GB2097630A - Ultrasonic transducers - Google Patents

Abstract

An ultrasonic piezoelectric transducer comprising an acoustic lens (3), a crystal or piezoelectric element (2), an element (9) for attenuating transverse distortions and a damper (7), characterised in that the lens consists of a resin in which metallic particles of small diameter are distributed, the lens being placed in direct contact with the piezoelectric crystal which, in turn, is enclosed in a damper consisting of a resin in which metallic particles of large diameter are distributed; and in that in the body of the damper is embedded the electrical contact end of the electrical signal phase wire, the damper and the crystal being surrounded by the attenuating element for transverse distortions. <IMAGE>

Description

SPECIFICATION Simplified performing ultrasonic transducers The present invention relates to a method of producing the constituents of ultrasonic transducers (resin and metal powder or metal oxide) having predetermined acoustic and electrical characteristics. The present invention seeks to provide a simplified method for the production of performing ultrasonic transducers having optimum qualities of reproducibility of both acoustic and electrical characteristics.

Certain elements of an ultrasonic transducer are constituted by resins which are charged (filled) to a greater or lesser extent with a metal powder. These generally comprise a damper for longitudinal waves, an attenuatorfortransverse distortions, and an acoustic impedance matching device which sometimes also acts as a focusing lens.

The performance of the transducer, and above all the reproducibility of its characteristics, depends to a large extent on the care with which the composite is prepared and, in particular, on the choice of the base materials used and on their proportions.

At present, a series of transducers originating from the same manufacturer with the same constituents generally exhibits a very troublesome diversity of characteristics which sometimes, are even unacceptable to the user.

Furthermore, each piezoelectric element (monocrystal or ceramic) has to be connected via two faces; thus necessitating the metallization of those faces and the welding thereonto of electrical conductors - an operation which is always very difficult.

To obviate the above-mentioned drawbacks, according to the present invention, a new type of transducer is proposed.

A double study, theoretical and experimental, of the propagation of ultrasonic waves in composite media has made it possible to define and make available to the manufacturer the equations of the following parameters, as a function of the concentration ofthe powderfilling or charge: 1) Voluminal mass M which is not dependent on the dimensions of the sample nor on the mode of propagation of the acoustic wave; 2) Propagation velocity of longitudinal waves C; 3) Characteristic acoustic impedance Zas a function of the two aforementioned parameters:: (1) Z = M x C 4) Absorption coefficient a, the value ofwhich mainly depends on the frequency and particle size of the charge for a given concentration; 5) Electrical conductivity of the composite, which is dependent upon the concentration and nature of the constituents (for a given concentration), and also on the particle size of the charge.

In accordance with the present invention there is provided an ultrasonic piezoelectric transducer comprising an acoustic lens, a crystal or piezoelectric element, an element for attenuating transverse distortions and a damper, characterised in that the lens consists of a resin in which metallic particles of small diameter are distributed, the lens being placed in direct contact with the piezoelectric crystal which, in turn, is enclosed in a damper consisting of a resin in which metallic particles of large diameter are distributed; and in that in the body of the damper is embedded the electrical contact end of the electrical signal phase wire, the damper and the crystal being surrounded by the attenuating element for transverse distortions.

Preferably the metallic particles consist of tungsten or aluminum, and are distributed in the resin according to a variable, predetermined concentration.

Desirably the particles dispersed in the resin are oxides, and the resulting composite being noncnductive.

The present invention also provides a method of manufacturing ultrasonic transducers of the invention in a conventional manner but characterised in that the propagation velocity of the ultrasonic wave in the medium and the density of all the elements constituting the medium are calculated as a function of the different concentrations, the product of the velocity and density giving the acoustic impedance for the different concentrations.

One embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a cross-sectional view of a conventional transducer, Figure 2 is a cross-sectional view of a transducer of the present invention, Figure 3 is a graph showing the variation of three transducer properties with varying concentrations of filler, and Figures 4 and 4b are graphs showing the variation of absorption with the size and concentration of filler.

Figure 1 illustrates a conventional transducer. The electrical signal transmitted by 1 on the piezoelectric crystal 2 is converted into an acoustic signal which, by means of the acoustic lens 3, is sent towards 4, the element to be analysed. The two faces 5 and 6 of the piezoelectric crystal 2 in contact with the damper 7 and the lens 3 are metallized. The transducer components are contained within a casing 8. The elements 2 and 7 are surrounded by a cylinder 9 which serves as an attenuator of transverse distortions.

