Piezoelectric transducer for ultrasonic waves - has transducer with polymeric piezoelectric element of higher acoustic impedance than reflector and half wavelength thickness

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PIEZOELECTRIC TRANSDUCER

The present invention relates to piezoelectric transducers intended to transmit or receive ultrasonic waves propagating in an environment of acoustic impedance close to that of skin. These piezoelectric transducers find use, for example, in non-traumatic test systems such as those used for diagnostic methods on biological tissues whether in the case of humans or animals.

These non-traumatic testing systems use this transducer as a transmitter or receiver. The transmitting transducer is used to excite longitudinal ultrasonic waves in biological tissues, the waves being emitted in a frequency band which extends from a few megahertz to several tens of megahertz. The receiving transducer works on the principle of the reverse piezoelectric effect and is used to collect the echoes after insonification of biological tissues. The comparison between the emitted pulse and the examined echo pulse makes it possible to visualize the structure for diagnostic purposes. To obtain an ultrasound with good resolution, it is important to have transducers with a wide frequency band and therefore with a short impulse response. On the other hand, the transducer must radiate the maximum ultrasonic energy in a hemispherical body, which requires the presence of an energy return on the face of the transducer not coupled to the medium to be insonified.

A detector of the prior art comprises an active piezoelectric element associated on its rear face with a reflecting structure of acoustic impedance greater than that of the material constituting the piezoelectric element. An impedance transformer is necessary to obtain a high impedance value brought to the rear face of the active piezoelectric element. It is advantageous to use as piezoelectric material piezoelectric polymers, such as for example polyvinylidene fluoride or PVF2, because the acoustic impedance of the polymers is closer to that of biological tissues than is that of ceramics . In addition, the ceramics are rigid, fragile and the edges in the event of breakage are sharp. The reflector assembly used is often a copper or bronze sheet associated with a solid epoxy resin, the acoustic impedance brought down on the rear face of the active element is approximately ten times that of the polymers. The copper foil behaves like an impedance transformer capable of detecting the impedance in a restricted frequency range or even for a central frequency of a working frequency band. This is why a transducer of the prior art has as main drawback a small working frequency band. Indeed, the transformer is an overvoltage element which works only in a narrow frequency range.

The present invention proposes to overcome this drawback by proposing a piezoelectric transducer intended for the emission or reception of ultrasonic waves endowed with a wide working frequency band.

To do this, the present invention provides a piezoelectric transducer intended for the emission or reception of ultrasonic waves in a medium (6) having an acoustic impedance close to that of water, said transducer comprising an active element ( 1) in piezoelectric polymer having two large equidistant faces (la, 1b) each carrying at least one electrode (4a, 4b), said electrodes (4a, 4b) forming a capacitor, said active element (1) being polarized so as to vibrate in a thickness mode, one of said large faces (la, lb) being coupled to said medium (6) and the other of said large faces (16) being coupled to a reflective medium (30), characterized in that the acoustic impedance of said reflective medium (3) is substantially lower than the acoustic impedance of said piezoelectric polymer; said active element (1) vibrating in half-wave at the central operating frequency of said transducer.

The present invention will be better understood from the description which follows and from the appended drawings:

Figure 1: Sectional view of part of a transducer of the prior art

Figure 2: Sectional view of part of a transducer according to the invention Figure 3: Sectional view of part of a transducer according to the invention provided on its front face with a quarter-wave layer , and on its rear face of a reflective medium made of porous material

Figure 4: Sectional view of a transducer according to the invention mounted on a hollow housing

Figure 5: Sectional view of a transducer according to the invention, the active element being fixed by its periphery to the wall of a hollow housing filled with porous material and its front face being a concave surface

Figure 6 î Perspective view of a transducer according to the invention provided on its front face with a set of electrodes.

FIG. 1 shows a sectional view of part of a piezoelectric transducer of the prior art comprising an active element 1 made of piezoelectric polymer, a reflecting medium 3 and an impedance transformer 2. The active element 1 is a piezoelectric polymer plate which is provided on each of the large faces la, ib with at least one electrode 4a, 4b. The reflective medium 3, disposed at the rear of the active element i, is a material with an acoustic impedance close to that of the polymer, it is for example bakelite or epoxy. To bring the highest possible impedance to the rear face 1b of the active element 1, the impedance transformer is a sheet 2, made of copper, for example, the thickness of which is calculated to maximize its effect at a frequency of operation. Under these conditions, the impedance brought to the rear face 1b of the active element 1 varies in the working frequency band. This effect, added to the fact that the active element 1 vibrates in a quarter-wave plate leads to a relatively narrow frequency band.

In Figure 2, there is shown a sectional view of part of a piezoelectric transducer according to the invention. The transducer comprising an active element 1 and a reflective medium 30 without any interposition of an impedance transformer between the two. The active element 1 is, for example, made of polyvinylidene fluoride or PVFg, it is provided on each of its large faces la, lb with at least one electrode 4a, 4b forming a capacitor. Said active element being polarized and excited so as to vibrate according to a thickness mode, it is according to the invention a half-wave plate of thickness:

e = (n + 1) where n is a natural integer and λ the wavelength. That is to say that the acoustic wave leaving the front face 1a and the acoustic wave emitted on the face and arriving after having crossed twice, once on the outward journey and once on the return, said active element 1 , have a phase shift of 2, i.e. are in phase for the central operating frequency of the working frequency band. The reflective medium 30 in contact with the rear face 1b is, unlike the prior art, chosen from materials having an acoustic impedance much lower than that of the piezoelectric polymer material. A particularly advantageous material since it has a substantially zero acoustic impedance is Even. This allows to have, unlike the previous part given the fact of the absence of sheet 2 impedance transformer, an impedance reduced on the rear face lb constant and substantially zero over the entire working frequency band. The active element 1 then works in a significantly wider frequency band than in the previous part, and therefore taking into account the reflective medium 30 with a high reflection coefficient.

