Synthesis and characterization of Na₂Ti₆O ₁₃ whiskers and their transformation to (1-x) Na_0.5Bi _0.5TiO₃-xBaTiO₃ ceramics

In this paper we report the synthesis of monoclinic Na₂Ti ₆O₁₃ whiskers and their transformation to lead-free 0.93Na_1/2Bi_1/2TiO₃-0.07BaTiO₃ (NBTBT) ceramics. Synthesis of Na₂Ti₆O₁₃ templates was performed using molten salt synthesis method. These whiskers were further characterized using XRD, SEM, TEM, and HR-TEM for structural analysis. Tape-casting method was used to align the whiskers in base matrix powder and subjected to various processing temperatures to elucidate the microstructure and texture evolution. For this, SEM, HR-TEM, AFM, and EDS analysis were used as principal tools. The sintering process can be understood by dividing it into three stages, namely (i) transformation of monoclinic whiskers in to NBTBT perovskite phase through topochemical reaction (<800°C), (ii) localized sintering confined on single whisker (800°-1050°C), and (iii) liquid-phase sintering as densification and grain growth occurs in the whole matrix (>1050°C). The concentric growth ledges observed on grain surfaces were found to be preferably confined on the corners of cubical grains indicating <111> growth direction. The Lotgering factor (f₁₀₀) for the sintered matrix was found to decrease with increase in sintering temperature. The longitudinal piezoelectric constant (d₃₃) of samples sintered for 20 h at 1175°, 1200°, and 1225°C was measured to be ∼153, ∼216, and ∼180 pC/N, respectively.