Investigation on the Dielectric Material Parameters and the Electric Conductivity of Syntactic Foam at the Liquid Nitrogen Temperature Range

Liquid nitrogen (LN₂)-based insulation systems for superconducting components of the electric distribution network are state of the art. As the dielectric strength of LN₂-based insulation systems can be significantly reduced if bubbles occur, an alternative insulation system could be a solid insulation system using LN₂ only for cooling but not as electrical insulation material. This paper discusses syntactic foam as a solid substitution of LN₂-based insulation systems. Syntactic foam consists of a polymer matrix with embedded hollow microspheres (HMS) that have diameters of several 10 μm. Compared to the pure matrix material, the HMS filled matrix features a lower density and a significantly reduced thermal contraction when being cooled to cryogenic temperatures. Several syntactic foams are investigated regarding their dielectric parameters (relative permittivity, loss factor) and electric conductivity at liquid nitrogen temperature (LNT). The results show that the investigated parameters of syntactic foam are almost constant in the temperature range of LNT. Furthermore, the loss factor and the relative permittivity at LNT are lower than at room temperature. These effects can be explained by the existence of a secondary glass transition of polymers. At cryogenic temperatures, a decrease of the electric conductivity of syntactic foam is detected presumably due to the rise in required energy to lift valence electrons to the conduction band of the polymer at lower temperatures.