Determination of temperature dependences of material constants for lead-free (Na0.5K0.5)NbO3-Ba2NaNb5O15 piezoceramics by inverse method

The enhancement of the piezoelectric, dielectric, and elastic properties of lead-free piezoceramics is essential to achieving a usable alternative to common lead-based piezoceramics. In this contribution, the temperature dependences of the material constants for 0.985(Na0.5K0.5)NbO3-0.015Ba2NaNb5O15 (NKN-1.5BNN) were characterized and compared with those of MnO-doped (Na0.5K0.5)NbO3 (NKN-Mn). The material constants were determined by the simulation-based inverse method. As a result, NKN-Mn and NKN-1.5BNN were found to show significant differences in the temperature behaviors of piezoelectric, elastic, and dielectric constants. In particular, for temperatures less than 40 °C, material constants that mainly affect shear mode vibration in NKN-1.5BNN gradually increased with increasing temperature, whereas those of NKN-Mn remained constant because of a different crystal structure. In addition, we explain the observed mechanical softness of NKN-1.5BNN in the shear direction on the basis of characteristic material constant relations, macroscopic (scanning electron microscopy), and crystal structure examinations (X-ray diffractometry).