EP0015886A1

EP0015886A1 - An improved electro-acoustic transducer element

Info

Publication number: EP0015886A1
Application number: EP80850028A
Authority: EP
Inventors: Hiroji Ohigashi; Toshiharu Nakanishi; Miyo Suzuki
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 1979-03-13
Filing date: 1980-03-12
Publication date: 1980-09-17
Also published as: AU5637080A

Abstract

An electro-acoustic transducer element includes a polymer, preferably polyvinylidene fluoride, piezoelectric film of an acoustic impedance Zo and at least one, preferably polymeric, additional layer of like acoustic impedance Z coupled to one side surface of the film, either directly or indirectly. Adequate adjustment in thickness of the additional layer enables free change in frequency characteristics of its acoustic effect without any change in dimensions of the polymer piezoelectric film.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an improved electro-acoustic transducer element, and more particularly relates to improvement in construction of an electro-acoustic element incorporating a polymer piezoelectric film, which is used for transmission and/or conversion of ultrasonic waves.
As a substitute for the conventional inorganic piezoelectric material, polymer piezoelectric material is advantageously used for ultrasonic vibrators in the filed of diagnostics and detection of internal defects in various articles for its easy production of large sized films, easiness in treatment and fine fit to curved surfaces.
The acoustic impedance of the polymer piezoelectric material is by far lower than that of the inorganic piezoelectric material and very close to those of water, and general organic materials. Thus, the polymer piezoelectric material functions as an excellent transmitter and receiver for ultrasonic waves which travel through these objects.
In connection with this, however, use of the polymer piezoelectric film in the construction of an ultrasonic transducer is in practice accompanied with various problems.
In the case of ultrasonic devices used for diagnostics and/or detection of internal defects, ultrasonic waves are mostly used with frequencies in the range from 1 to 10 MHz. It is well known that, in order to obtain high transmission efficiency, the resonant frequency of the vibrator has to match the frequency of the ultrasonic wave to be used for the process. In other words, the thickness of the piezoelectric film has to be chosen in accordance with the frequency of the ultrasonic wave to be used for the aimed process.
In the case of polyvinylidene fluoride which is a typical polymer piezoelectric material, its frequency constant f_ot_o is equal to 115 KHz·cm, f_o being the resonant frequency of a free thickness vibrator and to being the thickness of the film. In order to obtain high efficiency in transmission of the ultrasonic wave of 2.5 MHz frequency which is commonly used for diagnostic purposes, it is required for the film to have a thickness of 460µm for a half wave drive, and 230µm for a quarter wave drive.
A potential of about 10⁶ V/cm is needed for polarization of polymer for provision of piezoelectricity. Polarization of a polymer film of a large thickness is often accompanied with troubles such as aerial discharge, thereby disabling easy preparation of a thick polymer piezoelectric film. The available thickness under the present condition is 100µm or smaller. This is the first disadvantage of the conventional art.
In production of a polymeric piezoelectric film, it is very difficult to optimumly control the process in order to provide the resultant film with a thickness well suited for transmission of the ultrasonic wave of an aimed frequency. Such a polymer piezoelectric film is in most cases obtained by polarization of a material film after drawing. Depending on the process conditions in drawing and heat treatment, thickness of the resultant film varies greatly. Quite unlike the inorganic piezoelectric material, it is extremely troublesome and, consequently, almost infeasible to adjust the thickness of a polymer piezoelectric film by mean of mechanical cutting. This is the second disadvantage of the conventional art.
Dielectric constant of a polymer piezoelectric film is in general not so high as that of the inorganic piezoelectric material such as PZT. Therefore, increase in thickness of the film causes reduction in electric capacity. As a result, increased electric impedance of the vibrator does not well match that of the electric power source, thereby blocking smooth supply of energy to the vibrator from the electric power source. This is the third disadvantage of the prior art.

