RESUMEN
Thick films with nominal composition (K0.5Na0.5)0.99Sr0.005NbO3 (KNNSr) on porous ceramics with identical nominal composition were investigated as potential candidates for environmentally benign ultrasonic transducers composed entirely of inorganic materials. In this paper, the processing of the multilayer structure, namely, the thick film by screen printing and the porous ceramic by sacrificial template method, is related to their phase composition, microstructure, electromechanical, and acoustic properties to understand the performance of the devices. The ceramic with a homogeneous distribution of 8 µm pores had a sufficiently high attenuation coefficient of 0.5 dB/mm/MHz and served as an effective backing. The KNNSr thick films sintered at 1100 °C exhibited a homogeneous microstructure and a relative density of 97%, contributing to a large dielectric permittivity and elastic constant and yielding a thickness coupling factor kt of ~30%. The electroacoustic response of the multilayer structure in water provides a centre frequency of 15 MHz and a very large fractional bandwidth (BW) of 127% at -6 dB. The multilayer structure is a candidate for imaging applications operating above 15 MHz, especially by realising focused-beam structure through lenses to further increase the sensitivity in the focal zone.
RESUMEN
For transducer design, it is essential to know the acoustic properties of the materials in their operating conditions. At frequencies over 15 MHz, standard methods are not well adapted because layers are very thin and backings have very high attenuation. In this article, we report on an original method for measuring the acoustic properties in the 15-25 MHz frequency range, corresponding to typical skin-imaging applications, using a backing/piezoelectric multilayer structure. Onto a porous Pb(Zr0.53Ti0.47O3 (PZT) substrate, a piezoelectric PZT-based layer with a thickness of [Formula: see text] was deposited and directly used to excite an acoustic signal into water. Herein, the measured signal corresponds to the wave that is first reflected on a target in water, then propagates back to the multilayer structure, and is transmitted through the thick film and further to the rear face of the porous backing, where it is again reflected and returns to the piezoelectric thick film, thus avoiding overlap with the electrical excitation signal. Two types of PZT backings with similar porosity of ~20% and spherical pores with size of 1.5 and 10 [Formula: see text] were processed. The ultrasound group velocities were measured at ~3500 m/s for both samples. The acoustic attenuation of the backings with pore size of 1.5 and 10 [Formula: see text] were 12 and 33 dB/mm, respectively, measured at 19 MHz. This advanced measuring technique demonstrated potential for the simple measurements of acoustic properties of backing at high frequencies in operating conditions. Importantly, this method also enables rapid determination of the minimum required thickness of the backing to act as a semi-infinite medium, for high-frequency transducer applications.