Dielectric behavior of syntactic foams at low temperatures and frequencies

Syntactic foam is a lightweight composite consisting of hollow particles embedded in a polymer resin matrix. In this paper, results are presented of dielectric spectroscopy measurements on test specimens of syntactic foam with glass micro-spheres partially filled with SO₂ . This type of syntactic foam is intended to be used as a high performance insulating material for lightweight high-voltage DC applications. The dielectric spectroscopy results showed an anomalous non-Debye behavior in a temperature range from roughly -140degC to +40degC. With increasing temperature the losses decrease at first, until they suddenly increase again at about +40degC. It has to be noted that the overall losses were quite high, compared to the base material. It appears that either the increased dc conductivity, the interfacial polarization at the interfaces between glass and epoxy, or both are the reason for these overall high losses. It is pointed out that the anomalous relaxation peak shifts to lower temperatures when the frequency is increased, contrary to normal behavior. In addition, a merge of two relaxation peaks is visible for higher frequencies at around 70degC, tentatively explained as being caused by the phase transition of SO₂.