Composition and microstructure of Na_0.5Bi_0.5TiO₃ ceramics with excess Bi

Abstract

In the present work, Na0.5Bi0.5TiO3 (NBT) ceramics with the addition of excess Bi in two different ways – before calcination and before sintering – are considered, revealing how the excess Bi affects their microstructure and chemical content. Average grain size is seen to decrease, with the grain size distribution becoming less diffused at higher excess Bi concentrations. The reason for such a feature is the shift of the sintering temperature region where the abnormal grain growth starts to contribute towards higher temperatures. The influence of excess Bi is more pronounced in the case if it is added before calcination. It was discovered that a small amount of excess Bi helps to prevent the formation of Bi‐deficient inclusions. While, high concentrations of excess Bi induce the formation of Bi‐rich inclusions – most probably Na0.5Bi4.5Ti4O15. Possible mechanisms of formation of both types of inclusions are discussed in detail. Instead of Bi over‐stoichiometry, elevated Na content and slightly lower O content were detected in the matrix grains of the sintered NBT ceramics prepared with excess Bi. These deviations increase upon increasing the added excess Bi concentration. The presence of another, Na‐rich phase, is assumed, which could not be detected by X‐ray diffraction or by energy‐dispersive X‐ray analysis.

Free‐standing 0.9Na0.5Bi0.5TiO3‐0.1Sr0.7Bi0.2TiO3thick films produced by water‐based tape‐casting method

“…In our previous studies, we have also shown that deficiency of Bi, leading to formation TiO 2 inclusions in the inner part of the ceramics, can be prevented by addition of overstoichiometric amount of Bi to the raw materials. 28 However, as it follows from the obtained results, TiO 2 content in the volume of the thick films is low already, contrary to the large amount of TiO 2 inclusions on the free surface, which is subjected to the evaporation due to the contact with surrounding media. Addition of higher concentrations of overstoichiometric Bi at weighing raw materials before calcination could probably help to eliminate formation of TiO 2 on the film surface, but would also lead to formation of Bi-rich inclusions in the volume.…”

“…In our previous studies, we have also shown that deficiency of Bi, leading to formation of TiO 2 inclusions in the inner part of the ceramics, can be prevented by addition of overstoichiometric amount of Bi to the raw materials 28 . However, as it follows from the obtained results, TiO 2 content in the volume of the thick films is low already, contrary to the large amount of TiO 2 inclusions on the free surface, which is subjected to the evaporation due to the contact with surrounding media.…”

“…However, as it follows from the obtained results, TiO 2 content in the volume of the thick films is low already, contrary to the large amount of TiO 2 inclusions on the free surface, which is subjected to the evaporation due to the contact with surrounding media. Addition of higher concentrations of overstoichiometric Bi at weighing raw materials before calcination could probably help to eliminate formation of TiO 2 on the film surface, but would also lead to formation of Bi‐rich inclusions in the volume 28 . For this reason, another method was chosen: during the sintering process of the thick films, they were embedded within a Bi‐containing powder, with the intention of increasing concentration of Bi in the surrounding environment, thereby reducing the evaporation of Bi from the surface of the film being sintered.…”

“…Addition of higher concentrations of overstoichiometric Bi at weighing raw materials before calcination could probably help to eliminate formation of TiO 2 on the film surface, but would also lead to formation of Bi-rich inclusions in the volume. 28 For this reason, another method was chosen: during the sintering process of the thick films, they were embedded within a Bi-containing powder, with the intention of increasing concentration of Bi in the surrounding environment, thereby reducing the evaporation of Bi from the surface of the film being sintered. Usually, NBT powder is used to eliminate evaporation from bulk ceramics of NBTbased compositions.…”