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NANO STRUCTURED ALUMINUM OXIDE BLACK COATING FOR SOLAR PANELS: DOUBLE ANODIZATION USING MUCH IMPROVED ENERGY SAVING PROCESS

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Aluminium oxide coating with highly dense nanopores arranged in ordered close arrays was prepared. The recent two-steps anodization technique has been adopted for this purpose. An improved method for detaching the porous nonregular part from the barrier layer, resulting from the first anodization was suggested. More power saving have been assessed. It also ensures the use of non-toxic species. The nanoporous construction produced shows an extra durability represented by hardness values. Samples with developed nanoporous films were investigated by using SEM and AFM analysis. Plan views and side sections revealed the improvement in the surface morphology and topography. Deep black colored sample was produced using copper sulphate solution 40 g/l.
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VOL. 10, NO. 18, OCTOBER 2015 ISSN 1819-6608
ARPN Journal of Engineering and Applied Sciences
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1
NANO STRUCTURED ALUMINUM OXIDE BLACK COATING FOR
SOLAR PANELS: DOUBLE ANODIZATION USING MUCH
IMPROVED ENERGY SAVING PROCESS
M. F. Shaffei, A. M. Awad, H. S. Hussein and M. S. Mohammed
Department of Chemical Engineering and Pilot Plant, National Research Centre, Dokki, Cairo, Egypt
E-Mail: hala.hussein21@yahoo.com
ABSTRACT
Aluminium oxide coating with highly dense nanopores arranged in ordered close arrays was prepared. The recent
two-steps anodization technique has been adopted for this purpose. An improved method for detaching the porous non-
regular part from the barrier layer, resulting from the first anodization was suggested. More power saving have been
assessed. It also ensures the use of non-toxic species. The nanoporous construction produced shows an extra durability
represented by hardness values. Samples with developed nanoporous films were investigated by using SEM and AFM
analysis. Plan views and side sections revealed the improvement in the surface morphology and topography. Deep black
colored sample was produced using copper sulphate solution 40 g/l.
Keywords: composite materials, nanostructures, thin films, electrochemical techniques.
INTRODUCTION
Nanoporous anodic aluminium oxide (AAO) has
been used in several fields such as electronic, photonics,
energy storage and nanotechnology [1, 2]. The application
of nanoporous AAO for production of solar selective
surface extended to 1980 after black coloration, due to its
great structural characteristics [3]. The porous films find
wide applications as solar panels to heat water for housing,
industrial or agricultural uses [4]. Aluminium, as the
substrate, has low specific gravity and excellent heat
conductivity. So, its alloys may be considered as adequate
substrate for solar panels [5]. AAO is produced previously
by one-step anodization process. During the anodization
process, within an acidic medium, the aluminium surface
is oxidized, where the oxide layer consists of a non-
organized nanoporous structure. A two-steps anodization
process is adopted recently to regulate the nanoporous
structure. Since, irregular nanopores are formed after first
anodizationstep, well-ordered nanopores are obtained by
detaching almost of the oxide layer with irregular structure
and then performing the second anodization steps [6-8].
Hence, the structure of AAO is characterized by high pore
density, high structural regularity and uniform nanopores
dispersion [3], [9], [10], and [11]. In previous researches,
nanoporous AAO produced via double anodization
process, the duration time was extended to hours as
reported by Moghadam et al. [3], or the applied voltage
reached to high values as mentioned by Mukhurov et al.
[1] and Abd-Elnaiem et al. [12]. Moreover, Shih et al. [13]
and most of the other researchers used a solution including
chromic acid for detachment process, which is hazardous
material. The aim of this work is to find a method for
replacing the toxic-chemical detachment step before the
second anodization. Moreover, producing a porous layer of
efficient pore filling can be obtained. Also, it can lower the
highly current density, voltage, and duration time used
usually in the two steps process. The final product
obtained during processing before was evaluated through
SEM, AFM analysis and hardness measurements.
EXPERIMENTAL
Materials
Samples of dimension 10 x 3 cm were cut from
aluminium sheet 99.5 %. Lead electrode was used as
counter electrode in both anodization steps. For pre-
treatment steps, acetone, sodium hydroxide, nitric acid
were used in the appropriate concentrations. Sulphuric acid
170 g/l was prepared and used for anodization. A solution
containing 55% phosphoric acid and 14% sulphuric acid
was used for electrolytic detachment steps.
Set-up and measurements
Cooling system was designed for controlling
temperature of electrolytic solution during anodization.
The system consists of thermostat, compressor, copper
serpentine immersed in water for Freon circulation. DC
Power Supply GW Lab GPR-3030 was used as the source
of DC current during anodization.
