Effects of CIP compaction pressure on piezoelectric properties of K_0.5Na_0.5NbO₃

The effects of cold isostatic press (CIP) compaction pressures (100–350 MPa) on the piezoelectric properties of K_0.5Na_0.5NbO₃ (KNN) samples prepared as lead-free piezoelectric ceramic was investigated. The calcined powder of KNN samples were pressed at a pressure of 100 MPa using a hand press to produce green body pallets followed by compacting using the CIP. Pelleted samples were placed in a closed alumina crucible prior to sintering at 1080 °C for 2 h in air atmosphere. The sintered pellets underwent poling process at 120 °C in silicon oil and poling electric field of 4.0 kV mm⁻¹ for 15 min to facilitate the measurement of piezoelectric properties. The results show that both d₃₃ (piezoelectric charge coefficients) and k_p (piezoelectric coupling factor) were affected by the variation in CIP compaction pressure. Increasing CIP compaction pressure induced the split of (100) and (110) peaks which attributed to the structural phase transition from orthorhombic phase to tetragonal as shown by XRD analysis. This transition is responsible for the enhancement of their piezoelectric properties with the optimum value obtained from sample CIPped at 300 MPa (d₃₃ = 138 pC N⁻¹, k_p = 0.36, ρ = 4.47 g cm⁻³). The sample also shows smaller grain size (1.0–2.5 µm) and dielectric properties (ɛ_r = 475 and tan δ = 0.42).