Aberration‐corrected STEM and EELS investigations of grain boundaries in an optimised BaTiO3 based PTCR ceramic

Abstract

Positive temperature coefficient of resistivity (PTCR) effect is a property found in polycrystalline materials which can transform from a low resistance state to a high state of resistance in response to heat. Accordingly, this effect has found extensive applications in sensing technologies such as self‐regulating heating elements, current sensors and sensors for the detection of air flow, liquid level and temperature changes. [1] Among the various materials exhibiting PTCR effect to date, the most favoured material group is Barium Titanate (BaTiO 3 ) based or quasi‐BaTiO 3 based ternary perovskite compounds where the temperature at which this switch in behaviour occurs, near the ferroelectric‐paraelectric Curie transition temperature (T c ), and the magnitude of the switch can be controlled and optimised via the addition of different dopants and/or changes in the processing conditions. [2, 3] The role of grain boundaries in these ceramics has been strongly deliberated in previous studies with most of the experimental evidence towards the role of grain boundaries established by macroscopic studies, allowing the interpretation of grain‐boundary resistivity in terms of equivalent circuit diagrams. [4, 5] Yet, direct visualisation and mapping studies of the PTCR behaviour on the nanoscale has been missing. Here, we identify the grain boundaries as the pivotal region of interest by reporting clear evidence of non‐linear changes in electrical potential (via Kelvin probe force microscopy (KPFM)) observed locally across single grain boundaries, explicating their central role in this phenomenon. Several studies have suggested that chemical diffusion, and segregation at the grain boundaries could play a part in creating the PTCR effect, but attempts to provide evidence of this chemical heterogeneity have so far been unsuccessful. [6, 7] We employed aberration‐corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to investigate chemical inhomogeneity and electronic structure changes across grain boundaries on the atomic scale. These studies have revealed the presence of localised PbTiO 3 ‐like formation (Figure 1) and striking changes in Ti‐ L 2,3 and O‐ K ELNES features approaching the grain boundary (Figure 2). Due to the clear chemical and intrinsic change in symmetry identified, a quantitative analysis of the crystal field splitting (CFS) was carried out across the grain boundary. We present the remarkable CFS trend, suggestive of octahedral distortion, transitioning across single grain boundaries. The state‐of‐the‐art microscopy techniques involved in this investigation have allowed us to unravel the complexity of this PTCR ceramic. As a result, we provide a logic interpretation of the BaTiO 3 ‐like grain interior and PbTiO 3 ‐like grain boundary regions in terms of an enhanced normal polarisation component at the grain boundary interface, which according to the modified Heywang‐Jonker model [8, 9] augments electronic transport in the ferroelectric phase and enhances the magnitude of resistivity jump at T c . We demonstrate the idea that a confined grain boundary region which exists in such a strategic manner offers a novel route towards engineering better performing PTCR devices.