Grain Size Effects on the Ferroelectric and Piezoelectric Properties of Na0.5K0.5NbO3 Ceramics Prepared by Pechini Method

A series of lead-free ceramics of Na0.5K0.5NbO3 with different grain sizes (800–2000 nm) were synthesized by Pechini method, and the effects of grain size on the microstructure, dielectric, ferroelectric, and piezoelectric properties were investigated. It was found that all the ceramics are of single phase with orthorhombic crystal structure at room temperature. The Curie temperature, dielectric constant, ferroelectric polarization, and piezoelectric constant were all strongly dependent on the grain sizes. It was observed that all the samples exhibited two-phase transitions above room temperature: the paraelectric cubic to ferroelectric tetragonal phase transition (TC) and the ferroelectric tetragonal to orthorhombic transition (TO-T). As the grain size increased, the TC increased from 357°C to a maximum value of 402°C, and then it decreased slightly with the further increase in grain size. However, the TO-T exhibited an opposite variation. Moreover, the nature of the grain size effects on the electrical properties has been discussed.