Impedance Spectroscopy Of Nano-porous Anodic Alumina Structures

Mahmood, Tahir (2014) Impedance Spectroscopy Of Nano-porous Anodic Alumina Structures. Doctoral thesis, Pakistan Institute Of Engineering And Applied Sciences, Islamabad.

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Abstract

Nanoporous anodic alumina has been formed by two step anodizing at 25V in 0.3M sulfuric acid solution. Extremely high aspect ratio cylindrical pores, with a diameter of about 20 nm form a hexagonal pattern with a cell size of about 60 nm, extend through up to 200 ƒ�m-thick anodic alumina. Various structures of PAA samples have been prepared including bi-layered PAA meeting at sandwiched pierced barrier layer, and single layer PAA on aluminum with a sandwiched barrier type oxide of 5-30 nm thickness.The former has been formed by through thickness anodic oxidation of aluminum sheet until complete consumption of aluminum metal.The latter samples have been prepared by anodic oxidation to form a porous film with an underlying barrier layer, which has been subjected to electrochemical thinning. In order to ensure regularity and uniformity in structures two-step anodizing has been performed. XRD shows amorphous nature of the sample. FESEM and TEM confirm hexagonally ordered pores with uniform diameter and regular inter pore distance extending straight through the pores layer meeting the barrier layer at the pore tip.Uniform thickness of the barrier layer in the as-anodized condition is confirmed.Thinning of the barrier layer with conical pore tips or Y-type shapes of the pore tips has been noticed by FESEM.For the bi-layer structure, the sandwiched porous layer contains scarcely located aluminum nanoparticles. Impedance spectroscopy has been employed to determine electrical properties in dry conditions and different humidity levels of up to 90% RH in a temperature ranges of 30-90oC.The electrical properties, as affected by humidity and temperature at low and intermediate frequency ranges, has been explained on the basis of correlated barrier hopping (CBH) and quantum mechanical tunneling (QMT) models, respectively. Temperature dependent changes have been employed to determine activation energy for conduction and thermodynamic parameters for water adsorption at pore surfaces. The role of surface conduction has been found to be different at dry and wet conditions. Contribution of surface conduction continuously increases with the increasing temperature at dry conditions due to thermal activation of the charge carriers (activation energy~0.45 eV). However, at high humidities, its role first decreases due to decreasing concentration of charge carrier (protons) as a result of desorption of physisorbed water and then increases with further rise in temperature as observed at dry conditions. In order to explain the positively sloped linear regions of the Arrhenius plots for surface conduction at low and intermediate frequencies, a model has been proposed to correlates the positive slope of that region with thermodynamic properties of the adsorbed water.The value of standard change in enthalpy for desorption is about 0.45 eV.The role of barrier layer observed at low frequencies and pore walls at high frequencies has also been explained by equivalent circuit fitting.The electrical properties of the former depend on temperature while those of the latter remain temperature independent. Resistance of the barrier decreases with the increase in temperature while it is almost independent of humidity level. It is suggested that barrier layer exhibits semiconducting properties with activation energy ~ 0.35 eV. For very thin barrier layer, it was difficult to deconvolute the electrical response originating from different electroactive regions of the anodic alumina, especially at low temperatures and high humidities.It may possibly be due to the drastic changes in spectroscopic response of anodic alumina that result in varying time constants which could not be resolved for surface conduction and/or bulk conductance through wall interiors.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: Structures, Impedance, Porous, Alumina, Spectroscopy, Anodic, Nano
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Depositing User: Muhammad Khan Khan
Date Deposited: 20 Sep 2016 04:37
Last Modified: 20 Sep 2016 04:37
URI: http://eprints.hec.gov.pk/id/eprint/1342

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