Type of presentation: Poster

MS-5-P-2087 Grain boundary domain dynamics investigated by Transmission Electron Microscopy in modified BaTiO3 displaying PTCR effect

Holsgrove K. M.1, Douglas A. M.1, Einsle J.1, Gregg J. M.1, Arredondo M.1
1Centre for Nanostructured Media, Queen’s University, Belfast, U.K
kholsgrove04@qub.ac.uk

Positive temperature coefficient of resistivity (PTCR) materials are long-established in the electronics industry and used for many applications such as temperature control, current stabilisers and industrial sensing. Interestingly the PTCR effect can be observed in donor-doped barium titanate near the ferroelectric transition temperature TC. The most accepted model to explain the PTCR behaviour in modified BaTiO3 materials is the Heywang-Jonker model, whereby the PTCR behaviour is said to arise from the presence of a potential barrier at the grain boundaries; the barrier arising from the trapping of electrons by acceptor species at the grain boundary1. Much work has been done studying the structural aspects and grain boundary potential2, but still many important factors of the PTCR phenomena are yet to be explored in depth. Transmission electron microscopy (TEM) has been used here to investigate the relationship between domains and grain structure in BaTiO3 based ceramics. As shown in Figure 1 and 2, domains can be continuous across a grain boundary (allowing the material to behave more as a single crystal) and conversely domains can abruptly change close to the grain boundary. Also, regions of different domain variant can occur within grains where areas of fine domain patterns and changes in domain orientation can be observed (Figs. 1 and 3). Internal stresses and defects are factors that add to the complexity of this material, the thick domain walls displayed in Figure 1 reflect internal strain generated due to the paraelectric-ferroelectric phase transition. The structural intricacy of crystallographic axes dependence on domain dynamics will be discussed in further detail. In addition, conductive-atomic force microscopy (c-AFM) studies will be presented to relate the macroscopic PTCR behaviour in specific grains with domain dynamic behaviour. In-situ TEM experiments such as heating and electrical bias, along with high resolution imaging, are being undertaken in an attempt to bring further understanding in a complex phenomena; by exploring the strong and delicate interplay between macroscopic polarization, switching mechanism and crystal structure, both within grains and across grain boundaries.

1 W. Heywang, Solid State Electron., 3, 51, 1961
2 R.D. Roseman and N. Mukherjee. J. of Electroceramics, 10, 117-135, 2003

We would like to acknowledge the department for employment and learning.

Fig. 1: STEM image of domains continuing across a grain boundary. Fine-scale domains can also be seen in some regions where the domain configuration changes orientation.

Fig. 2: STEM image of domains abruptly changing at the grain boundary.

Fig. 3: STEM image of complex regions of different domain variant within a grain.