- Email: [email protected]
Structural, optical and thermal properties of nanoporous aluminum
Accepted Manuscript Title: Structural, Optical and thermal properties of nanoporous aluminum Author: Taher Ghrib PII: DOI: Reference:
S0040-6031(14)00536-X http://dx.doi.org/doi:10.1016/j.tca.2014.11.020 TCA 77077
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
24-8-2014 17-11-2014 20-11-2014
Please cite this article as: Taher Ghrib, Structural, Optical and thermal properties of nanoporous aluminum, Thermochimica Acta http://dx.doi.org/10.1016/j.tca.2014.11.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural, Optical and thermal properties of nanoporous aluminum filled with graphite and silicon Taher Ghrib1,2,*[email protected] 1
Laboratory of Physical Alloys (LPA), Science Faculty of Dammam, Dammam University, Saudi Arabia.
2
Photovoltaic Laboratory, Research and Technology Centre of Energy, Borj-Cedria Science and Technology
Park, BP 95, 2050 Hammam-Lif, Tunisia
Highlights
-
A simple electrochemical technique is presented and used to manufacture a porous
-
PT
aluminum layer. Manufactured pores of 40 nm diameter and 200 nm depth are filled by nanocrystal of
-
RI
silicon and graphite.
Dimensions of pores increase with the anodization current which ameliorate the
A new thermal method is presented which permit to determine the pores density and
U
-
SC
optical and thermal properties.
the layer thickness.
All properties show that the manufactured material can be used with success in solar
N
-
A
cells.
M
Abstract
D
In this work the structural, thermal and optical properties of porous aluminum thin
TE
film formed with various intensities of anodization current in sulfuric acid are highlighted. The obtained pores at the surface are filled by sprayed graphite and nanocrystalline silicon
EP
(nc-Si) thin films deposited by plasma enhancement chemical vapor deposition (PECVD) which the role is to improve its optical and thermal absorption giving a structure of an
CC
assembly of three different media such as deposited thin layer (graphite or silicon)/(Porous aluminum layer filled with the deposited layer)/(Al sample).
A
The effect of anodization current on the microstructure of porous aluminium and the
effect of the deposited layer were systematically studied by atomic force microscopy (AFM), transmission electron microscopy (TEM) and Raman spectroscopy. The thermal properties such as the thermal conductivity (K) and thermal diffusivity (D) are determined by the photothermal deflection (PTD) technique which is a non destructive technique. Based on this
full characterization, it is demonstrated that the thermal and optical characteristics of this films are directly correlated to their micro-structural properties.
Keywords Porous Aluminum, optical reflectivity, thermal conductivity
1. Introduction In view of their practical importance, aluminum porous materials were intensively
PT
studied in the last decades [1-11] due to their interesting electrical, optical and structural properties for the reason of its self organized pores that have quasi-cylindrical shape which
RI
constitute a technological revolution and find now many applications such as filtration
SC
membranes [12,13], as backing of fabrication nanowires [14-16], as photonic crystals [17,18], as humidity sensors [19,20], or as cathodes for organic light emitting diodes[21,22]. In the
U
literature many attempts are reported in order to find relationships between porosity, thermal properties [23,24], optical properties [11, 25,26], and this by exploring the effect of size, form
N
and density of the pores where whose fine tuning and its filling by various types of materials
A
gives cells for energy applications such as solar cells and heating panels. Method most
M
common, used in this last period to produce nanopores is the electrochemical in which the anodization current at a significant effect on the physical and structural properties of the
D
porous thin layer produced.
TE
The main purpose of this work is to present a simple method for producing thin porous aluminium layer by electrochemical anodization technique, which will be filled by two
EP
different elements such as crystalline graphite and silicon and try to study the anodization current effect in the microstructure of the porous aluminium layer (PAL) and the effect of the
CC
deposition element on its optical properties such as total reflectivity and thermal conductivity such as thermal conductivity and thermal diffusivity which are determined by the
A
photothermal deflection (PTD) technique [27-29] which the principle will be recalled. 2. Experiments processing High-purity aluminum foil (99.997%), 0.25 mm thick was used as a starting material. Before anodization the aluminum paper was rinsed in a CP4 solution which is a mixture of 64% of nitric acid, 20% acetic acid CH3COOH and 16% of fluorhydric acid to eliminate the impurities linked to the surface. The cleaned samples were treated by a mechanical polishing
with polishing machine model 920 working with different speed combined with a alumina abrasive in an alkaline solution and anodized during 25 min in a solution of diluted sulfuric acid with 66.66% H2SO4 and 33, 33% H2O2 by weight at room temperature and with three different anodization currents (250 mA ,300 mA , 350mA). On the PAL surface, c-Si was deposited by PECVD technique [12] at 50 °C using a gas mixture of silane and H2 at a total pressure of 0.5 m Torr. And also another set of three samples was sprayed for the same pressure and speed by a thin commercial graphite layer which contains a high level of pure and fine graphite powder. The surface morphology of the films was examined by AFM and TEM microscopy. Samples preparation for TEM analysis was performed in a standard way.
