Effect of vanadium substitution on electrical and piezoelectric properties of lead-free (K_0.5Na_0.5)NbO₃ ceramics

During the last decade, efforts have been made towards developing lead-free piezoelectric ceramics which can replace the currently dominant but highly superior lead-based piezoelectric materials such as PZT. (K_0.5Na_0.5)NbO₃-based piezoelectrics are one promising candidate because of their relatively high Curie temperatures and piezoelectric coefficients. In the present work, powders of (K_0.5Na_0.5)(Nb_1−x V _x )O₃ (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) were synthesized by conventional solid state reaction of K₂CO₃, Na₂CO₃, Nb₂O₅, and V₂O₅. The powders were high-energy milled in isopropyl alcohol medium using a Retsch PM 100 planetary ball mill at the speed of 200 rpm for 8 h. The milled powders were sintered at 1080 °C for 1 h in closed alumina crucible. The crystalline phase of the ceramics was found to be perovskite with orthorhombic symmetry. Increasing V⁵⁺ content in the ceramics from x = 0 to x = 0.03 gives rise to gradual increase in room temperature dielectric constant (ε _r ) from 346 to 432, remnant polarization (P _r ) from 6.2 to 9.6 μC/cm², electromechanical coupling factor (k _p ) from 0.35 to 0.39, and piezoelectric charge constant (d ₃₃) from 76 to 93 pC/N. The increase in these parameters is attributed to the associated increase in density of the ceramics with increasing V⁵⁺ content. Further increase in V⁵⁺ content to x = 0.05 results in decrease in ε _r , P _r , k _p and d ₃₃ to 390, 4.7 μC/cm², 0.33 and 67 pC/N respectively. The study reveals that an optimum concentration of V⁵⁺ can enhance the dielectric, ferroelectric and piezoelectric properties of (K_0.5Na_0.5)NbO₃ system.