Effects of Bi2O3 as sintering aid on the microstructure and electrical properties of La2Ti2O7 ceramics

Abstract

Bismuth oxide (Bi₂O₃) as a liquid-phase sintering agent is examined for the preparation of La_2-xBi_xTi₂O₇ ceramics. Bi₂O₃ volatilisation restricts the sintering temperature to 1300 °C, and increasing Bi content (x = 0–0.1) facilitates increasing the average grain size (~ 1–15 μm) in sintered ceramics. However, higher Bi content (x = 0.15 & 0.25) leads to excessive Bi₂O₃ segregation at grain boundaries. Phase analysis by X-ray diffraction reveals presence of single phase La₂Ti₂O₇ along with residual phases consisting of (anatase-TiO₂, rutile-TiO₂ and α-Bi₂O₃) to a varying extent with increasing Bi content. Raman spectroscopy reveals the incorporation of Bi³⁺ at La³⁺ sites in La₂Ti₂O₇, and the in-phase and symmetric stretching vibrational modes of La–O are affected with Bi incorporation. Composition analysis by EDS (energy dispersive spectrometry) reveals (1) partial entry of Bi into the lattice, and (2) increasing loss of Bi due to volatilisation which have been further confirmed through simultaneous heat flow and mass loss measurements. Increasing Bi content enhances the dielectric constant (εʹ) from 60 to 82 and reduces the dielectric loss (tan δ ~ 10^–4). Grain, grain boundary and electrode effects are analysed, and Cole–Cole impedance analysis explains the purely capacitive nature of the ceramics with high electrical resistivity (\({\rho }_{DC}\)~ 10¹³ Ω-cm). Simultaneous heat-flow and thermogravimetric measurements indicate the ferroelectric phase transition at ~ 780 °C, and ferroelectric to paraelectric phase transition at ~ 1250 °C, and improved piezoelectric coefficient (d₃₃ = 2.1 pC/N) is seen for an optimum Bi content (x = 0.075) in La_2-xBi_xTi₂O₇ in comparison to the low d₃₃ value (1.4 pC/N) for undoped La₂Ti₂O₇.