PT
The average pore diameter was carried out using an ionic milling of type (Dual Ion Mill Model 600).
RI
The thermal properties such as the thermal conductivity and the thermal properties are determined by the PTD technique [14,15]. This method (fig. 1) consists in heating a sample
SC
with a modulated light beam of intensity I I 0 (1 cos t) that will be absorbed on the
U
surface and generates a thermal wave. This thermal wave will propagate in the sample and in the surrounding fluid (air in our case) and will induce a temperature gradient and then a
N
refractive index gradient in the fluid. The fluid index gradient will cause the deflection of a
A
probe laser beam skimming the sample surface. This deflection may be related to the thermal
M
properties of the sample. The sample is heated by a halogen lamp light of Power 100W modulated by using a mechanical chopper at a variable frequency. An (He-Ne) laser beam
D
skimming the sample surface at a distance z, is deflected. This deflection can be detected by a
TE
four quadrant photo-detector and converted to an electrical signal which is measured by a lock-in amplifier. Through the intermediary of interfaces, the mechanical chopper and the
EP
Look-in amplifier a microcomputer will set the desired modulation frequency and read the values of the amplitude and phase of the photo-thermal signal and then draw their variations
A
CC
according to the square root modulation frequency.
3. Results and discussions
3.1. Microstructural characterization Fig. 2 shows AFM microscopy of these samples treated with three different anodization current intensities, we notice the presence of a quasi-arranged structure which contains pores with width vary according to the current from 24 nm for Ia=250mA to 73 nm
for Ia=350mA and with depth whose value are given in Table 1 which also gives the porosity percentage according to the anodization current, where it is shown that it increases with the anodization current of 42% to 51%. Fig. 3 shows plan view TEM of the PAL surface with various anodization current in which it is noted for all micrographs that they exhibit pores which appear almost arranged with diameters which increase with the current intensity from 38 to 46 nm whose cells seem to be in an approximately hexagonal structure. The deposition of the silicon thin layer on these prepared samples gave samples whose AFM images are shown in fig. 4 which present a nano-granular aspect with size estimated
PT
(using WS×M 4.0 software) varying from 5 to 20 nm as (Ia) increases, which can be named nanocrystallites (nc). In order to confirm the above mentioned results in term crystallinity and
RI
grain size a Raman spectroscopy was performed and presented in fig. 5 giving the Raman spectra for the porous aluminum filled by nc-silicon film versus various anodization current.
SC
By comparing their spectra with that of the film deposited directly on the untreated aluminum substrate, main feature of nc-Si Raman spectra is the transverse optical (TO) phonon mode
U
located in the range from 500–530 cm–1, in which we remark that the peak intensity increases
N
and its position shifts to low energies (redshift) with the anodization currents tending towards
A
a peak corresponding to monocrystalline Si (100) at 529 cm-1.
D
M
The crystallite size was calculated using the following formula [30].
a w A D
TE
Where w w cSi w n cSi and “D” are the shift of Raman peak position of the nanocrystallite compared to that of the monocrystalline silicon. "a " is the lattice constant of the silicon
EP
which is equal to 0.543nm and A=47.41 and γ =1.44 are constants. The crystallite size
CC
estimated from this method was found between 8 –20 nm when Ia varies from 250 to 350mA which are close to the values found by AFM.
A
3.2. Thermal characterization The modification of the structure and composition of sample influences the physical properties for this reason we opts to study the thermal properties variation with the anodization current by using the (PTD) technique . In the case of a uniform heating we can
use a 1-dimensional approximation, and the amplitude and phase
of the probe beam
deflection is given by: x
2 L dn T0 e f n f dTf
(1)
and
x 5 f 4
(2)
Where l is the width of the pump beam in the direction of the probe laser beam, n and f are respectively the refractive index and the thermal diffusion length and T f is the temperature of the fluid, T0 and θ are respectively the amplitude and phase [28,29] of the temperature at the
PT
sample surface which are function of the thermal properties of the different media. x is the
RI
distance between the probe beam axe and the sample surface.
Before the calculation of the probe beam deflection, one must know the expression of
SC
the surface temperature T0 which is calculated by writing the heat flow and temperature continuity at the interfaces x1 l 2 , x2 l 2 l1 , x 3 l 2 l1 l s between the various
N
U
layers constituting the sample (fig. 6), which gives the following expression: [29]
(3)
A
T0 ((1 b)4es ls (1 b)2es ls )E3 (r1 b) es ls E1 (1 b)1 es ls (1 b)3es ls
M
Where 1 , 2 , 3 , 4 , b , 1 , r1 , E1 and E 3 are real or complex parameters, which
D
depend on the optical and thermal properties of different layers.
