Anja Strauchs

The impact of water absorption on the dielectric properties of syntactic foam

This paper deals with experimental investigations on syntactic foam and its behavior at outdoor conditions. For the experiments, plate specimens of syntactic foam of a thickness of 3mm are used. Hollow glass microspheres with untreated and silane-coated surface as well as microspheres with a low alkali ion concentration are applied as filler. During a test period of 50 days the specimens are stored in deionized water, which is tempered to 50°C to accelerate the water absorption. The weight of every specimen is measured during the water storage to determine the increase of weight due to water ingress. The dielectric permittivity and the loss factor are measured in constant intervals during the whole test period. As a result of these investigations, a different behavior of the syntactic foam filled with different hollow microspheres is observed. The syntactic foam specimens with untreated hollow microspheres show a significant increase of their permittivity and loss factor during the test period, whereas the dielectric properties of the specimens with silane-coated microspheres and microspheres with low alkali ion concentration are not influenced by water absorption. The results show that syntactic foam can potentially be used as insulation material for outdoor applications if suitable microspheres are used as filler.

Effects of SiO 2 nanofiller on the properties of epoxy resin based syntactic foam

This paper focuses on the behavior of syntactic foam consisting of hollow glass microspheres embedded in an epoxy resin matrix. The hollow microspheres have wall thicknesses of 1 μm and a mean diameter of about 45 μm. Inserting microspheres in pure epoxy resin results in an increased viscosity of the liquid material during manufacturing process. In order to counteract this effect, SiO2 nanofillers are mixed to the liquid, uncured epoxy resin during the manufacturing process before the hollow microspheres are added. Viscosity measurements verify a significant reduction of the mixing viscosity up to 64 % by adding a small weight percentage (<; 5 wt.%) of nanofillers. Furthermore, the impact of the SiO2 nanofiller on the electrical, mechanical and thermal properties of syntactic foam is determined and discussed.

The impact of electrical field stress on the volume conductivity of syntactic foam

This paper deals with the experimental investigation on the volume conductivity of syntactic foam, a new composite insulation material consisting of a epoxy resin matrix filled with hollow glass microspheres. This investigation focuses on the impact of high electrical field stress on the volume conductivity. Plate specimens of syntactic foam with a thickness of 3 mm are produced for the experiments. Hollow microspheres as well as solid microspheres are applied as filler. In addition, pure epoxy is used as a reference material during the measurements. The volume conductivities of the different specimens are determined by shield electrode configuration according to DIN IEC 60093 applying electrical fields up to 16 kV/mm dc. For each material and each voltage level a time resolved conductivity characteristic is recorded as a function of the load duration. Specimens with solid microspheres show a similar characteristic to the one of pure epoxy, while the behavior of the specimens with hollow microspheres differs. The results are discussed with regard to the filler types (hollow or solid). The impact of the cavity inside the filler on the conductivity and the time resolved measurement curve is shown. The main result of the investigations ascertains that the material behavior is not dominated by the presence of filler but by the presence of hollow filler. Finally, design recommendations for the use of syntactic foam in high voltage dc components are provided.

Investigations on the partial discharge behavior of syntactic foam under uniform field stress

This paper deals with the partial discharge behavior of epoxy resin filled with hollow glass microspheres. The resulting material, called syntactic foam, is a new composite insulation material which combines advantages of solid and gaseous dielectrics. Specimens of syntactic foam are stressed with uniform ac field stress and the partial discharge activity is simultaneously measured. After the electrical stress the material structure is visualized by Scanning Electron Microscopy. As an essential result of this investigation a theory of the breakdown process of syntactic foam under uniform field stress is developed. Unexpectedly, the breakdown process differs from the one under non-uniform field stress described in [1].

Investigation on inverse volume effect of syntactic foam under uniform dc field stress

This paper deals with the experimental investigation on a new insulation material called syntactic foam. Syntactic foam consists of hollow glass or plastic microspheres casted into a polymeric matrix material in order to obtain a foam-like structure. The investigations carried out focus on the dependency between breakdown field strength and the volume of the stressed material under dc field stress. Specimens of syntactic foam with embedded sphere electrodes and electrode distances up to 8 mm are investigated in a voltage step test until breakdown. For distances of 1 mm and 3 mm a well-known volume effect of higher breakdown field stress on small electrode distances is observed. For distances above 3 mm an inverse effect can be found. The determined characteristic - called inverse volume effect - is ascribed to the specific material structure of syntactic foam.

The impact of different alkali ion concentrated hollow glass microspheres on the electrical breakdown mechanism of syntactic foam

This paper deals with the experimental investigation on syntactic foam, a new composite insulation material consisting of an epoxy resin matrix filled with hollow glass microspheres. The investigations carried out focus on the impact of the glass composition of the microspheres on the dc breakdown field strength of syntactic foam. Two different breakdown test procedures are used to analyze the breakdown process. The breakdown properties of syntactic foam with microspheres with low alkali ion concentration depend on the breakdown step test procedure used, whereas for syntactic foam with microspheres of high alkali ion concentration no dependency on step test procedure is observed. The determined behavior is ascribed to the microscopic field distribution of syntactic foam, which depends on the alkali ion concentration of the hollow glass microspheres.