In Situ Spectroscopic Ellipsometric Study On Anodization Of Aluminum In Oxalic Acid Solution

The fabrication and application of porous anodic alumina （PAA） films withhighly-regular nanoporous structures have attracted attention due to its wideapplications as one of the most common templates for the synthesis of various types offunctional nano-materials. However, the growth mechanism of PAA films is stillunclear so far. In this thesis, in situ spectroscopic ellipsometry （SE） as a high-sensitivitytechnique was applied to quantitatively monitor the formation process of PAA films. Byseveral reasonable models, the in situ spectroscopic ellipsometic spectra can beinterpreted very well, and the dynamics changing information of the thickness andcomposition about the interface layer, barrier layer and porous layer could be acquired.Based on above information, the initial growth process of PAA films can be detailedlystudied. The influence of applied anodization voltage and temperature, theconcentration of oxalic acid solution on the formation process of PAA films was alsodiscussed, which could provide the parameters of growth kinetics of PAA films for thefurther theoretical studies.The results of SE, EIS and FE-SEM were used to analysis the initial growthprocess of PAA films in H₂C₂O₄solutions. It is found that the in situ ellipsometricspectra can be interpreted very well by the established optical models, where the gradedEMA and anisotropic EMA can accurately describe the structures and characteristics ofthe interfacial layer and porous layer of the PAA films, respectively. Based on theapplied optical models and the calculated results, the anodization of aluminum processcan be clearly divided into four stages, viz., the barrier layer growth, pores initiation,pores development and the steady-state porous film growth. Moreover, in the last stagethe thickness of the porous layer increases linearly with time while the porosity isalmost constant. Additionally, the change trend of the thickness of barrier layer obtainedby EIS and the thicknesses of the barrier layer and porous layer acquired by FE-SEMmeasurement is greatly agreed with the results of in situ SE, which confirms that theoptical models and the calculations are accurate and physically realistic. Thus, weestablish an obvious method for the interpretation of the ellipsometric spectra.In situ SE was also used to study the influence of applied anodization voltage andtemperature, the concentration of oxalic acid solution on the formation process of PAAfilms. And the results indicate that the increase of any experimental condition can promote the growth of the PAA films, so that the duration of the preceding three stagesis gradually shortened. The above results illuminate that with the voltage, temperatureand concentration increasing, the rate of electrode reaction gradually increases, hencethe evolvement of the growth stages of the PAA films becomes more quickly, and thegrowth of PAA films more early enters the last stage. Moreover, the increasing of threeconditions hardly affects the thicknesses of the interface layer, barrier layer, as well asthe porosity of the porous layer, mainly affects the thickness of porous layer and itsgrowth rate.It is also found that the growth rate of the porous layer increases with appliedanodization voltage and temperature, the concentration of oxalic acid solutionincreasing. The growth rate of the porous layer （v） follows an exponential functionrelationship （R²=0.9995） with the anodization voltage （E）, and the equation is: v=0.19exp （E/15.26）-0.31. And the correlation between the growth rate of the porous layer （v）and anodization temperature （T） is approximately an exponential relationship（R²=0.9960）, and the equation is: v=2.43*10^-5exp（T/23.91）-2.11. And the relationshipbetween the growth rate of the porous layer （v） and the concentration of oxalic acidsolution （c） is approximately linearity （R²=0.9812）, and the equation is: v=3.46c+1.15.