Chemical and technological aspects of increasing the functional characteristics of hard piezoceramics

Objectives. Ferroelectrically hard piezoelectric ceramics are in demand for high-power applications in piezotransformers, ultrasonic emitters, and piezo motors, which requires a combination of high piezoelectric characteristics and mechanical quality factors in it. The aim of this research was to reveal the main regularities in the microstructure and functional characteristic formation of ferroelectrically hard piezoceramics based on two widespread chemical systems, Pb(Zr_xTi_1-x)O₃ and (Na_1-xK_x)NbO₃, through various technological modes of production. In this study, two fundamentally different technological ways of forming a dense microstructure on the example of above systems have been employed to obtain the best set of dielectric, piezoelectric, and mechanical parameters for practical applications. In the case of lead-containing ceramics, various sintering technologies have been used, including conventional ceramic, hot pressing, and spark plasma sintering.

Methods. The microstructure of the piezoelectric ceramics was investigated using electron microscopy, and the functional characteristics were assessed in terms of mechanical and piezoelectric properties. The density values were determined by hydrostatic weighing in octane, the relative dielectric permittivity was measured using an LCR meter, and the values of the piezoelectric coefficient and mechanical quality factor were gathered using the resonance–antiresonance method.

Results. This research has identified that spark plasma sintering technology makes it possible to obtain high-density samples, which contain a homogeneous microstructure and double the figure-of-merit values, for use in high-power piezoelectric devices that operate at piezoresonance frequencies. It also found that the addition of a small amount of CuNb₂O₆ (x = 0.025) to lead-free solid solutions leads to the formation of a liquid phase during sintering, thereby creating a compacted microstructure with relative density values (96%) that have practical limitations in conventional ceramic technology. An increase in both the piezoelectric and mechanical properties, which leads to a twofold increase in the values of the quality indicator, was also observed.

Conclusions. It is possible to increase, and even to double, the functional characteristics of both lead-containing and lead-free ferroelectrically hard piezoceramics by varying the technology used in the manufacturing process. By using spark plasma sintering technology with lead-containing ceramics, it is possible to reduce the optimum sintering temperature by 200 °C and the sintering time by more than 20 times, thus reducing production costs.