INFLUENCE OF THE FORMING ELECTROLYTE ON THE ELECTRICAL-PROPERTIES OF ANODIC HAFNIUM OXIDE-FILMS - AN AC-IMPEDANCE INVESTIGATION

The potentiodynamic oxidation of hafnium at v = 0.1 V s-1 was investigated at potentials in the range 0-12 V in the following electrolytes: H2SO4, HNO3, H3PO4 and NaOH. After film growth, the oxides were characterized by performing ac impedance measurements at a potential of 0.1 V/SCE and frequencies in the range 0.1 Hz-10 kHz. The ac impedance spectra of the oxides showed the presence of two time constants. This response was adequately simulated by a transfer function corresponding to a two-layer structure for the oxides, with a thick layer close to the metal and a thin layer close to the solution. The electrolyte strongly affected the nature of the superficial layer. The variation of the electrical parameters of the oxides (resistance, capacitance etc.) with the final formation potential is reported and the reasons for the complex film structure are discussed.