FIG. 3 shows a sectional view of part of a piezoelectric transducer according to the invention provided on the front face 1a, with the active element 1, with an impedance-adapting layer 5 and on the rear face 1b of a reflecting medium 30 made of porous material. A piezoelectric transducer being intended for the emission or reception of ultrasonic waves in a medium 6 having an acoustic impedance close to that of water, which is the case for biological tissues and for the gels interposed between the transducer and biological tissues, a layer 5 advantageously performs the impedance matching between the impedance of the medium 6 and the impedance of the polymer material of the active element 1. The impedance matching layer 5 is a quarter wave layer whose impedance of the material making it up is between that of the medium 6 and that of the piezoelectric polymer material. Preferably, the acoustic impedance of the material of said impedance-adapting layer 5 is substantially equal to the geometric mean of the impedance of said medium 6 and the impedance of the piezoelectric polymer. It is understood that the quarter-wave plate has a thickness given by the relation:

e = kv where k is an odd number and λ the wavelength. In the case of the transducer according to the invention, the impedance of the medium 6 being close to that of water and the active element 1 being made of PVF₂, the best results are obtained with an adaptive impedance polystyrene layer 5 quarter wave. The attached table allows you to better understand the characteristic values ​​of the materials used;

<td>Materials</td><td>Density P Qo³.Kg.m</td><td>Speed ​​of sound V [m ·⁵’¹]</td><td>Impedance Acoustic Z = PV _notQcg.m².SJ</td><td>Wave length at 5 MHz A [p «]</td>

<td>Water</td><td> 1</td><td> 1500</td><td> 1500</td><td> 300</td>

<td>pvf₂</td><td> 1,76</td><td> 2200</td><td> 3870</td><td> 440</td>

<td>Polystê-</td><td> 1,06</td><td> 2300</td><td> 2500</td><td> 470</td>

<td>rein</td><td></td><td></td><td></td><td></td>

<td>Air</td><td>io “³</td><td> 330</td><td> 0,330</td><td> 66</td>

If the central operating frequency is, by way of nonlimiting example, equal to 5 MHz, the thicknesses will be respectively for the active element 1, knowing that it is a half-wave plate made of polyvinylidene fluoride, of 220 μ m and for the impedance-adapting layer 5, knowing that it is a quarter wave polystyrene layer, of 118 μm. If the reflecti ve medium 30 is a polyurethane foam, the average diameter of the cells, which is equal to the wavelength of the radiation in air for the central frequency, will be 66 µm.

In one embodiment shown in FIG. 4, a transducer according to the invention is mounted on a hollow housing 7. The active element 1 made of piezoelectric polymer is fixed by its periphery to the wall of the hollow housing 7. The reflecting medium 30 is a gas which remains trapped in the cavity, it can advantageously be air. The active element 1 is provided with electrodes 4a, 4b, obtained by metallization, or by any other known means, of the front faces la and rear lb. A metal washer 8, made of copper for example, interposed between the hollow housing 7 and the active element 1 ensures electrical contact with the rear electrode 4b. In FIG. 4, the active element 1 has faces 1a, 1b which are equidistant and planar. Since the emission or reception of the ultrasonic waves takes place towards a medium 6 with an acoustic impedance close to that of water, a layer 5 in quarter wave ensures the impedance adaptation.

In another embodiment shown in FIG. 5, a transducer according to the invention is also mounted on a hollow housing 7, the active element 1 being fixed by its periphery to the wall of the hollow housing 7. The reflective medium 30, is in this embodiment a polyurethane foam, for example, which fills the cavity of said hollow housing 7 and comes into contact with said active element 1. Therefore, the active element can take a predetermined shape, as for example in Figure 5 one of the faces is concave and the other convex. The active element 1 conforms to the imprint left in the foam which ensures its maintenance. If the predetermined shape is, for example, an organ or a member, it is not necessary to have a hollow housing 7, the foam itself forming the support of the active element 1 while marrying the shape given to this active element 1.

According to another embodiment shown in FIG. 6, the active element 1, of a transducer according to the invention, is provided with several electrodes. For example, its front face la is provided with three electrodes 9a, 10a, 11a while its rear face lb receives a counter electrode 4b. Said electrodes 9a, 10a, 11a and the counter electrode 4b, cooperate in order to constitute the adjacent cells of a network of transducers which in FIG. 6 extends in one dimension, but which can also form several alignments. Similarly, a network can be formed by several transducers mounted on individual hollow boxes as described above to form bars.

As can be seen, the invention is not limited to the sole exemplary embodiments given, the person skilled in the art can modify certain characteristics described here or group together a network of transducers according to the invention without thereby departing from it.