TE
In order to determine the thermal properties evolutions with the anodization current and the deposited layer a study of the photothermal signal with the square root modulation
EP
frequency is carried out for different samples. The experimental variations of the amplitude and phase of photothermal signal are given respectively in fig. 7 and fig. 8 for the porous
CC
aluminum respectively filled by graphite layer and nanocrystallite-silicon layer. We use the model of three layers such as (deposited thin layer) /(Porous aluminum layer filled with the
A
deposited element)/(Al sample). We begin in a first step by introducing thermal properties of the deposited layer which are ( K 2 0.35W . K 1.m 1 , D2 1,45104 m2 . s 1 ) for the graphite and
( K 2 4.8 W . K 1 .m 1 , D2 0.19 m 2 . s 1 ) for the silicon which are amorphous layers where behave as feeble thermal conductors. The best fit between theoretical and experimental curves give theoretical values of the pair ( K1 , D1 ) of the porous layer filled with graphite and silicon whose values are given in Table 2.
Physically, the thermal conductivity and thermal diffusivity are defined respectively as the ability to conduct heat and the propagation speed of heat in the material, we can say that introduction of thermal isolator material such as graphite or amorphous silicon in porous aluminum moves it thermal properties toward those of semiconductor. By using obtained thermal conductivities of the deposited layer K 2 , of the porous layer filled with graphite or silicon K1 and of the pure aluminum we can calculate the percentage of pores in the treated sample by using the relationship (4), whose values are given in Table 2 where we remark its increase with anodization current.
K1 K Al ( 4) K 2 K Al
PT
RI
We remark that its values are in good agreement by using the results of silicon or graphite deposition; since these pores are specific to the treated aluminum and do not depend
SC
on the filling element and also in good agreement with the results obtained by AFM
U
technique.
N
3.3. Optical characterization
A
These materials can be used in solar cells and heat exchangers then they must have
M
optical properties that are suitable with its areas, for this reason we study the total optical reflectivity versus wavelength, which are shown in fig. 9 and fig. 10 respectively for samples
D
with graphite deposited layer and with silicon deposited layer.
TE
In figure 8 the measured reflection coefficient versus le incident wavelength of the storage, we remark oscillatory form whose amplitude increases with the wavelength from the
EP
visible to the near infrared wave and disappears for a length of around 2 m , these oscillations may be due to interference between the waves reflected at the interfaces of
CC
different nanolayers which are graphite layer-porous layer filled with graphite and porous layer filled with graphite-Aluminum sample. In others hand if we measure the mean distance
A
between two successive peaks, we note that are in order of 200 nm which is somehow; the thickness of the porous layer and this implies that the interference is due to the interference of rays reflected on the two surfaces of the porous layer having a thickness d but the external graphite layer whose thickness is equal to 2 m does not affect the interference but will have a significant effect on the reflected amplitude. The total reflection coefficient r can be calculated using the Fresnel equations as, [31]
r21 r1s e 2 i r (5) 1 r21 r1s e2 i Where r21 and r1s are the reflection coefficients corresponding to the interface between the porous layer (1) and graphite (2) , and between porous layer (2) and substrate (s) respectively,
l1 is the thickness porous layer. Also is the phase difference between the reflected lights in these two interfaces, and is the incident wavelength received at the input face of the porous layer whose expression is:
4 n ( ) l1 cos (6)
PT
reflection angle and finally the total reflectivity is given as:
SC
2
R r 100
RI
Where n ( ) is the refractive index that can be varied with wavelength and is mean
U
The overall decrease in the total reflectivity with the anodization current intensity can be related to the pores depth, in fact more they are deep more the internal reflections are multiple
N
and which will increase the absorption and reduce the reflection, where we note that the
A
elevation of anodization current from 250mA to 350mA saves about 20% in the reflected
M
energy.
For the aluminum porous aluminum filled with crystallite silicon we note a rapid
D
increase in reflectivity to a saturation value which decreases with the anodization current with
EP
infrared waves.
TE
oscillations due to interferences between reflected waves that are not very clear for near
4. Conclusion
CC
In this work, we have presented a method for producing nanoporous thin layer by
electrochemical anodization technique with different anodizations currents. The obtained
A
pores having 40 nm diameter and 200 nm depth are filled by nanocrystal of silicon and graphite. Structural, optical and thermal properties of the obtained structure were highlighted and it was shown that the increase in the anodization current increases the pores density, decrease its total optical reflectivity and moderate its thermal properties. The thermal properties are determined using the photothermal deflection technique which can be related to
the pores density in the obtained porous aluminum layer. The obtained material can be used in the fields of energy production and storage.
Acknowledgements This work was supported by the Scientific Research Deanship of University, under Project no. 2014264.
