Transport properties of zirconium alloy oxide films

The conductivity mechanism of oxide films on tubes of various zirconium alloys grown in water and steam was investigated by I–V measurements. Electrodes of Ga, Ag, graphite and Au gave different results. The current decrease at voltage application was due to formation of space charge, which could be extracted again as short-circuit current. The positive branch of the I–V characteristics could be fitted by a second-order polynomial. Neither the Schottky emission mechanism nor the Poole–Frenkel effect could be proved. A high invariable carrier concentration with extremely low, but temperature dependent mobility could be proved using the Mott–Guerney relation. The carriers originate from oxygen vacancies in a substoichiometric black oxide layer near the metal–oxide interface. The conduction mechanism is believed to be a hopping process of electrons between oxygen-vacancy traps. There exists a small ionic conduction part, building up an open-circuit voltage so that the I–V characteristics do not pass through the origin. The Meyer–Neldel rule applies.