Surface morphology was investigated after each
step, using scanning electron microscope device (SEM),
QUANTA FEG 250 (FEI). Surface topography was
analysed using Atomic Force Microscope (AFM),
Shimadzu SPM-9600 Non-Contact Mode, made in Japan.
Hardness of the surface was measured using Shimadzu-
Japan Type - M.
Procedure
Multi steps of nanoporous AAO preparation are
summarized in the flow chart shown in Figure-1.
Aluminium samples were firstly degreased by acetone for
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the disposal of grease or oils which may remain over the
surface, etching by immersion the sample in a solution of
sodium hydroxide 12 %, and then dismutting was done by
immersion the sample into a solution of nitric acid. First
anodization steps was carried out using electrolytic cell of
two electrodes system consisting of DC power supply,
connected to working electrode and lead counter electrode
in electrolytic solution of prepared H2SO4 170 g/l. Electric
potential 15V was applied and the solution was cooled to
17˚C before starting first anodization for 15 min.
Electrolytic detachment steps was used to remove almost
of the upper non-ordered irregular layer of anodic coating.
It was applied for 1 min. at low current density 0.015
A/cm2. Second anodization step was carried out at constant
time 30 min. for re-building up of a new regular anodic
film with nano size diameter. Finally, the sample was fixed
in the electrolytic coloring bath of CuSO4 (40g/l) at pH 2,
where Cu++ ions were reduced and deposited within the
nanoporous film. A homogeneous black color was formed.
Figure-1. Flow chart of two-step anodizing for preparation of nano porous alumina and colouring.
RESULTS AND DISCUSSIONS
Atomic Force Microscope analysis (AFM)
As shown in Figure-2(a), AFM analysis of Al
substrate reveals the presence of highly average micro
roughness 0.967μm and the crystalline structure of Al
metal was appeared. The transfer of the surface to
amorphous structure was obtained after first anodization as
shown in Figure-2(b) and the surface roughness becomes
about 0.96μm. The non-ordered porous film of anodic
oxide film was removed and regulated after electrolytic
detachment step (Figure-2c), where leveling of the surface
was improved with low thickness ordered layer, roughness
reached to 0.941μm. Shades of huge numbers of nanopores
through nanoporous AAO film shown in Figure-2(d)
where the roughness of second anodized sample was
0.991μm.
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Figure-2. AFM of different steps (2µm x 2µm) of nanoporous alumina preparation.
(a) Pre-treated Al (R=0.967μm) (b) 1st anodized (R=0.958 μm)
(c) Electrolytic detachment (R=0.941μm) (d) 2nd anodized (R=0.991μm)
Scanning Electron Microscope (SEM)
The surface morphology was strongly dependent
on the applied conditions and varied after each steps.
Aluminium sample surface was cleaned thoroughly by
pretreatment and plain granular surface of aluminium was
observed as in Figure-3(a). A layer of porous AAO film
was formed after first anodization step and a non-regular
porous film was observed by SEM as shown in Figure-3(b)
and the measured pore diameter was vacillated between 16
to 19 nm. Electrolytic detachment was used in the
subsequent step to eliminate almost of the non-ordered and
irregular oxide film, where an ordered and regular matrix
of thin porous oxide layer was predominated after
electrolytic detachment with pore diameter ranged between
16 and 21 nm as shown in Figure-3(c). Hence, after second
anodization, 3D nano architectural structure and porous
nano composite was appeared with measured pore
diameter ranged between 18 and 23 nm as shown in
Figure-3(d).
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(a)
(b)
(c)
(d)
Figure-3. SEM images of different steps for nano porous alumina preparation (a) Pre-treated Al
(b) 1st anodized (c) Electrolytic detachment (d) 2nd anodized
Surface characterization for second anodized Al
sample After elimination if non-ordered porous AAO
film, a new ordered film was built up in regular and
arranged architectural structure by applying second
anodization step. SEM analysis of cross section of second
anodization sample revealed the presence of regular
bundles and ordered distribution of nanoporous AAO over
the surface. Aluminium oxide nanotubes stems from the
bottom barrier layer towards the top of the surface as
shown in Figure-4(a). From Figure-4(a), the diameter is
narrow and minute in the top, while it seems to be wider in
the bottom. Hence, previous researchers immersed the
samples in phosphoric acid after second anodization for
widening the nanopore tip.
Highly focusing on second anodized sample (1 x
1 μm) by AFM, it reveals the presence of corrugated
surface with longitudinal sections of different planes of
nanoporous AAO film as shown in Figure-4(b), each
section contains bundles of nanopores.