Dammam
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PT
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RI
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SC
[3] A. C. Galca, E. S. Kooij, H. Wormeester, and C. Salm, Journal of applied physics, 7 (2003), 94.
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[4] W. J. Tseng, C. J. Tsai, Journal of Materials Processing Technology, 146 (2004), 289293.
N
[5] J. Ram, R. Singh, Applied Thermal Engineering 24 (2004), 2727–2735.
A
[6] A. Y. Ku, S. T. Taylor, W. J. Heward, L. Denault, S. M. Loureiro, Microporous and
M
Mesoporous Materials, 88 (2006) 214-219.
[7] P. K. Samantray, P. Karthikeyan, K.S. Reddy, International Journal of Heat and Mass
D
Transfer 49 (2006), 4209–4219.
1849–1854.
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[8] T. C. Wang, T. X. Fan, D. Zhang, G. D. Zhang, D. S. Xiong, Materials Letters, 61 (2007)
5045.
M.B. Murphey, J.D. Bergeson, S.J. Etzkorn, L. Qu, L. Li, L. Dai, A.J. Epstein,
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[10]
EP
[9] B. Alinejad, M. Zakeri, Journal of Materials Processing Technology, 209 (2009) 5042-
Synthetic Metals, 160 (2010) 235-237.
A
[11] M. Ghrib, M. Gaidi, N. Khedher, T. Ghrib, M. Ben Salem, H. Ezzaouia, Applied Surface Science, 257 (2011) 3998-4003. [12] W. Liu, N. Canfield, Journal of Membrane Science, 409 (2012) 113-126. [13] E. F. Barbosa, L. P. Silva, Journal of Membrane Sciencbe, Volumes 407 (2012) 128135.
[14] Grzegorz D. Sulka, Agnieszka Brzózka, Lifeng Liu, Electrochimica Acta, 56 (2011) 4972-4979. [15] X. Wang, C. Li, G. Chen, L. He, H. Cao, B. Zhang, Solid State Sciences, 13 (2011) 280284. [16] J. Huang, H. Ren, P. Sun, C. Gu, Y. Sun, J. Liu, Sensors and Actuators B: Chemical, 188 (2013) 249-256. [17] X. Hu, Y.J. Pu, Z.Y. Ling, Y. Li, Optical Materials, 32 (2009) 382-386. [18] S.S. Kurbanov, Z.Sh. Shaymardanov, M.A. Kasymdzhanov, P.K. Khabibullaev, T.W. Kang, Optical Materials, 29, (2007) 1177-1182.
Physical, 174 (2012) 69-74.
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[20] L. Juhász, J. Mizsei, Thin Solid Films, 517 (2009) 6198-6201.
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[19] M. Almasi Kashi, A. Ramazani, H. Abbasian, A. Khayyatian, Sensors and Actuators A:
A.M. Mozalev, Surface Science, 507 (2002) 593-597.
SC
[21] A.V. Kukhta, G.G. Gorokh, E.E. Kolesnik, A.I. Mitkovets, M.I. Taoubi, Yu.A. Koshin,
[22] K. P. Kim, K.S. Lee, T.W. Kim, D.H. Woo, J.H. Kim, J.H. Seo, Y.K. Kim Thin Solid
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Films, 516, (2008) 3633-3636.
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[23] T. C. Wang, T. X. Fan, D. Zhang, G. Zhang, D. S. Xiong, Materials Letters, 61
A
(2007) 1849-1854.
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[24] R. Dyga, S. Witczak, Procedia Engineering, 42 (2012) 1088-1099. [25] H. Zhuo, F. Peng, L. Lin, Y. Qu, F. Lai, Thin Solid Films, 519 (2011) 2308-2312.
D
[26] S. Stojadinovic, Z. Nedic, I. Belca, R. Vasilic, B. Kasalica, M. Petkovic, Lj. Zekovic,
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Applied Surface Science, 256 (2009) 763-767. [27] M. Bertolotti, G. L. Liakhou, A. Ferrari, V. G. Ralchenko, A. A. Smolin, E.
EP
obrastsova, K. G. Korotoushenko, S. M. Pimenov, V. I. Konov, J. Appl. Phys., 75 (1994) 7795-7798.
CC
[28] T. Ghrib, F. Bejaoui, A. Hamdi, N. Yacoubi, Thermochimica Acta 473 (2008) 86–91. [29] T. Ghrib, I. Gaied, N. Yacoubi, Nova publishers, (2010) 95-146.
A
[30] M. Ghrib, M. Gaidi, T. Ghrib, N. Khedher, M. Ben Salam, H. Ezzaouia, Applied Surface Science, 257 (2011) 9129-9134. [31] J. A. Kong, Electromagnetic Wave Theory, (1990).