SUMMARY OF THE INVENTION

It is the basic object of the present invention to provide an electro-acoustic transducer element incorporating a polymer piezoelectric film of a reduced thickness which enables transmission of ultrasonic waves having frequencies lower than its inherent resonant frequency with reduced transmission loss.
It is another object of the present invention to provide an electro-acoustic transducer element incorporating a polymer piezoelectric film of an ideal function without any noticeable damage on high flexibility, low acoustic impedance characteristics and easiness in treatment inherent to the polymer piezoelectric material.
In accordance with the basic aspect of the present invention, a polymer piezoelectric film is accompanied, at least on its one surface side, with an additional layer whose acoustic impedance (_Z) is equal or very close to the acoustic impedance (Z_o) of the polymer piezoelectric film.
In accordance with preferred embodiment of the present invention, a polymer piezoelectric film is accompanied, on its acoustic emanation side, with an additional layer whose acoustic impedance (Z) is equal or very close to the acoustic impedance (Z_o) of the polymer piezoelectric film.
In accordance with another preferred embodiment of the present invention, a polymer piezoelectric film is accompanied, on the side opposite to its acoustic emanation side, with an additional layer whose acoustic impedance (Z) is equal or very close to the acoustic impedance (Z_o) of the polymer piezoelectric film.
In accordance with the other preferred embodiment of the present invention, a polymer piezoelectric film is accompanied, on its both surface sides, with respective additional layers whose acoustic impedances are equal or very close to the acoustic impedance (Z_o) of the polymer piezoelectric film.
The additional layer may be either directly or indirectly disposed to the polymer piezoelectric film.
Any polymer film having piezoelectricity in the thickness direction as a result of polarization is usable for the present invention. Such a film is made of a polymeric material such as polyvinylidene fluoride, copolymers of polyvinylidene fluoride, polyvinyl chloride, acryloni- titrile polymers or ferroelectric ceramic including lead zirconate-titanate powder.
The term "acoustic emanation side'' refers to one of the two surface sides of a polymer piezoelectric film which faces an acoustic transmission medium through which the ultrasonic waves of an aimed frequency travel away from or towards the polymer piezoelectric film.
In the following description, this side of the film may be referred to "the front side" whereas the other side of the film opposite to this acoustic emanation side may be referred to "the rear side".
In accordance with the present invention, a polymer piezoelectric film is either directly or indirectly accompanied, on either of its front and rear sides, with an additional layer. That is, the additional layer may be placed either in a direct surface contact with the piezo-. electric film or in an indirect surface association with the piezoelectric film via any intervening layer such as an electrode. The additional layer may hereinafter referred to "the front additional layer" or "the rear additional layer".
Presence of such an additional layer on either side of the piezoelectric film is essential to the purposes of the present invention, and the additional layer should be made of a material whose acoustic impedance (Z) is equal or very close to the acoustic impedance (Z_o> of the piezoelectric film.
Preferably, the ratio Z/Z_o should be in a range from 0.2 to 2 exclusive. More preferably, the ratio Z/Z_o should be in a range from 0.3 to 2 exclusively. Further preferably, the ratio Z/Z_o should be in a range from 0.5 to 2 exclusive.
When water is used for the acoustic transmission medium, it is preferable that the ratio Z/Z_f should be 0.5 or larger, Z_f being the acoustic impedance of water. Further, a relationship Z_f<Z<Z_o should preferably exist among the acoustic impedances of water, the additional layer and the piezoelectric film.
Such an additional layer is preferably made of a polymeric material such as polyethylene telephthalate, polycarbonate, PMMA, polystylene, ABS, polyethylene, polyvinyl chloride, polyamide, aromatic polyamide, polyvinylidene fluoride or a mixture of such a polymeric material with an inorganic compound.
When any shape retainability is required for the additional layer, carbon fibers may be mixed to such a polymeric material. The additional layer may be given in the form of a film which is a mixture of such a polymeric material with thin metallic fibers such as stainless fibers whose diameter is by far smaller than the wave-lengths of sonic waves.
When flexibility should be accentuated in the assembled state of the additional layer with the polymer piezoelectric film, a nylon, rubber, polyurethane or silicone rubber sheet is usable for the additional layer.
In order to assemble the polymer piezoelectric film with the additional layer in an acoustically integral fashion, the material for the additional is first shaped into a film which is next bonded to the polymer piezoelectric film. It is also employable to coat one surface of the polymer piezoelectric film with the material for the additional layer. In the latter case, diehlorobenzene solution of PMMA of chlorobenzene solution of polyethylene telephthalate is preferably used for the solvent, which may be removed by evaporation after coating. The coating may be subjected to appropriate polymerization such as vapor phase polymerization.

BRIEF DESCRIPTION OF THE DRAWINGS.

Fig. 1 is a schematic view for showing the general construction of an electro-acoustic transducer element incorporating a piezoelectric film,
Fig. 2 is circuit diagram of the equivalent-circuit for driving the system shown in Fig. 1,
Fig. 3 is a circuit diagram of the four terminal mesh loop used for connecting the equivalent circuit shown in Fig. 2 to an electric power source having internal impedance,
Fig. 4 is a view for explaining the mode of power source energy distribution and consumption,
Fig. 5 is a block diagram for showing the construction of an apparatus used for measurement of the non-tuning conversion loss,
Fig. 6 is a block diagram for showing the construction of an apparatus used for measurement of the electric reflection loss,
Figs. 7A through 7F are sectional side views of various embodiments of the electro-acoustic transducer element in accordance with the present invention,
Fig. 8 is a sectional side view of an electro-acoustic transducer incorporating one embodiment of the transducer element in accordance with the present invention,
Fig. 9A is schematic side view of one embodiment of the transducer element in use in accordance with the present invention,
Fig. 9B is a graph for showing the relationship between the frequency of the ultrasonic wave used for the arrangement shown in Fig. 9A and its non-turning conversion loss,
Fig. 10A is a schematic side view of a conventional electro-acoustic transducer element in use,
Fig. 10B is a graph for showing the relationship between the frequency of the ultrasonic wave used for the arrangement shown in Fig. 10A and its non-tuning conversion loss,
Fig. 11 is a sectional side view of another electro-acoustic transducer incorporating one embodiment of the transducer element in accordance with the present invention,
Fig. 12 is a graph for showing the relationship between the frequency of the ultrasonic wave used for the arrangement shown in Fig. 11 and its electro-acoustic conversion loss,
Fig. 13 is a sectional side view of the other electro-acoustic transducer incorporating one embodiment of the transducer element in accordance with the present invention,
Fig. 14 is a graph for showing the relationships between the frequency of the ultrasonic wave used for the arrangement shown in Fig. 13 and its electro-acoustic conversion losses, both nominal and actual,
Fig. 15 is a graph for showing a like relationship for the arrangement shown in Fig. 13 with omission of the front additional layer of the present invention,
Fig. 16 is a graph for showing a like relationship for the arrangement shown in Fig. 13 but under a different condition,
Fig. 17 is a sectional side view a further electro-acoustic transducer incorporating one embodiment of the transducer element in accordance with the present invention,
Fig. 18 is a graph for showing the relationship between the frequency of the ultrasonic wave used for the arrangement shown in Fig. 17 and its estimated nominal electro-acoustic conversion loss,
Fig. 19 is a sectional side view of a still further electro-acoustic transducer incorporating one embodiment of the transducer element in accordance with the present invention,
Fig. 20 is a graph for showing the relationship between the frequency of the ultrasonic wave used for the arrangement shown in Fig. 19 and its electro-acoustic conversion loss,
Fig. 21A through 21C are sectional side views of various electro-acoustic transducers each incorporating a polymer piezoelectric film,
Fig. 22 is a graph for showing the relationships between the frequencies of the ultrasonic waves used for the arrangements shown in Figs. 21A through 21C and their electric reflection losses, respectively,
Figs. 23A through 23E are sectional side views of various embodiments of the electro-acoustic transducer elements in accordance with the present invention,
Fig. 24 is graph for showing the relationships between the frequency of ultrasonic waves used for various electro-acoustic transducers each incorporating a polymer piezoelectric film and their electro-acoustic conversion and electric reflection losses,
Fig. 25A is a sectional side view of an electro-acoustic transducer element incorporating a polymer piezoelectric film,
Fig. 25B is a graph for showing the relationships between the frequency of the ultrasonic wave used for the arrangement shown in Fig. 25A and its electro-acoustic conversion and electric reflection losses, respectively,
Fig. 26A is a schematic side view of a still further electro-acoustic transducer incorporating one embodiment of the transducer element in accordance with the present invention, and
Fig. 26B is a graph for showing the relationship between the frequency of the ultrasonic wave used for the arrangement shown in Fig. 26A and its electro-acoustic conversion loss.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

