Electrical conduction mechanism at high voltages and dielectric breakdown strength in bulk ceramic insulators

Electrical conductivity at high electric fields and dielectric breakdown of bulk ceramic insulators is not well understood. In order to gain more insight we performed current-voltage measurements on different ceramics to identify whether ohmic, space charge limited, Schottky, or Poole-Frenkel conduction, is the dominating conduction mechanism. Voltages up to 70 kV were applied and revealed that space charge limited conduction (SCLC) prevails at high electric fields in all investigated ceramics. As SCLC is a size dependent phenomenon the transition from ohmic to SCLC was determined as a function of the thickness of the disc-shaped samples. As a consequence electric field assisted sintering or breakdown models based on ohmic conduction must be critically regarded whether they can be applied. In a second part of this presentation we present the dielectric breakdown strength of these ceramics as a function of the sample thickness. It turns out that besides the well-known inverse square root dependence of the sample thickness there is also an inverse square root permittivity dependence of the dielectric breakdown strength. A Griffith type energy release rate dielectric breakdown model based on SCLC is presented, which is able to describe the size and permittivity dependence of dielectric breakdown

Betriebliche Altersversorgung für GmbH-Geschäftsführer: Insolvenzschutz nach dem BetrAVG und Grenzen steuerlicher Anerkennung

Klümpen-Neusel C. Betriebliche Altersversorgung für GmbH-Geschäftsführer: Insolvenzschutz nach dem BetrAVG und Grenzen steuerlicher Anerkennung. Berlin: Logos-Verl.; 2006

Size-dependence of the dielectric breakdown strength from nano- to millimeter scale

Dielectric breakdown decisively determines the reliability of nano- to centimeter sized electronic devices and components. Nevertheless, a systematic investigation of this phenomenon over the relevant lengths scales and materials classes is still missing. Here, the thickness and permittivity-dependence of the dielectric breakdown strength of insulating crystalline and polymer materials from the millimeter down to the nanometer scale is investigated. While the dependence of breakdown strength on permittivity was found to be thickness-independent for materials in the nm-mm range, the magnitude of the breakdown strength was found to change from a thickness-independent, intrinsic regime, to a thickness-dependent, extrinsic regime. The transition-thickness is interpreted as the characteristic length of a breakdown-initiating conducting filament. The results are in agreement with a model, where the dielectric breakdown strength is defined in terms of breakdown toughness and length of a conducting filament

Thickness-dependence of the breakdown strength: analysis of the dielectric and mechanical failure

The breakdown strength as well as the mechanical strength of ceramic materials decreases with increasing volume. The volume-effect of the mechanical strength can be explained by the Weibull theory. For the breakdown strength the same explanation has been often assumed. In order to validate this assumption breakdown strength and mechanical strength of alumina samples with defined porosities were compared. Differences in the Weibull moduli of breakdown and mechanical strength distributions indicate that the volume-effect cannot explain the thickness-dependence of the breakdown strength. In particular, the thickness-dependence of the breakdown strength always leads to a Weibull modulus of two which is not in agreement with the measured Weibull moduli for samples with constant thickness. It can be concluded that the thickness-dependence of the breakdown strength cannot be explained by the Weibull concept. A recently developed breakdown model which is based on space charge injection is able to explain the experimental results

Dielectric breakdown of alumina single crystals

The bulk breakdown behaviour of alumina single crystals with two different crystal orientations, {11-20}-plane (single crystal A) and {0001}-plane (single crystal C), have been studied. Therefor plan-parallel single crystal samples were electrically loaded until dielectric breakdown was achieved. For each crystal orientation, a characteristic breakdown channel direction through the sample could be defined. In C-oriented crystals the breakdown channel originated parallel to the c-axis. For A-oriented crystals however, the breakdown channel crossed the sample in an oblique direction; the angle between crystal surface and breakdown channel was 60°. Here, the breakdown channel crossed the sample along an A-plane. Although the breakdown channel paths of A and C crystals are different, the observed breakdown strength are identical within the scatter range