Low-Temperature Co-Fired Unipoled Multilayer Piezoelectric Transformers

The reliability of piezoelectric transformers (PTs) is dependent upon the quality of fabrication technique as any heterogeneity, prestress, or misalignment can lead to spurious response. In this paper, unipoled multilayer PTs were investigated focusing on high-power composition and co-firing profile in order to provide low-temperature synthesized high-quality device measured in terms of efficiency and power density. The addition of 0.2 wt% CuO into Pb_0.98Sr_0.02(Mg_1/3Nb_2/3)_0.06(Mn_1/3Nb_2/3)_0.06(Zr_0.48Ti_0.52)_0.88O₃ (PMMnN-PZT) reduces the co-firing temperature from 1240 °C to 930 °C, which allows the use of Ag/Pd inner electrode instead of noble Pt inner electrode. Low-temperature synthesized material was found to exhibit excellent piezoelectric properties (Qm = 1300, kp= 0.47, tan δ = 0.4 %, d33 = 218 pC/N, and Tc= 325 °C). The performance of the PT co-fired with Ag/Pd electrode at 930 °C was similar to that co-fired at 1240 °C with Pt electrode (25% reduction in sintering temperature). Both high- and low-temperature synthesized PTs demonstrated 5-W output power with >90% efficiency and 11.5 W/cm³ power density.