EP0142178B2 - Ultrasonic transducer - Google Patents

Description

The present invention relates to an ultrasonic transducer comprising a substrate constituting a rear medium, a layer of piezoelectric material and one or more adaptation layers, the values of the impedances of the layer of piezoelectric material, acoustic impedance adaptation layers and of the medium before propagation forming, considered in this order, a decreasing sequence.

An ultrasonic transducer essentially consists, in a conventional manner, of a substrate constituting a rear absorption or reflection medium, a layer of piezoelectric material equipped with electrodes on its front and rear faces and at least one layer. acoustic impedance matching, placed in front of the piezoelectric material, between it and the propagation medium. Transducers of this type are notably described in the article "The effects of backing and matching on the performance of piezoelectric ceramic transducers" by G. Kossoff, published in the journal IEEE Transactions on sonics and ultrasonics, volume SU-13, March 1966 , pages 20 to 30. The implementation of one or more of these adaptation layers has the main effect of improving the sensitivity of the transducers and also contributes to increasing their bandwidth.

It will be recalled here that the ultrasonic transducers used in ultrasound must combine two main qualities at the level of transduction: not only good sensitivity (because the increase in signal-to-noise ratio facilitates the processing of received signals) but also sufficient damping (because the brevity of the impulse response conditions the axial resolution).

The object of the invention is to propose an ultrasonic transducer which easily reconciles the requirements of sensitivity and damping.

To this end, the invention relates to an ultrasonic transducer as defined in the preamble to the description and further characterized in that the adaptation layers are placed in identical number on either side of the piezoelectric material, the layers located symmetrically two by two having the same acoustic impedance value and the same thickness, in that the rear medium has an acoustic impedance value substantially equal to that of the medium before propagation, and in that the thickness of the layer of piezoelectric material is equal to half the wavelength associated with the resonant frequency of the transducer, so that the structure is symmetrical with respect to the median plane of the layer of piezoelectric material.

The published European patent application No. EP-A-0015886 describes various embodiments of ultrasonic transducers which all comprise on the one hand a layer of piezoelectric material and on the other hand one or more so-called additional layers, placed just in front and / or behind said piezoelectric material and which have an acoustic impedance equal to or very close to that of this material. The so-called Cook-Redwood modeling, exposed for the first time by EG COOK, in 1956, in the communication "Transient and steady-state response of ultrasonic piezoelectric transducers", IRE Conv. Record, 4, 1956, pages 61-69, and generalized by M. Redwood, allows however to carry out the mathematical analysis of the structures proposed in this cited document and to show that these additional layers play a piezoelectric role. This analysis indeed shows that the regime of ultrasonic vibrations is established not only in the piezoelectric material, but in the overall cavity formed by this material and the additional layer or layers. These layers artificially increase the thickness of the piezoelectric material, and therefore lower the working frequency thereof, to make this frequency compatible with the range of frequencies within which locate medical applications. They therefore play a role unrelated to the damping role held by the acoustic impedance matching layers provided in the case of the present invention.

The features and advantages of this invention will now be described below in more detail with reference to FIG. 1, given by way of nonlimiting example and which shows an embodiment of a transducer according to the invention.

The embodiment represented in FIG. 1 consists of an ultrasonic transducer with vibration in thickness mode, totally symmetrical, composed of a substrate 10 constituting the rear medium of the transducer, of a layer 20 of piezoelectric material of thickness equal to the half the wavelength associated with the resonant frequency of the transducer and covered on its front and rear faces with metal sheets 21 and 22 constituting first and second electrodes (connected in a known manner to a polarization circuit not shown which provides the excitation potential), and of two acoustic impedance adaptation layers 30, 40 known as quarter-wave interference layers and situated on the one hand between the rear medium and the piezoelectric material and on the other hand between this material and the medium before propagation 50. In this structure, the values of the acoustic impedances form a decreasing sequence from that of the piezoelectric material and these impedance values as well as the thicknesses of the adaptation layers 30, 40 are symmetrical on either side of this material. From this symmetry of the structure, it follows that the deformations on the two faces of the piezoelectric material are identical (since these two faces are acoustically charged identically) and that, consequently, the deformation is zero in the median plane of this material. The part of the structure which is located on only one side of this median plane is therefore equivalent to an infinitely rigid rear medium, that is to say with zero deformation.

• (a) impédances (en kg/cm².sec x 10⁶) :
  • milieu arrière : 1,5
  • couches d'adaptation : 1,8 et 4
  • matériau piézoélectrique : 30
  • couches d'adaptation : 4 et 1,8
  • milieu avant de propagation : 1,5
• (b) résultats obtenus :
  • indice de sensibilité : -13 dB
  • largeur de bande relative à -6 dB = 53 %
  • durée de réponse à -20 dB = 7,79 τ
  • durée de réponse à -40 dB = 9,8 τ

