Partial discharge current measurement in high permittivity dielectrics and their meaning for quality control

Dielectric breakdown is one of the major problems of dielectrics with high permittivity. There are several intrinsic factors affecting breakdown strength of BaTiO₃ such as porosity, grain boundaries, and domain wall instability. But in general, the dielectric breakdown is caused by partial discharge within pores and cavities in the insulating ceramic body. The permittivity in these enclosures is usually much lower than the one of the BaTiO₃ matrix. Therefore, under operation of the ceramic capacitor the voltage across the pores may exceed the gas breakdown threshold and causes partial discharge in the pores according to the Paschen´s law. Although, the momentary effect of these discharges typically caused by currents in the microampere regime is little, their meaning for the long-term reliability of the component is crucial. In this paper we describe a new experimental set-up for determination of partial discharge phenomena in BaTiO₃ capacitors. The partial discharge current is monitored using a bridge circuit containing two branches with the same nonlinearity. In this way the basic harmonics of the bias current can be suppressed and the sensitivity of the interesting current peaks due to partial discharge is increased. Using this set-up we demonstrate the impact of mechanical defects on the partial discharge inception field strength. Moreover, the same capacitors were analyzed by means of metallographic cross sections. The ceramic-structure and fractures in the dielectric body due to unfavorable process parameters during manufacturing are analyzed. As a surprising result we show that capacitors even with significant fractures do not necessarily exhibit partial discharge even at an electrical field beyond 2 kV/mm. For this reason we conclude that conventionally applied electrical tests are insufficient for quality control.