Figures and Tables
Fig. 1: Experimental set-up used for PTD investigation: 1-Table of micrometric displacement, 2-Sample, 3Photodetector position, 4-Fixed laser source, 5-Halogen lamp, 6-Look-in amplifier, 7-Mechanical chopper, 8Computer.
Fig. 2: AFM image of Porous Aluminum layer (PAL) versus anodization current intensity (250,300,350) mA.
Fig. 3: TEM micrographs of porous aluminum fabricated with various anodization current intensities (250,300,350) mA.
PT
Fig. 4: AFM image of Porous Aluminum layer (PAL) filled with nanocrystalline silicon for various anodization current intensities (250,300,350) mA.
RI
Fig. 5: Raman spectra of PAL filled by crystalline silicon prepared with different anodization current intensities (250, 300, 350) mA
SC
Fig. 6: Schema of the stacked three layers
A
N
U
Fig. 7: Experimental and theoretical of the photothermal signals of the porous aluminum filled with the graphite for different ano
M
Fig. 8: Experimental and theoretical of the photothermal signals of the porous aluminum filled with silicon for different anodizatio
Fig.9 Total reflectivity of the porous aluminum filled with graphite for different anodization current intensities
D
(250, 300, 350) mA.
A
CC
EP
(250, 300, 350) mA.
TE
Fig. 10: Total reflectivity of the porous aluminum filled with silicon for different anodization current intensities
Table 1: Thickness and porous density of nanoporous aluminum layer for different anodization current intensities (250,300,350) mA.
Ia (mA)
l1 (nm)
P (%)
250
212.03± 3.80
41.79± 1.23
300
233.12± 4.66
47.71± 1.41
350
254.12± 4.66
51.12± 4.66
Table 2: Measured thermal properties and calculated porous density of the porous aluminum filled with silicon for different anodization current intensities Ia (250,300,350) mA. Porous aluminum filled with graphite
Porous aluminum filled with silicon
Ia
250mA
300mA
350mA
250mA
300mA
350mA
K1(W.m-1.K-1)
158
142
131
162
144.3
133.6
D1(cm .s )
0.59
0.46
0.39
0.63
0.42
0.38
(%)
43.1%
48.0%
52.1%
41.4%
48.0%
52.0%
2
-1
M
A
N
U
SC
RI
PT
Figures
A
CC
EP
TE
D
Fig. 1
Fig. 2
N
U
SC
RI
PT
Fig. 3
A
CC
EP
TE
D
M
A
Fig. 4
Fig. 5
D
M
A
N
U
SC
RI
PT
Fig. 6
A
CC
EP
TE
Fig. 7
Fig. 8
TE
EP
CC
A
PT
RI
SC
U
N
A
M
Fig. 10
D
Fig. 9
S0040-6031(14)00536-X http://dx.doi.org/doi:10.1016/j.tca.2014.11.020 TCA 77077
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
24-8-2014 17-11-2014 20-11-2014
Please cite this article as: Taher Ghrib, Structural, Optical and thermal properties of nanoporous aluminum, Thermochimica Acta http://dx.doi.org/10.1016/j.tca.2014.11.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural, Optical and thermal properties of nanoporous aluminum filled with graphite and silicon Taher Ghrib1,2,*[email protected] 1
Laboratory of Physical Alloys (LPA), Science Faculty of Dammam, Dammam University, Saudi Arabia.
2
Photovoltaic Laboratory, Research and Technology Centre of Energy, Borj-Cedria Science and Technology
Park, BP 95, 2050 Hammam-Lif, Tunisia
Highlights
-
A simple electrochemical technique is presented and used to manufacture a porous
-
PT
aluminum layer. Manufactured pores of 40 nm diameter and 200 nm depth are filled by nanocrystal of
-
RI
silicon and graphite.
Dimensions of pores increase with the anodization current which ameliorate the
A new thermal method is presented which permit to determine the pores density and
U
-
SC
optical and thermal properties.
the layer thickness.
All properties show that the manufactured material can be used with success in solar
N
-
A
cells.
M
Abstract
D
In this work the structural, thermal and optical properties of porous aluminum thin
TE
film formed with various intensities of anodization current in sulfuric acid are highlighted. The obtained pores at the surface are filled by sprayed graphite and nanocrystalline silicon
EP
(nc-Si) thin films deposited by plasma enhancement chemical vapor deposition (PECVD) which the role is to improve its optical and thermal absorption giving a structure of an
CC
assembly of three different media such as deposited thin layer (graphite or silicon)/(Porous aluminum layer filled with the deposited layer)/(Al sample).
A
The effect of anodization current on the microstructure of porous aluminium and the
effect of the deposited layer were systematically studied by atomic force microscopy (AFM), transmission electron microscopy (TEM) and Raman spectroscopy. The thermal properties such as the thermal conductivity (K) and thermal diffusivity (D) are determined by the photothermal deflection (PTD) technique which is a non destructive technique. Based on this
full characterization, it is demonstrated that the thermal and optical characteristics of this films are directly correlated to their micro-structural properties.
