Ageing of high held dielectric properties in BaTiO3-based piezoceramics

The dielectric ageing characteristics of a commercial cobalt-doped BaTiO3 ceramic have been determined over a range of applied held strengths (field amplitude E-0 from 0.1 to 2.0 kV mm(-1)) and temperatures (30, 60, 80 degrees C). P-E (polarization-electric field) hysteresis data were obtained, from which the internal bias held E-i, polarization amplitude P-0 and the effective high field dielectric coefficients epsilon(r)', epsilon(r)" and tan delta were determined. Near-saturated P-E loops were obtained at high field levels (E-0 = 2 kV mm(-1)); it was shown that E-i increased gradually during ageing to yield an ultimate value around 0.6 kV mm(-1) after ageing for 17 h at 60 degrees C. The ultimate E-i value was reduced to approximately 0.4 kV mm(-1) at a higher ageing temperature of 80 degrees C. The P-E 'sub-loops' obtained for E-0 < 0.5 kV mm(-1) showed a pronounced asymmetry and high loss at early ageing times, which were attributed to a combination of partial ferroelectric domain switching and domain wall vibration. These characteristics were effectively removed by domain stabilization during ageing, giving rise to a near-linear P-E relationship with apparently zero internal bias field at long ageing times. The effective dielectric coefficients epsilon(r)', epsilon(r)" and tan delta derived from these P-E data all reduced significantly during ageing, giving an almost loss-free dielectric response after ageing for 17 h. The dielectric behaviour at early ageing times was attributed to a combination of the intrinsic (ionic) response, ferroelectric domain wall vibration and partial domain switching. The two lossy domain-related polarization mechanisms were effectively 'frozen out' by domain stabilization processes during ageing, which enabled a quantitative determination of the three contributions to the complex permittivity epsilon(r)* at various ageing times, held amplitudes and temperatures.