Impact of Thermal Treatment on the Surface of Na0.5Bi0.5TiO3-Based Ceramics

Abstract

Thermal etching is a widely accepted surface treatment method for studying microstructure in Na0.5Bi0.5TiO3-based compositions. Surprisingly, besides the flat pattern of grains (suitable for evaluating ceramics’ microstructure), images illustrating well-expressed relief and even microstructure consisting of partly bonded cubic-shaped grains are also found among the micrographs presented in various publications. The present paper shows that this different surface character in Eu-modified Na0.5Bi0.5TiO3 can be obtained through thermal treatment across a wide range of temperatures. At higher temperatures, remarkable growth of cubic-shaped grains on the surface is observed. This growth affects the grain size distribution on the surface more than it does within the bulk of a sample. Such micrographs cannot be used to characterise the microstructure of dense ceramics. Intensive growth of TiO2 inclusions at high thermal treatment temperatures is also observed, revealing substantial evaporation of Bi and Na from the surface of a ceramic sample, but not from its core part.

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

“…This is in good accordance with the earlier observed characteristics for NBT bulk ceramics 7 . There is also an impression that the grain edges are sharper for the samples sintered at higher temperatures (Figure 2E), which is related to the growth mechanism of the grains on the surface 23 . Grain size of the reference bulk ceramic sample is slightly lower than the grain sizes of the thick films (Figure 2F).…”

“…Obviously, this explains the difference between the obtained thick film surfaces in The influence of the embedding powder is especially important in the case of fabricating thick films, as their surface is inseparable, while, in the case of bulk ceramics, the free surface being affected during the sintering is usually cut or polished off. Taking into account that the inner part of the ceramics is practically not influenced by the surrounding media, as it is shown by the results of this study and our previous research, 23 use of an embedding powder for sintering of the bulk ceramics seems to be useless, although it is frequently applied.…”

“…Figure7A,B and allows us to infer that Na 0.5 Bi 0.56 TiO 3 is more effective as the embedding powder than stoichiometric NBT.The influence of the embedding powder is especially important in the case of fabricating thick films, as their surface is inseparable, while, in the case of bulk ceramics, the free surface being affected during the sintering is usually cut or polished off. Taking into account that the inner part of the ceramics is practically not influenced by the surrounding media, as it is shown by the results of this study and our previous research,23 use of an embedding powder for sintering of the bulk ceramics seems to be useless, although it is frequently applied.Dielectric spectroscopy measurements for all studied compositions indicate that dielectric permittivity temperature-frequency dependence resembles behavior of pure NBT 7 with a diffused maximum and a shoulder at lower temperatures, as it can be seen from Figure9where examples of the measurements for several studied thick films are shown. In the temperature dependences of the real part of dielectric permittivity ε'(T), a maximum is observed around 300 • C and a shoulder-around 200 • C. While the ε'(T) shoulder has a corresponding anomaly also in ε''(T).…”

“…As it can be seen from Figure 5, the micro-Raman spectrum for one of the observed inclusions contains all the peaks characteristic of rutile TiO 2 , 26,27 while the spectrum for a matrix grain of the ceramic thick film has similar bands to those identified in NBT. 23 Upon increase in the sintering temperature, the first type of inclusions (Na 0.5 Bi 0.5 Ti 3 O 7 ) disappears, while the second type of inclusions (TiO 2 ) starts to dominate, what was earlier observed also in bulk NBT ceramics. 7 In order to better understand the conditions of appearance of the secondary phase, we have considered the microstructure of the studied samples in three different locations.…”

“…Specifically, no results on (1‐x)NBT‐xSBT thick films have been previously published. In our study, we focus on development of microstructure at various sintering temperatures and consequences of evaporation of Bi, earlier observed as a result of high‐temperature treatment of the surface of bulk NBT ceramics 23 . Our findings lead to a better understanding of the production method applied to NBT‐based ferroelectrics and the underlying physical processes; as a consequence, they help to find the best technological parameters to obtain the thick films of the highest quality.…”