Keywords Porous Aluminum, optical reflectivity, thermal conductivity
1. Introduction In view of their practical importance, aluminum porous materials were intensively
PT
studied in the last decades [1-11] due to their interesting electrical, optical and structural properties for the reason of its self organized pores that have quasi-cylindrical shape which
RI
constitute a technological revolution and find now many applications such as filtration
SC
membranes [12,13], as backing of fabrication nanowires [14-16], as photonic crystals [17,18], as humidity sensors [19,20], or as cathodes for organic light emitting diodes[21,22]. In the
U
literature many attempts are reported in order to find relationships between porosity, thermal properties [23,24], optical properties [11, 25,26], and this by exploring the effect of size, form
N
and density of the pores where whose fine tuning and its filling by various types of materials
A
gives cells for energy applications such as solar cells and heating panels. Method most
M
common, used in this last period to produce nanopores is the electrochemical in which the anodization current at a significant effect on the physical and structural properties of the
D
porous thin layer produced.
TE
The main purpose of this work is to present a simple method for producing thin porous aluminium layer by electrochemical anodization technique, which will be filled by two
EP
different elements such as crystalline graphite and silicon and try to study the anodization current effect in the microstructure of the porous aluminium layer (PAL) and the effect of the
CC
deposition element on its optical properties such as total reflectivity and thermal conductivity such as thermal conductivity and thermal diffusivity which are determined by the
A
photothermal deflection (PTD) technique [27-29] which the principle will be recalled. 2. Experiments processing High-purity aluminum foil (99.997%), 0.25 mm thick was used as a starting material. Before anodization the aluminum paper was rinsed in a CP4 solution which is a mixture of 64% of nitric acid, 20% acetic acid CH3COOH and 16% of fluorhydric acid to eliminate the impurities linked to the surface. The cleaned samples were treated by a mechanical polishing
with polishing machine model 920 working with different speed combined with a alumina abrasive in an alkaline solution and anodized during 25 min in a solution of diluted sulfuric acid with 66.66% H2SO4 and 33, 33% H2O2 by weight at room temperature and with three different anodization currents (250 mA ,300 mA , 350mA). On the PAL surface, c-Si was deposited by PECVD technique [12] at 50 °C using a gas mixture of silane and H2 at a total pressure of 0.5 m Torr. And also another set of three samples was sprayed for the same pressure and speed by a thin commercial graphite layer which contains a high level of pure and fine graphite powder. The surface morphology of the films was examined by AFM and TEM microscopy. Samples preparation for TEM analysis was performed in a standard way.
PT
The average pore diameter was carried out using an ionic milling of type (Dual Ion Mill Model 600).
RI
The thermal properties such as the thermal conductivity and the thermal properties are determined by the PTD technique [14,15]. This method (fig. 1) consists in heating a sample
SC
with a modulated light beam of intensity I I 0 (1 cos t) that will be absorbed on the
U
surface and generates a thermal wave. This thermal wave will propagate in the sample and in the surrounding fluid (air in our case) and will induce a temperature gradient and then a
N
refractive index gradient in the fluid. The fluid index gradient will cause the deflection of a
A
probe laser beam skimming the sample surface. This deflection may be related to the thermal
M
properties of the sample. The sample is heated by a halogen lamp light of Power 100W modulated by using a mechanical chopper at a variable frequency. An (He-Ne) laser beam
D
skimming the sample surface at a distance z, is deflected. This deflection can be detected by a
TE
four quadrant photo-detector and converted to an electrical signal which is measured by a lock-in amplifier. Through the intermediary of interfaces, the mechanical chopper and the
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Look-in amplifier a microcomputer will set the desired modulation frequency and read the values of the amplitude and phase of the photo-thermal signal and then draw their variations
A
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according to the square root modulation frequency.
3. Results and discussions
3.1. Microstructural characterization Fig. 2 shows AFM microscopy of these samples treated with three different anodization current intensities, we notice the presence of a quasi-arranged structure which contains pores with width vary according to the current from 24 nm for Ia=250mA to 73 nm
for Ia=350mA and with depth whose value are given in Table 1 which also gives the porosity percentage according to the anodization current, where it is shown that it increases with the anodization current of 42% to 51%. Fig. 3 shows plan view TEM of the PAL surface with various anodization current in which it is noted for all micrographs that they exhibit pores which appear almost arranged with diameters which increase with the current intensity from 38 to 46 nm whose cells seem to be in an approximately hexagonal structure. The deposition of the silicon thin layer on these prepared samples gave samples whose AFM images are shown in fig. 4 which present a nano-granular aspect with size estimated
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(using WS×M 4.0 software) varying from 5 to 20 nm as (Ia) increases, which can be named nanocrystallites (nc). In order to confirm the above mentioned results in term crystallinity and
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grain size a Raman spectroscopy was performed and presented in fig. 5 giving the Raman spectra for the porous aluminum filled by nc-silicon film versus various anodization current.
