Lj. Zekovic

The galvanoluminescence spectra of porous oxide layers formed by aluminum anodization in oxalic acid

This paper presents the result of the most recent investigations of the galvanoluminescence (GL) spectra of porous oxide layers formed by aluminum anodization in oxalic acid. A new experimental procedure for GL spectra recording was established. This method can be easily applied to GL spectral measurements on porous anodic films. The spectra were recorded for different values of temperature and anodization current density. Two spectral peaks at 446 and 485 nm, respectively, were observed, indicating the probable existence of two kinds of luminescence centers incorporated in the oxide. In addition, it was discovered that the intensity ratio for the two peaks changes with the current density. Those GL measurements, with further photoluminescent (PL) measurements should lead to a definitive explanation of the GL phenomena, as well as of the nature and distribution of luminescent centers in both porous and barrier oxide films on aluminum.

Investigation of anodic alumina by a photoluminescence method: II

Abstract Photoluminescence emission spectra of anodic oxide films on aluminium were measured under the conditions where an interference of the Lummer-Gehrke type takes place (the emission angle was approximately equal to the total reflection angle). Both barrier (obtained in 0.1 M ammonium tartrate) and porous (obtained in 0.1 M oxalic acid) oxide films were investigated. From the positions of sharp interference maxima and minima in the emission spectra, a very precise determination of film thickness d and anodization rate ϰ is possible. The ϰ values obtained (ϰ1 = 1.33 nm V-1 in ammonium tartrate; ϰ2 = 0.317 nm min-1 in oxalic acid) are in excellent agreement with the results of other workers and with our previous results obtained by other luminescence methods. This procedure is more precise than the photoluminescence method described previously and it does not need any assumption about the spatial distribution of photoluminescence centres.

The theory of galvanoluminescence in the anodic oxide films obtained by aluminum anodization in ammonium tartrate

Abstract The experimental as well as the theoretical aspects of galvanoluminescence (GL) during the anodization of aluminum in the barrier-layer forming electrolyte ammonium tartrate are considered in this paper. The experimental results are explained by the avalanche theory, where the electron trapping processes and the influence of interference effects on the measured GL intensity have been taken into account as well. It is assumed that the luminescence is produced by impact excitation of luminescent centers (LCs). The electric field strength F in the parts of the oxide film occupied by avalanches is calculated in addition to its influence on the ionization coefficient α, mean free path for electron capture by trap x0 and the number of the avalanche electrons. The formula for spectral intensity of GL is also derived.

Determination of the refractive index of porous anodic oxide films on aluminium by a photoluminescence method

It is shown that previously described photoluminescence (PL) methods can be used for simultaneous determination of the refractive index n and thickness d of porous anodic oxide films formed on aluminium. The measurement of n and d is based on the determination of the positions of interference maxima which appear in emission PL spectra. For anodic alumina formed in 0.1M oxalic acid at T = 25 ± 1 °C and j = 2.5–12.5 mA cm-2 we obtained n = 1.68 ± 0.02 in agreement with the result of Huber and Gaugler (1.65±0.02) obtained by another measurement method.

Investigation of the porosity of thin Anodic Al2O3 films by interference of photoluminescence radiation

Abstract Photoluminescence emission spectra of thin (1–5 μm thick) Al 2 O 3 films formed by d.c. anodization of pure aluminium in 0.1 M oxalic acid have been investigated under the conditions of Lummer-Gehrke-type interference (in which the emmission angle is approximately equal to the total reflection angle). In this case, both p and s components of polarized light give additional (doublet) interference maxima owing to the inhomogeneity (porous structure) of the films. From the position of these doublets it is possible to determine the porosity coefficients of oxide films. The results are in good agreement with data obtained by electron microscopy.

The role of interference in the electroluminescence of thin anodic oxide films

Abstract Spectroscopic and time-resolved investigation of the electroluminescence (EL) of thin anodic oxide films have shown an important interference effect. The phenomenon is well pronounced if the anode is a pure aluminum sample with a highly polished reflective surface and the electrolyte is an aqueous solution of a dibasic carboxylic acid (or its salt). The effect can be used for the determination of the oxide film thickness and growth rate, the mean electron avalanche range in the oxide etc. It is also important in the evaluation of the real shapes of time-resolved EL spectra.

New investigations of the interference effect in electroluminescence of anodic films on aluminum

Abstract It is definitively proved that the electroluminescence of thin anodic oxide films on aluminum exhibits a pronounced interference effect. This phenomenon can be used for determination of the anodizing rate and some other important characteristics of oxide films.

Photoluminescence and molecular orbital theory in anodic oxide films formed on aluminium

Abstract We use molecular orbital theory (MOT) to explain the existence of one or two excitation maxima in photoluminescence excitation spectra of Al 2 O 3 formed on Al in different aqueous electrolytes (H 2 SO 4 , H 3 PO 4 , H 2 CrO 4 , Na 2 CO 3 , H 2 C 2 O 4 and HCOOH). All oxides were formed at the same temperature and current density. Comparing experimental and calculated values of the maximum of the excitation wavelength λ max we concluded: (a) that structural change in the formed oxides occurred; (b) that differences in λ max are in connection with different types of incorporated anions or anion fragments.