| Abstract [eng] |
The aim of doctoral dissertation „Grain Size Effect on Dielectric Properties of Ferroelectrics and Relaxors“ by Maksim Ivanov is to investigate, how grain size of ceramics and powders of a few ferroelectrics and relaxors influences macroscopic dielectric properties. The studied materials are powders of a relaxor PbMg⅓Nb⅔O3 (PMN), ceramics of a relaxor with a spontaneous phase transition PbSc½Nb½O3 (PSN), ceramics of a ferroelectric 0.36BiScO3-0.64PbTiO3, and ceramics of Ba2SnO4, which were compared to a better investigated BaSnO3. Investigations were performed in broad frequency (100 Hz – 55 GHz) and temperature (30 K – 1000 K) ranges. Experimental investigations and modelling showed, that bulk properties of relaxor materials are heavily influenced by polar nanoregions, but they do not fully determine them. Morphology of the material (i.e. grain size of ceramics) determines growth and interactions of the nanoregions, thus influencing bulk properties. Moreover, effective medium approximation can explain evolution of dielectric properties of ferroelectrics and relaxors only if dependence of bulk properties on grain size is known. The most interesting result is, that there exist polar entities in ferroelectrics, which are different from ferroelectric domains and are similar to polar nanoregions in relaxors. Their contribution to dielectric permittivity can be comparable to all other contributions. Dimensions of these entities depend on grain size in accordance with Kittel's law. |
