Generation of very high pressure pulses at the surface of piezo-composite materials using electrical pre-stress

New clinical concepts in lithotripsy demand small shock heads. Reducing the size of piezoelectric shock heads will only be possible if the pressure generated at the surface of each transducer can be increased so that, the total pressure at the focus remains very high. So, we propose a new method allowing the generation of large surface pressures. It is well known that the piezoelectric plots in piezo composite material are more fragile in the extension mode than in the compression mode. For this reason, actuators are mechanically pre-stressed between two flasks. This method can not be used for transducers working at high frequencies, about 0.5 MHz. For this reason, we proposed to electrically pre-stress the piezoelectric material by applying high electric field in the opposite direction of the polarization. In a first mode we proposed to pre-stress in continuous mode the transducer. Unfortunately we noticed a rapid de-poling and re-poling in the inverse direction. In a second mode to reduce depolarization, this field was applied only during a short time just before the generation of the pulse which generate the compressive wave and in a third mode, the transducer was re-poled between two successive electrical pulses