Generation of higher pressure pulses at the surface of piezo-composite materials using electrical pre-strain

New clinical concepts in lithotripsy demand smaller shock heads. Reducing the size of piezoelectric shock heads requires increasing the surface pressure of each transducer so that the total pressure at the focus remains the same. A new method allowing generation of large surface pressures is described. The hypothesis is that piezoelectric plots in piezo-composite material are more fragile in extension than in compression. For this reason, actuators are mechanically prestressed between two flasks. This method cannot be used for transducers working at high frequencies, such as 0.5 MHz. So we tried to electrically prestrain compressively the piezoelectric material by applying a high-electrical field in the opposite direction of polarization. Three protocols were tested and compared to classically driven transducers. In the first protocol, prestrain is permanently applied, in the second protocol prestrain is applied for 100 micros before the compressive impulse, and in the third protocol prestrain is applied for 100 micros and followed by a bipolar field that allows the material to be repoled between two successive pulses. With the two first protocols, rapid depoling and repoling in the opposite direction was noticed. Only with the last protocol was it possible to increase the maximum surface pressure. This increase was approximately the same whether the material was hard or soft. Using this protocol, aging tests were conducted on three samples of each kind of material, and a pressure of 4 MPa was obtained over 10(6) shocks. This value seems to be enough to develop a piezoelectric shock-wave generator with a diameter of approximately 20 cm instead of the 45-cm commercially available.