The Investigation of the Permittivity of Syntactic Foam under varying Humidity

The dielectric properties of syntactic foam samples, with emphasis on the complex permittivity, were studied via dielectric spectroscopy. The investigated syntactic foam samples consist of an epoxy matrix, filled with hollow glass spheres of 60 microns diameter on average with a high filler rate of 50% of volume. In order to see the influence of adsorbed water, the filler of one sample type was stored under 80% humidity to create wet samples, while the other samples were constantly kept dry. Previous measurements showed increased DC conductivity for syntactic foam - up to two orders of magnitude compared to conventional filled epoxy. This was suspected to be due to the relative high conductivity of the glass, as well as due to the sheer amount of glass spheres and therefore plenty of interfaces. Recent measurement at lower frequencies (below 10^-2 Hz) showed quasi-dc-behavior and virtually no difference for both dry and wet samples at 70degC, while dry samples had generally higher values in permittivity for 40degC. It is shown that the glass spheres overshadow the influence of adsorbed water in the syntactic foam samples measured.