Bibliothek

Notizen: A new way to process the information obtained in space-charge distribution measurements is presented. It is applied to data given by the pressure wave propagation method. This method can be carried out with or without an applied voltage. It is shown however that it does not always allow, under dc stress, for quantitative evaluation of the charge density, if the material contains space charges of very different mobilities, and particularly of high mobility. The new processing technique described here overcomes this difficulty. It is illustrated by application to cross-linked polyethylene samples. This permits the discrimination of two different kinds of space charge. One is the normal space charge which has been reported many times, the second is a type that displays a high mobility. This charge is not directly visible even at very short times after the suppression of the applied voltage. In these measurements, this very mobile space-charge is always seen as heterocharge near the anode. © 1995 American Institute of Physics.

Notizen: The pressure wave propagation method, usually used to study electrical behavior of dielectric materials, is applied here to nondestructive detection and quantification of adhesion defects in a bilayer structure. This method relies on a very simple idea: as an electric field is created in a bilayer dielectric sample placed between short-circuited electrodes, the propagation of a pressure pulse induces an electric signal. If the field distribution is known, the signal leads to the pressure profile all along its propagation through the sample and therefore gives information on the interface. First, we extend the signal expression already established for a monolayer structure to a multilayer structure and consider particularly bilayer structures. After explaining the signal analysis in perfectly bonded and totally disbonded structures, a model is proposed to describe and analyze the signal in a partially disbonded structure and relate it to the percentage of totally disbonded area in the zone being tested by the pressure pulse. To assess this analysis, measurements were carried out on kapton (130 μm)–adhesive (98 μm) transparent samples. Some samples are perfectly bonded, some are totally disbonded and the others are partially disbonded. The pressure pulse is created by the impact of a laser pulse on an absorbing target coupled to the sample. The excellent agreement between the measurements carried out on perfectly bonded and totally disbonded samples and simulations assesses the correctness of the signal expression for a multilayer structure. In the case of partially disbonded structures, the value of the percentage of totally disbonded area, determined by measurement and simulation, is very close to that deduced from a photograph of the sample, only possible for transparent materials. The spatial resolution of the method is related to the spectrum of the pressure pulse, which extends up to 200 MHz. This method presents an excellent spatial resolution. If the transit time of the pressure pulse in each medium is superior to 11 ns, measurements and simulations show that owing to this method, it is possible to detect and localize in the structure submicron gaps and a percentage of totally disbonded area as low as 5% in the tested zone by analyzing the signals in the time domain in a very simple manner.© 1999 American Institute of Physics.

Notizen: This paper is the first of a series with the common theme of comparing thermal and acoustic pulse methods of measuring charge or polarization profiles across the thickness of slab-shaped samples that are representative of different types of materials. In this paper, thermal and pressure pulse measurements are reported of the polarization distribution in poled, ferroelectric ceramic samples. The results obtained from both methods are complementary so that there is a benefit to using both. The results also demonstrate that large deviations from uniform polarization can be induced by processing differences.