EFFECT OF STOICHIOMETRY ON THE DIELECTRIC-PROPERTIES AND LIFE PERFORMANCE OF (PB0.875BA0.125)[(MG1/3NB2/3)0.5(ZN1/3NB2/3)0.3TI0.2]O3 RELAXOR DIELECTRIC CERAMIC .2. LIFE PERFORMANCE

The accelerated life performance of a partially A-site substituted relaxor dielectric ceramic, (Pb0.875Ba0.125)A[Mg1/3Nb2/3)0.5(Zn1/3Nb2/3)0.3Ti0.2]BO3, was investigated for disk specimens with silver electrodes. The specimens were subjected to a humidity load life condition (85-degrees-C, 95% rh, 700 V), and the life performance was measured as a function of the stoichiometry of the relaxor. Stoichiometry was represented by the mole ratio A/B, which is the mole ratio of elements at A-sites to those at B-sites in the perovskite structure. The life performance was found to depend greatly on A/B, which was varied between 0.95 and 1.05. When A/B was equal to or less than 0.99, the failure rate at 500 h was 25% or below, while all the specimens failed within 350 h when A/B was equal to or greater than 1.00. All failed specimens had evidence of short-circuit failures. The life performance had a strong correlation with the microstructure of the dielectrics. Transmission electron microscopy revealed a PbO-rich secondary phase both at triple points and at grain boundaries in specimens with A/B greater than 1.00. Although a secondary phase was found at triple points in specimens with A/B = 1.00, it consisted of unreacted elements such as Pb, Ba, Nb, Zn, Mg, and Ti which did not form the perovskite structure. The results suggest that the lead component analyzed at triple points and grain boundaries is PbO, which has slight solubility in water. Thus, it appears that triple points and grain boundaries dissolve during the life test, and then migration of Ag ions through the dissolved grain boundaries leads to degradation.