Nature of Aging in Ferroelectric Ceramics

N THE ferroelectric and piezoelectric applications of BaTiOa I and Pb(Zr,Ti)O,, dependence on time of many of their physical properties is encountered; this phenomenon is usually detrimental but is sometimes useful. Since aging is commonly considered to be related to the mobility of the ferroelectric domain structure, information on the nature of the aging process can be obtained by studying the changes in the hysteresis loops which occur with aging, a s shown in Fig. 1. A “fresh” ceramic sample of BaTiO, (just cooled to a temperature below the Curie point) exhibits a normal loop which on aging constricts gradually, resulting in decreasing remanent polarization. The rate and amount of constriction during aging are very sensitive to impurities and to slight variations in the Ba-Ti ratio. In the extreme case, the remanent polarization vanishes. This behavior can easily be confused with the double hysteresis loops exhibited by BaTiO, and PbZrO, as a result of fieldenforced paraelectric-ferroelectric and antiferroelectric-ferroelectric transitions, respectively. The constriction disappears if the sample is subjected to strong ac fields for prolonged times; this process is called deaging. The extremely constricted loop can be viewed as the result of reestablishment of the aged domain pattern after each half cycle. Strong fields are needed to disturb this pattern; however, after this disturbance a memory of the original state is retained, demonstrating that aging involves stabilization of the domain pattern. There is a general agreement’ that such stabilization occurs, but several mechanisms are possible: (1) Gradual equilibration of the domain pattern with respect to sizes and types of domains, with minimization of elastic and dielectric free energy.’*’ (2) Segregation of impurities and vacancies on domain walls and crystal boundaries. (3) Ordering of impurities and vacancies inside the ferroelectric domains with respect to the polar axis. Microscopy of single crystals and ceramics has demonstrated that process (1) occurs; however, its role in stabilization of the domain pattern is unclear. In the present work, an experiment was conducted that demonstrated aging simply and indicated clearly that process (2) or (3) is its principal cause. The ceramic samples were solid