When a tension T and an electric field E is applied to a piezoelectric film in its thickness direction, the resultant strain on the thickness vibrator S and electric displacement D are represented as follows, respectively;
Here, C* is a complex modulus of elasticity (∂T/∂_SD) with the mechanical loss being taken into account, and represented as follows;
Whereas β* is a complex electric susceptibility (∂E/∂D_S) with the dielectric loss being taken into account, and represented as follows;

where Ψ = tan δ m mechanical loss tangent
ϕ = tan δ_e dielectric loss tangent
h piezoelectric constant (real quantity)

On the assumption that a piezoelectric body has a thickness t, a surface area A, a density ϕ and a characteristic sound velocity v, that loads F₁ and F₂ are applied to both surfaces of the body at velocities U₁ and U₂ (angular frequency ω), and that a current I_S flows between electrodes with an electric voltage V, the following matrix is theoretically established between these values.
where
For a non-piezoelectric body, h is equal to zero in the equation (1).
The general construction of an electro-acoustic transducer is shown in Fig. 1, in which the transducer includes, on the front side of a piezoelectric film 0, front non-piezoelectric layers 1, 2, -- n including a front electrode e and an acoustic transimission medium f such as water or organism. The transducer further includes, on the rear side of the piezoelectric film 0, non-piezoelectric layers 1', 2', --- m'. These rear side non-piezoelectric layers may include a bonding layer or layers, a rear electrode e^t, a protector film, a substrate, a reflector plate and the additional layer in accordance with the present invention. The real thicknesses and weights of the electrodes e and e' disregarded in the following consideration.
An equivalent circuit for driving the system shown in Fig. 1 can be designed on the assumption that loads and displacements are continuous at each boundary between adjacent layers and the real electric charge within the piezoelectric film is equal to naught. The equivalent circuit so designed is shown in Fig. 2.
Re the arrangement shown in Fig. 2, the following relationships are established.
Like relationships are established for Z_Ai, Z_Ci, Z_Aj and Z_Cj on the basis of Z =ϕv, v, t and Ψ for each layer. Φ is the winding ratio for the secondary coil and given by hCe.
The two-port nextwork shown in Fig. 3 is -used for connecting the equivalent circuit in Fig. 2 to an electric power source having an internal,impedance Z_S, the electric impedance of the transducer with respect to the power source being designated with Z_L.
The node of distribution and consumption of the energy supplied by the electric power source is schematically shown in Fig. 4, in which Pr is the reflection energy caused by inconsistency between Z_S and Z_in, P_T is the input energy to the transducer and given in the form of the difference between P_o and P_r, P_Af is the front acoustic emanation, P_Ab is the rear acoustic emanation energy and P_th is the internal consumption energy (heat) within the transducer, P_th being equal to P_T - (P_Af + P_Ab). Hence, the following equations are conducted.
On the basis of these equations, the losses are defined as follows;
Electric reflection loss
Electro-acoustic conversion loss (front side)
Electro-acoustic conversion loss (rear side)
Internal loss
Non-tuning conversion loss (front side)
Non-tuning conversion loss (rear side)
These definitions clearly indicates that TLf should be designed as small as possible and TLb as large as possible over a wide frequency band, in order to enhance utility of a transducer used for non-destructive ultrasonic detection.
Following measurement systems were employed in evaluation of the function of the electro-acoustic transducer element in accordance with the present invention.
Measurements of the non-tuning conversion loss (front side) CL_f, the electro-acoustic conversion loss TLf and the electric reflection of the produced transducer were carried out in the following fashion.
An arrangement used for measurement of the non-tuning conversion loss is shown in Fig. 5. In the measurement, a transducer including a PVDF piezoelectric film was used in a water bath in which a bronze block was placed as. reflector. The transducer was connected to a known high frequency pulse oscillator having an impedance of 50Ω. The generated ultrasonic pulses were emanated into the water bath and those reflected by the bronze block were received by the sane transducer. The received signals were indicated an a CRT synchroscope for visual observation after appropriate amplification and detection. Concurrently with this procedure, the exciting electric signals (voltage) were passed to the synchroscope for visual observation after appropriate attenuation, amplification and detection. The rate of attenuation was adjusted so that both indications on the synchroscope should meet. This procedure was repeated for various frequencies of the ultrasonic pulses. When the rate of attenuation is equal to L_mes, the non-tuning conversion loss (front side) CLf is given by the following equation;
where

L_ref; Reflection loss of the bronze block
L_W ; Loss caused by adsorption and dispersion of the ultrasonic waves in the water bath.
6dB ; Loss caused by the parallel connection of impedances at transmission and reception, characteristic to the pulse echo method.

