Change of material properties with time. Aging of piezoelectric ceramic occurs very rapidly in the first few hours after poling. After a few days the changes in the material properties are very small and decrease logarithmically. Indeed in ferroelectrics, the aging of piezoelectric properties is often logarithmic. E.g.:
Where to is an arbitrary time when the measurement has started, and A is the “aging rate” constant, specific to the composition, the microstructure, and the processing history of the element.
An origin of aging is in internal stresses such as those built in the material during poling. These stresses may be relaxed by domain reorientation during time. Another source of aging is point defects that exist in many piezoelectric ceramics (vacancies of lead and of oxygen) and are mobile with time and may build internal fields that restrict domain wall movements. Space charge in grain boundaries may also contribute to aging.
The blocking force and the free stroke that can be obtained at the maximum applicable voltage, are probably the two most important properties to be considered, when evaluating if a given
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