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By comparing their spectra with that of the film deposited directly on the untreated aluminum substrate, main feature of nc-Si Raman spectra is the transverse optical (TO) phonon mode
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located in the range from 500–530 cm–1, in which we remark that the peak intensity increases
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and its position shifts to low energies (redshift) with the anodization currents tending towards
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a peak corresponding to monocrystalline Si (100) at 529 cm-1.
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The crystallite size was calculated using the following formula [30].
a w A D
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Where w w cSi w n cSi and “D” are the shift of Raman peak position of the nanocrystallite compared to that of the monocrystalline silicon. "a " is the lattice constant of the silicon
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which is equal to 0.543nm and A=47.41 and γ =1.44 are constants. The crystallite size
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estimated from this method was found between 8 –20 nm when Ia varies from 250 to 350mA which are close to the values found by AFM.
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3.2. Thermal characterization The modification of the structure and composition of sample influences the physical properties for this reason we opts to study the thermal properties variation with the anodization current by using the (PTD) technique . In the case of a uniform heating we can
use a 1-dimensional approximation, and the amplitude and phase
of the probe beam
deflection is given by: x
2 L dn T0 e f n f dTf
(1)
and
x 5 f 4
(2)
Where l is the width of the pump beam in the direction of the probe laser beam, n and f are respectively the refractive index and the thermal diffusion length and T f is the temperature of the fluid, T0 and θ are respectively the amplitude and phase [28,29] of the temperature at the
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sample surface which are function of the thermal properties of the different media. x is the
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distance between the probe beam axe and the sample surface.
Before the calculation of the probe beam deflection, one must know the expression of
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the surface temperature T0 which is calculated by writing the heat flow and temperature continuity at the interfaces x1 l 2 , x2 l 2 l1 , x 3 l 2 l1 l s between the various
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layers constituting the sample (fig. 6), which gives the following expression: [29]
(3)
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T0 ((1 b)4es ls (1 b)2es ls )E3 (r1 b) es ls E1 (1 b)1 es ls (1 b)3es ls
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Where 1 , 2 , 3 , 4 , b , 1 , r1 , E1 and E 3 are real or complex parameters, which
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depend on the optical and thermal properties of different layers.
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In order to determine the thermal properties evolutions with the anodization current and the deposited layer a study of the photothermal signal with the square root modulation
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frequency is carried out for different samples. The experimental variations of the amplitude and phase of photothermal signal are given respectively in fig. 7 and fig. 8 for the porous
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aluminum respectively filled by graphite layer and nanocrystallite-silicon layer. We use the model of three layers such as (deposited thin layer) /(Porous aluminum layer filled with the
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deposited element)/(Al sample). We begin in a first step by introducing thermal properties of the deposited layer which are ( K 2 0.35W . K 1.m 1 , D2 1,45104 m2 . s 1 ) for the graphite and
( K 2 4.8 W . K 1 .m 1 , D2 0.19 m 2 . s 1 ) for the silicon which are amorphous layers where behave as feeble thermal conductors. The best fit between theoretical and experimental curves give theoretical values of the pair ( K1 , D1 ) of the porous layer filled with graphite and silicon whose values are given in Table 2.
Physically, the thermal conductivity and thermal diffusivity are defined respectively as the ability to conduct heat and the propagation speed of heat in the material, we can say that introduction of thermal isolator material such as graphite or amorphous silicon in porous aluminum moves it thermal properties toward those of semiconductor. By using obtained thermal conductivities of the deposited layer K 2 , of the porous layer filled with graphite or silicon K1 and of the pure aluminum we can calculate the percentage of pores in the treated sample by using the relationship (4), whose values are given in Table 2 where we remark its increase with anodization current.
K1 K Al ( 4) K 2 K Al
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We remark that its values are in good agreement by using the results of silicon or graphite deposition; since these pores are specific to the treated aluminum and do not depend
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on the filling element and also in good agreement with the results obtained by AFM
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technique.
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3.3. Optical characterization
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These materials can be used in solar cells and heat exchangers then they must have
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optical properties that are suitable with its areas, for this reason we study the total optical reflectivity versus wavelength, which are shown in fig. 9 and fig. 10 respectively for samples
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with graphite deposited layer and with silicon deposited layer.