Under the conditions employed in the later described examples of the present invention, the value of L_ref + L_W is almost equal to 1 dB.
For measurement of the electric reflection loss ML, the transducer was placed within a water bath in which reflection of ultrasonic waves was negligible. The impedance of the transducer was measured by an arrangement shown in Fig. 6 in terms of the reflection voltage and its phase. For this measurement, the transducer included a polyvinylidene fluoride film obtained by applying polarization for 1 hour at 10° v/cm and 120° C to an uniaxially drawn material film.
For theoretical evaluation of the transducer element in accordance with the present invention, the following characteristic values were used for the materials used for the measurements.
Various embodiments of the electro-acoustic transducer element in accordance with the present invention are shown in Figs. 7A through 7F, in which each transducer element includes a polymer piezoelectric film 11. In the illustration, the bottom side of the polymer piezoelectric film 11 corresponds to the above-described acoustic emanation or front side.
The transducer element 10 shown in Fig. 7A includes a a polymer piezoelectric film 11, an electrode 14b fixed to the rear side surface of the film 11, another electrode 14a fixed to the front side surface of the film 11, and an additional layer 12 coupled to the film 11 via the front side electrode 14a.
The transducer element 10 shown in Fig. 7B includes a polymer piezoelectric layer 11, a rear side electrode 14b, an additional layer 12 fixed directly to the front side surface of the film 11, and a front side electrode 14a fixed to the front side surface of the additional layer 12.
The transducer element 10 shown in Fig. 70 includes a polymer piezoelectric film 11, a front side electrode 14a, an additional layer 12a coupled to the front side of the film 11 via the front side electrode 14a, a rear side electrode 14b, and another additional layer 12b coupled to the film 11 via the rear side electrode 14b. The one additional layer 12a will hereinafter be referred to "a front side additional layer" and the other " a rear side additional layer".
The transducer element 10 shown in Fig. 7D includes a polymer piezoelectric film 11, a front additional layer 12a coupled to the front side surface of the film 11 via a front side electrode 14a, and a rear side electrode 14b coupled to the rear side surface of the film 11 via a rear side additional layer 12b.
The transducer element 10 shown in Fig. 7E includes a polymer piezoelectric film 11, a front side electrode 14a coupled to the front side surface of the film 11 via a front side additional layer 12a, and a rear side additional layer 12b coupled to the rear side surface of the film via a rear side electrode 14b.
The transducer element 10 shown in Pig. 7F includes a polymer piezoelectric film 11, a front side electrode 14a coupled to the front side surface of the film 11 via a front side additional layer 12a, and a rear side electrode 14b coupled to the rear side surface of the film 11 via a rear side additional layer.12b.
In either cases, the acoustic impedance Z of each additional layer 12a or 12b is designed equal or very close to the acoustic impedance of the polymer piezoelectric film 11.
Fig. 8 depicts a practical electro-acoustic transducer 20 in which one embodiment of the transducer element in accordance with the present invention. The transducer 20 includes a circular column shaped substrate 23, a rear side electrode 24b operating as a reflector plate also and disposed to the front face of the substrate 23, a cylindrical spacer 25 surrounding the rear side electrode 24b and made of a bond or the like, a polymer piezoelectric film 21 coupled to the front side surface of the rear side electrode 24b, a front side electrode 24a coupled to the front side surface of the film 11, an additional layer 22 made of polyvinylidene fluoride or the like and coupled to the front side surface of the front side electrode 24a, an annular electrically conductive wafer 26 surrounding the additional layer 22 and in contact with the front side surface of the front side electrode 24a, and an annular bond layer 27 covering the periphery of the additional layer 22 and the front surface of the conductive wafer 26. One lead 28b extends outsides from the rear side electrode 24b whereas another lead 28a extends outsides from the front side electrode 24a via the conductive wafer 26. The transducer 20 is placed in touch with an acoustic transmission medium ATM via the additional layer 22.
The substrate 23 is made of a material such as a polymeric material which has a relatively small acoustic impedance. Such a polymeric material is preferably chosen from a group consisting of PMEIA, PS, ABS, bakelite and epoxy resin. When flexibility is in particular required, a rubber- type elastomer such as natural rubber and silicone rubber is preferably used for this purpose.
The reflector plate, i.e. the rear side electrode 24b is made of a material whose acoustic impedance is by far larger than those of the polymer piezoelectric film 21 and the substrate 23. Metals such as Au, Cu and W are in general advantageously usable for this purpose. When the rear side electrode 24b is formed on the rear side surface of the polymer piezoelectric film in advance to formation of the transducer 20, an insulating material such as a PZT ceramic. plate may be added as a reflector plate.
Although the transducer 20 shown in Fig. 8 incorporates the transducer element 10 shown in Fig. 7A, different type of transducer element 10 shown in either of Figs. 7B through 7F if usable for a similar purpose.
Still further embodiments of the electro-acoustic transducer element in accordance with the present invention are shown in Figs. 23A through 23E.
The transducer element 30 shown in Fig. 23A includes a polymer piezoelectric film 31, a front side electrode 34a fixed to the front side surface of the film 31, and an additional layer 32 coupled to the rear side surface of the film 31 via a rear side electrode 34b.
The transducer element 30 shown in Fig. 23B includes a polymer piezoelectric film 31, a front side electrode 34a fixed to the front side surface of the film 31, and a rear side electrode 34b coupled to the rear side surface of the film 31 via an additional layer 32.
The transducer element 30 shown in Fig. 23C includes a polymer piezoelectric film 31, a front side electrode 34a fixed to the front side surface of the film 31, an additional layer 32 coupled to the rear side surface of the film 31 via a rear side electrode 34b, and an acoustic reflector plate 35 fixed to the rear side surface of the additional layer 32.
The transducer element 30 shown in Fig. 23D includes a polymer piezoelectric film 31, a front side electrode 34a fixed to the front side surface of the film 31, a rear side electrode 34b coupled to the rear side surface of the film 31 via an additional layer 32, and an acoustic reflector plate 35 fixed to the rear side surface of the rear side electrode 34b.
The transducer element 30 shown in Fig. 23 E includes a polymer piezoelectric film 31, a front side electrode 34a fixed to the front side surface of the film 31, an additional layer 32 coupled to the rear side surface of the film 31 via a rear side electrode 34b, and a substrate 33 fixed to the rear side surface of the additional layer 32.

