A comparative study is made of Al2O3 layers formed at 400℃ in molten
KNO3 and Al2O3 formed at room temperature in a common solution of ammonium pentaborate in ethylene glycol. At 400℃ and constant current
(0.5 mA/cm2 ) the linear anodization range is limited to 2.7 V by scintillation and local oxide breakdowns.
Nonporous 200 Å thick (0.4 μF/cm2) oxide layers were produced at 2.0 V (400℃) and 8 V (20℃). Electron
diffraction indicated a γ-Al2O3 structure at an anodization temperature of 400℃ and amorphous structure at
room anodization temp. The initial values of tan δ were 100 · 10−4 ± 40 · 10−4 and 400 · 10−4 ± 200 · 10−4
respectively.Capacitance and tan δ measurements during accelerated life test indicated that the films produced at 400℃ are
not superior to those formed at room temperature. In both cases a diffusion of metal at the metal-oxide interface
seems to be the main ageing mechanism.The internal electrical field measured by the C-V method was unchanged in γ-Al2O3 layers during the life test.