Influence of Coupling Agents on Partial Discharge and Electrical Breakdown of Syntactic Foams at LNT

In this paper, syntactic foam is investigated as an alternative solid insulation material for cryogenic applications. Compared to conventional insulation materials based on liquid nitrogen, syntactic foam offers advantages like higher dielectric strength and it maintains its dielectric properties even during quenching of superconductors. Being an inhomogeneous material, syntactic foam features numerous interfaces between materials with different dielectric constants, which might be temperature de-pendent. It has been shown in previous investigations that improving bonding quality between materials of syntactic foam can be beneficial for mechanical and electrical strength. A broader variety of foam modifications using coupling agents are investigated in electrical tests for breakdown voltage at both 293 K and 77 K. To examine the initiation of breakdown, partial discharge measurements are performed at both ambient and liquid nitrogen temperature (LNT), and the results are compared to the existing model of electrical breakdown. The application of coupling agents shows no significant overall improvement for ac- and dc-breakdown stress. At LNT, syntactic foams show a higher partial discharge inception field stress, when microspheres are being treated with a coupling agent compared to untreated microspheres.