Precipitation hardening in piezoceramics

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The application of piezoceramics for high-power applications demands a minimization of dielectric and piezoceramic losses. Otherwise, the ceramic will heat up in service and depole. This minimization is achieved using an acceptor doping, which introduces oxygen vacancies into the lattice. These form dipoles, which stabilize the domain structure and pin the domain walls. However, high temperatures and high power (heating up the material) prompt a mobility of the oxygen vacancies and destroy the hardening effect. High-power applications such as ultrasconic welding and ultrasonic motors therefore demand a mechanism, which pins domain walls. As these are the carriers of non-linear deformation, methods stemming from metal physics may be useful. Indeed, theory (and our first experiments) verify that, for example, dislocations pin domain walls. We also published recently, that a second phase, which is mixed to the matrix powder, stabilizes the domain structure. In this project the hardening of the domain structure by precipitates is contemplated. This option offers considerable flexibility and superior distribution of pinning centers in the lattice (and not only at grain boundaries like the admixed second phase). Further, precipitate hardening is expected to be stable at high temperature and high vibration rates. We propose to utilize barium titanate to fundamentally investigate the hardening of piezoceramics. This entails the processing technology including sintering and aging treatment. A complex property evaluation at temperature and vibration rate is required. This holds specifically for electromechanical quality and for the coupling factor next to more common properties such as piezocoefficient and obtained strains. In addition to electrical hardening the mechanical hardening through an investigation of the ferroelastic behavior is planned. Explorative investigations through introduction of dislocations at high temperature followed by precipitation at same dislocations along the dislocation networks are also considered. A fundamental investigation of the new mechanism is contemplated using in-situ TEM experiments as function of temperature (preliminary work provided) and as function of electric field. Similarly, in-situ piezo force microscopy is also planned. These methods offer the potential to visualize the formation of domain walls at dislocations as function of temperature and pinning of domain walls by dislocations as function of electric field. The investigations are contemplated on lead-free piezoceramics as their breakthrough is expected for hard piezoceramics. The obtained fundamental understanding, however, can similarly be applied to lead-containing piezoceramics, which then could be utilized at higher temperatures than before.