Figure 2 shows the transducer forming the subject-matter of the present invention (like reference numerals designate identical elements). Here the omission of metallization on the piezoelectric element 2 is apparant. The supply takes place (a) through an electrode 1, which is embedded in the damper 7, and the damper itself, and (b) through the metal casing 8 and the acoustic lens 3. The cylindrical attenuator 9 of transverse distortions is nonconductive and encloses the damper 7 and the element 2.

This solution makes possible a substantial simplification of the manufacture of transducers.

In designing a damped transducer (wideband), it is necessary to satisfy two conditions; namely that the acoustic impedance of the damper is as close as possible to that of the piezoelectric crystal, and that the wave propagated in the damper is attenuated as much as possible in this latter.

The diversity of characteristics which is generally noted in the same series of conventional transducers is caused, inter alia, by an arbitrary choice of resin, of concentration of the charge and of particle size of the latter. In other words, the acoustic impedance of the damper is reproducible only with difficulty and its attenuation capacity is even more difficult to reproduce.

Figure 3 shows the curves relating to the density M, in Kg/m3, the propagation velocity of the wave C in m/s as a function of the concentration K (metallic powder/resin); in accordance with the formula (1) there is obtained the curve Z for acoustic impedance.

Now, as a result of the present method and after measuring M and the propagation velocity C in a resin sample without charge, it will be possible to determine the variations in parameters and thus in their product Z, as a function of the concentration of the charge K.

If the manufacturer wishes to attain the same acoustic impedance with a different resin, he will only have to refer to the graph relating to this second resin so as to find the new concentration to which the Z required corresponds.

Figure 4 shows the variation in absorption a in db/mm as a function of the size of the metallic particles for a given frequency (in the case of Figure 4 it is 2 MHz) and with different concentrations K.

In Figure 4b it is evident that absorption decreases upon increasing the frequency (5MHz); the other parameters remaining the same.

In Figures 4 and 4b, the concentration refers to the use of a metallic powder of tungsten in Araldite.

With a charge particle size/wavelength ratio of S 1/100, there are no substantial variations in velocity as a function of the frequency for a given concentration; moreover, absorption complies with a relative simple law : a = + (x) for a given frequency and particle size.

Accordingly, it is possible for the manufacturer to design his damper in accordance with the graphs of Figures 3 and 4.

For the attenuator 9 of transverse distortions, the mode of production is analogous to that used previously with regard to the damper, but here selecting a non-conductive composite, for example Araldite and aluminium.

The impedance matching device 3 may be devised in two ways: by choosing its acoustic impedance to be homogeneous and equal to the geometrical mean of the impedance of the piezoelectric element and of the transmission medium, or with an acoustic impedance varying progressively from that of the piezoelectric element towards that of the transmission medium. The same graphs in Figures 3,4 and 4b are here used to satisfy the conditions of impedance, on the one hand, and of conductivity, on the other hand, and finally of minimum absorption (fine particle size). If the matching device also serves as an acoustic focusing lens, its characteristic velocity of propagation at the interface with the transmission medium will be plotted so as to define its concavity.

Claims (9)

1. An ultrasonic piezoelectric transducer comprising an acoustic lens, a crystal or piezoelectric element, an element for attenuating transverse distortions and a damper, characterised in that the lens consists of a resin in which metallic particles of small diameter are distributed, the lens being placed in direct contact with the piezoelectric crystal which, in turn, is enclosed in a damper consisting of a resin in which metallic particles of large diameter are distributed; and in that in the body of the damper is embedded the electrical contact end of the electrical signal phase wire, the damper and the crystal being surrounded by the attenuating element for transverse distortions.

2. A transducer as claimed in claim 1 wherein the metallic particles dispersed in the resin are distributed therein according to a variable, predetermined concentration.

3. A transducer as claimed in claim 1 or claim 2 wherein the metallic particles consist of tungsten.

4. A transducer as claimed in claim 1 or claim 2 wherein the metallic particles consist of aluminium.

5. A transducer as claimed in any one of the preceding claims wherein the particles dispersed in the resin are oxides.

6. A transducer as claimed in any one of the preceding claims wherein the attenuator element for transverse distortions comprises a resin in which metallic particles are distributed; the resulting composite being non-conductive.

7. A transducer as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in Figure 2 ofthe accompanying drawings.

8. A method of manufacturing ultrasonic transducers as claimed in claim 1 in a conventional manner but characterised in that the propagation velocity of the ultrasonic wave in the medium and the density of all the elements constituting the medium are calculated as a function of the different concentrations, the product of the velocity and density giving the acoustic impedance for the different concentrations.

9. A method as claimed in claim 8 substantially as hereinbefore described.