Hopping Electrical Conduction and Thermal Breakdown in Natural and Synthetic Mica

The electrical properties of thin mica crystals have been measured between 4 and 520°K, and have been related to optical absorption measurements and to electron microscope observations. The dc conductivity and breakdown strength depends on the concentration of iron and titanium in the mica. It is suggested that impurities act as localized centres for electrical conduction in mica and that the Poole‐Frenkel model applies at high temperatures while electrons tunnel between the centres at low temperatures. The conductivity of mica depends on the frequency of the applied field. Two types of breakdown have been found. The results are interpreted as avalanche breakdown in synthetic micas and thermal breakdown in natural micas.