Abstract
The use of porosity to modify the functional properties of piezoelectric ceramics is well known in the scientific literature as well as by the industry, and porous ceramic can be seen as a 2-phase composite. In the present work, examples are given of applications where controlled porosity is exploited in order to optimise the dielectric, piezoelectric and acoustic properties of the piezoceramics. For the optimisation efforts it is important to note that the thickness coupling coefficient kt will be maximised for some non-zero value of the porosity that could be above 20%. On the other hand, with a good approximation, the acoustic velocity decreases linearly with increasing porosity, which is obviously also the case for the density. Consequently, the acoustic impedance shows a rather strong decrease with porosity, and in practice a reduction of more than 50% may be obtained for an engineered porous ceramic. The significance of the acoustic impedance is associated with the transmission of acoustic signals through the interface between the piezoceramic and some medium of propagation, but when the porous ceramic is used as a substrate for a piezoceramic thick film, the attenuation may be equally important. In the case of open porosity it is possible to introduce a liquid into the pores, and examples of modifying the properties in this way are given.
Highlights
In many cases, functional materials are selected as a trade-off between a set of requirements
As will be apparent from this study, piezoelectric ceramics is a field where a number of significant advantages are obtained by introducing porosity
The purpose of this paper is to show two rather different cases of porous piezoceramics with important commercial applications
Summary
Functional materials are selected as a trade-off between a set of requirements. Rybyanets has performed extensive work on porous piezoceramics, including preparation methods, theory, modelling and functional characterisation, cf the review article [9]. The porosity of the thick films is a consequence of a reduced sintering temperature compared to the bulk case and the shrinkage being limited by adhesion to the substrate The thickness and characteristic properties of these films make them suited for ultrasonic imaging at frequencies in the range 10 MHz to 30 MHz, both single-element transducers and multi-element arrays When the former type is used for ultrasonic imaging, it needs to be moved (scanned) mechanically in order to produce a 2-dimensional image [14]. The present work includes examples of the design and performance of both single-element and multi-element transducers in pulse-echo measurements
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