Effect of sintering temperature on structural phase coexistence, microstructure, and electromechanical properties of (1 − x) (K0.5Na0.5)NbO3 – xBaTiO3 (x = 0.06) lead-free ceramics

Abstract

This work systematically examines the effects of spark plasma sintering (SPS) temperature on the phase structure, microstructure evolution, and multifunctional properties of (1 − x)(K_0.5Na_0.5)NbO₃ – xBaTiO₃ (x = 0.06) :KNN–BT ceramics sintered at 950 °C and 1050 °C lead-free ceramics near the morphotropic phase boundary (MPB). X-ray diffraction and Rietveld refinement revealed the coexistence of orthorhombic (Amm2) and tetragonal (P4mm) phases, with sintering at 1050 °C inducing enhanced crystallinity, increased average crystallite size (from ~ 138 nm to ~ 165 nm), and reduced microstrain (0.56% to 0.29%), indicative of lattice relaxation and defect reduction. Microstructural analysis showed near-full densification (> 99%) and controlled grain growth, yielding uniform grain size (~ 2.66 μm) and minimized porosity, which suppress grain boundary diffusion and enhance mechanical integrity. Dielectric characterization demonstrated a pronounced increase in relative permittivity (ε_r) from 3500 to 4800 and an elevation in Curie temperature (from ~ 280 °C to ~ 343 °C), reflecting improved polarizability and domain stability linked to enhanced grain connectivity and reduced domain wall pinning. The diffuseness parameter (γ) decreased from 1.83 to 1.71, indicating a transition toward sharper ferroelectric phase transitions with higher sintering temperature. Mechanical hardness improved substantially from 4.82 ± 0.15 GPa to 6.18 ± 0.12 GPa, correlating with densification and grain boundary strengthening. Piezoelectric performance metrics exhibited significant enhancement, with longitudinal coefficient d₃₃ increasing from 105 to 148 pC/N, planar coupling factor kₚ from 0.34 to 0.44, and the figure of merit rising from 2.51 to 3.05 pC·V/m·N², attributed to optimized domain alignment and reduced defects facilitating domain wall motion. The combined improvements in dielectric, mechanical, and electromechanical properties demonstrate that SPS processing at 1050 °C effectively tailors phase composition, microstructure, and functional responses, making these KNN–BT ceramics highly suitable for lead-free piezoelectric applications in sensors, actuators, and energy harvesters demanding high efficiency, thermal stability, and mechanical robustness.