Phenomenological Theory of High Permittivity in Fine-Grained Barium Titanate

The anomalously high permittivity in fine-grained ceramic barium titanate is explained by the absence of 90° twinning within the grains, giving rise to internal stresses as the ceramic cools below the Curie temperature. Using the Devonshire phenomenological thermodynamic method and a simplified model, the stresses involved are calculated. At these stress levels, the reduced strain calculated on this model agrees well with X-ray data for the fine-grained material. The limitations and possible developments of the proposed model are discussed.