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(a)
(b)
Figure-4. (a) SEM of cross section of anodized Al, (b) AFM 1x1 µm zooming graph of anodized Al
sample after second anodization step.
Durability and Hardness
Durability of aluminium samples at various stages
of processing was evaluated by hardness measurements.
Figure-5 represents the hardness values attained. The
hardness was increased appreciably by first anodization.
Then, a slight increase was observed after electro-
detachment step. Hence, a sudden increase was obviously
measured by the second anodization. The hardness values
of the four samples are respectively: 56, 110, 122 and 211
Kg/mm2. The durability of aluminium was doubled by first
anodization and binary doubled by the second anodization.
This is a great proof for the effect of the nanopores and
channels in reinforcement of the oxide matrix. Moreover,
it represents a new evidence for the great ordering and
reclamation of the nanotubes in the second anodization
[14-16].
Figure-5. Measurements of hardness of different samples of aluminium.
Energy consumption and saving
The most important factor in choosing any
process for commercial application is energy consumption
and consequently the cost. Many processes have been
invented for two -steps anodization. However, it depends
mainly on electric power. Electric energy consumption is a
function of applied potential, current density and duration
time. The goal of this research was mainly to reduce these
factors by replacement of the chemical etching by electro-
detachment between the two anodizing steps. Table-1 lists
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some ranges of the aforementioned factors used in
previous researches. According to the conditions applied,
the statistical data indicates that the cost of electro-
detachment step is highly favored to replace the chemical
etching methods. In addition, the total energy of the whole
steps comprising, first anodization, electrolytic detachment
and second anodization is 0.00227 KWh/cm2. This
indicates the economic possibilities and the process is
amenable to applications. The present research gives much
in more reduction in energy consumption factors as can be
deduced from Table-1. Extra refrigeration, required by
other researchers, represents additional process costs. The
use of the proposed step shows a considerable energy
saving, as the total average energy estimated was 0.00125
KWh/cm2 for first anodization step and 0.00102 KWh/cm2
for the second anodization step.
Table-1. Ranges of variables used in two- step anodization for previous researches.
Research author
(s) step
Year
V, volts
I, mA/cm2
t, min.
T, oC
Shih. H .H et al. [13]
2006
20-60
-
30
20
20-60
-
30
20
Zhu. Z et al. [17]
2011
40
-
3600
2
20-60
-
30-60
2
Abbas. M. M [18]
2013
8
-
60-180
18
18
-
60-180
18
Samantilleke. A. P et al.
[6]
2013
17-20
-
60
5
17-20
-
120
5
Mukhurov. N. I et al.
[1]
2014
Up to 110, 160
Up to 70, 80
10
5
Up to 140, 150
Up to 40, 70
25-45
5
Present Research
2015
15
166
15
17
15
203
30
17
Black coloration of AAO films
Figure-6 reveals the coloured sample with three
different regions, Al substrate, second anodized and
coloured regions. Highly ordered nanoporous AAO
coating was prepared by anodic oxidation of aluminium
surface in 170 g/l sulphuric acid. Figure-7 reveals the
variation of the surface aspects after each step; aluminium
surface was modified by multi steps including preliminary
treatment, first anodization, electrolytic detachment and
second anodization. Low cost and low hazardous
electrolytic detachment step was suggested to eliminate
almost of non-ordered porous AAO after first anodization
step. As shown in Figure-8, it is focusing on the blackened
sample which shows deep coloured film with homogenous
and smooth texture.
Figure-6. Different regions of coloured aluminium
sample.
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Figure-7. Different steps of prepared samples: (a) Aluminium sample (b) 1st Anodized sample
(c) Electro- detached sample (d) 2nd Anodized sample (e) electrolytic ally coloured sample.
(a)
(b)
Figure-8. (a) and (b) SEM and AFM of nanoporous coloured AAO.
CONCLUSIONS
A new step for nanoporous oxide film preparation
was developed to aluminium substrate in two-steps
anodization process. It was elucidated in an
electrochemical detachment of almost the porous oxide
film after first anodization. This proposed step causes a
great reduction in electric energy consumption in the
process of two steps anodization. More power saving was
affected in refrigeration requirements during the process.
The toxic chromic acid used previously for chemical
detachment step was avoided. Hence a great benefit in
environmental impact was attained. The nanoporous
construction produced shows an extra durability
represented by hardness values. Moreover, the prepared
nanoporous AAO film is suitable for extra quality deep
black coloration as efficient solar energy panels.
ACKNOWLEDGEMENT
We deeply thank Prof. Dr. Maha Farid Shaffei for
her effort and all the team work and we appreciate the
valuable help from services central lab in National
Research Centre, Dokki-Cairo.
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