| At this time there is a large and growing market for high field strength dielectrics. The high breakdown strength is needed because, for several applications, there is a continuing trend to reduce the thickness of the dielectric and to operate it at the high field levels. Ferroelectric relaxors are currently being considered for many of the applications.; The main goals of this work were to establish the breakdown mechanism in lead lanthanum zirconate titanate (PLZT) and to determine the role porosity plays in controlling the thickness dependence of the breakdown strength.; The breakdown experiments were conducted with both hot-pressed and conventionally sintered PLZT ceramics. The hot-pressed samples were essentially pore-free and the conventional samples had densities varying from 92.5% to 96% of the theoretical. Both types of samples showed an increase in breakdown strength with a reduction of thickness. The hot-pressed samples had a stronger thickness dependence than the conventional ceramics. This result is in qualitative agreement with the Gerson-Marshall model. Porosity is not a dominant defect controlling the thickness dependence of breakdown in PLZT relaxors.; To determine the mechanism of the breakdown in PLZT, the breakdown strength as a function of electrode material, voltage polarity, ramp rate, and temperature was studied. All of the results were consistent with electromechanical breakdown being the dominant mechanism.; Since the breakdown may be influenced by variables other than porosity, samples were characterized using SEM, x-ray diffraction, DSC, dielectric measurements, electro-optics, electrical conductivity, and ellipsometry. In the conductivity study it was found that the Schottky mechanism is operable at high fields and that the surface conductivity is affected more than the bulk conductivity by humidity. Dielectric measurements were extended to the ultra-low frequency range. It was determined that the relaxation extends into this frequency range but with a significant change in activation energy compared to the conventional frequencies. |