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In figure 8 the measured reflection coefficient versus le incident wavelength of the storage, we remark oscillatory form whose amplitude increases with the wavelength from the
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visible to the near infrared wave and disappears for a length of around 2 m , these oscillations may be due to interference between the waves reflected at the interfaces of
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different nanolayers which are graphite layer-porous layer filled with graphite and porous layer filled with graphite-Aluminum sample. In others hand if we measure the mean distance
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between two successive peaks, we note that are in order of 200 nm which is somehow; the thickness of the porous layer and this implies that the interference is due to the interference of rays reflected on the two surfaces of the porous layer having a thickness d but the external graphite layer whose thickness is equal to 2 m does not affect the interference but will have a significant effect on the reflected amplitude. The total reflection coefficient r can be calculated using the Fresnel equations as, [31]
r21 r1s e 2 i r (5) 1 r21 r1s e2 i Where r21 and r1s are the reflection coefficients corresponding to the interface between the porous layer (1) and graphite (2) , and between porous layer (2) and substrate (s) respectively,
l1 is the thickness porous layer. Also is the phase difference between the reflected lights in these two interfaces, and is the incident wavelength received at the input face of the porous layer whose expression is:
4 n ( ) l1 cos (6)
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reflection angle and finally the total reflectivity is given as:
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2
R r 100
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Where n ( ) is the refractive index that can be varied with wavelength and is mean
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The overall decrease in the total reflectivity with the anodization current intensity can be related to the pores depth, in fact more they are deep more the internal reflections are multiple
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and which will increase the absorption and reduce the reflection, where we note that the
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elevation of anodization current from 250mA to 350mA saves about 20% in the reflected
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energy.
For the aluminum porous aluminum filled with crystallite silicon we note a rapid
D
increase in reflectivity to a saturation value which decreases with the anodization current with
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infrared waves.
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oscillations due to interferences between reflected waves that are not very clear for near
4. Conclusion
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In this work, we have presented a method for producing nanoporous thin layer by
electrochemical anodization technique with different anodizations currents. The obtained
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pores having 40 nm diameter and 200 nm depth are filled by nanocrystal of silicon and graphite. Structural, optical and thermal properties of the obtained structure were highlighted and it was shown that the increase in the anodization current increases the pores density, decrease its total optical reflectivity and moderate its thermal properties. The thermal properties are determined using the photothermal deflection technique which can be related to
the pores density in the obtained porous aluminum layer. The obtained material can be used in the fields of energy production and storage.
Acknowledgements This work was supported by the Scientific Research Deanship of University, under Project no. 2014264.
Dammam
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Figures and Tables
Fig. 1: Experimental set-up used for PTD investigation: 1-Table of micrometric displacement, 2-Sample, 3Photodetector position, 4-Fixed laser source, 5-Halogen lamp, 6-Look-in amplifier, 7-Mechanical chopper, 8Computer.
Fig. 2: AFM image of Porous Aluminum layer (PAL) versus anodization current intensity (250,300,350) mA.
Fig. 3: TEM micrographs of porous aluminum fabricated with various anodization current intensities (250,300,350) mA.
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Fig. 4: AFM image of Porous Aluminum layer (PAL) filled with nanocrystalline silicon for various anodization current intensities (250,300,350) mA.
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Fig. 5: Raman spectra of PAL filled by crystalline silicon prepared with different anodization current intensities (250, 300, 350) mA
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Fig. 6: Schema of the stacked three layers
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Fig. 7: Experimental and theoretical of the photothermal signals of the porous aluminum filled with the graphite for different ano
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Fig. 8: Experimental and theoretical of the photothermal signals of the porous aluminum filled with silicon for different anodizatio
Fig.9 Total reflectivity of the porous aluminum filled with graphite for different anodization current intensities
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(250, 300, 350) mA.
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(250, 300, 350) mA.
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Fig. 10: Total reflectivity of the porous aluminum filled with silicon for different anodization current intensities
Table 1: Thickness and porous density of nanoporous aluminum layer for different anodization current intensities (250,300,350) mA.
Ia (mA)
l1 (nm)
P (%)
250
212.03± 3.80
41.79± 1.23
300
233.12± 4.66
47.71± 1.41
350
254.12± 4.66
51.12± 4.66
Table 2: Measured thermal properties and calculated porous density of the porous aluminum filled with silicon for different anodization current intensities Ia (250,300,350) mA. Porous aluminum filled with graphite
Porous aluminum filled with silicon
Ia
250mA
300mA
350mA
250mA
300mA
350mA
K1(W.m-1.K-1)
158
142
131
162
144.3
133.6
D1(cm .s )
0.59
0.46
0.39
0.63
0.42
0.38
(%)
43.1%
48.0%
52.1%
41.4%
48.0%
52.0%
2
-1
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A
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Figures
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Fig. 1
Fig. 2
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Fig. 3
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Fig. 4
Fig. 5
D
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A
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Fig. 6
A
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EP
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Fig. 7
Fig. 8
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EP
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A
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Fig. 10
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Fig. 9