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Variation of Anodizing Conditions
The main
parameters
affecting the anodic oxidation of Al and the prop-
erties of the resulting alumina template are the current density or
applied
voltage, the pH and type of electrolyte and the temperature of the bath.
They may be varied individually or in combination to yield specific di-
mensions and distributions of the subsequent nanopores. Although the an-
odization of Al has been an industry standard for many years, the
specific
manipulation of the anodization process for intended nanoporus
configura-
tions is a relatively recent development in the field of nanotechnology
over
the last ten years.
Variation of the total current density has been
shown to yield a linear
increase in alumina thickness, pore size and pore spacing.[5] The two
dimen-
sional frequency of pore occurrence, called the porosity (P), is a
function
of pore size (d) and spacing (s) is given as[5]
P = (pi*d2/4)/s2
The pore density (Np) is a function of pore spacing and is
defined as
Np = 1/s2
making it inversely proportional to the square of the applied
voltage.[5]
However anion absorption and transport also increases with increasing
voltage resulting in a greater occurrence of anionic impurities at
higher
voltages.[5]
The total current density is a function of applied
voltage and thus pro-
duces similar effects; however the electric field at the surface oxide
interface
is constant with steady state current density.[5] In the case of a
fixed poten-
tial the electric field decreases with time as the oxidation process
neutralizes
the surface charge distribution.[5] It has been observed that varying
the po-
tential incrementally as a function of time results in variation of the
inner
to outer pore diameter ratio.[5]
Increasing the electrolyte pH at constant current
density serves to de-
crease the dissolution rate thereby increasing thickness of the
oxidation
layer.[5] Additionally, the steady-state voltage also increases which
directly
effects the pore spacing; and subsequently the pore density is
reduced.[5]
However an increase in electrolyte pH at constant voltage results in a
de-
crease in oxidation current density to compensate for a decrease in
dissolu-
tion current density thus maintaining the steady-state voltage
condition.[5]
It has been shown that variational pore diameters are possible while
main-
taining pore density by variation of applied voltage along with
increasing
pH.[5]
At higher electrolyte temperatures similar effects to a
decreased pH are
observed.[5] Additionally there seems to be a decrease in anionic
impurities
however the ratio of inner to outer barrier thickness increases due to
the
tendency of crystalline oxide growth at elevated temperatures.[5]
P. D. Wallace
UAF Spring 09'
Steve's Templates
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