Graded Resistivity in Ceramic Matrix Composites

Abstract

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 because they can withstand the large thermal shock loads during switching. Additionally, during a switch off event a continuous resistivity increase in the switching distance reduces the electrical current while preventing an arc formation. As soon as the resistivity reaches 10^12 Ω·mm²/m the current comes to a standstill and the power is successfully switched off. To produce CMCs with graded resistivity a multi-layer material development is carried out. Every layer consists of a mix of short fibers, resin and fillers. The mix ratio is changed in every layer to change its resistivity. Multiple mixes are stacked in a mold and solidified via a hot-press cycle. 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. Afterwards a polishing procedure is applied to the contact surface to enhance the material durability and further reduce the risk of an arc formation. To verify every single resistivity a static conductive test is performed for samples consisting of one mix each. In order to verify the graded resistivity a dynamic load test simulating a power switch-off is carried out. For samples consisting of one mix a resistivity bandwidth from 10^1 Ω·mm²/m to above 10^12 Ω·mm²/m was successfully produced. For graded samples the resistivity bandwidth accomplished ranges from 10^3 Ω·mm²/m to 10^9 Ω·mm²/m. The graded samples suppressed successfully the arc formation during switching. To check proper matrix conversion SEM/EDX analysis is carried out. The layer thickness is monitored via computed tomography to ensure proper layer structure.