EXAMPLES

Example 1 and comparative example 1.
A Fig. 7A-type transducer element 10 shown in Fig. 9A. was used in the measurement. The polymer piezoelectric film 11 was made of polyvinylidene fluoride and 30 µ m in thickness and 0.92 cm² in surface area. Water was used as the acoustic transmission medium ATM.
Four types of test pieces were prepared. The first to third test pieces I to IV included polyvinylidene fluoride, either piezoelectric or non-piezoelectric, additional layers, one for each, of 7.5,15, 30 and 60 µm thicknes respectively. The fourth test piece IV included no additional layer. Non-tuning conversion loss CL_f=ML + TL_f was measured for ultrasonic waves of various frequencies and the results are graphically shown in Fig. 9B.
For a comparative example, a transducer element shown in Fig. 10A was prepared, which included a polyvinylidene fluoride piezoelectric film 11, front and rear side electrodes 14a and 14b, and a copper plate 15 of 66.5 µ m thickness fixed to the front side surface of the front side electrode 14a. was used for the acoustic transmission medium ATM. The piezoelectric film 11 was common in dimension to that used for the transducer element shown in Fig. 9A.
The resultant relationship between the non-tuning conversion loss and the frequency of the ultrasonic wave applied to the transducer element is shown in Fig. 10B, in which the solid line curve is for a film without mechanical and dielectric losses (Ψ = 0, ϕ = 0), and the dot line curve is for a film with mechanical and dielectric losses (Ψ = 0.1, ϕ = 0.25).
This outcome clearly indicates that, for the transducer element of this comparative example, the loss presents minimal peaks at frequencies fo/2, fo and 3fo/2 but upsurges at other frequencies. That is, the transducer element of this type has narrow frequency-band filtering characteristics. In the evaluation of the relationship, the mechanical loss of the copper plate was disregarded.
When the result in Fig. 9B (present invention) is compared with that in Fig. 10B (comparative example), the characteristic curve for the present invention extends over wider frequency band and no remarkable rise in the loss is noted.
It is further learned from the results shown in Fig. 9B that increased thickness of the front side additional layer results in a lower resonant frequency, i.e. a frequency corresponding to the minimum value of the loss TL_f. Consequently, resonant frequency of the transducer element in accordance with the present invention can be adjusted quite freely by means of appropriately changing the thickness of the front side additional layer 12 without any change in the thickness of the polymer piezoelectric film 11. This successfully precludes the above-described disadvantages inherent to the prior art.
Further, in the construction of the transducer element used for the present example (see Fig. 9A), the additional layer 12 is coupled to the front side surface of the piezoelectric film 11 via the front side electrode 14a, the latter is protected against any external attack, thereby assuring long life of the transducer element and blocking dangerous electric leakage outside the system.

Example 2

A different type of electro-acoustic transducer 40 in accordance with the present invention is shown in Fig. 11. Here, a Fig. 70-type transducer element is used. The transducer 40 includes a metallic housing 45 having a bottom opening, a polymer piezoelectric film 41 placed in the housing 45 whilst closing the bottom opening, and a pair of electrodes 44a and 44b placed in contact with both side surfaces of the film 41. In contact with the front side electrode 44a, a front side additional layer 42a is filled into the bottom opening and fixed to the housing 45 via an annular bond layer 47a. A rear side additional layer 42b is located on the rear side electrode 44b and surrounded by an annular metallic ring 46. The members 42b and 46 are fixed to the housing 45 via a bond layer 47b. A lead 48 extends outsides from the metallic ring 46 and the metallic housing. 45 is earthed. Water is used for the acoustic transmission medium ATM, which the front side additional layer 42a contacts.
A polyvinylidene fluoride film of 70µm thickness and 4.4 cm² surface area was used for the piezoelectric film 41, polyester films of 25 and 50 µm thickness (t_f) were used for front side additional layer 42a, and polyester films of 25 and 50 µm thickness (t_b) were used for the rear side additional layer 42b. A further transducer of like construction but without the rear side additional layer was prepared as a comparative example. Particulars of the test pieces are as follows.
The obtained result is shown in Fig. 12. This illustrated result clearly indicates that, just like the result in example 1, the characteristic curves for the present invention extend over a wide frequency band and the resonant frequency of the transducer can be adjusted as desired by accordingly changing the thickness of the rear side additional layer.

Example 3.

