Improved piezoelectric and strain performance of Na₂B₄O₇-doped (Li,K,Na)NbO₃ lead-free piezoceramics

A polymorphic phase transition effect has been employed to regulate the piezoelectric property of lead-free (K,Na)NbO₃-based materials, which intrinsically bear the disadvantage of the evident temperature sensitivity. In this research, Li_0.04[(K_0.49Na_0.51)]_0.96NbO₃–xNa₂B₄O₇ piezoceramics were prepared through a conventional solid-state reaction. A piezoelectric coefficient d₃₃ of about 285 pC/N and a unipolar strain of 0.14%@5 kV/mm were achieved in the ceramics with x = 0.5%. These improved properties can be ascribed to the coexistence of orthorhombic and tetragonal phase structure around room temperature. Besides, the field-induced unipolar strain varied less than 15% in the temperature region from room temperature to 180 °C. The stability of strain property could be rationally interpreted by a quantitative approach of electrostrictive coupling to the polarization amplitude.