Abstract
Understanding coupling of ferroic properties over grain boundaries and within clusters of grains in polycrystalline materials is hindered due to a lack of direct experimental methods to probe the behaviour of individual grains in the bulk of a material. Here, a variant of three-dimensional X-ray diffraction (3D-XRD) is used to resolve the non-180° ferroelectric domain switching strain components of 191 grains from the bulk of a polycrystalline electro-ceramic that has undergone an electric-field-induced phase transformation. It is found that while the orientation of a given grain relative to the field direction has a significant influence on the phase and resultant domain texture, there are large deviations from the average behaviour at the grain scale. It is suggested that these deviations arise from local strain and electric field neighbourhoods being highly heterogeneous within the bulk polycrystal. Additionally, the minimisation of electrostatic potentials at the grain boundaries due to interacting ferroelectric domains must also be considered. It is found that the local grain-scale deviations average out over approximately 10–20 grains. These results provide unique insight into the grain-scale interactions of ferroic materials and will be of value for future efforts to comprehensively model these and related materials at that length-scale.
Highlights
Understanding coupling of ferroic properties over grain boundaries and within clusters of grains in polycrystalline materials is hindered due to a lack of direct experimental methods to probe the behaviour of individual grains in the bulk of a material
Lead-based and lead-free single crystal materials have been shown to exhibit very large reversible strains under external electric field[2,3], difficulties and costs associated with crystal growth limit their applicability
Ceramic materials offer significant advantages in terms of processing, the total strain achievable in ceramics is much less than that obtainable in single crystals optimised for high strain applications
Summary
The 3D-XRD method allows for the indexing of many individual grains from a bulk polycrystalline material and provides direct access to grain orientation resolved information. It is shown that in a polycrystalline ceramic of (0.82)Bi0.5Na0.5TiO3 – (0.18)Bi0.5K0.5TiO3, the grain orientation relative to the applied electric field influences the resulting phase and domain structure of the electric-field-induced phase In this particular composition, most grains were found to transform to a rhombohedral symmetry, while grains with a < 100> direction within 7° of the applied field vector displayed a significant volume fraction that either remained in the cubic state or transformed partially to a tetragonal or other lower symmetry structure. The length-scale on which these local variations average out is likely of the order of 10–20 grains The origin of these deviations is suggested to result from complex interactions of grain-neighbour strain magnitudes, electric field magnitude inhomogeneities and the interaction of ferroic domains at the grain boundaries. These results are of critical importance when considering the validity of grain-scale modelling efforts, and provide additional considerations in the design of novel electro-mechanical materials
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