A further different type of electro-acoustic transducer 50 in accordance with the present invention is shown in Fig. 13. Here, a Fig. 7A-type transducer element is used. The transducer 50 includes a hollow metallic housing 55, a cylinder 56 screwed to the bottom of the housing 55, and a polyvinylidene fluoride piezoelectric film 51 placed within the housing 55 whilst closing-the end opening of the cylinder 56. The film 51 is backed by a PMMA substrate 53 via a rear side reflector plate 54b made of strainless steel. Here, the reflector plate 54b acts as a rear side electrode also, and 100 µm in thickness and 21.0 mm in diamter. In the end opening of the cylinder 56, a front side additional layer 52 is coupled to the front side surface of the film 51 via a front side electrode 54a. A lead 58 extends outwards- from the reflector plate 54b, i.e. the rear side electrode, and the metallic housing 55 is earthed.
The transducer 50 of the above-described construction was placed within a water bath for transmission of high frequency pulses of several µs periods and the ultrasonic waves reflected by a brass block immensed in the water bath were received by the same transducer. The resultant actual frequency characteristics of its electro-acoustic conversion loss TL_f are shown in Fig. 14 with nominal frequency characteristics theoretically estimated on the basis of the above-described equations (1) through (5). In the illustration, the solid line curve is for the actual frequency characteristics and the dot line curve for the nominal frequency characteristics.
It is clear from the illustration that the actual and nominal loss values roughly meet with difference of 2 to 3 dB, and differences in loss peak value are 0.5 MHz or smaller. Sufficient coincidence between the loss values is recognized.
For comparison, like frequency characteristics are shown in Fig. 15 for a comparative transducer without provision of the front side additional layer 52. The solid line curve is for the actual values and the dot line curve for the estimated nominal values. Appreciable coincidence is recognized to exist between both values in this case also. The difference in frequency characteristics between Figs. 14 and 15 is resulted from provision of the front side additional layer. By adding the additional of 25 µm thickness, the resonant frequency for the minimum peak loss value is lowered from about 10 to 8 MHz without any substantial narrowing in the frequency band which the characteristic curve extends over. This enables production of an electro-acoustic transducer having a relatively low consonant frequency or frequencies even with use of a relatively thin polymer piezoelectric film which can be produced rether easily. In addition, the front side electrode 54a is well protected against possible external attack by presence of the additional layer on its front side. The limited thickness of the epoxy bond layer is regarded the proximate cause of the difference between the actual and estimated nominal loss values.

Example 4.

An electro-acoustic transducer similar in basic construction as that used in the preceding example was used with some changes in demension of the elements.
Additional layers made of polyethylene telephthalate were disposed to the front sides of the associated polyvinylidene fluoride films 51, respectively in accordance with the present invention. For comparison, a transducer was prepared also, which included no front side additional layer. Particulars of the test pieces were as follows;
An uni-axially drawn polyvinylidene fluoride film of 76 µm thickness was used for the polymer piezoelectric film 51 and a copper plate of 168 µm thickness and 11.3 mm diameter was used for the rear side reflector plate 54b, i.e. the rear side electrode. In the case of the test pieces I and II (5 and 25 µm thickness), the additional layers 52. were coated with chlorophenol solution of polyethylene telephthalate.
The nominal results obtained by estimation are shown in Fig. 16, in which the eleetro-acoustic conversion loss TL_f and non-tuning conversion loss CLf are taken on the ordinate and the frequency of the ultrasonic wave used in the measurement is taken on the abscissa. It was confirmed that the difference in resonant frequency between the actual and nominal estimated values was 0.5 MHz and that in loss values was 3 dB or smaller. With increase in thickness of the additional layer 52, the resonant frequency shifted towards the lower side. The relative frequency band (Δf/fr) was 0.52, 0.54, 0.57 and 0.63 for 0, 5, 25 and 50 µm thickness, respectively. That is, the relative frequency band increased with increase in thickness of the additional layer 52.

Example 5.

Fig. 17 depicts a still different type of transducer 60 incorporating a polymer piezoelectric transducer element in accordance with the present invention. The transducer element used in this example is basically same in type with that used in Fig. 13 except that the entire construction is concave towards the acoustic emanation side, i.e. the front side. The transducer 60 includes a polycarbonate pipe 65 of 20 mm outer diameter, a polyvinylidene fluoride piezoelectric film 61 of 76 µm thickness and bonded to the bottom opening of the pipe 65 and a pair of aluminium electrodes 64a and 64b disposed to both side surfaces of the film 61 by evaporation. The front side electrode 64a is accompanied on its front side with a thin bronze phosphate ring 66 via electrically conductive bond. An additional layer 62 made of undrawn polyvinylidene film is bonded to the ring 66 in contact with the front side electrode 64a. A rearwardly converging copper substrate 63 of 13 mm maximum diameter is placed in the pipe 65 in contact with the rear side electrode 64b, which is in turn backed with an epoxy resin filler 67 in a manner such that the converging end of the substrate 63 projects rearwards. A front side lead 68a extends outsides from the front side electrode 64a through the pipe whereas a rear side lead 68b extends from the converging end of the substrate 63. The assembly is encased within a cylindical metallic housing 69.
Five sets of different test pieces I through V were prepared as follows;
Water bath was used for the acoustic transmission medium ATM.
The estimated nominal frequency characteristics of the electro-acoustic conversion loss TL_f are shown Fig. 18.
As in Example 4, the resonant frequency shifts towards the lower side with increase in thickness of the additional layer 62. Use of the additional layer enlarges the relative frequency band. Thus, the electro-acoustic transducers prepared were able to generate sonic waves of frequencies well suited for diagnostic applications whilst using thin polymer piezoelectric films.
A brass plate was placed in the water bath measurement of echo signals reflected by the brass block. It was confirmed that there was a good coincidence between the actual and estimated nominal values.
Using the transducer incorporating the additional layer of 38µm thickness, a high frequency coil L of about 5 µH was connected in series to the transducer. The transducer so prepared was used for obtaining resolution echogram of a fish in a water bath using an ultrasonic wave of 5MHz frequency. The resulting resolution was satisfactory for both depth and transverse direction.

