ANODIC OXIDE FILMS ON NIOBIUM: THICKNESS, DIELECTRIC CONSTANT, DISPERSION, REFLECTION MINIMA, FORMATION FIELD STRENGTH, AND SURFACE AREA

Abstract

The wavelengths of minimum specular reflectivity (at 11° incidence) due to interference were determined using a spectrophotometer for a series of films formed on chemically polished niobium. With a value of the refractive index n = 2.46 ± 1% at 4358 Å wavelength by the Abelès method (reported elsewhere), the spectrophotometric data give the refractive index as a function of the wavelength λ, n = 2.26 + 0.398/(λ/103 Å − 2.56)1.2. To analyze the spectrophotometric results, an auxiliary measure of thickness was required (though, with the chart given below, the thickness of a film may be determined directly from spectrophotometric measurements alone). A combination of measurements of the a-c. capacity and of the charge required to form the films gives a suitable measure of thickness (in terms of ρ/ε, where ε = dielectric constant and ρ = density) which is not dependent on a knowledge of the true surface area. The spectrophotometric data provide a calibration of this measure of thickness and thus give ε/ρ. With ρ = 4.36 g cm−3 (reported for the bulk amorphous oxide) this gives ε about 41 (compared with about 27.6 for Ta2O5). The effective surface area of the chemically polished metal was then found to be about 7% greater than the apparent area. At the ionic current density used to form the films (10 ma cm−2), the field strength in the oxide was estimated as 4.96 × 106 v cm−1 within a few per cent uncertainty. Because the field to produce a given ionic current is lower than with Ta2O5 films, the capacity of films formed to a given voltage at a given current density and temperature is not so much greater for niobium than for tantalum as the dielectric constants might lead one to expect. It is suggested that there may be a correlation between dielectric constant and ionic conductivity. The Nb2O5 films recrystallize like Ta2O5 films under an applied field but more readily, at least with the purity of metal now available.