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G. KATUMBA1, G. MAKIWA1, L OLUMEKOR1 & A FORBES2
1Physics Department, University of Zimbabwe, Harare, Zimbabwe2CSIR-National Laser Centre, Pretoria 0001, South Africa
SAIP 2006
SOLAR SELECTIVE ABSORBER FUNCTIONALITY OF CARBON
NANOPARTICLES EMBEDDED IN SiO2, ZnO and NiO MATRICES
Concept and motivation
0
200
400
600
800
1000
1200
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1
1 10
Pow
er d
ensi
ty (W
/m2 µµ µµm
)
Idea
l abs
orbe
r re
flec
tanc
e
Wavelength (µµµµm)
Solar (AM1.5)
Blackbody
Ideal selective absorber
100oC
200oC
300oC
Electromagnetic wave propagation
� Interaction of light with matter.� Complex refractive index.
� Magnetic field is not altered in optical and IR.
� Logarithmic reduction in intensity for a film thickness d is ln (I/I 0) = -αd = -4πkd/λ.
( ) ( )[ ]c/xntiexpc/xkexpEEm ωωω −−= 0
Tailoring materials� Factor kd/λ determines the efficiency of
a solar absorber surface.
� Proper combination of k and d.
� Homogeneous films – can only vary d.
� Composite films – change k! Practically constant for most materials used.
� Can also tune n by changing porosity.
�Therefore can only tune d to place the crossover at an appropriate wavelength.
�Reflectance and efficiency of absorber surface depends on k and d.
�Near-normal reflectance of bulk material is: 2
1
1
N
NR
+−=
Tailoring materials
Device structure
�Many designs are possible.
�A tandem device: a composite layer deposited on a metallic substrate.
Composite layer
Aluminium substrate
Effective medium approximations
�Bruggeman:
�Maxwell-Garnett:
2N=ε
( ) 02
12
=+−−+
+−
Brb
Brba
Bra
Braa ff
εεεε
εεεε
( )( )baaba
baababMG f
f
εεεεεεεεεε
−−+−++=
22
22
Theoretical optimisation
Vary fill factor, f.
EMAs – Brugeman andMaxwell-Garnett.
Variable fill factor, f.
MULTILAYER PROGRAM.Calculate theoretical reflectance, R.Variable fill factor, f and thickness, t.
Reflectance data treatment.Calculate theoretical solar absorptance, α
and thermal emittance, ε.
OPTIMUM?(qualitative)
END
Vary thickness, t.
Get n & k data.
No
Yes
No
Theoretical reflectance
�Bruggeman �Maxwell-Garnett
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tBR
=0.20 um
tBR
=0.50 um
tBR
=1.00 um
Wavelength (µµµµm)
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tMG
=0.20 um
tMG
=0.50 um
tMG
=1.00 um
R
Wavelength (µµµµm)
Experimental procedure
�Sol production �Tube furnace
Experimental procedure�Carbonization �Filtration of nitrogen
gas
Optical characterisation
�Varian Cary 500
�UV-VIS-NIR
�Bomem DA8
�NIR-IR
Non-optical characterisation
�SEM: Philips XL30
Surface mophology
�XRD: Philips PW1840 Diffractometer
Cu Kα, 35 keV, 30 mA
Crystal structure
Experimental results: SiO2 samples�Spin-coating speed�Carbon precursor
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2000 rpm (9:6)3000 rpm (9:6)4000 rpm (9:6)4000 rpm (9:12)5000 rpm (9:12)
R
Wavelength (µµµµm)
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6 g SUC9 g SUC11 g SUC12 g SUC
R
Wavelength (µµµµm)
Experimental results: IR spectrum
�FTIR reflectance spectrum
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1
5001000150020002500300035004000
R
Wavenumber (cm-1)
Si - O - Si
OHOH
Shoulder
Experimental results: defects
�Problem- cracked films
�Solutions to cracking
�MTES & Ac2O
Experimental results: defects
Experimental results: spectra
�Addition of MTES �Addition of Ac2O
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10 wt.% A c2O
15 wt.% A c2O
20 wt.% A c2O
15 wt.% A c2O (9:11)
0 wt.% A c2O
R
W avelength (µµµµm)
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10 wt.% MTES
20 wt.% MTES
30 wt.% MTES
0 wt.% MTES
R
Wavelength (µµµµm)
Experimental results: X-HRTEM� (a) TEOS only
� (b) TEOS + Ac2O
� (c) TEOS + MTES
� Uniform distribution
� Segregation7 nm
7 nm
a
bc
Experimental results: EELS� EELS mapping
� Carbon K peak
7 nm
7 nm
a
b
� (a) TEOS only
� (b) TEOS + Ac2O
� (c) TEOS + MTES
c
Experimental results: ZnO samples
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ZnO (A)
ZnO (B)
ZnO (C)
ZnO (D)
ZnO (F)
Ref
lect
ance
Wavelength (µµµµm)
Experimental results: NiO samples
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NiO (A)
NiO (B)NiO (C)NiO (D)
NiO (E)NiO (F)
NiO (G)NiO (H)
Ref
lect
ance
Wavelength (µµµµm)
XRD results: NiO and ZnO
Experimental results: Comparison
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1 10
NiO (E)ZnO (C)SOLKOTESiO
2
Ref
lect
ance
Wavelength (µµµµm)
Experimental results: XRD
2 0 3 0 4 0 5 0 6 0 7 0 8 0
2 θ ( ο )
Al
( 2 0 0 )( 1 0 4 )
ZnO
(A
) ( 1 1 0 )
( 1 1 0 )
ZnO
(B
)
( 1 1 0 )
ZnO
(C
)
( 1 1 0 )
NiO
(A
)
( 2 0 0 )
NiO
(B
)
Conclusions�∃ a possibility to produce low cost
selective solar absorbers with sol-gel technique.
�Addition of 15 wt.% Ac2O appeared to solve the problem of cracking in SiO2
samples better than 20 wt.% MTES.
Conclusions�New and interesting microstructure of the
sol-gel derived samples have been revealed:
A short chain-like structure of both a silica matrix and carbon nanoparticles is quite evident.
�Homogeneity of the coatings at nano-scale is very encouraging.
Conclusions
8.940.100.93C-NiO
6.290.140.89C-ZnO
2.900.310.90C-SiO2
α/εεαSample
Acknowledgement
�University of Zimbabwe.
�Uppsala University.
�NMMU.
�Defence Peace Safety & Security, CSIR.
�Rental pool.
�African Laser Centre.