Example 6.

A transducer shown in Fig. 19 was used for this Example and included a Fig. 7A-type transducer element in accordance with the present invention. The transducer 70 includes a polyvinylidene fluoride piezoelectric film 71 of 76 µm thickness and accompanied with a pair of aluminium electrodes 74a and 74b, an additional layer 72 coupled to the front side of the film 71 via a front side of the film 71 via a front side electrode 74a, a copper reflector plate 76 coupled to the rear side of the film 71, and a substrate 73. The additional layer 72 is made of an uniaxially drawn polyvinylidene fluoride non-piezoelectric film. The thickness of the reflector plate 76 is chosen so that it operates as a quarter wave-length plate in the vicinity of the resonant frequency.
Five sets of test pieces I through V were prepared as follows;
The frequency characteristics of the non-tuning conversion loss are shown in Fig. 20. It is noticed in the illustration that the lowest peak loss for the test piece IV (152 µm thickness) appears at 2.5 MHz frequency. In the case of the conventional transducer of a same resonant frequency, the piezoelectric film 71 is required to have a thickness of 230 µm, which is too large to produce easily. Further, this increased thickness causes lowering in electric capacity. In the case of the present invention, no lowering in capacity assures well match of the transducer to the electric power source.

Example 7.

Three sets of different transducers 80, 80' and 80" were prepared as shovm in Figs. 21A through 21C.
The transducer 80 (test piece I) shown in Fig. 21A includes basically a Fig. 7A-type transducer element in accordance with the present invention. That is, the transducer includes a polyvinylidene fluoride piezoelectric film 81 of 70 µm thickness, a polyethylene telephthalate additional layer 82 of 100 µm and coupled to the front side of the film 81 via a front side electrode 84a, a copper reflector plate 86 of 50µm thickness 1.0 cm2 surface area and fixed to the rear side surface of the film 81, and a bakelite substrate 83 (Z = 4.84 x 10⁶ kg/m².s) backing the reflector plate 86.
The transducer 80' (test piece E) shown in Fig. 21B includes basically a Fig. 7A-type transducer element in accordance with the present invention, also. That is, the transducer 81' includes a polyvinylidene fluoride piezoelectric film 81' of 140 µm thickness, a polyethylene telephthalate additional layer 82' of 25µm thickness and coupled to the front side of the film 81' via the front side electrode 84a, the copper reflector palte 86 fixed to the rear side surface of the film 81', and the bakelite substrate 83 backing the reflector plate 86.
The transducer 80" (test piece III) shown in Fig. 21C was prepared for comparison and conventional in construction. The transducer 80" includes a polyvinylidene fluorize - piezoelectric film 81" of 76 µm thickness, the front side electrode 84a, a copper reflector plate 86" of 40 µm thickness, and a PMMA substrate 83" backing the reflector plate 86".
Water bath was used for the acoustic transmission medium ATM and frequency characteristics of electric reflection loss MLf were measured and is graphically shown in Fig. 22.
It is clearly recognized in the graph that the resonant frequencies for the test pieces I and II are both shifted towards the lower side than that for the test piece III. In particular, the test piece I presents a further minimal loss peak at a double frequency approximately equal to 10MHz. The system may be driven for operation by means of short pulse excitation also.

Example 8.

An electro-acoustic transducer incorporating a Fig. 23A-type transducer in accordance with the present invention was used with a water bath as the acoustic transmission medium ATM.
A polyvinylidene fluoride piezoelectric film of 76 µm thickness and 10 cm² surface area was used for the polymer piezoelectric film, which was sandwitched by a pair of electrodes. This film was backed with three types of polyvinylidene fluoride additional layers in order to obtain test pieces I to III of the present invention. For comparison, a test piece IV without the additional layer was prepared also. Thus, particulars of the test pieces I to IV are as follows;
Frequency characteristics of electro-acoustic conversion loss TL_f were measured using the test pieces.
In addition, frequency characteristics of electric reflection loss ML_f was measured also using the test piece II, the result being disignated in the graph with a chain line curve.
For comparison purposes, a transducer such as shown in Fig. 25 A was prepared, which includes a like polyvinylidene fluoride piezoelectric film accompanied with a front side electrode, and, as a substitute for the polyvinylidene fluoride backing additional layer, a copper plate of 163µ m. In this case, the copper plate acts as a rear side electrode also. Frequency characteristics of electro-acoustic and electric reflection losses TL_f and ML_f were measured and are shown in Fig. 25B, respectively. A water bath was used for the acoustic transmission medium ATM, also.
In Fig. 25B, the minimal peak value for the electro-acoustic conversion loss TL_f appears about the frequency of 7 MHz whereas, in Fig. 24, the test piece III has an almost similar resonant frequency for the electro-acoustic conversion loss TLf.
In the case of the comparative example shown in Fig. 25A, presence of the rigid thick copper plate in back of the piezoelectric film greatly kills flexibility of polyvinylidene used for the piezoelectric film. In contrast to this, use of the polymeric additional layer in combination with the polyvinylidene fluoride film does never damage the flexible nature of the latter.
Fig. 24 further indicates that simple adjustment in thickness of the additional layer enable free choice of the resonant frequency.

Example 9.

