Investigations on the dielectric strength of syntactic foam at cryogenic temperature and the impact of the filler material on the volume shrinkage

This paper deals with syntactic foam as a solid substitution of liquid nitrogen (LN₂) based insulation systems for superconducting power cables. Syntactic foam consists of a polymeric matrix and gas-filled microspheres with a mean diameter of several 10 μm serving as filler material. As microspheres feature lower density and price compared to the matrix materials, such hybrid insulation systems become lighter and cheaper than pure materials. The microspheres are also able to reduce shrinking during cooling down to cryogenic temperature significantly. Epoxy resin and unsaturated polyester resin are investigated regarding their ability to serve as a suitable matrix as well as untreated glass, silanated glass and ceramic microspheres to serve as filler. Firstly, the impact of the hollow microsphere material on the volume shrinkage of syntactic foam is observed. Additionally, the dielectric strength under AC stress of several formations of syntactic foam at liquid nitrogen temperature (LNT) and at ambient temperature is determined. Finally, the results are compared to identify both the influence of the hollow microspheres´ material and the impact of temperature on the breakdown field strength of syntactic foam.