Extrinsic effects in twinned ferroelectric polycrystals

The structure of medium-grain piezoelectric ceramics often consists of alternate lamellae of non-180 degrees domains. In this work, the extrinsic effects of the electric and elastic fields on such structures are studied. A description of the extrinsic behavior of a single grain is given, and it is shown that the relations between piezoelectric and dielectric or elastic constants must be independent of the wall mobility, being solely dependent on the relation between spontaneous polarization and strain. By using an appropriate coordinate system, the conditions under which the intrinsic and extrinsic effects can be added are analyzed. The linear global behavior of a grain can then be described as a function of its orientation and of two additional parameters: the lack of equilibrium between domains alpha and its mean thickness d. The basis is established to describe the state of a ceramic through a distribution function that accounts for domain orientation and which depends on the poling, fatigue, and ageing of the sample. Finally, the goodness of the model is analyzed, although some aspects must be still modified in order to describe the overall behavior of the ceramic. (c) 2007 American Institute of Physics.