB) with respect to the power supply internal resistance of 50 (Ω). Note that the characteristic curve shown here has ω as a universal parameter. Using C, ω. Let us consider the case of C-0,02. Here, C is the damping capacity of the first vibrator.

The curve (1) in the figure is the characteristic obtained for the series connection configuration shown in FIG. 2(A), and is the characteristic when a coil is used as the additional impedance 11. The magnitude of the additional impedance is such that it resonates with the damping capacitor 1c of the first vibrator at a resonance angular frequency ω. The value that resonates at, ie.

Lo-1/ω. 'C is 0.4 times. The curve (2) in the figure corresponds to the characteristics when the additional impedance is made infinite with the same connection as (1). Furthermore, for comparison, the curve (3) shows the characteristics when ultrasonic waves are directly radiated from the PZT vibrator to the molten crystal. As you can see from the diagram,
By adding a coil to the two-layer structure, broadband characteristics with a fractional bandwidth of 100% are obtained.

In FIG. 4, the circumferential displacement direction in the case of the parallel connection configuration of FIG. 2(B) is reversed. In the figure, curve 9 (1) shows the case when a coil is added, and curve 1 (2) shows the case when a capacitor is added, and the respective sizes are 4Lo and 0.5G. Although these characteristics have a slight increase in loss compared to the characteristics shown in FIG. 3, broadband frequency characteristics are obtained.

Next, an example of characteristics when different types of piezoelectric materials are used is shown in FIG. Curve (1) shows the case where the first vibrator is PZT, the second vibrator is a polyvinylidene fluoride (PVDF) polymer piezoelectric film, and the thickness of each is 1:0.4, and the sound field medium is This is the case when water is taken up. Note that the connection of the electrode terminals corresponds to the parallel connection configuration shown in FIG. 2(B).

This is a frequency characteristic obtained when the impedance of the coil shown in series with the first vibrator in the figure is zero. For comparison, Figure 9 shows the characteristics of a piezoelectric transducer with a single layer structure, with curve 1 (2) and curve 2 (3) of PZT.
shows the frequency characteristics when ultrasonic waves are emitted directly into water from a PVDF membrane transducer, and as can be seen from the comparison, good characteristics with a much wider band and lower loss are obtained than with a single layer structure. I know that there is.

Now, although 9 and above have shown the case of a two-layer structure, the same structure can also be extended to the case of a multi-layer structure. FIG. 6 shows an example of a three-layer structure. As shown in Figure 1, the structure is such that all three layers are composed of PZT vibrators, a coil is connected in parallel to the second vibrator, and a capacitor is connected in parallel to the third vibrator. Although the characteristics are slightly undulating within the band, they exhibit very wide-band characteristics.

Effects As explained above, according to the present invention, in an ultrasonic transducer in which a piezoelectric vibrator has a multilayer structure, it is possible to provide a broadband frequency characteristic without reducing the electroacoustic conversion efficiency. Since electric impedances are externally connected in series or parallel to the electrode terminals of each piezoelectric vibrator, by making these electric impedances variable, it is possible to improve the It is possible to subsequently adjust the frequency characteristics of the electroacoustic transduction characteristics.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of a two-layer ultrasonic transducer.
FIG. 2 shows a connection circuit of the electrode terminals, in which (A) shows a series connection configuration and (B) shows a parallel connection configuration. Figure 3 and subsequent figures show specific examples. Figure 3 shows the characteristics of a series connection configuration using the same type of oscillators, Figure 4 shows the characteristics of a parallel connection configuration, and Figure 5 shows the characteristics of a parallel connection configuration using different types of oscillators. Parallel connection configuration when
FIG. 6 shows the characteristics of a three-layer structure of the same type of vibrator. 1.2... Piezoelectric vibrator, 3... Electrode, 4... Adhesive. 5...Sound field medium, 6...Back load medium, 7. 8.
9.10... Electrode terminal, 8'... Terminal that combines the two terminals of terminal 8.9, 11.14... Impedance element, 12.13°15, 16... Electrical terminal therapy 1 1:il Z product (A) *Z product (E5) Figure 3 * Figure 51

Claims (2)

[Claims] (1) To multiple piezoelectric vibrators made of the same type of piezoelectric material with different thicknesses, or to multiple piezoelectric vibrators made of different types of piezoelectric materials,
A pair of electrode terminals for applying an electric field is attached, and a plurality of piezoelectric vibrators are bonded together to form a multilayer structure, and ultrasonic waves are applied to the sound field medium from the surface of the outermost piezoelectric vibrator. In a multilayer piezoelectric ultrasonic transducer configured to radiate, a new terminal pair formed by connecting an electrical impedance element in series or parallel to one or more electrode terminal pairs is added to the electrode terminals of other piezoelectric vibrators. A high-frequency ultrasonic transducer connected in series or parallel with the pair and configured so that an electric field is applied to each piezoelectric vibrator so that the direction of vibration displacement of the piezoelectric vibrator in each layer is alternately reversed. . (2) The ultrasonic transducer according to claim 1, wherein in the multilayer structure piezoelectric ultrasonic transducer, an electric impedance element connected to the electrode terminal pair is variable.