Thin layer deposition for ceramic matrix composites for electrical applications

Kurzfassung

When it comes to electrical applications, CMCs are used as conductors or insulators with a fixed resistivity defined by the used fiber, matrix and production route. Therefore, electrical applications that need a graded resistivity could not be produced with CMCs so far. For electrical applications such as direct current switches CMCs with a wide range of conductive and insulating properties within the same material are necessary. Therefore, CMCs with graded resistivities are developed at the German Aerospace Center (DLR) because they can withstand the large thermal shock loads during switching. By stacking multiple layers of material mixes into a mold CMCs with graded resistivities are pressed and solidified via a hot-press cycle into a multi-material plate. After that a polymer infiltration and pyrolysis (PIP) cycle is carried out to convert the resin into ceramic matrix. This procedure is repeated until the open porosity is low enough to form a dense material. Every layer consists of a mix of short fibers, resin and fillers. The mix ratio is changed in every layer to change its resistivity. Until now the layer stacking is done manually which leads to inhomogeneous layer heights throughout the plate. To ensure consistent layer heights an automatic mix dispenser is developed. It consists of a funnel with a motorized toothed shaft, linear drive unit, sensors and a microcontroller. Manual and automated stacking is compared by analyzing the cross-section of the plate. Image processing algorithms are used to obtain quantitative values to describe the layer homogeneity. Multi-material plates with good layer homogeneity were successfully produced with a layer height less than 1 mm and tested for electrical behavior.