Aging and Characterization of Semiconducting Glazes
Doctoral thesis, 2004
Semiconducting glazes on ceramic insulators had been introduced as a solution for improving the performance of high voltage insulators especially under heavily polluted conditions. The major concern in using such glazes, despite their approved advantages, is the observed aging of the surface with exposure time, leading to reduction of conductivity and deterioration of the glaze itself, especially under dc voltage.
The work carried out focused on the aging of tin oxide based semiconducting glaze, in order to find the main factors and mechanisms leading to the observed surface deterioration. Model samples with different glazes and parameters were prepared and characterized electrically and structurally. Later, these samples were aged under natural conditions as well as in laboratory, and then characterized again. Finally, full-scale long-rod insulators with semiconducting glaze were manufactured and installed under ac and dc voltage. Their leakage currents were monitored and their surfaces scrutinized.
The surface of new samples was free of cracks. Crystalline tin oxide particles with a diameter of less than 1 μm inhomogeneously distributed in a glassy matrix were identified, using scanning electron microscopy (SEM) and x-ray diffraction (XRD). The voltage-current characteristics (I-V) followed a power law with an exponent n of 1.1 to 1.2. Dielectric spectroscopy in frequency domain (FDS) showed a clear relaxation polarization originating from the presents of a low-conductive glassy surface layer covering the bulk semiconducting glaze. This layer might also cause the observed non-linear I-V characteristic.
After the aging under field and laboratory conditions, the conductivity of the glaze was found to be reduced and structural and chemical changes were observed on the surface, when moisture and pollution were present. SEM investigations showed craters and cracks. A tin oxide rich layer was found, indicating migration of tin oxide particles from the glaze towards the surface during the aging. These surface changes were more severe for samples aged under dc voltage. Due to the phenomena observed, the investigated glazes cannot be recommended for the use in dc conditions in the present state. More work has to be carried out in order to improve or to protect the glaze.