On rappellera ici que la sensibilité est caractérisée par un indice de sensibilité dont l'expression en dB est du type 20 log V_S/V_REF où V_REF est, pour un générateur d'impédance interne adaptée à sa charge, la tension permettant l'émission d'une impulsion résonnante rectangulaire et où V_S est la tension crête-à-crête de la réponse, et que l'amortissement est généralement caractérisé par la largeur de bande relative à -6 dB dF/F du spectre fondamental, exprimée en % et dans laquelle dF est l'écart entre les points où l'amplitude électrique est à -6 dB sous le maximum et F la fréquence centrale correspondant audit maximum. Cependant, cette dernière information est insuffisante pour caractériser complètement l'amortissement puisqu'elle ne tient compte ni de la forme, qui peut être irrégulière, du spectre fondamental ni de la présence d'harmoniques supérieurs qui perturbent la fin des échos, et elle est complétée par deux autres indicateurs temporels qui sont les durées de la réponse électrique à -20 dB et à -40 dB étant définis par les instants auxquels l'amplitude crête-à-crête est devenue inférieure respectivement aux dixième et au centième de sa valeur initiale) à une impulsion résonnante resctangulaire de durée τ, ces durées étant normées c'est-à-dire exprimées par référence audit temps de vol τ. Dans le cas où le matériau piézoélectrique est du polyfluorure de vinylidène, on peut citer de même l'exemple suivant (à une couche d'adaptation d'impédance acoustique) :

• (a) impédances :
  • milieu arrière et avant : 1,5
  • couches d'adaptation arrière et avant : 1,8
  • matériau piézoélectrique : 4,6
• (b) résultats obtenus :
  • indice de sensibilité : -23,8 dB
  • largeur de bande relative à -6 dB = 75 %
  • durée de réponse à -20 dB = 5,63 τ
  • durée de réponse à -40 dB = 8 τ

The tests and simulations carried out with a structure thus constituted show that the spectrum (or module of the Fourier transform) of the electrical response in ultrasound mode to an electrical excitation of impulse type and of effective duration equal to the time of flight in the piezoelectric material (the flight time is the duration of the path of the ultrasonic waves from one face to the other of the piezoelectric material) vibrating according to its thickness equal to half the ultrasonic wavelength at the emission frequency of the transducer is Gaussian form; consequently, the envelope of the electrical response is also Gaussian and this response is quickly amortized. The tests carried out (under equal electrical conditions of emission and reception) showed the possibility of effectively obtaining various structures meeting the objectives of the invention (sensitivity and damping simultaneously satisfactory). In the case where the piezoelectric material is a ferroelectric ceramic of the PZT-5 type (piezoelectric material based on lead zirconate titanate: see the work "Physical Acoustics, Principles and Methods", by Warren P. Mason, Vol.1, part A, page 202), we can cite the following example (with two acoustic impedance matching layers) of such a structure called total symmetry:

• (a) impedances (in kg / cm².sec x 10⁶):
  • mid back: 1.5
  • adaptation layers: 1.8 and 4
  • piezoelectric material: 30
  • adaptation layers: 4 and 1.8
  • medium before propagation: 1.5
• (b) results obtained:
  • sensitivity index: -13 dB
  • relative bandwidth at -6 dB = 53%
  • response time at -20 dB = 7.79 τ
  • response time at -40 dB = 9.8 τ

It will be recalled here that the sensitivity is characterized by a sensitivity index whose expression in dB is of the type 20 log V _S / V _REF where V _REF is, for an internal impedance generator adapted to its load, the voltage allowing l emission of a rectangular resonant pulse and where V _S is the peak-to-peak voltage of the response, and that the damping is generally characterized by the relative bandwidth at -6 dB dF / F of the fundamental spectrum, expressed in% and in which dF is the difference between the points where the electrical amplitude is at -6 dB below the maximum and F the central frequency corresponding to said maximum. However, this latter information is insufficient to fully characterize the damping since it takes into account neither the shape, which can be irregular, the fundamental spectrum nor the presence of higher harmonics which disturb the end of the echoes, and it is supplemented by two other time indicators which are the durations of the electrical response at -20 dB and at -40 dB being defined by the instants at which the peak-to-peak amplitude has become respectively less than tenth and one hundredth of its initial value ) to a resectangular resonant pulse of duration τ, these durations being normalized, that is to say expressed by reference to said flight time τ. In the case where the piezoelectric material is polyvinylidene fluoride, the following example can also be cited (with an acoustic impedance matching layer):

• (a) impedances:
  • mid back and front: 1.5
  • rear and front adaptation layers: 1.8
  • piezoelectric material: 4.6
• (b) results obtained:
  • sensitivity index: -23.8 dB
  • relative bandwidth at -6 dB = 75%
  • response time at -20 dB = 5.63 τ
  • response time at -40 dB = 8 τ

Of course the present invention is not limited to the embodiments described, from which variants can be proposed without departing from the scope of the invention, in particular those in which a different number of layers of material would have been chosen. adaptation of acoustic impedance between the piezoelectric material and the extreme environments.

Claims (1)

1. An ultrasonic transducer, comprising a substrate (10) which forms a backing medium, a layer (20) of piezoelectric material, and one or more matching layers (30, 40), the impedance values of the layer (20) of piezoelectric material, of the acoustic impedance matching layers, and of the propagation medium (50) in front forming a descending progression in this order, characterized in that an equal number of matching layers (30, 40) is provided on both sided of the piezoelectric material (20), the pair-wise symmetrical situated layers having the same acoustic impedance value and the same thickness, in that the backing medium has an acoustic impedance value which is substantially equal to that of the propagation medium (50) in front, and in that the thickness of the layer (20) of piezoelectric material is equal to one half of the wavelength associated with the resonant frequency of the transducer, so that the structure is symmetrical with respect to the central plane of the layer of piezoelectric material.

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