Impact of K-compensation on phase fraction, dielectric permittivity, remnant polarization and piezoelectric constant of sodium potassium bismuth titanate ceramics

摘要

Sodium potassium bismuth titanate ceramics (NKBTx) has been considered as a potential replacement material to lead-based one due to its moderate piezoelectric constant and high Curie temperature. Recently, the piezoelectric constant and remnant polarization of NKBTx ceramics with x = 0.20 mol fraction has been improved by considering the hygroscopic nature and volatility of K_2CO_3. In the present work, detailed structural information of K-compensated NKBTx ceramics has been examined using Rietveld refinement program. Samples with three different mole fractions (x = 0.10, 0.20 and 0.30) were synthesized using solid-state reaction method. Not like in earlier reports, x = 0.10 and 0.30 mol fraction samples also have a mixture of rhombohedral and tetragonal phases which are revealed from Rietveld refinement. The presence of super-lattice reflection 1/2 {311}_(pc) near 38° indicated that the existing rhombohedral phase has R3c symmetry in x = 0.10 mol fraction. Where as in x = 0.20 and 0.30 mol fraction samples, satisfactory profile fitting is obtained with rhombohedral (R3m) + tetragonal (P4mm) mixed phase structural model due to the absence of superlattice reflection. Grain growth is suppressed with the addition of xmole fraction and it is evidenced from the microstructure. Reduction of dielectric permittivity and slight shifting of phase transition temperature toward high temperature with increase in frequency indicates, all the samples have relaxor in nature. Temperature at which the dielectric permittivity maximum (T_m) increased from 325 to 345 ℃ with increase in x-mole fraction from 0.10 to 0.30 respectively. It is revealed from the modified Curie-Weiss law that the degree of diffusion (γ) is reduced from 1.95 to 1.71 with increase in x-mole fraction. Large remnant polarization, relative permittivity and piezoelectric constant are obtained for x=0.20 composition and they are 21 μC/cm~2, 4530 (at 5 KHz) and 215 pC/N respectively. Consolidated results reveals the extension of MPB region is strongly attributed to the K~+ and Bi~(3+) compensation and the obtained electrical properties are associated with the phase fraction of end members as exhibited in lead-based piezoelectric ceramics.