Surface characterization of new and aged semiconducting glazes

A study is presented on changes in the structure and the chemical composition of antimony-doped semiconducting glazes before and after exposure to AC and DC electric fields in outdoor conditions and in the laboratory using the rotating-wheel-dip (RWD) test. Surface sensitive techniques such as scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX), Xray photoelectron spectroscopy (XPS) and secondary ion-mass spectrometry (SIMS) were employed to characterize the materials. Aging in the form of crater building, surface cracking and pitting as well as formation of a tin oxide rich layer took place under both AC and DC exposure. However, the damage was much more severe under DC voltage. Similar aging changes could be obtained in the laboratory after only a few days of the RWD test. In the field, leakage currents were monitored for evaluating the electrical performance of the samples. On new samples, the currents varied only slightly depending on weather conditions. However, during rainfall periods the leakage currents on samples exposed to DC voltage for a longer period were significantly increased compared to the new state, while the currents on AC exposed samples remained unchanged. No flashovers occurred during the 4-month testing. Thermal runaway on samples exposed to DC voltage in the field caused the appearance of black spots in the glaze. Their origin and chemical composition are still under investigation.