A comparative study is made of Al2O3layers formed at 400℃ in molten KNO3and Al2O3formed 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 (0.4μF/cm2) oxide layers were produced at 2.0 V (400℃) and 8 V (20℃). Electron diffraction indicated aγ-Al2O3structure at an anodization temperature of 400℃ and

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... s structure at room anodization temp. The initial values of tanδwere100 · 10−4 ± 40 · 10−4and400 · 10−4 ± 200 · 10−4respectively.Capacitance and tanδmeasurements 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 inγ-Al2O3layers during the life test.