Piezoelectric Materials for Ultrasonic Transducers : Review of Recent Developments

First, a brief review of available classes of piezoelectric materials (in particular emergent piezoelectric single crystals) and specific requirements for medical imaging applications is made. Several piezoelectric materials are retained for two specific applications (50 MHz high frequency single element transducer and high density 2D array) to simulate and compare the overall transducer performance. This is performed by combining the K.L.M model to calculate the parameters and an optimisation method to extract the best design which gives the transducer performance. Single crystals are shown to be promising, particular for high density arrays. INTRODUCTION Transducer performance requirements are more and more demanding due to a wide range of imaging modalities, such as flow and harmonic modes, which are now integrated in echographic systems. Transducer performance depends on its internal structure and on constitutive material properties, the most important of which is the active piezoelectric element. During the past fifty years, a succession of new piezoelectric material as been discovered, from Barium titanate (BaTiO3) in the 1950s to piezoelectric single crystal based on a relaxor (PZN or PMN) and PT solid solution, constantly improving the electromechanical properties [1]. This paper gives a brief review of the piezoelectric materials and pertinent parameters used for medical imaging applications. To study and quantify, the overall performance of a transducer integrating these different piezoelectric materials, it is very difficult to define a universal figure of merit for a large range of configurations or applications. So, two precise configurations have been retained. First a high frequency single element transducer with a centre frequency at 50 MHz will be simulated with six different piezoelectric materials. Many applications require such transducers as dermatology, ophthalmology or small animal imaging. Secondly, one element of a high density 2D array with a centre frequency at 3.5 MHz is studied. These results will allow to show which piezoelectric material delivers the highest transducer performance for a precise configuration. GENERAL AND SPECIFIC REQUIREMENTS The classical single-element transducer, in particular for medical imaging applications, is based on a piezoelectric plate or disc poled along the thickness direction, whose thickness defines the resonance frequency of the device. The plate, typically a ferroelectric ceramic, has an acoustic impedance (i.e. around 33 MRa) much higher than of biological tissues (close to that of water, i.e. 1.5 MRa). This large difference leads to an acoustical mismatch and a poor axial resolution. Consequently, other layers are added to the active layer. On the front (i.e. between the piezoceramic and the propagation medium), matching layers are used. The thickness of a matching layer is generally around a quarter-wavelength at the resonance frequency, and its acoustical impedance is intermediate between those of the piezoceramic and tissues. The use of a matching layer thus improves the sensitivity of the transducer. Moreover, since the acoustical energy can better flow towards the tissues, the duration of acoustical resonance in the active layer is decreased. Consequently, the matching layer also improves axial resolution. The use of multiple matching layers, based on the same principle, can further improve transducer performance. On the rear face of the active element, a thick layer is usually added. It is referred to as the backing which allows acoustic energy to flow by the rear face. The closer its acoustical impedance is to that of the active layer, the more energy is lost. The consequence is a lower sensitivity but a higher axial resolution. Thus, a trade-off has to be performed for each application. The attenuation coefficient and the thickness of the backing layer must be sufficient so that no energy can be radiated back to the active layer, which would produce parasitic echoes. For the piezoelectric material, in particular for medical applications, two of the most important material parameters for transducer applications are the effective electromechanical coupling coefficient kef f of the main vibration mode used and the acoustic impedance Z. The kef f factor represents the piezoelectric activity of the material in the considered mode of vibration. It should be as high as possible. This factor depends not only on the material properties but also on the geometry of the active element. In medical imaging applications, all vibration modes are longitudinal, i.e. the displacements are in the poling direction which defines the thickness dimension. For large plates or discs (thickness much lower than lateral dimensions), the thickness coupling factor kt is used. For bars or pillars (thickness higher than lateral dimensions), the factor is k33. For the intermediate case of an array element (one small and one large lateral dimensions with a thickness value between them), k’33 factor is defined. The value of the dielectric constant also has an important role on the electrical matching. COMMONLY USED PIEZOELECTRIC ELEMENTS Figure 1 represents the values of the thickness mode coupling factor (k t) versus the acoustic impedance (Z) for a wide range of available piezoelectric materials. It can be observed that no material allows to obtain both high coupling and acoustic impedance equal to that of biological tissues. For almost all medical transducer applications, PZT piezoceramics are used because of their high coupling factor, even through their acoustic impedance is high, which can be compensated by using acoustic matching layer in the transducer structures. Many types of PZT piezoceramics are available on the market, with different properties due to doping by additives in varying proportions and to specific fabrication processes. For a given application, properties such as dielectric constant and grain size allow to choose a specific material. A wide range of references can be found from relatively low (a few hundred) to very high (a few thousand) relative dielectric constants with grain sizes from one to ten micrometers. For large area devices such as single element transducers, a moderate dielectric constant allows good electrical matching to cables and electronics (which are typically at 50 to 80 ohms), while array elements require much higher dielectric constants. For high frequency devices or for arrays (whenever small dimensions must be achieved), fine grain materials are required. Considering these requirements, soft PZT materials are commonly used such as PZT5A (or Ferroperm Pz27) with moderate dielectric constant or PZT-5H (or Ferroperm Pz29) with higher dielectric constant. At the end of the 70’s, piezocomposites combining a high coupling piezoceramic (such as soft PZT) and a low acoustic impedance polymer (such as an epoxy resin) appeared. These materials allow to obtain a very good trade-off between a high coupling and a low acoustic impedance. For ultrasonic medical transducer applications, ceramic pillars in a polymer matrix (called 1-3 connectivity) are of great interest. A high thickness coupling factor kt, close to the value of k33 in the ceramic alone, is obtained even for relatively low ceramic contents, for which the acoustic impedance is around three to four times lower than that of pure ceramic. These properties allow both sensitivity and bandwidth to be increased in transducers, in comparison with classical piezoceramic devices. 0 10 20 30 40 50 60 70 80 90 100