Principles of piezoelectric ceramics for mechanical filters

Piezoelectric ceramics are prepared for fabricating the electromechanical transducers used in the mechanical frequency filters that find application in long-haul communications systems. These ceramics have to satisfy specifications that can only be met by utilizing all the possibilities offered by the physical effects of the ferroelectric material. The material properties are stabilized by modification with Mn. The required positive temperature coefficient of the frequency constant is realized with the aid of elastic anomalies in the region of ferroelectric phase transitions. Complex additives sharply increase permittivity while keeping mechanical losses low. Through the careful coordination of additives for softening and stabilization it is possible to prepare stabilized ceramics with the typically high piezoelectric coupling coefficient of soft ceramics accompanied by a high mechanical quality factor and effective resistance to aging. Moreover, any sharp increase in mechanical losses due to a rise in temperature is largely avoided.