Fig. 26A depicts an electro-acoustic transducer including a Fig. 23A-type transducer element in accordance with the present invention. That is, the transducer 90 includes a polyvinylidene piezoelectric film 91 of 76 µm thickness and 10 cm² surface area, a pair of electrodes 94a and 94b sandwiching the piezoelectric film 91, and a polyethylene telephthalate additional layer 92 in back of the rear side electrode 94b. The thickness of the additional layer is 25 µ m for a test piece I and 100 µm for a test piece IL. The additional layer 92 is backed by a copper reflector plate 96 of 168µm thickness, and further by a PMMA substrate 93. A water bath was used for the acoustic transmission medium ATM.
Frequency characteristics of electro-acoustic conversion loss TL_f were measured using the test pieces I and II and the obtained result is shown in Fig. 26B. The transducer used in this Example is different from that shown in Fig. 25A (comparative example) in that an additional layer 92 is interposed between the polyvinylidene fluoride piezoelectric film 91 and the copper reflector plate 96. Due to insertion of the additional layer, the resonant frequency is lowered from about 7 MHz (Fig. 25B) to about 5 MHz (Fig. 26B). Thus, the effect caused by presence of the additional layer is clearly indicated.

EFFECT OF THE INVENTION

The results of the above-described Examples clearly indicate that use of the additional layer in the present invention, either on the front or rear side of the polyvinylidene piezoelectric film, assures apparent lowering in resonant frequency and broader frequency band for loss values characteristic curves. Simple adjustment in thickness of the additional layer enables low losses even at high frequencies.
An electro-acoustic transducer with low losses can be produced without any increase in thickness of the polyvinylidene fluoride piezoelectric film whose mechanical loss is rather on the higher side. Presence of the additional layer gives reliable projection to the polyvinylidene film against a wide variety of thinkable external attacks.

Claims

1. An improved electro-acoustic transducer element comprising

a polymer piezoelectric film having an acoustic impedance Z_o, and

at least one additional layer coupled to at least one side surface of said polymer piezoelectric film and having an acoustic impedance Z which is equal or very close to said first-named acoustic impedance Z_o.

2. An improved electro-acoustic transducer element as claimed in claim 1 in which
said acoustic impedances are related to each other as follows;

3. An improved electro-acoustic transducer element as claimed in claim 2 in which
said second-named acoustic impedance Z is related to the acoustic impedance Z_f of water as follows;

4. An improved electro-acoustic transducer element as claimed in claim 1 in which
said additional layer is made of a polymer.

5. An improved electro-acoustic transducer element as claimed in claim 4 in which
said additional layer is made of a polymer same as that used for said polymer piezoelectric film.

6. An improved electro-acoustic transducer element as claimed in claim 4 in which
said polymer is chosen from a group consisting of polyethylene telephthalate, polycarbonate, PMMA, polystylene, ABS, polyethylene, vinyl chloride, polyamide, aromatic polyamide and polyvinylidene fluoride.

7. An improved electro-acoustic transducer element as claimed in claim 6 in which
said polymer includes inorganic compound.

8. An improved electro-acoustic transducer element as claimed in claim 6 in which
said polymer includes carbon fibers.

9. An improved electro-acoustic transducer element as claimed in claim 6 in which said polymer includes metallic fibers.

10. An improved electro-acoustic transducer as claimed in claim 1 in which
said additional layer is coupled to the front side surface of said polymer piezoelectric film.

11. An improved electro-acoustic transducer element as claimed in claim 10 in which

said additional layer is coupled to the front side surface of said polymer piezoelectric film via a front side electrode, and

said polymer piezoelectric film is backed with a rear side electrode.

12. An improved electro-acoustic transducer element as claimed in claim 10 in which

said additional layer is fixed to the front side surface of said polymer piezoelectric film, and

both, as an integral body, are sandwiched by a pair of electrodes.

13. An improved electro-acoustic transducer as claimed in claim 1 in which
said additional layer is coupled to the rear side surface of said polymer piezoelectric film.

14. An improved electro-acoustic transducer element as claimed in claim 13

said additional layer is coupled to the rear side surface of said polymer piezoelectric film via a rear side electrode, and

a front side electrode is fixed to the front side surface of said polymer piezoelectric film.

15. An improved electro-acoustic transducer element as claimed in claim 14 in which
said additional layer is backed with an acoustic reflector plate.

16. An improved electro-acoustic transducer element as claimed in claim 13 in which

said additional layer is fixed to the rear side surface of said polymer piezoelectric film, and

both, as an integral body, are sandwiched by front and rear side electrodes.

17. An improved electro-acoustic transducer element as claimed in claim 16 in which
said rear side electrode is backed with an acoustic reflector plate.

18. An improved electro-acoustic transducer element as claimed in claim 1 in which
two additional layers are coupled to both side surfaces of said polymer piezoelectric film.

13. An improved electro-acoustic transducer element as claimed in claim 18 in which
a front side additional layer is coupled to the front side surface of said polymer piezoelectric film via a front side electrode.

20. An improved electro-acoustic transducer element as claimed as claim 19 in which
a rear side additional layer is coupled to the rear side surface of said polymer piezoelectric film via a rear side electrode.

21. An improved electro-acoustic transducer element as claimed in claim 19 in which
a rear side addtional layer is fixed to the rear side surface of said polymer piezoelectric film and further backed with a rear side electrode.

22. An improved electro-acoustic transducer element as claimed in claim 18 in which
a front side additional layer is fixed to the front side surface of said polymer piezoelectric film and forwardly covered with a front side electrode.

23. An improved electro-acoustic transducer element as claimed in claim 22 in which
a rear side additional layer is coupled to the rear side surface of said polymer piezoelectric film via a rear side electrode.

24. An improved electro-acoustic transducer element as claimed in claim 22 in which
a rear side additional layer is fixed to the rear side surface of said polymer piezoelectric film and further backed with a rear side electrode.

25. An improved electro-acoustic transducer element as claimed in claim 1 in which
said polymer piezoelectric film and said additional layer are both forwardly concave.